Polymer Nanocomposite Membranes for Pervaporation (Micro and Nano Technologies) [1 ed.] 0128167858, 9780128167854

Table of contents :
Cover
Polymer Nanocomposite Membranes for Pervaporation
Copyright
Contents
List of contributors
Preface
1 Polymer nanocomposite membranes for pervaporation: an introduction
1.1 Introduction
1.2 Basic principles of pervaporation
1.2.1 Solution-diffusion model
1.2.2 Pore flow model
1.2.3 Permeability: a normalized flux
1.2.4 Selectivity: an intrinsic membrane properties
1.3 Membranes for pervaporation
1.3.1 Inorganic membranes
1.3.2 Mixed matrix membranes
1.3.3 Polymer membranes
1.4 Factors affecting the pervaporation
1.4.1 Pressure
1.4.2 Concentration polarization and partition coefficient
1.4.3 Temperature
1.4.4 Membrane thickness
1.5 Advantages of separation using pervaporation process
1.6 Conclusions
References
2 Nanocellulose/polymer nanocomposite membranes for pervaporation application
2.1 Introduction
2.1.1 Design and choice of membrane materials for pervaporation
2.2 Nanocellulose isolation methods
2.3 Nanocellulose/polymer nanocomposite membranes for pervaporation application
2.3.1 Cellulose-polydimethylsiloxane blends for pervaporation
2.3.2 Cellulose/poly(vinyl alcohol) membranes for pervaporation
2.3.3 Cellulose acetate/polyacrylonitrile membranes for pervaporation
2.3.4 C60-filled ethyl cellulose hybrid membranes for pervaporation
2.3.5 Cellulose acetate membrane filled with metal oxide particles for pervaporation
2.3.6 Ethyl cellulose reinforced with natural zeolite membranes for evaporation
2.3.7 Ethyl cellulose membranes for pervaporation of water, hydrazine, and monomethyl hydrazine
2.3.8 Blend membranes of sodium alginate and (hydroxyethyl) cellulose for pervaporation
2.3.9 Ethyl cellulose reinforced with TiO2 membranes for pervaporation
2.3.10 Bacterial cellulose/alginate blend membranes for pervaporation
2.4 Conclusions
References
3 Biobased (nanochitin, nanochitosan) polymer nanocomposite membranes and their pervaporation applications
3.1 Introduction
3.2 Pervaporation of chitin and chitosan membranes
3.3 Chitin membranes
3.4 Chitosan membranes
3.4.1 Modified chitosan membranes for pervaporation
3.4.2 Chitosan/organic membranes
3.4.2.1 Chitosan/polybenzoimidazole membrane
3.4.2.2 Chitosan/poly(n-vinyl-2-pyrrolidone) membrane
3.4.2.3 Chitosan/polyvinyl alcohol membrane
3.4.2.4 Chitosan/poly(acrylic acid) membrane
3.4.2.5 Chitosan/polyvinyl sulfate membrane
3.4.2.6 Chitosan/sodium alginate membrane
3.4.2.7 Chitosan/cellulose membrane
3.4.2.8 Chitosan/carrageenan membrane
3.4.2.9 Chitosan/gelatin membrane
3.4.2.10 Chitosan/glutaraldehyde membrane
3.4.2.11 Chitosan/polyaniline membrane
3.4.3 Chitosan/inorganic membranes
3.4.3.1 Chitosan/clay membrane
3.4.3.2 Chitosan/titanium dioxide membrane
3.4.3.3 Chitosan/ferric oxide membrane
3.4.3.4 Chitosan/functionalized graphene sheets membrane
3.4.3.5 Chitosan/NaY membrane
3.4.3.6 Chitosan/silica membrane
3.4.3.7 Chitosan/sulfosuccinic acid membrane
3.4.3.8 Chitosan/toluene-2,4-diisocyanate membrane
3.4.3.9 Chitosan/reduced graphene oxide membrane
3.4.3.10 Chitosan/phosphotungstic acid membrane
3.4.3.11 Phosphorylated chitosan membrane
3.4.3.12 Sulfonized chitosan membrane
3.4.3.13 Chitosan/multiwall carbon nanotube/silver membrane
3.4.3.14 Chitosan/Mxene membrane
3.4.3.15 Chitosan/boehmite membrane
3.4.4 Chitosan hybrid membranes
3.4.4.1 Sodium alginate/chitosan/multiwall carbon nanotube membrane
3.4.4.2 Chitosan/PVA/multiwall carbon nanotube membrane
3.4.4.3 Chitosan/PVA/Ag membrane
3.4.4.4 Chitosan/silica/polytetrafluoroethylene membrane
3.4.4.5 Chitosan/iron oxide/PAN membrane
3.4.4.6 Chitosan/silica/PAN/PEG membrane
3.4.4.7 Chitosan/aluminum-based metal organic framework membrane
3.5 Conclusion
References
Further reading
4 Pervaporation performance of polymer/clay nanocomposites
4.1 Introduction
4.1.1 Polymer nanocomposites
4.1.2 Structure of nanoclay
4.1.3 Organic modification of nanoclay
4.1.4 Polymer nanoclay composites
4.2 Pervaporation characteristics
4.2.1 Transport mechanism
4.2.2 Solution diffusion mechanism
4.2.3 Pore flow mechanism
4.3 Factors affecting membrane performance
4.3.1 Effect of nanoclay content in pervaporation process
4.3.2 Feed composition
4.3.3 Temperature
4.3.4 Concentration polarization
4.4 Conclusions
References
5 Carbon nanotubes-polymer nanocomposite membranes for pervaporation
Nomenclature
Abbreviations
Symbols
5.1 Introduction
5.2 Pervaporation
5.2.1 Solution-diffusion model
5.2.2 Separation characteristics of pervaporation membranes
5.3 Polymer nanocomposites
5.4 Carbon nanotubes
5.4.1 Carbon nanotubes functionalization
5.4.1.1 Purification and oxidation of carbon nanotubes
5.4.1.2 Noncovalent functionalization of carbon nanotubes
5.4.1.3 Covalent functionalization of carbon nanotubes
5.5 PV application of carbon nanotubes-polymer nanocomposite membranes
5.5.1 Dehydration of solvents and alcohols
5.5.2 Separation of organic–organic mixtures
5.5.3 Recovery of organics from aqueous solutions
5.6 Conclusions
References
6 Graphene-based polymer nanocomposite membranes for pervaporation
6.1 Introduction
6.2 Graphene
6.2.1 Structure and properties of graphene and its derivatives
6.2.2 Graphene membranes—synthesis and characterization
6.3 Graphene-based membranes for pervaporation
6.3.1 Graphene oxide-based membranes
6.3.2 Reduced graphene oxide membranes
6.3.3 Hybrid graphene oxide membranes
6.3.4 Functionalized graphene oxide membranes
6.3.5 Quantum dot membranes
6.4 Conclusions and future aspects
References
7 Fullerene and nanodiamond-based polymer nanocomposite membranes and their pervaporation performances
7.1 Introduction
7.2 Pervaporation
7.3 Membranes for pervaporation
7.4 Nanodiamond
7.5 Pervaporation performance of fullerenes-based nanocomposite membranes
7.6 Membranes modified with fullerenes and derivatives
7.6.1 Fullerene-based nanocomposites and its pervaporation
7.7 Conclusions
References
8 Polymer nanocomposite membranes for pervaporation desalination process
8.1 Introduction
8.2 Synthesis methods of polymer nanocomposite pervaporation membranes
8.2.1 Physical blending
8.2.2 Sol–gel synthesis
8.2.3 In situ polymerization
8.2.4 Self-assembly
8.3 Factors affecting the performance of pervaporation desalination membranes
8.3.1 Selectivity and nature of membrane material
8.3.2 Diffusivity and nature of the filler
8.3.3 Salt transport suppression
8.3.4 Operating temperature
8.4 Polymer membranes for pervaporation desalination
8.4.1 Cellulose acetate membranes
8.4.2 Polyacrylonitrile and polyvinyl alcohol-based membranes
8.4.3 Poly(vinyl alcohol)/polyvinylidene fluoride pervaporation membrane
8.4.4 PEBAX membrane
8.4.5 Tubular pervaporation membrane
8.4.6 Sulfonated poly(styrene-ethylene/ butylenes- styrene) block copolymer membrane
8.5 Polymer nanocomposite membranes for pervaporation desalination
8.5.1 Mixed matrix membranes for pervaporation desalination
8.5.2 Self-assembled membranes
8.5.3 Sol–gel synthesized membranes
8.6 Conclusion and future aspects
References
9 Polymer/polyhedral oligomeric silsesquioxane nanocomposite membranes for pervaporation
9.1 Introduction
9.2 Polyhedral oligomeric silsesquioxane
9.2.1 Different types of POSS
9.2.2 Synthesis of POSS
9.2.3 Properties of POSS
9.2.4 Applications of POSS
9.3 Pervaporation performance of polymer/POSS membranes
9.3.1 Separation of azeotropic mixtures and organic solvents
9.4 Factors affecting the pervaporation through polymer membrane
9.4.1 Effect of free volume
9.4.2 Nature of polymers
9.4.3 Nature of filler particles
9.4.4 Effect of temperature
9.4.5 Nature of penetrants
9.4.6 Degree of cross-linking
9.5 Applications of polyhedral oligomeric silsesquioxane-embedded polymeric systems
9.5.1 Dehydration of ethanol
9.5.2 Ethanol recovery
9.5.3 Separation of organic mixtures
9.5.4 Water treatment
9.5.5 Desulfurization of fuels
9.6 Challenges and future aspects
References
10 Nanometal and metal oxide-based polymer nanocomposite membranes for pervaporation
10.1 Introduction
10.2 Synthesis of polymer–nanometal nanocomposite
10.2.1 Synthesis of metal/metal oxide nanoparticles
10.2.1.1 Physical method
10.2.1.1.1 Mechanical grinding
10.2.1.1.2 Melt mixing
10.2.1.1.3 Evaporation
10.2.1.1.4 Laser ablation
10.2.1.1.5 Sputtering
10.2.1.2 Chemical methods
10.2.1.2.1 Chemical reduction
10.2.1.2.2 Chemical precipitation
10.2.1.2.3 Sol–gel technique
10.2.1.2.4 Hydrothermal method
10.2.1.2.5 Microemulsion technique
10.2.1.3 Biological method
10.3 Direct use of nanometal and metal oxides as membrane
10.4 Nanometal and metal oxide-based polymer–metal nanocomposites membranes
10.4.1 Grafting of nanoparticles to polymer
10.4.2 Incorporation of nanoparticles in polymer
10.4.3 Intermatrix synthesis technique
10.4.4 Silver nanoparticle-based polymer–metal nanocomposites membranes
10.4.5 Pervaporation using Ag polymer–metal nanocomposites membranes
10.4.6 Iron nanoparticle -based polymer–metal nanocomposite membranes
10.4.6.1 Pervaporation with iron polymer–metal nanocomposites membranes
10.4.6.1.1 Polyvinyl alcohol-based iron polymer–metal nanocomposite membranes
10.4.6.1.2 Alginate–iron nanoparticle nanocomposite
10.4.6.1.3 Chitosan–iron nanoparticle nanocomposite
10.4.6.2 Characterization of iron polymer–metal nanocomposites membranes
10.4.6.2.1 Characterization of polyvinyl alcohol–iron nanoparticle composite membrane
10.4.6.2.2 Characterization of alginate–iron nanoparticle composite membrane
10.4.7 Pervaporation performance of alumina nanoparticle -based PMNC membranes
10.4.7.1 Nanoalumina as membrane
10.4.7.2 Silver polymer–metal nanocomposites membranes
10.4.8 Pervaporation using titanium nanoparticle-based polymer–metal nanocomposite membrane
10.4.9 Gold nanoparticle-based polymer–metal nanocomposite membrane
10.4.9.1 Plasmon pervaporation
10.4.10 Polymer–metal nanocomposites based on nano-MgO and ZnO
10.5 Conclusions
Acknowledgment
References
11 Modified zeolite-based polymer nanocomposite membranes for pervaporation
11.1 Introduction
11.2 Water and alcohol-selective zeolites
11.3 Mechanism of water/alcohol separation in zeolite
11.3.1 The separation mechanism in zeolite particles
11.3.2 The separation mechanism through mixed matrix membranes and inorganic fillers incorporated composite membranes
11.4 Fabrication of zeolite-filled nanocomposite membranes
11.5 Zeolite–polymer compatibility
11.5.1 Predicting the combination of zeolite and polymer
11.5.1.1 Toward compatible zeolite–polymer mixed matrix membranes
11.5.1.2 Inorganic bridging agents
11.5.1.3 Organic bridging agents
11.5.1.4 Alternative strategies for improving compatibility
11.6 Zeolite–polymer membrane performances in pervaporation
11.7 Conclusions
References
12 Pervaporation and pervaporation-assisted esterification processes using nanocomposite membranes
12.1 Introduction
12.2 Esters and esterification
12.3 Combined esterification and reaction systems
12.4 A unique separation process: pervaporation
12.4.1 General mechanism and basic characteristics of pervaporation process
12.5 Pervaporation membrane reactor
12.6 Membranes for membrane reactors
12.7 Nanocomposite membranes
12.7.1 Nanocomposite membranes for pervaporation and pervaporation-assisted esterification
12.8 Conclusions and future recommendations
References
13 Polymer/metal-organic frameworks membranes and pervaporation
13.1 Introduction
13.2 Preparation methods
13.3 Hydrophobic polymer/metal-organic frameworks membranes for organics recovery
13.3.1 Polydimethylsiloxane/metal-organic frameworks membranes
13.3.2 Poly(ether-block-amide)/metal-organic frameworks membranes
13.3.3 PTMPS/metal-organic frameworks membranes
13.4 Hydrophilic polymer/metal-organic frameworks membrane for organics dehydration
13.4.1 Metal-organic frameworks/poly(vinyl alcohol) membranes
13.4.2 Metal-organic frameworks/polybenzimidazole membranes
13.4.3 Metal-organic frameworks/chitosan membranes
13.5 Challenges and perspectives
13.6 Final remarks
Acknowledgment
References
14 Computational modeling of pervaporation process
14.1 Definition
14.2 Pervaporation performance
14.2.1 Enrichment factor
14.2.2 Separation factor
14.3 Process conditions
14.4 Mass transfer in pervaporation
14.4.1 Pore flow model
14.4.2 Solution-diffusion model
14.4.2.1 Sorption
14.4.2.2 Diffusion
14.4.2.3 Desorption
14.4.3 Modified solution-diffusion model
14.4.4 Thermodynamics model
14.4.5 Maxwell–Stefan model
14.4.6 Computational model
14.5 Transport properties in pervaporation
14.6 Sorption of pure liquid i in an amorphous polymer
14.6.1 Time-dependence of sorption
14.6.2 Time-lag experiment
14.6.3 Inverse gas chromatography method
14.7 Pervaporation modeling
14.7.1 Determination of sorption coefficient (S)
14.7.2 Determination of diffusivity (D)
14.7.3 Determination of permeability (P)
14.7.4 Modified pervaporation process
14.8 Predictive model
14.8.1 Polarity and solubility parameter
14.8.2 Interfacial thermodynamics
14.8.3 Chromatographic property
14.8.4 Contact angle
14.8.5 Physicochemical properties-process conditions
14.9 Conclusion
References
15 Hybrid pervaporation process
15.1 Introduction
15.2 Distillation process
15.3 Hybrid process parameters
15.4 Hybrid distillation–pervaporation process
15.5 Simulations of hybrid distillation–pervaporation process
15.6 Other pervaporation hybrid processes
15.7 Advantages of hybrid pervaporation process
15.8 Conclusion
References
Index
Back Cover

Citation preview

P O LY M E R NANOCOMPOSITE MEMBRANES FOR P E R VA P O R AT I O N

P O LY M E R NANOCOMPOSITE MEMBRANES FOR P E R VA P O R AT I O N Edited by

SABU THOMAS International and Inter University Centre for Nanoscience and Nanotechnology, Mahatma Gandhi University, Kottayam, India

SONEY C. GEORGE Centre for Nano Science and Technology, Amal Jyothi College of Engineering, Kanjirapally, India

THOMASUKUTTY JOSE Department of Basic Sciences, Centre for Nano Science and Technology, Amal Jyothi College of Engineering, Kanjirapally, India

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 © 2020 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. British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library Library of Congress Cataloging-in-Publication Data A catalog record for this book is available from the Library of Congress ISBN: 978-0-12-816785-4 For Information on all Elsevier publications visit our website at https://www.elsevier.com/books-and-journals

Publisher: Matthew Deans Acquisitions Editor: Simon Holt Editorial Project Manager: Mariana C. Henriques Production Project Manager: Prasanna Kalyanaraman Cover Designer: Greg Harris Typeset by MPS Limited, Chennai, India

CONTENTS List of contributors .......................................................................................xi Preface .........................................................................................................xv

1 Polymer nanocomposite membranes for pervaporation: an introduction............................................................................................................1 Thomasukutty Jose, Soney C. George and Sabu Thomas

1.1 Introduction .......................................................................................... 1 1.2 Basic principles of pervaporation ....................................................... 2 1.3 Membranes for pervaporation ............................................................ 6 1.4 Factors affecting the pervaporation .................................................... 9 1.5 Advantages of separation using pervaporation process ................ 13 1.6 Conclusions ........................................................................................ 13 References ................................................................................................. 14

2 Nanocellulose/polymer nanocomposite membranes for pervaporation application ................................................................................17 Jithin Joy, Neenu George, Cintil Jose Chirayil and Runcy Wilson

2.1 Introduction ........................................................................................ 17 2.2 Nanocellulose isolation methods...................................................... 18 2.3 Nanocellulose/polymer nanocomposite membranes for pervaporation application.................................................................. 20 2.4 Conclusions ........................................................................................ 30 References ................................................................................................. 31

3 Biobased (nanochitin, nanochitosan) polymer nanocomposite membranes and their pervaporation applications.....................................35 Geetha Kathiresan and Naveen Rooba Doss M.

3.1 Introduction ........................................................................................ 35 3.2 Pervaporation of chitin and chitosan membranes .......................... 36 3.3 Chitin membranes.............................................................................. 37 3.4 Chitosan membranes ......................................................................... 37 3.5 Conclusion .......................................................................................... 75 References ................................................................................................. 77 Further reading ......................................................................................... 79 v

vi

Contents

4 Pervaporation performance of polymer/clay nanocomposites ...............81 Runcy Wilson and Gejo George

4.1 Introduction ........................................................................................ 81 4.2 Pervaporation characteristics ............................................................ 87 4.3 Factors affecting membrane performance ....................................... 90 4.4 Conclusions ...................................................................................... 100 References ............................................................................................... 100

5 Carbon nanotubes-polymer nanocomposite membranes for pervaporation.....................................................................................................105 Maryam Ahmadzadeh Tofighy and Toraj Mohammadi

Nomenclature.......................................................................................... 105 5.1 Introduction ...................................................................................... 106 5.2 Pervaporation ................................................................................... 108 5.3 Polymer nanocomposites ................................................................ 112 5.4 Carbon nanotubes............................................................................ 113 5.5 PV application of carbon nanotubes-polymer nanocomposite membranes....................................................................................... 116 5.6 Conclusions ...................................................................................... 129 References ............................................................................................... 129

6 Graphene-based polymer nanocomposite membranes for pervaporation.....................................................................................................135 M.B. Sethu Lakshmi and Bincy Francis

6.1 Introduction ...................................................................................... 135 6.2 Graphene .......................................................................................... 137 6.3 Graphene-based membranes for pervaporation ........................... 141 6.4 Conclusions and future aspects ...................................................... 147 References ............................................................................................... 148

7 Fullerene and nanodiamond-based polymer nanocomposite membranes and their pervaporation performances ................................153 Neetha John

7.1 Introduction ...................................................................................... 153 7.2 Pervaporation ................................................................................... 156

Contents

vii

7.3 Membranes for pervaporation ........................................................ 158 7.4 Nanodiamond................................................................................... 160 7.5 Pervaporation performance of fullerenes-based nanocomposite membranes....................................................................................... 162 7.6 Membranes modified with fullerenes and derivatives.................. 164 7.7 Conclusions ...................................................................................... 166 References ............................................................................................... 169

8 Polymer nanocomposite membranes for pervaporation desalination process .......................................................................................175 Deepak Roy George, Shalin Tyni, Asha Elizabeth and Abhinav K. Nair

8.1 Introduction ...................................................................................... 175 8.2 Synthesis methods of polymer nanocomposite pervaporation membranes....................................................................................... 177 8.3 Factors affecting the performance of pervaporation desalination membranes ................................................................. 178 8.4 Polymer membranes for pervaporation desalination ................... 179 8.5 Polymer nanocomposite membranes for pervaporation desalination ...................................................................................... 184 8.6 Conclusion and future aspects ........................................................ 195 References ............................................................................................... 197

9 Polymer/polyhedral oligomeric silsesquioxane nanocomposite membranes for pervaporation .......................................................................201 Valiya Parambath Swapna, Vakkoottil Sivadasan Abhisha and Ranimol Stephen

9.1 9.2 9.3 9.4

Introduction ...................................................................................... 201 Polyhedral oligomeric silsesquioxane ............................................ 205 Pervaporation performance of polymer/POSS membranes ......... 210 Factors affecting the pervaporation through polymer membrane......................................................................................... 214 9.5 Applications of polyhedral oligomeric silsesquioxaneembedded polymeric systems ........................................................ 218 9.6 Challenges and future aspects ........................................................ 220 References ............................................................................................... 221

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Contents

10 Nanometal and metal oxide-based polymer nanocomposite membranes for pervaporation .....................................................................231 Samit Kumar Ray, Amritanshu Banerjee, Swastika Choudhury and Debapriya Pyne

10.1 10.2 10.3 10.4

Introduction ...................................................................................231 Synthesis of polymer nanometal nanocomposite ....................233 Direct use of nanometal and metal oxides as membrane .........239 Nanometal and metal oxide-based polymer metal nanocomposites membranes.......................................................239 10.5 Conclusions ...................................................................................255 Acknowledgment.................................................................................. 257 References............................................................................................. 257

11 Modified zeolite-based polymer nanocomposite membranes for pervaporation ............................................................................................263 I.G. Wenten, K. Khoiruddin, G.T.M. Kadja, Rino R. Mukti and Putu D. Sutrisna

11.1 Introduction ...................................................................................263 11.2 Water and alcohol-selective zeolites............................................264 11.3 Mechanism of water/alcohol separation in zeolite .....................266 11.4 Fabrication of zeolite-filled nanocomposite membranes ...........275 11.5 Zeolite polymer compatibility.....................................................279 11.6 Zeolite polymer membrane performances in pervaporation ...287 11.7 Conclusions ...................................................................................292 References............................................................................................. 293

12 Pervaporation and pervaporation-assisted esterification processes using nanocomposite membranes .........................................301 Yavuz Salt, Berk Tirnakci and Inci Salt

12.1 Introduction ...................................................................................301 12.2 Esters and esterification................................................................302 12.3 Combined esterification and reaction systems...........................305 12.4 A unique separation process: pervaporation ..............................306 12.5 Pervaporation membrane reactor ................................................310 12.6 Membranes for membrane reactors ............................................313 12.7 Nanocomposite membranes ........................................................314 12.8 Conclusions and future recommendations .................................318 References............................................................................................. 321

Contents

ix

13 Polymer/metal-organic frameworks membranes and pervaporation.... 329 Peiyong Qin, Zhihao Si, Houchao Shan and Di Cai

13.1 Introduction ...................................................................................329 13.2 Preparation methods.....................................................................331 13.3 Hydrophobic polymer/metal-organic frameworks membranes for organics recovery ...............................................335 13.4 Hydrophilic polymer/metal-organic frameworks membrane for organics dehydration ..............................................................341 13.5 Challenges and perspectives........................................................345 13.6 Final remarks .................................................................................347 Acknowledgment.................................................................................. 347 References............................................................................................. 348

14 Computational modeling of pervaporation process...............................355 Muhammad Mujiburohman

14.1 Definition........................................................................................355 14.2 Pervaporation performance..........................................................356 14.3 Process conditions ........................................................................357 14.4 Mass transfer in pervaporation ....................................................359 14.5 Transport properties in pervaporation ........................................367 14.6 Sorption of pure liquid i in an amorphous polymer ..................369 14.7 Pervaporation modeling ...............................................................373 14.8 Predictive model............................................................................383 14.9 Conclusion .....................................................................................389 References............................................................................................. 389

15 Hybrid pervaporation process.....................................................................393 Thomasukutty Jose, Soney C. George and Sabu Thomas

15.1 Introduction ...................................................................................393 15.2 Distillation process........................................................................394 15.3 Hybrid process parameters ..........................................................394 15.4 Hybrid distillation pervaporation process .................................396 15.5 Simulations of hybrid distillation pervaporation process ........403 15.6 Other pervaporation hybrid processes ........................................405 15.7 Advantages of hybrid pervaporation process.............................406 15.8 Conclusion .....................................................................................406 References ............................................................................................... 406 Index........................................................................................................... 409

List of contributors Vakkoottil Sivadasan Abhisha Department (Autonomous), Devagiri, Calicut, India

of

Chemistry,

St.

Joseph’s

College

Amritanshu Banerjee Department of Polymer Science & Technology, University of Calcutta, Kolkata, India Di Cai National Energy R&D Center for Biorefinery, Beijing University of Chemical Technology, Beijing, P.R. China Cintil Jose Chirayil Newman College, Thodupuzha, India Swastika Choudhury Department of Polymer Science & Technology, University of Calcutta, Kolkata, India Asha Elizabeth Department of Chemical Engineering, Amal Jyothi College of Engineering, Kottayam, India Bincy Francis PG Department of Chemistry, St. Thomas College, Ranny, India Deepak Roy George Department of Chemical Engineering, Amal Jyothi College of Engineering, Kottayam, India Gejo George School of Pure & Applied Physics, Mahatma Gandhi University, Kottayam, India Neenu George St. Joseph’s College, Moolamattom, India Soney C. George Centre For Nanoscience and Technology, Amal Jyothi College of Engineering, Kanjirapally, India Neetha John Central Institute of Plastics Engineering & Technology (CIPET), Institute of Plastics Technology (IPT), Kochi JNM Campus, Udyogamandal, Kochi, India Thomasukutty Jose Department of Basic Sciences, Centre For Nanoscience and Technology, Amal Jyothi College of Engineering, Kanjirapally, India Jithin Joy Newman College, Thodupuzha, India G.T.M. Kadja Research Center for Nanosciences and Nanotechnology, Institut Teknologi Bandung, Bandung, Indonesia; Division of Inorganic and Physical Chemistry, Institut Teknologi Bandung, Bandung, Indonesia; Center for Catalysis and Reaction Engineering, Institut Teknologi Bandung, Bandung, Indonesia Geetha Kathiresan Nanotechnology Division, Department of Electronics and Communication Engineering, Periyar Maniammai Institute of Science and Technology, Vallam, Thanjavur, India

xi

xii

List of contributors

K. Khoiruddin Department of Chemical Engineering, Institut Teknologi Bandung, Bandung, Indonesia Toraj Mohammadi Department of Chemical, Petroleum and Gas Engineering, Center of Excellence for Membrane Research and Technology, Iran University of Science and Technology (IUST), Narmak, Tehran, Iran Muhammad Mujiburohman Department of Chemical Engineering, Muhammadiyah University of Surakarta, Surakarta, Indonesia Rino R. Mukti Research Center for Nanosciences and Nanotechnology, Institut Teknologi Bandung, Bandung, Indonesia; Division of Inorganic and Physical Chemistry, Institut Teknologi Bandung, Bandung, Indonesia; Center for Catalysis and Reaction Engineering, Institut Teknologi Bandung, Bandung, Indonesia Abhinav K. Nair Department of Chemical Engineering, Amal Jyothi College of Engineering, Kottayam, India Naveen Rooba Doss M. Nanotechnology Division, Department of Electronics and Communication Engineering, Periyar Maniammai Institute of Science and Technology, Vallam, Thanjavur, India Debapriya Pyne Department of Polymer Science & Technology, University of Calcutta, Kolkata, India Peiyong Qin National Energy R&D Center for Biorefinery, Beijing University of Chemical Technology, Beijing, P.R. China Samit Kumar Ray Department of Polymer Science & Technology, University of Calcutta, Kolkata, India Inci Salt Department of Chemical Engineering, Yildiz Technical University, Esenler, Istanbul, Turkey Yavuz Salt Department of Chemical Engineering, Yildiz Technical University, Esenler, Istanbul, Turkey M.B. Sethu Lakshmi Research and PG Department of Chemistry, N.S.S. Hindu College, Changanacherry, India Houchao Shan National Energy R&D Center for Biorefinery, Beijing University of Chemical Technology, Beijing, P.R. China Zhihao Si National Energy R&D Center for Biorefinery, Beijing University of Chemical Technology, Beijing, P.R. China Ranimol Stephen Department of Chemistry, St. Joseph’s College (Autonomous), Devagiri, Calicut, India Putu D. Sutrisna Department of Chemical Engineering, University of Surabaya (UBAYA), Surabaya, Indonesia

List of contributors

xiii

Valiya Parambath Swapna Department of Chemistry, St. Joseph’s College (Autonomous), Devagiri, Calicut, India Sabu Thomas International and Inter University Centre Nanotechnology, Mahatma Gandhi University, Kottayam, India

for

Nanoscience

and

Berk Tirnakci Department of Chemical Engineering, Yildiz Technical University, Esenler, Istanbul, Turkey Maryam Ahmadzadeh Tofighy Department of Chemical, Petroleum and Gas Engineering, Center of Excellence for Membrane Research and Technology, Iran University of Science and Technology (IUST), Narmak, Tehran, Iran Shalin Tyni Department of Chemical Engineering, Amal Jyothi College of Engineering, Kottayam, India I.G. Wenten Department of Chemical Engineering, Institut Teknologi Bandung, Bandung, Indonesia; Research Center for Nanosciences and Nanotechnology, Institut Teknologi Bandung, Bandung, Indonesia Runcy Wilson Department of Chemistry, St. Cyrils College, Adoor, India

Preface The new era of membrane technology is keen to develop efficient ways to reduce industrial pollution and energy consumption. Pervaporation is one of the most energy-efficient methods to develop sustainable separation and purification systems. Permeation and evaporation combine to get a costeffective and pollution-free alternative to conventional separation processes such as distillation. Polymer nanocomposite membranes give more viability to these membrane-based separation technologies. The polymer nanocomposite membranes and their application in pervaporation are the prime area of research to develop new energy-efficient and ecofriendly separation and purification strategies. The recent advancement in polymer nanocomposite membranes and the pervaporation process prompted us to summarize the results in a collective way. The book gives detailed insight into different polymer nanocomposite membranes and their role in pervaporation separation processes. It consists of 15 chapters including a brief introduction about the pervaporation process. The first four chapters exclusively deal with the 21st century nanomaterials such as nanocellulose, nanochitin, and nanoclay-based nanocomposite membranes and its pervaporation applications. The pervaporation performance of nanocomposite membranes with different nanoscale allotropes of carbon (graphene, carbon nanotubes, fullerene, and nanodiamond) is well explained in fifth to seventh chapters. Desalination is another significant area of research, and one chapter is for pervaporation-based desalination processes. Nanocomposite membranes with different nanomaterials such as POSS, nanometal and metal oxides, and modified zeolites and their pervaporation performance are explained in subsequent chapters. The chapters based on computational modeling of the pervaporation and hybrid pervaporation processes add attraction to the readers. The chapters provide detailed insights to young researchers and industrialist to know more about different nanocomposite membranes and their pervaporation applications. The enormous support and help of all the contributors to the book are well appreciated. We gratefully acknowledge the great efforts of all the reviewers who reviewed the chapters in

xv

xvi

Preface

the agreed time. A very special word of thanks to the editorial team members of Elsevier for their guidance and continuous support in this venture. We indebted to the support, guidance, and motivation of our management and colleagues. We hope that the book gives a wonderful experience to the readers who focused on theoretical and experimental aspects of pervaporation.

Polymer nanocomposite membranes for pervaporation: an introduction

1

Thomasukutty Jose1, Soney C. George2 and Sabu Thomas3 1

Department of Basic Sciences, Centre For Nanoscience and Technology, Amal Jyothi College of Engineering, Kanjirapally, India 2Centre For Nanoscience and Technology, Amal Jyothi College of Engineering, Kanjirapally, India 3International and Inter University Centre for Nanoscience and Nanotechnology, Mahatma Gandhi University, Kottayam, India

1.1

Introduction

One of the most significant membrane separation processes is pervaporation (PV). It is the only membrane process in which a liquid-to-vapor phase transition occurs through a dense or microporous membrane during the transport of solvents. The partial pressure difference exist between the membrane is the real driving force behind the phase transition and it can be due to the vacuum at the permeate side. The efficiency of separation is obtained by selective diffusion of components from a liquid mixture through the membrane. PV is an energy efficient separation method, especially for the separation of azeotropic, isomeric, and close boiling point liquids. The azeotropes or close boiling point liquids can be separated by a classical distillation process where separation is based on the differences in their relative volatility. The separation of different mixtures occurs on the basis of their affinity with the membrane materials in PV. Another prime condition for the PV separation of azeotrops and other close boiling liquids is that they have clear difference in their transport characteristics through membrane material. So, PV is a useful technique for the separation of azeotropic mixtures [1], close boiling point mixtures [2], and structural isomers [3]. PV is mainly applied for the separation and concentration of mixtures that are difficult to separate by distillation [4]. The transport properties of water through polymer membranes are much more different than that of organic Polymer Nanocomposite Membranes for Pervaporation. DOI: https://doi.org/10.1016/B978-0-12-816785-4.00001-X © 2020 Elsevier Inc. All rights reserved.

1

2

Chapter 1 Polymer nanocomposite membranes for pervaporation: an introduction

solvents. Thus the separation of water
organic mixtures by PV is getting more attention due to their difference in transport characteristics. Selection of proper polymers and preparation of a suitable membrane with high performance are the main factors in PV. Many researchers developed different polymeric membranes for the PV separation of different organic
water mixtures [5,6]. The use of PV for the separation of azeotropic mixtures is first introduced by Aptel et al. [7] in 1976 and after that PV is potentially applied for the separation of various liquid mixtures. However, the concept of PV is first introduced by Kober [8] in 1917 for the selective permeation of water from aqueous solutions of albumin and toluene through cellulose nitrate films. He observed the “permselective evaporation” of components through the membrane and was abbreviated as “pervaporation.” But the systematic and potential approach toward PV was first carried out by Binning et al. for the separation of hydrocarbon mixtures through dense polythene films and observed that the linear hydrocarbons permeate faster than branched isomers [9,10]. Then a large number of hydrophilic and organophilic membranes were developed and extensively applied for the dehydration of organic solvents and separation of organic solvents, respectively. Polymeric-based hydrophilic membranes are widely used for the PV dehydration of organic solvents such as alcohols, ketones, acids, and ethers [11]. Polydimethylsiloxane (PDMS)-based hydrophobic membranes were developed and used for the extraction of organic compounds from water mixtures [12,13]. Gesellschaft fur Trenntechnik introduced first commercialized PV membranes by coating poly(vinyl alcohol) (PVA) on a porous support of poly(acrylonitrile) [14].

1.2

Basic principles of pervaporation

PV of membranes is described basically by solution-diffusion mechanism, and mass transport in PV is explained by two models: (1) solution-diffusion model and (2) pore flow model.

1.2.1

Solution-diffusion model

The solution-diffusion model was first proposed by Thomas Graham to explain gas transport through the diaphragms. In this model, there are three consecutive steps in PV: (1) permeate selectively sorbed from the feed liquid to the membrane, (2) diffusion of the permeate through the membranes, and (3) desorption of the permeate component to the vapor phase

Chapter 1 Polymer nanocomposite membranes for pervaporation: an introduction

3

Figure 1.1 Schematic representation of solutiondiffusion model for pervaporation. Source: Adapted from Jose et al., Ind. Eng. Chem. Res. 2014, 53, 43, 16820
16831. Copyright r 2014, American Chemical Society.

(Fig. 1.1) [15]. The physicochemical interactions of the membrane materials and the permeating molecules play a vital role in PV separation. Sorption selectivity and/or diffusion selectivity of the permeating molecules are the key factors that affects PV performances. The permeating molecules which have more interaction with the membrane favors sorption selectivity and is basically identified by the solubility parameter of the membrane and the penetrants.

1.2.2

Pore flow model

Okada and Matsuura [15] proposed the pore flow model which is an alternative to investigate the mass transport in PV. According to this model, the PV taking place through the cylindrical pores exists in the membranes and micropores favors better PV separation. The main steps in pore-flow model are (1) the permeant transports through the liquid/vapor-filled portion of the pore and (2) the phase change (liquid to vapor) occur inside the pore. Fig. 1.2 describes the PV transport through the asymmetric hollow fiber [16]. The penetrating length, liquid-filled section of the pore for liquid-phase transport of the permeants, and vapor-filled section for vapor-phase transport are well explained in the figure. The PV performance is a complied effect of transport through the liquid and vapor phases, and the evaporation is occurring at the boundary region of liquid and vapor sections. The basic theory of PV is well explained by many researchers. Drioli and Giorno explained the mass transport through membranes in the book “Encyclopedia of membranes” [17]. Baker et al. [18], Neel et al. [19], Feng and Huang [20], etc. are prime scientists who studied and explained the different theoretical aspects of PV separation technology.

4

Chapter 1 Polymer nanocomposite membranes for pervaporation: an introduction

Figure 1.2 Schematic representation of the pore-flow model in pervaporation transport within asymmetric hollow fiber membranes. Source: Adapted from P. Sukitpaneenit, T.S. Chung, L.Y. Jiang, Journal of Membrane Science 362 (2010) 393
406, Copyright r 2010 Elsevier B.V. All rights reserved.

Recently in 2010 Baker et al. [18] introduced new terms called permeability, permeance, and selectivity to understand the real membrane intrinsic properties. The basic terms such as flux and separation factor lag the intrinsic, driving force normalized properties behind the separation. In this chapter, we correlated the flux and separation factor with permeability and intrinsic selectivity, respectively, to get more idea on real membrane intrinsic properties.

1.2.3

Permeability: a normalized flux

The efficiency of the membrane to separate a particular component from the mixture is determined by flux ( J ), permeability or permeance, and these factors are associated with the weight of the permeate (Q) in grams or kilograms. J5

Q At

ð1:1Þ

The effective area of the membranes (in m2) and time (t in h) used for separation are also influenced the permeation flux. Component flux is the another factor which depends on the permeate composition of one of the components and total flux. Ji 5 J Xi and Jj 5 J Yj

ð1:2Þ

Chapter 1 Polymer nanocomposite membranes for pervaporation: an introduction

Ji and Jj are the component fluxes, J is the flux, Xi and X are the permeate composition of the mixtures. Permeability of the membrane is component fluxes normalized for membrane thickness and are given from the following equation: Permeabilty ðPiG Þ 5 Ji

l Pi0 2 Pil

ð1:3Þ

Permeance is a thickness normalized component and is expressed as Permeance ðPiG =lÞ 5

Ji Pi0 2 Pil

ð1:4Þ

It is most commonly reported as gas permeation unit (gpu) (1 gpu 5 3.349 3 10210 mol m22 s21 Pa21).

1.2.4

Selectivity: an intrinsic membrane properties

Separation factor (α), of the PV is defined as 

XB =YB α5 XA =YA

 ð1:5Þ

XA and XB are the composition of mixture “X” in the feed and permeate, respectively. YA and YB are the composition of component “Y” in the feed and permeate, respectively. The preferential sorption of selective component increases the separation efficiency of the PV process. The enrichment factor is also used to describe the selective separation of preferentially sorbed component and is expressed as  β5

XB XA

 ð1:6Þ

The intrinsic membrane performance is analyzed by the parameter, selectivity, and it is the ratio of permeability of component i by component j. αij 5

PiG PjG

ð1:7Þ

The membrane intrinsic properties obtained in molar terms and are concentration dependent, even if the driving force contribution is removed. This is the main advantage of the selectivity, permeance, and permeability.

5

6

Chapter 1 Polymer nanocomposite membranes for pervaporation: an introduction

1.3

Membranes for pervaporation

The membranes used in PV process are usually dense or microporous polymeric membranes which allow the passage of selective components through it. The membrane material would be chemically and thermally stable in nature and able to transport selective components at elevated temperature. In this regard, the stiff and rigid polymer chains have been considered as potential candidate for PV application [21]. The selection of polymeric material for PV mainly depends on the desired application. The different application areas and selection of suitable membrane type are summarized in Fig. 1.3. PVA, chitosan, and sodium alginate are mainly used PV membranes for the separation of water from aqueous
organic mixtures [22,23]. Polyimides (PIs), polybenzimidazole (PBI), and PDMS-based membranes are used for hydrophobic or organophilic PV separation [24,25]. Kung et al. [26] reported PBI/PI blend membranes for the separation of toluene/isooctane mixtures. The hydrophobic PV is very useful in many fields and the numbers of polymeric hydrophobic membranes are used for the recovery of organics, but it looks unattractive due to their poor performance.

Figure 1.3 Pervaporation membrane types and its application areas.

Chapter 1 Polymer nanocomposite membranes for pervaporation: an introduction

The hydrophilic membranes are widely used for the PV separation of different mixtures. The PV membranes are broadly categorized into (1) inorganic membranes, (2) mixed matrix membranes (MMMs), and (3) polymeric membranes,

1.3.1

Inorganic membranes

Zeolites, silica, titania, and zirconia are the main inorganic membranes used for the PV separation of water from organic mixtures. The inorganic membranes offer superior thermal and chemical stability compared to polymers. Veen et al. [27] reported the dewatering of solvents such as ethanol, methylethylketone, DMF, THF, and ethyl acetate using silica membranes. Silica membranes exhibited a permeation flux of 1485 g m22 h21 with process selectivity of 350 for the dewatering of ethanol. Micorporous silica and doped silica membranes showed excellent alcohol dehydration [28]. The 10% zirconia doped silica membranes showed a flux of 0.86 kg m22 h21 with separation factor 300 for the dehydration of isopropanol/water (90/10 wt.%) mixtures. Zeolite membranes are one of the extensively studied inorganic membranes for PV process. Both the hydrophilic and hydrophobic zeolites exhibited better PV performance. Sodium alginate hydrophilic zeolite membranes effectively used for the dehydration of alcohols with higher separation factor [29,30]. The silicate-1 hydrophobic zeolites were used for the removal of organic solvents from water [31
33]. Matsufuji et al. [34] observed a separation factor of 270 for the separation of n-hexane/2,3-DMB using ZSM-5 zeolite membranes, but the separation flux decreases with membrane concentration and is due to the presence of Al2O3 support. The uniform, molecular-sized pores cause significant difference in the transport of various solvents, but the high cost of zeolite limited their applications [35].

1.3.2

Mixed matrix membranes

To combine the strengths of inorganic fillers and polymeric membranes, MMMs were used for the PV applications. The porous magnesium oxide (MgO) particle-incorporated Matrimid matrix membranes were used for the PV dehydration of IPA and the membranes showed higher selectivity with 15 wt.% MgO loading [36]. The SiO2-reinforced polyelectrolyte complexes (PECs) membranes showed good performance for the IPA dehydration [37]. The 5 wt.% SiO2-loaded PECs membranes exhibited a separation flux of 2.3 kg m22 h21 and separation factor of

7

8

Chapter 1 Polymer nanocomposite membranes for pervaporation: an introduction

1721 at 75 C with a feed composition of 10 wt.% water in IPA. Numerous inorganic fillers such as zeolite [38,39], silica [40], MgO, more recently polyhedral oligosilsesquioxane (POSS) [41,42], etc. were incorporated into the polymer matrix and the resultant membranes showed promising PV separation performance. Metal-organic frameworks (MOFs) are other membrane components with controllable pore sizes and high porosity, and they get considerable attraction in PV application due to its exceptional thermal and chemical stability [43,44].

1.3.3

Polymer membranes

Polymeric membranes are widely used for the PV separation of various mixtures because of its excellent chemical stability, ease of processability, better mechanical properties, thermal stability, etc. The polymeric membranes are used for the dehydration of organic solvents [45], the removal of organic components from water [46,47], biofuels from fermentation broth [48,49], the separation of organic liquid mixtures [50
52], and the desulfurization of gasoline [53]. The PV separation efficiency largely depends on the nature, size, and molecular structure of the polymers. The natural and synthetic polymers exhibited excellent PV performance. Chitosan and sodium alginate are two naturally occurring polymers that are widely used in PV process. Chitosan-based membranes are widely used for the dehydration of organics and removal of alcohols from organics due to its excellent film forming and hydrophilic nature [54
56]. The UV cross-linked chitosan/polyvinyl pyrrolidone blend membranes showed a PSI of 167 for the separation of water
ethanol mixtures [57]. Sodium alginate (NaAlg) is a naturally occurring hydrophilic polymer which is widely used in separation of water
organic mixtures [58,59]. But the main drawback of NaAlg is its high swelling which reduces the selectivity and mechanical strength, and hence it is not favorable for commercial applications. Synthetic polymers such as PIs [60,61], poly(ether amides) [62], polyurethanes [63], poly(methyl methacrylates) (PMMA) [64], PDMS [65], polyacrylates [66], polyesters, polyethylenimines [67], etc. are used for the PV separation of various mixtures. Ribeiro et al. [47] synthesized poly(siloxane-co-imide) membranes for the PV separation of toluene/n-heptane mixtures and found that the incorporation of siloxane in the polymer greatly improved the PV performance. The polypropylene-, polyvinylidene fluoride-, and PMMA-based membranes were used for the PV separation of benzene and cyclohexane

Chapter 1 Polymer nanocomposite membranes for pervaporation: an introduction

mixtures [68]. The PV dehydration of 90% ethanol was carried out using PVA-carboxylated chitosan and achieved a separation factor of 1231 at 30 C [69].

1.4

Factors affecting the pervaporation

The main factors that affect the efficiency of separation are as follows.

1.4.1

Pressure

Feed and permeate pressure are the main driving force for PV. The activity of the components at the downstream side of the membrane and the permeate pressure is in direct relation which strongly influence the PV performances. Eqs. (1.3) and (1.4) show the relationship between the partial pressure and the permeation flux. The maximum gradient can be obtained at zero permeate pressure. Moulik et al. [70] studied the effect of pressure on the PV process and observed that the driving force for methanol transportation decreases at low vacuum which slows down the desorption rate of molecules. Hence Separation factor and permeation decrease with low vacuum conditions.

1.4.2

Concentration polarization and partition coefficient

PV performances are also affected by the partition coefficient (Fig. 1.4). According to physical sciences, partition coefficient is the ratio of concentrations of a compound in a mixture of two immiscible solvents at equilibrium. But, in PV, it represents an equilibrium state of one solute (permeate) at the interphase of membrane phase and solution phase. Qiu et al. [71] studied the effect of partition coefficient on the PV separation of ethanol and observed that the ethanol flux was increased with partition coefficient. The increase in partition coefficient increased the mass transfer during PV which is mainly due to the strong polymer and solvent interaction. From Fig. 1.5, it is observed that the concentration polarization coefficient has much effect on flux. Higher concentration polarization coefficient has bad effect on increased flux. This could be overcome by choose a membrane with more solvent interaction.

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Chapter 1 Polymer nanocomposite membranes for pervaporation: an introduction

Figure 1.4 Effect of partition coefficient on ethanol pervaporation, ethanol concentration: 5 wt.%; temperature: 45˚ C. Source: Adapted from B. Qiu, Y. Wang, S. Fan, J. Liu, S. Jian, Y. Qin, et al. Sep. Purif. Technol. 220 (2019) 276
282. Copyright r 2019 Elsevier B.V. All rights reserved.

1.4.3

Temperature

The permeation performance can be described as a function of temperature that obeys following Arrhenius-type relationship:   Ea J 5 Jo exp 2 Rg T where J is the permeate flux (kg m22 h21), Ea is the activation energy (J/mol) associated with the permeate process, Rg is the gas constant (J mol21 K21), and T is the absolute feed temperature (K). As the feed temperature increases, the thermal motions of the polymer chains are stimulated and permeate molecules become more energetic, and hence permeation rate increases. Thus the permeation flux increases with rise in temperature. The activation energy of the process can be obtained from the semilogarithmic plots of the permeate fluxes against the reciprocal of the absolute temperature (1/T). From Table 1.1, it is observed that the permeation flux increases with increase in feed temperature. But, the separation factor decreases significantly with increase in temperature. This is due to the fact that

Chapter 1 Polymer nanocomposite membranes for pervaporation: an introduction

11

Figure 1.5 Total flux against PVA layer thickness for 10% and 40% feed ethanol mixtures at 25˚C. PVA, Poly(vinyl alcohol). Reproduced with permission from M.N. Hyder, R.Y.M. Huang, P. Chen, J. Membr. Sci. 318 (2008) 387
396, Copyright r 2008 Elsevier B.V. All rights reserved.

the random thermal motion increased the free volume available for permeation of the molecules. The increase in free volume in the membrane provides more space for the molecule to diffuse which in turn decrease the selective permeation of one of the component through the membrane. Hence, the permeation flux increases, but selectivity decreases significantly.

12

Chapter 1 Polymer nanocomposite membranes for pervaporation: an introduction

Table 1.1 Effect on feed composition, temperature, and thickness of the membrane on the PV characteristics. Membrane type

Feed composition Feed Thickness Flux temperature (µm) (kg m22 h21) (˚C)

Silicate 2 1PDMS

5 wt.% dilute ethanol 40 aqueous solution 50 60 50 Composite Acetonitrile
water 20 ceramic PDMS mixture membrane 55 30 Ternary azeotropic Buckypapermixture of ethyl supported acetate/ethanol/ ionic liquid/ water (9 wt.% water) PVA membrane 60 Ethanol
water 70 PVA-PSf mixture composite membrane 70

Separation Ref. factor

5

3.62

16.5

[72]

5 5 15


5.52 6.83 1.94 0.225

15.5 14.9 26.3 28

[72]





0.353 0.385

17 246.65

[72]


2.5

0.537 0.039

105.66

4

0.096

Notes: PDMS, Polydimethylsiloxane; PVA, poly(vinyl alcohol).

1.4.4

Membrane thickness

Membrane thickness is another important parameter which affects the PV performances. The permeation rate of a component is inversely proportional to the membrane thickness because as the membrane thickness increases, the permeation resistance increases. Thin membranes give higher permeation fluxes and is due to the fact that the permeate molecules have to cross less diffusive path inside the membrane and mass transfer become easier through a thinner membrane [72]. Fig. 1.5 shows that the total flux decreased with increase in membrane thickness.

Chapter 1 Polymer nanocomposite membranes for pervaporation: an introduction

1.5

Advantages of separation using pervaporation process

PV is an ideal alternative to conventional distillation process. The main advantages of the PV process are • The solvent mixture cannot be counted with their vapor
liquid equilibrium. • Azeotropes can be easily separated without adding any additional components. • Solvents with same volatility can be separated. The preferential sorption and diffusion of selective components through the membranes is the determination factor in the PV process rather than vapor
liquid equilibria. The recovery of the dissolved solids and their components are another important advantage of PV. Thus the recovery of solvents and their reuse reduce the production of industrial waste streams. Solvent recovery by conventional activated carbon adsorption on printing and coating operations results in recovered solvents heavily diluted with water. This causes disposal problems and it can also be reduced by PV process.

1.6

Conclusions

The present book is exclusively dealt with the PV performance of different polymer nanocomposite membranes. The current research interest in the field of nanocomposite membranes and PV is the focal theme of the book. The book consists of 15 chapters including brief introduction about PV process. Chapter 2, Nanocellulose/polymer nanocomposite membranes for pervaporation application is based on nanocellulose-based PNC and its PV performance. Nanocellulose is a versatile material in the 21st century and finds many applications in different research and industrial fields. Another biomaterials such as nanochitin/nanochitosan-based membranes and their PV performances are well explained in Chapter 3, Biobased (nanochitin, nanochitosan) polymer nanocomposite membranes and their pervaporation applications. Chapter 4, Pervaporation performance of polymer/clay nanocomposites explains the effect of nanoclay on the PV performance of different polymeric membranes. The following chapters provides detailed insights on the PV performances of carbon nanotubes, graphene, fullerene, nanodiamond, POSS, nanometal and metal oxide, and

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14

Chapter 1 Polymer nanocomposite membranes for pervaporation: an introduction

modified zeolite nanocomposite membranes. Other chapters deals with the desalination, theoretical modeling of PV, hybrid PV process, and PV-aided esterification processes. Thus the book gives a wonderful experience to the readers who focused on theoretical and experimental aspects of PV.

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396.

Nanocellulose/polymer nanocomposite membranes for pervaporation application

2

Jithin Joy1, Neenu George2, Cintil Jose Chirayil1 and Runcy Wilson3 1

Newman College, Thodupuzha, India 2St. Joseph’s College, Moolamattom, India 3Department of Chemistry, St. Cyril’s College, Adoor, India

2.1

Introduction

Pervaporation (PV) is an efficient membrane process for liquid separation. The past decades had witnessed substantial progress and exciting breakthrough in both the fundamental and the application aspect of PV. The thermodynamic approach of PV, featuring emphasizing membrane/species interactions, though gained great successes in the past decades, is now facing its toughest challenge in the org org separation. A kinetic era of PV, featuring emphasizing diffusion selectivity, as well as the synergy between the selective diffusion and sorption, is in the making, and this approach will eventually find solutions to the challenging org org separation [1 3].

2.1.1

Design and choice of membrane materials for pervaporation

For the practical application of PV, the membrane must have a high permeation rate and a large separation factor. To obtain a higher permeation rate, an improved permeability coefficient is necessary. Although a so-called trade-off relationship exists between permeability and selectivity, that is, high selectivity is generally accompanied by low permeability, acceptable membrane materials with both high permeability and high selectivity may be synthesized by polymer design. Generally diffusion of small molecules through a dense membrane is favored, and the solubility of a compound in a polymer is governed by the chemical affinity between the penetrant and Polymer Nanocomposite Membranes for Pervaporation. DOI: https://doi.org/10.1016/B978-0-12-816785-4.00002-1 © 2020 Elsevier Inc. All rights reserved.

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the membrane. Therefore when the difference in the molecular size of two components to be separated in a mixture is large, permeation through the membrane may be preferential for the small component, although the solubility of the big component in the membrane is large. For this reason, many polymers are preferentially permeable to water rather than organic components because the water molecule is much smaller than the organic molecules. An organic selective membrane must have very high solubility for organic molecules. Some work on the design of membrane materials has been published. It discussed the swelling ratio or concentration in a membrane and the separation behavior of an alcohol solution with dense membranes such as silicone rubber blends, neutral and charged copolymers composed of hydrophilic and hydrophobic monomers, crosslinked and modified PVC, etc. They found that the permeation rates vary in proportion to degree of swelling, and that the separation factors are independent of concentration in membranes in most cases [4].

2.2

Nanocellulose isolation methods

Managing biomass could transform waste into a valuable product. Moreover, plant waste is often burned, thus processing it avoids air pollution. The combination of a strong attraction between lignin layers, low chemical accessibility to the cellulose surface, and high cellulose crystallinity retard biomass digestion. A multistep biorefinery process can be used to degrade the noncellulosic content in the lignocellulosic biomass while preserving cellulose, followed by hydrolysis to produce the nanocellulose material. In practice, a combination of certain chemicals and mechanical treatments are commonly applied to rupture the biomass structure and obtain nanocellulose. Typically nanocellulose is isolated from biomass through mechanical, chemical or enzymatic treatments, all of these will be discussed in detail subsequently. However, it is necessary to draw attention to a new method, which is particularly environmental friendly. Poplar wood floor was mixed with different deep eutectic solvents and then subjected to microwave irradiation for 3 min [5]. The 80% of the lignin content was extracted, leaving behind a cellulosic residue that was 75% crystalline nanocellulose. The major benefits of this separation technique are the low energy consumption of the microwave treatment and the use of recyclable, biosourced deep eutectic solvents.

Chapter 2 Nanocellulose/polymer nanocomposite membranes for pervaporation application

To summarize nanocellulose isolation methods and the obtained products: • Sulfuric acid hydrolysis is the most frequently used technique to isolate CNC from biomass. • Acid hydrolysis is used in combination with a mechanical treatment to produce CNF. • CNCs range in diameter from 5 to 35 nm and have a length of a few hundred nanometers. CNFs on the other hand have larger diameters of between 3 and 100 nm and are micrometers in length. • Both CNCs and CNFs are extensively used as reinforcements in polymer matrices, high lighting their prominence in composite applications. Nanoscale cellulosic materials are extracted from various plant fibers (biomass) and bacterial sources. International research aims to extract a valuable resource, nanocellulose, while efficiently managing biowaste. All agricultural, forest crops and residue sources can be referred to as lignocellulosic biomass [6]. Lignocellulosic biomass is composed of cellulose, hemicellulose, and lignin of vegetal origin [7]. Lignocellulosic biomass contains 30 50 wt.% cellulose [8], 19 45 wt.% hemicellulos, and 15 35 wt.% lignin. Together these polysaccharides form a hetero matrix that varies in its composition and structure as a function of the biomass source. Cotton is an almost pure form of cellulose with .90% composition. Complex physicochemical interactions are responsible for forming the hierarchical lignocellulosic biomass structure and for shielding the cellulose, hemicellulose, and lignin from the environment. Lignin covalently cross-links cellulose and hemicelluloses via ester and ether linkages. This cross-linking restricts structural disintegration, referred to as lignocellulosic biomass recalcitrance. Lignocellulose is the most abundant natural biopolymer. Cellulose extracted from different sources including sugar beet [9 13], potato tuber cell [14,15], lemon and maize [16], coir [17,18], banana [19 21], bagasse [22,23], wheat straw [24], jute [25 27], hemp [28], kenaf fibers [29], aquatic weeds [30,31], oil palm [32], soy hull [33], pineapple leaf [34], mulberry [35], cassava [22], alpha fiber [36], rice husk [37,38], lotus leaf stalk [39], and cotton [40]. As well as various other renewable resources exhibited varying reinforcing ability indicating the strong correlation of the aspect ratio, crystallinity, morphology, and properties of cellulose with the raw material and the used extraction process [41,42].

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2.3

Nanocellulose/polymer nanocomposite membranes for pervaporation application

Nanocomposites are interesting class of materials in which nanomaterials are reinforced and the nanomaterial has at least one dimension up to 100 nm. Polymer nanocomposites have received great deal of interests among the scientific and industrial communities in recent years because these materials can achieve significant improvements in mechanical properties, dimensional stability, and solvent or gas barrier properties with respect to the matrix polymer at very low concentration of fillers as compared with continuous phase [43 45]. Nanosized cellulosic materials can be a good nanofillers for the fabrication in the polymer nanocomposites to improve the overall performance (in terms of mechanical properties and other functional properties viz. partial degradation, improvement of barrier properties, etc.) of nanocomposites. Nanocellulosic materials have been received significant research interest as potential nanofiller for the reinforcements in the polymer matrices due to its renewable in nature, readily availability, biocompatibility, inexpensive, excellent physical properties, tailorable surface properties, etc. [46 48]. The main goal for the addition of fillers to polymer focuses on the enhancement in the mechanical and electrical properties, heat resistance and permeation barrier properties of polymer [49,50]. Cellulose nanocrystals (NCC) have been successfully used as fillers in polymer matrices and some systems are being commercialized. Cellulose fibers are easily obtained from biomass and are abundant, renewable, biodegradable resource. NCC is the crystalline portion obtained after acid hydrolysis of cellulose [51,52]. Recently NCC have attracted much attention from researchers for their remarkable reinforcing abilities, and it also had been successfully used as fillers in polymer matrices for the application of barrier membranes. Here we discuss important nanocellulose/polymer nanocomposites membranes for PV applications.

2.3.1

Cellulose-polydimethylsiloxane blends for pervaporation

Peinemann et al. presented a novel method for the preparation of cellulose-polydimethyl siloxane (PDMS) blend membranes for organic solvent nanofiltration (OSN) and ethanol PV [53]. The elegance of this approach is on the use of trimethyl silyl cellulose (TMSC), a hydrophobic cellulose derivative, for

Chapter 2 Nanocellulose/polymer nanocomposite membranes for pervaporation application

the blend membrane fabrication followed by a simple hydrolysis to convert the TMSC back into cellulose. The use of TMSC does not only allow cellulose processing in the common organic solvents but also enables the creation of highly compatible cellulose-PDMS blend membranes. The proper composition of the blend membrane gave the best OSN performance with more than 10 L m22 h21 bar acetone permeance and around 750 Da molecular weight cutoff. The blend membrane can be used for OSN with any other solvents regardless their polarity. The same membrane also displayed an excellent PV performance with a separation factor of 14 and a flux of 1.6 kg m22 h21 flux using a 5 wt.% ethanol water mixture at room temperature. The blending of PDMS with cellulose resulted in a 100% increase of the separation factor when compared with pure PDMS.

2.3.2

Cellulose/poly(vinyl alcohol) membranes for pervaporation

Zhang et al. prepared PV membranes from poly(vinyl alcohol) (PVA) with different amounts of NCC as filler, and characterized by scanning electron microscopy [54]. The characterization results demonstrated that NCC particles dispersed homogeneously within the PVA matrix. Moreover, the PV performance of these membranes was investigated using the separation of ethanol water mixture as model system. Among all the prepared membranes, PVA/cellulose nanocomposite membrane containing 1 wt.% NCC exhibited the best PV performance, whose averaged permeation flux reduced slightly but separation factor was increased from 83 to 163 for 80% aqueous solution of ethanol at 80 C, respectively. PV performance of PVA/NCC hybrid membranes can be explained as follows. The NCC content effects on the PV performance for an 80 wt.% ethanol aqueous solution through the PVA/NCC nanocomposite membranes are shown in Fig. 2.1. The figure shows that the permeation rate decreased and the separation factor increased with the increasing NCC content when the NCC content is lower than 1 wt.% in the PVA/NCC nanocomposite membranes. However, the separation factor decreases sharply when the NCC content becomes higher than 1 wt.% in the PVA/NCC nanocomposites membranes. This is due to the fact that the large amount of hydroxyl groups on cellulose nanofiber surfaces would form strong hydrogen bonding with PVA matrix, resulting in good adhesion at the fiber/PVA interfaces. Moreover, the resistance of the molecular diffusion

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Chapter 2 Nanocellulose/polymer nanocomposite membranes for pervaporation application

600

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Figure 2.1 Effect of NCC content on pervaporation performance of PVA/NCC membrane [54]. NCC, Cellulose nanocrystal; PVA, poly(vinyl alcohol).

and tortuosity of the diffusion pathway, resulting from the decreasing of the degree of swelling, thus the permeation rate decreased and the separation factor increased. However, the separation factor decreases sharply while the flux increases when the NCC content higher than 1 wt.%, which was possibly due to agglomeration of the NCC and the consequent formation of a separate NCC phase and reduced NCC content in the matrix Operation temperature effect on PV performance can be explained as follows. The PV performance of membranes was influenced not only by the structure of membranes but also by the characteristics of separation components and operation parameters. To investigate the effect of operation temperature on the permeation flux and separation factor, PVA/NCC membrane with the best PV performance was chosen as model membrane. PV experiments were conducted in 80 wt.% aqueous solution of ethanol at 50 C 80 C, and the results are shown in Fig. 2.2. It can be seen that, the permeation flux of PVA/NCC membrane increased with the increasing of temperature, while the separation factor decreased remarkably. The mobility of PVA chains increased with the increase in temperature, which led to the increase in free volume in the membrane. This would result in quicker transportation of feed mixtures, and thus the increased permeation flux of the membrane. As for the separation factor of membrane, the diffusivity difference between water molecules and ethanol molecules played an important role. It was found that the diffusion coefficient of ethanol

Chapter 2 Nanocellulose/polymer nanocomposite membranes for pervaporation application

23

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350 300 300 200 250 100

0

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50

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Figure 2.2 Effect of operation temperature on the pervaporation performance of PVA/NCC nanocomposite [54]. NCC, Cellulose nanocrystal; PVA, poly(vinyl alcohol).

increased faster than that of water, therefore the separation factor decreased with the increase of feed temperature.

2.3.3

Cellulose acetate/polyacrylonitrile membranes for pervaporation

Xiao et al. prepared cellulose acetate/polyacrylonitrile (CA/ PAN) membranes and used to separate pyridine/water mixtures by PV [55]. The membranes were characterized through SEM. The effects of feed concentration, operation temperature, and downstream pressure on this partition performance were evaluated. Experimental results indicated the increase in operation temperature could raise the permeation flux and the separation factor. Under the conditions that pyridine solution was 99 wt.%, operation temperature was 323K and downstream pressure was 20 mmHg, the CA/PAN blend membrane showed its best separation performance. Fig. 2.3 shows the variation of flux and separation factor versus feed composition through CA/PAN blend membrane under the conditions of 298K and 30 mmHg. It could be seen that with increasing pyridine concentration in the feed, the total flux decreased; whereas, separation factor increased. Fig. 2.3 also shows that the flux of water decreases when the concentration of water decreases. The higher water concentration in the feed mixture, more swollen the amorphous regions of the membrane were and more flexible the polymer chains became. This made both

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Chapter 2 Nanocellulose/polymer nanocomposite membranes for pervaporation application

200

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J/g m–2 h–1

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140 120 100 80 60 40

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0.8

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0.7

0.8

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Pyridine in feed (wt.%)

Figure 2.3 Effect of pyridine concentration on separation performance [55].

water molecules and pyridine molecules more easily penetrate through membranes, so the total permeation flux and the partial fluxes increased.

2.3.4

C60-filled ethyl cellulose hybrid membranes for pervaporation

Kong et al. prepared C60-filled ethyl cellulose (EC) hybrid membrane for removing sulfur from model gasoline by PV [56]. The transmission electron microscopy (TEM) image of the microstructure of EC/C60 hybrid membrane showed the existence of C60 clusters in the hybrid membrane, which could resemble the path of electron-rich gasoline components through the membrane (Fig. 2.4). The result of the PV performance using the EC membrane and the EC/C60 hybrid membrane showed that permeation flux increased as the operating temperature increased, but the sulfur enrichment factor increased first and then decreased. The change of cross-linking time did not influence the variation trends of permeation flux and sulfur enrichment factor with operating temperature. In addition, the linear fitting results proved that sound linearity existed between the permeation flux and the operating temperature; the apparent PV activation energy of EC/C60 hybrid membrane was lower than that of the pure EC membrane. Therefore for the C60-filled EC hybrid membrane, the addition of C60 could contribute to increasing the flux, and the reasonable operating temperature was 75 C.

Chapter 2 Nanocellulose/polymer nanocomposite membranes for pervaporation application

Figure 2.4 Microstructure of the EC/C60 hybrid membrane [56].

2.3.5

Cellulose acetate membrane filled with metal oxide particles for pervaporation

Wang et al. prepared a new PV CA membrane filled with metal oxide particles to intensify the separation of methyl tertbutyl ether (MTBE)/methanol mixtures [57]. The SEM and Raman spectrometry revealed that the particles of metal oxide were evenly dispersed in the membrane matrix and made a denser membrane. When the content of Al2O3 and ZnO was 1.98 and 4 wt.%, respectively, both the permeation flux and separation factor of blended CA membrane were higher than those of pure CA membrane. In comparison with the pure CA membrane, the maximum flux of blended membrane filled with Al2O3 and ZnO improved 96.5% and 111.1%, respectively; the maximum separation factor improved 48.0% and 37.8%, respectively. In addition to the substantially improved permeation flux and selectivity, the CA membrane filled with metal oxide was found easy to prepare and with low cost. Effect of mass fraction of Al2O3 on separation performance can be explained as follows. The separation factors and flux of CA membrane filled with different mass fractions of Al2O3 are shown in Fig. 2.5. When the mass fraction of Al2O3 was 1.98%, the separation factors were 859.3, which was larger than the separation factor of pure CA membrane, 560.4. When the mass fraction of Al2O3 was 3.21%, the flux reached a maximum; when the mass fraction of Al2O3 was 1.98%, the flux was 2.50 kg m22 h21, which was nearly twice that of pure CA, 1.27 kg m22 h21. The adding of metal oxide particles made the membrane dense,

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Chapter 2 Nanocellulose/polymer nanocomposite membranes for pervaporation application

4

1000

Figure 2.5 Effect of Al2O3 weight fraction on separation performance [57].

800

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CA membrane filled AI2O3

0

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which slowed down the diffusion of methanol and MTBE. However, the methanol molecules has interaction with the Lewis acid site on the surface of Al2O3 particles, as a result, the solubility of methanol in the CA membrane filled with metal oxide become larger than that in pure CA membrane. The product flux is determined by solubility and diffusion rate, in this study a greater increase of solubility and a relatively lower decrease of diffusion rate were believed to obtain with the increase of metal oxide in the membrane. When the metal oxide content was too high, there were some faults in the structure of the blended membrane, which decreased the separation efficiency. It was found that the real flux value was within 5% of the error of the average value of three groups of data.

2.3.6

Ethyl cellulose reinforced with natural zeolite membranes for evaporation

PV of ethanol 2 water can be cost-competitive in the production of renewable biomass ethanol [58]. For the purpose of improving the PV performance of polymeric membranes, we prepared CA-filled Lampung Natural Zeolite (LNZ) membranes by incorporating LNZ into CA for PV separation of ethanol water mixtures. The characteristics and performance of these filled membranes in the varied ratio of CA:LNZ (30:0, 30:5, 30:10, 30:20, 20:20, and 40:10 wt.%) were investigated. The prepared membranes were characterized for PV membrane performance such as percentage of water content and membrane

Chapter 2 Nanocellulose/polymer nanocomposite membranes for pervaporation application

27

swelling degree. Further, the permeation flux and selectivity of membrane were also observed. The results of investigation showed that water content of membrane tends to increase with increase of LNZ content. However, the swelling degree of membrane decrease compared than that of CA control membrane. The permeation flux and the selectivity of membranes tend to increase continuously. The CA membrane with ratio of CA:LNZ 30:20 shows the highest selectivity of 80.42 with a permeation flux of 0.986 kg m22 h21 and ethanol concentration of 99.08 wt.%. Permeation and selectivity of membranes can be explained as follows. The results of permeation flux test and selectivity of membrane shows in Fig. 2.6. An increase of LNZ addition from 5% to 10% resulted in an increase of membrane total flux. In contrast to LNZ addition to 20% in the casting solution, decreases membrane flux. Similar results were obtained in the ratio of CA/LNZ 20:20 and 40:10. This result occurs due to the thickness of the synthesized membrane were reduced by increasing CA concentration. Increasing CA concentration from 20% to 40% and LNZ loading from 10% to 20% intensifies thermodynamic instability of the cast film solution and thus remixing of this concentrated solution can be performed with less amount of nonsolvent. It also observed from the experiment, viscosity values of the casting solutions are too high and thus it affects diffusivities of ingredients. Increase in the CA concentration 30% to 40% increases in viscosity values and intensive reduction of mutual diffusivities between the 3.0

2.5

30:10

2.0 1.5

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3.0

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Figure 2.6 Effect of addition of LNZ into CA solution in varied ratio of CA/LNZ to pervaporation performance [58]. CA, Cellulose acetate; LNZ, Lampung Natural Zeolite.

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Chapter 2 Nanocellulose/polymer nanocomposite membranes for pervaporation application

nonsolvent (water) and the solvent (acetone) in the system during solidification of the casting solution. Thus it influences the precipitation process and this leads to preparation of thinner and denser membranes. Differ with flux permeation value, the selectivity of membrane in ratio of CA/LNZ 30:10 decreases and it were decreasing the dehydration performance of ethanol (97.67%). However, when the ratio of CA/LNZ 30:20 the selectivity of membrane was higher than the separation factor of pure CA and other CA:LNZ ratio. Based on the results, it was observed that increase of LNZ loading into membrane casting solution made hydrophilicity of the membrane changes the membrane morphology in a manner that the flux permeation increase. This can be explained that the transport of ethanol molecules through the CA-filled LNZ membranes took place in a straight path through the zeolite pores and led to increase of the separation factor. Increase in separation factor has achieved when the LNZ content was 20 wt.%. The increase in separation factor may have been attributed to the pore size effect of LNZ and a close contact between the polymer and LNZ particles. When excess LNZ was added, more interfacial defects could be generated. Consequently more ethanol molecules got the opportunity to pass through the membrane.

2.3.7

Ethyl cellulose membranes for pervaporation of water, hydrazine, and monomethyl hydrazine

EC membrane has been selected on the basis of Hansen’s solubility parameter and Flory Huggins interaction parameter for the enrichment of hydrazine and monomethyl hydrazine (MMH) liquid propellants by PV [59]. An extensive study of the overall mass transfer resistance experienced by the permeants has been conducted by Khanb et al. The resistance values were quantitatively estimated by changing the membrane thickness and calculating the corresponding flux. Due to its lower sorption, and fewer interactions, the membrane showed least desorption resistance toward water and thus it is permselective with respect to water. Results of PV selectivity obtained in separation of water hydrazine and water MMH mixtures at azeotropic compositions have been correlated. Higher sorption of MMH and hydrazine did not result in preferential separation in spite of lower membrane

Chapter 2 Nanocellulose/polymer nanocomposite membranes for pervaporation application

resistance. Experimental results clearly showed that desorption resistance and diffusivity were predominant over the respective solubilities.

2.3.8

Blend membranes of sodium alginate and (hydroxyethyl) cellulose for pervaporation

In an effort to improve membrane performance of pristine sodium alginate (NaAlg) for 2-propanoldehydration, blend membranes of NaAlg with (hydroxyethyl) cellulose (HEC) (5, 10, and 20 mass %) were prepared [60]. Membranes were prepared by solution casting and cross-linked by a two stage process as confirmed by Fourier transform infrared spectroscopy. The blend membrane of NaAlg with 10 mass % HEC gave the highest selectivity of 63,000 for 5 mass % water in feed mixture (highest selectivity achieved so far in the literature) by removing 99.97% of water, giving a flux of 0.04 kg m22 h21. Incorporation of ZSM-5(40) zeolite in the blend membrane increased flux without affecting selectivity. Sorption and diffusion selectivity values were computed from experimental data, which were comparable with the theoretically calculated values obtained from thermodynamic treatment based on Flory Huggins theory. They also studied membrane solvent interactions.

2.3.9

Ethyl cellulose reinforced with TiO2 membranes for pervaporation

The TiO2 nanoparticles were incorporated into an EC matrix to improve the PV performance of the membrane for gasoline desulfurization by Hou et al. [61]. The microstructures of different EC membranes were characterized by scanning electron microscopy, Fourier transform infrared spectroscopy, X-ray, and TEM. The PV experiments showed that the hybrid membrane of EC/TiO2 demonstrated an improved permeation flux ( J ) of 7.58 kg m22 h21 and a sulfur enrichment factor (a) of 3.13 in comparison with the pure EC membrane, with a J of 3.73 kg m22 h21 and an a of 3.69. In addition, the effects of the operating conditions, including the operating temperature, layer thickness, cross-linking time, feed flow rate, and feed sulfur content level, on the PV performance of the EC/TiO2 membrane were investigated. Under a 100 mL min21 feed flow rate and a 85 lg g21 sulfur content, J of the 10 lm thick membrane increased to 7.58 kg m22 h21 compared to the pure

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Chapter 2 Nanocellulose/polymer nanocomposite membranes for pervaporation application

EC membrane (3.73 kg m22 h21, 3.69) at 80.8 C with 30 min of cross-linking time.

2.3.10

Bacterial cellulose/alginate blend membranes for pervaporation

Phisalaphong et al. prepared bacterial cellulose/alginate blend membranes for PV applications [62]. Bacterial cellulose and alginate in an aqueous NaOH/urea solution were used as substrate materials for the fabrication of a novel blend membrane. The blend solution was cast onto a Teflon plate, coagulated in a 5 wt.% CaCl2 aqueous solution, and then treated with a 1% HCl solution. Supercritical carbon dioxide drying was then applied for the formation of a nanoporous structure. The physical properties and morphology of the regenerated bacterial cellulose and blend membranes were characterized. The blend membrane with 80% bacterial cellulose/20 wt.% alginate displayed a homogeneous structure and exhibited a better water adsorption capacity and water vapor transmission rate. However, the tensile strength and elongation at break of the film with a thickness of 0.09 mm slightly decreased to 3.38 MPa and 31.60%, respectively. The average pore size of the blend membrane was ˚ with a 19.50 m2 g21 surface area. 10.60 A

2.4

Conclusions

Advances in nanotechnology have opened up new opportunities for the fabrication of novel nanocomposites with improved properties. Nanocellulose as novel nanofiller has potential for tailoring the structure and properties of matrix polymers. Current development of polymer nanocomposites based on NCC as potential nanofillers in polymer matrix has been reviewed and discussed. In this chapter, we attempted to provide an overview of various methods for nanocellulose-reinforced polymer nanocomposites for PV applications. The focus was on membranes for critical technologies with improved mechanical and thermal properties that have the necessary capabilities to solve the problems of a selective PV. For the purpose of directional changes in the parameters of membranes, the effects on their properties of the type, amount, and conditions of nanoparticle incorporation into the polymer matrix were reviewed. An influence of nanoparticles on the structural and morphological characteristics of the nanocomposite membranes was considered, as well as possibilities of forming transport channels for separated liquids

Chapter 2 Nanocellulose/polymer nanocomposite membranes for pervaporation application

are analyzed. Particular attention is paid to a correlation of nanocomposites structure transport properties of membranes, whose separation characteristics are usually considered within the framework of the diffusion sorption mechanism.

References [1] S. Zhang, E. Drioli, Pervaporation Membranes, Sep. Sci. Technol. 30 (2006) 1 31. [2] J. Neel, Fundamentals of pervaporation for ethanol/water separation, Membr. Alternative. Energy Implic. Ind. (1990) 59. [3] J.L. Rapin The Betheniville pervaporation unit, the first large-scale productive plant for the dehydration of ethanol, in: 3rd International Conference on Pervaporation in the Chemical Industry, 1988, pp. 364 378. [4] R. Hughes, Membrane science and technology, Membr. Sci. Technol. (1994) 467. [5] Y. Liu, W. Chen, Q. Xia, B. Guo, Q. Wang, S. Liu, et al., Efficient cleavage of lignin carbohydrate complexes and ultrafast extraction of lignin oligomers from wood biomass by microwave-assisted treatment with deep eutectic solvent, ChemSusChem 10 (2017) 1692 1700. [6] C.J. Chirayil, L. Mathew, S. Thomas, Review of recent research in nano cellulose preparation from different lignocellulosic fibers, Rev. Adv. Mater. Sci. 37 (2014) 20 28. [7] H.K. Sharma, C. Xu, W. Qin, Biological pretreatment of lignocellulosic biomass for biofuels and bioproducts: an overview, Waste Biomass Valoriz. 10 (2019) 235 251. ¨ ttermann, Sustainable global energy supply based on [8] J.O. Metzger, A. Hu lignocellulosic biomass from afforestation of degraded areas, Naturwissenschaften 96 (2009) 279 288. [9] E.D.M. Teixeira, T.J. Bondancia, K.B.R. Teodoro, A.C. Correˆa, J.M. Marconcini, L.H.C. Mattoso, Sugarcane bagasse whiskers: extraction and characterizations, Ind. Crop. Prod. 33 (2011) 63 66. [10] S.M. Costa, P.G. Mazzola, J.C.A.R. Silva, R. Pahl, A. Pessoa, S.A. Costa, Use of sugar cane straw as a source of cellulose for textile fiber production, Ind. Crop. Prod. 42 (2013) 189 194. [11] A. Mandal, D. Chakrabarty, Isolation of nanocellulose from waste sugarcane bagasse (SCB) and its characterization, Carbohydr. Polym. 86 (2011) 1291 1299. [12] M. Ghaderi, M. Mousavi, H. Yousefi, M. Labbafi, All-cellulose nanocomposite film made from bagasse cellulose nanofibers for food packaging application, Carbohydr. Polym. 104 (2014) 59 65. [13] M. Li, L.J. Wang, D. Li, Y.L. Cheng, B. Adhikari, Preparation and characterization of cellulose nanofibers from de-pectinated sugar beet pulp, Carbohydr. Polym. 102 (2014) 136 143. [14] D. Chen, D. Lawton, M.R.R. Thompson, Q. Liu, Biocomposites reinforced with cellulose nanocrystals derived from potato peel waste, Carbohydr. Polym. 90 (2012) 709 716. [15] S. Arun, K. a A. Kumar, M.S. Sreekala, Fully biodegradable potato starch composites: effect of macro and nano fiber reinforcement on mechanical, thermal and water-sorption characteristics, Int. J. Plast. Technol. 16 (2012) 50 66.

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[16] C. Rondeau-Mouro, B. Bouchet, B. Pontoire, P. Robert, J. Mazoyer, A. Bule´on, Structural features and potential texturising properties of lemon and maize cellulose microfibrils, Carbohydr. Polym. 53 (2003) 241 252. [17] D. Verma, P.C. Gope, A. Shandilya, A. Gupta, M.K. Maheshwari, Coir fibre reinforcement and application in polymer composites: a review, J. Mater. Env. Sci. 4 (2013) 263 276. [18] E. Abraham, B. Deepa, L.A. Pothen, J. Cintil, S. Thomas, M.J. John, et al., Environmental friendly method for the extraction of coir fibre and isolation of nanofibre, Carbohydr. Polym. 92 (2013) 1477 1483. [19] S. Elanthikkal, U. Gopalakrishnapanicker, S. Varghese, J.T. Guthrie, Cellulose microfibres produced from banana plant wastes: isolation and characterization, Carbohydr. Polym. 80 (2010) 852 859. [20] N.M. Ennig, M.K. Ottaisamy, S.A.B.U.T. Homas, A novel method for the synthesis of cellulose nanofibril whiskers from banana fibers and characterization, J. Agric. Food Chem. 56 (2008) 5617 5627. [21] B. Deepa, E. Abraham, B.M. Cherian, A. Bismarck, J.J. Blaker, L.A. Pothan, et al., Structure, morphology and thermal characteristics of banana nano fibers obtained by steam explosion, Bioresour. Technol. 102 (2011) 1988 1997. [22] E.D.M. Teixeira, D. Pasquini, A.A.S. Curvelo, E. Corradini, M.N. Belgacem, A. Dufresne, Cassava bagasse cellulose nanofibrils reinforced thermoplastic cassava starch, Carbohydr. Polym. 78 (2009) 422 431. [23] F. Zhu, Composition, structure, physicochemical properties, and modifications of cassava starch, Carbohydr. Polym. 122 (2014) 456 480. [24] J. Liu, Q. Li, Y. Su, Q. Yue, B. Gao, Characterization and swelling-deswelling properties of wheat straw cellulose based semi-IPNs hydrogel, Carbohydr. Polym. 107 (2014) 232 240. [25] N. Soykeabkaew, P. Supaphol, R. Rujiravanit, Preparation and characterization of jute- and flax-reinforced starch-based composite foams, Carbohydr. Polym. 58 (2004) 53 63. [26] U.S. Ishiaku, O.A. Khondker, S. Baba, A. Nakai, H. Hamada, Processing and characterization of short-fiber reinforced jute/poly butylene succinate biodegradable composites: The effect of weld-line, J. Polym. Env. 13 (2005) 151 157. [27] N. Kasyapi, V. Chaudhary, A.K. Bhowmick, Bionanowhiskers from jute: preparation and characterization, Carbohydr. Polym. 92 (2013) 1116 1123. [28] R. Moriana, F. Vilaplana, S. Karlsson, A. Ribes, Correlation of chemical, structural and thermal properties of natural fibres for their sustainable exploitation, Carbohydr. Polym. 112 (2014) 422 431. [29] S. Gan, S. Zakaria, C.H. Chia, H. Kaco, F.N.M. Padzil, Synthesis of kenaf cellulose carbamate using microwave irradiation for preparation of cellulose membrane, Carbohydr. Polym. 106 (2014) 160 165. [30] M. Thiripura Sundari, A. Ramesh, Isolation and characterization of cellulose nanofibers from the aquatic weed water hyacinth—Eichhornia crassipes, Carbohydr. Polym. 87 (2012) 1701 1705. [31] T. Mochochoko, O.S. Oluwafemi, D.N. Jumbam, S.P. Songca, Green synthesis of silver nanoparticles using cellulose extracted from an aquatic weed; water hyacinth, Carbohydr. Polym. 98 (2013) 290 294. [32] M.K.M. Haafiz, A. Hassan, Z. Zakaria, I.M. Inuwa, Isolation and characterization of cellulose nanowhiskers from oil palm biomass microcrystalline cellulose, Carbohydr. Polym. 103 (2014) 119 125.

Chapter 2 Nanocellulose/polymer nanocomposite membranes for pervaporation application

[33] W.P. Flauzino Neto, H.A. Silve´rio, N.O. Dantas, D. Pasquini, Extraction and characterization of cellulose nanocrystals from agro-industrial residue soy hulls, Ind. Crop. Prod. 42 (2013) 480 488. [34] B.M. Cherian, A.L. Lea˜o, S.F. de Souza, S. Thomas, L.A. Pothan, M. Kottaisamy, Isolation of nanocellulose from pineapple leaf fibres by steam explosion, Carbohydr. Polym. 81 (2010) 720 725. [35] R. Li, J. Fei, Y. Cai, Y. Li, J. Feng, J. Yao, Cellulose whiskers extracted from mulberry: a novel biomass production, Carbohydr. Polym. 76 (2009) 94 99. [36] D. Trache, A. Donnot, K. Khimeche, R. Benelmir, N. Brosse, Physico-chemical properties and thermal stability of microcrystalline cellulose isolated from Alfa fibres, Carbohydr. Polym. 104 (2014) 223 230. [37] N. Johar, I. Ahmad, A. Dufresne, Extraction, preparation and characterization of cellulose fibres and nanocrystals from rice husk, Ind. Crop. Prod. 37 (2012) 93 99. [38] S.M.L. Rosa, N. Rehman, M.I.G. de Miranda, S.M.B. Nachtigall, C.I.D. Bica, Chlorine-free extraction of cellulose from rice husk and whisker isolation, Carbohydr. Polym. 87 (2012) 1131 1138. [39] Y. Chen, Q. Wu, B. Huang, M. Huang, X. Ai, Isolation and characteristics of cellulose and nanocellulose from lotus leaf stalk agro-wastes, BioResources 10 (2015) 684 696. [40] E. Morais Teixeira, A.C. Correˆa, A. Manzoli, F. Lima Leite, C.R. Oliveira, L.H. C. Mattoso, Cellulose nanofibers from white and naturally colored cotton fibers, Cellulose 17 (2010) 595 606. [41] S.-Y. Lee, D.J. Mohan, I.-A. Kang, G.-H. Doh, S. Lee, S.O. Han, Nanocellulose reinforced PVA composite films: effects of acid treatment and filler loading, Fibers Polym. 10 (2009) 77 82. [42] A. Limayem, S.C. Ricke, Lignocellulosic biomass for bioethanol production: current perspectives, potential issues and future prospects, Prog. Energy Combust. Sci. 38 (2012) 449 467. [43] S. Mondal, Polymer nano-composite membranes, J. Membr. Sci. Technol. 5 (2014) 1. [44] S.V. Kononova, G.N. Gubanova, E.N. Korytkova, D.A. Sapegin, K. Setnickova, R. Petrychkovych, et al., Polymer nanocomposite membranes, Appl. Sci. 8 (2018) 1181. [45] V.E. Yudin, V.M. Svetlichnyi, Effect of the structure and shape of filler nanoparticles on the physical properties of polyimide composites, Russ. J. Gen. Chem. 80 (2010) 2157 2169. [46] O.Y. Golubeva, V.E. Yudin, A.L. Didenko, V.M. Svetlichnyi, V.V. Gusarov, Nanocomposites based on polyimide thermoplastics and magnesium silicate nanoparticles with montmorillonite structure, Russ. J. Appl. Chem. 80 (2007) 106 109. [47] I.V. Gofman, V.M. Svetlichnyi, V.E. Yudin, A.V. Dobrodumov, A.L. Didenko, I.V. Abalov, et al., Modification of films of heat-resistant polyimides by adding hydrosilicate and carbon nanoparticles of various geometries, Russ. J. Gen. Chem. 77 (2007) 1158 1163. [48] V.E. Yudin, J.U. Otaigbe, S.I. Nazarenko, W.D. Kim, E.N. Korytkova, A comparative study on the mechanical and barrier characteristics of polyimide nanocomposite films filled with nanoparticles of planar and tubular morphology, Mech. Compos. Mater. 47 (2011) 335. [49] A.K. Bledzki, J. Gassan, Composites reinforced with cellulose based fibres, Prog. Polym. Sci. 24 (1999) 221 274.

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[50] A. Chanachai, R. Jiraratananon, D. Uttapap, G.Y. Moon, W.A. Anderson, R.Y. M. Huang, Pervaporation with chitosan/hydroxyethylcellulose (CS/HEC) blended membranes, J. Membr. Sci. 166 (2000) 271 280. [51] T. Zimmermann, E. Po¨hler, T. Geiger, Cellulose fibrils for polymer reinforcement, Adv. Eng. Mater. 6 (2004) 754 761. [52] W.J. Orts, J. Shey, S.H. Imam, G.M. Glenn, M.E. Guttman, J.F. Revol, Application of cellulose microfibrils in polymer nanocomposites, J. Polym. Env. 13 (2005) 301 306. [53] T. Puspasari, T. Chakrabarty, G. Genduso, K.V. Peinemann, Unique cellulose/polydimethylsiloxane blends as an advanced hybrid material for organic solvent nanofiltration and pervaporation membranes, J. Mater. Chem. A 6 (2018) 13685 13695. [54] L. Bai, P. Qu, S. Li, Y. Gao, L.P. Zhang, Poly (vinyl alcohol)/cellulose nanocomposite pervaporation membranes for ethanol dehydration, Mater. Sci. Forum (2011) 383 386. [55] J.H. Lv, G.M. Xiao, Dehydration of water/pyridine mixtures by pervaporation using cellulose acetate/polyacrylonitrile blend membrane, Water Sci. Technol. 63 (2011) 1695 1700. [56] S. Sha, Y. Kong, J. Yang, The pervaporation performance of C60-filled ethyl cellulose hybrid membrane for gasoline desulfurization: effect of operating temperature, Energy Fuels 26 (2012) 6925 6929. [57] Y. Wang, L. Yang, G. Luo, Y. Dai, Preparation of cellulose acetate membrane filled with metal oxide particles for the pervaporation separation of methanol/methyl tert-butyl ether mixtures, Chem. Eng. J. 146 (2009) 6 10. [58] D.A. Iryani, N.F. Wulandari, A.W. Cindradewi, S.B. Ginting, E. Ernawati, U. Hasanudin, Lampung natural zeolite filled cellulose acetate membrane for pervaporation of ethanol-water mixtures, Earth Environ. Sci. (2018) 012013. [59] R. Ravindra, S. Sridhar, A.A. Khan, A.K. Rao, Pervaporation of water, hydrazine and monomethylhydrazine using ethylcellulose membranes, Polym. (Guildf.) 41 (2000) 2795 2806. [60] B.V.K. Naidu, T.M. Aminabhavi, Pervaporation separation of water/2propanol mixtures by use of the blend membranes of sodium alginate and (hydroxyethyl) cellulose: roles of permeate 2 membrane interactions, zeolite filling, and membrane swelling, Ind. Eng. Chem. Res. 44 (2005) 7481 7489. [61] Y. Hou, M. Liu, Y. Huang, L. Zhao, J. Wang, Q. Cheng, et al., Gasoline desulfurization by a TiO2-filled ethyl cellulose pervaporation membrane, J. Appl. Polym. Sci. (2017) 134. [62] M. Phisalaphong, T. Suwanmajo, P. Tammarate, Synthesis and characterization of bacterial cellulose/alginate blend membranes, J. Appl. Polym. Sci. 107 (2008) 3419 3424.

Biobased (nanochitin, nanochitosan) polymer nanocomposite membranes and their pervaporation applications

3

Geetha Kathiresan and Naveen Rooba Doss M. Nanotechnology Division, Department of Electronics and Communication Engineering, Periyar Maniammai Institute of Science and Technology, Vallam, Thanjavur, India

3.1

Introduction

Chitin is a polysaccharide of glucosamine having structure similar to cellulose [1]. It is the second most widely available polymer in the world next to cellulose, present in the exoskeleton of arthopods as microfibrils and in some fungi and yeast [2]. Strucutrally it is defined as poly(β-(1 4)-N-acetyl-D-glucosamine). The main difference from cellulose is the existence of acetamide groups (-NHCOCH3) at C-2 positions in chitins [3]. The structure of chitosan is shown in Fig. 3.1. About 10 billion tonnes of chitin is produced annually by the molluscs, crustaceans, fungi, algae, and insects [4]. Even though chitin as a biomaterial possess many high level properties such as biorenewabllity, biocompatibility, biodegrading, etc., the chelating or sequestrating property of chitin is highly recognized. This property enables the material to create stable water soluble metal complex which aids in several

Figure 3.1 Chemical structure of chitosan.

Polymer Nanocomposite Membranes for Pervaporation. DOI: https://doi.org/10.1016/B978-0-12-816785-4.00003-3 © 2020 Elsevier Inc. All rights reserved.

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Chapter 3 Biobased (nanochitin, nanochitosan) polymer nanocomposite membranes

applications such as separation of liquid mixtures. Another main property of chitin that makes it unique is its hydrophobicity which is attributed to the highly stretched hydrogen bonds in its structure [5]. This property ensures insolubility of chitin in most organic solvents and aqueous solvents. Chitosan is obtained by partial deactylation from chitin under alkaline conditions (or) by combination of deacetylation and depolymerization of chitin (or) by hydrolysis using enzymes along with chitin deacetylase [5]. The degree of acetylation ranges from 70% to 95% which has a direct effect on the number of free amine groups present in the structure. Unlike chitin, chitosan structure is more similar to glycosaminoglycan [6]. Similar to chitin, chitosan has more or less same advantages but significant differences do exist. The chemical structure of chitosan is primarily made up of linear β(1 4) glycosidic linkages and consists of amino and hydroxyl groups [7]. It is interesting to note that unlike chitin, chitosan is highly hydrophilic and highly crystalline in nature [8]. Other important properties of chitosan that make it applicable in wide areas are its film-forming ability, solubility in wide range of organic solvents, adsorbing nature, biocompatible, antimicrobial property, and ability to form complex structures on chemical activation. The important applications of chitosan include its use as membranes for dehydration of alcohols and other organic liquids, removal of metal ions from liquids, wound dressing, tissue engineering, food and feed material, and cosmetics [9 12].

3.2

Pervaporation of chitin and chitosan membranes

Pervaporation is a novel technique used for separation of liquid mixtures such as dehydration of alcohols, separation of organic organic liquid mixtures, separation of water from organic liquid, etc. The method is highly advantageous than distillation as the principle of separation of liquid mixtures is based on partial vaporization of liquid from other through a membrane. This is aided by differential pressure at entry and exit sides of membrane. In most of the cases the pressure at entry is equal to atmospheric pressure, whereas the pressure at exit is under vacuum. The separation can be best explained by sorption diffusion model, and the effective boundary layer formed during these steps also has an impact in the separation process [13 15].

Chapter 3 Biobased (nanochitin, nanochitosan) polymer nanocomposite membranes

The membranes used for pervaporation are generally classified into organic, inorganic, and organic inorganic membranes. The organic membranes have better functionality but lack stability, whereas the case is reverse for inorganic membranes. The hybrid organic inorganic membranes have better flux properties than both organic and inorganic membranes. Both chitin and chitosan membranes come under the category of organic membranes. Even though the latter is derived from the former, chitosan membranes possess better mechanical stability and film-forming character.

3.3

Chitin membranes

Chitin membranes were first prepared by Sannan et al. [16] using combination of dichloroacetic acid and formic acid. Several other solvents such as trichloroacetic acid and dichloroethane [17], N,N-dimethyl acetamide, N-methyl-2-pyrrolidone, and lithium chloride solutions (DMA/NMP/LiCl) have been used. Aiba et al. [18] studied chitin membrane prepared using the above solvents followed by coagulation in 2-propanol. The chitin membranes prepared possessed greater water absorption capacity than pristine chitin powder. The significant increase is due to the fact that the crystallinity of the chitin membranes decreased and has more amorphous nature. Also it is understood that during sorption study, the hydrogen bond present in the chitin structure was broken down by water and this led to increase in the sorption behavior. The permeability coefficients for the prepared membranes were studied and were compared with cuprophane. Of all the chitin membranes prepared and studied, the membrane prepared using DMA had the higher permeability coefficient. The tensile strength of the membranes did not differ much between them and has values in the range 2.8 3.3 MPa. The chitin membranes exhibited good properties but in the wet state, the properties declined. On the other hand, the chitosan membranes were significantly much better than chitin membranes.

3.4

Chitosan membranes

Chitosan membrane being a polymer-based membranes comes under the category of organic membranes used for pervaporation. Generally chitosan membranes are prepared by solution casting method. In this, the first step involves treatment of chitosan with acetic acid to form a solution, and the

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Chapter 3 Biobased (nanochitin, nanochitosan) polymer nanocomposite membranes

second step involves treatment in sodium hydroxide or ammonium hydroxide such that pH becomes B7.5. Finally the solution obtained is cast and cured for a particular period of time depending on the curing temperature. The acetic acid is selected because the degree of protonation caused is high and the viscosity of the chitosan solution remains constant even at high concentrations, whereas when the same solution was prepared with hydrochloric acid, there was change in viscosity and separation of salts were formed due to increase in ionic concentration [19]. Other weak acids such as citric acid, malic acid, and lactic acid have also been used as solvents for chitosan but the tensile strength of the membranes formed was less compared to the membrane formed using acetic acid [20]. The second step is carried out to create flocculation for membrane formation which is achieved by deprotonation effect produced by the bases and remove acetate formation. The degree of deacetylation of chitosan affects its crystallinity. The increase in degree of deacetylation increases crystallinity of chitosan but it is found that the separation of liquid is independent of degree of deacetylation as separation happens through the amorphous regions of the membrane [21].

3.4.1

Modified chitosan membranes for pervaporation

Chitosan membranes due to their high hydrophilicity absorb more water from liquid mixtures. This results in increase in degree of swelling of membrane and as a result permeation rate gets increased but the selectivity of membrane gets decreased. This affects the yield of the pervaporation process [22]. Clasen et al. [23] reported that the permeability of the membrane can be improved by introduction of micropores but this will bring down the mechanical strength of the membrane. High swelling character of chitosan membrane also leads to drop in mechanical strength. Hence, chitosan membranes used for pervaporation have been modified using several materials to produce cross-linking within the membrane and added material to compensate the loss in mechanical strength. Modified chitosan membranes (Fig. 3.2) have been reported to have better separation factors. One of the reasons behind this increase is that the change in structure effected by crosslinking, which results in restriction of movement of larger molecular sizes of liquids through the membrane (Fig. 3.3). This cross-linking of chitosan can be achieved by treating chitosan

Chapter 3 Biobased (nanochitin, nanochitosan) polymer nanocomposite membranes

39

Cross-linking

Organic modification

Inorganic modification Chitosan

Polyelectrolyte complex

Organic and inorganic modification

Figure 3.2 Various modification routes for chitosan.

Direction of permeation Other molecule Water Modified chitosan membrane Permeated water

with different organic, inorganic, or both materials. So, the modified chitosan membranes can be broadly classified accordingly as, 1. chitosan/organic membranes, 2. chitosan/inorganic membranes, and 3. chitosan hybrid membranes.

3.4.2

Chitosan/organic membranes

Chitosan membranes have been interlinked with many organic compounds, mostly polymers for improvement in membrane structural strength, hydrophilicity, and selectivity during pervaporation. This interlinking can be achieved by blending/mixing chitosan into another polymer but in most of the cases, the interlinking is effected by a third medium which

Figure 3.3 Illustration of permeation of water molecules through modified chitosan membrane.

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Chapter 3 Biobased (nanochitin, nanochitosan) polymer nanocomposite membranes

can be either a reactive compound or a coupling force. The strength of interlinking and the selection of interlinking polymer/component always depend on the nature of modifications in amine and hydroxyl groups in the chitosan. In some cases the interlinking will be observed only at the surface than on the whole membrane. Hence, the component is selected based on the area of application. The modification induced by polymer/organic compound in chitosan membranes is often studied by several routes.

3.4.2.1 Chitosan/polybenzoimidazole membrane Han et al. [24] studied chitosan polybenzoimidazole membrane for dehydration of isopropanol. In this study, the interlinking between chitosan and polybenzoimidazole was achieved by using 4-isocyanato-40-(3,30-dimethyl-2, 4-dioxo-azetidino) diphenyl methane (IDD). The isocyanate group in IDD gets attached to the secondary amine groups in polybenzoimidazole, whereas the azetidino-2,4-dino group in IDD gets attached to the primary amine groups in the chitosan. The chitosan gets grafted to the surface of the polybenzoimidazole membrane by the IDD. This chitosan polybenzoimidazole membrane has increased hydrophilicity which is observed by change in contact angle from 82 to 68 degrees. The permeability study using 70 wt.% of isopropanol/water mixture revealed that the permeability of chitosan polybenzoimidazole membrane was 35.8% more than polybenzoimidazole membrane with slight increase in selectivity values (10 21). The reason is because the dissolution rate of water is more than that of isopropanol due to the increased hydrophilicity due to the addition of chitosan. The chitosan adsorbed on the surface of the polybenzoimidazole membrane enables faster water transfer rate into the membrane when compared with pure polybenzoimidazole membrane. Hence improvement in selectivity of membrane increases about five times.

3.4.2.2 Chitosan/poly(n-vinyl-2-pyrrolidone) membrane Chitosan was blended with another polymer poly(n-vinyl-2pyrrolidone) (PVP) and has been studied for separation of methanol and methyl tert butyl ether (MeOH-MTBE) by Cao et al. [25]. This blend was possible due to intermolecular reaction between carbonyl group present in the PVP and the hydroxyl group present in the chitosan. As the PVP content in the chitosan membrane was increased, the intermolecular bond between carbonyl group and hydroxyl group increased

Chapter 3 Biobased (nanochitin, nanochitosan) polymer nanocomposite membranes

with the decrease in the intramolecular bonds between hydroxyl groups of chitosan. In this case the role of PVP is to aid in separation of methanol from MTBE in chitosan membrane. The addition of PVP to chitosan brings about a conformational change. In addition, due to the formation of intermolecular bonds as discussed above, the methanol gets selectively absorbed from methanol MTBE mixture through the blended membrane. The absorbed methanol leaves hydroxyl groups in the blended membrane which acts as barriers for MTBE to permeate into the other side of membrane. It is important to note that with more addition of PVP to chitosan, the blended membrane also becomes sensitive to temperature. The increase in temperature from 30 C to 50 C, the intermolecular bonds between carbonyl groups and hydroxyl groups dissociate and the partial flux of MTBE starts to increase as the released hydroxyl groups starts to interact with MTBE. In this study, it was observed that with increase in PVP content in the modified membrane, the permeation flux of MTBE increased greatly. Zhang et al. [26] prepared chitosan-blended PVP membranes using ultraviolet irradiation. In this study, chitosan and PVP was mixed together followed by deacetylation and the solution was cast on a glass plate. Then the glass plate was kept under UV irradiation for cross-linking at different exposure times (in minutes). The cross-linking happened due to the formation of PVP macroradicals by pyrrolidine substituents and polyamides in the PVP chain during UV exposure. These macroradicals interlink with each other and thus result in compact structure of the blended membrane. Along with this, the chitosan macroradicals are also formed during UV exposure which reacts with PVP macroradicals to form additional cross-link points within the structure. The resulting compact structure has improved tensile strength which is in direct relationship with the amount of irradiation time. The blended membrane had tensile strength of 80.6 MPa for irradiation times that of 8 min which is two times of the simple chitosan membrane. These membranes when used for pervaporation of ethylene glycol mixtures which is found that the cross-linking due to UV has produced more diffusion channels inside the blended membrane enhancing selectivity of the membrane in the process. In this case, permeation flux increases with increase of PVP content owing to increase in hydrophilicity but the same decreases with more irradiation time as more the radiation more cross-linking and more compact the membrane become.

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Chapter 3 Biobased (nanochitin, nanochitosan) polymer nanocomposite membranes

3.4.2.3 Chitosan/polyvinyl alcohol membrane The reason behind modification of chitosan membranes is to improve the mechanical stability of the membrane and the permeability of the component to be separated by it. For the same reasons, when chitosan was blended with PVA and made into a membrane, the overall selectivity and the flux improved in the membrane. PVA blended chitosan membrane was used to study the pervaporation of isopropanol water and tetrahydrofuran (THF) water mixture [27]. The content of PVA in chitosan was varied 20%, 40%, and 60% of the total weight of the membrane and the blended membrane was treated with urea formaldehyde sulfuric acid mixture (UFS) for effective crosslinking to attain good strength. The UFS used cross-links amine groups of chitosan and hydroxyl groups of PVA effectively. The blending of PVA with chitosan decreased overall crystallinity of the membrane and increased overall hydrophilicity of the membrane. The cross-linking action of UFS was confirmed by FT-IR characterization of the cross-linked membrane. The band at 702 cm21 corresponding to S-O-C linkage proved interactions between hydroxyl groups of PVA and amine groups of chitosan. With increasing PVA content in the blended membrane, the water flux increased resulting in more swelling of the membrane, whereas selectivity was effectively decreased. This is attributed to the increase in flexibility and free volume space of the blended membrane. In this study, the cross-linked chitosan membrane with 20% PVA exhibited increase in selectivity of about 116% than pure chitosan membrane for pervaporation dehydration of 5 wt.% water/isopropanol mixture. PVA-CS blended membranes were prepared and studied by Zhang et al. [28] for dehydration of n-butanol/water mixture and n-butanol/n-butyl acetate/water mixture to account the effect of the proportions of PVA added in the blend membranes. In this study, the blend membranes were prepared by coating blend solution of PVA and chitosan on polyvinylidene difluoride (PVDF) substrate. The percentage of chitosan in the blend membranes varied in the concentrations of 25%, 50%, and 75%. The FT-IR study of the blended membranes showed absorption peaks at 1142 cm21 (-C-C-) present in PVA membrane shifted to a higher frequency at 1153.07 cm21. Similarly the absorption peak at 1093 cm21 was shifted to a low frequency at 1091.48 cm21. The absorption peak 3300 cm21 (due to OH) shifted to a higher frequency at 3368.18 cm21. This clearly indicates that hydrogen bonds are existing between chitosan and PVA ensuring perfect compatibility. From DSC

Chapter 3 Biobased (nanochitin, nanochitosan) polymer nanocomposite membranes

43

studies, the melting temperature depression was found out to speculate the compatibility of the blend between PVA and chitosan. In this study, PVA membrane exhibits endothermic peak at 232.1 C (refer Fig. 3.4). The same value was slightly reduced in the blended membranes indicating the change in crystallinity of PVA. The XRD data of the blended membranes at room temperature exhibited slow disappearance of diffraction peak of PVA and growth of diffraction peak of chitosan. This result shows that there is strong intermolecular interaction between chitosan and PVA. Also with an increase of chitosan content in the blend membrane, the crystallinity decreases initially and then increases. The decrease of crystallinity may be caused by growth of intermolecular hydrogen bonding between chitosan and PVA. Sorption studies of the blended membranes indicated that the degree of swelling in water decreased with addition of chitosan at higher temperatures (60 70 C). The perfect miscibility of chitosan and PVA and the hydrogen bonding interactions between them can reduce the number of free hydroxyl groups and the chain movement of the polymers. Also the degree of swelling of water increases with rise in temperature. These reasons indicate that the blended membranes have larger affinity for water than n-butyl acetate. The pervaporation study of removing water from n-butyl acetate was carried out using n-butyl acetate solution containing 2 wt.% of water at 40 C. The permeation flux was almost constant for the blended membranes, although there was slight drop in the blended membrane containing 25 wt.% of chitosan. As the amount of chitosan in the membrane increases, the intramolecular hydrogen bonds in PVA are broken and intermolecular hydrogen bonds between PVA and chitosan are formed. This results

Endo down (µV·mg–1)

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350

400

Figure 3.4 DSC thermograms of PVA-CS blend membranes with different blend ratios: (a) PVA-CS0, (b) PVA-CS25, (c) PVA-CS50, (d) PVA-CS75, and (e) PVA-CS100 [28].

44

Chapter 3 Biobased (nanochitin, nanochitosan) polymer nanocomposite membranes

in rearrangement of backbones of chitosan and PVA. This results in chain relaxation and induces greater water diffusion than n-butyl acetate in the blended membranes. The selectivity of water in the blended membranes at 25 wt.% of chitosan was 27,000. This value is higher than the individual selectivity values of PVA and chitosan but beyond 25 wt.% of chitosan in the blended membranes, the selectivity value is decreased. The reason may be attributed to the formation of new hydrogen bonds between chitosan and PVA that are weaker than the hydrogen bonds present in PVA or chitosan. Due to this the energy barrier got reduced and dense structures are broken forming a network structure aiding an improved diffusion of water in the blend membranes. With an increase in the feed temperature (30 70 C), the total permeation flux increased from 0.3 to 0.86 kg m22 h21. This is because the thermal motion of the polymer chains increases with rise in temperature, leading to more permeation of water than n-butyl acetate. Hence an increase in the separation factor of the blend membranes was observed with increase in temperature. Zhu et al. [29] studied pervaporation experiments for dehydration of alcohols (ethanol and t-butanol) using PVA/chitosan membranes dip coated on ceramic supports made of ZrO2/ Al2O3 with tubular structure. These ceramic supports were of various pore sizes of 0.05, 0.2, and 0.5 μm. The membranes were made by dipping the ceramic supports into the mixture solution containing PVA and chitosan, dried and annealed at 120 C. Maleic anhydride was used as cross-linking agent and catalyst during membrane preparation. An interface layer was formed between the PVA/chitosan and ceramic support due to adhesion and this has been the reason for improvement in structural stability of the membranes. Of all the membranes, the one with 0.5-μm pore-sized ceramic support does not have dense structure, whereas the one with 0.05-μm pore-sized ceramic support had improper layer adhesion. Hence, the ceramic support with 0.2 μm was taken for further studies. The concentration of both PVA and chitosan was varied and the optimum concentration of PVA and chitosan in the ratio 40:60 for membrane preparation based on permeation flux and separation factor study on dehydration of t-butanol with 6 wt.% water at 60 C. Pervaporation study of the selected membrane for dehydration of ethanol/water mixture and t-butanol/water mixture revealed highest separation factor of around 1200 for 8 wt.% feed concentration of ethanol/water mixture and around 2000 for 14 wt.% of feed concentration of t-butanol/water mixture.

Chapter 3 Biobased (nanochitin, nanochitosan) polymer nanocomposite membranes

3.4.2.4

Chitosan/poly(acrylic acid) membrane

Chitosan being a cationic polymer can be made into a polyelectrolyte complex membrane by mixing with an anionic polymer such as poly(acrylic acid) at suitable proportions. In this study by Nam et al. [30], chitosan and poly(acrylic acid) were synthesized in different ratios of 94/06, 88/12, 83/17, 79/21, and 75/25, respectively, such that the molar ratios of amine and carboxyl groups were in the ratio of 83/17, 71/29, 63/37, 56/44, and 50/50, respectively. The polyelectrolyte complex formation was characterized by the observation of C 5 O peak in infrared spectrometry which is not available in pure chitosan membranes. Also the observed peaks start to diminish only at temperature above 200 C which is an indirect indication of the thermal stability of the membrane till that temperature. X-ray diffraction studies indicated that the amine groups in chitosan break their bonds from native hydroxyl groups and form complexes with carboxyl groups of the poly(acrylic acid). This results in decrease of crystallinity of the membrane complex. As expected, due to the formation of ionic complexes, the chitosan poly(acrylic acid) membrane exhibits increased tensile strength than the individual counterparts. The drawback in this type of membrane is that they are pH sensitive. At pH value 3, the carboxylate salt did not dissociate and at pH value 9, complete dissociation of carboxylate salt happens. Hence, it is essential to add some additives to compensate this nature at respective pH values but these additives will be a hindrance to the pervaporation process. The membranes were subjected to pervaporation of water alcohol mixtures and were found that the permeate flux decreases and swelling characteristics increases with increasing poly(acrylic acid) content but with increasing feed concentration permeate flux increased ($99.9 wt.%) for the 94/06 membrane during separation of methanol MTBE mixture.

3.4.2.5

Chitosan/polyvinyl sulfate membrane

Similar to the above study, Zheng et al. [31] prepared chitosan-based polyelectrolyte complex membranes made from poly(sodium vinylsulfate) and chitosan (PVS/CH). PVS/CH containing unreacted amino groups are further transformed to sulfated poly(sodium vinyl sulfate) (S-PVS/CH) containing water soluble-free acid groups through sulfation. The formation of the polyelectrolyte complex membranes was observed by broadening of peaks in FT-IR spectroscopy and the sulfation of the polyelectrolyte membrane (S-PVS/CS) was confirmed through X-ray photoelectron microscopy. Water contact angle measurements

45

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Chapter 3 Biobased (nanochitin, nanochitosan) polymer nanocomposite membranes

of S-PVS/CH membrane indicated lesser value than simple chitosan membrane which assures overall increase in hydrophilicity of the polyelectrolyte membrane. The membrane was then used for pervaporation studies for dehydrating 10% water in water/ethanol, water/isopropanol and water/n-butanol mixtures at 70 C. The results revealed that the water content in permeate was found to be 99.53% which is 7.03% more than the unmodified chitosan membrane for ethanol/water dehydration. The research was focused on the effect of degree of sulfation and feed temperature on ethanol/water dehydration since it is difficult to separate than other mixtures due to similar size of molecules. It was evident that the amount of flux was higher than unmodified chitosan membranes and this was attributed to the increase in the degree of sulfation. The flux rate was seen to improve with rise in temperature from 40 C to 70 C without compensating selectivity of the membrane. The reason behind this was the effective cross-linking between the ionic pairs of SO32 and NH31 groups in the polyelectrolyte membrane. In this S-PVS/CH membrane the pervaporation is more sorption dominated than diffusion which was found from swelling study of membrane. It is found that only 1.3% of the swelling is caused by ethanol and the remaining 98.3% swelling was dominated by water in the feed mixture. The performance of the S-PVS/CH membrane remained the same for water content .5% in mixture but the trend got slightly decreased when the water content was ,5% but sufficiently higher than unmodified chitosan membranes. It was reported that the performance of the membrane was unaltered even after 120 h of operation.

3.4.2.6 Chitosan/sodium alginate membrane Similar to chitosan, sodium alginate is another biocompound found extensively in seawater algae and weeds. It also has membrane-forming characteristics and has been used in membrane-aided separation applications. Its applications are restricted due to its high hydrophilicity and weak mechanical strength. Subba Reddy et al. [32] studied how chitosan and calcium can be effectively used in the presence of maleic anhydride to cross-link and improve sodium alginate membranes for the separation of water from 1,4-dioxane. The modified membrane was prepared by dissolving appropriate amounts of chitosan and sodium alginate in oxalic acid. After uniform mixing to attain homogeneous solution, the resulting solution was degased and cast into a membrane. It is then treated with calcium chloride solution to effectively replace sodium ions in the

Chapter 3 Biobased (nanochitin, nanochitosan) polymer nanocomposite membranes

membrane. Sodium alginate being a polymer possesses several rings in which these calcium ions are trapped. Further the membrane was cross-linked with maleic anhydride in a bath containing isopropanol with sulfuric acid catalyst. The membrane was characterized by FT-IR, XRD, and TGA. The FT-IR results confirmed the complex formation between the carboxyl groups inside the alginate chain and the protonated amine groups in chitosan chain. The XRD data revealed that the maleic alginate cross-linked sodium alginate/chitosan membrane (M-CA/CH) was amorphous in nature. Hence, more permeation is possible rather than solo membranes of chitosan and sodium alginate. The improvement in thermal stability of the modified membrane was confirmed by TGA with final decomposition at 450 C. The pervaporation studies were conducted with 1,4-dioxane/water mixture with feed concentrations varying from 5% to 30%. With increase in feed concentration, swelling of the membrane increased steadily and the selectivity of membrane 512 to 8.8 but the results were promising for the feed concentration between 5% and 20%. The pervaporation selectivity was not much affected with increase in thickness of the membrane from 45 to 150 μm.

3.4.2.7

Chitosan/cellulose membrane

Qiang Zhao et al. [33] studied pervaporation performance of membranes made from chitosan and sodium carboxymethyl cellulose (NCMC) polyelectrolyte complex such that the carboxylic groups present in it plays an important role in improving selectivity and hydrophilicity. The polyelectrolyte complex was formed between the carbonyl groups of NCMC and nitrogen in chitosan. The membranes (CH/NCMC) were prepared first by adding chitosan solution dropwise to NCMC solution in HCl (0.005, 0.007, and 0.009 M) until the polelectrolyte complex percipitates out. It was dried and dissolved in NaOH and finally cast on polysulfone membrane. The FT-IR study of polyelectrolyte complex before treating with NaOH reveals the presence of both ionized and unionized carboxylic acid groups with corressponding absorption bands at 1610 and 1730 cm21. The intensity of these bands was found to decrease significantly after treating with NaOH as ionization of the unionized groups happens. This concludes that the effect of ionic cross-linking in the membrane decreases with increasing concentration of HCl. The thermal stability of the membranes was slightly lower than that of chitosan and NCMC as the ionic cross-linking between NH3 and COONa groups has a negative impact. The crystallinity of

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Chapter 3 Biobased (nanochitin, nanochitosan) polymer nanocomposite membranes

the CH/NCMC decreased significantly due to the effective cross-linking bonds arresting chain movement in the membranes (refer Fig. 3.5). The SEM studies of prepared membranes showed nanosized needle-like structures spread throughout the membrane, whereas those that of chitosan and NCMC did not have any special structural features. This further confirms the formation of polyelectrolyte complex between chitosan and NCMC and indirectly proves that the membranes were not formed by blending. This significant structural feature is as a result of ionization of protonated carboxyl acid groups by NaOH and also due to the conformational linkage of chitosan and NCMC. Swelling studies of membranes were done using ethanol/water feed. It is found that the swelling degree increased exhibiting two stages. This is because the water molecules first get filled up in the chain spaces of chitosan and NCMC and later gets filled up into polyelectrolyte aagregates. The membrane prepared using lower concentration of HCl had lesser swelling degree than other due to effective ion crosslinking. The pervaporation performance of the membrane using ethanol/water mixture containing 10 wt.% of water gave a separation factor of 1062 at 70 C with considerably higher amount of permeation flux (1.17 kg m22 h21) for membrane prepared using high concentration of HCl. This is because that path provided by needle-like structures is less torturous than normal membranes and the polyelectrolyte complex effectively preserves selectivity also.

Figure 3.5 WAXD of chitosan and CH/NCMC (redrawn) [33].

Chapter 3 Biobased (nanochitin, nanochitosan) polymer nanocomposite membranes

3.4.2.8

Chitosan/carrageenan membrane

Konovova et al. [34] prepared polyelectrolyte complex pervaporation membranes based on chitosan and λ carrageenan (CARN)—an anionic polysaccharide containing sulfate groups. The modified chitosan membrane was prepared by layer-bylayer deposition. It is important to note that CARN acquire several conformations in solutions which aids in pervaporation applications. This layer-by-layer deposition helps to create more strengthened interactions between counterions present in chitosan and CARN. The layers of the membrane were made of chitosan, CAR and an insoluble PEC layer formed between them. The layer of CARN was removed using ammonia soltion containing ethanol and acetic acid and the rest PEC-CH membrane was taken for further studies. The AFM studies reveal that CARN interaction with chitosan resulted in crystallization in the PEC-CH membrane. Also the PEC-CH membrane exhibited smooth surface on both sides with spherical potrusions distributed on a even scale. The study was repeated after pervaporation and it is noted that the lateral depth and size of pores present in the membrane were unchanged on the CARN side but on the chitosan side, the spherical potrusions dissppeared. Pervaporation studies of these membranes were carried using ethanol/water mixtures with different water concentrations of 6 49.5 wt.% at 40 C. The concentration of water permeated through the membrane was found to be 99.98% for feed containing 6 19 wt.% of water. For the other higher concentrations, the water content in permeate was found to be 99.8% and this clearly indicates that selectivity of membrane was not affected by increase in the concentration of water in the feed content. This is because structuration of PEC-CH membrane occurs during pervaporation and this prevents swelling and reduction in selectivity as the surface roughness of membrane increased by a factor of 2.

3.4.2.9

Chitosan/gelatin membrane

Teli et al. [35] studied gelatin-based chitosan (GE/CH) membranes for perveporation dehydration of 1,4-dioxane. In this study, chitosan was blended with gelatin (10 and 20 wt.%) and then cross-linked with 90/10 volumetric mixture of 1 vol.% HCl as catalyst and 4 vol.% of glutaraldehyde. The prepared membranes have thickness in the range 40 45 μm. The membranes were named as GE-10/CH and GE-20/CH. FT-IR characterization studies revealed imine bond stretching which confirms cross-linking action by glutaraldehyde. The imine

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Chapter 3 Biobased (nanochitin, nanochitosan) polymer nanocomposite membranes

bonds formed via interactions between amino groups of chitosan and CHO of glutaraldehyde. The XRD data reveals semicrystalline nature of the modified membranes. This is due to the presence of decreased intensity in the blended form. The sorption studies of the membranes indicate that the modified GE/CH membranes swelled more than pure chitosan membranes. This is because the gelatin added to chitosan is also exhibit hydrophilicity, therefore overall increase in hydrophilicity. As expected GE-20/CH had more swelling than other membranes in study. Ion exchange capacity (IEC) of the blended membrane is 0.23 mequiv g21 which is 0.22 times less than the pure chitosan membrane. This further confirms cross-linking of chitosan by glutaraldehyde. The pervaporation performance of the pure and modified chitosan membranes was studied at feed containing 10, 15, 20, 25, and 30 wt.% of water in 1,4-dioxane at 30 C. The total permeation flux of the modified membranes was higher than pure chitosan membrane due to enhanced hydrophilicity by gelatin. The case was the same for all concentrations of feed but the selectivity decreased with increase in concentration. The higher selectivity value of 302 was attained for GE-10/CH at 10 wt.% of water in feed, whereas it was 133 for GE-20/CH for the same concentration. The reason is at high concentrations of water in feed, the modified memnbranes became more hydrphilic which led to more swelling and this resulted in more flexibility of the polymer structure and hence reduced selectivity. The results were in the same trend when the studies were conducted at higher temperatures (40 C and 50 C).

3.4.2.10

Chitosan/glutaraldehyde membrane

Katarzyna et al. [36] prepared chitosan membranes strengthened by glutaraldehyde and studied pervaporation of alcohol/ water mixtures. When chitosan is treated with glutaraldehyde (25 wt.%), cross-linking happens between the amine groups of chitosan and the aldehyde groups of glutaraldehyde. This crosslinking is affected by the formation of imine bonds between the above said groups. The cross-linking can be characterized by infrared spectroscopy but noticeable changes will occur only at significant concentrations of the glutaraldehyde in chitosan. Also there is not much change in the weight loss percentages between chitosan- and glutaraldehyde-strengthened chitosan membranes during thermogravimetric analysis. This indicates that the thermal stability of the chitosan membrane is clearly not affected by addition of glutaraldehyde. The membranes were then subjected to swelling studies which revealed that

Chapter 3 Biobased (nanochitin, nanochitosan) polymer nanocomposite membranes

both the membranes swelled to a great extent when treated with water, comparatively lesser in alcohol/water mixtures and very less in pure alcohols. In alcohol/water mixtures, the swelling was observed to be more in alcohols containing less carbon atoms such as methanol but lesser swelling in ethanol. Overall, it was concluded that the glutaraldehyde cross-linked membrane was highly permeable (flux value of 10,857 g μm h21m21 for chitosan/glutaraldehyde membrane and 7974 g μm h21m21 for chitosan membrane) but lesser selective than simple chitosan membrane as the separation factor for methanol/water mixture was about 5 for the latter and 4 for the former membrane.

3.4.2.11

Chitosan/polyaniline membrane

Varghese et al. [37] studied pervaporation performance of chitosan-grafted polyaniline membrane. It is found that polyaniline could impart chemical and thermal stability to the chitosan matrix. The membrane was prepared by mixing chitosan solution prepared in acetic acid and aniline solution prepared in hydrochloric acid. After a certain amount of stirring, the polymerization of aniline was affected by the addition of ammonium persulfate. This was followed stirring for complete polymerization, neutralization of acid by NaOH and then the membrane was casted and dried. Four different membranes were made with increasing concentration of aniline (0.275, 0.55, 1.55, and 2.583 g) with corresponding increased concentrations of both HCl and ammonium persulfate. The tensile strength studies done on the membranes revealed that the elongation of membrane at breaking point increased with increase in polyaniline grafting ratio. The FT-IR studies reveal that the stretching frequency corresponding to amine and hydroxy groups increased in intensity with increased concentration of aniline. Similarly the intensity of the band corresponding to C-O also increased with increase in grafting ratio due to the bending vibrations of N 5 C 5 N bond with shifting toward lower frequency. This lower shift confirms hydrogen bond formation between chitosan and imine groups of polyaniline. Membrane swelling studies revealed that the degree of swelling decreased with increase in grafting ratio. This is due to the formation of crystalline domains during grafting on polysaccharide chains. Pervaporation studies using isopropyl alcohol and water (10%) mixture at 30 C exhibited higher separation selectivity of 2092 and permeation flux of 1.19 3 1022 kg m22 h21 for membrane containing chitosan and polyaniline in the ratio of 1:3. The

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Chapter 3 Biobased (nanochitin, nanochitosan) polymer nanocomposite membranes

reasons are due to the increase in crystalline character and hydrophobic character imparted with addition of polyaniline to the chitosan matrix (Table 3.1).

3.4.3

Chitosan/inorganic membranes

Chitosan/inorganic membranes used in pervaporation applications have inherent strength and high temperature stability factors which are not much pronounced in chitosan/polymer membranes. With the incorporation of inorganic moeties inside the chitosan matrix, the path for sorption and diffusion for permeate molecules become more chaotic and tortorous. Also these inorganic additions possess higher thermal properties which makes them suited for high temperature applications in industries. The only drawback is the permeated flux may not be as large as chitosan/polymer membranes due to decreased sorption capacity of the chitosan/inorganic membranes.

3.4.3.1 Chitosan/clay membrane Choudhari et al. [38] studied nanocomposite membranes made with quarternized chitosan and clay sodium montmorillonite (CH/NaMMT). The clay possess negative charges and these interact with positve charges in chitosan which results in fine dispersion of clay within chitosan. The quarternization of chitosan with alkyl halides increases the permeation selectivity of water in chitosan and it is found to increase with increase in degree of quarternization. It also contributes to increase in swelling degree of chitosan to some extent. In this study, chitosan was quarternized with methyl iodide and the degree of quarternization was about 15% found by NMR spectroscopy. To this quarternized chitosan, clay was added in 0, 5, 10, and 15 wt.% and the membranes were prepared. The prepared membranes were subjected to IEC study and it is found that the value decrease from 0.342 to 0.110 mequiv g21 confirming the electrostatic interactions. FT-IR studies revealed that with increase in clay content, the C-O stretching frequency increased due to overlap between it and the Si-O-Si stretching of the clay. Also the amino bands were found to shift to a lower stretching frequency indicating possible hydrogen bond formation between the chitosan and clay particles. This is further highlighted by shifting of O-H stretching to a lower frequency. The XRD studies reveal decrease in d spacing, proving that nanocomposites of clay with chitosan were of intercalated nature. The TGA study revealed that the decomposition of

Table 3.1 Chitosan/organic membranes. S. no Membrane

Type

1.

Organic Interlinking using 30 wt.%/water isopropanol third compound Organic Blending 14.45 wt.% methanol/methyl tert butyl ether Organic Ultraviolet Methanol/ethylene glycol irradiation Organic Blending 10 wt.% water/isopropanol 5 wt.% water/tetrahydrafuron Organic Blending 2 wt.% water/n butyl acetate

30 30 40

0.113 0.0978 0.5

17,991 4203 27,000

[27]

Organic Blending

6 wt.% water/t-butanol

60

0.147

1350

[29]

Organic Polyelectrolyte complex Organic Polyelectrolyte complex Organic Blending

20 wt.% water/ethanol

80

0.277

4000

[30]

10 wt.% water/ethanol

70

1.980

-

[31]

28 wt.% water/1,4-dioxane

30

0.63

56.20

[32]

10 wt.% water/ethanol

70

1.17

1062

[33]

6 wt.% water/ethane

40

0.0027

156,510

[34]

10 wt.% water/1,4-dioxane 10 wt.% water/isopropanol

30 30

0.118 0.027

302 450

[35] [37]

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

Chitosan/ polybenzoimidazole Chitosan/poly(nvinyl-2-pyrrolidone) Chitosan/poly(nvinyl-2-pyrrolidone) Chitosan/poly vinyl alcohol Chitosan/polyvinyl alcohol Chitosan/polyvinyl alcohol Chitosan/ polyacrylic acid Chitosan/poly vinyl sulfate Chitosan/sodium alginate Chitosan/cellulose Chitosan/ carrageenan Chitosan/gelatin Chitosan/ polyaniline

Way of modification

Organic Polyelectrolyte complex Organic Polyelectrolyte complex Organic Blending Organic Grafting

Feed

Operation Permeation temperature flux (kg (˚C) m22 h21)

Separation Reference selectivity

25

124

[24]

155.6

[25]

0.184

30 50 60

[26]

[28]

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Chapter 3 Biobased (nanochitin, nanochitosan) polymer nanocomposite membranes

CH/NaMMT membranes were extended by 5 C 30 C than pure quarternized chitosan membranes. This confirms improvement in thermal stability of membranes due to addition of clay. The swelling of membranes in water showed a decreasing trend as the added clay particles in CH/NaMMT membranes decrease the free volume availablity inside the membrane along with prevention of interactions of quarternized chitosan with water. Pervaporation studies of CH/NaMMT membranes were carried out using feed containing 10 vol.% water in isopropanol/water mixture and it is clearly evident that permeation flux decreased and selectivity increased till 10 wt.% of clay. The reason behind decline in selectivity beyond 10 wt.% of clay is because of microphase separation of the clay and chitosan. With increase in water concentration in the feed, the membranes exhibited increase in total permeation flux but the selectivity got reduced exponentially for the same case. In this study, the CH/NaMMT membrane containing 10 wt.% of clay exhibited highest selectivity of 14,992 for 10 vol.% of water in isopropanol feed.

3.4.3.2 Chitosan/titanium dioxide membrane Yang et al. [39] studied pervaporation of ethanol/water mixture using chitosan membranes modified with titanium dioxide (TiO2). Titanium dioxide exhibits good stability, hydrophilicity, and has resistance to bacteria but incorporating them within polymer matrix leads to frequent generation of many non selective voids at the interfaces between the polymer and titanium dioxide. This can be overcome by in situ sol gel synthesis of titanium dioxide. In this study, CH/TiO2 membranes were prepared by mixing chitosan, tetrabutyl titanate and acetyl acetone in molar ratio of 1:0.7:4 in which wt.% of titanium dioxide where 2, 4, 6, 8, and 10. Also the same membrane was prepared by blending for comparative studies. The SEM studies reveal that the size of titanium dioxide was 100 nm at 2 wt.% and this increased to 1.2 μm at 10 wt.% for in situ prepared membranes, whereas all the blended membranes had micrometer-sized titanium dioxide particles within them and exhibited phase seperation. Hence the blended membranes were not taken for further studies. The FT-IR studies of pure chitosan and in situ prepared where done and results showed that new broad absorption band at 850 cm21 was found in CH/TiO2 membrane which confirms the presence of Ti-O-Ti groups. Also the bands corresponding to hydroxyl and amine groups in chitosan membrane have decreased intensity in the CH/TiO2 due to reaction with hydroxyl groups of titanium dioxide. The XRD data of CH/TiO2

Chapter 3 Biobased (nanochitin, nanochitosan) polymer nanocomposite membranes

membrane did not show any crystalline peak of TiO2. This indicates that titanium dioxide is present in amorphous form in CH/TiO2 membranes. This is caused due to the steric effect and hydrogen bonds formation between chitosan and titanium dioxide. The TGA study revaled that the amount ofTiO2 determined by the study is smaller than the TiO2 determined by theoretical calculation. Hence it can be concluded that some percentage of tetrabutyl titanate still remains in the CH/TiO2 membrane. This is because of the acetyl acetone used as chelating agent in the fabrication of CH/TiO2 forms complex with tetrabutyl titanate and prevents its complete conversion into TiO2. Swelling studies of CH/TiO2 membranes indicated increase in swelling characteristics of membranes with increase of TiO2 concentration in the chitosan because the regions of the membrane become amorphous due to introduction of TiO2 and also because of their interaction between the water molecules. Pervaporation studies of the modified membranes were conducted using feed containing 90% ethanol/water solution at 80 C. The CH/TiO2 membranes exhibited increased flux rate in propotion with TiO2 content but decreased when the concentration of TiO2 is $ 6 wt.%. The reasons for this increase is due to the interference of TiO2 in the packing order of polymer chains of chitosan but beyond the concentration of 6 wt.%. The TiO2 reduces its interaction with the chitosan chains as it starts to segregate and gets agglomerated. The separation factor of the CH/TiO2 membrane decreased with increase in concentration of TiO2 from 0 to 4 wt.% but increase for concentrations of 4 to 8 wt.%. The reason is attributed to the presence of nanovoids at lower concentrations which became less pronounced due to more size selective interactions of TiO2 with the chitosan membrane.

3.4.3.3

Chitosan/ferric oxide membrane

Dudek et al. [40] investigated the effect of the presence of ferric oxide nanoparticles synthesized by Das-modified method in chitosan membranes strengthened by glutaraldehyde and sulfuric acid separately and their influence on the transport properties of ethanol and water during pervaporation. The feed solution used for study contained 50 vol.% of ethanol in water. FT-IR characterization of the modified membranes revealed the formation of new imine bands in case of glutaraldehyde strengthening, whereas shift in amine bands was found in the case of sulfuric acid strengthening. The swelling experiments of the two different strengthened membranes gave 25% increase of degree of swelling in case of sulfuric acid strengthening,

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Chapter 3 Biobased (nanochitin, nanochitosan) polymer nanocomposite membranes

whereas only 10% in case of glutaraldehyde strengthening. This is mainly because of the hydrophobic character of the glutaraldehyde and hydrophilic character of the sulfuric acid. The addition of ferric oxide nanoparticles (2, 7, and 15 wt.%) to the membranes decreased the swelling character. Addition of 2 wt. % of ferric oxide nanoparticles has produced three times decrease in swelling nature. Also the introduction of ferric oxide nanoparticles increased the diffusion coefficient of water by 70 times and that of ethanol by 5 times with their respective solubility coefficients found to be decreasing. This has attributed to the increase in permeation flux of water, whereas the same for ethanol was found to have a reduction. The optimum concentration of ferric oxide in the membrane was found to be 7 wt.% based on the pervaporation studies and this was in good agreement with the glutaraldehyde-strengthened membranes. The researchers conclude that the reason for observed findings is due to the change in hydrophobic hydrophilic balance of the membranes due to incorporation of ferric oxide nanoparticles.

3.4.3.4 Chitosan/functionalized graphene sheets membrane Dharupaneedi et al. [41] prepared and studied functionalized graphene sheet (FGS)-based chitosan membranes for dehydration of ethanol/water and isopropanol/water mixture. First, they prepared FGS following modified Brodie method. The chitosan/FGS membranes were made by proper mixing of chitosan solution prepared in acetic acid and FGS suspended in water. First, 10% of chitosan solution was added dropwise into the FGS solution with continuous stirring for 12 h. Later, the remaining solution was added and mixed with little addition of glacial acetic acid to facilitate proper mixing. Then the solution mixture was cast and dried in bath containing water, acetone, glutaraldehyde and hydrochloric acid. The glutaraldehyde and hydrochloric acid enabled cross-linking between the chitosan polymer and the FGS. The amount of FGS in chitosan solution was varied (1, 2, 2.5, and 3 wt.%) and membranes were fabricated. It is observed that all the membranes were black in color and had a thickness of 84 μm. Comparative studies of pure chitosan membrane with that of chitosan/FGS membranes using wide angle X-ray diffraction revealed that reduction in intensity of peak observed with addition of FGS to the chitosan membrane. The hydrophilic character of the membrane increased with addition of FGS as the measured contact angle of the chitosan/FGS membranes decreased from 88 to 79 degrees. The thermal stability of the membrane increases with addition of

Chapter 3 Biobased (nanochitin, nanochitosan) polymer nanocomposite membranes

FGS to chitosan due to increase in electrostatic interactions between the FGS and chitosan with 20 30% weight loss occurring at 230 C. Swelling studies of chitosan/FGS membranes with ethanol and isopropanol showed that overall swelling is higher and much higher in ethanol than isopropanol. This is because of the high polar nature of the ethanol resulted in molecule formation with water and hence less permeation. Pervaporation studies using chitosan/FGS membranes concluded that permeability and selectivity values of these composite membranes increased with increase in the concentration of FGS loading up to 2.5 wt.%. The reason behind increase is established by several complicated pathways for the feed mixture created by the anionic charges and polar groups present in the FGS and the chitosan polymer matrix. Above 2.5 wt.% of FGS, the chitosan/FGS membranes exhibited increase in permeation flux but drop in selectivity due to aggregation of FGS within the chitosan matrix.

3.4.3.5

Chitosan/NaY membrane

Membranes made with zeolite exhibit higher flux as well as separation characteristics due to its sieve-like molecular structure and high adsorption rate. The drawback is that efficient preparation of these membranes without defects is a very difficult task. This can be overcome by attaching inorganic adsorbent to the zeolite and then incorporating them inside a polymer matrix but the mechanical strength often tends to be poor. Hence, cross-linkers have been used to improve the strength of the composite matrix. In case of chitosan being the polymer matrix, the most used cross-linkers are supfric acid, glutaraldehyde, diisocyanates, etc. Chitosan membranes crosslinked with diisocyanates have better separation factors but the flux rate is very poor. So, a composite membrane of zeolite, chitosan and diisocyanate will be beneficial for pervaporation application as disadvantage of one gets compensated by the other component. Premakshi et al. [42] prepared NaY Zeolite/ chitosan membranes cross-linked by hexamethylene diisocyanate. The membrane was made by preparing chitosan solution followed by addition of 40% of blocked diisocyanate and stirred for 2 h. Then, the NaY zeolite particles were added in the varying concentrations (10, 20, 30, and 40 wt.%) to the solution and stirred and sonicated till uniform dispersion of NaY is achieved. It is then casted on a glass plate and dried to get the membrane (CH/NaY). At higher concentrations ( . 40 wt.%), the membrane fabricated become more brittle. The thickness of the cast

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Chapter 3 Biobased (nanochitin, nanochitosan) polymer nanocomposite membranes

membranes was found to be approximately 40 μm. FT-IR study confirmed the presence of characteristic peaks of chitosan but their intensity slightly increased owing to additional hydrophilicity incorporated by the zeolite in the CH/NaY membrane. From XRD analysis, the characteristic peaks of chitosan started to diminish with increase in addition of NaY zeolite to the membrane. This indicated transition of semicrystalline nature of chitosan matrix into amorphous nature. The thermal stability of CH/ NaY membrane is higher than pure chitosan membrane as the melting point of the CH/NaY was 25 C more than that of pure chitosan membrane. Also the decomposition temperature extended in case of the CH/NaY membrane by 40 C than chitosan membrane found from TGA studies. The SEM images revealed homogeneous dispersion of NaY on the top of CH/NaY membrane, whereas in the cross-sectional portions of CH/NaY membrane (high concentrations), some cavities were present. The tensile strength of the membranes measured by Universal Testing Machine revealed marginal increase in strength for lower concentrations of zeolite taken in study and abrupt increase in 40 wt.% of chitosan which is primarily due to more amount of NaY. The swelling characteristics of the CH/NaY membranes were studied at 30 C and it was found that the swelling increased with increase in both zeolite concentration and feed concentration. The reason behind this is attributed to the presence of cage-like network in NaY structure and sodium ions which increase the overall electrostatic interactions between the water molecules and the membrane and ultimately improve the hydrophilicity. Pervaporation studies were conducted with 10 wt.% of water in isopropanol/water mixture at 30 C. The permeation flux was found to increase with increase in the concentration of NaY in the CH/NaY membrane and the selectivity was 11,241 for CH/ NaY (40 wt.%). The same was not repeatable at higher concentrations of feed mixture, since more water molecules decrease the interaction between zeolite and chitosan in the membranes and enhance reaction between water molecules and NaY.

3.4.3.6 Chitosan/silica membrane Varghese et al. [43] studied pervaporation performance of modified chitosan membranes using TEOS and ϒ-glycidosypro pyltrimethoxysilane (CH-TEOS-GPTS) in isopropanol/water mixture. TEOS when added to chitosan produces cross-linking but undergoes self condensation of silanol groups present in it which leads to phase seperation. This could be overcome by GPTS because it contains epoxy rings that allows formation of

Chapter 3 Biobased (nanochitin, nanochitosan) polymer nanocomposite membranes

hydroxyl groups which enables interactions with the chitosan. The modified membranes were prepared by mixing chitosan solution with TEOS contained in HCl. Both TEOS and GPTS were partially hydrolyzed in acid so that the epoxy rings of both the compounds would open and form C-O-C bond with chitosan hydroxyl groups. This results in formation of siloxane. To the mixture of chitosan and TEOS solution, GPTS is added and the solution is cast and dried at room temperature. The amount of TEOS was varied (0.75, 0.5, and 0.25 g) and the corresponding amounts of GPTS is taken as 0.25, 0.5, and 0.75 g. This is done to ensure that the total amount of silane compounds is 1 g. But the amount of chitosan is the same for all combinations. The membranes prepared were named as M-2, M-3, and M-4 and only chitosan/TEOS membrane was named as M-1 and taken for comparison. The thickness of the membranes were maintained at 40 μm. The membranes were subjected to increasing feed concentrations the membranes swelled in linear relationship with increase in water concentrations in the field. This is because in addition to hydroxyl and amine groups of chitosan, the hydroxyl groups produced due to addition of GPTS elevates overall hydrophilicity of the modified membranes. This results in and increase of permeation flux with increase in water content. But the reverse trend was exhibited by selectivity incase of increasing water content. The selectivity of all membranes were greater than pure chitosan membrane but among the modified membranes the selectivity decreased with increase in silica content. The highest selectivity of 18,981 was exhibited by M-2 membrane for a feed concentration with 0.05 wt.% of water at 30 C.

3.4.3.7

Chitosan/sulfosuccinic acid membrane

Unlu et al. [44] prepared and studied chitosan-based catalytic membranes for the production of ethyl levulinate, a bioadditive used in fuels. The catalytic membranes possess catalyst within them, and when a reactant comes in contact with the membrane, reaction happens and the product is formed. This product is selectively removed by the membrane through sorption and diffusion while the other by products remain on the upper layers of the membrane. In this case, the catalyst coated in chitosan is sulfosuccinic acid (SSA). Chitosan membranes are prepared by mixing chitosan solution and SSA during which cross-linking happens. The concentration of SSA is varied from 0.5, 1 and 2 mmol. After this, the solution is cast and dried followed by heating to further cross-link SSA with chitosan

59

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Chapter 3 Biobased (nanochitin, nanochitosan) polymer nanocomposite membranes

membrane. Ethyl levulinate is synthesized when mixture of levunic acid and ethanol comes in contact with the membrane. SSA acts as the catalyst and converts the mixture into ethyl levulinate and water. The water gets permeated through crosslinked SSA/chitosan membrane while the ethyl levulinate remains on the reaction side. The way by which pervaporation of water from product mixture happens. The SSA/chitosan membrane exhibited good thermal stability as the initial decomposition stage was 40 C 200 C during TGA study which is greater than normal chitosan membrane. SEM characterization reveal homogeneous surface of the membrane. The pervaporation characteristics of the SSA/chitosan membrane was studied with 5 20 wt.% water in ethanol/water mixtures during which an increase in water (wt.%) in the feed, and thereof the selectivity dropped and the permeation flux increased. The highest selectivity of 45.60 was found at lowest water concentration of the feed (5 wt.% at 70 C) and this was for the SSA/chitosan membrane with 2 mmol concentration of SSA. To understand the effect of temperature in the pervaporation performance, the feed mixture was supplied at 50 C, 60 C, and 70 C. The movement of polymer chains became flexible with increasing temperature and permeation flux increased with drop in selectivity.

3.4.3.8 Chitosan/toluene-2,4-diisocyanate membrane Anjali devi et al. [45] studied dehydration of isopropanol/ water mixture using chitosan membranes cross-linked with toluene-2,4-diisocyanate (TDI). The chitosan/TDI membranes were prepared by immersing chitosan membranes in a hexane bath containing TDI and dibutyl tin dilaurate as catalyst. The optimum immersion was found to be 6 h. After this, membranes were washed with acetone and dried. The IEC of the pure chitosan membrane and chitosan/TDI membranes were studied. It was found that the IEC in pure chitosan was 0.42 mequiv g21 and it decreased on cross-linking with TDI. The IEC of chitosan/TDI was 0.2 mequiv g21 and this confirms the interactions of amine groups of chitosan and TDI. As the IEC value dropped to half of initial value after cross-linking, it can be inferred that nearly 50% of the amine groups of chitosan crosslinked with TDI and formed urea linkages. The above urea linkage formation during chitosan was confirmed by the peak at 1640 cm21 in FT-IR characterizations. Also peaks corressponding to benzene ring and reactions between amine groups of chitosan with that of carboxyl groups of TDI was observed in the result.

Chapter 3 Biobased (nanochitin, nanochitosan) polymer nanocomposite membranes

The XRD data of pure chitosan and chitosan/TDI revealed that both the membranes exhibited semicrystalline nature but effec˚ was observed which further tive shrinkage in cell size of 4.27 A confirmed cross-linking. The data of dynamic mechanical thermal analysis showed that the glass transistion temperature (Tg) of chitosan membranes increased by 64.05 C after cross-linking. Hence, thermal stability of chitosan membrane has improved significantly with cross-linking with TDI. Pervaporation studies indicated that the chitosan/TDI exhibited higher selectivity factor of 523.33 for 4.4 wt.% of water in isopropanol/water feed but selectivity dropped with increase in water concentration of the feed. Also the pervaporation performance decreased with increase in thickness of the membranes due to reduction in diffusion-dominated separation process.

3.4.3.9

Chitosan/reduced graphene oxide membrane

Hung et al. [46] studied chitosan membranes incorporated with reduced graphene oxide (RGO) for pervaporation applications. Graphene oxide (GO), a derivative of graphite possess oxygen containing groups such as hydroxyl, diol, ketone, carboxyl, and epoxide. Due to this it exhibits strong hydrophilic character. In addition to that the hydrogen bonding and π-π interaction it creates nanochannels for better permeation. The major problem with GO is that they cannot be dispersed finely in the chitosan matrix. The aggolomeration of GO can be attributed to the presence of carboxyl groups. These carboxyl groups undergo electrostatic interaction with the postively protonated amine groups of chitosan forming an ionic complex (-COO2H31N-R). Hence, the GO is reduced hydrothermally to RGO at 90 C (for 0, 6, 12, 24, 48, and 72 h) and then dispersed into the chitosan matrix. The synthesized RGO has less thickness (0.78 nm) compared to GO (1.26 nm) indicating removal of certain oxygen containing groups in GO. The results of elemental analysis indicated that with increase in reduction time of GO, the RGO exhibited reduction in ratio of oxygen to carbon atoms from 0.62 to 0.12. The dispersion of RGO was comparatively better than that of GO. The expected reduction in hydrophilicity was confirmed by water contact angle measurements and FT-IR. The water contact angle of RGO/chitosan membrane is 77.2 degrees, whereas that of GO/chitosan is 56.7 degrees. The FT-IR study of both the membranes revealed shifting of peaks corressponding to carboxyl and amine groups in GO/ chitosan membranes to lower wavenumbers in RGO/chitosan membranes. Also the peak corressponding to OH group was

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severely weakened in RGO/chitosan membranes. This clearly indicates removal of carboxyl groups in RGO. Transmission electron microscopic studies confirmed uniform dispersion of RGO in chitosan matrix. It was found that the RGO formed highly ordered lamellar strucuture with the chitosan and the ˚ . The average space between the RGO sheets was about 6.7 A RGO/chitosan pervaporation study on methanol/water mixture revealed that the permeate contained 99.8 wt.% of water compared to 87% that of only chitosan membrane and the seperation was based on molecular sieves principle rather diffusion sorption separation. The increase in temperature of the feed from 30 C to 80 C resulted an increased permeation flux but there is a slight decrease of 3.8 wt.% of water on the permeate side, whereas in GO/chitosan, the water content on permeate side decreased to 65.6 wt.%.

3.4.3.10

Chitosan/phosphotungstic acid membrane

Rachipudi et al. [47] studied phosphotungstic acid-based chitosan membranes (PTA/CH) for pervaporation of isopropanols. It is found that PTA formed polyelectrolyte complex with chitosan caused by electrostatic interactions. The membranes for study were prepared by mixing of fixed concentration of chitosan solution with varying concentrations of PTA (0.015, 0.025, 0.035, and 0.045 N) at room temperature. The FT-IR studies revealed slight shift in the OH bond to 3380 cm21 due to the bond formation between hydroxyl group of chitosan and the PTA. Further the band intensity was found to increase with increase in the concentration of PTA. This leads to significant increase in the hydrophilicity of the polyelectrolyte membrane. The other bands corressponding to the C-O stretchings have an increased intensity suggesting that the geometry of PTA is in the form of keggin ion and this is another reason for increase in hydrogen bonding between chitosan and PTA. The XRD studies of PTA/CH membranes had semicrystalline peaks corressponding to chitosan but with an decreased intensity. This implies that the semicrystalline nature gets reduced due to PTA. The PTA/CH membrane with 0.035 and 0.045 N concentration of PTA exhibited more crystalline nature suggesting at these concentrations, the dispersion of PTA in the membranes was purely inorganic. The SEM studies did not show any clusters or voids in the PTA/CH membrane matrix for lesser concentrations of PTA. But the PTA/CH membrane having concentrations 0.035 and 0.045 N exhibited open voids and clusters. The swelling study of membrane with water at 30 C revealed degree of

Chapter 3 Biobased (nanochitin, nanochitosan) polymer nanocomposite membranes

swelling upto 50% for the 0.045N-PTA/CH membrane. Pervaporation studies of these composite membranes using isopropanol/water mixtures revealed higher flux and separation characteristics with increase in feed concentration for 0.015 and 0.025 N PTA/CH membranes. For higher concentrations of PTA in PTA/CH membranes, selectivity dropped very low but the flux was increasing with more feed. The reason is the decrease in ionic interaction of PTA and chitosan at these concentrations. For lower concentrations of feed, this drawback is not found. The highest separation selectivity value of 31,648 and flux of 4.4 3 1022 kg m22 h21 was found in 0.035N-PTA/CH membrane with 5 wt% of feed at 30 C.

3.4.3.11

Phosphorylated chitosan membrane

Sunitha et al. [48] prepared phosphorylated chitosan membranes using isopropyl alcohol, phosphoric acid, and water in the ratio 44:5:1. The cross-linking of chitosan membranes was dependent on membrane exposure time to the solution mixture (0.5 4 h). The net phosphorylation effect was analyzed by studying the IEC of the membrane. The study revealed that the IEC has reduced from 0.52 to 0.35 mequiv g21 confirming the reduction of active groups in membrane which in turn confirms crosslinking in the membrane. FT-IR results revealed that cross-linking happens due to the reaction between the hydroxyl groups of phosphoric acid and the amine groups of chitosan resulting in the formation of strong Columbic interaction between PO432 of acid and the NH31 of chitosan. The phosphorylated chitosan membrane exhibited good water sorption and the significant difference was present between ethanol sorption compared to that of water indicate good selection and sorption behavior after cross-linking. The increase in cross-linking time induced improvement in separation factor with reduction in permeation flux with the ideal time of phosphorylation found to be 2 h. When the water in the feed was increased from 0.63% to 10.23%, the permeation flux increased to 111.67% and the selectivity dropped heavily (183 6). This is because of the high increase in swelling with additional contribution due to plasticization effect.

3.4.3.12

Sulfonized chitosan membrane

Pandey et al. [49] studied sodium 2-formyl benzene sulfonate polysiloxane (SBAPTES) and N-o-sulfonic acidbenzyl chitosan (NSABC) hybridmembranes for pervaporation dehydration of ethanol/water mixture. Sulfonic acid functionalization of silica enables better interactions with chitosan improving

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Chapter 3 Biobased (nanochitin, nanochitosan) polymer nanocomposite membranes

Figure 3.6 TEM analysis of 50 nm microtome membrane at various magnification range [49].

permiselectivity on the whole. The membranes were prepared by sol gel method by mixing solutions of NSABC, SBAPTES, and PVA in the presence of HCl. PVA is used as a plasticizer in this study. Further the membranes were cross-linked with different amounts of formaldehyde for different periods of time and taken for study. The FT-ATR spectra showed stretching frequencies at 1120 and 1260 cm21 corressponding to HSO3 and SO3 confirming the successful grafting of suphonic acid in the membrane. Also stretching frequencies corressponding to Si-O-Si, Si-O-C, and C-O-C were found confirming cross-linking between SBAPTES

Chapter 3 Biobased (nanochitin, nanochitosan) polymer nanocomposite membranes

and NSABC. Optical studies such as SEM and TEM confirmed the presence and even distribution silica particles with size range 5 14 nm (refer Fig. 3.6). Compared to uncross-linked hybrid membranes, cross-linked membranes possessed two types of pores inside them for permeation. The first type of pores was formed in the membrane due to hydrophilic and hydrophobic regions, whereas the second type was formed due to silica inclusions. Pervaporation studies were carried out using 10 wt. % water/ethanol mixtures at 30 C and 50 C. It is found that hydrogen bond formation between silica and chitosan played at role in selection absorption and diffusivity of water in the membrane. The uncross-linked hybrid membrane exhibited a flux of 0.11 kg m22 h21 with selectivity of 2725, whereas the crosslinked hybrid membrane had improved flux of 0.59 kg m22 h21 with selectivity 5282 at 30 C.

3.4.3.13

Chitosan/multiwall carbon nanotube/silver membrane

Shen et al. [50] studied pervaporation of benzene/cyclohexane mixtures using chitosan membranes modified with addition of silver (I) ions and multiwall carbon nanotubes (CH/MWCNTAg). It is found that the silver ions are able to form benzene monovalent metal (I) coordinate complex and this is found to increase pervaporation performance. The MWCNTs used in this study have diameter in the range 20 30 nm. In this study, the MWCNTs were first loaded with silver ions and then dispersed into chitosan. The incorporation of Ag ions into MWCNT is done by treating MWCNT in mixture of acids (nitric acid and sulfuric acid) which introduces hydroxyl and carboxyl groups in MWCNT. It is then followed by treatment with isonicotinic acid and then finally washed with isopropanol. After this, silver nitrate was added to the washed MWCNTs and then stirred and ultrasonicated at appropriate conditions and subsequent washing with methanol was done. These modified MWCNTs were added to chitosan solution at different wt.% (0, 0.005, 0.01, 0.015, and 0.02) and then coated on PSF membrane using casting knife and dried. The XPS results of the modified MWCNTs confirmed the presence of Ag ions on MWCNT, whereas the EDS confirmed the dispersion of Ag ions on MWCNT to be homogeneous. From XRD studies, it was evident that the crystalline nature of chitosan was not significantly affected with incorporation of MWCNT-Ag as the crystallinity was observed to be reduced by 16.37% for the 1.5 wt.% MWCNTAg. Swelling studies of CH/MWCNT-Ag membranes revealed that swelling degree increased with increase of MWCNT-Ag

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concentration in chitosan. The main reason behind this was found to be increase in loosening of chains and cleavage of hydrogen bonds from chitosan matrix with incorporation of MWCNT-Ag. Also it was noted that the membrane swelling was more prominent in benzene compared to cyclohexane. Comparitive pervaporation studies using CH/MWCNT-Ag membranes and CH/membranes revealed that in both the cases permeation flux increased with increase in respective concentrations of MWCNT-Ag in former and MWCNT in latter for feed concnetrations containing 10, 20, and 30 vol.% benzene in benzene/ cyclohexane mixtures at 20 C. The case was different in selectivity as the values got increased till 1.5 wt.% of MWCNT-Ag in CH/ MWCNT-Ag and then decreased, whereas it dropped continously with increase in the concentration of MWCNTs in CH/MWCNT membranes. It is understood that the blending compatibility of MWCNT was increased due to Ag ions and this resulted in higher selectivity in CH/MWCNT-Ag membranes. The selectivity of CH/ MWCNT-Ag was found to have a maximum value of 8.92 for 10 vol.% of benzene in benzene/cyclohexane mixture at 20 C.

3.4.3.14

Chitosan/Mxene membrane

Xu et al. [51] prepared and studied Mxene-based chitosan composite membrane for dehydration of ethanol/water mixtures. The Mxene (Mn11XnTx) is a new class of 2D materials which contains a transition metal (M), carbon and/or nitrogen (X) and also surface groups (T) made of OH, H, or F. In this study, Mxene Ti3C2Tx nanosheets were prepared from LiF, HCl, and Ti3AlC2. These nanosheets were added to acetic acid with varying concentrations of 1, 3, and 5 wt.%. To these solutions, chitosan powder was added and was kept under stirring for 18 h. The membranes were made by spin coating the mixture on PAN substrates followed by cross-linking using sulfuric acid. The AFM and SEM characterizations revealed that the lateral size of Mxene nanosheets was in the range between 0.5 and 1 μm with thickness range of 1 2 nm. Also the dispersion was uniform indicating better compatibility between Mxene nanosheets and chitosan matrix till 3 wt.% of the former in the membrane. The EDX performed on the prepared membranes indicated the presence of carbon, oxygen, aluminum, and titanium elements, further confirming the incorporation of Mxene into chitosan matrix. The surface hydrophilicity was not much changed on addition of Mxene into chitosan matrix as it is speculated that both chitosan and Mxene have nearly the same hydrophilicity found by water contact angle measurements. Pervaporation studies performed at

Chapter 3 Biobased (nanochitin, nanochitosan) polymer nanocomposite membranes

50 C revealed that the permeation flux improved nearly 30% from 1.1 to 1.4 kg m22 h21, whereas separation factor improved from 407 to 1421. The increase is attributed to the Mxene sheets which acted as laminates providing interlayer channels between them for transport of water.

3.4.3.15

Chitosan/boehmite membrane

Esmaeili et al. [52] studied boehmite-incorporated chitosan nanocomposite (CH/B) membrane for pervaporation dehydration of ethanol water mixture. Boehmite is an aluminum hydroxide compound in which surface is dominated by hydroxyl groups. The structure is made up of octahedral hydroxyl chains with octahedral double sheets with aluminum ions at the center. The boehmite nanoparticles were synthesized from aluminum nitrate and sodium hydroxide. Then, these were dispersed in 2 vol.% of acetic acid solution. To this chitosan was added and stirred for 24 h. The membranes were prepared by film casting method with concentration of boehmite being 1 and 2 wt.%. Further the prepared membranes were treated in NaOH to remove any excess acid present in them. The FT-IR studies revealed that the stretching frequency of OH band in the composite membrane shifted to a lower frequency of 3400 cm21 confirming intermolecular hydrogen bond formation between OH groups of both boehmite and chitosan. Further the observed peak had increased intensity suggesting an overall increase in hydrophilicity of the CH/B membrane. It is also observed that the peak corresponding to amino groups in chitosan also shifted to lower frequency proving hydrogen bond formation between amino groups of chitosan and hydroxyl groups of boehmite. The SEM analysis showed no agglomeration of boehmite nanoparticles in chitosan matrix for 1 wt.% of former but showed nonuniform dispersion and agglomeration for 2 wt.%. Swelling studies of the prepared membranes showed decreased swelling behavior than that of pristine chitosan membrane. This attributed to the formation intermolecular hydrogen bonds which prevented molecular chain movement confirmed by the FT-IR studies. Pervaporation studies revealed that the both permeation flux and separation factor increased for composite membranes when compared to pure chitosan membrane. It is evident that the water molecules had travelled through the nanochannels created by the intermolecular hydrogen bonds within the polymer matrix, aiding in increase of separation factor. The trend was not similar with increasing feed concentration as only permeation flux increased and separation factor dropped (Table 3.2). MWCNT, Multiwall carbon nanotube

67

Table 3.2 Chitosan/inorganic membranes. S. no

Membrane

Way of modification

Feed

Operation temperature (˚C)

Permeation flux (kg m22 h21)

Separation selectivity/ separation factor

Reference

1. 2

Chitosan/clay Chitosan/TiO2

10 wt.% water/isopropanol 10 wt.% water/ethanol

30 80

0.1423 0.340

14,992 196

[38] [39]

3.

Chitosan/ferric oxide (glutaraldehyde) Chitosan/functionalized graphene sheets Chitosan/NaY Chitosan/silica

Cation exchange In situ sol gel process Solution mixing

50 wt.% water/ethanol

30

0.

3.02

[40]

Solution mixing

10 wt.% water/ethanol

Solution mixing In situ sol gel process Catalyst action

10 wt.% water/isopropanol 0.05 wt.% water/isopropanol

30 30

0.11 0.017

11,241 18,981

[42] [43]

8.4 wt.% water/isopropanol

30

0.079

472

[45]

Solution mixing

10 wt.% water/methanol

30

0.341

-

[46]

Polyelectrolyte complex Cross-linking Sol gel method

5 wt.% water/isopropanol

30

0.04

31,648

[47]

10 wt.% water/ethanol 10 wt.% water/ethanol

30 30

0.73 0.59

183 5282

[48] [49]

50 wt.% benzene/ cyclohexane

20

0.357

7.89

[50]

10 wt.% water/ethanol 20 wt.% water/ethanol

50 50

1.424 0.513

1421 676

[51]

4. 5. 6. 7.

10. 11.

Chitosan/toluene2,4-diisocyanate Chitosan/reduced graphene oxide Chitosan/phospotungstic acid Phosphorylated chitosan Sulfonized chitosan

12.

Chitosan/MWCNT/Ag1

14. 15.

Chitosan/Mxene Chitosan/boehmite

8. 9.

Surface functionalization and complexation Solution mixing Solution mixing

1093

Chapter 3 Biobased (nanochitin, nanochitosan) polymer nanocomposite membranes

3.4.4

Chitosan hybrid membranes

This class of membranes include both the characteristics of organic and inorganic modifications in the chitosan membrane. Previously this class of membranes was made up of polymers and their compatible inorganic materials. At present, current research is focussed toward producing this class of membranes using nanomaterials due to their lesser size and high surface area to volume ratio, so as to attain greater results.

3.4.4.1

Sodium alginate/chitosan/multiwall carbon nanotube membrane

Sajjan et al. [53] studied sodium alginate/chitosan-wrapped multiwall carbon nanotubes (MWCNT)-based membranes for pervaporation of isopropanol/water mixture. The membranes were prepared with varying concentrations of MWCNT (0.5,1.0, 1.5, and 2 mass%). The membrane preparation involved synthesis of separate solutions of sodium alginate and chitosan/ MWCNT in suitable solutions. Then, the chitosan/MWCNT solution was added to the sodium alginate solution which resulted in a black gel. This gel was redissolved using sodium hydroxide solution with continous stirring for 24 h followed by ultrasonication for 30 min and was cast on a glass plate. The thickness of the membranes was found to be 50 μm. The prepared membranes (SA/CH-MWCNT) were comparatively studied with pure sodium alginate membranes. FT-IR studies showed that the intensity of bands found in pure alginate membrane was increased in the case of SA/CH-MWCNT membranes. This is due to the enhanced hydrogen bonding between sodium alginate and chitosan-wrapped MWCNT. The significant increase was seen in COO band. Wide angle XRD studies revealed that with increase in the concentration of MWCNT in the composite membrane, the amorphous character of the membrane increased as the peaks in sodium alginate was found to be decreased and broadened. The differential scanning calorimetry studies revealed that the Tg of SA/CH-MWCNT membrane is 14 degrees more than pure sodium alginate membranes whose Tg is 95 C. The reasons toward this increase is attributed to restrictions in sodium alginate chain mobility caused by incorporation of MWCNT and enhanced hydrogen bonding interactions. The TGA study of pure sodium alginate and SA/CH-MWCNT indicated that the latter membrane has improved thermal stability. This was found from the 20 C higher difference between the two membranes during 50% weight loss. The information inferred from pervaporation studies

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containing 10 wt.% of water in the isopropanol feed is the total flux permeated in the membrane is nearly same as the amount of water flux entering the membrane and the increase in MWCNT % in the SA/CH-MWCNT membranes improved selectivity of the membrane. The increase in selectivity exhibited a linear relationship till 1.5 mass% and beyond that there was a drastic increase in selectivity of about 225%. This is highly attributed to the effective increase in hydrogen bonding formation as well as the reduction in crystalline area in the sodium alginate chain by the addition of MWCNT. This trend was found to decrease with increase in water content ( . 10 wt.%) in the feed mixture.

3.4.4.2 Chitosan/PVA/multiwall carbon nanotube membrane Langari et al. [54] studied pervaporation dehydration of isoprpoyl acohol using nanocomposite membrane consisting of chitosan, PVA, and aminofunctionalised MWCNTs. The amine functionalization enables better dispersion of MWCNTs in chitosan matrix. The membranes were prepared by adding calculated amount of MWCNT (mass ratio of 0, 5, 10, and 15) in chitosan solution and selected amount of PVA was added followed by subsequent cross-linking using glutaraldehyde and then heating and casting and drying. The contact angle studies on the membranes indicated decrease in the hydrophilicity upto 10 wt.% addition of MWCNT and then slight decrease at the 15 wt.% of MWCNT. Also the tensile strength of the prepared nanocomposite membranes was found to increase for 5 and 10 wt.% but beyond that it was found to decrease due to agglomeration and improper dispersion of MWCNTs in the chitosan matrix. As expected, all the membranes had lesser degree of swelling than pure chitosan membrane. This is because of the adhesion of the rigid structured MWCNT in the chitosan polymer chain rendering it with reduced flexibility and mobility. The degree of swelling increased with increase in the concentration of MWCNT is due to the agglomeration at higher concentration but overall degree of swelling was less than pure chitosan membrane. The membranes were subjected to pervaporation studies using isopropanol feed and it was found that with increasing cioncentrations of MWCNT the permeation flux increased and sepearation factor decreased. This is because at lower concentrations of MWCNT, lesser swelling happens and the size of isopropanol molecules was comparitively larger than the voids present inside the chain. The membrane with 10 wt.%

Chapter 3 Biobased (nanochitin, nanochitosan) polymer nanocomposite membranes

MWCNT showed overall good characteristics with its PSI being 10 times greater than pure chitosan membrane.

3.4.4.3

Chitosan/PVA/Ag membrane

Cheng et al. [55] studied silver-chelated chitosan strengthened PVA membranes cross-linked with glutaraldehyde for dehydration of isopropanol applications [52]. The membrane was fabricated by solution casting method in which appropriate amounts of PVA, chitosan, and silver nitrate solution were dissolved and dried. The membrane was characterized using XRD and the result was compared with PVA membrane, chitosan membrane, and PVA/chitosan membrane. The chitosan membrane and PVA membrane possessed weaker peaks due to its semicrystalline nature. The combined PVA/chitosan exhibited combined regions of peaks from their respective pristine membrane, whereas the silver chelated PVA/chitosan membrane (PVA/CH-Ag) possessed sharp peaks which confirmed the presence of silver ions in that membrane. The peak of the PVA/CHAg membrane after cross-linking with glutaraldehyde (PVA/GH/ CH-Ag) was comparatively weaker than noncross-linked membrane. This effect is pronounced due to the steric effect created by the glutaraldehyde in the polymer matrix, indicating a less compact structure. The FT-IR results showed enhanced hydrogen bonding confirming PVA and chitosan molecular interaction (refer Fig. 3.7). The chelating of silver ion to the PVA and chitosan was understood from the shift in the amide band from higher wavenumber to a lower wavenumber. It was also suggested that the formation of acetal group within the membrane structure is due to the cross-linking action produced by glutaraldehyde. Among the studied membranes, it was clearly found that the thermal stability of PVA/GH/CH-Ag was found to be higher from TGA studies as the decomposition range was expanded from 230 C to 250 C. Contact angle measurements revealed that the addition of Silver ions have improved the hydrophilic character of the membrane due to its polar nature. In terms of mechanical strength, the PVA/GH/CH-Ag showed higher strength of 150 MPa which is B780% more than PVA/CH membrane. This enormous increase is mainly due to the incorporation of silver ions as they penetrate and interact leading to a denser membrane with their contribution being 64.7% of the total strength increase. The PVA/ GH/CH-Ag membranes with varying concentrations of silver ions (2.4 3 1022 moles, 4.7 3 1022 moles, 7.1 3 1022 moles, 9.4 3 1022 moles, and 1.17 3 1021 moles) were studied for pervaporation dehydration of 10 wt.% of isopropanol/water mixture at

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a

T(%)

b

c

d

Figure 3.7 FT-IR spectra of (a) PVA, (b) chitosan, (c) PVA/CH, (d) PVA/CH-Ag and (e) PVA/ GH/CH-Ag [52].

e 4000

3500

3000

2500

2000

1500

1000

500

Wavenumber (cm–1)

30 C. It is found that the permeation flux increased up to 362% in PVA/GH/CH-Ag membrane when compared with PVA/chitosan membrane. Also the separation factor was 89,991 which is very high compared to PVA/chitosan membrane.

3.4.4.4 Chitosan/silica/polytetrafluoroethylene membrane Moulik et al. [56] studied pervaporation separation of methanol/toluene separation using TEOS cross-linked chitosan membrane coated on polytetrafluoroethylene substrate (TEOS/ CH-PTFE) [53]. The TEOS was added to chitosan solution in the presence of hydrochloric acid catalyst to the chitosan solution and then cast on PTFE substrate. The amount of TEOS used for cross-linking was varied from 0.04, 0.06, 0.08, and 0.12 g. The prepared membranes were characterized along with pure chitosan membrane for comparison. The presence of Si-O-SI and SiO-C bands in FT-IR studies reveals that due to incorporation of TEOS, the amine groups of chitosan reacted and formed ionicbonding between the silanol groups of chitosan. This resulted in enhanced strength and rigidity of the membrane due to TEOS in chitosan. From TGA studies, it was found that the glass transistion temperature, Tg, of chitosan was about 172 C, whereas that of TEOS/CH-PTFE was about 232 C. This significant difference confirms an increase in the thermal stability of the membrane. The effect of TEOS concentration on the mechanical properties of the TEOS/CH-PTFE membranes was analyzed and it was found that 0.12 g concentration of TEOS in the composite exhibited a gel-like structure rather than a

Chapter 3 Biobased (nanochitin, nanochitosan) polymer nanocomposite membranes

membrane. The maximum tensile strength of around 32 MPA was found in membrane with the second highest TEOS concentration (0.08 g) taken for further studies. The methanol flux permeation increased with increase in feed concentration (10 68 wt.% of methanol in methanol/toluene mixture), since methanol diffuses more due to its hydrophilic nature but selectivity decreased at very high concentrations. This is attributed to more swelling at high concentrations and leading to more volumes in the membrane. The highest seperation factor of 401.96 was found for lowest concentration of methanol and the lowest 58.4 for highest methanol concentration in the feed for TEOS/CH-PTFE membrane. These values are higher than that of pure chitosan membranes which gave separation factor value of 144.06 and 7.51 for the same conditions. Also the membrane thickness and pressure on the permeate side have significant impacts on permeation flux and separation factor of composite membrane. If the permeate side pressure gets increased, the flux and separation factor both decreases. This is same for the case of increase in thickness also. The reason is due to decrease in transportation force for the methanol to diffuse out of the membrane.

3.4.4.5

Chitosan/iron oxide/PAN membrane

Xing et al. [57] prepared modified chitosan membranes containing ferric oxide for pervaporation of ethanol/water mixtures. Fe3O4 and ϒ-Fe2O3 possess excelent thermal and chemical stability which makes them effective inorganic fillers in membranes. They also have excellent magnetic response which makes their distribution in polymer matrix controlled by an external magnetic field. The only drawback of these fillers is that these get agglomerated which makes them difficult to disperse uniformly. To overcome this, inorganic fillers are synthesized in situ by incorporating their precursors into polymer matrix. This is done by attaching chelating groups in polymer. Fe3O4 and ϒ-Fe2O3 are effectively dispersed into chitosan by this technique and were studied. The chelating groups of Fe3O4 and ϒ-Fe2O3 are hydroxyl and amine groups. These groups are already present in chitosan. The membranes were prepared by incorporating ferric chloride and ferrous chloride precursor into chitosan solution containing glutaraldehyde (cross-linker). The solution mixture was spin coated on polyacrylonitrile substrate and soaked in hydroxyl solution. Then the membranes were subjected to external magnetic field of 200 Mt. The membranes were desiginated as M-CS-Fe3O4(X)/PAN, where M represents magnetic field and X represents mass ratio of ferric oxide to

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Chapter 3 Biobased (nanochitin, nanochitosan) polymer nanocomposite membranes

chitosan. In this study, the mass ratio was varied between 0 to 15. The SEM results showed homogenous distribution of ferric oxide nanoparticles on the surface of the membranes. The XRD data of M-CS-Fe3O4(X)/PAN shows five new peaks compared to pure chitosan membrane. The peaks obseved in chitosan appeared with decreased intensity in the modified membranes. This is because the ferric oxide nanoparticles interfered in the packing of chitosan chains and further lowered the semicrystalline nature. The FT-IR data showed a characteristic vibration at 1078 to 1070 cm21 in the hybrid membrane confirming interaction between chitosan and ferric oxide nanoparticles. Another characteristic vibration at 607 cm21 corresponding to Fe-OH was found which further confirmed the presence of ferric oxide nanoparticles in the hybrid membrane. The pervaporation studies of the hybrid membranes were conducted in 90 wt.% of ethanol/water mixture at 350K. The separation factor of the M-CS-Fe3O4(X)/PAN membranes increased till the mass ratio 10 and then showed decrease for mass ratio 15. The reason is attributed to the random distribution of ferric oxide nanoparcticles within chitosan matrix. During the application of magnetic field, these ferric oxide nanoparticles drag the chitosan polymeric chains around themselves and extend diffusion path for permeation of molecules. This resulted in enhanced seperation factor for mass ratios 0 to 10. At higher mass ratio of 15 the ferric oxide nano particles agglomerated and facilitated formation of interface voids rigidified between polymer layers and chitosan matrix. Hence a decrease in seperation factor at higher mass ratio. The highest seperation factor of 674 was found for hybrid membrane containing mass ratio 10.

3.4.4.6 Chitosan/silica/PAN/PEG membrane Asghari et al. [58] studied pervaporation of ethanol/water mixture using nanosilica-incorporated chitosan membranes. The modified membranes were prepared on PAN/PEG substrate. The mixture of chitosan solution along with precursor solution consisting of 10 wt.% of either APTEOS or TEOS was mixed together and cast on the PAN/PEG substrate. It has been proved that the silica nanoparticles tend to reinforce mechanical strength in the chitosan membrane along with reducing plasticization effect. The FT-IR studies confirmed the SiO2 incorporation in the modified membrane. It was found that the intensity of band at 1231 cm21 increased indicating the esterification reaction between the hydroxyl groups and silicon dioxide particles. The AFM characterization studies revealed increase in surface

Chapter 3 Biobased (nanochitin, nanochitosan) polymer nanocomposite membranes

roughness of the chitosan membranes due to incorporation of silicon precursors. This is because of the interaction of SiO2 causing decrease in fluidity of the membrane and this was found to be more in APTEOS-added membrane. The XRD data revealed that the membranes became amorphous with addition of silicon dioxide precursors and this amorphous nature was more in APTEOS-modified chitosan membrane. Pervaporation studies of modified membranes were conducted at temperatures of 30 C to 70 C. It is observed that with increase in temperature of feed, the permeation flux increased and selectivity decreased. This is due to the increased motion of chains with increase in temperature. The membranes made with APTEOS exhibited separation factor from 380 at 30 C to 150 at 70 C and permeation flux increased from 850 to 1100 for the same conditions.

3.4.4.7

Chitosan/aluminum-based metal organic framework membrane

Vinu et al. [59] studied modified chitosan membranes using aluminum-based metal organic framework (Al-MOF). These MOFs are advantageous than zeolite since the pore size and surface functionality in the MOF can be easily controlled which is not possible in zeolites. The membranes were prepared by adding synthesized Al-MOF to chitosan solution prepared in acetic acid and were further cross-linked using glutaraldehyde. The prepared Al-MOFs used in this study can be differentiated by the presence of SO2 groups and CH2 groups. Of the prepared membranes, those with CH2 groups exhibited hydrophobic character, whereas SO2 groups exhibited hydrophilic character. In the latter, differences in hydrophilic nature supported by higher difference in contact angle proved that the hydrophilic character of the membrane was not only on functional groups but also dependent on the pore sizes of the framework in these membranes. The pervaporation study of ethanol water mixture at 25 C showed improved permeation fluxes and separation factor than pure chitosan membranes. This is highly due to the presence of functional groups, pore size and also the defects present in the membrane (Table 3.3). Al-MOF, Aluminum-based metal organic framework; MWCNT, multiwall carbon nanotube.

3.5

Conclusion

Chitosan membranes unlike chitin membranes have wide spread applications in pervaporation, and it is mainly attributed

75

Table 3.3 Chitosan hybrid membranes. S. no Membrane

Way of modification

Feed

Operation temperature (˚C)

Permeation flux (kg m22 h21)

Separation factor/ Reference selectivity

1.

Solution mixing

10 wt.% water/ isopropanol 30 wt.% water/ isopropanol 30 wt.% water/ isopropanol 10 wt.% methanol/toluene 10 wt.% water/ ethanol 10 wt.% water/ isopropanol 10 wt.% water/ ethanol

30

0.218

6420

27

0.795

30

0.62

30

0.13

144.06

[56]

77

1.042

674

[57]

30

0.85

380

[58]

25

0.458

2741

[59]

2

Sodium alginate/ chitosan/MWCNT Chitosan/PVA/MWCNT

Solution mixing

3.

Chitosan/PVA/Ag

Solution mixing

4.

6.

Chitosan/silica/ Solution mixing polytetrafluoroethylene Chitosan/iron oxide/PAN Solution mixing and spin coating Chitosan/silica/PAN/PEG Solution mixing

7.

Chitosan/Al-MOF

5.

Solution mixing

99.48 8991

[53] [54] [55]

Chapter 3 Biobased (nanochitin, nanochitosan) polymer nanocomposite membranes

to the significant difference in structural stability and permeation capability. Chitosan being a hydrophilic polymer can absorb and permeate water preferentially but indirect relationship between the rate and the purity of permeation was a concern. This difference could be negated by using several polymers, inorganic particles and compounds and also their combinations. All the above solutions are highly dependent on their interactions with the two main groups within the chitosan matrix—amine and hydroxy groups. The modifications are done either through grafting or surface adsorption, formation of polyelectrolyte complex or blending, etc. Also the role of other additional fillers in case of inorganic and hybrid composite membrane is significant as they provide numerous networked nanosized pathways for water to pass through them. Overall, the success mainly depends on the following factors when these organic and inorganic moieties are added to chitosan: • Intermolecular and intramolecular hydrogen interactions. • Hydrophilic and hydrophobic balance. • Change in crystallinity of membrane. • Way of dispersion of organic/inorganic fillers (responsible for effective permeation pathways). • Chemical interactions between fillers and with chitosan in case of hybrid membranes. More research needs to be directed to enrich chitosan membrane pervaporation performance at high temperature and feed concentrations with significant decline in the tradeoff between permeation flux and separation factor.

References [1] R.A.A. Muzzarelli, Chitin, Elsevier Science & Technology, University of California, 1977. [2] M. Rinaudo, Chitin and chitosan: properties and applications, Prog. Polym. Sci. 31 (7) (2006) 603 632. [3] P. kumar Dutta, J. Dutta, V.S. Tripathi, J. Sci. Ind. Res. 63 (1) (2004) 20 31. [4] V. Zargar, M. Asghari, A. Dashti, ChemBioEng Rev. 2 (3) (2015) 204 226. [5] S.M. Hudson, C. Smith, Polysaccharides: Chitin and chitosan: Chemistry and technology of their use as structural materials, in: D.L. Kaplan (Ed.), Biopolymers from Renewable Resources. Macromolecular Systems Materials Approach, Springer, Berlin, Heidelberg, 1998, pp. 96 118. [6] S. Yang, K.-F. Leong, Z. Du, C.-K. Chua, Tissue Eng. 6 (2004) 679 689. [7] R.A.A. Muzzarelli, F. Greco, A. Busilacchi, V. Sollazzo, A. Gigante, Carbohydr. Polym. 89 (3) (2012) 723 739. [8] V.K. Thakur, M.K. Thakur, ACS Sustain. Chem. Eng. 2 (2014) 2637 2652. [9] C. Jeon, W.H. Ho¨ll, Water Res. 37 (19) (2003) 4770 4780. [10] V. Dodane, V.D. Vilivalam, Pharm. Appl. Chitosan, PSTT 1 (6) (1998) 246 253.

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[11] Y.M. Lee, S.Y. Nam, J.H. Kim, Polym. Bull. 29 (1992) 423 429. [12] R.P. Yadav, M.K. Chauhan, Int. J. Pharm. Sci. Res. 2 (2) (2017) 6 11. [13] J.G. Wijmans, A.L. Athayde, R. Daniels, J.H. Ly, H.D. Kamaruddin, I. Pinnau, J. Membr. Sci. 109 (1) (1996) 135 146. [14] G. Jyoti, A. Keshav, J. Anandkumar, J. Eng. 2015 (2015) 1 24. Article ID 927068. [15] C.K. Yeom, K.-H. Lee, J. Appl. Polym. Sci. 63 (2) (1997) 221 232. [16] T. Sannan, K. Kurita, Y. Iwakura, Macromol. Chem. Phys. 176 (4) (1975) 1191 1195. [17] T. Uragami, Y. Naito, M. Sugihara, Polym. Bull. 4 (1981) 617. [18] S.-ichi Aiba, M. Izume, N. Minoura, Y. Fujiwara, Carbohydr. Polym. 5 (1985) 285 295. [19] M. Rinaudo, G. Pavlov, J. Desbrieres, Polymer 40 (1999) 7029 7032. [20] S.Y. Park, H.J. Park, X.Q. Lin, Y. Sano, Characterization of chitosan film and structure in solution, Hydrocolloids, Part 1, Elsevier, 2000, pp. 199 204. [21] A. Mochizuki, Y. Sato, H. Ogawara, S. Yamashita, J. Appl. Polym. Sci. 37 (1989) 3375 3384. [22] Tadashi Uragami, K. Takigawa, Polymer 31 (1999) 668 672. [23] C. Clasen, T. Wilhelms, W.-M. Kulicke, Biomacromolecules 7 (2006) 3210 3222. [24] Y.-J. Han, K.-H. Wang, J.-Y. Lai, Y.-L. Liu, J. Membr. Sci. 463 (2014) 17 23. [25] S. Cao, Y. Shi, G. Chen, J. Appl. Polym. Sci. 74 (1999) 1452 1458. [26] Q.G. Zhang, W.W. Hu, A.M. Zhu, Q.L. Liu, UV-crosslinked chitosan/ polyvinylpyrrolidone blended membranes for pervaporation, RSC Adv. 3 (2013) 1855 1861. [27] K.S.V. Krishna Rao, M.C.S. Subha, M. Sairam, N.N. Mallikarjuna, T.M. Aminabhavi, J. Appl. Polym. Sci. 103 (2007) 1918 1926. [28] S. Zhang, Y. Zou, T. Wei, C. Mua, X. Liu, Z. Tong, Sep. Purif. Technol. 173 (2017) 314 322. [29] Y. Zhu, S. Xia, G. Liu, W. Jin, J. Membr. Sci. 349 (2010) 341 348. [30] S.Y. Nam, Y.M. Lee, J. Membr. Sci. 135 (1997) 161 171. [31] P.-Y. Zheng, C.-C. Ye, X.-S. Wang, K.-F. Chen, Q.-F. An, K.-R. Lee, et al., J. Membr. Sci. 515 (2016) 220 228. [32] A. Subba Reddy, S. Kalyani, N. Siva Kumar, V.M. Boddu, A. Krishnaiah, Polym. Bull. 61 (2008) 779 790. [33] Q. Zhao, J. Qiana, Q. An, C. Gao, Z. Gui, H. Jin, J. Membr. Sci. 333 (2009) 68 78. [34] S.V. Kononovaa, A.V. Volod’kob, V.A. Petrovaa, E.V. Kruchininaa, Y.G. Baklaginaa, E.A. Chusovitinc, et al., Carbohydr. Polym. 181 (2018) 86 92. [35] S.B. Teli, G.S. Gokavi, T.-M. Tak, T.M. Aminabhavi, Sep. Sci. Technol. 44 (13) (2009) 3202 3223. ´ [36] K. Zielinska, W. Kujawski, A.G. Chostenko, Sep. Purif. Technol. 83 (2011) 114 120. [37] J.G. Varghese, A.A. Kittur, P.S. Rachipudi, M.Y. Kariduraganavar, J. Membr. Sci. 364 (2010) 111 121. [38] S.K. Choudhari, M.Y. Kariduraganavar, J. Colloid Interface Sci. 338 (2009) 111 120. [39] D. Yang, J. Li, Z. Jiang, L. Lu, X. Chen, Chem. Eng. Sci. 64 (2009) 3130 3137. [40] G. Dudek, M. Gnus, R. Turczyn, A. Strzelewicz, M. Krasowska, Sep. Purif. Technol. 133 (2014) 8 15. [41] S.P. Dharupaneedi, R.V. Anjanapura, J.M. Han, T.M. Aminabhavi, Functionalized graphene sheets embedded in chitosan nanocomposite

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[42] [43] [44] [45] [46] [47] [48] [49] [50] [51] [52] [53] [54] [55] [56] [57] [58] [59]

membranes for ethanol and isopropanol dehydration via pervaporation, Ind. Eng. Chem. Res. 53 (37) (2014) 14474 14484. H.G. Premakshi, K. Ramesh Mahadevappa, Y. Kariduraganavar, Chem. Eng. Res. Des. 94 (2015) 32 43. J.G. Varghese, R.S. Karuppannan, M.Y. Kariduraganavar, J. Chem. Eng. Data 55 (2010) 2084 2092. D. Unlu, N.D. Hilmioglu, J. Membr. Sci. 559 (2018) 138 147. D. Anjali Devi, B. Smitha, S. Sridhar, T.M. Aminabhavi, J. Membr. Sci. 262 (2005) 91 99. W.-S. Hung, S.-M. Chang, R.L.G. Lecaros, Y.-L. Ji, Q.-F. An, C.-C. Hu, et al., Carbon 117 (2017) 112 119. P.S. Rachipudi, A.A. Kittur, S.K. Choudhari, J.G. Varghese, M.Y. Kariduraganavar, Eur. Polym. J. 45 (2009) 3116 3126. K. Sunitha, S.V. Satyanarayana, S. Sridhar, Carbohydr. Polym. 87 (2012) 1569 1574. R.P. Pandey, V.K. Shahi, J. Membr. Sci. 444 (2013) 116 126. J.-nan Shen, Y.-xia Chu, H.-min Ruan, L.-guang Wu, C.-jie Gao, B.V. der Bruggen, J. Membr. Sci. 462 (2014) 160 169. Z. Xu, G. Liu, H. Ye, W. Jin, Z. Cui, J. Membr. Sci. 563 (2018) 625 632. M. Esmaeili, L. Rajabi, O. Bakhtiari, J. Macromol. Sci. A, Pure Appl. Chem. (2019) 1 13. A.M. Sajjan, B.K. JeevanKumar, A.A. Kittur, M.Y. Kariduraganavar, J. Membr. Sci. 425 426 (2013) 77 88. S. Langari, E. Saljoughi, S.M. Mousavi, Polym. Adv. Technol. (2017) 1 11. P.-I. Cheng, P.-D. Honga, K.-R. Lee, J.-Y. Lai, Y.-L. Tsai, J. Membr. Sci. 564 (2018) 926 934. S. Moulik, B. Vani, S.S. Chandrasekhar, S. Sridhar, Carbohydr. Polym. 193 (2018) 28 38. R. Xing, H. Wu, C. Zhao, H. Gomaa, J. Zhao, F. Pan, et al., Chem. Eng. Technol. 39 (2016) 969 978. M. Asghari, M. Sheikh, M. Afsari, M. Dehghani, J. Mol. Liq. 247 (2017) 7 16. M. Vinu, D.S. Raja, Y.-C. Jiang, T.-Y. Liu, Y.-Y. Xie, Y.-F. Lin, et al., J. Taiwan. Inst. Chem. E (2017) 1 9.

Further reading M. Goto, A. Shiosaki, T. Hirose, Sep. Sci. Technol. 29 (14) (1994) 1915 1923.

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Pervaporation performance of polymer/clay nanocomposites

4

Runcy Wilson1 and Gejo George2 1

Department of Chemistry, St. Cyrils College, Adoor, India 2School of Pure & Applied Physics, Mahatma Gandhi University, Kottayam, India

4.1 4.1.1

Introduction Polymer nanocomposites

Composite is a material which is formed from two or more components combined in a way that allows the individual materials to be physically distinct and identifiable. The major component of a composite system is the matrix and it controls the properties of the composite. The second component of a composite is the reinforcement material and can be in different forms such as small particles, fibers, whiskers, etc. The composites can be classified based on their reinforcement, for example, particle-reinforced (particulate composites), laminarreinforced (laminate composites), and fiber-reinforced (fibrous composites) [1]. Fiber-reinforced composites can be further subdivided on the basis of the nature of fiber used (natural, biofiber, or synthetic fiber). The biofiber composites can be again divided on the basis of the type of matrix used (nonbiodegradable matrix or biodegradable matrix) [2]. While considering the composites based on the reinforcement, the particles have no desired alignment, fibers are materials with one very long axis and other two axes either often circular or near circular and the whiskers have a preferred shape but are small both in diameter and in length when compared with fibers [3]. The composites can also be classified based on their physicochemical nature of the matrix phase, for example, polymer matrix composites, metal matrix composites, and ceramic matrix composites. The nature of reinforcement, concentration, distribution, orientation, size and shape of reinforcements, etc. are the parameters that control the final properties of the composite [4]. Polymer is the most widely used matrix material in composites and is lighter in weight, softer, and easier to process Polymer Nanocomposite Membranes for Pervaporation. DOI: https://doi.org/10.1016/B978-0-12-816785-4.00004-5 © 2020 Elsevier Inc. All rights reserved.

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Chapter 4 Pervaporation performance of polymer/clay nanocomposites

compared to metals or ceramics. Polymer nanocomposites are made up of polymer as the major constituent and filled with inorganic or organic fillers with at least one dimension of the filler outside the nanosale. A very high surface area-to-volume ratio is obtained as a result of the smaller size of the reinforcing material, leading to better bonding between the reinforcement and the matrix [5
7]. The fillers can be classified mainly into three types, depending on the dimensions in the nanometric range. In zero-dimensional (0D) nanomaterials, all the dimensions are measured within the nanoscale (no dimensions are larger than 100 nm). In one-dimensional (1D) nanomaterials, one dimension is outside the nanoscale (e.g., nanotubes, nanowires, nanorods, etc.). In two-dimensional (2D) materials, two dimensions are outside the nanoscale (e.g., graphene, nanofilms, nanolayers, and nanocoatings). Three-dimensional materials are not confined to the nanoscale in any dimension. This class can contain bulk powders, dispersions of nanoparticles, bundles of nanowires, and nanotubes as well as multinanolayers. The composites made by mixing polymer and nanoclay are known as polymer-layered nanocomposites and are obtained by the intercalation of the polymeric chain within the galleries of the layered structures [8
10].

4.1.2

Structure of nanoclay

Nanoclay is also referred as montmorillonite and the structure in general contains a number of platelets with an inner octahedral layer packed between two silicate tetrahedral layers. The octahedral layer has an aluminum oxide sheet and some of the aluminum atoms are exchanged with magnesium; negative charges are produced due to the difference in combining capacities of Al and Mg within the plane of the platelets that are balanced by sodium ions, positioned between the platelets [11]. The montmorillonite clay is made up of stacks of many platelets and the hydration of the sodium ions results in the clay to swell leading to these platelets to be fully dispersed in water. The structure of clay particles can be in layers; each layer is composed of two types of structural sheets: octahedral and tetrahedral. The tetrahedral sheet is composed of silicon-oxygen tetrahedra linked to neighboring tetrahedra by sharing three corners, resulting in a hexagonal network, remaining fourth corner of each tetrahedron forms a part to adjacent octahedral sheet. The octahedral sheet is usually composed of aluminum or magnesium in sixfold coordination with oxygen from the tetrahedral sheet and with hydroxyl group. The two sheets

Chapter 4 Pervaporation performance of polymer/clay nanocomposites

Figure 4.1 Smectite structure of a 2:1 clay mineral [12]. Source: Reprinted with permission from M. Ghadiri, W. Chrzanowski, R. Rohanizadeh, Biomedical applications of cationic clay minerals, RSC Adv. 5 (2015) 29467. Copyright (2015) with permission from RSC.

together form a layer, and several layers may be joined in a clay crystallite by interlayer cations, Vander Waals force, electrostatic force, or hydrogen bonding. The elementary structural units are silica tetrahedron and aluminum octahedral. The cation, Si14, is fourfold and possesses tetrahedral coordination with oxygen, while the cation, Al13, occurs in sixfold or octahedral coordination. Various clay minerals can be described by the arrangement of tetrahedral and octahedral sheets, that is, 1:1 clay mineral would have one tetrahedral and one octahedral sheet per clay layer; 2:1 clay mineral would contain two tetrahedral sheets and one octahedral sheet sandwiched between the two tetrahedral sheets (montmorillonites an example of a clay mineral having 2:1 sheet-structure); and 2:1:1 clay minerals are composed of an octahedral sheet adjacent to a 2:1 layer (Fig. 4.1) [12].

4.1.3

Organic modification of nanoclay

Organically modified clays created a great interest among researchers because of their wide industrial applications. The modification of clays can be done to modify the silicate surface from hydrophilic to organophilic and it improves the compatibility with less polar matrices (Scheme 4.1). To prepare an organically modified clay, sodium ions are replaced with different cationic surfactants such as quaternary alkyl ammonium compounds (organic functionalities). The long carbon chain alkyl quaternary ammonium compounds (cationic surfactants) such as dodecyl trimethyl ammonium bromide, hexadecyl trimethyl ammonium bromide, cetyl trimethyl ammonium bromide, and octadecyl trimethylammonium bromide are common

83

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Chapter 4 Pervaporation performance of polymer/clay nanocomposites

Scheme 4.1 Organic modification of nanoclay.

materials for nanoclay modification. By cationic exchange mechanism, cationic surfactants are easily penetrated into the MMT interlayer and intercalation state can occur. The ammonium cation may have a long hydrocarbon tail and is referred to as a “surfactant” owing to its amphiphilic nature. The cation exchange capacity denotes the extent of negative charge on the montmorillonite clay (CEC) [13]. The clay can also modify with nonionic surfactants such as polypropylene glycol and amphiphilic phospholipid [14]. Rodrı´guez-Cruz et al. [15] modified the montmorillonite, kaolinite, and palygorskite clay minerals and a clayey soil with cationic surfactant octadecyl trimethyl ammonium bromide (ODTMA). The results revealed that the use of reactive clay barriers can be used for decreasing the leaching of hydrophobic pesticides coming from point-like sources of pollution. These barriers would avoid the generation of elevated concentrations of these compounds in the soils due to their rapid washing. Clay modification can also be done by grafting of functional polymers to the surface of the clay, using ionic liquids such as imidozolium, pyridinium and phosphonium derivatives, 1,6-diamino hexane, etc. [16]. Bottino et al. [17] prepared polystyrene/montmorillonite nanocomposites using in situ bulk polymerization, sodium cations in the interlayer space of montmorillonite were exchanged using C12, C16, and C18. The interlayer spacing of the clay layers was examined using XRD and found that the d spacing is increased with the alkyl chain length (for MMT, d was 1.48 nm while it was 1.61, 1.80, and 1.88 nm for C12, C16, and C18/MMT) (Scheme 4.2). Surface modifications also help to reduce tendency of clay agglomeration during dispersion by reducing interlayer attractive forces between the platelets thereby enhancing dispersion [18].

Chapter 4 Pervaporation performance of polymer/clay nanocomposites

Scheme 4.2 Structures of C12, C16, and C18 used to treat MMT [17].

Scheme 4.3 Different forms of layered silicates.

4.1.4

Polymer nanoclay composites

Toyota research group in 1989 fabricated the first polymer clay nanocomposite with a loading of 5% clay [19,20]. Polymer
clay nanocomposites are typically hybrid composites made of a polymeric matrix containing dispersed clay nanoparticles in it. Nanoclay has been used commonly as an inorganic reinforcement in the polymeric matrix. Three different types of PLS nanocomposites can be thermodynamically achieved as per the strength of interfacial adhesion between the matrix and filler and are as follows: (1) microcomposite, (2) intercalated, and (3) exfoliated nanocomposites (Scheme 4.3). As a result of the high contact surface area, exfoliated system exhibits better properties [21]. The addition of nanoclay in the polymer matrix can lead to an increase in tensile strength [22], higher fire retardant properties [23], and low gas permeability [24]. The

85

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Chapter 4 Pervaporation performance of polymer/clay nanocomposites

enhanced properties of the composite by the addition of nanoclay are probably due to the nanoscale structure effects and the interaction between silicate layers and polymer matrix. Messersmith and Giannelis [25] prepared epoxy-organically modified clay nanocomposites and in their study, the effects of different curing agents (nadic methyl anhydride, benzyl dimethyl amine, or boron triflouride monoethyl amine) and curing conditions on the formation of nanocomposites were studied in detail. They have prepared nanocomposites using diglycidyl ether of bisphenol A epoxy as the matrix and MMT clay modified by bis(2-hydroxyethyl) methyl-hydrogenated tallow alkyl ammonium cation. It has been displayed that the organomodified nanoclay becomes thoroughly dispersed in the epoxy matrix after sonication. The dynamic mechanical analysis of the nanocomposite showed substantial increases in dynamic storage modulus by the incorporation of 4 vol.% of silicate. It was also noted that there is broadening and shift in Tg value, due to the good interfacial adhesion between the epoxy matrix and the silicate particles. The chemical bonding in between the silicate and epoxy matrix could reduce the relaxation mobility in the polymer segments, which leads to broadening and increase of Tg. Sun et al. [26] prepared organically modified nanocomposites via a solid-state method. The results revealed that the organically modified montmorillonite in the nanocomposite fasten the curing process and improved the mechanical properties of natural rubber matrix. It was also noted that the aging resistance of the samples was improved for the nanocomposites. The improvement in the properties was attributed mainly to the partial intercalation of the organophilic clay by natural rubber macromolecules. A lot of works have been reported based on thermoplastic clay [27
29], elastomer clay [30
32], thermoset clay nanocomposites [33
37], and so far. In this chapter, we are discussing about the transport properties of the polymer clay nanocomposites especially on the pervaporation (PV) process of polymer/clay nanocomposites membranes. Polymeric membranes have been widely used in various separation processes such as desalination of brine, oxygen enriched air, diary food processing, removal of water from organics, etc. The basic understanding behind these transport processes helps in the designing and fabrication of various equipments that can be used in industrial processes. For example, if there is an interaction between a solvent molecule and polymer, it creates local strains in the polymer and this might change the overall properties of that polymer [38]. On being exposed to atmospheric moisture or vapors for a long time, polymeric membranes may

Chapter 4 Pervaporation performance of polymer/clay nanocomposites

87

Table 4.1 Polymeric membrane-based process. Polymer membrane processes Pressure driven processes

Concentration driven processes

Electric potential gradient processes

Temperature gradient processes

Microfiltration Ultra filtration Nano filtration Reverse osmosis Gas separation Pervaporation

Dialysis Osmosis Forward osmosis

Electro dialysis Membrane electrolysis Electrophoresis

Membrane distillation

get degraded or modified. The transport functions in the polymers can be mainly divided into two: one is related to the transport properties of polymers themselves, and the other relates to their properties as a transport media for other small molecules. In the former case polymers serve as transport carriers and in the latter case they are used as permeable membranes. A membrane can act as a flexible selective barrier for regulating the transport of substances between the two partitions. The selective transport is attained based on the differences in the physical and/or chemical properties of permeating components across the membrane. The major advantage of membrane technology is its selectivity and the nonrequirement of any additives and its ability to perform isothermally at low temperature [39]. The membrane processes are widely used in the textile [40], pharmaceutical [41], pulp and paper [42], semiconductor [43], tanning and leather [44], food and beverage processing [45], and wastewater treatment industries [46]. Various polymeric membranebased processes are presented in Table 4.1.

4.2

Pervaporation characteristics

The general principle of PV is that the liquid mixture to be separated is placed in contact with one side of a membrane and the permeated product is removed as a low-pressure vapor from the other side by liquids selectively transporting through a homogeneous, nonporous membrane with the evaporation of permeates (Fig. 4.2). The important factors that control the efficiency of this process are the partial vaporization of feed

88

Chapter 4 Pervaporation performance of polymer/clay nanocomposites

Feed/retentate

Driving force

Membrane

Flux Permeate

Figure 4.2 Schematic diagram of membrane process.

molecules and a phase change of permeate. The separation efficiency achieved is dependent on the sorption and diffusion differences between the feed components, which in turn are controlled by the complicated interactions between the feed components, the membrane material, and permeate. The major advantages and disadvantages of PV process are mentioned in Table 4.2.

4.2.1

Transport mechanism

Mass and heat transfer are involved in the PV process because it contains liquid and gas separation. A phase transition occurs from the feed to the permeate side wherein the membrane behaves as a barrier between the liquid and vapor phase. Chemical potential gradient (fugacity gradient for each species) is the driving force behind the mass transport across the membrane, which can be produced by applying either a vacuum pump or an inert purge (normally air or steam) on the permeate side to maintain the permeate vapor pressure lower than the partial pressure of the feed liquid. Hence fugacity differences between permeates and by permeability differences of the membrane toward different permeates leads to the selectivity of the membrane. The ultimate separation performance to a liquid mixture is determined by three factors: (1) physicochemical properties of feed mixtures and their own interactions, (2) the affinities of permeates toward the macromolecules of the membrane material, and (3) the membranes physical structure. There are two approaches to describe the mass transport in PV (1) the solution diffusion model and (2) the pore flow model.

Chapter 4 Pervaporation performance of polymer/clay nanocomposites

89

Table 4.2 Advantages and disadvantages of pervaporation process. Advantages of pervaporation

Disadvantages of pervaporation

1. Pervaporation separation system requires a 1. It is an environmentally benign and energy efficient purified feed. technology. 2. Process is completely enclosed, thereby minimizing direct and 2. Temperature reduction in Pervaporation reduces the transmembrane flux. escape emissions. 3. System is compact, sectional and easily transportable. 4. Considerably reduces the energy demand because only the fraction of the liquid needs to be vaporized. 5. Opportunity for recovering concentrated organics.

Dissolution

Feed liquid

n

sio

u iff

Membrane

D

Evaporation

Permeate vapor

Figure 4.3 Mechanism of solution diffusion model.

4.2.2

Solution diffusion mechanism

The solution diffusion model [47,48] is generally used to explain the mechanism behind porous membrane. Conceptually this mechanism can be divided into three steps: (1) the sorption of permeate from the feed liquid to the membrane, (2) the diffusion of permeate in the membrane, and (3) desorption of the permeate to the vapor phase on the downstream side of the membrane (Fig. 4.3). It hypothesizes that solubility and diffusivity are the factors that govern or control the selectivity and permeation rate of the feed components permeating across the membrane. The chemical nature of the membrane material and the permeating molecules determines the

90

Chapter 4 Pervaporation performance of polymer/clay nanocomposites

Membrane L2

Feed

L1

Permeate (Vapor)

Figure 4.4 Mechanism of pore flow model.

solubility of a feed component in the membrane and may be qualitatively estimated using the solubility parameter [49], whereas the diffusivity depend on both chemical and physical factors such as the size and the shape of penetrate molecules as well as the mutual interactions between the penetrate molecules and the membrane material [50].

4.2.3

Pore flow mechanism

The pore flow model [51,52] is proposed by Okada and Matsuura and it assumes that there is a bundle of straight cylindrical pores of length L penetrating across the active surface layer of the membrane which is perpendicular to the membrane surface. Furthermore it also hypothesizes that the pores are filled with liquid from the pore inlet to a distance L2 along the cylindrical axis as shown in Fig. 4.4. The rest of pore is filled with vapor with the length L1. A liquid vapor phase boundary is believed to exist somewhere in the middle of the membrane pore. The mechanism of mass transport by the pore flow model can also be divided into three steps: (1) the liquid transports from the pore inlet to a liquid vapor phase boundary, (2) the liquid evaporates at the phase boundary, and (3) the vapor transports from the boundary to the pore outlet.

4.3

Factors affecting membrane performance

In PV process the feed liquid comes in direct contact with the membrane and hence, the interaction between the membrane and the feed liquid plays an important role on the membrane separation performance. The operating parameters such

Chapter 4 Pervaporation performance of polymer/clay nanocomposites

as downstream pressure, feed composition, and temperature are important factors that control the separation performance of membranes [53
55].

4.3.1

Effect of nanoclay content in pervaporation process

The selectivity of the membrane is directly affected by the concentration of the filler in the nanocomposite and is also dependent on the diffusion of the components. Kanti et al. [56] dehydrated ethanol using chitosan and sodium alginate blend membranes by PV process. It is found that the cross-linked blend membranes have very good selectivity of 436 accompanied with a flux of 0.22 kg m22 10 μm21 h21 for the azeotropic composition of 0.135 mol fraction of water. The membrane selectivity was increased with membrane pressure reduction and was found to be relatively constant for variable membrane thickness. Choudhari and Kariduraganavar [57] dehydrated isopropanol using chitosan membrane using quaternized chitosan and montmorillonite nanoclay (Na1-MMT) using PV process. In their study, they demonstrated that the membrane containing 10 mass % of nanoclay showed the highest separation selectivity of 14,992 with a flux of 14.23 3 1022 kg m22 h21 at 30 C and this was for 10 mass% of water in the feed. It was reported that the quaternized chitosan and Na1-MMT clay are highly selective toward water. Susheelkumar et al. [58] performed the PV process to dehydrate the aqueous mixtures of isopropanol and 1,4-dioxane through poly(vinyl alcohol) membranes. The results revealed that the PVA membranes loaded with 5% and 10% of nanoclay membranes when tested for 10 wt.% water-containing isopropanol feed, flux values of 0.051 and 0.075 kg m22 h21, respectively was observed. For the case of water and 1,4-dioxane feed mixtures, the separation factors were found to be moderately lower (216
369) with the corresponding flux values of 0.076 and 0.093 kg m22 h21, respectively (this was observed for 10 wt.% water in the feed). From this study, it can be concluded 99.20 and 99.60 wt.% water on the permeate side from water
isopropanol mixture, whereas from water
1,4-dioxane mixture 96.0 and 97.62 wt.% of water were removed for 10 wt.% watercontaining feed. Jose et al. [59] inspected the PV performance of PVA nanocomposites membranes for the separation of isopropanol and water mixture (azeotropic composition). From Fig. 4.5A, it can be clearly understood that the component flux increases by the

91

92

Chapter 4 Pervaporation performance of polymer/clay nanocomposites

Figure 4.5 (A) Variation of component flux with concentration of bentonite clay. (B) Variation of flux and separation factor with concentration of nanoclay for the separation of azeotropic composition of IPA 2 water mixture [59]. Source: Reprinted with permission from T. Jose, S.C. George, H.J. Maria, R. Wilson, S. Thomas, Effect of bentonite clay on the mechanical, thermal, and pervaporation performance of the poly (vinyl alcohol) nanocomposite membranes, Ind. Eng. Chem. Res. 53 (2014) 16820. Copyright (2014) American Chemical Society.

addition of 1 wt.% clay and the majority of total flux was for water than IPA. They found that PVA/bentonite clay membrane is an effective membrane for the dehydration of IPA from its azeotropic composition. Fig. 4.5B provides an idea about the separation factor of the membranes with different concentrations of nanoclay. The separation factor showed an increment upon the addition of 1 wt.% clay to PVA and then decreased with a further increase in the concentration of bentonite clay and this can be attributed to the hydrophilic attraction of PVA/ bentonite membranes toward water. The separation factor increased by 60% for 1 wt.% clay loading when compared with the pristine polymer. Morphology of the PVA/clay nanocomposite was analyzed from transmission electron micrographs. The images are displayed in Fig. 4.6 and it is clear that the dispersion of nanoclay increases with increase in the concentration of the nanoclay without any agglomeration. The most uniform and welldispersed structure was observed for 5 wt.% nanoclay membranes (Fig. 4.6C and D). On increasing the concentration of the clay beyond 5 wt.%, agglomeration of the nanoclay layers occurred in the structure of nanocomposite membranes (Fig. 4.6E). The TEM images of the nanocomposites support the

Chapter 4 Pervaporation performance of polymer/clay nanocomposites

93

Figure 4.6 Transmission electronic images of poly(vinyl alcohol)/clay nanocomposite membranes. (A) PVA with 1% of nanoclay, (B) PVA with 3% of nanoclay, (C and D) PVA with 5% of nanoclay, (E) PVA10% of nanoclay, and (F) intercalated structure of 1 wt.% PVA nanocomposite (20 nm) [59]. Source: Reprinted with permission from T. Jose, S.C. George, H.J. Maria, R. Wilson, S. Thomas, Effect of bentonite clay on the mechanical, thermal, and pervaporation performance of the poly (vinyl alcohol) nanocomposite membranes, Ind. Eng. Chem. Res. 53 (2014) 16820. Copyright (2014) American Chemical Society.

94

Chapter 4 Pervaporation performance of polymer/clay nanocomposites

intercalated network of the PVA/bentonite nanocomposite membranes with an interlayer distance of 2.04 nm (Fig. 4.6F).

4.3.2

Feed composition

The activity gradient of the components in the membrane is the major force that governs the PV. The PV properties are strongly dependent on the permeate pressure which is in turn directly related to the activity of the components at the downstream side of the membrane. The maximum gradient can be obtained for zero permeate pressure and hence the PV characteristics are influenced by the feed pressure at higher pressure conditions [60]. The feed composition is also a very significant factor in the PV process. Samanta and Ray [61] prepared ethanol selective mixed matrix membranes from the copolymer of butyl acrylate and styrene and an organomodified nanoclay used as a filler. From their study it was observed that around 1
6.5 mol.% ethanol in water (2.5
15 wt.%) is concentrated to 11.2
39.9 mol.% in the membrane without any filler and the ethanol concentration increases to the 13.1
44.3 mol.% in the membrane containing the 2 wt.% of the filler. They also studied the variation of the partial molar flux of ethanol and water with feed ethanol concentration. It is noticed that the flux of ethanol increases with increase in feed concentration of ethanol, whereas water flux decreases linearly with increase in feed ethanol concentration. Flux of both ethanol and water increases upon increasing the nanoclay content in the copolymer. Suhas et al. [62] utilized para-toluene sulfonic acid-treated clay-filled sodium alginate membranes to dehydrate isopropanol. An opposing effect on the PV performance was observed with increase in feed water composition. When the feed water composition increases from 10 to 40 wt.% for 5 wt.% p-TSAtreated clay-loaded NaAlg composite membranes (PCL-5), the total permeance increases from 3015 to 3937 GPU and was accompanied with a decrease in selectivity from 975 to 176 (Fig. 4.7). This is mainly due to the increase in the driving force shadowed by a subsequent increase in membrane swelling. Wilson et al. [63] studied the consequence of feed composition (chloroform/acetone mixtures) on the permeation rate and separation factor of poly(ethylene-co-vinyl acetate) nanocom˚ . Fig. 4.8 repreposite membranes reinforced with cloisite 25 A sents the effect of feed chloroform composition on total flux, component fluxes, and separation factor. It was noted that if the amount of concentration of chloroform in the feed is small, the membrane is very selective and separation factor is high

Chapter 4 Pervaporation performance of polymer/clay nanocomposites

4000

1000

3800

800

3600 600 3400

Selectivity

Total permeance (GPU)

PCL-5

400

3200

200

3000 10

20

30

40

Water in feed (wt.%)

Figure 4.7 Concentration dependence of permeance (GPU) and selectivity for PCL-5 at 30˚C [62]. Source: Reprinted with permission from D.P. Suhas, T.M. Aminabhavi, A.V. Raghu, Para-toluene sulfonic acid treated clay loaded sodium alginate membranes for enhanced pervaporative dehydration of isopropanol, Appl. Clay Sci. 101 (2014) 419. Copyright (2014) with permission from Elsevier.

70 Total flux Chloroform flux Acetone flux

80

60

50 60 40 40

30

20

20

0

10 20

30

40

50

60

70

80

Separation factor

Membrane flux (k g–2 h–1)

100

90

Chloroform in the feed (wt.%)

Figure 4.8 Influence of feed composition [63]. Source: Reprinted with permission from R. Wilson, T.S. Plivelic, P. Ramya, C. Ranganathaiah, M.Y. Kariduraganavar, A.K. Sivasankarapillai, et al., Influence of clay content and amount of organic modifiers on morphology and pervaporation performance of EVA/clay nanocomposites, Ind. Eng. Chem. Res. 50 (2011) 3986. Copyright (2011) American Chemical Society.

but the amount collected on the permeate side is low. When the concentration of chloroform in the feed is increased, flux increases and the separation factor goes down.

95

96

Chapter 4 Pervaporation performance of polymer/clay nanocomposites

This is because of the swollen amorphous region due to the higher concentration of chloroform in the feed. This in turn leads to decreased energy for diffusive transport as a result of polymer chains becoming more flexible. The increased swelling at higher feed composition of chloroform has an adverse impact on separation factor and is because of the swollen and plasticized upstream membrane layer being able to allow some acetone molecules to escape into the permeate side. The separation factor is also decreased at higher clay content due to agglomerated state of clay with increased concentration.

4.3.3

Temperature

The change in working temperature of the PV process may alter mass transport coefficient of each component as a result of variation in the membrane structure and mutual interaction between feed components. The free volume of the polymeric membrane is altered by the change in temperature and thereby leading to the swelling of the feed components and thermal expansion of the membrane. Aouinti al [64]. studied the separation of toluene-n-heptane mixtures by PV through poly(vinyl chloride) (PVC). From Fig. 4.9 it can be seen that the flux is also strongly improved by the temperature and the selectivity remains almost independent of temperature. Therefore any increase in temperature will lead to nanocomposite membranes possessing higher separation properties than the neat PVC membranes. (B) 1.6 PVC

Flux 10µm (J in k g–2 h–1)

Toluene in permeate (wt.%)

(A) 100 90 80 70 60

1.4 1.2 1 0.8 0.6 0.4 0.2 0

50 0

10

20

30

Nanocor content (wt.%)

40

0

10

20

30

Nanocor content (wt.%)

Figure 4.9 Effect of nanomaterial content on the permeate enrichment (A) and on the flux (B) of toluene (feed 5 50 wt.%). V: T 5 54˚C and x: T 5 74˚C [64]. Source: Reprinted with permission from L. Aouinti, D. Roizard, G.H. Hu, F. Thomas, M. Belbachir, Investigation of pervaporation hybrid polyvinylchloride membranes for the separation of toluene
nheptane mixtures—case of clays as filler, Desalination 241 (2009) 174. Copyright (2009), with permission from Elsevier.

Chapter 4 Pervaporation performance of polymer/clay nanocomposites

97

Table 4.3 Pervaporation data of PVA
PANI
III (3 mass% solution of PVA and 1.2 mL of aniline) nanocomposite membrane at different temperatures for 10 mass% water-containing feed mixture [65]. Temperature (˚C) 30 40 50

Water flux (kg m22 h21)

Mass percentage of water in permeate

Selectivity

0.069 0.144 0.158

98.43 95.26 92.87

564.2 180.9 117.2

Source: Reprinted with permission from B.V.K. Naidu, M. Sairam, K.V.S.N. Raju, T.M. Aminabhavi, Pervaporation separation of water 1 isopropanol mixtures using novel nanocomposite membranes of poly (vinyl alcohol) and polyaniline, J. Membr. Sci. 260 (2005) 142, Copyright (2005) with permission from Elsevier.

Water and isopropanol mixtures were separated by Naidu et al. [65] using poly(vinyl alcohol) and polyaniline membranes. A twofold increase in the permeation rate (for 10 mass% water and 90 mass% isopropanol feed mixture) is observed when the temperature was raised from 30 C to 40 C, however, it is accompanied by considerable loss in selectivity from 564 to 181 (Table 4.3). An increase in flux accompanied with a decrease in selectivity is observed on further increasing the temperature to 50 C. The increase in flux by the increase in temperature is due to the improved diffusion rate of the feed molecules and the small changes in the fractional free volume. Jullok et al. [66] prepared a membrane using polydimethyl siloxane/polyphenyl sulfone. The membrane showed hydrophobic character and it was achieved by coating the surface of membrane using conventional silicon material, which imitates the character of epicuticular wax of Prunus laurocerasus L. leaves. It was then applied as a separation medium in PV of diluted mixtures of ethyl acetate and aroma compounds. Considering the importance of feed temperature in PV, its effect on the biologically inspired membrane was studied. Burggraaf [67] used classical adsorption diffusion model to connect both of the activated microscopic models based on the configuration and surface diffusion with temperature dependence and is shown in Eq. (4.1) Ej 5 ΔHs 1 ED 1 ΔHvap

ð4:1Þ

where ΔHvap is the heat of vaporization of the permeant (ethyl acetate) through the membrane (32.33 kJ mol21). The term (ΔHs 1 ED) is the activation energy of permeance, EP/1

Chapter 4 Pervaporation performance of polymer/clay nanocomposites

In(P/I)ethyl acetate (mol m–2 h–1 bar–1)

98

–2 1 ppm 5 ppm 10 ppm

–2.5 –3 –3.5 –4 –4.5 –5 –5.5 3.1

3.15

3.2

3.25 3.3 1000/T (K)

3.35

3.4

Figure 4.10 Influence of temperature on ethyl acetate permeance for different concentrations [66]. Source: Reprinted with permission from N. Jullok, R. Martı´nez, C. Wouters, P. Luis, M.T. Sanz, B. Van der Bruggen, A biologically inspired hydrophobic membrane for application in pervaporation, Langmuir 29 (2013) 1510. Copyright (2013) American Chemical Society.

(kJ mol21), and when the permeate pressure is sufficiently low; thus Eq. (4.1) can be rewritten as follows: Ej 5 Ep=l 1 ΔHvap

ð4:2Þ

The relation of ethyl acetate permeance on the temperature is displayed as a logarithmic plot in Fig. 4.10. The EP/l (activation energy) for this application was determined to be 25.2 kJ mol21 for ethyl acetate. Increase in temperature leads to increased permeation flux indicated by positive activation energy, whereas negative values denote reduced permeability with increase in temperature. The EP/l has been determined and the enthalpy of vaporization of ethyl acetate (EJ) calculated as: EJ 5 EP/l 1 ΔHv. Hence, EJ 5 27.13 kJ mol21. Therefore it can be conclusively stated that the feed temperature was found to be a vital factor because of the increase in diffusivity and reduction in viscosity on heating. Avagimova et al. [68] studied the PV performance of methanol
toluene through nanocomposite membrane based on poly(phenyleneiso-phtalamide) and clay. It was reported that the nanofiller-dispersed composites had higher flux and separation factor than pure counterparts. With increasing nanoclay content in the membranes the permeability of both methanol and toluene increases. The increased permeability is attributed to the increase of free volume elements in PA/MMT membranes, which are available for the penetration of large toluene molecules.

Chapter 4 Pervaporation performance of polymer/clay nanocomposites

4.3.4

Concentration polarization

Various constituents in the feed mixture are allowed to permeate through the membrane at different rates during the PV process. Therefore at the membrane surface there is a difference in concentration of permeating and nonpermeating species and the concentration of the preferred molecules in the solution, adjacent to the membrane surface, becomes less than that in the bulk fluid. In the meantime, the solution becomes strengthened in the nonpermeating or less-permeating molecules leading to a decrease in fluid velocity from the bulk feed to the membrane. This may result in the formation of a concentration gradient in the membrane. Lowering of the flux by reducing the driving force across the membrane is the result of formation of concentration gradient [69]. Schnabel et al. [70] reported that when hollow fibers are positioned in a spiral way, it led to a centrifugal force being produced on the surface of a perforated partition tube, which in turn leads to enhanced permeation flux. This indicated that the resistance of the diffusion layer is much higher than that of the membrane itself, thereby limiting the mass transport through the PV membrane as a result of concentration polarization. So it can be concluded that the hydrodynamics of the system should be well improved before PV for reducing the effect. When a binary liquid mixture is permeating through a semipermeable membrane, with different individual component permeation rates, an increase of the less permeable component in the boundary layer near the membrane surface occurs when a binary liquid mixture is permeating through a semipermeable membrane. Concentration polarization can be defined as the concentration gradient between the more concentrated boundary solution and the less concentrated bulk. Feng and Huang [71] pursued a theoretical approach to describe the concentration polarization in PV processes. From this study, it was revealed that the importance of concentration polarization is determined not only by the membrane permeability and the hydrodynamic conditions but also by the membrane selectivity. The parametric analysis shows that the effect of concentration polarization is intensified by an increase in membrane permeability and/or selectivity and by a decrease in the mass transfer coefficient in the boundary layer. The effect of concentration polarization may be, but is not necessarily, more severe in the PV of dilute solutions at lower concentrations where the minor component permeates through the membrane preferentially; however, it is unlikely to pose a severe problem when the feed concentration is considerably high.

99

100

Chapter 4 Pervaporation performance of polymer/clay nanocomposites

4.4

Conclusions

Nanocomposites are gradually gaining wide acceptance in the mainstream of global plastics processing and are being promoted as the way of the future. The manufacturing of various nanocomposites for a particular application can be a revolution in the plastic industry. Comparing with the various nanomaterials that are currently being developed, the polymer/clay nanocomposites have already proved their great potential in automotive parts, barrier layers for the production of food wrapping, and fire retardant materials. One of the most potential areas for resolving various technical and commercial challenges that is coupled with the separation and purification technologies/industries/applications is nanotechnology. The processing and preparation of nanostructured materials with exceptional properties have changed the traditional separation methods and have led to new separation approaches that exceed the existing processes and methods. PV is a remarkable separation technique in chemical industry that could be applied to a great extent in the areas such as environmental protection, clean resources, food chemical, and pharmaceutical areas. These area/applications may benefit immensely from PV process, which is now considered as a remarkable protection method in the chemical industry. Among all available technologies, PV is considered to be the most cost effective and environment friendly technology for separation of azeotropic/close boiling mixtures, recovery of traces of impurities from aqueous solutions, and treating the heat-sensitive biomaterials.

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208.

Carbon nanotubes-polymer nanocomposite membranes for pervaporation

5

Maryam Ahmadzadeh Tofighy and Toraj Mohammadi Department of Chemical, Petroleum and Gas Engineering, Center of Excellence for Membrane Research and Technology, Iran University of Science and Technology (IUST), Narmak, Tehran, Iran

Nomenclature Abbreviations CNTs CS CVD ED FTIR MMMs MWCNTs MWCNTs-BP MWCNTs
PAH MWCNTs-PSS PAN PDMS PES PHB PMMA PNCMs PU PV PVA SA SEM SWCNTs TEM TGA

Carbon nanotubes Chitosan Chemical vapor decomposition Electrodialysis Fourier transform infrared Mixed matrix membranes Multiwalled carbon nanotubes MWCNTs-bucky paper Poly(allylaminehydrochloride)-wrapped MWCNTs Poly(sodium 4-styrenesulfonate) (PSS)-wrapped MWCNTs Poly(acrylonitrile) Polydimethylsiloxane Polyethersulfone Poly(3-hydroxybutyrate) Poly(methylmethacrylate) Polymer nanocomposite membranes Polyurethane Pervaporation Polyvinyl alcohol Sodium alginate Scanning electron microscopy Single-walled carbon nanotubes Transmission electron microscopy Thermogravimetric analysis

Symbols J α

Permeation flux Separation factor

Polymer Nanocomposite Membranes for Pervaporation. DOI: https://doi.org/10.1016/B978-0-12-816785-4.00005-7 © 2020 Elsevier Inc. All rights reserved.

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Q A t PSI PG i PG i =l Ji P i0 P il l αij

5.1

Quantity of the permeate in (kg) Effective area of the membrane surface (m2) PV time (h) PV separation index Membrane permeability Membrane permeance Permeation flux of component i Partial pressure of feed Partial pressure of permeate Membrane thickness Selectivity

Introduction

In recent years, membrane technologies as promising energy-saving alternatives over conventional separation processes in the industries have undergone rapid development and have been used in many industrial scale separations [1
3]. Membrane (thin sheet of natural or synthetic material) primarily functions as a barrier that allows selective transport of a specific component from one side to the other side. The component permeation through the membrane may be driven by chemical (pressure, temperature, and concentration) and/or electrical potential gradients. Based on the transport mechanism, membranes can be divided into porous and nonporous membranes. In porous membranes, separation takes place due to size exclusion that smaller molecules pass through the membrane pores leaving the larger molecules at the feed side under pressure driving force. Thus permeation flux of the porous membranes depends on the applied pressure and the membrane pore diameter. Porous membranes are mainly used for microfiltration (pores ranging from 0.1 to 10 μm) and ultrafiltration (pores ranging from 0.01 to 0.1 μm). One disadvantage of porous membranes is membrane fouling that causes permeation flux decline over time. In nonporous membranes, separation takes place based on the solution-diffusion model, where component permeation depends on its permeability. A component with higher diffusivity and solubility within the membrane material has higher permeability. Nonporous membranes are mainly used for pervaporation (PV), reverse osmosis, nanofiltration, and gas separation. One disadvantage of nonporous membranes is low permeation flux; therefore the nonporous film is usually made extremely thin and is deposited on top of asymmetric porous substrate. Based on membrane material, membranes can be divided into inorganic and organic (polymeric) membranes. Inorganic

Chapter 5 Carbon nanotubes-polymer nanocomposite membranes for pervaporation

membranes such as ceramic, zeolite, metallic, and carbon membranes have some advantages such as high selectivity, inertness to microbiological degradation, high chemical and thermal stability, and ease of cleaning after fouling compared to polymeric membranes. However, high membrane fabrication cost, brittleness, complex preparation method, and lack of technology to form defect-free and continuous membrane are the main disadvantages of inorganic membranes. Polymeric membranes have received considerable attention due to their relatively easy and economical processing capability (processability) and mechanical properties. However, low permeation flux, trade-off between selectivity and permeability and chemical and thermal instability are the main disadvantages of polymeric membranes. Limitations of inorganic and polymeric membranes have prompted the development of a new class of membranes as polymeric nanocomposite membranes (PNCMs) as well as mixed matrix membranes (MMMs). In these membranes, inorganic materials such as zeolites, silica, titanium dioxide (TiO2), clay, metal organic frameworks, and carbon nanomaterials incorporate as fillers into polymeric materials. Using this technique the advantages of both inorganic and polymeric membranes can be benefitted simultaneously [4,5]. PV is a vacuum-driven membrane process for liquid mixtures separation and follows the solution-diffusion model principles. PV applications include separation of organic
water mixtures such as dehydration of solvents and alcohols [6,7], organics recovery from dilute aqueous solutions [8], separation organic
organic mixtures [9], separation of azeotropes and close boiling mixtures [10], separation of thermal-sensitive compounds [11], and separation of isomers [12]. PV is considered as a promising alternative approach to conventional energy intensive technologies such as azeotropic or extractive distillation in separation of liquid mixture due to its high separation factor and permeation flux and also no requirement of any additional carcinogenic entrainer (e.g., benzene) as used in azeotropic distillation separation [13
17]. However, technical feasibility and separation overall efficiency of the PV process largely depends on the used membrane and its properties. It is well known that one of the key targets in PV research and design is membranes fabrication with high selectivity, permeability, and structural stability [2,7]. In recent years, PNCMs as well as MMMs with high separation performance have been utilized for the PV purposes [13,14]. Carbon nanotubes (CNTs) with unique structural and extraordinary physical and chemical properties have been identified as a new type of nanofillers for fabrication of new

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Chapter 5 Carbon nanotubes-polymer nanocomposite membranes for pervaporation

PNCMs. Significant properties of CNTs, which make them attractive for membrane preparation, are high aspect ratio, small dimensions, high surface area, tubular structure, and also remarkable smoothness of their interior graphitic walls [15,16,18]. Recently CNTs
PNCMs have been considered as attractive and innovative membranes for PV applications. In this chapter, current progresses in preparation and characterization of CNTs-PNCMs for PV applications are being reviewed.

5.2

Pervaporation

In 1917 Kober after observing water selective permeation through the collodion and parchment membrane defined the term of “pervaporation” from the abbreviation of “permeation” and “evaporation” [19]. By the 1980s, membrane technology development made it possible to build economically viable PV systems. The first industrial installation of a PV plant was reported in Brazil in 1983 for ethanol dehydration [2]. Gesellschaftfu¨r Trenntechnik first commercialized PV membrane that composed of a composite configuration [20]. PV is a membrane separation technology with high efficiency, selectivity, and energy-saving benefits, especially in separating azeotropic and close boiling point mixtures, thermally sensitive compounds, organic
organic mixtures, as well as dehydration of solvents and alcohols, and dilute organic compounds removing from wastewater. PV could also be an interesting alternative in chemical industry for aliphatic/aromatic mixtures separation that often composed of components with high or close boiling points or azeotropes. In this field, expensive and energy intensive processes such as extractive and azeotropic distillations and liquid
liquid extraction are conventional technologies competing with PV [21,22]. PV as a vacuum (or pressure)-driven process follows the solution-diffusion model principles which are explained in details later. In PV, feed liquid mixture (upstream side) is in contact with a semipermeable membrane surface at atmospheric pressure, while vacuum or a sweep gas is applied at the other side (permeate side or downstream side) of the membrane to generate the chemical potential difference for separation to occur and the membrane permeate is obtained in the vapor phase as shown in Fig. 5.1 [2,13]. Vapor pressure difference between the permeate vapor and the feed solution induces transport through the membrane. This vapor pressure

Chapter 5 Carbon nanotubes-polymer nanocomposite membranes for pervaporation

Figure 5.1 Schematic of general PV process. PV, Pervaporation.

difference can be supplied in several ways. Vacuum pump is usually used to draw a vacuum on the permeate side of the system in the laboratory scale. Industrially the permeate side vacuum is most economically generated by cooling the permeate vapor. This causes the vapor to be condensed and the condensation creates a partial vacuum spontaneously [1]. Thermodynamic vapor
liquid equilibrium limitation does not restrict PV. When compared with conventional separation technologies, PV consumes less energy (only latent heat of permeate evaporation). Also PV prevents the final product contamination with elimination the entrainer component that is usually required in extractive distillation or azeotropic distillation [2]. PV membranes can be tailored into either a water-selective membranes (water preferentially permeates through the membranes) or an organic-selective membranes (organics preferentially permeate through the membranes) by means of morphological engineering during the membrane fabrication process and material selection [2]. Membrane properties determine separation performance of the PV process. For example, for dehydration purposes, the membrane surface hydrophilicity is an important factor that determines the PV performance. Based on the solution-diffusion model, the water molecules dissolution within and diffusion across the membranes enhances with increase in the membrane surface hydrophilicity. Consequently higher water permeation flux and selectivity can be obtained in the dehydration processes [13
15].

5.2.1

Solution-diffusion model

The most widely accepted transport mechanism for PV process is the solution-diffusion model that has been developed for

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Chapter 5 Carbon nanotubes-polymer nanocomposite membranes for pervaporation

Figure 5.2 Schematic illustration of the solution-diffusion model [2].

describing the mixture separation using porous polymeric membranes with pores smaller than 1 nm and also nonporous polymeric membranes. In this model, permeating component transport through the membrane involves three constructive steps: 1. Sorption: the permeating component is absorbed from the liquid feed into the membrane surface. 2. Diffusion: migration of the permeating component through the membrane takes place. 3. Desorption: the permeating component is desorbed to the vapor phase on the downstream of the membrane. The illustration of the above mentioned three steps is shown in Fig. 5.2. Therefore separation performance of PV membranes can be improved via enhancements of the penetrants solubility selectivity and/or diffusivity selectivity across the membranes [2,23]. In non-porous polymeric membranes, permeating molecules through the membranes are located in the microvoids of the polymers that these voids are the result of suboptimal chain stacking and their sum is referred to polymers free volume. Due to thermally induced statistical movement of chain segments, microchannels are formed between microvoids. This allows molecules to move across the membranes from one void to another corresponding to the applied driving force [24].

5.2.2

Separation characteristics of pervaporation membranes

Separation characteristics of PV membranes are evaluated by separation factor (α) and permeation flux (J) that can be obtained from the following expressions, respectively [25]: α5

ðXB =YB Þ ðXA =YA Þ

ð5:1Þ

Chapter 5 Carbon nanotubes-polymer nanocomposite membranes for pervaporation

where XA an XB are compositions of the component with higher boiling point in the feed and permeate, respectively, and YA and YB are compositions of the component with lower boiling point in the feed and permeate, respectively. J5

Q A:t

ð5:2Þ

J is the membrane permeation flux in (kg m22 h21), Q is the permeate quantity in (kg), A is the membrane surface effective area in (m2), and t is PV time in (h). The overall PV performance can be evaluated from the PV separation index (PSI). PSI is calculated using the following expression [25]: PSI 5 J:ðα 2 1Þ

ð5:3Þ

where J is membrane permeation flux in (kg m22 h21) and α is separation factor. Component fluxes for separation of azeotropic composition of water and ethanol mixture can be calculated by the following equations [25]: JWater 5 J:XWater

ð5:4Þ

JEthanol 5 J:XEthanol

ð5:5Þ

where JWater and JEthanol are the component fluxes, respectively. Also XWater and XEthanol are the permeate compositions, respectively. The real driving force for transport of components through the membranes in PV is the partial pressure difference between the membranes either sides. The membranes real intrinsic properties are evaluated from their permeability, permeance, and selectivity values as calculated using Eqs. (5.6), (5.7) and (5.8), respectively.
PiG 5 Ji

l Pi0 2 Pil

ð5:6Þ


PiG =l 5

Ji Pi0 2 Pil

ð5:7Þ

where PiG is the membrane permeability, PiG =l is the membrane permeance, Ji is the component i permeation flux, Pi0 and Pil are the partial pressures of the membrane either sides (feed and permeate, respectively) and l is the membrane thickness.

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Chapter 5 Carbon nanotubes-polymer nanocomposite membranes for pervaporation

The membrane intrinsic selectivity is the ratio of permeabilities or permeances of the components i and j through the membrane and can be obtained from the following equation: αij 5

PiG PjG

ð5:8Þ

The swelling percentage degree can be calculated using the following equation: DSð%Þ 5

Ws 2 Wd 3 100 Wd

ð5:9Þ

where Ws and Wd are the weights of swollen and dried membranes, respectively.

5.3

Polymer nanocomposites

Polymer nanocomposites as foundation of nanotechnology have attracted a great scientific interest and found a wide range of applications from toys to aircraft. By definition, polymer nanocomposites are polymers that are reinforced with nanomaterials as nanofillers. In polymer nanocomposites, the most important topic to be considered is dispersion of the nanofillers in the bulk polymer matrix. Homogeneous nanofillers distribution results in improved properties of the obtained polymer nanocomposites. But tendency of nanomaterials for agglomeration due to the strong van der Waals forces between the particles results in deterioration in the polymer nanocomposites properties. It is now well established that for better dispersion of nanomaterials in polymer matrices, the nanomaterials can be surface modified and functionalized. Surface modification and functionalization of nanomaterials improve interfacial interaction or compatibility between the polymer matrix and the filler that this results in better dispersion to develop highperformance nanocomposites for advanced applications. Ajayan et al. [26] reported the first polymer nanocomposite using CNTs as nanofiller. Later, researches in the field of PNCMs containing CNTs were increased every year [27,28]. In recent years, CNTs have been identified as the main type of nanofillers for preparation of high-performance PNCMs due to their extraordinary physical and chemical and unique structural properties [29
35].

Chapter 5 Carbon nanotubes-polymer nanocomposite membranes for pervaporation

5.4

113

Carbon nanotubes

Since CNTs discovery in 1991 by Iijima [36], they have received great attention and their various applications have been continuously under study. CNTs are seamless macromolecules with a few nanometers radius and up to several micrometers in length. The ends of these tubes are usually capped by half fullerene like structures and their walls consist of the hexagonal lattice of carbon atoms of graphene sheets. The unique structures of CNTs can be divided as single-walled carbon nanotubes (SWCNTs), double-walled carbon nanotubes (DWCNTs), or multiwalled carbon nanotubes (MWCNTs) as shown in Fig. 5.3. SWCNTs and DWCNTs comprise of one and two concentric cylinders of graphene sheets, respectively, whereas MWCNTs consists of several cylindrical shells of graphene sheets with interlayers spacing close to that of graphene (about 0.34 nm) held together by the van der Waal’s bond [28]. Current synthesis techniques of CNTs including electric laser ablation [37], arc discharge [38], and chemical vapor decomposition (CVD) [39,40] are commercially used to produce large quantity of CNTs. CNTs as an important member of carbon family has many superior properties such as high specific surface area, high flexibility, high mechanical stiffness, low mass density, high aspect ratio (length to diameter ratio), onedimensional structure, frictionless surface, and effective π
π stacking interaction with aromatic compounds. Since CNTs are chemically inert and have tendency to form bundle, it is difficult to achieve homogeneous CNTs distribution in the polymer matrices. Therefore surface modification of CNTs is of great

Figure 5.3 CNTs structure (A) SWCNTs, (B) DWCNTs, and (C) MWCNTs [35]. CNTs, Carbon nanotubes; DWCNTs, double-walled carbon nanotubes; MWCNTs, multiwalled carbon nanotubes; SWCNTs, single-walled carbon nanotubes.

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Chapter 5 Carbon nanotubes-polymer nanocomposite membranes for pervaporation

significance to avoid CNTs agglomeration and facilitate the uniform CNTs distribution in the polymers [41,42]. CNTs tubular structure provides stable nanochannels with variable size for mass transport. High flux of CNTs-based membranes are contributed to the inherent smoothness of the interior CNTs and the molecular ordering phenomena inside the CNTs nanochannels. The smooth and frictionless CNTs interior surfaces result in weak carbon
permeating component attractive interaction and hence facilitate extremely the high fluid velocity. Uniform dispersion of CNTs with agglomeration tendency in the polymer matrix can significantly improve permeation flux and separation factor and is a critical issue in fabrication of CNTs-PNCMs. Therefore CNTs functionalization with specific functional moieties is necessary to reduce the van der Waals forces between CNTs and improves CNTs dispersion in the polymer matrix [43,44].

5.4.1

Carbon nanotubes functionalization

Functionalization is an effective way of preventing CNTs aggregation and helps to better CNTs dispersion and stabilization within the polymer matrix. There are several methods for CNTs functionalization including defect functionalization (purification and oxidation), and covalent and noncovalent functionalization as shown in Fig. 5.4, schematically [28].

5.4.1.1 Purification and oxidation of carbon nanotubes CNTs usually contain various impurities such as inactivated catalyst particles and carbonaceous species (amorphous carbon) that are unwanted in the CNTs applications. Therefore CNTs should be purified before using. Oxidative treatments are effective in removing CNTs impurities and also can open the ends of CNTs [28,46]. One of the first oxidation methods used for carbonaceous species oxidation is CNTs heating to high temperatures (above 700 C) in the presence of air that opens the CNTs ends (i.e., without half-fullerenes attached to each end). This shows that the CNTs ends are more reactive than the CNTs walls. Also amorphous carbon is more susceptible to oxidation than CNTs. Other gases include CO2, N2O, NO, NO2, O3, and ClO2 may also be used instead of air for the CNTs purification [47]. Purification and oxidation may also be done in solution with extremely strong acids such as sulfuric acid (H2SO4), nitric acid

Chapter 5 Carbon nanotubes-polymer nanocomposite membranes for pervaporation

Figure 5.4 Functionalization approaches of CNTs [45]. CNTs, Carbon nanotubes.

(HNO3), hydrochloric acid (HCl), or mixtures of acids such as a mixture of (HNO3 and H2SO4), (K2Cr2O7 and H2SO4), and (H2SO4 and KMnO4). The amount of inactivated catalyst particles decreases after acid treatment [48]. In oxidative methods, defects are mainly observed at the CNTs open ends. These methods generate several functional groups on the walls and defects of CNTs such as carboxyl, carbonyl, and hydroxyl groups. The oxidation degree depends on the oxidation reaction conditions (duration, temperature, etc.) and the nature of oxidizing agent [49].

5.4.1.2

Noncovalent functionalization of carbon nanotubes

CNTs non-covalent functionalization is especially important because it improves solubility and processability of CNTs without compromising their physical properties. This type of functionalization can be performed by biomacromolecules, surfactants, or wrapping with polymers. The non-covalent functionalization is based on the π 2 π interactions between the extended π electron of the CNTs sidewalls with the other molecules or the van der Waals interactions between the CNTs sidewall with the other molecules [50]. Preservation of the CNTs properties is the main advantage of this technique, while its disadvantage is the weak forces between CNTs and the coupled molecules, which can reduce the load transfer in the prepared nanocomposites [51,52].

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5.4.1.3 Covalent functionalization of carbon nanotubes CNTs are more reactive than flat graphene sheets and have an enhanced tendency to covalently attach to chemical species due to the π 2 π orbitals of the sp2-hybridized carbon atoms [53]. After the CNTs covalent functionalization, the sp2-hybridized carbon atoms change to sp3-hybridized carbon atoms, and the CNTs properties are influenced [54]. CNTs covalent functionalization can improve their dispersion in solvents and polymer matrices. Covalent functionalization can be done by coupling different chemical groups with oxygen containing functional groups of the CNTs defects such as carboxyl and hydroxyl groups formed during CNTs oxidation by air, oxygen, concentrated nitric acid, sulfuric acid, acid mixtures, and aqueous hydrogen peroxide. The extent of the induced oxygen containing functional groups depends on the procedures of oxidation and the nature of oxidizing agents. Also functional groups can attach covalently to the CNTs sidewalls that are associated with the hybridization change from sp2 to sp3. Covalent functionalization usually requires very reactive reagents, and thus high chemical functionalization degree is achievable [52]. CNTs functionalization with polymer molecules (covalent polymer grafting to CNTs) is particularly important for CNTs/ polymer nanocomposites processing. “Grafting to” and “grafting from” are two main approaches for the covalent grafting of polymers to CNTs. In “grafting to” approach commercially available or asprepared polymer molecules are attached to the CNTs surface by chemical reactions, such as radical coupling, esterification, amidation, etc. In this approach, the polymer must have suitable reactive functional groups for the composites preparation. The “grafting from” approach is based on the polymer attachment to the surface of CNTs by monomers in situ polymerization in the presence of CNTs supported initiators or reactive CNTs. Using this approach, high grafting density of the CNTs/polymer nanocomposites can be prepared [28].

5.5

PV application of carbon nanotubespolymer nanocomposite membranes

5.5.1

Dehydration of solvents and alcohols

Dehydration of solvents and alcohols can be achieved economically with PV process. Hydrophilic polymers such as chitosan (CS), polyvinyl alcohol (PVA), cellulose, alginate, etc., are

Chapter 5 Carbon nanotubes-polymer nanocomposite membranes for pervaporation

117

often used for preparation of PV membranes to dehydrate solvents and alcohols. These polymers would enhance water solubility selectivity toward the membranes through hydrogen bonding interactions. Since these membranes are susceptible to swell, to stabilize the membranes, crosslinking processes are generally used [6,55]. Shirazi et al. reported preparation of PVA
CNTs nanocomposite membranes for dehydration of isopropanol/water mixtures by PV process. They synthesized CNTs by CVD method and used nitric acid for CNTs purification and functionalization. As shown in Fig. 5.5, transmission electron microscopy (TEM) images demonstrated that acid treatment could remove encapsulated catalyst particles of CNTs. Acid treatment not only increases CNTs purity but also the obtained hydrophilic functional groups improves CNTs dispersion and stabilization within polymer matrix and prevents CNTs aggregation. FESEM characterization results showed that 2 wt.% modified CNTs loading is better dispersed in the polymer matrix and increasing modified CNTs loading more than 2 wt.% agglomerates CNTs as shown in Fig. 5.6. Furthermore the PV results demonstrated that incorporating of the modified CNTs into PVA matrix decreases swelling degree and increases significantly water selectivity due to the chains rigidification of polymer. Chain rigidity that can be found in glassy polymers decreases the polymer chains mobility and free volume and consequently selectivity increases and permeability decreases. The water selectivities for the neat PVA and

(A)

(B)

Figure 5.5 TEM images of (A) the raw CNTs and (B) the CNTs modified by 8 M HNO3 [16]. CNTs, Carbon nanotubes; TEM, transmission electron microscopy.

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(A)

(B)

(C)

(D)

(E)

Figure 5.6 FESEM surface images of the PVA
CNTs nanocomposite membranes with the modified CNTs incorporation of (A) 0 wt.%, (B) 0.5 wt.%, (C) 1 wt.%, (D) 2 wt.%, and (E) 4 wt.% [16]. CNTs, Carbon nanotubes; PVA, polyvinyl alcohol.

nanocomposite membrane with 2 wt.% modified CNTs loading were evaluated as 119 and 1794, respectively [16]. Amirilargani et al. reported preparation of poly(allylaminehydrochloride)-wrapped MWCNTs (MWCNTs
PAH) incorporated PVA membranes for dehydration of isopropanol/water mixtures by PV process. TEM image of the MWCNTs
PAH is presented in Fig. 5.7. As can be observed, PAH covers completely the

Chapter 5 Carbon nanotubes-polymer nanocomposite membranes for pervaporation

Figure 5.7 HR-TEM image of the PAH-wrapped MWCNTs [18]. PAH, poly (allylaminehydrochloride).

MWCNTs surface and is noncovalently adsorbed around the MWCNTs with the van der Waals interactions. The obtained results demonstrated that in comparison with raw MWCNTs, the modified MWCNTs were well dispersed in the PVA matrix. The prepared membranes swelling degree decreased by the modified MWCNTs addition into the PVA matrix. Furthermore the PV results demonstrated that the modified MWCNTs incorporation into the PVA matrix increases water selectivity significantly due to the polymer chains rigidification. The prepared membranes containing 1 wt.% of the modified MWCNTs exhibited excellent PV properties. Separation factor and permeation flux were 141.2 and 0.229 kg m22 h21 for the neat PVA and 948.4 and 0.207 kg m22 h21 for the membrane prepared with 1 wt.% of the modified MWCNTs, respectively [18]. Amirilargani et al. reported preparation of Poly(sodium 4-styrenesulfonate) (PSS)-wrapped MWCNTs (MWCNTs-PSS) incorporated into PVA solution for PVA/MWCNTs-PSS nanocomposite membranes preparation for dehydration of isopropanol/water mixtures by PV process. FESEM and HR-TEM images of MWCNTs-PSS are shown in Fig. 5.8. As observed, PSS chains are adsorbed noncovalently around the MWCNTs due to the van der Waals forces and PSS completely covers the outer surface of the MWCNTs. The obtain results demonstrated that wrapping MWCNTs with PSS leads to better MWCNTs dispersion in the PVA matrix. Upon addition of 0 2 3 wt.% modified MWCNTs to the PVA matrix, the membrane selectivity increased, whereas the membrane permeance decreased. For example, the water selectivity increased from 39.8 to 249.4, and the water permeance decreased from 4200.8 to 3243.9 when the modified MWCNTs content was increased from 0 to 3 wt.% [56].

119

120

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Chapter 5 Carbon nanotubes-polymer nanocomposite membranes for pervaporation

(B)

Figure 5.8 (A) FESEM and (B) HR-TEM images of MWCNTs-PSS [56]. MWCNTs-PSS, Poly(sodium 4-styrenesulfonate)-wrapped multiwalled carbon nanotubes.

Panahian et al. reported preparation of multilayer nanocomposite membranes containing MWCNTs, PVA, polyethersulfone (PES) and polyester as inorganic filler and selective top, intermediate and support layers, respectively for dehydration of ethanol/water mixtures by PV process. They synthesized MWCNTs by thermal CVD technique and then modified MWCNTs by incorporating titanium oxide (TiO2) nanocrystals and carboxyl functional groups. Their results demonstrated that the membranes containing the modified MWCNTs have higher crosslinking density and lower swelling degree, resulting in less total permeation flux in comparison to the membrane containing raw MWCNTs and the neat membrane. Furthermore by increasing the modified MWCNTs content, the membranes hydrophilicity, crosslinking and surface roughness were improved. All prepared nanocomposite membranes showed less total flux than the neat membrane due to the membrane top layer resistance increasing by incorporation of the modified MWCNTs filler. Finally it was observed that the MWCNTs modification leads to better MWCNTs dispersion in the polymeric matrix and improves the multilayer nanocomposite membranes separation performance. The total permeation flux and water separation factor of all the prepared membranes are shown in Fig. 5.9 [14]. Hu et al. reported preparation of composite membranes comprising of polyvinylamine-PVA separating layer incorporated

Chapter 5 Carbon nanotubes-polymer nanocomposite membranes for pervaporation

Figure 5.9 The total flux and water separation factor of all prepared membranes [14].

with CNTs supported on a microporous polysulfone (PSf) substrate for ethylene glycol dehydration by PV process. The CNTs incorporation into the membrane increased separation factor and permeation flux, and the separation performance improvement was particularly significant at lower feed water concentrations. At feed water concentration of 1 wt.%, separation factor of 1160 and permeation flux of 146 g m22 h21 were achieved at 70 C using the prepared membrane containing 2 wt.% of CNTs [57]. Sajjan et al. reported preparation of CS-wrapped MWCNTs incorporated sodium alginate (SA) membranes for dehydration of isopropanol by PV process. The preparation scheme of the CS-wrapped MWCNTs incorporated SA membrane is shown in Fig. 5.10. The obtained results demonstrated that the membrane containing 2 wt.% of the modified MWCNTs exhibits the highest separation selectivity of 6419 with a flux of 21.76 3 1022 kg m22 h21 at 30 C for 10 wt.% of water in the feed [58]. Choi et al. embedded MWCNTs into PVA membranes for dehydration of ethanol by PV process. The results revealed that the MWCNTs addition mitigates crystallinity, induces microorientation, and decreases PVA matrix-free volume. Despite the reduced free volume, water permeation of the membranes improved because tubular structure of MWCNTs provided

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Chapter 5 Carbon nanotubes-polymer nanocomposite membranes for pervaporation

Figure 5.10 Scheme for the CS-wrapped MWCNTs incorporated SA membranes development [58]. CS, Chitosan; MWCNTs, multiwalled carbon nanotubes; SA, sodium alginate.

stable nanochannels with smooth surface for permeating molecules with relatively less resistance. Their results demonstrated that permeation flux of the membrane increases with increase in the MWCNTs content, whereas separation factor is maintained up to 1.0 wt.% MWCNTs and beyond that, it reduces. Fig. 5.11 shows the effect of MWCNTs on the swelling degree of the prepared membranes in a water/ethanol (10/90 wt.%) mixture. As observed, with increase in the MWCNTs content, the prepared membranes swelling degree decreases and this may be due to the PVA molecules stiffened by the MWCNTs [59].

Chapter 5 Carbon nanotubes-polymer nanocomposite membranes for pervaporation

123

Figure 5.11 Swelling degree of the MWCNT/PVA membranes as a function of the MWCNTs content (in a 90 wt.% ethanol aqueous solution) [59]. MWCNTs, multiwalled carbon nanotubes; PVA, polyvinyl alcohol.

Yeang et al. functionalized MWCNTs with PVA to improve MWCNTs dispersion and compatibility in CS matrix. Glutaraldehyde was used for the PVA
MWCNT/CS nanocomposite membranes crosslinking. PV performance of the prepared membranes were evaluated in acetone dehydration. Their results demonstrated that compared to the crosslinked neat CS membrane, water permeance of the crosslinked nanocomposite membrane increases, while its selectivity decreases. Furthermore crosslinking was found to improve the membranes selectivity but reduce the water permeance [60]. Ong et al. functionalized MWCNTs with poly(3-hydroxybutyrate) (PHB) and then aligned them into a membrane filter template through a filtration process. A solution casting technique was then applied to cast CS onto the template to form PHBMWCNTs/CS nanocomposite membranes. Schematic diagram of the PHB-MWCNTs/CS nanocomposite membrane fabrication is shown in Fig. 5.12. TEM images showed that the polymer is wrapped on the MWCNTs surface as shown in Fig. 5.13. Presence of the PHB functional moieties helped to improve MWCNTs compatibility and dispersion in the CS matrix. The prepared nanocomposite membranes were applied in PV process of 1,4-dioxane dehydration. Compared to the neat membrane, the nanocomposite membranes showed a relatively higher selectivity and permeation flux [61]. Yee et al. reported preparation of asymmetric membranes by first forming MWCNTs-bucky paper (MWCNT-BP) structure and

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Chapter 5 Carbon nanotubes-polymer nanocomposite membranes for pervaporation

Figure 5.12 Schematic diagram of the PHB-MWCNTs/chitosan nanocomposite membrane fabrication [61]. MWCNTs, multiwalled carbon nanotubes; PHB, poly(3-hydroxybutyrate).

(A)

(B)

Figure 5.13 HR-TEM images of (A) the oxidized MWCNTs and (B) the PHB-MWCNTs [61]. MWCNTs, multiwalled carbon nanotubes; PHB, poly(3-hydroxybutyrate).

then coating the structure with a thin layer of PVA to form novel MWCNT-BP/PVA asymmetric nanocomposite membranes for dehydration of a multicomponent reaction mixture obtained from etherification by PV process. The asymmetric membranes showed improved mechanical properties compared to the neat

Chapter 5 Carbon nanotubes-polymer nanocomposite membranes for pervaporation

PVA membrane. The obtained results demonstrated that using the oxidized MWCNT-BP/PVA asymmetric membranes, two-and four-fold enhancements of permeation flux and separation factor are achieved, respectively, compared to those of the neat PVA membrane. This effect is due to the hydrophilic functional groups on the modified MWCNTs and the pre-selective layer nanochannels, which favor water molecules permeation [62]. Shameli et al. used citric acid as crosslinking agent and investigated performance enhancement of crosslinked PVA membrane by incorporation of modified MWCNTs. The prepared nanocomposite membranes were applied as water separating element in a membrane reactor to improve esterification of acetic acid with methanol. The results showed that the prepared nanocomposite membranes have good separating properties for removing water from the reaction mixture and the acid conversion rate was improved with increase in the modified MWCNTs content of the membranes. The final acid conversion was increased from 52.30% to 99.25% using the membrane with 2 wt.% modified MWCNTs instead of the neat membrane at the optimized condition and 4 h operation [63].

5.5.2

Separation of organic
organic mixtures

Due to the lack of robust membrane materials and modules that are able to withstand the long-term exposure of organic solvents, organic
organic separations via PV process are the most challenging and the least developed applications [9]. Peng et al. reported preparation of novel PVA
(CS wrapped MWCNTs) nanocomposite membranes by incorporating modified MWCNTs into PVA for separation of benzene/cyclohexane mixture by PV process. At first, molecular dynamics simulation was introduced to qualitatively analyze the contribution of MWCNTs incorporation on improving the nanocomposite membranes free volume characteristics. Second, the PV performance of the prepared nanocomposite membranes was investigated. Their results demonstrated that for the benzene/ cyclohexane (50/50, w/w) mixture at 323 K, separation factor and permeation flux of the neat PVA membrane were only 9.6 and 20.3 g m22 h21, respectively, while the corresponding values of the prepared nanocomposite with the modified MWCNT content of 1 wt.% were 53.4 and 65.9 g m22 h21 [64]. Shen et al. incorporated modified MWCNTs into CS membranes for benzene/cyclohexane mixtures separation by PV process. The raw MWCNTs were treated by acid mixture (H2SO4/HNO3) and then functionalized with isonicotinic acid.

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Chapter 5 Carbon nanotubes-polymer nanocomposite membranes for pervaporation

(A)

(B)

(C)

(D)

Figure 5.14 TEM images of (A and B) the raw MWCNTs and (C and D) the Ag1/MWCNTs [65].

The Ag ions were grafted to the pyridine rings on the modified MWCNTs by complexation reaction. TEM images of the raw MWCNTs and the Ag1/MWCNTs are shown in Fig. 5.14. The Ag1/MWCNTs/CS membranes were prepared on PSf membranes as support. The PV results showed that the separation performance of the Ag1/MWCNTs/CS nanocomposite membrane is better than the MWCNTs/CS nanocomposite membrane and the neat CS membrane. With increase in the Ag1/MWCNTs content, the membrane permeation flux increased and the membrane selectivity firstly increased and then decreased. The membrane selectivity reached 7.89 and the membrane permeation flux was 357.96 g m22 h21 for the feed of 50% benzene at 20 C when the Ag1/MWCNTs content was 1.5 wt.% [65].

Chapter 5 Carbon nanotubes-polymer nanocomposite membranes for pervaporation

Peng et al. reported novel PVA/CNTs nanocomposite membranes preparation for PV separation of benzene/cyclohexane mixtures. They dispersed CNTs in PVA matrix using β-cyclodextrin (β-CD). The characterization results demonstrated that the prepared nanocomposite membrane exhibits significant improvement in PV performance, thermal stability and Young’s modulus when compared with the neat PVA and the β-CD/PVA membranes. Separation factor and permeation flux and of benzene was 41.2 and 61.0 g m22 h21, respectively [66]. Wang et al. reported preparation of the poly(methylmethacrylate) (PMMA) and polyurethane (PU) membranes containing the raw and amino (NH2) functionalized MWCNTs. The nanocomposite membranes performance was evaluated for benzene/cyclohexane mixtures separation by PV process. Their results demonstrated that compared to the raw MWCNTs, the functionalized MWCNTs showed an improved distribution in the PU and PMMA nanocomposite membranes, and performance of the nanocomposite membranes containing the functionalized MWCNTs was better than those containing the raw MWCNTs as shown in Fig. 5.15 [67].

Figure 5.15 Effect of the raw and functionalized MWCNTs addition on separation factor of the prepared membranes for benzene/cyclohexane mixtures [67]. MWCNTs, multiwalled carbon nanotubes.

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Chapter 5 Carbon nanotubes-polymer nanocomposite membranes for pervaporation

5.5.3

Recovery of organics from aqueous solutions

PV becomes feasible in recovery of low concentration of organics from aqueous solutions. As opposed to dehydration, the dilute organic compounds removal from aqueous solutions via PV process requires hydrophobic (organophilic) membranes to increase the organic compounds affinity toward the membranes. Recovery of organics from industrial wastewaters is important for the environmental pollution prevention [68]. Xue et al. reported preparation of CNTs-polydimethylsiloxane (PDMS) nanocomposite membranes for ethanol recovery from model solutions as well as real solutions (fermentation by self-flocculating yeast). CNTs incorporation into the PDMS membrane led to ethanol recovery improvement, with maximum permeation flux of 128.7 g m22 h21 and ethanol titer of 615.1 g L21 in permeate. The inner smooth surfaces of CNTs could provide a flexible route for ethanol transport. Crosssectional SEM images of the neat PDMS and the CNTs/PDMS nanocomposite membranes are shown in Fig. 5.16. As can be observed, the CNTs were uniformly distributed in the PDMS matrix due to the excellent interface compatibility between the PDMS polymer and the CNTs [44]. Wang et al. incorporated chemically carboxyl-functionalized and physically hydroxyl-functionalized MWCNTs into HTPBbased PU membrane for phenol/water mixture separation by PV. Their results demonstrated that the chemically modified

(A)

(B)

Figure 5.16 Cross-sectional SEM images of (A) the neat PDMS and (B) the CNTs/PDMS membranes [44]. CNTs, Carbon nanotubes; PDMS, polydimethylsiloxane.

Chapter 5 Carbon nanotubes-polymer nanocomposite membranes for pervaporation

MWCNTs incorporated PU membrane with separation factor of 3.6 and permeation flux of 101.73 kg μm21 m22 h21 at 80 C shows the better PV performance than the physically modified MWCNTs incorporated PU membrane with separation factor of 2.8 and permeation flux of 175.56 kg μm21 m22 h21 at 80 C [69].

5.6

Conclusions

PV is a vacuum driven membrane process for liquid mixtures separation, especially close boiling point and azeotropic mixtures, organic
organic mixtures, thermally sensitive compounds as well as removal of dilute organic compounds from wastewater and dehydration of solvents and alcohols. In recent years, CNTs with unique structural and extraordinary physical and chemical properties have attracted significant attention as a new type of nanofillers for novel polymer nanocomposite membranes fabrication. Uniform dispersion of CNTs with agglomeration tendency in polymer matrix is a critical issue in fabrication of CNTs- PNCMs. Therefore CNTs functionalization with specific functional moieties is necessary to reduce the van der Waals forces between CNTs and improve CNTs dispersion in polymer matrix. In this chapter, current progresses in preparation and characterization of CNTs-PNCMs for PV applications were reviewed. More investigations are still required to bring a new insight into a wider application of the CNTs-PNCMs in practical industry to access the potential of these membranes for related separation problems.

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28.

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6

M.B. Sethu Lakshmi1 and Bincy Francis2 1

Research and PG Department of Chemistry, N.S.S. Hindu College, Changanacherry, India 2PG Department of Chemistry, St. Thomas College, Ranny, India

6.1

Introduction

In recent years, membrane separation technologies underwent rapid development due to its applications in various fields. The membrane provides a vital role in controlling the mass transfer through its selectivity. It increases the concentration of one of the components on the other side of the membrane and causes the separation or removal of the desired component. The wide range of applications involves analytical sample preparations of volatile/semivolatile organics, inorganics, and metals, large-scale applications such as desalination, ultrafiltration, gas separations, and pervaporation [1]. In this chapter, pervaporation separation using graphene-based composite membranes is highlighted. Pervaporation is a modern and potential separation technique which has a high efficiency in azeotrope mixture separations. As the name implies, the pervaporation process includes permeation and evaporation of one of the components in a mixture at a faster rate through the membrane than the other. The liquid stream mixture is placed in contact with one side of the polymeric dense (nonporous) membrane and vacuum is applied on the other side. Sorption of the particles on the surface takes place first followed by the permeation of the highest permeable ones through the membrane to evaporate to the other side of the membrane which depends on the selectivity of the polymer. The vapors collected (permeate) are condensed and the process is repeated to get pure sample from the mixture. The process highly depends on the selectivity and Polymer Nanocomposite Membranes for Pervaporation. DOI: https://doi.org/10.1016/B978-0-12-816785-4.00006-9 © 2020 Elsevier Inc. All rights reserved.

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permeability of the membrane and the species in the mixture, even trace amounts of a particle in the feed can be separated as permeate [2]. Thus it is a highly efficient method for the dehydration of organic solvents and also for trace removal of organics from aqueous solutions compared to other conventional separation methods. The schematic diagram for pervaporation process is shown in Fig. 6.1. Inorganic membranes with a dense surface were used earlier for dehydration applications. Gemert and Cupreous [4] used ceramic dense membranes for separation of organic mixtures by pervaporation. Van Veen et al. [5] performed pervaporation dehydration of organics using microporous silica membranes. Polymer membranes were used for pervaporation performance at early ages and were also available commercially. Modification

Figure 6.1 Experimental set-up for pervaporation: schematic diagram (1. preheating cell; 2. peristaltic pump; 3. pervaporation cell; 4. thermocouple; 5. needle valves; 6. liquid traps placed in Dewar flask; 7. McLeod; Pirani gauge, capillary column in parallel mode; 8. vacuum pump; and 9. condensor) [3].

Chapter 6 Graphene-based polymer nanocomposite membranes for pervaporation

of the membranes by introduction of crosslinking in polymer chains has also been done in early ages. Chitosan (CS)-based composite membranes are widely used for pervaporation dehydration of various organic solvents. Ionic liquid-based membranes were used for ternary azeotrope separation by Ong and Tan [6]. Graphene oxide (GO) pristine and composite membranes with different polymers are the future scope in the area of liquid separation by pervaporation.

6.2

Graphene

Graphene is an allotrope of carbon connected by sp2 hybridized bonds and belongs to hexagonal Bravais lattice. It is the basic building block for graphitic material of all dimensionalities. It can be wrapped up into 0D fullerenes rolled into 1D carbon nanotubes and stacked into 3D graphite. Novoselov et al. [7] extracted graphene in 2004 and got Nobel Prize for this discovery. After 2012 research on graphene plummeted and over 2000 patents involving this material awarded. Its flexibility and structure make it a leading candidate for next generation applications.

6.2.1

Structure and properties of graphene and its derivatives

Pristine graphene can be synthesized by various bottom-up processes such as chemical vapor deposition, arc discharge, epitaxial growth on SiC, chemical conversion, mechanical exfoliation using scotch tape, etc. However, bottom-down method offers a facile, cost effective, and scalable laborious peeling route for graphene synthesis which utilize GO as the intermediate. GO was first prepared over 150 years ago by Brodie [8] which was modified by Staudenmaier [9] and Hummers and Offeman later [10]. GO has layered structure similar to that of graphene, but the periphery of the lamella was decorated by oxygen-containing functional groups such as epoxide, OH, and COOH which improve hydrophilicity and expand the interlayer distance. Aromatic entities, double bonds, and epoxide groups give rise to flat carbon grid while carbon with hydroxyl groups shows wrinkling character. The functional groups lie above and below the carbon grid. Structural variation in the conversion of graphite to GO is illustrated in Fig. 6.2.

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Figure 6.2 Schematic illustration of structure variation of graphite to oxidized graphite [11].

Figure 6.3 FESEM images of (A) graphene oxide and (B) prepared graphene [11].

Several reduction methods such as chemical [12,13], thermal [14], microwave assisted, and flash induced reduction [15] can be made use for the conversion of GO to graphene. Complete reduction of GO to graphene is not yet observed and so the term modified graphene or reduced graphene is used. FESEM analysis of GO and the reduced GO (rGO) is shown in Fig. 6.3. Other reported methods for the reduction of GO are flame induced reduction [16], electrochemical reduction [17] and ion implantation [18,19]. Graphene is extraordinarily strong and supernaturally light. Graphene exhibits fascinating mechanical properties with a Young’s modulus (E) of 1.0 TPa and an intrinsic strength (τc) of 130 GPa which can be fine-tuned by heteroatom doping, polymer compositing, and thickness reduction. Graphene is found to be a semiconductor with zero bandgap from its energy band structure. According to Terrones et al. [20], other outstanding properties of graphene are quantum hall effect at room temperature, Klein paradox, zero field conductivity, and large mean free path. Good conductivity of 188 S cm21 was reported for graphite while the conductivity of GO decreases remarkably with increasing oxidation [21,22]. Graphene outstands graphite

Chapter 6 Graphene-based polymer nanocomposite membranes for pervaporation

139

in thermal properties also. Computational modeling suggests that isotopic disorder, strain, and chemical functionalization such as hydrogenation and fluorination lead to sensible reduction in thermal conductivity. Moreover graphene exhibits approximately 97% optical transparency in the visible range and offers chemically inert surface [23 31].

6.2.2

Graphene membranes—synthesis and characterization

High selectivity, throughput, high surface area, and low folding of graphene made it novel material for membrane chemistry. Even though graphene-based composite membrane is still in its infancy, numerous graphene polymer composites are developed using polysulfones, poly(vinyl alcohol), polyurethane, polycarbonate, polyaniline, and polystyrene. Graphene-based membranes can be prepared in various routes such as spin coating, layer-by-layer assembly, drop coating vacuum filtration, and blend method. Filtration-assisted method includes vacuum, pressure, and evaporation-assisted self-assembly of membranes and is widely used for the synthesis of free standing membranes [32]. Schematic representation of GO composite membrane preparation through filtrationassisted approach is shown in Fig. 6.4. Hung et al. [33] prepared GO layers on polyacrylonitrile substrate through pressureassisted self-assembly technique. This composite membrane shows 2047 g m21 h21 permeation flux for the pervaporation separation of 70 wt.% IPA/water mixture. Miculescu et al. [34]

Figure 6.4 Schematic diagrams of composite GO/mPAN membranes fabricated through PASA, VASA, and EASA [35]. GO, Graphene oxide.

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synthesized GO-methyl cellulose membranes via evaporationassisted assembly technique. Filtration-assisted method offers a potential route to control the membrane structure and thickness during synthesis. Very thin graphene oxide membranes (GOM) can be prepared using spin coating method [36]. Casting/coating-assisted method includes dip coating, spray coating, spin coating, drop casting, etc. Shen et al. [37] fabricated GO-polyether block amide (PEBA)mixed matrix membranes for gas separation by drop casting method. Polysulfone SiO2-GO nanohybrid composite membranes developed for water desalination via solution casting. Luo et al. [38] prepared graphene-based anion exchange membrane fuel cell by solvent casing method. Scalese et al. [39] incorporated GO flakes functionalized with 3-amino-1-propanesulfonic acid (GOSULF) to the support matrix Nafion through solvent casting method. Nafion-GOSULF membranes are efficient for the degradation of cationic dye, methylene blue. Sulfonic groups on GO enhance dye adsorption through electrostatic interactions. Hu and Mi [40] in their study incorporated GO to dopaminecoated polysulfone support through LBL method for water purification. Zhang et al. [41] also succeeded in depositing ultrathin graphene oxide framework (GOF) on a modified Torlon fiber support via layer-by-layer approach. In layer-by-layer approach membrane thickness can be controlled by altering the number of deposition cycles and also the electrostatic interaction in between the layers attribute stability to the membrane. Schematic diagram of LBL formation of GO-PAH/hPAN is shown in Fig. 6.5. Shear alignment method, evaporation-assisted method, and templating method are the additional membrane preparation techniques. The morphology, topography, crystallinity, and thermal stability of the graphene membranes can be characterized by

Figure 6.5 Schematic diagram of LBL assembly of GO-PAH bilayers on both sides of hPAN [42]. GO, Graphene oxide.

Chapter 6 Graphene-based polymer nanocomposite membranes for pervaporation

various techniques such as AFM, SEM, TEM, XRD, and TGA. Surface hydrophilicity can be measured using contact angle measurement. According to the XRD results, GO/PP membranes ˚ , while that of GO/polyacrylonitrile [43] has d spacing 8.18 A ˚ . FTIR, Raman spectra, and XPS ensure the (PAN) films is 7.36 A effect of functionalization and surface chemistry. In the comparative FTIR spectra of GO and NHGO, two new peaks appeared in NHGO’s IR center at 3300 3600 and 1580 cm21 which confirms the amine functionalization in NHGO membranes. The characterization results give an insight into the structure and separation property of the membranes. • • • • •

Advantages Simple membrane preparation from readily available resource. Reduced solvent loss. Durable and recyclable with extended lifetime. No chemical drying. No generation of waste water.

Applications • Dehydration or concentration of compound from aqueous media. • Desalination. • Separation of compounds that form azeotropes with water.

6.3

Graphene-based membranes for pervaporation

Research for an ideal material is now after the promising material graphene due to its inherent mechanical and chemical stability and frictionless surface. rGO with 2D lamellar structure can be derived from GO by removing the oxygen-carrying functionalities. This modification may be achieved by chemically, thermally, or hydrothermally. Nanofiller rGO differs from GO in the pore size which is B678 and is suitable for desalination application. rGO-based membranes can be prepared in various routes such as spin coating, layer-by-layer assembly, drop coating vacuum filtration, and blend method. This supreme filler can be incorporated with ceramic and polymer-supported membranes and can be used for various PV applications.

6.3.1

Graphene oxide-based membranes

In the field of separation techniques, graphene membranes were initially used for water purification by removal of dyes

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such as methylene blue and also for removal of heavy metal such as cadmium and chromium from waste water and are considered to be efficient. Later graphene-based polymer membranes were introduced, which offered high durability and easy recycling of the system. Graphene and its compounds such as GO and rGO were introduced into the pervaporation separation technique during the last 5 years and is now an emerging area in membrane development with superior qualities [44]. Pristine GOMs are not highly efficient for pervaporation applications due to its less endurance and hence needed a support whenever in use. In the pervaporation dehydration of butanol, Chen et al. [45] used AAO membrane support for GO fabrication. This problem was easily overcomed by the introduction of composite membranes with various polymer systems. The polymer matrix for GO composite membranes was selected based on the application. Due to the selectivity of the GO component for water removal, the composite membranes were highly recommended for dehydration and desalination applications. Cao et al. [46] studied the water permeation through sodium alginate-GO composite membranes and found to be highly efficient. Lecaros et al. [47] studied the dehydration of acetic acid using tunable interlayer spacing of GOF in PVA composite membranes. The addition of PVA made the GOF more stable and with 8 wt.% PVA ˚, loading based on GO has a d spacing at wet state of 8.95 A ˚ ). This indicates that the PVA which is lower than GO (10.7 A binds the GO nanosheets more closely and forms a framework. The GO-PVA8 wt.% has an enhanced permeation flux of 463.9 g m22 h21 and water concentration in permeate of 97.7% at 80 C. Recently ethylene glycol pervaporation dehydration was reported with GO polyelectrolyte complex (PEC) membranes [35]. The study of dehydration of alcohols using pervaporation is the vast area where GO composite membranes were used and tremendous research is still going on the modification of membranes for the same [48,49]. Another area in which the most recent pervaporation studies are focused is the desalination of water and GO and its modified forms are promising fillers in the composites used for desalination pervaporation [50 54]. Choudhari et al. [55] studied the removal of butyric acid from aqueous and anaerobic digestion solutions using PEBA-GO composite membranes. Biodiesel was produced by pervaporation-assisted esterification using GOCS composite membranes and was proved to be a technology for the future.

Chapter 6 Graphene-based polymer nanocomposite membranes for pervaporation

6.3.2

143

Reduced graphene oxide membranes

As rGO inherent excellent tolerance to organic solvents and chemical environments, rGO membranes are widely used for organic nanofiltration in the early stage and now extended to permeation and pervaporation studies. Hung et al. [56] described a comparative study of GO and rGO composite membranes with CS for methanol dehydration through pervaporation. Extreme membrane aggregation occurred in GO/CS membranes due to the interionic complexation between the negatively charged carboxylate ions of GO and the positively charged protonated amines of CS. But the hydrothermal reduction of GO to rGO prevents undesired ionic complexation. Consequently the dispersion improved and rGO/CS stack up and self-assembled to ordered lamellar structure. Fig. 6.6 illustrates the various interactions—hydrogen bonding, ionic bonding, and covalent bonding—between GO/CS (Fig. 6.6A) and rGO/CS (Fig. 6.6B). GO/CS membrane yields 91.5 wt.% water concentration on the permeate side of membrane. For rGO/CS membrane, pervaporation efficiency increases as a function of hydrothermal reduction time. When the reduction time was 72 h, the water concentration on the permeate side was 99.8 wt.% for the composite membranerGO72h/CS. Maya et al. [57] developed rGO/polychloroprene nanomembranes for the pervaporation separation of a zeotropic mixture chloroform/acetone.

Figure 6.6 Proposed chemical interactions in GO/CS (A) and rGO/CS (B) [56]. CS, Chitosan; GO, graphene oxide; rGO, reduced graphene oxide.

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Chloroprene loaded with 0.9 phr rGO membrane shows optimum flux rate and separation efficiency.

6.3.3

Hybrid graphene oxide membranes

Membrane materials for pervaporation can be generally categorized as inorganic, polymeric, and hybrid membranes. Polymeric membranes credit low cost, easy processing, and better transport properties while inorganic membranes are supreme for their chemical and thermal stability. Hybrid membranes can be prepared either by introducing thin sheets into polymer to get lamellar structure or by incorporating nanofillers into polymers which inherit superior attributes from both inorganic and polymer. The two possible structures are shown in Fig. 6.7. These mixed matrix membranes attributes 4M features: multiscale structure, multiple interactions, multiple functionalities, and multiphase. Different interactions such as hydrogen bond, π π interaction, van der Waals’ force, covalent bond, or ionic bond stabilize matrix and filler. The hybrid membranes containing more than one filler or one nanoparticle modified with other nanoparticles were studied by various researchers for different applications. In the pervaporation area, CNT and GO were mixed to get synergic effect on a PEC membrane and found to be effectively increased the separation factor, tensile properties, and operational stability. Wu et al. [59] selected sodium carboxymethyl cellulose (CMC) and poly(2-methacryloyloxy ethyl trimethyl ammonium chloride) (PDMC) as the oppositely charged polyelectrolytes in their work. The PDMC-CMC/GO-CNT pervaporation membrane showed improved separation performance in dehydrating 10 wt.

Figure 6.7 The two kinds of structures of hybrid membrane [58].

Chapter 6 Graphene-based polymer nanocomposite membranes for pervaporation

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% water/IPA at 40 C. PSI of PEC/GO-CNT was found to be 2 times than the pure PEC membrane. Cheng et al. [60] in their work combined the high permeability of electrospun PAN with the flexible GO in polyamide layer (PA-GO) to construct a high efficiency thin film nanocomposite membrane for PV application. The membrane yield a separation factor of 1491 and PSI of 98.2 3 105 at 70 C for 70 wt.% IPA feed concentration. Preparation and pervaporation process for PA-GO/PAN membrane is shown in Fig. 6.8.

6.3.4

Functionalized graphene oxide membranes

GO promises extensive progress in the field of membrane research. Due to weak adhesion, GO tend to aggregate in dope solutions and the resulting defective surfaces may cause decline

Figure 6.8 Schematic diagram for the preparation and water/IPA separation process of PA-GO/PAN pervaporation membranes [60]. PA-GO, Graphene oxide in polyamide layer; PAN, polyacrylonitrile.

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in mechanical, thermal, and electrical properties. However, oxygen-containing various functional groups (epoxide, hydroxyl, and carbonyl) in this flexible membrane offer infinite possibilities for the surface functionalization and modification. Graphene surface could be complimented with many additives such asdiamines, methyl nicotinamide chloride [61], and boranes [62]. But only a few were adopted for pervaporation application. Among such many attempts, zwitter-ionic polymers get incorporated into graphene successfully and procured antifouling property, enhanced mechanical property, microporosity, and separation performance. Zwitter-ionic polymers are neutral molecules with a mixture of positive and negative moieties within the same polymer chain. They (e.g., phosphobetaine, sulfobetaine, and carboxybetaine) could form hydration layers around hydrophilic functional groups through ionic solvation and thereby induce antifouling behavior to the membrane. Many techniques were applied to graft zwitter-ionic polymer onto membrane surface. Zhao et al. [63] modified GO with polysulfobetaine methacrylate using free radical polymerization technique and were introduced to sodium alginate membranes for efficient water permeation and water/alcohol separation. High-density zwitter-ionic groups in GO induce high water affinity and ethanol repellency and the hydration layer could mitigate concentration polarization of ethanol. Another comparative study with rGO SA membrane also resulted with superior permeation flux and separation factor. Ang et al. [64] in their work modified GO with zwitter-ionic copolymer poly(glycidyl methacrylate-sulfobetainemethacrylate) or poly(GMA-SBMA). Schematic diagram showing the interaction between GO sheet and zwitter-ionic copolymer is shown in Fig. 6.9. Among the hybrid membranes with varying zwitterion load, 0.6Z-GOM optimized as the high-performing membrane with a permeation flux of 1102 g m22 h21 on a 70 wt.% aqueous IPA solution. Psfsupported Z-GO membrane owed high PV performance on aqueous IPA solution and increased permeation flux. In another work Salehian et al. [65] designed high-performance ammonia functionalized GO, NHGO/6FDA-based polyimide MMMs for pervaporation dehydration of isopropanol. The theoretical NHGO loading in the mixed matrix membrane can be calculated by the following equation: WNHGO

mNHGO 3 100% mpolymer 1 mNHGO

where WNHGO is the particle loading and mNHGO and mpolymer represent the weights of NHGO and polyimide, respectively.

Chapter 6 Graphene-based polymer nanocomposite membranes for pervaporation

Figure 6.9 Reaction scheme between GO sheets and poly(GMA-SBMA) [64]. GO, Graphene oxide.

The NHGO particles have remarkable effect on diffusivity and on sorption selectivity of water/IPA. As the NHGO filler loading increasing from 0.1 to 0.5, water/IPA diffusion selectivity increases from 3128 to 5125. High-performing hybrid membranes can be developed by optimizing the surface modifiers.

6.3.5

Quantum dot membranes

With a very small size (less than 10 nm), quantum dots are one of the most efficient nanoparticles in the research field worldwide and consist mainly of two types: graphene quantum dots and carbon quantum dots [66]. Graphene quantum dots are not widely reported in the pervaporation applications. Recently Lecaros et al. [67] incorporated graphene quantum dots into polyvinyl alcohol and prepared a hybrid membrane for pervaporation application. The membrane incorporated with 300 ppm of GOQD (PVAx-GOQD300) showed the highest separation performance. It has a separation factor of 476.4 6 8.25, total permeation flux of 463.5 6 18.2 g m22 h21, and a PSI value of 2.20 3 105 g m22 h21. The membrane prepared was stable, the dehydration ability was found to be high for alcohols and the membrane showed different flux for isomers of propanol which is promising for the separation of alcohols.

6.4

Conclusions and future aspects

Pervaporation technology is the most promising technique in the field of liquid separation. It is the most efficient technology for the separation of azeotropic binary or tertiary mixtures. The technology provides ecofriendly, clean, and energy efficient

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mass transfer method which is considered to be the most economical one compared to other conventional mass transfer techniques. In the case of petroleum products and rocket propellants, where concentration conditions needed to be low temperature and low pressure, pervaporation provides the key for future. The challenges associated with the technique are the selection of polymer for membrane preparation. The membrane material must have features such as solvent resistance, perm selectivity, permeation ratio, and sustainability. Also the polymer should be sustainable. To overcome this problem, the polymer should be modified by crosslinking, blending, or composite making. Nanofillers are promising materials for polymer composites. Out of the large number of nanoparticles available today, graphene and its compounds are the most promising ones for the pervaporation applications as it provides large surface area and does not affect the stability of the membranes. GOMs are efficient in water separation or dehydration applications. Graphene and its modifications which are thermally and mechanically stable enough provide a good candidate for pervaporation membrane development in the future as it is an efficient and economical filler which can be prepared and incorporated in the matrix easily. The graphene-based polymer composites can be a suitable replacement for the current water treatment systems if studied and improved as it is the most sustainable and efficient membrane systems developed.

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[45]

[46]

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[50]

[51]

[52]

[53]

[54]

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[56]

[57]

[58] [59]

membranes for pervaporation ethylene glycol dehydration, J. Mem. Sci. 577 (2019) 104 112. X. Chen, G. Liu, H. Zhang, Y. Fan, Fabrication of graphene oxide composite membranes and their application for pervaporation dehydration of butanol, Chin. J. Chem. Eng. 23 (2015) 1102 1109. K. Cao, Z. Jiang, J. Zhao, C. Zhao, C. Gao, F. Pan, et al., Enhanced water permeation through sodium alginate membranes by incorporating graphene oxides, J. Mem. Sci. 469 (2014) 272 283. R.L.G. Lecaros, G.E.J. Mendoza, W.S. Hung, Q.F. An, A.R. Caparanga, H.A. Tsai, et al., Tunable interlayer spacing of composite graphene oxideframework membrane for acetic acid dehydration, Carbon 123 (2017) 660 667. P. Li, C. Cheng, K. Shen, T. Zhang, X. Wang, B.S. Hsiao, Enhancing dehydration performance of isopropanol by introducing intermediate layer into sodium alginate nanofibrous composite pervaporation membrane, Adv. Fibre Mater. (2019) 1 15. X. Cheng, W. Cai, X. Chen, Z. Shi, J. Li, Preparation of graphene oxide/poly (vinyl alcohol) composite membrane and pervaporation performance for ethanol dehydration, RSC Adv. 9 (2019) 15457 15465. S. Gahlot, P.P. Sharma, B.M. Bhil, H. Gupta, V. Kulshrestha, GO/SGO based SPES composite membranes for the removal of water by pervaporation separation, Macromol. Symposia 357 (2015) 189 193. 201. B. Liang, W. Zhan, G. Qi, S. Lin, Q. Nan, Y. Liu, et al., High performance graphene oxide/polyacrylonitrile composite pervaporation membranes for desalination applications, J. Mater. Chem. A 3 (2015) 5140 5147. B. Feng, K. Xu, A. Huang, Covalent synthesis of three-dimensional graphene oxide framework (GOF) membrane for seawater desalination, Desalination 394 (2016) 123 130. E. Halakoo, X. Feng, Layer-by-layer assembly of polyethyle neimine/ graphene oxide membranes for desalination of high-salinity water via pervaporation, Sep. Purif. Technol. 234 (2020) 116077. C. Cheng, L. Shen, X. Yu, Y. Yang, X. Li, X. Wang, Robust, Construction of a graphene oxide barrier layer on a nano fibrous substrate assisted by the flexible poly(vinyl alcohol) for efficient pervaporation desalination, J. Mater. Chem. A 5 (2017) 3558 3568. S.K. Choudhari, F. Cerrone, T. Woods, K. JoyCE, V.O. Flaherty, K.O. Connor, et al., Pervaporation separation of butyric acid from aqueous and anaerobic digestion (AD) solutions using PEBA based composite membranes, J. Ind. Eng. Chem. 23 (2015) 163 170. W.S. Hung, W. Song, S.M. Chang, R.L.G. Lecaros, Y.L. Ji, Q.F. An, et al., Fabrication of hydrothermally reduced graphene oxide/chitosan composite membranes with a lamellar structure on methanol dehydration, Carbon 117 (2017) 112 119. M.G. Maya, S.C. George, T. Jose, L. Kailas, S. Thomas, Development of a flexible and conductive elastomeric composite based on chloroprene rubber, Polym. Test. 65 (2018) 256 263. X. Chenga, F. Pan, M. Wanga, W. Lia, Y. Songa, Hybrid membranes for pervaporation separations, J. Mem. Sci. 541 (2017) 329 346. J.K. Wu, C.C. Ye, T. Liu, Q.F. An, Y.H. Song, K.R. Lee, et al., Synergistic effects of CNT and GO on enhancing mechanical properties and separation performance of polyelectrolyte complex membranes, Mater. Des. 119 (2017) 38 46.

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[60] C. Cheng, P. Li, T. Zhang, X. Wang, B.S. Hsiao, Enhanced pervaporation performance of polyamide membrane with synergistic effect of porous nano fibrous support and trace graphene oxide lamellae, Chem. Eng. Sci. 196 (2019) 265 276. [61] Y.P. Tang, J.X. Chan, T.S. Chung, M. Weber, C. Staudt, C. Maletzko, Simultaneously covalent and ionic bridging towards antifouling of GOimbedded nanocomposite hollow fibre membranes, J. Mater. Chem. A 3 (2015) 10573 10584. [62] J. Oh, Y.H. Mo, V.D. Le, S. Lee, J. Han, G. Park, et al., Borane-modified graphene-based materials as CO2 adsorbents, Carbon 79 (2014) 450 456. [63] J. Zhao, Y. Zhu, G. He, R. Xing, F. Pan, Z. Jiang, et al., Incorporating zwitterionic graphene oxides into sodium alginate membrane for efficient water/alcohol separation, J. ACS Appl. Mater. Interf. 8 (2016) 2097 2103. [64] M.B.M.Y. Ang, M.R. Gallardo, G.V.C. Dizon, M.R.D. Guzman, L.L. Tayo, S.H. Hang, et al., Graphene oxide functionalized with zwitterionic copolymers as selective layers in hybrid membranes with high pervaporation performance, J. Mem. Sci. 587 (2019) 117188. [65] P. Salehian, T.S. Chung, Thermally treated ammonia functionalized graphene oxide/polyimide membranes for pervaporation dehydration of isopropanol, J. Mem. Sci. 528 (2017) 231 242. [66] Y. Wang, A. Hu, Carbon quantum dots: synthesis, properties and applications, J. Mater. Chem. C. 2 (2014) 6921 6939. [67] R.L.G. Lecaros, K.M. Deseo, W.S. Hung, L.L. Tayo, C.C. Hu, Q.F. An, et al., Influence of integrating graphene oxide quantum dots on the fine structure characterization and alcohol dehydration performance of pervaporation composite, J. Mem. Sci. 576 (2019) 36 47.

Fullerene and nanodiamondbased polymer nanocomposite membranes and their pervaporation performances

7

Neetha John Central Institute of Plastics Engineering & Technology (CIPET), Institute of Plastics Technology (IPT), Kochi JNM Campus, Udyogamandal, Kochi, India

7.1

Introduction

Carbon is a material in the whole universe including living and nonliving things. Various developments occur out of carbon. In 19th century 1st carbon fiber invented by T.A. Edison, in 1950s introduced the carbon-reinforced materials, followed by polyacrylonitrile (PAN) fibers, C-whiskers, vapor-phase grown techniques. In 1985 there was discovery of 1D fullerenes and followed by 3D fullerenes. Fullerene is a curved structure built up of fused pentagons and hexagons. The smallest stable and the most abundant fullerene obtained by the usual preparation method is the symmetrical Buckminster fullerene C60. There are other stable fullerenes such as C70 to C84. Fullerenes, carbon nanotubes (CNTs), grapheme, and carbon black have the great potential to be used in many areas from medicine to engineering. Many researches are going on CNTs for the application in diverse areas such as electrochemical devices, field emission, sensors, and probes. It have also been used as additives in the thermoplastics used for 3D printing. The materials have poor adhesion and low miscibility with conventional organic solvents which prevent them from dispersed well in polymer matrices [1,2]. The membrane technologies are widely used in environmental remediation, green energy, food, chemical, and pharmaceutical sectors. Membranes are having permselective barrier property that allows particular species to pass through it while posing a partition for nonselective species. Nanotechnology has Polymer Nanocomposite Membranes for Pervaporation. DOI: https://doi.org/10.1016/B978-0-12-816785-4.00007-0 © 2020 Elsevier Inc. All rights reserved.

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been considered as one of the highest potential areas for resolving the technical challenges coupled with the separation and purification technologies. Fullerenes are produced by arc or combustion methods and subsequent purification of reaction products and are mixed with polymers. The production of fullerene is achievable by means of an irradiating laser beam on a graphite rod placed in a helium atmosphere. Different production methods have been developed for sufficient production of fullerene. The second proposed method is based on laser ablation of graphite in a helium atmosphere. In the laser ablation method, materials are removed from a solid surface by irradiating it with a laser beam. During the laser irradiation of graphite, materials are evaporated and their vapors are converted to plasma. On cooling the gas, the vaporized atom tends to combine and form fullerene. The arc discharge process is a method, in which the vaporization of the input carbon source is achieved by the electric arc formed between two electrodes [3,4]. Fullerenes are closed hollow cages consisting of carbon atoms interconnected in pentagonal and hexagonal rings. Each carbon atom on the cage surface is bonded to three carbon neighbors therefore is sp2 hybridized. The most famous fullerene is C60, known also by “buckyball.” It is an unusual form of carbon; dissolves in common solvents such as benzene, toluene, and hexane; and readily vaporizes in vacuum around 400
C, low thermal conductivity, pure C60 is an electrical insulator, and C60 doped with alkali metals shows electrical conductivity between insulator (K6 C60) to superconductor (K3 C60) [5,6]. Fullerenes can be made by vaporizing carbon within a gas medium. They could form spontaneously in a condensing carbon vapor. An electric arc is maintained between two nearly contacting graphite electrodes. Most of the power is dissipated in the arc and not in resistive heating of the rod. The entire electrode assembly is enclosed in a reaction kettle that is filled with 100 Torr pressure of helium. Black soot is produced, and extraction with organic solvents yields fullerenes [7]. No other element has such wonderful properties as carbon. Bucky balls are relatively cheap as carbon is everywhere. Even though each carbon atom is only bonded with three other carbons in a fullerene molecule, dangling a single carbon atom next to the structure is not strong enough to break the structure of the fullerene molecule. In fullerenes, 12 pentagonal rings are necessary and sufficient to affect the cage closure. Fullerenes contain carbon atoms arranged as a combination of 12 pentagonal rings and n hexagonal rings. The chemical formula is

Chapter 7 Fullerene and nanodiamond-based polymer nanocomposite membranes

˚ in diameter and are C2012n. Fullerene cages are about 715 A one carbon atom thick. Quite stable from chemical and physical conditions, breaking the balls requires temperatures of about 1000
C. It possesses highest tensile strength of any known 2D structure or element and highest packing density of all known structures. It is also impenetrable to all elements under normal circumstances, even to a helium atom with energy of 5 eV [8]. Fullerenes with material inside are called cage compounds or endohedral compounds. Exohedral compounds are those in which a wide variety of both inorganic and organic groups added to the exterior of the cage. Combination endo- and exohedral compounds have also been synthesized. An interesting example is Gd@C82 (OH)n [9]. There are many side reactions possible by fullerens and can form many by products out of it due to it reactivity. Each has many functionalities and applications that depend on its side groups [10]. The A3C60 phase is a conductor at room temperature due to the partial filling of the conduction band. Electrons can move between C molecules through the radiating-orbitals. At low temperature, A3C60 becomes superconducting. A6C60 phase is an insulator due to the complete filling of the t1u orbitals [11]. Photovoltaics are best known as a method for generating electric power by using solar cells to convert energy from the sun into a flow of electrons which can be made by fullerenes. The photovoltaic effect refers to photons of light-exciting electrons into a higher state of energy, allowing them to act as charge carriers for an electric current [12]. Hydrogen storage for fuel-cell powered vehicle is another major area where fullerenes are used. Organometallic molecules are used as Hydrogen storage which are based on C60 and boron-doped C48B12. Complexes of transition metals with hydrogen on pentadiene rings can store up to six dihydrogen species. It may polymerize when the hydrogen is removed, rendering the process irreversible. Arranging the complexes on buck balls, such as C60 [ScH2]12 and C48B12 [ScH]12, leads to stable species which can reversibly absorb additional hydrogen. Almost 9 wt.% can be retrieved reversibly and room temperature and near ambient pressure. Doping with boron reduces the fullerene weight, enhances the stability of the complex by increasing the binding energy, and allows the binding of an additional H2 molecule per Sc, increasing the amount of retrievable H2 [13,14]. Diamond below the size of 1 µm is termed as nanodiamonds (Fig. 7.1). It can be produced in large scale and can be functionlized easily which shows very good biocompatibility. It can be used in many electronic and biomedical applications.

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Figure 7.1 Nanodiamond cross section.

The structure of nanodiamond is considered with the overall shape, the core, and the surface. The shape of diamond is found to be spherical or elliptical. The core of nanodiamond is surrounded by a cage made of carbon. The core structure is similar to diamond and the surface looks like graphite. The surface consists of phenols, pyrones, sulfonic acid, carboxylic acid, and many functional groups. There are many structural defects also present. They are produced by explosion of meteoritic activity. Nanodiamond is synthesized by hydrothermal synthesis, ion bombardment, laser bombardment, microwave plasma chemical vapor deposition techniques, ultrasound synthesis, and electrochemical synthesis [15]. The functionalization of fullerenes is a broad topic; the major reason is the solubility of fullerene derivatives. Nanodiamond can be used in medicine as antioxidants as it gives water solubility also helps in the production of reactive oxygen species. Fullerenol can be a water soluble pharmaceutical additive for the treatment of oxidative stress-related disease and as biocompatible actuators. The OH-group-modified fullerenol provides better versions of chemical reactions and processes like catalysis [16].

7.2

Pervaporation

Pervaporation (PV) (Fig. 7.2) is one of the most active areas in membrane technology in the separating process of close boiling or azeotropic liquid mixtures, heat sensitive biomaterials, water, or organics from its mixtures that are inseparable by any other techniques. It consists of two stages, permeation and

Chapter 7 Fullerene and nanodiamond-based polymer nanocomposite membranes

Membrane

Feed

Permeate

Evaporation

Figure 7.2 Schematic representation of pervaporation.

evaporation. This technique is used in many purification processes. Membranes separate the liquid and vapor phases in the feed. Separation depends on the selective transport of the individual components through the membranes. The upstream side is at ambient temperature and downstream at vacuum. This makes the easy vaporization of selective components. The driving force for separation is the partial pressures on the two sides of the membranes. It can be used to separate waterethanol mixtures which can be explained with a solution-diffusion model. Hydrophobic membranes are used like polydimethylsiloxanes (PDMS) also hydrophilic types polyvinyl alcohol (PVA). Ceramic membranes are also widely used with macroporosity over which nanoporosity exists. It can retain larder molecules like ethanol a pore size and allows water to pass through of the ˚ . Zeolite is most common membrane. order of 4 A PV process is more economical compared to other distillation process. It gives high product purity and no environmental pollution. Multicomponent can be easily separated which has close distillation points. The feed supply can be either liquid or vapor. Low energy consumption can operate very small unit also. Very high flexibility possible and time consumption is set up and installation is very less and can easily operate, start, and shut down quickly. PV is a useful analytical tool that features simplicity, automation, and miniaturization capabilities. The actual potential of this technique for the analysis of solid, liquid, and slurry samples can be inferred from its intrinsic features and from available methods using a pervaporator. PV becomes a useful tool for routine environmental, pharmaceutical, food, and industrial analyses. The technique used to separate components by partial vaporization through porous or nonporous membranes. PV technique is mostly used for dehydration processes such as

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separation or removal of water from solvents, alcohols, ethers, ester, and ketones. It also uses the removal of organic from aqueous liquids and separation of components of mixtures. Separation is facilitated by PV membranes. It is nonporous dense membranes made of polymers or ceramic. The membranes show different permeability for different solvents. The feed is heated up to the operating temperature and comes in contact with the membranes. The membranes allow the better permeating components which passes through the membrane. There will be a concentration gradient across the membranes and that is the driving force behind separation [17]. PV allows separation of mixtures that are difficult to separate by distillation, extraction, adsorption, and absorption. PV has advantages in the separation of azeotrope mixtures, closeboiling mixtures, thermally sensitive compounds, molecules that are similar in shape or weight, and removing species present in low concentrations [18]. During PV, a phase change from liquid to vapor takes place. Processes involving phase changes are generally energy intensive, but PV process contains the phase change by two features [19,20] like phase change occur to minor components only and PV uses the most selective membranes. The major feature is that it effectively reduces the energy consumption of the process. Processes can be combination type or hybrid type [21,22] by integrating PV with other viable liquid-separating technologies and processes seem to be very promising for practical purposes. PV process is attractive for separations in biotechnology due to have several features such as low temperature, low pressures, high cross-flow velocities are not needed and additional chemicals are not required. There are four suggested classes of bioseparation for PV, which require membranes with different characteristics such as direct byproduct recovery, volatile byproduct removal, concentration of sensitive byproducts, and dehydration of low molecular organics. The concentration of sensitive byproducts requires primarily high water flux membranes, as the products are usually high molecular species, for example, amino acids and enzymes, which are non-permeable to PV membranes. In the last stage of downstream processing dehydration of low molecular byproducts by PV occurs.

7.3

Membranes for pervaporation

For PV, the hydrophilic membranes were the first one to achieve the industrial applications and were used for the

Chapter 7 Fullerene and nanodiamond-based polymer nanocomposite membranes

organic solvent dehydration. There are two main types of membrane material, namely polymeric and inorganic membranes. The main industrial applications in PV are the dehydration of organic liquids. These membranes are enabled to extract water with broad ranges of flux and selectivity by modifying the active layer of these membranes with different chemical compositions and structures [22,23]. Commercially available hydrophilic membranes are made of polymeric membrane materials are PVA, polyimides, polymaleimides, Nafion, and PAN. Presently there have been a number of investigators concerning R&D of the hydrophilic membrane, which can be cataloged into organic, inorganic, and organicinorganic hybrid membranes. Zeolite and silica are generally used as inorganic membranes [24,25]. Inorganic membranes are very suited for high temperature applications in harsh environments and generally these membranes are prepared by solgel method. It can be elaborated appropriately to thin and porous layers with controllable porosity on a wide range of chemically resistant macroporous substrates [26]. Inorganic membranes show high dehydration efficiency, industrialization process may be slowpaced due to the complicated large-scale preparation and high manufacturing cost. Several kinds of commercial organic membranes including PERVAP 2201 [27,28], PERVAP 1005 (GFT) [29], and GFT-1005 [30] have been introduced to the PV-esterification coupling process. Cross-linked PVA membranes have also been investigated, such as PVA with catalyst or (SO4)2  4H2O and Amberlyst [31,32], cast on polyethersulfone [33,34], and on PAN. Organicinorganic hybrid materials have been proposed to be the best choice which can have both the functionality of the organic components and the stability of inorganic components. Budd et al. [35] employed zeolite/polyelectrolyte [chitosan/poly (4-styrene sulfonate)] multilayer PV membranes to enhance the yield of ethyl lactate. Adoor et al. [36] prepared aluminum-rich zeolite beta incorporated sodium alginate PV membranes. Organicinorganic hybrid membranes showed improved performance of evaporative dehydration of solution, with better flux and retention. Hydrophobic membranes used as PV membranes are used to separate volatile organic compounds from the body of water. Hydrophobic membrane systems utilize molecules made from proprietary polymeric hollow fiber membranes. The membrane only permits the volatile organic compound and rejects water molecule due to its hydrophobic nature. PV integrated with membrane separation technology serves as an interesting

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subject for the separation of organic compounds such as pollutants and high-value products such as aroma compounds. There are many membranes used which are polymeric in nature [3740]. Ceramic membranes [4145] can also be very well used. When to recover the volatile polar organics it should come from their very dilute solutions in water [46] and the selectivity exhibited by polymeric membranes and ceramic membranes is not much higher [4749]. PDMS is the most applied polymeric materials for organic separation. Because of the flexible structure of PDMS, it exhibits high selectivity and permeability toward organic substances and therefore is preferred for the removal of organic compounds from water [50]. Organic liquid membranes of oleyl alcohol were also found to be demonstrating high selectivity for recovering species like n-butanol and acetone from simulated fermentation broths. Using tri-n-octylamine (TOA) as a supported liquid membrane (SLM), Thongsukmak and Sirkar [5154] prepared liquid membrane of TOA in the coated hollow fibers and demonstrated high selectivity for n-butanol, acetone, and ethanol from a very dilute solution representative of an acetone-n butanol-ethanol fermentation products. In case of hybrid configuration in the PV membrane reactor (PVMR), esterification is a widely applied in which PV is used for removal of the product or the byproduct of a reaction. Esterification reactions are found to be reversible and equilibrium-limited processes with ester and water as products. PVMRs have a selective membrane for removal of water from the esterification reaction mixtures and hence achieve a higher yield of ester.

7.4

Nanodiamond

Nature of interaction of nanocarbons with polymer matrix depends on the dimensions of filler components. Nanodiamond may interact with polymer and CNT interact over the length of polymer chain. Mechanical properties of polymer matrix can be very well improved by different type of nanocarbons. By incorporating two types of nanocarbons, the synergistic effect in the properties of final composite may be achieved in which each of the components interacts differently with the matrix [55]. In 1963 it was used by army for the first time, for the explosive named mixture. A huge concentration of nanodiamond produced in the smoke after the explosion was found although in the military and have been studied for a long time.

Chapter 7 Fullerene and nanodiamond-based polymer nanocomposite membranes

Nanodiamond was started with mass production until 1990s that has and has been widely used in basic researches. Recent applications of nanodiamonds are polymer composition, electrochemical coatings, and polishing, antifriction coatings, lubricants, imaging probes, biosensors, implant coating, and drug carriers. The absence of specific functional groups in ND surface limits its applications. Nanodiamond surface can be modified by variety of functional groups such as phenols, carboxylic acid, sulfonic acid, hydroxyl, epoxide groups, etc. Nanodiamonds have wide range of applications. As it shows the unique property of inertness and hardness, it can be used as medical applications such as drug carrier, coat implant materials, synthesis of biosensors, and biorobots. It has a very low cytotoxicity level and is biocompatible. Diamonds can be fluorescent, photostable, chemically inert, and extended life time can be successfully used in biocarriers. It is used in drug delivery, catalysis, skin care, surgery, and blood testing in medical fields. In electronics it acts as good sensors. The mechanical properties reveal the maximum tensile elastics strain for the nanodiamond compounds. Bhadra studied the detonation of nanodiamonds and its uses in hydrophobic membranes for desalination application [56]. DND were dispersed in polyvinydylene fluoride and injected through hydrophobic membranes. 2% DND in the membranes enhance the water vapor flux. Desalination process was found to be better with DND. Hoseinpour et al. [57] studied the property enhancement in the antifouling properties of PP-based membranes. Nanoparticles were carboxylated and the carboxy functional group in the surface of the nanoparticle was enhancing the flux through the membranes. Pulyalina et al. [58] studied the membranes for hydrogen separation for petrochemical streams. Hybrid membranes with ND and polyimide were also developed. Nanodiamond can reduce the antifouling characteristics as per Siddiqa et al. of PVDF membranes [59]. Avagimova et al. studied polyphenylene isophthalamide (PA) membranes modified with detonation nanodiamond particles. The solid-phase dispersion of powders of the components was used. The transport properties of dense film membranes containing up to 5 wt.% ND have been studied in the methanol and methyl acetate mass transfer processes using the sorption and PV methods [60]. Many researchers studied the relevance of the use of fullerene and its water-soluble derivatives to improve the transport properties of PV membranes as a modifier and a cross-linking agent.

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7.5

Pervaporation performance of fullerenesbased nanocomposite membranes

Polymer nanoinorganic particles composite membranes present an interesting approach for improving the physical and chemical, as well as separation properties of polymer membranes. They possess characteristics of both organic and inorganic membranes such as good permeability, selectivity, mechanical strength, thermal stability, etc. Penkova et al. studied the effect of incorporation of fullerenol nanoparticles in PVA-fullerenol mixed matrix membranes for alcohol dehydration by PV. Cross-linked hybrid membranes based on composites of PVA and low-hydroxylated fullerenol C60 (OH) 12 have been prepared [61]. Dmitrenko et al. derived a dense and supported mixed matrix membranes based on chitosan and poly(2,6-dimethyl-1,4-phenylenoxide) (PPO) with low-hydroxylated fullerenol C60(OH)12. It was found that the membranes were well stable and highly water-selective in spite of the different nature of polymers. A different fullerene-derivative carboxyfullerene—as a modifier and a cross-linking agent is developed by Penkova et al. [62]. This type of fullerene derivative can promote an esterification reaction with PVA, which leads to a new chemical structure of PVA with crosslinks on the polymer chains and with an increase in the membrane stability in the processes of dehydration and water purification. It should be noted that fullerene derivatives have high stability in a wide range of organic solvents including aggressive compounds [16,63] and can be used as perspective polymer modifiers [64,65]. Penkova et al. [65] illustrate the physicochemical features of nanocomposites of PPO and fullerene C60 with the purpose of using them as a selective layer of composite membranes in pervaporation-coupling esterification of acetic acid with ethanol to produce ethyl acetate [66]. Novel supported membranes based on PVA were developed using two strategies by Dmitrenko et al. [67]. The first is by the modification of the PVA network, via bulk modification, with the formation of the selective layer accomplished through the introduction of fullerenol and poly(allylamine hydrochloride). The second is by the functionalization of the surface with successive depositions of multilayered films of polyelectrolytes, such as poly(allylamine hydrochloride) and poly(sodium 4-styrenesulfonate) on the PVA surface. The modified PVA membranes were examined for their dehydration transport properties by the PV of

Chapter 7 Fullerene and nanodiamond-based polymer nanocomposite membranes

isopropyl alcoholwater. There is improvement marked which increase the permeability. It was found that the surface modification mainly give rise to a higher permeation flux but with a strong reduction in selectivity. Penkova et al. [68] focuses on the incorporation of fullerenol nanoparticles to get PVA-fullerenol mixed matrix membranes for alcohol dehydration by PV. Cross-linked hybrid membranes based on composites of PVA and low-hydroxylated fullerenol C60 (OH) 12 have been prepared. It was shown that the aggregate sizes increase with the increase of fullerenol content in the PVA and fullerenol composite solutions. Dmitrenko et al. [69] focus on the development of a green PV process for dehydrating a wide range of commercially significant mixtures such as concentrated and corrosive acetic acid blends using mixed matrix membranes based on PVA crosslinked by low-hydroxylated fullerenol (C60(OH)12). Markelove et al. [70] studied the transport properties of poly (phenylene oxide) in separation of quaternary mixture and binary mixture by PV. In this method a hybrid process of reaction plus PV to be carried out. This membrane was modified by star shaped macromolecules with fullerene C60 core and arms of different nature. They are 12-arm star consisted of six nonpolar arms of polystyrene (PS) and six polar arms of poly-2vinylpyridine that are covalently bonded to C60 core. Hybrid membranes based on composites of PVA and lowhydroxylated fullerenol C60 (OH) 12 have been assembled by Penkova et al. [71]. Two different methods are adopted for PVA membrane cross-linking a thermal treatment, with elevated temperatures and chemical treatment with the addition of maleic acid to the polymer matrix and thermal treatment in their study. Transport properties of hybrid membranes containing up to 5 wt.% fullerenol were studied by the PV of chemicalequilibrium quaternary mixtures of n-propyl acetate, acetic acid, n-propanol and water to identify the properties of the membranes used in the hybrid process of esterification plus PV, ternary azeotropic n-propyl acetaten-propanolwater mixtures. All the membranes were selective with respect to water and the optimal transport properties were obtained for the PVA-5% fullerenol membranes containing maleic acid. Polymer nanosized inorganic particle-based composite materials, herein defined as inorganic nanoparticles dispersed at a nanometer level in a polymer matrix, have been investigated in the recent years and possess the potential to provide a solution to overcome the problems of polymer membranes [72]. The polymer nanosized inorganic particle-based composite

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materials may combine the advantages of polymer and inorganic nanosized material. Separation performance of polymer nanosized inorganic particles composite membranes can be further enhanced by chemical modification. Introduction of organic functional groups on an inorganic particle surface contributes gives a better dispersion of the inorganic material in the polymer membrane. It also gives a better absorption and transportation of penetrants which results in favorable selectivity and permeability [73,74].

7.6

Membranes modified with fullerenes and derivatives

Wide variety of fullerenes is obtained by addition of functional groups resulting in nanoparticles with different physicochemical properties. Fullerene chemical modification by adding hydroxyl groups leads to a variety of new nanoparticles, called fullerenols or fullerols, which are less hydrophobic than the original molecules. Due to their unique characteristics, fullerenes have been intensively investigated to develop formulations with specific biological activities to be used in different biomedical fields, from diagnosis methods to therapeutic applications [75,76]. The production and use of fullerenes result in their release and for the most stable accumulation in the environment, the risk of biological systems to be exposed to high concentrations of these nanoparticles is increased [77]. Studies using atomistic molecular dynamics simulations suggested that fullerenes deeply incorporate into cellular membranes and fullerenols can barely penetrate the lipid bilayer [78]. The size, shape, surface chemistry, and surface charge were identified as the physicochemical properties of water-colloidal fullerenes that can influence biological interactions and consequently their toxicity. The disposition of carbon nanoparticles in biological systems from bacteria to human being necessarily involves the passage across cell membranes. This is the site of first contact between nanoparticles and organisms. It is important to find out the relationship between physicochemical properties of nanoparticles and their impact on cell membrane structure and dynamics. Because it can better predict the potential risks arising from the exposure to those compounds for health and for the environment. Several biological processes have been shown to be affected by membrane physical properties [79]. Modification by carbon nanoparticles results in occurrence of new properties of the polymer, for example, biological

Chapter 7 Fullerene and nanodiamond-based polymer nanocomposite membranes

activity. Biological activity of fullerenes is due to lipophilic properties. They can penetrate into the cell membrane and also electron deficit in it will be promoting to react with free radicals. It also has the capacity of excited C60 to generate active oxygen species. There are researches regarding the effects of fullerenes and fullerene derivatives on bacteria. Specifically cationic ammonium fullerene derivatives suppressed Escherichia coli growth, whereas an anionic derivative did not [80]. There is no enough studies on the biological activity of polymer and fullerene composites. It can be assumed that the insertion of fullerenes into a polymer matrix will result in creation of biocomposites, which may be used as agents for drug delivery and antiseptic preparations [81]. PS is one of the synthetic polymers capable to content nanocarbonic particles. Some features of the interaction of C60 with PS are known [82] the behavior of such composites containing small concentrations of fullerenes was studied effectively. Alekseeva et al. [83] studied how to fabricate PS and fullerene composite films containing up to 1 wt.% of C60. The research structure and antimicrobial activity of them were also analyzed. IR spectroscopy and X-ray diffraction measurements also describe the results of the tests that were performed to compare bacteriostatic effect and fungistatic effect of the PS films and PS and fullerene composite films.

7.6.1

Fullerene-based nanocomposites and its pervaporation

Fullerenols C60(OH)n, C60HzOx(OH)y are polar polyhydroxylated fullerene C60 derivatives and have a wide range of applications [71]. Fullerenol are water-soluble and was chosen as an inorganic filler for the introduction into PVA-CS matrix, creating a mixed matrix membrane shows its application as a modifier and a cross-linking agent for the PV PVA-based membranes in the previous works [65,71,8486]. The incorporation of fullerenol nanoparticles to get PVAfullerenol mixed matrix membranes for alcohol dehydration by PV is reported. Cross-linked hybrid membranes based on composites of PVA and low-hydroxylated fullerenol C60(OH)12 show the aggregate sizes of the particles increased with the increase of fullerenol content in the PVA/fullerenol composite solutions. It was observed that the incorporation of fullerenol and cross-linking with maleic acid led to a more uniform distribution of the amorphous PVA phase. The membrane

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Chapter 7 Fullerene and nanodiamond-based polymer nanocomposite membranes

transport characteristics found improving for the dehydration of n-butanol 2 water mixtures by PV. PVA membranes containing maleic acid and fullerenol exhibit very high transport properties [87]. Blending of PVA with chitosan and low hydroxylated fullerene C60 (OH) 12 were also studied to estimate the physical, chemical, and transport properties of PVA. The conditions of preparation and cross-linking of membranes based on nanocomposite PVAfullerenolchitosan were developed. The highly hydrophilic cross-linked PVAfullerenol membranes have been fabricated for dehydration of ethanol [68]. The incorporation of 5 wt.% low-hydroxylated fullerenol C60 (OH)12 and cross-linking with maleic acid helped for distributing the NPs uniformly into the amorphous PVA phase [88] (Table 7.1). Theory of diffusion: The transport of gas, vapor, or liquid through a dense nonporous membrane is described as [59,115] Permeability ðPÞ 5 solubility ðSÞ 3 diffusivity ðDÞ

ð7:1Þ

The two parameters used to define membrane performance in a PV process are flux Jp, and selectivity β. Jp 5 Wp =At

ð7:2Þ

where Wp is the mass of the permeate and At is the area of membrane. ðβ Þ 5

ðPw =Porg Þ F=Forg

ð7:3Þ

Pw and Porg are the mass% of water and organic phase, respectively, in the permeate; Fw and Forg are mass% of water and organic in the feed [116].

7.7

Conclusions

PV of organicorganic mixtures has the great potential of replacing conventional processes. Finding a suitable membrane is the most important hurdle in devising a PV system. The major thrusts in this field should be toward developing new membranes with high flux, selectivity, and stability. To achieve better commercialization high surface area modules must be developed. With emerging trends in membrane research and newer techniques such as asymmetric and composite membranes, PV will be the preferred process. The introduction of

Chapter 7 Fullerene and nanodiamond-based polymer nanocomposite membranes

167

Table 7.1 Membrane performance parameters of different pervaporation systems. No System 1

2 3

4

5

6 7 8 9

10

42

11

12 13 14

Membrane

Water/ethyl Glutaral cross-linked lactate chitosan (GCCS)/carbomer (CP)/polyacrylonitrile (PAN) Water/ethyl Glutaral cross-linked gelatin lactate (GCGE)/PAN Water/ethyl Glutaral cross-linked lactate hyaluronic acid (GCHA)/ hydrolysis modification (HM)-PAN) PDMS Water/ isobutyl acetate PERVAP 1201 Water/ isobutyl acetate Water/nPVA/PES butanol Water/ CS-TEOS methanol Ethanol/ Tri-n-octylamine (liquid water membrane) Surface-modified alumina Ethyl membrane butyrate/ water Ethyl Surface-modified alumina propionate/ membrane water Surface-modified alumina Ethyl membrane acetate/ water Cross-linked PDMS Isobutyl acetate/ water Water/ethyl Hydrophilic acetate Water/ethyl Hydrophobic acetate Water/ethyl PERVAP 1000 oleate

Temperature Jp, flux Selectivity Reference (˚C) (kg m22 h21) (β) 80

1.247

256

[89]

80

1.08

298

[89]

80

1.634

233

[89]

70

4.429

1.421

[90]

70

0.712

686.5

[90]

75

0.14

135

[91]

6080

0.0050.29

4501150

[92]

54

0.0598

100

[93]

40

0.377

120.5122.5

[94]

40

0.343

106.597.3

[94]

40

0.254

66.978.9

[94]

60

3.7

1.4

[95]

40

0.448



[95]

40

7.505

25

0.215

[96] 120

[97] (Continued )

168

Chapter 7 Fullerene and nanodiamond-based polymer nanocomposite membranes

Table 7.1 (Continued) No System

Membrane

Temperature Jp, flux Selectivity Reference (˚C) (kg m22 h21) (β)

15

PERVAP 2255-50

41

Polyvinyl alcohol

90100

Polyamide-6

16 17 18 19 20 21 22 23 24 25

26 27 28 29 30 31 32 33

n-Butyl acetate/ methanol Water/ ethanol Water/ ethanol Water/ dioxane Water/ acetic acid Water/ isopropanol Water/ isopropanol Acetone/ water Isopropanol/ water Butanol/ water Butyl acetate/ water Ethanol/ water Water/ acetic acid Water/ isopropanol Ethanol/ water Ethanol/ water Ethanol/ water Methanol/ isopropanol Water/nbutanol

3.5

[98]

02

502000

[99]

80

1.15

2

[100]

Polyamide-6

35

0.04

45

[101]

Polyamide-6/PAA

15

0.005

82

[102]

PESS Li 1

25

0.087

40

[103]

PESS K 1

25

0.026

60

[104]

Polypropylene

116

0.11.2

3

[105]

Silicone rubber

25

0.030.11

922

[106]

Silicone rubber

30

, 0.035

4565

[107]

PDMS

50

0.55

370

[107]

Zeolite NaA

95

3.35

5100

[108]

Pervatech

75

2.5

150

[109]

Pervatech

100

4

250

[109]

Polyacrylonitrile

70

0.03

12500

[110]

Polyvinyl alcohol

70

0.38

140

[110]

Polyhydrazide

70

1.65

19

[110]

Polypyrrole

58

0.004

2

[111]

PVA membrane

7090

0.1540.184

12.216.5

[112] (Continued )

Chapter 7 Fullerene and nanodiamond-based polymer nanocomposite membranes

169

Table 7.1 (Continued) No System

Membrane

Temperature Jp, flux Selectivity Reference (˚C) (kg m22 h21) (β)

34

PVA membrane

7090

0.150.196

432441

[112]

PVA membrane

7090

0.1760.192

9.010.8

[112]

Modified porous glass

79

0.1

1630

[113]

Polyimide

75

0.01

850

[114]

35 36 37

Water/nbutyl acetate Water/ acetic acid Water/ ethanol Water/ ethanol

An Overview:(Copyright r 2015 Ghoshna Jyoti et al.).

fullerenol into the polymer matrices-based membranes favors the rise of transport of liquids and creation of dehydration membranes with improved PV transport properties. It is very clear that the use of fullerenol as nanomodifier for membrane led to the increase in both flux and selectivity. The functionalized fullerene can make crosslinking to the matrices and enhance the performance of membranes. Nanodiamond is a nanocarbon with exceptional optical, electronic, and mechanical properties rapidly growing field of nanocomposite. Nanodiamond is promising nanofiller for technical applications demanding improved mechanical and thermal stability. It has been found that the sorption and diffusion characteristics of the membranes are improved due to the incorporation of nanodiamond particles in the matrix.

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173

Polymer nanocomposite membranes for pervaporation desalination process

8

Deepak Roy George, Shalin Tyni, Asha Elizabeth and Abhinav K. Nair Department of Chemical Engineering, Amal Jyothi College of Engineering, Kottayam, India

8.1

Introduction

Water is life and very precious to all living beings on earth. The growing population and rapid urbanization have led to overexploitation of our freshwater resources for the past several decades. Only 2% of the water on earth exists as freshwater and remaining is in saline form. The obvious possibility to meet this growing water demand is undoubtedly by desalination of seawater. Desalination has been successfully used to generate freshwater in the water-scarce Middle Eastern countries for nearly a century. Desalination technology has evolved over the decades from thermal processes ranging from distillation, flash distillation, and multiple effect distillations to membrane-based separation process including membrane distillation and reverse osmosis [1]. The thermal processes are energy intensive in nature which made their sustainability a challenge owing to the growing energy crisis. Many membrane-based isothermal processes such as reverse osmosis, nanofiltration, and forward osmosis have been developed to decrease the energy consumption [2,3]. Reverse osmosis is currently the most effective and established technology to generate potable water from seawater and is being extensively used globally due to its isothermal nature and lower footprint. Reverse osmosis works on the principle of osmotic pressure. To obtain fresh water, a pressure higher than the osmotic pressure of the saline water needs to be applied. It indicates that the pressure is a function of concentration and hence higher pressures are needed to process water with higher salt concentration or total dissolved solids Polymer Nanocomposite Membranes for Pervaporation. DOI: https://doi.org/10.1016/B978-0-12-816785-4.00008-2 © 2020 Elsevier Inc. All rights reserved.

175

176

Chapter 8 Polymer nanocomposite membranes for pervaporation desalination process

(TDS) [4]. Also like all membrane-based processes fouling is a major challenge at high concentration operation which makes continuous maintenance and membrane treatment a necessity and thereby increasing the overall operation cost [5,6]. Pervaporation (PV) is a membrane-based separation process used to separate liquids based on relative affinity or diffusivity across a membrane and was commonly used for the dehydration of organic solvents [7,8]. Although a thermally driven process PV can operate with much lower heat input when compared to distillation, PV process is driven by the difference in vapor pressure of a component across the membrane. Since the decrease in vapor pressure of saline water is much less compared to the increase in osmotic pressure with increasing TDS, PV can be used efficiently for the desalination of seawater with high TDS [9]. For desalination processes, a membrane with hydrophilic nature is used such that water can easily diffuse through and evaporate out at the exit leaving behind the salts providing excellent salt rejection. Another advantage of PV is that the feed after the process is nearly supersaturated and can be used for crystallization of salts, this can prevent the environmental hazards caused due to discharge of highly concentrated saline water [10]. Also unlike reverse osmosis, the need for pretreatment is minimal in the case of PV (Fig. 8.1). A few works have been done on PV desalination using inorganic membrane in the past two decades [11]. In this chapter, we focus on the developments in PV desalination using polymeric membranes especially the recently developed nanocomposite polymeric membranes.

Figure 8.1 Schematic representation of pervaporation desalination process.

Chapter 8 Polymer nanocomposite membranes for pervaporation desalination process

8.2

Synthesis methods of polymer nanocomposite pervaporation membranes

The synthesis methods to develop hybrid membranes for PV have been discussed meticulously by Cheng et al. [12] in their review article. Major methods are discussed below in brief for the benefit of the readers.

8.2.1

Physical blending

Blending of the nanofiller in the polymer matrix is the most common method used in the literature for the synthesis of mixed matrix membranes. This method is very versatile and can be used for a different type of fillers. Solution blending and melt blending are most commonly used for polymers. In solution blending, the polymer is dissolved in a suitable solvent and then the filler is dispersed into the polymer solution. Vigorous stirring and sonication modes are used to disperse the filler uniformly in the viscous polymer solution. In melt blending, the polymer is heated to molten form and then fillers are added and mixed. Carbon nanotubes (CNTs), graphene oxide (GO), and silica are some of the nanofillers reported to be used in PV membranes, they will be discussed in detail in the following portions of this chapter [13,14].

8.2.2

Sol gel synthesis

Sol gel synthesis is a common method for the synthesis of nanoparticles but it can be effectively used for the synthesis of composite membranes. The nanofiller is generated by the hydrolysis and subsequent condensation of the inorganic precursor. Usually alkoxides are used as the precursor for sol gel synthesis, for example, tetraethoxysilane (TEOS) is commonly used as a precursor for silica. For the synthesis of composite membranes, the precursor is dissolved in the polymer solution and allowed to undergo a sol gel process. The condensation of the precursor along with the polymer chains results in a composite structure. Some precursors such as 3amino-propyltrietheoxylsilane have shown to not only act as the inorganic source but also act as cross-linkers between the polymer and the filler, thereby improving the interfacial interaction [5,15].

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8.2.3

In situ polymerization

In situ polymerization method has been used to enhance interfacial interaction between polymer and filler. The process involves the mixing of polymer monomer and the inorganic precursor together enabling then to interact via physical or chemical means. Polymerization is initiated by the presence of various functional groups on the inorganic complex. The main advantage of this method is that the nanofillers are well dispersed within the polymer. The lack of suitable precursors and polymer monomers is a major retardant in using this method for different materials [16].

8.2.4

Self-assembly

Self-assembly method involves the development of functional layers on a substrate membrane forming a sandwiched structure. They are more of a structural composite than a 3D composite material. The deposition of the nanomaterials is facilitated mainly by the interaction between the substrate surface and the materials being deposited. Self-assembly enables good control over the thickness of coating generated on the substrate and is independent of the geometry of the substrate. Pressured assembly and spin coating can also be used to prepare assembled layers on membrane substrates [17].

8.3 8.3.1

Factors affecting the performance of pervaporation desalination membranes Selectivity and nature of membrane material

Based on solution-diffusion mechanism proposed for PV membranes, the selectivity of the membrane material plays a major role in the separation process. In desalination applications, the hydrophilicity of the membrane provides improved affinity of the membrane toward the water which enables easy removal of water from the solution. Therefore the hydrophilic nature of the nanofiller used will play a major role in determining the selectivity of the membrane [18,19].

8.3.2

Diffusivity and nature of the filler

Diffusivity of water through the membrane is a major parameter that affects the output flux of the membrane. Diffusivity is depended on free volume or void volume of the

Chapter 8 Polymer nanocomposite membranes for pervaporation desalination process

membrane. The nanofillers used are porous in nature and it can improve the free volume of the composite membrane, they can act as molecular sieves. If nonporous fillers are used, they tend to create torturous paths in the membrane, thereby reducing diffusivity but improve selectivity. The addition of filler can also increase the thickness of the resultant membrane. Any increase in thickness will further retard the transport of species across the membrane. Also at higher filler addition agglomeration can result in loss of performance. Therefore the amount of filler added should be optimized based on all the performance indicators [20,21].

8.3.3

Salt transport suppression

There exists a tradeoff between diffusivity and selectivity of the membrane or in simpler words between the membrane flux and salt rejection. When the free volume of the membrane is increased, faster diffusion is favored which can result in the passage of salt through the membrane along with the water molecule. When free volume is decreased, diffusion is retarded but salt is unable to diffuse through the membrane. The major challenge in developing novel materials for PV desalination membranes is in enhancing diffusion without compromising the salt transport across the membrane [22].

8.3.4

Operating temperature

The operating temperature of a PV process has a close association with diffusivity and selectivity. As temperature increases the vapor pressure of the feed increases and membrane material also experiences swelling since polymer chains swell and increase the free volume. These factors result in increased flux through the membrane. Operating temperature is also important since it leverages the energy efficiency of the process, hence an optimum operating temperature is preferable [23].

8.4

Polymer membranes for pervaporation desalination

Over the past two decades, desalination PV membranes have been developed using polymeric materials. Some of the major works have been described below.

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8.4.1

Cellulose acetate membranes

Cellulose acetate is a common polymer used for the synthesis of RO and UF membranes due to its hydrophilic nature. A cellulose acetate membrane for PV desalination was fabricated by Naim et al. [24] by the phase-inversion technique. A maximum flux of 6.11 L m22 h21 at a feed of 0.133 M NaCl content was obtained. The flux reported for an initial salt concentration of 40 g L21 is 5.97 L m22 h21 with a salt rejection of 99.7% which indicates the high performance of the membrane. Also for higher feed salt concentrations (as high as 140 g L21), the salt rejection approaches 100%. The flux is observed to be proportional to the PV temperature and the best performance was obtained at 70 C for feed concentration in range of 40 to 140 g L21.

8.4.2

Polyacrylonitrile and polyvinyl alcohol-based membranes

Polyacrylonitrile (PAN) and PVA are very common polymeric materials used for the preparation of PV membranes. PVA/PAN composite membrane cross-linked by pyromellitic dianhydride (PMDA) was developed by Zhang et al. [25] for desalination of wastewaters. Poly(vinyl alcohol) (PVA) cross-linked with PMDA was coated on top of the PAN ultrafiltration membrane to form the PVA/PAN composite membrane. When the PMDA content was increased from 5 to 20 wt.% the water flux also increased from 9.88 to 16.47 L m22 h21. The cross-linking temperature was varied from 70 C to 140 C, the water flux reduced from 35 to 32 L m22 h21, indicating that highly cross-linked PVA structure hinders the diffusion of water molecules. At 100 C, the salt rejection was estimated to be 99.95%. NaCl rejection of 99.98% was achieved for a feed containing 35,000 ppm NaCl solution at 70 C. A novel thin-film nanofiber PV composite membrane was prepared by Liang et al. [26] via sequential deposition using electrospraying/electrospinning. The first layer of PVA is electrosprayed on aluminum foil and PAN nanofibrous scaffold is deposited by electrospinning as mid-layer support. A polyethene terephthalate nonwoven top layer was added to the top to form the membrane and some membranes were soaked in glutaraldehyde (GA)/water/acetone solution to cross-link the PVA. Performance of the membrane was studied using NaCl solution at room temperature and 100 Pa on the permeate side. The flux obtained for NaCl solution of concentration 5000, 35,000, and 50,000 ppm at room temperature were 8.53, 7.36,

Chapter 8 Polymer nanocomposite membranes for pervaporation desalination process

and 5.81 L m22 h21 and salt rejections were 99.9%, 99.8%, and 99.8%, respectively. The thickness of the membranes could be varied by controlling electrospraying time and it was seen that the salt rejection remained the same, that is, . 99% but the water flux decreased from 9.04 to 0.15 L m22 h21 as the thickness changed from 0.62 to 12.9 μm. Another PAN/PVA-based PV desalination membrane was developed by Liang et al. [27] using a sulfonic acid functionalized PVA membrane supported on PAN by a dip-coating method. Porous PAN substrate and a dense selective layer of PVA were cross-linked with 4-sulfophthalic acid where sulfonic acid groups facilitated the transport of water molecules. The PV tests revealed that the membrane with thickness 800 nm gave a higher flux of 46.3 L m22 h21 with a salt rejection of 99.8% for 35,000 ppm NaCl feed solution at 70 C and a permeate side pressure of 100 Pa. For 100,000 ppm NaCl solution at 70 C a water flux of 22.1 L m22 h21 and 99.5% salt rejection was observed. Ultrathin PVA membrane supported on polysulfone hollow fiber were synthesized by the dip-coating method by Chaudhri et al. [28]. The preparation of PVA membranes over cylindrical polysulfone hollow fiber support of 1.1 mm diameter was done by dip-coating method. Dilute aqueous solution of PVA and maleic acid (MA) was taken as the coating solution. PV experiments were conducted at 298K 344K for feed concentrations 30,000 50,000 ppm NaCl. It was found that the flux decreased for greater MA contents in the membrane. Membranes of thickness in the range 100 1000 nm gave 99.9% salt rejection, and the thinnest membrane gave a flux of 7.4 L m22 h21 for a feed of 30,000 ppm NaCl at 71 C.

8.4.3

Poly(vinyl alcohol)/polyvinylidene fluoride pervaporation membrane

A composite PV membrane was prepared by Li et al. [29] using PVA and PVDF by a simple dip-coating method. A thin PVA layer approximately 0.3 μm had shown a flux of 16.38 L m22 h21 at an operating temperature of 80 C with MilliQ water. When the desalination performance was tested using 100 g L21 NaCl solution through PVA layer of thickness 0.3 μm, a flux of 13.7 L m22 h21 was observed, which was slightly lower than the one with Milli-Q water but higher than membrane with other thicknesses. The permeate conductivity was consistently around 1 1.5 μS cm21 for membrane with thickness 0.8

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and 2 μm even after 300 and 120 h of continuous operation and gave a very stable and high salt rejection (around 99.9994%). The average flux for the membrane with 2 μm thickness over different temperatures for 4 h operation increased with increase in temperature. The composite membrane’s excellent storage stability, antifouling property, and cleaning efficiency make it a good candidate for brine treatment.

8.4.4

PEBAX membrane

Polyether block amide membranes for PV desalination were studied by Ben Hamouda et al. [30] using its commercially available different grades, namely PEBAX 2533, 3533, and 1657. The material is a segment-elastomeric polyether-amide-blockcopolymer. Polyether-block-amide (PEBA) copolymer has a hard PA block which provides mechanical strength and a soft PE group that provide high permeability. A 20 wt.% copolymer solution was dissolved in dimethylacetamide at 100 C and cast on a heated glass plate and the solvent was evaporated in an oven at 60 C. PV flux increased from 0.00247 to 0.00367 L m22 h21 as feed temperature increased from 22 C to 28.7 C at 200 mL min21 flow rate. The PEBAX 1657 membrane gave a higher flux of 0.0073 L m22 h21 at 60 C which indicates its hydrophilic nature, whereas 2533 and 3533 indicated a hydrophobic nature. Studies also suggested that a hybrid material such as PEBAX 2533 1657 can improve water permeation flux. The optimum thickness for the membrane was found to be 100 μm and optimum temperature for PV was 50 C. Another PEBA membrane was prepared by Wu et al. [31] by solution casting method for potential applications in PV desalination. Membrane with a thickness 56 μm was considered to study the effect of feed concentration (1 20 wt.%) on permeation flux of water through the membrane. It was found that a decrease in flux to 50% was experienced when the salt concentration in feed increased. However, as the temperature was increased from 25 C to 65 C the pure water flux increased from 1.160 to 1.680 L m22 h21 and on increasing the membrane thickness from 39 to 88 μm at a feed temperature of 25 C, the permeation flux decreased from 1.620 to 0.79 L m22 h21 while the salt rejection was high ( . 99.9%). It was also established that no irreversible fouling during the desalination process occurred and the declining water flux after batch operations could be recovered by washing the membrane with deionized water.

Chapter 8 Polymer nanocomposite membranes for pervaporation desalination process

8.4.5

Tubular pervaporation membrane

The performance of a tubular, non-porous hydrophilic membrane which has a thermoplastic block copolymer of the polyester family was studied by Sule et al. [32]. The tube has an inner diameter of 19 mm and an outer diameter of 23 mm with a wall thickness 0.75 mm and ridge width 7 mm. A mass gain of 49%, 46%, and 37% at 30 C, 40 C, and 50 C was observed for the membrane in the presence of deionized water. When placed in media such as air, soil or sand, a higher flux trend was exhibited in the air (5.6 3 1022 L m22 h21) compared to silver sand (7.9 3 1023 L m22 h21) and top-soil (2 3 1022 L m22 h21) which is due to increased moisture content around the tube that reduced the driving force for permeation. Studies have shown that top soil is able to maintain a lower permeate vapor pressure and higher membrane driving force ( . 99.8% rejection of sodium chloride). Moreover, the flux reduction was more in the air as NaCl concentration increased. The membrane placed in sand exhibited flux values of 7.9 3 1023 and 5 3 1023 L m22 h21 for 70 g L21 salt and deionized water, respectively, but for a tube-air system, the values reduced to 5.6 3 1022 and 1.7 3 1022 L m22 h21 indicating that the salt content in the water has an effect on sorption and permeate flux. However, the tube thickness had no significant effect on salt rejection and flux. The results have suggested that pervaporative tubular membrane has potential in irrigation using saline water.

8.4.6

Sulfonated poly(styrene-ethylene/ butylenesstyrene) block copolymer membrane

Sulfonated poly(styrene-ethylene/butylene-styrene) (S-SEBS) polymer membranes were synthesized by Wang et al. [33] by grafting sulfonic groups onto SEBS triblock copolymer with acetyl sulfate as the sulfonating reagent. The highest sulfonation degree was achieved at a temperature between 40 degrees and 45 degrees. It was also seen that the highest sulfonation degree of 80.7% was obtained at 1.3:1 mole ratio of acetic anhydride to sulfuric acid when the designed sulfonation degree was 350%. When the sulfonation degree was varied from 0% to 54.1%, the water uptake rose from 0% to 189% while the contact angle decreased from 100 degrees to 23 degrees. The optimum feed temperature for 1 wt.% NaCl concentration feed with a flow rate of 16 L h21 to obtain a high flux of 22.87 L m22 h21 was reported as 63 C when using a membrane with sulfonation degree as

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54.1%. Moreover, the high tensile strength and simplicity in cleaning make it a promising innovation in PV desalination.

8.5

Polymer nanocomposite membranes for pervaporation desalination

Polymer composite membranes have gained popularity recently due to the significant performance enhancement attainable by addition of suitable fillers. Hydrophobic membranes can be incorporated with hydrophilic fillers and their overall hydrophilicity can be improved. The performance of hydrophilic polymers can also be upgraded.

8.5.1

Mixed matrix membranes for pervaporation desalination

Nigiz [34] synthesized a mixed matrix membrane using a PVDF as a hydrophobic matrix and poly(vinyl pyrrolidone) (PVP) as a hydrophilic polymer and zeolite 3 A as the water selective additive. The base polymers were taken in the PVDF/ PVP ratio of 3:2 and were dissolved in N,N-dimethylformamide solvent at 60 C. To the homogenous mixture, 3 A zeolites within a range of 0 40 wt.% of the weight of the dry polymer were added and various membrane samples were made by varying the zeolite concentration using the priming method. This is done by adding a small amount of the polymer mixture to the zeolite water mixture and stirring to ensure zeolite particles were covered by the ultrathin polymer film and the remainder polymer solution is added to the stirred mixture. The homogeneous zeolite polymer mixture thus obtained was poured on a Teflon plate and oven-dried resulting in a membrane of 180 μm average thickness. PV studies were performed on the membrane using Marmara seawater (conductivity 43,700 μs cm21) with a vacuum below 1000 Pa maintained downstream. All membranes exhibited a good salt rejection above 99.5%. At a temperature of 40 C the pristine PVDF/PVP membrane exhibited a low water flux of 1.1 L m22 h21 and on increasing the zeolite concentration, the water flux peaked at 2.5 L m22 h21 for 10 wt.% zeolite concentration and decreased as the zeolite concentration was increased. The increase in water flux can be attributed to the increased number of separation cages due to increasing zeolite concentrations. Beyond 10 wt.%, the zeolite particles would form agglomerates and the free passage of the zeolite would be

Chapter 8 Polymer nanocomposite membranes for pervaporation desalination process

blocked hence reducing the flow of water through the matrix. A similar increase decrease trend was observed in the case of salt rejection when the zeolite concentrations were varied. The salt rejection was low for a pristine membrane, whereas it was higher for membranes containing 5 30 wt.% zeolite and suddenly declined for membrane containing 40 wt.%. As discussed earlier, an increase in feed temperature increased water flux and the same trend was observed in this case for temperatures 40 60 C. The membrane seemed promising when compared to the previously developed membranes which lack either good salt rejection or water flux. Also the membrane remained stable during a 55-h desalination operation without any significant performance decline. Similar to the above work, another hydrophobic matrix, PEBA was incorporated with 3 A zeolite by Nigiz et al. [35]. The membranes were prepared by solution casting method. A premembrane base solution of PEBA-acetic acid consisting of 10 wt.% of PEBA was prepared and stirred for 5 h at 60oC to % the which zeolite 3 A within a range of 0 50 wt.% was added to polymer solution. PV desalination tests were conducted on the membrane using aqueous NaCl solutions and Marmara seawater samples (conductivity 43,700 μs cm21). On increasing zeolite amount, salt rejection increased and peaked at 99.63% for 20 wt.% zeolite concentrations at 40 C and 3 wt.% NaCl concentrations in the feed and then gradually decreased. A similar trend was observed with flux values increasing from 2.07 to 3.1 L m22 h21 with zeolite addition increasing from 0 20 wt.%. The salt concentrations in the range of 1 3 wt.% did not have significant effects on salt rejection, however, when NaCl concentrations were varied from 3 to 10 wt.%, flux values remarkably dropped from 3.1 to 1.98 L m22 h21. This is due to the change in driving force across the membrane due to increasing salt concentrations. An increase in temperatures in the range from 40 C to 60 C increased the flux from 3.1 to 4.33 L m22 h21 as higher feed temperature improves the driving force on the feed side. However, as with other composite membranes for PV, the increasing temperature negatively affects the NaCl rejection which declined from 99.65% to 99.16% when the temperature was increased from 40 C to 60 C. Upon conducting PV desalination studies using seawater, a good salt rejection as high as 99.81% along with a very good flux of 4.57 L m22 h21 was obtained when 20 wt.% zeolite 3 A incorporated membrane was used at 40 C. The flux of pure PEBA membrane for seawater desalination was found as 3.33 L m22 h21 under the same conditions.

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Another PEBA composite membrane was prepared using graphene nanoplates (GNPs) by Nigiz et al. [36]. GNP incorporation resulted in the improved stability of the membrane during the desalination process. The use of graphene in the membrane increased the separation capability and lifespan of the membranes due to the antimicrobial properties of the graphene as reported by Wang et al. Influence of GNP concentration on water flux and rejection were studied, from which the optimum concentration was found to be 2 and 3 wt.% GNPs, where the salt rejection was . 99.94% and water flux . 2.58 L m22 h21. Influence of temperature with constant GNP loading (3 wt.%), showed increased flux. Effect of membrane thickness and flux dependence was the same as predicted by Fick’s law. Stability studies showed 99.8% performance during 60 h of the studies. GO is a hydrophilic form of graphene with oxygen functionalities and has been widely used as a nanofiller in PV desalination membranes. Huang and Feng [37] reported a GO/ polyimide (PI) fiber membrane, prepared by the direct spinning of GO/PI suspension using the coaxial two-capillary spinning strategy. This addition of GO on the hollow PI fiber helped in the formation of a porous membrane, compared to the macropores present in the pure PI membrane, also it helped in the Tensile strength enhancement of the membrane. From the studies conducted it was found that the ion rejection of Na1, K1, Mg21, Ca21, F2, Cl2, and PO32 was 99.9%, 99.8%, 99.9% 99.9%, 99.8%, 99.9%, and 99.8%, respectively. Also the studies conducted at various temperatures proved that water flux increased with increasing temperature, while the rejection remaining high in all conditions. Another study conducted by varying the feed concentrations showed a decrease in flux with an increase in feed concentration, with no significant change in ion rejection. In another study by Qian et al. [38] GO was used to hybridize chitosan (CS) to fabricate a composite membrane for PV of saline water of NaCl concentrations ranging from 0 to 10 wt.%. CS, a multifunctional polymer with abundant reactive amino groups and hydroxyl groups on its backbone, has been widely used for PV because of its strong hydrophilicity, nontoxicity, and biochemical properties. Research shows that GO is a promising nanomaterial filler for composite membranes due to its large number of oxygen-containing groups, which provides a negatively charged surface and strong hydrophilicity which allows for high water permeation. On incorporating GO with CS, the reaction between epoxy groups on GO sheet and primary amino groups in CS chains lead to the formation of a

Chapter 8 Polymer nanocomposite membranes for pervaporation desalination process

covalent bonded cross-linked structure. Also strong electrostatic interactions and strong hydrogen bond network between the polar functional groups of CS could enhance the properties of CS membrane for desalination. GO (0.25 2 wt.% of GO mass fraction in the total mass of GO and CS) was dispersed in 2 wt. % acetic acid and CS powders were dissolved to prepare the cast solutions. The membranes were tested using pure water, 3.5 10 wt.% NaCl solutions with a feed flow rate of 16 L h21 within a temperature range of 40 C 85 C. The permeate side pressure was kept at 6 kPa vacuum. At an operating temperature of 81 C, the membrane containing 1 wt.% GO in the MMM, high fluxes of 30.0 and 27.6 L m22 h21 were attained for feed concentration of 5 and 10 wt.% aqueous NaCl solutions, respectively. The salt rejection remained constant at 99.99% irrespective of feed concentrations and temperatures (Fig. 8.2). It was observed that, as the GO content increased, the water flux first increased, with the maximum value obtained at 1 wt.% content of GO and then decreased regardless of the feed concentration (whether pure water or brine, the same trend was observed). The increase in water flux could be due to the improved hydrophilicity and water adsorption capacity of the membrane due to the addition of GO in small amounts. The further increase of GO content restricts the mobility of polymer chains due to the strong interaction between GO and CS, which decreases the water adsorption capacity of membrane thereby reducing the water flux at larger GO contents. The decline in water flux due to higher GO content can also be attributed to

Figure 8.2 Schematic of molecular interactions between chitosan and graphene oxide.

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more CS segments being cross-linked leading to the limitation of thermal motion of polymer segments. A comparative study of composites between different types of PVA and CNTs was done by Yang et al. [39] These membranes were produced by interfacial bonding. The CNTs provide low mass transfer resistance which enhances fast fluid transport. Three types of mixed matrix membranes were studied, that is, PVA/multiwalled CNT (MWCNT), PVA/carboxylic MWCNT (C-MWCNT), cross-linked PVA/C-MWCNT/MA. Each of the membranes exhibited varying properties during the desalination process. PVA/MWCNT forms bundles or entanglements leading to the reduction in surface area for separation, whereas PVA/C-MWCNTs have uniform dispersion in the polymer matrix which provides a large interfacial area for separation. Also high water flux was observed due to the presence of COOH group. This membrane is hydrophilic in nature which increases the water adsorption. Another type of membrane is cross-linked PVA/C-MWCNT/MA with high flux decline due to steric hindrance due to the presence of cross-links. The work emphasized on the structural features between the different types of membranes (Fig. 8.3). The use of clay materials in desalination membranes is a proven fact. Filiz and Hilmioglu [40] worked on developing a

Figure 8.3 Proposed schematic illustration of water permeation through PVA/C-MWCNT/MA membrane. ①: water transport through CNT central pore; ②: water transport along outside surface of CNT; and ③: water transport through PVA. CNT, Carbon nanotube; PVA/C-MWCNT/MA, polyvinyl alcohol/carboxylic multiwalled CNT/maleic acid.

Chapter 8 Polymer nanocomposite membranes for pervaporation desalination process

carboxymethylcellulose (CMC) membrane incorporated with bentonite clay for PV desalination of a NaCl solution. The solution-casting method based on phase inversion was used for synthesizing pure CMC membrane and bentonite loaded membranes. For PV desalination studies, the bentonite concentration in CMC was varied in the range 2.5 10 wt.%, feed temperature in the range 30 C 60 C and NaCl feed concentration in the range 1 4 wt.% while maintaining the permeate side pressure at 500 Pa. The membranes exhibited greater than 99% of salt retention with reasonable water flux values regardless of temperature. As bentonite loading was varied from 0 to 10 wt.%, the flux increased from 0.9 to 1.9 L m22 h21 at 40 C due to the high-water uptake capacity of the clay. Bentonite acted as the hydrophilic adsorptive filler for water (proven by the swelling results) and hence a higher flux was obtained. However, as bentonite concentration was increased beyond 10 wt.%, the water flux decreased due to the decrease in hydrophilicity owing to the restricted active sorption sites of the clay. The membrane with 7.5 wt.% bentonite content gave a high-water flux of 2.6 L m22 h21 with salt retention of 99.8% when fed with 3 wt.% NaCl solution at 60 C. Recently Selim et al. [41] incorporated laponite nanoclay into PVA membranes. Laponite, a synthetic nanoclay belonging to semetic clay family, consists of a layered structure with 30 nm diameter and 1 nm in thickness. The properties of laponite clay such as high biocompatibility, anisotropic, the great surface area along with its great ability for cationic exchange, and hydrophilicity impart the ability to enhance the properties of the nanocomposite polymer. The mixed matrix membranes were fabricated via exfoliation of the nanoclay in the polymer solution with GA as a cross-linking agent followed by casting procedure. A 5% PVA casting solution was mixed with laponite XLG suspension (0 10 wt.%). GA was added for cross-linking and the final solutions were cast onto Petri dishes and dried at room temperature followed by annealing. The average thickness of the resultant membranes was in the range of 120 160 μm. The PV desalination performance of the PVA and PVA/GA/laponite membranes were studied using feed solution of concentrations varying from 0 to 10 wt.% of NaCl aqueous solutions and by varying the temperature in a range of 40 70 C. The feed flow rate was maintained at 182 L h21 and a permeate side vacuum of 800 Pa. It was observed that at 40 C the membrane with 2 wt.% laponite XLG concentration had the maximum flux and salt rejection for all the feed solutions. In general, it was observed that increasing feed concentrations decreased flux

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values. The flux increased at 2 wt.% laponite content and then gradually decreased for all membranes. The low laponite content enables more permeation as the membranes were rougher and more hydrophilic whereas on increasing the laponite content, nanoplatelets restricting the chain mobility of PVA and the possible cross-linking by laponite decreased the water flux. However, the salt rejection of the membranes increased on increasing the amount of laponite and was above 99.9% when compared with 99.5% for the pristine PVA. For the pristine PVA membrane, salt rejections were above 99.5% for all the concentrations of feed in the temperature range of 40 C 70 C and the water flux increased with increasing temperature. In case of the membrane with 2 wt.% laponite XLG concentration, salt rejections were above 99.9% regardless of the feed temperature and exhibited relatively high water flux of 58.6 and 39.9 L m22 h21 for desalinating 3 and 10 wt.% NaCl solution, respectively, at 70 C. Similar to other membranes, it was observed that a simultaneous increase in the feed temperature along with an increase in the feed concentration causes the water flux to decrease significantly which is due to the fact that at higher temperatures (60 C 70 C), the vapor pressure of water is greatly influenced by feed concentration.

8.5.2

Self-assembled membranes

A GO/PAN-assembled membrane was reported by Liang et al. [42]. The membrane was synthesized by vacuum filtration assisted assembly of GO on PAN-UF membrane. High-water flux was achieved and the membrane could operate up to a salt concentration of 100,000 ppm. The PAN membrane, when modified using GO films, showed good selectivity towards the water. The presence of hydrogen bonds between COOH or CONH2 of PAN and hydroxyl and carboxyl groups of GO resulted in a stable coating. A flux increase was observed with increased GO loading but decreased at higher loading. From the experiments, it was concluded that minimum GO layer thickness should be preferred without losing the structural integrity for better results. The membrane performance at 30 C and 90 C with feed concentration of 35,000 ppm resulted in a rejection of 99.8% in both cases and a flux of 14.3 and 65.1 L m22 h21, respectively. Vacuum filtration method to assemble GO on-base membranes was adopted by Cha-Umpong et al. [43] to deposit a dense layer of GO nanosheets on porous polypropylene (PP) hydrophobic hollow fiber membrane. The GO deposition converted the PP membrane into an asymmetric composite

Chapter 8 Polymer nanocomposite membranes for pervaporation desalination process

membrane that can be effectively used for PV. It was also noted that the GO deposition provided nanocapillary effect for rapid water transport and prevents the passage of salt ions by providing selective molecular sieving. The studies were conducted on concentrated inland brine under different temperatures and were compared with vacuum membrane distillation. It was observed that the GO/PP membrane PV gives high water productivity. The studies also showed that the GO/PP membrane can be used efficiently in cases of frequent shutdowns and restarts, as the Ca21 and Mg21 in the inland brine solutions form stable cation-π interaction and cross-linking with the oxygen functional groups found in GO nanosheets. Investigations done on reusability of the membrane found that the use of 2 wt.% citric acid for 30 min after 48 h continuous use, almost fully restored the membrane capacity although a small loss of GO nanosheets was observed. A thin nanofibrous composite membrane with stable GO layer on the membrane surface by vacuum-filtration assisted assembly method was reported by Cheng et al. [44]. The membrane composed of an ultrathin GO layer, cross-linked using GA on an electrospun PAN nanofibrous substrate. The membrane showed high water flux and salt rejection due to the presence of the GO layer than the previously used nascent PVA/PAN membranes. Studies indicated the use of ultrathin GO skin layer on PAN ultrafiltration membrane could yield high PV performance, with good water flux and rejection of 99.8% at 90 C. Another study conducted on the GO/PAN membrane at 70 C, with an aqueous NaCl feed solution of concentration 35 g L21, showed a stable rejection of 99.9% and an excellent permeate flux of 69.1 L m22 h21. The incorporation of GO had remarkably increased the desalination performance of the previously used membranes. Discovery of graphene and GO has motivated the research in 2D materials which have a sheet-like structure with atomic thickness, micrometer lateral size and 2D transport channels as properly stacked which make them ideal for high-performance membranes for various separation applications. In 2011, a new family of 2D materials called MXene was discovered by Naguib et al. [45]. Its empirical formula is Mn11XnTx, where n is 1, 2, or 3, M stands for an early transition metal, X represents carbon and/or nitrogen, and T refers to the surface group (OH, O, or F). MXene has been identified to be hydrophilic which enables it to be used in desalination applications. Liu et al. [46] developed ultrathin MXene (Ti3C2Tx) membranes for PV desalination. The atomic-thin MXene nanosheets synthesized by selective etching followed by the delamination process were stacked into

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Figure 8.4 Schematic illustration of MXene nanosheet and mass transport through the ultrathin 2D membrane.

membranes by vacuum filtration of the MXene suspension through PAN substrate. The PV desalination performance of the MXene membranes with an effective area of 2.54 cm2 was studied by feeding 3.5 wt.% NaCl solution at 100 L h21 maintaining the permeate side pressure at 400 Pa. The pure PAN membrane had very high-water flux of 120.2 L m22 h21 but an ultralow NaCl rejection of 11.66%. For a deposition amount of 2.190 3 1026 g cm22 MXene nanosheets, the salt rejection improved but was still below 30% indicating that continuous MXene separation layer could not be formed under such low MXene loading. On increasing the MXene loading to 3.662 3 1026 g cm22, a very high salt rejection of 99.5% was achieved, indicating the formation of a defect-free layer. Also the very thin (thickness around 60 nm) defect-free MXene laminate yielded a very high-water flux of 85.4 L m22 h21. The hydrophilic surface, along with the fast and selective water transport channels provided by the MXene laminates contributes to the high-water flux and selectivity over salt. It was further found that increasing the deposition amount of MXene led to a gradual decrease in water flux with salt rejection remaining constant around 99.5%. This is due to the higher transport resistance that builds up in the membrane with increasing thickness of the MXene layer. On varying the temperature in the range of 30 C 65 C, the water flux increased from 48.2 to 83.5 L m22 h21 while salt rejection remained around 99.5% indicating that the water flux can be easily improved by increasing the feed temperature (Fig. 8.4).

8.5.3

Sol gel synthesized membranes

Xie et al. [47] developed a hybrid polymer inorganic membrane based on PVA, MA and silica by the sol gel method.

Chapter 8 Polymer nanocomposite membranes for pervaporation desalination process

TEOS was used as the silica precursor and MA served as an additional cross-linking agent. The hybrid membrane was synthesized by the aqueous route and was annealed at 140 C for 2 h to complete the polycondensation reaction of TEOS and the esterification between PVA and MA. The composition of MA in the prepared membranes was fixed as 20 wt.% of PVA however silica composition was varied between 0 25 wt. % of PVA. At a feed temperature of 22 C and a vacuum of 800 Pa in the permeate side, the PV desalination tests of the synthesized 5 μm thick membranes resulted in an overall good desalination performance with a water flux greater than 3.65 L m22 h21 and salt rejection greater than 95.5%. The effect of the composition of silica was studied and it was revealed that both salt rejection and water flux increased for a membrane with a silica content of 10 wt.%. However, as the silica content was increased from 10 to 25 wt.%, the salt rejection remained the same as that for a membrane with 10 wt.% silica content and the water flux decreased. The decrease in water flux is due to the negative impact of the increased silica content on the free volume of the membrane attributed to the increased degree of cross-linking. The PVA/MA/silica membrane containing 20 wt.% MA and 10 wt.% silica gave a high flux of 5.51 L m22 h21. From another set of experiments, it was observed that increasing the thickness of the membrane (containing 5 wt.% MA and 10% SiO2with respect to PVA) in the range of 6 110 μm had no significant effect on salt rejection (remained constantly . 99%) but the water flux gradually decreased from 6.93 to 0.82 L m22 h21. In another report, Xie et al. [48] further studied the effect of operating conditions on PV through hybrid membranes so as to optimize the parameters for a commercially viable process. The synthesis method was the same as that of the previous work and PVA/MA/silica membranes containing 5 wt.% MA and 10 wt.% SiO2 were synthesized and the following operating conditions were studied: Feed concentration (0.2 5.0 wt.% NaCl to represent the typical salt level of brackish water, seawater and brine stream, respectively), Temperature (20 C 65 C), Permeate pressure (266.65 5332.9 Pa) and Feed flow rate (30 110 mL min21). Irrespective of the operating conditions, the salt rejection remained high at 99.9%. This is due to the low volatility of NaCl, the preferential diffusion and permeation of water into the membrane and the more rigid, compact structure of the membrane due to the cross-linking among PVA, MA, and silica. The salt concentration in feed has a negligible effect on flux at room temperatures as this decrease or increase in water

193

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Chapter 8 Polymer nanocomposite membranes for pervaporation desalination process

concentration does not cause much variation in the water vapor pressure at a salt concentration range of 0.2 5.0 wt.%. However, at higher temperatures (50 C 65 C) the effect of salt concentration is significant. With an increase in the feed salt concentration from 0.2 5.0 wt.%, the amount of water in the feed decreases from 99.8 to 95.0 wt.%. Since diffusion in the membrane is concentration-dependent, the permeate flux is dependent on the bulk feed concentration of water. At the higher temperature, increasing salt concentration would lead to a decrease in diffusivity in the membrane due to the decreased water concentration. This is due to the exponential relationship of the vapor pressure with the temperature at higher temperature conditions. On varying the feed flow rate in the range of 30 150 mL min21, water flux remained constant at around 2.5 L m22 h21 and it seems that the feed flow rate has a negligible effect on the water flux. Upon increasing the permeate side pressure up to 2000 Pa, it was observed that the water flux dropped down to less than 0.5 L m22 h21. This is because the saturation vapor pressure of water is about 2266.50 Pa and increasing the permeate side pressure beyond 2000 Pa reduces the driving force for water vaporization and it approaches zero, leading to near-zero net evaporation. Increasing feed temperature increases the water pressure on the feed side. Since the permeate side vapor pressure is being held constant, the driving force for the PV process which is the partial vapor pressure difference of water on feed and permeate sides increases and as a result, the water flux increases. Also an increase in temperature increases the diffusion coefficient for transport through the membrane. It was experimentally observed that for feed flow rate of 30 mL min21 and a vacuum of 800 Pa on permeate side, increase in feed temperatures in the range 20 65 C, increased the water flux exponentially and a high water flux of 11.7 L m22 h21 was achieved at the feed temperature of 65 C. In an attempt to further improve the properties of the PVA/ MA/silica membrane, Xie et al. [49] synthesized hybrid membranes that were subjected to heat treatment. It is believed that heat treatment favorably affects the structural morphology and swelling of membranes which is very important for PV performance. The hybrid PVA/MA/silica membranes containing 5 wt. % MA and 10 wt.% SiO2 with respect to PVA were synthesized similar to the above membranes via an aqueous sol gel route, however, the annealing temperatures and times were varied. The resultant mixture was cast on Petri dishes and was air-dried and subjected to either 2 h of heating time at temperatures

Chapter 8 Polymer nanocomposite membranes for pervaporation desalination process

between 100 and 160 C or heating at 140 C for 2 24 h. A reference membrane sample for comparison was prepared at room temperature (21 C) without any heat treatment. The unheated reference membrane had a salt rejection of 95.5% and heat-treated hybrid membrane showed an increased salt rejection. This is attributed to the formation of a more compact structure due to heating which favors the esterification and polycondensation reactions involved in the cross-linking of the base materials of the membrane. However, the water flux decreased with increasing heating temperature and time due to the increased hydrophobicity which results in less sorption of water at the membrane surface (Table 8.1). CMC, carboxymethylcellulose; CS, chitosan; GNP, graphene nanoplate; GO, grapheme oxide; GA, glutaraldehyde; PI, polyimide; PVA/MA, polyvinyl alcohol/maleic acid; PVP, poly(vinyl pyrrolidone); PEBA, polyether-block-amide; PVDF, polyvinylidene fluoride.

8.6

Conclusion and future aspects

Pervaporative desalination has gained popularity due to its promising results in terms of water flux and salt rejection and its reduced thermal loading. The factors on which PV desalination performance depends are nature and selectivity of the membrane, nature and diffusivity of the filler and operating conditions (feed temperature and permeate-side pressure). The research works have proven that incorporating nanofillers in membranes improves the desalination ability of the different composite membranes. The tradeoff between water flux and salt rejection poses a major challenge in developing novel materials for PV desalination membranes. Also the economics of the long-term usage of such membranes has not given much importance in the studies. Different composite membranes require different operating conditions for its optimal performance; hence, the operating cost is a factor to be considered before commercialization of the membranes. The additional factors such as membrane integrity and operating conditions greatly affect the possibility of commercialization. Addressing the abovementioned challenges would definitely help to unlock the potential of PV membranes for desalination and hence provide solutions to the water crisis.

195

Table 8.1 Performance of polymer nanocomposite membranes in PV desalination. Membrane

NaCl (wt.%)

Feed Permeate temperature side vacuum (˚C) (Pa)

Membrane thickness (µm)

Flux Salt Ref. (L m22 h21) rejection (%)

PVA/MA (20%)/SiO2 (10%) PVA/MA (5%)/SiO2 (10%) Heated-treated PVA/MA (5%)/SiO2 (10%) at 100 C 1600 C for 2 24 h PVDF/PVP/zeolite 3A (10 wt.%)

0.2 5 0.2 0.2

22 65 22

800 800 800

5 5 10

5.51 11.7 6 to 4

. 95.5 99.5 99.5

[48] [49] [50]

Marmara seawaterconductivity 43,700 μs cm21 Marmara seawaterconductivity 43,700 μs cm21 3 3.5

40

1000

180

2.5

99.9

[35]

40

100

100

4.57

99.81

[36]

70 65

800 400

120 160 0.06

58.6 85.4

. 99.9 99.5

[42] [46]

81 60 35

6000 500 500

10 13

30 2.6 2.58

99.99 99.8 99.94

[39] [41] [37]

60

100,000

15.6

99.8

38

PEBA/zeolite 3A (20 wt.%)

PVA/GA/laponite (2 wt.%) Ultrathin PAN/MXene (3.662 3 10 6 g cm22) CS/GO (1 wt.%) CMC/bentonite (7.5 wt.%) PEBA/GNP (3 wt.%)

GO/PI

5 3 Marmara seawaterconductivity 43,700 μs cm21 2 10

120

Chapter 8 Polymer nanocomposite membranes for pervaporation desalination process

References [1] A. Subramani, J.G. Jacangelo, Emerging desalination technologies for water treatment: a critical review, Water Res. 75 (2015) 164 187. [2] U.M. Aliyu, S. Rathilal, Y.M. Isa, Membrane desalination technologies in water treatment: a review, Water Pract. Technol. 13 (2018) 738 752. [3] S.F. Anis, R. Hashaikeh, N. Hilal, Functional materials in desalination: a review, Desalination. 468 (2019) 114077. [4] M.C. Duke, S. Mee, J.C.D. da Costa, Performance of porous inorganic membranes in non-osmotic desalination, Water Res. 41 (2007) 3998 4004. [5] P.S. Goh, A.F. Ismail, A review on inorganic membranes for desalination and wastewater treatment, Desalination. 434 (2018) 60 80. [6] Z. Yang, X.H. Ma, C.Y. Tang, Recent development of novel membranes for desalination, Desalination. 434 (2018) 37 59. [7] M. Olukman, O. S¸ anli, E.K. Solak, Synthesis of magnetite in poly (vinyl alcohol) matrix and its use in separation of acetone/water mixtures via pervaporation, vapor permeation with and without temperature difference methods, Vacuum. 120 (2015) 107 115. [8] Y.K. Ong, G.M. Shi, N.L. Le, Y.P. Tang, J. Zuo, S.P. Nunes, et al., Recent membrane development for pervaporation processes, Prog. Polym. Sci. 57 (2016) 1 31. [9] E. Huth, S. Muthu, L. Ruff, J.A. Brant, Feasibility assessment of pervaporation for desalinating high-salinity brines, J. Water Reuse Desalin. 4 (2014) 109 124. [10] W. Kaminski, J. Marszalek, E. Tomczak, Water desalination by pervaporation comparison of energy consumption, Desalination. 433 (2018) 89 93. [11] Z. Xie, N. Li, Q. Wang, B. Bolto, Desalination by pervaporation, in: Emerg. Technol. Sustain. Desalin. Handb. 2018: pp. 205 226. [12] X. Cheng, F. Pan, M. Wang, W. Li, Y. Song, G. Liu, et al., Hybrid membranes for pervaporation separations, J. Memb. Sci. 541 (2017) 329 346. [13] M. Khayet, J.P.G. Villaluenga, J.L. Valentin, M.A. Lo´pez-Manchado, J.I. Mengual, B. Seoane, Poly (2,6-dimethyl-1,4-phenylene oxide) mixed matrix pervaporation membranes, Desalination. 200 (2006) 376 378. [14] S. Daer, J. Kharraz, A. Giwa, S.W. Hasan, Recent applications of nanomaterials in water desalination: a critical review and future opportunities, Desalination. 367 (2015) 37 48. [15] S. Roy, N.R. Singha, Polymeric nanocomposite membranes for next generation pervaporation process: strategies, challenges and future prospects, Membr. (Basel) 7 (2017). [16] W.J. Lau, A.F. Ismail, N. Misdan, M.A. Kassim, A recent progress in thin film composite membrane: a review, Desalination. 287 (2012) 190 199. [17] N. Wang, S. Ji, G. Zhang, J. Li, L. Wang, Self-assembly of graphene oxide and polyelectrolyte complex nanohybrid membranes for nanofiltration and pervaporation, Chem. Eng. J. 213 (2012) 318 329. [18] Z. Jia, G. Wu, Metal-organic frameworks based mixed matrix membranes for pervaporation, Microporous Mesoporous Mater. 235 (2016) 151 159. [19] G. Liu, W. Wei, W. Jin, N. Xu, Polymer/ceramic composite membranes and their application in pervaporation process, Chin. J. Chem. Eng. 20 (2012) 62 70. [20] N.R. Singha, M. Karmakar, P.K. Chattopadhyay, S. Roy, M. Deb, H. Mondal, et al., Structures, properties, and performances—relationships of polymeric membranes for pervaporative desalination, Membr. (Basel) 9 (2019) 58.

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[21] G.A. Polotskaya, A.V. Penkova, A.M. Toikka, Fullerene-containing polyphenylene oxide membranes for pervaporation, Desalination. 200 (2006) 400 402. [22] L. Li, J. Hou, Y. Ye, J. Mansour, Y. Zhang, V. Chen, Suppressing salt transport through composite pervaporation membranes for brine desalination, Appl. Sci. 7 (2017). [23] L. Bueso, M. Osorio-Galindo, I. Alcaina-Miranda, A. Ribes-Greus, Swelling behavior of pervaporation membranes in ethanol-water mixtures, J. Appl. Polym. Sci. 75 (2000) 1424 1433. [24] M. Naim, M. Elewa, A. El-Shafei, A. Moneer, Desalination of simulated seawater by purge-air pervaporation using an innovative fabricated membrane, Water Sci. Technol. 72 (2015) 785 793. [25] R. Zhang, X. Xu, B. Cao, P. Li, Fabrication of high-performance PVA/PAN composite pervaporation membranes crosslinked by PMDA for wastewater desalination, Pet. Sci. 15 (2018) 146 156. [26] B. Liang, K. Pan, L. Li, E.P. Giannelis, B. Cao, High performance hydrophilic pervaporation composite membranes for water desalination, Desalination. 347 (2014) 199 206. [27] B. Liang, Q. Li, B. Cao, P. Li, Water permeance, permeability and desalination properties of the sulfonic acid functionalized composite pervaporation membranes, Desalination. 433 (2018) 132 140. [28] S.G. Chaudhri, B.H. Rajai, P.S. Singh, Preparation of ultra-thin poly(vinyl alcohol) membranes supported on polysulfone hollow fiber and their application for production of pure water from seawater, Desalination. 367 (2015) 272 284. [29] L. Li, J. Hou, Y. Ye, J. Mansouri, V. Chen, Composite PVA/PVDF pervaporation membrane for concentrated brine desalination: salt rejection, membrane fouling and defect control, Desalination. 422 (2017) 49 58. [30] S. Ben Hamouda, A. Boubakri, Q.T. Nguyen, M. Ben Amor, PEBAX membranes for water desalination by pervaporation process, High. Perform. Polym. 23 (2011) 170 173. [31] D. Wu, A. Gao, H. Zhao, X. Feng, Pervaporative desalination of high-salinity water, Chem. Eng. Res. Des. 136 (2018) 154 164. [32] M. Sule, J. Jiang, M. Templeton, E. Huth, J. Brant, T. Bond, Salt rejection and water flux through a tubular pervaporative polymer membrane designed for irrigation applications, Environ. Technol. (UK) 34 (2013) 1329 1339. [33] Q. Wang, Y. Lu, N. Li, Preparation, characterization and performance of sulfonated poly(styrene-ethylene/butylene-styrene) block copolymer membranes for water desalination by pervaporation, Desalination. 390 (2016) 33 46. [34] F.U. Nigiz, Complete desalination of seawater using a novel polyvinylidene fluoride/zeolite membrane, Environ. Chem. Lett. 16 (2018) 553 559. [35] F.U. Nigiz, S. Veli, N.D. Hilmioglu, Deep purification of seawater using a novel zeolite 3A incorporated polyether-block-amide composite membrane, Sep. Purif. Technol. 188 (2017) 90 97. [36] F.U. Nigiz, Preparation of high-performance graphene nanoplate incorporated polyether block amide membrane and application for seawater desalination, Desalination. (2018) 164 171. [37] A. Huang, B. Feng, Synthesis of novel graphene oxide-polyimide hollow fiber membranes for seawater desalination, J. Memb. Sci. 548 (2018) 59 65.

Chapter 8 Polymer nanocomposite membranes for pervaporation desalination process

[38] X. Qian, N. Li, Q. Wang, S. Ji, Chitosan/graphene oxide mixed matrix membrane with enhanced water permeability for high-salinity water desalination by pervaporation, Desalination. 438 (2018) 83 96. [39] G. Yang, Z. Xie, M. Cran, D. Ng, S. Gray, Enhanced desalination performance of poly (vinyl alcohol)/carbon nanotube composite pervaporation membranes via interfacial engineering, J. Memb. Sci. 579 (2019) 40 51. [40] F.U. Nigiz, N.D. Hilmioglu, Bentonite-loaded carboxy methylcellulose membrane for pervaporative desalination, Desalin. Water Treat. 92 (2017) 20 26. [41] A. Selim, A.J. Toth, E. Haaz, D. Fozer, A. Szanyi, N. Hegyesi, et al., Preparation and characterization of PVA/GA/Laponite membranes to enhance pervaporation desalination performance, Sep. Purif. Technol. (2019) 201 210. [42] B. Liang, W. Zhan, G. Qi, S. Lin, Q. Nan, Y. Liu, et al., High performance graphene oxide/polyacrylonitrile composite pervaporation membranes for desalination applications, J. Mater. Chem. A 3 (2015) 5140 5147. [43] W. Cha-Umpong, G. Dong, A. Razmjou, V. Chen, Effect of oscillating temperature and crystallization on graphene oxide composite pervaporation membrane for inland brine desalination, J. Memb. Sci. 588 (2019) 117210. [44] C. Cheng, L. Shen, X. Yu, Y. Yang, X. Li, X. Wang, Robust construction of a graphene oxide barrier layer on a nanofibrous substrate assisted by the flexible poly(vinylalcohol) for efficient pervaporation desalination, J. Mater. Chem. A 5 (2017) 3558 3568. [45] M. Naguib, M. Kurtoglu, V. Presser, J. Lu, J. Niu, M. Heon, et al., Twodimensional nanocrystals produced by exfoliation of Ti3AlC2, Adv. Mater. 23 (2011) 4248 4253. [46] G. Liu, J. Shen, Q. Liu, G. Liu, J. Xiong, J. Yang, et al., Ultrathin twodimensional MXene membrane for pervaporation desalination, J. Memb. Sci. 548 (2018) 548 558. [47] Z. Xie, M. Hoang, T. Duong, D. Ng, B. Dao, S. Gray, Sol-gel derived poly (vinyl alcohol)/maleic acid/silica hybrid membrane for desalination by pervaporation, J. Memb. Sci. 383 (2011) 96 103. [48] Z. Xie, D. Ng, M. Hoang, T. Duong, S. Gray, Separation of aqueous salt solution by pervaporation through hybrid organic-inorganic membrane: effect of operating conditions, Desalination. 273 (2011) 220 225. [49] Z. Xie, M. Hoang, D. Ng, C. Doherty, A. Hill, S. Gray, Effect of heat treatment on pervaporation separation of aqueous salt solution using hybrid PVA/MA/TEOS membrane, Sep. Purif. Technol. 127 (2014) 10 17.

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Polymer/polyhedral oligomeric silsesquioxane nanocomposite membranes for pervaporation

9

Valiya Parambath Swapna, Vakkoottil Sivadasan Abhisha and Ranimol Stephen Department of Chemistry, St. Joseph’s College (Autonomous), Devagiri, Calicut, India

9.1

Introduction

Membrane separation technologies have so far been a rapidly developing area in the field of separation due to its economic and environmental advantages and their ability to overcome the limitations of conventional separation methods such as distillation. The separation process is the main component in the manufacturing processes in chemical industries. It mainly aims at the elimination of contaminants from factory effluents, raw materials, and purification of primary products. Membranes can be defined as a thin layer of semipermeable material allowing the selective transport of one or more chemical component in contact with it [1,2]. Membranes consist of distinct thin interfaces that manage the permeation of chemical substances through it. According to Mulder, the membrane is a selective barrier between two phases [3]. Conventional separation processes such as liquid
liquid extraction, distillation, adsorption, and solvent absorption are energy-exhausting processes that involve many complicated operations, produce many environmental, economic, and technical issues as well as ineffective in the separation of azeotropic mixtures. The membrane technology has demonstrated outstanding advantages over conventional separation methods, which include efficiency in separation, ease of processability, moderate cost, low maintenance necessity, and low energy consumption. The membrane technology is an environment-friendly method where no additives are required and offer contaminant elimination without generating any hazardous by-products [4
8]. Polymer Nanocomposite Membranes for Pervaporation. DOI: https://doi.org/10.1016/B978-0-12-816785-4.00009-4 © 2020 Elsevier Inc. All rights reserved.

201

202

Chapter 9 Polymer/polyhedral oligomeric silsesquioxane nanocomposite membranes for pervaporation

In the past few decades, polymeric membrane-based separation and purification technologies have gained considerable research attention because of its remarkable applications in various fields such as medical, electronics, and food industry. Physicochemical properties of the polymers and affinity of the membrane toward penetrants are the important factors that determine the separation performance of the membrane. The rate of transport of liquid or gases through polymers is governed by number of factors, which include driving forces (concentration gradients, temperature, and pressure), features of permeating substance (molecular size, chemical nature, and molecular shape), properties of polymer membrane (free volume, physical aging, crystalline/amorphous structure, and porous/nonporous structure), and the composition of the feed mixture [9
13]. The general features of the membrane to exhibit efficiency in gas and liquid separation are • presence of amide or hydroxyl functionality, • degree of polarity (e.g., nitrile, chloride, fluoride, acrylic, and ester), • high chain stiffness and crystallinity, • inertness to the diffusing substance, • strong attraction or bonding between polymer chains, and • high glass-transition temperature (Tg). Therefore polymers such as polyimides, poly(vinyl alcohol) (PVA), polysulfone, polyamides, polyaniline (PANI), poly (dimethyl siloxane), and polycarbonates are widely used for the fabrication of membranes for separation processes [14
20]. The simplicity and cost-effectiveness of the membrane separation process make it extremely attractive for industrialists and other scientific community. However, the efficiency of the membrane is a major concern for its end-use applications. For instance, membrane fouling is one of the major drawbacks associated with the membrane-based liquid mixture separation. Several kinds of fouling have been observed in the membrane system including crystalline, organic, and microbial. The attachment of solutes onto the surface or internal structure of the membrane results in fouling. Consequently these materials block the transport of solvent across the pores of the membrane, resulting in reduced selectivity, flux, and overall service life of the membrane [21
24]. Presently only a small number of pure polymer membranes are applied in industry for liquid and gas mixture separation even though quite a lot of polymers have been investigated for their separation performance. Therefore current researches in polymeric membranes are

Chapter 9 Polymer/polyhedral oligomeric silsesquioxane nanocomposite membranes for pervaporation

focused mainly on its performance, that is, better selectivity without sacrificing permeability and durability under the process conditions. Unfortunately intrinsic trade-off behavior between permeability and selectivity and decreased product purity are the major restrictions for the development of highperformance polymer membranes [25
28]. Moreover, plasticization and aging factors also significantly reduce the efficiency of separation. The development of high-performance membranes by embedding nanoscale inorganic moieties in organic polymers has been found to be a successful strategy to overcome the limitation. Nanoparticle incorporation in the polymer matrix has been an efficient path for enhancing the polymer membrane performance for a wide range of processes, such as gas separation and pervaporation (PV). As the particle size reduces, the number of atoms on the particle surface increases leading to the exceptionally high interfacial area in composites. The particles might enhance the permeability, diffusion rate, permselectivity, tensile strength, and fouling resistance of the polymer membrane [29
32]. Filler characteristics (size, volume fraction, surface area, and agglomeration), method of incorporation of nanoparticle in the polymer matrix, and the interaction between polymer matrix and filler particle are the crucial factors that determine the transport properties of polymer nanocomposites. The interaction existing between the nanoparticle and organic polymers may be strong chemical (covalent/coordination/ionic bonds) or weak physical (van der Waals force/hydrogen-bonds/hydrophilic
hydrophobic balance) interactions [33]. Schematic representation of different types of polymer
filler interactions is shown in Fig. 9.1. The interfacial interaction between the polymer and inorganic filler and the dispersion of nanofillers in the polymer matrix are the important factors that control the reinforcing efficiency of nanoparticle in the polymer matrix. One of the major issues encountered by researchers in the fabrication of inorganic
organic systems is in the mixing between two dissimilar phases because the nanoparticles usually tend to aggregate in a polymer matrix to reduce very high surface energy. This particle aggregation reduces the mechanical and other properties of the resulting material. This problem can be addressed by using functionalized polymers or surface modified inorganic particles with appropriate organic groups [34,35]. The surface modification of particles can be achieved through grafting and in- situ functionalizations during their development (Fig. 9.2). Organic modification of inorganic

203

204

Chapter 9 Polymer/polyhedral oligomeric silsesquioxane nanocomposite membranes for pervaporation

(A)

(B)

Figure 9.1 Schematic demonstration of various methods for the incorporation of inorganic materials in polymer systems, where organic and inorganic phases are linked via (A) covalent bonds and (B) van der Waals force or hydrogen bonds.

Figure 9.2 Surface modification of nanoparticle through grafting and in situ functionalization.

Chapter 9 Polymer/polyhedral oligomeric silsesquioxane nanocomposite membranes for pervaporation

nanoparticle made it more compatible with the polymer matrices. The improved interface between particle and polymer system leads to the improvement in mechanical, transport and other properties of polymer [36
39]. Khoonsap et al. synthesized a membrane by incorporating poly(2-hydroxyethylmethacrylate) grafted fumed silica (FS) in the PVA matrix [40]. The membrane has been used for the PV separation of water
acetone mixture and found that the membrane exhibit higher water permeation and selectivity over unmodified FS-doped PVA membrane. Semsarzadeh and Ghalei [41] synthesized silica particles through the in situ method using tetraethoxysilane precursor in the presence of PVA and cetyltrimethyl ammonium bromide (CTAB) as templating agents. The prepared particles were incorporated into a polyurethane (PU) membrane by the solution casting method. This membrane exhibited enhanced CO2 solubility in PU as compared with pristine PU due to the presence of polar hydroxyl groups in PVA template and in silica particle.

9.2

Polyhedral oligomeric silsesquioxane

Polyhedral oligomeric silsesquioxane (POSS) are promising nanostructured organic
inorganic hybrid molecule with the general formula [RSiO3/2]n or RnTn where n 5 6, 8, 10, and 12, and R is various polar or nonpolar organic substituents such as an aryl, alkyl, or any of their derivatives are attached to silicon atoms via covalent bonds. Among various POSS molecules, octahedral silsesquioxane is the most commonly used one ([RSiO3/2]8 or R8T8). The cage-like structured POSS molecules consists of cubic shaped inorganic silicon-oxygen inner cage (Si8O12) with 0.53 nm diameter and the organic groups attached to the corners of the cage determines its final properties. Depending on the nature and size of the functional groups, the size of POSS nanostructure ranges from 1 to 3 nm. It is zerodimensional (sphere-like structure), multifunctional, highly symmetrical, well-defined, and smallest possible nanosized particle of silica [42
44]. A schematic illustration of the anatomy of POSS molecules are given in Fig. 9.3. The siloxane core (Si-O bond energy is 444 kJ mol21) of POSS is very rigid and inert because of the robust overlapping of vacant d orbitals of silicon with the two lone pair electrons in the oxygen and it is intact up to 550 C. Recently POSS has been explored as molecular filler in the fabrication of polymer
inorganic nanocomposites. Nanosized

205

206

Chapter 9 Polymer/polyhedral oligomeric silsesquioxane nanocomposite membranes for pervaporation

(A)

(B) R 1–3 nm

0.53 nm

Octameric core (SiO1.5)8 Si-Si distance 0.53 nm R-R distance 1.5 nm V = 0.065nm3

R O

O Si O

Si O

Unreactive organic groups (R) for solubilization and compatibilization

V = 0.9 nm3

Si

Si O R O

O R Si O Si O O R Si O Si O R R

One or more reactive groups for grafting

Figure 9.3 (A) Sizes and volume details of POSS molecule and (B) schematic drawing of the anatomy of POSS molecule. POSS, Polyhedral oligomeric silsesquioxane.

POSS molecule can control the polymer chain mobility and effective reinforcement is possible through the chemical reactivity of functionalized POSS with polymer. The degree of dispersion and the interaction between polymer
POSS compounds are tailored by the nature of organic substituent attached to the corners of the siloxane core [45,46].

9.2.1

Different types of POSS

POSS materials are also referred to spherosiloxanes and it is included in the family of silsesquioxanes. The term silsesquioxane originated from its structure, where each tetravalent silicon atom (sil-) bound to one and a half (-sesqui-) of oxygen (ox) and one hydrocarbon (-ane) functional group. Based on the structure, silsesquioxane molecules are classified into noncaged and caged structures, as illustrated in Fig. 9.4. As shown in Fig. 9.4, there are three types of noncaged silsesquioxane molecule: (1) random, (2) ladder, and (3) partial cage structures [48]. The cage-structured silsesquioxanes are generally named as POSS and its spherical siloxane core is commonly denoted by the symbol Tn, based on the number of Si atom present in the vertices of the cage of POSS molecule. POSS materials exist as T8, T10 and T12 structures. Among these T8 type is the most commonly synthesized and studied one because of its Si4O4 ring, which is the most stable Si-O cyclic structure [47].

9.2.2

Synthesis of POSS

Scott was the first person who introduced the synthetic method of POSS synthesis with empirical formula (CH3SiO1.5)n by the thermolysis of polymeric product obtained by dimethyl chlorosilane and methyl trichlorosilane cohydrolysis.

207

Chapter 9 Polymer/polyhedral oligomeric silsesquioxane nanocomposite membranes for pervaporation

(A)

O

R R O O R R Si HO Si O O Si Si Si O O O O O OR O R R Si R O O Si Si O Si O Si Si O R R O OH O O O Si Si O Si Si O O O O R R

R

R O

O

O Si

SiH

Si

HSi

O

Si

HO

O

O

O

O Si

Si

Si

SiH O

O

O R

(1)

R R

R R

R O Si

R Si O

R

O O

R Si O Si O

R Si O Si O

O

O Si

O Si O O Si

R R O Si O R

O

Si

Si

R

O O R

Si O

Si

R

O Si O

O

O

O

Si

Si O

R

R

Si

R

Si

Si R

O

O O Si

R Si O Si O O

R R

RO O

Si

O

Si

R

Si O

O

R

Si

(3) R

(B)

OO

O O

(2)

OH

R Si

R

R

O Si O O O R Si Si R O O R O O Si R Si O O O O Si Si O O Si O Si R

R

R

R

R

(1)

(2)

(3)

Figure 9.4 Structures of silsesquioxanes (A) noncaged silsesquioxanes: (1) random, (2) ladder, and (3) partial caged structures and (B) caged silsesquioxanes: (1) T8, (2) T10, and (3) T12 structures. Source: Adapted from G.Z. Li,

L.C. Wang, H.L. Ni, C.U. Pittman, Polyhedral oligomeric silsesquioxane (POSS) polymers and copolymers: a review, J. Inorg. Organomet. Polym. 11 (2001) 123.

Even though Scott discovered the first POSS molecule ((CH3SiO1.5)n) in 1946 at US air force research laboratory for aerospace applications, only in the last 20 years scientists are widely studied and commercialized the extensive applications of POSS molecules. In 1991 Feher’s group and Lichtenhan have made significant effort for the development of POSSincorporated polymer systems [49,50]. Hybrid plastics is the POSS manufacturing company first established in 1998 at Fountain Valley and then in 2004, in Southern California and city of Hattiesburg, Mississippi. The company made dramatic enhancement in the production of large variety of POSS. Many scientists synthesized monofunctionalized and multifunctionalized POSS nanostructure through silane chemistry and it was found that many factors such as monomer concentration, solvent nature, catalyst, temperature, and the quantity of water addition affect the properties of the final product [51
60]. The most common method for the production of monofunctional POSS is the hydrolysis and condensation of trifunctional monomers RSiX3, where “X” is a very reactive substituent such as Cl or alkoxy group and R is chemically stable substituent

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such as methyl, phenyl, or vinyl. About 48% of monosubstituted product is obtained from the above mentioned method. Another method is the substitution reaction on the silicon atom with the retention of siloxane cage such as hydrosilylation reactions. Corner-capping reactions also produce fully condensed monosubstituted POSS starting from incompletely condensed molecule such as R7Si7O9(OH)3 with RSiCl3. Trifunctional organosilicon monomers produce incompletely and completely condensed POSS molecules. POSS
polymer nanocomposites can be classified into three based on the functionality of POSS or method used for the incorporation of POSS in polymer matrix; (1) star-like polymer nanocomposites: polymerization initiated from the surface of the multifunctional POSS, so it act as a microinitiator and produce star type macromolecule, (2) cross-linked polymer nanocomposites: polymerization of multireactive POSS with polymers forms a thickly cross-linked network, and (3) pendent type: polymerization of monofunctional POSS onto a polymer backbone produce pedant POSS cages contains polymer system. These three systems are presented in Fig. 9.5 [61].

9.2.3

Properties of POSS

With respect to other nanoparticles like clay, POSS chemistry is very flexible and can be functionalized with various organic groups onto the apex silicon atoms, which provides good compatibility and controlled dispersion with wide range of polymer matrices at the molecular level. Dispersion of POSS in polymer matrices is generally achieved through covalent bonding of the reactive organic substituent on the periphery of POSS with the polymer host. In the case of nonreactive substituent-attached POSS core, interaction occurs due to the similarity in the chemical structure in the periphery of POSS to the polymer host [58,62
64]. The interesting unique advantages of POSS nanofillers are its well defined structure, lower density, monodisperse molecular weight, and high thermal stability. Incorporation of POSS in polymer systems can substantially enhance the polymer properties [61,65,66].

9.2.4

Applications of POSS

POSS nanostructures are found to be a promising material for many applications from biomedical to aerospace technologies. The unique properties of POSS
polymer nanocomposite materials offer interdisciplinary technological applications (Fig. 9.6) [61,68,69].

Chapter 9 Polymer/polyhedral oligomeric silsesquioxane nanocomposite membranes for pervaporation

209

Figure 9.5 The three main types of POSS-based nanocomposites: (A) star-like polymer nanocomposites, (B) cross-linked nanocomposites, and (C) pendent type. Source: Adapted from F. Wang, X. Lu, C. He, Some recent developments of polyhedral oligomeric silsesquioxane (POSS)-based polymeric materials, J. Mater. Chem. 21 (2011), 2775
2782.

Therefore scientists explore the potential application in various fields, for instance, reinforcing agent or good nanofiller in polymer nanocomposites, mechanics, sensors, catalysts, photoresists, semiconductors, high temperature lubricants, and electron beam lithography. With respect to carbon nanofibers and nanotubes, POSS chemical is ecofriendly nanofiller; it is odorless, nonpoisonous, posses’ cytocompatible moieties, and does not produce any volatile organic components. Hence it is a good choice for the fabrication of biomaterials [70]. Material scientists explore the POSS
polymer systems in tissue engineering and biomedical fields. POSS form dental nanocomposites with methacrylate based polymers can overcome the weakness (lack of strength and toxicity) of dental monomer and methyl methacrylate [71,72]. POSS/polymethyl methacrylate (PMMA) composites dramatically enhance the strength of PMMA. POSS/poly(carbonate
urea) urethane nanocomposites are ideal materials in cardiovascular bypass grafts and

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Chapter 9 Polymer/polyhedral oligomeric silsesquioxane nanocomposite membranes for pervaporation

Figure 9.6 Various applications of POSS nanomaterial. POSS, Polyhedral oligomeric silsesquioxane. Source: Adapted from A.N. Frone, F.X. Perrin, C. Radovici, D.M. Panaitescu, Influence of branched or unbranched alkyl substitutes of POSS on morphology, thermal and mechanical properties of polyethylene, Compos. Part B, 50 (2013) 98
106 [67].

microvessels [73]. Ammonium
POSS-like cationic POSS finds great application in the field of gene delivery, drug delivery, and DNA detection [74]. POSS nanocomposite-based coating film holds advanced conductive, protective, and mechanical properties, so it found many future opportunities in habitation missions and lunar exploration [75,76]. The very low dielectric constant and high thermal stability of POSS
polymer nanocomposites bring its use in modern microelectronics. POSS/polyfluorene shows high thermal and optoelectronic properties, hence it is extensively used in electroluminescent devices [77]. Xiong et al. discovered PANI
POSS-based complementary electrochromic device with high coloration ability [78]. The US Airforce has recognized the space application of POSS. POSS-incorporated polymers show high atomic oxygen resistance. POSS form thin SiO2 network that resists further decay of polymers from atomic oxygen erosion. POSS
polyimide nanocomposites have high protective power from atomic oxygen erosion [79].

9.3

Pervaporation performance of polymer/ POSS membranes

PV is an important energy-efficient, chemical potential, or activity-driven membrane based green separation technique.

Chapter 9 Polymer/polyhedral oligomeric silsesquioxane nanocomposite membranes for pervaporation

It has found great application in many industrial scenarios for dehydration of organic solvents, separation of organic
organic mixtures, structural isomers, mixtures containing close boiling constituents or azeotropes, and water purification. In PV technique, liquid mixture is separated by partial vaporization using a dense nonporous or porous polymer membrane. Physical structure and intrinsic properties of membrane, feed mixture interaction, and affinity of feed components toward the membrane are the important parameters that control the PV separation. Azeotropic mixture separation through currently existing distillation technique is difficult, which can be made possible only by adding entrainers [12,80
84]. Nowadays, existing separation processes are not effective for the separation of water from organic solvents due to its high energy cost. Hydrophilic membranes are usually used for the solvent dehydration, which preferentially allows water diffusion across the membrane. Interaction between membrane and water is the key factor behind the separation of water. Membranes with glasstransition temperature above room temperature are usually employed for this purpose. To attain high water selectivity, it is necessary to have an active functional group in the polymer to interact with water. Dipole
dipole, hydrogen bonding, and ion
dipole interactions are the existing chemical interactions between the hydrophilic membranes and water [85,86]. Hydrogen bonding interactions are observed in polymers such as PVA, cellulose acetate and polyamides with water. The materials such as Nafion (ion exchange membranes), quarternized ammonium group containing materials, chitosan, and cellulose sulfate (polyelectrolyte complexes) exhibit ion
dipole interaction with water. Shan et al. synthesized high-performance superhydrophilic membrane using (poly(acrylic acid)/poly (ethyleneimine))n/polyacrylonitrile polyelectrolyte membrane through the biomineralization of calcium carbonate onto the membrane by spray-assisted technique. Ethanol
water mixtures can be separated using the membrane by overcoming the trade-off relation between selectivity and permeability [87]. The transport of liquid and gas mixtures across the nonporous dense polymeric membranes is based on the solution diffusion mechanism introduced by Thomas Graham and afterward by Paul and Koros [88,89]. The process occurs in three steps and is schematically presented in Fig. 9.7 [90
93]. 1. sorption of solute in the upstream face of the membrane surface, 2. diffusion of permeate across the membrane based on the gradient of concentration, and

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Figure 9.7 Schematic illustration of the solution diffusion mechanism.

3. desorption of permeate from the downstream surface of the membrane Chemical potential or concentration gradient between the upper and downstream side of the membrane is the basic driving force for separation, which is created by the application of partial pressure difference to the diffusing species across the membrane.

9.3.1

Separation of azeotropic mixtures and organic solvents

Ethanol, isopropanol (IPA), and tetrahydrofuran (THF) form an azeotrope with water at 96, 88, and 94.7 wt.% in mole fraction respectively. Chemicals such as cyclohexane are required in the conventional distillation process for the complete liquid mixture separation. The addition of carcinogenic chemicals as an entrainer can lead to impurity in one component and is difficult to isolate. Thus membrane-based PV has been turned as a promising alternative technique for the efficient separation of these azeotropes without requiring additional entrainers [94]. Stephen et al. have studied the PV separation of an azeotropic mixture of THF/water system using PVA/POSS and PVAPEO/POSS membranes [95,96]. These membranes exhibited excellent PV performance for the separation of THF
water azeotropic mixture when compared with pristine PVA. Table 9.1 demonstrated the percentage of water separation attained from THF
water azeotropic mixture by the membranes. Hydrophilic poly(ethylene glycol)-POSS (PEG-POSS) and anionic octa

Chapter 9 Polymer/polyhedral oligomeric silsesquioxane nanocomposite membranes for pervaporation

Table 9.1 The percentage of water separation attained from THF
water azeotropic mixture by the membranes [95,96]. Membranes

% of water separation attained from THF
water azeotropic mixture

P0 PVA/PEG-POSS PVA/octa-TMA-POSS PVA-PEO/PEG-POSS PVA-PEO/octa-TMA-POSS

50 97 98 98.1 96.7

(tetramethylammonium)-POSS (octa-TMA-POSS) were used for the fabrication of PVA/POSS membranes. PVA-PEO/POSS systems also showed high PV separation of THF
water azeotropic mixture due to its high hydrophilicity, improved free volume properties, and sustained crystallinity of PVA-PEO/POSS membranes. Remarkable enhancement in water selectivity and permeance was observed for CTAB modified POSS (m-POSS)incorporated PVA membrane for the separation of IPA
water azeotropic mixture [97]. octa-TMA-POSS, Octa(tetramethylammonium)-polyhedral oligomeric silsesquioxane; PVA, poly(vinyl alcohol); PEG-POSS, poly(ethylene glycol)-polyhedral oligomeric silsesquioxane; THF, tetrahydrofuran. Magalada et al. studied the PV separation performance of ethanol
water azeotropic mixture using phosphomolybdic acid (PMA)-loaded PVA
poly (vinyl pyrrolidone) blend membrane. The nascent membrane exhibits very low selectivity while on the addition of 4 wt.% of PMA its selectivity increases significantly [98]. IPA is a nontoxic and rapidly evaporating alcohol produced through microorganism’s fermentation as well as from water and propene reaction. It is an extensively used solvent in various industrial applications such as intermediate for the synthesis of vitamin B12, isopropyl acetate, and rubbing alcohol, used as an additive in disinfecting pads and as a powerful cleaner in electronic devices. Every day, a large quantity of IPA is produced as a by-product from various industries. Hence, recycling and reusing of IPA are found to be an essential factor [96]. Zhang et al. reported PV separation of IPA
water system using PVA-silicone hybrid membranes. γ-Aminopropyl-triethoxysilane (APTEOS) is used as a precursor for the preparation of PVA/silicone membrane. The membrane exhibited high water

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permselectivity and flux concomitantly as compared with pristine PVA. The membrane exhibited a separation factor of 1580 and permeation flux of 0.0265 kg m22 h21 at 5 wt.% of APTEOS for PV separation of IPA (90 wt.%)
water system [99]. Amirilargani et al. synthesized zeolitic imidazolate frameworks (ZIF-8) nanoparticle and it is introduced into the PVA matrix. The PVA/ZIF-8 membrane is used for the separation of IPA
water mixture. The membranes show improved permeance without much reduction in the separation factor [26]. THF is an expensive volatile polar aprotic solvent frequently used in many chemical processes, including pharmaceutical products, drug synthesis, adhesives, paints, and inks due to its high dissolving power for non-polar and polar species. Recycling of highly pure THF is very essential to overcome the economic and environmental challenges. It is very difficult to separate highly pure THF, especially from water. Usually it is separated by the multistage distillation process. The distillation process possesses many environmental, economic and technical challenges including explosion due to the reaction of THF with atmospheric oxygen. The distillation separation of THF
water azeotropic mixture needs a third component (entrainer), which lead to contamination during the separation of THF. The PV technique is a suitable alternative for distillation separation of THF. Zhang et al. studied the PV dehydration of THF (90 wt.%) using polysilsesquioxane (PSS)-incorporated PVA membrane. The PSS introduction in PVA membrane improves its hydrophilicity, reduce crystalline region and overcome the inverse relation between permeance and selectivity. At 2 wt.% loading of PSS, the membrane achieve maximum separation factor (1810) with improved flux [100,101].

9.4

Factors affecting the pervaporation through polymer membrane

Permeability and solubility of gases or liquids vary with the nature of the polymer and permeants. There are many key parameters that have great influence on the transport of gas or liquid molecule across the polymer membrane such as polymer chain mobility, amount of free volume within the membrane, crystallinity, molecular weight, Tg, nature of cross-links, penetrant feature, composition of the feed, casting solvents used for membrane fabrication, temperature, polarity of permeant, and affinity of membrane toward diffusing molecule [102
104].

Chapter 9 Polymer/polyhedral oligomeric silsesquioxane nanocomposite membranes for pervaporation

Yampolskii has reported the relation between structure and transport of various polymeric systems [105].

9.4.1

Effect of free volume

Free volume in a polymer can be defined as the volume of the total mass, that is not occupied by polymer chains themselves and hence diffusing molecules can be situated there. It can generally be said to the gap or pores occupied between the chains of polymers. The direct examination of the pore in the membrane is impossible because the gaps occur at molecular scale. The schematic representation of the free volume in a polymer is shown in Fig. 9.8. It is an intrinsic, transient, and dynamic property. The physical state and density of the polymer greatly influence the free volume properties of the membrane. The process of reduction of excess free volume in the membrane can be referred to physical aging, which originates from the lattice contraction and diffusion of the free volume from membrane interior to the surface [106]. The diffusion ability of the membrane can be described by free volume theory and molecular model. According to free volume theory, diffusion in polymer membrane is due to the random redistribution of free volume voids. On the other hand, the molecular model describes diffusion as a thermally activated phenomenon. This parameter reflects the movement of polymer chains and hence it has a strong influence on gas or liquid molecule transport and the overall performance of the membrane.

Figure 9.8 Schematic representation of the free volume in a polymer.

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The diffusing molecule can travel distances lesser than the diameter of the void itself. These voids are termed as free volume elements (FVEs) or “holes” and its sum is termed as free volume. FVE in the polymer can form randomly and die out because of the possibility of thermally activated chain motion or molecular oscillation in them. Free volume in the membrane decreases with increase in the intermolecular cohesion. The diffusing molecule can pass one FVE to another based on the size of the cavity sufficient to occupy the molecule [107].

9.4.2

Nature of polymers

The permeation properties of polymers differ from one another. The polymers internal composition is a key factor that influences the permeation process of polymers. Polar polymers show strong affinity or increased diffusion toward permeant gas with polar groups, it also causes swelling, partial dissolution, or crazing of the polymer. High degree of unsaturation in polymers results in reduced diffusion. Glass-transition temperature, crystallinity, and molecular weight of polymers play a vital role in the diffusion process. As the crystallinity, Tg, and molecular weight of the polymer increases the permeation decreases. The presence of crystalline phase opposes the diffusion of permeant species. A polymer contains both crystalline and amorphous regions, wherein the amorphous region shows higher permeability due to the presence of a higher amount of fractional free volume. However, in the crystalline region permeability is less due to uniform rigid chain packing. Both crystalline and semicrystalline materials are usually found in packaging applications due to their ability to prevent permeation of molecules. The distribution and number of voids in the polymers have great influence in the permeability [108
110]. Tg of the polymer has a significant influence on the transport properties. As Tg decreases the diffusion coefficient of the permeant increases due to high segmental motion, which leads to high fractional free volume. The number of chain ends represents the degree of discontinuity in the polymer chain, which means that chain ends decreases with increase in the molecular weight of the polymer. Therefore as the molecular weight increases, the sorption sites within the polymer decreases.

9.4.3

Nature of filler particles

Nanoparticles are usually added to polymers to improve properties. The nanofiller either improve or reduce transport

Chapter 9 Polymer/polyhedral oligomeric silsesquioxane nanocomposite membranes for pervaporation

properties of polymers, which depends on polymer
filler interaction and nature of filler. Uniform dispersion of fillers in the polymer matrix has a major role in the permeation of molecules because of the increase in the tortuosity of the path for permeating molecules and decrease in the permeation cross-section. Generally inorganic particles resist and organic particles facilitate permeation properties of the polymer membrane. The shape of the filler is another key factor that plays a vital role in influencing the barrier property. Platelet fillers exhibit high barrier property due to the long diffusion pathway. The parallel orientation of nanofillers with respect to diffusing molecule in the matrix will increase the permeation, whereas perpendicular orientation reduces the permeation [111
113]. Nanoparticle-embedded polymer matrix reduces their free volume and creates a tortuous path for the diffusing molecules. In the reverse case, incompatible nanoparticle-introduced polymer matrix creates voids at the interface and increases the free volume of the polymer matrix.

9.4.4

Effect of temperature

The temperature has a vital role in the permeation of gas/liquid molecule across the polymer membrane. As the temperature increases, mobility and flexibility of polymer chain increases, which will result in more free volume and easier diffusion of a molecule across the membrane. The temperature has a significant effect on selectivity; selectivity decreases with increase in temperature. As the temperature increases, free volume increases due to the thermal expansion and reduced density in the membrane, consequently permeation increases. The relationship between solubility coefficient (S) and the temperature is   ΔHS S 5 S0 exp 2 ð9:1Þ RT Similarly Arrhenius equations express the relation between temperature and diffusion coefficient (D)   ED ð9:2Þ D 5 D0 exp 2 RT where ED corresponds to energy barrier to diffusion.

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9.4.5

Nature of penetrants

Generally shape, size, molecular weight, and phase of the penetrants have a strong effect on permeation property of the polymer. As the size of the gas/liquid penetrant molecule increases, the diffusivity of molecule decreases. The diameter of the molecule in the gaseous or liquid mixture is also an important factor that influences the transport. It is usually observed that those components in the feed with small molecular dimension are highly permeable across the membrane [114,115].

9.4.6

Degree of cross-linking

It is an important factor that influences the membrane selectivity. Generally a higher level of cross-linking between the polymer chains results in higher membrane selectivity. The crosslinked polymer can attain high strength and stability due to the compact network structure, resulting in reduced swelling [116].

9.5

Applications of polyhedral oligomeric silsesquioxane-embedded polymeric systems

Dispersion of POSS molecules in polymer matrix modifies the physical properties of polymers and overcomes the intrinsic drawbacks of pristine polymer membranes. The most important applications of PV process are in the separation of water from organic
aqueous mixtures and separation of organics from organic
organic mixtures or organic
aqueous mixtures. POSS
polymer membranes in specific are used in fuel dehydration of ethanol or IPA, wastewater treatment, fuel desulfurization, etc. Very recently our group reported the PV separation of THF
water and IPA/water azeotropic mixture using PEG and anionic octa-TMA-functionalized POSS embedded PVA and PVA-PEO membranes. The membranes achieved excellent water selectivity and permeance due to the high affinity of the membrane toward water molecule [95
97].

9.5.1

Dehydration of ethanol

POSS-incorporated polymer nanocomposites are widely used to fabricate membranes for dehydration. Hydrophilic sidechains of POSS can add to the membrane surface hydrophilicity

Chapter 9 Polymer/polyhedral oligomeric silsesquioxane nanocomposite membranes for pervaporation

which aids PV dehydration of ethanol. Wang et al. [117] reported that the PEG-m-POSS increased the free volume of alginate nanocomposites and obtained permeation flux of 2500 g m22 h21. The PEG side group on the POSS significantly enhanced water affinity while it reduced the crystallinity of the membrane. A novel blend of polyimide and sulfonated polyimide incorporated with POSS exhibited superior flux and advanced separation factor for ethanol dehydration1 [118]. Similarly dual-layer hollow fiber membrane containing POSS also increases free volume size and diffusion selectivity. Most of the PV membranes obtained maximum permeation flux and free volume when 2 wt.% of POSS was incorporated [119].

9.5.2

Ethanol recovery

The hydrophobic side groups on POSS increase the interaction with organic components on the feed and the polymeric membrane as well as the separation efficiency. POSSincorporated Pebax membranes are employed for ethanol recovery. Le et al. successfully developed two types of POSS disilanolisobutyl (SO1440) and octa(3-hydroxy-3-methylbutyldimethylsiloxy) (AL0136)-incorporated polyether-block-amide (Pebax2533) system. They analyzed PV performance of the membrane for the dehydration of ethanol by varying the feed composition, POSS loading and temperature. They found that Pebax/AL0136 exhibit higher separation efficiency than Pebax/ SO1440. This can be attributed to the high affinity of Pebax/ AL0136 system toward ethanol molecule. The best performance of the membrane obtained at 2 wt.% filler loading. Membrane show improved flux and reduced selectivity on increasing the ethanol concentration in the feed [120]. Polydimethylsiloxane incorporated with synthesized octa [(trimethoxysilyl)ethyl] POSS also exhibited similar results. The hydrophobic side groups and enlarged free volume cavities improved ethanol diffusion selectivity and decreased the amount of water absorption [121].

9.5.3

Separation of organic mixtures

The hydrophobic property of POSS-incorporated polymeric membranes can be used to separate organic component from water
organic mixtures or to separate organic components from each other. POSS/PDMS membranes are effective in separating organic mixtures. Thiophene, benzene, and toluene

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can be separated from n-heptane using PDMS membranes with ,5 wt.% of octa methyl POSS [122].

9.5.4

Water treatment

The hydrophobic PV of POSS-polymer membranes finds great application in water treatment also. PV separation using POSS/polymer membranes is best suited for large scale water treatment as it offers a clean economically viable and environment-friendly method that can separate even small amounts of the organic component from water. POSSincorporated PDMS used for acetone removal from wastewater exhibits 212% improvement in separation factor compared to pristine PDMS [123].

9.5.5

Desulfurization of fuels

Gasoline contains sulfur-containing compounds such as mercaptans, thiophenes, sulfides, disulfides, and their derivatives. Emission of these compounds to the atmosphere causes severe environmental pollution and also affects the health of organisms. So desulfurization of fuels is an essential step in the petrochemical industry that demands to scale up. Desulfurization of fuels is currently using membrane based separation PV method. Glycidal POSS-copolyimide membranes were used for PV experiments of a mixture of benzothiophen and n-dodecane. The cross-linking of POSS on polymer chains resulted in lower fluxes but these type of membranes finds applications in desulfurization of methanol in direct methane fuel cells [124].

9.6

Challenges and future aspects

Pervaporative membrane technology is the cost-effective, pollution-free, and clean technology with minimum waste generation. It has future applications in a pharmaceutical, petrochemical, petrochemical refinery, environmental refinery, and food and dairy industry. The limitations of pervaporative membranes regarding stability, separation efficiency, homogeneous dispersion of nanoparticles, and scaling up are to be overcome to exploit its full potential in future applications. The flux and separation factor of the POSS-polymer membranes show a trade-off depending on the temperature and feed concentration. So the foremost challenge is to overcome this

Chapter 9 Polymer/polyhedral oligomeric silsesquioxane nanocomposite membranes for pervaporation

trade-off so that POSS
polymer membranes can be commercialized for various large-scale applications. PV dehydration using hollow fiber membranes has intrinsic advantages of large surface area per volume, self-contained vacuum channel, and self-supporting structure. Improved separation efficiency is obtained for IPA dehydration using hollow fiber membranes [125
127]. Only a few have studied ethanol dehydration performance of hollow fiber membranes containing POSS fillers [118]. More focus is required in this area to improve separation efficiency and flux of hollow fiber membranes by tailoring fabrication methods and effective incorporation of nanoparticles. Another important challenge is to fabricate PV membranes with stability for long
term application. POSS molecules act as cross-linking agents in the polymer matrix. The threedimensional network improves chemical stability and temperature resistance but flux is sacrificed. So it is high time to create pervaporative membranes with enhanced chemical stability and high separation efficiency. POSS
polymer membranes and PV technology together can replace all the existing separation techniques such as distillation considering the efficiency, economical viability easiness to scaling-up, etc. But there are some factors that are to be thoroughly studied to make use of all of its potential in future applications.

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10

Samit Kumar Ray, Amritanshu Banerjee, Swastika Choudhury and Debapriya Pyne Department of Polymer Science & Technology, University of Calcutta, Kolkata, India

10.1

Introduction

The very term “nano” originates from the Latin word “nannus” meaning “dwarf.” As per the international system unit (SI) prefix, nano means 1029 of an SI base unit. Nanoparticles (NPs) including nanometal and nanometal oxides are defined as nanosized structure with one or more of its “dimensions,” that is, length, width, or thickness in the nanometer range of 1 100 nm [1,2]. Siegel [3] showed nanomaterials with a size in (1) zero dimension (0D) where all three dimensions are within 50 nm, that is, NP, fullerenes (2) one dimension (1D) where 1D is above 100 nm, that is, nanowires, nanobelt, nanorods or nanotubes, (3) two dimensions (2D) with 2D . 100 nm and the remaining one within 100 nm, for example, nanosheet or nanofilm, and (4) three dimension (3D) where all three dimensions are . 100 nm, that is, clusters of NPs. Nanomaterials exist as “nanotubes,” “quantum dots,” “fullerenes,” or “dendrimers.” The scientific report on nanomaterials dates back to 1857 when Michael Faraday reported the synthesis of the nanosized colloidal gold particles, though its intense red or purple color was explained after 50 years in 1908 by Mie [4]. In the meantime for some 70 years, extensive works were carried out on nanostructured catalysts, and in early 1940s fumed and precipitated nanosilica was commercialized to use it in rubber, compounding as an alternative to carbon black. In 1960s and 1970s, metal nanopowders were developed for magnetic tape recording [5]. The advancement of nanomaterials continued Polymer Nanocomposite Membranes for Pervaporation. DOI: https://doi.org/10.1016/B978-0-12-816785-4.00010-0 © 2020 Elsevier Inc. All rights reserved.

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and, today, the nanoscience and engineering find various commercial products based on organic and inorganic nanosized metal and metal oxides. Among the various metals and its oxides, nanometals are mainly prepared from gold (Au), silver (Ag), copper (Cu), and iron (Fe) while nanometal oxides are prepared from oxides of aluminum (Al2O3), magnesium (MgO), zinc (ZnO), titanium (TiO2), nickel (NiO), iron (Fe2O3), zirconium (ZrO2), molybdenum (MoO3), cerium (CeO2), and yttrium (Y2O3). These nanometal and metal oxides are significantly different from bulk metal or metal oxides in many aspects. Because of very small sizes, these nanomaterials show much higher surface area to volume ratio and higher surface energies than bulk metal or metal oxides. The nanometal and metal oxides also show shortrange ordering with an increased number of kinks, plasmon excitation, quantum confinement, and a large number of low coordination sites for storing excess electrons resulting in specific chemical reactivity which is absent in its bulk counterpart. Thus by virtue of its unusual chemical, mechanical, electrical, and magnetic properties, nanometals and metal oxides are being extensively used in many products such as nanosized ceramics with much improved ductile properties at elevated temperature, nanostructured semiconductor showing luminescence due to quantum confinement, nanometallic powder in gas tight material and for porous coating, magnetic nanocomposites for high-density information storage and magnetic refrigeration, nanostructured metal clusters as heterogeneous catalysts for chemical reactions and electrocatalysis in fuel cells, nanometal oxide thin film for gas sensors of NOx, CO, and CO2, rechargeable battery, and nanometal and metal oxideincorporated polymer nanocomposites. Polymer metal nanocomposites (PMNCs) prepared by integrating functional homo/ copolymer or their blends and nanosized metal or metal oxides are widely used in automobiles, aerospace, injection molded products, packaging, optically integrated circuits, drug delivery, medical devices, sensors and also in membranes. The membrane is a selective barrier between two phases. Membranes (excluding cell membranes of living organism) are used for (1) desalination and water purification by reverse osmosis (RO), nanofiltration (NF), and ultrafiltration (UF), (2) dye desalting by NF, (3) dialysis and for separation of dissolved organic molecules by UF, (4) separation of biomolecules by UF and microfiltration (MF), and for (5) separating close boiling or azeotropic organic mixtures, removal/recovery of organics from water or dehydration of a solvent by

Chapter 10 Nanometal and metal oxide-based polymer nanocomposite membranes for pervaporation

pervaporation (PV). In the first generation membrane processes like RO, NF, UF, or MF, porous membranes prepared usually by phase inversion method is used while nonporous dense membranes prepared by solvent casting method are used in the second-generation membrane process like PV and gas separation. As membrane materials, inorganic metal/metal oxides, ceramic materials, or organic polymers are used for both porous and nonporous membranes. Inorganic membranes are highly selective with excellent mechanical stability. However, because of its structural rigidity, it is difficult to make a membrane module with high surface area from inorganic or ceramic material. In contrast, mechanical stability or chemical resistance of “organic” polymer membranes is much poorer than inorganic membranes, but polymers are more flexible and require much less processing cost for the fabrication of a membrane module. Thus polymer composite or organic inorganic “hybrid” membranes were developed where the inorganic metal or metal oxides are incorporated into a polymer matrix. Accordingly the flexibility of an organic polymer is combined with the high selectivity and stability of inorganic metal or metal oxides in these PMNC membranes. The separation potential of these membranes strongly depends on the compatibility of dissimilar “organic polymer” with “inorganic” metal/metal oxides. It has been reported that a small amount of nanosized metal or metal oxide in the polymer membrane matrix significantly improves its mechanical strength and durability, permeability, selectivity and resistance to flux decline, fouling, and microbial attack. In fact “nanotechnology” in membrane science and technology or “nanoscale-based membrane technology” consists of “nanostructured” membranes referring to membranes with a pore size of 1 nm (NF) or less (RO) and “nanoenhanced” membranes [6] corresponding to polymer membranes incorporated with nanosized particles, fibers, sheets, or rods of metal and metal oxides including several clay materials and carbon, for example, carbon nanotubes (CNTs) or graphene. The discussion of the present chapter is limited to the present status of research on these nanometal and metal oxides incorporated nanocomposite membranes (PMNCs) used for PV.

10.2

Synthesis of polymer nanometal nanocomposite

In recent times several nanometal- and nanometal oxideincorporated mixed matrix type PMNCs have been reported for

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NF, UF, gas separation, and PV. These PMNCs are prepared either by incorporating metal/metal oxide NPs into a polymer matrix or by in situ synthesis of these NPs in a polymer matrix.

10.2.1

Synthesis of metal/metal oxide nanoparticles

Metal/metal oxides NPs may be prepared by a physical, chemical, and biological process using “top-down” or “bottomup” method as shown in Scheme 10.1. In “top-down” method the bulk metal/metal oxide particles are mechanically ground, and the resulting NPs are stabilized with a colloidal protecting agent. The “bottom-up” processes starts from the atom of the metals or metal oxides by chemical processes such as sol gel, colloidal, chemical vapor deposition, plasma, laser pyrolysis, and atomic or molecular condensation [3,7]. Some of the wellknown processes are depicted below.

10.2.1.1

Physical method

10.2.1.1.1

Mechanical grinding

This physical method of synthesis of nanometal and metal oxide involves high energy ball milling where the mills with

Bottomup m etho dol ogy

Top-dow n methodol ogy

Bulk materials

Transformation involving mechanical grinding/attrition

Precursor molecules (organic/inorganic metallic salts)

Formation of radicals and atoms

Small fragments or powder

Etching out crystal planes from the fragments Exfoliation of stacked layers to attain a “monolayer”

Nan

o - m e t a l p a rti cl e

s

Scheme 10.1 Top-down and bottom-up synthesis of nanometal and metal oxides.

Clusters

Chapter 10 Nanometal and metal oxide-based polymer nanocomposite membranes for pervaporation

planetary, vibratory, rod or tumbler type design are used and filled with balls made from hard steel or tungsten carbide. The mills are filled with air or inert gas and the bulk metal/metal oxides such as aluminum, iron, silver, chromium or tungsten, cobalt or their oxides are rotated at high speed to obtain fine powders. 10.2.1.1.2 Melt mixing This is another physical method where the metals after melting are cooled at a very fast rate (105 106K s21) to form the amorphous “metallic glasses,” for example, a molten stream of Cu-B and Ti is mixed at high velocity to form NPs of TiB2 as amorphous “glass metals” [8]. 10.2.1.1.3

Evaporation

Nanometals and metal oxides are also prepared physically by this method. Here the bulk metal like W or Mo is used as filament in a vacuum chamber and heated to vaporize. The highdensity vapors of the metals with sizes , 5 nm are cooled with a “cold finger,” for example, liquid nitrogen cooled inert gas to force out the solid clustered NPs of the metals. Oxides, nitrides, or hydrides of the metals in nanosize are also prepared by contacting the evaporated metals with reactive cold fingers, that is, oxygen, nitrogen, hydrogen, or ammonia. 10.2.1.1.4 Laser ablation In this method the material is vaporized by a high power laser beam which generates atoms from the solid bulk metals. The particle size and particle size distribution of the resulting NP depends on the wavelength of the laser, ablation period, liquid medium, and the presence of salts such as sodium chloride, a surfactant, or a water-soluble polymer. 10.2.1.1.5

Sputtering

This technique is also used to prepare multilayer nanofilms on ceramic or alloy compounds. In this method the target is held at the cathode while the bulk metal from which nanofilm is to be coated on the target is held at the anode, and a high voltage (100 3000 V) is applied between cathode and anode at a very low pressure of , 10 Pa in the presence of an inert gas like argon. As the current flows between anode and cathode, a visible glow is observed with the formation of the nanofilm on the ceramic substrate.

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10.2.1.2

Chemical methods

Nanosized particles, plates, or fibers prepared by “chemical method” [9] exist as colloids, consisting of two or more phases (solid, liquid or gas) of same or different materials with a dimension of the colloid in nanometer ranges. Some of the widely used chemical methods for preparing nanometal and metal oxides are described in short below. 10.2.1.2.1

Chemical reduction

In this method metal salt or acid of gold, silver, copper, etc., is reduced to form colloidal NPs which are further stabilized with some capping molecule, for example, chloroauric acid (HAuCl4) is reduced with trisodium citrate (Na3C6H5O7) to form nanogold particles in red or magenta color in the colloid depending on its particle size and its distribution which is further stabilized with a thiol. Similarly an aqueous solution of silver salt, usually AgNO3 is reduced with sodium borohydride, sodium citrate, or ascorbate to form the silver nanoparticle (AgNP). The presence of a polymer such as polyethylene glycol (PEG) blocks copolymer further reducing the sizes of the AgNP. 10.2.1.2.2

Chemical precipitation

In this low-cost method a metal salt is precipitated in an alkaline medium, that is, concentrated ammonium hydroxide, ammonium or sodium bicarbonate followed by calcination of the vacuum dried metal hydroxide at an elevated temperature to produce the nanometal or metal oxide. Accordingly aluminum hydroxides were precipitated from its chlorides, nitrate, or sulfate salts in concentrated ammonia or sodium bicarbonate followed by calcination of the vacuum dried aluminum hydroxide at an elevated temperature of 800 C 900 C for some 1 h to produce nanoalumina [10]. The presence of a surfactant or PEG provides good control over particle sizes while instead of the aqueous medium, use of ethanol medium proves softer and finer NPs of gamma alumna [10]. 10.2.1.2.3

Sol gel technique

The essence of the sol gel technique [11,12] is alkaline hydrolysis of an organic ester precursor of the metal or metal salt with sodium hydroxide to produce metal hydroxide followed by its condensation to produce the metal oxide which is further calcined at low temperature to the metal or metal oxide NPs. Because of the requirement of low temperature and fewer

Chapter 10 Nanometal and metal oxide-based polymer nanocomposite membranes for pervaporation

chemicals, the sol gel technique is less expensive though it is difficult to prevent agglomeration of the resulting NPs. Accordingly ZnO NP was prepared by hydrolyzing zinc acetate [Zn(CH3CO2)2
2H2O] with sodium hydroxide followed by its condensation to produce ZnO which is calcined at low temperature, for example, at 80 C for 1 h to produce ZnO NP [13]. Similarly nanotitanium oxide is produced by hydrolyzing titanium tetrachloride (TiCl4), titanium oxygen dichloride (TiOCl2) or an alkoxide (TiOR) to produce “amorphous” nanotitanium oxide which cannot be prepared by the high temperature plasma synthesis. By sol gel technique, porous titanium dioxide membrane can be prepared. N. Agoudjil and T. Benkacem [14] reported the synthesis of mesoporous titanium dioxide membrane by a sol gel technique using titanium isopropoxide Ti(OC3H7)4 as a precursor and concentrated nitric acid as peptizer. Nanosized gamma alumina powder has been prepared by dissolving an inorganic salt such as nitrate or chloride or an organic isopropoxide of aluminum in a suitable solvent such as ethanol, diethylene glycol, ethyl acetate, tetrahydrofuran, and methyl ethyl ketone. and dehydrating the resulting “sol” to a “gel” followed by its drying to form the powder hydroxide which is calcined at 900 C 1000 C. Silver silica composite NPs were synthesized from the precursor silver nitrate and tetra ethyl ortho silicate using a sol gel technique by annealing these samples at 550 C for 30 min [15]. 10.2.1.2.4

Hydrothermal method

In general in the hydrothermal method, a metal or metal oxide with high vapor pressure near its melting point is crystallized at high temperature and vapor pressure in a single step process in an autoclave. Thus by hydrothermal method γ-alumina is produced from aluminum hydroxide in an autoclave at a temperature of around 400 C and a pressure of 1 15 MPa. Similarly zinc oxide NPs were synthesized by hydrolysis of zinc acetate with sodium hydroxide using a Teflon lined sealed stainless steel autoclave at 100 C 200 C for 6 12 h under autogenous pressure [16] 10.2.1.2.5 Microemulsion technique The nanometal and metal oxides are prepared in a microemulsion in a similar way as in sol gel technique, that is, the salt or organic ester of the metal is hydrolyzed with an alkali followed by calcination of the metal oxide or hydroxide in the microemulsion. However, the microemulsion technique

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provides better control over size and size distribution of the NPs because of the nanosized micelle where hydrolysis occurs [17]. For preparing nanoalumina in reverse microemulsion, a nonionic surfactant, for example, PEG tert-octylphenyl ether (Triton X-114) and AlCl3 is added in an oil phase, viz., cyclohexane. The AlCl3 present in this microemulsion is then hydrolyzed with dropwise addition of ammonia solution when the precursor Al2O3 is formed at a pH of 9.5. This aluminum oxide is precipitated in ethanol and reclaimed by centrifugation, washed, dried, and calcined in a tube furnace at around 1200 C to form the nanoalumina. Zinc oxide NPs may also be prepared by a reverse microemulsion method. The zinc precursor, for example, zinc acetate dehydrate, is taken in a microemulsion prepared by integrating sodium bis(2-ethylhexyl)sulfosuccinate [Aerosol OT (AOT)], glycerol, and n-heptane [18]. The ZnO NPs are formed by calcination of premature zinc glycerolate microemulsion product in the air at 300 C, 400 C, and 500 C. Metallic NPs are also prepared by this technique by mixing two microemulsions one containing the metal salt and the other a reducing agent such as sodium borohydrate, hydrazine, or ammonia. Thus for preparing nanosized titanium oxide, 1:1 weight ratio of poly (oxyethylene) pentyl (NP5) and nonyle (NP9) phenol nonionic surfactant was taken in cyclohexane as oil phase. Two separate microemulsions containing 0.5 M titanium tetrachloride (TiCl4) and 2.0 M ammonia, respectively, were mixed, leading to the formation of insoluble titania particles which was centrifuged, washed with acetone, and finally vacuum dried to form nano-TiO2 with an average size of 5 nm and narrow size distribution [19]. In microemulsion technique, nucleation and growth of the NP occur within the micelle, which acts as a nanoreactor. The size and size distribution of the resulting NPs depend on the rate of exchange of reactants between micelles, type of the continuous phase, the molar ratio of water to oil (termed as w), and the precursor content dissolved within the nanodroplets. As the chain length of the oil increases, micellar exchange rate increases with a decrease in the size of the NPs. Thus silver NPs were reported [20] to decrease in size as the oil length was increased from n-heptane to decane with a shift of the plasmon absorption peak towards shorter wavelengths (blue shift).

10.2.1.3

Biological method

Metal and metal oxide NPs are prepared by the use of microorganism, plant extracts, enzymes, DNA, membranes, or viruses [21]. Nanosized gold or silver can be prepared from its salts by

Chapter 10 Nanometal and metal oxide-based polymer nanocomposite membranes for pervaporation

treatment with lactobacillus strain or Fusarium oxysporum [21]. Similarly nanometal and nanometal oxides have also been synthesized from different plant extracts.

10.3

Direct use of nanometal and metal oxides as membrane

There are some reports on PV membranes from nanometal and metal oxides. Though these ceramic membranes possess good mechanical and chemical stability, the flux or selectivity obtained from a ceramic membrane is not up to the mark. Further, these membranes are expensive. Thus nanometal and metal oxide-based PMNC membranes were developed for PV separation.

10.4

Nanometal and metal oxide-based polymer metal nanocomposites membranes

Similar to other PMNCs, nanometal and metal oxide-based PMNCs are also prepared either by incorporating the nanometal and metal oxides in the matrix of a polymer/copolymer or polymer blend or by graft copolymerization of a functional monomer to the surface of the nanometal or its oxide. There should be uniform dispersion of the nanometal or metal oxides in the polymer matrix without any agglomeration along with high interfacial interaction between this inorganic filler and the organic polymer matrix. However this is difficult to achieve. In fact, in a polymer matrix, the micron size inorganic particle shows much better dispersion than its nanocounterpart. As the particle size decreases, specific surface area and hence surface energy of the particles increases. Thus NPs always tend to aggregate to reduce its surface energy. Further, any metal oxide contains hydroxyl groups on its surface also tend to form oxygen bridges by agglomeration. For preventing agglomeration, the surface of the nanometal/metal oxides is coated or grafted with surfactant molecules [10]. The adsorbed or bonded molecules prevent the formation of these oxygen bridges and agglomeration of the NPs. For preparing nanometal/metal oxides-based PMNCs for membranes, nanometal/metal oxides are grafted to a polymer or incorporated in a polymer matrix.

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10.4.1

Grafting of nanoparticles to polymer

The surface functionality of the nanometal or metal oxide is modified to prevent its agglomeration and also to make it hydrophilic or organophilic depending on the type of monomer taken for graft copolymerization. There are two kinds [12] of grafting: (1) grafting to and (2) grafting from. “Grafting to” process for polymerization is widely used for various applications including membranes for PV. In this process, a functional monomer is polymerized in the presence of a nanometal/ metal oxide. During polymerization, the radicals from the monomer also form a covalent bond with functional groups, that is, hydroxyl groups on the surface of the nanometal. In “grafting from” the initiator is immobilized on the surface of the metal or metal oxide, and the monomer propagates to growing radicals in situ forming a polymer brush on the metal surface.

10.4.2

Incorporation of nanoparticles in polymer

This is carried out by solution blending, sol gel, or by in situ polymerization method [22]. In solution blending the NPs are dispersed with mechanical stirring in a solvent usually water, and after sonication it is added to the polymer solution with continuous stirring till a good dispersion is achieved. The membrane is cast from this dispersion. In sol gel technique the monomer, oligomer, or polymer and the NPs are taken in a solvent, usually water where a network is formed between the NPs and the organic polymer at mild temperature while in “in situ” technique the monomer is allowed to polymerize in the presence of the NPs where the nanomaterials are also covalently bonded with its surface functionality through formation of macro radicals with the growing polymers.

10.4.3

Intermatrix synthesis technique

Intermatrix synthesis is a simple, low-cost method [23] of preparing PMNCs by surface modification of polymeric ion exchange matrices such as resins, fibers, and membranes. In this case, the metal/metal oxide NPs are formed in situ only on the polymer matrix since it cannot penetrate the ion exchange matrix of the membrane (e.g., polyetherether ketone membrane) because of like charge repulsion, and thus ion exchange properties of the matrix are not affected.

Chapter 10 Nanometal and metal oxide-based polymer nanocomposite membranes for pervaporation

10.4.4

Silver nanoparticle-based polymer metal nanocomposites membranes

Silver is a soft and lustrous noble metal possessing high electrical and thermal conductivity. AgNPs prepared from bulk silver or silver salt by physical, chemical, or biological methods are extensively used in medicine and water purification because of its antifungal, antiinflammatory, antimicrobial, and bactericidal properties. Biofouling of the membrane is eliminated in the presence of AgNPs. Hence AgPMNCs membranes have been widely studied for NF, UF, MF, gas separation, perstraction, and also for PV.

10.4.5

Pervaporation using Ag polymer metal nanocomposites membranes

Premakshi et al. [24] reported in situ synthesis of AgNP in polyvinyl alcohol (PVA) matrix by reducing silver sulfate in PVA solution with sodium borohydride. The water flux and selectivity of this glutaraldehyde crosslinked and AgNP incorporated PVA membrane increased with an increase in wt.% of AgNP in the membrane, and the membrane prepared with 2.5 wt.% AgNP showed an optimum flux and selectivity of 7.16 3 1022 kg m22 h21 and 634, respectively, for a feed containing 10 wt.% isopropyl alcohol in water. Selim et al. [25] reported the synthesis of AgNP by adding different concentrations of silver nitrate in PVA solution followed by heating at 70 C for 1 h. In this case, AgNPs are formed via the formation of Ag-PVA chelate [25]. This membrane showed a flux and selectivity of 12 3 1022 kg m22 h21 and 50, respectively, for PV dehydration of ethanol. In the presence of AgNPs, the flux increases significantly because of increased hydrophilicity and reduced crystallinity of the membrane. Chaudhury et al. [26] synthesized AgNPs by dissolving AgNO3 in an aqueous solution of PVA and adding polyacrylic acid (PAA). Initially a silver salt of polyacrylate is formed, and Ag is then transferred from polyacrylate to PVA (R-COO2Ag1 to R-O-Ag) and finally to AgNP. The membrane containing 2.25 wt.% AgNP showed the swelling of 67.5% for 80 wt.% acetic acid in feed with a total flux of 0.25 kg m22 h21. AgNP has also been incorporated in the organophilic membrane to enhance organic selectivity of the membrane. Li et al. [27] reported the synthesis of AgNP without any reducing agent. In this work polydimethylsiloxane (PDMS) rubber membrane was immersed in 10 mM AgNO3 solution in ethanol where the sorbed AgNO3 was

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Figure 10.1 (A) UV vis absorption of PVA-AgNP [23]. (B) TEM images of AgNP-PDMS (AgNPs is shown with a red circle) [25]. AgNP, Silver nanoparticle; PDMS, polydimethylsiloxane; PVA, polyvinyl alcohol.

reduced to AgNP inside the PDMS layer by its residual Si-H groups. This AgNP incorporated PMNC membrane showed ethanol selectivity from the ethanol water mixture by PV. The presence of AgNP in a PMNCs matrix is characterized by its absorption in the visible range at 410 445 nm in a UV visible spectrophotometer depending on the type of polymer and its interaction with AgNP corresponding to its surface plasmon resonance (SPR). As the amount of AgNP in the polymer matrix increases, the absorption intensity increases with the shift of absorption peak at lower values, indicating a decrease in size of AgNP in the polymer matrix as shown in Fig. 10.1A. AgNP incorporated PVA membrane showed this absorption peak at 410 nm [26] while AgNP incorporated PVA- PAA blend membrane showed this absorption peak at 445 nm which decreased to 430 nm as the amount of AgNP was increased in the polymer blend. TEM analysis shows the size, shape, and also the distribution of the particle size through a histogram of the NPs. PDMS rubber membrane containing AgNPs prepared by dipping PDMS rubber in AgNO3 solution for different periods [27] show a size range of 2 15 nm of AgNPs uniformly distributed in the PDMS matrix, as shown in Fig. 10.1B.

10.4.6

Iron nanoparticle -based polymer metal nanocomposite membranes

Iron nanoparticle (FeNP) exists as oxides of iron (either Fe3O4 or γFe2O3). Similar to other metal/metal oxide NPs, it is

Chapter 10 Nanometal and metal oxide-based polymer nanocomposite membranes for pervaporation

also prepared by physical, chemical, and biological methods. In addition to the high surface area, excellent electrical and mechanical properties similar to other metal/metal oxide NPs, FeNPs with particle size below 15 nm also show superparamagnetism [28]. In fact, because of nanosize, it shows a single magnetic domain resulting in this magnetic property for which it shows induced magnetism. This property of FeNPs is suitably used in many applications such as in magnetic resonance imaging, targeted drug delivery, hyperthermia, cancer therapy, and in separation processes including PV.

10.4.6.1

Pervaporation with iron polymer metal nanocomposites membranes

FeNP-based PMNCs membranes used in PV is prepared in situ from mixtures of hydrated ferrous and ferric salt, that is, FeSO4
7H2O and FeCl3
6H2O [29,30]or from anhydrous FeCl3 and FeCl2 [31] in 1:2 or 1:1 molar ratios, respectively. As polymer matrix, PVA, sodium alginate, and chitosan have been reported where FeNPs have been incorporated by in situ synthesis and subsequently used for PV separation. 10.4.6.1.1

Polyvinyl alcohol-based iron polymer metal nanocomposite membranes PVA membrane has already been commercialized for PV dehydration of solvents. However, the poor mechanical strength and low selectivity of PVA membrane are improved to a great extent by incorporation of iron oxide nanoparticles (FeNP) in its matrix which reduces the relaxation of the polymer chain of PVA during PV. Further, FeNPs show strong electrostatic interaction and hydrogen bonding with OH functionality of PVA to reduce its crystallinity. Thus both flux and selectivity increase in the presence of FeNPs [30]. For preparing PVA-based iron polymer metal nanocomposites (FePMNC) membranes the hydrated ferrous and ferric salt or anhydride ferrous and ferric salts in varied molar ratios, usually 1:1 to 1:2 is mixed in water, and the resulting hydroxide precipitate is removed. The filtrate is added to aqueous PVA solution (5 8 wt.% in water). After casting and drying, the membrane (PVA-Fe21/Fe31) is immersed in potassium hydroxide (4 molars in water) solution for the generation of the FeNPs within the PVA matrix as reported elsewhere [29 31]. The reaction between ferrous and ferric ion with OH groups of NaOH/KOH and PVA leads to the formation of black colored Fe3O4 for 1:1 molar ratio of ferrous/ ferric salts or brown colored Fe2O3 for 1:2 molar ratio of

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ferrous/ferric salts. Thus the membrane prepared with 10 wt.% FeNPs showed a flux, sorption selectivity, PV selectivity, and diffusion selectivity for the water of 12 g m22 h21, 876.46, 711.12, and 0.81, respectively, for PV dehydration of 90 wt.% acetonitrile which was dehydrated to 99.06% at 35 C [30]. The incorporation of FeNP engaged most of the OH groups of PVA, and thus crystalline PVA was converted to amorphous PVA FeNP composite. Sairam et al. [29] reported the synthesis of FeNP PVA in a similar way. The PVA FeNP composite membrane containing 4.5 wt.% FeNP showed a flux of 84 g m22 h21 and a separation factor for the water of 144 for 10 wt.% 1,4-dioxane which was dehydrated to 94.1% at 30 C. Olukman et al. [31] synthesized magnetite (Fe3O4)-filled PVA membrane in a similar way, which showed a flux of 0.015 0.091 kg m22 h21 and selectivity of 29 14,000 for 0 100 wt.% acetone in water. 10.4.6.1.2 Alginate iron nanoparticle nanocomposite Among the various biopolymers, sodium alginate-based hydrophilic membranes cross-linked with glutaraldehyde or calcium chloride have been reported for PV dehydration of solvents. To improve its membrane performance further, Zhao et al. [32] reported the synthesis of PAA FeNPs by adding an aqueous solution of one molar of FeCl2, 4 H2O to 5 wt.% PAA solution with constant stirring followed by alkaline hydrolysis at a pH of around 13 with one molar NaOH solution. The resulting black, red precipitate of PAA NPs was separated, dried, and added to sodium alginate (B3 wt.% in water) solution to form the PAA FeNPs incorporated sodium alginate membrane. This membrane showed higher selectivity than unfilled alginate or the alginate membrane filled with iron oxide NPs without PAA for dehydration of ethanol. Dudec [33] reported the synthesis of Fe3O4, Ag2O, and ZnO NPs from their respective salts by alkaline hydrolysis, and the resulting nanometal oxides were incorporated in the matrix of sodium alginate. Accordingly the Fe3O4 NPs were prepared by mixing FeCl2, 4H2O, FeCl2, sodium acetate, and ethylene glycol and by reducing this mixture with (ethylenedioxy)bis(ethyl amine) using Das’s modified method [34]. The resulting nanometal oxide-incorporated alginate membrane was used for dehydration of ethanol. The silver oxide-filled alginate membrane showed the highest flux while iron oxide-filled membrane showed the highest selectivity. Gao reported decorating of Fe3O4 NPs with multiwalled carbon nanotubes (MWCNT) [35]. For this, hydrated ferric and ferrous chloride (FeCl3, 6H2O, FeCl2, 4H2O) in 2:1 molar ratio was

Chapter 10 Nanometal and metal oxide-based polymer nanocomposite membranes for pervaporation

Figure 10.2 (A) Effect of CNT on total flux and separation factor for water. (B) TEM analysis of Fe3O-CNT [34]. CNT, Carbon nanotubes.

mixed in deionized water, and around 1 g of carboxylic MWCNT was dispersed in this aqueous solution by ultrasonic vibration for 1 h followed by reduction of the iron salts in the presence of the MWCNT by dropwise addition of ammonium hydroxide at a pH of 11 by vigorous mixing for around 30 min to generate iron oxide NPs modified with MWCNT. The synergistic effect of combining two NPs and incorporating the same in sodium alginate led to a loss in crystallinity and generation of micro channels in the alginate membrane. Initially with an increase in CNT content in the membrane, from 0 to 1.1 wt.%, flux increased from 1832 to 2211 g m22 h21, and water selectivity increased from 551 to 1870 at 76 C for PV dehydration of 90 wt.% ethanol (Fig. 10.2A) which might be due to decrease in hydrophilicity of the membrane by nano-CNT-iron oxide NPs but increase in fast-moving micro channel of CNT and its increased interaction with the water molecules in the nanopores. However, above 1.1 wt.% CNT the decrease in flux and separation factor might be due to the agglomeration of these NPs and its poor compatibility with the alginate matrix. 10.4.6.1.3

Chitosan iron nanoparticle nanocomposite

Chitosan or poly[β-(1,4)-D-glucosamine] is an aminopolysaccharide which is the deacylated form of chitin and the second most abundant biopolymer found in nature. Chitosan-based biopolymer has also been reported for PV dehydration. There are few reports on chitosan-based nanocomposite membranes filled with Fe3O4 NPs. Dudek et al. [36] showed that ferroferric oxide (Fe3O4) NPs, prepared from ferrous and ferric chloride by coprecipitation and reduction with (ethylenedioxy)bis(ethyl amine),

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enhanced the water permeability of the chitosan membrane cross-linked with glutaraldehyde (CHGA) and sulfuric acid (CHSA) while CHGA membrane showed higher permeation coefficient and separation factor than CHSA membrane [36].

10.4.6.2

Characterization of iron polymer metal nanocomposites membranes

10.4.6.2.1

Characterization of polyvinyl alcohol iron nanoparticle composite membrane The FTIR studies showed an additional peak at 594 cm21 corresponding to FeNPs in the PVA FeNP membrane [31]. The DSC studies showed an increase in glass transition temperature (Tg) from 102 C for PVA to 107 C for FeNP PVA because of increased resistance for the onset of segmental motion of PVA in the presence of FeO NPs [31]. DMA study at varying temperatures for PVA FeNP composite showed an increase in Tg from 72 C for virgin PVA to 85 C for PVA FeNP containing 24 wt.% FeNP [29]. Olukman et al. showed an increase in contact angle from 46.7 degrees for PVA to 61 degrees for PVA FeNP containing 20 wt.% FeNP, indicating an increase in hydrophilicity of the PVA membrane in the presence of Fe3O4 NPs [31]. Mandal et al. [30] showed TEM analysis of PVA FeNP composite membrane where nanosized iron oxides were observed to be uniformly distributed in the membrane matrix, and average particle sizes were less than used particle size. Similarly from XRD analysis, loss in crystallinity of PVA matrix is observed in PVA FeNP composite because of the loss of intermolecular hydrogen bonding of PVA molecules. 10.4.6.2.2

Characterization of alginate iron nanoparticle composite membrane The FTIR analysis showed the reduction of peak intensity at 1714 and 1198 cm21 of MWCNT in the membrane matrix signifying its interaction with iron oxide as well as alginate. Similarly iron NP incorporated membrane also showed an additional peak at 549 cm21 correspondings to stretching of Fe-O bond [35]. The same band of Fe-O was reported at 607 cm21 for PAA coated Fe3O4 [32]. The XRD analysis showed that the XRD peak of sodium alginate at two theta of 14 degrees is reduced significantly with an increase in the amount of PAA Fe3O4 in the membrane indicating the loss in crystallinity of alginate. PAA 2 Fe3O4 NPs interfere with the ordered packing of alginate and generate more free volume for increased flux. Dudek et al. [37] studied the structural morphology of Fe3O4 incorporated

Chapter 10 Nanometal and metal oxide-based polymer nanocomposite membranes for pervaporation

alginate membrane in terms of elongation factor, bulkiness, surface factor, and irregularity parameter. The Fe3O4 NPs were observed to be elongated and irregular from different aggregates of 40, 250, and 450 nm. These membranes were also characterized similarly. The iron NPs in the chitosan matrix were reported to show Raman shift at the 225, 292, 409 and 609 cm21 with 633 nm excitation wavelength. The TEM analysis showed fine dispersion with aggregations in size of submicron up to 4 mm in diameter and mean size of 20 nm as shown in Fig. 10.2B.

10.4.7

Pervaporation performance of alumina nanoparticle -based PMNC membranes

Depending on the composition of the naturally available bauxite ore, 1 ton of bulk aluminum oxide (Al2O3) or alumina is industrially prepared from around 1.9 to 3.6 ton of bauxite by Bayer process [38]. However, two major issues such as purity and particle morphology prevent it from many applications. In contrast, all properties of bulk alumina are significantly improved when it is converted to very fine nanosized alumina (both α and γ forms) of high purity by different methods such as chemical precipitation, sol gel, hydrothermal, and microemulsion techniques. The resulting nanosized porous αAl2O3 (particle size 4 14 nm) and mesoporous γAl2O3 (particle size 25 50 nm) formed by calcination at around 1200 C and 400 C, respectively are used for advanced applications such as efficient adsorbent for wastewater treatments and also as membrane materials [5]. Unlike other metal/metal oxide NPs, nanoalumina is also used as membrane material without any polymer support (ceramic membrane) apart from nanoalumina bases PMNC membranes.

10.4.7.1

Nanoalumina as membrane

Seculic et al. [39] reported the use of nanoalumina-based ceramic membrane for separation of alcohols from water by PV. For these applications, the porous αAl2O3 was used as a substrate while the mesoporous γAl2O3 was used as active separating top layer. Accordingly αAl2O3 macroporous support was made from αAl2O3 powder by colloidal filtration. The intermediate mesoporous γAl2O3 was coated on αAl2O3 by dip coating and then calcined at 600 C. A silica sol prepared by acid catalyzed hydrolysis of an alkoxide was coated on this mesoporous γAl2O3 followed by calcination at 400 C. The membrane with a

247

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Chapter 10 Nanometal and metal oxide-based polymer nanocomposite membranes for pervaporation

thickness of around 100 nm and pore size of 0.3 0.5 nm showed the highest separation factor (500) and lowest flux (1.7 kg m22 h21) for 2-butanol/water mixture followed by 2-propanol/water (100 and 2.2 kg m22 h21) and ethanol/water (20 and 1.3 kg m22 h21) which is in tune with the relative molecular size of the alcohols, that is, 0.55, 0.5, and 0.45 for butanol, propanol, and ethanol, respectively. This indicates that flux and separation factor in PV experiments is controlled by size exclusion principal as also reported elsewhere [40]. Yang et al. [41] used commercially available HybSi membrane, a γ-alumina supported organosilica membrane of Pervatec for separation of ammonia from water containing 50 mg L21 ammonia (pH 12) by PV at 45 C. The membrane showed a steady flux and separation factor of 4.3 kg m22 h21 and 12, respectively.

10.4.7.2

Silver polymer metal nanocomposites membranes

Similar to silver and iron oxide-based PMNCs membranes, nanoalumina-based PMNCs membranes are prepared by (1) encapsulation of nanoalumina in the polymer matrix or (2) by grafting a layer of a functional polymer on the surface of γAl2O3. For increased flux and selectivity, γAl2O3 was modified with PVA by adding aqueous PVA in the alumina sol which was subsequently dip-coated on an αAl2O3 substrate of porosity 0.3 and pore size 0.5 1 μm followed by calcination at 600 C. The average pore size of the resulting PMNCs membrane reduced to 4.7 nm, which is much bigger than both water and ethanol. However, this membrane showed a steady flux and water selectivity of 6.7 kg m22 h21 and 5.67, respectively for 95 wt.% ethanol in water [42]. This result indicates that the sieving mechanism is not followed in pervaporative separation with this membrane. Chen et al. [43] synthesized sulfonated polystyrene (SPS) by sulfonation of polystyrene with acetyl sulfate, and both sodium and magnesium counter ions were prepared by treating it with sodium hydroxide/ethanol or magnesium methoxide/ methanol. The sulfonated polymer, dissolved in DMSO was coated on alumina membrane and the resulting composite membrane SPS/Na/alumina membrane with 10.5 molar% sulfonation, showed a high water selectivity of 400 for PV dehydration of azeotropic ethanol. The γAl2O3 membrane prepared by similar sol gel technique and calcined for 1 h at 450 C was grafted with trifunctional organosilanes by immersing the membrane in toluene containing the silane and heating the same at 110 C for 48 h. The grafted membrane showed lower flux than the γAl2O3 membrane with similar water selectivity for PV of

Chapter 10 Nanometal and metal oxide-based polymer nanocomposite membranes for pervaporation

ethanol water mixture though the grafted membrane showed higher ethanol selectivity than untreated γAl2O3 for PV of ethanol cyclohexane mixtures [44]. Similarly γAl2O3 membrane was dip coated with a polyetharimide solution in dichloroethane, and the resulting polyetherimide alumina composite membrane showed moderate flux with a very high water selectivity (infinity) for acetic acid/water, 248 for ethyl acetate/water, and 74 for ethanol/water mixture [45]. The tubular alumina membrane was modified with a silane coupling agent, that is, perfluoro-alkylsilane to make it highly hydrophobic as evident from water drop contact angle which increased to 162 degrees (a membrane is termed as hydrophilic up to a maximum contact angle of 90 degrees). For a flavor ester water (ethyl acetate, ethyl propionate, ethyl butyrate) binary mixture with 0.15 0.6 wt.% flavor, the flavor flux (g m22 h21)/selectivity was 32 380/56 95, 53.7 482/75 114, and 55 512/89 131 for ethyl acetate, ethyl propionate, and ethyl butyrate, respectively, at a feed temperature of 30 C 50 C [46]. The nanoaluminabased ceramic membrane is characterized by X-ray photoelectron spectroscopy (XPS) with Ar 1 sputtering. The XPS study of silica (SiO2) coated alumina membrane showed the atomic concentration of silica and alumina in terms of Al 2p and silica 2p orbital as a function of depth from the surface to the inside layer (Fig. 10.3A). The shifting of Al 2p spectra from the bulk alumina layer towards the SiO2 surface indicates incorporation of Al atoms into the silica matrix [39]. The PMNCs membranes are characterized like other nanocomposites. Younssi et al. [44] characterized grafting of organosilane on γAl2O3 by comparing FTIR and TGA analysis of grafted and ungrafted γAl2O3 membrane. The functionalization of γAl2O3 by silanes is confirmed by the appearance of C-H band at 3000 cm21, Si-C band at 1300 cm21, and Si-O band at 950 cm21 due to di or trifunctional chloro, ethoxy, or methoxysilanes. These absorption bands were not found in the ungrafted membrane while DTA analysis showed the presence of an additional exothermic peak at 500 C corresponding to silane grafting.

10.4.8

Pervaporation using titanium nanoparticlebased polymer metal nanocomposite membrane

Titanium dioxide (TiO2) exists as rutile, anatase, or brookite polymorph. Rutile is the thermodynamically most stable phase. Anatase is most stable at a size of less than 11 nm, brookite is

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Chapter 10 Nanometal and metal oxide-based polymer nanocomposite membranes for pervaporation

Figure 10.3 (A) XPS depth profile of Al 2p and Si 2p spectra of alumina/silica interfacial region [36]. (B) Effect of TGDNM NPs on flux and separation factor of 90% alcohol for chitosan-based PMNC membrane [47]. NPs, Nanoparticles; PMNC, polymer metal nanocomposite; XPS, X-ray photoelectron spectroscopy.

most stable at sizes between 11 and 35 nm, while rutile is stable at sizes above 35 nm [48]. Bulk or micro titanium oxides are mainly used as a white pigment in paints, plastics, papers, and cosmetics while transparent nano or fine powder of titanium dioxide is used in many applications, for example, for purification of water and air because of its photocatalytic ability to decompose organics and kill bacteria [49]. It is also used as a part of ceramic and polymer membranes [6]. Nanotitanium dioxide is prepared industrially by chloride or sol gel process from naturally occurring mineral ore rutile (TiO2) or ilmenite (FeTiO3). Similar to nanoalumina-polymer composite, most of the titanium dioxide polymer nanocomposites is also prepared by (1) direct mixing of the nanotitanium dioxide particles with the polymer matrix in a solvent, (2) sol gel technique, or by (3) in situ polymerizations of a monomer in a solvent containing titanium dioxide NPs. Direct mixing causes agglomeration of the particles which is eliminated by its surface modification in a similar way as practiced for nanoalumina, that is, by treating with a surfactant molecule or forming a covalent bond. In comparison, the sol gel technique or in situ polymerization gives much better mixing with a uniform dispersion of the titanium dioxide NPs in the polymer matrix. TiPMNCs membrane has

Chapter 10 Nanometal and metal oxide-based polymer nanocomposite membranes for pervaporation

been reported to increase permeability and resistance to fouling for first generation membrane processes like NF, UF, and MF [6]. These membranes have also been used in second generation membrane processes such as gas separation and PV. Madaeni et al. [50] reported the selectivity of carbon dioxide/ methane and carbon dioxide/nitrogen as 8.6 and 3.4, respectively and carbon dioxide permeance of 188.7 GPU for a ternary feed mixture of carbon dioxide/nitrogen/methane through a polyethersulfone membrane embedded with 0.7 wt.% nanotitanium dioxide and coated with PDMS rubber. Sairam et al. [51] reported PVA TiO2 nanocomposite membrane for PV dehydration of isopropanol. The TiO2 NP, as well as polyaniline coated TiO2 NPs, was incorporated in PVA in its aqueous solution followed by crosslinking of PVA with glutaraldehyde. These membranes showed very high water selectivity though the total flux obtained for dehydration of 90 wt.% IPA was much less than with other reported works. The coating of TiO2 with polyaniline showed marginally higher flux than the membrane prepared with uncoated TiO2. Lokesh et al. [52] reported similar membrane for pervaporative dehydration of 1,4-dioxane where sodium alginate was used instead of PVA. Polyaniline coated nanotitanium dioxide increased the selectivity of the composite membrane significantly though the presence of the NP in the alginate decreased flux. Yang et al. [47] reported in situ synthesis of chitosan-nanotitanium dioxide PMNC membrane by sol gel technique with tetrabutyltitanate (TBT) as a precursor. In this method TBT, acetyl acetone and ethanol with a molar ratio of 1:0.7:4 were added to an aqueous solution (with 2 wt.% acetic acid) of chitosan. The mixture after stirring for 2 h was cast on glass plate and dried for 48 h at room temperature followed by annealing in an oven at 120 C for 2 h to obtain the nanotitanium dioxide chitosan membrane which showed a flux of around 110 g m22 h21 and a separation factor for water of around 250 for azeotropic ethanol water (B95 wt.% ethanol) mixture. Zhu et al. [53] incorporated nanotitanium dioxide into cellulose by adding cotton linters, nanotitanium dioxide, urea, and sodium hydroxide in water. The slurry obtained was stored at 2 10 C in a refrigerator for 24 h and then taken out to room temperature to cast the suspension on a glass plate. It was then immediately immersed in 5 wt.% sulfuric acid solution for phase inversion. The resulting regenerated cellulosenanotitanium dioxide hydrophilic membrane showed a high flux of 1.8 kg m22 h21 and a separation factor of 55,091 for water at 50 C for 50 wt.% caprolactam in water. Organophilic membranes were also prepared from nano-TiO2 polymer

251

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Chapter 10 Nanometal and metal oxide-based polymer nanocomposite membranes for pervaporation

composite. Organo titanium compound was used to prepare NP. Nanosized organo titanium compound containing hydrophilic carboxylic group, that is, titanium glycine-N,N-dimethylphosphonate (TGDMP) was synthesized by the reaction of titanium tetrachloride and N,N-bis(phosphonomethyl)-glycine in hydrochloric acid for 24 h. The precipitate of TGDMP was filtered, washed with distilled water till the pH of the filtrate was around 3 3.5, and it was then dried over P2O5. This nanoTGDMP was incorporated in the matrix of chitosan in aqueous acetic acid solution by stirring at 60 C for 3 h. It was then cast on a dry glass plate and dried at 50 C to obtain the membrane. This hydrophilic membrane containing 1.2 mass% of TGDMP demonstrated the highest separation factor of 1050 for water with a permeation flux of 7.37 3 1022 kg/m2 h at 30 C for a binary feed mixture of isopropanol water containing 10 mass% water as shown in Fig. 10.3B [54]. From the above results, it is evident that the presence of nanotitanium dioxide increases water selectivity at the cost of flux. However, both flux and selectivity increases when the same nanometal oxide is incorporated into an organophilic membrane, and the composite membrane is used for separation of an organic. Shi et al. [55] blended organophilic PDMS rubber with nanotitanium hydroxide and coated it on polyvinylidene fluoride UF membrane. The PDMS membrane showed a flux of 115.52 g m22 h21 and an organic selectivity of 10.66 from an aqueous solution of formaldehyde containing 1000 ppm formaldehyde which increased to a flux of 187.72 g m22 h21 and a formaldehyde selectivity of 11.25 for nanotitanium dioxide incorporated PDMS membrane. The particle size distribution of nanotitanium oxide, as well as its distribution in the polymer matrix, is evaluated by SEM and TEM analyses. FTIR analysis shows a change of intensity and wavelength of absorption of the functional groups of the polymer due to its interaction with available hydroxyl groups of titanium dioxide. Further, it shows the appearance of an additional absorption band at around 850 cm21 due to the vibration of Ti-O-Ti bond [54]. The crystalline structure of TiO2 is also lost as it is incorporated in a polymer matrix [47] where it interferes with the order packing of the polymer [56]. The surface roughness of the nano-TiO2filled polymer membrane is evaluated by AFM. As the wt.% of TiO2 in the membrane matrix increases, surface roughness also increases [53]. The DSC analysis shows shifting of onset of the endothermic peak of a crystalline polymer, that is, PVA to a lower value after incorporation of TiO2 due to a loss in its ordered structure [51].

Chapter 10 Nanometal and metal oxide-based polymer nanocomposite membranes for pervaporation

10.4.9

Gold nanoparticle-based polymer metal nanocomposite membrane

Similar to AgNP, AuNP is another noble NP which is different from other NPs such as semiconductor quantum dots, magnetic NPs, or polymeric NPs in that these noble NPs possess the unique property of single SPR which may be suitably used for many applications including PV. AuNP is synthesized by different methods [57] while colloidal gold is prepared by citrate reduction method [58].

10.4.9.1

Plasmon pervaporation

The SPR properties of nanosized noble metals such as silver or gold may be advantageously used in plasmon PV. In PV, the increase in feed temperature increases permeability at the cost of selectivity of the membrane. However, the local heating of the polymer nanocomposite membrane containing silver or gold NPs due to SPR can increase membrane flux without the need of heating the entire feed reservoir with little effect on solute rejection for NF [59] or selectivity for PV [60]. Gold nanocomposite (AuNCMs) was synthesized by mixing tetrachloroauric acid in varying concentrations (0.1%, 0.4%, 0.6%, and 0.75% Au by mass) in uncured PDMS rubber to promote Au reduction to AuNPs facilitated by the silicon-hydride active sites in the PDMS crosslinker [61]. Laser irradiation of Au incorporated PMNC in the plasmonic PV resulted in linear flux enhancements up to 117% which increased with increase in laser power and Au concentration in the membrane as shown in Fig. 10.4 [60].

10.4.10

Polymer metal nanocomposites based on nano-MgO and ZnO

There are also few reports on PV using zinc oxide and magnesium oxide-based PMNC membranes. Crystalline zinc oxide exists as hexagonal wurtzite or cubic zinc blend. Most of the nanozinc oxide is available as 1D nanorods, nanowires, nanobelts, nanoribbons, or nanocombs. Some of the nanozinc oxides also exist as 2D nanopellets, nanosheet, or nanoplate or 3D dandelion, snowflakes or flower. It is prepared by conventional sol gel, hydrothermal or microemulsion and also by metal organic chemical vapor deposition, controlled precipitation or by RF plasma synthesis technique [60]. Similarly nanocrystalline MgO is another important nanometal oxide because of its optical transparency, low heat capacity, and high chemical and

253

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Chapter 10 Nanometal and metal oxide-based polymer nanocomposite membranes for pervaporation

1.2 0.0% 0.1% 0.4% 0.6%

Fractional flux increase

1 0.8 0.6 0.4 0.2

Figure 10.4 Fractional increase of flux with laser power and gold concentration in the membrane [59].

0 0

500 250 Laser power (mW)

750

thermal stability. It is prepared by sol gel, hydrothermal, and microemulsion method [61,62]. Jiang et al. [63] reported the synthesis of MgO NP incorporated polyimide [Matrimid made from 3,3,4,4-benzophenone tetracarboxylic dianhydride and 5(6)amino-1,-(4-aminophenyl-1,3-trimethylindane) of Ciba Polymer, USA] mixed matrix membrane (MMM) with loading of MgO up to 50 wt.% where the generation of voids between the polymer and the inorganic particle as usually found in MMM was eliminated by heating the cast membrane at 170 C under partial vacuum by nitrogen purging for 1 h followed by peeling off the membrane to place it in between wire mesh and further heating at 200 C for 24 h. As the membrane was placed between two wire mesh, it was equally heated on both sides to eliminate nonuniform distribution of NPs and voids in the membrane. The PV results with dehydration of 82 wt.% isopropanol showed higher permeability for the same membrane prepared with conventional process, that is, heating the membrane with its one surface attached to the casting glass plate. However, the membrane prepared by keeping both side open in a wire mesh showed lower permeability but higher selectivity for water. Fast evaporation of the solvent at high temperature from a cast membrane leads to immediate vitrification of the polymer chains with tight and dense structure resulting in high selectivity. The rate of solvent evaporation is slow on the side attached to the casting plate which allows the polymer chains to coil and forming a loosely packed dense structure for conventional membranes [64]. This explanation may be confirmed by analysis of surface morphology by SEM and surface roughness by AFM.

Chapter 10 Nanometal and metal oxide-based polymer nanocomposite membranes for pervaporation

10.5

255

Conclusions

From the above discussion, it transpires that the presence of small amounts of nanometal oxides or its salts in a polymer membrane significantly improves its mechanical strength, permeability and separation potential. However, metal-polymer compatibility factor markedly affects the overall performance of the composite membranes. The performance of the various polymer metal nanocomposite membranes for PV separation of organic water and organic organic mixtures are shown in Table 10.1. It is evident from the results that these nanocomposite membranes show very high selectivity with moderate to high flux. In general, nanometal oxide particles of titanium, aluminum, and iron shows very high selectivity and high flux as also observed in Table 10.1. However, the comparison of the

Table 10.1 Pervaporation performances of various polymer-metal nanocomposite membranes. Mixture to Membrane material/ be thickness separated/ conc. (wt.%) IPA/water, 90/10 1,4-Dioxane/ water, 90/10 Acetonitrile/ water, 87/13 (azeotrope) Ethanol/water, 90/10 IPA/water, 30/70 1,4-Dioxane/ water, 90/10 Caprolactum/ water, 50/50 Ethanol/water, 95/5 Toluene/ heptane, 50/50

PVA-iron oxide/10 μm

Flux Selectivity Temperature Reference (kg m22 h21) ( ) (˚C)

0.079

470

30

[29]

0.084

144

30

[29]

PVA-iron oxide/normalized to 1 μm

2.4

272

35

[30]

PAA Fe3O4 2 sodium alginate/0.385 μm PVA polyaniline-TiO2/10 μm

1.63

1044

76

[32]

0.423

Infinity

30

[51]

Na-Alg/PANI coated TiO2

0.034

12,848

30

[52]

Regenerated cellulose-TiO2/ 40 μm TiO2/L-b-L Assembled PEI/PAA

1.8

55,091

55

[53]

0.865

17,254

60

[65]

0.25

20

40

[66]

Metal organic nanocage in Boltron w3000 hyperbranched polymer coating on alumina/ 30 nm

(Continued )

256

Chapter 10 Nanometal and metal oxide-based polymer nanocomposite membranes for pervaporation

Table 10.1 (Continued) Mixture to Membrane material/ be thickness separated/ conc. (wt.%) Toluene/ heptane, 50/50 Methyl tertbutyl ether/ methanol, 31/ 69 Methyl tertbutyl ether/ methanol, 50/ 50 Thiophene/noctane,0.13/ 99.87 Thiophene/noctane,0.13/ 99.87 Benzene/ cyclohexane, 50/50 Benzene/ cyclohexane, 50/50 Benzene/ cyclohexane, 50/50 Model gasoline with 85 μg g21 sulfur Toluene/nheptane, 10/90 IPA/water, 90/ 10

Flux Selectivity Temperature Reference (kg m22 h21) ( ) (˚C)

PVA-Cu salt/6 μm

0.133

18

40

[67]

Cellulose acetate-Al2O3 Cellulose acetate-ZnO/ normalized to 1 μm

2.8 4

400 170

40 40

[68]

Polyamide-6/Al2O3 Polyamide-6/TiO2 Polyamide-6/ZrO2/normalized to 1 μm Polyether block amide-Fe2O3/ Cu1/1.8 μm

0.476 0.327 0.400

20 11 46

30 30 30

[69]

1.1

60

[70]

13.4

Polydimethylsiloxane rubber-Cu salt/28 37 μm

6.06

5.2

40

[71]

Poly(methyl acrylate-costyrene) 1 AgCl/25 μm

1.94

27

30

[72]

Polyurethane-Ag/multiwalled carbon nanotube/nm

2.4

64.8

30

[73]

Polyimide-Ag-graphene oxide/ 25 μm

1.6

35

30

[74]

Ethyl cellulose 1 TiO2/10 μm

7.5

1.3

80

[75]

Polyether block amide-cobalt acetate/3 μm Polyamide-TiO2-ceramic thin film composite membrane/ 230 nm

0.77

7.75

40

[76]

6.44

. 12,000

60

[77]

PAA, Polyacrylic acid; PVA, polyvinyl alcohol.

Chapter 10 Nanometal and metal oxide-based polymer nanocomposite membranes for pervaporation

relative performance of these nanometal-based polymer composite membranes is difficult since flux or selectivity also strongly depends on the type of feed mixture to be separated, the polymer used for making the membrane and also the process conditions employed. Thus the same iron oxide-polyvinyl alcohol nanocomposite membranes show much higher water selectivity (470) for 90 wt.% IPA than for 90 wt.% 1,4-dioxane (176) with comparable flux under similar process condition as observed in Table 10.1. Similarly with 6 wt.% loading of each of the nanometal oxides of zirconium, aluminum, and titanium in polyamide membranes, ZrO2-based membrane showed the best results in terms of both flux and selectivity for PV separation of 50 wt.% methanol in MTBE as observed in Table 10.1. It is also evident that despite immense prospect some nanometal oxides like ZrO2, ZnO, MgO, and CuO are yet to be explored widely for making polymer nanocomposite membranes for PV. Similarly Plasmon PV with polymer nanocomposite membranes containing noble nanometals like gold or silver may reduce the cost because of increased flux and selectivity.

Acknowledgment The authors are grateful to the Department of Biotechnology (DBT, BT/ PR5757/PID/6/709/2012/63) and Council for Scientific and industrial research (CSIR, 22(0746)/17/EMR-II), and Government of India for financial supports.

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Chapter 10 Nanometal and metal oxide-based polymer nanocomposite membranes for pervaporation

[26] S. Chaudhari, Y.S. Kwon, M.J. Moon, M.Y. Shon, S.E. Nam, Y. Park, Bull. Korean Chem. Soc. (2016) 1 7. [27] Y. Li, T. Verbiest, R. Strobbe, I.F.J. Vankelecom, Improving the performance of pervaporation membranes via localized heating through incorporation of silver nanoparticles, J. Mater. Chem. A 1 (2013) 15031. [28] L. Mohammed, H.G. GomaaT, D. Ragab, J. Zhu, Magnetic nanoparticles for environmental and biomedical applications: a review, Particuology (2016). [29] M. Sairam, B.V.K. Naidu, S.K. Nataraj, B. Sreedhar, T.M. Aminabhavi, Poly (vinyl alcohol)-iron oxide nanocomposite membranes for pervaporation, dehydration of isopropanol, 1,4-dioxane and tetrahydrofuran, J. Memb. Sci. 283 (2006) 65 73. [30] M.K. Mandal, S.B. Sant, P.K. Bhattacharyaa, Dehydration of aqueous acetonitrile solution by pervaporation using PVA iron oxide nanocompositemembrane, Colloids Surf., A: Physicochem. Eng. Asp. 373 (2011) 11 21. [31] M. Olukman, O. Sanlı, E.K. Solak, Synthesis of magnetite in poly (vinyl alcohol) matrix and its use in separation of acetone/water mixtures via pervaporation, vapor permeation with and without temperature difference methods, Vacuum 120 (2015) 107 115. [32] C. Zhao, Z. Jiang, J. Zhao, K. Cao, Q. Zhang, F. Pan, High pervaporation dehydration performance of the compositemembrane with an ultrathin alginate/poly(acrylic acid) 2 Fe3O4 active layer, Ind. Eng. Chem. Res. 53 (2014) 1606 1616. [33] G. Dudek, M. Gnus, A. Strzelewicz, R. Turczyn, M. Krasowska, The influence of metal oxides on the separation properties of hybrid alginate membranes, Sep. Sci. Technol. 53 (8) (2018) 1178 1190. [34] M. Das, P. Dhak, S. Gupta, D. Mishra, T.K. Maiti, A. Basak, et al., Highly biocompatible and water dispersible, amine functionalized magnetite nanoparticle, prepared by a low temperature, air assisted polyol process: a new platform for bio-separation and diagnostics, Nanotechnology 21 (2010) 125103. [35] B. Gao, Z. Jianga, C. Zhao, H. Gomaac, F. Pan, Enhanced pervaporative performance of hybrid membranes containing Fe3O4@CNT Nanofillers, J. Membr. Sci. 492 (2015) 230 241. [36] G. Dudek, M. Gnus, R. Turczyn, A. Strzelewicz, M. Krasowska, Pervaporation with chitosan membranes containing iron oxide nanoparticles, Sep. Purif. Technol. 133 (2014) 8 15. [37] G. Dudek, M. Krasowska, R. Turczyn, M. Gnus, A. Strzelewicz, Structure, morphology and separation efficiency of hybrid Alg/Fe3O4 membranes in pervaporative dehydration of ethanol, Sep. Purif. Technol. 182 (2017) 101 109. [38] H.R. Andrew, S.K. Bhargava, C.S. Grocott, The surface chemistry of Bayer process solids: a review, Colloids Surf. A: Physicochem. Eng. Asp 146 (1 3) (1999) 359 374. [39] J. Sekulic, J.E. ten Elshof, D.H.A. Blank, Separation mechanism in dehydration of water/organic binary liquids by pervaporation through microporous silica, J. Membr. Sci. 254 (2005) 267 274. [40] T. Bowen, S. Li, R.D. Noble, J.L. Falconer, Driving force for pervaporation through zeolite membranes, J. Membr. Sci. 225 (2003) 165. [41] X. Yang, L. Ding, M. Wolf, F. Velterop, H.J.M. Bouwmeester, S. Smart, et al., Pervaporation of ammonia solution with γ-alumina supported organosilica membranes, Sep. Purif. Technol. 168 (2016) 141 151. [42] M.C. Duke, R. Campbell, X. Cheng, A. Leo, J.C. Diniz da Costa, Characterization and pervaporation study on ethanol separation membranes, Dry. Technol. Int. J. 27 (4) (2009) 538 541.

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[43] W.J. Chen, P. Aranda, C.R. Martin, Pervaporation separation of ethanol/ water mixtures by polystyrenesulfonate/alumina composite membranes, J. Membr. Sci. 107 (1995) 199 207. [44] S.A. Younssi, A. Iraqi, M. Rafiq, M. Persin, A. Larbot, J. Sarrazin, Alumina membranes grafting by organosilanes and its application to the separation of solvent mixtures by pervaporation, Sep. Purif. Technol. 32 (2003) 175 179. [45] B.G. Park, Pervaporation characteristics of polyetherimide/g-alumina composite membrane for a quaternary equilibrium mixture of acetic acid-ethanol-ethyl acetate-water, Korean J. Chem. Eng. 21 (4) (2004) 882 889. [46] K.H. Song, K.R. Lee, J.M. Rim, Pervaporation of esters with hydrophobic membrane, Korean J. Chem. Eng. 21 (3) (2004) 693 698. [47] D. Yang, J. Li, Z. Jiang, L. Lu, X. Chen, Chitosan/TiO2 nanocompositepervaporation membranes for ethanol dehydration, Chem. Eng. Sci. 64 (2009) 3130 3137. [48] H. Arora, C. Doty, Y. Yuan, J. Boyle, K. Petras, B. Rabatic, et al., Titanium dioxide nanocomposites, nanomaterials for the life sciences, in: C.S.S.R. Kumar (Ed.), Nanocomposites, vol. 8, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, 2010. [49] H. Choi, S.R. Al-Abed, D.D. Dionysiou, Nanostructured titanium oxide filmand membrane-based photocatalysis for water treatment, in: Savage, et al. (Eds.), Nanotechnology Applications for Clean Water, William Andrew, 2009, pp. 39 46. [50] S.S. Madaeni, M.M.S. Badieh, V. Vatanpour, N. Ghaemi, Effect of titanium dioxide nanoparticles on polydimethylsiloxane/ polyethersulfonecompositemembranes for gas separation, polym. Eng. Sci. 52 (12) (2012) 2664 2674. [51] M. Sairam, M.B. Patil, R.S. Veerapur, S.A. Patil, T.M. Aminabhavi, Novel dense poly(vinyl alcohol) TiO2 mixed matrix membranes for pervaporation separation of water isopropanol mixtures at 30 C, J. Membr. Sci. 281 (2006) 95 102. [52] B.G. Lokesh, K.S.V.K. Rao, K.M. Reddy, K.C. Rao, P.S. Rao, Novel nanocomposite membranes of sodium alginate filled with polyanilinecoated titanium dioxide for dehydration of 1,4-dioxane/water mixtures, Desalination 233 (2008) 166 172. [53] T. Zhu, Y. Lin, Y. Luo, X. Hu, W. Lin, P. Yu, et al., Preparation and characterization of TiO2-regenerated celluloseinorganic polymer hybrid membranes for dehydration of caprolactam, Carbohydr. Polym. 87 (2012) 901 909. [54] H.G. Premakshi, A.M. Sajjan, M.Y. Kariduraganavar, Development of pervaporation membranes using chitosan and titanium glycine-N, Ndimethylphosphonatefor dehydration of isopropanol, J. Mater. Chem. A 3 (7) (2015) 3952 3961. [55] D. Shi, P. Li, B. Cao, Preparation of PDMS/PVDF compositepervaporation membrane modified with hydrophobic TiO2 nanoparticles forseparating formaldehyde solution, Polym. Sci. 2 (1) (2016) 1 7. [56] G. Dhanuja, S. Sridhar, B. Smitha, Pervaporation of isopropanol/water mixtures through polyion complex membranes, Sep. Purif. Technol. 44 (2004) 130 138. [57] A.N. Madu, P.C. Njoku, G.N. Iwuoha, U.M. Agbasi, Synthesis and characterization of gold nanoparticles using 1-alkyl, 3-methyl imidazolium based ionic liquids Int, J. Physic. Sci. 6 (2011) 635 640.

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[58] P.C. Chen, S.C. Mwakwari, A.K. Oyelere, Gold nanoparticles: from nanomedicine to nanosensing, Nanotechnol. Sci. Appl. 1 (2008) 45 65. [59] K. Vanhercka, I. Vankelecoma, T. Verbiest, Improving fluxes of polyimide membranes containing gold nanoparticles by photothermal heating, J. Membr. Sci. 373 (2011) 5 13. [60] A. Russell, Plasmonic Pervaporation via Gold Nanoparticle-Functionalized Nanocomposite Membranes (Ph.D. thesis), University of Arkansas, Fayetteville ScholarWorks@UARK, 8-2012. [61] Q. Zhang, J. Xu, Y. Liu, H. Chen, In-situ synthesis of poly(dimethylsiloxane)gold nanoparticles composite films and its application in microfluidic systems, Lab. Chip 8 (2008) 352 357. [62] A.C. Mohan, B. Renjanadevi, Preparation of zinc oxide nanoparticles and its characterization using scanning electron microscopy (SEM) and x-raydiffraction (XRD), Procedia Technol. 24 (2016) 761 766. [63] L.Y. Jiang, T.S. Chung, R. Rajagopalan, Matrimid1/MgO mixed matrix membranes for Pervaporation, AIChE, J. 53 (2007) 1745 1757. [64] I. MI, I.A.M. Saleh, M.R. Abdelhamid, Synthesis of MgO nanoparticles from different organic precursors; catalytic decontamination of organic pollutants and antitumor activity, J. Mater. Sci. Eng. 6 (2017) 1 18. [65] L.L. Gong, L. Zhang, N.X. Wang, J. Li, S.L. Ji, H.X. Guo, et al., In situ ultraviolet-light-induced TiO2 nanohybrid superhydrophilic membrane for pervaporation dehydration, Sep. Purif. Technol. 122 (2014) 32 40. [66] C. Zhao, et al., Hybrid membranes of metal organic molecule nanocages for aromatic/aliphatic hydrocarbon separation by pervaporation, Chem. Commun. 50 (2014) 13921 13923. [67] Y. Zhang, N.X. Wang, S.L. Ji, R. Zhang, C. Zhao, J.R. Li, Metal-organic framework/ poly(vinyl alcohol) nanohybrid membrane for the pervaporation of toluene/nheptane mixtures, J. Membr. Sci. 489 (2015) 144 152. [68] Y. Wang, L. Yang, G. Luo, Y. Dai, Preparation of cellulose acetate membrane filled with metal oxide particles for the pervaporation separation of methanol/methyl tert-butyl ether mixtures, Chem. Eng. J. 146 (2009) 6 10. [69] R. Kopec, M. Meller, W. Kujawski, J. Kujawa, Polyamide-6 based pervaporation membranes for organic-organic separation, Sep. Purif. Technol. 110 (2013) 63 73. [70] H. Ding, F.S. Pan, E. Mulalic, H. Gomaa, W.D. Li, H. Yang, et al., Enhanced desulfurization performance and stability of Pebax membrane by incorporating Cu 1 and Fe2 1 ions co-impregnated carbon nitride, J. Membr. Sci. 526 (2017) 94 105. [71] S.N. Yu, Z.Y. Jiang, H. Ding, F.S. Pan, B.Y. Wang, J. Yang, et al., Elevated pervaporation performance of polysiloxane membrane using channels and active sites of metal organic framework CuBTC, J. Membr. Sci. 481 (2015) 73 81. [72] L. Zhou, X.Q. Dai, J.J. Du, T. Wang, L.G. Wu, Y.C. Tang, et al., Fabrication of poly(MMA-co-ST) hybrid membranes containing AgCl nanoparticles by in situ ionic liquid microemulsion polymerization and enhancement of their separation performance, Ind. Eng. Chem. Res. 54 (2015) 3326 3332. [73] T. Wang, Y.Y. Jiang, J.N. Shen, L.G. Wu, B. Van der Bruggen, C.Y. Dong, Preparation of Ag nanoparticles on MWCNT surface via adsorption layer reactor synthesis and its enhancement on the performance of resultant polyurethane hybrid membranes, Ind. Eng. Chem. Res. 55 (2016) 1043 1052.

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[74] S.Q. Dai, Y.Y. Jiang, T. Wang, L.G. Wu, X.Y. Yu, J.Z. Lin, et al., Enhanced performance of polyimide hybrid membranes for benzene separation by incorporating three-dimensional silver-graphene oxide, J. Colloid Interface Sci. 478 (2016) 145 154. [75] Y.F. Hou, M. Liu, Y.Q. Huang, L.L. Zhao, J.F. Wang, Q. Cheng, et al., Gasoline desulfurization by a TiO2-filled ethyl cellulose pervaporation membrane, J. Appl. Polym. Sci. 134 (2017) 9. [76] Y. Zhang, N.X. Wang, C. Zhao, L. Wang, S.L. Ji, J.R. Li, Co(HCOO)(2)-based hybrid membranes for the pervaporation separation of aromatic/aliphatic hydrocarbon mixtures, J. Membr. Sci. 520 (2016) 646 656. [77] X.W. Liu, Y. Cao, Y.X. Li, Z.L. Xu, Z. Li, M. Wang, et al., High-performance polyamide/ceramic hollow fiberTFC membranes with TiO2 interlayer for pervaporation dehydration of isopropanol solution, J. Memb. Sci.

Modified zeolite-based polymer nanocomposite membranes for pervaporation

11

I.G. Wenten1,2, K. Khoiruddin1, G.T.M. Kadja2,3,4, Rino R. Mukti2,3,4 and Putu D. Sutrisna5 1

Department of Chemical Engineering, Institut Teknologi Bandung, Bandung, Indonesia 2Research Center for Nanosciences and Nanotechnology, Institut Teknologi Bandung, Bandung, Indonesia 3Division of Inorganic and Physical Chemistry, Institut Teknologi Bandung, Bandung, Indonesia 4Center for Catalysis and Reaction Engineering, Institut Teknologi Bandung, Bandung, Indonesia 5Department of Chemical Engineering, University of Surabaya (UBAYA), Surabaya, Indonesia

11.1

Introduction

In pervaporation, the membrane is used as a selective barrier for separating feed and permeate streams which are in liquid and vapor phases, respectively. Component to be separated migrates across the membrane and vaporizes while reaches the permeate phase due to vacuum pressure. Pervaporation is mainly applied for the separation of water/organic mixtures, but there are also efforts to utilize this process for desalination [1
7]. As the key factor of the process, attempts have been made to synthesize membrane with desirable separation properties. Various types of membranes have been developed, such as polymeric, inorganic, and hybrid membranes [8
10]. Polymeric membranes offer numerous advantages, such as economically affordable as well as reasonably good permeability and selectivity. However, using these traditional membranes, one cannot overcome the polymer upper bound between selectivity and permeability which seems to have undergone “saturation” judging from the fact that it only gives a slight increase despite the extensive works which have been devoted for a long time [11
13]. On the other hand, zeolite membranes provide significantly higher permeability and selectivity with other advantages, including superior chemical and thermal stabilities. Polymer Nanocomposite Membranes for Pervaporation. DOI: https://doi.org/10.1016/B978-0-12-816785-4.00011-2 © 2020 Elsevier Inc. All rights reserved.

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Nevertheless, zeolite membranes face an economic barrier because of the higher cost and poor processability. Therefore there should be a breakthrough improvement which combines the advantages of polymer and zeolite inside mixed matrix membranes (MMMs). MMMs are inorganic
organic composites that consist of inorganic materials, in this discussion limited to zeolite, dispersed in an organic polymer matrix as the continuous phase. Aside from combining advantages of both constituents, the production of MMMs actually can be adapted in the current polymer membrane fabrication technology. Combining the advantages of zeolite and polymer into a membrane means to integrate the molecular sieving and surface diffusion mechanisms within the zeolite membranes with the solution-diffusion mechanism of polymer membranes. Hence, the selectivity and permeability of the MMMs can be further enhanced. It should be noted that the actual performance of MMMs is not a simple combination of the properties of each constituent, but rather it includes many factors which should be taken into account, for example, the particle size and surface area of zeolite and the hydrophobic and hydrophilic nature of zeolite and polymer. The main issue in the fabrication of the MMMs is how to obtain a suitable and strong interaction between zeolite and polymer [8,14,15]. This chapter summarizes the latest development of zeolite-based polymer nanocomposite membranes for pervaporation, including preparation methods, separation performances, water/alcohol separation mechanisms, and challenges of zeolite-filled polymeric membrane fabrication.

11.2

Water and alcohol-selective zeolites

Zeolite is one of the interesting materials for pervaporation since it has uniform pore sizes and tailorable hydrophilic
hydrophobic properties. The uniform molecular pore size allows zeolite to achieve molecular separation with high selectivity. Meanwhile, the tailorable hydrophilic
hydrophobic characteristic provides the possibility to improve the separation factor by increasing the selective sorption toward the permeated component. By combining molecular sieving and selective sorption, the zeolite may achieve an excellent separation performance for pervaporation process [16]. Linde type A (LTA) zeolite is the most used membrane material for pervaporation due to its high separation factor (B10,000 for water
ethanol separation) [17]. LTA zeolite comprises aluminosilicates with Si to Al ratio of 1:1, resulting in high

Chapter 11 Modified zeolite-based polymer nanocomposite membranes for pervaporation

hydrophilicity. The eight-membered oxygen ring creates a maximum pore size of 0.42 nm which is close to the size of a water molecule (kinetic diameter of 0.296 nm [18]). The combination of high hydrophilicity and suitable pore structure enables LTA zeolite for obtaining high water selectivity. Generally LTA zeolite membrane is synthesized by growing LTA zeolite layer on a ceramic support. LTA zeolite membrane can be prepared via primary and secondary growth methods [17]. In primary growth method, the support undergoes hydrothermal processing. Thus, zeolite layer is formed in situ. In the secondary growth, the support is firstly coated by zeolite crystals or seeds before the hydrothermal processing step. Even though this type of membrane has high durability and thermal stability, it suffers from several disadvantages, such as high manufacturing cost which is attributed to the cost of support and complex processing steps and low selectivity due to defects formation [17,19]. The application of LTA zeolite for dehydration is also hindered by its hydrothermal stability due to dealumination phenomenon when operated at high water content [20]. Furthermore, the LTA structure is also affected by pH values. Dealumination may occur at acidic environment while operation at basic condition may lead to membrane damage [21,22]. Hence, neutral pH is preferred. T-type zeolite is another type of zeolite which can be used for pervaporation membrane, especially for dehydration processes. T-type zeolite has Si to Al ratio of 3:1 and 4:1 for erionite and effretite crystals, respectively [23]. The lower content of Al (than LTA) is expected to increase its hydrothermal stability and acid stability, but it yields in lower hydrophilicity. The larger pore size of T-type zeolite framework (0.36 nm 3 0.51 nm) leads to lower water selectivity than LTA [24,25]. T-type zeolite membrane has been commercialized by Mitsui Engineering & Shipbuilding Co., Ltd. [26]. Chabazite (CHA) zeolite has a maximum 3D pore size of 0.38 nm or in between of those in KA and NaA LTA zeolites [18]. This yields in an excellent water selectivity and over 270,000 separation factor (Si/Al 5 B3; feed: 8% water; 77 C) for ethanol dehydration [27]. In the separation of water/ethanol mixture with 86% water content, CHA with Si/Al of 7.5 can achieve 500 separation factor [28]. CHA zeolite framework may have different Si to Al ratios with the highest of 11:1 [19]. The high Si to Al ratios is believed to improve its organic acid resistance; thus, it can be used for organic acid separation, which is not applicable for LTA. However, the high Si content decreases the water flux of the membrane. DDR is all-silica zeolite, which is believed as a good alternative for LTA membrane even it is more hydrophobic. The all-silica

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Table 11.1 Zeolite type and characteristics. Zeolite

Characteristics

Reference

LTA T-type CHA DDR ZSM-5

Hydrophilic; pore size: 0.42 nm Hydrophilic; pore size: 0.36 nm 3 0.51 nm Hydrophilic; pore size: 0.38 nm Hydrophobic; pore size: 0.36 nm 3 0.44 nm Hydrophobic; pore size: 0.51 nm 3 0.57 nm

[18] [24,25] [18] [30] [34]

CHA, Chabazite; LTA, Linde type A.

structure makes DDR membrane is more hydrothermally stable than LTA. A long-term pervaporation test on dehydration of acetic acid confirmed that the DD3R zeolite was stable under the presence of inorganic acid [29]. Also, the all-silica DDR membrane displayed good water permeability of 20 kg m22 h21 at 344K
398K and water/ethanol selectivity of 1500 at 373K (0.18 water) [30]. The high silica content and small pore size (0.36 nm 3 0.44 nm) suggest that the separation of water in DDR is based on size exclusion [30]. This is different when compared to the LTA membrane, where water adsorption also occurs. ZSM-5, which has a mordenite framework inverted (MFI)-type structure, has been studied in developing pervaporation membrane as reported in several works [16,31
34]. In the ZSM-5 framework, the straight channels are elliptical with a pore size of 0.51 nm 3 0.57 nm, while the sinusoidal channels have a diameter of 0.54 nm [34]. With that structure, ZSM-5 is also believed as a good selective separator for small molecules. ZSM-5 is alcoholselective since it has a high Si/Al ratio, which prefers to sorb less polar compound from an aqueous mixture [35]. Silicalite-1 is aluminum-free zeolite, which preferentially adsorbs alcohol [36]. The hydrophobicity of silicalite-1 is similar to ZSM-5 zeolite [36]. These types of zeolite, that is, ZSM-5 and Silicalite-1, are suitable for the preparation of pervaporation membrane used for alcohol removal from aqueous mixtures (Table 11.1).

11.3

Mechanism of water/alcohol separation in zeolite

The successful separation of water/alcohol mixture through the zeolite-filled nanocomposite membranes or MMMs is determined not only by the presence of zeolite particles and polymer

Chapter 11 Modified zeolite-based polymer nanocomposite membranes for pervaporation

matrices but also by the interaction between zeolite and polymer. To understand the thorough mechanism of water/ alcohol separation through the zeolite-filled nanocomposite membranes, the discussion of separation mechanism will be started from the mechanism of separation through the zeolite particles first and then followed by the explanation of separation mechanism through the composite membranes.

11.3.1

The separation mechanism in zeolite particles

The separation of components through zeolite particles can be elucidated by the adsorption of selected component(s) into the pores of zeolites and the diffusion of components along the surface of zeolite mechanisms [18]. In the adsorption process of components to the surface of the adsorbent, the adsorbates or components to be adsorbed are accumulated on the surface of the adsorbent through either physical or chemical adsorption. In the pervaporation process, in which the water/alcohol is mainly separated by composite membranes, physisorption, or physical adsorption dominates the adsorption mechanism of the component in zeolite particles [18]. According to physisorption mechanism, the extent of adsorption can be influenced by the interaction of adsorbent
adsorbate or adsorbate
adsorbate, the pores structure of the adsorbent, and the size of adsorbate molecules. The adsorption of adsorbate in zeolite particles is commonly quantified using the adsorption isotherm. The adsorption isotherm provides information about the amount of adsorbate adsorbed by adsorbent as the function of relative pressure, the pressure of adsorbate or fugacity at certain temperatures. The adsorption isotherm can be derived from the data of adsorption equilibrium, which is usually expressed in a mathematical model for adsorption equilibrium. Several models have been developed to explain the monolayer and multilayer adsorptions phenomena. For pure component adsorption: Henry’s law, Langmuir isotherm, and Freundlich isotherm are common models used to describe the adsorption behavior through the zeolite pores [18,37,38]. Henry’s law is usually utilized to designate the adsorption phenomenon at a relatively low pressure, where the adsorption loading is proportional to the adsorption pressure and can be formulated as: qi 5 KH;i pi ;

ð11:1Þ

267

268

Chapter 11 Modified zeolite-based polymer nanocomposite membranes for pervaporation

where qi is the surface coverage, pi is the partial pressure of component or adsorbate, and KH,i is the Henry constant for certain component in adsorbate. At low pressure, the surface coverage of adsorbent by the adsorbate is relatively low, thus produces a linear adsorption isotherm. At high pressure, the direct correlation between adsorption loading and pressure will not be valid. Hence Henry’s law is only applicable for low pressure of the adsorption process. Other models that can be applied to describe the adsorption isotherm through zeolite particles are Langmuir and Freundlich isotherms. The correlation for monolayer Langmuir isotherm is described in Eq. (11.2) [18,38]. qi 5

qsat;i  bi :fi ; 1 1 b i  fi

ð11:2Þ

where qsat,i is the saturation coverage, bi is an adsorption equilibrium constant, and fi is the fugacity of the component where can be substituted by the partial pressure of component for the case of gas adsorption. The equation for Freundlich isotherm is presented in Eq. (11.3) [18]. 1

qi 5 KF;i P n ;

ð11:3Þ

where KF,i and n are the constants that be governed by the nature of the adsorbent and adsorbate at a certain temperature. The adsorption process in zeolite particles, especially for the low molecular size of adsorbate component, generally follows the Langmuir adsorption isotherm as has been discussed thoroughly in Ref. [18]. For higher molecular size components, the model used to explain the adsorption process can be expanded by using the dual-site Langmuir model. While the models above have been used extensively to explain the single component adsorption through zeolite pores, the separation of water/alcohol mixture or multicomponent in zeolite pores requires a different model. For water/alcohol mixture, the adsorption mechanism can be elucidated by using extended Langmuir isotherm model, which is formulated in Eq. (11.4) [39], qi 5

qsat;i bi  pi : N P 11 bj pj

ð11:4Þ

j51

Eq. (11.4) can be applied in the case where the saturation loadings of the mixture are exactly the same. However, in many

Chapter 11 Modified zeolite-based polymer nanocomposite membranes for pervaporation

cases, the mixture of water/alcohol for separation in zeolite pores is not identical. Hence, for this case, other models should be employed, including the real adsorbed solution theory (RAST) and the ideal adsorbed solution theory (IAST). These two models are distinguished based on the ideality of the adsorbed phase [18,39]. In contrast to the adsorption process, the diffusion of adsorbates into the zeolite particles occurs mainly due to the potential difference within the pores. The extent of the diffusion process is induced by the interactions of the pores wall and the diffusing molecules. It is recognized that the diffusion of adsorbates into the pores of zeolite is a surface diffusion that is influenced by the gradient of the chemical potential of the adsorbates. Generally the diffusion process, along with the porous media, such as zeolite, can be explained using Fick’s first law as formulated in Eq. (11.5) [40]. J 52D

dC ; dx

ð11:5Þ

where J is the mass transfer flux, D is the diffusion coefficient or diffusivity, dC is the concentration gradient, and x is the position or length. The Fick’s first law shows the correlation between the diffusive flux and concentration under steady-state condition. The flux of component is influenced by the concentration or potential gradient along with the pores of porous media. When the adsorbate is adsorbed to the pores of zeolite, the mobility of the adsorbate could potentially make the adsorbate to jump to different active sites in the pores of the zeolite. This phenomenon requires the adsorbate molecules to surpass some energy barrier that can be represented by Eq. (11.6) [38]. ! dif 2Ei o Di 5 Di exp ; ð11:6Þ RT where Di is the diffusivity of component i, Dio is a preexponential factor, and Eidif is the activation energy of diffusion. The diffusivity represents the ability of the adsorbate molecules to move, and it can be different among adsorbates. In general, the diffusivity of adsorbate to the pores of zeolite depends on the molecular size of component or adsorbate. In addition to the adsorption and diffusion mechanisms, the separation of water/alcohol mixture can also be described by the molecular sieving mechanism. This postulation stems from the knowledge of the kinetic diameter of water and alcohol.

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Chapter 11 Modified zeolite-based polymer nanocomposite membranes for pervaporation

The kinetic diameter of water is 0.296 nm, while the kinetic diameters of some alcohol, such as ethanol and methanol are 0.430 and 0.380 nm, respectively [18,41]. The difference in molecular size between water and alcohol could potentially be exploited by finding suitable zeolite with suitable pore size.

11.3.2

The separation mechanism through mixed matrix membranes and inorganic fillers incorporated composite membranes

The water/alcohol separation using pervaporation process is conducted by employing polymeric, inorganic, or hybrid membranes [1,14,31
33,42
44]. In principle, the separation mechanism in pervaporation process is driven by several factors, such as the physical and chemical characteristics of the membranes and the feed solutions, the interactions among the components in the permeants, as well as the interactions between the permeant and membrane. The extent of the separation through the pervaporation membrane can be controlled by controlling the chemical potential gradient, which acts as the driving force for separation. The separation mechanism in pervaporation is mainly described by the solution-diffusion model. However, in some cases, the pore-flow model is also used to explain the separation mechanism [45
47]. In the solution-diffusion model, the separation and the interaction of permeant and membrane are modeled and described by three steps as presented in Fig. 11.1. The first step is the adsorption of liquid feed molecules onto the membrane surface at the feed side and then followed by the diffusion of penetrant through the membrane. The last step includes the desorption of the permeant or permeates at the permeate side [40,48]. In the desorption step, the permeate is in the vapor phase. The flux of the permeate through the membrane can be described by Fick’s first law as has been formulated by Eq. (11.5). The concentration gradient in Eq. (11.5) can be extended to ci0
civ, in which ci0 and civ are the concentration of component in the feed and permeate sides, respectively. The concentration of the component is the multiplication of vapor pressure (pi) and the solubility coefficient (Si) of the component in the membrane. Hence, Fick’s first law can be written as [46]: Ji 5

 Pi  0  Di :Si  0 pi 2 pvi 5 pi 2 pvi ; δ δ

ð11:7Þ

Chapter 11 Modified zeolite-based polymer nanocomposite membranes for pervaporation

Membrane

Feed liquid

271

Permeate [vapor]

Dissolution n sio f fu

Di

Evaporation

Figure 11.1 The schematic diagram showing the solutiondiffusion model.

d

where Pi 5 Di.Si, is the permeability coefficient. In the pervaporation process, the upstream or feed pressure is usually much larger than the permeate pressure, which is vacuum. Also, the concentration of penetrant is much smaller than feed concentration, and the upstream vapor pressure is the saturated vapor pressure (pio); hence Ji 5

Di :cii Pi :poi 5 : δ δ

ð11:8Þ

The successful separation in pervaporation process is measured as the separation factor (αi/j). The separation factor is basically the ratio of permeate concentration (yi/yj) and feed concentration (xi/xj) and can be calculated using Eq. (11.9) [32,43,44,49,50]. αi=j 5

yi =yj : xi =xj

ð11:9Þ

As can be seen from Eq. (11.7), the diffusivity and solubility coefficients play important rule to determine the extent of the pervaporation process. The solubility coefficient can be predicted by considering three factors, such as the hydrogen bonding interaction, the contribution of polar interaction, and the contribution of dispersion interaction. In water/alcohol separation, the hydrogen bonding interaction between water and alcohol is relatively high. Hence it is concluded that the separation of water from alcohol is feasible using the pervaporation process.

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In addition to the solution-diffusion model, the separation of water/alcohol mixture in the pervaporation process can also be explained by the pore-flow model. In contrast to the solutiondiffusion mechanism, the phase change in pervaporation is taken in to account in the pore-flow model or mechanism. The schematic diagram showing the pore-flow mechanism is depicted in Fig. 11.2. The pore-flow mechanism involves three consecutive steps, such as the transfer of liquid from the inner side of the pore to the interface between liquid and vapor, the evaporation process at the boundary of the phase, and the transfer of vapor from the boundary to the outer of the membrane pores. For the mathematical calculation of simple component, it is considered that the flux can be assumed the same in the liquid and vapor regions and can be formulated by using Darcy’s law as presented in Eq. (11.10) [51]. Jliquid 5

  Apore  liquid Bpore  sat p 2 psat 5 Jvapor 5 p 2 pvapor ; lliquid lvapor ð11:10:Þ

is determined by Darcy’s equation, B is calcuwhere A lated by simplified Henry’s law and monolayer adsorption, lliquid is the length of the pore-filled by liquid, psat is the saturated pressure, and pvapor is the length of pore-filled by vapor. pore

pore

Membrane

Feed liquid

Permeate [vapor]

d

da

db

Figure 11.2 The schematic diagram of the pore-flow mechanism.

Chapter 11 Modified zeolite-based polymer nanocomposite membranes for pervaporation

The extent of water/alcohol separation in pervaporation process is determined by the high values of permeate flux as well as the selectivity of the membranes. In principle, to obtain high flux, it is required to have a thin membrane. The thin membrane will reduce the resistance of mass transfer; however, as experienced by other membrane processes, there is also a trade-off between the flux and selectivity of the membranes. To produce a high flux through the membranes, we can employ a composite membrane that has a thin active layer and porous substrate. The thin layer acts as the separating layer and is usually in the form of a dense layer. On the other hand, the porous substrate provides an enhancement in the mechanical strength of the composite membrane. In recent years, the thin layer of pervaporation membrane has been synthesized by combining the polymer matrix and inorganic particles [14,43]. This combination tries to exploit the separation capabilities of both materials. In another line of research, the application of MMMs in pervaporation has just been started recently. In MMMs, the inorganic fillers, such as zeolite particles, are incorporated inside the polymer matrix. Hence, the separation mechanism inside the membranes will combine the separation mechanism of pure zeolite and pure polymeric materials. The successful pervaporation process using composite membranes and MMMs is determined mainly by the absence of voids and defects on the interface between the inorganic fillers and the polymer matrix. Voids and defects in polymer
particle combination in some cases can increase the flux through the membranes but will decrease the selectivity of the membranes. The formation of void and defect-free composite membranes and MMMs is therefore growing as an interesting field of research in the pervaporation process. The different morphologies of polymer
particles will determine the separation mechanism through the composite membranes, and MMMs are depicted in Fig. 11.3. The morphologies include ideal morphology, sieve-in-a-cage, the rigidification of polymer, and the blockage of particle pore [42,52,53]. These defects can be caused by several factors, such as the incompatibility between polymer and particle, the evaporation of solvent during membrane formation that stresses the interface of polymer
particle, and the weak adhesion between the particle and polymer [54]. As has been mentioned and presented in Fig. 11.3, four morphologies or cases in MMMs can be described as (1) idealized or “hard to obtain” morphology, (2) rigidified polymer layer morphology, (3) reduced permeability region within sieve morphology, and (4) voids at the interface morphology.

273

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Chapter 11 Modified zeolite-based polymer nanocomposite membranes for pervaporation

Figure 11.3 The morphologies and typical defects in MMMs. MMMs, Mixed matrix membranes.

Case 1 is usually explained by Maxwell model that is formulated by Eq. (11.11):   Pd 1 2Pc 2 2[d ðPc 2 Pd Þ Pmm 5 Pc ; ð11:11Þ Pd 1 2Pc 1 [d ðPc 2 Pd Þ where Pmm is the effective permeability of an MMMs, φ is the volume fraction, while c and d denote the continuous and dispersed phases, respectively. Case 2 demonstrates the formation of the rigidified region on the interface between the polymer matrix and inorganic particles. The rigidified region arises because of the stress experienced by two materials during the preparation of MMMs. When the rigidification of polymer occurs, it can be expected that polymer layer near the particle surface has low chain mobility that will improve the resistance to penetrants and will decrease the permeability and increase the membrane selectivity. This phenomenon can be observed by the increasing value of the glass transition temperature of the polymer. In case 3, the interface between polymer matrix and inorganic particles has a region with reduced permeability in the outer layer of the particles or the whole particles. In case 4 or sieve-in-a-cage morphology, membrane shows in increased permeability with the

Chapter 11 Modified zeolite-based polymer nanocomposite membranes for pervaporation

negligible change of the selectivity. Although this morphology is favorable, it will be difficult to synthesize such morphologies without a significant loss of selectivity.

11.4

Fabrication of zeolite-filled nanocomposite membranes

In the preparation of hybrid inorganic/organic membrane, the following suggestions should be considered [55,56]: • The inorganic filler should be small, well distributed in the polymer matrix, and no aggregation. In some cases, when the filler requires activation, the temperature should be suitable for the polymer stability. • The procedure for the preparation of polymer matrix should allow the particle to be easily dispersed in the polymer matrix. • Nanoparticles and polymer should have high compatibility to avoid the defect formation in the interfacial of nanoparticle and polymer. One of the important factors in the preparation of zeolitefilled nanocomposite membranes is zeolite loading. Zeolite membrane plays an important role in improving membrane selectivity. Thus, the selectivity would be proportional to zeolite content. However, more zeolite content will result in a loose membrane structure or more free volume in the membrane matrix [8]. The formation of more free volume may be associated with the weak interfacial adhesion between zeolites and the polymer matrix. This is undesirable because it will yield a considerable decrease in membrane separation factor even though the membrane will have a higher permeate flux. Inorganic/organic membranes can be prepared via several methods, such as blending, layer-by-layer self-assembly, in situ polymerization, sol
gel, and bioinspired methods [10,14,57
60]. In the blending method, inorganic particles were dispersed in a polymer solution before casting. Layer-by-layer assembly method consists of multilayer deposition of a selective layer on a support. In general, the layers are polyelectrolytes having positive or negative charges [61]. Polyelectrolytes are then deposited in multitimes on the membrane support. The compatibility between the layers is facilitated by electrostatic interaction of the charge. The sol
gel method involves the hydrolysis and polycondensation reactions of the inorganic precursors. These reactions take place in the polymer solution, and thus simultaneous polymer solidification and nanoparticles formation occur that form inorganic/polymer hybrid membrane.

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Unlike the blending method, in situ polymerization uses monomers instead of the polymer [31]. After the monomer and inorganic particles are mixed to form a homogenous solution, the solution undergoes polymerization. In this method, fillers are entrapped in the polymer matrix during the polymerization; thus, a well-dispersed filler is attained. Bioinspired method for fabricating organic
inorganic membrane involves the biomineralization process. For the biomineralization process, the biomineralizing agent made from biological or synthetic molecules and precursors such as inorganic salts or alkoxide molecules are used. Since biomineralization is carried out at the molecular level, uniform distribution of inorganic fillers can be attained. This also provides the possibility of controlling the inorganic filler size, structure, and chemical composition as well as interfacial interaction between the inorganic filler and polymer matrix. Among the methods above, blending is the most common and simplest method for fabricating inorganic-filled polymeric membrane. In general, different techniques to blend zeolite particles and polymeric solution include simple blending, zeolite blending followed by in situ polymerization, zeolite blending followed by cross-linking, and zeolite blending followed by heat treatment as illustrated in Fig. 11.4. Pervaporation membranes are typically fabricated into tubular and flat-sheets configurations. Zeolite nanoparticles can be incorporated into the membrane matrix by simply blending the particles into a polymer solution before casting to form a membrane. This method is suitable for the preparation of a flat sheet membrane. The membrane solution can be cast directly to form a free-standing membrane or cast on porous support to create a composite membrane. In the composite membrane, the cast membrane acts as the selective layer while the support improves the mechanical strength of the membrane—for instance, Guan et al. [62] fabricated zeolite/poly(vinyl) alcohol (PVA) membrane by preparing a suspension of zeolite/PVA aqueous solution. Fumaric acid, which acted as a cross-linking agent, was added to the suspension. After rigorous mixing, the suspension was cast on nonwoven fabric substrate fixed on a glass plate. When the phase inversion process was completed, the membrane was then heated in the oven at 150 C to induce cross-linking reaction. The pervaporation of 80% ethanol/water mixture at 60 C revealed that the cross-linking procedure successfully improved the membrane selectivity from 511 (for noncross-linked membrane) to 1297. The enhanced selectivity was attributed by the decrease of the polymer’s free volume due to the cross-linking reaction [8].

Chapter 11 Modified zeolite-based polymer nanocomposite membranes for pervaporation

277

Figure 11.4 The illustration of blending method. (1) Zeolite blending, (2) zeolite blending followed by in situ polymerization, (3) zeolite blending followed by cross-linking, and (4) zeolite blending followed by heat treatment.

The effect of polymer cross-linking on zeolite/polymer membrane was also investigated by Zhan et al. [31] for the synthesis of zeolite/PDMS membrane. They prepared a suspension containing zeolite, PDMS prepolymer, and n-hexane by stirring and ultrasonication treatment. Ultrasonication is usually aimed to improve the dispersion of particles and to avoid the agglomeration of nanoparticles during the preparation of the membrane casting solution [63
68]. Cross-linking agent, poly(phenyltrimethoxylsiloxane) (PTMOS), and catalyst, di-n-butyltin dilaurate, were added into the suspension. These reagents were used

278

Chapter 11 Modified zeolite-based polymer nanocomposite membranes for pervaporation

to induce the prepolymerization reaction of PDMS. The suspension was then cast on porous PVDF membrane support and dried subsequently. To obtain the complete cross-linking, the membrane was placed in an oven at 80 C for 5 h. One such problem that usually occurs during the fabrication of zeolitefilled membrane is the agglomeration of filler or zeolites. However, in this study, SEM characterization of the synthesized membrane showed no agglomeration of nanoparticles in the membrane matrix. Meanwhile, the cross-linking procedure could produce a membrane with better selectivity. However, the selectivity was dramatically reduced when the zeolite content was above 30%. They concluded that higher zeolite content would destroy the interfacial adhesion between zeolite particles and the polymer matrix. During the membrane fabrication process, heat treatment can be utilized to improve the separation properties of the synthesized membrane. Ahmad and Hä gg [69] examined the effect of pretreatment and posttreatment on the properties of zeolite 4A/polyvinyl acetate membrane. Zeolite-filled membrane prepared from calcined zeolite displayed improved permeability, selectivity, and thermal stability. The improved performance of produced membranes was attributed by the strong adhesion of zeolite/polymer due to the removal of adsorbed water from the zeolite during the calcination process at 500 C. They also found that the annealing of the prepared membrane could improve membrane selectivity but reduce its permeability. The loss of membrane permeability after annealing at higher temperature might be due to the formation of more rigid membrane structure. Therefore, they suggested to optimize the annealing temperature for improving membrane selectivity at reasonable flux. The preparation of zeolite/polymer membrane using the layer-by-layer method has been demonstrated by Kang et al. [61]. The layer-by-layer method was successfully used to synthesize zeolite-filled polymer membrane with relatively high particle loading between 30 and 60 wt.%. The membrane was prepared by the deposition of negatively charged poly(acrylic acid), positively charged polyethyleneimine, and LTA zeolite particles on polyacrylonitrile. The LTA zeolite was endowed with negative charge to improve zeolite/polymer compatibility. The electrostatic interactions between those components made the layer highly compatible. Tubular-type membrane, especially hollow fiber membrane, could provide higher packing density than other configurations. Hence pervaporation plant will have lower footprint. Hollow fiber membrane can facilitate better contact between the liquid

Chapter 11 Modified zeolite-based polymer nanocomposite membranes for pervaporation

phase in the feed and vapor phase in the permeate. Spinning technique can be used to prepare hollow fiber zeolite/polymer membrane. Ge et al. [70] synthesized zeolite/polymer-based hollow fiber membrane by firstly blending zeolite crystals into a polymeric solution. Then, polyethersulfone solution containing LTA zeolite crystals were spun to form hollow fiber membrane with 2.2 mm and 1.0 mm outer diameter and inner diameter, respectively. The composite hollow fiber membrane showed .10,000 selectivity and 9000 g m22 h21 in the pervaporation of 90% ethanol solution conducted at 60 C.

11.5

Zeolite
polymer compatibility

As the performances of zeolite/polymer composite membranes are affected by the compatibility of the zeolite particles and polymer materials, this section presents several aspects related to the issue of polymer
particles compatibility.

11.5.1

Predicting the combination of zeolite and polymer

To predict the permeability of the MMMs, several analytical models can be utilized. One of the most popular models is the Maxwell model or equation as presented in Eq. (11.12) [71,72].  Z  P P Z M P Pi 1 2Pi 2 2φðPi 2 Pi Þ Pi 5 Pi ; ð11:12Þ PiZ 1 2PiP 1 φðPiP 2 PiZ Þ where PiM , PiP , and PiZ are the permeability of component i in the MMMs, polymer, and zeolite, respectively, while φ is the volume fraction of the zeolite. This model assumes the zeolite particle to be spherical with a low to moderate concentration (φ , 0.3). Another model developed by Cussler considers the nonspherical shape of the zeolite which increases the tortuosity of the diffusion path with a moderate to high concentration (φ . 0.3). The Cussler model is shown in Eq. (11.13). 0 1 B PiM 5 PiP B @

 12φ1

1

C C A;

ð11:13Þ

1 ð1=φÞðPiZ =PiP Þ 1 4ðð1 2 φÞ=α2 φ2 Þ

where α is the zeolite aspect ratio or the ratio between the longest and the shortest dimension of zeolite particle. It should be stressed out that Cussler defines α as half of this ratio. Hence,

279

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Chapter 11 Modified zeolite-based polymer nanocomposite membranes for pervaporation

the equation differs accordingly. Cussler model is reasonably accurate when α is large. However, there are many combinations of α and φ in which Maxwell and Cussler model cannot successfully predict the MMMs performance. For this reason, the Cussler model is modified to fill other α and φ combinations, as depicted in Eq. (11.14). 0 1 B PiM 5 PiP B @

 12φ1

1

1 ð1=φÞðPiZ =PiP Þ 1 ðð1 2 φÞ=α2 φ2 Þ

C C A:

ð11:14Þ

In all models, the selectivity can be calculated as PiM =PjM . For example, we have a certain polymer, so-called polymer I, with a CO2 permeability and a CO2/CH4 selectivity of 20 Barrer and 20, respectively. We need to select the proper zeolite for the filler between zeolite C (CO2 permeability 5 60 Barrer, CO2/CH4 selectivity 5 100) and zeolite D (CO2 permeability 5 700 Barrer, CO2/CH4 selectivity 5 80). Suppose the volume fraction of zeolite to be 30%, we can predict the performance of the resulted MMMs, membrane I-C and I-D. Using the Maxwell model, it is seen that membrane I-C exhibits substantially higher selectivity than that of membrane I-D and pure polymer I (Fig. 11.5).

Figure 11.5 Selecting a decent zeolite for mixed matrix membranes based on the Maxwell equation. The black line indicates the polymer upper-bound limit.

Chapter 11 Modified zeolite-based polymer nanocomposite membranes for pervaporation

Hence, zeolite C is preferred than zeolite D as a filler to form MMMs with polymer I. Cussler and modified Cussler models can be used when another parameter, that is, the aspect ratio, is known.

11.5.1.1

Toward compatible zeolite
polymer mixed matrix membranes

The main concern in the preparation of zeolite
polymer MMMs is the compatibility between the zeolite and the polymer. The zeolite should be compatible with the continuous matrix so that the resulted MMMs are free of defects. This means full-coverage of the zeolite by the polymer as well as “just right” adhesion forces between these two phases, which is the ideal case. Incompatibility between the zeolite and the polymer could be, at least, three cases [53]. As depicted in Fig. 11.4, the first case (1) occurs through the densification of the polymer network at the interface resulted in a rigidified region. In this case, the permeability is reduced with or without increasing the selectivity according to the extent of rigidification. In the MMMs of polyethersulfone (PES) and zeolite A for O2 and N2 separation, it is predicted through the Maxwell equation that the permeability should increase with the addition of more zeolite A [73]. However, the actual experiments showed the opposite that the permeability decreased with the increase in zeolite loadings. It was found that the results were owing to the occurrence of polymer rigidification since the glass temperature (Tg) of the MMMs was higher than that of the pure PES. Zarshenas et al. [74] reported that in spite of the increase in selectivity, the polyether block amide (Pebax-1657)
nanozeolite X MMMs showed a decline in the gas permeability for the separation of CO2 from N2, and O2. This was also indicated by the increased of Tg after the addition of zeolite. The rigidification of polymer networks in the MMMs has also been reported by other researchers [75
78]. The second case (2) is when the polymer networks penetrate into the pore structure of zeolite, which block the pathway for the desired molecules. This situation leads to the suppressed molecular sieving functionality of zeolite, and hence the selectivity may be kept constant, but the permeability reasonably decreases. The first- and second cases are relatively difficult to be discriminated. Even, they are often found to occur simultaneously within the MMMs [73,79,80]. The third case (3), the most often to occur, is due to the low adhesion leading to the formation of defects and empty

281

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Chapter 11 Modified zeolite-based polymer nanocomposite membranes for pervaporation

interfacial voids which have a larger size compared to that of zeolite micropores. Consequently the penetrant molecules can easily pass through those voids, making no use of the zeolite. The permeability enhances significantly with the decrease in gas selectivity. Mahajan et al. [81] performed a comparative study on the preparation of MMMs using zeolite 4A as the filler and polyvinyl acetate (PVAc) and commercial polyimide (Matrimid) as polymer matrices. They found that PVAc could provide a full-coverage to zeolite A while Matrimid
zeolite A membranes displayed the observed leaky interface or the interfacial voids. The latter case was due to the incompatibility of Matrimid with the surface of zeolite 4A (Fig. 11.6). The nature of Matrimid is comparatively hydrophobic owing to the presence of both aliphatic and aromatic chains. On the other hand, zeolite 4A possesses hydroxyls (
OH) on its surface, which are relatively hydrophilic. Hence, their interaction is unfavorable, which leads to poor adhesion. It is crystal clear that the compatibility between zeolite, as the dispersed phase, and polymer, as the continuous phase, depends on the nanoscale morphology at the interface and dictates the performance of the MMMs (Fig. 11.7). The previously described analytical methods, that is, Maxwell, Cussler, and modified Cussler methods work accurately for the ideal case. Nevertheless, under the nonideal cases, they fail to accurately predict the performance of MMMs. The strategies to overcome the incompatibility issues rely on the use of either inorganic or organic agents to bridge the polymer and the zeolite.

1 µm PVAc–zeolite 4A

1 µm Matrimid–zeolite 4A

Figure 11.6 Cross-sectional SEM images of PVAc
zeolite 4A and Matrimid
zeolite 4A mixed matrix membranes. Source: Reprinted with the permission from R. Mahajan, R. Burns, M. Schaeffer, W.J. Koros, Challenges in forming successful mixed matrix membranes with rigid polymeric materials, J. Appl. Polym. Sci. 86 (2002) 881
890, Copyright John Wiley & Sons.

Chapter 11 Modified zeolite-based polymer nanocomposite membranes for pervaporation

283

Figure 11.7 The performance of mixed matrix membranes as a function of their morphology.

11.5.1.2

Inorganic bridging agents

The use of inorganic agents to bridge the polymer and the zeolite has attracted attention due to its simplicity and ability to enhance the performance of MMMs. Specifically MgOxHy nanoparticles, where 1 # x # 2 and 0 # y # 2, have been used in this purpose. There are, at least, four methods to grow MgOxHy nanoparticles on the surface of zeolite as shown in Fig. 11.8. Through the Grignard method, the Grignard reagent (alkyl magnesium bromide) is hydrolyzed to create MgOxHy nanoparticles on the surface of zeolite [82,83]. In this method, the zeolite would undergo a pretreatment either delamination or seeding using thionyl chloride or sodium chloride, respectively. Through the solvothermal method, MgOxHy nanoparticles are grown in the presence of a simple organic solvent, that is, ethylenediamine, water, and Mg21 ions at a high temperature and autogenous pressure [84,85]. This method could be modified using a bulkier organic solvent such as diethylenetriamine to prevent the zeolite micropores to be penetrated by the solvent [85]. The last method, that is, the ion-exchanged method, includes the ion exchange of Na1 ion residing in the zeolite structure with Mg21 at a neutral pH, followed by a hydrothermal treatment under Na1 solution at a basic pH (9.5) [86]. At the latter stage, reverse ion exchange takes places along with the formation of MgOxHy nanoparticles on the surface of the zeolite. Lydon et al. [56] have examined the four said methods to prepare MMMs from Matrimid and zeolite LTA. They showed that the MMMs prepared without bridging agents resulted in the formation of

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Chapter 11 Modified zeolite-based polymer nanocomposite membranes for pervaporation

Figure 11.8 Synthesis of MgOxHy nanoparticles on the surface of zeolite via (A) Grignard, (B) solvothermal, (C) modified solvothermal, and (D) ion-exchange methods. Source: Reprinted with the permission from M.E. Lydon, K.A. Unocic, T.-H. Bae, C.W. Jones, S. Nair, Structure
property relationships of inorganically surface-modified zeolite molecular sieves for nanocomposite membrane fabrication, J. Phys. Chem. C 116 (2012) 9636
9645, Copyright American Chemical Society.

interfacial voids while the ones prepared using inorganic bridging agents displayed full-coverage and good adhesion as depicted in Fig. 11.9. The nanoparticles could enhance the area of zeolite and played a role as interlocks to strongly bind the polymer and the zeolite. Shu et al. [82] prepared the MMMs using a poly(ether imide), so-called Ultem, and zeolite 4A. It was shown that the interfacial voids were formed due to the incompatibility between the dispersed and continuous phases, which caused a decrease in selectivity. Incorporating MgOxHy nanoparticles on the surface

Chapter 11 Modified zeolite-based polymer nanocomposite membranes for pervaporation

285

Figure 11.9 Matrimid/zeolite LTA mixed matrix membranes prepared (A) without the presence of bridging agents and with the presence of MgOxHy nanoparticles prepared via (B) Grignard, (C) solvothermal, (D) modified solvothermal, and (E) ion-exchange methods. Source: Reprinted with the permission from M.E. Lydon, K.A. Unocic, T.-H. Bae, C.W. Jones, S. Nair, Structure
property relationships of inorganically surface-modified zeolite molecular sieves for nanocomposite membrane fabrication, J. Phys. Chem. C 116 (2012) 9636
9645, Copyright American Chemical Society.

of zeolite 4A through the Grignard method led to the elimination of the interfacial voids. Consequently the permeability and selectivity substantially improved in the separation of O2/N2 as well as CO2/CH4. Ultem was also not compatible with zeolite MFI; however using MgOxHy nanoparticles as a bridge prepared via the solvothermal method, the defect-free MMMs could be realized [85]. The resulted MMMs showed an enhancement in gas permeability and selectivity during CO2/CH4 separation. From the thermodynamic point-of-view, the compatibility of the zeolite and the polymer is entropy-driven. In general, the polymer prefers the random coils as its conformation. When the polymer sticks or adsorbs on the surface of zeolite assumed to be flat and smooth, it should adapt by deforming into a more ordered shape. In this way, the entropy change (ΔS) is highly negative. If the enthalpy change (ΔH) is not negative enough to offset the
TΔS part, the Gibbs free energy (ΔG) will be .0. Thus the process cannot occur spontaneously. Note that the Gibbs free energy is expressed as ΔG 5 ΔH 2 TΔS. The presence of inorganic bridges at the surface of zeolite increases the surface heterogeneity so that the polymer does not have to deform the confirmation to a far extent. At this condition, ΔS is less negative compared to the adsorption on a smooth surface. Hence, ΔG could be .0, which means spontaneous adsorption.

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Chapter 11 Modified zeolite-based polymer nanocomposite membranes for pervaporation

11.5.1.3

Organic bridging agents

The most widely known that organic bridging agents are the organosilanes which work by covalently bound to the hydroxyl groups on zeolite surface through the silylation reaction to form siloxane bondings while the other functional groups either react to form covalent bonds or strongly interact with the polymer. In general, the organosilanes acting as organic bridging agents include 3-aminopropyl-triethoxy silane (APTES), N-β-(aminoethyl)-γ-aminopropyltrimethoxy silane, γ-glycidyloxypropyltrimethoxy silane, and 3-aminopropyl-dimethyl ethoxy silane [87]. Ismail et al. [87] prepared the PES
zeolite 4A MMMs using APTES as the bridging agent. The schematic diagram in Fig. 11.10 shows the bridging role of APTES. Without the use of organosilanes, the interfacial voids were observed while those voids were eliminated when the organosilanes were utilized (Fig. 11.11). The defect-free MMMs consisted of polyimide (6FDA-6FpDADABA) and APTES-functionalized zeolite L was prepared by Pechar et al. [88]. The performance test on the separation of N2/CH4 showed an increase in both permeability and selectivity. It should be stressed out that the use of organosilanes cannot be too excessive since they can penetrate into the zeolite porosity or create too many linkages which result in pore-blockage. Aside from organosilanes, other types of organic compounds could also be used with the prerequisite of being able to strongly interact with both zeolite and polymer, typically via the hydrogen bonds. Yong et al. [89] demonstrated the use of 2,4,6triaminopyrimidine as an effective bridge for defect-free

Figure 11.10 The role of organosilanes to bridge zeolite and polymer in the mixed matrix membranes. Source: Reprinted with the permission from A.F. Ismail, T.D. Kusworo, A. Mustafa, Enhanced gas permeation performance of polyethersulfone mixed matrix hollow fiber membranes using novel Dynasylan Ameo silane agent, J. Memb. Sci. 319 (2008) 306
312, Copyright Elsevier.

Chapter 11 Modified zeolite-based polymer nanocomposite membranes for pervaporation

287

Figure 11.11 Mixed matrix membranes of PES
zeolite 4A (A) without and (B) with organosilanes. Source: Modified and reprinted with the permission from A.F. Ismail, T.D. Kusworo, A. Mustafa, Enhanced gas permeation performance of polyethersulfone mixed matrix hollow fiber membranes using novel Dynasylan Ameo silane agent, J. Memb. Sci. 319 (2008) 306
312, Copyright Elsevier.

Matrimid-based membranes with various type of zeolites (Fig. 11.12). Matrimid-2,4,6-triaminopyrimidine
zeolite-13X showed a CO2/CH4 selectivity of 617, which was a remarkable enhancement since the selectivity of pure Matrimid was only 1.22. Matrimid-2,4,6-triaminopyrimidine
zeolite 4A also displayed another dramatical increase in CO2/CH4 selectivity of 133. However, both cases showed a decline in gas permeability.

11.5.1.4

Alternative strategies for improving compatibility

There are other strategies to prepare the defect-free MMMs aside from using the bridging agents. One of the simplest alternatives is priming method. In this method, zeolite is modified using a dilute polymer solution to introduce an ultrathin layer of either the same polymer or different to the matrix polymer [90]. This method could increase the adhesion and prevent the particle agglomeration, especially when the zeolite is in nanosized range. Another strategy is by annealing at the temperature higher than the polymer Tg to render the polymer to be more flexible for better contact and interaction with the zeolite [91].

11.6

Zeolite
polymer membrane performances in pervaporation

Pervaporation is generally used for organic solvent dehydration and alcohol removal from aqueous mixtures. Performances

288

Chapter 11 Modified zeolite-based polymer nanocomposite membranes for pervaporation

Figure 11.12 The bridging role of 2,4,6-triaminopyrimidine. Source: Reprinted with permission from H.H. Yong, H.C. Park, Y.S. Kang, J. Won, W.N. Kim, Zeolite-filled polyimide membrane containing 2,4,6-triaminopyrimidine, J. Memb. Sci. 188 (2001) 151
163, Copyright Elsevier.

of zeolite-based nanocomposite membranes in pervaporation are tabulated in Table 11.2. Suhas et al. [34] prepared a PVA mixed matrix membrane containing 7% H-ZSM-5 for pervaporation of water
ethanol and water
IPA mixtures. The zeolite particles had silica by alumina ratios of 38, 187, and 408. They observed that the membrane showed an excellent separation factor of 349 and 568 for water (4%)/ethanol and water (10%)/ IPA, respectively, at 30 C feed temperature and 133.3 Pa permeate vacuum pressure. Those separation factors were obtained for membrane with zeolite that contains the highest alumina content. The separation factor was increased with the increase

Table 11.2 Performances of zeolite-based polymer nanocomposite membranes in pervaporation. Membrane: zeolite (in wt.%)/polymer

Feed (in wt.%)

T (oC); % Ppermeate (Pa)

NaA (15%)/PBZ NaA (10%)/PAAS APTES-NaA (10%)/PAAS NaA (5%)/PVA NaA (85%)/PES KA (11%)/PVA CaA (11%)/PVA Zeolite-13X (20%)/polyimide H-ZSM-5 (7%)/PVA H-ZSM-5 (7%)/PVA ZSM-5 (5%)/PEBA ZSM-5 (30%)/PDMS/PES Silane-ZSM-5 (20%)/PDMS Chlorosilane-ZSM-5 (30%)/PDMS Silicalite-1 (30%)/PDMS Silicalite-1 (65%)/PDMS

Water (10%)
ethanol Water (10%)
ethanol Water (10%)
ethanol Water
butanol (4%) Water (10%)
ethanol Water (20%)
ethanol Water (20%)
ethanol Water (20%)
ethanol Water (4%)
ethanol Water (10%)
IPA Butanol (2.5%)
water Butanol (4.5%)
water Ethanol (10%)
water Ethanol (5%)
water Ethanol (4%)
water Acetone (0.5%)
butanol (1.0%)
ethanol (0.15%)
water Ethanol (5%)
water Ethanol (5%)
water

70; 30; 30; 25; 75; 50; 50; 35; 30; 30; 45; 31; 40; 40; 25; 50;

Ethanol (5%)
water

Silicalite-1 (67%)/PDMS Chlorosilane-silicalite-1 (50%)/ PDMS VTES-silicalite-1 (60%)/PDMS

1333 135 135 340
13.3 13.3 66.7 133.3 133.3 320 600
100 200


Separation factor (
)

Flux (g m22 h21)

Reference

100,000 313.2 435.7 24.14 .10,000 40 22.3 5118a 349 568 30.7 30.5 14.1 15.8 16.5 B50a

1071 440.8 533.2 1866 11,500 164 194 121 125 144 569 113 348 202.9 200 B20b

[92] [93] [93] [94] [95] [96] [96] [97] [34] [34] [98] [35] [99] [100] [36] [101]

60; 300 40;


15.5 19.9

5520 66.3

[102] [103]

50; 170
210

26

230

[104]

APTES, 3-Aminopropyltriethoxysilane; PAAS, poly(acrylic acid) sodium; PBZ, polybenzoxazine; PDMS, polydimethylsiloxane; PEBA, poly(ether-block-amide); PES, polyethersulfone; PVA, poly (vinyl) alcohol. a Selectivity. b Acetone.

290

Chapter 11 Modified zeolite-based polymer nanocomposite membranes for pervaporation

in alumina composition, which was associated with better zeolite
polymer interaction. The effect of Si to Al ratio on the separation properties of ZSM-5/polymer membrane was also examined by Xue and Shi [35]. By increasing Si/Al ratio from 25 to 300, they observed that the separation factor for an n-butanol aqueous solution was increased to 24.1 from its initial separation factor of 16.5. Those studies confirmed the tailorable hydrophilic/hydrophobic properties of zeolite that facilitate the improvement of zeolite
polymer membrane separation properties. Tan et al. [98] prepared and investigated the performance of ZSM-5/PEBA membrane for separation of n-butanol from water mixture. They found that 5% ZSM-5 was the optimum zeolite concentration and the higher concentration yielded in zeolite agglomeration. The membrane showed enhancing flux and selectivity with increasing temperature and butanol concentration. As the temperature increased, more rapid adsorption
desorption rate occurred in the membrane phase. According to the activation energy analysis, they found that the transport of butanol was much more temperature-sensitive than the water. It explained why the n-butanol permeated across the membrane faster than water when the operating temperature was shifted. The higher flux at the higher temperature was also associated with the increasing motion of the polymer segment of the PEBA matrix. Therefore, the enhanced flux at higher temperature was observed both in PEBA and ZSM-5/ PEBA membranes. Even though water and n-butanol have a smaller size than the pore of zeolite channels, the preferential sorption induces selective separation. The selective n-butanol transport can be due to the formation of preferential pathway created by zeolite in the membrane matrix, causing water to permeate through the polymer phase more than through the zeolite channels [32]. Silicalite-1/PDMS membrane was prepared by Yadav et al. [36] for pervaporation of ethanol in the water mixture. The membrane showed increasing separation properties, both in flux and separation factor, with the increase in zeolite loading (from 0% to 30%). Nanocomposite membrane with 30% silicalite-1 content could achieve 16.5 separation factor for ethanol (4%)
water solution, which was higher than those in pure PDMS membrane (8.0). The interesting feature of the zeolitefilled polymeric membrane is the enhanced separation factor. By introducing more zeolite content, an excellent selectivity can be achieved. A relatively high zeolite content, up to 67%, has

Chapter 11 Modified zeolite-based polymer nanocomposite membranes for pervaporation

been used in silicalite-1/PDMS membrane [102]. Improving membrane separation properties as the effect of increasing zeolite loading was also observed in the gas separation membrane [105]. It should be noted that too high zeolite loading may affect the membrane mechanical strength. Also high particles content may lead to selectivity loss due to the formation of more voids in the interface between polymer and particles [8]. Zeolite
polymer compatibility is one of important factors determining the separation properties of zeolite-based polymer nanocomposite membrane. To improve LTA zeolite-poly(acrylic acid) sodium compatibility, Wei et al. [93] modified LTA zeolite crystal with APTES. The reaction between LTA and APTES yielded in the surface-modified LTA crystal. They found that modified LTA/PAAS membrane exhibited better separation properties than unmodified LTA/PAAS membrane. Modification of LTA crystal resulted in better dispersion and more homogenous structure. On the other hand, the introduction of unmodified LTA crystal produced a membrane with more voids as the results of zeolite
polymer incompatibility. Excellent performance of zeolite/polymer membrane was demonstrated by Chuntanalerg et al. [92] in the dehydration process. They synthesized NaA-filled polybenzoxaine (PBZ) membrane for ethanol dehydration. The incorporation of 15% zeolite significantly improved the separation factor from 10,000 to .100,000 and flux from B25 to 1071 g m22 h21. This study confirmed the attractive combination of zeolite and polymer. Another important operating condition in pervaporation is feed composition. For instance, Gu et al. [106] prepared silicalite-filled PEBA for removing ethanol from an aqueous solution. PEBA membrane with 2% silicalite showed an increasing separation factor from B3 to B4 when ethanol concentration was increased from 2% to 10% at 40 C. Since silicalite zeolite is hydrophobic, ethanol flux is higher than water. Also, the increasing ethanol concentration would provide a more driving force for ethanol transport. As a result, the separation factor was increased. It was demonstrated by several studies that incorporating nanoparticles zeolite into a polymer matrix can successfully improve the separation properties of polymeric membrane. However, one should note that membrane separation properties also depend on zeolite
polymer compatibility. Therefore the method of the preparation of defect-free zeolite/polymer by improving zeolite
polymer compatibility is the crucial factor for obtaining high separation performance.

291

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Chapter 11 Modified zeolite-based polymer nanocomposite membranes for pervaporation

11.7

Conclusions

As the key factor of process efficiency, the development of highly permeable and selective membranes is crucial for industrial application of pervaporation. Even though polymeric membranes have been widely used, they still have some limitations such as the trade-off between permeability and selectivity as well as low durability against harsh conditions. The incorporation of nanoparticles into the polymeric membrane matrix has been considered as an effective way for improving the separation properties of the polymeric membrane. Zeolites are among the interesting materials for this purpose because they have welldefined structures which allow obtaining nanocomposite membrane with high separation factor. The tailorable hydrophilic
hydrophobic characteristic also allows one to improve the selective sorption of zeolite toward the permeated component so the zeolite-filled membrane can attain higher selectivity. The separation performance of zeolite-filled nanocomposite membranes or MMMs is determined by the presence of zeolite particles, polymer matrices, and interaction between zeolite and polymer. The separation mechanism inside the membranes will combine the separation mechanism of pure zeolite and pure polymeric materials. The separation of components through zeolite particles can be elucidated by the adsorption of selected component(s) into the pores of zeolites and the diffusion of components along the surface of zeolite mechanisms. In addition to the adsorption and diffusion mechanisms, the separation can also be described by the molecular sieving mechanism. The difference in molecular size between water and alcohol could potentially be exploited by finding suitable zeolite with suitable pore size. The extent of the separation through the pervaporation membrane can be controlled by controlling the chemical potential gradient, which acts as the driving force for separation. The separation mechanism in pervaporation is mainly described by the solution-diffusion model. However, in some cases, the poreflow model is also used to explain the separation mechanism. The effect of zeolite addition into polymer matrix on pervaporation membrane performance has been examined in numerous studies. It was reported that the incorporation of zeolite nanoparticles into polymer matrix could successfully improve the separation of organic
aqueous solutions. The separation performance increases with the increasing zeolite loading. However, too high zeolite loading may lead to the selectivity loss due to the formation of more free volume at the interface between zeolite and polymer matrix.

Chapter 11 Modified zeolite-based polymer nanocomposite membranes for pervaporation

Despite the excellent performance of the zeolite-filled membrane, fabricating defect-free zeolite-based nanocomposite membrane is quite challenging due to the poor zeolite
polymer compatibility and dispersibility. Several strategies to improve zeolite
polymer compatibility have been proposed, such as by employing inorganic or organic bridging agents, priming method, and annealing. By using those strategies, better zeolite
polymer compatibility, as well as improved membrane separation properties, can be obtained.

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509. Available from: https://doi.org/10.1021/ez500335q. [62] H.-M. Guan, T.-S. Chung, Z. Huang, M.L. Chng, S. Kulprathipanja, Poly (vinyl alcohol) multilayer mixed matrix membranes for the dehydration of ethanol
water mixture, J. Memb. Sci. 268 (2006) 113
122. Available from: https://doi.org/10.1016/j.memsci.2005.05.032. [63] S.S. Madaeni, S. Amirinejad, M. Amirinejad, S.M. Hosseini, A. Khodabakhshi, Cation exchange characterizations of phosphotungstic aciddoped polyvinyl alcohol/polyethersulfone blend membranes by sodium chloride solution, J. Polym. Eng. 33 (2013) 71
76. [64] F. Parvizian, S.M. Hosseini, A.R. Hamidi, S.S. Madaeni, A.R. Moghadassi, Electrochemical characterization of mixed matrix nanocomposite ion exchange membrane modified by ZnO nanoparticles at different electrolyte conditions “pH/concentration”, J. Taiwan. Inst. Chem. Eng. 45 (2014) 2878
2887. Available from: https://doi.org/10.1016/j.jtice.2014.08.017. [65] S.M. Hosseini, Z. Ahmadi, M. Nemati, F. Parvizian, S.S. Madaeni, Electrodialysis heterogeneous ion exchange membranes modified by SiO2 nanoparticles: fabrication and electrochemical characterization, Water Sci. Technol. 73 (2016) 2074
2084. [66] D. Ariono, Khoiruddin, S. Subagjo, I.G. Wenten, Heterogeneous structure and its effect on properties and electrochemical behavior of ion-exchange membrane, Mater. Res. Express. 4 (2017) 24006. Available from: https://doi. org/10.1088/2053-1591/aa5cd4.

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934. [68] Khoiruddin, D. Ariono, S. Subagjo, I.G. Wenten, Surface modification of ionexchange membranes: methods, characteristics, and performance, J. Appl. Polym. Sci. 134 (2017) 45540. Available from: https://doi.org/10.1002/ app.45540. [69] J. Ahmad, M.B. Ha¨gg, Effect of zeolite preheat treatment and membrane post heat treatment on the performance of polyvinyl acetate/zeolite 4 A mixed matrix membrane, Sep. Purif. Technol. 115 (2013) 163
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55. Available from: https://doi.org/10.1016/j.memsci.2005.03.019. [74] K. Zarshenas, A. Raisi, A. Aroujalian, Mixed matrix membrane of nanozeolite NaX/poly (ether-block-amide) for gas separation applications, J. Memb. Sci. 510 (2016) 270
283. Available from: https://doi.org/10.1016/ j.memsci.2016.02.059. [75] A. Ghadimi, T. Mohammadi, N. Kasiri, A. Novel, Chemical surface modification for the fabrication of PEBA/SiO2 nanocomposite membranes to separate CO2 from syngas and natural gas streams, Ind. Eng. Chem. Res. 53 (2014) 17476
17486. Available from: https://doi.org/10.1021/ie503216p. [76] B. Yu, H. Cong, Z. Li, J. Tang, X.S. Zhao, Pebax-1657 nanocomposite membranes incorporated with nanoparticles/ colloids/carbon nanotubes for CO2/N2 and CO2/H2 separation, J. Appl. Polym. Sci. 130 (2013) 2867
2876. Available from: https://doi.org/10.1002/app.39500. [77] H. Wu, X. Li, Y. Li, S. Wang, R. Guo, Z. Jiang, et al., Facilitated transport mixed matrix membranes incorporated with amine functionalized MCM-41 for enhanced gas separation properties, J. Memb. Sci. 465 (2014) 78
90. Available from: https://doi.org/10.1016/j.memsci.2014.04.023. [78] J. Ahmad, M.-B. Ha¨gg, Preparation and characterization of polyvinyl acetate/zeolite 4 A mixed matrix membrane for gas separation, J. Memb. Sci. 427 (2013) 73
84. Available from: https://doi.org/10.1016/ j.memsci.2012.09.036. [79] Y. Li, H.-M. Guan, T.-S. Chung, S. Kulprathipanja, Effects of novel silane modification of zeolite surface on polymer chain rigidification and partial pore blockage in polyethersulfone (PES)
zeolite A mixed matrix membranes, J. Memb. Sci. 275 (2006) 17
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Chapter 11 Modified zeolite-based polymer nanocomposite membranes for pervaporation

[80] X. Ding, X. Li, H. Zhao, R. Wang, R. Zhao, H. Li, et al., Partial pore blockage and polymer chain rigidification phenomena in PEO/ZIF-8 mixed matrix membranes synthesized by in situ polymerization, Chin. J. Chem. Eng. 26 (2018) 501
508. Available from: https://doi.org/10.1016/ j.cjche.2017.07.017. [81] R. Mahajan, R. Burns, M. Schaeffer, W.J. Koros, Challenges in forming successful mixed matrix membranes with rigid polymeric materials, J. Appl. Polym. Sci. 86 (2002) 881
890. Available from: https://doi.org/10.1002/ app.10998. [82] S. Shu, W.J. Husain, A. Koros, General strategy for adhesion enhancement in polymeric composites by formation of nanostructured particle surfaces, J. Phys. Chem. C. 111 (2007) 652
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14663. Available from: https://doi.org/10.1021/ja907435c. [85] T.-H. Bae, J. Liu, J.A. Thompson, W.J. Koros, C.W. Jones, S. Nair, Solvothermal deposition and characterization of magnesium hydroxide nanostructures on zeolite crystals, Microporous Mesoporous Mater. 139 (2011) 120
129. Available from: https://doi.org/10.1016/j.micromeso.2010. 10.028. [86] T.-H. Bae, Engineering Nanoporous Materials for Application in Gas Separation Membranes, Georgia Institute of Technology, 2010. [87] A.F. Ismail, T.D. Kusworo, A. Mustafa, Enhanced gas permeation performance of polyethersulfone mixed matrix hollow fiber membranes using novel Dynasylan Ameo silane agent, J. Memb. Sci. 319 (2008) 306
312. Available from: https://doi.org/10.1016/j.memsci.2008.03.067. [88] T.W. Pechar, S. Kim, B. Vaughan, E. Marand, M. Tsapatsis, H.K. Jeong, et al., Fabrication and characterization of polyimide
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202. Available from: https://doi.org/10.1016/J.MEMSCI.2005.10.029. [89] H.H. Yong, H.C. Park, Y.S. Kang, J. Won, W.N. Kim, Zeolite-filled polyimide membrane containing 2, 4, 6-triaminopyrimidine, J. Memb. Sci. 188 (2001) 151
163. Available from: https://doi.org/10.1016/S0376-7388(00)00659-1. [90] A.M.W. Hillock, S.J. Miller, W.J. Koros, Crosslinked mixed matrix membranes for the purification of natural gas: effects of sieve surface modification, J. Memb. Sci. 314 (2008) 193
199. Available from: https:// doi.org/10.1016/j.memsci.2008.01.046. [91] A.F. Ismail, R.A. Rahim, W.A.W.A. Rahman, Characterization of polyethersulfone/Matrimids 5218 miscible blend mixed matrix membranes for O2/N2 gas separation, Sep. Purif. Technol. 63 (2008) 200
206. Available from: https://doi.org/10.1016/j.seppur.2008.05.007. [92] P. Chuntanalerg, S. Kulprathipanja, T. Chaisuwan, P. Aungkavattana, K. Hemra, S. Wongkasemjit, Performance polybenzoxazine membrane and mixed matrix membrane for ethanol purification via pervaporation applications, J. Chem. Technol. Biotechnol. 91 (2016) 1173
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3397. Available from: https:// doi.org/10.1002/app.38555. [94] D. Oh, S. Lee, Y. Lee, Mixed-matrix membrane prepared from crosslinked PVA with NaA zeolite for pervaporative separation of water
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5370. Available from: https://doi.org/10.1080/19443994.2013.768810. [95] J. Li, J. Shao, Q. Ge, G. Wang, Z. Wang, Y. Yan, Influences of the zeolite loading and particle size in composite hollow fiber supports on properties of zeolite NaA membranes, Microporous Mesoporous Mater. 160 (2012) 10
17. Available from: https://doi.org/10.1016/j.micromeso.2012.04.039. [96] Z. Gao, Y. Yue, W. Li, Application of zeolite-filled pervaporation membrane, Zeolites. 16 (1996) 70
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112. Available from: https://doi.org/10.1002/app.38704. [99] L. Ji, B. Shi, L. Wang, Pervaporation separation of ethanol/water mixture using modified zeolite filled PDMS membranes, J. Appl. Polym. Sci. 132 (2015). Available from: https://doi.org/10.1002/app.41897. [100] X. Han, X. Zhang, X. Ma, J. Li, Modified ZSM-5/polydimethylsiloxane mixed matrix membranes for ethanol/water separation via pervaporation, Polym. Compos. 37 (2016) 1282
1291. Available from: https://doi.org/ 10.1002/pc.23294. [101] H. Zhou, Y. Su, X. Chen, Y. Wan, Separation of acetone, butanol and ethanol (ABE) from dilute aqueous solutions by silicalite-1/PDMS hybrid pervaporation membranes, Sep. Purif. Technol. 79 (2011) 375
384. Available from: https://doi.org/10.1016/j.seppur.2011.03.026. [102] H. Zhou, J. Zhang, Y. Wan, W. Jin, Fabrication of high silicalite-1 content filled PDMS thin composite pervaporation membrane for the separation of ethanol from aqueous solutions, J. Memb. Sci. 524 (2017) 1
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635. Available from: https://doi.org/ 10.1007/s10118-010-9136-4. [104] S. Yi, Y. Su, Y. Wan, Preparation and characterization of vinyltriethoxysilane (VTES) modified silicalite-1/PDMS hybrid pervaporation membrane and its application in ethanol separation from dilute aqueous solution, J. Memb. Sci. 360 (2010) 341
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86. Available from: https://doi.org/10.1016/0376-7388(94)00018-2. [106] J. Gu, X. Shi, Y. Bai, H. Zhang, L. Zhang, H. Huang, Silicalite-filled PEBA membranes for recovering ethanol from aqueous solution by pervaporation, Chem. Eng. Technol. 32 (2009) 155
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Pervaporation and pervaporation-assisted esterification processes using nanocomposite membranes

12

Yavuz Salt, Berk Tirnakci and Inci Salt Department of Chemical Engineering, Yildiz Technical University, Esenler, Istanbul, Turkey

12.1

Introduction

Esters are important chemical compounds in many areas such as chemistry, food, cosmetics, and pharmaceuticals [1]. Esters can be obtained as a result of a reaction between carboxyl acids and alcohols called esterification. Water formed as a by-product causes ester hydrolysis in a reversible reaction and esterification reaction reaches thermodynamic equilibrium, which results in a low level of reaction conversion. Increasing the degree of conversion requires shifting the reaction to the ester side. By integrating these reaction systems with separation processes, the by-product water can be removed and the esterification efficiency can be increased. Pervaporation membrane reactors (PVMRs) have many successful applications for this purpose [2
5]. Pervaporation (PV) is a membrane process that is used to separate liquid mixtures, particularly mixtures with close-boiling points, heat-sensitive materials, and azeotropic mixtures by using nonporous membrane based on solution
diffusion mechanism consisting of three consecutive steps: sorption, diffusion, and desorption. The PV process has significant potential in various industrial applications such as the separation of organic
organic mixtures, the dehydration of aqueous organic mixtures, and organic recovery from organic
water mixtures [6]. When the PV method and the esterification

Polymer Nanocomposite Membranes for Pervaporation. DOI: https://doi.org/10.1016/B978-0-12-816785-4.00012-4 © 2020 Elsevier Inc. All rights reserved.

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reaction are integrated as a PV-assisted esterification process, water can be removed from the reaction medium, and the reaction shifts to the ester side [3,7,8]. Separation efficiency can be increased using nanocomposite membranes prepared by incorporating nanoparticles (NPs) into the polymeric membrane matrix, resulting in higher conversion rates and lower reaction times for PV-assisted esterification process. The unique properties of NPs show promise in improving the performance of the membrane material and nanocomposites have great potential in membrane applications due to their separation performance and enhanced durability. Inorganic NPs such as TiO2, SiO2, Al2O3, Si, Ag, ZnO, and ZrO2 and organic NPs such as graphene oxide (GO), carbon nanotubes (CNTs), and carbon nanofibers (CNFs) are used in membrane technology as the nanosized fillers to provide thermal, physical, and chemical resistance and for improved performance [9
12]. This chapter discusses the PV and PV-assisted esterification processes and nanocomposite membranes used in these processes.

12.2

Esters and esterification

Esters are chemical substances that have been known for many years and usually derived from a reaction between carboxylic acids and alcohol. Fig. 12.1 shows their basic chemical formula [13]. Esters are polar molecules similar to other organic molecules such as aldehydes, ketones containing carbonyl groups in their structure and thus form dipole
dipole and van der Waals interactions. However since the ester
ester molecules do not form hydrogen bonds between them, their boiling point is lower than that of the carboxyl acids having the same number of carbon atoms. Hydrogen bonds can form between oxygen atoms in the structures of esters and the hydrogen atoms in water molecules, which makes esters slightly water soluble. Table 12.1 lists the physicochemical properties of certain carboxyl acids, alcohols, esters, and water [14
18].

Figure 12.1 General formula of an ester.

Table 12.1 The physicochemical properties of certain carboxyl acids, alcohols, esters, and water [14
18]. Name

Chemical formula

Molar weight (g mol21)

Density Molar volume (cm3 mol21, 20˚C) (g cm23, 20˚C)

Boiling Solubility (g/100 mL Solubility point (˚C) H2O, 20˚C) parameter (cal cm23)1/2

Methanol Ethanol Propanol i-propanol Butanol i-butanol Formic acid Acetic acid Butyric acid Propionic acid Water

CH3OH CH3CH2OH CH3CH2CH2OH CH3(CH3)CHOH CH3CH2CH2CH2OH (CH3)2CHCH2OH HCOOH CH3COOH CH3CH2CH2COOH CH3CH2COOH HOH

32.042 40.068 60.095 60.095 74.121 74.121 46.026 60.052 88.106 74.079 18.016

40.5a 58.4a 74.8a 76.5a 91.6a 92.5 37.7a 57.2a 92.0 74.6 18.05a

64.6 78.29 97.2 82.3 117.73 108.78 101.0 117.9 163.75 141.15 100.00

a

Values calculated from density at 20˚C. Values at 25˚C.

b

0.7914 0.7893 0.8037 0.7855 0.8095 0.8016 1.2196 1.0492 0.9582 0.9930 0.9982

Miscible Miscible Miscible Miscible 7.45b 10.0b Miscible Miscible Miscible Miscible


14.5 12.7 11.9 11.5 11.4 10.5 12.1 10.1 10.5 9.9 23.4

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Figure 12.2 General equation of esterification.

Esters are chemicals with an organic structure and are obtained by a typical condensation reaction between carboxyl acids and alcohols called esterification. Fig. 12.2 is a schematic representation of the basic mechanism of esterification reactions [19]. When a carboxyl acid is combined with alcohol, esterification takes place and an ester is formed. This basic reaction mechanism is the desired condition since it does not require heat, additives, or catalysts. However when the esterification reaction was carried out in this manner, it produced low ester conversions and esterification mechanisms involving various catalyst systems have been introduced to accelerate the process and increase the reaction’s efficiency. Esterification reactions are one of the best examples of acid-catalyzed reactions. The most well-known example of these is Fischer esterification, an acid-catalyzed esterification reaction. In this method discovered by Emil Fischer—an esterification reaction is carried out using a homogeneous or heterogeneous catalyst. In many previous studies, Fischer esterification was performed using Brønsted acids [20,21], Lewis acids [22,23], and solid acids [24,25]. Except for reactions using an acid catalyst, esters can be synthesized using base activators and carbodiimide activators or by several different methods such as the Mitsunobu reaction. The mechanism, kinetics, and industrial applications of these reactions between carboxyl acids and alcohols have been extensively studied by Otera and Nishikido [26], which can be examined for further information. Such reaction types are controlled by thermodynamic equilibrium, and as the reaction proceeds, water—the main by-product of esterification reactions—is formed. The resultant water causes ester hydrolysis and the reaction becomes reversible. Thus, the rate of esterification conversion remains low. Some technologies have been developed to increase the reaction efficiency by shifting the esterification reaction direction toward the product side. The fundamental aim of these technologies is to design more efficient reaction systems by removing water from the esterification medium.

Chapter 12 Pervaporation and pervaporation-assisted esterification processes

12.3

Combined esterification and reaction systems

Esterification reactions are a very good example of reversible reactions. When a carboxyl acid is combined with an appropriate alcohol, the esterification reaction takes place and water is released as a by-product beside the ester as a result of the reaction, as in many catalytic reactions. Since esterification reactions are thermodynamically restricted, the resultant water must be removed from the medium, and so that the reaction can be made to progress toward the product side. For this purpose, esterification systems can be integrated with appropriate separation processes. The basic logic behind this process is quite simple; the water that results from the reaction is continuously removed from the medium and the reaction is allowed to proceed by preventing the hydrolysis of the ester. In this manner, high conversion rates can be achieved with high-quality and low-cost products while avoiding the use of excess amounts of reactants. Many different technologies have been developed to date to remove by-product water from esterification reactions: • Reactive distillation [27
29] • Use of dry inert gas [30,31] • Use of salt hydrates [32] • Sorption [33] • PVMR [4] • Vapor permeation membrane reactors [34] Examining the literature, combining the distillation process—whose separation dynamics have been known for many years—with esterification reactions is the most widely used and studied technology among these methods. Esterification reactions are reversible in the liquid phase and generally exhibit azeotropic properties. To ensure the progression of these reactions, esterification reactions can be integrated with reactive distillation systems. Thus, low conversion rates can be avoided by disturbing the thermodynamic equilibrium that occurs in the reaction medium. The first example of an integrated system that includes this type of distillation and esterification, was designed and patented by Backhaus [35]. Fig. 12.3 gives a schematic representation of the esterification mechanism by reactive distillation [27,28,36]. In reactive distillation, the reaction and separation processes are carried out together by carrying out the reaction in the presence of a homogeneous catalyst or on a solid catalyst surface

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Figure 12.3 Schematic representation of a reactive distillation system.

that is in contact with the liquid in the medium, and thus the hydrolysis of esters is prevented, which allows the reaction to proceed in the direction of the products. The synthesis of many different ester products such as ethyl acetate [37], butyl acetate [38], ethyl levulinate [39], n-propyl propionate [40], and isopropyl acetate [41] were successfully performed by reactive distillation. The application of esterification with reactive distillation has many advantages such as the need for smaller equipment and plant size, less waste generation, lower capital cost, and higher esterification efficiency. However despite all these advantages, the system’s main disadvantages, such as its high energy requirement and control difficulties suggest the need to create an alternative to reactive distillation in esterification applications. Membrane processes are the best candidates when considering system designs with higher performance esterification systems and overcoming these disadvantages; PVMRs are promising in this regard.

12.4

A unique separation process: pervaporation

Membrane separation processes are used in many areas such as water technology, energy, food, chemistry, petrochemicals, and pharmaceuticals. The PV process, which has a distinct and important place among membrane processes, is a membrane separation process first named by Kober in 1917 by combining the words “permeation” and “evaporation” [42]. In 1950s Binning et al. used the PV process to remove liquid hydrocarbon mixtures using polymeric membranes and carried out

Chapter 12 Pervaporation and pervaporation-assisted esterification processes

pioneering studies in the progression of PV technology while working for American Oil [43]. After these studies, PV technology has been continually developed and successfully applied in many areas. PV is an effective method for the separation of liquid mixtures and has two fundamental important application areas: dehydration of organics [44
46] and separation of organic
organic mixtures [47,48]. With advances in membrane science and technological possibilities, desalination studies have accelerated with PV and successful studies have been carried out in this regard [49
51].

12.4.1

General mechanism and basic characteristics of pervaporation process

PV is a membrane process in which mass transfer and separation are performed using dense membranes. Mass transfer through the membrane is directly dependent on the interaction between the membrane material and the components to be removed. The better the interaction, the better the separation characteristics; therefore the membrane’s chemical nature and physical structure have an important place in realizing “successful” membranes. Fig. 12.4 gives a schematic representation of the separation process in PV. In the PV process, the liquid feed mixture is kept in contact with a dense membrane (upstream side) while the other side of the membrane (downstream side) is kept at low pressure. Keeping the downstream at low pressure can be achieved using a vacuum or sweeping gas. This creates a driving force on the two sides of the membrane that allows separation. The component toward which the

Figure 12.4 Schematic representation of a pervaporation system.

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Figure 12.5 Schematic representation of separation mechanism of pervaporation process.

membrane acts selectively diffuses through the membrane material and is obtained by evaporation on the other side [52]. Fig. 12.5 shows the separation in the PV that takes place preferentially according to the solution
diffusion mechanism; this is not limited by the thermodynamic vapor
liquid equilibrium (VLE) [53
55]. To examine this in more detail, PV separation generally takes place in three basic steps [55,56]. • Sorption: The first step is the partial dissolution of the desired feed components in the feed mixture that is in the membrane material. In the sorption step, the feed mixture is initially enriched. In addition, the chemical potential gradient needed for mass transfer is generated in this step due to the concentration difference in the absorbed component in the interface of the feed mixture with the membrane. Sorption is created due to possible interactions between the penetrant and the polymer. Solubility parameter theory is a general approach to explain the interaction between the polymer and the solvent. A solubility parameter value close to that of the polymer means that the solvent is suitable for the polymer. In addition, binary interaction parameters calculated from the excess Gibbs free energy data should be determined to explain the thermodynamic interaction in ternary systems [57,58]. • Diffusion: In the second step, the absorbed components diffuse through the membrane according to their chemical

Chapter 12 Pervaporation and pervaporation-assisted esterification processes

potential gradients. This step is important for general PV performance because the diffusion step has a limiting effect on the total mass transfer; since different components have different molecular sizes and different transport rates, the general PV selectivity is determined here. • Desorption: In the third and final step, the components diffusing through the membrane suddenly evaporate on the other side of the membrane. This step is not considered to have a limiting effect on the total mass transport and selectivity. The two steps that affect the overall PV performance are sorption and diffusion. Different parameters are available to characterize the PV performance [59]. The first one is the membrane mass transfer coefficient, which is also associated with the flux value and the amount of permeate per unit area of the membrane per unit of time, km (m s21). km 5

Pi : δ

Here Pi refers to the permeability of the i component and δ refers to the membrane’s thickness. The permeability value can be the molar or volumetric permeability. Another PV characterization parameter is β i, which is the enrichment factor. βi 5

Ci;p : Ci;f

Here, Ci,p and Ci,f represent the concentration of the i component in the permeate and feed, respectively. Apart from that, the selectivity rate is also important. The selectivity value, symbolized by αij, can be expressed as the extent of preferential permeation of the i component by the membrane compared to the j component. αij 5

Pi : Pj

Here, Pi and Pj represent the permeability of the i and j components, respectively. The separation factor in the PV is expressed as follows: Separation factor 5

Ci;p =Cj;p Ji =Jj 5 : Ci;f =Cj;f Ci;f =Cj;f

The pervaporation separation index (PSI) can be used to express the total PV performance in a manner that considers the

309

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trade-off relationship between the flux and separation factor as well as the selectivity. PSI 5 ½ðSeparation factorÞ 2 1: PV has some advantages compared to traditional separation processes in particular; distillation. The most important advantage of PV is that the energy requirement is very low compared to other processes. Moreover, PV provides highly effective solutions for the separation of the azeotropic mixtures and mixtures with close-boiling points. Furthermore one of the most important advantages of PV is that it can be used to separate heatsensitive substances since separation can be achieved at fairly reasonable temperatures [60]. The PV process can be easily integrated with many separation and reaction systems; for example, very high energy savings have been achieved in systems in which PV and distillation are combined [61,62]. The PV process can also be easily combined with thermodynamically restricted reversible reactions. Thus, hybrid systems in which separation and reactions take place in the same structure can be obtained. The PV process can be a solution in many industries due to its many advantages such as high efficiency, low energy requirement, high operational stability, requiring no additional chemicals, and ease of integration into industrial systems. With the PV systems integrated with esterification reactions, the reaction can be allowed to proceed by both taking these advantages of PV and by removing water from the medium.

12.5

Pervaporation membrane reactor

There are many studies in the literature on PVMR [63
65]. The main feature of membrane reactors is that the reaction system and separation process are performed together. Membrane reactors are divided into two basic groups according to the membrane’s role in the catalytic process: • Catalytic membrane reactors (CMR): In these systems, the membrane itself plays an active role in the catalytic process. CMR use catalytic active and permselective membranes. In catalytic nonpermselective membrane reactors (CNMR), the membrane is not permselective and only acts as a reaction medium. • Inert membrane reactors (IMR): These are systems in which the membrane has no function in the reaction other than the separation function. NMRs are noncatalytic and

Chapter 12 Pervaporation and pervaporation-assisted esterification processes

311

Figure 12.6 Membrane reactor classification. Source: Adapted from Vital, J., Sousa, J.M., 2013. Polymeric Membranes for Membrane Reactors, In A. Basile (ed.), Handbook of Membrane Reactors-Volume 1: Fundamentals Materials Science, Design and Optimisation (pp. 3
41). Cambridge, UK: Woodhead Publishing” with permission from Elsevier.

nonpermselective membrane reactors and the membrane in NMRs only acts as a reactant distributor. In a packed bed membrane reactor (PBMR), the membrane’s function is to extract products to proceed with the the esterification. In a packed bed reactor consisting of membrane-coated catalyst particles (PLMR), the reactants diffuse through the membrane material to the catalyst surface; through this, the reaction can be controlled. Fig. 12.6 gives the classification of membrane reactors [66
68]. These two types of membrane reactor can be classified according to the following application characteristics: • Extractor-type membrane reactors: This type is used as a CMR or IMR to increase the conversion of equilibriumrestricted reactions by selectively removing one or more components from the reaction medium. • Distributor membrane reactors: In these, the membrane’s role is to control the distribution of reactants to the reaction medium. • Contactor-type membrane reactors: These reactors are systems in which reactants are controlled by a membrane to diffuse the catalyst. For esterification reactions to proceed the water that results from the reaction must be removed from the medium; PVMRs

312

Chapter 12 Pervaporation and pervaporation-assisted esterification processes

Figure 12.7 Different configurations of membrane reactors: (A) external membrane reactors and (B) internal membrane reactors. Source: Adapted from Van der Bruggen, B., 2010. Pervaporation membrane reactors, In Drioli, E., Giorno, L. (eds.), Comprehensive Membrane Science and Engineering, Volume 3: Chemical and Biochemical Transformations in Membrane Systems (pp. 135
163), Elsevier” with permission from Elsevier.

can be used for this purpose. PVMRs can be both catalytic [69] and IMR [70]. PV-assisted esterification reactions are a good example of extractor-type membrane reactors; they are generally used in thermodynamically limited reactions to remove a specific component from the medium and allow the reaction to proceed. Lipnizki et al. [71] divided PVMR into two basic groups called R1 and R2. R1-type PVMRs extract the desired product from the system in the reaction system; in these, the membrane material is generally hydrophobic [5]. In R2-type membrane reactors, the water by-product is removed from the medium. Therefore the membrane is generally made of a polymeric material with hydrophilic properties [72]. Two integration configurations are available for both types of membrane reactor: (1) membrane reactor with an external membrane and (2) membrane reactor with an internal membrane [73]. Fig. 12.7 gives a schematic representation of these configurations. PVMRs have a great advantage in equilibrium-limited esterification reactions compared to traditional distillation methods. Since the separation process is not restricted by VLE in PVMRs, only in the permeate’s changing phase and the energy requirement is very low compared to distillation, PVMRs are the biggest alternative. However, benefitting from all these advantages requires preparing the most efficient, low-cost membrane that is suitable for the system. Therefore as with all membrane processes, the choice of appropriate membrane material is critical in PV-assisted esterification reactions.

Chapter 12 Pervaporation and pervaporation-assisted esterification processes

12.6

Membranes for membrane reactors

Membranes can be classified in many different ways according to their nature, geometry, and separation regimes. Specifially, the factor that distinguishes membranes is their materials; thus membranes are divided into three groups: organic, inorganic, and organic/inorganic composites, according to the materials used in their production [74]. Membrane selection depends on some basic process parameters such as efficiency, separation selectivity, membrane life, mechanical and chemical stability in operating conditions, and cost. Fig. 12.8 shows the membrane classification [75
78]. Membranes are divided into two basic groups according to their nature: natural and synthetic. The best example of natural membranes is biological membranes. They work at lower temperatures and limited pH ranges compared to synthetic membranes and are sensitive to microbiological activity. Synthetic membranes are divided into two groups: organic and inorganic. Inorganic membranes can be produced from ceramic, glass, metal, and zeolite materials. They can easily find application areas at high temperatures. Organic membranes are

Figure 12.8 Membrane classification and membrane applications.

313

314

Chapter 12 Pervaporation and pervaporation-assisted esterification processes

usually a group of membranes produced from polymeric materials. Polymeric membranes can operate in a wide temperature and pH ranges, can be easily processed, and are cost-effective compared to metallic or ceramic membranes. Membranes can be produced in various ways according to the membrane material’s structure and the specification of the separation process. Membranes are divided into three basic groups according to their structure: polymeric, inorganic, and composite membranes. Inorganic membranes have high permeability and selectivity values. In addition, they have high chemical and thermal stability. However they have disadvantages such as low mechanical strength, difficult production procedures, and high cost. Polymeric membranes are widely used in membrane processes. The fact that polymeric materials are easy to supply and process, that they can be used in wide operating ranges, and their relatively low cost make polymeric materials attractive in membrane technology and many different technological areas. However polymeric membranes also have some disadvantages such as low thermal and chemical stability and average level separation potential. When preparing membranes, total membrane performance can be increased by obtaining composite membranes using various organic or inorganic material fillings in the membrane matrix [79]. In composite membranes, polymeric matrices are generally combined with organic or inorganic materials and are referred to as mixed matrix membranes (MMM). MMMs are obtained by distributing organic or inorganic additives in a polymer matrix structure by various methods. Thus, the positive aspects of both polymeric material and filler material can be combined in a single material and the trade-off relationship between selectivity and permeability can be overcome. Developments in nanotechnology and engineering have made nanoscale contributions also prominent in membrane applications. As when using many different fillers, while preparing nanocomposite membranes, the homogeneous distribution of the filler material in the matrix and the compatibility between the polymeric matrix and the filler material is a critical point in preparing a successful membrane.

12.7

Nanocomposite membranes

Today polymeric membranes are used in many different application areas, thanks to their many advantages. However

Chapter 12 Pervaporation and pervaporation-assisted esterification processes

the trade-off relationship between permeability and selectivity still has a limited function to achieve higher separation performances. New methods must be developed to produce structures with higher permeability and selectivity, antipollution properties, and higher durability in each area that uses membrane separation processes. Polymer nanocomposite membranes are advanced materials obtained by homogeneously distributing NPs in polymeric matrices. Such membranes can be used to separate liquid
liquid, solid
liquid, and liquid
gas mixtures. The development of nanocomposite membranes started in 1990s to overcome Robeson’s upper-bound limit [80] in gas separation studies. After this, studies related to nanocomposite membranes were carried out on fuel cells [81
83], sensor applications [84,85], water treatment technologies [9,86], nanofiltration [87,88], reverse osmosis [89,90], and PV [91
93] besides gas separation [94]. Studies have shown the potential of nanocomposite membranes to overcome the trade-off relationship between selectivity and permeability, and nanomaterials are very important in the production of new generation highperformance membranes.

12.7.1

Nanocomposite membranes for pervaporation and pervaporation-assisted esterification

With the advantages of nanomaterials in membrane technology and other areas, the use of nanocomposite membranes in many different applications has gradually increased. Nanocomposite membranes have been used successfully in PV and PV-assisted esterification. Table 12.2 shows some recent PV studies that use nanocomposite membranes. Many different nanocomposite membranes have been used in systems where esterification and PV are coupled. Silicaincorporated poly(vinyl alcohol) (PVA) membranes were prepared by Torabi and Ameri [103] and PV-aided esterification studies were performed for methyl acetate production. Membranes were prepared by the solution-casting method using nanosilica with an average particle size of 15 nm. The esterification reaction between acetic acid and methanol was carried out at different temperatures (50 C, 60 C, and 70 C) for different alcohol:acid molar feed compositions (1:1, 2:1, 3:1, and 4:1), and the acid conversion was calculated as 42.2% for the alcohol: acid ratio of 4:1 at 70 C. However in PV-aided

315

Table 12.2 Nanocomposite membranes used for pervaporation applications. Membrane material

Filler

Size

Loading (%)

Application

Temperature Flux (kg Selectivity Reference (˚C) m22 h21)

PELSC PVA PEK-c

SiO2 g-C3N4 ZIF-8

20 nm Nanoporous 20
40 nm

2 4 4

51 75 50

1.23 6.332 1.48a

89 30.7 19731

[95] [91] [96]

PDMS SA PEI/SA 6FDA-NDA/ DABA co-PI PEBA/PES PVA PEBA

Nanoclay PEG@POSS Sodium silicate Disilanolisobutyl POSS ZSM-5 C-MWCNTs GNPs

, 13 µm (90.0%) Nano-sized cage 100 nm 2 nm

MeOH (14.3 wt%)/MTBE Ethanol (90 wt.%)/water Water (7.38 wt.%)/methanol (14.03 wt.%)/MTBE Toluene (50 wt.%)/methanol Ethanol (90 wt.%)/water Ethanol (90 wt.%)/water Ethanol (85 wt%)/water

10 30 5c POSS modification 7.5 Ethyl acetate (5 wt%)/water 1 NaCl solution (35,000 ppm) 3 Seawater

30 77 76 60

B0.65a 2.50 1.49 2.00

B5 1077 2050 237

[97] [92] [98] [99]

50 22 35

1.895 6.96 2.58

108.52 99.91b 99.94b

[100] [101] [102]

35 nm 2
5 nm


Notes: C-MWCNTs, carboxylic multiwalled carbon nanotubes; PDMS, poly(dimethylsiloxane); PEBA, polyether-block-polyamide; PES, polyethersulfone; PELSC, polyelectrolyte surfactant complex; PVA, poly(vinyl alcohol); SA, sodium alginate; GNPs, graphene nanoplates; PEG@POSS, poly(ethylene glycol)-functionalized polyoctahedral oligomeric silsesquioxanes; 6FDA-NDA/ DABA, copoly(1,5-naphthalene/3,5-benzoic acid 2,2-bis(3,4-dicarboxyphenyl))hexafluoro-propanedimide; PI, polyimide. a Values expressed in the unit kg µm m22 h21. b Rejection values, %. c Sodium silicate concentration, nM.

Chapter 12 Pervaporation and pervaporation-assisted esterification processes

esterification experiments conducted using pure PVA membranes, 52% acid conversion was obtained under the same conditions. In addition, nanocomposite membranes were prepared with different nanosilica ratios and it is determined that the reaction efficiency increased as the filler amount increased. The separation factor and water flux of the pure and nanocomposite membranes (20%) were increased from 160.34 to 1,107 and from 103.8 to 286.7 g h21 m22, respectively. Lin et al. [104] conducted PV-coupled esterification experiments for biodiesel production. Experimental studies were carried out using GO/chitosan (GO/CS) nanocomposite membranes that contain 1%, 2%, 3%, and 4% nanosized filler with ethanol: acetic acid ratio of 1:1, 2:1, and 3:1 (mol/mol) at different temperatures (50 C
70 C). It is reported that in PV-assisted esterification equilibrium conversion is higher compared to non-PV-assisted esterification reactions. In addition as the temperature increased, both the esterification rate and PV efficiency increased. The water flux of about 375 g h21 m22 at 50 C was up to 400 g h21 m22 at 70 C. The conversion rates in a non-PVcoupled and PV-coupled system at 50 C were 75% and 80%, respectively. The reaction conversion was around 80% for a non-PV-coupled esterification system at 70 C and 90% for a PVcoupled system at the same temperature. The conversion of PVassisted esterification at 50 C was found to capture the conversion of the non-PV-assisted esterification at 70 C and it can therefore be seen that the energy costs can be kept low by using PV-assisted esterification hybrid systems. In another study by Shameli and Ameri [105], cross-linked PVA-multiwalled carbon nanotube (MWCNT) nanocomposite membranes were prepared. The membrane’s performance was investigated using a membrane reactor system to increase the efficiency of the acetic acid
methanol esterification. Membranes were prepared by the solution-casting and solvent evaporation method. MWCNTs of average diameter 5
10 nm were incorporated into the PVA matrix at different concentrations (0.5%, 1%, 1.5%, and 2%). As a result of the experiments, the conversions for non-PV-aided esterification, PV-aided esterification with a pure PVA membrane, and PV-aided esterification with a nanocomposite PVA membrane were 42.2%, 52.3%, and 74.2%, respectively, at 70 C and a 4:1 alcohol: acid ratio. In addition, it is observed that the esterification conversion increased by increasing NP amount. The conversion of the membranes with 2% MWCNT approached 100% by the end of 300 min. They reported that this was caused by the increase in PV water flux due to the strengthening of the hydrophilic nature

317

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Chapter 12 Pervaporation and pervaporation-assisted esterification processes

of the membranes with MWCNT incorporation. To examine the effect of temperature by using PVA membranes containing 2% MWCNT, experiments were performed at different temperatures at the same feed rate. The conversion rates were calculated as 80.45%, 91.30%, and 99.25% for 50 C, 60 C, and 70 C, respectively. In addition, the reaction conversion increased upon increasing the alcohol:acid molar ratio of the feed from 1:1 to 4:1. Furthermore, higher acid conversions were obtained by increasing the amount of Amberlyst-15 used as a catalyst from 5% to 15% (based on the acid amount). Ma et al. [106] prepared the catalytic sulfonated poly(phthalazinone ether sulfone ketone)-polyethersulfone (PES)/TiO2 membranes and investigated the membrane’s mechanical properties, sorption degree, hydrophilic structure, and catalytic performance. The authors concluded that membranes with a high specific surface area (33.2
56.0 m2 g21) are geometrically stable at relatively high temperatures such as 80 C (swelling ratio # 1.7%), and that the membranes showed good mechanical strength and high catalytic properties (82% ethanol conversion). In the study by Lu et al. [107], multilayer perfluorosulfonic acid/SiO2 nanofibers and PVA catalytic membranes were prepared. PV-assisted esterification studies were carried out for acetic acid and ethanol esterification in a continuous membrane contactor and a 90% reaction conversion rate was obtained. They pointed out that the prepared membranes had high performance in PV-esterification processes.

12.8

Conclusions and future recommendations

The PV method is used to separate organic
organic liquid mixtures, recover organics from organic
water mixtures, and purify aqueous organic mixtures with nonporous (dense) membranes. Among these, the separation of organic
organic mixtures is an area of application with relatively less development because a suitable membrane material cannot be fully revealed; therefore, this warrants greater focus. In recovery and dehydration processes where there are low feed concentrations for the component that is desired to be separated, membranes are prepared using hydrophobic polymers [such as poly(dimethylsiloxane), poly(vinylidenefluoride), polypropylene, polytetrafluoroethylene] and hydrophilic character [such as poly(vinylalcohol), poly(acrylic acid), poly

Chapter 12 Pervaporation and pervaporation-assisted esterification processes

(ethylene oxide), polyamide, chitosan] and when necessary, by blending polymers with different characteristics. Improving the separation performance has been attempted in recent years with different additives such as inorganic NPs such as TiO2, SiO2, Al2O3, Si, Ag, ZnO, ZrO2, and organic NPs such as GO, CNTs and CNFs; these efforts are on going. However despite the increase in total flux due to the type of polymer used the type and amount of filler used, selectivity decreases due to the combination of flux coupling and the plasticization effect. This effect occurs more frequently as the temperature increases because of the increased segmental movements in the polymer chains, and the fact that these movements create more free volume for the passage of molecules. It should also be noted that- other than the decrease in selectivity with the increasing temperature- the need for hot utility will also increase, which will also negatively impact membrane life. In conclusion, although PV shows higher selectivity compared to other membrane processes the nonporous structure of the membrane used means that the flux values remain relatively low, which increases the need for membrane area and limits the industrial application areas. The results of some of the studies on the use of nanomaterials in the preparation of membranes in overcoming these limitations are promising and more studies are needed on this issue. These problems, which are seen in the PV method, become more complex in PV-assisted esterification processes because of the multicomponent mixture of acid, alcohol, ester, and water components and since separation and reaction are carried out simultaneously. In PV-assisted esterification reactions, the main approach is to remove the water or ester from the reaction mixture by using membranes with hydrophilic or hydrophobic characteristics to ensure that the reaction equilibrium constantly shifts to the ester direction. However the reaction mixture is in the form of an organic
water mixture. When attempting to remove ester or water from the reaction medium, it is not possible to reach high selectivity due to the high concentration of organic compounds or the high flux due to low water concentration, respectively. Here, it is worth emphasizing that increasing the studies on the separation of different ester
alcohol mixtures in particular will significantly contribute to PVMR studies. In PV-assisted esterification applications, the catalyst may be incorporated directly into the reaction medium, absorbed onto the membrane surface, or the membrane itself may have a catalytic effect. The type and amount of catalyst used in

319

320

Chapter 12 Pervaporation and pervaporation-assisted esterification processes

esterification reactions are important factors that affect the performance of PVMR separation. Studies to develop catalysts according to the esterification reaction to be used and how it is used will support PVMR applications. The basic polymers and additives and preparation methods for membranes presented both commercially and academically are evident. Considering membrane-focused studies, the membrane performance has evidently reached an upper limit in different applications that cannot be exceeded without specific approaches, using specific materials or forcing operational conditions. In this case, either the cost of the membrane increases or the investment and/or utility costs will increase to the extent that it will limit industrial applications. It is considered that the basic approaches and research into the effective and economical use of a membrane should focus on the combination and optimization of the following:
operation at minimum temperature and pressure,
reduction of membrane preparation steps,
decreasing the membrane cost,
improving the surface properties of the membrane,
developing the separation performance of known base polymers with different functional groups,
utilizing nanosized additives,
developing catalysts for PVMR studies, compatible with membrane material, based on the reaction and mode of application, and
evaluation of different process design possibilities. As a result, no matter which membrane process is considered, the key component in the separation performance is the membrane itself. Specific problems require specific solutions. However, to present a membrane as a solution to an industrial problem, it should not only be able to give the highest possible flux and selectivity values, it should also be chemically and mechanically durable, economical, and usable for a long time. High flux reduces the required membrane area while high selectivity provides high purity. Both lead to reductions in manufacturing costs. Considering the need to replace membranes excluding the requirements imposed by specific conditions, the cost of the unit membrane area is the decisive factor in its ability to transform into an industrial application. Therefore, some important aspects here are selecting the appropriate membrane material, method, membrane preparation steps, and optimizing operating conditions.

Chapter 12 Pervaporation and pervaporation-assisted esterification processes

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8156.

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13

Peiyong Qin, Zhihao Si, Houchao Shan and Di Cai National Energy R&D Center for Biorefinery, Beijing University of Chemical Technology, Beijing, P.R. China

13.1

Introduction

To develop a “green” chemical process, researches focused on economically viable and alternative separation technologies are in the ascendant. Pervaporation is a promising, energysaving, low-cost, and environmental-friendly separation technique for the volatility organics recovery from diluted aqueous solution and the organics dehydration from low-water containing mixtures [1 3]. As a key building block, membranes with high-separation efficiency are critical to the economic and technological possibilities of pervaporation processes. Among different types of pervaporation membranes, polymer membranes exhibit excellent comprehensive performance in permselectivity, stability, and production cost, which are extensively applied in laboratory and industry scale processes [4]. However, the permeability of pure polymer membrane, without any incorporation and modification, always could not completely meet the practical demand due to the lack of high-efficiency diffusion channel [5,6]. Consequently most of the current researches aim to prepare mixed matrix membranes (MMMs) in which porous particles are incorporated into the basic polymer membrane [7,8]. In this case, the exceptional transport properties of porous fillers can facilitate the separation performance of pervaporation membranes [9 11]. Metal-organic frameworks (MOFs) are a series of nanoporous materials composed of metal center and organic linker. Compared with inorganic fillers, MOFs have better compatibility with polymer matrix owing to the presence of organic ligands [12]. Besides, the permanent porosities, ultrahigh surface areas, Polymer Nanocomposite Membranes for Pervaporation. DOI: https://doi.org/10.1016/B978-0-12-816785-4.00013-6 © 2020 Elsevier Inc. All rights reserved.

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Chapter 13 Polymer/metal-organic frameworks membranes and pervaporation

Figure 13.1 The structures of the common MOFs used in pervaporation MMMs. MMMs, Mixed matrix membranes; MOF, metal-organic frameworks.

and tunable pore sizes of MOFs also promoted their application in the preparation of MMMs [13]. Typically there are three specific advantages of MOFs for preparing pervaporation membranes. First, different hydrophilic or hydrophobic MOFs generally showed good compatibility with the polymers that are always applied in the preparation of polymer pervaporation membranes, for example, polydimethylsiloxane (PDMS) [14 16], poly(ether-block-amide) (PEBA) [17], poly[1-(trimethylsilyl)-1propyne] (PTMPS) [18], poly(vinyl alcohol) (PVA) [19], polybenzimidazole (PBI) [20,21], and chitosan (CS) [22]. Second, there are various possibilities in chemically modifying the active groups, for example, hydroxyl groups and carboxyl group, on the surface of MOFs particles. Thus, the interaction force between MOFs and polymers can be enhanced, and the compatibility can also be improved [23,24]. Third, apart from traditional membrane preparation methods such as solution casting, dip-coating or spraying, the diversity of preparation technology can be explored based on the unique coordination structure of MOFs, for example, interfacial synthesis [25]. The common MOFs used in MMMs are shown in Fig. 13.1 and Table 13.1. The MOFs-based MMMs can be divided into two categories, namely, (1) chemically bonded membranes in which MOFs bond with the polymer and (2) physically doped membranes in which only noncovalent interactions (dispersion, polarity, and hydrogen bonds) exist between MOFs and polymer [12]. The structure and properties of MOFs largely affect the pervaporation performances of MMMs. Given the superiorities of MOFs, in this chapter, the combination methods of MOFs

Chapter 13 Polymer/metal-organic frameworks membranes and pervaporation

331

Table 13.1 The common MOFs used in pervaporation MMMs. Common name

Molecular structure

Ligands

ZIF-8 ZIF-71 ZIF-7 Co(HCOO)2 MIL-53 (Al) MAF-6 MOF-801 UiO-66 (Zr) ZIF-90

Zn Zn Zn Co Al Zn Zr Zr Zn

2-Methylimidazole 4,5-Dichloroimidazole Benzimidazole Formic acid 1,4-Benzenedicarboxylic acid 2-Ethylimidazole Fumaric acid 1,4-Benzenedicarboxylate Imidazole-2-carboxaldehyde

MMMs, Mixed matrix membranes; MOFs, metal-organic frameworks.

and polymers are summarized. As for the screening of suitable MOFs for MMMs, particle stability, hydrophobicity/ hydrophilicity, surface functional structure, particle morphology, and pores characteristics are compared and discussed. In addition, the breakthroughs and the current advances of MOFbased MMMs in pervaporation application are reviewed.

13.2

Preparation methods

Generally four methods are commonly used in the preparation of MOF/polymer MMMs. (1) Solution-casting. It has been widely used in the preparation of MMMs since it is easy to operate [35,36]. As shown in Fig. 13.2A, the casting solution is prepared via blending organic polymer, MOFs, cross-linking agent, and catalyst. Then, it is coated in the porous substrate followed by placed it in oven. However, because of the weak interaction between MOFs and polymer, it is difficult to obtain a homogeneous dispersion of MOFs, especially at high particles loading [37]. The voids formed in the membrane fabrication lead to the decrease of membrane performance [38]. As illustrated in Fig. 13.3, the interfacial gaps are attributed to the incompatibility of MOF with PDMS; and aggregation-induced voids are caused by the viscous polymer solution, which is difficult to penetrate the particle particle voids [38]. Therefore, to avoid the voids in preparation process using the solution-casting method, many approaches are taken into account, for example, (1) improving

Pore size (A˚) 3.4 4.8 3

7.6 6 3.5

Reference [26] [27] [28] [29] [30] [31] [32] [33] [34]

332

Chapter 13 Polymer/metal-organic frameworks membranes and pervaporation

Figure 13.2 Schematic diagram of MOF-based MMMs preparation methods. (A) Solution casting method, (B) dipcoating method, (C) spraying method, and (D) interfacial synthesis method. MMMs, Mixed matrix membranes; MOF, metal-organic frameworks.

Figure 13.3 Schematic diagram of the interfacial gaps and aggregation-induced voids.

Chapter 13 Polymer/metal-organic frameworks membranes and pervaporation

333

Figure 13.4 Schematic diagram of improving the MOFs-polymer compatibility by surface modification of (A) MOFs or (B) polymer. MOFs, Metal-organic frameworks.

the MOFs-polymer compatibility by surface modification to achieve covalently bonding between MOFs and polymer (Fig. 13.4) [15,23,24]; (2) enhancing hydrophobicity/hydrophilicity of MOFs to improve the dispersity in polymer matrix (Fig. 13.5); and (3) increasing particle size to reduce the voids caused by particles aggregation (Fig. 13.6) [39]. Additionally as shown in Table 13.2, the high-temperature and long-time curing treatment are required for the cross-linking of the active layer in MMMs. Thus, a serious of problems will limit the scale-up of MMMs fabrication. The casting solution cannot be cured in short time which is difficult for rolling-up (Fig. 13.7). (2) Dip-coating. As shown in Fig. 13.2B, after dipping a dilute casting solution, an ultrathin polymer layer is formed on the surface of porous substrate. This method is widely used in the

Figure 13.5 Schematic diagram of improving the dispersity in polymer matrix by enhancing hydrophobicity/ hydrophilicity of MOFs. MOF, Metal-organic frameworks.

Figure 13.6 Schematic diagram of reducing the voids caused by particles aggregation by increasing particle size.

Table 13.2 The curing treatment in the preparation of MOFs-based MMMs by the solution casting method. Membrane ZIF-71/PDMS ZIF-71/PDMS ZIF-8/PDMS ZIF-8/PDMS ZIF-8/PDMS ZIF-90/PDMS ZIF-8/PVA UiO-66/PVA MIL-53/PVA ZIF-8/PEBA ZIF-71/PEBA ZIF-7/CS ZIF-8/CS

The curing treatment 

110 C Room temperature Room temperature Room temperature Room temperature 60 C (12 h) Room temperature Room temperature 40 C (12 h) 70 C (24 h) Room temperature 45 C (12 h) Room temperature

(48 h) 1 100 C (15 h) (24 h) (24 h) (24 h) (12 h) 1 50 C (12 h) (12 h) 1 40 C (12 h)

(48 h) 1 70 C (24 h) (48 h)

Reference [39] [40] [41] [15] [42] [43] [23] [44] [24] [45] [17] [46] [47]

CS, Chitosan; MMMs, mixed matrix membranes; MOFs, metal-organic frameworks; PDMS, polydimethylsiloxane; PEBA, poly(etherblock-amide); PVA, poly(vinyl alcohol).

Chapter 13 Polymer/metal-organic frameworks membranes and pervaporation

335

Figure 13.7 Schematic diagram of the scale-up fabrication process using solution casting.

Table 13.3 MOFs-based MMMs prepared by dip-coating method for pervaporation. Membrane

Thickness (µm)

Separation system

Total flux (g m22 h21)

Reference

ZIF-7/PTMPS ZIF-8/PTMPS MAF-6/PEBA UiO-66/PEI

2.5 2.5 B5 10

Isobutanol/water (3/97) Isobutanol/water (1/99) Ethanol/water (5/95) Acetic acid/water (95:5)

6100 6400 4446 212

[18] [18] [31] [22]

CS, Chitosan; MMMs, mixed matrix membranes; MOFs, metal-organic frameworks; PTMPS, poly[1-(trimethylsilyl)-1-propyne]; PEBA, poly(ether-block-amide).

preparation of ceramic hollow fiber supported MMMs [48,49]. With a thin membrane thickness, the high membrane fluxes can be obtained when adopting this method (Table 13.3) [50]. (3) Spraying. As shown in Fig. 13.2C, the casting solution doping MOFs is uniformly sprayed on the substrate from pressure barrels. In comparison with solution casting method, the MOFs loading can be hugely increased [51 53]. (4) Interfacial synthesis. As shown in Fig. 13.2D, this method highly depends on the unique chemical structure of MOFs (metal 1 ligand). Metal and ligand meet at the interface. After crystallization, a MOF-based MMM can be obtained. This method significantly avoids the particle aggregation caused by mixing the presynthesized MOFs into polymer [25].

13.3

Hydrophobic polymer/metal-organic frameworks membranes for organics recovery

Hydrophobic MOF-based MMMs are mainly used in the volatility organics separation from diluted aqueous solution. To improve the compatibility between polymer and MOF, the hydrophobic MOFs, which showed substantial adsorption

336

Chapter 13 Polymer/metal-organic frameworks membranes and pervaporation

Figure 13.8 The common hydrophobic polymers used in pervaporation MMMs. (A) PDMS, (B) PEBA, and (C) PTMPS. MMMs, Mixed matrix membranes; PDMS, polydimethylsiloxane; PEBA, poly(ether-block-amide); PTMPS, poly[1-(trimethylsilyl)-1-propyne].

selectivity for organic solvents over water [54], are encouraged to match hydrophobic polymer. Fig. 13.8 summarized typical polymer structures as hydrophobic matrix, whereas Table 13.4 summarized the current advances for organics recovery using hydrophobic polymer/MOFs membranes.

13.3.1

Polydimethylsiloxane/metal-organic frameworks membranes

PDMS is known as one of the most representative polymer materials for solvent enrichment and separation from dilute aqueous solution [57]. It has excellent comprehensive performance in high hydrophobicity, high permselectivity, high chemical stability, good film-forming property, and low cost [4,58]. Due to the excellent properties, in the majority of MMMs appeared in recent years, PDMS was often selected as the basic polymer material for the preparation of pervaporation membrane [35,43,55,59 63]. As for the MOF-based fillers, hydrophobic ZIF-71 with pore size of 0.49 nm and high sorption capacity

Table 13.4 Current advances for organics recovery using hydrophobic polymer/MOFs membranes. Polymer MOF

Loading (wt.%)

Supports

Thickness (µm)

Preparation method Separation system

Feed temperature (˚C)

PDMS PDMS

ZIF-8 ZIF-8

10 10

PVDF PVDF

12 12

Solution casting Solution casting

55 55

PDMS PDMS PDMS PDMS PDMS PDMS PDMS PDMS PDMS PEBA PEBA

1 40

PVDF PS PVDF PVDF PVDF PVDF PVDF PVDF PVDF

20

PEBA PEBA

ZIF-8 ZIF-8 ZIF-8 ZIF-8 ZIF-71 ZIF-71 ZIF-71 ZIF-7 ZIF-90 ZIF-8 Co (HCOO)2 ZIF-71 MAF-6

PTMPS

ZIF-8

41.3

PTMPS

ZIF-7

41.3

PTMPS

ZIF-7

10

PTMPS

ZIF-8

10

3 20 40 40 20 2.5 10 4 20 7.5

Tubular ceramic PVDF Ceramic hollow fiber Stainlesssteel-meshes Stainlesssteel-meshes Alumina capillary Alumina capillary

10 5

20 50 6 3 B3

Model ABE ABE fermentation broth Solution casting Butanol/water Spraying self-assembly Butanol/water In situ fabrication Ethanol/water Solution casting Butanol/water Solution casting Ethanol/water Solution casting Ethanol/water Solution casting Butanol/water Solution casting Butanol/water Solution casting Ethanol/water Solution casting Phenol/water Dynamic pressure-driven Toluene/nassembly method heptane Solution casting Model ABE Dip-coating Ethanol/water

55 80 40 55 50 60 60 60 60 70 40

Feed concentration (wt.%)

Total flux (g Separation m22 h21) factor

11.3a 11.5a

1.5 1 5 1.5 5 2 2 1 5 8000e 10

1952.4 1853.7

Reference

20.2 20.7

[41] [41]

1039.04 4846.2 1778 1249.5 B1000 24,809b 123,045d 1689 10300 1310 771

56 81.6 12.1 53.1 B9.5 0.81c 5.64c 66 10.8 53 5.1

[15] [51] [25] [42] [39] [40] [40] [55] [43] [45] [29]

37 60

1f 5

520 4446

18.8 5.6

[17] [31]

Plugging filling

Furfural/water 80

1

900

53.3

[56]

Plugging filling

Furfural/water 80

1

670

35.9

[56]

2.5

Dip-coating

80

3

6100

32.7

[18]

2.5

Dip-coating

Isobutanol/ water Isobutanol/ water

80

1

6400

40.1

[18]

10 20 B5

ABE, Acetone butanol ethanol; MOFs, metal-organic frameworks; PDMS, polydimethylsiloxane; PS, polysulfone; PTMPS, poly[1-(trimethylsilyl)-1-propyne]; PVDF, poly(vinylidene fluoride). a Butanol concentration with the unit g L21. b Ethanol permeability with the unit Barrer. c Selectivity. d Butanol permeability with the unit Barrer. e Feed concentration are in the unit ppm. f Butanol concentration.

338

Chapter 13 Polymer/metal-organic frameworks membranes and pervaporation

for alcohols was always used in MMM preparation [40,60,64,65]. It evidenced that ZIF-71-based MMMs showed good performance in separating methanol, ethanol, and butanol from dilute aqueous solutions [17,39,66]. Typically the separation factor and molecules permeability of ethanol and butanol were widely increased with the increase of ZIF-71 loading [40]. Besides, ZIF-7 and ZIF-8, which have similar structure and chemical property with ZIF-71, were also loaded in hydrophobic polymer matrix. Compared with ZIF-71, the pore sizes of ZIF-7 (0.3 nm) and ZIF-8 (0.34 nm) accommodated more organic molecules, allowing better separation performance of the corresponding MMMs [12]. Besides, ZIF-7 forms a hexagonal symmetry opening window with SOD topology. Hence, the superhydrophobic ZIF-7 pore channels in PDMS-based MMMs facilitated the n-butanol permeation and improved its separation factor [55]. Moreover, ZIF-8 also exhibits high adsorption selectivity for alcohols such as n-butanol because of the hydrophobic pore surface [67]. After incorporating ZIF-8 into PDMS layer by solution casting, the prepared MMMs showed improved performances for organics recovery [68]. However, a common bottleneck of the hydrophobic MOFs-based MMMs is the poor filler dispersion due to the lack of the bridging between MOFs and polymer. To address this problem, many researches were proposed to improve the compatibility between ZIF-8 and PDMS. A simultaneous spray self-assembly technique was put forward to prepare the ZIF-8-PDMS MMM on sheet polysulfone (PS) substrate for maximizing the dispersion and loading of ZIF-8 particles [51]. Additionally the nucleation of MOF precursors can be adjusted by in situ generation of ZIF-8 nanoparticles in PDMS layer (Fig. 13.9). Within the active layer, the ZIF-8 nanoparticles provided the preferential channels for organics separation due to ultrahigh adsorption capacity and superhydrophobicity. Simultaneously ZIF-8/PDMS MMMs displayed the increases in permeation flux and separation factor for ethanol recovery [25]. Furthermore ZIF-8 can also be modified by polydopamine and silane coupling agents (n-propyltrimethoxysilane) to bond with hydroxy-terminated PDMS tightly

Figure 13.9 Schematic diagram of preparation procedure of ZIF-8/PDMS MMMs via interfacial synthesis. MMMs, Mixed matrix membranes; PDMS, polydimethylsiloxane.

Chapter 13 Polymer/metal-organic frameworks membranes and pervaporation

339

Figure 13.10 Schematic diagram of ZIF-8 hydrophobic modification.

(Fig. 13.10). The adsorption selectivity for n-butanol was effectively improved [15]. It is worth noting that after carbonization of ZIF-8 template, ZIF-8-derived nanoporous carbon also provided high-efficiency transport path in MMM, which also showed outstanding hydrophobicity inherited from ZIF-8 precursor [42].

13.3.2

Poly(ether-block-amide)/metal-organic frameworks membranes

PEBA is another potential polymer in the preparation of pervaporation membrane, which comprises of rigid polyamide (PA) segments and flexible polyether (PE) segments [69 72]. The rigid PA segments offer good mechanical strength, while the flexible PE segments, rich in high mobility of PE chains, provide high permeability [71,73]. In comparison with other polymers, such as PDMS [15,41,59], PTMSP [74,75], and poly(vinylidene fluoride) (PVDF) [76], pervaporation membranes prepared from PEBA have drawn immense attention as they can avoid timeconsuming cross-linking process [31]. In addition, the presence of PE segments in hydrophobic PEBA polymer makes it more appropriate for separating organics from aqueous solution by pervaporation [17,69,77]. ZIFs showed superior thermal and chemical stability and were promising for industrial applications, which can be doped into PEBA layer for MMMs [78]. For instance, both flux and separation factor of the PEBA membranes were remarkably improved by incorporating appropriate content of ZIF-71. The ZIF-71-filled PEBA MMMs showed a long-term stability up to 100 h of continuous pervaporation operation for n-butanol recovery [17]. Besides, due to the flexible pore of ZIF-8, phenols permeate flux and separation factor were increased by 54.6% and 52% compared with the pure PEBA membrane, respectively,

340

Chapter 13 Polymer/metal-organic frameworks membranes and pervaporation

when using ZIF-8/MMMs with 10 wt.% particle loading [45]. Additionally the incorporation of Zn(BDC)(TED)0.5 into PEBA matrix resulted in a decreased surface-free energy and improved mechanical properties [79]. A ceramic hollow fiber supported PEBA-based MMM containing MAF-6, a large-pore MOF with exceptional hydrophobic, was prepared via facile dip-coating [31]. Total flux of 4446 g m22 h21 and separation factor of 5.6 were achieved under the optimized conditions when separating 5 wt.% ethanol/water mixture. The low transport resistance and high packing density allow a high productivity for practical implementation of this new MAF-6/PEBA mixed matrix membrane. Co(HCOO)2 materials with different particle sizes were prepared by solvothermal method [29]. After mixed with PEBA solution for MMM, the resulted mixture was finally deposited on the outer surface of tubular ceramic substrate by dynamic pressure-driven assembling. These membranes were applied for recovering toluene from n-heptane mixtures and 771 g m22 h21 of permeate flux with a separation factor of 5.1 was obtained. The immobilization effect of the Co (HCOO)2 for Co(II) ions led to stable separation performances of Co(HCOO)2/PEBA membrane

13.3.3

PTMPS/metal-organic frameworks membranes

PTMPS, another type of silicone rubber that structurally similar to PDMS, is also used for pervaporation membrane. The presence of phenyl group in PTMPS provided stronger hydrophobicity performance. ZIF-8/PTMPS MMM and ZIF-7/PTMPS MMM with 10 wt.% of particles loading were obtained by dip-coating γ-Al2O3 modified tube. Unfortunately the performance of ZIF-7/PTMPS MMM was behind that of ZIF-8/PTMPS MMM for isobutanol recovery. This is attributed to the hydrophobic and tunable channels of ZIF-8. Each sodalite cage of ZIF-8 has the ability to accommodate six isobutanol molecules, proving a reversible gate-opening effect upon variation of the isobutanol pressure or temperature [18]. Moreover, a homogeneous ZIF-8/PTMPS nanocomposite membrane with high particle loading was fabricated on a hierarchically ordered stainless-steel-mesh (HOSSM) that prepared by “Plugging Filling” [56]. A HOSSM skeletonreinforced MMM was achieved relying on its space restriction and physical cross-linking effects. By virtue of ultrahigh adsorption selectivity, ZIF-8 nanoparticles in PTMPS matrix create

Chapter 13 Polymer/metal-organic frameworks membranes and pervaporation

preferential pathways for furfural molecules. The HOSSM-ZIF8-PTMPS membrane was promising in pervaporative separation of furfural from dilute solution with excellent stability [56].

13.4

Hydrophilic polymer/metal-organic frameworks membrane for organics dehydration

Hydrophilic membranes are typically used for the organics dehydration from low-water containing solution. To improve the dehydration performance of hydrophilic membranes, different MOFs were mixed with the hydrophilic matrix for MMMs. The hydrophilic MMMs with rich high sorption centers of water molecular usually have good structural rigidity and regularity for selective water diffusion [80]. Fig. 13.11 summarized typical hydrophobic polymers, whereas Table 13.5 summarized the current advances of hydrophilic polymer/MOFs membranes for organics dehydration.

Figure 13.11 The common hydrophilic polymers used in pervaporation MMMs. (A) PVA, (B) PBI, and (C) CS. CS, Chitosan; MMMs, mixed matrix membranes; PBI, polybenzimidazole; PVA, poly(vinyl alcohol).

341

Table 13.5 Current advances for organics dehydration using hydrophilic polymer/MOFs membranes. Polymer MOF

Loading Supports Thickness Preparation Separation Feed Feed Total flux (wt.%) (µm) method system temperature concentration (g m22 h21) (˚C) (wt.%)

PVA

ZIF-8

7.5

15

PVA

ZIF-8

7.5

15

PVA

UiO-66

1

PAN

28

PVA

MIL-53 (Al)

7.5

PAN

3.6

PVA

ZIF-90

PBI

ZIF-8

58.7

50 6 15

PBI

ZIF-8

58.7

50 6 15

PBI

ZIF-8

58.7

50 6 15

CS

ZIF-7

5

25

CS CS

MOF-801 ZIF-8

4.8 5

70 80

PAN

1 40 60

Solution casting Solution casting Solution casting Solution casting Solution casting Solution casting Solution casting Solution casting Solution casting Spin-coating Solution casting

CS, Chitosan; MOFs, metal-organic frameworks; PBI, polybenzimidazole; PVA, poly(vinyl alcohol). a Water permeance with the unit GPU, 1 GPU 5 1 3 1026 cm3 (STP) cm22 s21 cmHg21. b Selectivity.

Separation Reference factor

Ethanol/water

50

85

185

119

[23]

Isopropanol/ water Ethanol/water

40

85

112

1200

[23]

30

90

48.2

41.6

[44]

Ethanol/water

40

92.5

24641a

27b

[24]

Ethanol/water

30

90

268

1379

[81]

Butanol/water

60

85

226

698

[21]

Ethanol/water

60

85

992

10

[21]

Isopropanol/ water Water/ethanol

60

85

246

310

[21]

25

90

322

2812

[46]

Water/ethanol Water/IPA

70 30

90 85

1937 330

2156 806

[82] [47]

Chapter 13 Polymer/metal-organic frameworks membranes and pervaporation

13.4.1

Metal-organic frameworks/poly(vinyl alcohol) membranes

Among various types of hydrophilic polymers employed for the pervaporation dehydration, PVA is considered to be the ideal one due to its excellent hydrophilicity, outstanding physicochemical property, and remarkable membrane-forming ability [83,84]. To prevent the plasticization of PVA-based membranes, methods including cross-linking, thermal treatment, and blending have been adopted [1]. Similar with the preparation of hydrophobic MMMs, hydrophilic MMMs also suffered from the problems of particle aggregation and defect formation, which were mainly caused by the poor affinity between particles and the polymer matrix [12]. Various strategies have been explored to enhance the compatibility between inorganic particles and polymer matrix to fabricate MMMs with uniform distribution of MOF particles. Ethanediamine-modified ZIF-8 particles (ZIF-8-NH2) is synthesized and incorporated in the PVA matrix to prepare ZIF-8/ PVA MMMs for ethanol dehydration [23]. The modified ZIF-8 showed high hydrophilicity and affinity with the PVA matrix via hydrogen bonding between ZIF-8-NH2 molecules and PVA polymer chains. The PVA/ZIF-8-NH2 MMMs exhibited excellent membrane homogeneity and separation performance because of the higher hydrophilicity and restricted agglomeration of the particles compared to the corresponding MMMs using unmodified particles. Moreover, Zr-cluster-based MOFs are considered as potential candidates for MMMs owing to their outstanding thermal and chemical stability [85 87]. According to the flexible structure of MOFs, UiO-66, UiO-66-OH, and UiO-66-(OH)2 were prepared by altering the organic linkers (1,4-benzenedicarboxylate acid, 2-hydroxyterephthalic acid, and 2,5-dihydroxyterephthalic acid), which was expected to tune the interactions between Zr-MOFs and PVA matrix for ethanol dehydration [44]. With the increased hydrophobic constants of organic ligands in Zr-MOFs, the hydrophilicity of MMMs declines while the swelling degree of MMMs in 90 wt.% ethanol rises. UiO-66-(OH)2/ PVA MMMs overcame the tradeoff effect. Similar method for MOFs modification was also widely applied in preparation of other MOFs for PVA-based MMMs. Because of the exceptional structural stability in aqueous solution and acid, MIL-53(Al) is considered as another outstanding candidate fillers for MMMs [88]. For example, MIL-53 has been incorporated into PVA for pervaporation dehydration of 92.5 wt.% ethanol [24]. The pervaporation performance of PVA-based MMMs was adjusted by

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Chapter 13 Polymer/metal-organic frameworks membranes and pervaporation

surface functionization of MIL-53-NH2, for example, modification with formic acid, valeric anhydride and heptanoic anhydride. With the increased hydrophobic constant of the surface substituents on MIL-53-NH2 derivatives, the ethanol permeance increased while the separation factor and water permeance dropped. Except for MOFs modification, some other researches focused on alternative membrane preparation methods for high performance and stable hydrophilic membrane. A novel membrane preparation method was proposed to improve homogeneous dispersion of MOFs in PVA-based MMMs, namely the viscosity-driven in situ self-assembly method [81]. ZIF-90, containing aldehyde groups, has a favorable noncovalent interaction with the polymer matrix for MMMs preparation [34]. By adopting viscosity-driven in situ self-assembly method, ZIF-90 particles were in situ generated in a PVA matrix and crosslinked with PVA chains simultaneously. The resulted ZIF-90/ PVA MMMs provided high mechanical strength, swelling resistance, and high dehydration performance.

13.4.2

Metal-organic frameworks/ polybenzimidazole membranes

PBI is a high-performance aromatic polymeric material with donor and acceptor hydrogen-bonding sites [89], which is capable of participating in specific interactions [90]. Water can preferentially permeate the PBI membrane due to the strong water affinity with PBI molecules [91]. However, the issues of brittleness and low permeability of the corresponding pervaporation membrane are the major drawbacks for PBI application [92,93]. Recently there is a trend in incorporating inorganic materials into PBI to enhance permeability and mechanical toughness. However, the incompatibility between the inorganic and polymeric phases consequently engenders in the decreases of selectivity [94]. Thus, it is important to select suitable porous filler for PBI-based MMMs. It reported that nanosized ZIF-8 particles were incorporated into PBI to fabricate ZIF-8/PBI MMMs [21]. The water-induced swelling can be severely suppressed because of the hydrophobic nature and rigid structure of ZIF-8 particles. At the same time, the nbutanol-induced swelling was enhanced owing to the greater free volume in the PBI/ZIF-8 MMM [20]. As a result, the flux of the MMM was increased by four times when loading 33.7 wt.% of ZIF-8 in PBI matrix.

Chapter 13 Polymer/metal-organic frameworks membranes and pervaporation

13.4.3

Metal-organic frameworks/chitosan membranes

CS, the derivative of chitin, has reactive amino and hydroxyl groups for polymerization [95,96]. CS has been used as a typical pervaporation membrane material due to its good film-forming properties and chemical resistance [97,98]. Specifically the high water-permselectivity is mainly caused by the hydrophilic groups such as reactive amino and hydroxyl groups [96]. ZIF-7 was doped into CS for water/ethanol pervaporation. The obtained CS MMMs with 5 wt.% ZIF-7 showed excellent separation efficiency attributed to the rigidified polymer chain [46]. Besides, the hydrophilic MOF-801 incorporated CS matrix showed superior water adsorption ability for ethanol dehydration. In this MMM, the porous structure of MOF-801 can not only provide additional transport pathways for the transition of water molecules, but also twisted the ethanol transport pathways so as to improve the PV performance [82]. Furthermore ZIF-8 was ever incorporated into CS to fabricate MMMs. The permeation flux was significantly enhanced when dehydrating isopropanol. Unfortunately when ZIF-8 loading was up to 10 wt. %, the separation factor decreased significantly due to particles agglomeration [47].

13.5

Challenges and perspectives

Organophilic and hydrophilic polymer/MOFs pervaporation membranes have been widely used in organics recovery or dehydration. However, in the preparation of polymer/MOFs membranes, there are some challenges in keeping stability of MOFs. In the beginning of the preparation, MOFs particles often need to be presynthesized and further dried under vacuum to remove the residual guest molecules. Mechanical stability of MOFs is important for subsequent procedure. However, MOFs under vacuum may contribute to partial collapse of pore structure [99], which cannot play a role in sieving effect after doped into polymer matrix. Moreover, other report found that a strong pressure also may lead to a decrease in porosity [100]. To activate the pore structure of MOFs, a commonly used method is to use a lower surface tension solvent to exchange a higher one [101]. Then, solvent removal is beneficial for the activation of MOFs before used.

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Chapter 13 Polymer/metal-organic frameworks membranes and pervaporation

It is well known that the pervaporation process is always conducted at high operating temperature ( . 50 C) and water enriched environment as a response to the upstream process, such as the hydrolysis process of hemicelluloses [102] and acetone butanol ethanol (ABE) fermentation process [103]. However, in recent years, some studies have pointed out the drawbacks of poor stability of MOFs in these harsher conditions, for example, high temperature [104 107] and even waterrich environment [108]. ZIF-7 and ZIF-11 can generally remain their structures in water with the temperature of 20 C 2 50 C. However, some new crystalline substances were obtained when put them into boiling water [104]. James et al. suggested that at low temperatures (,200 C), ZIFs are stable in gas atmosphere, but unstable in aqueous solutions [107]. Similar to the above phenomenon, both ZIF-8 crystallites and membranes that an impurity crystalline structure was presented after hydrolyzed in water [109]. The procedure of MOF degradation in water is because the metal-coordinated linkers are replaced by water or hydroxide [110,111], which indicates the relatively poor strength of the coordination bonds between inorganic clusters and coordination groups. Meanwhile, in the practical application, the acidic feed solution is another challenge for keeping stability during pervaporation. Compared to neutral water molecules, proton easily attacks the MOFs. The degradation of MOFs in acid solution is mainly caused by the competition of proton and metal ion for the coordinating linkers. For instance, when ZIF-8 captured the protons in acid solution, which then neutralized the 2methylimidazole [112]. The performance decline of ZIF-8/ PDMS MMMs was also obvious when separating ABE separation from fermentation broth [41]. Because of the weakly acidic conditions, ZIF-8 framework in the PDMS matrix was depredated. In this case, nonselectivity defects (interfacial gaps and structure collapse-induced voids) are formed in dense layer (Fig. 13.12). Unfortunately the permeation molecules can preferentially pass through these defects instead of MOFs [18].

Figure 13.12 Schematic diagram of performance decline of ZIF-8/PDMS MMM in pervaporation. MMMs, Mixed matrix membranes; PDMS, polydimethylsiloxane.

Chapter 13 Polymer/metal-organic frameworks membranes and pervaporation

Based on aforementioned analysis, the instability of MOFs is one of the barriers for the actual practice and the commercialization of polymer/MOFs MMM. If this problem is not properly addressed, the application of MOF-based pervaporation membrane will be limited. To improve the stability of ZIF-8, Liu et al. reported a shell-ligand-exchange reaction for improving the hydrothermal stability of ZIF-8 [105]. Si et al. prepared ZIF-8-derived nanoporous carbon to improve the stability of ZIF-8-based MMM. After transferring the coordinating linkers of ZIF-8 into carbon skeleton, the protonmetal ion competition for the coordinating linkers in acid solution can be avoided [41,42]. Zhang et al. showed the stability improvement of ZIF-8 crystal and membrane in water by using a ligand exchange postmodification method. The methylimidazole ligands on the outer surface of ZIF-8 crystals or membranes was replaced by the more hydrophobic, bulkier 5,6-dimethylbenzimidazole [113]. Overall, challenges still exist in the stabilization of MOF-based membranes, some modification possibilities help to solve the problem to some extent.

13.6

Final remarks

This chapter introduces the application of hydrophobic and hydrophilic MOF/polymer MMMs in pervaporation. The striking features of the tunable nanostructures and permanent porosities of MOFs contribute to adequate affinity adsorption and separation for a variety of guest molecules. The unique chemical composition of MOFs creates great possibilities to modify MOFs in MOFs-based MMMs, which can improve the pervaporation performance. Besides, different types of MOF/polymer MMMs are also presented by describing the designations, preparations, modifications, and performances. Overall, MOF/polymer MMMs will be more viable in a near future.

Acknowledgment This work was supported by the National Key Research and Development Program of China (Grant No. 2018YFB1501703), the National Nature Science Foundation of China (Grant Nos. 21676014 and 21706008), and Beijing Natural Science Foundation (Grant No. 2172041).

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[35] S. Li, P. Li, D. Cai, H. Shan, J. Zhao, Z. Wang, et al., Boosting pervaporation performance by promoting organic permeability and simultaneously inhibiting water transport via blending PDMS with COF-300, J. Membr. Sci. 579 (2019) 141 150. [36] Z. Si, H. Shan, S. Hu, D. Cai, P. Qin, Recovery of ethanol via vapor phase by polydimethylsiloxane membrane with excellent performance, Chem. Eng. Res. Des. 136 (2018) 324 333. [37] J. Gascon, F. Kapteijn, B. Zornoza, V. Sebastian, C. Casado, J. Coronas, Practical approach to zeolitic membranes and coatings: state of the art, opportunities, barriers, and future perspectives, Chem. Mater. 24 (2012) 2829 2844. [38] S. Takahashi, D.R. Paul, Gas permeation in poly(ether imide) nanocomposite membranes based on surface-treated silica. Part 2: with chemical coupling to matrix, Polymer 47 (2006) 7535 7547. [39] L.H. Wee, Y. Li, K. Zhang, P. Davit, S. Bordiga, J. Jiang, et al., Submicrometer-sized ZIF-71 filled organophilic membranes for improved bioethanol recovery: mechanistic insights by Monte Carlo simulation and FTIR spectroscopy, Adv. Funct. Mater. 25 (2015) 516 525. [40] H. Yin, C.Y. Lau, M. Rozowski, C. Howard, Y. Xu, T. Lai, et al., Free-standing ZIF-71/PDMS nanocomposite membranes for the recovery of ethanol and 1-butanol from water through pervaporation, J. Membr. Sci. 529 (2017) 286 292. [41] Z. Si, D. Cai, S. Li, C. Zhang, P. Qin, T. Tan, Carbonized ZIF-8 incorporated mixed matrix membrane for stable ABE recovery from fermentation broth, J. Membr. Sci. 579 (2019) 309 317. [42] Z. Si, D. Cai, S. Li, G. Li, Z. Wang, P. Qin, A high-efficiency diffusion process in carbonized ZIF-8 incorporated mixed matrix membrane for n-butanol recovery, Sep. Purif. Technol. 221 (2019) 286 293. [43] S. Xu, H. Zhang, F. Yu, X. Zhao, Y. Wang, Enhanced ethanol recovery of PDMS mixed matrix membranes with hydrophobically modified ZIF-90, Sep. Purif. Technol. 206 (2018) 80 89. [44] G. Wu, Y. Li, Y. Geng, X. Lu, Z. Jia, Adjustable pervaporation performance of Zr-MOF/poly(vinyl alcohol) mixed matrix membranes, J. Chem. Technol. Biotechnol. 94 (2019) 973 981. [45] C. Ding, X. Zhang, C. Li, X. Hao, Y. Wang, G. Guan, ZIF-8 incorporated polyether block amide membrane for phenol permselective pervaporation with high efficiency, Sep. Purif. Technol. 166 (2016) 252 261. [46] C.-H. Kang, Y.-F. Lin, Y.-S. Huang, K.-L. Tung, K.-S. Chang, J.-T. Chen, et al., Synthesis of ZIF-7/chitosan mixed-matrix membranes with improved separation performance of water/ethanol mixtures, J. Membr. Sci. 438 (2013) 105 111. [47] S. Fazlifard, T. Mohammadi, O. Bakhtiari, Chitosan/ZIF-8 mixed-matrix membranes for pervaporation dehydration of isopropanol, Chem. Eng. Technol. 40 (2017) 648 655. [48] Z. Dong, G. Liu, S. Liu, Z. Liu, W. Jin, High performance ceramic hollow fiber supported PDMS composite pervaporation membrane for bio-butanol recovery, J. Membr. Sci. 450 (2014) 38 47. [49] G. Liu, W. Wei, H. Wu, X. Dong, M. Jiang, W. Jin, Pervaporation performance of PDMS/ceramic composite membrane in acetone butanol ethanol (ABE) fermentation-PV coupled process, J. Membr. Sci. 373 (2011) 121 129. [50] X. Zhuang, X. Chen, Y. Su, J. Luo, W. Cao, Y. Wan, Improved performance of PDMS/silicalite-1 pervaporation membranes via designing new silicalite-1 particles, J. Membr. Sci. 493 (2015) 37 45.

Chapter 13 Polymer/metal-organic frameworks membranes and pervaporation

[51] H. Fan, Q. Shi, H. Yan, S. Ji, J. Dong, G. Zhang, Simultaneous spray selfassembly of highly loaded ZIF-8-PDMS nanohybrid membranes exhibiting exceptionally high biobutanol-permselective pervaporation, Angew. Chem. Int. Ed. Engl. 53 (2014) 5578 5582. [52] R. Wang, L. Shan, G. Zhang, S. Ji, Multiple sprayed composite membranes with high flux for alcohol permselective pervaporation, J. Membr. Sci. 432 (2013) 33 41. [53] H. Tang, S. Ji, L. Gong, H. Guo, G. Zhang, Tubular ceramic-based multilayer separation membranes using spray layer-by-layer assembly, Polym. Chem. 4 (2013) 5621 5628. [54] J.Y. Lee, D.H. Olson, L. Pan, T.J. Emge, J. Li, Microporous metal-organic frameworks with high gas sorption and separation capacity, Adv. Funct. Mater. 17 (2007) 1255 1262. [55] X.L. Wang, J.X. Chen, M.Q. Fang, T. Wang, L.X. Yu, J.D. Li, ZIF-7/PDMS mixed matrix membranes for pervaporation recovery of butanol from aqueous solution, Sep. Purif. Technol. 163 (2016) 39 47. [56] X. Liu, H. Jin, Y. Li, H. Bux, Z. Hu, Y. Ban, et al., Metal-organic framework ZIF-8 nanocomposite membrane for efficient recovery of furfural via pervaporation and vapor permeation, J. Membr. Sci. 428 (2013) 498 506. [57] F. Lipnizki, S. Hausmanns, P.K. Ten, R.W. Field, G. Laufenberg, Organophilic pervaporation: prospects and performance, Chem. Eng. J. 73 (1999) 113 129. [58] S. Li, F. Qin, P. Qin, M.N. Karim, T. Tan, Preparation of PDMS membrane using water as solvent for pervaporation separation of butanol-water mixture, Green. Chem. 15 (2013) 2180. [59] S. Hu, W. Ren, D. Cai, T.C. Hughes, P. Qin, T. Tan, A mixed matrix membrane for butanol pervaporation based on micron-sized silicalite-1 as macro-crosslinkers, J. Membr. Sci. 533 (2017) 270 278. [60] H. Yin, A. Khosravi, L. O’Connor, A.Q. Tagaban, L. Wilson, B. Houck, et al., Effect of ZIF-71 particle size on free-standing ZIF-71/PDMS composite membrane performances for ethanol and 1-butanol removal from water through pervaporation, Ind. Eng. Chem. Res. 56 (2017) 9167 9176. [61] X. Zhan, J. Lu, H. Xu, J. Liu, X. Liu, X. Cao, et al., Enhanced pervaporation performance of PDMS membranes based on nano-sized Octa[ (trimethoxysilyl)ethyl]-POSS as macro-crosslinker, Appl. Surf. Sci. 473 (2019) 785 798. [62] H. Zhou, J. Zhang, Y. Wan, W. Jin, Fabrication of high silicalite-1 content filled PDMS thin composite pervaporation membrane for the separation of ethanol from aqueous solutions, J. Membr. Sci. 524 (2017) 1 11. [63] A. Khan, M. Ali, A. Ilyas, P. Naik, I.F.J. Vankelecom, M.A. Gilani, et al., ZIF-67 filled PDMS mixed matrix membranes for recovery of ethanol via pervaporation, Sep. Purif. Technol. 206 (2018) 50 58. [64] M.E. Schweinefuss, S. Springer, I.A. Baburin, T. Hikov, K. Huber, S. Leoni, et al., Zeolitic imidazolate framework-71 nanocrystals and a novel SODtype polymorph: solution mediated phase transformations, phase selection via coordination modulation and a density functional theory derived energy landscape, Dalton Trans. 43 (2014) 3528 3536. [65] K. Zhang, R.P. Lively, M.E. Dose, A.J. Brown, C. Zhang, J. Chung, et al., Alcohol and water adsorption in zeolitic imidazolate frameworks, Chem. Commun. 49 (2013) 3245 3247. [66] D. Xueliang, Y.S. Lin, Synthesis of an organophilic ZIF-71 membrane for pervaporation solvent separation, Chem. Commun. 49 (2013) 1196 1198. [67] J.C. Saint Remi, T. Re´my, H.V. Van, dP.S. Van, T. Duerinck, M. Maes, et al., Biobutanol separation with the metal-organic framework ZIF-8, Chemsuschem 4 (2011) 1074 1077.

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[68] Y. Bai, L. Dong, C. Zhang, J. Gu, Y. Sun, L. Zhang, et al., ZIF-8 Filled Polydimethylsiloxane Membranes for Pervaporative Separation of nButanol from Aqueous Solution, Sep. Sci. Technol. 48 (2013) 2531 2539. [69] Y. Li, J. Shen, K. Guan, G. Liu, H. Zhou, W. Jin, PEBA/ceramic hollow fiber composite membrane for high-efficiency recovery of bio-butanol via pervaporation, J. Membr. Sci. 510 (2016) 338 347. [70] A.E. Yildirim, N.D. Hilmioglu, S. Tulbentci, Separation of benzene/ cyclohexane mixtures by pervaporation using PEBA membranes, Desalination 219 (2008) 14 25. [71] T. Wu, N. Wang, J. Li, L. Wang, W. Zhang, G. Zhang, et al., Tubular thermal crosslinked-PEBA/ceramic membrane for aromatic/aliphatic pervaporation, J. Membr. Sci. 486 (2015) 1 9. [72] M.K. Djebbar, Q. Nguyen, R. Clement, Y. Germain, Pervaporation of aqueous ester solutions through hydrophobic poly (ether-block-amide) copolymer membranes, J. Membr. Sci. 146 (1998) 125 133. [73] L. Liu, A. Chakma, X. Feng, CO2/N2 separation by poly (ether block amide) thin film hollow fiber composite membranes, Ind. Eng. Chem. Res. 44 (2005) 6874 6882. [74] A.G. Fadeev, S.S. Kelley, J.D. McMillan, Y.A. Selinskaya, V.S. Khotimsky, V.V. Volkov, Effect of yeast fermentation by-products on poly[1(trimethylsilyl) 2 1-propyne] pervaporative performance, J. Membr. Sci. 214 (2003) 229 238. [75] A. Fadeev, Y.A. Selinskaya, S. Kelley, M. Meagher, E. Litvinova, V. Khotimsky, et al., Extraction of butanol from aqueous solutions by pervaporation through poly (1-trimethylsilyl-1-propyne), J. Membr. Sci. 186 (2001) 205 217. [76] P. Sukitpaneenit, T.-S. Chung, L.Y. Jiang, Modified pore-flow model for pervaporation mass transport in PVDF hollow fiber membranes for ethanol-water separation, J. Membr. Sci. 362 (2010) 393 406. [77] F. Liu, L. Liu, X. Feng, Separation of acetone-butanol-ethanol (ABE) from dilute aqueous solutions by pervaporation, Sep. Purif. Technol. 42 (2005) 273 282. [78] A. Phan, C.J. Doonan, F.J. Uribe-Romo, C.B. Knobler, M. O’keeffe, O.M. Yaghi, Synthesis, structure, and carbon dioxide capture properties of zeolitic imidazolate frameworks, Acc. Chem. Res. 43 (2010) 58 67. [79] S. Liu, G. Liu, J. Shen, W. Jin, Fabrication of MOFs/PEBA mixed matrix membranes and their application in bio-butanol production, Sep. Purif. Technol. 133 (2014) 40 47. [80] S.I. Semenova, H. Ohya, K. Soontarapa, Hydrophilic membranes for pervaporation an analytical review, Desalination 110 (1997) 251. [81] Z. Wei, Q. Liu, C. Wu, H. Wang, H. Wang, Viscosity-driven in situ selfassembly strategy to fabricate cross-linked ZIF-90/PVA hybrid membranes for ethanol dehydration via pervaporation, Sep. Purif. Technol. 201 (2018) 256 267. [82] Q. Li, Q. Liu, J. Zhao, Y. Hua, J. Sun, J. Duan, et al., High efficient water/ ethanol separation by a mixed matrix membrane incorporating MOF filler with high water adsorption capacity, J. Membr. Sci. 544 (2017) 68 78. [83] J.H. Chen, Q.L. Liu, A.M. Zhu, Q.G. Zhang, Dehydration of acetic acid by pervaporation using SPEK-C/PVA blend membranes, J. Membr. Sci. 320 (2008) 416 422.

Chapter 13 Polymer/metal-organic frameworks membranes and pervaporation

[84] S. Khoonsap, S. Amnuaypanich, Mixed matrix membranes prepared from poly(vinyl alcohol) (PVA) incorporated with zeolite 4A-graft-poly (2-hydroxyethyl methacrylate) (zeolite-g-PHEMA) for the pervaporation dehydration of water-acetone mixtures, J. Membr. Sci. 367 (2011) 182 189. [85] C. Jasmina Hafizovic, J. SøRen, O. Unni, G. Nathalie, L. Carlo, B. Silvia, et al., A new zirconium inorganic building brick forming metal organic frameworks with exceptional stability, J. Am. Chem. Soc. 130 (2008) 13850 13851. [86] M.J. Katz, Z.J. Brown, Y.J. Colo´n, P.W. Siu, K.A. Scheidt, R.Q. Snurr, et al., A facile synthesis of UiO-66, UiO-67 and their derivatives, Chem. Commun. 49 (2013) 9449 9451. [87] A. Schaate, P. Roy, J. Lippke, F. Waltz, M. Wiebcke, Modulated synthesis of Zr-based metal-organic frameworks: from nano to single crystals, Chemistry (Weinheim an der Bergstrasse, Germany). [88] L. Thierry, S. Christian, H. Clarisse, F. Gerhard, T. Francis, H. Marc, et al., A rationale for the large breathing of the porous aluminum terephthalate (MIL-53) upon hydration, Chem. Eur. J. 10 (2010) 1373 1382. [89] E.W. Neuse, Aromatic polybenzimidazoles. Syntheses, properties, and applications, Adv. Polym. Sci. 47 (1982) 1 42. [90] M. Jaffe, P. Chen, E.-W. Choe, T.-S. Chung, S. Makhija, High performance polymer blends, Adv. Polym. Sci. 117 (1994) 297. [91] Y. Wang, M. Gruender, T.S. Chung, Pervaporation dehydration of ethylene glycol through polybenzimidazole (PBI)-based membranes. 1. Membrane fabrication, J. Membr. Sci. 363 (2010) 149 159. [92] S.S. Hosseini, M.M. Teoh, S.C. Tai, Hydrogen separation and purification in membranes of miscible polymer blends with interpenetration networks, Polymer 49 (2008) 1594 1603. [93] T.S. Chung, A critical review of polybenzimidazoles: historical development and future R and D, J. Macromol. Sci. Rev. Macromol. Chem. Phys. C 37 (1997) 277 301. [94] T.S. Chung, Y.J. Lan, L. Yi, S. Kulprathipanja, Mixed matrix membranes (MMMs) comprising organic polymers with dispersed inorganic fillers for gas separation, Prog. Polym. Sci. 32 (2007) 483 507. [95] C. Xin, W. Li, Z. Wei, Y. Lu, T. Yu, pH sensitivity and ion sensitivity of hydrogels based on complex-forming chitosan/silk fibroin interpenetrating polymer network, J. Appl. Polym. Sci. 65 (2015) 2257 2262. [96] R.Y.M. Huang, G.Y. Moon, R. Pal, Chitosan/anionic surfactant complex membranes for the pervaporation separation of methanol/MTBE and characterization of the polymer/surfactant system, J. Membr. Sci. 184 (2001) 1 15. [97] R.Y.M. Huang, R. Pal, G.Y. Moon, Crosslinked chitosan composite membrane for the pervaporation dehydration of alcohol mixtures and enhancement of structural stability of chitosan/polysulfone composite membranes, J. Membr. Sci. 160 (1999) 17 30. [98] G.Y. Moon, R. Pal, R.Y.M. Huang, Novel two-ply composite membranes of chitosan and sodium alginate for the pervaporation dehydration of isopropanol and ethanol, J. Membr. Sci. 156 (1999) 17 27. [99] O.K. Farha, J.T. Hupp, Rational design, synthesis, purification, and activation of metal-organic framework materials, Acc. Chem. Res. 43 (2010) 1166 1175.

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Computational modeling of pervaporation process

14

Muhammad Mujiburohman Department of Chemical Engineering, Muhammadiyah University of Surakarta, Surakarta, Indonesia

14.1

Definition

The term “pervaporation” (PV) originates from two words “permeation” and “evaporation.” The term was introduced by Kober [1] due to the fact that during PV there was permeation of liquid permeant through the membrane, followed by the phase change of the liquid permeant into vapor. At least two hypotheses come up regarding to the point where the evaporation takes place: on the membrane surface of permeate side as desorption step (1) and somewhere inside the membrane (2). The schematic process of PV is given in Fig. 14.1. All membrane-based separation processes have similar schema, consisting of three streams around the membrane, that is, feed, permeate, and retentate. The unique condition applied in PV is that the permeate side is kept vacuum. The main product may be permeate, like in the removal of volatile organic compounds from aqueous solutions [2], or retentate as found in the dehydration of alcohols [3]. The membrane used in PV is a dense or nonporous membrane, which is generally arranged with a porous medium, socalled composite. The dense surface acts as a selective layer where the separation undergoes, while the porous medium supports the mechanical strength of the dense layer. Most membrane materials are polymers, which can be synthesized through various modifications. Based on the types of permeant to be preferentially permeated, membrane is classified into two, that is, hydrophilic or organophobic membrane (more permeable to water), and hydrophobic or organophilic membrane (more permeable to organic compounds). To provide

Polymer Nanocomposite Membranes for Pervaporation. DOI: https://doi.org/10.1016/B978-0-12-816785-4.00014-8 © 2020 Elsevier Inc. All rights reserved.

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Chapter 14 Computational modeling of pervaporation process

Feed (liquid phase)

Dense membrane Membrane chambe r

Permeate (vapor phase)

Retentate (liquid phase)

Figure 14.1 A schematic representation of PV process. PV, Pervaporation.

membrane area accepted in industrial applications, the membrane is packaged in a compartment so-called module. Several membrane modules have been developed including plateframe, tubular, spiral wound, and hollow fiber. The commercial PV has been established, particularly for dehydration of alcohols (ethanol, iso-propanol; most by GFT Company), and some for removal of volatile organic compounds from water (most by Membrane Technology and Research).

14.2

Pervaporation performance

Similar to other separation methods, PV has parameters to measure the separation performance of membrane, that is, permeation flux and selectivity. The permeation flux denotes the capacity of permeation, expressed in terms of the amount of permeant (usually in mass) passing through the membrane per unit of time per unit of cross-sectional area of membrane, or mathematically follows [4]: J5

Q Am t

ð14:1Þ

where J is the permeation flux of permeant, Q is the amount of permeant i collected at certain period of time, t is the period of time of operation, and Am is the effective cross-sectional area of membrane.

Chapter 14 Computational modeling of pervaporation process

The selectivity reflects the capability of membrane to selectively separate the components in the feed. The selectivity of membrane may be stated in the following terms:

14.2.1

Enrichment factor

Enrichment factor (β) is the ratio of concentration of permeant in the permeate stream (Ci,P) over that in the feed (Ci,F), or mathematically [4]: βi 5

Ci;P Ci;F

ð14:2Þ

The enrichment factor does not take into account directly the existence of other components (less permeable) in the stream. However, the simple correlation of enrichment factor is useful when more than two components exist in the feed mixture.

14.2.2

Separation factor

The separation factor (α) considers the existence of less permeable components in both permeate and feed streams. For ternary system (two permeants i and j and one membrane), the separation factor is given by [4]:
 αij 5


Ci;P Cj;P Ci;F Cj;F



ð14:3Þ

In addition to the above selectivity parameters, some selectivity terms are also used such as sorption selectivity, mobility selectivity, and permeability selectivity.

14.3

Process conditions

Process conditions are any conditions that can be set to influence the PV performance, including feed concentration, temperature, feed pressure, thickness of membrane, permeate pressure, and types of membrane. The followings are the effects of each process condition on the PV performance: • Feed concentration Generally speaking, the higher feed concentration increases significantly both permeation flux and selectivity. This makes sense since the controlling rate in PV is diffusion through membrane, which is driven by the concentration

357

358

Chapter 14 Computational modeling of pervaporation process

gradient between the two sides of membrane surfaces (detailed mass transfer in PV is discussed in the next section). The higher feed concentration causes higher concentration of preferentially sorbed permeant on the membrane surface of feed side; while that on the membrane surface of permeate side is relatively low due to vacuum condition kept in the permeate side. • Temperature The increase in temperature increases the frequency of thermal motion of polymeric membrane and thus promotes the diffusion. As a matter of fact, the promoted diffusing permeants are not only the preferentially sorbed permeant but also the others. Those having smaller molecular sizes diffuse more easily. Generally speaking, the higher temperature increases significantly the permeation flux but slightly the selectivity. • Feed pressure In PV operation the feed pressure is typically atmospheric. Increasing feed pressure increases the feed chemical potential or effective feed concentration; this inevitably increases the driving force of permeation since the permeate pressure is kept low. Such treatment, however, is not commonly applied. • Thickness of membrane The diffusion flux is inversely proportional to the thickness of membrane. To maximize the permeation flux in PV, the thickness of membrane is made as thin as possible. In practice, to provide high permeation flux as well as selectivity, the dense selective membrane is made extremely thin, supported by larger porous media. • Permeate pressure The driving force in PV may be expressed in term of the chemical potential gradient between the feed and the permeate sides. For a constant feed pressure, lowering the permeate pressure increases the chemical potential gradient, and thus the permeation flux. In practice, the permeate pressure is made as low as possible (vacuum condition), depending on the performance of equipment. • Type of membrane Various membrane materials and membrane preparation methods have been developed to enhance the PV performance. The most significant finding in membrane preparation was invented by Loeb and Sourirajan [5] where a thin selective dense layer serried with larger porous layer could be made. All commercial dense membranes use such pattern. The development of

Chapter 14 Computational modeling of pervaporation process

membrane materials (polymers) is directed to the invention of new membrane materials through polymerization or blending that fulfill the following criteria: accepted permeation and selectivity, good chemical and thermal resistance, and cheap. The selection of membrane material primarily depends on the preferential sorbed permeant, the first step of mass transfer in PV.

14.4

Mass transfer in pervaporation

To have a comprehensive insight of mass transfer in PV, various models of mass transfer through membrane are discussed in this section. All membrane-based separations involve either porous or nonporous membranes. These membrane structures affect the mechanism of mass transfer of permeant through the membrane, as illustrated in Fig. 14.2. In principle, any transfer rate is proportional to a driving force, something that initiates the transport process, and a conductance or such kind of proportionality coefficient. A qualitative expression to express the transfer rate may be written as Eq. (14.4). Transfer rate 5 Conductance 3 Driving force

ð14:4Þ

The mass transfer rate of permeant through membrane also follows Eq. (14.4) and is elucidated in the following models.

14.4.1

Pore flow model

Pore flow model is applied for porous membrane where the permeant passes through the membrane due to the driving force of pressure drop across the membrane. It is similar to the flow of fluid through a pipe, and thus Darcy’s equation [Eq. (14.5)] can be applied [6]. (A)

(B)

Figure 14.2 Mass transfer of permeant through porous (A) and nonporous (B) membranes.

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Chapter 14 Computational modeling of pervaporation process

Ji 5 2 K 0 :Ci :

dP dx

ð14:5Þ

where dP/dx is the pressure gradient existing in the pores of membrane, acting as the driving force; while Ci and K0 are the concentration of permeant i in the membrane and a constant reflecting the nature of the medium, respectively. Some researchers classified further the pore flow model based on the size of pores, as follows (illustrated in Fig. 14.3): • Convective flow Convective flow applies for large pores (larger than 0.1 μm), and no separation occurs. In such condition, Eq. (14.5) well describes the permeation flux. • Knudsen diffusion Knudsen diffusion may occur when the diameter of pores (d) is typically smaller than 0.1 μm, similar size or smaller than the mean free path (l) of molecule, but still larger than the molecular size of permeant. The separation may take place partially. • Molecular sieving or surface diffusion

Figure 14.3 The pore flow model with different sizes of pores (AC) and the solution-diffusion model (D). Source: Courtesy H. Yin, A.C.K. Yip, A Review on the production and purification of biomass-derived hydrogen using emerging membrane technologies, Catalyst 7 (2017) 297 [7].

Chapter 14 Computational modeling of pervaporation process

361

˚ ), The pores are extremely small (in the range of 520 A which are very close to the molecular sizes of the smaller compounds. The separation results in better purity than that by Knudsen diffusion. The porous medium is rigid so that larger molecules cannot be absorbed and diffuse through the medium like in dense membrane.

14.4.2

Solution-diffusion model

The solution-diffusion model is well accepted to describe the mass transfer of permeant through a dense membrane as found in PV (Fig. 14.3D) [812]. In PV the movement of permeant from the feed (bulk solution) to the permeate side undergoes three consecutive steps, that is, sorption, diffusion, and desorption, as illustrated in Fig. 14.4.

14.4.2.1

Sorption

Sorption is an interphase mass transfer where the permeant in the bulk feed moves onto the surface of membrane. Using concentration difference as driving force, the mass flux of sorption may be written as:   Ji 5 kc : Ci;F 2 Ci;0 ð14:6Þ where kc, Ci,F, and Ci,0 are the mass transfer coefficient and the concentration of permeant i in the feed bulk, and that on the Feed side (liquid phase) PF = PM Preferential permeant

Sorption Nonpreferential permeant Diffusion

PP Desorption

CF,i CsF,i CsP,i

CP,i

Permeate side (vapor phase) lm Membrane thickness

Figure 14.4 Schematic representation of solutiondiffusion model in PV. PV, Pervaporation.

362

Chapter 14 Computational modeling of pervaporation process

membrane surface of feed side, respectively. Once the permeant is sorbed on the surface of membrane, it will swell the membrane and increase the free volume of polymeric membrane.

14.4.2.2

Diffusion

Diffusion is a molecular mass transfer taking place within one phase (in this case, membrane medium) due to the driving force of concentration gradient. The increase of free volume due to membrane swollen facilitates the sorbed permeant to diffuse through the membrane, down to the permeate side which has lower concentration. The diffusion flux of permeant i (Ji) through the membrane follows Fick’s law [8]: Ji 5 2 Di

dCi dx

ð14:7Þ

The algebraic equation of the diffusion flux form Eq. (14.7) can vary depending on the diffusivity dependence. For the simplest case where the diffusivity Di is considered constant (PV is operated at constant temperature), and the diffusivity is independent on concentration (Ci) [13], the diffusion flux in Eq. (14.7) is simply integrated to be,   Ci;0 2 Ci;l Ji 5 Di ð14:8Þ l where Ci,l, and l are the concentration of permeant i on the membrane surface of permeate side and the thickness of membrane, respectively.

14.4.2.3

Desorption

Desorption is antithesis of sorption, where the permeant on the membrane surface of permeate side moves into permeate stream. Since PV applies vacuum pressure in the permeate side, when reaching the membrane surface of permeate side, the permeant will quickly move into permeate stream as vapor. Both sorption and desorption occur very fast compared to the diffusion step. It may be stated that the controlling rate of mass transfer in PV is the diffusion step. Eq. (14.8), however, is not practically used because the concentrations of permeant on both membrane surfaces are difficult to measure. The concentration of permeant on the membrane surface of feed side can be correlated with that in the bulk feed, and generally follows Henry’s law in which the concentration of permeant on the

Chapter 14 Computational modeling of pervaporation process

membrane surface of feed side (Ci,0) is linearly dependent on that in the bulk feed (Ci,F), or mathematically: Ci;0 5 Si :Ci;F

ð14:9Þ

where Si is the sorption coefficient, which is obtained from sorption experiments. Due to vacuum condition, the concentration of permeant on the membrane surface of permeate side (Ci,l) is much smaller than that of feed side (Ci,0). Thus Eq. (14.8) can be simplified: Ji 5

Di :Si Pi Ci;F 5 Ci;F l l

ð14:10Þ

where Pi is the permeability of permeant through the corresponding membrane. Eq. (14.10) reveals that the permeation flux in PV can be increased by: • increasing the feed concentration, • reducing the thickness of membrane as thin as possible, and • increasing the permeability, which is essentially affected by the physicochemical properties of permeants and membrane material.

14.4.3

Modified solution-diffusion model

Fujita [14] and Huang [4] modified the solution-diffusion model by taking into account the volume fraction of liquid (permeant) in the membrane (vi), as follows: Ji 5 2

Di dCi ð1 2 vi Þ dx

ð14:11Þ

The volume fraction vi represents the free volume of the membrane that can be occupied by the permeant. Eq. (14.11) reveals that the higher volume fraction in the membrane gives higher permeation flux. The diffusivity itself may be affected by the volume fraction of permeant, the permeant activity (ai, an effective concentration), and the thermodynamic diffusivity (DT ) Di 5 DT ;i ð1 2 vi Þ with

 DT ;i 5 RTAd;i exp 2

dlnai dlnvi

Bd;i f ð1 2 Φc Þ

ð14:12Þ  ð14:13Þ

363

364

Chapter 14 Computational modeling of pervaporation process

where R is the gas constant, Ad and Bd are the parameters related to the shape and size of the permeant, f is the fractional free volume affected by temperature (T), and Φc is the crystallinity of the membrane. Eqs. (14.12) and (14.13) reveal that the diffusivity increases as the temperature increases, as the molecular size decreases, as the fractional free volume increases, and as the crystallinity of the membrane decreases or the membrane tends to be rubbery. The permeant activity can be determined using the FloryHuggins thermodynamics correlation [15]:   d lnai vi 512 12 ð14:14Þ vi 2 2χim vi vm d lnvi vm where vi and vm are the molar volumes of permeant and membrane, respectively. χim is the interaction parameter between the permeant and the polymeric membrane, which is defined by: χ52

½lnð1 2 vm Þ 1 vm  2 vm

ð14:15Þ

The modified solution-diffusion model proposed by [4,14] essentially correlates the diffusivity with the free volume, effective concentration (activity), temperature, shape and size of permeant, and the permeantmembrane interaction.

14.4.4

Thermodynamics model

Another approach to describe the mass transfer of permeant through membrane is derived from the thermodynamic point of view. Thermodynamically the driving force of mass transfer of permeant through membrane is a chemical potential gradient across the membrane (dμi/dx), or mathematically [5], J i 5 2 Li :

dμi dx

ð14:16Þ

where Li is the coefficient of proportionality. The chemical potential itself is affected by concentration (in mole fraction, ni), pressure (p), temperature (T), and molar volume (vi) as follows:   ð14:17Þ dμi 5 RTdln γ i ni 1 vi dp

Chapter 14 Computational modeling of pervaporation process

where γ i and R are the activity coefficient and the gas constant, respectively. In PV the pressure inside the membrane is considered constant, similar to the higher pressure of the feed side. Considering the permeant as an ideal fluid (γ i 5 1), substituting Eq. (14.17) into (16) gives: Ji 5 2 RTLi

d lnni RTLi dni 52 : dx ni dx

ð14:18Þ

The mole fraction ni can be replaced by concentration Ci to give Ji 5 2

RTLi dCi : Ci dx

ð14:19Þ

The term (RTLi/Ci) basically represents a proportionality coefficient and is similar to diffusivity Di in Fick’s law [Eq. (14.7)].

14.4.5

MaxwellStefan model

Due to the fact that the molecular interactions of permeantpermeant and permeantmembrane affect the movement behavior of permeant inside the membrane, Mason and Viehland [16] proposed a model, so-called generalized MaxwellStefan (GMS). The model was derived from the statistical mechanics, correlating the driving force of permeation, that is, the chemical potential gradient [dμi/dx; similar to Eq. (14.16)], with the friction resistances of the mixture, as follows: n X   RT dμi 5 xj vj 2 vi 0 dx Dji j51

ð14:20Þ

where x and v are the mole fraction and the local velocity of permeant, respectively. The term (RT/D0ji) represents a friction coefficient taking into account the frictional effect exerted by permeant j on permeant i. Heintz and Stephan [17] then modified the GMS, replacing the mole fraction x with mass fraction w0, and considering Dij 5 Dji (coupled diffusion coefficient). The modified MS basically modifies the solution-diffusion model by taking into account the nonideal multicomponent solubility effect,

365

366

Chapter 14 Computational modeling of pervaporation process

nonideal diffusivity of all permeants, concentration dependent density of the membrane, and diffusion coupling. In case of ternary system (permeant 1, 2, and membrane M), the equation of modified MS follows: 0

J1 J2

1 0 1 0 0 0 0 w1 D2M 1 D12 dw1 w1 D2M @ A @ AρM dw2 1 D1M 5 D1M ρM 0 0 0 0 D12 1 w1 D2M 1 w2 D1M dx D12 1 w1 D2M 1 w2 D1M dx 0 1 0 1 0 0 0 0 w2 D1M 1 D12 w2 D1M AρM dw2 1 D2M @ AρM dw1 5 D2M @ 0 0 0 0 dx dx D12 1 w2 D1M 1 w1 D2M D12 1 w2 D1M 1 w1 D2M

ð14:21Þ where ρM is the average density of the swollen membrane, which is defined by: ð w0 iF  0 0 ρM wi dwi ρM 5

0

wiP

0

0

wiF 2 wiP

ð14:22Þ

The subscripts F and P represent the membrane surface of feed side and permeate side, respectively.

14.4.6

Computational model

A computational simulation has been made to figure out the movement of permeant inside the nonporous membrane. It is conceived that a permeant molecule initially occupies a micro˚ in diameter) called free volume of polymer cavity (typically 5 A matrix. At certain period of time the permeant molecule just moves around in the microcavity, and jumps into other cavities when the free volume moves due to thermal motion, as illustrated in Fig. 14.5 [18]. Such mechanism undergoes repeatedly until the permeant molecule reaches the membrane surface of permeate side. The simulation considers the frequency of movement and jump of permeant molecule is affected by the molecular size. ˚ ) moves more frequently and For description, helium (2.55 A ˚ ) and methane (3.76 A ˚ ) as larger jump than oxygen (3.47 A shown in Fig. 14.6 [19]. Unfortunately the diffusivity obtained by this simulation is far different from the apparent diffusivity obtained by experiments. Among the various models discussed above, most researchers consider the solution-diffusion is the most well accepted model to describe the mass transfer of permeant through dense membrane as applied in PV.

Chapter 14 Computational modeling of pervaporation process

367

Figure 14.5 Description of movement of permeant through a dense membrane. Source: Courtesy E. Smit, M.H.V. Mulder, C.A. Smolders, H. Karrenbeld, J. van Earden, D. Feil, Modeling of the diffusion of carbon dioxide in polyimide matrices by computer simulation, J. Membr. Sci. 73 (1992) 247.

14.5

Transport properties in pervaporation

Solubility and diffusivity are two primary transport properties constituting the solution-diffusion model. Solubility denotes the maximum capacity of membrane to absorb permeant; in other words, it is the saturated or maximum concentration of permeant in the membrane, generally expressed in terms of mass of permeant sorbed per mass of dry membrane. Solubility relates directly to the sorption coefficient where the higher sorption coefficient shows higher solubility. The combination of sorption coefficient and diffusivity then

368

Chapter 14 Computational modeling of pervaporation process

Figure 14.6 The movement (rotation and jumping) of permeant with different molecular sizes. Source: Courtesy S.G. Charati, S.A. Stern, Diffusion of gases in silicone polymers: molecular dynamic simulations, Macromolecules 31 (1998) 5529.

constitutes permeability (Eq. 14.10). Experimentally for binary system of permeant i and membrane, the solubility of an individual permeant (pure component) in a dense membrane can be determined through the following experimental procedure: • A certain weight of dry membrane is prepared. • The dry membrane is then soaked in the permeant solution; the permeant is absorbed and swells the membrane. • The solubility is obtained when the weight of swollen membrane has been relatively constant (considered to be achieved at t 5 N, infinite time). For ternary system, the experimental work to determine the solubility of permeant i in the presence of permeant (solvent) j is similar to above procedure, but the initial concentration of permeant i in the solution is varied. If the concentration of permeant i in the membrane is linearly dependent on that in the bulk solution, a constant of sorption coefficient (Si) is obtained, which is the slope of the line (Fig. 14.7). The sorption equilibrium may also be expressed in term of volume fraction of the swelling of membrane (1 2 φp) as derived in the FloryHuggins thermodynamics [15]. The volume fraction of the swelling of membrane is affected by the nature of permeant and membrane; several cases are correlated subsequently.

Concentration in the membrane

Chapter 14 Computational modeling of pervaporation process

0.3 0.25 0.2 0.15 0.1 0.05 0 0

0.1

0.2 0.3 0.4 0.5 Concentration in the solution

0.6

0.7

Figure 14.7 The typical experimental data of sorption experiments; the x axis denotes the concentration of permeant in the bulk solution, and the y axis is that in the membrane. The slope of line is the sorption coefficient of corresponding system.

14.6

Sorption of pure liquid i in an amorphous polymer

 
 Δμi 1 5 ln ai 5 ln 1 2 φp 1 φp 1 2 1 χip φ2p 5 0 ni RT

ð14:23Þ

where ni is the molar volume ratio between the polymer and permeant i (Vp/Vi), φp is the volume fraction of polymeric membrane, and χ is the interaction parameter of permeantmembrane. In swollen membrane the values of χ are higher than 0.5. A simplified relationship has been proposed to evaluate the interaction parameter of permeantmembrane [Eq. (14.15)]. With given value of interaction parameter, the volume fraction of the swelling of membrane can be determined. • Sorption of binary liquid mixture (permeant 1 and 2) in an amorphous polymer p      
    ϕ1 v1 φ ln 2 ln 5 ðl 2 1Þln 2 2 χ12 ðv1 2 v2 Þ 1 ϕ2 1 ϕ1 2 ϕp χ1p 2 lχ2p ϕ2 v2 v2 ð14:24Þ where v and l are the specific volume of permeant and the thickness of membrane, respectively. • Sorption of pure liquid i in cross-linked polymer (to restrict the swelling of membrane)  
 ρp
1=3 ϕp  1 ln 1 2 ϕp 1 ϕp 1 2 ϕ 2 1 χip ϕ2p 1 Vi 50 ni Mc p 2 ð14:25Þ

369

370

Chapter 14 Computational modeling of pervaporation process

where ρp and Mc are the density of polymer and the molecular weight between two crosslinks, respectively. Diffusivity or diffusion coefficient reflects the ability of permeant to diffuse through the membrane. As discussed in the computational model, in addition to concentration gradient, the movement of permeant through the membrane is also caused by the movement of free volume of polymer matrix due to thermal motion. Experimentally the diffusivity can be determined using various methods:

14.6.1

Time-dependence of sorption

The data of sorption experiments can also be used to determine the diffusivity. Once the permeant is sorbed on the two sides of membrane surfaces, the permeant swells the membrane and diffuses towards inside the membrane. The changes of membrane weight (mass) as the sorption time runs basically represents how much the permeant diffuses through the membrane. From the primary data of sorption experiments, that is, mass of swollen membrane (M) versus time (t), modified data correlating the ratio of mass of swollen membrane at any time over that at infinite time (Mt/MN) and the squared root time (t1/2) give Fig. 14.8. From Fig. 14.8, the diffusivity may be obtained by one of the following three conditions: 1. At early sorption or small values of t1/2 At early sorption the curve shown in Fig. 14.8 tends to be linear and follows [20,21]:   Mt 4 D 1=2 1=2 5 : :t ð14:26Þ l π MN

1 Mt /M∞ 0.5

Figure 14.8 The modified data of sorption experiments, that is, (Mt/MN) versus (t1/2).

0

t

t1/2

Chapter 14 Computational modeling of pervaporation process

The term of [4/l.(D/π)1/2] is the slope of the early line; with given thickness of membrane l the diffusivity D can be determined. 2. At half time (Mt/MN 5 0.5) In case Mt/MN 5 0.5, executing Eq. (14.26) gives D5

0:04919
τ  : l2

ð14:27Þ

where τ is the observed time when the mass of swollen membrane is half of the equilibrium or saturated mass. 3. At late times 4. At late times (Mt/MN . 0.5) the curve in Fig. 14.8 increases exponentially and can be written as [20]:   Mt 8 π2 :D:t 5 1 2 2 exp 2 2 ð14:28Þ π l MN Eq. (14.28) can be modified to be linear, and the diffusivity is evaluated from the slope of the straight line:     Mt π2 :D 8 ln 1 2 5 2 2 t 1 ln 2 ð14:29Þ l π MN

14.6.2

Time-lag experiment

The method of time-lag experiment basically uses the primary data obtained from PV experiments from starting up at unsteady state to the stable condition or steady state, as illustrated in Fig. 14.9. The interval time to achieve the steady state from the initial time is called time lag (L, Fig. 14.9 shows L 5 20 h), which is affected by the thickness of membrane (l) and the diffusivity of permeant (D), as formulated by [2022]: L5

l2 : 6D

ð14:30Þ

The steady state is generally achieved at operating time higher than 3L. When the steady state occurs, Eq. (14.10) can be applied, and with given sorption coefficient (Si) obtained from separated experiments (i.e., sorption experiment), the diffusivity can be determined using regression method. However, it must be noticed that the time-lag method is only valid when the diffusivity has small dependence or independent on concentration.

371

Chapter 14 Computational modeling of pervaporation process

Cumulative amount penetrated (mg)

372

120 100 80 60 Steady-state slope

40 20

LAG-time

0 0

20

40 Time (h)

60

80

Figure 14.9 The typical data of time-lag experiments.

14.6.3

Inverse gas chromatography method

In case the amount of sorbed permeant is extremely low, some researchers [23,24] used inverse gas chromatography (IGC) to determine the diffusivity. The measurement of diffusivity by IGC is based on partitioning of a substance between a mobile gas phase and a stationary polymer phase (membrane). As a matter of fact, the diffusivity through membrane is affected by several factors including molecular size, concentration, temperature, and type of membrane material. The presence of other permeants so-called coupling effect is also believed to influence the diffusivity. All these factors have been accommodated in the modified solution-diffusion model proposed by [4,14]. Equating Eqs. (14.7) and (14.19), the diffusivity is found to be concentration dependence. Instead of constant diffusivity [13], linearly dependent diffusivity on concentration was proposed by [9,10]:   ð14:31Þ Di 5 D0i : Aii Ci 1 Aij Cj where A denotes the permeant interaction or such kind of plasticizing coefficient; in most cases Aii is taken one, and Aij is zero for pure component (no coupling effect). The most accepted correlation of concentration dependence of diffusivity follows Arrhenius’s relationship [23]:   ð14:32Þ Di 5 D0i :exp Aii Ci 1 Aij Cj As shown in Eq. (14.10) the combination of diffusivity and sorption coefficient gives a constant so-called permeability (P).

Chapter 14 Computational modeling of pervaporation process

Both solubility and diffusivity are affected by temperature, and thus permeability as well. To find the solubility and diffusivity temperature dependence, the sorption and PV experiments are carried out at various temperatures. The effect of temperature (T) on diffusivity (D), permeability (P), and permeation flux (J) generally follows the Arrhenius’s relationship [24]:   Ed  Di 5 D :exp 2 ð14:33Þ RT   Ep P 5 P :exp 2 RT

ð14:34Þ

  E J 5 J  :exp 2 RT

ð14:35Þ



where Ed, Ep, and E are the activation energy of diffusion, permeation, and overall activation energy of PV, respectively. Ep and E are essentially same, and can be correlated with Ed as follows: Ep 5 Ed 1 ΔH

ð14:36Þ

where ΔH is the permeantpolymer heat of solution, such kind of activation energy for sorption.

14.7

Pervaporation modeling

The models or equations related to mass transfer in PV and its relevant parameters have been discussed in the previous section. This part gives detailed procedure in determining the apparent values of transport properties including sorption coefficient, diffusivity, and permeability. As well, some empirical correlations to predict the PV performance and modified designs of PV are presented.

14.7.1

Determination of sorption coefficient (S)

The primary data are obtained from sorption experiments, of which procedure has been given in the previous section. For ternary system of permeant i, j and membrane m where i is the preferential sorbed permeant, the typical primary data are as presented in Table 14.1. The equilibrium concentration is achieved at infinite time (tN) when the concentration of

373

374

Chapter 14 Computational modeling of pervaporation process

Table 14.1 The typical primary data of sorption experiments by measuring the concentration of permeant in the solution. The initial concentration i is varied, constant mass of dry membrane, and constant temperature No.

Sorption time

Concentration of permeant I in the Solution

1 2 3 4

t0 5 0 t1 t2 t3 . . .. tN

C0,1 5 the initial concentration 1 C1,1 C2,1 C3,1 . . .. CN,1 5 the equilibrium concentration 1

1 2 3 4

t0 5 0 t1 t2 t3 . . .. tN etc.

C0,2 5 the initial concentration 2 C1,2 C2,2 C3,2 . . .. CN,2 5 the equilibrium concentration 2

permeant i in the solution is relatively constant. This can also be indicated with the relatively unchanged mass of the swollen membrane. To obtain the equilibrium concentration of permeant i in the membrane, a mass balance of permeant i can be derived: Mass sorbed in the membrane 5 Mass at initial  Mass at equilibrium ð14:37Þ Considering the volume of solution (V) is constant, the mass sorbed in the membrane (mi,m) can be calculated: mi;m 5 ðC i;0 2 C i;N Þ:V

ð14:38Þ

The concentration of permeant in the membrane is usually expressed in term of mass of sorbed permeant per mass of dry membrane. A graph to correlate the equilibrium concentration in the solution and in the membrane then can be built as shown in Fig. 14.7. The slope of the line represents the sorption coefficient Si.

Chapter 14 Computational modeling of pervaporation process

14.7.2

375

Determination of diffusivity (D)

Instead of measuring the concentration of permeant in the solution, the sorption experiments may also measure the mass of swollen membrane at any times until the mass is relatively unchanged, as shown in Table 14.2. The primary data can be further processed to obtain the secondary data, the ratio of mass of swollen membrane at any time (Mt) over that at equilibrium (MN) or (Mt/MN). Correlating the (Mt/MN) and the squared root time (t1/2), a graph of time-dependence of sorption is obtained as shown in Fig. 14.8, and the diffusivity can be determined using Eqs. (14.26), (14.27), (14.28), or (14.29). The diffusivity can also be determined using experimental data of PV. The time-lag experiments measure the volumetric permeation flux (Q) versus the operating time from the initial operation (unsteady state) until the steady state is achieved, and the diffusivity is determined using Eq. (14.30). If the diffusivity is assumed to be independent on concentration or constant diffusivity, using the experimental data of PV at steady state, the diffusivity can also be determined using Eq. (14.10). In this case, the PV experiments vary the feed concentration of

Table 14.2 The typical primary data of sorption experiments by measuring the mass of swollen membrane. The initial concentration i is varied, constant mass of dry membrane, and constant temperature No.

Sorption time

Mass of swollen membrane

1 2 3 4

t0 5 0 t1 t2 t3 . . .. tN t0 5 0 t1 t2 t3 . . .. tN etc.

M0,1 5 the mass of dry membrane, initial concentration C0,1 M1,1 M2,1 M3,1 . . .. MN,1 5 the equilibrium mass of swollen membrane with initial concentration C0,1 M0,1 5 the mass of dry membrane, initial concentration C0,2 M1,2 M2,2 M3,2 . . .. MN,2 5 the equilibrium mass of swollen membrane with initial concentration C0,2

1 2 3 4

376

Chapter 14 Computational modeling of pervaporation process

Table 14.3 The typical primary data of steady-state PV experiments and the regression method to determine diffusivity. Constant feed and permeate pressure, thickness of membrane l, constant temperature with sorption coefficient Si, and membrane are Am Primary data No. 1 2 3 ...

Feed concentration, Ci,F . . .. . . . . .. . . . . .. . . . . .. . .

Processed data Permeation Flux, Ji . . .. . . . . .. . . . . .. . . . . .. . .

Ci,F. Ji . . .. . . . . .. . . . . .. . . . . .. . . P (Ci,F . Ji)

(Ci,F)2 . . .. . . . . .. . . . . .. . . . . .. . . P (Ci,F)2

permeant (Ci,F), and measure the steady state permeation flux (Ji). Because many data may be collected from the PV experiments, the determination of diffusivity uses regression method, which in principle minimizing the sum of squared error (SSE), as exampled in Table 14.3. To apply regression method, Eq. (14.10) is modified to be Ji 5 aCi;F 1 b

ð14:39Þ

where a and b are the constants of regression, each represents the slope and the intercept of the line, respectively. Equating Eqs. (14.39) and (14.10), the constant a equals to (Di.Si/l), while b is zero. To obtain the value of constant a, regression method applying SSE is used,  2  2   2  SSE 5 Ji;data 2Ji;calc 5 Ji;data 2 a:Ci;F 1b 5 Ji;data 2a:Ci;F ð14:40Þ The best constant a is the one that gives the minimum SSE. To minimize the SSE, the first differentiation of SSE to constant a must be zero, or dSSE 50 da

ð14:41Þ

Differentiating Eq. (14.41) gives the value of constant a:  P Ci;F :Ji a5 P ð14:42Þ 2 Ci;F

Chapter 14 Computational modeling of pervaporation process

The value of constant a is also easily determined graphically by a solver software; even MS Excel can do it. The diffusivity is then determined by, Di 5

14.7.3

a:l Si

ð14:43Þ

Determination of permeability (P)

Once the sorption coefficient Si and the diffusivity Di are known, the multiplication of both gives the permeability Pi [Eq. (14.10)].

14.7.4

Modified pervaporation process

To enhance the PV performance, several efforts have been conducted including modifications of process conditions, membrane materials, and process design. The effects of process conditions on the PV performance have been discussed in the previous section (Section 14.3). There are modifications of process design, but still limited. It is understood that to perform PV separation, it requires low pressure in the permeate side. In laboratory scale, the low pressure is conditioned by a vacuum pump, but it seems too costly for industrial scale. Some alternatives may be applied to replace the use of vacuum pump such as: the application of cooling system to condense the permeant vapor, and sweeping the permeate stream with counter-current flow of carrier gas, either noncondensable or condensable gas, either immiscible or miscible liquid after condensing. Another process design is the application of permeate decantation and water phase recycle to enhance the recovery of organic compounds from aqueous solutions. Such process designs are illustrated in Fig. 14.10. In particular to the application of permeate decantation and water phase recycle (Fig. 14.10F, modified in Fig. 14.11), Mujiburohman and Feng [25] have simulated the effects of two parametric studies, that is, the ratio of the amount of aroma compound in the feed over the membrane area (F0/Am), and the solubility of aroma compound in the water, on the extent of recovery of aroma compound, operated in a batch process. The simulated membrane was poly(ether-block-amide) (PEBA) to preferentially permeate the aroma compound of propyl propionate, an ester compound. Using mass balance surrounding each unit as well as overall unit, and the permeation rate in the membrane chamber, some model equations can be formulated

377

Figure 14.10 Various process designs in PV. PV, Pervaporation. Source: Courtesy R.W. Baker, Membrane Technology and Applications, second ed., John Wiley & Sons, Ltd., 2004.

Figure 14.11 Batch PV of aroma compound recovery from aqueous solutions with permeate decantation and water phase recycle. PV, Pervaporation.

Chapter 14 Computational modeling of pervaporation process

and are resumed in Table 14.4, with a list of nomenclature given in Table 14.5, and quantitative parametric studies given in Table 14.6. The column “One recycle” represents the model equations derived based on the conventional PV, as comparison. PV, Pervaporation; PEBA, poly(ether-block-amide). The simulation shows that when compared with the conventional PV process, the application of water phase recycle is able to enhance the recovery of aroma compounds in various extents, depending on the operating time and aroma solubility. At a given (F0/Am), the longer operating time increases the improvement in aroma compound recovery. However, the utilization of a larger membrane area [or smaller (F0/Am)] does not affect the maximum aroma recovery that can be achieved, but only shortens the operating time. The higher the solubility of aroma compound in the water is, the application of water phase recycle is strongly recommended due to its significant improvement in recovery. Another advantage offered by this modified PV system is that practically there is no strict restriction dealing with the operating time to obtain as much as aroma recovery where the conventional PV system must be care of it. In the modified PV system, the aroma compound recovered in the product increases permanently; whereas, there is an optimum operating time to obtain the maximum recovery in the conventional PV system. To attain the maximum recovery, the conventional operation must be stopped when the permeate concentration reaches its solubility limit. The two modes of operation show very distinctive performance, especially for higher aroma compound solubility. This makes sense that the higher solubility of aroma compound in water means more aroma compound exists in the water phase, and thus water phase recycling becomes more indispensable. Note that a higher solubility of aroma compound in water can reduce the recovery of aroma compound that can be achieved for both systems. This is especially important when dealing with the selection of operating temperature. Generally speaking, a higher operating temperature increases the permeation flux and thus more aroma compound can be collected; however, the solubility of aroma compound in the water phase will increase as well, and thus reduce the recovery of aroma compound. In this sense, it is expected that there will also be an optimum operating temperature in obtaining the maximum recovery of aroma compounds from aqueous solutions. The main simulation results are graphically shown in Figs. 14.12 and 14.13.

379

Table 14.4 Model equations [Eqs. (14.4414.53)] for recovery of low solubility aroma compounds from aqueous solutions by PV with two and one recycle streams. Two recycles

Remarks

dF dt dxF Rt ðxR 2 xF Þ 1 W ðxW 2 xF Þ 5 F dt X Am X Am ρm X Di CsF ;i 5 Si Di xF ;i Ji A m 5 PV 5 lm lm xP 5 βxF Rt 1 W 2 FV 5

Rt 5 FV 2 PV xF ðFV 2 βPV Þ xR 5 Rt ðβxF 2 xG ÞPV W5 ð xW 2 xG Þ dG PV 2 W 5 dt or

G 5 F0 2 F βxF PV 2 xW W 5

PV, Pervaporation.

d ð xG G Þ dt

One recycle

ð14:44Þ

To determine the mass in the feed tank as a function of timeTo determine the concentration of aroma in the feed tank as a ð14:45Þ function of time

dF 5 Rt 2 FV dt dxF Rt ðxR 2 xF Þ 5 F dt

ð14:46Þ To determine the mass flow rate of the permeate stream as a ð14:47Þ function of timeTo determine the concentration of aroma in the permeate stream as a function of time ð14:48Þ To determine the mass flow rate of retentate stream as a function ð14:49Þ of timeTo determine the concentration of aroma in the retentate stream as a function of time ð14:50Þ To determine the mass flow rate (or mass collected) of water phase in decanter as a function of time ð14:51Þ To determine the mass of permeate (organic phase) collected in the decanter as a function of timeTo determine the mass of aroma ð14:52Þ compound recovered as a function of time

Same

ð14:53Þ

Same

dW ðβxF 2 xG ÞPV 5 dt ðxW 2 xG Þ dG ðβxF 2 xW ÞPV 5 dt ð xG 2 xW Þ or G 5 F 2 F0 2 W Morg;recovered 5 xG G

Chapter 14 Computational modeling of pervaporation process

381

Table 14.5 List of nomenclature of model equations for recovery of low solubility aroma compounds from aqueous solutions by pervaporation with two and one recycle streams. Symbol

Remark

Am Di F, F0 FV J lm P PV R Si T W xF, xG, xP xR, xW β ρm

Cross-sectional area of membrane, m2 Diffusivity coefficient of component i, m2 h21 Mass of the mixture in the feed tank at any time, at initial, g Mass flow rate of feed stream, g h21 Total permeation flux through the membrane, g m22 h21 Membrane thickness, m mass of organic phase collected in the permeate collector, g Mass flow rate of permeate stream, g h21 Mass flow rate of retentate stream, g h21 Sorption coefficient of component i Operating time, h Mass flow rate (or mass) of water phase stream (or water phase), g h21, g Mass faction of aroma compound in the feed tank, organic phase, permeate stream Mass fraction of aroma compound in the retentate stream, water phase stream Enrichment factor Density of membrane, g m23

Table 14.6 Process conditions and other parameters used in model calculation for propyl propionatewater recovery. Parameter

Quantity

Initial mass of feed per membrane area, F0/Am Initial feed concentration Solubility of propyl propionate in water phase Feed circulation, FV Solubility coefficient of propyl propionate in PEBA membrane, Si Solubility of water in PEBA membrane as a function of mass fraction of propyl propionate Diffusivity of propyl propionate in PEBA membrane (from PV experiment data, average constant), Di Diffusivity of water in PEBA membrane (from PV experiment data, average constant) Enrichment factor, β Thickness of membrane, lm Solubility of water in propyl propionate phase Density of PEBA membrane, ρm

375, 150, and 15 kg m22 1000 ppm 5600 ppm (0.56 wt.%) [26] 1.6 kg min21 4.6014 g (g membrane ppm)21 1.784 Xpp 1 0.0078 g water g membrane21 6.5 3 10211 m2 s21 1.4 3 10210 m2 s21 185 25 μm 1 wt.% [27] 1.010 kg m23 [28]

382

Chapter 14 Computational modeling of pervaporation process

Figure 14.12 Mass of product (organic phase) collected and recovery of propyl propionate as a function of time. Conv., Conventional PV; Mod., modified PV; PV, pervaporation; sol., aroma solubility in water (in wt.%).

Chapter 14 Computational modeling of pervaporation process

Figure 14.13 Recovery of aroma compound as a function of time at various aroma solubility in water (F0/ Am 5 375 kg m22).

14.8

Predictive model

In addition to the process conditions, the PV performance is basically affected by the physicochemical properties of permeants to be separated and the membrane material [27]. To reduce the number of experimental works, a predictive model needs to develop, which is arranged either empirically or mechanistically. Some empirical approaches to predict the preferential permeant have been proposed based on the physicochemical properties of permeants and membrane.

14.8.1

Polarity and solubility parameter

Based on the polarity and solubility parameter, the closer distance of solubility parameter between the permeant p and the membrane m (Δpm) gives better sorption. The distance of solubility parameter is defined as [28], qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2  2  2ffi δd;p 2δd;m 1 δp;p 2δp;m 1 δh;p 2δh;m Δpm 5 ð14:54Þ

383

384

Chapter 14 Computational modeling of pervaporation process

where δd, δp, and δh are the components of solubility parameters of dispersion forces, polar forces, and hydrogen bonds, respectively; these components can be obtained from the literature [29]. As illustration, if (Δim/Δjm) , 1 means the permeant i is preferentially sorbed on the membrane m over the permeant j. This approach, however, is not valid for nonpolar systems. Zellers [30] tried to improve the prediction by adding the weighing factor (ω), as follows: qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi  2  2  2 Δpm 5 ω0 δd;p 2δd;m 1 ω1 δp;p 2δp;m 1 ω2 δh;p 2δh;m ð14:55Þ

14.8.2

Interfacial thermodynamics

This approach was initially proposed by van Oss et al. [31], and took into account the total Gibbs free energy (ΔGimj) as interfacial tension dependence (σ), ΔGimj 5 σij 2 σim 2 σjm

ð14:56Þ

The large value of ΔGimj reveals that the permeant i and j are potential to separate in the membrane m. To evaluate the interfacial tension, Neuman et al. [32] proposed the following empirical equation:
2 0:5 σ0:5 2σ i j σij 5 ð14:57Þ 0:5 1 2 0:015:σ0:5 i :σj

14.8.3

Chromatographic property

This approach was first proposed by Matsuura and Sourirajan [33], using liquid chromatography in which the carrier or mobile phase is liquid, while the stationary phase is the membrane material. A good affinity or interaction of permeantmembrane is indicated from the delay of elution.

14.8.4

Contact angle

The preferential sorption is measured from the contact angle between the permeant droplet and the membrane surface. The smaller contact angle gives better sorption [34]. The illustration of this approach is given in Fig. 14.14.

Chapter 14 Computational modeling of pervaporation process

γL θ

γs

γs L

γs ··· Solid surface tension γL ··· Liquid surface tension γs L ··· A solid and liquid boundary tension γs ··· γ L • cos θ + γs L

Figure 14.14 Contact angle approach: the smaller contact angle, the better sorption.

14.8.5

Physicochemical properties-process conditions

The predictive approaches in Sections 14.4.114.4.4 take into account only one step of transport mechanism in PV (i.e., sorption or diffusion), and ignore the desorption step. These approaches, however, are valid for certain systems but may not be valid for other systems. Therefore in this approach a hypothesis is withdrawn that taking into account all the steps of transport mechanism in PV may give more accurate prediction on the PV performance [35]. Using the concept of transport phenomena, at steady state the permeation flux in PV may be expressed as       Ji 5 2 ksorp;i μsF;i 2 μF;i 5 2 kdiff;i μsP;i 2 μsF;i 5 2 kdesorp;i μP;i 2 μsP;i

ð14:58Þ

where μF,i, μsF,i, μsP,i, and μP,i are the chemical potentials of permeant i in the feed bulk, on the membrane surface of feed side, on the membrane surface of permeate side, and in the permeate bulk, respectively. The parameters ksorpt,i, kdiff,i, and kdesorp,i are the permeation conductance of permeant i at sorption (sorption conductance), diffusion (diffusion conductance), and desorption (desorption conductance), respectively. In case an overall permeation conductance (koverall,i) is used, Eq. (14.58) can be simplified, Ji 5

1 ksorp;i

1

1 1 kdiff;i

 1

1

   μF;i 2 μP;i 5 koverall;i μF;i 2 μP;i

kdesorp;i ð14:59Þ

385

386

Chapter 14 Computational modeling of pervaporation process

Table 14.7 The physicochemical properties of permeantmembrane and process conditions affecting the permeation conductance of sorption, diffusion, and desorption. Symbol

Remark

koverall,i Dim lm ΔSolij, ΔSolim vi , vm Tgm T Tb,i P i0 ΔHvap,i CF,i PP P

Overall permeation conductance Diffusivity of permeant i through the membrane m Thickness of membrane Solubility difference of permeantpermeant, permeantmembrane Molar volume of permeant i, membrane m Glass-transition temperature of polymeric membrane Operating temperature Boiling point of permeant i Vapor pressure of permeant i Heat of vaporization of permeant i Concentration of permeate i in the feed Permeate pressure Density

Because of the vacuum condition in the permeate side, the chemical potential of permeant i in the permeate side may be ignored to give simpler equation, Ji 5 koverall;i μF;i

ð14:60Þ

The chemical potential in the feed bulk is proportional to the feed concentration (CF,i), and thus Eq. (14.60) becomes Ji 5 koverall;i CF;i

ð14:61Þ

Eq. (14.61) is more general than Eqs. (14.10) and (14.11) because the coefficient of proportionality is affected by various physicochemical properties of permeant and membrane as well as process conditions. The properties and process conditions that affect prominently the sorptiondiffusiondesorption step are identified and resumed in Table 14.7, and to be correlated empirically with the overall permeation conductance (koverall,i), as follows:        m4   ΔSolij m1 vi m2 Tgm m3 Pi0 koverall;i lm 5Φ Dim ΔSolim vm T ΔSolim 2 m6  m8   m5    m7 ΔHvap;i Tb;i PP v C m F;i T ΔSolim 2 vm ΔSolim 2

Chapter 14 Computational modeling of pervaporation process

where ΔSolim 5 jδm 2 δi j

  ΔSolij 5 ΔSolji 5 δj 2 δi 

CF;i 5 xF;i ρF 5

o Pi;j

 5 exp Ai;j 2

Bi;j Tref 1 Ci;j

xF;i ρi ρj   xF;i ρj 1 1 2 xF;i ρi 

ðAntoine0 s equationÞ

ð14:62Þ

The values of empirical constants (Φ, m1, m2, . . ., m8) are obtained from regressing a lot of experimental data (in this simulation, 122 data points collected from 12 PV systems) using Polymath software, as follows: Nonlinear regression (L-M) Model: kDim 5 Φ*(dSolij/dSolim)^m1*(vi/vm)^m2*(Tgm/T)^m3*(Pi/ dSolim^2)^m4*(Tbi/T)^m5*(dHvi/(dSolim^2*vm))^m6*(Cfi*Vm) ^m7*(Pp/dSolim^2)^m8 Variable Φ m1 m2 m3 m4 m5 m6 m7 m8

Ini guess 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5

Value 1.487E 2 07 9.8038826 3.8140146 3.5258736 21.0074883 213.840064 22.8161196 0.0554761 20.360851

Nonlinear regression settings Max # iterations 5 300 Precision R^2 5 0.9721616 R^2adj 5 0.9701908 Rmsd 5 0.0145858 Variance 5 0.0280223 General Sample size 5 122 # Model vars 59 # Indep vars 5 12 # Iterations 5 279

95% confidence 1.017E 2 07 0.2277103 0.1560243 0.4407409 0.0608951 1.2054992 0.1239057 0.0232483 0.0660869

387

388

Chapter 14 Computational modeling of pervaporation process

Figure 14.15 The comparison of permeation fluxes between experiments and model predictions.

The permeation flux is then predicted by substituting Eq. (14.62) into (14.61): 13:526 0 13:814 0 121:008 0 T P v gm i i @ A A @ A @ A Ji 5 1:487 310 ΔSolim vm T ΔSolim 2 0 1213:840 0 122:816 0 120:361 0 1  0:056 ΔH T P D vap;i P im b;i @ A @ A @ A @ ACF;i vm CF;i T lm ΔSolim 2 vm ΔSol2im 0

27 @ ΔSolij

19:804 0

ð14:63Þ

It can be seen that the relative ratio of molecular interactions of permeantpermeant over that of permeantmembrane (ΔSolij/ΔSolim) and the operating temperature (T) have very significant effect on the permeation flux. The groups of (ΔSolij/ ΔSolim), a combination of (Tgm/T) and (Tb,i/T), and [ΔHvap,i/ (ΔSolim2.vi)] dominate the permeation conductance at sorption, diffusion, and desorption steps, respectively. The dependency of permeation flux on the group [ΔHvap,i/(ΔSolim2.vi)] justifies that to some extent the desorption step can influence the permeation rate. In general, the proposed model gives fairly good predictions on the permeation flux of tested systems, as shown in Fig. 14.15. The model may be improved by increasing the number of reliable data set to be regressed. Instead of using historical data conducted by different researchers with different purposes,

Chapter 14 Computational modeling of pervaporation process

a series of designed experiments with respect to investigating the effects of selected variables on the PV performance is important to carry out.

14.9

Conclusion

As closing remarks, to the author’s opinion, the models related to the mass transfer of permeant through membrane in PV as well as the predictive models on PV performance have been satisfactory. It comes to conclusion that the solutiondiffusion is the best basic model explaining the transport phenomena in PV. Due to limited applications of PV, the study in this area should pay attention on the expansion of PV applications. As the permeation flux is typically low, PV is very potential to break the azeotropic point. A hybrid separation technology combining distillation and PV is very prospective to handle the separation of azeotropic solutions, as compared with the existing technologies. The principles are similar to the fixed adsorptive distillation method [36], a hybrid method combining distillation and adsorption.

References [1] P.A. Kober, Pervaporation, perstillation and percrystallization, J. Am. Chem. Soc. 39 (1917) 955. [2] G. Cox, R.W. Baker, Pervaporation for the treatment of small volume VOCcontaminated waste water streams, Indust. Wastewater 6 (1998) 35. [3] H.E.A. Bruschke, State of art of pervaporation, in: R. Bakish (Ed.), Proceedings of Third International Conference on Pervaporation Processes in the Chemical Industry, Bakish Materials Corp, Englewood, NJ, 1998, pp. 211. [4] R.Y.M. Huang (Ed.), Pervaporation Membrane Separation Processes, Elsevier, Amsterdam, 1991. [5] S. Loeb, S. Sourirajan, Seawater demineralization by means of as osmotic membrane, in saline water conversion-II, Adv. Chem. Ser., Am. Chem. Soc. 28 (1963) 117. [6] R.W. Baker, Membrane Technology and Applications, second ed., John Wiley & Sons, Ltd, 2004. [7] H. Yin, A.C.K. Yip, A Review on the production and purification of biomassderived hydrogen using emerging membrane technologies, Catalyst 7 (2017) 297. [8] R.B. Long, Liquid permeation through plastic films, Ind. Eng. Chem. Fundam. 4 (1965) 445. [9] F.W. Greenlaw, W.D. Prince, R.A. Shelden, E.V. Thompson, Dependence of diffusive permeation rates on upstream and downstream pressures: I. Single component permeant, J. Membr. Sci. 2 (1997) 141.

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Chapter 14 Computational modeling of pervaporation process

[10] F.W. Greenlaw, W.D. Prince, R.A. Shelden, E.V. Thompson, Dependence of diffusive permeation rates on upstream and downstream pressures: II. Two component permeant, J. Membr. Sci. 2 (1997) 333. [11] J.P. Brun, C. Larchet, G. Bulvestre, B. Auclair, Sorption and pervaporation of dilute aqueous solutions of organic compounds through polymer membranes, J. Membr. Sci. 25 (1985) 55. [12] A. Heintz, W. Stephan, A generalized solution-diffusion model of the pervaporation process through composite membranes. Part I. Prediction of mixture solubilities in dense active layer using the UNIQUAC model, J. Membr. Sci. 89 (1994) 143. [13] C.H. Lee, Theory of reverse osmosis and some other membrane permeation operations, J. Appl. Polym. Sci. 19 (1975) 83. [14] H. Fujita, Diffusion in polymer-diluent systems, Adv. Polym. Sci. 3 (1961) 1. [15] J. Flory, Principles of Polymer Chemistry, Cornell University Press, Ithaca, NY, 1953. [16] E.A. Mason, L.A. Viehland, Statiscal-mechanical theory of membrane transport for multicomponent system: passive transport through open membrane, J. Chem. Phys. 68 (1978) 3562. [17] A. Heintz, W. Stephan, A generalized solution-diffusion model of the pervaporation process through composite membranes. Part II. Concentration polarization, coupled diffusion of the porous support layer, J. Membr. Sci. 89 (1994) 153169. [18] E. Smit, M.H.V. Mulder, C.A. Smolders, H. Karrenbeld, J. van Earden, D. Feil, Modeling of the diffusion of carbon dioxide in polyimide matrices by computer simulation, J. Membr. Sci. 73 (1992) 247. [19] S.G. Charati, S.A. Stern, Diffusion of gases in silicone polymers: molecular dynamic simulations, Macromolecules 31 (1998) 5529. [20] J. Crank, G.S. Park, Diffusion in Polymers, Academic Press, London and New York, 1968. [21] J. Crank, The Mathematics of Diffusion, second ed., Clarendon Press, Oxford, 1975. [22] J. Comyn, Polymer Permeability, Elsevier Applied Science Publishers, London and New York, 1985. [23] J.S. Vrentas, J.L. Duda, Diffusion in polymer-solvent systems. II. A predictive theory for the dependence of diffusion coefficients on temperature, concentration, and molecular weight, J. Polym. Sci. Polym. Phys. Ed. 15 (1977) 417. [24] C.A. Pawlisch, J.R. Bric, R.L. Laurence, Solute diffusion in polymers 2. Fourier estimation of capillary column inverse gas chromatography data, Macromolecules 21 (1998) 1685. [25] M. Mujiburohman, X. Feng, Simulation of recovery of aroma compound from aqueous solutions by batch pervaporation coupled with permeate decantation and water phase recycle, J. Membr. Sep. Technol. 5 (2016). [26] M.K. Djebbar, Q.T. Nguyen, R. Clement, Y. Germain, Pervaporation of aqueous ester solutions through hydrophobic poly(ether-block-amide) copolymer membranes, J. Membr. Sci. 146 (1998) 125133. [27] X. Feng, R.Y.M. Huang, Preparation and performance of asymmetric polyetherimide membranes for isopropanol dehydration by pervaporation, J. Membr. Sci. 109 (1996) 165. [28] R. Ravindra, S. Sridhar, A.A. Khan, Separation studies of hydrazine from aqueous solution by pervaporation, J. Polym. Sci. Polym Phys. Ed. 37 (1999) 1969.

Chapter 14 Computational modeling of pervaporation process

[29] Hannsen and Beerbower, Encyclopedia of Chemical Technology, Supplement Volume, 1971. [30] E.T. Zellers, Three-dimensional solubility parameters and chemical protective clothing permeation. I Modelling the solubility of organic solvents in Viton s gloves, J. Appl. Polym. Sci. 50 (1993) 513. [31] C.J. van Oss, J. Visser, D.R. Absoom, S.N. Omenyi, A.W. Neumann, The concept of negative hamaker coefficients. II. Thermodymanics, experimental evidence and applications, Adv. Colloid Interface Sci. 18 (1983) 113. [32] A.W. Neumann, R.J. Good, C.J. Hope, M. Sejpal, An equation of state approach to determine surface tension of low-energy solids from contact angels, J. Colloid Interface Sci. 49 (1974) 291. [33] T. Matsuura, S. Sourirajan, Properties of polymer-solution interfacial fluid from liquid chromatographic data, J. Colloid Interface Sci. 66 (1978) 589. [34] A. Nabe, E. Staube, G. Belfor, Surface modification of polysulfone ultrafiltration membrane and fouling by BSA solutions, J. Membr. Sci. 133 (1997) 57. [35] M. Mujiburohman, A. Elkamel, K.A. Mahdi, Predictive model of pervaporation performance based on the physicochemical properties of permeants-membrane material and process conditions, J. Membr. Sci. 381 (2011). [36] M. Mujiburohman, W.B. Sediawan, H. Sulistyo, A preliminary study: distillation of isopropanol-water mixture using fxed adsorptive distillation, J. Sep. Purif. Technol. 48 (2006) 8592.

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Hybrid pervaporation process

15

Thomasukutty Jose1, Soney C. George2 and Sabu Thomas3 1

Department of Basic Sciences, Centre for Nanoscience and Technology, Amal Jyothi College of Engineering, Kanjirapally, India 2Centre for Nanoscience and Technology, Amal Jyothi College of Engineering, Kanjirapally, India 3International and Inter University Centre for Nanoscience and Nanotechnology, Mahatma Gandhi University, Kottayam, India

15.1

Introduction

Membrane-based separation processes can offer many advantages than conventional separation processes. In industries, large number of membrane-based separation process is currently being used. But, due to certain limitations especially individual use of membrane separation processes face some drawbacks. To overcome these limitations, hybrid processes have been developed to enhance the separation efficiency and minimize the cost. Hybrid process is a process in which two or more processes are combined together to get better outcomes. In hybrid pervaporation (PV) PV combines with other processes and those combinations enhance the product outcomes. Lipnizki et al. [1] focused on the PV-hybrid processes and gave detailed insight about the process design, economical advantages, and different applications. Hybrid process can offer low capital cost and production costs, and it is energy efficient in nature. Pressly and Ng [2] developed a general scheme for process synthesis based on distillation
membrane hybrid process for azeotropes and close boiling binary liquids. Stephan et al. [3] provide a general methodology to design criteria, different configuration comparison, and optimization of process conditions. The possibility of PV and vapor permeation in combined processes for the dehydration of organic solvents is successfully demonstrated by Sommer and Melin [4]. The commercially available simulations and self-developed membrane routine exhibited a flexible and efficient tool for process design. The investment and operation cost reduced to more than 40% and energy saving 85% for the purification of the azeotropic distillate stream.

Polymer Nanocomposite Membranes for Pervaporation. DOI: https://doi.org/10.1016/B978-0-12-816785-4.00015-X © 2020 Elsevier Inc. All rights reserved.

393

394

Chapter 15 Hybrid pervaporation process

There are number of PV-assisted hybrid processes such as PV
distillation, PV
esterification, PV
adsorption, PV
reverse osmosis, PV
vapor permeation, etc. Gonzalez and Ortiz [5] analyzed the hybrid distillation
PV for the purification process in MTBE production. Arpornwichanop et al. [6] extensively analyzed the hybrid reactive distillation (RD) to enhance tert-amyl ethyl ether production and achieved significant improvement in process optimization using simulation results. The recovery of dilute acetic acid from waste stream is an important issue as environmental concern. The simulation study shows that the hybrid PV
RD requires lower energy for the conversion of acetic acid than conventional RD process [7]. This chapter exclusively deals with different hybrid PV processes and its process design. The simulation used for process design is well explained in the chapter. Apart from distillation, other hybrid processes are also included in the chapter.

15.2

Distillation process

In this process, the liquid or vapor
liquid feed mixture of two or more components is separated into two or more products. The distillate composition is differing from that of the feed in the distillation process. The effectiveness of separation needs (1) both liquid and vapor phases are in contact with each other in separation columns, (2) difference in their relative volatilities, and (3) two phases can be separated by any means. The capital cost of distillation is less compared to PV, but it is an energy intensive process. Azeotropic or close boiling point mixture separation and purification by distillation is very much expensive and needs additional solvents. These limitations can be overcome with the implementation of hybrid processes. A schematic representation of distillation process is shown in Fig. 15.1A.

15.3

Hybrid process parameters

Book by Douglas gives a detailed idea about different process parameters used in hybrid processes [8,9]. 1. The total annual cost (TAC) is the important parameter which depends on the efficiency and economical advantages of the hybrid processes. The TAC analysis is also the prime component in the process optimal design. The TAC is calculated using Eq. (15.1)

Chapter 15 Hybrid pervaporation process

395

Figure 15.1 Schematic diagram (A) conventional distillation (B) hybrid distillation
PV process. PV, Pervaporation.

TAC 5 AOC 1

TCC Payback period

ð15:1Þ

It is the combination of annual operating cost and total capital cost. 2. Cost of membrane module   A 0:3 Membrane module cost 5 125; 550 ð15:2Þ 324 where A is effective membrane area (m2) for each module. 3. Height of a column LC ð ftÞ 5 2:3 3 ðNT 2 1Þ

ð15:3Þ

where NT is the total number of trays in the column. 4. Cost of column Column cost 5

M&S 3 101:9 3 DC1:066 L0:802 3 ð2:18 1 3:67Þ ð15:4Þ C 280

where DC (ft) is the diameter of the column. 5. Cost of tray Tray cost 5

M&S 3 4:7 3 D1:55 C LC 3 ð1 1 1:8 1 1:7Þ 280

ð15:5Þ

6. Reboiler heat transfer area AR ð ft 2 Þ 5

QR UR ΔTR

ð15:6Þ

QC UC ΔTC

ð15:7Þ

7. Condenser heat transfer area AC ð ft 2 Þ 5

396

Chapter 15 Hybrid pervaporation process

15.4

Hybrid distillation
pervaporation process

Low energy consumption, high selectivity, moderate cost, etc. give much attention to the membrane-based separation processes. But, the lower permeation rate and reduced flux are still challenging in the conventional PV process. Researches are investigating to eliminate the usual tradeoff between flux and selectivity. The hybrid process such as distillation
PV process is better alternative to increase the permeation rate and energy consumption compared to conventional PV and distillation, respectively. A schematic representation of hybrid PV
distillation process is shown in Fig. 15.1B. The efficiency of the hybrid process depends on two factors: (1) configurations of hybrid process and (2) choice of configurations [10]. The PV module can be configured in three different ways as follows: 1. PV module after the distillation column

2. PV module before distillation column

Chapter 15 Hybrid pervaporation process

3. PV module on a side stream of distillation column

The choice of the configurations is depends on the type of membrane and the composition that to be treated by the membrane. Servel et al. [10] explained the configuration based on the vapor
liquid equilibrium (VLE) curve to select either a hydrophilic or a hydrophobic membrane for the separation of acetic acid/water mixtures. The VLE of water/acetic acid is shown in Fig. 15.2 and it describes that the relative volatility of the water and acetic acid is very low. Hence it needs large number of separation stages in the distillation columns. But, the distillation is not effective in high water content and the energy consumption is also higher. The hydrophilic PV module at the bottom stream of the distillation column is an alternative solution in high acetic acid content and it minimizes acetic acid loss and reduced the energy consumption. Khazaei et al. [11] studied the energy consumption in a hybrid PV process for the separation of toluene and ioctane. The hybrid PV is applied for low toluene concentration stream and purified toluene is collected from the bottom. There are two stages for PV section to collect the products. Retentate of the first stage is routed to the second stage and retentate of the second stage is the purified i-octane. Permeate stream contains high i-octane content and not mixed with toluene product and achieved reduced energy consumption to 56% compared to extractive distillation process (Fig. 15.3). Paredes et al. [12] described the hybrid process configuration design and its economical advantages for the separation of

397

398

Chapter 15 Hybrid pervaporation process

1

Vapor mass fraction of water

0.9

Hydrophobic membrane

0.8 0.7 0.6 Hydrophilic membrane

0.5 0.4

VLE (1 atm)

0.3 0.2 0.1 0 0

0.2

0.4 0.6 Liquid mass fraction of water

0.8

1

Figure 15.2 VLE curve for hydrophilic
hydrophobic membrane for the separation of acetic acid/water mixtures. VLE, Vapor
liquid equilibrium. Source: Reprinted with permission from C. Servel, D. Roizard, E. Favre, D. Horbez, Ind. Eng. Chem. Res. 53 (2014) 7768 2 7779. Copyright r 2014, American Chemical Society.

methanol
methyl acetate azeotropic mixtures. The methanol and methyl acetate form low boiling azeotropes with a methanol composition 0.336 mol mol21. The hybrid process is configured with two distillation columns and a PV unit (Fig. 15.4). The hybrid process is explained on the basis of following parameters: the main stream flow rate F0, composition (x0) and is separated into two (one rich with methanol and other rich with methyl acetate). D1 and D2 are distillate streams with compositions xD1 and xD2, respectively, at both sides of the azeotropes. Main feed stream F with composition xF is fed into the stream. From PV unit, methyl acetate-rich retentate stream (xR) and methanol-rich permeate stream (xP) are obtained. The retentate stream is fed to column C1 to obtain pure methyl acetate and permeate stream is to column C2 to get almost pure methanol. The selection of configuration depends on the composition of the main stream and configuration II is preferred over others because it is feasible over entire compositions. The hybrid process depends on the nature of the membranes used. The optimal solution of the retentate composition is very different for each membrane, and the overall cost for the designs also depends on the membranes. For example, the overall cost of

Chapter 15 Hybrid pervaporation process

Figure 15.3 The process of hybrid distillation
pervaporation for separation of toluene and i-octane.

the separation using PolyAl TypM1 and Pervap 2256 membranes is 13.4 3 106 and 12.1 3 106 US dollar per year, respectively. The overall cost of separation for configurations I*, II*, and III* is much higher than that of I, II, and III, respectively, for each type of membranes. Thus the performance of the membrane can be analyzed based on the conceptual design of different PV
distillation hybrid process configurations. Wang et al. [13] studied the different combinations of pressure swing reactive distillation (PSRD) with PV for the transesterification processes to produce n-propyl acetate. In PSRD transesterification process, the higher pressure for the RD column is required to the distillate composition. The rise in pressure depends on temperature of trays (below 120 C) and temperature difference between the overhead vapor of the RDC and the bottom liquid of the methanol recovery column (15 C). This condition is essential to implementing heat integration to the process [14]. The flowsheet for PV-assisted PSRD process is shown in Fig. 15.5. The PV membranes are set at 1.5 bars and 50 C and the retentate stream is removed as methanol product. The permeate stream is pumped to the methanol recovery column with fixed pressure 0.05 bar after condensing from vapor to liquid. The residual MeOH is obtained at the bottom, and the distillate from the column is recycled into the fresh feed of MeAc in the RDC.

399

400

Chapter 15 Hybrid pervaporation process

Figure 15.4 Hybrid process configurations depending on the location of the main feed stream, a
f represents different configurations of hybrid process.

Chapter 15 Hybrid pervaporation process

401

Figure 15.5 (A) Partial-assisted PV-PSRD process and (B) full PV-RD process with three PV units in series. PV-RD, Pervaporation
reactive distillation; PV-PSRD, pervaporation
pressure swing reactive distillation.

The results obtained by PV-PSRD process are given in Table 15.1, and it is clear that the PV-assisted PSRD with partial heat integration is the most competitive configuration. The energy consumption, CO2 emission, and economic cost are much promising for PV-assisted PSRD process rather than full PV-assisted RD process. It is due to the fact that less energy consumption is taken place for higher MeOH concentration in the distillate stream. Harvianto et al. [15] studied reactive hybrid distillation process with highly selective PV membranes for butyl acetate production via transesterification Butanol 1 Methyl acetate-Butyl acetate 1 Methanol

402

Chapter 15 Hybrid pervaporation process

Table 15.1 Comparison of RD, PSRD, and PV-assisted PSRD processes [13]. PSRD

Number of rectifying trays in reactive distillation (NR) Number of reactive trays in reactive distillation (NRX) Number of stripping trays in reactive distillation (NS) MeAc flow rate of recycle rate (kmol h21) MeOH flow rate of recycle rate (kmol h21) Conversion pre pass of MeAc (%) Total annual cost saving (%) CO2 emission saving (%)

PV-assisted PSRD

Full PV-assisted RD Series

Parallel

42

35

36

42

33

34

36

33

5

6

3

5

24.36 12.64 66.89 52.4 59.5

15.44 8.83 69.05 58.15 67.32

24.73 12.57 66.59 53.04 65.51

36.28 34.44 57.63 46.65 56.62

PV, Pervaporation; PSRD, pressure swing reactive distillation; RD, reactive distillation.

The use of highly selective PV membrane is essential to eliminate the need of required methanol distillation column in the hybrid process. The hybrid process is effectively used to produce butyl acetate with a 60% reduction in annual cost compared to conventional methods. The operating variables such as catalyst weight and methyl acetate purity in the retentate affect the process outcomes. The amount of catalyst plays a slight positive effect on reducing the boiler duty of the RD, but the minimum TAC is obtained with minimum amount of the catalyst to attain specific product outcomes. As the MeAc concentration in the retentate increases to unity, the TAC of the hybrid process decreases. Thus pure MeAc concentration is needed to proceed the process in a cost-effective manner. Apart from large membrane area; significant reduction of condenser and reboiler duty and the RD column diameter are also required to attaining purity of the MeAc and in effect, it affects the cost. Lv et al. [16] investigated a new approach to enhance the EtAc purity in which the membrane is located in the bottom stream to selectively remove water from the reboiler. The selective removal of water enhanced the purity of the separated components. Luyben [17] studied hybrid column PV process to

Chapter 15 Hybrid pervaporation process

produce 99.77 wt.% ethanol from ethanol/water azeotropic mixtures. The permeate pressure and retentate temperature play a significant role in permeation properties. Pemeate flux is increased with decreasing permeate pressure or increasing retentate temperature. A novel EtAc production process using hybrid RD and PV is configured by Lee et al. [9]. The hybrid process is designed as a target of 99 mol.% of EtAc synthesized with 87 mol.% ethanol and 95 mol.% HAc fed into the bottom RD column. The rectifying section of the RD column can draw 96 mol.% EtAC and it is fed to the membrane module to enhance the EtAC purity to 99 mol.%. This type of two column configuration can be used to achieve 13% energy saving compared to conventional processes.

15.5

Simulations of hybrid distillation
pervaporation process

Aspen Plus (Aspen Technology, Cambridge, United States) is used as a tool to simulate and design chemical process. It can be used to simulate various hybrid processes. The software is updated frequently to reduce the complexity of stimulations programming. The membrane model can be programmed with the help of the programming language Visual Basic for Applications (VBA) in Excel. The basic performance of any PV process depends or expressed in terms of flux and separation factor. Hence, two types of models (1) based on Fick’s binary diffusion and (2) Maxwell
Stefan theory are most relevant for PV processes [18,19]. The Fick’s theory does not work if two or more components are present inside the membrane. The VBAAspenPlus programming is used to simulate the hybrid PV process based on the above theories [20]. The first part of the software is a simulation program to simulate a flowsheet containing a PV unit. In this step, user can specify which model can be used to calculate the membrane process. User can also vary the specified variable in the way the user wish to do the process. The second part mainly designs program, which consist of number of membrane units or total membrane area required to attain a particular separation condition. The information direction shared between different programming languages are as follows [21]: AspenPlus$Excel$Matlab

403

404

Chapter 15 Hybrid pervaporation process

Figure 15.6 (A) Hybrid process design and (B) Aspen Plus flowsheet for correct simulation of the hybrid process.

AspenPlus and VBA establish a data exchange to use the simulations for hybrid processes. First Aspen Plus flowsheet has to be made for the entire process and VBA program is connected to this flowsheet. Verhoef et al. [20] established some rules to avoid data lossage due to the software connection problem. The rules are as follows: • The flowsheet must contain membrane unit and feed for the membrane unit, otherwise disconnection of the program occurred in Aspen Plus. • No blocks to represent a membrane unit and the flowsheet should be work without the membrane blocks. • VLE must be set to the program to obtain vapor
liquid equilibrium data. The hybrid process design and corresponding Aspen Plus flowsheet representation for simulation are shown in Fig. 15.6. An input flow corresponding to FEED2 is defined in the Aspen Plus flowsheet with equal input and output streams. The retentate of the membrane is selected as a feed for the distillation column. Product stream of the distillation column comes from another membrane unit or feed streams. The retentate and permeate streams can be selected using VBA programme. All VLE data can be analyzed by the Aspen Plus which contains already set vapor
liquid equilibrium data. The VBA is based on either Fick’s or Maxwell Stefan model to calculate the process parameters. The initial flows and stream compositions of all membranes can be obtained from mass balances. It is assumed that

Chapter 15 Hybrid pervaporation process

the retentate contains perfectly mixed fluid, and based on these assumption the driving force for the separation can be obtained from mass fractions of retentate and permeate using Eq. (15.8) vap;ret

ret Δpi 5 χret i γ i Pi

perm perm vap;perm γi Pi

2 χi

ð15:8Þ

ret pi is the driving force, χret is the activity i molar fractions, γ i coefficient, and Pi is the vapor pressure of component. The accuracy of flux can be improved by subdividing the membrane modules into equal areas.

15.6

Other pervaporation hybrid processes

Bukusoglu et al. [22] investigated a new and novel approach for the dehydration of the solvent mixture using hybrid PV
liquid phase adsorption process. The adsorption process can reduce the water content of the solvent mixture below 0.1 wt.% for a feed solution of 1.5 wt.% water at a process condition of 72.8 g zeolite, feed flow rate of 1.0 mL min21, and room temperature. The lower water feed concentration slows down the process in the single-stage PV dehydration. Hence, the hybrid PV
adsorption process is an efficient alternative for single-stage PV dehydration. Cai et al. [23] studied PV-coupled fermentation process for butanol recovery. The hybrid PV system contains 2 L high pressure tank in which SSB was taken and hydrolyzed using acetic acid. The hydrolyzed liquid was pumped to first PV module with 0.5 L min21 velocity rate. After PV, the furfural liquid was condensate. Furfural free hydrolysate was pumped into a fermentor (5 L capacity with working volume 3.5 mL). To reach the required working volume, nine batch hydrolysis is needed. Then culture was sterilized with laccase addition. A microfiltration membrane is used to remove strain after fermentation. Then recirculate the fementation broth from the fermentor over PV modules (0.5 L min21) using a pump. A heat exchanger and liquid nitrogen were used to warm the broth and condense the permeate solvent, respectively. The PV-coupled laccase treatment gives better separation efficiency. It can reduce furan to 94.5% and phenolic compounds to 87.5%, and as a result 138.5 g L21 furfural and 201.9 g L21 butanol were separated from SSB hydrolysate and its fermentation broth. This type of hybrid process must be a promising alternative to industrial production of biobutanol and fufural.

405

406

Chapter 15 Hybrid pervaporation process

15.7

Advantages of hybrid pervaporation process

Hybrid process are very promising alternative to conventional processes. The main advantages are the • high product purity, • energy efficiency, • high degree of separation, • cost effectiveness, • any type of combinations can be possible in hybrid PV process, • the use of highly selective membranes can reduce the cost of separation.

15.8

Conclusion

The widely accepted membrane technologies can offer many advantageous over conventional separation processes. The hybrid processes are much promising with its prime process advantages. The limitations of solemn membrane-based PV separation processes can be overcome by different hybrid PV processes. More recently lot of developments took place in the area of hybrid PV processes, and some of them are commercialized. The chapter gives a detailed outline about different hybrid PV processes and its process conditions. The hybrid PV processes are much promising in its environmental and economical concerns.

References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]

F. Lipnizki, R.W. Field, P.K. Ten, J. Membr. Sci. 153 (1999) 183. T.G. Pressly, K.M. Ng, AIChE J. 44 (1998) 93
105. W. Stephan, R.D. Noble, C.A. Koval, J. Membr. Sci. 99 (1995) 259
272. S. Sommer, T. Melin, Ind. Eng. Chem. Res. 43 (2004) 5248
5259. B. Gonzalez, I. Ortiz, J. Chem. Technol. Biotechnol. 77 (2002) 29
42. A. Arpornwichanop, U. Sahapatsombud, Y. Patcharavorachot, S. Assabumrungrat, Chin. J. Chem. Eng. 16 (2008) 100
103. A. Arpornwichanop, K. Koomsup, S. Assabumrungrat, J. Ind. Eng. Chem. 14 (2008) 796
803. J.M. Douglas, Conceptual Design of Chemical Processes, McGraw- Hill, New York, 1988. H. Lee, S.Y. Li, C.L. Chen, Ind. Eng. Chem. Res. 55 (2016) 8802
8817. C. Servel, D. Roizard, E. Favre, D. Horbez, Ind. Eng. Chem. Res. 53 (2014) 7768
7779. A. Khazaei, V. Mohebbi, R.M. Behbahani, S.A.A. Ramazani, Chem. Eng. Process. 128 (2018) 46
52.

Chapter 15 Hybrid pervaporation process

[12] D.A.F. Paredes, D.S. Laoretani, J. Zelin, R. Vargas, A.R. Vecchietti, J. Espinosa, Sep. Purif. Technol. 189 (2017) 296
309. [13] C. Wang, Z. Zhang, X. Zhang, J. Gao, B. Stewart, Sep. Purif. Technol. (2019). Available from: https://doi.org/10.1016/j.seppur.2019.03.074. [14] W.L. Luyben, Principles and Case Studies of Simultaneous Design, John Wiley & Sons, New York, 2012. [15] G.R. Harvianto, F. Ahmad, M. Lee, J. Membr. Sci. 543 (2017) 49
57. [16] B. Lv, G. Liu, X. Dong, W. Wei, W. Jin, Ind. Eng. Chem. Res. 51 (2012) 8079
8086. [17] W.L. Luyben, Ind. Eng. Chem. Res. 48 (2009) 3484
3495. [18] R.B. Bird, W.E. Stewart, E.N. Ligthfoot, Transport Phenomena, second ed., Wiley, New York, 2002. [19] A.W. Verkerk, P. Van Male, M.A.G. Vorstman, J.T.F. Keurentjes, J. Membr. Sci. 193 (2001) 227
238. [20] A. Verhoef, J. Degreve, B. Huybrechs, H. van Veen, P. Pex, B.V. Bruggen, Comput. Chem. Eng. 32 (2008) 1135
1146. [21] J. Fontalvo, Using user models in Matlabs within the Aspen Pluss interface with an Excels link. Ing. Investig. 34 (2) (2014) 39
43. [22] E. Bukusoglu, H. Kalipcilar, L. Yilmaz, Ind. Eng. Chem. Res. 57 (2018) 2277
2286. [23] D. Cai, T. Zhang, J. Zheng, Z. Chang, Z. Wang, P. Qin, et al., Bioresource. Technol. 145 (2013) 97
102.

407

Index Note: Page numbers followed by “f” and “t” refer to figures and tables, respectively.

A Acetone-butanol-ethanol (ABE) fermentation process, 346 Acetyl acetone, as chelating agent, 54 55 Acetyl sulfate, sulfonating reagent, 183 184 Acid catalyzed hydrolysis, 247 248 Acid hydrolysis, 19 Acid treatment, 117 118 A3C60 phase, 155 Adsorption diffusion model, 97 98 Adsorption isotherm, 267 268 Aerosol OT (AOT), 237 Aging factors, 202 203 Alginate, 116 117 Alginate-iron nanoparticle composite membrane, 246 247 Alginate-iron nanoparticle nanocomposite, 244 245 Alkaline hydrolysis, 244 245 Alkyl quaternary ammonium compounds, 83 84 Alumina, 236 237, 247 nanoparticle -based PMNC membranes, pervaporation performance of, 247 249 nanoalumina as membrane, 247 248 silver polymer, 248 249 Aluminum-based metal organic framework (Al-MOF) membrane, 75 Aluminum nitrate and sodium hydroxide, 67 Aluminum oxide (Al2O3), 247

ϒ-Aminopropyl-triethoxysilane (APTEOS), 213 214 Ammonium hydroxide, 236 Amphiphilic phospholipid, 83 84 Anionic octa (tetramethylammonium)POSS (octa-TMA-POSS), 212 213 Anionic polysaccharide, 49 Aqueous 2 organic mixtures, 6 7 Arc discharge, 113 114, 137, 154 AspenPlus, 404 Azeotropes, 108 separation, 107 Azeotropic distillation separation, 107 Azeotropic mixture separation, 2, 201, 210 211 and organic solvents, 212 214 Azetidino-2,4-dino group, 40

B Bacterial cellulose/alginate blend membranes, 30 Bentonite clay membrane, 91 92, 92f concentration in CMC, 188 189 Benzene, carcinogenic, 107 Benzene/cyclohexane mixtures separation, 127 raw and functionalized MWCNTs effect, 127f Benzene 2 monovalent metal (I) coordinate complex, 65 66 Benzyl dimethyl amine, 86

Binary liquid mixture, 99 Biocompatibility, 20, 35 36, 155 156, 189 190 Biodegradable matrix, 81 Biodegrading, 35 36 Biofiber composites, 81 Biomass, 18 19 Biomineralization process, 275 276 Biorefinery process, 18 Biorenewability, 35 36 Bisphenol A epoxy, diglycidyl ether of, 86 Black soot, 154 Blend inorganic-filled polymeric membrane, 275 276, 277f membranes phosphomolybdic acid (PMA)-loaded PVA 2 poly (vinyl pyrrolidone), 213 214 of sodium alginate and (hydroxyethyl) cellulose, 29 Boehmite membrane, 67 68 Boranes, 146 Boron triflouride monoethyl amine, 86

C Cage-structured silsesquioxanes, 206 Calcination of metal oxide, 237 238 Calcium chloride, 244 245 Capping molecule, 236 Carbon black, 153 Carbon nanotubes (CNTs), 107 108, 113 116, 233

409

410

Index

Carbon nanotubes (CNTs) (Continued) covalent functionalization, 116 functionalization, 114 116, 115f noncovalent functionalization, 115 purification and oxidation of, 114 115 structure, 113f total flux and separation factor for water, 245f tubular structure, 114 Carbon nanotubes-polymer nanocomposite membranes carbon nanotubes, 113 116 FESEM surface images, 118f inorganic and organic (polymeric) membranes, 106 107 pervaporation, 108 112 applications, 107 organic 2 organic mixtures, separation, 125 127 organics from aqueous solutions, recovery, 128 129 schematic, 109f separation characteristics, 110 112 solution-diffusion model, 109 110 solvents and alcohols, dehydration of, 116 125 polymer nanocomposites, 112 porous and nonporous membranes, 106 Carboxymethylcellulose (CMC) membrane, 188 189 Carrageenan membrane, 49 Catalytic membrane reactors (CMR), 310 Cationic exchange mechanism, 83 84 Cationic surfactants, 83 84 Cellulose, 116 117 acetate membranes, 23 24, 180

filled with metal oxide particles, 25 26 fibers, 20 membrane, 47 48 polyacrylonitrile membranes, 23 24 poly(vinyl alcohol) membranes, 21 23 Cellulose nanocrystals (NCC), 20 Cellulose-polydimethyl siloxane blends, 20 21 Ceramic matrix composites, 81 Ceramic membranes, 156 157, 159 160 Cetyltrimethyl ammonium bromide (CTAB), 83 84, 205 C60-filled ethyl cellulose hybrid membranes, 24 Chabazite (CHA) zeolite, 265 Characterization, 21 alginate 2 iron nanoparticle composite, 246 247 enrichment factor, 309 FESEM, 117 118 graphene membranes, 139 141 iron polymer-metal nanocomposites membranes, 246 247 SEM, 59 60 Chemical affinity, 17 18 Chemical potential gradient, 88 Chemical precipitation, 236 Chemical reduction, 236 Chemical vapor decomposition (CVD), 113 114 Chitin and chitosan membranes, 36 37 membranes, 37 Chitosan-based membranes, 8 Chitosan-blended PVP membranes, 41 Chitosan (CS), 6 7, 116 117, 245 246, 329 330 FGS membranes, 56 57 and graphene oxide, 186 187, 187f

hybrid membranes, 76t aluminum-based metal organic framework membrane, 75 iron oxide/PAN membrane, 73 74 PVA/Ag membrane, 71 72 PVA/multiwall carbon nanotube membrane, 70 71 silica/PAN/PEG membrane, 74 75 silica/ polytetrafluoroethylene membrane, 72 73 sodium alginate/chitosan/ multiwall carbon nanotube membrane, 69 70 macroradicals, 41 membranes, 37 75 chemical structure of, 35f, 36 hybrid membranes, 69 75 inorganic membranes, 52 68 interlinking, 39 40 modification routes for, 39f modified, 38 39 organic membranes, 39 52 water molecules, permeation of, 39f Chitosan (CS)-based composite membranes, 136 137 Chitosan/inorganic membranes, 68t boehmite membrane, 67 68 clay membrane, 52 54 ferric oxide membrane, 55 56 functionalized graphene sheets membrane, 56 57 multiwall carbon nanotube/ silver membrane, 65 66 Mxene membrane, 66 67 NaY membrane, 57 58 phosphorylated chitosan membrane, 63

Index

phosphotungstic acid membrane, 62 63 reduced graphene oxide (RGO) membrane, 61 62 silica membrane, 58 59 sulfonized chitosan membrane, 63 65 sulfosuccinic acid membrane, 59 60 titanium dioxide membrane, 54 55 toluene-2,4-diisocyanate membrane, 60 61 Chitosan membranes using TEOS and ϒ-glycidosypro pyltrimethoxysilane (CHTEOS-GPTS), 58 59 Chitosan/organic membranes, 53t carrageenan membrane, 49 cellulose membrane, 47 48 gelatin membrane, 49 50 glutaraldehyde membrane, 50 51 poly(acrylic acid) membrane, 45 polyaniline membrane, 51 52 polybenzoimidazole membrane, 40 poly(n-vinyl-2-pyrrolidone) membrane, 40 41 polyvinyl alcohol membrane, 42 44 polyvinyl sulfate membrane, 45 46 sodium alginate membrane, 46 47 Chitosanpoly(acrylic acid) membrane, 45 Chitosan 2 polybenzoimidazole membrane, 40 Chromatographic property, membrane material, 384 Citric acid, crosslinking agent, 125 Clay crystallite by interlayer cations, 82 83

materials in desalination membranes, 188 189 membrane, 52 54 modification, 83 84 CMC. See Carboxymethylcellulose (CMC) membrane CMR. See Catalytic membrane reactors (CMR) CNTs-polydimethylsiloxane (PDMS) nanocomposite membranes, 128 Colloids, 236 238 Column, hybrid process parameters cost, 395 height, 395 Commercial polyimide (Matrimid), 281 282 Component flux, 4 5 Composite membranes, 94, 266 267, 314 isopropanol/water mixtures, 62 63 membranes structures, 284 285, 314 mixed matrix membranes and inorganic fillers incorporated, 270 275 polymer nanoinorganic particles, 162 polyvinylamine-PVA separating layer, 120 121 sol 2 gel synthesis, 154 155 water/alcohol separation, 267 Composites, 81 biofiber, 81 ceramic matrix, 81 fiber-reinforced, 81 membranes, 314 metal matrix, 81 nanofiller-dispersed, 98 polymer matrix, 81 Computational modeling, pervaporation (PV), 366 Concentration gradient, 99 Concentration polarization, 99 and partition coefficient, 9

411

Condenser heat transfer area, hybrid process parameter, 395 Contact angle, 384 Contactor type membrane reactors, 311 Convective flow, 360 Conventional separation processes, 201 Corner-capping reactions, 207 208 Coupling effect, 372 Covalent functionalization, 116 carbon nanotubes, 116 Covalent grafting, 116 Crosslinked and modified PVC, 17 18 Cross-linked hybrid membranes, 162 Cross-linked membrane, 64f Cross-linked nanocomposites, 208, 209f Cross-linked PVA/C-MWCNT/ MA, 188, 188f Cross-linked PVA-multiwalled carbon nanotube (MWCNT) nanocomposite membranes, 317 318 Cross-linkers, 57 58 Cross-linking agent, 161, 277 278 Crystallinity, 19, 37 38, 216 CS-wrapped MWCNTs incorporated sodium alginate (SA) membranes, 121, 122f CTAB. See Cetyltrimethyl ammonium bromide (CTAB) Cussler model, 279 281 CVD. See Chemical vapor decomposition (CVD) C-whiskers, 153 Cyclohexane, 65 66

D DDR, 265 266 Dealumination, 264 265 Dehydrated ethanol, 91 Dehydrated isopropanol, 91 Dehydration, 265 266, 291

412

Index

Dendrimers, 231 232 Dense polythene films, 2 Desalination of brine, 86 87 technology, 175 176. See also Pervaporation desalination membranes Desorption, 110, 309, 362 363 Desulfurization, 220 Diamines, 146 1,6-Diamino hexane, 83 84 Dichloroethane, 37 Diffusion, 110, 308 309, 362 coefficient, 217, 370 flux, 358 selectivity, 2 3 Diffusion-dominated separation process, 60 61 Diffusivity, 367 368, 370 determination, 375 377, 375t, 376t Diisocyanates, 57 58 Dimensional stability, 20 Dimethylacetamide, 182 1,4-Dioxane dehydration, 123 Dip-coating, 333 335 Distributor membrane reactors, 311 Dodecyl trimethyl ammonium bromide, 83 84 Doped silica membranes, 7 Double-walled carbon nanotubes (DWCNTs), 113 114 DWCNTs. See Double-walled carbon nanotubes (DWCNTs)

E EC/C60 hybrid membrane, 24, 25f Electrical insulator, 154 Electric laser ablation, 113 114 Electron-rich gasoline components, 24 Electrostatic force, 82 83 Energy efficiency, 406 Enrichment factor, 5, 357 Esterification, 160 pervaporation-assisted, 142

pervaporation-coupling, 162 processes using nanocomposite membranes, 318 320 combined esterification and reaction systems, 305 306 esters and, 302 304 membrane reactors, 313 314 nanocomposite membranes, 314 318 pervaporation and pervaporation-assisted esterification, 315 318 pervaporation membrane reactor, 310 312 PV-aided applications, 319 320 separation process, 306 310 mechanism and basic characteristics, 307 310 and reaction systems, combined, 305 306 application, 306 by-product water removal, 305 membrane processes, 306 pervaporation system, schematic representation, 307f reactive distillation system, schematic representation, 305, 306f Esters and esterification, 302 304 acid-catalyzed reactions, 304 Fischer esterification, 304 formula, 302f hydrogen bonds, 302 Ethanediamine-modified ZIF-8 particles (ZIF-8-NH2), 343 344 Ethanol, 7 dehydration, 108

polyhedral oligomeric silsesquioxane (POSS), 218 219 PV, 20 21 water dehydration, 45 46 water mixture, 63 65 Ethyl acetate permeance, 98, 98f Ethyl cellulose (EC) hybrid membrane, 24 membranes, 28 29 reinforced with natural zeolite membranes, 26 28 reinforced with TiO2 membranes, 29 30 Ethylene glycol pervaporation dehydration, 142 Ethyl levulinate, 59 60 Evaporation, 235, 306 307 permeates, 87 88, 88f solvent, 273 Exfoliated nanocomposites, 85 86 Extractor type membrane reactors, 311

F Feed concentration, PV performance, 357 358 Feed pressure, PV performance, 358 Ferric oxide membrane, 55 56 nanoparticles, 55 56 Fiber-reinforced composites, 81 Fischer esterification, 304 Flexible selective barrier, 86 87 Flory 2 Huggins thermodynamics, 29, 368 correlation, 363 364 Flux, 4 5 reduction and NaCl concentration, 183 Free volume elements (FVEs), 215 216 Free volume of polymer matrix, 366 Freundlich isotherm, 268 Fuel-cell powered vehicle, 155

Index

Fuels desulfurization, 220 Fugacity gradient, 88 Fullerene-derivative carboxyfullerene, 162 Fullerenes, 231 232 biological activity, 164 165 buckyball, 154 cage compounds, 155 chemical formula, 154 155 closed hollow cages, 154 A3C60 phase, 155 endohedral compounds, 155 exohedral compounds, 155 functionalization, 156 membranes modified with, 164 166 pervaporation (PV), 156 158 membranes for, 158 160 schematic representation, 157f production, 154 Fullerenes-based nanocomposite membranes performance parameters of different pervaporation systems, 167t pervaporation performance, 162 164 Fullerenols, 164 165 Fullerols, 164 Functionalization, 138 139 amine, 140 141 carbon nanotubes, 114 116 covalent, 116 fullerenes, 156 sulfonic acid, 63 65 surface, 145 146 Functionalized graphene oxide membranes, 145 147 Functionalized graphene sheet (FGS)-based chitosan membranes, 56 57 Fusarium oxysporum, 238 239 FVEs. See Free volume elements (FVEs)

G Gas permeation unit (gpu), 5

Gelatin-based chitosan (GE/ CH) membranes, 49 50 Gelatin membrane, 49 50 Generalized Maxwell 2 Stefan (GMS), 365 GFT-1005, 159 Glacial acetic acid, 56 57 Glass metals, 235 Glass-transition temperature, diffusion process, 216 Glutaraldehyde (GA), 55 58 membranes, 50 51, 55 56 water/acetone solution, 180 181 Glycidal POSS-copolyimide membranes, 220 GNPs. See Graphene nanoplates (GNPs) GO. See Graphene oxide (GO) GO/chitosan (GO/CS) nanocomposite membranes, 317 Gold nanoparticle-based polymer, 253 plasmon pervaporation, 253 GO-PAH/hPAN, layer-by-layer approach membrane thickness, 140, 140f Graft copolymerization, 239 Grafting ratio, 51 52 Graphene, 233 derivatives, 137 139 FESEM images graphene oxide, 138f prepared graphene, 138f graphene oxide (GO) pristine, 136 137 graphite to oxidized graphite, structure variation, 138f mechanical properties, 138 139 membranes for pervaporation, 141 147 synthesis and characterization, 139 141 quantum hall effect, 138 139 sheets, 113 114

413

structure and properties, 137 139 Graphene-based anion exchange membrane, 140 Graphene-based membranes for pervaporation, 141 147 functionalized graphene oxide membranes, 145 147 graphene oxide-based membranes, 141 142 hybrid graphene oxide membranes, 144 145 quantum dot membranes, 147 reduced graphene oxide membranes, 143 144 Graphene nanoplates (GNPs), 186 Graphene oxide-based membranes, 141 142 Graphene oxide framework (GOF), 140 Graphene oxide (GO), 61 62, 186 FESEM images, 138f flakes functionalized with 3amino-1-propanesulfonic acid (GOSULF), 140 GO/polyimide (PI) fiber membrane, 186 in polyamide layer (PA-GO), 145 reduction, 138 Graphene oxide membranes (GOM), 140 “Green” chemical process, 329

H Hexadecyl trimethyl ammonium bromide, 83 84 Hierarchically ordered stainlesssteel-mesh (HOSSM), 340 341 Hollow fiber membrane, 278 279 Homogeneous nanofillers distribution, 112

414

Index

Hybrid graphene oxide membranes, 144 145 PA-GO/PAN membrane, 145, 145f structures, 144f Hybrid plastics, 206 207 Hybrid processes, 393 advantages, 406 distillation 2 pervaporation process, 396 403, 399f distillation process, 394, 395f hybrid PV 2 liquid phase adsorption process, 405 parameters, 394 395 PV-coupled fermentation process, 405 PV-coupled laccase treatment, 405 simulations, 403 405 Hybrid PV 2 liquid phase adsorption process, 405 Hybrid reactive distillation (RD), 394 Hydrogen bonding, 82 83 interactions, 210 211 Hydrophilic and organophilic membranes, 2 Hydrophilicity, 241 242 Hydrophilic membranes, 341 Hydrophilic polymers, 116 117 MOFs membranes, 337t, 342t Hydrophobic cellulose derivative, 20 21 Hydrophobic 2 hydrophilic balance, 55 56 Hydrophobic membranes, 128, 156 157, 159 160 Hydrophobic MOF-based MMMs, 335 336 Hydrosilylation reactions, 207 208 Hydrothermal method, 237 Hydroxyethyl cellulose (HEC), 29

I

IAST. See Ideal adsorbed solution theory (IAST)

Ideal adsorbed solution theory (IAST), 268 269 Imidozolium, 83 84 IMR. See Inert membrane reactors (IMR) Inert membrane reactors (IMR), 310 311 Inorganic adsorbent, 57 58 Inorganic and organic (polymeric) membranes, 106 107 Inorganic bridging agents, 283 285 Inorganic membranes, 7, 159, 233, 275 276, 314 In situ polymerization, polymer nanocomposite pervaporation membranes, 178 Intercalated nanocomposites, 85 86 Interfacial thermodynamics, 384 Intermatrix synthesis technique, 240 Intramolecular hydrogen bonds, 42 44 Intrinsic membrane performance, 5 Inverse gas chromatography method, 372 373 Ion exchange capacity (IEC), 49 50 Iron nanoparticle -based polymer, 242 247 Iron nanoparticle (FeNP), 242 243 Iron oxide/PAN membrane, 73 74 Iron polymer-metal, characterization, 246 247 4-Isocyanato-40-(3,30dimethyl-2, 4-dioxoazetidino) diphenyl methane (IDD), 40 Isopropanol/water mixture, 60 61

K Kaolinite, 83 84 Kinks, nanometal and metal oxides, 232 Klein paradox, graphene, 138 139 Knudsen diffusion, 360

L

Lactobacillus strain, 238 239 Lampung Natural Zeolite (LNZ) membranes, 26 28, 27f Langmuir adsorption isotherm, 268 Laponite nanoclay into PVA membranes, 189 190 Laser ablation, 235 Layer-by-layer assembly method, 275 276 Lignocellulosic biomass, 19 recalcitrance, 19 Linde type A (LTA) zeolite, 264 265 Liquid-to-vapor phase transition, 1 2 Lithium chloride solutions, 37

M Maleic acid (MA), 181 Maleic alginate cross-linked sodium alginate/chitosan membrane (M-CA/CH), 46 47 Maleic anhydride, 44, 46 47 Mass transfer coefficient, 99 in PV, 359 366 computational model, 366 Maxwell 2 Stefan model, 365 366 modified solution-diffusion model, 363 364 pore flow model, 359 361 solution-diffusion model, 361 363 thermodynamics model, 364 365 resistance, 28 29, 188

Index

Mass transport and membranes, 88, 90, 99 Matrimid 2 zeolite A membranes, 281 282 Maxwell 2 Stefan model, 365 366 Mean free path, graphene, 138 139 Mechanical grinding, 234 235 Melt mixing, 235 Membrane, 232 233 fouling, 202 203 isothermal processes, 175 176 module cost, 395 permeability, 111 permeation flux, 111 processes fouling, 175 176 reactors classification, 311f membranes for, 313 314 selectivity, 112 separation applications, 46 47 processes, 393 technology, 108, 159 160 surface effective area, 111 thickness pervaporation, 12 PV performance, 358 type, 358 Metal matrix composites, 81 Metal/metal oxide nanoparticles, synthesis, 234 biological method, 238 239 chemical methods, 236 238 chemical precipitation, 236 chemical reduction, 236 hydrothermal method, 237 microemulsion technique, 237 238 sol-gel technique, 236 237 physical method evaporation, 235 laser ablation, 235 mechanical grinding, 234 235 melt mixing, 235 sputtering, 235

Metal nanocomposites membranes, 239 254 alumina nanoparticle -based PMNC membranes, pervaporation performance of, 247 249 nanoalumina as membrane, 247 248 silver polymer, 248 249 gold nanoparticle-based polymer, 253 plasmon pervaporation, 253 iron nanoparticle -based polymer, 242 247 iron polymer-metal, characterization, 246 247 pervaporation with iron polymer, 243 246 nanoparticles in polymer, incorporation, 240 to polymer, grafting, 240 pervaporation using Ag polymer, 241 242 using titanium nanoparticle-based polymer, 249 252 polymer-metal nanocomposites based on nano-MgO and ZnO, 253 254 silver nanoparticle-based polymer, 241 Metal-organic frameworks (MOFs), 7 8, 329, 330f Methanol and methyl tert butyl ether (MeOH-MTBE), 40 41 Methanol 2 MTBE mixture, 40 41, 45 Methylene blue removal, 141 142 Methylethylketone, 7 Methyl nicotinamide chloride, 146 Methyl tertbutyl ether (MTBE)/ methanol mixtures, 25 Micorporous silica, 7

415

Microcomposite nanocomposites, 85 86 Microemulsion technique, 237 238, 253 254 Microfiltration, 106 Mitsunobu reaction, 304 Mixed matrix membranes (MMMs), 7 8, 106 107, 188, 253 254, 263 264, 270 275, 314, 329 331 morphologies and typical defects in, 113f, 273 for pervaporation desalination, 184 190 PES 2 zeolite 4A, 287f Mobility selectivity, 357 Modified pervaporation process, 377 382 Modified solution-diffusion model, 363 364 Modified zeolite-based polymer nanocomposite membranes compatibility, 279 287, 291 performances in pervaporation, 287 291, 289t solution-diffusion mechanism, 264 water/alcohol separation in zeolite, mechanism, 266 275 mixed matrix membranes and inorganic fillers, 270 275 zeolite particles, 267 270 water and alcohol-selective zeolites, 264 266 zeolite-filled nanocomposite membranes, fabrication, 275 279 Modifier, PV membranes, 161 MOFs-based MMMs, 334t Molecular sieving, 360 Molecular weight, diffusion process, 216 Monomers, hydrophilic and hydrophobic, 17 18 Monomethyl hydrazine (MMH) liquid propellants, 28 29

416

Index

Montmorillonite, 82 84 Mordenite framework inverted (MFI)-type structure, 266 Multiwalled carbon nanotubes (MWCNTs), 65 66, 113 114 MWCNTs-bucky paper (MWCNT-BP) structure, 123 125 Mxene-based chitosan composite membrane, 66 67 MXene membranes, 66 67, 191 192 laminates, 191 192 nanosheet and mass transport, 192f

N NaA-filled polybenzoxaine (PBZ), 291 Nadic methyl anhydride, 86 Nafion-GOSULF membranes, 140 Nanoalumina as membrane, 247 248 Nanocellulose, 18 isolation methods, 18 19 polymer nanocomposite membranes bacterial cellulose/alginate blend membranes, 30 blend membranes of sodium alginate and (hydroxyethyl) cellulose, 29 cellulose acetate membrane filled with metal oxide particles, 25 26 cellulose acetate/ polyacrylonitrile membranes, 23 24 cellulosepolydimethylsiloxane blends, 20 21 cellulose/poly(vinyl alcohol) membranes, 21 23 C60-filled ethyl cellulose hybrid membranes, 24

ethyl cellulose membranes, 26 30 membrane materials, design and choice of, 17 18 nanocellulose isolation methods, 18 19 pervaporation application, 20 30 Nanoclay clay particles, structure, 82 83 octahedral sheet, 82 83 organic modification of, 83 84, 84f silicon-oxygen tetrahedra, 82 83 Nano-CNT-iron oxide, 244 245 Nanocomposite membranes for pervaporation, 6 9, 6f, 314 318 advantages, 13 esterification, 315 318 factors affecting, 9 12 concentration polarization and partition coefficient, 9 membrane thickness, 12 pressure, 9 temperature, 10 11 inorganic membranes, 7 mixed matrix membranes, 7 8 polymer membranes, 8 9 principles, 2 5 permeability, normalized flux, 4 5 pore flow model, 3 4 selectivity, intrinsic membrane properties, 5 solution-diffusion model, 2 3 Nanocrystals, 20 Nanodiamonds, 155 156, 160 161 applications, 160 161 biocompatibility, 155 156 concentration in smoke, 160 161

cross section, 156f in hydrophobic membranes, 161 mass production, 160 161 Nanofibrous composite membrane, 191 Nanofiller-dispersed composites, 98 Nanofillers, 20, 107 108, 112, 141, 144, 148 Nanofiltration, 106, 143 144, 175 176, 315 Nanomaterials in membrane applications, 315, 318 319 surface modification and functionalization, 112 Nanometals, 232 and metal oxides as membrane, 239 metal nanocomposites membranes, 239 254 synthesis, 234f nanocomposite, 233 239 Nanoparticle-embedded polymer matrix, 217 Nanoparticles in polymer, incorporation, 240 to polymer, grafting, 240 Nanoscale cellulosic materials, 19 20 Nanostructured semiconductor, 232 Nanotechnology, 153 154, 233, 314 Nanotitanium oxide, 236 237 Nanotubes, 231 232. See also Carbon nanotubes (CNTs) carbon nanotubes (CNTs), 107 108, 113 116 multiwalled carbon nanotubes (MWCNTs), 65 66, 113 114 single-walled carbon nanotubes (SWCNTs), 113 114 NaY membrane, 57 58

Index

Neutral and charged copolymers, 17 18 N-methyl-2-pyrrolidone, 37 N,N-dimethyl acetamide, 37 N,N-dimethylformamide solvent, 184 185 Nonbiodegradable matrix, 81 Noncaged silsesquioxane molecule, 206, 207f Noncovalent functionalization, carbon nanotubes, 115 Nonporous membranes, 106 Non-porous polymeric membranes, 110 N-o-sulfonic acidbenzyl chitosan (NSABC) hybridmembranes, 63 65

O Octadecyl trimethyl ammonium bromide (ODTMA), 83 84 Octa(tetramethylammonium)POSS (octa-TMA-POSS), 212 213 OH-group-modified fullerenol, 156 Oleyl alcohol, organic liquid membranes, 160 Oly[β-(1,4)-D-glucosamine], 245 246 One-dimensional (1D) nanomaterials, 81 82 Organically modified clays, 83 84 Organically modified montmorillonite, 86 Organic bridging agents, 286 287 Organic chemical vapor deposition, 253 254 Organic functionalities, 83 84 Organic 2 inorganic hybrid membranes, 159, 233 Organic membranes, 159 Organic mixtures, separation, 219 220 Organic polymer membranes, 233 Organics dehydration, 329

hydrophilic polymer/MOFs membranes for, 341 345, 342t Organic-selective membranes, 17 18, 109 Organic solvent nanofiltration (OSN), 20 21 Organic 2 water mixtures, 107 Organometallic molecules, 155 Organosilanes, 286f Osmosis, forward, 175 176

P Palygorskite clay minerals, 83 84 Para-toluene sulfonic acidtreated clay-filled sodium alginate membranes, 94 PEBA-acetic acid, 185 PEBAX membrane, 182 Pendent type nanocomposites, 208, 209f Permeability, 4 5, 372 373 coefficient, 270 271 determination, 377 normalized flux, 4 5 selectivity, 357 and selectivity, 17 18 Permeance, 4 5 Permeate pressure, PV performance, 358 Permeation flux, 42 44, 59 60, 110 111, 356, 358 Permeation performance, 10 Permeation rate, 12, 377 379 Permselective evaporation, 2 PERVAP 2201, 159 PERVAP 1005 (GFT), 159 Pervaporation, 135 136, 176, 232 233, 329 advantages, 13 chitin and chitosan membranes, 36 37 definition, 355 356 experimental set-up for, 136f factors affecting, 9 12 concentration polarization and partition coefficient, 9

417

membrane thickness, 12 pressure, 9 temperature, 10 11 with iron polymer, 243 246 mass transfer in, 359 366 computational model, 366 Maxwell 2 Stefan model, 365 366 modified solution-diffusion model, 363 364 pore flow model, 359 360 solution-diffusion model, 361 363 thermodynamics model, 364 365 membranes, 6 9, 6f inorganic membranes, 7 mixed matrix membranes, 7 8 polymer membranes, 8 9 mixed matrix membranes (MMMs), 242f modeling, 373 382 diffusivity determination, 375 377 modified pervaporation process, 377 382 permeability determination, 377 sorption coefficient determination, 373 374 performance, 356 357 enrichment factor, 357 separation factor, 357 plasmon, 253 polymer-metal nanocomposite membranes, 255t predictive model, 383 389 chromatographic property, 384 contact angle, 384 interfacial thermodynamics, 384 physicochemical properties-process conditions, 385 389, 386t

418

Index

Pervaporation (Continued) polarity and solubility parameter, 383 384 principles, 2 5 permeability, normalized flux, 4 5 pore flow model, 3 4 selectivity, intrinsic membrane properties, 5 solution-diffusion model, 2 3 process conditions, 357 359 pure liquid sorption, 369 373 inverse gas chromatography method, 372 373 time-dependence of sorption, 370 371 time-lag experiment, 371 transport properties in, 367 368 using Ag polymer, 241 242 using titanium nanoparticlebased polymer, 249 252 Pervaporation desalination membranes factors affecting, 178 179 filler, diffusivity and nature, 178 179 membrane material, selectivity and nature, 178 operating temperature, 179 salt transport suppression, 179 polymer membranes for, 179 184 cellulose acetate membranes, 180 PEBAX membrane, 182 polyacrylonitrile and polyvinyl alcohol-based membranes, 180 181 poly(vinyl alcohol)/ polyvinylidene fluoride pervaporation membrane, 181 182 sulfonated poly(styreneethylene/ butylenes-

styrene) block copolymer membrane, 183 184 tubular pervaporation membrane, 183 polymer nanocomposite membranes, 184 195 chitosan and graphene oxide, schematic of molecular interactions, 187f mixed matrix membranes for pervaporation desalination, 184 190 self-assembled membranes, 190 192 sol 2 gel synthesized membranes, 192 195 Pervaporation desalination process, polymer nanocomposite membranes physical blending, 177 schematic representation, 176f self-assembly method, 178 in situ polymerization, 178 sol 2 gel synthesis, 177 synthesis methods, 177 178 Pervaporation membrane reactors (PVMRs), 301 302, 310 312 membrane classification and membrane applications, 313f Pervaporation separation index (PSI), 309 310 Phosphonium derivatives, 83 84 Phosphorylated chitosan membrane, 63 Phosphotungstic acid membrane, 62 63 Physical blending, polymer nanocomposite pervaporation membranes, 177 Physicochemical propertiesprocess conditions, 385 389, 386t Plasmon excitation, 232

Plasmon pervaporation, 253 Plasticization, 202 203 effect, 63 PMDA. See Pyromellitic dianhydride (PMDA) Polarity and solubility parameter, 383 384 Polar polymers, 216 Polyacrylates, 8 9 Polyacrylic acid (PAA), 45, 241 242 membrane, 45 Polyacrylonitrile (PAN) fibers, 153 and polyvinyl alcohol-based membranes, 180 181 Poly(allylaminehydrochloride)wrapped MWCNTs (MWCNTs 2 PAH), 118 119, 119f Polyaniline membrane, 51 52 Polybenzimidazole (PBI), 6 7, 329 330 Polybenzoimidazole membrane, 40 Poly(2,6-dimethyl-1,4phenylenoxide) (PPO), 162 Polydimethyl siloxane/ polyphenyl sulfone, 97 Polydimethylsiloxanes (PDMS), 8 9, 156 157, 160, 329 330 hydrophobic membranes, 2 membranes, 6 7 rubber membrane, 241 242 Polyelectrolytes, 275 276 aggregates, 47 48 complex, 47 48 complex formation, 45 Polyesters as inorganic filler and selective top, 120 polyethylenimines, 8 9 Poly(ether amides), 8 9 Poly(ether-block-amide) (PEBA), 329 330, 377 379 copolymer, 182 Polyethersulfone (PES), 120, 278 279

Index

Polyethylene glycol (PEG) blocks copolymer, 236 Poly(ethylene glycol)-POSS (PEG-POSS), 212 213 Polyhedral oligomeric silsesquioxane (POSS), 7 8, 205 210. See also Polymer/ POSS membranes, pervaporation performance applications, 208 210, 210f challenges and future aspects, 220 221 as molecular filler, 205 206 nanomaterial, applications, 210f polymer/POSS membranes, pervaporation performance, 210 214 azeotropic mixtures and organic solvents, separation, 212 214 cross-linking, 218 filler particles, nature, 216 217 free volume effect, 215 216 nature of, 216 penetrants nature, 218 temperature effect, 217 POSS-based nanocomposites, types, 209f POSS-embedded polymeric systems, 218 220 ethanol, 218 219 fuels desulfurization, 220 organic mixtures, separation, 219 220 water treatment, 220 properties, 208 silsesquioxanes, structures, 207f sizes and volume details, 206f synthesis, 206 208 THF 2 water azeotropic mixture, 212 213, 213t types, 206 Poly(3-hydroxybutyrate) (PHB), 123 Polyimides (PIs), 6 7 Polymer

free volume, 110 membranes, 8 9 nanoclay composites, 85 87 nanoinorganic particles, 162 nanosized inorganic particlebased composite materials, 163 164 pervaporation membranes, 329 330 Polymer-based membranes, 37 38 Polymer/clay nanocomposites, 81 82 membrane performance, factors affecting, 90 99 concentration polarization, 99 feed composition, 94 96 nanoclay content effect, 91 94 temperature, 96 98 nanoclay organic modification of, 83 84, 84f structure of, 82 83 pervaporation characteristics, 87 90 advantages and disadvantages, 89t membrane process, schematic diagram, 88f polymeric membranebased process, 87t pore flow mechanism, 90 solution diffusion mechanism, 89 90 transport mechanism, 88 polymer nanoclay composites, 85 87 Polymer 2 filler interactions, 203 205 Polymeric-based hydrophilic membranes, 2 Polymeric membranes, 86 87, 87t, 106 107, 263 264, 313 314 Polymeric nanocomposite membranes (PNCMs), 106 107

419

high-performance, 112 Polymerization, 178 Polymer-layered nanocomposites, 81 82 Polymer matrix, 85 86, 129, 239 composites, 81 Polymer-metal nanocomposites (PMNCs), 232 234 based on nano-MgO and ZnO, 253 254 membranes, 255 257 Polymer/metal-organic frameworks membranes, 329 331 hydrophilic polymer/metalorganic frameworks membrane for organics dehydration, 341 metal-organic frameworks/ chitosan membranes, 345 metal-organic frameworks/ polybenzimidazole membranes, 344 metal-organic frameworks/ poly(vinyl alcohol) membranes, 343 344 hydrophobic polymer/metalorganic frameworks membranes, organic recovery, 335 341 polydimethylsiloxane/ metal-organic frameworks membranes, 336 339 poly(ether-block-amide)/ metal-organic frameworks membranes, 339 340 PTMPS/metal-organic frameworks membranes, 340 341 preparation methods, 331 335 Polymer nanocomposite membranes pervaporation, 108 112 applications, 107

420

Index

Polymer nanocomposite membranes (Continued) organic 2 organic mixtures, separation, 125 127 organics from aqueous solutions, recovery, 128 129 schematic, 109f separation characteristics, 110 112 solution-diffusion model, 109 110 solvents and alcohols, dehydration of, 116 125 pervaporation desalination process physical blending, 177 schematic representation, 176f self-assembly method, 178 in situ polymerization, 178 sol 2 gel synthesis, 177 synthesis methods, 177 178 Polymer nanocomposites, 20, 81 82, 112, 208 star-like, 208, 209f Polymer nanometal nanocomposite, synthesis of, 233 239 Polymer poly(n-vinyl-2pyrrolidone) (PVP), 40 41 Polymer/POSS membranes, pervaporation performance, 210 214. See also Polyhedral oligomeric silsesquioxane (POSS) azeotropic mixtures and organic solvents, separation, 212 214 cross-linking, 218 filler particles, nature, 216 217 free volume effect, 215 216 nature of, 216 penetrants nature, 218 temperature effect, 217

Poly(2-methacryloyloxy ethyl trimethyl ammonium chloride) (PDMC), 144 145 Poly(methyl methacrylates) (PMMA), 8 9 membranes, 127 Poly(n-vinyl-2-pyrrolidone) membrane, 40 41 Poly(phenyleneiso-phtalamide), 98 Poly(phenyltrimethoxylsiloxane) (PTMOS), 277 278 Poly(phthalazinone ether sulfone ketone)polyethersulfone (PES)/TiO2 membranes, 318 Polypropylene glycol, 83 84 Poly(sodium 4styrenesulfonate) (PSS)wrapped MWCNTs (MWCNTs-PSS), 119, 120f Poly(sodium vinylsulfate) and chitosan (PVS/CH), 45 46 Polystyrene/montmorillonite nanocomposites, 83 84 Polysulfone (PS), 120 121 SiO2-GO nanohybrid composite membranes, 140 substrate, 336 339 Poly[1-(trimethylsilyl)-1propyne] (PTMPS), 329 330 Polyurethane (PU) membranes, 8 9, 127 Polyvinyl acetate (PVAc), 281 282 Polyvinyl alcohol-based iron polymer, 243 244 Poly(vinyl alcohol) (PVA), 2, 116 117, 156 157, 241 242 Ag membrane, 71 72 bentonite clay membrane, 91 92, 92f carboxylic MWCNT (CMWCNT), 188 clay nanocomposite membranes, 93f cross-linked with PMDA, 180 membrane, 42 44

multiwall carbon nanotube membrane, 70 71 polyvinylidene fluoride pervaporation membrane, 181 182 Polyvinylamine-PVA separating layer, 120 121 Poly(vinylidene fluoride) (PVDF), 339 substrate, 42 44 Poly(vinyl pyrrolidone) (PVP), 184 185 Polyvinyl sulfate membrane, 45 46 Pore-flow mechanism, 90 schematic diagram, 272, 272f Pore flow model, 3 4, 90, 90f, 359 361 Porous and nonporous membranes, 106 POSS. See Polyhedral oligomeric silsesquioxane (POSS) POSS-incorporated polymer systems, 206 207 POSS 2 polymer nanocomposites, 208 Predictive model, PV, 383 389 chromatographic property, 384 contact angle, 384 interfacial thermodynamics, 384 polarity and solubility parameter, 383 384 Pressure, pervaporation, 9 Pressure swing reactive distillation (PSRD), 399 Pristine graphene, 137. See also Graphene Protonated carboxyl acid groups, 47 48 PSRD. See Pressure swing reactive distillation (PSRD) Pure liquid sorption, 369 373 inverse gas chromatography method, 372 373 time-dependence of sorption, 370 371 time-lag experiment, 371

Index

PV. See Pervaporation PVA. See Poly(vinyl alcohol) (PVA) PVA-CS blended membranes, 42 44, 43f PV-assisted esterification reactions, 312 PV-coupled fermentation process, 405 PV-coupled laccase treatment, 405 PV-esterification coupling process, 159 PV membrane reactor (PVMR), 160 PV separation index (PSI), 111 Pyridine solution, 23 24, 24f Pyridinium, 83 84 Pyromellitic dianhydride (PMDA), 180

Q Quantum confinement, 232 Quantum dots, 231 232 membranes, 147 Quantum hall effect, graphene, 138 139 Quaternary alkyl ammonium compounds, 83 84 Quaternized chitosan and montmorillonite nanoclay (Na1-MMT), 91

R Raman spectrometry, 25 RAST. See Real adsorbed solution theory (RAST) Real adsorbed solution theory (RAST), 268 269 Reboiler heat transfer area, hybrid process parameter, 395 Reduced graphene oxide (RGO) membrane, 61 62, 143 144 Reverse microemulsion, 237 238 Reverse osmosis, 175 176 RF plasma synthesis technique, 253 254

S Scanning electron microscopy, 21 Selectivity, intrinsic membrane properties, 5 Selectivity rate, 309 Self-assembled membranes, 190 192 Self-assembly method, polymer nanocomposite pervaporation membranes, 178 Separation efficiency, 143 144, 301 302 Separation factor, 5, 110 111, 271, 357 Silica, 7 incorporated poly(vinyl alcohol) (PVA) membranes, 315 317 membranes, 7, 58 59 PAN/PEG membrane, 74 75 polytetrafluoroethylene membrane, 72 73 sol, 247 248 Silicalite-1, aluminum-free zeolite, 266 Silicalite-1/PDMS membrane, 290 291 Silicate-1 hydrophobic zeolites, 7 Silicone rubber blends, 17 18 Silver nanoparticle-based polymer, 241 Silver polymer-metal nanocomposites membranes, 248 249 Silver salt of polyacrylate, 241 242 Simulations, hybrid pervaporation process, 403 405 Single-walled carbon nanotubes (SWCNTs), 113 114 SiO2-reinforced polyelectrolyte complexes (PECs) membranes, 7 8 Si-O-Si stretching, 52 54 S-O-C linkage, 42

421

Sodium alginate, 6 8, 46 47 chitosan/multiwall carbon nanotube membrane, 69 70 membrane, 46 47 Sodium carboxymethyl cellulose (NCMC), 144 145 polyelectrolyte complex, 47 48 Sodium 2-formyl benzene sulfonate polysiloxane (SBAPTES), 63 65 Sol 2 gel synthesized membranes, 177, 192 195 esterification and polycondensation reactions, 194 195 heat-treated hybrid membrane, 194 195 heat treatment, 194 195 hybrid polymer inorganic membrane, 192 193 operating conditions, 193 194 TEOS, 192 193 Sol-gel technique, 159, 236 237, 253 254 Solubility, 367 368 coefficient, 217, 271 Solution-casting method, 188 189, 331 333 Solution-diffusion mechanism, 89 90, 178 schematic illustration, 212f Solution-diffusion model, 2 3, 89 90, 89f, 107 110, 110f, 361 363 schematic diagram, 270, 271f water/alcohol mixture separation, 272 Sorption, 110, 308, 361 362 coefficient determination, 373 374, 374t equilibrium, 368 selectivity, 2 3, 357 Sp2-hybridized carbon atoms, 116 Spinning technique, 278 279 Spraying, 335 Sputtering, 235

422

Index

Star-like polymer nanocomposites, 208, 209f Sulfation of polyelectrolyte membrane (S-PVS/CS), 45 46 Sulfonated poly(styreneethylene/ butylenes-styrene) (S-SEBS) block copolymer membrane, 183 184 Sulfonation of polystyrene, 253 Sulfonic acid functionalization, 63 65 Sulfonized chitosan membrane, 63 65 Sulfosuccinic acid (SSA) membrane, 59 60 Sulfuric acid, 55 58 hydrolysis, 19 strengthening, 55 56 Superhydrophobic ZIF-7 pore channels, 336 339 Supported liquid membrane (SLM), 160 Surface diffusion, 360 Surface hydrophilicity, 140 141 Surface plasmon resonance (SPR), 241 242 Surfactant, 83 84, 235 SWCNTs. See Single-walled carbon nanotubes (SWCNTs) Swelling percentage degree, 112 Synthesized sulfonated polystyrene (SPS), 253 Synthetic membranes, 313 314 Synthetic polymers, 8 9

T

TAC. See Total annual cost (TAC) Temperature, pervaporation, 10 11, 358 TEOS cross-linked chitosan membrane coated on polytetrafluoroethylene substrate (TEOS/CH-PTFE), 72 73 Tetrabutyltitanate (TBT), 253 Tetraethoxysilane (TEOS), 177

Tetrahydrofuran (THF), 42, 212, 214 Thermal-sensitive compounds, 107 Thermal stability, 59 60, 253 254 Thermodynamics model, 364 365 Thermodynamic vapor 2 liquid equilibrium limitation, 109 Thermoplastic block copolymer, 183 THF. See Tetrahydrofuran (THF) 3-aminopropyltrietheoxylsilane, 177 Three-dimensional materials, 81 82 Time-dependence of sorption, 370 371 Time-lag experiments, 371, 372f Titania, 7 Titanium dioxide (TiO2) membrane, 54 55, 253 Titanium glycine-N,Ndimethylphosphonate (TGDMP), 253 Titanium tetrachloride, 253 Toluene-2,4-diisocyanate (TDI) membrane, 60 61 Toluene-n-heptane mixtures, 96 Total annual cost (TAC), 394 395 Total dissolved solids (TDS), 175 176 Trade-off relationship, 17 18 Transmission electron microscopy (TEM), 24 Transport mechanism, polymer/clay nanocomposites, 88 Transport properties in pervaporation (PV), 367 368 Tray cost, hybrid process parameter, 395 2,4,6-Triaminopyrimidine, 288f Trichloroacetic acid, 37 Trifunctional organosilicon monomers, 207 208

Trimethyl silyl cellulose (TMSC), 20 21 Tri-n-octylamine (TOA), 160 T-type zeolite, 265 Tubular pervaporation membrane, 183 Tubular-type membrane, 278 279 Two-dimensional (2D) materials, 81 82

U Ultrafiltration, 106 Ultrasonication, 277 278 Ultrathin PVA membrane, 181 Uncross-linked hybrid membrane, 63 65 Universal Testing Machine, 57 58 Urea formaldehyde sulfuric acid mixture (UFS), 42 UV-vis absorption of PVA-AgNP, 242f

V Vacuum-driven membrane process, 107 Vacuum filtration method, GO deposition, 190 191 Vacuum pump, 108 109 Vander Waals force, 82 83 Vapor 2 liquid equilibrium (VLE), 13 curve, 397 water/acetic acid, 397, 398f Vapor-phase grown techniques, 153 Vapor pressure difference, 108 109 VBA. See Visual Basic for Applications (VBA) Visual Basic for Applications (VBA), 403 405

W Water/alcohol separation in zeolite, mechanism, 266 275

Index

alcohol-selective zeolites, 264 266 mixed matrix membranes and inorganic fillers, 270 275 zeolite particles, 267 270 Water-colloidal fullerenes, 164 Water removal, 304 305 Water-selective membranes, 109 Water-soluble polymer, 235 Water treatment polyhedral oligomeric silsesquioxane-embedded polymeric systems, 220

X X-ray diffraction, 45 X-ray photoelectron microscopy, 45 46 X-ray photoelectron spectroscopy (XPS), 253

Z Zeolite, 7, 57 58 aspect ratio, 279 280

chabazite (CHA), 265 DDR, 265 266 Linde type A (LTA), 264 265 particles, separation mechanism in, 267 270 and silica, 159 T-type, 265 type and characteristics, 266t water and alcohol-selective, 264 266 ZSM-5, 266 Zeolite 3A, 184 185 Zeolite-based polymer nanocomposite membranes compatibility, 279 287, 291 performances in pervaporation, 287 291, 289t solution-diffusion mechanism, 264 water/alcohol separation in zeolite, mechanism, 266 275 mixed matrix membranes and inorganic fillers, 270 275

423

zeolite particles, 267 270 water and alcohol-selective zeolites, 264 266 zeolite-filled nanocomposite membranes, fabrication, 275 279 Zeolite-filled polymeric membrane, 264, 275 279 Zeolite 2 polymer MMMs, 281 282 Zeolite/poly(vinyl) alcohol (PVA) membrane, 276 Zeolitic imidazolate frameworks (ZIF-8) nanoparticle, 213 214 Zero-dimensional (0D) nanomaterials, 81 82 Zero field conductivity, graphene, 138 139 Zero permeate pressure, 9 ZIF-7/PTMPS MMM, 340 ZIF-8/PTMPS MMM, 340 Zinc oxide NPs, 237 238 Zirconia, 7 Zn(BDC)(TED)0.5, 339 340 ZSM-5, 266 Zwitter-ionic polymers, 146