Supported Ionic Liquids E-Book
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- Herausgeber: Wiley-VCH
- Kategorie: Wissenschaft und neue Technologien
- Sprache: Englisch
Technology with supported ionic liquids is a fundamental new approach to modifying the surfaces of porous solids by dispersing a thin film of ionic liquid onto it. The technology allows introducing new and unique material properties and attains long-term stable materials. Divided into four parts, this book introduces the topic of supported ionic liquid materials and fundamentals, covering ionic liquids, porous supports, synthesis and characterization. The main part covers various applications, such as the catalytic production of bulk and fine chemicals, environmental processes, biotechnology, energy production and gas separation. In each case, the most pertinent authors available describe here the underlying research. The final part discusses the perspectives and outlook of these materials as well as describing some real life applications. This book is aimed at organic and inorganic chemists, industrial chemists, catalytic chemists, chemical engineers, biotechnologists, materials scientists and libraries
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Seitenzahl: 821
Veröffentlichungsjahr: 2013
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Table of Contents
Related Titles
Title Page
Copyright
Preface
List of Contributors
Chapter 1: Introduction
1.1 A Century of Supported Liquids
1.2 Supported Ionic Liquids
1.3 Applications in Catalysis
1.4 Applications in Separation
1.5 Coating of Heterogeneous Catalysts
1.6 Monolayers of IL on Surfaces
1.7 Conclusion
References
Part I Concept and Building Blocks
Chapter 2: Concept and Building Blocks
2.1 Introduction
2.2 Preparation
2.3 Liquid Range
2.4 Structures
2.5 Physical Properties
2.6 Effects of Ionic Liquids on Chemical Reactions
2.7 Ionic Liquids as Process Solvents in Industry
2.8 Summary
References
Chapter 3: Porous Inorganic Materials as Potential Supports for Ionic Liquids
3.1 Introduction
3.2 Porous Materials – an Overview
3.3 Silica-Based Materials – Amorphous
3.4 Layered Materials
3.5 Microporous Materials
3.6 Ordered Mesoporous Materials
3.7 Structured Supports and Monolithic Materials
3.8 Conclusions
References
Chapter 4: Synthetic Methodologies for Supported Ionic Liquid Materials
4.1 Introduction
4.2 Support Materials
4.3 Preparation Methods for Supported Ionic Liquids
4.4 Summary
References
Part II Synthesis and Properties
Chapter 5: Synthesis and Properties
5.1 Example I: [EMIM][NTf2] on Porous Silica
5.2 Example II: SCILL Catalyst (Commercial Ni catalyst) Coated with [BMIM][OcSO4]
Acknowledgments
Symbols
Abbreviations
References
Chapter 6: Transport Phenomena, Evaporation, and Thermal Stability of Supported Ionic Liquids
6.1 Introduction
6.2 Diffusion of Gases and Liquids in ILs and Diffusivity of ILs in Gases
6.3 Thermal Stability and Vapor Pressure of Pure ILs
6.4 Vapor Pressure and Thermal Decomposition of Supported ILs
6.5 Outlook
Acknowledgments
Symbols
Abbreviations
References
Chapter 7: Ionic Liquids at the Gas–Liquid and Solid–Liquid Interface – Characterization and Properties
7.1 Introduction
7.2 Characterization of Ionic Liquid Surfaces by Spectroscopic Techniques
7.3 Orientation and Properties of Ionic Liquids at the Solid–Liquid Interface
7.4 Comments
References
Chapter 8: Spectroscopy on Supported Ionic Liquids
8.1 NMR-Spectroscopy
8.2 IR Spectroscopy
References
Chapter 9: A Priori Selection of the Type of Ionic Liquid
9.1 Introduction and Objective
9.2 Methods
9.3 Usage of COSMO-RS to Predict Solubilities in IL
9.4 Results of Reaction Modeling
9.5 Perspectives of the A Priori Selection of ILs
References
Part III Catalytic Applications
Chapter 10: Supported Ionic Liquids as Part of a Building-Block System for Tailored Catalysts
10.1 Introduction
10.2 Immobilized Catalysts
10.3 Supported Ionic Liquids
10.4 The Building Blocks
10.5 Catalysis in Supported Thin Films of IL
10.6 Supported Films of IL in Catalysis
10.7 Advantages and Drawbacks of the Concept
10.8 Conclusions
Acknowledgments
References
Chapter 11: Coupling Reactions with Supported Ionic Liquid Catalysts
11.1 Introduction
11.2 A Short History of Supported Ionic Liquids
11.3 Properties of SIL
11.4 Application of SIL in Coupling Reactions
11.5 Conclusion
References
Chapter 12: Selective Hydrogenation for Fine Chemical Synthesis
12.1 Introduction
12.2 Selective Hydrogenation of α,β-Unsaturated Aldehydes
12.3 Asymmetric Hydrogenations over Chiral Metal Complexes Immobilized in SILCAs
12.4 Conclusions
References
Chapter 13: Hydrogenation with Nanoparticles Using Supported Ionic Liquids
13.1 Introduction
13.2 MNPs Dispersed in ILs: Green Catalysts for Multiphase Reactions
13.3 MNPs Immobilized on Supported Ionic Liquids: Alternative Materials for Catalytic Reactions
13.4 Conclusions
References
Chapter 14: Solid Catalysts with Ionic Liquid Layer (SCILL)
14.1 Introduction
14.2 Classification of Applications of Ionic Liquids in Heterogeneous Catalysis
14.3 Preparation and Characterization of the Physical Properties of the SCILL Systems
14.4 Kinetic Studies with SCILL Catalysts
14.5 Conclusions and Outlook
Acknowledgments
Symbols Used
Greek Symbols
Abbreviations and Subscripts
References
Chapter 15: Supported Ionic Liquid Phase (SILP) Materials in Hydroformylation Catalysis
15.1 SILP Materials in Liquid-Phase Hydroformylation Reactions
15.2 Gas-Phase SILP Hydroformylation Catalysis
15.3 SILP Combined with scCO2 – Extending the Substrate Range
15.4 Continuous SILP Gas-Phase Methanol Carbonylation
15.5 Conclusion and Future Potential
References
Chapter 16: Ultralow Temperature Water–Gas Shift Reaction Enabled by Supported Ionic Liquid Phase Catalysts
16.1 Introduction to Water–Gas Shift Reaction
16.2 Challenges
16.3 SILP Catalyst Development
16.4 Building-Block Optimization
16.5 Application-Specific Testing
16.6 Conclusion
References
Chapter 17: Biocatalytic Processes Based on Supported Ionic Liquids
17.1 Introduction and General Concepts
17.2 Biocatalysts Based on Supported Ionic Liquid Phases (SILPs)
17.3 Biocatalysts Based on Covalently Supported Ionic Liquid-Like Phases (SILLPs)
17.4 Conclusions/Future Trends and Perspectives
Acknowledgments
References
Chapter 18: Supported Ionic Liquid Phase Catalysts with Supercritical Fluid Flow
18.1 Introduction
18.2 SILP Catalysis
References
Part IV Special Applications
Chapter 19: Pharmaceutically Active Supported Ionic Liquids
19.1 Active Pharmaceutical Ingredients in Ionic Liquid Form
19.2 Solid-Supported Pharmaceuticals
19.3 Silica Materials for Drug Delivery
19.4 Factors That Influence the Loading and Release Rate of Drugs
19.5 SILPs Approach for Drug Delivery
19.6 Conclusions
References
Chapter 20: Supported Protic Ionic Liquids in Polymer Membranes for Electrolytes of Nonhumidified Fuel Cells
20.1 Introduction
20.2 Protic ILs as Electrolytes for Fuel Cells
20.3 Membrane Fabrication Including PIL and Fuel Cell Operation
20.4 Proton Conducting Mechanism during Fuel Cell Operation
20.5 Conclusion
Acknowledgments
References
Chapter 21: Gas Separation Using Supported Ionic Liquids
21.1 SILP Materials
21.2 Supported Ionic Liquid Membranes (SILMs)
21.3 Conclusion
References
Chapter 22: Ionic Liquids on Surfaces – a Plethora of Applications
22.1 Introduction
22.2 The Influence of ILs on Solid-State Surfaces
22.3 Layers of ILs on Solid-State Surfaces
22.4 Selected Applications
22.5 Sensors
22.6 Electrochemical Double Layer Capacitors (Supercapacitors)
22.7 Dye Sensitized Solar Cells
22.8 Lubricants
22.9 Synthesis and Dispersions of Nanoparticles
References
Part V Outlook
Chapter 23: Outlook – the Technical Prospect of Supported Ionic Liquid Materials
23.1 Competitive Advantage
23.2 Observability
23.3 Trialability
23.4 Compatibility
23.5 Complexity
23.6 Perceived Risk
References
Index
Related Titles
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Series Editor: Anastas, P. Volume Editors: Leitner, W., Jessop, P. G., Li, C.-J., Wasserscheid, P., Stark, A.
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The Editors
Prof. Dr. Rasmus Fehrmann
Technical University of Denmark
Department of Chemistry
Building 207
2800 Kgs. Lyngby
Denmark
Dr. Anders Riisager
Technical University of Denmark
Department of Chemistry
Building 207
2800 Kgs. Lyngby
Denmark
Dr. Marco Haumann
FAU Erlangen-Nürnberg
LS für Chem. Reaktionstechnik
Egerlandstr. 3
91058 Erlangen
Germany
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Preface
In recent years, the concept of supported ionic liquids has been utilized as an innovative and widely applicable technology to design new catalysts, absorbents, and other functional materials. The technology offers enormous potential to obtain materials with unique surface properties such as great uniformity, high specificity, and tunable chemical activity. These materials can show significantly enhanced efficiencies when applied in processes and products, leading to substantial cost savings and greatly improved performance. In 2012 a gas purification process based on 60 tons of supported ionic liquid phase (SILP) absorber material has been reported by a petrochemical company, constituting the first large-scale application of this technology in industry.
We anticipate that the concept of ionic liquids on surfaces has great potential to establish a new and promising field of material science in the future. For improved material development the profound knowledge of ionic liquid and solid interactions and the development of sophisticated synthetic methodologies for new and large-scale production become significant. Reliable characterization methods as well as a priori tools for fast and efficient selection of the most suitable ionic liquids are also a key factor in this development. This book addresses these topics in the first two parts while catalysis with supported ionic liquid material is the focus of part three. Special applications will be described in part four, including sensor technology, lubrication, gas purification, and pharmaceuticals.
This book has been written by different authors, being at the forefront of the particular field, and the reader will find differences in style and notation. We are convinced that this variety does not harm the scientific impact and that the reader will be able to get a coherent broad knowledge to this new and exciting research field.
Copenhagen and Erlangen
October 2013
Rasmus Fehrmann, Anders Riisager,
and Marco Haumann
List of Contributors
Chapter 1
Introduction
Rasmus Fehrmann, Marco Haumann, and Anders Riisager
1.1 A Century of Supported Liquids
Natural and synthesized solid materials are generally characterized by a nonuniform and undefined surface. The surface contains face atoms, corner atoms, edge atoms, ad-atoms, and defect sites, which together determine the surface properties of the material [1]. In many applications, these different sites display different properties, for example, with respect to their chemical activity. Often, only certain sites are advantageous with regard to the specific application of the material as in the case of, heterogeneous catalysts and adsorbents. Future development of more efficient catalysts and adsorbents in industrial processes will depend on the design of solid surfaces that allow all surface atoms to be most effective. At the same time, new technologies are required, which will lead to the design of completely new surface properties within solids [2].
One possible way to achieve a uniform surface is by coating the solid support material with a thin liquid film, thereby defining the material properties by the liquid's properties. Such supported liquid phase (SLP) materials date back a 100 years ago till 1914, when BASF introduced a silica-supported V2O5-alkali/pyrosulfate SO2 oxidation catalyst for sulfuric acid production (see Figure 1.1) [3]. This catalyst, which is still the standard system for sulfuric acid production today, can be described as a supported molten salt, as it consists of a mixture of vanadium alkali sulfate/hydrogensulfate/pyrosulfate complexes that are present under reaction conditions (400–600 °C) [4].
Figure 1.1 Historical development of supported liquids in catalysis.
The concept of supported liquid catalysis is not restricted to liquid salts. In order to apply the concept of uniform surface properties and efficient catalyst immobilization, several authors investigated the SLP concept during the 1970s and 1980s [5–11]. However, later studies revealed that the evaporation of the loaded liquid cannot be avoided completely during operation. This is especially a problem when using water as the liquid phase [12–17]. In these supported aqueous phase (SAP) systems, the thin film of water evaporated quickly under reaction conditions, making the concept applicable only for slurry-phase reactions with hydrophobic reaction mixtures.
1.2 Supported Ionic Liquids
The supported ionic liquid phase (SILP) technology is a fundamental, new approach to obtain liquid containing solid materials that do not evaporate, made through surface modification of a porous solid by dispersing a thin film of ionic liquid (IL) onto it, as depicted in Figure 1.2 [18, 19]. ILs are salts consisting completely of organic cations and inorganic or organic anions (for further details see Chapter 2) [20]. Their better charge distribution and larger ion size compared to classical inorganic salts result in melting points below 100 °C. Owing to the extremely low vapor pressure of ILs, the surface of SILP materials is coated permanently, even under elevated reaction conditions. By variation of anions and cations, solubility, reactivity, and coordination properties of the ILs can be changed according to the special requirements of the given application.
Figure 1.2 Schematic representation of an ionic liquid film supported on a porous material.
With respect to material and surface design, ILs are characterized by a highly pre-organized, homogeneous liquid structure with distinctive physicochemical characteristics and these – often unique – characteristics are exclusively governed by the combination of ions in the material [20]. Hence, by an appropriate choice of the ions (and eventually additives) contained in the IL material, it is possible to transfer specific properties of the fluid to the surface of a solid material by confining the fluid to the surface. Thus, the SILP concept allows custom-making of solid materials, resulting in uniform and well-defined surface topologies with definite properties and a controlled chemical reactivity. Importantly, the SILP concept thereby constitutes an attractive methodology to circumvent the lack of uniformity of solids in traditional material science. In addition, the approach provides a great potential to create materials with new surface properties, as the transfer of specific IL properties to solid surfaces may result in “designer surfaces” with properties that are impossible to realize with any present synthetic approach.
In principle, all ILs can be contacted with a solid surface and therefore, looking at the tremendous numbers of publications in the field of “ILs,” exceeding 6700 in the year 2012, it is anticipated that the concept of “supported ILs” will benefit from this scientific input.1
A common method to immobilize ILs on surfaces is the covalent anchoring of a monolayer of IL onto a support – usually pretreated – as shown in Figure 1.3a. Here, the IL becomes part of the support material, thereby losing certain bulk phase properties such as solvation strength, conductivity, and viscosity. The IL can contain a certain functionality (e.g., acidity, hydrophobicity) that will render the support surface.
Figure 1.3 Categorization of materials based on supported ionic liquid films according to the phase behavior of the supported ionic liquid: (a) covalently attached monolayer and (b) multilayers of ionic liquid.
If multilayers of IL are immobilized onto a support, the bulk properties of the IL can be retained. In such SILP systems, depicted schematically in Figure 1.3b, functionalities can be incorporated by dissolving, for example, metal salts, acids, transition metal complexes, and nanoparticles.
Various efficient and recyclable systems based on the latter category have been developed, including supported ionic liquid catalysis (SILC), supported ionic liquid catalysts (SILCA), solid catalyst with ionic liquid (SCIL), solid catalysts with ionic liquid layer (SCILL), supported ionic liquid nanoparticles (SILnPs), supported ionic liquid phase (SILP), supported ionic liquid phase catalyst (SILPC), ionic liquid crystalline-SILP (ILC-SILP), structured SILP (SSILP), supported ionic liquid-like phase (SILLP), polymer-supported ionic liquid (PSIL), and supported ionic liquid membrane (SILM). All of these concepts try to use the intrinsic properties of IL bulk phases and can be regarded as derivatives of the general SILP concept, which itself is a branch of the “SLP-tree.”
The synthesis of SILP materials is usually straightforward and the thin film of IL is fixed on the surface mainly by physisorption, and in a few cases by chemisorption [21]. The IL is mixed with the support and the catalyst complex (if applied) in a low-boiling solvent. The solvent is then removed by evaporation or freeze-drying, yielding a dry, free-flowing powder as the SILP catalyst. Depending on the amount of IL and the pore structure of the support material, film thicknesses between 3 and 30 nm can be accomplished. Detailed descriptions of support materials and synthetic methodologies are given in Chapters 3 and 4 while the structure and stability of these materials are discussed in Chapters 5 and 6. Solid-state NMR studies of different amounts of IL on silica support indicated that below a critical value of 10 vol% IL loading, small islands of ILs exist on the support [22]. At values higher than 10 vol%, complete surface coverage with IL was observed, which resembled the characteristics of the bulk IL. This is an important prerequisite for the efficient immobilization of homogeneous catalyst complexes that would lose activity and, more importantly, selectivity upon interaction with the support surface or in a constrained environment. Spectroscopic studies of SILP materials are summarized in Chapters 7 and 8 while Chapter 9 introduces tools for a-priori selection of suitable ionic liquids.
Form an engineering point of view these SILP materials offer some advantages compared to classical gas–liquid or liquid–liquid systems, especially
a high surface area supplied by the support structure
a thin film of liquid that circumvents mass transport problems
adjustable solvent properties, for example, solubility
thermal stability of most ILs up to 200 °C
application of fixed-bed or fluidized-bed reactor technology
efficient catalyst immobilization in defined environment.
1.3 Applications in Catalysis
In SILP catalysis, work is focused mainly on the immobilization of homogeneous transition metal complexes within the thin IL film. Homogeneous catalysts, in contrast to their heterogeneous counterparts, have a uniform molecular structure and can easily be modified by the use of dedicated ligands in terms of reactivity, selectivity, and stability [23]. The main drawback of homogeneous catalysis is the elaborate recycling of the dissolved catalyst from the reaction mixture, usually accomplished by distillation or extraction. This issue, which currently limits more applications of homogeneous catalysts in continuous processes, can be circumvented by the SILP technology.
The use of SILP systems in catalysis has been reviewed recently, including both liquid and gas-phase applications [21, 24]. With respect to the application of these solid materials in liquid phase slurry reactions, the leaching of IL from the support is the most crucial issue. The smallest cross-solubility of the IL in the liquid substrate or product phase will cause rapid removal of the thin film accompanied by leaching of the catalyst complex, resulting in lower catalyst activity.
This problem can be circumvented in a very elegant manner if the reaction is performed in SILP gas-phase contact. Since the IL does not have any technically relevant vapor pressure, it is not removed via gas-phase leaching, and catalyst stabilities have been found to be very high [25]. Moreover, the gas-phase has no solution power for the catalyst, which means that catalyst immobilization in SILP gas-phase systems does not require any dedicated ligand modification.
As this approach builds on the volatility of the reaction products it is clearly limited to feedstock and products with considerable vapor pressure. Note that every molecule that can be analyzed by gas chromatography is in principal accessible for SILP gas-phase reactions. The removal of high-boiling reactants from the SILP catalyst requires, however, a high amount of gas stripping, which is economically less attractive at least for the production of bulk chemicals. A suitable alternative for performing continuous reactions with high-boiling substrates is the combination of SILP catalysis with a supercritical fluid as the mobile extraction phase, in particular scCO2 [26, 27]. A summary of catalytic gas and liquid phase applications is given in Chapters 10 to 18.
1.4 Applications in Separation
For gas purification, chloro-tin- and chloro-zinc-containing ILs have been immobilized on alumina supports to reversibly absorb sulfur compounds from gas streams [28]. These ILs have been tested initially for liquid–liquid biphasic extraction of sulfur from diesel or gasoline. Owing to the intrinsic high viscosity of all ILs, mass transport within these liquids is usually slow, making large-scale applications not feasible as large extractor and regenerator volumes would be required [29].
By applying a thin film of the IL onto a silica or alumina support, mass transport could be enhanced by orders of magnitude because of the large interfacial exchange area on the one hand and the small diffusion time in the thin film of IL on the other. The sulfur content of the gas-condensate feed could be reduced below 10 ppm and the then-loaded SILP catalyst regenerated in vacuum. This loading–unloading procedure could be repeated several times without significant loss of performance, resulting in overall time-on-stream of 600 h [28]. Combinations of ILs can extend this flexibility spectrum even further, making SILP absorbers a promising alternative for gas-mask filters, off-gas purification (e.g. NO, SO2), and CO2 capture technology [30–33]. In refinery technology, the first commercial SILP process for mercury removal from hydrocarbon feed has been reported recently [34], while the important separation of ene/ane mixtures, for example, propene/propane, might be facilitated by the use of SILP materials or SILP-based membranes [35, 36]. Applications and future trends are highlighted in Chapter 22.
1.5 Coating of Heterogeneous Catalysts
In a strong analogy to the SILP technology, a concept called solid catalysts with ionic liquid layers has been discussed in the literature [37]. In this case, a solid heterogeneous catalyst is coated with a thin film of IL. In contrast to SILP catalysts, the support material itself is catalytically active and no homogeneous catalyst or dissolved nanoparticle is involved. It has been experimentally demonstrated that such systems may exhibit better selectivity and even higher activities than their uncoated analogs [38]. However, the origins of such selectivity and activity effects are yet unclear. The IL may influence the catalytic performance in a twofold manner. On the one hand, it can directly interact with the active centers comparable to the behavior of a ligand. These so-called cocatalytic effects have been extensively reported for catalytically active metal nanoparticles. Such interactions may even lead to decomposition of the IL under reaction conditions, with the co-adsorbed decomposition products further modifying the catalytic properties. On the other hand, the IL can modify the effective concentrations of the substrates and intermediates at the active sites, so that the solubility of liquids or gaseous reactants in the IL differs in an appropriate manner from that in the liquid organic phase, causing a “physical solvent effect.” In addition, the IL can compete with the substrates for active sites on the catalyst surface, thereby blocking sites that lead to unwanted by-product formation [39]. The SCILL technology has been successfully applied in various hydrogenation reactions, resulting in better selectivity and enhanced activities. Examples of SCILL catalysis involving metal nanoparticles can be found in Chapter 14.
1.6 Monolayers of IL on Surfaces
The amount of IL can be reduced further compared to SILP and SCILL systems, in the extreme case, to only a monolayer or islands of IL coating the support [40]. The role of IL in these systems is to transfer a certain functionality of the IL to the support surface.
Such thin films of IL can obviously have no significant influence on substrate solubility. The IL is usually anchored onto the support via chemisorption, involving a surface reaction between the IL's cation and the surface. Other procedures have been reported in the literature and are highlighted in Chapter 4.
1.7 Conclusion
The field of ILs on surfaces is highly multidisciplinary, attracting experts from material sciences, synthetic chemistry, physical chemistry, chemical engineering as well as pharmaceutical sciences, electrochemistry, and bioengineering.
In summary, surface coating of solid materials with IL thin films constitutes a versatile and broadly applicable technology. However, the main markets for supported IL materials are expected in the fields of catalysis and separation as depicted in Figure 1.4.
Figure 1.4 Fields of application for ionic liquids on surfaces.
Considering these benefits, it is estimated that SILP materials will contribute a substantial part of the catalyst and adsorbent markets within the next 10 years. A market share of 5% for SILP catalysts, having significant advantages compared to classical heterogeneous or homogeneous systems, seems realistic. A similar share can be expected for adsorbents.
References
1. Schüth, F., Sing, K.S.W., and Weitkamp, J. (eds) (2002) Handbook of Porous Solids, Wiley-VCH Verlag GmbH, Weinheim.
2. Ozkan, U.S. (ed.) (2009) Design of Heterogeneous Catalysts, Wiley-VCH Verlag GmbH, Weinheim.
3. Blum, E. (1914) Swiss Patent CH71326, assigned to BASF. Kontaktsubstanz zur Erzeugung von Schwefelsäureanhydrid.
4. (a) Lapina, O.B., Balzhinimaev, B.S., Boghosian, S., Eriksen, K.M., and Fehrmann, R. (1999) Catal. Today, 51, 469–479.(b) Eriksen, K.M. and Fehrmann, R. (1999) Catal. Today, 51, 469–479;(c) Frazer, J.H. and Kirkpatrick, W.J. (1940) J. Am. Chem. Soc., 62, 1659–1660;(d) Topsøe, H.F.A. and Nielsen, A. (1948) Trans. Danish Acad. Tech. Sci., 1, 3–17.
5. Acres, G.J.K., Bond, G.C., Cooper, B.J., and Dawson, J.A. (1966) J. Catal., 6, 139–141.
6. (a) Rony, P.R. (1969) J. Catal., 14, 142–147;(b) Rony, P.R. (1968) Chem. Eng. Sci., 1021–1034.
7. (a) J. Hjortkjaer, M.S. Scurell, P. Simonsen, J. Mol. Catal. 1979, 6, 405–420;(b) Hjortkjaer, J., Scurell, M.S., and Simonsen, P. (1981) J. Mol. Catal., 12, 179–195.
8. Strohmeier, W., Marcec, R., and Graser, B. (1981) J. Organomet. Chem., 221, 361–366.
9. Gerritsen, L.A., van Meerkerk, A., Vreugdenhill, M.H., and Scholten, J.J.F. (1980) J. Mol. Catal., 9, 139–155.
10. (a) Gerritsen, L.A., Herman, J.M., Klut, W., and Scholten, J.J.F. (1980) J. Mol. Catal., 9, 157–168;(b) Gerritsen, L.A., Herman, J.M., and Scholten, J.J.F. (1980) J. Mol. Catal., 9, 241–256;(c) Gerritsen, L.A., Klut, W., Vreugdenhill, M.H., and Scholten, J.J.F. (1980) J. Mol. Catal., 9, 257–264;(d) Gerritsen, L.A., Klut, W., Vreugdenhill, M.H., and Scholten, J.J.F. (1980) J. Mol. Catal., 9, 265–274;(e) de Munck, N.A., Verbruggen, M.W., and Scholten, J.J.F. (1981) J. Mol. Catal., 10, 313–330;(f) de Munck, N.A., Verbruggen, M.W., de Leur, J.E., and Scholten, J.J.F. (1981) J. Mol. Catal., 11, 331–342;(g) Pleit, H.L., van der Lee, G., and Scholten, J.J.F. (1985) J. Mol. Catal., 29, 319–334;(h) Pleit, H.L., Brockhus, J.J.J.J., Verburg, R.P.J., and Scholten, J.J.F. (1985) J. Mol. Catal., 31, 107–118;(i) Pelt, H.L., Gijsman, P.J., Verburg, R.P.J., and Scholten, J.J.F. (1985) J. Mol. Catal., 33, 119–128;(j) Scholten, J.J.F. and van Hardeveld, R. (1987) Chem. Eng. Commun., 52, 75–92;(k) Herman, J.M., Rocourt, A.P.A.F., van den Berg, P.J., van Krugten, P.J., and Scholten, J.J.F. (1987) Chem. Eng. J., 35, 83–103.
11. Brüsewitz, R. and Hesse, D. (1992) Chem. Eng. Technol., 15, 385–389.
12. (a) M. E. Davis, in Aqueous Phase Organometallic Chemistry, Chapter 4.7, B. Cornils, W. A. Herrmann (Eds.), Wiley-VCH Verlag GmbH, Weinheim, 1998, pp. 241–251;(b) M. E. Davis, ChemTech 1992, 22, 498–502.
13. Anson, M.S., Leese, M.P., Tonks, L., and Williams, J.M.J. (1998) J. Chem. Soc., Dalton Trans., 3529–3538.
14. (a) Arhancet, J.P., Davis, M.E., Merola, J.S., and Hanson, B.E. (1989) Nature, 339, 454–455;(b) Arhancet, J.P., Davis, M.E., Merola, J.S., and Hanson, B.E. (1990) J. Catal., 121, 327–339;(c) Arhancet, J.P., Davis, M.E., and Hanson, B.E. (1991) J. Catal., 129, 94–99;(d) Arhancet, J.P., Davis, M.E., and Hanson, B.E. (1991) J. Catal., 129, 100–105.
15. Zhu, H., Ding, Y., Yin, H., Yan, L., Xiong, J., Lu, Y., Luo, H., and Lin, L. (2003) Appl. Catal. A: Gen., 245, 11–117.
16. Riisager, A., Eriksen, K.M., Hjortkjaer, J., and Fehrmann, R. (2003) J. Mol. Catal. A: Chem., 193, 259–272.
17. (a) Tóth, I., Guo, L., and Hanson, B.E. (1997) J. Mol. Catal., 116, 217–229;(b) T. Bartik, B. Bartik, I. Guo, B. E. Hanson, J. Organomet. Chem. 1994, 480, 15–21.
18. Mehnert, C.P., Cook, R.A., Dispenziere, N.C., and Afeworki, M. (2002) J. Am. Chem. Soc., 124, 12932.
19. Riisager, A., Fehrmann, R., Flicker, S., van Hal, R., Haumann, M., and Wasserscheid, P. (2005) Angew. Chem. Int. Ed., 44, 185.
20. Wasserscheid, P. and Welton, T. (eds) (2007) Ionic Liquids in Synthesis, 2nd edn, Wiley-VCH Verlag GmbH, Weinheim.
21. van Doorslaer, C., Wahlen, J., Mertens, P., Binnemans, K., and de Vos, D. (2010) Dalton Trans., 39, 8377–8390.
22. Haumann, M., Schönweiz, A., Breitzke, H., Buntkowsky, G., Werner, S., and Szesni, N. (2012) Chem. Eng. Technol., 35, 1421.
23. van Leeuwen, P.W.N.M. (ed.) (2005) Homogeneous Catalysis, Springer, Dordrecht.
24. Gu, Y. and Li, G. (2009) Adv. Synth. Catal., 351, 817.
25. (a) Jakuttis, M., Schönweiz, A., Werner, S., Franke, R., Wiese, K.D., Haumann, M., and Wasserscheid, P. (2011) Angew. Chem. Int. Ed., 50, 4492;(b) Haumann, M., Jakuttis, M., Franke, R., Schönweiz, A., and Wasserscheid, P. (2011) ChemCatChem, 3, 1822–1827.
26. Hintermair, U., Zhao, G., Santini, C.C., Muldoon, M.J., and Cole-Hamilton, D.J. (2007) Chem. Commun., 1462–1464.
27. Hintermair, U., Höfener, T., Pullmann, T., Franciò, G., and Leitner, W. (2010) ChemCatChem, 2, 150–154.
28. Kohler, F., Roth, D., Kuhlmann, E., Wasserscheid, P., and Haumann, M. (2010) Green Chem., 12, 979.
29. Kuhlmann, E., Haumann, M., Jess, A., Seeberger, A., and Wasserscheid, P. (2009) Chem. Sus. Chem., 2, 969.
30. Zhang, X., Liu, Z., and Wang, W. (2008) AIChE J., 54, 2717–2728.
31. Jessop, P.G., Heldebrant, D.J., Li, X., Eckert, C.A., and Liotta, C.L. (2005) Nature, 435, 1102.
32. Huang, J. and Rüther, T. (2009) Aust. J. Chem., 62, 298–308.
33. Zhang, Z., Wu, L., Dong, J., Li, B.-G., and Zhu, S. (2009) Ind. Eng. Chem. Res., 48, 2142.
34. Abai, M., Atkins, M.P., Cheun, K.Y., Holbrey, J., Nockemann, P., Seddon, K., Srinivasan, G., and Zou, Y. Process for removing metals from hydrocarbons, WO 2012/046057.
35. Mokrushin, V., Assenbaum, D., Paape, N., Gerhard, D., Mokrushina, L., Wasserscheid, P., Arlt, W., Kistenmacher, H., Neuendorf, S., and Göke, V. (2010) Chem. Eng. Technol., 33, 63–73.
36. Lozano, L.J., Godínez, C., de los Ríos, A.P., Hernández-Fernández, F.J., Sánchez-Segado, S., and Alguacil, F.J. (2011) J. Membr. Sci., 376, 1–14, and references therein.
37. Kernchen, U., Etzold, B., Korth, W., and Jess, A. (2007) Chem. Eng. Technol., 30, 985–994.
38. (a) Arras, J., Steffan, M., Shayeghi, Y., Ruppert, D., and Claus, P. (2009) Green Chem., 11, 716–723;(b) Arras, J., Paki, E., Roth, C., Radnik, J., Lucas, M., and Claus, P. (2010) J. Phys. Chem. C, 114, 10520–10526;(c) Arras J., Steffan M., Shayeghi Y., and Claus P. 2008, Chem. Commun., 4058–4060;(d) Arras J., Ruppert D., and Claus P. 2009, Appl. Catal. A: Gen.371, 73–77.
39. Sobota, M., Schmid, M., Happel, M., Amende, M., Maier, F., Steinrück, H.-P., Paape, N., Wasserscheid, P., Laurin, M., Gottfried, J.M., and Libuda, J. (2010) Phys. Chem. Chem. Phys., 12, 10610–10621.
40. Cremer, T., Killian, M., Gottfried, J.M., Paape, N., Wasserscheid, P., Maier, F., and Steinrück, H.-P. (2008) ChemPhysChem, 9, 2185–2190.
1 Literature search using SciFinder including the term “ionic liquid”, March 2013.
Part I
Concept and Building Blocks
Chapter 2
Introducing Ionic Liquids
Tom Welton
2.1 Introduction
The last two decades have seen an explosion of interest in ionic liquids [1]. Their use as solvents has been the subject of widespread academic study [2] and they have been applied in a number of commercial processes [3]. Much of the interest in ionic liquids has centered on their possible use as “green solvents” [4]. However, this has been the subject of much controversy [5], and the concept of a “green solvent” itself is now somewhat dated. There have been many reviews of ionic liquids. Some of these have focused on particular applications, for example, analysis [6], biocatalysis [7], catalysis [8], electrochemical devices [9], or engineering fluids [10]. Others have concentrated on particular subgroups of ionic liquids, for example, task-specific ionic liquids [11]. This chapter summarizes what is known about the physicochemical properties that are of particular interest for supported ionic liquid phases (SILPs).
2.2 Preparation
Any salt that is sufficiently thermally stable will form an ionic liquid when it melts. However, it is with the introduction of low-melting, air- and moisture-stable ionic liquids that the explosion of interest in these began [12]. These ionic liquids mostly have cations that are alkylated amines, with a smaller number of phosphonium salts used (Figure 2.1) with a variety of polyatomic anions (Figure 2.2). The cations can be relatively simply prepared as their halide salts by alkylation of one of the wide range of commercially available amines (Scheme 2.1) or phosphines. Throughout this step, air and moisture should be rigorously excluded and the temperature must be well controlled to prevent runaway reactions [13, 14]. The desired ionic liquid can then be prepared by metathesis of the halide salt with a metal or ammonium salt or the conjugate acid of the required anion (Scheme 2.2). For hydrophobic ionic liquids, this can be done in aqueous solution [12]; for hydrophilic ionic liquids, the metathesis is usually performed in a water-immiscible organic solvent [15]. The resulting ionic liquid is then separated from the by-product salt and organic solvent and if necessary decolorized [16]. Of course, the wide variety of ionic liquids means that the preparations of different salts are not all the same in detail and there are many ionic liquids that are prepared using different techniques [2].
Figure 2.1 Some commonly used cations for ionic liquids, with the used notation.
Figure 2.2 Some commonly used anions for ionic liquids, with the used notation.
Scheme 2.1 Synthesis of [CnC1im]X (X = Cl or Br).
Scheme 2.2 Formation of ionic liquid from a halide (X) precursor.
2.3 Liquid Range
Perhaps the most important property of an ionic liquid is whether it is a liquid or not in the temperature range at which you are operating. While it is very well known as to which molecular properties influence the melting points of ionic liquids, their accurate prediction is still not possible. One reason for this is that many ionic liquids are glass-forming materials that do not display a well-defined melting point. Also, the melting point of any substance is a consequence of the structure of both the solid and the liquid phases. Ionic liquids are highly and differently ordered liquids (see below). Consequently, most discussions of ionic liquid melting points are only semiquantitative.
Reasonable correlations have been found between molecular properties and the melting points of some ionic liquids, using methods such as quantitative structure–property relationships (QSPR) [17, 18]. These have identified contributing factors (see below) and given general trends, but have not been able to provide precise predictions of the melting points of individual ionic liquids. Group contribution methods have also been used to predict ionic liquid melting points, with a good fit similar to the general trends, but lack of precision for any individual ionic liquid [19].
The general principles of which factors contribute to determining the melting points of ionic liquids have been known for many years [20–22]. The dominance of coulombic forces in determining the melting points of ionic liquids has led to the vast majority that are in general use being simple 1 : 1 salts of singly charged cations and anions. Larger ions have weaker coulombic attraction for each other and so lower melting points. There is a contrasting trend that larger ions have greater van der Waals attractions and so for any homologous series of ionic liquids there is usually some alkyl chain length at which the coulombic interactions are low, but the van der Waals interactions have not yet become significant, that gives a minimum melting point for the series. Delocalization of the charge on the ions over several atoms also reduces the coulombic attraction between the ions. Breaking the symmetry of the ions prevents close contact of the ions and so also reduces coulombic attraction.
Notwithstanding recent results regarding the formation of ionic liquid vapors [23], ionic liquids do not boil under normal atmospheric conditions. Hence, the upper operating limit of an ionic liquid is given by its thermal decomposition. These arise as a consequence of both the kinetics and thermodynamics of the decomposition reactions, and so are sensitive to the measurement conditions, particularly the rate of temperature increase in the experiment. It is now generally accepted that reported decomposition temperatures are usually higher than temperatures at which no decomposition occurs if a sufficient time is given [24].
For ionic liquids with protic cations decomposition occurs most easily by proton transfer from the cation to the anion to produce the acid and base from which the ionic liquid was prepared. The temperatures at which this occurs have been related to the difference in the pKa values of these parent acids and bases [25–27]. For fully alkylated ionic liquids, two major decomposition routes have been identified (Scheme 2.3). The first is dealkylation by nucleophilic attack of the ionic liquid anion on the cation, and can be correlated with the nucleophilicity of the anion [28]. Quantum chemical calculations have been used to calculate activation energies for the SN2 dealkylation of the cation by the anion and rates for [C4C1im]X (X = Cl, [N(CN)2], [BF4], [PF6], or [NTf2]) and correlated with experimental decomposition temperatures [29]. This nucleophilic substitution competes with Hoffman elimination, in which the anion acts as a Brønsted base and abstracts a proton from the β-carbon of one of the alkyl chains. However, this reaction is often suppressed in imidazolium ionic liquids, because a third decomposition mechanism via deprotonation at the C2 carbon of the imidazolium ring is preferred. When the ionic liquid anion is very non-nucleophilic and nonbasic decomposition of the anion itself may occur first [30].
Scheme 2.3 Possible decomposition routes for [CnC1im]X.
2.4 Structures
The strength and long-range of Coulomb forces between ions lead to simple salts having infinite ionic lattices, which are among the most highly ordered of all chemical structures. This is, of course, why most of these have very high melting points. When a simple halide salt such as NaCl melts, the phase transition is accompanied by a sharp increase in conductivity; at 800 °C (solid) κ = 1 × 10−3 Ω−1 cm−1, at 900 °C (liquid) κ = 3.9 Ω−1 cm−1 [31]. This increase arises because of the increased mobility of ions in the liquid salt. The molar volumes of the halides also increase upon melting, for example, NaCl, 23% and KBr, 22% [32]. Clearly, the structure of the salt is breaking up in some way and space is being introduced. Perhaps surprisingly, these changes are not accompanied by large differences in either the closest ion distances or the coordination numbers, which can even show greater short-range ordering than the crystal [33]. A recent comparison of the high-energy X-ray diffraction patterns of solid and liquid [C2C1im]Br has shown just this effect, with the Br− ions being shown to be closer to the cation ring atoms and more symmetrically distributed around the ring than in the crystal and even having a significant component of the cation–cation partial distribution function indicating closer contacts between ring centers [34].
It should be noted that these liquid “structures” are time-averaged views of the liquids and any local structures that do exist in the liquids will break apart in time to be replaced by others. So, similar to other liquids, simple ionic liquids are dynamic systems that have short-range order, but not long-range order. Hence, it is not surprising that low-melting ionic liquids are at this level the same [35], with both length and time scales being important considerations. One consequence of the dynamic nature of ionic liquid structures is that these change with the temperature at which the measurements are made, particularly the difference between the measurement temperature and the glass transition temperature of the ionic liquid [36]. This is typical of materials that exhibit glass-forming behavior.
Coulombic interactions between ions give rise to the largely periodic behavior (alternating cation–anion structures) that is seen in ionic liquids [34, 35, 37]. It has also been demonstrated crystallographically [38], by neutron diffraction [35, 37], NMR spectroscopy [39], vibrational spectroscopy [40], and mass spectrometry [41], and theoretically [42] that hydrogen bonding is an important structure-forming factor in many pure ionic liquids [43] and that the degree of these short-range cation–anion interactions is dependent upon which ions the ionic liquid is composed of. The final structure-generating interactions that have been identified are those between the alkyl chains of the cations of the ionic liquids [44]. These can lead to the formation of liquid crystalline phases [45], and they have been proposed to lead to the formation of hierarchical structures, sometimes called nanostructuring, in ionic liquids in which ionic and nonpolar domains separate [46, 47], even in systems with alkyl chains as short as in ethylammonium nitrate [48]. However, in other systems with longer alkyl chains, such as [C4C1im][PF6], no such nanostructuring occurs [49]. How and why these differences arise is yet to be fully determined. While this alkyl chain effect has primarily been studied with respect to cations, similar effects can be seen when the anions contain alkyl chains [37a].
2.4.1 The Liquid/Solid Interface
The use of ionic liquids as a thin layer supported on a solid surface greatly increases the importance the interfaces between both the liquid and the support and the liquid and the gas. Hence, it is even more important than usual that the structures of these interfaces are properly understood. In order to understand how ionic liquids interact with solid surfaces it is necessary to have an understanding of how they organize and structure themselves at the interface. This area is not yet highly developed, but some interesting results are beginning to appear [50]. Most studies to date have concentrated on charged surfaces. This, of course leads to an attraction to oppositely charged ions and a repulsion of like charged ions, which in turn leads to structural ordering at the surface. Hence, the electrical double layer formed by ionic liquids has been shown to be very different to that formed in dilute solutions of electrolytes in molecular solvents. This was first demonstrated for [C2NH3][NO3] and its aqueous solutions by Horn et al. [51] by measuring the force between mica surfaces immersed in the solution being tested. At low concentrations, the solutions showed typical force/separation distance profiles for 1 : 1 electrolytes, but as the concentration of the ionic liquid increased to above 50% (v/v) clear evidence of layers, each of 0.5–0.6 nm thickness, of the ionic liquid forming at the mica surface is seen, which repeat to a lessening degree eight or nine times before the ionic liquid is indistinguishable from its bulk. The authors reasoned that because the mica surface is negatively charged, the first layer must be enriched with cations. They also concluded from the fact that the ionic liquid is autophobic (it initially spreads over the surface and then retracts again) that these cations must be orientated with the charged ammonium group closest to the surface and the alkyl chain extending into the bulk liquid. Atkin et al. have used atomic force microscopy to study the surface structuring of ionic liquids. Their results for [C2NH3][NO3], the closely related [C3NH3][NO3], and the structurally dissimilar [C2C1im][CH3CO2] on a mica surface all showed good agreement with those of Horn [51, 52]. The small differences between the results for the different ionic liquids were attributed to differences in the size of the ions composing the ionic liquids and the increased compressibility of the layers: [C2NH3][NO3] < [C2C1im][CH3CO2] < [C3NH3][NO3]. Their subsequent study of [C2C1im][NTf2], [C4C1pyrr][NTf2], and [C4C1im][PF6] again demonstrated multiple layering.
The same technique has more recently been applied to the study of [C2C1im][C2SO4] [53] and [CnC1im][NTf2] (n = 4 or 6) [54]. Again, the formation of layers was seen. In the latter of these studies, the interlayer separation for [C6C1im][NTf2] was twice the size that was expected. This was interpreted to mean that whereas for [C4C1im][NTf2] layering took the form of simple repeating CACA… (Figure 2.3a) as seen in the studies described above, for [C6C1im][NTf2] the structure was of the form AACCAACC… (Figure 2.3b,c), showing the changeover from a structure in which the layers are essentially the product of imperfect charge screening to one in which interactions between alkyl chains of the cation begin to be important [55].
Figure 2.3 (a–c) Ordering of ions of [CnC1im][NTf2] (n = 4 or 6) at a mica surface.
In terms of the SILP concept, silica is a more relevant substrate. Silica has a reduced surface charge and an increased surface roughness in comparison to mica. In their study of [C2NH3][NO3], [C3NH3][NO3], and [C2C1im][CH3CO2] Atkin and Warr [52] found that the peaks and troughs in their separation/force profiles were smoothed out in comparison to the mica results. Again, the distance apart of the layers was approximately 0.5 nm, except for the first layer, which was only half this thickness. They interpreted this to be due to the silica being covered by a first layer only composed of cations, or at least a layer highly enriched with cations, with the alkyl chain projecting away from the surface into the ionic liquid. This is in general agreement with the findings of a vibrational spectroscopy study of [CnC1im][NTf2] and [CnC1im][BETI] (n = 6, 8, or 10; BETI = bis(perfluoroethylsulfonyl)imide) [56]. A study of [C4C1im][NTf2] and [C4C1im][BF4] showed, once again, the pattern of layers of ionic liquid that decay until the bulk liquid is reached at about 10 nm for [C4C1im][NTf2] and 7 nm [C4C1im][BF4] [57].
An X-ray reflectivity study of [C4N][FAP] and [CnC1pyrr][FAP] (n = 4, 6 FAP = tris(pentafluoroethyl)trifluorophosphate) on a charged sapphire (α-Al2O3) substrate also showed repeating layers of alternating cations and anions, with the layers broadening and decaying as they got farther from the surface [58]. Importantly, this technique was able to differentiate between “checkerboard” structures with both cations and anions in the same layer and truly alternating layers of cations and anions and clearly demonstrated the presence of the latter.
2.4.2 The Liquid/Gas Interface
The structure of an ionic liquid's interface with a gas, such as any enrichment with one or the other of the component ions, the orientation of the surface species, the density of packing of ions at the surface, and so on will determine both thermodynamic properties such as surface tension and dynamic properties such as its permeability for gases. Again, knowledge and understanding of the structure of the ionic liquid/gas interface is necessary to be able to design successful supported ionic liquid processes [59].
Fortunately, the ability to handle ionic liquids under ultrahigh vacuum allows their surfaces to be studied by a range of techniques that would usually only be used with solid materials [60]. X-ray photoemission spectroscopy (XPS) was one of the first of these to be applied [61]. XPS is capable of giving elemental analysis, and by varying the data collection angle the depth to which the spectrometer probes can be altered. Different experiments, using different ionic liquids, have given different results for enrichment of the surface layer with one of the components of the ionic liquid. The presence of a long (n ≥ 4) alkyl chain on the cation leads to its enrichment in [CnC1im][NTf2] (n = 2–16), with this effect being greater with longer chain, but not for [C2C1im][NTf2] [62]. This is in good agreement with work on [CnC1im][BF4] (n = 4–8) [63], which did show surface enrichment with the alkyl chain and on [C2C1im][C2OSO3] [64], which did not. This alkyl chain effect is not restricted to the cation. For [C2C1im][C8OSO3], surface enrichment with the alkyl chain was also observed [62]. While the alkyl chain effect is dominant, it has been noted that for the same cation, [C8C1im]+, the enrichment of the surface layer decreases as the anion gets larger [65]. This effect is greatest when the anions are small, when the difference between their sizes is relatively large, and diminishes as the anions become larger.
Surface enrichment with the alkyl chain does not necessarily mean that the surface layer is enriched with the ion of which it is part (see Figure 2.4). For instance, for an [CnC1im]X salt it is possible that the anions are closely associated with the charged imidazolium ring and that the alkyl chain then protrudes into the vacuum. This would still give a surface enrichment of the alkyl chain, but with no surface enrichment of the cation itself. This is what one might expect from the bulk structures of ionic liquids [46, 47]. In fact, the available data supports the presence of such structures [66], even to the extent of showing examples that have a surface enrichment of anions and of the cation alkyl chain [67]. It is probably best to think of the surface layer containing cations and anions, but with any lengthy (n > 4) alkyl chains directed into the vacuum.
Figure 2.4 Ionic liquid/gas (or vacuum) interface with surface enhancement of alkyl chains and (a) surface enhancement of cations, (b) no surface enhancement of cations or anions, and (c) surface enhancement of anions.
There is also X-ray reflectivity evidence that the surface structure has several layers that maintain some degree of structural regularity before displaying the bulk structure. For instance, in [(C8)3C1N][BETI] [68] the surface layers repeat at least four times into the bulk of the ionic liquid. Similar layers have been proposed for [CnC1im][BF4] (n = 4 or 8), with the layers being made up of distinct regions of alkyl chains and ionic groups [69]. However, these data are not unambiguous, and often more than one structure could fit the observations [70].
2.5 Physical Properties
There are many liquid properties that are important to the performance of a reaction solvent – heat capacities, viscosities, and so on. These for ionic liquids have been very well reviewed elsewhere and are not detailed here [1, 71]. These properties are controlled by the selection of both the cation and the anion. This has led to the concept of ionic liquids being “designer solvents” [72]. However, achieving this requires not just a post hoc rationalization of ionic liquids' properties, but the ability to predict these as well.
Molecular volume data have been used to predict a number of physical properties of ionic liquids, such as densities [73–75] and viscosities [76]. Given the potential importance of molecular volume data for predicting physical properties of ionic liquids, it is useful that they have also been the subject of prediction using a variety of methods [21, 76].
When an ionic liquid is supported on a surface as a fine layer, such as in SILP, the surface properties, both at the liquid/solid support interface and the liquid/gas interface, will increase in their importance in comparison to when using the same ionic liquid as a bulk liquid.
2.5.1 The Liquid/Solid Interface
For SILP to be successful, the first requirement is that the ionic liquid covers (wet) the support material sufficiently well to produce a stable liquid film. The ability to wet a surface depends upon competing interactions between the liquid and the solid, the liquid surface with its bulk, and both the liquid and solid with the gas phase. The final configuration is the one leading to the minimum surface energy [77]. Thus, the ability of an ionic liquid to wet a particular surface depends upon the relative strengths of the interactions of its ions with that surface, its own bulk, and the gas phase. For example, it has been recently shown that for a silica surface this is dependent upon the nature of both the cation and the anion, with wetting angles decreasing (greater wetting) in the order 51°, 48°, and 35° found for [C4C1im][PF6], [C6C1im][PF6], and [C4C1im][NTf2] [78]. However, little detailed systematic work has been reported in this area to date. It is generally true that, all other variables being equal, liquids with low surface tensions will wet a surface better than those with high surface tensions.
2.5.2 The Liquid/Gas Interface
There has been more work on the liquid/gas interface and surface tensions, and these have been recently reviewed [79]. These measurements, although relatively simple to make, can be very sensitive to impurities in the ionic liquids, particularly those that have a tendency to concentrate at the liquid/gas interface [80]. For example, secondary ion mass spectrometry (SIMS) of [C2C1im][NTf2] showed the presence of poly(dimethylsiloxane) – commonly used to lubricate ground-glass joints [81]. This makes it difficult to compare across the work of different research groups, who have used different, often only partially described, synthesis and purification techniques. However, some general trends can be seen.
First, ionic liquid surface tensions are unremarkable and lie in the range of conventional molecular liquids [79]. The surface tension of a series of [CnC1im]X (n = 2, 4, or 6; X = [OTf]− or [BF4]−) ionic liquids has been shown to decrease as the alkyl chain length increases [82]. This would be expected from standard relations that show that surface tension is inversely proportional to molecular volume [79]. Similar results were found for the N-alkylpyridinium ionic liquids [Cnpyr][NTf2] (n = 2, 4, or 5) [83]. Rebelo et al. [84] found that for the ionic liquids [CnC1im][NTf2] (n = 1, 2, 4, 6, 8, 10, 12, or 14) this trend held, whereas Maier et al. [85] found that for many of the same ionic liquids [CnC1im][NTf2] (n = 1, 2, 4, 6, 8, 10, or 12) this trend held up to n = 8, after which the surface tensions leveled. Although these results appear to contradict each other, closer inspection of the Rebelo results shows a considerable reduction of the change in the surface tension on increasing the alkyl chain length as the chain gets longer, and the difference between the two interpretations is not as dramatic as might be supposed at first glance.
Comparisons of [C4C1im]X and [C4C1pyrr]X (X = [NTf2], [SCN], or [N(CN)2]) have shown that the surface tensions of the pyrrolidinium salt is higher than that of the imidazolium salt [85, 86]. This is perhaps a little counterintuitive, given that the greater hydrogen bonding possible between imidazolium cations and their counter anions would be expected to be greater than that between the pyrrolidinium cations and these. However, it must be remembered that the surface tension results from the surface structure as well as interactions in the bulk, and that for [C4C1pyrr][NTf2] a considerably lower surface enrichment with alkyl chains was observed than for [C4C1im][NTf2] [85].
This effect of reduced surface tension with increasing alkyl chain length also holds when the alkyl chain is on the anion as in [C4C1im][CnOSO3] (n = 1, 2, 3, 4, or 8) [87]. Otherwise, the effect of the anions on the surface tension of ionic liquid is not so clear-cut. Again, there is the possibility of competing effects from bulk interactions and surface structure. This leads to there being no simple rule that can be applied across all possible ionic liquid anions. Standard models would predict that with increasing molecular volume and decreasing cation–anion interaction the surface tension should decrease. This has been found to hold for some ionic liquids, for example, when comparing [C4C1im][PF6] to [C4C1im][NTf2] [88], [C4C1im][BF4] to [C4C1im][PF6] and [C4C1im][OTf] to [C4C1im][NTf2] [89], and [C4C1im][BF4] to [C4C1im][PF6] and [C8C1im][OTf] to [C8C1im][NTf2] and [C8C1im][NPf2] ([NPf2] = bis[(pentafluoroethyl)sulfonyl]imide) [85]. However, for others, for example, [C8C1im][BF4] to [C8C1im][PF6] [85, 89] and [C8C1im]X (X = Cl, Br, or I) [85], the reverse trend has been found. Clearly, the effect of changing the anion depends upon the alkyl chain length of the cation, which again implies the importance of surface structures in determining the differences between these ionic liquids.
