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Handbook of Green Chemistry

Volume 6
Ionic Liquids

Edited by

Peter Wasserscheid and Annegret Stark

Wiley Logo

Ionic Liquids and Green Chemistry – an Extended Preface

Green Chemistry, or Sustainable Chemistry, deals with the development of chemicals (auxiliaries, intermediates, solvents, consumer products) and processes with the aim of designing for minimal environmental impact while providing maximum technical performance, as outlined in the Principles of Green Chemistry by Paul Anastas [1]. As such, Green Chemistry is part of Sustainability, meaning economically profitable production at lowest possible ecological intrusion, taking into account the social development of society (Three Pillars of Sustainability). Due to the globalization of markets, the social effects of industrial actions are difficult to assess, as many soft factors, such as politics, lobbying activities, funding strategies and artificial market regulations impact strongly, and are known to be unpredictable. Hence natural scientists have opted to concentrate on the consolidation of the equilibrium of two pillars only, i.e. ecology and economy.

Ionic liquids are salts which are characterized – due to their special distribution of charges and due to their special shape of ions – by melting points below 100 °C. Figure 1 displays typical cations and anions forming ionic liquids.

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Figure 1 Typical cations and anions forming ionic liquids.

Ionic liquids represent a new class of non-molecular, liquid materials with unique property profiles. These unique properties originate from a complex interplay of Coulombic, hydrogen bonding and van der Waals interactions. To understand and to utilize these complex ion interactions is the heart of ionic liquid science and leads to the optimization of ionic liquid structures for specific applications. Figure 2 displays a number of properties that can be combined in ionic liquids in a unique manner.

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Figure 2 Properties of ionic liquids that can be exploited to design new and greener processes and products.

In addition to the large number of modified and task-specific ions forming ionic liquids, the concept of using ionic liquids brings along some general features that have to do with the general properties of ionic liquids. Thus, some properties of ionic liquids are tunable in a wide range and others are more or less intrinsic to the approach. Table 1 shows an overview of typical ionic liquid property ranges. It gives typical values for selected properties and also known upper and lower limits. This overview is intended to give the less experienced reader quick access to some important ionic liquid facts. Much more detailed information about the specific properties of ionic liquids can be found, for example, in reference [2].

Table 1 Typical property ranges of ionic liquids [2]

Property Lower limit example Typical range of most ionic liquids Upper limit example
Density [C6C1pyr][DCA] = 0.92 g l−1a 1.1–1.6 g l−1a [C2mim]Br–AlBr3d:1/2 = 2.2 g l−1a
Viscosity [C2mim]Cl–AlCl3d:1/2 = 14 mPa sa 40–800 mPa sa [C4mim]Cl (supercooled) = 40 890 mPa sa
Thermal stability [C2mim][OAc] ≈ 200 °Cb 230–300 °Cb [C2mim][NTf2] = 400 °Cb
Hydrolytic stability [BF4], [PF6] Also heterocyclic cations can hydrolyze under extreme conditions [NTf2], [OTf], [CH3SO3]
Base stability [Al2Cl7], [HSO4] All 1,3-dialkylimidazolium ionic liquids are subject to deprotonation [PR4]+, [OAc]
Corrosion [NTf2], [OTf] Most ILs are corrosive towards Cu; additives available Cl, HF formed from [MFx] hydrolysis
Price [HNR3][HSO4] ≈ 3 € kg−1c 25–250 € kg−1 [C4dmim][NTf2] ≈ 1000 € kg−1
aAt room temperature.
bTGA experiments at 10 K min−1.
cEstimation made for a production scale of 1000 kg and for a purity >98%.
d[C2mim]Br–AlBr3: 1/2 or [C2mim]Cl–AlCl3 denotes the ratio of 1-ethyl-3-methylimidazolium halide and aluminium halide.

From the early 1990s, the development of ionic liquids as solvents for organic synthesis and homogeneous catalysis evolved and these studies went in parallel with efforts to reduce the environmental impact of chemical synthesis and production (Green Chemistry). As the growing understanding of the unique properties of ionic liquids justified expectations that some improvements to existing technology could be made, the development of ionic liquids and Green Chemistry over the last 20 years went hand in hand.

The term Green Chemistry has been used extensively in the literature, and even a journal bearing this title has been established. However, upon examination of the literature, it becomes clear that in most cases synthetic strategies have been labeled with the term Green Chemistry rather uncritically. Ionic liquids are a prime example of this practice: due to their generally low vapor pressure (reducing the risk of exposure of workers and gaseous emissions), low flammability (reducing the risk of explosion) and often low toxicity when compared with conventional, volatile organic solvents, ionic liquids were often touted as Green Solvents.

From today's point of view, some of the early expectations proved to be unfounded, or at least not valid for the whole class of ionic liquids. Of course, it was found that just like any other physico-chemical property, toxicity, explosivity and volatility can also be designed into the structure of ionic liquids, but likewise, they can be carefully avoided.

Thus ionic liquids are not intrinsically green! They are a class of fascinating, new liquid materials providing unique combinations of properties. By making use of those unique properties, more efficient and greener processes and products can be realized. As a consequence of this statement, the greenness of an ionic liquid always has to be defined by its performance in a specific application –as also for any other performance chemical. The performance of an ionic liquid in a given application is highly dependent upon the structure of the ions forming the liquid. That means that some ionic liquids may be “green” in a given application whereas others, less favorably chosen ones, may be “not green”. In addition to this performance argument, it must be noted that the greenness of any chemical is a function of many additional factors, including quantity, risk of release into the environment, location and type of environment, toxicological properties and biodegradation. These factors cannot be assessed a priori, but only in the context of a specific technical application at a given scale.

Designing green ionic liquids and also processes, applications and products based on ionic liquid technology requires a profound knowledge of a broad base of multidisciplinary aspects. In many cases that are discussed today, research is still at an early stage. This is because the use of ionic liquids in general is a very young scientific topic that has attracted massive scientific attention only in the last 10 years. Given the huge number of proposed Green Chemistry concepts involving ionic liquids and in the light of this short period, it is understandable why there are often too few data available to predict with certainty whether a certain application will really profit in its greenness from the use of an ionic liquid.

To illustrate this statement, one can look at ionic liquids in organic chemistry: Most organic reactions have been tested in ionic liquids to date, but this has often been done by simply transferring traditional reaction conditions to this new class of solvents. In many cases, interesting results were obtained; in others, the outcome was disappointing; in yet other cases, effects were first attributed to the ionic liquid solvent that later turned out to be due to impurities in the ionic liquid or due to ionic liquid decomposition products. From all these studies, it is evident, however, that the full utilization of the ionic liquid's potential in organic synthesis requires a detailed knowledge of the ionic liquid–substrate interactions which influence significantly rates and/or selectivities. This detailed knowledge is just about to be generated from ongoing and recent fundamental kinetic and spectroscopic work.

It should also be noted that it is not easy to derive the green advantage of a certain ionic liquid in a given application from an initial “proof of concept” research. Much information that is necessary to evaluate the greenness of an ionic liquid application is only generated in a more advanced state of development in which attention can be given to aspects such as recycling efficiency, ionic liquid recovery or the degree of ionic liquid degradation over time. However, this type of information is absolutely crucial for evaluating the greenness of every ionic liquid application.

As opposed to existing reviews and monographs, this book aims to summarize the current state of the art of Green Chemistry using ionic liquids. Authors from industry and academia have contributed their points of view on various aspects, starting with a thorough assessment of the synthesis of ionic liquids by Maggel Deetlefs and Ken Seddon of the Queen's University of Belfast Ionic Liquid Laboratories. Using SWOT analysis, the synthesis (by conductive heating, microwave or ultrasound-assisted) and purification of ionic liquids is analyzed with respect to the 12 Principles of Green Chemistry.

A large section of the book is dedicated to the application of ionic liquids to synthesis, with subsections dealing with organic synthesis, transition metal catalysis, the conversion of saccharides to platform chemicals and the processing of cellulose by direct dissolution in ionic liquids. In their contribution on organic synthesis, Peter Wasserscheid and Joni Joni of the University of Erlangen-Nuremberg discuss important aspects of green organic synthesis with ionic liquids and exemplify the latter for Friedel–Crafts alkylation reaction. In the section on transition metal catalysis, Peter Wasserscheid details the advantages of multiphase transition metal catalysis in ionic liquids. Both liquid–liquid biphasic catalysis and supported ionic liquid phase (SILP) catalysis are highlighted with special emphasis on catalytic hydroformylation as an illustrative example. A summary of the state of the art of saccharide transformation to 5-hydroxymethylfurfural, a potential platform chemical of future biorefineries, is presented by Annegret Stark and Bernd Ondruschka of the Friedrich-Schiller University in Jena, showing the advantages of biphasic liquid–liquid processing. Uwe Vagt of BASF SE details the achievements of direct dissolution and processing of cellulose from ionic liquids, and compares them with currently used production technology.

It is clear that Green Chemistry with ionic liquids is much more than just synthesis and catalysis. Improved separation processes, more efficient devices or optimized products can be realized by using ionic liquids and this can add to a greener chemistry as the ionic liquid applied may help to e.g. save, generate or store energy. Moreover, some non-synthetic aspects of Green Chemistry come naturally into play when evaluating the overall sustainability of a chemical process that includes not only reaction steps but also product purification and the operation of process machinery. Hence the second large section of this book is dedicated to Green Chemical Engineering. Here, Wytze Meindersma, Ferdy Onink and André de Haan summarize various separation techniques involving ionic liquids, including extraction of various liquid, solid and gaseous solutes, desulfurization methods, and separation over membranes. The use of ionic liquids in greener electrochemical applications, in particular dye-sensitized solar cells, are presented by Will Pitner and colleagues of Merck in Germany and Japan. Ionic liquids have also shown great promise for energy saving as advanced lubricants, as highlighted by Marc Uerdingen of Solvent Innovation. Matthias Seiler and Peter Schwab of Evonik Degussa demonstrate the potential of ionic liquids in smart heat pumps when compared with conventional fluids as working pairs for absorption chillers.

A very detailed knowledge of the toxicological properties of ionic liquids is required not only to adhere to REACH requirements for the introduction of novel ionic liquids into the market, but also for the conscious design of ionic liquid structures to reduce risk of environmental damage in the case of spillage. The team of authors headed by Stefan Stolte of the University of Bremen introduce the T-SAR concept and a test kit approach for the broad assessment of various toxicological properties and degradation pathways of ionic liquids to derive a structure–activity relationship, from which conclusions are drawn on how to design innocuous ionic liquids.

The assessment of the eco-efficiency of ionic liquids is discussed from both academic and industrial points of view. The group of Dana Kralisch of the Friedrich Schiller University at Jena has developed a tool which allows for a simplified life-cycle analysis (ECO method), which has been applied to the optimization of the synthesis of ionic liquids and their application in the Diels–Alder synthesis. Peter Saling and colleagues of BASF SE focus on the comparison of the performance of ionic liquids in the BASIL process, in which the formation of an ionic liquid as a side-product of the preparation of dialkoxyphenylphosphines increases the rate of reaction and facilitates phase separation.

In this book, we have tried to give a broad readership easy access to Green Chemistry studies involving ionic liquids. In the course of editing this book, we have realized that the number of different abbreviations and nomenclatures for ionic liquids is often a problem for the the non-expert to enter the field. Therefore, we have decided to use in this book a uniform system of abbrevations throughout all contributions to avoid any confusion in this respect. The list of abbreviations is given in Table 2.

Table 2 Abbreviations for ionic liquid ions used in this book

Anion Name Cation Name
[OAc] Acetate [Cnmim]+ 1-Alkyl-3-methylimidazolium
[CnC1im]+
[CF3CO2] Trifluoroacetate [Cndmim]+ 1-Alkyl-2,3-dimethylimidazolium
[OTf] Trifluoromethanesulfonate [Allylmim]+ 1-Allyl-3-methylimidazolium
[CF3SO3]
[OTs] p-Toluenesulfonate [H-mim]+ 1-H-3-Methylimidazolium
[CH3SO3] Methanosulfonate [1-C4py]+ N-Butylpyridinium
[C4F9SO3] Nonafluorobutylsulfonate [1-C4,4-C1py]+ 1-Butyl-4-methylpyridinium
[NTf2] Bis(fluoromethanesulfonyl)amide [1-C4,3-C1py]+ 1-Butyl-3-methylpyridinium
[N(SO2CF3)2]
[DCA] Dicyanamide [H-Py]+ N-H-Pyridinium
[N(CN)2]
[BF4] Tetrafluoroborate [Pw,x,y,z]+ Tetraalkylphosphonium
[TCB] Tetracyanoborate [Nw,x,y,z]+ Tetraalkylammonium
[B(CN)4]
[BBB] Bis[1,2-benzenediolato(2–)-O,O′]borate [Me3NH]+ Trimethylammonium
[SbF6] Hexafluoroantimonate [Et3NH]+ Triethylammonium
[HSO4] Hydrogensulfate [Sxyz]+ Trialkylsulfonium
[PF6] Hexafluorophosphate [Cnthia]+ N-Alkylthiazolium
[FAP] Tris(pentafluoroethyl)trifluorophosphate [CnC1pyr]+ N-Alkyl-N-methylpyrrolidinium
[PF3(C2F5)3]
[(RO)2PO2] Dialkylphosphate [C4C1pip]+ 1-Butyl-1-methylpiperidinium
[DMP] Dimethylphosphate [Cnquin]+ N-Alkylquinolinium
[DEP] Diethylphosphate [Cnpicol]+ N-Alkylpicolinium
[H2PO4] Hydrogenphosphate [CnC1morph]+ N-Alkyl-N-methylmorpholinium
[TCM] Tricyanomethide [TGA)]+ Tetramethylguanidinium
[C(CN)3]
[CTf3] Tris(trifluoromethanesulfony)methide
[MeSO4] Methylsulfate
[CH3SO4]
[EtSO4] Ethylsulfate
[C2H5SO4]
[C8H17SO4] Octylsulfate
[n-C8H17OSO3]
[SCN] Thiocyanate

This book has been produced in a large and cooperative effort by many contributors. First, we would like to thank all authors for having brought in their expertise. Likewise, our thanks go to the Wiley-VCH staff for their support, their patience and their encouragement.

We are confident that this book will offer useful information for a broad and diverse readership interested in realizing Green Chemistry with the help of advanced solvent concepts. It is our hope that the book may contribute to the further promotion of ionic liquids in Green Chemistry both in industry and in academia, but also to a more critical assessment of this approach in the various potential fields of application. The book aims to stimulate further research and to deepen the collaboration between chemists, chemical engineers, mechanical engineers and toxicologists in the field of Green Chemistry with ionic liquids. We are convinced that ionic liquids as a scientific tool can help to realize a Greener Chemistry but much remains to be achieved and to be developed towards this goal. We hope that this book will help to attract young scientists to this important field of research for our future.

References

  1. 1. Anastas, P.T. and Kirchhoff, M.M. (2002) Acc. Chem. Res., 35, 686.
  2. 2. Wasserscheid, P. and Welton, T. (eds) (2007) Ionic Liquids in Synthesis, Wiley-VCH Verlag GmbH, Weinheim, (two volumes, 724 pages).

October 2009

Annegret Stark

Jena

Peter Wasserscheid

Erlangen

About the Editors

Series Editor

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Paul T. Anastas joined Yale University as Professor and serves as the Director of the Center for Green Chemistry and Green Engineering there. From 2004–2006, Paul was the Director of the Green Chemistry Institute in Washington, D.C. Until June 2004 he served as Assistant Director for Environment at the White House Office of Science and Technology Policy where his responsibilities included a wide range of environmental science issues including furthering international public-private cooperation in areas of Science for Sustainability such as Green Chemistry. In 1991, he established the industry-government-university partnership Green Chemistry Program, which was expanded to include basic research, and the Presidential Green Chemistry Challenge Awards. He has published and edited several books in the field of Green Chemistry and developed the 12 Principles of Green Chemistry.

Volume Editors

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Annegret Stark studied pharmaceutical chemistry at the University of Applied Sciences in Isny, Germany. She conducted her diploma thesis in 1997 in the labs of R.D. Singer at St. Mary's University in Halifax, Nova Scotia, who inspired her to take up a researcher's career in the field of ionic liquids. After finishing her PhD in K.R. Seddon's research group at the Queen's University of Belfast, Northern Ireland, in 2001, she moved on to South Africa for a SASOL-sponsored post-doc in the group of H.G. Raubenheimer at Stellenbosch University (2001–2003).

Since 2003, she heads her own research group at the Institute for Technical Chemistry and Environmental Chemistry (B. Ondruschka) of the Friedrich-Schiller University in Jena, Germany. Her research focus lies, on the one hand, on the elucidation of structure-induced interactions between ionic liquids and solutes, and the resulting effects on the reactivity of these. On the other hand, she is interested in the application of microreaction technology, e.g. in the conversion of highly reactive intermediates. Both, ionic liquids and microreaction technology, are exploited as tools with the goal to provide sustainable chemical and engineering concepts.

Since October 2009, she has been an interim professor for Technical Chemistry at the TU Chemnitz, Germany.

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Peter Wasserscheid studied chemistry at the RWTH Aachen. After receiving his diploma in 1995 he joined the group of Prof. W. Keim at the Institute of Technical and Macromolecular Chemistry at the RWTH Aachen for his PhD thesis. In 1998 he moved to BP Chemicals in Sunbury/GB for an industrial post-doc for six months. He returned to the Institute of Technical and Macro-molecular Chemistry at the RWTH Aachen where he completed his habilitation entitled “Ionic liquids – a new Solvent Concept for Catalysis”. In the meantime, he became co-founder of Solvent Innovation GmbH, Cologne, one of the leading companies in ionic liquid production and application (since December 2007 a 100% affiliate of Merck KGaA, Darmstadt). In 2003 he moved to Erlangen as successor of Prof. Emig and since then is heading the Institute of Reaction Engineering. In 2005 he also became head of the department “Chemical and Bioengineering” of the University Erlangen-Nuremberg. P. Wasserscheid has received several awards including the Max-Buchner-award of DECHEMA (2001), the Innovation Award of the German Economy (2003, category “start-up”) together with Solvent Innovation GmbH and the Leibniz Award of the German Science Foundation (2006). His key research interests are the reaction engineering aspects of multiphase catalytic processes with a particular focus on ionic liquid reaction media. The Wasserscheid group belongs to the top research teams in the development and application of ionic liquids in general, and in developing the ionic liquid technology for catalytic applications in special. For various reaction types the group has successfully demonstrated greatly enhanced performance of ionic liquid based catalyst systems vs. conventional systems.

Peter Wasserscheid has a scientific track record of more than 130 publications in peer-reviewed scientific journals plus many papers in the form of proceedings. Moreover, he is a co-inventor of more than 40 patents, most of them in the field of ionic liquids.

List of Contributors

Jürgen Arning

University of Bremen

UFT – Centre for Environmental Research and Technology

Department 3: Bioorganic Chemistry

Leobener Strasse

28359 Bremen

Germany

Maggel Deetlefs

The Queen's University of Belfast

The QUILL Research Centre

David Keir Building

Stranmillis Road

Belfast BT9 5AG

Northern Ireland, UK

André B. de Haan

Eindhoven University of Technology

Department of Chemical Engineering and Chemistry/Process Systems Engineering

5600 MB Eindhoven

The Netherlands

Bernd Jastorff

University of Bremen

UFT – Centre for Environmental Research and Technology

Department 3: Bioorganic Chemistry

Leobener Strasse

28359 Bremen

Germany

Joni Joni

Friedrich-Alexander-Universität Erlangen-Nürnberg

Lehrstuhl für Chemische Reaktionstechnik

Egerlandstrasse 3

91058 Erlangen

Germany

Kentaro Kawata

Merck Ltd Japan

New Technology Office

4084 Nakatsu

Aikawa-machi

Aiko-gun

Kanagawa 243-0303

Japan

Peer Kirsch

Merck Ltd Japan

New Technology Office

4084 Nakatsu

Aikawa-machi

Aiko-gun

Kanagawa 243-0303

Japan

Dana Kralisch

Friedrich-Schiller-Universität Jena

Institut für Technische Chemie und Umweltchemie

Lessingstrasse 12

07743 Jena

Germany

Matthias Maase

BASF SE

67056 Ludwigshafen

Germany

Marianne Matzke

University of Bremen

UFT – Centre for Environmental Research and Technology

Department 10: Ecology

Leobener Strasse

28359 Bremen

Germany

Wytze (G. W.) Meindersma

Eindhoven University of Technology

Department of Chemical Engineering and Chemistry/Process Systems Engineering

5600 MB Eindhoven

The Netherlands

Bernd Ondruschka

Friedrich-Schiller-Universität Jena

Institut für Technische Chemie und Umweltchemie

Lessingstrasse 12

07743 Jena

Germany

Ferdy (S. A. F.) Onink

Eindhoven University of Technology

Department of Chemical Engineering and Chemistry/Process Systems Engineering

5600 MB Eindhoven

The Netherlands

Denise Ott

Friedrich-Schiller-Universität Jena

Institut für Technische Chemie und Umweltchemie

Lessingstrasse 12

07743 Jena

Germany

William R. Pitner

Merck KGaA

PC R&D Ionic Liquids

Frankfurter Strasse 250

65293 Darmstadt

Germany

Johannes Ranke

University of Bremen

UFT – Centre for Environmental Research and Technology

Department 3: Bioorganic Chemistry

Leobener Strasse

28359 Bremen

Germany

Peter Saling

BASF SE

67056 Ludwigshafen

Germany

Peter Schwab

Evonik Degussa GmbH

Care and Surface Specialties

Goldschmidtstrasse100

45127 Essen

Germany

Kenneth R. Seddon

The Queen's University of Belfast

The QUILL Research Centre

David Keir Building

Stranmillis Road

Belfast BT9 5AG

Northern Ireland, UK

Matthias Seiler

Evonik Degussa GmbH

Process Technology and Engineering

Rodenbacher Chaussee 4

63457 Hanau

Germany

Hiromi Shinohara

Merck Ltd Japan

New Technology Office

4084 Nakatsu

Aikawa-machi

Aiko-gun

Kanagawa 243-0303

Japan

Annegret Stark

Friedrich-Schiller-Universität Jena

Institut für Technische Chemie und Umweltchemie

Lessingstrasse 12

07743 Jena

Germany

Stefan Stolte

University of Bremen

UFT – Centre for Environmental Research and Technology

Department 3: Bioorganic Chemistry

Leobener Strasse

28359 Bremen

Germany

Marc Uerdingen

Solvent Innovation GmbH

Nattermannallee 1

50829 Köln

Germany

Uwe Vagt

BASF SE

67056 Ludwigshafen

Germany

Peter Wasserscheid

Friedrich-Alexander-Universität Erlangen-Nürnberg

Lehrstuhl für Chemische Reaktionstechnik

Egerlandstrasse 3

91058 Erlangen

Germany

Part I
Green Synthesis