Novel Catalytic and Separation Processes Based on Ionic Liquids [1st Edition] 9780128020555, 9780128020272

Table of contents :
Content:
Front-matter,Copyright,About the Authors,AbbreviationsEntitled to full textChapter 1 - Introduction, Pages 1-11
Chapter 2 - Preparation and Characterization of Ionic Liquids, Pages 13-44
Chapter 3 - Properties of Ionic Liquids, Pages 45-110
Chapter 4 - Catalytic Reaction in Ionic Liquids, Pages 111-191
Chapter 5 - Separation Science and Technology, Pages 193-202
Chapter 6 - Biomass Utilization, Pages 203-220
Chapter 7 - Synthesis of Fine Chemicals, Pages 221-232
Chapter 8 - Ionic Liquid Gating of Thin Films, Pages 233-243
Index, Pages 245-258

Citation preview

Novel Catalytic and Separation Processes Based on Ionic Liquids

Novel Catalytic and Separation Processes Based on Ionic Liquids Dickson Ozokwelu US Department of Energy’s Office of Energy Efficiency & Renewable Energy, Washington, DC, United States

Suojiang Zhang Institute of Process Engineering, Chinese Academy of Sciences, Beijing, China

Obiefuna C. Okafor Corning Incorporated, Corning, NY, United States

Weiguo Cheng Institute of Process Engineering, Chinese Academy of Sciences, Beijing, China

Nicholas Litombe Harvard University, Cambridge, MA, United States US Department of Energy’s Office of Energy Efficiency & Renewable Energy, Washington, DC, United States

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 r 2017 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: 978-0-12-802027-2 For Information on all Elsevier publications visit our website at https://www.elsevier.com/books-and-journals

Publisher: Cathleen Sether Acquisition Editor: Kostas Marinakis Editorial Project Manager: Sarah Jane Watson Production Project Manager: Mohanapriyan Rajendran Designer: Matthew Limbert Typeset by MPS Limited, Chennai, India

About the Authors Dr. Dickson Ozokwelu has more than 40 years of diversified experience in chemical engineering and management, equally well in entrepreneurship, government, industry, and academia, including 16 years at the US Department of Energy. Currently, he is Lead Technology Manager for the Chemicals’ portfolio at the United States Department of Energy’s (DOE) Office of Energy Efficiency and Renewable Energy. In this position, Dr. Ozokwelu plays significant roles in reshaping the US government’s energy policy as well as in developing and deploying energy-efficient and renewable energy technologies in key manufacturing industries in the United States, including Chemicals, Petroleum Refining, and Pulp and Paper Industries. Dr. Ozokwelu was an invited author and online editor for Toluene Manufacturing Technology and Economics in the Kirk-Othmer Encyclopedia of Chemical Technology. In the last 12 years at US DOE, Dr. Ozokwelu championed the focusing of R&D on ionic liquids by funding and managing several R&D projects in ionic liquids as part of the Chemicals portfolio. Subsequently, he pioneered the inclusion of Ionic Liquids into Topical Conference series at the American Institute of Chemical Engineers, AIChE Annual Meetings Program. Dr. Ozokwelu’s expertise is in process design, process economics and process synthesis, separation technologies, energy efficiency, and renewable energy. For more than 10 years prior to his joining the US DOE, Dr. Ozokwelu held several technical and managerial positions in the US chemical industry. He was Engineering Associate (second highest rank in technical ladder) at BP North America, formerly BP-Amoco and Senior Research Chemical Engineer at Eastman Chemicals Company. He is widely recognized (in the United States and abroad) as an expert in separations technology and won the OIT-DOE First Technology Vision 2020 Award for the Chemical Industry in Separations. Prior to US industrial experience, Dr. Ozokwelu has more than 10 years of university teaching and accomplishments. Dr. Ozokwelu is a Fellow of the American Institute of Chemical Engineers. He received PhD and MS degrees in Chemical Engineering from Oklahoma State University in 1981 and 1978, respectively; a BS in Chemical Engineering from University of Ife, Nigeria in 1975; and an MBA in Planning and Management from the University of Memphis in 1984. To date, Dr. Ozokwelu has been a licensed Professional Engineer (PE) in the State of Tennessee in the United States since 1984.

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About the Authors

Prof. Suojiang Zhang is Professor & Director General of Institute of Process Engineering (IPE), Chinese Academy of Sciences (CAS), Member of CAS, Director of Beijing Key Laboratory of Ionic Liquids Clean Process, and Director of Professional Committee of Chinese Chemical Society Ionic Liquids and Green Engineering. His main research interests are ionic liquids and green process engineering, including molecular simulations, properties, preparation, and applications of ionic liquids for catalysis. Several green processes based on ionic liquids have been developed, such as a large-scale preparation process of ionic liquids, an innovative cleaner process of methyl methacrylate, a new process for the synthesis of ethylene glycol via hydrolysis or alcoholysis of ethylene carbonate, a new process for carbon dioxide capture, polyethylene terephthalate degradation, a novel method for production of biogasoline from biomass, and a new electrolyte. To date, Prof. Suojiang Zhang has published more than 550 papers in journals, and 450 papers of those were published in Science Citation Index (SCI) journals. He has edited 4 books and 2 chapters and has filed over 110 patents for inventions. Dr. Obiefuna Okafor is a Senior Project Leader at Corning Incorporated, where he currently leads development and implementation of new technologies in the Environmental Technologies division. He is a licensed Professional Engineer and Project Management Professional with over 14 years of strong, diversified experience in Research, Development, and Engineering. Prior to his current role, he worked in a Corporate Engineering capacity, delivering project results via lead technical and project management roles in development and engineering projects involving process simulation, design, scale-up and optimization; cost estimation; development of characterization techniques; and engineering upgrades to various divisions including Emerging Innovations Group, Environmental Technologies, and Life Sciences. Dr. Okafor received his Bachelor of Engineering in Chemical Engineering from the Federal University of Technology, Owerri, Nigeria, where he graduated with First Class Honors (Summa Cum Laude). He received his Master of Engineering and PhD in Chemical Engineering from the Stevens Institute of Technology, Hoboken, New Jersey, where he specialized in Microreactor Technology applications for the specialty chemicals industry. Prior to joining Corning Incorporated, he worked as a Process/Systems Engineer in the oil and gas industry, as a Risk Manager in the finance industry and as a Faculty member teaching undergraduate chemistry courses. He serves as a local section leader of American Institute of Chemical Engineers, as well as on the board of several non-profit organizations. He has two published patents, three pending patent applications, and a trade secret. Prof. Weiguo Cheng earned his Ph.D. degree from Dalian University of Technology in 2005, majoring in Industrial Catalysis. He served as a visiting scholar at the University of Waterloo (Canada). He was appointed professor by the Institute of Process Engineering, at the Chinese Academy of Sciences, in 2014. He focuses on revealing the essential

About the Authors xi relationship between the catalyst structure and the catalytic performance by using surface analytical techniques, particularly in-situ vibrational and optical spectroscopies. He specializes in the design of novel catalytic materials and catalytic process. His current research interests include synthesis of ionic liquid catalysts, mechanisms and kinetics of catalytic reaction, carbon dioxide utilization, and a new catalytic process for the synthesis of ethylene glycol based on ionic liquids. To date, he has presided over one national hightech research and development program, has served as the chief scientist at the National Natural Science Fund, and has participated in many projects, including the National Basic Research Program. He has published 35 peer-reviewed papers, including one in the Journal of Catalysis (2008;255:343), and submitted 21 invention patent applications, of which 12 have been granted. Dr. Nicholas Litombe received his PhD (2015) from Harvard University in experimental condensed matter physics. His research focused on lithographic patterning and characterization of high-temperature superconductors in the cuprate 214-family into nanostructures to probe dimension-limited superconductivity through transport measurements. His expertise is in low-temperature, low-noise measurements of strongly correlated systems. From 2015 to 2016, he was postdoctoral fellow in Physics at the Department of Physics at Harvard University. Currently, he is a postdoctoral associate of the department. He is also a Science and Technology policy fellow at the Department of Energy’s Advanced Manufacturing Office under the Office of Energy Efficiency and Renewable Energy (EERE). In this role, he supports a portfolio focused on clean technology energy and innovation for energy-intensive manufacturing sectors in the United States. He holds a bachelor’s degree in Applied Physics from the School of Engineering and Applied Sciences from Columbia University, where he has also received training in Applied Mathematics.

Abbreviations Techniques AFM ATR DSC ES EXAFS ESI-MS FAB FAB-MS HRTEM IR spectroscopy L-SIMS MALDI-MS MBE MRI MS NMR pH QSPR SANS SPC STM UV
VIS XRD XPS

atomic force microscopy attenuated total reflectance differential scanning calorimetry electrospray mass spectrometry extended X-ray absorption fine structure electrospray ionization mass spectrometry fast atom bombardment fast atom bombardment mass spectrometry high-resolution transmission electron microscopy infrared radiation spectroscopy liquid secondary ion mass spectrometry matrix-assisted laser desorption/ionization mass spectrometry molecular beam epitaxy magnetic resonance imaging mass spectrometry nuclear magnetic resonance spectroscopy measure of the acidity or alkalinity of a solution quantitative structure
property relationships small-angle neutron scattering spectrophotometric colorimetry method scanning tunneling microscope ultraviolet
visible X-ray powder diffraction X-ray photoelectron spectroscopy

Miscellaneous ˚ A δ 1 H NMR spectroscopy

˚ ngstrom 5 10210 m 1A Chemical shift Proton Nuclear Magnetic Resonance Spectroscopy

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Abbreviations

13

C NMR spectroscopy F NMR spectroscopy 35/37 Cl NMR spectroscopy UV UV/vis VOCs v/v w/w wt% ρ r γ X p m V Vm M Nρ κT T Mio VmE Vm;i η Δη γN β S R o Tfus;1 Δfus H1o Δfus cop;1 19

MD Td pKa Pa EDL(G) IL(G) FET MOSFET

Carbon-13 Nuclear Magnetic Resonance Spectroscopy Fluorine-19 NMR spectroscopy Chlorine-35/37 Nuclear Magnetic Resonance Spectroscopy ultraviolet ultraviolet
visible spectroscopy volatile organic compounds volume for volume weight for weight weight per cent mass density the bond length surface tension molar fraction pressure mass volume molar volume molar mass of ILs, Avogadro’s constant isothermal compressibility temperature molar mass of the pure copmponent i excess molar volume molar volume of component i of the mixture viscosity viscosity deviations activity coefficients at infinite dilution solute distribution ratio selectivity gas constant fusion temperature of pure solid at p 5 0.1 MPa fusion enthalpy of pure solid at p 5 0.1 MPa isobaric heat capacity change due to fusion of pure solid at p 5 0.1 M molar concentration molecular dynamics decomposition temperature acid dissociation constants Pascals Electric double layer (gating) Ionic liquid (gating) Field effect transistor Metal oxide semiconductor field effect transistor

Abbreviations xv MIT SIT DC BCS HTSC

Metal insulator transition Superconductor insulator transition Direct current Bardeen Cooper Schrieffer High-temperature superconductor

Ionic Liquids [mim]1 [Hmim]1 [C1mim]1 [C2mim]1 [C3mim]1 [C4mim]1 [C5mim]1 [C6mim]1 [C7mim]1 [C8mim]1 [C9mim]1 [C11mim]1 [C12mim]1 [C13mim]1 [C14mim]1 [C15mim]1 [C16mim]1 [C17mim]1 [C18mim]1 [Amim]1 [Omim]1 [C2mmim]1 [Ceim]1 [C2py]1 [C3py]1 [C4py]1 [C6py]1 [C2mβpy]1 [C4mβpy]1 [C4mγpy]1 [C4mpyr]1 [C4mpip]1 [C4ebim]1 [C4ebt]1

N-methylimidazolium cation 1-methylimidazolium cation 1-methyl-3-methylimidazolium cation 1-ethyl-3-methylimidazolium cation 1-propyl-3-methylimidazolium cation 1-butyl-3-methylimidazolium cation 1-pentyl-3-methylimidazolium cation 1-hexyl-3-methylimidazolium cation 1-heptyl-3-methylimidazolium cation 1-octyl-3-methylimidazolium cation 1-nonyl-3-methylimidazolium cation 1-nonyl-3-methylimidazolium cation 1-dodecyl-3-methylimidazolium cation 1-tridecyl-3-methylimidazolium cation 1-tetradecyl-3-methylimidazolium cation 1-pentadecyl-3-methylimidazolium cation 1-hexadecyl-3-methylimidazolium cation 1-heptadecyl-3-methylimidazolium cation 1-octadecyl-3-methylimidazolium cation 1-allyl-3-methylimidazolium cation 1-methyl-3-methylimidazolium cation 1-ethyl-2,3-dimethylimidazolium cation 1-ethyl-3-ethylimidazolium cation 1-ethylpyridinium cation 1-propylpyridinium cation 1-butylpyridinium cation 1-hexylpyridinium cation 1-ethyl-3-methylpyridinium cation 1-butyl-3-methylpyridinium cation 1-butyl-4-methylpyridinium cation 1-butyl-1-methylpyrrolidinium cation N-butyl-N-methylpiperidium cation 1-butyl-3-ethylbenzimidazolium cation 1-butyl-3-ethylbenzotriazolium cation

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Abbreviations

[C4vim]1 [C2C6pip]1 [P6 6 6 14]1 [Pi(444)1]1 [N6 6 6 14]1 [N1111]1 [N2222]1 [N3333]1 [N4444]1 [C1OC2mim]1 [C2OHmim]1 [HC2mim]1 [HC2im]1 [HEMMor]1 [C2mmor]1 [CQC2mmor]1 [CQC2mpip]1 [C2ebim]1 [Dim]1 [P14,6,6,6]1 [C4dmim]1 [dibmim]1 [DEP]2 [CF3(CF2)6COO]2 [FAP]2 [CH3COO]2 [(CH3O)2PO2]2 [SCN]2 N½CN2 2 [methide]2 [TfO]2 [DCA]2 [NO3]2 [dca]2 [NTf2]2 [OTf]2 [BF4]2 [PF6]2 [TOS]2 [OAc]2 [SbF6]2 [PF3(CF2CF3)3]2

1-butyl-3-vinylimidazolium cation 1-ethyl-1-hexylpiperidinium cation trihexyl(tetradecyl)phosphonium cation tri-iso-butyl(methyl)phosphonium cation trihexyl(tetradecyl)ammonium cation tetra-n-methylammonium cation tetra-n-ethylammonium cation tetra-n-propylammonium cation tetra-n-butylammonium cation 1-methoxyethyl-3-methylimidazolium cation 1-hydroxyethyl-3-methylimidazolium cation 1-(2-hydroxyethyl)-3-methyl imidazolium cation 1-(2-hydroxyl-ethyl)-imidazolium cation N-(2-hydroxyethyl)-N-methyl morphorinium N-ethyl-N-methylmorpholium cation N-allyl-N-methylmorpholium cation N-allyl-N-methylpiperidium cation 1,3-diethylbenzimidazolium cation dialkylimidazolium tris(p-hexyl)-tetradecylphosphonium cation 1-butyl-2,3-dimethylimidazolium cation 1-(4-diacetoxyiodobenzyl)-3-methylimidazolium cation diethylphosphate anion pentadecafluorooctanoate anion trifluorotris(pentafluoroethyl)phosphate anion acetic acid anion alkyl phosphate anion thiocyanide dicyanimide tris(trifluoromethylsulfonyl)methide trifuoromethanesulfonate dicyanamide nitrate dicyanamide anion bis{(trifluoromethyl)sulfonyl}amide anion trifluoromethanesulfonate anion tetrafluoroborate anion hexafluorophosphate anion tosylate anion acetate anion hexafluoroantimonate anion trifluorotris(pentafluoroethyl)phosphate

Abbreviations xvii [CF3SO3]2 [N(CF3SO2)2]2 [PtCl6]22 [Rh(CO)2I2]2 [SbCl6]2 [Sn2Cl5]2 [Ace]2 [CF3COO]2 [Cl]2 [ClO4]2 [I]2 [HSO4]2 [EtSO4]2 [H2PO4]2 [p-MeC6H4SO3]2 [MeSO3]2 [ZnCl3]2 [AlCl4]2 [C1mim][Cl] [C4mim][Cl] [C8mim][Cl] [C4mpy][Cl] [C12C12im][Cl] [C2mim][Br] [C4mim][Br] [C6mim][Br] [C8mim][Br] [C10mim][Br] [C12mim][Br] [C16mim][Br] [Cnpim][Br] [C4mim][I] [C4mim][BF4] [C6mim][BF4] [C8mim][BF4] [C4mim][PF6] [C4mim][NTf2] [C4mim][OTf] [C4mim][O2CC6H5] [C4mim][O2CC7H15] [C4mim][OH]

trifluoromethanesulfonate anion bis(trifluoromethanesulphonyl) imide anion hexachloroplatinate anion monsanto catalyst anion hexachloroantimonate anion chlorostannate (II) anion acesulfamate anion trifluoroacetate anion chloride anion perchlorate anion iodide anion hydrosulfate anion ethyl sulfate anion dihydrogen phosphate anion p-toluenesulfonate anion methanesulfonate anion trichlorozincate anion aluminum chloride anion 1-methyl-3-methylimidazolium chlorine 1-butyl-3-methylimidazolium chlorine 1-octyl-3-methylimidazolium chlorine 1-butyl-3-methylpyridinium chlorine 1,3-bis(dodecyl)imidazolium chloride 1-ethyl-3-methylimidazolium bromide 1-butyl-3-methylimidazolium bromide 1-hexyl-3-methylimidazolium bromide 1-octyl-3-methylimidazolium bromide 1-decyl-3-methylimidazolium bromide 1-dodecyl-3-methylimidazolium bromide 1-hexadecyl-3-methylimidazolium bromide 1-(2,4,6-trimethylphenyl)-3-alkylimidazolium bromide 1-butyl-3-methylimidazolium iodide 1-butyl-3-methylimidazolium tetrafluoroborate 1-hexyl-3-methylimidazolium tetrafluoroborate 1-octyl-3-methylimidazolium tetrafluoroborate 1-butyl-3-methylimidazolium hexafluorophosphate 1-butyl-3-methylimidazolium bis{(trifluoromethyl)sulfonyl} amide 1-butyl-3-methylimidazolium trifluoromethanesulfonate 1-butyl-3-methylimidazolium benzoate 1-butyl-3-methylimidazolium octanoate 1-dodecyl-3-methylimidazolium hydroxide

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Abbreviations

[C4mim][OMs] [C4mim][HCOO] [C4mim][CH3CHOHCOO] [C4mim][HOCH2COO] [C4mim][HSCH2COO] [C4mim][(C6H5)COO] [C4mim][H2NCH2COO] [C4mmim][PF6] [C2mim][PF6] [C6mim][PF6] [C8mim][PF6] [C10mim][PF6] [C12mim][PF6] [C3mpy][NTf2] [C4mpy][NTf2] [C6mpy][NTf2] [C8mpy][NTf2] [C10mpy][NTf2] [C2mim][NTf2] [C6mim][NTf2] [C8mim][NTf2] [C10mim][NTf2] [C2mmim][NTf2] [C8H4F13mim][NTf2] [C1mim][MeSO4] [C1mim][DMP] [C2mim][Gly] [C2mim][B(CN)4] [C2mim][EtOSO3] [C2mim][OTf] [C4mim][(iC8)2PO2] [C4mim][C12H25SO4] [C4mim][MeSO4]

1-butyl-3-methylimidazolium methanesulfonate 1-butyl-3-methylimidazolium formate 1-butyl-3-methylimidazolium lactate 1-butyl-3-methylimidazolium glycollate 1-butyl-3-methylimidazolium thioglycollate 1-butyl-3-methylimidazolium benzoate 1-butyl-3-methylimidazolium aminoethanic acid salt 1-butyl-2,3-dimethylimidazolium hexafluorophosphate 1-ethyl-3-methylimidazolium hexafluorophosphate 1-hexyl-3-methylimidazolium hexafluorophosphate 1-octyl-3-methylimidazolium hexafluorophosphate 1-decyl-3-methylimidazolium hexafluorophosphate 1-dodecyl-3-methylimidazolium hexafluorophosphate 1-propyl-3-methylpyridinium bis{(trifluoromethyl)sulfonyl} amide 1-butyl-3-methylpyridinium bis{(trifluoromethyl)sulfonyl} amide 1-hexyl-3-methylpyridinium bis{(trifluoromethyl)sulfonyl} amide 1-octyl-3-methylpyridinium bis{(trifluoromethyl)sulfonyl} amide 1-decyl-3-methylpyridinium bis{(trifluoromethyl)sulfonyl} amide 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide 1-hexyl-3-methylimidazolium bis{(trifluoromethyl)sulfonyl} amide 1-octyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide 1-decyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide 1-ethyl-2,3-dimethylimidazolium bis(trifluoromethylsulfonyl) imide 1-(3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluorooctyl)3-methylimidazolium bis(trifluoromethylsulfonyl)imide 1,3-dimethylimidazolium methylsulfate 1,3-dimethylimidazolium dimethylphosphate 1-ethyl-3-methylimidazolium glycine 1-ethyl-3-methylimidazolium tetracyanoborate 1-ethyl-3-methylimidazolium Ethanesulfonic acid 1-ethyl-3-methylimidazolium trifluoromethanesulfonate 1-butyl-3-methylimidazolium bis(2,2,4-trimethylpentyl) phosphinate 1-butyl-3-methylimidazolium dodecylsulfate 1-butyl-3-methylimidazolium methylsulfate

Abbreviations xix [C4mim][SCN] [C4mim][FeCl4] [C6mim][FEP] [C8mim][OTf] [C10mim][Gly] [C4mβpy][TOS] [C4mβpy][BF4] [C2mpyr][NTf2] [C4bim][Br] [C4mim][OCSO4] [C2mim][EtSO4] [Bzmim][BF4] [C2mβpy][EtSO4] [Amim][Cl] [N1444][NTf2] (n 5 4, 6, and 8) [N1666][NTf2] (n 5 4, 6, and 8) [N1888][NTf2] (n 5 4, 6, and 8) [P4444][TFA] [P66614][Pro] [P66614][Met] [P66614][Gly] [P66614][Ala] [P66614][Sar] [P66614][Ile] [aP4443][Ala] [aP4443][Arg] [aP4443][Asn] [aP4443][Asp] [aP4443][Cys] [aP4443][Gln] [aP4443][Glu] [aP4443][Gly]

1-butyl-3-methylimidazolium thiocynate 1-butyl-3-methylimidazolium tetrachloroferrate(III) 1-hexyl-3-methylimidazolium tris(pentafluoroethyl) trifluorophosphate 1-octyl-3-methylimidazolium trifluoromethanesulfonate 1-decyl-3-methylpyridinium glycine 1-butyl-3-methylpyridinium tosylate 1-butyl-3-methylpyridinium tetrafluoroborate N-methyl-N-ethyl-pyrrolidinium bis{(trifluoromethyl)sulfonyl} amide 1,3-di-n-butylimidazolium bromide 1-butyl-3-methylimidazolium octylsulfate 1-ethyl-3-methyl-imidazolium ethylsulfate 1-benzyl-3-methyl-imidazolium tetrafluoroborate 1-ethyl-3-methylpyridinium ethylsulfate 1-N-allyl-3-methylimidazolium chloride tributyl(methyl)ammonium bis{(trifluoromethyl)sulfonyl}amide trihexyl(methyl)ammonium bis{(trifluoromethyl)sulfonyl} amide trioctyl(methyl)ammonium bis{(trifluoromethyl)sulfonyl}amide tetra-n-butylphosphonium acetate trihexyl(tetradecyl)phosphonium prolinate trihexyl(tetradecyl)phosphonium methioninate trihexyl(tetradecyl)phosphonium glycinate trihexyl(tetradecyl)phosphonium lanate trihexyl(tetradecyl)phosphonium sarcosinate trihexyl(tetradecyl)phosphonium isoleucinate (3-Aminopropyl)tributylphosphonium L-α-aminopropionic acid (3-Aminopropyl)tributylphosphonium L-α-amino-5guanidinovaleric acid (3-Aminopropyl)tributylphosphonium L-α-aminosuccinamic acid (3-Aminopropyl)tributylphosphonium L-α-aminosuccinic acid (3-Aminopropyl)tributylphosphonium L-α-amino-3mercaptopropropionic acid (3-Aminopropyl)tributylphosphonium L-α-aminoglutaramic acid (3-Aminopropyl)tributylphosphonium L-1-aminopropane1,3-dicarboxylic acid (3-Aminopropyl)tributylphosphonium aminoethanoic acid

xx Abbreviations [aP4443][His] [aP4443][Ile] [aP4443][Leu] [aP4443][Lys] [aP4443][Met] [aP4443][Phe] [aP4443][Pro] [aP4443][Ser] [aP4443][Thr] [aP4443][Trp] [aP4443][Tyr] [aP4443][Val] [P6 6 6 14][OAc] [Pi(444)1][TOS] [P4 4 4 4][TOS] [MG][LAC] [ppg][BF4] [aemmim][Tau] [tmg][L] [pabim][BF4] [P(C4)4][Gly] [C2H5NH3][NO3] [Admim]Br [NH23’3’][C7CO2] [Pyrr][C8CO2] [Im][C7CO2] [TEtA][OTf] [CNC2Him][DOSS] [C2mmim][NTf2] [NEPP][CH3CH2COO]

(3-Aminopropyl)tributylphosphonium L-α-amino4-imidazolepropionic acid (3-Aminopropyl)tributylphosphonium L-α-amino3-methylvaleric acid (3-Aminopropyl)tributylphosphonium L-α-amino4-methylvaleric acid (3-Aminopropyl)tributylphosphonium L-α-diaminocaproic acid (3-Aminopropyl)tributylphosphonium L-α-amino-g(methylthio)butyric acid (3-Aminopropyl)tributylphosphonium L-α-aminohydrocinnamic acid (3-Aminopropyl)tributylphosphonium (S)-2-pyrrolidinecarboxylic acid (3-Aminopropyl)tributylphosphonium L-α-amino3-hydroxypropionic acid (3-Aminopropyl)tributylphosphonium L-α-amino3-hydroxybutyric acid (3-Aminopropyl)tributylphosphonium (S)-a-amino-1H-indole3-propanoic acid (3-Aminopropyl)tributylphosphonium L-α-aminop-hydroxyhydrocinnamic acid (3-Aminopropyl)tributylphosphonium L-α-aminoisovaleric aci trihexyl(tetradecyl)phosphonium acetate tri-iso-butyl(methyl)phosphonium tosylate tetra(n-butyl)phosphonium tosylate hexamethylguanidinium lactate N,N,N0 ,N0 ,Nv-pentamethyl-Nv-propylguanidinium tetrafluoroborate 1-aminoethyl-2, 3-dimethylimidazolium amino acid taurine 1,1,3,3-tetramethylguanidinium lactate 1-n-propylamine-3-butylimidazolium tetrafluoroborate tetrabutylphosphonium glycinate ethyl-ammonium nitrate 1-N-allyl-2,3-dimethylimidazolium bromide diisopropylethylammonium octanoate pyrrolidinium nonanoate imidazolium octanoate triethylamine trifluoromethanesulfonate 1-hexyl-3-propanenitrileimidazolium dioctysulfosuccinate 1-ethyl-2,3-dimethylimidazolium bis{(trifluoromethyl)sulfonyl} amide N-ethyl piperazinium propionate

Abbreviations xxi [Epy][EtSO4] [HMEA][HCOO] [C2OHmim][FeCl3Br] [C2OHmim][Br] [TMSP(Benzo15C5)HIM] [N(SO2CF3)2] [A(Benzo15C5)HIM][N (SO2CF3)2] [AEMP][OH] [AEMP][A] [APMIM][Br] [(CH2)2COOHmim] [HSO4] [Apmim][BF4] [MnIIITTMAPP][PF6]5

ethyl pyridinium ethylsulfate mono-ethanolammonium formate 3-(2-hydroxyethyl)-1-methyl imidazolium bromotrichloro ferrate 3-(2-hydroxyethyl)-1-methyl imidazolium bromide 1-(trimethoxysily) propyl 3-(60 -oxo-benzo-15-crown-5 hexyl) imidazolium bis(trifluoromethanesulphonyl) imide 1-allyl-3-(60 -oxo-benzo-15-crown-5 hexyl) imidazolium bis (trifluoromethanesulphonyl) imide 1-(2-aminoethyl)-1-methylpiperazin-1-ium hydroxide 1-(2-aminoethyl)-1-methylpiperazin amino acid 1-aminopropyl-3-methylimidazolium bromide 1-(2-carboxylic acid)ethyl-3-methylimidazolium bisulfate

1-(3-aminopropyl)-3-methylimidazolium tetrafluoroborate manganese tetrakis-(4-N-trimethylaminophenyl)porphyrin hexafluorophosphate [Hmim][TFA] 1-methylimidazolium triflouroacetate N-ethyl-4-dimethylaminopyridinium dicyanamide [C24DMAPy][N(CN)2] N-butyl-4-dimethylaminopyridinium dicyanamide [C44DMAPy][N(CN)2] N-hexyl-4-dimethylaminopyridinium dicyanamide [C64DMAPy][N(CN)2] [(CH2)2COOHPy][HSO4] N-carboxyethylpyridine hydrosulphate [CH2COOHPy][HSO4] N-carboxymethylpyridine hydrosulphate [BsAIm][OTf] 1-allyl-3-(butyl-4-sulfonyl) imidazolium trifluoromethanesulfonate [MEA]L monoethanolaminium lactate 1-butyl-3-methylimidazolium methylselenite [C4mim][SeO2(OCH3)] tetrapropylammonium perruthenate [nPr4N][RuO4] TEMPO 2,2,6,6-tetramethyl-1-piperidinyloxy TCCA trichloroisocyanuric acid DMAP 4-dimethylaminopyridine IBX iodoxybenzoic acid NHPI N-Hydroxyphthalimide NADH Nicotinamide adenine dinucleotide HPW phosphotungstic acid POM peroxopolyoxometalate DBT dibenzothiophene ODS oxidative desulfurization dhp dihydrogen phosphate TSIL task-specific ionic liquid IPILs imidazolium perrhenate ionic liquids PILs polymeric ionic liquids

xxii

Abbreviations

[Rh(cod)2][BF4] [C4mim]3[Co(CN)5] [Ru(η6-p-cymene)(η2TRIPHOS)][Cl2] [C4mpyr][NTf2] [NMP][HSO4] [Et3NH]Cl/AlCl3 [Hmim][SbF6] [C2mim][Cl]/AlCl3 [HNMe3][Cl]/AlCl3 DEME DEME-TFSI DEME-BF4 DEME-BETI

Bis(1,5-cyclooctadiene) rhodium(I) tetrafluoroborate Tris(1-butyl-3-methylimidazolium) cobalt pentxacyanide η2-1,3,5-triaza-7-phosphaadamantane-η6-p-cymene ruthenium bromide 1-butyl-1-methylpyrrolidinium bis{(trifluoromethyl)sulfonyl} amide N-methyl-2-pyrrolidone hydrogen sulfate triethylamine hydrochloride aluminum chloride 1-methylimidazolium hexafluoroantimonate 1-ethyl-3-methylimidazolium aluminum chloride trimethylammonium aluminum chloride N,N-diethyl-N-methyl(2-methoxyethyl)ammonium [DEME]-bis(trifluoromethylsulfonyl)imide [DEME]-tetrafluoroborate [DEME]-bis(pentafluoroethanesulfonyl)imide

CHAPTER 1

Introduction 1.1 Importance of Ionic Liquids The discovery of ionic liquids (ILs) can be dated back to the work of Paul Walden, who formulated his thesis with Wilhelm Ostwald in 1891. However, the modern definition of ILs was provided in 1992 by the groups of Seddon, Hussey, and Chauvin, who discovered air- and water-stable ionic liquids; ILs were new solvents that could not be considered merely as some kind of molten salts [1]. Over recent years, ILs have attracted considerable attention in various fields [2], such as materials science, chemical engineering, biomass, electrochemistry, and so on. Scientists and engineers have documented a variety of interesting features because of the diversified and unique phenomena demonstrated by ILs. A diverse group of novel sciences and engineering has been developed using ILs, which have unique features, e.g., air-stable forms, lack of vapor pressures, ability to dissolve various materials, and high ionic conductivities. An infinite number of ILs designed with various functions can be achieved by combining different constitutive cations and anions. This effort has now become a frontier of science. And id one of the strategies of science and technology that has been adopted in many countries. ILs, as a new class of compounds, have become a fascinating new topic. The publication of papers on this topic has grown almost exponentially. Since the second generation of ILs, such as BF4 2 , PF6 2 , and other anions, appeared in 1992, the publication of papers increased from dozens every year 20 years ago to more than 6000 every year now (Fig. 1.1). The number of papers increased rapidly, especially in chemistry and chemical engineering journals, such as Nature, Science, Chemical Reviews, JACS, AIChE, JCES, and IECR. The patent literature has also shown equally explosive growth (currently .10,000). Meanwhile, papers and patents from electrochemistry initially to catalysis, separation, materials, polymer, energy, and environment have been widely published. The importance of ILs is elucidated by many existing scientific programs and laboratories, e.g., the Center of Ionic Liquids and Green Engineering in the Chinese Academy of Sciences, the U.S. Department of Energy, the Queen’s University Ionic Liquid Laboratories (QUILL), the Center for Green Manufacturing in the University of Alabama, the Japanese “Science of Ionic Liquids,” the Power, Environmental & Energy Research Center (PEER), and the German priority program “Ionic Liquids.” More important, the Ionic Liquids

Novel Catalytic and Separation Processes Based on Ionic Liquids. DOI: http://dx.doi.org/10.1016/B978-0-12-802027-2.00001-7 © 2017 Elsevier Inc. All rights reserved.

1

2

Chapter 1

6000

Number of papers

5000

4000

3000

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0 1992 1994 1996 1998 2000 2002 2004 2006 2008 2010 2012 2014 Year

Figure 1.1 The trends of publications on ionic liquids over years.

Specialized Committee of China, which was established in 2012, is a professional organization. It promotes fundamental research and application of ILs through a collaboration of scientists. Achievement of the goals of the committee will make significant contributions to society. These laboratories and centers cooperate with industrial members, including CNPC, Sinopec, Shell, BASF, MERCK, and P&G. The research-funded industrial members bridge the gap between the academic and industrial research goals. The importance of ILs is seen in the rapid advances in numerous applications with some extant processes, e.g., BASF (BASIL), Institut Franc¸ais duPe´ trole (Difasol), Degussa (paint additives), Linde (hydraulic ionic liquid compressor), Pionics (batteries), and G24i (solar cells) [3]. One of breakthroughs was BASF’s introduction of the first application on a commercial scale: the BASIL process. The yield per unit volume time increased significantly from 8 to 690,000 kg m23 h21 in this process. At the same time, the Institute of Process Engineering, Chinese Academy of Sciences, has explored the development of a new ethylene glycol process using ILs as catalysts. The 20,000 t/a industrial demonstration plant is now in progress, based on 1000 h of a successful continuous pilot plant test. The novel technology is very competitive due to its high single conversion and selectivity, low energy consumption, and simple process. Conferences on ILs have attached great importance to academic exchange. International conferences (e.g., International Congress on Ionic Liquids, Asia-Pacific Conference on Ionic Liquids and Green Processes, Gordon Research Conferences Ionic Liquids,

Introduction 3 Conference on Molten Salts and Ionic Liquids, and International Conference on Ionic Liquids in Separation and Purification Technology) have highlighted many of the emerging themes important worldwide and represent opportunities for discussion of major topics in this field among experts from around the world. Among the conferences, the 366th Xiangshan Science Conferences, officially supported by the Ministry of Science and Technology of China and the Chinese Academy of Sciences, have given priority to the scientific frontiers and future development of ILs. Conferences have made significant contributions to the development of fundamental research and industrialization of ILs. ILs have aroused widespread interest in the international community. ILs were voted the UK innovation most likely to shape the 21st century in a nationwide poll run by science museums and learned societies [4]. Recently, ILs have been voted one of 20 prospective materials in future due to their special features and widespread use (Fig. 1.2) [5]. Professor Robin D. Rogers won the Presidential Green Chemistry Challenge Award for his contribution on ILs in 2005.

1.2 Alternatives to Traditional Catalysis and Separation ILs are considered alternative catalysts and solvents in different reaction systems based on their tenability, stability, and recyclability. Great interest has been shown in their applications to promote reaction systems. Reviews and books have been published by such groups as MacFarlane [6], Ohno [7], Seddon [8], Welton [9], and other groups, summarizing IL applications in different areas. For example, the research groups Ohno [10] and Rogers [11] have written reviews on cellulose processing in ILs. In 2009, Ohno pointed out that there is a dramatic increase in studies on the solubilization of nearly insoluble compounds, including cellulose, which is impossible to dissolve using conventional molecular liquids [10]. Compared with the chloride salts, a series of carboxylate-based ILs were shown to be more effective at dissolving cellulose. However, carboxylate ILs have relatively poor thermal stability. Ohno suggested that more ILs need to be examined in this area. Later, in 2012, Rogers summarized data for a wide range of different cations and anions and concluded that ILs do provide a new and powerful platform for the conversion of biomass into valuable chemicals, either as a “pretreatment process” or as a conversion solvent [11]. ILs, indeed, are opening up new research areas, driven by applications and providing new methodologies that are different from traditional chemical processes. Here, we focus on the most recent and potential applications of ILs in catalysis and separation processes.

1.2.1 Ionic Liquids in Catalysis The tenability of ILs has enabled their applications in catalytic reactions. They can be finely designed by carefully choosing the main cation and anion groups and further modified by

Electrolyte in batteries Metal plating Solar panels Fuel cells

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spectrometry Gas chromatography

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Figure 1.2 Features and applications of ionic liquids.

Nanochemistry Multiphase reactions and extractions

columns Stationary phase for HPLC

Introduction 5 introducing different functional groups to meet the requirements of the catalytic systems. This often leads to the formulation of task-specific ILs as well as to process control over the most appropriate solute-solvent system after screening and optimization. In an IL-involved catalytic reaction, the interactions among the IL and the catalyst, the IL and the reactant, and the solubility of the reactant and the catalyst can determine the outcome of the reaction. The complexity of the systems has challenged current research groups to attempt to understand the mechanisms, which has led to the growing interest in the study of ILs in catalysis. A collection of highly informative books and reviews from specialists in this area can provide strong fundamental and practical knowledge to inexperienced as well as experienced readers [12]. The IL-enhanced reactions can be not only a homogeneous system or a heterogeneous liquid liquid biphase system but also a hetergeneous liquid-solid system through immobilized ionic liquid catalyst on the support. Although an immobilized catalyst on the support is preferable for the separation and recyclability of the catalyst, the approach can be limited by low activity and selectivity. Therefore, both homogeneous and heterogeneous systems need to be evaluated for specific cases. The interest in the catalysis in ILs is centered on the fact that ILs are generally nonvolatile reagents providing low vapor pressure. This makes them attractive alternatives to conventional solvents in catalytic reactions under ambient conditions. The products can be extracted or distilled from the reaction mixture after the reaction. Under a liquid liquid biphasic system, ideally, the catalyst can be dissolved in the ionic phase, which is miscible with the reactant phase, and the product would be hardly dissolved in the ionic phase. This can allow for an efficient process and easy separation by decantation. For example, in the sulfide oxidation process, the ILs have been used as both reaction media and extractants, which can dissolve the formed sulfones [13]. In a typical system, there are two layers: an upper layer, the oil; and a lower layer, the IL. The sulfide in the upper layer can be transferred into the sulfone and moved into the ionic phase. Then the product is separated from the IL by extraction using CCl4. A new approach using an immobilized catalyst involving ILs has emerged, especially in combination with metal catalysts. Strategies such as supported ionic liquid phase (SILP) and supported catalyst with ionic liquid layer (SCILL) have been developed. The SILP is the immobilization of a catalyst into an IL, followed by impregnation onto a support, whereas in the SCILL, the traditional heterogeneous catalysts are impregnated with an IL layer. ILs have become the alternatives for molecular solvents. Extensive types of systems have been established using ILs as solvent, support and catalyst, or both. Moreover, the hazardous reactions occurring in intensive reaction conditions have been replaced by the introduction of the new reaction systems and processes.

6

Chapter 1

1.2.2 Ionic Liquids in Separation ILs have enabled application of the new concept in separation processes exhibiting low vapor pressures. However, the application of ILs is not straightforward. Some of the ILs decompose at lower temperatures compared with conventional molecular reagents, and some of the ILs are flammable. Therefore, for specific separation systems, cation and anion compositions, indeed, need to be optimized and further studied in the system. Task-specific ILs can be designed, which will allow for separation as a specialty. Currently, the use of ILs is mainly studied in gas separation, liquid liquid extraction, and other separation technologies, such as gas chromatography, mass spectroscopy, and high performance liquid chromatography (HPLC). Initially, ILs were considered for liquid liquid extractions because of their stability, adjustable miscibility, and polarity. The toxicity of the IL while being used in the separation process is also crucial and needs to be evaluated before establishing the system. ILs are under investigation in the extraction of metals, metal oxides, metal ions, and noble organometallics from several systems. For example, large quantities of metal oxides can be dissolved in the ammonium bistriflamide salts with carboxylic acid functionalities. The metals are separated from ILs by adding acidic aqueous solution. The ionic liquid can be recycled after the metal ions have been transferred to the aqueous phase. Moreover, ILs have demonstrated a potential for the noble organometallic Wilkinson’s and Jacobsen’s catalyst extraction from a homogeneous organic phase. Functionalized ILs are screened, and those with an amino acid based anion, glycinate, or methionate are identified. Pereiro et al. reviewed the contributions of ILs to liquid liquid extraction and extractive distillation [14]. They reported that in the azeotropes reviewed, the alkyl sulfate anion based ILs showed the greatest potential as solvents in separation by liquid liquid extraction. [C2mim][EtSO4] is the most frequently studied in this extraction process, and [C1mim][MeSO4] and [C4mim][MeSO4] were used as azeotrope breakers in the laboratoryscale liquid liquid extraction process. [C1mim][MeSO4] has the highest extraction efficiency. Additionally, IL regeneration and recycling are simple for this process. ILs have also been proposed as gas separation media. Their cations and anions can be tailored to meet absorption requirements. The supported IL membranes are especially considered in the separation of CO2, H2S, and SO2 from gas gas or gas organic compound mixtures. For example, an adequate porous support material, a polymer film, was coated with ILs and was successfully used for the separation of CO2 from CO2, N2, and CH4 gas mixtures [15]. ILs have also drawn considerable attention recently in the field of analytical sciences, including gas chromatography and HPLC. 1-vinyl-3-hexylimidazolium PF6-/NTf2- were

Introduction 7 grafted onto the polymethylsiloxane capillary by Wei et al. [16], and the separation ability was evaluated. Poole et al. [17] employed ILs as additives to the mobile phase. A series of alkylammonium-based ionic salts were synthesized with nitrate and isocynate anions, four of which were liquid at room temperature. These ILs were added and mixed with organic solvents as the mobile phases in HPLC. The first generation of anion exchange media, imidazolium-based ionic liquids immobilized on silica, were reported by Liu et al. [18]. These IL-based columns have successfully separated different analytes, such amines, nucleotides, and anions.

1.3 Developments and Trend of Ionic Liquids in Chemical Engineering From the engineering point of view, ILs have provided a platform for chemical engineering by offering new catalysis and separation systems. However, challenges are still encountered while these novel Il-promoted techniques are transferred to industrial scales, e.g., the largescale production of SILP. Such uses are under investigation, as well as a novel reaction system and engineering design to generate clean and sustainable processes.

1.3.1 Current Focus and Development of the Application of Ionic Liquids Alongside the promotion of chemical reactions, the synthesis of ILs has to be evaluated for a truly environment-friendly system. In 2001, Varma’s group prepared a series of 1-alkyl-3methylimidazolium halide ionic liquids using the microwave heating method [19]. 1-Methylimidazole was treated with haloalkanes under solvent-free conditions, and different conversions were obtained in relation to microwave input power and irradiation time. For example, 1 mmol 1-butyl bromide was mixed with 1 mmol 1-methylimidazole and heated for 30 s. The reaction mixture was then taken out, mixed again for 10 s, and heated at the same power level for an additional 15 s. This step was repeated three times, providing a yield of 81%. The combination of an IL solvent and microwave heating provides an easy and efficient synthetic procedure. In 2003, Deetlefs and Seddon reported a wide range of nitrogen-containing, heterocyclicbased ILs synthesized in multimode microwave reactors [20]. Small-, medium-, and largescale amounts of ILs were prepared in minutes, compared with the use of the thermal heating method, in which the reaction takes hours to complete. For example, it was reported in the paper that the mixture of 1-methylimidazole and 1-chlorobutane was made and irradiated at 1200 W, programmed to ramp to 80 C over 5 min, and then irradiation was continued for a total of 5.5 h, providing an 85% yield of 1-butyl-3-methylimidazolium chloride. A faster and greener process was developed, and it offered a flexible, small- to largescale approach to prepare ILs in microwave reactors.

8

Chapter 1

A recent review of the “greenness” of IL synthesis was performed by using the 12 principles of green chemistry, and a SWOT (strengths, weaknesses, opportunities, and threats) analysis: microwave-induced synthesis was found to be the second greenest methodology, second only to the use of micro-reactors [21,22]. The high decomposition temperatures and good microwave absorption during the heating processes have allowed ILs to be used more efficiently integrated with microwave reactors than micro-reactors. Recently, Holbrey and his group synthesized a group of halide-free ILs with methylcarbonate anion by using a microwave reactor [23]. The ILs were directly synthesized by heating the reaction mixture under microwave irradiation for 1 2 h at 140 170 C, and relatively high yields were gained. The initial set input power of the microwave should be adjusted to achieve a steady increase in the set temperature, rather than overshooting, which would lead to excess pressure from the mixture in the reactor, causing an explosion. The reaction temperature and time were optimized, and the use of the microwave reactor proved to be a convenient and practical route for synthesis [24]. One of the main applications of ILs is their utilization in industrial catalysis. ILs, alone or together with traditional catalysts, are usually immobilized on different materials, including silica, alumina, and titania. This has become a widely researched area, since more variables, apart from the tuning of cations and anions, need to be studied. For the supporting materials, wettability, pore diameter, surface area, pore volume, and surface reactivity require detailed measurement and full description. The most accepted IL-involved supported catalysts are the SILP and the SCILL, as mentioned in Section 1.2.1. Many types have been designed, prepared, and tested in a laboratory experiment successfully; the scalable production methodologies have attracted the attention of chemical engineers. Recently, a scalable preparation method of the SILP and the SCILL has been reported; it applies a fluidized-bed spray coating to disperse the IL film onto the support surface ([25], Fig. 1.3A). The uncoated support or catalyst is fluidized by temperature-controlled flow of gas, such as air, nitrogen, or argon. Once the support material is fluidized, the solution of the ionic liquid in an auxiliary solvent is sprayed onto the material through a nozzle (Fig. 1.3B). The helper solvent is rapidly evaporated because of the gas flow to achieve good dispersion of the catalyst-containing IL onto the support (Fig. 1.3C). The fluidized bed with its inherent mixing pattern ensures that the support material is well mixed and evenly coated by the IL film. In general, the ILs are developed on the basis of their clean production and convenient use in industries. Application-oriented research has enhanced the applications of ILs in chemical engineering and has been promoted by chemical engineering designs and technologies.

Introduction 9

Figure 1.3 (A) Schematic drawing of the toroidal movement of particles in Aircoater IAC5; (B) photograph of the spraying nozzle; (C) cross-section of the nozzle, indicating the two flows (bottom). Adapted from S. Werner, N. Szesni, M. Kaiser, M. Haumann, P. Wasserscheid, A scalable preparation method for SILP and SCILL ionic liquid thin-film materials, Chem. Eng. Technol. 35 (11) (2012) 1962 1967.

1.3.2 Challenge and Trend of Ionic Liquids in Chemical Engineering 1.3.2.1 Ionic liquids in metallic nanoparticle catalysis ILs have become the suitable media to immobilize catalysts on supports (SILP and SCILL). In addition, they have acted as electrostatic or electrosteric stabilizers for the metallic nanoparticles (MNPs),which are generally considered core-shell systems, mainly involving electrostatic and electrosteric repulsion forces. The anions interact with the electrophilic surface of the nanoparticles, forming a layer at the surface of the MNP. They have been considered in the cases of MNPs, such as iron, ruthenium, cobalt, and so on. In some cases, the leaching of atoms located at low coordination positions at the metallic surface can be avoided by using ILs. The results in this area seem promising and can lead to the industrial application of MNPs. 1.3.2.2 Ionic liquids in biomass transfer A recent review raised the question “Are ILs the proper solution to current environmental challenges?” The review summarized the use of ILs in “hot” fields with regard to their utilization as solvents, e.g., in cellulose dissolution, chitin and keratin dissolution, waste plastic recycling, metal recovery, and CO2 capture, some of which have already been

10

Chapter 1

discussed in Section 1.2. The fact that ILs have unique solvent power has attracted the attention of research groups to study the dissolution of lignin in ILs, which requires further investigation taking into consideration the structural complexity of lignin. Current research suggests that ILs with anions of strong H-bond basicity have high solubility to lignin. However, for biomass transformation, the structural design of ILs for selective dissolution is still a challenge, which leads to the question of tailoring of ILs by functional groups. ILs have been found to play a special role in the pharmaceutical industry because of the constant demand for effective purification and extraction processes. Moreover, the applications of ILs in protein stabilization and bio-preservation have attracted the attention of the pharmaceutical field. However, the toxicity of ILs in this application needs to be fully evaluated in this area. The large diversity of ILs has inspired both scientists and engineers. Basic studies in ILs have led to a good understanding of their applications, and their industrial uses have motivated laboratory research. In this book, the preparation, characterization, and properties of ILs will be introduced in Chapter 2, Preparation and Characterization of Ionic Liquids and Chapter 3, Properties of Ionic Liquids, and a detailed study of their applications in catalysis, separation, biomass utilization, and synthesis of fine chemicals will be discussed in Chapters 4 7. Finally, the use of ILs in tuning electronic phase changes in materials for potential application in field effect transistors is explored in Chapter 8, Ionic Liquid Gating of Thin Films.

References [1] K.R.K. Seddon, Ionic liquids for clean technology, J. Chem. Technol. 68 (Iii) (1997) 351 356. [2] R.D. Rogers, K.R. Seddon, Ionic liquids—solvents of the future? Science 302 (5646) (2003) 792 793. [3] M. Petkovic, K.R. Seddon, L.P.N. Rebelo, C.S. Pereira, Ionic liquids: a pathway to environmental acceptability, Chem. Soc. Rev. 40 (3) (2011) 1383 1403. [4] E. Stoye, Ionic liquids win Great British Innovation Vote, 28 March 2013 [Online]. Available from: ,http://www.rsc.org/chemistryworld/2013/03/great-british-innovation-vote-ionic-liquids.. [5] The New Material, The twenty most potential new materials in the future, 2014 [Online]. Available from: ,http://202.96.155.158/html/weikandian/lidianchi/2014/1027/1128.html.. [6] J. Stoimenovski, D.R. MacFarlane, K. Bica, R.D. Rogers, Crystalline vs. ionic liquid salt forms of active pharmaceutical ingredients: a position paper, Pharm. Res. 27 (4) (2010) 521 526. [7] K. Fujita, K. Murata, M. Masuda, N. Nakamura, H. Ohno, Ionic liquids designed for advanced applications in bioelectrochemistry, RSC Adv. 2 (10) (2012) 4018. [8] M.J. Earle, K.R. Seddon, Ionic liquids. Green solvents for the future, Pure Appl. Chem. 72 (7) (2000) 1391. [9] T. Welton, Room-temperature ionic liquids: solvents for synthesis and catalysis. 2, Chem. Rev. 111 (2011) 3508 3576. [10] H. Ohno, Y. Fukaya, Task specific ionic liquids for cellulose technology, Chem. Lett. 38 (1) (2008) 2 7. [11] H. Wang, G. Gurau, Ionic liquid processing of cellulose, Chem. Soc. Rev. 41 (2012) 1519 1537. [12] V.I. Paˆrvulescu, C. Hardacre, Catalysis in ionic liquids, Chem. Rev. 107 (6) (2007) 2615 2665.

Introduction 11 [13] D. Betz, P. Altmann, M. Cokoja, W.A. Herrmann, F.E. Kuhn, Recent advances in oxidation catalysis using ionic liquids as solvents, Coord. Chem. Rev. 255 (13 14) (2011) 1518 1540. [14] A.B. Pereiro, J.M.M. Arau´jo, J.M.S.S. Esperanc¸a, I.M. Marrucho, L.P.N. Rebelo, Ionic liquids in separations of azeotropic systems—a review, J. Chem. Thermodyn. 46 (2012) 2 28. [15] J.E. Bara, R.D. Noble, D.L. Gin, Effect of “free” cation substituent on gas separation performance of polymer-room-temperature ionic liquid composite membranes, Ind. Eng. Chem. Res. 48 (9) (2009) 4607 4610. [16] Q. Wei, M. Qi, H. Yang, R. Fu, Separation characteristics of ionic liquids grafted polymethylsiloxanes stationary phases for capillary GC, Chromatographia 74 (9) (2011) 717 724. [17] C.F. Poole, B.R. Kersten, S.S.J. Ho, M.E. Coddens, K.G. Furton, Organic salts, liquid at room temperature, as mobile phases in liquid chromatography, J. Chromatogr. A 352 (1986) 407 425. [18] H. Qiu, S. Jiang, X. Liu, L. Zhao, Novel imidazolium stationary phase for high-performance liquid chromatography, J. Chromatogr. A 1116 (1 2) (2006) 46 50. [19] R.S. Varma, V.V. Namboodiri, An expeditious solvent-free route to ionic liquids using microwaves, Chem. Commun. 7 (2001) 643 644. [20] M. Deetlefs, K.R. Seddon, Improved preparations of ionic liquids using microwave irradiation, Green Chem. 5 (2) (2003) 181 186. [21] P.T. Anastas, P. Wasserscheid, A. Stark, Handbook of Green Chemistry, Volume 6, Green Solvents, Ionic Liquids, John Wiley & Sons, Inc., Hoboken, NJ, 2014. [22] M. Deetlefs, K.R. Seddon, Assessing the greenness of some typical laboratory ionic liquid preparations, Green Chem. 12 (1) (2010) 17 30. [23] J.D. Holbrey, R.D. Rogers, S.S. Shukla, C.D. Wilfred, Optimised microwave-assisted synthesis of methylcarbonate salts: a convenient methodology to prepare intermediates for ionic liquid libraries, Green Chem. 12 (3) (2010) 407 413. [24] E`. Boros, K.R. Seddon, C.R. Strauss, E`. Boros, C.R. Strauss, Chemical processing with microwaves and ionic liquids, Chim. Today 26 (2008) 28 30. [25] S. Werner, N. Szesni, M. Kaiser, M. Haumann, P. Wasserscheid, A scalable preparation method for SILP and SCILL ionic liquid thin-film materials, Chem. Eng. Technol. 35 (11) (2012) 1962 1967.

CHAPTER 2

Preparation and Characterization of Ionic Liquids 2.1 Introduction Room temperature ionic liquids (ILs) are regarded as environmentally benign solvents and have been widely investigated for their many unique properties, including extremely low vapor pressure, tunable structure, high thermal and chemical stability, and excellent solvent power for organic and inorganic compounds. In recent years, more and more kinds of ILs have been synthesized, and the structures of ILs, as well as their synthesis routes, have become more complex. Particularly, the cost of ILs is very high because of the complex synthesis approaches and purification methods. It is necessary to summarize the preparation, purification, characterization, and largescale production of ILs for their applications, especially industrial ones. In this chapter, first, our aim is to provide a comprehensive overview of the synthetic methods currently available. According to the reported synthetic methods, a researcher who wants to synthesize a new IL could easily design and evaluate the synthetic methodology that might be applicable. Second, we also discuss a number of purification methods and characterization issues that arise in the synthesis process of ILs. It is very difficult to avoid these issues, which may result from the inherent properties of ILs. Finally, we review the largescale production of ILs and point out the corresponding problems, which may be helpful for the applications of ILs in the future.

2.2 Preparation of Ionic Liquids ILs, in principle, have become more sophisticated and are aiming at more complex compounds. Different families of ILs have to be synthesized by different methods. The reactions of these processes generally include neutralization, quaternization, and ion exchange reaction.

Novel Catalytic and Separation Processes Based on Ionic Liquids. DOI: http://dx.doi.org/10.1016/B978-0-12-802027-2.00002-9 © 2017 Elsevier Inc. All rights reserved.

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Chapter 2

2.2.1 Protic Ionic Liquids Protic ionic liquids (PILs) are formed through the transfer of a proton from a Brønsted acid (A) to a Brønsted base (B), according to the following reaction [1
3]. B 1 HA-HB1 1 A2 Angell and co-workers synthesized a series of PILs, including organic or inorganic anions (Scheme 2.1), and they pointed out the general process of the reactions: Equimolar acid and base were reacted together, either neat or in an aqueous solution; since these reactions are very exothermic, the acid was dropped into the cooling base solution, the mixture was then reacted at room temperature for several hours; and to ensure a complete reaction, a slight excess amount of base was used and then removed by evaporator for bases with small molecular weights or additional purification procedures for bases with higher molecular weights; the products were then dried at 80 C to allow enough time in a vacuum oven containing phosphorus pentoxide (P2O5) to remove any water. Additionally, in recent years, a series of PILs containing complicated heterocyclic cations were synthesized by neutralization of organic tertiary amines with organic acids or inorganic acids (Scheme 2.2) [3
6].

NH3

NH3

NH3

NH3

NH4

NH3

H2 N

H N

N H2 H N

NH3

F

R

F3CSO3

CH3SHO3

F3 C

O

H N

O

H N

N

N H

N

BF4

O F3C S NH

H N

NO3

Fn(F2C)

NH3

N H2

H N

O

O

NH

H N

HSO4

O

O

COO

NH3

O

H2PO4 O2 S

N

O2 S

(CF2)nF

Scheme 2.1 Representative anions and cations of PILs prepared by Angell et al.

Preparation and Characterization of Ionic Liquids 15

NH

N

N

NH

HN

H 2N

NH

N N NH N NH 2

NH

NH

N H H N

N

H 2N N

N

N N

NH

Scheme 2.2 Cation of new protic PILs synthesized in recent years.

Using stronger acids and/or stronger base can improve the proton-transfer process. The pKa values of acids and bases can be used to evaluate the possibility of proton transfer from acid to a base, although it must be noted that the pKa can only be used in an aqueous solutions [1,3]; ILs can be distinctly volatile, and proton transfer between acid and base is not complete unless ΔpKa for the acid/base pair is high (.10) [7,8].

2.2.2 Aprotic Conventional Ionic Liquids ILs are usually synthesized by quaternization of imidazoles, alkylamines, or phosphines, using alkyl halides as alkylating agents, and other anions can be obtained by ion exchange reaction [3]. Synthesis of the aprotic ILs are shown as follows: The vast majority of traditional ILs were designed and synthesized with 1,3dialkylimidazolium as cations because the unique imidazole framework can be adjusted through control of substituent chain length or modification [5]. These cations were readily formed by quaternization of methylimidazole and alkyl halides, and halide-free ILs could be prepared by the metathesis reaction of the corresponding halide salt, either with its metal or ammonium salts, or the free acid of the anion (Scheme 2.3) [9
12]. Alkylpyridinium-based ILs can be synthesized by similar methods with quaternization and metathesis reactions (Scheme 2.4). For example, equimolar amounts of pyridine or picoline and alkyl halide were reacted at 70 C for 24 h, and white precipitates were obtained; then the white precipitate further reacted with the corresponding anion-based metal salts at room temperature in water, and the corresponding ILs were obtained through extraction with dichloromethane [13
15]. A series of homologous ammonium-based ILs with (CH3CH2)3N1(CnH2n11) (n 5 2, 4, 6, 8) as cations and [BF4]2, [PF6]2, or [SbF6]2 as anions were synthesized by quaternization and metathesis reactions (Scheme 2.5) [16]. To obtain high purity ILs, Cai and co-workers proposed a combined method, according to which potassium chloride was removed by the metathesis of halide salts with potassium hydroxide in ethanol, and then the ion exchanges in cation exchange and anion exchange resin columns were used to purify the hydroxide

16

Chapter 2 N

N

+

R

n

N

X

N X

n³0

R

MA

N

N A

R

A=BF4 ; PF6 ; CH3COO, CF3COO, SCN, Tf2N

X=Cl; Br; I

Scheme 2.3 Synthesis of imidazolium-based ionic liquids.

+ N

R

n

X

MA

n³0

N X

N

R

R

A

A=BF4 ; PF6 ; CH3COO, CF3 COO, SCN, Tf2N

X=Cl; Br; I

Scheme 2.4 Synthesis of pyridinium-based ionic liquids.

N

R-Br EtOH, 80°C,12h inert atmosphere

N R Br

NH4 BF4 , KPF6, or NSbF6 Acetone, H2O, or CH2Cl2

N R A

R= Ethyl, Butyl, Hexyl, Octyl; A= BF4, PF6, SbF6

Scheme 2.5 Synthesis of ammonium-based ionic liquids.

intermediates [17], and a series of ILs with high purity were prepared by the neutralization reaction of the resulting hydroxide intermediates with free acids of desired anions [17]. Phosphonium-based ILs can be synthesized by direct reactions of phosphines with sulfates, tertiary phosphines, or imidazoles with alkylating agents, or phosphines with acids (Scheme 2.6) [3,18,19]. Nucleophilic addition of tertiary phosphones to haloalkanes is a typical method for preparing asymmetric tetralkylphosphonium halides. For example, phosphonium halides were prepared by quaternization of PR3 (R 5 pentyl, hexyl, octyl) with 1-chloro- or 1-bromotetradecane, and other anions of phosphinate, carboxylate, tetrafluoroborate, and hexafluorophosphate could be prepared through ion exchange with phosphonium halides. Direct reactions of tertiary phosphines with alkylating agents, such as alkyltosylates, benzenesulfonate, trialkylphosphates, and dialkylsulfates, also can be used to form phosphonium-based ILs with no halogen. Asymmetric tertiary phosphines (RR0 2P or R2RvP) can be obtained through free radical addition to olefins from primary and secondary alkylphosphines (RPH2, R2PH), and these resulting phosphonium cations have generic formulas of RR0 2RvP1 and R2R0 RvP1.

Preparation and Characterization of Ionic Liquids 17 O

+

PR'3

RO

S

R

R'

P

OR

+

O

R'

OR

PR'3

O

R'

OR

OR R

R'

R'X

S

R'

P

X

R' O

X =

O

P

O

OR

O

OR

+

PR'3

P R

O OR

R

R R'

HA

P

O

R

R'

P

A

R'

A=BF4; PF6; CH3COO, CF3COO, SCN, Tf2N

Scheme 2.6 Synthesis of phosphonium-based ionic liquids [3]. R1 N

N

R1

R3SO3R2

N

N

R2

R3 =Me, n-Bu, 2-Bu, Et R2 =Me, n-Bu, (CH2)2OMe

R3 R1 N

R2 N

O

O S O

O

R1

MX

R3SO3

N

N

R2

X

X=BF4, PF6, PF3(CF2CF3)3, CF3SO3, N(CF3SO2)2

R3

R2 R1

N

N

R2

O

O S

O

O

R1 =Me, Et, Pr, Bu R2

R2 =Me, Et R3 =H, CH3

Scheme 2.7 Synthesis of sulfur-based ionic liquids.

The synthesis of sulfur-based ILs refer to sulfonate- and sulfate-based ILs (Scheme 2.7) [3]. 1,3-Dialkylimidazolium alkanesulfonates were synthesized through the reaction of N-alkylimidazole with alkyl sulfonates at room temperature in high yields, and the alkanesulfonate anion could be easily substituted with a series of other anions ([BF4]2, [PF6]2, [PF3(CF2CF3)3]2, [CF3SO3]2 and [N(CF3SO2)2]2) through the reaction of anions,

18

Chapter 2

salts, or acid at room temperature [20,21], which is a typical method for synthesizing halogen-free ILs. The reaction of 1-alkylimidazoles with dimethyl sulfate and diethyl sulfate can be used to synthesize low-cost ILs containing methyl- and ethyl-sulfate anions under ambient conditions. Alkylation should conducted in a solvent because the solvent acts as a diluent and heat sink to moderate and control the reactivity. It is much faster and requires low-cost processing equipment when using dialkyl sulfates as alkylating agents [22,23].

2.2.3 Functionalized Ionic Liquids ILs in which a functional group is covalently tethered to the cation, the anion or a zwitterion are defined as functionalized ILs, where the cation typically bears reactive moieties. The usual method to incorporate functionality into the IL was displacement of halide from an organic halide containing the functional group by a parent precursor (Scheme 2.8). The functional groups usually include 2 OH, 2 OR, 2 SH, NH2, 2 PPh2, 2 Si(OR)3, 2 SO3H [3,24
26]. Recently, many new functionalized ILs were synthesized by similar methods. (Table 2.1) Amino acid
functionalized ILs were synthesized by neutralization between hydroxyl salt and amino acids [27
29]. N-functionalized imidazole derivatives with α-amino acid residues were synthesized and subsequently used for synthesizing new optically pure imidazolium ILs. Long-chain amide-functionalized ILs can be synthesized in two steps by using standard methodology, including alkylation and ion exchange reactions. A novel paramagnetic IL, 3-(2-hydroxyethyl)-1-methyl imidazolium bromotrichloro ferrate (III) ([C2OHmim]FeCl3Br), was prepared through the reaction of equimolar amounts of FeCl3  6H2O and [C2OHmim]Br in dry methanol [30]. Pyridine-containing anionfunctionalized ILs were synthesized through neutralization of various hydroxypyridine or 4azabenzimidazole compounds in an ethanol solution of trihexyl(tetradecyl)phosphonium hydroxide [31]. ILs functionalized by two sol-gel reactive crown ether with [N(SO2CF3)2]2 as anions, namely, 1-(trimethoxysily) propyl 3-(60 -oxo-benzo-15-crown-5 hexyl) imidazolium bis(trifluoromethanesulphonyl) imide ([TMSP(Benzo15C5)HIM][N(SO2CF3)2]) and 1-allyl-3-(60 -oxo-benzo-15-crown-5 hexyl) imidazolium bis(trifluoromethanesulphonyl) imide ([A(Benzo15C5)HIM][N(SO2CF3)2]) were synthesized by quaternization and metathesis reactions. A series of functional guanidinium-based ILs containing both Lewis N

N

+

X-(CH2)n-FG

N

N

(CH2)n

X

FG

MA

FG= -OH, OR, -SH, -NH2, -PPh2, -Si(OR)3, -Urea & Thiourea, Metal complex, -SO3H, SO3Cl

Scheme 2.8 Synthesis of functionalized ionic liquids.

N

N

(CH2)n

A

FG

Table 2.1: Ion structures of functionalized ionic liquids. Cations

Anions + N

N

FeCl3 Br2

OH

2 Cl2 ; Br2 ; BF2 4 ; PF6

R + NH N

N

C6H13 + P

C6H13

OCH3

C6H13 N

C14H29 OCH3 H3CO Si OCH3

N

+

N

N

O

+

N

O

P

N H

R N

N

O +

N

S O

O O

O

O

O

O

N

O

N

N

NHðSHO2 CF3 Þ2

O O O

O

O

H3C

N

PF2 6

NTf 2 2

Br2

NTf 2 2

+ H N C12 25

CH3

R=H, CH3 (Continued)

Table 2.1: (Continued) Cations

Anions Me

+ (Et)3P

Si

O

Si

Me

+

11

+

N

HN

Me

Me

NTf 2 2

H2 C

R3P

NTf 2 2

Me

P

O

NH2

O

O O O O

PF2 6

+ N R

+ N

N +

NTf 2 2 + N

+ N

CN CN

CN

N+ CN

CN

NH2

BF2 4

R

NTf 2 2

Preparation and Characterization of Ionic Liquids 21 acid and basic sites were also prepared by using a similar method. Liang synthesized 16 imidazolium-type cyano-functionalized Brønsted acidic ILs, with [HSO4]2 and [H2PO4]2 as anions through a two-step method. A novel CMPO (carbamoylmethylphosphine oxide)
based functionalized IL with an [NTf2]2 counter-anion was synthesized in two steps through the reaction of diphenyl phosphite with ethyl bromoacetate and subsequent reaction of the formed product with 3-(1H-imidazol-1-yl)propan-1-amine. Sulfone-functionalized imidazolium ILs were synthesized from direct nucleophilic substitution for the first time by Weng [32]. Novel amino-functionalized ILs of 1-(2-aminoethyl)-1-methylpiperazin amino acid ([AEMP][A]) were obtained by neutralizing 1-(2-aminoethyl)-1-methylpiperazin-1-ium hydroxide ([AEMP][OH]) with natural amino acids, and [AEMP][OH] was obtained from quaternization and anion exchange reactions. Phosphine-functionalized phosphonium ILs were synthesized by quaternization of the corresponding phosphine (R 5 methyl, butyl, octyl) with undecenyl bromide, followed by an anion exchange using Li[NTf2]. Acrylatefunctionalized ILs based on tetraalkylammonium salts with terminal acrylates and methylacrylates were synthesized through quaternization reaction of the amine with an alkyl halide, which was followed by anion exchange of halide ions through salt metathesis with [PF6]2, [CF3SO3]2, [(CF3SO3)2N]2. Series of poly (ethylene glycol) (PEG)-functionalized ILs were synthesized according to Scheme 2.9 in four steps. Disiloxane-functionalized phosphonium-based IL [P222Si][NTf2] was prepared by quaternization of triethylphosphine (P(CH2CH3)3 with iodomethylpentamethyldisiloxane (ICH2SiMe2OSiMe3) and followed the anion exchange reaction of [P222Si][I] with Li[NTf2]. Two series of 1-alkylpyridinium and N-alkyl-N-methylpiperidinium ILs functionalized with a nitrile group at the end of the alkyl chain have been synthesized by Lethesh, also by using quaternization and metathesis methods [33].

H

O

OH

O

+

NaOH THF/H2O

S Cl O

n

O S O

O

EtBr

Et2NH, NaOH N

O

reflux,80° C

N n

N

Br

anion exchange

O S O

O n

N

OAc

O

O

N n

Br

N n

AcO

Scheme 2.9 Synthesis of poly (ethylene glycol) (PEG)-functionalized ionic liquids [34].

22

Chapter 2

Additionally, di-functional ILs have also been reported in recent years. Dan reported the Brønsted acidic
functionalized ILs with two Brønsted acid sites of 2 COOH, HSO4, or H2PO4 [3]. Similar carboxyl-functionalized benzidimazolium-based ILs were synthesized by Muskawar [35].

2.2.4 Chiral Ionic Liquids Chiral ILs have great potential for a wide range of applications including asymmetric synthesis, nuclear magnetic resonance (NMR) spectroscopy shift reagents, enantioselective separation processes, and liquid crystals. Chiral imidazolium ILs with a (1R, 2S, 5R)-(-)-menthoxymethyl substituent have been directly obtained from the natural chiral pool of (1R, 2S, 5R)-(-)-menthol (Scheme 2.10), followed by the metathesis reaction of the chloride salts with KBF4, NaClO4, KI, NaPF6, K (Ace) and NaCF3CO2, and the corresponding chiral imidazolium ILs have been obtained as crystalline materials with sharply defined melting temperatures. Jesu´s and co-workers reported a series of camphorpyrazolium-based chiral ILs, some of which have functional trihalide and cobaltate anions [36]. Chiral pyrazolium halides were synthesized by the reaction of N-phenyl-camphorpyrazole with BrC4H9, IC2H4OCH3, and IC2H4OC2H4OC4H9 in solvent-less conditions. The anions of the formed chiral pyrazolium halides can be modified either by trihalide formation with Br2 and I2, or by salt metathesis with Li[NTf2] and NaCo(CO)4 (Scheme 2.11).

O

Cl R1

N

R1

N R2

N

O

N R2

Hexane, rt

X MX H2O/MeOH

R1

N

N

O

R2

X= BF4; ClO4; PF6; I; Ace; CF3OO

Scheme 2.10 Synthesis of chiral imidazolium-based ionic liquids.

Preparation and Characterization of Ionic Liquids 23 R

N

X3

N Ph

R= C4H9; X= Br R= C2H4OC2H4OC4H9; X= I

X2 no solvent

N N Ph

R-X no solvent 125 ° C

R

N N Ph

LiN(Tf)2 H2O/AcMe/ CHCl3

X

N N

R

X

Ph

R= C4H9; X= Br R= C2H4OCH3; X= I R= C2H4OC2H4OC4H9; X= I

Na[Co(CO)4] CH2Cl2

N N

R

[Co(CO)4]

R= C2H4OCH3; R= C2H4OC2H4OC4H9

Ph

Scheme 2.11 Synthesis of camphorpyrazolium-based chiral ionic liquids [36].

2.2.5 Polymerized Ionic Liquids Polymerized ILs, or Poly(ionic liquid)s, as a subclass of polyelectrolytes, have attracted much attention because of their combined properties of both ionic moieties and polymers. The ionic moieties of PILs generally contain bulky cations (alkylammonium, alkylphosphonium, N,N0 -dialkylimidazolium, N-alkylpyridinium, 1,2,3-triazolium, etc.) and noncoordinating anions (i.e., bis(trifluoromethylsulfonyl)imide (NTf22), BF42, PF62, etc.) (Scheme 2.12). These various PILs have been synthesized by direct polymerization of IL monomers or through postmodification of precursor polymers [37
41]. In recent years, many functionalized polymeric ILs have been synthesized. Wei Zhou and co-workers synthesized polymerized quaternary ammonium salts of N,N-dimethyl-2-[(2methylacryloyl)oxy] ethanaminium 5-carboxy-2,4-bis-benzolate by neutralizing reaction. Bi-functionalized PEG1000 IL [imim-PEG1000-TEMPO][CuCl2] was synthesized according to Scheme 2.13. Similar PEG chain
functionalized N-dodecylimidazolium dichlorides,

24

Chapter 2 n

AIBN DMF/H2O 65 ° C R X R

R

R

A

X

X=N; P R=Methyl; Ethyl;Propyl;Butyl A=Cl; BF4;Tf2N; PF6; TFSI

R

A

R

Scheme 2.12 Synthesis of poly(ionic liquid)s [37
41].

HO HO

Cl

O

O

CuCl

O

n

n

O

OH

O

N

SO2Cl

Cl

N

N O

N

O

n

O

O

N

O

O

n

N

O

Cl

Cl N

N

N O

O

n

O

O

N O

CuCl2

Scheme 2.13 Synthesis of bi-functionalized PEG1000 ionic liquids [45].

[salox-PEG1000-DIL][PF6] have also been prepared in recent years [42,43]. Fluorofunctionalized polymeric ILs with imidazolium cations and bromide or chloride anions were synthesized by two-step alkylation. Hydroxyl-functionalized PILs were synthesized by radical co-polymerization from their corresponding precursors [44].

2.2.6 Metal Based Ionic Liquids Metal-based ILs are attractive candidates for magnetic liquids, magnetorheologic fluids, or catalyst. Metal-based ILs can be divided into three groups: transition metal salts, p-block

Preparation and Characterization of Ionic Liquids 25 C6H13

C6H13 C6H13 P C14H29

FeCl4

MnCl42

O N H

N

CoCl42

N

N

C10H21 N

OH

N O

GdCl63

PtCl6

Rh(CO)2I2

Sn2Cl5

Scheme 2.14 Ions of metal based ionic liquids.

metal salts, and f-block metal salts (Scheme 2.14). Transition metal
based ILs can be synthesized through the reactions of trihexyl(tetradecyl)phosphonium ([PR4]1), 1-decyl-3methylimidazolium ([C10mim]1), or 1-butyl-3-methylimidazolium ([C4mim]1) halides with the corresponding metal halides, or metathesis with alkali salts of metal-based anions. The starting material of metal halide (FeCl4, CoCl2, MnCl2 or GdCl3) was added directly to PR4Cl in the absence of solvent, and the corresponding metal-based ILs were formed [46
49]. Series of other transition metal
based ILs and p-block metal
based ILs, such as [PtCl6]22 [50], [Rh(CO)2I2]2 [51], [SbCl6]2 [52], and [Sn2Cl5]2, have also been synthesized by using similar methods. Series of lanthanide-containing ILs based on the 1-butyl-3-methylimidazolium cation and lanthanide thiocyanate anions of the general formula [Ln(NCS)x(H2O)]32x (x 5 6
8; y 5 0
2; Ln 5 Y, La, Pr, Nd, Sm, Eu, Gd, Tb, Ho, and Yb) could be synthesized by ion exchange reaction from equimolar amounts of lanthanide (III) perchlorate, ammonium thiocyanate, and thiocyanate salts with the corresponding imidazolium cations [53,54].

2.2.7 Energetic Ionic Liquids In recent years, energetic ILs, which are composed of high nitrogen organic cations and bulky anions with one or more energetic groups, have received great attention [5]. Synthetic methods of energetic ILs are similar to the traditional ILs. Neutralization, quaternization, and ion exchange reaction are the fundamental routes [4,55]. For example, a series of energetic ILs comprising substituted 1,2,4-triazolium cations and 4,5-dinitro-imidazolate or 5-nitrotetrazolate anions were synthesized by neutralization (Scheme 2.15) [56]. According to Scheme 2.16, 1,1-dimethylhydrazine could alkylate with bromoacetonitrile easily, and this was followed by the metathesis reaction to give a new family of energetic ILs based on the 1-cyanomethyl-1,1-dimethylhydrazinium cation and nitrate, perchlorate, azide, 5-aminottetrazolate, 5,50 -azobistetrazolate, and picrate anions [57]. A series of energetic salts of 1-(chloromethyl)-1,1-dimethylhydrazine were synthesized by using similar methods [58].

26

Chapter 2 N3 N

N

N N

N

N N

N

N

N3

N

N

N

Pr O 2N

O 2N

N

O2N

O 2N

N N

N

N N N

N

N N O2N

N NO2 N

Scheme 2.15 Ions of energetic ionic liquids. H3 C N H3C

NH2

Br

CN

CH3 H3C N NH2

Xa

CN

Br

AgX

CH3 H3C N NH2

CN

Xa a

= NO3 , ClO4 , N3 , [NH2-CN4] , SO42

Scheme 2.16 Synthesis of 1-cyanomethyl-1,1-dimethylhydrazinium
based energetic ionic liquids [57].

2.2.8 Nonconventional Preparation The typical procedures for the synthesis of ILs are excessively time consuming and unsuitable for largescale production, and this restricts its industrial application. Aiming at the key process in the preparation of ILs, several nonconventional techniques, such as microwave, ultrasound, microwave-ultrasound, and micro-reactor, have been developed. Varma and co-workers proposed a solvent-free, microwave-assisted protocol for the synthesis of ILs in open containers, using an unmodified household microwave oven. The reaction time for this solvent-free method was only a few minutes in contrast to several hours needed under conventional heating conditions [59,60]. Varma and Namboodiri studied homogeneous and neat ultrasonic irradiation
assisted preparation of some ILs, and the yields and reaction time were significantly improved [61]. Le´veˆque also reported similar results and developed the one-pot method. Different families of nitrogen-bearing ILs can be obtained in a solvent-free or aqueous medium by using ultrasonic irradiation, and the overall process is much greener. Reaction in terms of space-time-yield could be enhanced by using a micro-reactor because the micro-reactor could offer the possibility of applying higher cooling temperatures because

Preparation and Characterization of Ionic Liquids 27 of the high surface-to-volume ratios and short diffusion lengths. Waterkramp reported the high-temperature production of the imidazolium-based IL of [Bmim][Br] by using a continuously operating micro-reactor system and demonstrated that high-quality ILs could be prepared even under harsh thermal conditions [62,63]. Other similar micro-reactions also have been reported, such as the synthesis of [Bmim][Br] in micro-channel reactor. Recently, Yu developed a ship-in-a-bottle strategy for preparing the [APPMIM]Br@NaY host
guest system. The small precursors of ILs were allowed to diffuse through zeolite pore apertures and reacted inside the supercages of zeolite (host), and larger IL products (guest) formed in situ were entrapped inside the cavities. According to this method, 1-aminopropyl-3-methylimidazolium bromide ([APMIM][Br]) was encapsulated in situ in the NaY supercages.

2.3 Purification of Ionic Liquids During the synthesis of ILs, some organic solvents and water will be used, so the impurities cannot be avoided. Usual grades of purity are as follows: higher than 95% for “synthesis,” higher than 99% for “high pure,” and higher than 99.9% for “ultra pure.” The physical and chemical properties of ILs can be significantly influenced by small amounts of impurities. Moreover, the catalytic activity and the electrochemical properties of ILs are highly dependent on the purity of the ILs. Impurities should be removed before the ILs are used. However, it is difficult to purify ILs by using the traditional crystallization and distillation techniques because of the ILs’ special property of low or nonexistent freezing points and negligible vapor pressure. In the following section, the purification methods for removing different kinds of impurities will be discussed. Typical impurities are organic starting materials, halide impurities and other ionic impurities from metathesis reactions, colored impurities, and water.

2.3.1 Unreacted Organic Starting Materials and Solvents The volatile organic compounds in ILs are composed of unreacted organic starting materials and solvents added during the reaction process. Theoretically, volatile organic compounds could be removed simply by vacuum distillation. However, in practice, the content of the impurity in ILs and the boiling point of the impurity will affect the ease or complexity of removal of the organic compounds. Generally, after synthetic reactions, ILs are washed several times by a low-boiling-point organic solvent (e.g., ethyl acetate, diethyl ether) that is immiscible with ILs and could extract the impurities. Then, the organic phase is separated from ILs. The trace amount of volatile organic compounds in ILs is removed by vacuum distillation. However, it is difficult to remove the high-boiling-point organic compounds (e.g., 1-methylimidazole, bp

28

Chapter 2

198 C) by conventional vacuum distillation. The best method is to make the high-boilingcompounds react completely through control of the molar ratio of reactants, temperature, and other reaction conditions. 1-Methylimidazole is one of the major impurities in ILs based on 1-methylimidazolium. In particular, the presence of unreacted starting material 1-methylimidazole in ILs from incomplete alkylation causes potential downstream problems as a result of catalyst poisoning and introduction of protic impurities [64]. John et al. developed a simple and rapid technique to monitor the levels of 1-methylimidazole in the final product. The levels of unreacted 1-methylimidazole contamination (,0.2 mol%) could be detected by the spectrophotometric colorimetry (SPC) method because 1-methylimidazole could react with CuCl2 and form [Cu(MeIm)4]21complexation, which has an intense blue color. For example, the color of the solution will change from blue to yellow as the content of 1-methylimidazole decreases gradually in the mixture of [C2mim]Cl and ethanol [64].

2.3.2 Halide Ions Halide impurities can influence the physical properties of ILs and the transition metal
catalyzed reactions in the ILs. Seddon et al. [65] found that the viscosity of ILs increases dramatically with the concentration of halide in the ILs. Dyson et al. [66] indicated that chloride impurities caused transition metal
catalyzed reactions, such as hydrogenation, as halides could coordinate to the transition metal centers of catalysts. In the case of hydrophobic ILs, halide impurities can be removed through washing with water, which is very simple and effective. However, in the case of hydrophilic ILs, it is difficult to remove the halide impurities dissolved in ILs. Generally, hydrophilic ILs can be dissolved in an organic solvent that is immiscible with water, such as dichloromethane, and then washed with a small amount of water. The organic phase was washed with deionized water until no halide ions remained. The halide ions were tested by the addition of a saturated AgNO3 solution into the washed aqueous phase. This method will result in low yields of ILs, while the loss in yield can be reduced by using very cold water. In addition, metathesis with a silver salt could yield ILs with high purity but produces a molar equivalent of silver chloride and is comparatively expensive. The acid-base method also leads to chloride-free media, but residual acidity can be a problem. For instance, in the case of the ILs containing nitrates, it was found that they contain dissolved HNO3 [65]. The reaction conditions of the metathesis need to be validated to ensure quantitative conversion.

2.3.3 Alkali Metal Salts Alkali metal salts usually remain in ILs as byproducts of the metathesis process. Usually, alkali metal salts have no effect on reaction in the ILs but greatly influence the physical properties of ILs, such as density and viscosity. The method of removing the alkali metal

Preparation and Characterization of Ionic Liquids 29 salts is similar to that for halide impurities. In the case of hydrophobic ILs, alkali metal salts can be removed through washing with water. In the case of hydrophilic ILs, the ILs can be dissolved in a water immiscible solvent, such as dichloromethane, and then washed with a small amount of chilled water. In addition, the organic solvent used in the metathesis process for synthesis of the ILs should be fully dried. Even trace amounts of water in the organic solvent could significantly increase the solubility of these salts in ILs. In addition, ILs could also be passed through silica gel to reduce alkali metal salts [67].

2.3.4 Colored Impurities Pure ILs based on 1-methylimidazole or pyridinium should be colorless and clear liquids. In fact, most synthetic methods for preparing ILs lead to final products with a yellowish or brown color. The impurities that cause this discoloration are reported to be below the detection limit of NMR spectroscopy, and apparently, these impurities do not affect the organic or catalytic reactions carried out in ILs. However, colored impurities play a key role in influencing the spectroscopic applications of ILs, for example, photochemical reactions and photophysical measurements [68,69]. Welton and co-workers reported that these colored impurities may be the products of side reactions occurring at high temperature in the alkylation reaction when ILs halides are synthesized [70]. Some strategies have been proposed to obtain colorless ILs, and these can be classified as four approaches: purification of starting materials, control of conditions for quaternization reaction, ion exchange, and purification of ILs. For example, 1-methylimidazole should be dried by NaOH and then redistilled prior to use. The alkyl halide can be washed by sulfuric acid until the acidic phase appears colorless. Then the NaHCO3 water solution is used to neutralize the acid remaining in 1-methylimidazole. Finally, alkyl halide is redistilled prior to use. In the case of dark-colored ILs, absorbents can be used to remove colored impurities. Seddon and co-workers used a column filled with carbon and silica to remove colored impurities from a range of ILs with [NTf2]2, [BF4]2, [PF6]2 anions [71]. Burrell et al. developed a method for producing high-quality ILs at kilogram scale by decolorizing with active charcoal [72].

2.3.5 Absorbents During the removal of colored impurities from ILs, absorbents, such as activated carbon, silica, and alumina, are used. It has always been assumed that absorbents are easily removed from ILs through simple filtration. Some researches indicated that these absorbents could dissolve in ILs as nanoparticle contamination. Before 200 nm filtration, the nanoparticle contamination showed a high content of 190 ppm; after 200 nm filtration the absorbents could be reduced to very low levels (,5 ppm). That is, ILs exposed to absorbents are contaminated with ppm levels of absorbent particles, even after 200 nm

30

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filtration. The complete removal of these absorbent particles may require advanced synthetic techniques, such as distillation or zone melting [73].

2.3.6 Water Water is the most common impurity in ILs, mainly caused by two factors. First, some solvents containing certain amounts of water are used in the synthesis of ILs. Second, ILs, whether they are hydrophilic or hydrophobic, can absorb water. The residual water in ILs not only reduces the purity of ILs but also influences their physicochemical properties. For example, the presence of water can change the viscosity, density, and conductivity of ILs [65]. Moreover, the presence of water can make the catalysts in ILs inactive and affect the resultants [74]. Therefore, water should be removed before ILs are used in some applications. At present, there are three main methods to remove water from ILs. (1) Reagents pretreatment: All the solvents or reaction solvents should be dried before synthesis of ILs. The chemical drying agents (e.g., potassium metal, magnesium chloride, magnesium sulfate) can be very effective for removal of water from the solvents. In addition, ILs can be synthesized in a dry atmosphere (e.g., glove box). However, this procedure may be inappropriate for largescale production of ILs. (2) Vacuum drying: Water can be removed by heating ILs under vacuum for several hours because of the low volatility of most ILs. This method has some disadvantages in the purification of ILs. For example, the duration of this process is at least 12 h to even 24 h or more. Degradation of ILs would occur when the temperature is held at an elevated level for a long period (about 10 h) [75]. This method is not suitable for many protic ILs because the conjugate acid and base species may also be volatile, and loss of one of the species may take place. (3) Sweeping the water with nitrogen: This procedure is carried out at an ambient pressure by using nitrogen as the sweeping gas at a certain temperature. By this method, water can be removed in just a few hours, or in less than 0.5 h, and the water mass fraction of hydrophilic ILs can be reduced to about 0.1%. Purification can be achieved in a short time by increasing the temperature and the nitrogen flow rates. This method can save much energy and avoid the degradation of ILs because of the short time required for purification of ILs [76].

2.3.7 Clean Synthesis As mentioned, it is difficult to purify the ILs because of their intrinsic properties. If some precautions were taken during the synthesis process to avoid impurities from either starting materials or unwanted side reactions, ILs can be synthesized as cleanly as possible. Especially for ILs containing a quaternary nitrogen (e.g., ILs based on imidazolium, pyridinium, pyrrolium), many impurities result from byproducts of the quaternization

Preparation and Characterization of Ionic Liquids 31 process. The precautions to avoid impurities from either starting materials or unwanted side reactions are as follows: 1. Impurities remaining in the starting materials can lead to side reactions or produce colored compounds. All the starting materials should be as pure as possible. The starting materials should be purified just before the quaternization reaction is carried out because starting materials often degrade over time. 2. During the synthesis process, the reaction temperature should be kept at a minimum value to reduce any unwanted side reactions. For reactions using halide-based reagents, the following temperatures should preferentially be used: chloride salts , 80 C, bromide salts , 40 C, and iodide salts , 0 C. It should be pointed out that this general trend might not be appropriate for all reactions. Some quaternization reactions may need to be carried out in an ice bath, which should be allowed to warm slowly while monitoring progress of the reaction to determine the minimum temperature required. However, longer reaction time will be required because of the lower temperature used. 3. Quaternization reactions should be carried out in an inert atmosphere. The oxygen in air could cause the occurrence of colored compounds during the quaternization reaction process. 4. Quaternization reactions are usually exothermic, and it is difficult to avoid local hotspots within the reaction mixture. So, small-scale reactions (,0.3 mol) are suitable for obtaining ILs with high purity [77]. If the above precautions are insufficient to achieve the required purity, the quaternization products (intermediates of ILs) should be purified by using traditional purification methods, such as distillation and recrystallization. With the increase in the number of studies on ILs, many novel kinds of ILs have started to appeared. Given the growing importance of ILs, it is vital to develop low-cost production methods, including efficient techniques for purification and ultra-purification.

2.4 Characterization of Ionic Liquids Molecular structure determines the properties and applications of ILs [78]. A clear understanding of the relationship between the structure and properties of ILs would be helpful for designing new ILs with the desired properties. Moreover, because impurities can have a profound impact on subsequent IL applications, all reported data on ILs should be accompanied by an analysis report on the presence of common IL impurities. A variety of experimental techniques that have been used to investigate IL structures and interactions include NMR spectroscopy, mass spectrometry, infrared spectroscopy (IR), and Raman spectroscopy, and these will be introduced here.

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2.4.1 Nuclear Magnetic Resonance Spectroscopy In organic synthesis, the information given by a combination of various NMR spectroscopic techniques is important, and this method is unmatched by any other spectroscopic method. Since the properties of ILs are predominantly determined by the molecular structure and impurities, the applications of NMR techniques in the research of IL structures, properties, purity, interactions of cations with anions of ILs, and interactions between ILs and other compounds have become very important. Although there are some difficulties in the NMR measurements of ILs, such as high viscosity, less deuterated form, and entirely ion component, many common ILs have been investigated using NMR techniques [79
84]. For ILs, NMR spectroscopic measurements are generally used to identify the chemical structures of cations because many anions, such as BF42 and PF62, cannot be characterized by NMR spectroscopy. Chemical shifts can be affected by many factors, such as kinds of anion and substituted alkyl chain, interaction with co-solvents, impurities, and so on. Studies on the relationship between chemical shifts and anion types and substituted alkyl chain have been studied by Lin et al., based on a series of methylimidazolium, as shown in Fig. 2.1 and Table 2.2 [85]. They observed high sensitivity in chemical shift for the C-2proton of the imidazolium ring with a change of anions from Br2 to PF62 and increase in

Figure 2.1 Structure of 1-alkyl-3-methylimidazolium salts. Table 2.2: 1H and Cation C2MIm C2MIm C2MIm C4MIm C8MIm

Anion 2

Br PF62 BF42 BF42 BF42

13

C chemical shift values of 1-alkyl-3-methylimidazolium salts in CD3OD at 298 K.

H2 9.04 8.79 8.89 8.87 8.85

H4

H5

C2

C4

C5

7.71 7.59 7.66 7.62 7.61

7.63 7.52 7.58 7.55 7.55

137.91 137.50 137.86 137.83 137.80

125.54 124.87 125.19 124.92 124.90

123.62 123.41 123.54 123.61 123.59

Preparation and Characterization of Ionic Liquids 33 the alkyl chain length from ethyl to octyl. This can be interpreted as hydrogen-bonding effects. In 2002, Headley and Jackson first studied the relationship between chemical shift and kinds of solvents [86]. After systematically comparing the NMR chemical shifts of two imidazolium-based ILs measured in nine deuterated solvents, they found that the imidazolium ring-protons of the PF62 salt were more sensitive to solvation compared with the ones of the BF42 salts. They suggested that the smaller BF42 resulted in a stronger ionpair interaction with the cation, and thus this salt was less sensitive to solvation effects. Furthermore, the effects of halide ions as a typical impurity in ILs were studied by the group of Seddon in 2000. High concentrations of chloride ions led to downfield shifts in 1H NMR, which was attributed to the formation of stronger hydrogen bonds between chloride and the imidazolium protons. It is well known that hydrogen bonding causes the proton chemical shift to move downfield [87].

2.4.2 Mass Spectrometry Mass spectrometry is one of the most universally used analytic techniques covering applications in almost all fields in chemistry. The most outstanding properties of this technique are its low sample consumption, the capability for automatization allowing a high sample throughput, and the capability of coupling to separation techniques, such as gas or liquid chromatography. Together with the results of the cation structure observed with NMR and other spectroscopy, mass spectrometry can provide information on molecule weight, including information on cation and anion. It is helpful to determine the entire chemical structure. Thus, in the field of ILs, mass spectrometry also has become a valuable tool for analyzing the structure of anions and cations, impurities, and byproducts in ILs. There are many kinds of mass spectrometry, such as electrospray ionization mass (ESI-MS), fast atom bombardment mass (FAB-MS), matrix-assisted laser desorption/ionization mass (MALDI-MS), and so on, which can be used to characterize ILs. For example, the anion structures of chloroaluminate ILs could be determined directly from the molten salts by using FAB-MS spectrometry. Abbott et al. observed the presence of complex zinc chloride ions [ZnCl3]2, [Zn2Cl5]2, and [Zn3Cl7]2 by using FAB-MS spectrometry [88]. Yeon et al. determined the existence of 1-(2-hydroxyethyl)-3-methyl imidazolium ([HEMIm]1) and N-(2-hydroxyethyl)-N-methyl morphorinium ([HEMMor]1) cations based on mass results [89]. Furthermore, positive and negative electrospray mass spectrometry was used to identify cations and sulfonate anions by observing P1n1 and N6nnn1 cations and p-MeC6H4SO32 or MeSO32 [90].

2.4.3 Infrared Spectroscopy Most spectral information comes from the mid-IR region, and IR spectroscopy research on most ILs is mainly concentrated on this region [82,91
94]. IR spectroscopy can be used to

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Chapter 2

identify and study the stretching and bending vibrational levels, and rotational levels in the chemical molecules. The close relationship that exists between chemical bond vibration frequencies of IR spectra and peak strength is discussed below. IR spectroscopy can be used to observe the structures of cations and anions through functional groups, and it also plays important roles in investigating the hydrogen bond network structure, π
π stacking structure, ionic pairs, and ionic aggregation. The peak shifts and intensity changes of IR spectra is a sensitive probe of hydrogen bonding interactions in the IL system. IR was first used for studying the hydrogen bonding behavior of ILs. For example, in 1988, Dymek et al. studied the ionic structure and interactions in 1-methyl-3-ethylimidazolium chloride-AlCl3 molten salts through IR spectral analysis [95]. They found that the hydrogen bond effect not only exists between Cl2 and C2-H in the imidazolium ring but also between Cl2 and C4-/C5-H because of the π
π stacking structure. (The numbering of carbon in the imidazole here is the same as in Fig. 2.1.) Furthermore, In 2007, Katsyuba et al. studied the possible ionic-pair molecular structures of imidazolium-based ILs using IR spectroscopy in combination with density functional theory [96]. They found that the formation of ionic-pair molecular structure mainly influences stretching and out-of-plane vibrations of the imidazolium C-H groups, whereas the hydrogen bond effect is relatively weak. Carper et al. studied the vibrational spectra changes by changing the alkyl chain on the imidazole from ethyl to butyl and found the existence of the hydrogen bond between cations and anions. The results also showed good agreement with Raman spectroscopy and theoretical calculations [97]. IR spectroscopy is also an important tool to study the interactions between IL and cosolvent. Takamuku et al. studied the state of water in room temperature IL, 1-ethyl-3methylimidazolium tetrafluoroborate ([C2mim][BF4]), by using attenuated total reflectance IR spectroscopy and the 2H NMR relaxation method. They found that individual water molecules that are hydrogen-bonded to the anions predominate in the solutions at Xw # B0.2, whereas B30% of water molecules are hydrogen-bonded among them in the solutions at Xw $ B0.3. In addition, the activation energies for the rotational motion of a water molecule estimated from the 2H NMR relaxation rates have indicated that the motion of water molecules in [C2mim][BF4]-D2O solutions gradually becomes more free with increasing water content from Xw 5 0.10
0.30, but is retarded again at Xw 5 0.33. The results of 2H NMR is in good agreement with that in the ATR-IR experiments [98].

2.4.4 Raman Spectrometry Raman spectroscopy is a spectroscopic technique used to observe vibrational, rotational, and other low-frequency modes in a system. IR absorption and Raman scattering are both commonly used to study and identify substances based on the compound’s characteristic internal vibrations. For a vibrational transition to be IR active, there must be a change in

Preparation and Characterization of Ionic Liquids 35 the dipole moment of the molecule during the vibration. If the vibration affects the polarizability of the molecule, then the vibrational mode will be Raman active. Raman spectroscopy is commonly used in chemistry to provide a fingerprint by which molecules can be identified [99]. IR and Raman spectra are complementary. Recently, researchers observed the configuration changes of ILs, microstructures, and interactions between ILs and co-solvents by using Raman spectroscopy. Hauashi et al. studied the structure of the [C4mim]1 cation in liquid state by using Raman spectra. The Raman spectrum showed that the trans and gauche conformations coexist in [C4mim] halide ILs. The relative intensities of the peaks at 600 and 625 cm21 differ, depending on the halogen anion. These peaks are assigned to the vibrational modes of the imidazolium ring; the 625 cm21 band corresponds to the trans conformation of the C7
C8 bond of the n-butyl group, and the 600 cm21 peak corresponds to the gauche conformation [100]. Hamaguchi et al. elucidated the molecular structure and arrangement of 1-butyronitrile-3-methylimidazolium halide, in the presence or absence of the intruded water molecule by using single-crystal X-ray crystallography and near-IR Raman spectroscopy. Water molecule is found to change the conformation of the nbutyronitrile chain of the cation. The hydrogen-bonding interaction between the anion and the water molecule, leading to loose molecular packing, is most likely to be responsible for the change. They proposed that hydrogen-bonding network between water molecules and anions displaces the anion position, thereby disturbing the stable molecular structure and arrangement [101]. Rubim et al. studied anionic structures formed in mixtures of [C4min] [Cl] with different amounts of niobium pentachloride (NbCl5) or zinc dichloride (ZnCl2) by using Raman spectroscopy [102]. Passerini et al. investigated the interaction between Nmethyl-N-propylpyrrolidinium bis(trifluoromethanesulfonyl)imide ([PYR13][NTf2]) IL and the lithium salt LiN(SO2CF3)2 by using Raman spectroscopy [103].

2.5 Largescale Production of Ionic Liquids Producing large quantities of ILs of high-quality is not easy [104]. The cost of raw materials, reaction time, purification of ILs, and so on need to be taken into account. The Aachen University of Applied Sciences established a 30 L [C4mim][Cl] synthesizer. Largescale production of [C4mim][Cl] was implemented by BASF and MERCK. In comparison with ILs with nitrogen, the techniques for manufacturing alkylphosphine ILs were developed early by Cytec Industries Inc. were named CYPHOS® 3653 and CYPHOS IL 101 [105]. In 2005, the Institute of Process Engineering (IPE) of the Chinese Academy of Science (CAS) signed a cooperation agreement with Henan Lihue Pharmaceutical Co. Ltd. and established a set of general technologic processes for the largescale preparation of ILs, and as a result of this, 200 tons of ILs can be produced per year, as shown in Fig. 2.2. The general chemical reactor includes the mixing reactor, crystallizing separator, extraction separator, and so on, but it cannot meet the synthesis process of different kinds of ILs.

36

Chapter 2

Figure 2.2 The production line for largescale production of ionic liquid by Henan Lihue Pharmaceutical Co. Ltd. [106].

To solve such issues, a multifunction reactor for the preparation of different kinds of ILs was designed, as shown in Fig. 2.3. The reactor with different modes of operation, including compression and decompression, heating, cooling, stirring, fluid, crystallization, suction filtering, drying, and solid
liquid separation, can process heterogeneous or homogeneous reactions and the subsequent product separation and purification. The design improved the reaction conversion and equipment utilization ratio, where the reaction conversion can reach more than 90%. It also reduced energy consumption, equipment investment and operating costs, environment pollution, and possible contamination by other impurities and raw materials. The multifunction reactor plays an important role in the industrial application of ILs [106]. The technologic process of largescale IL preparation, for example, the production process of [C4mim][Cl] and [C4mim][BF4], has been designed to proceed in four steps, as follows: Step 1 Synthesis of [C4mim][Cl] A certain molar ratio of methylimidazole and butyl chloride is placed into the reactor connected with a reflux condenser and heated at 70 C under N2 atmosphere. After stirring for 24 h, the mixture is changed to a colorless viscous liquid. Step 2 Purification of [C4mim][Cl] The colorless viscous liquid obtained is left to stand for stratification. The unreacted chlorobutane in the superstratum can be recycled. The desired [C4mim][Cl] is immediately precipitated at a low temperature. The product is washed with ethyl acetate and recrystallized for several times. The white solid of [C4mim][Cl] is obtained.

Preparation and Characterization of Ionic Liquids 37

Figure 2.3 The multifunction reactor in production of ionic liquid [106]. 1. Reaction chamber; 2. Handspike or man hole; 3. Solid discharging mouth; 4. Export jacketed medium; 5. Jacket; 6. Lifter; 7. Import jacketed medium; 8. Vacuum tube; 9. Vacuum suction filter tube; 10. Liquid inlet; 11. Gas supply pipe; 12. Speed regulating motor; 13. Temperature sensor; 14. Pressure sensor; 15. Endoscopy; 16. Condensate return pipe; 17. Strainer; 18. Clip; 19. Stirring blade; 20. Sample connection; 21. Combitips plus; 22. Liquid discharging mouth.

Step 3 Synthesis of [C4mim][BF4] Fluorin boric acid sodium saturated solution and CH2Cl2 are added dropwise to the [C4mim][Cl] IL by stirring with a magnetic bar. After stirring for 24 h, the obtained product is separated from water. Step 4 Purification of [C4mim][BF4] The organic phase is washed with CH2Cl2 for 3
4 times and then distilled to obtain the product. It is then washed with water to remove Cl2, which is then evaporated at 80 C. The [C4mim][BF4] product is dried for 48 h. The process is optimized as shown in Fig. 2.4.

38

Chapter 2

Figure 2.4 Flow diagram of the preparation of [C4mim][BF4] [106]. S1: Raw materials C4H6N2; S2: Raw materials C4H9Cl; S3: [C4mim][Cl] reaction mixture; S4: [C4mim][Cl]reaction mixture; S5: [C4mim][Cl] initial products; S6: CH3COOC2H5; S7; [C4mim][Cl] 1 CH3COOC2H5 mixture; S8: CH3COOC2H5 Returned Logistics; S9: Qualified [C4mim][Cl]products; S10: Raw materials NaBF4; S11: C4H2Cl2 (Extractant); S12: Unreacted C4H9Cl; S13: [C4mim][BF4] 1 C2H2Cl2 mixture; S14: [C4mim][BF4]; S15: C2H2Cl2 Returned logistics; S16: Unreacted C4H9Cl.

The IPE also established the system integration of IL preparation. For example, the system integration for the preparation of [C4mim][Br] and [C4mim][BF4] is shown in Fig. 2.5. The total integrated system is divided into three subsystems, namely, subsystem of IL synthesis, waste bromine water recovery and subsystem of bromine ethane synthesis, and waste water treatment subsystem. The three parts are constitute a network structure, which can help keep waste to a minimum. The total system is a clean process, which realizes zero release and the circular economy principle. On the basis of data from the largescale production of ILs, many researchers have described the applications of ILs in the chemical industry, and their reports may help accelerate largescale production of ILs. Milota and his co-workers first designed a pilot scale system for IL absorption [107]. The absorption system, which uses tetradecyl(trihexyl)phosphonium dicyanamide IL, was tested as an alternative to thermal oxidation of pollutants in the dryer and the press exhaust. Their results indicated that the total hydrocarbon reduction ranged from 50% to 80%. The reduction in most of the hazardous air pollutants tested was much better, often higher than 90%. ILs with good electrochemical properties can be used as electrolyte. Wang et al. reported that the heteroleptic Z-907Na sensitizer showed a relatively long lifetime of its oxidized state, which is highly desirable for practical application [108].

2.6 Summary and Prospects In summary, the preparation, purification, characterization, and largescale production of ILs were reviewed in this chapter. Although some progress has been made in recent years, there

Preparation and Characterization of Ionic Liquids 39

Figure 2.5 System integration of largescale production of ionic liquid [106].

is still some work to be done. (1) As the family of ILs probably includes more than 1018 compounds, the synthetic strategies available will continue to expand, depending on the kinds of ILs. (2) Obtaining “ultra pure” ILs remains a challenge for researchers. Novel purification methods to obtain “ultra pure” ILs should be developed in the future. (3) The purity of ILs is just qualitative description according to the existing methods of characterization. Quantitative characterization methods should be investigated to determine the purity of ILs accurately.

40

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[89] S.-H. Yeon, K.-S. Kim, S. Choi, H. Lee, H.S. Kim, H. Kim, Physical and electrochemical properties of 1-(2-hydroxyethyl)-3-methyl imidazolium and N-(2-hydroxyethyl)-N-methyl morpholinium ionic liquids, Electrochim. Acta 50 (27) (2005) 5399
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6856.

CHAPTER 3

Properties of Ionic Liquids 3.1 Introduction Ionic liquids (ILs) are special molten salts at low temperatures (usually ,100
C), composed entirely of positive and negative ions. As the cations and anions can be varied, ILs have tunable properties, such as density, viscosity, electrochemical window, microstructures, and so on. Compared with conventional solvents, ILs also have some special properties, such as low volatility, high viscosity, high conductivity, wide electrochemical window, long-term thermal stability, and so on [14]. Their designable properties have allowed ILs to become alternatives to traditional organic solvents as a novel and promising medium in catalytic reactions and green separation processes. The research on molecule level of ILs is very important to understand ILs’ effects in catalytic reactions and separation processes for industrial applications. The micro-interactions play key roles in IL-involved reactions and processes. It is known that the interaction of hydrogen bonds is one of the most important forces in ILs [5,6]. Aggregation behavior or clusters are also found in neat ILs and ILs solutions [7,8], which are important for certain applications. ILs are also potential candidates for carbon dioxide (CO2) capture [911] and cellulose dissolution [12,13] because of the unique structures of ILs and their interactions with CO2 and cellulose. Meanwhile, with progress of IL research, data on the reliable physical properties of substances are essential to design their industrial applications [14].

3.2 Micro-structure and Interaction 3.2.1 Interaction and Hydrogen Bond of Ionic Liquids Generally, ILs consist solely of anions and cations, and in simple terms, ILs can be classified into classic molten salts. As most ILs are liquid in room temperature, the low melting points of ILs have demonstrated that the earlier classification of ILs is not quite right. As fluid solvents, ILs have many special properties, for example, high viscosity and good chemical stability. What are the characteristics that make these ILs so special? Why do modern ILs deserve such increased interest and curiosity? These questions have spurred on interest in the internal structures and the interactions of these “ionic fluids.”

Novel Catalytic and Separation Processes Based on Ionic Liquids. DOI: http://dx.doi.org/10.1016/B978-0-12-802027-2.00003-0 © 2017 Elsevier Inc. All rights reserved.

45

46

Chapter 3

3.2.1.1 Application of ILs In 1914, the first IL [C2H5NH3][NO3] that was liquid at room temperature (mp 12
C) was identified [15]; however, further investigation of ILs was withheld for a long time. Many ILs have been exploited as catalysts and solvents [16,17]. ILs are sensitive to moisture, and this limits their industrial applications [17,18]. Since air-stable ILs containing organic anions, such as [PF6]2, [BF4]2, and [TFSA]2, were synthesized in the 1990s [19,20], studies on ILs grew exponentially, and numerous papers have been published annually on various aspects of their chemistry and physics. In 2012, around 104 papers were published on the subject of “ionic liquids.” The “popularity” of ILs is closely related to their properties [21,22]. Typical properties of ILs include the following: •

• •

ILs have large organic ions (at least one of them, usually the cation) with anisotropic and asymmetric shapes. The features that principally distinguish them from inorganic molten salts are that the Coulombic interactions are weaker in their range of action, whereas their complicated shapes do not allow the ions to easily form stable, ordered crystal structures [23]. Many ILs have low volatility because of electrostatic interactions [24,25]. ILs can be designed by choosing different anions and cations and mixed with each other, and thus they are called “designer solvents” [26].

Because of these properties, ILs have been applied in many areas, including the following: • • • •

• •

Catalyst for homogeneous and surface catalysis [4,27,28]. Extractants for purification of metals [29], proteins and colloids [21], and biomass conversion products [12]. Media for electrochemical reactions, electrocatalysis, and electrodeposition [3032]. Electrolytes for power sources and generators [33], such as supercapacitors [32], [3437], batteries [3840], solar cells [41,42], and, prospectively, fuel cells (once room-temperature ILs (RTILs) with high proton conductance are found) [43]. Chemical and electrochemical sensing [44]. The organic liquid component for self-assembled nanomaterials [45].

3.2.1.2 The structures of ILs The typical cations and anions are shown in Fig. 3.1. For cations, the 1,3-dialkylimidazolium ([R1R2im]1,) and N-alkylpyridinium ([Rpy]1) are mostly documented in recent reports. For R1 5 methyl, R2 5 ethyl, the cation is abbreviated as [C2mim]1. The tetraalkylammoniumbased ([N1234]1) and tetraalkylphosphonium ([P1234]1) cations are also interesting, and their related properties have been described in recent reports [4,27]. The corresponding abbreviations of the cations are provided in the Appendix. The simplest anions are halide

Properties of Ionic Liquids 47

Figure 3.1 The structures of typical cations and anions. Cation: 1,3-dialkylimidazolium ([Cnmim]1, when R2 5 methyl), N-alkylpyridinium ([Npy]1), 1,2,3trialkylimidazolium, 1,2,4-triazolium, 1,1-dialkylpyrrolidinium, tetramethylammonium ([N1234]1), tetraalkylphosphonium ([P1234]1), 1,1,3,3-tetramethylguanidinium, 1,2,3-trialkylsulfonium. Anion: tetrafluoroborate ([BF4]2), hexafluorophosphate ([PF6]2), nitrate ([NO3]2), bis (trifluoromethylsulfonyl)imide ([NTf2]2), thiocyanide ([SCN]2), dicyanimide (N[CN]22), trifluoromethanesulfonates ([OTf]2), trifluoroacetyltrifluoromethanesulfonamide, methylsulfate ([CH3SO3]2), halide-based (Cl2, Br2, and I2).

(A)

(B)

(C)

(D)

(E)

Figure 3.2 Representation of [C4mim]1 cation electronic structure.

based (e.g., Cl2, Br2, and I2) with only an atom. The [BF4]2 and [PF6]2 anions possess high geometric symmetry, and [BF4]2 is Td and [PF6]2 is Oh [5]. The [Tf]2 and [OTf]2 anions show analogous geometry, and O atoms distribute the more negative charges. [NTf2]2 anion shows analogous geometry and N atoms distribute the more negative charges. [SCN]2 2 2 and N½CN2 2 anions show different geometries. [SCN] is linear, but in N½CN2 , the middle N atom possesses a lone pair of electrons, leading to nonlinearity [46,47]. 3.2.1.3 The electronic structure of imidazolium cation The electronic structures of imidazolium-based cations are investigated extensively, and shown in Fig. 3.2. Hunt et al. performed a detailed analysis by using theoretical calculation [48]. The imidazolium ring is assumed to be aromatic, and the charges are fully delocalized on the ring (Fig. 3.2A). The dominant resonance structures (Fig. 3.2B and C) have the

48

Chapter 3

positive charge “formally” carried by the quaternary nitrogen atoms. A minor resonance structure with C2 carrying a positive charge is also possible (Fig. 3.2D). The electron distribution may be represented as shown in Fig. 3.2E. The imidazolium cation can be written as A-H1, where A is the N-heterocyclic carbene. For the carbenic structure, two models have been proposed. One model supported by charge densities and topologic analysis proposes that there is substantial electron transfer from the C atom to the adjacent N atoms so that the carbene is kinetically stabilized; this results from electron repulsion between the lone pair of the carbene and those on nitrogen atoms, which show little evidence of cyclic delocalization [49]. The second model supported by thermodynamic and structural data proposes that p-electron donation from the nitrogen lone pairs into the empty carbene-type carbon pπ orbital is dominant [50]. Although the electronic structure of carbenes can be drawn by analyzing imidazolium cations, the cations themselves maybe neglected. In experiment, the electronic properties of ILs have also been investigated by using spectroscopic probe methods. The polarizability (p ), hydrogen-bond donor (acidity, α), and acceptor (basicity, β) of imidazolium cation have been correlated with the KamletTaft parameters [51]. Nevertheless, The electronic nature of the imidazolium cation has not been investigated in any depth [48,51,52]. By carrying out the theoretical calculations, Hunt et al. provided an elaborate description about the electronic structure of the [C4mim]1 cation [48]. 3.2.1.3.1 Charge distribution

The natural atomic orbital (NAO) charges for the lowest-energy [C4mim]1 cation at B3LYP/ 6-31 1 1 G theoretical level are listed in Table 3.1. A primary feature is that most of the positive charge is located on the peripheral H atoms, significant negative charge is located on the N atoms, and C4/5 atoms remain essentially neutral. Of particular interest is the “acidity” of the H atoms on the ring because contradictory results have been reported. Bu¨hl et al. Table 3.1: NAO charges of [Bmim]1 cation at B3LYP and MP2 levels [48]. H H H

C6 H

H H H C4 C5 H H + C C10 N1 N3 8 C7 C9 C2 H H H H H H

N1 N3 C2 C4 C5 C6 C7 C10 C2-H C4-H C5-H

B3LYP/6-31 1 1 G**

MP2/6-31 1 1 G**

2 0.341 2 0.348 0.268 2 0.039 2 0.035 2 0.449 2 0.241 2 0.661 0.536 0.239 0.242

2 0.324 2 0.327 0.235 2 0.035 2 0.037 2 0.421 2 0.225 2 0.636 0.499 0.238 0.236

Properties of Ionic Liquids 49 obtained similar charges on H (C4/5) and H(C2), which simply indicated that C2-H is no more or less acidic than C4/5-H [53]. However, some experimental results showed that the C2-H hydrogen atom is the most acidic in the ring [51,54,55]. Nevertheless, the H atoms are connected to essentially neutral C atoms at C4/5, and the H atom is bound to a positively charged C atom at C2, and when we analyzed the C-H moiety, it was found that the moiety charges of q(C2-H), q(C4-H), and q(C5-H) on ring were 0.536, 0.239, and 0.242, respectively, as shown in Table 3.1. Thus, the relative acidity of the ring hydrogen atoms should not be associated with the H atoms alone but referred to the charge on the “C-H” moiety. The carbon atoms (C10) of the alkyl chain are negatively charged, except for those (C6 and C7) adjacent to the imidazolium ring, but summing the positive charges of hydrogen atoms into the atoms shows roughly neutral alkyl chains. The NAO charge at MP2/6-31 1 1 G level is very similar to that obtained at B3LYP level, indicating that the inclusion of dispersion effects is not crucial to providing a good representation of the charge density. 3.2.1.3.2 Natural bond orbital (NBO) analysis

NBO analysis on ring determines a double bond between C4 and C5, and thus there remains a three-center (N1-C2-N3) four-electron π system. Clearly, the last two electrons must enter an antibonding π orbital. However, the different electronegativity on the nitrogen atoms and the carbon atoms ensures that electrons are not equally distributed along N1-C2-N3. The σ induction “pushes” more electron density onto the more electronegative nitrogen atoms (about 63% σ bonds are nitrogen based). The corresponding π bonds (πC5N) are about 70% nitrogen based. However, there is also significant occupation of the C2-N1 σ bond (0.54e), which “pulls” electron density away from the nitrogen atoms. Thus, σ induction and π delocalization form “pushpull” effects on the electron density and results in the substantial delocalization through the N1-C2-N3 [48]. The effect of delocalization can be estimated by the second-order perturbation energy (ΔE2) based on the occupation of the donating orbital, the orbital coupling, and the energy difference between the coupled orbitals by using the NBO analysis, and the primary interactions are listed in Table 3.2. The largest components of delocalization are from the N3 Table 3.2: Second order perturbation energy (ΔE2) between the coupled orbitals by NBO analysis [48]. From N3-lp N3-lp πCQN π*CQN πCQN πCQC

To

ΔE2 (kJ mol21)

π*CQN π*CQC N3-lp π*CQC π*CQC π*CQN

77.80 29.83 22.77 17.09 16.87 15.24

50

Chapter 3

Figure 3.3 Contour map of electron density Laplacian [C2mim]1 cation projected onto the N1-C2-N3 plane. Solid lines denote positive, and dashed lines denote negative. The electron densities were obtained at B3LYP/6-31 1 1 G** level, and the contour map was drawn by Multiwfn [56].

lone pair (N3-lp). The total delocalizing energy is 101.6 kJ mol21 within the N1-C2-N3 section, whereas delocalization to the C4-C5 section is 79.0 kJ mol21, indicating that delocalization is occurring around the entire ring, and it is more extensive over the N1-C2-N3 component. 3.2.1.3.3 Electron density

A contour plot of the Laplacian of [C4mim]1 cation is shown in Fig. 3.3. Dashed lines indicate negative curvature of the electronic density and areas where the local density is increasing. Conversely, the solid contours and gaps between the dashed contours indicate areas where the local electrons were depleted. The wells on either side of the alkyl groups or behind the imidazolium ring indicate decreasing electron density in the areas, which are favored positions for the negatively charged anion. The shallowness of the well between C4 and C5 indicates that the anion will be less stable in the position. 3.2.1.4 Structure and interaction of paired cation and anion (ion pair) 3.2.1.4.1 The structures of the typical ion pairs

Ion-pair is starting point for understanding ILs. The opposed charges pull the cation and the anion together to form ion pairs, but charge delocalization results in many energy minima on the potential energy surface (PES), in which the anion locates different positions around the cation. Fig. 3.4 shows the lowest-energy conformers of the typical ion pairs by theoretical calculations, and other conformers, as shown by Hunt et al. [56]. and Tsuzuki et al. [57].

Properties of Ionic Liquids 51

[C2mim][CF3CO2]

[C2mim][BF4]

[C2mim][OTf]

[C2mim][NTf2]

[C2mim][PF6]

[C2py][BF4]

[(C2H5)(CH3)3N][BF4]

[C2mpro][BF4]

Figure 3.4 The lowest-energy geometries of the ion pairs that are involved in different cations and anions [57]. Acknowledgments to be used by ACS authors.

In the [C2mim][CF3CO2] ion pair, the [CF3CO2]2 anion is close to the ring C2-H group of the imidazolium cation. The 2 CO2 group of [CF3CO2]2 has a closer contact with the C2-H. This preference is explained by the larger negative charge on the 2 CO2 group (20.78 e) compared with that on the 2 CF3 group (20.22 e). A similar ion pair is [C2mim] [OTf], the 2 SO3 group of [OTf]2 is close to the C2-H, and two oxygen atoms of 2 SO3 have close contact with the C2-H group. This preference is explained by the charge distributions of [OTf]2, where the 2 SO3 group has a much larger negative charge (20.82) compared with the 2 CF3 group. For [C2mim][BF4] ion pair, nine local minima were obtained, and the [BF4]2 anion favors locating above or below the plane of the imidazolium ring. In the lowest-energy conformer of Fig. 3.4, the [BF4]2 has close contact with the C2-H of the imidazolium cation, and the ˚ . For the other conformers, the C2-H?B distances are C2-H?B distance is 2.46 A ˚ 2.452.49 A. On the other hand, the [BF4]2 also has close contact with other hydrogen atoms of the imidazolium ring (i.e., C4-H and C5-H). Similar geometries were calculated for [C3mim]1 and [C4mim]1 complexes with [BF4]2. Molecular dynamics simulations also showed that anions prefer to locate around the C2-H group, both above and below the plane of the imidazolium ring [5860]. The C2-H group has a larger positive charge compared with the C4-H and C5-H groups, as shown in Table 3.1, which would cause this preferential interactions. The C2-H?F distance that close contact with two fluorine atoms are 2.12 and ˚ , respectively, and the C2-H?F angles are 134.3 and 120.9
, respectively. Similarly, 2.26 A eight local minima were obtained for the [C2mim][PF6] ion pair, in which the [PF6]2 anion close contacts with the C2-H group and locates above or below the plane of the ˚ , and the shortest C2-H?F imidazolium ring. The C2-H?P distance is 2.812.91 A

52

Chapter 3

˚ . The hydrogenfluorine radial distribution functions obtained from distance is 2.092.21 A molecular dynamics simulations of [C4mim][PF6] IL show that the first maximum location ˚ ) is shorter than those of C4-H?F and C5-H?F (2.53 A ˚ ) [58]. of C2-H?F (2.23 A For the complex [NTf2]2 anion, there are 24 local minima in optimization. For the lowestenergy conformer in Fig. 3.4, the C2-H group of imidazolium cation has close contact with the N atom or 2 SO2 group of the anion. One 2 SO2 group is located above the imidazolium ring. Pyridinium-based ILs are an important species in the IL family. Taking the [C2py][BF4] ion pair as an example, 12 local minima were obtained by geometry optimization, in which the [BF4]2 anion has close contact with the nitrogen atom of pyridinium cation, and the N?B ˚ . The lowest-energy conformer in Fig. 3.4 has Cs symmetry. In the distance is 3.213.62 A conformers in which the [BF4]2 is close to the C2-H group, the N?B distance is ˚ , and the C2-H?B distance is 2.452.82 A ˚ . In the conformer where the 3.734.16 A 2 [BF4] is close to the C3-H and C4-H groups, the C3?B and C4?B distances are 2.64 and ˚ , respectively. The positive charge of [C2py]1 is distributed mainly on the nitrogen 2.84 A atom, C2-H, C4-H, C6-H, and methylene groups. This charge distribution would cause the stability. For the [(C2H5)(CH3)3N][BF4] ion pair with the complex cation, eight stable conformers ˚ , respectively. In were obtained. In two conformers, the N?B distances are 3.92 and 3.94 A ˚ and the furthest is 4.42 A ˚. other stable conformers, the N?B distances are 4.124.28 A For the [C2mpro][BF4] ion pair, 19 local minima were obtained by the geometry ˚ in these complexes. optimization. The N?B distances lie between 3.87 and 4.36 A 3.2.1.4.2 The interaction energy (ΔE)

Besides the molecular structure and symmetry, the intermolecular noncovalent interaction is important to determine the properties of ILs. In the classic molten salts, such as NaCl, the dominating interaction is electrostatic force and polarization. However, it seems that ILs are not that simple, and thus in-depth investigation of the general properties of ILs is required. An effective approach to better understanding of ILs is to determine the dominating intermolecular forces and compare them with those of molten salts. The total ΔEs between the typical cation and anion at their equilibrium distance from ab initio calculations range from 2300 to 2400 kJ mol21; however, the energies have not exhibited a good correlation with the properties of ILs, especially the melting point (K), as shown in Fig. 3.5A [23,61], which indicates that ILs are not identical to classic molten salts and that electrostatic force is not dominant. However, what are the forces in ILs that mostly represent the interactions? Using the symmetry-adapted perturbation theory (SAPT), Stefan et al., deconstituted the total ΔE of 1,3-dimethylimidazolium chloride ([C1mim][Cl]) and NaCl into different contributions, analogous to a multiple expansion, as shown in Fig. 3.5B [61]. Note that the

Contribution of interaction /%

Properties of Ionic Liquids 53

T/K

Interaction energy (ΔE), kJ/mol (A)

Deviation from equilibrium distance /pm

(B)

Figure 3.5 (A) The total interaction energies plotted against the melting points for the typical ion pairs of ILs. 1: [C2mim][AlCl4], 2: [C1mim]Cl, 3: [C2mim][BF4], 4: [C2mim][Cl], 5: [C2mim][DCA], 6: [C2mim][SCN]. (B) Contributions of different to the total attractive interaction energy. Circles: electrostatic, stars: exchange, squares: induction, diamonds: dispersion [61]. Acknowledgments to be used by Wiley authors.

equilibrium distance is set to zero in order to provide comparability. For the NaCl pair (red diamonds), the dispersion term is negligible, whereas this contribution is comparable in magnitude with the induction term for the two conformers of [C1mim][Cl] (M1A [blue diamond] and M1B [green diamond]). The main contribution to the total energy stems from the electrostatic interaction for all species (circles). The total energy of NaCl consists of main electrostatic and small part exchange contributions, but the induction contribution is mostly zero at the equilibrium position. Although the main contribution of the total ΔE is also electrostatic interaction, the exchange and induction contributions have a larger proportion for the conformers of [C1mim][Cl] at respective equilibrium positions. Therefore, it has been indicated that other forces besides Coulombic interaction must play a role in ILs. The different contributions partly compensate each other, resulting in a very shallow potential energy curve. Consequently, the system is highly flexible and liquid-like and is able to adjust easily to different situations, which might also explain the good solvating properties of ILs. For the ILs with the same [C2mim]1 cation, the magnitude of ΔE follows the order: [CF3CO2]2 . [BF4]2 . [CF3SO3]2 . [NTf2]2 . [PF6]2, which coincides with the order of the electrostatic contribution (Ees), as summarized in Table 3.3. The large negative charge on oxygen atoms (20.68 e) of [CF3CO2]2 results in the large Ees value. Comparatively, the Ees of the [C2mim][CF3SO3] is substantially smaller, and the C2-H?O

54

Chapter 3 Table 3.3: Interaction energy (ΔE) between paired cationanion and physical properties of the ILs.

ILs [C2mim][CF3CO2] [C2mim][BF4] [C2mim][OTf] [C2mim][NTf2] [C2mim][PF6] [C2py][BF4] [(C2H5)(CH3)3N][BF4] [C2mpro][BF4]

ΔEa

Eesa

mpb

ρc

Dd

ΛNMRe

Λimpf

2 375.72 2 356.48 2 345.60 2 329.70 2 328.03 2 346.44 2 353.97 2 353.13

2 359.82 2 348.95 2 333.46 2 315.05 2 323.00 2 336.81 2 345.18 2 338.90

2 14 11 29 2 15 62 26 19 2 15

1.22 1.20 1.30 1.44 1.37 1.44 1.39 1.40

3.23 2.72 3.02 4.75 1.21 3.91 2.34 3.12

1.25 1.04 1.10 1.82 0.46 1.44 0.91 1.19

0.63 0.69 0.64 1.10 0.31 0.94 0.61 0.82

Energies in kJ mol21. Melting point (
C). c Density (g cm23) at 298K. d Self-diffusion coefficient (cation 1 anion) at 298K (1027cm2 s21). e Molar conductivity obtained from ion diffusivity measurement by NMR (S cm2 mol21) at 298K. f Molar conductivity obtained from impedance measurement (S cm2 mol21) at 298K. a

b

˚ ) are shorter than those in [C2mim][CF3CO2] (2.15 and 2.17 A ˚ ). distances (2.02 and 2.10 A The steric repulsion from the sulfur atom is the main reason for the larger separation and the smaller Ees value. The ΔE of [C2mim][PF6] is much smaller than that of the [C2mim] [BF4] complex. A larger intermolecular separation is the main reason for the smaller electrostatic energy, further resulting in the smaller interaction energy. For the [NTf2]2 anion, negative charge is focused on one nitrogen atom and four oxygen atoms, whereas the nitrogen atom has closer contact with the C2-H compared with the oxygen atoms because of steric repulsion. This structural feature would result in the smaller Ees and ΔE values. The calculated ΔEs of [C2mim] complexes are compared with the experimental properties (melting point, density, self-diffusion coefficient, and molar conductivity) of RTILs as shown in Table 3.3. No close relationship between the energies and their properties has been found, which indicates that these properties are not determined solely by the size of the ΔE. However, Dong et al. found a linear relationship between ΔEs of dialkylimidazolium ILs with the [Cl]2, [BF4]2, and [PF6]2 anions and their melting points, as shown in Fig. 3.6. It was found that the melting points decrease when the N-alkyl side chains increase. The linear trend is pronounced for the [Cl]2 and [BF4]2 series. A similar trend is taken from the [PF6]2 series but is very gross. Thus, it is indicated that the more atoms the anions include, the more complex are the interactions between cations and anions [62]. The molar conductivity is an important property of ILs in their applications as electrolytes for electrochemical devices. It is found that the molar conductivity ratio from impedance measurement (Λimp) and ion diffusivity measurement by nuclear magnetic resonance (NMR) (ΛNMR), Λimp/ΛNMR can be correlated with ΔE. The round-shaped anions ([BF4]2 and [PF6]2) have larger Λimp/ΛNMR compared with those composed of other rod-shaped anions.

Properties of Ionic Liquids 55 450 400 350 Mp(K)

300 250 200 150 100 50 0 –380

–370

–360

–350 –340 E(KJ/mol)

–330

–320

–310

Figure 3.6 The correlation of melting points (K) and interaction energies (kJ mol21) for dialkylimidazolium ILs. (▲) [Cl]2 series; (K)[BF4]2 series; (◆) [PF6]2 series. Figure adapted from [62]. Acknowledgments to be used by ACS authors.

3.2.1.4.3 Interaction from experimental determination

Electrospray ionization mass spectrometry (ESIMS) has been used to determine the strength of the cationanion interaction [63]. The order of the interaction with [Br]2 anion was derived as: [C2mim]1 . [C4mim]1 . [C1C2morph]1 . [C6mim]1 . [C8mim]1 . [C1C4morph]1 . [C4py]1 . [C4mpyr]1 . [C4dmim]1 . [N4444]1. By using the refractive index data, it has been found that the interactions increase linearly with increasing alkyl chains of the cations: [Cnmim]1 , [Cndmim]1 , [Cnpy]1 and with increasing molecular sizes of the anions: [BF4]2 , [BTA]2 , [CF3SO3]2 [6466]. 3.2.1.5 The hydrogen bond (H-bond) between paired cation and anion ILs were classified as salts because of their ionic attributes. Although the ΔE between counter-ions is comparable with the 545.0 kJ mol21 of NaCl, ILs have shown differences from these classic salts. The large size and structural asymmetry result in the different ion pack in solid and liquid states, even in the gas state. Besides electrostatic force, the H-bond is responsible for the ion pack and phase transfer. 3.2.1.5.1 Halide-based ILs with an atomic anion

In the early period of research on ILs, imidazolium-based ILs with halide anions were extensively studied, and the solid structures were determined by X-ray powder diffraction (XRD). Tait and Osteryoungs described the H-bonds between the hydrogen on the C2 carbon atom of the ring and the chloride anion [67]. In contrast, Wilkes and co-workers exhibited a model in which the liquid consists of oligomeric chains held together by ionion interactions, each cation being associated with two anions, one above and one

56

Chapter 3

below the plane of the imidazolium ring, and these authors explicitly stated that no hydrogen bonding is possible in this system [68]. Seddon et al. have provided structural evidence based on XRD and indicated that there are discrete H-bonded ion pairs in [C2mim]I crystals, in which the distance between the hydrogen atom on the C2 atom of the imidazolium ring and the iodide anion is 0.293 nm and the distance the hydrogen atom on the C4 atom of the methyl chain is 0.33 nm. Moreover, the iodide ion is not situated above the imidazolium ring, which is itself planar and conventionally aromatic [69]. Further evidence for the presence of strong H-bond in the liquid state is obtained from infrared (IR) spectroscopy. Tait and Osteryoung assigned a band at 3050 cm21 to CaH?Cl H-bonding in the [C2mim]Cl-A1Cl3 IL system. Seddon et al. found that a similar band is present at 3080 cm21 in the IR spectra of all the salts [C2mim]X (X 5 C12, Br2, or I2) [69]. The presence of this band, both in the liquid and solid states, may thus be regarded as diagnostic of the presence of a strong, discrete CaH?Cl hydrogen-bond. Furthermore, orthorhombic and monoclinic crystal structures of [C4mim]Cl have been determined [70]. It was found that solid anions interact in-plane with the H atoms peripheral to the imidazolium ring, primarily C2-H, C4-H, and C5-H, and tend to remain in the ring plane. The two monoclinic forms show significant differences in interaction between the cation and the anion. The liquid-phase structure of [C4mim]Cl has been determined by neutron diffraction. The first maximum radial distribution function (RDF) ˚ (to the center of the imidazolium ring), and each cation is surrounded (on occurs at 4.5 A ˚ ) [71]. The most probable position of the chloride is average) by six anions (within 6.4 A around the imidazolium ring and forms a hydrogen bond with C2/4/5 above and below the imidazolium ring. At lower probability levels, the chloride is found in a broad cylinder passing above and below the plane of the imidazolium ring with the methyl groups protruding from the open ends of this cylinder and at a much lower probability density broadening of the cylinder toward the methyl groups. The calculation evidence about H-bond in [C4mim]Cl has also been presented by Hunt et al. [56]. The most stable conformer of [C4mim]Cl is the Cl2 anion, which localizes in front of the C2-H, consistent with the conformer in the monoclinic crystal structures. These ˚ ), to the most acidic H (C2-H) of the conformers all form a strong H-bond (r  2.0 A imidazolium ring, and a weaker interaction with a hydrogen atom on the methyl group C6˚ ). Anion can form Cl2?π interaction with imidazolium ring for the top H (r  2.632.71 A conformer. It is known that the imidazolium rings can form π?π interactions, as these have been observed in the crystal structures of other ILs [72]. Subsequently, according to geometric criteria, an interaction is considered a strong H-bond if the CaH?Cl distance ˚ and a weak H-bond if 2.3 , r , 2.75 A ˚ ; the H-bond geometry is also different r , 2.3 A 2 when Cl locates on different positions of cation. In optimized ion pairs, the Cl2 anion prefers to form two basic types of interaction: a shorter-range H-bond with an imidazolium ˚ ; and another is the weaker ring H atom, and Cl?H distances range from 2.00 to 2.18 A

Properties of Ionic Liquids 57 interaction with the H atom from one of the alkyl groups and Cl?H distances range from ˚ . Both interactions are important, as no stable conformer with only one 2.35 to 2.72 A H-bonding interaction has been found. The calculated ion pairs present the interactions and geometries in the gas phase; however, in the liquid and solid phases, each ion interacts with ˚) multiple ions of opposite charge, and hence, the tightest H-bonds in the crystalline (2.52 A ˚ ) are generally longer than those found for the gas-phase ion pairs and liquid phases (2.78 A ˚ (2.10 A). 3.2.1.5.2 Fluoro-anionbased ILs

The main defect of halide-based ILs is that they are very sensitive to water, which prohibits their application. The fluoro-anion ILs (e.g., [BF4]2, [PF6]2, [AsF6]2, [CF3CO2]2, [CF3SO3]2, [NTf2]2, etc.) are typically air stable and have received more attention in many chemical processes. Coulombic attractive forces in ILs must be diffused to inhibit or suppress crystallization. The length or degree of branching of alkyl chain of imidazolium cation can dramatically influence the melting points. This has attributed to the rotational freedom of the alkyl chain and the influence on the ability of the cations to pack efficiently (or not) within the crystalline cell. By using X-ray crystallographic analysis, Reichert et al. examined the solid state of 1,3-diakylimidazoium hexafluorophosphate ([Dmim][PF6]) ILs with different N-alkyl chains [73]. In general, it is seen from the local structures that the [PF6]2 anions closely contact with imidazolium cation by the CaH?F H-bonds, although the length of the N-alkyl chains have the remarkable effect on the ions packing. When N-alkyl chains are short, Cn # 4, the [PF6]2 anions make contact with not only the heads (ring) of the cation but also the tail (alkyl chains); however, when N-alkyl chains are long, Cn . 4, [PF6]2 anions make contact with only the heads of the cation. It is also found, from the side views of the cations, that [PF6]2 anions localize above and below the imidazolium ring and have close contact with the C and N atoms of the rings. The direct interactions between the delocalized positive charge of the imidazolium ring and the anion further indicate the Coulombic nature of ILs. Although the electrostatic force is dominant between the cation and the anion, the close contacts with a wide angular distribution indicates the CaH?F H-bonded interactions. In crystal structures of salts containing [CF3SO3]2 or [CF3CO2]2 anions, for example [C4mim][CF3SO3], the imidazolium ring and the SO3 group of the anion are connected by the CaH?O H-bonds to form a two-dimensional NaCl-like arrangement in each layer, and the butyl chain of the cation and the CF3 group of the anion protrude from the layer in the same direction. Therefore, this structure contains a layer of the butyl and CF3 group sandwiched between the two layers. For [Dim][NTf2] ILs, both two crystallographically distinct [NTf2]2 anions adopt a cis-conformation [74]. The crystal lattice of [Dim][NTf2] consists of alternating two-dimensional sheets with an AABAA pattern.

58

Chapter 3

The interaction and H-bond of fluoro-anionbased ILs in liquid state have been studied with experimental spectroscopy (IR, Raman, NMR, neutron scattering, etc.), as shown in Table 3.4 [6,75]. Neutron diffraction studies on some ILs have indicated that charge ordering endures in the liquid state. In the [C1mim][Cl] and [C1mim][PF6] ILs, the molecular packing in the first two or three solvation shells are similar both in the crystal state and in the liquid state, although the atomic distances are altered [7176]. In comparison, the [C1mim][NTf2] IL presents smaller charge ordering [77] because of the diffuse charge density and larger size of the [NTf2]2 anion. For [C4mim][CF3SO3] IL, this more ordered structure is also not obvious in the liquid state, but except the CaH?O H-bonds between the cation and the 2 SO3 group of the anion, the 2 CF3 group of the anion can connect with the other cation by CaH?F H-bond [78]. Nevertheless, it remains clear that ILs form “quasimolecular” structures through three-dimensional H-bonded networks. The network is maintained to a great extent even in solutions having low dielectric media, making the RTILs highly organized media. Ludwig et al. [7981]. have provided evidence for the H-bond by substituting the hydrogen atom of ring for a 2 CH3 group, and the bands associated with stretches or bends of H-bonds disappear. With the increasing abilities and strengths of H-bonds in [C2mim][NTf2] IL, the [NTf2]2 anion was found in the cis conformation. Switching off these local interactions leads to the energetically favored trans conformation. These structural effects caused by H-bonds should have significant influence on IL properties, such as melting points and viscosities, as discussed later. Moreover, the vibration frequencies of H-bonds themselves can be observed directly by the far-IR spectrum even for the complex [Dim][NTf2] IL, as listed in Table 3.4. Table 3.4: The experimental spectroscopic studies of H-bonds on some typical fluoro-anion-based ILs [6]. a

HBDA(α)c [85,86,279]

ΔEd [57,62]

Mpe Tdf

ILs

Spectra

H-Bond

[Dim][BF4] [Dim][PF6]

X-ray, IR[88] X-ray [70], Neutron [71,76], IR[89] X-ray [90], NMR [91], IR[92] X-ray, NMR[91]

CaH?F CaH?F

0.61 0.64

356.47 319.00

11 10

412 375

CaH?O, CaH?F

0.60

345.60

29

140

CaH?O, CaH?F

0.56

375.72

214 150

X-ray [81], THz, IR [79], CaH?N, CaH?F, Neutron CaH?O X-ray, Raman[93]

0.66

313.60

215 455

[Dim][OTf] [Dim] [CFCO2] [Dim][NTf2] [Pyr][NTf2] a

b

1

1

0.48

[Dim] represents the 1,3-dialkylimidazolium cation, [Pyr] represents the pyridinium cation. The possible hydrogen bonds between cation and anion in crystal and liquid phase. c Hydrogen-bonded donation abilities (only for [Bmim]1 cation). d The average binding energies were calculated by the DFT and MP2 methods (only for [Emim]1 cation). e Melting points. f The measured thermal decomposition temperature (for [Emim]1 cation). b

26

Properties of Ionic Liquids 59 Wavenumbers above 150 cm21 were assigned to the intramolecular bending modes of cations and anions in the ILs, and wavenumbers below 150 cm21 were assigned to the bending and stretching vibrational modes of H-bonds [82]. The solvent properties of these ILs have been investigated using chromatographic techniques [83,84]. It is generally found that ILs may be considered polar phases, with the solvent properties being largely determined by the ability of the salt to act as an H-bond donor and/or acceptor. Table 3.4 lists the H-bonded donation abilities (HBDA, α) to qualitatively donate the acidity of ILs. The α of the pyridinium-based ILs and the quaternary amino-ILs are obviously smaller than that of imidazolium cation, which indicates that the interactions of H-bonds are not remarkable in these ILs [8593]. The H-bonds have been investigated by using certain calculations. Fig. 3.7 shows the optimized [C2mim][BF4] ion pairs by ab initial density functional theory (DFT) method [5]. The initial structures are ethyl-front, methyl-back, back, methyl-front, and ethyl-back when [BF4]2 anion can localize at five positions of the cation. The structures were optimized at

ethyl-front

F2ĂH(C2)=2.02Å F2ĂH(C6)=2.54Å F3ĂH(C2)=2.29Å F3ĂH(C8)=2.21Å

meth-front

F2ĂH(C2)=2.03Å F2ĂH(C6)=2.27Å F3ĂH(C8)=2.32Å

[Emim][BF4](II)

meth-back

[Emim][BF4](I) ethyl-back

F2ĂH(C5)=1.91Å F3ĂH(C6)=2.05Å F4ĂH(C7)=2.40Å

[Emim][BF4](III) back

Figure 3.7 The optimization of [C2mim][BF4] ion pairs with the anion located at five different positions of the cation. The arrows noted the initial locations of the anion. The dashed lines indicate the H-bonds [5]. Acknowledgments to be used by ACS authors.

60

Chapter 3

B3LYP/6-31 1 1 G theory level. Three lowest energy geometries were obtained, as shown by the ball-stick model in Fig. 3.7. [C2mim][BF4](I) was obtained from the ethylfront, methyl-back and back structures, in which the [BF4]2 anion moved to the upper part of ring from the lateral part and localized near the C2-H group. [C2mim][BF4](II) was obtained from the methyl-front structure, and the [BF4]2 anion also moved to the upper part of ring. [C2mim][BF4](III) was obtained from the ethyl-back structure, but the [BF4]2 anion only localized near the C5-H group. According to the criterion for forming a H-bond, that the distance between H on the donor atom and the basic acceptor atom, RH?A, is less than the sum of their respective Van der ˚ [75], four F?H H-bonds are to form in [C2mim][BF4](I). Two Waals radii of 2.670 A H-bonds are formed between the F atoms, labeled as (1) the F2 and H(C2) atom of the ring, ˚ and 2.54 A ˚, (2) and the H(C6) atom of the alkyl chain. The distances are 2.02 A respectively. The other two occur between the F3 atom and the H(C2) atom of the ring, and ˚ and 2.21 A ˚ , respectively. A the H(C8) atom of the alkyl chain. The distances are 2.29 A very similar H-bonded structure can be found in [C2mim][BF4](II), but there is no H-bond between the F3 atom and the H(C2) atom. When the [BF4]- anion localizes at the ethyl back of imidazolium ring as [C2mim][BF4](III), only two H-bonds are formed, and one is between the F2 atom and the H(C2) atom of the ring, and another is between the F3 atom ˚ , respectively. and the H(C6) atom of the alkyl chain. Their distances are 1.91 and 2.05 A NBO analysis found that the occupation on the N3 orbital always increases, whereas the occupations on πC25N1 and πC45C5 orbitals vary with the positions of anion in optimized [C2mim][BF4] ion pairs [5]. In the [C2mim][BF4](I) and [C2mim][BF4](II) ion pairs, the occupation on the πC25N1 orbital decreases, but it increases on the πC45C5 orbital. Furthermore, molecular orbital analysis has provided an important insight into ion pairs. Before pairing, the highest occupied molecular orbital (HOMO) of the isolated anion consists of the p orbitals (E 5 2 0.256), and the LUMO of the cation is the anti-π (π ) bond orbital (E 5 2 0.189). After pairing, there are no pπ orbital interaction, but the HOMO of the [C2mim][BF4](I) is the highest occupied p orbital of anion and the lowest empty π (C2-H) orbital of the cation, and the lowest unoccupied molecular orbit (LUMO) of [C2mim][BF4](I) exhibits an anti-σ (σ ) bond orbital (E 5 2 0.0522). The electrostatic attraction drives the anion to approach the cation and finally localizes at the largest positive charge position of cation. Although the repulsion of the electron between the anion and the cation increases the total energy, the orbital effective overlap and the partial electrons transfer result in an energy reduction and further stabilize the ion pairs. As reported, the cluster by the ionic close contacts can reflect the structural arrangement in the bulk phase of ILs [8,9496]. The clusters may become microscopic models to describe the structures of the bulk ILs or their solutions. Fig. 3.8 shows an experimental Fourier transform IR (FTIR) spectrum of [C2mim][BF4] IL and the calculated IR vibrational

Properties of Ionic Liquids 61

Figure 3.8 Experimental FTIR spectrum of [C2mim][BF4] IL compared with the vibrational modes of the corresponding ion clusters ([C2mim][BF4])n with n 5 2, 3, 4, 5 calculated by DFT at B3LYP/631 1 G** level (all of the calculated bands were corrected by the factor 0.9640.967) [5]. Acknowledgments to be used by ACS authors.

frequencies for the ([C2mim][BF4])n ion clusters with the different number, n 5 2, 3, 4, 5 at B3LYP/6-31 1 G level. The strong agreement on major bands has provided the opportunity to explore detailed structural information on [C2mim][BF4] IL in the liquid state, and these experimental vibrations can also be assigned rationally. The 30003500 cm21 band is assigned to the symmetric stretches of three CaH bonds of the imidazolium cation. Two peaks at 1600 and 1500 cm21 are assigned to the breadths of the rings and the scissors of the alkyl chains. The peaks at 1208 cm21 are assigned to the interplanar wagging vibrations of the CaH bonds of the ring. The strongest peak occurs at 1136 cm21 and is assigned to the stretches of the BaF bonds of the [BF4]2 anion, whereas these weak peaks around 750880 cm21 are the out-of-planar wagging vibrations of the rings. Investigation into the structures of clusters has found that ion arrangements are ordered and similar to the crystal structure of X-ray [97,98]. It was found that these ions are connected by the H-bonds together to form a three-dimensional network, in which ion packing is influenced very little by intramolecular bonding structures, and the imidazolium cations are arranged by alternative layers, and the [BF4]2 anions are sandwiched between two cations but do not localize above the rings and prefer to localize near the CaH groups ˚ between H(C2) of rings in the clusters [62]. The distance of cationcation is around 5.10 A

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atom and the imidazolium ring center. No strong interaction between the ring proton and π-electron cloud of the imidazolium ring has been observed, and the ethyl groups protrude from the imidazolium ring planes (C2N1C6C7 torsion angle is around 74
).

3.2.2 Anisotropic Structure and Aggregation in Ionic Liquids 3.2.2.1 Definition of IL cluster In a solution of ILs, the term IL cluster refers to the complex, heterogeneous phase, and in the neat system, IL cluster always refers to the nanosegregation structure. The terms aggregates, nanostructures, local structures, heterogeneity, and so on are also used to describe the clusters in the IL systems. It is accepted that IL cluster is defined as an aggregate of ions, numbering over three. Among all of the ILs, 1,3dialkylimidazoliumbased ILs are widely used, and they are reported to be represented as nanostructures of the type [(DAI)m(X)m_n)]n1[(DAI)m_n(X)m)]n2, where DAI is the cation, X is the anion, and n $ 3. In fact, most of the ILs’ structural organizations are reported to be between classic liquids and liquid crystals [99]. 3.2.2.2 IL cluster in solutions The self-aggregation behavior of ILs in solution was reported earlier than 2000. Through visual observation and small-angle X-ray diffraction, amphiphilic association structures have been found in the system composed of [C4mim][PF6], water, and Laureth 4 [100]. Besides, the imidazolium cation with its long tail has been proven to facilitate the emulsification of fluoroalkanes in conventional ILs and that it acts as a surfactant [101]. The aggregation structures in the binary mixture of water and [C10mim][Br] have been described by Juliette et al. [102]. 1-butyl-3-methylimidazolium octyl sulfate and 1-methyl3-octylimidazolium chloride acting as surfactants and micelle formation under certain conditions were proposed by La´szlo et al. [103]. Many related works about the nano-clusters or the local heterogeneity have been reported by different groups recently. Thermodynamic methods [102,104,105] spectrum [106109], and molecular simulation [110,111] have been used to investigate the structures and behaviors of ion clusters in the IL solution. Researchers use conductivity, volume, fluorescence, dynamic light scattering, and transmission electron microscopy to investigate the aggregation behavior of [C8mim] (X 5 Cl2, Br2, [NO3]2, [CH3COO]2, [CF3COO]2, [CF3SO3]2, and [ClO4]2), 1-octyl-4-methylpyridinium bromide, and 1-methyl-1octylpyrrolidinium in aqueous solutions. Structures of anions and cations have been found to have very weak effects on morphology, but they do affect aggregate sizes [112]. The aggregation behavior of the IL 1-(2,4,6-trimethylphenyl)-3-alkylimidazolium bromide [Cnpim][Br] (n 5 10, 12, and 14), in aqueous solutions was systematically explored by surface tension, electrical conductivity, and NMR spectroscopy [113]. Self-aggregation of

Properties of Ionic Liquids 63 the ILs [C4mim][Cl], [C8mim][Cl], [C4mim][BF4], and [C4mpy][Cl] in aqueous solution has been investigated through NMR spectroscopy and steady-state fluorescence spectroscopy [108]. Small-angle neutron scattering (SANS) and NMR spectroscopy were used to investigate the cluster formation of 1-dodecyl-3-methylimidazolium bis (trifluoromethanesulfonyl)amide in benzene solutions [114]. Molecular dynamics (MD) simulations were also performed for the aqueous solutions of [Cxmim][Br] (x 5 10, 12, 14, 16). Quasi-spherical polydisperse aggregates with the cation alkyl tails buried deep inside the aggregates have been observed and the aggregation numbers steadily increase with the chain length [115]. A snapshot of a part of the aqueous [C16mim][Br] solution is shown in Fig. 3.9. MD simulations for aqueous 1-n-alkyl-3methylimidazolium bromide ([Cnmim][Br], n 5 2, 4, 6, 8) solutions have been performed by Bhargava and Michael [116]. The computed cation distributions were found to be inhomogeneous, with the degree of organization of their tails increasing with the length of the alkyl chain. Cation diffusion decrease with increasing chain length partly because of the formation of aggregates. [C2mim][Br] was found to remain isotropic even at high concentrations, [C4mim][Br] associated weakly to form small clusters, [C6mim][Br] formed small aggregates, and definite aggregates formed in the [C8mim][Br] solution. The distribution of cations into an aggregate of size N is shown in Fig. 3.9. They found the aggregates are poly-disperse, and their size varies from 10 to 25 cations for the [C8mim] [Br] solution. Bhargava and Klein performed MD simulations on an aqueous solution and its vaporliquid interface of the di-cationic ionic liquid 1,3-bis(3-decylimidazolium-1-yl) propane bromide. The system, starting from a uniform distribution of cations, was found to evolve spontaneously, forming crosslinked cationic micellar aggregates. Alkyl tails were typically found buried inside the aggregates, whereas the polar head groups were exposed to water. Anions were found to be not strongly bound to the cations, and cationic micellar aggregates were interconnected by head groups, which is different from monocationic IL solutions [117]. Besides, they also performed coarse-grained MD (CGMD) simulations for aqueous solutions of the gemini di-cationic IL 1,5-bis(3decylimidazolium-1-yl) pentane bromide to study structure and organization in the solution. The 40% (w/w) aqueous mixture evolved to a hexagonal structure, starting from a random distribution of ions. Unlike the spherical aggregates of mono-cationic ILs, a near-hexagonal arrangement of the hydrophobic cores connected to each other by hydrophilic head groups was observed in the di-cationic IL solution displayed. Anions were found to be present close to the polar head groups [118]. Arun Venkatnathan et al. employed MD simulations to characterize the influence of various water concentrations on the nano-structural properties of [C6mim][NTf2]. They found small regions of water molecules surrounded by several cation 2 anion pairs at low water concentrations, cation tail aggregation starts, phase separation between the IL and water observed at medium

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Figure 3.9 Snapshot of aggregates from aqueous [C16mim][Br] solutions: (A) largest aggregate observed; (B and C) another aggregate from different viewing angles. Tail group atoms are shown in dark color and headgroup atoms in light color. Hydrogen atoms of the cations are not shown. Water molecules, anions, and cations not belonging to the shown aggregate are removed for ease of visualization. Reprinted with permission from Ref. B.L. Bhargava, M.L. Klein. Initial stages of aggregation in aqueous solutions of ionic liquids: molecular dynamics studies. J. Phys. Chem. B 113 (2009) 94999505. Copyright American Chemical Society.

water concentrations, and increasing cationic tail aggregation leading to micelle formation at high water concentrations [119,120]. The aggregation behaviors of two trisiloxanetailed surface active ILs in water were investigated by CGMD simulation. CGMD simulation was performed to understand the formation process of micelles when dissolving ILs in water. Vesicles were observed, and their partially truncated views and density profiles were obtained to describe the structure [121].

Properties of Ionic Liquids 65

Figure 3.10 Snapshot of [C12mim][Br] in aqueous solution (x 5 0.59 mol L21), H2O and cation molecule are omitted for clarity. Dark color: Polar group composed by imidazolium ring and the connected atoms, Light color: Nonpolar group composed by the longer alkyl chain.

Recently, Liu et al. performed MD simulations for [C12mim][Br] in aqueous solution (x 5 0.59 mol L21), and the formation and structure of rodlike micelle in [C12mim][Br] aqueous solution has been studied on the basis of the united-atom force field. The simulated ion cluster for cations is shown in Fig. 3.10. Several separate simulations were performed by using both liquid droplet and random starting conditions, and self-assembly of cations into the rodlike micelle was observed. A more detailed analysis and comparison between the solution and the neat IL were performed by using radial distribution functions to demonstrate the influence factors of the micelles, the structure, and the interaction between ions and water. Besides, the intermolecular energy and hydrogen bond number and pattern were also analyzed to further reveal the nature of the micelles [122]. 3.2.2.3 IL cluster in neat systems It is not surprising that self-aggregation exists in IL solution because of the amphiphilic nature of the cations, but this has caused controversy with regard to the existence or non-existence of clusters in the pure liquid state composed entirely of ions. In pure ILs, cluster structures were first proposed through MD study. Voth et al. used a CG model and identified the segregated domains in ILs in 2005 [8]. United-atom simulation was performed by Urahata and Ribeiro, and signs of intermediate-range order by structure factors were reported [123]. All-atom simulation was performed by Lopes et al., who suggested that nanosegregation exists between polar and nonpolar domains [124]. It is indicated through the above simulation that the existence of nano-clusters in ILs is not a result of differences in models [125]. Furthermore, theoretical simulations by Lopes and Padua showed the presence of inhomogeneities that have a few ions sizes, but they also reported that for [Cnmim][PF6] (n 5 2, 4, 6, 8, 10, 12) ILs with alkyl side chains longer than or equal to C4, polar/nonpolar domains can be formed with the length scale from 1.1 to 2.0 nm [124].

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Recently, many researchers have reported their simulation results. Cummings performed MD simulations and compared the results with X-ray scattering experimental results for [CnMPy][NTf2] (n 5 3, 4, 6, 8, 10). Through analyzing radial distribution functions and structure functions, they found that the scattering peaks in structure functions generally shifted to lower Q values with increased temperature for all of the liquids in this series. The first peak in the longer alkyl chain liquids displayed a marked shift to higher Q values with increasing temperature. However, they also observed the alkyl chain-dependent ordering of the polar groups and increased tail aggregation with increasing alkyl chain length in partial pair correlation functions and structure functions [126]. Carbone et al. developed a CG model to investigate the structure and dynamic properties of [Cnmim][PF6] (n 5 4B10). It was observed that different ways of grouping atoms into CG beads differently affect the structure and dynamics of the liquid. They reported that upon increasing the length of the alkyl tail, the diffusion coefficients of the cations decreased as expected, whereas the anion diffusion became slightly faster; the reduced dynamic heterogeneity of liquids at low temperatures was caused by a decrease in the number of the slow particles only. At the timescale where the models show their highest dynamic heterogeneity, the cross-over displacement, after which part of the anions show fast dynamics, is consistently higher in C10 than in C4 [127]. Wu et al. found that a new double-layer stacking formation of the [C4mim][PF6] can be triggered by the surface negative charge, and the double-layer formation induced by the surface charge thoroughly extended into the bulk phase. The simulation results also indicated that the double-layer formation in the bulk phase was caused by rapid structural transition. Different IL formations, including the conventional adsorption layer and the double-layer formation, can be achieved in sequence by increasing the surface negative charge. Besides, the diffusion ability of the new double-layer formation in the bulk phase is much lower compared with that observed in its original uncharged condition [128]. Lopes et al. studied the nanostructure of [N1nnn][NTf2] (n 5 4, 6, and 8). They proposed that ILs with sufficiently large alkyl side chains were nanostructured media composed of polar and nonpolar domains and that the complex structure can be further subdivided according to different classes of morphology (globular, filamentous, stratified). These different topologies were a result of the specific ionic frames and interactions, as well as the characteristics of each type of cation and anion present in the homologous series [129]. Biswas et al. investigated the heterogeneity effects on reorientation correlation time in [C4mim][PF6]. The simulation at high temperatures (450K) captures the entire decay. The ratio of the simulated reorientation time constants differs considerably from 3K to 450K, indicating the presence of significant heterogeneity effects even at high temperatures. The observation corroborates well with the simulated non-Gaussian parameter at 450K [130]. Feng et al. investigated the distinctive structural organization of di-cationic ILs (DILs) with different alkyl chain lengths. The long-chain DILs were found to display a relatively insignificant prepeak and low heterogeneity order parameter (HOP), which are accompanied by the less evident structural heterogeneity, and the predominant role of anion

Properties of Ionic Liquids 67 type in the structure of DILs was also verified. They found that the different nanoscale organizations in DILs and MILs were rationalized by the relatively unfavorable straight and folded chain models proposed for the nanoaggregates in DILs and the favorable micelle-like arrangement for those in MILs [131]. Through coarse grained MD simulations, Wang et al. found that when the cationic side chain was sufficiently long, the structure of ILs goes through a transition from spatially heterogeneous to liquid crystalline-like due to the stronger van der Waals interactions between the side chains. In the liquid crystal-like phase, the cationic side chains tend to be parallel to each other, while the cationic head groups and anions still form a continuous polar network, although being perpendicular to the direction along the side chains [132]. Lopes et al. used large simulation boxes to simulate the mesoscopic segregation in the ILs of [Cnmim][NTf2] (2 # n # 10). Through analysis the whole family of the corresponding structure factors, the periodicity of the polar network of the IL and its intermediate low-q peak equivalence, and the statistical functions, comprised aggregate size distributions, average number of contact neighbors within an aggregate, neighbor distributions, distributions of aggregate maximum length, and distributions of aggregate volume probed the existence and characterize the polar network and the nonpolar aggregates [133]. Computer simulations and neutron diffraction for four primary alkylammonium protic ILs (ethylammonium hydrogen sulfate, ethylammonium formate, ethylammonium thiocyanate, and butylammonium thiocyanate) were determined and compared with ethylammonium nitrate and propylammonium nitrate. It was found that they arranged into a sponge-like bi-continuous nanostructure consisting of polar and apolar domains, and anion has comparatively little effect on structure [134]. By performing allatom MD simulations for 1-(n-hydroxyalkyl)-3-methylimidazolium nitrate, Wang et al. found that both the nonpolar region and the flexibility of cationic tails increased with increasing side-chain length. The larger nonpolar region pushes both the charged groups and nonpolar groups to become more organized, while the increasing tail flexibility allows the hydroxyl terminals to retain a relatively uniform distribution [135]. The distinctive structural organizations of di-cationic ILs (DILs) with varying alkyl linkage chain lengths were systematically investigated. In comparison with the mono-cationic ILs (MILs), the DILs with short linkage chains exhibited almost identical structural features regardless of the anion types, whereas the long-chain DILs displayed a relatively insignificant pre-peak and low HOP, which was accompanied by less evident structural heterogeneity [131]. Lopes et al. performed a systematic MD simulation with large simulation boxes to analyze the mesoscopic segregation behavior of [CnCmim]-[NTf2] (2 # n # 10, 2 # m # n). The analysis included a discussion of the structure factors in the low-q range (1.6 # q/nm 2 1 # 20), the periodicity of the polar network and intermediate peaks of the IL, and the characterization of the polar network and the nonpolar regions. They found that the percolation of the nonpolar regions into a continuous domain occurs when the total number of carbon atoms in the alkyl chains exceeds six and the longer alkyl chain contains more than a critical alkyl length (CAL) of five carbon atoms [136].

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(A) [C2mim][Gly]

(B) [C6mim][Gly]

(C) [C10mim][Gly]

Figure 3.11 Snapshot of imidazolium amino acid-based ILs.

Liu et al. also performed MD simulations for a series of imidazolium- and phosphonium-based ILs [137], and the ion clusters were found, and the snapshots in imidazolium amino acidbased ILs are shown in Fig. 3.11. It is obvious that no aggregation can be found in [C2mim][Gly]; however, heterogeneity is easy to identify for [C6mim][Gly] and [C10mim][Gly]. Watanabe et al. suggested the existence of clusters in bulk ILs, based on their investigation on the viscosity and conductivity of a series of [Cnmim][NTf2] salts [138]. Hamaguchi et al. reported on their Raman studies on a series of [Cnmim][BF4] and [Cnmim][Cl] salts and ˚ might exist in these materials found that a local ordering or structures smaller than 100 A [139]. However, the above studies are indirect evidences of ion clusters existing in neat ILs. In 2007, the experimental evidence of the existence of clusters inside bulk ILs from X-ray scattering was reported by Triolo et al. [140]. This technique allows probing of electron density fluctuations over a spatial scale from Angstroms to several nanometers; thus, it is an ideal tool to probe ILs’ complex morphology. Recently, Chen et al. directly observed the clusters in IL thin film by means of high-resolution transmission electron microscopy (HRTEM) (Fig. 3.12), with the aid of a specially designed carbon nanotube grid to obtain an ultrathin freestanding film [141]. The thickness of as-obtained freestanding film is only several-tens nanometers, which ensures the high resolution imaging on the IL film by HRTEM. The cluster structure inside an imidazolium-based IL [C4mim][I] was directly observed for the first time. The size of clusters inside ILs has been studied by many groups. The existence of clusters has been suggested by electrospray ionization mass spectrometry (ESI-MS), and the ILs can form supramolecular aggregates [142]. Hamaguchi et al. investigated three common imidazolium ILs [Cnmim][PF6] (n 5 4, 6, 8) and found that the cluster size is in the range of 10100 nm, which increases with the alkyl chain length [143]. By using X-ray diffraction, Triolo et al. provided experimental evidence of the existence of nanoscale heterogeneities in neat ILs and supercooled ILs, such as 1-alkyl-3-methyl imidazolium-

Properties of Ionic Liquids 69

Figure 3.12 HRTEM images of the freestanding [C4mim][I] IL thin films. (A) TEM image of the [C4mim][I] IL thin film supported by the nanosized holes on a carbon nanotube network. The blue circles marked the location of the freestanding [C4mim][I] thin film on the grid. (B) High-resolution TEM image shows the existence of well-ordered clusters in [C4mim][I] thin film. Reprinted with permission from Ref. S.M. Chen, K. Kobayashi, R. Kitaura, Y. Miyata, H. Shinohara. ACS Nano 5 (2011) 49024908, 2011. Copyright American Chemical Society.

based salts [140]. It is also reported that ILs with an alkyl chain on their cation or anion segregate into polar and nonpolar domains over an intermediate length scale of typically ˚ . The type of cation and the length of the alkyl chain determine the segregation to 820 A be detectable or not [140,144]. For ILs with long alkyl chains ( . 512 carbons), a smectic A phase intermediate between the crystalline and liquid phases was reported to form through segregation of the alkyl chains [7,145]. Besides, theory equations were also used to study the aggregation of neat ILs. For example, Zherenkova and Khalatur applied the theory of integral equations to investigate the formation of structures in ILs. They studied the effect of temperature and the length of the cation tails on the structural properties. They observed the distribution of the polar domains having the shape of a three-dimensional net coexisting with nonpolar domains. The characteristic scale of intermediate ordering is shown to increase as a power function without disturbing the shape of the distribution of polar domains as the length of the cation tails grew [146]. 3.2.2.4 Ionic cluster at interface The micro-structure, especially the cluster effect on the behavior of ILs, plays an important role on the performances of ILs at interfaces. Many applications of ILs are dependent on the micro-structure of the IL-vacuum interface or the solid-IL interface. Experimentally, this aspect of ILs has been investigated by a number of groups, utilizing techniques, such as sum-frequency generation vibrational spectroscopy, direct recoil spectroscopy, neutron reflectivity, and X-ray photoelectron spectroscopy.

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Smith et al. studied 1-ethyl-3-methylimidazolium ethylsulfate [C2mim][EtOSO3] and solutions of Pd(PPh3)2(OAc)2 in this solvent with X-ray photoelectron spectroscopy (XPS) [147]. The Pd 3d XP spectra reveal that Pd clusters are not stable in the solvent but are slowly reduced to Pd0. Yoshimura et al. found that in imidazolium-based salts, the highest occupied orbitals in the IL are located on the cation based on experiment and DFT calculations [148]. It has been proven by Voth et al. that the region probed by XPS has the same average composition as the volume by studying the XPS and low energy ion scattering of [C4mim][NTf2], whereas the outermost layer shows an excess of fluorine atoms, indicating the formation of the clusters at the gasliquid interface of an IL [149]. Besides, some studies have focused on the properties of ILs on solid surfaces, in which clusters show some influences on the behavior of ILs. In these studies, the solid layers were shown to be very stable, and their orientations had a correlation with the mica lattice, indicating the ILs bearing an interface-induced ordering process, and that the ion clusters are formed at the ILsolid interface. The same group further studied the micro-structure of ILs [C4mim][PF6] on graphene by molecular dynamic simulation [150]. Atkin et al. found that ILs could form several remarkably strongly adhering solvation layers on solid surfaces based on their atomic force microscopy (AFM) studies [151]. Endres et al. showed molecularly resolved scanning tunnel microscope (STM) images of an IL monolayer by temperature-dependent ultra-high vacuum (UHV)-STM [152]. Liu et al. found the coexistence of the liquid and solid phases of the IL [C4mim][PF6] on mica surface by using AFM [153]. [C4mim][PF6] was reported to ˚ thickness on the graphite surface. form a stable solidlike IL bottom layer of about 6 A 3.2.2.5 IL in confined space Studies on the IL in confined space and the micro-structure and effects of ionic clusters have attracted great attention. Despite these technologic developments, the fundamental aspects of the structure and behavior of IL clusters in confined space have been seldom reported, especially on the cluster influence on the geometry and behavior of confined ILs. This is partly due to the recent emergence of the field and, more importantly, the great difficulty in setting up well-defined model experiments. The effects of confinement on the physico-chemical properties of ILs also have been widely studied. Kanakubo et al. investigated the intriguing melting-point depression of 1, 3dialkylimidazolium-based ILs, which was confined in controlled-pore glasses [154,155]. Sha et al. reported a drastic phase transition in [Dmim][Cl] IL confined between two graphene walls [156]. In addition to these studies, Chen et al. reported when [C4mim][PF6] was confined in multiwalled carbon nanotubes (CNTs), [C4mim][PF6] transited from liquid to a high-melting-point crystal [157]. Besides, they also investigated the morphology and melting behavior of a zinc-containing, quaternary ammonium-based IL (ChZnCl3) inside single-walled carbon nanotubes by HRTEM [158]. It was found that different morphologies of the IL clusters formed in the channel of nanotubes, including single-chain, double-helix,

Properties of Ionic Liquids 71

Figure 3.13 The packing arrangement of ChZnCl3 IL inside SWNTs of different nanotube diameters. The observed (left side) and simulated (right side) HRTEM images of four typical morphologies of ChZnCl3 (single-chain (A), double-helix (B), zigzag tubes (C), and random tubes (D)) are shown. Based on the TEM images, structural models (center) were constructed, and the diameter of the SWNTs suitable for each configuration were determined (center right). The calculated tube diameters for the single-chain, double-helix, zigzag tubes, and random sizes are 1.2 nm, 1.4 nm, 1.8 nm, and 2.1 nm, respectively. Reprinted with permission from Ref. S. Chen, K. Kobayashi, Y. Miyata, N. Imazu, T. Saito, R. Kitaura, et al. J. Am. Chem. Soc. 131 (2009) 1485014856. Copyright American Chemical Society.

and zigzag tubes and random tubes (Fig. 3.13). They investigated the melting of the solidlike ChZnCl3 into “nanofluid” inside single-walled nanotubes (SWNTs) by in situ TEM electron beam irradiation and compared with a high-temperature heat treatment. It is found that the thermal-decomposition temperature of the IL ChZnCl3 confined in the SWNTs was much higher than in the bulk system. The structure and behavior of the ILs clusters in confined ILs could be studied by spectroscopic techniques, such as NMR, IR, Raman, and so on, which indicated that the cluster structure may influence the performance of ILs. The line shape analysis of the temperature-dependent NMR spectra of bulk ILs and their confined state indicates that the behavior of ILs in confinement is similar to the behavior of non-ionic guest molecules in the mesoporous silica. They also found that the anion and cation of the IL exhibit the same dynamic behavior in confinement. IR spectroscopy on silica-based ionogels indicated no obvious difference between the bulk and confined ILs when the molecular size of the IL is small [159]. Solid-state NMR spectroscopy gives detailed information on the molecular dynamics of confined ILs. Waechtler et al. reported (2)H and (19)F solid-state NMR studies of the perdeuterated IL N-ethylpyridinium-bis(trifluoromethanesulfonyl)amide confined in mesoporous silica materials [160]. Raman spectra reveals that the liquid range of [C4mim] [Cl] becomes larger than in the bulk system after being confined in mesoporous silica,

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indicating that steric constraints prevent cluster organization of ILs at low temperatures [161]. In the case of imidazolium ILs with long alkyl chains, the shifts of the bands assigned to Bn(CH2) revealed some changes in the conformational order of the alkyl chain upon confinement, which may be a clue to the formation of clusters [162]. Dong et al. performed MD simulations for [C4mim][PF6] and provided direct structural evidence that the IL crystallizes in a carbon nanotube. The ordered ionic arrangement in both the radial and axial directions can be observed inside the channels of the CNTs to induce the formation of crystallites. The ionic stacking and distributing can be determined by the sizes of the CNTs. H-bonds remain the dominant interactions between cations and anions when the IL enters the CNT from the bulk phase. It is found that it is difficult for a single anion to enter the channel of the CNT spontaneously. A more favorable way is through an ion pair, in which a cation “pulls” an anion to enter into the channel of the CNT together. It is predicted that other ILs that possess similar structures, including the pyridinium-based ILs, can show higher melting points when confined in CNTs [163].

3.2.3 Interaction of Ionic Liquids and CO2 Recently, ILs have been given much attention and regarded as potential candidates for CO2 capture due to the unique physico-chemical properties of ILs and because CO2 is highly soluble in some ILs, whereas these ILs are not measurably soluble in CO2 [911]. A lot of studies have been focused on CO2ILs systems [164204]. However, deep research on the structure and properties of ILsCO2 systems has not been performed; this is the obstacle to practical application of ILs, and thus it is very important to design new ILs for capturing CO2. In this section, we mainly focus on the interactions of conventional ILsCO2 and task-specific ILsCO2. 3.2.3.1 Conventional ILsCO2 3.2.3.1.1 Effect of anions

It is generally accepted that the anion of an IL has stronger effect on the solubility of CO2 compared with the cation [10,205208]. Kazarian et al. studied CO2 solubility in 1-butyl-3methylimidazolium hexafluorophosphate ([C4mim][PF6]) and 1-butyl-3-methylimidazolium tetrafluoroborate ([C4mim][BF4]) by attenuated total reflectance (ATR)-IR spectroscopy. They found that there exist a weak Lewis acidbase interaction between CO2 and the anion, with the axis of the OQCQO orienting toward the anion in perpendicular arrangement to PaF or BaF bonds. CO2 solubility in [C4mim][PF6] and [C4mim][BF4] is governed by interactions with the anions [209]. Molecular dynamics simulations and experiments were used to study the mechanism of the high solubility of CO2 in six imidazolium-based ILs, that is, [C4mim][PF6],

Properties of Ionic Liquids 73 1-butyl-2,3-dimethylimidazolium hexafluorophosphate ([C4mmim][PF6]), [C4mim][BF4], [C4mmim][BF4], 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C2mim] [NTf2]), and 1-ethyl-2,3-dimethylimidazolium bis(trifluoromethylsulfonyl)imide ([C2mmim] [NTf2]) by Cadena et al. [210]. It was found that the presence of the hydrogen on the C2 carbon slightly affects CO2 solubility. The solubility of CO2 is dominated by the nature of the anion, and the cation plays a secondary role. [NTf2]2 anionbased ILs can absorb much more CO2 than those with either the [PF6]2 or the [BF4]2 anion. Simulation results revealed that CO2 molecule organizes strongly above the [PF6]2 anion in a tangent-like configuration. The structure of ILs being almost unperturbed with the addition of 10 mol% CO2 is attributed to the strong Coulombic interactions, which helps to form a network in which CO2 is forced to accommodate in the interstices [210]. However, when the mole fraction of CO2 is 0.6, the ˚ , and the hexyl chain on the cation extended distance of cationanion increases by about 1 A slightly [211]. On the other hand, at 10 bar, CO2 has a minimum effect in the global mobility of the IL but would result in faster rotational motions of the C10 methyl carbon atom [201]. Infrared and Raman spectra have also shown that the structure of [C4mim][PF6] does not change strongly with the addition of CO2 at 150 bar [212]. Huang et al. analyzed the reason for the structure of the IL not changing significantly with the addition of high concentrations of CO2. They reported that CO2 is accommodated in the larger cavities generated by small angular rearrangements of the [PF6]2 anion, as shown in Fig. 3.14. Therefore, the volume of ILs does not change significantly even upon dissolution of high concentrations of CO2 [213].

Figure 3.14 The relative positions of [C4mim] cation and [PF6] anion in the neat IL. The arrows approximately indicate the directions in which anions displace to accommodate CO2 in solution and the most probable location of CO2. Reprinted with permission from Ref. X.H. Huang, C.J. Margulis, Y.H. Li, B.J. Berne. Why is the partial molar volume of CO2 so small when dissolved in a room temperature ionic liquid? Structure and dynamics of CO2 dissolved in [Bmim(1)] [PF6-]. J. Am. Chem. Soc. 127(50) (2005) 1784217851.

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The effect of the anions on the CO2 solubility was further investigated by Aki et al. through experiment [214]. They found that with the same [C4mim]1 cation, the solubility of CO2 decreases in the order of tris(trifluoromethylsulfonyl)methide ([methide]2) . [NTf2]2 . trifluoromethanesulfonate ([TfO]2) . [PF6]2 B [BF4]2 . dicyanamide ([DCA]2) . nitrate ([NO3]2). The higher solubility of CO2 in ILs with [methide] and [Tf2N]2 anions may be due to the acidbase interactions between CO2 and fluorous alkyl chains in the anions. Shi et al. also reported that the high CO2 solubility in tetra-n-butylphosphonium acetate ([P4444][CH3COO]) results from the stronger interactions of CO2[CH3COO]2, which are much stronger than that of CO2[P4444]1 [215]. Zhang et al. found that tris(pentafluoroethyl)trifluorophosphate ([FEP]) anion based ILs can absorb more CO2 and that the cation has limited influence on the solubility of CO2 in [FEP]-based ILs by predicting the Henry’s Law constants of CO2 in 408 ILs by Conductor-like Screening Model for Real Solvents (COSMO-RS) and experiment. For example, the solubility of CO2 in 1-hexyl-3-methylimidazolium tris(pentafluoroethyl) trifluorophosphate ([C6mim][FEP]) is about 15% higher than that in [C6mim][NTf2] [216]. By studying the mechanism for CO2 absorption in [C6mim][FEP] and [C6mim] [PF6], they found that the size and shape of anion have a significant effect on the mechanism of CO2 absorption in ILs. As seen in Table 3.5, for CO2-[FEP]- anion interaction, van der Waals energies are about three times larger than electrostatic energies, but for CO2[PF6]2 anion interaction, electrostatic energies are nearly twice as large as van der Waals energies. However, van der Waals energies are higher than electrostatic energies for interaction of CO2[C6mim]1 cation both for [C6mim][FEP] and [C6mim][PF6]. The results indicate that the smaller and symmetric [PF6]2 anion attracts CO2 strongly rely on the electrostatic interactions, whereas the larger and asymmetric [FEP] anion absorbs CO2, mainly depending on the van der Waals interaction. Moreover, the introduction of six CF2/CF3 groups into [FEP] results in a larger molar volume, leaving more free volume to host CO2 [217]. The behavior of CO2 solubility in ILs was investigated by using the COSMO-RS method. Anions were demonstrated to play an important role and cations play a secondary role in CO2 solubility. The effect of the anions on the CO2 solubility based on [C6mim]1 cation was analyzed by calculating the Henry’s Law coefficients, using the COSMO-RS/[CA]GAS method. It was found that attractive van der Waals interactions between CO2 and anion dominate the solubility of CO2 in ILs, whereas repulsive electrostatic interactions play a secondary contribution [218]. The interactions of CO2 with various anions by density functional theory calculations were studied by Bhargava et al. The anionCO2 complexes seem to be dominated by Lewis acidbase interaction, the strength of which was found to be directly

Properties of Ionic Liquids 75 Table 3.5: The electrostatic (ELEC) and van der Waals (VDW) energies per mole of solute due to the solute (S), the cations (C), and the anions (A) at different temperatures, pressures and mole fraction x of CO2 in [hmim][FEP] and [hmim][PF6] by using both Monte Carlo (MC) and molecular dynamics (MD) simulations. Also shown in the parentheses are last digits for the standard deviation obtained from block averages. T (K)

P(bar)

x

S-C (kJ mol21)

S-A (kJ mol21)

VDW

ELEC

VDW

ELEC

0.135 0.333 0.511 0.092 0.247 0.330

210.6 (1) 211.59 (4) 210.89 (3) 212.01 (8) 211.46 (8) 212.13 (4)

24.0 (1) 23.14 (4) 23.35 (4) 22.6 (3) 23.3 (2) 22.79 (4)

213.8 (1) 212.76(8) 212.01(4) 212.6 (1) 212.80(4) 211.97(4)

24.6 (1) 24.60(8) 24.31(4) 24.1 (3) 24.10(8) 24.44(4)

0.511

210.7 (4)

23.04 (4)

211.4 (4)

24.10(3)

0.511

217.1 (3)

20.46 (3)

25.31(11)

29.81(5)

CO2-[hmim][FEP], MC 298.2 298.2 298.2 323.2 323.2 323.2

2.5 10 20 2.5 10 20

CO2-[hmim][FEP], MD 298.2

20

CO2-[hmim][PF6], MD 298.2

35

Reprinted with permission from Ref. X.C. Zhang, F. Huo, Z.P. Liu, W.C. Wang, W. Shi, E.J. Maginn, Absorption of CO2 in the ionic liquid 1-n-Hexyl-3-methylimidazolium Tris(pentafluoroethyl)trifluorophosphate ([hmim][FEP]): a molecular view by computer simulations. J. Phys. Chem. B 113(21) (2009) 75917598.

proportional to the basicity of the anion [219]. CO2 prefers to localize near the anion, and its carbon atom can interact with two electronegative atoms of the anion. They also found that the solubility of CO2 in ILs is inversely proportional to the calculated binding energies [219]. Cabaco et al. studied the solubility of CO2 in [C4mim][CH3COO] by using Raman spectroscopy and DFT calculations. It was shown that most CO2 molecules are aggregated in the voids existing in the ions pairs of ILs, and some weakly interact with [CH3COO]2 anion through a weak charge transfer interaction between the C atom of CO2 acting as a Lewis acid and the O atom of the COO group of [CH3COO]2 anion acting as a Lewis base. The short-range local structure of [C4mim][CH3COO] is not significantly disturbed by accommodating a large amount of CO2. It also pointed out that the local structure of ILs with addition of CO2 is similar to aggregation phenomena reported for ILs; only the role of the increasing alkyl chain is replaced by the increasing concentration of CO2 [220]. The effect of side groups in ILs on CO2 capture was studied by experimental and simulation method by Yan and coworkers. CO2 interacts more strongly with the anion than with the cation, and CO2 interacts more strongly with the side CF3 groups than with the

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central N atom and the SO2 group of the [NTf2]2 anion. On the other hand, CO2 molecules distribute randomly with respect to the cation, and CO2 interacts strongly with the methyl side group of cations. Therefore, CO2 molecules can distribute between the methyl side group and the anion to maximize their interaction with anions [221]. The [C4mim][PF6] and [C4mim][PF6]-CO2 mixtures were studied by molecular dynamics and ab initio molecular dynamics simulation. It was reported that the solubility of CO2 in [C4mim][PF6] is predominantly facilitated by the anion rather than the cation and that CO2 is tangential to the [PF6]2 sphere. In addition, the H-bond between CO2 and the ring hydrogen could play a role in the absorption of CO2 [222]. CO2 molecules interact with at least two of the [PF6]2 anions, and anions tend to move away from each other with increased CO2 concentration [223]. Besides the imidazolium-based ILs, the properties and interactions between CO2 and hexamethylguanidinium lactate ([HMG][LAC]) were studied by molecular dynamics simulations. As seen in Fig. 3.15, CO2 molecules prefer to distribute above the [LAC]2 anion molecular plane opposite the COO group. The interaction energy results showed that the interaction of CO2 with the [LAC]2 anion is slightly stronger than that with [HMG]2 cation, and the van der Waals energy is larger than Coulombic energy both for

Figure 3.15 Probability distribution of CO2 for equimolar mixtures of CO2 1 [HMG][LAC] at 298K and 2.5 MPa. Surfaces in orange show three times the bulk number density of CO2, and surfaces in white show eight times the bulk number density. Reprinted with permission from Ref. S. Aparicio, M. Atilhan. Computational study of hexamethylguanidinium lactate ionic liquid: a candidate for natural gas sweetening. Energy Fuels 24 (2010) 49895001.

Properties of Ionic Liquids 77 CO2-cation and CO2-anion interactions. Cavities in [HMG][LAC] are too small to accommodate CO2; therefore, the absorption of CO2 should lead to a rearrangement of these cavities to form larger ones without large volume expansion [224]. The microscopic structure, interaction and properties of N,N,N0 ,N0 ,Nv-pentamethyl-Nvpropylguanidinium tetrafluoroborate ([ppg][BF4])-CO2 systems were studied by molecular dynamics simulations and ab initio calculations. It was found that the anions tend to be far away from each other with increasing CO2, whereas H?F interactions gradually become weaker with increasing CO2. By comparing the radial distribution functions of CO2 with [ppg]1 cation and CO2 with [BF4]2 anion with the addition of 0.6 mole fraction of CO2, it was found that CO2 molecules prefer to distribute around [BF4] anions. The interactions of anionanion and cationanion decreased with increasing CO2 concentration. However, the presence of 0.6 mole fraction of CO2 in [ppg][BF4] has a negligible effect on the structure of anionanion and cationanion [225]. 3.2.3.1.2 Effect of cations

Although the solubility of CO2 in ILs is dominated by anions, cations are believed to play a secondary role. It is agreed that the long alkyl chain on the imidazolium ring generally slightly increases CO2 solubility [167,226228]. Aki et al. reported that CO2 solubility increases with an increase in the alkyl chain length at all pressures, especially at higher pressures [214]. It is generally thought that ILs with a longer alkyl chain show a larger free volume; therefore, the solubility of CO2 increases with increasing length of the alkyl chain in the cation. CO2 solubility in ILs containing the alkyl imidazolium cation and the [NTf2] anion were measured by Baltus et al. The results showed that CO2 solubility increases as the length of the alkyl side chain on the imidazolium increases and that it is higher in ILs with [NTf2] anion than [PF6]2 anion. These authors also found that CO2 solubility could significantly increase in an imidazoliumbased IL with a fluorine-substituted octyl side chain [229]. The effect of the proton in the C2 position on CO2 solubility was studied through experiments and molecular dynamics simulations for several ILs with C2-H and their counterparts with a C2-methyl substituent [210,214]. At low pressures, replacing the protic hydrogen at the C2 position with a methyl group has a negligible effect on CO2 solubility. However, at the highest pressure, ILs with C2-H show slightly higher solubility than ILs with a methyl group in the C2 position. Probably, CO2 is distributed relatively far away from the cations and are primarily interact with the anions at low pressures. However, at higher pressures, a larger amount of CO2 is absorbed, and some CO2 may distribute the secondary locations near the cations; thus, some CO2 can localize closer to the imidazolium rings with C2-H than to the methyl group in the C2 position [210].

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The effects of fluorination and size of the alkyl side chain on the solubility of CO2 in 1-octyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C8mim][NTf2]), 1-decyl-3methylimidazolium bis(trifluoromethylsulfonyl)imide ([C10mim][NTf2]), and 1(3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluorooctyl)-3-methylimidazolium bis (trifluoromethylsulfonyl)imide ([C8H4F13mim][NTf2]) were investigated by Almantariotis et al. It was shown that the solubility of CO2 in [C8H4F13mim][NTf2] is higher than that in [C8mim][NTf2]. The reason is that there is stronger interaction between CO2 and the terminal carbon of the alkyl side chain in [C8H4F13mim][NTf2] than in [C8mim][NTf2], and that the free volume in [C8H4F13mim][NTf2] is larger than that in [C8mim][NTf2]. It was also found that the CO2 solubility increases less in ILs with the number of carbon atom in the alkyl chain higher than 8, which can be attributed to the balance of the interaction of CO2IL and the structure of the solvent surrounded by the solute [230]. Zhu et al. investigated the halogen-bonding interactions between brominated ion pairs and CO2 molecules by quantum chemical calculations. The results showed that the cations of the brominated ILs can interact with CO2 molecules through C-Br. . .O halogen bonds, and the solubility of CO2 could be enhanced with halogenated ILs as a result of the formation of halogen bonds between carbon-bound halogens in cations and CO2. It was revealed that cations may also make an important impact on the physical solubility of CO2 in brominated ILs [231]. 3.2.3.1.3 Effect of interaction of cationsanions

The interaction of cationsanions in ILs affects the free volume, and a weaker interaction results in more free volume being available. Moreover, just as discussed above, the more free volume the ILs have, the more CO2 could be absorbed. For example, although the solubility of CO2 in [C4mim][PF6] is higher than that in [C4mim][BF4], spectroscopic studies have shown that the interaction of CO2[C4mim][BF4] is stronger than that of CO2[C4mim][PF6]. The higher solubility of CO2 in [C4mim][PF6] could be explained as resulting from the more free volume in [PF6]2 anion [209]. Liu et al. studied the solubility, diffusivity, and permeability of CO2 in 1-ethyl-3methylimidazolium tetracyanoborate ([C2mim][B(CN)4]), [C2mim][NTf2] and [C2mim] [BF4] through alchemilcal free energy calculations with Bennett acceptance ratio analysis by molecular dynamics simulations. They reported that reason for the higher solubility of CO2 in [C2mim][B(CN)4] seems to be the weaker cationanion interaction, higher fraction of larger cavity, larger free volume, and more favorable interaction with CO2 [232]. The interaction of [ppg]1 cation[BF4]2 anion was found to be slightly weaker than that of [C4mim]1 cation[BF4]2 anion; therefore, there is more free volume in [ppg][BF4] than in [C4mim][BF4]. Thus, more CO2 molecules could easily localize around [ppg][BF4]. Moreover, it is found that the distribution of CO2 around [BF4]2 anions does not compete with the distribution of [ppg]1 cations but are

Properties of Ionic Liquids 79 complementary to each other. Hence, [ppg][BF4] could effectively absorb CO2 [225]. Damas et al. evaluated the interactions between gases (CO2, SO2, and H2S) and anioncation from ILs, as well as cationanion interactions by quantum chemistry. The results suggested the strength of the cationanion interaction is responsible for generating voids in the structures of ILs to accommodate more gases. Thus, comparing the binding energy of aniongas and cationgas, the cationanion interactions are more important to evaluate the solubility of gas. The anion is more available to interact with gas when cationanion interaction becomes weaker [233]. Deschamps et al. reported that the solubilities of CO2 in ILs can be explained by the balance creation of a cavity in the solvent capacity of hosting CO2 molecule (free-volume contribution) and activation of the CO2ILs interactions [234]. It is worth keeping in mind that most studies are focused on imidazolium-based ILs, and research on other cations, such as ammonium [235], phosphonium [236239], pyrrolidinium [216], pyridinium [240], thiouronium [216], and guanidinium [224,225,241], is rather sparse. 3.2.3.2 Task-specific ILsCO2 Bates et al. were the first to report the CO2 solubility in a task-specific IL (TSIL; also called functionalized IL) with a free amine functional group in alkyl chain on an imidazolium cation ([NH2p-bim][BF4]). CO2 capacity could dramatically increase in ILs at low partial pressures. The mechanism of the absorption of CO2 in a proposed IL is implemented through chemical absorption, which is similar to the reaction between an organic amine and CO2. CO2 capacity reaches 0.5 mol CO2 per mol IL. The process is reversible, and CO2 can be regenerated at 80100
C under vacuum. The CO2 uptake by [NH2p-bim][BF4] is similar to the compounds used in the amine process [242]. Based on the above research, a series of anion-tethered functionalized ILs were synthesized and used to absorb CO2 [243246]. For example, trihexyl(tetradecyl)phosphonium prolinate ([P66614][Pro]) and trihexyl(tetradecyl)phosphonium methioninate ([P66614][Met]), could react with CO2 in a 1:1 stoichiometry. The mechanisms determined by experiment and ab initio calculations have been provided. It was shown that the anion contained amine favored to form the carbamic acid to lead to the instability of the di-anion product, while the cation contained amine favored the formation of the carbamate, indicating the electrostatic stability of zwitterions product [247]. Solubility of CO2 in trihexyl(tetradecyl) phosphonium glycinate ([P66614][Gly]), trihexyl(tetradecyl)phosphonium lanate ([P66614] [Ala]), trihexyl(tetradecyl)phosphonium sarcosinate ([P66614][Sar]), and trihexyl(tetradecyl) phosphonium isoleucinate ([P66614][Ile]) could reach more than 0.5 mole at CO2 pressures of less than 1 bar. CO2 reacts with one IL to form a carbamic acid, and it could further react with another IL to make a carbamate [248].

80

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Moreover, dual amino-functionalized ILs were synthesized for improving CO2 absorption capacity. A series of 20 kinds of dual amino-functionalized phosphonium ILs has been reported. The ILs are (3-Aminopropyl)tributylphosphonium L-α-aminopropionic acid ([aP4443][Ala]), (3-Aminopropyl)tributylphosphonium L-α-amino-5- guanidinovaleric acid ([aP4443][Arg]), (3-Aminopropyl)tributylphosphonium L-α-aminosuccinamic acid ([aP4443] [Asn]), (3-Aminopropyl)tributylphosphonium L-α-aminosuccinic acid ([aP4443][Asp]), (3Aminopropyl)tributylphosphonium L-α-amino-3-mercaptopropropionic acid ([aP4443][Cys]), (3-Aminopropyl)tributylphosphonium L-α-aminoglutaramic acid ([aP4443][Gln]), (3Aminopropyl)tributylphosphonium L-1-aminopropane-1,3-dicarboxylic acid ([aP4443][Glu]), (3-Aminopropyl)tributylphosphonium aminoethanoic acid ([aP4443][Gly]), (3-Aminopropyl) tributylphosphonium L-α-amino-4-imidazolepropionic acid ([aP4443][His]), (3Aminopropyl)tributylphosphonium L-α-amino-3-methylvaleric acid ([aP4443][Ile]), (3Aminopropyl)tributylphosphonium L-α-amino-4-methylvaleric acid ([aP4443][Leu]), (3Aminopropyl)tributylphosphonium L-α-diaminocaproic acid ([aP4443][Lys]), (3Aminopropyl)tributylphosphonium L-α-amino-g-(methylthio)butyric acid ([aP4443][Met]), (3-Aminopropyl)tributylphosphonium L-α-aminohydrocinnamic acid ([aP4443][Phe]), (3Aminopropyl)tributylphosphonium (S)-2-pyrrolidinecarboxylic acid ([aP4443][Pro]), (3Aminopropyl)tributylphosphonium L-α-amino-3-hydroxypropionic acid ([aP4443][Ser]), (3Aminopropyl)tributylphosphonium L-α-amino-3-hydroxybutyric acid ([aP4443][Thr]), (3Aminopropyl)tributylphosphonium (S)-a-amino-1H-indole-3-propanoic acid ([aP4443][Trp]), (3-Aminopropyl)tributylphosphonium L-α-amino-p-hydroxyhydrocinnamic acid ([aP4443] [Tyr]), and (3-Aminopropyl)tributylphosphonium L-α-aminoisovaleric acid ([aP4443][Val]). The absorption of CO2 reaches 1 mol CO2 per mol IL [249]. 1-Aminoethyl-2, 3dimethylimidazolium amino acid taurine ([aemmim][Tau]) was also synthesized to capture CO2 with amino-functionalized imidazolium cation and taurine anion. CO2 absorption capacity can reach about 0.9 mol CO2 per mol of IL at 303.15K and ambient pressure [250]. Recently, the dual amino-functionalized cation-tethered IL (1,3-di (20 -aminoethyl)-2methylimidazolium bromide [DAIL]) was investigated for capturing CO2. DAIL exhibits good thermal stability up to 521.6K and can absorb 18.5 wt% CO2. The CO2 capture capacity remains steady after four recycles, and absorption equilibrium can be reached within 4 min. The absorption mechanism is similar to other amino-functionalized ILs [251]. The frontier molecular orbital interactions between CO2 and NH2 in TSILs, 1,1,3,3tetramethylguanidinium lactate ([tmg][L]) and 1-n-propylamine-3-butylimidazolium tetrafluoroborate ([pabim][BF4]), have been studied. The results have shown that CO2 capture ability of amine-functionalized ILs relies on molecular orbital interactions between the HOMO on 2 NH2 group in the functionalized ILs and the lowest unoccupied molecular orbital of CO2 [252]. However, amine-functionalized ILs systems usually show much higher viscosity in the pure state and after absorbing CO2 [236,247249,253]. The high viscosity of the pure TSILs

Properties of Ionic Liquids 81 composed of 1-(aminoethyl)-3-methylimidazolium cation could be attributed to the interaction between the anion and the NH2 group of the cation [254]. The dramatically increased viscosity after reaction with CO2 may result from the formation of hydrogen-bonded networks when CO2 reacts with the functionalized cation [243]. Based on this mechanism, a series of aprotic heterocyclic anions (AHAs)based TSILs, such as the ILs combining the trihexyl (tetradecyl)phosphonium cation with either 2-cyanopyrrolide or 3-(trifluoromethyl) pyrazolide anion, could react with CO2 in a 1: 1 stoichiometry, but the viscosity almost remained unchanged upon reaction with CO2 [255] as no hydrogen-bonded network was formed [256]. Luo et al. reported that CO2 solubility can be improved by using pyridine-containing anionfunctionalized ILs. Solubility could reach 1.60 mol of CO2 per mol of the IL, and excellent reversibility were achieved with the phenolate- and imidazolate-based anions. Multiple site cooperative interactions between two kinds of interacting sites in the anion and CO2 have been demonstrated by using quantum chemical calculations, spectroscopic investigations, and calorimetric data to explain the high CO2 capacities, which originated from the p-electron delocalization in the pyridine ring [257]. Teague et al. investigated the interaction energies between CO2 and a series of oxygencontaining Lewis-base anions (including cyclohexanolate and phenolate and their respective derivatives) by the computation method. As shown in Fig. 3.16, there is good correspondence between the anionCO2 interaction energy and the O charge in the anion. For chemical interactions, O partial charge indicates the driving force for the nucleophilic

Figure 3.16 Variation of anionCO2 interaction energy ΔE with the calculated Mulliken or NBO charge on the O atom in the free anion at the DFT level with the BP functional. Linear fits for each trend are shown; R2 values are 0.961 (Mulliken) and 0.976 (NBO). Reprinted with permission from Ref. C.M. Teague, S. Dai, D.E. Jiang. Computational investigation of reactive to nonreactive capture of carbon dioxide by oxygen-containing Lewis bases. J. Phys. Chem. A 114(43) (2010) 1176111767.

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attack on CO2, whereas for physical interactions, O partial charge can polarize the CO2 molecule and attract the partially positive C atom of the CO2 molecule. Therefore, the partial charge on the O atom in the anion is a very good indicator of the anionCO2 interaction energy across a range of interactions encompassing both chemical absorption and physical absorption [258]. Zhang et al. developed a ReaxFF force filed for the CO2-tetrabutylphosphonium glycinate ([P(C4)4][Gly]) system to account for both chemical and physical interactions. It was found that there are strongly interactions between CO2 and [Gly]2 anion and that the interactions are stronger in the product state than that in the reactant state. A minimum energy pathway for the reaction of CO2 with [P(C4)4][Gly] was obtained. There are two energy barriers. The first barrier height is 0.7 kcal mol21, which involves proton transfer from the N atom in [Gly]2 to one of the O atoms in the carboxylic group in [Gly]2. The next barrier height is 2 kcal mol21, which involves movement of the proton to a position where it is shared by the O atom from CO2 bound to the N atom in [Gly]2 and the O atom from the carboxylic group in the anion [239].

3.2.4 Dissolution of Cellulose in ILs The energy sustainability and environmental issues have become important challenges to in recently years. Utilizing renewable resources is a good way to solve these problems. At present, using biomass to get high valueadded products has become important research areas in many countries. The objects of fuel industry are changing from “hydrocarbon’ to renewable “carbohydrate” [9,252,259262]. Straw and wood are the most abundant biomass resources in nature, and the biomasscellulosebiologic gasoline constitutes the virtuous cycle of energy. The main ingredient of most biomass is cellulose, which has a complex structure. The main obstacle of the further application of cellulose is the lack of green and powerful solvent systems. With a new generation of green-medium ILs with both good dissolving capacity and catalytic activity, breakthroughs in reaction mechanism and kinetics, as well as innovation brought into traditional and conventional catalytic processes, will be important. ILs have shown great potential in biomass conversion and have demonstrated good prospects in the separation and transformation of biomass components. The dissolution of cellulose in ILs has become an important research area worldwide, and dissolution processing has shown great potential in industrial application. Because of the polarity, adjustable solubility, and their nonvolatile and catalytic properties, ILs offer advantages in biomass utilization, including their mild conditions, low energy consumption, low cost, easy product separation, and so on. More details on the interactions of ILs and cellulose are covered in Chapter 6.

Properties of Ionic Liquids 83

3.3 Physical Properties ILs have many interesting properties and are widely used. For example, in some chemical reactions, the IL plays the role of a green solvent and catalyst at the same time and has shown that the unique chemical effect not only can accelerate the rate of a chemical reaction but also can improve reaction selectivity and yield. Overall, this results from the unique properties of ILs, such as low volatility, high viscosity, high conductivity, wide electrochemical window, long-term thermal stability, designable structures, and so on. Knowledge of these properties can help understand the role of ILs in practical applications. Thus, since the advent of ILs, many reports have described their various properties, including the most common physical and chemical properties, for which investigations of simple fitting using empirical or semi-empirical models are expanded to predictions of properties based on structure. Meanwhile, with the progress of research in ILs, thousands of ILs have been synthesized, data on the physical properties of IL have increased rapidly, several ILs databases have been established, and properties have been analyzed. Reliable data on the physical properties of ILs are essential for the design of industrial processes. Density, viscosity, and phase equilibrium of some IL systems are discussed here.

3.3.1 Density Density (ρ) is one of most important physical parameters of a substance. Mathematically, density is defined as mass divided by volume: ρ5

m v

(3.1)

where m is the mass, and V is the volume. To use ILs more effectively, knowledge of density is needed because many properties are calculated on the basis of density for formulating industrial designs. Thus, the density of ILs is one of the most reported properties. Up to now, the total number of reports on the density of pure ILs in the IPE database is around 11,000 [14]. Usually, the density of ILs lies in the range of 1.122.4 g  cm23. However, for some ILs, such as [DIPEA][C7CO2] (0.8585 g  cm23, 298.15K) [263], [P666,14][OAc] (0.891 g  cm23, 298.15 K) [264], [Pyrr] [C8CO2] (0.9315 g  cm23, 298.15K) [265], [Im][C7CO2] (0.987 g  cm23, 298.15K), density values are smaller than 1 g  cm23 [266]. Impurities in ILs can affect the accuracy of density data. The presence of chloride impurity in an IL causes density to decrease [267]. The influence of water in ILs on density varies: for most ILs, the presence of water can lower the density [268271]; however, a slight amount of water in an IL can also result in increase of density in a few ILs [272274].

84

Chapter 3

Figure 3.17 The densities of ILs as a function of chain length of cations. ’: [CnMIM][BF4] at 298.15K, K: [Cnpy][TFSI] at 298.15K, ▲: [P1n][TFSI] at 303.15K, ★: [CnMIM][TFSI] at 298.15K [14].

The density of ILs is related to temperature, the structures of cations and anions, and so on. Generally, the values of density decrease with increasing temperature. The alkyl chain length of cations also affects the density of ILs. In general, with the same anion, the values of density decrease with increasing alkyl chain length (Fig. 3.17). The influence of the anion on the density of ILs is more remarkable. For the same cation, there is appreciable difference in the density of ILs with different anions. In general, the bigger the volume of anions, the greater is the density of an IL [275]. ILs with large volumes and weak anions usually have relatively higher densities, which has nothing to do with the cation. So, when choosing ILs within a particular scope, the convenient method is to choose anions to determine the approximate range of densities and to get the needed IL by judicious selection of the cation. The molar volume (Vm) of ILs can be obtained from density. The equation is as follows: [276] Vm 5

M Nρ

(3.2)

where M is molar mass of ILs, Nρ is the Avogadro’s constant. The isothermal compressibility (κT) and the thermal expansion coefficient (αp) can also be calculated from density. Equations are as follows:
 1 @ρ κT 5 (3.3) ρ @p T

Properties of Ionic Liquids 85 1 αp 5 2 ρ




@ρ @T

 (3.4) p

where T and p are temperature and pressure. Since it is difficult to measure the densities of all ILs under different conditions, it is necessary to use certain methods to predict the densities of ILs. Ye and Shreeve [277] reported a method to estimate the densities of ILs by developing volume parameters of groups. For 59 of the most common ILs, the mean absolute deviation (MAD) of the densities is 0.007 g cm23. However, the method is not straightforward and can only be used at atmospheric pressure and room temperature. Trohalaki et al. [278]. proposed a quantitative structure 2 property relationships (QSPR) method to estimate the densities of ILs, whereas it was only useful for one kind of IL. Jacquemin et al. [279]. proposed a group of contribution methods to estimate the densities of ILs by the sum of the effective molar volume of the component ions. Qiao et al. [280]. also presented a group contribution method to estimate the densities of ILs. They collected 7381 density data points of 123 pure ILs and developed 51 groups. Calculating formulas are equations 3.53.8. Valderrama et al. [281]. use the Group Contribution and Artificial Neural Networks simultaneously to predict the densities of ILs, and the accuracy is acceptable. ρ=gUcm23 5 A 1 Bðp=MPaÞ 1 CðT=KÞ A5

51 X

(3.5)

ni ai 1 a0

(3.6)

ni bi 1 b0

(3.7)

ni ci 1 c0

(3.8)

i51

B5

51 X i51

C5

51 X i51

where ρ, p, and T are density, pressure, and temperature, respectively. A, B, and C are three parameters. ai, bi, and ci are contributions of group i for A, B, and C, respectively. ni is the number of group i in the compound. In a practical application, ILs are used together with other solvents. Thus, studies on the thermodynamic properties of IL mixtures are also likely to be needed urgently. Generally, the densities of IL mixtures are related to temperature, pressure, and components, and so on. The values of the densities of IL mixtures decrease with increases in temperature, but the values increase with increases in pressure. Arce et al. [282]. found that the density of [C8mim]

86

Chapter 3

[BF4] mixtures decrease in the order of methanol , ethanol , 1-propanol , 2-propanol at the same temperature and concentration. The same conclusion could also be obtained from the [Mmim][DMP] mixtures [283]. Densities of IL mixtures at different temperatures for the whole range of composition are obtained by using the linear equation or the second-order polynomial given by: ρ5

n X

Ai T i

(3.9)

i50

where ρ is the density of mixtures, A is coefficients of equation, T is the temperature, and n 5 1 or 2. The molar volume (Vm) of IL mixtures can be calculated from measuring the density data of mixtures. The equation is as follows: Vm 5

x1 M1o 1 x2 M2o ρ

(3.10)

where Mio is the molar mass of the pure component i, and xi is the mole fraction of component i of the mixture. The excess molar volume (VmE ) can be calculated from molar volume by the following equation: 2 X VmE 5 Vm 2 xi Vm;i

(3.11)

i51

where Vm,i is the molar volume of component i of the mixture. The values of VmE can be fitted by the Redlich-Kister type polynomial equation:[268,281,284,285] VmE 5 xð1 2 xÞ

n X

Ai ð2x21Þi

(3.12)

i50

where x is the mole fraction of the one component of mixture, Ai are the adjustable parameters, and n is the order of polynomial equation, normally, n 5 4 or 5.

3.3.2 Viscosity Viscosity is also an important physical property for the design of processes and equipment. For instance, it determines the force and energy required to transfer and mix the IL with other substances [286]. It also affects the size of heat exchangers, pipelines, and distillation columns [287,288]. Viscosities of most ILs are several orders of magnitude higher than those of water and organic solvents. For example, the viscosity of water is only 0.90 cP at

Properties of Ionic Liquids 87 room temperature, whereas it is 10 cP for [TEtA][TfO] [289] and even as high as 19896.3 cP for [CNC2Him][DOSS] [290]. ILs used as solvents usually have low viscosity, which can reduce pumping costs and increase mass transfer rates [291]. Moreover, the viscosity of an IL could influence the solubility of biomass because high viscosity impedes the dissolution of the biomass, sugars, or cellulose in ILs [292]. Therefore, numerous research studies are focusing on the discovery of new ILs with low viscosity, and it is crucial for the future development of IL science and cleaner technologies [293]. ILs used as lubricants and lubricant additives or in membrane processes usually have relatively high viscosities [291]. It must be noted that the viscosities of ILs are highly influenced by temperature and impurities. The viscosities of ILs decrease dramatically as the temperature increases. It also appears that the presence of water and chloride impurities dramatically decreases the viscosities of ILs [267,294]. The viscosities of ILs are associated with short-range van der Waals interactions, hydrogen bonding, and Coulombic interactions. Generally, the viscosities of ILs increase with the increasing numbers of carbon atoms in the linear alkyl group in the cations of ILs [275]. Furthermore, the more the branched chains and fluorine atoms in the alkyl group of ILs, the higher are their viscosities. (See Table 3.6.) The reason is that such ILs have stronger van der Waals interactions. The H-bonding is also a factor affecting the viscosities of imidazolium ILs. For example, substituting the H-bond at the C(2)H position through methylation leads to a significant increase in viscosity [295]; thus, the viscosity of [C2mmim][NTf2] (88 mPa  s, 298.15K) is higher than that of [EMIM][NTf2] (28 mPa  s, 298.15K) [296,297]. Such factors as anion type, anion size, and fluorination of the alkyl chain in the anion can all influence the viscosities of ILs. As for the same cation, the bigger the size of anions, the higher are the viscosities of ILs. In general, the viscosities of ILs with a shared cation increase in the order of [dca]2 ,[NTf2]2 ,[TfO]2 , [BF4]2 , [PF6]2 , Cl2 [14,298300]. Because of an improved charge distribution, and thus the weakness of interionic electrostatic interaction, the fluorinated anion causes lower viscosity [301]. Data on the viscosity of ILs are the basis for theoretical studies, and several models have been used for the correlation and prediction of the viscosities of ILs. QSPR is an effective approach to find a quantitative relationship between the physico-chemical property under investigation and one or more descriptive parameters related to the molecular structure [302]. Chen et al. Table 3.6: The viscosities of some ILs [14]. ILs

η/mPa  s

T/K

ILs

η/mPa  s

T/K

[C4MIm][TFSI] [C4MIm][BF4] [N1113][TSAC]

52 77.1 45

293.15 298.15 298.15

[i-C4MIm][TFSI] [i-C4MIm][BF4] [N1113][TSAC]

83 118.5 108

293.15 298.15 298.15

88

Chapter 3

[303]. proposed a high correlated and simplified QSPR equation for calculating the viscosities of imidazolium-based ILs. The correlation coefficient between the data reported in the literature and the predicted values of viscosity was R2 5 0.9888. Gardas and Coutinho [304,305] proposed a new Group Contribution (GC) method (a similar “cation 1 anion 1 side chain” idea) based on the Vogel-Tammann-Fulcher (VTF) equation, namely: ln η 5 Aη 1

Bη ðT 2 T0η Þ

(3.13)

where η is viscosity in Pa  s units, T is temperature in K, and Aη, Bη, and T0η are adjustable parameters. T0η is similar for all the ILs. Aη and Bη can be obtained by a group contribution method according to: Aη 5

k X

ni ai;η

(3.14)

ni bi;η

(3.15)

i51

Bη 5

k X i51

where ni is the number of groups of type i, k is the total number of different groups in the ILs. Gharagheizi et al. [306]. proposed a reliable group contribution method to estimate the viscosities of ILs at atmospheric pressure. The equation is as follows: logðηÞ 5

Na X i51

Nai ηai 1

Nc X i51

Nci ηci 1 AT 1

B D 1 CT 2 1 2 1 η0 T T

(3.16)

where Nai, Nci, ηai, ηci, and η0 are the number of occurrence of ith substructure of anions and cations, the contribution of the ith substructure of anions and cations, and the intercept of the equation, respectively. A, B, C, and D are the temperature coefficients. Several researchers used the Artificial Neural Network (ANN) model to correlate and predict the viscosities of ILs. For instance, Valderrama et al. [291]. used the ANN and the concept of mass connectivity index to predict the viscosities of ILs. They studied different topologies of a multilayer feed-forward ANN and then determined the optimal architecture. To improve the quality of predicted data, the above models could be used in a combination. Doma´nska et al. [293]. employed a new model based on the GC strategy and an ANN-based machine-learning algorithm (GC-ANN) to predict the viscosities of ILs. The results they calculated were shown to be more accurate than those obtained with the best GC model for calculating the viscosities of ILs as described in the literature. The viscosities of IL mixtures are much lower than those of neat ILs; therefore, ILs are used as solvents, together with other solvents, in practical applications. In addition to temperature,

Properties of Ionic Liquids 89 the structures of ILs and the viscosities of IL mixtures are also related to the size of the solvent and the interaction between the solute and the solvent. For N-ethyl piperazinium propionate ([NEPP][CH3CH2COO]) mixtures, Shao et al. [307] found that the values of viscosities increase as the alkyl chain of alcohol solutions increases at the same temperature. The viscosity deviations (Δη) depend on molecular interactions as well as on the sizes and shapes of the molecules [308]. It can be calculated by using the following relation: Δη 5 η 2

n X

xi ηi

(3.17)

i51

where η and ηi are the viscosity of mixtures and pure components i, respectively. The sign of Δη can be determined by the competition between the H-bonds and van der Waals interactions present in mixtures containing a molecular solvent and an ionic species [266]. For most of the IL mixtures, the values of Δη are negative over the whole composition range because the mixture is dominated by van der Waals interactions, such as [C4mim][MeSO4]-C2H5OH mixtures [309], [C4mim][SCN]-CH3OH mixtures [310], [Epy] [ESO4]-C3H7OH mixtures, and so on [311]. However, for some IL mixtures [266,308], the values of Δη are positive. The reason is the breakup of the self-association through H-bonding. As for binary IL mixtures, the values of Δη can be obtained by using the Redlich-Kistertype polynomial equation:[312] Δη 5 x1 x2

n X

Ai ð122x2 Þi

(3.18)

i50

where x1 and x2 are the mole fraction of the components 1 and 2, Ai are the adjustable parameters, and n is the order of polynomial equation.

3.3.3 Phase Equilibrium Knowledge of the thermodynamic behavior of IL systems is essential for many industrial applications. Moreover, it is also necessary to design any process and equipment involving ILs on an industrial scale. Generally, the phase behavior of IL mixtures includes vaporliquid equilibrium (VLE), liquidliquid equilibrium (LLE), and solidliquid equilibrium (SLE). 3.3.3.1 Vaporliquid equilibrium VLE data are basic data for the development of new products, new technology to reduce energy consumption, and waste handling in the chemical industry. It is also important for establishing separation parameters of liquid compound in the chemical industry.

90

Chapter 3

VLE data are generally measured at constant temperature or constant pressure. There are different methods for the determination of VLE. Generally, according to the measured data of T, P, X, and Y, these methods can be classified into two categories: direct method and indirect method. The direct method [313] can be divided into the distillation method, the circulation method, the static method, the bubble point method, and the flow method. The indirect method can be divided into the saturated vapor pressure method and the ebulliometer method. However, the most commonly used method for measuring VLE is headspace gas chromatography (GC). In industry, ILs are usually used as entrainer for a special distillation or extraction process. For example, Gmehling et al. [314]. measured VLE data for ternary systems (hexane 1 benzene), (hexane 1 cyclohexane), (benzene 1 cyclohexane), and (ethanol 1 water) with an IL as entrainer for extractive distillation by GC. They found that the addition of [Hmim][BTI] and [C8mim][BTI] as entrainer could improve the separation factor for the systems of (hexane 1 benzene), (hexane 1 cyclohexane), and (benzene 1 cyclohexane), whereas [C4mim][OTF] and [C8mim][OTF] had no significant effect on the separation factors for the system (ethanol 1 water). Li et al. [315]. measured VLE data for the pseudo-binary mixtures of (water 1 ethanol), (water 1 methanol), and (methanol 1 ethanol) containing an alkanolammonium-based IL by using a quasi-static ebulliometer. They found that the relative volatilities of ethanol to water and ethanol to methanol are enhanced by the addition of [HMEA][HCOO]. Therefore, [HMEA][HCOO] can be used to separate ethanolwater mixture by special rectification. Actually, if data on activity coefficients at infinite dilution (γ N)are known, VLE data over the entire concentration range can be calculated both rapidly and at a reduced cost by using the excess Gibbs energy model [316]. It is generally known that γ N describes the behavior of a single-solute molecule completely surrounded by solvent and that it indicates a maximum nonideality [317]. It is very important in chemical engineering and can be used in the reliable design of thermal separation processes and the synthesis and design of new technologies [318]. GC is the most widely used method for the measurement of γN. Since ILs have negligible vapor pressure and high thermal stability, GC can be used as stationary phase to measure γ N [319]. At present, there are many reports on the measurement of activity coefficients at infinite dilution of ILs [320], representing the growth trend in recent years. So far, many thermodynamic models have been used to correlate the VLE, for example, the equations of Wilson, of NRTL, UNIQUAC, UNIFAC, and the most widely used NRTL [321323]. 3.3.3.2 Liquidliquid equilibrium If the concentration of some component in the two liquid phases is no longer changing after the two liquid phases contact, indicating that the positive and negative transfer rates of

Properties of Ionic Liquids 91 these components are equal in the two liquid phases, there is liquidliquid equilibrium. Liquidliquid equilibrium is fundamental for ILs to be effectively used as solvents in extractive distillation or liquidliquid extraction. In recent years, the study of LLE about IL mixtures mainly include ILs (e.g., [C2mim] [PF6], [C2mim][(CF3SO2)2N], [C4mim][BF4], [C4mim][PF6], [C4mim][CF3SO3], etc.) 1 water, aliphatic alcohol, polyhydric alcohol, alkane, olefins, aromatics and chlorinated hydrocarbon, ether, etc.[324,325]; ILs (e.g., [C2mim][NTf2], [C8mim][PF6]) 1 water 1 alcohols; ILs (e.g., [C4mim][BF4]) 1 water 1 nonionic surfactant; and ILs (e.g., [C2mim][PF6], [C4mim]I, [Hmim][BF4] [326], [C8mim]Cl) [327] 1 benzene 1 alkane, etc [328330]. In addition, other kinds of LLE about ILs 1 organics have also been reported, such as [bmPy][TOS] 1 aliphatic alcohol, alkane, aromatics hydrocarbon [322,331], [mmim][MeSO4]/[C4mim][MeSO4] 1 hexane 1 ethanol [332], [C4mim][CH3SO4] 1 hexane 1 benzene [333], [bmPy][BF4] 1 cyclohexane 1 aromatics hydrocarbon [334], [C1C2pyrr][NTf2] 1 benzene [335], [empy][ESO4] 1 benzene 1 alkane [336], etc. In general, (ILs 1 aliphatic alcohol) binary mixtures show LLE with an upper critical temperature (UCST). An increase in the alkyl chain length of the alcohol resulted in an increase in the UCST. The UCST decreased with increasing of alkyl chain length on the imidazolium ring [337]. Crosthwaite et al. [338]. found that the UCST increased with the replacement of the hydrogen at C2 position of the ring with the methyl group. There are also reports on lower critical solution temperature (LCST), such as ([C4mim][SCN] 1 benzene/toluene/ tetrahydrofuran) binary mixtures [339]. Many factors affect the LLE of IL mixtures. Ferreira et al. [340]. found that miscibility with a contraction of the immiscibility region increased with increasing temperature, which is mostly visible in the IL-rich phase. The structure of the IL cation and anion could affect the solubility of hydrocarbons in ILs. Yang et al. [341]. found that the solubility of IL in alcohols increased with increasing alkyl chain length in IL molecules. With the same anion ([CF3SO3]2 and [TOS]2), the increasing miscibility of n-alkanes in ILs follows the cations trend: [1,3-C4mpy]1 , [C4mpyr]1 , [C4mim]1 ,, [Pi(444)1]1 [331,342344]. The nature of the IL anion has a slight impact on the mutual solubilities of IL mixtures. For [C4mim]-based ILs, the solubility of n-alkanes in ILs decreases in the following order: [MeSO4]2 . [SCN]2 . [CF3SO3]2 . [BF4]2 . [PF6]2 . [NTf2]2 [345,346]. The order is related to the H-bond basicity of the IL anion. As for the influence of solvents, Doma´nska et al. [339]. found that the increase of the nalkane chain length increases the immiscibility of IL mixtures. The solute distribution ratio (β) and the selectivity (S) are two important parameters in LLE. They can be defined as follows: β5

xIIi xi

(3.19)

92

Chapter 3

S5

xII2 xI1 xI2 xII1

(3.20)

where xIi and xIIi are mole fractions of compound i (i 5 1 or 2) in phase I and II, respectively. A number of models have been applied to the correlate of LLE. The NRTL and UNIQUAC models are the most commonly used. In addition, the COSMO-RS model, the Lattice-Fluid Model, and the Analytical solution of groups (ASOG) model are also used in the description of LLE. 3.3.3.3 Solidliquid equilibrium Solidliquid equilibrium (SLE) can be divided into two categories. One is the solid solubility in the solvent. Its feature is that the boiling points of the solid and the solvent are significantly different and can be usually expressed in solubility. The other is the melt balance of substances that have similar boiling points. However, the first one is the most common SLE. Generally, there are three methods for the determination of SLE: balance method, dynamic method, and thermal analysis. SLE is a theoretical basis for chemical separations. The studies of SLE define a limiting separation for crystallization separation process and provide the basic data for the design of the equipment structure size and determination of operating conditions [347]. Specifically, knowledge of SLE is of paramount importance for the design of cooling, evaporation, and antisolvent crystallization. The study of the SLE is at a younger stage compared with those on VLE and LLE. However, Doma´nska systematically studied the SLE of IL mixtures [339,348350]. Numerous factors influence the SLE of IL mixtures. Generally, the solubility of ILs in solvents increases with increasing temperature and decreases with increasing alkyl chain length in alcohols [351,352]. The higher the melting points of ILs, the smaller is the solubility of ILs in solvents. That is, the smaller solubility of ILs corresponds to higher melting enthalpy [350]. Doma´nska et al. [344]. found that the solubility of 1-dodecanol in [i-B3MP][TOS] is slightly lower than in [B4MP][TOS] at lower temperature because of the larger exposure of the positive charge on the phosphorus atom of the [i-B4MP]1 cation. On the other hand, the ILs are also used as solvents. For example, Santos et al. [353]. found that the solubility of coumarin in ILs decreases in the following order: [C10mim][NTf2] . [C6mim][NTf2] . [C4mim][NTf2] . [C2mim][NTf2] . [C4mim][TfO] . [C2mim][TfO]. The SLE in a mixture of a solid 1 in a liquid may be expressed in a very general manner by the following equation: 

 o o Δfus cop;1 Tfus;1 Tfus;1 Δfus H1o T 2 ln a1 5 lnx1 γ 1 5 2 12 o 12 1 ln (3.21) Tfus;1 RT R T T

Properties of Ionic Liquids 93 o o where Tfus;1 , Δfus H1o , and Δfus Cp;1 are basic thermal properties of pure solid at p 5 0.1 MPa, namely, fusion temperature, fusion enthalpy, and isobaric heat capacity, change as a result of fusion. R is the gas constant, and x1 is the solid solubility (in mole fraction) at temperature T.

So far, the models used for correlating of SLE are from VLE or LLE, with a few small modifications. The commonly used models are activity coefficient equation, equation of state, λh equation, topologic method and so on. Recently, Verma et al. [354]. used the COSMO-RS model to predict the solubilities of IL mixtures, and the deviations are satisfactory when compared with the experimental values.

3.4 Summary and Prospects The structures and properties of ILs are very important and are the foundation for designing new ILs and developing IL-based technologies for specific applications. As discussed above, some progress has been made on understanding ILs, but a lot of work needs to be done. For H-bond and clusters, systematic investigation and mastering of the mechanism of forming and controlling H-bond and cluster in ILs are necessary for developing new theories and models based on ILs. Much work remains to be done on the interactions of ILsCO2 and ILscellulose for establishing the structureproperty relationship and a method for quickly and accurately screening suitable ILs from numerous candidates. As for prediction of the physical properties of ILs, more accurate instruments need to be set up, and reliable data need to be measured.

Acknowledgments This work was supported by the National Basic Research Program of China (No.2015CB251401, 2014CB239701), the National Natural Science Foundation of China (21106146, 91434111, 21276255, 21376242, 21343004), and the Beijing Natural Science Foundation (2142029).

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CHAPTER 4

Catalytic Reaction in Ionic Liquids 4.1 Introduction The special properties of ionic liquids (ILs), such as high stability and negligible vapor pressure, make them promising applications in chemical reactions, one of which is their applicability as reusable catalysts. ILs are highly designable so that catalytic activity for specific reactions could be formulated by changing the structure and combination of cations and anions. Additionally, ILs could be used as solvents for synthesis of metal catalysis. Because of their the good solubility of inorganics as well as organics, large molecular size, and ionic nature, ILs will play versatile roles in the synthesis, e.g., as solvents, protecting agents, and structure-directing agents. Moreover, specific ILs own the ability to coordinate with metal center, thus facilitating the metal complex-catalyzed reaction with the presence of ILs as solvents or functional ligands. In this chapter, several kinds of important reactions catalyzed by ILs or IL containing systems are discussed. The versatility of ILs is demonstrated by their catalyzing reactions in different disciplines, such as (1) acid
base reactions (e.g., alkylation, esterification, and cycloaddition) based on the inherent catalytic activity of ILs; and (2) redox reactions (e.g., hydrogenation, oxidation, and hydroformylation) using ILs as additives to enhance the performances. The pioneering and potential applications of IL catalysis in the chemical industry are also highlighted to encourage largescale and practical utilization of ILs in future efforts in chemical engineering.

4.2 Oxidation Reactions in Ionic Liquids Oxidation reactions have been widely studied in ILs [1
7], and in many cases, the solvent cannot be considered inert, as the formation and stability of radical species can be strongly affected by the presence of an ionic environment [5,6]. The rates of electron transfer are closely related to the ILs’ viscosities [8
10] but not limited to such a factor. For example, although it is still being debated vigorously, the structure-depended “solvent cage” [11,12], inherent electrostatic forces, and potential hydrogen bonding [13,14] possessed by ILs all have been shown to play crucial roles in radical activity. On this basis, ILs are interesting alternatives to classic solvents for oxidation reactions. Currently, a wide variety of ionic or peroxide oxidants can be used in the formulation of ILs, and many ILs thus exhibit Novel Catalytic and Separation Processes Based on Ionic Liquids. DOI: http://dx.doi.org/10.1016/B978-0-12-802027-2.00004-2 © 2017 Elsevier Inc. All rights reserved.

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112 Chapter 4 increased stability. A variety of catalysts have shown good or even enhanced activities when applied in ILs. In addition, organometallic, organic, and inorganic catalysts are being developed and can be applied in systems consisting of ILs.

4.2.1 Ionic Liquids as Solvents for Oxidation Reactions The organic reaction products can be easily extracted from ILs containing charged catalysts or a bi-phasic catalysis system using ILs as solvents. The ILs and the catalyst systems can be recycled and reused. 4.2.1.1 Aerobic oxidation using ionic liquids as solvents The oxidation of primary alcohols to their corresponding aldehydes or carboxylic acids and of secondary alcohols to ketones are fundamental synthetic transformations. Among the oxidation reactions using ILs, oxidation of alcohols is the most studied. ILs play important roles in these reactions, as solvents, catalysts, oxidants, and, in some cases, have bi-functions. ILs are also good solvents for other salts and are immiscible with many organic solvents. Seddon et al. [15] reported on the oxidation of alcohols to aldehydes catalyzed by palladium. They demonstrated that improvements in both the rate of partial oxidation of benzyl alcohol and the ease of isolation of the product benzaldehyde occur when the reaction is carried out in ILs instead of dimethyl sulfoxide (DMSO). The catalyst/IL system can be recycled, depending on the presence of chloride and the water content of the IL. The formation of [PdCl4] causes the formation of dibenzyl ether as a byproduct, and an excess of water leads to overoxidation to benzoic acid [15]. A recyclable system has been developed, in which there is no need for an extracting solvent and oxidation of nonactivated alcohols to ketones occurs with molecular oxygen in the presence of palladium(II) acetate in [C4mim][BF4]. Complete miscibility with the alcohol substrate at reaction temperature and clear phase separation of the derived ketone product at room temperature were observed. The IL mixture functions as an immobilization medium for the catalyst, which allows efficient recycling of the catalyst [16]. Farmer and Welton reported on [nPr4N][RuO4]/O2/CuCl in ILs based on a substituted imidazolium cation system for the selective oxidation of aliphatic and aromatic alcohols to aldehydes and ketones [17]. Products could be easily removed from the reaction mixture by extraction with diethyl ether [17]. Wolfson et al. reported on Ru(PPh3)3Cl2 catalysts in a two-ammonium-salt IL system. It does not require the addition of any cocatalyst, such as TEMPO (2,2,6,6-Tetramethyl-1-piperidinyloxy) or CuCl [18]. de Souza et al. developed a RuCl3 catalyst dissolved in the [C4mim][CF3(CF2)6COO] IL system [19]. A TEMPO-CuCl-catalyzed aerobic oxidation of primary and secondary alcohols to the corresponding aldehydes and ketones in the IL [C4mim][PF6] with no trace of overoxidation

Catalytic Reaction in Ionic Liquids 113 to carboxylic acids has been developed [20]. Jiang reported on a three-component catalytic system consisting of acetamido-TEMPO, copper(II) salt, and 4-dimethylaminopyridine (DMAP) with an IL as the solvent [21]. Oxidation of 2,3,6-trimethylphenol to trimethyl-1,4-benzoquinone with CuCl2 as catalyst in the IL medium of [C4mim][Cl] and n-butanol as cosolvent has been developed [22]. An oxotetracuprate [Cu4(μ4-O)Cl10]24 is isolated as an active species [23]. Selectivity and conversion are significantly influenced by the anions of ILs [24]. The employment of a volatile and flammable organic solvent is eliminated [25]. Gold nanoparticles and Nhydroxyphthalimide in the IL [C4mim][BF4] is applied to oxidation of 1-phenylethanol [26]. Shen et al. reported on a series of photocatalysts HPW/MCM-41 with enhanced activity in the presence of ILs [27]. Jiang developed aerobic oxidation of alcohols into their corresponding aldehydes or ketones by using a two-component system—VO(acac)2/DABCO—in the IL [C4mim][PF6] [28]. However, in the presence of Cu(II)2-ethylhexanoate as cocatalyst in [Hmim][OTf] solvent, activated alcohols can be selectively oxidized into acids [29]. The transition-metal-free aerobic oxidation of benzylic alcohol is accelerated by a [C4mim] [PF6]/PhCF3 bi-phasic system and Cs2CO3 to obtain a good yield of the corresponding ketones [30]. Ren et al. reported on a task-specific IL (TSIL), monoethanolaminium lactate ([MEA]L), used to study the absorption of SO2 and oxidation of SO2 by O2 in simulated flue gases with and without ash and activated carbon in [MEA]L [31]. Singh et al. reported on the oxidative coupling of alkyl, aryl, and heteroaryl thiols with atmospheric oxygen. This methodology utilizes [C4mim][BF4] as a recyclable solvent and does not require support materials or metal salts. Excellent yields of symmetric disulfides are obtained[32]. Thurow et al. reported the use of [C4mim][SeO2(OCH3)] in the synthesis of symmetric disulfides starting from thiols. This method is common for aromatic, aliphatic, and functionalized thiols to obtain disulfide [33]. Chauhan et al. reported on the procedure for catalyst solubility, recycling, and easy product isolation in the oxidation of thiols to disulfides with molecular oxygen catalyzed by cobalt(II) phthalocyanines dissolved in an IL at room temperature [34]. Pomelli reported that in the reaction of the singlet oxygen (1O2) with thioethers, 1,3-dialkylimidazolium cations are able to stabilize the persulfoxide intermediate by hydrogen bonding on its peroxy appendage, thus characterizing them as protic-like species. Such a stabilization suppresses the unproductive intersystem crossing process, thus favoring the competitive sulfoxide product formation [35]. Lu et al. reported on a nonmetal catalytic system consisting of N,N,Nw, -trihydroxyisocyanuric acid (THICA) and dimethylglyoxime (DMG) for the selective oxidation of toluene derivatives with dioxygen in a PEG-1000-based di-cationic acidic IL to

114 Chapter 4 corresponding acids under mild conditions [36]. Fig. 4.1 shows a possible reaction mechanism. Meng et al. reported on the liquid-phase oxidation of toluene. ILs with strong polarity substantially improved the reaction performance by increasing both toluene conversion and benzaldehyde selectivity. In contrast, the conversion of toluene was low when hydrophobic ILs, such as[C4mim][PF6], were used as reaction media [37]. Chrobok reported on an efficient method for the synthesis of lactones, which involves the application of an oxygen/benzaldehyde system as the oxidant and ILs as solvents [38]. N-Hydroxyphthalimide (NHPI) and its ionic derivative, 3-pyridinylmethyl-Nhydroxyphthalimide have better performance in the IL [C4mim][PF6] compared with conventional organic solvents for cobalt-catalyzed aerobic oxidation of N-alkylamides to imides [39]. The aerobic oxidation of 5-(hydroxy-methyl)furfural is performed over solid ruthenium hydroxide catalysts in ILs. Several different catalyst supports have been tested in combination with various ILs. The best result was obtained in [C2mim][OAc] at 100 C with 30 bar of oxygen over Ru(OH)x/La2O3 which afforded 48% of 2,5-furandicarboxylic acid and 12% of 5-hydro-xymethyl-2-furancarboxylic acid [40]. Several aromatic aldehydes are oxidized by using the catalyst [Ni(acac)2] and dioxygen at atmospheric pressure in the IL [C4mim][PF6]. The catalyst and the IL could be recycled after extraction of the carboxylic acid product [41]. Lignin is a component of lignocellulosic biomass from which important aromatic compounds can potentially be obtained. Alcell and soda lignin can be dissolved in the IL [C2mim][DEP] and subsequently oxidized by using several transition metal catalysts and molecular oxygen. CoCl2  6H2O proves particularly effective for the oxidation. The catalyst rapidly oxidizes benzyl and other alcohol functionalities in lignin but leaves phenolic functionality and

Figure 4.1 Possible reaction mechanism for the catalytic cycle of the THICA/DMG system [36].

Catalytic Reaction in Ionic Liquids 115 phenylcoumaran linkages intact [42]. Spectroscopic investigations of the complexes involved during this process have been conducted using in situ attenuated total reflection
infrared (ATR-IR) spectroscopy, Raman spectroscopy, and ultraviolet
visible (UV
Vis) spectroscopy. The reaction proceeded via the coordination of alcohol-containing substrates to the Co followed by formation of a Co-superoxo-species. The presence of hydroxide is necessary for coordination of the alcohol to occur. Hydrogen peroxide (H2O2) that forms as a reaction byproduct undergoes rapid disproportionation to yield water and molecular oxygen. The properties of the IL greatly influence the catalytic activity both by stabilizing reactive intermediates and by favoring the coordination of the substrate to the cobalt over the direct oxidation of the cobalt without substrate [43]. A clean and environmentally friendly strategy for overall utilization of lignin and preparation of an aromatic aldehyde has been developed by using an IL as a reversible medium coupled with the separation process, which prevents the aromatic aldehyde products from oxidizing and increases their yields [44]. 4.2.1.2 Oxidations using chemical oxidants using ionic liquids as solvents Alcohols are oxidized with H2O2 in various ILs catalyzed by a phosphotungstate complex [45], IL-supported selenium reagents [46], salicylaldehyde amino acid Schiff-based manganese ligand [47], and TEMPO [48]. In addition, the combination of ILs and microwaves can activate H2O2 as an alternative for transition metal-catalyzed oxidation reactions [49]. With hypervalent iodine reagents, alcohols undergo oxidation in ILs at room temperature. With iodoxybenzoic acid (IBX) as oxidant in the [C4mim][Cl]/water system, alcohols give corresponding carbonyl compounds [50]. With IBX or with Dess-Martin-Periodinane (DMP) in hydrophilic [C4mim][BF4] or hydrophobic [C4mim][PF6], oxidations are faster compared with oxidations with conventional solvents [51]. Recycling and reuse of oxidants and ILs have also been reported. Qian synthesized a new oxidant, ion-supported hypervalent iodine(iii) reagent 1-(4-diacetoxyiodobenzyl)-3-methylimidazolium tetrafluoroborate, and demonstrated that it could selectively oxidize alcohols to aldehyde using the IL [C2mim][BF4] as solvent [52] (Fig. 4.2). Using sodium hypochloride as an oxidant, benzyl alcohols can be oxidized in ILs, such as [C2mim][BF4] [53], [C4mim][BF4], and an IL containing cyclic guanidinium cation [54]. These ILs act as both phase-transfer catalysts and solvents, and the catalytic system can be recycled and reused. Using N-bromosuccinimide (NBS) in [C4mim][BF4], oxidation of benzyl alcohols to the corresponding carbonyl compounds are carried out with [55] or without a base [56]. In addition to the above oxidant, KIO4 [57], KMnO4 [58], peracetic acid (PAA) [59], n-Bu4NHSO5 [60] and t-BuOOH [61
63] in various ILs can also oxidize alcohols to the corresponding carbonyl compounds.

116 Chapter 4

Figure 4.2 Selective oxidation by [dibmim][BF4]2 in [C2mim] [BF4] of alcohol substrates to carbonyl compounds [52].

Zhu et al. reported on surfactant-type polyoxometalate-based ILs, such as [(n-C12H25)3NCH3]3[PO4[MoO(O2)2]4], [(n-C8H17)3NCH3]3[PO4[WO(O2)2]4], and [(n-C12H25)3NCH3]3[PO4[WO(O2)2]4], for oxidative desulfurization using H2O2 [64]. Lu¨ et al. reported on a deep desulfurization process for diesel, based on extraction using [C4mim][PF6] [65]. Yansheng et al. reported that dibenzothiophene (DBT) could be effectively extracted from a model oil using the acidic IL 1-ethyl-3-(4-sulfobutyl)imidazolium bis (trifluoromethanesulfonyl)imide ([EimC4SO3H]NTf2) as the extractant and catalyst in the presence of H2O2 or NaClO as oxidant [66]. Li et al. reported on oxidative desulfurization of DBT in n-octane by the IL Me3NCH2C6H5Cl  2ZnCl2. DBT in the oil phase was extracted into the IL phase and then oxidized to its corresponding sulfone by H2O2 and equal volume of acetic acid [67]. Zhu et al. reported on a temperature-responsive magnetic IL [C4py][FeCl4] for oxidative desulfurization of fuels using H2O2 [68]. Mota et al. reported that VO(X-acac)2 complexes have high solubility in ILs and are good solvatochromic probes [69]. Zhu et al. reported that the extraction desulfurization (EDS) and extraction combined with oxidation desulfurization (EODS) for the removal of DBT, benzothiophene (BT), and 4,6-dimethyldibenzothiphenein, a model oil, were carried out in Fenton-like ILs, such as [Et3NHCl]FeCl3, [Et3NHCl]CuCl2, [Et3NHCl]ZnCl2, [Et3NHCl] CoCl2, [Et3NHCl]SnCl2, and [Et3NHCl]CrCl3 [70]. Zhu et al. reported that a tungstencontaining TSIL [(C6H13)3PC14H29]2W6O19 was synthesized and applied in the desulfurization process of a DBT-containing model oil with aqueous H2O2 [71]. Zhao et al. reported on a photochemical oxidation and [C4mim][PF6] extraction coupling technique for deep desulfurization of light oil in the presence of photo oxidant H2O2 [72]. Zhang et al. reported on phosphotungstic acid supported ceria (HPW-CeO2) combined with [C8mim] [BF4], for the removal of DBT by using H2O2 as the oxidant under mild reaction conditions, which could reach a sulfur removal of 99.4% [73]. Lo et al. reported on the room temperature ILs (RTILs) [C4mim][PF6] and [C4mim][BF4] used for extraction of sulfur-containing compounds in light oils into RTILs [74]. Ma et al. reported on an

Catalytic Reaction in Ionic Liquids 117 oxidative desulfurization technique via dielectric barrier discharge plasma oxidation by using MnO2 catalysts and a combination of [C4mim][OAc] extraction [75]. Zhang et al. reported on the use of the Bro¨nsted acidic ILs [CH2COOHPy][HSO4] and [(CH2)2COOHPy][HSO4] as the extractant and catalyst for the extraction
oxidation desulfurization of model oil [76]. Tang et al. reported on chemo-selective oxidation and chlorination of aryltrifluoromethylsulfide using trichloroisocyanuric acid (TCCA) in an IL [77]. Zhang et al. reported on oxidation of sulfides to sulfoxides at room temperature in the absence of a catalyst in the IL [C4mim][BF4] using aqueous H2O2 (35%) as oxidant [78]. Hajipour et al. reported on the oxidation of sulfides to the corresponding sulfoxides using ceric ammonium nitrate in the presence of methyl imidazolium hydrogen sulfate as Bro¨nsted acidic IL ([Hmim][HSO4) as the solvent under mild conditions [79]. Hajipour et al. reported on the oxidation of thiols to the corresponding disulfides using potassium persulfate (K2S2O8) in the IL [C4mim][Br] at 65
70 C [80]. Cimpeanu et al. reported on Ti-SBA-15 and UL-TS-1 catalysts with 1% and 1.5% Ti for the liquid-phase sulfoxidation of 4,6-dimethyl-2-thiomethyl-pyrimidine in a range of water-miscible and water-immiscible ILs and molecular solvents [81]. Cimpeanu et al. reported on oxidation of various aliphatic, aromatic, and heteroaromatic thioethers carried out in a range of ILs, [C4mim][CF3COO], [C4mpyr][NTf2], [C4mim][NTf2], [C4mim][BF4], and [C4dmim][NTf2], using sol-gel prepared mixed oxide silica tantalum and tantalum grafted MCM-41 catalysts [82]. Ternois et al. reported that oxidation of 2-(benzhydrylthio)acetic acid and its derivatives was performed with various catalytic and stoichiometric enantio-selective reagents and that the best results was obtained with stoichiometric chiral oxaziridine. The use of [C4mim][PF6] as solvent with oxaziridine gave slightly higher yields [83]. Cimpeanu et al. reported on heterogeneous catalytic oxidation of a series of thioethers (2-thio-methylpyrimidine, 2thiomethyl-4,6-di-methyl-pyrimidine, 2-thiobenzylpyrimi-dine, 2-thiobenzyl-4,6dimethylpyrimi-dine, thioanisole, and n-heptyl methylsulfide) in ILs by using MCM-41 and UVM-type mesoporous catalysts containing Ti, or Ti and Ge, and using anhydrous H2O2 or the urea H2O2adduct [84]. Liu et al. reported that ILs not only can be used as solvent for the reuse of the catalyst but also can be beneficial for epoxidation of olefins catalyzed by [C4mim]3PW12O40 and H2O2 [85]. Chatel et al. reported on the H2O2/NaHCO3/imidazole/Mn(TPP)OAc oxidation system and MOPyrroNTf2 IL combined under ultrasonic irradiation to give an exceptionally favorable environment for Mn(TPP)Oac-catalyzed olefin oxidations [86]. Teixeira et al. reported on enantio-selective epoxidation of 6-cyano-2,2-dimethylchromene (Chrom) catalyzed by the Jacobsen catalyst, using sodium hypochlorite (NaOCl) as the oxygen source, at room temperature, in a series of 1,3-dialkylimidazolium and tetra-alkyldimethylguanidium-based ILs [87]. Owens et al. reported on the kinetics of oxygen atom transfer from the peroxo-complexes of methyltrioxorhenium (MTO) to alkenes in ILs [88]. Brito et al. reported on monometallic dioxomolybdenum complexes, containing bis

118 Chapter 4 (oxazoline) and oxazolinyl-pyridine as catalytic precursors for alkenes epoxidation [89]. Bortolini et al. reported on epoxidation of electron-deficient olefins, in particular vitamin K3 and its analogs, with aqueous basic solutions of H2O2 in different ambient-temperature ILs, such as [C4mim][BF4] [90]. Saladino et al. reported on the efficient and high-yielding domino-epoxidation
methanolysis of glycals under environment-friendly conditions by oxidation with urea H2O2 adduct (UHP) and H2O2 in ILs catalyzed by methyltrioxorhenium and different heterogeneous methyltrioxorhenium derivatives ([OTf]2, [PF6]2, [NTf2]2) [91]. Kumar reported on epoxidation of alkenes with H2O2 catalyzed by a new tungsten catalyst, 1-methyl-3butylimidazolium decatungstate, in the IL [C4mim][BF4] [92]. Herbert et al. reported on molybdenum(VI) compounds as good catalysts in the epoxidation of cyclooctene with the UHP in the IL [C4mim][PF6] [93]. Pinto et al. reported on symmetric epoxidation of limonene using Jacobsen’s catalyst dissolved in the IL [C4mim][BF4] and with H2O2 oxidant [94]. Li et al. reported on oxidation by using an electron-deficient manganese(III) porphyrin catalyst in combination with iodobenzene diacetate in an RTIL [95]. Tangestaninejad et al. reported on epoxidation of alkenes with sodium periodate catalyzed by tetraphenylporphyrinatomanganese (III) chloride, Mn(TPP)Cl, and octabromotetraphenylporphyrinatomanganese(III)chloride, Mn (Br8TPP)Cl, using the IL[C4mim][BF4] [96]. Crosthwaite et al. reported on ILs used as cosolvents for the N-alkyl-3,4-dihyroisoquinoliunium-catalyzed epoxidation of alkenes [97]. Li et al. reported on catalytic epoxidation of alkenes by iodobenzene diacetate and manganeseporphyrin in the environmentally benign and ambient-temperature IL [C4mim][PF6] [98]. Chiappe et al. reported on a series of hydrophilic N,N-dimethylpyrrolidinium- and N, N-dimethylpiperidinium-based ILs for the oxidation of styrene with H2O2 using PdCl2 as catalyst [99]. Chen et al. found that the IL [C8mim][Cl] could effectively intensify cyclohexanol oxidation, which resulted in 100% conversion of cyclohexanol with 100% selectivity to cyclohexanone using H2O2 as oxidant and WO3 as catalyst [100]. Yongqi et al. reported on heterogeneous oxidation of cyclohexane by tert-butyl hydroperoxide (TBHP) carried out over ZSM-5 catalysts with different Si/Al ratios in ILs and organic molecular solvents. Higher yield and selectivity of the desired products were found in ILs than in molecular solvents [101]. Jianying et al. reported on oxidation of cyclohexane with TBHP could be catalyzed by titanium silicalite-1 (TS-1) in the ambienttemperature IL [C2mim][BF4] [102]. Wang et al. reported on metal-containing ZSM-5 (MZSM-5) molecular sieves for cyclohexane oxidation with TBHP in the IL [C2mim][BF4] under mild conditions [103]. Gago et al. reported on a CuCl2 complex with a pyridylethanimine ligand prepared and examined as a catalyst for the oxidation of ethylbenzene with TBHP at 30 C, using acetonitrile or the IL [C4mim][PF6] as solvent [104]. Kotlewska et al. reported on epoxidation and Baeyer
Villiger oxidation of olefins and (cyclic) ketones in hydrogen-bond-donating (HBD) ILs, using a lipase-catalyzed cascade and

Catalytic Reaction in Ionic Liquids 119 H2O2 as the terminal oxidant [105]. Conte et al. reported on Pt(II)-catalyzed Baeyer
Villiger oxidation of cyclohexanone with H2O2 in a two-phase system, H2O
IL [106]. Panchgalle et al. reported on oxidation of aryl ketones to esters at room temperature with 30% aqueous H2O2 and catalytic Sn-b-molecular sieve in an IL [107]. Baj et al. reported on cyclic ketones oxidized with H2O2 using acidic ILs as solvents [108]. Chrobok reported on cyclic and linear ketones oxidation with oxone at 40 C with ILs as solvents [109]. Baj et al. reported on lactone synthesis with bis(trimethylsilyl) peroxide as oxidant and ILs as solvents [110]. Rodrıguez et al. reported on the use of an isolated thermostable Baeyer
Villiger monooxygenase (phenylacetone monooxygenase [PAMO]) in the presence of ILs as an oxidative enzyme [111]. Baj et al. reported on silyl peroxides as oxidants in the Baeyer
Villiger oxidation of cyclic ketones in chloroaluminate(III) ILs [112]. Dake et al. reported on mild and selective oxidation of aryl halides to corresponding aldehydes using iodoxybenzoic acid as oxidizing agent [113]. Hu et al. reported on oxidation of organic halides to aldehydes and ketones with H5IO6 in the IL [C12mim][FeCl4] [114]. Khumraksa et al. reported on conversion of organic halides into carbonyl derivatives via microwave-assisted oxidation with N-methylmorpholine N-oxide in an IL [115]. Water-soluble iron(III) porphyrins and phosphotungstic acid in the IL [C4mim][BF4] are effective catalysts for the H2O2-mediated oxidation of the C 5 NOH bond in Nhydroxyarginine and other oximes. The oxidation products, carbonyl compounds, can be easily isolated, and the catalyst immobilized in the IL can be easily recycled and reused [116]. Two cyclic acetals, 2-methoxytetrahydropyran (MTP) and 2-ethoxytetrahydro-furan (ETF), are oxidized to hydroxy ester in the IL [C4mpyr]N[CN]2 by ozone [117]. Oxidation of hydroxylated and methoxylated benzaldehydes and acetophenones to the corresponding phenols by H2O2/methyltrioxorhenium is achieved in the ILs [C4mim][BF4] and [C4mim][PF6] [118]. The use of imidazolium ILs as solvents for organic transformations, with tetravalent cerium salts as oxidizing agents, has been evaluated. Good solubility is has been found for ammonium hexanitratocerate(IV) (cericammonium nitrate, CAN) and cerium(IV) triflate in 1-alkyl-3-methylimidazolium triflate ILs. Anhydrous cerium(IV) triflate transforms benzyl alcohol into dibenzyl ether, whereas hydrated cerium(IV) triflate yields benzaldehyde as the main reaction product. Reaction of ammonium hexanitratocerate(IV) with 1,4-hydroquinone results in 1,4-quinone, whereas anisole and naphthalene are nitrated [119]. [C4mim][Br] as an IL selectively promotes oxidation of alkyl arenes and alcohols to the corresponding carbonyl compounds with NaBrO3 at 70 C [120]. Oxidation of alkanes and alkenes are obtained by using two manganeses(III) porphyrin catalysts in combination with iodobenzene diacetate oxidant in the RTIL [C4mim][PF6].

120 Chapter 4 The effects of various organic solvents on the reactions and the role of axial ligands have been examined. A high-valent manganese-oxoporphyrin complex (MnV 5 O) is considered a reactive oxidation intermediate according to investigation by stopped-flow rapid spectroscopy [121]. The oxidation of various substrates by H2O2 with microwave activation in the presence of vanadium complexes in IL solvents have been reported [122]. Oxidation of cellobiose is realized by cellobiose dehydrogenase (CDH) in the hydrated IL choline dihydrogen phosphate (DHP). Both inter- and intraelectron transfers of CDH are observed in the hydrated choline dhp, suggesting a potential candidate of a nonaqueous solvent for enzymatic treatment of a depolymerized biomass [123]. Chloroperoxidase (CPO) enzyme from Caldariomyces fumago catalyzes oxidation of indole and thioanisole in reaction mixtures containing up to 40% (v/v) of different ILs. ILs containing tosylate, trifluoroacetate, chloride, and methylsulfate anions are suitable cosolvents for these transformations [124]. Oxidation of organophosphorus pesticides methyl-parathion with CPO enzyme in the presence of ILs as cosolvent is also reported. Among ILs with the same cation, those with PF6 exhibited strongest inhibition toward CPO [125]. A method for oxidation of flavanones to flavones has been developed by using [hydroxy (tosyloxy) iodo]benzene (HTIB) in the RTIL [C4bim][Br] [126]. Biomimetic oxidation of E- and Z-guggulsterones has been studied with H2O2 catalyzed by iron(III) porphyrins in [C4mim][BF4] [127]. 4.2.1.3 Electro-oxidations using ionic liquids as solvents TEMPO-mediated catalytic electrocatalytic oxidation of alcohols in the ILs [C4mim][PF6] [128], [C4mpy][NTf2] [129], and [C4mpyr][NTf2] [130] has been studied. TEMPO undergoes reversible redox in the IL medium, generating the catalytically active oxoammonium species that oxidizes alcohols to the corresponding aldehyde or ketone products. In the protic IL diethylmethylammonium trifluoromethanesulfonate [dema][OTf], The electrochemical oxidation of methanol (CH3OH) has been studied at Pt electrodes. The oxidation of trace H2O, which is unavoidably present in this liquid, provides the adsorbed oxygen species necessary for complete oxidation of CH3OH [131]. Kunihiro et al. reported that CH3OH could be oxidized to dimethoxymethane and proton at a Pt electrode in the hydrophobic IL [C4mpyr][NTf2] without water [132]. Electro-oxidation of benzyl alcohol is conducted by electrolysis in an IL/supercritical CO2 (IL/SCCO2) two-phase system in an undivided cell. It has been demonstrated that benzyl alcohol could be efficiently electro-oxidized to benzaldehyde. [C4mim][BF4] is a more effective medium than [C4mim][BF4] for the electro-oxidation of benzyl alcohol [133].

Catalytic Reaction in Ionic Liquids 121 Sun et al. reported on electro-oxidation of benzyl alcohol to benzaldehyde in homogeneous SCCO2/[C4mim][PF6]/MeCN solutions [134]. O’Mahony et al. reported on electrochemical oxidation of hydrogen sulfide gas (H2S) at a platinum microelectrode (10 μm diameter) in five RTILs): [C4mim][OTf], [C4dmim][NTf2], [C4mim][PF6], [C6mim][FAP], and [P14,6,6,6][FAP]. In four of the solvents, no clear oxidative peak was observed. In [C4mim][OTf], an adsorptive stripping peak was observed, followed by a steady-state signal at a more positive potential. No signal was observed on the reverse sweep [135]. Kuroiwa et al. reported on the spectroscopic and electrochemical properties of chlorophyll (Chl), an aggregate in mixed solvents of acetonitrile and the IL [C2mim][BF4] [136]. Gaillon et al. reported on electroassisted activation of molecular oxygen by Jacobsen’s epoxidation catalyst in an RTIL and observed the formation of the postulated high-valent manganese
oxoactive intermediate [137]. Ho et al. reported H2O2 generated in RTILs by electrolysis, which was then used for epoxidation of lipophilic alkenes under a carbon dioxide-saturated environment and in the presence of a catalytic amount of manganese salt [138]. Zhang et al. reported on the oxidation behavior of iodide by linear sweep voltammetry and cyclic voltammetry at a platinum electrode in the RTIL [C4mim][BF4] [139]. Yu et al. reported on the anodic processes of bromide and chloride ions on platinum electrodes in the IL [C4mim][PF6] hexafluorophosphate under the same conditions [140]. Rogers et al. reported on electrochemical oxidation of [C4mim][I], which has been investigated by cyclic voltammetry at a platinum microelectrode at varying concentrations in the RTIL [C4mim][NTf2] [141]. Allen et al. reported on oxidation of bromide by linear sweep and cyclic voltammetry at platinum electrodes in the RTIL [C4mim][NTf2], and the traditional solvent acetonitrile [142]. Zhang et al. reported on the oxidation behavior of chloride ions on platinum electrodes in the natural IL [C4mim][BF4] in the presence of high concentrations of [C4mim][Cl] [143]. Electrochemical oxidation of primary amine in the IL [C2mim][NTf2] is conducive to the modification of the electrode by the attachment of the organic layer onto the electrode surface. The use of ILs as media for the grafting leads to decrease of the surface concentration of the grafted layer in comparison with classic solvents, such as acetonitrile, and could provide better control of the grafted layer [144]. The IL [C2mim][N(Tf)2] has been proven to be a new electrochemical solvent. The direct electrochemical oxidation of ammonia has been examined in dimethyl formamide (DMF) and [C2mim][N(Tf)2]. The steady-state linear sweep voltammetric response for the direct oxidation of ammonia in both DMF and [C2mim][N(Tf)2] produces a linear response of limiting current against vol% ammonia over the range studied, thereby demonstrating the

122 Chapter 4 possible analytic utility of the approach [145]. A mechanistic study of the direct oxidation of ammonia has been reported in several RTILs, namely, [C4mim][BF4], [C4mim][OTf], [C2mim][NTf2], [C4mim][NTf2], and [C4mim][PF6]. Cyclic voltammetric analysis suggests that ammonia is initially oxidized to nitrogen and that protons, which are transferred to an ammonia molecule, form NH41 via the protonation of the anion(s) in four of the RTILs studied. In [C4mim][PF6], NH41 is formed first, followed by the protonated anion(s) [146]. The electrochemical oxidation of [C4mim][NO3] has been studied by cyclic voltammetry in the RTIL [C2mim][NTf2] [147]. The electrochemical oxidation of potassium nitrite has been studied in the RTIL [C2mim][NTf2] by cyclic voltammetry [148]. The electrochemical oxidation behaviors of hydroxypivalaldehyde have been studied in the ILs [C4mim][PF6], [C4mim][BF4], and [C8mim][PF6] at glass carbon electrode; it can be concluded that the electrochemical oxidation of hydroxypivalaldehyde consists of two successive one-electron irreversible reactions. The diffusion coefficients are determined and the minimum diffusion coefficient is in [C8mim][PF6] because of the higher viscosity [149]. Electrochemical oxidation of silver in the IL [C4mim][Br] has been studied. Two electrode processes irreversibly proceed on the silver electrode in the potential range: the formation of compound [C4mim][AgBr2], which is soluble in [C4mim][Br] and difficultly soluble in AgBr [150]. Electrochemical oxidation of copper in the IL [C4mim][Br] monohydrate has been studied. Two irreversible electrode processes occur in the potential range under study on a copper electrode in [C4mim][Br]  H2O. These processes are accompanied by formation of a well-soluble compound, [C4mim][CuBr2], which is rather stable under experimental conditions and is oxidized to CuBr2 at more positive potentials and upon interaction with atmospheric oxygen [151]. Electrochemical oxidation of metals in ILs can be regarded as a possible way to obtain ILs with complex metal-containing anions. Another study showed that [NTf2]2 can be easily decomposed during anodic oxidation of copper in the IL [C4mpyr][NTf2] at 70 C leading to formation of CuF2 [152]. The electrochemical behavior of ruthenocene, RuCp2, has been investigated in six different RTILs of varying viscosities. A process involving the one-electron oxidation of RuCp2 to the [RuCp2]1 monocation, followed by dimerization to form the [Cp2Ru
RuCp2]21 species has been speculated. It is believed that an ECE (electron transfer
chemical reaction
electron transfer) mechanism proceeds in which the electrogenerated [RuCp2]1 monocation undergoes a nucleophilic reaction with the anions of the IL media, in a chemical step that facilitates the transfer of a second electron [153]. The IL [C4mim][PF6] is utilized as electrolyte for the direct anodic oxidation electropolymerization of fluorene. The corresponding electroactive polyfluorene films have shown good redox activity and structural stability [154]. The electrochemical oxidation of hydrogen has been studied in a range of ILs [155
157]. The appearance and position of the reverse (reduction) peak on the voltammograms is

Catalytic Reaction in Ionic Liquids 123 thought to depend on three factors: the stability of the protonated anion, the position of equilibrium of the protonation reaction (related to the pKa), and any unusual follow-up chemistry (e.g., dissociation or reaction of the protonated anion) [158]. Results from studies have revealed diffusion coefficients (D) and solubilities of hydrogen in each IL, and no obvious relationship has been found between D and viscosity, suggesting that H2 is too small a molecule for the Stokes
Einstein relation to apply [159]. The detection of nicotinamide adenine dinucleotide (NADH) at the bare Pt electrode in RTILs has been achieved. The oxidation of NADH is examined in two RTILs, [C4mim] [PF6] and [C2mim][NTf2]. In [C2mim][NTf2], no oxidation for NADH is observed, which is attributed to the lack of interaction between the RTIL anion [NTf2]2 and the proton released if NADH is to be oxidized; in [C4mim][PF6], the two-electron oxidation of NADH to NAD1 is observed at 1.4 V versus Ag, where NADH is oxidized to form the radical cation NADH•1; [PF6]2 is thought to react with this species to form NAD• and HPF6, which likely degrades to HF and PF5 [160]. The cyclic voltammetry and the electrochemical impedance spectroscopy responses of pdoped poly(3,4-ethylenedioxythiophene) (PEDOT) electrodeposited on platinum electrode surface have been studied in the RTIL [C2mim][NTf2] [161].

4.2.2 Ionic Liquids as Catalysts for Oxidation Reactions 4.2.2.1 Homogeneous ionic liquids as catalysts for oxidation reactions IL-supported TEMPO is synthesized and applied as catalyst for selective aerobic oxidation of aromatic alcohols. With bis(acetoxy)iodobenzene (BAIB) as oxidant and IL-CLICKTEMPO as catalyst, alcohols are oxidized in CH2Cl2 under mild conditions. The advantages of IL-CLICK-TEMPO over free TEMPO are simplified workup procedure and easy recovery and recycling [162] The bi-magnetic IL [Imim-TEMPO][FeCl4] with cooperative functionalities is also applied for selective aerobic oxidation of aromatic alcohols with molecular oxygen (Fig. 4.3) [163]. IL-immobilized TEMPO (TEMPO-IL) catalyst and CuCl as a cocatalyst system for oxidation of alcohols to the corresponding aldehydes or ketones with molecular oxygen under solvent-free conditions has been developed [164]. A temperature-dependent catalytic system composed of a TEMPO-functionalized imidazolium salt [Imim-PEG600-TEMPO][OMS]/NaNO2/O2 in mixed solvent of cyclohexane and carbon tetrachloride is applied to the selective oxidation of alcohols. The homogeneous catalyst [Imim-PEG600-TEMPO][OMS] can be recovered by simple decantation after reaction [165]. Alcohol oxidation catalyzed by an L-aspartic acid-coupled, imidazolium-based IL is achieved by using H2O2 [166]. Nonvolatile and odorless organosulfur compounds anchored onto an imidazolium-IL scaffold has been synthesized. Sulfoxides can be used for the oxidation of primary allylic and benzylic alcohols to aldehydes and secondary alcohols to

124 Chapter 4

Figure 4.3 Fe/TEMPO-based bi-magnetic IL with cooperative functionalities.

ketones under Swern oxidation conditions, and the corresponding sulfides can be recovered and recycled [167]. The oxidation reaction of benzylic alcohols with trichloroisocyanuric acid (TCCA) and [C4mim][BF4] in water has also been reported [168]. P- or N-containing ligand-functionalized, imidazolium-based ILs are used in RuCl3  3H2O catalyzed aerobic oxidation of various alcohols in imidazolium-based IL solvents [169]. An imidazolium-IL-grafted 2,20 -bipyridine ligand can be employed in the copper-catalyzed selective oxidation of alcohols to the corresponding carbonyl compounds in the IL [C4mim] [PF6] [170]. The aerobic oxidation of 1-phenylethanol to acetophenone over a carbon nanotubesupported palladium catalyst is improved with the IL additive [C2mim][NTf2]. The enhanced solubility of both alcohol substrate and gaseous O2 in [C2mim][NTf2] leads to a better substrate
catalyst contact [171]. An imidazolium-IL bearing two C11 alkenyl chains with terminal CQC double bonds is a monomer that undergoes self-assembly in water and gives cell-like polymeric liposomes after polymerization. The internal location of the imidazolium units and their ion pairs with AuCl2 4 ions form gold nanoparticles inside the polymeric structure. The resulting material exhibits notable catalytic activity for the selective aerobic oxidation of 2-hydroxybenzyl alcohol to salicylaldehyde [172]. Cross-linked poly(1-butyl-3-vinylimidazolium bromide) microspheres with the diameter of about 200 nm are synthesized via mini-emulsion polymerization and used as support to synthesis Pt nanoparticle hybrids. The catalytic performance of PIL/Pt shows better electrocatalytic activity toward electro-oxidation of methanol compared with pure Pt nanoparticles. Furthermore, they are effective and easily reusable catalysts for the selective oxidation of benzyl alcohol in aqueous reaction media [173]. Zhao et al. reported on ILs based on pyridinium cations as phase-transfer catalysts (PTCs) for phase-transfer catalytic oxidation of DBT dissolved in n-octane [174]. Zhu et al. reported that a extraction and catalytic oxidative desulfurization (ECODS) system

Catalytic Reaction in Ionic Liquids 125 composed of VO(acac)2, 30% H2O2, and [C4mim][BF4] was suitable for the deep removal of DBT in model oil at room temperature [175]. Wang et al. reported that three 4dimethylaminopyridinium-based ILs, [C24DMAPy][N(CN)2], [C44DMAPy][N(CN)2], and [C64DMAPy][N(CN)2], were synthesized and demonstrated to be efficient for extraction of aromatic sulfur compounds from fuels [176]. Liu et al. reported on a halogen-free TSIL of [(CH2)2COOHmim][HSO4] applied as a catalyst and reaction medium for deep oxidative desulfurization of real diesel [177]. Nejad et al. reported on the use of [C4mim][OCSO4] and [C2mim][EtSO4] for the removal of aromatic sulfur compounds, such as benzothiophene and thiophene, from a model of gasoline [178]. Liang et al. reported on acetic acid-based ILs as both catalyst and extractant for oxidative desulfurization [179]. Reddy et al. reported on the quantitative oxidation of organic sulfides to sulfones with 30% aqueous H2O2 (3:1 molar ratio of H2O2/sulfide) at room temperature by using the known titanium alkoxide Ti4[(OCH2)3CMe]2(i-PrO)10 as a catalyst [180]. Wang et al. reported on a bi-functional IL, bis-[N-(propyl-1-sulfoacid)-pyridinium] hexafluorotitanate, as a recyclable catalyst for room temperature sulfoxidation of sulfides using H2O2 as oxidant [181]. Zhang et al. reported on imidazolium perrhenate ionic liquids (IPILs) as efficient catalysts for the oxidation of sulfides to sulfones with aqueous H2O2 under mild conditions. Good to excellent yields of various sulfones have been obtained. IPILs are stable and can be reused at least 10 times without detectable activity loss [182]. Bigi et al. reported that catalytic ILs containing chiral tungstate(VI) anions give enantiomeric excesses up to 96% in sulfide oxidation to sulfoxides [183]. Li et al. reported on the use of peroxopolyoxometalate-based RTIL catalyst for efficient epoxidation of various olefins [184]. Zhang et al. reported that an ionic metalloporphyrin of manganese tetrakis-(4-N-trimethylaminophenyl)porphyrin hexafluorophosphate ([MnIIITTMAPP][PF6]5) residing in the mixed ILs of [Bzmim][BF4] and [C4mim][BF4] proved to be an efficient and recyclable catalytic system for styrene (derivative) epoxidations without the involvement of the auxiliary axial ligands [185]. Tan et al. reported on polymeric IL-functionalized chiral salen ligand as catalyst in the enantioselective epoxidation of styrene [186]. Lu et al. reported on dialkyl imidazolium-metal chloride ILs as catalysts in selective oxidation of toluene with H2O2 as oxidant. Their catalytic activities were compared in the absence of any organic solvents [187]. Xu et al. reported on oxidation of 1,3diisopropylbenzene to corresponding hydroperoxides and its derivatives by using the IL [C4mim][OH] [188]. Liu et al. reported on the functionalized IL combined with cationic (tetrakis(N-methyl-4-pyridinium)porphyrina-to)manganese(III) and anionic catalyst for ethylbenzene (derivative) oxidations without involvement of the auxiliary axial ligands. A synergetic catalytic effect between the cations and the counteranions is observed in terms of activity and stability in this functionalized IL [189]. Chrobok et al. reported on the IL 1-methyl-3-(triethoxysilylpropyl)imidazolium hydrogen sulfate as acidic catalyst for the Baeyer
Villiger reaction [190].

126 Chapter 4 A three-component condensation reaction between 2-aminobenzophenone derivatives, formaldehyde or aromatic aldehydes, and ammonium acetate efficiently provides substituted quinazolines in a one-pot reaction in the presence of the Bro¨nsted acidic IL [Hmim][TFA] in conjunction with aerobic oxidation. The IL is separated from the reaction mixture by simple extraction and is recycled three times without considerable loss in activity [191]. A new kind of polyoxometalateionic liquid ([(CH3)3NCH2CH2OH]5PV2Mo10O40) has been synthesized by using a precipitation/ion exchange method, with choline chloride and H5PMo10V2O40 as precursors. The produced salt turns out to be an original IL catalyst for the oxidation of starch. Elevated temperature leads to the miscibility of catalyst and substrate, and when the temperature is decreased, the catalyst precipitates and becomes a heterogeneous form that is separated automatically from the reaction mixture [192]. 4.2.2.2 Supported ionic liquids as catalysts for oxidation reactions Several supported IL heterogeneous catalysis systems have been introduced and applied to the selective oxidation of alcohols. The advantages of these systems are not only high conversion and selectivity but also easy recovery and good reusability. A tungsten-containing POM [α-PW12O40]32 immobilized on IL-modified polystyrene resin beads to heterogeneous catalyst has been developed. In this heterogeneous catalytic system, alcohols can be efficiently oxidized to corresponding carbonyl groups with 30% aqueous H2O2 as oxidant in a CH3CN solution [193]. A di-cationic IL-modified phosphotungstate hybrid catalyst 1,10 -(butane-1,4-diyl)-bis(3-methylimidazolium) phosphotungstate [[C1mim]1.5PW10O40] has been developed as catalyst for H2O2-based oxidation of alcohols [194]. Various organic
inorganic hybrid compounds, [C4mim] 3 [PW11MO39].3H2O [M 5 V(IV), Cr(III), Mn(II), Fe(III), Co(II), Ni(II), and Cu(II), Zn(II)], are prepared and used as catalysts in oxidation of various alcohols with H2O2 in CH3CN. The effect of the transition metal on the catalytic activity is in the order: Zn . Fe . Ni . Cr . Co . V . Mn . Cu [195]. A pseudo-homogeneous process that makes the recovery and reuse of catalyst achievable and efficient has been reported for oxidation of alcohols. In this, a novel magnetic silica-supported bi-functional hybrid material combining TEMPO-based ILs and POMs moieties is used [196]. Heteropolyacid immobilized on IL-modified mesoporous silica SBA-15 catalyst (V2ILSBA) catalyzes aerobic oxidation of alcohols in CH3CN. The activities and product selectivities are comparable with those of the homogeneous analog [197,198]. A supported IL catalyst system of TEMPO-IL/CuCl2/silica has been developed and used for aerobic alcohol oxidation. CuCl2 acts as a homogeneous catalyst which is dissolved in a small quantity of IL and dispersed in the form of film on the solid silica support. [C4mim] [OCSO4] has been observed to be the most active “catalyst-philic” phase [199].

Catalytic Reaction in Ionic Liquids 127 A silica-gel-supported TSIL catalyst, TEMPO-IL/CuCl2/SiO2, is prepared by using the solgel technique and has been proven effective for aerobic oxidation of alcohols. The immobilization of ILs results in a more intimate bi-phasic system. The solid matrix forms a porous prison, which prevents ILs or transition metal catalysts from leaching but allows the free transportation of reactants and products [200]. Silica-supported IL catalyst doped with perruthenate is applied to aerobic oxidation of alcohols in SCCO2 [201]. The SBA-15-functionalized, TEMPO-confined IL [C4mim]Br is reported to be an efficient catalyst system for transition-metal-free aerobic oxidation of alcohols [202]. The strong physical confinement of the IL inside the mesochannels of SBA-15-supported TEMPO improves selectivity in the aerobic oxidation of allylic alcohols. Tang et al. reported on the [C4mim]PW/HMS catalyst synthesized through impregnation of the hexagonal mesoporous silica (HMS) support by HPW and the IL [C4mim]HSO4 [203]. Shi et al. reported on an environmentally benign catalyst (peroxotungstates immobilized on multilayer IL brushes-modified silica) for the selective oxidation of sulfides to corresponding sulfoxides and sulfones with dilute H2O2 [204]. Zhao et al. reported that the imidazolium POM salts are very active and selective heterogeneous catalysts for oxidations of sulfides, with the additional advantages of convenient recovery, steady reuse, simple preparation, and flexible composition [205]. Zhao et al. reported on a novel heterogeneous process for selective oxidation of sulfides with aqueous H2O2, catalyzed by alkyl-tethered, imidazolium-IL-based POM salt catalysts [205]. Xian-Ying reported on recyclable catalysts of peroxotungstate immobilized on IL-modified silica, which provide mild reaction condition and excellent chemo-selectivity and thus are easy catalytic systems for the selective oxidation of sulfides to their corresponding sulfoxides and sulfones with commercially available 30% aqueous H2O2, an ideal oxidant [206]. Tan et al. reported on chiral oxovanadium (IV) Schiff base complex covalently grafted with [Apmim][BF4] for the enantio-selective oxidation of methyl aryl sulfides to sulfoxides with H2O2 as oxidant. Of note, the IL-functionalized complex could be recovered conveniently by simple precipitation with addition of hexane and reused for at least six cycles without loss of activity and enantio-selectivity [207]. Doherty et al. reported on a peroxophosphotungstate-based, polymer-immobilized IL phase as an efficient and recyclable catalyst system for the epoxidation of allylic alcohols and alkenes, with only a minor reduction in performance on successive cycles [208]. Du et al. reported on direct vapor-phase epoxidation of propylene in the presence of hydrogen and oxygen, and this system was studied at a space velocity of 7000 mL/h/g cat: over gold catalysts with varying gold and titanium contents prepared by an IL-enhanced immobilization method, in which biosynthesized gold nanoparticles (GNPs) were immobilized onto the titanium silicalite-1 (TS-1) supports through the assistance of a small amount of [C4mim][BF4] [209]. Yamaguchi et al. reported on peroxotungstate immobilized on dihydroimidazolium-based IL-modified

128 Chapter 4 SiO2 as an efficient heterogeneous epoxidation catalyst with H2O2 [210]. Hajian reported on a recyclable catalyst based on immobilization of vanadium polyoxometalate on IL-modified MCM-41, an efficient catalyst for epoxidation of alkenes with tert-BuOOH [211]. Liu reported on the ionic manganese porphyrin with a pyridinium tag embedded in the pyridinium-based IL [C4py][BF4] for oxidation of styrene and its derivatives under mild conditions [212]. Xuehui et al. reported on PdCl2/TSILs for catalytic oxidation of styrene to acetophenone with H2O2 through the combination of PdCl2 and TSILs of N-carboxylappended imidazolium cations with various anions [213]. Luo et al. reported on an oligomer of an IL containing imidazolium and disulfide groups, which is miscible with the IL [C4mim] [PF6] for the epoxidation of styrene [214].

4.3 Hydrogenation Reaction Hydrogenation is one of the most important reactions in chemistry [215]. However, the solubility H2 in most ILs is quite low, much lower than for molecular organic solvents, and is in the same range as that for water [216]. Despite the low solubility, several types of successful reactions have been found by employing ILs as the catalysts [5,217
220]. In most of these reports, the homogeneous nature of the catalyst was assumed. In this part, the hydrogenation reactions are summarized as three parts, heterogeneous reactions, homogeneous reactions and asymmetric hydrogenation and transfer hydrogenation reactions.

4.3.1 Heterogeneous Hydrogenation Reactions Catalysts are important for the hydrogenation reaction. As shown in Fig. 4.4, soluble and stable transition metal nanoparticles can be easily prepared in ILs by four methods, as summarized by Dupont et al. [222]: (1) reduction of compounds dissolved in ILs; (2) (A)

(B)

(C)

(D)

Figure 4.4 Examples of metal nanoparticle synthesis in ILs: (A) reduction of compounds dissolved in ILs; (B) decomposition of organometallic complexes in the formal zero oxidation state dissolved in ILs; (C) bombardment of bulk metal precursors with deposition onto ILs; (D) phase-transfer of preformed nanoparticles in water or organic solvents to ILs [221].

Catalytic Reaction in Ionic Liquids 129 decomposition of organometallic complexes in the formal zero oxidation state dissolved in ILs; (3) bombardment of bulk metal precursors with deposition onto ILs; and (4) phasetransfer of preformed nanoparticles in water or organic solvents to ILs [221]. (1) and (2) are the simplest methods to get metal nanoparticles of different sizes, since they involve the simple reaction of a metal compound precursor dissolved in an IL by hydrogen [223]. By using these two approaches, irregular metal nanoparticles are obtained, depending on factors such as reaction conditions (temperature, metal concentration, time, etc.) [224], the type of metal precursor and reducing agent [225], and ILs [226]. (3) appears to be one of the cleanest, since only the metal and the IL are used [227,228]. By this method, the size and shape of nanoparticles depend mainly on the surface structure of the IL and the current used [229]. The size of the nanoparticles increases with an augmentation in the current and also with the concentration of the nonpolar domains at the IL interface with the vacuum [230,231]. Well-defined sizes and shapes of nanoparticles can be prepared by (4), water or organic solvents [232], and then transferred to ILs for hydrogenation. It is believed that the surface of metal nanoparticles is electron-deficient. The electron-rich species present in the media will interact preferentially. The interactions of ILs with the metal nanoparticle surface are characterized by various kinds of devices [233
235] and indicate that ILs interact with the metal surface as aggregates rather than as isolated ions. Additionally, theoretical studies have suggested that metallic nanoparticles in ILs are preferentially surrounded by the charged moieties of ions, with an interface layer that is one ion thick [236]. Because of the low solubility of hydrogen in ILs, the catalytic hydrogenation performed in ILs usually contain at least two phases. One of the problems of catalytic processes in ILs is mass controlling. ILs can be used as immobilizing agents instead of solvents [237]. By this, simple thin films of ILs could give the desired properties for the catalyst and significantly reduce mass transfer problems, such as “solid catalysts with an IL layer,” as shown in Fig. 4.5 [238]. Such a system takes advantage of the tailor-made physico-chemical properties of ILs and their distinct potential to chemically interact with supported catalytic nanoparticles. High-selective hydrogenation of 1,3-butadiene to 1-butene by Pd nanoparticles immobilized in a thin film of [C4mim][BF4] or [C4mim][PF6] ILs has been reported [239]. The selectivity effects by employing solid catalysts with an IL layer are as yet unclear. Steinru¨ck thought [238] that the IL film may act as a differential reactant collector, which, depending on the solubility of individual reactants and products, may either enhance or suppress specific reaction channels. However, ILs may directly interact with the active catalytic sites, similar to what a ligand would do. One of the successful examples of hydrogenation employing metal nanoparticles is the hydrogenation of alkenes and arenes. It was thought that metal nanoparticles in ILs usually behave as typical homogeneous-like catalysts for hydrogenation [224,240,241]. Metal nanoparticles for the reaction can be formed by reduction of the metal center or

130 Chapter 4

Figure 4.5 Solid catalysts with an IL layer [238].

decomposition of transition metal compounds. As reported [242], cyclohexanone hydrogenation in ILs requires more time to complete the conversion because of the characteristic bi-phasic nature of catalysis as a result of the mass-controlled limitation. However, the system could be reused for up to 15 recharges without loss of catalyst activity. Although long reaction times are needed in bi-phasic catalysis when using ILs, the possibility of recovering and reusing the catalyst several times is an attractive aspect. Transmission electron microscopy (TEM) analysis of the particles dispersed in [C4mim] [NTf2] shows the presence of Ru nanoparticles of 2.1
3.5 nm in diameter. A 90% conversion of toluene has been observed in the nanoparticles generated in [C10mim][NTf2], which is a strong indication of heterogeneous catalysis [243].

4.3.2 Homogeneous Hydrogenation Reactions Since Wilkinson discovered the complex tris(triphenylphosphine)rhodium chloride (RhCl (PPh3)3), homogeneous catalytic hydrogenation has begun to attract great attention. When Knowls first developed homogeneous chiral catalyst and applied it in asymmetric synthesis in industrial scale successfully, research on homogeneous catalytic hydrogenation has advanced significantly [244]. Homogeneous catalytic hydrogenation is not only of importance in producing a large number of fine chemicals but also is very helpful to study catalysis and mechanism of hydrogenation reactions because the structures of homogeneous catalysts are usually much simpler and better defined than those of heterogeneous catalysts. Homogeneous catalytic hydrogenation has provided elementary knowledge and principles for understanding catalysis phenomena. However, for the traditional homogeneous hydrogenation system, difficulty in the separation and recycling of the catalyst limits its

Catalytic Reaction in Ionic Liquids 131 largescale application [245]. To solve this problem, bi-phasic solvent systems for homogeneous catalytic hydrogenation, such as aqueous
organic [246] or fluorous
organic, have been introduced. These bi-phasic solvent systems typically consist of a lower-phase solvent that dissolves the catalyst and an upper-phase solvent that carries the substrate into the reaction vessel and the products out. Later research has shown that these systems do not meet environmental requirements, that trace amounts of organic compounds in water are notoriously difficult to remove, and that the fluorous
organic systems also have problems with partial solubility of the catalyst in the organic phase. Thus, the ideal solvent system should be environmentally friendly, should be able to dissolve the homogeneous catalyst, and should be easily removable from the products. ILs are a type of novel green solvent that meet the above-mentioned conditions. The homogeneous hydrogenation catalysts used in ILs, first reported by Suarez, were based on rhodium complexes, such as RhCl(PPh3)3 and [Rh(cod)2][BF4](cod 5 cyclooctadiene), in hydrogenation of C 5 C bonds [247]. The two catalysts are completely soluble in [C4mim] [BF4] and [C4mim][PF6] and can catalyze the hydrogenation of cyclohexene at 10 atm and at room temperature, with turnovers up to 6000 h21. Then, [C4mim][BF4] continued to be used in hex-1-ene, cyclohexene, and 1,3-butadiene hydrogenation by the complex RuCl2(PPh3) and K3Co(CN)5, resulting in moderate activities and selectivities [248]. Use of [C4mim]3[Co(CN)5] to hydrogenate 1,3-butadiene leads to 100% selectivity for but-1-ene. It is important to note that the hydrogenation reaction product is removed from the IL system by simple decantation and that the transition metal complex catalysts are almost completely retained in the ILs. It is also worth mentioning that the recovered ionic purple solutions of the used catalyst show the same efficiency (activity and selectivity) upon reuse (at least four cycles). Subsequently, four different transition metal carbonyl cluster anions, [HFe (CO)11]2, [HWOs3(CO)14]2, [H3Os4(CO)12]2, and ½Ru6 CðCOÞ16 2 2 , were also evaluated as catalysts/precatalysts for the hydrogenation of alkenes in the IL [C4mim][BF4], octane, and methanol, and it was found that the activity of certain clusters immobilized in ILs is up to 3.6-fold faster than that observed in organic solvents [121]. Use of high-pressure nuclear magnetic resonance (NMR) spectroscopy has made it possible to trace the improvements in activity to the increased stability of the cluster species in the IL. The IL also gives rise to higher regio-selectivity in the hydrogenation of cyclic dienes to monoenes compared with that observed in organic solvents. The hydrogenation of arenes is an important industrial process, particularly for the generation of cleaner diesel fuels [249]. Although the catalytic system for this reaction is dominated by heterogeneous catalysts, homogeneous catalysts also have been studied to a significant extent, especially in the IL systems [250]. A complex cluster-IL system was designed and investigated for hydrogenation of arenes (benzene, toluene, cumene). The Rucluster ½H4 Ru4 ðη-C6 H6 Þ4 1 2 is a catalyst precursor, which can oxidatively add hydrogen to give the real hydrogenating species ½H6 Ru4 ðη-C6 H6 Þ41 2 and was found to hydrogenate

132 Chapter 4 arenes in [C4mim][BF4] with slightly higher time of flights (TOFs) compared with aqueous systems under mild reaction conditions [251]. Boxwell described another arene hydrogenation catalyst [Ru(η6-p-cymene)(η2-TRIPHOS)][Cl2], which is active in dichloromethane but considerably more active in the IL [C4mim][BF4] and, remarkably, can hydrogenate the arene ring of allylbenzene without hydrogenating the alkene bond. However, it is essentially inactive toward arenes with α-alkene substituents, such as styrene and 1,3-divinylbenzene [252]. The most typical homogeneous hydrogenation reaction is asymmetric hydrogenation because most of the chiral catalysts are metal complexes. Thus, some successful use of ILs has also been reported for the asymmetric hydrogenation of α-enamide esters [253
255], β-ketoesters [256,257], and aromatic ketones [258
260]. A key concern in asymmetric hydrogenation using ILs is the effective dissolution and immobilization of the chiral catalyst in the ILs to avoid catalyst loss while maintaining high catalytic activity and high stereo-selectivity [255]. Therefore, Guernik [253] immobilized the chiral metal complexes Rh-MeDuPHOS in ILs and hydrogenated methyl α-acetamidoacrylate and methyl α-acetamidocinnamate at 25 C and 2 atm H2 for 20 min. In the first run, the enantioselectivities obtained for the two substrates in [C4mim][PF6] (93% and 96%) were similar to those obtained in isopropanol (97% and 99%), but it should be noted that for further cycles, the activity of the catalyst in isopropanol decreased a lot, whereas it was still remarkable in [C4mim][PF6]. On the basis of the predecessors, Hu [257] designed and synthesized a family of highly tunable and enantio-selective Ru catalysts for the asymmetric hydrogenation of a wide range of β-aryl ketoesters by taking advantage of the remarkable effects of 4,40 -substituents on binap. It is evident from the results that the Ru catalyst based on 4,40 -TMS-binap successfully hydrogenated a variety of β-aryl ketoesters in the [C4mim] [BF4]/methanol system with complete conversions and very high ee (enantiomeric excess) values of 97.8
99.6% under 1400 psi of hydrogen gas at room temperature for 20 h. The asymmetric hydrogenation of α,β-unsaturated ketones catalyzed by the achiral ruthenium monophosphine complex di(triphenylphosphine trisulfonic acid sodiumruthenium) chloride complex [RuCl2(TPPTS)2] modified by (S, S)-DPENDS [disodium salt of sulfonated (S, S)1,2-diphenyl-1,2-ethylene-diamine] was investigated in the IL [RMIM][Ts](R 5 ethyl, butyl, octyl, dodecyl) by Wang et al [260]. Chemo-selectivity of 100% and 75.9% ee was obtained for benzalacetone under optimized conditions. The resulting products can be easily separated from the catalyst immobilized in the IL [C2mim]Ts by extraction with n-hexane, and the catalyst can be reused seven times without loss of catalytic activity and enantioselectivity. Of note, addition of water can improve the performance of the catalyst. For homogeneous hydrogenation in ILs, the reaction products often extracted adopt organic solvents from the catalyst-ILs system. Nevertheless, most organic solvents are volatile and/ or flammable, which may result in the “green solvents system” of ILs not being “green.” To address this issue, SCCO2 has received attention as a versatile, environmentally benign

Catalytic Reaction in Ionic Liquids 133

Figure 4.6 Metal-catalyzed organic reactions conducted in an IL and supercritical or compressed CO2 [262].

solvent partner with nonvolatile and fairly polar ILs for a variety of applications [261], especially in several metal-catalyzed organic reactions (Fig. 4.6) [262]. The success of this SCCO2/ILs system is based on the solubility of SCCO2 in the IL, which is controlled by pressure, but IL is not soluble in SCCO2. Liu et al. examined the use of SCCO2 extraction on the [C4mim][PF6]-mediated hydrogenation of but-1-ene and cyclohexene using the Wilkinson’s catalyst RhCl(PPh3)3 [263]. For facile hydrogenations, no activity advantages were found when using SCCO2 compared with hexane, but the protocol of using SCCO2 as the extractant was demonstrated and enabled the catalyst/IL system to be recycled four times with no loss in activity. From the above, it is evident that imidazole ILs, such as [C4mim][PF6] and [C4mim][BF4], are the most widely used ILs in transition metal catalytic reaction. This is mainly based on the advantages of [PF6]2 or [BF4]2 ILs, that is, their relatively large polarity and weak coordination. However, for [C4mim][BF4], it is difficult to completely eliminate impurities, such as chloride ions, in the preparation process by using the ion exchange method. Sometimes, the presence of chloride ions can lead to deactivation of transition metal complexes. Thus, the design and synthesis of different kinds of ILs to homogeneous catalysis hydrogenation are crucial.

4.4 Hydroformylation Olefin hydroformylation, which is also known as oxo-synthesis, is the simultaneous addition of hydrogen and carbon monoxide molecules (the mixture of hydrogen and carbon monoxide is known as syngas) across a carbon
carbon double bond of alkene. As a pure addition reaction, the hydroformylation reaction possesses 100% atom efficiency, which meets the requirement of “green chemistry.” The products are linear or branched aldehydes, which have one more carbon atom than the original olefin compound. Since first introduced by Otto Roelen in the late 1930s [264], hydroformylation has become one of the largest applications of homogeneous catalysis in industry. Today, over 10,000,000 tons of

134 Chapter 4 chemicals are produced by hydroformylation. The oxo-products, including aldehydes and alcohols, have been used as raw materials in a wide range of fine chemical products, such as plasticizers, surfactants, solvents, flavors, and so on. Most of the catalysts used in hydroformylation are transition metal complexes. The first generation of hydroformylation catalysts (BASF, ICI, Ruhrchemie) were cobalt-based complexes, and the reaction was performed at a relatively high temperature (150
180 C) and pressure (200
350 bar) [265]. However, cobalt-based processes result in large amounts of byproducts, such as alkanes. Therefore, the chemo-selectivity and regio-selectivity are relatively low. In the 1960s, Wilkinson developed hydroformylation at mild conditions catalyzed by rhodium complexes [RhCl(PPh3)3], which showed high chemo-selectivity and regio-selectivity. Subsequently Rhodium-based catalysts had received great attention, and a great number of research has been devoted to developing effective catalytic systems for the hydroformylation process. Because of their superior catalytic performances, cobalt-based catalysts have been gradually replaced by rhodium-based catalysts in industrial applications. Nevertheless, because of the high cost of rhodium, more oxo-products are still being produced using cobalt as catalyst. Moreover, some complexes based on other transition metals, including Ru, Pt, Ir, Pd, Fe, and Os, are capable of catalyzing hydroformylation reactions to give corresponding aldehyde products [266]. As a homogeneous process, volatile organic solvents have to be used in the hydroformylation reaction. This will lead to serious environmental problems, especially when the process is expanded to an industrial scale. One of the alternative strategies is employing water as the solvent; however, the olefins that can be utilized are limited to C2
C5 chain lengths because of solubility issues [5]. Moreover, separation between the products and the catalysts is always difficult in a homogeneous catalytic system. Therefore, to finding solutions for these problems in hydroformylation is a significant and urgent issue. The unique properties of ILs help meet the demands in hydroformylation. For example, (1) the negligible vapor pressure of ILs avoid the inconvenience cause by volatilization of the solvent; (2) the good solubility of the ligands in ILs guarantee that catalysts are highly stable in ILs; and (3) the immiscibility of ILs and organic solvents are relatively low so that subsequent separation can proceed easily. With the above advantages, ILs have shown promise in hydroformylation reactions [1]. The first example of hydroformylation in ILs can be traced back to 1972, when Parshall reported on the platinum-catalyzed hydroformylation of ethane in molten salt tetraethylammonium trichlorostannate [267]. The reaction was conducted under harsh conditions, with 90 C and 400 bars of syngas. Knifton reported on hydroformylation of internal and terminal alkenes in tetra-n-butylphosphonium bromide melts, in which ruthenium- and cobalt- based complexes were used as catalysts [268]. It was found that the ionic medium was beneficial in stabilizing the catalytic species, thus improving the lifetime

Catalytic Reaction in Ionic Liquids 135 of the catalysts. These early result have demonstrated the great potential of ILs as reaction media for hydroformylation reactions.

4.4.1 Homogeneous Hydroformylation Bi-phasic hydroformylation is considered one of the ideal catalytic systems that facilitate separation while reducing undesired leaching of the catalyst. The first liquid
liquid bi-phasic hydroformylation was illustrated by Chauvin et al. in 1995 [269]; in their study, 1-pentene was converted to n- and iso-hexanal. Using a bi-phasic system, substrates, complexes, and ligands that are poorly soluble or unstable in water can be employed for hydroformylation reactions. Wasserscheid and coworkers studied the hydroformylation of methyl-3-pentenonate (M3P) in [C4mim][PF6] [270]. Controlling product selectivity is important in this reaction. The activity and stability of the catalyst were greatly enhanced by using ILs as solvents. In [C4mim][PF6], the turnover numbers (TONs) could reach 6640 after 10 cycles, and the catalyst was deactivated after the third cycle. M3P was also hydroformylated by using the platinum-based complex catalyst Pt(TPP)2Cl2 in Lewis acidic ILs with chlorostannate as the anion [271]. The Pt catalyst showed much higher activity in chlorostannate ILs than in the organic solvent CH2Cl2. Olivier-Bourbigou and coworkers systematically investigated the influences of IL solvents and ligands in the hydroformylation of 1-hexene [272]. A wide range of ILs varying in the nature of the cations (e.g., 1,3-dialkylimidazolium, 1,2,3trialkylimidazolium, and N,N-dialkylpyrrolidinium) and the nature of the anions (e.g., [BF4]2, [PF6]2, [CF3CO2]2, [OTf]2, and [NTf2]2) were prepared and studied. By changing the length of the alkyl chain in the cation and the species of the anion, the Rh-catalyzed hydroformylation could achieve optimized performances. In nonaqueous IL media, phosphite ligands that are unstable in an aqueous two-phase system can be used. Anderson and coworkers employed high-melting phosphonium tosylates as ionic solvents in hydroformylation reactions catalyzed by Rh-based catalysts. The salt melted at 100 C, the temperature at which the reaction was carried out. After cooling the temperature, the solvent turned into solid again, which facilitated the separation of the products. Recycling experiments have shown that catalysts and solvents can readily be recovered and reused several times without loss of activity and metal components. 4.4.1.1 Hydroformylation of lower olefins Hydroformylation of lower olefins is a great challenge in catalysis because of the low solubility of gaseous olefin substrates in solvents. Compared with conventional organic solvents, ILs possess some obvious advantages, such as good solubility of reactants and negligible vapor pressure. Therefore, ILs can be used as alternative solvents in hydroformylation to simplify the separation process. Moreover, using ILs as solvents will significantly enhance overall productivity and the catalyst’s lifetime. In hydroformylation of

136 Chapter 4 ethylene catalyzed by HRh(CO)(PPh3)3 using imidazolium-based ILs as the solvent [273], the structures of ILs have an important influence on the activity and stability of the Rh catalyst. The increased length of the chains in the N positions of the imidazole ring reduces the catalytic activity of the catalyst, and the anions of ILs also have a significant influence on the activity of the catalyst. [C4mim][BF4] has been shown to be the best solvent after careful selection. The [C4mim]1 cation interacts as a ligand with the Rh catalyst to form a new active catalytic site for hydroformylation, which is essential for stabilization of the Rh catalyst and for prevention of the formation of low-active Rh clusters. A high TOF (10,627 h21) has been achieved under mild conditions, and the Rh catalyst can be reused with the IL solvent without obvious loss of catalytic activity. As one of the most important and widely used processes in chemical industry, hydroformylation of lower olefins has showed great potential in the production of various key chemicals. One of its typical applications is in the production of methyl methacylate (MMA). The coal-based MMA route is composed of three steps: hydroformylation of ethylene, production of methylacrolein (MAL) with formaldehyde, and subsequent oxidation to MMA. In the hydroformylation step, homogeneous Rh complex is used as the catalyst, and a large amount of solvents has to be used. In every pass, the conversion has to be reduced to ensure that the product propanal could be removed from the system by the gas flow of the substrates. The product is separated from the substrates to be recycled into the next passes. Additionally, solvents and catalysts should preferably stay in the reaction system. Obviously, this is a difficult process if conventional solvents are used, as they are inevitably volatized from the reaction system and have to be separated and recycled before reuse. The unique characters of ILs facilitate the separation of propanal from the solvent while effectively reducing the loss of solvent caused by volatility and thus the possibility of loss and deactivation of the catalysts. An experimental study using different kinds of ILs as solvents revealed that both cations and anions had a great impact on the results. Comparing the best IL and toluene as the solvent revealed that using an optimized IL as the solvent would result in a conversion and selectivity as high as with toluene as the solvent. The TOF value in IL is slightly higher than that in toluene, and the amount of Rh catalyst is reduced as well. The special structures of ILs make them good stabilizers for unstable Rh-H motifs. An electrospray ionization
mass spectrometry (ESI
MS) analysis revealed that [C4mim] cations could act as ligands for a homogeneous Rh catalyst, forming [Rh(C4mim)(CO) (PPh3)2]1 active sites, reducing aggregation of Rh and enhancing reactivity. Moreover, in the step wherein MAL is produced using propanal and formaldehyde, the selected IL could act as a highly efficient catalyst, and the reaction could be conducted under mild conditions (40 C, 1 atm). The “one-step” approach with ILs has simplified the procedure while yielding good catalytic performances. Recycling experiments have shown that the catalyst is stable and that the yield of MAL was maintained at .90% after scaling up to the kilogram scale. Additionally, using ILs as absorbents in some separation procedures in this process, the operation was easier, and energy consumption was significantly reduced.

Catalytic Reaction in Ionic Liquids 137 4.4.1.2 Hydroformylation of higher olefins and other substrates Compared with lower olefins, hydroformylation of higher olefins ($C8) and other substrates containing vinyl groups has been more widely studied because both reactants and products are in the liquid-phase. Sometimes, functionalized ILs have more roles in the reaction beyond that as solvents, for example, as ligands for Rh catalysts, significantly improving the catalytic activity and stability. Moores and coworkers [274] synthesized a series of phosphine-functionalized phosphonium ILs (PFILs) and used as ligands in hydroformylation of higher alkenes. Combining with Rh(I) precursor, [Rh(acac)(CO)2], the PFIL-Rh(I) complexes have shown good activity and excellent selectivity. The P-alkyl chain length in PFILs has influence on the stability, catalytic activity, and selectivity of the catalyst. Similar concepts were also adopted by Chen et al., who synthesized ionic phosphine ligands for the Rh(III) complex [275]. Because of their specific properties inherited from ILs, “ionic ligands” are insensitive to moisture and air and show good immiscibility to IL solvents. Therefore, their activity and stability are greatly enhanced [276]. Chen et al. demonstrated that the ion-pair effect of phosphine-imidazolium salts will have a great impact on the catalytic performances of Rh(III)-complex-catalyzed hydroformylation of 1-octene in water suspension [277]. The hydrogen bonding networks between the cation and the anion of ion pairs has been shown to be important for hydroformylation. Other organic compounds containing CQC double bonds, such as vinyl acetate [278] and methyl-3-pentenoate [271], are utilized as substrates for hydroformylation in IL media. According to the designability of ILs, various ILs with special tags can be synthesized and used as reaction media in bi-phasic hydroformylation, which will greatly enhance the reaction rate and facilitate the separation after the reaction. Jin and coworkers [279,280] demonstrated the hydroformylation of olefins in a thermoregulated IL/organic bi-phasic system. As shown in Fig. 4.7, at the reaction temperature, the IL containing polyethylene

Figure 4.7 Thermoregulated phase transition property of IL/cyclohexane bi-phasic system with IL-stabilized Rh nanoparticles observed at (A) room temperature before heating, (B) Miscibility temperature and (C) room temperature after cooling [280]. Copyright Springer Co.

138 Chapter 4 glycol (PEG) fragment was homogenized with organic solvent, which enables a homogeneous reaction. After cooling to room temperature, the solvent and IL go through two phases and facilitate the separation. The catalyst is dispersed in the IL phase and can be used several times without loss of activity. It is necessary to point out that most of hydroformylation reactions reported in the open literature refer to terminal monounsaturated alkenes, and the reactivity toward hydroformylation of internal alkenes is dramatically lower. By dissolving the metal complex or ligands into ILs, reactivity toward internal alkenes is obviously enhanced. Suarez and coworkers reported that an IL-based bi-phasic catalytic system used for hydroformylation of soybean biodiesel contains internal CQC double bonds and other functional groups [281]. The employed IL and catalyst precursor were 1,3-dialkylimidazolium hexafluorophosphate and HRhCO(PPh3)3, respectively. The IL-stabilized the metal complex in the form active for hydroformylation, bringing about positive effects on both conversion and selectivity. The yield for aldehyde reached 100%. For industrial application, multiphase reactors are often used for hydroformylation of higher olefins. Besides optimizing the catalysts and reaction conditions, improving the reactor designation will further increase the efficiency. Peschel et al. [282]. demonstrated a methodology for design of multiphase reactors for hydroformylation of 1-octene using a biphasic IL system with triphenyl phosphine trisulfonate (TPPTS)-modified Rh catalyst. With the use of static mixers, advanced cooling, and discrete 1-octene dosing, the selectivity of the derived technical reactor is 9.1% higher compared with an optimized reference case. Furthermore, the proposed methodology is suitable for the design of tailor-made multiphase reactors, and this will lead to significantly higher efficiency compared with conventional reactor concepts.

4.4.2 Supported Ionic Liquid-Phase (SILP) Catalysts for Heterogeneous Hydroformylation Because of their negligible volatility and stale chemical properties, ILs can be supported on the solid catalyst support and form a thin film on the surface of the solids. The supported IL phase catalyst was first developed by Mehnert’s group in 2002 [283,284]. Supported IL phase (SILP) catalysts can be considered a pseudo-homogeneous-heterogeneous catalyst, which is prepared by dissolving a homogeneous transition metal catalyst within a multilayer of ILs on the support. They combined the inherent characteristics of both ILs and the solid support materials. SILP catalysts were widely used in hydroformylation reactions. Wasserscheid and coworkers [285
288] have shown that SILP catalysts can be prepared by dispersing the catalyst complex in an IL and supported on the internal surface of a porous solid. The SILP catalysts combined the advantages of homogeneous and heterogeneous catalysts; thus, metal complex catalysts can be used in heterogeneous reactions, still

Catalytic Reaction in Ionic Liquids 139 retaining their structure and high activity. Such a SILP catalyst has been demonstrated to be highly effective for propene hydroformylation. Shylesh et al. systematically discussed the factors that influence the activity and stability of Rh-based SILP catalysts for the gas-phase hydroformylation of propene [289] and suggested that interactions of the IL and metal complex with the support are required to yield a stable catalyst. The metal complex has been proven to be stable in the IL layer of the SILP catalysts. Blum and co-workers demonstrated the regio-selective hydroformylation of vinylarenes in aqueous media using Rh complex [Rh(cod)Cl]2 immobilized in IL-modified silica [290]. No transformation of the organometallic catalyst into nanoparticles was detected despite the use of H2 in the reaction. Because of their high activity and heterogeneous nature, SILP catalysts can be applied to continuous, gas-phase hydroformylation of lower olefins. Ferhmann and coworkers studied the characterization and parameters of Rh-TPPTS SILP catalysts for ethylene hydroformylation [291]. Rh-TPPTS SILP catalysts with relatively low IL loading were shown to be stable and highly active for the reaction. The catalytic activity, BET surface area, and pore morphology of the catalysts were strongly dependent on the content of ILs. It was demonstrated that catalysts with high IL loading content showed deactivation at high reaction temperatures, which was possibly caused by redistribution of the IL out of the pores under those conditions. Continuous-flow gas-phase hydroformylation of propene was performed using SILP catalysts by the same group [285]. Rh complexes with the biphosphine ligand were immobilized in the IL layer. The SILP catalysts proved to be more regio-selective than the analogous IL-free catalysts, and the performance of the catalysts was generally strongly influenced by the composition of the catalyst. Hanna et al. investigated the kinetics of propene hydroformylation in the gas-phase over silica-supported Rh-sulfoxantphos complex stabilized by the IL [C4mim][C8SO4] [292]. According to their study, the kinetics for forming n- and iso-butabal are functions of the reaction temperature, which suggests that the rate-determining step (RDS) was also dependent on the reaction temperature. Furthermore, SILP catalysts can be also used in the tandem hydroformylation and hydrogenation reaction [293]. The tandem catalytic conversion of propene and synthesis gas to butanol is then carried out using an SX-Rh SILP catalyst to promote the hydroformylation of propene to butanal and Shvo/SiO2 to promote the hydrogenation of butanal to butanol. The rate expressions describing the kinetics of each of the catalysts are then used to predict operating conditions required to achieve high conversion of propene to butanol. Under the most favorable conditions examined (H2/CO 5 10), an overall yield of 13% to butanol was achieved with 15% propene conversion and 90% aldehyde conversion at a temperature of 413K. Until now, the SILP system has not been implemented in largescale industrial catalytic applications. Indeed, the SILP technology-based catalytic process is being currently tested and upscaled for continuous gas-phase hydroformylation of industrial mixed C4-feedstocks

140 Chapter 4

Figure 4.8 (A) scheme of the toroidal movement of particles in the reactor; (B) photograph of the spraying nozzle (top) and cross-section of the nozzle, indicating the two flows (bottom) [295]. Images are courtesy of Innojet Herbert Hu¨ttlin.

[294]. Hence, the use of SILP technology in largescale industrial applications can be expected in the near future. Despite the effectiveness of SILP in the above-mentioned application, its inherent issues, such as recyclability and lack of long-term stability as a result of slow but steady leaching, need to be addressed. The scalable preparation for SILP catalysts were demonstrated by Wasserscheid and co-workers [295]. Different types of support materials, such as powders, spheres, agglomerates, and extrudates, were successfully impregnated by the novel fluidized-bed impregnation method, as shown in Fig. 4.8. The IL together with the dissolved catalyst is sprayed onto the fluidized support material by means of a helper solvent. Depending on the temperature and pressure inside the fluidized-bed, the helper solvent can be evaporated immediately or slowly, giving rise to either shell-type or complete coating of the support with the IL. This technique will help introduce the promising SILP technology into industrial processes in the near future.

4.4.3 CO2 as the Substitute for CO in Hydroformylation CO2 sequestration and its use as a feedstock in industrial processes are major challenges in the development of alternative greener and sustainable processes [296]. Among the various approaches, the substitution of CO by CO2 in carbonylation reactions may allow for new applications with broader use for this important and abundant, but poorly reactive, substrate. As hydroformylation reactions use CO and H2 as the gaseous substrates and CO could be obtained by hydrogenation of CO2, CO2 could be used as the CO source in

Catalytic Reaction in Ionic Liquids 141 hydroformylation. Dupont and coworkers demonstrated Ruthenium-catalyzed hydroformylation of alkenes in the presence of ILs [297]. The reaction of [C4mim]Cl or [C4C1mim]Cl with Ru3(CO)12 could generate Ru-hydride-carbonyl-carbene species in situ that are efficient catalysts for a reverse water gas shift/hydroformylation/hydrogenation cascade reaction. The hydrogenation of CO2 to CO has been facilitated by the addition of H3PO4, and di-substituted alkenes are easily functionalized to alcohols through sequential hydroformylation/carbonyl reduction. It is believed that using inexpensive CO2 as a source of CO for carbonylation reaction processes, such as hydroformylation reactions, will open a new window for achieving sustainability in the chemical industry.

4.5 Cycloaddition Reaction of CO2 and Epoxides As a class of carbon
carbon addition reaction, cycloaddition reaction of CO2 is of great interest in connection with the development of a truly eco-friendly and environmentally benign process as well known. Although, there are many possibilities for CO2 to be used as a safe and cheap C1 building block in organic synthesis and potential fuel preparation [298], in this context, the synthesis of five-membered cyclic carbonates via the coupling of CO2 and epoxides is still one of the most promising methodologies. These carbonates are valuable as precursors for polymeric materials, such as polycarbonates, aprotic polar solvents, and pharmaceutical/fine chemical intermediates, and in many biomedical applications [299]. In the past decades, numerous catalyst systems have been developed for this transformation process. Investigations of ILs as catalysts has gained growing attention because of their many unique advantages. The increasing eco-friendly awareness has brought more attention to the applications of ILs as green media. Indeed, ILs find versatile applications in the fields of organic synthesis, catalysis, material synthesis, and electro-chemistry. Of note, much progress has been made with regard to the application of ILs in cycloaddition reactions. These advances have revealed that ILs have high advantages compared with traditional chemicals. The presence of ILs further makes the process eco-friendly, with high efficiency, low-cost, high selectivity, and less wastage. Also, the quick development of ILs has promoted investigations on the corresponding theory and mechanism. The recent application of ILs in cycloaddition reactions as substantive media is summarized below. This critical review just briefly described the recent application of ILs as substantive media in the cycloaddition reactions of CO2 with epoxides, and thus, it is not meant to cover every published reference or aspect of the processing of cycloaddition reaction using ILs; rather, our opinions on some fundamental questions about the role of ILs are provided. The corresponding advances related to the cycloaddition reaction of CO2 and epoxides to produce cyclic carbonates focus on the utilization of ILs as catalysts for the synthesis of cyclic carbonates, wherein ILs are used in combination with various metallic compounds or

142 Chapter 4

Figure 4.9 IL-based cycloaddition of CO2 with epoxides.

alone. Only a small amount of examples have been provided below, divided into two sections: metallic catalysis process and nonmetallic catalysis process (Fig. 4.9).

4.5.1 Metallic Catalysis Process 4.5.1.1 Metallic complex/ionic liquid Recently, the metallic complex/IL binary systems have attracted much attention for the cycloaddition reaction of CO2 and epoxide. This is not only because the presence of a small amount of metallic complex can obviously improve the activity of ILs but also because ILs offer more advantages compared with other catalysts. As a result, this kind of binary system has been predominant in the past decades and is still flourishing. Although hundreds of metallic complexes or compounds have been developed for this transformation process, as discussed in the literature, the mechanism of synergetic catalysis for the high efficiency of the catalytic system made by Lewis acid (i.e., metallic complex) and Lewis base (i.e., IL) is well accepted. Fig. 4.10 summarizes the reported typical IL structures. The electrophilic coordination between the metallic cation and the oxygen atom of the epoxide causes the polarity of the epoxide ring, which is one factor for breaking the CaO bond of the epoxide. This is because without the additional nucleophilic attack coming from the anion of the IL, it is still difficult to open the epoxide ring. That is why the synergetic catalysis made by

Catalytic Reaction in Ionic Liquids 143 Cation: R1 N

N+

R2

N+

P+

N +H 2 N+ R

H 2N

NH 2

Anion: Cl –, Br –, I –, BF 4–, PF 6–

Figure 4.10 Typical structures of cation and anion used for ILs.

Lewis acid (i.e., metallic complex) and Lewis base (i.e., IL) is crucial to accelerate the breaking of the CaO bond; as a result, opening of the ring becomes much easier. To the best of our knowledge, ZnBr2/imidazolium salt was the earliest reported metal complex/IL binary system for the cycloaddition reaction of CO2. In 2003, a zinc bromide complex composed of ZnBr2 and [C4mim][Br] was reported with good results for the conversion of CO2 with propylene oxide (PO) and ethylene oxide (EO) [300]. The presence of a small amount of zinc salt obviously improved the activity of the imidazolium salt. And the corresponding TOF was as high as 1683 h21, which was much higher than imidazolium-based IL alone. Before this kind of system was used for this reaction, although imidazolium-based ILs have been introduced as effective catalysts for the synthesis of propylene carbonate (PC) from the coupling reaction of CO2 and PO, their catalytic activities expressed as TOF were not very high (TOF 5 10.63 for [C4mim][Cl], 14.98 for [C4mim][BF4] at 110 C) [301]. Thereafter, a low-reactive bulky epoxide, such as styrene oxide (SO), and its conversion to styrene carbonate (SC) in the presence of zinc bromide and IL under mild conditions were reported in 2004 [302]. And it was found that ZnBr2 and [C4mim][Cl] system could exhibit a 93% SC yield at a low temperature of 80 C and 4 MPa for 1 h. More importantly, the authors found that the combination of zinc bromide with [C4mim]Cl exhibited a markedly high activity: when the ZnBr2: [C4min]Cl ratio was changed from 1:2 to 1:1, the SC yield decreased from 93% to 82%. However, further changing the ratio to 1:4 affected the SC yield very little. Based on the results, the optimal ratio of ZnBr2:[C4mim]Cl was set as 1:2, which was in accordance with the results reported by Kim et al. [300]. These authors revealed that the type of metal cations had a strong effect on the carbonate yield, which was in the activity order of Zn21 . Fe31 . Fe21 . Mg21 . Li1 . Na1, in accordance with the order of Lewis acidity of the metal cations [302]. Almost simultaneously, asymmetric synthesis of optically active PC from racemic epoxides and CO2 was discovered at near room temperature by using simple and highly efficient chiral SalenCo-(III)/quaternary ammonium halide catalyst systems (Fig. 4.11) [303].

144 Chapter 4 O H 3C

O

O

+ H 3C

n-Bu 4NY (Y=Cl, Br, I)

+ CO2 H

H

H N O

M X

N

H Major O

O

(1R, 2R)(t-Bu)2SalenMX

O

O + H 3C

O H 3C H

O H 3C H

O

O + H 3C H Minor

Figure 4.11 Chiral Salen Co-(III)/quaternary ammonium halide catalyzed asymmetric synthesis of optically active PC [303].

Interestingly, it was found that the anion of quaternary ammonium salts in the binary catalyst systems had a great effect on enantiomeric purity and reaction rate. For example, the use of tetra-n-butylammonium chloride ([N4444][Cl]) was more beneficial for improving the enantiomeric purity of the products but had a pronounced negative effect on the rate. Another finding revealed that an axial X-group of chiral Salen Co (III)X complexes was essential for attaining high enantio-selectivity of the resulting PC. By using a sterically bulk axial X-group, such as p-toluene-sulfonate, the formed chiral Salen Co (III)X complexes could exhibit high enantio-selectivity of PC in this reaction [303]. In 2006, an efficient system of quaternary phosphonium halide ([PR1R2R3R4]X; X 5 Cl2, Br2 and I2) combined with ZnCl2 was developed with high selectivity for the coupling reaction of CO2 and epoxide under the mild conditions [304]. The effects of reaction temperature, CO2 pressure, various compositions of the catalysts have been investigated systematically. It was found that a 96% conversion of PO and high TOF value (4718.4 h21) could be achieved in the presence of ZnCl2/PPh3C6H13Br (molar ratio 5 l:6) at a low pressure 1.5 MPa, 120 C and 1 h, the catalyst was also proved to be applicable to other terminal epoxides. Additionally, the catalyst could be reused with little loss of catalytic activity after five times. In 2006, a highly active hexabutylguanidinium salt/zinc bromide binary catalyst for the coupling reaction of CO2 and epoxides was also developed with significant catalytic activity (e.g., TOF values as high as 6600 h21 for SO and 10100 h21 for epichlorohydrin) under 130 C, 3 MPa and 1 h (Fig. 4.12) [305]. This catalyst system also offers the advantages of recyclability and reusability. The special steric and electrophilic characteristics of hexabutylguanidinium bromide IL have resulted in the prominent performance of this novel catalyst system. As an example, in 2008, catalytic performance of pyridinium-based ILs in the synthesis of cyclic carbonate from CO2 and butyl glycidyl ether (BGE) was reported [306]. It was found

Catalytic Reaction in Ionic Liquids 145 R2

R1 N

O R

R3

N

R4

X N

R5

ZnBr2

O

R6

O

O

130 oC, 3 MPa, 1 h

CO 2

R

Figure 4.12 Hexabutylguanidinium salt/ZnBr2 catalyzed coupling reaction of CO2 [305].

that the structure of pyridinium ILs affected the conversion of BGE. For example, the BGE conversion after 6 h increased from 68.2% to 78.6%, with the size of cations increasing from 1-ethylpyridinium chloride ([C2Py][Cl]) to 1-propylpyridinium chloride([C3Py][Cl]), possibly because a bulky IL has longer distance between the cation and the anion and thus may have higher anion activation capacity [307]. However, for more bulky ILs, such as 1butylpyridinium chloride ([C4Py][Cl]) and 1-hexylpyridinium chloride([C6Py][Cl]), the increase of BGE conversion is very small, possibly because of the steric hindrance of these bulky ILs compensating for the increase of the anion activation ability with increasing alkyl chain length [306]. The synergistic catalysis role of the metallic compound and ILs has been widely discussed, including the references presented in this review. In a typical catalytic cycle, the metallic compound first acts as a Lewis acid coordinating the epoxide and activating it for ringopening by the anion of the IL to generate an active species, which could coordinate CO2 to give an active alkylcarbonate. Finally, the formed alkylcarbonate transforms into a cyclic carbonate by the intramolecular substitution of the halide in the following step. During the catalytic process, the nucleophilic nature of the IL and the electrophilic nature of the metallic compound work together to make the ring-opening of epoxide much easier than each by itself. Based on the discussion above, we can call it the “E 1 N” mechanism. 4.5.1.2 Metallic-based material/ionic liquid Although the high activities exhibited in the cycloaddition reaction of CO2 make metallic compounds the preferred catalysts combined with ILs, facilitating the separation process is still a big challenge because of the difficult separation natures of these homogeneous catalysts. As one of the protocols, metallic-based materials have been developed, including supported metallic compounds, metal-organic frameworks (MOFs), metal-coordinated conjugated polymers, and so on. Metallic compounds supported by Organic or inorganic materials have been widely used for the synthesis of cyclic carbonates. Generally, impregnation is used as the main protocol to achieve this immobilization target. Because of the coordination interaction between the

146 Chapter 4 transition metallic cation and the IL, the formed catalytic system can be reused several times as the heterogeneous catalyst [308
311]. However, because of the weak interactions, loss of the metallic catalysis site cannot be avoided, and thus their stabilities still need to be improved. MOFs formed by copolymerization of organic molecules with metal ions or metal ion clusters have zeolite-like properties, including high internal surface area, wellordered porous structures, and high absorption capacity. They have various applications in gas storage [312], separation [313], chemical sensors [314], and catalysis [315]. It was discovered that the combination of MOFs and ILs could be used as efficient catalysts for the synthesis of cyclic carbonates. For example, MOF-5 and quaternary ammonium salts, that is, tetra-n-methylammonium chloride ([N1111][Cl]), tetra-n-methylammonium bromide [N1111][Br], tetra-n-ethylammonium bromide [N2222][Br], tetra-n-propylammonium bromide ([N3333][Br]), and tetra-n-butylammonium bromide ([N4444][Br]), had an excellent synergetic effect in promoting the synthesis of cyclic carbonate from epoxides and CO2 (Fig. 4.13) [316]. First, the optimal temperature for the reaction could be operated around 50 C by using MOF-5/n-[N4444]Br as the best efficient catalytic system. A proposed mechanism described that the coupling reaction was initiated by the activation of the epoxy ring under the coordination of the Zn4O clusters (Lewis acidic site) in MOF-5 with the oxygen atom of the epoxide. Second, the Br2 generated from n-[N4444]Br attacked the lesshindered carbon atom of the coordinated epoxides, followed by the same procedure as the widely discussed “E 1 N” mechanism. Third, the reaction temperature could occur at room temperature with high efficiency under 1 atm pressure by crystal engineering of the nbo

Zn Zn

O

Zn

O

O

Zn

O

O

O

n-Bu 4N +Br – R

R

n

O O O

n-Bu 4N +Br –

O

R

O n-Bu4N +– O Br

–O

O

CO 2

R

n-Bu 4N + Br

R

Figure 4.13 The plausible reaction mechanism for the cycloaddition of CO2 with epoxides catalyzed by MOF-5 and n-[N4444]Br [316].

Catalytic Reaction in Ionic Liquids 147

N

N Zn

O

O R1

tBu

R2

O tBu

Zn-CMP

BuN 4Br

O n

O R1

O R2

CO 2

Figure 4.14 Synthesis of organic carbonates from CO2 and epoxides by Zn-CMPs [318].

MOF platform with a custom-designed azamacrocycle ligand [317]. The use of MOFs provides a protocol for the efficient conversion of CO2 by heterogeneous metal-based materials. As another type of metal-based materials, zinc-coordinated conjugated microporous polymers (Zn-CMPs) (Fig. 4.14), prepared by linking salen zinc and 1,3,5triethynylbenzene, exhibit extraordinary activities (TOF of 11,600 h21), broad substrate scope, and group tolerance for the synthesis of functional organic carbonates by coupling epoxides with CO2 [318]. The catalytic activity of Zn-CMP is comparable with those of homogeneous catalysts and superior to those of other heterogeneous catalysts. This catalyst can be reused more than 10 times without significant decrease in performance.

4.5.2 Nonmetallic Catalysis Process Metallic compounds have dramatically improved the activity of ILs. However, it must be noted that some of these catalytic systems have innate limitations in the case of aqueous conditions because of the water sensitivity of metallic compounds. To solve this problem, some efforts have been made to develop nonmetallic catalytic processes. As discussed about the ‘E 1 N’ synergistic mechanism, electrophilic activation of epoxide ring by Lewis acid is one of the important preconditions for the ring to be further reacted with CO2. Thus, new type of interactions needs to be discovered before Lewis acid-based systems have substituted other candidates. In this section, the corresponding advances can be divided into three aspects: IL-based nonmetallic system, TSILs, and supported ILs. 4.5.2.1 Hydrogen bond donor/ionic liquids Recently, water was found to play a role in accelerating this kind reaction combined with traditional ILs [319]. The result with the presence of water was surprising, with five to six times higher activity discovered for traditional ILs, compared with the result without water. More interestingly, besides water, other hydroxyl group-containing solvents, such as

148 Chapter 4 H

H-Bond

H O R

O

O C e O

O H

H

O-

IL

R

O H O

IL

R

O

O H

H

H

OIL

O R

O O IL

Nucleophilic attack

Figure 4.15 Hydrogen bond/IL synergistic catalysis process [319].

ethanol, PEG, acid, and phenol could also greatly promote the reaction [319]. However, nonhydroxyl group solvents, such as N,N-dimethylformamide, propylene carbonate, acetone, acetonitrile, cyclohexane, dimethyl carbonate, and dichloromethane, did not work. Based on their results and previous reports [320,321], the authors proposed a new mechanism portraying the probable sequence of events (Fig. 4.15). It was emphasized that CaO bonds of epoxide are polarized by the hydrogen bond (H-bond) between water and epoxide, and the synergistic nucleophilic attack of the halide anion made on the less sterically hindered β-carbon atom of the epoxide occurs simultaneously, thus facilitating the ring-opening of the epoxide. With respect to the “E 1 N” mechanism, this nonmetal-based process could be explained by an “H 1 N” mechanism (H, hydrogen bond). Thereafter, the H-bond donor-promoted fixation of CO2 and epoxides into cyclic carbonates was further investigated through experimental and density functional theory studies [322]. Three reaction mechanisms of the [N4444][Br]/H2O-catalyzed process, noncatalytic process, and the [N4444][Br]-catalyzed process for the fixation of CO2 with EO were investigated. The discrete Fourier transform (DFT) results demonstrated that the activation and ringopening of the epoxides were the key steps toward making the catalytic process efficient, and the hydrogen bond, indeed, played an important role in these key steps [322]. The [N4444][Br]/H2O-catalyzed fixation reaction proceeded with a much lower barrier for the rate-determining step compared with the noncatalytic process or the [N4444][Br]-catalyzed process. This notable activity for the H2O-promoted process possibly originated from the cooperative actions of H2O and [N4444][Br], which helped activate the EO and more easily stabilize the intermediates and transition states through H-bond interactions, making the cycloaddition reaction to much easier [319,322]. Beside water, H-bond donors reported to accelerate the reactions, found such as cellulose [323,324], chitosan [325], lignin [326], β-cyclodextrin [327], formic acid [328], amino acid [329,330], and so on, have been so far. 4.5.2.2 Task-specific ionic liquids Single traditional ILs have been tested for the synthesis of cyclic carbonates [301,331
333]; however, their activities are still unsatisfactory because of their role in poor

Catalytic Reaction in Ionic Liquids 149 synergistic catalysis, as mentioned above. As one of the efficient protocols to resolve these problems, TSILs functionalized with functional groups, such as hydroxyl [321], amino/ amino acid [334
336], and carboxyl group [337
339], were developed. Here, we have provided and discussed some typical examples (Fig. 4.16). Inspired by the synergistic catalysis role of hydrogen bond, recently, a series of simple hydroxyl-functionalized ILs (HFILs) were synthesized and found to have efficient reactivity and reusability toward the coupling of epoxide and CO2 without any additional cocatalyst and organic solvent [321]. Highest activity and selectivity were achieved in the presence of 1-(2-hydroxyl-ethyl)-3-methylimidazolium bromide (HC2mim[Br]) (Fig. 4.18, 1a-b, 2a-b) in comparison with other similar catalysts investigated. With the presence of a hydroxyl group in the cation of the imidazolium-based IL, an obvious enhancement of activity could be obtained compared with that of the traditional 1-ethyl-3-methylimidazolium bromide ([C2mim][Br]) (PO conversion:99% vs 83%). As another kind of functionalized ILs, amino/amino acid-functionalized ILs, were developed and were shown to have high activity for the reaction (Fig. 4.18, 3a-f, 4a-e, 5a-d) [334
336]. Amines are well known to be able to react with CO2, forming carbamate salts, which can be regarded as activated forms of CO2. At the same time, the additional 2 NH2 group in amino acid-based ILs may also activate the epoxy ring through a H-bond, also resulting in enhanced reaction [335]. Thus, the high activities of these TSILs can be attributed to the dual functions of the amino group in the activation of epoxide ring and CO2 [334,335].

HO

N+

N

N +R

3

2a: R=C 4H 9, X=Br; 2b: R=C 2H 5, X=Cl.

Hydroxyl functionalized ILs

HO O-

H 2N 5a: R=CH 3; 5b: R=CH 2OH; 5c: R=(CH 2) 4OH; 5d: R=(CH 2) 2COOH.

R

N

N R

X6a: R=CH 2, X=Br; 6b: R=CH 2, X=Cl; 6c: R=CH(CH 3), X=Br; 6d: R=(CH 2) 3, X=Br; 6e: R=(CH 2) 3, X=Cl.

Amino acid functionalized ILs

N

Br -

N

4a:R=NH2; 4b:R=COOH; 4c: R=OH; 4d: R=CH3; 5e: R=H. Functionalized guanidium ILs

O OH

O

N

X-

NH 2

Amino functionalized ILs

O

O

N

3a-c: R=CH 3, X=Cl, Br, I; 3d: R=C 2H 5, X=Cl; 3e-f: R=C 4H 9, X=Br, l.

Hydroxyl functionalized ILs

R N

R N

X-

X1a: X=Br; 1b: X=Cl.

R

+HN

HO

Dicarboxyl functionalized ILs

OH N

N R

N+

OC

X7a: R=CH 2, X=Br; 7b: R=CH 2, X=Cl; 7c: R=(CH 2) 3, X=Br.

8a: Betaine

Carboxyl functionalized ILs

Betaine

Figure 4.16 Some typical functionalized ILs [321,334
339].

O

150 Chapter 4 However, as acidic and stronger H-bond donors than hydroxyl groups, carboxyl groupfunctionalized ILs were synthesized (Fig. 4.18, 6a-e, 7a-c, and 8a) [337
339]. Compared with hydroxyl-functionalized ILs, these acidic ILs could exhibit higher activities in the conversion of CO2, although via a similar activation role on the epoxide ring. In addition, because of their acidic nature, this kind of ILs can also found potential applications in esterifications and transesterfications. 4.5.2.3 Supported ionic liquid As catalysts, ILs can also be supported onto organic and inorganic supports to facilitate their separation. Because of the organic nature of the cations of ILs, several methods can be used to realize the chemical immobilization of most of the ILs, and the resulting heterogeneous catalysts have been proven to have good recycling performance compared with physical-supported ILs. Here are some typical IL-supporting protocols. 4.5.2.3.1 Using of alkylating agent

Increased utilization of mechanically stable synthetic matrices, particularly functionalized silica as a solid support and its surface modification by covalent grafting through various ligands or organic reagents, have been highlighted in recent years. The active silica surface with large specific surface area is of great importance, and its immobilization results in a great variety of silylating agents, allowing pendant functional groups in the inorganic framework [340]. By using silylating agents, such as (3-chloropropyl) triethoxysiliane, many kinds of silica-supported ILs can be synthesized, and they show the great advantages in separation and activity [341
347]. Fig. 4.17 summarizes some typical structures of silica-supported ILs. 4.5.2.3.2 Functional polymer

As an attractive type of carrier for IL catalysts, polymer materials have gained much attention because of their low-cost, easy separation, and versatile surface functionalization. By grafting ILs onto polymers, a variety of heterogeneous catalysts have been developed in recent years. As one example, 1-(2-hydroxyl-ethyl)-imidazolium-based ILs ([HC2im][X], X 5 Cl, Br, I), which have both acidic and basic characteristics, were covalently anchored onto a highly cross-linked polystyrene resin (Fig. 4.18), and their activities on the synthesis of cyclic carbonates via cycloaddition reaction of CO2 with epoxides was studied [348]. Because of the presence of chloromethyl functional groups in the resin, ILs could be easily chemically anchored. The results demonstrated the H-bond donor-promoted mechanism once again, and a high yield (80
99%) and excellent selectivity (92
99%) of cyclic carbonates could be achieved at mild conditions (2.5 MPa, 120 C and 4 h) without any cosolvent. In addition, the catalyst could be reused for as many as six times without loss of catalytic activity.

Catalytic Reaction in Ionic Liquids 151 2

1 O

Si

O

N

O OH O Si (CH ) 2 3 N

NR X-

OEt

C 4H 9

N + N C 4H 9 C 4H 9 C 4H 9 C 4H 9 Cl -

RX: CH 3Cl; C 2H 5Br, C 3H 7Br, C 4H 9Br, etc

Silica supported imidazolium based ILs

Silica supported hexaalkylguanidium chloride

3

4 O Si O OEt

N N

P +(n-Bu)3 X-

N R X-

X=Cl, Br, I

Si R:CH 2CH 3; CH 2CH 2OH, CH 2CH 2COOH, X=Cl, Br, I

SBA-15 supported triazolium based ILs

Silica supported phosphonium based ILs

5

6 IO O

O O Si O

N+

Si OH

N

Cl R N+ R R1

R, R1 =C 2H 5, C 6H13, C 2H 4OH

Silica supported aminopyridinium iodide

MCM-41 supported trialkylamine based ILs

Figure 4.17 Some typical silica/molecular sieve supported ILs [341,342,344
347].

HO

Cl

Cl

Cl

i) Imidazole HO

Cl ii) X

X– N

N

N+

X– N+ N

N

OH

X– N+

OH

X=Cl–, or Br – Cl

X– N+

X– + N N

Cl

N

X– N+

OH

HO

Figure 4.18 Functional polymer supported ILs [348].

4.5.2.3.3 Polymerization of IL

Porous cross-linked polymer matrix synthesized by suspension polymerization has been a novel method to obtain supported ILs with uniform size, permanent pore structure, and tunable texture properties (pore size and specific surface area) [349,350]. Several methods have been reported for the synthesis of poly(ionic liquids) (PILs).

O H

152 Chapter 4 Polymerization of the IL monomer with other unsaturated monomers is one commonly used method. For example, a highly cross-linked polymer-supported IL was prepared, in which 1-butyl-3-vinylimidazolium chloride ([C4vim][Cl]) was covalently anchored on divinylbenzene (DVB)-cross-linked polymer matrix [349]. It was demonstrated that the catalytic activity was comparable with or even better than those of the liquid catalysts ([C4vim][Cl. After easily separated from the products, the catalyst could be used at least five times without loss of activity. By tuning the molar ratio of DVB to N-vinylimidazole, the surface area of the synthesized, highly cross-linked porous poly(N-vinylimidazole-codivinylbenzene) (PVIm) could be more than 900 m2/g [350], and comparable with silicabased supports. Following a simple polymerization procedure using NH4X (X 5 Cl, Br and I) and dicyandiamide, functionalized dicyandiamide
formaldehyde polymers (Fig. 4.19A) were developed as efficient heterogeneous catalysts for conversion of CO2 into organic carbonates [351]. Among the four polymers, the ammonium bromide modified dicyandiamide
formaldehyde polymer (ABMDFP) was the optimal catalyst and was applicable for various substrates. It was proposed that ABMDFP could play a bi-functional NH 2 XH N

N+

R

R

R

NH 2

N H OH

R

OH n

N+ X-

N

N R

R

N+ X-

R

n

R

X=Cl, Br, I R=F or H; X=Cl, Br.

ACMDFP (X=Cl), ABMDFP (X=Br), AIMDFP (X=I). (A)

(B)

N+

N O O

R2

O

N n H

O R1

N H

O

O-

O

O O

O N H

R1

N H

N H

PUDA BMIM (C)

Figure 4.19 Some reported poly(ionic liquids) [351
353].

NH m

Catalytic Reaction in Ionic Liquids 153 role on the activations of reactants, that is, ring-opening of epoxides assisted by the hydroxyl group and activation of CO2 induced by amine groups [351]. The catalyst is stable and can be easily recovered. As the second example, fluoro-functionalized polymeric ILs (F-PILs) with imidazolium cations and bromide or chloride anions (Fig. 4.19B) were designed for cycloaddition reactions of CO2 with epoxides [352]. It was found that F-PIL-Br showed three times higher activity for CO2 reacting with styrene oxide compared with nonfluorous PIL-Br because the fluorine content in F-PILs significantly influenced the catalytic activity of the catalysts [352]. Also, it was found that F-PIL-Br could be extended to catalyze a broad range of reactants under 1 MPa CO2 pressure, producing a series of cyclic carbonates in excellent yields (93
99%). Recently, a new kind of urethane-based PILs was synthesized by the addition of diisocyanate to a di-functional polyol and diol mixture, which then went through an exchange reaction with [C4mim][Cl] [353]. Such PILs were used as catalysts in the cycloaddition of CO2 to epoxides. The results demonstrated that the variation of PILs compositions influence their physical and thermal properties, along with its catalytic activity. A higher CO2 conversion was obtained by extending PIL with ethylene-diamine (PUEA BMIM) (Fig. 4.19C) at mild conditions of 25 bar, 110 C, and 6 h, which can be easily separated and recycled without the loss of catalytic activity [353].

4.6 Esterification Reaction Esters, ranging from aliphatics to aromatics, are important organic compounds holding broad applications in bulk chemicals, fine chemicals, natural products, and polymers [354]. Consequently, esterification is one of the most significant industrial processes and has attracted much interest from academic and industrial communities. Traditionally, esters are produced by reacting a carboxylic acid with an alcohol in the presence of liquid mineral acids, such as H2SO4, HF, and H3PO4, as homogeneous catalysts (Fig. 4.20). However, these acids are extremely corrosive and contaminative and need to be neutralized at the end of the reaction. Meanwhile, because of the unfavorable position of the esterification equilibrium, removal of water formed in the esterification process or utilizing reactive carboxylic acid derivatives are often required to achieve high yields. For these reasons, the O R1

OH

+ R2OH

O

H+ R1

OR2

+ H2O

Figure 4.20 Esterification of carboxylic acids with alcohols.

154 Chapter 4 replacement of traditional esterification process with a more environmentally benign alternative has become urgent. ILs have been identified as green and environmentally friendly reaction media and catalysts because of their unique properties, such as negligible volatility, remarkable solubility, high thermal stability, and tunable structures [298]. Much effort has been devoted to an IL-based esterification process to replace traditional mineral acid-based technologies, and many practical strategies have been proposed [6,355]. Below, we will discuss the esterification process based on IL in detail.

4.6.1 Ionic Liquids as Catalysts/Solvents for Esterification 4.6.1.1 Acidic ionic liquids as catalysts/solvents for esterification In 2001, ILs were first reported to catalyze the esterification of acetic or anchoic acids with alcohols [356]. In this work, the acidic chloroaluminate, [C4py][AlCl4], was used as the catalyst. Although the moisture sensitivity of [C4py][AlCl4] made it an impractical catalyst/ solvent for esterification reactions, the introduction of ILs into esterification process is still considered a milestone work. Since then, many relevant investigations have been conducted [357
359]. Park et al. reported on esterification of a number of primary, secondary, and tertiary alcohols with acetic acid and acetic anhydride in [C4mim][PF6] using metal catalysts [360]. They found that these reactions proceeded well initially, but the catalytic system became ineffective when the IL/catalyst was recycled. It was deduced that the dissociation of [PF6]2 to phosphate and fluoride ions under aqueous conditions led to catalytic deactivation. Ultrasound was used in the acetylation of a range of alcohols in [C4mim]Br IL [361]. Generally, a 5- to10-time enhancement in the reaction rate is observed compared with the equivalent silent reactions. Han et al. found that the chemical equilibrium of p-toluene sulfonic acid (pTSA)-catalyzed Fischer esterification can be enhanced from 63% to 73% by using ILs, such as [C2mim][PF6] and [C4mim][PF6], as solvents, which opens up a new way to improve equilibrium conversion using green solvents [362,363]. The selective esterification of carboxylic acid with alkyl halides was developed by using ILs (1-butyl-3methyl-4,5-dihydroimidazolinium hexafluorophosphate) as solvents in the presence of triethylamine with high ester yields [364]. Hayashi and Hamaguchi discovered a magnetic IL ([C4mim][FeCl4]) and reported that the [FeCl4]2 anion displayed magnetic behavior [365]. This magnetic IL was used to catalyze esterification of oleic acid with methanol to produce biodiesel [366]. Under optimal conditions, a methyl oleate yield of 83% was achieved. Kinetic experiments showed that the reaction followed a pseudo-first-order reaction with an activation energy of 17.97 kJ mol21, which is lower than that of homogeneous and heterogeneous catalysts for oleic acid esterification.

Catalytic Reaction in Ionic Liquids 155 Davis and co-workers reported that the first Brønsted acidic ILs with an alkane sulfonic acid group covalently tethered to the IL cations catalyzed Fischer esterification of acetic acid with ethanol [367]. During the course, they observed temperature-controlled liquid
solid separation phenomenon. Since then, alkane sulfonic acid-functionalized ILs have attracted increasing interest. Xing et al. prepared a series of water-stable ILs with the alkane sulfonic acid group in a pyridinium cation and tested their catalytic activity for esterification of benzoic acid with methanol, ethanol, and butanol [368]. They found that the catalytic activity of each IL was dependent on its anion, that increasing the anion Brønsted acidity improved the catalytic activity of ILs, and that the IL with [HSO4]2 as the anion showed the highest efficiency. Liu et al. also found that double SO3H-functionalized ILs showed high efficiency to catalyze esterification of glycerol with acetic acid [369]. Liang et al. synthesized a multiSO3H-functionalized IL using hexamethylenetetramine and 1,4-butane sulfonate as starting substrates [370]. The novel IL was an efficient catalyst for esterification of free fatty acids (FFAs) and transesterification of rapeseed oil. Because of its insolubility in the organic phase, recycling of the IL catalyst was very convenient. After being recycled 10 times, no deactivation was observed. Wang et al. reported a series of “reaction-induced self-separated” IL catalysts composed of propylsulfonic acid-functionalized organic cations and heteropolyanions for various esterification reactions, with one of the reactants being polycarboxylic acids or polyols [371]. The good solubility in the strong polar polycarboxylic acids or polyols, nonmiscibility with apolar ester products, and high-melting points of the heteropolyanionbased IL catalysts facilitate switching from homogeneous catalysis at the beginning to heterogeneous catalysis at the end, making the recovery and reuse of these catalysts very convenient. After recycling four times, only a slight decrease in the catalytic activity was observed. Fu and coworkers synthesized a di-cationic IL ([C6mim][HSO4]) and used the IL to catalyze the esterification reaction of organic acids in bio-oil with ethanol at room temperature to upgrading bio-oil [372]. They found that no coke and deactivation were observed after reactions and that the yield of upgrading oil was about 49% with significantly improved properties. Similar work was also reported by Fang and coworkers [373]. They found that some di-cationic quaternary ammonium salt ILs were efficient and recyclable catalysts for the production of biodiesel from long-chain fatty acids or their mixture with low-molecularweight alcohols. The ester yields reached 93
96%. Hu et al. used Brønsted acidic amino acid ILs to catalyze the esterification reaction of valeric acid with ethanol to produce ethyl valerate (EV) [374]. They found that under the optimized reaction conditions, proline bisulfate (ProHSO4) IL provided the best performance, with an EV yield over 99%. The produced EV was identified as a promising biofuel candidate. Qin et al. designed a tailed

156 Chapter 4 long-chain Brønsted acidic IL, 3-(N,N-dimethyldodecylammonium) propanesulfonic acid p-toluene-sulfonate, and used the as-synthesized IL to catalyze the esterification of FFA with methanol [375]. They found that the tailed long-chain Brønsted acidic IL exhibited high efficiency with good methyl ester yields of 93
97% and that the IL catalyst could be recycled more than nine times without deactivation. Thus, the tailed long-chain Brønsted acidic IL has good potential for the production of biodiesel from low-cost waste oils. Yuan et al. synthesized a series of N-methyl-2-pyrrolidone (NMP) and caprolactam (CP)-based Brønsted acidic ILs and found that the relative acidity of the prepared lactam-based ILs followed the order of H0 value: [NMP][BF4] (20.25) . [CP][BF4] (20.22) . [NMP] [CH3SO3] (0.95) . [CP][CH3SO3] (0.98) . [NMP][NO3] (2.83) . [CP][NO3] (2.94) . [NMP] [CF3COO] (4.46) . [CP][CF3COO] (4.56) [376]. Obviously, anions played a crucial roles in tuning the IL acidity. Using esterification of acetic acid with n-butanol as a model reaction, they found that ILs with a methyl sulfonate anion afforded the highest activity, indicating that the acidity and immiscibility of Brønsted acidic ILs have a synergistic effect on the esterification performance. 4.6.1.2 Basic ionic liquids as catalysts/solvents for esterification The IL [C2mim][PhCO2] was used in the benzoylation of glucose with benzoic anhydride (Fig. 4.21) [6]. The reaction was found to work well with anhydride but failed when acid chlorides were employed. It was stated that the basicity of the benzoate ion was responsible for promoting the reaction. A similar work was reported by Forsyth et al. using [C4mim] [N(CN)2] as the catalyst and acetic anhydride as the acylation reagent [377]. The milky basicity of bicyanamide ion promote the esterification reaction. Welton et al. investigated the IL solvent effect on esterification rate by using a linear salvation energy relationship based on the Kamlet-Taft solvent scales and found that the H-bond basicity of the solvent was the dominant factor in determining the reaction rate and that the highest rate was achieved in solvents with low basicity [378]. Homogeneous esterification of Xylan-rich hemicelluloses with maleic anhydride was conducted by Ren and coworkers using LiOH as the catalyst and [C4mim]Cl as the solvent, which provided an efficient approach to modify biopolymers for functional biomaterials [379]. Chen et al. used a Lewis basic IL, 1,8-diazabicyclo[5.4.0]undec-7-en-8-ium acetate, as the catalyst to conduct esterification reactions of phenols and in primary, secondary, and benzylic alcohols OCOPh

OH HO HO

(PhCO)2O

O OH OH

[C1C2mim][PhCO2]

PhOCO PhOCO

O OCOPh OCOPh

Figure 4.21 Esterification of glucose with benzoic anhydride in [C1C2mim][PhCO2] [6].

Catalytic Reaction in Ionic Liquids 157 with acetic hydride and good to excellent ester yields were obtained [380]. They also found that compared with traditional organic solvents, such as 1,2-dichloroethane, acetonitrile, toluene, tetrahydrofuran (THF), and n-hexane, the highest ester yield and efficiency were achieved under solvent-free conditions, which made the present esterification reaction an environmentally friendly process and suitable for industrial applications. Ambika and coworkers reported a NaHCO3-catalyzed chemo-selective esterification of salicylic acid with dimethyl sulfate using ILs as solvents [381]. In comparison with organic solvents (DMF, DMSO, and THF), NaHCO3 showed much better catalytic performance in the presence of [C4mim][PF6]. 4.6.1.3 Ionic liquids as solvents for lipase-catalyzed esterification ILs often lead to improved process performance and increase in enzyme activity, stability, and selectivity [382
391]. In 2000, Sheldon’s group reported the first example of ester biosynthesis catalyzed by Candida antarctica lipase B (CaLB) in [C4mim][PF6] and [C4mim][BF4] IL media [392]. They found that the reaction rates in ILs were equal or even faster than in a conventional organic reaction medium, t-BuOH. CaLB also showed good activity for transesterification in ILs based on dialkylimidazolium or quaternary ammonium cations associated with perfluorinated or bis(trifluoromethyl)sulfonyl amide anions. Rios et al. found that in water-immiscible ILs, such as [C4mim][PF6], [C4C10mim][PF6], [C6mim][PF6], [C8mim][PF6], [C2mim][NTf2], [C4mim][NTf2],[C6mim][NTf2], and [C8mim][NTf2], CaLB exhibited higher transesterification activity than in hexane for the synthesis of butyl butyrate [393]. Regarding anion composition of water-immiscible ILs, CaLB showed higher activity in ILs containing the [PF6]2 anion than that in [NTf2]2. It was stated that the higher tendency of the most nucleophilic anion [NTf2]2 changed the conformation of CaLB by interacting with the positively charged sites in the enzyme structure and resulted in the low CaLB activity. In contrast, in water-miscible ILs, such as [C2mim][BF4], [C4mim][BF4], [C4C10mim][BF4], [C6mim][BF4], [C2mim][dca], [C4mim] [dca], [C8mim][dca], [C4mim][NO3], [C4mim][OAc], [C4mim][OcSO4], and [C4mim] [MDEGSO4], the enzymatic activity was lower than in hexane. The reason was attributed to the direct interaction of the anion with the enzyme molecules leading to protein denaturation. The dissolution of CaLB in water-miscible ILs, such as [C4mim][NO3], [C4mim][OAc], and [C4mim][dca], also resulted in deactivation. Besides CaLB, other lipases, such as Pseudomonas cepacia lipase (PcL), Candida rugosa lipase (CrL), Candida antarctica lipase A (CaLA), Thermomyces lanuginosus lipase (TLL), Rhizomucor miehei lipase (RmL), Pseudomonas fluorescens lipase (PfL), Pig pancreas lipase (PpL), and Alcaligenes sp. lipase (AsL), have been often used as biocatalyst for ester synthesis in ILs [394
399]. Salunkhe et al. found that PcL exhibited a relatively high extent for transesterification of 2-hydroxymethyl-1,4-benzodioxane with vinyl acetate in [C4mim][PF6] in comparison with CH2Cl2 [398]. Kim et al. reported that CrL was more

158 Chapter 4 rapid and selective to catalyze the esterification of carbohydrates in ILs than in organic solvents [400]. Plazl and coworkers reported on a lipase-catalyzed esterification process in a 1-butyl-3methylpyridinium dicyanamide/n-heptane two-phase system for the production of isoamyl acetate with a micro-reactor [401]. The amphiphilic properties of CaLB lipase made it an efficient catalyst for esterification and product removal. CaLB-catalyzed esterification of lactic acid with ethanol was conducted by Gubicza et al. using commercially available Cyphos 104 and Cyphos 202 ILs as solvents and microwave irradiation as the heating source [402]. The combination of microwave, enzymes, and ILs enhanced the esterification process, and an ethyl lactate yield of 105% was achieved after reacting for 7 h. Gubicza and coworkers also reported that in the presence of the IL [C4mim][PF6] Candida rugosa lipase showed high activity for enantio-selective esterification of (R,S)-2-chloropropanoic acid with butan-1-ol [403]. They found that the polarity and the hydrophobicity of the medium influenced the enzyme activity dramatically. Liu et al. immobilized Candida rugosa lipase on Fe3O4 magnetic nanoparticles-supported ILs through ionic adsorption [404]. They found that the supported lipase showed better catalytic activity and stability for esterification between oleic acid and butanol. The activity of the supported lipase was 118.3% compared with that of the free enzyme. Additionally, the supported Candida rugosa lipase retained 60% of its initial activity after eight recycles, whereas complete deactivation was observed for the native lipase after six recycling tests.

4.6.2 Supported Ionic Liquids as Catalysts for Esterification Although homogeneously catalyzed esterification reactions using ILs as catalysts show high efficiency, inevitably, there are some inherent drawbacks of ILs. For instance, certain solubility of ILs in some organic compounds, especially polar molecules, resulted in the loss of catalyst. Furthermore, the high viscosity of ILs increased the mass transfer resistance and limited their industrial application. In recent years, much effort has been made to solve these problems. Immobilization of ionic liquids on solid supports provides a promising choice [405
410]. Supported IL catalysts with a catalytic amount of ILs immobilized on solid supports combine the advantages of both heterogeneous catalysts, in terms of their easy separation, and homogeneous catalysts, in that the IL films on the support surfaces provide a homogeneous environment for catalytic reactions. Silica, alumina, polymers, and magnetic materials are commonly used supports to immobilize ILs [411,412]. The preparation of supported IL catalysts was mainly conducted by using two different immobilization methods. The first one immobilized ILs by depositing the IL phase on the surface of the support. The second one involves the covalent bonding of ILs to the support surface. The first supported Lewis acidic IL catalyst was prepared by impregnating a previously dried silica gel with a chloroaluminate ionic liquid (Fig. 4.22) [413]. After removal of

Catalytic Reaction in Ionic Liquids 159

Si OH +

N

N Si O

(AlCl3)xCl

(AlCl3)x N

+ HCl

N

Figure 4.22 Immobilization of chloroaluminated-based IL via impregnation [413].

excess IL via Soxhlet extraction, the resulting IL was anchored onto the silica surface via covalent bonds between the aluminum atoms and the surface OH groups. Unfortunately, the catalytic activity of the resulting catalyst for esterification was not tested. Hashemi et al. immobilized a task-specific Brønsted acidic IL, [Bzmim][HSO4], onto a commercially available silica gel (Davisil grade 635) with high surface area of 480 m2 g21 through physisorbed confinement [406]. The prepared IL@silica catalyst showed higher catalytic activity compared with the homogeneous IL system for Fischer esterification of 3-phenylpropionic acid, lauric acid, palmitic acid, and stearic acid. The ester products were produced with high yields, and the IL@silica catalyst could be easily separated from the reaction mixture by simple filtration. However, deactivation of the supported catalyst was detected. After four recycles, the yield of ester decreased from the initial 92
81%. Karimi et al. incorporated a hydrophobic Brønsted acidic IL, [C8mim][HSO4], inside the nanochannel of propylsulfonic acid-functionalized SBA-15 (SBA-15-Pr-SO3H) by impregnating SBA-15-Pr-SO3H with an acetone solution of [C8mim][HSO4], and the obtained solid catalyst (IL@SBA-15-Pr-SO3H) showed excellent catalytic performance in direct esterification of various carboxylic acids with alcohols at room temperature under solvent-free conditions [414]. They found that the synergistic effect between grafted propylsulfonic acid and the immobilized [C8mim][HSO4] IL accounted for the improved catalytic efficiency of IL@SBA-15-Pr-SO3H. Unfortunately, leaching of confined ILs was detected. After recycling four times, the catalyst loss reached 7%, and the ester yield dropped from 100% to 89%. Yokoyama et al. immobilized 1-allylimidazolium containing acidic ILs on 3mercaptopropyltrimethoxysilane modified silica via radical chain transfer reaction between the allyl and sulfhydryl groups (SH) (Fig. 4.23) [415]. They found that the immobilized IL showed nearly the same activity as its homogeneous counterparts in the esterification of ethanol with acetic acid, and the supported catalyst could be recycled more than three times without loss of catalytic activity. The same protocol was also used to prepare the [BsAIm] [OTf]/SiO2 supported IL catalyst by Zhen and co-workers [409]. After immobilization, the acidic IL 1-allyl-3-(butyl-4-sulfonyl) imidazolium trifluoromethanesulfonate ([BsAIm]

160 Chapter 4 O O

O O

OMe Si

OMe Si

CF3SO3– SH

+

N

AIBN

MeCN

30 h

reflux

N (CH2)4

SO2Cl

CF3SO3– S

N

N (CH2)4

SO2Cl

Figure 4.23 Immobilization of IL on modified silica [415].

[OTf]) also showed high catalytic activity for esterification of oleic acid with methanol, and the oleic acid conversion increased with the increment of [BsAIm][OTf] loading. In contrast, in the transesterification of glycerol trioleate with methanol, the substrate conversion decreased with the increase of [BsAIm][OTf] loading. Li and co-workers immobilized [BsAIm][OTf] onto magnetic mesoporous silica (SCF) via free radical addition [416]. They found that the prepared [BsAIm][OTf]/SCF catalyst exhibited some shapeselective catalytic behavior in the esterification of oleic acid with straight-chain alcohols. When the carbon chain became longer, the conversion of oleic acid decreased because of the mass transfer resistance of the reaction system. Guan et al. used tetraethoxysilane (TEOS) as the silica source, P123 as the template, and 3-mercaptopropyltrimethoxysilane as the linker to prepare mesoporous silica-supported Brønsted acidic IL ([HSO3(CH2)3C2H3im] [HSO4]) catalyst [417]. The supported IL catalyst showed good to excellent yield for the esterification of C1-C3 carboxylic acids with various aliphatic and aromatic alcohols. Using the esterification of acetic acid with n-butanol as a model reaction, the authors measured the kinetic parameters, and they found that the activation energy catalyzed by supported ILs was a little higher than that of its homogeneous counterparts (38.2 vs 33.9 kJ mol21). However, the immobilized ILs could be easily separated and recycled. After seven recycling tests, the yield of n-butyl acetate could still achieve 89%. They also used a chloromethyl polystyrene as the linker to immobilize a Brønsted acidic IL ([HSO3(CH2)3Him][HSO4]) onto silica gel and prepared the supported IL catalyst IL/PS-SG [418]. The heterogeneous catalyst showed high catalytic activity for esterification of acetic acid and propanoic acid with methanol, ethanol, n-propanol, and benzyl alcohol. Excellent ester yields of 85
94% were achieved. Additionally, the catalyst could be recycled more than nine times without deactivation. A magnetic nanoparticles-supported, dual-acidic IL catalyst was prepared by immobilizing [HSO3(CH2)3Him][HSO4] IL onto the surface of core-shell structured Fe3O4@SiO2 nanoparticles using 3-chloropropyltrimethoxysilane as the chemical linker [419].

Catalytic Reaction in Ionic Liquids 161 The synthesized catalyst showed good behavior in esterification of oleic acid with shortchain alcohols and transesterification of soybean oil. The magnetic catalyst could be easily separated from the reaction mixtures and recycled more than eight times. Qi and coworkers synthesized a series of strong acidic IL-functionalized, ordered, and stable mesoporous polymer catalysts (OMR-ILs) using a high temperature hydrothermal synthetic method [420]. The obtained OMR-ILs catalysts showed much higher catalytic efficiency than that of Amberlyst-15, SBA-15-[C3mim][OTf], H-USY, and H-Beta for esterification of acetic acid with cyclohexanol and transesterification of tripalmitin with methanol [421]. The unique features of OMR-ILs, such as excellent catalytic efficiency, reusability, and thermal stability, will be important for their industrial applications.

4.6.3 Ionic Liquid Polymer as Catalysts for Esterification Besides loading ILs on solid supports, copolymerization of ILs with styrene or divinylbenzene represents another strategy to prepare IL-based heterogeneous catalysts. Liang prepared a novel solid acidic polymeric IL by copolymerizing the Brønsted acidic IL [SO3H(CH2)3VIm][HSO4](Fig. 4.24) with divinylbenzene [422]. To form ions clusters and enhance the interaction between ions, [SO3H(CH2)3VIm][HSO4] monomer was first polymerized to form oligomers. Then, prepared oligomers were reacted with divinylbenzene to form an IL-based co-polymeric catalyst (PIL). The hydrophobic property of the polydivinylbenzene surface improves mass transfer efficiency and prevents leaching of the acid sites. Therefore, the prepared catalyst showed high activity and stability for esterification of FFAs with methanol to produce biodiesel. The total yield of 99.1% was achieved after 12 h for the one-step biodiesel synthesis. Recycling tests showed that the PIL-based catalyst could be used more than six times without deactivation. Table 4.1 summarizes the results of a comparative study on the catalytic performance of PIL with other commonly used catalysts. For the Lewis acid IL catalyst [Et3NH]Cl AlCl3, after reacting for 24 h, the methyl ester yield was merely 79%. It is well known that Lewis acid ILs are sensitive to water. Therefore, generated water in the esterification course may decompose the catalyst and make the catalyst unrecyclable. For the SO3H-functionalized IL [HSO3C2mim][HSO4], acidic sites interacted well with methanol as well as water. The generated byproduct, water, might cause hydrolysis of ester products and decrease the

+N

HSO4– SO3H

Figure 4.24 Chemical structure of [HSO3(CH2)3VIm]HSO4 [422].

162 Chapter 4 Table 4.1: Catalytic performance of different catalysts for the production of biodiesel.

a

Catalyst

Catalyst Amount (mg)

Reaction Time (h)

Yield (%)a,b

PIL [Et3NH]Cl AlCl3 [HSO3C2mim][HSO4] H2SO4 H3PO4 Amberlyst-15

50 60 60 70 150 500

12 24 16 18 24 18

99 79 95 90 80 81

Reaction conditions: waster oil 5 g, methanol 2.91 g, 70 C. The yield was calculated on gas chromatography (GC) using an internal standard.

b

O N N

S

O

*

*

O N N

273K, 12 h

N

N 333 K, MeOH

N

*

* H3PW12O40

N

AIBN, 24 h

PW12O403-

SO3

SO3

VMPS

Poly(VMPS)

SO3H

3

Poly(VMPS)-PW

Figure 4.25 Preparation of the acidic polymeric hybrid poly(VMPS)-PW [423].

product yield. H2SO4 and H3PO4 are traditionally used homogeneous catalysts. However, for this reaction system, product yields were 90% and 80% for H2SO4 and H3PO4 catalysts, respectively. The commercially available resin Amberlyst-15 showed much lower activity because of low acidity. The novel PIL-based catalyst showed the highest efficiency in the screened catalysts. For the PIL catalyst, the active sites are located in the pores of hydrophobic surface, which facilitates the separation of water from the catalyst. Therefore, high activity for the synthesis of biodiesel was obtained. Leng et al. prepared a SO3H-functionalized heteropolyacid-based polymeric hybrid catalyst through coupling SO3H-functionalized polymeric IL cations with heteropolyanions (Fig. 4.25) [424]. The prepared poly(VMPS)-PW catalyst showed superior performance on esterification of acetic acid with n-butanol to referenced catalysts (Table 4.2). They found that the featured structure of the polymeric IL cations endowed the hybrid catalyst as an insoluble solid
liquid bi-phasic system for esterification and the acidic SO3H groups in the polymeric IL cations accounted for the excellent catalytic activity. Compared with the liquid MimPS-PW catalyst, poly(VMPS)-PW could be recycled more than six times without deactivation for esterification of acetic acid with n-butanol, demonstrating steady reusability.

Catalytic Reaction in Ionic Liquids 163 Table 4.2: Esterification of acetic acid with n-butanol by poly(VMPS)-PW and referenced catalystsa.

a

Catalyst

Phenomenon

Con (%)

Sel (%)

Poly(VMPS)-PW MimPS-PW Poly(VMPS)-HSO4 H3PW12O40 Blank

Liquid
Solid Liquid
Liquid Liquid
Solid Homogeneous


97.4 94.5 52.3 88.2 37.7

100 100 100 100 100

Reaction conditions: catalyst (0.2 g), acetic acid (30 mmol), n-butanol (36 mmol), 110 C, 1.5 h.

4.7 Alkylation Reaction ILs have been used in a wide range of catalytic alkylation reactions as catalysts, ligands, or solvents. Considerable amount of literature on IL-catalyzed alkylation reactions have been reported in detail, and there are several reviews on catalysis in ILs, including alkylation systems [5,425]. In the liquid-phase catalytic systems, such as alkylation of benzene [426], m-cresol and p-cresol [427,428], amines [429], ketones, and aldehydes [430], ILs are mainly added straight into the reaction mixtures and recycled by distilling the solvent out from the residue [431]. In a heterogeneous system, ILs can be supported on a solid phase leading to a clean and simple industrial application [432]. In this section, the selective reports of alkylation reactions with ILs in liquid-phase and heterogeneous system with supported catalysts will be reviewed, and the ILs involved industrial alkylation processes will also be summarized.

4.7.1 Liquid-Phase Systems Ru¨ther et al. reported on N-alkylation of the heterocyclic compound N-heterocycle 3azabicyclo[3.2.2]nonane catalyzed by [C4mim][PF6] or [C4mpyr][NTf2] in conjunction with potassium hydroxide [423]. Alkyl halides were used as alkylation reagent at 80 C in 20.5 h generating the yields of 70
90%. The reaction mixture then formed a three-phase system consisting the IL, product, and aqueous phase after quenching (Fig. 4.26). Followed by facile isolation, the IL could be reused in the next reaction circle. Imidazolium-based ILs are widely used in alkylation reactions, as catalysts or solvents in absence of other catalysts. The common effective anions can be halides and [PF6] is reported in the literature. In a facile cyclization of the arene process, [C4mim][PF6] showed the best performance compared with the ILs with the anions of [BF4], [OTf], [SbF6], and [NTf2] and other molecular solvents [433]. Similar results was also gained by Natrajan et al. in the N-alkylation of aridine esters [434]. In most of the cases, the imidazolium salts were recycled through distillation or extracted in toluene followed by vacuum drying.

164 Chapter 4

Figure 4.26 The three-phase reaction mixture formed by Ru ¨ther et al. [10].

Monopoli et al. reported a group of quaternary ammonium salts in N-monoalkylation of arylamines [435]. The imidazolium and pyridinium ILs with halide, [BF4] and [PF6] anions were also studied in the same system. Good conversions and selectivities were generated by using the ammonium halides. Moreover, the question of how the ILs promoted the reaction system was addressed. The authors pointed out that the nature of the anions played a predominant role in their system. At a certain temperature (90 C), the data revealed anion 2 effects as coupling efficiency in the following order: Br2 . Cl2 . . BF2 4  PF6 , which reflects the H-bond basicity of anions. Hydrogen sulfate salts have been also reported as a series of effective catalysts in phenol alkylations by Qurishi et al. [436]. N-methyl-2-pyrrolidone hydrogen sulfate ([NMP] [HSO4]) was used to promote the reaction in a solvent-free system. More details of hydrogen sulfate salts were studied by Cui et al. with good diversity of the cations [437]. The functionalized ILs with SO3-H have also been applied in several alkylation cases. N-(4sulfonic acid) butyl triethylammonium, N-(4-sulfonic acid) butylpyridinium and N-(4sulfonic acid) butyl-3-methylimidazolium hydrogen sulfate were used in p-cresol alkylation [438]. The catalysts were recovered by extracting with toluene, vacuum dried for hours and repeatedly used five times without major loss of the conversion and selectivity. Wasserscheid’s group used the same series of functionalized ILs but with a different anion [OTf] in the alkylation of phenol and anisole [439]. The ILs were also recycled through a similar method as above. Metal catalysts have been combined with ILs to form metal complexes to catalyze alkylation reactions. Fuente et al. reported an efficient catalytic system in allylic alkylation with ILs based on Pd/phosphine-imidazoline [440]. This system affords very good results (ee’s up to 95%) under microwave conditions. The product was removed by extraction and the IL (e.g., [C4mim][BF4]) left was vacuum dried for next circle. Seddon’s group also

Catalytic Reaction in Ionic Liquids 165 demonstrated a [cation]-Cl
InCl3 catalyst system for alkylation of hydroxyarenes [441]. In their system, a selection of hydroxyarenes and gaseous/liquid alkenes were examined, and a selective alkylation was achieved for the active aromatic compounds. The [cation]Cl
InCl3 catalyst system (mole fraction 5 0.667) was used, and the cations [C2mim], [C4mim], [C2dmim], [C4dmim], [C7dmim], and [C6py] were investigated. For example, the alkylation of catechol was performed using diisobutene with above IL
metal catalyst mixture, high yields of the desired products were obtained (.80%). Moreover, the kilogram scale was completed, which also generated 80% yield of the desired product. In these homogeneous or liquid systems, the recycling of ILs has either not been reported or been carried out through extraction or distillation. In the perspective view of industrial applications, supported ILs have been proposed for future study in some of above areas. This will allow an easy and energy-saving separation process of catalyst recycling, which opens up another IL catalysis area in combination with material science.

4.7.2 Heterogeneous Systems with Supported Catalysts Acidic ILs, including protic ILs and Bronsted acidic ILs, have shown good performance in alkylation catalysis. For example, the specifically designed pyridinium protic ILs were used in tert-butylation of phenol by Duan et al. [442]. It was shown that a range of pyridinium protic ILs could be easily produced and can be modified, leading to different acidities, catalytic activities, and, hence, selectivities and conversions for this reaction type. In general, ILs with HSO2 4 have also led to comparable selectivities and conversions for alkylation as the protic ILs. Recently, Sheng et al. reported supported acidic IL in o-xylene alkylation. Phosphotungstic acid (H3PW12O40) has been successfully loaded onto sulfonate-functionalized, IL-modified mesoporous silica SBA-15 forming the anion [H2PW12O40] [443]. It was prepared starting from imidazole and 3-triethoxysilylpropylchloride to form triethoxysilylpropylimidazole. Then 1,3-propane sulfone was added to generate 1-(propyl-3-sulfonate)-3triethoxysilylpropylimidazolium. The supported catalyst was obtained after reaction with SBA-15 and phosphotungstic acid. The immobilized IL could be reused without significant loss of catalytic activity even after recycling six times. There is a limited number of reports on supported IL catalyst in alkylation; however, many groups have, indeed, pointed out that development of immobilized IL catalysts would be included in their future work, and this indicates a great potential for both IL catalysts and green alkylation processes.

4.7.3 Industrial Alkylation Processes ILs have been applied in several important industrial production methods, mostly because of their relatively good tunability, activity, and stability. They are used in the alkylation of

166 Chapter 4 olefins, ethylbenzene production, linear alkylbenzenes production, and acylation of aromatic hydrocarbons, which are discussed in detail below. 4.7.3.1 Alkylation of olefins with isobutane Isobutane alkylation is one of the most important processes to produce reformulated gasoline. Sulfuric acid or hydrogen fluoride are used as catalysts commercially in alkylation plants. Both of the catalytic systems are liquid-phase systems, with limitations with regard to productivity, alkylate quality, safety, and operating costs. Meanwhile, a solid catalyst has been evaluated as an alternative, but some problems, such as catalyst deactivation and regeneration, must be solved before its industrial application. Several research groups have developed acidic chloroaluminate ILs into alkylation of isobutane with 2-butene. Jess’s group studied the influence of the acidity of modified acidic chloroaluminate-based ILs [444]. The additives transition metal salts, water, and acidic cation exchange resins were used to optimize the total conversion and selectivity of the desired products. A high content of the desired trimethylpentanes and, thus, a high research octane number of the alkylate were gained by using triethylamine hydrochloride aluminum chloride ([Et3NH]Cl/AlCl3) (molar fraction of AlCl3 is 0.6). Later, the same group reported on a designed batch reaction system for the application of this series of ILs that contain HCl [445]. Xing et al. observed in their studies a dramatic enhancement of the catalytic effects by using the acidic IL [Hmim][SbF6]. The C8 selectivity was 80%, and the research octane number was 95 [446]. Relatively good results were also obtained by other groups using acidic ILs [447,448]. A continuous-flow pilot plant demonstrated with the IL [pyridine/HCl]/AlCl3 (1:2 molar ratio) was the best candidate in the case of ethylene [449]. The reaction can be run at room temperature and provides good-quality alkylate. Relatively low temperatures and fine tuning of the IL acidity were required to avoid cracking reactions and heavy byproduct formation. The continuous butene alkylation has been performed for more than 500 h with no loss of activity and stable selectivity. Petrochina demonstrated the high stability of chloroaluminate-IL with CuCl through an 8month ageing test before a 60-day pilot scale operation [425]. The alkylation reaction was performed at 15 C and 0.4 MPa. During the pilot test period, olefin conversion was more than 99%. It was highlighted that the noncorrosive nature of the IL catalyst allows the use of less costly material for the design of the pilot (carbon steel reactors, piping, tanks, pumps, and valves). 4.7.3.2 Ethylbenzene production The investigation of chloroaluminate ILs was also carried out. They were used as liquid acid catalyst for the alkylation of benzene with ethylene to produce ethylbenzene.

Catalytic Reaction in Ionic Liquids 167 Industrially, AlCl3-red oil was used as the acid catalyst, which is defined as a mixture of AlCl3 with a polyalkylate, such as diethylbenzene. The liquid red oil formed a bi-phasic mixture, with the reaction products initially providing a potential system that can be separated easily. Unfortunately, during the reaction, some of the AlCl3 was gradually lost in the reaction product, which rapidly rendered the system monophasic and made the catalyst recycling very complicated and not economically applicable. Then, the chloroaluminate ILs based on the imidazolium cation was studied in detail to search for bi-phasic alternatives. BP demonstrated the potential of ILs, which compared favorably with the industrial red oil. The catalytic system of [C2mim][Cl]/AlCl3 or [HNMe3][Cl]/AlCl3 in a 1:2 molar ratio remains the bi-phasic character of the mixture, which facilitates the recycling of the catalyst by gravity separation. However, the limitation is the high cost of producing and using the IL catalyst. This would not be an application barrier if certain technical targets could be met according to the overall outcome of the laboratory BP evaluation. For example, the deactivation rate must not exceed 15% per cycle, and the production cost of the IL catalyst should be no more than $5000/t. 4.7.3.3 Linear alkylbenzenes (LAB) production The alkylation of benzene with linear olefins (C10-C14) is a well-established industrial process. The linear alkylbenzenes produced are used as intermediates in the manufacture of surfactants and detergents. Traditional processes use AlCl3 and HF as acid catalysts, which suffer from both poor catalyst separation and recycling. Hope et al. developed specific ILs based on triethylamine hydrochloride and aluminum chloride, which can be inexpensive alternatives to imidazolium-based salts and applied in a similar manner. These ILs were specially applied to the alkylation of benzene with 1-dodecene. It appeared that higher 2-dodecylbenzene yields were obtained in the IL than with the conventional HF process. Similar as the ethylbenzene production, alkylbenzenes are poorly miscible in ILs. The reaction proceeds in a bi-phasic mode making catalyst recovery and recycling easier. In catalyzation by traditional catalysts, consecutive polyalkylation reactions may occur, since the alkylated benzene hydrocarbons are more reactive than the monoalkylated starting material. The polyalkylation can be limited in the bi-phasic IL mode. The polyalkylation reactions are not favored, since alkylated benzenes are less soluble in the catalytic phase compared with monoalkylated benzenes. Analogous ILs were also evaluated and supported on solids, such as silica alumina or zirconia, providing improved activity and selectivity for monoalkylated products. 4.7.3.4 Acylation of aromatic hydrocarbons ILs have been largely used in acylation to enhance reaction rates, conversion, and selectivity. The solvation properties of ILs, which can aid in solubilizing the reactants, allows for a reduction in the number of steps in synthesis. However, in this process, even if

168 Chapter 4 the ILs have demonstrated their benefits, the main issue remains in the separation of products from the ILs and the IL recycle.

4.7.4 Main Process Challenges and Issues As reported in the selected examples previously cited both in the liquid-phase and heterogeneous systems with supported ILs in alkylation, it is obvious that ILs can contribute to certain significant improvements. However, ILs must meet the specific requirements to be applied in industrial processes. Even considering long-term advantages for the industry, changing an established industrial process for a novel technology using ILs is not easy. It requires willingness to accept the risk associated with implementation of new approaches and products. New environmental regulations and competition may force the chemical industry to take the plunge. The novelty and the price/production cost of ILs are probably one of the main barriers. The IL cost must be related to the process performance and to overall economy, as discussed under “ethylbenzene production.” The major issues relate to IL life times (e.g., their chemical and thermal stability, their loss in the process, their recovery, and recycling). The novelty of ILs raises a certain questions with regard to their toxicity, their reliable supply, their material compatibility, and their notification requirements. Their reliable supply is currently not a major concern, as IL production and commercialization have resulted in the development of new companies or businesses. 4.7.4.1 IL stability, lifetime and recyclability Generally, ILs display high chemical and thermal stability, the latter being mostly dependent on the nucleophilic character of the anion. However, very often, valuable information concerning the long-term stability of ILs is lacking. In some cases, thermal gravimetric analysis (TGA) provides these data, without reporting the decomposition products. The thermal and chemical stability of ILs also depend on the presence of impurities from their synthesis and on their exposure to process conditions, reactants, and products. In multiphase liquid
liquid systems, pretreatment of feedstocks can be important to protect the catalytic system from polar impurities that can accumulate in the IL phase. For example, in the Difasol “stand-alone” process, the lower the content of feedstock pollutants, the longer will be the lifetime of the chloroaluminate-IL. The best approach to remove these poisons as thoroughly as possible includes a water wash with condensed or feed boiling water, followed by processing with a water removal device. The dry feed is then treated by proper molecular sieves to remove both oxygenates and sulfur compounds. It should be noted that all these feedstock treatments are also recommended to minimize classic Dimersol catalyst consumption, but using stand-alone Difasol would lead to more severe treatments and therefore higher investments and chemical expenses.

Catalytic Reaction in Ionic Liquids 169 As discussed earlier, some of the cases, indeed, demonstrated a “simple” separation process because the products hardly dissolved in the catalyst. However, the partial miscibility of ILs in organic reaction products, even at the trace level, can be the cause of IL loss and product contaminations, which can be dramatic issues for the overall economy of the process and product quality when the systems are scaled up. To solve this issue, different techniques have been envisaged, such as the use of an organic cosolvent, which can also be nonconverted recycled reactants or supported IL processes, which use less IL-immobilized on solid supports and can be applied in some specific cases. 4.7.4.2 Safety and environmental issues As the alternatives of the traditional catalysts in alkylation reactions, ILs have been claimed as “green solvents.” To evaluate this claim, some ILs have been the subject of toxicity and ecotoxicity studies, and data are now available on a larger variety of organisms (bacteria, fungi, fish, algae.). Most studies have been carried out on imidazolium- and pyridiniumbased ILs, with alkyl or alkoxy side chains. The variety of anions studied is limited mainly to bromide, chloride, hexafluorophosphate, and tetrafluoroborate. It has been established that side chains on the imidazolium cations have a stronger influence on IL toxicity. The longer and the more branched the side chain, the more toxic is the IL. Effects of anions are generally more complicated. In an IL-catalyzed alkylation process, not only economics but also sustainability must be considered in the evaluation. Sustainability concerns the whole process from raw materials, the catalytic system, manufacturing, purification of the final products, disposal of waste, transport, and storage. In this context, the whole life cycle of ILs in the reaction system, including the disposal, must be taken into account.

4.8 Summary and Prospects In summary, the works cited in this chapter indicate that ILs have been demonstrated to be a promising catalyst because of their unique physical and chemical properties. The high structural versatility of ILs points to more opportunities. Various functional groups or active species could be introduced on to the IL structure by synthetic methods, which can be tailored for desired reactions. Moreover, integration of ILs with other catalysts generates wide applications in chemical reactions. The addition of ILs has shown great potential in catalysis for sustainability, such as cooperative effects, reduction of activation energy, and modification of the reaction pathway. Thus, despite the still relatively high costs and the insufficient toxicity data on ILs, their potential has been recognized worldwide. Future studies should focus on analyzing the mechanism of the IL-catalyzed reaction, investigating the structure
property relationship, and developing the application of ILs in chemical

170 Chapter 4 industry. Scientists and engineers have been working toward the advancement of largescale production and modification of ILs to making them feasible in practical applications.

Acknowledgments This work was supported by the National Basic Research Program of China (No.2015CB251401, 2014CB239701), the National Natural Science Foundation of China (21106146, 91434111, 21276255, 21376242, 21343004), and the Beijing Natural Science Foundation (2142029).

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CHAPTER 5

Separation Science and Technology 5.1 Introduction Many chemical processes have solvent mixtures as a typical byproduct. Efficient recycling of these solvent mixtures is crucial not just for cost implications but also for environmental considerations. Distillation is a traditional and still the most commonly used separation technique for separating solvent mixtures in the chemical industry. Distillation systems have been developed to a high extent over the last couple of decades; however, distillation systems still usually account for more than half the total energy consumption by the chemical industry and about a third of the total capital cost [1]. These huge energy consumption and the subsequent impact on the infrastructure continue to be targets for industry-wide optimization by many governmental agencies. Distillation works by exploiting the differences in the compositions of coexisting vapor and liquid phases during phase equilibrium. The order of the components’ compositions or relative volatilities does not change over the entire range of mixture compositions for many mixtures. However, some mixtures exhibit one or more points at which the compositions of coexisting liquid and vapor phases are the same: azeotropes. Consequently, separation of these mixtures into their individual components is not possible with simple distillation alone. Azeotropes can either be homogeneous, consisting of only one liquid-phase, or heterogeneous, consisting of two liquid phases. Heterogeneous azeotropes can usually be separated by decantation, followed by distillation processes that exploit both liquid
liquid and vapor
liquid equilibrium points. Some homogenous azeotropes are pressure sensitive, with appreciable changes in the azeotropic compositions of the components at different pressures. Such azeotropic compositions can therefore be separated by pressure-swing distillation, which works by using two or more distillation columns that operate at different pressures, together with appropriate recycling strategies to achieve the desire separation. If, however, the change in azeotropic compositions is small with pressure changes, pressureswing distillation will require very large recycling flow rates, subsequently leading to an uneconomical separation process [1]. All other forms of azeotropic compositions usually require the addition of an extraneous component (known as an entrainer or mass-separating agent) to the azeotropic mixture to facilitate separation by distillation—extractive or azeotropic distillation. However, it is much less energy intensive to be able to facilitate

Novel Catalytic and Separation Processes Based on Ionic Liquids. DOI: http://dx.doi.org/10.1016/B978-0-12-802027-2.00005-4 © 2017 Elsevier Inc. All rights reserved.

193

194 Chapter 5 separation by the addition of extraneous components to enable separation at room temperatures—liquid
liquid extraction. The use of ionic liquids (ILs), with their unique characteristics offers an enhanced separation process [2]. Several separation processes for separating azeotropes are currently available, including extractive distillation, pressure-swing distillation, liquid
liquid extraction, adsorption, and membranes.

5.2 Extractive Distillation Extractive distillation is currently the most common process for separating one component from an azeotropic system at or close to its azeotropic point. Entrainers, or mass-separating agents, are heavy chemical compounds (high boiling point with low volatility) used for extractive distillation and work by altering the relative volatilities of the components of the azeotropic system. Entrainers typically used for extractive distillation have a higher boiling point than that of either of the pure components and does not form an azeotrope with any of the pure components. Azeotropic distillation requires that both the entrainer and the components be vaporized. Azeotropic distillation also usually requires large amounts of entrainer, leading to higher energy consumption compared with extractive distillation [3]. Unlike in azeotropic distillation, entrainers used in extractive distillation do not need to be vaporized. Used entrainer can therefore be easily separated in a secondary distillation column. An IL-based extractive distillation separating pure components using an entrainer is shown in Fig. 5.1 [4]. However, conventional extractive distillation typically results in

Anhydrous ethanol IL

Water IL+ Water

Feed (ethanol, water) Recycling IL

Figure 5.1 An extractive distillation sequence for separating (A) and (B) using entrainer (E) requires a double-feed extractive column [4].

Separation Science and Technology 195 low levels of product purity as the entrainer usually exits from the bottom of the solvent recovery column and contains impurities that affect the separation effect. Conversely, extractive distillation utilizes more energy compared with liquid
liquid extraction, so typical separation processes combine extractive distillation with other separation processes, such as azeotropic distillation and liquid
liquid extraction. A great example of the use of combined separation processes is the dehydration of isopropanol using distillation followed by pervaporation with a ceramic membrane, which has been shown to result in a total cost reduction of approximately 49% [5]. Typical entrainers include organic solvents, inorganic salts, and hyperbranched polymers. ILs possess unique properties that make them suitable for use as entrainers in extractive distillation. The negligible vapor pressure of ILs at ambient temperature and pressure minimize vaporization into the atmosphere, thereby leading to a more environmentally friendly operation, which also reduces exposure of chemicals to workers, increases ease of reuse and recycling by striping evaporation or drying, and consequently reducing total operating costs [6]. ILs are more convenient than solid salts, as such issues as transport, dissolution, and reuse do not arise. The purity of the distillate produced when using ILs is also improved, as there are no ILs in the distillate because of their nonvolatility. ILs can also be tailored for specific applications, can be easily regenerated, and can be inexpensive [7].

5.3 Aqueous Azeotropic Systems The most studied aqueous azeotropic system is the ethanol 1 water system [6]. Other aqueous azeotropic systems that have been studied using ILs as a mass-separating agent include the water 1 2-propanol, water 1 1-propanol, water 1 2-methyl-2-propanol, water 1 1-nutanol, and water 1 ethanol 1 ethyl acetate systems [8]. Ethanol for use in alcoholic beverages has been produced by the fermentation of sugars for a very long time. The major method of industrial production of ethanol is the acid-catalyzed hydration of ethylene. Ethanol has many industrial uses, the largest single use being a motor fuel and fuel additive. Ethanol may also be utilized as rocket fuel, for household heating, as a base chemical for other compounds, as antiseptic, and as a solvent. Separating the ethanol 1 water azeotropic system typically involves the use of several distillation columns. The ethanol 1 water azeotropic system has been extensively studied, mostly because of the ease of availability of its constituents but also because of its industrial importance. The ethanol 1 water azeotrope is formed at an ethanol concentration of 95.60 wt% at a temperature of 78.15 C at 760 mmHg [9]. Table 5.1 shows some ILs that have been studied as azeotrope breakers for the ethanol 1 water system. Various factors, such as the concentration of the IL, the alkyl chain length, and nature of the anion and the cation, affect the effectiveness of the separation. The relative volatility (αVLE ij ) and VLE selectivity (S ) can be calculated from vapor
liquid equilibria (VLE) data.

Table 5.1: Ionic liquids used as azeotrope breakers, as reported in literature, for the separation of water 1 ethanol, ethanol/water selectivity from LLE data (SLLE), ethanol/water relative volatility from VLE data (αVLE) at azeotropic composition, and ethanol/water separation factor from membranes data (αMEM) [6,10
16]. Ionic Liquid [C2mim][OTf] [C2mbpy][EtSO4] [C4mim)[Cl] [C4mim][Otf] [Omim][OTf] [C6mim][Cl] [C4mim][MeSO4] [C2mim][EtSO4] [C2mim][BF4] [C4mim][BF4] [C2mim][N(CN)2] [C4mim][N(CN)2] [C2mim [Cl] [C4mim][Cl] [C2mim][OAc] [C4mim][OAc] [C6mim][Cl] [C4mim][Cl] [C1mim][(Me)2PO4] [C2mim][(Et)2PO4] [C4mim][Br] [C4mim][Cl] [C4mim][PF6] [C2mim][BF4] [C4mim][BF4] [C4mim][Cl] [C2eim][(Et)2PO4] [C2mim][EtSO4] [C2mim][(Me)2PO4] [C1mim][(Me)2PO4] [C1mim][(Me)2PO4] [C3mim][Br] [(EtOH)NH3][Oac] [(EtOH)3NH][Oac] [(EtOH)2NH2][Cl] [P6 6 6 14][N(CN)2] [P6 6 6 14][((Me)3Pe)2PO2] [C6mim][NTf2] [C4mim][PF6] [C6mim][PF6] [Omim][PF6] [C4mim][PF6]

SLLE

αVLE

a

αMEM

1.27 2.40b 1.38 1.71c 0.95d 1.31 2.38b 2.02b 1.37 1.37 1.36 1.53 1.68 1.41 1.63 1.46 0.39 1.87b 1.17e 1.41e 1.06e 1.25e 1.19e 1.44 1.34 1.55 n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a.

b

VLE VLE VLE VLE

data data data data

at 100 kPa at 101.3 kPa at 30, 40, 100 kPa at 314.2 and 331.7 K

VLE VLE VLE VLE

data data data data

at 100 kPa at 101.3 kPa at 101.3 kPa at 100 kPa

VLE data at 101.3 kPa VLE data at 101.3 kPa VLE data at 101.3 kPa

VLE data at 363.15 K

Vapor pressure data Vapor pressure data Vapor pressure data Vapor pressure data Vapor pressure data Vapor pressure data Boiling temperature at 101.3 kPa

n.a. n.a. 10f 0.40g 0.33h 0.29i

LLE data at 298.15 K LLE data at 295 K LLE data at 298.15 K

4
6

Ethanol molar fraction  0.95 and IL molar fraction  0.10. Ethanol molar fraction  0.55 and IL molar fraction  0.10. c Ethanol molar fraction  0.90 and IL molar fraction  0.50. d Ethanol molar fraction  0.95 and IL molar fraction  0.80. e Ethanol molar fraction  0.95 and IL molar fraction  0.05. f Ethanol molar fraction in organic phase 5 0.04. g Ethanol distribution coefficient at ethanol molar fraction 5 0.97. h Ethanol distribution coefficient at ethanol molar fraction 5 0.48. i Ethanol distribution coefficient at ethanol molar fraction 5 0.60. a

Remarks

Supported liquid membranes: vapor permeation

Separation Science and Technology 197 αVLE 5 ij SVLE 5

ðyi =xi Þ ðyj =xj Þ

(5.1)

IL αwith ij

(5.2)

IL αwithout ij

Extraction efficiency indicates the ability of the solvent to remove solute from the azeotropes and can be calculated either from (liquid 1 liquid) equilibria (LLE) data, ELLE, or from extraction process data, EEP, as shown in Eqs. (5.3) and (5.4). ELLE 5

ILPhase IL Phase CSolute  VSolute  100 Mix  V Mix CSolute Solute

EEP 5

(5.3)

Raffinate wFeed inert 2 winert

(5.4)

Equilibrium wFeed inert 2 winert

IL Phase Mix is the concentration of solute in IL phase (LLE data); CSolute is the where CSolute IL Phase concentration of solute in the initial azeotropic mixture; VSolute is the volume of solute in Mix is the volume of solute in the initial azeotropic mixture (LLE IL phase (LLE data); VSolute Feed data); winert is the mass fraction of the inert in feed stream (extraction process data); wRaffinate is the mass fraction of the inert in raffinate stream (extraction process data); and inert

wEquilibrium is the mass fraction of the inert in one equilibrium stage (theoretical data). inert The ILs that have been studied have mostly worked by enhancing the relative volatility of the ethanol to the point of removing the azeotrope at a specific IL composition. Table 5.2 shows some ILs that have been studied for their effect on the water 1 ethanol system. However, the water 1 ethanol system behaves differently according to each IL. Zhang et al. [10] proposed that these differences may exist as a result of the balance of interactions between IL and water and IL and ethanol. An analysis of the dependence of the relative volatility of the water 1 ethanol system on the anion used is shown in Fig. 5.2 for the imidazolium cation at an ethanol concentration of Table 5.2: Concentrations and references of ionic liquids (ILs) studies in water 1 ethanol systems [10
12,17,18]. ILs

Concentration

References

[C2mim][OTf] [C2mim][BF4] [C4mim][BF4] [C1mim][(Me)2PO4] [C2mim][(Et)2PO4] [C4mim][Cl] [C6mim][Cl]

0.05, 0.1, 0.2 0.1, 0.3, 0.5 0.1, 0.3, 0.5 0.1, 0.2 0.1, 0.2 0.1, 0.2, 0.4, 0.6, 0.8, 0.98 0.1, 0.2, 0.4, 0.6, 0.8, 0.98

[11] [17] [17] [18] [18] [12] [10]

198 Chapter 5

Ethanol/water relative volatility (α αVLE)

2

1.5

1

0.5

[C

6]

]

F

Tf im

][P

P e) 2

M

4m

im

][O

(C N

] O4

][(

[C

m

2

2m

[

im

m 2 [C

[C

][(

im

][N

m

C2

][B

im

E

im

m

2

] F4

]

4

2m

[C

PO t) 2

A

][O

[C

c]

l]

][C

im

)2 ]

0

im

[C

r]

][B

im

[C

m

4

m

1

Ionic Liquid

Figure 5.2 Effect of the nature of the anion in ionic liquids (ILs) based on the imidazolium cation, taking into account the relative volatility for the (ethanol 1 water) system at xethanol  0.95 and xIL  0.1 [6].

0.95, and an IL concentration of 0.1. Separation factors are much larger (B10) at low ethanol concentrations, compared with separation factors at high ethanol concentrations [5]. Anions also tend to have a stronger effect in interaction with a given solvent compared with cations. Studies of the dependence of the water 1 ethanol system on the nature of the entrainer cation are few. A comparison shows that at low ethanol concentrations, the use of [C2mim][EtSO4] leads to a higher relative volatility than [C2mbpy][EtSO4]. However, at an ethanol concentration of 0.2, both [C2mim][EtSO4] and [C2mbpy][EtSO4] had a similar effect on the relative volatility of the system [12,16]. Studies on the effect of the IL cation chain length on the effectiveness of the IL as an entrainer for the water 1 ethanol system using various combinations of [C2mim]1, [Hmim]1 and [C4mim]1 as cations and [OAc]2, [N(CN2)]2, [Cl]2 and [BF4]2 anions showed that an increase in the IL cation chain length led to a less effective entrainer [10,13,17
21]. This suggests that smaller cations possess stronger IL
water interaction compared with large cations, which forces the ethanol into the vapor phase and increases the relative volatility. There was, however, no noticeable dependence on the relative volatility on the IL cation’s chain length at IL concentrations lower than 0.03 (Tables 5.3
5.5).

Separation Science and Technology 199 Table 5.3: Azeotropes, as reported in the literature, regarding separation of alcohols 1 esters. SLLE

Azeotrope

Ionic Liquids

Methanol 1 methyl acetate Ethanol 1 ethyl acetate

[C2mim][OTf] [C2mim][BF4] [C4mim][BF4] [Omim][BF4] [C2mim][OTf] [C4mim][Cl] [C6mim][Cl] [Omim][Cl] [Amim][Cl] [Amim][Br] [C2mim][BF4] [(EtOH)mim][BF4] [C2mmim][BF4] [(EtOH)mmim][BF4] [mim][H2SO4] [C2mim][H2SO4] [C4mim][H2SO4] [C2mim][H2SO4] [C4mim][PF6] [C6mim][PF6] [C1mim][MeSO4] [C2mim][H2SO4]

1-propanol 1 1-propyl acetate 2-propanol 1 ethyl acetate 1-butanol 1 1-butyl acetate

a

αVLE

b

Comments

1.85 1.83 1.45 2.35 1.57

VLE data at 100 kPa VLE data at 101.32 kPa VLE data at 101.32 kPa VLE data at 100 kPa LLE data at 298.15 K

89c 75c 97c 84c 81c 1.56 2.51 1.54 1.93 30.75 19.61 10.26 21.51 3.29 2.65 6.7 17.25

LLE data at 298.15 K

LLE data at 313.15 K

LLE data at 313.15 K LLE data at 298.15 K

LLE data at 313.15 K

Ionic liquids (ILs) used as azeotrope breakers. Alcohol/ester selectivity from LLE data (S volatility from VLE data (αVLE) [6,22
31]. a Alcohol molar fraction in organic phase  0.10. b Azeotropic composition and IL molar fraction  0.30. c Extraction efficiencies from Eq. (5.4).

LLE

) and acetate/alcohol relative

Table 5.4: Azeotropes, as reported in the literature, regarding separation of alcohols 1 ketones. a

Azeotrope

Ionic Liquids

SLLE

Methanol 1 acetone Ethanol 1 2-butanone

[C2mim][OTf] [C4mim][PF6] [C1mim][MeSO4] [C4mim][PF6] [C1mim][MeSO4]

1.64c 2.81d 4.14c 1.53d

2-propanol 1 2-butanone

αVLE 1.52

b

Comments

References

VLE data at 100 kPa LLE data at 298.15 K LLE data at 298.15 K LLE data at 298.15 K

[31] [32] [33] [32]

Ionic liquids (ILs) used as azeotrope breakers. Solute/inert selectivity from LLE data (SLLE) and acetone/methanol relative volatility from VLE data (αVLE) [6,32
34]. a Alcohol molar fraction in organic phase  0.10. b Azeotropic composition and IL molar fraction  0.36. c 2-Butanone/ethanol selectivity. d Ethanol/2-butanone selectivity.

200 Chapter 5 Table 5.5: Azeotropes, as reported in the literature, regarding separation of alcohols 1 alkanes. Azeotrope

Ionic Liquids

SLLE

Comments

Methanol 1 heptane Ethanol 1 hexane

[Omim][Cl] [C2mim][EtSO4] [C6mim(EtOH)N][BF4] [C4mim][MeSO4] [C1mim][MeSO4] [C6mim][PF6] [Omim][PF6] [C2mim][EtSO4] [C1mim][MeSO4] [Omim][PF6] [C4mim][MeSO4] [C1mim][PF6] [Omim][Cl]

401 3631 1584 482.1 9427 330.7 115.2 2343 5592 229.8 22,326 2861 457

LLE data LLE data LLE data LLE data LLE data LLE data LLE data LLE data LLE data LLE data LLE data LLE data LLE data

Ethanol 1 heptane

at 298.15 at 298.15 at 298.15 at 298.15 at 298.15 at 298.15 at 298.15 at 298.15 at 298.15 at 298.15 at 298.15 at 298.15 at 298.15

K K K K K K K K K K K K K

Ionic liquids (ILs) used as azeotrope breakers. Alcohol/alkane selectivity from LLE data (SLLE) at 0.01 alcohol molar fraction in organic phase [6,35
40].

5.4 Summary ILs can also be used as advanced materials in solid-phase extraction and separation [41], liquid-phase microextraction technology [42], and analytical separations [43]. Analytical separation techniques, including gas chromatography (GC), liquid chromatography (LC), and electrophoretic methods (capillary electrophoresis [CE]) have been investigated. In LC and CE, ILs are not used as pure solvents but, rather, are diluted in aqueous solutions. In this situation, ILs are just salts that are dual in nature. Although there has been a lot of interest in the use of ILs as azeotrope breakers, the fact that some of them hydrolyze in water and at high temperatures to form hydrofluoric acid has restricted their use to applications at moderate temperatures that do not have water present [44,45]. However, ILs based on alkyl sulfate anion have shown the most potential as solvents in LLE or the azeotropes presented.

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49.

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1008. ´ . Domı´nguez, Vapor-liquid equilibria for the ternary system [16] N. Calvar, B. Gonza´lez, E. Go´mez, A ethanol 1 water 1 1-ethyl-3-methylimidazolium ethylsulfate and the corresponding binary systems containing the ionic liquid at 101.3 kPa, J. Chem. Eng. Data 53 (3) (2008) 820
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2330. [26] D.L. Zhang, Y.F. Deng, C.B. Li, J. Chen, Separation of ethyl acetate - ethanol azeotropic mixture using hydrophilic ionic liquids, Ind. Eng. Chem. Res. 47 (6) (2008) 1995
2001.

202 Chapter 5 [27] X. Hu, Y. Li, D. Cui, B. Chen, Separation of ethyl acetate and ethanol by room temperature ionic liquids with the tetrafluoroborate anion, J. Chem. Eng. Data 53 (2) (2008) 427
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1703.

CHAPTER 6

Biomass Utilization 6.1 Introduction Biomass is defined as organic matter available on a renewable basis and includes agricultural crops and wastes, forest and mill residues, wood and wood wastes, animal wastes, livestock operation residues, aquatic plants, fast growing trees, and municipal and industrial wastes [1]. Biofuels from plant biomass, such as bio-alcohols (ethanol, butanol, etc.), biodiesel, bio-oils, and syngas derivatives, are the only sustainable class of liquid fuels [2
5]. Biomass is typically converted via chemical decomposition or biological digestion. Biomass conversion technologies for utilizing biomass can be separated into four basic categories: direct combustion processes, biochemical processes, thermochemical processes, and agrochemical processes [6]. Table 6.1 lists the various ways biomass is typically converted into biofuels and bio-products. Historically, research on the use conversion of biomass into liquid transportation fuels seems to have originated during the first energy crisis of October 1973 in the United States [7]. This energy crisis was a consequence of the Yom Kippur war and the OPEC embargo. Since then, global demand for petroleum products continues to increase steadily, with an increase from 57 3 106 barrels/day in 1973 to 82 3 106 barrels/day in 2004 and a projected increase of about 50% from the 2004 levels by 2025 [1,8]. With the 1979 Iranian revolution, diminishing oil supplies, national security issues arising from reliance on imported crude oil, the climate implications of burning fossil fuels, the environmental hazards of drilling, and the risk of severe environmental disasters, such as the Gulf of Mexico BP blowout/leak, there has been a global demand for the development of renewable energy resources that are not based on crude petroleum oil. Sources of biomass for fuels production include: • • • •

Palm and vegetable oils Sucrose, derived from sugar cane and sugar beets Starch-containing crops, such as corn Lignocellulosic biomass, including woody biomass, such as trees, shrubs, and grasses (e.g., Micanthus and switchgrass)

Novel Catalytic and Separation Processes Based on Ionic Liquids. DOI: http://dx.doi.org/10.1016/B978-0-12-802027-2.00006-6 © 2017 Elsevier Inc. All rights reserved.

203

204 Chapter 6 Table 6.1: Biomass conversion processes [6]. Advantages Direct Combustion Boiler combustion Stove combustion Biochemical Processes Anaerobic digestion Alcoholic fermentation G

G G

Easy processing Proven technology

Disadvantages G

Low thermal efficiency

G

G

G

G

G

G

High product yields Low energy consumption Modest reaction conditions

G G

G

Thermochemical Processes Gasification Pyrolysis Supercritical fluid extraction Direct liquefaction Agrochemical Processes G

G G

Fewer processing steps Shorter processing time

G G

G

G

G

G

Slow rates of conversion Secondary treatment of waste required Difficult mass production of enzymes High oxygen content Poor thermal stability Complex components Low energy efficiency

G

G G

G G

Reduction in greenhouse gases Reduction in particulate emissions High degradability Energy supply security

G

High raw material cost

Use of these biomass sources is dictated by their availability, cost, and sustainability of production. Palm and vegetable oils can be converted to biodiesel, which can be used on its own as a transportation fuel or blended with other diesels. Sugars are converted to ethanol by microbial fermentation and blended with gasoline. Most modern cars can run on gasoline with low concentrations of ethanol (5
15%), and flex-fuel cars are able to utilize fuels with any concentration of ethanol. Brazilian sugar cane ethanol can save 80% of greenhouse gas emissions compared with gasoline with current technology [9]. However, other ethanol options, such as corn ethanol, provide more modest savings on greenhouse gas emissions because of the more energy-intensive cultivation of these crops. The fact that these ethanol sources for fuel production are also food sources is a legitimate concern that makes their adoption untenable. Lignocellulosic biomass, however, is the most abundant plant material on Earth, with much higher yields per area of land, and is therefore available at lower cost than starch- or sucrose-based materials [9]. Estimated global biomass production is 1.0 3 1011 tons per year [10]. The use of lignocellulosic biomass as biofuel feedstock is also likely to provide much higher carbon dioxide emission savings [11]. Lignocellulose is a composite material that consists of three major biopolymers (cellulose, hemicellulose and lignin, making up approximately 90% of the dry matter). Lignocellulose contains about 60
70 wt% sugars/ carbohydrates and is typically not for human food production. The major drawback in the use of lignocellulose commercially for the production of fuels and chemicals is the lack of cost-effective solutions in the processing of lignocellulose. The

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Figure 6.1 Conversion of first-generation and second-generation feedstocks into ethanol via the fermentation route [9].

complexity of biomass processing increases from sucrose to starch to lignocellulose. Sucrose can be directly utilized for fuel production via yeast fermentation, whereas starch has to be hydrolyzed (depolymerized) to glucose by using enzymes, before fermentation (Fig. 6.1). The interwoven linkages among the major biopolymers of lignocellulosic biomass, however, result in a natural recalcitrant composite. Biomass requires an additional deconstruction step (also commonly called pretreatment) to bring the sugar polymers into a form suitable for hydrolysis and subsequent fermentation. This additional processing consists of a number of steps: feedstock comminution, the actual feedstock deconstruction, conditioning of the treated biomass and the hydrolysate (e.g. detoxification and neutralization), hydrolysis/ depolymerization of the polysaccharides. The multiple steps required for deconstruction of lignocellulose suggest that deconstruction will account for a sizeable portion of the energy requirement for biomass processing [12]. Proper utilization of the noncarbohydrate fraction of lignocellulose (lignin) is also required to ensure optimization of the deconstruction step and profitability for processing centers/bio-refineries [9,13].

6.2 Dissolution and Fractionation of Biomass The objective of biomass pretreatment is to improve the enzymatic digestibility of lignocellulosic biomass. Many substrate factors, such as substrate accessibility, lignin content, and particle size, affect the difficulty of the pretreatment process. Lignocellulosic biomass is a composite that consists mostly cellulose (35
50 wt%) and hemicellulose (25 wt%), both of which are polymeric carbohydrates, as well as lignin, which is an aromatic polymer (Fig. 6.2). Lignocellulosic biomass also contains smaller amounts of inorganic compounds, proteins, pectins, and extractives, such as waxes and lipids. Lignocellulosic biomass has varied compositions, depending on the species, plant tissue, and growth conditions. Hemicellulose is thought to bind noncovalently to the surface of

206 Chapter 6

Figure 6.2 Spatial arrangement of cellulose hemicellulose and lignin in the cell walls of lignocellulosic biomass [1].

cellulose fibrils and hold them in place, as indicated in Fig. 6.2. Hemicellulose is amorphous, so its noncrystalline nature makes it more susceptible to depolymerization compared with cellulose, especially in acidic conditions. Lignin, in contrast, is a waterinsoluble polymer that becomes part of the lignocellulose composite after plant growth has ceased. Lignin provides structural reinforcement, water-proofing, and resilience to biological and physical attack compared with the all-carbohydrate cell walls of immature plant tissues. Hemicellulose and lignin are entangled and covalently cross-linked [14]. Cellulose is composed of linked linear glucose chains by numerous inter- and intramolecular hydrogen bonds, or it could be seen as a linear polymer of cellobiose that consists of two glucose sugar units that are linked by glucosidic linkages (C-O-C) [14
19] The degree of cellulose polymerization decides the size of its chains, and the very stable β(1- . 4)-link of glucose repeating units is reinforced by intrachain hydrogen bonds (H-bonds) (Figs. 6.3 and 6.4). It is very difficult for cellulose to be dissolved in water and most of the common organic solvents with numerous inter- and intramolecular H-bonds, and the dissolution processes suffer from environmental, energy, and other problems. When Rogers et al. [20] discovered in 2002 that ionic liquids (ILs) were capable of dissolving cellulose, international academia and industry began to pay attention to the research of ILs for biomass utilization. ILs have demonstrated good prospects for the separation and conversion of biomass [21
24], and this has given rise to growing interest in the study of this field [25
28].

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Figure 6.3 Intermolecular and intramolecular hydrogen bonds in cellulose.

Figure 6.4 Structure and intramolecular hydrogen bonds of cellobiose.

6.3 Interaction of Ionic Liquids and Cellulose In the studies by Rogers [20], it was found that the cellulose could be directly dissolved in [C4mim][Cl] without any pretreatment. It is easy to precipitate cellulose from the IL solution and regenerate it by the addition of water or other solutions, such as ethanol, without significant change in the degree of polymerization. Since then, many studies have used ILs to dissolve cellulose, and these ILs are mainly composed of halogen [29,30], acetate, formate, acetic acid, and phosphonic acid ester [31
33] as the anion and imidazole [34,35], pyridine [36], choline [37], and phosphonic ion [38] as the cation. However, research shows that ILs can dissolve cellulose effectively, probably because ILs can destroy the H-bonds between the molecular chains of cellulose but this does not explain the micro interaction between ILs and cellulose. Most of the biomass research in ILs has focused on the development of technology to dissolve different kinds of biomass (e.g., cotton, silk,

208 Chapter 6 chitin, etc.), but few studies have investigated the real mechanism of ILs dissolving biomass (cellulose). To effectively separate the lignin, cellulose, and hemicelluloses in lignocelluloses, it is important to pay more attention to the interaction between ILs and the components in biomass and identify the structure
activity relationship and the dissolution mechanism. In general, there are two viewpoints regarding the interaction between ILs and cellulose in the process of cellulose dissolution, and the main difference between these viewpoints is the role of the cation and the anion in the dissolution. The first one is that the interaction between the anion and cellulose determines the whole dissolution process, and there is no clear action of the cation [39
41]. Another viewpoint is that in the process of dissolving cellulose, the main driving force is hydrogen bonding interaction of the hydroxyl group with both the cation and anion of ILs [42
44]. Both these views are supported by experimental and theoretical results. On the basis of these two opinions, it could be deduced that both the anion and the cation are involved in the dissolution process; however, the main point of contention is the role of the cation in the dissolution process [45,46].

6.3.1 Interaction of Cellulose and Anions At present, most of the experimental results have shown that the interaction between the anions of ILs and cellulose plays a leading role in the process of dissolution of cellulose. It was pointed out [20] that ILs incorporating anions, which are strong H-bond acceptors, were the most effective and that cellulose is dissolved through formed hydrogen bonding between hydroxyl functions and the anions. High-resolution 13C nuclear magnetic resonance (NMR) studies have indicated [41] that there is obvious interaction between the chloride anions and the cellulose hydroxyl groups in the IL [C4mim][Cl]. Remsing et al. [40] investigated the dissolution mechanism of carbohydrates in [C2mim][CH3COO], [C4mim] [Cl], and [Amim][Cl] by using 13C and 35/37Cl NMR relaxation and 1H pulsed field gradient stimulated echo diffusion measurements. The results of the NMR spectrum test, such as cellulose oligomer dissolved in the IL [C4mim][Cl], also showed that t dissolved cellulose, mainly destroying its intramolecular or intermolecular H-bonds and that the dissolving of cellulose was mainly a result of the specific interactions of the anion with carbohydrates. The ability of the anion to dissolve cellulose is influenced by many factors, and it is mainly dependent on the ability to form H-bonds. These factors include H-bond type and structure of the receptor, the nature of electronic absorption chemical group in ILs, the interaction between the anion and the cation, and so on. It was found that ILs with anions that are strong H-bond acceptors were the most effective solvents [20]. ILs containing chloride have a high ability to dissolve cellulose, whereas anions [BF4]2 and [PF6]2 do not. Other research [31,47] has indicated that the IL 1-N-ethyl-3-methylimidazolium acetate [C2mim] [CH3COO] with a lower melting point and viscosity has high ability to dissolve cellulose.

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Xu et al. [33] investigated the effect of the anion structure on the solubility of cellulose using 1H NMR, 13C NMR spectra, and solvatochromic ultraviolet
visible (UV/vis) probe. Several kinds of ILs were studied by coupling [C4mim]1 cation with different anions, including [CH3COO]2, [HSCH2COO]2, [HCOO]2, [(C6H5]COO]2, [H2NCH2COO]2, [HOCH2COO]2, and so on. It was found that cellulose solubility in ILs decreases in the order: [C4mim][CH3COO] . [C4mim][HSCH2COO] . [C4mim][HCOO] . [C4mim][(C6H5) COO] . [C4mim][H2NCH2COO] . [C4mim][HOCH2COO] . [C4mim][CH3CHOHCOO]. The electron-withdrawing groups (e.g., OH, SH, NH2, or CH3OH) to replace hydrogen in the CH3COO- anion will decrease the solubility, and the authors attributed this to the fact that electron-withdrawing groups decrease the H-bonds formation ability of CH2XCOO2 (X 5 OH, SH, NH2, and CH3OH) with the hydroxyl protons of cellulose. The [CH3COO]2 anion has the strong H-bond-accepting ability, so its solubility of cellulose is high, even better than that of the Cl2 anion [14]. These results suggest that the H-bond-accepting ability of the anions strongly dominates the capacity of ILs for the dissolution of cellulose, and the solubility of cellulose will increase along with the increasing H-bond-accepting ability of anions in ILs, as disruption of the intermolecular H-bond will be helpful for the dissolution process.

6.3.2 Interaction of Cellulose and Cations Zhang’s group [48] investigated the dissolving process of cellulose in the IL 1-allyl-3methylimidazolium chloride ([Amim][Cl]) and proposed that both the anion and the cation of [Amim][Cl] would form a strong H-bond with the hydroxyls of cellulose. These interactions of the cation and the anion disrupted the H-bond in cellulose. Subsequent work by Zhang et al. [43] focused on dissolution of cellobiose in [C2mim][CH3COO] ILs in different concentrations and temperatures by using NMR spectrum. It was found that H-bonds were formed between the acetate anion and the hydrogen atoms of the hydroxyls of cellobiose. At the same time, H-bonds also exist between the protons on the aromatic ring of the [C2mim]1 cation, especially H2 acidic protons, and the oxygen atoms on cellobiose hydroxyls. This suggests that the dissolution of cellulose is determined by ion interaction between the cation and the anion of ILs, although the anionic hydrogen bonding interaction with cellulose probably plays a leading role. However, Lindman et al. [49] thought that hydrophobic interactions between cellulose and the cations of ILs are probably the predominant factor, after they analyzed several kinds of interactions involved in the dissolution of cellulose. The role of the cation in the process of dissolution of cellulose is complex, and it is difficult to give an accurate description by using experimental measurements. It was thought that the role of the cation in the dissolution process is minor and related to size and hydrophobicity [49]. The most common and effective cations for dissolving cellulose are usually based on the imidazolium and pyridinium ILs with ethyl, butyl, or allyl side chains

210 Chapter 6 [15], although we cannot reach the conclusion that these cations are superior to the other cations, such as quaternized ammonium or phosphonium [14]. The high cellulose dissolution ability of pyridinium and imidazolium ILs was suspected to be related to the aromatic nature of these cations. The aromatic rings are not only more polarizable but also have lower relative strength of interaction between cations and anions with the charge delocalization of aromatic rings. Rogers [40], in his work on imidazolium ILs, found that for linear-chain alkyl imidazole chlorine ILs, the longer-chain substituted ILs ([C6mim][Cl] and [C8mim][Cl]) appear to be less efficient than [C4mim][Cl] in dissolving cellulose, which is attributed to the reduced effective chloride concentration within these liquids. The solubility of cellulose in these ILs decreases in the order: [C4mim][Cl] . [C2mim][Cl] . [C6mim][Cl] . [C8mim][Cl] . [C10mim][Cl] [14]. It can be seen that solubility decreased as the length of the solvent carbon chain increased. Erdmenger [50] investigated a series of 1-alkyl-3methylimidazolium chloride ILs with alkyl chain length varying from ethyl to decyl, as shown by elemental analysis and 1H NMR spectroscopy, and a distinct odd
even effect was found for chain lengths: ILs with even numbers of carbon atoms in the side chain show higher dissolution ability compared with those with odd numbers of carbon atoms. It is interesting to find this odd
even effect for the small alkyl chains, whereas the IL [C4mim] [Cl] has maximum dissolution power, and the reason for this is thought to be the additional polarity in the heteroatomic substituents on the imidazolium ring. For imidazole cation ILs, the ability for dissolution of cellulose is effectively increased by introducing functional groups into the side chain. Ren et al. [51] synthesized 1-N-allyl-3methylimidazolium chloride ([Amim][Cl]) ILs by appending the allyl (2CH2 2 CH 5 CH2) group on the imidazole cation. This IL with more double bonds than [C4mim][Cl] is effective, and the strong polarity of the double bond is thought be essential and to have good properties for dissolution of cellulose. Zhang [48] also reported that [Amim][Cl] is a powerful solvent for cellulose and that microcrystalline cellulose in [Amim][Cl] will dissolve quickly without the need for the IL to be activated or pretreated. The presence of a double bond
containing side chain is thought to reduce the viscosity of ILs [15]. Heinze et al. [52] investigated the change of degree for polymerization of cellulose in the process of dissolution by using several kinds of ILs, such as [C2mim][Cl], [C4mim][Cl]), 3-methyl1-butyl-N-pyridine chloride ([C4mpy][C1]), and 1-N-allyl-2,3-dimethylimidazolium bromide ([Admim][Br]), and found that all of these ILs were good cellulose solvents under mild conditions. Lu et al. [53] performed a systematic experiment on the effect of cationic structures on cellulose dissolution by using 13 kinds of ILs with a fixed anion [CH3COO]2 and varied cations. These cations with different backbones and alkyl chains included 1-butyl-3methylimidazolium [C4mim]1, 1-methoxyethyl-3-methylimidazolium [C1OC2mim]1,

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1-hydroxyethyl-3-methylimidazolium [C2OHmim]1, 1-butyl-2,3-dimethylimidazolium [C4dmim]1, 1-benzyl-3-methylimidazolium [phC1mim]1, N-ethyl-N-methylmorpholium [C2mmor]1, N-allyl-N-methylmorpholium [C 5 C2mmor]1, N-allyl-N-methylpiperidium [C 5 C2mpip]1, N-butyl-N-methylpiperidium [C4mpip]1, N-butyl-N-methylpyrrolidinium [C4mpyr]1, 1-butyl-3-ethylbenzimidazolium [C4ebim]1, 1,3-diethylbenzimidazolium [C2ebim]1, and 1-butyl-3-ethylbenzotriazolium [C4ebt]1. After investigating the solubilities of cellulose in these ILs at different temperatures, they found that the acidic protons on the heterocyclic rings of the cations in ILs are important for dissolution of cellulose because these protons may form C-H-O H-bonds with oxygen on the cellulose structure leading to an increase in the solubility of cellulose. However, the strong interaction between cations and anions and the steric hindrance effect of a large-size group would decrease cellulose solubility. Thus, it is a complex interaction based on structures and different influencing factors. The van der Waals interaction of the cation with cellulose is found not to be important in these cases. On the basis of all of the above experimental results, it can be deduced that the dissolution of cellulose is determined by both anions and cations in ILs. The H-bond interaction between the anion and cellulose plays a leading role. Although the cation plays a minor role in the dissolution process, it cannot be ignored. It is possible to control the ability of ILs for dissolving cellulose by designing and changing the structure of the cation.

6.3.3 Dissolution Mechanism of Cellulose with Interactions From Anions and Cations Most of the above reports indicated that hydrogen bonding is the major reason for dissolution cellulose in ILs [54]. ILs break the H-bonds in and between cellulose structures, and anions form strong direct hydrogen bonding with cellulose, whereas the cation has been suggested to play an indirect role in the ability of ILs to dissolve cellulose [55]. The proposed mechanism of dissolution of cellulose in ILs [56] (Fig. 6.5) shows that the oxygen and hydrogen atoms on the hydroxyl groups of cellulose form hydrogen bonding as electron donor
electron acceptor with the corresponding species of ILs. The formation of new H-bonds between the hydroxyl group with the anion or the cation leads to breakage of the H-bonds connecting the hydroxyl groups of different chains and then the dissolution of cellulose. The separation of hydroxyl is thought to occur between the C-6 and C-3 hydroxyl groups of neighboring cellulose chains [57]. In addition to the separation of the cellulose chains, there are other possibilities of structural changes occurring in the process of cellulose dissolution. Some experimental studies identified the side reaction of cellulose chains. When Heinze et al. [58] investigated the mechanism of cellulose dissolution in [C2mim][CH3COO] ILs with NMR studies, they found that the C-1 signal of the glucose unit disappeared after dissolution. On the basis of the result, they suggested that a covalent carbon
carbon bond formed between the carbon of the reducing end

212 Chapter 6

Figure 6.5 Proposed dissolution mechanism of cellulose in ionic liquids [56].

of the glucose and the carbon of the imidazolium core. This hypothesis was verified by Ebner et al. [59] through 13C-isotopic labeling and fluorescence labeling experiments. It was observed that in ILs with the 1-alkyl-3-methyl-imidazolium cation and the acetate anion, the cation reacted with cellulose at its reducing end to form a carbon
carbon bond. However, a similar reaction could not be found when cello-oligomer was dissolved in [C2mim][Cl] [58]. Ohno et al. [60] investigated the reaction behavior of cellulose in [C2mim][Cl] at different temperatures. It was found that the solubilized cellulose was depolymerized into various low-molecular-weight compounds, including cellobiose, cellobiosan, glucose, and levoglucosan, but the carbon
carbon covalent bond mentioned above could not be detected. These results suggest that the dissolution of cellulose in ILs may lead to not only the separation of chains but also complex reactions based on the different reactivities of ILs with various anions and cations to the biopolymer.

6.3.4 Interaction Study by Simulation and Theoretical Study Theoretical study is a reliable and powerful tool to understand the interaction of cellulose with ILs and the dissolution mechanism. Research using theoretical methods to study interaction and mechanism of cellulose in ILs has been reported increasingly. Most of these works are based on molecular dynamics simulation studies, and some of them are carried out using quantum chemical theoretical methods (Fig. 6.6). Youngs et al. [42,61] investigated the dissolution process of glucose in ILs, including 1,3dimethylimidazolium chloride [C1mim]Cl, [C2mim]Cl, and [C2mim][CH3COO] by using molecular dynamics simulation. It was found that hydrogen bonding (electrostatic) interaction between the oxygen in anion and the hydroxyl in sugar plays the main role in

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Figure 6.6 Interaction of cellobiose and anion/cation of ionic liquid. (A) Interaction of cellobiose and anion Cl2. (B) Interaction of cellobiose and cation C4mim 1.

dissolution and makes major contributions and that the interaction of the cation with the sugar is weak. They thought that the secondary effect of the cation around the sugar resulted from the interaction between the cations and chloride anions bonding to the glucose by hydrogen bonding. A small but favorable energy contribution was made by the van der Waals interaction between the sugar and cations, so it was thought that the adjustment of cations could improve the dissolution of cellulose in ILs. Liu et al. [44] investigated the behavior of cellulose in the IL 1-ethyl-3-methylimidazolium acetate [C2mim][OAc] by using molecular dynamics simulations. A series of glucose oligomers with a degree of polymerization 5, 6, 10, and 20 were examined with a developed all-atom force field for the IL. Similar simulations were carried out on cellulose oligomers in water and methanol for comparison. The interaction energy between the polysaccharide chain and the IL was found to be stronger than that for water or methanol. The authors detected interaction between the cation and polysaccharides, which was attributed to the hydrophobicity of the cation, whereas the anion acetate was found to form strong H-bonds with the hydroxyl groups of cellulose. The cation was also thought to play a significant role in the dissolution of cellulose according to the theoretical results. In general, the results indicate that anions form H-bonds with cellulose, whereas cations have van der Waals interactions with cellulose. Cho et al. [62] tried to explain why cellulose can dissolve in ILs and not in water by using molecular dynamics simulations. They built a cellulose microfibril and investigated the process of peeling off an 11-residue glucan chain from it in the IL 1-butyl-3methylimidazolium chloride ([C4mim][Cl]) and water (Fig. 6.7). The free-energy costs were calculated as well. Because the model compound involved in the simulations had 36 glucan

214 Chapter 6

Figure 6.7 Simulation model of cellulose deconstruction [62].

chains and about 100,000 atoms, they used a coarse graining scheme to dissect the interactions in the simulation models. It was found that water couples to the hydroxyl and side-chain groups of glucose but lacks interaction with sugar rings and linker oxygens. As for [C4mim] [Cl], anions strongly interact with hydroxyl groups, and the cations couple to side chains and link oxygens. Thus, the IL demonstrated versatility in targeting glucose residues. Gross et al. [63] investigated dissolution of different moieties of glucose residues in the IL [C4mim][Cl] by using all-atom molecular dynamics simulations. They analyzed thermodynamic driving forces and interactions of glucose with anions and cations. The simulations performed at two extreme states of cellulose dissolution were in a crystalline microfibril and a fully separated dissociated state. The simulations in water and in the IL [C4mim][Cl] were compared, and the solvent effects were analyzed. The results revealed that both anions and cations interact with the moieties of glucan residues. The Cl2 anions were found to form H-bonds with the hydroxyl groups of glucan chains, and [C4mim]1 cations were also found as the important compensator based on calculated density profiles. These two works had similar results with regard to the difference between water and ILs solvents. Zhao et al. [64,65] systematically investigated the influence of anions and cations on the process of dissolution of cellulose by using molecular dynamics simulations. A series of ILs with different anions and the same cation [C2mim]1 were selected to be simulated to study the effect of anionic nature, and these ILs included F2, Cl2, Br2, I2, [H(CH2)nCOO]2, [HOCH2COO]2, [(CH3O)2PO2]2, [SCN]2, and [PF6]2. It was shown that H-bonds were formed between the anions of the ILs and the hydroxyl protons of cellulose. Among all of the anions, the Cl2 anion and the O atom of [CH3COO]2 and [(CH3O)2PO2]2 were found to be better H-bond acceptors. Guided by the analysis of the results, it was concluded that for dissolution of cellulose, the stronger the electro-negativity of the H-bond-accepting ability of the anions and the shorter the alkyl chain of the anions, the better is the solubility. As for the effect of the cationic structure on cellulose dissolution, eight different kinds of

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ILs, ([Cnmim]Cl (n 5 2,3,4,6,8,10), [C4mpy]Cl, and [Amim]Cl), were simulated. On the basis of the analysis of radial distribution functions, interaction energies, and the number of H-bonds, the simulation suggested that the heterocyclic structure with high polarity and shorter alkyl chain leads to more efficient dissolution of cellulose, and electron-withdrawing groups in alkyl chain of the cations are also useful. With the method of molecular dynamics simulation and quantitative calculation from the molecular level, it was found that the basis for the mechanism of cellulose dissolution in ILs is O-H. . .anion H-bond formation between the anion and cellulose, whereas cations interact with cellulose by formation of weaker H-bonds and van der Waals forces. Quantum chemistry theory and approaches are also good tools to investigate the interactions between ILs and cellulose, with the advantage of mechanism study and high accuracy. Novoselov et al. [54] performed quantum chemical calculations on systems, including 1butyl-3-methylimidazolium chloride and cellobiose, used as a model of cellulose. Structure, energy, and charges were used to characterize the interaction of cellobiose with ion pairs. H-bonds were found to be formed between hydroxyl hydrogen atom on cellobiose with negatively charged chloride. It was thought that bonding of the cation with cellobiose is relatively difficult because of steric hindrances. Guo et al. [66] reported on interactions between the cellobiose molecule and several imidazolium ILs with anions, including acetate [CH3COO]2, alkyl phosphate [(CH3O)2PO2]2, tetrafluoroborate [BF4]2, and hexafluorophosphate [PF6]2 by using density functional theory calculations. They found that H-bonds exist between anions and cellobiose and that the strength of interactions between anions and cellobiose is in the following order: acetate anion . alkyl phosphate anion . tetrafluoroborate anion . hexafluorophosphate anion. The anion Cl2 would form H-bonds with the hydroxyl groups of cellulose as well. They also studied the interactions between cellulose and the IL [C4mim][Cl] by using quantum chemistry calculations [67] and found that H-bond interaction occurs between the [C4mim]1 cation and cellobiose, although the interaction between cation and cellobiose is weaker than that between cellobiose and the chloride anion. According to the research work of Payal [68] on cellobiose and xylan in solvents of water, methanol and 1,3-dimethylimidazolium acetate IL by using density functional theory calculations, the inter- and intramolecular H-bonds play an important role in the dissolution process. They also found weak H-bond interaction between the cation and cellobiose. It was thought that the high hydrogen-donating and accepting nature of ion pairs leads to the formation of strong intermolecular H-bonds and also breaks the intramolecular H-bond of cellobiose. Xu et al. [69] performed both quantum chemistry calculations and molecular dynamics simulations on cellulose dissolution models (i.e., cellulose oligomers with degrees of

216 Chapter 6 polymerization n 5 2, 4, and 6) in the IL 1-butyl-3-methylimidazolium chloride . They also found that the intramolecular H-bond in the oligomer is broken by the combined effect of anions and cations. However, anions occupy the first coordination shell of the oligomer and form more and stronger H-bonds with oligomer than do cations. Some theoretical work has been performed on cellulose dissolution in cosolvents, and this provided valuable information for enhancement of dissolution [70]. In general, these results showed that anions play a critical role and that cations also make a contribution to the cellulose dissolution process. Focusing on the interaction of ILs and cellulose, it was realized that it is important to develop a structure relationship and a molecular design method to achieve effective dissolution and separation of cellulose with ILs as solvent and catalyst. The study on the mild catalytic conversion process of biogasoline preparation will provide scientific basis for energy-oriented use of biomass and establish a research platform to investigate ILs, from fundamentals to applications. In general, it is thought that ILs can effectively destroy the H-bond of cellulose molecules. It is necessary to design and synthesize stable and effective functional ILs to achieve dissolution, separation, and catalytic conversion of biomass. At present, deep theoretical research has not been performed on the dissolution and catalytic conversion of cellulose in ILs, and there is no clear and systematic explanation for the mechanism of cellulose dissolution. It is therefore difficult to provide a theoretical basis for effective separation of cellulose, hemicellulose, and lignin in biomass. The interaction of ILs with various components of biomass, the liquid-phase catalytic conversion mechanism in an IL medium, and a new separation
catalysis
conversion technology based on ILs need to be further researched and developed.

6.4 Enzymatic Catalysis in Ionic Liquids Research on ILs as nonaqueous media for enzymatic reactions has been growing dramatically since the last decade [71]. Properties of ILs that have made them suitable candidates for enzymatic catalysis are mostly their high thermal stability and low vapor pressure, which directly resolve the problem of emission of conventional organic solvents. The fact that the physicochemical properties of ILs can be tuned makes them very attractive, as these properties can be tuned to the stability and activity of such enzymatic reactions [72
74]. Notable research has already shown that ILs can be used to improve thermal stability, biocatalyst activity, enantio-selectivity, and reusability [75
80]. In particular, some enzymatic reactions that have been carried out with the use of ILs include: • • •

Ketone reduction using whole cells of Baker’s yeast [81] Transesterification using α-chymotryspin [82
84] Synthesis of Z-aspartame using thrmolysin, alcoholysis, ammoniolysis, and perhydrolysis using lipase [85
87]

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Mu et al. [72], in their review, showed that a number of factors, including polarity, alkyl chain length in the cation, anions, hydrophobicity, viscosity, pH of medium, cosolvent, and halide impurity, could have an effect on the stability, activity, and conformational/structural dynamics of enzymes. However, they concluded that although ILs have a hydrophobic nature, their low viscosity, kosmotropic anion, and chaotropic cation usually enhance the activity and stability of enzymes; a general correlation could not be established as the results were conflicting. This implies that further understanding of the interaction between ILs and the structural and conformational dynamics of proteins is required to engineer or design specific ILs for specific enzymatic reactions, protein preservation, and other bio-applications.

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8401.

CHAPTER 7

Synthesis of Fine Chemicals 7.1 Introduction

Numbr of patents with "ionic liquid" in their claims

The use of ionic liquids (ILs) in areas of chemical industry has been reported extensively. ILs are also important candidates for solving classic issues, such as clean and efficient energy, through the development of a broad swath of energy technologies, such as advanced batteries, dye-sensitized solar cells, double-layer capacitors, actuators, fuel cells, thermocells, and water splitting. Most of these technologies are related to highly efficient carbon capture and storage technologies and resource efficiency [1]. Apart from electrochemical and chemical research, many industrial applications exploit the physical properties of ILs, rather than their chemical properties. In such applications, ILs are used as functional parts of devices, equipment, and machinery in such industries as automotive, air, textile, electronics, machinery, and energy industries [2]. The exponential increase of patent literature in the last decade shows that the applications of ILs are moving from research laboratories to industry (Fig. 7.1). Notable companies, such as BASF, Degussa, Arkema, Chevron Philips, Scionix, Eli Lily, Air Products, and Linde, have implemented several pilot and commercial-scale processes, such as acid scavenging, extractive distillation, chlorination, olefin dimerization and oligomerization, hydrosilylation, fluorination, and electroplating. In such industrial applications, ILs act as solvents, extractants, catalysts, electrolytes, and performance additives [2]. Major industrial applications of ILs are in the pharmaceutical, electrochemical capacitor, and fuel cell fields (Table 7.1).

3500 3000 2500 2000 1500 1000 500 0 1991

1994

1997

2000

2003

2006

2009

Publication year

Figure 7.1 Exponential increase in number of patents with “ionic liquid” in their claims [3].

Novel Catalytic and Separation Processes Based on Ionic Liquids. DOI: http://dx.doi.org/10.1016/B978-0-12-802027-2.00007-8 © 2017 Elsevier Inc. All rights reserved.

221

2012

222 Chapter 7 Table 7.1: ILs in chemical processes. Chemical Process/Synthesis BASILt Process (Bi-phasic acid scavenging utilizing ionic liquids) used for synthesis of alkylphenylphosphines (photoinitiators used to cure coatings and printing inks by exposure to ultraviolet light)

Role in Synthesis Acid scavenging

Benefits G G G G G G

Extractive distillation of azeotropes

Entrainer

G G G

Chlorination of alcohols with “nucleophilic HCl”

Solvent

Cleavage of ethers

Catalyst/ Reagent

G G

G

G

Dimerization of olefins

Solvent

G G

Oligomerization of olefins

Catalyst

G

Hydrosilylation

Catalyst

G

G

G

Fluorination

Catalyst

G G

G G

Synthesis of pigment phases

Compatibilizers

G G

No handling of solids Better heat transfer Higher chemical yield Higher space
time yield Lower investment cost Higher sustainability of process Breaking of azeotropes Lower energy consumption Reduced equipment cost HCl substitutes for phosgene High selectivity at high concentrations HCl can be used as a cheap cleaving agent of aromatic methoxy groups Lower cost Higher catalyst activity and stability Higher selectivity Provides product with a unique viscosity profile useful for application as lubricant Recovery and reusability of catalyst phase Shorter reaction times because of higher catalyst loadings Improvement in product quality Higher catalyst activity Higher stability of catalyst to reductive deactivation Avoidance of chlorine co-feed Higher selectivities towards perfluorinated product Stabilize pigments in pigments pastes Provide truly universal water-based pigment pastes suitable for water and solvent-based paints and coatings

Adapted from content [2].

7.2 Pharmaceutical Applications Synthesis of pharmaceutical compounds typically requires efficient and cost-effective extraction/purification processes, where 20
50% of the total production costs are associated with these separation processes [1]. Most of these extraction/purification processes are typically carried out with the use of organic solvents because of the ease of

Synthesis of Fine Chemicals 223 handling and the cost-effectiveness of the processes. However, the use of such organic solvents result in organic contamination of the final pharmaceutical products. Guidelines for acceptable limits of contamination of such organic solvent residues (also known as “residual solvents” or “organic volatile impurities”) are specified in pharmacopoeias and the International Conference on Harmonization of Technical Requirements for Registration of Pharmaceuticals for Human Use. These guidelines distinguish four classes of residual solvents in drug products: solvents to be avoided, solvents to be limited, solvents with low toxic potential, and solvents without adequate toxicologic data [4]. Apart from direct toxicity to humans, the use of organic solvents by the pharmaceutical industry also has environmental implications. Sheldon proposed the introduction of the E(nviromental)-factor in the late 1980s to assess the environmental impact of waste generation in manufacturing processes. The Sheldon Efactor is defined as kg waste/kg product. This parameter has played a major role in focusing the attention of the chemical industry worldwide and, in particular, the pharmaceutical industry, to waste minimization. The Sheldon E-factor shows that contrary to public opinion, the pharmaceutical industry has the highest Sheldon E-factor (25
100) compared with oil refining (,0.1) or the bulk (,1
5) and fine chemicals (5
50) industries [5]. Therefore, with the progressive discovery of the side effects of chemical compounds to human health and the environment, it is crucial that options to replace the use of organic hazardous solvents with greener solvents with lower volatility and flammability be explored for use in pharmaceutical manufacturing. The ability to independently modify the cationic and anionic properties of ILs [6], as well as their other properties, such as viscosity, density, solvent miscibility, and melting point, results in flexibility in the design of new functional materials [7
9]. ILs have therefore been suggested to be “environmentally friendly” and ideal replacements for volatile organic solvents [6,10]. The use of ILs in pharmaceutical synthesis is desirable because reactions in ILs are usually faster compared with those in conventional solvents, and it requires no special methods or apparatus. The most commonly used ILs in pharmaceutical synthesis usually consists of organic salts with the 1,3-dialkylimidazolium and N-alkylpyridinium cation and a noncoordinating anion [11]. In spite of this commonality, no universal catalytic system exists for synthesis using ILs, with each system requiring its own unique solution [4] (Fig. 7.2). Tri-phasic mixtures with alkanes, alkylated aromatic compounds, and water can be formed with ILs containing the hexafluorophosphate anion (PF6). Cull et al. [12] describe the advantages of multiphasic behavior of ILs over other multi-phase processes performed with conventional mixtures, in the use of [C4mim] [PF6] in the liquid
liquid extraction of the antibiotic erythromycin-A, and in Rhodococcus R312 catalyzed biotransformation of 1,3dicyanobenzene (1,3-DCB) in a liquid
liquid, two- phase system.

224 Chapter 7

Figure 7.2 Most widely used ILs in chemical synthesis (A) 1,3-dialkylimidazolium and (B) N-alkylpyridinum cation; where R and R0 are alkyl groups, and X2 is an anion like BF42, PF62, CF3SO32, and CF3COO2.

Figure 7.3 Antiviral drug trifluridine (5-trifluoromethyl-20 -deoxyuridine [TFT]).

The syntheses of nucleoside-based antiviral drugs, such as brivudine, stavudine, and trifluridine, using such ILs as 1-methoxyethyl-3-methylimidazolium methanesulfonate, 1-methoxyethyl-3-methyl-imidazolium trifluoroacetate, and 1-butyl-3-methylimidazolium trifluoroacetate ([C4mim][TFA]) as reaction media, have been performed successfully. The synthesis of high-purity trifluridine (5-trifluoromethyl-20 -deoxyuridine [TFT]; Fig. 7.3) with a 10-fold decrease in solvent consumption compared with the standard reaction medium, pyridine/DMAP or acetonitrile/Et3N/DMAP was reported by Kumar and Malhotra [13]. The synthesis of another drug with high antiviral activity, 3-amino-imidazo[1,2-α]pyridines, has also been shown to benefit from the use of ILs. The IL 1-butyl-3-methylimidazolium bromide ([C4mim][Br]) was used for the synthesis of 3-amino-imidazo[1,2-α]pyridines rather than the commonly used organic solvents. Synthesis of 3-amino-imidazo[1,2-α]pyridines was achieved at room-temperature with relatively high yields of 70
99%, and the removal of [C4mim][Br] from

Synthesis of Fine Chemicals 225 the reaction media was performed by washing with water and evaporating the solvent under vacuum [14] (Fig. 7.4). Other successes with the syntheses of antiviral, antileishmanial, and antiparasitic agents have also been reported. Fan et al. [15] reported the synthesis of “hybrid” compounds, pyrimidine nucleosides combined with pyrano[3,2
c]pyridines, and pyrimidine nucleosides combined with pyrano[4,3
c] pyranes, by using ILs (Fig. 7.5). This potential antiviral and antileishmanial agents were synthesized by using [C4mim][BF4] without any catalyst and relatively high yields were achieved with the possibility of easy recovery and reuse of the solvent. Zhang et al. [16] also reported the synthesis of a potentially antiparasitic drug, a series of pyrimidine nucleoside-thiazolini-4-one hybrids, by using 1-butyl-3-methyl-imidazolium hexafluorophosphate [C4mim][PF6] as the reaction medium without the use of volatile and poisonous conventional organic solvents (Fig. 7.6).

Figure 7.4 Synthesis of 3-amino-imidazo[1,2-α]pyridines by three-component condensation using [C4mim] [Br] as the reaction media [14].

Figure 7.5 Synthesis of pyrano[3,2
c]pyridone or pyrano[4,3
b]pyran nucleoside hybrids [15].

Figure 7.6 Synthesis of pyrimidine nucleoside-thiazolini-4-one hybrids using 1-butyl-3-methyl-imidazolium hexafluorophosphate [C4mim][PF6] as the reaction medium [16].

226 Chapter 7 The synthesis of other types of pharmaceutical drugs have also been shown to benefit from the use of ILs. Categories reported in the literature include: •

Anticancer drugs: A comparable 91.65% yield of caffeic acid phenethyl ester analogs with antitumor properties was obtained when Candida antarctica lipase B (Novozyme 435) in 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C4mim] [NTf2]) was used as a solvent rather than isooctane [17]. The ILs [C4mim][BF4] and [C4mim][PF6] were also used in the synthesis of L-4-boronophenylalanine, a clinically approved boron neutron capture therapy drug, which targets tumor tissue by using a suitable boron carrier. The use of ILs, rather than conventional volatile solvents, enabled the synthesis of L-4-boronophenylalanine with a good yield (82
89%) after 20 min [18,19] (Figs. 7.7 and 7.8).

Figure 7.7 Production of caffeic acid phenethyl ester analogs in the transesterification reaction of methyl caffeate with various alcohols using Candida antarctica lipase B (Novozyme 435) as the catalyst and [C4mim][NTf2] as the reaction medium. Yields obtained for R 5 3-cyclohexylpropyl (93.8%), 5-phenylpentyl (84.0%), 2-cyclohexylethly (97.6%), and 4-phenylbutyl (96.7%) [17].

Figure 7.8 Synthesis of L-4-boronophenylalanine using [C4mim][PF6] (82% yield) and [C4mim][BF4] (89% yield) [19].

•

Antidepressant drugs: (S)-3-chloro-1-phenyl-1-propanone is often used as a substrate in the synthesis popular antidepressant drugs, such as fluoxetine and atomoxetine. (S)-3chloro-1-phenyl-1-propanone is obtained from enantio-pure (S)-3-chloro-1-phenyl-1propanol. Asymmetric synthesis of such chiral compounds is usually performed via enantio-selective enzymatic reduction by using a variety of reductases and dehydrogenases. However, the low solubility of (S)-3-chloro-1-phenyl-1-propanol in the aqueous phase results in low yields of (S)-3-chloro-1-phenyl-1-propanone. Various ILs

Synthesis of Fine Chemicals 227

•

were tested in an aqueous bi-phasic system to increase the solubility of (S)-3-chloro-1phenyl-1-propanol, and [C4mim][NTf2] was shown to increase the yields of (S)-3chloro-1-phenyl-1-propanone with an enantiomeric excess of more than 99% [20]. Nonsteroidal anti-inflammatory drugs (NSAIDs): Ibuprofen, from isobutylphenylpropanoic acid, is a nonsteroidal anti-inflammatory drug (NSAID) used for pain relief, fever relief, and reduction of inflammation. Although ibuprofen may be considered a weaker antiinflammatory compared with other NSAIDs., it may have fewer side effects, such as gastrointestinal bleeding, making it one of the most popular NSAIDs. Ibuprofen is produced industrially as a racemate. However, the (S)(1)-enantiomer is about 160 times more active than the (R)-(
)-enantiomer [21]. In an effort to increase enantio-selectivity, several forms of alternative biosynthesis have been attempted. Lipases are enzymes used in several industrial applications for their ability to recognize enantiomers of a racemate, resulting in an enantio-pure compound. These lipases are typically used in organic solvents that are volatile and toxic to the environment because such solvents enable synthetic reactions. Contesini et al. [22] conducted a study that involved four commercially available lipases and two native lipases for the resolution of (RS)-Ibuprofen in systems containing the ILs [C4mim][PF6] and [C4mim][BF4]. They reported that Candida rugosa and Aspergillus niger lipases showed the highest enantio-selectivity and esterification activity after 96 hours of reaction for a bi-phasic system (1:1) containing [C4mim][PF6] and isooctane, rather than pure isooctane. Other NSAIDs that have benefited from the use of ILs for synthesis include pravadoline [23] and (S)-naproxen [24].

7.3 Electrochemical Capacitors Electrochemical capacitors store energy by using either ion adsorption (electrochemical double-layer capacitors [EDLCs]) or fast surface redox reactions (pseudo-capacitors). They can complement or replace batteries in electrical energy storage and harvesting applications, when high power delivery or uptake is needed [25]. Electrochemical capacitors have a variety of commercial applications, such as consumer electronics, automotive, telecom, and industrial applications, notably in “energy smoothing,” momentary-load devices, and applications where extremely fast charging is a valuable feature, such as pulse power, bridge power, main power, and memory backup. The cell voltage of electrochemical capacitors is limited by the electrolyte decomposition at high potentials. Consequently, the larger the electrolyte stability voltage window, the higher is the electrochemical capacitor cell voltage. The use of organic electrolytes, rather than aqueous electrolytes, has allowed for increased cell voltages from 0.9 V to 2.5
2.7 V for EDLCs. Considerable research effort has been focused on the design of highly conducting, stable electrolytes with a wider voltage window, mostly because the energy density is proportional to the voltage squared (Eq. 7.1). The most advanced EDLCs currently use organic electrolyte solutions in

228 Chapter 7 acetonitrile or propylene carbonate, with propylene carbonate becoming more popular because of its lower flash point and toxicity compared with acetonitrile. 1 E 5 CV 2 2

(7.1)

The use of ILs as electrolytes for both batteries and EDLCs has been widely discussed [26
38]. Because ILs are room-temperature, liquid solvent
free electrolytes, their voltage window stability is only driven by the electrochemical stability of the ions. As such, proper selection of the cation and the anion can allow for high voltage ultracapacitors [39]. However, current use of ILs is limited by the fact that current solutions are available outside the 230 to 1 60 C temperature range, which is typical for batteries and supercapacitors. At these temperature ranges, ILs typically have relatively low ionic conductivity. Strides are, however, being made in making ILs more functional for EDLC application. Palm et al. [33] have shown that the operating ranges of the temperatures can be tailored. In their study, Palm et al. showed that melting point of an [C2mim][BF4] electrolyte could be reduced from 13.5 C to 26 C by adding a relatively small concentration of [C4mim][BF4]. This increased temperature range can thus allow for the use of ILs as EDLC electrolytes at temperatures lower than 20 6 1 C [33]. The exceptional life cycle of electrochemical capacitors and their increasing importance will continue to motivate research on designer ILs that can overcome existing challenges. EDLCs are already available in cordless tools, such as screwdrivers and electric cutters, as well as for use in memory backup in toys, cameras, video recorders, and mobile phones [25].

7.4 Fuel Cells Fuel cells are devices that convert chemical energy from a fuel into electricity through a chemical reaction of positively charged hydrogen ions with oxygen or another oxidizing agent. Fuel cells are different from batteries because they require a continuous source of fuel and oxygen or air to sustain the chemical reaction. Fuel cells, unlike batteries, can therefore produce electricity continuously for as long as both fuel and oxygen or air are supplied. The major drawback hindering the commercialization of fuel cell technologies is the availability of commercial membrane materials having high proton conductivity that is almost unaffected by temperature and relative humidity [40]. Polybenzimidazole (PBI), a commercially available membrane, has been extensively studied and used in membranes doped with various strong inorganic acids for high-temperature applications up to 200 C [41]. Among the main strategies to improve the performance and stability of current PBI membranes, the most studied are ionic cross-linking of polymeric

Synthesis of Fine Chemicals 229 acids and polymeric bases; use of covalently cross-linked acids or halides; and composite organic
inorganic membranes from PBI and inorganic fillers [42]. The use of protic ILs as proton transport carriers in the polymer membrane of fuel cells is becoming an attractive alternative because of their high proton conductivity, low water sorption, thermal stability, and low viscosity [40,43]. ILs are able to overcome operational limitations at temperatures above 100 C. ILs are able to transport protons because of their acid
base character and their capability to form complex or intermolecular hydrogen bonds (H-bonds) [44]. The main challenge hindering the use ILs as proton conductor in fuel cells is the phase separation process that takes place between the polymer phase and ILs, resulting in inhomogeneous membranes. However, researchers have demonstrated a way to work around this challenge of conducting PBI systems containing ILs by dissolving the polymer and IL in a common solvent and casting the film for operations at temperatures up to 150 C [45]; and thermal stability has been improved compared with H3PO4/PBI systems and ionic conductivities up to 16 mS cm21 at 250 C under anhydrous conditions [46]. Van de Ven et al. [43] demonstrated use of the IL 1-H-3-methylimidazolium bis (trifluoromethanesulfonyl) imide ([C6mim] NTf2) as conductive filler in a tailor-made PBI support for high-temperature ( . 100 C) fuel cell applications. With a macrovoid free porous structure volume porosity of 65% and a pore size of approximately 0.5 μm, the pores were filled with the IL by direct immersion of the PBI support into molten IL at 50 C. After impregnation, the proton conductivity of the PBI/IL membrane reached a value of 1.86 mS cm21 at 190 C. Fuel cell performance of these membranes exceeded that of Nafion 117 at temperatures above 90 C, with a power density of 0.039 W cm22 obtained at the intended operation temperature of 150 C. Eguizabal et al. [40], studied various conducting fillers based on 2-hydromethyl trimethylammonium dimethyl phosphate (IL1), N,N-dimethyl-N-(2-hydroxyethyl) ammonium bis(trifluoromethanesulfonyl)imide (IL2), and 1-H-3-methylimidazolium bis (trifluoromethanesulfonyl)imide (IL3) encapsulated in large-pore zeolites (NH4BEA and NaY) for the preparation of high-temperature proton exchange membranes using PBI casting solution. They showed that the addition of 1-H-3-methylimidazolium bis (trifluoromethanesulfonyl)imide (IL3) imbibed in commercial NaY type zeolite to the PBI casting solution, in 3 wt%, exhibited conduction behavior and H1/H2 transport selectivity that outperformed pristine PBI and zeolite-PBI counterparts at 50, 100, and 150 C. They also validated this optimal composite membrane in a H2/O2 single cell under nonhumidified conditions up to 180 C as a proof of concept demonstration. With continued research and development, PBI/IL membranes can be considered a serious candidate for hightemperature fuel cell applications (Table 7.2).

Table 7.2: Main properties of conducting fillers used by Eguizabal et al. [40]. Composite Fillers 21

Ionic Liquid σ (mS cm



) at 100 C

Zeolite Host (limiting channel dimensions) Name ˚ ˚ ˚ ˚ BEA (7.6 A 3 6.4 A)(5.5 A 3 5.5 A) IL1-BEA

IL Encapsulated (wt%)

σ (mS cm21) At 100 C

29

65 (100% RH) 5 ðyH2 O 5 5%Þ

˚ 3 6.4 A ˚ )(5.5 A ˚ 3 5.5 A ˚ ) IL2-BEA BEA (7.6 A

5

˚) NaY (7.4 A

17

61 (100% RH) 0.15 (ðyH2 O 5 5%Þ 34 (100% RH) 6 ðyH2 O 5 5%Þ

IL1

IL2-NaY

IL2

˚) NaY (7.4 A

IL3

77 (100% RH) 0.25 ðyH2 O 5 5%Þ IL3-NaY

19

Synthesis of Fine Chemicals 231

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2846.

CHAPTER 8

Ionic Liquid Gating of Thin Films 8.1 Introduction 8.1.1 Conventional Field-Effect Transistor The transistor, which has been one of the remarkable technological advances in human history, utilizes a third gate voltage terminal to tune the conductive properties of the channel between the source and the drain electrodes. It does so across a solid-state gate dielectric, which is limited by a dielectric breakdown voltage and cannot accumulate nearly the charge carrier concentration to tune across disparate electronic phases of the material [1]. It is most commonly employed in a field-effect transistor configuration (Fig. 8.1). In a metal-oxide semiconductor field-effect transistor (MOSFET), a gate voltage is applied through an intermediary oxide or more generally, a dielectric to modulate the resistive properties of a semiconductor channel connecting source and drain terminals. By varying the voltage at the gate (G), electrical transport through the device can be effectively switched ON and OFF through modulating the oxide channel resistance. Q 5 C Vg

(8.1)

AA d

(8.2)

C5

C A 5 A d

(8.3)

An applied gate (G in Fig. 8.1) voltage, Vg, to a capacitor with capacitance, C results in an accumulation of charge at the channel surface, Q (Eq. 8.1). The capacitance between the gate and the channel follows a parallel plate capacitor equation (Eq. 8.2). The separation between the gate and the channel is given by the oxide layer thickness, d, over a geometric area A, of the channel surface under influence of the electric field. E is then the dielectric permittivity of the intervening oxide layer and includes the relative permittivity and vacuum permittivity contributions. For surface charge accumulations, it is useful to express an areal capacitance given by Eq. 8.3. An induced surface charge density will thus be proportional to the areal Novel Catalytic and Separation Processes Based on Ionic Liquids. DOI: http://dx.doi.org/10.1016/B978-0-12-802027-2.00016-9 © 2017 Elsevier Inc. All rights reserved.

233

234 Chapter 8

Figure 8.1 Side view of a typical field-effect transistor (FET) device. Source (S) and Drain (D) electrodes are shown. A gate (G) voltage is used to apply a transverse electric field across the oxide channel sitting atop a substrate. Adapted from [2].

capacitance and the applied gate voltage. Because of Thomas-Fermi screening of charges in the material, charge accumulation is limited to the channel material’s surface. Since charging becomes a surface effect, using thin films becomes strategic so that the underlying shunt resistance beneath the charged surface layer does not dominate electronic transport. This three-terminal, FET operation underlies most of modern electronic devices and technology through its application in implementing device logic gate systems [3]. However, there exist intrinsic limitations to the MOSFET. Solid-state dielectric gating with conventional oxide dielectrics are limited in the density of induced charge because of intrinsic dielectric breakdown. In addition, the solid-state gate dielectric has to be latticematched with the underlying substrate material [4], a requirement that might be limiting for gating non lattice-matched materials. Advances in novel channel materials and gated dielectrics are therefore needed if Moore’s scaling should continue. Studies of two-dimensional (2D) materials with significant mobility using ILs as field-effect dielectrics have been proposed. There is a limit to the miniaturization of MOSFET transistors for high-density packing of transistors per chip. This deviation from Moore’s law requires industry to contend with alternative pathways including novel materials, configurations, and technologies to combat short transistor channel effects, current leakages, defect density, deteriorating mobility, device fabrication issues, and the low dimensional finite-size effects from approaching the quantum limit. Alternative pathways are one thrust of research in recent years seeking to continue Moore’s scaling in addition to carrying out fundamental research in the basic sciences to explore electronic phases of materials. Currently, there are certain limitations, which include long relaxation times, operational temperatures, and possible electrochemical interactions beyond electrostatic charge transfer effects.

Ionic Liquid Gating of Thin Films

235

8.2 Electric Field-Induced Gating with Ionic Liquids ILs, which are salts with a low melting point and are liquid at room temperature [5], have found a wide range of applications both in industry and basic experimental sciences. They are used in catalytic processes and play a role in fashioning potential high-energy-density storage solutions, such as with supercapacitors and batteries [6]. The utility of ILs in catalysis and energy storage highlight their great importance in addressing sectors of demand not only in clean energy generation but also in energy storage to help mitigate climate change. In this chapter, we will focus on the use of ILs in materials research applied to thin films. ILs have been used more fundamentally, to gate materials and control charge carrier densities in condensed matter systems by several orders of magnitude, making it possible to tune through different electronic phases. Ionic liquid gating (ILG) uses an IL electrolyte as a gate dielectric (Fig. 8.2) instead of a solid-state dielectric (e.g., SiO2, SrTiO3, HfO2, Pb(ZrxTi12x)O3-PZT, AlOx etc.). A tunable and reversible means of varying a material’s charge carrier concentration across its insulating, semi-conducting, and metallic phases provides possibilities for new technological devices. Doing so by simple variation of an applied voltage makes different electronic phases of a material accessible with little or no additional disorder introduced.

Figure 8.2 (A) shows a cartoon schematic of the device geometry of a device gated with ionic liquid (DEME-BF4) and (B) shows a photograph of the pattern device of a KTaO3 thin film under study. An ionic liquid droplet covers the device under test (DUT) and is used to modify its transport characteristics. Lateral or mesh gates are often used. Adapted from [7].

236 Chapter 8 Much like the conventional FET, when a gate voltage is applied to an IL (Fig. 8.2), mobile ions move to electrodes of opposite polarity. Electric double layers (EDLs) form at the electrodes, including at the liquid solid interface. The EDL is effectively a parallel plate capacitor composed of a sheet of ions in the electrolyte with an image sheet of charges in the solid. This yields large interfacial areal capacitance (C/A) when ILs are applied to a material surface. These EDLs form capacitors with nanosized gaps or separation (d , 1 nm). High electric fields are generated across at the interface because of the atomic scale size of the separation between EDLs, d. With this large interfacial capacitance from small separation length scales, they can accumulate high charge carrier densities. ILG systems and devices can thus attain carrier densities (Q/A) far exceeding even those produced by high-k solid-gate dielectrics. High charge density accumulation results in the so-called space charge region and Helmholtz [8] layer within the solid. Electric double layer gating (EDLG) can be used in an FET configuration, applied to the body of the transport channel, between the source and drain to modulate the physical properties and electronic states of the material. Surface resistance changes of the thin film result from carrier concentration changes. EDLG using ILs can be utilized to drive electronic phase transitions of solid matter from extremely insulating, through metallicity and even into a superconducting phase. Additional control that can finely tune charge carrier concentration close to a critical point allows for more sensitive studies of phase transitions as well as for scaling analyses [9,10]. Electric field-induced electronic phase transitions are of great interest not only because of their materials discovery and engineering potential, but also for their application to electronics.

8.3 Experimental Methods Multiple ILs (DEME-TFSI, DEME-BF4, DEME-BETI, etc.) are employed for ELDG of thin films and single crystals. Here, we use one of the more commonly used ILs to explain the experimental methods used for typical measurements. N,N-diethyl-N-(2-methoxyethyl)N-methylammonium is-trifluoromethylsulfonylimide (DEME-TFSI) is an IL popular for its high electrochemical stability window. DEME-TSFI is also appealing, since it has higher ionic conductivity and dielectric constant compared with electrolyte dielectrics [11]. ILs that can be fashioned by chemists and chemical engineers to have higher electrochemical windows would prove very useful to the condensed matter physics community; spurring further research and pushing the boundaries and tenability of higher carrier densities needed to explore novel electronic ordering degrees of freedom. DEMETFSI has a melting point of approximately 220 K. Above this temperature, the ions are mobile but are frozen when the temperature goes below the melting point. The ILG

Ionic Liquid Gating of Thin Films

237

charging procedure consists of applying a gate voltage to the IL dielectric above the melting point, which is called the charging temperature. This temperature is chosen so that the ions remain mobile and can induce surface charge accumulation. Above the melting point, the gate voltage is applied for a given time, called the charging time. It may depend on the material system under study or the given experiment. The charging gate voltage is then maintained as the system is cooled to below the melting point. Samples are usually patterned in a Hall bar configuration to measure transport characteristics of the IL gated material. IL is applied in micro-drops to the body of the material or the channel. In a Hall bar device geometry, longitudinal and Hall resistances can be measured, yielding device resistivity and charge carrier density and mobility, respectively. Samples are often fabricated and patterned with photolithography or electron beam (ebeam) lithography. Source, drain, and other device electrodes defined by lithography are then deposited with contacts of several nanometer thickness by using e-beam or thermal evaporation processes. The choice of metal for contacts, typically Ti/Au, are chosen on the basis of the material to maintain as minimal contact resistance as possible. Transport measurements are carried out in either current- or voltage-biased modes, depending on the sample conductivity. For highly insulating materials, voltage-biased measurements are used, with a source-drain current measured to yield the resistance and ultimately resistivity of the material after considering the device geometry. Resistance versus temperature curves are measured with a gate voltage held fixed by using the charging scheme described. To change the gate voltage, and thus the induced charge carrier density, the sample is warmed up above the melting point, and a new gate voltage set point is utilized. Low-temperature operation (e.g., 220 K) of ILG devices are also preferred to diminish activation energies, thereby suppressing chemical reactions. This enhances the electrochemical window leading to an increased range of the charge accumulation possible.

8.4 Gating of Semi-conducting and Insulating Systems It is widely recognized that tuning charge carrier density in materials allows for exploration of new functionalities and behaviors of the material electronic state. Charge carrier density has typically been tuned by chemical doping. Substitutional doping of one elemental component of the compound for another introduces induced mobile charge. Chemical doping of materials and compounds has several drawbacks. One of them is the unavoidable introduction of system disorder, which adds a degree of freedom that may not be controllable and varies in ways that are not obvious a priori, from sample to sample.

238 Chapter 8 First, the disorder landscape could change because of thermal cycling within the same sample during different measurement runs. The disorder can mask or hinder the understanding of intrinsic physics, which is better probed within a more pristine system. Second, chemical doping usually is time consuming and requires synthesizing multiple samples if a gallery of doping concentrations is to be investigated. That is, each sample must be grown or synthesized with a fixed dopant concentration. Although there are methods to increase throughput, such as utilizing combinatorial libraries on the same sample by growing materials with a dopant concentration gradient, such techniques remain cumbersome. EDLG with ILs does not suffer innately from the setbacks of material disorder from chemical doping or from synthesizing a plethora of samples at different carrier concentrations. ILG can be used to tune carrier concentration by several orders of magnitude without introducing additional disorder. ILG devices show higher mobility, indicating pristineness and a lower degree of disorder. IL devices are more effective at band bending comparable with solid-state dielectric gating techniques. This ensures that ILs can more easily realize and span beyond hackneyed device characteristics. As such, EDLG using ILs has been used to study a vast array of material systems. Studies seeking to modulate the physical properties of materials, including insulators, semiconductors, and even superconductors, have been published. More commonly, quasi2D materials are studied because their top layer in the EDLG configuration dominates the transport properties as a result of the higher carrier concentration induced closest to the solid liquid interface. Subsequent layers besides the top layer have significantly lower charge accumulation, which falls off exponentially with the screening length, as the characteristic length scale beyond which the electric field is screened out by the induced charge. The gated quasi-2D materials studied are typical obtained by cleaving thicker films down to a single layer or few number of layers of atomically flat films via mechanical exfoliation. Mechanical exfoliation became prominent in obtaining graphene from graphite and has been applied to other layered compounds whose interlayer couplings consist of weak van der Waals forces. A list of materials that have been gated and studied with ILs include, but are not limited to: MoS2 [12], La22xSrxCuO4 [9], YBCO [10], Bi-2212, VO2 [13], ZnO [14], black phosphorous [15], SrTiO3, ZrNCl [16], and WO3 [17]. Below, we summarize a few of the typical results obtain in ILG studies. ZnO is a B3.3 eV wide band gap insulator, which has found use in laser diodes and LEDs, rubber vulcanization, sunscreen and UV blocking, methane reforming, food additives, paint pigments, and anticorrosive coatings. ZnO thin films have been gated with the IL DEMETSFI [14]. Measurements of DEME-TSFI-gated ZnO show an increase of electron accumulation by up to two orders of magnitude achieving a concentration of

Ionic Liquid Gating of Thin Films

239

4.5 3 1014 cm22 at room temperature. A maximum carrier concentration of 8 3 1014 cm22 at 220 K with an applied gate voltage of 5.5 V was reported. Oxide dielectric gated ZnO devices were only able to achieve less than 1013 cm22. Si is the one of the most important semiconductors, and certainly the most important elemental semiconductor. Si is ubiquitous in the semiconductor industry as a crucial material underpinning modern technology. Additional control over its properties could have great ramifications for the industry. Typically, solid-state dielectric electrostatic gating in MOSFET is carried out using SiO2 native oxide grown on Si. However, needing much higher charge carrier density change and motivated by the search for a superconductor insulator transition (SIT) in Si, as well as a metal insulator transition (MIT), Nelson et al. [18] used the IL DEME-TFSI to modulate the transport properties of 2D hole gases in low-mobility p-type ,100 . Si wafers. A low-temperature (,15 K) MIT was driven by introducing additional holes via gating. A metallic carrier density up to 1.4 3 1013 cm22 (T 5 2 K) was realized with an insulator density approximately 2.6 3 1012 cm22 at T 5 2 K. Gating InOx thin films with 1-ethyl-2-methylimidazolium bis(trifluoromethyl- sulfonyl) imide (EMI-Beti), can change the resistance by a factor of 104 [11]. The carriers in InOx are electrons. Applying a voltage of 21 V suppresses conductivity driving the system into an insulating state. The areal capacitance induced by IL gating (11 V) is an order of magnitude larger than for a solid-gate dielectric, AlOx. An increase in electron mobility under IL gating is also reported: 20.6 cm2 V21 s21 compared with a typical value of 3.0 cm2 V21 s21 with solid dielectrics.

8.5 Gating of Superconductors with Ionic Liquids 8.5.1 Overview of Superconductivity Superconductivity is one of the interesting effects observed in experimental condensed matter physics. When cooled below their critical temperatures, superconductors undergo a phase transition into a zero direct current (DC) resistance, lossless state. In addition to zero resistivity, superconductors act as perfect diamagnets in the so-called Meissner state, screening out any applied magnetic fields from the bulk. The societal impact of superconductivity cannot be overstated: from potentially lossless transmission of electricity across the power grid, high-speed levitated trains, magnetic resonance imaging (MRI), and even future realizations of qubits for quantum computers, in addition to other nanotechnology applications [19,20]. Superconductivity was discovered in 1911 by Kamerlingh Onnes [21] during experiments to measure the zero temperature limits of the resistances of metals, such as mercury. The

240 Chapter 8 phenomenon was enabled by the liquefaction of helium, which made attaining low sub-5 K temperatures possible. That a true thermodynamic state defines superconductivity was further elucidated by the observations of Meissner and Oschenfeld [22] in 1933 investigating magnetic responses of superconductors. The London equations were later put forth to summarize both observations that have defined superconductors thus far: zero resistance and diamagnetic screening of magnetic fields. While the macroscopic London equations explained some aspects of the electrodynamics of superconductors, the microscopic quantum origins were only appreciated after the theory of Bardeen, Cooper, and Schrieffer in 1957 [23], for which they won the Nobel Prize. Today, the theory is known by the initials of its inventors (or philosophically, discoverers) as BCS theory. In it, electrons close to the Fermi level of the metal could pair up into so-called Cooper pairs by a weak attractive electron phonon coupling interaction. Early superconductors consisted of metals with critical temperatures typically below 10 K. For technological applications, this temperature scale was much too low. The holy grail in the field of high-temperature superconductivity has been to discover or engineer materials that have critical temperatures at room temperature. The applications for such materials would have paramount importance in energy storage and power distribution. Room temperature superconductors applied to power transmission lines will not only obviate the need for cryogens but will reduce the amount of electricity that would need to be generated. Later, binary alloys of the so-called A-15 class with A3B stoichiometry attained Tc in 20s K range. Nb-based compounds had the highest Tc with Nb3Ge reaching as high as 23K. Organic superconductors and Heavy Fermion superconductors, all with unremarkable Tc, were also synthesized. However, it was not till 1986 when Bednorz and Muller synthesized films of complex oxides, or cuprates (La2BaCuO4), that critical temperatures of 40 K were realized; even more surprising, was that these were non-metallic ceramic material systems [24]. A year after the Bednorz and Muller discovery, Wu et. al. reported record critical temperatures of 90 K in a then, newly grown yttrium-based complex oxide, YBCO [25]. At the time of this writing, the record ambient pressure critical temperature measured in the cuprates has been Hg-based, HBCCO at 134 K—somewhat of a historical irony. The cuprates exhibit a Perovskite crystal structure and are known to be layered compounds with copper oxide planes separated by charge reservoirs. It is now understood that superconductivity is dominated by the transport in the copper oxygen planes. The year 2008 saw the discovery of yet another family of superconductors, which were not based on copper but, instead, on iron, thereby ushering in the iron age of superconductors (another irony). The critical temperatures for these compounds have been modest compared with that of the cuprates but their significance in showcasing another template for

Ionic Liquid Gating of Thin Films

241

superconductivity cannot be overstated. Recently, in 2015, hydrogen sulfide (H2S) has been reported to have a transition temperature of 203 K (at 90 GPa) [26]. So far, a complete theoretical framework does not exist for superconductivity in complex oxides. BCS theory falls short in explaining the microscopic origins of high-temperature cuprate superconductivity. Specifically, understanding the nature of the pairing mechanism remains a hotbed of research activity, although it is generally agreed that it does not result from electron phonon coupling as is the case with conventional superconductivity.

8.5.2 Gating of Superconductors IL gating techniques applied to superconductors is a subfield, which is teeming with activity. DEME-TFSI as a gate dielectric has been used to shift the critical temperature (Tc) of cuprate superconductors by up to 30 K [9]. YBCO [10] and LSCO [9] are but two of several cuprates that have been studied. The undoped parent compound of the cuprates is an antiferromagnetic Mott insulator. That is, classical band theory indicates that the cuprate parent compound should not be insulating. The observed insulating behavior, however, arises from the strongly correlated electron interactions. When chemically doped (e.g., substitutional doping), there exists a critical doping concentration where the insulating cuprate gives way to a superconducting phase. In La22xSrxCuO4, it occurs at approximately x 5 0.05 dopant concentration. Starting with LSCO thin films with dopant concentrations controlled to various values of x, Bollinger et al. [9] were able to tune across the insulator superconductor transition by using electrostatic doping with ILs while keeping x fixed for each sample. Achieving large shifts in Tc not only requires ILG to induce high electric fields resulting in high charge accumulation, but also requires that pristine ultrathin films used for measurement be properly synthesized. Because ILG is predominantly a surface effect, the films used were confirmed by atomic force microscopy (AFM) to be atomically smooth. Films of 1- to -2-unit-cell thickness grown by molecular beam epitaxy (MBE) were used. ILG of such thin films ensures electronic transport is not shunted by the bulk but instead is dominated by the top layer under the influence of the IL-induced electrostatic field. The observed electric field-effect was ambipolar and reversible and resulted from both charge accumulation and depletion, corresponding to the polarity of the gate voltage applied. The resistance of the LSCO film was modulated by at least four orders of magnitude with ILG. Such studies illustrate the possibility of utilizing ILG to tune the cuprate electronic property across the entire superconducting phase diagram, or “dome.” From tuning the electronic properties of HTSC using ILG, one can peek into the microscopic mechanisms at play. BCS theory showed that the Fermi sea is unstable in the presence of Cooper pair formation. This instability implies that the material would rather be

242 Chapter 8 in a lower energy state: the superconducting state. With induced charge carrier density as a tunable parameter, studies have sought to peel back one more layer of the HTSC mystery.

8.6 Summary Investigations into ILG have come to appreciate tradeoffs in electrochemical and electrostatic effects. Oxygen vacancies were found to play a role in the charge carrier accumulation observed in VO2 beyond electrostatic charge transfer effects [27]. Applied gate voltage and operational temperature can play a role in electrochemical effects. Hightemperature operation of the ILG transistor could activate electrochemical processes, whereas lower temperatures may freeze out the electrochemical processes altogether, allowing for true electrostatic charge transfer. Charging at high temperatures is still needed to take advantage of the ion mobility in ILs above the IL glass transition temperature. For now, further investigation is needed before ILG devices can find widespread technological use. Current state-of-the-art ILG technology, however, offers insights into tuning material properties with charge density accumulation and paves the way for potential future applications.

References [1] K. Ueno, S. Nakamura, H. Shimotani, Ohtomo, N. Kimura, T. Nojima, et al., Electric-field-induced superconductivity in an insulator, Nat. Mater. 7 (11) (2008) 855 858. [2] C.H. Ahn, J. Triscone, J. Mannhart, Electric field effect in correlated oxide systems, Nature 424 (August) (2003) 1015 1018. [3] S.M. Sze, Semiconductor Devices: Physics and Technology, Wiley, New York, 1985. [4] A.M. Goldman, Electrostatic gating of ultrathin films, Annu. Rev. Mater. Res. 44 (1) (2014) 45 63. [5] M. Galinski, A. Lewandowski, I. Stepniak, Ionic liquids as electrolytes, Electrochim. Acta 51 (26) (2006) 5567 5580. [6] P. Simon, Y. Gogotsi, Materials for electrochemical capacitors, Nat Mater 7 (2008) 845 854. [7] K. Ueno, S. Nakamura, H. Shimotani, H.T. Yuan, N. Kimura, T. Nojima, et al., Discovery of superconductivity in KTaO3 by electrostatic carrier doping, Nat. Nanotechnol. 6 (7) (2011) 408 412. [8] H. von Helmholtz, Ueber einige Gesetze der Vertheilung elektrischer Strome in korperlichen Leitern, mit Andwendung auf die thierisch-elektrichen Versuche, Ann. Phys. 165 (6) (1853) 211 233. [9] A.T. Bollinger, G. Dubuis, J. Yoon, D. Pavuna, J. Misewich, I. Boˇzovi´c, Superconductor-insulator transition in La22xSrxCuO4 at the pair quantum resistance, Nature 472 (2011) 458 460. [10] X. Leng, J. Garcia-Barriocanal, S. Bose, Y. Lee, A.M. Goldman, Electrostatic control of the evolution from a superconducting phase to an insulating phase in ultrathin YBa2Cu3O72x films, Phys. Rev. Lett. 107 (2) (2011) 6 9. [11] R. Misra, M. McCarthy, A.F. Hebard, Electric field gating with ionic liquids, Appl. Phys. Lett. 90 (5) (2007) 1 4. [12] M.M. Perera, M. Lin, H. Chuang, B.P. Chamlagain, C. Wang, X. Tan, et al., Improved carrier mobility in few-layer MoS2 field-effect transistors with ionic-liquid gating, ACS Nano (5) (2013) 4449 4458. [13] H. Ji, J. Wei, D. Natelson, Ionic liquid as a gating medium, Nano Lett. vol. 12 (2012) 2988 2992.

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[14] H. Yuan, H. Shimotani, A. Tsukazaki, A. Ohtomo, M. Kawasaki, Y. Iwasa, High-density carrier accumulation in ZnO field-effect transistors gated by electric double layers of ionic liquids, Adv. Funct. Mater. 19 (7) (2009) 1046 1053. [15] M.V. Kamalakar, B.N. Madhushankar, A. Dankert, S.P. Dash, Effect of high-k dielectric and ionic liquid gate on nanolayer black-phosphorus field effect transistors, Appl. Phys. Lett. 107 (11) (2015) 1 5. [16] J.T. Ye, S. Inoue, K. Kobayashi, Y. Kasahara, H.T. Yuan, H. Shimotani, et al., Liquid-gated interface superconductivity on an atomically flat film, Nat. Mater. 9 (2) (2010) 125 128. [17] X. Leng, J. Pereiro, J. Strle, G. Dubuis, A.T. Bollinger, A. Gozar, et al., Insulator to metal transition in WO3 induced by electrolyte gating, (to be published); 2017. [18] J. Nelson, A.M. Goldman, Metallic state of low-mobility silicon at high carrier density induced by an ionic liquid, Phys. Rev. B - Condens. Matter Mater. Phys. 91 (24) (2015) 1 4. [19] N.E. Litombe, A.T. Bollinger, J.E. Hoffman, I. Boˇzovi´c, La22xSrxCuO4 superconductor nanowire devices, Phys. C 4 (2014) 2 6. [20] A. Gozar, N.E. Litombe, J.E. Hoffman, I. Bozovi´c, Optical nanoscopy of high-Tc cuprate nanoconstriction devices patterned by helium ion beams, to be Publ.; 2017. [21] D. Van Delft, P. Kes, The discovery of superconductivity, Phys. Today 63 (9) (2010) 38 43. [22] W. Meissner, R. Ochsenfeld, Ein neuer effekt bein eintritt ber supraleitfahigkeit, Naturwissenschaften 21 (September) (1933) 787 788. [23] J. Bardeen, Theory of superconductivity, Physica 24 (1958) S27 S34. [24] J.G. Bednorz, K.A. Mu¨ller, Possible high Tc superconductivity in the Ba-La-Cu-O system, Zeitschrift fu¨r Phys. B Condens. Matter 64 (2) (1986) 189 193. [25] M.K. Wu, J.R. Ashburn, C.J. Torng, Superconductivity at 93 K in a new mixed phase YBCO compound system at ambient pressure, Phys. Rev. Lett. 58 (9) (1987) 908 910. [26] A.P. Drozdov, M.I. Eremets, I.A. Troyan, V. Ksenofontov, S.I. Shylin, Conventional superconductivity at 203 Kelvin at high pressures in the sulfur hydride system, Nature 525 (7567) (2015) 73 76. [27] J. Jeong, N. Aetukuri, T. Graf, T.D. Schladt, M.G. Samant, S. Parkin, Suppression of metal-insulator transition in VO2 by electric field-induced oxygen vacancy formation, Science 339 (6126) (2013) 1402 1405.

Index Note: Page numbers followed by “f ” and “t” refer to figures and tables, respectively

A ABMDFP. See Ammonium bromide modified dicyandiamide
formaldehyde polymer (ABMDFP) Absorbents, 29
30 Acetate (CH3COO)
, 215 Acetophenone, 124 Acid catalyst, 166
167 Acid
base reactions, 111 Acidic ILs, 118
119, 165 as catalysts/solvents for esterification, 154
156 immobilized 1-allylimidazolium containing, 159
160 Acylation of aromatic hydrocarbons, 167
168 [Admim][Br]. See 1-N-Allyl-2,3dimethylimidazolium bromide ([Admim][Br]) Aerobic oxidation, 112
115 AFM. See Atomic force microscopy (AFM) Aggregation in ILs IL cluster, 62 in neat systems, 65
69 in solutions, 62
65 IL in confined space, 70
72 ionic cluster at interface, 69
70 AHAs. See Aprotic heterocyclic anions (AHAs) Air-stable ionic liquids, 1 Alcaligenes sp. lipase (AsL), 157
158 AlCl3-red oil, 166
167

Alcohol(s), 115 oxidation, 123
124 oxidized with H2O2, 115 separation of alcohols 1 alkanes, 200t separation of alcohols 1 esters, 199t separation of alcohols 1 ketones, 199t Alkali metal salts, 28
29 Alkane sulfonic acidfunctionalized ILs, 155 Alkanes oxidation, 119
120 Alkenes oxidation, 119
120 Alkyl chains, 210
211 Alkyl halides, 15, 163 Alkyl phosphate ((CH3O)2PO2)
, 215 1-Alkyl-3-methylimidazolium chloride ILs, 210 Alkylation, 17
18, 163. See also Esterification reaction of olefins with isobutane, 166 reaction, 163
169 heterogeneous systems with supported catalysts, 165 IL stability, lifetime and recyclability, 168
169 industrial alkylation processes, 165
168 liquid-phase systems, 163
165 process challenges and issues, 168
169 safety and environmental issues, 169

245

N-Alkylpyridinium, 23 Alkylpyridinium-based ILs, 15 Alkyltosylates, 16 Allyl groups, 159
160 1-N-Allyl-2,3dimethylimidazolium bromide ([Admim][Br]), 210 1-Allyl-3-(60 -oxo-benzo-15-crown5 hexyl) imidazolium bis (trifluoromethanesulphonyl) imide ([A(Benzo15C5)HIM] [N(SO2CF3)2]), 18
21 1-Allyl-3-(butyl-4-sulfonyl) imidazolium trifluoromethanesulfonate ([BsAIm] [OTf]), 159
160 [Amim][Cl]. See IL 1-allyl-3methylimidazolium chloride ([Amim][Cl]) Amine-functionalized ILs systems, 80
81 Amino acid-functionalized ILs, 149 Amino functionalized ILs, 149, 149f phosphonium ILs, 80 3-Amino-imidazo[1,2-α]pyridines, 224
225, 225f 1-(2-Aminoethyl)-1methylpiperazin amino acid ([AEMP][A]), 18
21 1-(2-Aminoethyl)-1methylpiperazin-1-ium hydroxide ([AEMP][OH]), 18
21

246 Index 1-Aminopropyl-3methylimidazolium bromide ([APMIM][Br]), 27 Ammonium bromide modified dicyandiamide
formaldehyde polymer (ABMDFP), 152
153 Ammonium hexanitratocerate(IV), 119 Analytical solution of groups (ASOG), 92 Anion(s), 32
33, 198. See also Cation(s) effect of, 72
77 cations
anions interaction, 78
79 dissolution mechanism of cellulose with interactions from anion and cation, 211
212, 212f groups, 3
5 for ILs, 143f interaction of cellulose and, 208
209 Anisotropic structure in ILs IL cluster, 62 in neat systems, 65
69 in solutions, 62
65 IL in confined space, 70
72 ionic cluster at interface, 69
70 ANN model. See Artificial Neural Network model (ANN model) Anticancer drugs, 226 Antidepressant drugs, 226
227 Aprotic conventional ILs, 15
18 synthesis of ammonium-based ILs, 16f synthesis of imidazolium-based ILS, 16f synthesis of pyridinium-based ILS, 16f Aprotic heterocyclic anions (AHAs), 80
81 Aqueous azeotropic systems, 195
199 ILs, 196t nature of anion in ILs, 198f separation of alcohols 1 alkanes, 200t

separation of alcohols 1 esters, 199t separation of alcohols 1 ketones, 199t water 1 ethanol systems, 197t Areal capacitance, 233
234, 236, 239 Arenes hydrogenation, 131
132 Aromatic aldehydes, 114 Aromatic hydrocarbons, acylation of, 167
168 Artificial Neural Network model (ANN model), 88 AsL. See Alcaligenes sp. lipase (AsL) ASOG. See Analytical solution of groups (ASOG) Asymmetric tertiary phosphines, 16 Atomic force microscopy (AFM), 70, 241 ATR. See Attenuated total reflectance (ATR) Attenuated total reflectance (ATR), 72 Attenuated total reflectanceinfrared spectroscopy (ATR-IR spectroscopy), 72, 114
115 Azeotropes, 193, 197 distillation, 193
195 in separation of alcohols 1 alkanes, 200t in separation of alcohols 1 esters, 199t in separation of alcohols 1 ketones, 199t Azeotropes, 193, 197 distillation, 193
195 in separation of alcohols 1 alkanes, 200t in separation of alcohols 1 esters, 199t in separation of alcohols 1 ketones, 199t

B Baeyer
Villiger reaction, 125 BAIB. See Bis(acetoxy) iodobenzene (BAIB)

Bardeen, Cooper, and Schrieffer theory (BCS theory), 240
242 Basic ionic liquids as catalysts/ solvents for esterification, 156
157 BCS theory. See Bardeen, Cooper, and Schrieffer theory (BCS theory) Benzenesulfonate, 16 Benzothiophene (BT), 116
117 Benzylic alcohol, 113 BGE. See Butyl glycidyl ether (BGE) Bi-functionalized PEG1000 IL [imim-PEG1000-TEMPO] [CuCl2], 23
24, 24f Bi-phasic hydroformylation, 135 Bi-phasic system, 135 Biodiesel, 203
204 Biofuels, 203 Biogasoline preparation, 216 Biomass, 203 conversion of first-generation and second-generation, 205f process, 203, 204t dissolution of, 205
206, 206f, 207f enzymatic catalysis in ILs, 216
217 fractionation of, 205
206, 206f, 207f pretreatment, 205
206 sources, 203
204 transfer ILs, in, 9
10 utilization, 203 Bis(acetoxy)iodobenzene (BAIB), 123 Brazilian sugar cane ethanol, 203
204 N-Bromosuccinimide (NBS), 115 BT. See Benzothiophene (BT) Butyl glycidyl ether (BGE), 144
145 (1-Butyl-3-methyl-4,5dihydroimidazolinium hexafluorophosphate), 154

Index 1-Butyl-3-methylimidazolium bromide ([C4mim][Br]), 224
225 1-Butyl-3-methylimidazolium cation, 25 1-Butyl-3-methylimidazolium trifluoroacetate ([C4mim] [TFA]), 224 1-Butyl-3-vinylimidazolium chloride ([C4vim][Cl]), 152

C [C2mim][BF4] ion pair, 51
52, 120
121 [C4mim][Br], 122 [C4mim][Cl]. See IL 1-butyl-3methylimidazolium chloride ([C4mim][Cl]) [C4mim][OTf], 121 [C4mpy][C1]. See 3
Methyl
1
butyl
N
pyridine chloride ([C4mpy][C1]) Caffeic acid phenethyl ester analogs, 226f CAL. See Critical alkyl length (CAL) CaLA. See Candida antarctica lipase A (CaLA) CaLB. See Candida antarctica lipase B (CaLB) Caldariomyces fumago (C. fumago), 120 CAN. See Cericammonium nitrate (CAN) Candida antarctica lipase A (CaLA), 157
158 Candida antarctica lipase B (CaLB), 157, 226 Candida rugosa lipase (CrL), 157
158 Caprolactam (CP), 155
156 Carbamoylmethylphosphine oxide (CMPO), 18
21 Carbon dioxide (CO2), 45 conventional ILs
CO2 effect of anions, 72
77 effect of cations, 77
78 cations
anions interaction, 78
79

cycloaddition reaction of, 141
153 IL-based cycloaddition of CO2, 142f metallic catalysis process, 142
147 nonmetallic catalysis process, 147
153 hexabutylguanidinium salt/ ZnBr2 catalyzed coupling reaction of, 145f IL-based cycloaddition of, 142f interaction of ILs and, 72
82 conventional ILs
CO2, 72
79 task-specific ILs
CO2, 79
82 plausible reaction mechanism for cycloaddition of, 146f as substitute for CO in hydroformylation, 140
141 Carbon nanotubes (CNTs), 70
71 Carbon
carbon addition reaction, 141 bond, 211
212 CAS. See Chinese Academy of Science (CAS) Catalysis, 3
7 ILs in, 3
5 Catalyst(s). See also Solvents catalyst/IL system, 112 ILs as catalysts/solvents for esterification acidic ionic liquids as, 154
156 basic ionic liquids as, 156
157 ionic liquids as solvents for lipase-catalyzed esterification, 157
158 ILs for oxidations reactions homogeneous ionic liquids as, 123
126 supported ionic liquids, 126
128 Catalytic reaction in ILs alkylation reaction, 163
169 cycloaddition reaction of CO2 and epoxides, 141
153

247

esterification reaction, 153
162, 153f hydroformylation, 133
141 hydrogenation reaction, 128
133 oxidation reactions in ionic liquids, 111
128 Cation(s), 32
33, 198. See also Anion(s) effect of, 77
78 cationic structure, 214
215 cations
anions interaction, 78
79 dissolution mechanism of cellulose with interactions from anions and cations, 211
212, 212f groups, 3
5 for ILs, 143f interaction of cellulose and, 209
211 CDH. See Cellobiose dehydrogenase (CDH) Cellobiose, 206, 215 oxidation, 120 Cellobiose dehydrogenase (CDH), 120 Cellulose, 3, 206 dissolution in ILs, 82 interaction of ionic liquids and, 207
216 and anions, 208
209 and cations, 209
211 dissolution mechanism of cellulose with interactions from anions and cations, 211
212, 212f interaction study, 212
216, 213f, 214f microfibril, 213
214 oligomers, 215
216 polymerization, 206 Ceramic membrane, 194
195 Cericammonium nitrate (CAN), 119 CGMD. See Coarse-grained MD (CGMD) Chaotropic cation, 217 Charge distribution, 48
49 Charging temperature, 236
237

248 Index Charging time, 236
237 Chemical doping, 237
238 Chemical engineering, 111 challenge and trend ILs in biomass transfer, 9
10 ILs in MNPs catalysis, 9 developments and trend of ILs in, 7
10 focus and development of application of ILs, 7
8 toroidal movement of particles in Aircoater IAC5, 9f Chemical engineers, 236
237 Chemical oxidants, 115
120 Chinese Academy of Science (CAS), 35 Chiral ILs, 22 synthesis of camphorpyrazolium-based, 23f Chiral imidazolium ILs, 22, 22f Chiral Salen Co-(III)/quaternary ammonium halide catalyzed asymmetric synthesis, 144f Chl. See Chlorophyll (Chl) (S)-3-Chloro-1-phenyl-1propanone, 226
227 Chloroaluminate ILs, 166
167 on imidazolium cation, 167 1-Chlorobutane, 7 1-(Chloromethyl)-1,1dimethylhydrazine, 25 Chloroperoxidase (CPO), 120 Chlorophyll (Chl), 121 Classic band theory, 241 Clean synthesis, 30
31 Cluster, IL, 62 in neat systems, 65
69 in solutions, 62
65 system, 131
132 CMPO. See Carbamoylmethylphosphine oxide (CMPO) CNTs. See Carbon nanotubes (CNTs) CO in hydroformylation, CO2 as substitute for, 140
141 Coal-based MMA, 136 Coarse-grained MD (CGMD), 63
64

Colored impurities, 29 Conductor-like Screening Model for Real Solvents (COSMO-RS), 74 Conventional extractive distillation, 194
195 Conventional field-effect transistor (Conventional FET), 233
234, 234f, 236 Conventional ILs
CO2 anions effect, 72
77 cations effect, 77
78 interaction effect of cations
anions, 78
79 COSMO-RS. See Conductor-like Screening Model for Real Solvents (COSMO-RS) CP. See Caprolactam (CP) CPO. See Chloroperoxidase (CPO) Critical alkyl length (CAL), 66
67 Critical doping, 241 Critical temperatures, 240
241 CrL. See Candida rugosa lipase (CrL) Crystalline microfibril, 214 Cuprates, 240
241 Cyclic voltammetric analysis, 121
122 Cycloaddition reaction. See also Hydrogenation reaction of CO2 and epoxides, 141
153 IL-based cycloaddition of CO2, 142f metallic catalysis process, 142
147 nonmetallic catalysis process, 147
153

D DBT. See Dibenzothiophene (DBT) DC. See Direct current (DC) 1,3-DCB. See 1,3-Dicyanobenzene (1,3-DCB) Delocalization, effect of, 49
50 DEME-TFSI, 241 DEME-TSFI-gated ZnO, 238
239 Density, 83
86 Density functional theory (DFT), 59
60, 215

Designer solvents, 46 Dess-Martin-Periodinane (DMP), 115 Device under test (DUT), 235f DFT. See Density functional theory (DFT); Discrete Fourier transform (DFT) DHP. See Dihydrogen phosphate (DHP) Di-cationic IL ([C6mim][HSO4]), 155
156 Di-cationic ILs (DILs), 66
67 Di(triphenylphosphine trisulfonic acid sodiumruthenium chloride complex ((RuCl2(TPPTS)2), 132 Dialkyl imidazolium-metal chloride ILs, 125 1,3-Dialkylimidazolium alkanesulfonates, 17
18 N,N0 -Dialkylimidazolium, 23 Dialkylsulfates, 16 Diamagnets, 239 Dibenzothiophene (DBT), 116
117 Dichloromethane, 28 1,3-Dicyanobenzene (1,3-DCB), 223 Dielectric permittivity, 233 N,N-Diethyl-N-(2-methoxyethyl)N-methylammonium istrifluoromethylsulfonyli mide (DEME-TFSI), 236
237 Dihydrogen phosphate (DHP), 120 DILs. See di-cationic ILs (DILs) Dimethyl formamide (DMF), 121
122 Dimethyl sulfoxide (DMSO), 112 N,N-Dimethyl-2-[(2methylacryloyl)oxy] ethanaminium 5-carboxy-2, 4-bis-benzolate, 23
24 4,6-Dimethyl-2-thiomethylpyrimidine, 116
117 N,N-Dimethyl-N-(2-hydroxyethyl) ammonium bis (trifluoromethanesulfonyl) imide, 229

Index 4-Dimethylaminopyridine (DMAP), 112
113 4,6-Dimethyldibenzothiphenein, 116
117 3-(N,N-Dimethyldodecylammonium) propanesulfonic acid ptoluene-sulfonate, 155
156 Dimethylglyoxime (DMG), 113
114 1,1-Dimethylhydrazine, 25, 26f Direct current (DC), 239 Discrete Fourier transform (DFT), 148 Disiloxane-functionalized phosphonium-based IL [P222Si][NTf2], 18
21 Disodium salt of sulfonated (S,S)1,2-diphenyl-1,2-ethylenediamine ((S,S)-DPENDS), 132 Disparate electronic phases, 233 Dissolution, 206 of biomass, 205
206, 206f, 207f of cellulose in ILs, 82 with interactions from anions and cations, 211
212, 212f interaction of cellulose and anions, 208
209 of cellulose and cations, 209
211 of ionic liquids and cellulose, 207
216 study, 212
216, 213f, 214f Distillation, 193 Divinylbenzene (DVB), 152 DMAP. See 4Dimethylaminopyridine (DMAP) DMF. See Dimethyl formamide (DMF) DMG. See Dimethylglyoxime (DMG) DMP. See Dess-MartinPeriodinane (DMP) DMSO. See Dimethyl sulfoxide (DMSO)

Dual-acidic IL catalyst, 160
161 DUT. See Device under test (DUT) DVB. See Divinylbenzene (DVB)

E

“E 1 N” mechanism, 145
147 e-beam lithography. See Electron beam lithography (e-beam lithography) ECE mechanism. See Electron transfer
chemical reaction
electron transfer mechanism (ECE mechanism) ECODS system. See Extraction and catalytic oxidative desulfurization system (ECODS system) EDLCs. See Electrochemical double-layer capacitors (EDLCs) EDLG. See Electric double layer gating (EDLG) EDLs. See Electric double layers (EDLs) EDS. See Extraction desulfurization (EDS) ELEC energies. See Electrostatic energies (ELEC energies) Electric double layer gating (EDLG), 236, 238 Electric double layers (EDLs), 236 Electric field-induced electronic phase transitions, 236 Electric field-induced gating with ILs, 235
236, 235f Electrical transport, 233 Electro-oxidations, 120
123 of benzyl alcohol, 120
121 Electrochemical capacitors, 227
228 Electrochemical double-layer capacitors (EDLCs), 227
228 Electrochemical interactions, 234 Electrochemical windows, 236
237 Electrodynamics, 240 Electron beam lithography (e-beam lithography), 237

249

Electron density, 50 Electron transfer
chemical reaction
electron transfer mechanism (ECE mechanism), 122 Electron-rich species, 129 Electronic phases, 235 transitions, 236 Electronic structure of imidazolium cation, 47
50 charge distribution, 48
49 electron density, 50 NBO analysis, 49
50, 49t Electron
phonon coupling, 241 interaction, 240 Electrospray ionization mass spectrometry (ESI
MS), 33, 55, 68
69, 136 Electrostatic doping, 241 Electrostatic energies (ELEC energies), 75t EMI-Beti. See 1-Ethyl-2methylimidazolium bis (trifluoromethyl-sulfonyl) imide (EMI-Beti) Energetic ILs, 25 Energy smoothing, 227
228 Entrainer agent, 193
195 Enzymatic catalysis in ILs, 216
217 EO. See Ethylene oxide (EO) EODS. See Extraction combined with oxidation desulfurization (EODS) Epoxides, cycloaddition reaction of IL-based cycloaddition of CO2, 142f metallic catalysis process, 142
147 nonmetallic catalysis process, 147
153 Equimolar acid, 14 ESI
MS. See Electrospray ionization mass spectrometry (ESI
MS) Esterification reaction, 153
162, 153f. See also Alkylation— reaction IL polymer as catalysts for esterification, 161
162

250 Index Esterification reaction (Continued) ILs as catalysts/solvents for esterification, 154
158 supported ionic liquids as catalysts for esterification, 158
161 Esters, 153
154 ETF. See 2-Ethoxytetrahydro-furan (ETF) Ethanol, 195 Ethanol 1 water azeotrope, 195
197 2-Ethoxytetrahydro-furan (ETF), 119 N-Ethyl piperazinium propionate ([NEPP][CH3CH2COO]), 88
89 Ethyl valerate (EV), 155
156 1-Ethyl-2-methylimidazolium bis (trifluoromethyl-sulfonyl) imide (EMI-Beti), 239 1-Ethyl-3-(4-sulfobutyl) imidazolium bis (trifluoromethanesulfonyl) imide ([EimC4SO3H]NTf2), 116
117 1-Ethyl-3-methylimidazolium bromide ([C2mim][Br]), 149 1-Ethyl-3-methylimidazolium tetrafluoroborate ([C2mim] [BF4]), 34 Ethylbenzene production, 166
167 Ethylene oxide (EO), 143 1-Ethylpyridinium chloride ([C2Py][Cl]), 144
145 EV. See Ethyl valerate (EV) Excess molar volume, 86 Extraction and catalytic oxidative desulfurization system (ECODS system), 124
125 Extraction combined with oxidation desulfurization (EODS), 116
117 Extraction desulfurization (EDS), 116
117 Extraction efficiency, 197 Extractive distillation, 194
195, 194f

F F-PILs. See Fluoro-functionalized polymeric ILs (F-PILs) Fast atom bombardment mass (FAB-MS), 33 Fenton-like ILs, 116
117 Fermentation, 204
205 FFAs. See Free fatty acids (FFAs) Field-effect dielectrics, 234 Field-effect transistor (FET) device, 234f operation, 234 Fine chemicals, synthesis of electrochemical capacitors, 227
228 fuel cells, 228
230 pharmaceutical applications, 222
227 Flavanones oxidation, 120 Flavones oxidation, 120 Flex-fuel cars, 203
204 Fluorin boric acid sodium saturated solution, 37 Fluoro-anion
based ILs, 57
62, 58t Fluoro-functionalized polymeric ILs (F-PILs), 153 Fourier transform IR (FTIR), 60, 61f Fractionation of biomass, 205
206, 206f, 207f interaction of ionic liquids and cellulose, 207
216 dissolution mechanism of cellulose with interactions from anions and cations, 211
212, 212f interaction of cellulose and anions, 208
209 interaction of cellulose and cations, 209
211 interaction study, 212
216, 213f, 214f Free fatty acids (FFAs), 155 FTIR. See Fourier transform IR (FTIR) Fuel cells, 228
230 Fuels production, 203

Functionalized ILs, 18
22, 18f, 19t, 79, 137, 149 N-Functionalized imidazole derivatives, 18
21

G Gas chromatography (GC), 6, 90 Gate voltage, 233 Gating InOx thin films, 239 of semi-conducting, 237
239 charge carrier density, 237 disorder landscape, 238 of superconductors, 241
242 superconductivity, 239
241 GC. See Gas chromatography (GC) GC method. See Group Contribution method (GC method) GC strategy and ANN-based machine-learning algorithm (GC-ANN), 88 Global biomass production, 204 Gold nanoparticles (GNPs), 113, 127
128 Green chemistry, 133
134 Green solvents system, 132
133 Greenhouse gas emissions, 203
204 Group Contribution method (GC method), 87
88

H H-bond. See Hydrogen bond (Hbond) H-bonded donation abilities (HBDA), 59 H-bonds, 206, 208
209, 211, 215
216 basicity of solvent, 156
157 H-bonds, 206, 208
209, 211, 215
216 basicity of solvent, 156
157 Halide ions, 28 Halide-based ILs with an atomic anion, 55
57 Hall bar configuration, 237 device geometry, 237

Index Hall resistances, 237 Haloalkanes, 16 HBD. See Hydrogen-bonddonating (HBD) HBDA. See H-bonded donation abilities (HBDA) Heavy Fermion superconductors, 240 Helium liquefaction, 239
240 Hemicellulose, 205
208 N-Heterocycle 3-azabicyclo[3.2.2] nonane, 163 Heterocyclic cations, 14 Heterocyclic rings, 210
211 Heterocyclic structure, 214
215 Heterogeneity order parameter (HOP), 66
67 Heterogeneous with supported catalysts, 165 systems, 5 azeotropes, 193 hydroformylation, SILP catalysts for, 138
140 hydrogenation reactions, 128
130 methyltrioxorhenium derivatives, 118 Hexafluorophosphate (PF6)
, 16, 215, 223 Hexagonal mesoporous silica (HMS), 127 Hexamethylguanidinium lactate ([HMG][LAC]), 76
77, 76f HFILs. See Hydroxylfunctionalized ILs (HFILs) High electric fields, 236 High performance liquid chromatography (HPLC), 6 High-resolution transmission electron microscopy (HRTEM), 68, 69f Higher olefins, hydroformylation of, 137
138 Highest occupied molecular orbital (HOMO), 60 [HMG][LAC]. See Hexamethylguanidinium lactate ([HMG][LAC])

HMS. See Hexagonal mesoporous silica (HMS) HOMO. See Highest occupied molecular orbital (HOMO) Homogeneous azeotropes, 193 hydroformylation, 135
138 of higher olefins and substrates, 137
138 of lower olefins, 135
136 hydrogenation reactions, 130
133 ionic liquids, 123
126 Fe/TEMPO-based bi-magnetic IL, 124f systems, 5 Homologous ammonium-based ILs, 15
16 HOP. See Heterogeneity order parameter (HOP) HPLC. See High performance liquid chromatography (HPLC) HRTEM. See High-resolution transmission electron microscopy (HRTEM) HTIB. See [Hydroxy (tosyloxy) iodo]benzene (HTIB) HTSC, 241
242 Hydroformylation, 133
141 CO2 as substitute for CO in, 140
141 of higher olefins and substrates, 137
138 homogeneous hydroformylation, 135
138 of lower olefins, 135
136 SILP catalysts for heterogeneous hydroformylation, 138
140 Hydrogen atoms, 211 electrochemical oxidation, 122
123 Hydrogen bond (H-bond), 45, 147
148, 229 donor/ILS, 147
148 synergistic catalysis process, 148f ILs electronic structure of imidazolium cation, 47
50

251

interaction and, 45
62 structure and interaction of paired cation and anion, 50
55 interaction, 208 network structure, 34 between paired cation and anion, 55
62 fluoro-anion
based ILs, 57
62, 58t halide-based ILs with an atomic anion, 55
57 Hydrogen fluoride, 166 Hydrogen peroxide (H2O2), 114
115 Hydrogen sulfide (H2S), 121, 240
241 salts, 164 Hydrogen-bond-donating (HBD), 118
119 Hydrogenation reaction, 128
133. See also Cycloaddition reaction; Oxidation reactions heterogeneous hydrogenation reactions, 128
130 homogeneous hydrogenation reactions, 130
133 metal nanoparticle synthesis in ILs, 128f metal-catalyzed organic reactions, 133f solid catalysts with IL layer, 130f 2-Hydromethyl trimethylammonium dimethyl phosphate, 229 Hydrophobic interactions, 209 [Hydroxy (tosyloxy) iodo]benzene (HTIB), 120 5-(Hydroxy-methyl)furfural, 114 1-(2-Hydroxyethyl)-3-methyl imidazolium ([HEMIm]1), 33 N-(2-Hydroxyethyl)-N-methyl morphorinium [(HEMMor]1), 33 1-(2-Hydroxyl-ethyl)-3methylimidazolium bromide (HC2mim[Br]), 149

252 Index 1-(2-Hydroxyl-ethyl)-imidazoliumbased ILs, 150 Hydroxyl-functionalized ILs (HFILs), 149, 149f Hydroxyl-functionalized PILs, 23
24 N-Hydroxyphthalimide (NHPI), 113
114 Hydroxypivalaldehyde, 122 Hypervalent iodine reagents, 115

I Ibuprofen, 227 (RS)-Ibuprofen, 227 IBX. See Iodoxybenzoic acid (IBX) IL 1-allyl-3-methylimidazolium chloride ([Amim][Cl]), 209
210 IL 1-butyl-3-methylimidazolium chloride ([C4mim][Cl]), 213
214 IL 1-ethyl-3-methylimidazolium acetate [C2mim][OAc], 213 IL-based co-polymeric catalyst (PIL), 161 IL-functionalized, ordered, and stable mesoporous polymer catalysts (OMR-ILs), 160
161 IL-immobilized TEMPO (TEMPOIL), 123 ILG. See Ionic liquid gating (ILG) ILs. See Ionic liquids (ILs) 3-(1H-Imidazol-1-yl)propan-1amine, 18
21 Imidazole cation ILs, 210 Imidazolium amino acid-based ILs, 68f Imidazolium cation, 197
198 electronic structure, 47
50 charge distribution, 48
49 electron density, 50 NBO analysis, 49
50, 49t Imidazolium ILs, 119, 209
210 Imidazolium perrhenate ionic liquids (IPILs), 124
125 Imidazolium-based ILs, 124, 163 of [Bmim][Br], 26
27

Immobilization of chloroaluminated-based IL via impregnation, 159f of IL on modified silica, 160f Industrial alkylation processes, 165
168 acylation of aromatic hydrocarbons, 167
168 alkylation of olefins with isobutane, 166 ethylbenzene production, 166
167 LAB production, 167 Infrared spectroscopy (IR spectroscopy), 31, 33
34, 56 Inorganic compounds, 13 Inorganic materials, 145
147 Institute of Process Engineering (IPE), 35 Insulating materials, 237 Insulating systems, 237
239 charge carrier density, 237 disorder landscape, 238 Intrinsic physics, 238 Iodoxybenzoic acid (IBX), 115 Ion pair H-bond between paired cation and anion, 55
62 fluoro-anion
based ILs, 57
62, 58t halide-based ILs with an atomic anion, 55
57 interaction energy, 52
54, 54t interaction from experimental determination, 55 structure and interaction of paired cation and anion interaction energy, 52
54, 54t interaction from experimental determination, 55 structures of typical ion pairs, 50
52 structures, 50
52 Ionic cluster at interface, 69
70 Ionic liquid gating (ILG), 235
238

Ionic liquids (ILs), 1, 4f, 13, 45, 163, 165
168, 194
195, 206, 221, 221f application, 46 in catalysis, 3
5 as catalysts/solvents for esterification acidic ionic liquids as catalysts/solvents for esterification, 154
156 basic ionic liquids as catalysts/solvents for esterification, 156
157 ionic liquids as solvents for lipase-catalyzed esterification, 157
158 cation chain length, 198 characterization, 31
35 IR spectroscopy, 33
34 mass spectrometry, 33 NMR spectroscopy, 32
33 Raman spectrometry, 34
35 in chemical processes, 230t in chemical synthesis, 224f and CO2 interaction, 72
82 conventional ILs
CO2, 72
79 task-specific ILs
CO2, 79
82 developments and trend in chemical engineering, 7
10 electrolyte, 235 enzymatic catalysis in ILs, 216
217 gating of thin films conventional FET, 233
234, 234f electric field-induced gating with ILs, 235
236 experimental methods, 236
237 semi-conducting and insulating systems, 237
239 superconductors with ionic liquids, 239
242 IL stability, lifetime and recyclability, 168
169 IL-based extractive distillation, 194
195

Index interaction of ILs and cellulose, 207
216 dissolution of cellulose with interactions from anions and cations, 211
212 interaction of cellulose and anions, 208
209 interaction of cellulose and cations, 209
211 interaction study, 212
216, 213f, 214f largescale production of ILs, 35
38, 36f, 39f micro-structure and interaction, 45
82 in pharmaceutical synthesis, 223 physical properties, 83
93 density, 83
86 phase equilibrium, 89
93 viscosity, 86
89, 87t polymer as catalysts for esterification, 161
162 preparation, 13
27 aprotic conventional ILs, 15
18 chiral ILs, 22 energetic ILs, 25, 26f functionalized ILs, 18
22, 18f, 19t metal based ILs, 24
25, 25f nonconventional preparation, 26
27 PILs, 14
15 polymerized ILs, 23
24, 24f purification, 27
31 absorbents, 29
30 alkali metal salts, 28
29 clean synthesis, 30
31 colored impurities, 29 halide ions, 28 unreacted organic starting materials and solvents, 27
28 water, 30 in separation, 6
7 structures, 46
47 trends of publications on ILs over years, 2f

IPE. See Institute of Process Engineering (IPE) IPILs. See Imidazolium perrhenate ionic liquids (IPILs) IR spectroscopy. See Infrared spectroscopy (IR spectroscopy) Iranian revolution, 203 Isobutane alkylation, 166 alkylation of olefins with, 166 Isothermal compressibility, 84
85

K Kosmotropic anion, 217

L L-4-boronophenylalanine, 226f LAB production. See Linear alkylbenzenes production (LAB production) Largescale production of ILs, 35
38, 36f, 39f multifunction reactor in production of ILs, 37f preparation of [C4mim][BF4], 38f LCST. See Lower critical solution temperature (LCST) Lewis acid, 142
143 Lewis acid IL catalyst [Et3NH]Cl AlCl3, 161
162 Lewis acidic ILs, 135 Lignin, 114
115, 205
208 Lignocellulose, 204
205 Lignocellulosic biomass, 204
206 Linear alkylbenzenes production (LAB production), 167 Linear-chain alkyl imidazole chlorine ILs, 210 Lipase-catalyzed esterification, ionic liquids as solvents for, 157
158 Liquid transportation fuels, 203 Liquid-phase catalytic systems, 163 Liquid-phase systems, 163
165 (Liquid 1 liquid) equilibria data (LLE data), 197

253

Liquid
liquid equilibrium (LLE), 89
92 Liquid
liquid extractions, 6, 194
195 Lithography, 237 LLE. See Liquid
liquid equilibrium (LLE) LLE data. See (Liquid 1 liquid) equilibria data (LLE data) London equations, 239
240 Lower critical solution temperature (LCST), 91 Lower olefins, hydroformylation of, 135
136 Lowest unoccupied molecular orbit (LUMO), 60 LSCO, 241 LUMO. See Lowest unoccupied molecular orbit (LUMO)

M MAD. See Mean absolute deviation (MAD) Magnetic resonance imaging (MRI), 239 MAL. See Methylacrolein (MAL) MALDI-MS. See Matrix-assisted laser desorption/ionization mass (MALDI-MS) Mass spectroscopy, 6, 31, 33 Mass-separating agent, 193 Matrix-assisted laser desorption/ ionization mass (MALDIMS), 33 MBE. See Molecular beam epitaxy (MBE) MD. See Molecular dynamics (MD) (MEA)L. See Monoethanolaminium lactate ((MEA)L) Mean absolute deviation (MAD), 85 Mechanical exfoliation, 238 Meissner state, 239 Metal based ILs, 24
25, 25f catalysts, 164
165 Metal-containing ZSM-5 (MZSM-5), 118

254 Index Metal-organic frameworks (MOFs), 145 Metal-oxide semiconductor fieldeffect transistor (MOSFET), 233 miniaturization of, 234 Metal
insulator transition (MIT), 239 Metallic catalysis process. See also Nonmetallic catalysis process cation and anion used for ILs, 143f chiral Salen Co-(III)/quaternary ammonium halide catalyzed asymmetric synthesis, 144f metallic complex/IL, 142
145 metallic-based material/IL, 145
147 Metallic complex/IL, 142
145 Metallic nanoparticles (MNPs), 9 catalysis, ILs in, 9 ILs in MNPs catalysis, 9 synthesis in ILs, 128f Metallic phases, 235 Metallic-based material/IL, 145
147 2-Methoxytetrahydropyran (MTP), 119 Methyl methacylate (MMA), 136 N-Methyl-2-pyrrolidone hydrogen sulfate ([NMP] [HSO4]), 164 N-Methyl-2-pyrrolidone (NMP), 155
156 1-Methyl-3-(triethoxysilylpropyl) imidazolium hydrogen sulfate, 125 N-Methyl-N-propylpyrrolidinium bis (trifluoromethanesulfonyl) imide ([PYR13][NTf2]), 34
35 3
Methyl
1
butyl
N
pyridine chloride ([C4mpy][C1]), 210 Methylacrolein (MAL), 136 1-Methylimidazole, 7

1-H-3-Methylimidazolium bis (trifluoromethanesulfonyl) imide ([C6mim] NTf2), 229 Methyltrioxorhenium (MTO), 117
118 Micro-structure and interaction anisotropic structure and aggregation in ILs, 62
72 dissolution of cellulose in ILs, 82 interaction and H-bond of ILs, 45
62 interaction of ILs and CO2, 72
82 Microscopic quantum origins, 240 Mild catalytic conversion process, 216 MILs. See Mono-cationic ILs (MILs) MIT. See Metal
insulator transition (MIT) MMA. See Methyl methacylate (MMA) MNPs. See Metallic nanoparticles (MNPs) Model oil, 116
117 MOFs. See Metal-organic frameworks (MOFs) Molar volume, 84, 86 Molecular beam epitaxy (MBE), 241 Molecular design method, 216 Molecular dynamics (MD), 63, 72
73 simulation, 215 Molecule level of ILs, 45 Mono-cationic ILs (MILs), 66
67 Monoethanolaminium lactate ((MEA)L), 113 Moore’s law, 234 Moore’s scaling, 234 MOSFET. See Metal-oxide semiconductor field-effect transistor (MOSFET) MRI. See Magnetic resonance imaging (MRI)

MTO. See Methyltrioxorhenium (MTO) MTP. See 2Methoxytetrahydropyran (MTP) Multiphase liquid
liquid systems, 168 Multiple ILs, 236 MZSM-5. See Metal-containing ZSM-5 (MZSM-5)

N [N4444][Br]/H2O-catalyzed process, 148 NADH. See Nicotinamide adenine dinucleotide (NADH) NAO. See Natural atomic orbital (NAO) Natural atomic orbital (NAO), 48
49, 48t Natural bond orbital analysis (NBO analysis), 49
50, 49t, 60 NBO analysis. See Natural bond orbital analysis (NBO analysis) NBS. See N-Bromosuccinimide (NBS) NHPI. See N-Hydroxyphthalimide (NHPI) Nicotinamide adenine dinucleotide (NADH), 123 Niobium pentachloride (NbCl5), 34
35 NMP. See N-Methyl-2-pyrrolidone (NMP) NMR spectroscopy. See Nuclear magnetic resonance spectroscopy (NMR spectroscopy) Nonconventional preparation, 26
27 Nonmetallic catalysis process, 147
153. See also Metallic catalysis process hydrogen bond donor/ILS, 147
148, 148f supported ionic liquid, 150
153 task-specific ionic liquids, 148
150

Index Nonsteroidal anti-inflammatory drugs (NSAIDs), 227 Nonvolatile organosulfur compounds, 123
124 NSAIDs. See Nonsteroidal antiinflammatory drugs (NSAIDs) Nuclear magnetic resonance spectroscopy (NMR spectroscopy), 22, 31
33, 54, 208, 210 structure of 1-alkyl-3methylimidazolium salts, 32f, 32t Nucleophilic addition, 16 Nucleoside-based antiviral drugs, syntheses of, 224

O Odorless organosulfur compounds, 123
124 Olefin(s) alkylation with isobutane, 166 hydroformylation, 133
134 hydroformylation of higher olefins and substrates, 137
138 hydroformylation of lower olefins, 135
136 OMR-ILs. See IL-functionalized, ordered, and stable mesoporous polymer catalysts (OMR-ILs) “One-step” approach, 136 Organic compounds, 13 Organic materials, 145
147 Organic solvents, 206 residues, 222
223 Organic superconductors, 240 Organic volatile impurities. See Organic solvent— residues Oxidation reactions, 111
128. See also Hydrogenation reaction ILs as catalysts for homogeneous ionic liquids as, 123
126 supported ionic liquids, 126
128

ILs as solvents for, 112
123 aerobic oxidation, 112
115 chemical oxidants, 115
120 electro-oxidations, 120
123 selective oxidation, 116f THICA/DMG system, 114f Oxygen atoms, 211

P PAA. See Peracetic acid (PAA) PAMO. See Phenylacetone monooxygenase (PAMO) PBI. See Polybenzimidazole (PBI) PC. See Propylene carbonate (PC) PcL. See Pseudomonas cepacia lipase (PcL) PEDOT. See Poly(3,4ethylenedioxythiophene) (PEDOT) PEER. See Power, Environmental & Energy Research Center (PEER) PEG. See Polyethylene glycol (PEG) Peracetic acid (PAA), 115 PES. See Potential energy surface (PES) PFILs. See Phosphinefunctionalized phosphonium ILs (PFILs) PfL. See Pseudomonas fluorescens lipase (PfL) Pharmaceutical applications, 222
227 antiviral drug trifluridine TFT, 224f ILs in chemical synthesis, 224f production of caffeic acid phenethyl ester analogs, 226f synthesis of other types of pharmaceutical drugs, 226
227 synthesis of pharmaceutical compounds, 222
223 use of ILs in pharmaceutical synthesis, 223 Phase equilibrium, 89
93 LLE, 90
92 SLE, 92
93

255

VLE, 89
90 Phase-transfer catalysts (PTCs), 124
125 N-Phenyl-camphorpyrazole, 22 Phenylacetone monooxygenase (PAMO), 118
119 1-Phenylethanol, 124 Phosphine-functionalized phosphonium ILs (PFILs), 18
21, 137 Phosphonic acid ester, 207
208 Phosphonium-based ILs, 16, 17f Phosphorus pentoxide (P2O5), 14 Phosphotungstic acid (H3PW12O40), 119, 165 Photolithography, 237 Physical properties of ILs, 83
93 density, 83
86 phase equilibrium, 89
93 viscosity, 86
89, 87t Pig pancreas lipase (PpL), 157
158 PIL. See IL-based co-polymeric catalyst (PIL) PILs. See Poly(ionic liquids) (PILs); Protic ionic liquids (PILs) PO. See Propylene oxide (PO) Poly(1-butyl-3-vinylimidazolium bromide) microspheres, 124 Poly(3,4-ethylenedioxythiophene) (PEDOT), 123 Poly(ionic liquids) (PILs), 23
24, 24f, 151, 152f Poly(N-vinylimidazole-codivinylbenzene) (PVIm), 152 Poly(VMPS)-PW catalyst, 162, 163t Polybenzimidazole (PBI), 228
229 Polyethylene glycol (PEG), 18
21, 21f, 137
138 Polymerized ILs. See Poly(ionic liquids) (PILs) Polyoxometalateionic liquid, 126 Polysaccharides, 213 Potassium persulfate (K2S2O8), 116
117 Potential energy surface (PES), 50

256 Index Power, Environmental & Energy Research Center (PEER), 1
2 PpL. See Pig pancreas lipase (PpL) Pressure-swing distillation, 193 Pretreatment, 204
205 Proline bisulfate (ProHSO4), 155
156 1-n-Propylamine-3butylimidazolium tetrafluoroborate ([pabim] [BF4]), 80 Propylene carbonate (PC), 143 Propylene oxide (PO), 143 1-Propylpyridinium chloride ([C3Py][Cl]), 144
145 Protic ionic liquids (PILs), 14
15, 14f, 15f, 229 Proton, 77 proton-transfer process, 15 Pseudomonas cepacia lipase (PcL), 157
158 Pseudomonas fluorescens lipase (PfL), 157
158 Pt catalyst, 135 PTCs. See Phase-transfer catalysts (PTCs) p-toluene sulfonic acid (pTSA), 154 pTSA. See p-toluene sulfonic acid (pTSA) PVIm. See Poly(N-vinylimidazoleco-divinylbenzene) (PVIm) Pyrano[3,2
c]pyridone nucleoside hybrids, 225, 225f Pyrano[4,3
b]pyran nucleoside hybrids, 225, 225f Pyridine-containing anionfunctionalized ILs, 18
21 Pyridinium-based ILs, 52, 209
210 3-Pyridinylmethyl-Nhydroxyphthalimide, 114 Pyrimidine nucleoside-thiazolini-4one hybrids, 225f

Q Quantitative structure
property relationships (QSPR), 85 Quantum chemistry calculations, 215
216 theory and approaches, 215

Quasi-2D materials, 238 “Quasimolecular” structures, 58
59 Quaternization reactions, 31 Queen’s University Ionic Liquid Laboratories (QUILL), 1
2

R Radial distribution function (RDF), 56 Raman spectrometry, 34
35, 114
115 Rate-determining step (RDS), 139 RDF. See Radial distribution function (RDF) RDS. See Rate-determining step (RDS) “Reaction-induced self-separated” IL catalysts, 155 Recyclable system, 112 Redox reactions, 111 Residual solvents. See Organic solvent—residues Rhizomucor miehei lipase (RmL), 157
158 Rhodium-based catalysts, 134 RmL. See Rhizomucor miehei lipase (RmL) Room temperature superconductors, 240 Room-temperature ILs (RTILs), 46, 116
117 Ruthenocene (RuCp2), 122

S SANS. See Small-angle neutron scattering (SANS) SAPT. See Symmetry-adapted perturbation theory (SAPT) SC. See Styrene carbonate (SC) Scanning tunnel microscope (STM), 70 SCILL. See Supported catalyst with ionic liquid layer (SCILL) Separation alternatives to traditional, 3
7 ILs in separation, 6
7 science and technology

aqueous azeotropic systems, 195
199, 196t, 197t, 198f, 199t, 200t azeotropes, 193 distillation, 193 extractive distillation, 194
195, 194f separation
catalysis
conversion technology, 215
216 SH. See Sulfhydryl groups (SH) Sheldon E-factor, 223 Shunt resistance, 233
234 Silicon (Si), 239 SILP. See Supported ionic liquid phase (SILP) Silver, electrochemical oxidation of, 122 Silylating agents, 150 Simulation study, interaction of ILs and cellulose, 212
216 Single-walled nanotubes (SWNTs), 70
71 Singlet oxygen (1O2), 113 SIT. See Superconductor
insulator transition (SIT) SLE. See Solid
liquid equilibrium (SLE) Small-angle neutron scattering (SANS), 62
63 SO. See Styrene oxide (SO) SO3H-functionalized heteropolyacid-based polymeric hybrid catalyst, 162 SO3H-functionalized IL [HSO3C2mim][HSO4], 161
162, 161f Sodium hypochloride, 115 Sodium hypochlorite (NaOCl), 117
118 Solid catalysts with IL layer, 130f Solid-state gate dielectric, 233
234 Solid
liquid equilibrium (SLE), 89, 92
93 Solubilized cellulose, 212 Solvent(s). See also Catalyst(s) ILs for oxidations reactions aerobic oxidation, 112
115

Index chemical oxidants, 115
120 electro-oxidations, 120
123 selective oxidation, 116f THICA/DMG system, 114f mixtures, 193 Spectrophotometric colorimetry method (SPC method), 27
28 STM. See Scanning tunnel microscope (STM) Strengths, weaknesses, opportunities, and threats analysis (SWOT analysis), 8 Styrene carbonate (SC), 143 Styrene oxide (SO), 143 Substitutional doping, 237, 241 Sugar polymers, 204
205 Sulfhydryl groups (SH), 159
160 Sulfone-functionalized imidazolium ILs, 18
21 Sulfoxides, 123
124 Sulfur-based ILs, synthesis of, 17
18 Sulfuric acid, 166 Superconductivity, 239
241 Superconductor
insulator transition (SIT), 239 Superconductors gating, 241
242 heavy Fermion, 240 organic, 240 room temperature, 240 Supported catalyst with ionic liquid layer (SCILL), 5 Supported catalysts, heterogeneous systems with, 165 Supported ionic liquid phase (SILP), 5 catalysts, 138
139 for heterogeneous hydroformylation, 138
140 Supported ionic liquid(s), 126
128, 150
153 alkylating agent, 150 as catalysts for esterification, 158
161 functional polymer, 150

functional polymer supported ILs, 151f polymerization of IL, 151
153 silica-supported ILs, 151f Supported Lewis acidic IL catalyst, 158
159 Surface resistance, 236 SWNTs. See Single-walled nanotubes (SWNTs) SWOT analysis. See Strengths, weaknesses, opportunities, and threats analysis (SWOT analysis) Symmetry-adapted perturbation theory (SAPT), 52
53 Synergistic catalysis, 145 Syngas, 133
134

T Task-specific IL (TSIL), 79, 113, 148
150 TSIL
CO2, 79
82 TBHP. See Tert-butyl hydroperoxide (TBHP) TCCA. See Trichloroisocyanuric acid (TCCA) TEM. See Transmission electron microscopy (TEM) TEMPO-CuCl-catalyzed aerobic oxidation, 112
113 TEMPO-IL. See IL-immobilized TEMPO (TEMPO-IL) TEMPO-IL/CuCl2/silica, 126
127 TEOS. See Tetraethoxysilane (TEOS) Tert-butyl hydroperoxide (TBHP), 118 Tetradecyl(trihexyl)phosphonium dicyanamide IL, 38 Tetraethoxysilane (TEOS), 159
160 Tetrafluoroborate (BF4), 16, 169, 215 Tetrahydrofuran (THF), 156
157 1,1,3,3-Tetramethylguanidinium lactate ([tmg][L]), 80 TFT. See Trifluridine (5trifluoromethyl-20 deoxyuridine) (TFT)

257

TGA. See Thermal gravimetric analysis (TGA) Theoretical study, interaction of ILs and cellulose, 212
216 Thermal expansion coefficient, 84
85 Thermal gravimetric analysis (TGA), 168 Thermal stability, 216
217 Thermodynamic state, 239
240 Thermomyces lanuginosus lipase (TLL), 157
158 THF. See Tetrahydrofuran (THF) THICA. See N,N,Nʹʹ,Trihydroxyisocyanuric acid (THICA) Thin films, 233
234, 236, 241 Thomas-Fermi screening, 233
234 Three-phase reaction mixture, 164f Time of flights (TOFs), 131
132 Titanium silicalite-1 (TS-1), 118, 127
128 TLL. See Thermomyces lanuginosus lipase (TLL) TOFs. See Time of flights (TOFs) TONs. See Turnover numbers (TONs) TPPTS. See Triphenyl phosphine trisulfonate (TPPTS) Transistor, 233 Transition metal
based ILs, 24
25 Transmission electron microscopy (TEM), 129
130 Tri-phasic mixtures, 223 Trialkylphosphates, 16 1,2,3-Triazolium, 23 Trichloroisocyanuric acid (TCCA), 116
117 Triethylamine hydrochloride aluminum chloride ([Et3NH]Cl/AlCl3), 166 Trifluridine (5-trifluoromethyl-20 deoxyuridine) (TFT), 224, 224f

258 Index Trihexyl(tetradecyl)phosphonium hydroxide, 18
21 N,N,Nʹʹ,-Trihydroxyisocyanuric acid (THICA), 113
114 THICA/DMG system, 114f Trimethoxysily) propyl 3-(60 -oxobenzo-15-crown-5 hexyl) imidazolium bis (trifluoromethanesulphonyl) Trimethyl-1,4-benzoquinone, 113 2,3,6-Trimethylphenol, 113 Triphenyl phosphine trisulfonate (TPPTS), 138 Tris(triphenylphosphine)rhodium chloride (RhCl (PPh3)3), 130
131 TS-1. See Titanium silicalite-1 (TS-1) TSIL. See Task-specific IL (TSIL) Turnover numbers (TONs), 135 Two-dimensional materials (2D materials), 234

U Ultra-high vacuum (UHV), 70 UHV-STM, 70

Ultraviolet
visible spectroscopy (UV
Vis spectroscopy), 114
115, 209 Upper critical temperature (UCST), 91

V van der Waals energies (VDW energies), 75t Vapor pressure, 216
217 Vapor
liquid equilibrium (VLE), 89
90, 195
197 VDW energies. See van der Waals energies (VDW energies) Viscosity, 30, 86
89, 223 amine-functionalized ILs systems, 80
81 deviations, 88
89 of ILs, 45, 87t, 158 VLE. See Vapor
liquid equilibrium (VLE) Vogel-Tammann-Fulcher equation (VTF equation), 87
88 Volatile organic compounds, 27 VTF equation. See VogelTammann-Fulcher equation (VTF equation)

W Water, 30 water-soluble iron(III) porphyrins, 119 water-stable ionic liquids, 1 water 1 ethanol system, 197
198

X X-ray photoelectron spectroscopy (XPS), 70 X-ray powder diffraction (XRD), 55
56

Y YBCO, 238, 240
241

Z Zinc dichloride (ZnCl2), 34
35 Zinc-coordinated conjugated microporous polymers (Zn-CMPs), 147 ZnBr2/imidazolium salt, 143 ZnO, 238
239