Green Sustainable Process for Chemical and Environmental Engineering and Science: Carbon Dioxide Capture and Utilization 0323994296, 9780323994293

Table of contents :
Cover
Half title
Title
Copyright
Contents
Contributors
Chapter 1 Carbon dioxide capture and its utilization towards efficient biofuels production
1.1 Introduction
1.2 Utilization of captured carbon dioxide for biofuel production
1.2.1 Photosynthesis and photo oxidation of water
1.2.2 Bio-sequestration of CO2
1.3 Conclusion and future perspectives
References
Chapter 2 Deep eutectic liquids for carbon capturing and fixation
2.1 Carbon dioxide emissions
2.2 Deep eutectic liquids
2.3 Types of deep eutectic liquids
2.4 Preparation of DELs
2.5 Authentication of DELs
2.6 DEL based CO2 absorption
2.7 Carbon capture efficiency of various HBDs
2.7.1 Urea
2.7.2 Glycerol
2.7.3 Glycerol^^c2^^a0 + ^^c2^^a0L-arginine
2.7.4 Natural organic acids
2.7.5 Dihydric alcohols
2.7.6 Amines
2.7.7 Levulinic acid
2.7.8 Guaiacol
2.7.9 Azoles
2.7.10 Miscellaneous HBD
2.8 CO2 absorption in aqueous solution of DELs
2.9 CO2 absorption in ternary DELs
2.9.1 Alkanolamines
2.9.2 Superbases
2.9.3 Hybrid
2.10 Ammonium-Based DELs
2.10.1 Carboxylic acids
2.11 Phosphonium based DELs
2.12 Azole based DELs
2.13 Bio-phenol derived superbase based DELs
2.14 Hydrophobic DELs
2.15 Non-ionic DELs
2.16 DEL supported membranes
2.17 DELs with multiple sites interaction
2.18 Conclusion and future prospects
Acknowledgment
References
Chapter 3 Cookstoves for biochar production and carbon capture
3.1 Introduction
3.2 Cookstoves designed for biochar production
3.2.1 Top-lit updraft \(TLUD\) stove
3.2.2 Development of TLUD-Akha architecture design
3.2.3 Origins of TLUD-Biochar ^^e2^^80^^98Ecosystem^^e2^^80^^99
3.2.4 Composition of biochar produced from biochar cookstoves
3.2.5 Rural women in carbon capture
3.3 Biochar production and climate-change implications
3.3.1 Biochars and their applications for carbon capture and others
3.3.2 Challenges of biochar cookstoves in rural developing countries
3.4 Conclusion
References
Chapter 4 Metal support interaction for electrochemical valorization of CO2
4.1 Introduction
4.2 Metal supports for ECR of CO2
4.2.1 Carbon and graphene-based support systems
4.2.2 Titanium nanotubes
4.2.3 Foam electrode
4.2.4 Mesoporous electrode
4.2.5 Hydrogel and aerogel
4.2.6 Gas diffusion electrode
4.3 Conclusion
Acknowledgment
References
Chapter 5 Utilization of carbon dioxide as a building block in synthesis of active pharmaceutical ingredients
5.1 Introduction
5.2 NNucleophile-triggered CO2-incorporated carboxylation to form C^^e2^^80^^93N bonds
5.2.1 Synthesis of carisoprodol
5.2.2 Synthesis of felbamate
5.2.3 Synthesis of furaltadone
5.2.4 Synthesis of oxadiazon
5.2.5 Synthesis of oxazolidinone
5.2.6 Synthesis of toloxatone
5.2.7 Synthesis of doxazosin, bunazosin, and prazosin
5.2.8 Synthesis of zenarestat and KF-31327
5.2.9 Synthesis of tipifarnib
5.2.10 Synthesis of MAO-B inhibitor
5.2.11 Synthesis of URB602
5.2.12 Synthesis of alpha-alanine
5.3 NNucleophile-triggered CO2-incorporated methylation to form C^^e2^^80^^93N bonds
5.3.1 Synthesis of butenafine
5.3.2 Synthesis of methylephedrine
5.3.3 Synthesis of naftifine
5.4 ONucleophile-triggered CO2-incorporated carboxylation to form C^^e2^^80^^93O bonds
5.4.1 Synthesis of atorvastatin
5.5 CO2-catalyzed oxidation of alcohols to form C^^e2^^80^^93O bonds
5.5.1 Synthesis of DMU-212 and combretastatin A-4
5.6 C-Nucleophile-triggered CO2-incorporated reductive carboxylation to form C^^e2^^80^^93C bonds
5.6.1 Synthesis of methionine hydroxy analog
5.6.2 Synthesis of naproxen
5.7 C-nucleophile-triggered CO2-incorporated direct C^^e2^^80^^93H carboxylation to form C^^e2^^80^^93C bond
5.7.1 Synthesis of aspirin
5.7.2 Synthesis of 4-aminosalicylic acid
5.7.3 Synthesis of diflunisal
5.7.4 Synthesis of gentisic acid
5.8 C-nucleophile-triggered CO2-incorporated organozinc-mediated carboxylation to form C^^e2^^80^^93C bonds
5.8.1 Synthesis of tamoxifen
5.8.2 Synthesis of \(E\)^^e2^^88^^923-Benzylidene-2-indolinone
5.8.3 Synthesis of ibuprofen
5.9 C-nucleophile-triggered CO2-incorporated organolithium-mediated carboxylation to form a C^^e2^^80^^93C bond
5.9.1 Synthesis of repaglinide
5.9.2 Synthesis of flurbiprofen
5.9.3 Synthesis of epristeride
5.9.4 Synthesis of mefloquine
5.9.5 Synthesis of amitriptyline
5.9.6 Synthesis of methantheline bromide
5.9.7 Synthesis of garenoxacin
5.9.8 Synthesis of englitazone
5.10 C-Nucleophile-triggered CO2-incorporated organomagnesium-mediated carboxylation to form a C^^e2^^80^^93C bond
5.10.1 Synthesis of enadoline
5.10.2 Synthesis of loxoprofen
5.10.3 Synthesis of lamotrigine
5.10.4 Synthesis of felbinac
5.10.5 Synthesis of spironolactone
5.10.6 Synthesis of finafloxacin
5.11 Conclusion
References
CHAPTER 6 Electrochemical Carbon Dioxide Detection
6.1 Introduction
6.2 Capture technologies of CO2
6.2.1 Adsorption
6.2.2 Absorption
6.2.3 Separation by membranes
6.2.4 Chemical capture
6.2.5 CO2 sensors
6.3 Fundamentals of electrochemistry
6.3.1 Voltammetry
6.3.2 Potentiometric methods
6.4 Direct potentiometric methods
6.4.1 Potentiometric titrations
6.4.2 Amperometric methods
6.4.3 Conductometric methods
6.4.4 Coulometric analysis methods
6.4.5 Electrodes
6.4.6 Reference electrode
6.4.7 Auxiliary electrode
6.4.8 Potentiometric electrodes
6.4.9 Indicator electrodes
6.4.10 Electrochemical gas sensors
6.4.11 Potentiometric gas sensors
6.4.12 Electrochemical applications
6.5 Summary and conclusion
References
Chapter 7 Carbon dioxide injection for enhanced oil recovery and underground storage to reduce greenhouse gas
7.1 Introduction
7.1.1 Global carbon management concerns
7.1.2 CO2 availability
7.1.3 Options available for CO2 storage
7.1.4 Comparison of available storage methods
7.2 Oil recovery using CO2
7.2.1 Hydrocarbon miscibility
7.2.2 CO2 miscible injection method
7.2.3 Injection and storage facilities required
7.2.4 Storage capacity calculations
7.2.5 Impact on economics and tax incentives
7.3 Underground storage of CO2 in unconventional reservoirs
7.4 Current status, challenges and future directions
7.5 Conclusions
Acknowledgment
References
Chapter 8 Ionic liquids as potential materials for carbon dioxide capture and utilization
8.1 Introduction
8.2 Types of ILs
8.2.1 Conventional ionic liquids \(CILs\)
8.2.2 Functionalized ionic liquids \(FILs\)
8.2.3 Reversible ionic liquids \(RILs\)
8.2.4 Polymeric ionic liquids \(PILs\)
8.2.5 Supported ionic liquids \(SILs\)
8.2.6 Magnetic ionic liquids \(MILs\)
8.2.7 Task specific ionic liquids \(TSILs\)
8.2.8 Multiphasic ionic liquids \(MILs\)
8.2.9 Switchable polarity ionic liquids \(S-Polymeric ionic liquids\)
8.2.10 Thermoregulated ionic liquids \(TRILs\)
8.2.11 Ionic liquids gel
8.3 Future applications of IL and GR-based IL
8.4 Conclusion
References
Chapter 9 Recent advances in carbon dioxide utilization as renewable energy
9.1 Introduction
9.2 CO2 utilization technologies
9.2.1 Mineralization
9.2.2 Beverage and food processing
9.2.3 Biological utilization
9.2.4 Oil recovery enhancement, coal bed methane and fracking of CO2
9.2.5 Fuels and chemicals
9.2.6 Principal and favorable utilization technologies
9.3 Developments in worldwide CO2 utilization projects
9.3.1 United states
9.3.2 China
9.3.3 Germany
9.3.4 Australia
9.4 Market scale and value
9.5 Regulation and policy
9.6 Conclusion and future prospects
References
Chapter 10 Metal Organic Frameworks as an Efficient Method for Carbon dioxide capture
10.1 Introduction
10.2 Metal organic framework \(MOF\)
10.2.1 Conventional synthesis route
10.2.2 Microwave synthesis technique
10.2.3 Sonochemical synthesis
10.2.4 Mechanochemical synthesis
10.2.5 Electrochemical synthesis
10.3 Synthesis of some MOFS
10.4 Properties of MOFs
10.4.1 Chemical and thermal
10.4.2 Mechanical
10.4.3 Thermal conductivity
10.5 CO2 capture using MOF
10.6 Adsorption of carbon dioxide in metal organic frameworks
10.7 Methods to enhance CO2 adsorption
10.8 Methods to enhance MOF stability
10.8.1 Chemical stabilities
10.8.2 Thermal stabilities
10.8.3 Mechanical stability
10.9 Conclusion
References
Chapter 11 Industrial carbon dioxide capture and utilization
11.1 Introduction
11.1.1 Commercial capturing processes of carbon dioxide gas
11.2 CO2 collection systems based on liquid
11.2.1 Amine-type liquid solvents for capturing CO2 gas
11.2.2 Basic working principle of absorbents based on liquid amines
11.2.3 Advances in amine-type liquid absorbent materials
11.2.4 Mixtures of amine solvents
11.2.5 Overview and prospects for liquid amine-based absorbents
11.3 CO2 capturing with ionic liquid solvents
11.3.1 Working principle of ionic liquid-based absorbents
11.3.2 Advancement in ionic solvents
11.3.3 Overview and prospects of ionic liquid-based solvents
11.4 Applications, implementation and challenges
11.5 Solid CO2 adsorbents for low-temperature applications
11.5.1 Impact of impurities
11.5.2 Solid amine-based adsorbents: introduction and future prospects
11.6 Carbon adsorbents
11.6.1 Tuning of carbon textural properties
11.6.2 Carbon surfaces with chemical modification
11.6.3 Carbon-based hybrid composites fabrication
11.7 Zeolite adsorbents
11.7.1 Adaptations through cation exchange
11.7.2 Amine impregnation
11.7.3 Fabrication of zeolite-based hybrid materials
11.7.4 Overview and prospects for zeolite-based adsorbents
11.8 Adsorbents of the MOF \(metal^^e2^^80^^93organic framework\) type
11.8.1 Functional component integration
11.8.2 Regulation of intrinsic properties
11.8.3 Overview and prospects for MOF-based adsorbents
11.9 Adsorbents predicated on carbonate-based alkalis
11.9.1 Post-combustion applications, difficulties and implementation
11.9.2 Solid CO2 adsorbents for intermediate temperature applications
11.10 Layered double hydroxides \(LDHs\)-based adsorbents
11.10.1 The influence of LDHs' chemical composition and manufacturing methods
11.11 Adsorbents made of magnesium oxide \(MgO\)
11.11.1 Mesoporous structure fabrication
11.11.2 Transformation of molten salts
11.11.3 Overview and prospects for MgO type adsorbent materials
11.12 Solid CO2 sorbents for high-temperature applications
11.12.1 Calcium oxide \(CaO\) sorbents
11.12.2 Improvements in CO2 collecting efficiency
11.12.3 Modifications in sintering-resistance
11.12.4 CaO generated from discarded materials
11.12.5 Granulation of powder
11.12.6 Overview and future prospects for CaO adsorbents
11.13 Pre-combustion applications, implementation and problems
11.14 The utilisation of CO2 in industrial processes
11.14.1 Conversion of CO2 to energy
11.14.2 Thermochemical method for CO2 methanation
11.14.3 The thermochemical method for dry CO2 and methane reforming
11.14.4 RWGS \(reverse water-to-gas shift\) reaction thermo -- chemical methodology
11.14.5 Methanol is produced by the thermochemical electrolysis of water of carbon dioxide
11.14.6 hydrogenation of CO2 to hydrocarbons through a thermochemical process
11.14.7 Carbon dioxide \(CO2\) photochemical conversion
11.14.8 Photocatalytic CO2 reduction perspectives and prospects
11.14.9 A sorting oxidant: CO2
11.5 Conclusions and prospects
References
Chapter 12 Ionic liquids for carbon capturing and storage
12.1 Introduction
12.2 CO2 capture technologies
12.3 Ionic liquids \(ILs\)
12.4 Features of ILs
12.5 IL as absorbents for CO2 capture
12.5.1 Conventional ionic liquids
12.5.2 ILs based hybridized solvents
12.6 IL hybrids as adsorbents for CO2 capture
12.7 IL hybrids with membranes for CO2 capture
12.8 Ionic liquid supported membrane
12.9 Poly ILs membrane
12.10 Composite membranes
12.11 Conclusion and future insights
References
Chapter 13 Advances in utilization of carbon-dioxide for food preservation and storage
13.1 Introduction
13.2 Utilization of carbon-dioxide in food preservation
13.2.1 Beverage drink preservation
13.2.2 Drying of vegetables and fruits
13.2.3 Food preservation using dry ice
13.2.4 Animal stunning procedure
13.2.5 Tanning of animal skin
13.3 Utilization of carbon-dioxide in food storage
13.3.1 Control of storage microsphere
13.3.2 Storage equipment disinfection
13.4 Prospects and conclusion
References
Chapter 14 An insight into the recent developments in membrane-based carbon dioxide capture and utilization
14.1 Introduction
14.2 Carbon dioxide capture technologies
14.3 A brief about membrane technology
14.4 CO2 separation using membranes
14.4.1 Pre-combustion CO2 capture using membranes
14.4.2 Oxy-fuel combustion CO2 capture using membranes
14.4.3 Post-combustion CO2 capture using membranes
14.4.4 Future considerations for membrane-based CO2 capture
14.5 CO2 utilization using membranes
14.6 Conclusions
References
Chapter 15 Carbon dioxide to fuel using solar energy
15.1 Introduction
15.2 CO2 reduction onto semiconductor surface
15.3 Major bottleneck for CO2 reduction
15.4 Different types of photo catalyst
15.4.1 Homogeneous photo-catalysts
15.4.2 Cu based photo-catalysts
15.5 Reduction of CO2 to methanol using Cu2O as photo catalyst
15.6 Reduction of CO2 to methanol using Cu2O as electro catalyst
15.6.1 Reduced graphene-oxide, Cu2O and amine compounds composite photo catalysts for CO2 reduction
15.7 Benefits of using RGOin the composite catalyst
15.8 Conclusions
Acknowledgment
References
Chapter 16 Adsorbents for carbon capture
16.1 Introduction
16.2 Carbon capture processes
16.2.1 Pre-combustion carbon capture
16.2.2 Post-combustion carbon capture
16.3 Adsorbents for CO2 capture
16.3.1 Materials derived from biomass
16.3.2 Clays
16.3.3 Zeolites
16.3.4 Metal-organic frameworks \(MOFs\)
16.3.5 Covalent-organic frameworks \(COFs\)
16.4 Future perspective and conclusion
References
Chapter 17 Carbon dioxide capture and utilization in ionic liquids
17.1 Introduction
17.2 Capture of CO2 in ILs
17.2.1 Conventional ionic liquids
17.2.2 CO2 capture by functionalized ionic liquids
17.2.3 Capture CO2 by metal coordination-based \(chelate-based\) ionic liquids
17.2.4 CO2 capture by ILs based mixtures
17.2.5 Polyionic liquid membranes
17.2.6 CO2 captures by supported ionic liquid membranes
17.3 Electroreduction of CO2 in ILs
17.3.1 Electrochemical reduction of CO2 to CO
17.3.2 Electrochemical reduction of CO2 to HCOOH
17.3.3 Electroreduction of CO2 to CH3OH
17.3.4 Electrochemical reduction of CO2 to cyclic carbonate
17.3.5 Electrochemical reduction of CO2 to ketone compounds
17.3.6 Electroreduction of CO2 to urea
17.3.7 Electroreduction of CO2 to carbamate
17.3.8 Electroreduction of CO2 to amides and methylamines
17.3.9 Electrochemical reduction of CO2 to other compounds
17.4 Conclusions
Acknowledgments
References
Chapter 18 Hydrothermal carbonization of sewage sludge for carbon negative energy production
18.1 Introduction
18.2 Sludge as a potential source of alternate energy
18.3 Hydrothermal \(HT\) treatments for the production of fuel
18.3.1 Thermal hydrolysis
18.3.2 Hydrothermal carbonization
18.3.3 Hydrothermal liquefaction
18.3.4 Hydrothermal gasification \(HTG\)
18.4 Hydrothermal carbonization^^c2^^a0+^^c2^^a0gasification^^c2^^a0+^^c2^^a0ccs
18.5 Conclusion
Acknowledgement
References
Chapter 19 Utilization of supercritical CO2 for drying and production of starch and cellulose aerogels
19.1 Introduction
19.2 CO2 application -- Supercritical drying
19.2.1 How does supercritical drying work?
19.3 Starch aerogel and CO2 utilization
19.3.1 Starch specific aerogels
19.3.2 Hybrid starch aerogels
19.3.3 Mechanical properties of starch aerogels
19.3.4 Topology and morphology of starch aerogels
19.4 Cellulose aerogels and CO2 utilization
19.4.1 Cellulose specific aerogels
19.4.2 Cellulose aerogels as thermal insulators
19.4.3 Hybrid cellulose aerogels
19.5 Conclusions
Author contributions
Ethical approval
Declaration of competing interest
Acknowledgment
References
Chapter 20 Advances in carbon bio-sequestration
20.1 Introduction
20.2 Carbon sequestration methods
20.3 Limitations of carbon sequestration methods
20.4 Overview of biological sequestration \(Cycle/Mechanism\)
20.5 Bioresources for carbon bio-sequestration
20.6 Cyanobacteria
20.7 Microalgae
20.8 Plants
20.9 Bacteria
20.10 Nanomaterials in carbon sequestration
20.11 Future perspectives
20.12 Conclusion
References
Chapter 21 Photosynthetic cell factories, a new paradigm for carbon dioxide \(CO2\) valorization
21.1 Introduction
21.2 Carbon capture, utilization and storage mechanism
21.2.1 Pre-combustion capture
21.2.2 Post-combustion capture
21.2.3 Oxy-fuel combustion
21.2.4 Carbon capture by microalgae
21.3 Biological mechanism of carbon capture
21.4 Products from CCU
21.5 Challenges and opportunities
21.5.1 Pre-Combustion technology
21.5.2 Post-Combustion capture
21.5.3 Oxy-fuel combustion
21.5.4 Bio-carbon capture by microalgae
21.6 Future perspectives and conclusions
Funding information
References
Chapter 22 Carbon dioxide capture and sequestration technologies ^^e2^^80^^93 current perspective, challenges and prospects
22.1 Introduction
22.2 Carbon capture and sequestration \(CCS\) technologies
22.2.1 Carbon capture strategies
22.2.2 Carbon capture technologies
22.3 CO2 transportation, storage and opportunities/applications for CCS technologies
22.3.1 Transportation
22.3.2 Carbon storage
22.4 Current perspective and policies of CSS technologies in various countries throughout the world
22.4.1 Review of CCS policies
22.4.2 Artificial intelligence \(AI\) applications in carbon capture
22.5 Challenges and socio-economic implications of CCS technologies
22.5.1 Post-combustion capture challenges
22.5.2 Geologic storage challenges
22.5.3 Gasification challenges
22.5.4 Environmental impact of CCS technologies
22.5.5 Socio-economic impact of CCS technologies
22.6 Applications and opportunities for CCS techniques
22.6.1 Electricity power generation
22.6.2 Industrial application
22.6.3 Application of CCS techniques in CO2 capture from exhaust gases capture
22.6.4 Application of CCS techniques in CO2 capture from natural gas
22.7 Prospects and future work considerations for CCS approaches
22.8 Conclusion
References
Chapter 23 Microbial carbon dioxide fixation for the production of biopolymers
23.1 Introduction
23.2 Sources of CO2 emission
23.3 Sequestration methods of CO2
23.4 Carbon concentrating mechanisms
23.5 Advancements in carbon capture and storage & carbon capture utilization
23.6 Carbon dioxide fixation pathways
23.6.1 Calvin cycle
23.6.2 Reductive TCA cycle
23.6.3 Wood-Ljungdahl pathway
23.6.4 Dicarboxylate^^e2^^80^^914-hydroxybutyrate cycle
23.6.5 Malyl Co-A/3-hydroxypropionate pathway \(3-hydroxypropionate bicycle\)
23.6.6 Hydroxy propionate-hydroxybutyrate cycle
23.7 Factors affecting the carbon dioxide biofixation
23.8 Production of biopolymers/bioplastics
23.9 Conclusion
References
Chapter 24 Carbon dioxide capture and its enhanced utilization using microalgae
24.1 Introduction
24.2 Photosynthesis and CO2 fixation using microalgae
24.2.1 Photosynthesis
24.2.2 CO2 fixation
24.3 Cultivation systems for carbon dioxide capture by microalgae
24.3.1 Physico-chemical properties and carbon dioxide sources
24.3.2 CO2 capture prospects for microalgae cultivation
24.3.3 The impact of cultivation methods on biomass production
24.3.4 Microalgae culture system for CO2 capture
24.4 CO2 capture improvement strategies
24.4.1 CO2 capture can be improved by genetic engineering and metabolic changes
24.5 Conclusion
References
Chapter 25 Supported single-atom catalysts in carbon dioxide electrochemical activation and reduction
25.1 Introduction
25.2 CO2ERR products
25.3 Single-Atom catalysts efficiency descriptors
25.4 Single-Atom catalyst supports
25.4.1 Two-dimensional \(2D\) metal oxides
25.4.2 Two-dimensional \(2D\) metal chalcogenides
25.4.3 Metal carbides, nitrides \(MXenes\)
25.4.4 Metal-Organic frameworks
25.5 Mechanisms for CO2ERR on single-atom catalysts
25.6 Conclusion
References
Chapter 26 Organic matter and mineralogical acumens in CO2 sequestration
Abbreviations
26.1 Overview
26.2 Introduction
26.3 Geo-sequestration
26.4 Bio-sequestration
26.5 Mechanisms of carbon capture
26.5.1 Pre-combustion
26.5.2 Post-combustion
26.5.3 Oxyfuel combustion
26.6 Transport of carbon dioxide
26.7 Mechanism of carbon accommodation
26.8 Carbon dioxide sequestration in organic matter
26.8.1 Carbon dioxide sequestration in coal
26.8.2 Carbon dioxide sequestration in shale
26.9 Mineralogical acumen of carbon sequestration
26.9.1 An overview
26.9.2 Clay minerals
26.9.3 Swelling properties of clay minerals
26.9.4 Carbon protection capacity of clay minerals
26.9.5 Methods of organic carbon protection by clays
26.9.6 Adsorption of carbon dioxide on clays
26.9.7 Supercritical carbon dioxide sequestration in clays: an additional chronicle
26.9.8 Adverse influences of carbon dioxide sequestration in clays
26.10 A note on CO2 disposal in basalt formations
26.11 Summary
References
Index

Citation preview

GREEN SUSTAINABLE PROCESS FOR CHEMICAL AND ENVIRONMENTAL ENGINEERING AND SCIENCE

GREEN SUSTAINABLE PROCESS FOR CHEMICAL AND ENVIRONMENTAL ENGINEERING AND SCIENCE Carbon Dioxide Capture and Utilization Edited by

Inamuddin Department of Applied Chemistry, Zakir Husain College of Engineering and Technology, Faculty of Engineering and Technology, Aligarh Muslim University, Aligarh, Uttar Pradesh, India

Tariq Altalhi Department of Chemistry, College of Science, Taif University, Taif, Saudi Arabia

Elsevier Radarweg 29, PO Box 211, 1000 AE Amsterdam, Netherlands The Boulevard, Langford Lane, Kidlington, Oxford OX5 1GB, United Kingdom 50 Hampshire Street, 5th Floor, Cambridge, MA 02139, United States Copyright © 2023 Elsevier Inc. All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Details on how to seek permission, further information about the Publisher’s permissions policies and our arrangements with organizations such as the Copyright Clearance Center and the Copyright Licensing Agency, can be found at our website: www.elsevier.com/permissions. This book and the individual contributions contained in it are protected under copyright by the Publisher (other than as may be noted herein). Notices Knowledge and best practice in this field are constantly changing. As new research and experience broaden our understanding, changes in research methods, professional practices, or medical treatment may become necessary. Practitioners and researchers must always rely on their own experience and knowledge in evaluating and using any information, methods, compounds, or experiments described herein. In using such information or methods they should be mindful of their own safety and the safety of others, including parties for whom they have a professional responsibility. To the fullest extent of the law, neither the Publisher nor the authors, contributors, or editors, assume any liability for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions, or ideas contained in the material herein. ISBN: 978-0-323-99429-3 For Information on all Elsevier publications visit our website at https://www.elsevier.com/books-and-journals Publisher: Susan Dennis Acquisitions Editor: Anita Koch Editorial Project Manager: Franchezca A. Cabural Production Project Manager: Swapna Srinivasan Cover Designer: Christian J. Bilbow Typeset by Aptara, New Delhi, India

Contents Contributors

xi

Acknowledgment References

Chapter 1 Carbon dioxide capture and its utilization towards efficient biofuels production 1

Chapter 3 Cookstoves for biochar production and carbon capture 53 Mashura Shammi, Julien Winter, Md. Mahbubul Islam, Beauty Akter, and Nazmul Hasan

Abhinay Thakur, and Ashish Kumar

1.1 Introduction 1.2 Utilization of captured carbon dioxide for biofuel production 1.3 Conclusion and future perspectives References

1

3.1 Introduction 3.2 Cookstoves designed for biochar production 3.3 Biochar production and climate-change implications 3.4 Conclusion References

5 13 13

Chapter 2 Deep eutectic liquids for carbon capturing and fixation 17

2.8 2.9 2.10 2.11 2.12 2.13 2.14 2.15 2.16 2.17 2.18

Carbon dioxide emissions Deep eutectic liquids Types of deep eutectic liquids Preparation of DELs Authentication of DELs DEL based CO2 absorption Carbon capture efficiency of various HBDs CO2 absorption in aqueous solution of DELs CO2 absorption in ternary DELs Ammonium-Based DELs Phosphonium based DELs Azole based DELs Bio-phenol derived superbase based DELs Hydrophobic DELs Non-ionic DELs DEL supported membranes DELs with multiple sites interaction Conclusion and future prospects

53 54 62 64 65

Chapter 4 Metal support interaction for electrochemical valorization of CO2 69

Zainab Liaqat, Sumia Akram, Hafiz Muhammad Athar, and Muhammad Mushtaq

2.1 2.2 2.3 2.4 2.5 2.6 2.7

49 49

17 19 19 20 21 22

Abinaya Stalinraja, and Keerthiga Gopalram

4.1 Introduction 4.2 Metal supports for ECR of CO2 4.3 Conclusion Acknowledgment References

24

69 70 80 80 81

Chapter 5 Utilization of carbon dioxide as a building block in synthesis of active pharmaceutical ingredients 85

40 41 42 44 44 45 45 46 46 47 48

Muhammad Faisal

5.1 Introduction 5.2 N–Nucleophile-triggered CO2 -incorporated carboxylation to form C–N bonds 5.3 N–Nucleophile-triggered CO2 -incorporated methylation to form C–N bonds

v

85

87

95

vi

Contents

5.4 O–Nucleophile-triggered CO2 -incorporated carboxylation to form C–O bonds 96 5.5 CO2 -catalyzed oxidation of alcohols to form C–O bonds 97 5.6 C-Nucleophile-triggered CO2 -incorporated reductive carboxylation to form C–C bonds 97 5.7 C-nucleophile-triggered CO2 -incorporated direct C–H carboxylation to form C–C bond 99 5.8 C-nucleophile-triggered CO2 -incorporated organozinc-mediated carboxylation to form C–C bonds 101 5.9 C-nucleophile-triggered CO2 -incorporated organolithium-mediated carboxylation to form a C–C bond 102 5.10 C-Nucleophile-triggered CO2 -incorporated organomagnesium-mediated carboxylation to form a C–C bond 108 5.11 Conclusion 111 References 113

Chapter 6 Electrochemical Carbon Dioxide Detection 119 S. Aslan, C. I¸sık, and A.E. Mamuk

6.1 Introduction 6.2 Capture technologies of CO2 6.3 Fundamentals of electrochemistry 6.4 Direct potentiometric methods 6.5 Summary and conclusion References

119 121 126 128 139 141

Chapter 7 Carbon dioxide injection for enhanced oil recovery and underground storage to reduce greenhouse gas 149 Shubham Saraf, and Achinta Bera

7.1 Introduction 7.2 Oil recovery using CO2 7.3 Underground storage of CO2 in unconventional reservoirs 7.4 Current status, challenges and future directions 7.5 Conclusions Acknowledgment References

149 156 167 169 170 172 172

Chapter 8 Ionic liquids as potential materials for carbon dioxide capture and utilization 177 Md Abu Shahyn Islam, Mohd Arham Khan, Nimra Shakeel, Mohd Imran Ahamed, and Naushad Anwar

8.1 Introduction 8.2 Types of ILs 8.3 Future applications of IL and GR-based IL 8.4 Conclusion References

178 179 191 192 193

Chapter 9 Recent advances in carbon dioxide utilization as renewable energy 197 Muhammad Hussnain Siddique, Fareeha Maqbool, Tanvir Shahzad, Muhammad Waseem, Ijaz Rasul, Sumreen Hayat, Muhammad Afzal, Muhammad Faisal, and Saima Muzammil

9.1 Introduction 9.2 CO2 utilization technologies 9.3 Developments in worldwide CO2 utilization projects 9.4 Market scale and value 9.5 Regulation and policy 9.6 Conclusion and future prospects References

197 198 204 205 205 206 206

Chapter 10 Metal Organic Frameworks as an Efficient Method for Carbon dioxide capture 211 Bharti Kataria, and Christine Jeyaseelan

10.1 10.2 10.3 10.4 10.5 10.6

Introduction Metal organic framework (MOF) Synthesis of some MOFS Properties of MOFs CO2 capture using MOF Adsorption of carbon dioxide in metal organic frameworks 10.7 Methods to enhance CO2 adsorption 10.8 Methods to enhance MOF stability 10.9 Conclusion References

211 212 215 217 217 219 219 222 227 227

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Contents

Chapter 11 Industrial carbon dioxide capture and utilization 231

12.11 Conclusion and future insights References

Uzma Hira, Ahmed Kamal, and Javeria Tahir

11.1 11.2 11.3 11.4

Introduction CO2 collection systems based on liquid CO2 capturing with ionic liquid solvents Applications, implementation and challenges 11.5 Solid CO2 adsorbents for low-temperature applications 11.6 Carbon adsorbents 11.7 Zeolite adsorbents 11.8 Adsorbents of the MOF (metal–organic framework) type 11.9 Adsorbents predicated on carbonate-based alkalis 11.10 Layered double hydroxides (LDHs)-based adsorbents 11.11 Adsorbents made of magnesium oxide (MgO) 11.12 Solid CO2 sorbents for high-temperature applications 11.13 Pre-combustion applications, implementation and problems 11.14 The utilisation of CO2 in industrial processes 11.15 Conclusions and prospects References

231 233 242 244 245 246 248 250 252 254 255 257 260 262 268 270

12.7 12.8 12.9 12.10

Adeshina Fadeyibi

13.1 Introduction 13.2 Utilization of carbon-dioxide in food preservation 13.3 Utilization of carbon-dioxide in food storage 13.4 Prospects and conclusion References

297 298 302 305 305

Chapter 14 An insight into the recent developments in membrane-based carbon dioxide capture and utilization 311 Pritam Dey, Pritam Singh, and Mitali Saha

14.1 Introduction 14.2 Carbon dioxide capture technologies 14.3 A brief about membrane technology 14.4 CO2 separation using membranes 14.5 CO2 utilization using membranes 14.6 Conclusions References

311 312 314 316 321 322 323

Srijita Basumallick

Faizan Waseem Butt, Hafiz Muhammad Athar, Sumia Akram, Zainab Liaqat, and Muhammad Mushtaq

Introduction CO2 capture technologies Ionic liquids (ILs) Features of ILs IL as absorbents for CO2 capture IL hybrids as adsorbents for CO2 capture IL hybrids with membranes for CO2 capture Ionic liquid supported membrane Poly ILs membrane Composite membranes

Chapter 13 Advances in utilization of carbon-dioxide for food preservation and storage 297

Chapter 15 Carbon dioxide to fuel using solar energy 327

Chapter 12 Ionic liquids for carbon capturing and storage 279

12.1 12.2 12.3 12.4 12.5 12.6

291 291

279 280 281 281 283 289 289 290 290 290

15.1 Introduction 15.2 CO2 reduction onto semiconductor surface 15.3 Major bottleneck for CO2 reduction 15.4 Different types of photo catalyst 15.5 Reduction of CO2 to methanol using Cu2 O as photo catalyst 15.6 Reduction of CO2 to methanol using Cu2 O as electro catalyst 15.7 Benefits of using RGOin the composite catalyst 15.8 Conclusions

327 327 328 329 330 330 331 333

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Contents

Acknowledgment References

333 333

Chapter 16 Adsorbents for carbon capture 337 Vijay Vaishampayan, Mukesh Kumar, Muthamilselvi Ponnuchamy, and Ashish Kapoor

16.1 Introduction 16.2 Carbon capture processes 16.3 Adsorbents for CO2 capture 16.4 Future perspective and conclusion References

337 338 338 342 342

Chapter 17 Carbon dioxide capture and utilization in ionic liquids 345 Guocai Tian

17.1 Introduction 17.2 Capture of CO2 in ILs 17.3 Electroreduction of CO2 in ILs 17.4 Conclusions Acknowledgments References

345 349 391 406 407 407

Chapter 18 Hydrothermal carbonization of sewage sludge for carbon negative energy production 427 Milan Malhotra, Anusha Sathyanadh, and Khanh-Quang Tran

18.1 Introduction 18.2 Sludge as a potential source of alternate energy 18.3 Hydrothermal (HT) treatments for the production of fuel 18.4 Hydrothermal carbonization + gasification + ccs 18.5 Conclusion Acknowledgement References

427 431 432 437 437 438 438

Chapter 19 Utilization of supercritical CO2 for drying and production of starch and cellulose aerogels 441 Jeieli Wendel Gaspar Lima, Clara Prestes Ferreira, Jhonatas Rodrigues Barbosa, and Raul Nunes de Carvalho Junior

19.1 Introduction 19.2 CO2 application – Supercritical drying 19.3 Starch aerogel and CO2 utilization 19.4 Cellulose aerogels and CO2 utilization 19.5 Conclusions Author contributions Ethical approval Declaration of competing interest Acknowledgment References

441 442 445 447 448 449 449 449 449 449

Chapter 20 Advances in carbon bio-sequestration 451 Nigel Twi-Yeboah, Dacosta Osei, and Michael K. Danquah

20.1 Introduction 20.2 Carbon sequestration methods 20.3 Limitations of carbon sequestration methods 20.4 Overview of biological sequestration (Cycle/Mechanism) 20.5 Bioresources for carbon bio-sequestration 20.6 Cyanobacteria 20.7 Microalgae 20.8 Plants 20.9 Bacteria 20.10 Nanomaterials in carbon sequestration 20.11 Future perspectives 20.12 Conclusion References

451 452 453 454 455 456 457 457 458 458 459 459 460

Chapter 21 Photosynthetic cell factories, a new paradigm for carbon dioxide 463 (CO2 ) valorization Bijaya Nag, Abdalah Makaranga, Mukul Suresh Kareya, Asha Arumugam Nesamma, and Pannaga Pavan Jutur

21.1 Introduction 21.2 Carbon capture, utilization and storage mechanism 21.3 Biological mechanism of carbon capture 21.4 Products from CCU 21.5 Challenges and opportunities 21.6 Future perspectives and conclusions Funding information References

463 465 469 470 472 475 476 476

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Contents

Chapter 24 Carbon dioxide capture and its enhanced utilization using microalgae 531

Chapter 22 Carbon dioxide capture and sequestration technologies – current perspective, challenges and prospects 481 Ifeanyi Michael Smarte Anekwe, Emmanuel Kweinor Tetteh, Stephen Akpasi, Samaila Joel Atuman, Edward Kwaku Armah, and Yusuf Makarfi Isa

22.1 Introduction 22.2 Carbon capture and sequestration (CCS) technologies 22.3 CO2 transportation, storage and opportunities/applications for CCS technologies 22.4 Current perspective and policies of CSS technologies in various countries throughout the world 22.5 Challenges and socio-economic implications of CCS technologies 22.6 Applications and opportunities for CCS techniques 22.7 Prospects and future work considerations for CCS approaches 22.8 Conclusion References

481 484

493

497 501 504

Tuba Saleem, Ijaz Rasul, Muhammad Asif, and Habibullah Nadeem

Introduction Sources of CO2 emission Sequestration methods of CO2 Carbon concentrating mechanisms Advancements in carbon capture and storage & carbon capture utilization 23.6 Carbon dioxide fixation pathways 23.7 Factors affecting the carbon dioxide biofixation 23.8 Production of biopolymers/bioplastics 23.9 Conclusion References

24.1 Introduction 24.2 Photosynthesis and CO2 fixation using microalgae 24.3 Cultivation systems for carbon dioxide capture by microalgae 24.4 CO2 capture improvement strategies 24.5 Conclusion References

531 532 533 541 541 541

Chapter 25 Supported single-atom catalysts in carbon dioxide electrochemical activation and reduction 547 Amos Afugu, Caroline R. Kwawu, Elliot Menkah, and Evans Adei

507 508 509

Chapter 23 Microbial carbon dioxide fixation for the production of biopolymers 517 23.1 23.2 23.3 23.4 23.5

Pinku Chandra Nath, Biswanath Bhunia, and Tarun Kanti Bandyopadhyay

25.1 Introduction 25.2 CO2 ERR products 25.3 Single-Atom catalysts efficiency descriptors 25.4 Single-Atom catalyst supports 25.5 Mechanisms for CO2 ERR on single-atom catalysts 25.6 Conclusion References

517 519 519 520

549 551 554 556 557

Chapter 26 Organic matter and mineralogical acumens in CO2 sequestration 561

521 521 527 527 529 529

547 549

Santanu Ghosh, Tushar Adsul, and Atul Kumar Varma

26.1 26.2 26.3 26.4

Overview Introduction Geo-sequestration Bio-sequestration

562 562 563 563

x 26.5 26.6 26.7 26.8

Mechanisms of carbon capture Transport of carbon dioxide Mechanism of carbon accommodation Carbon dioxide sequestration in organic matter 26.9 Mineralogical acumen of carbon sequestration

Contents

564 566 566

26.10 A note on CO2 disposal in basalt formations 26.11 Summary References

587 587 588

566 573

Index

595

Contributors Evans Adei Department of Chemistry, Kwame Nkrumah University of Science and Technology, Kumasi, Ghana

Muhammad Asif Department of Bioinformatics and Biotechnology, Government College University Faisalabad, Faisalabad, Pakistan S. Aslan Department of Chemistry, Faculty of Science, Mugla Sitki Kocman University, Mugla, Turkey Hafiz Muhammad Athar Department of Chemistry, Government College University, Lahore, Pakistan Samaila Joel Atuman School of Chemical and Metallurgical Engineering, University of the Witwatersrand, Johannesburg, South Africa; Department of Chemical Engineering, Faculty of Engineering, Abubakar Tafawa Balewa University Bauchi, Nigeria Tarun Kanti Bandyopadhyay Department of Chemical Engineering, National Institute of Technology Agartala, Jirania, Tripura, India Jhonatas Rodrigues Barbosa Institute of Technology (ITEC), Faculty of Food Engineering (FEA), Federal University of Para (UFPA), Rua Augusto Corrêa S/N, Guamá, Belém, PA, Brazil Srijita Basumallick Asutosh College, University of Calcutta, Kolkata, India Achinta Bera Department of Petroleum Engineering, School of Energy Technology, Pandit Deendayal Energy University, Gandhinagar, Gujarat, India Biswanath Bhunia Department of Bio Engineering, National Institute of Technology Agartala, Jirania, Tripura, India Faizan Waseem Butt Department of Chemistry, Government College University, Lahore, Pakistan Michael K. Danquah Department of Chemical Engineering, University of Tennessee, Chattanooga TN, United States of America Pritam Dey Department of Chemistry, National Institute of Technology Agartala, Tripura, India

Tushar Adsul Coal Geology and Organic Petrology Laboratory, Department of Applied Geology, Indian Institute of Technology (Indian School of Mines) Dhanbad, Jharkhand, India Amos Afugu Department of Chemistry, Kwame Nkrumah University of Science and Technology, Kumasi, Ghana Muhammad Afzal Department of Bioinformatics and Biotechnology, Government College University Faisalabad, Faisalabad, Pakistan Mohd Imran Ahamed Department of Chemistry, Faculty of Science, Aligarh Muslim University, Aligarh, UP, India Stephen Akpasi Green Engineering Research Group, Department of Chemical Engineering, Faculty of Engineering and the Built Environment, Durban University of Technology, Durban, South Africa Sumia Akram Division of Science and Technology, University of Education Lahore, Pakistan Beauty Akter Department of Environmental Sciences, Jahangirnagar University, Dhaka, Bangladesh Ifeanyi Michael Smarte Anekwe School of Chemical and Metallurgical Engineering, University of the Witwatersrand, Johannesburg, South Africa Naushad Anwar Department of Chemistry, Faculty of Science, Aligarh Muslim University, Aligarh, UP, India Edward Kwaku Armah School of Chemical and Biochemical Sciences, Department of Applied Chemistry, C. K. Tedam University of Technology and Applied Sciences, Navrongo, Upper East Region, Ghana

xi

xii

Contributors

Adeshina Fadeyibi Department of Food and Agricultural Engineering, Faculty of Engineering and Technology, Kwara State University, Ilorin, Kwara State, Nigeria Muhammad Faisal Creative Research Center for Brain Science, Brain Science Institute (BSI), Korea Institute of Science and Technology (KIST), Seoul, Republic of Korea; Division of BioMedical Science & Technology, KIST School, Korea University of Science and Technology (UST), Seoul, Republic of Korea; Department of Chemistry, Quaid-i-Azam University, Islamabad, Pakistan; Institute of Plant Breeding and Biotechnology, MNS-University of Agriculture, Multan, Pakistan Clara Prestes Ferreira Institute of Technology (ITEC), Faculty of Food Engineering (FEA), Federal University of Para (UFPA), Rua Augusto Corrêa S/N, Guamá, Belém, PA, Brazil Santanu Ghosh Coal Geology and Organic Petrology Laboratory, Department of Applied Geology, Indian Institute of Technology (Indian School of Mines) Dhanbad, Jharkhand, India; Organic Geochemistry Laboratory, Department of Earth Sciences, Indian Institute of Technology Bombay, Mumbai, Maharashtra, India; Department of Geology, Mizoram University, Aizwal, Mizoram, India Keerthiga Gopalram Department of Chemical Engineering, SRM Institute of Science & Technology, Kancheepuram, Tamil Nadu, India Sumreen Hayat Departmant of Microbiology, Government College University Faisalabad, Faisalabad, Pakistan Nazmul Hasan The United Graduate School of Agricultural Sciences, Kagoshima University, Kagoshima, Japan; Fruit Science Laboratory, Saga University, Saga, Japan Uzma Hira School of Physical Sciences (SPS), University of the Punjab, Lahore, Pakistan Yusuf Makarfi Isa School of Chemical and Metallurgical Engineering, University of the Witwatersrand, Johannesburg, South Africa Md. Mahbubul Islam Bangladesh Biochar Initiative, Dhaka, Bangladesh

Md Abu Shahyn Islam Interdisciplinary Nanotechnology Centre, ZHCET, Aligarh Muslim University, Aligarh, UP, India C. I¸sık Department of Chemistry, Faculty of Science, Mugla Sitki Kocman University, Mugla, Turkey Christine Jeyaseelan Department of Chemistry, Amity Institute of Applied Sciences, Amity University, Noida, Uttar Pradesh, India Raul Nunes de Carvalho Junior Institute of Technology (ITEC), Faculty of Food Engineering (FEA), Federal University of Para (UFPA), Rua Augusto Corrêa S/N, Guamá, Belém, PA, Brazil Pannaga Pavan Jutur Omics of Algae Group, Industrial Biotechnology, International Centre for Genetic Engineering and Biotechnology, New Delhi, India Ahmed Kamal School of Physical Sciences (SPS), University of the Punjab, Lahore, Pakistan Ashish Kapoor Department of Chemical Engineering, Harcourt Butler Technical University, Kanpur, Uttar Pradesh, India Mukul Suresh Kareya Omics of Algae Group, Industrial Biotechnology, International Centre for Genetic Engineering and Biotechnology, New Delhi, India Bharti Kataria Department of Chemistry, Amity Institute of Applied Sciences, Amity University, Noida, Uttar Pradesh, India Mohd Arham Khan Interdisciplinary Nanotechnology Centre, ZHCET, Aligarh Muslim University, Aligarh, UP, India Ashish Kumar NCE, Department of Science and Technology, Government of Bihar, India Mukesh Kumar Discipline of Chemistry, Indian Institute of Technology, Gandhinagar, Gujarat, India Caroline R. Kwawu Department of Chemistry, Kwame Nkrumah University of Science and Technology, Kumasi, Ghana Zainab Liaqat Department of Chemistry, Government College University, Lahore, Pakistan Jeieli Wendel Gaspar Lima Institute of Technology (ITEC), Faculty of Food Engineering

Contributors

xiii

(FEA), Federal University of Para (UFPA), Rua Augusto Corrêa S/N, Guamá, Belém, PA, Brazil

Technology, Potheri, Kattankulathur, Tamil Nadu, India

Abdalah Makaranga Omics of Algae Group, Industrial Biotechnology, International Centre for Genetic Engineering and Biotechnology, New Delhi, India

Ijaz Rasul Department of Bioinformatics and Biotechnology, Government College University Faisalabad, Faisalabad, Pakistan Mitali Saha Department of Chemistry, National Institute of Technology Agartala, Tripura, India

Milan Malhotra Department of Energy and Process Engineering, Norwegian University of Science and Technology, Trondheim, Norway A.E. Mamuk Department of Physics, Faculty of Science, Mugla Sitki Kocman University, Mugla, Turkey Fareeha Maqbool Department of Bioinformatics and Biotechnology, Government College University Faisalabad, Faisalabad, Pakistan Elliot Menkah Department of Chemistry, Kwame Nkrumah University of Science and Technology, Kumasi, Ghana Muhammad Mushtaq Department of Chemistry, Government College University, Lahore, Pakistan

Tuba Saleem Department of Bioinformatics and Biotechnology, Government College University Faisalabad, Faisalabad, Pakistan Shubham Saraf Department of Petroleum Engineering, School of Energy Technology, Pandit Deendayal Energy University, Gandhinagar, Gujarat, India Anusha Sathyanadh Department of Energy and Process Engineering, Norwegian University of Science and Technology, Trondheim, Norway Nimra Shakeel Department of Chemistry, Faculty of Science, Aligarh Muslim University, Aligarh, UP, India

Saima Muzammil Departmant of Microbiology, Government College University Faisalabad, Faisalabad, Pakistan

Mashura Shammi Hydrobiogeochemistry and Pollution Control Laboratory, Department of Environmental Sciences, Jahangirnagar University, Dhaka, Bangladesh

Habibullah Nadeem Department of Bioinformatics and Biotechnology, Government College University Faisalabad, Faisalabad, Pakistan

Tanvir Shahzad Departmant of Environmental Sciences and Engineering, Government College University Faisalabad, Faisalabad, Pakistan

Bijaya Nag Omics of Algae Group, Industrial Biotechnology, International Centre for Genetic Engineering and Biotechnology, New Delhi, India

Muhammad Hussnain Siddique Department of Bioinformatics and Biotechnology, Government College University Faisalabad, Faisalabad, Pakistan

Pinku Chandra Nath Department of Bio Engineering, National Institute of Technology Agartala, Jirania, Tripura, India

Pritam Singh Department of Chemistry, National Institute of Technology Agartala, Tripura, India

Asha Arumugam Nesamma Omics of Algae Group, Industrial Biotechnology, International Centre for Genetic Engineering and Biotechnology, New Delhi, India

Abinaya Stalinraja Department of Chemical Engineering, SRM Institute of Science & Technology, Kancheepuram, Tamil Nadu, India

Dacosta Osei Chemical and Petroleum Engineering Department, University of Kansas, KS, United States of America Muthamilselvi Ponnuchamy Department of Chemical Engineering, College of Engineering and Technology, SRM Institute of Science and

Javeria Tahir School of Physical Sciences (SPS), University of the Punjab, Lahore, Pakistan Emmanuel Kweinor Tetteh Green Engineering Research Group, Department of Chemical Engineering, Faculty of Engineering and the Built Environment, Durban University of Technology, Durban, South Africa

xiv

Contributors

Abhinay Thakur Department of Chemistry, Faculty of Technology and Science, Lovely Professional University, Phagwara, Punjab, India

Vijay Vaishampayan Department of Chemical Engineering, Indian Institute of Technology, Ropar, Punjab, India

Guocai Tian State Key Laboratory of Complex Nonferrous Metal Resources Clean Utilization, Faculty of Metallurgical and Energy Engineering, Kunming University of Science and Technology, Kunming, Yunnan Province, China

Atul Kumar Varma Coal Geology and Organic Petrology Laboratory, Department of Applied Geology, Indian Institute of Technology (Indian School of Mines) Dhanbad, Jharkhand, India

Khanh-Quang Tran Department of Energy and Process Engineering, Norwegian University of Science and Technology, Trondheim, Norway

Muhammad Waseem Departmant of Microbiology, Government College University Faisalabad, Faisalabad, Pakistan

Nigel Twi-Yeboah Operations Department, Ghana National Gas Company, Western Region, Ghana

Julien Winter Private consultant, Cobourg, ON, Canada

C H A P T E R

1 Carbon dioxide capture and its utilization towards efficient biofuels production Abhinay Thakur a and Ashish Kumar b a

Department of Chemistry, Faculty of Technology and Science, Lovely Professional University, Phagwara, Punjab, India b NCE, Department of Science and Technology, Government of Bihar, India

1.1 Introduction When released into the atmosphere, carbon dioxide (CO2 ), a foremost greenhouse gas, retains heat by reflecting infrared light back to the Earth’s surface. As a result, increased CO2 emissions are a worldwide problem because they are one of the primary causes of climate disruption [1–4]. Furthermore, worldwide CO2 emission patterns indicate an annually rise, which is accompanied by an annually rise in overall warming. The rise in CO2 concentration was 2.46 0.26 ppm y−1 in October 2021, as per current statistics, but the temperature has enhanced at an annualized level of 0.08 C per decade since 1980, as per National Oceanic and Atmospheric Administration (NOAA) 2020 annual climate disclose. The result is progressive warming and withering of the environment, which, among other things, is creating enormous and disastrous wildfires around the world, which in turn emit enormous volumes of CO2 into the environment, making carbon releases even more of a problem [5–7]. In actuality, rising CO2 levels in the environment have a variety of other environmental consequences, including adjustments in the hydrogeological process, the enhanced incidence of numerous severe climate occurrences, sea-level rise, speciation migratory, harvest losses, and enhanced events that occurred of infectious diseases, and so on. From 2010 to 2040, consumption for fossil fuels is expected to increase by 40 percent. As a result, alternate energy sources have been and continue to be investigated in order to meet our energy requirements. Renewable energy resources include sunlight, air, and biomass. In the last several years, biomass, which is formed through a physiological origin, has been exploited to generate biofuels and bio-products. There are 4 phases of biofuels, based on the sort of biomass. Biodiesel, bioethanol, bioethanol,

Green Sustainable Process for Chemical and Environmental Engineering and Science: Carbon Dioxide Capture and Utilization DOI: https://doi.org/10.1016/B978-0-323-99429-3.00008-4

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c 2023 Elsevier Inc. All rights reserved. Copyright 

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1. Carbon dioxide capture and its utilization towards efficient biofuels production

biohydrogen, and bioethers are examples of biofuels. Bioethanol and biodiesel, which both constitute the first generation of biofuel technologies, are the most common biofuels, as per the Department of Energy. The United States of America, Australia, and the European Union have all financed biofuel experiments. The United States provided financing to New Mexico (2009), Arizona (2008), Florida (2013), and Massachusetts (2011) while the European Union provided funds for four experimental initiatives, three of these operated during 2011 to 2015/16 and the remaining between 2012 and 2017. Biomass is considered as diametrically opposed to the usage of fossil fuels, and hence prevents the release of fresh CO2 into the environment. Utilizing biomass (and biofuels produced from it) is regarded an operation that does not add CO2 to the environment from fossil fuels. Although though it isn’t precisely so, burning biomass or biofuels is regarded a zero-emission solution for power generation and consumption. Despite contrast, comprehensive life cycle analysis investigations demonstrate that, in the existing production–utilization–accounting framework, the usage of biofuels is a carbon transmission through subsurface subsoil to the environment, equivalent to the usage of fossil fuels, albeit considerably less intensively. In truth, biomass is made up of CO2 from the environment, and when burnt, it is thought to restore the equivalent quantity of CO2 to the environment, as if the process were a component of the biological process. In reality, unlike in reality, the cycle is not truly ended. In essence, in contrast to the carbon dioxide produced by combustion processes, one must also evaluate the quantity of CO2 released by numerous human actions that precede biomass generation and processing, as well as the soil carbon depletion induced by agricultural methods [8–15]. By combining telmisartan and a suitable tin(IV) chloride, Hadi et al. [16] was able to create unique, permeable, extremely aromatic organotin(IV) structures. The surface area of the produced mesoporous organotin(IV) complexes was 32.3–130.4 m−2 g−1 , the pore capacity was 0.046–0.162 cm−3 g−1 , and the pore diameter was roughly 2.4 nm, according to Brunauer– Emmett–Teller (BET) calculations. Tin complexes with a butyl group were found to be more effective as carbon dioxide storage devices than those with a phenyl group. At a regulated temperature (323 K) and compression, the dibutyltin(IV) molecule offer the greatest BET interface region (128.871 m−2 g−1 ), the highest quantity (0.162 cm−3 g−1 ), and termed to be effective for CO2 retention (8.3 wt percent) (50 bars). The sorption of compounds was investigated under a specified temperature (323 K) and strain. The H2 and CO2 adsorption isotherms in the presence of compounds are shown in Fig. 1.1. Complexes absorbed a lot of CO2 , which might own because of intense van der Waals contact among them and CO2 . For complexes, the amount of absorbed CO2 was 17.9, 21.2, 15.7, and 34.9 cm−3 g−1 . Evidently, these structures have the maximum Co2 absorption aperture (6.9 wt percent) of the organotin(IV) compound, that could be due to the fact that they have the biggest BET interface region (128.871 m−2 g−1 ). Furthermore, within the organotin(IV) complexes, significant dipole-quadrupole encounters in CO2 or H2 bonding and heteroatoms may occur. When compared to other gases like nitrogen and methane, highly permeable organic polymers having nitrogen, oxygen or sulphur atoms are efficient at preferentially absorbing CO2 . Furthermore, in similar circumstances as those employed for CO2 absorption, complexes display very little H2 adsorption (0.5–1.1 cm−3 g−1 ). It’s possible that this behavior is owing to minimal contact. Similarly, Nasir et al. [17] used the partial pressures of methane and CO2 , as well as the proportions of several membrane materials (polymer, amine, and filler), to link three optimal results in a unified model: CO2 permeance, CH4 permeance, and CO2 /CH4 selectivity.

1.1 Introduction

3

FIGURE 1.1 CO2 and H2 adsorption isotherms for complex (Adapted from Ref. [16]) MDPI 2019. Published in accordance with Creative Common attribution License CCBY 4.0.

These variables aided in forecasting membrane efficiency and influencing secondary variables including membrane life, effectiveness, and product quality. For CO2 permeability, CH4 permeability, and CO2 /CH4 selectivity, the model findings accord with experimental data having an relative deviation of 5.9 percent, 3.8 percent, and 4.1 percent, approximately. The findings suggest that the model could forecast values under a variety of membrane formation configurations. Scholes et al. [18] investigated the capacity of a covalently linked polyether-polyamide block copolymer (PEBAX 2533) and polyethylene glycol diacrylate to extract carbon dioxide through N2 and CH4 in a basic and integrated gas circumstances, as well as when 500 ppm H2 S was involved. The Lennard Jones well depth was found to be a stronger predictor of gas solubility within these polymers than essential temperature. Due to competing sorption from CH4 or N2 , CO2 penetration was decreased in dry mixed gas circumstances relative to single gas measurements. Both polymers, though, maintained CO2 selectivity. Water in the feed caused the PEG membrane to expand, leading in a considerable improvement in CO2 penetration as compared to the gas (dry) environment. Interestingly, the sensitivity was maintained even when the supply gas was moist. The inclusion of H2 S reduces CO2 penetration via both membranes merely slightly. Jiang et al. [19] conducted extensive research on the effects of calcination degrees upon the system architectures of organosilica films. The precursor Bis(triethoxysilyl)acetylene (BTESA) was chosen for membrane manufacturing using the sol-gel method. Calcination degrees influenced film porous width and silanol density, as indicated by TG, FT-IR, N2 adsorption, and molecule tunable gas permeation measurements. The disintegrated acetylene bridges resulted in a loose architecture in the BTESA membrane, which had an extreme high CO2 permeation of 15,531 GPU but a limited CO2 /N2 sensitivity of 4.1. BTESA membranes showed remarkable potential for CO2 extraction applications when they were calcined at 100 °C, with a CO2 permeability of 3434 GPU and a N2 /CO2 sensitivity of 21. FE-SEM was used to analyse BTESA composite membranes that had been calcined at 100 °C in order to learn more about their chemistry, as shown in Fig. 1.2.

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1. Carbon dioxide capture and its utilization towards efficient biofuels production

FIGURE 1.2 SEM images of the BTESA-100 porous film. (Adapted from Ref. [19]) MDPI 2022. Published in accordance with Creative Common attribution License CCBY 4.0.

The BTESA-100 membrane was used to separate CO2 /N2 mixtures for analytical purposes. A study for the long-term operating durability of binary CO2 /N2 (14/79) segregation conducted at 323K to demonstrate its durability of its extraction efficiency, and the findings are presented in Fig. 3. In a constant operation lasting up to 26 h, negligible discernible loss in CO2 /N2 extraction efficiency for CO2 /N2 sensitivity and CO2 permeability. During a long extraction experiment, the BTESA-100 membrane was proven to be dependable, and it has a lot of possibilities in CO2 collection applications. However, owing to the presence of moisture in the operational CO2 /N2 separation procedure, the membrane stability in humidified conditions must also be evaluated. Furthermore, Su et al. [20] investigated the effect of pollutants in the flue gaseous, like H2 O vapor, O2 and SO2 for the sorption of CO2 /N2 integration in carboxyl doped CNT matrix and carbon nanotubes (CNTs) using a large canonical Monte Carlo simulation. The most effective inhibitor of CO2 adsorption when a solitary unclean gas SO2 was introduced, while water only had a significant impact at low pressures (0.1 psi), when a 1D lattice of H2 -bonded monomers formed. Furthermore, O2 was discovered to have no effect on CO2 purification and segregation. With three contaminants in flue gas, SO2 performed a key function in suppressing CO2 adsorption by drastically lowering the adsorption quantity. This was due to the fact that SO2 exhibited a greater affinity with carbon walls than CO2 . Because of correlations among distinct entities, the inclusion of three contaminants in flue gas increased the adsorption intricacy. The CNT matrix’ external adsorption region was heavily dominated by H2 O, which hydrophilic carboxyl groups modified, and SO2 effectively adsorbs CO2 inside the duct. These two impacts restricted CO2 adsorption while increasing CO2 /N2 selectivity, and the contest among them controlled the CO2 adsorption pattern within and without the tube. Furthermore, it was discovered that in the existence of impurity gas, carbon nanotube consistently retained the optimum CO2 /N2 sorption and segregation efficiency, in both single CNT and CNT array situations. A considerable amount of water molecules are absorbed and accumulated among tubes to create chain formations, as per the molecular image of water molecules deposited in (7, 7)

1.2 Utilization of captured carbon dioxide for biofuel production

5

CNT array in Fig. 4, although water molecule adsorption in tubes is scarcely detected. At the same time, as tube radius increases, the adsorption rate of water molecules reduces. Measuring the weight fraction of interfering carboxyl reveals that carboxyl concentration has a significant impact on water molecule adsorption capability. The mass percentage of carboxyl group drops as the width of the tube increases, resulting in a reduction in the adsorption capability of water molecules. The absorption of SO2 in small-diameter nanotube arrays was aided by the existence of water molecules.

1.2 Utilization of captured carbon dioxide for biofuel production Worldwide climate warming and rising green-house gas emissions, and the exhaustion of traditional fuel sources, have become an increasing source of concern in recent decades. Coal, oil, and natural gas burning release upwards of 6 billion tonnes of CO2 into the environment each year [5,21–29]. In this context, physiological CO2 reduction is increasing interest since it results in the production of energy through biomass generated by CO2 fixation via photosynthesis. Because it is energy economical, durable, and ecologically friendly, photosynthetic CO2 fixation is regarded to be a viable technique. Green plants may capture CO2 via photosynthesis, which is a natural process. Furthermore, due to the mitigated rates of growth of traditional land plants, CO2 collection through sustainable natural sources predicted to be just 4–7 percent of fossil fuel outputs [12,30–33]. Microalgae, on either hand, could present a possibility because to its quantity and rapid development proportion. Rapidly maturing single celled microbes called microalgae have a 10–50 percent greater capacity to absorb photovoltaic radiation over bryophytes simultaneously fixing CO2 . Carbonic anhydrase (CA), an extracellular zinc metalloenzyme, aids in the absorption of CO2 from the environment by microalgal cells. CA catalyzes the transformation of CO2 to bicarbonates, that are absorbed by microalgal cells via transporter. The CO2 collected by microalgae is retained as carbohydrates, lipids, or proteins, based on the genus. It may be possible to extract CO2 from microalgae lipid stores and use it as a biofuel. One of the least studied methods for capturing CO2 is the biological pathway via microalgae, in which CO2 is instantly converted to biomass via single source discharges in specially designed platforms like photobioreactors. Phototrophic algae’s carbon fixation has the ability to reduce CO2 emissions into the environment, hence reducing global warming. Microalgal CO2 biofixation in photobioreactors is a potential method for producing more biomass and ethanol. The usage of photobioreactors for CO2 capture by microalgae has several benefits, including increased microalgal production owing to regulated atmospheric factors and enhanced area or volumetric utilization, resulting in more effective utilization of expensive land. Microalgae might thus serve a dual purpose by lowering greenhouse gases through CO2 sequestration and supplying cleaner energy to meet the expanding need for energy.

1.2.1 Photosynthesis and photo oxidation of water Photosynthesis is known to be a biological activity that is performed out by bacteria, algae, and elevated plants. It relates to the process through which species turn light energy to the chemical energy through gathering light and using it for fuel by CO2 adsorption.

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1. Carbon dioxide capture and its utilization towards efficient biofuels production

FIGURE 1.3 At 50 °Celsius, a extended durability experiment of CO2 /N2 (14/79) mixture segregation for the BTESA-100 porous film was performed. (Adapted from Ref. [19]) MDPI 2022. Published in accordance with Creative Common attribution License CCBY 4.0.

(A)

(B)

FIGURE 1.4 At 1.0 bar, 300 K, a molecular image of the (7, 7) CNT array in cross-sections (A) and axial axis (B). (Adapted from Ref. [20]) MDPI 2022. Published in accordance with Creative Common attribution License CCBY 4.0.

Carbon gets transferred from the environment into biomass in this manner. The watersplitting process, which results in the creation of oxygen, is a bonus element of algae’s photosynthesis. The photosynthesis reaction occurred in chloroplasts, which are specialized organelles. The physicochemical and biological processes are the two series of steps that make up photosynthesis. The biophysical processes take occur in the chloroplasts’ thylakoid discs [34–39]. The absorbing of light photons by essential pigments such as like xanthophylls and carotenes is referred to as photon absorption. The water is oxidized, releasing oxygen (Fig. 1.3). The reduction of nicotinamide adenine dinucleotide phosphate and production of adenosine triphosphate (ATP) are both aided by the electrons released from water molecules

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FIGURE 1.5 Photosynthesis and photolysis pathways of photoautotrophic bacteria are depicted schematically. (Adapted from Ref. [40]) Springer 2017. Published in accordance with Creative Common attribution License CCBY 4.0.

(NADPH). The energy produced while NADPH and ATP are in their active states is used in the dark processes to bind CO2 . The stroma is where the metabolic response occurs, and the end metabolites are primarily sugar molecules and a few other chemical compounds required for metabolic activity and cell function.

1.2.2 Bio-sequestration of CO2 CO2 from the environment is absorbed during the photosynthetic cycle, that is carried out by microalgae to produce feed. The C3 and C4 routes are the two processes through which green plants assimilate CO2 from the environment. Around 250,000 species of C3 plants and 7500 kinds of C4 plants have been identified. For CO2 fixation, many algae utilize the C3 pathway (Calvin Cycle). CO2 is mixed using a 5-carbon molecule to produce dual 3-carbon chemicals in this process. Ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCo) is referred to be an enzyme which catalyzes this process. Most algae are photoautotrophs, which means they can obtain all of their energy from photosynthesis and most of their carbon through carbon dioxide absorption. Diatoms are categorised as C4 plants because it could absorb CO2 in a different way than terrestrial agricultural plants like corn, cotton, and wheat. C4 plants combine CO2 using a tri-carbon molecule to create a tetra-carbon molecule instead using

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RuBisCo to create dual three-carbon molecules, limiting photorespiration loss and improving the efficiency of CO2 fixation. C4 plants are believed to possess double the photosynthetic rate of C3 plants, though that this advantage has become fewer noticeable when CO2 levels are sufficient. The absorbed CO2 is retained as carbohydrates and lipids in the algal cells. The Hatch Slack phase is used by C4 plants in complement to the Benson Calvin process. In this additional cycle, the phosphoenolpyruvate carboxylase (PEPcase) enzyme achieves a pre-acquisition of carbon dioxide in the form of a tetra-carbon molecule. The byproducts of this process are employed to increase the level of CO2 at the location wherein RuBisCO (the carboxylation enzyme of the Benson Calvin cycle) is active, preventing photorespiration. The extracellular carbonic anhydrase (CA) enzyme aids in the absorption of CO2 by microalgal cells. It’s thought to be the carbon concentrating mechanism’s likely main enzyme. The enzyme is involved in a broad variety of macro and microalgal organisms. It aids in CO2 absorption by catalysing the interaction between HCO3 and CO2 . It has been discovered that intracellular CA can happen in the identical cell. The genes code for the CA isoforms are controlled by the inorganic carbon in the media. As a result, the action of CA rises as the amount of inorganic carbon in the media decreases. In order to convert CO2 into HCO3 in the cytosol during C4 photosynthesis and furnish substrates for PEP carboxylase, CA is required. The information gathered from the research conducted with CA inhibitors has proved the presence of CA. Although CA activity has been studied in a variety of micro and macroalgae, investigations of the standard green alga Chlamydomonas reinhardtii have provided the majority of the present knowledge of the function of CA in algae.

r Enhancing passive and active carbon absorption from the atmosphere. r Reducing CO2 escape from high-CO2 -concentration areas within the cell. Montazersadgh et al. [41] decided to generate a novel electrochemical system for producing low-carbon e-biofuels using multipurpose electrosynthesis and integrating CO2 covalorization of biomass resources. Drop-in fuels were produced by reducing CO2 near the cathode, whereas value-enhanced chemicals were produced near the anode. In this study, a mathematical analysis of a continuous-flow architecture was established to evaluate the most technoeconomically viable combinations based on energy effectiveness, environmental effect, and economical ideals. After then, the reactor architecture was tweaked using parametric study. A constant electrolytic cell was designed and confirmed analytically. The algorithm was then utilized in combination with multiple cell kinetics to estimate the optimal cell architecture for distinct e scenarios. A selection of organic compounds with at least one interaction from each group were used to generate the kinetics. The most current developments in biomass oxidation for biofuel generation and CO2 electroreduction dynamics and are also covered in this research. The overall performance of the cell is improved by using a non-water solvent because HER was not predominant at the cathode. Whereas the energy content of the primary commodity primarily determines the energy effectiveness of the cell, properly choosing the reactor kinetics could significantly increase the efficiency of CO2 conversion. When contrasted to certain other manufacturing applications, the cumulative environmental impact (E-factor) is considerable. This is owing to the solvent’s huge quantity in comparison to the output, and it could be minimized by cycling the solvent through the process. The byproducts at the anode for the specified reaction mechanism influence the cell’s economically additional value one of the most. Because both compounds possess a relatively high Gibbs energy output,

1.2 Utilization of captured carbon dioxide for biofuel production

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FIGURE 1.6 For several half-cell reactions, (A) CO2 capture ratio and (B) E-factor. (Adapted from Ref. [41]) Springer 2021. Published in accordance with Creative Common attribution License CCBY 4.0.

the increased cellular energy performance was enhanced to 340 percent. CO2 transformation frequency of 69.3 percent, present efficacy of 56.7 percent with E-factor of 704 are some of the other cell effectiveness parameters. The CO2 conversion rate is also another crucial productivity component that could be improved (see Fig. 1.6). Because the HER was not present at the cathode, DMF was presumed to be the solvent. Similarly, Zdeb et al. [42] discussed the empirical findings of incorporating carbon dioxide as a reagent in the valorization process in coal gasification. Three basic setups featuring varied modeled waste heat use situations were tested on a batch process moving bed gasifier. CO2 , O2 , and a combination of 30 percent CO2 in O2 were utilized as gasification reagents at

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(A)

(B)

FIGURE 1.7 Configuration on a lab scale with a rolling bed reactor and a gasification reagent pre-heating system: (A) a perspective and (B) a graphic illustration. (Adapted with Ref. [42]) MDPI 2019. Published in accordance with Creative Common attribution License CCBY 4.0.

temperatures of 700, 800, and 900 °Celsius. The cumulative influence of processing parameters on coal treatment efficiency of gas productivity, content, and calorific range was investigated, and the empirical value was analyzed utilizing Principal Component Assessment. In the controlled situations used, the trials confirmed the possibility of producing gas with a calorie content of 4–6 MJ/m3 by pyrolysis with a carbon dioxide-containing gasifying agent. Even though encouraging in the development of energy-efficient and low-carbon footprint processes, the concept of carbon dioxide valorization and waste heat utilization in coal gasification requires much further breakthroughs in relation to working assimilation as well as cost-competitiveness metrics until it can be regarded for widespread application. Similarly, Ahmad et al. [43] created a system of data-based soft sensors that uses an ensemble technique called boosting to forecast the content, amount, and grade of fatty acid methyl esters (FAME) in the biofuel synthesis procedure using the oil of several vegetables. The non-intrusive polynomial chaos expansion (PCE) technique was added into the sensitive detectors design to evaluate how ambiguity affected the results. In each of the elements, flow rate and cetane, a unique model (soft sensor) was created. The anticipated results of Methyl-Li, -O, -M, -P, -S, FAME transmission rate, and cetane frequency were 0.27479, 0.32227, 2.41208, 0.1651, 0.82135, 0.96546, and 0.97013 with 1 percent variation in all supply parameters of the sensitive detectors were 0.27479, 0.32227, 2.41208, 0.1651, The sensors are extremely precise at predicting and quantifying ambiguity, making them ideal for practical uses. Zhang et al. [44] focused on the technical and economical configuration of solid-oxide electrolysis for the manufacture of green methanol by Hydrogenation of carbon dioxide. System unification, technical and economical analysis, and multi-objective management are carried out successfully for a research project. The results show a trade-off between energy efficiency and the cost of generating CH3OH. The assessed example’s annual methanol production was 100 kton, with a quality of 98.6 percent weight and a carbon dioxide usage of 150 kton,

1.2 Utilization of captured carbon dioxide for biofuel production

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offering it an annual retention capacity of 800 GWh sustainable energy. Methanol production costs approach 560 $/ton having an electric cost of 74.26 $/MWh, making it commercially unworkable with an usable life of over 13 years, despite the performance being about 70 percent and varying within a small range. When the price of energy is reduced to 47 dollars per megawatt hour and subsequently to 24 dollars per megawatt hour, the cost of producing methanol falls to 365 and 172 dollars per ton, respectively, with a 4.6 and 2.8-year economic success. The cost of power has a considerable influence on project execution. The cost of power varies by country, resulting in varied payback times in various places. Esteves et al. [45] examined at the effects of different light frequencies on biomass production, carbon dioxide reduction, and nutrient removal through a synthetic discharge in Tetradesmus obliquus, Chlorella vulgaris and Neochloris oleoabundans. Light-emitting diodes (LEDs) having varied wavelengths were used in the experimentations: 620–750 nm (red), 380–750 nm (white) and 450–495 nm (blue). N. oleoabundans with white LEDs had the highest specific growth rate (0.264 0.005 d−1 ), while C. vulgaris had the highest biomass output (14 4 mg CO2 L−1 d−1 ) and CO2 fixation rates (12.5 mg CO2 L−1 d−1 ). The three microalgae investigated had the greatest nitrogen and phosphorus extraction efficiency when exposed to white light. Molino et al. [46] developed Scenedesmus almeriensis into a green microalga on a benchscale to trap CO2 and produce lutein. In a vertically hydrodynamic cavitation photo-bioreactor with a steady stream of a mixture of gases of N2 , O2 and CO2 with the former having a concentration of 0.0–3.0 percent v/v, heterotrophic growth of S. almeriensis was carried successfully. Batching was used in the liquid phase. The development of S. almeriensis was optimized. Furthermore, lutein separation was conducted out at 59 °C and 9 MPa utilizing rapid solvent separation using C2 H5 OH to be a Generally Recognized as Safe (GRAS) substrate. Utilizing a carbon dioxide concentration of 2.9 percent v/v, the highest biofuel productivity of 129.24 mgL−1 d−1 was attained during in the development, allowing for a lutein concentration of 8.54 mgg−1 , that was 5.6-fold greater than the similar procedure performed with out CO2 . The ion chemistry analysis of the growing medium revealed that rising CO2 concentration progressively boosted nutrient intake throughout the growth stage. Because it focuses on pigment creation from a natural origin while also capturing CO2 , this research could be of relevance for lutein harvesting at an industrial level. Fig. 1.8 shows the influence of CO2 concentration on nutrient absorption as assessed at the conclusion of S. almeriensis’ development. The results revealed a full phosphate ion consumption, that would impede cellular proliferation. The efficiency of nutrient intake improved as CO2 level raised. The extended culture periods (i.e., 20 days for CO2 = 0.5 percent v/v; 16 days for CO2 = 1.5 percent v/v; 13 days for CO2 = 3.0 percent v/v) did, though, help to increase nutritional intake. During in the development period, nitrate and phosphate were the most heavily absorbed nutrients. A proposed reason for this phenomena is that protein production requires a nitrogen supply, and lutein occurs in microalgae as a nitrogenous macromolecule. The current investigation showed that NO3 and PO4 −3 ions are the most essential nutrient for cell growth in microalgae development. Furthermore, with CO2 levels of 0.0, 0.5, 1.5, and 3.0 percent v/v, the absorption of NO3 ions was 5.0, 59.88, 77.26, and 87.22 percent, correspondingly, throughout development. This finding could be explained by a restricted carbon source, that causes strain in microalgae growth cells, resulting in reduced biological nutrient absorption. In contrast to the intake of other nutrients, there was a reduced intake of both Na+ and Cl ions at the conclusion of the

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FIGURE 1.8 Throughout the development of S. almeriensis, the impact of CO2 levels on nutrient consumption effectiveness was investigated. (Adapted from Ref. [46]). MDPI 2022. Published in accordance with Creative Common attribution License CCBY 4.0.

growth, with an intake under 25 percent. When CO2 level was increased from 0.5 percent v/v to 3.0 percent v/v, the intake of Cl ions reduced, which might be accounted by a shorter cultivation period. Following the growth of Chlorella vulgaris and production of high yield biofuel after successive CO2 capture, the similar findings were reported. Valdovinos-García et al. [47] goal was to assess the techno-economics of microalgae biomass generation whereas only examining methods that could be scaled up to industrial levels. The criterion for the assessment are energy usage and operational costs. Furthermore, the absorption of CO2 by a thermoelectric system was investigated to be a feedstock of carbon for microalgae production. 24 scenarios were created by combining raceway pond cultivars, the primary extract with 3 distinct coagulants, the intermediate extract using samples centrifuged and 3 different filtration technologies, and finally rinsing using Mist and Drum Dryers. The cultivated area was estimated to absorb 102.13 tonnes of CO2 /year with a moderate biomass production of 12.7 g/m2 /day. The situations that featured spinning and vacuum filtering were the ones that used the most energy. The operational costs per kilogram of dry biomass ranging from $4.75 to $6.55 USD. The ideal situation is determined by the final usage of biomass. Ye et al. [48] developed computer simulation algorithms to examine efficient energy and predict manufacturing costs depending on their innovative technique, which catalytically converts glycerol into acrylic acid (C3 H4 O2 ) in a dual-phase method using CO2 as a reactive substrate. The research was carried out using publicly available data from a conventional, intermediate-sized biodiesel plant, with the goal of determining the viability of manufacturing C3 H4 O2 in real time scenario of a regenerative financial system. Variables assessment in reaction to glycerol conventional price, carbon dioxide recycling supply and price, and variations in process scalability and circumstances are also reported. The findings revealed

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it to be an eco-benign CO2 source to the C3 H4 O2 factory is critical for future exploration and advancement.

1.3 Conclusion and future perspectives The majority of the investigations that have been published so far have been carried out on laboratory-scale production under well-regulated circumstances. Several aspects, including as the availability of sufficient CO2 , fertilizers, and light, must be explored and improved in order to apply the optimal parameterization circumstances in the commercial generation of biofuels on a wide level. Technological viability has been demonstrated on a limited level, and tiny quantities of usable biofuel have been generated, but economical viability has yet to be determined. Microalgal-based biofuels should be price competitive with petroleumbased fuels in order to be commercially viable. The process could be made more affordable by incorporating the use of CO2 via direct origin exhaust gas outputs, sewage treatment, or the separation of essential components for use in various industries. Several studies on CO2 biosequestration and biofuel synthesis using organic elements have been conducted, but more investigation is warranted to satisfy the growing utilization of energy. Researchers predict that throughout the future, biofuel would mostly replace fossil fuels, reducing atmospheric CO2 amounts and averting global warming. As a result, a significant effort in the advancement of this technology, as well as technical skills in this field, are still essential until biofuel could become an actuality. The expansion of biofuel companies will undoubtedly be financially and ecologically advantageous, while also creating a great amount of employees at various levels.

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C H A P T E R

2 Deep eutectic liquids for carbon capturing and fixation Zainab Liaqat a, Sumia Akram b, Hafiz Muhammad Athar a and Muhammad Mushtaq a a b

Department of Chemistry, Government College University, Lahore, Pakistan Division of Science and Technology, University of Education Lahore, Pakistan

2.1 Carbon dioxide emissions The emission of various greenhouse gases (carbon dioxide, methane, nitrous oxide, water vapor, and fluorinated gases) has a very substantial effect on our environment. Among all gases, CO2 stands as the most emitted gas due to human activities [1]. High emissions of CO2 cause numerous environmental problems, particularly global warming due to the rise in global temperature. Global warming causes serious problems like, snow clads melting, glaciers melting rise in sea levels, and severe weather conditions. Therefore, it has become a worldwide challenge to reduce CO2 and other greenhouse gases (GHGs) and control global warming [2]. The combustion of fossil fuels (coal, natural gas, and oil) ranks as the most challenging and indispensable emission source of CO2 and other GHGs. Fig. 2.1 further elaborate the contribution of the various sectors toward the global CO2 burden. It is obvious that around 25 percent of CO2 emissions are produced worldwide during the production of electricity and heat for other uses [3]. Recently, CO2 capture has gained globally among researchers. A lot of technologies are already in practice to reduce the combustion of fossil fuels or minimize the emissions of GHGs [4]. Intergovernmental Panel on Climate Change-IPCC has declared Carbon-dioxide Capture & Storage (CCS) as the most impactful method to reduce the post-combustion CO2 burden. CCS depends on the efficiency of the employed process, materials and design, and overall cost [5]. The technologies explored for CO2 capture fall into three categories i.e., pre-combustion, post-combustion, and oxyfuel-combustion.

Green Sustainable Process for Chemical and Environmental Engineering and Science: Carbon Dioxide Capture and Utilization DOI: https://doi.org/10.1016/B978-0-323-99429-3.00007-2

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2. Deep eutectic liquids for carbon capturing and fixation

FIGURE 2.1 Sectoral Contribution (percent) of Greenhouse CO2 Emission in United States due to fossil fuels (https://www. epa.gov/ghgemissions/inventory-usgreenhouse-gas-emissions-and-sinks).

i. In the pre-combustion methods, CO2 is captured from oxidized fuel gas, i.e., synthesis gas/syngas, which consists of CO and H2 . This in turn reacts with steam and converts CO into CO2 and H2 from where it is captured. ii. In the post-combustion methods, after the combustion of fossil-fuel, CO2 is captured from the flue gases in a nitrogen-rich environment. iii. In oxy-fuel combustion, fossil fuel is burnt in an oxygen-rich environment. The resulting flue gas consists of CO2 and vapors which are condensed and pure CO2 is captured through the outlet. Most of the commercial plants employ pre-combustion and post-combustion technologies, but oxy-fuel technology is still under process [1]. Among all these technologies, postcombustion capturing has been widely employed due to its flexibility [4]. A variety of catalysts, liquid/solid adsorbents, and membranes are utilized for CO2 removal [6]. The predominant methods for CO2 capture are absorption, adsorption, and membrane-based separation. Captured CO2 is further stored and consumed for various purposes [7]. Out of three methods, adsorption is the most employed method due to its versatility, as this is suitable for both, pre and post-combustion techniques. Adsorption involves a high rate of heat and mass transfer and can be used either physically or chemically based on different reaction factors [8]. Solvents selection is one of the key points in absorption/adsorption methods [9]. According to the fifth rule of “Green Chemistry”, the use of safe solvents and auxiliaries should be emphasized to reduce the by-products [10]. Henceforth, the use of green solvents for desired applications and with least or zero environmental impact has gained the attention of researchers during last couple of decades [11]. For the sake of CO2 capture, many attempts

2.3 Types of deep eutectic liquids

19

have been made to achieve high efficiency [12]. Recently, a novel class of solvents similar to ionic liquids (ILs), called Deep Eutectic Liquids (DELs), has gathered much attention from scientists as a promising liquid for CO2 capture, particularly from an industrial point of view [13]. The present chapter covers the key features of deep eutectic liquids and opurtunities regarding the use of these liquids in carbon dioxide capturing. The chapter also contains the case-studies regarding the use of deep eutectic liquids and conditions for their application as CO2 absorber and capacitor. Finally, readers interested in challenges regarding the use of deep eutectic liquids for CO2 capturing and conversion may get useful information.

2.2 Deep eutectic liquids The term “deep eutectic solvent” has been initially notified in 2001 [14], and finally coined in 2003 [15], for the liquids that melt below their original melting points. The title eutectic actually originsfrom the Greek word “ευτ ηκτ ος ” which meansfacile melting [16]. Deep eutectic liquids (DELs) are not pure compounds rather these exist as combinations of various elements that come from a hydrogen bond donor (HBD) and a hydrogen bond acceptor (HBA). The mixture prepared under a specific stoichiometric ratio may have freezing points lower than either of the two components. The presence of strong hydrogen bonding and other intermolecular interaction renders the majority of these solvents non-volatile, nonflammable, and in certain cases liquid at room temperature. The principles of green chemistry [17] categorize these liquids as green for their non-volatile character. These liquids have become potential candidates to replace toxic conventional organic solvents and expensive ionic liquids (IL) for analytical, synthesis, and electrolytic solutions. Besides, their low cost, ease of raw material availability, biodegradability, and biocompatibility make them more viable as compared to ionic liquids. Another cite-worthy opportunity regarding DELs rises when we change the HBA/HBD ratios (more than 1 million different possible combinations) [18,19]. These liquids have the potential to revolutionize the fields of environmental, polymer, biological and material science. DELs may rank as one of the most important discoveries of the 21st century. Before we proceed towards the utilization of DELs for CO2 capturing, it is important to highlight various types of DELs and the fundamental mechanism involved in their formation or existence. Once becoming familiar with the molecular interactions present in DELs, we can easily customize the various physicochemical features of these liquids for their uses as adsorber or solvent for CO2 dissolution.

2.3 Types of deep eutectic liquids The deep eutectic liquids fall into five major classes based on HBD and HBA involved (Fig. 2.2) in their formation. The most frequently reported and the first generation deep eutectic liquids (labeled as type-I) are prepared by the combination of quaternary ammonium salts (HBA) and metal chlorides (HBD). The next generation (Type II) compose of similar HBD and HBA yet metal chlorides are hydrated which increases the scope and range of produced combinations. Another perspective aspect of type II liquids rises due to the ease of

20

2. Deep eutectic liquids for carbon capturing and fixation

FIGURE 2.2 Types of Deep Eutectic Liquids (DELs).

preparation as metal chlorides coupled with their inherent air/moisture can be used. The third type of deep eutectic liquids also contain similar quarternary ammonium compounds like choline chloride as HBA but here HBD may be alcohol, amine, or carboxylic acid. These low melting liquids are easy to prepare, economical, and non-reactive to biomolecules. Besides, the presence of a large number of organic hydrogen bond donors offers a wide range of combinations, adaptability, and application of this class of liquids. Another interesting class of low melting liquids (type IV) have been prepared by the direct combination of metal chlorides with hydrogen bond donors like urea. It had been generally believed that transition metal salts will not ionize in organic solvents until 2001 Abbott, Capper [14] found that ZnCl2 can form a eutectic mixture with urea, ethylene glycol, acetamide, and 1,6 hexanediol. Some fellow researchers also prepared type IV liquids by mixing HBD and hydrated metal chlorides [20]. Such low melting liquids generally stands as hydrophilic (because of the ionic components) which restricts their application in hydrated samples. The interesting features of non-ionic or hydrophobic DELs emerged as a feasible cure in this concern which have strong interactions among the components of type V of deep eutectic liquids [21]. Commonly, exclusive hydrophobic mixtures utilize long chain fatty acids, alcohols, and monoterpenes which work well for the preparation of these liquids. Recently, Choi et al. prepared natural deep eutectic liquids from natural HBD and HBA and claimed that many of such combinations already exist in living cells and provide a natural medium to various physiological phenomena [22]. A growing trend has appeared regarding the preparation and utilization of natural deep eutectic liquids for a large number of available combinations. Natural deep eutectic liquids possess extremely good solvation capacity along with low volatility and melting point [23]. Besides, the natural deep eutectic liquids prepared by mixing organic acid and glucose (1:1) have been found to be non-toxic, and biocompatible.

2.4 Preparation of DELs As stated earlier, deep eutectic solvents are customized molecular liquids, so their preparation may vary with the nature (melting and boiling points, and stabilities) of HBD and HBA, personal preference, and end-use applications. In general, the formation of deep-eutectic solvent involves the development of intermolecular interactions between HDB and HBA so the

2.5 Authentication of DELs

21

word synthesis misfit for their productions rather than “preparation or formation” are more suitable terminologies. Moreover, prolonged and abrupt heating may degrade the constituents of the solution to limit the effectiveness of the end-user application. In General, deep eutectic liquids have been prepared via three common procedures i.e. (i) thoughly heating the compounds with specific molar ratio in a properly sealed flask at 323–363 K with continuous stirring for 30–90 min till the production of a homogenous transparent solution [23], (ii) vacuum drying the compounds in a rotary evaporator at 323 K followed by addition of water, afterwards, drying the resultant mixture in a desiccator with silica or silica gel [24]. The vacuum drying approach seems to be more suitable for the preparation of natural deep eutectic solvents. Freeze-drying is another way (iii) to prepare low melting liquids which involves the mixing of HBD and HBA in a stoichiometric ratio, followed by dilution with distilled water to get transparent solutions which are subsequently freeze-dried for 2–24h [25]. The liquids prepared via the freeze-drying approach often contain micelle, vehicles, or nonreactor capsules which render them more suitable for biological applications. In the majority of mechanical methods, the HBD and HBA are either dried under vacuum until there is no further weight loss and then subjected to grinding or extrusion to get a clear low melting solution [26,27]. This strategy seems to be more suitable for thermally labile HBD and HBA components. Gomez et al. have applied microwave heating for the formation of low melting liquids and noted that microwaves can reduce energy and time consumption[28]. Likewise, Santana et al. have applied ultrasound waves to facilitate the formation of required liquids [29].

2.5 Authentication of DELs A set of fundamental molecular characterization approaches have been employed to authenticate the formation of DELs [30]. For example, Nuclear Magnetic Resonance (NMR) Spectroscopy works well to monitor the hydrogen bond transfers and determine the composition and structure of DELs [31]. In NMR or H–NMR, the proton environm of the DEL is compared with that of constituents HBDs and HBAs, however in combination with other techniques, it can provides the information about what is present in the mixture, water content, and impurities. Fourier Transform-Infrared (FT-IR) can also identify the shifts and adjustments in molecular structure of DELs in various molar ratios and compositions. Another important spectroscopy associated with molecular vibrations and rotation is Raman Spectroscopy and in this techniquethe observed transitionsare further analyzed through various techniques for detection of “what is present” and “how it is present” in various deep eutectic liquids. These all techniques are often applied to prove DEL formation or composition and purity of studied liquids with high accuracy. Beside spectroscopic characterization, thermogravimetric analysis (TGA) and differential scanning colorimetery (DSC) can help us to study the behavior of DELs in various applications. TGA is widely utilized to gain baseline data quickly for any unknown deep eutectic system, especially volatility. This technique delivers more detailed insights into physicochemical properties. DSC regulate the required heat for any change in temperature particularly latent heat transformation (like phase change or glass transition). Different thermodynamic parameters

22

2. Deep eutectic liquids for carbon capturing and fixation

such as melting point, entropies and enthalpies of fusion, thermal stability, and heat capacity of both the pure components and their resultant mixture, thus making it suitable for identifying anomalous behavior of DELs [32].

2.6 DEL based CO2 absorption The polar nature of DELs raises an opurtunity for these liquids to be used as absorbents in CO2. Besides, the physico-chemical features of these liquids are tunable. During last couple of decades, a great deal of deep eutectic liquids has been explored for their potential in CO2 capturing and absorbption. HBA has more significant impact on CO2 solubility. Interestingly, the widely used HBAs in DEL formation have been also found to be suitable in CO2 capturing experiments. Moreover, the solvents formed economical, easily available, and eco-friendly in their nature [33]. The high thermal stability and vapor pressure these liquids are the features of high importance for their application in CO2 capturing devices at variable temperature. For example, monoethanolamine (MEA), a commonly used absorbent, has an absorption capacity of almost 0.5 mol CO2 /mol MEA, but its vapor pressure is quite high which causes degradation on exposure to heat and oxygen. The degradation of MEA is the main disadvantage in terms of absorbent reusability. Comparatively, ChCl:urea based DELs having low vapor pressure and high thermal stability remain stable over the similar temperature range. The amine functional group is advantageous regarding CO2 capture; however, it is impossible to attach this functional group with any conventional solvent/absorbent. Above all, these liquids can be tuned by selecting various HBA and HBD to find out more suitable absorbent. Regardless of extenses associated with these liquids, they face a drawback of having high viscosity which can be solved by dilution. Dilution not only lowers the viscosity of DELs but improves the polarity and conductivity which results in increased CO2 uptake [34]. Intermolecular interactions in deep eutectic liquids after capturing CO2 were very first time reported by Shukla and Mikkola [35] through gravimetric analysis of ammoniumbased eutectic liquids composed of various HBA (monoethanolammonium chloride, 1methylimidazolium chloride, and tetra butyl ammonium bromide) and amine and aminoalcohol based HBD in different molar ratios at 298.15 K. The prepared liquids were checked in terms of polarity, viscosity and absorption. It was observed that the absorption of CO2 doesn’t only dependent on HBD basicity but also affected by the strength of interaction between HBA and HBD. In most of the DELs, an increase in the intermolecular forces increased the CO2 absorption but at the same time stronger intermolecular may increase the visocosity and reduce the mass transfer rate. The FT-IR and 1 H & 13 C NMR results reveal the formation of carbamte by showing peaks at 1558, 1292 cm−1 and at 3.12, >164 ppm, respectively. In addition, a peak at 1350–1505 cm−1 and below 160 ppm confirms the formation of carbonate/bicarbonate groups. The addition of water in studied deep eutectic liquids may reduce CO2 uptake due to increase in acidity and/or decrease in basicity. Another important point regarding the utilization of DELs is to understand how CO2 gets absorbed/adsorbed or dissolve in deep eutectic liquids. Many studies in this regard [36, 37] confirm that CO2 intract physically with the DEL. For example, Ullah, Atilhan [37] report the CO2 absorption mechanism in ChCl-levulinic acid-based deep eutectic liquids which don’t show a significant change in intramolecular interaction after absorption. This reveals

2.6 DEL based CO2 absorption

23

FIGURE 2.3 CO2 Absorption Mechanism in DELs made up of Amine based HBAs. (courtesy to Wibowo, Susanto [36]).

that the CO2 is absorbed physically and not chemically. In physical absorption, molecules of CO2 reside on the liquid surface for some period and then move towards the bulk phase. Wang et al. [38] described the effect of molar ratios and type of HBA and HBD on CO2 solubility. It was observed that HBA cation plays an important role in CO2 absroption and retention. The CO2 molecules are clustered around the molecules of HBA in spatial distribution function isosurfaces. To check the visualization of interactions between CO2 and HBA/HBD, Reduced Density Gradient-RDG, isosurfaces were utilized, which reveal strong van der Waals interactions. These interactions usually turn out in reversible type of sorption. In amine-based DELs, chemisorption may also takes place along with physical sorption, if it happens it can offer more CO2 solubility. In physical absorption, the van der waals interactions and H-bonding between different HBA and HBD also control the solubility of CO2 . While chemsorption take place due to the formation of carbamates as shown in Fig 2.3 [39], in which carbamate formation also occurs along with van der Waals forces between CO2 and HBA. Thus, it is assumed that amine-based deep eutectic liquids are highly efficient in CO2 capturing but the regeneration process can be complicated [36]. In another study, Cheng, Wu [40] projected the mechanism for CO2 absorption by protic ionic liquid-based DEL made up of monoethanolamine + imidazole-ethylene glycol in a 1:3 molar ratio at 298.15 K and 1 atm pressure. It was found that CO2 molecules bind with the amine group of monoethanolamine (carbamate formation) and deprotonate ethylene glycol (carbonate formation). It was supposed that either deprotonation of protonated monoethanolamine cation by imidazole anion results in the formation of neutral monoethanolamine, which in turn reacts with CO2 forming carbamate or EG is deprotonated by imidazole, which then reacts with CO2 to form carbonate. Ishaq, Gilani [41] evaluated the performance of synthesized poly deep eutectic liquidsbased on supported liquid membranes (PDEL-SLM) for pure gases. It was observed that the solution-diffusion mechanism controls CO2 transport across the membrane. Solutiondiffusion is a three-step process, in the first step CO2 is dissolved in DELs, in the second step it is diffused through the bulk phase, and in the last step it is desorbed towards the permeate side

24

2. Deep eutectic liquids for carbon capturing and fixation

FIGURE 2.4 The pathway for carbonate formation during Carbon Capturing by ternary DELs.

of the membrane, to form a concentration gradient. In SLM, only CO2 molecules permeate into the membrane, mostly liquids couldn’t pass from the pores because of the hydrophobicity of the membrane [42]. Recently, Wang, Wang [43] noted that DELs formed by bio-phenol derived ionic liquid and ethylene glycol can capture up to 1.0 mol of CO2 /mol of DEL. The mechanism of CO2 capture in superbase-derived phenol-based DELs is quite different and occurs in two stages (Fig. 2.4). In first stage, anion (Car− ) deprotonate the EG in an acid-base reaction, which results in the formation of HO–CH2-CH2-O− . In the second stage, the anion formed of EG further reacts with CO2 molecules to form carbonate .

2.7 Carbon capture efficiency of various HBDs In general the carbon atom in the vicinity of a more electronegative atom (like O in the case of CO2 ) tends to develop van der waal’s attractions or Hydrogen bond for the formation of Carbonic Acid (Fig. 2.5). In this context, it can be predicted that liquids containing O, N, S, and halogens (X) can offer good platform for the absorption or capturing of CO2 . The similar

2.7 Carbon capture efficiency of various HBDs

25

FIGURE 2.5 The Molecular Orientation CO2 undergo during Dissolution/Absorbtion.

kind of behavior has been observed by the researchers who undertaken the capture of CO2 by DELs or ILs. Besides, the presence of electronegative atom in the neighbourhood of C makes it more electropositive that turn into strong interaction between the carbon and Cl of ChCl (a universal HBA in DELs). The subsequent section highlight the research work undertaken where C of CO2 works as HBA.

2.7.1 Urea Recently, Li, Hou [44] explored the CO2 absorption capacity of deep eutectic liquids consisting of ChCl:Urea in 1:1.5, 1:2, and 1:2.5 molar ratios at various temperatures and pressures. The temperature was studied in the range of 313–333 K with 10 K intervals and pressure was applied upto 13MPa. It was observed that temperature, pressure, and mole ratios significantly (p ≤ 0.05) affected the solubility of CO2 in ChCl:Urea. The solubility of CO2 in these liquids varied abruptly in the low-pressure range and usually increased with increasing pressure. At high temperatures, solubility decreases regardless of the pressure. For the molar ratio of ChCl:Urea mixtures, 1:2 molar ratio exhibited the highest solubility (0.27 mol/kg) than the other two mixtures at 11.1 MPa pressure and 323 K temperature. Likewise, Xie, Dong [45] also studied the CO2 solubility in ChCl:Urea based DELs at different temperatures (308–328 K) with 10 K interval, and at a pressure of up to 45 bars. It was noted that the solubility of CO2 , within the investigated ranges, increases at high pressure and low temperature. The highest solubility of CO2 was 0.195 molar at 308 K and 44 bars. The CO2 solubility was further estimated in dry and aqueous ChCl-urea mixtures. It was noted that solubility increases in an aqueous mixture up to Wwater < 0.5 molar and then it becomes constant. An aqueous mixture will reduce the required solvent amount and viscosity, hence lowering the energy and cost of the process.

2.7.2 Glycerol Leron and Li [46] measured the CO2 solubility in ChCl:Glycerol based DEL in a 1:2 molar ratio at temperature 303–343 K, with 10 K interval, and at the pressure of up to 63 bars. It was again noted that CO2 solubility in ChCl-Glycerol based DEL mixture increases with increasing pressure and decreases with increasing temperature. The highest captured mole fraction of CO2 was observed up to 3.126 mol/kg at 303 K and 58 bars. The solubility data was validated

26

2. Deep eutectic liquids for carbon capturing and fixation

by Henry’s law (CO2 solubility as a function of pressure and temperature) with a 1.61 percent absolute deviation. Overall, glycerol based HBD when combined with ChCl offered the carbon capturing capacity comparable with sugar based DELs (Table 2.1). Alok, Dawn [47] reported the absorption of CO2 by preparing crude glycerol and choline chloride-based deep eutectic liquids in 1:1, 1:2, 2:1, 1:3, 3:1, 1:4, and 4:1 molar ratios. This mixture chemically absorbs the CO2 and forms carbamate, from the reaction between CO2 and their HBD units. Out of various crude Gly-ChCl ratios, the highest CO2 solubility (0.377 wt percent) was observed in 2:1 molar ratio after 24hrs. The 2:1 molar ratio was observed further for optimization of temperature, time, and water for CO2 absorption. At 343.15 K, maximum absorption was observed (0.331 wt percent). The optimized time for maximum CO2 absorption (0.123 wt percent) was found to be 20 mins. The absorption of CO2 increases with moisture content up to 10mL (0.381 wt percent) and then become reduces with the addition of water. the FT-IR analysis shows the peak at 2927 cm−1 , which indicates the formation of carbamate. The TGA-DSC analysis indicates the degradation of the respective mixture at 318 °C. The authors found crude glycerol-based solvent systems worth exploring in the future.

2.7.3 Glycerol + L-arginine Chemat, Gnanasundaram [48] evaluated the CO2 solubility in ChCl-glycerol-l-arginine based ternary deep eutectic liquids mixed with l-arginine at 303.15 K and 6–20 bar in different molar ratios. The ChCl-glycerol + L-arginine (1:2:0.1 molar ratio) showed the highest CO2 solubility (5.23 mol CO2 /kg DEL), which was further evaluated on a temperature range of 303–323 with 70 bar pressure. The results show that the uptake of CO2 increases when pressure increases and temperature decreases. l-arginine has a significant effect on CO2 solubility. The higher CO2 solubility is might be due to the presence of amine groups in l-arginine which interact with CO2 . At a high ratio of l-arginine, CO2 solubility decreases due to the high viscosity of deep eutectic liquids. Haider, Jha [49] observed the CO2 solubility in both amine and glycol-based deep eutectic liquids at 303.15 K temperature and 0.1–2 MPa pressure. Choline chloride (ChCl) and tetra butyl ammonium bromide (TBAB) were used as HBA and ethylene glycol (EG), diethylene glycol (DEG), methyldiethanolamine (MDEA), and diethanolamine (DEA) as HBDs. Overall, amine-based systems exhibited higher solubility than glycol-based systems due to the more interactions between the amine group and CO2 molecules. It was observed that in a glycolbased system, HBD has a very pronounced effect on the solubility of CO2 as it decreased from EG to DEG due to the oversaturation, which caused a reduction in free volume. An increase in the DEG mole ratio results in improved solubility due to a decrease in H-bond strength. In FT-IR after absorption spectra, a peak at 2340 cm−1 is due to asymmetric stretching of O = C = O, other spectra remain unchanged confirming the physical absorption of CO2 in MDEA-based systems. Whereas, in after absorption spectra of DEA, a peak at 1441 cm−1 is associated with C = O vibration of carbamate confirming chemisorption. DEA forms carbamate due to the availability of one hydrogen bond to be replaced with CO2 , while there is no hydrogen bond available in MDEA.

TABLE 2.1 A Comparison of Carbon Capturing Potential of different Deep Eutectic Liquids under different conditions. Thermodynamic Conditions (K)/(MPa)∗

Solubility (mol kg−1 )

Remarks

References [44,45]

HBD

Choline Chloride

Urea

1:1.5

1. 398

0.20

Economical

1:2

2. 1

0.27

Low CO2 solubility

1:2.5

1. 308

0.20

Thermally unstable

1:2

2. 4.5

0.19

Not easily recycleable

1:2

1. 303

3.12

Economical

Glycerol

2. 5

[46]

Low CO2 solubility Thermally stable Not easily recycleable

Crude glycerol

1:1

1. 353

0.22

Economical

1:2

1. 368

0.10

Low CO2 solubility

0.10

Thermally unstable

0.34

Not easily recycleable the addition of water enhance CO2 solubility

1:3 1:4

1. 343

2:1

0.38

3:1

1. 313

0.14

4:1

1. 368

0.10

[47]

2.7 Carbon capture efficiency of various HBDs

HBA

Molar ratio HBA:HBD

(continued on next page)

27

28

TABLE 2.1 A Comparison of Carbon Capturing Potential of different Deep Eutectic Liquids under different conditions—cont’d

HBA

Thermodynamic Conditions (K)/(MPa)∗

Solubility (mol kg−1 )

Remarks

References

Fructose

1:1

1. 298

4.24

Economical, non-toxic

[38]

2. 5.0

4.52

High CO2 solubility

4.22

Thermally unstable

Lactic acid Malic acid

Not easily recycleable Solubility increases with the bulkier group on HBA 1,2-propanediol

1,4-butanediol

2,3-butanediol

Monoethanolamine

1:3

1. 293

0.16

Low CO2 solubility

1:4

2. 0.5

0.15

Thermally unstable

1:3

1. 293

0.15

Not easily recycleable

1:4

2. 0.5

020

1:3

1. 293

0.18

High molar ratio and pressure can show higher solubility

1:4

2. 0.50

0.18

1:5

1. 333

0.32

Economical

1:6

2. 1.5

0.31 g/g

Good CO2 solubility

1:8

1. 313

0.33 g/g

Thermally unstable

1:10

2. 0.02

0.34 g/g

Not easily recycleable

[50]

[52]

(continued on next page)

2. Deep eutectic liquids for carbon capturing and fixation

HBD

Molar ratio HBA:HBD

Diethanolamine

Methyldiethanolamine

0.14 g/g

1:8

0.15 g/g

1:10

0.16 g/g

1:6

0.02 g/g

1:8

0.02 g/g

1:10

0.02 g/g

High temperature and pressure

[51]

1:2

1. 293

2.316

Economical, non-toxic

1:3

2. 3.0

0.25

Good CO2 solubility

1:4

1. 303

0.27

Thermally unstable

1:5

2. 0.57

0.28

Not easily recycleable

Phenol

1:2

1. 293.15

0.20

Toxic

Diethylene glycol

1:3

2. 0.50

0.20

Low CO2 solubility

Triethylene glycol

1:4

1. 293

0.21

Thermally more stable

1:3

2. 0.50

0.16

1:4

1. 293.15

0.18

Not easily recycleable An increase in pressur and mole ratio improves CO2 solubility

[37,56]

[59]

2.7 Carbon capture efficiency of various HBDs

Levulinic acid

1:6

(continued on next page)

29

30

TABLE 2.1 A Comparison of Carbon Capturing Potential of different Deep Eutectic Liquids under different conditions—cont’d

HBA

HBD

Molar ratio HBA:HBD

Thermodynamic Conditions (K)/(MPa)∗

Solubility (mol kg−1 )

1:3

2. 0.50

0.19

1:4

References

Solubility decreases from EG to DEG due to oversaturation, both chemical & physical absorption occurs

[49]

Functionalization with amines results in improved absorption, difficult regeneration

[54]

0.19

1:2

1. 303

0.045 mol−1

Diethylene glycol

1:3

2. 1.0

0.037 mol−1

1:4

0.038 mol−1

Diethanolamine

1:6

0.098 mol−1

Methyldiethanolamine

1:6

0.180 mol−1

1:7

0.195 mol−1

1:7:1

1. 298

1:7:5

2. 2.0, 2.2

0.22

Ethanolamine/Amino ethyl piperazine

0.26

Ethanolamine/ Piperazine

0.360

Ethanolamine/ Methyldieth-anolamine

0.259 0.250 (continued on next page)

2. Deep eutectic liquids for carbon capturing and fixation

Ethylene Glycol

Ethanolamine/ Diethanolamine

Remarks

Monoethanolamine/ Diethanolamine

1:2

1. 298

2.67

Glyceline/ Diethanolamine

0.86

Piperazine activated glyceline/ Diethanolamine

1.53

1:2:0.1

1. 303

5.23

Economical, non-toxic

1:2:0.2

2. 2.0

4.01

High CO2 solubility

1:3:0.1

3.99

1:3:0.2

3.75

High ratio of l-arginine can decrease CO2 solubility due to high viscosity

1:4:0.1

3.76

1:4:0.2

3.40

Urea + 20 percent water

1:2:9.97

1. 313

0.342

Low cost

Ethylene glycol + 20 percent water

1:2:12.64

2. 0.10

0.289

Non-toxic

1:2:10.14

0.249

Addition of water reduces solubility, low viscosity

Glycerol + 20 percent water

1:2:14.65

0.246

1:1:10.76

0.219

[48]

[33]

2.7 Carbon capture efficiency of various HBDs

Glycerol/L-arginine

[34]

Levulinic acid + 20 percent water DL-Malic acid + 20 percent water (continued on next page)

31

32

TABLE 2.1 A Comparison of Carbon Capturing Potential of different Deep Eutectic Liquids under different conditions—cont’d

HBA

HBD Monoethanolamine + 50 percent water

Molar ratio HBA:HBD

Thermodynamic Conditions (K)/(MPa)∗

Solubility (mol kg−1 )

1:5

1. 293

0.461

2. 1.5

References

Thermally stable, highly selective, efficient

[52]

High solubility than ILs, efficient, regeneration at high temperature

[63]

0.612 1. 323.15

0.51 mol−1

Glycerol/1,5diazabicyclo[4.3.0]-non5-ene

1:2:3

Glycerol/1,5diazabicyclo[4.3.0]-non5-ene

1:2:6

0.42 mol−1

Glycerol/1,5diazabicyclo[4.3.0]-non5-ene

1:2:7

0.41 mol−1

Glycerol/1,5diazabicyclo[4.3.0]-non5-ene

1:2:8

0.39 mol−1

Glycerol/1,5diazabicyclo[4.3.0]-non5-ene

1:2:10

0.39 mol−1

Glycerol/1,8diazabicyclo[5.4.0] undec–7-ene

1:2:6

0.14 mol−1

(continued on next page)

2. Deep eutectic liquids for carbon capturing and fixation

Monoethanolamine + 75 percent water

Remarks

1:2:6

Urea/ Monoethanolamine

1:2

0.40 mol−1

1. 301

0.70 mol−1

2. 0.10

0.58 mol−1

Urea/Triethanolamine

0.22 mol−1

Urea/2methylaminoethanol

0.95 mol−1

Urea/2-amino-2methyl-1-propanol

1.00 mol−1

Imidazole

1:3

1. 293

0.17

Low CO2 Solubility

1:4

2. 0.516

0.17

1:5

0.18

High levels of guaiacol increases solubility, low dissolution enthalpy

1:3

0.21

Easy Desorption

1:4

0.22

1:5

0.23

1:3

0.18

1:4

0.19

1:5

0.19

1:2

1. 303

[62]

Functionalization with amines enhances the CO2 good recyclability

Urea/Diethanolamine

Guaiacol

Good CO2 solubility

0.2607

Difficult to handle

[57]

2.7 Carbon capture efficiency of various HBDs

Diethylamine hydrochloride Acetylcholine chloride

Glycerol/7-methyl1,5,7-triazabicyclo [4.4.0]dec–5-ene

[58] (continued on next page)

33

34

TABLE 2.1 A Comparison of Carbon Capturing Potential of different Deep Eutectic Liquids under different conditions—cont’d

HBA

HBD

Molar ratio HBA:HBD

Thermodynamic Conditions (K)/(MPa)∗

Solubility (mol kg−1 )

1:3

2. 57, 567, 572, 587

0.2940 0.22

1,2,4-triazole

1:1

0.21

Levulinic acid

1:3

1. 303

0.30

Tetraethylammonium bromide

1:3

2. 0.54

0.27

Tetrabutylammonium chloride

1:3

0.24

1:3

0.30

Tetraethylammonium chloride

Tetrabutylammonium bromide

Ethylene glycol

1:3

1. 303

0.26

1:2

2. 1.0–2.0

0.05 mol−1

1:3

0.05 mol−1

1:4

0.05 mol−1

References

High imidazole content has a positive impact on solubility, thermally stable Bulkier cations show high solubility, exothermic

[65]

Solubility decreases from EG to DEG due to oversaturation, both chemical & physical absorption occurs, amines show higher solubility

[49]

(continued on next page)

2. Deep eutectic liquids for carbon capturing and fixation

2:3

Remarks

0.11 mol−1

1:3

0.09 mol−1

1:4

0.09 mol−1

1:3

0.25 mol−1

1:4

0.29 mol−1

Diethanolamine

1:6

0.10 mol−1

Octanoic acid

1:2

Methyldiethanolamine

Decanoic acid DL-menthol

Dodecanoic acid

2:1

Tetraethylenepentamine chloride

Thymol

1:3

Triethylenetetramine chloride

Diethylene glycol

Methyltriphenylphosphonium Bromide

1. 293

0.59

2. 6.0, 6.6, 3.56

0.75

[68]

Low volatility, synergism of physical and chemical absorption, efficient at high thymol content

[73]

Basic, efficient, good regeneratability

[67]

DEG show reasonable CO2 solubility due to low viscocity, good regeneration, pressurised absorption

[4]

0.39 1. 313

1.355 mol−1

2. 0.10

1.298 mol−1

1:2

1. 313

1.42 mol−1

Ethylene glycol

1:3

2. 0.10

1.46 mol−1

Diethylene glycol

1:4

1. 303

0.053

Glycerol

1:4

2. 10.19, 9.21, 10.39

0.062

1:3

Hydrophobic, good solubility

2.7 Carbon capture efficiency of various HBDs

1:2

Diethylene glycol

0.059 (continued on next page)

35

36

TABLE 2.1 A Comparison of Carbon Capturing Potential of different Deep Eutectic Liquids under different conditions—cont’d

HBA

HBD

Molar ratio HBA:HBD

Monoethanolamine hydrochloride

Methyldiethanolamine

1:3

Solubility (mol kg−1 )

1. 298

0.1158 g/g

2. 0.10

0.1082 g/g

Methyldiethanolamine hydrochloride Imidazole

1,5-diazabicyclo[4.3.0]non-5-ene N-methylthiourea

References

Efficient, dilution lowers viscosity and improve solubility, good recycability

[55]

High pressure and imidazole content show high solubility, non-spontaneous, Recycleable

[72]

Improved absorption on higher mole ratio of EU, high viscosity of DMLU based system, good recyability

[64]

Increased absorption in [DBNH]2 [DTU]:EG due to multiple site interaction, thermally stable, chemisorption results in carbamate and carbonate formation

[76]

0.0594 g/g p-toulene sulfonic acid

3:1

1. 303

1.0059

3.5:1

2. 1.44, 1.49, 1.48

1.0642

4:1 1,5-Diazabicyclo [4.3.0]non-5-ene

Remarks

Ethylene urea

2:1 3:1

1.0959 1. 318

1.75 mol−1

2. 0.1

mol−1

2.01

1,3-dimethylurea

2:1

1.32 mol−1

Dimethylolurea

2:1

0.38 mol−1

Ethylene glycol

1:1

1. 313 2. 0.10

0.145 g/g

(continued on next page)

2. Deep eutectic liquids for carbon capturing and fixation

Diethanolamine hydrochloride

Thermodynamic Conditions (K)/(MPa)∗

TABLE 2.1 A Comparison of Carbon Capturing Potential of different Deep Eutectic Liquids under different conditions—cont’d

HBA

HBD

Molar ratio HBA:HBD

Thermodynamic Conditions (K)/(MPa)∗

1:2

1,5-diazabicyclo[4.3.0]non-5-ene N-dimethylthiourea Malic acid

Betaine

Lactic acid

1:1

Remarks

References

Solubility increases with bulkier group on HBA, non-toxic, ease of availability

[38]

Low cost, excellent regeneration, good absorption, heat resistant

[74]

Comparatively low solubility than other DELs, PAA show highest solubility due to π -π interactions among these, low regeneration

[69]

0.173 g/g

1. 298

4.14 mmol/g

2. 5.0

4.30 mmol/g 4.26 mmol/g

Carvone

Thymol

Cineole

Menthol

1:1

1. 298

0.48

2. 4.0

0.50

Thymol

0.51 0.50

N,N,N-trimethylglycine

Oxalic acid dihydrate

1:2

1. 333

1.72 mg/g

2. 4.0

∗

Glycolic acid

1.68 mg/g

Phenylacetic acid

32.0 mg/g

2.7 Carbon capture efficiency of various HBDs

β-alanine

Solubility (mol kg−1 )

1. Temperature in Kelvin rounded off to 3 significant figure, 2. Pressure (mPa or state) round off to 3 significant figures.

37

38

2. Deep eutectic liquids for carbon capturing and fixation

2.7.4 Natural organic acids Altamash, Amhamed [38] investigated the effect of HBD and HBA on CO2 in natural deep eutectic liquids. The tested HBAs included ChCl, betaine, and β-alanine, whereas HBDs applied comprise Fructose (Fr), Citric acid (CA), Malic acid (MA) and Lactic acid (LA). It was noticed that the no. of hydroxyl groups in HBD controlled the solubility of CO2 . In general, higher the number of hydroxyl groups in HDB the more will be the solubility. This is due to the inter and intramolecular H-bonding between HBD and CO2 . There are 5, 4, 3, and 2 hydroxyl groups in Fr, CA, MA, and LA, respectively. In the case of HBA, an increase in the polarity or size of groups attached to the N atom can increase the solubility of CO2 in resultant DEL. The results of the subject study confirms that natural deep eutectic liquids made up of amino acids (HBA) and sugars/organic acids (HBD) offer more fascinating opurtunities. However, nature of interactions in natural deep eutectic liquids are more complex and are still undisclosed. Untill now, the development of strong van der Waals interactions can be speculated between molecules of CO2 and natural DEL.

2.7.5 Dihydric alcohols The various combinations of ChCl with dihydric alcohols like 1,2-propanediol, 1,4butanediol, and 2,3-butanediol were prepared and tested for their CO2 capturing potential by Chen, Ai [50]. The solubility of CO2 was monitored at different temperatures (293–323 K) with 10 K interval and 6 bars pressure under the isochoric conditions. The solubility of CO2 in dihydric alcohols based DEL mixtures increases as the applied pressure increases and the temperature decreases. It has been observed that DEL contain ChCl and 2,3-butanediol in a 1:4 molar ratio shows the highest solubility of CO2 among all others. The highest mole fraction of absorbed CO2 was observed up to 0.1915 mol/kg at 293.2 K and 508.5 kPa. The enthalpies of all solutions were observed as negative in all conditions.

2.7.6 Amines Three different amine-based DELs were employed in three different molar ratios using ChCl as HBA. The amine based deep eutectic liquids have the advantage of high solubility for CO2 over conventional eutectic liquids and aqueous amine solutions. The highest solubility of CO2 (0.150 g CO2 /g of solvent) was observed in ChCl-monoethanolamine in 1:8 molar ratio. With increasing concentration of monoethanolamine (MEA), CO2 solubility also increases. This higher solubility can be attributed to the simultaneous presence of two electronegative functional group i.e. NH2 and OH. The N–H stretching and O–H broadening in the FT-IR spectrum indicate the formation of H-bonds between these two before absorption. The peaks at 1950 and 2200 cm−1 due to carbamate formation confirmed the chemical absorption in these liquids [51]. Yan, Huan [52] and Wibowo, Liao [53] also undertaken the CO2 capturing from biogas by using ChCl-MEA based DELs comprising 1:5 molar ratio of HBA and HBD. The effect of temperature, pressure, and water content on the absorption capacity and selectivity of CO2 was evaluated by ANOVA and response surface methodology (RSM). The highest absorption was recorded in ChCl-MEA with 75 percent water i.e., 0.609 mol CO2 /mol liquid at 293.15 K temperature and 1.50 MPa pressure. Whereas, pure ChCl-MEA based system showed highest capacity at higher temperature of 333.15 K temperature due to high viscosity. The

2.7 Carbon capture efficiency of various HBDs

39

average absorption capacity of this system was 0.481 mol CO2 /mol solvent. With increasing pressure and water content, the selectivity and absorption capacity increases, while at higher temperature, both the parameter decrease. As the temperature has less influence on the absorption rate than pressure and water content, it confirms the physical absorption in this system. ChCl-MEA based liquids and its aqueous solutions showed pronounced effect on CO2 solubility from biogas, hence, provided potential system for future research. Sarmad, Nikjoo [54] investigated the effect of novel ternary DEL ChCl-ethanolamine (EA) in 1:7 molar ratio functionalized with amine type 1, 2, 2, and 3, as 1-(2- aminoethyl) piperazine (AEP), piperazine (Pz), and diethanolamine (DEA), methyldiethanolamine (MDEA), respectively. These liquids were prepared at 29.15 K temperature and 2 MPa pressure and have their melting points in the range of 264–275 K. The FT-IR and 13 C NMR confirm the carbamate formation due to the chemisorption. The association model was used to explain the thermodynamic characteristics and the categories of chemical bonding set were AB2, and AB (A = DEL, B=CO2 ). The results further revealed that the solubility of CO2 in ChCl-EA-amine based system increases in the order of Pz > AEP > MDEA > DEA. The piperazine (Pz) containing DEL system exhibited the CO2 solubility equivalent to 0.360 mol CO2 /kg DEL. The CO2 capturing efficiency of methyldiethanolamine based DELS was checked along with regeneration capability. Three different DEL with methyldiethanolamine (MDEA) as HBD and different HBA [Monoethanolamine hydrochloride (MEAHCl), Diethanolamine hydrochloride (DEAHCl), and Methyldiethanolamine hydrochloride (MDEAHCl)] were prepared in 1:3 molar ratio at 298.15 K and at CO2 flow rate of 10.1 mL/min. Density Functional Theory and Molecular Dynamics were utilized to evaluate the characteristics of these liquids and their interaction with CO2 molecules. The absorption of CO2 is mainly concerned with HBA. MEAHCl-MDEA based DEL exhibited the highest CO2 absorption (0.115 g CO2 /g of DEL) which was further improved (0.145 g CO2 /g of DEL) by adding 13.50 wt percent of MEA at same temperature in 1:2:0.5 molar ratio. DFT and Molecular Dynamics showed that the interaction of MEAHCl-MDEA with CO2 is through amine group and hydroxyl group in MEAH and MDEA, respectively, also approved by NMR and FT-IR. Addition of primary or secondary amine to aqueous solution of tertiary amine improves the CO2 uptake efficiency. Ternary DEL is cost effective and efficient system which has low desorption energy and shows tremendous regeneration capacity of about five cycles [55].

2.7.7 Levulinic acid The CO2 absorption capacity DEL comprising ChCl and levulinic acid in a molar ratio of 1:2 was estimated at 293 K, 298 K, 308 K, 318 K, and 323 K, and at the pressure up to 30 bars. The highest CO2 solubility (2.316 mmol/gram of DEL) was observed at 293 K temperature and 50 bar pressure [37]. In another report, choline chloride was combined with levulinic acid and furfuryl alcohol in 1:3, 1:4, and 1:5 molar ratios and applied for CO2 absorption in the range of 303.2–222.2 K and 6 bars under isochoric-saturation condition. The obtained results show that CO2 has higher solubility in levulinic acid-based DEL than in furfuryl alcohol-based one. The highest solubility was observed as 0.33 mol CO2 /kg of DEL. Likewise other system described above, the solubility of CO2 in these DELs was found to be function of temperature, pressure, and molar ratio [56].

40

2. Deep eutectic liquids for carbon capturing and fixation

2.7.8 Guaiacol The presence of the benzene ring and –C–O-C- group in guaiacol makes it an efficient candidate for CO2 capture. Liu, Gao [57] prepared guaiacol-based DELs using quaternary ammonium salts like ChCl, acetyl ChCl, and DEA/Cl as HBA and guaiacol as HBD in 1:3, 1:4, and 1:5 molar ratio. The subject DELs were tested for CO2 absorption at 293.15–323.15 K temperature and 0.6 MPa pressure by isochoric saturation method. With the increasing molar ratio of guaiacol, the solubility of CO2 increased. The absorption of CO2 in the studied system was declared to be physical dissolution. DEA/Cl- guaiacol based system in 1:5 molar ratio outperformed the best solubility of CO2 (0.2321mol CO2 /kg DEL) at 293.15 K and 0.522 MPa. Low dissolution enthalpy is required for desorption of CO2 .

2.7.9 Azoles Recently, various azole-based DELs were utilized by Li, Liu [58] to capture CO2 efficiently in isochoric-saturation method. Acetyl ChCl as HBA was mixed with imidazole (Im) and 1,2,4triazole (Tri) as HBD in 1:2, 1:3, and 2:3 molar ratios at 303.15–333.15 K and pressure up to 6 MPa. Solubility of CO2 increases with increasing pressure and decreasing temperature i.e., CO2 is absorbed by physical dissolution. Henry’s law was used to correlate solubility data and thermodynamic properties. Acetyl ChCl-Im system in 1:3 molar ratio showed the highest solubility of CO2 (0.2940 mol CO2 /kg DEL), while Acetyl ChCl-Tri in 1:1 molar ratio had the least CO2 solubility (0.2190 mol CO2 /kg DEL) at 303.15 K and 0.567 and 0.587 MPa pressure, respectively. It is confirmed from the results that high imidazole (Im) content has a positive impact on the solubility of CO2 . Henry’s constant of solubility increases with an increase in temperature.

2.7.10 Miscellaneous HBD Choline chloride was combined with different hydrogen bond donors like phenol, diethylene glycol, and triethylene glycol in 1:2, 1:3, and 1:4 molar ratios. The solubility of CO2 was measured at a temperature range of 293.2–323.2 K, with a 10 K interval and 6 bars pressure. The results showed that the solubility of CO2 increases as the pressure increases and decreases with the increase in temperature. The enthalpies of all solutions were observed as negative in all conditions. Furthermore, choline chloride-triethylene glycol-based liquid mixture in 1:4 molar ratio shows the highest CO2 solubility (0.1941 mol/kg) among others [59].

2.8 CO2 absorption in aqueous solution of DELs Reline-based eutectic liquids, consisting of ChCl: Urea mixture in a 1:2 molar ratio, has the lowest known melting point among known DELs. Reline is hygroscopic as most of the ChCl-based liquids must contain water in trace amounts. This water affects the H-bonding interactions and subsequently their physical/chemical properties. Most of the eutectic liquids and their aqueous mixtures have a low density at high temperatures and high densities at high pressure (1–500 bars) in 1:2 molar ratios. This behavior of density is due to the dependence of

2.9 CO2 absorption in ternary DELs

41

H-bonding on the temperature. It also reduces the molecular distance and hence free volume. In eutectic liquids, water molecules act as an anti-solvent which blocks the absorption of CO2 . Therefore, the hygroscopic behavior of DEL can significantly affect the CO2 solubility at lower pressure [60]. Sun, et al. studied the effect of water on CO2 solubility in reline solution. He investigated that CO2 absorption becomes exothermic with water concentration of less than 0.231 molar ratio. Above this ratio the absorption process becomes endothermic. This outcome is only concerned when the swing-absorption stripping process is considered for CO2 capture. Hsu et al. investigated the solubility of CO2 in a binary solution of aqueous reline (ChClurea) and ternary solution (ChCl-urea-monoethanolamine) in 1:2 molar ratio. The obtained results demonstrated that the solubility of CO2 in binary aqueous solution can be enhanced by combining it with monoethanolamine [61]. Aravena, Lee [33] studied the absorption of CO2 in aqueous deep eutectic liquids based on ChCl as HBA and urea, ethylene glycol, glycerol, malic acid, levulinic acid as HBD, and diluted in 20 percent wt water to improve mass transfer. ChCl-urea-water system was proved to be an efficient system for CO2 solubility in place of alkanolamine based absorbents. At high temperatures, solubility decreases due to the dominant desorption phenomenon. Due to the addition of water, the surface tension of the prepared liquids was increased while the viscosity and density were decreased. The rate of absorption decreases with the addition of water. FT-IR and NMR results revealed that both chemisorption and physical absorption occur in the ChCl-urea-water system, with the former due to the amino groups in urea.

2.9 CO2 absorption in ternary DELs 2.9.1 Alkanolamines The absorption capacity of CO2 in numerous alkanolamines was assessed in deep eutectic liquids and an aqueous medium by Muthu et al. The amines used were monoethanolamine (MEA), diethanolamine (DEA), triethanolamine (TEA), 2-methylaminoethanol (MAE), and 2-amino-2-methyl-1-propanol (AMP). These amines were employed in ChCl:urea medium in optimized molar ratio. The solubility of CO2 in these amines-based eutectic liquids was higher as compared to the aqueous medium. 2-amino-2-methyl-1-propanol-based liquids exhibited higher solubility of CO2 among all the solvents [62].

2.9.2 Superbases Sze, Pandey [63] investigated the effect of different superbases i.e., 1,5-diazabicyclo[4.3.0]non-5-ene (DBN), 1,8-diazabicyclo[5.4.0]undec–7-ene (DBU), and 7-methyl-1,5,7-triazabicyclo [4.4.0]dec–5-ene (MTBD) on the solubility of CO2 . These superbases, when combined with ChCl-Gly-based DEL in different molar ratios can deprotonate the OH groups of ChCl and Gly. The 13 C NMR before and after mixing with CO2 indicates the phenomenon of chemical absorption of. The DBN (an economically avialble base) containing DEL offered an exceptional results. Different molar ratios (ChCl-Gly-DBN 1:2:x, whereas x = 3,6,7 or 8) of DBN-DELs were studied for CO2 solubility and it was observed that 1:2:7 molar ratio gave the highest solubility of CO2 i.e., 105mg/g DEL. These modified liquids have higher solubility values

42

2. Deep eutectic liquids for carbon capturing and fixation

even to those of ILs. Another advantage of these mixtures is that they can be regenerated within 35 mins of heating at 333.15 K in a nitrogen-rich environment. Functionalized DELs (or ternary DELs) are proved as a promising alternatives to conventional solvents or other low melting liquids. In this regard, Jiang, Ma [64] prepared a novel class based on acylamidesuperbase in 1:2 molar ratio. 2-imidazolidone/ethyleneurea (EU)−1,5-Diazabicyclo[4.3.0]non5-ene (DBN) based system exhibited the highest CO2 solubility of 1.751 mol CO2 /mol DEL (upto 23.03 percent wt) at 318.15 K temperature and 0.1 MPa pressure. The absorption capacity of EU-DBN is due to its ring structure, which causes low steric hindrance. When the molar ratio of DBN is increased from 1:2 to 1:3, absorption capacity increases from 1.80 to 2.01 mol CO2 /mol DEL. The absorption capacity increases upto 318.15 K temperature, after that it starts decreasing. Acylamide-superbase-based system showed good recyclability after five successive absorption/desorption cycles. FT-IR and 13 C NMR analysis revealed strong multiple site interactions due to chemisorption between the nitrogen atom of 2-imidazolidone and CO2 resulting in carbamate formation. This is an excellent method to optimize acylamidesuperbase-based DEL to achieve high CO2 capturing efficiency.

2.9.3 Hybrid Choline chloride was combined with monoethanolamine, glycerine and piperazineactivated glyceline in 1:2 molar ratio. All these prepared deep eutectic liquids were added to a diethanolamine solution of 10–30 wt percent. The solubility of CO2 was studied according to the mass transfer. The addition of eutectic liquids in diethanolamine solution increases the conc. of electrolyte resulting in reduced bubble diameter and improved gas holdups. The replacement of water with ChCl-MEA or ChCl-Gly/Pz mixtures results in improved CO2 solubility, mass transfer and thermal stability. While ChCl-glyceline based liquids can negatively affect the solubility of CO2 . The lower stability of ChCl-glyceline and ChCl-Gly/Pz after CO2 absorption provides easy separation of CO2 [34].

2.10 Ammonium-Based DELs Five different deep eutectic liquids were synthesized from levulinic acid as HBD and various ammonium salts (acetylene choline chloride, tetraethylammonium chloride, tetraethylammonium bromide, tetrabutylammonium chloride, and tetrabutylammonium bromide) as HBA in 3:1 molar ratio. The absorption of CO2 in these liquids was studied at 303.2, 313.2, 323.2, and 333.2 K with pressure up to 6 bars. The results indicated the increase in solubility with decreasing temperature and increasing pressure. The cations in ammonium salts play a key role in CO2 absorption, the larger the cation, the more the solubility. Tetrabutylammonium bromide-levulinic acid mixture shows higher solubility (0.043 mol/kg) in 1:3 molar ratio at 303.2 K and 5.6 bars. The enthalpies were exothermic [65]. A comprehensive evaluation of various eutectic liquids for CO2 capture was performed by Luo, Liu [66] through both, experimental and simulation. At first, a hydrophobic low melting liquid comprising of TBAB as HBA and DA as HBD with 1:2 molar ratio was checked by COSMO-SAC model. After that, quantum chemistry models were utilized to screen the interactions between DEL and CO2 molecules. The results revealed that these

2.10 Ammonium-Based DELs

43

interactions are mainly based on weak H-bonding and van der Waals forces. Then, the liquidgas equilibrium method was carried out to check the effect of the pressure and temperature, molar ratios of HBA and HBD, and the type of HBA and HBD, on the absorption of CO2 . The results reveal that the absorption of CO2 follows the Henry’s law which approves the reliability of the applied model. With the decrease in temperature and increase in pressure, CO2 solubility increases. The values of entropy and enthalpy are negative indicating the exothermicity of the process. While positive values of Gibbs free energy indicate that the absorption process is non-spontaneous and requires high pressure. Finally, for low melting liquid-based post-combustion capture of CO2 , a rigorous rate model (RRM) was simulated. RRM was applied to check the environmental sustainability and the life cycle of the studied liquids. Ammonium based DELs have been found to be more efficient for CO2 absorption. Triethylenetetraamine hydrochloride ([TETA]Cl) has been utilized as HBA and EG and DEG as HBD at various mole ratios. Among all the designed liquids, [TETA]Cl-EG based system in 1:3 molar ratio exhibited the highest absorption capacity of 1.457 mol CO2 /mol solvent (17.50 percent wt) at 313.15 K temperature and 0.01 MPa pressure. The molar ratio of HBA:HBD and partial pressure positively affects the solubility while temperature and chloride ion has a negative effect. These liquids showed good renewability even after five successive absorption-desorption cycles as the absorption capacity is not affected. Both the HBD, EG and DEG, improves the basicity of studied liquids by activating –NH/NH2 group on [TETA]Cl. FT-IR results confirmed the presence of carboxylate which is due to chemical absorption between [TETA]Cl and CO2 molecules [67].

2.10.1 Carboxylic acids The CO2 solubility DELs containing carboxylic acids as HBD was studied by Rabhi, Mutelet [68], who used tetrabutylammonium bromide (TBAB) and DL-menthol as HBA and Octanoic acid (OA), Decanoic acid (DA), and Dodecanoic acid (DDA) as HBD in 1:2 and 2:1 molar ratio. The CO2 solubility has been studied under different thermodynamic conditions, i.e. 293–373 K temperature and 12.29 MPa pressure. The DEL comprising TBAB-DA in 1:2 molar ratio exhibited the highest CO2 solubility (2.529 mol CO2 /kg DEL) at 313.15 K temperature and 4 MPa pressure. It was noticed that carboxylic acid (CA) based eutectic liquids have lower efficiency than phosphonium-based DEL but more than ILs. The effect of various zwitter-ion containing natural deep eutectic liquids (NADELs) comprising of N,N,N-trimethylglycine (TMG), and carboxylic acids like oxalic acid (OA), glycolic acids acid (GA), and phenylacetic acid (PAA) were studied by Siani, Tiecco [69]. The solubility of CO2 was estimated gravimetrically at 29–333 K temperatures and pressure upto 4 MPa. The highest CO2 uptake equal to 45.5 mg CO2 /g NADES was observed in PAA-TMG based system in 2:1 molar ratio at 313.15 K temperature and 4 MPa pressure. The presence of π -π interactions in these DELs was accounted for the absorption of CO2 . As the studied NADELs are all acidic in nature and CO2 shows more affinity for basic solvent system, so, CO2 solubility is comparatively lower in these liquids. PAA being less acidic among all the studied NADELs, shows more affinity for CO2 which results in high physical absorption of CO2 . Physical absorption of CO2 was confirmed by FT-IR results which show no difference in spectrum

44

2. Deep eutectic liquids for carbon capturing and fixation

before and after absorption. The regeneration ability of studied NADELs was also less than other DELs.

2.11 Phosphonium based DELs Haider, Maheshwari [4] reported three different deep eutectic liquids based on methyltriphenylphosphonium bromide (MTPPB) as HBA and glycerol (Gly), ethylene glycol (EG) and diethylene glycol (DEG) as HBD, for CO2 solubility. The absorption of CO2 in these liquids was studied at 303, 313, and 323 K with pressure upto 1200 kPa. The highest CO2 solubility (0.062 mol CO2 /mole of solvent) was exhibited by MTPPB-DEG based mixture due to its low viscosity. These liquids can be regenerated at higher temperatures, during the decarbonization, for reuse. The solubility of CO2 increases with increased pressure. Phosphonium and ammonium based deep eutectic liquids were designed with different HBD in specific molar ratio and their viscosity and capability of CO2 absorption was checked by Sarmad, Xie [70]. Both the solubility and viscosity are the key factors while designing efficient absorbents. The CO2 solubility was measured on a vapor-liquid setup at 29.15K temperature and upto 2.0 MPa pressure. With the increase in pressure, CO2 solubility also increases as it follows Henry law according to which solubility and partial pressure are proportional to each other. The type of HBD also effects the solubility of CO2 due to interactions between them. It follows the order Triethylmethylammonium chloride-Lactic acid > Triethylmethylammonium chloride-Levulinic acid > Triethylmethylammonium chlorideacetic acid. Due to strong interactions between TEMA-LA, it is difficult for them to interact with molecules of CO2 , hence results in lower solubility. While H-bond interactions are weakest in TEMA-AC, therefore, molecules of AA interact more easily with CO2 molecules resulting in highest solubility. In case of HBA, the length of alkyl chain also effects the CO2 solubility in positive direction. As the viscosity increases with CO2 absorption, this problem was coped by addition of water as a coordinating solvent. The viscosity is decreased by addition of 0.11 mol of water resulting in more pronounced solubility. However, the addition of water can negatively reduce the CO2 solubility and similar are the cases with an increase in temperature to reduce the viscosity. These liquids can further be functionalized in future to improve the absorption capacity.

2.12 Azole based DELs A new class of deep eutectic liquids was reported by Cui, Lv [71] based on azole as HBA [(P2222 )(Im), (P2222 )(Triz), (N2222 )(Im), and (N2222 )(Triz)] and EG as HBD in 1:2 molar ratio at 298.15 K temperature and 0.1 MPa pressure. [N2222 ][Im]-EG in 1:2 molar ratio showed the highest solubility i.e., 0.129 g CO2 /g DEL, while [P2222 ][Triz]–EG and [P2222 ][Im]–EG had the least solubility (0.118 g CO2 /g DEL). FT-IR results indicated the formation of carbonate through the hydroxyl group of EG. Regeneration capability of [P2222 ][Triz]–EG was evaluated as regeneration of any solvent is a key point for industrial/practical applications. Results revealed that this system can be completely recycled at 343.15 K temperature in nitrogen enriched environment.

2.14 Hydrophobic DELs

45

Imidazole based deep eutectic liquids were employed by Qin, Song [72] to cope the problem of global warming by capturing CO2 . Imidazole was mixed with p-toulene sulfonic acid; PTSA in 3:1, 3.5:1, and 4:1 molar ratios at 303.15–333.15 K temperature and 0.11–1.5 MPa pressure. COSMO-RS and Jou-Mather model was used to predict the solubility of CO2 in studied DELs. Results reveal that improving the pressure and Imidazole content can improve the solubility while the temperature has the opposite effect. The lower values of enthalpy of dissolution suggest their easy regeneration capacity, while positive Gibb’s free energy of dissolution reveals non-spontaneity of process. Jou-Mather model predicted the results more accurately (5.1 percent deviation) than COSMO-RS, which predicts qualitative results more accurately than quantitative.

2.13 Bio-phenol derived superbase based DELs Bio-phenols can be derived from plants and follow the principles of “green chemistry”. A unique combination of bio-phenols like thymol (Thy) and carvacrol (Car) functionalized with superbases (1,8-diazabicyclo [5,4,0]undec–7-ene; DBUH) was mixed with EG in 1:2, 1:3, 1:4 molar ratios at 298.15 K and 0.01 MPa to check their performance for CO2 solubilities. [DBUH][Thy]-EG exhibited the highest CO2 capacity of 1.0 mol CO2 /mol DEL in 1:4 molar ratio at room temperature and atmospheric pressure. The DBUH based ILs exhibited much lower capacity for CO2 due to their high viscosity than the studied DELs. Studied DELs show much higher capacities as the addition of EG lower their viscosities. Interestingly, EG molecules only interact with CO2 molecules through H-bonding and not with anions (CAR− , Thy− ). FT-IR and NMR analysis reveal the chemisorption of CO2 . The appearance of signals at 3.49, 3.77 ppm and 61.2, 65.9, 157 ppm in after absorption spectra of 1 H and 13 C NMR, respectively, indicates the formation of carbonate. HMBC also confirms the observed results. In FT-IR spectra the peaks at 1640 and 1282 cm−1 in after absorption spectra are also due to the carbonate formation. These liquids can be desorbed in nitrogen rich environment at 343.15 K and showed good recyclability [43].

2.14 Hydrophobic DELs Gu, Y. et al. reported a class of hydrophobic deep eutectic liquids composed of functional polyamine-hydrochloride as hydrogen bond acceptor and monoterpene (thymol) as hydrogen bond donor to check the solubility of CO2 from flue gases. These liquids showed hydrophobic behavior even after CO2 absorption. The increased conc. of monoterpene results in decreased density and viscosity. The key advantage of these liquids is their low volatility due to the H-bonding between HBA and HBD groups. Monoterpene based deep eutectic liquids could even capture CO2 at low pressure, and the highest CO2 solubility (1.36 mol CO2 /mol DEL) was observed for [TEPA] Cl and thymol in 1:3 molar ratio at 313.2 K and 103 kPa. The solubility of CO2 increases with decreasing temperature and increasing pressure. The terpene based eutectic liquids can be used more than five times, which indicates their high stability. The FT-IR analysis showed that high solubility of CO2 is due to its interaction with amine resulting in the formation of carboxylate. Both chemical and physical absorption put up the overall CO2

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absorption. It was found that these polyamine-terpene based deep eutectic liquids exhibited much higher absorption capacities than hydrophobic ILs (0.020 mol CO2 /mol) [73]. Al-Bodour, Alomari [74] investigated the effect of hydrophobic NADELs on CO2 solubility through isochoric saturation process. The NADELs were prepared by mixing HBA like carvone (CAR), and cineole (CIN) and HBD including thymol (THY), and menthol (MEN) in different molar ratios at 298 and 308 K temperatures and upto 4 MPa pressure. Hydrophobic terpenes based NADELs exhibited high physical absorption on high pressure and low temperature. CIN-MEN system showed the highest solubility of CO2 (0.51 and 0.48 mol/kg), while CAR-THY system had the least CO2 solubility (0.45 and 0.43 mol/kg) at 298 K and 308 K temperature, respectively. Absorption capacity of CIN-MEN system is 6.50 percent higher at 298 K and 10.50 higher at 308.15 K than CAR-THY system. Regeneration capacity of these systems is also excellent thus reducing the energy and operating cost. These solvents can be proved as best candidates for carbon management in future.

2.15 Non-ionic DELs The non-ionic low melting liquids comprising of phenolic alcohols were prepared by Alhadid, Safarov [75] in preselected molar ratios and applied CO2 capturing application. It was revealed that phenolic alcohol structure/symmetry positively effect the CO2 solubility. Hence, high molar ratio of phenolic alcohol accelerates the CO2 solubility. The choice and molar ratio of constituents was strictly limited by melting temperature of these liquids, which was maintained by the addition of l-menthol to lower the melting point. The absorption of CO2 was evaluated in l-menthol-thymol and 2,6-xylenol-thymol based DEL in 1:2 and 1:1 molar ratio, respectively. The CO2 solubility in preselected liquids was also measured experimentally by isochoric method at different temperature and pressure. l-menthol-thymol based liquids exhibited the highest solubility in both COSMO-RS and experimental (0.308 mol CO2 /kg). In these systems, CO2 solubility is higher as compared to ionic deep eutectic liquids or IL, which makes them an excellent, cost effective and simple solvent for CO2 absorption.

2.16 DEL supported membranes The different combinations of the well known DEL system comprising choline chloride and urea, when infused into the micro pores of membrane composed of polyvinylidene-fluoride (PVDF) can work more effectively for gases absorption and purification. The deep eutectic liquid-supported liquid membranes (DEL-SLM) were employed to check the pure and mixed CO2 gas (N2 /CO2 & CH4 /CO2 ) solubilities along with separation. The solubility was found to be function of H-bonding and basicity of DEL. The interaction energies (Ei ) of reported liquids for CO2 , CH4 and N2 were −29.4, −13.01, −9–91kJ/mol, respectively. These energy values of CO2 indicates its strong interaction with DEL-SLM than others which was counter confirmed by DFT results. The DEL-SLM consisting of ChCl:urea in 2:1 molar ratio exhibited the highest permeability for CO2 (45.6 barrer). DEL-SLM showed the same effect of temperature as that of SILM, permeability of CO2 increases with decreasing temperature due to low density. Another advantage of DEL-SLM is their mechanical stability and reusability [5].

2.17 DELs with multiple sites interaction

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Due to the high efficiency of amine based deep eutectic liquids, Ishaq, Gilani [39] incorporated these into the microporous PVDF membrane. ChCl as HBA was mixed with different HBD such as MEA, DEA, and TEA in 1:6 and 1:8 molar ratios. Resulting DELs were confirmed by FT-IR along with their physiochemical properties. Consequently, prepared liquids were incorporated in membrane (SLM) to check its absorption capability. This novel system exhibited tremendous CO2 selectivity in CO2 /N2 and CO2 /CH4 as 78.87 and 70.46, respectively. CO2 shows chemical absorption and high selectivity over N2 and CH4, which is due to the basic nature of these liquids. The transport of CO2 gas through membrane is based on solution-diffusion mechanism. The CO2 permeability increases upto 9 barrer with increase in temperature from 298 to 338 K due to low viscosity. At higher conc. of CO2 , selectivity is decreased from 7.87 to 68.1 due to saturation of SLMs which resulted in low transport. These SLM showed excellent CO2 capturing performance when compared to ILs based SLM due to low viscosity and can be used at industrial level. Another class of designer liquids based on ChCl as HBA, and polyacrylic acid (PAA) & polyacrylamide (PAM) as HBD were prepared in 15:1 and 20:1 molar ratio and impregnated in micropourous membrane PVDF for CO2 capture. Ishaq, Gilani [41] screened the performance of the synthesized PDEL-SLM with pure and mixed gases. It exhibited the highest selectivity for CO2 /N2 and CO2 /CH4 as 60 and 55.4 barrer, respectively due to small kinetic diameter of CO2 , basic character, strong H-bonding and molar free volume of PDELs. The permeability of CO2 is increased with increasing temperature due to low viscosity, activation energy and more free volume of PDELs. FT-IR results confirm the presence of H-bonding between ChCl and HBD. These green, low cost, and efficient solvents are the best replacement of PILs in CO2 capture which can be further modified into task specific liquids.

2.17 DELs with multiple sites interaction Deep eutectic liquids having multiple sites for interaction play a key role in improved CO2 absorption efficiency. Keeping this fact in view, Fu, Sang [76] prepared such DELs comprising of superbase ILs (1,5-diazabicyclo[4.3.0]-non-5-ene N-methylthiourea [DBNH][MTU]) as HBA and EG as HBD. [DBNH][MTU]:EG showed absorption capacity of 0.142 g CO2 /g DEL with single site interaction while [DBNH]2 [DTU]:EG exhibited absorption of 0.173 g CO2 /g DEL with double site interactions at 313.15 K which were higher than their respective ILs. This high CO2 absorption is attributed to the synergistic interactions between DEL and CO2 , resulting in carbonate and carbamate formation. EG plays significant role in CO2 absorption as DBNH+ activate the EG then both capture the CO2 through their imino and –OH groups, respectively. The viscosity of single site interaction system decreases with increase in temperature and EG content. While, in double site interaction system, it is independent of temperature but increases with increase in EG content. Activation energies also have impact on solubility as single site system had lower activation energies. The solubility of CO2 increases upto 313.15 K, after then desorption occurs in both systems. Addition of water in both systems results in decrease of CO2 absorption as water molecules form H-bonds with DEL which in turn reduces the interaction sites for CO2 .

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2.18 Conclusion and future prospects An increase in the atmospheric concentration of CO2 and other green house gases is believed to cause various disasters and severe climate changes. Therefore, it has been the biggest challenge of 21st century to reduce the emission of CO2 through switching towards energy efficient technologies and environmental friendly energy source. However, the relentless increase in CO2 can be exclusively managed by pre-combustion technologies, rather there is an urgent need of post-combustion carbon dioxide conversion or reduction. In this context, amine scrubbing technology which utilizes an aqueous solution of alkanolamines has been already in practice, but these methods are associated with the certain drawbacks like solvent evaporation, equipment corrosion, and toxic effluents. Recently, deep eutectic liquids (DELs) have captured much attention due to their incredible properties i.e., non-volatile and nontoxic character, tunable density and viscosity, and above all biodegradability, and recyclability. Deep eutectic liquids have been also proved to be an efficient solvent than ionic liquids in terms of CO2 capturing. Various combinations of HBA and HBD were evaluated to check their performance for CO2 absorption/solubility. Table 8.1 provides a comparison of CO2 sorption potential and green or thermodynamic credential of various DEL system tested so far. A critical review of collected data infers that DEL system made up of nitrogen and oxygen containing hydrogen bond donors work more efficienly as compared to others. Overall, amine based DELs are found to be most efficient ones due to their synergistic (both physical and chemical) CO2 absorption. The chemical absorption gives the best value of solubility as compared to physical absorption because the later involves the formation of H-bonding between HBA and HBD which reduces the available sites for CO2 molecules. The desorption behavior in chemical absorption is challenging in terms of solvent recyclability and degradation, as high temperature is required along with nitrogen rich environment. In amine based DELs, high CO2 solubility (up to 2.67 mol CO2 /kg DEL) is due to formation of carbamate. High viscosity of DELs is their biggest limitation which can be coped by diluting with water or any other solvent in specific molar ratio (upto 30 wt percent). Dilution not only decreases the viscosity but also improves its conduction and polarity. Increased level of dilutions can affect negatively as it reduces the available sites for CO2 interaction and desorption occurs. The highest solubility is observed at high pressure and low temperature as at high temperature kinetic energy of DEL molecules dominates the binding energy. After amine based DELs, EG based systems also showed good efficiency, as EG absorb CO2 molecules through chemical absorption resulting in carbonate formation. DELs can be functionalized into ternary DELs to improve the absorption efficiency. The functionalization with super-bases results in increased CO2 solubility when compared with other DELs. Acylmide-superbase based DELs show highest CO2 absorption capacity upto 2.01 mol CO2 /kg DEL. A novel method of CO2 absorption is DEL supported liquid membranes with amine based DEL impregnated in microporous PVDF membrane. SLMs exhibits highest absorption, selectivity and regeneration. Finally, DELs have the potential to become the most versatile and prominent solvents for CO2 capture at industrial level due to their low cost, tunability and other characteristics. These can surely replace the other solvents including ionic liquids by careful selection of HBA and HBD. Further research can be focused on the novel methods to lower the viscosities of DELs in order to enhance its efficiency.

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Acknowledgment The first author wants to acknowledge the moral and financial support of her parents Mr. and Mrs. Liaqat Ali.

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C H A P T E R

3 Cookstoves for biochar production and carbon capture Mashura Shammi a , Julien Winter b, Md. Mahbubul Islam c, Beauty Akter d and Nazmul Hasan e,f a

Hydrobiogeochemistry and Pollution Control Laboratory, Department of Environmental Sciences, Jahangirnagar University, Dhaka, Bangladesh b Private consultant, Cobourg, ON, Canada c Bangladesh Biochar Initiative, Dhaka, Bangladesh d Department of Environmental Sciences, Jahangirnagar University, Dhaka, Bangladesh e The United Graduate School of Agricultural Sciences, Kagoshima University, Kagoshima, Japan f Fruit Science Laboratory, Saga University, Saga, Japan

3.1 Introduction Traditional biomass burning is the only energy source for food-preparation in developing nations that consume most household energy. Near about 40 percent of global homes use conventional biomass fuel and cooking equipment which is not clean energy [1,2]. Biomass fuel sources in rural developing countries are usually agricultural crop residues such as rice straws, husks, sugarcane, other cereals, cow dung, litter, other manure, and biomass crops agroforestry products [3,4]. Most consumers want cookstoves that cook rapidly and effectively while using as little fuel as feasible [1]. The most common biomass cookstoves are three-stone fires (TSF) [5]. However, in waste biomass fuels, the high nitrogen content is usually correlated with high nitric oxide (NO) emissions, while fast-cooking is linked with carbon monoxide (CO) emissions for a given cooking task [1]. Consequently, particulate matter and gaseous emissions, as well as poor indoor air quality, are the main contributors to the serious health issue from biomass burning. [6,7]. Improved cookstoves have aided in reducing adverse environmental and health consequences in rural areas [8]. Still, they also had drawbacks in terms of labor for fuel preparation, lighting, and

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replenishing [9,10]. Therefore, changing the design and architecture of cookstoves was shown to reduce smoke, fuelwood savings, and char generation. Cookstoves’ thermal and physicochemical operating parameters can significantly control the selected gaseous emissions of unburned products such as CO, NO, VOC, soot, and particulate matter [1,2]. Nonetheless, worldwide attention has addressed environmental challenges by improving energy efficiency and lowering carbon emissions in these countries. In many developing countries, a simple and efficient cookstove design can be an essential item in reducing the amount of domestic energy and indoor pollution [10]. Biochar-bioenergy can help solve various pervasive economic, public health, and environmental issues that must be addressed [11]. For thousands of years, pyrolysis has been the traditional method that produces char or charcoal from biomass [12]. However, ’biochar’ is a comparatively current research topic [13]. Biochar is derived from the pyrolysis or thermal conversion (>350 °C) of organic biomass such as plant or animal-based waste under limited or oxygen-deficient conditions [11,13-16]. So the final biochar product is a pyrogenous organic material with aromatized carbon structure [17]. Biochar is a steady carbon enriched natural product [13,16]. From modest home cook burners to bigger commercial pyrolysis facilities, biocharbioenergy systems come in a variety of sizes [18]. However, biochar has a high probability of scaling up as a sustainable cooking technology using alternative fuels (clean solid energy for cookstove) in developing countries [12]. Biochar cookstoves also offer other advantages to sustainable development, such as directly enhancing yields in tropical agriculture [9]. Additional applications for it include air filtration, water treatment, activator for biogas generation, carbon storage, and reducing environmental effects [12]. Because of these reasons, cookstoves that pyrolyze or gasify their fuels have gotten a lot of interest in developing countries. Biochar cookstoves are modest devices designed to provide clean, affordable, and renewable energy in these regions. It is important to remember that the seventh sustainable development goal (SDG7) calls for ensuring that everyone has access to cheap, dependable, sustainable, and modern energy. Moreover, the thirteenth sustainable goal (SDG 13) is about immediate action to fight climate change and its impacts. Biochar cookstoves can complement these both goals simultaneously by providing clean and affordable fuel while capturing carbon for climate action. This chapter, therefore, aims to review the role of cookstoves (SDG 7) in carbon sequestration or carbon capture (SDG 13) and simultaneously contribute to climate change mitigation.

3.2 Cookstoves designed for biochar production Around the world, 2.9 billion people lack access to technology and secure, inexpensive, clean and efficient fuels. Numerous research and development projects have tried to device and implement efficient cookstoves. Unfortunately, several attempts typically fail because the cook’s requirements or preferences are not satisfied due to issues with the stove’s architecture, availability to fuels, or management problems [19]. This situation usually happens when the cook, usually woman, is left out of the participation. A common traditional cooking appliance that uses charcoal, wood chips, and biomass briquettes is the charcoal stove and its modified variant [4]. It is well known that conventional biochar-making technologies such as clay, brick, and steel kilns typically release lots of VOCs. So changes in stove design can minimize pyrolysis fuel usage or enhance airflow rate, lowering

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CO emissions. Boosting natural convection, increasing stove height, insulating the furnace or chimney to raise internal gas temperature can also increase airflow and reduce CO emissions. Another way to lower fuel use is to alter the construction or geometry of the pyrolysis chamber [1]. The traditional pyrolysis process is usually the sluggish one characterized by low temperature (∼300 °C), a long retention period (about 10–30 min to 25–35 h) with a slow heating rate (0.1–0.8 °C/S) [12]. According to research conducted in Kenya, the gasification burner was mostly utilized to prepare food items that required quick cooking. Fifty households were given free gasifier stoves and monitored for 2–3 months of use. 96 percent of the households used the stove at varying frequencies, while 40 percent used it daily. It was wellreceived by all users since it saved fuels, made fewer smokes and provided chars for soil amendment [20]. Typically, the feedstock is carefully selected based on factors such particle size, lignocellulose composition, moisture content, etc. The biochar yield is then measured by a variety of different parameters, including temperatures, heating time, inert gas flow rate, residence duration, etc. [12]. The biochar cookstoves are generally made of steel, metal sheet, structural steel, etc. They are lightweight, portable, and transportable in isolated regions [21]. A combination of locally resourced materials such as clay, brick, and steel can be used to make the structure of the stoves which can have a tinier carbon footprint. For example, Howell and colleagues produced biochar from two agricultural wastes, cottonseed and pecan shells, using simple and low control top-lit updraft (TLUD)-microgasifiers made from paint cans [22].

3.2.1 Top-lit updraft (TLUD) stove A question may arise on what is a top-lit updraft method? A fan for air supply is a feature of the top-lit updraft (TLUD) cookstove, which is an improved version of the biomass cookstove [2,4]. Compared to traditional stoves, this gasifier stove delivers better energyexergy efficiency. The TLUD cookstove’s efficiency is credited to the primary and secondary funnel-shaped central air-inlet that takes airflow into the combustion chamber for burning and results in high combustion temperatures [4]. Because of higher energy-exergy efficiency, TLUD cookstoves require less woody biomass consumption and consequently less harmful emissions. In operation, the top cover (28 cm diameter) of TLUD supports the cooking pot. Usually, the chosen biomass is filled into the combustion chamber (40 cm long) from the top and fired. The top cover is positioned back, and a steady fire is allowed for cooking. During refueling, the stove cover is removed and refilled with biomass fuel [4]. As fuel feedstock for TLUD stoves, wood chips, rice husk briquette and coconut shell performed well [5]. However, it is essential to remember that the locally modified TLUD method commonly has a low biochar yield than industrial pyrolysis reactors. In the TLUDs temperature reaches above 450 °C, and it is frequently difficult to maintain precise temperatures in the devices [22]. The biochar is highly microporous at this temperature, with a larger surface area and fewer functional groups. Higher ash content is found in the more carbonized biochar and has lower residual cellulose crystallinity. TLUD gasifiers are vertical, cylindrical stoves that are filled with a ‘batch’ small piece of wood or compressed biomass. The fuel is ignited on top, and the combustion proceeds downward to the bottom of the cylinder. A grate at the base of the cylinder acts as a conduit for "primary air," which supports the combustion above. We call the downward movement

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of the combustion an ‘ignition front’. It is also known as a ‘migrating flaming pyrolysis front’ (MFPF). At the MFPF there is not enough primary air to burn the fuel to completion, so as the MFPF moves downward, it leaves unburned char above. At the MFPF, heat is generated that causes thermal degradation of the fuel, known as ‘pyrolysis’. Pyrolysis creats two classes of products; char and volatiles. Char is a carbon-rich organic complex that is low in hydrogen and oxygen. It is chemically quite stable (i.e. biochar), so requires high temperatures burn (>1000 C°), and a steady supply of oxygen (because it is mostly carbon). Think of hot charcoal fires. The temperature at the MFPF can range from 550 to 1200 °C in relation to the primary air velocity. Below 550 °C flaming pyrolysis tends to fail, and we have only smoldering combustion (Winter, unpublished data). The volatiles, on the other hand, are quite flammable. However, once the MFPF consumes all of the oxygen in the main air, the volatiles ascend through the unburned char as a white smoke. Volatiles are combined with secondary air entering at the top of the reactor, and are mixed together in a gas burner. This creates most of the heat for cooking. Volatiles are composed of gases (CO, H2 , and light hydrocarbons such as methane), droplets of non-gaseous tars, and fine particles of soot. The volatiles are also known as ‘wood gas.’ Heat generated at the MFPF radiates downwards and raises the temperature of unburned fuel below to its ignition temperature. There are two modes of supplying the primary air; forced draft and natural draft. Forced draft TLUDs (FD-TLUD) use a fan to increase the velocity of air flow. Compared to natural draft stoves, the secondary air can be driven through the burner’s tiny apertures at a considerably faster velocity since it is under pressure. The high momentum of secondary air entrains and mixes with the volatiles into a well-mixed efficient flame. As the velocity of the primary air increases, the downward velocity of the MFPF will increase. Too much primar air will cool the MFPF and slow its downward velocity. FD-TLUD stoves often burn pelleted fuel (5–6 mm diameter and 1–3 cm long). The fan is power by a battery or a thermal-electric generator (that uses heat from the hot surface of the stove). Air movement in natural draft TLUD (ND-TLUD) is driven solely by the buoyancy of gasses that are hotter than the ambient air in the room. Buoyancy is generated both at the MFPF and in the gas burner. Buoyancy forces are usually much smaller that with forced air. Consequently, the fuel burns slower, but the results for cooking are perfectly acceptable. ND-TLUDs are typically used to burn chunks of wood, chips, and briquettes, and their reactors are tallers and wider than FD-TLUDs. ND-TLUDs can burn pellets, but they would require a shorter or modified reactor (because it only takes a fuel bed of pellets half as high or less to burn as long as wood pieces). In order to generate enough draft in a ND-TLUD, there has to be a ‘riser’ or cylinder for the gas flame to pass through before hot gases reach the bottom of the cooking pot. The height of the riser, or vertical distance between the point where the secondary air enters above the reactor, to the top of the riser should be at least 15 cm. If the riser is too short, there will not be sufficient mixing between the secondary air and the volatiles. The gasification rate is controlled by restricting the flow of primary air through a small orifice. It takes very little primary air for gasification, so the orifice should close down to only 2–5 percent of the cross-sectional area of the rector. The orifice can be opened up for more complete combustion of woody fuel and charcoal. It is important not restrict the primary air too much in a NDTLUD, because if the flaming pyrolysis stops, and the gas flame goes out, it is hard to get it flaming pyrolusys started again. It is easier to re-start a FD-TLUD, because you just turn up

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the fan; you are not dependent on natural draft. If the household is not burning pellets, then a ND-TLUD is fine, and you are not reliant on a working fan. Fuels make a difference to the performance of TLUDs, especially ND-TLUDs. Pellets are an ideal fuel, because a very narrow, well defined MFPF froms in the fuel bed, and moves downward as a flat, uniform front regardless of the velocity of primary air. Wood chips burn well if the velocity of primar air is moderate to high, but when primary air is restricted, the MFPF channels down the sidewalls of the reactor. When the fuel are small sticks or chunks of wood, a clear MFPF will not form, and the flameing pyrolysis may occur throughout most of the reactor at the same time. It is best that the fuel is less than 15 percent moisture. Since pellets are only 5–6 mm in diameter, they pryolize to completion as the MFPF moves down. With thicker chunks of wood, it can take several minutes to the center of the fuel to reach pyrolysis temperatures (ca. 400 °C), which is also why a cleay MFPF does not form. Vertical stems of coarse grass or jute sticks will burn in a TLUD (but because of low density don’t burn for long). Loose biomass such as straw and leaves do not burn properly in a TLUD. Wet wood will not burn in a TLUD

3.2.2 Development of TLUD-Akha architecture design The mechanism of the Akha TLUD followed design principles pioneered in the 1990s by Paal Wendelbo in Norway and independently by Dr Tom Reed and Dr Paul Anderson in the USA. Their novel idea was to create a gasification reactor out of a vertical metal cylinder by placing a grate at the bottom to feed a little quantity of “primary” air. The cylinder was loaded with a batch of fuel and ignited on top. The primary air below supported flaming pyrolysis but was insufficient for the combustion of most of the volatiles (white smoke) and char. Volatiles released by gasification rose upwards and were combined with secondary’ air forming a gas flame above the fuel at the top of the cylinder and passing that flame through a circular aperture called a concentrator ring [23]. Their stoves were made entirely of metal. The reaction cylinder was contained within an outer cylinder. Secondary air was preheated by passing upwards through the annular space between the two cylinders. The concentrator ring was a type of gas burner. Other more efficient gas burners exist, but they are more complicated to make. The outer cylinder is concrete in the Akha and forms the stove body. TLUD technology is not very difficult for stove designers to understand, and TLUD stoves can be adapted and scaled for specific uses. The Akha’ Chula’ was designed for rural Bangladesh and its plans are free for the general public to copy, modify and improve as they please as an open-access patent by Winter and Islam [24]. However, specific architectural guidelines should be followed so that the stoves perform cleanly (Fig. 3.1(A)). Commercial variations should be tested for emissions at certified stove-testing laboratories. Certain architectural guidelines should be followed so that the stoves perform cleanly. (1) The base of the stove should be close to air-tight so primary air can be regulated through an orifice that closes down to about 0–3 percent of the crosssectional area of the reactor (Fig. 3.1(B-C)), and (2) the gas burner and riser should be 15 cm tall to provide sufficient velocity of secondary air (Fig. 3.1(D)). Finally, (3) the burner should not restrict the flame or else the flame will pulse and emit soot. Bangladesh has a history of using concrete for combustion cookstoves. The Akha Chula used that experience and a wise strategy to decentralize stove-making by using existing

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FIGURE 3.1 (A) The Akha Chula is composed of several functional modules. It is a prototype still under development. (B) The base module serves as the stove’s supporting base. It is constructed of reinforced, clay-coated, heatresistant concrete. It also functions to receive char dumped from the reactor through a hinged grate, and to regulates “primary air” flow into the stove. Char is cleaned out through a side door. Primary air is regulated by cracking the side door open. (C) On top of the base sits a metal reactor cylinder. Around the reactor are concrete rings that form the stove body. The reactor is held in place by loose sand around its base in the space between the reactor and the lower stove body ring. Through openings in the bottom stove body, secondary air enters and is heated as it flows upward toward the reactor. There are multiple tiny holes in the reactor’s sidewall that control the quantity of air that enters the reactor to prevent the gasification process from stopping. They also control the gas flame so that it doesn’t extinguish (which would create a lot of smoke). (D) The concentrator ring burner is made of reinforced concrete coated with clay. Secondary air moved up and over the top of the reactor cylinder and under the concentrator ring burner. The flame exits through the concentrator orifice. In order to prevent the flow of burning gas from being impeded, the nozzle orifice’s aperture needs to be appropriately large. Laminar flames should therefore fill around 34 of the aperture diameter. The flame will show a prolonged pulse in height at a low frequency between 1–3 Hz if the aperture is too tiny. This results from releases of pressure that have built up beneath the burner. Usually, the pulsation may be heard. The height of the burner module should be at least 15 cm so that the secondary air has sufficient momentum to mix with the volatiles. This burner is based on the original metal burners of the ‘Peko Pe’ and ‘Champion’ Stoves (Diagram source: authors).

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businesses that cast concrete for sanitary installations to make concrete components for stoves. Artisans who work in metal and concrete can be found in most market towns. Dr Md. Kalequzzammen and E. Otto Gomm of the Deutsche Gesellschaft für Internationale Zusammenarbeit (GIZ), Dhaka, created the “Bondhu Chula,” a concrete combustion burner, in 2009. A concrete combination of sand, clay brick fragments, and Portland cement (3:2:1 by volume), reinforced with wire netting, and water cured for seven days, was suggested (Gomm, pers. com., 2020). If (when) the concrete cracks, reinforcing keeps the structure intact (even refractory concrete can crack with uneven heating and cooling.) After assembly, clay was applied to the concrete parts to shield them from extreme heat. The Akha Chula used the same mixture ratio.

3.2.3 Origins of TLUD-Biochar ‘Ecosystem’ The momentum for biochar and TLUD research in Bangladesh got a boost in 2013 with the "Colloquium on Biochar in Bangladesh" hosted by Mr Joyanta Adhikari (Executive Director) of the Christian Commission for Development in Bangladesh (CCDB). CCDB invited three Canadian scientists (Dr Jugesh Vig (agronomist), Dr Sunal Mustafa (economist), Dr Julien Winter (soil scientist) to tour the countryside, observe cooking practices and the availability of biomass, and discuss the potential for biochar with academics and extension agronomists. Afterward, the participants were invited to a colloquium at the CCDB, Dhaka, to discuss their impressions. From the Colloquium came several recommendations: 1. Since many Bangladeshi soils are low in soil organic matter [25-28], biochar could significantly increase soil quality and agricultural productivity. Yields will increase and/or inputs of commercial fertilizers will fall. This will help Bangladesh buffer the impact of climate change and land lost to rising sea levels. 2. However, the practical problem was not how to use biochar but how to make it. Rural households were under energy stress because the demand for wood as cooking fuel was much greater than the supply [29,30]. Although wood is an excellent feedstock for biochar, any method for making biochar in Bangladesh should in no way jeopardize the energy security of households. 3. TLUD stoves may be the solution for making biochar because they will make it a byproduct of cooking. Even when char is collected from the TLUD stoves, TLUDs still use half as much fuel as traditional stoves and reduce women’s exposure to smoky flue emissions. Biochar made in a stove is placed directly into the hands of households. 4. TLUD stoves and biochar are synergistic technologies, so they must be introduced to villages simultaneously rather than separately. For over forty years, the adoption of improved cookstoves by Bangladeshi households has been slow. The game will change when a stove is more than just a cooking device; when a stove makes biochar, and the biochar increases household income. Thus, for TLUD acceptance by households, villagers must have first-hand of experience of biochar effects on soil productivity. In villages target for an intervention, ‘TLUD-biochar ecosystems’ should be established. 5. Developing TLUD-biochar ecosystems will be a multidisciplinary intervention. Villagers will have to learn how to use and prepare fuel for a new type of stove, and use biochar in gardening, crop, and livestock agriculture. Consequently (a) the collaboration of various

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professions and organizations will be needed, and (b) to ensure that the technologies become appropriately established, efforts should be focused on building the knowledge capacity of villagers in selected rural communities rather than diffusely over a broad area. Once established, nodal villages with practical wisdom can help spread the technologies to the surrounding countryside. Introducing TLUDs and biochar to rural villages should be a reciprocal process where researchers and villagers learn from each other. Locations differ in climate, vegetation, crops, livestock, soil, access to urban markets, and cultural conditions. Villagers have detailed, historical, and practical knowledge of their environment. Since there are many uses for biochar, the technologies can be adapted accordingly. TLUD stoves should be manufactured within Bangladesh rather than imported because imported goods are expensive for rural households. In addition, importing will increase the carbon footprint. That includes minimizing the use of imported raw materials such as metal or electrical components. Some metal components can be replaced with concrete and clay. However, in India, the all-metal ‘Champion Stove,’ invented by Dr Paul Anderson [23], has been successfully manufactured and accepted by households. Natural draft (NDTLUD) stoves are preferred to forced draft TLUDs because they don’t use an electric fan to move air. Local manufacturing makes it possible to tailor TLUD stoves to consumer preferences and employ artisans found in most market towns. They can manufacture TLUD components and provide spare parts. ATLUD-biochar ecosystem combats climate change because TLUD biochar contains about 35 percent of the original carbon in fuelwood. Biochar added to soil sequesters the carbon in soil organic matter. It was recommended that CCDB facilitate the formation of a ‘Bangladesh Biochar Initiative’ (similar to the US Biochar Initiative) as a association of research, extension education, and industry professionals for networking, exchanging ideas and as a source of educational material for the public.

Following the Colloquium, CCDB was provided with a 200-L TLUD reactor to make biochar for agronomic trials, plans for small agricultural trials, and instructions on how to make TLUDs. The Akha Chula was developed over the next two years. In 2016, Kerk in Actie (Netherlands) began funding the Akha-Biochar Project at CCDB [31]. With advice from agricultural universities and the Bangladesh Agricultural Research Institute, CCDB established ‘TLUD-biochar ecosystems’ in small clusters of villages in the Manikganj, Naogaon, and Dinajpur Districts. A survey of 111 households in 2018 showed that enthusiasm for biochar became a strong incentive for using the Akha Chula (beyond the other benefits of saving fuel and clean cooking).

3.2.4 Composition of biochar produced from biochar cookstoves The conversion of biomass to biochar (carbon-rich materials) using cookstoves is an inexpensive way to remove atmospheric C by incorporating it into the land. Carbon sequestration is related to the slow-release of carbon dioxide (CO2 ) from the soil and storing CO2 in the soil for a long time, or C in any other forms that help reduce CO2 concentration [32]. Biochar

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cookstoves have the potential to sequester carbon and provide other benefits for sustainable development, containing the establishment of energy security and improving agricultural yields [9]. Biomass is abundantly available and considered a green energy source [12]. It is essential to determine the composition of biochar to get the maximum benefits through the customization of biochar from different substances. Biomass materials and production conditions are mainly responsible for the varying composition of biochar [32]. International Biochar Initiative (IBI), a platform for fostering biochar standards over many years of research and industry collaboration, published Biochar Standards Version 2.1 in 2015. It reported that the minimum requirement for organic carbon (OC) should be 10 percent [33,34]. The biochar is also classified into three types of which, Class 1 biochar containing ≥60 percent OC, Class 2 biochar ≥30 percent to 7: 1.

5.3.3 Synthesis of naftifine Naftifine 69, an API used in Exoderil, is an allylamine antifungal medicine that is used for treating tinea corporis, tinea cruris, and tinea pedis. This compound exhibits anti-inflammatory, antibacterial, and antifungal properties [59]. Dyson et al., synthesized 69 in 58 percent overall yield in two catalytic steps (Scheme 5.15) [60]. (E)-N-(Naphthalen-1ylmethyl)−3-phenylprop-2-en-1-amine 68 was prepared in good yield (70 percent) from the

SCHEME 5.14 One-pot synthesis of methylephedrine starting from 2-(methylamino)−1-phenylpropan-1-ol.

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SCHEME 5.15 Two-step synthesis of naftifine starting from naphthalen-1-ylmethanamine.

reaction of cinnamyl alcohol 67 with naphthalen-1-ylmethanamine 66 in the presence of bis[(2-diphenylphosphino)phenyl] ether (DPEphos) and dichloro(cycloocta-1,5-diene) platinum(II) [Pt(cod)Cl2 ]. Then, the obtained amine 68 was N-methylated using CO2 as a C1 source combined with hydrosilane as the reducing agent and N-heterocyclic carbene 70 to obtain 69 in 83 percent yield.

5.4 O–Nucleophile-triggered CO2 -incorporated carboxylation to form C–O bonds 5.4.1 Synthesis of atorvastatin Atorvastatin 79 is an API in Lipitor, an HMG CoA reductase inhibitor, and belongs to a class of inhibitors called “statins” or HMG CoA reductase inhibitors. It decreases the levels of triglycerides and LDLs (low-density lipoproteins), sometimes called “bad” cholesterol in the blood, whereas it increases the levels of HDLs (high-density lipoproteins), sometimes called “good” cholesterol. Rádl et al., described a multistep synthesis of ester intermediate 77, a key precursor for the formation of 79 [61] The treatment of heptadienol 71 with nbutyllithium, CO2 , and iodine in tetrahydrofuran provided ketal 72, which was reacted with p-toluenesulfonic acid in acetone to afford acetonide 73 (Scheme 5.16). The treatment of 73 with potassium cyanide in hot DMSO provided the cyano compound 74, which was

SCHEME 5.16 Synthesis of atorvastatin starting from hepta-1,6–dien-4-ol.

5.6 C-Nucleophile-triggered CO2 -incorporated reductive carboxylation to form C–C bonds

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SCHEME 5.17 Two-step synthesis of DMU-212 and combretastatin A-4 starting from (3,4,5-trimethoxyphenyl) methanol.

functionalized at the terminal double bond with Me2 S and O3 , or NaIO4 and OsO4 , to afford aldehyde 75; this was then oxidized by the Jones reagent to afford carboxylic acid 76. Following this, the acid was esterified with tert–butanol using 4-dimethylaminopyridine and N,N-dicyclohexylcarbodidimide (DCC) in methylene chloride, affording the required ester intermediate 77. According to the report by Alcantara et al., precursor 77 can be easily transformed into 79 using a three-step reaction protocol [62].

5.5 CO2 -catalyzed oxidation of alcohols to form C–O bonds 5.5.1 Synthesis of DMU-212 and combretastatin A-4 DMU-212 84 is a methoxylated resveratrol analog and an API in certain anti-cancer formulations. This compound has significant anti-cancer activity and selectively targets tumor cells. Moreover, combretastatin A-4 85 is a powerful vascular-damaging and microtubuletargeting agent that targets tumor vasculature to inhibit angiogenesis. Das et al., reported a CO2 -mediated transition-metal-free oxidative approach to synthesize of 84 and 85 from (3,4,5-trimethoxyphenyl)methanol 80 (Scheme 5.17) [63]. That is to say, (3,4,5trimethoxyphenyl)methanol 80 was oxidized to the respective aldehyde 81 in 84 percent yield using CO2 in the presence of K2 PO4 and DMSO. For 84 (96 percent), the obtained aldehyde 81 was treated with 1-(bromomethyl)−4-methoxybenzene 82 in the presence of triethylphosphine in aqueous NaOH, while for 85 (90 percent), aldehyde 81 was treated with hydroxy 83 in the presence of KOH in EtOH.

5.6 C-Nucleophile-triggered CO2 -incorporated reductive carboxylation to form C–C bonds 5.6.1 Synthesis of methionine hydroxy analog Methionine hydroxy analog 87 (MHA) is an API in Microquel Pig Starter often named 2–hydroxy-4-methylsulfanylbutyric acid. MHA represents a significant technical product

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SCHEME 5.18 Reductive carboxylation of 3-(methylthio)propanal to methionine hydroxy analog.

largely employed to feed animals. Additionally, MHA displays a higher bioavailability than the essential amino acid methionine. Moreover, MHA reduces urolithiasis and avian kidney damage caused by excess dietary calcium [64]. The preparative electroorganic synthesis of MHA 87 is illustrated in Scheme 5.18. Polycrystalline boron-doped diamond (BDD) is used for the reductive carboxylation of aldehyde 86 to 87 in the presence of a CO2 atmosphere [65]. In the electrochemical approach, a Mg sacrificial anode and a BDD cathode can be used (conversion 66 percent and current efficiency, CE, 22 percent). However, one drawback of this reaction is the formation of impurity by direct reduction of aldehyde 86 to alcohol 88 (Scheme 5.18).

5.6.2 Synthesis of naproxen Naproxen 94, an API used in Naprosyn, is a non-steroidal anti-inflammatory medication used for the treatment of fever and inflammatory diseases such as rheumatoid arthritis, menstrual cramps, and pain. The multistep approach for the synthesis of 94 is illustrated in Scheme 5.19. Methylation of naphthalen-2-ol 89 using Me2 SO4 , followed by the Friedel-Crafts acylation of 90 afforded 2-acetyl-6-methoxynaphthalene 91, which was subjected to electrochemical carboxylation and subsequent treatment with acid to afford acid 92 [3,66]. In the next step, acid 92 was dehydrated using a suspension of fused potassium acid sulfate, dilauryl thiodipropionate (DLTDP) and 2,6-di–tert–butyl–p-cresol (DBPC) in chlorobenzene to produce the corresponding α-arylpropenoic acid 93. Then, 93 was asymmetrically hydrogenated using an asymmetric hydrogenation catalyst (ruthenium complex of chiral phosphine) at low temperatures to furnish 94.

SCHEME 5.19 Synthesis of naproxen starting from naphthalen-2-ol.

5.7 C-nucleophile-triggered CO2 -incorporated direct C–H carboxylation to form C–C bond

99

SCHEME 5.20 Industrial synthesis of aspirin.

5.7 C-nucleophile-triggered CO2 -incorporated direct C–H carboxylation to form C–C bond 5.7.1 Synthesis of aspirin Acetylsalicylic acid 98, commonly known as aspirin, is an API of various commercial drugs such as Bayer Aspirin, Ecotrin, and Aspir 81, and it is one of the most widely used medications worldwide. Aspirin is a popular medicine utilized for treating inflammation, fever, or pain. Specific inflammatory conditions in which acetylsalicylic acid is employed include rheumatic fever, pericarditis, and Kawasaki disease [67]. The industrial synthesis of 98 is illustrated in Scheme 5.20. Sodium hydroxide is used to deprotonate phenol 95, forming the expected sodium phenoxide 96, which reacts with electrophilic CO2 at 125 °C to furnish sodium salicylate through the Kolbe-Schmitt process. This is followed by acid treatment to afford salicylic acid 97. Ultimately, 97 is treated with acetic anhydride to furnish 98. This synthetic approach is being used in the industries for the synthesis of 98 since 1874 [68]. When CO2 is introduced under pressure (5–7 bars), the yield increases from approximately 50 percent to 90 percent [69].

5.7.2 Synthesis of 4-aminosalicylic acid 4-Aminosalicylic acid 100, also known as para-aminosalicylic acid is an API in Paser, which is an antibiotic primarily used to treat drug-resistant tuberculosis [70]. The one-step industrial formation of 100 is illustrated in Scheme 5.21. Treatment of 4-aminophenol 99 with CO2 in a gas phase under high pressure and temperature affords 100 in 80 percent yield [71]. This process is short and environmentally friendly.

SCHEME 5.21 Industrial synthesis of 4-aminosalicylic acid.

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SCHEME 5.22 Synthesis of diflunisal starting from 2,4-difluoroaniline.

5.7.3 Synthesis of diflunisal Diflunisal 106 is an API in Dolobid and a popular non-steroidal anti-inflammatory drug (NSAID) with pharmacological properties comparable to other prototypical NSAIDs. Additionally, this compound possesses antipyretic, analgesic, and anti-inflammatory activities and is utilized for short and short-lasting symptomatic relief of moderate to low pain in rheumatoid arthritis and osteoarthritis [72]. Jones et al. synthesized 106 in five steps with an overall yield of 70.4 percent (Scheme 5.22) [73]. The synthesis was initiated by the reaction of 2,4difluoroaniline 101 with a diazotizing reagent (isoamyl nitrite) to afford 2,4-difluorobiphenyl 102, which was then acylated via a Friedel-Crafts reaction to furnish ketone 103. Subsequently, ketone 103 was oxidized to ester 104 using maleic anhydride. The ester product was then hydrolyzed via standard alkaline saponification or acid hydrolysis, followed by acidification to afford alcohol 105. Ultimately, alcohol 105 was carboxylated under the Kolbe process for 4–8 h at 200–260 °C and 1400–1100 psi of CO2 , in the presence of K2 CO3 to afford 106.

5.7.4 Synthesis of gentisic acid Gentisic acid 109 is an API used in Codopalm that exhibits antioxidant, anti-inflammatory, and antibiotic activities. This compound is produced by carboxylation of hydroquinone 107 using CO2 and K2 CO3 , followed by acid treatment of the obtained salt 108 (Scheme 5.23) [74].

SCHEME 5.23 Two-step synthesis of gentisic acid starting from hydroquinone.

5.8 C-nucleophile-triggered CO2 -incorporated organozinc-mediated carboxylation to form C–C bonds

101

SCHEME 5.24 Synthesis of tamoxifen starting from 4-iodophenol.

5.8 C-nucleophile-triggered CO2 -incorporated organozinc-mediated carboxylation to form C–C bonds 5.8.1 Synthesis of tamoxifen Tamoxifen 115, an API used in Soltamox and Nolvadex, is employed to cure breast cancer in men and women, and to prevent breast cancer in women [75]. The multistep synthetic sequence leading to 115 starts from the alkylation of 4-iodophenol 110, followed by condensation of the obtained 111 with phenylacetylene using a palladium catalyst, which leads to 112 (Scheme 5.24) [76]. Then, 112 is subjected to bis(cyclooctadiene)nickel-catalyzed arylative carboxylation using, followed by reaction with acid and then with diazomethane to afford 113. Treatment of 113 with DIBAL-H (diisobutylaluminum hydride) affords alcohol 114. Ultimately, the Dess–Martin reaction, followed by Wittig reaction and subsequent hydrogenation afford 115.

5.8.2 Synthesis of (E)−3-Benzylidene-2-indolinone (E)−3-Benzylidene-2-indolinone 120 is a potent anti-proliferative drug; it is broadly studied for its chemopreventive effects in inducing NQO1 potency [77]. The multistep synthesis initiates with the Sonogashira reaction between 1-amino-2-iodobenzene 116 and ethynylbenzene, affording 117 in excellent yield (95 percent) (Scheme 5.25) [78]. Then, the amino group is protected with TTFA (trifluoroacetic anhydride) to deliver 118 in 79 percent yield. Subsequently, the bis(1,5-cyclooctadiene)nickel-catalyzed carboxylation under 1 atm of CO2 proceeds smoothly to provide acrylic acid 119 in 96 percent yield. Ultimately, 120 is obtained in an overall yield of 56 percent after the removal of the protecting moiety, viz., COCF3 on the amino group of 119, and the reaction of the unprotected intermediate with EDCl.

5.8.3 Synthesis of ibuprofen Ibuprofen 124 is an API in the most common NSAIDs, such as Nurofen, Motrin, Advil, and Brufen, and it is utilized treating inflammation or pain induced by many conditions, such as

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SCHEME 5.25 Synthesis of (E)−3-benzylidene-2-indolinone starting from 2-iodoaniline.

SCHEME 5.26 Synthesis of ibuprofen starting from 1-(4-isopropylphenyl)ethanone.

minor injury, menstrual cramps, arthritis, back pain, toothache, and headache. Ibuprofen operates by reducing hormones that cause pain and inflammation in the body [79]. Knochel et al., disclosed a novel four-step approach for the synthesis of 124 with an overall yield of 59 percent through improvement by the addition of MgCl2 (Scheme 5.26) [80]. Briefly, the reduction of commercially available ketone 121 with sodium borohydride and subsequent chlorination using SOCl2 affords 1-(1-chloroethyl)−4-isopropylbenzene 122 in excellent overall yield (94 percent). The corresponding mineral-salts-based complex 123 (Zn species complexed with MgCl2 species) is readily obtained in 70 percent yield via the treatment of chloride 122 with Mg/ZnCl2 /LiCl. Reagent 123 is appropriately reactive to afford 124 in excellent yield (89 percent) by the addition of CO2 .

5.9 C-nucleophile-triggered CO2 -incorporated organolithium-mediated carboxylation to form a C–C bond 5.9.1 Synthesis of repaglinide Repaglinide 130 is an API in Prandin, an oral anti-hyperglycemic drug used to treat type 2 diabetes mellitus. This compound helps to regulate the levels of blood sugar by causing the pancreas (digestive juices) to produce insulin. The multistep synthesis of 130 is shown in Scheme 5.27. The reaction of 2–hydroxy-4-methylbenzoic acid 125 with 1-bromopropane in the presence of K2 CO3 in dimethyl sulfoxide affords 2-ethoxy-4-methylbenzoic acid ethyl ester 126, which is carboxylated by a reaction with LDA (lithium diisopropylamide) and CO2

5.9 C-nucleophile-triggered CO2 -incorporated organolithium-mediated carboxylation to form a C–C bond

103

SCHEME 5.27 Synthesis of repaglinide starting from 2-hydroxy-4-methylbenzoic acid.

in THF/1,3-dimethylperhydropyrimidin-2-one to furnish the key intermediate [3-ethoxy-4(ethoxycarbonyl)phenyl]acetic acid 127 [81]. Then, condensation of phenyl acetic acid derivative 127 with enantiomerically pure amine analog 128 in the presence of DCC, followed by saponification of the resulting amide derivative 129 affords 130 [82].

5.9.2 Synthesis of flurbiprofen Flurbiprofen 135 is an API of many non-steroidal anti-inflammatory formulations such as Ansaid. This compound has analgesic and anti-pyretic potencies and is employed to reduce joint stiffness from arthritis, swelling, and pain [83]. The synthesis of flurbiprofen 135 starts with the deprotonation of 1-fluoro-3-methylbenzene 131 by a Schlosser’s base (Scheme 5.28). When a blend of potassium t-butoxide and t-butyllithium is employed as the mixed-metal mixture, the selectivity increases considerably. The deprotonation of complex 132 takes place at the position adjacent to fluorine because it is the least-hindered position. The trapping of the organolithium complex with fluorodimethoxyborane-diethyl etherate followed by hydrolysis provides phenylboronic acid, which is subjected to the Suzuki–Miyaura cross-coupling reaction to produce 133 [84,85]. Another superbase metalation of 133 using a mixture of potassium tert-butoxide and lithium diisopropylamide and subsequent carboxylation with CO2 and acid

SCHEME 5.28 Synthesis of flurbiprofen starting from 1-fluoro-3-methylbenzene.

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SCHEME 5.29 Total synthesis of epristeride starting from methyl 3-oxo-4-androstene-17β-carboxylate.

treatment furnishes acid 134. A second metalation, followed by a reaction with iodomethane affords flurbiprofen 135.

5.9.3 Synthesis of epristeride Epristeride 141, an API used in Chuanliu and Aipuliete, is used to treat enlarged prostate in china. This compound is a potent human dihydrotestosterone (DHT) blocker (5-ARI) and operates by reducing the formation of DHT, an androgen sex hormone, in certain areas of the body such as the prostate gland [86]. Baine et al., reported an improved synthetic pathway of 141 in four steps with an overall yield of 44 percent using 136 (methyl 3-oxo-4-androstene17β-carboxylate) as a starting material (Scheme 5.29) [87]. Briefly, ketone 136 was converted to bromide 137 in good yield (85 percent) via a reaction with phosphorous(III) bromide in glacial ethanoic acid. Saponification of bromide 137 with KOH in MeOH afforded acid 138, which was transformed to the main precursor amide 139 in good yield (75 percent) by acid chloride. The latter was in situ generated by the reaction between tert–butylamine and oxalyl chloride and quenching into excess tert–butylamine. The reaction of amide 139 required an organomagnesium reagent (EtMgCl) to exchange the halide at P-3 and produce ethyl chloride. Then, the introduction of s-BuLi resulted in the formation of the desired lithio analog 140 [88]. Ultimately, carbonation at P-3 occurred by a reaction with CO2 , resulting in 141.

5.9.4 Synthesis of mefloquine Mefloquine 147 is an API in Lariam used for the treatment of malaria and the protection of travelers who visit zones where malaria is present. Thus, this drug belongs to the family of antimalarials [89]. The multistep synthetic route to 147, starting from 2-trifluoromethylaniline 142, is outlined in Scheme 5.30 [90]. Treatment of 147 with ethyl trifluoroacetylacetate (TFAAE), followed by bromination of 143 with POBr3 furnishes bromide 144. Acid 145 is obtained by lithiation and carboxylation with CO2 . The reaction of acid 145 with 2-pyridyllithium, generated from n-butyllithium and 2-bromopyridine, affords 146, which is converted into 147 by hydrogenation with a platinum catalyst.

5.9 C-nucleophile-triggered CO2 -incorporated organolithium-mediated carboxylation to form a C–C bond

105

SCHEME 5.30 Synthesis of mefloquine starting from 2-(trifluoromethyl)aniline.

SCHEME 5.31 Synthesis of amitriptyline starting from 1–bromo-2-(bromomethyl)benzene.

5.9.5 Synthesis of amitriptyline Amitriptyline 155, an API used in Elavil, is a tricyclic antidepressant normally employed in the treatment of depression and anxiety; however, low concentrations of it can stop or reduce pain. Kirschning and Kupracz proposed a multistep synthesis of 155 using a flow reactor system, which included three consecutive halogen-lithium exchange reactions (Scheme 5.31) [91]. The Li-Br exchange of 2-bromobenzyl bromide 148 with n-butyllithium affords o-bromobenzyllithium 149, which couples with the unchanged substrate 148 to afford 1–bromo-2-[2-(2-bromophenyl)ethyl]benzene 150. Another Li-Br exchange provides bromide 151. Treatment with CO2 , followed by a third Li-Br exchange furnishes ketone 153 in 76 percent overall yield. Then, the reaction of [3-(dimethylamino)propyl] magnesium chloride 154 with ketone 153 and subsequent acid treatment affords 155.

5.9.6 Synthesis of methantheline bromide Methantheline bromide 160 is an API in the most common antispasmodic drugs, such as Vagantin, and it is used for the treatment of stomach ulcers, intestinal ulcers, pancreatitis,

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SCHEME 5.32 Synthesis of methantheline bromide starting from 2-phenoxybenzoic acid.

gastritis, biliary dyskinesia, intestinal problems, pylorospasm, and urinary problems [92]. A multistep synthetic approach for 160 is illustrated in Scheme 5.32. Briefly, a Friedel–Crafts cyclization of acid 156 using concentrated H2 SO4 results in the formation of 9H-xanthen-9one 157 [93]. The reaction of ketone 157 with metallic Na in EtOH affords the intermediate benzhydrol. The OH unit of this intermediate is further reduced to afford 9H-xanthene 158. The reaction of CO2 with the anion of precursor 158 provides 9H-xanthene-9-carboxylic acid 159 after acid treatment. Ultimately, the carboxylate salt of acid 159 is alkylated with 2–chloroN,N-dimethylethanamine, and treated with bromomethane to afford 160.

5.9.7 Synthesis of garenoxacin Garenoxacin 170 is a quinolone antibiotic used as an API in Geninax to treat respiratory tract infections, for instance tonsillitis, pneumonia, pharyngitis, bronchitis, and pharyngitis [94]. The synthesis of Garenoxacin 170 initiates with the methylation of 161, followed by carbonation with CO2 and conversion of the obtained acid to a methyl ether with diazoniomethanide to afford ester 162. Then, the cleavage of the methyl ether using tribromoboron furnishes phenol 163, which is treated with chlorofluromethane in the presence of K2 CO3 to afford 164. The obtained ester 164 reacts with NaN3 , followed by catalytic hydrogenation and saponification to yield acid 165. Subsequently, the amine unit of 165 is diazotized using copper(I) bromide and sodium nitrite, and the diazonium salt is treated with HBr to obtain bromide 166. The condensation reaction of bromide 166 with the Mg salt from ethylpropanedioate and subsequent treatment with DMF-DMA (dimethylformamid-dimethylacetal) and cyclopropylamine affords ketone 167. The reaction of ketone 167 with potassium carbonate closes the ring to give quinolone 168, which is subjected to a Suzuki cross-coupling reaction with the boronic acid from dihydroisoindole 169, leading to the formation of the coupling entity (Scheme 5.33). Ultimately, treatment with HCl removes the trityl protecting group from the isoindole nitrogen atom to afford garenoxacin 170 [95,96].

5.9.8 Synthesis of englitazone Englitazone sodium is the sodium salt-form of Englitazone 176, an agent belonging to the glitazone class of anti-diabetic agents with anti-hyperglycemic activity. Englitazone 176

5.9 C-nucleophile-triggered CO2 -incorporated organolithium-mediated carboxylation to form a C–C bond

107

SCHEME 5.33 Synthesis of garenoxacin starting from 2,6-difluorophenol.

also appears to decrease triacylglycerol levels in animal studies [97]. Clark et al., proposed a multistep synthetic sequence leading to Englitazone 176, which starts from the reaction of reality available chromanol 171 with benzylchloromagnesium; this reaction results in an addition to the latent aldehyde and the development of intermediate dialcohol 172 (Scheme 5.34) [98]. Treatment of the crude product 172 with toluenesulfonic acid affords chroman 173. Then, the organolithium substrate from the halogen-metal exchange reaction of chroman 173 with n-BuLi reacts with CO2 to afford acid 174. Following this, acid 174 is resolved by isolating the diastereomeric salts produced with a chiral base. The reduction of the acid provides aldehyde 175 as a single enantiomer. Ultimately, aldol condensation with 2-thioxo-4-thiazolidinone and subsequent reduction using H2 and Pd/C affords 176 [99].

SCHEME 5.34 Synthesis of englitazone starting from 6-bromochroman-2-ol.

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SCHEME 5.35 Synthesis of enadoline starting from 1–methoxy-2,3-dimethylbenzene.

5.10 C-Nucleophile-triggered CO2 -incorporated organomagnesium-mediated carboxylation to form a C–C bond 5.10.1 Synthesis of enadoline Enadoline 184 is a medicine that behaves as a highly potent and selective κ-opioid receptor agonist, and it exhibits anti-allodynic and anti-hyperalgesic properties [100]. Huang et al., synthesized 184 in seven steps from 2,3-dimethylanisole 177 (Scheme 5.35) [101]. 2-Methoxy-6-methylbenzaldehyde was obtained by refluxing 2,3-dimethylanisole 177 in a suspension of copper sulfate and potassium persulfate. Subsequently, the obtained alcohol was transformed into the respective aldehyde using manganese dioxide. De-O-methylation was performed with tribromoboron in dichloromethane to afford 6-methylsalicylaldehyde 178. Ethyl 4-methylcoumarilate 179 was obtained in the next step via the treatment of aldehyde 178 with ethyl bromomalonate in potassium carbonate and butanone. Then, the reaction of ester 179 with NBS (N-bromosuccinimide) in the presence of dichloroethane and AIBN (azobisisobutyronitrile) afforded ethyl 4-bromomethylcoumarilate, which was subjected to saponification using K2 CO3 , leading to the formation of acid 180. Heating of acid 180 in leucoline led to a decarboxylation product, which was then stirred with triphenylphosphine and N-chlorosuccinimide in THF to afford 4-chloromethylbenzofuran 181. Following this, 181 was gradually introduced to a stirred magnesium-anthracene mixture at −45 °C, and the solution was bubbled with CO2. Afterward, thionyl chloride and 183 were added to the mixture, which resulted in 184.

5.10.2 Synthesis of loxoprofen Loxoprofen 191, an API used in Loxonin, is an NSAID; its tablets are widely used as painkillers or anti-inflammatory agents. The multistep synthesis of loxoprofen 191 initiates with the reaction of 1-(1-chloroethyl)−4-methylbenzene 185 with magnesium turnings and then with gas CO2 (Scheme 5.36) [102]. After being carboxylated, the obtained acid 186 is treated with NBS (N-bromosuccinimide) in the presence of azobisisobutyronitrile or benzoyl peroxide to afford bromide 187, which is then subjected to methoxylation towards ester

5.10 C-Nucleophile-triggered CO2 -incorporated organomagnesium-mediated carboxylation to form a C–C bond

109

SCHEME 5.36 Synthesis of loxoprofen starting from 1-(1-chloroethyl)−4-methylbenzene.

SCHEME 5.37 Synthesis of lamotrigine starting from 1,2-dichloro-3-iodobenzene.

188 [103]. Next, ester 188 is treated with NaIO4 to construct aldehyde 189. Subsequently, the reaction of aldehyde 189 with 4-(cyclopent-1-en-1-yl)morpholine in HCl, followed by treatment with hydrogen furnishes 191.

5.10.3 Synthesis of lamotrigine Lamotrigine 195, an API used in Lamictal, is an anticonvulsant drug used to treat bipolar disorder and epilepsy. A multistep process described in the patent for preparing 195 is illustrated in Scheme 5.37 [104]. Carboxylation of 1,2-dichloro-3-iodobenzene 192 using CO2 affords acid 193, which is treated with SOCl2 and then with CuCN to produce 194. Then, the reaction of compound 194 with aminoguanidine, followed by saponification furnishes 195.

5.10.4 Synthesis of felbinac Felbinac 198, an API in Traxam and Nabolin, is an NSAID widely used to cure arthritis and muscle inflammation. The two-step synthetic route to 198 starting from 1,1 biphenyl 196 is outlined in Scheme 5.38. Chloromethylation of 1,1 -biphenyl 196 affords 4(chloromethyl)−1,1 -biphenyl 197. Subsequently, the Grignard reagent of 197 is formed and subjected to carboxylation towards 198 [105].

5.10.5 Synthesis of spironolactone Spironolactone 206 is an API in Aldactone, i.e., the drug used to treat fluid build-up owing to kidney disease, liver scarring, or heart failure [106]. The industrial synthesis of 206 consists

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SCHEME 5.38 Two-step synthesis of felbinac starting from 1,1 -biphenyl.

SCHEME 5.39 Industrial synthesis of spironolactone.

of seven linear steps, starting from dehydroepiandrosterone 199 (Scheme 5.39). The reaction of the Grignard reagent of diol 200, which is obtained from 199, with CO2 affords an alkyne 201. Then, the triple bond of alkyne 201 is hydrogenated using Lindlar’s catalyst in pyridine and dioxane. The reaction of acid 202 with tosylic acid furnishes unsaturated lactone, which is hydrogenated to saturated lactone 203 using H2 over palladium on carbon. Following this, the Oppenauer oxidation delivers 204. Afterward, carbon-6 is unsaturated via a reaction with chloranil as an oxidizing agent. Ultimately, the resulting diene 205 reacts with thiacetic acid, providing 206 in a total yield of 40 percent (Scheme 5.39) [107,108].

5.10.6 Synthesis of finafloxacin Finafloxacin 215 is an API in the fluoroquinolone-based antibiotic marketed by Novartis under the trademark Xtoro. This drug is used to treat acute otitis externa (swimmer’s ear) induced by the bacteria Staphylococcus aureus and Pseudomonas aeruginosa [109]. A multistep synthetic route to 215, starting from 2-trifluoromethylaniline 207, is outlined in Scheme 5.40.

5.11 Conclusion

111

SCHEME 5.40 Synthesis of finafloxacin starting from 2,6-dichloro-3-fluorobenzonitrile.

Treatment of fluoride 207 with a brominating agent in concentrated sulfuric acid affords bromide 208, which, in the next step, is treated with i-PrMgCl to generate a Grignard reagent. This Grignard reagent is further subjected to carboxylation by CO2 , and subsequent acidification to furnish acid 209 with a good yield of 60 percent [110]. Subsequently, treatment with SOCl2 delivers acid chloride 210 in excellent yield (95 percent). Treatment of this acid chloride 210 with ethyl 3-(dimethylamino)prop–2-enoate in the presence of Et3 N in toluene furnishes the acrylate precursor 211 in good yield (80 percent) [109]. The introduction of cyclopropylamine leads to the β-ketoacrylate ester 212, which is readily cyclized to quinolone product 213 using K2 CO3 (Scheme 5.40). Ultimately, 213 bonds with chiral N-Boc pyrrolidine 214 to form an ester. Hydrolysis of this ester and subsequent Boc-deprotection affords the desired 215.

5.11 Conclusion The book chapter highlights advances on the incorporation of carbon dioxide as both an oxygen and carbon source through carboxylation, methylation, and oxidation in the production of important APIs and key intermediates of APIs. Summarily, the last three decades have seen tremendous advances in carbon dioxide capture and its in situ transformation (Fig. 5.2). Such achievements on the transformation of carbon dioxide are of great importance in the development of alternative schemes to APIs because the present academic organic syntheses, as well as industrial chemical formation, rely mostly on the application of fossil-mediated carbon sources. Additionally, carbon dioxide is an environmentally benign, renewable, inexpensive, and abundant source of oxygen and carbon and offers numerous technological schemes for the processing of pharmaceuticals that could afford contaminant-free APIs; this could lead to a considerably reduced usage of conventional liquid and solid reagents and the development of sustainable chemical industry. However, establishing a chemical industry

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FIGURE 5.2 Potential applications of CO2 as a building block (carbon and oxygen source) in the production of APIs by constructing new C–C, C–O, and C–N bonds.

mediated on carbon dioxide as a raw material (building block) in the production of APIs is a long-standing challenge because of the following issues and demands: (a) some catalytic transformations of carbon dioxide in a homogeneous phase discussed in this book chapter involve harsh conditions (such as high pressure of CO2 , i.e., 3400 psi in the K2 CO3 -based approach, and application of organomagnesium and organolithium at −78 °C); therefore, the development of novel approaches that operate under mild reaction conditions, for instance room temperature or low pressure of CO2 , is a significant objective; (b) the current loading of catalysts and ligands is too high for large-scale implementation. Also, the requirement of more than 1 equiv. of organometallic reagents is undesirable concerning both the economy and the environment. Consequently, the development of more efficient reagents and catalytic systems constitutes a demanding goal; (c) some reactions contain highly moisture- and airsensitive compounds; hence, research on alternative stable synthetic approaches is essential; (d) in few cases, the product distribution is not selective; thus, research on the improvement of the chemo- and regioselectivity of APIs is mandatory; (e) the elucidation of CO2 -based reaction mechanisms is vital to further construct cost-effective catalytic systems for the formation of APIs. Computational approaches have assisted in understanding the mechanisms of numerous organic reactions. This method will play an increasingly significant part in understanding the reactions; consequently, it can promote the formation of new catalytic systems. Therefore, computational chemistry must be employed for mechanistic findings; (f) fundamental knowledge on the thermophysical processes is vital for rational scale-up and

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design; (g) the development of highly recyclable catalysts, such as nanomagnetic catalytic systems, and the optimization of the stability of catalysts for their long-term performance are required; (h) challenges facing effective implementation of the technology also involve the continuous formation of particles with a required and reproducible drug quality; and (i) currently, in many processes for the formation of APIs, a huge number of steps are included, which increase the quantity of waste and the manufacturing costs. Therefore, the current objective is to reduce the number of steps and improve the productivity of the reaction steps. The construction of eco-friendly synthetic procedures for the transformation of CO2 is necessary to accomplish this goal. Particularly, chemists must work on microwave-assisted, ionic-liquid-catalyzed, ultrasonic-promoted, and enzyme-catalyzed synthetic approaches to transform CO2 into value-added chemicals. With this book chapter, we aim to inspire extensive research on CO2 capture and its in situ conversion to APIs.

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C H A P T E R

6 Electrochemical Carbon Dioxide Detection S. Aslan a, C. I¸sık a and A.E. Mamuk b a

Department of Chemistry, Faculty of Science, Mugla Sitki Kocman University, Mugla, Turkey b Department of Physics, Faculty of Science, Mugla Sitki Kocman University, Mugla, Turkey

6.1 Introduction Since life on the planet depends on the carbon element, the carbon dioxide (CO2 ) cycle is among the principal biochemical cycles on the earth [1]. In the cycle, plants use CO2 and water in the atmosphere during the photosynthesis process and convert them into sugars such as glucose because of CO2 fixation [2]. Decomposition of organic matter, cellular respiration, and combustion return CO2 to the atmosphere, unlike photosynthesis. For this reason, it is extremely important to keep the quantity of CO2 in the inspired air in balance with the CO2 cycle for life on Earth to continue [3]. CO2 is a type of gas whose existence is vital with both advantages and disadvantages. Since it is one of the adjuvant greenhouse gas that significantly affects climate change by the increase in emissions into Earth’s atmosphere, recently, CO2 has become a crucial global warming factor in nature [4–7]. The utilizing fossil fuels has risen significantly in the 1900s due to the increase in population and the increasing need for energy as a consequence of technological developments [8]. By the rising of utilizing fossil fuels, the quantity of CO2 in the Earth’s atmosphere has increased tremendously. As a result of this situation, the quantity of CO2 in the air has come dimensions that cannot be kept in balance with the CO2 cycle [9,10]. The excessive CO2 emission has caused negative effects in many areas, especially on human health, energy security, life cycle, air quality, and global climate change [11]. The contribution of the CO2 to the increase in global warming is specified as 76 percent, also because of the human-induced CO2 emissions, the CO2 amount in the atmosphere has been increasing by nearly 1.5 ppm per year for the last two centuries. It was specified that

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the concentration of CO2 was about 280 ppm and 370 ppm at the beginning of the Industrial Age and in 2000, respectively. The present amount is approximately 412 ppm. As a result of CO2 being categorized as the major heat-trapping gas, the increment of CO2 amount in the air increases territorial and nautical temperature, which has been determined as the highest in June 2019 since 1880, also causing ocean acidification [12–14]. For all mentioned reasons, the increment of CO2 amount in the air becomes one of the major climate change boosters. Hence, because of the concerns about global warming and greenhouse effects, the research on sensing, capturing, and monitoring CO2 has very excessively come into prominence. Moreover, apart from important concerns about the CO2 rate in the outdoor environment, the existence of high CO2 concentration is possibly fatal or may cause some unhealths such as headache, tiredness, respiratory problems, loss of consciousness, etc. for most the creatures especially humans in the atmosphere, as well. In pursuant of the American Society of Heating, Refrigerating and Air Conditioning Engineers (ASHRAE) Standard, 1000 ppm of the CO2 concentration should be the upper limit in enclosed buildings. The main source of CO2 concentration in enclosed places is human breathing whose level is generally quite more than approximately 450 ppm of the average CO2 level in an outdoor atmosphere. Once again according to ASHRAE standards, 350 to 1000 ppm of the CO2 concentration in closed places is considered a healthy environment. Besides, the existence of CO2 is a fundamental agent for photosynthesis, thus, plants and algae can convert CO2 into glucose and O2 . Therefore, determination and monitoring of the CO2 level, which causes substantial cost, also is quite momentous for both healthcare and organizing life in closed places [13,15]. Alongside harmful effects of CO2 gas on human and environmental health, a part of the atmosphere, CO2 gas, can become a beneficial substance. One of the most known CO2 -using industries is food packaging in a modified atmosphere. The main scope of this field is the prohibit food spoilage and extend the shelf life of packaged food products. The freshness of packaged food can be specified by monitoring the CO2 level, and if there is a decrease in its concentration it is considered that is probably proof of a symptom of leakage in a food package [16,17]. Also, with intrinsic properties, CO2 gas can be used in many industries such as biotechnological processes, producing some chemicals, and also the fabrication of carbonated liquids, cooling and welding systems, fire-extinguishers, and water treatment systems [5,15,18]. Thus, it is due to occupy an important place in daily life, monitoring and sensing the CO2 gas takes an important place in the manufacturing process of such materials. In recent years, more renewable energy technologies such as photovoltaics and wind power have been used instead of fossil fuels in transportation, energy generation, and industry to prevent climate change and reduce its effects. Despite this situation, the Energy Information Administration (EIA) states that fossil fuels will continue to become the fundamental part of useable energy in the near future and the amount of CO2 in the air will maintain to increase [19]. Therefore, it is necessary to develop alternative methods besides renewable energy systems such as wind and photovoltaics in order to reduce CO2 emissions [20–22]. It is a quite effective approach proposed to diminish the amount of CO2 in the air in recent years is carbon capture technology [23]. This technology is divided into two categories, Carbon Capture and Storage (CCS) and Carbon Capture and Use (CCU). CCS means to the capture of refuse CO2 using several technologies and its transport and storage to mineral, geological and oceanic sequestrations. CCS is not preferred because energy is required to extricate, convey, and reservoir CO2 in the air and this energy is supplied from fossil fuels, and also because of limitations such as uncertainty regarding the permeability of CO2 on the storage source

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after storage [24–26]. The CCU method is more advantageous than the CCS method because it can be recycled for later uses instead of storage immediately after the CO2 is captured in the atmosphere. While both methods diminish the quantity of CO2 in the air, CCU uses CO2 as a renewable resource, providing an additional benefit for resource and potential energy recovery [27,28]. It is stated that CCU is a quite suitable method to diminish the quantity of CO2 by taking the natural carbon cycle as an example to generate complex using CO2 in the air [29]. Beyond the CCU methods, electrochemical ways are outstanding. Electrochemistry basically studies the electrons formed due to a chemical change. Electrochemical methods are used to determine the presence or amount of a chemical that causes a change depending on electron exchange, current, concentration, or voltage difference [30]. Electrochemical methods basically include potentiometric, amperometric, voltammetric, conductometric, and coulometric methods. Many sensors developed using these methods find a wide application area [31]. Electrochemical gas sensors are one of those fields [32]. There is a chemical reaction that occurs between the ambient gas and a part of the sensor for an electrochemical gas sensor. At the end of the reaction, electrons are released. The liberated electrons create a current, allowing a current value to be read in the output. Thus, the amount of gas in the environment can be measured according to the flow at the outlet [33]. In the present chapter the former, a bunch of the most used general methods, the latter, electrochemical detection, and reduction of CO2 have been detailed and exampled by the studies reported by divergent groups in the manner of catalysis or electrode development.

6.2 Capture technologies of CO2 CO2 , which is the most important reason for global warming, is one of the topics of great interest both industrial and academic. It is stated that the amount of this gas in the atmosphere is increasing day by day owing to human activities and that the efforts of countries with large economies are not sufficient to prevent global warming, and it is expected that be possible with environmental policies to reduce CO2 emissions of each country [34,35]. It has been stated that the amount of CO2 in the atmosphere can be reduced by using methods such as traditional use (e.g., solvent), fuel generating (e.g. raw materials for diesel and gasoline), biological imposition (e.g., food of carbon for microalgae culture) and chemical production (e.g., raw material for carbonates and polymers) [36]. These methods for reducing the amount of CO2 have an environmentally friendly and economical approach, as methods contribute to the formation of innovative materials and products since CO2 is used as a raw material in industrial processes [34,36,37]. The capture of CO2 from gas flows is carried out using a great variety of technologies. Although these technologies could be adapted to many industrial fields, the technologies are not still appropriate for large-scale production applications. These technologies can be divided into adsorption, absorption, separation by membrane, and chemical capture [38].

6.2.1 Adsorption Adsorption is the process of binding a substance to a solid substance by shifting it from the gas phase with the help of electrostatic or van der Waals forces. Adsorption is the process of linking material to solid material by changing it from the gas state with the help of electrostatic

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or van der Waals forces [39]. The carbon capture process with the adsorption method takes place by attaching CO2 from a gas flow to an adsorbent with supreme CO2 selectivity. The adsorbent is then desorbed (reproduced) by pressure or thermal fluctuations or by applying a potential difference to the adsorbent [40]. These procedures cause the links between the solid material and the gas phase to break, allowing for continuous reuse of the adsorbent [41]. The performance of solid adsorbents is determined by the form and range of the pores, their magnitude, surface area, and interaction between the surface and the CO2 molecules [42]. The adsorption process as a CO2 capture technology has some limitations as it involves complex processes, low efficiency, and requires a big deal of energy to sustain the procedure [43].

6.2.2 Absorption The absorption process takes place in the presence of material that can absorb this gas after CO2 has been captured and transport it from one phase to another. In the process, when the CO2 in the gas phase comes into contact with the solution in the liquid phase, it dissolves into the liquid phase and allows it to be separated from the other components [44,45]. Also, temperature change is very important as it affects the solubility of the gas in the solution. Therefore, the energy requirement during the process to control temperature changes is also quite high. Absorption technologies are one of the most interesting and studied technologies by researchers due to their advantages [46,47].

6.2.3 Separation by membranes In the capture process by the membranes, the CO2 passes through the membranes and is separated from the other compounds. The basis of this method is based on the selectivity of gases against the materials that make up the membrane [48]. The success of this method depends on variables for instance grain size, pore volume, permeability, and selectivity of the membranes. However, since these parameters are difficult to optimize during the capture process, this is considered to be the biggest challenge in the development of studies aimed at capturing CO2 with membranes [44–49].

6.2.4 Chemical capture Among the currently developed CO2 capture technologies, the chemical capture method stands out due to its features such as providing added value to industrial applications and offering more sustainable production systems. This method is based on the principle of obtaining a final output as a consequence of the reaction of CO2 in the atmosphere with organic or inorganic chemicals [50,51]. There is a great interest in the technology because the organic or inorganic chemicals used in this method are biodegradable, have low toxicity, and have high boiling points [40]. The most important factors limiting chemical capture technology are the stability of the chemicals used against oxidation and reduction during repeated cycles and the cost of the process [52,53].

6.2.5 CO2 sensors The sensor means that an apparatus detects or monitors the change in a physical/chemical quantity in the target environment and, though not always, converts the information using

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software and control electronics. Fundamentally, depending on being electronic or nonelectronic, the sensors may contain some important members such as a sensing unit, a transducer, data processing electronics, and a display for giving outcomes [16,54]. A gas sensor works, basically, by relying on specifying the molecules of the target gas and converting that information into beneficial and considerable information. That converting can take place with and without an electronic network. Non-electronic sensors generally give the sensing information with the change in optical visualization. In electronic sensors, on the other hand, a sensing unit generates a change in a physical quantity such as conductivity, capacitance, resistance, etc. of the sensed material in accordance with the chemical reaction between the analyte gas and sensing unit. Plenty of studies have been carried out related to sensors or materials that can be used as sensors with different working principles for the measurement of CO2 change in a certain environment [16,54]. Gas chromatography (GC) and Mass Spectrometer (MS) are one of the common detecting systems for CO2 gas. GC enables the division and specification of the ingredients of a mixture. It consists of a chromatograph with several columns and detectors [55]. The running of GC is based on selective adsorption and the driving of the analyte gas molecules in the column of the system. Choosing the right column and detector leads to good detection of analyte gas. GC consists of various fragments, e.g. thermal conductivity detector (TCD), and electron capture detector (ECD) which are quite beneficial for detecting the CO2 gas [55–57]. The main advantage of GC is being cost-effective. However, analyzing the analyte, for instance, CO2 , takes a long time in GC, thus it is the main disadvantage of the system. The running of MS, besides, relies on the principle that the analyte gas is exposed to high-energy electron beams and thus the gas molecules are split. The split molecules are motioned benefitting from the having charge of them, thus, the molecules can be separated through their charge and mass. However, MS enables to analysis of CO2 gas in an almost instant, it is a quite pricey system. Due to mentioned disadvantages of both GC and MS, using them is not advisable and the methods need to be solved the drawbacks such as miniaturization, sampling issues, the price [4,16,55]. Severinghaus electrode which is a type of CO2 sensor a glass electrode. This electrode is covered by a thin membrane that is selective-permeable, namely, however it enables the transition of CO2 molecules, it prevents the transition of electrolytes. On the other hand, the glass electrode contains a dilute bicarbonate solution. The Severinghaus electrode sensor is based on a working principle that carbonic acid, which is resulted from CO2 gas decomposed into HCO3 − and a p+ , thus, due to the case, the pH of the electrolyte changes. In this method, CO2 gas cannot directly be sensed, however, it can be determined while it forms carbonic acid in an electrolyte, so this case builds the main disadvantage of the system. Besides, while measuring the CO2 , other volatile compounds and gases exist in the measuring environment can affect the pH of the electrolyte [16,17,58]. Infrared detectors are the members of the CO2 detectors that generally possess less response time, portable size, and low-cost. Nondispersive infrared detectors are the most preferred detectors among infrared detectors and exhibit long-term determination, high fidelity, and great gas specificity. Because gases, of course also, CO2 , absorb infrared light energy depending on the quantized vibrating energy of relevant gas, the running principle of the infrared detectors relies on the energy absorption characteristics of analyte gas. Also, the narrow bands of the wavelength of infrared radiation of CO2 are 2700, 4300, and 15,000 nm, thus, it means that

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most of the heat-producing radiation escapes CO2 [16,59–61]. In infrared CO2 detectors, the analyte gas, which is CO2 here, is detected by comparing it with another reference (control) gas that does not absorb infrared. The radiation from the infrared source is let to penetrate both the CO2 and the control gas to reach a radiation sensor. The amount of CO2 is determined in regard to the infrared absorption of the analyte gas compared to the control gas. The main disadvantage of non-dispersive infrared detectors is that non-target gases such as water vapor and carbon monoxide (CO), which are in the environment to be measured and have absorption in the infrared region, may affect the CO2 measurement results [16,17,60,62]. Nanomaterials which can involve organic, inorganic fragments or merging them are a good alternative to sense CO2 gas. Although organic materials have some beneficial features such as mass transport, chemical reactivity, etc. as regards CO2 sensing, inorganic materials have different advantages such as mechanical stability, conductivity, and optical properties as well. Since metal oxides have some outstanding features such as taking small space, simply generating, high stability and sensitivity, although taking high working energy and show low selectivity, metal oxide gas detectors are preferable sensors for sensing and monitoring the CO2 gas as a nanomaterial-based sensor [7,15,63,64]. Due to being a producible large surface area, which increases the adsorption of gas molecules and contributes to surface reaction with the gas phase, various metal oxide materials such as La2 O3 [7], CdO [65], Y-doped ZnO: CdO [66], SnO2 [67] are some of the favorable sensing materials for the manufacture of gas sensors. Metal-oxide gas detectors had the main disadvantage owing to generating process conditions that exhibit impurity during the generating films [7,15,68]. Other crucial nanomaterials for generating a CO2 sensor are carbon-based semiconductors. Carbon-based semiconductors are interesting and utilizable materials for sensing systems with their important features such as being tough, high carrier mobility, etc. Carbon nanotubes (CNTs) are one of the members of carbon-based semiconductors and CNTs are preferred sensing carbon-based semiconductor materials for gas sensing systems owing to possessing a large surface area to sense the gas molecules. Depending on the aligning form of CNTs, the response time of the sensor is significantly changed [13,69]. It is reported that at room temperature, horizontally aligned CNT-based CO2 sensor shows a faster response time, which may be related to the lacking of intertube links, while randomly aligned CNT-based CO2 sensor has a slower response time, which may be also due to existence of bending of CNTs [70]. Another member of carbon-based semiconductors is graphene. Pure graphene, graphene oxide, and reduced graphene oxide, which are derivatives of graphene are favorable nanomaterials for generating a gas sensor. While a graphene absorbs the analyte gas molecules, however, the amount of electrons in the graphene is a trace of change, there is a significant change in the conductance of graphene, thus, graphene becomes a quite suitable sensing material of a gas. A graphene-based CO2 sensor shows quite a short response time in comparison with CNTbased CO2 sensors at room temperature and also at over room temperature [13,71]. While the analyte gas, namely CO2 , is detected, the other gas/gases can affect the right measurement, therefore this case reveals the main disadvantage of graphene-based CO2 sensors. Apart from the nanomaterials, which are given above, various nanomaterial structures such as semiconductors, and polymers such composite structures can be used for sensing CO2 . In recent years liquid crystals (LCs) have been strong prospective photonic structures with simple adaptable, low energy consumption for being a gas sensor. Liquid crystals (LCs) are

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materials that exhibit both rheologic features of isotropic liquids, namely ordinary liquids, and features of molecular order of solid crystals. This unique feature for LCs reveals at a specific temperature interval which is called mesophase [72,73]. To open a new door for the technology of capturing and storing carbon or carbon-based materials, scientists have focused on CO2 sorption in LCs. It is reported that LC could absorb fundamental gasses such as CO2 , and N2 , and it was especially shown that CO was the best solute among the gasses used [74]. As is known, however, CO2 weakly dissolves in ordered liquids such as LCs, it is well dissolved in isotropic liquids which do not show any molecular order [75–78]. It is possibly done that CO2 gas is solved in an LC material at isotropic phase then decreasing the temperature of the LC a few degrees at constant pressure, thus, the LC changes its phase to mesophase and due to that new state, CO2 is released. This CO2 capturing and releasing process needs very less energy than other known CO2 capturing systems because a few degrees change in temperature of an LC material is enough to trigger the phase transition [79]. De Groen and co-workers reported the change in phase behavior of LC+CO2 [75,76] and LC mixture+CO2 [78] in nematic mesophase and separated the CO2 gas by considering a three-phase equilibrium. Moreover, some amphiphilic block copolymers consist of blocks such as poly(propylene oxide) (PPO), and poly(ethylene oxide) (PEO), and such blocks can be potentially used for CO2 capture [80]. A copolymer based on PPO, and PEO self-assemble and change their form as a lyotropic LC in a dilute solution. Lyotropic LCs are a type of LC whose features can be changed with the change in temperature and concentration [72]. Rodriguez-Fabia et al. studied on CO2 absorption of lyotropic LCs which consisted of PEO-PPO blocks and exhibited lamellar and hexagonal phases, depending on the viscosity of the LCs, and it was showed that the ionic strength of the LC solutions could affect the CO2 absorption [80]. In addition, cholesteric (Ch) LCs have the potential to be used as an optical sensors so as to detect gases [81–83]. (Ch LCs are more symmetrical than nematic LCs. The centroids of the molecules and the n director, which characterizes the orientations of the molecules, lie in a plane. The orientation of the director is not constant and changes as it moves from one plane to the other. Thus, a spiral step which is generally called as the pitch is formed. The pitch can be changed by an external stimuli such as temperature, electric, and magnetic fields. The helical orientation direction of Ch LC molecules is determined by the right- or left-handed chiral dopant depending on its helical twisting power (HTP) [72,84,85] If the light coming into the Ch LC has the same wavelength as the length of the pitch and is circularly polarized in the same direction as the pitch, the light beam is held by Ch. On the contrary, if the light is oppositely polarized, Ch allows it to pass through the structure, thus Ch LCs is identified as selective transparent photonic material. The helical pitch can be varied by stimuli such as CO2 gas, therefore the color of transmitted light changes compared to its non-CO2 -exposed state. So, the HTP of the chiral is changed and the presence of CO2 can be specified according to the change in color of transmitted light which can be observed with necked eyes [82]. It was reported the real-time monitoring of CO2 using an E7 nematic mixture doped with a high HTP chiral [81]. Effect of CO2 in the chiral structure revealed in both optical observations and structural analyses. In the IR spectra of chiral dopant, stretching vibrations of the N–H which belongs to the amino group (3377 and 3355 cm−1 ) was not observed after exposing it with CO2 , while a carbonyl group stretching band was obvious at 1650 cm−1 [81]. As a new approach to detecting volatile organic compounds, electrospun LC fibers are a substantial prospective structures with the ability which is based on varying scattering of transmitted light while the electrospun fiber

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is exposed to volatiles. Electrospinning is one of the versatile and cost-effective methods to obtain phase-separated fiber, [86–92]. Lagerwall and co-workers stated that electrospun coresheath LC/polymer fibers with various morphologies could sense the toluene in the gas phase as a non-electronic gas sensor [93]. In the beaded (non-homogenous) LC/polymer fibers, exposed to toluene, the scattering degree of the LC decreased since the toluene vapor reached the LC and reduced the phase transition temperature of LC due to behaving as an impurity. So, the isotropic liquid-nematic phase transition temperature of the LC decreases up to room temperature depending on the concentration amount of vapor. Thus, the LC inside the fiber as a core formed isotropic liquid, and the disappearance of light scattering could be observed with necked eyes without the need for any polarizer. When closing the exposing toluene, toluene molecules left from the LC and LC formed an anisotropic phase again. By benefiting from the effect of volatile organic compounds on LC birefringence, a similar study was taken place for detecting toluene, cyclohexane, and isopropanol with electrospun fibers involving LC mixture [94]. It was stated that the detecting process was reversible and repeatable, Also, the response time was quite short (a few seconds). Lagerwall et al. reported such kind of LC/polymer core-sheath fibers might be used for detecting hazardous gases and would be potentially useful materials for wearable sensor [93].

6.3 Fundamentals of electrochemistry Electrochemistry is a branch of chemistry that studies the electrical behavior of substances and the relationships between electrical energy and chemical reactions between conductive interfaces [95]. The methods used to analyze the electrochemical properties of substances are also called electroanalytical methods. Electroanalytical methods have many advantages over other analytical methods [33,96]. Among them, the simultaneous detection of species with different oxidation levels is easy, and the equipment and software that enable the application of the methods are much cheaper and more useful than chromatographs and spectrophotometers [97]. Moreover, there are multi-purpose ones that enable the applicability of many electroanalytical methods in the same device [98]. A collective representation of the electroanalytical methods is given in Table 6.1.

6.3.1 Voltammetry Voltammetry is an electroanalytical method that examines the relationship between the measurement of different current values at varying potential values of a polarized working electrode with a small surface area (usually less than 1 cm2 ) and the analyte concentration [99]. In voltammetry, different types of working electrodes are used, as well as different types of voltage sources. Voltammetric methods are named according to the types of current used and the working electrode [100]. Such as direct current voltammetry, direct current polarography, alternating current voltammetry, alternating current polarography, square wave current voltammetry, pulse voltammetry, differential pulse voltammetry, and cyclic voltammetry [101]. Two or three-electrode systems can be used in voltammetric measurements, threeelectrode systems are commonly used. A simple representation of a three-electrode system is given in Fig. 6.1.

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6.3 Fundamentals of electrochemistry

TABLE 6.1 A collective representation of electroanalytical methods. Electroanalytical Methods Methods throughout the analysis environment

Interface Methods Static methods (I = 0)

Dynamic methods (I ˃ 0)

Potentiometry Potentiometric Controlled potential titrations

Conductometry

Conductometric titrations

Constant current

Fixed electrode Coulometric titrations potential coulometry Voltammetry Amperometric titrations

Electrogravimetry

Electrogravimetry

FIGURE 6.1 A simple illustration of a three-electrode system.

Different voltages can be applied separately to the E1, E2, and E3 input resistors, or superimposed voltages can be obtained by applying different voltages to each end. The three fundamental quantities that characterize a voltammogram are residual current, limit current, and half-wave potential [102]. As seen in Fig. 6.2, each voltammogram has two digits. The first step is called the residual current (ia) step. The current value of the second digit is also called the limit current (il). Residual current is the sum of currents due to two reasons. The first is the capacitance current resulting from the double layer formed between the electrode and the solution. The second is the faraday current resulting from small levels of electroactive impurities that can be found in the supporting electrolyte apart from the analyte. The limit

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6. Electrochemical Carbon Dioxide Detection

FIGURE 6.2 A voltammogram and its components.

current is the current value between the first digit and the second digit of the voltammogram. This current is the current value when the concentration of the electroactive sort on the electrode surface goes to zero and is almost independent of the potential. The half-wave potential is the potential value corresponding to half of the limit current (il/2 ) and is denoted by E1/2 . Techniques to eliminate residual current have been developed to increase sensitivity (e.g. normal pulse voltammetry/polarography, voltammetry of square wave, pulse voltammetry, differential pulse voltammetry, cyclic voltammetry). In these methods, pulse-shaped voltages are used [103].

6.3.2 Potentiometric methods In potentiometric measurements, a reference electrode, an indicator electrode, and a potential measuring device are required. In the process, the potential of the electrochemical cells is measured without drawing a significant amount of current (Fig. 6.3). Potentiometric methods can be defined into two groups [104].

6.4 Direct potentiometric methods Direct potentiometric measurements can be made with an indicator electrode. The method is simple, the potential generated at an indicator electrode inserted into the sample precursor is compared with the potential generated when the same electrode is inserted into a standard precursor. Since the ion to be detected by the electrode is special, no pre-separation is required. Direct potentiometric measurements also allow continuous and automatic monitoring of analytical parameters [105]. In addition to the great convenience of direct potentiometric measurements, some obligatory errors in the structure of the method should also be known and considered. The most important of these is the "liquid coupling" potential found in many potentiometric measurements. The coupling potential imposes a limitation on the accuracy of the measured values [106].

6.4.1 Potentiometric titrations The equivalence point of a potentiometric titration is determined by the potential of a suitable indicator electrode. In a potentiometric titration, different information is obtained than in direct potentiometric measurements. For example, direct potentiometric measurements of acetic acid of 0.100 M and hydrochloric acid of 0.100 M solutions with a pH-sensitive electrode

6.4 Direct potentiometric methods

129

FIGURE 6.3 A general illustration of a potentiometric cell.

give very different pH values, because hydrochloric acid is completely dissociated while acetic acid is partially dissociated in solution. However, for the neutralization of equal volumes of solutions of these two acids, equal amounts of the standard bases are used. The endpoint determination method with potentiometric titration gives much more accurate results than the endpoint determination with the indicator. The method is particularly successful in working with colored and turbid solutions and in the determination of known ions in the solution. However, it takes more time than with the indicator [107].

6.4.2 Amperometric methods Amperometric methods are used to determine the equivalent points of titrations. It is essential here that at least one of the substances leaving the reaction is oxidized or reduced in a microelectrode. In the method, the current at a constant potential is measured as a function of the volume of the titrant (or time if the substance is created by a constant-current coulometric process). The results are analyzed by graphing. Data on either side of the equivalence point give the slopes a different line; The turning point is found by extrapolating the point where these lines intersect (Fig. 6.4) [108].

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6. Electrochemical Carbon Dioxide Detection

(A)

(B)

(C)

FIGURE 6.4 Typical amperometric titration curves.

An amperometric titration is more sensitive than voltammetric titrations and is less dependent on the properties of the microelectrode and supporting electrolyte. It is desirable that the temperature be constant, but tight control is not required. Also, the substance to be determined does not have to be electroactive; It is also sufficient for the titrant or product to be active on the electrode [105].

6.4.3 Conductometric methods In an electrolyte solution, basically, the positively charged particles and the negatively charged particles migrate to the cathode and anode electrodes, respectively. This process is defined as electrical conduction. Even conductivity is identified as a measure of current and it is known that it is linearly related to the amount of charged particles in the solution. Depending on the mobility talent of a particle and its concentration in the media, the current can be produced [109]. Peculiarities of the particles restrict the carrying out of the analyzes which relied on direct conductivity measurements. In a solution that involves blends of ions, direct conductivity measurement is not selective because all the ions in the solution, which promote conductivity, affect the whole solution’s conductivity. Although, the method is significant for some applications owing to its sensitivity being high. One of the most applications of this method is the controlling of the refinement of deionized water [110]. Conductivity measurements are applied to determine the concentrations of solutions containing only one strong electrolyte (such as alkalis or acids). Calibration curves are used in this type of analysis. The measurement can be made in solutions containing up to 20 percent by weight of the substance. The salinity of seawater can also be determined by conductivity measurements. With conductivity measurements, information can be obtained about the association and dissociation properties of aqueous solutions containing one or more ionic particle [111]. Utilizing the method for aqueous solutions and systems in which the reaction is outstanding is the most significant benefit of the method. For example, however, carrying out the analysis of dilute phenol (Ka »10−10 ) solution is impossible by using methods which are the potentiometric or indicator endpoint, it could be carried out with the method. While the whole electrolyte concentration rises, the precision of

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6.4 Direct potentiometric methods

TABLE 6.2 A general classification of voltammetric working electrodes. Mercury based electrodes

r r r

Dropping mercury: Gravity forced, Mechanical Hanging mercury Mercury film

Solid electrodes

r r r

r

Platinum Gold Carbon: Graphite, Carbon paste, Glassy carbon or carbon paste, Carbon felt, Carbon fiber, etc.… Bismuth

Modified electrodes

r r

Composite Chemically modified: Polymer membrane coated, Surface adsorption, Covalent bonding, etc.…

Rotating electrodes

r r

Disc Ring-disc

the method diminishes. The change in current when titrant is added is observed when the concentration of salt of solution is rich [112].

6.4.4 Coulometric analysis methods The coulometer encompasses a group of analytical methods that measure the electricity (in coulons) required to quantitatively switch the analyte to another oxidation state. There is a proportionality constant, subtracted from known physical constants, between the weight of the measurand and the analyte; therefore, there is no need for a calibration and standardization step. Coulometric methods are as accurate as gravimetric or volumetric processes; it is also faster and more useful than gravimetric determinations [113]. Two general techniques are applied in the coulometric analysis. In the first technique, the potential of the working electrode is kept at a level that does not affect the less reactive particles in the solution during the quantitative oxidation or reduction of the analyte. Here, the current is initially high, but drops rapidly and decreases to zero as the analyte leaves the solution. The energy required is measured with a chemical coulometer. In the second technique, a constant current is applied until the indicator signal indicates that the reaction is complete. When the equivalence point is attained, the amount of electricity is calculated from the amount of current. This second technique is called “coulometric titration”, its application area is wider than the first method [114].

6.4.5 Electrodes 6.4.5.1 Working electrodes Both the chemical and electrochemical properties of the electrodes used in voltammetry are important. Therefore, a limited number of polarizable electrodes are used in voltammetry. These are mercury liquid, platinum, gold, bismuth, and carbon-based solid electrodes and modified electrodes grouped in Table 6.2. The potential working range (working window) of each of these electrodes, which can be used as stationary or rotated, is different. This range depends on many parameters such as electrode type, solvent, type of electrolyte used, and pH. The formation of hydrogen or the reduction of the supporting electrolyte determines the cathodic limit, while the oxidation of the electrode material or solvent determines the anodic limit [115].

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6.4.5.2 Carbon-based electrodes Carbon-based electrodes are fast-responsive, economical, and easily formed electrodes in different configurations and diameters. Carbon is an ideal electrode material in terms of many properties such as having a large anodic potential interval, weak electrical resistance, low electrical resistance, low residual current, and renewable surface structure. Various forms such as carbon fiber, glassy carbon, graphite paste, and carbon film can be used in electrochemical applications [116]. Glassy carbon (GC) and carbon paste (CP) are the most used carbon electrodes. CPE has advantages such as very low ground current, and composite nature with ease of regeneration and modification, while GC electrode has significant electrochemical reactivity, good mechanical stiffness, and negligible porosity. These important properties of carbon electrodes are of great importance in the development of carbon-based electrochemical sensors. GC electrodes, one of the carbon electrode types, are obtained by reducing the pore size with a special method. GC is formed because of the thermal decomposition (degradation) of some polymers at approximately 1800 °C. Since these materials are hard, the surface of the GC electrodes must be polished before each trial. The GCs get ready for use by applying a pre-polarization process at +0.65 V. The working potential limits of the GC electrodes are approximately +1.00 V and −0.75 V. GC electrodes are generally available as stationary and rotating discs. In addition, these electrodes are widely used as detectors in high performance liquid chromatography and fluid systems [117]. 6.4.5.3 Composite carbon electrodes CP electrodes are prepared by mixing powdered graphite with an organic liquid such as mineral oil. After the cake is prepared, it is filled by squeezing into a tube (eg Teflon tube) [118]. A platinum or copper wire is used for the electrical connection. CP electrodes, which have a wide potential range, are short and easy to make and replace and have a sufficiently low ground current [119]. CP electrodes are frequently used in the production of modified composite electrodes. Electrodes can be prepared by adding and mixing the modifying chemical directly to the conductive electrode material. These electrodes are called composite electrodes. For example, the modifier (complexing agent, adsorbent, catalyst) is used by making a paste together with carbon powder and nujol. In addition, electrodes can be made by compressing with carbon and turning it into pellets [120].

6.4.6 Reference electrode For this purpose, non-polarized metal-metal ion electrodes of the second class which are mentioned in the following potentiometric electrodes section, are used. These electrodes are also non-polarized only at small current intensities. As the current intensity increases, the electrodes deviate from the ideal position. The most used ones are calomel and Ag/AgCl electrodes [121].

6.4.7 Auxiliary electrode The non-polarized electrode becomes over-polarized at high currents because current flows along the above mentioned electrodes in two-electrode systems. In addition, if the solution resistance is high, the potential (iR) required to overcome this resistance increases to

6.4 Direct potentiometric methods

133

FIGURE 6.5 The potentiometric electrodes applied in the field of the gas sensing.

a significant level. For these two reasons, the polarization potential of the working electrode is perceived incorrectly. As a result, the i = f (E) curves become flat, and the steps or peaks disappear after a certain point. This issue is resolved by using a third electrode in the system. The current is passed through the working electrode and the auxiliary electrode pair, and the potential of the working electrode is determined under zero current against the comparison electrode. Since the current passes through the auxiliary electrode, these electrodes must be noble metals [122].

6.4.8 Potentiometric electrodes The most preferable potentiometric electrode is the pH electrode. Ion-selective electrodes are commonly preferred over the redox electrodes to selectively measure certain ions. For example, while the fluoride measurements, in dental care products, ion-selective electrodes are extensively useable in applications since fluoride cannot be simply determined or else. Using ion-selective electrodes, the electrolytes sodium, potassium, lithium, and calcium in the blood are determined by clinical analyzers [123]. Such an electrode has a metal in contact with a solution involving the cation of the mentioned metal. One of the leading example is the silver electrode which is inserted into a solution of silver nitrate (Fig. 6.5).

6.4.9 Indicator electrodes In potentiometric measurements, a reference electrode, an indicator electrode, and a potential measuring device are required. In the process, the potential of the electrochemical cells is measured without drawing a significant amount of current. Application areas include

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TABLE 6.3 A brief of the potentiometric electrodes. Metallic electrodes

Membrane electrodes (special or ion-selective)

First order electrodes: Used for cations Second order electrodes: Used for anions 1)Ag/AgI, 2)Ag/AgCl, 3)Mercury Inert redox electrodes: Fe2+/3+ and Ce4+

Glass membrane pH electrodes Liquid-membrane electrodes Solid state-precipitation electrodes Gas-sensitive membrane electrodes: CO2 , NH3 , O2

FIGURE 6.6 General working principle of an electrochemical gas sensor.

titrations based on endpoint determination, ion determinations with ion selective membrane electrodes, pH measurement and determination of thermodynamic equilibrium constants such as Ka, Kb, and Ksp [124]. Indicator electrodes are electrodes that are selected in the appropriate response according to the analyte and detect analyte activity. The salt bridge is used to prevent the components in the analyte solution from blending with the reference electrode; A potential arises along the fluid connection at both ends. An ideal electrolyte for salt bridge is potassium chloride, since the motions of K+ and Cl- ions are almost equal [125]. Indicator electrodes used in potentiometric measurements are collected in Table 6.3.

6.4.10 Electrochemical gas sensors Electrochemistry studies the electrons formed due to a chemical change. Electrochemical methods, on the other hand, determine the chemical that causes this change depending on electron exchange, current, concentration, or voltage difference. Electrochemical methods include potentiometric, amperometric, voltammetric, conductometric, and coulometric methods. Many sensors developed using these methods find a wide application area. Electrochemical gas sensors are one of them. In the working principle of an electrochemical gas sensors, a chemical reaction occurs between the ambient gas and the active centre in the sensor. At the end of this reaction, electrons are released. The liberated electrons create a current, allowing a current value to be read at the output. Thus, the amount of gas in the environment can be measured according to the flow at the outlet [126]. Electrochemical sensors are generally used to detect toxic gases and oxygen, especially CO, ammonia, and chlorine (Fig. 6.6). Briefly, there are two or three electrodes in the structure of electrochemical sensors. CO gas in the environment passes through the membrane and reaches the electrodes. Electrodes react chemically with CO gas and an electron current is formed. The amount of the gas concentration

6.4 Direct potentiometric methods

(A)

135

(B)

FIGURE 6.7 Schematic view of two gas-sensing electrodes.

is linearly proportional to the amount of the current. The detector sends an alarm signal according to the level of current generated in the sensor [127].

6.4.11 Potentiometric gas sensors Potentiometric gas sensors have been improved relied on the potential difference of different gases under different physical conditions. Generally, zirconium-based sensors based on measurement at high temperatures are well-developed examples in this field. It is widely used in the measurement of environmentally harmful nitrogen oxides and similar gases in the automotive industry [128]. A schematic view of a well-known gas-sensing electrode is given in Fig. 6.7. The electrode composes of three parts; a reference electrode, a special ion electrode, and an electrolyte solution contained in a cylindrical plastic tube. A thin gaspermeable membrane is attached (replaceable) to one end of the tube to separate the inner electrolyte solution from the outer sample solution. The membrane is a thin microporous film made from a hydrophobic plastic; Due to its water-repellent property, water and electrolyte are prevented from entering and leaving the pores of the film. The pores only allow air or other gases with which the membrane is in contact to pass. If the solution in which the membrane is immersed contains a gas, for example, CO2 passes from the solution to the pores of the membrane, as seen in the following reaction. CO2(aq) outer solution

↔

CO2(g) Member pores

Since the number of pores is very large, the equilibrium position is reached quickly. The CO2 in the pores is also in contact with the inner solution and a second equilibrium reaction

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6. Electrochemical Carbon Dioxide Detection

easily occurs. CO2(g) Member pores

↔ CO2(aq) Inner solution

A glass-reference electrode pair immersed in the inner solution film detects the pH change. The total reaction of the described process is found by adding up the three chemical Eqs. CO2(aq) + 2H2 O ↔ H3 O+ +HCO3 − outer solution

Inner solution

The reaction’s equilibrium constant is represented by K,    H3 O+ HCO− 3  K=  CO2(aq) outer

(6.1)

The HCO3 − concentration in the inner solution is not significantly affected by the HCO3 − concentration that is formed by the CO2 passing through the pores from the outer solution if the initial concentration is kept very high; thus, Kg, K  Ka =  HCO3 − aq   H3 O+  Ka =  CO2(aq) outer and A1 the hydrogen ion activity in the inner solution is written.     A1 = H3 O+ = Kg CO2(aq) outer

(6.2) (6.3)

(6.4)

The potential of the electrode system in the inner solution is dependent on A1 according to Eq. (6.4). Substituting Eq. (6.4) in Nernst’s Eq., the following Eq. is derived.   E = L + 0.0592.logKg. CO2(aq) outer (6.5) or

  E = L + 0.0592.logKg. CO2(aq) outer 

L = L + 0.0592.logKg

(6.6) (6.7)

Accordingly, the potential of a cell containing an internal reference and indicator electrode is determined by the CO2 concentration in the outer solution. Here, none of the electrodes meet the sample solution; therefore, it would be more accurate to call the system a gas-sensing “cell” rather than a gas-sensing electrode. Substances that prevent measurement are other gases dissolved in the sample that can pass through the membrane and affect the pH of the inner solution. The selectivity of gas-sensitive electrodes can be increased by using an inner electrode that is sensitive to some ions other than the hydrogen ion; For example, a nitrate-sensitive electrode should be used to prepare a nitrogen dioxide-sensitive cell. Here the equilibrium reaction is as follows. 2NO2(aq) + H2 O ↔ NO2 − + NO3 − + 2H+ outer solution

Inner solution

6.4 Direct potentiometric methods

137

FIGURE 6.8 Field effect transistors and gas detection mechanism.

This electrode allows NO2 determination in the existence of gases e.g. SO2 , CO2 , and NH3 , which can raise the pH of the inner solution [33,105].

6.4.12 Electrochemical applications Kelvin probes are also devices based on work function measurement. Species that create a charge change on the electrode surface shows a physical capacitance vibration effect by changing the dielectric value. These measurements, especially made using field effect transistors, are interesting in terms of adapting to small-scale studies in analytical chemistry. FET-based gas sensors (Fig. 6.8), which are effective and have a wide range of selectivity, contain a metallic sensing surface [129]. In one of the studies, a FET sensor developed using a mixture of platinum and gold was used to detect ozone at ppb level at high temperature. [130]. In recent studies, it has been reported that sensors used at room temperature are more useful and preferred. For example, a screen-printed electrode (SPE) imprinted with barium carbonate was used as a Kelvin probe for CO2 determination [131]. Although it works at room conditions, its sensitivity to humidity besides CO2 emerges as the sides that need to be developed. It has been reported that porphyrin film was used in other FET-based studies. In these studies, the fact that the porphyrin is a single layer has shown that it is a suitable ground as a gas sensor, but it is a subject that is open to development. There are also studies to identify toluene using conductive polymer-containing FETs [132,133]. Porous and membranecontaining electrodes, where metal, electrolyte, and gas are in contact, providing direct oxidation of the gas, were used as amperometric gas sensors. In another example, peak separations of CO2 , O2 and N2 O gases could be easily seen using gold microdisk electrodes [134]. In general, the peaks obtained from the gases in the studies performed in blood gas analysis overlap. Therefore, economical electrodes such as vitreous carbon or metal electrodes with the

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6. Electrochemical Carbon Dioxide Detection

high surface area are being developed [135]. But gold, silver or platinum electrodes possess better sensitivity and selectivity performances. Apart from this, different alternatives can be used as electrolyte medium. In a study using a gold working electrode and a solid polymer electrolyte membrane, it was reported that CO2 was determined, and alternative studies with Nafion and poly(benzimidazole) for the use of polymer electrolyte were also successfully applied [136–138] Similar electrochemical systems, in which O2 and aromatic gases are determined apart from CO2 , allowing the gasses to be determined by adsorbing them to the surface and are known as all-solid-state sensors [139]. Here the indicator stage is the desorption of gases individually according to their electrochemical desorption potentials. Clark electrode can be utilized for gas-sensing gas mixtures in this type of medium achieving minimized Faradaic currents [137]. Electronic noses are a developing area in the analytical chemistry, generally, polymer modified composite detectors are used to capture gases [140]. Aromatic gases which have different odors can be identified near to the human senses. This type of a sensor gives a better result by using modified electrodes because of combining more than one detector. In particular, chemiresistive sensors combined with quartz crystals have been reported [141]. Although these devices are for multiple determination, studies for the determination of individual gases are also continuing. Detection of nerve gases is one of them and can be detected up to 0.05–0.2 mg/m3 LOD values. Also, other chemiresistive gas-sensing semiconductive materials including SnO2 metal-oxide are reported widely Thus, changes can be made to increase the sensitivity of the measurement [142]. Especially atmospheric gases are reported in this detection method. Electrochemical determination was made with ceramic-type sensors containing Na2 SO4 and aluminum composites for the determination of CO2 in an inert atmosphere containing CO2 and O2 . In this study, it has been reported that a sensor that can be used continuously for weeks at high temperatures such as 300 to 600 °C has been developed [143]. Sometimes ionic liquids are used to support electrode structure to enhance the catalytic performance of CO2 detection. In such study, bismuth electrodes were combined with various ILs to reduce CO2 electrochemically [144]. Copper based electrodes are maintaining a significant contribution to this field and several modifications are reported to enhance catalytic CO2 reduction ability. In the same manner, the effect of the O2 on the catalytic CO2 reduction of Cu-based electrode was examined. Because of the selectivity and the hindrance effects due to the O2 affinity to Cu two main problems are studied in detail by [145]. The authors reached certain findings such as subsurface O2 presence facilitates the CO2 conversion in different ways with additional supports like intermediate binding and optimization of the adsorption way [145]. The advantage of catalytic activity obtained using noble metals creates a great disadvantage as they are not economical at the same time. For this reason, studies on the use of semi-noble metals in the field have accelerated, as in Cu. In general, metal-based catalysts used in CO2 determination and reduction are divided into types such as metals, partially oxidized metals, metal oxides, metal sulfides, frameworks, and metal-loaded carbons. In electrochemical CO2 determination and reduction studies using these metals and their derivatives, it has been observed that the final and intermediate products vary greatly according to the electrode material used, the medium, the method, and the applied potential. It has been reported that some semi-metal oxides such as Sn, Bi, and Co perform better in the composite form [146].

6.5 Summary and conclusion

139

Starting from the point of economy, the choice of electrode material mostly tends towards carbon-based materials. The use of carbon-based electrodes modified with the mentioned metals rather than metal electrodes is also preferred because of their long life. In addition, carbonbased electrodes provide high overpotential for hydrogen generation, which facilitates CO2 reduction. Metal modification can be done by physical, chemical, or electrochemical methods. While electrochemical and physical methods are economical and fast methods, the chemical method creates more stable structures. It is a disadvantage that the chemical method requires more research in multi-step and synthesis steps. In general, an electrochemical reduction is preferred for the modification of electrodes with metals, unless a simpler chemical method is available. Sometimes a combination of these methods can be used. Activation of the electrode surface is done electrochemically, and the metal modification can be physical or chemical (Table 6.4).

6.5 Summary and conclusion Keeping the CO2 cycle in balance is a very important issue for life. With the increase in the use of fossil fuels, this balance has deteriorated significantly, and its negative effects are felt at a serious level in many areas such as health, energy, air quality, global warming, and climate change. Therefore, determination and monitoring of the CO2 level, which cause substantially cost, also is quite momentous for both healthcare and organizing life [13,15] In recent years, more renewable energy technologies e.g. photovoltaics and pinwheels have been used instead of fossil fuels in transportation, energy production and industry to prevent climate change and reduce its effects. The most significant and beneficial method proposed to diminish CO2 emissions in recent years is carbon capture technology [23]. These technologies can be divided into adsorption, absorption, separation by membrane, and chemical capture [38]. However, the technologies show some disadvantages. The adsorption process as a CO2 capture technology has some limitations as it involves complex processes, low efficiency, and requires large amounts of energy to sustain the process [43]. Absorption technologies are one of the most interesting and studied technologies by researchers, but temperature change is very important as it affects the solubility of the gas in the solution. Therefore, the energy requirement during the process to control temperature changes is also quite high [46,47]. Since the particle size, pore volume, permeability, and selectivity parameters are difficult to optimize during the capture process, the membrane usage is also challenging in capturing CO2 [44–49] The most important factors limiting chemical capture technology are the stability of the chemicals used against oxidation and reduction during repeated cycles and the cost of the process [52,53]. Beyond the CCU methods, electrochemical ways are outstanding. Electrochemistry studies the electrons formed due to a chemical change [32]. In the working principle of an electrochemical gas sensor, a chemical reaction occurs between the ambient gas and the active center in the sensor which can be developed by the addition of different nanomaterial, metal, metal oxide, carbon-based materials, and so on [33]. Until now, nondispersive infrared detectors, nanomaterials which can involve organic, inorganic fragments, gas chromatography, and mass spectrometers are utilized in CO2 detection and determination or as sensor detectors. Although taking high working energy and showing a low selectivity, due to the features such as taking small space, simple generating, high stability and sensitivity

140

6. Electrochemical Carbon Dioxide Detection

TABLE 6.4 Electrochemical CO2 detection and reduction studies. Utilized material

Method

Electrolyte

Purpose

Ref.

BaTiO3 –CuO thin-film

Impedance

–

Detection

[147]

SnO2 thin films

Conductometry

–

Detection

[148]

Cu NPs on Glassy carbon plate electrode

Voltammetry

0.1 M KHCO3

Catalysis/Reduction

[149]

Zn dendrite on Zn foil

Potentiometry

0.5 M NaHCO3

Catalysis/Reduction

[150]

nano-SnO2 /graphene on GC electrode

Voltammetry

0.1 M NaHCO3

Catalysis/Reduction

[151]

Zn–BTC MOFs on carbon paper

Controlled potential electrolysis

Ionic liquid

Catalysis/Reduction

[152]

Polyethylenimine/ Nitrogen-doped carbon nanotubes on GC electrode

Controlled potential electrolysis

0.1 M KHCO3 /CO2 -saturated water

Catalysis/Reduction

[153]

Cu modified boron doped diamond electrode

Linear Sweep voltammetry

0.5 MKOH

Catalysis/Reduction

[154]

Ag NP and MWCNTs with Gas diffusion electrode

Impedance

1 MKOH

Catalysis/Reduction

[155]

Au NPs on mesoporous carbon

Controlled potential electrolysis

0.5 M KHCO3

Catalysis/Reduction

[156]

PdCu/C on carbon paper electrode

Controlled potential electrolysis

0.1 M KHCO3

Catalysis/Reduction

[157]

SnO2 /MWCNT on carbon paper

Linear Sweep Voltammetry (LSV) and Chronoamperometry (CA)

0.5 M NaHCO3

Catalysis/Reduction

[158]

NiO on MWCNT with carbon paper

Linear Sweep Voltammetry (LSV) and Chronoamperometry (CA)

0.5 M NaHCO3

Catalysis/Reduction

[159]

metal oxide gas detectors are preferable sensors for sensing and monitoring the CO2 gas as a nanomaterial-based sensor [7,15,63,64]. On the other hand, CNTs are the most popular member of carbon-based semiconductors and pure graphene, graphene oxide, and reduced graphene oxide which are derivatives of graphene are favorable nanomaterials for generating a gas sensor [13,69]. CO2 sensors are not only designed as solid-state platforms but also an electrolyte including, gas sensors, Clark type sensors, solid carbon transducers, and FET sensors that use divergent modification materials. In the recent years LCs have been strong prospective photonic structures with simple adaptable, low energy consumption for being a gas sensor. Electroanalytical methods have many advantages over other analytical methods [33,94]. Among them, the simultaneous detection of species with different oxidation levels is easy,

References

141

and the equipment and software that enable the application of the methods are much cheaper and more useful than chromatographs and spectrophotometers [95]. Electrochemical sensors are generally used to detect toxic gases and oxygen, especially CO, ammonia, and chlorine. Besides CO2 detecting and reducing electrochemical sensors occupy an important place. Especially modified electrodes possess enhanced advantages on the catalytic activity obtained using noble metals. However, the electrodes show a great disadvantage as the electrodes are not economical. For this reason, studies on the use of semi-noble metals in the field have accelerated, as in Cu. In general, metal-based catalysts used in CO2 determination and reduction are divided into types such as metals, relatively oxidized metals, metal oxides, metal sulfides, frameworks, and metal-loaded carbons. In electrochemical CO2 determination and reduction studies using these metals and their derivatives, it has been observed that the final and intermediate products vary greatly according to the electrode material used, the medium, the method, and the applied potential. It has been reported that some semi-metal oxides such as Sn, Bi, and Co perform better in the composite form [144]. In conclusion, no matter which type of electrode or method is used there is no edge for the sustainable development of electroanalytical sensor devices. The selectivity, sensitivity, and detection limits can be modified as the researchers’ demands.

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C H A P T E R

7 Carbon dioxide injection for enhanced oil recovery and underground storage to reduce greenhouse gas Shubham Saraf and Achinta Bera Department of Petroleum Engineering, School of Energy Technology, Pandit Deendayal Energy University, Gandhinagar, Gujarat, India

7.1 Introduction Changes in the energy balance of the Earth can lead to global warming. This is called “climate forcing.” Also, greenhouse gas emissions are increasing, which leads to climate change. The term “greenhouse effect” refers to substances that absorb and radiate infrared energy in the same wavelength range as the Earth [1]. Almost 0.1 percent of the Earth’s atmosphere is made up of greenhouse gases such as carbon dioxide (CO2 ) (0.04 percent), methane (0.012 percent), nitrous oxide (0.08 percent), and ozone (0.012 percent) [2]. Fig. 7.1 shows how these gases and the contribution of carbon dioxide emissions in the atmosphere from different countries interact. For a large percentage of carbon dioxide emissions, natural gas, oil, and coal are the primary sources, whereas other contributions come from deforestation, cement, other land-use changes, and fertilizer [3]. Greenhouse gases affect the environment and health by generating smog, trapping heat, and causing lung disease in humans from air pollution. Furthermore, food shortages, wildfires, and extreme weather are caused by greenhouse gas emissions [4]. From 1990 to 2019, the effects of greenhouse gases that humans put into the air grew by more than 40 percent [5]. Moreover, oil production around the world is decreasing, and massive oil resources are not being found quickly enough to compensate. Thus, the tertiary recovery technique must be used to maintain and develop existing oil fields worldwide. Fig. 7.2 shows how top countries’

Green Sustainable Process for Chemical and Environmental Engineering and Science: Carbon Dioxide Capture and Utilization DOI: https://doi.org/10.1016/B978-0-323-99429-3.00002-3

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7. Carbon dioxide injection for enhanced oil recovery and underground storage to reduce greenhouse gas

(A)

(B)

Worldwide Greenhouse Gases Emission by Different Gas Nitrous oxide 6%

Top CO2 Emitting Countries, 2020

Flue gases 2%

Methane 16%

India 3% Carbon Dioxide by Fossil fuel and Industria l process 65%

Carbon Dioxide by Forestry and other land use 11%

Rest of the countries 16%

United States 34%

Japan 5% United Kingdom 6% Germany 8%

China 19%

Russia 9%

FIGURE 7.1 (A) Worldwide greenhouse gas emission percentage and (B) higher rate of CO2 emitting countries in 2020 [102].

Worldwide Oil Produce, Export, and Consume (in percentage) by Different Countries in 2021 Crude oil producing 20

Crude oil exporting

Crude oil consuming

20.48 12 6.6 2.85

United State

3.19

Saudi Arabia

13.5

13.07

11 4.6 3.6

Russia

6 3.04 2.53

Canada

5

4 1.3

China

2.1 3.23

Brazil

5

4.84 2

0.5

India

1.5

Rest of the countries

FIGURE 7.2 Top countries’ oil production, exports, and consumption in 2021 [103].

oil production, exports, and consumption were distributed in 2021. Tertiary recovery techniques can increase oil recovery by 40 to 50 percent [6]. Tertiary recovery strategies are known as enhanced oil recovery (EOR) methods. Over the past ten years, nitrogen gas, natural gas, and carbon dioxide gas injections have been used as part of an enhanced oil recovery strategy to reduce miscibility pressure, improve displacement efficiency, and lessen the effects of global warming. Furthermore, re-pressurizing oil reserves using CO2 gas infusion and displacing residual oil with long-term storage has been utilized to minimize CO2 emissions [7]. With CO2 gas

7.1 Introduction

151

injection, most oil is recovered [8]. Because CO2 mixes with oil, the oil floats and becomes less dense [9]. As a result, CO2 gas injection has been one of the most effective strategies for improving oil recovery and storing greenhouse gases in geological contexts for a long time, thereby helping to mitigate the effects of global warming.

7.1.1 Global carbon management concerns Nowadays, human progress would be impossible without energy. So, improving human growth and industrial development while addressing the global climate disaster is the greatest challenge of the 21st century [10]. As industries move towards a low-carbon future, the changeover will be difficult. Therefore, the worldwide need for energy will necessitate a large amount of time and ongoing investment in alternative energy sources. In the current policy scenario, global CO2 emissions and economic development remain strongly linked [11]. Fig. 7.3 depicts how emissions will continue to rise to around 36 Gt of CO2 in 2040 as primary energy demand rises. In addition, there will be a rise in the cost of power generation without government subsidies. Government initiatives that encourage sustainable growth can reduce the global energy demand from 19 Gtoe (giga tonne oil equivalent) to 14 Gtoe in 20 years [12]. In addition, direct CO2 emissions from industry and transportation will rise by about 20 percent by 2040. As a result, carbon capture and storage (CCS) is an essential low-carbon strategic tool used to reduce CO2 emissions in order to improve oil recovery. According to Adu et al. [13], oil and gas companies tend to prefer CO2 -EOR in depleted oil and gas reservoirs as a strategy of CO2 sequestration and to enhance oil production. Furthermore, capturing,

FIGURE 7.3 Worldwide energy demands related to CO2 emissions by New Policies, Current Policies, and Sustainable Development Scenarios [11].

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transporting, and storing excess CO2 due to increased oil production can be mitigated to some extent if multiple techniques for capturing CO2 are used in CO2 -EOR operations. Therefore, CCS with CO2 -EOR supports a strategy that improves oil recovery and mitigates climate change problem [13]. CO2 has many different effects on oil production, such as the mechanisms used to inject CO2 into a depleted oil reservoir to increase production rate by oil swelling and viscosity reducing for the infusion of immiscible fluids (at low pressures) to entirely miscible displacement in high-pressure operations [14]. Furthermore, some of the CO2 injected comes with the oil and is frequently separated and re-injected back into the reservoir in order to reduce operational expenses. For instance, CO2 -based EOR projects are still ongoing in Brazil, Canada, Croatia, Hungary, Trinidad, and Turkey, and CO2 -based EOR increases the amount of oil that can be extracted from a well by an average of 13.2 percent in the United States [15]. Thus, planning and allocating CO2 are crucial in maximizing the economic benefits via EOR strategies and minimizing environmental damage through this process.

7.1.2 CO2 availability Carbon dioxide levels in Earth’s atmosphere are presently around 412 parts per million (ppm) and going up [16]. Fig. 7.4 shows the percentages of different gases present in the air on the Earth. So, many CO2 capture and storage systems have been developed. For example, Marchetti [17] mentioned many options for CO2 capture from power stations and heat exchangers and recommended storing the CO2 in the sea. In addition, Fu and Gundersen [18] stated that CO2 be separated from fuel or exhaust gas before or after combustion by using membranes, adsorption, hybrid applications, cryogenic separation, and absorption. Therefore, there are three main methods considered to capture CO2 during combustion: oxyfuel combustion, post-combustion capture, and pre-combustion capture. Firstly, post-combustion capture (PCC) traps and separates CO2 from flue gases after burning natural gas, coal, or oil in the atmosphere, but primarily charcoal [13]. A post-combustion

FIGURE 7.4 Different gases with their percentage present in the atmosphere [16].

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7.1 Introduction

TABLE 7.1 Differences between CO2 capturing methods (post, pre, and oxy-fuel combustion capture) [13]. Capture methods

Strategy

Advantages

Disadvantages

Application

Post combustion

Using air to separate CO2 from exhaust gas

Technique is easily retrofittable

CO2 levels are extremely low, at less than 15 percent

Mostly coal-fired power plants

Pre-combustion

Air, steam, or oxygen can be used to separate CO2 from fuel.

CO2 range 15–60 %

Additional capturing equipment costs

Power plants IGCC

Oxy-fuel combustion

To separate CO2 , fuel is burned with pure oxygen

CO2 concentration over 80 %

Technique is costly due to its high oxygen demand

Oxygen-combusting power plant

capture method was used to absorb CO2 at the Boundary Dam CCS Project, which started operating in 2014 [19]. Secondly, pre-combustion capture extracts CO2 before combustion, and this method is typically used in integrated gasifier combined cycle (IGCC) coal power plants. Also, Blomen et al. [20] stated that pre-combustion with IGCC could make controlling pollutant emissions more efficient and less expensive. Thirdly, an oxy-fuel combustion method uses an air separation unit (ASU) to separate the oxygen flow from the gasoline channel, which is then used in a furnace to combust [13]. After combustion, flue gas contains carbon dioxide, water vapour, and extra oxygen. Then, CO2 congregation in the oxygen-blown combustion exhaust gas is about 80 percent, which simplifies CO2 separation [21]. There have been recent advancements in oxy-combustion that make it an extremely efficient solution for coal power plant CO2 capture, both in terms of customizing ancillary equipment, boilers, and ASU to existing facilities as well as its large CO2 purity of greater than 99 percent [22]. Thus, absorption systems remove CO2 from flue gases at a lower cost and require less energy than PCC. Table 7.1 summarizes the differences between post, pre, and oxy-fuel combustion capture. Finally, gas pipelines are used to transfer the captured CO2 from power plants and manufacturing plants to the sequestration location, where it will either be injected into subsurface formations or used for EOR. In addition, anthropogenic CO2 must be transported under various compositional controls to ensure its safety [23]. But transporting CO2 made by humans through pipelines might not be a big problem. In fact, CO2 pipelines can be structured and work properly within certain limits [24]. Also, pure CO2 is delivered through pipes in a supercritical gas state at a pressure of 7.38 MPa and a temperature of 31.18 ºC, well above its critical point due to the need to increase density while reducing pressure gradients and eliminating multiple flows [25]. A phase diagram of CO2 can be described in Fig. 7.5 to understand the phase behaviour of CO2 at different pressures and temperatures.

7.1.3 Options available for CO2 storage Significantly, the aim of CCS projects is the successful and long-term storage of captured and transported CO2 , in order to reduce the greenhouse effect. There are three major CO2

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7. Carbon dioxide injection for enhanced oil recovery and underground storage to reduce greenhouse gas

FIGURE 7.5 The CO2 supercritical fluid region as depicted in the temperature-pressure phase diagram [104].

FIGURE 7.6 Various methods for storing carbon dioxide [9].

storage concepts: ocean storage, mineralization, and geological storage [26]. Fig. 7.6 illustrates different types of CO2 storage methods in the underground. Firstly, geological CO2 storage sites need to be found across the territory. So, CO2 can be stored in a plethora of geological contexts, including deep coal seams, saline aquifers, and depleted oil and gas fields [27]. Mohamed et al. [28] also stated that saline aquifers had the maximum storage capacity of any

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7.1 Introduction

type of subsurface storage reservoir. Secondly, mineralization is the process of transforming CO2 from a soluble phase into a solid mineral phase through chemical reactions with minerals and natural materials [29]. For instance, the formation of carbonates as a reaction of aluminosilicate minerals can be a crucial storage process, but the time scale is in the hundreds to thousands of years [30]. Thirdly, it is possible to store CO2 in the oceans for hundreds of years by injecting CO2 into the subsurface of seawater, which then forms hydrates under the ocean’s subsurface or deep within the ocean’s layers [13]. As a result of the ocean acidification caused by the interaction between the leaked CO2 and salty water, the quality of seawater changes, and marine life is impacted [31]. Furthermore, when compared to other methods of storing energy, mineral carbonation has two key advantages: First, the amount of metal oxides in Earth’s silicate rocks exceeds the amount needed to fix all the CO2 created by fossil fuel combustion [32]. Second, there is almost no practical limit on how long CO2 can be stored [14]. However, mineral carbonation is associated with significant constraints. Thus, the huge amounts of CO2 make handling the gas difficult.

7.1.4 Comparison of available storage methods Here, several technologies have been proposed to enable and expand CO2 storage to ameliorate climate change’s effects. Table 7.2 summarises the comparison between ocean storage, mineral carbonation, and geological storage of CO2 for the long term. Theoretically, injected CO2 in a geological formation will be trapped by many mechanisms, including residual, structural/stratigraphic, mineral, and solubility. It is shown in Fig. 7.7 that TABLE 7.2 Summary of the comparison of several CO2 storage strategies [14]. CO2 Storage Methods

Methodology

Merits

Demerits

Applications

Ocean storage

Injection of captured CO2 into the deep ocean

Store more CO2 than terrestrial vegetation

Leakage of CO2 can harm marine organisms

Anthropogenic CO2 has no physical limit

Mineral Carbonation

Storage fixes CO2 as inorganic carbonate minerals

No practical limitation in storage time (Long-term storage)

Expensive and energy intensive resulting

Low temperatures systems are more favorable

Geological storage: Depleted oil and gas reservoir

Miscible and/or Immiscible CO2 -EOR method

Re-pressurize oil fields and displace residual oil

CO2 Transportation rises exploration costs

To enhance oil recovery and mitigate greenhouse gas effect

Geological storage: Deep saline aquifer

Super-critically injected CO2 as a gas or a heavy fluid

There would be an extremely low leakage rate

Process is very time consuming

Store CO2 for long-time period

Geological storage: Coal-bed methane

Injected CO2 binds to the coal

Sequestered CO2 permanently

–

Extract methane gas

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FIGURE 7.7 The level of CO2 storage security over time by various CO2 trapping methods [9].

CO2 storage security is increasing with time due to different CO2 trapping techniques. Firstly, the structural/stratigraphic trapping mechanism refers to the capture of supercritical CO2 beneath low-permeability cap-rock [33]. Depending on the rock type, this trapping technique may take 20–40 years. Secondly, the relative permeability and capillary force impacts of residual trapping also play a significant role in the transition of the CO2 injected into an immobile state [29]. In addition, depending on the type of rock, this sequestration mechanism might not take action for hundreds of years after the injection. Thirdly, solubility trapping occurs when CO2 dissolves in brine and sinks to the formation’s bottom over time, enhancing the security of the CO2 trap [34]. Finally, mineral trapping is the process of converting CO2 into a solid mineral component through chemical processes that take place in the production of minerals and organic matter [30]. However, CO2 -EOR could be used to store CO2 underground in depleted oil reservoirs as residual trapping, which can increase oil production too.

7.2 Oil recovery using CO2 CO2 injection for oil recovery can be a miscible or immiscible displacement process. Kamali et al. [35] determined that CO2 -EOR is one of the most effective EOR technologies because of the chemical and physical properties of CO2 , the CO2 oil system, and technological progress. The viscosity, density, and interfacial tension of oil in a reservoir can be lowered by adding CO2 . Also, CO2 injection makes heavy to medium hydrocarbon oil more viscous. In conditions

7.2 Oil recovery using CO2

157

FIGURE 7.8 Continuous injection of CO2 during the CO2 -EOR procedure.

of miscibility, the manifestation of these effects is more prominent than in conditions of immiscibility [36]. Furthermore, single-phase flow occurs at pressures between the minimal miscibility pressures (MMP) and fracture pressures. So, when the pressure in the reservoir is below MMP, immiscible flooding is used [37]. It is also important to mention that Moghanloo et al. [38] conducted experiment on CCS for reducing CO2 emissions and continuous injection of CO2 -EOR for improving oil recovery and reducing carbon footprints as shown in Fig. 7.8. Different computational and experimental techniques exist for determining the MMP. Many laboratory experiments use one of these three methods: the rising bubble strategy, the slim tube test, or the vanishing interfacial tension technique [39]. In addition, according to Ekundayo and Ghedan [40], the slim tube test is the most effective method for determining MMP because it models the 1-D displacement of reserve crude oil by CO2 injection, ultimately accounting for thermodynamic processes in the CO2 -oil system within a slim tube coil. Furthermore, the literature recommends long coil lengths and slow injection rates to standardized slim tube studies by avoiding component fluctuations, fingering, and ensuring a constant thermodynamic front [41]. Fig. 7.9 shows an MMP measurement apparatus utilizing slim tubing. The coil in that setup is 60 to 80 ft long since the researchers suggested using a coil with a length greater than 40 ft to obtain a consistent displacement [42]. Consequently, it takes at least five weeks to determine MMP using the slim tubing approach when the recovery factor is obtained at just four different pressures [43]. A lower rate of CO2 injection is used to displace the oil when the pressure and temperature have been stabilized in the system. To estimate the recovery factor, discharges are monitored throughout the experiment. For each pressure, two zones are recognized: the “immiscible” region (where recovery variables are strongly dependent on pressure) and the “miscible” region (where recovery factors are less dependent on pressure) [43]. Fig. 7.10 depicts the MMP as the junction of these two regions.

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7. Carbon dioxide injection for enhanced oil recovery and underground storage to reduce greenhouse gas

FIGURE 7.9 Schematic of the MMP measurement equipment setup utilizing a slim tube [42].

FIGURE 7.10 A plot for determination of MMP at various pressures using a 60 ft coil [43].

Several CO2 -EOR systems, namely CO2 water alternating gas (WAG), continuous CO2 flooding, and huff-and-puff, have been implemented and developed [44]. In Table 7.3, a comparison and contrast of the benefits and drawbacks of different methods to inject CO2 into hydrocarbon reservoirs is depicted. However, continuous CO2 flooding has disadvantages such as early breakthrough due to viscous fingering effects, gravity override, and poor volumetric sweep efficiency, although field cases demonstrate its efficacy [45]. In addition,

159

7.2 Oil recovery using CO2

TABLE 7.3 Positives and negatives sides of a variety of CO2 injection methods [107]. Method

Methodology

Advantages

Disadvantages

Continuous gaseous CO2 injection

It is applied on a large scale to displace residual oil in a reservoir with continuous CO2 injection.

It is possible to get a larger oil displacement ratio with this technology than the other CO2 injection methods.

Gravitational separation and a considerable decrease in sweep efficiency are possible.

Injection wells with CO2 in a sequence

Low-permeability, heterogeneous reservoirs may be developed through the cyclic injection of CO2 .

Increased oil displacement by CO2 and reservoir sweep, and reduce gas breakthrough in producing wells may be possible.

This method is inefficient, expensive, and impractical.

Injecting CO2 and water alternately

Better CO2 -EOR efficiency can be obtained by injecting the appropriate amount in small portions, alternatively or concurrently with water.

It is an efficient method for increasing oil recovery with limited permeability.

Injection rates are limited in some reservoirs due to low permeability and inadequate pore-space connectivity.

Injection of CO2 -foam

These approaches employ foams to control the movement of CO2 .

Foam displaces oil in a better way than the water-flooding or CO2 injection.

CO2 -foam technique is time-consuming.

continuous CO2 flooding in several field applications requires considerable capital and operational expenses [46]. Furthermore, it is possible to significantly enhance the production of oil and extraction from some reservoirs using CO2 -EOR. Also, there are financial benefits to using CO2 -EOR, such as the minimal cost of CO2 , and the fact that it makes good-quality oil that can be supplied and recycled [47]. However, CO2 -EOR has been used for a long time in the United States [48]. Since the 1980s, it is the only oil recovery method that has grown. It is estimated that 50 Mt per annum of CO2 is being used for oil recovery in North America, which accounts for more than 5 percent of the country’s oil production [49]. Fig. 7.11 shows how much of the current demand for CO2 is required by different ways of extracting energy. In addition, oilfields may be extended for decades, and millions of barrels of oil can be recovered with the CO2 -EOR method. Oil extraction from these reservoirs can also be improved by 4–15 percent, which means a small percentage of the trapped oil may be recovered using CO2 -EOR techniques, while a larger portion cannot be recovered at all. Even then, developers are still interested in the CO2 -EOR [50]. Hydrocarbon industries have generated substantial economic returns due to the huge majority of EOR projects depending on low-cost CO2 sources. Fig. 7.12 shows how CO2 -EOR advancements have increased oil recovery around all the reservoirs. Approximately 50 percent of the CO2 injected into the reservoir can be retained at the CO2 breakthrough if reinjection is not considered [51]. However, EOR and the storage of expensive anthropogenic CO2 are constrained economically, despite the widespread recognition that there are no significant technical difficulties.

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FIGURE 7.11 The percentage of current CO2 demand in the different applications [49].

Different EOR Methods for Oil Recovery 60.00%

15.00%

CO2 Miscible

CO2 Immiscible

12.00%

Steam

8.00%

5.00%

HCCombustion Immiscible

3.00%

Micellar

FIGURE 7.12 Performances of most efficient CO2 -EOR approaches for oil production [105].

7.2.1 Hydrocarbon miscibility The extraction of crude oil from a rock’s pores by a solvent action that prevents the creation of interfaces between the driven and driving fluids is known as miscible displacement [52]. Thus, to establish whether a displacement process is miscible flooding or not under reservoir circumstances, the MMP, which determines the lowest pressure at which a miscible phase may be generated, is an essential quantity [53]. An empirical calculation is preferable to the time-consuming and complex laboratory measurement of MMP for engineering design of CO2 flooding, particularly during the feasibility study phase. Furthermore, first-contact miscibility operations at reservoir pressures greater than the MMP are required to produce oil through CO2 miscibility. It is possible to achieve multiple contact miscibility by freezing and vaporising gas drive mechanisms when infused fluid and reserve oil come into contact at a relatively greater reservoir pressure than MMP [54]. But injecting CO2 at a high pressure above the fracture pressure of the reservoir will cause fractures, so this cannot be done forever [37].

7.2 Oil recovery using CO2

161

Moreover, miscibility in petroleum reservoirs is the physical condition of two or more fluids that allows them to mix in any proportion without forming an interface [52]. For example, if two fluid phases occur after adding a little amount of one fluid to another, the fluids are deemed immiscible [55]. Thus, there are two ways to obtain miscibility during CO2 flooding: the first-contact approach and the frequent contact method. The circumstances of first and multiple contact miscibility at a given pressure are determined by the temperature of the infused gas and oil [56]. But because of the limits on injection pressure, it is hard to get CO2 and reservoir oil to mix on first contact. Instead, multiple contact miscibility processes are common in the field [37].

7.2.2 CO2 miscible injection method Miscible CO2 -EOR is the most common type of gas injection-based EOR method. In addition, injected CO2 and reservoir oil combine in any proportion to form a single-phase [57]. The injection must be done at a high pressure to ensure that the injected gas is compatible with the reservoir crude oil. MMP refers to the pressure at which miscibility occurs [58]. MMP is a critical element during miscible CO2 flooding since displacement efficiency is heavily reliant on it. Hence, a process known as multiple-contact or dynamic miscibility is used when reservoir pressure exceeds the MMP, causing intermediate and larger molecular weight asphaltenes in the reservoir oil to vaporise into CO2 (the vaporised gas-drive method) and some of the injected CO2 to volatilise into the oil (the condensed gas-drive technique) [59]. This movement of molecules between oil and CO2 helps to create an oil-and-CO2 -miscible transition zone [60] by making it possible for the two phases to mix without a boundary. Moreover, when miscible solvents are mixed with reserve oil in full proportions, the combination remains in one phase. While multiple contacts are made, the amount of oil that can be recovered is significantly increased [61]. So, inserting CO2 into the oil in the reservoir causes the intermediate-molecular-weight petroleum to evaporate in place, which makes the oil dynamically miscible [62]. Holm [63] noted that dynamic miscibility can be achieved by condensing hydrocarbons of moderate molecular weight from a thick solvent into a thin reservoir of oil. Therefore, CO2 injection can be miscible or immiscible with oil, depending on the composition, pressure, and temperature of the reservoir. Since miscible and immiscible CO2 -EOR methods are used in advanced CO2 -EOR utilization, the two methods are shown in Table 7.4 [64]. Furthermore, a high-pressure core flooding device is utilized to conduct CO2 -EOR flood experiments at the laboratory, which is schematically depicted in Fig. 7.13. A continuous flow pump is operated to inject brine, crude oil, and CO2 via a core plug contained within a high-pressure core holder. In addition, a syringe pump is applied to maintain a pressure of 2–3 MPa greater than the infusion pressure on the core plug [110]. The components listed above have been warmed in an air bath. The temperature of the air bath is maintained at the reservoir temperature by way of a temperature controller. A backpressure regulator is deployed during the core flooding test to set the appropriate production pressure. The volume of produced oil is measured using a measuring cylinder, and the volume of generated gas is measured with a gas flow meter. During each test, low-rate CO2 is injected to move the crude oil at a specific reservoir temperature and insertion pressure [9]. The injection and production pressures are continually observed and recorded during the experiment. It is possible to evaluate and record

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7. Carbon dioxide injection for enhanced oil recovery and underground storage to reduce greenhouse gas

TABLE 7.4 Comparison between miscible and immiscible CO2 -EOR technique [64]. Miscible flooding

Immiscible flooding

It is applicable in deep reservoirs with heavy oil and a pressure greater than the MMP for mixing of oil and CO2 .

In this method, oil and CO2 are not mixed (shallow reservoirs with heavy oil or low reservoir pressure).

In presence of CO2 , oil condenses into a single liquid phase.

It is related to the partial dissolution of CO2 in petroleum.

It can utilize the water-injection infrastructure.

After long-term CO2 injection, oil output rises.

It is a high economically efficient process.

It is not an economically feasible process in some cases.

This method can be used on a small reservoir.

This method is only used on large reservoirs (part of the reservoir).

FIGURE 7.13 The schematic of core flooding apparatus demonstrating the high-pressure CO2 injection process [9].

the total volume of oil and gas generated using a camera and a gas flow meter. Each core flooding test collects the oil and gas generated, and gas chromatography is used to look at the oil and gas components. The determined oil recovery factor (ORF) in the core flooding test at various injection pressures and reservoir temperatures is shown in Fig. 7.14 as a function of injected CO2 vol [110]. When high concentrations of CO2 are added, the ORF increases until no further oil can be

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7.2 Oil recovery using CO2

FIGURE 7.14 Effects of CO2 injection

Oil recovery factor (%)

100

pressure on oil recovery factor [110].

80 60 Injection Pressure 13.5 Mpa

40

Injection Pressure 16.2 Mpa 20 0

Injection Pressure 19.5 Mpa Injection Pressure 22.1 Mpa 0

0.5

1 PV of injected CO2

1.5

2

extracted. In the early stages of the operation, the ORF of lower infusion pressure is higher than that of high pump pressure. Due to the reduced solubility, a smaller fraction of the inserted CO2 is absorbed into the light crude oil at a lower injection pressure, and a moderate fraction of the injected CO2 plays a key role in dispersing the crude oil [110]. Then, since CO2 and crude oil have a higher interaction as injection pressure rises, the eventual ORF at medium pore volume increased. Additionally, the interfacial tension between the oil and gas-phase disappears as miscibility occurs between the injected CO2 and residual oil [58]. As a result, the mixture is pushed as a single-phase from porous rock to oil wells. When using miscible displacement, efficiency is increased, and IFT is reduced, while residual oil saturation is reduced and overall production is increased [58]. But the only problem with the miscible displacement method is that it costs a lot to run.

7.2.3 Injection and storage facilities required Surface facilities at the infusion site include storage capabilities, a distribution manifold at the end of the transportation pipeline, transmission pipelines to wellheads, further compression facilities, monitoring and control systems, and insertion wells [65]. So, CCS and CO2 -EOR projects require significant investments in infrastructure. For financial reasons, it is important to have a reliable, continuous source of CO2 . Therefore, depleted oil and gas reserves offer a potential location for the storage of carbon dioxide [66]. Due to phase-behaviour difficulties, low reservoir pressure will pose a considerable obstacle for CO2 injection. This tendency will make CO2 injection settings much more difficult to change at the beginning [66]. 7.2.3.1 Onshore facilities The facilities needed for onshore CO2 -EOR are almost the same as those required for water-flooding. These are also essential for developing the systems for CO2 -EOR operations, including gas-phase gathering pipelines, CO2 meters, and distribution systems [67]. However, there are three critical differences between the two procedures. These are [68]:-

r An innovation in producing wells has made CO2 separator gas more abundant, making the extraction of CO2 gas easier.

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7. Carbon dioxide injection for enhanced oil recovery and underground storage to reduce greenhouse gas

FIGURE 7.15 Progression of onshore CO2 -EOR facilities [106].

r Once CO2 has been taken out of the separator gas and dried out before being compressed, it must be processed for infused uses.

r As a result of compression, CO2 is given a higher injection pressure. Also, onshore CO2 -EOR facilities should plan to add flue gas CO2 restoration facilities, a CO2 contraction unit, CO2 pipelines, CO2 insertion wells, and a facility for separating CO2 from associated gas [68]. Fig. 7.15 illustrates the development of CO2 -EOR onshore facilities. 7.2.3.2 Offshore facilities The offshore facility upgrade and modification activities required for CO2 -EOR do not differ much from onshore designs. Because of the increase in reservoir pressure, the initial CO2 capture pressure of about seventy to eighty bars will not be enough to keep the required injection rate steady [66]. Pumps are needed for higher discharge pressures from the trunk line to be transferred to an injection facility [66]. Fig. 7.16 depicts the development of CO2 EOR offshore facilities. During the CO2 -EOR method, the current crude launcher, which was created to deliver crude from an offshore platform to the oil terminal, will be transformed into a water injection receiver [69]. The mixer obtained from the onshore plant will be injected straight into the platform’s water injection facilities through coarse filters. The objective is to collect pipeline-produced suspended particles larger than eighty microns [69]. Furthermore, it may be necessary to use an offshore pressure enhancer in cases where the onshore compressed air station’s pressure drops too early to allow direct injection into the reservoir [70]. Therefore, the pressure discharge of an offshore pumping system must be greater than the bubble point in order to avoid serious damage and cavitations [71].

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7.2 Oil recovery using CO2

FIGURE 7.16 CO2 -EOR offshore facilities development [69].

7.2.4 Storage capacity calculations In a CO2 -EOR project, the CO2 storage potential is linked to the planned incremental oil recovery. According to the initial assumption, the potential amount of CO2 that can be stored in oil reservoirs is equal to the volume of the generated oil and water. Therefore, more precise estimations can be made by using accurate mathematical reservoir simulations, which can correspond to the gravity segregation, impacts of water intrusion, reservoir heterogeneity, and CO2 dissolving in the formation water during the simulation process [72]. Then, to figure out how much CO2 is trapped in the reservoir, researchers need the following content composition (mole): Inj

produced

brine residual + Mmobile + Mmineral + MCO2 MCO2 = Moil CO2 + MCO2 + MCO2 CO2 CO2

(7.1)

residual is where, Moil CO2 represents the proportion of CO2 that has been dissolved in oil, MCO2 brine the volume of CO2 stored due to hysteresis in relative conductivity, MCO2 defined as the quantity of CO2 that’s been dissolved in brine, Mmobile refers to the quantity of CO2 that has CO2 produced

relates to the quantity of CO2 also been fundamentally trapped in the subsurface, MCO2 generated, Mmineral applies to the amounts of CO stored as a result of crystalline admixture, 2 CO2 Inj

brine and MCO2 refers to the quantity of CO2 injected [73]. Moil CO2 and MCO2 can be transported from the simulator specifically, but Mresidual and Mmobile should be measured based on residual gas CO2 CO2

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7. Carbon dioxide injection for enhanced oil recovery and underground storage to reduce greenhouse gas

saturation due to hysteresis on each grid block as follows: n   = Vmg (i)∗ fCO2 (i)∗ Sg (i) − Sgr (i) ∗PV (i) Mmobile CO2 i=1  n Mresidual = VmCO2 (i)∗ fCO2 (i)∗ SCO2 ,r (i)∗PV (i) CO2 i=1

(7.2) (7.3)

In this case, fCO2 the CO2 mole fraction, Vm ,g denotes the gas-phase molar mass, Sg the gas saturation, PV the net porous volume, Sgr the residual gas saturation, and n the total number of grid frames. In the short term, Mmineral is insignificant [74,75]. CO2 Recently, only light oil and gas fields are considered for CO2 storage in research study. The following sources can be used to estimate a gas field’s CO2 storage capacity [76,77]: mCO2 = ρCO2 × OGIP × Bg × R

(7.4)

where, mCO2 is the mass of the CO2 stored in kg; ρCO2 is the CO2 density at reservoir conditions in kg/m3 ; OGIP is the original gas-in place at standard conditions in m3 ; Bg is the gas formation volume factor in fraction; R is the primary recovery factor in percentage. If a gas field includes gas condensate, the remaining condensate in the reservoir after primary depletion can be recovered by means of CO2 – enhanced gas recovery [78]. By adding an extra CO2 – enhanced gas recovery factor, it can be estimated how much CO2 can be stored (Asia Pacific Economic [79]): mCO2 = ρCO2 × OGIP × Bg × (R + RCO2 )

(7.5)

where, RCO2 is the additional recovery factor by CO2 – enhanced gas recovery. Using CO2 EOR on a light oil field is appropriate if the oil has gravity greater than or equal to API 27 [51]. After secondary recovery and more recovery with CO2 -EOR, a recovery factor can be used to estimate the CO2 storage capacity (Asia Pacific Economic [79]): mCO2 = ρCO2 × OOIP × Bo × (R + RCO2 )

(7.6)

where, OOIP is the initial oil-in-place at standard conditions in m3 ; Bo is the oil formation volume factor in rm3 /Sm3 , R is the recovery factor after secondary recovery and RCO2 is the recovery factor for CO2 -EOR. The following Eq. (Asia Pacific Economic [79]) can be used to predict CO2 storage in saline aquifers: mCO2 = ρCO2 × A × h × ∅ × E

(7.7)

where, A is the aquifer size in m2 , h is the net sand thickness in m, ∅ is the formation porosity of in percent, E is the CO2 storage efficiency factor in percent. In order to connect one or more CO2 sources to the closest CO2 sink, researchers execute a source sink mapping effort [80]. For example, long-term CO2 storage capacities in different sites in India are summarised in Table 7.5, which shows the differences between various efficiency parameters.

7.2.5 Impact on economics and tax incentives CO2 capture, transportation, and storage in conjunction with EOR are frequently regarded as a promising technique to ensure cost-effective avoidance of CO2 emissions into the atmosphere [81]. The fact that a reservoir is geologically suitable for CO2 -EOR does not mean that CO2 -EOR will be cost-effective. Generally, the cost of crude, the store tax credit, and

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7.3 Underground storage of CO2 in unconventional reservoirs

TABLE 7.5 Capacities of CO2 storage in various Indian reservoir fields. CO2 Storage Capacity (Mt) Reservoir Type

Low

Mid

High

Percent (%)

Reference

Ocean

105,226

412,650

1,134,788

99.15

[108]

Depleted Gas Field

1696

1850

1995

0.44

[109]

Depleted Oil Field

1425

1702

1969

0.41

[80]

Total

108,347

416,202

1,138,752

100.00

the price of CO2 are three external factors that affect the cost of a CO2 -EOR operation [82]. However, the most expensive part of an EOR project is the purchase cost of CO2 . In addition, an EOR oil barrel’s production costs can be increased by up to 25–50 percent just by the upfront expenditures of supplying, injecting, and recycling CO2 [48]. Because of the recycling of CO2 , oil exploring industries plan the EOR project in such a way that the most CO2 can be used and the least amount of CO2 is purchased again. Also, achieving economic goals in CO2 EOR reservoirs requires a cutting-edge, more significant CO2 -EOR strategy as well as tax or other government subsidies for storing greenhouse gases [83]. Thus, it has been shown that the utilization of the CO2 displacement mechanism impacts the oil production rate, the CO2 consumption percentage, and the CO2 recycling rate [84]. In general, long-term CO2 storage has not been considered while designing CO2 -EOR activities. A CO2 -EOR site must also go through additional storage-focused activities, known as monitoring programs, before, during, and after CO2 injection to demonstrate and ensure that 99 percent of the CO2 is stored for an extended period of time [85]. As a result, the Intergovernmental Panel on Climate Change (IPCC) desires a well-selected site capable of storing 99 percent of CO2 in a reservoir for a thousand years [86]. However, CO2 -EOR regulatory authorities may not run or allow injection sites to reduce greenhouse gas emissions [87]. This is because monitoring programs add costs to CO2 -EOR operators that, if not offset by compensatory measures, will affect the economics of the project [88].

7.3 Underground storage of CO2 in unconventional reservoirs Conventional reservoirs can be found in separate basins for CO2 storage. Consequently, the hydrocarbons can be easily extracted using conventional exploration methods using vertical or horizontal wells [68]. But there are two types of oil or gas reservoirs: those that require unconventional methods of recovery and those that require conventional methods of recovery [89]. The primary reasons for the increased interest in unconventional reservoirs are the decline of conventional sources of energy and rising energy demand. Unconventional reservoirs for CO2 storage are depicted in Fig. 7.17. Gas and oil shales, tight-gas sands, heavy oil, coalbed methane, gas-hydrate, and tar sands deposits are some of the most anticipated reservoirs [68]. These reservoirs frequently demand complex recovery strategies, such as stimulation or thermal recovery techniques, as well as specialised process facilities. Pilcher et al. [90] stated that these methods need to be technically and, more importantly, financially possible.

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7. Carbon dioxide injection for enhanced oil recovery and underground storage to reduce greenhouse gas

FIGURE 7.17 Some of the most common unconventional reservoirs.

Shale deposits can be discovered all over the world, and traditional assets can be easily obtained. However, the vast change in recent years by operators toward producing unconventional reservoirs has left many such formations available for CO2 sequestration [91]. So, the extremely low permeability of shale reservoirs may appear to be a deficiency, but shale fracture surfaces can absorb a substantial quantity of CO2 . Therefore, shale reservoirs with natural and hydraulic fracture networks are ideal for CO2 storage [92]. Furthermore, some of the greatest oil reserves in the world are found in heavy oil fields. Large oil deposits can be found in more than thirty countries around the world. These deposits have the same amount of oil as the Middle East’s greatest conventional oil reserves, but few of them have been fully developed [68]. Thus, there is a problem with the heavy oil reservoir’s asphaltenic crude precipitation [93]. Due to the big difference in viscosity between heavy oil and CO2 , heavy oil is hard to get out of the reservoirs. Moreover, in these unconventional reservoirs, oil and gas extraction and CO2 sequestration entail a hierarchical network of sophisticated movement and transport of nano-pores, cracks, and micro-fractures. Some of the mechanisms employed include the creation and degradation of hydrates; adsorption and absorption in shale and coal seams; thermal cracking of shale oil; CO2 replacement; and other methods. The following are some of the possible approaches, but these are not comprehensive [94]:

r CO2 can be stored underground in a way that makes it easier to get oil and gas, move gas and liquid, and move through porous media.

7.4 Current status, challenges and future directions

r r r r

169

Gas absorption and desorption in shale gas and coal-bed methane reservoirs. CO2 helps to recover gases from gas hydrates, shale gas, and coal-bed methane. Unconventional oil/gas and CO2 geological storage micro-fluidic technology. Geothermal CO2 Storage

7.4 Current status, challenges and future directions Economically, CO2 -EOR is attractive, but there is disagreement about how much CO2 it reduces because CO2 is released back into the environment when oil products are burned. As a result, calculating the ultimate total feasible storage of CO2 is crucial in order to thoroughly analyze the carbon storage potential of CO2 -EOR [84]. CO2 emitters from various sources can be found, as well as oil reservoirs large enough to store the CO2 produced by these emitters [95]. Additionally, it is a simple way for the general public to learn about the benefits of CO2 -EOR. After that, CO2 -EOR will rise in popularity and gain support from the general population. However, the current status of CO2 -EOR and carbon storage in India can be stated as follows [96]:

r The National Program on Carbon Sequestration (NPCS) Research was created by the Department of Science and Technology (DST) in 2007.

r At Ankleshwar, a depleted oil and gas reservoir, ONGC Ltd has been working to establish a pilot prototype EOR project using CO2 (40 MMSCMD of sour gas per day) from the Hazira gas production site. This CO2 would be recompressed and injected to increase crude oil recovery. r A combined funding scheme for Indian-Norwegian climate research projects, including CCS, has been launched by the DST and the Research Council of Norway (RCN). According to the Agreement on Scientific and Technological Cooperation between the governments of India and Norway, this program is being carried out. To implement CCS, three challenging steps must be accomplished: first, CO2 must be captured from large air pollutants like power plants; second, it must be transported to a storage place; and third, it must be injected into the storage unit, which is frequently a deep geological context [9]. When it comes to CCS systems, there is no unique way to distinguish them from each other [97]. Fig. 7.18 depicts the global capacity and state of CCS in 2020 (Global CCS Institute, 2021). Worldwide, more than a hundred and thirty-five large-scale CCS facilities are currently under construction [98]. These projects can be different types as follows:

r There are currently 27 CCS projects in operation, with an annual capture capacity of 36.6 Mt CO2 , or less than 1 percent of the 5.4 Gt CO2 expected in 2050.

r 4 are still in the planning stages. r 58 projects are in significant progress with a specific front-end structural engineering approach in place (in contrast to only 13 in 2020).

r Early stages of 44 projects are under consideration (compared to 21 in 2020). r Two CCS project initiatives have been put on hold.

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CCS Capacity (Mtpa)

100.00 80.00

Operational

60.00 Planned 40.00 20.00 0.00

FIGURE 7.18 Capacity and status of CCS around the world in 2020 (Global CCS Institute, 2021).

The important criteria considered in technical consideration are usually the residual oil in place, MMP, reservoir depth, formation dip angle, and oil API gravity [99]. However, with offshore areas, additional aspects must be considered. Firstly, if the CO2 source is not close to the field, the best thing is to do the separation of CO2 from the flue gas. When there is a lot of CO2 , it is required to find a suitable place and a way to process the gas [100]. This is due to the fact that CO2 turns the water in the formation into an acidic state as it is injected, which, in turn, leads to the corrosion of coastal equipment. Therefore, facilities must be compatible with acid that may be generated if CO2 -WAG procedures are used in the fields to prevent corrosion in the facilities [101]. Moreover, there will be a need to improve reservoir simulations and modelling studies in the future to develop new strategies for exploration. There is still much work to be done to properly understand such interactions and their impact on the storage reservoir’s quality [95]. Efforts should be made to create new CO2 flooding methods. Mineral dissolution should be addressed as a future CO2 emission control strategy. More research is being conducted to make CO2 monitoring and leakage detection sensors and technologies that are quick, accurate, and extremely sensitive [100]. Finally, optimization studies integrating mathematical simulation, modelling, and programming are needed.

7.5 Conclusions During the first few years of operation, CO2 -EOR has the potential to decarbonize. The timing of the lowering in EOR carbon intensity is an important consideration, given the urgency with which global warming must be halted. Therefore, CO2 -EOR is the only economically established emission utilization option that provides large-scale continuous preservation for captured CO2 . Still, CCS has been the only technique that can decarbonize industries like

7.5 Conclusions

171

metallurgy, cement, and petroleum products. As a result, the summary and conclusions of the current study include a few of the most important points:

r CO2 flooding is more interesting than water flooding, and huff-and-puff supplies N2. As

r r

r

r

r

r

r

r

discovered by the EOR strategy, the only other liquid that may mix with oil is CO2 (a 90 percent mole part with CO2 in the oil stage is possible). As a result, the solubility of CO2 in oil increases and the oil viscosity is reduced. Using CO2 -reinjected petroleum framework MMP data, an optimized CO2 -EOR structure MMP relationship was found to improve the sweep efficiency and CO2 use of the MMP, which is a key part of miscible flooding. For CO2 -EOR projects, the difference between the MMP values produced using the fast slim tube approach and the standard method proved insignificant. An effective mixing region in the movement of light crude oil with CO2 can be accommodated in a 40 ft coil when the displacement velocity is modest enough for transverse dispersion to eliminate viscous fingering. After careful assessment of exploratory tests and prototype simulations, the gas injection method was found to be more practical than water flooding. A CO2 miscible flood must be done with the computational model to validate and store CO2 in an appropriate study of oilfield test parameters such as oil viscosity, depth, reservoir porosity and permeability, API gravity, oil saturation, and gross pay thickness. The results suggest that CO2 injection is the best acceptable EOR approach for 75 percent of the reservoirs examined (60 percent as miscible and 15 percent as immiscible). Based on the MMP requirements, it is also suggested that CO2 injection can be used to increase oil recovery from hydrocarbon reserves by 15 percent after secondary recovery. CO2 -EOR processes with increased injection pressure usually show a greater dependence on the CO2 extraction factor. As a result, CO2 retrieved and extracted more light components, but more heavy components, including asphaltene, were left in the reservoir rock. In addition, the CO2 miscible flooding phase of the asphaltene precipitation in the reservoir rock creates a significant problem. Optimizing the CO2 recycling system can reduce project CAPEX and OPEX by minimizing the need for conversion of surface facilities as well as investing in CO2 -EOR for residual oil zones. It has been found that CO2 -EOR could be used to get a lot more oil out of the remaining oil zones below the oil and water contacts in oil reservoirs. The miscibility impact of CO2 over all other gaseous injections makes CO2 infusion the best for oil and gas production. While gas reinjection can enhance the reservoir’s storage capacity by around 60 percent, it is possible to store CO2 in the reservoir at a CO2 breakthrough. Consequently, India has 420 Gt of CO2 storage capacity in 6 large gas sources, 37 significant oil fields, and saline aquifers in 22 geological formations. Also, CO2 from the electricity and industrial sectors could be stored in these reservoirs for more than 200 years. Finally, one of the most essential strategies for slowing or halting the effects of climate change is the use of carbon capture and storage (CCS). As a result, the CCS chain needs to build an extensive network of pipes with lower transport costs as the amount carried increases. It also needs to expand CCS projects, reduce the energy cost of capturing CO2 from power plants, and do many other things.

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Acknowledgment The authors would like to acknowledge Shell Energy India Private Limited, Hazira, Gujarat for funding the project related to CO2 -EOR and underground storage in the Indian context under the theme of CO2 capture, utilization, and storage (CCUS) and the School of Energy Technology, Pandit Deendayal Energy University, Gandhinagar, Gujarat, India for providing the required facilities to conduct the project.

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C H A P T E R

8 Ionic liquids as potential materials for carbon dioxide capture and utilization Md Abu Shahyn Islam a, Mohd Arham Khan a, Nimra Shakeel b, Mohd Imran Ahamed b and Naushad Anwar b a

Interdisciplinary Nanotechnology Centre, ZHCET, Aligarh Muslim University, Aligarh, UP, India b Department of Chemistry, Faculty of Science, Aligarh Muslim University, Aligarh, UP, India

List of Abbreviations ILs [EMIM][BF4 ] [BMIM][BF4 ] CO2 [HMIM][NTf2 ] [BMIM][PF6 ] [EMIM][NTf2 ] [OMIM][PF6 ] [HMIM][PF6 ] [OMIM][BF4 ] [N-BuPy][BF4 ] [BMIM][NO3 ] [EMIM][EtSO4 ] [EMIM][Ac] [BMIM][DCA] [C4 mim] [CF3 CF2 CF2 CF2 SO3 ] GO SILM [P6,6,6,14] [CoCl4 ] [P6,6,6,14][FeCl4 ] [P6,6,6,14][MnCl4 ]

Ionic liquids 1-ethyl-3-methylimidazolium tetrafluorobarate 1–butyl–3-methylimidazolium tetrafluorobarate Carbon dioxide 1-hexyl-3-methyl imidazolium bis(trifluoromethylsulfonyl)imide 1–butyl–3-methyl imidazolium hexafluorophosphate 1-ethyl-3-methyl imidazolium bis(trifluoromethylsulfonyl)imide 1-octyl-3-methyl imidazolium hexafluorophosphate 1-hexyl-3-methyl imidazolium hexafluorophosphate 1-octyl-3-methyl imidazolium tetrafluorobarate N-butylpyridinium tetrafluorobarate 1–butyl–3-methyl imidazolium nitrate 1-ethyl-3-methyl imidazolium ethylsulfate 1-ethyl-3-methyl imidazolium acetate 1–butyl–3-methyl imidazolium dicynamide 1-n–butyl–3-methylimidazoliumnonafluorobutylsulfonate Graphene oxide Supported ionic liquid membrane Phosphonium tetrachlorocobalt Phosphonium tetrachloroferrate Phosphoniumtetrachloromanganese

Green Sustainable Process for Chemical and Environmental Engineering and Science: Carbon Dioxide Capture and Utilization DOI: https://doi.org/10.1016/B978-0-323-99429-3.00004-7

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c 2023 Elsevier Inc. All rights reserved. Copyright 

178 [P6,6,6,14] [GdCl6 ] [P6,6,6,14][NTf2 ] [N2224][CH3 COO] [N1111][Gly] [N2222][Gly] [N1111][Lys] [N2222][Lys] [N4444] [Gly] [N1111][Gly] [aN111][Gly] PAMPS

8. Ionic liquids as potential materials for carbon dioxide capture and utilization

Phosphonium hexachlorogadolinium Phosphonium Triethylbutylammonium acetate Tetramethylammonium glycinate Tetraethylammonium glycinate Tetramethylammoniumlysinate Tetraethylammonium lysinate Tetrabutylammonium glycinate Tetramethylammonium glycinate 1,1,1-trimethylhydrazinium glycinate Poly(2-acrylamido-2-methyl-1-propanesulfonic acid)

8.1 Introduction Ionic liquids (ILs) are low melting point ( propylene > propane > CO2 according to their decreasing value of Henry’ s constant. Table 8.1 [19,36,46–49] represents the specific ILs and their solubility at different temperature and pressure.

8.2.7 Task specific ionic liquids (TSILs) TSILs is such type of ILs which has imidazole-based compound which help in selective adsorption of different gases specially CO2 . 8.2.7.1 CO2 capture by only TSILs TSILs is the most attractive approach of for the separation of target compound from a mixture of in a gas stream is selective absorption into a liquid. It has both synthetic and separation application [38,51,52]. Cation of TSILs consists of an imidazole ion to which a primary amine ion moiety is covalently bonded and we bifluoride ion very basic bicarbonate ion from CO2 and water. The solubility of CO2 in pyridinium and imidazolium liquids which are ionic in nature was reported by Brennecke et al. and Xie et al. [52,53]. It was observed by them that the CO2 solubility improves in the case of ILs which contains chains of fluoroalkyl either on anion or on the cation in comparison to ILs which are less fluorinated. These Ionic Liquids which are

187

8.2 Types of ILs

TABLE 8.1 ILs and their solubility at different temperature and pressure. ILs

Mol fraction

gCO2 /g

Pressure/Temperature (bar/K)

Reference

[BMIM][BF6 ]

0.36

0.087

29.5/313

[19]

[NBuPy][BF4 ]

0.243

0.063

28.6/313

[19]

[BMIM][NO3 ]

0.276

0.052

12.8/298

[19]

[EMIM][EtSO4 ]

0.192

0.032

20/313

[19]

[BMIM][DCA]

0.1582

0.04

28.3/313

[20]

[BMIM][BF4 ]

0.137

0.031

12.7/313

[46]

[EMIM][BF4 ]

0.160

0.026

8.8/298

[47]

[P(14,6,6,6)][NTf2 ]

0.6309

0.098

14.2/313

[48]

[EMIM][Ac]

0.39

0.165

20/323

[50]

[BMIM][Ac]

0.373

0.132

[BMIM][NTf2 ]

0.4

0.07

28.3/298

[50]

[HMIM][NTf2 ]

0.2535

0.033

27.4/313

[50]

[EMIM][NTf2 ]

0.26

0.0395

22/313

[49]

[DMIM][NTf2 ]

0.562

0.112

13.7/313

[51]

[50]

fluorinated have low reactivity and high stability and hence render them several outstanding properties. The CO2 solubility in Task Specific Ionic Liquids [N2224][CH3 COO] was studied by Wang et al. [54] and the hydrated complexes which are derived from it. For dried TSILs, the mechanism of CO2 capture involves mainly the interactions between Lewis’s acids and bases, in case of the CO2 , hydrated complexes, the acetate component and H2 O reacted to form the acetic acid and bicarbonate ion. At 1 bar pressure and room temperature, the reported mole fractions range from 0.05 to 0.39. 8.2.7.2 CO2 capture by TSILs based nanomaterials In this case of TSILs based nanomaterials, authors have use 1-butylimidazoliumion and 2-bromopropylaminehydrobromide in ethanol for the cationic part. Brennecke and his coworkers [55] have been observed that intrinsic solubility of CILs phase [HMIM][PF6 ] increase the mass transfer rate on the exposure of CO2 . The mechanism involving Task Specific ILs-amine which is functionalized with carbon dioxide was designed by Bates et al. [56]. The mechanism of the reaction results in the maximum concentration of 0.5 mol of carbon dioxide captured with TSILs being 1 mol (1:2 mechanism).

8.2.8 Multiphasic ionic liquids (MILs) MILs are basically ionic liquids which has primary amine group and different type of gases are adsorb on the ILs surface by a phenomenon called liquid or gas adsorption.

188

8. Ionic liquids as potential materials for carbon dioxide capture and utilization

8.2.8.1 CO2 capture only by MILs In case of MILs four ILs, [N1111][Gly], [N2222][Gly], [N2222] [Lys]and [N1111] [Lys]were studied and it is mixed with N-methyl diethanolamine (MDEA) or water for the capture of CO2 [57]; the result of the experiment showed that ILs–MDEA in aqueous media shows a greater rate of absorption and higher capacity of uptake than MDEA solutions in aqueous media, which means that when an ILs is added, it can boost the capacity of absorption of CO2 . Same kind of results were observed for the MDEA mixture with three kinds of ILs [BMIM][DCA], [BMIM][Ac] and [BMIM][BF4 ], respectively [46]. In case of these three types ILs [BMIM][BF4 ] has the highest capability to adsorb CO2 .ThoughCO2 solubility in [BMIM][BF4 ] at 8.8 bar and 298 K is 0.026 gCO2 /g in case of [BMIM][Ac] at 20 bar and 323 K is 0.132 gCO2 /g [58] and in case of [BMIM][DCA], it is 0.04 gCO2 /g at 14.2 and 313 K [46]. 8.2.8.2 CO2 capture by MILs based nanomaterials In case of MILs base nanomaterials two different phases consist of liquid and gas are separated by gas adsorption technique. Liquid flow nanotechnologies play an important role in nanotechnologies, ranging from graphene oxide membrane for gas separation [55] and supercapacitor for energy storage [59,60]. In the practical application the materials which are two dimensional like sheets of GO and graphene which self-assemble in the structure which is like a paper with intra layer or inter layer at nanometer range, liquids are in relative order layer in the system [61].

8.2.9 Switchable polarity ionic liquids (S-Polymeric ionic liquids) Switchable ILs is such type of ILs which are activated on the presence of some special type of gases and it work as on off phenomena of switch so it is called switchable polar ionic liquid when gases are present near the IL it activates and adsorb the gases on the surface. 8.2.9.1 CO2 capture only by S-Polymeric ILs S-Polymeric ILs are triggered and activated by CO2 , COS or CS2 with a very smaller concentration and under very light condition. Such triggers which are on molecular make it easier to solve the problem of recycling the conventional ILs and hence, help in the solute’s preparation. For the preparation of the solution the equimolar mixture of 1-hexanol and 1,8 triazabicyclo(5,4,0)–undec-7-ene may be used as the solvents with switchable polarity. Using carbon dioxide as a trigger by charging the polarity Decane was elucidated from the SPS. It was reported by Jessop et al. [62] and Feng et al. [63] that the mixture of 1-hexanol and 1,8-di aza-bi-cyclo [5.4.0]–undec-7-ene (DBU) which is equimolar in nature can be employed as an SPS. SPILscan be developed by using phenol, fluoro alcohol, pyrrolidone or imidazole to neutralize 7-methyl-1,5,7-triazabicyclo[4,4,0] dec–5-ene (MTBD) by Wang et al. [64]. 8.2.9.2 CO2 capture by S-Polymeric ILs based nanomaterials In the case of S-Polymeric ILs based graphene nanomaterials, authors have been electrochemical switchable CO2 capture scheme has been proposed as highly selective and reversible CO2 for bare h-Bn nanomaterial. Especially CO2 is weakly absorbed on neutral h-Bn material [65]. By the process of Density functional theory and ejecting an extra electron there can be

8.2 Types of ILs

189

huge increment in the adsorption by using chemisorption system which is charge induced. It all depends on the band gap between the layer of the h-Bn and due to its insulating character. The capture of CO2 which is switchable is possible by taking into the account the g-C4 N3 nanosheets which are conductive in nature, of which there is a scope of modification experimentally in the charge states due to large mobility of electron and higher electrical conductivity. It was observed by using the first principal calculations that the CO2 adsorption energy on g-C4 N3 nanosheets can be significantly increased from 0.24 to 2.52 eV by injection of some extra electrons into the adsorbent. When capture coverage of CO2 reaches its saturation, the negatively charged g-C4 N3 nanosheets accomplish the capture capacities of CO2 up to 73.9 × 1013 cm−2 or 42.3 wt. percent.

8.2.10 Thermoregulated ionic liquids (TRILs) TRILs are such type of ILs which are activate on a very high temperature and it conserve energy and work at an optimum temperature and CO2 and some other gases selectively adsorb on the surface. Fig. 8.7 [48] [EMIM][NTf2 ], [BMIM][NTf2 ], [HMIM][NTf2 ] based ILs, how react when increase in the temperature and how it affects CO2 absorption. It is observed that the absorption rate of [EMIM][NTf2 ] based ILs has higher CO2 absorption rate at lower temperature and when we increase temperature absorption of CO2 decrease and in case of [BMIM][NTf2 ] it is also first increase then decrease the rate of CO2 absorption with increase in temperature. 8.2.10.1 CO2 capture only by TRILs TRILs used in lowers the electrical and thermal energy costs and several ILs used in selective capturing liquids used in purpose of separation of CO2 separation and purification from gas mixtures [66]. Albo et al. have been reported that [HMIM][PF6 ] has a strong has a strong dissolving ability for CO2 . After many experiments it can be seen that the smaller ratio of water is favorable for CO2 solvation. In contrast CO2 solubility lowers in the presence of high concentration of water [67]. Solubility measurements of CO2 in the novel ILs [C4 MIM] [CF3 CF2 CF2 CF2 SO3 ] were performed with a high-pressure view-cell technique in the temperature range from 293.15 to 343.15 K and pressures up to about 4.2 MPa whereas, solubilities of H2 , N2 , and O2 in the ILs were also measured at 323.15 K via the same procedure. The mole fraction solubility of a single gas in [C4 MIM][CF3 CF2 CF2 CF2 SO3 ] was expressed as Henry’s constant, as deduced from the Krichevsky-Kasarnovsky equation. 8.2.10.2 CO2 capturing by TRILs based nanomaterials In the case of thermoregulated ILs based nanomaterials we use C4 N3 and C3 N4 type of 2D sheet in which carbon nitride nanosheets with attractive bandgaps and surface engineered application in both energy and environment related topics, catalysis for water splitting [64,67], Hydrogen evolution [68], CO2 reduction [69], organo-synthesis [70] and two kinds of nanosheets used by cross linking nitride containing anions in ILs[76]. By switching on and off the voltage and providing voltage we can initiate the reaction. OS-based carbon was able to adsorbs 4.8, 3.0, and 0.7 mmol CO2 g − 1 at the temperatures of 0, 25, and 100 °C, respectively. Generally, CO2 solubility in [EMIM][NTf2 ] at 12.8 bar and 298 K is 0.0395 gCO2 /g [49] and CO2 solubility in [BMIM][NTf2 ] at 22 bar and 313 K is 0.07 gCO2 /g [46] and in case

190

8. Ionic liquids as potential materials for carbon dioxide capture and utilization

FIGURE 8.7 Absorption of CO2 in the presence of different ILs in different temperature [48].

8.3 Future applications of IL and GR-based IL

191

of [DMIM][[NTf2 ], CO2 solubility at 28.3 bar and 298 K is 0.112 gCO2 /g [36] (highest among all) and in case of [HMIM][NTf2 ], CO2 solubility is 0.033 gCO2 /g [46] at 13.7 bar and 313 K.

8.2.11 Ionic liquids gel ILs gel is a composite material which consists of inorganic material or polymer matrix with ILs. ILs gel used to capture CO2 and other gases in the mixture like H2 , O2 , N2 and CO2 . ILs gel has a stationary solid phase and mobile liquid phase. 8.2.11.1 CO2 capture by only ILs gel To capture CO2, we use Amino acid based Ionic liquids AAILs based facilitated transport mechanism [53]. To resolve the problem of CO2 capturing a large amount of Amino acid ILs (AAILs) must be added into AAILs gel membranes which help in decreasing the mechanical strength as well as pressure resistance of AAILs gel membrane. In order to capture CO2, authors create DN gel network. As well as the molecular size of AAIL decreases, by changing the substituents in the ammonium-based cation, and by reducing the fractional free volume, CO2 absorption increases. The increase of the humidity up to 20 percent caused the decrease of the AAIL viscosity of about 2 mPa s after the CO2 absorption which was related to the loosely formed hydrogen bond network in the AAIL [62]. Kasahara et al. [70] investigated AAILs containing [N4444][Gly], [N1111][Gly] and [aN111][Gly] for CO2 adsorption. 8.2.11.2 CO2 capture by ILs gel-based nanomaterials In case of ILs gel-based nanomaterials, researchers have been used two layered double network (DN) [55] gel matrix that increases the resistance of pressure and gel membrane containing a large amount of AAILs help in fabrication of a thin membrane with high CO2 permeability. First report on the AAILs fabrication based polymeric gel membrane showiing excellent CO2 permeability and CO2 /N2 ratio selectivity as well as outstanding stability under pressurized condition. By the experimental result we found that poly vinyl pyrrolidine and poly dimethylacrylamide have good compatibility with phosphonium based ionic liquid gel. First network of the DN gel, PAMPS, is a rigid polyelectrolyte, was used because it provides high osmotic pressure inside the gel during immersion of it in water. AAIL-based DN ion gel membranes were prepared by 3-step process as follows: Firstly, preparation of DN hydro-gel, and impregnated it on AAILs/water mixture of the DN matrix, and finally, removal of water from the gel using evaporation techniques. Marr and co-workers combine to for the metal nanoparticles in ILs to prepare catalytic gel. Pd(OAc)2 and PPh3 were heated in [BMIM][NTf2 ] to form a suspension of nanoparticles.

8.3 Future applications of IL and GR-based IL In recent times due to global warming concentration of CO2 rises and to minimize the global warming, capturing of CO2 using nanotechnology is a trending topic now mostly we used graphene-based structure which is formed in multilayer and CO2 stored in the interfacial surface of graphene structure [71]. The more the thickness of graphene structure increases less the absorption and when the thickness decreases it increase the CO2 absorption [72]. Mainly

192

8. Ionic liquids as potential materials for carbon dioxide capture and utilization

we capture the CO2 using ILs like [EMIM][BF4 ] or [BMIM][PF4 ]. Due to their low melting point we use these types of ILs [73]. With the help of simulation technique and umbrella sampling [74], with a proper set of desirable traits like Canonical, grand canonical and micro canonical structure we can form the experimental setup. With a set of variables and constant in different types of system in canonical system like N, V,E and N,V,T in grand canonical system μ,V,T in micro canonical structure we can form the desirable structure to study how the reaction is proceed and set up is monitored computationally. ILs are used extensively in the adsorption of CO2 and some other harmful gases because it is cost effective, eco-friendly, less harmful and no harmful side products obtained after the CO2 adsorption, ILs are used in association with graphene because it is extremely important for the reduction of viscosity between the different layer of graphene or graphene like polymeric layer system. Nanotechnology is used in CO2 capturing extensively and it has a vast use in the absorption of harmful gases in atmosphere. Generally, we use multilayer like structure like graphene and graphene oxide which can absorb CO2 in its cage or interfacial surface of different graphene layer [75]. Sometimes GO absorbs CO2 directly. Different kind of nanomaterial is also used to absorb CO2 . In the place of single layer graphene functional nano porous graphene with high porosity can selectively permeable gas molecule [15]. The space occurs between the plane of GO and the composition of intercalated H2 O during the membrane facilitate the process of gas permeation. The GO planes are mainly composed of oxygen rich functional groups which mainly absorb CO2 and carbonyl group and carboxyl group present in the edge plane. Oxygen atom which is highly polarized in the carboxylic group plays as negative center which helps in the stacking of GO sheets [68,76]. Most of the GO polymer mixed matrix membrane polymer needs to be soluble and miscible with aqueous GO solution. ILs mixed with GO are very selective layer [77]. The absorption rate of gas increases by the selectivity of ILs by the addition GO nano-sheet with molecular sieving channel. Among all the type of absorbing component graphene oxide has the highest rate of absorption capacity is of 1.52 mmol·g−1 at 273 K and 1 bar [76]. In the case of nanomaterial supported graphene base structure we use PAN supported membrane.

8.4 Conclusion In modern day due to global warming the atmospheric CO2 increases the temperature of the environment and to tackle the problem we use the CO2 capturing technique using ILs with the help of graphene-based structure and in that case, graphene helps to capture CO2 in the interfacial surfaces and ILs lowers the viscosity between the two surfaces. CO2 absorption ability mainly depends on the interfacial distances between the two-layers and its thickness. When thickness decreases more quantity of CO2 adsorb in the interfacial surfaces. Many functional groups mainly amine based functional group like mono ethanolamine help as a functional ILs and in different type of ILs we use mainly [BMIM][BF4 ], [EMIM][BF4 ], [BMIM][Ac], [EMIM][Ac] and [BMIM][PF6 ], [EMIM][PF6 ] type of ILs because they are very reactive than others and CO2 absorption is also dependent on temperature if we increase or decrease the temperature rate of CO2 absorption also affected mainly CO2 absorption decrease when we increase the temperature. In recent years, PILs, SILs, TRILs and ILs gel extensively are extensively used for CO2 capturing.

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C H A P T E R

9 Recent advances in carbon dioxide utilization as renewable energy Muhammad Hussnain Siddique a, Fareeha Maqbool a, Tanvir Shahzad b, Muhammad Waseem c, Ijaz Rasul a, Sumreen Hayat c, Muhammad Afzal a, Muhammad Faisal d and Saima Muzammil c a

Departmant of Bioinformatics and Biotechnology, Government College University Faisalabad, Faisalabad, Pakistan b Departmant of Environmental Sciences and Engineering, Government College University Faisalabad, Faisalabad, Pakistan c Departmant of Microbiology, Government College University Faisalabad, Faisalabad, Pakistan d Institute of Plant Breeding and Biotechnology, MNS-University of Agriculture, Multan, Pakistan

9.1 Introduction Environment and energy are two most important issues of the present era. Rapid economic growth in many countries has a negative impact on pollution and environment and this problem is becoming more severe worldwide. Therefore, there is a need to find out the novel techniques so that survival of present and future generations could be ensured. One of the major problems facing the environment today is the production of greenhouse gases (GHG) at a high rate and other air pollutants. Major part of our energy comes from the combustion of fossil fuels. The combustion of fossil fuels produce the greenhouse gases [1]. Around about 56 percent of CO2 emits from the industrial sectors while burning fossil fuels. CO2 is the most anthropogenic greenhouse gas leading towards serious environmental issues. The increase of carbon dioxide release by the consumption of fossil fuels lead towards the climate change and global warming. In 2018 other sources of CO2 emission are power generator sectors, transportation vehicles and industries. While in 2019 the increase emission

Green Sustainable Process for Chemical and Environmental Engineering and Science: Carbon Dioxide Capture and Utilization DOI: https://doi.org/10.1016/B978-0-323-99429-3.00032-1

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of CO2 is due to the sustainable use of natural gas and oil. According to Earth System Research Laboratory concentration of CO2 in atmosphere has reached to 411 ppm that is a high record while the 350 ppm is the safe level of CO2 concentration. It is noticed that the increase in temperature will continue in the future because of economic growth and industrial development. In the past CO2 is removed from the environment through plants and crops which absorb the CO2 and sunlight in a process of photosynthesis and release the oxygen. While today due to increase in population, industrialization is increasing steadily that’s why the concentration of CO2 is increasing with a high rate. So plants are not able to remove such a sufficient amount of CO2 naturally [2]. Therefore, Intergovernmental Panel on Climate Change (IPCC) recognized that there is a need to reduce CO2 concentration and relevant policies and regulations should be introduced. Following measures can reduce the carbon emissions: a) b) c) d)

improvement of fuel energy efficiency CO2 capture carbon storage CO2 conversion

Furthermore, the creation of fresh processes, products, and industries could be aided by the introduction of new technologies for converting CO2 to valuable items, hence reducing global warming [3–5]. In CCUS (carbon capture and sequestration), CO2 is the seized from the exhausted gases which are released during the burning of fossil fuels and later on filtered to attain better quality of CO2 . This purified CO2 gas is used to produce valuable products that are economically, socially and environmentally beneficial. CO2 is also used in various fields of biological, chemical and food industries. Although all these efforts are made to reduce the GHG but unfortunately we are failed to do that. In this chapter we discuss carbon dioxide utilization technologies including energy storage, biological usage, mineralization, drinks and food utilization and chemical synthesis. Likewise, we discuss existing CDU research and development projects around the world. Finally, we talk about the CDU market, policy, and difficulties.

9.2 CO2 utilization technologies CO2 is used in different ways especially in mineralization, beverage and food industries, fuel and chemical production etc.

9.2.1 Mineralization In Mineralization processes also known as accelerated carbonation, CO2 emissions through industries is used to form the valuable products. CO2 mineralization processes involves the use of feedstock, natural silicates ores [6] and natural alkaline solid waste [7,8]. Mineralization process in which alkaline residues are used to decrease CO2 emissions from industries and power plants become more effective [9,10]. Mineralization has two main advantages 1) having high capture capacity using natural ores 2) low feedstock cost when alkaline solid waste is used. Alkaline solid residues used for mineralization include mineral and mining processing

9.2 CO2 utilization technologies

199

waste [11,12], cement and concrete waste [13], fossil fuel residues [14,15] and paper industry waste [16,17]. These residues contain large amount of calcium and magnesium that’s why considered as best feedstock for CO2 mineralization. Mineralization is achieved by using 4 main approaches [18]. 1) carbonation curing: a process in which CO2 is used to enhance the durability and strength of cement based products 2) electrochemical mineralization: a process in which CO2 is mineralized through electrochemical cell and electricity is produced 3) Indirect carbonation: a process in which different ions are extracted to produce highly purified chemicals 4) direct carbonation: a process in which CO2 and alkaline slurry are mixed together in same step Mineralization of CO2 through alkaline solid waste is also useful to control air pollution for several industries and power plants. Pei et al. [18]. were used fly ash to control the pollutants from petrochemical industries and results showed the successful removal of CO2 was 96 percent, for NOx it was 99 percent, and for particulate matter it was 83 percent. The products obtained from CO2 mineralization can also have a variety of environmental applications [7]. Additional mineralization methods (indirect carbonation) can yield high-value minerals e.g., abiotic catalysts, geopolymers, soil conditioners and calcium carbonate precipitates as well as glass ceramics. Furthermore, electrochemical mineralization technology can be used to restore gaseous CO2 from a different industrial processes whereas also producing electrical energy [19]. The amount of mineralized carbon dioxide is little as compared to global CO2 emissions, it is advantageous in terms of alkaline solid waste remediation and the production of high-value products.

9.2.2 Beverage and food processing CO2 is consumed as acidifying agent in beverages and food industries [20]. CO2 purity is considered as main factor during the gasification processes because contamination occur from benzene, COS and H2 S. CO2 is especially used in producing carbonated drinks, water which is deoxygenated, products made from milk and food preservatives. A large quantity of liquid CO2 is used in making sparkling wines, beer and soft drinks so it is necessary CO2 come from renewable resources. For food preservation, mechanical refrigerators are used for CO2 storages and transportation. Liquid CO2 and dry ice (solid form CO2 ) are used for food that require freeze drying. For the production of flavors, essentials oil and coffee decaffeination, CO2 is utilized through supercritical fluid extraction method which is important in removal of volatile, heat subtle and oxidizing compounds. In this technology agents used for extraction have following advantages [21–23]. a) non-toxic b) non-corrosive c) stable chemically d) better permeability e) reused after decompression in sense to save energy (Fig. 9.1). In 1978, Germany introduced the first SFE technology at industrial scale by converting the coffee beans into caffeine [24]. Nowadays this technology is routinely used for daily life purposes as described in Fig. 9.2 [25]. The considerable applications of SFE technology includes fat and oil extraction [26–30], cholesterol and lipids [31,32], production of natural colors [33], antioxidants [34,35], hops [36] as well as decaffeination of coffee and tea [37–39]. A lot of essential oils are extensively consumed in cosmetics and food industries. Importantly, SCE have

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9. Recent advances in carbon dioxide utilization as renewable energy

FIGURE 9.1 Represent the CO2 mineralization process through alkaline residues.

better extraction capacity when compared with other removal equipment. Conde-Hernandez et al. [40] compared the SCE technology with steam and hydro distillation approaches and found that SCE technology had better antioxidant activity and oil yield. Nowadays SFE techniques in combination with other methods like enzymes [41], ultrasound [42] and microwave [43–45] have been widely reported. However, there are a lot of hurdles in the development of SCE technology because of high costs at large scale production, sophisticated equipment is required and difficulties in continuous production which make low use of this technology (Fig. 9.3, Table 9.1).

9.2.3 Biological utilization CO2 is naturally fixed by plants and autotrophic microorganisms through a process of photosynthesis. This fixation is safe and cost effective. The use of microorganism is advantageous because of rapid production rate, high photosynthetic activity, require small volume, easily adaptable to any environment and easily integrated with other technology. Microalgae attain a great attention of researchers because it can easily replace the fossil fuels and used as an alternative energy source. Moreover, 1kg biomass of microalgae can fix the 1.83 kg of CO2 [46–48]. The sources of carbon obtain from microalgae are inorganic carbon that is [49,50] dissolved in water. The type of microalgae used depends on the application, such as CO2 fixation from flu gas. Microalgae not only need to be able to fix CO2 efficiently, but they also need to be able to withstand high temperatures, high concentrations of CO2, NOx and

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9.2 CO2 utilization technologies

FIGURE 9.2 Represent the daily used food obtained through supercritical fluid extraction.

FIGURE 9.3 Represent the process of CO2 -EOR. TABLE 9.1 Represent the CO2 fixation using different algae. Algae sp

Temperature (K scale)

% level of CO2

CO2 fixation rate g/(L d)

References

Anabaena sp.

309

11

1.02

[57]

Botryococcus braunii

291

6

0.496

[58]

Chlorella vulgari

297

5.1

0.251

[58]

Dunaliella tertiolecta

292

5.2

0.271

[58]

Botryococcus sp.

302

10.4

0.258

[59]

Chlorella pyrenoidosa

293

10.1

0.25

[60]

Chlorella sorokiniana

297

4.2

0.252

[61]

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9. Recent advances in carbon dioxide utilization as renewable energy

SOx. [49,50]. Following physiochemical parameters should be considered during the fixation of CO2 : 1) Culture temperature is a considerable parameter for the photosynthetic bio-fixation of CO2 assisted by microalgae. Microalgae can grow at 291–298k temperature. Fluctuations in temperature can hinder the growth of microalgae by dropping the Rubisco enzyme activity level that is important for CO2 fixation [20]. 2) Light is another important parameter that play significant role in the growth of microalgae, system operation and reactor design and provide the energy for activation and assimilation of some enzymes that play important role in the process of photosynthesis [51]. 3) Without the addition of CO2 , the pH can reach a very high level of 10 during high-density production. As a result, such an environment has a negative impact on the photosynthetic reaction process and nutritional salt absorption in cells, resulting in a drop in microalgae biomass output. As a result, during algae cultivation, the pH should be kept neutral [52]. 4) CO2 concentration is very critical parameter that seriously affect the microalgae growth rate. Concentration of CO2 should be kept between 10–20 percent [53]. It is feasible to directly utilize flue gas from fossil fuels. Microalgae can also thrive in higher CO2 environments. Chlorococcum littorale and Synechocystis aquatilis, for example, grow well at a concentration of 40 percent. The fixation of CO2 by different algae has been reported in Table 9.1. These findings might aid in the development of microalgae tolerance to high CO2 concentrations from fossil fuel combustion exhaust gas [54–56].

9.2.4 Oil recovery enhancement, coal bed methane and fracking of CO2 Carbon dioxide storage of earth is accomplished by introducing collected CO2 into subterranean pools under appropriate conditions, where it is kept for more than 10,000 years [62]. However, by injecting CO2 in gas and oil reservoirs, it is feasible to boost fossil fuel output. CO2 injections into depleted oil reservoirs improve oil recovery (EOR), shale formations improve shale gas recovery (ESGR), and un-mineable coal seams improve coal bed methane recovery. It is believed that CO2 will be used as a fracturing fluid in the future, potentially replacing water [63]. The EOR method is utilized for collection of crude oil from oilfields that has been moderately removed by principal and subordinate recovery methods. Chemical, heat and gas injections are the most common procedures utilized for EOR [64–66]. CO2 -EOR projects are mostly active in the United States and Canada, where there are also available CO2 sources. More than 90 percent of the world’s oil reserves may be eligible for CO2 EOR technology [67]. The following key considerations should be kept in mind for the ongoing deployment of CO2 -EOR technology: 1) checking of subterranean and released discharges 2) enhancing on site risk management 3) Refining the usage of unrestrained fields.

9.2.5 Fuels and chemicals CO2 can be used for the fuels and chemicals production. The commonly used products obtained from CO2 are salicylic acid, methanol, formic acid, cyclic carbonates and urea [68]. Formic acid play great role in the CO2 emission reduction [69].

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203

9.2.5.1 Electrocatalytic conversion Different types of pathways are used for CO2 conversion into fuels and chemicals e.g. biochemical, photochemical, electrochemical and thermochemical [70]. Different types of electrochemical methods are used to convert the CO2 to valuable products such as methanol [71], methane, carbon monoxide [72] and hydrocarbons [73]. Although a lot of studies are reported which focused on the electro-reduction of CO2 on different catalytic sites but still there are many challenges that limits the use of this technology [74–76]. 9.2.5.2 Plastics Polymers used for the synthesis of plastics are mostly CO2 based and they are environmental friendly. Plastics are synthesized through a process known as copolymerization in which hydrocarbons and CO2 (31–50 percent) are used and the use of petrochemical is reduced [77,78]. In 1969 Inoue et al. [79] first time used the CO2 with epoxide in a polymerization process. Copolymerization process is catalyzed by the catalyst. So, the production of cost efficient and highly reactive catalyst is necessary to future development. CO2 is used to form the fabricated polyoxymethylene (POM) through polycondensation process [80].

9.2.6 Principal and favorable utilization technologies In order to enhance oil recovery CO2 injection method is working in different areas. However, following problems are associated with the conventional methods including leakage and acidification of water supply and the energy used for the transportation of CO2 . So to encourage this technology issues associated with the transportation and corrosion of CO2 should be solved [81]. However, different energy inputs are required for CO2 conversion while the renewable energy resources like solar and wind power, are the favorable sources for this process, and the energy from these processes cannot be directly used for electricity generation. However, if the charges of renewable energy resources down continuously in this manner, the usage of renewable energy for CO2 conversion will increase. Microalgae is one of the finest options for producing liquid fuels and reducing CO2 emissions [82]. The quick rise in lipid content with low energy usage and minimal CO2 emission during biofuel conversion are major hurdles for such technology. It is also critical to recognize that such technologies are spatially specialized for early production and are tailored to local resources and circumstances. To summarize, the synthesis of basic industrial chemicals in huge quantities will be more beneficial if the objective of these processes is to diminish the released anthropogenic CO2 . The combined systems are utilizing various usage methods for example the methanol production is linked with the increased gas retrieval, are potentially interesting solutions [83,84]. Before the commercialization of CO2 utilization techniques, it is necessary to identify optimal use routes, which is accomplished through the use of Life Cycle Assessment (LCA) or Techno-Economic Analysis (TEA). According to von der Assen et al. [85], Even at the early stages of development, LCA can suggest environmentally advantageous avenues for CDU. Several extensive evaluations, on the other hand, have concentrated on reviewing the environmental consequences of several CDU technologies in depth, which will aid in the future in making appropriate CO2 utilization paths in certain cases. As a result, the optimum

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9. Recent advances in carbon dioxide utilization as renewable energy

CO2 utilization technique has the following properties. a) little extra energy requirement b) simple methods c) large size and worth of future market.

9.3 Developments in worldwide CO2 utilization projects In the following countries the CO2 utilization technologies are operated and constructed at different stages:

9.3.1 United states The united states are working in EOR technology. In the United States three following technologies are working on the CO2 utilization technology which focused on increasing commodity market including 1) mineralization 2) chemicals 3) polycarbonate plastics. It is expected that in 2030 these methods will work on large scale with a lot of application. Initially in United States carbon tax was $10/t in 2008, which will increased to $50/t for saline storage and $35/t for EOR use by 2026 [86].

9.3.2 China CO2 usage in China is mostly subsidized by the government and carried out by enterprises backed by universities or research organizations [87]. China National Petroleum Company operates the first large-scale CO2 utilization technology in 2018 [88]. Furthermore, the participation of Chinese government in the major Clen Energy conference, other international structures and Carbon Sequestration Leadership Forum as well as secondary national research companies and institutes are involved in mutual cooperation projects. China now has the most carbon storage, sequestration and experimental facilities in procedures, structure, or development [87].

9.3.3 Germany Ever since 2002, German government decided to decrease the GHG emission by 40 percent upto 2020 and 80 percent upto 2050 so that change in climate can be combat [89]. In 2015 they set a target as “New High Tech Approach” that describe the upcoming direction. 33 Carbon dioxide utilization projects were granted in Germany from 2010–16. Government paid €100 million and different universities paid €50 million for research projects [90].

9.3.4 Australia The government of Australia is included in several international forums that work on promoting the development and construction of CO2 reduction technologies. These forums includes Australia-China Combined Organization Assembly and others inside and elsewhere in Asian region [91].

9.5 Regulation and policy

205

9.4 Market scale and value The market for CO2 consumption differs across countries which affect the environment benefits obtained the CO2 utilization methods. The demand of the yields obtained by using CO2 is increasing day by day. The market for CO2 show a great growth rate >13 percent/year by 2022. CO2 is not used an alternate for storage because a large volume of CO2 is required for storage. CO2 assisted EOR market is mainly funded through gas and oil industries. Four points should be kept in mind about the CO2 utilization market: 1) 2) 3) 4)

Building materials (concrete, carbonate aggregates) Chemical intermediate (formic acid, methanol etc.) Fuels (methane) Polymers

Methane contributed 3–4 trillion cubic meter per year to current market that is expected to be 4–5 trillion cubic meter per year in 2030. Following obstacles are associated with this technology 1) low cost catalyst 2) merged process for carbon conversion, storage and renewable energy. High cost of this technology limit the market growth rate [92]. The methods involved in the utilization of CO2 require more expansion on vast scale by enhancing the size of market and producing ability in experimental to viable plants.

9.5 Regulation and policy There are still failure risks in the present CCS industry. As a result, in the absence of a well-made strategy, the private division will not invest in carbon release and storage at the scale which is required to accomplish climate change alleviation goals. A solid monitoring framework is required to reduce and evade the destructive consequences of failure while also maximizing economic profit on investment. As a result, well-thought-out CO2 use laws are crucial for creating and expanding markets. In many circumstances, products must fulfil current industry standards in order to be accepted. These standards are usually produced by consensus-based and voluntary groups under the supervision of government and industry members. There are currently little incentives to update or amend existing standards. Even where there is a willingness to change, regulatory frameworks move slowly and to minimize the CO2 release [93]. Development and investment is a major problem today due to shortage of information, the self-motivated market and technology, and a unstable government landscape. Discrete approaches regarding CO2 consumption techniques are usually optimistic, owing to the superficial objective of minimizing CO2 emissions. To foster public interest and confidence, governments must make suitable information available to the public. Governments should interact with principles backgrounds to minimize intervals in the market release of these items and to broaden a creative agenda for CO2 consumption. Only a few countries, most notably Norway, UK, US, China, Japan and Canada have specific policies in place to assist CCS deployment [94]. In the time-consuming track, the guidelines of government are crucial to enhance the technologies organization, without them, CO2 consumption will not contribute much to attaining climate objectives. We must match CO2 product purchases with climate policy if we are serious about meeting the objectives.

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9.6 Conclusion and future prospects Rising CO2 levels in the atmosphere are considered to worsen climate change. Despite the fact that CO2 collecting technologies are fairly advanced, using captured CO2 remains a big challenge that will necessitate more future study. There are currently several restrictions to emerging CDU, including as water and energy usage, the use of costly chemical agents, and gas substructure concerns. Economical CO2 consumption technologies are especially needed, also commercial adoption requires trials on larger scale. It is obvious that a detailed understanding of various CO2 usage methods is useful in understanding processes and selecting appropriate ways for CO2 capture. For bridging gaps in present industrial applications, potential integrated technologies are preferred. Furthermore, displaying CDU R&D projects and assessing CO2 consumption markets can help determine the viability of full-scale adoption of these technologies. Economic viability is critical for the industrial practicality of CDU technology, whether accomplished via technological improvement or regulatory reforms. As a result, future significant research objectives should focus on CDU and CCS guidelines, policies, and evaluations, as well as the incorporation of CO2 consumption with other measures to decrease energy depletion and expenses, particularly on a pilot scale. In the meantime, public education and exposure of CCUS should be prioritized, and international partnerships should be expanded to promote public knowledge of the environmental consequences. It is also worth mentioning that CDU is not a replacement for CCS, but rather a supplement to it, and that without CCS, we would fall short of our environment goals. Governments should boost their commitment to CCUS and play a vital role in assisting its implementation in order to limit the global mean temperature increase below 1.5 °Celsius. Encourage private sector participation in larger-scale demonstration programs, as well as in the commercialization of CDU technology. CO2 may become a resource sought after by numerous sectors of the global economy in the near future, influencing regulation and policy in the market for CO2 -based products.

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C H A P T E R

10 Metal Organic Frameworks as an Efficient Method for Carbon dioxide capture Bharti Kataria and Christine Jeyaseelan Department of Chemistry, Amity Institute of Applied Sciences, Amity University, Noida, Uttar Pradesh, India

10.1 Introduction Elevation in industrialization and population leads to excessive energy consumption. Due to advanced technology and ready availability of non-renewable fossils more than three fourth of energy needs are pillared by burning these fuels as a corollary of which the rate of greenhouse gases majorly carbon dioxide is increasing at high pace [1]. On the basis of previous studies the concentration of CO2 has shown a steep escalation and reached 408 ppm in 2019 which eventually will have adverse impact on the climate [2]. Increased level of CO2 will proliferate the earth’s temperature therefore, implementation of carbon dioxide capture and storage (CCS) techniques might help to reduce its emission to some extent [3]. CCS is an efficient way to mitigate the concentration of CO2 in atmosphere. Out of the steps involved in the above mention technique capturing of carbon dioxide gas is the most difficult task and to achieve this new advanced materials are produced for the same. Metal-Organic Frameworks are latest materials that will reliably serve the purpose of providing a platform for evolution of next generation materials possessing benignant properties like high gas adsorption capacity and their chemical and structural tenability [7]. The selection of the components of the framework depends upon the attraction potential of the internal pore surface with regards to carbon dioxide gas. Many technologies have been invented which works on the principle of adsorption, absorption or membrane separation [8]. Out of all methods amine scrubbing is found to be most effective as it can mitigate CO2 levels by 98 percent but it has some major demerits which includes high energy usage, corrosive nature and the huge amounts of absorber used [9]. In the past few decades various studies have been carried out and published explaining applications of different MOFs. In this review, the studies focus on various techniques used by

Green Sustainable Process for Chemical and Environmental Engineering and Science: Carbon Dioxide Capture and Utilization DOI: https://doi.org/10.1016/B978-0-323-99429-3.00009-6

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MOFs to capture carbon dioxide gas selectively [9]. Different synthesis methods for preparing metal organic frameworks have been discussed. Besides all the properties [10] showed by MOF, modification in frameworks via ligand functionalization is attracting more and more scientists, they provide- one with the advantage of synthesizing desired pore size MOF for various applications. The synthesis methods involve conventional routes, microwave synthesis, sonochemical routes, electrochemical and mechanochemical routes, out of all the conventional method which includes generation of frameworks via Solvothermal methods are the most readily used one. Synthesis of some MOFs for CO2 selective adsorption are also discussed which involves MOFs like Mg-MOF-74, MOF-801-Zr, MIL-608(In)NH2 . Some basic properties of MOFs are also explained. Principles and methods involved in gas separation are studied, four mechanisms- thermodynamic equilibrium separation, kinetic effect, molecular sieving effect, and quantum sieving effect are explained in detail. Selection of adsorbent plays important role in designing frameworks, the selection procedure involves two major methods that is adsorption process and nature of pores of the adsorbent. Among various factors enthalpy of adsorption and adsorption capacity are two most considerable factors while explaining capturing of this gas on frameworks via adsorption. Various methods like opening pore sites, pre synthetic and post synthetic procedure and size tenability are some methods to proliferate adsorption capacity for carbon dioxide gas which are briefed in this chapter. Along with that stability of metal organic frameworks also plays crucial character in amelioration of its properties, hence in this chapter we reported various methods which can be used for both existing frameworks and unknown frameworks respectively. These methods includes stabilities in terms of chemical, mechanical and thermal aspects.

10.2 Metal organic framework (MOF) Metal Organic Framework are compounds formed by combining inorganic metal ions or nodes connected via organic linkers hence classified under porous organic-inorganic hybrid constituents. They are crystalline coordinate compounds having 1D, 2D, 3D structures. They show various unique properties like low density, uniform channels, adjustable chemical functionalities, internal surface area [11], high porosity [12] and many more. They have attracted attention due to their applications in various fields like separation and capturing gases, drug delivery, medical imaging, sensors, biomedical applications, heterogeneous catalysis.

10.2.1 Conventional synthesis route It is among the most popular route for synthesizing MOFs and the needed energy is produced by conventional methods only. Solvothermal reactions are generally carried out in sealed reactors at very high temperature between the ranges 80 to 260 °C which is more than the boiling point of the solvent being used at autogenous pressure conditions. On the other hand, non-Solvothermal reactions are carried out at normal temperature which is either under or around the solvent’s boiling point being used [13]. The major pros attached to these reactions is that many frameworks can be prepared by just mixing the primer materials at room temperature hence precipitation take place in very small time gap therefore it is also known

10.2 Metal organic framework (MOF)

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FIGURE 10.1 Solvothermal synthesis process.

FIGURE 10.2 Microwave synthesis technique.

as direct preparation method which saves reaction time as many reactions are recorded to be completed in days or weeks [14] (Fig. 10.1).

10.2.2 Microwave synthesis technique It is a renowned method for the synthesis of MOF materials. The method depends upon the interaction of electromagnetic wave with the electric charge produced by ions or electrons present in the solution and in case of solids electric current is produced by the electric resistance offered by solid materials. The reactions were carried out in an oven by maintaining the reaction with respect to temperature and pressure [15]. The reaction time is usually less than one hour and the reaction is usually achieved at 100 °C. This method results in the formation of highly crystalline material with nano size which makes it more adaptable and useful than conventional method. Using this method, we can mitigate synthesis time and alter the shape and size of the crystals accordingly [16] (Fig. 10.2).

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FIGURE 10.3 Sonochemical synthesis process.

10.2.3 Sonochemical synthesis The synthesis reaction in this type of method initiates in presence of high ultrasonic waves and the frequency used for the waves is higher than hearing range of humans that is 20 kHz to 10 MHz. The process begins by passing ultrasonic waves into the solution which will creates variable pressure zones inside the solution say rarefaction or compression which leads to the formation of bubbles also refer as alternate cyclic regions as a corollary of which the pressure in the lower region starts mitigating towards reactants and solvent vapor pressure which results in formation of small size cavities. The bubbles tends to achieve their critical size which is not stable, leads to failure. The whole process of synthesis, maturing and collapsing occurs in very short span of time but high attention needs to be paid for selecting synthesis materials vapor pressure, viscosity and the equipment temperature, frequency along with majorly considering the selection of solvent for the process because organic solvents did not prove themselves to be useful as they impact the collision cavities intensity which will directly affect the temperature and pressure [17] of the reaction. In case of solid particles cavitation is not the preferable phenomenon hence microjet is considered suitable in which whenever the bubbles come in contact with the particle, bubble’s erosion takes place which results in activated particle surface. This method is used specifically for generation of organic substances and nano range materials [18]. This method possesses various advantages over and above the mentioned methods because it environment friendly, very fast [19] and most important being energy efficient (Fig. 10.3).

10.3 Synthesis of some MOFS

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FIGURE 10.4 Mechanochemical synthesis.

10.2.4 Mechanochemical synthesis This method does not fall with all other methods because in this there is no use of solvent, it is a solvent free method. Process initiates with normal mixing of organic linkers with metal salt by ball mining process. Among all the above methods, it is the most convenient and had many merits being environment harmless, less reaction time, solvent free reaction [20], size of particles obtained is small. In some cases, when metal is replaced by metal oxides in this process it will produce water molecule as side product of the reaction [21]. Reactivity of the reaction can be enhanced by choosing organic linker with low melting point and metals which release salt during the reaction due to which the movement of reactants become easy which will speed up the rate of reaction [21] hence metal carbonates and oxides are highly used for the same. The outside addition of small volume of solvent, the process called liquid-assisted grinding (LAG) was found to be impactful in proliferating mobility of reactants [22] (Fig. 10.4).

10.2.5 Electrochemical synthesis Reaction in this method is carried out by flow of electric current produced by electric transfer of molecules. Cell construction involves cathode, anode, and electrolyte solution made up of conducting salt and linker molecules dissolved in it. Instead of using direct metal salts into the reaction, metals ions were added occasionally to avoid deposition of metal ion on cathode, to avoid protic compounds like acrylonitrile, acrylic, and maleic esters were used. This method is usually used for large scale production of materials because of high end point yield offered and used by researchers for producing zinc and copper metal organic frameworks [23] (Fig. 10.5).

10.3 Synthesis of some MOFS I. Mg-MOF-74 This material is synthesized using Solvothermal method which initiates with reaction between magnesium nitrate with DOT (2,5-dihydroxyterepthalic acid) in presence of

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FIGURE 10.5 Electrochemical synthesis process.

deionized water and DMF (N,N–diethylformaide) and ethanol [24]. The procedure followed was reported in literature is as follows: 0.712 g of magnesium nitrate was mixed with 0.167 g DOT via sonication and then it is dissolved in a mixture of DMF (67.5 ml), ethanol (4.5 ml), and deionized water (4.5 ml) in the (v/v/v) ratio 15:1:1. The prepared solution is then transferred to an autoclave which is made up of stainless steel with a Teflon lining inside it. It is then heated at autogeneous pressure and 125 °C temperature for 26 h then taken out and cooled to RT. The mother liquor is then poured out and replaced with methanol followed by removal of guest molecules in vacuum for 15 h at 250 °C which results in the preparation of dark yellow crystals of Mg-MOF-74. II. MOF-801-Zr 0.06 g of zirconium chloride (ZrCl4 ) and (0.36 g) fumaric acid were dissolved in 45 ml DMF solution followed by addition of extra 20 ml [25] of fumaric acid and stirring of the mixture for 10 min. The mixture was then transferred to an autoclave, which is then sealed and heated in an oven for 16 h at 403 k temperature. The precipitates obtained after cooling and filtering the mixture were dipped in DMF (30 ml) for one more day then washed with methanol various times and after drying it at 333 k in vacuum white powder of MOF-801-Zr was obtained. III. MIL-68(In)-NH2 The procedure followed has some variation in comparison to published literature in [26], 1.92 mmol of In(NO3 )3 .xH2 O and 0.645 mmol of NH2 -BDCH2 were dipped in 6.2 ml DMF in Teflon autoclave. The mixture was then stirred for 15 min after addition of pyridine, the autoclave was sealed and heated at 125 °C for 5 h The obtained yellow powder was then washed with DMF and dried.

10.5 CO2 capture using MOF

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10.4 Properties of MOFs 10.4.1 Chemical and thermal In comparison to other zeolites and inorganic porous solids their thermal and chemical stability is shorter because of the weak bond of coordination which is holding the metal and ligands together. Most of them are moisture or air sensitive hence need extra care and generally used under inert atmospheric conditions. To illustrate, MOF-5 is highly sensitive to air and leads to quick degradation of crystal structure of the material and mitigation of surface area due to hydrolysis of the present bonds of Zn-O, [27] whenever they came in contact with the same.

10.4.2 Mechanical For application like capturing gases the mechanical stability of material should be considerably high so that it can permit the heavy packaging of adsorbent bed without overlooking the structure. To elucidate, in case of Cu3 (BTC)2 when large pressure is applied it results in decrease of lattice volume by 10 percent [28].

10.4.3 Thermal conductivity This parameter is important in identifying heating efficiency of the adsorption bed and the time required to regenerate temperature swing adsorption based capture process. According to published work by Yaghi [29] the thermal conductivity of MOF-5 declines when temperature is low in the range (20 k–100 k) but remain almost stable above 100 k.

10.5 CO2 capture using MOF MOF as Adsorbent M M organic frameworks, MOFs due to their porosity, 3D structure and modular nature are able to sustain their framework with minimum damage. They showcase many attractive properties like excellent surface area and their functionality [30], variability in size of pores [31]. These functional groups allow MOF to modify [32] the sizes of the pores. Due to these numerous properties, they attract the attention of researchers and are being used for a number of applications including biosensors, biomedical applications, catalysis, separation and storage of gases [32] and many more. The role of MOFs as an adsorbent to capture carbon dioxide has been discussed. 5.1. Methods for gas separation: Separation is a procedure of differentiating components of any mixtures, which usually needs large amount of energy [33]. Separation can be achieved by various separation methods like separation adsorption in which the components present on the adsorbent surface have different affinity towards the guest molecules. Gas separation methods involves adsorption and absorption-based technologies [34], cryogenic distillation, and membrane based technologies but out of all adsorption separation is the most convenient because of the advantages it provides in comparison to the above mentioned methods which includes being eco-friendly [34], low energy

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10. Metal Organic Frameworks as an Efficient Method for Carbon dioxide capture

consumption, and easy maintenance. The whole process consist of two main steps which includes adsorption followed by desorption. In the former step the gaseous mixture is allowed to pass through a column filled with adsorbents. Then desorption is carried out to replace all the components which get struck on the adsorbent bed for reusable purpose. There are mainly four methods which are generally used for gas separation like vacuum swing adsorption (VSA), electric swing adsorption (ESA), temperature swing adsorption (TSA) and pressure swing adsorption (PSA). Out of above-mentioned methods TSA and PSA are most commonly used ones. In TSA the deposition starts with heating the adsorbent. Due to heating or cooling it lasts up to few hours or sometimes up to a day. While in PSA the rejuvenation takes place by decreasing the pressure of the adsorbent. It is usually more preferable over TSA due to short time periods which varied from seconds to minutes and its non-temperature dependent process. VSA is also attracting the interests of researchers because it works in range of ambient pressure, as the stream pressure of many flue gases is close to atmospheric one [35]. 5.2. Mechanism for specific adsorption: There are basically four types of mechanisms for selective gas adsorption separation for porous materials5.2.1. Thermodynamic equilibrium separation: Also known as dynamic separation. In this method the size of the adsorbent pores needs to be big enough as a corollary of which the constituents of the gaseous amalgam can easily sweep into the adsorbent. In this situation the extent of reaction between the adsorbate molecules and the surface of adsorbent play crucial role in selective adsorption quality. This interaction strength is directly proportional to dipole moment, polarizability, magnetic susceptibility and quadrupole moment [36]. 5.2.2. Quantum sieving effect: This is mainly used for separation of isotopes like D2 /H2 [37]. The adsorption is achieved on the basis of rate of diffusion of the guest molecules and pore diameter compatibility with de-Broglie wavelength. 5.2.3. Molecular sieving effect: Also known as shape and size exclusion. In this adsorption depends upon cross sectional size that is kinetic diameter (the minimum distance between two molecules with null kinetic energy and they have a collision) and shape of the adsorbate [37]. 5.2.4. Kinetic effect: This decoupling is implied when equilibrium dissociation is not achievable. The major problem faced while using this method is maintenance of pore size of the adsorbent between the kinetic diameters of the molecules that needs to be separated. Example includes separation of carbon dioxide and methane gas using this effect [36]. 5.3. Adsorbent selection and criterion: This is one of the most important step for separation process. Adsorbents having high adsorbing capacity and selectivity for selective molecules are considered to be ideal for use, therefore, the following things need to consider while choosing adsorbent: r Adsorption process r Nature of pores of the adsorbent The separation of large amount of gas, in case of detachment process, the adsorbents are sorted out on the basis of how quickly desorption can occur. While when we are not considering the selection process then the factor which should be considered is nature of adsorbent including the shape and size of its molecules, polarizability, and dipole moment

10.7 Methods to enhance CO2 adsorption

219

along with quadruple moment. To elucidate, if the targeted molecule possesses higher quadrupole moment then adsorbent with high electric field gradient and high surface area is suitable for separation. Similarly, if molecules possess high dipole moment then adsorbent molecules must contain high polarized surface [37].

10.6 Adsorption of carbon dioxide in metal organic frameworks This section explains the most important aspects required to be considered during adsorption of CO2 within metal organic frameworks. 6.1. Adsorptive capacity for CO2 : This is one of the critical factors for adsorption technology and is based on surface area of the adsorbent molecules. Because of ultra-high surface area, MOFs are capable enough to capture carbon dioxide in comparison to zeolites and other compounds of activated carbon. Other than surface area, there are some factors like adsorption, pressure, temperature, and interaction between the adsorbent and adsorbate molecules which plays vital role in CO2 grasp capacity [38] of adsorbent material. Many researches have been published in this context but the main focus in most of them is on gravimetric capacity which relies upon the weight percent of grasped gas over total system’s weight [39]. According to the study carried out by Yaghi and group [40], they used 9 different metal frameworks with distinct geometries (MOF-177, MOF-505, Cu3 (BTC)2 , IRMOF-11, MOF-2, IRMOFs-3, MOF-74, IRMOF-6, IRMOF-1) to find the relation between surface area of MOFs and the adsorption capacity of CO2 at temperature-298 k and pressure-35 bar The results conclude that out of all MOFs mentioned above MOF-177 found to have higher surface area hence higher carbon dioxide adsorptive capacity. When pressure is increased from 35 bar to 50 bar then another MOF called MOF-210 showed best CO2 uptake capacity as reported by another group. It was found out that by increasing the size of organic linker this capacity can be enhanced, as reported when the organic linker in case of MOF-177 is changed to BBC 4,4’,4’-(benzene-1,3,5-triyl-tris(benzene-4,1-iyl)) from BTB: 1,3,5 benzenetribenzoate, MOF-200 was obtained with proliferation in adsorptive capacity. Enhancement of surface area along with volume of pores can also be used to boost the capacity, this can be obtained by expanding the structure of the material during synthesis or by using huge organic ligands [40]. 6.2. Enthalpy for adsorption: Another important factor which impact the broadband of MOF application in gas capturing is enthalpy of adsorption also referred as isoelectric or adsorption heat. The magnitude of isoelectric heat indicates the extent of interaction between the CO2 and pore surface which in turn explains the amount of heat needed for desorption and details of adsorption selection [41].

10.7 Methods to enhance CO2 adsorption Various methods have been designed to proliferate the adsorption of carbon dioxide gas. This chapter provides a detailed explanation of all the methods follows: 7.1. OMS-open metal sites: metal sites possess significant role in elevating selectivity and captivity of carbon dioxide gas over all other gases. During preparation of MOF some metal ions get involved with solvent molecules leading to mitigating adsorption capacity

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10. Metal Organic Frameworks as an Efficient Method for Carbon dioxide capture

of MOF towards guest CO2 . This can be reduced by varying temperature conditions which leads to remove all the extra solvent molecules and the created vacant space known as open metal sites, which will result in increased adsorption of gas. These open metal sites hold on the binding between the guest and the host molecules hence, creation of more active sites results in more trapping of guest gas [42]. Most of the d-block elements can be used for preparing MOFs as they have vacant d-orbitals which are used for interaction with other molecules and atoms. In order to generate metal frameworks with excellent separation and adsorption properties it is essential to understand the nature of interaction between the host and the guest entity. Transition metals play vital role in interaction between open metal sites and the guest molecule, as they maintain the balance between various important factors like van der Waals forces, hybridization of orbitals, Pauli repulsions and electrostatics. According to the results published in literature [43] it is found that MOF-74 shows highest binding enthalpy with vanadium forming V-MOF-74 which resulted in enhancing the bonding interactions between gas molecules and OMS. 7.2. Pre-synthetic procedure: MOF and separating gas (CO2 ) can be improved by implying ligand functionalization. Variety of various functional groups and the ease of modification makes this most suitable and applicable for gas storage purpose. CO2 molecules exert quadrupole moment, hence when the framework contain polar groups it will show higher interaction towards guest gas molecules carbon dioxide over other gases. There are many ways for ligand functionalization mentioned below: 7.2.1. Nitrogen site opening: Usually presence of ONS i.e. open nitrogen sites goes in favor of enhancing MOF properties of selection and adsorption of carbon based gases, examples include tetrazole. When CPF-6 MOF is coordinated, the tetrazole ligand having high amount of uncoordinated nitrogen sites, formed a compound that possess high adsorption for CO2 [44] even in absence of OMS (open metal sites). The adsorption ratio varies with different open nitrogen sites, in some cases their presence enhances adsorption while sometimes decreases the same. To elucidate, MOFUTSA-49 [Zn(mtz)2 ], the tetrazolate ligand that is attached to it contains open nitrogen sites via (5-methyl-1H-tetrazole) and CH3 groups. The interaction between zinc metal and mtz− ligand leads to formation of 2 free nitrogen which are present adjacent to the methyl groups hence the remaining uncoordinated nitrogen on the pores of surface will help in efficient capturing of CO2 that is found to be 13.6 weight percent at 1bar and 298 k [45]. Another class of ligands involve adenine groups because of number of nitrogen groups presents at various positions allow variety in MOF structure; also, adenine groups are coplanar hence leads to π –π interaction between ligands [46]. Furthermore, another study reported the effects of size of chain length of ligand attached to the MOFs for carbon dioxide capture, results states that larger the size of chain lower will be the adsorption due to chain entanglement leading to less OMS [47]. 7.2.2. Functionalization via amine groups: High affinity of amine ligands for carbon dioxide ligands make them the first method to opt for CO2 selectivity and adsorption. It is evident that amines fall in category of Lewis bases while carbon dioxide act as Lewis acid [48]. According to the results published by Pachfule [49] he discovered two amine functionalized MOFs i.e. Cd-ANIC-1 and Ca-ANIC-2 where ANIC stands for 2-amino-isonicotinic acid. The capacity for capturing CO2 gas in each case is very high that is found to be 4.72 mmol/g and 4.22 mmol/g respectively.

10.7 Methods to enhance CO2 adsorption

221

It has been found that along with merits possessed by amine groups in capturing of gases there are some demerits as well, according to which whenever a high amine group containing ligand is attached to the metal framework the excessive cluster of these ligands will cause interlaces on the surface of framework leading to reduction in adsorption capacity. Also, chain length of amine groups needs to be considered in context to the same research team [50]. Experiments by using different chain lengths, the results suggested that groups with shorter length shows greater adsorption in comparison to the other with larger chains. 7.2.3. Sulfonates and phosphates functionalization: With relation to adsorption enhancement along with stability of frameworks the above mentioned two groups need to be taken into account. They enhance framework stability towards water at a considerable rate [51]. It has been found out that presence of water in MOFs impact their stability towards adsorption of gases [52]. According to the first study carried out by marco, he synthesized two phosphate ligand coordinated MOF, [(Cu3 (H2 L2 ) (bipy)2 ·11H2 O), H2 L2 and [N,N,N΄,N΄tetrakis(phosphonomethyl) hexamethylenediamine]. The results explained that the above MOF showed high carbon dioxide absorptivity over N2 gas i.e. 77 cm3 /g at 10 bar Reasons stated for selectivity includes quadrupole moment of CO2 causing attraction to ligand molecules and no vacant pores were left to absorb N2 . On the basis of other publications, CALF-30 and CALF-25 [53] are two MOF which have shown no structural changes even when exposed to water conditions (90 percent) humidity at 298 k and 273 k respectively, this may be due to hydrophobic framework which is generatesd due to steric groups present on the ligand. According to another recent study [54], MOFCALF-33 found to proliferate water stability and adsorption due to the ester groups present on the ligand which increases the active sites for trapping of gas. 7.2.4. Functionalization using other groups: groups like hydroxyl, carbolic, alkyls and many more can be used for enhancement of adsorption of gases on the surface of MOFs. Examples from the studies include functionalization of ligand [Zn3 (bpdc)3 (bpy)].(DMF)4 .(H2 O) and produced [Zn3 (bpdc)3 (2,2’ -dmbpy). (DMF)x (H2 O)y and [Zn3 (bpdc)3 (3,3’ dmbpy)].(DMF)4(H2 O) 1 . In both the cases the volume of pores 2 and the surface area decreases in comparison to the non-functionalized MOF while the adsorption is higher in first and lower in second. This is due to the fact that, if the thrust of carbon dioxide affinity is controlled on space losses, it leads to decrease in adsorption and in second one the reason can be the elevation in affinity for CO2 is not up to the mark for surface loss and volume of pores [55]. 7.3. Post synthetic procedure: Another method for altering the adsorption capacity of metal organic frameworks for adsorption of greenhouse gas carbon dioxide is post synthetic method. The principle followed behind this method is to modify MOF with high selectivity and adsorptive coefficient with minimum energy usage. It differs from presynthetic procedure as in pre-synthetic we usually insert functionalized ligands into the already existing frameworks while in post-synthesis method there is chance of reactions between pre-existed metal sites and ligands functional groups which will make some changes in crystal structure followed by the product. Other drawbacks of pre-synthesis methods involve high attention during Solvothermal process especially due to behavior of function groups in varying reaction conditions; also, solubility of functional groups in used solvent along with hindrances caused by them leads to production of reaction side

222

10. Metal Organic Frameworks as an Efficient Method for Carbon dioxide capture

products hence, post synthesis methods [56] has proved useful in comparison to a certain extent. The most exclusive functionalized group used for post synthetic treatment is ethylenediamine symbolized as en. One example is synthesis of en-MOF-74 which shows good adsorption towards CO2 , Moreover it shows remarkable stability in various humid conditions [56]. The first method for post synthetic procedure was introduced by Gadipelli and Guo that is thermal annealing for synthesizing MOF-5 (Zn4 O[BDC]3 ). In this the framework is treated in temperature conditions which are below the decomposition temperature of the framework results in removal of solvent molecules from pore surfaces. Results shows double adsorption in context to unmodified MOF [57]. 7.4. Tuning pore size: As already explained size of pores of metal organic frameworks impact their adsorption capacity and this phenomenon is termed as size exclusion effect. The adsorption capacity of MOF depends upon the kinetic diameter of CO2 molecules and pores of surface of frameworks. Generally, molecules with diminished size can pass through the pores while others cannot. The idea diameter range for CO2 over N2 and methane is 3.3A0 – 3.6A0 . The relative size can be obtained via metal and ligand exchange. According to publications, short length ligands, small molecules of metal, heavy linker are perfect for the same [58]. This effect can be achieved via PS-exchange process. To exemplify, the exchange of metal ion in MOF UiO-66(Zr) to UiO-66(Ti) was discovered by Kim [59] and its carbon dioxide adsorption properties were evaluated by Lau [60]. The outcomes stated that due to small size of Ti than Zr the size of the pores of framework reduces which is more harmonious with the kinetic diameter of carbon dioxide gas, hence capturing enhanced two times along with increase in isoelectric heat. Another way to mitigate pore size of MOF is interlinking between different frameworks by using bulky ligands, also called as interweaving or interpenetration networking [61]. What actually happens in this case is self -assembly [62] of frameworks causing networks hence overall stability will proliferate which ultimate leads to pore size reduction hence capturing capacity will increase for adsorption of gases. When this theory was tested with some MOFs it was found that at high pressure due to reduction of pore size, adsorption capacity decreases while at lower pressure it increases due to availability of open sites. According to publications, a new 2-folded Cd-MOF was synthesized by Qin and his team who found out that this framework shows great selectivity of carbon dioxide [63] gas over methane. Furthermore, Geo and coworkers represented another 4-fold interpenetrated network framework i.e. MMPF-18. This MOF at ambient temperature showed good adsorption selectivity for CO2 [64].

10.8 Methods to enhance MOF stability As the practical applications of MOFs is burgeoning, the most important factor is stability of MOFs. There are different ways to improve the MOF stability i.e. by modifying ligand configuration, coordinate bond property, and metal node characteristics. The methods which will be discussed in this section are: 1. Chemical stabilities 2. Thermal stabilities 3. Mechanical stabilities

10.8 Methods to enhance MOF stability

223

FIGURE 10.6 Chemical stabilities of MOFs.

10.8.1 Chemical stabilities Despite the fact that many MOFs are sensitive to medium atmospheric environment which leads to structural degradation because of the coordination bonds between ligands and metal ions. There are two approaches to improve the chemical stabilities. First is to improve the stability of unknown MOFs and second one is to enhance the stability of known or existing MOFs [65] as shown below in Fig. 10.6. i. Improving the stability of unknown MOFs: It is believed that there are two reasons for destruction of MOFs i.e. breaking of metal ligand bond and formation of more stable product compared to starting MOFs. Factors on which chemical stability of MOFs depends are basicity, internal structure of MOFs, charge density of metal ions, configuration, and connectivity of metal ions and hydrophobicity of ligands.

r High-valent metal containing MOFs: According to hard soft acid base principle, in the similar coordinate environments, hard acids also known as high-oxidation states metals which have high density of charge such as Cr3+ , Fe3+ , Zr4+ , Al3+ etc., [65] which form the coordinate bond ligand and act as donor ligands known as hard bases which form MOFs with good coordination bonding and resulting in chemical stability. To balance the pKa value (carboxylate linkers have low pKa value) high oxidation states metal ions coordinate with great attachment to stabilize the charges. It results into a large amount of connections of metal clusters which also improve the chemical stableness of metal organic framework. r Low-valent metal containing MOFs: Except from the high oxidation states metal ions, soft acids also known as low valent metal ions such as Ni2+ , Zn2+ , Ag+ , Co2+ ,Fe2+ etc., form the

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10. Metal Organic Frameworks as an Efficient Method for Carbon dioxide capture

balanced magnetic organic framework with N-containing linkers considered as soft bases. Azoles large value of pKa and high coordination bonds represent the stability of MOFs in basic solutions. There is directly proportional relation between the pKa value of N-donor ligand and stability of MOFs under the humid conditions. Example – zeolite imidazolate framework (ZIF)−8 also known as MAF-4. r Mixed metal MOFs: A new strategy for enhancement of the chemical balanced metal organic frameworks (MOFs) is to introduce the more than two different metal ions into the MOFs. These MOFs are more stable as compared to single metal MOFs [65]. Different reasons can be considered for this like resulting in strong coordination bonds, increasing the metal clusters inertness and improve the surface hydrophobicity [66]. Its main principle is to replace the existing metal ions to inert species. For example, in MOF-5 the Zn2+ metal ion was replaced by Ni2+ to improve the hydro stability. r Hydrophobic ligand: Hydrophobic groups are attached or created near the metal nodes so that hydrophobic interaction properties of metal MOFs are improved by reducing the affinity of MOFs towards the water or any moisture [65]. The fragile coordinate covalent bonds from the strike of hydro molecules are protected by the hydrophobic functional groups by placing near the metal clusters and it improves the stability of MOFs in water. r Interpenetrated framework: Term interpenetration also known as framework catenation is an alternative method to enhance the MOFs stability. The presence of free spaces in a larger amount in extended porous frameworks leads to high energy which results in instability of MOFs. Interpenetration is defined as the entanglement or interweaving of the two or more frameworks which can be identical or independent [65]. Interpenetration reduces the pore size and increases the wall thickness of the framework. Interpenetration does not allow the movement of the ligand by fixing it on one place within the framework which leads to more stable MOFs structures formation [66]. ii. Improving the chemical stability of existing MOFs: The method used for improving the chemical stability using unknown MOFs has some limitations so another strategy were introduced i.e. enhancing the chemical stableness of already known MOFs. This has major four approaches which includes, post synthetic modification, post synthetic exchange, composite fabrication and hydrophobic surface treatment [65].

r Post-synthetic modification (PSM): PSM is an important technique for the formation of new MOFs from the existing MOFs to better the chemical properties of the parent metal organic frameworks. Combination of metal clusters and organic linkers in MOFs offers major chances to a wide scope of chemical modifications. It states that inserting a specific functional groups through the (PSM) method can change the pore conditions of MOFs [67]. PSM is further classified into four parts i.e. metal based modification of MOFs, ligand based, metal and ligand based and guest based [70]. Metal based modification of MOFs includes the metal exchange where breaking of the coordinate bonds and formation of new bonds takes place, epitaxial growth on the surface, metal incorporation or also known as metal doping. Ligand based modification of MOFs includes ligand exchange in MOFs, ligand installation and ligand labialization or ligand removal [70].

r Post- synthetic exchange (PSE): It is the method which can improve both chemical and physical properties of MOFs through linker exchange in which the replacement takes place

10.8 Methods to enhance MOF stability

225

of counter ions present in MOFs and metal ion metathesis. The process PSE has the ability to expand the toughness of bonds of unstable SBUs coordinate covalent bonds and adjust the MOFs water protection without disorganizing the framework structure [65]. r Hydrophobic surface treatment: The introduction of functional groups in MOFs, the adsorption and porosities properties gets reduced. Hydrophobic surface treatment technology was created to solve this issue. To prevent the MOFs from water this technique is used on the exterior plane of MOFs that leads to protection of porosity to a greater extent. Hydrophobic surface treatment included three processes, post synthetic annealing, post synthetic surface moderation and surface coating. Surface water repellent coating protect from the attack of water for example (PDMS) polydimethylsiloxane coating improve the stability of MOFs [65]. r Composite: the compatibility and porosity of MOFs are the two factors which allows the hybridization of MOFs with different materials like graphite oxide, polymers, carbon nanotubes etc. [68] by inserting the MOFs on the pores exterior either inside the holes or by inserting different substances into the left out space of MOFs resulting in MOF composites. MOF composites can be prepared with merged property or show new outcomes, hydrostability, hydrophobicity, mechanical properties etc.

10.8.2 Thermal stabilities Breakage of node and linker bond resulting in thermal deterioration of MOFs is the most common cause which is followed by linker combustion. As a result, number of linkers and bond strength of node-linker are both related to thermal stability. Melting, amorphization, graphitization or linker de-hydrogenation (anaerobic), node-cluster dehydration are all example of thermal degradation. The degradation products are useful materials in the linkergraphitization process. The majority of MOFs contains divalent cations for example: Cd(II), Co(II), Zn(II), Cu(II) and N-donating linkers or carboxylate. Higher valency metal centres for example Ti(IV), Zr(IV), Al(III), Ln(III) [69] with the presence of oxyanion terminated linkers improves thermal stability as we increases the metal ligand bond strength. Changing the composition of linker pendent groups is another technique to enhance thermal stability, increasing bond strength. Framework catenation or interweaving or interpenetration of networks can improve the thermal stability fostering positive framework-framework interactions. As we increase the crystal density the absolute energy of polymorphs decreases according to computational results [69]. By applying mechanical stress more stable compounds can be form by changing MOFs phase. Catenation process can also be used but not all the time. It is useful for limited application. Catenation can improve the intriguing properties of MOFs for example huge pore columns. Despite the fact that this has yet to be investigated, by changing the MOF node structure, thermally driven dehydration occurs at temperatures below those for framework disintegration and it affect directly on the MOFs structures catalytic activity. Direct loss of water ligands or by deflating the hydro ligands into molecular water and oxo ligands can both cause dehydration [69]. There are two types of techniques which can measure the thermal stability of MOFs i.e. in situ PXRD and thermogravimatric analysis (TGA). PXRD gives the more detached information, but TGA is used for screening.

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10. Metal Organic Frameworks as an Efficient Method for Carbon dioxide capture

TABLE 10.1 Some MOFs used for CO2 capture. Name Of MOFs

CO2 Uptake

Pressure

Temperature

References

MIL-102

3.4

3 MPa

298 k

[74]

Zn2 (ndc)2 (dpni)

4.3 mmol/g

1750 KPa

298 k

[74]

MIL-96

3.7 mmol/g

3.5 bar

303 k

[75]

MIL-53-Al, Cr

10 mmol/g

30 bar

304 k

[76]

ZIF-20

70 ml/g

760 torr

273 k

[77]

Cu(fam)(4–4’-bpe)0.5

100 ml/g

760 torr

195 k

[78]

MIL-53

7.5 mmol/g

20 bar

304 k

[79]

Cu(dhbc)2 (4–4’-bpy)

70 ml/g

.4–8 atm

298 k

[80]

Ni2 (cyclan)2 (mtb)

57 ml/g

1 atm

195 k

[81]

Ni-MIL-74

1 percent

1 bar

298 k

[82]

UiO(bpdc)

8 percent

1 bar

303 k

[83]

CPM-33b

1 percent

1 bar

298 k

[84]

DGC-MIL-101

1 percent

1 bar

298 k

[85]

Co/DOBDC

2 percent

1 bar

298 k

[86]

HKUST-1

1 percent

1 bar

313 k

[87]

ZIF-7-R

8 percent

1 bar

303 k

[88]

10.8.3 Mechanical stability Another important factor is mechanical stability of MOFs for industry and practical applications. Under vacuum, MOF pore structure changes its phase or partial collapse of pores occurs which leads to the instability [71]. To avoid these kinds of problems two methods were introduced i.e. solvent evacuation and solvent exchanges. By exchanging the solvents of higher surface tension with the lower surface tension ones which includes n-hexane, CH2 Cl2 , and liquid CO2 followed by solvent removals helps in activating the MOFs [73]. Under mechanical pressure, MOFs show low stability. Another way to improve the mechanical stability is based on the connectivity of the bonding topology. A major role play in mechanical stability is stiffness/hardness of the materials which can be defined by the non-bonded interactions and introducing functional groups which leads to extra framework connectivity through non-bonded interactions [72]. Non-bonded secondary network interactions can improve the mechanical stability of MOF. According to the researcher’s, mechanical stability of MOF (nano porous material) can be improve by two strategies i.e. by modifying the secondary and primary network. Table 10.1 summarizes the various MOF’s used for CO2 capture.

References

227

10.9 Conclusion Research from last 20 years proved to be useful in alleviating the concentration of harmful greenhouse gas carbon dioxide with the help of metal organic frameworks. They are the best opted option till now for the same due to their functionalities which includes excellent surface areas, tunable size of pores, disparity in structures, and their eco-friendly synthesis methods. MOFs proved to be the excellent material for capturing of gases over other zeolites and active carbon compounds due to the possibility of changing its properties via making alteration in the metal ions and organic ligands. Functionalization of frameworks via altering ligand properties by changing attached groups proved to enhance the carbon capturing capacity of MOFs to a considerable value and also enhance their selectivity towards the same. Various research has been going on how to augment the open metal sites by reducing the entanglement caused by large chains to burgeon the adsorption capacity of these frameworks. This area of study needs to be explored more because of its existing potential applications in order to find the solutions of various problems in field of adsorption of gases, sensors, batteries, and many more.

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C H A P T E R

11 Industrial carbon dioxide capture and utilization Uzma Hira, Ahmed Kamal and Javeria Tahir School of Physical Sciences (SPS), University of the Punjab, Lahore, Pakistan

11.1 Introduction Despite the rapid growth of alternative and unconventional energy sources, fossil fuels remain the world’s principal energy source, and their replacement seems unlikely in the future decades [1,2]. CO2 emissions are steadily increasing as a result of increased nonrenewable fossil fuel use and their industrial uses and are considered a major contributor to the escalating challenge of climate change [3]. According to Earth’s CO2 , the average amount of atmospheric CO2 has increased intensely, from 172–300 (parts per million) before the industrial revolution to 416.47 parts per million on 30th May 2020 [4,5]. The disastrous implications of ever-increasing CO2 emissions, such as global warming, climate change disasters, as well as the associated energy and environmental issues, have been a source of significant concern in recent decades. As a result, many investigations on capturing and utilization (CCU) of the carbon dioxide concept are encouraged and suggested. The capturing and utilization (CCU) of carbon dioxide case study is widely recognized as a viable approach to reducing anthropogenic carbon emissions. Capturing carbon dioxide is one of the cheapest methods to decrease the amount of carbon which is being emitted from the chemical industries. The useful compounds and oil recovery can be done by the conversion of raw substances using this captured CO2 through successive utilization processes [6]. Hence, CCU technologies are crucial for solving global emission challenges and carbon-intensive industry in general. During the last few years, substantial improvement has been made in the area of possible industrial carbon dioxide capturing and utilization (CCU) materials and their use in CO2 emission reduction (See Fig. 11.1). Due to the shortage of an appropriate assessment of the progress of favorable CCU methods, particularly from the 2017 to the present, providing a satisfactory and a brief overview of these sophisticated approaches with a thorough understanding is critical. This study seeks to offer a detailed, authoritative, and critical assessment of important CCU achievements, which has gotten a lot of attention due to its great risk of speeding up global warming. The properties and all features of solid and liquid type carbon

Green Sustainable Process for Chemical and Environmental Engineering and Science: Carbon Dioxide Capture and Utilization DOI: https://doi.org/10.1016/B978-0-323-99429-3.00023-0

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FIGURE 11.1 Industrial CO2 capturing and utilization scheme.

dioxide capturing substances were studied briefly under various operational settings, as well as prospective orders were also suggested for the improvement of their ability to capture CO2 and CO2 utilization developments in various domains.

11.1.1 Commercial capturing processes of carbon dioxide gas Carbon dioxide capture is excessively acknowledged as an important step in reducing carbon emissions from fossil fuel use [7]. To keep atmospheric CO2 levels at 350 ppm, by the end of the century, at least 550 giga tones (Gt) of CO2 will have been eliminated [8]. CO2 can be trapped in a variety of ways, for example, pre, oxy-fuel and post-combustion [9]. After burning of fossil fuels, CO2 emitted from carbon-containing materials is adsorbed during the post-combustion process. CO2 is separated from fossil fuels before they are burned in precombustion technology [10]. The oxy-fuel combustion method minimizes the amount of N2 used in the combustion process and replaces it with pure O2 [11]. Post-combustion carbon (PCC) capture needs the least amount of modification of existing facilities, making it the most efficient method for reducing carbon emissions. The low temperature based solid CO2 capturing systems and the liquid CO2 capture material system are the most often used CO2 capture systems for PCC. Meanwhile, in some sectors, hightemperature solid CO2 capture materials systems might be utilized. Liquid amine- and ionic liquid-based CO2 capture systems are the two most common types of liquid-based CO2 capture systems. Carbon, solid-amine, zeolite, alkali carbonate-based compounds are common low-temperature solid CO2 capture adsorbents. However, the low CO2 partial pressure in flue gas remains a significant separation problem, resulting in insufficient driving power for CO2 collection and a large amount of gas to be handled [12].

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The pre-combustion process may be used for a variety of industrial process like in the hydrogen power plant, and coal-fired station. The generally used solid CO2 capturing compounds can be separated into high temperature-based CO2 sorbents and medium-temperature adsorbents depending on the temperature at which certain industrial processes operate. However, the most pressing issues for present pre- and post-combustion CO2 collection include optimizing high regenerability, high CO2 reversible uptake, kinetics optimization and moisture effect.

11.2 CO2 collection systems based on liquid Given the seriousness of the worldwide heating problem, it is critical to advance costeffective and practical CO2 collection methods at the commercial level. CO2 collection Process is usually linked with CO2 /N2 separation in fossil fuel combustion exhaust flue gas. Chemical absorption, solid adsorption, and membrane separation are the three basic methods used in PCC. Chemical absorption takes advantage of CO2 ’s high reactivity to provide a high collection efficiency and rapid absorption rate. Furthermore, the use of a solvent/absorbent based on liquid in the absorption process might be simply incorporated into existing fuel-fired power stations, making it a very promising short-to-medium-term CO2 capture method. According to statistics, absorption-based procedures account for over 60 percent of PCC technologies [13]. Because of their high rate of absorption, huge CO2 capacity, and chemical/thermal durability, low viscosity, solvents based on amines, such as methylethanolamine (MEA), are the most commonly used absorbents [14]. Despite this, amine-based CO2 absorption systems have high restoration energy requirements, amine decomposition and equipment corrosion during operation. The development of innovative liquid-based CO2 absorbents has sparked a lot of attention in the scientific community. From the point of molecular engineering, a full overview of recent advancements in CO2 capture processes based on liquids, including ionic liquids and solvents based on liquid amines, materials should be provided.

11.2.1 Amine-type liquid solvents for capturing CO2 gas Most commonly used liquid absorbents for carbon dioxide capturing and utilization (CCU) are liquid amines because of their large absorption rate, low viscosity, and large CO2 potential, and thermal/chemical stability [14]. Despite this, amine-based CO2 absorption systems have high restoration energy requirements, amine decomposition and equipment corrosion during operation. The filtered gas exits the absorber with a CO2 removal effectiveness of ∼ 90 percent in most cases. Thermal fluctuation process is then used to liberate the trapped CO2 from the enriched CO2 solution at an elevated heat range between 100 and 120°C. Chemical absorption’s great selectivity allows for an almost pure CO2 as a product at the stripping exit. Amine-based absorbents have a significant advantage in CO2 absorbing at reduced pressure and dilute flue gas cascade, to make scrubbing of amine an appropriate method on a wide view, CO2 emission sources with a fixed point of emission. Therefore, due to high desorption energy consumption, chemical absorption’s reactivity offers resistance behavior. The energy generally utilized in the CO2 inclusion/exclusion process (See Fig. 11.2) covers reboiler renewal energy, electrical energy is provided to the pumps and compressor for CO2 collection process. The considerable regeneration heat required in the reboiler is the

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FIGURE 11.2 The characteristic absorption/desorption procedure for industrial CO2 capturing through using liquid solution and solvents, which is favorable for low energy consumption [15].

greatest impediment to utilizing this “wet-scrubbing” technology. There are three sources which regenerate the energy: (1) Energy absorbed or released is used for providing heat to the solvent, (2) Energy produced during reaction used for CO2 capturing, and (3) latent heat used for solvent evaporation step, in a particular example, water is used for an aqueous amine solvents. The inherent features of amine solvents are represented by these three sections. Low regeneration energy consumption is a benefit of the solution’s qualities. Low stripping volume, high cycle capacity and low heat capacity are the first three requirements. A solvent with a high cycle capacity usually has an enlarged assimilation potential capacity for the assimilation process and a high dissimilation rate potential for dissimilation process. Secondly, for the reduced reaction energy, a reduced enthalpy rate for CO2 desorption is advantageous. The low value of reaction enthalpy, on the other hand, may be linked to a high stripping-steam needed and a high value of latent heat. Thirdly, getting low latent heat requires a small amount of stripping-steam and low evaporation enthalpy. When researching new solvents, various factors such as equipment corrosion, amine decomposition, reaction kinetics, cost, viscosity, and environmental friendly options should be considered [15] (Fig. 11.3).

11.2.2 Basic working principle of absorbents based on liquid amines Methylethanolamine (MEA) has been used commercially as the most robust amine absorbent, to capture carbon for over 50 years. Moreover, many other liquid amines, such as N-MDEA and DEA, are also commonly used and studied. These materials use a reversible process to absorb and desorb CO2 . The CO2 absorption performance is mostly determined by the chemical structure of the chosen amines. The quantity of hydrogen atoms bound to nitrogen atom in amines are divided into the three categories: PA (Primary Amine) with –NH2 ligand. SA (Secondary Amine) with the –NH ligand TA (Tertiary Amine) with the –N ligand

11.2 CO2 collection systems based on liquid

235 FIGURE 11.3 Representation of the probable reactions happening in the molten alkalibased metal nitrates-covered magnesium oxide.

In the given situation, binding energy of certain liquid based-amines differs depending on their basicity. The following is the order of binding energy between CO2 and amines: PA < SA < TA

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The net reactions of different amines with CO2 are as follows: a) For primary amines/secondary amines: CO2 + 2R1 R2 NH  R1 R2 NH2 + + R1 R2 NCOO– b) For tertiary amines: CO2 + H2 O+R1 R2 R3 N  2R1 R2 R3 NH+ + HCO3 – R1 and R3 represent to the alkyl/alkanol group whereas for primary amines, R2 denotes the hydrogen atom, whereas for secondary and tertiary amines, R2 denotes the alkanol/alkyl substituent. The zwitterion reaction mechanism is widely assumed to govern CO2 absorption in primary amines and secondary amines [16,17]. Following the CO2 reaction, these amines create a carbamate, due to a strong C–N bond formed in the carbamate, it has a quick absorption rate but a high regenerative energy [18]. The enthalpy value of absorption reaction of the CO2 for PA is typically 80.0–90.0 kJ/mol CO2 and 70.0–75.0 kJ/mol CO2 in secondary amines. A base-catalyzed hydration process absorbs CO2 into tertiary amines, in which tertiary-amines increase CO2 hydrolysis to bicarbonate production, as a consequence, the CO2 capability is large and the regenerative heat is low, but the absorption rate is very sluggish. Furthermore, the value of enthalpy of CO2 absorption rate in tertiary-amines ranges from 40 kJ/mol to 55 kJ/mol. As a result, there is a barter in rate of absorption and regeneration energy for CO2 collection with help of single amine. Absorption of CO2 by a single amine is plainly insufficient to produce all of the desired outcomes. One of the most successful approaches for CO2 extraction from energy plants that ignite natural gas or coal is the absorption process employing liquid amine solvents. Nonetheless, amine-scrubbing techniques encounter a number of difficulties. Due to demand of high energy for the regeneration of solvent, most significant disadvantage is the high running cost, regardless of the fact that it is the cheapest PCC technology available [19]. During carbon capture, the energy-intensive regeneration process has 60.0 percent higher consumption of the total energy. MEAs renewal energy (almost 4.0 GJ t1 CO2 ), in instance, far surpasses both the theoretical and tolerable limits (0.40 and 0.72 GJ t1 CO2 , respectively). Second, amine degradation is seen as a significant impediment to CO2 collection utilizing absorbents based on amines, because it increases the overall CO2 collection cost by 10 percent of total and creates possible secondary pollutants, it is not recommended. Furthermore, several decompositions of the products have been demonstrated to enhance corrosion in stainless steel and carbon steel systems. Some of the amines and amine intermixes with high degree resistance of disintegration have recently been explored to reduce operational costs and amine loss caused by amine decomposition. Several other inhibitors can also be utilized as oxygen scavengers to slow down the breakdown of amines. Third, the corrosion of the equipment is a problem in the absorption process of CO2 because an electrochemical reaction is involved with wet CO2 , which limits life expectancy of machinery and even creates unplanned plant downtime and safety mishaps. Amine-scrubbing apparatuses, which are often composed of carbon based-steel, are easily corroded, especially in high-temperature locations like the regenerator and heat exchanger. CO2 concentration,

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temperature, specific breakdown of the products, the amine utilized, and the oxygen content all influence corrosion rates. Furthermore, heat stable salts (HSS) are typically generated when amines react irreversibly with degradation of the materials or the other contaminants in the input gas [20]. They are unable to soak up CO2 or disintegrate along with the heating solvent renewal process. The accumulation of HSS in the mechanism of absorption can cause a variety of operational concerns, including a loss in CO2 capacity, excessive corrosion, foaming, and fouling [21]. In general, the most common procedures for removing blended HSS from used weak amine absorbents are thermochemical reclamation (distillation), electro-dialysis (ED), and ion exchange. Thermal reclamation is the distillation process of the separated amine which is away from the bottoms that contain HSS. ED uses a DC electric field to separate HSS that requires no extra substances. Thus, it is an energy-saving technique having a tiny imprint [22]. Nonetheless, in the DC field, the ED process suffers from significant amine loss. HSS anions can be changed into hydroxide ions using ion (basic) exchange resins that contain OH– from KOH or NaOH. This maintains the lost amines’ capacity to absorb CO2 . However, this procedure uses a lot of hydroxides of sodium or potassium (NaOH or KOH), and the resulting useless water that is alkaline, pollutes the environment severely [23]. As a result, these restrictions encourage more research into chemical absorption for CO2 capture.

11.2.3 Advances in amine-type liquid absorbent materials The principal disadvantages of the amine-exfoliating method, namely high renewable energy, amine disintegration, and machinery erosion, are induced by intrinsic properties of the particular amines, including poor thermal permanency, destructive character, and more CO2 desorption enthalpy value. As a result of these factors, there has been a lot of research into finding alternative solvents to conventional amines. The development of absorbents based on liquid amines have received the most consideration due to the good CO2 absorption capability of amine solvents. Several absorbents based on liquid amines, such as solvents of mixed amines, solvents based on non-aqueous amines, and bi-phasic solvents, have been developed to improve amine scrubbing’s carbon capture ability and eliminate the downsides. Additionally, catalyst-assisted regeneration is offered as a feasible method for lowering CO2 desorption below 100 o C while also reducing energy demand. As a result, the next section of this chapter contains a full status report on these four types of absorbents based on liquid amines.

11.2.4 Mixtures of amine solvents A mixture of all types of amines have been created to solve the constraints of employing a single amine for CO2 capture. The advantages of different amines include their rapid absorption rates (originating from primary/secondary amines), low extrusion reaction enthalpies (originating from tertiary/sterically-hindered amines), and high absorption capacities, can be reconciled by blended amine solvents, resulting in a novel amine-based regeneration CO2 astringents. A bi-amine mix is created by combining specific primary/secondary amines with dissolved tertiary amines to achieve optimal efficiency. Blending a little amount of MEA with aqueous MDEA, for example, increased CO2 absorption accelerates MDEA while lowering

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heat recovery as compared to solo MEA. Furthermore, raising the MEA/MDEA ratio can improve kinetics of reaction and mass transit rates. Increasing the number of components to generate tri/quad-amine integrates may aid to maximize CO2 intake/release efficiency while reducing regeneration heat. The overall performance of blended amine solvents incorporating multi-components has shown considerable increases in reaction kinetics, Stability, solubility of CO2 , corrosion problems, mass transfer issue, and regenerative heat. CO2 absorption response mechanism is complicated because of the intricate constituents of mixed amine absorbents. Liu, et al., applied the concept of mixed amines based on non-aqueous solutions and used 13 C NMR to compare the reaction mechanisms of (TETA–AMP) in both non-aqueous solvents and aqueous solvents [24]. During CO2 absorption, carbamate was produced initially, followed by CO3 –2 /HCO3 – ions. As a result, CO3 –2 /HCO3 – was absorbed subsequent to desorption of carbamate. In contrast, the non-aqueous counterpart generated carbamate and C2 H5 OCO2 . 11.2.4.1 Amine-based solvents that are non-aqueous The deionized water in a conventional aqueous amine absorbent causes a number of issues. Its large heat capacity and vaporization heat make energy-efficient regeneration difficult. Furthermore, aqueous conditions are undoubtedly linked to equipment corrosion. Novel possible non-aqueous absorbents have recently received a lot of scientific attention. Several organic compounds, such as EG (ethylene glycol) and di-EG, can be used as water substitutes because of their poor heat potential, stripping-temperature value and enthalpy of evaporation [24]. Because of the comparatively poor heat potential and the value of enthalpy of evaporation for organic solvents, the majority of the needed regenerative energy is utilized to decompose the CO2 -related materials, whereas sensible-heat value for solvents and heat of vaporization for solvents are greatly reduced. As a result, non-aqueous amine solvents allow for lower energy use while keeping aqueous amines’ CO2 absorption reactivity [25]. Furthermore, low range of stripping-temperatures can decrease losses of amines, oxidative and thermal decomposition, and corrosion of machinery when compared to typical aqueous solvents [26]. New non-aqueous absorbents with higher performance have recently been the focus of intense research efforts. The non-aqueous solvent AMP–AEEA–NMP, for example, was produced with an influx of 1.65 mmolg–1 for CO2 [26]. With a total regenerative heat of 2.09 giga-Joule per tones of CO2 , or roughly half that of the aqueous 5 molar MEA solution, NMPs poor heat potential was appropriate for its low value of heat and its low value of enthalpy of evaporation was appropriate for its lower latent heat. Through the production of carbonate species, 2-piperidineethanol–ethylene glycol a non-aqueous solvent recently achieved potential of 0.97 mol of CO2 mol–1 and CO2 desorption occurred at 50 °C, presenting a possible replacement for conventional amine solvents [27]. The CO2 absorption feature of many absorbents, which were non-aqueous and aqueous, was compared by Barzagli et al., [28]. In the amine/CO2 /solvent systems, 13 C NMR spectroscopy was found to be a useful approach for determining reaction mechanisms and distribution of species. In the aqueous solution, There was the creation of carbamate and bicarbonate Only carbamate has been produced in non-aqueous conditions. In non-aqueous conditions, the absorption rate was higher and the CO2 loading was lower. in place of water, two different types of glycol-ether have recently been used to dilute methyl ethanolamine [29]. Therefore, the solvents which were non-aqueous, only carbamate was formed, while 13 C NMR revealed both carbamate and bicarbonate. Non-aqueous solvents outperformed

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aqueous 5 M MEA in terms of desorption effectiveness, vaporization heat, and cyclic capability of 1.45 mmol g–1 . As a result, when matched to the aqueous 5 molar methylethanolamine classification, the non-aqueous solvents low value of regenerative energy by 55 percent. The viscosity of amines which was non-aqueous, however, showed a dramatic increase in value when CO2 is added. The quick increase in viscosity during absorption of CO2 in non-aqueous absorbents based on amines has been identified as a critical parameter for commercial use, as it impairs mass transfer and liquid transportation. Liu et al., [30] projected a new technique to minimize the Carbon- dioxide mixed in non-aqueous solution viscosity based on the structural correlation. As non-aqueous CO2 capture solvents, a series of ethylene di-amine derivatives were developed. Lower viscosities and high regeneration characteristic were achieved in range of 50–80 °C. Furthermore, using organic-diluent having high volatility alcohols might result in significant solvent loss [31]. Some non-aqueous solvents based on amines have also been found to have poor regeneration ability without N2 sweeping. Various small-scale experiments are needed to evaluate amazing total carbon sequestration capability before utilizing non-aqueous solvents based on amines in industrial applications. 11.2.4.2 Biphasic solvents Biphasic solvents have caught a lot of attention because of their low stripping volume and low regeneration energy consumption [32]. They are classified as: liquid & solid and liquid & liquid biphasic-based solvents that depend on the CO2 -rich transition state. The homogenousabsorbents may be divided into two separate layers by CO2 absorption or temperature change. The CO2 -lean layer is one, and the CO2 -rich layer, contains the large quantity of the captured carbon dioxide gas. Only the CO2 -rich transition state, which is related with a flow of restricted volume and very high concentrated CO2 , requires a stripping-process, allowing for lower heat of vaporization, value of sensible heat, and CO2 pressure. As a result, a much more effective regenerating procedure, and less CO2 work on compression might be achievable [33]. Using the thermomorphic biphasic solvent DMXTM , IFP Nouvelles Sources of power initially created a DMX-process. The desorption heat is only 2.1 GJt–1 CO2 , whereas the reproduced heating for the traditional 5 molar MEA material is up to 4.0 GJ t–1 CO2 [34,35]. Mixed amine-based biphasic solvents, which typically contain all types of amine, have recently received a lot of attention. The absorption accelerator in the CO2 absorption reaction is a primary/secondary amine-based –NH2 or –NH groups, such as MEA and DETA, which forms protonated-amine and carbamate with high value of ionic strength. Liquid–liquid phase separation is aided by unreacted tertiary amines with a high hydrophobicity, for example diethylenthanolamine (DEEA), N,N-dimethylcyclohexylamine. Blended-amine solvents such as DEEA/DETA, TETA [36], and pentamethyldiethylenetriamine/ DETA, for example, showed change in transition state after CO2 absorption. The MEA–DEEA and AEEA–DEEA blended amine-based biphasic solvents have a large cyclic potential (∼ 0.64 mol of CO2 mol–1 ), a quick split time of transition state, and a low value of regenerative energy (2.56 gigajoule per tones of CO2 ). Higher value of the viscosity is frequently linked with CO2 mixed in biphasic solvents, particularly because the CO2 -rich phase is highly concentrated. With the use of AMP, the excessive viscosity might be reduced [37]. Furthermore, the mixed amine biphasic solvent had an abnormal strong CO2 injection and a good value of phase volume ratio, which was harmful

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to lowering regeneration energy usage [38]. Later, (1-methyl-2-pyrrolidinone) chemical was utilized to partially replace H2 O in the mixed amine biphasic solvent to improve the phasesplitting property [39]. Some organic solvents, for example 1-propanol and sufolane, have recently been observed to cause amine absorbent phase separation because of hydrophilicity differences [40,41]. The mechanism/procedure of carbon dioxide absorption in the biphasic solvent methyethanolamine–sulfolane is depicted. Sulfolane facilitated phase splitting of the methyethanolamine solvent during CO2 absorption, producing a hydrophilic CO2 -enriched stage and a hydrophobic CO2 -depleted phase. The CO2 absorption rate increased due to the significant interaction between sulfolane and CO2 . Furthermore, the regeneration energy needed to remove such a CO2 -rich stage was restricted to ∼ 2.67 GJ/t CO2 . Sulfolane was also used to fine-tune the DEEA–TETA solvent’s phase-splitting characteristics [42]. The addition of sulfolane to the absorbent increased the hydrophobic composition, lowering the percent volume of the hydrophilic CO2 -enriched state[42]. The quantity of energy prerequisite to regenerate the CO2 -enriched stage was decreased to 1.81 gigajoule per ton of CO2 . Developing innovative low regenerative heat biphasic solvents, quick rate of absorption, big CO2 absorption potential, low value of viscosity, higher stability, and lower value of volatility has remained a key problem to date. Furthermore, existing CO2 absorption systems utilizing biphasic solvents must be optimized in order to decrease energy intake and operational budgets for CO2 arrest. 11.2.4.3 Regeneration with catalysts Catalyst-assisted regeneration is currently in its early stages of development as a new technique. Catalysts for solid acids, such as metal-oxide compounds and molecular sieves with functional properties, have been added into absorbents based on amines in this scenario to enhance carbamate degradation and at that time eject the absorbed CO2 at lower temperature [43]. As a result, solvent regeneration at lower temperatures was possible, resulting in significant reductions in stripping steam and regeneration energy. Furthermore, some disadvantages linked with higher temperatures, including corrosion and decomposition, would be alleviated. Various solid- acids have been designed and tested for their catalytic desorption capability CO2 at lower temperature values and decrease the energy intake in energy recovery. The γ -Al2 O3 and solid-acid (HZSM-5) have been shown to successfully maximize CO2 rate of desorption and reduce regenerative energy. For solid catalysts to enable CO2 desorption, acid power, and the Bronsted to Lewis-acid ratio are also crucial. The TiO(OH)2 nano-catalyst has recently been shown to improve the rate of carbon dioxide desorption from the absorbent methyenthanolamine by more than 45,00 percent, allowing for desorption at a lower value of temperature [41]. The CO2 desorption efficiency, cyclic capacity, and desorption rate were reported to improve by 32 percent, 56 percent, and 54 percent, respectively, with ZrO2 and ZnO [43]. At 80 °C, Ag2 O–Ag2 CO3 showed better catalytic activity in solvent regeneration, increasing CO2 desorption rate by 1000 percent [44]. Metal-oxide compounds and molecular sieves with metal modifications are among the solid acid catalysts produced by Liang and Tontiwachwuthikul’s group. The energy required for regeneration was lowered by up to 40 percent [45–48]. The basic and acidic sites, mesopore shape, and more surface area to volume ratio are the essential catalyst characteristics that impact CO2 desorption performance. Strong basic and acidic sites, in particular, could

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considerably improve the catalytic mechanism. The catalytic process benefits from a higher value of Bronsted to Lewis acid ratio. The massive mesoporous SSA also boosts the active site availability for catalytic reactions. Furthermore, catalytic activity has been found for catalysts with two sites containing both basic and acidic sites, for example SZMF [45–48]. The SZMF catalyst has been presented as a potential desorption pathway. The MEA regeneration process was controlled by the zwitterion mechanism, which consisted of two steps: carbamate breakdown and protonated amine deprotonation. The metal atoms such as Fe, Zr and acidsites, on the other hand, assisted the transition of MEA-carbamate to MEA and CO2 . Lowtemperature CO2 desorption was made possible by the innovative method, which was energyefficient. The catalyst-assisted regeneration method has a lot of promise for lowering the necessary regeneration heat duty. However, being a newly created strategy during early stages of progress, catalyzed regeneration research area is confined to a few different types of catalyst compounds, certain of them are costly, hard to detached, and unstable, making catalyzed CO2 desorption a pipe dream. Designing solid acid catalyst compounds with good catalytic activity, higher stability, cheap, and facile fragmentation is critical. Furthermore, future industrialization will require the development of a viable the absorption–desorption cycle using solid-acid catalysts.

11.2.5 Overview and prospects for liquid amine-based absorbents CO2 capture through liquid amines, finding new effective absorbents and designing a CO2 capture method with adequate heat integration remain important problems. Blended amine solvents, which are among the promising solvents based on amines under study, are maintaining good CO2 adsorption capability, including quick absorbing speed and low regenerate heat, they are viable for utilization in commercial application in the coming future. Furthermore, the classic amine absorption procedure may be employed virtually straight without modification. Blended amine solvents have been validated as viable alternatives to conventional single amines in lab and pilot plant research. To investigate blended amine solvents on an industrial scale, not only performance-limited aspects like cyclic potential, desorption, Kinetics of absorption, value of enthalpy should be considered, but also various additional factors like volatility, viscosity, corrosion, toxicity, decomposition, and market value. Screening analysis on biphasic solvent systems have been carried out frequently in the last decade. They have the ability to reduce low regeneration heat without losing CO2 capacity or rate of absorption. However, the issues of higher regenerative temperature and high viscosity in combination with biphasic solvents must be further investigated. For further scale-up pilot experiments, biphasic-solvents that operate well overall, such as lower regenerative heat, lower degradation, fast absorption rate, high CO2 absorption potential, lower corrosivity, low value of viscosity, and lower volatility, are urgently needed. Furthermore, process development and optimization are critical for achieving industrial-scale low-energy CO2 capture. Conditions that are not aqueous We are still in the early stages of developing better amine absorbents & catalyst-aid production. Prior to industrial use, their viability should be assessed. Non-aqueous amine absorbents offer excellent chances to strip CO2 at lower temperatures with less regeneration heat, minimizing amine loss and equipment corrosion.

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The investigation of the CO2 absorption/desorption mechanism and solvent screening are still ongoing. The quick increase in viscosity during CO2 absorption, significant volatility while using alcohol as an organic-diluent, as well as low restoration efficiency without N2 sweeping, should also be considered. Production through catalysts is still being studied in the lab for the layout of effective catalysts and the process of catalytic desorption, but it could be industrialized in the near future. A few solid-acid catalyst materials have been suggested so far for the use only. Designing efficient solid acid catalysts with outstanding stability and cheap cost for low-energy CO2 desorption is very desirable. Furthermore, customized methods for each technological choice should be developed in order to drastically decrease energy intake and operational price.

11.3 CO2 capturing with ionic liquid solvents Various absorbents for classic solvents based on amines have been the subject of extensive investigation. ILs are organic salts with a designable organic cation and anion. They have intrinsic structural adaptability, lower corrosion, good thermal stability, low volatility, and higher solubility of CO2 [49] making them prospective applicants for CO2 collection with low value of volatility, lower corrosivity, higher degree of stability, low degradation, and low regenerative energy. The primary benefit of ionic liquids is CO2 capturing, that is why they have good potential to absorb CO2 while using less regeneration heat than solvents based on different amines.

11.3.1 Working principle of ionic liquid-based absorbents Physical absorption of functional ionic liquid is commonly differentiated based on the structural properties and the interaction between ionic liquid Atmospheric CO2 absorption through absorbents & chemical processes. Imidazolium-based ionic liquids, for example, were initially proposed as a method of capturing CO2 emissions. The fluorination of the anionic part and an increase in the alkyl group chain lengths could increase CO2 solubility in physical ILs. The interaction between ILs and CO2 (van der Waals forces) is usually minimal, resulting in lower regenerative energy but restricted absorption speed of CO2 , selectivity, and potential. Absorption process of CO2 through ionic liquids is also found to be restricted by restricted mass transfer instead of solubility of CO2 , because of their high viscosity [50]. Physical ILs have a number of disadvantages that make large-scale industrial application difficult. In physical ionic liquids, mass transfer might be solved by changing suitable anions like [DCN]−1 dicyanamide, or [TCM]−1 tri-cyanomethanide, [50]. Functional ionic liquids, on the other hand, have been anticipated as a system to advance CO2 collection efficiency by adding different active groups for a synthetic absorption. Because of the functional tenacity of polymer electrolytes, it’s indeed possible to customize anion groups to achieve specific features like low value of viscosity, low value of reaction enthalpy, and higher capacity for CO2 . Their absorption enthalpy, in particular, may be easily controlled by the anion that influences the regeneration heat of ionic liquids. As a result, these chemisorption sites of functional ionic liquids showed increased CO2 absorption performance, making them suitable amine absorbent replacements. Furthermore, ILs may directly absorb

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CO2 without the use of a water as diluent agent, and the regenerative portion was streamlined as a result of the ionic liquid’s lower vapour pressure, which reduced regeneration energy even more when compared to the traditional amine-scrubbing procedure [51].

11.3.2 Advancement in ionic solvents To solve the difficulties with physical ILs stated above, certain structural alterations can be made by providing functional groupings for certain tasks. The first functionalized IL was created by substituting a primary amine moiety on the imidazolium cation [52]. IL absorbed CO2 in a stoichiometry ratio of 1: 2, just like MEA, and generated ammonium-carbamate salts. Based on carboxylate- or acetate pioneering ionic liquids for CO2 chemisorption were suggested and extensively researched. When generating novel ILs, the benchmark is frequently [Bmim][acetate]. With the carboxylate reaction product, it has a large CO2 capacity and a low reaction enthalpy. The high temperature deterioration problem and inadequate high viscosity for mass transfer, however, linger unaddressed. Amino acid-Ionic liquids have recently grabbed a lot of attention because of their nontoxic biodegradability and significant CO2 capacity. According to the findings, functional AA-ionic liquids with amino acid or an amine can enhance solubility for CO2 [53]. The long side chain chains of the amino-acid influenced CO2 potential, which was highly connected to reactive amine groups [54]. However, the CO2 -mixed in Amino acid-Ionic liquid solution has a hydrogen bond network that causes significant viscosity that may be reduced through separating the amino–based group in the anion, lowering hydrogen bonding probabilities. [55]. Higher CO2 solubility with stoichiometric ratio of 1:1, higher stability, higher rate of reaction, and AHA-Ionic liquids, which primarily have ([CNPyr]) 2-cyanopyrrole anions, are promising options for CO2 chemisorption [56]. The planned ILs’ viscosity was very low. The reaction processes between [DETAH][AHA] and CO2 are simple to comprehend, with [DETAH]+ enabling rapid CO2 absorption and [AHA] ensuring huge absorption capacity and minimal regeneration heat. The deprotonation of the alkyl chain, which was found by the basic character of the anion along with the lengthy alkyl chains, was confirmed to be a major determinant in CO2 absorption. Despite their promising CO2 collection capabilities, functional ILs are limited in their industrial application due to their low mass transfer for gas and liquid, and expensive price caused by their high value of viscosity. Co-solvent support, encapsulation, and use, in addition to changing the composition of ionic-liquid, are recommended as ways to lower viscosity and alleviate transport problems. 1-propanol–water co-solvent with Dilute double-functional ionic liquids such as ([DETAH][Tz]) resulted in the creation of a biphasic solvent [57]. The massive cost and viscosity of Ionic liquids, as well as their significant value of volatility and energy penalty, severely restrict their usage in carbon collection. The performance of amines and ionic liquids can be effectively preserved by combining ordinary amine solutions with functional ILs as activators. This technique provides a potential absorbent with lower regeneration costs than the traditional amine process along with good economics and higher absorption rates of CO2 than ionic liquids. Liang and co-workers [58] recently investigated CO2 absorption in low viscous binary-absorbents containing imidazoline ionic-liquid and amines. The blended absorbent such as MDEA–[BEIM][BF4 ] had a higher CO2 capability, low viscosity,

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and strong reproduction. Furthermore, when compared to the traditional methylethanolamine with 30 percent by weight of the solvent, the [bpy][BF4]–MEA binary solvent was observed to save 15 percent energy and 7.44 percent money. Furthermore, increasing the concentration of [bpy][BF4] would lower total energy and operational costs even more. Overall, ILs and amine mixes are attractive options for CO2 capture that saves energy and money.

11.3.3 Overview and prospects of ionic liquid-based solvents Because of their low value of volatility, higher stability, lower corrosivity, minimal decomposition, and low regenerative heat, ILs are potential candidates for CO2 absorption. However, various barriers, such as high price and higher viscosity, prevent them from being widely used. The high viscosity would generate a mass transmission delinquent and additional drive exertion for liquid transmission. To maximize the viscosity, reaction enthalpy, and cost of the ionic-liquids, task-specific ILs must be designed. To filter ionic-liquids and examine the effect of various anions and cations, molecular modelling can be utilized. Pure ionic-liquids are currently finding it challenging to compete with traditional amine solvents. By mixing the ionic-liquids with the amine solution or co-solvent, the viscosity can be reduced, and the price problem can be offset by the abridged restoration of heat. Before ionic-liquids can be used on a large scale, a thorough feasibility and economic analysis is required.

11.4 Applications, implementation and challenges At the small scale, a wide range of absorption methods based on amines have been examined. Some topologies and process integration, such as split-flow of solvent, absorber intercooling, stripper inter-heating, and flash-stripper have been thoroughly researched to enhance performance in absorption and lower demand of energy of CO2 collection from power plant exhausts [59,60]. Though the integration process would result in greater plant efficiency, the additional cost is unavoidable because of augmented intricacy of the procedure, [61] It should be technologically and economically assessed before commercialization. A promising alternative for CO2 absorption is the mixture of a solvent with a membrane. CO2 solvent placed on the permeate side can absorb permeating CO2 instantly; after the CO2 molecules have diffused over the membrane, high CO2 removal rates result. Many universities and businesses have recently conducted pilot plant tests and achieved CO2 absorption capacities of up to 80 t CO2 per day [62]. The energy penalty for regeneration is substantially smaller for URCASOLTM (DOW and ALSTOM), IHI, and RS2TM solvents than for MEA [63]. Demonstration scale experiments have recently been carried out in order to offer references for future commercial deployment. Two major technology providers such as Shell CansolvTM and MHI for large-scale CO2 capture based on amines among other PCC technology providers. At the commercial demonstration scale, the liquid-amine based CO2 capture technique has been used in the Petra-Nova and Boundary-Dam projects. Since 2014, the Boundary-Dam project in Saskatchewan, A 160 MW petroleum power plant in Canada has already been established to capture CO2 emissions. As of 2017, a 240 MW petroleum power station in Houston, USA, has been generating 1.4 million tons of Carbon dioxide per year through the Petra Nova project. Unlike the traditional CO2 capture procedure that uses an absorbent based

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on monophasic amine, few absorption procedures for recently found biphasic solvents have been created. A blending unit, such as a decanter unit, is deployed before to the stripper unit in the demixing process of the solvent (DMXTM , IFP Nouvelles Energies).The CO2 rich and CO2 lean different stages are detached in this unit, and the CO2 rich phase is revived thermally [64]. In general, liquid-based carbon capturing and storage (CCS) technology is intended to absorb CO2 from fossil-fuel-related plants. According to CO2 emission reduction targets, the demand for industrial scale carbon capturing and storage technology is increasing. By 2050, carbon capturing and storage technologies are predicted to reduce 30 percent of overall CO2 emission reductions. Despite the fact that the There is still a long way to go before CCS technology can be commercialised. constrained by economic and technical issues. The significant energy consumption related to CO2 capture is the biggest roadblock. For example, when the cutting-edge amine unit is deployed, the power station’s thermal efficiency will be lowered by 18 to 30 percent and electricity costs would climb by 45 to 70 percent, providing a substantial hurdle to CCS technology progress [64,65]. To reduce energy costs and attain a commercially viable price, liquid-based CO2 capture technology still requires significant advancements in solvent chemistry and design of the process. Furthermore, given the current high cost of CCS, its complete deployment will be impossible without appropriate governmental initiatives, for example carbon cost, an acceptable exhausts trading system, a global carbon pricing tax, performance requirements, and CO2 storage restrictions [63].

11.5 Solid CO2 adsorbents for low-temperature applications Solid amine-type adsorbent compounds are good CO2 capture materials, with good capacity at lower CO2 partial pressure ∼ 10–15%percent, low restoration temperature ( Mg > Na sequence. The greater energy of sorption of this chemical revealed that it aids in the carbonation of CaO. CO2 chemisorption, surface reactivity with O2– to generate CO3 2– , and lastly the formation of CaCO3 are all part of the CaO carbonation process. CO2 diffusion and O2– mobility are important elements in CO2 capture. Guo et al. [163] proposed a Zr–Ce-additive with improved CO2 capture of the CaO-based sorbent based on these findings. Oxygen vacancies were formed as a result of the synthesis of Ce2 Zr2 O7 , allowing CO2 and O–2 diffusion and mobility to be facilitated. Huang et al. [164] created an (oxysalt) eutectic doped CaO with a low melting point. At the initial carbonation procedure, loading Calcium oxide with alkali carbonate molten salt increased its CO2 uptake from 3.26 to 10.94×103 mol g–1 , which can be associated with the molten salts high O2– concentration and O2– conductivity.

11.12.3 Modifications in sintering-resistance For CaO-based sorbents, capacity loss is acknowledged as the most critical issue. In this sense, a great effort has gone into reducing sintering and boosting cycle stability. Incorporating CaO particles with inert materials is one of the most feasible options. Supporting materials such as Al2 O3 , MgO, Y2 O3 , CuO, CeO2 , TiO2 , KMnO4 , SiO2 , Cex Zry Oz , La2 O3 , LaAlx Mgy O3 , and CaZrO3 have been completely explored and reviewed in previous studies, but all the good works still need to be highlighted in order to disclose some new edges. According to prior studies, the performance and economy of CaO-based materials are more important. Due to the excellent stability of their derivatives such as Ca12 Al14 O33 , Ca9 Al6 O18 , MgAl2 O4 , and CaZrO3 , additives such as Al2 O3 , MgO, ZrO2 , and their combinations are chosen. Sintering resistance of stabilized CaO sorbents is determined by the degree of dispersion of inert supports. As a result, the study focus is on adopting multiple synthetic methods, such as sol–gel, co-precipitation, hydrothermal, freeze/spray drying, flame spray pyrolysis and milling, to ensure effective and efficient dispersion of the supporting components in the CaO or Ca precursors. Using various precursors synthesized using the aforementioned procedures, stabilized Calcium oxide with particular structure, shape and mixing degree can be obtained. Armutlulu et al. [165] also described an ALD technique for making shell-comprising nanoparticles having an Al2 O3 coating on CaO nanoparticles. Sintering among the particles was efficiently prohibited by enclosing hollow Calcium oxide in a thin Al2 O3 film (< 3 nm). After 30 cycles, CaO with 10 ALD cycles had a CO2 absorption of 55 wt percent (80.50 percent capacity retention), which is greater than untreated manufactured Calcium oxide (40.0 wt percent with 53.40 percent capacity retention); both samples had considerably excellent uptake as compared to limestone’s performance (11.0 wt percent with about 20.30 percent capacity retention). The good morphological properties of Al2 O3 -stabilized CaO were related to its

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great cyclic stability, and the hollow (spherical) structure was widely retained during the cyclic process. The structure and morphology of CaO, particularly the porous structure created by the calcination of organic precursors, have been hypothesized to be affected by calcium precursors. As a result, in many mixing procedures, such as calcium acetate, organic calcium has been used as the CaO precursor. Huang et al. [166] used calcium acetate as a precursor and used a simple non-solvent/solvent approach to make CaO/MgO. The CaO/MgO composite was made at a nm mixing level and showed a CO2 uptake of 59.20 wt percent after 7 cycles (with 95.30 percent of its (initial) capacity retained).

11.12.4 CaO generated from discarded materials Despite the fact that synthetic stabilized CaO has a higher CO2 uptake and is more stable, Because of its high price, it can’t be used in more constructive ways. A growing number of calcium byproducts or inert additions are being made from solid waste or minerals, both of which are economically advantageous. Metallurgical slag, eggshell, seashell, and limestone are all reasonably priced calcium wellsprings. Good example of additives that includes Si, Al, and Mg include Sepiolite, bauxite gravels, ash, fruit bunch, and cement. As a result of the use of these low-cost scrap/raw materials, solid waste management becomes much more feasible. Unlike the chemical reagent-based direct synthesis of stabilised calcium oxide, the extraction of a Ca doped and source is required prior to the formation of sorbent. Mechanical grinding, drying, and mixing are typical procedures for preparing Ca supplies and dopant precursors.

11.12.5 Granulation of powder The studies outlined above are primarily focused on the production and characterization of powdered calcium oxide sorbents. Particles in white powder cannot be used directly for implementation due to elutriation from heat exchanger and carbonator. CaO sorbents are fixed using granulation and resource that enables in the manufacture of moulding materials. While low CaO loading significantly reduces sorption capacity, the converse is also true. Another method is to extrude or spheronize calcium oxide into granules, use the mechanical forces to shape the granules into pellets. The porous structural properties of powder materials are always destroyed when using this method, however [167]. In order to increase the pore size of CaO-based pellets, cellulose [168], hemicellulose [167], plastic wastes [169], lignin [169] and urea [170] were commonly used as pore-forming templates. Polymer is the most commonly used template among these. Calcium oxide pellets were mixed with 30.0 percent microcrystalline cellulose for 20 cycles, and uptake (34.0 wt percent) was increased by 48.0 percent as compared to the untreated materials/pellets, as according Li and colleagues [168]. Porosity promoter’s burning resulted in a significant increase in the number of cavities and gas channels formed by the release of gas and removal of (microcrystalline) cellulose. In recent years, researchers have focused on improving the properties of calcium oxide (CaO), such as CaL, and reducing production costs by developing inexpensive precursors. CaL had already been tested in aircraft and bench plants in the power sector as an outlay choice for scrubbing amine for post-combustion capture or as a catalyst for IGCC pre-combustion. Ca looping for CO2 capture from power systems was tested on a pilot plant by Hanak et al. [159].

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Because of the better penetration for clean energy H2 , more calcium oxide is being used for adsorptive of enhanced H2 production, such as glycerol radical reform, methane reforming [171], ethanol and tar reforming, and gasification of biomass [172]. It is common to use reversible processes like WGS and SR in these hydrogen production reactions. When CO2 is removed from the mixture, a larger amount of H2 is produced, pushing the equilibrium to the right. CO2 capture helps the endothermic reaction because calcium oxide cementation is an exothermic process. The SEMR becomes an exothermic process as a result of this modification, as well. CaO-supported catalysts (bi-functional) such as Nickel/CaO have been developed for sorption-enhanced hydrogen production. Although sintering reduces Bioavailability in bulk CaO, the most common solution is to add additional support, as initially noted. Addition of a Cao carrier and immobile substances (amphoteric oxides) to Ni-based motivators blocks the development of coke, and is the most significant thing with Ni-based catalysts because it makes rapid activity decay. Transition metals, like Ni, have a greater catalytic activity than Pd, Rh, and Pt [173]. Despite this, transition metals are thought to have a greater research trend. Catalysts for simultaneous steam reforming of glycerol and CO2 removal have been characterised by Charisiou et cetera. [171]. Increasing the scattering and grain size of nickel lifeforms resulted in greater resistance to coke formation and calcium oxide sintering in modAl, resulting in more effective glycerol conversion and higher purity H2 production.

11.12.6 Overview and future prospects for CaO adsorbents Before CaO-based sorbents can be used in industrial applications, their costing, activity, stability, anti-crush strength, and shape must all be considered. Because of their higher cost but also lack of stability, chemically synthesized CaOs CO2 sorption capacity and long cycle life have improved significantly. After granulation, CaOs ability to absorb CO2 is significantly reduced, making its activity unsatisfactory, and more importantly. Future research should therefore concentrate on the following areas: 1) preparation of Calcium oxide technology on a large scale from mineral/solid waste, with greater capacity and stability for sorption-enhanced hydrogen production, along with catalyst loading techniques; 2) separation and purification of carbon dioxide and material regeneration; 3) direct conversion/utilization of Calcium oxide sorbents to fix CO2 .

11.13 Pre-combustion applications, implementation and problems The pre-combustion photographing CO2 process extracts CO2 from syngas or gas restructured at high energies (2.0–7.0 MPa) and medium air temperature (200.0–450.0 °C). Carbon monoxide is enzymatically converted to CO2 and H2 in the capture facility, which is site construction after the WGS reactor. However, it can also be consolidated with WGS to change this same equilibration. It is possible to use high-temperature CO2 adsorbents like CaO and Li2 SiO3 to improve H2 production by capturing CO2 in situ during the process of reforming at elevated heat (4450 1C). The PSA technique is preferred for pre-combustion CO2 capture because this can take benefit of great CO2 concentrations (e.g. 15 percent to 60 percent). Precombustion low-temperature classification technique are still confined to laboratory experiments because of the complex processes, expensive tools costs, and energy requirements

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(TRL3). When compared to WGC, which removes CO2 from shifted gas immediately at high temps without or before [174], it has received considerable attention. Weak chemisorbents with reversible CO2 capture work well with WGC systems. Sulfur removal, WGS catalysis, and CO2 capture all take place in one stage in SEWGS technology. This is a pretty standard WGC pre-combustion CO2 capture method. Using commercialised K2 CO3 supported hydrotalcites as CO2 adsorbents, the ECN has been working to expand SEWGS since 2005. The technical validity of SEWGS must have been confirmed in during two-year CCP [175]. Following feed, a co-current CO2 rinse and counter-current vapour purge step were added to the six, ten-step PSA process to reduce loss of H2 in the product gas. A steam rinse was used instead of CO2 rinsing in the successive European FP6 construction CACHET [176] because it reduced productivity and consumed CO2 compression energy. A European FP7 operation CAESAR sought to reduce steam consumption during the rinsing as well as purging stages through implementation parameter optimization [177]. A significant reduction in steam intake could be achieved by accounting for H2 O co-adsorption during flow and rinse processes [178]. According to ECN, the SEWGS innovation was ready to be scaled up in 2013. As of now, SEWGS is used in the Systematic project to capture CO2 from BFG, with the aim of reaching an 85 percent CCR with 60 percent less energy consumption or a 25 percent lower collect cost. TDA developed another PSA-based WGC method for pre-combustion CO2 capture at 190–260 °C using functionalized mesoporous carbon. No degradation in performance was observed after 1500 cycles of adsorption and desorption at the United Nations Carbon Sequestration Core [179–182]. For example, a 10-step PSA predicated on adsorbents patented by TDA can make maximum CCR (>90 percent) and CP (>99 percent) and with little fuel (0.34 MJe kg1), according to recent virtual environment research. The State - owned oil Yangzi Petrochemical Plant in China is where TDA plans to build a pilot-scale PSA facility. On a smaller scale (10 kg CO2 h1), TDA is also developing a poor WGS–PSA consolidated PSA technique [183]. When compared to a single PSA process, the integrated WGS–PSA process improved performance of the system by 0.5 percent. A preliminary study by Tsinghua University found that the ET–PSA method uses 20.3– 26.6 cents less fuel than the Selexol mechanism [184]. Over the course of a five-year project (2011AA050601), an ET–PSA prototype capable of handling 6Nm3 h1 of CO2 /H2 /H2 S mixed gas at 400 1C but also 30 bar was developed [185]. After 75 h of total operation and a total of 1089 working days of cumulative operation, the lab-scale proposed system achieves a CCR of 95,7–98,6 percent and an H2 S elimination ratio of more than 99 percent. Since 2016, researchers have been focusing on high Vapour synthesis with in-situ CO2 separation. Hydrogen sanctity (HP, 99.9994 percent) and hydrogen restoration ratio (HRR, 97.51 percent) from shifted gas have recently been improved in a 2-train Sameera reflux structures [186]. Even by end of 2020, a Shanxi Treasury project aims to construct an ET–PSA component with the a computational power of 5000 N m3 h-1 for the production of high-purity H2 from a petroleum plant (MH2 01506). High-temperature CO2 adsorbents can be used to capture CO2 from methane, biomass, glycerol and bio-oil [187] in a cost-effective manner. It eliminates need for a distinct WGS fission and H2 purification process by absorbing CO2 at the production point. Endothermic reforming activities can be supplied with thermal energy via the linked carbonation process. Other than traditional fixed-bed structures, the DFB reactor was used for regeneration of

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saturated adsorbents, where excess heat from combustion is used. Pure O2 and repurposed flue gas are needed to cool the CO2 calcination process. In the last few years, SER pilot farms have already been successfully built. In reality, the vast bulk of pilot-scale initiatives have relied on SER technology primarily to control the H2 -to-CO ratio for upstream synthesis methods and to lessen reforming power consumption, with the harvested CO2 being released back into flue gas without really being concentrated. The IFK Stuttgart’s SER tests with pine biogas as the fuel supplier for oxyfuel combustion in a 200 kWth DFB reactor are an excellent example. When the calcination mode was changed from air to O2 combustion, the CO2 level in the flue gas rose from 26.7 vol cents to up to 95 vol percent. Finally, the PSA-based WGC procedure is a promising pre-combustion CO2 collection method. A number of pilot-scale WGC plants (TRL 6) are currently being built around the globe. Pre-combustion CO2 collection through the use of adsorption is described in research papers. In addition, a number of issues have been discovered at the adsorbent, nuclear power plant, and overall system levels. Post-combustion CO2 capture is still a challenge for intermediate and high-temperature CO2 adsorbents, despite significant progress in their development over the past few years. For pre-combustion CO2 collection, molten salt endorsed MgO has incredible capacities that can only be tested through prototypes. Current WGC devices still use a number of excess steam to get high purity and recovery. However, multitrain PSA can reduce energy consumption while increasing operational complexity but rather capital expenditure. The separation of contaminants at extreme heat, such like H2 S, COS, HCl, and metals, must also be taken into consideration in addition to the CO2 requirement. It is possible to use WGC procedures in not only IGCC, but also in other high-pressure, hightemperature processes, like NGCC, biomass gasification energy plants, inclusive steelworks, and diesel fuel chemical industries. SER hydrogen manufacturing processes based on hightemperature CO2 adsorbents with pure O2 generated by an actual air separating funnel for the calcination are feasible for pre-combustion CO2 capture; however, a thorough technoeconomic assessment should be undertaken to assess the capture cost. As a result of comparing different CO2 capture technologies, this paper provides an indepth look at how well different technologies absorb and desorb CO2 , as well as the enthalpy and capacity of each technology’s absorption and adsorption processes. There is a wide range of applications for liquid solvent filtration systems in post-combustion processes. Capture materials with high adsorption performance and excellent regenerability are essential for the pre-combustion process. Pilot-scale demonstrations for real world applications should be included in future research priorities.

11.14 The utilisation of CO2 in industrial processes One of the most important strategies in combating climate change is to close the anthropogenic carbon cycle. This requires the effectual coupling of Carbon dioxide capture with subsequent conversion [188–190]. The net cost of releasing Carbon dioxide and reducing emissions could be reduced if CO2 was used to produce meaningful products [191]. Looking to diversify their power production and increasing their energy security could be achieved through the use of CO2 -based fuels in countries that do not have access to fossil fuel production [192–194]. Diluting and NOx pollution from fuels could also be mitigated by CCU

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products [195–197]. Traditional chemical production methods must be compared to CO2 based production, as well as the impact on the environment. Hydrocracking reactions have emerged as perhaps the most good potential conversion technologies for trying to mitigate the effects of global warming on the life cycle of polyol and formic acid production [199]. For the fuels sector, the CO2 utilisation potential is 10 percent, while the chemical products sector’s is between 1 and 7 percent, according to recent numbers Chemical compounds such as urine (70 Mt CO2 per a Mt product, annualised demand 100 Mt), methanol (14 Mt CO2 used per Mt device, annual market clamour 40 Mt per year), and organometallic trace elements (over 110 Mt CO2 used per Mt brand, annualised mandate 40 Mt every year) (30 Mt CO2 or per Mt product, annualised clamour 80 Mt per year) have been made from CO2 . It is possible to reduce annual Emissions of carbon dioxide by v3.7 Gt, — roughly 10 percent of the current annual CO2 emissions, by implementing a variety of CO2 utilisation methods. Aside from a few materials, the market would be overwhelmed if they were implemented on a large scale [198]. CO2 utilisation hot topics include: thermochemical CO2 , photochemical CO2 , electrical electrochemical CO2 , photoelectrochemical CO2 , thermal photothermal CO2 , and CO2 as soft superoxide anion for dehydrogenation, all of which are systematically summarised in this section.

11.14.1 Conversion of CO2 to energy This method of CO2 utilisation, thermo-catalytic CO2 conversion into chemicals, is a promising strategy for CO2 utilisation which not only helps reduce environmental issues characterized by excessive Dioxide (CO2 ) emissions, but it also delivers a long-term alternative for producing essential substances or materials. This means that CO2 to console fuels and contaminants could be rapidly industrialised on a world basis, accelerating human efforts to reduce CO2 emissions. In order to significantly reduce CO2 molecules, large-scale clean fuel conversion plants will be built. Because of the recent spike in oil barrel price levels, or at least the uncertainty that has accompanied it, many scientists are focussed on the photochemical, electrical conduction, catalytic, or genetical conversion of CO2 . Another important driver for the conversion of CO2 to fuels is the ongoing energy transition, which aims to change the way power is generated by using techniques that emit less CO2 . Power-to-X processes, which first store excess electricity as hydrogen and then combine it with CO2 to form chemicals or fuels, allow the decarbonized sources inside the energy mix to expand. This has a major effect on the reduction of CO2 emissions as a result. When it comes to making valuable compounds, activating CO2 is the most important step because it is both stable thermodynamically and kinetically. Due to the high energy content of CO2 , another useful reactant is often reacted to it. When it comes to the most commonly used reducing agent, H2 is the most widely used because it can be developed utilising renewable energy and also the well before water electrolysis technology [200]. However, due to the "apparent inertness" of chemical compounds, extremely harsh conditions (such as high temperature and pressure) are absolutely essential to quicken increase the rate of chemical or alter chemical equilibrium. A viable option would therefore appear to be CO2 hydrogenation processes to start producing methanogenesis products like CO2 , methanol and other hydrocarbons, as discussed in the following section. It is important to consider the long history of chemical reactions as well as the numerous publications that have addressed these topics when summarising the various technologies and methods that will be

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used to fully explain reaction mechanisms and catalyst synthesis strategies. Supplementary importance is attached to the use of transition metals such as Fe, Ni, Cu, and Co, which are less expensive than noble metals. A global assessment of industrial CO2 emissions will also be conducted.

11.14.2 Thermochemical method for CO2 methanation Methane, the primary component of natural gas, is used not only in industrial activities but also as a primary source of cooking and heating throughout Europe. Methane may also be simply liquefied and safely stored in huge amounts. The presence of several existing infrastructures makes transportation and storage of this high energy density alkane extremely convenient. Therefore, the Sabatier–Senderens reaction, which is also known as the methanation of CO2 (CO2 + 4H2 CH4 + 2H2 O), is critical for both industrial and consumer applications. To help astronauts survive and provide energy on Mars, NASA has studied the possibility of using a reduction agent (H2 ) produced by splitting water to convert CO2 in the Atmospher into CH4 and H2 O. The dehydrogenation of CO2 takes place in the Tremp procedure at low pressures (5.0– 20.0 bar) and high temperatures (200–500 °C to 800 °C). When operating CO2 methanation reactors, controlling the exothermicity of the reaction is critical to maintaining favourable conditions and avoiding the deactivation of the catalyst due to sintering under hydrothermal conditions. A wide range of catalytic systems for the Sabatier reaction have been developed, including those based on Rh and Ru endorsed on traditional oxide telcos (Al2 , SiO3 , ZrO2 , TiO2 , but also CeO2 ), but also their mixtures. In terms of sensitivity, exercise, and stability, nickel-based catalysts have now been identified as being the most popular materials for methanogenesis of CO2 at a low cost. The addition of activators at binding sites and the modification of the support by tinkering with metal–support interactions have both been used to increase levels or improve methane specificity [201–205]. Electrochemical interactions between support and its materials, which are motivated by changes in the toughness of covalent bonding of the chemisorbed species, have a significant impact on the performance of the catalyst As a result of the vacancies of oxygen being created during the process of reduction, reducible supports are recommended to provide suitable active sites for CO2 activation.

11.14.3 The thermochemical method for dry CO2 and methane reforming Dry Methane, the primary component of natural gas, is used not only in industrial activities but also as a primary source of cooking and heating throughout Europe. Methane may also be simply liquefied and safely stored in huge amounts. The presence of several existing infrastructures makes transportation and storage of this high energy density alkane extremely convenient. Therefore, the Sabatier–Senderens reaction, which is also known as the methanation of CO2 (CO2 + 4H2 CH4 + 2H2 O), is critical for both industrial and consumer applications. To help astronauts survive and provide energy on Mars, NASA has studied the possibility of using a reduction agent (H2 ) produced by splitting water to convert CO2 in the Atmospher into CH4 and H2 O. The transformation of CO2 takes place in the Tremp method at low pressures (5.0–20.0 bar) and high temperatures (200–500 °C to 800 °C). When operating CO2 methanation reactors,

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controlling the exothermicity of the reaction is critical to maintaining favourable conditions and avoiding the deactivation of the catalyst due to sintering under hydrothermal conditions. A wide range of catalytic systems for the Sabatier reaction have been developed, including those based on Rh and Ru maintained on traditional oxide bearers (Al2 , SiO3 , ZrO2 , TiO2 , etc. CeO2 ), but also respective mixtures. In terms of selective, activity, and stability, nickel-based catalysts have already been identified as perhaps the most popular materials for methanation of CO2 at a low cost. The addition of activators at catalytic activity and the modification of the support by tinkering with metal–support interactions have both been used to increase participation or improve methane selection [202,203]. Electrochemical interaction between support and also its elements, which are controlled by variations as in strong of covalent bonding of the adsorbed species, have a significant impact on the performance of the catalyst As a result of the vacancies of oxygen being created during the process of reduction, reducible supports are recommended to provide suitable active sites for CO2 activation.

11.14.4 RWGS (reverse water-to-gas shift) reaction thermo – chemical methodology This method of converting CO2 to CO2 has long been considered a viable option because the resulting CO is a chemically active compound and considered as an essential intermediate in numerous chemical industrial processes, such as carbonylation and upgrading, as well as the synthesis of acids such as acetic acid and methanol, hydrocarbons, and dimethylether. RRM and AM are two of the most widely accepted mechanisms for the RWGS reaction. Without used in the intermediate, H2 serves as a corrosion inhibitor in RRM [206]. Because CO2 has the potential to completely oxidise all of the partially reduced financial backing, RRM position is dependent on this possibility. CO is created by the break – down of a base – catalyzed intermediate formed when H2 and CO2 react, according to AM [207]. Another theory put forth by scientists was that the RWGS reaction is mediated by carbonates. They are most commonly used because of their rising activity and selectivity in this catalyzed reaction. A high selectivity for CO is achieved by using noble iron metal carbides [208]. Aside from these active metals (Ni, Co and Fe), the RWGS reaction has a good performance when other metals are tuned. A common active metal in the methanation procedures is Ni. Bimetallic Ni–Cu compositions have been used by Reina et al. in order to reduce methanation [209].

11.14.5 Methanol is produced by the thermochemical electrolysis of water of carbon dioxide Methanol is a major chemical product that is utilized as a solvent, fuel or fuel additive for fuel cells. Methanol is also a necessary component in the synthesis of high-value-added compounds including acetic acid, anhydrides, formaldehyde, monomers like methyl methacrylate and methyl esters. Global consumption is steadily increasing, from over 40 million tonnes in 2007 to nearly 100 million tonne in 2019 [210]. For its first industrial methanol plant, BASF used liquid fuels as the biodiesel in 1923. With Zn metal acting like a physical spacer, Cu/Zn-based

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precursors or chemistries have been thoroughly examined in the production of CH3 OH from hydrogen evolution Reaction. A motivator for the process has been proposed to use the metal of Cu, Cu–ZnOx contact, or CuZn alloys, while the concern of “in what life forms operates as the exact and authentic active sites” remains open.

11.14.6 hydrogenation of CO2 to hydrocarbons through a thermochemical process Hydrocarbons are critical components in bulk chemistry and include light (C2 –C4 ) olefins, fresh herbs, and liquid alkyl and alkenes. There are a variety of other solvents can be used to power engines and vehicles, such as gasoline, jet fuel, and diesel (C5–C20). Fischer–Tropsch synthesis, catalysed by Fe, Co, and Ru, has been used for decades to produce hydrocarbons from syngas [211–213]. CO and H2 dissociation and C–C chain formation are greatly facilitated by the metallic nanoparticle surface. However, selectivity management is a major challenge because product distribution generally follows the ASF rule. Metal oxide and zeolite bifunctional catalysts have recently been examined, and their unique properties of willfully violating the ASF distribution, a problem for decades, attracted considerable attention. Because of this, an oxide-zeolite catalytic system was developed with the goal of bypassing the traditional Fischer–Tropsch synthesis’s hydrocarbon selectivity limitations Chemical coupling was carried out by the acid sites (derived from MSAPO) in this ZnCrOx/MSAPO catalyst by Bao et al. [214], resulting in an 80 percent preference for C2 – C4 olefins. Using a Zr–Zn/SAPO-34 hybrid catalyst, Wang et al. [215] generated 70 percent selectivity for C2 –C4 olefins. It is possible to activate CO2 through the RWGS reaction and then convert it to hydrocarbons through the use of the ASF distribution, but these are two separate processes. Direct conversion of CO2 into gasoline was achieved by Sun et al. [216] using a cross catalyst with three different catalytic activity (Fe3 O4 , Fe5C2, but also acid sites). After CO2 was converted into a-olefins by RWGS at Fe3 O4 sites, it was reduced to CO again at Fe5C2 sites by the FT synthesis. This second route requires CO2 hydrogenation to benzene or DME, accompanied by C–C coupling catalysed by Bronsted acid sites, and the electrophilic required to produce diffused forward towards the porous acid sites. With a selectivity of 79 percent for gasoline range hydrocarbons, the Zhong et al. [217,218] bifunctional catalyst for CO2 hydrogenation was developed. As CO2 as well as H2 were activated on the oxygen - containing functional groups of In2 O3 , methanol was produced, followed by C–C coupling reactions in the zeolite pores. When using the bifunctional launching pad, several copper alloys have been investigated for their potential use in the CO2 to methanol/DME conversion process, which would include Zn–Cr oxide [218] and Cu–Zn–Zr [219] as well as iron-zinc oxide (Fe–Zn–Zr) and zinc oxide (ZnO–ZrO2 ). In all these oxide–zeolite catalysed systems, the length between the oxide and nanoclay particles remains a critical factor in the targeted product selectivity. For example, poisoning of the Halide acid sites due to close communication between two features could lead to the production of CH4 as the primary end product.

11.14.7 Carbon dioxide (CO2 ) photochemical conversion The use of solar energy for photolysis CO2 conversion is one of the most practical and widely available renewable sources, and it is both clean and beneficial to the environment.

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FIGURE 11.4 Presentation of three basic charge kinetic steps during conversion of carbon dioxide.

photochemical

CO2 conversion bolstered by an endless photovoltaic fuel source is enriched by the simple structure and economic viability of an electrocatalytic array, which is promoting the tech’s ongoing development. In order to overcome the fluctuation of sunlight whereas the generating energy, chemical fuels can be stored and used on sales, thanks to the highly endothermic process of CO2 reduction by water [220,221]. The growth of long-term CO2 transformation systems feedstock has gained considerable traction by moving away from use of fossil fuels [122,124].

11.14.8 Photocatalytic CO2 reduction perspectives and prospects The use of solar energy for CO2 photocatalytic conversion into precious chemical fuels is particularly enticing because it makes use of renewable resources. When compared to other methods, photochemical conversion uses less heat and energy because it can be done at or near room temperature and pressure. Several approaches have been devised to resolve the major challenges of developing CO2 reduction portrait with high kinetics potentials. A heterogeneous system, which includes a heterogeneous photo-catalyst with a combined effect of complexes, is more exciting in terms of scaling up. A few key characteristics should be present in a photo catalyst for chemiluminescence CO2 conversion. To begin, a narrow band with a high percentage should be utilised. Fig. 11.4 shows an example of this. Good charge intrinsic properties that increase charging migration while reducing charging recombination are also important. Ultimately, the system must have all the good features of ground that lead to specific interactions between surfaces. High adsorption capacity for Molecules and adequate band potentials reduce competition only with H2 evolution reaction, among other things (water reduction reaction). There are many ways to increase CO2 and H2 O transformation after lighting, but product accuracy must also be taken into account. There has been little control over control devices and tunability due to the possibility of a wide selection (CH4 , CO, C2 H4 , C2 H5 OH, and CH3 OH) and a lack of perception of awareness of reaction

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pathways and processes. A definitive understanding of photochemical CO2 conversion’s reaction pathways and product distributions has been achieved as a result. Because of lower power in comparison to the conventional, there is still an urgent the need investigate and develop new photo-catalytic materials for greater CO2 photo-conversion efficiency. In their current state, photocatalytic CO2 reduction’s low production of solar incites energy is unfeasible. There is a need for extensive mechanistic investigation, both experimental and predicated upon the first principles, in order to discover new photo precursors and gain an understanding of when to capture the entire range of sunlight. Photo-thermal catalysis (detailed in the previous section) could benefit from this new understanding, which could be applied to other types of catalysis.

11.14.9 A sorting oxidant: CO2 CO2 has primarily been employed in conjunction with a reducing agent, primarily H2 . Though, the present main source of H2 depends heavily on co-production of CO from the reforming of methane, naphtha, and low carbon alcohols and oxygenates like dimethyl ether. If renewable H2 is not utilized, the conversion of CO2 into chemicals and fuels becomes uneconomical. It has been suggested that CO2 reduction could be performed utilizing various saturated hydrocarbons such as alkyl aromatics and light paraffin, rather than just hydrogen. At the same time, these hydrocarbons can changed into olefin compounds like ethylene, butenes (including butadiene), propylene and styrene which are main industrial chemicals and can be made through simple dehydrogenation, cracking and oxidative dehydrogenation with molecular oxygen. These olefins, on the other hand, have been reported to be formed by using oxidative dehydrogenation with CO2 , in which CO2 acts as an oxidative agent which directly remove H2 from the saturated bond and/or capture H2 molecules, which are produced via dehydrogenation as follows: Cn H2n+2 + CO2 → Cn H2n + CO + H2 O Cn H2n+2 → Cn H2n + H2 CO2 + H2 → H2 O + CO CO2 use, as a soft oxidative, accelerates all the catalytic oxidative changes and is experiencing significant expansion in different industries. It has the potential to contribute to C emission reduction ,energy-efficient and cost-effective chemical manufacture [225,226].

11.5 Conclusions and prospects Carbon collection and utilization offers a lot of promise when it comes to lowering carbon releases. This chapter provided a complete assessment of long-term growth of enhanced CO2 capture and usage, which has gotten a lot of consideration since its significant prospective to exacerbate worldwide increasing temperature. The processes before and after ignition/combustion have gained global consideration and developed as a two primary strategies for CO2 capture. CO2 -EOR and CO2 hydrate are both effective CO2 utilization strategies. In 2030, CCU has the technical capacity to reduce annual CO2 discharges about 3.5 Gt. CO2 -eq [195,227,228]. The administration of all countries should frame targeted funding schemes for CO2 usage [229]. Despite the enormous difference concerning the little carbon recognition rate and extraordinary CO2 reduction price, incorporating a power plant using

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CCU technological features into discharge transaction structure might significantly reduce economic funding burden [230]. The national carbon market is set to go live in 2020, conferring to a scheme released in public (through the China administration, not just the global opinion or illustrations), with first stage focusing primarily on power generation industries, such as cogeneration source of electrical and heat power plants. The process of CO2 hydrogenation might be proceed in oil refineries via electrolytic procedure of hydrogenation or through coking plants via the use of residual gas. It could also combine with renewable sources without producing waste hydrogen. The excess energy could be used for electrocatalytic CO2 conversion and hydrogen production via water electrolysis. The hydrogen energy created from renewable sources could be used in other ways. The carbon resources market value and their movement have developed as a dynamic field for engineers and researchers throw-out the globe in order to achieve tremendous minimization of active carbon release. The significance of carbon budgets as a direct prize for ecofriendly businesses has been established through the investigation of carbon allowances as a straight incentive for a good little-carbon financial enlargement. Corporations that have received a carbon recognition prizes are required to provide specified allocations on everyday base for jumble sale of the carbon market in order to increase market liquidity. The cost-effective development of carbon-free hydrogen generating technology is critical in the context of resourceful CO2 consumption, particularly CO2 hydrogenation process for the manufacture of useful chemical compounds. Aside from the high-rate H2 synthesis by water electrolysis, additional present manufacturing H2 fabrication technologies, for example H2 fabrication from natural resources such as (coal & natural gas), generate a significant quantity of CO2 . Recent research has concentrated on solar photocatalytic hydrogen generation through hydrolytic process, and breakdown of hydrogen sulphide (H2 S) for H2 fabrication, among other things, in order to develop inexpensive, carbon-free hydrogen sources. CO2 -ECBM and CO2 -EOR are the two most common methods for exploiting CO2 , indicating the critical relevance of maintaining and increasing the output of most oilfields, as well as increasing the extraction and usage capacity of coal-bed methane. The widespread use of CO2 -EOR might result in significant carbon storing capacity and increased oil fabrication, which would be beneficial to the gas and oil engineering’s economic benefits in addition to mitigating the energy security threat posed by expanding foreign dependency on the oil [231]. The rate of the demonstration of CCUS scheme is comparatively significant, which stymies the CCUS process’s expansion. The demonstration of the present CCUS scheme is expected to cost too much, and carbon capturing might add an additional 140–600 RMB per (CO2 ) to the operation cost. Given existing CCUS technologies, implementing CCUS would raise primary energy utilization by 10 to 20 percent while sacrificing productivity, which is a main roadblocks to widespread CCUS adoption. CCUS technology’s comprehensive method flow, incorporation, and ascendable skill development might be implemented in phases and technology demonstrations in a range of industries. The cumulative expertise gained through several tests and the identification of important methodologies will gradually speed up the standardization and rate discount of CCUS expertise, which is critical for attaining longstanding profitable positioning of the CCUS. For coal chemical companies, demonstration projects combining low rate carbon capturing with EOR are advised to be broadly adopted. Oil and gas companies are urged to take the lead in launching a CO2 -EOR demonstration in order to broaden their engineering skills. The CCUSs relevant premeditated design and system scheme should be reinforced in order to boost governmental support and economic motivation,

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laying a firm groundwork for the CCUS to perform a vital part in long-standing carbon release drop. Controlling overall carbon emissions should be stepped up, and quantification of restricted aims for numerous productions should be defined and recommended in order to raise awareness of little quantity carbon growth and the need for CCUS distribution for businesses. Ground-breaking inducement strategies that assist the development of CCUS, such as tax exemptions and differential subsidies, are urged to be investigated and implemented. The CCUS demonstration project is well supported by the commercialized speculation and bankrolling procedure with incorporation of the administration and the marketplace, with optimistic exploitation of several means like green economics, environment bond, low-carbon funds, and so on. Overall, the impending research and growth in the CCU area are discussed.

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C H A P T E R

12 Ionic liquids for carbon capturing and storage Faizan Waseem Butt a, Hafiz Muhammad Athar a, Sumia Akram b, Zainab Liaqat a and Muhammad Mushtaq a a b

Department of Chemistry, Government College University, Lahore, Pakistan Division of Science and Technology, University of Education Lahore, Pakistan

12.1 Introduction The rapid increase in population and urbanization has not only declined the worldwide CO2 sinks (landscapes, forests, and soil) but also increased the consumption of fossil fuels. The most concerned outcome is the exponential increase in levels of carbon dioxide which makes up nearly 86% of the green-house gases responsible for global warming, extreme heat episode, and other climate crisis [1]. A report “Global warming of 1.5 °C” (2018), stated “the worldwide temperatures were 1.0 °C more than the pre-industrial levels and predictions are that it will reach 1.5 °C by 2030” [2]. The mandatory initiative to eradicate the effect of global warming is to reduce the emissions of greenhouse gases most importantly CO2 . The signing of Kyoto Protocol has offered initiatives to researchers to reduce CO2 emissions especially from fossil fuels. Carbon capture and storage (CCS) is a great initiative that has revolutionized the field of environmental chemistry as an effective pathway to reduce CO2 atmospheric concentration on a large scale [3]. CCS is concerned with lowering of CO2 emissions from various processes like cement manufacturing, ammonia production, and fossil fuel plants (the main contributor) etc. [4]. A report suggests that an approximate amount of 236 billion tons of CO2 can be captured and stored if CCS is executed to its full potential probably by the end of 2050 [5]. The best approach towards CO2 capturing is its separation from coal beds, saline deposits, depleted oil, and gas reservoirs and other suitable sedimentary formations of this sort [6,7,8,9]. The keygoal is to introduce a technology which will help us in fulfilling the assigned task in a well efficient way while keeping a check on environmental impact as there is a huge risk

Green Sustainable Process for Chemical and Environmental Engineering and Science: Carbon Dioxide Capture and Utilization DOI: https://doi.org/10.1016/B978-0-323-99429-3.00018-7

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c 2023 Elsevier Inc. All rights reserved. Copyright 

280

12. Ionic liquids for carbon capturing and storage

FIGURE 12.1 The Generic layout of Pre, Post, and Oxyfuel-combustion capture technologies.

of leaking of stored CO2 [10-12]. Manchao et al. [13] evaluated the risk assessment of stored CO2 in geological features with focal emphasis on dumped coal seams as well as mines. An alternate and safe mainstream approach would be conversion of CO2 into another product of high value which is termed as carbon capture and usage (CCU) [14,15].

12.2 CO2 capture technologies The materials that interact with CO2 can be utilized for capturing and requirement of other materials depend entirely on the processes in which the gas is constrained (Fig. 12.1). The three common processes adopted for large scale CO2 capturing are (i) pre-combustion and (ii) the post-combustion. The first involves the conversion of hydrocarbon fuels into CO and H2 . The CO is then converted into CO2 utilizing water shift conversion followed by separation from H2 [16,17]. The post-combustion process involves sequestration of CO2 after the combustion of hydrocarbon fuels utilizing air as an oxidant [16,17]. It is one of the most commonly used process for CO2 capture and storage. In another strategy, the Oxyfuel-Combustion utilizes pure O2 as an oxidant to convert hydrocarbon fuels into CO2 and steam followed by separation of CO2 [16,17]. Before we proceed towards the application of Ionic Liquids in carbon capturing and storage it is necessary to review certain terms. Some of the techniques that capture CO2 may use cryogenic separation, bio-fixation, absorption, adsorption, membrane separation, and chemical looping [18]. Absorption utilizes physical interaction or chemical bonding to interact and react with CO2 . Chemical absorbents form covalent bonds while physical absorbents form Van der Waals interactions with CO2 obeying Henry’s law (which states that the solubility of the gas has direct relation with the partial pressure of gas at constant temperature). The solvent thus formed can be regenerated and CO2 can be released. The regeneration process is carried out using high temperature and low pressure [19]. Adsorption utilizes only the surface of the material for the interaction (either chemical or physical). A bed of adsorbers placed in the path of flue gas capture CO2 only while flue gas is sent to a clean bed followed by regeneration of the saturated bed [20]. However, the limited selectivity of adsorbents has restricted the usage of adsorption technique for a large-scale application. Membrane separation involves the physical or chemical interaction of gases with a permeable membrane. An advantage of this process is that the membranes can easily be modified according to need and conditions

12.4 Features of ILs

281

and can widely be used on large scale industrial processes. The best approach is to couple membrane separator with a liquid that imparts selective retention and/or capturing of CO2 [21]. Cryogenic Separation involves cooling and condensation to produce liquid CO2 which helps a lot in transportation. High concentration gases as in pre-combustion and oxyfuelcombustion are better suited for this process as low concentration gases require high amounts of energy for cooling process [22].

12.3 Ionic liquids (ILs) The previous decade has seen many classes of novel materials that are synthesized specifically to increase the efficiency of CO2 capture and storage. One such promising class in this regard is Ionic Liquids (ILs). Ionic liquids are molten salts comprising of cations and anions [23]. Ionic liquids may contain very large number of ions and neutral molecule and show remarkable ionic interaction additionally to van der Waals interaction which is present in many liquid states. These remarkable ionic interactions develop very low vapor pressures, thermal stability, miscibility, and high viscosity. The word ionic liquid firstly used by Walden for Ethylammonium nitrate which have very low melting point below 50°C. The ionic liquid which are liquid at room temperature or have very low melting point often consist of large size organic cations for example 1-alkyl-3-methylimidazolium, 1-alkylpyridinium [24]. Cations of ionic liquid are generally very large size organic compounds (Fig. 12.2) whereas anion may have smaller size as compared to cation and thus the final structure formed may be of inorganic nature. The difference between the size of cation and anion might be responsible for the development of weak interaction in ILs. The general final structure of ILs is very similar to salt but in salt there are strong interaction between cation and anion responsible of crystalline structure and very high melting point (Fig. 12.3).

12.4 Features of ILs The major advantages of these liquids over others are that they have extremely low vapor pressure, high chemical and thermal stability, and a large electrochemical window. In addition to that, ILs are highly modifiable having flexible functional groups leading to creation of green functional solvents [25,26]. The ionic liquids usually comes in with higher densities as compared to water and organic solvents, however, tetraalkylborates having shorter alkyl chain length are lighter in density. In general, the density of IL changes with the size and nature of alkyl chain length and similar might the case with anion [27]. The ionic liquids are good conductor and polar in nature, their polarity may fall closer to the alcohols with short alkyl chain [28]. Recently, scientists have coupled ILs with membranes and various membrane processes to modify their use in numerous purposes. The most useful modification is the immobilization of ILs on membrane, which deliver excellent recovery and reusability of ILs along with minimum loss of these liquids out of the system. Furthermore, immobilization of ILs establishes the minimal use of active phase needed for carbon capture or retention. Besides, the matrix used for immobilization of ILs may act as a barrier between both the phases (receiving phase and feeding phase). Membrane based ILs can be employed on industrial level

282

12. Ionic liquids for carbon capturing and storage R

R

+

N

N

R

R

+

N

R

R

R

R

R

S

R

F

O –

N

R

S

O

R R +

P

+

N

R

R

O

R

O

R

F

S

R

O

C

O

+

R

–

S

N

N

R O

R

S

+

N

O

O

O

R

O

R

O

R

–

B

+

F

O

O R

-

N

–

R

O

R

S

R

P

R

R

R

R

R

O

N

+

B

–

O

O

R +

N

R

S

R

R

-

R

O

OH

O

O

N

O

O

S

–

O

O

O

O R

O

S

–

O

+

N R

R

–

O

O

FIGURE 12.2 The Generic structure of well-known cations and anions used for the preparation of ionic liquids.

for the absorption of CO2 . The ionic liquids come in good solvation power and higher ionic conductivity [29–32]. Finally, the ionic liquids may comprise of what has been termed as “ionic cluster” and “hydrogen bond” and viscosity of ILs changes with the strength of hydrogen bond. The nature and size of cluster in addition to viscosity and density affects the features of these solvent like dissolution, alkalinity, acidity [33,34]. Overall, ionic liquids can be prepared from bulky organic cations and relentless number of anions and these combinations can have countless number of arrangements, that is why these liquids are also known as ‘Designer Solvents’ [29,35,36]. The key barriers towards the use of ILs in various application and specifically carbon capturing are higher density, viscosity, and synthesis cost of these liquids. However, opportunities are available regarding cost-effective synthesis, density, and viscosity tuning. For example, tetraalkylborates containing ILs are lighter and less viscous. Besides, the cation-alkyl chain length can be altered to adjust the density/viscosity, same is the case with the nature and

12.5 IL as absorbents for CO2 capture

283

FIGURE 12.3 The reaction mechanism CO2 follow during its absorption by Amine group containing Ionic Liquids.

alkyl chain length of anions [27]. Another opportunity involves the hybridization of ILs with other solvents. Likewise, modification of ILs with various membranes and processes, and immobilization can resolve the density and cost challenges. The immobilization of ILs on solid supports (supported ILs) not only improve their reusability and recycling but also minimize the loss of these liquids out of the system [29-32].

12.5 IL as absorbents for CO2 capture The traditional ammonia alcohol solutions like monoethanolamine (MEA) that are used as absorbents for CO2 capture face challenges in terms of large solvent loss and high energy consumption. According to the experimental data, the reaction between CO2 and amine are associated with larger enthalpy changes, which simply mean larger amount of heat energy will be consumed during the liberation of captured carbon. For example, 90% absorption of one ton CO2 by 30% aqueous MEA is associated with 2.5–3.6 Gigajoule energy [37-39]. Therefore, it is necessary to introduce alternative substitutes with strong absorption capacity and little to no solvent loss/energy loss. Several generations of ILs offer similar properties with promising results, some of these have been cited in the subsequent sections.

12.5.1 Conventional ionic liquids It was reported in 1999, that CO2 is highly soluble in a hydrophobic IL 1-Butyl-3methylimidazolium hexafluorophosphate BMIM-BF6 [40]. A lot of studies have been conducted since then to investigate the role of these designer liquids in CO2 capture. The properties of ILs can be easily fine-tuned by modifying functional-groups and resulting “task

284

12. Ionic liquids for carbon capturing and storage

specific” can be applied for CO2 absorption. It has been reported that ILs based on pyridine and imidazole have been used for carbon capture and storage (CCS) [39]. Bates et al. [37] designed imidazolium ion containing primary amine group based ILs which showed CO2 capture level of 0.5 mol CO2 /mol for a 3 hrs exposure. The special chemical structure of amino acids has shown to provide great attachment sites and thus they have been incorporated in the preparation of ILs [41]. It was proven by Sistla and Khanna [42] that ILs that have multiple attachment sites (amino acid based ILs) show much higher CO2 absorption as compared to other absorbents. Lv et al. [43] comprehensively investigated the mechanism involved in carbon dioxide absorption by amine containing ILS. It has been noticed that initially CO2 reacts with [APmim]/[Gly] exothermally to form carbamate, the CO2 first react with anion (Gly−1 ) and then with cation (APmim+1 ). There were no evidences of the formation of zwitterions. Meanwhile, the concentration of carbamate increases initially, then decreases due to decrease in pH. In the subsequent stage, the concentration of HCO3 −1 /CO3 −2 due to the hydration of CO2 . The presence of amine functional-groups in ILs combination resulted in CO2 capture level of 1.23 mol CO2 /mol. Bhattacharya et al. [44] also formulated a novel IL absorbent based on choline-amino acid that had low viscosity with a remarkable CO2 capture level of 1.62 mol CO2 /mol. The density functional theory (DFT) analysis also verifies the mechanism of action of interaction among CO2 and amine groups. In addition to amines, several super-bases have been used for formulation of novel ILs and it was reported that the basicity led to increased CO2 capturing. A new class of ILs based on imidazole anions and 1,8-diazbicyclo[5.4.0]–undec-7-ene (DBU) cations was developed by Zhu et al. [45]. It was reported that the addition of super-base resulted in CO2 capture level of 1 mol CO2 /mol. Xu et al. [46] referred the highly absorption of CO2 to increased basicity of ILs due of addition of DBU. Moreover, the CO2 absorption can also be increased significantly by tuning the alkyl chain length and anions. Aki et al. [47] demonstrated a remarkable increase in CO2 absorption by changing the alkyl chain length from butyl to a longer octyl chain. Sharma et al. [48,49] evaluated the efficacy of various anions on CO2 absorption and listed them in the following order: BF4 − < DCA− < PF6 − < TfO− < Tf2 N− . It has been reported that a CO2 capture level of more than 2 mol CO2 /mol can be achieved by introduction of particular anions on various reaction sites [50]. The economic value and feasibility of ILs over traditional absorbents is the key concern to declare them superior in addition to other properties. Ma et al. [51] found that carbon capture and storage process from a power plant based on ILs and traditional organic solvents (based on alcohol) and evaluated that the process based on IL absorbent was 30.01% more cost effective in terms of energy consumption. In another report, Ma et al. [52] have also found that ILs based on ([Bmim][PF6 ]) and ([Bmim][BF4 ]) showed remarkably less energy consumption by 24.8 and 26.7% respectively as compared to alcohol ammonia solutions. De rivaet et al. [53] optimized the process and reported a remarkable reduction in overall energy consumption to 1.4 Gigajoule/ton CO2 .

12.5.2 ILs based hybridized solvents Another promising approach towards carbon capture is formulation of novel biphasic solvents consisting of two or more solvents hybridized together in a fuse. Table 12.1 compare

TABLE 12.1 Summary of frequently used Ils for carbon capturing.

Ionic liquids (Ils)

Acronym

3-(n-aminoalkyl)−1,2-dimethyl imida- [aamim][MtF3 SO2 ][NH2 ] zoliumbis((trifluoromethyl)sulfony)

Conditions1 T(K) P(mP) 298.15

Capture capacity XCO2 /mol of IL2

References

0.348 mmol/g Water miscible, solubility increases with increasing chain length, IL with BF4 1.18 mmol/g anion is more efficient

[66]

[67]

[aamim][BF4 ]

298.15

1-butyl-3-methylimidazolium tetrafluoroborate

[bmim][BF4 ]

320, 330 43, 49

0.60

1-hexyl-3-methylimidazolium tetrafluoroborate

[hmim][BF4 ]

330 35

0.67

1-butyl-3-methylimidazolium hexafluorophosphate

[bmim][PF6 ]

298

0.20

1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide

[bmim][Tf2 N]

298

0.19

1-n-octyl-3-methylimidazolium hexafluorophosphate

[C8 -mim][PF6 ]

313 9.26

0.75

1-n-octyl-3-methylimidazolium tetrafluoroborate

[C8 -mim][BF4 ]

313 9.29

0.70

323 9.26

0.53

1-ethyl-3-methylimidazolium ethyl sulfate

[emim][EtSO4 ]

333 9.46

0.46

N-butylpyridinium tetrafluoroborate

[N-bupy][BF4 ]

323 9.23

0.58

Good solubility with bulkier alkyl group at low pressure, hydrophobic

Absorption capacity decreases [68] with BF4 as anion, quaternary ammonium based IL was found efficient, hydrophobic Solubility changes with both cations and anions, less soluble in water

[69]

285

(continued on next page)

12.5 IL as absorbents for CO2 capture

3-(n-aminoalkyl)−1,2-dimethyl imidazoliumtetrafluoroborate

1-n-butyl-3-methylimidazolium nitrate [bmim][NO3 ]

Remarks

Conditions1 T(K) P(mP)

286

TABLE 12.1 Summary of frequently used Ils for carbon capturing—cont’d Capture capacity XCO2 /mol of IL2

Remarks

References [70]

Acronym

1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide

[hmim][Tf2 N]

298 1.30

0.58

Low water miscible

bis(trifluoromethylsulfonyl)imide

[C6 H4 F9 mim][Tf2 N]

298 1.30

0.35

hydrophobic

1-butyl-3-methylimidazolium dicyanamide

[bmim][DCA]

313, 333 11.8, 11.2

0.58 0.52

1-butyl-3-methylimidazolium nitrate

[bmim][NO3 ]

313, 333 9.80, 9.20

0.50 0.43

DC and Tf2 N anion based IL are [71] insoluble in water, CO2 absorption reduces with increasing temperature and decreasing pressure, fluorinated anions exhibited good absorption capacity

1-butyl-3-methylimidazolium trifuoromethanesulfonate

[bmim][TfO]

313, 333 15.0 11.6

0.64 0.55

1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide

[bmim][Tf2 N]

313, 333 13.2, 13.0

0.74 0.72

Methyltrioctylammonium nicotinate

[N8881 ][NIA]

303

0.53

Methyltrioctylammonium formate

[N8881 ][For]

1.02, 1.02, 1.03

0.35

Methyltrioctylammonium acetate

[N8881 ][Ac]

Polyurethane-imide tetrabutylammonium bromide

HPIL-02-TBA

303

33.1 mg/g

Polyurethane-imide 1-Butyl-3-methylimidazolium

HPIL-06-BMIM

0.02

27.8 mg/g

Polyurethane-imide tetrabutylphosphonium

HPIL-06-TBP

28.7 mg/g

Polyurethane-imide 1-butyl-1-methylpyrrolidinium chloride

HPIL-06-BMPYRR

26.4 mg/g

0.31

[N8881 [NIA] has lowest viscocity [72] and higher absorption, also it showed good regeneration capacity, functionalization improves efficiency TBA and TBP cations were most [73] efficient, improved mechanical properties

(continued on next page)

12. Ionic liquids for carbon capturing and storage

Ionic liquids (Ils)

[BTMA][Tf2 N]

298

0.19

Methyltrioctylammonium trifluoromethanesulfonate

[MTOA][OTf]

1.0

0.27

Diethylmethylammonium methanesulfonate

[DEMA][METS]

0.15

Diethylmethylammonium trifluoromethanesulfonate

[DEMA][OTf]

0.10

ButyltrimethylammoniumMethyltrioctylammonium bis[(trifluoromethyl)sulfonyl]imide trifluoromethanesulfonate

[BTMA][MTOA][Tf2 N][OTf]

0.27

ButyltrimethylammoniumDiethylmethylammonium bis[(trifluoromethyl)sulfonyl]imide methanesulfonate

[BTMA][DEMA][Tf2 N][METS]

0.15

ButyltrimethylammoniumDiethylmethylammonium bis[(trifluoromethyl)sulfonyl]imide trifluoromethanesulfonate

[BTMA][DEMA][Tf2 N][OTf]

0.15

MethyltrioctylammoniumDiethylmethylammonium trifluoromethanesulfonate methanesulfonate

[MTOA][DEMA][OTf][METS]

0.23

MethyltrioctylammoniumDiethylmethylammonium trifluoromethanesulfonate

[MTOA][DEMA][OTf]

0.24

Diethylmethylammonium methanesulfonate trifluoromethanesulfonate

[DEMA][METS][OTf]

0.12

Physical absorption, absorption [74] capacity reduced minimally with low cost regeneration

12.5 IL as absorbents for CO2 capture

Butyltrimethylammonium bis[(trifluoromethyl)sulfonyl]imide

287

(continued on next page)

288

TABLE 12.1 Summary of frequently used Ils for carbon capturing—cont’d

Ionic liquids (Ils)

Acronym

Conditions1 T(K) P(mP)

Capture capacity XCO2 /mol of IL2

Remarks

References

IL based Hybrid Solvent for Carbon Capturing [Hopmim][NO3 ] + H2 O

318 2.33 bar

0.09

Low carbon capturing capacity [75]

(tri-isobutyl(rnethyl)phosphonium tosylate) + H2 O (4:96)

[iBu3 MeP][TOS] + H2 O

289 4.5 bar

0.03

Low carbon capturing capacity [76]

triethylbutylammonium acetate + H2 O (1:2)

[N2224][CH3 COO] + H2 O

298 0.1 bar

0.19

Low carbon capturing capacity [77]

trihexyl(tetradecyl)phosphonium prolinate + H2 O (95.5:4.5)

[P66614][2-CNPyr] + H2 O

295 0.1 bar

0.91

Highest carbon capturing capacity

1-hexyl-3-methyl-imidazolium bis(trifluoromethylsulfonyl) imide + MEA (1:1)

[Hmim][NTf2] + MEA

313 0.1 bar

0.50

Good carbon capturing capacity [79]

1-ethyl-3-methylimidazolium acetate + [Emim][Ac] + [Emim] [TFA] + MEA 1-ethyl-3-methylimidazolium trifluoroacetate + MEA(49.98:50.02)

323 0.1 bar

0.12

[80]

hydroxyl imidazolium ionic liquids [Cl] + MEA (1:2)

308 0.1 bar

0.40

[81]

imidazolium-based poly(ionic liquid) Grafting on Titanium nanotubes 298 0.2 bar

2.43 mmol g − 1 Efficient and recyclable

[61]

tetramethylammonium glycinate ([N1111][Gly]) + MDEA (1:1)

[N1111][Gly] + MDEA

298 0.1 bar

0.56

tetraethylammonium lysinate ([N2222][Lys])+MDEA (1:1)

[N2222][Lys] + MDEA

298 0.1 bar

0.74mol

“1” Denotes temperature in Kelvin and 2.

[C3 OHmim][Cl] + MEA

[78]

Good carbon capturing capacity [82]

12. Ionic liquids for carbon capturing and storage

hydroxypropylmethylimidazolium nitrate + H2 O (95.89:4.11)

12.7 IL hybrids with membranes for CO2 capture

289

the carbon capturing potential of various solvents in combination with ILs. The crux of this process is that there will be two distinct phases after liquid absorption: one with a high CO2 level and the other one with poor CO2 levels. The regeneration process will be very economical using these hybrid solvents as there would be no need for desorption in CO2 poor phase. Hasib ur Rahman et al. [54] formulated a biphasic solvent based on diethanolamine and 1alkyl-3-methylimidazoliumbis imide with high levels of CO2 absorption. It was reported that the separation was made easier as CO2 interacted with DEA to form a carbonate. Zhang et al. [55] prepared a novel solvent based on DMEE and ([N1111] [Gly]) and reported the formation of two separate phases. Huang et al. [50] blended amine group containing ILs with ethanol and water to prepare a hybrid solvent that separated into two distinct phases after CO2 absorption.

12.6 IL hybrids as adsorbents for CO2 capture The adsorption process for CO2 capture has some advantages: The adsorbent consumes less energy than absorbing liquid and is much easy to regenerate. The cost of utilizing IL hybrids as adsorbents is much less as compared to absorbing liquids. The potential of adsorbent is greatly enhanced using ILs. The high viscosity disadvantage of ILs is eradicated when it is hybridized with the adsorbent. There are two methods for preparation of IL hybrids that can be used as powerful adsorbents. First is the impregnation of these liquids into the adsorbents. Uehara et al. [56] utilized impregnation method for CO2 capture through a porous silica material and amino acid-based IL ([EMIM][Lys]). A CO2 capture capacity of 1.2 mmol/g was observed using this IL hybrid. Zhang et al. [57] impregnated various mesoporous materials on a variety of IL combinations and reported that the MCM/ILs showed the superior results in terms of CO2 capture capacity. Cheng et al. [58] demonstrated that hybridized ILs impregnated onto various sieves had significant advantages over unsupported ILs. The second method for the preparation of IL hybrids is grafting. Nkinahamira et al. [59] reported that the adsorption selectivity of the adsorbent can be increased greatly by the introduction of ILs onto mesoporous materials. Zhu et al. [60] evaluated that grafting imparts stable functionality to the ILs leading to great retention and effective adsorption. Yuan et al. [61] demonstrated that grafting of amine based ILs onto titanate nanotubes improved the adsorption capability of the adsorbent up to 2.46 mmol/g.

12.7 IL hybrids with membranes for CO2 capture A widely used technology for separation and capturing of CO2 is membrane technology. The two fundamental factors related to this technology are selectivity and permeability [62]. These two factors working antagonistically have a trade-off effect and thus the main focus in this technology is on improving selectivity by modifying materials leading to effective permeability [63]. It has been reported that the CO2 capturing ability of ILs is much superior [64] and provides a viable root to introduce CO2 separation membranes [65]. Three different types of ILs hybrid membranes exist which are: (i) ionic liquids supported membranes, (ii) poly ionic liquids membrane, and (iii) composite membrane.

290

12. Ionic liquids for carbon capturing and storage

12.8 Ionic liquid supported membrane A membrane technology based on incorporation of ILs on a solid inorganic support to achieve CO2 separation is termed is supporting ionic liquid membrane (SILM). The separation process involves absorption of gas molecules by the layer of absorbing liquid followed by diffusion of captured gas inside the liquid and desorption of the gas. The overall process is time saving and the solvent usage is reduced to a great extent as well [83]. The introduction of ILs lead to a wide operating temperature due to their greater thermal stability [84]. Karunakaran et al. [85] formulated novel membranes based on graphene oxide and [EMIM][BF4 ] ILs for CO2 separation which showed remarkable results as compared to traditional methods. The immobilization of ILs into the pores is a difficult task which is carried out either via vacuum filtration [86] or with the help of an autoclave [87]. Lan et al. [88] stabilized hollow fiber membranes with vacuum treatment which showed significantly improved results in terms of CO2 capturing. Zhang et al. [55] demonstrated that introduction of water serves as an additional support in transport mechanism for CO2 suggesting that the selectivity can be improved under humid conditions. Liu et al. [89] introduced 2D nanosheets for additional delivery channels for CO2 transport leading to high-pressure and temperature resistant membrane with long term durability. Hwang et al. [90] reported that the application of electric fields can increasing the overall performance of the membranes by 2–5 times. Jie et al. [91] demonstrated that the membranes can be modified using grafting amine groups which makes use of the fact that amine groups show great affinity with CO2 .

12.9 Poly ILs membrane The large-scale application of SILM is restricted due to poor mechanical properties and pressure stability of IL membranes [92]. One way to stabilize ILs in the membranes is to prepare poly ILs membranes which will impart mechanical strength to the layer owing to the mechanical properties of the polymeric substance [93]. The formation of poly IL membranes involve addition of poly-ILs in a volatile solvent followed by assembly of ILs molecules after the solvent is removed [94]. Tang et al. [95] studied the effect of poly-ILs on CO2 absorption and reported very promising superior results in terms of absorption capacity and desorption rates. Vollas et al. [94] formulated a composite membrane based on PILs-ILs and reported that this membrane was much efficient as compared to ILs monomers. Nellepalli et al. [96] studied the efficacy of PILs based polymers in terms of stability and reported superior results as compared to pure membranes.

12.10 Composite membranes Using polymer as a matrix and ILs as an additive, a novel composite membrane can be prepared with unique mechanical and separation properties. The first choice to formulate such membranes is to crosslink ILs and polymers. The second option is to mix inorganic porous material, polymer, and ILs leading to a mixed matrix membrane (MMM). Halder et al. [97] demonstrated that composite membranes based on [C2 mim] [Tf2 N] and a copolymer

References

291

showed significantly improved CO2 absorption results. Lu et al. [98] reported a relatively high affinity between CO2 and ILs in a ([Bmim][TFSI]) based membrane with a very good performance for CO2 capturing. The blended of ILs with other polymers to create a composite can result in improved the selectivity of membranes [99]. Dai et al. [100] reported that humidification significantly improves the absorption capacity of membranes formed by mixing polymers with ILs. Hao et al. [101] formed a ILs/zeolite based MMM for CO2 capture and reported high perm selectivity of CO2 due to introduction of inorganic porous materials providing better membrane separation effect. Huang et al. [102] utilized graphene oxide customized with ILs as a filler and paired it with a polymer matrix i.e., poly(ether-blockamide), for the formulation of a new MMM. A uniform system with great hydrogen bonding between filler and polymer matrix was reported as a result. Ahmad et al. [103,104] evaluated that the introduction of ILs in the composite membranes lead to enhanced permeability and selectivity by strengthening the affinity among the polymer matrix and the filler. Vu et al. [105] prepared MMMs utilizing ILs-coated ZIF particles and reported that the interface defects were significantly reduced by the introduction of hydrogen bonding of ILs.

12.11 Conclusion and future insights Large scale global warming due to increased CO2 concentration by anthropogenic activities has led to undesired air pollution. A very popular option to eradicate the amount of this greenhouse gas is carbon capture and storage (CCS) which is hindered by various economical and environmental prospects along with the solvents used in the process. A wide range of solvents have gained momentum is that regard having certain advantages and pitfalls and ILs have evidenced to be the excellent candidate at present. The non-volatile and flexible nature of ILs impart an appealing benefit yet their application on a large scale is not implemented as of now due to their non-biodegradable nature and cytotoxic profile. The conventional ILs comprising of 1-n-octyl-3-methylimidazolium cations and hexafluorophosphate and tetrafluoroborate offered good carbon capturing capacities. The mixing of IL trihexyl(tetradecyl)phosphonium prolinate with water further improved the effectiveness of the subject liquids as CO2 adsorbent. The grafting of amine based ILs onto titanate nanotubes also can improve the adsorption capability of the ILs. The experimental conditions and parameters also need significant improvements overall. The key barriers regarding the use of ILs as adsorbent i.e. high cost, difficult density, and more viscosity, can only be handled via integrating ILs into solid supports, membranes or other solvents.

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[103] Ahmad N, Leo C, Mohammad A, Ahmad A. Interfacial sealing and functionalization of polysulfone/SAPO34 mixed matrix membrane using acetate-based ionic liquid in post-impregnation for CO2 capture. Sep Purif Technol 2018;197:439–48. [104] Ahmad N, Leo C, Ahmad A. Effects of solvent and ionic liquid properties on ionic liquid enhanced polysulfone/SAPO-34 mixed matrix membrane for CO2 removal. Microporous & Mesoporous Mater 2017;133:13–21. [105] Vu M-T, Lin R, Diao H, Zhu Z, Bhatia SK, Smart S. Effect of ionic liquids (ILs) on MOFs/polymer interfacial enhancement in mixed matrix membranes. J Membr Sci 2019;587:117157.

C H A P T E R

13 Advances in utilization of carbon-dioxide for food preservation and storage Adeshina Fadeyibi Department of Food and Agricultural Engineering, Faculty of Engineering and Technology, Kwara State University, Ilorin, Kwara State, Nigeria

13.1 Introduction Quality control is an integral part of food preservation and storage [1]. Different approaches have been used to maintain product quality and extend shelf-life by inhibiting microbial magnification [2–5], maintain physiological conditions such as moisture and temperature, and modifying the storage atmosphere [6,7]. The bottom line is to ensure microorganisms, like the bacteria and yeast, are prevented from multiplying to such a level that can cause deterioration or affects shelf-life [6–8]. By this, wastages can be minimized, and the costs of materials and resources will be reduced thereby enhancing efficiency of the system for environmental sustainability [9,10]. Typically, the CO2 and other acid oxides are commonly used as preservatives or additive agents because of their ability to retard the magnification of bacteria and yeast in stored products [11–13]. Current and future applications of the CO2 gas in post-harvest value chain were discussed in this study. For example, the solid CO2 gas, also referred to dry-ice, is an important refrigerant and abrasive agent useful in food equipment cleaning [14–17]. It is a robust industrial substance especially applied in stunning, tanning, decaffeination of coffee, chemical feedstock, and solvent for controlled drying of food [18]. The gas is added to beer, wine, soft-drink, and drinking water to aid effervescence and prevent short-term spoilage [19]. It is also an effective material in the CSA storage of grains and horticultural crops [20–22]. Also, it was suggested to investigate the influence of the dry ice pretreatment on microstructural properties, vibration loadings and rheological stability of the stored foods as a way-forward for further studies. Besides, the CSA technology maybe limited in application during transportation and retail storage of the food due the involvement of big and sophisticated equipment. Thus, this study

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proposed the use of simplified equipment to advance the potential of the CO2 technology in the design of CSA storage facilities. This will help advance development and management of the food products during their storage and preservation.

13.2 Utilization of carbon-dioxide in food preservation 13.2.1 Beverage drink preservation The ability of the CO2 gas to be absorbed easily into a liquid to form tiny bubbles is an advantage for the beverage industries [19]. This provides protection against the activities of microorganisms thereby retaining the beverage quality and extending its shelf-life [23]. The gas is also effective in inactivating enzymes such as pectin methylesterase, polyphenol oxidase, and lipoxygenase, which cause undesirable chemical and physical changes in the food [24,25]. Lima et al. [26] reported an effective preservation of a coconut water and acerola fruit juice beverage drink for 6 months at ambient air temperature. The authors reported acceptable microbiological, color and taste stabilities during period of storage. Also, according to Campos et al. [27], the CO2 has been used to inactivate the enzyme activities in the coconut water beverage thereby effectively reducing the carbonic acid pH and extending the shelf-life of the product. A high-pressure- CO2 technique has been used to inactivate the enzymes without heating that may otherwise cause nutrient denaturing by exerting a minimal impact on the quality but extends shelf-life and inactivates the enzymes activities [11,24,28–30]. A typical flow process for beverage drinks pretreatment using a continuous dense-phase CO2 system can be described in Fig. 13.1 [31]. The system was able to pasteurize the beverage by causing the heating of the product through electrostatic effects. Unlike the actual pasteurization the heat is supplied directly via external sources, this technology obtains its heat by temperature build-up as current passes through an electric conductor. In fact, Porto et al. [31] concluded that the process can minimize quality loss of the beverages even at low pressure CO2 concentration.

13.2.2 Drying of vegetables and fruits Drying is a convenient way of extending the shelf-life of vegetables and fruits by removing excess moisture through mass and heat transfer [32,33]. But this is often associated with a loss in nutrients and structure thereby affecting the physical nature of the food. By using a pressurized CO2 drying technology, the physical nature of the food products can be regulated by maintaining its color and shape, so they appear natural and fresh. However, only few research demonstrates the ability of this technique for preserving the physical and biochemical characteristics or preventing the quality degradation of the dried samples of the vegetables and fruits. A high pressure micronization technique, which uses the high-pressure CO2 drying processes, was reported by Weidner [34] for the drying and particle size reduction of the vegetables and fruits. The system was able to preserve the quality thereby adding value to the product and enhancing its shelf-life. A combination of the high-pressure CO2 pretreatment and drying can control the quality degradation of the food. Wei et al. [13] reported the effectiveness of this technology for controlling the activities of pathogenic microorganisms in the food system. During this process, Balaban et al. [13] reported that Listeria monocytogenes

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FIGURE 13.1 Typical flow process for beverage drinks pretreatment using a continuous dense-phase CO2 system [31].

and other microorganisms, for example, are destroyed when the food was treated with the CO2 at 6.18 MPa at 35°C for 2h, and the nutritional and other chemical quality parameters of the product are preserved after drying. Although the CO2 application pretreatment alone is effective for quality control during food drying, combining the process with novel ultrasound, magnetic field, electric field, nonionizing effects and so on, can better enhance the system performance. For example, the quality and microstructure of fresh honeydew melon can be preserved by freeze drying the product after it has been pretreated with a combination of the CO2 and ultrasound technique, as shown in Fig. 13.2 [35]. According to Jiang et al. [35], the melon fruits pretreated with the ultrasound assisted CO2 show an insignificant drip loss during thawing compared to the untreated products. To the best of the writer’s knowledge, there are no reported research on the application of the combination of the CO2 gas and the magnetic field, electric field, and nonionizing radiations as pretreatment effects on the quality the vegetables and fruits. Therefore, there is a need to further research in these areas.

13.2.3 Food preservation using dry ice The dry ice is a solid form of the CO2 and is suitable for maintaining the freshness of such as meat and dairy products during long trips or overnight shipping [36]. The dry ice has its temperature ranging from −78.3°C or −61.1°C and does not melt, but rather sublimates

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FIGURE 13.2 A high-pressure CO2 - ultrasound pretreatment for freeze-drying of vegetables and fruits [35].

into carbon dioxide gas thereby leaving no liquid mess to clean up [18]. It is produced in the solid blocks, slices, pellet, and rice size pieces depending on the customer’s demand [37]. The attribute of many food products including a bovine muscle [38], seer fish [14,16], arctic charr fillets [39], meat [40], and shrimp [41] has been reportedly enhanced by storing them in the dry ice crystals. Also, according to Jo et al. [15], eggs treated with a chitosan coating and stored with the dry ice will have a reduce loads of Salmonella Typhimurium on its surface than those without treatment with the dry ice at ambient temperature. It will also be limited in terms of the moisture loss, CO2 emission, and pH increase, thereby helping to maintain the freshness and shelf-life of the eggs. Thus, the dry ice has great potential to inhibit the quality loss in the food since it can control the activities of the deterioration microorganisms, pH, and the moisture content of the produce. There is however no available information on the effect of the dry ice treatment on the microstructural properties of the stored food. It is therefore necessary to investigate the influence of the dry ice pretreatment on microstructural properties, vibration loadings and rheological stability of the stored foods as a way-forward for further studies.

13.2.4 Animal stunning procedure The CO2 gas is used to induce unconsciousness on farm animals in preparation for slaughtering [42–45]. As the animal inhales the CO2 gas, the oxygen level in the blood decreases resulting in a loss of brain function and eventual brain death. A typical CO2 stunning procedure is presented in Fig. 13.3 [46]. The moment the animal enters the stunning room, the gas is passed to it in cyclic form as it moves via the turn tables, it becomes unconscious as it exits the other end as shown. According to most literatures [47–50], the loss of consciousness or awareness will normally begin after 1 minute inhalation of about 85% CO2 gas by the animal [22]. During this process, Pedro et al. [22] and Macron et al. [51] reported that the animal will experience a decrease in the brain activity which affected the blood flow and concentration. This permits easy preservation since the animal is now unaware of its surrounding. A group

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FIGURE 13.3 A CO2 stunning procedure for pigs [46].

stunning of the animals, with minimal restraint, less handling or stress can also be maintained in the process. The dependance on people and equipment is also minimal with this type of stunning technology. Despite the advantages of using the CO2 for stunning, the procedure is associated with a lot of challenges. A high concentration of the CO2 can cause fear, pain, and distress before the animals become unconscious [50]. Also, as an agent that stimulates the respiratory organ, it can cause irritation of the nasal mucosa membranes and suffocate the animals before losing consciousness. For these reasons, improvement in the application of the CO2 stunning technique or alternative methods are often sought after in the food industry. So far, available research has shown no clear alternative improvement in the application of this stunning technique in animal husbandry. Thus, it is necessary to focus research in this area to find alternative for the CO2 stunning technology or at least improve it.

13.2.5 Tanning of animal skin Tanning refers to the treatment of animal skins to produce leather. The tanning involves a procedure that permanently alters the protein structure of the skin, thereby making it less susceptible to degradation. However, this process is rigorous and takes time to complete. A compressed CO2 can shorten tanning times, to reduce water effluents and pollution and to save leather-finishing-fats. In conventional tanning process, chemicals are applied for cleaning, but this usually leads to the release of toxic substances like the chromium and sulphate into wastewater tanneries. However, according to Hu and Deng [21] who reviewed the application of a supercritical CO2 for the preservation of leather, this technique can be improved by pressurizing the gas to clean the skin product without involving chemicals, as shown in Fig. 13.4. The high amount of the CO2 required in this process has makes it economically unattractive, but it presents the cleanest approach for tanning of the leather products. For this reason, a synergy of both methods is sometimes employed to reduce cost and ensure effective tanning. The findings of Prokein et al. [52] proposed the production of a high-quality lather

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FIGURE 13.4 A high-pressure supercritical CO2 for tanning of leather [21].

using a CO2 -intensified tanning at 60 bars in combination with a 50 wt% of the tanning agent applied at separate stages. This is a better cleaning alternative, but it does not eliminate the problem since there are still traces of chemicals that may affect aquatic life in the receiving water bodies. Hence, it is necessary to further research on the application of the CO2 gas alone for the tanning of the leather and skin products without the chemical addition at low gas concentration and pressure.

13.3 Utilization of carbon-dioxide in food storage 13.3.1 Control of storage microsphere A CSA is a technology that is used to regulate the gaseous atmosphere in a food storage environment to prolong shelf-life and maintain quality [20]. Agricultural materials like the grains, legumes and oilseed can be stored in a CSA primarily to control insect pests [53] since the system could create an unpleasant condition for their survival. The CSA system are normally designed to cause a reduction in the level of atmospheric oxygen and elevates the CO2 gas concentration. This can be achieved by adding pure CO2 or nitrogen to the bulk grains at 25 °C to ensure the concentration is raised above 35 percent and maintained for 15 days [54]. This is sufficient to cause the insects and pests to suffocate and are killed. A CSA storage of different grains species had been described by Kaspersson et al. [55] and Pelhate [56] in their work on the safety and analysis of the microbial loads of stored barley by determining the proportion of the adenosine triphosphates in the grains stored in a silo [57]. The addition of

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FIGURE 13.5 A hematic storage system for grain storage [83].

CO2 to the grains, according to the authors, increases the safe storage time by several months. A combination of the CO2 and nitrogen fillets was reported to improve the performance of unpolished rice grains in storage [58]. This will create a reduce oxygen and rich CO2 atmosphere for respiration and microbial activities thereby enhancing the product’s storability. A hematic CSA system which is based on the depletion of the oxygen concentration and enrichment of the CO2 gas is suitable for controlling the microorganisms, insect, and pests in the storage, as shown in Fig. 13.5 [83]. This scenario is created via a natural process metabolism of the insect present in the storage and the respiration of the microorganisms within the system, especially if the grains contain high amount of moisture content. It is also effective for extending the shelf-life of meat, cheese, and other animal food products by retarding the bacterial and mold magnifications [59,60]. Consequently, this will increase both the lag phase and the generation time of the spoilage microorganisms; but the bacteriostatic effect of the CO2 gas on the magnification of the microbes and the molds has not yet been reported. Moreover, the CSA storage technology is well proven for preserving the quality of the vegetables and fruits [61]. The mechanism involves a careful material selection and special technique to produce a smart system [62–64], which can control the level of the gases in the storage atmosphere like the CSA system of storing grains in the silos. When fruits or vegetables are stored or packaged in these systems [65–67], they utilize the available oxygen gas and exhale the CO2 gas into the micro-atmosphere [68–71]. The excess of this gas can be removed using a CO2 scrubber or the system maybe specially designed using a nanoparticle [72–74], which can help to remove the excess gas, to balance the atmosphere. However, the products stored using the CSA approach may likely deteriorate faster when it is exposed to the

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FIGURE 13.6 A fouling occurring due to overcrowding of microbes [86].

ambient or physiological temperatures [75–78]. Also, the involvement of big and sophisticated equipment in the CSA may limit its application during transportation and retail storage of the products [79–82]. There is therefore the need for a simplified equipment that can advance the potential of this technology for food storage application in small farms and industries.

13.3.2 Storage equipment disinfection The CO2 are also used as disinfectants for animal feed to destroy microorganisms. The substance sublimates on contact and expand to generate the pressure needed to sweep the microorganisms. It can be applied to dissolve hydrocarbon contaminants, especially those found as residues in large storage tanks because of its low temperature. The dry ice crystals can also be forced through contaminated pipes to remove unwanted substances at the temperature rises in the pipes [84]. This is obviously a relatively new area of research requiring urgent attention because of the contaminants can malfunctioning of the equipment and halt operation [85]. In most cases, the food equipment maybe affected by overcrowding of the microorganisms around the metal (Fig. 13.6), thus resulting in a condition called fouling [86]. When this happens, disinfectants like the CO2 can be applied at high pressure to kill the microbes and enable the equipment for further healthy engagement. To the best of the writer’s knowledge, the commercial application of this technology has not been fully harnessed.

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13.4 Prospects and conclusion The commercial and industrial utilization of the CO2 preservation technology is partly dependent on its solubility in water and partly on its biochemical functionalities, which allows it to exhibit a disinfectant property. In food preservation, the substance is applied to perverse quality during drying and inactivate enzymes, such as polyphenol oxidase, and lipoxygenase, which are responsible for loss of color and flavor in beverages. It is also applied for plant and animal management, including facility inspection and control, cleaning of hide and skin products, and protection of grains and horticultural crops against insects, pest, and microbes’ attacks. However, to best of the writer’s knowledge, there are no reported research on the application of the combination of the CO2 gas and the magnetic field, electric field, and nonionizing radiations as pretreatment effects on the quality the vegetables and fruits. There is also no available information on the effect of the dry ice treatment on the microstructural properties of the stored food. Also, it was suggested to investigate the influence of the dry ice pretreatment on microstructural properties, vibration loadings and rheological stability of the stored foods as a way-forward for further studies. So far, available research has shown no clear alternative improvement in the application of this stunning technique in animal husbandry. Thus, it necessary to focus research on the improvement of the CO2 stunning technology to minimize the residual chemical generated into the wastewater bodies for a better cleaning alternative. Finally, the food products stored using the CSA technology may likely deteriorate rapidly when exposed the ambient or physiological temperatures. Also, the involvement of sophisticated facilities in this procedure may limit its application during transportation and retail storage of the products. Therefore, a simplified equipment was recommended to facilitate management of the storage system.

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C H A P T E R

14 An insight into the recent developments in membrane-based carbon dioxide capture and utilization Pritam Dey, Pritam Singh and Mitali Saha Department of Chemistry, National Institute of Technology Agartala, Tripura, India

14.1 Introduction Carbon dioxide (CO2 ) is the primary greenhouse gas that has the highest contribution toward global warming potential, due to a rise in Earth’s temperature. CO2 is produced when a carbon-rich fuel is completely burnt with the help of oxygen. Fossil fuels are considered the primary sources of energy, amounting to around 80 percent of the total global energy in 2019 [1]. Thus, the burning of fossil fuels is the primary reason behind CO2 production, while industrial and vehicular exhausts are the primary emitters of CO2 in the atmosphere. In the last century, uncontrolled and unrestricted use of such fossil fuels has led to an unprecedented increase in the CO2 levels in the Earth’s atmosphere. In this regard, fossil fuel utilization for energy needs and industrial purposes has resulted in more than 35 gigatons of worldwide CO2 emissions in the last year itself, which is the highest to date [2]. The rise in CO2 levels in the atmosphere is a real concern for our society. Implementation of stricter emission norms, utilization of cleaner energy sources, reduction in the dependency on fossil fuels, and encouragement toward afforestation are a few ways with which the issue of rising CO2 levels can be kept under control [3,4]. With the growing impetus toward the reduction of CO2 concentrations in the atmosphere to thwart the rising problems of global warming and climate change, scientists and researchers have suggested the following three strategies [5]: (1) To reduce energy intensity – which aims at minimal and efficient use of energy (2) To reduce carbon intensity – where the primary energy sources have to be shifted towards non-fossil fuels based and renewable energy

Green Sustainable Process for Chemical and Environmental Engineering and Science: Carbon Dioxide Capture and Utilization DOI: https://doi.org/10.1016/B978-0-323-99429-3.00012-6

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FIGURE 14.1 Carbon dioxide capture strategies.

(3) To enhance CO2 sequestration – which involves technological up-gradation for enhanced CO2 capture and utilization In recent times CO2 has emerged as one of the important gaseous raw materials in several industries. CO2 is generally added in compressed form in soft drinks via a process known as carbonation. CO2 finds application as an important industrial gas in the welding sector [6]. CO2 has immense application in the synthesis of alternative fuels, where it goes through an electrochemical reduction process to produce fuels and polycarbonates [7]. CO2 is the raw material in the production of synthetic gas or syngas and also in Fischer-Tropsch liquid fuel [8]. CO2 can also be used for producing biofuels using CO2 -concentrating bacteria, which utilizes biochemical pathways to synthesize hydrocarbons from CO2 and water [9]. Moreover, other uses of CO2 include enhanced recovery of oil and gas [10], energy storage [11], and chemical industry – for synthesizing methanol [12,13]. Thus, capturing a problematic gaseous emission (CO2 ) and utilizing it for the production of valuable products (chemicals and fuels) is a crucial method to mitigate the high levels of CO2 in the atmosphere which in turn would alleviate the issue of global warming.

14.2 Carbon dioxide capture technologies CO2 capture has become an economically significant and environmentally decisive way to mitigate global warming. In this regard, various CO2 capture technologies have evolved in the last few decades. The CO2 capture process is usually coupled with technology that governs either its storage or utilization (Fig. 14.1). Thus, the two types of CO2 capture technologies fall into the domains of carbon capture and storage (CCS) and carbon capture and utilization (CCU) [14]. In CCS, the carbon is captured as gaseous CO2 emission at the point of generation, compressed, and transported to a storage spot. The storage usually involves placing the compressed CO2 in underground geological formations [15]. The CCS technologies

14.2 Carbon dioxide capture technologies

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mostly target point emissions coming from burning fossil fuels for power generation and transportation, two of the most power-intensive sectors and also major contributors to global CO2 levels [6]. Although technologies to capture CO2 from the flue gas of coal-fired power plants can cut emissions, these technologies still cannot reduce the threat posed by the CO2 that has been already added to the air. In contrast to the mitigation of CO2 from industrial flue gas, CCS is advantageous, as not all the CO2 needs to be removed from the air [16]. On the other hand, CO2 capture and utilization (CCU) is considered a significant mitigation strategy to utilize CO2 as a source for the production of biofuels or other hydrocarbon compounds [8,17]. In CCU, the captured CO2 undergoes chemical or bio-chemical modifications which upgrade the CO2 into hydrocarbon-based fuels or other valuable chemicals. The CO2 capture can be categorized into three types, depending upon the place and method of CO2 capture. These three processes are pre-combustion CO2 capture, oxy-fuel combustion CO2 capture, and post-combustion CO2 capture [6]. Fig. 14.1 illustrates the three CO2 capture methods. As the name suggests, in pre-combustion CO2 capture, the CO2 gas is captured before the actual combustion of fuel (specifically, coal) takes place. As soon as the coal is gasified, carbon monoxide (CO) is released. A CO2 reformer, causes a steam-based reaction of CO at high pressures to yield hydrogen gas and CO2 . The evolved CO2 is removed using physical scrubbers or chemical solvents, while the hydrogen is used for generating power and electricity in the thermal plant. The removed CO2 is compressed and stored for further use. Physical absorption, adsorption, chemical looping, and non-polymeric membrane-based techniques are used to capture CO2 in the pre-combustion capture process. In oxy-fuel combustion, the fuel is burnt in an oxygen-rich environment (with above 85 percent oxygen saturation in the gas stream) instead of air to enhance the combustion efficiency of the unit. As we know that complete combustion of fuel results in only CO2 release and no formation of CO, thus this method generates a huge quantity of unadulterated CO2 upon combustion of fuel. An air separator unit separates pure oxygen and sends it to the boiler/ combustion unit. After combustion, CO2 remains as the primary flue gas (with a very high concentration in the emitted gas), which can be easily captured, compressed, and stored. An advantage of the oxy-fuel combustion process is that other gaseous emissions like oxides of nitrogen, particulate matter, CO, etc. are emitted at very trace amounts, making CO2 capture convenient [5]. Adsorption, chemical looping, and cryogenic air separation are usually employed for the oxy-fuel combustion CO2 capture. In the post-combustion capture method, CO2 capture takes place after the fuel is burnt in the air. As a result, CO2 , along with other gaseous emissions are liberated from the combustion unit. CO2 is captured from the flue gas after flue gas cleaning or scrubbing, sulfur, and particulate matter reduction, and CO2 separation from the flue gases advanced gas separation technologies [6]. Chemical absorption, adsorption, low-temperature solid-gas reactions, cryogenic CO2 anti-sublimation, calcium looping, and polymeric membrane-based techniques are used for capturing post-combustion CO2 . Physical pre-combustion or chemical post-combustion CO2 capture technologies; like absorption, adsorption, cryogenic separation, chemical looping, etc., with their elaboratelystudied designs and a better understanding of the operational parameters, have led to their wider implementation [18,19]. Chemical absorption using amine solvents is the conventional method to capture post-combustion CO2 . However, hazardous compounds in the atmosphere are due to the high volatility of such amine solvents.to form [20]. As a result, the focus has

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shifted towards novel sorbent materials. Also, cryogenic technologies are very costly, while chemical looping requires excessive amounts of reactive chemicals. Modern technologies are developed with a promise to be sustainable and cost-effective. Likewise, recent innovations in the development and up-gradation of existing CO2 capture technologies should focus on being more efficient, cost-effective, and sustainable for being acceptable in the modern era [21]. A handful of review articles are present in the recent literature which systematically describe the technologies available for CCS and CCU. Conventional technologies like absorption, adsorption, and bio-sequestration are generally highlighted in such reviews. The emergence of membrane technology for CO2 capture is a recent phenomenon, which came into the light only after advancements were made in materials for membrane production and processing. As a result, the niche of CO2 capture using membranes was not exploited to a greater extent. A lack of experimental literature is an implication of that. Hence, this chapter shall prove effective in summarizing the recent developments in membrane technology for CO2 capture, storage, and utilization. The subsequent headings deal with a brief description of the concept of the membrane technology behind CO2 capture, some examples of this technology being used throughout the world, and the advantages and limitations associated with this technology.

14.3 A brief about membrane technology A membrane is a partition (or a thin film) between two different fluid phases through which an active transport of specific solute molecules takes place depending upon the type of gradient which exists between them. Membranes are very much in use for the advanced separation of valuable products (solutes) from a mixed stream (solution). In membrane separation, the target (solute) is selectively permeated from one side of the membrane (packed inside a module) to its other side, while the bulk fluid (retentate) is discarded on the former side. The affinity of a solute molecule toward the membrane and the selectivity of the membrane are two important parameters that govern the separation efficiency of the membrane [6]. Fig. 14.2 provides a simple illustration on the working of a membrane in selective separation of a target molecule. Different types of membrane modules exists, in which membranes are fabricated and placed together in series or parallel formation to yield a membrane system or network. Such membrane networks are designed as per the requirements of an industry or the intent of separation (the purity to be achieved and the rate of separation to be followed) [21]. Earlier naturally-obtained membranes were considered for preparing membrane modules. But, their longevity and purity raised serious questions. As a result, synthetic membranes came into the picture and are now dominating the membrane industry. Synthetic membranes are of two types: polymeric membranes (in which organic membranes as well as organic-inorganic coupled membranes are present), and inorganic membranes. However, both polymeric and inorganic membranes can have either porous or non-porous nature, depending upon the material and method of fabrication. Separation through a porous membrane occurs due to Knudsen diffusion of the target molecules through the membrane structures. Whereas, in an non-porous membrane, separation occurs due to atomic, molecular, or ionic-diffusion mechanism coupled with a facilitated transport of the target molecules from the feed to the

14.3 A brief about membrane technology

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FIGURE 14.2 Mechanism of membrane separation.

permeate side. As a result, inorganic membranes are usually linked with superior performance and stability. Now-a-days much research and development are focused on developing mixed-matrix and composite membranes. Mixed-matrix membranes (MMM) are fabricated by embedding inorganic fillers into a polymeric matrix to get a combination superior performance of inorganic materials with the facile handling properties of polymeric materials. Composite membranes possess an asymmetric structure with a thin and dense top layer embedded on a thicker but porous support layer. Both the layers can be fabricated separately, which increases the complexity and cost of such membranes [21]. However, both the layers can be optimized separately for improvements in the selectivity, permeability, and stability of the composite membrane. The placement of membrane in a module plays an important role in determining the overall performance and durability of a membrane system. Three most widely used membrane modules are: Plate and frame module, spirally wound module, and Hollow fiber module [22]. The effective membrane area, packing volume, and membrane cost per area are dependent on the material for membrane fabrication and the type of membrane module [21]. The performance of the membrane operation also depends upon the type of feed flow and membrane placement. The two commonly used modes of module operation are: cross-flow and counter-flow modules. For a fixed feed flow rate, feed pressure, retentate composition and permeate pressure, a counter-flow mode requires lesser effective membrane area and power consumption as compared to a cross-flow mode. Also, a higher mass fraction of the target gas molecule can be achieved in the permeate stream by using the counter-flow mode of membrane operation [23]. The classification of membranes and their modules are illustrated in Fig. 14.3. Few benefits of using membranes are: reasonably minor carbon footprint associated with its usage, no change in the phase of the target is requited, not a cumbersome mechanical system to operate, operates generally under steady-state conditions, an ease to scale up, and flexibility in operations [21]. Still, some disadvantages of membranes are: cost of membrane-modules

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FIGURE 14.3 Membrane classification (with examples) and types of membrane modules.

increases with an increase in the selectivity for a particular molecule (target), a compression system is required for generating the required inlet pressure, membrane fouling occurs during continuous operations, and issue of membrane wetting [22]. Much research and innovations are still required to make membrane technology a mature one for thorough industrial use.

14.4 CO2 separation using membranes Gas separation using membranes came into the picture in 1980s, when Cellulose acetate membranes were used to separate CO2 from natural gas stream. During the 1990s selective separation of N2 from air using polymeric membranes became a thrust area in membrane research [23]. However, deployment of membrane systems for gas separation at commercial scales was not successfully carried out in the past. More emphasis was given on research and development of membrane systems for CO2 capture and storage. Only recently, a few industrial applications of gas separation using membrane systems were performed. In 2015, Abanades et al. [21] has listed out a detailed review recent innovations in CO2 capture technologies in which recent developments in membrane modules and systems for CO2 separation were also presented. Very recently, a couple of articles were published which reviewed latest star-of-the-art in research and development as well as industrial applications of CO2 capture, storage, and utilization technologies [6,17,21,24]. However, there is a deficit in the thorough reviews only on membrane processes for CO2 capture and utilization in the recent literature [25,26]. Thus, in this chapter the authors intend to highlight the recent developments in membrane-based CO2 capture and utilization.

14.4.1 Pre-combustion CO2 capture using membranes As mentioned earlier the air with steam is first passed through a CO2 reformer or a coal gasifier, which converts the two into CO2 and H2 , also called syngas, using a water-shift

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reaction. This CO2 is then removed from the mixture of gases, using a membrane system. Membrane-based pre-combustion CO2 capture is the least talked-about and worked-upon CO2 capture technique [23]. Researchers are usually more concerned about capturing the postcombustion CO2 . However, the cost of CO2 capture during pre-combustion is significantly lower than that for post-combustion process. One reason for this is the requisite of adding an extra equipment, like compressor or vacuum pump for pressurizing the feed stream [6]. The membranes for the pre-combustion CO2 capture are usually of two types: H2 -selective membranes and CO2 -selective membranes. The H2 -selective membranes permeates H2 gas, while CO2 is present in the retentate. Whereas, the CO2 -selective membranes have high selectivity for CO2 and preferentially permeates the same. Usually, metallic or inorganic membranes are utilized for CO2 capture from the air before it is sent to the combustion chamber. Metallic membranes are mostly considered for pre-combustion CO2 capture due to their high thermal stabilities. Zeolite-based membranes and membranes involving metalorganic frameworks are also considered for H2 /CO2 separation [27]. Additionally, highly efficient absorbent coupled with a membrane contactor were also investigated for successful H2 or CO2 separation from pre-combustion process. Ionic liquids as absorbent species were reported to have improved the performance of an absorptive membrane separation of precombustion CO2 [28]. Recently, mixed matrix membranes (MMM) and ceramic-composite membranes were tested for pre-combustion CO2 capture [26]. Such membranes were reported to have enhanced CO2 permeability as well as H2 /CO2 selectivity. Table 14.1 presents a few selected recent advancements in pre-combustion CO2 capture using advanced membrane systems. In each case, the feed was H2 and CO2 which came out of the steam reformer. Most of the membrane-based gas separation technique were aimed at permeating H2 from the feed so that high purity CO2 can be collected from the retentate stream. CO2 capture of around 90 percent was reported in most of the recent literature, which indicates superior performance of MMMs and Pd-based composite membranes.

14.4.2 Oxy-fuel combustion CO2 capture using membranes The major issue with post-combustion CO2 capture is the presence of N2 in the flue gas coming out of the combustion chamber. In oxy-fuel combustion process, pure oxygen in suppled for the combustion of fuel, thus there is no N2 present in the exhaust gas. The combustion of fuel results in the liberation of CO2 and water vapor. While water vapor can be condensed out of the exhaust stream, the remaining the CO2 -rich gas can be easily compressed, stored, and utilized. Industrially, in oxy-fuel combustion process, membranes are used for oxygen separation or O2 /N2 separation from air. Thus, O2 -selective membranes are generally employed in oxy-fuel combustion CO2 capture. In general, fluorite-based and perovskite-based membranes are used for selective O2 separation from air [27]. This separation can be performed with polymeric membranes at ambient temperature or with ceramic oxygen transport membranes at elevated temperature [36]. Usually, most of the work in the literature under membrane-based oxy-fuel combustion process had dealt with removing O2 or N2 from the air so that combustion of fuel can near its completion, which can result in the emission of pure CO2 in the exhaust stream. Ji and Zhao [27] and Sunarso et al. [37] have provided extensive literature review on oxy-fuel combustion CO2 capture process using membrane, prior to 2015. Table 14.2 provides a few selected recent

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TABLE 14.1 Selected recent membrane technologies for pre-combustion CO2 capture. % CO2 removal/CO2 flux/ CO2 permeability/H2 /CO2 selectivity

Membrane type

Operating conditions

References

Cu-based organic framework blended with polybenzimidazole to form MMM

Feed = H2 +CO2 Feed pressure = 2–5 bars CO2 membrane adsorption at 308 K, and 333 K Time = 5 h

CO2 permeability = 15.1 Barrers H2 /CO2 selectivity = 29–39

[29]

Pd-porous ceramic composite membrane

Feed = H2 +CO2 with N2 sweep Feed pressure = 2.6 bar Temperature = 673 K Time = 3600 h Counter-current flow

90 percent CO2 capture achieved H2 /CO2 selectivity = 500

[30]

Butyl-3-methlyimidazolium tricyanomethanide as CO2 absorber

Feed = CO2 +He Feed pressure = 20 bar Temperature = 353 K

10.4–24.9 × 10−6 molm−2 s−1

[31]

Pd-(23 percent Ag) alloy composite membrane

Feed = H2 +CO2 with N2 sweep Feed pressure = 15–20 bar Temperature = 573 K Time = 36 h

90 percent CO2 capture achieved

[32]

ZIF-8 nanoparticles in polybenzimidazole MMM

Feed = H2 +CO2 Feed pressure = 6 bar Temperature = 523 K

CO2 permeability 0.765 GPU H2 /CO2 selectivity = 22.3–35.6

[33]

3-stage membrane module with CO2 selective membranes

Feed = H2 +CO2 Feed pressure = 35–110 bar Temperature = 343–423 K

90 percent CO2 capture achieved H2 /CO2 selectivity = 20–50

[34]

Butyl-3-methlyimidazolium tricyanomethanide as CO2 absorber in a shell-and-tube membrane contactor

Feed = CO2 +He Feed pressure = 1–20 bar Temperature = 293–353 K

3.04 × 10−5 molm−2 s−1

[35]

advancements in oxy-fuel combustion CO2 separation using membranes. In recent literature, O2 recovery from air using ion-transport ceramic membranes at high operating temperature (> 1173 K) were mostly presented. Also, O2 recovery of above 90 percent were reported by most authors. The innovations in the field of membranes for O2 /N2 separation in an oxy-fuel combustion process is still at its developmental stage, as no commercialization has been reported till date. Although, lot many literature is already present which showcases the capabilities of such membranes at lab scale. When compared to the cryogenic air separation, the most common CO2 capture technique during oxy-fuel combustion process, membrane-based CO2 capture is still unfavorable owing to high temperature requirement for its operation and requirements of costly membranes for the process [27]. Also, problems of high temperature sealing as well as chemical and mechanical stability of the membrane set-up are still some technical issues that

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TABLE 14.2 Selected recent membrane technologies for oxy-fuel combustion CO2 capture process. % O2 or CO2 recovery/ O2 flux / net efficiency

References

Membrane type

Operating conditions

Ion-transport ceramic, non-porous membrane

Feed = Air Feed pressure = 26.3 bar Temperature = 1173 K

85 percent CO2 recovery Net efficiency = 68 percent

[38]

Ta-doped SrCo0.8 Fe0.2 O3-δ O2 transport membranes in parallel tubes

Feed = Air Temperature = 1223 K Time = 70 h

92 percent O2 recovery Net efficiency = 31.8 percent

[36]

Oxygen transport membrane (non-porous, ceramic)

Feed = Air with water vapor as sweep gas Temperature = 1173 K Pressure = 40 bar

97.5 mol%percent O2 recovery Net efficiency = 85 percent

[39]

La0.6 Sr0.4 CoO3−δ hollow fiber membrane reinforced with stainless steel

Feed = Air Temperature = 1273 K

O2 flux = 2. 29 mlcm−2 min−1

[40]

needs to be addressed before commercializing the oxy-fuel combustion CO2 capture using membrane systems.

14.4.3 Post-combustion CO2 capture using membranes Most membrane operations for carbon capture are primarily linked with the postcombustion CO2 capture condition. CO2 from the industrial flue gases are separated from other components of flue gas, like H2 and N2 , mostly using polymeric membranes. Commonly used polymeric membranes are: cellulose acetate, polysulfone, polyethersulfone, polyvinyl alcohol, polyvinylidene difluoride and polyimide membranes. Polyimides are known to retain the best performance CO2 separation owing to their good thermal, mechanical, and chemical stabilities, and varying CO2 permeability [24]. Whereas, inorganic membranes have higher selectivity, lower CO2 permeability, superior chemical and thermal stability. Recently various mixed matrix membranes (MMM), ceramic porous membranes, metal-organic frameworks, and composite membranes were developed which enhanced the CO2 separation capacities of membrane operations [36] Addition of inorganic filers of micro- or nano-scale into polymeric matrix were found to have increased the thermo-mechanical stability of the membrane. Alami et al. [6], Chao et al. [24], and Kárászová et al. [41] have provided extensive reviews on post-combustion CO2 capture using polymeric membranes. These reviews highlighted the trends in post-combustion CO2 capture using membranes. In recent times, along with the traditional membranes many commercial and advanced polymeric separation membranes like: Pebax® , Matrimid® , PolyActiveTM , PolarisTM , PermSelectTM , PRISMTM , sulfonatedpolyether-ether-ketone (SPEEK), and sulfonated-polystyrene-b-poly(ethylene-r-butylene)-bpolystyrene (S-SEBS) were in great demand for preparing MMMs as well as composite membranes for post-combustion CO2 capture. Moreover, polymer intrinsic microporosity (PIM) films and PolyILs (polymeric membranes infused with ionic liquids) are current materials for

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TABLE 14.3 Selected recent membrane technologies for post-combustion CO2 capture process. % CO2 recovery/ CO2 /N2 selectivity/ CO2 flux / net efficiency

References

Membrane type

Operating conditions

Poly-amine fixed site carrier membrane

Feed = Flue gas from coal-fired plant Temperature = 323 K Pressure = 1.1 bar Counter-current mode

90 percent CO2 recovery CO2 permeance = 3.8 × 10−5 kmolm−2 bar−1 s−1

[20]

Poly(vinylidene fluoride) fibers in Polydimethylsiloxane thin film composite hollow fiber membrane contactor

Feed = CO2 +N2 Temperature = 298 K Pressure = 0.4–1.4 bar Counter-current mode

38 percent CO2 recovery CO2 /N2 selectivity = 27 CO2 permeance = 2049 GPU

[42]

Nano-porous single-layer graphene membrane

Feed = CO2 +N2 +H2 O Two stage cross-flow mode of membrane operation

90 percent CO2 recovery CO2 permeance = 10,000 GPU CO2 /N2 selectivity = 30

[43]

Polyvinylamine-based facilitated transport membranes

Feed = Flue gas Temperature = 313 K Pressure = 1.5 bar 2–3 membrane stages

90 percent CO2 recovery CO2 /N2 selectivity = 30 CO2 /O2 selectivity < 10 CO2 permeance = 2.18 Nm3 m−2 h−1 bar−1

[44]

Polydimethylsiloxane-Trimesoyl chloride-PolydimethylsiloxanePolysulfone multi-layered composite membrane

Feed = CO2 /N2 mixture Temperature = 298 K Pressure = 1.5 bar 2 membrane stages

90 percent CO2 recovery CO2 /N2 selectivity = 13-CO2 permeance = 6000–10,500 GPU

[45]

active research on membrane-based post-combustion CO2 separation. Table 14.3 provides selected recent advancements in last two years in post-combustion CO2 capture using membrane systems. A recent review by Han et al. [46] has discussed the emerging polymeric membranes for post-combustion CO2 separation, with in depth analysis of the theory involved in the working of such polymeric membranes. Also, the advancements in the field of thin film membrane synthesis and their utilization in post-combustion CO2 capture were reviewed by Liu et al. [47], which needs mentioning. Recently, hybrid systems comprising of membranes and other separation process were proposed and developed for increasing the capture efficiency of post-combustion CO2 . Membranes systems coupled with solid adsorbents or with strong absorbents or with chemical looping technique are under research and developmental stage for enhancing CO2 capture [6]. However, the manufacturing as well as operating cost of such hybrid systems are a cause of concern before being commercially utilized in industrial plants for large-scale post-combustion CO2 capture.

14.4.4 Future considerations for membrane-based CO2 capture The advancements in manufacturing membranes and membrane systems for CO2 capture should aim for high CO2 permeability and selectivity. However, a trade-off exists between the

14.5 CO2 utilization using membranes

321

two in terms of membrane operation which needs to be balanced in the future for enhancing the membrane performance. Membranes used for CO2 enrichment, as well as for N2 , H2 , and O2 separation, should possess long term thermo-mechanical and chemical stability, and should be efficient and cost-effective [41]. Polymeric membranes need to withstand membrane ageing and plasticization, while inorganic membranes need to have enhanced flexibility in operation. Also, with the availability of membrane performance data, developments in the membrane manufacturing can be fast-tracked in the future [21]. All membranes need to be manufactured in a way such that they possess least fabrication complexities, can be easily scaled-up, and are produced from cheapest possible precursors. The cost of membranes and their modules is still an issue, which dictates the overall process economics. Many of the membranes and membrane systems currently under development are likely to decrease in cost over time with technological advancements in production and installation techniques and also a general rise in the market demand and competition [21]. Recent trends of utilizing bio-based “green composite” materials can be looked into for manufacturing of advanced composite membranes for specific gas separation. Green composites are a current thrust area of research for manufacturing of a variety of advanced materials which have lover carbon footprints and are sustainable than most commercially available materials [48].

14.5 CO2 utilization using membranes The concept behind CO2 utilization is to use the emitted CO2 for synthesizing new substances, either as a raw material or as a catalyst. The captured CO2 is linked with reduction in the carbon footprint of the Earth. The captured CO2 can be utilized in many ways. The direct utilization of CO2 is performed in many food-beverage and chemical industries [26]. Microalgae production has recently being considered as a major CO2 sink after a wide variety of applications have been realized. Some of the products of microalgae production include pharmaceuticals, livestock food, and biofuels [7,8]. Other major uses of CO2 are as commercial refrigerant [49], in the synthesis of acetate using molecular H2 gas [50], methanol synthesis [12,13], urea generation [51], electrochemical synthesis of formic acid [52], and enhanced oil recovery by means of CO2 flooding or displacement process [53]. Some interesting CO2 utilization pathways include desalination of seawater [54] and photocatalytic removal of dyes from aqueous solutions [55,56]. Fig. 14.4 illustrates the various utilization pathways for capture CO2 . Among the above-mentioned CO2 utilization pathways, membrane-based CO2 capture and utilization for algal production is a sustainable approach because the captured CO2 is directly utilized by the algae for biomass formation and synthesis of various metabolites [9,16]. The use of membrane-based photo-bioreactor is an example of example of the same. In such bioreactors, the membrane module helps in permeation of pure CO2 from the entering air, which is utilized by micro-algal culture supported behind the membrane module. The micro-algae utilizes this CO2 by cellular sequestration and formation of organic compounds (metabolites) with the release of O2 gas into the environment. Thus, membrane-based algal bioreactors not only sequestrate the ever increasing CO2 in the air, but also releases O2 in the environment. Also, the algal biomass can be easily converted into bio-fuels by various

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FIGURE 14.4 Pathways for CO2 utilization.

processes [9]. Some recent and innovative membrane bioreactors [14] and membrane microalgal reactors [57] were fabricated to capture CO2 from air and utilize it for microbial biomass growth. Such systems can prove very beneficial in the coming future. However, further research and development in this field is imperative to enhance the CO2 permeability of such membranes.

14.6 Conclusions There is an eminent need to mitigate the rising carbon dioxide (CO2 ) concentrations in the Earth’s atmosphere. In this regard, CO2 capture technologies, before and after the combustion of fuel, have become paramount in the modern era. Within the CO2 capture technologies, membrane-based CO2 capture has emerged as an efficient and sustainable alternative. With tremendous advancements in membrane technologies, CO2 capture using membranes has become a hassle-free affair. Metallic or metal-ceramic composite membranes with high N2 /CO2 selectivity are mostly investigated for pre-combustion CO2 separation. Such membranes have high thermo-mechanical stability. During oxy-fuel combustion process, fluoride or perovskite-based membranes have demonstrated high O2 selectivity, thus resulting in a high percent weight CO2 in the exhaust stream. Whereas, polymeric membranes are used for post-combustion CO2 capture. Now-a-days, commercial membranes and multilayered polymeric membranes, having high CO2 permeance and CO2 /N2 selectivity are utilized for CO2 capture from industrial flue gases. Still more advancements and innovations are required for making the membranes more affordable, stable, and high-performing. The captured CO2 can be utilized by different pathways, for material synthesis, enhanced oil

References

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recovery, pharmaceuticals, and bio-fuel production. Membrane-based micro-algal bioreactors are a sustainable technology for CO2 capture and utilization. Such systems not only sequester CO2 and provide O2 to the atmosphere, but also provide valuable bio-materials as well as biofuels. Such systems are also used for water remediation and wastewater treatment. Thus, it can be concluded that membrane-based CO2 capture and utilization can become a sustainable technology in the coming future for global warming abatement.

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C H A P T E R

15 Carbon dioxide to fuel using solar energy Srijita Basumallick Asutosh College, University of Calcutta, Kolkata, India

15.1 Introduction Green plants convert CO2 to bio-fuel glucose using sun light and chlorophyll. But the mechanism of photo-synthesis is not yet fully known, it is not yet possible to replicate photosynthesis in laboratory. It is known photolysis of water at light phase and reduction of CO2 in the dark phase are the key processes of photosynthesis. Again, it is known natural photo catalyst NADP plays an important role in Kelvin cycle, where CO2 is reduced. In fact, conversion of CO2 to glucose is thermodynamically unfavourable, a uphill reaction, as free energy change of this reaction if positive, the thermodynamical requirement is fulfilled using solar energy. This reaction is also kinetically slow, because of requirement of high activation energy. Thus, requirement of efficient photo-catalyst is important for conversion of CO2 to simple fuel like methanol, ethanol etc. In the recent years, immense interests are noted [1–10] in development of photo-catalysts and electro-catalysts for carbon dioxide (CO2 ) reduction. It is known CO2 is a greenhouse gas causing global warming. Natural photosynthetic way of reducing CO2 pollution (as stated above) is not enough to restore the ecological CO2 balance because of rapidly increasing global carbon emission (exceeding gig ton, annually). Thus, photo-chemical or electro-chemical reduction [1–10] of CO2 is important to restore CO2 balance.

15.2 CO2 reduction onto semiconductor surface In general, photo reduction of carbon dioxide is carried out using a semiconductor with band gap energy compatible to solar light. Upon irradiation electron is excited to conduction band leaving behind hole in the valence band. This is illustrated by our work on reduction of carbon dioxide and formation of formic acid. We have carried out an experiment where chitosan coated Cu2 O quantum dots were drop casted on boron doped Si-wafer. Chitosan

Green Sustainable Process for Chemical and Environmental Engineering and Science: Carbon Dioxide Capture and Utilization DOI: https://doi.org/10.1016/B978-0-323-99429-3.00033-3

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FIGURE 15.1 Probable mechanism of CO2 reduction on Chitosan coatedCu2 O dispersed Si-water.

provided stretched film with well separate and exposed Cu2 O quantum dots that helps adhere Si-wafer as confirmed from AFM images. Increased -OH and -C = O peak indicated formic acid formation. The quantum yield was low when catalyst surface was kept inside glass jar containing H2 O which acts as hole scavenger. Formation of formic acid as photo-reduction product of CO2 was also supported from a similar study that was done by Liu et al. [9] using TiO2 catalyst. They reported formation of formic acid as photo-catalytic reduction product of CO2 on to TiO2 surface. In case of chitosan coated Cu2 O quantum dot dispersed Si-wafer we have proposed a probable mechanism of formation of formic acid (shown in Fig. below) [11]. Interestingly, unlike photo synthesis here CO2 reduction occurs in the presence of light (Fig. 15.1). Recently photo electrochemical reduction of carbon dioxide is also been reported, here semiconductor is anchored to an electrochemical cathode surface where photo reduction is enhanced many fold. Thus carbon dioxide reduced onto semiconductor surface accepting this photo excited electron in two different ways, first simple photo-catalytic (PC) reduction and secondly, photo electro-catalytic (PEC) reduction [12]. In both these processes overall action happens through different steps. In case of PEC photo catalyst present in cathode reduce carbon dioxide adsorbed on the surface. Whereas the hole migration takes place at counter electrode [12] or anode side. This hole in anode is scavenged by water or sacrificial electron donors like amine as well as other whole scavenging material. It is important to note that band gap of chosen semiconductor catalyst should straddle reduction and oxidation potentials of carbon dioxide as well as water or sacrificial electron donor or hole scavenging material. Now due to band bending in the interface of semiconductor and electrolyte or vacuum, the band gap of semiconductor catalyst on the surface becomes much less than bulk band gap. As a result net band gap must be greater than the oxidation and reduction potential of carbon dioxide and particular anode reaction as mentioned above [13].

15.3 Major bottleneck for CO2 reduction Among several products of CO2 reduction, methanol, ethanol, methane and ethane formation is favourable. But it is kinetically highly unfavourable as it goes through intermediate radical anion CO2 ◦− formation. Conversion of CO2 to CH3 OH requires 6e- as shown in Eq. (15.1). This makes it even more kinetically unfavourable to reduce. Free energy of formation of CO2 ◦−

15.4 Different types of photo catalyst

329

radical anion (1 e- transfer to CO2 ) is higher compared to other products as it requires bending of linear CO2 [14]. CO2 + 6H+ + 6e = CH3 OH + H2 O E0 = −0.38 V vs NHE at pH 7

(15.1)

Another problem of photo-reduction of CO2 onto semi conductor surface is recombination of hole-electron pair. For that rate of electron and hole separation and migration through catalyst should be comparable to the rate determining step of reduction of CO2 . This step may be either rate of mass diffusion of product or reactant. It is probable that water splitting reaction taking place onto hole site and plays an important role. Due to band bending modified and band gap issue water splitting reaction become competent with CO2 reduction. Both of these reactions got same anodic oxidation of O2 − (H2 O) to O2 at +1.23 V and CO2 reduction potential varies from −0.24 V (CH4 formation) to −0.89 V (HCHO formation) vs NHE at pH 7, 1 atm and 25 °C depending on the products, where as in case of water splitting at cathode proton adsorption and hydrogen evolution takes place at −0.41 V vs NHE at pH 7. Not only that CO2 ◦− can form CO2 back. Hole generated in the semiconductor catalyst produce oxygen, OH radical, also hole itself and free proton from water all of which can take back electron from the intermediate species like CO2 ◦− to form CO2 back through a reverse reaction. As already discussed bending of CO2 during adsorption on catalyst surface is a prime step for carbon dioxide reduction reaction. This can be done by several ways like increase in surface area, increasing defect, introducing basic sites or metal co catalysts. It is noted that some crystalline faces facilities CO2 adsorption like 101 of TiO2 . Whereas 001 face of TiO2 has higher active oxygen as a result high photo catalytic activity is seen. At the same time many mesoporous zeolite particles with catalytic sites inside pore surface shows better catalytic activity for selective methanol formation from carbon dioxide reduction. This can be explained due to formation of stable charge transfer complex (Ti3+ -O− )∗ and trapped hole centres on O− sites in TiO2 on zeolite surface. Introduction of basic sites either inorganic or organic like amines or calcium oxide, magnesium oxide works towards better adsorption of carbon dioxide. Wang and co-workers found addition of MgO on TiO2 as well as Pt-TiO2 enhances CO and CH4 formation respectively. Crystal defect play important role in adsorbing carbon dioxide. Thus oxygen and sulphur vacancies introduced in several semiconductors show reduction in activation energy with increase in adsorption of CO2 . Co catalysis of noble metals as Ru play an important role as it can trap electrons and reduce the recombination process as well as desorption of carbon monoxide help reduce poisoning of catalyst surfaces.

15.4 Different types of photo catalyst 15.4.1 Homogeneous photo-catalysts When homogeneous catalyst is used rate of CO2 reduction becomes faster due to less kinetic barrier for electron transfer reaction and formation of easy adduct/complexation. Again heterogeneous catalysts undergo more poisoning compared to homogeneous one [14]. Thus water dispersible catalyst might produce better yield in CO2 reduction due o increase in net surface area or more adsorption of CO2 on to catalyst surface. Some amine compounds are highly soluble in water and enhance CO2 reduction. Since one-electron reduction of

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CO2 requires strong reducing agents that are generally difficult to obtain by photochemical methods [15]. It is known amines enhance solubility of CO2 [15] which can help in better yield too. Sacrificial electron donor amines like TEOA (triethanolamine) or tertiary amine is used [16,17] in combination with another photo sensitizer or alone [16,17]. But after electron donation to CO2 , it undergoes secondary reactions leading to the formation of by-products i.e. oxidized amines [18,19]. Theoretical models for renewable sacrificial electron donor amines have been proposed [17], where once used, by-product can be hydrogenated to return amine for one more CO2 ◦− radical anion generation, there by working as a catalyst. Major problem associated with this approach is that a well divided compartment for CO2 reduction as well as amine regeneration site is essential for practical purpose. Especially they form homogeneous mixture in water and it is difficult to separate amine and products of CO2 reduction. But this remains unsolved. Among all possible heterogeneous photo catalyst semiconductor photo catalysts are most important. There are several semiconductor systems that has band gap fitting to the required potential. Examples are TiO2 , Cu2 O, CdS, ZnO, GaP, SiC, WO3 etc. Among which TiO2 and Cu2 O is most important. In case of semiconductors corrosion is a major issue to be identified.

15.4.2 Cu based photo-catalysts Among different photo- catalysts, Cu2 O is a unique semiconductor catalyst because it absorbs in the visible light (sunlight), its CO2 adsorption efficiency is really high and last but not the least certain crystal faces as well as copper oxygen cluster helps bending CO2 reducing free energy of formation CO2 ◦− . It is known [5–7] Cu2 O is an efficient catalyst for CO2 reduction in sun light or using electrical energy. This is because Cu2 O has compatible crystal structures [20] where CO2 molecules can easily adsorbed. Calculated values of heat of adsorption show favourable interaction [20] of CO2 molecules on Cu2 O catalyst. In spite of several unique features, major problem associated with Cu2 O photo catalyst is that it undergoes photo-corrosion.

15.5 Reduction of CO2 to methanol using Cu2 O as photo catalyst Even though the CH3 OH formation has huge kinetic barrier due to 6e- reduction process but CH3 OH is a better fuel than that of CH4 or C2 H4 . Surprisingly, due to the unique binding of CO2 onto Cu2 O surface as well as its H binding capacity that helps stabilizing the intermediate which is required for methanol production.

15.6 Reduction of CO2 to methanol using Cu2 O as electro catalyst In case of electrochemical reduction current density directly relates to the formation of by products as well as faraday efficiency of CO2 reduction to methanol. But higher current density helps 6 electron transfer reaction on inefficient catalyst surface. Several electro catalysts like Mo [21] and different types of Ru catalyst [22,23] have low current densities (60 percent). But among all other Cu shows best faradaic

15.7 Benefits of using RGOin the composite catalyst

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efficiency 100 percent [22] for current density up to 33 mA cm−2 . Faradaic efficiencies have been calculated on the basis of six-electron transfer reduction of CO2 and efficiencies greater than unity suggest electrochemical-chemical (EC or CE) mechanisms [7,22,24].

15.6.1 Reduced graphene-oxide, Cu2 O and amine compounds composite photo catalysts for CO2 reduction A composite catalyst where a heterogeneous catalyst is grafted with a sacrificial amine is a better choice because catalyst purification from reaction mixture is easy and it leads to recovery of sacrificial amine too. In another approach, H2 O oxidation mediated H2 generation is coupled with CO2 reduction. But amines are protonated at lower pH. Very interestingly this can help reducing hydrogen evolution reaction that can compete with CO2 reduction. Though it compromise its activity as sacrificial electron donor [25]. If Cu2 O is attached to RGO it can act as a hole or electron trap and reduce the rate of hole-electron recombination [26]. Key idea behind this proposed scheme is to combine amine functionalized GO with copper (I)-oxide to make a composites that will be water dispersible and this composites will reduce activation energy of CO2 reduction onto Cu(I)-Oxide surface. In particular Cu2 O grafted reduced graphene oxide can be prepared following literature [27,28]. We envisage the amine fictionalization will help in chelating Cu2+ ions. This will help in better growth in crystalline Cu2 O nano particles [25,29] on top of reduced graphene oxide sheet. Besides this amine can act as sacrificial electron donor [30] to neutralize hole generated in Cu2 O due to photo excitation in semiconductor band gap. Particularly pentane amine derivative of graphene oxide might be interesting. Recently Carpenter et al. [17] has shown γ proton containing amine can act as sacrificial electron donor can be regenerated instead of stoichiometric west generated in reaction medium. Formation of composite photo-catalyst comprising of amino functionalized reduced graphene oxide and Cu2 O is schematically shown below (Fig. 15.2) [31].

15.7 Benefits of using RGOin the composite catalyst Mechanistic study of this catalytic reduction process will definitely provide new insight of photo-reduction mechanism of CO2 onto this catalyst composite system. The objective is to synthesis highly water dispersible amine functionalized graphene oxide (GO) – Cu2 O (AGO–Cux O) nano composite catalyst for CO2 reduction. This is because GO has the ability of forming an ultra thin film [31] of large surface area. Pt loaded GO composites have been successfully prepared and applied in fuel cells. In recent years, GO and partially reduced Graphene Oxide (rGO) have been focused as important catalyst for CO2 reduction due to their large specific surface area having unique graphitized basal plane with excellent electrical, mechanical and thermal properties. Though studies on synthesis of Pt loaded graphene/rGO and their applications in different catalytic reactions are well doccumented, synthesis of Cu2 O loaded GO is limited. Recently, [28] reduced GO (rGO)-Cu(I) oxide composite catalyst has been used in the study of photo- reduction of CO2 . The purpose of this study is to provide a catalyst support with high surface area. But we propose to explore catalytic activity of

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FIGURE 15.2 Scheme of amino-rGO–Cu2 O composite and their photo-catalytic reaction for CO2 reduction.

amino-functionalize GO along with its enhanced water dispensability. Thus, our proposed composite catalysts are expected to have multi functional activities. Very recently, we have reported [25] chitosan-Cu2 O catalyst composite for electro reduction of CO2 and showed that this novel catalyst not only help reducing catalyst loading by its film forming ability, but also retard H2 evaluation reaction at high cathodic potential in aqueous medium, a major problem of CO2 electro reduction in aqueous media. DFT calculations [20,32] on CuO and Cu2 O have shown that Cu2 O is a better catalyst for CO2 reduction than CuO. Our objective is to obtain liquid fuels like methanol and formaldehyde as reduction products, by optimising reaction conditions and catalyst preparation conditions. R&D activities on electro reduction of CO2 to methanol/hydrocarbon have gained tremendous momentum during the recent years.

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Japanese groups, particularly group lead by Hori et al. studied exhaustively electro reduction of CO2 using mainly Cu based catalysts. They have reported different reduction products and concluded that reduction products varies widely from hydrocarbons like methane, ethane etc. to methanol depending on experimental conditions. This group has analyzed reduction products formed underdifferent conditions. These are documented under references [1–5]. In USA different groups are actively engaged in electro-reduction of CO2 using Cu based catalysts, the work of Krishna Rajeswar Rao of Texax A&M may be specially mentioned here, he used Cu based catalysts, prepared by elecro-chemical method to study elecro-reduction of CO2 in aqueous solution and showed that methanol is a major product. Works of John Flake et al. [33,34] on CO2 reduction deserves special mentioned here, Their work is mainly on Cu nano electro-catalysts and they have identified the efficiency of different crystal faces on CO2 reduction reactions. They have shown that with thin catalyst layer, the Faradiac efficiency enhanced to double. In France, Jean-Michel Save´ant group working on electrochemical reduction of CO2 , particularly with iron based catalysts but there work is cited here for their excellent review article on this subject published in chem. Soc review of RSC. Theoretical works [32] on copper oxides catalysts on CO2 reduction are interesting and provide an understanding of mechanism of this reduction reactions, particularly, on the adsorption of CO2 on different surfaces and calculated values of interaction energies. Electrochemical reduction of CO2 to methanol is mentioned here as electricity generation through sunlight is a viable path.

15.8 Conclusions Based on our discussions in this chapter, we conclude that instead of simple semiconductor catalysts, composite catalysts, particularly combined with reduced graphene oxide has a promise in development of future efficient photo-catalyst.

Acknowledgment The author wishes to thank Head of the Department of Chemistry and Principal, Asutosh College under Calcutta University for their cooperation.

References [1] Costentin C, Robert M, Savéant J-M. Catalysis of the electrochemical reduction of carbon dioxide. Chem Soc Rev 2013;42(6):2423–36. [2] Takeda H, Ishitani O. Development of efficient photocatalytic systems for CO2 reduction using mononuclear and multinuclear metal complexes based on mechanistic studies. Coord Chem Rev 2010;254:346–54. [3] Windle CD, Perutz RN. Advances in molecular photocatalytic and electrocatalytic CO2 reduction. Coord Chem Rev 2012;256(21):2562–70. [4] Keith JA, Carter EA. Theoretical Insights into Pyridinium-Based Photoelectrocatalytic Reduction of CO2 . J Am Chem Soc 2012;134(18):7580–3. [5] Yoshio H, Katsuhei K, Shin S. Production of CO and CH4 in electrochemical reduction of CO2 at metal electrodes in aqueous hydrogencarbonate solution. Chem Lett 1985;14(11):1695–8. [6] Yoshio H, Katsuhei K, Akira M. Shin S. production of methane and ethylene in electrochemical reduction of carbon dioxide at copper electrode in aqueous hydrogencarbonate solution. Chem Lett 1986;15(6):897–8.

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[7] Hori Y. Electrochemical CO2 Reduction on Metal Electrodes. In: Vayenas CG, White RE, Gamboa-Aldeco ME, editors. Modern Aspects of Electrochemistry. New York, NY: Springer New York; 2008. p. 89–189. [8] Xiong Z, Zheng M, Liu S, Ma L, Shen W. Silicon nanowire array/Cu2 O crystalline core–shell nanosystem for solar-driven photocatalytic water splitting. Nanotechnology 2013;24(26):265402. [9] Liu D, Fernandez Diez Y, Ola O, Mackintosh S, Maroto-Valer M, Parlett CMA, et al. On the impact of Cu dispersion on CO2 photoreduction over Cu/TiO2 . Catal Commun 2012;25:78–82. [10] Ogura K. Electrochemical reduction of carbon dioxide to ethylene: mechanistic approach. Journal of CO2 Utilization 2013;1:43–9. [11] Basumallick S. Chitosan Coated Copper-Oxide Film onto Si-wafer:a Novel Photo Catalyst for CO2 Reduction. Journal of Multidisciplinary Engineering Science and Technology (JMEST) 2014;1(4):70–2. [12] Chang X, Wang T, Gong J. CO2 photo-reduction: insights into CO2 activation and reaction on surfaces of photocatalysts. Energy Environ Sci 2016;9(7):2177–96. [13] Hisatomi T, Kubota J, Domen K. Recent advances in semiconductors for photocatalytic and photoelectrochemical water splitting. Chem Soc Rev 2014;43(22):7520–35. [14] Fu Z-C, Mi C, Sun Y, Yang Z, Xu Q-Q, Fu W-F. An Unexpected Iron (II)-Based Homogeneous Catalytic System for Highly Efficient CO(2)-to-CO Conversion under Visible-Light Irradiation. Molecules 2019;24(10):1878. [15] Singto S, Supap T, Idem R, Tontiwachwuthikul P, Tantayanon S, Al-Marri MJ, et al. Synthesis of new amines for enhanced carbon dioxide (CO2 ) capture performance: the effect of chemical structure on equilibrium solubility, cyclic capacity, kinetics of absorption and regeneration, and heats of absorption and regeneration. Sep Purif Technol 2016;167:97–107. [16] Reithmeier R, Bruckmeier C, Rieger B. Conversion of CO2 via Visible Light Promoted Homogeneous Redox Catalysis. Catalysts 2012;2(4). [17] Richardson RD, Holland EJ, Carpenter BK. A renewable amine for photochemical reduction of CO(2). Nat Chem 2011;3(4):301–3. [18] Carpenter BK. Computational Study of CO2 Reduction by Amines. J Phys Chem A 2007;111(19):3719–26. [19] Cohen SG, Parola A, Parsons GH. Photoreduction by amines. Chem Rev 1973;73(2):141–61. [20] Le MTH Electrochemical reduction of CO2 to methanol: louisiana State University and Agricultural and Mechanical College; 2011. [21] Summers DP, Leach S, Frese KW. The electrochemical reduction of aqueous carbon dioxide to methanol at molybdenum electrodes with low overpotentials. J Electroanal Chem Interfacial Electrochem 1986;205(1):219– 32. [22] Frese SL W. Electrochemical reduction of carbon dioxide to methane, methanol, and CO on Ru electrodes. J Electrochem Soc 1985;132(1):259–60 23. [23] Bandi A. Electrochemical Reduction of Carbon Dioxide on Conductive Metallic Oxides. J Electrochem Soc 1990;137:2157. [24] Gattrell M, Gupta N, Co A. A review of the aqueous electrochemical reduction of CO2 to hydrocarbons at copper. J Electroanal Chem 2006;594(1):1–19. [25] Basumallick S, Santra S. Chitosan coated copper-oxide nano particles: a novel electro-catalyst for CO2 reduction. RSC Adv 2014;4(109):63685–90. [26] Linsebigler AL, Lu G, Yates JT. Photocatalysis on TiO2 Surfaces: principles, Mechanisms, and Selected Results. Chem Rev 1995;95(3):735–58. [27] Tran PD, Batabyal SK, Pramana SS, Barber J, Wong LH, Loo SCJ. A cuprous oxide–reduced graphene oxide (Cu2 O–rGO) composite photocatalyst for hydrogen generation: employing rGO as an electron acceptor to enhance the photocatalytic activity and stability of Cu2 O. Nanoscale 2012;4(13):3875–8. [28] An X, Li K, Tang J. Cu2 O/reduced graphene oxide composites for the photocatalytic conversion of CO2 . ChemSusChem 2014;7(4):1086–93. [29] Togashi T, Hitaka H, Ohara S, Naka T, Takami S, Adschiri T. Controlled reduction of Cu2 + to Cu+ with an N,O-type chelate under hydrothermal conditions to produce Cu2 O nanoparticles. Mater Lett 2010;64:1049– 1051. [30] Pellegrin Y, Odobel F. Sacrificial electron donor reagents for solar fuel production. CR Chim 2017;20(3):283–95. [31] Basumallick S. Design and Synthetic Scheme of Water Dispersible Graphene Oxide-Coumarin Complex for UltraSensitive Fluorescence Based Detection of Copper (Cu2 +) Ion in Aqueous Environment. Graphene. 2014;03:45– 51.

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[32] Bendavid LI, Carter EA. CO2 Adsorption on Cu2 O(111): a DFT+U and DFT-D Study. J Phys Chem C 2013;117(49):26048–59. [33] Andrews E, Ren M, Wang F, Zhang Z, Sprunger PT, Kurtz R, et al. Electrochemical Reduction of CO2 at Cu Nanocluster/(100) ZnO Electrodes. J Electrochem Soc 2013;160:H841–H8H6. [34] Le M, Ren M, Zhang Z, Sprunger PT, Kurtz RL, Flake JC. Electrochemical Reduction of CO2 to CH3OH at Copper Oxide Surfaces. J Electrochem Soc 2011;158(5):E45.

C H A P T E R

16 Adsorbents for carbon capture Vijay Vaishampayan a, Mukesh Kumar b, Muthamilselvi Ponnuchamy c and Ashish Kapoor d a

Department of Chemical Engineering, Indian Institute of Technology, Ropar, Punjab, India b Discipline of Chemistry, Indian Institute of Technology, Gandhinagar, Gujarat, India c Department of Chemical Engineering, College of Engineering and Technology, SRM Institute of Science and Technology, Potheri, Kattankulathur, Tamil Nadu, India d Department of Chemical Engineering, Harcourt Butler Technical University, Kanpur, Uttar Pradesh, India

16.1 Introduction Technological advancement and rapid growth in industrialization paved the way for the global industrial revolution. Developing countries aspire to generate and utilize more energy to match the demand and supply ratio of energy economics. Unfortunately, most developing nations rely on conventional energy sources, leading to enormous atmospheric pollution and release of especially CO2 . The excessive release of greenhouse gases has resulted in climate change [24,26]. Researchers are trying to find solutions through CCS, which could be a viable solution to mitigate the issues that arise due to global climate change. The minimization of CO2 concentrations is now more focused on the research of innovative storage strategies and new materials such as Metal-organic frameworks (MOFs), Covalent-organic frameworks (COFs), highly porous carbonaceous materials, biomass-based materials, and so on. These new methodologies could contribute to achieving the United Nation’s declared sustainable development goals (SDGs) and a healthy future for our mother earth. CCS technologies hope to reduce carbon footprints but host a few techno-economic challenges that must be addressed for more effective implementation. The environmental risk factors, energy utilization, etc., could be addressed before the implementation of CCS projects. High equipment cost, capital expenditure, and less explored highly porous materials make the projects less economically viable to implement in developing countries. [12,27].

Green Sustainable Process for Chemical and Environmental Engineering and Science: Carbon Dioxide Capture and Utilization DOI: https://doi.org/10.1016/B978-0-323-99429-3.00026-6

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This chapter discusses the various processes and strategies used for the CCS, along with the advantages and disadvantages. Furthermore, the various classes of the materials are discussed and the future perspective for the CCS are provided.

16.2 Carbon capture processes Today, various commercially available technologies already exist to capture CO2 from the mixture of gases. These technologies are majorly implemented in the purification stage of downstream industries, chemical processing units, oil and gas processing plants. The process selection depends on the pollution parameters, chemical, geographical, and physical conditions [20,21]. The carbon capture methods can be categorized into pre-combustion and post-combustion carbon capture.

16.2.1 Pre-combustion carbon capture The initial fuel is partially combusted in the pre-combustion carbon capture process to get H2 and CO/CO2 . Later, the gaseous feed is supplied to the CO2 separator employing physical or chemical adsorption methods. In the last stage, a clean fuel without containing the CO2 is provided for energy conservation. The capital cost of such plants is higher, but it’s a more effective process to reduce CO2 pollution with less efficiency. These processes are also easy to incorporate into the existing processing of the plant.

16.2.2 Post-combustion carbon capture Power generation plants produce flue gases contributing to carbon emissions. The postcombustion carbon capture methodology in the power plants is implemented for CCS. After the combustion process, the energy and flue gases are generated. This generated energy is utilized for the various unit processes in the plant. These flue gases are rich in nitrogen, followed by carbon dioxide. Chemisorption or physisorption processes can capture the CO2 from the mixture of gases to meet the pollution control standards. These processes are easy to accommodate in the matured process plant. Along with these techniques, there are several approaches for CC. Researchers have used adsorption, absorption, membrane separation, etc., for CCS. The selection of these processes depends on various factors such as the type of material used for the CCS, the material’s chemical and physical properties, the number of active sites present in the material, and so on. Various metal oxides, MOFs. COFs, biomass-derived activated carbon materials, zeolites, and clays are used for the CCS as showed in the Fig. 16.1 [5,11,16,25].

16.3 Adsorbents for CO2 capture 16.3.1 Materials derived from biomass The biomass derived materials can be useful sources to manufacture the carbonaceous materials. Further, various chemical modifications could be performed to enhance the functionalities by activating the prepared carbons. Farooq Sher et al. synthesized an activated

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FIGURE 16.1 Various adsorbent materials for carbon capture.

carbon from the three different biomass materials using a single-step chemical activation process followed by heating at 750 °C. Potassium hydroxide was utilized to activate carbonaceous materials in different mass ratios. This strategy was applied to form a high surface area, enhance porosity, and increase the number of active sites, which helped increase CO2 uptakes [23]. Yafei Guo et al. developed a MgO-loaded adsorbent using biomass waste of coffee grounds, rice husk, sugarcane bagasse, and sawdust. The calcination was performed to make the adsorbent. The adsorption behavior of the prepared adsorbent was studied with the MgO loadings using a fixed-bed reactor. The CO2 adsorption performance varied with the different supporting materials due to changes in physicochemical properties. MgO in the nano-crystallized form could provide more active sites for CO2 adsorption and thus improves the utilization of MgO. The CO2 adsorption capacity was decreased in the range of 10–40 wt percent while increasing the MgO loading. The pore blockages caused an increase in the diffusion resistance, thus hampering the performance of the prepared material [12]. Similarly, Zhang et al. developed carbon material from the black locust and activated chemically using KOH with ammonia solution for the adsorption of CO2 . Surface characteristics were studied using the N2 adsorption isotherm. The prepared activated carbon exhibits a high surface area of 2511 m2 /g. As adsorption temperature increased, the CO2 adsorption onto the various activated catalyst decreased. The adsorption results revealed that the activated carbon using KOH material could be one of the ways for CC [4].

16.3.2 Clays The most crucial and expensive step in CCS is CO2 capture. Clay minerals are a good option for CO2 adsorption due to their low cost, high availability, stability, and ease of modification. Pozueloa et al. developed an amine-functionalized clay material for CO2 capture. They have used a series of inexpensive clay materials such as montmorillonite, bentonite, saponite, sepiolite, and palygorskite as support to introduce amine-containing sorbents for CO2 capture.

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Firstly these clay materials were hydrated and then functionalized with a different method - (a) grafting with aminopropyl (AP) and diethylenetriamine (DT) organosilanes; (b) impregnation with polyethyleneimine (PEI); and (c) double functionalization by impregnating previously grafted samples. The maximum uptake of CO2 was observed in the case of grafted and impregnated samples as 61.3 and 67.1 mg CO2 /g. The low adsorption in double functionalization material was probably due to high organic loading, which resulted in pore size reduction [10]. Ahmed Hamza et al. studied sandstone rocks which consist of different amounts and types of clay for CO2 adsorption in the temperature range from 50 to 100 °C and pressure up to 20 bars. Sandstone rocks contain minerals such as Quartz, Illite, Kaolinite, Chlorite, Plagioclase, Feldspar, Calcite, and Dolomite. Maximum CO2 adsorption was observed for sandstone rocks associated with high swellable clay content, such as illite at a pressure of 20 bars and temperature of 50 °C. With an increase in temperature to 75 °C, adsorption uptake of CO2 decreases, but a further increase in temperature to 100 °C improves the CO2 adsorption due to a change in crystallinity. The adsorption of CO2 was monolayer at a lower temperature (50 °C), and multilayer adsorption was observed in the temperature range 75 and 100 °C [13]. Cecilia et al. worked on two inexpensive clay minerals sepiolite and palygorskite, as potential adsorbents for CO2 capture. The microwave-assisted acid treatment was done to enhance the textural properties (surface area and pore volume) of sepiolite and palygorskite, and then they were functionalized by the ammine group using different methods of grafting, impregnation, and double functionalization. The introduction of the amine group to clay minerals increases the interaction of CO2 with the amine group, and adsorption mainly takes on the outer surface of the clay mineral. Since the adsorption of CO2 is purely due to the chemical interaction of amine with CO2 so, the double functionalization leads to the maximum adsorption due to a more number of amine groups [3].

16.3.3 Zeolites Zeolites belong to the category of aluminosilicates which are crystalline and have wide applications in commercial adsorbents and catalysis. They have tetrahedrally coordinated Al and Si, which are joined together by oxygen (O). They are very effective as CO2 adsorbents at temperatures up to 250–300 °C. Murge et al. synthesized a low-cost Y-type zeolite adsorbent from gasified rice husk waste for adsorption of CO2 . The adsorption capacity and performance of the zeolite material were checked in a fixed-bed flow reactor. The material’s adsorption capacity depends upon Si/Al ratio, pore size distribution, adsorption temperature, reactor pressure, and moisture presence. The maximum adsorption capacity was around 114 and 190 mg of CO2 /g of the sorbent at 1 and 5 bar pressure, respectively, at 30 °C [18]. Lithium low silica X type (LiLSX) zeolite as a potential CO2 sorbent for post-combustion carbon system was synthesized by Rasmus Kodasm et al. Sorption study of Li-LSX-zeolite was done with the help of fixedbed configuration and TGA instrument. The maximum adsorption capacity and selectivity (CO2 /N2 ) were observed at 60 °C, which was 4.43 mmol/g and 85.7, respectively. With an increase in the calcination temperature from 60 to 300 °C, the adsorption ability of zeolite for CO2 was decreased by 10 mol percent due to the reduction of the micropore surface. Also, the increase in partial pressure of CO2 enhances the adsorption rate of CO2 because of facilitated CO2 diffusion processes, and the diffusion can be calculated by the rate-limiting step for CO2

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adsorption [15]. Wang et al. synthesized rare earth metal zeolites of La and Ce by ion exchange in X zeolite, which was prepared from rice hull ash (RHA). It was observed that the chemical and crystal structure of the modified zeolites did not change, but the microscopic properties and pore size changed. The maximum amount of CO2 adsorption exhibited by NaX and LaLiX was 6.14 and 4.36 mmol/g, respectively, at atmospheric pressure. The cyclical regeneration performance studies and desorption curve data suggest that it was physical adsorption and easy to regenerate [28]. Farid Akhtar et al. synthesized binderless zeolite NaX laminates using pulsed current processing method for CO2 capture. The thickness of the laminates ranges from 310 to 750 μm. The NaX laminates exhibit very high CO2 adsorption capacity and high selectivity of CO2 -over-N2 and CO2 -over-CH4 . It was observed that with the increase in laminate thickness, the adsorption rate decreased, for the laminate of thickness of 310 μm, 40 percent of maximum uptake reached in only 24 s, and thickness of 750 μm reached 40 percent of maximum uptake in around one minute [1].

16.3.4 Metal-organic frameworks (MOFs) The metal-organic framework is highly used for CO2 capture due to its high porosity and chemical tunability. Many studies support that MOF is a significant material in carbon capture. Like other adsorbents such as silica, zeolites, and activated carbon, the CO2 adsorption in MOF is physisorptive due to weak interaction between CO2 and the pore, but the CO2 uptake in the case of MOF is much higher than other materials due to their ultra-high surface area. Mei-Hui Yu et al. synthesized a novel multistage-based MOF using a mixed-ligands strategy. It was reported that four kinds of cages would selectively adsorb CO2 over other gases based on angle-directed and face-directed strategies. Due to the very high surface area (BET and Langmuir are 2111.2 and 2307.2 m2 /g) of MOF, the large amount of CO2 adsorption 113 cm3 /g is possible, which is comparatively very high as compared with other MOF [31]. Behnam Ghalei et al. prepared a mixed matrix membrane (MMM) by introducing a nanosized metal-organic framework (MOF) to enhance the adsorption of CO2 . Nano sizing the MOF by water modulated synthesis helps in its better dispersion over the matrix, which allows for the reduction of non-selective microvoid formation around the particle. Interaction of MOF with polymer matrix was increased by amination, which also helped rigidification and enhanced the selectivity of the overall composite [9]. Liang et al. synthesized a fluorinated MOF dptzCuTiF6 by interpenetration approach to effectively capture CO2 from flue gas at 298 K. It was reported that MOF dptz-CuTiF6 showed remarkable volumetric and gravimetric CO2 uptakes at 10 percent CO2 and 298 K. It was also reported that the MOF is also required significantly less amount of energy for the regeneration as compared with reference aqueous amine technique (38 kJ/mol versus 105 kJ/mol) and dptz-CuTiF6 achieves complete CO2 desorption at 298 K under inert gas purging. The single-crystal studies revealed high CO2 adsorption capacity, moderate CO2 heat of adsorption, and high CO2 –N2 selectivity because of optimal packing of the CO2 molecules within the MOF and favourable thermodynamics and kinetics from cooperative host-guest interactions [17].

16.3.5 Covalent-organic frameworks (COFs) COFs became one of the most suitable candidates due to their unique properties, such as high porosity, predetermined structure, thermal stability, and structural diversity with a

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lower density [7,30]. Similarly, COFs shared some properties with the organic polymers for the uncomplicated fabrication of thin films, synthesis, tunability in structure and chemicals, growth, and user-friendly functionalization [6]. These properties make COFs useful for applications like chemical sensing, gas adsorption, catalysis, energy storage, etc. [19,22,29]. Y B Apriliyanto et al. designed the 2D COFs for the carbon dioxide and nitrogen gas adsorption by integrating density functional theory (DFT) and force field-based molecular dynamics (MD) simulations. The COF was modeled using a 1,3,5-tris(chloromethyl)benzene-based building unit and p-diaminobenzene and hydrazine as linkers. The adsorption sites and energies were explored, and the capacity to uptake gas, permeability, and adsorption isotherm was studied. The adsorption isotherm suggested a more robust CO2 adsorption than N2 [2]. Bin Han et al. synthesized 2D-COFs using a solvothermal process with the help of phthalocyanine-based building blocks. Tetraanhydrides of 2,3,9,10,16,17,23,24-octacarboxyphthalocyaninato cobalt(II) with 1,4- phenylenediamine and 4,4 -biphenyldiamine were taken as a precursor materials. The synthesized material was highly porous with excellent chemical and thermal stability [14]. Qiang Gao et al. built 2D COFs using a solvent-directed divergent synthesis method. The central core for COF was constructed using tetraphenylethane (TPE). During the process, solvent control formed two separate COF structures, TPE-COF-I and TPE-COF-II. These structural variations were unable to predict via conventional COF synthesis methods. These variations were initiated from solvent-influenced crystallization chemistry. TPE-COF-II exhibits a superior carbon dioxide adsorption performance than TPE-COF-I due to its enhanced surface area and high CO2 uptake capacity. This solvent-influenced modification paved the way for multifunctional tunability during the COF synthesis [8].

16.4 Future perspective and conclusion The sequestration of the produced CO2 would play an essential role in shaping the global ecosystem in the near future. These approaches must adapt and run on a multi watt scale to sequester most of the generated carbon emissions. The utilization of sequestrated CO2 could be utilized for the production of other value-added materials. These processes must be safe, green, and stable for wider acceptance. Compared to an unabated plant, carbon capture plants of almost all designs have a few extra possibilities to hold energy by time-shifting energy-intensive processes. The industrial sector must try decarbonizing to achieve global CO2 emissions targets. Numerous industries such as paper and pulp, iron and steel manufacturing, petroleum refining could be the prime sides to implement the next generation CCS. CCS is regarded to be a viable method for lowering CO2 emissions from industrial activities. Each industrial process has a unique set of physical characteristics, chemical makeup, and gas volume fluxes. On the basis of these stream parameters, such as CO2 concentration and moisture content, the suitability and selection of a CCS system would be determined.

References [1] Akhtar F, Ogunwumi S, Bergström L. Thin zeolite laminates for rapid and energy-efficient carbon capture. Sci Rep 2017;7(1):3–8. https://doi.org/10.1038/s41598-017-10518-4.

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[21] Roy J, Some S, Das N, Pathak M. Demand side climate change mitigation actions and SDGs: literature review with systematic evidence search. Environ Res Lett 2021;16(4). https://doi.org/10.1088/1748-9326/abd81a. [22] Sang SH, Furukawa H, Yaghi OM, Goddard WA. Covalent organic frameworks as exceptional hydrogen storage materials. J Am Chem Soc 2008;130(35):11580–1. https://doi.org/10.1021/ja803247y. [23] Sher F, Iqbal SZ, Albazzaz S, Ali U, Mortari DA, Rashid T. Development of biomass derived highly porous fast adsorbents for post-combustion CO2 capture. Fuel 2020;282(August):118506. https://doi.org/ 10.1016/j.fuel.2020.118506. [24] Singh G, Lakhi KS, Sil S, Bhosale SV, Kim IY, Albahily K, et al. Biomass derived porous carbon for CO2 capture. Carbon N Y 2019;148:164–86. https://doi.org/10.1016/j.carbon.2019.03.050. [25] Skorjanc T, Shetty D, Mahmoud ME, Gándara F, Martinez JI, Mohammed AK, et al. Metallated isoindigoporphyrin covalent organic framework photocatalyst with a narrow band gap for efficient CO2 conversion. ACS Appl Mater Interfaces 2022;14(1):2015–22. https://doi.org/10.1021/acsami.1c20729. [26] Vishal V, Chandra D, Singh U, Verma Y. Understanding initial opportunities and key challenges for ccus deployment in india at scale. Resour Conserv Recycl 2021;175(January):105829. https://doi.org/10.1016/j. resconrec.2021.105829. [27] Vishal V, Chandra D, Singh U, Verma Y, Guo Y, Tan C, et al. Assessment of the pre-combustion carbon capture contribution into sustainable development goals SDGs using novel indicators. Int J Greenhouse Gas Control 2021;153(January):111710. https://doi.org/10.1016/j.resconrec.2021.105829. [28] Wang Y, Jia H, Chen P, Fang X, Du T. Synthesis of la and ce modified x zeolite from rice husk ash for carbon dioxide capture. J Mater Res Technol 2020;9(3):4368–78. https://doi.org/10.1016/j.jmrt.2020.02.061. [29] Xu F, Xu H, Chen X, Wu D, Wu Y, Liu H, et al. A squaraine-linked mesoporous covalent organic framework. Angewandte Chemie - International Edition 2013;130(13):6814–18. https://doi.org/10.1002/anie.201501706. [30] Yang H, Du Y, Wan S, Trahan GD, Jin Y, Zhang W. Mesoporous 2D covalent organic frameworks based on shapepersistent arylene-ethynylene macrocycles. Chem Sci 2015;6(7):4049–53. https://doi.org/10.1039/c5sc00894h. [31] Zhou X, Liu D, Zhong R, Dai Z, Wu D, Wang H, et al. Determination of SARS-coronavirus by a microfluidic chip system. Electrophoresis 2004;25(17):3032–9. https://doi.org/10.1002/elps.200305966.

C H A P T E R

17 Carbon dioxide capture and utilization in ionic liquids Guocai Tian State Key Laboratory of Complex Nonferrous Metal Resources Clean Utilization, Faculty of Metallurgical and Energy Engineering, Kunming University of Science and Technology, Kunming, Yunnan Province, China

17.1 Introduction With the rapid development of economy and society, people are more and more dependent on natural resources, and the consumption rate is also increasing. As one of the most important primary energy sources, fossil energy consumption is increasing at a very alarming rate in recent years. In 2017, the International Energy Agency (IEA) noted in its annual outlook that the world’s energy structure will still be dominated by traditional fossil energy before 2035. According to preliminary estimates, with the development of economy and society, the total global energy demand will increase by more than 34 percent, and the demand for fossil energy will still account for more than 80 percent of the total energy [1–3]. However, the continuous growth of energy consumption have also brought many negative impacts and pressure to the ecological environment on which human beings depend, and has become the focus of global attention. At the same time, it is considered to be among the most severe challenges for mankind since the 21st century. Since the industrial revolution, the combustion of fossil fuels has continuously increased carbon dioxide emissions, leading to serious greenhouse effect [1,2]. By 2018, global carbon dioxide emissions have increased to 3.71 billion tons [1–3]. In recent years, climate change caused by the greenhouse effect has led to various extreme weather events, which have a more and more serious impact on human production, life and life health. For example, glacier retreat, frozen soil melting, sea level rise, rainfall increase, ocean acidification, biological system disorder, bio-diversity reduction, and even have a negative effect on agricultural production and food security. Tsunami, earthquake and other natural disasters have caused significant harm to human life. This will further aggravate the economic gap and geopolitical conflicts in the whole world [4–8]. In recent years, the content of carbon dioxide in the

Green Sustainable Process for Chemical and Environmental Engineering and Science: Carbon Dioxide Capture and Utilization DOI: https://doi.org/10.1016/B978-0-323-99429-3.00019-9

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17. Carbon dioxide capture and utilization in ionic liquids

atmosphere has increased at a rate of 2 ppm per year. By 2019, the volume fraction of CO2 in the atmosphere has reached 4.11 × 10−4 [1–3]. In 2020, affected by the 2019 coronavirus epidemic, global CO2 emissions decreased by nearly 2 × 108 tons. However, with the economic recovery, CO2 emissions rise again [6,7]. The global energy-related CO2 emissions in December 2020 increased by 2 percent compared with December 2019. According to BPs energy outlook, carbon dioxide emissions from energy consumption will continue to increase in the coming years. Under the changing transition scenario, it will increase by about 10 percent in 2040 [8]. The rising global carbon dioxide emissions have attracted extensive attention from the business community, the public, the government and academic groups [1–9]. In order to cope with these rapid growth and achieve the 1.5 °C temperature control target proposed by IPCC (Intergovernmental Panel on climate change) [9], it is particularly important to develop reasonable methods in time to solve the CO2 problem. According to IPCC prediction, by 2100, the global temperature is expected to rise by about 1.4–5.8 °C, which greatly exceeds the ecological environment load, causing serious global climate problems. How to reduce the concentration of CO2 in the atmospheric environment is extremely important to deal with global warming. In order to alleviate the pressure brought by CO2, good separation and recovery and effective comprehensive utilization are one of the important topics of current research. Therefore, capturing, fixation and conversion of carbon dioxide is the urgent task. Carbon capture and storage (CCS) is the most direct measure taken by people to control carbon dioxide emissions. CO2 capture mainly includes three aspects: Capture before combustion, capture during combustion and capture after combustion [1–3]. Before combustion, the captured flue gas flow is small, the partial pressure of CO2 is high, the separation difficulty is low, and the cost is low, but the operation is complex, the stability is poor, and the requirements for gas turbines are high. Capture technology in combustion mainly includes oxygen enriched combustion technology and chemical combustion technology. Oxygen enriched combustion carbon capture technology is generally applicable to newly planned coal-fired power plants. It is relatively low-cost and easy to scale. It is considered to be among the CCS technologies that are easiest to be popularized and commercialized on a large-scale. The main problems of this technology are enormous investment and high energy consumption. The advantage of chemical combustion carbon capture technology (CLC) is that it can capture CO2 at the source without adding separation devices [1–3]. Making the fuel burns gradually, and uses the energy step by step. In the process, there is no flame and the temperature is relatively low, which can reduce NOx emissions. Its disadvantage is that the investment of the device is large, and it is not suitable for the transformation of existing thermal power units. The power-generation system based on CLC capture technology is still under continuous development and research. The post combustion capture system is usually installed downstream of the pollutant removal device of the existing power plant, which has no impact on the structure and energy utilization mode of the power plant. It has the characteristics of a mature process and simple principle. The post combustion capture technology can meet the requirements of the existing flue gas characteristics of the system and is not difficult to operate. Because of its small workload, simple operation and no change to the existing mainstream process of the plant, it is considered to be the most feasible CO2 emission reduction method. However, the coal-fired flue gas flow is large, the flue gas composition is complex, the CO2 partial pressure is low, and the energy consumption in the capture process is very high. At present, available post combustion collection technologies mainly include solvent absorption, adsorption, membrane separation

17.1 Introduction

347

and low temperature separation. Chemical absorption technology is basically applicable to most large-scale carbon capture projects that have been brought into operation in the world. Among them, amine solvents such as diethanolamine (DEA) and monoethanolamine (MEA) have become one of the most mature and commonly used CO2 capture methods in industry owing to their good solubility and fast CO2 absorption rate. Although amine solvents have outstanding advantages, they have problems such as large solvent volatilization loss and serious equipment corrosion; In addition, due to the large absorption enthalpy of CO2 in amine solvents, the energy consumption of a solvent regeneration process is high [1–3]. Therefore, looking for new green solvents to absorb CO2 and developing related processes has become a research hotspot of carbon capture technology. On the other hand, as a C1 resource, CO2 resource is rich, cheap, accessible, green, non-toxic and renewable. Transforming its chemistry into high value-added chemicals, energy products and material are part of the effective ways to realize its resourcefulness and solve energy and environmental problems, which is of far-reaching significance for sustainable development [1–6]. Utilization of CO2 resources mainly includes two aspects: the direct application of CO2 and the transformation and utilization of CO2 . Carbon dioxide can be directly used in industrial processes, such as soft drinks, food, fire extinguishers, welding and foaming agents. It can also be pressurized to form a new green solvent supercritical CO2 , which can be used for the synthesis and separation of nanoparticles and composites [2]. As a more promising way of CO2 resource utilization, converting CO2 into high value-added chemicals and fuels cannot only effectively reduce its emissions and mitigate the greenhouse effect, but also bring economic benefits and meet energy needs [2]. At present, CO2 conversion technology mainly includes thermal catalytic hydrogenation, biological conversion, photocatalytic conversion and electrochemical reduction. So far, people have conducted extensive research on the resource utilization of CO2 . Various high value-added chemicals and chemical intermediate with CO2 as raw materials, such as urea, formic acid, methanol, formamide, methylamine, carbonate, polycarbonate, polyurea and benzimidazole, have been synthesized by designing efficient catalytic systems or reacting with high-energy substances [1–8]. However, as the highest valence oxide of carbon, CO2 has a high degree of thermodynamic stability and kinetic inertia. CO2 reaction system usually needs to use specific metal catalysts for catalysis under high pressure and/or high-temperature conditions. There are also many challenges in the electrochemical conversion of CO2 . In order to effectively carry out CO2 chemical conversion, CO2 molecules must be activated first, and the key to activate CO2 molecules is to establish an efficient catalytic system. At present, people have developed a variety of suitable systems for catalytic activation and conversion of CO2 , such as metal containing porous materials, metal/organic ligand systems, hindered acid-base pairs, phosphine ylide CO2 adducts, nitrogen heterocyclic carbene systems, ionic liquids (ILs) systems, etc. [2]. In particular, development of ionic liquids provides a new system for efficient and clean CO2 conversion, and important progress has been made in the utilization of CO2 resources under mild conditions. Ionic liquids (ILs) are a kind of substance that are liquid at or near room temperature, which is composed of larger organic cations and smaller inorganic or organic anions [9–12]. Common cations include quaternary phosphate salt ions, pyridine salt ions, quaternary ammonium salt ions, and imidazole salt ions. Anions include halogen ions, hexafluorophosphate ions, tetrafluoroborate ions, etc. Ionic liquids have low melting point, extremely low vapor

348

17. Carbon dioxide capture and utilization in ionic liquids

pressure, nonvolatile, wide liquid temperature range, nonflammability, good thermal and chemical stability, strong solubility, wide electrochemical window, adjustable structure and performance, and can be recycled. More importantly, ILs has higher designability. Through the changes of cations and anions, ionic liquids can be equipped with one or more specific functional groups to meet different application needs [10–12]. They have broad application prospects in replacing traditional organic solvents, so they are extremely valued by academia and industry [10–12]. The reason why ionic liquids can become one of the research hotspots in 1 Wide liquid recent years is closely related to their unique physical and chemical properties.  temperature ranges. From near or below room temperature to above 300 °C, it is conducive to kinetic control and has high chemical and thermal stability. Generally, its structure remains 2 small vapor pressure and basically non-volatile, so it will not unchanged within 300 °C.  evaporate during storage and use. It can be utilized in high-vacuum system to eliminate the environmental pollution caused by the volatilization of organic compounds during operation. 3 high conductivity At the same time, it can be recycled by a simple ion exchange method.  and wide electrochemical window. It can be used as an electrolyte in many electrochemical 4 adjustable physical and chemical properties of ionic liquids. The solubility of processes.  inorganic matter, water, organic matter and polymer can be adjusted by selecting different cations or anions. From hydrophobic to hydrophilic, and sensitive to water in the air, when it is an immiscible solvent, it can provide a non-aqueous, polar adjustable two-phase system. Hydrophobic ionic liquids can be used as the immiscible polar phase of water and can adjust 5 good solubility in many inorganic and organic substances, salts, solids and the pH value.  liquids. It has a wide variety and a wide range of choices. Often, ionic liquids have the dual functions of solvent and catalyst, so they can be used as catalyst support or solvent for various reactions. Ionic liquids are generally considered as ideal green solvents due to their unique properties, and are widely used in separation and extraction processes and organic synthesis reactions [10–12]. Ionic liquids have broad application prospects in CO2 capture, fixation and conversion. In recent years, significant progress has been made in related researches [13–40]. In the research field of CO2 capture and fixation, ILs are mainly used as solvents to absorb CO2 , or applied as adsorbents to achieve the purpose of CO2 fixation or separation [13–34]. On the one hand, ILs have a high physical adsorption capacity for CO2 . On the other hand, the basic groups in ILs can also react with CO2 for chemical adsorption. Therefore, the adsorption of CO2 by ILs will be higher than 1:1 (mass ratio of substances). In the field of CO2 conversion, ionic liquids are widely used in the electrochemical reduction of CO2 because of their efficient dissolution and activation of CO2 , precise regulation of products and strong conductivity [35–40]. Among them, the solubility of CO2 in ILs is much higher than that of aqueous solutions. Therefore, ionic liquid electrolytes have high CO2 concentration, which improves mass transfer efficiency and CO2 conversion. In addition, hydrogen bonds, electrostatic and cluster interactions in ionic liquids make it easier to combine with CO2 , thus effectively activating CO2 molecules. Under mild conditions, the electrochemical conversion of CO2 in ionic liquid systems is the frontier and hotspot of CO2 conversion and utilization. In recent years, many valuable achievements have been made in the capture, fixation and conversion of CO2 by ionic liquids [1–40]. In this chapter, we will discuss the progress and prospects of ionic liquids as adsorbents and catalysts for CO2 capture, fixation and conversion. We mainly introduces the advantages and disadvantages of various ionic liquids such as functionalized ionic liquids, supported

17.2 Capture of CO2 in ILs

349

ionic liquids and ionic liquid polymers as CO2 adsorbents and catalysts in the capture and conversion process, and discus the research prospects in related fields, providing a reference for the systematic research of ionic liquids in the capture, fixation and conversion of CO2 in ionic liquids [13–40].

17.2 Capture of CO2 in ILs 17.2.1 Conventional ionic liquids Compared with the traditional organic solvent system, the solubility of CO2 in ILs is very large, although its absorption process is a physical adsorption [41,42]. According to the structural characteristics of ILs and the CO2 fixation or absorption mechanism, ionic liquids that absorb CO2 can be divided into conventional ionic liquids (such as imidazolium salt, pyrrolidine salt, ammonium salt, sulfonate plasma liquid) and functionalized ionic liquids. Among them, the interaction between conventional ionic liquids and CO2 is mainly physical interaction, so compared with other ionic liquids, conventional ionic liquids can absorb or fix CO2 less [41–99]. Table 17.1 displays the relevant data of CO2 absorption by conventional ionic liquids. 17.2.1.1 Imidazolium ionic liquid Imidazole is highly alkaline, easy to salt, and easy to alkylate in an alkaline environment. Conventional imidazole ILs have low viscosity and good fluidity. Therefore, various imidazole based ILs have been extensively applied in CO2 absorption. The most commonly used imidazole ILs for CO2 absorption are formed with imidazole cation and the anions of PF6 - , Ntf2 - and BF4 - , such as [C8 mim]PF6 , [Bmim]PF6 , [Bmim]BF4 , [Hmim]Ntf2 and so on [41–99]. Compared with conventional organic solvents, imidazole ILs have a large absorption capacity for carbon dioxide even a physical absorption process. Blanchard et al. [52] measured the CO2 solubility in a series of imidazolium ILs such as [Bmim]NO3 , [Bmim]PF6 , [Emim]EtSO4 , [C8 mim]BF4 , [C8 mim]PF6 , [n-[Bupy]BF4 within the pressure range of 0.1∼10 MPa. It showed that the order of CO2 solubility is: [Bmim]PF6 /[Omim]PF6 > [Omim]BF4 > [n-Bupy]BF4 > [Bmim]NO3 > [Emim]EtSO4 at 313 K and 0∼9.5 MPa. It was found that the CO2 solubility in ILs with fluoride anions is greater than that in other anion based ILs. The CO2 solubility in ILs is greatly affected by anions, but less by cations. Carvalho et al. [71] studied the solubility of CO2 in phosphonium-based ionic liquids. The gas–liquid equilibrium of trihexyltetradecylphosphonium chloride and trihexyltetradecylphosphonium bis(trifluoromethylsulfonyl)imide, in a wide range of pressures and temperatures. It showed that phosphonium ionic liquids can dissolve even larger amounts of CO2 (on a molar fraction basis) than the corresponding imidazolium-based ILs. Kazarian et al. [75] showed that the bending vibrational spectrum of CO2 in [Bmim]PF6 and [Bmim]BF4 split to vary degrees, which may be due to the Lewis base interaction between F atom in ionic liquid anion and CO2 . Therefore, they speculated that the role of CO2 and ionic liquid anions is that the O–C-O axis is vertically arranged around the P-F and B-F bonds Aki et al. [76,77] studied the solubility of CO2 in ten imidazole based ILs. It is found that solubility of CO2 increases with increasing the pressure, while it decreases with the

350

17. Carbon dioxide capture and utilization in ionic liquids

TABLE 17.1 Adsorption capacity of CO2 in conventional ILs. Entry

System

T(K)/P(bar)

mol CO2 per mol IL

Ref.

1

[Aemim]BF4

303/1

0.41

[43]

2

[Aemim]DCA

303/1

0.42

[43]

3

[Aemim]PF6

303/1

0.46

[43]

4

[Aemim]Ntf2

303/1

0.49

[43]

5

[Aemim]TfO

303/1

0.47

[43]

6

[Amim]Ntf2

313/1

0.1777

[44]

7

[Apbim]BF4

295/1

∼0.5

[45]

8

[Bmim]Ac

323/20

0.373

[46]

9

[Bmim]ATZ

293/0.1

0.1228

[47]

10

[Emim]ATZ

293.15/0.1

0.1150

[47]

11

[Bmim]BF4

313.15/3.161

0.2734

[48]

12

[Bmim]OTf

313.15/0.2441

0.0373

[49]

13

[Hmim]Ntf2

313.15/0.3371

0.0801

[49]

14

[Bmim]SCN

313/16

0.126

[50]

15

[Bmim]DCA

298.2/59.43

0.5149

[51]

16

[Bmim]Met

298.3/61.73

0.741

[51]

17

[Bmim]NO3

298.2/56.21

0.4073

[51]

18

[Bmim]Ntf2

298.2/46.94

0.6491

[51]

19

[Hmim]Ntf2

298.2/60.91

0.7396

[51]

20

[Hmmim]Ntf2

298.2/56.56

0.6845

[51]

21

[Omim]Ntf2

298.2/60.13

0.7516

[51]

22

[Bmim]PF6

313/29.5

0.36

[52]

23

[Omim]BF4

313/28.9

0.319

[52]

24

[Omim]PF6

313/29.5

0.353

[52]

25

[Emim]EtSO4

314/28.1

0.146

[52]

26

[n-Bupy]BF4

313.15/26.8

0.243

[52]

27

[BPy]BF4

323/92.35

0.581

[52]

28

[Bmmim]PF6

298.15/6

0.092

[53]

29

[Emim]Ntf2

298/6

0.15

[53]

30

[Emim]TFA

323/20

0.2

[53]

31

[Emmim]Ntf2

298/6

0.137

[53] (continued on next page)

351

17.2 Capture of CO2 in ILs

TABLE 17.1 Adsorption capacity of CO2 in conventional ILs—cont’d Entry

System

T(K)/P(bar)

mol CO2 per mol IL

Ref.

32

[BMP]Ntf2

313/19.3

0.35

[54]

33

[C12 mim]BF4

333.15/0.94

0.1271

[55]

34

[C12 mim]PF6

333.15/0.99

0.1371

[55]

35

[C12 mim]Ntf2

333.15/0.76

0.1286

[55]

36

[Hmim]BF4

298/9.0

0.163

[56]

37

[Emim]BF4

298/8.8

0.106

[56]

38

[Hmim]PF6

313/25.7

0.299

[57]

39

[C9 mim]PF6

298/19.2

0.357

[58]

40

[DEA]Bu

313/0.926

0.1010

[59]

41

[Dmim]Ntf2

298/28.3

0.562

[60]

42

[EimH]CuCl2

303.2/0.1

0.0050

[61]

43

[Emim]CuCl2

303.2/0.1

0.0177

[61]

44

[Emim]C2 N3

313/28.1

0.9896a

[62]

45

[Emim]DCA

313.2/0.0528

0.0040

[63]

46

[Emim]SCN

313.2/0.0528

0.0023

[63]

47

[Emim]TCM

313.2/0.0497

0.0082

[63]

48

[HOPmim]NO3

315/21.4

0. 100 4

[64]

49

[N1114 ]NTf2

298/0.995

0.1948

[65]

50

[P14,6,6,6 ]Ntf2

313/27.4

0.6309

[66]

51

[Emim]Ac

298/0.994

0.1171

[67]

52

[PMPy]DCA

298/0.994

0.1218

[67]

53

[TBMP]Formate

298/0.99

0.1705

[67]

54

[Bmim]MeSO4

303/10

0.119

[68]

55

[MBPy]BF4

303/10

0.1443

[68]

56

[MBPy]DCA

303/10

0.1436

[68]

57

[MBPy]SCN

303/10

0.0962

[68]

58

[MBPyrr]DCA

303/10

0.1204

[68]

59

[MeBuPyrr]SCN

303/10

0.0971

[68]

60

[MeBuPyrr]TFA

303/10

0.1674

[68]

61

[Emim]TfO

303.85/149

0.626

[69]

62

[Bmim]TfO

303.85/11.5

0.273

[69] (continued on next page)

352

17. Carbon dioxide capture and utilization in ionic liquids

TABLE 17.1 Adsorption capacity of CO2 in conventional ILs—cont’d Entry

System

T(K)/P(bar)

mol CO2 per mol IL

Ref.

63

[Hmim]TfO

303.85/12.5

0.288

[69]

64

[Omim]TfO

303.85/180

0.741

[69]

65

[Hmim]FAP

298.1/19.99

0.493

[70]

66

[Bmim]TFES

298/19.9

0.285

[70]

67

[Bmim]TMA

298/19.9

0.431

[70]

68

[TBP]For

298.1/19.9

0.348

[70]

69

[THTDP]Cl

302.55/149.95

0.8

[71]

70

[THTDP]Ntf2

296.58/721.85

0.879

[71]

71

[BMPyrr]FEP

283.5/18

0.498

[72]

72

[HEA]Ac

298.15/15.15

0.1076

[73]

73

[HHE]MEA

298.15/15.42

0.0761

[73]

74

[BHEA]Lac

298.15/15.12

0.0835

[73]

75

[HHEME]Lac

298.15/15.23

0.0776

[73]

76

[HE]For

303/78.9

0.3083

[74]

77

[HE]Ac

303/90.1

0.4009

[74]

78

[HE]Lac

303/82

0.2422

[74]

79

[THEA]Ac

303/82.5

0.2561

[74]

80

[THEA]Lac

303/70.9

0.4617

[74]

81

[HEA]For

303/72.8

0.1907

[74]

82

[HEA]Ac

303/65.7

0.486

[74]

83

[HEA]Lac

303/73.2

0.264

[74]

84

[HEA]Lac

303/12.4

0.0704

[74]

increase of the temperature. To study the effect of anions, seven ILs fromed by [Bmim]+ and with different anions of NO3 - , trifluoromethanesulfonate (TFO- ), dicyandiamide (DCA- ), bis (trifluoromethylsulfonyl) imide (Ntf2 - ), tetrafluoroborate (BF4 - ), hexafluorophosphate (PF6 - ), and tris (trifluoromethylsulfonyl) formamide) were studied. It is found that the solubility of CO2 strongly relies on the anions. The order of the solubility of CO2 follows NO3 - > DCA- > BF4 - > PF6 - > CF3 SO3 - > Ntf2 - > formamide- . It was shown that the more fluorine atoms in the anion, the higher the CO2 solubility is. The effects of the numbers and lengths of alkyl chains on cations were studied, including 1-hexyl-3-methylimidazole bis (trifluoromethylsulfonyl) imide ([Hmim]Ntf2 ), 1-octyl-3-methylimidazole bis (trifluoromethylsulfonyl) imide ([Omim]Ntf2 ) and 2,3-dimethyl-1-hexyl imidazole bis (trifluoromethylsulfonyl)

17.2 Capture of CO2 in ILs

353

imide ([Hmmim]Ntf2 ). It was shown that the solubility of CO2 would be slightly increased with increasing the length of alkyl chain on cations. The solubility capacity of CO2 in 1,1,3,3-tetramethylguanidine lactic acid ([TMG]L) and 1–butyl–3-methylimidazolium hexafluorophosphoric acid ([C4 mim]PF6 ) at 0 to 11 MPa and 297 K to 328 K was studied by Zhang et al. [78]. The experimental results noted that the CO2 solubility in [TMG]L is slightly larger than that in [C4 mim]PF6 . At 5.73 MPa and 319 K, the CO2 solubility in [TMG]L and [C4 mim]PF6 is 2.77 mol/kg and 2.65 mol/kg, respectively. The selectivity of [TMG]L for CO2 is much better than that of other gases such as O2, N2 , H2, and CH4 . Shariati et al. [79,80] measured the equilibrium relationship between CO2 and [Emim]PF6 , [Bmim]PF6 , [Bmim]BF4 , [Hmim]BF6 and [Hmim]PF6 ionic liquids under high pressure (up to 97 MPa), and analyzed the effects of anions and alkyl chain lengths on the CO2 solubility in ILs. It was showed that the CO2 solubility in ILs increases with increasing the length of alkyl chain at the same pressure. It is showed that the increase of the length of alkyl chain lead to the decrease the point pressure of bubble, which causes to the higher CO2 solubility in ILs. It also shows that the CO2 solubility in ILs with PF6 - is greater than that in ILs with BF4 - , suggesting that the interaction of PF6 - with CO2 is more strong than that of BF4 - . Shiflett et al. [81] measured the CO2 solubility in [Bmim]Ac, [Bmim]PF6 and [Bmim]BF4 at 283 ∼ 384 K and up to 2.0 MPa, and established a state equilibrium theory to correct the experimental data. It was showed that the gas solubility in ILs increases with the increase of the pressure and decreases with increasing the temperature. Kumelan et al. [82,83] measured the CO2 solubility in [Bmim]PF6 , [Bmim]CH3 SO4 , and [Hmim]Ntf2 at 293–393 K and 0–9.7 MPa. By using the generalized Henry’s law to correlate the experimental data, the thermodynamic properties of the system such as dissolution enthalpy, dissolution entropy and dissolution Gibbs free energy, was calculated. Jacek et al. [84] showed that the CO2 solubility in [Hmim]Ntf2 can reach about 4.7 mol/kg at a pressure less than 10 MPa and 293–413K. Schilderman et al. [85] obtained the same conclusion when measuring the CO2 solubility in [Emim]Ntf2 under the conditions of the CO2 mole fractions of 12.3 percent–59.3 percent, at a pressure less than 15 MPa and 310–450 K and. Anderson et al. [86] revealed the effect of cationic fluorination on the CO2 solubility in ionic lqiuids. They found that CO2 solubility in [C8 H4 F13 mim]Ntf2 was larger than that in [C6 H4 F9 mim]Ntf2 , while solubility of CO2 in [C6 mim]Ntf2 was the lowest. The CO2 solubility increases with increasing the amount of fluorine in the side chain of alkyl, but this trend is not obvious. Mark et al. [87] studied the CO2 solubility in [Bmim]Ac under the conditions of pressure less than 2 MPa and 283–384 K. It was found that when CO2 mole fraction reached to 20 percent in the system, there was almost no vapor pressure, indicating that CO2 and ILs formed a nonvolatile or extremely low vapor pressure molecular complex, with strong mutual attraction between molecules and obvious formation of molecular complexes. The formation of the complexes is reversible and the ILs do not degenerate. Kumelan et al. [82] measured solubility of CO2 in [Bmim]PF6 , [Hmim]Ntf2 and [Bmim]CH3 SO4 at 293–393 K and 0–9.7 MPa. They calculated the thermodynamic properties of the system, such as dissolution Gibbs free energy, entropy and enthalpy by using the Henry’s law. Soriano et al. [88,90] measured the solubility of CO2 in [Bmim]PF6 , [Emim]BF4 and

354

17. Carbon dioxide capture and utilization in ionic liquids

[Emim]TFO at 303.2∼343.2 K and medium pressure (within 6.5 MPa) by thermogravimetric microbalance method. Palgunadi et al. [89] studied the CO2 solubility in two dialkyl imidazolium dialkyl phosphate ILs [Emim]Et2 PO4 and [Bmim]Bu2 PO4 near atmospheric pressure and 313 ∼ 333 K. It showed that the dissolution mechanism of CO2 is the same as that of other ILs. Jung et al. [91] synthesized an imidazole methyl sulfonate [Dbim]MeSO3 , and measured the absorption capacity of CO2 together with another three imidazole methyl sulfonates [Emim]MeSO3 , [Bmim]MeSO3 , [Dmim]MeSO3 . It was found that the absorption capacities of CO2 in four ILs from small to large is [Dmim]MeSO3, [Emim]MeSO3 , [Bmim]MeSO3 and [Dbim]MeSO3 . Brennecke et al. [92] measured the absorption capacity of CO2 in 10 imidazolium ILs at 25– 60 °C and the pressure of 1 ∼ 15 MPa. For the same cation ([Bmim]+ ), the absorption capacity of CO2 with different anions at the same temperature and pressure is from small to large as DCA- , NO3 - , PF6 - , BF4 - , Ntf2 - , TFO- and methide- . Shannon [93] and Manic [94] measured the absorption capacity of CO2 in ILs with different lengths of carbon chain. It was found that the increase of the carbon chain was caused to the enhancement of CO2 absorption. When anions of ILs are same, the solubility enhances slightly with increasing the carbon chain of substituents on cations. 17.2.1.2 Pyridine and pyrrolidine ILs Anthony et al. [92] found that the gas solubility in ILs with pyrrolidine cation is larger than that in the ammonium based ILs. The solubility of gas in [C1 C2 im]Ntf2 changes significantly with temperature, but the solubility in ionic liquid [C1C4 Pyrr]Ntf2 changes little. Honga et al. [96] revealed the effect of cation change in ionic liquid on gas solubility. The solubility of CO2 and C2 H6 was measured out in three Ntf2 - containing ILs with cations [C1 C2 im]+ , [C1 C4 Pyrr]+ and [N1132 –OH]+ at 300∼345 K. It was shown that changing cations have a little effect on the CO2 solubility, but it is obvious. The effects of changing cations on the solubility of two gases in ionic liquids are similar, but a more significant effect on the solubility C2 H6 was found than that on CO2 . The order of the effect of ILs on the solubility of CO2 follows [C1 C4 Pyrr]Ntf2 > [C1 C2 im]Ntf2 > [N1132 –OH]Ntf2 , and the solubility reduce with increasing the temperature. Although the effect of changing cations on solubility is smaller than that of changing anions, it is worth noting. Anderson et al. [97] measured the solubility of CH4 , N2 , CO2 and other gases in 1-hexyl3-methylpyridinium bis (trifluoromethyl) amine ([HmPy]Ntf2 ), which was compared and analyzed with the existing solubility data. They proposed many ways to improve the CO2 solubility in ILs. It concluded that the anions in ILs can affect CO2 solubility more than the cations. The solubility of CO2 can be improved by introducing fluorine atoms instead of hydrogen atoms into the anions or changing the types or introducing functional groups of anion. The effect is more obvious when introducing fluorine atoms into anions, and increases with the increase of fluorine substitution degree. The CO2 solubility can also be enhanced by introducing ether groups or long-chain alkyl groups to increase the free volume of ILs or the affinity with CO2 . Moganty et al. [98] studied CO2 solubility of in 1-hexyl-3-methylimidazolium hexafluorophosphate ([Hmpy]BF6 ), 1-butyl-3-methylpyridinium tetrafluoroborate([Bmpy]BF4 ),

17.2 Capture of CO2 in ILs

355

1-octyl-3-methylpyridinium ([Ompy]BF4 ), [Hmim]Ntf2 , [Emim]Ntf2 , 1-ethyl-3methylimidazolium bis(pentafluoroethyl sulfonyl) imino ([Emim]BETI), 1-ethyl3-methylimidazolium trifluoro-acetate ([Emim]TFA), 1-ethyl-3-methylimidazolium trifluoromethanesulfonate ([Emim]TFO) at atmospheric pressure and at 10 °C, 25 °C and 40 °C. They calculated the corresponding Henry’s constants. In combination with the Henry’s constants in the four ILs reported in the literature, the solubility parameters of ILs were calculated according to the viscosity data of ILs. The Henry’s constants in the 12 ILs for CO2 were predicted by RST. The deviation between the experimental results and the predicted results is within ±2.5 percent, which indicates that the prediction method is basically reasonable and reliable. 17.2.1.3 Sulfonate ionic liquids Recently, CO2 solubility in sulfonate ILs have also been studied. Zhang et al. [99] studied the CO2 solubility in sulfonate ILs such as [P66614 ]C12 H25 PhSO3 and [P66614 ]MeSO3 at a pressure of 4∼9 MPa and a temperature of 305∼325 K. It was shown that the CO2 solubility in the two sulfonate ILs varies small, but the CO2 solubility in [P66614 ]MeSO3 is larger than that in [P66614 ]C12 H25 PhSO3 . The CO2 solubility in sulfonate ILs is related to Henry’s constants, which increases with the increase of temperature, which is consistent with Alvaro’s results. It is also showed that the CO2 solubility in sulfonate ILs is generally lower than that in imidazolium ILs.

17.2.2 CO2 capture by functionalized ionic liquids Under ambient temperature and pressure, imidazole type, sulfonate type, ammonium salt type, pyrrole type ILs and other conventional ILs have limited CO2 absorption capacity, so it is necessary to develop new ILs with specific functions for CO2 fixation/conversion. The emergence of functionalized ILs has greatly improved the absorption capacity of CO2 in ILs. Functionalized ionic liquids is an ILs that uses the structural designability of ILs to introduce one or more specific functional groups into the cation and anion ions or the cation and anion ions of ionic liquid have a specific structure and give ionic liquid some special functions or characteristics. Functionalized ILs can obviously break through the limitations of conventional ILs and solve the shortage of commercial absorbents such as amino-based solutions [100–187]. Table 17.2 shows the CO2 absorption in functionalized ILs. Bates et al. [45] reported that the functionalized ionic liquids of [pabim]BF4 was appied to capture CO2 . The saturated CO2 concentration in ILs can reach to 7.4 percent. The capture mechanism was that CO2 molecules attacked the free electron in N atoms to form a new CO–O radical. At the same time, another NH2 radical of [Pabim]+ accepted an H+ and became an NH3 + radical. Zhang et al. [116] successfully prepared tetrabutylphosphonium amino acid ILs ([TBP]amino acids). They also used the ionic liquid as an absorbent for CO2 to conduct CO2 absorption tests. It was found that the quaternary phosphonic amino acid ionic liquid has a high absorption capacity of CO2 with 0.5 mol CO2 /mol IL. However, due to its high viscosity and high cost, it does not have the capacity of large-scale production. Yu et al. [118] synthesized fifteen novel amino acid ionic liquids (AAILs) by the combination of several tetraalkylammonium cations with four amino acid anions ([Gly], [L-Ala], [β-Ala]

356

17. Carbon dioxide capture and utilization in ionic liquids

TABLE 17.2 The capture of CO2 in various functionalized ILs. Systems

T(K)/P(bar)

mol CO2 per mol IL

Ref.

1

[NH2 p-bim]BF4

295/1

∼0.5

[45]

2

[APMim]DCA

303/10

0.29

[68]

4

[APMim]Ntf2

303/10

0.27

[68]

5

[AEMPyrr]BF4

303/10

0.28

[68]

6

[APMim]BF4

303/10

0.32

[68]

7

[MeImNet2 ]BF4

303/4

0.09

[68]

8

[Emim]Ac

313.2/0.1

0.28

[100]

9

[Emim]Ala

313.2/0.1

0.38

[100]

10

[DETAH]Lys

313.15/0.1

2.13

[101]

11

[TETAH]Lys

313.15/0.1

2.59

[101]

12

[DETAH]Gly

313.15/0.1

1.81

[102]

13

[DETAH]Im

313.15/0.1

2.04

[102]

14

[DETAH]Py

313.15/0.1

1.95

[102]

15

[DETAH]Tz

313.15/0.1

1.74

[102]

16

[DMAPAH]2-F-PhO

303.2/0.1

0.67

[103]

17

[DMAPAH]3-F-PhO

303.2/0.1

0.73

[103]

18

[DMAPAH]3,5-F-PhO

303.2/0.1

0.82

[103]

19

[DMAPAH]4-F-PhO

303.2/0.1

0.86

[103]

20

[VBTMA]Ala

298/0.1

0.29

[105]

21

[VBTMA]Arg

298/0.1

0.83

[105]

22

[VBTMA]Gly

298/0.1

0.47

[105]

23

[VBTMA]Hist

298/0.1

0.46

[105]

24

[VBTMA]Lys

298/0.1

0.66

[105]

25

[VBTMA]Pro

298/0.1

0.38

[105]

26

[VBTMA]Ser

298/0.1

0.39

[105]

27

[VBTMA]Tau

298/0.1

0.44

[105]

28

[P2228 ]2-CNPyr

333.15/0.1

0.92

[106]

29

[P2228 ]6-BrBnIm

333.15/0.15

0.88

[106]

30

[P2228 ]BnIm

333.15/0.1

0.97

[106]

31

[P4444 ]Ala

298/1

∼0.67

[106]

32

[P4444 ]Gly

298/1

∼0.6

[106]

Entry

(continued on next page)

357

17.2 Capture of CO2 in ILs

TABLE 17.2 The capture of CO2 in various functionalized ILs—cont’d Systems

T(K)/P(bar)

mol CO2 per mol IL

Ref.

33

[P4444 ]β-Ala

298/1

∼0.6

[106]

34

[DMAPAH]Formate

298.15/0.1

0.2

[111]

35

[DMAPAH]Octanoate

298.15/0.1

0.46

[111]

36

[DMEDAH]Formate

298.15/0.1

0.32

[111]

37

[Emim]Benzoate

333/5.3

0.129

[113]

38

[Emim]Lac

333/5.3

0.113

[113]

39

[Emim]Piv

333/5.3

0.275

[113]

40

[PEG150 MeBu2 NLi]Ntf2

298/1

0.66

[114]

41

[PEG150 MeNH2 Li]Ntf2

298/1

0.45

[114]

42

[PEG150 MeTMGLi]Ntf2

298/1

0.89

[114]

43

[P4442 ]Cy-Suc

293.15/0.1

2.21

[118]

44

[P4442 ]Suc

293.15/0.1

1.85

[118]

45

[P4442 ]pH-Suc

293.15/0.1

1.4

[118]

46

[N2222 ]β-Ala

313

∼0.50

[118]

47

[N2224 ]Ala

313

∼0.48

[118]

48

[TMGH]Im

313.15/0.1

0.64

[122]

49

[TMGH]PhO

313.15/0.1

0.05

[122]

50

[TMGH]Pyrr

313.15/0.1

0.66

[122]

51

[DEEDAH]Ac

293/1

0.3

[122]

52

[DEEDAH]HCOO

293/1

0.47

[122]

53

[DMAPAH]Ac

293.15/1

0.33

[122]

54

[DMAPAH]HCOO

293.15/1

0.28

[122]

55

[DMEDAH]HCOO

293/1

0.38

[122]

56

[P66614 ]Ala

295/1

0.66

[122]

57

[P66614 ]ILe

295/1

0.97

[122]

58

[P66614 ]Sar

295/1

0.91

[122]

59

[P66614 ]Lys

295/1

1.37

[122]

60

[P66614 ]Tau

295/1

∼0.8

[122]

61

[P4442 ]DAA

293.15/0.01

1.25

[123]

62

[P4442 ]Suc

293.15/0.01

1.65

[123]

63

[N2222 ]Ala

313

∼0.45

[123]

Entry

(continued on next page)

358

17. Carbon dioxide capture and utilization in ionic liquids

TABLE 17.2 The capture of CO2 in various functionalized ILs—cont’d Systems

T(K)/P(bar)

mol CO2 per mol IL

Ref.

64

[N66614 ]Asn

295/1

2

[123]

65

[N66614 ]Gln

295/1

1.9

[123]

66

[N66614 ]His

295/1

1.9

[123]

67

[N66614 ]Lys

295/1

2.1

[123]

68

[N66614 ]Met

295/1

1

[123]

69

[Bmim]2-Op

303.15/0.1

1.02

[125]

70

[BMmim]2-Op

303.15/0.1

1.06

[125]

71

[BMPyr]2-Op

303.15/0.1

1.17

[125]

72

[N4442 ]2-Op

303.15/0.1

1.24

[125]

73

[P4442 ]2-OP

303.15/0.1

1.4

[125]

74

[P4442 OH]2-Op

303.15/0.1

0.94

[125]

75

[pH–C8 eim]2-Op

293.15/0.1

1.69

[125]

76

[Bmim]Ala

298/2

0.39

[125]

77

[Bmim]Arg

298/2

0.62

[125]

78

[Bmim]Gly

298/2

0.38

[125]

79

[Bmim]His

298/2

0.45

[125]

80

[Bmim]Leu

298/2

0.38

[125]

81

[Bmim]Lys

298/2

0.48

[125]

82

[Bmim]Met

298/3

0.42

[125]

83

[Bmim]Pro

298/2

0.32

[125]

84

[Bmim]Val

298/2

0.39

[125]

85

[Me2 N(CH2 CH2 OH)2 ] Gly

313/1

0.48

[125]

86

[Me2 N(CH2 CH2 OH)2 ]Pro

313/1

0.54

[125]

87

[Me2 N(CH2 CH2 OH)2 ]Gly

313/1

0.53

[125]

88

[Me2 N(CH2 CH2 OH)2 ]Tau

313/1

0.5

[125]

89

[N1112 ]Pro

313

0.74

[125]

90

[P66614 ]Beta-Ala

303.15/0.1

1.1

[127]

91

[P66614 ]MA-Tetz

303.15/0.1

1.13

[127]

92

[P66614 ]Gly

303.15/0.1

1.2

[127]

93

[P66614 ]2 Asp

303.15/0.1

1.96

[127]

94

[Emim]Arg

313/1

0.52

[127]

Entry

(continued on next page)

359

17.2 Capture of CO2 in ILs

TABLE 17.2 The capture of CO2 in various functionalized ILs—cont’d Systems

T(K)/P(bar)

mol CO2 per mol IL

Ref.

95

[P4444 ]Ala

298/5

1.1

[127]

96

[P4444 ]Bic

298/5

0.44

[127]

97

[P4444 ]dmGly

298/5

0.24

[127]

98

[P4444 ]Gly

298/5

1.02

[127]

99

[P4444 ]ILe

298/5

1.07

[127]

100

[P4444 ]Pro

298/5

1.01

[127]

101

[P4444 ]Val

298/5

1.07

[127]

102

[AP4443 ]Ala

298/1

0.92

[127]

103

[AP4443 ]Gly

298/1

0.94

[127]

104

[N1111 ]Lys

303

0.39

[128]

105

[Cho]Pro

308/1

∼0.6

[128]

106

[P66614 ]o-AA

303/1

0.6

[128]

107

[P66614 ]o-ANA

303/1

0.56

[128]

108

[P66614 ]p-AA

303/1

0.94

[128]

109

[P66614 ]p-ANA

303/1

0.78

[128]

110

[MTBDH]2 HFPD

296/1

2.04

[128]

111

[P66614 ]2-CN-Pyr

295/1

0.9

[129]

112

[P66614 ]3-CF3 -Pyra

295/1

0.9

[129]

113

[P66614 ]2-SCH3 BnIm

295/1

0.73

[130]

114

[P66614 ]6-BrBnIm

295/1

0.9

[130]

115

[P66614 ]BnIm

295/1

0.91

[130]

116

[P22212 ]2-CN-Pyr

295/0,15

0.73

[131]

117

[P2224 ]2-CN-Pyr

295/0.15

0.8

[131]

118

[P44412 ]2-CN-Pyr

295/0.15

0.72

[131]

119

[P44418 ]2-CN-Pyr

295/0.15

0.64

[131]

120

[P66614 ]2-CN-Pyr

295/0.15

0.62

[131]

121

[DBUH]Phth

298/1

0.98

[132]

122

[TMGH]Phth

298/1

0.98

[132]

123

[P66614 ]1-Naph

303/1

0.89

[133]

124

[P66614 ]2,4,6-Cl-PhO

303/1

0.07

[133]

125

[P66614 ]2,4-Cl-PhO

303/1

0.48

[133]

Entry

(continued on next page)

360

17. Carbon dioxide capture and utilization in ionic liquids

TABLE 17.2 The capture of CO2 in various functionalized ILs—cont’d Systems

T(K)/P(bar)

mol CO2 per mol IL

Ref.

126

[P66614 ]2-Cl-PhO

303/1

0.67

[133]

127

[P66614 ]2-Naph

303/1

0.86

[133]

128

[P66614 ]3-Cl-PhO

303/1

0.72

[133]

129

[P66614 ]3-NMe2 -PhO

303/1

0.94

[133]

130

[P66614 ]4-CF3 -PhO

303/1

0.61

[133]

131

[P66614 ]4-Cl-PhO

303/1

0.82

[133]

132

[P66614 ]4-MeO-PhO

303/1

0.92

[133]

133

[P66614 ]4-NO2 -PhO

303/1

0.3

[133]

134

[P66614 ]4-Me-PhO

303/1

0.91

[133]

135

[P66614 ]4-H-PhO

303/1

0.85

[133]

136

[DBUH]TFE

298/1

1.01

[134]

137

[P4444 ]3-F-PhO

313/1

0.74

[135]

138

[P4444 ]PhO

313/1

0.77

[135]

139

[P4444 ]2-F-PhO

313/1

0.67

[135]

140

[P4444 ]4-F-PhO

313/1

0.84

[135]

141

[Bmim](CH3 )2 CHCOO

298/1

0.28

[136]

142

[Bmim](CH3 )3 CCOO

298/1

0.31

[136]

143

[Bmim]CH3 CH2 COO

298/1

0.28

[136]

144

[Eeim]Ac

298/1

0.32

[137]

145

[Apmim]Im

313/1

0.75

[138]

146

[Bis(mim)C2 ]Im2

313/1

0.75

[138]

147

[Bis(mim)C4 ]Im2

313/1

0.95

[138]

148

[Bmim]Im

313/1

∼0.54

[138]

149

[Emim]Im

313.5/1

∼0.54

[138]

150

[HO-emim]Im

313/1

∼0.55

[138]

151

[N1111 ]Gly

298/1

0.17

[139]

152

[C2 (N112 )2 ]Gly2

298/1

0.89

[140]

153

[C2 (N114 )2 ]Gly

298/1

0.81

[140]

154

[Bis(mim)C2 ]Gly2

313/1

∼0.8

[141]

155

[Bis(mim)C2 ]Pro2

313/1

∼0.9

[141]

156

[Bis(mim)C4 ]Gly2

313/1

∼0.9

[141]

Entry

(continued on next page)

361

17.2 Capture of CO2 in ILs

TABLE 17.2 The capture of CO2 in various functionalized ILs—cont’d Systems

T(K)/P(bar)

mol CO2 per mol IL

Ref.

157

[Bis(mim)C4 ]Pro2

313/1

∼0.8

[141]

158

[C1 C4 Pyrro]Ac

353/0.98

0.01

[142]

159

[N2222 ]PhO

323

0.64

[143]

160

[TETA]NO3

288.15/1

1.49

[144]

161

[ApaeP444 ]Ala

298/1

1.14

[145]

162

[ApaeP444 ]Asp

298/1

1.07

[145]

163

[ApaeP444 ]Gly

298/1

1.29

[145]

164

[ApaeP444 ]His

298/1

1.01

[145]

165

[ApaeP444 ]Lys

298/1

1.73

[145]

166

[ApaeP444 ]Ser

298/1

1.19

[145]

167

[AEMP]Ala

298/1

1.57

[146]

168

[AEMP]Gly

298/1

1.5

[146]

169

[AEMP]Leu

298/1

1.47

[146]

170

[AEMP]Pro

298/1

1.54

[146]

171

[OH-emmim]PhO

298/1

1.58

[147]

172

[P66614 ]2-Op

293/1

1.58

[148]

173

[P66614 ]3OCH3 -2-Op

303/1

1.65

[148]

174

[P66614 ]3-Op

303/1

1.38

[148]

175

[P66614 ]4-ABI

303/1

1.6

[148]

176

[P66614 ]4-Op

293/1

1.49

[148]

177

[P66614 ]4-CHO-Im

303/1

1.24

[149]

178

[P66614 ]4-CHO-PhO

303/1

1.01

[149]

179

[P66614 ]4-EF-PhO

303/1

1.03

[149]

180

[P66614 ]4-Kt-PhO

303/1

1.04

[149]

181

[P66614 ]Met

295/1

∼0.9

[150]

182

[P66614 ]Pro

295/1

∼0.9

[150]

183

[Aemmim]Tau

303/1

0.9

[151]

184

[MTBDH]Im

296/1

1.03

[152]

185

[MTBDH]TFE

296/1

1.13

[152]

186

[MTBDH]PhO

296/1

0.49

[152]

187

[MTBDH]Pyrr

296/1

0.92

[152]

Entry

(continued on next page)

362

17. Carbon dioxide capture and utilization in ionic liquids

TABLE 17.2 The capture of CO2 in various functionalized ILs—cont’d Systems

T(K)/P(bar)

mol CO2 per mol IL

Ref.

188

[(P2 -Et)H]Im

296/1

0.96

[152]

189

[MTBDH]TFPA

296/1

0.39

[152]

190

[(P2 -Et)H]Pyrr

296/1

0.86

[152]

191

[(P2 -Et)H]PhO

296/1

0.45

[152]

192

[(P2 -Et)H]TFE

296/1

1.04

[152]

193

[Me2 N(CH2 CH2 OH)2 ]Tau

310/2.5

∼0.4

[153]

194

[Me2 N(CH2 CH2 OH)2 ]Tau

310/4

0.92

[153]

195

[P66614 ]Bentriz

296/1

0.17

[154]

196

[P66614 ]Im

296/1

1

[154]

197

[P66614 ]Ind

296/1

0.98

[154]

198

[P66614 ]Oxa

296/1

0.91

[154]

199

[P66614 ]PhO

296/1

0.5

[154]

200

[P66614 ]Pyr

296/1

1.02

[154]

201

[P66614 ]Tetz

296/1

0.08

[154]

202

[P66614 ]Triz

296/1

0.95

[154]

Entry

and [Val]). The capacity of CO2 absorption was measureed at 40 °C under ambient pressure. It was found that [N2224 ][LAla] (N2224 : triethylbutylammonium) has the lowest viscosity (29 mPa.s/25 °C), and the adsorption of CO2 in this ILs can reach the theoretical equilibrium value (0.5mol CO2 /mole ILs) within 30min. The absorbed CO2 can be completely desorpted at 60 °Cand 0.1 kPa. Gurkan et al. [122] synthesized Trihexyl(tetradecyl)phosphonium Methioninate ([P66614 ]Met) and Trihexyl(tetradecyl)phosphonium Prolinate ([P66614 ]Pro), and studied CO2 absorption in these ionic liquids. It is showed that there is intramolecular proton transfer in ILs, that is, the sterically hindered large phosphonium cation prevents the intermolecular proton transfer process. Then six amine functionalized anionic tethered ILs was synthesized such as Trihexyl(tetradecyl)phosphonium sarcosine ([P6614 ]Sar), Trihexyl(tetradecyl)phosphonium glycinate ([P66614 ]Gly), Trihexyl(tetradecyl)phosphonium alanine ([P66614 ]Ala), Trihexyl(tetradecyl)phosphonium valine ([P66614 ]Val), Trihexyl (tetradecyl)phosphonium leucine ([P66614 ]Leu) and Trihexyl(tetradecyl)phosphonium isoleucine ([P66614 ]Ile) and studied the CO2 absorption in these ionic lqiuids. They found that the capacity of CO2 adsorption in these ILs was higher than 0.5 mol CO2 /mol IL under the pressure less 0.1 MPa. The order of the decomposition temperatures of ionic lqiuid were observed to decrease as Tau- > Gln- > Pro- > Met- > Lys- > Thr- > Asn- . Wang et al. [127] fixed three kinds of amino acid based ILs of [Emim]Ala, [Emim]Gly, and [Emim]Arg on nano-porous material of PMMA, and tested their CO2 adsorption capacity at

17.2 Capture of CO2 in ILs

363

different adsorption temperatures. The results show that the higher the temperature is, the lower the absorption capacity is. The absorption capacities of CO2 in these three ILs were 1.38 mmol/g, 1.53 mmol/g and 1.01 mmol/g at 40 °C, respectively. Li et al. [128] used polyethylene glycol 200 (PEG-200) as a solvent to dissolve [choline]Proto obtain a low viscosity TSILs solution, which was used to capture CO2 near atmospheric pressure. The results show that the addition of inert PEG-200 can greatly shorten the CO2 absorption and desorption time in [choline]Pro (at 323.15k, the CO2 absorption and desorption time in pure [choline]Pro-are 240 and 260 min respectively; when PEG-200 of the same quality as [choline]Pro is added, the CO2 absorption and desorption time is reduced to 50min at 308.15K. In addition, phenolic anion functionalized ILs have also attracted much attention. Hu et al. [135] reported some functionalized ILs with fluorophenol anions. The results showed that these ILs had low viscosity. The order of capacity of CO2 absorption was [P4444 ]4-F-PhO > [P4444 ]3-F-PhO> [P4444 ][2-F-PhO]. Ma et al. [140] designed and synthesized amino acid ILs [C2 (N112 )2 ]Gly2 and [C2 (N114 )2 ]Gly2 , measured the capacity of CO2 absorptionin the two ILs at different concentrations. They mixed the ionic liquid with aqueous solution of MDEA to study the CO2 absorption capacity of the compounded system. It was shown that the amount of CO2 absorbed by ILs is decreased with increasing their concentration. The CO2 absorption capacity of 15 percent [C2 (N112 )2 ]Gly2 +15 percent MDEA mixed solution can reach 1.02 mol CO2 /mol IL,when the ionic liquid is mixed with MDEA aqueous solution, while the pressure at this time is only 0.25 MPa. Zhang et al. [141] synthesized four kinds of amino acid based ILs of [Bis(mim)C4 ]Pro2 [Bis(mim)C2 ]Pro2 , [Bis(mim)C2 ]Gly, [Bis(mim)C4 ]Gly2 , and two dumbbell ILs of [bis(mim)C6 ](Ntf2 )2 and ([N111 –C6 -mim](Ntf2 )2 which contains two groups of anions and anions. The physical properties of the six ILs and their absorption of CO2 were tested. It was found that the CO2 absorption in six ILs increased with increasing the pressure. At 1 MPa and 40 °C, the absorption of CO2 by the four amino acid ILs exceeded 1 mol CO2 /mol IL, and the efficiency of absorption was much larger than that of the other two dumbbell ILs. Peng et al. [146] synthesized four amino acid ILs of [Aemp]Gly, [Aemp]Ala, [Aemp]Leu, [Aemp]Pro-with [Aemp]+ cation. Due to its high viscosity, they dissolved the ionic liquid in water, ethylene glycol and SiO2 gel respectively to test its CO2 absorption capacity. The results show that ionic liquid + SiO2 has higher CO2 absorption capacity than that dissolved in water or ethylene glycol. When m(IL): m (SiO2 ) = 1ࢼ4 (mass ratio), the capacity of CO2 absorption can reach to 1.5 mol CO2 /mol IL When only one amino-group is introduced into the cation, and the increase of the capacity of CO2 absorption is limited. Therefore, some scholars began to introduce multiple amino groups. Zhang et al. [155] synthesized 20 bifunctional ILs by using amino acid as the anion and (3-aminopropyl) tributylphosphonium [AP4443 ]+ as the cation. Four ILs with anions Gly- , Ala- , Aal- and Leu- were selected for atmospheric CO2 capture experiments. It showed that when directly used these ionic liquid for CO2 capture, the CO2 absorption can remain at 0.2 CO2 /mole ILs after 2h, since the viscosity of ILs will rise sharply after absorption of CO2 . However, when supported on porous SiO2 to form a supported ILs liquid film, the resistance formed by high viscosity can be effectively overcome, and the theoretical absorption amount of 1.0 mol CO2 /mole ILs can be reached within 80 min.

364

17. Carbon dioxide capture and utilization in ionic liquids

Wu et al. [156] synthesized an amino containing ILs of 1-(1-aminopropyl)−3methylimidazolium bromide ([NH2 p-mim]Br), and found it can effectively absorb CO2 . At 40 °C and 10.6 MPa, the capacity of CO2 absorption can reach 0.444 mol CO2 /mol IL. Han et al. [157] studied the absorption of CO2 by urea/choline chloride, 1,1,3,3-tetramethylguanidinium perchlorate ([TMG]ClO4 ) and 1-aminoethyl-3-methyl imidazolium tetrafluoroborate ([Aemim]BF4 ), It is found that adding atmospheric CO2 to IL can significantly reduce its alkalinity, and the alkalinity of IL can be easily recovered by bubbling N2 through the solution to remove CO2 . Meindersma et al. [158] measured the CO2 absorption in [Bmim]BF4 amino functionalized 1-(3-aminopropyl)−3-methylimidazolium tetrafluoroborate ([Apmim]BF4 ). It was found that the amount of CO2 absorbed depends on the concentration of [Apmim]BF4 . A new diamino ionic liquids 1-aminoethyl-2,3-dimethylimidazolium taurine([Aemmim] Tau) was synthesized with a dual amino ionic liquid with amino-functionalized imidazolium cation and taurine anion by Zhang et al. [151]. At 303.15 K and 0.1 MPa, the CO2 absorption capacity of this ionic liquid can reach to 0.9 mol CO2 /mol IL at 303.15 K and 0.1 MPa. The dissolved CO2 can be easily deabsorbed under higher temperature or vacuum. They found that [Aemmim]Tau can be recycled, and no significant loss of performance has been observed after six cycles. ILs containing amino groups can greatly improve the absorption capacity of CO2 , and ILs containing amino groups on both anion and cation have better absorption effect. It was shown that when the ionic liquid containing amino group is mixed with conventional imidazole ILs, the effect of imidazole ILs on CO2 absorption can be improved [159], which provide a potential foundation for large-scale application in the future. Natural amino acids and their derivatives in amino-acid ILs can act as both anions and cations of ILs, such as glycine nitrate ([Gly]NO3 ), alanine boron tetrafluoride ([Ala]BF4 ), 1-ethyl-3-methyl-imidazole glycine ([Emim]Gly), 1ethyl-3-methyl-imidazole valine ([Emim]Val), etc. Amino acid ILs have high thermal stability, negligible vapor pressure, and wide liquid stability range. At the same time, amino acid ILs have many unique properties. For example, amino acid ILs have a strong hydrogen-bond network structure and can dissolve many life substances, such as DNA, cellulose, etc. It can replace the traditional organic solvent medium for chemical reaction and realize the greening of the reaction process [159]. Wang et al. [160] used Task-Specific ionic liquids (TSILs) to capture CO2 with (1-(2hydroxyethyl)−3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([Im21 OH][Ntf2 ]) and 2-hydroxyethyl(dimethyl)isopropylammonium bis(trifluoromethylsulfonyl)imide ([Nip,211 OH]Ntf2 ) with hydroxyl groups on the cationic side chains. A new CO2 capture solution is formed by mixturing with equimolar super bases DBU (1,8-diazabicyclo[5.4.0]undec– 7-ene), MTBD (1,3,4,6,7,8-hexahydro-1-methyl-2H-pyrimido[1,2-a]pyrimidine), BEMP(2–tertbutylamino-2-diethylamino-1,3-dimethylperhydro-1,3,2-diazaphosphorine) and ETP2 (DMA) (1-ethyl-2,2,4,4,4-pentakis(dimethyl amino)−2λ5 ,4λ5 -catenadi(phosphazene)) with TSILs. It is found that it can be achieved near equimolar absorption of CO2 under ambient pressure and 20 °C within 30 min. The capture of CO2 by [Im21 OH]Ntf2 –DBU occurs rapidly, and the capacity of CO2 capture is more than 1 mol per mole of superbase, which is superior to those captured by convetional ILs. Sharma et al. [161] combined the amino cation [2-Aemim]+ with six different anions, and studied the CO2 absorption capacity in these six ILs at 30 °C, 50 °C and 0.16 MPa. It showed that the absorption capacity of ILs containing amino groups to CO2 is greatly enhanced under

17.2 Capture of CO2 in ILs

365

ambient pressure, up to 0.49 mol CO2 /mol IL. The the capacity order of CO2 capture from small to large in different anions based ILs follows as BF4 - , PF6 - , DCA- , Ntf2 - , and TfO- . Daiyue et al. [162] designed a series of symmetrical amino acid based ILs and measured the performance of CO2 absorption. It was shown that these ILs had high fast absorption rate, absorption capacity and good repeated absorption performance. At the same time, [N2222 ]Ala, [N111 ]Gly, [N2224 ]CH3 COO, which are functional ILs with low viscosity, short synthetic route and fast absorption rate, was compounded with MDEA aqueous solution to study the absorption properties of the mixed solutions with different proportions. Among them, the absorption capacity of 2.5 percent [N111 ]Gly is the largest, and the amount ratio of substances reaches 1.058 at 0.94 MPa. Amino acids have low cost, excellent biodegradability and bioactivity, a wide variety and stable chiral centers. Amino acid ILs have excellent absorption properties for CO2 , and have high absorption at room temperature and less than 1 MPa. Wang et al. [163] designed various phenolic anion functionalized quaternary ILs with large π -bond structure. It was found that this functionalized ILs have a higher absorption capacity of CO2 and easier desorption than those functionalized by phenol anion (PhO− ). For example, at 20 °C and 1 bar, the absorption capacity of [P66614 ]PhO is 0.73 mol CO2 /mol IL, and the desorption residue is 0.17 mol CO2 /mol IL. Under the same conditions, the CO2 absorption capacity of the functionalized quaternary ionic liquid containing phenolic anions with large π bond structure is greater than 0.9 mol CO2 /mol IL, and desorption residue is less. Because the structure of phenol can be adjusted by substituents, the phenolic functionalized ILs can also be adjusted by substituents of anions, so as to improve the interaction between ILs and CO2 and regulate the CO2 absorption capacity. The phenol anion of short-chain quaternary ammonium ionic liquid [P2222 ]PhO with phenol as anion removes H atom connected to C2 in cation, and then easily interacts with CO2 to produce phosphorus of phenol [164], since its electronegativity, the carboxylic acid radical can also react with CO2 . For example, Tao et al. [165] used formic acid, acetic acid, propionic acid and butyrate as anions and [P4444 ]+ as cations to form quaternary carboxylate ILs, and studied the absorption performance of CO2 in these ILs. It is shown that due to the electron donating property of alkyl group, the CO2 absorption in ILs with butyrate as an anion can reach 0.4 mol CO2 /mol IL Wang et al. [166] measured the CO2 absorption performance of quaternary ILs functionalized with a series of cinnamic acid-based anions, indicating that the capacity of CO2 absorption in these ILs is close to the amount of equivalent substances, and when the substituent on the anion benzene ring is an electron donor substituent, the CO2 absorption capacity of ILs will increase, and the electron withdrawing substituent will lead to the reducing of CO2 absorption capacity. Tao et al. [167] synthesized [P4442 ]2 Ida ionic liquid by removing two protons from iminodiacetic acid Ida2− anions. It was found that the absorption capacity of this ionic liquid for CO2 was 1.69 mol CO2 /mol IL at 40 °C and 1 bar. Zhu et al. [168–170] studied the interaction between substituted [Im]+ and CO2 , indicating that [Im]+ can react to equimolar with CO2 , and the electron withdrawing group and electron pushing a group on the anion reduce the CO2 capture capacity of ILs. Because SO2 is more acidic than CO2 , the basic azolyl anion can interact with both CO2 and SO2 . Cui et al. [171] revealed the selective absorption of SO2 and CO2 gas by substituted imidazole anion functionalized ILs. It is shown that the substituents with

366

17. Carbon dioxide capture and utilization in ionic liquids

electron withdrawing reduced the alkalinity of ILs, thus improving the selectivity of SO2 /CO2 . This shows that low alkaline (or acidic) ILs can effectively separate SO2 from CO2 , and have potential application prospects for the selective separation of acid gases. Wang et al. [172] adjusted the structure of imidazole functionalized ionic liquids by imidazole anion by adding substituents to imidazole cation, reduced the yield of carbene CO2 and improved the CO2 capture capacity of anion. Wang et al. [173] designed a kind of ILs functionalized with imide anions, indicating that the [P4442 ]Suc ILs which containing succinimide anions constructed by pre organization can efficiently absorb low concentration CO2 through three site synergy. For example, at 20 °C, the molar absorption and mass absorption of [P4442 ]Suc reach 1.65 mol CO2 /mol IL and 22 percent (mass fraction), respectively. The viscosity of the system decreases after CO2 absorption. The implantation of electron withdrawing phenyl group in anion decreases the absorption capacity of CO2 in ionic liquid, while the implantation of electron pushing cyclohexyl group increases the absorption capacity of CO2 . The development and application of the above functionalized ionic liquids of azo anions, carboxylate anions and phenol anions have enriched and developed the types of functionalized ionic liquids that can be used to capture CO2 , as well as people’s understanding of the interaction between functionalized ionic liquids and CO2 . In terms of CO2 capture capacity, these ionic liquids are usually similar to that of other substances. How to further improve the capture efficiency and utilization of ionic liquids, so that each mole of ionic liquids can capture multiple moles of CO2 , and ionic liquids are easy to recover, is still one of the important goals of developing functional ionic liquids. Hao et al. [174] found that [P4444 ]2-OP has the advantages of low viscosity of 193 cP, and the absorption capacity of CO2 is 1.2 mol CO2 /mol IL. Macfarlane et al. [175] studied the interaction mechanism between amino functionalized hydroxypyridine anion and CO2 . It showed that the CO2 absorption in ILs was 0.87 ∼ 0.99 mol CO2 /mol IL. The NMR further showed that amino was easy to interact with CO2 , while the interaction between oxygen negative sites and CO2 were secondary. It can be seen that different positions and structures of functional groups on anions will have different effects on their interaction with CO2 . In addition, Liu et al. [176] synthesized [P4444 ]3 2,4-Opym-5-Ac ionic liquid by removing three protons from 2,4-dihydroxypyrimidine-5-carboxylic acid as a 3-valent anion, with a capacity of 1.46 mol CO2 /mol IL for CO2 absorption at ambient temperature and pressure. This result shows that it is limited to using multivalent anions to improve the capacity of CO2 absorption in ILs. Xu et al. [177] used the alkanolamine-based dual functional ILs with multidentate cation coordination and pyrazolide anion in a molar ratio of 2:1 to obtain a chelating ionic liquid containing both amino functionalized cations and pyrazole anions. The capacity of CO2 absorption in this ILs is 0.5 ∼ 0.8 mol CO2 /mol IL at 1 bar and 80 °C. It showed highly efficient CO2 capture at relatively high temperature. Taylor et al. [178] measured the absorption of SO2 and CO2 by benzimidazole anion functionalized ILs, indicating that the ionic liquid can replace CO2 by absorbing SO2 after absorbing CO2 , otherwise it cannot replace SO2 . Two kinds of amido-containing anion-functionalized ionic liquids (ILs) were designed and synthesized to capture CO2 , where the anions of these ILs were selected from deprotonated succinimide (H-Suc) and o-phthalimide (pH-Suc) [179]. The relationship between the structure of this kind of amido anion-functionalized ionic liquid and its CO2 capture performance provides new enlightenment for the development and design of new materials and new

17.2 Capture of CO2 in ILs

367

methods for reversible and efficient CO2 capture and separation. It was shown that these anion-functionalized ILs exhibited high CO2 solubility (up to 0.95 mol CO2 /mol IL) and low energy desorption, and enthalpy change was the main driving force for CO2 capture by using such ILs as absorbents. MacFarlane et al. [180] showed that the proton type ionic liquid synthesized by dimethylaminoethylamine or dimethylaminopropylamine with azolide compounds or fluorophenol compounds in a molar ratio of 1:1, although the cation has tertiary amine functional groups, the amount of CO2 absorbed by such substances occurs on the active site of the anion, not on the tertiary amine or quaternary ammonium functional group of the cation. It shows that tertiary amine has no obvious contribution to CO2 capture by ionic liquid. Wang et al. [181] prepared quaternary ILs with triazolide as anion (Triz- ) and azobenzene group on the cation through acid-base neutralization reaction. It was found that these ILs can contact with CO2 in a molar ratio of 1:1 to form carbamates through the interaction of anions. The in-depth study shows that compared with ILs containing CIS-azo groups, ILs containing trans-azo groups have higher capacity of CO2 absorption. The effect of entropy in the system is the key factor in the interaction between these ILs and CO2 , and the entropy changes greatly when the ILs containing trans-azobenzene interact with CO2 . Brennecke et al. [182] synthesized [Emim]2-CNPyr ionic liquid and studied the interaction between CO2 and ILs. It is found that this ionic liquid can absorb CO2 through two parallel paths of 2-CNPyr-CO2 and carbene-CO2 . Hu et al. [183] found that the carbine-CO2 structure produced by the interaction of [Bmim]PhO and CO2 can be transferred to the anion to form PhO-CO2 . Similarly, umecky et al. [184] showed that ILs with acetylacetone as anion can also absorb CO2 through chemical absorption. Sharma et al. [185] introduced acidic groups (-COOH) into the carbon chain to obtain a series of ILs [Emmim]X, and measured their CO2 absorption capacity at 30 and 50 °C and atmospheric pressure. It showed that these ILs have good absorption capacity at atmospheric pressure, about 1.0 mol CO2 / mol IL, and have excellent regeneration performance, which can be used repeatedly. Mahurin et al. [186] introduced benzyl groups into cations to synthesize a new type of ionic liquid, which showed stronger selectivity for CO2 absorption. Liu et al. [187] added Zn2+ to ionic liquid [Emim]TFSI and mixed it in the amount ratio of 1:1. The absorption capacity of the resulting mixed solution for CO2 was greatly enhanced, reaching 8.2 percent (mass fraction) at 40 °C and 0.1 MPa. Although TSILs has the advantage of large absorption capacity, it also has the inherent disadvantage of high viscosity of ILs. In particular, the formation of new bicarbonate by reacting with CO2 will rapidly increase the viscosity, and the whole system will even change into gel, greatly reducing the mass transfer efficiency and absorption efficiency. At the same time, it is also necessary enhance the energy consumption and temperature in the regeneration stage of ILs. Compared with traditional ILs, the TSILs have the following characteristics. (1) The introduced groups are diverse. There are various groups introduced into functional ILs, which can be designed by scholars, so many different functional ILs have been produced. (2) It has good regeneration. After multiple absorption desorption, the absorption capacity is almost the same as that of the first time, so it can be reused. (3) Higher production cost and higher viscosity. Because the functionalized ILs are designed and synthesized by scholars, the cost and time required are huge, and they do not have the conditions for application. (4) The research is relatively scattered, lacking integrity and systematisms.

368

17. Carbon dioxide capture and utilization in ionic liquids

The reason for the rapid increase of the viscosity of the system after the capture of CO2 by TSILs is pointed out. On the one hand, the TSILs with free amino group has a large density and a small free volume, which will slow down the rotation of ions in the system and increase the viscosity; On the other hand, there is a hydrogen bond between carbonate and amino group after CO2 capture, which is easy to form a complex hydrogen bond network structure, resulting in a sharp increase in the viscosity of the system. To overcome the above difficulties and further improve TSILs, we need to start from the following aspects: (1) look for TSILs with large capture capacity, low viscosity, low desorption temperature and energy consumption, especially the development of aprotic heterocyclic anions with CO2 capture function, which will not produce acidic protons after CO2 capture, and the viscosity of the system will not increase [45]. (2) By mixing TSILs with ordinary ILs or molecular solvents, the carbonate (hydrogen) salt generated in the absorption process is dissolved or extruded into the system to reduce the viscosity of the system. (3) Adopt process enhancement means, including high gravity field, microwave, ultrasound, etc., to enhance mass and heat transfer in the absorption and desorption process and reduce energy consumption.

17.2.3 Capture CO2 by metal coordination-based (chelate-based) ionic liquids Metal coordination-based ionic liquids are molten salts formed by the coordination of metal ions with inorganic or organic ligands at low temperatures. When metal ions and ligands have multiple coordination sites, they are usually called metal-chelated ionic liquids. Metal coordination (chelation) ionic liquids have been widely used in the field of organic catalysis and gas absorption, showing good absorption properties. In recent years, it has been applied to CO2 capture, fixation and conversion, and has made some progress [188–202]. Table 17.3 shows the main progress of CO2 capture by metal-coordinated ionic liquids. In this section, we will summarize the absorption performance and influencing factors of CO2 in this metal ionic liquid. It can be seen from Table 17.3 that the absorption of CO2 in metal ion coordination ionic liquids depend on the type of ligands and metal ions. In general, when the cations are the same, the CO2 absorption of metal anion coordination ionic liquids increases with the increase of the volume of coordination anions. Although the CO2 absorption of metal anion coordination ionic liquids is slightly higher than that of traditional ionic liquids, the CO2 absorption of metal anion coordination ionic liquids at an atmospheric pressure is still relatively small compared with functional ionic liquids, which may be because the physical interaction between metal anion coordination ionic liquids and CO2 is still dominant. The viscosity of metal anion coordination ionic liquids is usually large, which greatly limits its application in carbon capture. Metal coordination-based ILs used for CO2 absorption mainly include metal anioncoordination-based ILs, metal cation-coordination-based ILs and metal anion/cationcoordination-based ILs. Metal-anion-coordination ILs are prepared by the reaction of conventional ILs with metal salts (such as iron salt, aluminum salt, zinc salt, etc.) with the same anion. They have the characteristics of simple synthesis and low raw material cost [187–193]. Compared with the conventional ILs, the metal anion coordination ILs has a certain improvement in CO2 absorption performance. Liu et al. [187–192] compared the CO2 absorption of [Emim]Ntf2 and metal-anion-coordination ILs of [Emim]Zn(Ntf2 )3 at 313.15 K and 0.1 MPa. It was found that the CO2 absorption of [Emim]Ntf2 was 0.0347 mol CO2 /mol IL,

369

17.2 Capture of CO2 in ILs

TABLE 17.3 The capacity of CO2 absorption in metal coordination-based ILs. Entry

Systems

T(K)/P(bar)

mol CO2 per mol IL

Ref.

1

[Li(AEE)]Ntf2

313/1

0.55

[102]

2

[P(C4 )4 ]Gly-SiO2

298/1

0.6

[116]

3

[P(C4 )4 ]Ala]-SiO2

298/1

0.67

[116]

4

[aP4443 ]Gly-SiO2

298/1

1.0

[127]

5

[aP4443 ]Ala]-SiO2

298/1

1.0

[127]

6

K[Sacc]

298/1

0.38

[132]

7

K[Phth]

298/1

0.96

[132]

8

Na[Phth]

298/1

0.96

[132]

9

[Na(MDEA)2 ]Pyr

353.15

0.75

[177]

10

[Emim]Zn(Ntf2 )3

313.15/0.1

1.8953

[187]

11

[Na(CyNH)]Gly

298/1

0.68

[188]

12

[Na(iPrNH)]Ala

298/1

0.73

[188]

13

[Na(iPrNH)]Gly

298/1

0.91

[188]

14

[Na(NH2 )]Gly

298/1

0.43

[188]

15

[Na(nPr2 N)]Gly

298/1

0.48

[188]

16

[Na(nPrNH)]Gly

298/1

0.59

[188]

17

[Na(tBuNH)]Gly

298/1

0.85

[188]

18

[Na(β-iPrNH)]Ala

298/1

0.65

[188]

19

[K(18-crown-6)]Ala

298/1

0.71

[189]

20

[K(18-crown-6)]Gly

298/1

0.75

[189]

21

[K(18-crown-6)]Leu

298/1

0.68

[189]

22

[K(18-crown-6)]Met

298/1

0.75

[189]

23

[K(18-crown-6)]PhO

298/1

0.90

[189]

24

[Li(12-crown-4)]PhO

298/1

0.75

[189]

25

[Na(15-crown-5)]PhO

298/1

0.84

[189]

26

[K(18-crown-6)]Pro

298/1

0.99

[189]

27

[Na(15-crown-5)]Pro

298/1

0.89

[189]

28

[K(18-crown-6)]Ser

298/1

0.63

[189]

29

[K(18-crown-6)]Thr

298/1

0.73

[189]

30

[K(18-crown-6)]Val

298/1

0.79

[189]

31

[Li(12-crown-4)]PhO

298.15

0.75

[189] (continued on next page)

370

17. Carbon dioxide capture and utilization in ionic liquids

TABLE 17.3 The capacity of CO2 absorption in metal coordination-based ILs—cont’d Entry

Systems

T(K)/P(bar)

mol CO2 per mol IL

Ref.

32

K[Triz]

283/1

0.96

[190]

33

Li[Triz]

283/1

0.90

[190]

34

Na[Triz]

283/1

0.96

[190]

35

DAIL

303/1

1.05

[191]

36

[Li(DETA)]Ntf2

353.15

0.66

[192]

37

[Li(TEG)]Ntf2

353.15

0.01

[192]

38

[Li(TETA)]Ntf2

353.15

0.7

[192]

39

[Li(TTEG)]Ntf2

353.15

0.02

[192]

40

[Bmim]FeCl4

343.15/1.5

0.5086

[193]

41

[Bmim]Zn(Ac)3

343.15/1.5062

0.7181

[193]

42

[Bmim]ZnCl3

343.15/1.5478

0.2448

[193]

43

[Li(DEA)]Ntf2

313.15

0.52

[194]

44

[Li(DGA)]Ntf2

313.15

0.55

[194]

45

[Li(DOBA)]Ntf2

333.15

0.9

[194]

46

[Li(EA)]Ntf2

313.15

0.54

[194]

47

[Li(HDA)]Ntf2

313.15

0.88

[194]

48

[K(DEA)2 ]Im

333.15

1.11

[195]

49

[K(MDEA)2 ]Im

333.15

0.81

[195]

50

[K(DGA)2 ]Im

333.15

1.37

[195]

51

[Na(DGA)2 ]Im

333.15

1.23

[195]

52

[Li(DGA)2 ]Im

333.15

0.73

[195]

while that of [Emim]Zn(Ntf2 )3 was 1.8953 mol CO2 /mol IL, indicating that the introduction of Zn2+ was conducive to the improvement of CO2 absorption. They noted that [Emim]Zn(Ntf2 )3 can absorb CO2 through chemical interaction, but it has not been confirmed. Peng et al. [193] synthesized three metal anion coordination ILs by reacting [Bmim]Cl, [Bmim]Ac with equimolar ZnCl2 , FeCl3 and ZnAc2 . They found that the density, viscosity and CO2 absorption of new synthesized ILs were measured and compared with pure ILs. It showed that the CO2 absorption and density of [Bmim]ZnCl3, [Bmim]FeCl4 , and [Bmim]ZnAc3 are greater than that of [Bmim]Cl and [Bmim]Ac under the same conditions. The viscosity of [Bmim]ZnCl3 and [Bmim]ZnAc3 is much higher than that of the corresponding [Bmim]Cl and [Bmim]Ac, while the viscosity of [Bmim]FeCl4 is much lower than that of [Bmim]Cl. Metal chelated-ILs used for CO2 absorption are mainly metal cation-chelated ionic liquids. Metal cation-chelated ILs is a new type of functionalized ionic liquid. Its cations are formed

17.2 Capture of CO2 in ILs

371

by multi-point coordination between metal ions and ligands (such as amine, alcohol amine, crown ether, etc.). Its cation not only retains the reactivity of alcohol or amine with CO2 , but also effectively improves the thermal stability of alcohol or amine in the system [196,197]. In 2012, Wang et al. [194] first reported the work of capturing CO2 with metal cation-chelated ionic liquids. These ionic liquids are prepared by the equimolar reaction of LiNtf2 with different alcohol amines. It can be seen from Table 17.3 that with the increase of the number of coordination atoms in alcohol amine, the absorption of CO2 by metal chelating cation ILs also increases, in which [DOBA] has the largest number of coordination atoms. [Li(DOBA)]Ntf2 still maintains good absorption performance at higher absorption temperature, and its absorption capacity is significantly better than that of other metal cation chelated ionic liquids. By comparing IR and 13 C NMR before and after CO2 absorption by metal cation chelated ionic liquids, they found that alcohol amines in chelated cationic ions can contact CO2 to form carbamic acid, which indicates that CO2 absorption by ionic liquids is a chemical action [194]. The adsorbed CO2 can be decomposed in nitrogen atmosphere, and the metal chelated cationic ionic liquid can repeatedly absorb CO2 . Li et al. [198] calculated and optimized the configurations of [Li(DOBA)]Ntf2 , [Li(HDA)]Ntf2 , [Li(DEA)]Ntf2 and [Li(EA)]Ntf2 by density functional theory (DFT). The interaction energy of anions and cations was calculated. It showed that lithium ion in metal cation-chelated ILs has multi-points coordination with alcohol amine, and the coordination intensity increases with the increase of the number of coordination atoms. The interaction energy order of anions and cations is [Li(EA)]Ntf2 > [Li(DEA)]Ntf2 > [Li(HDA)]Ntf2 > [Li(DOBA)]Ntf2 , which is opposite to the coordination intensity of CO2 absorption and chelating cation. The above results prove theoretically that the coordination strength of chelated cation and the interaction energy of cation and anion are directly related to the carbon capture performance of ionic liquids. The greater the coordination strength of cation, the weaker the interaction energy between cation and anion is, which is conducive to the interaction between coordination cation and CO2 , so as to improve the absorption rate. The chemical interaction between chelating cation and CO2 are an important reason to improve the trapping ability [194], but the anion Ntf2 - in the above metal cation-chelating ionic liquid does not participate in CO2 capture. Therefore, in subsequent studies, functional anions such as amino acids [200], oxazolyl [108–169] and hydroxypyridine [201,202] were introduced to further enhance the CO2 capture performance of metal cation-chelated ionic liquids. Da et al. [189] synthesized a series of metal cation chelated ionic liquids from alkali metal amino acid salts, phenol salts and crown ethers, and studied their performance of CO2 capture at room temperature and pressure. It showed that [K(18-crown-6)]Pro-has the best absorption performance. Due to the interaction between Pro- and CO2 , equimolar absorption is achieved within 10 min. In addition, it can be seen from Table 17.3 that anion type is the key factor affecting CO2 absorption performance, and the absorption amount of Pro- is better than that of PhO- .The absorption capacity of [Li(12-crown-4)]PhO, [Na(15-crown-5)]PhO and [K(18crown-6)]PhO are basically the same as those of [P66614 ]PhO ILs, indicating that capturing CO2 in crown-ether metal cation-chelated ILs is mainly through the chemical interaction between anions and CO2 , while crown ethers in chelated cation ions lack the ability to react with CO2 . Quan et al. [177] developed a metal cation-chelated ionic liquid with anion and cation bifunctionalization by introducing azo anion with CO2 reaction activity on the basis of retaining chelated cation to react with CO2 . It is found that the ionic liquid still has good absorption performance at high temperature. For example, [Na(MDEA)2 ]Pyr is prepared by the reaction

372

17. Carbon dioxide capture and utilization in ionic liquids

of pyrazole sodium (NaPyr) and N-methyldiethanolamine (MDEA) in a molar ratio of 1:2. This ILs can reach absorption equilibrium within 6 min at 0.1 MPa and 353.15 K with an absorption capacity of 0.75 mol CO2 /mol ILs. The viscosity of the system (323.2 K) decreased from 1310 MPa.s before absorption to 713.9 MPa.s [177–192]. IR and 13 C NMR studies showed that both chelate-cation and pyrazole anion reacted with CO2 . In order to further study the effects of alcohol amine ligands and central metal ions on absorption properties and clarify the reaction sequence between anions and cations and CO2 . Su et al. [195] designed and synthesized 9 kinds of metal catio-chelated ionic liquids. These metal cation-chelated ILs come from alkali metal (Li+ , Na+ and K+ ) imidazoliums and alkanolamines of primary amine (DGA), secondary amine (DEA) and tertiary amine(MDEA). Combined with CO2 absorption experiment, DFT calculation and spectrum, a detailed study was carried out for these ILs. It showed that the CO2 capture performance of metal cationchelated ILs is related to the central metal ions when the alcohol amine ligands are the same. The order of CO2 absorption is K+ > Na+ > Li+ . When the central alkali metal ions remain unchanged, the carbon capture performance of ionic liquids is related to the alcohol amine level, and the absorptioncapacity follows DGA > DEA > MDEA. Among them, [K(DGA)2 ]Im has the best CO2 absorption performance, reaching 1.37 mol CO2 /mol ILs in 15 min at 333.15 K and 0.1 MPa, and has good circulation performance. IR and 13 C NMR results showed that Im- reacted with CO2 preferentially over [K(DGA)2 ]+ . DFT calculation results showed that the binding energy of chelating cation depends on the polydentate coordination ability of central metal ions and alcohol amines, which is consistent with the results of Li et al. [198]. The binding energy of chelated cation is consistent with the CO2 absorption of ionic liquids, but opposite to the viscosity of ionic liquids. The binding energy of different ionic liquids has a linear relationship with absorption or viscosity, because with the increase of chelating cation coordination strength, the interaction between cations and anions weakens, and the viscosity of the system decreases. It is conducive to the chemical interaction between ionic liquids and CO2 and chelating cation, and improves the absorption performance. Wang’s group [192] developed a strategy to reduce the viscosity of the system by mixing ILs, realizing the capture of CO2 in high-temperature flue gas. They mixed the metal cation chelating ILs ([Li(DETA)]Ntf2 , [Li(TETA)]Ntf2 , [Li(TEPA)]Ntf2 ) with three polyamines (DETA, TETA, TEPA) as ligands with the metal cation chelating ILs ([Li(TEG)]Ntf2 , [Li(TTEG)]Ntf2 ) with two Polyols (TEG, TTEG) as ligands at the molar ratios of 1:1, 1:2 and 1:3. The CO2 absorption capacity of above 6 mixed ILs were measured at different molar ratios. It was shown that all of absorption capacities for these 6 ILs were large than 1.0 mol CO2 /mol IL. [Li(TEPA)]Ntf2 /[Li(TEG)]Ntf2 mixed system has the best absorption performance. When the molar ratio of the two is 1:1, 1:2 and 1:3, the capacity of CO2 absorption in the mixed system is 1.61, 1.95 and 2.01 mol CO2 /mol IL respectively. The capacity of CO2 absorption in the mixed system increases with increasing the molar ratio of [Li(TEG)]Ntf2 or [Li (TTEG)]Ntf2 . Because both polyamine ligand metal cation chelating ILs and polyol ligand metal cation chelating ILs have good thermal stability, the above mixed system shows good cyclic absorption performance at higher absorption temperature. After 8 absorption desorption cycles, the absorption capacity remains basically unchanged. Therefore, the above efficient and reversible method has the potential application value of capturing CO2 from flue gas in industry. Metal cation-chelated ILs is a kind of excellent CO2 capture solvent which can be used at higher temperature. Compared with metal anion-coordinated ILs, metal cation-chelated ILs

17.2 Capture of CO2 in ILs

373

can capture carbon through the chemical interaction of anion, cation and CO2 , so it has more excellent carbon capture ability. In addition, changing the types of alcohol amines, central metal ions, and anions can effectively regulate the absorption performance of metal cation chelated ILs, so as to ensure that it can still maintain good carbon capture capacity at higher absorption temperature. It is similar to metal anion coordination ILs, and the viscosity of metal anion coordination ILs is relatively large. It can effectively reduce the viscosity of the system by coordinating with other ILs with similar structure to form a mixed system, So as to improve the carbon capture capacity of the system.

17.2.4 CO2 capture by ILs based mixtures The CO2 adsorption in conventional ILs is mainly physical adsorption process, which is much weaker than the chemical adsorption in alcohol amine systems [203–239]. The main disadvantages of ILs is that they have higher cost and larger viscosity than other solvents. Therefore, one of the most direct methods is to mix the ILs with organic solvents or water to form a mixture solution to reduce the viscosity and the cost for CO2 absorption [203–239]. Table 17.4 shows the CO2 absorption in ILs/solvents mixture. It showed that the nonvolatile ionic liquid can overcome the volatility of alcohol amine aqueous solution. At the same time, compared with a single absorbent, the mixture absorbent can provide more unique physical properties [209,218–220]. Camper et al. [209] used mixture formed with equimolar alkyl ethanolamine and imidazole ILs for CO2 capture. It is shown that the mixture of [Hmim]Ntf2 and monoethanolamine (MEA) can absorb 0.5 mol CO2 with per mole of MEA, and the product of the reaction of CO2 and MEA was insoluble in the compound solvent and separated from the system, which accelerated the capture process. Zhang et al. [218] pointed out that imidazole ILs are expensive and difficult to be economically accepted for large-scale application. The ILs/solvents with a equi-mass fraction of Nmethyldiethanolamine (MDEA), [MDEA]BF4 , H2 O and a little piperazine (PZ) was used to capture CO2 . At the optimal conditions (303.15K and 1.5MPa), the capture capacity of CO2 in mixture is 0.15 g CO2 /1 g solvent. They also used other complex solvents containing ILs prepared by neutralizing alkyl alcohol amines with inorganic acids, such as [MDEA]BF4 , [MEA]BF4 , [MDEA]HSO4 , [MDEA]Cl and [MDEA]H2 PO4 for CO2 capture. They pointed out that the ILs/solvents is easy to prepare and has lower corrosion to the equipment than the traditional amine solution, so it has the prospect of industrial application. Zhang et al. [220] added tetramethylammonium glycine, tetraethylammonium glycine, tetramethylammonium lysine and tetraethylammonium lysine as activators into MDEA aqueous solution to form a new CO2 absorbent. They studied the effects of ILs types and dosage on the absorption performance of the mixed absorbers, analyzed the mechanism, absorption capacity and regeneration efficiency. It is showed that TSILs addition can significantly improve the CO2 absorption rate in MDEA aqueous solution. The more TSILs is added, the faster absorption rate is. The glycine/MDEA aqueous solution has the maximum absorption capacity and regeneration efficiency. Lei et al. [221] used acetone and [Emim]BF4 to form a ILs/solvent to capture the CO2 . It is showed that the ILs/solvent has a good solubilty of CO2, which combines the respective

374

17. Carbon dioxide capture and utilization in ionic liquids

TABLE 17.4 CO2 capture in ILs/solvents mixture (Rma : mass ratio, Rmo : mole ratio, xA : molar fraction of A,wA: mass fraction of A). Entry

Systems

P (bar) /(K)

mol CO2 per mol IL

Ref.

1

H2 O + [P66614 ]2-CNPyr (Rma = 4.5:95.5)

0.01/295

0.9

[130]

2

[Emim]TFA + [Emim]Ac (Rmo = 50.02:49.98)

0.01/323

0.124

[137]

3

[Emim]Ac + [Emim]EtSO4 (Rmo = 1:1)

0.393/298

0.19

[137]

4

[Eeim]Ac+H2 O (xH2 O = 0.208)

2/300

0.27

[137]

5

[Eeim]Ac+H2 O (xH2 O = 0.208)

0.75/300

0.21

[137]

6

[Eeim]Ac+H2 O (xH2 O = 0.208)

0.07/300

0.10

[137]

7

[Eeim]Ac+H2 O (xH2 O = 0.35)

0.10/300

0.08

[137]

8

[Eeim]Ac+H2 O (xH2 O = 0.533)

0.16/300

0.06

[137]

9

[Emim]TFA + [Emim]Ac (Rmo = 50.02 :49.98)

1.0/323

0.124

[137]

10

[Hmim]Ntf2 + MEA (Rmo = 1 : 1)

1.01/313

0.5

[137]

11

[N1111 ]Gly+H2 O (wH2 O = 0.2)

0.63/298

0.25

[139]

12

[N1111 ]Gly+H2 O (wH2 O = 0.35)

0.63/298

0.31

[139]

13

H2 O + [N1111 ]Gly (wH2 O = 0.5)

0.63/298

0.40

[139]

14

H2 O + [N1111 ]Gly (wH2 O = 0.7)

0.64/298

0.60

[139]

15

H2 O + [C1 C4 Pyrro]Ac (xH2 O = 0.35)

0.09/303

0.12

[142]

16

H2 O + [C1 C4 Pyrro]Ac (xH2 O = 0.6)

0.15/303

0.06

[142]

17

H2 O + [C1 C4 Pyrro]Ac (xH2 O = 0.85)

0.64/303

0.01

[142]

18

[BHEA]Ac + H2 O (Rma = 8 : 2)

3.7/298

0.1078

[203]

19

MEA +[BHEA]Ac + H2 O (Rma = 1 : 1 : 3)

3.92/298

0.6628

[203]

20

H2 O+[Bmim]BF4 (Rma = 8 : 2)

3.6/298

0.1155

[203]

21

[Bmim]BF4 + MEA + H2 O (Rma = 1 : 1 : 3)

3.6/298

0.6489

[203]

22

H2 O + MDEA (Rma = 3 : 7)

1.4/313

0.911

[204]

23

H2 O + MEA (Rma = 15.3 : 84.7)

1.2/313

0.722

[204]

24

[P4444 ]Gly–H2 O ( wH2 O -0.01)

1/298

∼1.0

[204]

25

[P4444 ]Gly+H2 O (wH2 O = 0.01)

1/298

∼1.0

[204]

26

[Omim]Ntf2 + [Bmim]BF4 (Rma = 1:1)

0.222/313

0.34

[205]

27

[Omim]Ntf2 + [Emim]BF4 (Rma = 1:1)

0.218/313

0.32

[205]

28

[Im21 OH]Ntf2 +BEMP

1/293

0.81

[206]

29

[Im21 OH]Ntf2 +DBU

1/293

1.04

[206]

30

[Im21 OH]Ntf2 +MTBD

1/298

1.02

[206]

31

[Nip,211 OH]Ntf2 +DBU

1/293

1.02

[206] (continued on next page)

375

17.2 Capture of CO2 in ILs

TABLE 17.4 CO2 capture in ILs/solvents mixture (Rma : mass ratio, Rmo : mole ratio, xA : molar fraction of A,wA: mass fraction of A)—cont’d Entry

Systems

P (bar) /(K)

mol CO2 per mol IL

Ref.

32

[N1111 ]n-Bu2 NEtCOO+CH3 OH

1/313

1.01

[207]

33

[N1111 ]n-Bu2 NEtCOO+EG

1/313

0.98

[207]

34

[N1111 ]n-Bu2 NEtCOO+H2 O

1/313

0.95

[207]

35

[N1111 ]n-Bu2 NEtNHEtCOO+CH3 OH

1/313

1.02

[207]

36

[N1111 ]n-Bu2 NEtNHEtCOO+EG

1/313

1.03

[207]

37

[N1111 ]n-Bu2 NEtNHEtCOO+H2 O

1/313

1.01

[207]

38

[N1111 ]n-BuNHEtCOO+CH3 OH

1/313

0.97

[207]

39

[N1111 ]n-BuNHEtCOO+EG

1/313

0.96

[207]

40

[N1111 ]n-BuNHEtCOO+H2 O

1/313

0.94

[207]

41

[Bmim]BF4 +DBU

1/296

0.80

[208]

42

[Bmim]PF6 +DBU

1/296

0.75

[208]

43

[Bmim]Ntf2 +(P2 Et)

1/296

1.00

[208]

44

[Bmim]Ntf2 +DBU

1/296

0.99

[208]

45

[Bmim]Ntf2 +MTBD

1/296

1.08

[208]

46

[Emim]Ntf2 +DBU

1/296

0.93

[208]

47

[Hmim]Ntf2 + MEA (Rmo = 1:1)

0.01/313

0.5

[209]

48

[N1111 ]Gly + MDEA (Rma = 1:1)

0.0097/298

0.56

[210]

49

[N1111 ]Lys + MDEA (Rma = 1:1)

0.0097/298

0.69

[210]

50

[N2222 ]Gly + MDEA (Rma = 1:1)

0.0097/298

0.64

[210]

51

[N2222 ]Lys + MDEA (Rma = 1:1)

0.0097/298

0.74

[210]

52

MDEA + [POHmim]Cl (Rmo = 2:1)

0.01/308

0.396

[211]

53

[C3 OHmim]BF4 + MEA (Rmo = 1 : 1)

1/308

0.378

[211]

54

MDEA + [Bmim]DCA (Rma = 3:1)

0.0233/313

0.652

[212]

55

MDEA + [EOHmim]DCA (Rma = 3:1)

0.0185/313

0.638

[212]

56

PEG-200+ [Choline]Pro (Rma = 1:1)

0.0107/333

0.520

[213]

57

MDEA + [Bmim]Gly (wIL = 0.1)

1/303

0.78

[214]

58

MDEA + [Bmim]Gly (wIL = 0.15)

1/303

0.73

[214]

59

MDEA + [Bmim]Gly (wIL = 0.05)

1/303

0.80

[214]

60

[Bmim]NO3 + H2 O (Rmo = 90.82 : 9.18)

13.2/315

0.1

[215]

61

H2 O + [HOPmim]NO3 (Rmo = 4.11:95.89 )

21.3/313

0.099

[215]

62

[Bmim]BF4 + H2 O + MDEA (Rma = 44.86 : 14:44.15 )

1/313

1.26

[216]

63

[Bmim]DCA +MDEA + H2 O (Rma = 37.94 : 44.06: 18)

1/313

0.97

[216]

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advantages of ILs and organic solvents, and is expected to replace the existing industrial acetone CO2 capture process. Xie et al. [222] added [C4 mim]Cl into the chitosan or chitin as a absorbent for CO2 . It was showed that mixture has a cellulose like polymer structure, chitin has two hydroxyl groups, and chitosan has an additional amino group. For chitosan/[C4 mim]Cl mixtures, although CO2 is physically absorbed in these liquids, the capacity of CO2 absorption exceeds the theoretical capacity. Physical absorption process of chitin/[C4 mim]Cl mixture was also observed. Although this method cannot give higher absorption capacity as that of pure functionalized ILs, it does have the advantage of low corrosion, fully recyclable, and non-volatile CO2 absorbent The effect of water on the viscosity of ionic liquids and absorption capacity of CO2 was systematically revealed by Ventura et al. [223]. It is showed that the addition of water can decrease the viscosity of ILs, and reduce the absorption capacity of CO2 . Ventura et al. [223] showed that adding polyethylene glycol can decrease the IL viscosity and enhance the CO2 desorption or absorption rate, but the solubility of CO2 decreases. Aroma et al. [224] showed that the capacity of CO2 in the MDEA/ IL mixture improved with increasing the pressure of CO2 and reduced with increasing the temperature. With increasing the ILs concentration, the absorption capacity of CO2 decreased significantly but the decrease of CO2 capacity in the solution containing [C4 mim]BF4 was less than that of other types of imidazole IL. Wang et al. [225,226] combined dicyclamidine (DBU) with hydroxyl modified IL as a super base to capture protons generated during CO2 adsorption, which proved to be an effective method to achieve equimolar adsorption. Under the same condition, the equilibrium adsorption molar ratio was close to 1:1, super alkali metal ILs were further synthesized Ionic liquid/water mixture is a widely studied system, and the mechanism of functionalized ILs and CO2 is different under the conditions of non-water, or more-water containing and less-water containing system. For amino-functionalized ILs, Jing et al. [227] revealed the absorption behavior of CO2 in tetramethylammonium glycine ([N1111 ]Gly)/aqueous solutions. It showed that the CO2 absorption process did not simply follow the CO2 absorption mechanism of conventional organic amines, but that Gly-amino reacted with CO2 to form carbamates. The carbamate is then hydrolyzed to form glycine and HCO3 - . However, Yang et al. [228] and Filippov et al. [229] studied the CO2 absorption in choline amino acid ionic liquid/water system.They believed that water reacts with CO2 to form carbonic acid and dissociate into H+ and HCO3 - , and then the amino group is protonated into ammonium. In general, the final form of CO2 absorbed by aqueous solutions of ILs is HCO3 - . Guo et al. [230], Jing et al. [101], and Ding et al. [231] also obtained similar conclusions when studying the CO2 absorption in aqueous solutions of bifunctional ILs formed with amino groups on both anions and cations. The mechanism of CO2 absorption in the mixed system of amino-functionalized ionic liquid/alcohol amine aqueous solution is similar [232–234]. For non-amine anion functionalized ILs, Wang et al. [235] studied the effect of water concentration on CO2 capture by succinimide anion (Suc- ) based functionalized ILs. It showed that few water (about 3.3 percent) in [P4442 ]Suc ILs did not change the CO2 absorption mechanism and absorption capacity, but only accelerated the rate of absorption. However, when the water concentration in ILs is increased (such as 8.8 percent and 17.6 percent), more HCO3 - and succinimide molecules are generated after CO2 absorption, and the absorption capacity is reduced. It showed that the water content in ionic liquid has a great influence on

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the absorption mechanism of CO2 in ILs/water mixture. When water content in ionic liquid is higher, the CO2 will changed to be HCO3 - instead of CO2 itself. DuPont et al. [236] found that the mixture of imidazole ILs and acetate, imidazole ions and CO2 can form bicarbonate through reversible reaction. Three types of CO2 adsorption were observed in these ILs/Water solutions: physical, CO2 imidazole adduct, and bicarbonate formation (up to 1.9 mol of molybdenum bicarbonate). In an ILs/Water (1:1000) system, the total absorption rate of CO2 relative to imidazole anion is 10:1 (molar ratio). Under similar experimental conditions, these adsorption values are higher than those of classical alkanolamine or even alkaline aqueous solutions. Brennecke et al. [237] studied CO2 adsorption in three aprotic heterocyclic anion (AHA) ILs/water mixture, such as triethylphosphonium benzimidazolide [P2222 ]BnIm, (triethyl(octyl)phosphonium 2-cyanopyrrolide [P2228 ]2-CNPyr, and triethyl(octyl)phosphonium indazolide [P2228 ]Inda). They proposed a reaction mechanism in which, in addition to the reaction of anions with CO2 to produce carbamates, anions also react with water (i.e. hydrogen ions in water produced by formation of carbonic acid or natural dissociation of water) and are re adjusted to make CO2 react with hydroxide to produce the bicarbonate. The reaction products and the mechanism in [P2228 ]2-CNPyr and [P2228 ]2-CNPyr system were identified and confirmed by 13 C NMR, heteronuclear multi bond correlation NMR, 1 H NMR, heteronuclear single quantum correlation NMR. However, unlike the other two ILs, [P2228 ]Inda only shows anion rearrangement in contact with water. For example, carbon dioxide does not need to form carbonic acid to increase protons concentration in water. The reversibility of the readjustment reaction was studied by measuring CO2 solubility in [P2222 ]BnIm and [P2228 ]2-CNPyr with few water by using the volumetric method and performing several cycles. They showed that the reaction of CO2 in the absence of water with AHA to form carbamates is reversible. In addition, in the presence of water, CO2 solubility recoverability measurement were performed on encapsulated [P2222 ]Bmim, and [P2228 ]2-CNPyr. Both reactions between AHA ILs and CO2 seem to be completely reversible in the presence of water. It showed that good capture capability of CO2 and good recyclability in the presence of few water in ILs. New bifunctional ILs/water system with AHA and [DETAH]+ (diethylenetriamine cations) was explored by Lv et al. [102] to achieve efficient and reversible CO2 capture by increasing the amino number in ILs. The CO2 capture in pure [DETAH]Im, [DEPTH]Tz and [DEPTH]Py are 11.91, 10.10 and 11.36 mol CO2 /kg IL respectively. The regeneration efficiency remained above 90 percent after the fifth cycles. According to 13 C NMR and the DFT calculation results, the mechanism of CO2 absorption into [DETAH]AHA solution was clarified. The amino group of [DETAH]+ reacts with CO2 to produce [DETAH]+ -carbamate, ensuring a high absorption rate. At the same time, [AHA]- - can also react with CO2 and other molecules to form carbamate, which is then hydrolyzed to HCO3 - /CO3 2- . It is worth noting that [AHA]is protonated to [AHA]-H which can react with [DETAH]-carbamate to generate [DETAH]+ during CO2 absorption, which helps to recover the active components of the absorbent and further improve the CO2 capacity and regeneration capacity of the absorbent. The novel double film is an effective candidate material for CO2 capture. When ILs with high melting points are mixed with molecular solvents such as ethylene glycol and glycerol, the melting point of the system is lower than that of pure ILs and molecular solvents. Wu et al. [238] used a series of amino functionalized ILs based on triethylene chloride tetramine salt as hydrogen bond acceptors and ethylene glycol or diethylene glycol as

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hydrogen bond donors to form a low melting solvent, and chemically absorbed CO2 through the molar ratio of amino to CO2 is 2:1. Yang et al. [239] used a series of eutectic solvents composed of quaternary and quaternary ammonium, with the imidazole anions and triazole anions as hydrogen bond acceptors and ethylene glycol as hydrogen bond donors for CO2 absorption. It showed that the strongly basic azolyl anions pull out the hydrogen atom on the hydroxyl group of ethylene glycol under the action of CO2 to generate neutral imidazole molecules and triazole molecules, so that the oxygen negative site on ethylene glycol reacts with CO2 to generate carbonate. Ionic liquids/ionic liquids mixture as absorbent can overcome the volatility of water and alcohol. Huang et al. [239] mixed the high viscosity ionic liquid containing imidazole amino acid anion with the low viscosity of ionic liquid containing imidazole acetic acid radical to prepare a low viscosity mixed absorbent. The absorption experiment shows that the rate of CO2 absorption is accelerated, and the two components can interact with CO2 as a single component without interference. In summary, the viscosity of ILs based mixture is smaller than that of pure ionic liquid, and it is easier to recover than traditional alcohol amines system. Therefore, this may be another potential option for carbon dioxide capture in the future. In general, functionalized ILs (especially those containing amino groups) have high viscosity, resulting in slow absorption of CO2 . If water, alcohol and other molecular solvents or other low viscosity ILs are added to pure ILs, the kinetics of the absorption process can be significantly improved. The CO2 absorption in ILs has the following characteristics. (1) The absorption capacity of CO2 in conventional ILs and functionalized ILs reduces with the increase of temperature, and increases with increasing the pressure. The increase of alkyl chain and fluorine-containing group is beneficial to the absorption of CO2 . (2) The CO2 absorption in conventional ILs is greatly limited by pressure. Under ambient temperature and pressure, the absorption rate of azole ILs for CO2 is less than 0.1 mol CO2 /mol IL [72]. Due to the introduction of various functional groups, functional ILs can obtain a higher absorption rate under ambient temperature and pressure. (3) Mixing ILs with organic solutions (especially alcohol amine solutions) can enhance the capacity of CO2 absorption in ILs and reduce production costs, which will be a good development direction and application prospects.

17.2.5 Polyionic liquid membranes Polyionic liquid (PIL) membranes are manufactured from the polymerizable IL monomers and are considered as a type of polyelectrolytes, which are polymers with anions and cations on repeating units. PIL membranes have good conductivity and processability. It can be used to prepare membranes with good selective permeability or ionic conductivity, and can be used for gas separation or ionic conductive materials. PIL membranes have been widely applied in electrolytes, gas separation and conductive materials due to their good selectivity, stability and conductivity [240–275]. Although the polymeric ILs membrane has many advantages, the membrane has strong intermolecular force and close polymer chains, resulting in low permeability and poor mechanical properties. In order to solve this problem, researchers began to blend polymers and ILs to improve the membrane permeability, or blend polymers, ILs and inorganic porous materials to enhace the mechanical properties of the membrane.

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Compared with ILs monomer, PIL membranes has higher capcity and faster rate of desorption and absorption for CO2 , and is a promising CO2 separation material [240–245]. The polymers of ionic liquid monomers, such as poly [1-(4-styryl)−3–butyl imidazolium tetrafluoroborate] (PBBIT), poly [1-(4-styryl)−3–butyl imidazolium tetrafluorophosphate] (PVBIT), poly [2-(1–butyl imidazolium-3)-ethyl methacrylate tetrafluoroborate] (PVBIH), were prepared by radical polymerizetion of corresponding ionic liquid monomers in DMF solution with AIBN as initiator at 333 K [241]. It has stronger capacity of CO2 absorption than pure [Bmim]BF4 . With the increase of absorption capacity and the rate of absorption/desorption of polymeric ILs, a large number of polymer ions in solid state at room temperature play a important role in the absorption of CO2 . The absorption experiment of CO2 /N2 /O2 gas mixture shows that the polymer only selectively absorbs CO2 . Moreover, the rate of CO2 absorption and desorption in ionic polymers is faster than that of ILs, and desorption/adsorption is completely reversible. These characteristics make the polymeric ILs have a good potential as solid absorbers and membrane materials to separate CO2 . According to the position of the charged group in the chain segment, PIL membranes can be divided into main chain cationic type and branch chain cationic type. According to the gas permeation transport mechanism of PIL membranes, cationic PIL membranes have better CO2 permeability and selectivity. The research on the preparation of CO2 separation membranes based on this material is the most common. Cationic PIL membranes can be divided into main chain cationic and branch chain cationic according to the position of the charged group in the chain segment. Moreover, the CO2 permeation separation performance of these two kinds of cationic PIL membranes shows completely different trends. Hu et al. [242] introduced PEG chain segments into two PIL membranes containing VBTMA (vinylbenzyltrimethylammonium) and matma (2-(methylacyloyloxy) ethyl trimethylammonium) ionic groups. The former had CO2 permeability coefficient of 17 barre and permeability selectivity of CO2 /CH4 with 22, while the latter had CO2 permeability coefficient of 39 barrer and permeability selectivity of CO2 /CH4 with 31. The selectivity and permeability were significantly improved. Compared with the main chain cationic PIL membranes, the branched chain cationic PIL membranes have obvious structural advantages for CO2 separation: the polymer main chain has a low degree of constraint on the cationic groups, has better mobility, and can better accelerate the osmotic mass transfer of CO2 , so the application research is more extensive. So far, a variety of main chain structures have been successfully used in the synthesis of branched cationic PIL membranes, including common PIL membranes such as polystyrene [243–254], polyacrylate [243–254], polyethylene [255–260], polyphenylene ether [252], polyacetylene [256], polyimide [272], polysulfone [274] and cellulose acetate [275]. Among them, the CO2 permeability coefficient is up to 1113 barrer, and the corresponding CO2 /CH4 selectivity is 45, which is completely different from the main chain cationic PIL material [274]. Sakaguchi et al. [256] preliminarily studied the effect of anion size ((Br- pyrrole > imidazole > pyridine. Contrary to the change trend of solubility coefficient, increasing the polarity and binding force of cationic groups will reduce the jump and migration of CO2 between groups, and then inhibit its diffusion coefficient. The order of influence of cationic groups in pill materials on CO2 diffusion coefficient is pyridine > imidazole > pyrrole > quaternary ammonium > choline. On the basis of material synthesis, they respectively mixed 10 percent (mass fraction) homologous ILs (with the same cationic groups) into the above PIL materials to prepare a series of PILIL blend membranes, and then tested the permeability coefficients of CO2 , CH4 and N2 in the membranes. The gas permeability test showed that the order of cationic groups affecting CO2 permeability was pyridine > imidazole ≥ pyrrole > quaternary ammonium > choline. The selectivity α CO2 /CH4 and permeability coefficient showed almost the opposite trend: choline > quaternary ammonium > pyrrole > imidazole ≈ pyridine. Jeffrey et al. [264] connected isopropyl, isobutyl, cyclopropyl methyl and cyclopentyl functional groups with different sizes on the imidazole group of PIL membranes by alkylation. In PIL membrane materials, anions and cations are indispensable. Especially in the wet environment, there is a competitive binding cation group relationship between anions and carbonate (formed after CO2 dissociation with water), which has a great impact on the CO2 solubility coefficient. Shaplov et al. [266] functionalized polyimide, converted amide groups into benzimidazole cationic groups through alkylation, and prepared a main chain cationic

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PIL membranes. The CO2 permeability coefficient measured at 35 °C was 38.9 barrer, but the corresponding CO2 /N2 selectivity was only 5.0, which was difficult to achieve efficient separation. In 2016, Cowan et al. [267] designed and synthesized three kinds of PIL materials ([Pmmm VB]Ntf2 ,m=4, 6,8) with quaternary cation groups (including three butyl, three hexyl and three octyl groups respectively), and compared the effect of cationic group size on permeability coefficient [267]. The CO2 permeability coefficient of butyl based PIL membrane material is 51 barrer, and the selectivity of CO2 /N2 is 18.9. The CO2 permeability coefficient in hexyl quaternary PIL membrane material is 120 barrer, and the selectivity of CO2 /N2 is 14.1; The CO2 permeability coefficient of octyl quaternary PIL membrane material is 186 barrer, and the selectivity of CO2 /N2 is 15.5. It was shown that the CO2 permeability enhaced with the increase the length of alkyl chain, and the selectivity of CO2 /N2 decreased with the increase of the length of carbon chain. Li et al. [268] synthesized a branched chain cationic PIL membranes with polysulfone as the main chain through two steps of chloromethylation and imidazolization, and then obtained imidazole PIL membranes containing different anions (chloride ion, tetrafluoroborate, acetate and carbonate) through ion exchange. The results show that the acidity and alkalinity of anions have a significant effect on the performance of separation by affecting the CO2 binding capacity of cationic groups. When the anion is acetic acid radical with moderate acidity, the imidazolized PIL membrane material has high permeability and selectivity. The CO2 permeability coefficient reaches 69 barrer, and the CO2 /N2 selectivity reaches 119. When the anion is carbonate with weaker acidity, the CO2 permeability coefficient is 31 barrer and the selectivity is 42. When the anion is tetrafluoroborate with slightly stronger acidity, the CO2 permeability coefficient is 60 barrer and the selectivity is 75. When the anion is strongly acidic chloride ion, the CO2 permeability coefficient is 41 barrer and the selectivity is about 75. Mittenthal et al. [269] prepared the main chain cationic PIL membranes through the polycondensation of imidazole group and polyimide chain segment. The CO2 permeability coefficient was lower, only 0.871 barrer, but the CO2 /N2 selectivity reached 32, which was improved. To solve this problem, Shaplov et al. doped the PIL material with [1-Me-3-Etim]TFSI ionic liquid. The CO2 permeability coefficient measured at 35 °C exceeded 100 barrer, and the corresponding selectivity of CO2 /N2 increased to 34.2. Jourdain et al. [270] synthesized polyurethane containing triazole group, which is also a main chain cationic PIL membranes. Its CO2 permeability coefficient is only 2.31 barrer, which is far lower than people’s expectations for PIL membranes. In a word, although the main chain cationic PIL membranes is still an ionic liquid from the perspective of charged ionic groups, it has lost the excellent CO2 permeation and separation performance shown by ILs due to the poor mobility of cationic groups from the perspective of CO2 permeation and mass transfer. The above studies show that increasing the polarity of cations cannot be effectively transformed into improving the CO2 permeability and selectivity of PIL membranes. In this case, some researchers proposed to modify the cationic groups, and then design and synthesize branched chain cationic PIL membranes with multiple polarity sites to improve the CO2 separation performance. The polar cyano functional groups into branched chain cationic PIL membranes containing imidazole groups, and the CO2 /CH4 permeation selectivity increased from 22 to 37. When the introduced polar group is ether oxygen functional group, the selectivity is 33. Both of them have obvious enhancement effect. Gye et al. [272] designed and synthesized a

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17. Carbon dioxide capture and utilization in ionic liquids

branched cationic PIL membranes with polyimide as the main chain, and its functional group is peg grafted imidazole group [272]. Because PEG and imidazole groups have appropriate CO2 binding strength, the solubility coefficient is increased without significantly reducing the diffusion coefficient. This PIL membranes has a higher selectivity and permeability. It was found that the membrane material with 80 percent cation group substitution degree has a permeability coefficient of 72 barrer for CO2 , a permeability selectivity of 37.1 for CO2 /N2 , and a CO2 /CH4 permeability selectivity of 42.0. Aiming at the problem that the CO2 binding force of strong polar cationic groups is too strong (PIL membranes of polysulfone grafted with quaternary-ammonium groups). In many studies on branched chain cationic PIL membranes, many researchers use polyimide, polysulfone and other glassy polymers as the main chain structure [272–274]. It is well known that the larger the free volume of glassy membrane, the greater the gas permeability coefficient affected by this. Qu et al. [273] introduced water molecules in the process of osmotic separation, hydrated with quaternary ammonium groups to form a hydrogen bond network, dispersed the charge, and improved the diffusion capacity of CO2 through swelling, so that the CO2 permeability coefficient of the membrane material reached 1109.4 barrer, and the CO2 /CH4 selectivity reached 29.70, significantly improving the separation performance. Qu et al. [274] used poss nanoparticles as sacrificial templates to improve the continuity of CO2 transfer channels. It showed that the CO2 permeability coefficient reached 1113 barrer and the CO2 /CH4 selectivity reached 45. The preparation of PIL membranes requires that the ionic liquid monomer must contain special polymerizable groups such as double bond and epoxides group, and the synthesis process of polyionic liquid is complex. Moreover, the strong interaction between molecules leads to the close polymer chains, which limits the gas permeation flux, resulting in the gas permeability far lower than that of ILs supported liquid membrane.

17.2.6 CO2 captures by supported ionic liquid membranes ILs is a research hotspot in CO2 fixation and separation because of their non-volatile, adjustable structure and good CO2 absorption performance. However, the large viscosity of ILs is the main disadvantage of its use as CO2 capture solvent. In particular, the viscosity of Task-Specific ILs (TSILs) will further increase after CO2 absorption, resulting in increased mass transfer resistance of gas in the liquid and reduced absorption efficiency. High viscosity and high cost limit its industrial application. In order to overcome the above shortcomings, it is an ideal strategy to load ILs onto the surface of inert porous inorganic materials or polymers with high surface area to form a layer of supported ionic liquid membranes (SILMs). Compared with common solvents, the low vapor pressure and high viscosity of ILs can also decrease the loss of liquid membranes. The supported ionic liquid membranes has higher gas separation performance than the traditional polymer membrane, but its stability is poor. After long-term use, the separation performance decreases, which is mainly caused by the volatilization of organic solvents in the liquid phase of the membrane. ILs are non-volatile, which can avoid solvent loss and reduce membrane performance. Combining ionic liquid with gas separation membrane materials, the new separation membrane materials have the advantages of both ILs and membrane, and become one of the trends in the research of ionic liquid. SILMs are formed by loading ILs on supports made of inert porous inorganic

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materials or polymers with high surface area by impregnation or other methods. The membrane has good permeability and can be used in organic matter separation, gas separation and other fields [276–319]. Ionic liquid composite membranes include polymer ionic liquid (PIL) composite membranes and polymer/ionic liquid/inorganic porous material (PIL/MOF) composite membranes. PIL composite membrane has good selective permeability, but its mechanical strength is poor. The PIL composite membrane is doped with some inorganic porous materials with high permeability, good mechanical properties, heat resistance and corrosion resistance, such as SiO2 and molecular sieve, to inprove the mechanical strength of the composite membrane, so that the membrane has good mechanical strength and high selective permeability at the same time. In recent years, there have been many studies on the CO2 separation in SILMs, mainly focusing on conventional supported ionic liquid membranes and functionalized supported ionic liquid membranes. The ILs involved include imidazole, quaternary, amino, etc. Similar to polymers, the gas separation behavior of SILMs also follows the Robeson curve rule. Although the SILMs have high gas permeability and separation selectivity, its stability is poor. Under a large transmembrane pressure difference (0.25∼0.3 MPa), the membrane liquid will overflow seriously, resulting in the loss of ILs and the decline of membrane performance [276,277]. If the liquid ionic liquid is "solidified", for example, the ionic liquid containing polymerizable groups such as double bonds is changed from liquid to solid through polymerization reaction to form a membrane, it can not only avoid the loss of membrane liquid and improve the stability of the membrane, but also use the high affinity of ionic liquid to CO2 to improve the membrane separation performance [278]. Another way to enhance the gas separation behavior of ILs supported liquid membrane is to use functionalized ionic liquids. Compared with conventional ILs, functionalized ILs have higher CO2 solubility. For example, CO2 absorption in ILs containing amino functional groups is more than 0.5 mol CO2 /mol IL [276–312], because the chemical interaction between CO2 molecules and amino functional groups, which is completely different from the physical absorption mechanism in conventional ILs. Moreover, the compound formed by the partial reaction of CO2 with the amino group of ionic liquid ionizes CO2 and promotes the transfer of CO2 in the membrane, that is, the CO2 transfer in this kind of SILMs conforms to the promotion transfer mechanism. The permeation behavior of gas in the conventional supported ionic liquid membranes conforms to the dissolution diffusion mechanism, that is, the selectivity and permeability of the membrane are closely related to the gas diffusion and solubility in ILs, and the diffusion is also affected by the viscosity of ILs [289]. The dissolution of CO2 by conventional ILs is physical absorption, and its ability to dissolve CO2 directly affects the separationbehavior of gas in ionic liquid supported liquid membranes. Scovazzo et al. [277] fixed imidazole ILs of [C2 mim]X containing different anions as carriers in the pore structure of the membrane instead of traditional solvents to obtain SILMs. The permeability coefficient of the supported liquid membranes for CO2 can reach 1000 barrer, and the ideal selectivity for CO2 /CH4 and CO2 /N2 are 20 and 61 respectively. The performance of gas separation exceeds the upper limit of Robeson. It is found that the permeability coefficient of ionic liquid supported liquid membrane to CO2 is conticive to the anion of ILs, and the order is Ntf2 - > CF3 SO3 - > DCA> Cl- , which is similar to the law of CO2 absorption and solubility of pure ionic liquid of Ntf2 - > DCA- ≈ CF3 SO3 - > Cl- .

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The combination of FIL and SILMs may be a good choice for aborption or capture CO2 under high pressure and temperature [290]. In the CO2 /CH4 system, the permeation mechanism of CH4 is a simple solute diffusion process. The permeation coefficient of CH4 in [C3 NH2 mim]Ntf2 and [C3 NH2 mim]CF3 SO3 is lower than that in [C4 mim]Ntf2 . Even at a very low pressure of CO2 , the selectivity of CO2 /CH4 is high. When the CO2 concentration is 2.5 kPa, the selectivity of SILMs based on [C3 NH2 mim]Ntf2 and [C3 NH2 mim]CF3 SO3 is about 100 and 120 respectively. Baltus et al. [278] obtained a good capacity of CO2 absorption by fixing [C4 mim]Ntf2 on the porous Al2 O3 . [C4 mim]PF6 like materials was supported into the porous (zeolite or ceramic) materials by placing pressurized gas on one side and collecting CO2 exhausted gas flow downstream of porous media. Tang et al. [279] synthesized a series of PILs, such as P[VBBI]BF4 (VBBI: 1- (pvinyl phenyl)−3–butyl imidazolium), P[VBBI]Ntf2 , P[VBBI]PF6 , P[VBTMA]BF4 , poly(p-vinyl phenyl trimethylammonium (VBTMA). Compared with polymerized monomers, PILs has stronger CO2 capture ability, faster absorption and desorption rate and is completely reversible. In contrast to the simple ILs effect, the PILs with PF6 - anion showed higher CO2 capture capacity than those with BF4 - or Ntf2 - . Compared with ILs containing only alkyl chains, the CO2 solubility in ILs containing fluoroalkyl groups is improved [283,280], so the separation behavior of supported ionic liquid membranes prepared from such ILs may be improved, but the increase of ionic liquid viscosity will reduce the diffusion of gas in the membrane, thus affecting the performance of the membrane. The PILs is more viscous at ambient temperature, so it can be easily used as a membrane material. Therefore, if we combine the polymerized ILs with SILMs, the CO2 absorption capacity will be higher than that of small molecular ILs. To further improve the gas solubility and separation performance of PILs. In the past, the supported ionic liquid membranes has been used to inject the commercial porous polymer membrane into the ionic liquid. When Jeffery et al. [281] separated CO2 from He gas, the permeability and selectivity of SILMs is far exceeded that of most existing membranes. For example, when the CO2 /He selectivity is 8.6 at 37 °C, the permeability of CO2 reaches 744 bar. The higher the permeability of the liquid phase, the advantages of the supported ionic liquid membranes will surpass the polymer membrane. Because the ionic liquid is not volatile, it can overcome the traditional problems of the supported ionic liquid membranes (solvent volatilization and carrier failure). The CO2 solubility in [Hmim]Ntf2 is larger than that of He gas. However, the greater the solubility and selectivity, the more limited the ILs. In the presence of ILs, the stability of polymer support will be enhanced at higher temperatures. Bara and others have made great contributions to PILs [287]. They took the lead in synthesizing a series of polystyrene or polyacrylic acid ILs and measuring their properties. It was found that the permeability of CO2 increases, and the selectivity of CO2 /CH4 and CO2 /N2 increases with the growth of the alkyl chain connected to the imidazole cation. At the same time, the supported ionic liquid membranes [282] was prepared by placing the crosslinked nylon scaffold in the container and depositing [H2 NC3 H6 IM]Ntf2 with a pipette on the top of the membrane. Mahurin et al. [295] used the ionic liquid containing nitrile group in the anion to prepare the supported ionic liquid membranes to improve the selectivity of CO2 /N2 of the membrane. Because the nitrile group is a polar functional group, its existence can improve the selectivity

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of the ionic liquid itself for CO2 /N2 [284]. The research results show that the selectivity of the obtained [Emim]B(CN)4 supported ionic liquid membranes is improved. Myers et al. [285] immobilized an amino functionalized ionic liquid ([H2 NC3 H6 mim]Ntf2 ) on the cross-linked nylon-66 polymer membrane. This supported ionic liquid membranes can promote the transfer of CO2 under high temperature and dry conditions, and the CO2 permeability coefficient can reach 1000 barrer. Water has a greater impact on some functional ILs, since that can react each other. For example, Hanioka et al. [286] studied two amino functional ILs supported ionic liquid membranes [C3 NH2 mim]CF3 SO3 and [C3 NH2 mim]NTf2 . It is showed that the mass transfer of CO2 in the membrane can be accelerated either few water presented. The permeability coefficient of CO2 reaches more than 2500 barrer and the selectivity of CO2 /CH4 reaches more than 120. Neves et al. [296] found that although the water vapor improved the permeability of CO2 , the selectivity of CO2 /CH4 and CO2 /N2 decreased, which was related to the formation of small water clusters in ILs. Bara et al. [287] studied the separation of CO2 and H2 in ILs with amino functional groups ([C6 mim]Ntf2 as a reference). The results show that when the temperature enhances above 85 °C, the promoted transport membrane based on ionic liquid will have lower CO2 permeability, which is similar to the results of Hanioka. The reason is that the stability of carbonate rocks was affected by high temperature, and the diffusion process is the dominant. Considering the permeability and selectivity, the performance of [C6 mim]Ntf2 is relatively good, but lower than that of [H2 NC3 H6 mim]Ntf2 . For the hydrophilic composite membrane, the flue gas moisture affects the performance of CO2 separation. Compared with the dry feed, the wet feed seems to enhance the permeability by 35 times, and no detectable selectivity loss of CO2 /H2 or CO2 /N2 . Bara et al. [288] introduced free ILs into PIL membranes to form PIL supported ionic liquid membranes. By screening the monomers of free ILs and PIL, the composite membrane obtained has better performance than the SILMs mentioned above. Compared with the membrane without [Emim]Ntf2 , the permeability of CO2 increased by about 400 percent, the CO2 /N2 selectivity increased by ∼25 percent, and the CO2 /CH4 selectivity decreased by about 33 percent. The type of free ions also affects the performance of the composite membrane. Imidazolium containing alkyl, alkoxy and fluoroalkoxy groups has little difference in performance (the CO2 permeability is 50 bars, the CO2 /N2 selectivity is about 37, and the CO2 /CH4 selectivity is about 26 under STP index temperature and pressure). However, the performance of imidazolium containing cyano group (the lowest permeability of CO2 is 33 bars, the highest selectivity CO2 /CH4 and CO2 /N2 is 40 and 28 respectively) and imidazolium containing siloxane (the highest CO2 permeability of 55 bars, the lowest CO2 /N2 and CO2 /CH4 selectivity of 33 and 20 respectively) is quite different. SILMs with excellent performance and high stability are prepared by selecting ILs with different structures and appropriate supporting materials. The main research focus is to explore the influence of ILs structure and supporting material properties on the stability, selectivity and permeability of the membrane [298,299]. Park et al. [292] coated [Bmim]BF4 on polyvinylidene fluoride membrane for natural gas sweetening treatment. Under optimized process conditions, the selectivity of CO2 /CH4 can be reached as 25∼45.

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Bara et al. [293] used hydrophilic or hydrophobic polyvinylidene fluoride (PVDF) as support, and [Bmim]PF6 , [Bmim]BF4 , [Omim]PF6 , [Dmim]BF4 and [Bmim]Ntf2 as membranesforming materials to prepare SILMs for O2 , H2 , CH4 , N2 and CO2 separation. The results showed that the hydrophobic PVDF membranes had better stability. With the increase of alkyl chain or viscosity in ILs, the interaction between ILs and all gases increases, and the permeability of gases increases. All supported ionic liquid membranes have good selectivity for CO2 . Especially when used for CO2 /CH4 separation, the behavior of the membrane has exceeded the upper limit of “rebeson”. The transmembrane pressure difference and the water vapor in the feed gas also have a great effect on SILMs performance. The CO2 /N2 permeability and selectivity of SILMs increased with the increase of the transmembrane pressure difference [294]. The vapor of water in the feed gas can significantly improve the membrane permeability. For the liquid membrane formed by hydrophilic ILs, the selectivity of CO2 /N2 and CO2 /H2 does not decrease significantly [295]. For the liquid membrane formed by hydrophobic ILs, water vapor will form water clusters in the liquid membrane, greatly reducing the selectivity of CO2 /CH4 and CO2 /N2 . As mentioned above, It can be found that although the SILMs have many disadvantages, such as thicker membrane (less than 150 μm or thicker) [282], unable to separate CO2 from flue gas under high pressure, but it is considered as a potential CO2 capture technology. In order to find more efficient and low-cost SILMs, it is necessary to deeply study the role of anions/cations in optimizing the structure of carrier materials and the molar volume of ILs, to develop more permeable, more stable but thinner membranes. Mahurin et al. [295] used the ionic liquid containing nitrile group in the anion to prepare the SILMs to improve the CO2 /N2 selectivity of the membrane. Because the nitrile group is a polar functional group, its existence can improve the selectivity of the ionic liquid itself for CO2 /N2 [284]. The research results show that the selectivity of the obtained [Emim]B(CN)4 supported ionic liquid membranes is improved. With increasing the temperature, the viscosity of ILs decreases, the mass transfer of gas accelerates, and the permeability of gas in ILs increases. However, with the increase of temperature, the CO2 solubility in ILs decreases, and the selectivity of CO2 decreases. Therefore, for SILMs, low operating temperature is more conducive to the separation of CO2 . In addition, water is an important factor affecting the diffusion and solubility of CO2 in ILs, thus affecting the performance of supported ionic liquid membranes. Generally, humidifying the gas will improve the CO2 separationbehavior of the supported ionic liquid membranes to a certain extent [296,308,317]. Zhao et al. [297] prepared [Bmim]BF4 -PES supported liquid membrane, and found that the permeability of CO2 is increased from 11.5 GPU (1 GPU = 10−6 .cm3 (STP)/(cm2 .s.cmHg)) to 13.8 GPU, and the selectivity of CO2 /N2 from 50 to 60, when a few water is added. Neves et al. [302] measured separate behavior of CO2 /CH4 and CO2 /N2 gases in SILMs. It was showed that SILMs sythetized with the most hydrophobic carrier is more stable than that prepared with hydrophilic carrier, and has a better affinity for CO2 than other gases. The study also shows that the vapor of water in the gas stream increases the permeability of gas in SILMs, but significantly reduces selectivity of CO2 /CH4 and CO2 /N2 . The decrease of selectivity is due to water clusters formation in the membrane, because this influence is more significant for conventional IL with low hydrophobicity.

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SILMs mainly rely on intermolecular forces to immobilize ILs in supporting materials. When the gas flow rate is too high or the pressure across the membrane is too large, the loss of ILs will lead to the decline of membrane performance. After polymerization, the molecular weight, viscosity and adhesion of ILs can be greatly improved, and the shortcomings of the common supported ionic liquid membranes formed by ILs may be overcome [219]. Shishatskiy et al. [301] supported n-aminopropyl-3-methylimidazole trifluoromethylsulfone ([C3 NH2 mim]CF3 SO3 ), n-aminopropyl-3-methylimidazole bis (trifluoromethylsulfonyl) imide ([C3 NH2 mim]Ntf2 ), and 1–butyl–3-methylimidazole (trifluoromethylsulfonyl) – imide ([C4 mim]Ntf2 ) on porous hydrophilic polytetrafluoroethylene (PTFE) membranes for separating CO2 and CH4 mixtures. It is showed that the CO2 permeability coefficient in [C3 NH2 mim]Ntf2 and [C3 NH2 mim]CF3 SO3 is larger than that in [C4 mim]Ntf2 , because CO2 permeates into SILMs through a simple solution diffusion mechanism in [C4 mim]Ntf2 , while CO2 permeates through a chemical reaction mechanism as a transport carrier for CO2 penetration in [C3 NH2 mim]Ntf2 and [C3 NH2 mim]CF3 SO3 . These SILMs have high stability after 260 days of operation with no detectable loss. The high temperature of the system further prevents the chemical reaction between amine and CO2 , so CO2 permeability coefficient decreases with the enhancement of temperature [302]. Jindaratsamee et al. [303] immobilized a series of imidazole ILs on polyvinylidene fluoride polymer to prepare a supported ionic liquid membranes. Its CO2 /N2 selectivity can reach 86, but the highest CO2 permeability is 445 barrer. Santos et al. [315] prepared the supported ionic liquid membranes containing acetic acid anion and tested the performance of gas separation. The CO2 permeability coefficient can reach 2114 barrer, while the selectivity of CO2 /N2 is only 39. Kim et al. [304] supported [C2 mim]Ntf2 on the top of polymer hollow-fiber, and showed that the structure of supported [C2 mim]Ntf2 has a great impact on the stability of the membrane, and the stability of the sponge structure is larger than that of the finger structure. Compared with the structure of cylindrical pore, the scaffolds structure composed of dense polymer chains (i.e. twisted structure) shows a stronger compressive resistance. Gonzalez Miquel et al. [305] studied the gas separation in SCN based ILs. It was showed that these ILs have low solubility of gas, but the CO2 solubility can be improved by enhancing the van-der Waals interaction between ILs and CO2 and no affinity for N2 . The reason is that the selectivity of CO2 /N2 relates to the anti-anion structure, it makes possible to select its alkyl substitutes and the cation family to adjust the CO2 /N2 selectivity and other relevant characteristics of ILs, such as viscosity or density, to optimize the separation of CO2 /N2 in SILMs. Kasahara et al. [307] immobilized two kinds of amino acid ILs into hydrophilic polytetrafluoroethylene microporous membrane to form a supported ionic liquid membranes. The CO2 permeability coefficient of [Emim]Gly-supported ionic liquid membranes reached 8300 barrer, and the selectivity of CO2 /N2 can reach 146. In addition, the author also studied the influence of different cation sizes of amino acid ILs on the gas separation performance of SILMs. It was shown that the permeation of CO2 was mainly to promote the transfer, while that of N2 was mainly to dissolve and diffuse. Moreover, the permeability coefficient of N2 could be reduced by reducing the size of cations to improve the selectivity of CO2 /N2 . Huang et al. [316] prepared dicarboxylic acid functionalized supported ionic liquid membranes supported by polyethersulfone. The permeability coefficient can reach to 2147∼2840 barrer at 0.01 MPa,

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and the selectivity of CO2 /CH4 and CO2 /N2 reached 178∼265 and 98∼221 respectively. The transport of CO2 in such SILMs also conforms to the mechanism of promoting transport, because [N2224 ]2 malonate and [N2224 ]2 maleate ILs have reversible chemical reactions with CO2 , which promotes the separation and transport of CO2 in the membrane. Operating conditions such as pressure, temperature and moisture also have some effects on the performance of gas separation in SILMs. Generally speaking, the pressure has little effect on the permeability and selectivity of SILMs. The structure and performance of ILs are the main factors that affect SILMs. The important thing is that ILs have structural design-ability. Therefore, the performance of ILs can be altered by varing the composition and structure of anions and anions of ILs and introducing appropriate functional groups, such as improving the CO2 solubility of ILs and reducing the viscosity of ILs, so as to enhance the performance of gas separation in SILMs. SILMs has poor pressure resistance [309]. In order to ensure its stability, supported ionic liquid membranes is usually thick. Therefore, how to reduce the thickness of membrane and ensure the stability of the membrane while ensuring high gas separation performance is a challenge in the development and research of supported ionic liquid membranes. Hao et al. [311] prepared P[VBim]Ntf2 /ZIF-8/[Emim]B(CN)4 composite membrane for separation of CO2 with P[VBim]Ntf2 , zeolite imidazole skeleton ZIF-8 and 1-ethyl-3methylimidazole tetracyanoborate ([Emim]B(CN)4 ). The effects of different contents of ZIF-8 on the selectivity of CO2 /N2 and permeability of CO2 in the composite membrane were studied. The results showed that the composite membrane containing 25 percent ZIF-8 had the best CO2 /N2 selectivity and CO2 permeability, which were 1062 barrer and 24, respectively. Clara et al. used titanium silicate molecular sieve ETS-10, [Emim]Ac, deacetylated chitosan biopolymer (CS) as raw materials to prepare CS/[Emim]Ac/ETS-10 composite membrane. Compared with polymer membrane and polymer IL membrane, the selectivity of the composite membrane for CO2 /N2 can be as high as 38.48, which can effectively separate CO2 /N2 . Zhang et al. [313] used [Bmim]PF6 as solvent and P(VDF-HFP) (poly (vinylidene fluoride hexafluoropropylene)) as polymer matrix to prepare P(VDF-HFP)/[Bmim]PF6 composite membrane by thermally induced phase separation. When [Bmim]PF6 content increases, the crystallization temperature of the system decreases, the structure of the composite membrane becomes loose, and the CO2 permeability increases significantly. When the content of [Bmim]PF6 is 60 percent, the best permeability of CO2 in the composite membrane can obtained. Sun et al. [318] used 2, 2-azodiisobutyronitrile as initiator to prepare a series of poly (1-vinyl-3-ethylimidazole) (P[ViEtim]X) containing different anions by radical polymerization, where X is PF6 - , BF4 - or Ntf2 - . PIL/IL composite membrane was prepared by blending P[ViEtim]X with 1–butyl–3-methylimidazole ionic liquid of [Bmim]X. It showed that the composite membrane permeability to CO2 is related to anions, and the composite membrane P[ViEtim]Ntf2 /[Bmim]Ntf2 has the best permeability to CO2 . It is also found that the permeability of CO2 in the composite membrane is related to the content of P[ViEtim]Ntf2 in the composite membrane, and the composite membrane containing 5 percent∼10 percent (mass fraction) P[ViEtim]Ntf2 has the best CO2 permeability, indicating that the CO2 permeability is closely related to the membrane structure. Scovazzo et al. [277] found that the CO2 permeability coefficient of [Emim]Ntf2 - supported ionic liquid membranes decreased with the increase of pressure, while the permeability coefficient of other supported ionic liquid membranes remained basically unchanged. Scovazzo

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et al. [277] evaluated the permeability and selectivity of this conventional IL membrane to CH4 , CO2 , and N2 in supported ILs on porous hydrophilic polyether sulfone (PES). They also studied the effects of conventional IL membranes with four water stable anions of Ntf2 - , CF3 SO3 - , Cl- and DCA- . The experimental results show that the CO2 permeability is 2.6 × 10−8 for Cl- to 7.5 × 10−8 for Ntf2 - based SILMs, the ideal CO2 /N2 selection rate is 15 for Clto 61 for DCA- based SILMs, and the ideal CO2 /C2 H4 selection rate is 4 for Cl- to 20 for DCA- based SILMs. Note that the above convention IL membranes have relatively better permeability/selectivity than polymer membranes, or even better. Wu et al. [316] showed that the difference of the transmembrane pressure has little impact on the CO2 permeability of [N2224 ]propionate, [N2224 ]acetate and [N2224 ]Ntf2 SILMs, while the CO2 permeability coefficient of [N2224 ]2 malonate and [N2224 ]2 maleate based SILMs decreases with the increase of transmembrane pressure difference. This is because ILs of [N2224 ]2 malonate and [N2224 ]2 maleate have strong alkalinity and are very easy to react with CO2 . Under high transmembrane pressure difference. There is almost no free ionic liquid to transport CO2 , and the high viscosity of the compound formed after the reaction further affects the diffusion of CO2 , so the permeability of SILMs is reduced. The influence of temperature on the separation behavior of SILMs is mainly based on the effect of temperature on the viscosity and CO2 solubility of ionic liquid. The researchers investigated the temperature effect on the selectivity and permeability of SILMs [315,316,319]. In summary, PIL/IL membrane and SILMs have good CO2 /N2 selectivity and CO2 permeability. The separation performance of ILs supported liquid membrane is related to their structure and performance of ILs. ILs supported liquid membrane has poor pressure resistance, so how to enhace the membrane stability and ensure the separation performance of the membrane is a challenge in the development of SILMs. The preparation of PIL membrane is complex, with strong intermolecular force, compact structure, and low CO2 permeability and easy to rupture. The introduction of ionic liquid can improve the permeability of the membrane to CO2 to a certain extent. Ionic liquid composite membranes include polymer/PL composite membranes and polymer/IL/inorganic porous material composite membranes. In polymer/IL composite membrane, the absorption of CO2 by ILs is mainly used to improve the permeability of the membrane to CO2 . Therefore, blending ILs with high capacity of CO2 absorption with polymer membranes may improve the performance of separation in polymer membranes. The mechanical properties of polymer/IL composite membrane are poor. The introduction of inorganic porous materials into polymer/IL composite membrane can improve the separation performance of CO2 and mechanical properties, which is a promising research direction.

17.3 Electroreduction of CO2 in ILs Over the years, with the massive consumption of fossil fuels, carbon dioxide emissions have increased sharply, causing global ecological environment and social problems. However, carbon dioxide is also a cheap renewable carbon resource, which can be applied as raw materials for the production of alcohol, ether, acid, ester and other important chemicals. Among many attractive CO2 utilization ways, electro-reduction and CO2 fixation technology, as a clean and controllable reaction process, has unique advantages in the production of

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chemicals under mild conditions. ILs have been widely applied in the electro-reduction of CO2 due to their unique properties [320–440]. The research status of CO2 electro-reduction to prepare related products in ILs was reviewed. The present chapter mainly introduces the preparation of CO, esters, formic acid, alcohols, ketones, urea, amines, carbamates and other compounds by electrochemical reduction of CO2 in ILs, and prospects the research direction.

17.3.1 Electrochemical reduction of CO2 to CO Syngas (CO, H2 ) is known as the “cornerstone of the synthesis industry”. In the FischerTropsch process, CO and H2 can react to synthesize liquid fuels such as alkanes and olefins, and can also synthesize a series of important chemical products such as methyl ether and methanol according to different proportions and under certain conditions. In addition to water gas transformation, CO can also be obtained by CO2 reduction technology. In the process of CO2 electrochemical reduction, the potential generated by hydrogen proton reduction is more correct than that generated by most of the CO2 electrochemical reduction products, and the energy required for hydrogen formation is also less. Therefore, H2 is often found in the gas products from the electrochemical reduction of CO2 . It is particularly important and challenging to find suitable electrode materials/dielectrics to efficiently reduce CO2 to CO So far, the applications of various conventional and functionalized ILs in the electroreduction of CO2 have been reported Rosen et al. [320] used [Emim]BF4 as electrolyte and obtained co product with selectivity up to 96 percent at very low over potential (0.17 V) on Ag electrode. It is speculated that the compound intermediate state of [Emim]+ -CO2 -BF4 is formed in the electrolysis process, which significantly reduces the reaction overpotential. Brennecke’s group [321] used tetraethyl ammonium perchlorate (TEAP) as electrolyte on Pb electrode to electro-reduce CO2 . The main product is oxalic acid. In the same system, 1ethyl-3-methylimidazole bis trifluoromethylsulfonimide ([Emim]Ntf2 ) ionic liquid was used as electrolyte, and the reduction product of CO2 was CO, indicating that the presence of ionic liquid changed the reaction pathway. Choi et al. [322] used iron porphyrin and [Bmim]BF4 as a CO catalysts for the electroreduction of CO2 , and found that the Faraday current efficiency of CO generation was as high as 93 percent. The addition of ILs in the reduction process effectively reduced the reduction over-potential of CO formation. When ILs were present, the generation of CO was detected at −1.36 V (vs NHE), while when ILs were not added, CO was not generated until −1.51 V (vs NHE). Barrosse Antle et al. [323] investigated the stability of the process of electroreduction of CO2 by ILs. [Bmim]OAC ILs were used for the CO2 electro-reduction. It was shown that the ILs still showed high reaction performance after 15 cycles. Snuffin et al. [324] investigated the process of CO2 electrochemical reduction using Pt rotating disc electrode (RDE) and a new ionic liquid of [Emim]BF3 Cl. The Cyclic Voltammogram measurement showed that the current density was obtained as 5.7 mA.cm−2 at −1.8 V. Yang et al. [325] studied the CO2 electro-reduction reaction on Au electrode in [Bmim]CF3 SO3 and [Bu4 N]CF3 SO3 by using traditional H–Cell. Compared with [Bu4 N]CF3 SO3 , the over-potential of CO2 electro-reduction in [Bmim]CF3 SO3 was reduced by 0.24 V, indicating that [Bmim]+ cation showed higher CO2 electroreduction performance than [Bu4 N]+ . Zhao et al. [326] investigated 13 different ILs and found that imidazole based ILs showed the best catalytic performance for the electro-reduction of CO2 . Through long-time

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electrolysis, it was found that it was helpful to maintain the reaction stability of imidazole based ILs when a few water was added. A large number of literatures have reported that imidazole cations have higher CO2 reduction activity than other ionic liquid cations, while anions have little effect on the reduction activity. The mechanism of different cations ([Bmim]+ , [Pmim]+ , [Emim]+ , [BmPyrr]+ ) on CO2 electroreduction with Ntf2 - as anion on silver working electrode was systematically studied. It showed that different cations can reduce CO2 at different initial reduction potentials, among which [Bmim]Ntf2 shows the best catalytic effect, and the formation of [Bmim]+ -CO2 complex intermediate makes CO2 reduce to CO at a low over-potential. At the same time, they investigated the mechanism of anions on CO2 electro-reduction, and studied the effects of BF4 - , Ntf2 - , FAP- anions on CO2 absorption and electro-reduction performance with [Bmim]+ as cation. It showed that the change of anions has no significant effect on the CO2 electro-reduction reaction. Among them, [Bmim]FAP has the strongest ability to dissolve CO2 , but the current density of CO2 electro-reduction is low. It is speculated that the cationic structure formed on the electrode surface has a more significant effect on CO2 electro-reduction. Although some progress has been made, the specific effects of anions and anions in ILs on the CO2 electroreduction should be studied in-depth combined with in-situ characterization technology, and quantitative calculation. The application of ILs combined with novel nano microelectrode materials in CO2 electroreduction is a hot research topic. Hu et al. [327] doped MoO2 particles on Pb substrate as working electrode, and investigated the promoting effect of [Emim]Cl, [Emim]PF6 , [Bmim]PF6 on CO2 electroreduction reaction. It showed that CO2 can be reduced to CO at an overpotential of 40 mV in [Emim]PF6 . It is found that the CO2 selectivity in electroreduction products changes greatly with the change of ionic liquid, which confirms the catalytic role of ionic liquid in the process of electroreduction of CO2 . In 2016, Asadi et al. [328] established an artificial photosynthesis system that converts sunlight, CO2 and water into liquid fuel by using two-dimensional Nano-thin-layer material WSe2 as working electrode and [Emim]BF4 as electrolyte. The conversion efficiency of the system can reach 10 percent using pure CO2 . It showed that when the overpotential formed by CO is only 54 mV, the TOF value can reach 0.28 s−1 , in which the role of ILs is mainly to rapidly transfer CO2 from the bulk phase to the surface of the reaction electrode to maintain the CO2 concentration on the electrode surface. The reduction of CO2 to CO in ILs combined with non-noble metals as electrodes is also a research hotspot. For example, Salehi et al. [330] used N-doped carbon nanofibers and [Emim]BF4 for CO2 reduction reaction to obtain 98 percent CO Faraday efficiency. They speculated that it is the result of the joint effect of [Emim]+ -CO2 intermediate and reducing carbon atoms in carbon nanofibers. At the same time, Han et al. [331] used the metal free N, P-codoped carbon aerogels as the working electrode in the H–Cell to boost CO2 electroreduction. A good CO2 reduction performance in [Bmim]PF6 can be obtained. The current density of the system is up to 143.6 mA.cm−2 which can be attributed to the synergistic effect of ionic liquid and carbon materials containing N and P doping. Imidazole ILs have higher CO2 reduction activity than other ILs. The adsorption behavior of ILs forming a monolayer on the surface of electrode is the main reason for promoting the reduction reaction [332,333]. It was shown that the cations play an important role in the process of CO2 reduction, which may be related to the cation structure of imidazole ILs adsorbed on the electrode surface, which is affected the selectivity of the reaction products and the reduction

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over-potential of CO2 [334–339]. In addition, some studies have shown that imidazole ILs are like surfactants at low concentrations. With the increase of concentration, the number of cations on the electrode surface increases. When the concentration increases to a certain extent, the cations are spread on the electrode surface and adsorb the anions in the solution through electrostatic interaction, which promotes the reduction reaction. However, the active sites of CO formation from imidazole ionic liquid cations and CO2 still need to be further explored. Rosenthal et al. [335] found that when [Bmmim]BF4 was used as electrolyte, the C2 site in imidazolium cation was replaced, the current density and Faraday efficiency of electro-reduction of CO2 were lower than that of [Bmim]BF4 . It believed that the C2 site of imidazolium cation was the active site, which promoted the CO2 reduction in imidazolium ILs with different substituents. Dyson et al. [340] found that when the C2 position was methylated, the reduction activity of CO2 was not affected, but when the C4 and C5 positions of imidazole were replaced, the reduction activity of CO2 decreased significantly. It was speculated that the hydrogen protons in C4 and C5 of imidazole could stabilize the intermediate state of the reaction, while the C2 position maintained the high stability of the reaction system. Although the experimental results show that imidazole ILs as electrolytes have good CO2 reduction activity, and some intermediate states have been observed by in-situ characterization, the reduction mechanism is still unknown. The mechanism of CO2 reduction promoted by C2 substituted imidazole ILs still needs to be further studied in combination with molecular simulation and quantitative calculation methods.

17.3.2 Electrochemical reduction of CO2 to HCOOH HCOOH is also a common reduction product in the electrochemical reduction of CO2 , and its commercial application value is greater. As a reaction raw material for chemical production, HCOOH can also be used as a hydrogen storage material. Compton et al. [341] reducted CO2 in [Emim]Ntf2 electrolyte using the pre-oxidized Pt electrode to generate HCOOH under ambient temperature and pressure. In 2015, Hardacre et al. [342] studied the CO2 electr-reduction in the super alkali of [P66614 ]124triz. It showed that a formic acid with a Faraday efficiency of 93 percent was obtained by using Ag working electrode. The overpotential of the reaction is 0.17 V. They proposed two different reaction paths for CO2 reduction in this ionic liquid. One possible path is that the 124triz- of [P66614 ]124triz can form a strong chemical bonds with inert CO2 molecules, in which the linear CO2 molecular structure is bent, and the generated [124triz]CO2 intermediate is reducted to formic acid radical under extremely low over-potential, and finally formic acid molecules are formed, or deep reduction reaction takes place to generate 4electron reduction product of formaldehyde. Another possible path is that CO2 molecules can also form CO2 -[P66614 ]+ through physical interaction with [P66614 ]+ . The intermediate receives electrons and hydrogen protons to reducte to methanol, CO and other products. Bocarsly et al. [343] used the binary mixed system of [Emim]TFA and water as electrolyte, and investigated the effect of indium, lead, tin and other metals on the electro-reduction performance of CO2 . Compared with CO2 reduction system in aqueous solution, CO2 reduction with low over-potential and high current density is realized in water/ionic liquid binary system. The formation rate of formic acid on Indium electrode is as high as 3 mg.h−1 .cm−2 .

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It is also found that lead and tin metals also exhibit catalytic activity for the CO2 electroreduction to form formic acid. Different from the reduction on tin and indium electrodes, the Faraday efficiency of formic acid formation on the lead electrode increases with the reduction potential. It is speculated that the reason may be that ILs form a stable intermediate complex with CO2 , reducing the over-potential of CO2 reduction reaction, and more CO2 is dissolved in the binary mixed system. The transfer speed of CO2 from the bulk electrolyte to the reaction electrode is accelerated, and a higher current density of electro-reduction of CO2 is obtained. The traditional bulk copper electrode showed high reactivity and conversion efficiency when CO2 was electro-reduced to CO and hydrocarbons in ionic liquids. Han et al. [344] designed and synthesized an alkaline ionic liquid of [Mammim]OTf, which was coupled with heterogeneous catalyst “Si” –CH2 -3-NH-RuCl3 -PPh3 . Under mild conditions, it synergistically catalyzed CO2 hydrogenation in aqueous solution, realizing high selectivity and high efficiency to prepare formic acid. The ionic liquid can form adducts with formic acid to push the reaction equilibrium; The adduct can decompose and release formic acid at 130 °C to regenerate the ionic liquid for recycling. Srivastava et al. used amino-functionalized ILs to promote the CO2 catalytic hydrogenation to produce formic acid [345,346,348]. DuPont et al. [349] used RuFe nanoparticles as catalyst and conventional imidazole based ionic liquid [Bmim]Ac as reaction medium to effectively realize the CO2 catalytic hydrogenation to produce formic acid at a low pressure of 1 MPa CO2 and 2 MPa H2 , in which the conversion number (tons) and conversion frequency (TOF) of formic acid were as high as 400 and 23.52 H−1 , respectively. It was shown that the high efficiency is mainly due to the cage structure of ionic liquid around RuFe nanoparticles and its alkalinity. Dupont et al. [350] further designed and synthesized a multifunctional alkaline ionic liquid, which was used as Ru catalyst stabilizer and acid buffer, so that the reaction could be carried out efficiently under the conditions of 70 °C, 2 MPa H2 , 2 MPa CO2 , and few H2 O and DMSO presence. The conversion number to generate formic acid was as high as 17,000, and the concentration of formic acid solution was as high as 1.2 mol.L−1 . Wu et al. [351] constructed a Pd/C+[Bmim]OAc catalytic to efficiently prepare free formic acid by reacting CO2 with H2 at 40 °C. It is found that [Bmim]OAc activates CO2 by generating [Bmim]-CO2 intermediate in the reaction process, then modifies Pd nanoparticles, promotes the activation of H2 by adjusting its electron density, and finally stabilizes formic acid products through hydrogen bonding. In addition to precious metals, Han et al. [352] studied the process of CO2 electro-reduction in the ILs/acetonitrile/water ternary electrolyte system using the commercial Pb/Sn metal as the working electrode. The results showed that [Bmim]PF6 30 percent (mass)]/ACN–H2 O[5 percent (mass)] showed the best reaction performance, and the highest current density of 37.6 mA·cm−2 for producing the formic acid, and obtained Faraday efficiency is 91.6 percent. Moreover, the long-time electrolysis experiment shows that the electroreduction system maintains the reaction stability. Combined with electrochemical analysis technology and small angle XRD, due to the weak interaction between anions and anions in the ternary system, a small-sized micro-cluster structure is formed in the ionic liquid, which improves the CO2 solubility of the electrolyte to a certain extent. Hu et al. [327] doped MoO2 on Pb electrode, electro-reducted CO2 in [Bmim]PF6 , obtained 60 percent HCOOH Faraday efficiency, and its over-potential was only 0.04 V. At present, the current density of CO2 reduction is generally low, and the used working electrodes are generally precious metal electrodes (such as Pt,

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Au, Ag) and two-dimensional thin-layer nano-materials that are difficult to obtain. Therefore, more and more researchers pay attention to the study of obtaining high current density CO2 electro-reduction products on inexpensive and easily available non-noble metal electrodes. Fontecave group [353] reported a porous dendritic nano-copper electrode for CO2 reduction. The Faraday efficiency of HCOOH detected in [Emim]BF4 electrolyte was 90 percent, and the current density was large than 10 mA.cm−2 . It was speculated that the high solubility of CO2 in ILs electrolyte and the porous dendritic Cu electrode provide multi-active sites. This porous dendrimer copper material provides a new research direction and idea for the development of high-efficiency catalyst for catalytic conversion of CO2 . Wu et al. [354] obtained 95.5 percent HCOOH Faraday efficiency and 40.8 mA on metal oxide PbO2 electrode in [Bzmim]BF4 electrolyte. It was found that ILs containing F atoms have better CO2 reduction performance. It was proved that the C2 position of imidazole cations can form carbene groups, and then carbene groups attack the C nucleophile in CO2 to form carboxylate at C2 position, and finally form the HCOOH [356–358]. In addition, the effect of cations on the CO2 reduction, and the role of anions has attracted more attentions. In the process of reduction, the super basic anions can form chemical bonds with CO2 molecules through chemical interaction, which changes the linear shape of CO2 molecules. The interaction between super basic ILs and CO2 is stronger than that of other conventional ILs. The strong interaction leads to significant variation in the molecular configuration of CO2 , and the bond and angle are much smaller than that of other ILs. Therefore, higher current density and selectivity of HCOOH can be obtained in the super alkali ILs system.

17.3.3 Electroreduction of CO2 to CH3 OH At present, in addition to reducte CO2 to two-electron products such as CO and HCOOH, it is also a current research hotspot to use ionic liquid electrolyte to obtain CH3 OH, CH4 and CH3 COOH with high Faraday efficiency and high current density on inexpensive non-noble metal electrodes [359]. Han et al. [360] used Mo/Bi bimetallic sulfide nano-sheet as cathode to reducte the CO2 in [Bmim]BF4 . They found that the Faraday efficiency of CH3 OH product can reach 71.2 percent, and the current density increased with the enhacement the concentration of [Bmim]BF4 . However, when the concentration is larger than 0.5 mol.L−1 , the current density will decrease, which is related to the conductivity of ILs. With the increase of the number of ions in the electrolyte, the conductivity will increase, but the viscosity will decrease. Therefore, the design of ILs with low viscosity is particularly important. Lu et al. [361,362] used [Bmim]BF4 as electrolyte and Pd83 Cu17 bimetallic aerogel electrode to reducte CO2 to CH3 OH. A Faraday efficiency of up to 80 percent of with a current density of 31.8 mA.cm−2 for the reduction process was obtained. It was also found that when the electrolyte does not contain ionic liquid, the CO2 reduction products are mainly H2 and a small amount of HCOOH, which indicates that the addition of ionic liquid can enhance the selectivity of reduction products. Kang et al. [363] used Zn-MOF as catalyst and [Bmim]ClO4 as electrolyte to reducte CO2 . Under a low potential of 0.25 V, the Faraday efficiency of 88.3 percent for CO2 reduction to CH4 was reached. However, when [TBA]BF4 was used as electrolyte, the Faraday efficiency of CH4 was extremely low. Although ILs can be used as electrolytes to efficiently and deeply reducted CO2 to CH3 OH, CH4 and other products, the

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research on the reduction mechanisms is extremely scarce and will become a difficult and hot spot in the field of CO2 electro-reduction in the future.

17.3.4 Electrochemical reduction of CO2 to cyclic carbonate In recent years, a series of achievements have been made in the catalyzed preparation of cyclic carbonates by the cycloaddition reaction of CO2 with epoxides in ionic liquids. Conventional ILs such as imidazole, quaternary ammonium, pyridine and quaternary phosphonium, functional ILs such as hydroxyl, amino and carboxyl groups, and heterogeneous ILs catalysts supported by inorganic or organic materials such as SiO2 , polymer, MOF and graphene oxide have been widely reported to be used for highly efficient catalytic preparation of cyclic carbonate from CO2 [368]. Peng et al. [369] used ILs (such as [Bmim]BF4 , [Bmim]PF6 , [BPy]BF4 ) as catalysts for the first time to achieve efficient catalytic preparation of cyclic carbonate from CO2 . Zhao et al. [370] synthesized [Cn Cm IM]HCO3 to catalyze the reaction of CO2 and methanol to produce the DMC at room temperature. They found that the ILs will form adducts with CO2 , and then adducts can combine with the by-product water to form imidazole bicarbonate. Therefore, the ILs can be used as catalyst and dehydrating agent at the same time, which not only breaks through the limitation of chemical equilibrium, but also simplifies the subsequent separation of dehydrating agent, realizing the efficient conversion of CO2 and methanol. Hu et al. [371] proposed a two-step strategy for convention of CO2 into dialkyl carbonates under ambient temperature and pressure. CO2 is firstly adsorbed and activated in [Cm C1 Im]An(m = 2, 4; An = OAc- , HCO3 - , Triz- ), forming the adducts of Nheterocyclic carbene (Cm C1 Im–CO2 ), then CO2 is mixed with alcohols (ROH, R = C2 H5 , CH3 ) and alkyl halides to synthesize dialkyl carbonates. A conversion of 40.2 percent CO2 and a high selectivity up to 99.9 percent can be obtained. Anthofer et al. [372] synthesized ten imidazolium-based compounds [R1 R2 R3 im]Br (R1 = H, CH3 , benzyl (Bz), 1-(2,3,4,5,6-pentafluoro)benzyl (BzF5 ); R2 = H, CH3 , C2 H5 ; R3 = n-butyl, noctyl) and used them as catalysts for the cycloaddition of carbon dioxide and propylene oxide (PO) to produce propylene carbonate (PC).The effect of substituents such as ion pairing and steric effects on the catalytic activity was revealed. It showed that 1-BzF5 -3-n-octylimidazolium bromide is the most active catalyst, which gives very good conversions up to 91 percent and selectivity of propylene carbonate exceeds 99 percent at 70 °C and below 5 bar CO2 pressure. It is the lowest temperature for the synthesis of PC from PO and CO2 using metal-free catalysts. It can be reused and recycled at least 10 times, without loss of yield and activity. Xiao et al. [373] prepared an inexpensive protic ionic liquids for cycloaddition of epoxides and CO2 to form cyclic carbonates without using any organic solvent and co-catalyst. The effects of the structure of protic ionic liquid and acidity and on the catalytic performance were studed. It showed that the catalyst can reused at least five times without catalytic activity loss and can still achieve high selectivity and yield. The proposed mechanism is the synergistic effects of cation and anion in protic ionic liquids. Girard et al. [374] explored the CO2 absorption capacity of a series of imidazole ILs and used them in the synthesis of cyclic carbonates. They found that proper amount of water was beneficial to improve the yield of cyclic carbonate, and non by-product diol was produced. However, excessive water will lead to ring opening of epoxides substrate and alkoxide hydrolysis, and accelerate the formation of by-product diols.

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Toda et al. [375] studied tetraarylphosphoniumsalts (TAPS)-catalyzed carbon dioxide fixation at atmospheric pressure for the coupling reaction with epoxides. Five-membered cyclic carbonates including enantio–enriched carbonates can be obtained. The mechanism is that the origin of the behavior of TAPS to be the in situ formation of an active species by TAPS addition to epoxides via halohydrin intermediates. Wu et al. [376] reported series of tetrabutylphosphonium ([Bu4 P]+ )-based ILs with multiple-site for CO2 absorption and activation, It showed that these ILS could efficiently catalyze the cyclization reaction of CO2 with propargylic alcohols at ambient conditions. The [Bu4 P]3 [2,4-OPym-5-Ac], which has three interaction sites for attracting CO2 , has the best activity for produce a series of α-alkylidene cyclic carbonates. The yield of 91 percent was achieved for 20 h reduction, and it has good catalytic activity for a variety of propargyl alcohol substrates.The IL served as a bifunctional catalyst with anion simultaneously activating CO2 via multiple-site cooperative interactions and the C≡C triple bond in propargylic alcohol via inductive effect to produce the α-alkylidene cyclic carbonates. Meng et al. [377] prepared DBU bifunctional protic ionic liquids (DB/PILs) to catalyze the cycloaddition reaction of CO2 with epoxides. After reacting in simulated flue gas (15 percent CO2 , 85 percent N2 ) for 6 h, the conversion of epoxides reached 92 percent. It should be noted that after the ILs was recycled for 4 times, the yield of cyclic carbonate was reduced from 99 percent to 77 percent, mainly because the addition of epoxides substrate to ILs reduced the activity of active sites. It showed that the DBPILs could activate both CO2 and epoxides by alkoxy anions and powerful hydrogen-bonding, which was well consistent with experiments. Mujmule et al. [378] prepared the [EvimOH]HSO4 and [EvimOH][Cl] ionic liquids and formed with DBU as binary catalytic system of [EvimOH]HSO4 /DBU and [EvimOH]Cl/DBU for efficient cycloaddition of CO2 and epoxides.They found that DBU is highly alkaline and the steric hindrance near the active nitrogen is small, which is easy to activate CO2 . the propylene oxide conversion was 99.8 percent and the product selectivity was more than 99 percent in [EVIMOH]Cl/DBU for 1 h at 120 °C and 2 MPa. In this system, the synergistic effect among the hydroxyl functional group, the active proton on C2 of imidazole ring and the N-containing basic site can improve the CO2 absorption and promote the reaction activity. The catalytic activity did not reduce after 5 times of recycling. Zhang et al. [379] prepared [DMAPH]Br ionic liquids to catalyzed CO2 and epoxides to form the cyclic carbonates. An excellent selectivity and conversions of terminal epoxides were achieved under solvent-free conditions with only 1 mol%percent of [DMAPH]Br. The ILs could be reused over five times without appreciable activity loss. It is attributed to the increasing synergistic interplay of bromide and acidic proton to CO2 and epoxides by delocalization of the positive charge on the cation. Zhao et al. [380] synthesized the hydroxyl–functionalized ionic liquids such as dihydroxyethyl immidazolium hydroxide, tetrahydroxyethyl ammonium hydroxide and alkylhydroxyethyl, dialkyl (or alkyl) di-(tri-) hydroxyethyl ammonium hydroxide and used as catalysts for direct producing the dimethyl carbonate (DMC) with carbon dioxide and methanol. The effect of hydroxyl groups on the activation of CO2 and methanol. It showed that BzMDH has the best selectivity and catalytic activity to DMC. It is proved that the synergistic effect of alcohol hydroxyl and OH- plays an important role in the capture and activation of CO2 . In addition, it is also found that the conjugation effect brought by benzyl substituents on cations is conducive to the carbonylation of methanol. After the ILS was recycled for 4 times, the methanol conversion rate decreased significantly, which may be due to the mass loss during

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recovery. Sun et al. [381] prepared the hydroxyl–functionalized ILs and used as the catalyst in the reaction of epoxide and CO2 . Highest selectivity and activity were obtained for [Hemim]Br in all ILs studied. The conversion rate of propylene oxide can be reached to 99.2 percent and the selectivity of propylene carbonate was 99.8 percent under 125 °C and 2 MPa for 1 h. They proposed the relationship between OH-functional group and the higher catalytic reactivity. Sun et al. [382] prepared the acid–base bifunctional catalysts (ABBCs) with a Lewis-basic site in the anionic part and one or two Brønsted acidic sites in the cationic part, and used for cycloaddition of CO2 without the use of other co-solvent and co-catalyst. The effects of the reaction parameters and catalyst structures the performance were revealed. They concluded that the strong acid performance of acid-base-bifunctional ILs catalyst could enhance the ring opening ability of cations, but also weaken the nucleophilicity of anions, and ultimately reduce the yield of propylene carbonate. After being recycled for 5 times, the catalytic activity did not decrease significantly, the conversion was still 97 percent and the selectivity was 99 percent. Xiao et al. [383] found that weak acids are conducive to improving catalytic activity. When the acidity is too strong, strong hydrogen bonds will be formed to hinder the insertion of CO2 and reduce the yield of target products. Meng et al. [384] found that the solubility of carboxylic ILs in the reaction system can be adjusted by controlling the alkyl chain length of carboxylic ILs. Homogeneous phase was formed under heating conditions, which showed maximum catalytic efficiency. After the reaction, the temperature was reduced, and the ILs was separated from the product, which was easy to separate. Amino functionalized imidazole ILs have been widely used to effectively catalyze the cycloaddition reation of epoxides and CO2 due to their ability to activate and capture CO2 . Yue et al. studied the CO2 reduction in [APBim]X (X: I- , Br- and Cl- ) ionic liquids [385] at 120 °C and 1.5 MPa for 1.5 h, the yield of propylene carbonate was 94.3 percent. They pointed out that the longer alkyl chain on the cation helps to improve the catalytic activity of ILs, while the catalytic activity order of halogen anions is I- >Br- >Cl- , which agrees well with the results obtained by Liu et al. [386]. In addition, amino-functionalized ILs of [HEBim]Ala, [HEMim]Glu, [HEBim]Asp, [HEBim]His, [HEMim]Asp can not only react with CO2 to generate carbamate to activate CO2 , but also activate epoxides through hydrogen bond between proton on amino group and oxygen atom of epoxides, so as to realize dual activation of CO2 and epoxides. Yue et al. [387] applied the bifunctional imidazolium ILs with different amino acids as anions to catalyze the cycloaddition reaction between CO2 and epoxides at 90 °C and 0.25 MPa. After 12 h of reaction, the highest yield of cyclopropylene carbonate was 99 percent. The amino acid ILs would not decompose at 200 °C, and the catalytic activity decreased slightly after being recycled for 4 times. In recent years, it has been found that the addition of alkaline earth metal halide catalysts can effectively improve the activity of the catalytic reaction. Lferov et al. [388] found that a series of imidazolium ILs can efficiently catalyze CO2 to prepare cyclic carbonate under relatively moderate conditions. They found that the catalytic activity of imidazolium ILs decreased with increasing the alkyl chains of imidazolium cation substituents. Meng [389] and Wang [390] used pyridine, quaternary ammonium salt and quaternary phosphonium ILs to efficiently catalyze CO2 to prepare cyclic carbonate. Yuan et al. [391] designed and synthesized quaternary phosphonium ILs (such as [P4444 ]2-OP) containing pyridine anions, which can catalyze the reaction of propylene oxide and CO2 efficiently to prepare cyclic carbonate without metal and halogen additives. The research shows that the o site on the [2-OP] anion

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activates CO2 by forming carbonate-CO2 , while propylene oxide is activated by coordinating with P in [P4444 ]+ , and then the cation and anion synergistically catalyze the reaction to produce the cyclic carbonate. Liu [392,393] and others used the Lewis acid N-heterocyclic compound/ZnBr2 ionic liquid catalytic system to efficiently catalyze the reaction of CO2 and propylene oxide. Zhang [394], Yue [395] and others successively designed a series of highefficiency ionic liquid catalysts ([Apbim]I) for hydroxyl, carboxyl and amino-functionalization to catalyze the reaction to produce cyclic carbonate. In 2016, Yang et al. [396] introduced hydroxyl into DBU to prepare hydroxyl containing ionic liquid of [HEDUB]Br, which was used to catalyze the reaction of epoxide and CO2 . The reaction occurs at p(CO2 ) = 2.0 MPa and 100 °C for 4 h, and the yield of the product reached almost 100 percent. The nitrogen-containing heterocyclic ionic liquid [HDBU]Cl designed and synthesized by He et al. [397] It can catalyze the rapid reaction between CO2 and propylene oxide at 140 °C and 1 MPa CO2 , in which the conversion rate of propylene oxide can be as high as 99 percent in 2 h. In addition, Pescamona [398], Kim et al. [399,400], Dharman [401], Zhang [402], Zhang [403], designed and synthesized a series of heterogeneous ionic liquid catalysts supported on SiO2 , carbon materials, molecular sieves for the efficient production of cyclic carbonate. These catalysts have the advantages of easy separation and recycling and can be reused for many times. In recent years, a series of Ag or Cu based ionic liquid composite catalytic systems have been widely used to catalyze the cyclocarboxylation of CO2 with alkynol to synthesize unsaturated cyclic carbonate [399–402]. The ILs/metal salt systems most used in this kind of reaction mainly include [Emim]Ac/AgI [401], [Emim]Ac/AgAc [401], [P4444 ]La/CuI, [P4444 ]La/AgAc [402], [P66614 ]DEIm/AgAc [403], and [(n-C7 H15 )4 N]Br/AgAc [403]. It is found that the alkalinity of ILs has a great influence on the yield. Adjusting the pKa value in the range of 8–10 can achieve the highest transmission rate and yield. Han [409] et al. reported that AgCl/[Bmim]OAc catalytic system can synergistically catalyze the reaction of propargyl alcohols with CO2 to efficiently prepare asymmetric carbonates. Chen et al. [410] reported that [P66614 ]DEIm/AgAc system can catalyze the reaction of propargyl alcohol with CO2 at atmospheric pressure and room temperature, and the product yield is as high as 91 percent, which provides an idea for designing and synthesizing metal anion coordination ILs for direct chemical conversion of CO2 . Kim et al. [411] prepared a series of metal cation coordination ILs [M([Rim]2 )]X2 (M = Fe, Cu, Zn; R = methyl, ethyl, butyl; X = Cl, Br, I) from alkylimidazolium and metal halide salts, and used them to catalyze the reaction of propylene oxide (PO) with CO2 to form propylene carbonate (PC). They found that the central metal ion of the coordination cation could interact with PO, and cooperate with nucleophilic halogen anions to attack PO ring opening, so that the above metal cation coordination ILs showed good catalytic activity. Yang et al. [412] compared the performance of multifunctional onecomponent halogenated zinc based ILs [ATU]I and [NTU]I as homogeneous catalysts for efficient catalytic conversion of CO2 and epoxides to cyclic carbonates. It showed that the catalytic performance of [ATU]I was better than that of [NTU]I due to the synergistic effect of multi-functional components in atui. Under the conditions of 110 °C, 1.0 MPa, 4.0 h or even mild, the yield of propylene carbonate could reach 95 percent. The catalytic activity is related to the number of hydrogen bond donors in the ligand. Luo et al. [413] developed a series of metal cation chelating ILs whose cation is Li+ chelated with alcohol amine ligand and anion is halogen or Ntf2 - , and successfully simulated the conversion of CO2 and epoxides

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styrene so to styrene carbonate (SC) under the condition of flue gas (volume ratio of CO2 and N2 is 1:9). Among them, [Li(TETA)]I and [Li(DOBA)]I can efficiently catalyze the reaction of CO2 and so at 373.15 K and 0.1 MPa, and the yields of SC are 86 percent and 78 percent respectively.However, the catalytic activity of [Li(TEG)]I was relatively low, the yield of SC was only 15 percent, but [Li(DOBA)]Ntf2 had no catalytic activity. Although these systems can efficiently catalyze the reaction of propargyl alcohol with CO2 to prepare carbonates under mild conditions, Ag salts are often required as CO catalysts. Therefore, the development of efficient metal free ionic liquid catalytic system is of great significance. Zhao et al. [414] synthesized azole-anion-based aprotic ILs with the deprotonation of weak proton donors (e.g., 2,4-dimethylimidazole,4-methylimidazole, 2-methylimidazole, and [Bu4 P]OH). They found that [Bu4 P]2-MIm can activate CO2 by carbamates formation and carbamate intermediates could further react with various substrate, including 2aminobenzonitriles, propargylic alcohols, 2-aminothiophenol, and ortho-phenylenediamines to form benzimidazolones, α-alkylidene cyclic carbonates, benzothiazoline, and quinazoline2,4(1H,3H)-diones. The yield of α-alkylene cyclic carbonate can reach 79 percent. When the pressure of CO2 was enhanced to 2 MPa and the reaction time was 5 h, the yield was 91 percent. It is found that the anions of ILs can simultaneously adsorb and activate CO2 , activate the hydroxyl and alkyne bonds in propargyl alcohol through hydrogen bonding and induction effect, and then the catalytic reaction can be conducted efficiently at atmospheric pressure and room temperature.

17.3.5 Electrochemical reduction of CO2 to ketone compounds 17.3.5.1 Oxazolidinone Oxazolidinones are one of ketone compounds which are important heterocyclic compounds. The cyclization of propargylamine with CO2 is a green way to prepare these compounds efficiently. Gu et al. [415] successfully catalyzed propargyl alcohol, primary amine and CO2 in the CuCl/[Bmim]BF4 and synthesized 5-methylene oxazolinone with external double bond through one-pot reaction. Wang et al. [416] found that Cu(II) – substituted polyoxometalate ILs, such as [(n-C7 H15 )4 N]6 [a-SiW11 O39 Cu], can efficiently catalyze the cyclization of alkynylamine with CO2 and synthesize oxazolidinone in high yield. Subsequently, the research group designed a series of Lewis bases [417] and proton salt [340] ILs to catalyze the reaction. For example, DABCO ionic liquid [418] was used to realize the cycloaddition reaction of CO2 and azacyclopropane to prepare oxazolinone under solvent-free conditions. However, the reaction conditions are relatively harsh and need to be carried out under CO2 high pressure of 3–9 MPa. Han et al. used [Bmim]Ac ILs to catalyze the reaction of propargylamine with CO2 under ambient pressure and metal free conditions to obtain 3,4,5-trisubstituted oxazolidinone [420]. It is found that the double ionic liquid composed of [Bmim]OAc and [Bmim]Ntf2 can greatly improve the reaction efficiency. Then they designed and synthesized a new proton type ionic liquid [DBUH]MIm [421], and realized the reaction of CO2 and alkynylamine to synthesize oxazolidinone under ambient pressure and 60 °C. The yield of the target product can be as high as 90 percent. The catalytic activity of ionic liquid was not significantly reduced after 5 times of recycling. Theoretical calculations show that the anions of [DBUH]MIm seize the protons on alkynylamine to promote carbamate formation from CO2 and alkynylamine,

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and then [DBUH]+ and 2-methylimidazolium can give protons to promote the intramolecular cyclization of carbamate, that is, the anions of [DBUH]MIm catalyze the reaction of CO2 and alkynylamine by seizing and giving protons. Metal-coordination ILs can not only catalyze the reaction of CO2 with epoxides compounds, but also catalyze the one-pot reaction of propargyl alcohol and amine with CO2 to prepare oxazolidinone. For example, Qiu et al.[421]. found the one-pot three component reaction of propargylic alcohols, anines and CO2 that can proceed by combination of CuI and [P4444 ]Im as a catalyst. It was found that Cu/[P4444 ]Im has the best catalytic performance under atmospheric pressure, and the yield of the product is as high as 91 percent, and the performance is far better than that of single Cu(I) or [P4444 ]Im. 1 H NMR and 13 C NMR results showed that [P4444 ]Im and Cu(I) activate -OH and C≡C of propargyl alcohol. Although they did not directly propose that Cu (I) and [P4444 ]Im form metal anion coordination ILs, according to the literature, when Cu (I) and [P4444 ]Im are equimolar, it is easy to form metal anion coordination ILs in situ. Therefore, this kind of metal salt /ILs system is also classified as metal anion coordination ILs. 17.3.5.2 Quinazoline-2,4(1H,3H)-diones Lu et al. [422] studied the production of quinazoline-2,4(1H,3H)-diones from and 2aminobenzonitriles and CO2 in various ILs. They found that [Bmim]Ac could act as both catalyst and solvent for efficient converting other 2-aminobenzonitriles into their corresponding quinazoline-2,4(1H,3H)-diones in high yields of 92 percent at atmospheric pressure. Moreover, the separation of the products from the IL was very easy, and the IL could be reused at least five times without considerable loss in catalytic activity. Zhao et al. [423] prepared bifunctional ionic liquid through neutralization reaction of organic base DBU and weak donor TFE, [HDBU]TFE to realize the first metal free catalytic conversion of CO2 to produce the quinazoline-2,4(1H,3H)-diones under ambient temperature and pressure through the synergistic action of cation and anion. They found that the ILs can absorb CO2 up to 1.01 mol.mol−1 . In the reaction process, TFE anion can activate CO2 molecule, [HDBU]+ and TFE- and jointly activate o-aminobenzonitrile substrate molecule through hydrogen bonding, and finally the product is formed. The catalyst has good universality and can catalyze a series of reactions of o-aminobenzonitrile containing different substituents with CO2 to obtain quinazoline-2,4(1H,3H)-diones products. Shi et al. [424] found that the alkalinity of [HDBU]TFE cations has a serious impact on the efficiency of the conversion of carbon dioxide to quinazoline-2,4(1H,3H)-diones and cationic hydrogen bonding can improve the reaction efficiency. Theoretical calculations show that the absorption of carbon dioxide by [HDBU]TFE occurs before its activation process. However, Dyson recently commented that based on the above reports, the role of cations and anions in catalytic reactions is not very clear, and cations play a secondary role in substrates activating and physical properties controlling. In addition, the PKa of anions is linear with the reaction rate, and anions determine the catalytic activity. 17.3.5.3 Benzimidazolone Benzimidazolones and their derivatives are a kinds of heterocyclic compounds with O and N atoms in their molecules. They are widely used in chemical production. Traditional preparation methods mostly use phosgene, CO and dimethyl carbonate as raw materials to react with o-phenylenediamine substrates[424,425]. Therefore, the direct preparation of

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benzimidazolone by the reaction of CO2 with o-phenylenediamine has an important green and sustainable development prospect. Yu et al. [431] used bifunctional ionic liquid [HDBU]OAc as catalyst to realize the reaction of CO2 with o-phenylenediamine compounds, and obtained a series of benzimidazolinone compounds in high yield. The ionic liquid can also realize the reaction of CO2 with omercaptoaniline to prepare benzothiazolone. The results showed that the cation activated CO2 , the anion activated amine, and the final catalytic product was formed under the synergistic action of anion and cation of ionic liquid.

17.3.6 Electroreduction of CO2 to urea Urea is an important carbonyl containing compound and reaction intermediate, which is widely used in the production of drugs, dyes, agricultural chemicals and antioxidants in gas transportation and additives in plastic processing [426]. Dehydration coupling of amine with CO2 is an important way to synthesize urea compounds. Deng et al. Designed the CsOH-[Bmim]Cl catalytic system, realized the reaction of CO2 with aliphatic amines and aromatic amines without dehydrating agent, and efficiently prepared N,N-disubstituted symmetric urea and its derivatives [355]. In addition, due to the non-volatile and selective solubility of ILs, the products can be extracted by simple filtration and drying. Moreover, the activity of CsOH-[Bmim]Cl catalytic system did not decrease significantly after being recycled for 5 times. Han et al. [75] used alkaline ionic liquid [Bmim]OH as catalyst to avoid the use of dangerous and relatively expensive CS oh. Under solvent-free conditions, they directly synthesized bisubstituted symmetric urea from amine and CO2 without dehydrating agent.

17.3.7 Electroreduction of CO2 to carbamate Choi [429] and others prepared proton ILs [HDBU]OAc catalyst derived from super base by ion exchange method with DBU and acetic acid as starting materials, realizing the direct production of carbamate from amine, silicate and CO2 . Under the conditions of 150 °C and 5 MPa CO2 pressure, using acetonitrile as solvent and 10 percent (mole fraction) [HDBU]OAc catalyst, the separation yield of carbamate can reach 96 percent. The ILs has good universality of amine substrates, and aromatic amines also show good reactivity [429]. Moreover, other functional groups of amine are hardly activated in the catalytic system, which provides an effective path for the selective activation and conversion of amine. The mechanism was that strong hydrogen bonds were formed between aniline and basic acetate anions, and the reaction was catalyzed by protonated cations and basic anions. Of course, the use of acetonitrile solvent and harsh reaction conditions of 150 °C and 5 MPa CO2 greatly limit the application of this reaction. Therefore, we believe that it is of great importance and application prospect to directly synthesize carbamate from amine, CO2 and silicate under milder conditions by designing a new type of special functional metal free ionic liquid.

17.3.8 Electroreduction of CO2 to amides and methylamines The formylation and methylation formed by the reaction of CO2 and amines are important reactions in the chemical conversion of CO2 , and the formamide compounds generated are

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important chemicals and intermediates. For example, DMF is a widely used polar solvent and chemical reagent in organic synthesis. Dong et al. [432] used the post synthesis strategy to load [Et4 N]Br ionic liquid on the pore wall of organic framework material (COF) to prepare ionic [Et4 N]Br 50 percent-Py-COF catalytic material, which not only has high CO2 adsorption capacity (164.6 mg.g−1 ,0.1 MPa, 273 K), but also can effectively catalyze the reaction of amine with CO2 and pH-SiH3 to prepare formamide [432] under mild conditionsof 30 °C and 0.1 MPa CO2 . Although the reaction conditions were mild enough, DMF was inevitably used as solvent, which was not conducive to the separation of products. Hao et al. used [Bmim]Cl) as catalyst to realize the formylation of amine compounds with hydrosilane and CO2 [433,434] at room temperature and without solvent, and obtained a series of amide compounds in high yield. It is shows that [Bmim]Cl ionic liquid has a dual-functional catalyts. The Si-H bond was activated in the reaction, so that CO2 reacts with silane to form silicon formate intermediate. At the same time, the N–H bond of the substrate amine can be activated by hydrogen bonding. Then the anions and cations of ILs jointly catalyze the reaction of substrate and CO2 to produce formamide. Subsequently, on the basis of the above system. They used [Bmim]Cl to efficiently catalyze the reaction of CO2 , aldehyde and primary amine to prepare asymmetric N, N-disubstituted formamide (NNFA) [433]. Recently, Ke et al. has prepared a [Bmim]BF4 -Pd/C catalytic system to realize the reaction between cyclic amine and H2 /CO2 . Formamide, 1,2-bis (N-heterocyclic) ethane and methylamine can be selectively synthesized by changing the temperature [434]. It is found that ILs activate amines through the hydrogen bond at low temperature. At higher reaction temperature (such as 140 °C), formamide can react with [Bmim]+ to form formamide-[Bmim]+ adduct. Therefore, the combination with Pd/C can catalyze the MC formamide Murry coupling to produce 1,2-bis (N-heterocyclic) ethane in the presence of H2 . Moreover, Pd/C-[Bmim]BF4 can further catalyze the hydrogenolysis process of 1,2-bis (N-heterocyclic) ethane to break the C–C bond and produce the methylamine.

17.3.9 Electrochemical reduction of CO2 to other compounds Dupont et al. [350] used RuFe as catalyst and [Bmim]Ntf2 as solvent to realize CO2 selective hydrogenation to prepare hydrocarbons at a total pressure of 8.5 MPa and 150 °C to prepare formic acid. The mechanism study shows that the hydrocarbon preparation process in this system is divided into two steps.At first, CO2 is converted to CO on the RuFe surface through the Reverse Water Gas Shift Reaction, and then C-chain growth is carried out through FischerTropsch synthesis. Branco et al. [346] used Ru nanoparticles as catalyst and [Omim]Ntf2 as solvent to prepare CH4 by CO2 selective hydrogenation under the conditions of H2 pressure of 4–6 MPa, total pressure of H2 and CO2 of 8 MPa and 150 °C. The yield can be as high as 69 percent. In this system, the yield of CH4 and ton can be improved by increasing reaction temperature and the amount of catalyst. Inspired by the above work, Wang et al. [436] has designed a Pd(PtBu3 )2 -FeCl2 bimetallic catalytic system, which is coupled with [Bmim]PF6 ionic liquid with Xanthos ligand as promoter to realize the synergistic catalytic selective hydrogenation of CO2 to prepare low chain alkanes. At 180 °C, the total pressure of CO2 and H2 is 9 MPa, and the reaction time is 12 h, the space-time yield of low chain alkanes is as high as 1.08 Cmmol.

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Wang et al. [437] selected Li salt (LiI and LiBF4 ) additives and Ru-Rh bimetallic catalysts (Ru3 (CO)12 and RhI3 ) to couple with ILs such as [Bmim]Cl, and to realize the reaction of CO2 /H2 and lignin to prepare acetic acid for the first time in 120–180 °C and under low pressure. 1 g of lignin can produce 0.17 g of acetic acid. The mechanism study showed that [Bmim]+ stable RhI3 and I-complex Ru(CO)n (n = 1–4) were the main catalytic active species. In addition, the catalytic system is stable and can be recycled for many times. Bediako et al. [438] used [Bmim]Cl as the solvents, Ru3 (CO)12 as the metal catalyst, LiI as the promoter, to realize the reaction of methanol, CO2 and H2 to prepare ethanol under relatively mild conditions (120 °C). The proposed mechanism is that the ionic liquid and Ru catalyst could form Ru-CO-carbene-hydride and other complex active species in situ during the reaction, which promoted the high efficiency of the reaction. In 2015, Xie et al. [439] designed two metal-cation-anion coordination ILs of [Cu(Im12 )2 ]CuBr2 (Im12 = 1-dodecyl imidazolium) and [Cu(Im12 )2 ]CuCl2 to catalyze the carboxylation reaction between CO2 and terminal alkynes. It showed that [Cu(Im12 )2 ]CuBr2 and [Cu(Im12 )2 ]CuCl2 can both catalyze the reaction of CO2 with 1-iodobutane and phenylacetylene. The yields of the products are 99 percent and 94 percent respectively, which are larger than those of single copper salt or metal anion-coordination ILs. Moreover, [Cu(Im12 )2 ]CuBr2 can also catalyze 1-iodobutane and other different types of terminal alkynes to react with CO2 , indicating that metal anion coordination ILs have good catalytic activity For the synthesis of benzothiazole compounds by the reaction of 2-aminophenyl mercaptans with hydrosilane and CO2 under mild conditions. Gao et al. developed an efficient catalytic system of imidazolium ILs. Using [Bmim]OAc as catalyst, under the conditions of room temperature, low pressure (0.5 MPa), no metal and no additives, the cyclization of 2-aminophenyl mercaptan with CO2 /hydrosilane was realized for the first time, and a series of benzothiazole compounds with different substituents were obtained, with a product yield of 99 percent [440]. The proposed mechanism is that [Bmim]OAc activates 2-aminophenyl mercaptan through hydrogen bonding, and at the same time activates hydrosilane and CO2 respectively to form the active intermediate of silicon formate, which further reacts with the activated 2-aminophenyl mercaptan to form the target product. From the above, it can be seen that ILs show unique advantages in catalyzing and promoting the electrochemical conversion of CO2 . It is found that when the ionic liquid is an electrolyte, it will adsorb on the electrode surface to form a monolayer, and the interaction between the CO2 molecules and ionic liquid can reduce the overpotential of CO2 generation, and finally affect the reaction performance by cooperating with the electrode Ionic liquid electrolytes, as new media and materials, show higher CO2 electrochemical reduction activity than traditional systems in some cases, which further indicates that ILs have advantages and application prospects in the research field of CO2 reduction. They can not only be used as catalysts to realize the chemical conversion of at metal free and mild conditions, but also can be coupled with metal catalysts to realize its directional conversion, so as to obtain a new method for the synthesis of high value-added chemicals. In ionic liquid catalytic system, the coexistence of various interactions improves the efficiency of CO2 activation and conversion, which makes it have broad application prospects. However, some ILs have some problems such as difficult separation and poor stability, which need to be paid more attention.

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17.4 Conclusions In recent years, the adsorption and electrochemical conversion of CO2 by ILs have been studied. The results show that ILs have a high capacity for CO2 and other gases adsorption. Most ILs have good desorption and adsorption effects, and can still maintain higher adsorption rate after many cycles. ILs show unique advantages in the absorption and fixation of carbon dioxide, catalysis and promotion of carbon dioxide electrochemical conversion. They can be used as adsorbents and catalysts to realize the CO2 conversion at metal-free and mild conditions, and can be coupled with metal catalysts to realize its directional absorption, fixation and conversion, to obtain a new method for producing high value-added chemicals, showing a bright application prospect. These studies provide useful support and help for the industrial application of ionic liquids to fix or convert CO2 efficiently. However, due to the complex characteristics of the ionic liquid system. The research results obtained are far from meeting the needs of industrial applications. For example, some ILs have problems such as high viscosity, difficult gas separation, and poor stability, high cost, purposeful design and screening of high-efficiency ILs, adsorption mechanism and so on. In addition, there are still many important problems need to be solved in terms of fixation and absorption of CO2 in ILs and the improvement of selective CO2 absorption efficiency and selectivity in mixed gas. The reaction activity of electrochemical reduction of CO2 in ILs is closely related to the local environment on the electrode surface. Ionic liquid can adsorb on the surface of electrode to form a monolayer. The interaction between CO2 molecules and ionic liquid can reduce the overpotential generated by CO2 , and ultimately affect the reaction performance by cooperating with the electrode. It can be found that ionic liquid electrolytes, as new media and new materials, show higher CO2 electrochemical reduction activity than conventional systems in some cases, which further indicate that ILs have advantages and application prospects in the field of CO2 electrochemical reduction However, ILs have numerous problems, such as high viscosity, poor stability, high cost, adsorption and electrochemical reduction mechanism, which need to be paid more attention. Focusing on the ionic liquid system, the resulting research will be carried out in the future. (1) Establishing the relationship between structures and properties of ILs. The relationship between the structure of ILs and CO2 reaction activity should be constructed through the combination of First Principle calculation, molecular simulation, high-throughput calculation and experimental methods.Various interaction laws in the ILs systems should be revealed. The quantitative structure-activity relationship (QSAR) between the performance and structure of ILs should be established so as to provide guidance and basis for the design or select the ILs for adsorption and electrochemical convertion of CO2 . (2) Study on the relationship between the structure of ILs and reduction activity of CO2 and revealing the reaction mechanism. By using molecular simulation and First Principle calculation, and in-situ experimental techniques, the micro-mechanism of CO2 absorption in ILs and the adsorption characteristics and regularity of ILs, CO2 and active substances on the electrode surface during electrochemical reduction should be explored. The synergistic mechanism between the proton/electron transfer process in ionic liquid system and the interface structure of catalytic materials need to be clarified. The reduction law of CO2 on

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the electrode surface and the reaction mechanism should be revealed to provide a scientific basis for the final formation of CO2 conversion technology. (3) Constructing standards, methods and criteria for molecular design of ILs suitable for specific needs. The research and development of ILs with low price, good gas absorption performance, low viscosity, stable structure, and high selectivity is committed as part of the research priorities. Depending on the structural characteristics and direct conversion of CO2 , ILs with easy separation and stable performance should be designed and synthesized. The ionic liquid electrolyte system with multi-component synergistic catalytic reaction system for CO2 electro-reduction to produce multi-carbon products need to be developed. The Functionalized ionic liquids electrolyte system that can activate CO2 molecules and promote C–C coupling in the intermediate state of the reaction need to be designed, as well as the matching electrode materials with excellent reduction performance, especially the non-noble metal reduction system need to be studied. Providing a good reaction environment for CO2 electro-reaction in an ionic liquid electrolyte system with high efficiency and low energy consumption for CO2 reduction to synthesize multicarbon products need to be carried out. (4) Building large-scale synthesis and purification technology and equipment for ionic liquids. Establishing a platform for CO2 to be involved in the synthesis of different types of high value-added chemicals under mild conditions based on ionic liquid. The largescale synthesis technology, purification methods and equipment of low-cost, high-purity, fast and efficient ILs, realize the large-scale integrated manufacturing of ILs should be improved to reduce the use cost of ILs, expand new reactions and methods for preparing chemicals and energy products by CO2 conversion, so as to obtain more green routes and methods for preparing high value-added chemicals and energy products, and promote the industrialization of ILs related applications. (5) Revealing the characteristics of flow mass transfer and stability in CO2 reduction device in ionic liquids. The effects of electro-reduction devices with different structures on the properties of CO2 reduction chemicals need to be studied, and the internal structures of the electrolytic cell, electrode structure and electrolyte flow type need to be optimized to provide a good reaction site for CO2 electrochemical reduction. It should provide guidance for the industrialization of liquid electro-reduction of CO2 to produce highvalued products.

Acknowledgments The partial work in this chapter is supported by National Natural Science Foundation of China (NSFC) with the grant of No.51774158 and 51264021. The author woud like to thank the financial support from NSFC and Department of Science and Technology of Yunnan Province.

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C H A P T E R

18 Hydrothermal carbonization of sewage sludge for carbon negative energy production Milan Malhotra, Anusha Sathyanadh and Khanh-Quang Tran Department of Energy and Process Engineering, Norwegian University of Science and Technology, Trondheim, Norway

18.1 Introduction Due to rapid industrialization, infrastructure development and large-scale use of fossil fuels, humans have disrupted the natural cycle of the earth’s climate. Even with advances in sciences and technology, the dependency on fossil fuels continues to increase. This dependency on non-renewable energy sources has accelerated global temperature rise, melting glacier ice cover and loss of species/biodiversity [1]. The historical contribution of rich and developed countries to the global emissions which led us to this current climate change is undeniable [2]. With an ever-increasing frequency of extreme climatic events, the countries that have contributed the least are most vulnerable to droughts, floods, forest fires and intense heat waves [3]. Climate change is usually defined as a significant long-term change in the climate parameters like air temperature, precipitation, windspeed etc. that occur over time scales of decades or centuries or longer. The major root cause can be natural variability (eg: variations in solar cycle) or human activity. The Inter-governmental Panel on Climate Change (IPCC) described climate change as “a change in the state of the mean climate variables that persist for a prolonged period that can be identified using statistical tests”. However, according to the United Nations Framework Convention on Climate Change (UNFCCC), the reason for climate change cannot be due to natural climate variability observed over comparable periods. Instead, it is solely attributed to anthropogenic activity, which alters the composition of the global atmosphere. Nevertheless, climate change is a reality, and it poses dangerous consequences to the planets

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and the entire living species. Anthropogenic human activity related to rapidly increasing demand for burning fossil fuels (e.g., coal, oil, and natural gas) plus the other anthropogenic emissions resulting from the huge energy demand of the world associated with the expansion of the global economy leads to the buildup of GHGs (greenhouse gas) in the atmosphere. The World Meteorological Organization (WMO) made it clear that the accumulated greenhouse gases in the Earth’s atmosphere alter the radiative properties of the atmosphere [4] as they absorb some of the Earth’s outgoing longwave heat radiation and reradiate it back towards the surface resulting in the warming of the Earth’s surface and the lower atmosphere. The major greenhouse gases that contribute to global warming are carbon dioxide, methane, and nitrous oxides. Global warming is an increase in the combined surface air and sea surface temperatures averaged over the globe over 30 years (IPCC Fifth Assessment Report (AR5). Unless otherwise specified, warming is the long-term heating expressed relative to the period1850–1900, used as an approximation of pre-industrial temperature level. Global warming has already crossed 1.1 °C compared to pre-industrial levels. The current warming rate is at 0.2 °C (between 0.1 °C and 0.3 °C) per decade due to past and ongoing emissions. According to the recent report by IPCC [5], “the current state of climate change has already caused widespread damage to ecosystem and displaced the lives of billions of people across the world, despite of all the efforts that has been made to cut down the emissions and reduce the consequences”. The world now must face unavoidable climate dangers for at least the coming two decades with the current warming rate itself. Exceeding this warming level can result in additional severe impacts which we are not prepared for and some of which will be potentially long lasting or irreversible. Risks for society will increase with global warming and accompanied climate change, but not proportionately as mentioned earlier. It should also be noted that warming is not uniform, it is much greater in many land regions and during certain seasons compared to the global annual average. For instance the Arctic belt is warming three times higher than the global average known as the Arctic/ Polar Amplification [6]. Several changes in regional climate have been noticed with the current global warming and more are expected to occur proportional to the warming rate, including extreme temperature events (heatwaves and cold waves) in many regions and heavy precipitation events (with increase in frequency and intensity) leading to floods and droughts, especially in tropical regions [7]. Projections of global sea-level rise show an acceleration towards 2100 (around 0.6 to 1.1 m) with 1.5 °C warming and is expected to continue beyond that due to the long response times of ice sheets and ocean. This sea rise’s magnitude and rate highly depend on future greenhouse gas emission pathways. Climate-related risks to the overall wellbeing of the planet and humans are projected to amplify with 2 °C warming, compared to 1.5 °C. Since the climate-related risk is high for under-developed countries and vulnerable communities, capping the global mean temperature increase at 1.5 °C, before it goes to 2 °C, can save several hundred million people by 2050. But it is doubtful to reach net zero emission by 2050 as it demands fast and deep emissions reductions across all sectors. Immediate cuts in CO2 growth rates will require a substantial reduction in fossil fuel use, widespread decarbonization of electricity, improved energy efficiency, and use of alternative fuels (such as green hydrogen and carbon neutral biomass). IPCCs Fifth Assessment Report introduced Climate-resilient development pathways (CRDPs). They are the key development trajectories that combine mitigation and adaptation to realize the goal of sustainability at multiple scales. It highlights the efforts to

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eradicate poverty through societal and systemic transitions and transformations for ensuring universal well-being [8]. Carbon dioxide accounts for almost three-fourths of the global anthropogenic emissions and is the largest contributor to climate change. The process by which carbon continually moves within and between its four major reservoirs (the atmosphere, the oceans, land, and fossil fuels) is called the carbon cycle. It is driven by geologic processes (volcanic eruptions), photosynthesis, and plant and animal respiration. Most carbon on earth is stored in rocks, and sediments and the rest are spread across the ocean, atmosphere and in living organisms. Carbon is released back into the atmosphere when organisms die, fires blaze, volcanoes erupt, and fossil fuels (such as coal, natural gas, and oil) are burnt and through a variety of other mechanisms. The carbon cycle combines all the processes through which carbon moves from the atmosphere to earth and viceversa, it ensures a balanced concentration of carbon in the different reservoirs on the planet [9]. But when the amount of carbon changes in one reservoir, it perturbs the entire system. Henceforth it is widely recognized that we are in a climate urgency and the world needs to act quickly so that we can either avoid or equip ourselves to face the worst impacts of climate change. COVID-19 pandemic showed us the possibility of reducing global emissions as lockdown reported an annual decline of emissions by 5.8 percent in 2020, that is equivalent to the largest dip in emission ever, almost 2 Gt CO2 [10]. However, this decline in 2020 emissions didn’t contribute much unfortunately. The economic revival following the Pandemic led to CO2 reaching its highest average annual concentration which means the global emissions rebound to their highest level in 2021, more than what is reversed by the pandemic-induced emission decline. This resulted in the CO2 concentration of 420 parts per million today, 50 percent higher relative to the period of industrial revolution as shown in Fig. 18.1. Meanwhile, renewables-based energy generation also reached an all-time high in 2021 due to the ongoing sustainable development activities on the other hand. Coal power plants have helped build economies worldwide and are responsible for about 40 percent of global GHGs emissions from fossil fuel use. Renewable energy production from solar, wind, water and other sources are growing in regions which are ideal for it. But their growth rate is not enough to satisfy the energy demand of the growing world nor the availability of commercial storage technologies. Hence the amount of coal used for energy production is still difficult to tackle. Coal, being the most inexpensive and abundant source, serves as an integral part of power generation in most of the emerging and developing economies to satisfy the rapid growth in electricity demand. 30 percent of the global CO2 emissions hence attributed to coal-fired electricity generation. Overall energy production from different types of fossil fuels is shown in Fig. 18.2. To overcome the challenges related to global climate change and temperature rise, we need to move towards greener alternative energy sources. However, considering our energy production and distribution infrastructure, a quick transition to alternative energy sources such as solar PV, wind energy, and geothermal is not feasible within a shorter period. Hence, using biomass for energy production combined with carbon capture and storage (CCS) can help transition toward a net carbon-negative energy production. In the present chapter, we have compiled recent studies on hydrothermal treatment and its application for fuel production,

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FIGURE 18.1 Change in atmospheric carbon dioxide concentration with time.

FIGURE 18.2 Global fossil fuel consumption [11].

18.2 Sludge as a potential source of alternate energy

431

FIGURE 18.3 Typical schematic of wastewater treatment Reference [16].

especially from wet feedstock. We have focused on sewage sludge as a potential feedstock for sustainable energy production and various challenges associated with it.

18.2 Sludge as a potential source of alternate energy The rapid growth of population and increase in economic activities throughout the world has led to a rise in the water demand and consequently, wastewater generation has also increased. Typically the generated wastewater is collected through a sewerage system and finally treated in sewage treatment plants (STPs). STPs use various physicochemical and biological treatment unit processes to treat sewage. In STPS, a large amount of sludge is generated at various stages of treatment as shown in Fig. 18.3, and the characteristics of sludge depend upon the type of treatment. With an ever-increasing wastewater generation, the amount of sludge generation is projected to increase rapidly. Due to high moisture content and large volume, sludge management/safe disposal is a major waste management challenge for WWTPs operators. Moreover, sludge management can cost 40 to 60 percent of the total operational cost of WWTPs [12]. WWTPs without sludge treatment facilities typically send sludge to agricultural fields as a fertilizer source or to landfill/open dumping [13,14]. However, this practice of sludge disposal poses severe environmental risks to soil and groundwater due to heavy metals, micropollutants, and pathogens [15] in sludge. Anaerobic digestion (AD) is suggested as one of the sustainable options for SS management. It has various advantages such as sludge stabilization, reduction of solids, and removal of pathogens along with biogas generation. The major disadvantages of the process include high retention time (20–30 days), partial degradation of organic matter, and large reactor volume requirements [17]. Besides, a significant amount of digestate is generated, which is often difficult to dispose off [18]. Apart from these, several systems do not work properly due to poor operational practices and varying influent characteristics. Hence, there is a need to find

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FIGURE 18.4 Conventional thermochemical treatment of sludge [16].

other alternative methods promoting resource recovery from SS and assisting in sustainable disposal. Many studies have evaluated the potential of thermochemical treatment for energy recovery from sludge [19,20]. The primary products obtained from different thermochemical treatments of sludge are shown in Fig. 18.4. However, the requirement of dry feedstock is a major limiting factor for the scaleup of these processes. Moreover, drying results in lowering final energy recovery efficiency from the biomass [21].

18.3 Hydrothermal (HT) treatments for the production of fuel Hydrothermal treatment utilises water’s properties at elevated temperature and pressure conditions in a closed and inert environment to enhance the biomass characteristic [22]. Typically, the moisture present within the biomass is used as a water source for the HT reactions. As most biomass already has a significant amount of inherited moisture, hydrothermal treatment becomes the preferable choice for managing the waste. In addition, HT treatment eliminates the need to dry the biomass, which is one of the most energy-intensive processes [21,23,24]. The end products and pathway of chemical reactions during HT are dependent on the solvent properties of water, reaction pressure, temperature and use of catalyst. The dielectric constant of water is inversely proportional to the reaction temperature, as shown in Fig. 18.5. In hydrothermal temperature range of 180–280 °C, the properties of water are similar to

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18.3 Hydrothermal (HT) treatments for the production of fuel

FIGURE 18.5 Influence of temperature on water properties [24]. FIGURE 18.6 Typical

reaction temperature for various hydrothermal techniques reference [25,31–34].

methanol/acetonitrile (polar organic solvent) [25]. This reduced dielectric constant results in higher dissolution of organic molecules and enhanced reactivity [26]. Amongst multiple parameters such as pH, pressure, reaction time, temperature and catalyst that influence HT processes, temperature is the most crtitcal factor affecting the properties of final products. Different hydrothermal techniques can broadly be categorized into four subcategories depending on the reaction temperature, as shown in Fig. 18.6. At the lowest end of the

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temperature spectrum of HT treatment is thermal hydrolysis (TH). TH is typically used as a pretreatment method before AD to reduce retention time, improve dewatering properties and enhance biogas production [27–29]. Hydrothermal carbonization (HTC) is usually performed at a higher temperature compared to TH to improve the fuel properties of hydrochar (HC) produced during hydrothermal treatment [12,30]. Hydrothermal liquefaction (HTL) results in conversion of biomass into liquid product i.e., bio-oil. Above 350 °C temperature, the supercritical phase of water generates flue gas rich in hydrogen or methane.

18.3.1 Thermal hydrolysis TH is mainly used as pretreatment to enhance sludge solubilization, enhance dewatering properties and improve biogas production during anaerobic digestion (AD). Furthermore, based on reaction temperature, TH can be categorised into two categories i.e., (1) lowtemperature thermal hydrolysis (LTTH), typically performed under 100 °C temperature, and no additional vapour pressure and (2) high-temperature thermal hydrolysis, which is performed at 100 °C and above temperature. During TH, biomass is treated at elevated temperature (120–180 °C) in a closed system under saturated pressure, resulting in organics solubilization from solid to liquid phase. No oxygen is added to the reaction system. At elevated temperature and pressure, water molecules break the chemical bonds of complex materials, thus converting them into simpler molecules that can be treated easily. TH is primarily used as a pretreatment before AD to enhance the biogas production rate, thereby improving its overall efficiency. Typical steps likely to occur during TH of sewage sludge are shown in Fig. 18.7.

18.3.2 Hydrothermal carbonization HTC is a hydrothermal process which is similar to the natural process of coalification. Bergius discovered it in early 1900 for the conversion of cellulose to a material similar to coal. HTC is typically performed in the temperature range of 180–240 °C under self-generated water pressure and the reaction time can vary from few minutes to several hours. Under high temperature and pressure, the properties of water tend to shift toward non-polarity thus making it a suitable solvent for dissolving organic and organic reactions. After HTC of biomass, the carbon content is distributed into two-phase i.e., solid phase termed as hydrochar (HC) and liquid fraction (LF). The latter is also known as process wastewater. A small fraction (>5 percent) of carbon also escapes in gaseous phase. In the subcritical temperature range of 200–280 °C, water simultaneously behaves as weak base and weak acid. It has highest ionic product and thus can catalyze various hydrolysis reactions [36]. The HC produced during HTC has various advantages over dried sludge such as improved calorific value, mesoporous texture, higher aromaticity and biologically sterilized [37]. HTC is an efficient way of producing fuel with higher energy density and carbon content than raw feed. The major uses of HTC are CO2 sequestration, fuel generation, soil conditioning, and anode/cathode material for batteries [38]. The use of HC in soil provides various advantages such as rise in pH, improved buffer capacity, water holding capacity, and cation exchange capacity [39]. The primary benefit of HTC process of biomass over other

18.3 Hydrothermal (HT) treatments for the production of fuel

435

FIGURE 18.7 Sequence of steps expected to occur during TH of SS (Modified from [35].

thermal processes such as pyrolysis and gasification are the lower reaction temperature & energy requirements, and enhancement of dewatering properties. The biomass can directly be used as received without any prior treatment. The lower sensitivity of HTC process towards the source of biomass also makes it a preferable method for handling wet biomass such as SS, food waste etc. over conventional treatment such as AD. The HC generated after the process has enhanced characteristics in terms of fuel properties, aromaticity, energy densification, and higher hydrophobicity and the liquid fraction can for biogas production.

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FIGURE 18.8 Simplified pathway of HTL (Modified from [43]).

18.3.3 Hydrothermal liquefaction Hydrothermal liquefaction (HTL) is typically pefroemd in a temperautre range of 280– 370 °C and vapour pressufde of 10–25 MPa. Majority of biomass is converted to liquid biocrued while a smaller fraction is converted to gas, aqueous and char by-products [40]. HTL process can be comparable to pyrolysis in hot liquid water. In the recent past, various catalysts have been used to favour base-catalyzed condensation reaction, resulting in enhanced aromatic oil production compared to unwanted char. The yield of biocrude depends on the reaction temperature; usually, the biocrude yield increases with temperature but decreases beyond a specific optimum temperature. For instance, [41] reported that for the HTL runs performed above 300 °C resulted in a reduction in heavy oil yield. The optimum temperature for maximum oil yield also depends on the initial composition of waste feedstock. Zhong et al. [42] compared the HTL of four types of woods: Cunninghamia lanceolata, Fraxinus mandshurica, Pinus massoniana Lamb. And Populus tomentosa Carr. It was observed that the highest heavy oil yield of 23.78 (Cunninghamia lanceolata), 20.60 (Pinus massoniana Lamb), 26,43 (P. tomentosa Carr.) and 30,72 (Fraxinus mandshurica) was achived at HTL temparture of 320, 340, 300 and 300 °C, respectvly. Hence, the optimum condition may vary with the chemical composition of the initial feedstock (Fig. 18.8).

18.3.4 Hydrothermal gasification (HTG) HTG is a biomass to gas conversion process typically performed in a temperature range of 350 to 500 °C resulting in syngas generation. During HTG process, various complex reactions occur simultaneously depending on the chemical composition of feedstock, reaction conditions and use of catalyst [44]. Onwudili et al. [45] proposed a simplified HTG pathway, the first step is rapid solubilization and the second step is perllel reactions of dehydration or

18.5 Conclusion

437

FIGURE 18.9 Simplified pathway of HTL.

gasification. A higher dehydration rate during the HTG process results in the production of unwanted char and tar formation, as depicted in simplified Fig. 18.9 [25]. In contrast higher gasification rate results in the production of syngas.

18.4 Hydrothermal carbonization + gasification + ccs To achive a sustainable energy source from wet biomass with lower CO2 emission, a combination of various technologies will be needed. Combining HTC pretreatment followed by gasification and carbon capture and storage could help reduce carbon emissions. In a recent study, Cheng et al. 2020 [46] evaluated the feasibility of hydrothermal treatment in combination with CCS vs conventional bioenergy + CCS (BECCS) approach. Based on life cycle assessment (LCA) approach, it was reported that the HTC+CCS approach might be a much better negative emission technology. However, the final amount of emission depends on the feedstock characteristics and processing conditions used. Erlach et al. [47] reported enhanced efficiency in syngas produced during gasification of hydrochar compared to initial feedstock. However, the overall energy efficiency from biomass to electricity was 27.7 percent and 28.6 percent for HTC-IGCC–CCS and IGCC–CCS, respectively.

18.5 Conclusion The dependency on coal for energy production will persist in the coming decades, especially in developing countries. Hence, there is an urgent need to utilize carbon-neutral and sustainable biomass for energy production. Sewage sludge is one of the most widely available biomass which can potentially be used for energy production. Hower conventional thermochemical processes require an energy-intensive pre-drying step, making these processes commercially non-viable. Hydrothermal treatment can help mitigate the problem associated

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with the utlization of wet biomass with less energy than drying. A combination of HTC process with gasification and carbon capture and storage may result in a negative emissions.

Acknowledgement The authers acknowledge the funding from the Norway Grants 2014–21 via the National Center for Research and Development. Article has been prepared within the frame of the project: “Negative CO2 emission gas power plant” – NOR/POLNORCCS/NEGATIVE–CO2 –PP/0009/2019–00 which is co-financed by programme “Applied research” under the Norwegian Financial Mechanisms 2014–21 POLNOR CCS 2019 – Development of CO2 capture solutions integrated in power and industry processes.

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19 Utilization of supercritical CO2 for drying and production of starch and cellulose aerogels Jeieli Wendel Gaspar Lima, Clara Prestes Ferreira, Jhonatas Rodrigues Barbosa and Raul Nunes de Carvalho Junior Institute of Technology (ITEC), Faculty of Food Engineering (FEA), Federal University of Para (UFPA), Rua Augusto Corrêa S/N, Guamá, Belém, PA, Brazil

19.1 Introduction With the advancement of environmental awareness, the term circular bioeconomy has been used as an alternative to the current economic pattern, and represents a paradigm shift, which values nature, and reuses process by-products, adding value and reducing environmental impact [1]. The change from current industrial processes to ecologically correct processes represent one of the most relevant aspects of the bioeconomy, which in the long term should positively impact economic development, generation of wealth, preservation of biodiversity, and adding value to undervalued by products [2]. Current sustainability challenges include the gradual replacement of polymers such as plastic with natural polymers, which are biodegradable and environmentally friendly. It is worth highlighting the need for sophisticated, ecologically correct, and safe technologies to be applied to the production of new products and inputs [3]. Natural polymers, which include starch and cellulose, are considered promising raw materials and should, in the long term, revolutionize the development of new products. These polymers have high density, depending on the size of their molecules, are complex in terms of a chemical structure, and have large amounts of functional groups such as hydroxyl, which facilitates the generation of bridges such as hydrogen bonds and strong intermolecular bonds [4]. Here, we revisit the use of polymers such as starch and cellulose for the production of aerogels, and validate the use of supercritical CO2 for drying and also the impregnation

Green Sustainable Process for Chemical and Environmental Engineering and Science: Carbon Dioxide Capture and Utilization DOI: https://doi.org/10.1016/B978-0-323-99429-3.00024-2

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FIGURE 19.1 Representation of the production of starch aerogels, fixed with ketoprofen and its controlled release process.

of bioactive and drugs in aerogels. Initially, we approach drying with supercritical CO2 , and the technology used. Next, we address the production of starch and hybrid starch aerogels, including their properties and drug and bioactive impregnation. Finally, we also discuss the production of cellulose aerogels, some applications, properties, and important functions.

19.2 CO2 application – Supercritical drying The drying process includes two methods: supercritical drying and lyophilization. Drying using supercritical CO2 is an efficient method for producing aerogels with small pores, with minimal damage to the microstructure, ideal for encapsulating bioactive. Supercritical drying produces aerogels with larger surface areas and small pores below 50 nm. The replacement of solvents in consecutive steps, followed by supercritical drying (10 MPa, 40 °C) for 4 h is the most used technique for the production of wheat nanostarch, used for crystalline phytosterol fixation. Ketoprofen, a drug from the non-steroidal anti-inflammatory class with analgesic, anti-inflammatory, and antirheumatic effects, has been fixed in starch aerogels, with promising results in preclinical studies [5]. In (Fig. 19.1), we bring a representation of the production of starch aerogels, fixed with ketoprofen and its controlled release process. In the flowchart below (Fig. 19.2), we demonstrate equipment used for dynamic drying with supercritical CO2 . -Alcogel-and ketogels are placed inside the drying vessel. The system is heated, and pressurized, when the necessary conditions for drying are reached, the drying process is started. Generally, after 2 h, the system is slowly depressurized to atmospheric pressure, where biphasic separation of the CO2 gas takes place [6]. The continuous flow of supercritical CO2 removes the solvent, making its way through the starch molecules, forming

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FIGURE 19.2 Equipment used for dynamic drying with supercritical CO2 .

small pores, and increasing the surface area of contact. The drying process is carried out in cycles and can be carried out with either a dynamic or a static system, or a combination of the two. The use of aerogels in microspheres is faster than in cylindrical aerogels because the increase in the surface area of the microspheres facilitates the diffusion path of supercritical CO2 [7].

19.2.1 How does supercritical drying work? 19.2.1.1 Critical point and supercritical fluids The critical point of a pure substance is defined by thermodynamics as a physical state of condensed matter, which occurs under specific conditions of temperature and pressure. When a given pure substance has reached the vapor-liquid phase limit, it is no longer possible to physically distinguish the differences between the phases. The critical point of a pure substance represented in the simple phase diagram (Fig. 19.3A), between pressure – temperature, is identified at the extreme of the boundary between the liquid – gas phases. The green dotted lines represent the thermodynamic behavior of water, which passes through solid, liquid and gaseous phases. Look at the phase diagram on the right, following the blue line, up to the limit (critical point), for water, this is the boundary between the liquid and gas phase. After this point, exactly (647 K and 374 °C), these two phases no longer exist, what we have is called a supercritical fluid [5]. When approaching the critical conditions, the conversion of phases occurs, which converge in a single and new physical state, homogeneous and highly dense, called homogeneous supercritical fluid. Under these extreme conditions, the fluid has zero heat of vaporization, so scientists know that there is no definition between the previous thermodynamic phases.

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(B)

FIGURE 19.3 (A) A phase diagram showing the critical point. (B) Reaction between hydroxyl groups on the solid surface of silica aerogel with carbon dioxide, forming methoxy groups.

After the fluid passes the critical point, any infinitesimal change in any process conditions (pressure or temperature) drastically affects other conditions of a physical and chemical nature, including an increase in fluid density, changes in electron centers and density, and in polarity [5]. Supercritical fluids are substances with strange properties, highly dense and conduct heat like liquids, expand and compress like gases. They have the ability to dissolve a variety of substances, including polar to nonpolar, or even fluorinated hydrocarbons such as Teflon. The most relevant property of supercritical fluids is the density [3]. Small changes in pressure and temperature dramatically change density. Especially pressure, pressure increases with constant temperatures raise the density of supercritical fluids, especially biomolecular fluids such as CO2 . The density gradient of supercritical fluids follows the weight of the fluid, pushing the center of mass downwards against gravity. The displacement of the center of mass undergoes changes as the molecules rotate around their axis, and with each displacement, the electron cloud is disturbed, changing the polarity of the molecule [4]. 19.2.1.2 High temperature supercritical drying Supercritical drying of aerogel at high temperature requires special attention when using organic solvents. Especially solvents that can be flammable like ethanol and methanol. Safety precautions are essential for the development of aerogels that require the use of solvents in which supercritical conditions are very high, above 300 °C and 100 atm [7]. Silica aerogels and other inorganic aerogels typically use methanol as the drying solvent. When methanol reaches the critical point (Fig. 19.3B), it can react with hydroxyl groups on the solid surface of the silica aerogel, forming methoxy groups. Changing functional groups on silica aerogel causes change in chemical properties, especially making the aerogel more hydrophobic. Aerogels produced by the methanol technique have different properties and generally have higher structural quality [4,7].

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Supercritical drying of aerogels by applying the high temperature technique of organic solvents helps by reducing the shrinkage of the gel, which allows drying of low-density gels. The important point highlighted is that organic aerogels such as cellulose and starch cannot be produced using the high temperature technique of organic solvents. This occurs because organic polymers react with organic solvents at the critical point, or end up degrading due to high temperatures. Although the production of pure organic aerogels is not feasible, there is an expectation regarding hybrid aerogels, which may be produced [3]. 19.2.1.3 Supercritical drying of low temperature aerogels Here, the process used replaces flammable organic solvents with supercritical CO2 , nontoxic solvent that has less severe process conditions (critical temperature and pressure). In this process, the solvent present in the gel is gradually replaced by the passage of supercritical CO2 . After depressurization in the micrometer valve, the CO2 and organic solvent are removed, leaving only the aerogel [6–8] The low temperature drying technique is applied to a variety of polymers for the production of aerogels, including aerogels from natural polymers. However, certain aerogels, such as those formed by metallic oxides, cannot be produced using the CO2 drying technique, react to form metallic carbonates. Aerogels drying can be done using liquid CO2 , or in the supercritical state [7]. After exchanging solvents for liquid CO2 , the gel formed passes by a volume reduction, due to the loss of water and the organizational redistribution of the polymeric network, to accommodate the new interactions formed between the original solvent and the liquid CO2 . After drying begins, the structure passes by a drastic change, with the departure of CO2 and the original solvent, resulting in an aerogel with distinct pores. Solvent exchange using CO2 in the supercritical state is equivalent to the aforementioned process, although there is no drastic reduction in gel volume [8].

19.3 Starch aerogel and CO2 utilization 19.3.1 Starch specific aerogels Starch aerogels are produced from raw materials rich in starch, especially corn, rice, and wheat, applying different preparation protocols. Both processes have in common the step of starch gelatinization in distilled water with stirring at temperatures ranging from 60 °C to 95 °C and an estimated preparation time of 15 to 20 min. Gelatinization is the swelling of granules in heated water. The starch granules are broken, releasing amylose chains to the aqueous medium and, later, amylopectin, causing all free water to be absorbed, forming a viscous paste [8]. The recrystallization or retrogradation of starch occurs after solubilization, the amylose chains, faster than the amylopectin ones, aggregate forming crystalline double helices stabilized by hydrogen bonds. The process is done at low temperature, followed by a step to replace the water in the gel with alcohol or acetone. Replacing water with low molecular weight, highly volatile solvent is called crosslinking. If alcohol is used, we have alcohol gels, and if acetone is used, we have ketogels, which will later be dried. Finally, freeze-thaw crosslinking

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is also used. In this step, water crystals are formed, which are removed during drying, forming pores [9]. In the lyophilization process, the aerogels produced by crosslinking (freezing and thawing) are dried at low temperatures and pressure. The lyophilization process results in the formation of large pores, since during drying the pores collapse, which does not occur in supercritical drying. Lyophilization is an economical method compared to supercritical fluid technology. However, each technology produces aerogels with different properties, and aerogels developed through the freeze-drying process have a low surface area compared to supercritical fluid-dried aerogels [10]. Wheat starch was gelatinized (10 percent) in a closed high-pressure reactor, then retrograded at 4 °C for 48h and supercritical drying at 40 °C and 10 MPa for 4h The aerogels produced had a large surface area (61 m2 /g) and small pores of 0.18 nm [11]. Corn starch was gelatinized (15 percent) in an autoclave at 120 °C, 0.1–0.2 MPa, retrograded at 4 °C for 48h, and dried by supercritical drying (scCO2 ) for 4h at 40 °C and 11.0– 12.0 Mpa. The aerogels produced had a large surface area (83 m2 /g) and micropores of 100–500 Å [12]. Finally, potato starch was mixed in an emulsion of gelatinized oil-starch solution (5, 7, 7, 10, 15 percent) (phase ratio 2:1) at 120 °C, 0.1–0.2 MPa, retrograded at 4 °C for 48h, and dried by supercritical drying (scCO2 ) for 4h at 40 °C and 11.0Mpa. The aerogels produced had the large surface area (204–230 m2 /g) ever produced among the aerogels compared (to wheat and corn starch [13].

19.3.2 Hybrid starch aerogels The addition of other biopolymers in the aerogel formulation results in the development of aerogels with properties distinct from conventional starch-specific aerogels. Starch hybrid aerogels are produced using techniques similar to conventional starch, with the simple difference in the addition of other polymers [14]. This process modifies its structure, mainly the pores, changing the number of pores formed and the physical characteristics. The added biopolymers modify the barrier structure by acting as structural reinforcement, making the aerogel more elastic and resistant [15]. Physically cross-linked aerogels based on wheat starch and poly (ethylene oxide) were developed and applied as water absorbers for food packaging. Starch poly (ethylene oxide) aerogel was developed with a 6 percent PEO solution as a dispersion. The aerogels showed improved physical and chemical properties compared to the starch aerogel. High degradation temperature, water absorption capacity, and physical integrity were observed. The addition of poly (ethylene oxide) increased the water absorption capacity and reduced the hardness and cohesion of the resulting aerogels [16]. Starch-based aerogels incorporated with agar or microcrystalline cellulose were prepared and dried by supercritical CO2 . The incorporation of agar or cellulose, modulates the properties of starch aerogels, improving properties such as the water absorption capacity and stability in the aqueous medium [15].

19.3.3 Mechanical properties of starch aerogels The mechanical properties, such as flexural strength, compression, and compression modulus, depending on the microstructure of the aerogels, which includes pore size, density, surface area, and porosity. In the case of starch aerogels, good mechanical strength is achieved when

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we have a uniform pore distribution and regularity in size, which is best achieved using the supercritical CO2 drying technique [17]. The addition of crosslinking agents helps to improve the mechanical properties of aerogels, especially by forming a more interconnected, compression-resistant gel network. The crosslinking of starch with glutaraldehyde reinforces the primary structure of the gel, forming hard aerogels, resistant to solubilization in water and also to oven temperatures [18].

19.3.4 Topology and morphology of starch aerogels Starch aerogels, prepared by different techniques, vary in topology and morphology and can be monoliths, films, and microspheres. The size also varies from nanometers to micrometers. The monoliths are obtained by the prefabricated mold, where gelatinized starch is added, acquiring the shape of the mold, followed by drying. The films are also manufactured in molds, however, glass plates are used, where the gelatinized starch is filled, followed by casting. Microspheres are manufactured by the emulsification method [19]. Aerogels generally have structures in homogeneous fibrils, formed during the retrogradation process of the starch hydrogel. The morphology varies depending on the amount of starch used in the production of the aerogel and the gelatinization and drying temperature. Temperature is a critical factor because high temperatures can disrupt the gel network and, during drying, destroy the pore network. SEM images of aerogels show that starch has thick, fibrillar, and homogeneous structures, while aerogels are porous and homogeneous [20].

19.4 Cellulose aerogels and CO2 utilization 19.4.1 Cellulose specific aerogels Cellulose aerogels are formed from a cellulose solution or suspension that forms a gel through polymer agglomeration. Cross-linking is mainly achieved by cellulose chains that contain abundant hydroxyl groups. There are two types of crosslinking, physical and chemical. Physical crosslinking is carried out under temperature and pressure. Chemical cross-linking is done with epichlorohydrin (ECH) and N, N-methylene bisacrylamide (MBA). Drying using carbon dioxide is the most applied, and it works the same way for drying starch. The supercritical fluid causes the dragging of the solvent (alcohol or ketone) opening pores, replacing the solvent with gas, preserving the porosity and superior physical properties of cellulose aerogels [21]. The fixation of compounds in cellulose aerogels is carried out by different techniques. In the case of fixing essential oils and some drugs such as Acetylsalicylic Acid, they are loaded in the drying phase via Supercritical CO2 . Another technique used to fix mainly more alcohol-soluble compounds is the transport carried out by the supercritical fluid itself. In this technique, the bioactive compound is solubilized in ethanol, and only after that, it is dried with supercritical CO2 [21]. Cellulose specific aerogels can be fixed and impregnated by mixing the chemical component with the hydrogel while still in the gelatinization phase. Compounds more sensitive to high temperatures need to be mixed at relatively low temperatures to avoid degradation of

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bioactive. However, the gelatinization of the hydrogel still needs to occur, already with the bioactive compound added, then retrograded and dried [22].

19.4.2 Cellulose aerogels as thermal insulators Cellulose aerogels have flame retardant properties, delaying ignition and slowing down the burning rate. They are applied to reduce the intrinsic flammability of materials, increasing combustion resistance. The flame retardants used by the industry today are halogenated and brominated compounds, materials that are toxic to humans. Cellulose aerogels showed potential to be incorporated in this area as they have the advantages of being biodegradable, non-toxic, and environmentally safe [23].

19.4.3 Hybrid cellulose aerogels Cellulose hybrid aerogels are those that have other biopolymers in the aerogel formulation. First, the cellulose is mixed with distilled water and mechanically stirred to form a clear fluid, then other polymers are mixed into the formulation. Samples are cooled and rectified with alcohol and then freeze-dried or dried with supercritical CO2 . These aerogels have improved properties such as a higher absorption coefficient and still act as a thermal insulator. Cellulose/chitosan aerogel has improved physical holding properties. The addition of chitosan helps in the formation of cross-links, which results in the supporting force of the aerogels, increasing its mechanical resistance to compression [24]. Cellulose hybrid aerogels prove to be useful for use with shielding material, thermal insulation in computer hardware, aerospace construction, and buildings [25]. Another application is in water treatment, acting in the removal of chemical substances such as industrial dyes. With population growth and industrial development, contamination of water resources with dyes has been increasing. One of the most used dyes in the textile industry is Methyl Orange (MO), with carcinogenic properties. One of the techniques used is the use of hybrid cellulose aerogels, which have adsorption capacity due to their high surface area and micropores [26].

19.5 Conclusions Drying using supercritical CO2 is a sophisticated technique that produces unique aerogels with improved properties and better impregnation yields. The quality of the pores produced is one of the most promising characteristics of drying with supercritical CO2 . Some of the most relevant characteristics of supercritical CO2 drying that are better when compared to other techniques are the production of stable micropores, high surface area, high impregnation rate, and formation of more compact polymeric networks. Finally, the use of supercritical CO2 is an environmentally friendly technology, it does not harm the structure of the aerogel, has no solvent residues, and can still be scaled up and replicated.

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Author contributions Jeieli Wendel Gaspar Lima: Conceptualization, Writing and proofreading. Clara Prestes Ferreira: Conceptualization, Writing and proofreading. Jhonatas Rodrigues Barbosa: Conceptualization, Writing, proofreading, supervision and editing. Raul Nunes de Carvalho Júnior: Supervision.

Ethical approval This Commentary does not contain any studies with human or animal participants performed by any of the authors.

Declaration of competing interest The authors declare that they are not aware of competing for financial interests or personal relationships that may have influenced the work reported in this manuscript.

Acknowledgment The authors thank the Federal University of Pará (UFPA), for the space for research development and intellectual production. We also thank the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior – CAPES.

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[13] García-González CA, Uy JJ, Alnaief M, Smirnova I. Preparation of tailor-made starch-based aerogel microspheres by the emulsion-gelation method. Carbohydr Polym 2012;88(4):1378–86. [14] Wu K, Wu H, Wang R, Yan X, Sun W, Liu Y, et al. The use of cellulose fiber from office waste paper to improve the thermal insulation-related property of konjac glucomannan/starch aerogel. Ind Crops Prod 2022;177:114424. [15] Dogenski M, Navarro-Díaz HJ, de Oliveira JV, Ferreira SRS. Properties of starch-based aerogels incorporated with agar or microcrystalline cellulose. Food Hydrocoll 2020;108:106033. [16] da Silva FT, de Oliveira JP, Fonseca LM, Bruni GP, da Rosa Zavareze E, Dias ARG. Physically cross-linked aerogels based on germinated and non-germinated wheat starch and PEO for application as water absorbers for food packaging. Int J Biol Macromol 2020;155:6–13. [17] Nasri-Nasrabadi B, Mehrasa M, Rafienia M, Bonakdar S, Behzad T, Gavanji S. Porous starch/cellulose nanofibers composite prepared by salt leaching technique for tissue engineering. Carbohydr Polym 2014;108:232–8. [18] Wang L, Sánchez-Soto M, Abt T, Maspoch ML, Santana OO. Microwave-crosslinked bio-based starch/clay aerogels. Polym Int 2016;65(8):899–904. [19] Zheng Q, Tian Y, Ye F, Zhou Y, Zhao G. Fabrication and application of starch-based aerogel: technical strategies. Trends Food Sci Technol 2020;99:608–20. [20] Ubeyitogullari A, Ciftci ON. Formation of nanoporous aerogels from wheat starch. Carbohydr Polym 2016;147:125–32. [21] Liu Z, Zhang S, He B, Wang S, Kong F. Synthesis of cellulose aerogels as promising carriers for drug delivery: a review. Cellulose 2021;28(5):2697–714. [22] García-González CA, Sosnik A, Kalmár J, De Marco I, Erkey C, Concheiro A, et al. Aerogels in drug delivery: from design to application. J Control Release 2021;332:40–63. [23] Huo S, Song P, Yu B, Ran S, Chevali VS, Liu L, et al. Phosphorus-containing flame retardant epoxy thermosets: recent advances and future perspectives. Prog Polym Sci 2021;114:101366. [24] Rong N, Chen C, Ouyang K, Zhang K, Wang X, Xu Z. Adsorption characteristics of directional cellulose nanofiber/chitosan/montmorillonite aerogel as adsorbent for wastewater treatment. Sep Purif Technol 2021;274:119120. [25] Zong Z, Ren P, Guo Z, Wang J, Chen Z, Jin Y, et al. Three-dimensional macroporous hybrid carbon aerogel with heterogeneous structure derived from MXene/cellulose aerogel for absorption-dominant electromagnetic interference shielding and excellent thermal insulation performance. J Colloid Interface Sci 2022;619:96–105. [26] Hasanpour M, Motahari S, Jing D, Hatami M. Investigation of operation parameters on the removal efficiency of methyl orange pollutant by cellulose/zinc oxide hybrid aerogel. Chemosphere 2021;284:131320.

C H A P T E R

20 Advances in carbon bio-sequestration Nigel Twi-Yeboah a,†, Dacosta Osei b,† and Michael K. Danquah c a

b

Operations Department, Ghana National Gas Company, Western Region, Ghana Chemical and Petroleum Engineering Department, University of Kansas, KS, United States of America c Department of Chemical Engineering, University of Tennessee, Chattanooga TN, United States of America

20.1 Introduction The emission of greenhouses gases by both natural and human activities into the atmosphere has become one of the largest contributors to global warming. The rate at which industries are multiplying worldwide has exponentially increased these emissions leading to greater depletion of the ozone layer of the earth. This has contributed more to global warming and climate change. According to the Natural Resource Defense Council (NRDC), from the time of the Industrial Revolution, the global annual temperature There has been about a 1 °Celsius or 2 Fahrenheit increment in the average annual temperature worldwide. Accurate recordkeeping for global temperature began in 1880, and from that year till 1980, there was a 0.07 °Celcius rise per decade, after which the rate of increase doubled: There has been a 0.18 °Celcius (0.32° Fahrenheit) annual rise in temperature per decade, worldwide, for the past four decades [1]. As these emissions continue, global warming and climate change intensify, leading to adverse environmental and health conditions. Disasters such as heat waves, floods, droughts and storms are worsened as temperatures go higher and these may lead to loss of lives, properties and agricultural productivity [2]. Respiratory diseases such as asthma and heart diseases are more likely to worsen during high temperatures and dry weathers, as the cardiovascular systems of patients would have to work harder to regulate the body temperature [3].

†

Equal contribution as co-first authors

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TABLE 20.1 Amount of CO2 emissions from 1960 to 2020 (Source of data: carbonbrief.org .pdf) [5]. Carbon Emissions in Gigatonnes of CO2 (GtCO2 ) Location ↓

1960

1970

1980

1990

2000

2010

2020

China

0.80

0.81

1.49

2.48

3.44

8.62

10.67

India

0.11

0.18

0.29

0.58

0.98

1.68

2.44

USA

2.90

4.34

4.81

5.11

6.01

5.68

4.71

European Union

2.10

3.33

4.07

3.86

3.60

3.43

2.60

Rest of World

3.48

6.24

8.83

10.71

11.20

13.95

14.39

Currently, the greenhouse gas that has drawn the highest level of attention is carbon dioxide (CO2 ), not because it is the most toxic gas, but because it is the most emitted greenhouse gas. In the year 2022, statistics released by the United States Environmental Protection Agency indicated that carbon dioxide formed 79 percent of the greenhouse gases released into the atmosphere [4]. The sources of CO2 emissions by sector are mainly the energy sector, agriculture, forestry and land sector, waste sector and industry sector. Table 20.1 below gives a clear image of the significant surge in worldwide statistics of CO2 emissions from 1960 to 2020.

20.2 Carbon sequestration methods There is a general notion worldwide that the complete turnaround in source of energy should be made in the direction of cleaner, alternative energy, since there is close to zero emissions of carbons, associated with their operations. However, the constant increase in global population will still lead to a significant demand for products from petroleum industries both now and in the future. Due to this, the most effective and immediately available method, is carbon sequestration which involves the capture, utilization and storage of CO2 in order to reduce net emissions of the greenhouse gas. Carbon sequestration can be executed using three methods namely, geological carbon sequestration, technological carbon sequestration, and biological carbon sequestration [6]. To begin with, carbon sequestration can be executed through geological means. Geological sequestration involves the storage and utilization of CO2 in rock formations, mainly utilized in the petroleum industries. Through this method, CO2 is stored in reservoir sites deep under the subsurface. This method can either be applied only for storage purposes, or for enhanced oil and gas recovery methods. Carbon dioxide can be injected in storage sites such as unmineable coal seams, saline aquifers and depleted oil and gas reservoirs for storage purposes only [7]. On the other hand, geological sequestration of CO2 may also involve utilization of the gas to recover more oil from reservoirs after the primary production stage. Currently, CO2 enhanced oil recovery methods include gas injection, carbonated water injection, water alternating gas injection, surfactant gas co-injection/ surfactant alternating gas injection, etc. Under both methods, CO2 gas can be trapped through mechanisms namely, stratigraphic

20.3 Limitations of carbon sequestration methods

453

trapping (trapping of mobile gas under an impermeable structure called a seal or cap rock), capillary trapping (trapping the gas residually by drainage and imbibition hysteresis), dissolution trapping (applying the solubility of the gas in formation water/brine to form one phase) and mineral trapping (geochemical reactions between the gas and rock minerals to produce new minerals to trap the gas) [8]. Moreover, carbon sequestration can be executed through technological means as carbon is captured from emissions, stored, transported and used as feedstock for the production of other materials. Carbon is captured from flue gases and syngases from industries using chemical solvents (diethanolamine, methyldiethanolamine, potassium carbonate), physical solvents (glycol, methanol, propylene carbonates, n-Methyl-pyrolidone), physical sorbents (zeolites, activated carbon), and membranes (polymer and ceramic membranes) [6]. After capture, CO2 is stored in pressurized vessels and transported to sites where it can be used as feedstock for other industries for the production of products or for other applications. For example, in the Chemical Industry, CO2 is used for the production of methanol and urea. It can also be used in the food and beverage industry for chilling and refrigeration [9]. Furthermore, another method for sequestering carbon is through biological means. In this method, carbon dioxide is extracted from the atmosphere by living and non-living organisms existing as part of the ecosystem. Agents of biological sequestration include plants [10], soil [11], macro and microorganisms [12] and water bodies [13]. Plants absorb CO2 from the atmosphere for photosynthesis. Macro and microorganisms use CO2 for their organic activities. When these organisms die, they decompose and release carbon either back into the atmosphere or into the soil depending on their proximity to the surface. Carbon dioxide can also be sequestered into oceans through diffusion.

20.3 Limitations of carbon sequestration methods Despite how essential the methods of carbon sequestration are in the reduction of CO2 emissions, there are some limitations and challenges associated with these methods. In geological sequestration of CO2 , the process is met with issues of early gas breakthrough or viscous fingering during gas injection as an enhanced oil recovery method. This occurs as a result of a high mobility ratio since CO2 gas is less viscous and more permeable than oil [14]. Also, there is the issue of gravity segregation which also leads to low sweep efficiency of CO2 resulting in greater production of the gas than the amount stored which makes the process uneconomical. This happens due to the significant difference in gravity between the gas and the oil. Conformance control of the gas is then applied to properly sequester CO2 while producing more oil [15]. In saline aquifers, dissolution of CO2 in brine creates carbonic acid which can lead to mineral dissolution of the reservoir and possibly the cap rock [16]. This occurrence, if not controlled, could lead to leakage of the stored CO2 under high pressure. This can create serious environmental conditions right from the reservoirs to the water bodies above them. Technological carbon sequestration also comes with its challenges. One of the main issues involved in industrial carbon sequestration is capital intensity during carbon capture, storage and utilization as feedstock for product manufacturing. Due to this, there has to be a significant generation of income for companies involved in technological sequestration of carbon thus,

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large markets for products from such industries need to be constantly active for this method to thrive [17]. Another great limitation to the technological carbon sequestration method falls in the aspect of transportation. Since CO2 is usually kept in pressurized containers or transported through pipelines under pressure, a sharp change in environment poses great danger to the location for storage or the path for transportation [18]. Leakages due to poor maintenance of storage vessels and transportation pipelines and adverse weather conditions can also pose danger to the health of people and animals in the surroundings. The biological method has its share of challenges as well when it comes to carbon sequestration. Taking sequestration of CO2 in oceans for instance, a high concentration of CO2 in water bodies leads to the creation of carbonic acids which may not be suitable for aquatic lives mostly animals. For example, increasing ocean acidification creates more bonds between carbonate ions present in the water bodies and hydrogen ions, leaving behind fewer ions for calcifying organisms to construct their structures such as shells and skeletons. Further acidification may lead to dissolution of already existing carbonate structures in these organisms [19]. Aquatic plants however benefit more from higher concentrations of CO2 in water bodies as they are required for photosynthesis. Other biological agents of CO2 sequestration, mainly living organisms, have the tendency to re emit the gas back into the atmosphere after expiration and decomposition. However, this issue is not so significant since other living organisms recapture the released gas for other purposes, forming a cycle. Since the biological sequestration process mostly occurs through natural means, there is an almost negligible impact on the environment.

20.4 Overview of biological sequestration (Cycle/Mechanism) As mentioned in previous chapters, biological carbon sequestration occurs mainly due to acts of nature creating the carbon cycle. This cycle consists of natural activities by plants, animals, soil, water bodies, microorganisms, etc. that exist in the ecosystem. Plants are autotrophs, meaning, they prepare their nutrition and other organic materials using inorganic substances. Plants use carbon dioxide present in the atmosphere to prepare their own food through photosynthesis. The stomata of the leaves absorb CO2 from the environment while the chlorophyll absorbs sunlight. These are combined with mineral salt and water for the production of glucose and oxygen [20]. Below is a balanced chemical Eq. for the conversion of CO2 to glucose and oxygen through photosynthesis. 6CO2 + H2 O + Light energy → C6 H12 O6 + 6O2 Also, living organisms (plants and animals) undergo respiration which is one of the life processes. Humans and animals use their respiratory systems for exchange of gases whereas plants use the stomata in their leaves [21]. Respiration, the breakdown of food substances to release energy, comes in two forms namely, aerobic (with oxygen as a reactant) and anaerobic respiration (without oxygen as a reactant) to breakdown food substances (glucose) with energy being released as a product, in the form of adenosine triphosphate [22]. The amount of energy released during aerobic respiration is in large quantities whereas the amount released in anaerobic respiration is in smaller quantities. In both mechanisms, carbon dioxide is released as a product, hence contributing to carbon emissions by biological means. The following are

20.5 Bioresources for carbon bio-sequestration

455

balanced chemical equations for both aerobic and anaerobic respiration processes. C6 H12 O6 + 6O2 → 6CO2 + 6H2 O + Energy Aerobic Respiration C6 H12 O6 → 2C2 H5 OH + 2CO2 + Energy Anaerobic Respiration Again, through photosynthesis processes, plants can store CO2 in the soil as well. Other autotrophs such as algae, bacteria, and fungi, present in the soil also require carbon dioxide for metabolic activities such as respiration and nutrition. When living organisms such as plants and animals expire, they decompose and release carbon back to the atmosphere. Living organisms that reside in soils may release carbon into the soil after expiration and decomposition, depending on how deep they are in the soil subsurface [23]. The National Oceanic and Atmospheric Administration (NOAA) reports that, about 25 percent of carbon dioxide released into the atmosphere is absorbed into certain parts of the ocean. The colder parts of the ocean have a higher tendency to absorb and store CO2 than the warmer parts of the ocean. The absorbed carbon dioxide is stored in the ocean in a dissolved state to form carbonic acids, bicarbonates, and carbonates. The chemical equations. below show the conversion of CO2 into these dissolved states [24]. CO2(aq ) + H2 O → H2 CO3 − − − − − − − Carbonic Acid H2 CO3 ↔ H+ + HCO3− − − − − − − − Bicarbonate HCO3− ↔ H+ + CO2− 3 − − − − − − − Carbonate The exchange of CO2 between water bodies and the atmosphere occurs as a result of concentration differences of CO2 between both media. This is often described as a flux. A flux can either be termed as a negative flux, which implies the absorption of atmospheric carbon dioxide into the oceans or a positive flux, which implies the release of previously dissolved carbon dioxide in the oceans, to the atmosphere and surroundings. Aquatic lives also serve as agents of CO2 exchange in water bodies. Respiration by these living organisms release CO2 into the environment whilst CO2 is absorbed in dissolved forms by aquatic plants to be used for photosynthesis [25]. Fig. 20.1 below shows the cycle of carbon transfer in the biological sphere (Fig. 20.2).

20.5 Bioresources for carbon bio-sequestration Geo-microbiological research has prioritized CO2 sequestration from the atmosphere by various bioresources by virtue of its potential as an alternative to other known sequestration processes to alleviate climate or global warming. This process could be upgraded by the use of Precambrian to sequester carbon through the production of minerals referred to as biomineralization. In this regard biomineralization of Mg and Ca carbonates by the formation of stromatolites by cyanobacteria, which qualifies as one of the bioresources for bio-sequestration process. Microalgae, plants and bacteria are equally relevant bioresources to alleviate the huge amount of atmospheric carbon dioxide.

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FIGURE 20.1 Biological carbon sequestration cycle.

FIGURE 20.2 Sequestration sentation of carbon microalgae [30].

repredioxide by

20.6 Cyanobacteria Cyanobacteria is one of the most promising fields of geo-microbiological research. It aims at capturing and sequestering the high amounts of at atmospheric carbon dioxide. Ganta et al. (2018) investigated the sequestration of carbon dioxide using eubacteria, a cyanobacteria strain to be specific. The specie, Synechococcus, a cyanobacteria that has the ability to photolyze H2 S and H2 O was used for the sequestration process specifically, NIT18. Due to the effectiveness

20.8 Plants

457

of sodium carbonate to initiate a bioconversion process of CO2 and a higher production of biomass, it was used as the source of inorganic carbon. Their work focused on sequestering CO2 with the use of the Synechococcus sp. NIT18, a strain of cyanobacteria. To understand the impact of different conditions and sensitivity of factors involved, this process was undertaken under different conditions and/or factors. Some factors that were considered were concentration of CO2 which ranged from 5–20 percent and pH range of 7–11 [26]. The concentration of CO2 in flue gas has been reported to be within the range of 10–15 percent and sequestration by the cyanobacteria sp. occurred at an initial CO2 concentration of 10 percent. The bacteria could be of great importance moving forward to alleviate the excess CO2 in the atmosphere.

20.7 Microalgae For exploitation of the energy sector and its development, the use of microalgae for carbon dioxide sequestration has been identified and looks encouraging for carbon bio-sequestration processes. Microalgae are microorganisms that have been grouped in various groups and vary with respect to their physiology and morphology and have the ability to grow at different environmental conditions [27]. Low productivity in this field, however, restricts its large-scale development. A study of the microalgae’s effectiveness to initiate the hatch-slack pathway or carbon fixation is imperative for a breakthrough in this research. First and foremost, biochemical mechanism of microalgae carbon is studied to give its biological properties, specifically methods in genetic engineering to enhance the process of sequestration. This process of carbon dioxide sequestration by microalgae is worthwhile and among the most potent sequestration methods in the world that is equally economically feasible and reliable fixation of carbon dioxide [28]. One proven sustainability support to this research is the conversion of CO2 into renewable energy that is produced by microorganisms via the process of photosynthesis. This is an indication of the fact that microalgae can alter the production of the CO2 and fix it [29]. Microalgae has been a proven bioresource for carbon bio-sequestration. Its use is thought of as a method of conversion of energy by biological properties. This process is referred to as a method of capture of carbon dioxide from various springs. While the process is ongoing solar energy is absorbed and converted to biomass [30].

20.8 Plants Plant as a bio-resource has provided a promising source to reduce CO2 emissions into the atmosphere and sequester a significant amount. Bhattacharya et al. (2020) investigated the sequestration of carbon dioxide using the plant, Nicotiana benthamiana, a close relative of tobacco, in India. They realized how pollution caused by transport vehicles has brought about the importance to escalate concerns regarding carbon dioxide emissions and proposed a trapping process through a range of soil conditions that impacts the lives of organisms in the soil. These factors are often referred to as bio-edaphic. To estimate the plant’s carbon available and some physiochemical properties, different samples of soil were used which were taken at

458

20. Advances in carbon bio-sequestration

various degree deep into the soil. Also, Plants with different diameters were used which were measured at a point around its trunk [31]. It has been reported that Nicotiana benthamiana, colloquially known as benth or benthi Is commended for the sequestration of carbon dioxide. This is due to its ability to store the most carbon and also being a tolerant specie. Plants as a bioresource for carbon biosequestration like other microbiological approaches undoubtedly look appealing but factors in this research play a vital role for upscaling. Environmental conditions under which the Nicotiana benthamiana or benthi will thrive for maximum CO2 sequestration is vital for future work and upscaling.

20.9 Bacteria The use of enzymes for carbon capture processes has become a good technological alternative for bio-sequestration of carbon dioxide. This is a promising aspect of research in a sense that, it is environmentally friendly and therefore has great support from EPA. However, these technologies used for reducing the carbon dioxide emissions into the atmosphere are not sufficient to sequestrate the desired amount. In view of these, recent improvement has been done on the sequestration of atmospheric CO2 using a gram-positive bacterium. The bacterium used was Bacillus tequilensis 401 [32]. Current technologies to alleviate the amount of atmospheric carbon dioxide has sprouted and give rise to the search of some bacteria to effectively undertake this method. The bacillus used was singled out and made to adjust to an environment. This process was targeted at producing urease and carbon anhydrase which is an enzyme for the carbon capture process. The enzyme production was to speed up the process of sequestration. Various factors were considered for the optimization of the sequestration process. Amongst these factors were, percentage of carbon dioxide, concentration of the bacteria used and temperature. The outcome gave a clear indication of a significant efficacy of the carbon dioxide sequestration method and a highest carbonation was recorded at a carbon dioxide percentage of 20 [32]. This process indicated effective sequestration process by the use of Bacillus tequilensis 401. At optimal carbonation depth of 20 percent CO2 , there is a good indication of sequestration of releases within such concentration.

20.10 Nanomaterials in carbon sequestration Nanomaterials that are local and renewable often referred to as green nanomaterials have become a stand-in for other sequestration processes which may not be regarded as environmentally friendly and sustainable. The use of nanomaterials like some bioresources have gained attention for their contribution towards green environment. This method provides sustainable route for reducing the huge amounts of carbon dioxide emitted into the atmosphere. Green materials have been a good source for adsorbents. The use of solid-phase adsorbents has been of great focus to capture atmospheric carbon dioxide and sequester them. These adsorbents include a wide range of solid-phase adsorbents. Some of these are zeolites and mesoporous silica. Also, the inclusion of nanotubes from halloysite and carbonic

20.12 Conclusion

459

anhydrase (CA) are some significantly studied nanomaterials for carbon dioxide capture and sequestration applications [33]. With industrialization geared towards minimum energy requirements and maximum capacity, these processes have been modified through synthesis, functionalization, characterization and carbon dioxide (CO2 ) absorption and/or desorption performances.

20.11 Future perspectives Regardless of the feasibility and sustainability of carbon bio-sequestration processes, there are some shortcomings and future prospects and perspective that need to be studied to demonstrate on largescale basis. The use of microalgae for carbon dioxide capture and sequestration is considered an external activity. In view of this, many external factors which are uncertain could affect outdoor processes, for example, maintaining the temperature of the culture and lightning activity to see to its steadiness. In view of this, understanding the growth of these microalgae naturally will be imperative to this research. That is to say, the adaptation of change in external conditions to microalgae needs to be studied to replicate this process on industrial basis and not restricted to the labs. There is an interdependence of macro and micro factors that need to be considered and delved into to help replicate this research for upscaling to industrial level. Again, the use of Bacillus tequilensis 401 showed significant sequestration process of CO2 with optimization in temperature, concentration and concrete density, however these were done on a small-scale basis. Future economic analysis should be delved into to analyze the cost involved on industrial basis. This economic analysis and possible equipment replacement value analysis, where possible would help juxtapose these carbon bio-sequestration processes with known geological sequestration and others. The use of plants for carbon bio-sequestration requires careful economic analysis. Moreover, environmental factors are recommended to be considered in this regard to know and understand the conditions under which a plant with good carbon dioxide potential will thrive.

20.12 Conclusion Geological sequestration of carbon dioxide has been one of the most applicable methods [34]. However, it is evident that other alternative technologies would be imperative to meet or maintain EPA standards for CO2 emissions. According to reports, carbon dioxide emissions recorded an increment from 300 to 400 ppm which represents 25 percent has been reported and research anticipate a rise in some years to come. These statistics prove a vital need for alternatives to sequester some amount of atmospheric CO2 . Geo-microbiological research has prioritized bioresources as an alternative to known sequestration methods. The use of bioresources like bacteria e.g., the Bacillus tequilensis was optimized by temperature, concentration CO2 percentage and concrete density to capture CO2 . Cyanobacteria was also demonstrated to be a favorable bioresource for capture of carbon dioxide and its sequestration with its optimum capture at 10 percent concentration of carbon dioxide which is a good representation for capture of flue gas with a reported carbon dioxide

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concentration range of 10–15 percent. Plant, specifically, the Nicotiana benthamiana or benth has been commended to be planted. This is due to the fact that, benth is reported to have a very good storage ability of carbon dioxide. Again, the use of Nicotiana benthamiana was suggested due its tolerance according to research. Finally, microalgae have also shown promising capture in this technological phase of carbon dioxide capture and sequestration. Carbon bio-sequestration has proven to be environmentally friendly and sound alternative to carbon dioxide capture and sequestration.

References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]

[17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27]

Dauncey G. The Climate challenge: 101 Solutions to Global Warming, 1. New Society Publishers; 2009. Denchak, M. Natural Resource Defense Council; 2022. Kasotia, P. United Nations Chronicle; 2020. EPA, U. Greenhouse Gas Emissions; 2021. Hausfather, Z. Carbon Brief, Clear on Climate; 2021. Muradov N. Low-carbon production of hydrogen from fossil fuels. Compendium of Hydrogen Energy. Woodhead Publishing Series in Energy; 2015. p. 485–522. Kim TW, et al. Assessment of oil and gas fields in California as potential CO2 storage sites. Int. J. Greenhouse Gas Control 2022;114. Sun Q, et al. Assessment of CO2 trapping mechanisms in partially depleted oil-bearing sands. Fuel 2020;278. Nexair. Carbon Dioxide in Food and Beverage; 2021. Oberle B. Carbon Relations, the Role in Plant Diversification of. Encyclopedia of Evolutionary Biology 2016:260–6. Heckman K, Rasmussen C. Role of Mineralogy and Climate in the Soil Carbon Cycle. Developments in Soil Science 2018;35:93–110. Cristina S, et al. Soil microbe contributions in the regulation of the global carbon cycle. Microbiome Under Changing Climate 2022:69–84. Prairie YT, Cole JJ. The Carbon Cycle in Lakes: a Biogeochemical Perspective. Encyclopedia of Inland Waters (Second Edition) 2022;2:89–101. Massarweh O, Abushaikha AS. A review of recent developments in CO2 mobility control in enhanced oil recovery. Petroleum 2021. Pal N, et al. Carbon dioxide thickening: a review of technological aspects, advances and challenges for oilfield application. Fuel 2022;315. Pearce JK, et al. A combined geochemical and μCT study on the CO2 reactivity of Surat Basin reservoir and caprock cores: porosity changes, mineral dissolution and fines migration. Int J Greenhouse Gas Control 2019;80:10– 24. Monteiro J, Roussanaly S. CCUS scenarios for the cement industry: is CO2 utilization feasible? Journal of CO2 Utilization 2022;61. Ruby J. CO2 separation, capture, and transport issues for the west coast regional carbon sequestration partnership. Greenhouse Gas Control Technologies 2005;II:1345–50 7. Bennett, J. Ocean; 2018. Dubey R. Photosynthesis in Plants under Stressful Conditions. Handbook of Photosynthesis 2018:21. Kimball, JW. Gas Exchange in Plants; 2022. Abedon ST, et al. Respiration. Encyclopedia of Ecology 2008:3010–20. Panchal P, et al. Soil carbon sequestration by root exudates. CLIMATE CHANGE AND SUSTAINABILITY II 2022;II. Feely RA, et al. Uptake and Storage of Carbon Dioxide in the Ocean: the Global CO2 Survey. Oceanography 2001;14. NOAA. Ocean-Atmosphere CO2 Exchange; 2015. Ganta, U, et al. Sequestration of carbon dioxide and production of biomolecules using cyanobacteria; 2018. 218: p. 234–244. Shovon, M and M Nirupama. Bioenergy Research: advances and Applications; 2014.

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[28] Sialve B, Bernet N, B O. Anaerobic digestion of microalgae as a neccessary step to make microalgal biodiesel sustainable. Biotechnol. Adv. 2009. [29] Polakovicova, G, P Kusnir, and S Nagyova. Process integration of algae production and anaerobic digestion; 2012. [30] Xianzhen, X, et al. Progress, challenges and solutions of research on photosynthetic carbon sequestration efficiency of microalgae; 2019. [31] Bhattacharya, A, et al. Carbon sequestration in the bio-edaphic ecosystem of National Highway-27 in Guwahati, Assam, India; 2020. 6(9). [32] Abdulla, FA, et al. Optimisation of carbon dioxide sequestration into bio-foamed concrete bricks pores using Bacillus tequilensis; 2021. 44(101412). [33] Yurdacan, HM and MS Mufrettin. Functional green-based nanomaterials towards sustainable carbon capture and sequestration; 2021: p. 125-177. [34] Pan, SY. CO2 capture by accelerated carbonation of alkaline wastes; 2012: p. 770-791.

C H A P T E R

21 Photosynthetic cell factories, a new paradigm for carbon dioxide (CO2) valorization Bijaya Nag, Abdalah Makaranga, Mukul Suresh Kareya, Asha Arumugam Nesamma and Pannaga Pavan Jutur Omics of Algae Group, Industrial Biotechnology, International Centre for Genetic Engineering and Biotechnology, New Delhi, India

21.1 Introduction It has been estimated that the global population will grow from ∼8 billion in 2020 to 10 billion by 2050, such an increase is predicted to increase the energy demand by 80 percent and food demand by more than 65 percent [1,2]. Overutilization of natural reserves and change of biological cycles that support life in the biosphere have fuelled the global economy over the past two centuries [3]. Such an exploitation could have a detrimental impact on food availability, increase health risks, and increase the likelihood of natural disasters, among other things such as degraded ecosystems may never improve naturally [4]. In addition, greenhouse gases (GHGs), especially CO2 emission, has led to a considerable increase in the global warming, and efforts are required to reduce the emission of GHGs [5]. The recent surge in petroleum resource usage and deforestation is a consequence to the tension to meet rising requirement for energy sources, food, and other goods. These unsustainable behaviours increase the emissions of GHGs from human activities, which are the main contributors to climate change. [6,7]. GHG emissions are predicted to rise by more than 50 percent by 2050, owing primarily to a 70 percent increase in energy-related CO2 emissions [8]. Nevertheless, natural resources are limited, as a result, a shift to sustainable energy production is required [9]. It is critical to minimise fossil fuel and carbon emissions while enhancing segregation of carbon and fixation in aquatic and other ecosystems in order to attain carbon neutrality and achieve sustainable development goals (SDGs) laid by the United Nations (UN) [10,11].

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21. Photosynthetic cell factories, a new paradigm for carbon dioxide (CO2 ) valorization

More than 24 gigatons [11] of CO2 are produced annually as a result of the use of fossil fuels, resulting in a large increase in atmospheric CO2 concentration over the last century. The 21st Conference of Parties (COP21) of the United Nations Framework Convention on Climate Change (UNFCCC) has become a significant turning point in the global community’s efforts to cut CO2 emissions by 2030 [12,13]. Moreover, each of the negative emission technologies (NETs) must be assessed for its practicality, cost, acceptability, and utility in reducing global warming and its impact on the ecosystems [2,11,14]. Various potential solutions for increasing carbon capture and utilization (CCU) from the surroundings through chemical technologies and carbon storage in terrestrial and aquatic ecosystems are studied to counteract climate change [14,15]. As a result, renewable and sustainable energy is needed to meet the world’s expanding energy demands while also cleaning the environment by lowering CO2 levels in the atmosphere. Due to their high CO2 collecting efficiency, photoautotrophic microorganisms, particularly microalgae and cyanobacteria, have appeared as a viable means to sustain an equilibrium between nutrition, fuel, and the atmosphere [14–16]. The energy generated by these photosynthetic cells that produce oxygen does not necessitate a change in land usage. Notwithstanding the circumstance that it promises to provide energy in an ecologically ethical manner, commercialisation of such non-exhaustible energy remains elusive due to its high industrial costs [14,17,18]. As a result, to tackle the current issue of rising CO2 levels as a substitute supply for lowering the usage of fossil fuels, it is vital to develop sustainable solutions [19–21]. Microalgae have been around for more than 190 million years and are crucial for carbon fixation and the development of environmentally friendly products. Biological CO2 capture utilising microalgae has been considered as a viable new strategy for lowering CO2 emissions, compared to chemical absorption and geologic sequestration [22]. Several microalgae species have shown to be effective at reducing carbon dioxide emissions. Furthermore, light, pH, temperature, CO2 concentration, and cultural circumstances influence microalgal CO2 uptake for the production of biomass, pigments, proteins, lipids, and all high-end compounds [23– 25]. Microalgal cell factories play an essential part in recycling and absorbing natural CO2 , and convert it into biomass, because to their rapid growth rates, exceptional environmental adaption, and use of non-arable soil. Carbon makes over half of microalgal biomass, and one kg of biomass can fix 1.83 kg of CO2 . Microalgae can also consume CO2 and create biomass for use in value-added products and biofuels [26,27]. For example, supplementing Chlorella vulgaris with ∼5 percent (v/v) CO2 increased biomass productivity from 830 mg L−1 d−1 to 1510 mg L−1 d−1 [28]. The growth of microalgae in flue gas is usually more challenging than in the atmosphere. Flue gas CO2 concentrations in power plants are typically 10–20 percent, although the ideal CO2 concentration for algae development is less than 10 percent [29]. Microalgae evolve their physiological mechanism and the cross-talk of metabolic pathways for cellular growth, i.e., 3–5 percent photosynthetic efficiency to fix CO2 and 6–12 percent distributed carbon to synthesize the energy resources, even though the supplementation of CO2 is adequate in the media [30,31]. Numerous tactics have been used for the carbon storage and utilization in microalgae, such that the carbon flux is dynamically diverted in these photosynthetic cell factories. Microalgae have been grown in photoautotrophic conditions, especially, at higher CO2 concentrations to stimulate the uptake of excess CO2 (mainly at industrial scale) [31]. Supplementation of flue gas containing 3–18 percent CO2 into bioreactors and ponds has been shown to have various

21.2 Carbon capture, utilization and storage mechanism

465

impacts on different microalgae species [32,33]. The characteristic biological mechanism of these photosynthetic cell factories of fixing the carbon in various cellular molecules such as proteins, lipids, carotenoids, etc., has been utilized tremendously in various studies for the carbon capture and utilization [14,18,34–37]. Furthermore, according to reports, Chlorella vulgaris is an ideal option for carbon sequestration, capable of fixing approxiamtely 38.4 mg L−1 d−1 at a CO2 concentration of 6 percent and producing 0.2 g L−1 of biomass. Another study found that Scenedesmus obliquus could fix CO2 and produce roughly 2.3 g L−1 of biomass while sustaining around 15 percent CO2 [14,35,38]. The chapter focusses on different strategies for the process of capturing, storage and utilization of CO2 . Certain physical and chemical processes for the carbon capture while having a primary emphasis on biological mechanism i.e., photosynthetic microalgal CO2 utilization has also been outlined. The mechanism of CCU and products obtained from photosynthetic mechanism also outline the challenges and future perspectives of CCU. Technological breakthroughs, techniques, and tactics for a deeper comprehension of molecular processes and their mechanism to boost algal productivity through CCU has been highlighted. In conclusion, the chapter demonstrates that the global bioeconomy needs to be dynamic in a sustainable process in order to maximize carbon utilization to minimize carbon emissions and accomplish the roadmap of SDGs.

21.2 Carbon capture, utilization and storage mechanism In order to achieve the global goals, almost every international climate change issue, under the 2015 Paris Agreement reflects the need for a massive uplift in the process of CCU and storage technologies [1,3,5,12,20,25,33]. In order to reduce the amount of CO2 in the atmosphere, industrial carbon management must acquire CO2 emissions from fossil fuel power plants and the cement industry. [39,40]. Carbon capture is beneficial in three distinct ways, which can be outlined as economically stimulating within the carbon markets, saving the expenses from environmental as well as humanitarian catastrophe caused by rampant climatic change and converting a harmful waste product into value-added products and sustainable energy [41] (Fig. 21.1). This chapter focuses on the different mechanisms that can harness carbon dioxide emitted in the atmosphere, namely; pre-combustion capture, post-combustion capture, and oxy-fuel combustion capture applicable in large power plants [39,41]. Carbon capture occurs through mainly three stages, i.e., carbon capture at the power plants, CO2 transportation and CO2 sequestration. In all the above-mentioned strategies different combustion temperature, pressure, varied oxygen concentration, are applied, and each of these has a powerful effect on the quantity of CO2 emitted from the power plant [5,39,41].

21.2.1 Pre-combustion capture In pre-combustion technique, carbon is separated from the fuel before it gets fully combusted. The conversion of carbon containing fuel into syngas (mixture of CO and H2 ) marks the important step of pre-combustion. The carbon fuel is partially oxidized with air or O2 with or without steam under high temperature (200–450 °C) and pressure of 14–17 bar for the

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21. Photosynthetic cell factories, a new paradigm for carbon dioxide (CO2 ) valorization

FIGURE 21.1 Carbon capture and utilization through chemical and biological carbon sequestration.

formation of syngas [25,39,40]. The syngas is cooled down and the other dissolved impurities like H2 S, HCl, Hg and COS (carbonyl sulphide) [42,43]. This process is followed by water gas shift reaction (Eq. (21.1)) by altering CO with steam with formation of CO2. The target is to raise the percentage of CO2 so that it can be readily separated in the later stages along with production of H2 . This additional H2 can serve as a decarbonised fuel, or to generate electricity through a gas turbine and also expended in thermal power plants or manufacturing units as a resource of green energy. H2 O is produced as the final combustion product. The high CO2 concentration with this method facilitates successful CO2 capture and lastly dehydrated and compressed for further transport and storage [42–44]. This technique of carbon capture is called Integrated Gasification Combined Cycle (IGCC). CO + 3H2 O CO2 + H2 (H2 = −41 kJ mol−1 )

(21.1)

A brief discussion of the other processes in pre-combustion are given below: 21.2.1.1 Physical solvent process Physical solvents like Selexsol, Rectisol or Propylene Carbonate are used to selectively absorb CO2 without any chemical treatment under high pressure. It results in the formation of a weakly associated complex from which CO2 can be readily stripped out by lowering the pressure. A number of factors like temperature, solvent, CO2 partial pressure in the gas stream

21.2 Carbon capture, utilization and storage mechanism

467

influences the absorption of CO2 , where low temperature and high partial pressure is favoured [45,46]. 21.2.1.2 Pre-combustion sorbent This procedure is applicable for treating carbon dioxide entrapment from syngas via gasification of hydrocarbon (coke, coal, natural gas etc.). Lithium silicate materials are employed as sorbents for CO2 under high temperature (250–550 °C) and pressure 0–20 atm and at CO2 concentration of 2–20 percent . Though this process is still under development, it shows promising hopes of securing more than 90 percent of CO2 acquisition from the produced syngas. Another advantage of this technique is that lithium silicate can withstand high pressure and has commendable regenerative power [47,48]. 21.2.1.3 Chemical looping combustion Chemical looping combustion is a potential technology for burning fossil fuels that employs solid-state oxygen carrier materials, most often transition metal oxides. CO2 and water vapour are the end products of combustion, out of which CO2 can be easily concentrated [49].

21.2.2 Post-combustion capture The post-combustion pathway is the process of capturing and separating dilute CO2 from flue gas in an oxidant environment [50,51]. The system for post-combustion capture can be retrofitted to prevailing huge point sources of fossil fuel power plants, cement producing companies, or refineries due to the fact that these supplies are the primary source of carbon dioxide emissions in the environment [52]. Before CO2 capture, exhaust fuel gas emissions (NOx, SOx, water vapour and particulate matter) are denitrified and de-sulphurised, as well as cooling off and eliminating dust are carried out to avert solvent deterioration [53]. The flue gas is cooled to 40–60 °C and treated with the solvent (usually amine based) in a countercurrent manner in the absorber. The CO2 gets specifically absorbed by the amines as much as above 85 percent , emitting the residual oxygen and nitrogen into the atmosphere. The CO2 is stripped from a CO2 -rich amine with steam, allowing the lean amine to be returned to the absorber while providing a concentrated CO2 stream. The renewal of the solvent necessitates a process which is exhaustive in nature because of its characteristic stability. Monoethanolamine is a widely used solvent in commercialized process of post combustion. A CO2 recovery of 80–90 percent can be possible through this process [54]. The CO2 is then transferred for accommodation into geological reservoir or saline aquifers [43]. One disadvantage of this chemisorption is that it imposes heavy operational costs, membrane separation can be used as an alternative method as it consumes lower energy, low capital costs, reduces the carbon footprint and easy retrofitting and scaling up with present power plants. Other methods, like solid sorbents such as calcium oxide, pressure swing adsorption, have all been experimented. 21.2.2.1 Pressure swing adsorption PSA is a cyclic adsorption method that permits for the constant partitioning of a specific component from gas stream under the effect of pressure, based on molecular properties and affinity for an adsorbent material. Though it comprises a number of steps, columns, and

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21. Photosynthetic cell factories, a new paradigm for carbon dioxide (CO2 ) valorization

cycle time, the main four steps are: pressurization, feed, blowdown and purge [55]. Initially, CO2 from a mixture of gases passing through the packed bed at high pressure and low temperature is adsorbed on the surface of the adsorbent, with the adsorption process reaching its equilibrium conditions at the exit point of the packed bag. The beds are restored for further cycles of adsorption [43,55]. 21.2.2.2 Temperature swing adsorption TSA is a process that switches the temperature of the bed between the adsorption and desorption processes by passing flue gas through the bead. At first specific adsorption of CO2 takes place on the adsorbent till equilibrium state is achieved, later on desorption of flue gas takes place at high temperatures with heat supplied. The regenerated bed is cooled down before the start of another round of cycle. TSA costs higher than PSA due to heat requirement, industries usually use a combination of both for better output [39,40]. 21.2.2.3 Calcium looping The term calcium looping refers to a group of CO2 capture systems that use CaO as a solid sorbent and are based on reversible reactions. 

CaO + CO2 → CaCO3 Hr = −178.8 kJ mol−1 The CO2 is captured into the carbonator where it connects with the CaO introduced in the calciner, along with CaO back to the carbonator to end the looping. An uninterrupted stream of CaCO3 is supplied into the structure to counterbalance for the deterioration in the CO2 carrying capacity of the CaO with each subsequent carbonation-calcination cycle; a liquidate is also removed to prevent inert gases from accumulating [56].

21.2.3 Oxy-fuel combustion Oxy-fuel combustion is the process of burning fossil fuel in the presence of pure oxygen, resulting in nitrogen-free flue gas generation with only CO2 and H2 O. The use of O2 instead of air in the combustion chamber improves combustion efficiency, reduces energy utilization, and creates a high CO2 concentration stream (higher than 80 percent ) that allows for easier refinement via., mechanical separation based on the alteration in reducing temperatures rather than absorption-based separation [57]. High flame temperature is required in oxy-fuel carbon capture, in order to bring down the temperature to reasonable levels to prevent damage to the equipment some of the flue stream is recirculated. By condensation of the exhaust steam, concentrated CO2 can be further processed to compress, transport and stored [58].

21.2.4 Carbon capture by microalgae Microalgae can be categorised as eukaryotic microbes capable of performing photosynthesis, which naturally contributes to bring down the CO2 level in the atmosphere. Microalgae is gaining popularity as a medium of mitigating CO2 emissions by power plants, automobiles, industries and volcanic eruptions [14,16,18,23,24,35]. By virtue of their normal diffusivity; these photosynthetic organisms have acquired carbon concentrating mechanism (CCM), that is beneficial for biologically induced carbon capture. The three pathways discovered in microalgae for this carbon capture are: dynamic, direct absorption of HCO3 − , an active transport

21.3 Biological mechanism of carbon capture

469

process, and by several enzymes external to the plasma membrane [27,29,36,41]. A number of factors affect the photosynthetic efficiency, and hence carbon fixation by microalgae like light, CO2 concentration and nutrients. Certain physico-chemical aspects such as pH, temperature, salinity and turbidity also influence carbon capture by microalgae. Microalgal photosynthesis, cell composition, and metabolic pathways are all greatly influenced by light intensity and photoperiod [36,41]. CO2 concentration tolerance varies from one species to another. Usually at higher concentration of CO2 , growth of microalgae is affected by acidification of medium. pH plays a central role as it has been studied that most microalgae species grow well within the pH range of 7–8.4 when most of the inorganic carbon is present as bicarbonate ions, thus regulating its nutrient uptake, photosynthetic activity and carbon sequestration ability [23,24,33]. Alkaline pH accelerates the CO2 absorption by making free CO2 available in the medium, suitable of high-CO2 tolerant microalgae [59]. Temperature is another vital factor that is inversely related with carbon dissolution. Optimal range for microalgae growth is around 0–30 °C, above 40 °C the metabolic activity and photorespiration gets hampered whereas lower temperature disrupts the carbon sequestration process [16,35]. The CO2 utilised by the microalgae as an inorganic source of carbon is converted into organic carbon constituents like lipids, proteins, carbohydrates, pigments and phenols photosynthetically. The carbon molecule in CO2 is ultimately getting stable to the molecular structure of these materials, and utilized during various biological functions and help in biomass production [37]. Furthermore, microalgae can fulfil its nutritional requirements (CO2 , NOX, SOX, inorganic and organic carbon, N and P) from unused gases from flue steam and other contaminants from farming, engineering and drainage wastewater transforming them into bioenergy sources, value-added products and environment-friendly products [14,18,29,38]. Various recent studies have projected the constructive influence of cultivating microalgae under high absorptions of Ci in the kind of CO2 without any impurity, actual or reproduced flue steam or bicarbonate, documenting higher bio-fixation and biomass generation [60,61]. Microalgae is reported to theoretically transform 513 tons of CO2 per hectare per year into ∼300 tons of dry biomass per year, utilising around 10 percent photosynthetically active radiation to sequester CO2 in open as well as closed culture system [60,61].

21.3 Biological mechanism of carbon capture The progressive rise of CO2 in today’s has also amplified the development appropriate sequestration technology to reduce many greenhouse gases from various sources, ranging from biochemistry of CO2 alleviation methods are energy-intense and expensive procedures and the carbon credits trading incentive was recently ratified to be created under the Kyoto Protocol [62–64]. Green plants can capture CO2 through photosynthesis. As a result of the slow growth rates of conventional terrestrial plants, CO2 capture potential of terrestrial plants is estimated to be only 3–6 percent of total conventional fossil fuel emissions. As a matter of fact, there is no one sustainable approach to extract CO2 by the current technologies available [62,63]. On the other hand, microalgae have attracted much attention as a strategic alternative that associates both environmental and economic interest [65–67]. The possibility of transforming biochemical content from microalgal biomass into biodiesel and bioethanol has already been reported in several studies. Microalgae have the capacity to assimilate CO2 while sequestering sunlight with an efficacy of 15–45 times and doubling their biomass at

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21. Photosynthetic cell factories, a new paradigm for carbon dioxide (CO2 ) valorization

much higher rate than plants in less than one day [62–64]. Other processes, namely, geological and chemical, have their own limitations due to safety and environmental concerns during CO2 capture from flue gas and the atmosphere. Recent researches are focussed on using CO2 as the sole carbon source for the growth of microalgae. The same can be further used in industrial application (biomass, biofuels, biodiesel, pigments, biofuels, biobutanol, and value-added products) because of their relatively higher conversion rate of solar energy into biochemical and bioenergy products than terrestrial plant. Green plants fix carbon mainly by using C3 and C4 pathways. Most plant species are using C3 compared to C4 pathways described species. For CO2 fixation microalgae also use the C3 pathways where by CO2 reacts with Ribulose Bisphosphate or RUBP (a 5-carbon compound) to produce two molecules of 3phosphoglyceric acid or PGA (a 3-carbon compound). Ribulose-1,5-bisphosphate carboxylaseoxygenase (RuBisCo) catalyses this reaction step. The majority of algae are photoautotrophs, which implies that microalgae can meet all of their energy needs through photosynthesis and all of their carbon demands from fixing CO2 [62,64–66].

21.4 Products from CCU Studies using carbon dioxide supplementation have been conducted done in cultures of eukaryotic microalgae at varying concentrations with the goal of increasing biomass, biofuels, biodiesel, pigments, biofuels, biobutanol, and value-added products [14,16–18,34,61,65]. Several studies report that eukaryotic microalga are subjected to various concentrations of CO2 to increase biomass and value-added products. In Chlorella saccharophila UTEX247 and Microchloropsis gaditana NIES 2587, results demonstrate that biomass with high CO2 (HC) (30,000 ppm, or 3 percent v/v) was improved ∼1.5-times as compared to VLC furthermore, changes were observed in cellular components. Additionally, it was shown that cultures treated with HC demonstrated total carotenoid productivity was ∼2-fold higher than VLC [65,66]. In another study, Singh et al., reported that by subjecting Monoraphidium sp. to 3 percent CO2, tocopherol yields increased manifold[67].. Mariam et al., stated that biomass productivity (i.e., 100 mg L–1 D–1 ) of an industrially relevant strain, Botryococcus braunii, was improved along with enhanced carotenoid content of 0.18 percent dry cell weight (DCW) when cultivated in 3 percent CO2 concentration [34]. The best results for both kinetics and carbon fixation rate were reported to be high when microalgae is cultivated in a photobioreactor. Morais and Costa reported the effect of CO2 supplementation in two different conditions, one in flask and the other in Photobioreactor for different microalgal species such as C. kessleri, C. vulgaris and Scenedesmus obliquus and Spirulina sp. For every instance, the vertical tubular photobioreactor provided the best results for both kinetics and rate of carbon fixation. In general, the Spirulina sp. was found to be most efficient for such studies. C. vulgaris, Scenedesmus obliquus and Spirulina sp. were cultivated with as much as 18 percent of CO2 [68]. Haematococcus pluvialis mutant induced with high light intensity demonstrated 1.7-fold higher than a wild strain on astaxanthin yield when supplemented with 15 percent CO2 [69]. In addition, transcriptome annotations mutant indicated phytoene synthase, beta-carotene desaturase, lycopene betacyclase involved in beta-carotene biosynthesis in of Haematococcus pluvialis mutant cells were high upregulated compered a wild strain when subjected to 15 percent CO2 supplementation [69] (Table 21.1).

TABLE 21.1 Production of Biomass, biofuels and biorenewables (B3 ) as final products with reference to valorization of CO2 by different microalgal strains. Initial CO2 concentration (percent)

CO2 fixation rate (g L–1 day–1 )

References

Cultivation system

Products

Chlorella saccharophila UTEX247

Erlenmeyer flask

Biomass, Biofuels & Biorenewables

0.03–3

–

[65]

Microchloropsis gaditana NIES 2587

Erlenmeyer flask

Biomass, Biofuels & Biorenewables

0.03–3

–

[66]

Monoraphidium sp.

Erlenmeyer flask

Tocopherol

3

–

[67]

Botryococcus braunii

Erlenmeyer flask

Biomass & Carotenoid

3

–

[34]

Synechococcus elongatus PCC7942

Roux culture bottles

Biodiesel (isobutyraldehyde and isobutanol)

5

–

[95]

Scenedesmus obliquus CNW-N

Photobioreactor (PBR)

biodiesel and bioethanol

10

0.3–1

[71]

Chlorella pyrenoidosa

Photobioreactor (PBR)

Biomass

0.03–15

1.5

[96]

Haematococcus pluvialis

Erlenmeyer flask

Astaxanthin

15

[69]

Chlamydomonas reinhardtii

Flat-oblong glass vessels

Gametogenesis & hydroxyproline glycoproteins

3

[97]

Chlorella vulgaris LEB 12, Chlorella kessleri LEB 15 and Scenedesmus obliquus LEB 22

Erlenmeyer flask and vertical tubular photobioreactors (VTPs)

Biomass

0.4–18

[68]

21.4 Products from CCU

Microalgal species

471

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21. Photosynthetic cell factories, a new paradigm for carbon dioxide (CO2 ) valorization

FIGURE 21.2 Carbon sequestration and products obtained from microalgal carbon capture.

The principal reason of microalgal biomass gaining considerable attention as potential feedstock is its constituents. Microalgal biomass accounts for lipids 7–23 percent lipids, 6– 71 percent proteins and 5–64 percent carbohydrates, varying upon diverse species and culture settings [61]. To improve major feedstocks of microalgae constituents different carbon concentrations supplementation in culture have been reported for generation of various bioenergy products such as biodiesel, biohydrogen, biobutanol, biogas, and bioethanol. In one of the studies C. vulgaris MBFJNU was supplemented with 3 percent CO2 and compared with a control set in a volume of 5 m3 open raceway. The results observed were promising as there was a 21.3 percent hike in total fatty acid content, increased lipid (9.1 mg L–1 D–1 ) and CO2 bio-fixation (63.2 mg L–1 D–1 ) than the control group [70]. CO2 uptake capacity and lipid productivity of Scenedesmus obliquus CNW-N reported by Ho et al. demonstrates that a twostage cultivation strategy indicated the CO2 utilization rates being 292.50 mg L–1 D–1 , 78.73 mg L–1 D–1 (38.9 percent lipid content/dcw (dry cell weight)), and 549.90 mg L−1 D−1 , respectively [71] (Fig. 21.2).

21.5 Challenges and opportunities 21.5.1 Pre-Combustion technology Due to application of high temperature, material degradation can take place. The high flow rate may lead to embrittlement of materials. Large equipment size increases the capital cost of the gasification plant. This technique is deemed expensive relative to classical supercritical plants and pre-combustion is tough to retrofit to the existing plan [47–49].

21.5 Challenges and opportunities

473

21.5.2 Post-Combustion capture Flue gas obtained from burning of fossil fuel has low CO2 concentration and pressure. This generates a low driving force for separation of CO2 along with production of large amount of flue gas to be treated further. A significant level of energy consumption, as well as a huge equipment footprint and a high expense deploys huge challenges for large scale implementation of post combustion capture [20,39]. Techniques such as membrane separation calls for high selectivity, large membrane space and low tolerance capacity for the flue gas contaminants and requires to be managed [72].

21.5.3 Oxy-fuel combustion Oxygen production through cryogenic air separation unit [2] is highly expensive and needs high refrigeration service and usually 3–4 percent energy cost is imposed as penalty for the oxygen production through ASU [57] (Table 21.2).

21.5.4 Bio-carbon capture by microalgae Microalgae have showed great potential as bio-carbon capture system; however, some challenges are still there like its cultivation regarding the small value majority components such as protein source in food supplements and fatty acid for nutraceuticals seems economically unattainable [73]. Currently, all the advancements in capturing of carbon dioxide by these photosynthetic cell factories has been carried out in laboratory phase under experimentally controlled conditions for carbon capture is necessarily an outdoor mechanism, and various factors in the outer environment may influence the process [74,75]. The production of biomass by microalgae may be hampered by slight changes to the growth environment. To stabilize the culture conditions and have negligible uncertain factors, researchers and engineering technicians need to collectively work upon [76]. Recent algal research aims to better comprehend the impacts of different light intensities, CO2 levels, temperature variation, changes in pH, etc. on carbon assimilation by microalgae by creating instruments to evaluate the amount of carbon fixation in microalgae. Moreover, by investigating the process of CO2 assimilation in these microalgae cell factories, these research hope to better understand the impacts of these factors. Research procedures must be improved in order to comprehend algal growth patterns and choose algae species that are effective and appropriate [14,18]. The growth kinetic parameters, irradiation, and physiological processes makes the culturing conditions of microalgae complicated. Refining the theory and designing the operating equipment to be cheaper, simple, and convenient to use will help to reduce the high economic cost and difficult tasks associated with the cultivation of these cell factories for carbon capture. Theoretical studies that illuminate the CO2 assimilation and fixation process close to the cell membrane are few and far between. Setting up experiments that can provide data for industrial scale production process is still under trial. The elimination of waste gas from manufacturing complexes and areas must be demonstrated practically in order to ensure that all potential economic and other barriers are avoided. The value-added products obtained from microalgae needs to be commercialized, but through monitoring of the quality of the products made from industrial waste has to be done. There can be a chance of contamination of microalgae used for carbon fixation by other heterotrophic microbes which can bring down the yield considerably

474

TABLE 21.2 Challenges and opportunities of chemical and biological carbon capture.

Post Combustion capture

Oxyfuel Combustion Capture

Microalgal Carbon Capture

Technologies used

Drawbacks

References

r Reduced capital investment due

r Chemical and physical absorption. r Physical adsorption. r Membrane separation r Chemical looping r Cryogenic distillation

r Carbon capture efficiency is low. r Regeneration energy required to remove

[30,39,41]

to minimal operating expenses. r Easier to carry out in modern day industrial power plants. r Simplest approach among the three mechanical carbon captures.

r Well-developed process with

r Chemical and physical

r Expensive due to high capital investment

efficient carbon capturing capacity and high CO2 concentration. r Appropriate for treating fossil fuel. r Pure CO2 can be obtained for further processing.

absorption. r Physical adsorption. r Hydrate-based separation.

and operating expenses. r Heat transfer management issue. r Reactions activate under high temperature and pressure. r Complex infrastructure.

r Chemical looping r Cryogenic distillation

r System efficiency is hindered by the

r It can operate in a wide range

r Metabolic process. CO2 is

r The cost involved in managing the

of CO concentration. r Grows2 faster than plants. r Do not compete with large stretches of arable land. r Simultaneous production of food, feedstock, value-added products, biofuels.

absorbed by carbonic anhydrase present in the microalgal cells.

r Highest concentration of is CO2 produced. r NOX emission is relatively lower than other mechanical combustion techniques.

the solvent involved in the process.

r Excess water resource required. r Captured CO2 needs to be compressed before transportation. [28,30,41,43]

[28,43]

presence of ASU (Air Separation Unit). r Pure oxygen evolved during the process can corrode the equipments.

culture systems is high, downstream processing is critical to maintain. r Microalgal cultivation needs rigorous monitoring being sensitive to NOX, Sox (present in flue gas), turbidity, trophic modes, culture media nutrients, light intensity. tolerate narrow range of pH, temperature and salinity. r Highly dependent on the species.

[60,61,63,73]

21. Photosynthetic cell factories, a new paradigm for carbon dioxide (CO2 ) valorization

Pre-Combustion Capture

Benefits

21.6 Future perspectives and conclusions

475

which is not desirable at industrial scale [14,18]. Presently, progress and developments in identification of new strains, culturing techniques and optimization of parameters for the process has made us enthusiastic about the future of microalgal biorefinery [60]. Regardless of the challenges related to bio capture of carbon by microalgae, crude algal oil was used in an internal combustion engine technique, which resulted in fewer NOx emissions. Design of new dewatering and biomass conversion processes can lower the huge energetic expenses and carbon footprint of dewatering [60,77]. A number of genetic and molecular alteration techniques are carried out to improve the photosynthetic efficiency of microalgae, such as modification of the truncated light-harvesting antenna, resulting in biomass accumulation [78–80]. Strategies like overexpressing the Rubisco gene or the variants have shown to improve the activity of these microalgal cell factories and hence scope for increased biomass [14,17,27,29,34,37,38]. Simple developments in downstream processing, notably harvesting, cell disruption, and extraction, which can reduce the expenditure at a biorefinery scale, as well as system incorporation, thus having the best chances of commercial success [26,81–83].

21.6 Future perspectives and conclusions Terrestrial ecosystems have been critical in removing around one-third of anthropogenic GHG emissions over the last 50 years [84]. Currently, the ocean environment sequesters onefourth of the annual CO2 released by human activities. To avoid such a permanent environmental deterioration, the earth must boost the product output and supply while reducing the emissions of GHGs, extract CO2 from the environment and create a reservoir of organic carbon in the atmosphere, and contribute to carbon separation [11]. In the present scenario, carbon is important in the textile industries for various reasons. Burning carbon such as coal produces both heat and electricity [85]. While carbon can be substituted for energy-related applications, it is more difficult to employ carbon for material reasons. Combustion processes for power generation should be replaced with renewable energy supplies such as solar and wind energy [86]. Carbon dioxide as a supplement of carbon for use as a material appears to be a potential alternative. To begin with, the feasibility has been thoroughly described and demonstrated in studies. There are several technological alternatives for using CO2 in combination with additional produced H2 for the generation of a range of organic compounds [87,88]. In addition, post-combustion process is relatively technologically simple in the initial stages of CCU technology advancement. CCU demonstration projects frequently employ this technology and it has been observed that such a mechanism has the prospective capability to decrease CO2 production in the short run [11,60,88]. Furthermore, despite substantial efforts over the last few years, converting CO2 into fuels and chemicals remains difficult due to both thermodynamic and kinetic hurdles [89]. Consequently, CCU based on microalgae are obviously intriguing, but their successful deployment has to be seen [61]. The concept of operating carbon dioxide as a raw material is as longstanding as industrialization, although there have been few implementations. The initial successful catalysis systems simulated by natural biological cycles were developed in the late 1960s, when investigators and researchers initially achieved success in creating the catalysis systems [21,90]. CCU initiatives are categorized as a process of high risk. Furthermore, such processes have substantial impact and ultimately affect the interests and benefits of several multi-national corporations. As a result, achieving

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21. Photosynthetic cell factories, a new paradigm for carbon dioxide (CO2 ) valorization

a collective authorization from society is required to reduce and prevent the social hazards of technology deployment and to boost the efficiency of specific initiatives [91]. The physiological pathway in microalgae, wherein inorganic carbon is biologically fixed into the cells from CO2 supplementation is one of the most understudied ways for CCU [92]. In addition, simultaneous biofuel production process with the extraction of other useful biomolecules from the biomass has been the principal interest of microalgal biorefineries for the CCU which will further lead to the economical production of biofuels [93]. The use of photobioreactors for CCU by microalgae has several benefits, including improved growth of these photosynthetic cell factories as a result of regulated culturing techniques and enhanced area for growth, resulting in reducing the usage of land. As a result, these cell factories provide two important functions: reducing global warming by CCU while providing sustainable energy to meet rising energy demand [14,35,77,94]. Recent studies have demonstrated the efficiency of these photosynthetic cell factories to capture the inorganic carbon, however, few constraints still govern leading to certain bottlenecks. Invariably, the CCU through microalgal machineries has led to consistent efforts and the potential of such mechanism is emphasized in these studies. In conclusion, microalgae lead the way for the process of CCU and strategies need to be evaluated for the rational engineering of the circular bioeconomy.

Funding information Support for this research was provided from the Department of Biotechnology, India, Grant BT/PB/Center/03/2011Phase II.

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C H A P T E R

22 Carbon dioxide capture and sequestration technologies – current perspective, challenges and prospects Ifeanyi Michael Smarte Anekwe a, Emmanuel Kweinor Tetteh b, Stephen Akpasi b, Samaila Joel Atuman a,c, Edward Kwaku Armah d and Yusuf Makarfi Isa a a

School of Chemical and Metallurgical Engineering, University of the Witwatersrand, Johannesburg, South Africa b Green Engineering Research Group, Department of Chemical Engineering, Faculty of Engineering and the Built Environment, Durban University of Technology, Durban, South Africa c Department of Chemical Engineering, Faculty of Engineering, Abubakar Tafawa Balewa University Bauchi, Nigeria d School of Chemical and Biochemical Sciences, Department of Applied Chemistry, C. K. Tedam University of Technology and Applied Sciences, Navrongo, Upper East Region, Ghana

22.1 Introduction The constant rise in gaseous emissions is a huge ecological concern that affects the entire world population, and it must be addressed immediately. With the rapid technological advances and the growth of the world population, there has been an enormous rise in the usage of fossil fuels, which are only accessible in constrained amounts and have a negative influence on the ecosystem [1,2]. Climate change and global warming are the consequence of the emission of carbon dioxide methane, chlorofluorocarbons, and nitrogen oxide into the environment [3,4]. Global warming is a serious concern for the majority of research institutes

Green Sustainable Process for Chemical and Environmental Engineering and Science: Carbon Dioxide Capture and Utilization DOI: https://doi.org/10.1016/B978-0-323-99429-3.00034-5

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c 2023 Elsevier Inc. All rights reserved. Copyright 

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22. Carbon dioxide capture and sequestration technologies – current perspective, challenges and prospects

and governmental organisations around the globe [5–7]. It occurs because of excessive levels of CO2 in the environment. Presently, many nations throughout the globe continue to depend largely on fossil fuels, which emit considerable levels of CO2 , for energy production, with fossil fuels accounting for about 85 percent of all electricity produced around the world [6,8]. In the foreseeable future, a complete replacement of old power plants with alternative clean energyproducing mediums that emit no CO2 will be practically unattainable. Due to the prevalence of fossil fuels, the majority of attempts to address the threat of greenhouse gases are focused on CO2 capture techniques [9]. The increased global greenhouse gas contents in the earth’s atmosphere from the year 1965 to 2021 were rather worrisome, which is almost 142 percent of the predicted CO2 level prior to the industrial revolution era with the year 2020 recording a 5 percent increase from the previous year (Fig. 22.2) (EIA, 2020). However, it was determined that the decrease in CO2 uptake in the biosphere was the root cause of this phenomenon. Since 2013, the rise in greenhouse gas (GHG) emissions, which has been attributed to increased growth in population density, commercial activity, and anthropogenic activities, has resulted in unexpected consequences associated with environmental contamination to health problems, water pollution, eco-degradation, the extinction of aquatic animals, and unfavourable climate conditions [10]. A total of 195 nations initiated a negotiated agreement on the first legally binding memorandum of understanding on climate issues in December 2015 at the United Nations Climate Change Conference in Paris (i.e., the COP21), where it generally consented that the world temperature would be maintained at an average rise of 20 wt percent)

Corrosion potential control system

Require high compression power

Prevent the formation of foaming No consumable cost required Stirling cooler Stirling cooler

Atmospheric

Exergy loss

Simultaneous removal of H2 O and CO2 Frost layer removal difficulty Low energy penalty

At Laboratory stage

Energy storage potential

elements [91,94,95]. Cryogenic carbon capture utilising a moving bed requires three important units for its smooth duty. 1. The chiller unit: this cools down the flue gas and also reduces the water content in it. 2. The recuperation cooler drier unit: The remaining water vapour content is removed and also cools the flue gas further. 3. Carbon capture unit: CO2 is captured as frost on the moving bed leaving clean gas escaping from the unit into the atmosphere.

22.3 CO2 transportation, storage and opportunities/applications for CCS technologies

493

22.2.2.4 Adsorption technology Adsorption is one of the post-combustion capture technologies that are of interest in recent times [97,98]. According to Oschatz and Antonietti [99], the performance requirement of a good adsorbent for CO2 capture are: (a) having high CO2 selectivity (b) having mild thermal conditions for CO2 desorption (c) having high CO2 working capacity (d) low cost (e) have fast adsorption and desorption kinetics (f) have high resistance against impurities and moisture, high stability, and high mechanical strength. CO2 adsorption in porous materials has attracted a lot of interest due to its lower energy penalty compared to absorption using chemical solvent and also because the process is clean and can easily be regenerated after capture [99]. CO2 capture using adsorbent has its drawbacks such as (1) lower carbon dioxide selectivity (2) lower CO2 adsorption capacity (3) carbon dioxide removal efficiency is lower compared to absorption and cryogenic technologies (4) adsorbent reusability and regeneration challenge. Fig. 22.9 shows three parallel adsorption columns for CO2 capture from flue gas streams. Flue gas was first dried to remove any moisture content and then compressed before sending it to the adsorption chamber. The three columns were packed with a solid adsorbent (i.e. metalorganic frameworks (MOFs), Zeolite, and activated carbon) for use in the adsorption of CO2 . The process has two or three adsorption columns, one column will be used for adsorption while the second column will be desorbing the captured CO2 using either an electrical or thermal energy source and the third column will be on stand-by [100]. 22.2.2.4.3 The role of catalyst/adsorbents

The growing concern on maintaining the global surface temperature to well below 2 °C temperature rise has led to many researchers looking for options that will efficiently reduce the release of greenhouse gases to the environment, especially from large point sources. Adsorbent capture technology over the years has been developed to find suitable catalyst/adsorbents that can efficiently capture CO2 and can easily be regenerated without loss in capture quality. Park et al. [101] study nanoporous adsorbents for efficient application in pre-combustion and PCC, and the study shows that porous carbon adsorbent has a large surface of 3010 m2 /g with a specific pore volume of 1.506 cm3 /g. The adsorbent shows high selectivity towards CO2 and also high storage capacity. Gutierrez-Ortega et al. [97] reported a performance assessment of binderless zeolite and carbon molecular sieve for CO2 capture application. Looking at the key performance index of the two classes of adsorbents it was discovered that binderless zeolites performed better than the carbon molecular sieves. Graphene and monolith molecular sieves have achieved a lot of progress in the area of carbon capture [100,102–105]. The newly developed nanocomposite by Aquatar et al. [105] has shown that it has better adsorption capacity as compared to zeolite derivatives, MOFs and carbons.

22.3 CO2 transportation, storage and opportunities/applications for CCS technologies 22.3.1 Transportation Cleaned and compressed CO2 is usually transported and stored in saline aquifers, oceans, depleted oil, and natural gas fields) after they are cleaned and compressed [106]. Transporting

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22. Carbon dioxide capture and sequestration technologies – current perspective, challenges and prospects

Dryer

Compressor

Flue gas Flue gas fan

Electrical/ thermal source for desorption

Condense water

Column 1

Column 2

Column 3

CO2 transport to storage or utilization FIGURE 22.9 Three parallel columns for CO2 removal from flue gas stream by adsorption (adopted from [96]).

of captured CO2 is the drive between the CCS system, which links CO2 sources to the storage location by way of pipelines, ships, trucks, and trains [107]. In essence, H2 O must be removed before the CO2 captured is transported to limit acid production that can corrode pipes and other equipment used for transportation [108]. In most cases, the process of dehydrating the CO2 gas stream that has been captured is carried out in conjunction with either the compression or the refrigeration processes [109]. Compressing CO2 to a dense phase (over 7.4 bar and 87.8 F) for pipeline transportation or defrosting it to a liquid phase makes it easier to transport large amounts of captured CO2 [107,110]. However, this comes with a high cost. In 2020, the estimated CO2 compression and dehydration costs for a CCS project on the US Gulf Coast were $23.45/t CO2 for 0.4 Mt CO2 /annual with a captured CO2 gas stream comprising 200 ppm H2 O at 15 MPa [109]. These estimated transport costs usually vary due to the transporting medium from the CO2 source and the considerable storage locations.

22.3 CO2 transportation, storage and opportunities/applications for CCS technologies

495

22.3.1.1 Pipeline transportation Onshore or offshore pipeline transport may depend on the source-to-storage site, whereby transporting via pipeline offshore is more costly than transportation on land. Generally, industrial pipelines for CO2 capture are operated at pressures between 8.6–15.2 MPa [106,107]. This condition is mostly observed to make transportation cheaper and easier by avoiding twophase regimes and a rise in the CO2 density [109]. Besides, CO2 being denser than air can cause health risks if pipeline bursts Also, to avoid corrosion and external coated pipe damage, temperatures between 13–43 °C are usually considered [106]. Mode, capacity and distance affect transportation mostly increasing the costs of pipeline systems. However, the cost of CO2 pipeline transportation can be minimised by constructing large-scale CCS networks [106,108,109]. Here, the CO2 pipelines can keep the compressed CO2 captured above the critical pressure even beyond distances of 150 km [106]. 22.3.1.2 Ship transportation For large-scale CO2 transport across long distances or globally, shipping CO2 may be more cost-effective than building new long-distance pipelines or repurposing existing gas pipes [106]. Besides, Food and beverage sectors may transport CO2 by ship differently due to their advanced technologies. Food and beverage sectors may transport CO2 by ship differently due to their advanced technologies. [108]. Comparatively, shipping is cheaper than pipelines since ships can be directed to several storage sites [106,109]. 22.3.1.3 Truck and rail transportation This is a possible technology for delivering modest amounts of CO2 , although pipelines and ships are cheaper and more scalable for long-term transport [106]. It is anticipated that there would be an increase in the cost of liquid CO2 that is conveyed by a truck when the source diameter exceeds 482 km [106].

22.3.2 Carbon storage 22.3.2.1 Geologic storage CO2 storage in geological, particularly depleted oil and gas reservoirs, is the most viable short-term option for CCS systems [107,108]. Despite environmental risks being unknown, geology appears to be less detrimental than ocean storage and depletion wells when compared to saline aquifers [108,110]. Storage locations (saline aquifers, oceans, exhausted basins) for CCS development are extensive and projected to exceed the demand as it can cover over 70 percent of the earth’s surface [107]. Some of these techniques involve pumping pressurized liquid CO2 down a subsea pipeline and storing it for millions of years. Porous and permeable formations with impermeable rock capping are commonly used in storing CO2 , especially where wells have deteriorated their reservoir seals [109,110]. Ageing oil–gas fields are geologically suitable for storage after millions of years of retention [108,109]. This makes CO2 to be trapped by (i) sedimentary and morphological and via primary trapping, behind seals of low permeability rocks; (ii) solubility via residual gas trapping; (iii) residual via trapped in rock pores and water capillary pressure and (iv) mineralization via altering the pore-space topology and connectivity [107].

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22.3.2.2 Ocean storage The potential ability of the world’s oceans to store CO2 is by far the highest of any other natural environment. Despite ocean storage having a vast potential capacity it still got environmental risks and tailbacks [108,109]. However, the negative environmental impact of ocean storage is very unpredictable to have a significant tendency of being disastrous. In essence, about 90 percent of present atmospheric emissions can be absorbed by the ocean leading to the rising of its acidity [2,3]. This becomes a great concern, as the uprising of the acidity can result in corrosion and detrimental risks to aquatic life. As there is a possibility that changes in CO2 or pH could have significant repercussions on the ecosystems that are found in the deep sea [107]. Meanwhile, water column injection technology including injecting dilute CO2 into the deep ocean through diffusers can change the biogeochemical cycles or chemistry of the ocean [106,108]. Consequentially, the application of ocean storage technology for CO2 is still limited due to its techno-economic and environmental risk bottlenecks which warrant improvement [106,111]. Thus, deep-ocean sequestration requires a long residence period, which can alter the phytoplankton abundance and community structure as well as generate unpredictable levels of eutrophication [107].

22.3.2.3 Other options Captured CO2 instead of storage may also have little potential for immediate commercial usage options. This includes:

22.3.2.3.4 Direct use

The direct use of about 80 percent of the CO2 captured by the industry can contribute to the net decrease in CO2 emission [107]. Thus, the trapped CO2 can be utilised as raw material to produce polymers and inorganic carbonates [107,112]. Herein, valorisation of the CO2 captured can be an open market for new inventions and research in the bioprocessing, chemistry, and engineering fields [107,112]. However, this market cannot withstand the massive quantities and application of CO2 made available by the CCS [108,109]. Chemically, converting CO2 is often energy-intensive, and in most cases (though not all), results in more net CO2 emissions [108,113]. Some of these setbacks as mentioned limits the direct utilisation of CO2 for industries application.

22.3.2.3.5 Conversion to carbonates

To safeguard the environment, by minimising challenges associated uprising ocean acidity and leakage from geologic reserves posing threat to human and aquatic life, necessitates the conversion of CO2 into carbonates [107,108]. This involves the combination or reaction of CO2 with alkaline rocks to generate highly stable and nontoxic carbonate chemicals [111]. Also, the notion of converting CO2 into carbonate materials is to speed up its storage process into a more stable form before injection into the environment. Moreso, industrial applications of this process are very rapid as compared to a natural mechanism which can take millennium years to get accomplished [107,111].

22.4 Current perspective and policies of CSS technologies in various countries throughout the world

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22.4 Current perspective and policies of CSS technologies in various countries throughout the world In the deployment of CCS on fossil-fuel-related operations, the majority of regulations have been developed that are concentrated on the transportation and storage of CO2 , together with some advancements including the application of artificial intelligence in CCS. In accordance with the International Energy Agency (IEA) [114], the European Union (EU) has the greatest number of connections with CCS technologies. There are two primary sections to the concepts of CCS techniques: the first recognises CCS as a tool for reducing emissions (the United Nations Framework Convention on Climate Change, the Kyoto Protocol), and the second pertains to policies on oceanic storage of collected CO2 (the International Convention on Oceanic Sequestration; United Nations Convention on the Law of the Sea, London Convention) [7].

22.4.1 Review of CCS policies 22.4.1.1 The united states and Australia Research efforts in CCS have increased dramatically in recent decades in the United States, owing to a government initiative to allocate 2.4 billion USD to promote CCS operations and studies [7]. In line with the American Clean Energy and Security Act, the installation of CCS should result in a decrease of around 26 percent in emissions from industrial sources [115]. The CCS state league was responsible for the development of the geological salt-water reservoir. Australia also had a role in the development of the CCS flagship project, which contributed 2 billion USD for studies into CCS. This later resulted in the establishment of the Global CCS Institute, whose primary mission is to further the advancement of CCS technologies [116]. Table 22.2 shows the United State CCS policies which are based on three key pillars including lower cost, infrastructure buildout and streamlined permitting. 22.4.1.2 European Union The EU is deeply committed to combating climate change, and as a result, it has demonstrated a significant commitment to the progress of the CCS technique, as shown in Table 22.3. Even though the EU considers CCS to be one of its high developmental goals, several EU member states are still battling to meet the benchmarks and requirements that have been established by the EU. The European Union has established regulatory guidelines for the growth of CCS technologies [118,119]. When it comes to emissions generated by fossil fuel operations, the European Commission (EC) believes that CCS is a viable remedy. In line with the commission, CCS represents the prospect of the energy sector with reference to energy security and addressing global warming issues. The European Commission (EC) applied the regulation on Geological CO2 storage in 2008 [119]. This finally establishes the legal grounds for the geological sequestration of CO2 . In accordance with the strategy, all new power facilities should be capable of capturing emitted CO2 , and existing coal-fired power facilities must be combined with CCS techniques in the coming years. There are also other regulations published by the European Commission, such as the necessity for CCS systems, the need for storage sires, and the need for renewable energy sources [119]. The European Commission (EC) revised the European Union Emission Trading Scheme in 2012 to incorporate CCS technology which

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TABLE 22.2 US CCS Policies (Source: Global CCS Institute [117]). Categories

Policies

Build-out Infrastructure

Storing CO2 and Lowering Emission (SCALE Act)

Cost Reduction

Extension and Expansion of 45Q Tax Credit

Federal Funding in Research, Development and Demonstration (RD&D)

Streamlined Permitting

Class VI Program Reform

Description

r A substantial expansion of CO2 transportation and infrastructure is required to achieve extensive implementation. r Linking storage sites and emitters, obtaining scale economies, and establishing a carbon control market, makes it possible to capture increased CO2 . r A programme to assist in the development of infrastructure is established by the SCALE Act. r FEED (Front-End Engineering Design) studies are required for an effective CCS system r Loan and Grant Assistance Program r Expands on the Department of Energy’s carbon sequestration programme

r The 45Q tax credit is widely regarded as the most effective

instrument available for encouraging the advancement of CCUS initiatives. r Utility could be enhanced by boosting the credit value, enabling direct payment, lowering minimum capture limits, and widening the credit period. r Several propositions are presently being considered in Congress.

r Investment in research and development is essential to

lowering the cost of carbon capture and facilitating new projects. r Novel development is also required for difficult-to-decarbonize areas. r The Energy Act of 2020 called for an unparalleled surge of innovative commercial-scale technological demonstrations in a quick timeframe to minimize the cost of carbon capture.

r In geologic storage, Class VI wells should be utilized to

pump CO2 into subsurface rock formations for the aim of long-term underground sequestration. r Class VI specifications are a critical consideration in the cost-effective implementation of CCS. Presently, there are only two functioning Class VI wells, each of which took six years or more to secure the necessary licences. r The streamlined permission procedure can be modified to be more efficient while still protecting the ecosystem. r Allowing state priority will help to expedite the deployment process.

is being applied in the present day. Some of these revisions dealt with the relevance of CCS technology, the clarification of auctions and quotas, and the inclusion of a new funding system in CCS research and development [119]. Evolving regulatory documents for CCS deployment in poor and developing nations were also established over the same period. Some of the

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TABLE 22.3 EU CCS Policies (Source: European Commission (EC) [119]). Policies

Description

Climate Change-based Interventions

According to the EC, the incorporation of CCS into the electricity grid will assist the European Union in meeting 15 percent of its emissions objectives. To be accomplished if CCS receives the required funding and intervention. The panel is conducting various feasibility studies on CCS, with the primary goal of expanding studies into this unique innovation. Moreover, the European Commission is providing financial assistance under the EU regulatory structure to aid in the rapid large-scale application of CCS technologies.

Geological Storage of CO2

Several laws and regulations concerning the security and safety of storage facilities have also been adopted by the European Union.

Energy technology strategy

The EU has policies in place to encourage the production of environmentally friendly, safe, and inexpensive energy. In addition, the Union encourages the rapid commercialization of low-carbon technologies by funding research efforts targeted toward CCS. Another key objective of the European Union is the development of effective capturing (ENCAP) systems, which is now underway.

Research fellowships

Coal in the United Kingdom emits almost no emissions in response to the near-zero-emission coal (NZEC) policy

Zero-emission platform

According to future projections of the European Union, the majority of carbon emissions would come from India and China, necessitating the requirement for close cooperation between the EU and these nations. Global interactions will aid in the mitigation of climate change and the improvement of carbon capture and sequestration technology

Analysis of sustainable development scheme

According to the EU, implementing policies relating to renewable energy and CCS will necessitate raising community understanding and swift decision.

Environmental evaluation regulations

The methodology used in the assessment of the deliberate ecological implications of CCS is still precarious.

Environmental impact analysis

To assess the particular project, an environmental impact evaluation is performed, and it has been used to assess CCS pilot projects.

EU’s incentivized programmes, such as the reduction of CCS operating costs (EC (European Commission), have been implemented from 2012 till the present [118,119]. The EU has several rules pertaining to carbon capture and storage (CCS), but these policies confront several obstacles. Because CCS technology has not yet reached complete commercialization, resolving the problems associated with certain of the policies will be challenging. The EU continues to face difficulties in the maintenance of storage locations, particularly with regard to risk evaluation [118,119]. The regulation on the geological sequestration of CO2 specifies that whenever there is a leakage of CO2 , the licence of any organisation will be cancelled; however, the policy does not specify what safeguards will be implemented during the incident of leakage. The EUIAB (European Union Impact Analysis Board) reported that strategies to assess liability for a corporation’s carelessness in the case of leakage are still being developed, and this has a significant impact on storage sites. Because of the difficulties

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associated with storage locations, the capture of CO2 , transportation, and storage cannot be assured, thus, limiting the likelihood of CCS being commercially viable [7,118].

22.4.2 Artificial intelligence (AI) applications in carbon capture Currently, the application of AI in CCS has contributed to the advancement and modification of CCS technologies for effective carbon capture. As a result of the speedy growth in computer technologies over decades, an arithmetic application using a computer for operations has developed more pertinent and appreciated in many sectors. Many researchers are currently examining artificial intelligence (AI) techniques, particularly machine learning [120]. Large-scale CO2 capture sites which are in parts of Norway and Canada, producing huge quantities of operational process data are employed in developing knowledge for CO2 capture operations which are highly efficient [121]. Artificial neural networks (ANNs) have been classified as one of the well-known and mostly applied AI systems for CO2 capture. This is because of the numerous variables which include the reason that they operate simply and are easy to execute. Within a shorter time, ANN prediction models including a variety of variables are created. This analysis is quite supple and frequently, yields more precise results other than arithmetic simulations and correlations [122]. Depending on this application, an ANN model could consist of correlations between input and output data. This system, which is inspired by “neurons” consists of an extensive variety of parallel processing units. The incoming data is distributed and evaluated inside a network of these neurons. In the field of CO2 capture, well-known networks; the back propagation neural networks (BPNN) and the radial basis function neural networks (RBFNN) are the two commonly applied ANN approaches. For BPNN, the hidden layers are individually responsible for establishing a connection between the input variables and the output variables. Network computers consider errors between the expected and real data through the back propagation of the BPNN network [123]. A noticeable adventure is that the training algorithm of RBFNN differs from that of BPNN. This arises from the fact that it is based on the spaces between the input and the network’s centre. An alternative to this is the use of the Adaptive Neuro-Fuzzy Inference System (ANFIS) which incorporates ANN and fuzzy inference systems (FIS) to increase the ANN’s efficiency, fault tolerance, stability, and adaptability [124]. The ANFIS has been employed to depict changeable non-linear interactions between parameters, construct fuzzy inference systems and develop very precise correlations between input and output parameters. However, ANFIS has several drawbacks. These include the need for expertise in designing the fuzzy member function. To address the shortcomings of the ANN and ANFIS, Helei 2021 [120] created the Piecewise Linear Artificial Neural Network (PWL-ANN) technique with the aim of revealing the ANN’s modelled correlations from a dataset of a carbon capture system. The advancement of AI has made it possible to use this technique in the following aspects of trapping CO2 and storing it in processes such as the AI application to the research of solubility and the physical properties of materials. In such instances, the physicochemical characteristics of CO2 and possibly, amines such as the reaction rate and heat capacity could significantly impact the efficiency and performance of a carbon capture approach [125]. Secondly, the AI and the compressibility factor. In this case, the compressibility factor is considered a carbon capture approach. The compressibility factor, Z is established to explain the thermodynamic association between

22.5 Challenges and socio-economic implications of CCS technologies

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variables of a real gas which is represented as PV=ZnRT; where P, V, n, R, and T represent pressure, volume, and amount of substance, molar gas constant and temperature, respectively. Various research has used the AI technique because it often solves non-linearly difficult problems which are quicker and gives more accurate results than other approaches. Nonetheless, in the last 15 years, researchers have focused on using AI to simulate mass transfer and the efficiency of CO2 capture systems to produce a good and realistic mass transfer rate calculation [126]. This CCS strategy, when paired with the use of biomass as fuel, could potentially result in a carbon emission approach. This renders a negative carbon footprint if CO2 is stored in applications such as buildings, where the likelihood of CO2 exiting the environment becomes avoided.

22.5 Challenges and socio-economic implications of CCS technologies It is difficult to advance carbon capture technology due to its high cost. Carbon capture technology is not currently available, so reducing carbon dioxide in energy generation will require an expensive method [127]. Nearly 30 percent of the total cost of tackling climate change can be reduced by CCS. The capturing costs, which make up about 75 percent of the overall cost, determine the total cost of CSS technology. It also raises electricity prices by between 30 and 90 percent [128]. This increase in energy demand is due to the capture and transport and injection of CO2 under compression. It’s important to note that electric power generated by a new plant with CCS is more expensive than electrical power generated by an old plant that has been retrofitted with CCS.

22.5.1 Post-combustion capture challenges Cost and energy penalties are the most important characteristics of this type of CCS. Post-combustion capture makes electricity expensive when used in coal power plants. This technology is expected to expand the Levelized cost of electricity by 80 percent. Moreover, i. Existing plants can be retrofitted for a fraction of the cost of designing a novel coal-fired power station void of post-combustion pollutants. Coal power plants are also subject to a high-efficiency penalty. The heat required for modern post-combustion capture solvents and pressure used to condense CO2 from exhaust stacks can reduce a plant’s capacity by 30 percent. Because of these inefficiencies, more coal is used to generate the same amount of electricity, resulting in higher plant cooling requirements. ii. Following initial demonstrations on a commercial scale, PCC processes will likely become more efficient and cost-effective. Until now, there has been little incentive for technology developers to process configurations and optimize solvents and fields.

22.5.2 Geologic storage challenges In terms of sequestration, combating climate change at a larger scale remains a huge challenge. Despite the widespread use of enhanced oil recovery in recent years, several sites have experimented with injecting large volumes of carbon dioxide into brine formations in

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geologic formations [129,130]. It is necessary to conduct larger field demonstrations globally. Science and industry agree that sequestration is safe if it is conducted at a suitable location. Knowledge sharing, proactive project coordination, and R&D support are required for controlling the injection of numerous sources into a single sedimentary basin. Commercial and public sequestration sites, as well as environmental protection, will require the development of permitting, monitoring, and long-term care programs. The development of these institutions requires a strong public policy framework [130–133].

22.5.3 Gasification challenges CO2 capture from a newly constructed gasification facility is less expensive than postcombustion capture from a coal-fired power plant [134]. The challenges of gasification are listed below. i. The power generation facilities and gasifier of an integrated gasification combined cycle (IGCC) must operate simultaneously. Despite gasification and power generation being old technologies, utilities still encounter difficulties integrating them. ii. Construction of the required infrastructure is also a significant concern for this technology. The cost of constructing IGCC without CCSis higher than that of pulverized coal without a CCS [135]. Many recent plants are not designed to capture or store carbon dioxide because of the difficulty in securing the mandates, market prices, and regulatory frameworks. iii. Altitude and coal type also affect the cost of IGCC plants. At high altitudes, it is more expensive to run these plants.

22.5.4 Environmental impact of CCS technologies It is imperative to mitigate the environmental impact of CCS regardless of its benefits. Capturing and storing carbon emissions reduces atmospheric carbon emissions, however, there are significant environmental challenges outlined in the following sections: 22.5.4.1 Effect on water resources As pipelines are constructed into water, spillage, and release from them is the most significant environmental concern. Carbon capture plants that pump the wastewater into aquatic environments are also thought to be toxic and harmful to aquatic life. CO2 leakage can also cause the water to become acidic; when these spills and leaks reach the groundwater, they become contaminated. CO2 leakage from wells, faults, and the caprock can also pollute the groundwater during the operational phase. Capillary, diffusion processes and fracture network faults contribute to groundwater contamination. Water pollution can also be caused by water outflow. With CO2 injections, brine water that had been present is replaced with CO2 . For instance, in the United States, water production from outflows ranges between 25 and 400 barrels/day, however, these waters are not suitable for consumption, as a result of high salinity, which requires additional treatment [135]. Other important factors that influence the concentration of these toxic chemicals include metabolism, aquifer recharge, and geological conditions.

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22.5.4.2 Effect on air, land, and climate During the operation and construction phases of the CCS project, the air quality will likely be affected. It is primarily due to dust suspension, machine fuel consumption, and shipping. Carbon dioxide raises the pH of the soil when it is released into the atmosphere, resulting in heavy metal mobilization. Air quality can be adversely affected by emissions from plants that capture CO2 . In addition to poor well sealing, other factors can affect air quality. To address this issue, the wellbore seal must be in good condition to prevent CO2 leakage. 22.5.4.3 Induced seismicity Several studies have shown that high volumes of fluids injected deep underground may result in seismicity or geomechanical effects. Data collected from oil and gas experts were used to compile the study. Generally, the Coulomb theory contends that shear cohesion, pore pressure, and normal stress are the major causes of faults in formations. When pressured fluids are injected into subsurface strata, they cause pore pressure to change, causing any faults that already exist to be affected. Several factors, including mine subsidence, depletion of oil and gas fields, and secondary oil recovery, can cause induced seismicity. There are also numerous factors to be considered, including deep drilling programs, wastewater disposal, reservoirs, and geothermal systems [136]. Induced seismicity is known to be significantly affected by fluid injections. There are also concerns about injecting CO2 into caprock, which has the potential to rupture, posing a risk to project personnel and the property [136,137].

22.5.5 Socio-economic impact of CCS technologies 22.5.5.1 Social aspects of CCS Most fossil-fuel-related projects using CCS require even more skilled workers to remain operational. As a result, more jobs will be created, improving people’s living conditions. Hands-on time will be needed daily at the plants, and constant observation and monitoring will be required at the storage site. The job opportunity provided by the CCS will continue to be very rewarding for an extended period. Construction of infrastructure will also create employment, although for a limited period. In addition, it will increase the number of people trained in local communities. As a result, the global economy will be stabilized [138]. Longterm jobs will become available, allowing workers to improve their living conditions. Setting up a project in an area with a high unemployment rate will improve the community’s standard of living. As a result, it will be used to alleviate poverty. Once more jobs are created, the poverty rate will drop dramatically. Upon project participants’ agreement to follow health and safety procedures, the work environment will be safer as a result of the technology implementation. 22.5.5.2 Economic implications of CCS techniques Natural resource production can help most countries, particularly developing countries, support their energy sectors and improve their economies [139]. Utilizing CCS for fossil fuel-related projects will considerably help the country where the project is located combat climate change. This energy typically comes from clean technologies [140]. Energy sources that are more environmentally friendly will be more appealing in the energy industry. Energy systems incorporating CCS may be eligible for greater incentives than projects excluding the

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technology. In addition to speeding up projects related to CCS, it will also boost the economy. As reported in the literature, incorporating CCS in energy-related projects will increase the appeal of the sector to the market, leading to increased investment. Because CCS technology is integrated into the system, the carbon footprints of fossil products are reduced. This is one of the most important contributions of the CCS application that persuades investors to invest in this novel technology. Conclusively, CCS is both a tool for reducing global warming and an effective method of meeting the world’s energy needs [141].

22.6 Applications and opportunities for CCS techniques The most cutting-edge technology for industrial and commercial energy plants is postcombustion CO2 capture. A solvent is necessary for the capture of CO2 via the post-combustion method. Researchers are now looking into different types of solvents, and designs, and integrating solvent designs for CO2 capture. In other studies, many different predictive methods have been used to determine the optimum solvent design and selection for the post-combustion CO2 capture [142,143]. During the investigation, several computational and statistical strategies were employed. According to Zhang [144] and Joel et al. [145], fluid theory family approaches and quantitative structure-property relationships were both utilized in the investigation. The quasi-chemical method of functional group activity coefficients has been used to develop systems that capture carbon dioxide [146]. Other researchers have made an effort to use the solvent selection method with CO2 capture [147–154]. Power plants that are retrofitted using post-combustion CO2 capture are the most viable option. A thorough investigation of this method for improving equipment performance has been conducted. As previously stated, the approach has been used in several numerical studies and modeling research projects [155]. Low gas volume, high pressure, and high CO2 content are necessary for pre-combustion CO2 capture. Also, this method requires less water and energy consumption than the post-combustion CO2 method. Hydrogen/syngas is an alternative fuel that is generated before the pre-combustion process [155]. In comparison to the other two methods, oxyfuel combustion is thought to be more environmentally friendly. It is suitable for capturing carbon dioxide from a wide range of coal fuels despite not requiring chemical processes [156–160]. The oxyfuel method is easier to renovate than other types, such as the post-combustion capture system. In terms of carbon capture efficiency, this method is very effective. With its small equipment size, advanced air separation technology, and ability to take advantage of steam cycles with fewer modifications, it is suitable for conventional use, efficient steam cycles with a small infrastructure, and the fact that NOx control and carbon dioxide separation have been removed makes it very advantageous [161,162].

22.6.1 Electricity power generation Decarbonization of fossil fuels and migration towards a hydrogen economy could potentially provide CCS with equal prospects. Coal might be gasified to generate synthesis gas (syngas - a blend of CO and H2 ), similar to CCS [107]. Conversely, syngas in the presence of catalysts and steam can undergo a series of reactions to produce H2 and CO2 [107,163]. Whereby, upon separation of the H2 , a clean stream of CO2 can be compressed and stored.

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Moreso, in power-to-fuel technology, captured CO2 are mostly compressed with hydrogen before being injected into the catalytic reactor. This mechanism ends up generating valuable sources of energy such as hydrogen, syngas, and liquid fuels as well as other chemicals and their derivatives [107,113]. Methanol technology in the energy, steel, and chemical industries has the potential to reduce CO2 emissions and balance grid services [107,108]. The use of by-products and off-gases by the industry’s combined power (CHP) and heat units linked to the grid could also assist in meeting their electricity demand [109,110]. Furthermore, the majority of coal-burning power plants employ conventional turbines (gas/steam-driven) to generate energy [109]. Absorption technology has been utilized to either directly create CO2 from fossil fuel burning or to recover CO2 that was produced as a by-product. This involves the use of chemicals such as monoethanol amine (MEA) [113,164]. MEA is currently among the efficient technology for capturing CO2 from flue gas streams because it can form bonds with CO2 . Since CO2 is acidic gas, upon its extraction after the absorption process, MEA and other alkaline solvents are regenerated for reuse by subjecting them to heat [113].

22.6.2 Industrial application CO2 capture from industry’s flue gases is theoretically viable, as about 81 percent of energy-related emissions mostly account for industries requiring a lot of energy including petroleum refining, petrochemicals, and steel and iron industries [107,109]. As a result, CO2 can be captured via methods (pre-combustion, post-combustion, and oxyfuel combustion capture systems) in industrial process streams (including natural gas purification, synthesis of ammonia, cement production, and steel production) [108,163]. Also, natural gas has variable concentrations of CO2 which are generally released into the atmosphere, therefore warrants value addition (capturing of CO2 ). Therefore, implementing CCS for CO2 capture offers low-cost potential for several utilization including hydrogen, chemical, ethanol and fertilizer production, waste-to-energy, natural gas processing, and cement as well as iron and steel manufacture [113,114,164]. Conventionally, the opportunities for carbon capture vary, since it may be simple to construct or retrofit a manufacturing plant such that it can handle carbon capture, while in other instances, these adjustments may not be compatible with the manufacturing processes [106,108,109]. Post-combustion technologies are used in cement manufacturing plants to capture carbon dioxide (CO2 ) released during the calcination of limestone which is fuelled by fossil fuel combustion [107,110]. In flue gases, this ranges from about 15 percent to 30 percent by volume (vol). In a cement factory, an oxygen-fuel combustion capture technique can be implemented through the substitution of the air in the cement kiln for pure oxygen [108]. Currently, this has been one of the game-changing cost-effective CO2 capture technologies employed in capturing CO2 from raw limestone [106].

22.6.3 Application of CCS techniques in CO2 capture from exhaust gases capture For power plants that use carbon, there is always (15 – 30) percent capturing energy penalty, which accounts for nearly 85 percent of the carbon capture and storage costs [165]. Carbonfired plants with a 33 percent efficiency require a reduction of 1/3 in power output, which

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raises capital expenditure to around 77 percent [128]. There are varying amounts of carbon dioxide emitted by carbon-fired power facilities as a result of a wide variety of fuels that are utilized, however, coal-fired power plants produce emissions of 669 gCO2 /kWh with 50 percent efficiency and 1116 gCO2 /KWh at 30 percent efficiency [134]. Although coal is a carbon-intensive fuel, capacity development demonstrates that carbon mitigation measures are inadequate in comparison to the economic subsidies for a fairly affordable fuel. Regarding capital expenses, natural gas-powered plants require half the capital expenditure of coalpowered plants, so they are more efficient than coal-fired plants [166]. Capital expenditures are highly uncertain, with an estimated 40 percent value, however, variability has minimal impact on the Levelized cost of energy (LCOE) [166]. Therefore, operations costs (OPEX) are the primary driver of total carbon capture and storage costs. Researchers have found that chemical absorption for post-combustion CO2 capture is a promising and valuable technique for CO2 capture and storage [167]. As a result, operating costs and power capacity are reduced. The main issue is heat demand. It is possible to boost the effectiveness of carbon-fired power plants by using hybridization and solar-assisted post-combustion. Modern membrane permeation is less effective than chemical absorption at capturing carbon dioxide from exhaust gases [132]. The use of coal-fired plants can reduce the number of fossil fuels burned by substituting fossil fuels with renewable energy. Owing to the sporadic nature of sustainable energy, the unit for capturing carbon must be adaptable to make it economically viable. By keeping the solvent in storage and eliminating carbon dioxide capture from the procedure, flexibility is achieved to meet peak energy prices [168]. Capturing units’ flexibility allows for a 28 percent reduction in capital expenditure [132]. Capturing and dispatching variables based on the energy demand reduces the capture energy penalty, which often results in increased capacity and network efficiency [133]. One relevant example is that the cost of a time-average condition-sized absorber is around 4 percent lower than one suited for peak energy generation [133].

22.6.4 Application of CCS techniques in CO2 capture from natural gas A precise physical absorption is the dominant feature of natural gas, like that of a postcombustion [169]. Floating Production Storage and Offloading (FPSO) natural gas processing differs slightly from other natural gas processing. Because of its small footprint and modularity, membrane permeation creates a technology niche for FPSO natural gas processing. For example, the first FPSO began operations in the last decade for the pre-salt oil and gas field in Brazil [169], and it used membrane separation to separate the carbon dioxide. In 2016, seven PFSOs were operational [170], with six of them using membrane permeation to process around 4–7 MMscmd of natural gas with a CO2 content of almost 20 percent [171]. Natural gas, as measured by its partial pressure of carbon dioxide, as well as the location of the plant, are important considerations when choosing a natural gas processing technology. Chemical absorption is best performed with low carbon dioxide feed (less than 20 percent), owing to increased solvent recirculation and heat load due to higher carbon dioxide levels. The membrane permeation system is more suitable for middle to high partial pressures of carbon dioxide compared to the chemical absorption system. Pre-salt fields in Brazil’s offshore pre-oil fields (Jupiter: 78 percent, Libra: 48 percent,) and the La Barge gas field in Wyoming use cryogenic distillation and are therefore high in carbon dioxide. Unlike many of these

22.7 Prospects and future work considerations for CCS approaches

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projects, these projects produce liquid carbon dioxide, which is more easily transported via pipeline. But this advantage has its drawbacks. If the temperature is low and the pressure is high, the carbon dioxide may freeze out, requiring a more complicated method like the Ryan Holmes process [132,133,172,173]. An example of how the scientific community explores new methods of transporting gas and liquids is the Ormen Lang project, where two underwater 120 km pipelines are used to deliver natural gas and mono ethylene glycol as anti-hydrates [174]. It is possible to pipe natural gas to an onshore facility for fractionation of natural gas liquids and CO2 separation, with the CO2 , then piped offshore. A cryogenic distillation process is commonly used in large separation processes to reduce carbon dioxide composition, enabling physical or chemical absorption [172]. Further, membrane permeation was studied as a method for removing natural gas chemically [173].

22.7 Prospects and future work considerations for CCS approaches Carbon capture systems (CCS) have been presented as a probable solution, allowing the application of power plants which run on fossil fuels. This averts CO2 emissions upon reaching the atmosphere [175]. The importance of saving gas, coal (and biomass)-fired power plants in the energy mix is that they can accommodate fluctuations in demand more quickly than other energy sources. Most of the various components of the CCS chain have been demonstrated at or near a commercial scale, from capture to storage. However, integration into a single process has been found as a major engineering issue although this hurdle can be overcome [176]. To attain net-zero emissions of CO2 beyond 2050, a variety of cost-effective technologies are required to capture CO2 in the application including coal and natural gas power generation; production of fertilizer, ethanol, chemical and hydrogen; waste-to-energy; cement, iron and steel production [106,107,110]. Moreover, switching from coal to natural gas and improving energy efficiency seems to be the most low-cost options for decreasing the emission of moderate CO2 [106]. The prospects of CO2 capture from energy generation and some commercial sources have proven to be environmentally sustainable. This includes economic growth with storage in geologic formations (depleted oil and gas reserves, and deep aquifers) [106,107,177]. Consequently, energy conservation and the utilization of fossil fuels are shifting to low carbon-intensive energy sources. It is very prudent to thoroughly explore carbon sequestration portfolio options for addressing global climate change [177,178]. However, more work is required to prove the economic and technological sustainability of large-scale CCS [109]. This warrants environmental risk assessment to address the barriers and opportunities presented by legislation, regulation, and popular sentiment on CCS adoption in a country [107,109]. The application of AI has been suggested as a means to enhance the overall efficiency of CO2 capture [179]. In that study, the Bootstrap Aggregated Neural Network (BANN), a technique that will involve the direct incorporation of several neural networks to forecast the aftermath of a CO2 capture process using chemicals. When this is compared to a traditional ANN, the BANN technology is foreseen to deliver more accurate and reliable results. The BANN also has the advantage of being able to be implemented into an optimization framework. Overall, it can be concluded that using AI to calculate operational factors for carbon capture can result in a favourable outcome [176].

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Moreso, modification of existing technologies such as cryogenic separation, physiochemical absorption (MEA), adsorption, (zeolite), membrane separation and algae seeding could necessitate improvement of CO2 sequestration efficiency [109,112,113]. For instance, the use of physiochemical solvents such as MEA or ionic liquids via absorption for CO2 sequestration can make CO2 eco-friendlier than other greenhouse gases [108,180]. Aside, from the application of ionic liquids and other solid adsorbents, membranes with a high level of thermal, hydrological, and chemical stability can also be employed. Furthermore, the high solubility of gases in certain ionic liquids can be functionalized in conjunction with other CO2 sequestration techniques to enhance the performance and lower the cost of CO2 capture systems [112,113,178]. Similarly, engineered materials such as zeolite and activated carbon can have great selectivity for target gaseous components. In order to achieve high functionality of membranes for selective CO2 sequestration warrants doping with nanoparticles (Zeolite) or activating with ionic liquids [178]. Meanwhile, most of the hydrogen used in ammonia manufacturing, oil refining, and other industries is produced from decarbonizing fossil fuels. This also produces a by-product stream of CO2 , which offers low-cost potential for CCS for hydrogen production [106–108,110]. Above all, the prospects of CCS have great potential to mitigate the earth’s carbon footprint.

22.8 Conclusion The increase in the average earth’s temperature is the most significant factor in global warming caused by CO2 emissions. The majority of the blame for global warming may be attributed to anthropogenic activity. The combustion of fossil fuels for electricity generation and industrial purposes is the most common act linked with this topic. Currently, this activity is the most significant source of CO2 released into the environment. This set of underlying variables is what ultimately causes climate change and its adverse consequences. A transition in the amount of CO2 emitted into the environment has become unavoidable. The CCS from industrial facilities has proven critical in minimizing CO2 emissions. The deployment of CCS technology to the industrial sector is predicted to trigger a decrease in overall emissions of the industry. CCS is thus demonstrated to be extremely important in guaranteeing energy security. It is possible to decarbonize the power sector, which accounts for tremendous CO2 emissions by including carbon capture and storage (CCS) techniques, particularly in locations that rely heavily on fossil fuels for their electricity production. Although significant progress has been achieved, future research must focus on developing fresh technologies to enhance the current CCS to minimise the current operational cost. The improvement of the CCS technique will also necessitate the recognition of the environmental impact in each stage of the capture operation, from the capture to the transport and eventually to the CO2 storage. The most effective option for greenhouse gas capture remains the development of optimised hybrid capture technologies, which include one or more combinations of CCS technologies for the efficient collection and storage of GHGs. A thorough understanding of the fundamental principles that contribute to carbon sequestration in these systems or their modified variants will be beneficial in tailoring the characteristics of these technologies to make them suitable for capturing other gases, given that CO2 capture accounts for the largest proportion of their application. More importantly, since most of these gases are highly soluble in specific ionic liquids, these liquids can be

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C H A P T E R

23 Microbial carbon dioxide fixation for the production of biopolymers Tuba Saleem, Ijaz Rasul, Muhammad Asif and Habibullah Nadeem Department of Bioinformatics and Biotechnology, Government College University Faisalabad, Faisalabad, Pakistan

23.1 Introduction The anthropogenic activities have ruined the climate. The concentration of greenhouse gases (GHGs) has been disturbed due to unstoppable activities of human beings to meet the needs of daily life. Deforestation, transportation, burning of fossil fuels and industrialization are the major activities at present to mitigate the human needs. Thus, these activities have led an increase in the concentration of GHGs and the major increase in the emission of carbon dioxide (CO2 ) which causes global warming. At present, the concentration of CO2 in atmosphere is 419 ppm. Industrial revolution has raised the CO2 concentration up to 43 percent and it is estimated that it will increase up to 60 percent by 2100. The climate is changing due to the change in sea levels, ocean acidification, and thermohaline circulation etc. Burning of coal is the major reason for increase in CO2 concentration. Almost, 1 ton of coal burning leads to 2.5 tons of CO2 production which is a huge amount. The primary source of CO2 emission is the burning of fossil fuels accounting 77 percent of the total emissions [1]. The equilibrium state of carbon in the atmosphere has been disturbed due to the large emission of CO2 leading to global climatic change and this is the major environmental problem that is being faced by the world today. Therefore, the novel environmental challenge is to reduce the concentration of CO2 in the atmosphere via different approaches. Development of realistic approaches to capture, store and sequester the CO2 is the basic aim to avoid the adverse effects of global climate shift [2]. Increased concentration of carbon dioxide can be controlled by i. The replacement of fossil fuel based products, making the current engine technologies better ii. The reduction of CO2 emission in environment, and iii. Carbon dioxide capture and sequestration (CCS).

Green Sustainable Process for Chemical and Environmental Engineering and Science: Carbon Dioxide Capture and Utilization DOI: https://doi.org/10.1016/B978-0-323-99429-3.00015-1

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Hence the last stated approach is the best suited method [3,4]. Carbon sequestration is the capturing of atmospheric CO2 followed by the storage through mitigation processes including physical, chemical and biological processes [5]. The physical process involves the use of industrial plants sowed deep down in oceans emitting carbon dioxide or in the land surface where wells of old drained oil is present (geological strategy) known as underground CO2 storage. The chemical process is the formation of mineral carbonates. The biological mitigation process is the biotransformation of CO2 into organic biomass/industrial materials. However, physical and chemical processes have some drawbacks, and the biological process is used mostly. Bio-fixation of CO2 is carried out by photosynthetic (bacteria, algae and cyanobacteria) and non-photosynthetic organisms (chemolithotrophs). The photosynthetic organisms fix CO2 in terrestrial plants and chemolithotrophs oxidize inorganic compounds to form ATP. CO2 fixation by autotrophic biota and other microorganisms like cyanobacteria, algae, chemoautotrophic bacteria is one of the best mitigation options as these microorganisms has CO2 fixation mechanism assisted by some enzymes [6]. The synthetic plastic is used worldwide, and it was estimated in 2019 that the production process releases almost more than 850 million tons CO2 in the atmosphere that account 2 percent global output of CO2 . It is due to the use of fossil fuels for the synthesis of synthetic plastic and the major portion of CO2 emission account for production process and 9 percent is due to the incineration of plastic waste [7,8]. CO2 capture, conversion and utilization by photosynthetic microorganisms into bio-plastics is an ideal approach. It also contributes to the sustainable development goals (SDGs) inhibiting the use of petroleum based resources for plastic production and avoiding the use of fossil fuels. At the same time, it reduces the burden of plastic waste and the cost of plastic waste disposal in the environment [9]. Microorganisms use six pathways to assimilate CO2 with the assistance of carbonic anhydrase (CA) and RuBisCo (ribulose-1,5-bisphosphate carboxylase oxygenase) enzymes. Carbon capture and utilization (CCU) is an approach to reduce the increased atmospheric concentration of CO2 leading to an eco-friendly and sustainable environment. The production of value added products (biomaterials) through CCU approach is a welcoming step to make the environment free from plastic waste pollution [10]. Microorganisms accumulate polyhydroxyalkanoates (PHAs) as energy reserves in the form of intracellular granules to obtain food and to survive in the environment in response to stress conditions exerted by environment. Prokaryotic bacterial class of gram positive and negative can synthesize PHA. A number of pathways has been reported in microorganisms developed naturally for the accumulation of PHA depending upon their habitat, energy and survival requirements [11]. PHAs are biopolymers produced from microorganisms in nutrients stress condition in the form of intracellular granules when carbon is in excess and nitrogen, phosphorus, sulphur are in limiting concentrations. The carbon atoms present in the monomer structure classifies into 3 classes. One class of PHA is the short chain length (SCL) polymer consisting of 3–5 carbon atoms, second is the medium chain length (MCL) PHA of 6 to 14 carbon atoms and the third one is long chain length (LCL) polymers containing more than 15 carbon atoms. PHAs share same properties as of synthetic plastic. Therefore, synthetic plastic can be replaced with PHA (bioplastic). They are being used in packaging, pharmaceutical industry, agricultural sector and drug delivery agents [12].

23.3 Sequestration methods of CO2

Fossil fuels burning CO 2 emission

519

Ship 2.25% Road 16%

Air 2.55% Industrial process Cement Iron and steeal Residential buildings 14.67% Chemicals and fisheries (2.42 and 1.07%) Land use CO2 Incineration 2.42% emission Land of crops 1.8% Land of forest 2.96%

FIGURE 23.1 CO2 emission through different anthropogenic activities.

This chapter covers the sources of carbon dioxide emission, carbon concentrating mechanism (CCM), CO2 sequestering pathways in microbes, and the production of PHAs, a value added product important for environmental stability.

23.2 Sources of CO2 emission Anthropogenic activities and natural system are the leading sources of CO2 emission, which leads to global warming and a disturbance in greenhouse gases. The natural system involves the sinks, oceans, respiratory and volcanic activities. Of all these, oceans contribute up to 42.84 percent emission of carbon dioxide. Oceans have more CO2 concentration as compared to atmosphere and a number of CO2 diffusion activities are carried out in oceans which release CO2 . It was reported that the global emission of CO2 by oceans is 330 giga tons. The CO2 generation from construction work and manufacturing processes was 6.1 bn tons in 2016. In addition, 220 giga tons of carbon dioxide emission is carried by respiration activities of animals and plants which is the 28.56 percent amount of natural system. Beside these sources, microbial respiration, soil animal’s respiration and plant’s roots respiration are the other natural sources. The decomposition activities of soil accounts for the CO2 emission. And the volcanic activities lead to 0.26 billion tons CO2 emission annually [13,14]. In addition to the natural sources, anthropogenic activities cause tremendous addition of CO2 to the environment, increasing industrialization, use of fossil fuels; increasing population levels and the deforestation (land use for agricultural and industrial processes) are the main causes of CO2 emission. It has been noted that the CO2 emission by anthropogenic activities is 66.6 percent than the GHG emission globally which is terrific amount. Fig. 23.1 describes the amount of CO2 emission by different anthropogenic activities [15].

23.3 Sequestration methods of CO2 CO2 sequestration is the process of capturing and fixing the atmospheric CO2 to any form which do not contribute to the global warming. There are different biological and

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CO2 Sequestration methods

Biological method

Microbial sequestration

Non-biological method

Terrestrial sequestration

Geological sequestration

Oceanic sequestration

Chemical sequestration

FIGURE 23.2 Types of CO2 sequestration.

non-biological methods of CO2 sequestration [16]. Biological methods include terrestrial and microbial processes and non-biological methods include the geological, chemical and oceanic or physical processes. These methods are used to reduce the CO2 concentration to control the climate change. Oceanic sequestration of CO2 is the process in which CO2 is stored deep down in oceans for a long time period. Geological sequestration includes the capturing of anthropogenic CO2 into deep underground rocks before their release to the atmosphere and the conversion of atmospheric CO2 into the stable carbonated form is chemical sequestration method [17]. Terrestrial sequestration is the capturing and storage of CO2 into the soil and plant’s biomass (roots and stem) being the biological method. The microbial sequestration is the process of carbon seizer and storage using microbes. Microbes trap the inorganic carbon and convert them to organic form. A number of autotrophic microorganisms like algae, archaea, bacteria and cyanobacteria seize the anthropogenic CO2 through photosynthesis. Some of the microorganisms are capable of trapping and reacting with atmospheric CO2 naturally. The enzymes which are used in biological sequestration process are carbonic anhydrase, phosphoribulokinase (PRK) and RuBisCO [18]. Sequestration methods are enlisted in Fig. 23.2.

23.4 Carbon concentrating mechanisms Carbon is a very important part of life on earth. It is found in two forms on earth, one is the CO2 gas and the other is the oxidized carbonate minerals. These oxidized forms of carbon can be changed by using autotrophic microbial species into more suitable rich organic form of biomass. The changes being made in environment or atmosphere has changed the O2 and CO2 levels which made the autotrophic microbial species to evolve as carbon concentrating mechanism (CCM) organisms. CCM can concentrate the CO2 by 1000 times more in RuBisCOs active site. A microbial micro component i.e., carboxysomes act as key component for CCM that

23.6 Carbon dioxide fixation pathways

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uses RuBisCO and carbonic anhydrase (CA) enzymes. Carboxysomes containing microbes are found in the environment deprived of carbon dioxide concentration or the lesser amount than the Michaelis constant (Km ) of RuBisCO. The need of the carbon is fulfilled by the CCM which accumulates the Ci in cytosolic pool or in the form of bicarbonates, the extracellular concentration. At times, RuBisCO has failed to concentrate the bicarbonates, so the cytosolic bicarbonate is concentrated to CO2 by the help of CA enzyme in carboxysomes. The activity of CA for the dehydration of bicarbonates and vice versa is an efficient role for CCM [3]. Carbonic anhydrase and carboxysomes are the key players of carbon concentrating mechanism (CCM) [6].

23.5 Advancements in carbon capture and storage & carbon capture utilization Carbon capture and storage (CCS) is the technology of carbon capture and their storage in an appropriate way. The main purpose of designing the CCS is the long-term storage of carbon dioxide to inhibit its escape from atmosphere. It is considered that CO2 storage in deep oceans and underground geological points formation is the best approach [19]. But it can be risky due to the leakage of CO2 and ocean acidification can happen. So, the escape of CO2 from underground geological storage points is the drawback of CCS. The CCS method is modified to CCUS which uses the concentrated CO2 built at CCS units [20]. Advancement in carbon capture is the CCU, which is the utilization of CO2 captured from emission points and its conversion to value added products (biomaterials). This process reduces the level of CO2 emission and also relegate the utilization of fossil-based resources [21].

23.6 Carbon dioxide fixation pathways Microorganisms trap atmospheric CO2 and from other sources to produce extra cellular substances and for cell growth in the form of organic biomass doing bio fixation of CO2 . A number of microbial species have tendency of CO2 bio fixation, and not mere restricted to the photosynthetic microbial community like cyanobacteria, algae and plants. This categorization of microbial community is classified into aerobic and anaerobic groups based on their ability of oxygen uptake and enzyme machinery. Autotrophic microorganisms belong to the archaeal (single-celled prokaryotes) and bacterial domain. Algae are the noteworthy microorganisms which can perform photosynthesis and can fix CO2 . They are mostly eukaryotic in nature and found all over the biosphere. Cyanobacteria are another group of microalgae used for bio fixation of CO2 with unique characteristics. There are six metabolic pathways reported for the bio fixation of atmospheric CO2 in microorganisms [22]. Fig. 23.3 describes the common CO2 fixation pathways, sequestration methods and biopolymer production with biotechnological scope.

23.6.1 Calvin cycle This is the first autotrophic carbon dioxide fixation pathway found in plants identified by Melvin Calvin, Andrew A. Benson and James A. This cycle is also known as reductive pentose

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CCMs Atmospheric CO2

Inorganic carbon transporters

Industry released CO2

Pyrenoids

Biomass burning CO2

Carbonic anhydrases Carboxysomes C4photosynthesis

Malate HCO3-

Carbon fixation pathways

CO2

Acetyl Co-A Succinyl Co-A

Carbon utilization

Biomaterials

Pyruvate

FIGURE 23.3 Overview of carbon dioxide fixation utilizing carbon sequestering mechanism for the production of biomaterials.

pathway and has been reported in many eukaryotes and prokaryotes. Algae have evolved the carbon concentrating mechanism (CCM) which maintains the imbalance between increasing inorganic carbon and low CO2 concentration. This mechanism makes the use of energy for maintaining increasing CO2 concentration. This mechanism is assisted by anhydrases found in RuBisCO having sub-cellular micro-compartments. This pathway fixation cycle consists of 13 enzymatic reactions. In the first reaction, carboxylation of ribulose-1,5-bisphosphate secreting two 3-phosphoglycerate (3-PGA) molecules with the help of RuBisCO enzyme. Then the reduction of 3-phosphoglycerate takes place and it is converted to glyceraldehyde 3-phosphate through an intermediate of 1,3-diphosphoglycerate using ATP molecules. In next reaction, glyceraldehyde 3-phosphate is converted to ribulos-1,5-bisphosphte (RuBP) through phosphoribulokinas (PRK) enzyme while the intermediate formed in this reaction was ribulose 5-phosphate. RuBP is the primary CO2 acceptor, which is mainly formed through PRK enzyme, the necessary one of CCM mechanism. Sedoheptulose bisphosphate formation step is also unique to this cycle. In these 13 enzymatic reactions, 1 is the fixation of CO2 producing two molecules of 3-PGA (phosphoglycerate), which needs RuBP, CO2 , and H2 O for reaction completion. The remaining 12 reactions includes the generation of RuBP, and three molecules of RuBP are formed by the fixation of three CO2 molecules, and as a result six molecules of 3-PGA are formed. Among these, five molecules of 3-PGA are used for the formation of RuBP again to fix further CO2 and the remaining one molecule of 3-PGA is used in the synthesis of cellular biomaterials. Hence this Calvin cycle is completed mainly by three enzymes named RuBisCO, sedoheptulose bisphosphate and PRK [23]. Fig. 23.4A depicts the cyclic pathway of calvin cycle.

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23.6 Carbon dioxide fixation pathways

(A)

CO2

(B)

Acetyl Co-A

H 2O

H 2O

Glycerald ehyde 3phosphate

H 2O

CoASH

FIGURE 23.4 Overview of carbon dioxide fixation pathways (A) Calvin Benson cycle, (B) Reductive TCA cycle, (C) Wood-Ljungdahl pathway, (D) Dicarboxylate–4-hydroxybutyrate cycle, (E) Malyl Co-A/ 3-hydroxypropionate pathway, and (F) Hydroxy propionate-hydroxybutyrate cycle.

23.6.2 Reductive TCA cycle This carbon dioxide fixation pathway came to known in 1966, an autotrophic pathway. The name tells us that it is a reductive direction pathway and the reverse form of citric acid cycle. It is also termed as the alternative of Calvin cycle and both bacterial and archaeal domain can carry this reaction type. Mostly, green sulphur bacterium (Chlorobium thiosulphatophilum) has been known for this reaction. The starting reaction of this pathway is the lysis reaction of citrate in to two molecules (acetyl Co-A and oxaloacetate) by the usage of ATP-citrate lyase (ACL). Thus, the previously formed oxaloacetate is reduced to malate in the presence of malate dehydrogenase enzyme which is further converted to fumarate in presence of fumarate dehydrogenase. In next step, fumarate reductase enzyme carries the reduction of fumarate to succinate, which is further reduced to succinyl Co-A by the action of succinyl Co-A synthase enzyme. Then the reductive carboxylation of succinyl Co-A takes place through 2-oxoglutarate; ferredoxin oxidoreductase (OGOR) to form 2-oxglutarate. In the final step, isocitrate is formed and citrate is regenerated from isocitrate through aconitase enzyme, and isocitrate itself is formed by 2-oxoglutarate by the action of isocitrate dehydrogenase enzyme. Acetyl Co-A is the end product of citrate and cleavage converted to pyruvate by a ferredoxin dependent carboxylating enzyme, pyruvate; ferredoxin oxidoreducatse POR) [18]. Fig. 23.4B is the diagramatic representation of this cycle.

23.6.3 Wood-Ljungdahl pathway It is the reductive acetyl Co-A pathway carried out in microbes. Mainly this pathway is used for the production of methane or acetic acid, as autotrophic acetogens and methanogens use

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(C)

CO2

(D) CoASH

Acetyl Co-A

CO2

CoASH

H2O HCO3-

CoASH

CoASH

CoASH

FIGURE 23.4, cont’d.

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23.6 Carbon dioxide fixation pathways

(E) Glyoxylate

Pyruvate Methylmalyl Co-A

Mesoconyl-C1-Co-A

1st stage CO2 fixation

Citramalyl Co-A

2nd stage glyoxylate Mesoconyl-C4-Co-A accumulation

(F)

FIGURE 23.4, cont’d.

this pathway and synthesize the acetic acid and methane by bio fixation of CO2 . Mostly anaerobic microorganisms work best in this cycle. Some of them are Proteobacteria, Spirochaetes, Planctomycetes, and Euryarchaeota. This pathway does not assist the regeneration of primary CO2 acceptor unlike other pathways. This pathway sometimes makes hydrogen use as electron donor and CO2 as electron acceptor by microbes. In addition, they are also used in the synthesis of cellular materials as the building blocks.

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This pathway is started by the simultaneous oxidation–reduction reaction of two CO2 molecules. Both of them enter to the pathway by unfamiliar path which are termed as western and eastern branches of pathway. Carbon monoxide dehydrogenase (CODH) is the main enzyme in this pathway which carries four crucial functions. One is the oxidation of CO to CO2 , other is the reduction of CO2 to CO, third is the C1 intermediate synthesis from CO and lastly, the formation of acetyl Co-A by the condensation of C1 intermediate with methyl and Co-A group. The CO2 at eastern branch is reduced to methyl group of acetate molecule hence the branch is also termed as methyl branch. Meanwhile the reduction of CO2 at methyl branch is completed in a number of steps including various cofactors and enzymes. Western branch is known as carbonyl branch consisting of the incorporation of CO2 into acetate carboxylic group. The following cycle is shown is Fig. 23.4C.

23.6.4 Dicarboxylate–4-hydroxybutyrate cycle This pathway is known in the anaerobic members of Desulfurococcales. This pathway starts by the dehydration of 4-hydroxybutyryl Co-A producing crotonyl Co-A by the action of 4hydroxybutyryl Co-A dehydratase enzyme. Then the two molecules of acetyl Co-A are formed by the β-oxidation of crotonyl Co-A [24,25]. the whole pathway has been drawn in Fig. 23.4D.

23.6.5 Malyl Co-A/3-hydroxypropionate pathway (3-hydroxypropionate bicycle) This is the fifth reported carbon dioxide bio fixation pathway which is famous for the renaissance of acetyl Co-A, the preliminary substrate of the 3-hydroxypropionate pathway. This pathway was first recognized by Holo and Sirevag when they were characterizing the assimilation mechanism of CO2 in Chloroflexus aurantiacus OK-70. This pathway is the collection of 13 reactions using 13 enzymes, all are oxygen insensitive. The first reaction is the carboxylation reaction. Acetyl Co-A is carboxylated to form malonyl Co-A, and the reaction is speed up by the ATP dependent acetyl Co-A carboxylase enzyme. The second reaction is the reduction of previously formed malonyl Co-A to form 3-hydroxypropionate by 3-hydroxypropionate dehydrogenase. This second reaction is a two-step reaction involving the formation of intermediate malonate-semialdehyde product by malonate-semialdehyde dehydrogenase enzyme. The next three steps include the translation of 3-hydroxypropionate to propionyl Co-A. 3-Hydroxypropionate Co-A and acryloyl Co-A are intermediates in these reactions catalyzed by three enzymes (3-hydroxypropionate Co-A ligase, 3-hydroxypropionyl Co-A dehydratase, and acryloyl Co-A reductase). The reductive conversion of hyroxypropionate to propionyl Co-A is carried out by propionyl Co-A synthase. In the next reactions, methyl malonyl Co-A and succinyl Co-A are formed by the action of propionyl Co-A carboxylase, methyl malonyl Co-A emiperase and methyl malonyl Co-A mutase. Further succinyl Co-A is converted to malyl Co-A by the action of succinyl-Co-A: malate Co-A transferase. After this succinyl Co-A is converted to succinate by transferring its Co-A group to malate. In the next step succinate is transformed into malate by succinate dehydrogenase and making fumarate as an intermediate by the action of fumarate hydratase. In the last step, the previously Co-A released group of succinyl Co-A bound to malate is transformed to malyl Co-A which further cleaves to form acetyl Co-A and glyoxylate by the help of malyl Co-A lyase. So, the

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acetyl Co-A is again used as a starter molecule of this pathway and glyoxylate is used in the formation of acetyl Co-A and pyruvate by making association with propionyl Co-A. Pyruvate act as the precursor molecule for all the metabolic reactions [26]. Fig. 23.4E describes the cyclic representation of stated pathway.

23.6.6 Hydroxy propionate-hydroxybutyrate cycle This is the cyclic pathway in aerobic and autotrophic cells. This pathway was located in the cell extract for the first time of a model organism (Metallosphaera sedula). This cycle is completed in two parts, first part is the followed by the formation of succinyl Co-A by the transfer of acetyl Co-A and two bicarbonate molecules, and second part is the formation of two acetyl Co-A molecules from succinyl Co-A. The first part follows the same pathway steps as of 3hydroxypropionate, but the enzymes involved in catalyzation are different. This pathway has been reported in archaeal domain of Crenarchaeota. This cycle is an autonomous process also known as archaeal cycle. Fig. 23.4F indicates the reactions in a mannered way.

23.7 Factors affecting the carbon dioxide biofixation Several abiotic factors affect the microbial activity which is involved in the CO2 fixation. To expand the knowledge and making the technology advanced, it is necessary to study these abiotic factors to enhance the microbial activity. Light is needed for the Calvin Benson reaction for the efficient working of RuBisCO, ATP, and NADPH during photosynthesis. So, it is necessary to have proper light during the reaction. The extra amount of light can damage the proteins which are responsible for electron transfer in photosynthesis, consequently it reduces the carbon dioxide fixation rate [27]. Temperature and pH are also the crucial factors which affect the microbial activity as the microbes need specific temperature for their growth and proper working in CO2 fixation. The lower temperature affects the oxygenase activity of RuBisCO enzyme which is a key enzyme of metabolic pathways of CO2 fixation. Hence the carbon concentrating mechanism is also get retarded. Temperature is also necessary for cell size, metabolic reaction kinetics and cell membrane components [28].

23.8 Production of biopolymers/bioplastics The worldwide production of synthetic plastics was gone beyond 360 million tons in 2020 and its waste is not disposed of properly. So, the need of hour is to replace the synthetic plastics due to their harms caused to environment and human health. The solution to this problem is the production of bioplastics which is totally bio-based and biodegradable. Biobased plastics are generated from renewable resources (plant biomass or organic material). Bioplastics are considered environment friendly, sustainable and non-toxic. One of the main classes of bioplastics is polyhydroxyalkanoates (PHA). PHA is the intracellular granules in microbes acting as energy and carbon reserves (Faizan [29,30]). PHA production from CO2 is an efficient way of accumulating CO2 from atmosphere and makes a decrease in GHG accumulation. The

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23. Microbial carbon dioxide fixation for the production of biopolymers

Pathway III

Pathway II Pathway I

Fatty acids Denovo synthesis

Acyl-CoA

EOxidation

R-3-HydroxyacylACP

Acetoacetyl-CoA

R-3-HydroxyacylCoA

PhoG

R-3Hydroxyacyl-CoA

3-HydroxybutyrylCoA

PHA synthase

PHA synthase PHA synthase MCL-PHA MCL-PHA

SCL-PHB

SCL-MCL-PHA

FIGURE 23.5 Biosynthesis pathways of PHA.

worldwide production of polymers is surpassing almost 200 million tons annually. Carbon dioxide is used as a primary carbon source for the synthesis resins of urea-formaldehyde and melamine-formaldehyde. The transesterification of CO2 and dimethyl carbonate or the CO2 reaction with epoxides produces polyalkylene carbonates which are further used in the synthesis of polyurethanes, elastomers, adhesives and varnishes etc. Besides this, the reaction of CO2 with formic acid and 1,3,5-trioxane can generate polyoxymethylene that can replace polypropylene, polyethylene and a strong competitor of polyolefins [31]. PHA is a unique class of bioplastics which have remarkable properties like biodegradability, sustainability, non-toxic and biocompatibility. It covers the thermal and mechanical characteristics as of petrochemical based plastics. A community of microorganisms can produce PHA, this community involves extremophiles, algae, recombinant bacteria, cyanobacteria and transgenic plants which can lead to a higher production rate of bioplastics. A number of waste carbon sources are used in the synthesis of PHA which proved to be the cheap carbon source accommodating the economic value of PHA [32]. PHA are used in the medical applications like drug delivery, tissue engineering and dressings [33].

References

529

Autotrophic microorganisms such as purple bacteria, cyanobacteria and hydrogen oxidizing bacteria have shown the ability to synthesize PHA using CO2 as inexpensive carbon source. Researchers are keen to produce PHA using CO2 . A study was conducted to check the efficiency of microbial mutualism of CO2 fixation. A co-culture of Azotobacter vinelandii and engineered Synechococcus elongates PCC7942 was used [34]. Polyhydroxyalkanoates are produced by different pathways using microbes under nutrients limitation. These pathways are carried out by different PHA-enzymes which rapidly synthesizes the PHA polymer [35]. The main player in these pathways is the acetyl Co-A which is converted to PHA through a number of steps undertaking different enzymatic reactions [36]. Different metabolic biosynthesis pathways of PHA are explained in Fig. 23.5.

23.9 Conclusion This review mainly targets the microbial cells to act as capturing components of carbon dioxide to mitigate the CO2 concentration in atmosphere. It is a biological process of carbon dioxide fixation. In future it might be possible that microbial cells work as natural factories to reduce the CO2 concentration. The bacterial cells have ability of bio fixation of carbon dioxide and to sequester it using different enzymes in different autotrophic pathways under nutrients limiting conditions. Microbes can be used for CO2 sequestration and can be beneficial for biopolymers production. It is an eco-friendly and sustainable approach for carbon dioxide sequestration through microbial cells. Now the recent developments in technology and metabolic engineering have led the advancement in CO2 capturing, storage and utilization (CCS & CCU) which in turn serve as producing factory of value added products.

References [1] Desai M, Harvey RP. Inventory of US greenhouse gas emissions and sinks: 1990–2015. Fed Regist 2017;82:10– 1002. [2] Mistry AN, Ganta U, Chakrabarty J, Dutta S. A review on biological systems for CO2 sequestration: organisms and their pathways. Environ Prog Sustain Energy 2019;38:127–36. [3] Effendi SSW, Ng I-S. The prospective and potential of carbonic anhydrase for carbon dioxide sequestration: a critical review. Process Biochem 2019;87:55–65. [4] Sharma T, Sharma S, Kamyab H, Kumar A. Energizing the CO2 utilization by chemo-enzymatic approaches and potentiality of carbonic anhydrases: a review. J Clean Prod 2020;247:119138. [5] Abu-Khader MM. Recent progress in CO2 capture/sequestration: a review. Energy Sources Part A 2006;28:1261– 79. [6] Bharti S, Pattanaik KK. Dynamic distributed flow scheduling for effective link utilization in data center networks. J High Speed Netw 2014;20:1–10. [7] Rosenboom J-G, Langer R, Traverso G. Bioplastics for a circular economy. Nat Rev Mater 2022:1–21. [8] Zheng J, Suh S. Strategies to reduce the global carbon footprint of plastics. Nat Clim Change 2019;9:374–8. [9] Miyasaka H, Okuhata H, Tanaka S, Onizuka T, Akiyama H. Polyhydroxyalkanoate (PHA) production from carbon dioxide by recombinant cyanobacteria. Environ Biotechnol New Approaches Prospect Appl 2013:197– 215. [10] Singh J, Dhar DW. Overview of carbon capture technology: microalgal biorefinery concept and state-of-the-art. Front. Mar Sci 2019;6:29.

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[11] Kumar M, Gupta A, Thakur IS. Carbon dioxide sequestration by chemolithotrophic oleaginous bacteria for production and optimization of polyhydroxyalkanoate. Bioresour Technol, International Conference on New Horizons in Biotechnology (NHBT-2015) 2016;213:249–56. [12] Muneer F, Rasul I, Azeem F, Siddique MH, Zubair M, Nadeem H. Microbial polyhydroxyalkanoates (PHAs): efficient replacement of synthetic polymers. J Polym Environ 2020;28:2301–23. [13] Saito T, Fang X, Stohl A, Yokouchi Y, Zeng J, Fukuyama Y, et al. Extraordinary halocarbon emissions initiated by the 2011 Tohoku earthquake. Geophys Res Lett 2015;42:2500–7. [14] Zhou X, Chen Z, Cui Y. Environmental impact of CO2 , Rn, Hg degassing from the rupture zones produced by Wenchuan M s 8.0 earthquake in western Sichuan. China Environ Geochem Health 2016;38:1067–82. [15] Ritchie H, Roser M, Rosado P. CO2 and Greenhouse Gas Emissions. Our World Data 2020. [16] Nogia P, Sidhu GK, Mehrotra R, Mehrotra S. Capturing atmospheric carbon: biological and nonbiological methods. Int J Low-Carbon Technol 2016;11:266–74. [17] Zhu Q. Developments on CO2 -utilization technologies. Clean Energy 2019;3:85–100. [18] Maheshwari N, Thakur IS, Srivastava S. Role of carbon-dioxide sequestering bacteria for clean air environment and prospective production of biomaterials: a sustainable approach. Environ Sci Pollut Res 2022:1–22. [19] Daneshvar E, Wicker RJ, Show P-L, Bhatnagar A. Biologically-mediated carbon capture and utilization by microalgae towards sustainable CO2 biofixation and biomass valorization – A review. Chem Eng J 2022;427:130884. [20] Kamkeng AD, Wang M, Hu J, Du W, Qian F. Transformation technologies for CO2 utilisation: current status, challenges and future prospects. Chem Eng J 2021;409:128138. [21] Vickers NJ. Animal communication: when i’m calling you, will you answer too? Curr Biol 2017;27:R713–15. [22] Maheshwari N, Kumar M, Thakur IS, Srivastava S. Production, process optimization and molecular characterization of polyhydroxyalkanoate (PHA) by CO2 sequestering B. cereus SS105. Bioresour Technol 2018; 254:75–82. [23] Kajla S, Kumari R, Nagi GK. Microbial CO2 fixation and biotechnology in reducing industrial CO2 emissions. Arch Microbiol 2022;204:1–20. [24] Li L, Yang Y, Lv Y, Yin P, Lei T. Porous calcite CaCO3 microspheres: preparation, characterization and release behavior as doxorubicin carrier. Colloids Surf B Biointerfaces 2020;186:110720. [25] Sharma T, Sharma S, Kamyab H, Kumar A. Energizing the CO2 utilization by chemo-enzymatic approaches and potentiality of carbonic anhydrases: a review. J Clean Prod 2020;247:119138. [26] Kumar M, Sundaram S, Gnansounou E, Larroche C, Thakur IS. Carbon dioxide capture, storage and production of biofuel and biomaterials by bacteria: a review. Bioresour Technol 2018;247:1059–68. [27] Morales M, Sánchez L, Revah S. The impact of environmental factors on carbon dioxide fixation by microalgae. FEMS Microbiol Lett 2018;365:fnx262. [28] Gani P, Sunar NM, Matias-Peralta HM, Apandi N. An overview of environmental factor’s effect on the growth of microalgae. J Appl Chem Nat Resour 2019;1. [29] Muneer F, Nadeem H, Arif A, Zaheer W. Bioplastics from Biopolymers: an Eco-Friendly and Sustainable Solution of Plastic Pollution. Polym Sci Ser C 2021;63:47–63. [30] Filho WL, Barbir J, Abubakar IR, Paço A, Stasiskiene Z, Hornbogen M, et al. Consumer attitudes and concerns with bioplastics use: an international study. PLoS One 2022;17:e0266918. [31] Gupta R, Mishra A, Thirupathaiah Y, Chandel AK. Biochemical conversion of CO2 in fuels and chemicals: status, innovation, and industrial aspects. Biomass Convers Biorefinery 2022:1–24. [32] Angra V, Sehgal R, Gupta R. Trends in PHA Production by Microbially Diverse and Functionally Distinct Communities. Microb Ecol 2022:1–14. [33] Sharma V, Sehgal R, Gupta R. Polyhydroxyalkanoate (PHA): properties and Modifications. Polymer (Guildf) 2021;212:123161. [34] Smith MJ, Francis MB. A designed A. vinelandii–S. elongatus coculture for chemical photoproduction from air, water, phosphate, and trace metals. ACS Synth Biol 2016;5:955–61. [35] Khatami K, Perez-Zabaleta M, Owusu-Agyeman I, Cetecioglu Z. Waste to bioplastics: how close are we to sustainable polyhydroxyalkanoates production? Waste Manag 2021;119:374–88. [36] Pakalapati H, Chang C-K, Show PL, Arumugasamy SK, Lan JC-W. Development of polyhydroxyalkanoates production from waste feedstocks and applications. J Biosci Bioeng 2018;126:282–92.

C H A P T E R

24 Carbon dioxide capture and its enhanced utilization using microalgae Pinku Chandra Nath a, Biswanath Bhunia a and Tarun Kanti Bandyopadhyay b a

Department of Bio Engineering, National Institute of Technology Agartala, Jirania, Tripura, India b Department of Chemical Engineering, National Institute of Technology Agartala, Jirania, Tripura, India

24.1 Introduction The accumulation of greenhouse gases in the ecosystem as a result of human activity and development has been identified as the primary source of climate change and as one of the most pressing global issues confronting humanity [1]. Although major efforts have been made to reduce such emissions, CO2 catch is a potential option [2]. Acien et al. (2012) [3], proposed in the 1960s that microalgae could play a role in CO2 sequestration because of their higher solar energy yield, environmental tolerance, and year-round cultivation potential compared to higher plants [4,5]. By recycling CO2 into biofuels [6], or producing enhanced goods from flue gases, microalgae can help decrease global warming emissions. In addition, microalgae can minimize CO2 emissions from the treatment of wastewater by reducing the required energy during oxygenation [7] or biogas upgrading [8]. Nevertheless, the commercialization of 15,000 tonnes [9] of microalgal biomass has increased global CO2 emissions (around 30,000 tonnes of CO2 ). As a result, this relatively low contribution from designed processes highlights the need to improve efficiency and enhance and upgrade the latest resources to produce bigger amounts of algal biomass, because photosynthetic carbon fixation requires natural sunlight, large-scale outdoor growing of microalgae remains the only commercially feasible alternative for cell and biofuel synthesis [10–14]. It has been suggested that microalgal CO2 fixation could become commercially viable once production expenses drop less than $500 t−1 ha−1 y−1 [15].

Green Sustainable Process for Chemical and Environmental Engineering and Science: Carbon Dioxide Capture and Utilization DOI: https://doi.org/10.1016/B978-0-323-99429-3.00031-X

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c 2023 Elsevier Inc. All rights reserved. Copyright 

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24. Carbon dioxide capture and its enhanced utilization using microalgae

CO2 removal efficiency, biomass output, and CO2 fixation capacities of a wide range of microalgae operating under various operational settings have all been demonstrated in a comprehensive evaluation [16] are dependent on the microalgae species, light, CO2 source, and bioavailability, photobioreactor design, and operating conditions [17–20]. The photosynthetic capacity factor, which is the quantity of light energy converted into chemical energy and serves as the foundation for the anabolic reactions that lead to biomass synthesis, is an important element in biomass productivity. Theoretically, the most solar energy that can be turned into biomass is about 12 percent [21–23]. Because the photosynthetic system can only absorb 27 percent of the photo-synthetically active radiation, this estimate assumes that just 47 percent of the solar radiation spectra are accessible for respiration (PAR). Phototransmission losses provide an extra 10 percent to the overall reduction. However, in outdoor settings [12,24], PE values of 1.5 percent were found for open systems and 3.5–6 percent for photobioreactor (flat panel or tubular), respectively, and a PE value of 6 percent was reported in the laboratory [24]. A greater PE ought to make it possible to achieve larger productivities at lower prices [25,26].

24.2 Photosynthesis and CO2 fixation using microalgae 24.2.1 Photosynthesis Photosynthesis permits microalgal CO2 fixing and the formation of organic compounds that sustain life. When sunlight hits chlorophyll and carotenoids in the photosystems (I and II), its converted the absorbed energy to ATP/NADPH, which are high-energy and reducing molecules that are used in the light-dependent cycle to produce molecular oxygen. These are then utilized in the Calvin Benson cycle to collect CO2 via the Rubisco (bisphosphate carboxylase-oxygenase) enzyme. Shao et al. (2014) [27] and Hugler et al. (2011) [28], provide information regarding the scale time of these processes. The low specificity of Rubisco is made up for by the CO2 concentrating mechanism (CCM), which increases the CO2 concentration by up to 1000 times compared to the CO2 in the fluid medium [29] and slows down photosynthesis [30].

24.2.2 CO2 fixation Even though microalgae eventually create CO2 overnight, the net CO2 absorption remains positive. The efficiency with which microalgae utilize light and the density of their cells impact the rate at which they fix CO2 . Anthropogenic CO2 can be employed as a carbon source in the photoautotrophic growth of microalgal CO2 fixation. To evaluate a microalgal cultivation system’s ability to remove CO2 directly, measurements of biomass and growth rates are essential [16,31]. The ratio of the CO2 concentration of the incoming effluents to the CO2 concentration of the effluents being discharged can be utilized to observation the CO2 removal efficiency of a photobioreactor that is used for microalgal culture. The effectiveness of removal expressed as a percentage can be calculated by the equation [32]. Efficiency (%) =

CO2 influent − CO2 effluent × 100 CO2 influent

(24.1)

24.3 Cultivation systems for carbon dioxide capture by microalgae

533

In a closed culture, the amount of carbon dioxide that can be removed or fixed based on the (a) algal strains, (b) amount of carbon dioxide, (c) the layout of the photobioreactor, and (d) the operational characteristics [32,33]. Chlorella vulgaris has the highest CO2 extraction value of 54.5 percent at 0.20 percent CO2 in a membrane-bioreactor. Scenedesmus obliquus and Spirulina sp. have the highest CO2 percentage removal of 10–15 percent and 30–40 percent, respectively, in a three serial tubular-photobioreactor [16], while their CO2 fixation value was reduced to 8.2 percent and 5.6 percent under 13 percent carbon dioxide aeration [34]. Although, the efficiency of CO2 separation and fixation depends on the strains of microalgae and their physiological states like their ability to grow cells and their carbon dioxide metabolism. This is the case even though all microalgae are capable of removing CO2 from the atmosphere. Carbon dioxide (CO2 ) fixation value can be enhanced by the quantity of carbon contained within a microalgal cell [35]. The following is the formula that was used to get the fixation rate:   MCO2 (24.2) RCO2 = CC × μL × MC Where, RCO2 : rate of fixation ((carbon dioxide (g)/m3 ·h)); μL: rate of growth (g/m3 .h); MCO2 : CO2 (molecular weight); MC : elemental C; and CC : concentration of carbon

24.3 Cultivation systems for carbon dioxide capture by microalgae A utilize microalgal to extract CO2 requires a wide range of concurrent operations. It beings with gaseous CO2 transfer to the growth media, where algae develop and continue unless carbon dioxide is fixed as biomass. Finding strains that are good at capturing CO2 and a proper cultivation system, taking into account configuration and operational variables like photosynthetic rate, heat, pH, and mixing, are two major problems when trying to maximize carbon fixation efficiency. Table 24.1 shows optimal microalgae growing conditions.

24.3.1 Physico-chemical properties and carbon dioxide sources CO2 is used by microalgae as a source of carbon. Without CO2 , these microorganisms won’t be able to grow. Most of the time, productivity is limited by not having enough CO2 . So, reducing CO2 in the air with microalgal photosynthesis is thought to be safe and good for the ecosystem [36]. The growth overall performance of a few microalgae (Chlorella sp.), may be negatively suffering from CO2 at a better than 5 percent (v/v) concentration [37]. However, a few algal strains can develop below a flue gasoline CO2 concentration limit (10 to 15 percent), but the carbon fixation and biomass manufacturing charges are less than that underneath decreasing carbon dioxide attention. Few numbers of microalgae strains are capable of tolerating extraordinarily excessive carbon dioxide levels of as much as 80 percent (Chlorella ZY-1) and 100 percent (Chlorella T-1). For most microalgal species, the ultimate Concentration of CO2 is normally suggested to be 0.037–15 percent; for example, most utilization became found at 2.6 percent CO2 for algae (Chlorella sp.) [38], 7 percent for Chlorella kessleri, and Scenedesmus obliquus [34].

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24. Carbon dioxide capture and its enhanced utilization using microalgae

TABLE 24.1 Optimal microalgae growing conditions. Microalgae

pH

CO2 Tolerance (%)

Temperature (°C)

Irradiance (μmol·m−2 ·s−1 )

Mixing

References

Chlorella sp.

7

40

27

100

Aeration

[80,81]

Spirulina platensis

9

10

30

330

Aeration

[82,83]

Spirulina maxima

9

10

35

320

Aeration

[84,85]

Euglena gracilis

7

45

28

100

Fermenter

[86–88]

Haematococcus pluvialis

7

34

27

90

Aeration

[89,90]

Scenedesmus obtusiusculus

8

15

35

300

Aeration

[91]

Dunaliella sp.

7

10

25

100

Aeration

[92,93]

Scenedesmus almeriensis

7

30

35

200

Aeration

[94,95]

Phaeodactylum tricornutum

8

15

25

10

Aeration

[96,97]

24.3.2 CO2 capture prospects for microalgae cultivation CO2 is effortless to be had in the environment with the attention of 0.04 to 0.07 percent (v/v). Another elevated supply of carbon dioxide originated from zero-fee flue gas, which can also offer 5 to 20 percent (v/v) of CO2 [39]. These will be the critical CO2 resources used for algal growth to constantly produce algae for carbon sequestration extraction. CO2 fixation and biomass yield vary according to algal genus features, biophysical technique outcomes, and cultural structure outcomes 24.3.2.1 Nutrient effects The most essential nutrients for biomass invention are phosphorus, carbon, and nitrogen. Microalgae need nitrogen and phosphorus from the medium to carry out their metabolism, which can be provided by ambient air or carbon dioxide sparging. For algae, nitrogen in the state of ammonia is the most essential source of nitrogen [7,40]. Phosphorus is also required for respiration, metabolism, DNA synthesis, ATP production, and cellular membrane formation. Phosphorus is present in the medium as phosphate and is usually given in bulk as it is not widely accessible. To encourage the successful photosynthesis process, other inorganic materials and essential minerals, such as metals and nutrients, are routinely introduced to the media. Microalgae are an important bioremediation agent because of their ability to access organic material and micronutrients from effluents for development. These are the most prominent biotics discovered in effluent treatment plants. An algae pond, as opposed to the usual activated sewage treatment, may eliminate contaminants more efficiently, especially N and P [41]. Pseudokirchneriella subcapitata, Chlorella vulgaris, Microcystis aeruginosa, and Synechocystis salina obtained a nitrogen elimination rate of 100 percent in wastewater under high radiance supply conditions [42]. Although nutritionally effluents can be used as a medium, issues such as the N/P and C/N ratios still exist. An adequate nutritional composition of wastewater is

24.3 Cultivation systems for carbon dioxide capture by microalgae

535

requisite for effective algae farming for carbon dioxide biosequestration. In terms of absorbing nutrients from wastewaters and producing considerable biomass outputs, Scenedesmus and Chlorella sp. are the foremost amazing algae [43]. 24.3.2.2 Effects of pH pH is another significant physicochemical parameter that influences CO2 solubilization. This parameter’s value defines the relative abundance of CO2 , HCO3− , and CO3 2− in comparison to total inorganic carbon concentration [44]. pH levels [7–10] of microalgae cultures are typically neutral or alkaline [45]. When CO2 is introduced into a microalgae culture, it produces H2 CO3 , whereas alkalization is related to the formation of hydroxyl radicals, which is normally due to NO3 − and HCO3 − absorption. Alkaline conditions are commonly used for microalgae cultivation because they have a positive influence on the CO2 dissolution rate, which may help to reduce culture contamination. 24.3.2.3 Effects of temperature Microalgae thrive best at temperatures between 16 and 25°C [46]. The low temperature of the culture media suppresses RuBisCo (ribulose bisphosphate carboxylase oxygenase) enzyme activity, foremost in a drop in the rate of photosynthesis. Warm temperatures, on the other hand, decrease the metabolism of microalgae and reduce CO2 absorption [47]. The rate of photosynthesis occurs when the RuBisCo oxidase binds to oxygen rather than carbon dioxide, resulting in a 20–30 percent drop in the rate of carbon bioconversion [39]. The log phase was extended due to a deviation in culture temperature from the ideal range. After freezing and scouring, high-temperature flue gas is frequently suitable to be acceptable for cultivating thermal properties of microalgae and these are grown at 35 °C and 40 °C temperatures, respectively [46]. Chlorella may stay alive and produce in thermal springs with temperatures above 45 °C and CO2 concentrations over 40 percent (v/v) [7]. 24.3.2.4 Effects on the intensity of light and light/dark cycle The intensity of light varying between 10 and 40 mmol·m2 ·s−1 is required for microalgae cultivation [48,49]. Increases in intensity of light to 450 mmol·m2 ·s−1 can also increase the metabolism of phototrophic algal cells [50]. Most cyanobacteria and light-harvesting microalgae favour low (15 mmol·m2 /s) and high (50–90 mmol·m2 /s) light intensities, respectively [7,49]. Scenedesmus and Chlorella sp. were cultivated in a light environment of 205 mmol·m2 ·s−1 . Higher illuminance permits light can permeate the high-density culture since the lightharvesting components absorb less light, allowing for a better rate of photosynthesis because of less cell injury of respiratory components [51,52]. Fernandez et al. [53], came up with many models to explain the connection between the amount of light and how fast microalgae grow. The amount of light also changes how much lipids microalgae store and what kinds of fatty acids they have. The low light intensity can help the body make more polar lipids, while high light intensity can make less polar lipids and more neutral lipids. Excessive light intensity hinders lipid accumulation, hence raising and maintaining light intensity may promote neutral lipid synthesis [54]. According to Li et al. [55], saturated fatty acids like C16:0 and C-18:0 increased in proportion to the intensity of light, but unsaturated fatty acids

536

24. Carbon dioxide capture and its enhanced utilization using microalgae

like C-18:2, C-16:1, C-16:2, and C-16:3 decreased. The final applications of the collected lipids are determined by the content of the microalgae lipids. The light and dark periods can also have an effect on the growth of algae. Jacob et al. [56], observed the influence of light and dark periods on Aphanothece microscopica production and carbon dioxide fixation in an airlift-bioreactor. The finding indicates that the day and night periods can have a considerable impact on the amount of carbon dioxide that is stored. Under steady light, the microalgae’s potential for fixing carbon achieved 99.69 percent. Tetraselmis tetrathele growth rate reduced with a longer light/dark cycle, according to Natrah et al. [57]. Only in day periods lasting between 6 and 18 hrs did the cell concentration, chl-a, and crude protein reach their maximum levels. A light cycle that was either excessively high or too shorter impeded cell growth, which resulted in a decline in cell concentration, chlorophylla content, and crude protein. This finding indicates that an ideal light cycle is necessary for photosynthesis and the improvement of microalgae. Abdel et al. [58], evaluated the development of Chaetoceros gracilis (24:0) and Isochrysis galbana (12:12) under day and night cycles and found that Isochrysis galbana grew at twice the rate of Chaetoceros gracilis. Similar growth rates were observed between the two microalgae species, demonstrating that algae maintained not only regulated by day/night periods, it also depends on light intensity.

24.3.3 The impact of cultivation methods on biomass production Now-a-days, open or closed methods have been used to cultivate microalgae. The cultivation method is built on the premise of creating a high surface area per volume ratio, which allows for better light penetration and CO2 gaseous transfer. To enhance energy efficiency for biomass production, the design, scale-up, and operating systems must incorporate carbon dioxide transmission, aeration, and illumination. Fig. 24.1 shows a microalgae-mediated carbon dioxide bio-mitigation and system integration process diagram. 24.3.3.1 Aeration and mixing Mass transfer from the gas to water phase is delayed due to carbon dioxide’s limited solubility in the solvent. As a result, there was a lack of carbon dioxide dispersion in the medium, which may have limited microalgal development. Optimized mixing is necessary to optimize carbon dioxide supply, as does oxygen stripped to reduce oxygen inhibition of respiration. Mechanical stirring techniques such as paddle wheels and baffles, gas injection systems such as bubble diffusers, and membrane-sparged systems have all been employed in various cultivation linked with mixing tactics.

24.3.4 Microalgae culture system for CO2 capture A sufficient amount of sunlight, CO2 , and nutrients is required for successful microalgae production. The purpose of a comprehensive culture system design should provide the optimal microalgal cell growth and thereby improve biomass production. Algal biomass cultivating methods are still chosen based on factors like price, carbon dioxide capture sources, desired product type, and availability of nutrients. Fig. 24.2 shows the newly created multilayer bioreactor design.

24.3 Cultivation systems for carbon dioxide capture by microalgae

537

FIGURE 24.1 Microalgae-mediated carbon dioxide bio-mitigation and system integration process diagram.

24.3.4.1 Open system Because of its inexpensive cost and ease of use and maintenance, the open method is widely utilized for huge-scale microalgae farming. It is frequently utilized in industrial settings to generate large quantities of things for commercial reasons at a minimal cost. For commercial microalgal production, an open pond with a width of 0.25 m and a surface size of 0.2–0.5 hectares is frequently used [59]. Furthermore, the open pond approach is commonly employed when the supply of nutrients is effluent mixed with carbon dioxide from flue gas. It also provides effectively treated wastewater. However, the open pond system has some drawbacks, including the fact that it takes up a lot of space, the culture is vulnerable to pollution, and evaporating water loss could be severe based on an open layout [60]. As a result, most ponds are coated with translucent material to promote microalgae development, reduce evaporate losses, and improve carbon dioxide distribution.

538

24. Carbon dioxide capture and its enhanced utilization using microalgae

FIGURE 24.2 The newly created multi-layer bioreactor design.

24.3.4.2 Closed system Microalgae could be developed in an isolated system under precisely sterile environments, like the amount of light used, the amount of space required, and the carbon dioxide concentration [61]. Some of the difficulties connected with open methods could be addressed by using a closed method. A photobioreactor is a form of controlled pond method that uses algal biomass to maintain temperature, reduce evaporation, and reduce CO2 outputs while preventing contamination from undesired algae, mold, and bacteria [62,63]. Table 24.2 shows microalgae cultivations in the photobioreactor. However, while photobioreactors prevent the development of competing microalgae weeds, they do not prevent the formation of pollutants. This system has two disadvantages: it is complex to construct and operate, as well as being costly. Some closed systems for the cultivation of microalgae strains are described below:

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24.3 Cultivation systems for carbon dioxide capture by microalgae

TABLE 24.2 Microalgae cultivations in the photobioreactor. Photobioreactor types

Name of the strain

Capacity (L)

Concentration of biomass

References

Tubular

Spirulina platesnsis

6.5

0.63 g/L

[98]

Phaeodactylum tricornutum

80

1.35 g/L

[99]

Flat plate

Airlift

Bubble column

g/m2 /d

Spirulina platensis

65

32.5

Phaeodactylum tricornutum

200

1.19 g/L

[101]

Phaeodactylum sp.

5

1.38 g/L

[102]

Dunaliella sp.

3.4

1.5 g/L

[103]

Chlorella vulgaris

3.0

0.045 g/L

[104]

Nannochloropsis sp.

200

0.225 g/L

[105]

g/m3 /d

[100]

Botryococcus branuii

3

2.31

Haematococcus pluvialis

3

4.09 g/L

[107]

Chlorella vugaris

2

0.89 g/L/d

[108]

Chaetoceros sp.

170

0.80 g/L

[109]

Chlorella vulgaris

1.8

1.41 g/L

[33]

Spirulina sp.

3.5

4.13 g/L

[33]

Cyanobium sp.

1.9

0.075 g/L

[110]

g/m3 /d

Monoraphidium sp.

4.6

25

Monodus sp.

64

0.20 g/L

[106]

[111] [112]

24.3.4.2.1 Tubular-photobioreactor

The tubular-photobioreactors are made of polymers and glass (Fig. 24.3), with minimum monitoring and controls, and run aseptically. By maintaining submerged or float tubes, temperature control is achievable. Carbon dioxide can either be delivered through tubes that already contain a medium that is saturated with carbon dioxide or it can be delivered by continuously bubbling carbon dioxide into the medium once a carbon dioxide line has been inserted along the internal tube wall. Culture broth is circulated through to the tubes using either liquid pumps or an airlift system [64–66]. Since there is no mechanical pumping involved, there are no operational components in the tubes, and there is minimum shear damage to the algal cells, pumping and airlifting is an ideal method for removing carbon dioxide and oxygen from a liquid medium [67]. The surface/volume ratio and lighting uniformity are affected by increasing reactor diameter. In photosynthesis, air pockets prevent carbon dioxide accumulation, and tube length increases gradients in CO2 and pH, as well as time for liquid retention [68,69]. Static mixers may increase mass transport in tubular photobioreactors. Even at the pilot scale, the sustainability of tubular-photobioreactors remains a dubious prospect [70–73].

540

24. Carbon dioxide capture and its enhanced utilization using microalgae

FIGURE 24.3 Schematic configurations of a tubular-photobioreactor.

24.3.4.2.2 Flat plate photobioreactor

In comparison to tubular photobioreactors, flat plate photobioreactors have a smaller footprint due to their narrow U-turns and laminar topology, surface to volume ratio, and more lighting surface area. However, the lack of dissolved O2 production indicates a decreased metabolic activity [74]. The plate has a perforated tube at the below through which carbon dioxide is injected. This tube only lets air move in one direction, parallel to the plates. This causes large dead spots where algal species might grow, but only a small amount of CO2 is taken up. The research that was conducted on the utilization of angled plates and the many ways in which they might be arranged demonstrated the optimal angle and position to face to receive the most sunshine to grow more feedstock [75,76]. However, researchers believe that these conditions alter the flow of CO2 in the reactor, resulting in low cell CO2 sequestration. A few considerations to make before deciding on this configuration are its incompatibility with sterilization, algal wall adhesion, and a higher power supply-demand (54 W/m3 ) than a bubble column’s (42 W/m3 ), as well as the need for unusual temperature sensors and controllers. Due to its lower power consumption, this photobioreactor is chosen over the tubular model (Fernendez et al., 2001). Additionally, increased aeration causes significant shear stress, which is harmful to the cells [77]. Unusually, a study on vertical flat plate photobioreactor’s semi-continuous operation revealed high biomass accumulation (85– 90 g/L), however, the researchers think the fixation rate of 15.6 g/l per day is excessive for biomass accumulation in the 1.5 L bioreactor [78]. Even though the pH of the medium changed when CO2 was added, adding 30–35 percent CO2 -rich air did not slow down the growth of the bacteria.

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541

24.4 CO2 capture improvement strategies 24.4.1 CO2 capture can be improved by genetic engineering and metabolic changes Aside from using amplified carbon dioxide through flue fuel to improve algal biomass production, genetic modification may be used to biologically edit algae to improve photosynthesis performance and carbon fixation effectiveness. There are studies underway for transgenic production and gene suppression in algae, which could have a greater economic application [50]. Microalgae strains had been modified by lowering the scale of light-harvesting chlorophyll antenna so that you can soak up more mild for photosynthesis and reduce photoinhibition [50,51,79]. In a light-saturated condition, Lee et al. (2002) [51], found that an antenna-deficient variant of Chlamydomonas sp. carried out higher carbon dioxide photoassimilation than a wild-type species. Furthermore, over-expression of the lipid biosynthesis genes would enhance microalgal lipid metabolism, but it may also result in cellular division decrease and less biomass synthesis. To control lipid anabolism in algal cells, an inducible promoter must be activated at the microalgal boom desk-bound segment [50]. Furthermore, knocking down enzymatic genes that cause lipid hydrolysis is effective in boosting lipid storage in cells.

24.5 Conclusion Carbon dioxide fixation through microalgae can provide substantially a discount on the greenhouse impact. This review discussed a high-level view of important aspects of carbon dioxide sequestration by microalgal species, as well as a few challenges and significant advancements that the clinical society is developing to set up modern approaches that include organic changes via molecular genetics tools, as well as technical challenges to maximize carbon dioxide sequestration strategies.

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C H A P T E R

25 Supported single-atom catalysts in carbon dioxide electrochemical activation and reduction Amos Afugu, Caroline R. Kwawu, Elliot Menkah and Evans Adei Department of Chemistry, Kwame Nkrumah University of Science and Technology, Kumasi, Ghana

25.1 Introduction Owing to the increasing dependence on carbonaceous energy systems, as more populations develop and industrialize [1–4], the management of greenhouse gases on earth has become one of the pivotal issues in science and society. Anthropogenic activities like agriculture, transportation, heating, cement production, etc. continuously cause the relocation of greenhouse gases (carbon dioxide (CO2 ), nitrous oxide, and perfluorocarbons) into the atmosphere including from the underground. The net continuous accumulation of CO2 in the atmosphere is due to the low natural fixation of CO2 compared to the amounts being released. Despite the devastating implications of the accumulations of these toxic gases on the atmospheric temperature, carbon dioxide emissions into the atmosphere contribute the most to climate change effects, and tackling CO2 atmospheric levels can significantly reduce the impact of climate change. CO2 valorization into fuels is projected to tackle both the impacts of climate change and provide a means to sustainably meet the world’s rising energy needs ([5–9]. Although CO2 is an important C1 source for chemical synthesis, its conversion is energetically challenging as it is a very stable linear molecule with 2 sp2 hybridized carbon-oxygen bonds, equivalent to an sp bond strength. CO2 is the most oxidized form of carbon, due to its high stability and chemical inertness, catalysts are vital for CO2 initial activation and reduction into hydrocarbons and carbon fuels [9].

Green Sustainable Process for Chemical and Environmental Engineering and Science: Carbon Dioxide Capture and Utilization DOI: https://doi.org/10.1016/B978-0-323-99429-3.00010-2

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25. Supported single-atom catalysts in carbon dioxide electrochemical activation and reduction

Catalysts reduce CO2 by electron addition to its lowest unoccupied molecular orbital (LUMO), this introduces changes in the shape and activity of the molecule, this important step is known as the CO2 activation step. Transition metals especially the late transition metals, Fe, Co, Ni, and Zn have shown excellent activity for CO2 activation, and quantification of the bond elongation of C–O has become extensive in the literature using scientific computing [10–14], as CO2 activation has been identified as the key intermediate state and limiting step in many CO2 transformation reactions [15,16]. Of the many catalytic approaches available for CO2 transformation, biochemical, thermochemical, photoelectrochemical, and electrochemical, the last two options involving the electrochemical conversion of CO2 are the most attractive as excess electricity from renewable energy (solar, tidal, hydro) can be harnessed to drive CO2 conversion, this approach does not add to the net carbon economy. Electrochemical processes help to forestall intermittency issues that exist with seasonal renewable energy sources by storing the excess energy generated in chemical bonds through the process of electrolysis (mainly the electrolysis of water and CO2 ). Electrochemical processes proceed in the absence of heat, under ambient conditions and the efficiency of this process can be controlled by varying the electrodes (electrocatalysts), electrolyte ions, pH, cell design, and the external voltage [17,18]. In CO2 electrolysis, two separate processes occur at two electrodes, CO2 reduction to hydrocarbon occurs at the cathode, and water oxidation to oxygen via the oxygen evolution reaction (OER) occurs at the anode. In acidic media, the hydrogen produced at the anode migrates to combine with anionic CO2 species at the cathode, leading to the production of desired products. In basic media, anions of CO2 species produced at the cathode migrate to combine with protons at the anode leading to the formation of the desired products. CO2 electrochemical reduction occurs via the general reaction of; aCO2 + bH2 O → product + CO2

(25.1)

The CO2 reduction stepwise mechanisms generally for both thermal and electrochemical reactions have been reported to proceed via the CO or non-CO pathways leading to hydrocarbon fuels on transition metals. These have been discussed earlier in our previous review, the various reaction mechanisms and paths leading to methane, ethane, CO, and formic acid formation have been highlighted [19]. The desired cathodic material should have high selectivity, and activity and should operate at high current density and low over-voltage [20]. High catalyst activity is the measure of amounts of the starting reactants the catalyst converts to products. High catalyst selectivity is the measure of the ability of a catalyst to promote the formation of a single product over others. High current density shows the amount of current flowing through a cross-section of the conductor. Low overpotentials are the measure of the amount of deviation of the theoretically predicted required potential and the actual experimental potential applied at the electrodes for an electrolytic reaction to occur, as a result of other high energy barrier processes occurring in the electrochemical cell e.g., gas diffusion (diffusion overpotential) and charge transport (ohmic overpotential). These desired properties have structural relationships and the structure-performance relationships of catalysts need to be understood to predict more robust and efficient materials. Controllable properties of the electrode include surface topology, morphology (shape), composition (defects), and geometry (atomic coordination), as these, can influence the electrode performance.

25.3 Single-Atom catalysts efficiency descriptors

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Single metal atoms immobilized on conductive materials have become very attractive for photoelectrochemical and electrochemical conversions as they open the avenue to design cheaper and more reactive materials for energy storage. Both experimental studies and theoretical predictions have shed some light on the rate-limiting steps, onset potentials, and the main intermediates involved in the reactions on various surfaces. The study of surfaces is crucial as the efficiency of these technologies and processes depend strongly on the performance of the catalysts employed. Single transition metals used extensively on 2 dimensitional (2D) supports are earthabundant transition metals, for example, the late 3d metals i.e., Ni, Co, Fe, Cu, and Zn [21] as well as some noble metals including Pt [22,23]. Supports used for single metal catalysts have been carbon materials like nanotubes, graphene, and amorphous carbon due to their high surface area (for high single-atom catalyst loading) and high conductivity (for fast electron transport). Single-atom catalysts have applications in several heterogeneous processes including electrocatalysis, photocatalytic water splitting, and air disinfection, Recently, a review on single atom binding and effects on CO2 ERR has been extensively explored on carbon materials [21], herein we focus on other large surface area supports like porous and other 2D materials. For example, the metal chalcogenides, metal oxides, MXenes, and metalorganic frameworks (MOFs) equally offer a large surface area for catalyst loading and good conductivity as observed for carbon materials.

25.2 CO2 ERR products CO2 ERR occurs through several multi-electron and multi-proton steps resulting in the production of a wide range of carbon-based products [24,25]. These products include syngas (CO/H2 ), formic acid, propanol, ethanol, acetaldehyde, coal, methane, methanol, etc. Among these products mostly considered in the literature, methanol, ethanol and formic acid are promising due to their high energy density, safety, easy storage, and their direct usage in fuel cells. Other products such as methane and ethylene also have enormous value [26,27].

25.3 Single-Atom catalysts efficiency descriptors SACs or atomic catalysts with reduced size, provides single nuclei with amazing properties such as quantum Hall effects due to quantum confinement and electron localization, low metal coordination and low valencies and oxidation states, which results in excellent catalytic activity and selectivity towards electrochemical processes. SACs were first synthesized by the coprecipitation method, by considering an isolated single Pt atom uniformly dispersed on iron oxide nanocrystals. This single-atom Pt catalysts show excellent catalytic activity and stability for CO oxidation and selective oxidation of CO in H2 . The excellent catalytic activity stems from the charge transfer from Pt to the iron oxide surface leaving the monodispersed Pt atom in a low coordination environment. Single-atom catalyst synthesis methods and characterization techniques have been well discussed [21]. Here we discuss the surface describers and factors that account for the high activity of single-atom catalysts, compared to their bulk counterparts.

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25. Supported single-atom catalysts in carbon dioxide electrochemical activation and reduction

Among the various catalytic chemical reactions, heterogeneous catalysis has enormous applications in industries due to the ease of catalyst separation from reactants and products after use. Heterogeneous catalysis has been widely used in petrochemical industries, environmental protection, and new energy generation mostly in the area of electrochemistry, in technologies like fuel cells, batteries, supercapacitors, and electrolyzers. Uniquely late 3d metals have high activity for CO2 activation. Copper and platinum have a good ability to activate CO2 molecule and convert it into CO, HCOOH, CH3 OH, or CH4 [28–30]. However, these transition metal electrocatalysts suffer a series of problems, such as poor durability, low catalytic efficiency, high overpotential, and low selectivity. In addition, noble metals like Pt are very expensive and exist in low natural abundance, therefore they cannot meet the increasing demand [31,32]. Therefore, novel inexpensive catalysts with high catalytic activity, high selectivity, and high stability remain a scientific issue in the field of CO2 ERR, hence the need to develop single atom catalysts [33], that are heterogeneous with more open metal centers and quantum effects obtained in homogeneous metal complexes. Reducing the size of these CO2 ERR active noble metals from bulk into a single atom, serve as a cost-effective way of maximizing atomic efficiency. Single atoms do not only increase the conductivity of conductors in an electrochemical cell but have also improved the efficiency of photocatalysts when loaded onto a semiconductor, for example, noble metals (Pd, Pt, Rh, and Ru) were used as single atoms to improve the photo-efficiency of TiO2 for water splitting through the hydrogen evolution reaction. Supports do not only provide stability and sites for metal dispersion but also help to finetune the coordination number, oxidation state, and reactivity of the single-atom catalysts. Single metals are undercoordinated with a lower oxidation state, for instance, monodentate +1 metal atoms, hence their reactivity is increased to attain higher oxidation states rendering them electrochemically active materials. Controlling the oxidation number by the introduction of axial ligands is also effective for the tuning of the metal d-center and activity. In this regard low coordination has been reported to reduce oxidation number and increased the activity of the metal. Mou et al., observed an excellent CO Faradaic efficiency of 90.2 percent on Ni and N-co-doped graphene, the free energies for the formation of intermediate COOH on the unsaturated site of Ni-N are significantly lower than that on the Ni-N4 sites, suggesting outstanding activities of CO2 electroreduction on the coordinatively unsaturated Ni-N sites [34]. Hence the introduction of electron rich ligands or nucleophiles like N, acts as oxidants of the SAC and this is seen to have a detrimental impact on reactivity at the active site. One of the factors of single-atom catalysts considered to control its performance is the d-band center and width of the SAC metal active center and how the d-orbitals interact strongly with the support and the reactants H+ /CO2 or intermediates [35]. The type of metal center and its d-band edge characteristics affects its stability to the support and reactivity to the adsorbate. Among the considered SACs, Ni and Pt showed a limiting potential of −0.41 V and −0.27 V for CH3 OH production, while Os and Ru both showed higher limiting potentials of −0.52 V for CH4 production. The density of state (DOS) analysis showed that the excellent catalytic activity is a result of the strong electronic interactions between d-orbitals of the metal atom and the p-orbital of graphene [36]. Adsorption of first-row single atom transition metals (Sc to Zn) on tetracyanoquinodimethane (TCNQ) monolayer showed that large adsorption energies were an indication of stable dispersion of atoms on support. The

25.4 Single-Atom catalyst supports

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catalyst systems proved efficient for CO2 ERR and the main product formed on V, Cr, Mn, Ni, and Cu is HCOOH, selectively CO was preferred on Zn [37]. Hence strong orbital interactions and low coordination of SAC to its support are desirable for efficiency and selectivity. The high d-orbital vacancy of the SAC is seen to improve its reactivity towards CO2 . For example, introducing Pt into oxygen vacancy defective Fe2 O3 , showed high catalytic activity. Attributed to a high vacancy in the d orbital and high oxidation state of Pt atoms compared to bulk Pt surface. This reduced CO adsorption energies and the activation barriers for CO oxidation [38]. The d-orbital occupation level also correlated to strong binding of SAC to the surface and reduced reactivity. There is a net electron loss from the SAC to the support as reported by Qiao et al., where the SACs act as reductants. The larger the atom, the lower its electronegativity as seen for Mn and the higher its ability to act as a reductant, leading to the highest oxidation state of Mn on support. The attributed Pauling electronegativity of the TM ad-atom is the same as the trend observed by Wang et al. [39]. Metals with the least proton number and d-orbital electrons in the period, i.e., Mn, as seen for Pt ad-atom, has the least electronegativity, high oxidation state, and reactivity towards CO2 . Many SACs showed high selectivity for CO2 electroreduction over competitive hydrogen evolution reaction (HER) due to favorable adsorption of nucleophiles i.e., carboxyl (COOH∗ ) or formate (OCHO∗ ) over electrophilic proton (H∗ ). Studies were done by Cui et al. on MC2 N (M = Ti, Fe, Ni, Co, Mn, Cu, Ru, Rh) as efficient SACs for CO2 ERR. COOH or OCHO formation was reported to be more favorable than the formation of H2 . Also, the formation of various CO2 ERR intermediates is thermodynamically favorable as it gave more negative Gibbs free energy than that of HER. Hence an efficient catalyst system for CO2 ERR. Ni, Co, Ru, and Fe single atoms form CH2 OH intermediate which is selective towards the production of methanol. Ti and Mn single atoms also formed CH3 O intermediate which is selective towards methane production with lowered overpotentials (0.58V – 0.80 V) [40]. To increase single-atom catalyst binding strength onto the host, defects (vacancies and impurities) are introduced into the support to stabilize atoms. Nitrogen doping divacancy site on graphene plays a crucial role in anchoring SACs leading to a relatively strong binding strength due to the strong hybridization between the d-orbital of metal atom and 2-p orbital of nitrogen. Co, Rh, and Ir single atoms anchored individually on divacancy nitrogen-doped graphene (Ngraphene) and exhibit high catalytic activity for CH3 OH generation via a six-electron pathway with an overpotential of 0.59 eV, 0.36 eV, and 0.29 eV respectively. Moderating defect sites is also seen to improve efficiencies, the electronic configuration of mononuclear Ni centers promoted the CO2 activation through facile electron transfer with improved electroreduction activity [41].

25.4 Single-Atom catalyst supports Despite the excellent catalytic performances of SACs, reducing the size of metal particles to a single-atom level increases the surface free energy, which is good for reactivity and bad for the thermal stability under reaction conditions. This highly increased surface energy leads to the aggregation and agglomeration of particles back to bulk or cluster forms and the decrease of catalytically active sites in the preparation and the catalysis process. This has

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become the bottleneck in the practical preparations and applications of single-atom catalysts [38,41,42]. Researchers in recent years have developed numerous ways to obscure their aggregation by adopting some high-surface-area support materials such as porous metals, metal oxides, and carbon-based entities which can be modified to promote metal dispersion and inhibit agglomeration. Unique physical and chemical properties coupled with a large surface area of porous materials, (metal-organic frameworks) and 2D monolayer materials, germanene, silicene, stanene, MXenes, metal oxides, metal chalcogenides, graphene, graphitic carbon nitride (g-C3 N4 ) and hexagonal boron nitride (h-BN) have been investigated as support for singleatom catalyst [43–49]. Reducible oxides such as TiO2 , CeO2 , In2 O3 and ZrO2 have also been used as adequate transition metal oxide supports for single-atom catalysts, oxygen vacancies on these supports are reported to improve the catalytic activity towards CO2 activation and hydrogenation [50]. Generally it is seen that finely dispersed SACs e.g., metals form strong chemical interactions to support materials that originate from the net charge lost from the metal species to the reducible supports. Aside single anion vacancies, SV, double vacancies DV, in BN 2D layers are also reported to impact on CO2 electrochemical reduction positively [84].

25.4.1 Two-dimensional (2D) metal oxides Oxide supports were used for many reactions with improved catalytic activities of electrocatalysis. Using both experimental characterizations and density theory functional calculations, Ren et al., sort to understand the effect of metal coordination on its reactivity. Their work investigated Pt on Fe2 O3 and found that low coordination to surface oxygen, corresponded to a low oxidation number of Pt, less empty d-orbitals, and high reactivity towards H2 oxidation to 2H∗ [22]. In 2018, Su et al., investigated the single atom Ir catalyst on TiO2 , for thermal and electrochemical CO2 conversion into CO and formate products. This catalyst in a reaction medium ratio of 1:1 CO2 to H ratios produced great capability towards CO2 conversion and showed excellent selectivity towards CO and not formic acid [51]. Pt1 was anchored on FeOx for CO oxidation, and the high activity of the catalyst was ascribed to the presence of a positively charged Pt, as a result of empty d-orbitals of Pt [38]. Ir1 /FeOx single-atom catalyst has been synthesized and used for the water gas shift reaction (WGS), the activity of the SAC is 1 order of magnitude higher than Ir cluster or nanoparticle counterparts and is even higher than those of the most active Au- or Pt-based catalysts for the WGS reaction [52]. Co3 O4 as support for Au SACs with very low Au loading of 0.05 wt percent and the as-prepared catalysts shows high catalytic activity for CO oxidation and exhibited complete conversion of CO at room temperature. The high catalytic activity of Au single atom was ascribed to distribution of the isolated Au atoms over the Co3 O4 nano-crystallites [53].

25.4.2 Two-dimensional (2D) metal chalcogenides Two-dimensional (2D) transition metal dichalcogenides (e.g., MoS2 , WS2 ) are increasing of interest to the scientific communities as a result of their diverse physical and chemical properties. In the family of transition metal dichalcogenides (TMDCs) as efficient support for SACs, MoS2 is the most studied. Characterization on doped MoS2 shows that metal

25.4 Single-Atom catalyst supports

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ad-atom interacts with the support via 3 main approaches; (1) ad-atom anchors to the S atom, (2) ad-atom substitutes the S atom, and (3) ad-atom substitutes the Mo atom when deposited [54]. 5 percent niobium on MoS2 in ionic liquid exhibits 1 order of magnitude higher CO formation turnover frequency (TOF) over the pristine MoS2 at an overpotential range of 50– 150 mV. The TOF is also two orders of magnitude higher than that of Ag nanoparticles over the entire range of studied overpotentials (100–650 mV) [55]. Transition metals Ni, Pd, Rh, Nb, and Re have been anchored on In2 Se3 for CO2 selective electrochemical reduction against the HER, reaction pathways and limiting potentials for CO2 reduction were effectively controlled [56] whereby Rh demonstrated the best activity. Eleven single metal atoms (Au, Co, Cr, Cu, Fe, Ir, Mn, Ni, Pd, Pt, or Sn) were adsorbed on MoS2 monolayer at different adsorption sites and their suitability for activating CO2 molecules were evaluated. The reported results show strong metal, monolayer adsorption, and interactions for most of the considered metals at the metal site and net charge accumulation on support. They reported through their calculations that the binding energy corresponds to the Mo-TM bonds [42]. Most of the TMs prefer to be adsorbed above the Mo atom, however, Sc, Ti, and Mn prefer the hollow site [39]. On MoS2 , high Pt loading of 7.5 percent is reported to enhance SAC activity. Li et al., reported the synergetic interaction between two neighboring Pt adatoms which greatly reduced the activation energy for CO2 hydrogenation, relative to the isolated Pt adatom. It was also reported that isolated Pt favors the conversion of CO2 into methanol without the formation of formic acid, however after Pt agglomeration, CO2 is hydrogenated stepwise into formic acid and methanol [5]. Lu et al. studied for the first time the thermoreduction of CO2 into methanol on the Co atom supported on MoS2 using spin-polarized DFT-GGA calculations. They found that the cobalt atom prefers to disperse and anchor on the MoS2 support as a single atom. Transition state calculations and the reaction rate constants calculated show the preferable CO2 reduction pathway as the reverse water gas shift (RWGS) conversion and subsequent CO hydrogenation. The rate-determining step for the CO hydrogenation into formyl (HCO) is reported to be through a barrier of 1.11 eV [32].

25.4.3 Metal carbides, nitrides (MXenes) Two-dimensional (2D) transition-metal carbides and nitrides, known as MXenes with the general formulae of M3 ×2 (where X is carbon or nitrogen), have been investigated for CO2 conversion with well-resolved density functional theory calculations. The new and growing family of two-dimensional (2D) materials i.e., transition metal carbides, nitrides, and carbonitrides and their synthesis and modification methods have been explained in detail [57]. Through computer screening of MXenes, Cr3 C2 and Mo3 C2 have been predicted as promising candidates for CO2 conversion into methane [58]. Alkali metal (Na, K, Rb, and Cs) doped on Mo2 C increased the electron density on the Mo host atom leading to improved activity for CO2 dissociation into CO [59]. The reaction mechanisms of CO oxidation catalyzed by the single atom Ti/Ti2 CO2 was investigated by using first-principles calculations. The calculation results indicated that the Ti2 CO2 substrate could prevent Ti atoms from agglomeration owing to the high diffusion barriers [60].

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Furthermore, the photocatalytic reduction of CO2 at the O vacancy site on Ti2 CO2 , V2 CO2 , and Ti3 C2 O2 was explored by the first-principles DFT calculations [61]. Ti2 CO2 was seen to be the most reactive surface. In another similar work by Low et al., TiO2 nanoparticles are in situ formed on conductive Ti3 C2 to develop TiO2 /Ti3 C2 hybrids for CO2 electrochemical methanation [62].

25.4.4 Metal-Organic frameworks Coordination polymers like metal organic frameworks (MOFs) are formed with an open metal site called a metal node undercoordinated to organic molecules that can be thermally converted to single metal nanocarbon materials. Single metals are formed for catalysis by different approaches, which include the use of the pristine metal node site, substituted metal node sites, and the incorporation of metal sites at the ligand or node sites [63]. MOFs offer a large surface area due to their porosity and are attractive owing to their structural diversity and functionality. With powder X-ray diffraction patterns, Zhao et al., in 2017, synthesized and characterized Ni single atom on metal-organic framework sites using the ionic exchange between Zn nodes and Ni for application in CO2 electrochemical reduction. Their single-atom catalyst exhibited an excellent TOF of 5273 h −1 with a high Faradaic efficiency of over 71.9 percent and a high current density of 10.48 mA cm–2 at an overpotential of −0.89 V [64]. Very recently, the unprecedented electrochemical reduction of CO2 on MOF modified with Cu and Ni single atoms has shown superior Faradaic efficiencies towards methanol production when in an intermetallic alloy form [65]. Bismuth-based MOF has been utilized in their thermal decomposed (charr) form, which led to the atomization of Bi nanoparticles for CO2 reduction with an activity for CO for up to 97 percent with a high turnover frequency of 5535 h-1 at a low overpotential of 0.39 V versus the RHE. DFT simulations showed a lower onset potential for the conversion of BiN4 /C compared to the carbon layer [59]. Aside from single metal atom control in MOFs other approaches like ligand design, halogen-modified and enzyme-induced structures have also been considered in review [66]. This review considered all applications of MOFs including supercapacitors, rechargeable batteries, fuel cells, water electrolyzers, and carbon dioxide/nitrogen reduction reactions.

25.5 Mechanisms for CO2 ERR on single-atom catalysts The reaction products formed from CO2 , have different equilibrium potentials to be applied at electrodes, where potentials of the cathodic reaction are relative to the reversible hydrogen electrode (RHE) at pH 7, 1 atm, and 25 °C in aqueous solution [67,25]. The use of catalysts lowers the potentials (i.e., the overpotentials or onset potentials for the given reactions). To understand the intermediates leading to products, and the potentials required on a given catalyst. Although when it comes to CO2 ERR, SACs share similar reaction pathways with their normal bulk metals, stability, selectivity, and catalytic efficiency differ. For example, under CO2 ERR conditions, Ni, Pt, and Fe in their bulk generate H2 (i.e., HER) due to their poor catalytic efficiency for CO2 ERR. However, reducing the size of these metals to a single atom

25.5 Mechanisms for CO2 ERR on single-atom catalysts

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enormously suppressed H2 production whiles CO and HCOOH are generated as intermediates leading to the production of methanol and other C1 products such as methane [68]. Computationally, the DFT method can be employed to study the onset potentials for the CO2 ERR using the thermodynamic computational hydrogen electrode (CHE). The onset potentials are the applied potential at which all the energy barriers go downhill on the energy profile diagram. Proton-electron pairs are added to the carbon species in a stepwise fashion. Within the CHE model, therefore, the chemical potential of a proton-electron pair is defined in equilibrium as half the free energy of gaseous H2 at an applied potential of 0V [69]. The free energy for the formation of each intermediate is determined using the formula below; G = E + ZPVE − TS − eU; when no pH effect is considered

(25.2)

G = E + ZPVE − TS − eU + kbT · ln 10 · pH; with pH effect

(25.3)

For an elementary reaction step: A + H → AH; E = E(AH∗) − E(A∗)

1 E(H2 ) 2

(25.4)

UL = −Gmax /e Where E are energies obtained from DFT simulations for individual reaction steps (see Eq. (25.4)), EZPVE is the zero-point vibrational energy, S is the entropy and T is the temperature (298K), e is the total elementary positive charge transferred for a given reaction when an external potential U is applied [70,71]. Zero-point vibrational energies, entropies, and heat capacities are calculated by vibrational mode analysis within the harmonic oscillator approximation model with the DFT simulations. UL is the onset potential and the potential at which all barriers go downhill, ࢞Gmax is the maximum positive free energy change during the reaction steps. Details for the activation barriers for all elementary steps are derived with a transition state search where all transition states are first order saddle points and characterized with a single imaginary frequency, the energy barrier and rate k are then estimated for the rate-limiting steps using Eqs. (25.5) and (25.6) respectively. Ea = ETS − E(A∗)

(25.5)

k = KB T/h · e(−Ea /RT )

(25.6)

Ea is the activation energy, ETS is the energy of transition state structure, E(A∗ ) is the energy of the species leading to the intermediate, and KB , T, h, R and T are constants i.e., 1.1.3806 × 10−23 JK−1 , 298.15 K, 6.6262 × 10−34 Js, 1.987 cal/mol K respectively. To shed insights into the reaction mechanisms for CO2 ERR on catalytic surfaces, research has been widely investigated using in-situ experimental spectroscopic means [54,72–75] and/or by computational studies based on density functional theory [20,76]. Reactants are considered to react via two possible paths, the Langmuir-Hinshelwood (LH) for co-adsorbed reactants and the Eley-Rideal (ER) mechanism for a single reactant adsorbed state. The hydrogenation steps can then be explained by the ER path where hydrogen is abstracted from water molecules. In the LH path hydrogen is first adsorbed on the catalyst surface to react

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with carbon species. In the LH approach, the adsorbed H occupies an additional active site on the catalytic surface, this must be near the adsorbed carbon species, to aid hydrogenation [77]. Proton reduction to hydrogen (instead of CO2 reduction) via the hydrogen evolution reaction (HER) (2H+ + 2e− → H2 ) at the cathode, is known to compete and interfere with the production of specific products and therefore affect the activity and selectivity of electrocatalysts [78,79]. Also, the direct outer-sphere single-electron reduction of CO2 to form the CO2 − anionic radical is energetically demanding (Eo = −1.90V vs. NHE), which has been attributed to the high reorganizational energy needed to bend the linear CO2 [80]. Although many reaction pathways have been proposed by researchers, adsorbed CO is recognized as one of the key reaction intermediates which play a crucial role in the formation of carbon one (C1 ) products. CO2 is either reduced directly into the desired product (called the non-CO pathway) or via the CO pathway where CO species must be formed. The formation of C1 products has been studied to proceed via either CO or non-CO (formate) pathways. One of the two possible intermediates of CO2 first protonation in the presence of an electron is formate (HCOO∗ ), the two oxygen atoms in the formate bind to the catalyst surface, the isolated nature of single-atom catalysts limits the formation of formate on most SACs [81]. CO formation path is mostly reported and proceeds via proton-coupled electron transfer (PCET) through a carboxylate (COOH∗ ) where only carbon binds to the SACs [82]. The adsorption energy of the CO intermediate formed is crucial in the determination of whether it would desorb from the surface as the main product or undergo further reduction to give other different products. Studies have shown that the adsorption of CO on Pt and Ni is so strong that it results in catalytic poisoning of the active sites leading to HER as the most plausible reduction reaction. CO is also reported to be the main product form on Au and Ag as CO is loosely adsorbed on these metal surfaces and desorbs from the surfaces after formation [83]. For carbon two products (C2 ) formation, C–C coupling becomes the most crucial step to be overcome. The C–C bond formation proceeds mainly via dimerization of CO to form OC–CO or coupling of deoxygenated carbonaceous species such as CH2 , C, and CH to form H2 C–CH2 , C–C, and HC–CH respectively [21]. Very recently, Linghu et al., studied the mechanisms of CO2 electrochemical methanation over 3d-5d transition metals as modified 1T-MoS2 monolayers. Pt and Ru single atoms showed the best activity for methanation and Zn and Re performed most for formic acid preparation. CO2 is reduced via the carboxylate, CO, and C sequential hydrogenation into methanol and methane with CO hydrogenation being the rate limiting step. The hydrogen evolution reaction is competitively formed on all metals except Zn showing the reduction of CO2 will be hindered [85]. CHO step has been reported on other SAC surfaces as the preferred path over the carbide mechanisms, which sequential C hydrogenation into methane and methanol, overall, the CHO step is the slowest step and uphill in CO2 reduction [36,86].

25.6 Conclusion SAC has been extensively used as a catalytic material for electrochemical processes, due to its quantum confinement and improved reactivity, selectivity, high current densities, and low overpotentials. However much of the studies on SAC have focused on homogeneous transition metal complexes with open d centers as well as single atom metals supported on

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carbon conducting materials. The earlier use of SAC based on metal dichalcogenides has utilized SACs for the HER and not CO2 ERR despite the potential shown by DFT predictions that they can bind SACs for CO2 activation. Metal-organic frameworks have also received considerable attention for SAC applications as it offers open d centers in the framework and provides versatility i.e., the opportunity to tune the framework ligands and metal nodes with unique guest transition metals, offering unique properties. Thermal deoxygenation of MOFs provides a carbon-based SAC which is also being applied for CO2 ERR. Structure activity descriptors shown to be good indicators for CO2 reduction are the low d-band center and width of the active metal, the strong low coordination to support, introduction of nucleophilic ligands, increased charge transfer and hybridization to support. Charge transfer is facilitated by large size of SAC, low electronegativity, high oxidation state on support due to high charge loss to its support, high conductivity, strong mixing of d orbitals with orbitals of the support, high affinity of SAC for nucleophiles (CO2 anionic species) over electrophiles (proton), anionic vacancies in support. Reactions are seen to proceed mainly by the carboxylate, and CO pathways due to the low coordination site provided by the SAC and the low coordination required by the carboxylate molecule compared to formate. Controlling supports and metal center dispersions have an influence on the overall activity, selectivity and overpotentials encountered on these surfaces.

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C H A P T E R

26 Organic matter and mineralogical acumens in CO2 sequestration Santanu Ghosh a,b,c, Tushar Adsul a and Atul Kumar Varma a a

Coal Geology and Organic Petrology Laboratory, Department of Applied Geology, Indian Institute of Technology (Indian School of Mines) Dhanbad, Jharkhand, India b Organic Geochemistry Laboratory, Department of Earth Sciences, Indian Institute of Technology Bombay, Mumbai, Maharashtra, India c Department of Geology, Mizoram University, Aizwal, Mizoram, India

Abbreviations Å Al+3 BET C=O Ca+2 CaCO3 CBM CH4 cm cmol(p+ )kg–1 CO CO2 COOH Cs d(001) F– Fe+3 g Gt H+ H2 H2 O HCO3 – IPCC K

Angstrom Aluminum trivalent cation Brunauer–Emmett–Teller carbonyl group Calcium bivalent cation Calcium carbonate/calcite Coal bed methane Methane gas Centimeter Centimoles kilogram−1 of cation exchange efficiency Carbon monoxide Carbon dioxide gas Carboxyl group Cesium Interplanar spacing in [001] crystallographic direction Fluoride anion Ferric (trivalent iron) cation Gram Gigaton Hydrogen monovalent cation (hydron) Hydrogen gas Water Bicarbonate monovalent anion Intergovernmental Panel on Climate Change Kelvin

Green Sustainable Process for Chemical and Environmental Engineering and Science: Carbon Dioxide Capture and Utilization DOI: https://doi.org/10.1016/B978-0-323-99429-3.00016-3

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c 2023 Elsevier Inc. All rights reserved. Copyright 

562 K+ Km Li+ m2 g–1 m3 Mg+2 MgCO3 Mn+2 Mpa Na+ —NH2 NH4 Ni+2 Nm NMR NO OH– OM P(CO2 ) Pb+2 Si2 O5 –2 SiO4 −4 SOC TMA TOC TOT USA

26. Organic matter and mineralogical acumens in CO2 sequestration

Potassium (kalium) monovalent cation Kilometer Lithium monovalent cation Square meter per gram Cubic meter Magnesium bivalent cation Magenesium carbonate/magnesite Manganese bivalent cation Megapascal Sodium (Natrium) monovalent cation Amine group Ammonium Nickel monovalent cation Nanometer Nuclear magnetic resonance Nitrous oxide Hydroxyl monovalent anion Organic matter Partial pressure of carbon dioxide Lead (plumbum) bivalent cation Sheet Silicate divalent anion Silicate tetravalent anion Soil organic carbon Tetramethylammonium Total organic carbon Tetrahedral-Octrahedral-Tetrahedral United States of America

26.1 Overview The IPCC (Intergovernmental Panel on Climate Change), an intergovernmental body of the United Nations, has well accepted the consequences of rising global temperature and has advised restricting warming at 1.5 °C to achieve ’net zero’ by 2050. The rising emission of anthropogenic carbon dioxide (CO2 ) has direct effects on the global climate. Hence, it is desirable to cut down the fossil fuel consumption, the most abundant greenhouse gas sources. However, due to rapid industrialization and urban growth, the demand and the use of energy are ever-increasing, leading to the consumption of fossil fuels and emission of greenhouse gases. Therefore, to balance the global emission budget, CO2 sequestration becomes one of the imperative steps to keep the global temperature within limits. It is a very efficient process to bring down atmospheric CO2 levels (both natural and anthropogenic) by capturing carbon and storing it in a safe place beneath the earth’s surface and ultimately alleviating global warming. Emitted CO2 from various industries can be captured efficiently either by pre- or post-combustion processes and thereby keeping it inaccessible from the atmosphere for a long time using different stowage mechanisms. This chapter explores different approaches through which CO2 can be trapped at its entry point to the atmosphere. Moreover, diverse pathways of CO2 sequestration under geo-sequestration category are addressed.

26.2 Introduction The atmospheric CO2 levels are rising from 280 ppm during the commencement of the industrial evolution to 420 ppm in present-day modern industrialization. The emission of

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greenhouses gases (GHGs) from the consumption of conventional fossil fuels and rapid industrialization elevates the concentration of methane (CH4 ), carbon dioxide (CO2 ), nitrous oxide (NO), and chlorofluorocarbons in the atmosphere [1]. These gases, most notably carbon dioxide (CO2 ) [2–4] traps heat and causes global warming and consequently inflicts catastrophic climatic calamities. The earth’s warming has significant negative influences on the global climate cycle and heat budget [5]. Moreover, derogatory effects on the natural ecosystem and economy cannot be ruled out. Some notable effects of global warming include polar ice cap thawing, uprise of sea-level, thermal expansion of seawater, increased precipitation in the northern hemisphere, and droughts in the parts of Asia. Therefore, it calls urgent attention from all intergovernmental bodies to address the issue and make advances related to CO2 emission and reduction. In this regard, the IPCC suggested that to achieve net-zero emission by 2050 global anthropogenic CO2 emission must be down to 45 percent by 2030 from 2010 levels across all sectors [6]. The elevated anthropogenic CO2 levels are worrisome and require special attention from all bodies. There is a dare need to halve CO2 emissions and decrease atmospheric CO2 levels. Earth’s natural carbon sink can only remove the equivalent quantity of carbon produced by its natural sources. Hence, to offset anthropogenic CO2 , carbon capture and sequestration is a fruitful way in which a carbon not only from its sources, i.e., thermal power plants, fossil fuel refineries, steel industries, and cement plants but also from the atmosphere can be resourcefully captured and stored in various safe places beneath the earth (sub-surface geological formations) [2]. CO2 sequestration is considered a crucial key to mitigate global warming crises [4,7] because it prevents the discharge of waste CO2 from industries into the atmosphere. It is a three-step process involving the capture of large quantities of carbon from the atmosphere and its industrial sources (carbon capture), followed by transport to a desirable location and finally, storage of captured carbon in protected carbon sinks beneath the earth for prolonged isolation from the environment [8,9]. It is assumed that if CO2 capture and accommodation expertise are acquired and employed competently, there will be a 45 percent reduction in CO2 emissions.

26.3 Geo-sequestration Geo-sequestration is a promising and secure carbon capture and accommodation method to prevent CO2 emanations from industrial sources. This process involves CO2 internment and injection directly into the deep subsurface geological formations. The geological routes where CO2 can be efficiently stored involve absorption and adsorption on clay minerals, injection of CO2 in abandoned coal formations or deep surface unmineable coal seams, sedimentary formations including sandstone and sandstone-shale formations, deep saline aquifer systems, depleted oil and gas fields, and oceanic storages [10–12] (Fig. 26.1).

26.4 Bio-sequestration Bio-sequestration promotes many different mechanisms to bring down the atmospheric CO2 levels. Increased plantation, genetic maneuvering in plants, enhanced soil quality, and algal bio-synthesis are some of the effective ways to trap atmospheric CO2 . It also involves

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26. Organic matter and mineralogical acumens in CO2 sequestration

FIGURE 26.1 Geo-sequestration of carbon dioxide.

using methanogenic bacteria in coal beds that, upon reaction, produce biogenic methane to capture CO2 from the atmosphere.

26.5 Mechanisms of carbon capture It is necessary to understand various methods by which carbon can be trapped at its entry point to the atmosphere. Efficient carbon capture from various sources is crucial before it reaches the atmosphere. Carbon capture involves the capture of CO2 produced by various industries to prevent its exposure to the atmosphere. The CO2 can be efficiently captured from industries before, during, and after burning the source materials (coal, biomass, oil). Present methods of carbon capture involve mainly three mechanisms: (a) pre-combustion, (b) postcombustion, and (C) oxyfuel combustion (Fig. 26.2).

26.5.1 Pre-combustion In this process, the initial source material is turned into a gaseous fuel using a hightemperature conversion process. The yielded gas contains various concentrations of hydrogen (H2 ), carbon monoxide (CO), carbon dioxide (CO2 ), and other hydrocarbons, such as methane (CH4 ). These gases then undergo reaction series, wherein carbon dioxide is captured by

26.5 Mechanisms of carbon capture

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FIGURE 26.2 Flow chart of CO2 capture mechanisms following Leung et al. [4]. (This Fig. is free to use under creative common license).

converting it into some other form of carbon-free (hydrogen-rich) fuel. The CO2 is then prepared for transport, and remaining CO2 -free gases are burnt for various purposes, i.e., generation of electricity. In this technique, the hydrogen is separated from hydrocarbons before combustion takes place. The advantage is that it recovers hydrogen and produces only water when burning takes place. Coal gasification plants use this carbon capture technique as it is technologically more mature than other alternatives.

26.5.2 Post-combustion In the post-combustion process, the solid gaseous fuel is burnt in a traditional way. It involves the removal of CO2 from post-combustion products, such as flue gas using an absorption process. It is based on the preferential affinity of specific compounds for CO2 . In post-combustion capture technology, amines are mixed with water to capture carbon dioxide from combusted gases selectively. The CO2 from the exhaust is then recovered and prepared for transport. This method has been efficiently carried out at small-scale plants. However, one of the major concerns of this technique is the relatively large parasitic load imposed by carbon capture procedure on power plant, especially when more energy is needed to regenerate the solvent [4]. This method is mostly employed by coal and gas fire plants.

26.5.3 Oxyfuel combustion It is the most straightforward approach to capture CO2 in power plants. In oxyfuel combustion, almost pure oxygen intercalated with the recycled flue gas combusts the fuel. It reduces the nitrogen content in the resulting flue gas that affects the later estrangement process. It is a modified combustion process of carbon capture. Oxygen gas of high purity is assorted with the flue gas (recycled) before the burning process so that the resultant flue gas is dominated by CO2 and water. Following the water removal through condensation, the remaining CO2 is purified. Although this method is technically viable, it utilizes substantial quantity of pure oxygen, which necessitates an energy-intensive processing unit, resulting in a higher operational cost.

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26.6 Transport of carbon dioxide The safe transport of separated CO2 from flue gas marks an important step in carbon storage and sequestration. Depending on the volume of CO2 separated, transport pathways may be utilized, i.e., road, rail, tankers, ship, or pipeline. To effectively transport large amounts of CO2, it is transferred either in liquid or in supercritical states. Transport by pipeline is favored when carrying carbon dioxide in supercritical conditions maintaining 32.1 °C temperature at 72.9 atmospheric pressure [4,13]. Care must be taken while carrying CO2 from the tanker and pipeline for any accidental leakage. This can be done by checking pipelines and tanker seals periodically.

26.7 Mechanism of carbon accommodation Carbon storage indicates prolonged storing of the seized atmospheric CO2 in different geological formations beneath the surface. The storage of CO2 is the most crucial step in the carbon capture and sequestration process. The CO2 is compressed using high pressure and pumped deep underground as a liquid in the storage mechanism. This liquid CO2 then gets trapped in the pore spaces by several means. The captured carbon can be stored in various geological sinks or converted into other valuable products. The principle aim of carbon sequestration is to keep CO2 isolated for the long term from the atmosphere. There are few mechanisms to store captured carbon in natural sinks. Presence of some structural traps is favorable in CO2 storage because of the greater buoyancy of CO2 than water, the pumped CO2 rises upward through the porous spaces until it reaches the impermeable layer of cap-rock such as shale, where it becomes trapped for a long time. Meanwhile, in reservoir storage, the waste CO2 is stored in depleted reservoir rocks that act like a rigid sponge, saturated with residual air within the pore spaces. However, when liquid CO2 is pumped into these rock formations, much of it becomes stuck within the pore spaces and does not move (Fig. 26.3A). This mechanism is also known as residual trapping. On the other hand, when CO2 is pumped in deep saline aquifer containing salty waters, it becomes dense and settles down to the bottom of the aquifer. It reacts with the minerals of the surrounding rock formations, and precipitates new minerals. Hence, this process locks dissolved CO2 in the new mineral phase for thousands of years. This entire mechanism is known as dissolution storage (Fig. 26.3B).

26.8 Carbon dioxide sequestration in organic matter 26.8.1 Carbon dioxide sequestration in coal Coal is a flammable carbonaceous sedimentary rock produced by a prolonged burial of vegetation remains for millions of years under the influence of time, temperature, and pressure [14]. It contains most of the elements mentioned in the periodic Table, but the majority of coal comprises carbon, hydrogen, nitrogen, sulfur, and oxygen, along with other mineral matter, depending on the conditions of deposition.

26.8 Carbon dioxide sequestration in organic matter

(A)

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(B)

FIGURE 26.3 (A) Residual trapping of CO2 in the pore spaces (B) Mineral trapping of CO2 by dissolution and precipitation.

FIGURE 26.4 Schematic representation of cleat and pore system in coal seam following Zhang et al. [22]. Reuse of this Fig. is permitted by Elsevier and Copyright Clearance Center; License Number: 5,304,790,477,214; dated: ninth May 2022).

The three-dimensional macromolecular structure of coal comprises fused/cross-linked aromatic moieties as crystalline carbon bridged by aliphatic and hetero-aliphatic functionalities and defects induced by aliphatic amorphous carbon and heteroatom-containing functional groups. Coal has a well-defined network of cleat systems and a highly heterogeneous pore structure consisting of macro- meso– and micropores [15,16]. Micropores are predominant within the coal matrix and are preferred sites of CO2 storage. In contrast, macropores refer to the inherent natural fracture called “cleats” that forms in response to escaping volatile gases and later to local structural forces [17]. The cleats are well-developed fractures classified as face and butt cleats that are orthogonally oriented, and master cleats orienting across the coal surface. Face cleats occur perpendicular in section and are more continues than butt cleats, whereas butt cleats are irregular and terminate at the face cleat (Fig. 26.4). The pore and cleat

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network facilitate the flow of gases/fluids in coal. Coal has a preferential affinity to adsorb more CO2 than other gases and hence, is preferred for CO2 sequestration [15,18–21]. The concept of CO2 storage in coal seams was proposed by MacDonald in 1991 [23,24]. Coal seams situated deep within the crust have the capacity to hold large quantity of gases due to their high hydrostatic pressure. However, CO2 sequestration in coal depends on various parameters, including cleat system, micropores, rank, and micropetrographic compositions. The main CO2 sequestration mechanisms in coal seams are 1) structural and stratigraphic strapping, 2) trapping of CO2 in minerals, 3) hydrodynamic trapping, and 4) adsorption [25]. Adsorption plays a vital role in CO2 trapping in coal, and this mechanism accounts for more than 90 percent of total CO2 storage [23,26]. During diffusion, CO2 moves through a large interconnected pore and micro-cleat systems. On the other hand, in adsorption, CO2 is adsorbed onto the internal surfaces, within the organic molecular structures, and clay minerals. During adsorption, the molecular structure of coal binds CO2 molecules. However, it depends on the gas retention capacity and sufficient permeability factors. The injected CO2 in deep-seated coal beds will be preferentially confined through sorption on macromolecular surface of coal and physically within cleats and micropores. If that coal is never exposed to the atmosphere, then the CO2 stored should be permanently locked in coal seams if we assume the retention time of coal seams on the order of millions of years. Unmineable coal seams are the ones that are set deep within the earth and hence are inaccessible for economic exploitation [3]. However, these coal seams have a good potential to store large amounts of CO2 (total storage capacity 100–300 Gt of CO2 ) [27] and are considered as a promising site for CO2 sequestration as CO2 is well adsorbed on coal surface. In this process, it releases methane which can be efficiently recovered. It is also noted that under certain pressure-temperature conditions CO2 adsorption capacity of coal is significant with simultaneous recovery of CH4 from these coals [23,28]. Therefore, coal seams are ideal sites for CO2 sequestration. Moreover, it is noted that coal has greater affinity towards CO2 than methane, which results in adsorption of twice the CO2 than methane in the coal matrix [2,26,28]. This observation assumes that two CO2 molecules can be accommodated for each CH4 molecule present [23,29]. In addition, Stanton et al. [30] suggested that some low-rank coals in the United States may store ten times the CO2 than CH4 . Therefore, it is believed that the deep unmineable coals seams are potential sites to store large volumes of CO2 . For example, Dutch coals with a capacity of about 8 Gt of CO2 [31] and deep coal seams from Alberta, Canada, with a capacity of 20 Gt of CO2 [4,32,33], New Mexico, the USA [4,34], Qinshui Basin [35,36], and Yaojie Coalfield [37] are some of the significant sites of the world where CO2 is being efficiently stored in deep coal mines. Coal is a heterogeneous material with pores of different sizes. The gas retention capability of coal depends mostly on the pore structure system. The IUPAC (International Union of Pure and Applied Chemistry classification system,1994) has categorized pores into micropores (50 nm). The symmetry and the distribution of pores in coal are crucial to understand coal’s gas absorption capacity to store CO2 . The porous surfaces of coal possess an inherent affinity for carbon dioxide adsorption onto their surfaces [35]. It is essential to understand the behavior of pores on exposure to CO2 for efficient sequestration in deep coals seams. It is observed that, due to its low maturity, the lignite rank coal is non-sensitive with slender changes in meso– and macropores when exposed to CO2 , whereas in bituminous coal, the carbon dioxide intercalation raises the mesopores, and

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macropores, and the fractures. The main factors responsible for increased porosity may include swelling and subsequent cracking, as well as, mineral and maceral dissolution. This enhanced porosity may provide an additional pathway for carbon storage [38]. The role of macerals (homogenous organo-micropetrographical constituents) in CO2 sorption was ambiguous until recent. These micropetrographical constituents of coal have recently been found to enhance their CO2 adsorption capacity. Karacan and Mitchell [39] found that vitrinite (lignocellulosic plant product) swells during CO2 adsorption. In some studies, vitrinite content and gas adsorption capacity were positively correlated [7,40,41]. Moreover, inertinite shows the highest adsorption capacity due to the presence of macro- and mesopores. Also, coals rich in the vitrinite and inertinite mixture effectively adsorb CO2 [42]. Likewise, Weishauprova et al. [43] reported the domination of liptinite and inertinite macerals in coals was responsible for the highest CO2 /CH4 adsorption ratio. In addition, the macerals associated with the mineral matter, especially clay, have a very profound gas adsorption rate. The mineral-maceral association of inertinite clay enables the gas molecules to pass in the meso– and macro pores easily, which enhances the CO2 adsorption capacity of coals effectively. 26.8.1.1 Factors controlling the CO2 injectivity and storage in coal seam 26.8.1.1.1 Temperature effect

The temperature of the CO2 being injected may be different from the temperatures of the reservoir. This could lead to the non-isothermal flow of gas and affect the CO2 injection efficiency and the reservoir’s potential to store gas. Moreover, temperature of CO2 itself influences the CO2 injection and storage capacity. CO2 becomes supercritical above 304.2 K or 31.05 °C (critical temperature) and 7.38 megapascal (MPa) or 73.8 bar (critical pressure). Under such supercritical conditions, more amounts of CO2 are stored in coals. 26.8.1.1.2 Pressure effect

Removal of overburden and drilling for production/injection of gas may affect the stress regimes around the coal strata. Consequently, the porosity and permeability of coal seams may alter, affecting injectivity potential. 26.8.1.1.3 Precipitation effect

Due to some geochemical reactions between injected CO2 and minerals from reservoir rock, there are chances of forming precipitates along the cleat or pore spaces. This can be considered as both merit and demerit of CO2 sequestration. Forming precipitate consumes and converts injected CO2 into inorganic minerals; however, it also leads to the blockage of permeability, thus affecting CO2 injectivity and storage. 26.8.1.1.4 Structural setting

Successful CO2 sequestration in coals seam is possible if the seam is deep-seated, laterally continues, and vertically isolated from surrounding strata. This type of setting is favorable for CO2 injection because it keeps waste CO2 isolated from the atmosphere and prevents gas seepage.

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26.8.1.2 Advantages of CO2 storage in coal

r CO2 captured from various sources alleviates the CO2 exposure to the ambient atmosphere. r Owing to its structural advantages, the coal seam offers a massive potential for CO2 storage. Coal has a greater capacity to store CO2 than methane, resulting in comparatively more excellent sorption of CO2 by displacing methane. The methane desorbs from coal due to weaker Van der Waals force acting between the coal-CH4 than the coal-CO2 during CO2 sequestration. This process stimulates the enhanced methane recovery from coal seams. This method is commercialized in the San Juan Basin and Powder River Basin of the United States of America. r Injection of CO2 increases effective and absolute permeability of coal seam, further increasing the CO2 storage efficiency into low permeable seams.

26.8.1.3 Disadvantages of CO2 storage in coal

r Proper identification of unmineable coal seams and reach to these coal seams may incur additional cost of operation.

r Due to the swelling properties of some petrological constituents of coal (vitrinite) [23,44],

r r r r r

the carbon dioxide adsorption alleviates permeability owing to the closure of the pore system, reducing the efficiency of coal for further CO2 injection. Moreover, coal seams seated deep within the surface suffer a reduction in permeability due to increased lithostatic stress, making them unsuitable for CO2 injection. The water solubility of CO2 is more than that of CH4, which causes the dissolution of minerals and acidification of pore waters. Dual porosity nature of some coalbeds may result in the multiphase flow of the injected CO2 that may cause gas leakage and adversely affect the entire carbon storage process. Possibility of re-sorption of the pre-adsorbed CH4 in the coal matrix, once diffused by the injected gas influencing the CO2 injectivity Possibility of matrix shrinkage and decrease in permeability because of desorption of gas. CO2 accommodation raises the possibility of diffusion of dissolved CO2 into caprock. Consequently, this diffusion would trigger geochemical reactions between caprock minerals and dissolved CO2 , affecting sealing capacity. A checklist of essential parameters for successful CO2 storage is shown in Fig. 26.5.

26.8.2 Carbon dioxide sequestration in shale Shales are usually organic-poor, very fine-grained sedimentary rocks formed by the compaction of fine sediments under quite low energy depositional environments. Due to very finegrained sediments and low permeability, shales are the suitable cap rocks for hydrocarbon trapping. Moreover, depleted shale gas reservoirs are now being used as potential sites for CO2 storage. Sequestration of carbon dioxide in reservoirs of shale gas accounts for the amplified recovery of shale gas and contributes to alleviate excess atmospheric CO2 content. The primary process responsible for CO2 storage in shale formations is adsorption. The CH4 adsorbed on shale is desorbed by CO2 injection, making room for CO2 adsorption with simultaneous recovery of shale gas (CH4 ) [45]. However, before commencing the CO2 injection in shale reservoirs, it is necessary to get acquainted with their adsorption behaviors. Carbon

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FIGURE 26.5 Parameter survey for the CO2 sequestration in coal seams.

dioxide injection in shales triggers an interaction between solid and gaseous phases due to the intermolecular force of attraction between them. So, if this attractive intermolecular force overcomes the intramolecular force between molecules of gas, the gas will accumulate on the solid surface, which is known as the adsorption of a gas onto a solid. [5,46]. In reservoirs containing shale, the adsorption capacity is shallow because of the compact nature of shale. Therefore, for effective adsorption of injected CO2 on shale, hydraulic fracturing is obligatory to augment reservoir permeability. This will allow gas to flow from free pores leading to depressurizing the system and resulting in the desorption of gases. The shale reservoir with increased permeability is a suitable site for CO2 injection because of its greater affinity towards CO2 than the other gases. Carbon dioxide is accommodated as free phase in pores and an adsorbed phase on shale matrix. 26.8.2.1 Adsorption behavior of shale for CO2 sequestration Gas adsorption is marked by a surface phenomenon induced by attractive intermolecular forces. In shale formations, CO2 is accommodated as both free gas within pores and as an adsorbed gas, in which the adsorbed phase accounts for greater than 50 percent. The gas volume, a reservoir can hold, depends upon the reservoir pressure, temperature, particle size,

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26. Organic matter and mineralogical acumens in CO2 sequestration

FIGURE 26.6 Gas transport and storage mechanism in pore matrix and fractures following Merey and Sinuyac [5]. (This Fig. is free to use under creative common attribution license).

type of the reservoir rock and its natural fracture system, and total organic carbon (TOC) content [5,47,48]. CO2 sorption takes place on the surface of the shale matrix and in fracture and pore spaces as a free gas. At the solid-gas interface, the Van der Waals type interaction increases the gas molecular concentrations near the shale surface, where the densities turn higher than liquid. Therefore, shale reservoir holds considerable CO2 than other reservoirs (Fig. 26.6) [5,49]. Shale Formations possesses higher CO2 adsorption capacities due to a more substantial dispersion effect between the surficial molecules of shales and carbon dioxide molecules than the methane molecules. The CO2 contains 16 electrons compared to 10 electrons of CH4, enhancing the dispersion force of CO2 . Further, during the CO2 adsorption, CH4 desorbs faster from shales [50]. Therefore, CO2 adsorption is higher in shale formations. 26.8.2.2 Potential of shale for carbon accommodation Shale gas resources are globally abundant and are recently proven to support carbon storage. Depleted shale gas formations are preferred sites for CO2 accommodation due to the sorption efficiency of shale. The adsorption efficiency of CO2 on shale and the extent to which it can displace pre-adsorbed CH4 is critical to understanding storage potential shale formations. The adsorption efficiency of shale is influenced by its pore structure, mineral content, and total organic (TOC) carbon content. Many authors have concluded that the TOC of shale is an essential controlling factor in the sorption mechanism of shale [51]. Moreover,

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the type of clay minerals in shale significantly affects the sorption capacity. Smectite can adsorb more CO2 than the kaolinite, illite, chlorite, palygorskite and other clay minerals, which is elaborated in the section dealing with the mineralogical acumens of CO2 sequestration. Some important shale reservoirs with the potential for CO2 storage around the world are Western Canadian Sedimentary Basins [52], Dutch shale resources [53], Devonian black shales of Eastern Kentucky [54], Mudrong shales in Australia [11,55].

26.9 Mineralogical acumen of carbon sequestration 26.9.1 An overview Soil organic carbon (SOC), measured by soil respiration, contributes a significant amount of carbon dioxide (CO2 ) to the environment [56]. Meanwhile, if properly managed, soil can act as the sink of CO2 instead of being its source [57]. Annually, soil can absorb ∼20 percent of anthropogenic carbon emanations. The clay minerals in the soil play major roles in capturing the carbon from the atmosphere [58]. Owing to their highly reactive nature, the clay minerals directly or indirectly influence the physicochemical properties of soils. Further, the types of clay minerals affect the carbon protection in soil [57] during the carbon sequestration. Smectite-rich soils can protect more organic carbon compared to soils enriched in kaolinite clay [59,60]. Moreover, organic carbon is effectively stabilized when organic amendments in soil are co-composted with the clay minerals. These clays offer large specific surface areas and both mutable and perpetual surface charges, which are vital in assessing the organic carbon protection in soils. The organic carbon is adsorbed on the clay mineral surfaces by hydrophobic /electrostatic attraction, π -bonding, exchange of ligands, etc. These mechanisms inhibit the microbial degradation of the organic carbon [61,62] in soil.

26.9.2 Clay minerals Clays are the phyllosilicate group of minerals comprising interconnected six-membered rings of silica (SiO4 −4 ) tetrahedra, which expand outward in infinite sheets. From each silica tetrahedron, three out of the four oxygen atoms are shared with the other tetrahedron, leading to a basic structural arrangement represented by Si2 O5 –2 . The phyllosilicates consist of hydroxyl (OH– ) ion, which is positioned at the center of the six-membered rings of the tetrahedra leading to the structural unit represented by Si2 O5 –2 (OH) –3 . The cations (Mg+2 , Al+3 , Fe+3 ) are octahedrally coordinated with the apical oxygen and the hydroxyl ions of the silica tetrahedral layer. Kaolinite, smectite, illite, chlorite, vermiculite, etc., are the major clay minerals encountered on the earth’s crust. More specifically, the clay minerals in soil belong to the chain and layer silicates, sulfates, carbonates, and sesquioxides (metal hydroxides, metal oxides, imogolite, allophane, etc.) [57]. The layer silicate structures of clay are distinguished into diphormic (1:1) clay, triphormic (2:1) clay, and tetraphormic (2:1:1) clay varieties based on the number of the silica tetrahedra and magnesium hydroxide/alumina octahedra [63]. Excessive degree of leaching and subsequent eliminations of K+ , Na+ , and Ca+2 cations in solution during chemical weathering episodes of orthoclase feldspar in humid and warm climatic conditions source diphormic kaolinite (1:1). Hence, kaolinite can be used as the marker

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FIGURE 26.7 Layer silicate structure of kaolinite (following Kotal and Bhowmick [68]; reuse of this Fig. is permitted by Elsevier and Copyright Clearance Center; License Number: 5,302,470,099,928; dated: sixth May 2022).

of the humid tropical to subtropical climatic conditions. Moreover, kaolin shows hydrophilic characteristics leading to its aquatic dispersal with a chemical dispersant [64,65]. Further, weathering of feldspar and some mica (muscovite) under high pH conditions in temperate climates forms the illite. Also, illite is a dominant constituent in marine shales. Alterations of mafic igneous and Ca, Mg-rich metamorphic rocks in temperate climatic conditions source triphormic smectites (2:1) [65]. Vermiculites are the alteration products of chlorite and some mica. Chlorite (2:1:1) is found mainly in temperate or cool and arid soils. Palygorskite shows the chain silicate structure and consists of hydrated Mg-Al silicates.

26.9.3 Swelling properties of clay minerals Clays have both swelling (expanding clays) and non-swelling (non-expanding clays) properties. In the diphormic clays, like in kaolinites, the tetrahedral-octahedral-tetrahedral (T-OT) sheets are bordered by alumina, silicon, and oxygen. Interlayer hydrogen bonding leads to a fixed and rigid structure, which barely allows any interlayer expansion when the clays are hydrated (Fig. 26.7). Thus, kaolinite does not swell when exposed to water/moisture [65–67]. In kaolinite, the structural units hinder the incorporation of water molecules and cations between the crystal structures. However, the exterior surface of this mineral undergoes a small amount of isomorphic substitution of aluminum for silicon and iron for aluminum [64]. This leads to a small efficiency of cationic exchange in the kaolinite structure. This low cationic exchange capacity coupled with a smaller surface areal extent results in little cationic absorption on the kaolinite exterior surface [64]. Triphormic clays (2:1) are mainly characterized by smectite, vermiculite, and illite. Smectite swells during hydration. Swelling happens when the internal surface of clay surpasses the external surface during hydration. Smectites can intercalate additional foreign molecules within the interlayer spaces and alter the distance of repetition along the z-crystallographic direction, a mechanism known as the swelling [69,70]. In montmorillonite, the principal clay mineral of the smectite group, magnesium ion (Mg+2 ), replaces aluminum ion (Al+3 ) in some of the octahedral sites leading to the imbalance in charge. Further, an additional charge imbalance of 0.66/unit cell is imposed by the replacement of the silicon atoms with aluminum

26.9 Mineralogical acumen of carbon sequestration

575 FIGURE 26.8 Layer silicate structure of smectite (following Kotal and Bhowmick [68]; reuse of this Fig. is permitted by Elsevier and Copyright Clearance Center; License Number: 5,302,470,099,928; dated: sixth May 2022).

at the tetrahedral sites [66]. This charge imbalance leads to the high cation exchange capacity, shrinkage, and swelling characteristics [65]. The charge imbalance is often compensated by the exchange of cations between the unit layers and edges. High cation exchange efficiency, layer charge, and large surface area make smectite a potential adsorbent of water, oil, gas, and other chemicals. Smectite can intercalate polar water molecules [69–74], as well as non-polar carbon dioxide in gaseous/liquid and supercritical states (Fig. 26.8) [11,75–85]. Furthermore, the vermiculite structure is characterized by di-octahedral and tri-octahedral cationic assemblies. Water molecules and magnesium are intensely adsorbed on the interlayer spaces of the vermiculite crystal, and these adsorbed materials hold the structural units together instead of splitting them apart. However, this leads to a lesser degree of swelling of vermiculite during hydration than the smectite [65]. Hence, vermiculite is a restricted expanding clay having higher swelling/shrinking properties than kaolinite but less than the smectite. On the other hand, illite, and palygorskite, are the non-expanding clays. Illite comprises less aluminum content compared to silica and lacks alkalis in its crystal structure, which result in its non-expanding/non-swelling property. The tetraphormic clays (chlorite) barely adsorb water molecules between the layers leading them to exhibit non-swelling property.

26.9.4 Carbon protection capacity of clay minerals Clay minerals strongly affect the fixation of organic carbon in soils due to their physicochemical properties and crystalline structure. Clay-rich soils can preserve more organic carbon

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from mineralization or conversion to carbon dioxide. Franzluebbers [86] suggested that twelve times enhancement of clay content in soils can alleviate ∼40 percent soil respiration to form carbon dioxide. Further, Wang et al. [87] reported that twenty-three times augmentation in clay content in soils could decrease ∼50 percent soil respiration rate. Triphormic clays adsorb larger amounts of dissolved organic carbon in soil compared to the diphormic clays due to some intrinsic physicochemical properties [88,89]. Triphormic smectite has a greater external surface area of 15 – 160 m2 g–1 compared to the diphormic kaolinite (6 – 40 m2 g–1 ) [57,63,90,91]. The smaller mean particle size of smectite provides a greater surface area per unit mass than kaolinite. As discussed above, the replacement of silica by aluminum and iron in the tetrahedral sites and/or isomorphous cation substitution in the octahedral sites are more significant in the case of smectite than the kaolinite. This results in a larger surface charge and subsequently more effective adsorption sites on the smectite surface than the kaolinite, which elevates the tendency to adsorb organic carbon [57]. Additionally, the higher cation exchange efficiency of smectite (160 cmol (p+ ) kg–1 ) than the kaolinite (0–10 cmol(p+ )kg–1 ) augments the organic carbon adsorption in soil enriched in triphormic smectite. Allophane in the sesquioxide minerals comprises a large external surface area ranging from 700– 1500 m2 g–1 . The oxide/hydroxide minerals also possess a substantial external surface area. These minerals, therefore, adsorb larger amounts of organic carbon compared to the clay minerals [57,90,92–94]. Additionally, Kleber et al. [95] reported that these minerals undergo particular chemical interactions with the organic matter and hinder microbial decomposition. Besides, the charge of the external surface area of these clay minerals influences the organic carbon adsorption. The triphormic smectite possesses a negative surface charge, whereas the diphormic kaolinite shows a low surface charge value, which may vary with the pH conditions. The layer charge of the allophane is also pH-dependent [57]. Organic functionalities in the soil can also influence the surface charge of the clay minerals. These organic moieties form organo-clay complexes in the soil and affect their intercalations with the freshly added organic matter. Further, the elimination of primary organic matter from these clay minerals may enhance their external surface area and subsequently boost their capacity to adsorb organic carbon. Moreover, the coating of the sesquioxides around these clay minerals may amend their surface area and surface charge. These coatings are formed by the precipitation of the secondary oxides/hydroxides of the Al+3 and Fe+3 ions liberated during the chemical weathering of primary oxide minerals in soils [90,96]. Also, the mingling of the sesquioxide with the clay minerals offers additional organic interactions through exchange of ligands and bridging of polyvalent cation, which ultimately augment the adsorption capacity of the soil organic matter and dissolved organic carbon [57]. This is evident by the positive correlation between the soil organic matter and the sesquioxide compounds [61,95,97]. On the other hand, Saidy et al. [90], Singh et al. [91], and Kahle et al. [98] reported that the exclusion of these sesquioxide compounds from the clay minerals initially lowers their external surface area, and subsequently, the organic carbon adsorption/protection. Besides, allophane forms stable associations of organo-mineral components by physical protection techniques coupled with the innersphere complexation process. This stable organo-mineral assemblage in allophane enhances the average residence time of various organic carbon components in soils [97,99]. Volcanic ash-rich soil (Andisol) comprises a large amount of allophane, which helps retention of more organic matter than any other kind of soil in terms of average residence time and equilibrium organic matter content [100,101]. Additionally, allophane-rich soils consist of free

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iron and aluminum, which form precipitation complexes and hinder microbial decomposition of the dissolved organic carbon [102,103]. Also, poorly available nutrients (most importantly phosphorus) to the soil microbial communities coupled with acidic conditions preferentially preserve/protect the organic carbon in allophane-rich soils [97,101].

26.9.5 Methods of organic carbon protection by clays The variability in the clay content in different types of soils leads to dissimilar responses to anthropogenic carbon dioxide emissions. Clay minerals deaccelerate the soil respiration or organic matter mineralization through interrelated chemical, physical, and biological procedures [57]. Stable assemblages sourced through the amalgamation of colloids in soil develop the physical carbon protection pathway. Organic matter cements the individual particles to form the aggregates. During this process, a part of organic matter may often enter the pore spaces of the individual particles and remain unreachable to the microbial communities [59,61,96,104]. Besides, adsorption of organic matter on the exterior surfaces of the clays through hydrogen bonding, anion exchange, cation bridging, ligand exchange, simultaneous precipitation of organo–metal complexes, hydrophobic bonding interactions, and van der Waals force [89], develops the chemical protection of the soil organic matter. The physical adsorption of the organic molecules onto the clay surfaces takes place through the van der Waals force and that is often supplemented by a little change in entropy in the reaction system. Neutral microsites on the triphormic smectites along with the nonexpanding kaolinite and palygorskite clays adsorb nonpolar and electrically neutral organic functionalities. Further, hydrophobic attraction may also adhere the nonpolar organic functionalities to the clay surfaces when the clays are coated with the native hydrophobic organic matter [57]. Physical adsorption also takes place by the development of hydrogen bonds. Here, the clay minerals with the oxygen surfaces interact with the organic functional groups, like carbonyl (C = O), carboxyl (COOH), amines (—NH2 ), nitrogen heterocycles, and phenolic OH [105]. Chemical sorption of organic carbon onto the clay surfaces occurs through the development of inner or outer sphere complexes. Inner sphere complex forms during the molecular adsorption at the particular sites on the surface of the minerals that may supersede the molecular electrostatic interface with the mineral surface. This mechanism is also called the ligand exchange pathway [57]. This exothermic reaction occurs through an exchange of anion where the anionic organic functionalities synchronize with the OH moieties available on the mineral external surfaces. Hydroxyl functionalities of the sesquioxides also participate in this process, which may be accompanied by an elevation in the pH of the system as the hydroxyl ions are transferred to the solution phase. Additionally, edge hydroxyl functionalities of the clay minerals may also partake in the ligand exchange reactions. Moreover, this mechanism leads to the strong adsorption of organic carbon on the imogolite and allophane mineral surfaces. Organic matter enriched in aliphatic acids, aliphatic/phenolic hydroxyl groups, nitrogen heterocycles, and amines are adsorbed onto the clay surfaces through this mechanism, forming stable organo-mineral assemblages [57,106]. However, a repulsion force may originate to hinder this chemi-adsorption as both the organic molecules and the clay surfaces are negatively charged. Meanwhile, variations in the system pH level may alter the surface charge of the clay minerals to less negative and even positive. In such circumstances,

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electrostatic attractions may supplement the ligand exchange reaction. Besides, zwitterionic or cationic organic molecules, at low pH conditions (0.7 nm).

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Coals adsorb CO2 in ultramicropores (2 nm diameter, water was adsorbed on the first layer, but a weak second layer adsorption was developed by CH4 and CO2 molecules. The carbon dioxide adsorbed layer was stronger compared to that of the methane. In low water and pure dry conditions, CO2 may form multilayer adsorption at high pressure, which may felicitate augmented sorption of this gas with the rising pressure. Meanwhile, the tiny presence of moisture in the exterior domain of the nanopore networks may considerably lower the CO2 adsorption in clay minerals. Further, Jeon et al. [122] investigated the kinetics and equilibrium of CO2 adsorption at nanoscale interfaces of clay mineral surfaces from subcritical stage to supercritical level. They utilized the gravimetric technique to assess the kinetics and sorption equilibrium of CO2 on sepiolite, illite, and montmorillonite clays. Sepiolite had the largest surface area and hence, exhibited the highest excess carbon dioxide adsorption isotherm in comparison to the other clay minerals throughout the experimental states. However, illite depicted the smallest amount of CO2 adsorption. Hence, the carbon dioxide adsorption efficiency declined from sepiolite (446.6 wt percent) through montmorillonite (165.0 wt percent) to illite (43.16 wt percent). Moreover, after the high-pressure adsorption of CO2 , montmorillonite revealed considerable positive hysteresis, whereas the sepiolite and illite exhibited weaker hysteresis. Near the critical pressure, in all the clay minerals, excess adsorption isotherms displayed a maximum. The absolute adsorption isotherms moved towards the saturation over the critical density of carbon dioxide. Besides, all the clay minerals under the investigation revealed a sharp alleviation in the Brunauer–Emmett–Teller (BET) surface area after the high-pressure carbon dioxide adsorption tests. Montmorillonite depicted a less significant declination in the surface area compared to the non-expanding clays (illite and sepiolite). Meanwhile, owing to their exposure to high-pressure carbon dioxide, the clay minerals underwent irreversible deformation of the pore structure. Therefore, the clay mineralogy of the reservoir and cap rocks are crucial for carbon dioxide sequestration as the properties and sorption efficiencies of the clay minerals are different from each other at the high-pressure CO2 exposure. 26.9.6.5 Smectites with various exchangeable cations Rother et al. [120] investigated the interactions of Na-saturated montmorillonite comprising a sub single water layer in its interlayer with the supercritical carbon dioxide through neutron

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diffraction and gas adsorption mechanisms. The excess adsorption isotherms depicted maxima at the bulk carbon dioxide densities of around 0.15 gcm–3 . The excess adsorption declines to zero and even negative values with the rising bulk density of carbon dioxide. The neutron diffraction tests revealed that restricted amounts of carbon dioxide were adsorbed into the interlayer spaces of the clay resulting in the alleviation of the interlayer peak intensity and an elevation of d(001) crystallographic spacing by 0.5 Å At a given temperature, the CO2 density in the pore networks of the clay minerals remained stable over a broad range of pressures. Further, swelling of interlayer spacing of the sub-single layer hydrated Na-montmorillonite may seal the small cracks in the caprocks, boost up the security of storage and thereby lower the leakage possibility. Meanwhile, Botan et al. [75] stimulated the CO2 intercalation at geological reservoir conditions (25 bar, 75 °C). They spotted that the carbon dioxide was intercalated within the hydrated interlayer spaces of Na-montmorillonite without imparting any shrinkage or swelling effects. Further, Schaef et al. [123] studied the interplay between the ammonium (NH4 )–, cesium (Cs) –, and sodium (Na)–montmorillonite with the weakly polar dry supercritical carbon dioxide. The Cs–and NH4 –montmorillonite clays adsorbed the carbon dioxide and swelled. However, although the cation solvation energy in carbon dioxide usually suggests a sturdy interaction with Na, the Na-montmorillonite did not expand. Molecular dynamic simulation experiments exhibited a large endothermicity during the interaction between the Namontmorillonite and carbon dioxide, but there was barely any energy barrier for the Cs–and the NH4 –montmorillonite clays. Besides, they reported that the interaction strength between the aluminosilicate sheets and the interlayer cation affected the CO2 -induced clay swelling at a low dielectric constant. Thus, the swelling of clay in the weakly polar solvents is subtle to the surface layer charge and its disposition between the octahedral and the tetrahedral sites.

26.9.6.6 Na-hectorite Loganathan et al. [124] reported the effects of fluoride (F− ) for hydroxyl ion (OH− ) substitution on the partitioning of carbon dioxide into the interlayers of smectite clay (Na– hectorite). Their experiments revealed that replacing OH− ion with F− ion made the clay surfaces hydrophobic. Computer molecular modeling dealing with the intercalation nature between the water and carbon dioxide revealed that the rising F− /(F− + OH− ) index in the octahedral sites gradually augmented the hydrophobicity of the Na–hectorite. This incident accompanied the fluorination of zeolites, metal-organic frameworks, and silica surfaces. At 90 bar pressure, 323 K temperature, and water-saturated carbon dioxide condition, a rising degree of hydroxyl substitution by fluoride ion led to the alleviation in the net carbon dioxidewater interaction, the elevation of the CO2 /(CO2 + H2 O) index in the smectite interlayers, and augmentation of the energy barrier of the carbon dioxide-water intercalation. Carbon dioxide was mostly intercalated at the monolayer basal spacings. Considering Na+ as an exchangeable ion, some water molecules crack the smectite interlayers for the incorporation of CO2 . The results of this investigation advised that the substitution of hydroxyl ion by the fluoride ion coupled with the alleviated structural charge and interchange with low charge cations would elevate the capacity of smectite to include hydrophobic elements like CH4 , CO2 , H2 , and other organic functionalities.

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26.9.6.7 Laponite (synthetic smectite) Bowers et al. [125] observed whether supercritical carbon dioxide reacts with clay minerals at reservoir pressure and temperature conditions (90 bar and 323 K) to source carbonate phases through dissolution-reprecipitation and ion exchange-precipitation mechanisms. The dissolution-reprecipitation reaction was performed employing the Cs–, Ca–, and tetramethylammonium (TMA+) laponite. This is a synthetic smectite clay with small particles and has a large concentration of edge sites where two oxygen atoms are shared with the other tetrahedra in the silica sheet (Q2 sites). These specifications make laponite an outstanding material for investigating the function of T-O-T edges. On the other hand, Pb–exchanged lowFe smectite was used for the ion exchange-precipitation reaction. The spectral data revealed that bicarbonate (HCO3 – ) ions generated in all the laponites at low water content. Carbonate anions formed at low water concentration when Ca+2 became the exchangeable cation. Also, in vacuum-dried clay samples, the Nuclear magnetic resonance data exhibited the development of amorphous CaCO3 . Then, the laponite samples were equilibrated at 100 percent relative humidity, followed by exposure to supercritical carbon dioxide. This resulted in forming a larger number of mobile bicarbonate ions and an amorphous/poorly crystalline hydrous magnesium bicarbonate/carbonate phase from the Mg+2 ion discharged by the dissolution of clay. The 100 percent relative humidity sample also precipitated vaterite, aragonite, and calcite phases in the presence of the Ca+2 exchangeable cation. Their experiments suggested the requirement of a large edge site Q2 fraction for the occurrence of the dissolution−reprecipitation mechanism in a short time period. On the other hand, the exposure of Pb–exchanged hectorite to the supercritical carbon dioxide formed cerussite (PbCO3 ) at the critical humidity of 78 percent. This accompanied the interlayer substitution of Pb+2 ions by the HCO3 – ion generated from the reaction between water and CO2 on the surface of the clay mineral. However, this reaction was barely observed with the natural smectites and Na– or Ca–exchanged natural hectorite on a similar time scale. 26.9.6.8 Fluorohectorite Michels et al. [116] employed fluorohectorite (synthetic smectite) to study the CO2 intercalation and retention governed by the interlayer cations. Synthetic clays comprise extremely low amounts of impurities and possess homogenous charge distribution than the natural ones. They noticed that the CO2 gas molecules intercalated within the nano-space in the interlayers of the fluorohectorite near the ambient temperature and pressure settings. The intercalation rate and the retention of CO2 depended on Na+ , Li+ , and Ni+2 interlayer cations. The Li–fluorohectorite retained carbon dioxide up to 356 °C at ambient pressure. This CO2 was released by heating the clay beyond that temperature. The authors found that smectite with the interlayer cations considered in their investigation was efficient in capturing almost the same mass of carbon dioxide (0.23 ton CO2 /m3 of the sample) as in comparison to the zeolite minerals (0.29 ton CO2 /m3 of the sample) or metal-organic frameworks (0.32 ton CO2 /m3 of the sample). 26.9.6.9 Na-montmorillonite and na-fluorohectorite Hwang et al. [126] investigated the carbon dioxide and methane sorption on dry Namontmorillonite over a broad temperature and pressure range by applying a gravimetric

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method. Their experiments advised that carbon dioxide was more strongly adsorbed than methane on Na–montmorillonite as quantified by the sorption kinetics and the selectivity parameter. Sozzani et al. [127] applied the hyperpolarized xenon Nuclear magnetic resonance (NMR) technique to investigate the nanoporosity of hectorites (smectite) pillared with tetraethylammonium. The clay interlayers were swelled by big organic cations. This expansion opened up the nanopore space, which was accessible to benzene, methane gas, and carbon dioxide. The authors noted an 18 percent uptake per wight of clay for carbon dioxide sorption at 600 Torr and –78 °C. However, in the pillared clays, the adsorption of the guest molecules varied from molecular intercalation in the non-modified clay minerals with small exchangeable cations. In the latter case, the molecular intercalation required significant mechanical work to expand the interlayer spaces. Hemmen et al. [79] reported that the carbon dioxide intercalated within the interlayer gallery of Na–fluorohectorite (synthetic smectite) almost near to the ambient conditions (5 bar, –20 °C). However, under similar conditions, neither the nitrogen gas nor the water vapor intercalated within the smectite. At 15 bar pressure and –20 °C temperature, the distance of the interlayer repetition upon CO2 intercalation was observed to be 12.5 Å Also, the timescale of CO2 intercalation within the Na–fluorohectorite interlayer was faster at 15 bar pressure and –20 °C temperature than at 5 bar pressure. Further, the carbon dioxide intercalation rate was several magnitudes slower compared to that of water at ambient temperature and pressure situations. They suggested that the kinetics of intercalation was governed by the temperature, intercalant’s partial pressure, and the affinity of the cations present in the interlayer or clay surfaces for the intercalating substance. Tsiao et al. [128] investigated the interlayer spacing of montmorillonite and Ca–bentonite (unmodified and pillared) and observed that the Xenon gas penetrated into both cases without any interlayer expansion. 26.9.6.10 Ni-fluorohectorite Cavalcanti et al. [129] further detailed the utility of the nano-silicate structure of Ni– fluorohectorite, which could adsorb around 0.79 t of carbon dioxide/m3 of host substance. Their study advocated that the high capture/sorption efficiency of this fluorohectorite was accompanied by the valence and types of cations present in the interlayer, as well as a large charge density that was nearly twice that of montmorillonite. So, this Ni–fluorohectorite could capture the largest amount of CO2 among the natural clay minerals. Higher layer charge in montmorillonite promotes the entry of H2 O molecules to the interlayers and alleviates the adsorption of CO2 , whereas a low layer charge enhances the CO2 adsorption [130]. Hunvik et al. [131] employed the Ni–fluorohectorite to study the impact of layer charge on CO2 adsorption and subsequent swelling of the clay interlayers. They investigated the CO2 adsorption efficiency in three separate layer charges in the Ni–fluorohectorite. They observed that the CO2 adsorption rose with the alleviating interlayer charge of the Ni–fluorohectorite, whereas the threshold pressure for sorption and subsequent swelling in response to carbon dioxide lowered. The authors advised that the inverse relationship between the CO2 uptake in the Ni–fluorohectorite interlayer and its layer charge originated due to the elevation in the available effective surface area along with the CO2 sorption sites with the decreasing layer charge. Attractive electrostatic forces between the clay interlayers induced weaker cohesion that led to slight onset pressure for the lower layer charge clay. They calculated the excess

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CO2 adsorption efficiency of the Ni–fluorohectorite was largest (8.6 percent) in the lowest layer charge. Additionally, the highest layer charge clay retained the largest amount of carbon dioxide upon its release. Besides, the layer charge for the natural montmorillonite accords with the prime layer charge for the Ni–fluorohectorite. Further, as montmorillonite is the principal component of bentonite, the findings of Hunvik et al. [131] would be favorable for employing bentonite in commercial carbon dioxide sequestration.

26.9.7 Supercritical carbon dioxide sequestration in clays: an additional chronicle Carbon dioxide is sequestrated initially in the supercritical state [120] within the geological reservoirs, but the prolonged quarantine of CO2 depends mostly on the interactions with the clays in the caprocks [132]. Confinement of CO2 within the interlayer galleries of smectites may alter the porosity and permeability properties of caprocks due to their expansion and shrinkage. Molecular simulations advised that after the supercritical CO2 injection within the subsurface reservoirs, the intercalation and confinement of CO2 within the smectite interlayer spaces depend mainly on the quantity and thermodynamic characteristics of water in the system. Lee et al. [133] investigated the thermodynamic and mechanistic implications of the differentially hydrated Ca–montmorillonite exposure to the supercritical carbon dioxide and carbon dioxide-sulfur dioxide mixtures under the reservoir conditions. They observed that interlayers swelled 8 – 12 percent in sub to single-hydrated montmorillonites upon intercalation with the supercritical carbon dioxide, while bi-hydrated montmorillonites shrank or remained mostly unaltered. The structural study of the sub to single-hydrated montmorillonites exhibited a larger number of calcium carbonate contacts and fractional modulation to vertically confined carbon dioxide molecules compared to the bi-hydrated ones. Further, the alleviated coefficient of diffusion of the intercalated carbon dioxide and water molecules suggested that the monohydrated Ca–montmorillonites are the better candidates for CO2 capture. Moreover, sulfur dioxide was found to hinder the intercalations of the water molecules with the cations and thereby augmenting the interactions between the Ca and the CO2 and capturing carbon dioxide through additional reduction of carbon dioxide diffusion. Besides, Kadoura et al. [134] conducted the Monte Carlo simulation study to document the comprehensive molecular procedure of interaction between carbon dioxide and Ca–, Mg–, and Na–montmorillonite exposed to erratically hydrated supercritical carbon dioxide at 90 bar pressure and 323.15 K temperature conditions. They noticed that the CO2 intercalation with the montmorillonite clays depended on the relative humidity. Carbon dioxide intercalation was hindered in the desiccated interlayer, which was followed by interlayer swelling owing to the uptake of carbon dioxide and water as the relative humidity rose. The intercalation of CO2 depleted with the rising relative humidity in hydrated clays due to progressively weaker interaction between the clay and carbon dioxide with the increasing relative humidity. So, at the low relative humidity, Mg– and Ca–montmorillonites were found to capture CO2 efficiently. The coefficient of diffusion of carbon dioxide rose with the growing relative humidity due to the concurrent interlayer expansion of these clays. Their findings indicated that the clay minerals having a large concentration of charged cations

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are more efficient in CO2 capturing in conditions almost similar to those of the subsurface reservoirs. Bowers et al. [135] researched the molecular-level interactions between the hectorite and the supercritical carbon dioxide at 50 °C temperature and 90 bar pressure employing the nuclear magnetic resonance method, X-ray diffraction technique, and infrared spectroscopy. They suggested that supercritical carbon dioxide is adsorbed in the interlayer spaces and on the exterior surfaces of hectorite when the interlayer CO2 uptake is preferred at small water content, and basal spacing is comparable to a hectorite’s monolayer hydrate unit. Charge balancing cations having large hydration energies, smaller radii, low polarizabilities delve water molecules from supercritical carbon dioxide or preserve the water held by the clay interlayers prior to the exposure to supercritical carbon dioxide, expanding instinctively to a bi-hydrated state, which hinders the cation-carbon dioxide interactions and thus, influences the clay-carbon dioxide intercalations. Contrastingly, cations having larger polarizabilities, larger radii, and smaller hydration energies quickly associate the carbon dioxide with the energetics facilitating the synchronicity of water and carbon dioxide in the clay interlayers over a broad humidity range of supercritical CO2 . Their investigation revealed if hectorite is equilibrated with a large quantity of supercritical carbon dioxide at 90 bar pressure and 50 °C temperature conditions, it leads to desiccation of the interlayers where the degree of this dehydration correlates with the hydrophilic properties of the charge balancing cations. Makaremi et al. [136] studied the molecular dynamics and multiphase Gibbs ensemble Monte Carlo simulations to calculate the free energy of expansion for Na-beidellite and Namontmorillonite intercalating with carbon dioxide and water at subsurface reservoir pressure and temperature settings. Their results implied that the reservoir conditions favor the adsorption of pure water in the clay interlayers over pure carbon dioxide. For the Na–beidellite–CO2 – H2 O associations, monolayer swelling took place for pure carbon dioxide and water. However, for the Na–montmorillonite–CO2 –H2 O association, the free swelling energy revealed two minima associated with the monolayer and bilayer organizations of the interlayer species. This difference may come from the strong attractive force between the negatively charged clay layers and the cations. Their experiments advised that the CO2 intercalation into the diphormic clays at the reservoir conditions expands the interlayer spaces, raises the clay volume, and affects the porosity and permeability of the caprocks. The large degree of solubility of carbon dioxide in the smectite interlayer galleries further promotes smectites as potential candidates for carbon dioxide geo-sequestration. Meanwhile, geological reservoirs and caprocks comprise an intimate association of clay minerals and organic matter. So, investigating CO2 geo-sequestration separately on clay minerals and the organic matter may leave some pits resulting in incomplete information about this crucial process. The variabilities in the nature of organic matter and clay minerals may lead to ambiguous results in CO2 adsorption, cationic discharge, and release of water and carbon dioxide molecules, among other aspects. So, a comprehensive investigation of the supercritical CO2 intercalation with the clay-organic matter composite may solve these issues a bit. Hence, Bowers et al. [137] performed a 13 C and 23 Na nuclear magnetic resonance experiment on a hectorite-humic acid (clay-natural organic polymer) composite exposed to CO2 reservoir conditions (50 °C, 90 bar) at the supercritical state. Their investigation revealed stronger interaction between the supercritical carbon dioxide and the clay-organic composite compared to only hectorite. Also, this intercalation between the clay-organic composite and

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supercritical carbon dioxide did not form any carbon-bearing functionalities for several days of exposure to the raised CO2 content. Supercritical carbon dioxide also raised the 23 Na peak intensity, alleviated the peak width at the half maxima, enhanced the basal width, augmented the 23 Na T1 rates of relaxation, as well as promoted a transition to more positive resonance frequencies. Also, the hectorite-humic acid composite exhibited greater amendments than the basic hectorite. These transformations took place possibly due to the rise in the hopping rate of Na+ sorption sites owing to the exposure to the supercritical CO2 , the abundance of new Na+ sites in the presence of the humic acid, as well as a concurrent surge in the Na+ cation numbers keenly included in the site hopping. Hence, the involvement of organic materials on the clay surfaces or in the interlayers may substantially influence the interaction of supercritical carbon dioxide along with the ion mobility in the reservoir rocks.

26.9.8 Adverse influences of carbon dioxide sequestration in clays The CO2 adsorption and sequestration affect the polymer-like coal structure resulting in the matrix swelling, which induces strains between the macro-pore networks [138] and the adsorbed CO2 gas molecules. This leads to the pore space declination for CO2 carriage and alleviates the coal permeability [139,140]. Further, interaction with CO2 induces a swelling effect on Na–montmorillonite at 22 – 47 °C temperature and ≤ 50 bars of partial pressure [77]. The primary water content of the montmorillonite clay influences its swelling amount. The peak expansion took place to 12.3 Å in a montmorillonite sample having a primary d(001) spacing of 11.3 Å at the 57 bars of partial pressure of carbon dioxide. Negligible to no swelling occurred for the samples having primary d(001) spacing of ≤10 Å and d(001) = 12.3 – 12.5 Å. The CO2 interaction was completed by P(CO2 ) of ∼50 bars, and a further increase in partial pressure did not additionally alter the d(001) spacing. Hence, this study suggests that the smectite-rich rock that caps a carbon dioxide reservoir may considerably be altered (swelled) during the carbon dioxide interaction. Supercritical CO2 infusion into the geological reservoir displaces the formation water. Consequently, the pore spaces adjoining the caprocks become dominated by dry to water-saturated supercritical CO2 . Dry supercritical CO2 may desiccate the hydrous minerals, while wet supercritical CO2 is involved in hydrating and carbonating particular minerals. As these geochemical mechanisms influence the porosity, permeability, and solid rock volume, the enduring integrity of the caprock overlying the CO2 reservoir may be affected. Loring et al. [141] studied the expansion and shrinkage of montmorillonite, exposing it to the supercritical CO2 with varying water concentrations at 90 bar pressure and 50 °C temperature. The swelling or shrinkage of montmorillonite depends on the hydration/dehydration mechanism as well as on the intercalation with CO2 . These pathways are governed by the quantity of water present in the supercritical carbon dioxide at a given pressure and temperature. The CO2 and H2 O intercalation induce swelling of montmorillonite and, consequently, raises the solid rock volume that leads to the sealing of the fractures in the caprocks. However, montmorillonite shrinks during desiccation and consequently alleviates the solid rock volume but augment the caprock porosity and permeability. That’s how the expansion and contraction of smectite clay affect the structural integrity of caprocks overlying the CO2 reservoir. Caprocks enriched in triphormic expandable smectite clay may dehydrate in direct contact with a dry plume of carbon dioxide resulting in the development of desiccation cracks. These

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cracks may offer pathways for the insertion of CO2 and accelerate dehydration and penetration of the caprock by supercritical carbon dioxide. Concurrently, the interlayer swelling of montmorillonite may seal the fractures or alleviate their apertures and consequently improve the structural integrity of the caprock. An additional plausible complication may arise from leaking of CO2 through wellbore annuli developed between host rock and the cement due to improper cementing or thermal influences. If dry carbon dioxide migrates upwards along these types of annuli, pore water in the caprock may be dissolved and transferred to shallower reservoirs or to the surface. This may lead to further development of desiccation cracks amplifying the CO2 leakage. However, partial healing of these fractures may be possible if subjected to normal stress. Smectite aligned at these fracture surfaces would swell to seal/minimize the fractures and consequently decrease flux rates of carbon dioxide. Meanwhile, the expansion of smectite upon intercalation with the CO2 may develop swelling pressure, which may significantly alter the local stress regime and, in extremely adverse situations, lead to shear failure of the caprock. Consequently, this may activate any structural deformation if the impact area is large. Hence, a proper geological investigation of a study area is mandatory before assigning any geological reservoir for the carbon dioxide sequestration and storage.

26.10 A note on CO2 disposal in basalt formations Layered basalt formations, sometimes part of large igneous provinces, such as Columbia River Basalt in the USA, the Deccan Traps in India, the Siberian Traps in Russia, and many others, offer storage sinks for captured CO2 but, more importantly, a means of low-temperature mineralization. When CO2 is injected into basalt, a reaction occurs wherein water saturated with CO2 reacts with minerals present in basalt, releasing Mg from pyroxene and olivine that further combines with carbonates to form highly stable MgCO3 (magnesite). On the other hand, water with lower amounts of CO2 reacts with plagioclase and release Ca, which combines with carbonates to form CaCO3 (Calcite) [142,143]. In this way, CO2 is consumed in the system and is locked in newly formed mineral phases. Ca2+ + CO2 + H2 O −→ CaCO3 + 2H+ (Calcium) (Carbon dioxide) (Water) (Calcium carbonate) Mg2+ + CO2 + H2 O −→ MgCO3 + 2H+ (Magnesium) (Carbon dioxide) (Water) (Magnesite)

26.11 Summary Carbon dioxide geo-sequestration is presently one of the most efficient way to culminate the excess amounts of atmospheric CO2 and thereby, limit the global warming and its dreadful consequences. Organic matter in the bio-sedimentary rocks, like coals and shales can adsorb substantial amounts of carbon dioxide within their micropores and mesopores. The adsorption of CO2 displaces the methane, which desorbs from the pores and is recovered from the wells. Hence, the CO2 sequestration in coal seams and shale beds promotes the enhanced recovery of methane, the preferred alternative of the conventional hydrocarbons. Therefore, deep-seated

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coal seams, unmineable coal beds, shale reservoirs, impermeable shale caprocks, etc. offer potential sites for adsorbing CO2 and keeping it secluded from the atmosphere for a long time. Apart from the organic matter, clay minerals play significant roles in carbon dioxide sequestration. Moreover, clay minerals, especially smectite, allophane, and sesquioxides, can stabilize organic carbon in soil and prevent mineralization and conversion to CO2 through chemical, physical, and biological procedures, making soil a sink for carbon dioxide. Clay minerals offer accessibility of the huge effective surface area, which favors CO2 adsorption. Clay minerals can swell/expand during the intercalation with water and carbon dioxide molecules, depending on multiple factors. Low cationic exchange capacity, smaller surface area, and interlayer hydrogen bonding hinder the diphormic kaolinite from swelling. However, triphormic smectite can intercalate additional foreign molecules (H2 O, CO2 , CH4 , etc.) within the interlayer spaces and swell. High cation exchange efficiency, layer charge, large effective surface area, environmental friendliness, low cost, and its large abundance in caprocks lead smectite as a potential adsorbent of CO2 . The volumetric capacity of CO2 adsorption in smectite clays is higher than the other clay minerals. This marks both natural and synthetic smectite clays as the most potential candidates for CO2 sequestration. The chapter reviews the mechanism, amounts, and variations of CO2 adsorption in natural and synthetic smectite clays at different temperature and pressure ranges and even close to reservoir conditions. Those particulars have further highlighted the expediency of smectite in CO2 capture and storage mechanisms. Meanwhile, the swelling and shrinking of smectite during CO2 adsorption may also cause some adverse effects on caprock integrity, which may require further extensive investigations to mitigate. Thus, if these adversaries are alleviated, anthropogenic CO2 sequestration in geological reservoirs will alleviate the global warming and offer a sustainable environment and a greener future.

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Index Page numbers followed by “f” and “t” indicate, figures and tables respectively.

A Absorption, 122 Accelerated carbonation, 198 Active pharmaceutical ingredient (API), 86 Adenosine triphosphate (ATP), 5 Adsorption, 121 Adsorption isotherms, 580 Adsorption technology, 493 Aerogel, 77 Air separation unit (ASU), 152 Alpha-alanine synthesis, 94 Alternating current voltammetry, 126 American Society of Heating, Refrigerating and Air Conditioning Engineers (ASHRAE), 119 Amine, 38 impregnation, 249 solvents, mixtures, 237 type liquid solvents, for capturing CO2 , 233 Amine-exfoliating method, disadvantages, 237 Amino acid ionic liquids (AAIL), 191, 355 4-aminosalicylic acid synthesis, 99 Amitriptyline synthesis, 105 Ammonium-based deep eutectic liquids, 42 carboxylic acids, 43 Amperometric methods, 129 Anaerobic digestion (AD), 431 Andante, 90 Animal stunning procedure, 300 Aqueous deep eutectic liquids, 40 Artificial intelligence (AI), 500 Aspirin synthesis, 99 Atorvastatin synthesis, 96 Auxiliary electrode, 132 Azole, 40 based deep eutectic liquids, 44

B Back propagation neural networks (BPNN), 500 Bacteria, 458 Benson Calvin process, 7 Benzimidazolone, 402 Beverage and food processing, 199 Beverage drink preservation, 298

Biobased plastics, 527 Biochar production biochar cookstoves, 60 carbon capture, 63 climate-change implications, 62 cookstoves designed, 54 rural developing countries, 64 rural women in carbon capture, 61 TLUD-Akha architecture design, 57 TLUD-Biochar ‘ecosystem,’, 59 top-lit updraft stove, 55 Biodiesel, 1 Bioethanol, 1 biohydrogen, 1 Bioether, 1 Biofuel, phases, 1 Biofuel production, captured carbon dioxide, 5 Biological sequestration, 454 future perspectives, 459 Biological utilization, 200 Biomass, 1 Bio-phenol derived superbase based del, 45 Bio-sequestration, 563 Bio-sequestration, of carbon dioxide, 7 Hatch Slack phase, 7 Biphasic solvents, 239 Bis(triethoxysilyl)acetylene (BTESA), 3 SEM images, 4f Brunauer–Emmett–Teller (BET) calculations, 2 Bunazosin synthesis, 90 Butenafine synthesis, 95

C Calcium looping, 468 Calvin Benson cycle, 532 Calvin cycle, 521, 523 Capturing carbon dioxide, 231 Carbon adsorbents, 246 carbon textural properties, 247 chemical modification, 247 hybrid composites fabrication, 248 Carbon and graphene-based support systems, 70 Carbonate-based alkalis, 252

595

596 post-combustion applications, 253 solid CO2 adsorbents, 253 Carbonation curing, 199 direct, 199 indirect, 199 Carbon-based electrodes, 132 Carbon bio-sequestration, bioresources, 455 Carbon capture, 338, 465 bio-carbon capture, 473 biological mechanism, 469 calcium looping, 468 challenges and opportunities, 472 chemical looping combustion, 467 future perspectives, 475 microalgae, 468 oxy-fuel combustion, 468, 473 physical solvent process, 466 post-combustion, 338, 467, 473 pre-combustion, 338, 465, 467, 472 pressure swing adsorption, 467 products, 470 temperature swing adsorption, 468 Carbon capture and storage (CCS), 120, 151, 279, 312, 346, 484, 521, 564 challenges and socio-economic implications, 501 environmental impact, 502 perspective, 497 policies, 498t, 499t status in 2020, 170f technique, 484, 506 technologies, 484 Carbon capture and utilization (CCU), 312, 518 Carbon capture strategies, 485 Carbon capture systems (CCS), 507 Carbon capture technologies, 488 Carbon concentrating mechanism (CCM), 522 Carbon concentrating mechanisms, 520 Carbon dioxide absorption, 122 adsorption, 121 alkanolamines, 41 amine-based solvents, 238 amines, 38 amine solvents mixtures, 237 amine-type liquid absorbent materials, 237 amine-type liquid solvents, 233 animal stunning procedure, 300 application, 442 applications, implementation and challenges, 244 in aqueous deep eutectic liquids, 40 availability, 152 azoles, 40 based mixture, 373

Index

beverage and food processing, 199 beverage drink preservation, 298 for biofuel production, 5 biological utilization, 200 bio-sequestration, 7 biphasic solvents, 239 bottleneck, 328 capture, 338 capture, in IL, 349 capture and storage, 17, 211 capture technologies, 121, 280, 312 carbon capture efficiency, 24 cellulose aerogels, 447 cellulose specific aerogels, 447 chemical capture, 122 clay, 339 collection systems, 233 commercial capturing processes, 232 conventional ionic liquid, 349 conversion to energy, 263 covalent-organic frameworks, 341 Cu based photo-catalysts, 330 current status, challenges and future directions, 169 and DEL, 22 dihydric alcohols, 38 drying of vegetables and fruits, 298 electrocatalytic conversion, 203 emission, 1, 17 and environmental problems, 17 favorable utilization technologies, 203 fixation, 541 pathways, 521 in food preservation, 298 food preservation using dry ice, 299 in food storage, 302 fuels and chemicals, 202 functionalized ionic liquids, 355 future perspective, 13, 206, 320, 342 glycerol, 25 guaiacol, 40 homogeneous photo-catalysts, 329 hybrid cellulose aerogels, 448 hybrid deep eutectic liquids, 42 hybrid starch aerogels, 446 hydrogenation, 266 imidazolium ionic liquid, 349 impact of impurities, 245 in industrial processes, 262 injection methods, 159t ionic liquid-based absorbents, 242 ionic liquid solvents, 242 ionic solvents, 243 levulinic acid, 39 liquid amine-based absorbents, 241

Index

liquid amines, 234 low-temperature applications, solid, 245 market scale and value, 205 materials derived from biomass, 338 membrane technology, 314 metal coordination-based ionic liquids, 368 metal-organic frameworks, 341 and methane reforming, 264 microalgae, 5 mineralization, 198 miscellaneous hbd, 40 miscible injection method, 161 natural organic acids, 38 oil recovery, 156 oil recovery enhancement, 202 oxy-fuel combustion, 317 photochemical conversion, 266 photosynthesis and photo oxidation, 5 plastics, 203 polyionic liquid membranes, 378 post-combustion, 319 pre-combustion, 260, 316 projects, 204 pyridine and pyrrolidine, 354 reduced graphene-oxide, 331 reduction onto semiconductor surface, 327 reduction perspectives and prospects, 267 reduction to methanol, 330 regeneration with catalysts, 240 regulation and policy, 205 reverse water-to-gas shift reaction thermo, 265 rising, 1 sensors, 122 separation by membranes, 122, 316 sequestration, 566, 570 solid amine-based adsorbents, 246 sorting oxidant, 268 starch aerogels, 446, 447 starch specific aerogels, 445 storage, 153 storage equipment disinfection, 304 storage microsphere, 302 sulfonate ionic liquids, 355 superbases, 41 supported ionic liquid membranes, 384 tanning of animal skin, 301 in ternary deep eutectic liquids, 41 thermal insulators, 448 thermochemical electrolysis, 265 thermochemical method, 264 underground storage, 167 urea, 25 utilization, 198, 204, 321 worldwide emissions, 1

597

zeolites, 340 Carbonic anhydrase, 5, 7 Carbon nanotubes, 4, 124 Carbon sequestration, 60, 485 methods, 452 limitations, 453 Carbon storage, 566 Carisoprodol synthesis, 87 Cathodic material, 541 Cellulose specific aerogels, 447 Chemical looping combustion, 467 Choline chlorideD, 26 Christian Commission for Development in Bangladesh (CCDB), 59 Clay, 339 minerals, 573, 575, 577 Clay-rich soils, 575 Climate-change implications and biochar production, 62 Climate change mitigation techniques, 484 Climate forcing, 149 Coal, 568 Composite carbon electrodes, 132 Composite catalyst reduced graphene-oxide, 331 RGO, 331 Composite membranes, 290 Computer molecular modeling, 581 Conductometric methods, 130 Conventional ionic liquids (CIL), 179, 180, 283, 349 Cookstoves designed, for biochar production, 54 Coordination polymers, 554 Coulometric analysis methods, 131 Covalent-organic frameworks (COF), 337, 341 Crosslinking, 445 Cryogenic capture technology, 490 Cryogenic carbon capture (CCC), 491 Culture broth, 539 Culture temperature, 200 Cyanobacteria, 456 Cyclic voltammetry, 126

D Deep eutectic liquids (DEL), 19 ammonium-based, 42 authentication, 21 azole based, 44 based CO2 absorption, 22 carbon capturing potential, 27 carboxylic acids, 43 future prospects, 48 hydrophobic, 45 with multiple sites interaction, 47 non-ionic, 46 phosphonium based, 44

598 preparation and authenticaiton, 20 supported liquid membranes, 46 supported membranes, 46 types, 19, 20f Density functional theory (DFT), 284 Department of Science and Technology (DST), 169 Designer solvents, 282 Diethanolamine (DEA), 26 Diethanolamine hydrochloride (DEAHCl), 39 Diethylene glycol (DEG), 26 Differential pulse voltammetry, 126 Differential scanning colorimetery (DSC), 21 Diflunisal synthesis, 100 Dihydric alcohols, 38 Dilauryl thiodipropionate (DLTDP), 98 Direct current voltammetry, 126 Direct potentiometric methods, 128 amperometric methods, 129 auxiliary electrode, 132 carbon-based electrodes, 132 composite carbon electrodes, 132 conductometric methods, 130 coulometric analysis methods, 131 electrochemical applications, 137 electrochemical gas sensors, 134 electrodes, 131 indicator electrodes, 133 potentiometric electrodes, 133 potentiometric gas sensors, 135 potentiometric titrations, 128 reference electrode, 132 working electrodes, 131 DMU-212 synthesis, 97 Doxazosin synthesis, 90 Dry ice, in food preservation, 299 Dynamic miscibility, 161 Dynamic separation, 218

E (E)-3-Benzylidene-2-indolinone synthesis, 101 Electricity power generation, 504 Electric swing adsorption (ESA), 217 Electrocatalytic conversion, 203 Electrochemical applications, 137 Electrochemical gas sensors, 134 Electrochemical reduction (ECR), 70 carbon and graphene-based support systems, 70 foam electrode, 74 gas diffusion electrode, 79 hydrogel and aerogel, 77 mesoporous electrode, 76 metal supports for, 70 titanium nanotubes, 73 Electrochemical synthesis, 215

Index

Electrochemistry, 126 potentiometric methods, 128 voltammetry, 126 Electrodes, 131 auxiliary, 132 carbon-based, 132 composite carbon, 132 indicator, 133 potentiometric, 133 reference, 132 working, 131 Electron capture detector (ECD), 123 Electro-reduction, 391, 404 to amides and methylamines, 403 benzimidazolone, 402 to carbamate, 403 CO2 to CH3 OH, 396 CO2 to CO, 392 CO2 to cyclic carbonate, 397 CO2 to HCOOH, 394 oxazolidinone, 401 quinazoline-2,4(1H,3H)-diones, 402 to urea, 403 Electrospinning, 124 Enadoline synthesis, 108 Energy Information Administration (EIA), 120, 482 Englitazone synthesis, 106 Enhanced oil recovery (EOR), 149 Epichlorohydrin (ECH), 447 Epristeride synthesis, 104 European Union, 497

F Faradaic efficiency, 71 Fatty acid methyl esters (FAME), 10 Felbamate synthesis, 88 Felbinac synthesis, 109 Finafloxacin synthesis, 110 Flat plate photobioreactor, 540 Fluorohectorite, 582 Flurbiprofen synthesis, 103 Foam electrode, 74 Food preservation, dry ice, 299 Fossil-fuel-related project, 503 Fossil fuels consumption, 1 Fouling, 304 Functionalized ionic liquids (FIL), 180, 355 Furaltadone synthesis, 88

G Garenoxacin synthesis, 106 Gas adsorption, 571 Gas chromatography (GC) advantage, 123

Index

Gas diffusion electrode (GDE), 79 Generally Recognized as Safe (GRAS) substrate, 11 Gentisic acid synthesis, 100 Geological carbon storage, 579 Geologic storage, 495 Geo-sequestration, 563, 564f Global carbon management concerns, 151 Global warming, 481 Global warming and carbon dioxide emission, 17 Glycerol, 25 Good cholesterol, 96 Graphene-based hydrogels (GH), 79 Greenhouse effect, 149 Greenhouse gas, 17, 197, 482 Guaiacol, 40

H Heat stable salts (HSS), 237 Helical twisting power (HTP), 124 High cation exchange efficiency, 575 Hollow fiber module, 315 Homogeneous photo-catalysts, 329 Hybrid cellulose aerogels, 448 Hybrid deep eutectic liquids, 42 Hybrid starch aerogels, 446 Hydrocarbon miscibility, 160 Hydrogel, 77 Hydrogen bond acceptor (HBA), 19 Hydrogen bond donor (HBD), 19 carbon capture efficiency, 24 Hydrophobic deep eutectic liquids, 45 Hydrothermal carbonization, 434, 437 Hydrothermal gasification (HTG), 436 Hydrothermal liquefaction (HTL), 436 Hydrothermal (HT) treatments for fuel production, 432 hydrothermal carbonization, 434 hydrothermal gasification, 436 hydrothermal liquefaction, 436 thermal hydrolysis, 434 Hydroxy propionate-hydroxybutyrate cycle, 527

I Ibuprofen synthesis, 101 Imidazolium ionic liquid, 349 Indicator electrodes, 133 Infrared detectors, 123 Integrated gasifier combined cycle (IGCC), 152 Inter-governmental Panel on Climate Change (IPCC), 167, 198, 427 International Biochar Initiative (IBI), 60 International Energy Agency (IEA), 345 Ionic cluster, 282

Ionic liquid (IL), 18, 19, 178, 281 as adsorbents, 289 advancement in ionic solvents, 243 for carbon capturing, 285 cations, 281 CO2 capture, 283 composite membranes, 290 conventional, 179, 283, 349 features, 281 functionalized, 180, 355 future applications, 191 gel, 191 hybridized solvents, 284 imidazolium, 349 magnetic, 185 with membranes, 289 membranes, supported, 384 metal coordination-based, 368 multiphasic, 187 polymeric, 181 principle, 242 prospects, 244 pyridine and pyrrolidine, 354 reversible, 181 solvents, CO2 capturing, 242 sulfonate, 355 supported membrane, 290 switchable polarity, 188 task specific, 186 thermoregulated, 189 types, 179 Ionic liquids, 489

K Kolbe-Schmitt process, 99 Kyoto Protocol, 279

L Lamotrigine synthesis, 109 Laponite (synthetic smectite), 582 Layered double hydroxides (LDH), 254 chemical composition, 254 Lennard Jones well depth, 3 Levulinic acid, 39 Life cycle assessment (LCA), 203, 437 Light-emitting diodes (LED), 11 Limit current, 127 Liquid amine-based absorbents, 241 Liquid-assisted grinding (LAG), 215 Liquid crystals (LC), 124 Low-temperature technologies, 492t benefits, 492t limitations, 492t

599

600 Low temperature thermal hydrolysis (LTTH), 434 Loxoprofen synthesis, 108

M Magnesium oxide (MgO), adsorbents, 255 mesoporous structure fabrication, 255 molten salts transformation, 256 prospects, 256 Magnetic ionic liquids (MIL), 185, 186 Mass Spectrometer (MS), in CO2 detection, 123 Mechanochemical synthesis, 215 Medium chain length (MCL), 518 Mefloquine synthesis, 104 Membrane technology, 490 Mesophase, 124 Mesoporous electrode, 76 Metal carbides, 553 Metal coordination-based ionic liquids, 368 Metallic titanium, 73 Metal organic framework (MOF), 212, 337, 341, 549, 554 as adsorbent, 217 adsorbents, 250 adsorbent selection and criterion, 218 carbon dioxide adsorption in, 219 chemical and thermal, 217 chemical stabilities, 223 composite, 225 conventional synthesis route, 212 electrochemical synthesis, 215 functional component integration, 251 functionalization, 220, 221 gas separation methods, 217 hydrophobic surface treatment, 225 intrinsic properties, 251 kinetic effect, 218 mechanical, 217 mechanical stability, 226 mechanochemical synthesis, 215 Mg-MOF-74, 215 microwave synthesis technique, 213 MIL-68(In)-NH2 , 216 MOF-801-Zr, 216 molecular sieving effect, 218 nitrogen site opening, 220 open metal sites, 219 post- synthetic exchange, 224 post-synthetic modification, 224 post synthetic procedure, 221 pre- synthetic procedure, 220 properties, 217 prospects, 251 quantum sieving effect, 218 sonochemical synthesis, 214 stability, 222

Index

sulfonates and phosphates functionalization, 221 synthesis, 215 thermal conductivity, 217 thermal stabilities, 225 thermodynamic equilibrium separation, 218 tuning pore size, 222 Metal supports, for electrochemical reduction, 70 carbon and graphene-based support systems, 70 foam electrode, 74 gas diffusion electrode, 79 hydrogel and aerogel, 77 mesoporous electrode, 76 titanium nanotubes, 73 Methantheline bromide synthesis, 105 Methionine hydroxy analog synthesis, 97 Methyldiethanolamine, 26, 39 hydrochloride, 39 Methylephedrine synthesis, 95 Methylethanolamine (MEA), 233, 234 Methyl ethyl ketone (MEK), 87 Methyltriphenylphosphonium bromide (MTPPB), 44 Microalgae, 457, 534, 538 Microalgae cultivations, 539t Microalgae culture system, 536 Microalgae-mediated carbon dioxide bio-mitigation, 537f Micropores, 567 Microwave synthesis technique, 213f, 213 Migrating flaming pyrolysis front (MFPF), 55 Mineralization, 198 advantages, 198 carbonation curing, 199 direct carbonation, 199 electrochemical mineralization, 199 indirect carbonation, 199 Minimal miscibility pressures (MMP), 156 determination, 157 rising bubble strategy, 157 slim tube test, 157 vanishing interfacial tension technique, 157 Miscible displacement, 160 Mixed matrix membrane (MMM), 290, 315, 317 Monoethanolamine (MEA), 22, 38 Monoethanolamine hydrochloride (MEAHCl), 39 Monte Carlo simulation, 4 Multiphasic ionic liquids (MIL), 187, 188

N Naftifine synthesis, 95 Nanomaterials, in carbon sequestration, 458 Naproxen synthesis, 98 National Oceanic and Atmospheric Administration (NOAA), 1, 455 National Program on Carbon Sequestration (NPCS), 169

Index

Natural deep eutectic liquids (NADEL), 43 Natural organic acids, 38 Natural Resource Defense Council (NRDC), 451 Natural resource production, 503 Negative emission technologies (NET), 463 Nitrogen removed mesoporous carbon (NRMC), 76 Nondispersive infrared detectors, 123 Non-ionic deep eutectic liquids, 46 Non-steroidal anti-inflammatory drug (NSAID), 100 Nuclear Magnetic Resonance (NMR) Spectroscopy, 21 Nucleophile-triggered CO2 -incorporated methylation, 95 butenafine synthesis, 95 methylephedrine synthesis, 95 naftifine synthesis, 95

O Ocean storage, 496 Oil recovery, 156 CO2 miscible injection method, 161 economics and tax incentives, 166 hydrocarbon miscibility, 160 injection and storage facilities required, 163 offshore facilities, 164 onshore facilities, 163 storage capacity calculations, 165 Oil recovery factor (ORF), 162 Organization for Economic Cooperation and Development (OECD), 482 Oxadiazon synthesis, 89 Oxazolidinone, 401 synthesis, 89 Oxyfuel combustion, 565 Oxy-fuel combustion, 317, 468, 473 Oxy-fuel combustion separation technique, 486

P Phosphoenolpyruvate carboxylase (PEPcase) enzyme, 7 Phosphonium based deep eutectic liquids, 44 Photobioreactors, usage, 5 Photo catalyst homogeneous, 329 reduced graphene-oxide, 331 types, 329 Photo-catalytic (PC) reduction, 328 Photo electro-catalytic (PEC) reduction, 328 Photolysis, of water, 327 Photosynthesis, 532 Photosynthesis and photo oxidation, of water, 5 Pipeline corrosion, 251 Pipeline transportation, 495 Plate and frame module, 315 Poly deep eutectic liquids based on supported liquid membranes (PDEL-SLM), 23

601

Polyhydroxyalkanoates, 527 biosynthesis pathways, 528f Polyionic liquid (PIL) membranes, 378 Polymeric ionic liquids (PIL), 181 Polynomial chaos expansion (PCE) technique, 10 Polyvinylidene-fluoride (PVDF), 46 Post-combustion capture technology, 488f Post-combustion carbon capture (PCC), 232, 487 technique, 487 Post-combustion process, 565 Post-combustion techniques, 486 Post- synthetic exchange (PSE), 224 Post-synthetic modification (PSM), 224 Potentiometric electrodes, 133 Potentiometric gas sensors, 135 Potentiometric methods, 128 Potentiometric titrations, 128 Power plants, 504 Prazosin synthesis, 90 Precipitation effect, 569 Pre-combustion process, 485, 564 Pre-combustion technology, 486f Pressure effect, 569 Pressure swing adsorption (PSA), 217, 467 Protic ionic liquid (PIL), 90 Pulse voltammetry, 126 Pyrolysis, 55

Q Quantum sieving effect, 218

R Radial basis function neural networks (RBFNN), 500 Raman Spectroscopy, 21 Reference electrode, 132 Repaglinide synthesis, 102 Research Council of Norway (RCN), 169 Residual current, 127 Response surface methodology (RSM), 38 Reverse water-to-gas shift (RWGS) reaction, 265 Reversible hydrogen electrode (RHE), 554 Reversible ionic liquids (RIL), 181 Rural developing countries, biochar cookstoves, 64 Rural women, in carbon capture, 61

S Sabatier reaction, 264 Screen-printed electrode (SPE), 137 Severinghaus electrode sensor, 123 Sewage treatment plants (STP), 431 Shale gas resources, 571 Single-atom catalysts (SAC), 71, 551 Single-atom catalyst synthesis methods, 549

602 Sludge, 431 Soil organic carbon (SOC), 573 Solid carbon dioxide adsorbents impurities, impact, 245 for low-temperature applications, 245 solid amine-based adsorbents, 246 Solid carbon dioxide sorbents, 257 calcium oxide sorbents, 257 future prospects, 260 granulation of powder, 259 sintering-resistance, 258 Solvent-based chemical absorption, 488 Solvothermal reactions, 212 Sonochemical synthesis, 214 Specific surface area (SSA), 247 Spirally wound module, 315 Spironolactone synthesis, 109 Square wave current voltammetry, 126 Starch specific aerogels, 445 21st Conference of Parties (COP21), 463 Storage equipment disinfection, carbon-dioxide, 304 Sulfonate ionic liquids, 355 Supercritical drying, 442 critical point and supercritical fluids, 443 high temperature, 444 low temperature aerogels, 445 Supported ionic liquid (SIL), 184 membranes, 384 Supporting ionic liquid membrane (SILM), 290 Sustainable biochar cookstoves, 63 Sustainable development goal (SDG), 54 Switchable polarity ionic liquids (S-Polymeric ionic liquids), 188 Syngas, 74, 316

T Tamoxifen synthesis, 101 Task specific ionic liquids (TSIL), 186, 187 Techno-Economic Analysis (TEA), 203 Temperature swing adsorption (TSA), 217, 468 Ternary deep eutectic liquids, 41 alkanolamines, 41 hybrid deep eutectic liquids, 42 superbases, 41 Tetra butyl ammonium bromide (TBAB), 26 Thermal conductivity, 217 Thermal conductivity detector (TCD), 123 Thermal hydrolysis (TH), 434 Thermal insulators, 448 Thermodynamic equilibrium separation, 218 Thermogravimetric analysis (TGA), 21 Thermoregulated ionic liquids (TRIL), 189

Index

Three-electrode system, 127f Three-stone fires (TSF), 53 Tipifarnib synthesis, 92 Titanium nanotubes, 73 Toloxatone synthesis, 90 Top-lit updraft (TLUD) Akha architecture design, 57 Biochar ‘ecosystem,’, 59 Top-lit updraft (TLUD) stove, 55 Triphormic clays, 574 Truck and rail transportation, 495 Tubular-photobioreactor, 539 Schematic configurations, 540f Twin Screw Extruder, 21

U United Nations Framework Convention on Climate Change (UNFCCC), 427, 463 United states, CO2 projects, 204 URB602 synthesis, 94 Urea, 25

V Vacuum swing adsorption (VSA), 217 Vegetables and fruits drying, 298 Voltammetry, 126 alternating current, 126 cyclic, 126 differential pulse, 126 direct current, 126 pulse, 126 square wave current, 126

W Water alternating gas (WAG), 158 Wet-scrubbing technology, 233 Wood gas, 56 Wood-Ljungdahl pathway, 523 Working electrodes, 131

X X-ray diffraction patterns, 554

Z Zenarestat synthesis, 91 Zeolite adsorbents, 248 adaptations through cation exchange, 249 amine impregnation, 249 fabrication, 249 prospects, 250 Zeolite-based hybrid materials, 249 Zeolites, 340 Zwitterion reaction mechanism, 236