Green Sustainable Process for Chemical and Environmental Engineering and Science: Switchable Solvents 9780128198506

Table of contents :
Cover
Half Title
Green Sustainable Process for Chemical and Environmental Engineering and Science: Switchable Solvents
Copyright
Contents
Contributors
1. Switchable solvents for bio-refinery applications
1. Introduction
2. Concept of biorefinery
3. Switchable solvents for biorefinery
3.1. Switchable polarity solvents
3.2. Switchable hydrophilicity solvents (SHSs)
3.3. Switchable water (SW)
4. Concluding remarks and future perspectives
References
2. Polarity-changing solvents for CO2 capture
1. Introduction
1.1. Switchable hydrophobicity solvents (SHSs)
1.2. Switchable water
1.3. Switchable polarity solvents
1.4. CO2-binding organic liquids (CO2-BOL)
2. Switchable solvent for CO2 capture
2.1. Investigation of thermodynamic and molecular CO2 capturing process using switchable solvent
2.2. SPS solvent for CO2 capture
2.2.1. Use of SPS as solvent for CO2 capture from flue gas
2.2.2. Use of SPS as solvent for CO2 capture from streams with high pressure
2.3. CO2-BOL solvent for CO2 capture
3. Switchable ionic liquid solvent for CO2 capture
4. Conclusions
References
3. Applications of switchable solvents in science and technology
1. Introduction
1.1. Switchable polarity solvents (SPSs)
1.2. Switchable hydrophilicity solvents (SHSs)
1.2.1. Criteria of selection for SHS
2. Switchable water (SW)
3. Technological and analytical applications of switchable solvents
3.1. Applications of SPSs
3.1.1. SPS as extraction media
3.1.2. SPSs in CO2 detection
3.1.3. SPSs as CO2 capture
3.1.4. SPSs as CO2 capture for high-pressure streams
3.2. Applications of SHSs
4. Conclusions
References
4. Switchable solvents for CO2 capture
1. Introduction
2. Environmental challenges due to CO2
3. Switchable solvents for the detection of CO2
4. Switchable solvents for CO2 extraction
4.1. Traditional ionic liquids versus switchable solvents
4.2. CO2 capture from flue gases
4.3. CO2 capture under high pressure
5. Reversible ionic liquid solvents for the capture of CO2
6. Conclusions
Acknowledgments
References
5. Switchable water
1. Introduction
2. Switchable polarity solvents
3. Switchable hydrophilicity solvents
4. Switchable water
5. Conclusion
References
6. Switchable solvents as alternative solvents for green chemistry
1. Introduction
2. Discovery of switchable solvents
3. Types of switchable solvents
3.1. Switchable-polarity solvents (SPS)
3.2. SHS (switchable-hydrophilicity solvents)
3.3. Switchable water
3.4. Synthesis/preparation of SS
4. Chemistry and development of SS
4.1. SPS
4.2. SHS
4.3. SW
5. Generally desirable properties of SS [76]
6. Acceptability of switchable solvents for green chemistry
7. Applications of switchable solvents as green alternatives [46, 78, 79]
7.1. Water treatment [80]
7.2. Soybean oil extraction
7.3. Solid particles cleaning
7.4. Residual motor oil recovery
7.5. SS as a reaction medium
7.6. Polystyrene recovery from polystyrene foam
7.7. Other applications
8. Future prospects and conclusions
References
7. Nanomaterials synthesis in switchable solvents
1. Introduction
2. Ionic liquids as switchable solvents
3. Reverse micelles for nanoparticles synthesis
4. Reverse micro-emulsions for nanoparticle synthesis
5. Reversible capping agents based on amines
6. Conclusion
References
8. CO2-triggered switchable polarity solvents and their advancements
1. Introduction
2. Chemistry of CO2-triggered SPS
3. CO2-triggered SPS
3.1. SPS as CO2 capture for production of syngas
3.2. SPS acts as a catalyst for the production of biodiesel
3.3. SPS as a ligand for catalyst removal in ATRP
3.4. SPS as solvent for microextraction of Cd(II) ions
3.5. SPS as solvent for determination of amino acids in biological samples
3.6. SPS as catalysts for the synthesis of 16-Arlydiene steroids
4. Challenges and future considerations
5. Conclusion
References
9. Switchable green solvents for lipids extraction from microalgae
1. Introduction
2. Green solvents
2.1. Green/renewable solvents
2.2. Lipid extraction by green solvents
3. Microalgae lipid and extraction process
3.1. Downstream processing
3.2. Lipid extraction paths
3.2.1. Biomass preference
3.2.2. Biomass processing
3.2.3. Nonsolvent-Mechanical approach
Homogenization under high pressure (HPH)
Expeller pressing
Bead beating or bead mills
Sonication
Microwave
Pulsed electric field (PEF)
Process of enzymatic hydrolysis
Supercritical fluid extraction
Subcritical water hydrolysis
Chemical hydrolysis
3.3. Solvent-based extraction: Selection of extraction solvent
3.3.1. Folch method
3.3.2. Bligh and dyer method
3.4. Lipid estimation methods
3.4.1. Gravimetric
3.4.2. Nile red staining
3.4.3. Modified Nile red
3.4.4. Sulfo Phospho Vanillin method (SPV) method
3.4.5. TLC/HPLC method
3.4.6. Comparative report on different analytical methods
4. Perspective and conclusion
References
Further reading
10. CO2 triggered switchable and tunable solvents for biocatalysis
1. Introduction
2. Tunable solvents for biocatalysis
3. Switchable solvents for biocatalysis
3.1. Switchable hydrophilicity solvents (SHS)
3.1.1. SHS in catalysis
3.2. Switchable water (SW)
3.3. Switchable polarity solvent (SPS)
3.3.1. SPS in biocatalysis
4. Conclusions and future perspectives
Acknowledgments
References
11. Basic synthesis and solvatochromic parameters in switchable solvents
1. Introduction
2. Chemical synthesis in a switchable solvent
3. Theory on solvatochromism process
4. Solvatochromic parameters
4.1. Molar electronic transition energies; ETN
4.2. Measurements of Kamlet-Taft parameters: Bipolarity (π*); hydrogen bond-donating (HBD) acidity (α); hydrogen bond-acc ...
5. Conclusion
References
12. Switchable solvents for catalysis
1. Introduction
2. Switchable hydrophilic solvents
2.1. Synthesis of hydrophilic switchable solvents
2.2. SHS selection criteria
3. Solvents with switchable polarity
3.1. Synthesis of hydrophilic switchable solvents
4. Switchable water
5. Surface operation (switchable cationic/anionic surfactants)
6. Materials that respond to stimuli
7. Application of switchable solvents as catalyst
7.1. Application of SHS to lipids from Botryococcus braunii to extract
7.2. Direct catalyzed transesterification of biomass of insects for production of biodiesel
8. Switchable solvents for biocatalysis
8.1. SPS in biocatalysis
8.2. SHS in biocatalysis
9. Conclusions
References
Index
Cover back

Citation preview

GREEN SUSTAINABLE PROCESS FOR CHEMICAL AND ENVIRONMENTAL ENGINEERING AND SCIENCE

GREEN SUSTAINABLE PROCESS FOR CHEMICAL AND ENVIRONMENTAL ENGINEERING AND SCIENCE Switchable Solvents Edited by

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

RAJENDER BODDULA CAS Key Laboratory of Nanosystems and Hierarchical Fabrication, National Center for Nanoscience and Technology, Beijing, China

ABDULLAH M. ASIRI Department of Chemistry, Faculty of Science, King Abdulaziz University, Jeddah, 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 © 2022 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. Library of Congress Cataloging-in-Publication Data A catalog record for this book is available from the Library of Congress British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library ISBN: 978-0-12-819850-6 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: Emerald Li Production Project Manager: Kumar Anbazhagan Cover Designer: Christian J. Bilbow Typeset by STRAIVE, India

Contents

Contributors

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1. Switchable solvents for bio-refinery applications

1

Muhammad Zubair and Aman Ullah 1. Introduction 2. Concept of biorefinery 3. Switchable solvents for biorefinery 4. Concluding remarks and future perspectives References

1 2 4 15 16

2. Polarity-changing solvents for CO2 capture

21

Zeynab Rezaeiyan, Shokufeh Bagheri, Mohammad Amin Sedghamiz, and Mohammad Reza Rahimpour 1. Introduction 2. Switchable solvent for CO2 capture 3. Switchable ionic liquid solvent for CO2 capture 4. Conclusions References

3. Applications of switchable solvents in science and technology

21 27 32 34 35

39

Mohammad Faraz Ahmer and Qasim Ullah 1. Introduction 2. Switchable water (SW) 3. Technological and analytical applications of switchable solvents 4. Conclusions References

4. Switchable solvents for CO2 capture

39 45 47 54 54

61

Satish Kumar 1. 2. 3. 4. 5.

Introduction Environmental challenges due to CO2 Switchable solvents for the detection of CO2 Switchable solvents for CO2 extraction Reversible ionic liquid solvents for the capture of CO2

61 64 66 67 90

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6. Conclusions Acknowledgments References

5. Switchable water

94 94 94

101

Dr. Elsa Cherian 1. Introduction 2. Switchable polarity solvents 3. Switchable hydrophilicity solvents 4. Switchable water 5. Conclusion References

6. Switchable solvents as alternative solvents for green chemistry

101 101 103 104 106 106

109

Divya Bajpai Tripathy, Anjali Gupta, Anuradha Mishra, M.A. Quraishi, Mohammad Luqman, and Mohd. Farhan Khan 1. Introduction 2. Discovery of switchable solvents 3. Types of switchable solvents 4. Chemistry and development of SS 5. Generally desirable properties of SS 6. Acceptability of switchable solvents for green chemistry 7. Applications of switchable solvents as green alternatives 8. Future prospects and conclusions References

7. Nanomaterials synthesis in switchable solvents

109 111 111 120 122 123 124 127 127

133

Anjali Gupta, Divya Bajpai Tripathy, Meenu Aggarwal, Mohammad Luqman, and Mohd. Farhan Khan 1. Introduction 2. Ionic liquids as switchable solvents 3. Reverse micelles for nanoparticles synthesis 4. Reverse micro-emulsions for nanoparticle synthesis 5. Reversible capping agents based on amines 6. Conclusion References

8. CO2-triggered switchable polarity solvents and their advancements

133 134 136 141 143 144 145

149

Pinki Chakraborty, Anupama Sharma, Mohammad Luqman, Mohd. Farhan Khan, and Karthikay Sankhydhar 1. Introduction 2. Chemistry of CO2-triggered SPS

149 150

Contents

3. CO2-triggered SPS 4. Challenges and future considerations 5. Conclusion References

9. Switchable green solvents for lipids extraction from microalgae

151 154 155 155

157

Debanjan Sanyal, G. Venkata Subhash, Nishant Saxena, Wriju Kargupta, Ajit Sapre, and Santanu Dasgupta 1. Introduction 2. Green solvents 3. Microalgae lipid and extraction process 4. Perspective and conclusion References Further reading

10. CO2 triggered switchable and tunable solvents for biocatalysis

157 158 158 170 170 174

177

K.K. Athira and Ramesh L. Gardas 1. Introduction 2. Tunable solvents for biocatalysis 3. Switchable solvents for biocatalysis 4. Conclusions and future perspectives Acknowledgments References

11. Basic synthesis and solvatochromic parameters in switchable solvents

177 178 180 187 187 187

191

Naushad Anwar, Nimra Shakeel, and Mohd Imran Ahamed 1. Introduction 2. Chemical synthesis in a switchable solvent 3. Theory on solvatochromism process 4. Solvatochromic parameters 5. Conclusion References

12. Switchable solvents for catalysis

191 192 193 194 197 197

201

Saurabh Jain, Shashank Sharma, Mohammad Luqman, and Mohd. Farhan Khan 1. 2. 3. 4.

Introduction Switchable hydrophilic solvents Solvents with switchable polarity Switchable water

201 202 207 209

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5. Surface operation (switchable cationic/anionic surfactants) 6. Materials that respond to stimuli 7. Application of switchable solvents as catalyst 8. Switchable solvents for biocatalysis 9. Conclusions References Index

212 214 215 216 217 217 225

Contributors

Meenu Aggarwal Department of Chemistry, Aggarwal College, Faridabad, Haryana, India Mohd Imran Ahamed Department of Chemistry, Aligarh Muslim University, Aligarh, Uttar Pradesh, India Mohammad Faraz Ahmer Department of Electrical and Electronics Engineering, Mewat Engineering College, Nuh, Haryana, India Naushad Anwar Department of Chemistry, Aligarh Muslim University, Aligarh, Uttar Pradesh, India K.K. Athira Department of Chemistry, Indian Institute of Technology Madras, Chennai, India Shokufeh Bagheri Department of Chemical Engineering, Shiraz University, Shiraz, Iran Pinki Chakraborty School of Basic & Applied Sciences, Galgotias University, Greater Noida, India Dr. Elsa Cherian Department of Food Technology, SAINTGITS College of Engineering, Kottayam, Kerala, India Santanu Dasgupta Reliance Industries Limited, Jamnagar, Gujarat, India Ramesh L. Gardas Department of Chemistry, Indian Institute of Technology Madras, Chennai, India Anjali Gupta Department of Chemistry, School of Basic and Applied Sciences, Galgotias University, Greater Noida, India Saurabh Jain Department of Biotechnology, MGIMT, Banthra, Lucknow, India Wriju Kargupta Monash University, Melbourne, VIC, Australia

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Contributors

Mohd. Farhan Khan Nano Solver Lab, Department of Mechanical Engineering, Z. H. College of Engineering & Technology, Aligarh Muslim University; Department of Science, Gagan College of Management and Technology, Aligarh, India Satish Kumar Department of Chemistry, St. Stephen’s College, University Enclave, Delhi, India Mohammad Luqman Chemical Engineering Department, College of Engineering, Taibah University, Yanbu Al-Bahr, Kingdom of Saudi Arabia Anuradha Mishra Department of Applied Chemistry, SoVSAS, Gautam Buddha University, Gautam Budh Nagar, India M.A. Quraishi Center of Research Excellence in Corrosion, Research Institute, King Fahd University of Petroleum and Minerals, Dhahran, Saudi Arabia Mohammad Reza Rahimpour Department of Chemical Engineering, Shiraz University, Shiraz, Iran Zeynab Rezaeiyan Department of Chemical Engineering, Shiraz University, Shiraz, Iran Karthikay Sankhydhar School of Basic & Applied Sciences, Galgotias University, Greater Noida, India Debanjan Sanyal Reliance Industries Limited, Jamnagar, Gujarat, India Ajit Sapre Reliance Industries Limited, Navi Mumbai, Maharashtra, India Nishant Saxena Reliance Industries Limited, Jamnagar, Gujarat, India Mohammad Amin Sedghamiz Department of Chemical Engineering, Shiraz University, Shiraz, Iran Nimra Shakeel Department of Chemistry, Aligarh Muslim University, Aligarh, Uttar Pradesh, India Anupama Sharma School of Basic & Applied Sciences, Galgotias University, Greater Noida, India

Contributors

Shashank Sharma Department of Chemistry, SBAS, Galgotias University, Greater Noida, India G. Venkata Subhash Reliance Industries Limited, Navi Mumbai, Maharashtra, India Divya Bajpai Tripathy Department of Chemistry, School of Basic and Applied Sciences, Galgotias University, Greater Noida, India Aman Ullah Department of Agricultural, Food and Nutritional Science, University of Alberta, Edmonton, AB, Canada Qasim Ullah Physical Sciences Section, School of Sciences, Maulana Azad National Urdu University, Hyderabad, Telengana, India Muhammad Zubair Department of Agricultural, Food and Nutritional Science, University of Alberta, Edmonton, AB, Canada

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CHAPTER 1

Switchable solvents for bio-refinery applications Muhammad Zubair and Aman Ullah∗

Department of Agricultural, Food and Nutritional Science, University of Alberta, Edmonton, AB, Canada ∗Corresponding author. e-mail address: [email protected]

1. Introduction In the present century, human beings are facing numerous challenges such as huge demand of supplies due to the ever-growing world population, climatic changes as a result of rapid industrialization, exhaustion of fossil fuel feedstocks, and, most importantly, geopolitical apprehensions effecting supply of raw materials across the globe. Moreover, concerns regarding the environment pollution have led to an increase in the demand of shift of global economy towards low cost, efficient, renewable, and sustainable feedstock [1, 2]. It has been predicted that the industry in coming decades will be heavily based on state of the art routes derived from renewable raw materials and providing chemicals with at least same or more innovative features than their petroleumbased counterparts [3, 4]. However, the production of bio-based products from sustainable resources poses certain challenges for an eco-based economy. The most suitable approach is the use of renewable biomass feedstock to provide a continuous supply chain that is not only the viable sustainable option to substitute for fossil fuel resources but also a source of organic compounds with a short span of life and infinite supply. These attributes make them an attractive candidate for research and development of biomass-based materials in a sustainable way [5]. The concept of biorefinery is considered an excellent opportunity to replace the petro-based materials as biorefinery utilizes biomass as a cheap feedstock to produce materials in both chemical as well as biological industries. There is a growing research interest in the value of bio-based materials derived from residual biomass. It is essential to focus on the extraction, recovery, and/or synthesis of bio-based products for industrial applications to implement sustainable approach in a future bio-derived economy. The efficient and effective use of biomass such as cellulose, hemicellulose, lignin, proteins, lipids has become attractive starting materials for the synthesis of various chemicals and materials to replace the fossil fuel-derived materials [6, 7]. These large-scale applications are evolving from innovative approaches that are being designed to advance the biorefineries for particular bio-derived feedstocks [5]. The focus Green Sustainable Process for Chemical and Environmental Engineering and Science https://doi.org/10.1016/B978-0-12-819850-6.00005-X

Copyright © 2022 Elsevier Inc. All rights reserved.

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Green sustainable process for chemical and environmental engineering and science

of the biorefineries is the extraction of valuable chemicals from the biomass to develop food additives, pharmaceuticals, fragrances, biofuels, dyes, coating, nutraceuticals, and other commodities [8–14]. In the synthesis and extraction process, solvents are ubiquitous auxiliary substances and play the most vital role in terms of their efficiency, cost, availability, and mostly importantly environmental impact. For the last two decades, research efforts of scientists have shifted towards study of the harmful impacts of solvents utilized in various industrial processes to produce diverse materials. Specifically, the introduction of green chemistry 12 principles signified the importance to diminish the hazards associated with human health, environment, and safety due to the tremendous use of traditional chemical processes [15, 16]. Similarly, in biorefinery various solvents/reagent have been used during the extraction of valuable components and production of materials [17, 18]. Thus, the progress in the utilization of environmentally benign solvents is presently a favorable choice in this rapid technological era, especially in biorefineries. Green solvents are being used in biorefineries as they have less harmful impacts on the health, safety, and environment as well as short life cycle. Ionic liquids derived from bio-based materials or natural eutectic solvents, supramolecular solvents, supercritical substances, and switchable solvents are few examples of greener solvents [19–22]. Amongst green solvents, switchable solvents, also called smart solvents, are one of the suitable choices to replace the traditional solvent systems. They have excellent ability to alter both their physical and chemical properties reversibly under the influence of external stimulus [23, 24]. For example, addition or removal of CO2 or a change in temperature results in the formation of switchable solvent. The solvent’s behavior can be switched by exposing them to triggers such as light, gas, and heat and ultimately used them for decontamination of solute in a separation process. They also facilitate their reuse and thus reduce the generation of waste [22]. Each of the trigger has advantages, but their most important characteristics are being environmentally benign and economically feasible with a straightforward reversible mechanism [25–28]. So, this chapter presents the existing switchable solvents used in the industrial biomass processing applications within a biorefinery context and highlights the concept of biorefinery.

2. Concept of biorefinery International Energy Agency Bioenergy Task 42 defines biorefinery and its most accepted definition in the scientific community. It states biorefinery as a “sustainable processing of biomass into a spectrum of marketable products (food, feed, materials, chemicals) and energy (fuels, power, heat)” [29]. In other words, biorefinery concept is to develop wide range of chemicals, materials, fuels at least similar or superior in properties than petroleum-based materials at industrial scale from biomass in a sustainable manner [30] as summarized in Fig. 1. The main goal of the future biorefineries is to extract

Switchable solvents for bio-refinery applications

Feedstock Biomass Lignocellulosic biomass Dedicated crops, woody biomass, and agricultural /agri waste biomass Marine biomass Aquatic plants, macroalgae and microalgae

Biorefinery Processes Thermochemical (pyrolysis, gasification) Biochemical Electrochemical Separation processes Catalytic and noncatalytic CO2 capture

Products Oil refineries Bioenergy Materials Biodiesel production Chemicals production Feed Food Fuel cell systems

Fig. 1 Concept of biorefinery.

valuable components that are present in the biomass and transform them into the bioderived materials, bioenergy, and other commercially viable commodities. Most importantly, biorefineries only be feasible if the extraction of chemicals from biomass and conversion efficiency into other materials is exploited maximum using proper production techniques [31]. Biorefinery is basically a conversion process of biomass feedstock into commercial products such as fuels, chemicals, and materials similar to petroleum resources [32]. In biorefinery processes, proper utilization of feedstock is essential to produce low-cost products while maintaining the concept of sustainable economy [33]. Biomass has the inherent property of diverse and complex molecular structures that makes them ideal candidates to transform into different chemicals and materials with excellent properties. Corn, sugar cane, corn stover, switch grass, plam oils, and microalgae are the major sources of biomass feedstocks which consist of mainly lignocellulosic and lipids components. These components are being extracted and converted into a variety of materials with multiple functions properties based on their composition. These biomass-derived materials at industrial scale can be categorized into energy, molecules, and materials [34]. The energy products can be synthesized using direct combustion or thermochemical conversion processes [35]. These conversions methods utilize the whole biomass fractions for the synthesis of biocrude or biogas. Similar to petroleum refineries, produced biocrude can be further fractionated into different hydrocarbons and syngas [36, 37]. In terms of molecules, extracted portion can be used either as chemical solvents or as basic unit to produce other chemicals [34]. Furthermore, the extracted components can be utilized to synthesize polymers or fibers with inherent properties of strength and biodegradability. The biomass can be processed to obtain chemicals or materials through different techniques including mechanical or physicochemical, fractionation, and conversion reactions, however these methods require large amount of energy. In addition, these processing steps involve solvents such as hexane, methanol, and dimethyl sulfoxide. Most of the organic solvents are inherently hazardous due to high toxicity and inflammability. In addition, at the end of the process, solvent separation from final product involves energy and ultimately more cost for the product production [38]. Hence, the better

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processing methods, especially the ones related to the use of suitable solvents, are essential to determine economic viability, cost and environmental, health, and safety impacts of the biorefineries [29].

3. Switchable solvents for biorefinery The unique properties of switchable solvents such as easy conversion into hydrophilic or hydrophobic phase, less energy for separations as well as recovery/recycling of solvent and catalysts, make them suitable for biorefinery operations [39]. There are numerous data available in the literature where switchable solvents are studied for the pretreatment, extraction, and fractionation of valuable components from lignocellulosic biomass such as pretreatment with butadiene sulfone [40], phenols extraction [41], and spruce wood fractionation with ionic liquids [42]. Despite the fact that switchable solvents have promising future in various biomass applications, optimization of parameters are extremely necessary such as solvent-biomass ratios, solvent recovery, and purity of product [43]. The utility of switchable solvents is mainly dependent on their properties. For example, switchable hydrophilicity solvents should have specific basic values as they tend to behave hydrophilic/hydrophobic at different values and can be efficiently triggered with CO2. In this section, different switchable solvents are discussed for their use with focus in the biorefinery.

3.1 Switchable polarity solvents Switchable polarity solvents are of low polarity that is changed to high polarity resulting in the dramatic change in the solubility of many solutes. The low polarity molecular liquid is switched to a higher polarity ionic liquid as CO2 is bubbled from a switchable polarity solvents as shown in Fig. 2 [23]. The molecular liquids often have equimolar mixtures of amines and alcohols as a nucleophilic solvents and guanidines, amines, or amidines, as an organic base [44, 45]. When CO2 binds to the nucleophilic moieties, it chemically results in the formation

Fig. 2 Exposure of CO2 results in polarity change [28].

Switchable solvents for bio-refinery applications

Fig. 3 Guanidine, amidine, and secondary amine (from top to bottom).

of ammonium carbonate and carbamate salts. In some cases, same molecule act as a nucleophile and a base in one-component switchable solvent system. In some cases, same molecule act as a nucleophile and a base in one-component switchable solvent system [46, 47] as shown in Fig. 3. So far, many switchable polarity solvents have been reported including a low-polarity liquid mixture of an alcohol [23], binary liquid mixtures such as glycerol-amidine [48] or amidine-primary amine mixtures [45, 49–51] and single component switchable polarity solvents, i.e., diamines [52], secondary amines, primary amines, etc. [53]. A study reported on the use of N,N-dimethylcyclohexylaminethe for the lipids extraction and recovery, directly from wet samples having about 80% water contents. They used microalgal strains and reported lipids extraction without any cell disruption from dilute cultivation media and avoided the use of volatile organic solvents [54]. Anugwom et al. reported switchable ionic liquids prepared from hexanol or butanol and CO2 along with amidine (1,8-diazabicyclo-[5.4.0]-undec-7-ene (DBU)) to investigate the effect of solvent on dissolution as well as fractionation of native spruce and preextracted spruce woody material for 5 days at 55°C. The study concluded that after the 5-day treatment, 38 wt% less hemicelluloses was observed in the undissolved fraction in comparison with native spruce. Less energy was required for this process in comparison with existing wood treatments technologies since dissolution/fractionation occurred at 55°C. Moreover, the ionic liquid gave more chances of recycling or reuse due to the possibility of return to their molecular components [42]. It is noteworthy that switchable polarity solvents, particularly alcohols, incline to be water sensitive because it is not easy for solid bicarbonate products to return into their

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neutral form. Furthermore, bicarbonate formation is thermodynamically more favorable than the carbamate salts formation. Thus, dry conditions are required to obtain better separations or chemical reactions while using alcohol containing switchable polarity solvents. The condition is critical particularly in organic reactions where water was produced as a by-product. Then, it is necessary to be dried before switching the solvent. Samori et al. reported the method to overcome this issue. They did extraction of lipids from microalgae using 1,8-diazabicyclo-[5.4.0]-undec-7-ene (DBU) and an alcohol. Dried and water-suspended samples of the microalga were used for the study. The nonionic nature of the DBU/alcohol SPS for nonpolar compounds was used to extract hydrocarbons from algae. While the DBU-alkyl carbonate form, which is in ionic character and obtained by the addition of CO2, was used for the recovery of hydrocarbons from the switchable-polarity solvents. The comparative study was performed for DBU using alternatively octanol or ethanol. The results indicated that extraction efficiency using octanol showed the maximum yields of extracted hydrocarbons from both freeze-dried and liquid algal samples [55]. SPS having amine nucleophiles are more stable in the presence of water since formation of carbamate is faster than bicarbonate formation. Organic reactions such as Michael and Heck have been performed in the switchable polarity solvents [56]. Functional groups present in the switchable solvents are not always chemically inert. Therefore, selection of appropriate reactants or reagents is necessary to avoid this problem. Equimolar quantities of alcohol are not required in one component systems still they have potential reactive amine moieties. In addition, in one component systems 2 equivalent of amine are needed for 1 equivalent of CO2 for anion-cation pair. Therefore, in one component systems, large quantity of switchable polarity solvents may involve contingent to the chemical process. The switching of polarity exhibited by the switchable polarity solvent is very critical for extractions/separations of the components predominantly in the complex materials. A transformation of polarity from high to low or low to high, resulted in phase separation or precipitation of a dissolved solute. Hence, switchable polarity solvents have been utilized in numerous applications, for example, oil extraction from soybeans [57], heavy metals recovery [58], pretreatment of lignocellulosic biomass, etc. [59, 60]. Phan and co-workers extracted soy oil from soybean flaked by an amidine as a switchable solvent to overcome the issue related to hexane harmful environmental impacts and cost. Later, the solvent was removed from the soy oil using carbonated water. They evaluated the oil separation ability of different solvents, a secondary amine, an amidine/alcohol, a combination of an amidine and excess water, and use of dioxane. A combination of amidine and excess water showed higher separation of solvent/oil, appropriate extraction of oil, and insensitive to water. Further, this method exhibited advantage of easy switching with CO2 application or removal.

Switchable solvents for bio-refinery applications

There are studies reported on the use of switchable ionic liquids for the extraction of lignin from the lignocellulosic materials as they are water insensitive and easy to handle [59, 60]. Monoethanol amine (MEA), an amidine, DBU were used as switchable ionic liquids along with CO2 and SO2 as triggers for the dissolution of one/two major components of the birch (B. pendula) wood. The study indicated that an excellent lignin extraction was obtained, i.e., 80 wt% and 50 wt% using MEA-SO2 and MEA-CO2, respectively. Moreover, pectin and uronic acids (90 wt%) of the wood were dissolved. Using MEA-CO2 (40 wt%) or MEA-SO2 (44 wt%) as switchable ionic liquids gave good weight reduction after a period of 24 h with vigorous agitation at a temperature of 120°C. Gel permeation chromatography results showed less degradation of cellulose by MEA-CO2 as compared to MEA-SO2 switchable ionic liquids [60]. In another study, reported by the same group, DBU as a switchable ionic liquid with glycerol or monoethanol amine as well as carbon disulfide or carbon dioxide were used for the fractionation of nordic birch. The results indicated that hemicelluloses (64 wt%) and lignin (70 wt%) were present in nondissolved parts after treatment. However, longer processing or consecutive short period treatments using switchable ionic liquids reduced the amounts of hemicelluloses (12 wt%) and lignin (15 wt%). The derivatization of part containing mainly cellulose was performed using partially dissolution in a base with an ionic liquid. The study demonstrated that acylation of the cellulose was achieved with/without catalyst to cellulose acetates with various degree of substitutions, which is a promising way to synthesis cellulose ester under mild conditions [59]. Soudham and co-workers have revealed that switchable polarity solvents are very effective for a variety of lignocellulosic materials [61]. They found that DBU-MEASO2 is better solvent, in case of spruce and pine (softwood substrates) for the lignocellulose pretreatment. While CO2 showed similar or better performance than SO2 for birch and reed canary grass. Biodiesel can be obtained by transesterification of triglycerides and ethanol. Xue et al. reported the optimization for this process and indicated that around 98% yield was obtained from ethanol to soy bean oil (12:1) [62]. Furthermore, activation energy (Ea) of this reaction was found to be lower (16.8 kJ mol
1) than other reactions used for transesterifications (33.6–84.0 kJ mol
1). Bao et al. produced biodiesel by transesterification of oil using DBU with methanol. They observed that both reactants and products were partially soluble during the production process. The authors studied the product system in terms of catalyst dosage and effect of temperature on the phase equilibrium. The results indicated that the methanol, DBU, and glycerol distribution in the biodiesel-rich phase were enhanced (1–15 wt% of oil) by increasing the dose of DBU and temperature while reducing the glycerol-rich phase accordingly. However, an increase in the dosage and temperature improved the distribution as well as solubility in the phase consists of mostly biodiesel that was ascribed to van der waals forces and hydrogen bonding [63]. Cao and co-workers studied production of fatty acid methyl esters (FAMEs)

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from yeast lipids. They observed 95.2% conversion efficiency with a methanolysis efficiency of 21.9%. Moreover, complete extraction of glycerol from FAMEs were observed with high purity. The study presented a simple method for the integrated biodiesel and glycerol production from soybean and microbial lipids [64]. Because biodiesel of highest purity is required, hence many researches are exploring the cheap production ways of biodiesel fuels with highest purity and efficiency [65–67]. The switchable polarity solvents can extract the components from raw material for various materials developments and convert them into final products at the same time. Switch polarity solvents may play an important role in future biodiesel production and many other material productions in the biorefinery. Nevertheless, benefits of switchable solvent polarity solvents are greatly reduced because of the additional treatments for the final use of synthesized materials such as washing, solvent removal, and catalyst recovery.

3.2 Switchable hydrophilicity solvents (SHSs) Switchable hydrophilicity solvents (SHSs) are a class of solvents that can alter their behavior from hydrophilic to hydrophobic as well as hydrophobic to hydrophilic. They have very low miscibility with water (CO2 absence) and form two phases, i.e., hydrophobic and aqueous. While in the presence of CO2 (1 bar), shows complete miscibility with water and single phase is formed due to the protonation of organic base due to carbonic acid as represented in Fig. 4 [68, 69]. Switchable hydrophilicity solvents are an excellent choice to advance the current extraction of liquid to solid and liquid to liquid [70]. The complete extraction of both hydrophilic and hydrophobic molecules has been reported with a switchable hydrophilicity solvent using solvent’s opposite-hydrophilicity states [71]. The hydrophobic form of solvent is typically an amidine, a tertiary amine, or a bulky secondary amine while solvent in hydrophilic form is the respective bicarbonate salt [39]. Alkylated amidines or secondary and tertiary amines are the commonly used switchable hydrophilicity solvents

Fig. 4 Miscibility of water due to polarity difference [28]. (Reproduced with the permission of American Chemical Society.)

Switchable solvents for bio-refinery applications

Fig. 5 Amidine and tertiary amine (from top to bottom).

functional groups. These groups act as bases for the deprotonation of carbonic acid/ hydrated CO2 as shown in Fig. 5. These unique attributes of switchable hydrophilicity solvents make them capable of being used in green biorefinery operations as the processing of biomass can be achieved from the wet state. Thus, a simple pretreatment is sufficient and energy requirements are also reduced for biomass processing. Guanidines, which are hydrophobic in nature, can easily be converted into watersoluble bicarbonates and have high basicities. Thus, higher amount of energy is required for the release of CO2 to act as switchable hydrophilicity solvents [69]. Switchable hydrophilicity solvents exhibited significantly larger polarity switch in comparison with switchable polarity solvents. For instance, amidine shows absorbance with Nile red dye at λmax ¼ 510 nm but the resulting solution exhibits absorbance of λmax ¼ 570 nm by the introduction of CO2 [68]. Fig. 6 shows the polarity switch relative to traditional solvents as well as switchable polarity solvents. Switchable polarity solvents in their hydrophobic states can be used for the dissolution or extraction of organic compounds such as lignin pyrolysis oil [41], crude oil, bitumen [70], polystyrene [72], algae oil [39, 54, 73]. After the extraction of the required component, the solvent gets washed away using solvent into which the solvent (carbonated water as its bicarbonate salt) is easily soluble. Later, it can be recovered from the carbonated water by the removal of CO2 from the solution. Thus, solvent goes back to its hydrophobic form and a second phase above the water is formed. Boyd and co-workers studied efficiency of N,N-dimethylcyclohexylamine to extract lipids from Botryococcus braunii (microalgae). The results indicated that solvent had lipids extraction efficiency up to 22 wt% in comparison with the freeze-dried cell weight. Furthermore, study presented, presence of high concentrations of long chain acylglycerols with no phospholipids. As compare to conventional organic solvents, this approach does not involve distillation or use of any toxic organic solvents. The solvent present in the

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Fig. 6 Wavelengths range with the dye. Green (light gray in print version): solvents in presence of air and Red (dark gray in print version): in CO2 presence [68]. (Reproduced with the permission of Royal Society of Chemistry.)

extract was removed using water (carbon dioxide saturated) under atmospheric pressure and later mixture de-carbonation led to the recovery from the water. Most importantly, lowering of lipid amount in the remaining recovered amine is desirable for the practical consideration of this technology [43]. SHSs are used in hydrophobic form for the extraction of hydrophobic solutes. However, there is a possibility of the presence of hydrophilic components as well as debris material in the treated biomass that are insoluble in either hydrophobic or hydrophilic solvents. Cicci et al. proposed the dual use of switchable hydrophilicity solvent both in conceptual and experimental approach as it can enhance the effectiveness for the extraction process and solvent itself. N,N-dimethyl-cyclohexylamine was used as a SHS for microalga via two distinct systems. They treated biomass first in contact with hydrophobic or hydrophilic state of the solvent and in second contact with the opposite state as shown in Fig. 7A and B. During this process, the solvent behaves as an additional solvent and did a second extraction, which is not possible with the usual solvents [71]. Durelle et al. described the triggered switching CO2 mathematically for two/three component mixtures and optimized the processes parameters in terms of both being extrinsic and intrinsic [74]. In this study, they used the ratio of concentration in both phases, with and without CO2, to explain the soybean oil extraction [65] and separations processes in switchable hydrophilicity solvents. They also suggested that amidines as a switchable hydrophilicity solvent is expensive to prepare and is not suitable for solvent applications on a large scale. As N-H bonds in secondary amines are available, so carbamate salts may form under the exposure of CO2 exposure contingent to the presence of the alkyl chain substituents [75].

Switchable solvents for bio-refinery applications

Fig. 7 Two stage extraction: (A) forward mode and (B) backward mode [71]. (Reproduced with the permission of Royal Society of Chemistry.)

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Thus, energy requirement are increased substantially due to the higher temperature and more time is needed to reform the nonpolar organic phase [76]. The carbamate formation by the branched alkyl substitution can be avoided. In turns, less energy is required for the CO2 removal [69]. Consequently, use of either secondary amines (sterically hindered) and tertiary amines are favorable in case of CO2-triggered solvents. Amines have been forced to show behavior similar to switchable hydrophilicity solvent either increase in the volume ratio of water to amine or pressure rises (1–10 bar) [77]. With these conditions, amines can easily return to the immiscible behavior with water. The consideration of performance and environmental impact, health, and safety at a time makes it difficult to select a suitable solvent. For example, secondary amino ester switchable hydrophilicity solvents are environmentally benign but its recycling ability limit its use as compare to other solvents [69]. Wang et al. studied the solubility and recovery performance of diesel and dimethylcyclohexylamine (DMCHA) as a switchable hydrophilicity solvent from oil-based drill cuttings. Switchable hydrophilicity solvent showed a good recovery, and 93.6% conversion rate was observed for DMCHA from carbonated water at 70°C. The kinetic study on DMCHA recovery was studied and the effect of temperature and aeration rate was tested. The results indicated that recovery rate was improved with the increase in temperature and aeration rate [78]. In terms of energy requirement for extraction process, several solvents such as hexane, supercritical carbon dioxide has been used for algal biomass and were compared with the N-ethyl butylamine as a switchable solvent. Switchable solvents showed the requirement of minimum energy for drying and extraction from biomass since switchable solvents have the ability to extract from wet biomass. Also, solvent can be carried out with nominal amount of energy [79]. However, this study is limited as they did not evaluate the production of solvent or reuse of super critical CO2.

3.3 Switchable water (SW) Switchable water (SW) can be defined as a stimulus-responsive aqueous solution, where trigger dramatically changes the ionic strength of the solution. For example, introduction of CO2 (at 1 atm) in an aqueous solution having amine or polyamine, also called the ionogen, resulted in a large increase of the solution’s ionic strength [39, 80–82]. Similar to switchable hydrophilicity solvents, addition of CO2 to switchable water gives salts of ammonium bicarbonate. In this case, an amine is most often used as an organic base which is water miscible. Therefore, system present in a single phase in the absence of CO2 [28] as shown in Fig. 8. Here, the role of CO2 as a trigger caused a significant change in ionic strength of the solution instead of shift in polarity and miscibility behavior of water. Therefore, organic compounds show less solubility in switchable water. In this way, these systems have the

Switchable solvents for bio-refinery applications

Fig. 8 An ionic strength of the switchable water increases with the exposure of CO2 [28]. (Reproduced with the permission of ACS.)

ability to remove organic pollutants from water with less energy using salting out phenomenon [80]. Switchable water presents as an alternative to the traditional expensive methods of disposal or recycling of salt solutions [83]. Other physical changes such as increase in conductivity and viscosity of switchable water occur with bubbling of CO2 [84]. In comparison with switchable hydrophilicity solvents, switchable water is not much studied. Most importantly, any phase change is less likely in the switchable water due to the presence of a trigger. Thus, it signifies a simple system that is easier to alter than switchable hydrophilicity solvent. Switchable water has many advantages and cannot be used only as a medium for the synthesis or extraction purposes but is also utilized for various applications. First, polymeric amines with CO2 are effective flocculants and provide aid to settle the clay suspensions in water. Also, low amount of ionogen is needed (10 ppm by mass) and flocculants can be switched off with the removal of CO2 [82]. Second, common available ionic surfactants are not CO2-responsive in normal aqueous solutions, however, in switchable water they are CO2-responsive [85]. Therefore, surfactants have the ability to stabilize emulsions or suspensions that can be “switched off” later by CO2 addition. As a result, emulsions can break while suspensions can settle down. Third, amine (concentrated solution) in carbonated switchable water can be used as a draw solvent in forward osmosis to purify the wastewater or sea water. The draw solution has higher osmotic pressure, hence, it draws water from wastewater or seawater from the other side of the membrane. As a result, seawater or wastewater becomes concentrated while draw solution is diluted. Now, water can be used that has been drawn into the draw solution after the removal of amine and CO2. Finally, switchable viscosity of aqueous solutions can be obtained using switchable water by two ways. In the first case, switchable water ionogen is utilized to affect the micelle formation of a nonswitchable surfactant. A study reported that sodium octadecylsulfate solution as a surfactant in switchable water (dimethylaminoethanol or a diamine as the ionogen) showed a higher viscosity without CO2 that was ascribed to the presence

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of worm-like micelles and can be finished by the addition of CO2 [84]. Another study reported the similar kind of changes in viscosity with a combination of sodium dodecylsulfate and a diamine [86]. In the second case, switchable water that can self-associate in neutral but not when it is charged. This approach is used by the Zhao’s group and presented that polymeric tertiary amines (aqueous solutions) can be changed from solutions to gels and vice versa under the exposure of CO2 [87, 88]. Composite switchable water was also reported by Li and co-workers, for the washing of oil sands. They used N,N-dimethylaniline (hydrophobic tertiary amine) and N,N-diethylethanolamine. The study indicated that contents of heavy hydrocarbons from residual oil sands was decreased from 4.64 wt% to 0.872 wt%. This new composite switchable water expands the utilization of switchable solvent for the removal of oil from oily contaminant solid [89]. In another study, an environment benign method was developed for the recovery of oil substances from waste oil-based drill cutting. CO2-triggered switchable water additive (tertiary amine N,N,N0 , N0 -tetramethyl-1,6-hexanediamine (TMHDA)) was used for the diesel emulsification-demulsification. They switched the ionic strength of aqueous solution by the CO2 addition or removal. The application of diesel and naphthenic acid mixture were tested, and authors concluded that acid value of diesel and volume ration (oil to water) were the key parameters for emulsion stability. The emulsion of diesel can be possible with an acid value 1.76 mg KOH/g and oil and water volume ratio 1:1. When the oil contents in water were 143.65 mg L
1, a good demulsification was observed. These showed excellent recycling performances by changing oil: water volume ratio into the solution or by adding TMHDA. Last, they suggested the plausible mechanism for this process as shown in Fig. 9 [90]. From the discussion it is clearly demonstrates that addition of CO2 trigger changes solvent properties quickly and offer many benefits over the traditional solvents for chemical processes especially related to separations and isolation of products. On the other hand, switchable solvents (neutral or ionic) may get lost in the system during processing, and additional energy is required for the removal of solvent from either final product or left over. As amines structural changes occur dramatically, in turn their toxicity changes [91–93]. Predictive models are of significant importance for the development of switchable polarity solvents, switchable hydrophilicity solvents, and switchable water. In addition to that, environmental impact of amine is not fully understood [94, 95] while present progress in post combustion CO2 capture has brought about a great attention in the emissions of amines [96]. Here, it is important to mention that switchable solvents are fairly new and their detailed life cycles assessment is still not understood. A conclusive comparison cannot be drawn with other solvents because of the lack of studies in evaluation of green metric for switchable solvent processes. However, predictive models have vital role in the development of switchable solvent related to their ESH concerns and solvent’s performance for each specific application.

Switchable solvents for bio-refinery applications

Fig. 9 Mechanisms: (A) emulsion, (B) demulsification, and (C) emulsification-demulsification reaction [90]. (Reproduced with the permission from Elsevier.)

4. Concluding remarks and future perspectives The chapter has discussed the utilization of switchable solvent in biorefineries and showed great potential in terms of product development, fractionations, and isolations of valuable components from different biomass. However, the concept is still in the early growing phase and have technical challenges. It is of utmost importance to optimize the reaction condition such as solvent to reactant volume ratio, temperature, and proper understanding of switchable solvent final fate in the environment. The development of switchable solvents in biorefineries depends on their performance and feasibility for biomass processing. Extraction efficiency, recovery or reuse of solvent, yield, and separation ability are the determining factors for the solvent compatibility with the biomass feedstocks. Besides, biomass feedstocks have significant portion of water contents so switchable solvents would ideally perform well in the presence of water. Other properties such as

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viscosity of solvent and compatibility with the reactant considerably affect the applications. Solvent and solute separation depends on their concentrations as well as physicochemical properties and are key factors for the recovery of the product recovery and recycling of the solvent. Switchable solvents have better properties in comparison with traditional solvents. Though, cradle-to-grave LCAs in necessary to draw any conclusion for any chemical process. The development of sustainable and robust switchable solvents in the biorefineries is the key for a successfully integrated production of material in the future at least on similar price or less in comparison with the fossil fuel-derived products. The main goal of using switchable solvents in biorefinery is to address the issues related to environmental, health, and safety (EHS) of traditional solvents. Although switchable solvents are already in use on a larger scale, technological improvements are necessary to obtain more efficient and economically viable processing. As a biorefinery it is seeking to offer low-cost products with numerous functional properties in an integrated manner with low or zero waste. The utilization of switchable solvents is highly desirable to demonstrate advances in terms of cost, EHS, and process metrics. The development and implementation of new solvent systems is very critical while these are still in their early stage of growth.

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[39] P.G. Jessop, Switchable solvents as media for synthesis and separations, Aldrichim. Acta 48 (2015) 18–21. [40] J.A. De Frias, H. Feng, Pretreatment of furfural residues with switchable butadiene sulfone in the sugarcane bagasse biorefinery, Green Chem. 16 (2014) 2779–2787. [41] D. Fu, S. Farag, J. Chaouki, P.G. Jessop, Extraction of phenols from lignin microwave-pyrolysis oil using a switchable hydrophilicity solvent, Bioresour. Technol. 154 (2014) 101–108. [42] I. Anugwom, P. M€aki-Arvela, P. Virtanen, S. Willf€ or, R. Sj€ oholm, J.-P. Mikkola, Selective extraction of hemicelluloses from spruce using switchable ionic liquids, Carbohydr. Polym. 87 (2012) 2005–2011. [43] A.R. Boyd, P. Champagne, P.J. McGinn, K.M. MacDougall, J.E. Melanson, P.G. Jessop, Switchable hydrophilicity solvents for lipid extraction from microalgae for biofuel production, Bioresour. Technol. 118 (2012) 628–632. [44] L. Phan, D. Chiu, D.J. Heldebrant, H. Huttenhower, E. John, X. Li, P. Pollet, R. Wang, C.A. Eckert, C.L. Liotta, Switchable solvents consisting of amidine/alcohol or guanidine/alcohol mixtures, Ind. Eng. Chem. Res. 47 (2008) 539–545. [45] T. Yamada, P.J. Lukac, M. George, R.G. Weiss, Reversible, room-temperature ionic liquids. Amidinium carbamates derived from amidines and aliphatic primary amines with carbon dioxide, Chem. Mater. 19 (2007) 967–969. [46] V. Blasucci, R. Hart, V.L. Mestre, D.J. Hahne, M. Burlager, H. Huttenhower, B.J.R. Thio, P. Pollet, C.L. Liotta, C.A. Eckert, Single component, reversible ionic liquids for energy applications, Fuel 89 (2010) 1315–1319. [47] L. Phan, J.R. Andreatta, L.K. Horvey, C.F. Edie, A.-L. Luco, A. Mirchandani, D.J. Darensbourg, P.G. Jessop, Switchable-polarity solvents prepared with a single liquid component, J. Org. Chem. 73 (2008) 127–132. [48] I. Anugwom, P. M€aki-Arvela, P. Virtanen, P. Damlin, R. Sj€ oholm, J.-P. Mikkola, Switchable ionic liquids (SILs) based on glycerol and acid gases, RSC Adv. 1 (2011) 452–457. [49] T. Yamada, P.J. Lukac, T. Yu, R.G. Weiss, Reversible, room-temperature, chiral ionic liquids. Amidinium carbamates derived from amidines and amino-acid esters with carbon dioxide, Chem. Mater. 19 (2007) 4761–4768. [50] T. Yu, T. Yamada, G.C. Gaviola, R.G. Weiss, Carbon dioxide and molecular nitrogen as switches between ionic and uncharged room-temperature liquids comprised of amidines and chiral amino alcohols, Chem. Mater. 20 (2008) 5337–5344. [51] G.V. Carrera, M.N. da Ponte, L.C. Branco, Synthesis and properties of reversible ionic liquids using CO2, mono-to multiple functionalization, Tetrahedron 68 (2012) 7408–7413. [52] D.J. Heldebrant, P.K. Koech, M.T.C. Ang, C. Liang, J.E. Rainbolt, C.R. Yonker, P.G. Jessop, Reversible zwitterionic liquids, the reaction of alkanol guanidines, alkanol amidines, and diamines with CO 2, Green Chem. 12 (2010) 713–721. [53] V. Blasucci, C. Dilek, H. Huttenhower, E. John, V. Llopis-Mestre, P. Pollet, C.A. Eckert, C.L. Liotta, One-component, switchable ionic liquids derived from siloxylated amines, Chem. Commun. (2009) 116–118. [54] C. Samorı`, D.L. Barreiro, R. Vet, L. Pezzolesi, D.W. Brilman, P. Galletti, E. Tagliavini, Effective lipid extraction from algae cultures using switchable solvents, Green Chem. 15 (2013) 353–356. [55] C. Samorı`, C. Torri, G. Samorı`, D. Fabbri, P. Galletti, F. Guerrini, R. Pistocchi, E. Tagliavini, Extraction of hydrocarbons from microalga Botryococcus braunii with switchable solvents, Bioresour. Technol. 101 (2010) 3274–3279. [56] R. Hart, P. Pollet, D.J. Hahne, E. John, V. Llopis-Mestre, V. Blasucci, H. Huttenhower, W. Leitner, C.A. Eckert, C.L. Liotta, Benign coupling of reactions and separations with reversible ionic liquids, Tetrahedron 66 (2010) 1082–1090. [57] L. Phan, H. Brown, J. White, A. Hodgson, P.G. Jessop, Soybean oil extraction and separation using switchable or expanded solvents, Green Chem. 11 (2009) 53–59. [58] F. Shah, T.G. Kazi, H.I. Afridi, A.R. Khan, S.S. Arain, M.S. Arain, A.H. Panhwar, Switchable dispersive liquid–liquid microextraction for lead enrichment: a green alternative to classical extraction techniques, Anal. Methods 8 (2016) 904–911.

Switchable solvents for bio-refinery applications

[59] V. Eta, J.-P. Mikkola, Deconstruction of Nordic hardwood in switchable ionic liquids and acylation of the dissolved cellulose, Carbohydr. Polym. 136 (2016) 459–465. [60] I. Anugwom, V. Eta, P. Virtanen, P. M€aki-Arvela, M. Hedenstr€ om, M. Hummel, H. Sixta, J.P. Mikkola, Switchable ionic liquids as delignification solvents for lignocellulosic materials, ChemSusChem 7 (2014) 1170–1176. [61] V.P. Soudham, D.G. Raut, I. Anugwom, T. Brandberg, C. Larsson, J.-P. Mikkola, Coupled enzymatic hydrolysis and ethanol fermentation: ionic liquid pretreatment for enhanced yields, Biotechnol. Biofuels 8 (2015) 135. [62] D. Xue, Y. Mu, Y. Mao, T. Yang, Z. Xiu, Kinetics of DBU-catalyzed transesterification for biodiesel in the DBU–ethanol switchable-polarity solvent, Green Chem. 16 (2014) 3218–3223. [63] J. Bao, Y. Liu, R. Parnas, B. Liang, H. Lu, Inter-solubility of product systems in biodiesel production from Jatropha curcas L. oil with the switchable solvent DBU/methanol, RSC Adv. 5 (2015) 8311–8317. [64] X. Cao, H. Xie, Z. Wu, H. Shen, B. Jing, Phase-switching homogeneous catalysis for clean production of biodiesel and glycerol from soybean and microbial lipids, ChemCatChem 4 (2012) 1272–1278. [65] D.Z. Troter, Z.B. Todorovic, D.R. Đokic-Stojanovic, O.S. Stamenkovic, V.B. Veljkovic, Application of ionic liquids and deep eutectic solvents in biodiesel production: a review, Renew. Sustain. Energy Rev. 61 (2016) 473–500. [66] D.Y. Leung, X. Wu, M. Leung, A review on biodiesel production using catalyzed transesterification, Appl. Energy 87 (2010) 1083–1095. [67] G. Lourinho, P. Brito, Advanced biodiesel production technologies: novel developments, Rev. Environ. Sci. Biotechnol. 14 (2015) 287–316. [68] P.G. Jessop, L. Phan, A. Carrier, S. Robinson, C.J. D€ urr, J.R. Harjani, A solvent having switchable hydrophilicity, Green Chem. 12 (2010) 809–814. [69] J.R. Vanderveen, J. Durelle, P.G. Jessop, Design and evaluation of switchable-hydrophilicity solvents, Green Chem. 16 (2014) 1187–1197. [70] A. Holland, D. Wechsler, A. Patel, B.M. Molloy, A.R. Boyd, P.G. Jessop, Separation of bitumen from oil sands using a switchable hydrophilicity solvent, Can. J. Chem. 90 (2012) 805–810. [71] A. Cicci, G. Sed, P.G. Jessop, M. Bravi, Circular extraction: an innovative use of switchable solvents for the biomass biorefinery, Green Chem. 20 (2018) 3908–3911. [72] P.G. Jessop, L.N. Phan, A.J. Carrier, R. Resendes, D. Wechsler, Switchable hydrophilicity solvents and methods of use thereof, Google Patents, 2014. [73] C. Samorı`, L. Pezzolesi, D.L. Barreiro, P. Galletti, A. Pasteris, E. Tagliavini, Synthesis of new polyethoxylated tertiary amines and their use as switchable Hydrophilicity solvents, RSC Adv. 4 (2014) 5999–6008. [74] J. Durelle, J.R. Vanderveen, P.G. Jessop, Modelling the behaviour of switchable-hydrophilicity solvents, Phys. Chem. Chem. Phys. 16 (2014) 5270–5275. [75] A. Alshamrani, J. Vanderveen, P. Jessop, A guide to the selection of switchable functional groups for CO 2-switchable compounds, Phys. Chem. Chem. Phys. 18 (2016) 19276–19288. [76] F. Bougie, M.C. Iliuta, Sterically hindered amine-based absorbents for the removal of CO2 from gas streams, J. Chem. Eng. Data 57 (2012) 635–669. [77] J. Durelle, J.R. Vanderveen, Y. Quan, C.B. Chalifoux, J.E. Kostin, P.G. Jessop, Extending the range of switchable-hydrophilicity solvents, Phys. Chem. Chem. Phys. 17 (2015) 5308–5313. [78] S. Wang, C. Zheng, J. Zhao, X. Li, H. Lu, Extracting and recovering diesel from oil-based drill cuttings using switchable hydrophilic solvents, Chem. Eng. Res. Des. 128 (2017) 27–36. [79] Y. Du, B. Schuur, S.R. Kersten, D.W. Brilman, Opportunities for switchable solvents for lipid extraction from wet algal biomass: an energy evaluation, Algal Res. 11 (2015) 271–283. [80] S.M. Mercer, P.G. Jessop, “Switchable water”: aqueous solutions of switchable ionic strength, ChemSusChem: Chem. Sustain. Energy Mater. 3 (2010) 467–470. [81] S.M. Mercer, T. Robert, D.V. Dixon, C.-S. Chen, Z. Ghoshouni, J.R. Harjani, S. Jahangiri, G.H. Peslherbe, P.G. Jessop, Design, synthesis, and solution behaviour of small polyamines as switchable water additives, Green Chem. 14 (2012) 832–839. [82] T. Robert, S.M. Mercer, T.J. Clark, B.E. Mariampillai, P. Champagne, M.F. Cunningham, P.G. Jessop, Nitrogen-containing polymers as potent ionogens for aqueous solutions of switchable ionic

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strength: application to separation of organic liquids and clay particles from water, Green Chem. 14 (2012) 3053–3062. S.M. Mercer, T. Robert, D.V. Dixon, P.G. Jessop, Recycling of a homogeneous catalyst using switchable water, Cat. Sci. Technol. 2 (2012) 1315–1318. X. Su, M.F. Cunningham, P.G. Jessop, Switchable viscosity triggered by CO 2 using smart worm-like micelles, Chem. Commun. 49 (2013) 2655–2657. X. Su, T. Robert, S.M. Mercer, C. Humphries, M.F. Cunningham, P.G. Jessop, A conventional surfactant becomes CO2-responsive in the presence of switchable water additives, Chem. A Eur. J. 19 (2013) 5595–5601. Y. Zhang, Y. Feng, Y. Wang, X. Li, CO2-switchable viscoelastic fluids based on a pseudogemini surfactant, Langmuir 29 (2013) 4187–4192. D. Han, O. Boissiere, S. Kumar, X. Tong, L. Tremblay, Y. Zhao, Two-way CO2-switchable triblock copolymer hydrogels, Macromolecules 45 (2012) 7440–7445. D. Han, X. Tong, O. Boissie`re, Y. Zhao, General strategy for making CO2-switchable polymers, ACS Macro Lett. 1 (2011) 57–61. X. Li, H. Lu, D. Liu, B. Wang, Preparation of composite switchable water with hydrophobic tertiary amine for washing oil sands, J. CO2 Utilizat. 29 (2019) 254–261. S. Wang, D. Liu, J. Zhao, X. Li, D. Xu, Y. Gu, H. Lu, Use CO2-triggered switchable water additive to reversibly emulsify and demulsify diesel, J. CO2 Utilizat. 26 (2018) 239–245. H. Greim, D. Bury, H. Klimisch, M. Oeben-Negele, K. Ziegler-Skylakakis, Toxicity of aliphatic amines: structure-activity relationship, Chemosphere 36 (1998) 271–295. R. Benigni, L. Passerini, Carcinogenicity of the aromatic amines: from structure–activity relationships to mechanisms of action and risk assessment, Mutation Res./Rev. Mutation Res. 511 (2002) 191–206. K.-T. Chung, L. Kirkovsky, A. Kirkovsky, W.P. Purcell, Review of mutagenicity of monocyclic aromatic amines: quantitative structure–activity relationships, Mutation Res./Rev. Mutation Res. 387 (1997) 1–16. A.E. Poste, M. Grung, R.F. Wright, Amines and amine-related compounds in surface waters: a review of sources, concentrations and aquatic toxicity, Sci. Total Environ. 481 (2014) 274–279. D. Lee, A.S. Wexler, Atmospheric amines–part III: photochemistry and toxicity, Atmos. Environ. 71 (2013) 95–103. S.A. Mazari, B.S. Ali, B.M. Jan, I.M. Saeed, S. Nizamuddin, An overview of solvent management and emissions of amine-based CO2 capture technology, Int. J. Greenhouse Gas Contr. 34 (2015) 129–140.

CHAPTER 2

Polarity-changing solvents for CO2 capture Zeynab Rezaeiyan, Shokufeh Bagheri, Mohammad Amin Sedghamiz, and Mohammad Reza Rahimpour∗ Department of Chemical Engineering, Shiraz University, Shiraz, Iran ∗Corresponding author. e-mail address: [email protected]

Abbreviations BOL DBU SHS SPS SW SWIL

binding organic liquid diazabicyclo[5.4.0]-undec-7-ene switchable hydrophobicity solvent switchable polarity solvent switchable water switchable ionic liquids

1. Introduction Carbon dioxide is regarded as one of the most influential greenhouse gases from flue gas streams that can exert detrimental effects over the environment. To cushion the devastating effects of CO2 emission into the environment, various techniques have been developed including membrane, cryogenic, and absorption [1]. Among the mentioned methods, one of the most effective ones is the chemical absorption of CO2 using alkanolamine solutions. This conventional method has demonstrated satisfactory results for different industrial applications with various CO2 partial pressures. Nevertheless, this process suffers from some shortcomings that bring about high operating costs [2], originating from: 1. High energy requirements connected to the regeneration of the solvents through stripping. 2. Solvent degeneration during the regeneration of absorption processes (thermal and oxidative degradation mechanisms). 3. Maintenance of apparatuses that suffer from corrosion owing to side reactions during the regeneration of the solvent. Regarding these mentioned faults, many researchers have concentrated on improvements of conventional solvents or substituting innovative ones [3]. In this regard, some scientists have channeled their efforts into using ionic liquids (ILs) considering their extremely low water pressure [4, 5]. Unfortunately, this type of solvent has some Green Sustainable Process for Chemical and Environmental Engineering and Science https://doi.org/10.1016/B978-0-12-819850-6.00003-6

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disadvantages such as its relatively high toxicity and viscosity along with huge costs for industrial processes [6]. Industrial absorption processes are commonly comprised of two or more steps; one of them may need a solvent with a polar chemical structure, while the other may require a nonpolar one. Conventionally, it was believed that, to deal with the mentioned issue, one specific solvent should be added to the first stage and afterword removed, which was followed by adding another solvent to the second stage. Actually, it was not previously considered to use a solvent that can alter itself into another solvent suitable for the second stage. Nevertheless, thanks to switchable solvents, the need for adding two different types of solvents can be eliminated [7]. In fact, this type of solvent can reversibly be changed into another type that is different in terms of both chemical and physical properties including dielectric constant, hydrophobicity, and ionic strength [7, 8]. The transformation of switchable solvents can generally be carried out by removing or adding a trigger such as light, heat, or gases, mostly at atmospheric pressure [9, 10]. Despite the beneficial features of each type of trigger, their use should be essentially evaluated with respect to safety, cost, waste production, and the possibility of reversible solvent conversion [7, 11, 12]. In this regard, CO2 is known as one of the most appropriate green candidates considering its abundance, low price, and low toxic production during its removal, to say nothing of CO2 mitigation by applying it as a trigger. As a matter of fact, CO2 is commonly regarded as a waste product in many industrial processes, which contributes substantially to global warming. Thus, its usage as a trigger of switchable solvents can also diminish its damaging effects on the environment [13]. Switchable solvents have been classified by Jessop et al. [7] into three different types according to their functions: (1) switchable hydrophilicity solvents (SHS), (2) switchable polarity solvents (SPS), and (3) switchable water (SW). One of the most significant advantages of these solvents is associated with their capability of changing their polarity between polar and nonpolar, i.e., varying from a high dielectric constant to a low one (SPS), hydrophobicity between hydrophilic and hydrophobic (SHS), and ionic strength between high and low levels (SW) (Fig. 1). Therefore, this chapter is aimed at reviewing the current research and development in switchable solvents, specifically for capturing CO2 [7]. +CO2

Solvent property 1

Switchable solvents

-CO2

Fig. 1 Switchable solvents.

Solvent property 2

Polarity-changing solvents for CO2 capture

1.1 Switchable hydrophobicity solvents (SHSs) This sort of solvent, in fact, can reversibly convert from one solvent, having relatively warm civility with water, to another with a high miscibility with water. By adding CO2 to the solvent, it changes to a hydrophilic form, and after treating with heat or gas, it can convert back to its initial form [14]. SHSs or biphasic solvents are comprised of an aqueous layer along with a hydrophobic amine. However, under one bar of CO2 exposure, the dissolved hydrated CO2 in addition to carbonic acid as a solution, which is a weak acid, protonates the organic base to yield a single phase [15–17] (Fig. 2). Following that, for the reverse reaction, sonic heating and bubbling is used to remove CO2 from the mixture. As a result, SHSs possess comparatively low solubility in water under the absence of CO2 while it is reversed under the presence of CO2 [13]. Alkylated amidines or secondary and tertiary amines, which are the common functional groups in SHS structure, are responsible for the protonation of hydrated CO2 or carbonic acid [14, 15, 18, 19] (Fig. 3). Even though some hydrophobic guanidines can easily be converted to water-miscible bicarbonates, their high level of basicity suggests that the CO2 release is extremely energy-intensive to be used as an SHS [13]. There are some basic principles proposed for selection and design of ideal SHSs. Basically, to ensure switchability behavior of SHSs, log KOW should be located in the range of

Fig. 2 Switchable hydrophilicity solvents experience a change in polarity that allows the miscibility with water to be switched.

N

H2O, CO2

N

O

Bu

H

Bu

O Bu2N

Bu2N

Bu

OH

Bu

O R

R

H2O, CO2

O

N

N R

R

R

OH

H R

Fig. 3 Amidine (top) and tertiary amine (bottom) switchable hydrophilicity solvents (SHS).

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1.0–2.5 [15], PKaH should be between 9.5 and 11, and the ratio of C/N needs to be between 6:1 and 12:1 [15, 20]. Aside from the internal factors mentioned before, external parameters can heavily affect the switchability of SHS; these parameters include water volume, pressure, and temperature [16, 17, 21]. The ecofriendliness of SHSs with minimum toxic production is a key factor that should necessarily be analyzed prior to industrialization of the process. Even though nearly all of SHSs possess flashing and boiling points much greater than hydrocarbon solvents, e.g., such as toluene and n-hexane, they are generally regarded more dangerous with respect to toxicity indexes, such as eutrophication potential [15]. To tackle this issue, several scientists have suggested introduction of functional groups with relatively low toxicity into the structure of SHSs to make them more ecofriendly, but there is still a lack of sufficient knowledge concerning the effects of chain structures and functional groups [15, 19, 22].

1.2 Switchable water Similar to SHS, adding CO2 gas to water contributes to the production of ammonium bicarbonate salts. Nevertheless, amine as the organic base is miscible in water, and thus the system is single phase without adding CO2 (Fig. 4) [13]. Regarding its switchability, waste of water is minimized since it can be recycled several times. Adding the CO2 gas as the trigger therefore causes a considerable change in the ionic strength instead of changing the polarity and miscibility of water. As a result, organic compounds’ solubility declines significantly in SW, so it can be efficiently used for the purification or removal of organic materials, which are soluble in water through salting out effects [23]. Considering that the salt solution disposal is not cost-effective, SW has been proposed as a renewable substitute for conventional salt solutions for the purpose of recycling and postreaction purification [24]. Moreover, other physical properties of SW such as its viscosity and conductivity increase through CO2 bubbling [25]. Therefore, regarding all the advantageous properties of switchable water solution, it can be considered as an excellent candidate for CO2 capture and solvent recovery application [26–29]. In addition, it has several applications in the petrochemical industry as an ideal solution. For example, it can be used for the washing of oily sludge and oil sands, although its weak miscibility with oil confines further enhancement in petroleum recovery applications [30].

Fig. 4 The ionic strength of SWs increases reversibly after adding CO2.

Polarity-changing solvents for CO2 capture

1.3 Switchable polarity solvents By bubbling CO2 into a SPS, it is converted from a low polarity liquid to an IL with high polarity (Fig. 5) [31]. There are various solvent systems known as SPS including a mixture of guanidine/acidic alcohol, guanidine alcohol amidine/amine, guanidine alcohol along with diamine, and secondary amines [7]. Nucleophilic groups build a chemical bond with CO2 molecules, producing carbamate or ammonium carbonate salts. When a suitable solvent is only composed of one component, the same molecule functions as both base and nucleophile [32, 33] (Fig. 6). The switched solvent is then converted back to a molecular liquid by sonicating, heating, and inert gas bubbling. It should be noted that bicarbonate salts can be produced through the reaction of organic bases of SPSs with water [34]. This is why SPSs that contain alcohol are regarded as water-sensitive solvents since bicarbonate products in solid form cannot easily be

Fig. 5 The polarity of SPSs increases reversibly after adding CO2.

Fig. 6 Examples of SPSs including: (1) guanidine/alcohol, (2) amidine/amine, and (3) secondary amine.

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converted back to a neutral form. Besides, bicarbonate production is thermodynamically more probable relative to carbamate salt. Hence, the chemical reactions and separation of alcohol-containing SPSs need to be carried out under dry conditions to give the best performance. For this reason, before switching the solvent, water produced as a result of organic reactions needs to be dried [35], increasing the number of processing steps. In contrast, SPSs containing amine nucleophiles exhibit higher water stability because carbamate production happens much more rapidly than bicarbonate production. Several organic reactions can presumably occur in SPSs such as Heck reactions, Michael additions, and polymerization [36]. Nonetheless, functional groups that are switchable should also be considered when choosing reagents or reactants. Onecomponent SPSs do not need an equimolar amount of alcohol although they contain amine functional groups, which are potentially reactive (Fig. 6). Moreover, for one equivalent of CO2, two equivalents of amine are needed to yield one pair of anion-cation. Consequently, greater amounts of these one-component systems may be required based on the chemical process. The polarity changes occurring in SPSs can be advantageous for separation and extraction processes. In fact, these switches can bring about precipitation of the dissolved solute or phase separation. By way of illustration, SPSs have a vast range of industrial applications involving soybean oil separation from soybeans [37], extraction of heavy metals [38], CO2 capture, as well as lignocellulose pretreatment [39, 40].

1.4 CO2-binding organic liquids (CO2-BOL) BOL stands for CO2 binding organic liquids, which are generally based on switchable solvents of Jessop. A typical example of BOLs is a liquid containing guanidine or amidine bases and alcohol, which produces guanidinium or amidinium alkylcarbonate salts after binding CO2 chemically (Fig. 7) [33, 41]. According to the literature, CO2-BOLs have exhibited a relatively high volumetric and gravimetric capacities in reversible CO2 binding. It should be noted that CO2-BOLs are known as liquids that are able to eliminate the need for superfluous inert solvents that lower the volumetric capacity and weight of the trapping agents.

Fig. 7 Reversible binding of CO2 with an amidine (DBU) and alcohol [34].

Polarity-changing solvents for CO2 capture

In this sort of organic solvent, CO2 binds the liquid to yield alkylcarbonate instead of carbamate or bicarbonate salts in conventional amine solvents. Carbamate and bicarbonate have a strong bond with CO2 with high hydrogen bonding. In contrast, the CO2 bond is weaker in an alkylcarbonate salt partially due to decreased hydrogen bonding. As a result, a lower amount of energy is needed for thermal CO2 stripping. Even in some cases, CO2 can be released at near room temperature, even though under these conditions the reaction rate is relatively low [41]. Chemical and physical properties of BOLs can be altered by either changing the base/ alcohol pairs or chemical modification of pairs. In this respect, nearly all primary and secondary alcohol types can be applied, while for the bases, amines, phosphazines, amidines, and guanidines are typically used; some examples are depicted in Fig. 8 [34].

2. Switchable solvent for CO2 capture Conventionally, gas sweetening and CO2 capturing processes have mainly been conducted using amines, while the solvent has been water. Nevertheless, using water as the solvent lowers the efficiency because the regeneration of water requires a great deal of energy. Besides, ILs are regarded as hygroscopic in the natural environment, thus the existence of water in natural gas can probably decrease process efficiency. Consequently, the need for employing a type of solvent that can work effectively in the presence of water is highlighted. However, this should not create any further energy requirement [42] while improving the gravimetric and volumetric capacities of CO2 capturing process [34]. To address the need for such a solvent, the switchable solvents have come into sharp focus recently as promising alternatives [42].

Fig. 8 Four examples of bases: (1) diazabicyclo [5.4.0]-undec-7-ene(dbu); tetramethylguanidine (TMG); (3) Bartons base; and (4) Hunigs base.

(2)

1,1,3,3

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2.1 Investigation of thermodynamic and molecular CO2 capturing process using switchable solvent CO2 capturing and stripping processes through nonaqueous solvents are conducted based on a chemical conversion and generally consist of at least two separate stages as depicted schematically in Fig. 9 [43]. In the first place, CO2 is solved in the organic solvents, which is followed by CO2 chemical binding. In the solvent containing a low amount of water, CO2 reactivity contributes to the production of azoline carboxylates or carbamates and alkyl carbonates, in addition to other base/acid reactions such as proton dehydration or transfer. The capturefree energy is calculated by the summation of both solvation and binding-free energies as in Eq. (1): ΔGðcaptureÞ ¼ ΔGðsolvationÞ + ΔGðbindingÞ

(1)

The binding must be sufficiently to be in favor of CO2 capturing at low temperatures (i.e., ΔG [binding] should be around 10–20 kJ/mol under a temperature of 40°C), and also CO2 release at nearly low temperatures (i.e., lower than 100°C) [44].

2.2 SPS solvent for CO2 capture SPS is known as a nonaqueous solvent for the CO2-capturing process with superiority over aqueous solutions in terms of lower heat capacity, higher gravimetric capacity, and comparable reaction enthalpies. They also give better performance relative to ILs, which are chemically modified with amine. It should be mentioned that the heat capacity of SPSs are widely important since the CO2 release process, and subsequently regeneration of the solvent, requires thermal energy. In previous research, the CO2 capacity in different sorbents with different CO2-binding modes were investigated. Results indicated that the CO2 capacity for different sorbents was in the range of 1–28 wt% [7].

Fig. 9 Schematic diagram of overall CO2 capture process.

Polarity-changing solvents for CO2 capture

2.2.1 Use of SPS as solvent for CO2 capture from flue gas Heldebrant et al. pioneered applying SPS for the separation of CO2 from flue gas. In this respect, they used CO2-binding organic liquids [34, 45, 46], which are known as alkylcarbonate SPS solvents containing guanidine and amidine in addition to alcoholic components [44]. Alkanolguanidines and alkanolamidines in the ionic form behave identical to twocomponent systems in terms of alkylcarbonate-binding mode (i.e., CO2 binding) although they are zwitterionic. In relation to two-component systems, they have the advantage of lower volatility, producing ILs with higher viscosity, in addition to less CO2 uptake due to their solidification or extremely high viscosity that hinders CO2 diffusion during CO2 sparging, and higher loading of CO2 can be obtained at higher pressures. Relative to alkylcarbonate SPSs for CO2 capture process, carbamate SPSs are more cost-effective and demonstrate greater water tolerance. By way of illustration, siloxylated ethanolamines were introduced for the first time as carbamate SPS systems for the purpose of reversible flue gas CO2 capture [32, 36]. The system, however, was understood to be vulnerable toward hydrolysis of SidO while the stripping process was carried out at relatively high temperatures. Interesting modifications on alkylcarbonate SPSs with declined volatility have been presented by other scientists. For example, Park et al. [44] developed dimeric alkanolamidines as solid sorbents for the CO2 capture processes. They reached the conclusion that solid alkanolamidines are weak in CO2 uptake regarding the low rate of CO2 diffusion into the solid. They also realized that dispersion of alkanolamidines in porous silica with 3% loading could boost the efficiency as a result of the larger surface area [7]. 2.2.2 Use of SPS as solvent for CO2 capture from streams with high pressure For the low-pressure CO2 capture including flue gas streams, alkylcarbonate and carbamate-based SPSs are well-known options, but they are not effectively feasible for high-pressure applications owing to the thermal energy needed for the regeneration of the sorbents. For high-pressure CO2 capture purposes, physical sorbents are typically desirable whereby the CO2 release process can be conducted more economically by a decrease in pressure [7]. For the first time, Heldebrant et al. employed a hybrid pressure-activated SPS that was chemically selective to separate CO2 under high pressure. The proposed system brought the benefits of high capacity, chemical selectivity, and efficient release of CO2 by a pressure drop. They demonstrated that any type of tertiary amine and alcohol, specifically anhydrous tertiary alkanolamines, could be carboxylated at pressures lower than 500 psi, and following that, dissolve more CO2 physically. Also, they are able to decarboxylate CO2 molecules with chemical bonds and release CO2 simultaneously by the pressure drop (Fig. 10) [47]. Theoretically, anhydrous alkanolamines involving

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Fig. 10 Pressure-activated SPS [7].

N,N-dimethylethanolamine (DMEA) can contain up to 25 wt% of chemical-bound CO2 and also act similarly to common ILs to dissolve CO2 physically. Despite being advantageous from a theoretical point of view, they suffer the shortcoming of low CO2 selectivity, which is most presumably associated with the degree of alkanolamine carboxylation. Thus, a greater rate of ionic conversion can boost CO2 selectivity since it can be modified to a more polar liquid, thereby separating CO2 more efficiently from streams containing high concentrations of nonpolar constituents including CH4 and N2 [7].

2.3 CO2-BOL solvent for CO2 capture Generally, switchable solvents work based on the CO2 reaction with amidine bases including DBUs (diazabicyclo[5.4.0]-undec-7-ene) as well as polymeric amidines [48–50]. According to the literature [49–51], polymer-based amidines have been introduced as CO2-capturing agents on the presumption that they are able to bind to CO2 chemically as stable zwitterionic adducts. Nonetheless, there was no firm evidence confirming the presence of such CO2 adducts with amidines, and later it was realized that those products were in fact bicarbonate salts [48]. Thus, researchers reached the conclusion that the CO2 capturing ability of DBUs and polymer-bound DBUs is confined to the CO2 amount that can be dissolved or absorbed physically in addition to the trapped content by adventitious water. CO2-BOLs, however, seems superior to other switchable solvents owing to their stoichiometric alcohol content that contributes to chemical CO2 binding without depending on adventitious water [34]. If a considerable amount of water exists, bicarbonate salts may undesirably be produced, but since the specific heat of CO2-BOL is lower than that of water, the generated salts can probably break down more easily relative to alkanolamine systems [52]. Typically, it is known that bicarbonate and carbonate salts generating from the alkanolamine reaction with CO2 contains less hydrogen bonds relative to alkylcarbonate ones produced from the reaction of CO2-BOLs and CO2 [42, 53]. This suggests that regeneration of CO2-BOLs needs a lower amount of energy in comparison with conventional amine systems including MEAs, proving the superiority of CO2-BOLs over amines for the CO2-capturing process. Particularly, a higher boiling point, lower vapor pressure,

Polarity-changing solvents for CO2 capture

lower thermal capacity, as well as lower corrosion probability in relation to water can be exemplified as the outstanding merits of CO2-BOLs [53]. According to gravimetric analysis, 1-hexanol CO2BOL DBU is able to capture 1.3 mol CO2/mole of DBU—the extra 0.3% is most likely absorbed physically not chemically—yielding 19 wt% absorbed CO2 and 147 g CO2/1 L liquid. Thus, chemical and physical absorptions collectively yield greater volumetric and gravimetric capacities for CO2-BOLs relative to aqueous ethanolamines in water. Moreover, by applying bases and alcohols having lower molecular weights, the CO2 gravimetric capacity of BOLs can be further improved [34]. Prior to introducing CO2-BOLs for the CO2-capturing process, they were known as “reverse ionic liquids”, and CO2 was conventionally used to alter their polarity [53]. In fact, CO2 is capable of changing the polarity of nonionic liquids (solvents) to yield ILs. This can be performed through passing CO2 through the solvent, recycling it to a polarizer, and stripping the solvent under a temperature much lower than its boiling point [54]. Conventional amine-based ILs have demonstrated to be good agents for CO2 scrubbing, yielding moderate weight capacity [55]. For example, Davis et al. proposed aminetethered imidazolium IL as an amine-based IL and indicated that it was able to absorb CO2 as a carbamate salt (0.5 M equivalents, or 7.4 wt% of CO2) with a saturation time of around 3 h [34]. In contrast, CO2-BOLs are regarded as ILs only if CO2 is chemically bonded to the liquid [52]. Additionally, there is no trapping functional groups in their structure, providing a higher CO2 weight capacity [34]. For the first time, Jessop et al. employed a CO2-BOL (Fig. 11) whereby they could capture 1.3 mol of CO2/DBU under a pressure of 1 atm. Accordingly, it was revealed that the capacity of CO2-BOLs are nearly two to three times greater than that of aqueous alkanolamines. Thus, it can be concluded that these solvents are fundamentally different from alkanolamines [31]. A few studies have dealt with capturing H2S, CO2, and SO2 by BOLs. Heldebrantet al. indicated that CO2-BOLs are able to capture and release CO2 for neat mixtures, which do not need any solvent for dissolution of CO2, with relatively high volumetric and gravimetric capacities, i.e., 147 g CO2/L liquid and the absorbed amount of CO2 around 19 wt% [34]. Also, it was revealed that, for concentrated and diluted

Fig. 11 First-generation CO2-BOL, where R ¼ (CH2) nCH3; n ¼ 0–5.

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streams, CO2-BOLs act selectively, which simplifies its use. In the case of precombustion and postcombustion, CO2 reacts with CO2-BOLs and form comparatively weak chemical bonds, leading to production of carbonate salts that are less stable than carbamate ones, and thus need a lower amount of energy for CO2 release in comparison with aqueous MEA systems. The low CO2-binding energy can be attributed to its weak ion pairs, not solely a function of pKa of bases [34]. They also noticed in another investigation that advanced nonvolatile SO2-BOLs and CO2-BOLs (one component) are superior to the first generation of CO2-BOLs regarding their lower synthesizing costs, lower energy requirement for solvent regeneration, as well as higher capacity. Alkanolamines and alkanolguadines are capable of capturing 10.2 wt% of CO2, but in the meantime, their viscosity goes up as a result of combination with CO2. The advanced SO2-BOLs (e.g., tertiaryalkanolaminediazabicyclo [5.4.0]-undec-7-ene) are more cost effective with capacity of capturing 35 wt% of SO2, which can be regenerated at temperatures lower than 70°C [54].

3. Switchable ionic liquid solvent for CO2 capture Switchable ILs, which are known as novel solvents containing nitrogen, can be directly applied as CO2 absorbents without needing any dilution [34, 56]. Due to the weak CO2 binding with switchable ILs, a lower amount of energy is needed to break down the structure of alkylcarbonate salts during the reversible process [22]. Switchable solvents or switchable ILs are regarded as a novel sort of ILs [57–59]. Among the switchable ILs, switchable polarity type is one of the most important types that can be converted from a molecular liquid to an ionic one [7, 59]. In fact, initially they are molecular liquids with low polarity that are modified by CO2 bubbling to ILs with high polarity. This type of solvent is mainly comprised of only one or two components involving primary amine, secondary amine, alkanolguanidine [53], amidine/alcohol [20], guanidine/alcohol [33], or amidine/amine [60]. The binary systems such as DBU/alcohol [45] are typically known as water-sensitive solvents as the presence of water is thermodynamically in favor of solid bicarbonate production instead of alkylcarbonate production with alcohol [7]. Production of bicarbonate is not favorable since it can make a switchable IL less recyclable [13]. Thus, the presence of alcohol in switchable ILs favors the low polarity nonaqueous extraction. However, it should be noted that high volatility of alcohols can affect the switchable performance negatively, and thus they are appropriate for a limited range of practical applications. On the contrary, the switchable ILs with one component made up of a mixture of alcohols and bases to give a single molecular structure can alleviate this problem. Actually, after the combination, the molecular weight of the SWILs increases, leading to lower volatility and higher viscosity of ILs. As noted previously, along with the SWILs that are able to alter polarity, there are other

Polarity-changing solvents for CO2 capture

types that can modify other properties including miscibility, basicity, and chemical structure [61–63]. However, it remains to be asked whether the structure of SWILs is exactly the same as conventional ILs or not. In response, it is presumed that the structure of SWILs is neither totally nonionic nor totally ionic but something fluctuating between the two. Consequently, it cannot conclusively deduced that SWILs have precisely the same characteristics as conventional ILs to give high performance. In fact, SWILs were introduced on the presumption they are similar to conventional ILs in terms of structure at relatively high ionicity (high loading of CO2), contributing to a template effect. Nonetheless, it is not distinguishable how the structure of the solvent would be changed toward alteration of ionicity. Likewise, it is not obvious how the thermodynamic and physical properties are affected by the changes in ionicity of the SWILs [64]. By way of illustration, to minimize waste of alcohol through evaporation, Heldebrant et al. [45, 65] applied one-component SWILs for the CO2 capture, revealing that the capacity of CO2 binding (6–9 wt%) was less than its binary counterparts (two-component SWILs) [65]. This can be attributed to lower CO2 mass transfer rate in single-component SWILs arising from its higher viscosity relative to two-component ones. To further improve the CO2 capture performance, some innovative single-component SWILs including alkanolguanidines have been introduced [66] whereby the CO2 capacity was approximately 7–12 wt% [41, 67, 68]. In another study by Zhu et al. [69], two-component SWILs were fabricated through a combination of various imidazoles and DBU. The charge distribution in the imidazole ring as well as reaction of CO2 and ILs are affected by various substitute positions and structure. The powerful electron-withdrawing effect exerted by the imidazole ring with a phenyl ring contributes to the least CO2 capacity. The steric hindrance increases due to substitute groups near the nitrogen anion, resulting in lower CO2 capacity and reaction rate. This can be used as a criterion to select an efficient SWIL for the purpose of CO2 capture [69]. Different silylamines involving 3-(aminopropyl)trimethoxysilane (TMSA), 3- (aminopropyl)triethoxysilane (TESA), 3-(aminopropyl)triethylsilane (TEtSA), and 3- (aminopropyl)tripropylsilane (TPSA) were examined for CO2 capture by Rohan et al. [68] They understood that the obtained CO2 absorption was higher than the conventional carbamate stoichiometry (i.e., 0.5 mol CO2/mole amine) [70]. In fact, the obtained CO2 uptake was 0.6 mol/mole of amine, which can most presumably be ascribed by the production of carbamic acid or physical absorption. They also reported that the CO2 binding enthalpies were in the range of 92 to 81 kJ/mole CO2, which were nearly in agreement with that recognized by Mathias. Also, the viscosity of silyamines went up with CO2 uptake, all equal or lower than 1300 cP at a temperature of 40°C and fillloading [71].

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Additionally, 13 other types of silylamines were studied by Eckert and Liotta [43] for CO2 capture, giving comparable performance to the four types mentioned before. Results indicated that the CO2 uptake was in the range of 0.33–0.63 mol CO2/mole of amine under a temperature of 25°C and 1 atm CO2, respectively. Also, methyl-3(triethylsilyl) propylamine (βMe-TEtSA) and TEtSA could absorb 0.6 mol CO2/mole amine under 1 atm CO2 and a temperature of 40°C (Fig. 12). The highest CO2 absorption capacities were related to trans-TEtSA and 1-(aminomethyl) triethylsilane (TEtSMA). The heat of reaction for trans-TEtSA and TEtSMA was 85 and 76 kJ/ mol, correspondingly, with viscosity of 362 and 625 cP under a temperature of 40°C after CO2 capturing [43].

4. Conclusions Synthesizing and using novel solvents for the absorption of acidic gases such as SO2 and CO2 have come into sharp focus recently. Specifically, for the purpose of CO2 capture through physical or/and chemical absorption, developing an efficient method such as using switchable solvents has captured scientists’ attention. Thus, in this chapter, we have dealt with describing and reviewing the most practical switchable solvents and their influential role as environmental-friendly and cost-effective solvents in CO2 capture processes. Despite encouraging results that have been presented in this respect, more comprehensive studies are needed to improve the efficiency of using such solvents for CO2 absorption processes. This type of solvent can bring great advantages to industrialists who deal with separation processes as their properties can be changed by adding a trigger such as CO2 gas, which is commonly regarded as a waste product. There are three types of switchable solvents: (1) those that change their polarity between polar and nonpolar (SPS); (2) those with hydrophobicity between hydrophilic and hydrophobic (SHS); and (3) those with ionic strength between a high and low levels (SW). Among the SWLs, CO2-BOLs have a wide range of practical applications for pre- and postCO2 capture processes. In addition, a variety of bases and alcohols can be selected to synthesize BOLs to further improve their chemical and physical properties. Generally, CO2-BOLs have demonstrated considerable potential to be efficient solvents for CO2 capture processes. Therefore, further research is needed to assess and improve their performance to make them more cost-effective and ecofriendly.

Fig. 12 TEtSMA and trans-TEtSA derivatives for CO2 separations [43].

Polarity-changing solvents for CO2 capture

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CHAPTER 3

Applications of switchable solvents in science and technology Mohammad Faraz Ahmera and Qasim Ullahb,∗ a

Department of Electrical and Electronics Engineering, Mewat Engineering College, Nuh, Haryana, India Physical Sciences Section, School of Sciences, Maulana Azad National Urdu University, Hyderabad, Telengana, India ∗Corresponding author. e-mail address: [email protected] b

1. Introduction Switchable solvents, developed to facilitate both technological advancements and reaction product separations constitute a unique group of solvents aligned with the concept of green chemical technology. A solvent may be considered as switchable if it is capable to change own physical properties (polarity, water-miscibility, ionic strength, viscosity, surface activity, osmotic pressure, fluorescence, and solubilizing ability) swiftly on exposure to carbon dioxide (CO2) at one atmospheric pressure and the transformed solvent can easily revert to its native state on the removal of CO2. Switchable solvents, thus, eliminate the necessity of using multiple solvents during analysis and products purification because of their capability to dissolve both organic and inorganic constituents of reaction. Various chemical triggers such as acids, bases, oxidants or reductants, and CO2 have been used to convert ordinary solvents into switchable solvents. Besides these chemicals, light and voltage have also been reported as trigger agents, but these have been found suitable only for switchable surfaces and not appropriate for dark, opaque, and nonconductive bulk solutions. Chemical triggers like acids or oxidants are also not generally found to be useful because of their possible use in stoichiometric ratio. Thus, CO2 has been considered the best trigger among all the reported alternatives because it is inexpensive, potent electrophile, benign, and useful to opaque systems. The switchable species resulted out due to the use of CO2 as trigger, can change the polarity, ionic strength, hydrophilicity, viscosity, surface charge, and chemical or catalytic reactivity. Furthermore, the use of waste CO2 as the trigger increases energy efficiency and reduces green-house gas emissions. Contrary to other solvents, a switchable solvent has twin sets of solvatochromic variables, one set for each of its form as shown in Table 1. In this table, three Kamlet-Taft solvent variables α, β, and Π* that enumerate the distinguishing features of acidity, basicity, and polarizability respectively along with λmax value for the dye Nile red are presented only for limited systems. In case of equimolar intermixture of propanol and DBU, CO2 brings about a substantial increase in polarizability (Π*) with no change in basicity (β). However, the value of α as expected to be more than zero because of the presence of 1-propanol falls below Green Sustainable Process for Chemical and Environmental Engineering and Science https://doi.org/10.1016/B978-0-12-819850-6.00009-7

Copyright © 2022 Elsevier Inc. All rights reserved.

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Table 1 Solvatochromic variables for CO2-triggered (+) switchable solventsa [1]. Variable Nile red λmax (nm)

α

β

Π*

S. no.

Solvent system

-CO2

+CO2

-CO2

+CO2

-CO2

+CO2

-CO2

+CO2

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.

DBU-1-propanol DBU-1-butanol DBU-1-hexanol DBU-1-octanol DBU-1-decanol TMBG-methanol TMBG-1-butanol TMBG-1-hexanol TMBG-1-octanol TMBG-1-decanol PhCH2NHMe CyNMe2 (dry) CyNMe2–H2O

542 538 537 537 537 538 532 531 530 528 534 – n

552 548 544 542 540 554 544 545 540 537 543 – 588.3


0.11


0.21

1.04

1.00

0.71

0.98

0.59 1.14

0.84 0.74 0.57

0.69 0.23 0.66 1.06

N

0.71 0.81 0.68

a

Liquid systems are equimolar mixtures.

0 0.47 1.13

1.09 1.09

Applications of switchable solvents in science and technology

(i)

(ii)

(iii)

HPS Single Phase

HPLS Single Phase

HISS Single Phase

Amidinium alkyl carbonate

Bicarbonate salt of CyNMe2

Bicarbonate salt of polyaniline

+CO 2

-CO2

LPS Single Phase

+CO 2

-CO2

Bi-Phasic HPBS Bi-phasic HPBS

+CO 2

-CO2

LISS Single Phase

Phase I Polyamine plus water

Amidine plus alcohol Cy NMe2 Plus H2 O Phase II

Fig. 1 Switchable solvent systems in different variants (LPS ¼ Low-polarity solvent, HPLS ¼ Hydrophilic solvent, LISS ¼ Low-ionic strength solvent; HPS ¼ High-polarity solvent, HPBS ¼ Hydrophobic solvent, HISS ¼ High-ionic strength solvent).

zero possibly due to strong basic nature of amidine and its hydrogen bond formation tendency with alcohol. Thus, a considerable change in polarity of switchable hydrophilicity solvents is attributed to their merging with an aqueous phase in the presence of CO2. Moreover, aqueous solution of switchable ionic strength (i.e., switchable water) does not show much change in solvatochromic variables when exposed to CO2. Classification: The CO2-triggered switchable solvent systems can be broadly classified into three main categories as mentioned below and delineated in Fig. 1: (i) Switchable polarity solvents (SPSs) (ii) Switchable hydrophilicity solvents (SHSs) and (iii) Switchable water (SW).

1.1 Switchable polarity solvents (SPSs) Philip G. Jessop et al. [2] published a one-page report on a novel polarity inter conversion (nonpolar to polar) solvent system, termed as “Switchable Polarity Solvent” in the

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prestigious journal “Nature” in 2005. They developed a new CO2-triggered switchable polarity system using the equimolar mixture of hexanol and DBU (1,8-diazabicyclo-[5.4.0]-undec-7-ene) as nonionic system that has capability to convert into ionic liquid system (i.e., a salt in liquid state) upon exposure to the stream of CO2 at one atmospheric pressure and room temperature. This mixture readily switched back to original nonionic form when exposed to N2 or argon gas at room temperature (or complete removal of CO2). The reaction was exothermic and caused a marked increase in the viscosity of the liquid. The phase changes from nonionic to ionic-liquids and vice versa were confirmed by polarity measurements of the solvent before and after treatments with CO2 using proton NMR spectroscopy and solvatochromic measurement. Thus, this SPS has been found most applicable in analytical chemistry. The dynamic polarity changes of the developed switchable solvent system were demonstrated by examining the solubility of decane (nonpolar) that was miscible with liquid in the presence of N2, but not with liquid under CO2 (Fig. 2). Thus, nonionic alcohol/amidine mixtures that easily transformed into amidinium alkyl carbonate ionic liquids were the first switchable polarity solvents to be used as extractants, reaction media, and CO2-capturing agents. SPSs based on amidine and guanidine alkyl carbonates show relatively higher polarity in their nonpolar state compared to CDCl3, within smaller polarity variation range. However, carbamate SPSs exhibit lower polarity range because alcohols are usually more polar then primary and secondary amines [3]. In single-component switchable solvents, the same molecule behaves simultaneously both as base and the nucleophile for example secondary amines [4], primary amines [5], diamines [6], hydroxyamidines, and hydroxyl guanidines [6] whereas in two-component switchable solvents, one molecule acts as nucleophile and the other molecule acts as a base. The mixtures of amidine with aliphatic primary amines [7] or amino alcohols [8] are the examples of two-component SPSs that have been found less sensitive in the presence of water because the reaction between primary amine and CO2 results in the formation of a carbonate salts, which is thermodynamically more stable than the bicarbonate and alkyl carbonate salts. Secondary amines as nucleophile can be used for the preparation of cheaper SPSs [4]. Low cost SPS systems can be obtained with only a single organic component without using amidine and guanidine bases. In fact, secondary amines were used as the first single component SPSs followed by the use of primary amines later on. The primary amine group containing ionic liquids have shown switchable basicity rather than switchable polarity.

1.2 Switchable hydrophilicity solvents (SHSs) Switchable hydrophilicity solvents are normally hydrophobic liquids that are capable of forming distinct two-phase system in contact with water. However, in the presence of CO2 at one atmospheric pressure, these solvents become highly hydrophilic and completely mixed with water forming a single phase, but on complete elimination of CO2 these revert back to form a two-phase system, as shown in Fig. 3 [9]. Compared

Applications of switchable solvents in science and technology

Fig. 2 Role of CO2inprotonation and deprotonation of DBU (1,8-diazabicyclo-[5.4.0]-undec-7-ene) in the presence of an alcohol and the CO2-triggered separation of decane.

CO2 low polarity solvent

-CO2

High polarity solvent

Fig. 3 Illustration of CO2-triggered switchable polarity solvents [9]: Alcohol plus secondary amin or amidine (single phase, low polarity) system changed into high polarity system through reaction with CO2. The process reversed on complete elimination of CO2.

to SPSs, SHSs have been more attractive because these are more stable and less expensive. Additionally, SHS can be completely recovered from SHS-water systems after CO2 removal. The amidine, N,N,N0 -tributylpentanamidine was the first SHS used as a substitute for volatile solvents and easily removed without distillation from organic products (such as soybean oil) by extraction with carbonated water. Most often, functional

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Bu N

Bu2N

H2O, CO2

H + Bu N Bu2N

Bu

O -

O

OH

Bu O

R

R

H2O, CO2

N

N R

R

-

+

R

O

OH

H

R

Fig. 4 Representation of the amidine and tertiary amine as SHSs [10].

groups include alkylated amidines or secondary and tertiary amines that act as bases to deprotonate carbonic acid or hydrated CO2 have been used SHS. Among these, tertiary amines have been found most suitable as SHS (Fig. 4). The polarity switches displayed by SHSs are considerately larger as compared to SPSs. For example, with Nile red dye; amidine exhibits maximum absorbance at 510 nm (λmax). However, in the presence of CO2 the resulting solution showed has maximum absorbance at 570 nm (λmax) [10]. This polarity switch along with polarity ranges of traditional solvents and SPSs is shown in Fig. 5. The SHSs have interesting applications in the field of solvent recovery. In industries, distillation has been the method of choice for removing solvent from product, but for distillation purpose the solvent has to be volatile and therefore flammable, smog-forming, and risky to workers. In case of solvents that could be removed from products and recycled without distillation, then such nonvolatile organic solvents can be used, reducing the risk to the environment and the workers. The SHSs can be easily removed by extraction using carbonated water, rather than distillation. This property makes these solvents very promising and applicable in the field of extraction technology particularly in extractions of soybean oil [10], algae oil [11], and bitumen from oil sands [12]. Following a new approach, Ohno et al. [13] generated an ionic liquid that acted as an SHS in contrary way to that of Jossep’s SHS. The proposed ionic liquid, tetrabutylphosphonium N-trifluoromethanesulfonyl leucine, was completely dissolved in water, but on exposure to CO2, it separated out from water because the protonation of the carboxylate group was effectuated by the pH change in the aqueous solution. After exclusion of CO2, the ionic liquid became miscible in water. 1.2.1 Criteria of selection for SHS (i) Traditionally, various amines have been examined as SHS, but all amines and their derivatives are not found to be suitable for use as SHS. For example, amines like diisopropylamine, diethylamine, butylethylamine, and triethanolamine are not suitable as SHS because they are highly soluble in water and unable to form two-phase extraction systems.

520

530

540

550

ethylene glycol

CHCl3

MeOH

510

DMF

500

achlene MeCH

ether

toulene

Applications of switchable solvents in science and technology

560

570

BuN=C(Bu)NBu2/H2O NHEtBu

TMBG/MeOH BDU/PrOH

MeNHCH2Ph Fig. 5 Comparison of the polarity ranges of several SPSs and one SHS in contact with an equal volumes of water and the same mixture after contact with CO2 [10].

(ii) Amines such as dihexylamine, trioctylamine, and butylisopropylamine are also not suitable as SHS due to their restricted solubility even in acidic aqueous media. Among amidine and guanidine as the SHS, tributylpentanamide has been considered most favorable. The use of guanidine as SHS has restricted because of strongly basic nature whereas the hydrolysis tendency of amidines limits their use. (iii) Some of the secondary amines show in switchable hydrophilicity, but their switching require energy to expel CO2 from aqueous solution of ammonium carbamate that generated during reaction of amine with CO2 [14]. (iv) Tertiary amines which are commercially available and less susceptible to hydrolysis are more suitable as SHS. (v) While choosing suitable SHS, properties such as stability, volatility, toxicity, and bioaccumulation be taken into consideration. (vi) Similar to amines, solvents like saturated fatty acids can be used as SHS. Andruch et al. [15] have successfully utilized hexanoic acid switchable solvent in the determination of ofloxacin in urine. (vii) The pKa value of conjugate acid must be taken into consideration while selecting SHS. For most of the water miscible amines, the pKa values remain close to 9.5 [16].

2. Switchable water (SW) Switchable water is defined as an aqueous solution that is capable of reversibly switching ionic strength between an initial ionic strength and the enhanced ionic strength. The interchangeability of ionic strength from lower to higher can be accomplished by bubbling with CO2, CS2, or treatment with Bronsted acids. On the other hand, switching

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from higher to lower ionic strength can be acquired by injecting inert gas/air, heating, or introducing partial or complete vacuum. For example, aqueous solutions of a diamine with practically negligible ionic strengths can be transformed into high ionic strength solutions simply by treating with CO2. This change is reversible and on removal of CO2, the solution returns to its initial state. CO2 is used as trigger agent in SW to control viscosity, surface tension, osmotic pressure, emulsion/suspension stability and solubility of organics in water. SM Marcer and PG Jessop [17] have admirably utilized the switchable ionic strength of water by direct addition of amines. They successfully used the solutions of N,N,N,N-tetramethyl-1,4-diamminobutane to dissolve sufficient quantity of tetrahydrofuran (THF) which on exposure to CO2 experienced a change in ionic strength. The dramatic change in ionic strength resulted in the separation of THF from water (Fig. 6). After complete separation of THF by decantation, the SW solution in its low-ionic strength form can be re-used. This approach represents a unique example of reversible methodology for “salting-out” organic compounds from water. An aqueous solution of a polyamine has negligible ionic strength and can dissolve organic substances because the polyamine acts as a hydrotrope. However, on bubbling CO2 in switchable water, the polyamine converted into a salt which enhances the ionic strength to a large extent followed by the dramatically changes in properties of the water. This SW has been found most applicable in analytical chemistry. The conductivity or viscosity of SW also increases on introducing CO2 in it. When triggered, switchable water does not experience any phase changes and thus symbolize a simplified system that can be modeled in a better way than SHS. For instance, Alshamrani et al. [18], using equilibrium calculations have predicted the required basicities for the high conversion of monobasic amines into bicarbonate salts and showed the concentration-dependent percent protonation of a base in air. The change in solvent properties with the introduction of a CO2 offers several avenues for chemical processes, involving separations purification and product isolations. However, these advantages become irrelevant if switchable solvents (ionic or neutral) Single phase system

Biphasic system

CO2 THF + Water + Amine (additive)

THF

Air

Water plus amines

Fig. 6 Separation of THF from water with the use of salting-out effect of SW.

Applications of switchable solvents in science and technology

are lost gradually during processing as additional energy required to remove solvent contaminants. Because the toxicity of amines dramatically depends upon their structures, the predictive models are important component of SPS, SHS, and SW development systems. Though, the environmental consequences of amines are not well understood, but current growth in postcombustion CO2 capture has resulted in increased interest in amine emissions. SW has been successfully used to stabilize emulsions and suspensions with or without the use of CO2 [19, 20]. SW is capable to make ionic surfactants CO2-responsive, even if the surfactant remains unaffected by CO2 [20]. SW as companion with CO2 has been an excellent source for the recovery of fresh water from wastewater or seawater. After the removal of CO2, the amine can be easily recovered, leaving behind fresh water. Further, SW can be used as the basis for aqueous solutions of switchable viscosity. To sum up, SW has the following applications (i) Reversible salting-out of organic products (ii) Extraction of water-soluble compounds (iii) Salting of suspensions (iv) Breaking of emulsions/foams (v) Desalination of sea water (vi) Dewatering of wastewater

3. Technological and analytical applications of switchable solvents 3.1 Applications of SPSs Though the development of the SPS technology is in its infancy, but SPSs are expected to be utilized in various fields of chemical technology in future. Some of the important applications of SPSs are discussed below to justify their analytical potential as green solvent systems. 3.1.1 SPS as extraction media SPSs have been considered most useful extractants because these are capable to remove the solute in their native state and released it on switching to its other form as shown in Fig. 7. For example, Samori et al. [21] used DBU/alcohol SPS for extracting fatty acids and lipids from algae. The DBU/1-octanol SPS system was efficient to extract 16% of the total lipids present in freeze dried algae samples and 8.2% of total lipids from nonfrozen samples, both extractions were superior to conventional extraction with n-hexane which extracted 7.8% from freeze-dried samples and 5.1% from liquid sample. Compared to n-hexane (toxic solvent)/the DBU/1-octanol system (a benign solvent) was less effective for recovering fatty acids.

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Fig. 7 Extraction of an oil (hydrophobic) from solid matrix [3].

3.1.2 SPSs in CO2 detection Generally, CO2-sensors perform well at lower concentrations. However, Tang and co-workers have reported a chemo-sensor with fluorescence that was capable to detect CO2 content in the air up to 100% by volume [22]. Some nonluminescent dyes show aggregate-induced emission on accumulation in polar solvents and hence are useful in the detection of CO2. 3.1.3 SPSs as CO2 capture SPSs have excelled as CO2-capturing nonaqueous liquids due to comparable enthalpies of reaction, lower heat capacities and higher gravimetric CO2 capacity than CO2 capture aqueous solutions or task-oriented ionic liquids. Table 2 summarizes important features of different SPSs used for CO2 capture. 3.1.4 SPSs as CO2 capture for high-pressure streams The SPSs based on carbamate and alkyl carbonate have been good options for separations of CO2 in low pressure gas streams such as flue gas. However, these are not favorable for high-pressure separations because of the necessity of heating to regenerate the sorbents. High-pressure CO2 separations, therefore, require physical sorbents, which are capable to stimulate an economical release of CO2 upon pressure reduction. Heldebrant and co-workers [23] were the first to develop a chemically selective pressure-activated “hybrid” SPS for CO2 separations at high pressures. The advantages

Applications of switchable solvents in science and technology

Table 2 CO2-capture efficiency of SPSs [3]. S.no.

Sorbent

CO2 capacity (wt%)a

CO2 binding mode

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.

DBU/1-hexanol TMG/1-hexanol 1 2 4 5 6 7 8 9 10

15 17 28 16 18 1–5 10 12 12 1 8

Alkylcarbonate Alkylcarbonate Carbamate Alkylcarbonate Alkylcarbonate Alkylcarbonate Alkylcarbonate Carbamate Carbamate Alkylcarbonate Alkylcarbonate

a

Chemically bound CO2 only, observed capacity at 25°C and 1 atm CO2.

N

OH

CO2 > 100 psi

DMEA

H N

O

O + O

–

DMEA-CO2

Fig. 8 Pressure-activated SPS.

of this system were unique chemical selectivity, reasonable CO2 capacity and effective CO2 release upon drop in pressure. The authors observed that anhydrous tertiary alkanolamines carboxylated under the pressure (