Supported ionic liquids fundamentals and applications

Rasmus Fehrmann, Marco Haumann, and Anders Riisager1.1 A Century of Supported Liquids 1 1.2 Supported Ionic Liquids 2 Part I Concept and Building Blocks 11 2.4.1 The Liquid/Solid Interfa

Edited by Rasmus Fehrmann, Anders Riisager, and Marco Haumann

Supported Ionic Liquids

Serp, P., Philippot, K (eds.)

Nanomaterials in Catalysis

2013

ISBN: 978-3-527-33124-6

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Edited by

Rasmus Fehrmann,

Anders Riisager, and

Marco Haumann

Supported Ionic Liquids

Fundamentals and Applications

Prof Dr Rasmus Fehrmann

Technical University of Denmark

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1.1 A Century of Supported Liquids 1

1.2 Supported Ionic Liquids 2

Part I Concept and Building Blocks 11

2.4.1 The Liquid/Solid Interface 17

2.4.2 The Liquid/Gas Interface 19

2.5 Physical Properties 20

2.5.1 The Liquid/Solid Interface 21

2.5.2 The Liquid/Gas Interface 21

2.5.3 Polarity 22

2.5.4 Chromatographic Measurements and the Abraham Model of

Polarity 24

2.5.5 Infinite Dilution Activity Coefficients 24

2.6 Effects of Ionic Liquids on Chemical Reactions 26

2.7 Ionic Liquids as Process Solvents in Industry 29

References 31

3 Porous Inorganic Materials as Potential Supports for Ionic Liquids 37

Wilhelm Schwieger, Thangaraj Selvam, Michael Klumpp, and

3.5.3 Hierarchical Porosity in Zeolite Crystals 55

3.6 Ordered Mesoporous Materials 56

3.6.1 Silica-Based Classical Compounds 58

3.6.3 Mesoporous Carbons 61

3.6.4 Other Mesoporous Oxides 61

3.6.5 Anodic Oxidized Materials 62

3.7 Structured Supports and Monolithic Materials 63

3.7.1 Monoliths with Hierarchical Porosity 64

3.7.2 Hierarchically Structured Reactors 65

3.8 Conclusions 66

References 66

4 Synthetic Methodologies for Supported Ionic Liquid Materials 75

Reinout Meijboom, Marco Haumann, Thomas E M¨uller, and

Normen Szesni

4.1 Introduction 75

4.2 Support Materials 76

4.3 Preparation Methods for Supported Ionic Liquids 77

4.3.1 Incipient Wetness Impregnation 77

4.3.2 Freeze-Drying 79

4.3.3 Spray Coating 80

4.3.4 Chemically Bound Ionic Liquids 82

4.3.5 IL–Silica Hybrid Materials 89

References 91

Contents VII

Part II Synthesis and Properties 95

5 Pore Volume and Surface Area of Supported Ionic Liquids Systems 97

Florian Heym, Christoph Kern, Johannes Thiessen, and Andreas Jess

5.1 Example I: [EMIM][NTf2] on Porous Silica 98

5.2 Example II: SCILL Catalyst (Commercial Ni catalyst) Coated with

6.2.1 Diffusivity of Gases and Liquids in ILs 106

6.2.2 Diffusion Coefficient of Evaporated ILs in Gases 108

6.3 Thermal Stability and Vapor Pressure of Pure ILs 109

6.3.1 Drawbacks and Opportunities Regarding Stability and Vapor Pressure

Measurements of ILs 109

6.3.2 Experimental Methods to Determine the Stability and Vapor Pressure

of ILs 110

6.3.3 Data Evaluation and Modeling Methodology 110

6.3.3.1 Evaluation of Vapor Pressure and Decomposition of ILs by Ambient

Pressure TG at Constant Heating Rate 110

6.3.3.2 Evaluation of Vapor Pressure of ILs by High Vacuum TG 114

6.3.4 Vapor Pressure Data and Kinetic Parameters of Decomposition of

Pure ILs 116

6.3.4.1 Kinetic Data of Thermal Decomposition of Pure ILs 116

6.3.4.2 Vapor Pressure of Pure ILs 116

6.3.5 Guidelines to Determine the Volatility and Stability of ILs 118

6.3.6 Criteria for the Maximum Operation Temperature of ILs 118

6.3.6.1 Maximum Operation Temperature of ILs with Regard to Thermal

Decomposition 118

6.3.6.2 Maximum Operation Temperature of ILs with Regard to

Evaporation 120

6.4 Vapor Pressure and Thermal Decomposition of Supported ILs 120

6.4.1 Thermal Decomposition of Supported ILs 121

6.4.2 Mass Loss of Supported ILs by Evaporation 123

6.4.2.1 Evaporation of ILs Coated on Silica (SILP-System) 123

6.4.2.2 Evaporation of ILs Coated on a Ni-Catalyst (SCILL-System) 132

6.4.2.3 Evaluation of Internal Surface Area by the Evaporation Rate of

Supported ILs 132

6.4.3 Criteria for the Maximum Operation Temperature of Supported

7 Ionic Liquids at the Gas–Liquid and Solid–Liquid Interface –

Characterization and Properties 145

Zlata Grenoble and Steven Baldelli

7.1 Introduction 145

7.2 Characterization of Ionic Liquid Surfaces by Spectroscopic

Techniques 146

7.2.1 Types of Interfacial Systems Involving Ionic Liquids 146

7.2.2 Overview of Surface Analytical Techniques for Characterization of

Ionic Liquids 146

7.2.3 Structural and Orientational Analysis of Ionic Liquids at the

Gas–Liquid Interface 147

7.2.3.1 Principles of Sum-Frequency Vibrational Spectroscopy 147

7.2.4 Cation-Specific Ionic Liquid Orientational Analysis 148

7.2.5 Anion-Specific Ionic Liquid Orientational Analysis 154

7.2.6 Ionic Liquid Interfacial Analysis by Other Surface-Specific

Techniques 157

7.2.7 Ionic Liquid Effects on Surface Tension 162

7.2.8 Ionic Liquid Effects on Surface Charge Density 163

7.3 Orientation and Properties of Ionic Liquids at the Solid–Liquid

Interface 165

7.3.1 Surface Orientational Analysis of Ionic Liquids on Dry Silica 165

7.3.2 Cation Orientational Analysis 166

7.3.3 Alkyl Chain Length Effects on Orientation 167

7.3.4 Competing Anions and Co-adsorption 168

7.3.5 Computational Simulations of Ionic Liquid on Silica 168

7.3.6 Ionic Liquids on Titania (TiO2) 170

Contents IX

8.1.2 Spectroscopy of the Catalyst 183

8.2 IR Spectroscopy 186

References 189

9 A Priori Selection of the Type of Ionic Liquid 191

Wolfgang Arlt and Alexander Buchele

9.1 Introduction and Objective 191

9.2.1 Experimental Determination of Gas Solubilities 192

9.2.1.1 Magnetic Suspension Balance 192

9.2.1.2 Isochoric Solubility Cell 194

9.2.1.3 Inverse Gas Chromatography 195

9.2.2 Prediction of Gas Solubilities with COSMO-RS 196

9.2.3 Reaction Equilibrium and Reaction Kinetics 197

9.3 Usage of COSMO-RS to Predict Solubilities in IL 198

9.4 Results of Reaction Modeling 201

9.5 Perspectives of the A Priori Selection of ILs 202

References 205

Part III Catalytic Applications 209

10 Supported Ionic Liquids as Part of a Building-Block System for Tailored

Catalysts 211

Thomas E M¨uller

10.1 Introduction 211

10.2 Immobilized Catalysts 212

10.3 Supported Ionic Liquids 214

10.4 The Building Blocks 215

10.4.1 Ionic Liquid 215

10.4.2 Support 216

10.4.3 Catalytic Function 218

10.4.3.1 Type A1 – Task Specific IL 219

10.4.3.2 Type A2 – Immobilized Homogeneous Catalysts and Metal

Nanoparticles 219

10.4.3.3 Type B – Heterogeneous Catalysts Coated with IL 221

10.4.3.4 Type C – Chemically Bound Monolayers of IL 221

10.4.4 Additives and Promoters 222

10.4.5 Preparation and Characterization of Catalysts Involving Supported

ILs 222

10.5 Catalysis in Supported Thin Films of IL 222

10.6 Supported Films of IL in Catalysis 223

10.6.1 Hydrogenation Reactions 224

10.6.2 Hydroamination 225

10.7 Advantages and Drawbacks of the Concept 228

10.8 Conclusions 229

Acknowledgments 229

References 229

11 Coupling Reactions with Supported Ionic Liquid Catalysts 233

Zhenshan Hou and Buxing Han

11.4.1.5 Suzuki Coupling Reactions 237

11.4.1.6 Heck Coupling Reactions 239

12 Selective Hydrogenation for Fine Chemical Synthesis 251

Pasi Virtanen, Eero Salminen, P¨aivi M¨aki-Arvela, and Jyri-Pekka Mikkola

12.1 Introduction 251

12.2 Selective Hydrogenation ofα,β-Unsaturated Aldehydes 251

12.3 Asymmetric Hydrogenations over Chiral Metal Complexes

Immobilized in SILCAs 257

12.4 Conclusions 261

References 261

13 Hydrogenation with Nanoparticles Using Supported Ionic Liquids 263

Jackson D Scholten and Jairton Dupont

13.1 Introduction 263

13.2 MNPs Dispersed in ILs: Green Catalysts for Multiphase

Reactions 264

13.3 MNPs Immobilized on Supported Ionic Liquids: Alternative Materials

for Catalytic Reactions 267

13.4 Conclusions 275

References 275

Contents XI

14 Solid Catalysts with Ionic Liquid Layer (SCILL) 279

Wolfgang Korth and Andreas Jess

14.3.1 Preparation of SCILL Catalysts 283

14.3.2 Nernst Partition Coefficients 284

14.3.3 Pore Volume and Surface Area of the SCILL Catalyst with

[BMIM][OcSO4] as IL 287

14.4 Kinetic Studies with SCILL Catalysts 287

14.4.1 Experimental 287

14.4.2 Hydrogenation of 1,5-Cyclooctadiene (COD) 288

14.4.2.1 Reaction Steps of 1,5-COD Hydrogenation on the Investigated Ni

14.4.4 Hydrogenation of Citral with SCILL Catalysts 298

14.5 Conclusions and Outlook 300

Andreas Sch¨onweiz and Robert Franke

15.1 SILP Materials in Liquid-Phase Hydroformylation

Reactions 307

15.2 Gas-Phase SILP Hydroformylation Catalysis 311

15.3 SILP Combined with scCO2– Extending the Substrate

Range 319

15.4 Continuous SILP Gas-Phase Methanol Carbonylation 322

15.5 Conclusion and Future Potential 323

References 324

16 Ultralow Temperature Water–Gas Shift Reaction Enabled by Supported

Ionic Liquid Phase Catalysts 327

Sebastian Werner and Marco Haumann

16.1 Introduction to Water–Gas Shift Reaction 327

17 Biocatalytic Processes Based on Supported Ionic Liquids 351

Eduardo Garc´ıa-Verdugo, Pedro Lozano, and Santiago V Luis

17.1 Introduction and General Concepts 351

17.1.1 Enzymes and Ionic Liquids 351

17.1.2 Supported ILs for Biocatalytic Processes 353

17.1.3 Reactor Configurations with Supported ILs for Biocatalytic

Part IV Special Applications 385

19 Pharmaceutically Active Supported Ionic Liquids 387

O Andreea Cojocaru, Amal Siriwardana, Gabriela Gurau, and

Robin D Rogers

19.1 Active Pharmaceutical Ingredients in Ionic Liquid Form 387

19.2 Solid-Supported Pharmaceuticals 389

19.3 Silica Materials for Drug Delivery 389

19.4 Factors That Influence the Loading and Release Rate of Drugs 391

19.4.1 Adsorptive Properties (Pore Size, Surface Area, Pore Volume) of

Mesoporous Materials 391

19.4.1.1 Pore Size 391

19.4.1.2 Surface Area 392

19.4.1.3 Pore Volume 392

19.4.2 Surface Functionalization of Mesoporous materials 392

19.4.3 Drug Loading Procedures 394

19.4.3.1 Covalent Attachment 394

19.4.3.2 Physical Trapping 394

19.4.3.3 Adsorption 395

19.5 SILPs Approach for Drug Delivery 395

19.5.1 ILs Confined on Silica 395

19.5.2 API-ILs Confined on Silica 396

19.5.2.1 Synthesis and Characterization of SILP Materials 396

19.5.2.2 Release Studies of the API-ILs from the SILP Materials 399

19.6 Conclusions 402

References 402

20 Supported Protic Ionic Liquids in Polymer Membranes for Electrolytes

of Nonhumidified Fuel Cells 407

Tomohiro Yasuda and Masayoshi Watanabe

20.1 Introduction 407

20.2 Protic ILs as Electrolytes for Fuel Cells 409

20.2.1 Protic ILs 409

20.2.2 Thermal Stability of Protic IL 410

20.2.3 PILs Preferable for Fuel Cell Applications 411

20.3 Membrane Fabrication Including PIL and Fuel Cell Operation 411

20.3.1 Membrane Preparation 411

20.3.2 Fuel Cell Operation Using Supported PILs in Membranes 414

20.4 Proton Conducting Mechanism during Fuel Cell Operation 415

22.2 The Influence of ILs on Solid-State Surfaces 445

22.3 Layers of ILs on Solid-State Surfaces 446

22.4 Selected Applications 446

22.5 Sensors 447

22.6 Electrochemical Double Layer Capacitors (Supercapacitors) 449

22.7 Dye Sensitized Solar Cells 451

Preface

In recent years, the concept of supported ionic liquids has been utilized as aninnovative and widely applicable technology to design new catalysts, absorbents,and other functional materials The technology offers enormous potential to obtainmaterials with unique surface properties such as great uniformity, high specificity,and tunable chemical activity These materials can show significantly enhancedefficiencies when applied in processes and products, leading to substantial costsavings and greatly improved performance In 2012 a gas purification processbased on 60 tons of supported ionic liquid phase (SILP) absorber material has beenreported 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 Forimproved material development the profound knowledge of ionic liquid and solidinteractions and the development of sophisticated synthetic methodologies for newand large-scale production become significant Reliable characterization methods

as well as a priori tools for fast and efficient selection of the most suitable ionicliquids 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 sensortechnology, lubrication, gas purification, and pharmaceuticals

This book has been written by different authors, being at the forefront of theparticular field, and the reader will find differences in style and notation We areconvinced that this variety does not harm the scientific impact and that the readerwill be able to get a coherent broad knowledge to this new and exciting researchfield

Copenhagen and Erlangen Rasmus Fehrmann, Anders Riisager,

List of Contributors

Wolfgang Arlt

Universit¨at Erlangen-N¨urnberg,

Department of Chemical &

Biochemical engineering (CBI)

Universit¨at Erlangen-N¨urnberg,

Department of Chemical &

Biochemical engineering (CBI)

AL 35487USA

David J Cole-Hamilton

University of St AndrewsEaStCHEM

School of Chemistry

St Andrews, FifeKY16 9ST, ScotlandUnited Kingdom

Jairton Dupont

UFRGSLaboratory of Molecular CatalysisInstitute of Chemistry

Av Bento Gonc¸alves, 9500Porto Alegre 91501-970 RSBrazil

Rub´en Duque

University of St AndrewsEaStCHEM

School of Chemistry

St Andrews, FifeKY16 9ST, ScotlandUnited Kingdom

The University of Alabama

Center for Green Manufacturing

Marco Haumann

FAU Erlangen-N¨urnberg

LS f¨ur Chem ReaktionstechnikEgerlandstr 3

91058 ErlangenGermany

Florian Heym

University BayreuthChair of Chemical EngineeringFaculty of Engineering ScienceUniversit¨atsstraße 30

D-95440 BayreuthGermany

List of Contributors XIX

Andreas Jess

University Bayreuth

Chair of Chemical Engineering

Faculty of Engineering Science

D-95440 Bayreuth

Germany

Christoph Kern

University Bayreuth

Chair of Chemical Engineering

Faculty of Engineering Science

Chair of Chemical Engineering

Faculty of Engineering Science

Biskopsgatan 8FI-20500, Turku/ ˚AboFinland

Reinout Meijboom

Faculty of ScienceDepartment of ChemistryUniversity of JohannesburgAuckland Park

2006 JohannesburgSouth Africa

Jyri-Pekka Mikkola

˚Abo Akademi UniversityProcess Chemistry CentreLaboratory of IndustrialChemistry and ReactionEngineering

Biskopsgatan 8FI-20500, Turku/ ˚AboFinland

and

Ume ˚a UniversityTechnical ChemistryDepartment of ChemistryChemical-Biological CenterOlof Palmes gata 29, SE-90323Sweden

Thomas E M¨ uller

CAT Catalytic CenterRWTH Aachen UniversityWorringerweg 1

52074 AachenGermany

The University of Alabama

Center for Green Manufacturing

˚Abo Akademi University

Process Chemistry Centre

Peter S Schulz

University Erlangen-NurembergDepartment of Chemical andBioengineering

Institute of Chemical ReactionEngineering

Egerlandstr 3

91058 ErlangenGermany

Wilhelm Schwieger

Lehrstuhl f¨ur ChemischeReaktionstechnikUniversit¨at Erlangen-N¨urnbergEgerlandstr 3

91058 ErlangenGermany

Thangaraj Selvam

Lehrstuhl f¨ur ChemischeReaktionstechnikUniversit¨at Erlangen-N¨urnbergEgerlandstr 3

91058 ErlangenGermany

Amal Siriwardana

The University of AlabamaCenter for Green ManufacturingDepartment of ChemistryTuscaloosa

AL 35487USA

List of Contributors XXI

Chair of Chemical Engineering

Faculty of Engineering Science

D-95440 Bayreuth

Germany

Pasi Virtanen

˚Abo Akademi University

Process Chemistry Centre

Yokohaa National University

Department of Chemistry and

Sebastian Werner

Friedrich-Alexander-Universit¨atErlangen-N¨urnberg

Department Chemie- undBioingenieurwesen (CBI)Lehrstuhl f¨ur ChemischeReaktionstechnik (CRT)Egerlandstraße 3D-91058 ErlangenGermany

Tomohiro Yasuda

Yokohama National UniversityCooperative Research andDevelopment Center79-5 TokiwadaiHodogaya-kuYokohama, 240-8501Japan

A Century of Supported Liquids

Natural and synthesized solid materials are generally characterized by a nonuniformand undefined surface The surface contains face atoms, corner atoms, edge atoms,ad-atoms, and defect sites, which together determine the surface properties of thematerial [1] In many applications, these different sites display different properties,for example, with respect to their chemical activity Often, only certain sites areadvantageous with regard to the specific application of the material as in the case

of, heterogeneous catalysts and adsorbents Future development of more efficientcatalysts and adsorbents in industrial processes will depend on the design of solidsurfaces that allow all surface atoms to be most effective At the same time, newtechnologies are required, which will lead to the design of completely new surfaceproperties within solids [2]

One possible way to achieve a uniform surface is by coating the solid supportmaterial with a thin liquid film, thereby defining the material properties by the liq-uid’s properties Such supported liquid phase (SLP) materials date back a 100 yearsago till 1914, when BASF introduced a silica-supported V2O5-alkali/pyrosulfate

SO2oxidation catalyst for sulfuric acid production (see Figure 1.1) [3] This lyst, which is still the standard system for sulfuric acid production today, can bedescribed as a supported molten salt, as it consists of a mixture of vanadium alkalisulfate/hydrogensulfate/pyrosulfate complexes that are present under reactionconditions (400–600◦C) [4]

cata-The concept of supported liquid catalysis is not restricted to liquid salts Inorder to apply the concept of uniform surface properties and efficient catalystimmobilization, several authors investigated the SLP concept during the 1970s and1980s [5–11] However, later studies revealed that the evaporation of the loadedliquid cannot be avoided completely during operation This is especially a problemwhen 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 hydrophobicreaction mixtures

Supported Ionic Liquids: Fundamentals and Applications, First Edition.

Edited by Rasmus Fehrmann, Anders Riisager, and Marco Haumann.

c

 2014 Wiley-VCH Verlag GmbH & Co KGaA Published 2014 by Wiley-VCH Verlag GmbH & Co KGaA.

ic acid cat

CuCl 2 -MCI /alumin

um silic at

(Deacon cat

CuCl 2 -PPh 2 /CuCI-KCI/silica

um silicat

e

(SLP h ydr

(SCILL h

ydr ogenation cat

Figure 1.1 Historical development of supported liquids in catalysis.

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 throughsurface 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 oforganic cations and inorganic or organic anions (for further details see Chapter 2)[20] Their better charge distribution and larger ion size compared to classicalinorganic salts result in melting points below 100◦C Owing to the extremely lowvapor pressure of ILs, the surface of SILP materials is coated permanently, even

-+ +

-

-Figure 1.2 Schematic representation of an ionic liquid film supported on a porous material.

1.2 Supported Ionic Liquids 3

under elevated reaction conditions By variation of anions and cations, solubility,reactivity, and coordination properties of the ILs can be changed according to thespecial requirements of the given application

With respect to material and surface design, ILs are characterized by a highlypre-organized, homogeneous liquid structure with distinctive physicochemicalcharacteristics 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 totransfer specific properties of the fluid to the surface of a solid material by confiningthe fluid to the surface Thus, the SILP concept allows custom-making of solidmaterials, resulting in uniform and well-defined surface topologies with definiteproperties and a controlled chemical reactivity Importantly, the SILP conceptthereby constitutes an attractive methodology to circumvent the lack of uniformity

of solids in traditional material science In addition, the approach provides a greatpotential to create materials with new surface properties, as the transfer of specific

IL properties to solid surfaces may result in ‘‘designer surfaces’’ with propertiesthat 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 benefitfrom 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 bulkphase properties such as solvation strength, conductivity, and viscosity The IL cancontain a certain functionality (e.g., acidity, hydrophobicity) that will render thesupport surface

1) Literature search using SciFinder including the term ‘‘ionic liquid’’, March 2013.

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 beendeveloped, including supported ionic liquid catalysis (SILC), supported ionic liquidcatalysts (SILCA), solid catalyst with ionic liquid (SCIL), solid catalysts with ionicliquid layer (SCILL), supported ionic liquid nanoparticles (SILnPs), supportedionic liquid phase (SILP), supported ionic liquid phase catalyst (SILPC), ionicliquid crystalline-SILP (ILC-SILP), structured SILP (SSILP), supported ionic liquid-like phase (SILLP), polymer-supported ionic liquid (PSIL), and supported ionicliquid 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 alow-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 3and 30 nm can be accomplished Detailed descriptions of support materials andsynthetic methodologies are given in Chapters 3 and 4 while the structure andstability of these materials are discussed in Chapters 5 and 6 Solid-state NMRstudies of different amounts of IL on silica support indicated that below a criticalvalue of 10 vol% IL loading, small islands of ILs exist on the support [22] Atvalues higher than 10 vol%, complete surface coverage with IL was observed, whichresembled the characteristics of the bulk IL This is an important prerequisitefor the efficient immobilization of homogeneous catalyst complexes that wouldlose activity and, more importantly, selectivity upon interaction with the supportsurface or in a constrained environment Spectroscopic studies of SILP materialsare summarized in Chapters 7 and 8 while Chapter 9 introduces tools for a-prioriselection of suitable ionic liquids

Form an engineering point of view these SILP materials offer some advantagescompared 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

The use of SILP systems in catalysis has been reviewed recently, including bothliquid and gas-phase applications [21, 24] With respect to the application of thesesolid 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 liquidsubstrate 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 isperformed in SILP gas-phase contact Since the IL does not have any technicallyrelevant vapor pressure, it is not removed via gas-phase leaching, and catalyststabilities have been found to be very high [25] Moreover, the gas-phase has nosolution power for the catalyst, which means that catalyst immobilization in SILPgas-phase systems does not require any dedicated ligand modification

As this approach builds on the volatility of the reaction products it is clearlylimited to feedstock and products with considerable vapor pressure Note that everymolecule that can be analyzed by gas chromatography is in principal accessiblefor SILP gas-phase reactions The removal of high-boiling reactants from the SILPcatalyst requires, however, a high amount of gas stripping, which is economicallyless attractive at least for the production of bulk chemicals A suitable alternative forperforming continuous reactions with high-boiling substrates is the combination ofSILP catalysis with a supercritical fluid as the mobile extraction phase, in particularscCO2[26, 27] A summary of catalytic gas and liquid phase applications is given

immobi-By applying a thin film of the IL onto a silica or alumina support, masstransport could be enhanced by orders of magnitude because of the large interfacialexchange 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 reducedbelow 10 ppm and the then-loaded SILP catalyst regenerated in vacuum Thisloading–unloading procedure could be repeated several times without significantloss 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 CO2capture technology [30–33] In refinery technology, the first commercialSILP process for mercury removal from hydrocarbon feed has been reportedrecently [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 inChapter 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 demonstratedthat such systems may exhibit better selectivity and even higher activities thantheir uncoated analogs [38] However, the origins of such selectivity and activityeffects are yet unclear The IL may influence the catalytic performance in atwofold manner On the one hand, it can directly interact with the active centerscomparable to the behavior of a ligand These so-called cocatalytic effects have beenextensively reported for catalytically active metal nanoparticles Such interactionsmay even lead to decomposition of the IL under reaction conditions, with theco-adsorbed decomposition products further modifying the catalytic properties Onthe other hand, the IL can modify the effective concentrations of the substratesand intermediates at the active sites, so that the solubility of liquids or gaseousreactants in the IL differs in an appropriate manner from that in the liquid organicphase, causing a ‘‘physical solvent effect.’’ In addition, the IL can compete with thesubstrates for active sites on the catalyst surface, thereby blocking sites that lead tounwanted by-product formation [39] The SCILL technology has been successfullyapplied in various hydrogenation reactions, resulting in better selectivity andenhanced activities Examples of SCILL catalysis involving metal nanoparticles can

be found in Chapter 14

1.7 Conclusion 7

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 thesupport surface

Such thin films of IL can obviously have no significant influence on substratesolubility The IL is usually anchored onto the support via chemisorption, involving

a surface reaction between the IL’s cation and the surface Other procedures havebeen reported in the literature and are highlighted in Chapter 4

1.7

Conclusion

The field of ILs on surfaces is highly multidisciplinary, attracting experts frommaterial 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 forsupported IL materials are expected in the fields of catalysis and separation asdepicted in Figure 1.4

Considering these benefits, it is estimated that SILP materials will contribute asubstantial part of the catalyst and adsorbent markets within the next 10 years A

Ionic liquids on surfaces Ionic liquids

Supports

Process Spectroscopy Synthesis

Coating techniques

Figure 1.4 Fields of application for ionic liquids on surfaces.

market share of 5% for SILP catalysts, having significant advantages compared toclassical heterogeneous or homogeneous systems, seems realistic A similar sharecan be expected for adsorbents.

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Part I

Concept and Building Blocks

Supported Ionic Liquids: Fundamentals and Applications, First Edition.

Edited by Rasmus Fehrmann, Anders Riisager, and Marco Haumann.

c

 2014 Wiley-VCH Verlag GmbH & Co KGaA Published 2014 by Wiley-VCH Verlag GmbH & Co KGaA.

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 ionicliquids that the explosion of interest in these began [12] These ionic liquids mostlyhave cations that are alkylated amines, with a smaller number of phosphoniumsalts used (Figure 2.1) with a variety of polyatomic anions (Figure 2.2) The cationscan be relatively simply prepared as their halide salts by alkylation of one of the widerange of commercially available amines (Scheme 2.1) or phosphines Throughoutthis step, air and moisture should be rigorously excluded and the temperature must

be well controlled to prevent runaway reactions [13, 14] The desired ionic liquidcan then be prepared by metathesis of the halide salt with a metal or ammoniumsalt or the conjugate acid of the required anion (Scheme 2.2) For hydrophobic ionicliquids, this can be done in aqueous solution [12]; for hydrophilic ionic liquids, themetathesis is usually performed in a water-immiscible organic solvent [15] The

Supported Ionic Liquids: Fundamentals and Applications, First Edition.

Edited by Rasmus Fehrmann, Anders Riisager, and Marco Haumann.

c

 2014 Wiley-VCH Verlag GmbH & Co KGaA Published 2014 by Wiley-VCH Verlag GmbH & Co KGaA.

resulting ionic liquid is then separated from the by-product salt and organic solventand if necessary decolorized [16] Of course, the wide variety of ionic liquids meansthat the preparations of different salts are not all the same in detail and there aremany ionic liquids that are prepared using different techniques [2].

CH3+

1-Alkyl-3-methylimidazolium 1-Alkylpyridimium 1-Alkyl-1-methylpyrrolidinium[CnC1im] +

+

N R +

Tetraalkylammonium Tetraalkylphosphonium

Figure 2.1 Some commonly used cations for ionic liquids, with the used notation.

Bis(trifluoromethylsufonyl)imide

Hexafluorophosphate Tetrafluoroborate Dicyanamide

RSO4−O

O

Trifluoromethylsulfonate Alkylsulfate triflate, [OTf ]−

O S O O

F B

MX HX

Scheme 2.2 Formation of ionic liquid from a halide (X) precursor.

Also, the melting point of any substance is a consequence of the structure of boththe solid and the liquid phases Ionic liquids are highly and differently orderedliquids (see below) Consequently, most discussions of ionic liquid melting pointsare only semiquantitative

Reasonable correlations have been found between molecular properties andthe melting points of some ionic liquids, using methods such as quantitativestructure–property relationships (QSPR) [17, 18] These have identified contribut-ing factors (see below) and given general trends, but have not been able to provideprecise predictions of the melting points of individual ionic liquids Group contri-bution methods have also been used to predict ionic liquid melting points, with agood fit similar to the general trends, but lack of precision for any individual ionicliquid [19]

The general principles of which factors contribute to determining the meltingpoints 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 tothe vast majority that are in general use being simple 1 : 1 salts of singly chargedcations and anions Larger ions have weaker coulombic attraction for each otherand so lower melting points There is a contrasting trend that larger ions havegreater van der Waals attractions and so for any homologous series of ionic liquidsthere is usually some alkyl chain length at which the coulombic interactions arelow, but the van der Waals interactions have not yet become significant, that gives aminimum melting point for the series Delocalization of the charge on the ions overseveral atoms also reduces the coulombic attraction between the ions Breakingthe symmetry of the ions prevents close contact of the ions and so also reducescoulombic attraction

Notwithstanding recent results regarding the formation of ionic liquid vapors [23],ionic liquids do not boil under normal atmospheric conditions Hence, the upperoperating limit of an ionic liquid is given by its thermal decomposition These arise

as a consequence of both the kinetics and thermodynamics of the decompositionreactions, and so are sensitive to the measurement conditions, particularly therate of temperature increase in the experiment It is now generally accepted thatreported decomposition temperatures are usually higher than temperatures atwhich no decomposition occurs if a sufficient time is given [24]

For ionic liquids with protic cations decomposition occurs most easily byproton transfer from the cation to the anion to produce the acid and base fromwhich the ionic liquid was prepared The temperatures at which this occurs

have been related to the difference in the pKa values of these parent acidsand bases [25–27] For fully alkylated ionic liquids, two major decomposition

N N CH3

CH3

CH3

CH3+

Substitution

+

R

Scheme 2.3 Possible decomposition routes for [CnC1im]X.

routes have been identified (Scheme 2.3) The first is dealkylation by nucleophilicattack of the ionic liquid anion on the cation, and can be correlated with thenucleophilicity of the anion [28] Quantum chemical calculations have been used

to calculate activation energies for the SN2 dealkylation of the cation by the anionand rates for [C4C1im]X (X= Cl, [N(CN)2], [BF4], [PF6], or [NTf2]) and correlatedwith experimental decomposition temperatures [29] This nucleophilic substitutioncompetes with Hoffman elimination, in which the anion acts as a Brønsted baseand abstracts a proton from theβ-carbon of one of the alkyl chains However,this reaction is often suppressed in imidazolium ionic liquids, because a thirddecomposition mechanism via deprotonation at the C2carbon of the imidazoliumring is preferred When the ionic liquid anion is very non-nucleophilic and nonbasicdecomposition of the anion itself may occur first [30]

2.4

Structures

The strength and long-range of Coulomb forces between ions lead to simple saltshaving infinite ionic lattices, which are among the most highly ordered of all chem-ical 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 × 10−3Ω−1cm−1, at

900◦C (liquid)𝜅 = 3.9 Ω−1cm−1[31] This increase arises because of the increased

mobility of ions in the liquid salt The molar volumes of the halides also increaseupon 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 Perhapssurprisingly, these changes are not accompanied by large differences in either theclosest ion distances or the coordination numbers, which can even show greatershort-range ordering than the crystal [33] A recent comparison of the high-energyX-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 moresymmetrically distributed around the ring than in the crystal and even having asignificant component of the cation–cation partial distribution function indicatingcloser contacts between ring centers [34]

It should be noted that these liquid ‘‘structures’’ are time-averaged views of theliquids and any local structures that do exist in the liquids will break apart in time to