Ionic liquids and catalysis

Applied Catalysis A: General 373 (2010) 1–56

Review

Ionic liquids and catalysis: Recent progress from knowledge to applications

H. Olivier-Bourbigou *, L. Magna 1, D. Morvan 2

IFP LYON, Departement Catalyse Moleculaire, Rond-point de l’echangeur de Solaize, BP3, 69360 Solaize, France

Contents

1. General introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

2. Ionic liquids: properties, evolution and next generations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

2.1. Properties of ionic liquids. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

2.2. A widening range of ionic liquids available. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

2.2.1. General remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

2.2.2. Protic ionic liquids (PILs). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

2.2.3. (Multi)-functional ionic liquids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

2.2.4. Chiral ILs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

2.2.5. Switchable-polarity solvents (SPS) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

2.2.6. ILs at the frontier between organic and inorganic materials. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

A R T I C L E I N F O

Article history:

Received 20 May 2009

Received in revised form 11 September 2009

Accepted 6 October 2009

Available online 12 October 2009

Keywords:

Ionic liquids

Biphasic catalysis

Supported Ionic Liquid Catalysis (SILC)

Task specific ionic liquids (TSIL)

Protic ionic liquids (PILs)

Thermoregulated ionic liquids

Biomass

Lignocellulose

Cellulose

A B S T R A C T

This review gives a survey on the latest most representative developments and progress concerning ionic

liquids, from their fundamental properties to their applications in catalytic processes. It also highlights

their emerging use for biomass treatment and transformation.

� 2009 Elsevier B.V. All rights reserved.

Abbreviations: IL(s), ionic liquid(s); [BMI]+, 1-butyl-3-methylimidazolium; [BMMI]+, 1-butyl-2,3-dimethylimidazolium; [MMI]+, 1-methyl-3-methylimidazolium; [HMI]+, 1-

hexyl-3-methylimidazolium; [OMI]+, 1-octyl-3-methylimidazolium; [AMI]+, 1-allyl-3-methylimidazolium; [AEI]+, 1-allyl-3-ethylimidazolium; [MI]+, 1-methyl-3-H-

imidazolium; [BMPy]+, N-butyl-3-methylpyridinium; [BPy]+, N-butylpyridinium; [PrMI]+, 1-propyl-3-methylimidazolium; [BMP]+, N-butyl-N-methylpyrrolydinium;

[PMP]+, N-propyl-N-methylpyrrolidonium; [NTf2]�, bis(trifluoromethylsulfonyl)amide (CF3SO2)2N�; [OTf]�, trifluoromethylsulfonate CF3SO3�; [OMs]�, mesylate CH3SO3

�;

[Fm]�, formate HCOO�; [Ac]�, acetate CH3COO�; TPPTS, triphenylphosphine trisulfonate sodium salt; TPP, triphenyphosphine; PEG, poly(ethylene glycol); LAB, linear alkyl

benzene; COD, 1,3-cyclooctadiene; SWNT, single wall carbon nanotube; TSIL, task specific ionic liquid; SILP, supported ionic liquid phase catalysis; PSIL, polystyrene

supported ionic liquids; SPS, switchable polarity solvent; PTC, phase transfer catalysis; LSER, linear solvation energy relationship; MD, molecular dynamics; DFT, density

functional theory; QSAR, quantitative structure–analysis relationship; HDS, hydrodesulphuration process; MAO, methylaluminoxane; CIL, chiral ionic liquid; PIL, protic ionic

liquid; DMSO, dimethylsulfoxide; DMAc, dimethylacetamide; PPN, bis(triphenylphosphorylidine)ammonium cation; DBU, 1,8-diazabicyclo-[5.4.0]-undec-7-ene; DABCO,

1,4-diazabicyclo[2.2.2]octane; REACH, registration, evaluation and authorisation of chemicals; EINECS, european inventory of existing commercial chemical substances;

EXAFS, extended X-ray absorption fine structure; ESI-MS, electrospray ionization-mass spectrometric; NOESY, nuclear overhauser enhancement spectroscopy; HOESY,

heteronuclear version of the NOESY experiment; ROESY, rotating frame overhauser effect spectroscopy.

Contents lists available at ScienceDirect

Applied Catalysis A: General

journa l homepage: www.e lsev ier .com/ locate /apcata

* Corresponding author. Tel.: +33 4 78 02 28 89; fax: +33 4 78 02 20 66.

E-mail addresses: [email protected] (H. Olivier-Bourbigou),

[email protected] (L. Magna), [email protected] (D. Morvan).1 Tel.: +33 4 78 02 28 86; fax: +33 4 78 02 20 66.2 Tel.: +33 4 78 02 38 74; fax: +33 4 78 02 20 66.

0926-860X/$ – see front matter � 2009 Elsevier B.V. All rights reserved.

doi:10.1016/j.apcata.2009.10.008

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H. Olivier-Bourbigou et al. / Applied Catalysis A: General 373 (2010) 1–562

2.3. Latest advances in the preparation and purification of ILs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

2.3.1. The different ways of ILs preparations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

2.3.2. Purification of ILs and analysis of trace impurities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

3. Structure and self-organisation of ILs at the supramolecular level . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

3.1. Solvent properties and solvent effect. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

3.2. Structure and organisation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

3.3. Toward a mesoscopic organisation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

3.4. Solute-ILs interactions: what impact on organic reactions? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

3.4.1. Interaction with water . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

3.4.2. Interaction with aromatic hydrocarbon. Clathrate behaviour . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

3.4.3. Interaction with chiral substrates: induction of chirality? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

3.4.4. Interaction with acid and base: toward new scale of acido-basicity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

3.5. Molecular modelling. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

4. How the ILs can affect the catalytic reactions pathway?. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

4.1. Some ‘‘unexpected’’ effects of ILs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

4.1.1. Effect of ILs impurities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

4.1.2. Effect of water and acidic protons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

4.1.3. Effect of bases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

4.1.4. ILs as additives: surprising effect! . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

4.2. When ionic liquids are involved in the formation of metal complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

4.2.1. Complex formation involving anions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

4.2.2. Complex formation involving cations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

4.3. ILs specially designed for catalysis. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

4.3.1. Change in mechanism pathway by stabilisation of charged transition state, active species or ligands . . . . . . . . . . . . . . . . . 25

4.3.2. Solvent for non-charged catalysts. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

4.3.3. Solvent/stabiliser for nanoparticles. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

4.4. Ionic liquids as medium for ‘‘in situ’’ spectroscopic investigations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

4.5. Removing sulfur from refinery streams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

5. Concepts for using ILs in homogeneous catalysis. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

5.1. Multiphasic IL systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

5.1.1. Some challenges and opportunities of multiphasic systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

5.1.2. Use of scCO2 as the transport vector for substrates and products . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

5.1.3. Demonstration of continuous catlytic performances . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

5.2. Supported ionic liquid phase system (SILP). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

5.2.1. ILs supported on solid inorganic solid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

5.2.2. ILs supported on hybrid organic–inorganic material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

5.2.3. ILs supported on organic polymers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

5.3. Switchable polarity solvents. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

5.4. Thermoregulated ILs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

5.5. Phase transfer catalysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

6. Overview of industrial applications and economic issues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

6.1. Selected examples of industrial/pilot scale applications of ILs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

6.1.1. Dimerization and oligomerisation of olefins: IL as solvent and Ni-co-catalyst. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

6.1.2. Friedel-Crafts alkylation and acylation of aromatic hydrocarbons: IL as solvent and catalyst. . . . . . . . . . . . . . . . . . . . . . . . . 37

6.1.3. Alkylation of olefins with isobutane: IL as solvent and acid catalyst . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

6.1.4. Chlorination and fluorination reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

6.1.5. Ether cleavage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

6.1.6. Acid scavenging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

6.1.7. Hydrosilylation: IL as solvent and nanoparticle stabiliser . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

6.1.8. Isomerisation: IL as a solvent . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

6.1.9. Methanol carbonylation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

6.1.10. Other examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

6.2. Main process engineering challenges and issues. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

6.2.1. IL stability, lifetime and recyclability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

6.2.2. Safety and environmental issues: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

7. ILs application in the biomass transformation into fuel and chemicals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

7.1. Processing of lignocellulosic and cellulosic materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

7.1.1. Direct solvent for dissolution of cellulose and sugars . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

7.1.2. Treatment of lignocellulosic materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

7.2. Applications of the use of ILs in the dissolution of ligno-cellulosic materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48

7.2.1. An improvement in the analysis of lignocellulosic material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48

7.2.2. Transformation of poly-saccharides in sugars using ILs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48

7.2.3. Catalytic transformation of sugars . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

7.3. Transformation of vegetable oils . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50

7.3.1. Transesterification of triglycerides: biodiesel production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50

7.3.2. Methyloleate metathesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

8. General conclusion and perspectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

H. Olivier-Bourbigou et al. / Applied Catalysis A: General 373 (2010) 1–56 3

1. General introduction

Ionic Liquids (ILs) have attracted rising interest in the lastdecades with a diversified range of applications (Fig. 1). The typesof ionic liquid available have also been extended to include newfamilies and generations of ionic liquids with more specific andtargeted properties. This expanding interest has led to a number ofreviews on their physico-chemical properties, the design of newfamilies of ionic liquids, the chemical engineering and the widerange of arrangements in which ILs have been utilised (liquidphase, multiphase, immobilized on supports, . . .) and pilot orindustrial developments [1].

Why ILs have attracted so much attention in the last few decades?

In addition to the fact that they are now commercially available,there is a better understanding of the effect of ionic liquids(chemical and physical properties as well as engineering fluids).Consequently, ionic liquids have been used more widely andefficiently, with better control over the overall process. Theintroduction of structural functionalities on the cationic or anionicpart has made it possible to design new ILs with targetedproperties [2]. More recently, ILs appear to be the subject offundamental publications aimed at improving the understandingof these solvents, predicting their physico-chemical properties andpublications describing their use in increasingly diverse applica-tions such as sensors, fuel cells, batteries, capacitors, thermalfluids, plasticizers, lubricants, ionogels, extractants and solvents inanalysis, synthesis, catalysis and separation, to name just a few.Some new applications, such as energetic compounds or pharma-ceutical ILs, are still emerging. ILs can be used as more than just aalternative ‘‘green’’ solvents. They differ from molecular solventsby their unique ionic character and their ‘‘structure and organisa-tion’’ which can lead to specific effects. They are tuneable,multipurpose materials.

When reading papers on ILs, one of the key words is diversity.Diversity of anion–cation combinations, diversity of modes ofpreparation, modes of purification and nature of impurities(quality), diversity of properties, diversity of mode of use, diversityof applications. This is one of the reasons why it is so difficult tomake generalisations about their physical properties or their use.The contribution ILs make to homogeneous catalysis has more todo with the enhancement of catalytic performances (activity,selectivity or new chemistry) and the possibility of catalystseparation and recycling by immobilization in the IL-phase thanwith environmental concerns. They can act as solvents, asmultifunctional compounds like solvents and ligands, solventsand catalysts, stabilising agents for the catalysts or intermediates.

Fig. 1. Evolution of

The performance of an IL will strongly depend on the technology inwhich it is implemented. They can be utilised in very differentways: homogeneous, multiphase, heterogeneous, in bio transfor-mations or in organo-catalysis. They play a specific role in all theseapproaches.

Even more than diversity, another keyword for the end-user isprediction. When will it be possible to move ahead to rational design

of ionic liquids? Is it possible to predict which ionic combinationresults in a given set of properties? Most work towards under-standing and knowledge has been achieved on imidazoliumcations, certainly the most popular cation but not the only one.New families of ILs with various other cations have been developedthese last decades. ILs are not trivial. They are generally composedof asymmetric and flexible ions, with components of highlydifferent sizes and shapes, and involve different types of dominantinteractions. Theoretical treatment and interpretations are com-plicated. However, it is important to have a better understanding ofneat IL’s properties, and their properties and interactions withother species such as molecular species or metal complexes tobetter understand their role in catalysis.

The aim of this review is not to provide an exhaustive list (or stateof the art) of the wide range of catalytic reactions occurring in ILs.Several good, recent reviews have already illustrated that point(Table 1). This review focuses mainly on recently published material.We have restricted ourselves to give a survey on the latest, mostrepresentative developments and progress on ionic liquids andcatalysis. This review also covers the different aspects of ILs, from theknowledge we have of these media to the use of their properties forcatalysis, catalytic processes and engineering. More particularly, thefollowing are reported: (i) the design of new generations of ILs: theevolutions and key events (a general history of the ILs is described byJ. Wilkes [3]). (ii) fundamental properties of ILs: structure andorganisation, IL-solute interactions, (iii) the IL’s effect: how ILs canimpact the outcome of the reaction and how it is possible to controlthe reaction process, (iv) the diversity of IL use in catalytic processes:homogeneous, multiphase, heterogeneous, (v) comments onindustrial applications and commercial aspects of ILs: barriers toovercome? (vi) key events in environmental catalysis: this lastchapter focuses on the role that ILs can play in the treatment andtransformation of bio-resources and in bio-processes.

2. Ionic liquids: properties, evolution and next generations

2.1. Properties of ionic liquids

Considering the broad range of ILs and applications [46], it isdifficult to generalise their properties and to report generaltendencies. Sometimes the authors emphasise their differences

IL generations.

Table 1General reviews on ILs (from 2003 to 2008).

Year Ref.

General (catalysis)

Task-specific ILs 2004 [4]

ILs in catalysis 2004 [5]

Catalysis in ILs 2006 [6]

Homogeneous catalysis in ILs 2007 [7]

Catalysis in ILs 2007 [8]

Transition metal-catalysed reactions in

non-conventional media

2007 [9]

The path ahead for ILs 2007 [10]

Applications of ILs in the chemical industry 2008 [11]

Catalysts with ionic tag and their use in ILs 2008 [12]

Specific reaction/topic (catalysis)

Polymerization processes in ILs 2004 [13]

Supported ionic liquid phase (SILP) catalysis 2006 [14]

Oxidations of organic compounds in ILs 2006 [15]

Functionalised imidazolium salts for TSILs

and their applications

2006 [16]

Olefin hydroformylation in ILs 2007 [17]

Brønsted acids in ILs 2007 [18]

Enantioselective catalysis in ILs 2007 [19]

Asymmetric synthesis in ILs 2007 [20]

Lanthanides and actinides in ILs 2007 [21]

ILs in separations 2007 [22]

Olefin metathesis in ILs 2008 [23]

Palladium-catalysed reactions in ILs 2008 [24]

ILs towards supercritical fluid applications 2007 [25]

ILs in heterocyclic synthesis 2008 [26]

Applications of chiral ILs 2008 [27]

Synthesis and application of chiral ILs 2008 [28]

Electrochemical reactions in ILs 2008 [29]

Bio-catalysis/biomass

Biocatalysis in ILs—advantages beyond green technology 2003 [30]

Biocatalytic transformations in ILs 2003 [31]

ILs: Green solvents for nonaqueous biocatalysis 2005 [32]

Chemical and biochemical transformations in ILs 2005 [33]

Biocatalysis in non-conventional media (ILs, scFluids.) 2007 [34]

Biocatalysis in ILs 2007 [35]

Ionic green solvents from renewable resources 2007 [36]

Oxidoreductase behaviour in ILs 2008 [37]

Biotransformations and organocatalysis with ILs 2008 [38]

Dissolution and functional modification of cellulose in ILs 2008 [39]

Synthesis (inorganic & organic)

Metal-containing ILs and ILs crystals based on

imidazolium moiety

2005 [40]

ILs solvent properties and organic reactivity 2005 [41]

Application of zeolites in supercritical fluids and ILs 2007 [42]

The phosphorus aspects of green chemistry 2007 [43]

Application of ILs in polymer science 2009 [44]

Analysis

ILs in chromatographic and electromigration techniques 2008 [45]


and not their similarities. Some of the properties described someyears ago are now subject to controversy: e.g. electrochemicalwindow; long-term thermal stability (thermal stability wascertainly overestimated in the past); polarity; volatility (someILs are distillable under certain conditions [47]). Why all of theseconflicting results? Because an evolution toward a better under-standing of these media, better characterization with improvedknowledge and quantification of their impurities (ion chromato-graphy, ICP-MS) which are well-known to affect the thermo-physical properties of ILs, have been achieved in recent years. Thedifferent experimental techniques used and the estimation of datauncertainty may also have influenced the discrepancies in terms ofphysico-chemical properties. However, ILs have widely acceptedgeneric properties. They consist entirely of ions (Scheme 1). Forexample, in [BMI][PF6] which melts at 12 8C, the ionic concentra-tion is 4.8 mol/L. The melting point of ILs should be less than100 8C, even if this is an arbitrary temperature limit, and theirionicity should be >99%. All these generic properties have been

described in open literature and can be easily found in a gooddatabase (e.g. ‘‘ILThermo—managed by the US National Institute of

Standards and Technologies)’’ [48]. They will not be reported indetail in this review—only a list of critical remarks is given below.

� Melting point: Data must be considered with caution as themelting point of many ILs may be uncertain as they can undergosupercooling and because of the potential presence of impurities.� Volatility: For typical ILs, normal boiling temperatures (Tb), which

correlate with their vapour pressure at 1 atmosphere, cannot beexperimentally determined as ILs decompose at a lowertemperature. It has nevertheless been reported that ILs can bedistilled at 200–300 8C but under significantly reduced pressureand at very low distillation rate (<0.01 g h�1) [47]. The questionis how ionic are ILs? The ionic nature (or ionicity) of ILs canpartially explain their negligeable vapour pressure in the liquidstate, which distinguishes them from molecular solvents.Quantitative descriptions of the ionicity would be a usefulindicator for characterizing ILs. This has tentatively been doneusing the effective concentration of ions [49].� Non flammability: Much of the interest for ILs has been centred on

their possible use as ‘‘green’’ alternatives to volatile organicsolvents, mainly because ILs are considered as non-volatile andconsequently non-flammable at ambient and higher tempera-tures. However there are many other potential solvents thatmeet these criteria but that have not been subject to suchinterest. It is worth mentioning that it is not because they arenon-flammable that ILs can be used near a heat source. ILs arecombustible. They even can be fine-tuned for energetic contentand replace hydrazine and its derivatives [50,51].� Thermal and chemical stability: The onset of thermal decomposi-

tion calculated from fast thermogravimetric analysis (TGA)indicates high thermal stability for many ILs, generally >350 8C.However, lower values are found for long-term stability which isimportant to consider when ILs are used in catalytic processes.Phosphonium ILs with [NTf2]� or [N(CN)2]� anions decomposecompletely to volatile products in a single step. The degradationproducts indicate that Hofmann elimination process and/ordealkylation reactions occur. On the contrary, ILs based onnitrogen cations do not decompose completely and generate charresidue (cyano groups are prone to polymerization) [52].� Conductivity and electrochemical window: ILs conductivity is an

interesting property to consider as ILs can play the role of bothsolvents and electrolytes in electrochemical reactions. ILs exhibitbroad range of conductivities spanning from 0.1 to 20 mS cm�1. Ingeneral higher conductivities are found for imidazolium-based ILsin comparison with the ammonium ones. Many factors can affecttheir conductivity, such as viscosity, density, ion size, anioniccharge delocalization, aggregations and ionic motions [29]. Strongion-pair associations have been invoked in the case of [NTf2]�

based ILs, to understand their lower conductivity in comparisonwith [BF4]� based ILs [53]. Concerning their electrochemicalwindow, it is typically found in the range 4.5–5 V, which is similarto or slightly larger than that found in conventional organicsolvents, but larger than that of aqueous electrolytes. Quaternaryammonium is generally more stable toward reduction thanimidazolium which can lead to the formation of N-heterocycliccarbenes. The challenge here is still to design ILs with wideelectrochemical window along with good electrical conductivity.� Density: A considerable amount of data on the density of ILs are

available in the literature [54]. ILs are generally denser thaneither organic solvents or water, with typical density valuesranging from 1 to 1.6 g cm�3. The density of ionic liquids versuspressure and temperature has also been modelled [55].� Viscosity: From the engineering aspect, the viscosity of ILs can

affect transport properties such as diffusion and may be an issue

Scheme 1. Main cations and anions described in literature.

Fig. 2. Typical polarity and volatility characteristics of alternative solvents.


in practical catalytic applications. It plays a major role in stirring,mixing and pumping operations. The viscosity of many ILs isrelatively high compared to conventional solvents, one to threeorders of magnitude higher. For a variety of ILs it has beenreported to range from 66 to 1110 cP at 20–25 8C. The design ofless viscous ILs is still a challenge for many applications [56].� Polarity: The polarity is one of the most important properties for

characterizing the solvent effect in chemical reactions [57]. It isalso the property which has probably been the most widelydiscussed in the case of ILs. Why? Because there is no singleparameter and direct measurement that can characterize ILpolarity. Solvatochromic dyes can be used to determine empiricalpolarity parameters but these parameters (Kamlet-Taft equation)are probably not truly independent on the probe molecule used.The difficulty in the case of ILs is to find a suitable soluble probewhich measures the polarity parameters as independently aspossible of the other influences of the solvent [58,59].� Toxicity and biodegradability [50,60]: The early claims of the low

toxicity and biodegradability of ILs has often been reduced totheir negligible vapour pressure which, of course, is not realistic.It has been confirmed that commonly used ionic liquids are noteasily biodegradable. But should this be a major limitation totheir use on industrial cases?� Surface tension: It has been the topic of a relatively minor number

of studies. ILs have relatively moderate surface tensionscompared to organic solvents [61].

For industrial implementation, some IL properties must be investi-gated under real process conditions. A screening of some propertiessuch as compressibility has been examined under long-termconditions and under high pressure [62]. How can they be comparedto conventional solvents? Fig. 2 gives a tentative qualitative descriptionof ILs compared to alternative solvents, in terms of polarity andvolatility.

2.2. A widening range of ionic liquids available

2.2.1. General remarks

The number of ILs has expanded exponentially recent years. Acompilation of all the described cations and anions is not possible.The main reviews can provide an overview. Many diversemotivations can explain the design of new IL families. Some ofthem are described in the following points.

(1) A great deal of attention has been devoted to (multi)functionalILs (often termed task-specific ILs) aimed at using synergic

‘‘chemical’’ properties. Protic ILs and Brønsted and/or Lewis ILscan be used as acid catalysts and solvents. Basic ILs have alsorecently been reported as playing a dual role of solvent andbase-catalyst with a particular interest and potential forcellulose acetylation [63]. ILs bearing a function (phosphorous,nitrile, imine, amine, alkyne) have been applied as both ligandsand supports for immobilizing and recycling transition metalhomogeneous catalysts [64] or as protective agents andsolvents for the stabilisation of metal nanoparticles. ILssupported organo-catalysts (such as proline as a chiral catalystin asymmetric synthesis) have been developed to improve therecovery of the catalyst which is often used in substantialquantities [65]. Chiral ILs, such as solvents and chiral inductingagents, have been modified in various ways, the chirality beingincorporated on the cationic or anionic part of the ILs.

(2) The tuneabiliy of combinations of cations and anions and thepossibility to achieve modification of the cation and/or theanion part offer access to ILs with targeted properties. Forexamples, the hydrophilicity/hydrophobicity flexibility, thedecrease of ILs viscosity, and the increase of ILs stability are stillchallenging targets. [NTf2]� and [N(CN)2]� anions alreadyappear as good candidates to get ILs with lower viscosity. Thereplacement of alkyl group on the imidazolium by moreflexible ether group was also a way to decrease both viscosityand melting point (Scheme 2). The replacement of alkyl groupsby oligoether groups has been shown to decrease the ionicliquid’s viscosity significantly. This effect has been demon-strated both for substituents at the anion (such as sulfate) [66]

Scheme 3. New methimazole based ILs.

Scheme 2. ILs with targeted properties (decrease viscosity and density).


and for substituents at the cation (such as PEG-functionalisedimidazolium dialkylphosphates) [67]. Novel ILs with Sisubstituted cations were also reported and present a reductionof viscosity thanks to a more flexible side chain than an ether[68]. This may be important since mass transfer may beimportant, reaction rate can be increased by reducing theviscosity of the ILs [69].

There has been an increased interest for ILs that present betterinertness under reaction conditions. The hydrolysis of [PF6]� or[BF4]� anions to generate HF in situ has been the object ofnumerous reports. Reactions catalysed by protic acids have oftenbeen described in [PF6]� based ILs, probably thanks to the presenceof HF. The formation of transition metal fluoride under certainconditions has also been observed [70,71]. The ionic liquid basedon the [(C2F5)3PF3]� anion has been recently proposed as a morechemically stable alternative to [PF6]� [72].

The activation of the C(2)-H of the imidazolium to lead to theN-heterocyclic carbene (NHC) in presence of base is also largelydescribed [73]. Consequently, increased interest has been foundin phosphonium ILs because of their higher stability underbasic conditions, such as in Grignard reactions [74]. To protectthe acidic C(2)-H, 2-methylimidazole based ILs are often used.By analogy, methimazole based ILs have been described inwhich the C(2) proton is replaced by a thiol linkage (Scheme 3)[75].

The latest applications of ILs concern ILs with biologicalproperties (Scheme 4) [76]. Hypergolic fuels in which hydrazine

is replaced with ILs based on dicyanamide anions, have beenproposed. It is expected that these ILs can be fine-tuned for betterenergy content and physical properties [51]. ILs such asdialkylimidazolium formate were produced as liquids havingstrong hydrogen bond accessibility. They are good solvents forpolysaccharides dissolution [77] under mild conditions and highconcentrations (Section 7.1.1).

(3) Cost and biodegradability have also been a main concern andnew families of ILs derived from renewable feedstock or from‘‘low cost’’ starting materials have been described (Scheme 5).These ‘‘Bio-ILs’’ are entirely composed of biomaterials [78]. Anexample is given by the development of the ‘‘deep eutecticmixtures’’ liquid systems based on chloline chloride [79] forwhich the qualification of ‘‘ionic liquids’’ is still the subject ofcontroversies. Choline can be used as alternative cation incombination with suitable anion to generate ILs (cholinesalicylate melts at 50 8C and was described in 1960). Thephysical properties (viscosity, melting point, thermal stability,polarity) of different carboxylate anions such as acetate, tartrate,lactate, succinate, glycolate, maleate coupled with choline cationhave been described. Surprisingly glycolate presents a Tm of38 8C. The thermal stability range of the series is 183–223 8C[78]. The maleate gives moderate viscosity. Other interesting ILsbased on choline cations have been prepared by directneutralisation of choline hydroxide with different aromatic orcyclic aliphatic carboxylic acids (Scheme 6). Surprisingly, someof these ILs show low Tg and Tm. The biodegradability properties

Scheme 5. Cost-effective ILs.

Scheme 4. Examples of ILs with targeted functions.


of these ILs have been reported [80]. Very recently, it was shownthat the incorporation of ester side chain moiety on pyridiniumor nicotinium cation could lead to biodegradable ILs contrary tothe pyridinium analogues ILs [81].

(4) New materials have been developed using imidazolium asbackbone to access to functional silica gels or carbon nanotubeswith flexible properties [82]. The IL is immobilized on the solidsupport by covalent bonds generally between the silyl groupand the imidazolium cation. The immobilization of metal ionson silica surface offers a novel class of materials where theenvironment of the metal is comparable to that found in of thetype [BMI]2[MX4] [83].

Scheme 6. ‘‘Bio’’ ILs.

(5) ILs recyclability [84] becomes one of the main issues whenprocess developments are envisioned: distillable ILs (underrelatively normal pressure and temperature conditions) or ILspresenting low thermal stability have been designed. These ILscan contain a weakly basic anion and a cation formed from atertiary amine and an exchangeable proton (Scheme 7). Bydistillation, the neutral acid or base (if volatile enough) can beseparated from the ionized species. They can subsequently berecombined to reform the IL. There is a vast number of cation–anion combination of such protic ILs. Carbamate based ILs formanother class of distillable ILs (see switchable solvents). Eachapplication requires specific properties, there is no ILs that cansatisfy all of them. We will focus here on the last developments.

2.2.2. Protic ionic liquids (PILs)

While one of the first IL, described in 1914 by Walden [85], was of‘‘the protic type’’ [EtNH3][NO3] (with a mp = 12.5 8C, described innearly all reviews on ILs!), aprotic ILs largely dominate the openliterature due to their inertness relative to organometallic com-pounds and their potential of applications, particularly in catalysis.However, there has been a resurgence of interest for these Protic ILsessentially because of their great potential for proton transferapplications in fuel cell technologies. A review was written by Pooleincluding the use of these Protic ILs in chromatography [86]. Some ofthese ILs present low melting points (well below 100 8C) and highconductivities (over 10�2 S cm�1 at 130 8C) [87]. Most of the non-protic ILs are synthesised by transferring an alkyl group to the basicnitrogen site through SN2 reactions. Protic ILs are formed throughdirect proton transfer from a Brønsted acid to a base (or a Brønsted

Scheme 8. N,N-dimethylethanolammonium formate ILs.

Scheme 7. Protic ILs synthesised by direct protonation (X� = [NTf2]�, [CF3SO3]�, [CF3CO2]�, [CH3SO3]�, [HCOO]�, [HSO4]�, [H2PO3]�).


base). They present the advantage of being cost-effective and easilyprepared as their formation does not involve the formation ofresidual by-products. Examples of Protic ILs are given in Scheme 7[88,89]. Many of these Protic ILs involve very strong acids, such asHNTf2, and hence the equilibrium is heavily shifted to the right thusproducing ILs as completely ionic salts. These ILs are generally liquidat room temperature.

NMR measurements show that the N–H proton is not labile,which tends to indicate that these ILs cannot be really consideredas Brønsted acids [90]. The acidic properties of these ILs couldrather be ascribed to the presence of residual acid in the mediumcoming from the synthesis. The purity has to be checked properlyby a more sensitive mean than NMR. In the case of weak acid, suchas acetic acid, the neutralisation reaction will reach a point ofequilibrium. The ‘‘complete or not’’ ionicity of these mixtures havebeen discussed by different groups and the challenge is still toprovide an unambiguous measurement of this degree of ionization,since the values for equilibrium constants are not known undernon-aqueous conditions [91]. These liquids can probably be bestdescribed as ‘‘liquid mixtures’’ of ionic and neutral species. It hasbeen suggested that to be classified as ILs, according to a formaldefinition, the products must be >99% ionized, and thus a carefulselection of acids and bases (based on pKa) is required [92].

The case of protic bases (such as dialkylamines) has also beenstudied (Scheme 8). In 1:1 mixtures, the boiling point is usuallymuch higher than the average value of the acid and the baseprecursors. This may suggest that significant and fast protontransfer between acid and base molecules occurs [93].

One can predict a growing interest in near future in ILs with‘‘dissociable protons’’ not only as potential solvents but also fortheir different properties and behaviours, their ability to form H-bonds (proton donor and acceptor) and their use to build ahydrogen-bond network [94]. However, a limitation of theseprotonated imidazolium salts is that they decompose at relatively

Fig. 3. ILs based dendrimer polymers base (reprinted with permis

low temperatures compared with their alkylated homologueswhile this property has been advanced as an advantage forrecycling.

Another interesting example of Protic ILs is based on the use ofhydrophilic monodispersed and hyperbranched dendrimer poly-

mers base such as polyamidoamine (Fig. 3). Protonation of thispolymer with Brønsted acid, followed by metathetic exchange ofanion with [NTf2]� leads to the formation of an hydrophobic IL thelow Tm of which being ascribed to the flexible nature of thedendritic backbone (Tm = �2.5 8C). Conductivity and thermaldegradation (near 350 8C) were determined. This IL, beside itsuse as proton conductive electrolytes, could be well suited forparticles encapsulation [95].

Brønsted ILs can be classified besides the PILs. An overview onthese ILs incorporating carboxylic esters and acid groups and theirzwitterionic counterpart has been written in 2004 [96]. Anotherrecent review gives an overview on the different Brønsted ILs andtheir applications in organic synthesis and catalysis [18].

2.2.3. (Multi)-functional ionic liquids

Recently, ILs based on different cations and anions bearingfunctional groups have been the object of several recent reviews[12,16,96–100].

2.2.3.1. Solvent and acid or base function. Acidic and basic ILsrepresent new classes of ILs (Scheme 9). The acid or basic functioncan be attached either on the anion, either on the cation [63]. ILs

sion from [95]. Copyright 2009 American Chemical Society).

Scheme 10. Examples of basic ILs.

Scheme 11. Example of switchable Lewis basic ILs.

Scheme 9. Acid and basic ILs.


containing polynuclear metallic anions such as chloroaluminates,have been known for a long time for their potential Lewis acidityand superacidity in presence of protons. They have been extendedto other polynuclear anions that are stable in presence of water andoxygen such as chloroferrate or chlorozincate. The chloroferrateanions have been associated with [NEt3H]+ to generate cheap andeasy to make acidic catalysts. Interestingly, Brønsted acidity canalso be introduced by addition of Brønsted acids such as HF or HClinto halide based ILs. This is a way to reduce the volatility of theacid by supporting it in the ILs through the formation of X(HX)n

type anion ([X]� = [F]� or [Cl]�) [101]. The species [HCl2]�,[H2Cl3]�, and [H3Cl4]� are also known to form when Brønstedacids are dissolved in chloride-rich chloroaluminate ionic liquids[102,103]. But some leaching of the acid in presence of an organicphase can be expected.

Alkane sulfonic or carboxylate acid groups have been covalentlytethered to different cations such as imidazolium, benzimidazo-lium [104], pyridinium [105], ammonium, or phosphoniumtethered sulfonic acid tosylate [106]. An interesting IL has beendescribed with the acidity linked to a quaternary ammonium([Me3N-(CH2)2-CO2H][X]), associated with the [NTf2]� anion, thiscompound has a melting point of 57 8C. It is known to solubilisemetal oxide [107].

The hydrogen atom in the C(2)-position of the dialkylimidazo-lium cation can also be proposed as source of acidity. For example, N-heterocyclic carbenes have been electrogenerated by cathodiccleavage of the C(2)-hydrogen bond of imidazolium-based room-temperature ionic liquids. These carbenes proved to be quite stablebases that can be used for the deprotonation of bromoamines [108].

Basic ILs have been less developed than acidic ones. Amineorganic bases have been tethered to IL cations. These functional ILswere firstly synthesised to capture CO2 [109,110]. Mono-chargeddiamine based ILs which incorporate Lewis basicity site (DABCOtype) on the cation with both thermal stability and low meltingpoint can be obtained when associated with [NTf2]� anion (Scheme10) [111,112].

One interesting concept has been described to switch thebasicity of ILs. This is achieved by using amino-group containing

ILs, either on the cation, either on the anion (Scheme 11). By addingCO2 pressure to the solutions, the basicity can be significantlyreduced. The basicity can be repeatedly recovered by removing CO2

by bubbling N2 to the ILs. This simple and reversible method couldhave potential applications in different fields [113]. Further, somebasic ILs containing biodegradable components, including ILsderived from natural amino acids have been developed [113].

Finally, ILs synthesised by the reaction of [RMI][OH] withdifferent poly-acids such as oxalic acid, malic acid, phthalic acid,tartaric acid. can be mentioned. According to the quantity of acidadded, these ILs can display a certain acidity level. They have beenused as buffers in pH sensitive catalytic reaction for controlling theacidity in non-aqueous media. The interest is that they maypresent solubilisation properties in organic solvents [114].

In the case of these acid or basic ILs, it is worth emphasising thatthe presence of impurities, such as water, halide, organic bases oracids or traces of solvents, mainly coming from the synthesis of the

Scheme 12. Functional ILs as ligands and supports for transition metal complexes.


ILs, can dramatically modify their acido-basic properties. Thedetermination of the level of acidity of these ILs has most of timenot been determined which can sometimes lead to misunder-standing in the role of the ILs.

2.2.3.2. Full-size image.

Solvent and ligand. In multiphase catalysis, the main challenge is torecycle the catalyst, to maintain the transition metal in the IL phaseand to prevent its loss by leaching on workup. The number ofdescribed tagged ligands is huge and their field of applicationscover nearly all catalytic reactions. Great progress, especially in theorganic synthesis of tagged ligands, has been achieved (Scheme12). Ionic phosphorus ligands are the subject of ongoing researchfor different catalytic reactions. ILs open a new field for the use P–Obased ligands, rarely used in water because of their sensitivity tohydrolysis. Almost a dozen of cationic phosphite ligands have beenrecently described, some of them could have been produced onquite large scale [115].

Scheme 13. Exampl

2.2.4. Chiral ILs

The number of publications dealing with chiral ILs (CILs)grew rapidly [28]. The source of chirality can be providedeither on the cation, on the anion or both on anion and cation(Scheme 13).

A large range of CILs have been prepared based on chiral amino-acids anions and ammonium, imidazolium and phosphoniumcations [116,117]. Beside imidazolium, guanidinium cations havealso opened the opportunity to create a new family of chiral ILsbased on natural chiral anions. The applications of these CILs can befound in asymmetric catalysis, but also in spectroscopic andchromatographic applications. In asymmetric synthesis, it is oftenbelieved that CILs can be used as chiral solvents and as sole inducerof chirality due to their polymer-like behaviour and potential highdegree of organisation. However, very few results are reportedwhich demonstrate such potential. The first result was reported bythe group Vo-Thanh in the Baylis-Hillman reaction [118] The ILused is based on the chiral ephedrinium cation (Scheme 14). The

es of chiral ILs.

Scheme 14. Chiral ephedrinium based ILs.

Scheme 15. Example of silica supported CIL.

Scheme 17. Two-steps formation of Ag based hydrophobic ILs (L = olefin or

diolefin).


hydroxy functionality of the cation plays an important role in theenantioselectivity.

IL-supported chiral ligands are also largely described. These ILshave tentatively been supported on inorganic materials but veryoften with a loss of enantioselectivity. For example, highly orderedmesoporous functional organosilicas incorporating chiral cam-phorsulfonamide entities were synthesised by a hydrolysis–polycondensation involving chiral imidazolium precursors andtetraethoxysilane (TEOS) [119] (Scheme 15).

2.2.5. Switchable-polarity solvents (SPS)

These solvents can be described as neutral liquids that can bereversibly converted to polar ionic liquids when exposed to CO2

(Scheme 16). This conversion is reversible. The viscous ionic liquidcan be converted back to neutral liquids in presence of N2 or argongas or heat. By a judicious choice of the liquid amine, ionic liquidcarbamate salts can be formed. Secondary amines have beenrecently described. These solvents have been described as a post-reaction separation of the product from a homogeneous catalyst.The example describes the polymerization of cyclohexene withCO2 catalysed with Cr(salen)Cl without solvent. At the end of thereaction, the polymer and the catalyst are dissolved in the NHEtBuamine. By bubbling CO2 the amine is converted into the polarcarbamate salt in which the polymer precipitates. The catalystremains mainly in the solution [120–122]. The limitation of thesesolvents is the reactivity of the base. This class of ILs is furtherdeveloped in Section 5.3 of the review.

Scheme 16. Switchable Polarity Solvents (SPS).

2.2.6. ILs at the frontier between organic and inorganic materials

2.2.6.1. Inorganic cations. A new methodology to synthesise ILs isby complexing inorganic cation such as Ag or Zn with neutralorganic ligands such as olefin, amide and amine compounds(Scheme 17). The associated anion can be subsequently changed bymetathesis reaction, such as [NTf2]�, to generate less viscous andhydrophobic ILs (mp < �10 8C for most of the ILs studied, highconductivity and low viscosity but low decomposition tempera-ture for [Ag(olefin)x][NTf2] with the olefin being 1-hexene, 1-pentene, 1-isoprene or ethylene). For example, [Ag(1-bute-ne)2][BF4] has a melting point of about 37.5 8C. It is well-knownthat unsaturated hydrocarbons can form reversible p-complexeswith the metallic cations (Ag+ or Cu+). In these new ILs, silver is notintroduced as a solute but is in the structure of the ILs itself, thusmaking its content quite high. These ILs have been applied forseparation process of olefin/paraffins. They combine the propertiesof ILs, liquid and solid membranes [123].

2.2.6.2. Deep eutectic solvents (DES). Recently, some deep eutecticmixtures with properties similar to those of ILs, have beendescribed. These mixtures can simply be obtained by mechanicallymixing two different components with no emission and massefficiency (Table 2).

2.2.6.3. Metal salts anions. Many ionic liquids based on metal ionshave been developed [40,129]. Work has been first focused onchloroaluminates associated with imidazolium or pyridiniumcations. A variety of different anions are formed in solution([AlCl4]�, [Al2Cl7]�, [Al3Cl10]�) the ratio of which vary withchanging aluminium chloride composition. These ideas havemore recently been extended to other chlorometalate salts. Theresultant molten salts have the advantage that they are not watersensitive, although they are in general, more viscous than theiraluminium analogues: to name just a few examples, [FeCl4]�,[CuX3]�, [InCl4]�, [AuCl4]�, [CoCl4]2�, [NiCl4]2�, [PdCl4]2�,[Co(CO)4]�. Ionic liquids with Cr or Mo based anions (Cr(O)3Cl)or Mo(O)2(NCS)4 have been applied as self-supported catalyst foroxidation, as well as polytungstate imidazolium complexes. Thelanthanide based [BMI]3[Ln(NCN)6(H2O)2] compound proved tobe low melting (Ln = lanthanide). These examples are scarce andstill quite exotic. In catalysis, these metal salts have mainly beenused as potential Lewis acids. Polynuclear anions have beendescribed in some cases such as [Zn2Cl5]� [130], [Zn3Cl7]�;[Fe2Cl7]�; [Sb2F11]�, [Sn2Cl5]�. These complex anions can be Lewisacids strong enough and not air and moisture sensitive (the cationcan be based on choline cation). The ‘‘soft’’ indates based ILs haveexhibited interesting Lewis acid properties for Friedel-Craftacylations and chlorozincates for Diels-Alder reactions or alkyla-tion reactions [131,132].

The liquid version of the Monsanto catalyst for MeOHcarbonylation can be cited: [BMI][RhI2(CO)2] obtained by reactionof [BMI][I] with [Rh2I2(CO)4] [133]. The introduction of the metalions inside the ILs is an interesting way to immobilize catalystswhile taking part of the potential ordered structure of the ILs.Although less studied, these metals containing ILs could be used forthe preparation of nanomaterials. Several liquids based onferrocenated imidazolium have been reported [134]. Their mainapplications are found in the domain of electrochemistry [135]. Areview on ILs crystals is also available [136].

Table 2Examples of deep eutectic solvents.

Compound 1 Compound 2 Selected characteristics Ref

Eutectic for 3:7 molar ratio [124]

Eutectic point at 56 8CViscosity: 22.5 cP at 60 8CConductivity: up to 5.2�10�2 S cm�1

Electrochemical window: about 3 V


Eutectic point at 12 8C, which is lower than choline chloride

(Mp = 302 8C) and urea (Mp = 133 8C)

Carboxylic acids Dependent upon the number of acid functionalities [79]

Li+ NTf2�

Eutectic for 4.8:1 molar ratio [126]

Eutectic points at �37.6 8CConductivity of urea/LiNTf2(3.6:1) is 2.3�10�4 S/cm at 25 8C

Acetamide Li+ NTf2� Eutectic for 4:1molar ratio [127]

Eutectic points at �67 8CThe acetamide/LiNTf2 is a liquid at room temperature

between the molar ratio of 2:1 and 6:1


Liquid at 50 8CViscosity: 69.2 cP

Conductivity: 5.3 S cm�1


2.3. Latest advances in the preparation and purification of ILs

2.3.1. The different ways of ILs preparations

Different routes for the synthesis of ILs are described, each ofthem presenting advantages and drawbacks. They can besummarised as follows (Scheme 18):

(1) Metathetic exchange of anion (path A): This is probably the mostused pathway for the synthesis of ILs. The production ofalkylimidazole is industrial. The metathetic exchange of anionsoften produces halide by-products (MX) which may be difficultto eliminate by filtration especially for hydrophilic ILs.

(2) Neutralisation of base with Brønsted acids (path B) or direct

alkylation of alkylimidazole (path C): This route is interestingbecause it avoids the presence of halide (atom efficiency).However, in the case of the direct reaction of Brønsted acid(HX), it may be difficult to obtain ILs with high purity. Traces ofalkylimidazole or acid may be present in the final ILs. Thealkylation reaction is limited to the reactivity and availability ofthe alkylating agents. This method has been described for thepreparation of sulfate, phosphate or sulfonate based ILs.

(3) The carbonate route (path D): The use of dimethylcarbonate(DMC) as a clean methylating agent to replace alkyl halides hasproved to be a new interesting route to avoid the presence ofhalide and other by-products [137]. However this method is

limited by the availability of the acid (HX) or [NH4]+ salts. TheseILs have been produced on an industrial scale by Proionics/BASF (Proionics is a PME specialised in the synthesis of ILs fromcarbonate intermediates).

Synthesis of ILs using non conventional activation method(microwaves or ultrasounds) has also been described [138].However, ILs can decompose under sono-chemical conditions[139]. Recently [BMI][BF4] has been synthesised with a yield of 87%by a rapid one-pot solvent-free synthesis in a batch-mode reactorusing a microwave irradiation (frequency of 5.8 GHz) [140]. A newenvironmentally benign process for the production of [EMI][OH] insolution has been reported by an electrodialysis set-up [141]. Thisprocess could be used for the purification of ‘‘spent’’ ILs. It is worthemphasising here that N,N-dialkylimidazolium hydroxide ILs arenot stable as pure compounds or when they are concentrated inaqueous solution. Formation of N-heterocyclic carbenes mayoccur. They have to be kept in diluted solutions. A direct accessto anion-functionalised ILs consists in the one-step ring-openingreaction of sultones (Scheme 19). The reaction leads to theformation of zwitterions which have in general high melting points(e.g. R1 = Bu, R2 = H, mp = 158 8C). These zwitterions can then reactwith acids or LiNTf2 to generate new functionalised ILs [99].Interestingly, this mixture is liquid at room-temperature althoughvery viscous. It is also ion conductive. The reaction of sultones hasbeen extended to the reaction of the nucleophilic chloride anion of

Scheme 19. Direct access to functionalised ILs.

Scheme 18. General route for ILs synthesis: (path A) metathetic exchange of anion, (path B) neutralisation of base with Brønsted acids, (path C) direct alkylation of

alkylimidazole, (path D) the carbonate method.


an ionic liquid. However the stability of these ILs remains limited attemperatures lower than 150 8C [142].

2.3.2. Purification of ILs and analysis of trace impurities

It has very often been demonstrated that the physical andchemical properties of ILs and their catalytic activity can besignificantly influenced by the presence of small amounts ofimpurities. The ‘‘quality’’ of ILs has become an importantconsideration and purifications methods have been developed.Typical impurities come from the incomplete synthesis of ILs. Theycan be volatiles, alkylating agents, inorganic halides, proticimpurities, organic amines, water. Few purification proceduresare proposed in the literature. Recently, melt-crystallisation hasbeen described for ultra-purification of ILs and sample of 10 kgcapacity of [EMI][Cl] have been produced [143]. Decolourization ofILs has been reported using activated charcoal [144], or by treatingthe ILs with silica [145] or alumina [146]. However, it is also worthmentioning the possible contamination of ILs when treated withsolid sorbents [147,148]. Acid impurities can be difficult to removefrom hydrophilic ILs as water washing is not possible. Acidneutralisations by a column were reported [149]. Analysis andtraces analysis of impurities are still challenging and are field offundamental research. A number of analytical protocols with their

limits of detection (when available) have been compiled [150].Using the 1H NMR chemical shift of water taken as an NMRimpurity indicator, 1H NMR appears as a highly sensitive analyticalmethod for detecting Brønsted acid impurities [151]. Very lowconcentrations of halide (<20 ppm and down to �5 ppm), notcurrently measurable by ion chromatography, have been quanti-fied in a wide range of ILs, using a special automated microfluidicdevice designed for electrochemical studies [152].

As a recent another example, X-ray photoelectron spectroscopy(AR-XPS) has been proposed as a suitable method to determineimpurities down to an extremely low level if those impurities showat least some surface activity in the IL system [153,154].

3. Structure and self-organisation of ILs at the supramolecularlevel

3.1. Solvent properties and solvent effect

A solvent is generally characterized by macroscopic physicalconstants (‘‘bulk properties’’) such as vapour pressure, boilingpoint, density, cohesive pressure, relative permittivity er (‘‘dielec-tric constants’’), surface tension, refractive index. A large numberof studies have been devoted to the characterization of ILs ‘‘bulk’’

Scheme 20. Schematic representation of the different type of interactions present in imidazolium-based ILs.


physico-chemical properties such as viscosity, density, surfacetension. These data are available in reviews and databases and willnot be reported and discussed here in detail [48].

It is just worth emphasising here that the convenient methodfor determining dielectric constants fails in ILs because of theirhigh electrical conductivity. However, it could be measured for aseries of imidazolium-based ILs using microwave dielectricspectroscopy. ILs can be classified as moderately polar solvents.Dielectric constant e values are found in the range of 8.8–15.2,decreasing with increasing the length of the alkyl chain on theimidazolium cation, it varies little compared to the wide range ofvalues covered by molecular solvents [155–157]. The dielectricconstants were found to depend mainly on the nature of the ILsanions with the following trend [OTf]� > [BF4]� � [PF6]�. How-ever, the abundant different interactions acting together in ILsmake them very complex (Scheme 20), so that it is not surprisingthat a single physical parameter such as the dielectric constant isincapable of adequately modelling the solvent–solute interactions.This parameter has often failed in correlating solvents effectsqualitatively and quantitatively. An example is given by thesolvent effect study on nucleophilic reactions in ILs compared tomolecular solvents [158] where Hugues-Ingold viewpoint usingdielectric constant as a measure of solvent polarity, proved to beinadequate to describe the IL system. Nevertheless, for a rationaldesign and a better choice of ILs, better understanding of theirproperties is required.

Fig. 4. Normalised solvent polarity scale (ET(30) = 0.00 for Me4Si and ET(30) = 1.00

If we focus on conventional solvents, they are also characterizedby molecular-microscopic properties such as dipolarity/polariz-ability expressed as the Kamlet-Taft parameter (p*), polarity, H-bond donating acidity (HBD or Kamlet-Taft a parameter), H-bondaccepting basicity (HBA or Kamlet-Taft b parameter), electron pairdonor or acceptor forces, to name just a few. Solvatochromicbetaine such as zwitterionic betaine dye 2,6-diphenyl-4-(2,4,6-triphenylpyridinium-1-yl)phenolate, called standard betaine dye(N0 30), or Reichardt’s dye, has been used to establish UV/visspectroscopically a comprehensive set of empirical parameters ofsolvent polarity, called the ET(30) or ET

N scale [159,160]. This isprobably the scale that has been applied to the greatest number ofILs [58,161]. For the same IL, different significant values of ET(30)reported in literature can be found. Several reasons are responsiblefor these deviations, comprising the use of different solvatochro-mic probes. One of them is the presence of impurities in ILs,especially water, that can considerably alter the ET(30) values andchange the ‘‘polarity’’ of ILs. The ET(30) values for about 80 ILs havebeen reported. This work reveals that ionic liquids behave not assuperpolar solvents. The ET(30) values range fits quite well into theexisting empirical solvent polarity scale for molecular solvents. Ithas also been found that the polarity decreases with increasingtemperature, while it increases with increasing pressure. TheET(30) values of ILs are mainly controlled by the ability of the IL toact as a hydrogen bond donor (cation effect) moderated by itshydrogen bond acceptor ability (anion effect). The phenoxide

for H2O)—reproduced by permission of The Royal Society of Chemistry [58].

Scheme 21. Possible location of the anions (represented as dotted circles) with

respect of the [RMI] imidazolium cation.


oxygen of the betaine, which is anionic, acts as a good hydrogenbond acceptor. The result is a consequence of the existence of acompetition between the IL anion and the Reichardt’s dye solutefor the proton. The Reichardt’s dye scale thus emphasises the roleof the IL cations (Fig. 4). For example, ILs containing the 1,3-dialkylimidazolium cation can be divided into those containing aC(2)-H, which display a higher ET(30), and those substituted on theC(2) which act as weaker H-bond donors and then are less polar.This indicates that H-bonding occurs mainly through this C(2)-Hgroup.

The parameter of Kamlet, Abboud and Taft (KAF parameter: p*,a and b) and the Gutmann donor number (DN), all threedetermined UV/vis spectroscopically by means of carefullyselected solvatochromic reference compounds, could be beneficialto better understand the ILs solvent strength and treating themultiple interacting solvent effects. These values have beenreported in different papers for ILs [162] and for ILs/organic co-solvents mixtures [163] and more recently for a series of [BMI]+

based ILs with different anions [149]. The authors found anexcellent correlation between the a (measured values) and the b(values independently measured): the value of a significantlydecreases with increasing the H-bond accepting strength of the ILanion. Correlation of the 1H chemical shift of the proton in 2-position of the imidazolium ring with the a value has also beenestablished.

3.2. Structure and organisation

Coulombic interactions are the dominant interactions betweenthe ions. But a simplified picture of ILs just consideringelectrostatic interactions, as can be the case in molten salt (suchas in NaCl), would be very restrictive and not adequate to explainsome experimental results. Molecular interactions such as H-bonding, p–p stacking and other dispersive forces such as van derWaals interactions are also present. As an indication of the strengthof the different energies: the energy of H-bonds are generallyaround 40 kJ/mol (for water), the van der Waals forces around40 kJ/mol (for n-pentane), whereas coulombic interactions (ion/ion) in ILs can be up to 600 kJ/mol.

The relationship between the crystal and the liquid structure ofrepresentative ILs have been reported [164]. A wide range ofexperimental techniques have been used to investigate the liquidstructure of ILs (Neutron Diffraction, X-Ray scattering, EXAFS,NMR. just to name a few). A close correlation between the solidstructure and the liquid structure may be found. Probable locationsof the anion (such as Cl�) around the imidazolium cation have beenproposed with a higher probability for the position closer to theC(2) of the imidazolium ring (Cl� is a good H-bond acceptor).Larger anions such as [PF6]� or [NTf2]� are preferably located overthe centre of the imidazolium ring with in the case of [NTf2]�

increased delocalization of the charge in the anion and softer ionicbonding (Scheme 21). The chloride anion effectively has a highlycharged density, it is symmetrical, and forms a more regularnetwork than [NTf2]� which is highly disordered and can displaydifferent conformations with possible small cluster formation.[MMI][PF6] shows quite strong ordering in the liquid phase asfound for the chloride based IL, despite the difference in H-bondingability of these two ILs.

However, H-bonding between imidazolium cations and anionsis still the subject of controversial debates. It is widely agreed thatthis H-bond depend on the nature of the anions. The existence of H-bond can be crucial for understanding the solvation of transitionstates in chemical reactions (competition for the ions between theadded species and the counter-ion). One may think it is possible tocontrol the solvation capability of ILs by changing the nature of theanion [165]. The case of C(2)-alkyl imidazolium ILs is an interesting

example [166]. The [BMI] cation forms stronger H-bonds than thephosphonium cation with [Cl]� as demonstrated by the exothermof mixing imidazolium and phosphonium ILs [167].

The strength of anion–cation interaction inside the ILs([RMI]+ + [A]� $ [RMI][A]) has been investigated by ESI-MS (bythe abundance of fragments originating from C� � �A� � �C, with cationnamed C and anion named A). Two classes of ILs have beenproposed: those with tightly associated anion to the 1,3-dialkyli-midazolium cation such as: [CF3COO]�, [Br]�, [N(CN)2]� and [BF4]�

and those in which the anion is loosely interacting with theimidazolium cation such as [OTf]�, [PF6]�, [NTf2]�. Among the ILsinvestigated, [NTf2]� is the least interacting [168]. The weak [BMI]+

and [NTf2]� interaction may have an important impact on metalsolvation. 1,3-Dialkylimidazolium [NTf2]� ILs may be the solvents ofchoice in catalytic systems that involve a chloride dissociation stepwhile being less solvating than water. For example, the ionicstrength of interaction has also been estimated by ab initiocalculation of the dissociative energies of different [BMI]+ or[EMI]+ anion ion-pairs. For the [BMI]+ cation, it decreases in theorder [Cl]� > [BF4]� > [NTf2]�. For the [EMI]+ cation, the inter-molecular interaction energies of nine ion-pairs were studied.The calculated interaction energies follow the trend[CF3CO2]� > [BF4]� > [CF3SO3]� > [NTf2]� � [PF6]� (energies liebetween �78.4 and �89.8 kcal/mol) and the ectrostatic interactionis mainly responsible of the attraction. For the [BF4]� anion, theinteraction energies with pyrrolidinium or ammonium cation is notsignificantly different, demonstrating that the hydrogen bond withC(2) of the imidazolium cation is not essential for the attraction.Comparison with experimental ion conductivities show that themagnitude and directionality of the interaction energy between ionsplay also a crucial role in the dissociation and association dynamicsin ILs [49,169]. We have seen that for ILs containing the [EMI]+

cation, one of the strongest interactions is the C–H� � �anioninteraction via the proton in position 2. An interesting simpleway to determine the strength of this interaction is by 1H NMRmeasurements. To exclude disturbing influences on the ion–ioninteractions, all measurements must be carried out in neat ILs, in theabsence of other deuterated solvents. The 1H and 13C NMR shiftsobtained in that way show a good correlation with the calculatedion-pair stabilisation energies (Fig. 5).

It has been shown that the presence of a co-solvent canchange the ion-pair strength. PGSE (Pulsed-Gradient Spin-Echo)diffusion and HOESY NMR techniques have been applied forstudying inter-ionic interactions in ILs. For neat ILs such as[BMI][BF4] and [BMI][NTf2], the diffusion constants, D-coefficientvalues, are quite similar for the anion and the cation of the sameIL, and relatively small, but different for [BF4]� and [NTf2]� ILs.Addition of methanol, as a co-solvent, results in an increase of theD-values (also due to decreasing viscosity) up to a maximumwhich may correspond to the completely solvated salt. Themethanol tends to separate the ions. On the contrary, indichloromethane, the anions and cations show strong HOESYcontacts which suggests that the cation and the anion formstrong ion-pairs in this solvent [171].

Scheme 22. Enantioselective hydrogenation of N-(30-oxobutyl)-N-methylimidazolium (R)-camphorsulfonate. Effect of ion-pair interactions in the IL.

Fig. 5. Correlation between calculated ion-pair stabilisation energies and NMR-shifts for the proton and carbon at the C-2 position (reprinted with permission from [170].

Copyright 2009, American Chemical Society).


The most surprising effect of ion-pairing is probably demon-strated by the transfer of chirality in Michael type reaction(Scheme 22). The hydrogenation of N-(30-oxobutyl)-N-methyli-midazolium (R)-camphorsulfonate using heterogeneous Ru oncharcoal in ethanol at 60 8C and 60 bar of hydrogen gives thehydroxybutyl derivative with quantitative yield and enantioselec-tivities up to 80% ee. The correlation between the imidazoliumconcentration and the enantioselectivity highlights the impor-tance of ion-pair interactions. This chirality transfer approach canbe interesting, taking into account the large possibilities ofattaching substrates on imidazolium cations [172].

3.3. Toward a mesoscopic organisation

An understanding of the nano-structural organisation andinter-ionic interactions of ILs is also crucial to understand theirsolvent effect. The interaction of ILs with reactants, products,activated molecules or complexes is another important concernthat needs to be taken into account to understand the solvent effecton the outcome of the reaction. Different experimental andtheoretical methods have been reported to try to describe theseinteractions. Three-dimensional supramolecular polymeric net-works of cations and anions connected by H-bonds have beenseveral times evidenced (X-ray diffraction, NMR, neutron diffrac-tion) (Scheme 23) [173,174]. The ability of ILs to give supramo-lecules has also been suggested by gas phase mass spectroscopyexperiments.

It is now proposed that ILs present supramolecular structuralorganisation. Experimental and theoretical methods tend to

evidence the presence of aggregates in ILs. It is then difficult totransfer the models developed for molecular solvents that aredescribed as a continuum with the properties of a macroscopicphase. For example, the sole presence of interionic interactions inILs is not sufficient to explain all the features of OpticalHeterodyne-Detected-Raman Induced Kerr Effect Spectroscopy(OHD-RIKES OHD-RIKES) [175]. Spectra show that these interac-tions may be responsible for a nanostructural organisation with athree-dimensional ionic network and the presence of clusters ofalkyl chain into non-polar domains. A consequence of thisorganisation is the existence of inhomogeneities in densities ofILs evidenced by temperature dependence of Optical Kerr Effect(OKE) spectra as a function of the size of the anions. NOESY NMR[176] demonstrates the existence of inter and intramolecularcontacts in the case of [BMI][BF4], while only intramolecularcontacts are observed in the case of [NTf2]� ILs. NOESY experi-ments evidenced cation–cation interactions either as p-stacking oras perpendicular T-shape assembly. These interactions could beresponsible for the aggregations. The size of these aggregates hasbeen measured by Raman Scattering signals as being in the rangeof 10–100 nm. These aggregates increase with the alkyl chainlength of the cation. X-ray diffraction [177] confirms the existenceof organisation and heterogeneities in neat and supercooled ILs.

Padua et al. developed molecular force field [178,179] for themolecular simulation of ILs based on 1,3-dialkylimidazoliumcations and extended to trialkylimidazolium and alkoxycarbonylimidazolium cations [180]. Molecular simulations show evidenceof the aggregation of side alkyl chain of the cations in non-polardomains [181]. This confirms the presence of hydrophilic domains

Scheme 23. Representative scheme of extended three-dimensional network of H-

bonds in [EMI][Cl] ionic liquid.


that are formed by the head groups of cations and anions and non-

polar domains that are formed by alkyl chains on the imidazoliumcations. It can be seen as if the liquid structure of IL has largecavities. The behaviour of ILs in viscosity, diffusion coefficient, andionic conductivity, can now be attributed to the presence ofmicrodomains, specially the break in the trends observed foralkylmethylimidazolium cation having alkyl chain longer thanbutyl [182]. The heterogeneity of the ILs can also explain their dualbehaviour when they are used as stationary phases for gaschromatography (they can separate polar and as well as non polaralkane compounds) [183]. The question now is: could the structureof ILs affect catalytic reactions? should channels favour diffusion ofsmall molecules? This will be discussed in section 4. But, there isstill few data available in the literature.

3.4. Solute-ILs interactions: what impact on organic reactions?

Many studies have focused on the cation–anion interactions(solvent–solvent interactions) rather than ions–solute interactions(solvent–solute interactions or solvent solvation). In conventionalmedium, solvent–solute interactions are generally predominantwhile in ILs, interactions inside the solvent can become moreimportant. The Diels-Alder reaction is an interesting examplebecause it is a key step in many syntheses used to prepare cyclicstructures, and because the reaction performances in term reactionrate and selectivity have been widely studied and are highlysolvent dependent. In the case of the reaction of cyclopentadiene

Scheme 24. Nucleophilic SN2

and methyl acrylate, the ability of the IL to act as H-bond donor(cation effect) appeared to be a key criteria to explain theenhancement of reaction rate and endo-selectivity [184]. Thiseffect has to be moderated by the H-bond acceptor ability of the IL(anion effect). ILs with strong H-bond interaction between thecation and the anion (contact pair-ions) are poor solvents for Diels-Alder due to competition between the anion and the H-bondacceptor dienophile for H-bonding with the cation. It is notsurprising that low yields have been reported for dialkylimidazo-lium bromide and trifluoroacetate ionic liquids [185]. This H-bonding with the substrate can be influenced by p-stacking of theimidazolium cations and H-bonding interactions between thecation and the anion of the IL. It can be manifested either with theIR with the C(imidazolium)-H� � �A stretch which is around3126 cm�1 for [BMI][PF6] and 3060 cm�1 for [EMI][Cl] or by the1H NMR shift of the C(2)-H proton of the imidazolium. The highestendo:exo selectivity for cyclopendiene/methylacrylate reaction isobtained for the [NTf2]� series which displays the least strong H-bonding interaction between the cation and the anion. When thecation is functionalised with H-bond donor (e.g. hydroxy group),even better selectivity can be reached [186]. The rationalization ofthe solvent effect in the Diels-Alder reaction of cyclopentadienewith three different dienophiles (acrolein, methyl acrylate andacrylonitrile) has been reported using multiparameter linearsolvation energy relationships (LSER). This work provides evidencethat the reaction performances (reactivity and selectivity) aredependent on the solvent but also upon the nature of thedienophile. In the case of acrylonitrile, a non-carbonyl containingdienophile, the effects on selectivity are mainly influenced by thehydrogen bond acceptor ability of the solvent and other factorsthan the hydrogen bond donor ability of the solvent as it is the casein the carbonyl-containing dienophiles [187].

The nucleophilic substitution reactions provide another goodexample of model reaction to examine the IL effect. In molecularsolvents, the Hughes-Ingold qualitative model describes the solventeffect considering the pure electrostatic interactions between ionsor dipolar molecules in initial and transition state (solvent polarity).This model does not take into account the H-bonds interactions andproved to be limited to describe ILs effect. Kamlet-Taft linearsolvation energy relationship has also been utilised to describe ILseffect on nucleophilic reactions. The characteristic values ofa,b, andp* have been collected for ILs [165]. The solvent’s hydrogen donorability (a value) appears as the dominant effect in reducing thenucleophilicity of the nucleophile and slowing the reaction rate (e.g.reaction of amines with methyl-p-nitrobenzenesulfonate) [188].Hard and soft natures of the nucleophile proved to be also important,the [BMI]+ cation of the ILs acting as a hard ‘‘solvent’’ in interactingmore strongly with hard anions (e.g. [Cl]�) than soft ones (e.g [CN]�).For example, the nucleophilicity of halide anions (e.g. [Br]�) in theSN2 reactions of methyl-p-nitrobenzenesulfonate (Scheme 24) wasrather reduced in ILs relative to molecular solvents (the reaction is15 times slower in [BMP][NTf2] than in dichloromethane, and it isroughly 2 times slower in [BMI][NTf2] than in [BMP][NTf2]). This canbe ascribed to the existence of strong H-bond between thenucleophile (the anion, particularly the chloride) and the [BMI]+

cation. This difference arises largely from the a value (Table 3).Another systematic study on nucleophilicity of a series of anions onthe substitution reaction of methanesulfonic group was conductedin different ILs and compared with that obtained in organic solvents

substitution reactions.

Table 3Quantitative kinetic studies of nucleophilic SN2 substitution reactions of [Br]�with

methyl p-nitrobenzenesulfonate [188].

k (M�1 s�1) a b p*

[BMI][NTf2] 0.0195 0.617 0.243 0.984

[BMP][NTf2] 0.0296 0.427 0.252 0.954

CH2Cl2 0.460 0.042 �0.014 0.791


(PhCl, DMSO and MeOH). The results emphasise the predominantrole of water for hydrophilic anions ([Cl]�, [PhCO2]�). In this case, theinteraction of the anion with the imidazolium has a lower effect.These results confirm that water molecules creates H-bond with theimidazolium cation replacing the cation–anion interactions presentin the ILs [189]. Higher reactivity is also observed in C(2)-protectedimidazolium-based ILs.

The case of charged electrophiles within the framework of SN2reactions is also of fundamental interest since many catalyticcentres carry positive charges. The effect of b (H-bond acceptingability) is dominant: low b values lead to acceleration of reactionrate. This is the case for less basic [NTf2]� based ILs. However, nounexpected special effect of ILs was observed [158].

The reaction of chloride ion with an ionic electrophile(sulfonium associated with [NTf2]� or [OTf]�) was studied inseveral molecular solvents and ionic liquids (Scheme 25). Thenucleophilic substitution reaction does not take place in eitherstrong dissociating molecular water solvents or in methanol. Innon-dissociating solvent, the reaction is supposed to occur via asolvated ion pairs. The behaviour of this reaction in ILs isdifferent from that in molecular solvents, the kinetic experi-ments in ILs are in favour of reaction via dissociated ions. Theauthors conclude that ILs can be considered as super-dissociat-ing solvents, this effect arising from the fact that ILs are at thesame time liquid and ionic. The reaction mechanism wouldproceed through a true SN2 reaction of free solvated ions ratherthan with a ion-pair mechanism seen in molecular solvents[190].

In the case of the esterification reaction of methoxyacetic acidwith benzyl alcohol in ILs, it appears that the IL basicity,characterized by the b value, is the dominant parameter in theLSER, and determines the reaction rate. The best rates are obtainedin low basicity solvents. In other words, a high b value correspondsto a high proton affinity and thereby low proton availability to alow reaction rate, as observed. The IL plays the role of levelling ofprotic acids [191].

With these model reactions, we can see that the interactionsbetween ILs and different species can occur in diverse and complexways and can significantly modify their reactivity. In the chapterbelow, we will discuss in more detail the type of interactions of ILswith selected solutes such as water and aromatic hydrocarbons.

3.4.1. Interaction with water

The role of water (or other substrates) in modifying IL propertieshas been a central focus of interest these last years [41,182,192]. Butthis is still controversial. The IL effect has been illustrated by theexamination of water solvation at low and high concentrations. At

Scheme 25. Reaction of chloride ions with ionic

low concentrations, spectroscopic measurements such as IR [193]and dielectric constant [194] provide evidence that water ismolecularly dispersed in 1-alkyl-3-methyl imidazolium-based ILs([H2O < 2 M]. When the water concentration is increased, smallwater aggregates form which lead to the formation of a well-definedwater hydrogen-bonds network [194,195]. The detailed nature ofwater interactions with highly diluted in 1-alkyl-3-methyl imida-zolium-based ILs with [BF4]� and [PF6]� anions has recently beeninvestigated combining vibrational spectroscopy based on IRabsorption, Raman scattering and DFT calculations [196]. It is foundthat the local organisation between ions precludes any specificinteractions between water and the proton of the imidazoliumcation. Water would be doubly hydrogen-bonded with two anions,in symmetric 2:1 [Anion� � �H–O–H� � �Anion] structures.

PCl3 and POCl3 show unexpectedly high hydrolytic stability inwet ILs. For example, in the [NTf2]� based ILs, PCl3 was soluble atconcentrations up to 0.20 M and in [BMP][NTf2] was found to behydrolytically stable for weeks, even when stirred in air andwithout drying the IL. The ability of even wet ILs to stabilisehydrolytically unstable solutes may be understood by consideringthe interaction of water in the IL. The nucleophilicity of water, andtherefore its hydrolysis activity, can be reduced due to itsinteraction with IL anions. In hydrophilic ILs, the higher watercontent results in higher rate of hydrolysis [197]. Similarstabilisation of reagents with respect to hydrolysis has also beendescribed in catalytic reactions in ILs [198,199].

Interestingly, ILs have been used to improve the solubility ofhydrophobic compounds in water. For example, the solubility ofacetophenone in aqueous solution can be increased by a factor of10 by addition of [BMI][BF4] (the same effect is observed for[MMI][MeSO4], which can be important for application in bio-catalysis, for example. This phenomenon can be explained by theability of ILs to form small aggregates which are solventdependent. ILs can behave as hydrotropes [200].

3.4.2. Interaction with aromatic hydrocarbon. Clathrate behaviour

Aromatic hydrocarbons show unusual high solubility in ILs incomparison to aliphatic compounds. This solubility decreases withan increase of the molecular weight of the hydrocarbon but thedifferences of solubilities of o-, m- and p-xylenes are not significant.It has been reported that imidazolium-based ILs can form liquidclathrates in presence of aromatic hydrocarbons [201]. Dialkylimi-dazolium cations are able to form specific and oriented interactionswith arenes (and chloroalkanes). For example, in the salt crystal[BMI][PF6], 0.5 benzene, a three-dimensional network has beenobserved with H-bonds between anion and cation. This results in theformation of channels containing the benzene molecules. Shortinteractions between methyl hydrogen of the cation and aromatichydrogen are present [202]. Interactions between p-aromaticsystems and inorganic cations (Li+, Na+, K+, or Ag+) or organiccations (ammonium) are already well-known as the ‘‘p-cationinteraction’’, important in biochemistry, and experimentally evi-denced [203]. A detailed study conducted by NMR (ROESYexperiments) and by molecular simulation shows difference ofinteraction of toluene with the ILs as a function of the substitution ofthe C(2) of the imidazolium cation. In the case of the [BMI] cation,

sulfonium electrophile (A� = NTf2� or TfO�).


toluene is located closer to the methyl group at the end of the butylchain, whereas in the case of [BMMI]+ cation, toluene was closer toC(2)-Me of the imidazolium. The H-bonding association between[BMI]+ and the [NTf2]� is too strong to be cleaved by toluene. In thecase of C(2)-Me cation, the less strongly bonded IL network renderspossible the penetration and interaction of toluene [204].

3.4.3. Interaction with chiral substrates: induction of chirality?

Some Chiral ILs have been designed and synthesised, they havealready been applied in different fields such as asymmetric synthesis(see reviews), stereoselective polymerization, chiral chromatogra-phy, liquid crystals, chiral resolution and as NMR shift reagents.Chiral solvents have been reported in asymmetric syntheses.However, low enantioselectivities are most of time obtained. Inthe Baylis-Hillman reaction of benzaldehyde and methyl acrylate inpresence of a base, chiral ILs (Scheme 26) demonstrate their ability inthe transfer of chirality, even if the enantiomeric excesses are stillmoderate. The presence of an alcohol function on the N-alkyl-N-methylephedrinium is primordial and acts as a fixing point of the CILon the reactants. It is indeed assumed that the OH group is connectedwith a carbonyl function of the substrate (from either benzaldehydeor methyl acrylate) via H-bonding. However, with N-methylephe-drine, very low ee are obtained which also shows that theammonium group plays a crucial role in the chirality induction.Even if not directly demonstrated, it seems that the key of effectiveasymmetric induction is the existence of both strong intermolecularinteractions, like electrostatic attraction and hydrogen bonding,between ionic solvents and intermediates or transition states of thediastereoselective reaction step. The need of H-bonding in thetransfer of chirality has also been confirmed in the case of boratebased CIL bearing maleic acid functions. In this latter case, byincorporating the acidic centre into the chiral anion of the solvent,the IL offers the possibility of establishing a bifunctional interaction,which allows monofunctional achiral nucleophiles to be used ascatalysts [205].

3.4.4. Interaction with acid and base: toward new scale of acido-

basicity

The interest of ILs as solvents to perform acid–base reactionshas been recently increasing. A simple way to generate andmodulate the acidity is to add a Brønsted acid into the ionic liquid.In that case a new scale of acidity can be obtained either by varyingthe acid concentration in the IL or by changing the nature of theionic liquid [206]. If quite a lot of acid-catalysed reactions havebeen reported in ILs, very few studies have been devoted to thequantification of the acidity level of the proton in these media.Nevertheless, as the acidity of protons is mainly determined bytheir solvation state, the properties of protons will depend stronglyon the nature of the IL and the nature and concentration of the acid.A first apparent relative estimation of the proton acidity level hasbeen reported using the determination of the Hammett acidityfunctions, by UV–vis spectroscopy [207]. For a similar content ofadded strong acid, the anion of ILs plays a fundamental role; theacidity levels are in the order: [PF6]� > [BF4]� > [NTf2]� > [OTf]�

thus implying that the solvating power (or basicity) of the anionsfollows the reverse order. The presence of basic impurities in the

Scheme 26. Baylis-Hillman in N-alkyl-N-methylephedrinium. First example of transfe

enantiomeric excess have been obtained.

ILs can also have a dramatic effect. Even if absolute acidities cannotbe determined with this method, global acidity must be (much)higher than that observed in water.

3.5. Molecular modelling

Because of the potential number of cation–anion combinations,experimental investigations of all ILs properties are very difficult,quite impossible. A molecular-based understanding of theirproperties is important for their rational use. If moleculardynamics (MD) calculations have been developed for inorganicmelts, liquid salts based on organic ions were not so extensivelyexplored. ILs are indeed not simple fluids. The unusual complexityof intra and intermolecular interactions in ILs renders interpreta-tions very difficult and gives rise to controversies speculations.Computer simulations have played an important role in theprediction of physico-chemical properties of ILs starting from themolecular structure of ions. The structure-properties relationship,called QSPR (Quantitative Structure-Property Relationship), drivesmost of theoretical studies. Many properties, useful for catalyticapplications, have been predicted requiring a combination ofseveral theoretical models and approaches (IL melting point [208];solubility and partition coefficient of organic solutes in ILs [209];viscosity [210]; surface tension [211]; ionic conductivity [49].

Cation–anion interactions can be obtained using quantumchemical calculations in gas phase. This calculation gives details onion-pair stability, intramolecular geometry, and orientation of ionsin the pair and allows a better understanding of H-bonds andcharge transfer between ions. One limitation is the size ofdialkylimidazolium cations. Different correlations could beobtained between the energy of ion pairs (ion-pair associationenergy) and the different structure of ILs and their melting points[212]. Based on DFT calculations, the high viscosity and the lowpressure of ILs could be rationalized with the location of the anionand the possible H-bonds with the dialkylimidazolium cation. Thetransport properties in the liquid are affected by the presence ofthe anion which influences the barrier for rotation of the alkyl (e.gbutyl) present on the imidazolium cation.

Ab initio techniques raise some issues: the poor performance ofDFT in dealing with systems bearing delocalized charge; theproblem associated with the calculation of a meaningful chargeon each atom in a delocalized molecular ion. Ab initio moleculardynamics, which combines electronic structure calculations withconventional MD (called AIMD), is the only technique able to predictthe intermolecular structure of ILs (under specific thermodynamicconditions). However, a major limitation of such simulation is thelength and time scale that can reasonably be explored and the needfor extensive computational resources. Long range organisation ofILs cannot easily be explored with this method. These studies haveessentially been performed on dialkylimidazolium associated withhalide or [PF6]� anions. They clearly identify the existence of H-bondbetween C(2)-H and the chloride anion [213]. The significance of thesimulation crucially depends on the quality of molecular force fieldused. Different groups tried to develop and refine force fields for ILs[214]. The growth of computational studies in ILs was driven by thedevelopment of force fields for a wide variety of ILs [215]. The

r of chirality. Because of the strong interactions between the IL and the reactants


validation with experimental results (Raman spectroscopy) wasmade in some cases. Force fields have been extended to othercations than imidazolium such as pyridinium, tetraalkylammo-nium, guanidinium, tetraalkylphosphonium. Atomistic MD simu-lations with empirical force fields are used to describe liquid statestructure and dynamic properties of ILs. For example, the localelectrostatic interaction of benzene with [MMI][Cl] or [MMI][PF6]IL was found to be one of the possible reasons for the highersolubility of aromatic hydrocarbons in ILs [216]. The interaction ofCO2 and with [BMI][PF6] could also be studied [217]. Monte Carlosimulations can be used to calculate solubility of gas, but thismethod remains limited due to the complexity of ILs [218]. Toevaluate mass transfer in IL biphasic systems, molecular dynamicshave been developed to study the interface involved in systemssuch as 1-hexene and ILs used in hydroformylation reactions[219]. The number of published data on the solvent-ILs equilibriais still limited although this information is of prime importancewhen ILs are involved in reactions and separation processes. TheCOSMO-RS thermodynamic model has been used to describe thesolvent-ILs systems. It has been able to evaluate the separationability of ILs for a given separation task such as ethanol–water orhexene–hexane separations [220].

An interesting properties of ILs is that they can exhibitnanostructural organisation. This long-range ordering can beascribed to the presence of the (long) alkyl chain on theimidazolium cation which can generate dominant van der Waalsinteractions. Evidence of this ordering was found by differentgroups and could be experimentally confirmed (X-ray scatteringexperiments) [177]. Coarse grain models (CG models) havetentatively been used but it remains challenging. All these methodsare computational time-consuming and need large experimentaldatabases. They are at the moment mainly developed on ‘‘simple’’ILs often taken as model such as [RMI][PF6] or [RMI][X] which arenot the ILs under study. It was recently found that the molecularvolume of ILs can be a useful and powerful tool to predict somefundamental physical properties of ILs such as melting points ordielectric constants. The molecular volume Vm of a salt has beendefined as the sum of the ionic volume of its constituents. Whennot described, the molecular volume of ions can be calculated (byquantum chemical calculations) with quite good accuracy.Correlations with ILs viscosity, conductivity and density havebeen established. This relationships are available only on pure ILs[221]. The modelling of the reactivity of ILs has been examined byDFT (and DFT/MM) calculation for a specific reaction (SN2intramolecular rearrangement). The energy barrier has beendescribed in ILs and compared to that calculated in otherenvironments. A simple model is proposed to explain the solventeffect [222].

In conclusion, we can consider ILs as a new class of solvents thechemical properties of which can be rationalized with multi-parameter linear solvation energy relationships and correlate withtheir effect on chemical reactions. But this approach is notsufficient. Imidazolium-based ILs display a pronounced self-organisation in the solid state as well as in the liquid phase andthen can also be regarded as ‘‘liquid supports’’ in which theintroduction of other molecules may occur with the formation ofinclusion-type compounds. We will try to illustrate below, withselected examples, how ILs can behave differently than organicconventional solvents.

4. How the ILs can affect the catalytic reactions pathway?

ILs proved to be very complex solvents. They can solvate polarand non-polar species, they can behave as polar or non-polarsolvents. Besides their ‘‘chemical’’ characteristics, their physicalproperties such as an elevated viscosity can affect the diffusion and

reduce reaction rates. The solubility of gas or the selectivesolubilisation of reactants relative to the products can also changethe reaction selectivity. The formation of primary reactionproducts can be favoured by their selective extraction from theIL catalytic phase in an organic upper phase. It is then difficult torationalize the IL effects on chemical and catalytic reactions.Nevertheless, it would be of primary importance to understandhow their physico-chemical properties can affect the outcome ofcatalytic reactions in order to be able to choose the best ILs for agiven reaction. Solvation properties, interactions with solutes,substrates, transition states, metal complexes, reactants, theircohesive pressure, their degree of organisation and their viscosityare all to be considered when ILs are used as solvents. To date, theILs’ effects have been best described and rationalized on chemicalreactions rather than on catalytic reactions involving transitionmetal complexes. We will see below that some effects of ILs are notexpected and then not under control. In some cases, the generationof the active catalyst has been dependent on the nature of the ILs.ILs can inhibit or promote the formation of the active species. Theycan also dramatically affect the outcome of reactions [223].

4.1. Some ‘‘unexpected’’ effects of ILs

4.1.1. Effect of ILs impurities

Water [224], halides, bases and metals are the most prevalentimpurities present in ILs. While water can be accumulated in ILs byabsorption of moisture (ILs are very hygroscopic), the otherimpurities mainly come from ILs mode of preparation. Theimpurities have been recognised as affecting both physical andchemical ILs properties [225]. Most of time, they have a poisoningeffect on transition metal catalysed reactions. For example,chloride anions present in [BMI][BF4] has been detected as acause of deactivation of the cluster [H4Ru4(h6-C6H6)][BF4]2 used aspre-catalyst in the hydrogenation of arenes [226]. In the case of 1-octene metathesis catalysed by ruthenium complexes (Grubbs orHoveyda type precursors), the purity of ILs proved to be veryimportant for the reproducibility of the results. An extensive studyshows that catalyst deactivation by impurities increases in theorder of water < halide < 1-methylimidazole, but no deactivationmechanism is described [227]. This result again underlines theimportance of the characterization of the ILs and the identificationof possible impurities.

4.1.2. Effect of water and acidic protons

Many papers describe the use of protonated imidazolium(Protic ionic liquids) as acid catalysts for organic synthesis. In thesesystems it is often not clearly identified if the system is completelyanhydrous. It is worth noting that H(OTf)2

� can be a stable acidwhich may be formed in the presence of water. This acid hasalready been described as stable [18,91]. Protic ILs based on alkylimidazolium cation have also been used as promotors (protonreservoir) for proton and metal-assisted catalytic reactions such asPd or Rh catalysed dimerisation of methyl acrylate and Rucatalysed ring closing metathesis N,N-diallyltosylamide. In bothcases, protons are known to enhance the reaction performances.The use of protic ILs leads to significant improvements both inactivity and selectivity. As the level of acidity of the N–H proton ofthe IL is very low, it is probable that the effect of the ILs is better dueto the presence of residual acid coming from the IL preparation.The acidity level of the proton, even present at very lowconcentration, can be exalted in the ILs [228].

Another interesting example of the effect of ILs is when chloridedissociation from the transition metal is a key step in themechanism of activation. This is the case of the hydrogenationreactions of arenes catalysed by ruthenium(II)-arene diphosphinecomplexes in biphasic aqueous systems [229]. The enthalpy of

Scheme 27. Dissociation of the chloride anion in [BMI][Cl].


interaction of the chloride anion with the 1-butyl-3-methylimi-dazolium has been estimated by variable-temperature 1H NMRmeasurements (Scheme 27).

In [BMI][OTf], the solvation enthalpy of the chloride[DHsolv = �46.2 kJ/mol], is about eight times lower than thesolvation enthalpy of chloride in water. The strong coordinationability of the chloride anion in 1-butyl-3-methylimidazolium ILscan be related to the low solvation enthalpy of chloride in suchionic liquids (the energy of interaction of the chloride with [BMI]+

is relatively weak: �15 kJ/mol). This can explain that chloridedissociation from a transition-metal complex can be thermo-dynamically disfavoured in ionic liquids and might be inhibited inthese solvents. An example is given by the catalytic activity ofcationic ruthenium(II)-arene diphosphine complexes in differentILs systems compared to water [230]. In ILs, without any water, theRu(II) precatalyst is inactive. Addition of water to the IL([BMI][OTf]:H2O 50:50) results in activation of the Ru complexand formation of an active hydride species (Scheme 28). Thisexample shows the crucial role that water can play in facilitatingthe solvation of the dissociated chloride.

Now in neat and dried ILs, the activation of the ruthenium(II)catalyst precursor occurs via an unexpected mechanism, which isdifferent from the activation mechanism that takes place aspreviously described in water or in water-ILs for the same reaction[231]. Both the cation and the anion of the IL display an importanteffect (promotor or inhibitor) on the rate of styrene hydrogenationwhich can be correlated with the differences in capability of the ILto solvate chloride.

4.1.3. Effect of bases

A key step in the synthesis of most ILs is the alkylation of 1-methylimidazole with the corresponding 1-haloalkane. ILs have

Scheme 28. Proposed mechanism of formation of the Ru active specie

been described as solvents able to increase amine basicity withrespect to conventional solvents [232]. Residual unreacted N-methylimidazole can remain in ILs and act as a base in promotingsome organic reactions [233]. For example, it has been observedthat bases such as L-proline or piperidine could be better basicorgano-catalysts for Michael additions in ILs than in dichloro-methane [234].

The presence of impurities may play a significant role onnanoparticle stability in ILs. Water or halides are the most cited.However, attention must also be paid to N-methylimidazole. Inrecent work, it has been demonstrated that when gold nanopar-ticles were synthesised in base free [BMI][PF6] via reduction ofHAuCl4 with NaBH4, aggregation of Au particles was observed aftera short period of time. On the contrary, when 1-methylimidazole ispresent, even at low concentration, particles are stabilised. PdAubimetallic nanoparticles stabilised in a similar way in IL have beenapplied to the hydrogenation of allyl alcohols. This result canexplain the discrepancies in the literature concerning the stabilityof nanoparticles. This also shows than highly pure ILs can in somecases be detrimental [235]. The lack of basic entity in phospho-nium based ILs, when associated with low nucleophilic anion suchas [NTf2]�, can prevent some base-mediated side reactions orinhibition, but these ILs present other limitations such as theirviscosity and long alkyl chain on the cation [236].

4.1.4. ILs as additives: surprising effect!

This effect has been described for hydroformylation reaction ofethyl vinyl acetate to yield ethyl lactate, the branched (noted b)product is the desirable isomer (l = linear isomer). In ILs superior b/l ratio can be obtained compared to the selectivity in toluene. Thisincreased selectivity is dependent on the level of IL in the reactionmedium, thus demonstrating a surprising effect of ILs. But moresurprisingly, this effect can be maintained when low level of ILs areblended with an organic solvent which offers a good compromisebetween rate enhancement and selectivity. The selectivity ismainly driven by the electronic properties of the ligand. Phosphiteligands give the best results in terms of activity and selectivity. Thiseffect of ILs is not really understood [237].

Another example of a nice effect of IL is given in theenantioselective rhodium-catalysed hydrogenation of dimethylitaconate and methyl N-acetamido acrylate using Binap typeligand in presence of chiral IL [238]. The combination of a racemicligand and a CIL either as reaction medium, or as additive gives

s for the hydrogenation of arene in water and in presence of ILs.

Scheme 29. Enantioselective Rh-Binap-catalysed hydrogenation. Scheme 31. Example of [NTf2]� coordination to Ti and Fe complexes through the O

and N atoms, respectively.


enantioselectivities identical to that obtained with enantiopureligand (Scheme 29). Even more, the association of CIL with theenantiomerically pure ligand leads to enhanced enantioselectiv-ities with an inverted absolute configuration in the productcompared to those obtained in organic solvents. Experimentsprovide evidence that the key role of the CIL is as a ‘‘chiralpoisoning’’ in blocking the catalytic cycle for one of the twoenantiomers of the catalyst.

Ionic liquids have been recognised as promising solvents forenzymatic reactions. Used as ‘‘pure solvents’’ they have been alsofound to completely inhibit cellulase from Trichoderma reesei

[239], laccase C from Trametes sp. [240] or lipase from Candida

Antarctica [241,242]. However used as additives (small percen-tages of ILs in an organic solvent) can efficiently improvebiocatalytic processes [243,244]. In a more recent paper, dopingthe reaction mixture with 1-methylimidazole or [BMI][BF4] resultsin both case in a notable improvement in lipase-catalysedtransesterification activity [245]. The effect of 1-methylimidazolecan be surprisingly compared to that of [BMI]+, but no rationalexplication can be drawn.

4.2. When ionic liquids are involved in the formation of metal

complexes

4.2.1. Complex formation involving anions

Most papers dealing with organometallic and ILs perceive ILs asbeing ‘‘chemically inert’’, assuming that the IL anions do notcoordinate with the metal. But when ‘‘stronger ligands’’ are absent,even weak coordinating anions can complex to metals. Examplesare now reported in which the supposed non-coordinating anions,such as [NTf2]�, bind indeed to the metal centre, even under mildconditions [246,247]. In Scheme 30, different binding mode of[NTf2]� are proposed: monodentate nitrogen or oxygen coordina-tion and/or bidentate oxygen–oxygen or nitrogen–oxygen coordi-

Scheme 30. Coordination modes of t

nation, the mode of coordination depending on the softness of themetal centre [248].

For example, the titanocene Cp2TiMe2 can be stabilised bycoordination to the [NTf2]�. It coordinates to two [NTf2]� anionstrough a monodentate metal-oxygen binding mode. Soft metalcentre are expected to prefer nitrogen coordination over oxygen.This is the case of iron complex in Scheme 31. Surprisingly, thereaction of YbI2 with the [PMP][NTf2] ionic liquid leads to theformation and isolation of [PMP]2[Yb(NTf2)4]. This can maybesometimes explain why inorganic compounds are soluble in ILs[249]. Metal complexes stabilised by the [NTf2]� anion arerendered significantly more electrophilic compared to analogoushalide species.

[NTf2]� can even be considered as nucleophilic anion when it isfound to be more reactive than [Br]� in heterolytic dediazonationreactions [250]. The coordinating ability of [NTf2]� anion could alsoexplain the inhibiting effect observed in the oligomerisation ofethylene catalysed by di-imine Ni(II) complexes activated withMAO. When only one equivalent of [BMI][NTf2] relative to theNi(II) complex is added, activity drops down significantly. Withaddition of 10 equivalents, the system is nearly inactive,demonstrating the poisoning effect of the ionic liquid on the Nicatalyst. However, by using 10 equivalents of a very weaklycoordinating anion such as [B(3,5-(CF3)2C6H3)4]� named [BArF]anion, the activity is less decreased. It can be assumed that theinteraction of the [BArF]� anion with the cationic nickel or with thealuminium centre of the activator is weaker than with the [NTf2]�

anion [251].In chloroaluminates, Ni-catalysed olefin oligomerisation is now

well understood. The influence of the chloroaluminate composi-tion on the overall mechanism of the catalysis has been establishedusing Raman spectroscopy. The resulting ionic liquids play the dualrole of solvents and nickel activator. The nature of the anionspresent in the chloroaluminate IL influences the activity of the

he [NTf2]� to metal centre (M).

Scheme 32.

Scheme 34. Possible formation of phosphonium salt by reaction of phosphine

ligand with pyridinium.


system. In presence of chloride anions, anionic Ni species areformed which proved to be completely inactive in presence ofaluminium alkylating agents (Scheme 32). On the contrary, acidicalkylchloroaluminates activate the Ni(II) precursor, the activity ofwhich being dependant on the composition of the IL (AlCl3 andEtAlCl2 to [BMI][Cl] molar ratio) [252].

Formation of anionic [PdX4][BMI]2 metal complexes are alsodescribed when PdCl2(COD) are put in [BMI][X] (X = Cl� or Br�).Chloride anions inhibit the Pd activity for the methoxycarbonyla-tion [253]. Wilkes reported that Cp2TiCl2 (Cp = cyclopentadienyl)associated with EtxAlCl3�x catalyses the polymerization ofethylene in ([EMI][Cl]/AlCl3) acidic chloroaluminate (the molarratio of AlCl3 to imidazolium chloride is greater than 1) [254].Analogous zirconium or hafnium complexes display no activity.The lack of activity of Zr and Hf complexes may be ascribed to thecomplexation of [AlCl4]� anion to the metal centre resulting in astrong M–Cl–Al bond precluding formation of the M–R activecatalyst (Cp2MCl2 + [Al2Cl7]� $ Cp2MCl(AlCl4) + [AlCl4]�). The M-Cl bond strength was shown to increase according to the order Ti-Cl < Zr-Cl < Hf-Cl. The coordination of [AlCl4]� on titanium is weakenough to allow its alkylation with the alkylaluminium derivativeand to form the suspected Cp2TiR(AlCl4) active species. Thisexample demonstrates the importance of the solvation propertiesof the anion.

Research on ILs with non-halogenated and non-coordinatinganions continues to be a field of investigation. New anions such as[Al(ORf)4]� (with Rf = perfluoroalkyl group) are now proposed butthey lead to relatively high transition temperature and room-temperature viscosity. However, it is interesting to note that thesalt [NBu4][Al(OC(H)(CF3)2)4] has a melting point of 42 8C but itdecomposes above 190 8C [255]. The salts [NBu4][B(C6F5)4] and[NBu4][B(C6H3(CF3)2)4] have also been reported both to be verystable [256,257].

4.2.2. Complex formation involving cations

A key of understanding of the properties of imidazolium salts isthe acidity of the C(2)-H. The presence of a base is not alwaysneeded for the formation of the corresponding N-heterocycliccarbenes [258]. It is worth noting that the protection of C(2) acidicposition is not necessarily sufficient to avoid carbene formation.Oxidative addition with 1,2,3-trialkylimidazolium salts has beenobserved with Pt(0) complexes at C(4) and C(5) positions [259]. Itis well known now that imidazolium cation may react with Pd

Scheme 33. Telomerisation of butadiene

complexes to form carbene-Pd that can be in some cases goodcatalysts for Heck or Suzuki reactions [260]. The in situ formationof mixed Pd phosphine/imidazolylidene Pd complexes has beendemonstrated, and these are active species. The formation of 1,3-dialkyl-2-arylimidazolium salts has also been observed and thesesalts can act as source of arene in the reaction. This shows thatboth oxidative addition and reductive elimination of dialkyl-2-arylimidazolium salts from and to the palladium can occur[261,262].

In some other cases, the formation of such carbenes can have adetrimental effect on the catalytic performances. An example isthe case of the telomerisation of butadiene catalysed by Pd(II)complex associated with TPP (TPP/Pd = 3) with MeOH as thetelogen [263]. The reaction leads to the formation of the 2-methoxy-octadiene isomers and to the butadiene dimers(Scheme 33).

In this reaction, the catalyst cycle is thought to pass through alow coordination Pd(0) intermediate, the general problem is theseparation of the catalyst and the formation of Pd black. In [BMI]+

or [EMI]+ based ILs, butadiene conversion is very low, surprisinglythere is no particle formation. It was established that the catalystdeactivation was attributed to stoichiometric reaction between thedialkylimidazolium salt and Pd. Addition of stoichiometricamounts of [BMI][NTf2] proved to be enough to deactivate thecatalytic system. In N-butylpyridinium or dialkylimidazoliumsubstituted in position C(2), the activity of the system wasrecovered. However in pyridinium salt, Pd black is formed and thesystem rapidly deactivated. This deactivation can be attributed tothe decoordination of phosphine ligand from Pd to form aphosphonium salt (Scheme 34).

The formation of metal-carbene can also occurs ‘‘in situ’’ by thedirect oxidative addition on electron rich Ni(0), Pd(0) or Pt(0)complexes [251,264,265]. In the case of ethylene dimerization

with Pd complex in [BMMI][NTf2].

Scheme 35. Oxidative addition of the imidazolium IL to Ni(0). Mechanism of catalyst deactivation.


catalysed by Ni(0) in [BMI]+ based ILs, active cationic [(carbene)-Ni(II)-H]+ catalyst can be generated without any added co-activator by the direct oxidative addition of the [BMI]+ to theNi(0). But deactivation occurs rapidly after insertion of ethylene bya reductive elimination process leading to the formation of 2-alkylated imidazolium salt (Scheme 35).

A direct synthesis of NHC-iridium from neat IL has been alsorecently reported [266]. The 1,3-dialkylimidazolium halides ILsinhibit the methoxycarbonylation of iodobenzene with all Pdprecursors used. However with C(2) substituted dialkylimidazo-lium chloride, the reaction proceeds in a similar way than in [BMI]+

based ILs containing the weaker coordinating anions [BF4]� or[PF6]�. A deactivation process has been proposed which involvesthe formation either of a Pd(bis-carbene) with phosphine free Pdprecursors, or the formation of a phosphonium salt if phosphinesare present with the Pd precursors used [253]. The formation of[BMI]2[PdCl4] salts in [BMI][Cl] or Pd(bis-carbene) can also preventthe nanoparticles to growth and limit activity for C–C Heckcoupling reactions. Other evidence of carbene formation is duringthe synthesis of nanoparticles of Ir(0) from [Ir(1,5-COD)(CH3CN)2][BF4] under D2 and in presence of [BMI][NTf2],acetone and proton sponge. H/D labelling and 2H NMR reveal theformation of N-heterocyclic carbene and its coordination with theIr(0) nanoparticles [267]. D/H exchange reactions at the imidazo-lium-d3 cation (80% at the C(2) position and 4% at the C(4) and C(5)have also been observed in hydroformylation reactions of 1-octenecatalysed by Rh(acac)(CO)2(xantphos phosphine ligand) in[BMI][NTf2] at 75 8C and under 5 atm of CO/H2. This resultsuggests the in situ formation of heterocyclic carbenes. Bothhydroformylation and H/D exchange are catalysed essentially by

Scheme 36. Isomerisation of pentenenitri

the Rh-Xanphos catalyst in the IL. The probability of the carbeneformation is enhanced by the presence of weak bases such asethanol [268].

Ni(0)phosphine catalysts have been investigated for theisomerisation of 2-methyl-3-butenenitrile under biphasic IL-organic solvent conditions (Scheme 36). As the active species issupposed to be charge free, charged phosphorous ligands arenecessary to anchor the nickel in the IL. Here again, conversionswere much higher with C(2)-methyl versus C(2)-H imidazoliumcation, attesting to the probable formation of a Ni-carbene less-active species in the case of on-methylated imidazolium [269].

Other evidence of the formation of N-heterocyclic carbenes noton the C(2) position but on the less acidic C(4) and C(5) positions ofthe imidazolium are described in hydrogenation reactions. Thisdefinitively shows that imidazolium-based ILs cannot be con-sidered as innocent solvents and that the use of protected C(2)imidazolium is not a guarantee that the carbene will not be formed[270]. In Baylis Hillman reaction, [BMI]+ cation was found to reactwith the aldehyde in the presence of a base (Scheme 37) [271]. Theadduct formation is dependent on the nature of the anion presentin the ILs, being more favoured in the case of the [Cl]� and [PF6]�

anion than with [NTf2]�.In conclusion, all these findings strongly show that ILs based on

imidazolium cations, the ones most employed, may act not only assolvent but also as reagent in forming N-heterocyclic carbeneswith many transition metal complexes. Therefore, it must benoted that, in several catalytic processes, the ‘‘effect of ILs’’compared to organic solvents can be explained by the possible insitu formation of NHC ligand and then new metal complexes withnew activity.

les catalysed by Ni complexes in ILs.

Scheme 37. Side reaction of the [BMI]+ cation with the aldehyde in presence of DABCO base.


4.3. ILs specially designed for catalysis

Can the changeover from conventional solvents to ILs lead tosignificant influence on, or even a modification of, the reactionmechanism? Most catalytic reactions have been evaluated in avery limited selection of ILs despite the potential huge number ofILs (including functionalised ILs). Until now, ILs have most beemployed on a trial and error basis rather than on the basis of awell-founded mechanistic understanding. The influence of ILs isnot always understood. They often have been designed to fulfill aspecial task. They even can have multi-functions in the catalysis,such as solvent, solvent and co-catalyst or catalyst, solvent andsupport, solvent and ligands.

4.3.1. Change in mechanism pathway by stabilisation of charged

transition state, active species or ligands

The non innocent role of ionic liquids can be exemplified withthe study of J. Mo et al. on the Pd-catalysed Heck arylation ofelectron-rich olefins [272]. This reaction has been performed inimidazolium ionic liquids with a wide range of aryl bromides andiodides (Scheme 38). Interestingly, the reaction proceeds veryefficiently in ionic liquids giving remarkable selectivities comparedwith conventional organic solvents (toluene, dioxane, acetonitrile).Under similar conditions, quantitative conversion were obtained in[BMI][BF4] to give quite exclusively the a-arylated branchedproduct (>99/1) whereas mixture of linear and branchedregioisomers were obtained in organic solvents (from 24/76 inDMAc to 86/14 in DMSO). The authors identified two reactionpathways involving neutral or ionic intermediates assuming thatthe ionic environment provided by the ionic liquid promotes thecationic pathway and the selective formation of branched product[273]. Though a great number of catalytic reactions have beenperformed in ILs, this is still one of the rare examples whichdemonstrates that the ILs are capable of altering the reactionpathway and then its selectivity.

Another typical example of the influence of the ionic liquid onthe outcome of chemical reactions is given by the study of thereaction of toluene and nitric acid [223]. Depending on the ionic

Scheme 39. Cation exchange for

Scheme 38. Pd-catalysed Heck arylation of bu

liquid used, nitration of toluene (in [BMI][OTf]), or oxidation oftoluene to benzoic acid (in [BMI][OMs]) or halogenation (in[BMI][Cl]) were observed. The influence of the IL anion was alsodemonstrated in the case of the isomerisation of 2-methyl-3-butenenitrile (2M3BN) into 3-pentenitrile catalysed by ‘‘Ni(COD)2/[Ph2P(C6H4SO3][Na]’’ [274]. In [BMMI][Cl], [BMMI][SnCl3],[BMMI][ZnCl3] or [BMMI][AlCl4] ILs, poor conversion and selec-tivity were obtained, while in [BMMI][PF6] or [BMMI][NTf2] goodperformances were achieved. It was demonstrated by solid state CPMAS 23Na NMR that an exchange between the sodium in[Ph2P(C6H4SO3][Na] and the cation of the IL took place and thatthis exchange was governed by the Hard and Soft Acid and Baseprinciple. ‘‘Hard’’ anions [A]� tend to preferentially associate withthe ‘‘hard’’ [Na]+ cation (Scheme 39). This reaction is driven to theright in presence of chloride or metallochloride based anions. Itwas further demonstrated that the [BMMI][Ph2P(C6H4SO3)]phosphine gave bad catalytic results.

ILs can also change the reaction selectivity by the promotion ofspecific ‘‘substrate–solvent’’ interactions. This behaviour wasobserved for the selective hydrogenation of a,b-unsaturatedaldehydes (cinnamaldehyde and citral) catalysed by supportedpalladium catalyst (Scheme 40) [275]. Depending on the ionicliquid nature and reaction conditions, large variations of selectiv-ities were observed. Under identical conditions, [BMI][PF6]produces hydrocinnamaldehyde with a selectivity of 100% whilewith [BMI][OTf] and [BMI][Ac] this selectivity goes down to 91%and 78%, respectively. The mechanism by which the ionic liquidworks may be associated with the strong interaction of thecarbonyl group with the ionic liquid [276,277]. This interactionleads to a protection of the carbonyl function, and then a selectivehydrogenation of the double bond. Both the hydrogen bond donorability and the nucleophilicity will contribute to any interactionwith the carbonyl. Proper choice of the IL, as well as reactionconditions are then very important.

Another example is the Ring Closing Metathesis (RCM) in ILsusing the cationic Ru allenylidene complex as catalyst precursor(Scheme 41). It was already shown that the catalytic transforma-tion of diallyltosylamide in organic solvents was very sensitive to

ionic ligand operating in ILs.

tyl vinyl ether by 4-bromobenzaldehyde.

Scheme 40. Selective hydrogenation of cinnamaldehyde in ILs.

Scheme 41. Metathesis of diallyltosylamine in ILs catalysed by cationic Ru-allenylidene.


the nature of the counter anion of the catalyst salt. The reactioncould give the N-tosyldihydropyrole 2 together with the cycloi-somerisation and isomerisation by-products 3 and 4 (Scheme 41).Therefore the catalytic transformation of the diallyltosylaminewas investigated with complexes [(p-cymene)RuCl(P-Cy3)55C55C55CPh2][X] in which the anion was varied (X = [PF6]�,[BF4]�, [OTf]�) in different [BMI]+ based ILs. One of the firstquestions that was asked when investigating catalysis in ILs was: isthe species that is actually present in ILs the same as the one thatwas originally used. These experiments demonstrate that thecatalytic RCM transformation depends on the nature of the counteranion of both the catalyst and the IL. This suggests that an anionexchange rapidly takes place and that the catalytic system involvesthe anion of the ionic liquid used rather than that of the initialruthenium precursor. It was found that the [BMI][OTf] IL affordsthe best results in selectivity for 2 as it was the case with triflateruthenium complex using toluene solvent [278].

To understand the IL’s effect, investigations were performed onthe model process of ligand susbtitution on a well-known Pt(II)complex in ILs, the entering molecule being thiourea named TU(Scheme 42). The reaction was followed using UV/vis spectroscopy.In general, as there is an increase in dipole moment in going fromthe reactant to the transition state, a decrease in solvent polarityresults in a decrease in the rate of the reaction. In general ILs, thesubstitution mechanism was found to follow an associativesubstitution similar to that in conventional organic solvents.However, ILs have a significant influence on the course of

Scheme 42. Competing coordination to the Pt centre between thiourea and IL anion

(TU = thiourea and ppp = terpyridine).

the reaction, which differs from that in conventional solvents.Slower substitutions are found in ILs: rate constant follows[EMI][NTf2] > [EMI][N(CN)2] > [EMI][OTf] � [EMI][EtSO4]. Thestudy provides evidence of the existence of interaction betweenthe anion of the IL and the positively charged Pt complex [279].

4.3.2. Solvent for non-charged catalysts

In contrast to charged organometallic species which can remainimmobilized in ILs without any change, the low affinity of neutralcatalytic species with ILs requires to change the catalyst or thecatalyst precursor to anchor it into the ILs. In these cases, the majorefforts have been dedicated to the modification of catalysts withthe aim of increasing their affinity for ILs without altering theirperformances. The addition of an ionic tag, cationic or anionic, onconventional ligands (chiral or achiral) has been largely developed.The development of ILs incorporating a functional group (ether,alcohol, nitrile.) can also offer new opportunities to be used as bothsolvents and ligands. The best example of this strategy is certainlygiven by the hydroformylation of olefins in ILs. Recent reviews byMagna [280] and Haumann et al. [14] describe in detail thedifferent approaches to immobilize the well-known, non charged,[HCo(CO)4] or [HRh(CO)4] active hydroformylation catalysts indifferent ILs. Besides the development of hydroformylation, the useof ILs as catalyst-carrying solvents to perform Ru-catalysedmetathesis reactions appear as an approach allowing both (i)the recycling of the Ru catalyst and (ii) the minimization of Rucontamination of products [23]. The Ru pre-catalyst is modified byintroducing an ionic tag on the ligand. Most reported examplesconcern the modification of the isopropoxystyrene of the Hoveydatype catalysts, although it is assumed to be the active leavingcarbene group. However, in most of the examples, high catalystloadings are employed (in general more than 2 mol% relative to thefeed) so that it is still difficult to estimate the real potential thesebiphasic IL systems.


4.3.3. Solvent/stabiliser for nanoparticles

ILs have received attention as alternative solvents and stabilisersfor nanomaterial synthesis, due to their general ease of synthesis,stability (nonflammable, thermally stable), and low vapour pres-sures [281,282]. Ionic liquids exhibit low interfacial tension thatallows them to adapt to the surrounding reaction media, and theirrelative solubility may be tuned by varying their anionic and cationiccomponents. The controlled and reproducible synthesis of definedand stable metal nanoparticles (NPs) is of high importance. The NPsynthesis in ILs have been described using different techniques. Thechemical reduction of metal salts or organometallic complexes arethe most extensively described while the decomposition (thermal oralternatively photochemical decomposition) of the di- and tri-nuclear metal carbonyls (Fe, Os, Ru) in [BMI]+ based ILs can also leadto uniform and very small-size nanoparticles [283].

ILs such as [BMI][PF6] and [BMI][BF4] have been usedparticularly to synthesise Rh, Ru, Ir, Pd and Ni nanoparticles withcontrol of size, near-monodispersity, and stabilisation. Usually inapplications involving nanoparticles in catalytic reactions, it isnecessary to use an additional stabiliser or a solid support material.Here ILs play a dual role of solvent and protective agent to avoidnanoparticle aggregation. It is assumed that the bulkiness of Ilimidazolium cations favour the electrosteric stabilisation ofnanoparticles, but no concrete information has been availableuntil now about their possible stabilisation role [284]. However, itis found that agglomeration can still occur with loss of catalyticactivity, indicating that IL-stabilisation alone can have somelimitations. A good balance between activity, stability andrecyclability of nanoparticles must be found for catalytic applica-tions. Different organic compounds, such as polyvinyl pyrrolidonepolymer (PVP) [285] or N-donor ligands such as phenanthroline[286] or 2,20-bipyridine [287,288], triazine or pyrazine derivatives[289] have proved their efficiency as protective agents of Pd (forphenanthroline) or Rh nanoparticles. The N-donor ligand stabilisedRh(0) nanoparticles have been successfully applied and recycled indifferent hydrogenation reactions involving ILs. Anion effects haveproven to be important. ILs have been put forward to increase thesolubility of PVP in ILs media, hydroxyl-functionalised (Scheme43). The combination of these hydroxyl-ILs with PVP has helpedstabilise Rh nanoparticles and provides effective and highly stablecatalytic systems for biphasic hydrogenation reactions of styrene.

Other functionalised groups (for instance nitrile, thiol, or ether)at the N-imidazolium side chains have also been described tostabilise metal(0) nanoparticles. An example is given with Au(0)nanoparticles which are characterized by their higher agglomera-tion tendencies. The stabilisation strength of the standard [BMI]+

based ILs seems to be insufficient. Ether functionalised ILs provedto be efficient stabilisers and prevent gold nanoparticles fromagglomeration. The surface-enhanced Raman scattering (SERS)was used as a tool to characterize the species adsorbed onnanostructured gold surfaces and discloses both the parallelcoordination mode of the imidazolium cation and its stabilisationrole. Surprisingly, no interaction could be detected between themethylsulfonate anions and the surface of nanoparticles [290].

Very interestingly, it has been recently demonstrated that ILscould impact the size of nanoparticles during their synthesis. Acorrelation between the size of nanoparticles generated by

Scheme 43. Examples of hydroxyl-functionalised ILs us

hydrogen reduction in ILs from organometallic complexes (Ni orRu) and the ‘‘self-organisation’’ of the IL has been established. Thedecomposition of Ru(COT)(COD) (COD = 1,5-cyclooctadiene;COT = 1,3,5-cyclooctatriene) under H2 was performed at differenttemperature (0–75 8C) with and without stirring in [BMI][NTf2]releasing cyclooctane. Nanoparticles with an extremely narrowsize distribution (ca. 1 nm) are obtained at lower T and larger sizeat 75 8C (ca. 2–2.5 nm) without stirring, suggesting that the ILslimit the crystal growth NPs.

The effect of the temperature on the size of RuNPs is theopposite in ILs to that normally observed in a THF–MeOH mixturein which low temperatures favour large particle size and,conversely, high temperatures favour small particle size. It isassumed that the segregation of Ru(0) occurs exclusively inside thecyclooctane pockets generated during synthesis; their sizedecreases with increasing temperature [291].

So far, the most widely used metal for catalysis in ILs ispalladium, with an increasing number of publications on Stille,Heck, Suzuki, Sonogashira-Higihara reactions [24]. The main issueswith palladium-catalysed reactions include difficulties in catalystrecycling, poor catalyst stability, decomposition to Pd black andloss of metal, product separation and isolation and post-reactionwork-up, to cite just a few. ILs could offer great advantages overconventional solvents. From a practical point of view, adding waterto the reaction mixture can lead to a triphasic system; the salt(base.HX), formed during the reaction could then be extracted intothe aqueous layer.

In Heck reactions, the use of ILs can be combined withmicrowave or ultrasonic irradiation to accelerate the reaction[292]. Another interesting example is the efficiency of the systembased on the functional ionic liquid [BMI][TPPMS] (TPPMS = di-phenyl(3-sulfonatophenyl) phosphine) together with [BMI][Ac],which acts as a base and a solvent, and PdCl2(CH3CN)2 in the Heckreaction of bromobenzene to ethyl cinnamate. The main advantageof this system is the synergic effect of imidazolium and the absenceof accumulation of salt as a by-product [293]. In Stille cross-coupling reactions, ILs based on nitrogen anions such as [N(CN)2]�

and [NTf2]� could ensure high catalytic efficiency and facilitateligandless reactions. Nitrile functionalised pyridinium cation couldimprove catalyst stability and reduce metal leaching [294].

It is difficult to draw succinct conclusions on the impact of ILson C–C coupling reactions with palladium catalysts, as the effect ofILs depends on the Pd precursor used, the presence and nature of abase, and the reactants. It is worth mentioning that ILs ofteninteract as a ligand in forming either anionic Pd(II) salt such as[PdX4]� in the case of halide based ILs, or in stabilising the Pd(0)nanoparticle formed in situ. It has been proposed that the ILs couldact as a ‘‘reservoir of catalytically active Pd species’’. It may behighly probable that the reaction proceeds through the oxidativeaddition of aryl halide on the Pd nanoparticle surface, and theoxidized Pd species thus formed are detached from the surface andenter the main catalytic cycle as a an active molecular species[295–297].

In fact, ILs may display multi-tasks. In the Suzuki reactioncatalysed by PdCl2, it was established that the hydroxyl-imidazolium functionalised ILs could facilitate the generation/stabilisation of the active species, favour the activation of the C–X

ed to solubilise PVP and stabilise Rh-nanoparticles.

Table 4Some examples of concepts developed for sulfur removal from hydrocarbon

streams.

Concept Added

reactant

Type of ILs Ref.

Photochemical oxidation H2O2 [BMI][PF6] [308]

Oxidative desulfurisation H2O2 Protic IL, [MI][BF4] [309]

Extractive desulfurisation No Phosphoric acid-

functionalised ILs

[310]

Extractive desulfurisation No Fe-containing ILs [311]

Alkylation of sulfur

derivative

Alkylating

agent

Non-cloroaluminate [312]


bond by H bonding with the X atom and could aid in the solvationof the salts generated during the reaction, thereby preventingcatalyst poisoning [298].

4.4. Ionic liquids as medium for ‘‘in situ’’ spectroscopic investigations

The analysis of catalytic systems directly in pure IL or in ILreaction mixtures, without the addition of a solvent, is ratherscarce although it may be very important to understand reactionmechanisms in these solvents and to characterize the activespecies. Some investigations have been made using mass spectro-scopy, infrared spectroscopy and NMR. However, ILs exhibit amultitude of signals throughout the whole spectral range ofprotons that precludes the use of selective pulses for completesuppression of the solvent signals. Very interestingly, usingDiffusion-Ordered NMR spectroscopy (DOSY), the IL solventsignals can be suppressed in NMR. The comparatively highviscosities of ILs can be used advantageously to separate thesolvent and the solute signals. Here, the slower moving moleculesare filtered. This method allows for complete removal of ILs solventsignals from the 1H NMR of solutes. RMN spectroscopy can be usedwithout requiring special preparation or deuteration of ILs. Thismay become a very useful tool for in situ studies of reactionsperformed in ILs [299]. This technique has been applied topalladium nanoparticle systems dispersed in ILs. Nanoparticlescannot be detected by NMR, but the determination of diffusioncoefficients of a solvent, such as methanol, and of ILs, and theirchanges in presence of the nanoparticles could give someinformation about nanoparticle organisation in ILs. For example,the decrease of IL diffusion coefficients in presence of thenanoparticles and a basic ligand compared with diffusioncoefficients obtained with ligand-free nanoparticles in ILs wasassumed to evidence for the presence of a Lewis base on themetallic surface [300]. Raman spectroscopy [252] and infraredspectroscopy were also shown to be interesting non-intrusivetechniques for in situ characterization of interactions betweencatalytic species and ILs. For example, solution of Wilkinsoncatalysts [HRh(PPh3)3] were investigated in [EMI][Ac] at differentconcentration levels. This study reveals the presence of stronginteractions of the [EMI]+ cation with the catalyst complex, whichwas assumed to be through H-bonding of the chloride ligand withthe C(2)-H of the imidazolium [301]. Infrared spectroscopy wasalso used to characterize, under variable CO/H2 pressures andtemperatures, the different catalytic intermediates involved in theCo-catalysed hydroformylation reaction. This study also empha-sises the potential of this technique to get better insight of reactionmechanisms in ILs [302]. Moreover, due to their very low vapourpressure, ILs enable the application of physical techniquestraditionally restricted to solid state chemistry. It is possible toinvestigate the interactions and behaviour of molecular, biologicaland macromolecular species in solution using physical andchemical methods that require special conditions such as high-vacuum and have been traditionally used only in solid statechemistry. The use of TEM to characterize nanoparticles directly inthe ILs without having to separate and precipitate the nanopar-ticles is another example.

4.5. Removing sulfur from refinery streams

In the near future, one may anticipate that the trend will bereinforced to produce sulfur-free gasoline and diesel. In gasolinehydrodesulfurisation (HDS) the challenge is to selectively convertalmost all the sulfur-bearing molecules while leaving the olefinicuntouched. In diesel, the main issue is the low reactivity of highlyaromatic sulfur species (thiophene, benzothiophene, dibenzothio-phene). Above and beyond the large efforts made to improve HDS

catalysts and processes, a great deal of work has been conducted ona variety of possible alternatives. These alternatives involveoxidative desulfurisation, adsorption, extraction, alkylation, orcomplexation. Among these methods, extractive desulfurisation(EDS) has been covered in many reports. Since the polarity ofaromatic sulfide compounds is close to that of sulfur free aromatichydrocarbons, the key to an EDS process is to find an extractantcapable of selectively removing the sulfur compounds withoutlosing a high volume of feed. In this context, ILs have been reportedas potentially interesting extractants [303]. Compared to mole-cular solvents, some ILs based on alkylphosphate or alkylsulfateanions have shown rather high extractability for sulfur derivatives(Table 4). For aromatic S-compounds, the desulfurisation ability ofILs is dominated by the cation [304]. But main issues still remainunsolved like the cross-solubility of hydrocarbons and limitedefficiency of ILs. The oxidation of sulfur is one way of improvingextraction selectivity [305]. Commercially available molybdiccompounds can be dissolved in ILs to oxidise S-compounds withH2O2 under moderate conditions while the ILs play both the role ofextractant and catalyst solvent [306]. Even though the use of ILsmay have some advantages (no need of H2), ILs do not appear tohave an edge over more traditional extractants [307].

5. Concepts for using ILs in homogeneous catalysis

Molecular catalysis is widely used in chemical industry as forexample in oxidation, metathesis, hydroformylation and carbo-nylation, hydrocyanation, oligomerisation. Some of these reactionshave no heterogeneous counterpart (hydroformylation, hydro-cyanation.). However despite its well-established advantages suchas, at least theoretically, using a single-site well-defined catalyst,high selectivity and activity compared to heterogeneous catalysis,it suffers from a serious drawback, the separation and recycling ofthe catalyst. Catalyst recovery in an active form suitable forrecycling is generally not feasible and the products may becontaminated with catalyst residues. This is all the more importantas molecular catalysts tend to become more structurally sophis-ticated. This situation often leads to expensive purificationprocedures which disagree with the development of moresustainable processes. Therefore, there is a need for systems thatcan combine the advantages of homogeneous catalysis withstraightforward separation, recovery and reuse of the catalyst. Thissituation is common to enzymatic, organometallic and organocatalysis. Different approaches have been employed to achieve thisgoal. The catalyst can be immobilized or contained in either a‘‘solid matrix’’ or in a ‘‘liquid phase’’ which forms a differentimmiscible phase with the reaction products. If gaseous reagentsare present, triphasic or multiphase mixtures may be encountered.But in the latter case, the key issue is the suitable choice of thecatalyst liquid phase. Many alternative non-conventional solventshave been developed in which it was possible to take advantage ofmolecular engineering to tailor polarity, viscosity, thermal stabilityand solubilising power [313,314]. Without being complete, one


can cite perfluorinated solvents, supercritical fluids and nonaqueous ionic liquids [315]. ILs have been used in variousimmobilization strategies: as ‘‘liquid supports’’ in multiphasecatalysis or in heterogeneous systems (SILP). In these differentuses, ILs have played specific and different roles. In this part of thereview, we will describe the different strategies and concepts of ILuse in catalytic applications, bearing in mind the possibility ofapplying these processes in a continuous mode on an industrialscale. We will evaluate the scope but also the limitations of thesedifferent approaches. We will see that some limitations canprovide opportunities for new developments or new chemistry.Rather than describing all the reactions performed in ILs, theauthors have preferred to describe selected examples.

5.1. Multiphasic IL systems

In homogeneous catalysis, the catalyst separation and recyclingis an important issue. The recycling can be operated by chemicaltransformation or by direct distillation, depending on the catalystand its stability. Its recycling can also be performed using a biphaseliquid/liquid system [316]. Initially developed for the aqueousbiphase system, this concept was further extended to other mediathan water including ILs. Most organic substrates generally do nothave sufficient solubility in the catalyst phase, particularly in water,to give practical reaction rates in catalytic applications, or in manycases there are incompatibilities between the catalyst and thesolvent. Thanks to the wide range of available ILs (cation–anioncombinations), one may find that ILs offer an attractive option toimprove the reactant’s solubility in the catalyst phase. In addition, itis often possible to find a biphasic IL/organic system for which thecatalyst is dissolved and immobilized in the IL. The ideal situation isobtained when the IL displays partial miscibility with the substratesand when the products have negligible miscibility with the IL (Fig. 6).Separation is then obtained by decantation which simplifies theprocess scheme and limits the risks of catalyst decomposition duringdistillation. This option can also provide opportunities for newchemistry, for example, by shifting equilibria through in situ

extraction or by improving selectivity for primary reaction productswhen there is a preferential solubility of one reactant in the catalystphase and then in situ extraction of reaction intermediates in theother phase. This can be a way to operate separative catalysis andprocess intensification. This improvement of selectivity has beenexemplified for transformations where consecutive reactions suchas olefin oligomerisation or selective diene hydrogenation need to beavoided.

5.1.1. Some challenges and opportunities of multiphasic systems

One of the main issues of the applicability of this concept is theimmobilization of the catalyst in the ionic liquid phase. When

Fig. 6. The IL-liquid/liquid-biphase concept (M = monomer, M-M = dimer, M-M-

M = trimer).

active catalysts are charged species (cationic or anionic), this goalcan be achieved without the need for specially designed ligandprovided that the active species remain charged during all thecatalytic cycle. However in some cases, cationic complexes couldexist in equilibrium with molecular species. For non chargedcatalysts, an efficient immobilization generally requires the use ofligands-bearing ionic tags. This adjustment of ligands to ILs canresult in a modification of the catalytic system performance whichis not always easy to anticipate (see Section 4 of this review).Furthermore, whatever the ionic ligand used, deactivation mayoccur by displacement of the ionic ligand with one of the reactants.This may result in a deactivation process due to leaching of theactive species in the organic phase. To prevent such metal loss,large amounts of ligand are used, which often has a detrimentaleffect on the reaction rate.

Multiphase systems involve not only chemical interactions ofthe catalyst with the solvent, but also main issues such assolubility, partition coefficient, mass transfer (viscosity of ILs isgenerally much higher than that of organic solvents) which can belimiting steps in kinetics and can define some thermodynamicconstraints. For example, the solubility of gases in ILs is animportant parameter for reactions involving gaseous reactants(hydrogenation, hydroformylation, oxidation.). Hydrogen [317],oxygen and carbon monoxide display, in general, very lowsolubility in ILs with the gas mole fraction of the order of 10�4

near ambient conditions [318].When the products are partially or totally miscible in the ionic

phase, such as aldehydes or alcohols in hydroformylation,separation is much more complicated. One advantageous optionmay be to perform the reaction in a single phase, thereby avoidingdiffusional limitation, and to separate the products in a further stepby extraction. We have recently applied this concept to the cobalt-catalysed hydroformylation of olefins associated with an originalcatalyst recycle [319]. In this work, the recycling of the catalyst isbased on equilibria between neutral and ionic species whendifferent reaction conditions (T, P) are applied. In the absence ofCO/H2 and under atmospheric pressure, the ‘‘Co2CO8/Pyridine’’system used is essentially present in its ionic forms [Co(Py-r)6][Co(CO)4]2 and [PyrH][Co(CO)4] [320,321]. While increasingpressure and temperature, [HCo(CO)4] is generated. Owing to itshigh solubility in heptane, one might expect it to be extracted inthe upper organic phase where it operates without any masstransport limitation. Reducing the operating conditions from100 bar CO/H2 and 130 8C to atmospheric CO/H2 pressure androom temperature decreases the stability of [HCo(CO)4]. The latterspecies tends to dimerize into [Co2(CO)8] or reacts with pyridine toform [PyH][Co(CO)4] by direct neutralisation. The [Co2(CO)8] dimercan then react with free pyridine to produce another ionic[Co(Pyr)6][Co(CO)4]2 species. Because of their high affinity for theionic medium, both these ionic species are extracted andimmobilized in the ionic phase. The products can then be separatedby decantation, thanks to the addition of a non solvent in thedecantation section. From a practical viewpoint, the addition of anon-solvent can result in cross-contamination, and it has to beseparated from the products in a supplementary step (distillation).More interestingly, unreacted organic reactants themselves (hereweakly polar olefins) can be recycled to the separation step and canbe used as the extractant co-solvent. The ionic liquid containingthe cobalt catalyst precursors can be recycled into the reactionsection and reactivated under CO/H2 pressure (Fig. 7).

5.1.2. Use of scCO2 as the transport vector for substrates and products

The use of a co-solvent poorly miscible with the IL (water ororganic solvent) to extract the reaction products diminishes theoverall simplification of the recycling and can go as far as causingcontaminating the IL. The use of supercritical fluids in particular

Fig. 9. Continuous hydrosilylation loop reactor (reproduced from [332] with

permission of Wiley).

Fig. 7. Cobalt-catalysed hydroformylation of olefins. A new concept for catalyst

recycle (1: reaction, 2: pressure/temperature decreasing, 3: separation section).

Fig. 8. Continuous flow homogeneous catalysis using a supercritical fluid–ionic

liquid biphasic system (reproduced by permission of the Royal Society of Chemistry

[323]).


CO2 (scCO2) proved to be an interesting alternative to commonorganic solvent because of its nontoxic nature, its recoverabilityand ease of separation. It has been experimentally demonstratedthat a wide variety of substrates could be removed from [BMI][PF6]with scCO2 [276]. ScCO2 dissolves quite well in ILs while ILs cannotdissolve in scCO2, which provides a means of ILs recycling withoutproduct contamination. Continuous-flow catalytic systems basedon the combination of IL [BMI][PF6] and scCO2 employed as thetransport vector for the substrates and products, was reported forthe first time for 1-octene Rh-catalysed hydroformylation usingionic phosphine [322]. In the continuous flow process, thesubstrates are transported into the reactor and the productsremoved using scCO2 (Fig. 8). But it was also discovered that highreaction rates could be obtained only for alkenes exhibiting goodsolubility in ILs and under very high pressure, to make scCO2 a goodsolvent for the products. The solubility of alkenes in ILs dependedconsiderably on the length of the alkyl chain on the IL’simidazolium cation. 1-octyl-3-methylimidazolium bis(trifluoro-methyl)sulfonamide [OMI][NTf2] was found to give optimumperformance when associated with [1-alkyl-3-methylimidazo-lium][Ph2P(3-C6H4SO3)] as catalyst ligand [323].

The concept of ILs/scCO2 was extended to other reactions suchas hydrogenation [324,325], hydrovinylation [326], as well as tobio-catalysed reactions [327]. Another recent example of con-tinuous reaction using ILs as a stationary reaction phase, and usingscCO2 as a mobile non reactive phase is the acylation of aromatichydrocarbons (such as anisole) catalysed with In(OTf)3, a ‘‘soft’’Lewis acid [328]. The [1-Butyl-4-methyl-pyridinium][NTf2] IL ischosen because of its negligible extraction in scCO2. One advantagebesides that of IL recycling and product extraction is that thepresence of compressed CO2 decreases the IL viscosity, which mayfacilitate mass transfer during catalysis. Another possible oppor-tunity associated with the use of CO2 is to bring about theseparation of water from hydrophilic ILs such as [BMI][BF4], byadjusting the pressure and temperature. However, the separationof water cannot be complete and the reaction of CO2 with water toform carbonate is probable and may induce a salting-out effectresponsible for aqueous phase separation. It has been experimen-tally shown that the addition of a salt such as Na2CO3 (0.28 g) inwater (1.25 mL) to the hydrophilic [BMI][BF4] IL (2.5 mL) leads to abiphasic system below 50 8C with the inorganic salt remainingpreferably in the aqueous layer. This is a way of removing the saltby-products generated in the Pd-catalysed Suzuki reactions [261].Other interesting ideas have been proposed for the recovery ofhydrophilic ILs from their mixture with water [84].

5.1.3. Demonstration of continuous catlytic performances

Batch-mode screening experiments are very often used toidentify the best combination of IL, catalyst precursor, ligand,operating conditions and product separation. However, dataconcerning the recycling IL catalyst phase, the lifetime of the

catalytic system (leaching aspect), the stability of the IL towardsfeedstock, products and process conditions, which are crucial forbiphasic system development, cannot be obtained with batch-mode tests. Continuous experiments must be designed to obtainthis information. Very few examples of continuous flow rateexperiments have been described in literature.

Continuous flow reactions in biphasic IL systems have also beendemonstrated in a well-stirred reactor (Section 6.1.1). One mightalso mention the reaction performed in loop reactor withintegrated IL separation (Fig. 9). This experimental tool has beenused at bench scale to demonstrate the feasibility of differentreactions performed in biphasic systems (for Ni-catalysed dimer-ization see [329]). The last example is the multiphasic hydro-silylation of olefins. This reaction has been covered in severalpublications, using ionic liquids as catalyst solvent. But theindustrial attractiveness was recently demonstrated for thehydrosilylation of 1-hexadecene with oligosiloxanes using pyr-idinium tetrafluoroborate as the IL. Average conversions of >86%(with K2PtCl4) and >82% (with PtCl2(PPh3)2) could be achieved[330,331]. Another example is given with the hydrosilylation ofallyl chloride with trichlorosilane to form trichloro(3-chloropro-pyl)silane. This industrial reaction is technically challengingbecause of the use of trichlorosilane and the formation ofundesirable side products such as tetrachlorosilane and propylenewhich can undergo consecutive reactions. This has been describedin continuous mode using a loop reactor, PtCl4 as Pt source, and[NTf2]� based ILs (i.e. [EMMI][NTf2]) due to its stability againstchlorosilane. The removal of reaction heat was achieved thanks to ahigh heat exchange surface to reactor volume ratio. In this system,the reactor is fully back-mixed and can be considered as acontinuous stirred-tank reactor (CSTR). The biphasic mixture (ILand reaction products and substrates) is circulated in the loop at a

Fig. 10. Preparation of acidic IL anchored on silica.


high flow rate. Decantation and separation of the products from theIL phase takes place in a gravity separator built in the loop. Usingthis reactor concept, it could be demonstrated that very goodimmobilization of Pt in IL could be achieved without any specificligand. The selectivity to the desirable product was stabilisedaround 62–71% for conversions ranging from 50 to 70% [332].

Numerous other examples of IL-liquid/liquid-biphase systemscan be described. Some of them are detailed in the first part of thisreview. For an exhaustive list, readers may refer to the recentreview by Parvulescu and Hardacre which covers this aspect verywell [8].

5.2. Supported ionic liquid phase system (SILP)

5.2.1. ILs supported on solid inorganic solid

In parallel to the development of ILs for biphasic liquid/liquidcatalysis, strategies for immobilizing ILs on a solid support havebeen addressed. This concept was developed, among other things,to minimize the amount of sometimes expensive ILs and to allowthe applicability of ILs in continuous-flow-operated fixed-bedprocessing.

Acidic chloroaluminates were first supported on solid inorganicmaterial. The immobilization of the IL consisted in the addition of apre-formed chloroaluminate IL to a previously dried and calcinatedsupport (500 8C for 3 h). The excess of IL on the support was theneliminated by extraction with dichloromethane [333]. Afterdrying, this material was applied for alkylation of aromatics(benzene, toluene, naphthalene and phenol) with 1-dodecene.Some catalyst deactivation occurred as the supported IL systemwas operated in continuous mode. This loss of conversion withtime was assumed to be the effect of heavy products which blockthe active sites of the solid material from further reaction. Analternative method for supporting acidic chloroaluminate ILs is tochemically bond the Lewis acid on an inorganic support alreadyfunctionalised with an ammonium, imidazolium or pyridiniumchloride moieties [334–336]. This approach can improve the

Fig. 11. Supported Ionic Liquid Phase (SILP) catalysis concept for olefin hydroformylation.

permission from [337]. Copyright 2009 American Chemical Society).

efficient immobilization of the IL especially when the catalyticreaction is performed in a liquid phase (Fig. 10).

In 2002, C. P. Mehnert at ExxonMobil developed quite a similarapproach for olefin hydroformylation [337]. The immobilizationstrategy involves a support material (silica gel) that is modifiedwith a monolayer of covalently anchored fragments of 1-n-butyl-3-[3-(triethoxysilanyl)propyl]-4,5-dihydroimidazolium. Treat-ment of this surface with additional ionic liquid results in theformation of a multiple layer of free ionic liquid which serves as thereaction phase in which the homogeneous catalyst is dissolved(Fig. 11). Batch 1-hexene hydroformylation experiments wereperformed using Rh(CO)2(acac) and ionic tagged ligand([TPPTS][Na], [TPPTS][BMI]) as catalyst precursor. For the differentsupported ionic liquid systems studied, increased reaction rateswere observed due to higher concentration of the active rhodiumspecies at the interface and the larger interface area of the solidsupport in comparison to the biphasic system. This work wasfurther extended to olefins hydrogenation with cationic rhodiumcatalyst [338,339]. The resulting catalysts exhibited high activityand outstanding stability. However, no solution was proposed forcatalyst regeneration.

Similar studies were conducted in continuous mode for the gas-phase hydroformylation of propene [340]. But in this case thesupported system was prepared by direct impregnation of aunmodified silica gel with a methanol solution containingRh(CO)2(acac), the ligand (sulfonated Xantphos, L/Rh = 10–20)and the ionic liquid ([BMI][n-C8H17OSO3]). The IL coatingconstitutes only a thin film which is confined to the surface ofthe solid by physisorption (Fig. 12). The performance of theoptimised catalytic system remained stable up to 5 h before adecrease in activity and selectivity was observed. This work waslater improved by a careful choice of both the IL, the ligand and theproperties of the support [14,341]. The Rh/SILP catalyst performedsimilarly to a homogeneous catalyst with demonstrated long-termstability. This long-term stability appeared to decrease when liquidsubstrates were used [14]. In that configuration, hydroformylation

The IL is confined on the surface of silica through covalent anchoring (reprinted with

Fig. 12. Schematic representation of SILP: the IL is confined on the solid support by

physisorption (example of the MeOH carbonylation)—reproduced from [343] by

permission of The Royal Society of chemistry.


activities drop due to the limited diffusion of H2 and CO into theliquid filled pores of the support and to the solubility of the IL in thereaction products which produced gradual leaching of both the ILand Rh catalyst. A concept using the combination of SILP and scCO2

was envisaged to circumvent these limitations [342]. Resultsclearly demonstrate that scCO2 improves the diffusion of both thesubstrate and CO/H2 within the supported ionic liquid. It alsopermits the continuous extraction of the by-products. The use ofSILP technology has been extended to several reactions and solidsupports such as methanol carbonylation catalysed by rhodiumcomplexes [343] or Friedel-Crafts alkylation [344]. Despite somedemonstrated advantages such as minimization of mass transferlimitations due to very short diffusion distances in the supportedionic liquid films, maximum utilisation of IL and catalyst, fixed bedtechnology applicability, it still suffers from some limitations. Tocite just a few, one may mention that substrates and productsshould preferably be gaseous, accumulation of heavy by-productsin the solid is possible and can lead to catalyst deactivation, no easysolutions for the regeneration of the solid catalyst bed areenvisioned.

5.2.2. ILs supported on hybrid organic–inorganic material

Hybrid organic–inorganic silica materials containing imidazo-lium and Si–C covalently bonded moiety have been synthesisedaccording to conventional sol–gel procedures in the presence ofsurfactant template and tetraethylorthosilicate [345]. These meso-structured materials have recently been used as supports toimmobilize transition metal complexes such as Pd for Suzuki cross-couplings with aryl bromide reactions. In the latter reaction, in situ

formation of NHC-stabilised nanoparticles are suspected [346].

5.2.3. ILs supported on organic polymers

The development of other supports than silica was alsoinvestigated with notably the use of a polystyrene functionalisedresins [347]. These supported ILs were synthesised from Merrifieldresins, with many opportunities for the variation of the linker

Fig. 13. Polystyrene suppor

length, the loading level of the IL portion and the nature of the ILanion (Fig. 13). The catalytic properties of this new material wereexamined through the study of nucleophilic fluorination andbromination of 2-(3-methanesulfonyloxypropyl)naphthalene as amodel compound. Best results were obtained with the PSIL(Polystyrene supported ionic liquids) presenting the longest linker(hexyl or dodecyl) with BF4

� as counteranion. The desiredfluoroalkane and bromoalkane were produced almost quantita-tively. The effect of the PSIL loading (mmol of IL per gram of PSresin) was also investigated. It appeared that large PSIL loadingproduces the best result for fluorination reaction (matrix effect)whereas for bromination reaction, the reverse tendency isobserved (both matrix and site isolation effects). Interestingly,no reaction occurred when PS with no IL portion is used.

Poly(ethylene glycol) (PEG) polymers have received risinginterest as a reusable solvent medium for organic synthesis andcatalytic process. The combination of PEG with ILs was realised bythe synthesis of PEG-functionalised ILs. These ILs have proved todisplay interesting chemical and physical properties [348]. Theirapplication in catalysis is not very developed but may offerinteresting opportunities [349]. PEG-supported ILs have also beenshown to be efficient media for catalytic reactions such as C–Ccoupling with Pd(OAc)2, thus providing a ligandless recyclablesystem [350].

5.3. Switchable polarity solvents

Some specially-designed solvents may reversibly switch theirpolarity from a low polarity form to a higher polarity form when atrigger is applied. In this concept, the modification of the solventpolarity induces solubility changes of products and/or catalysts,making their separation and purification feasible. For the conceptof ‘‘switchable solvents’’, the polarity change is performed byaddition of CO2 to a mixture of two liquid components, eitheramidines and an alcohol [351] or primary amine/amidine mixture[352] or secondary amines (without amidines) [120]. In both casesa carbamate salt forms which can then be assumed as an ionicliquid (Fig. 14).

This concept has been applied recently as a post-treatment stepin the alternating polymerization of cyclohexene oxide with CO2

[120]. The polymerization reaction is performed using [PPN]-N3 asco-catalyst and Cr(salen)Cl as catalyst under 35 bar of CO2 withoutsolvent (Scheme 44). After the polymerization section, the CO2 wasreleased and the polymer and the catalyst dissolved in the NHEtBu.When CO2 is bubbled through this new mixture, the secondaryamine is converted into the polar carbamate salts in which thepolymer precipitates. The colour of the solution indicates that thecatalyst remains mainly in the solution. Slight colouration of thepolymer is nevertheless observed but can be corrected by severalSPS cycles. Remarkably, the chromium catalyst can be recovered bydistillation of CO2 and the amine, and finally reused forpolymerization.

ted ionic liquids (PSIL).

Fig. 14. Switchable solvents.

Scheme 44. Alternating polymerization of cyclohexene oxide with CO2.


In an equivalent approach, Jessop et al. reported the poly-merization of styrene using DBU and 1-propanol as the SPS system[121]. In that case, CO2 and N2 at 1 bar were used as polymermiscibility and immiscibility triggering agents (Fig. 15). Interest-ingly, the molar ratio of DBU and 1-propanol was 1:2.5, the excessof 1-propanol reducing the viscosity of the polar form (75 � 7 cP)which then facilitates filtration of the polymer. This solvent can beused several times (4 cycles) with nevertheless the need for freshsolvent addition in order to compensate losses during filtration.

Recently a new class of one-component, thermally reversible,neutral to ionic liquid solvents were described [353]. Its structureis based on siloxylated amines which introduce weak Lewis acid

Scheme 45. Example of supporte

Fig. 15. Styrene polymerization in

functionality (Scheme 45). The ionic liquids produced under a CO2

atmosphere are reversed to their molecular precursors at moderatetemperatures (around 120 8C). This new switchable solvent systemwas applied to the recovery of alkanes from heavy crude oil. For theexample described, a mixture containing 50% wt of crude oil inTESA (triethoxysilylpropylamine) is used. The single-phase homo-geneous system obtained is then transformed by CO2 bubbling. Theviscosity increases as the carbamate ionic liquid forms. Centrifu-gation is then needed to separate the ‘‘purified crude oil’’ (topphase) from the IL containing the oil’s impurities (bottom phase).Heating the IL phase up to 120 8C regenerates the TESA to itsneutral form, which can then be recycled.

d switchable solvent system.

DBU/PrOH switchable solvent.

Fig. 16. Thermoregulated catalysis using perfluorinated solvent.


5.4. Thermoregulated ILs

Successfully introduced for biphasic catalysis using perfluori-nated solvents, the concept of thermoregulated catalysis has beenthe subject of many variations and extensions [354]. In the initialconcept, the catalyst is dissolved in a fluorous solvent where thesubstrates are not soluble if the reaction is maintained at roomtemperature. By heating the mixture, a single-phase homogeneoussystem is created making possible the formation of productswithout any mass transfer limitations. Finally, the separation of theproducts from the catalytic fluorous phase can be operated bycooling down the mixture (Fig. 16).

Thermoregulated IL-based systems were developed in analogyto this concept. The temperature-dependent reversible phaseseparation of ILs was exploited for this purpose. This phenomenoncan be obtained either for ‘‘ionic liquid-aqueous’’ system [355], or‘‘ionic liquid-organic liquid’’ systems [202,356]. In 2001, the study ofthe transition-metal-catalyse hydrogenation of a water-solublesubstrate was described [357]. The ionic liquid used ([OMI][BF4]),containing [Rh(h4-C7H8)(PPh3)2][BF4] catalyst forms a separatelayer to water containing 2-butyne-1,4-diol. Under reactionconditions (60 atm, 80 8C) a single phase forms. On cooling toroom temperature, two phases reform, with the ionic liquid phasecontaining the catalyst and the aqueous phase containing amixture of 2-butene-1,4-diol and butane-1,4-diol products thatcan be removed simply without catalyst contamination.

Olefin hydrosilylation was also described on the basis of thethermoregulated IL concept [358,359]. In this work, the Wilk-inson’s catalyst [RhCl(PPh3)3] was used in association with variousN-alkylpyridinium or N,N-dialkylimidazolium ILs (Scheme 46). Allthe ILs used were solid at room temperature. Before reaction, thecatalytic system is prepared by mixing the IL with [RhCl(PPh3)3] at

Scheme 46. Hydrosilylation of olefins with triethoxysilane (R55C6H5(styre

Scheme 47. Thermoregulated ionic liquid cata

100–120 8C. After cooling down to room temperature, a solid wasground up for use as the catalyst. Under hydrosilylation reactionconditions, the ‘‘Rh/IL’’ solid becomes a liquid and the reaction canbe conducted as a liquid–liquid biphasic system. After completionof the reaction, the solid ‘‘Rh/IL’’ reforms and can then be efficientlyseparated and recycled.

Recently, hydroformylation of 1-dodecene was also investi-gated [360]. In a classical IL biphasic system, the transformation of1-dodecene remains quite challenging as the solubility of 1-dodecene in ILs is low even under process conditions. In thiscontext, thermoregulated ILs can clearly bring some advantages. Inthe work of Tan et al., several ILs derived from quaternaryammonium alkylsulfonates with polyether chains were synthe-sised (Scheme 47). In combination with toluene and heptane, theseILs can form thermoregulated IL systems. For example, when thehydroformylation of 1-dodecene is conducted in ILPEG750, n-heptane and toluene, the phase containing ‘‘Rh/TPPTS’’ complexis immiscible with the upper organic phase at room temperature.The miscibility of the system is 108 8C. On heating the reactionmixture above this value, the system becomes monophasic and thereaction proceeds homogeneously. The conversion of 1-dodeceneis increased sharply if compared to reactions performed at 100 8C.The system switches back to two phases on cooling down to roomtemperature. Under optimum conditions, the conversion of 1-dodecene and yield of aldehyde are 99% and 97%, respectively. Inaddition, the catalyst could be easily separated from products byphase separation and efficiently recovered.

5.5. Phase transfer catalysis

One of the essential roles of classical phase transfer catalyst is todisplace an inorganic reagent from the aqueous phase into theorganic phase, thus enabling the organic substrate to react with thetransferred anion and form the product in the organic phasereaction. ILs can play this role with the unequivalent property ofbeing totally tunable for the targeted reaction. For example, Wanget al. described a [BMI][PF6]/water biphasic system for the phasetransfer epoxidation of electron-deficient a,b-unsaturated carbo-nyl compounds [6,361]. The reaction takes place in the presence ofNaOH as a base and hydrogen peroxide as an oxidant. The processcan be described following the mass transfer model shown inFig. 17. Under the mild conditions applied, the [BMI][PF6]/watersystem was more efficient than the traditional CH2Cl2/watersystem. By optimising the reaction conditions, including reactiontime, temperature, the amount of oxidant and sodium hydroxide,100% conversion and 98% selectivity could be achieved in theepoxidation of mesityl oxide.

ne), R55C4H9 (1-hexene), R55C6H13 (1-octene), R55C9H19 (1-undecene)).

lytic process for olefin hydroformylation.

Scheme 48. Enantioselective Michael addition of dimethyl malonate.

Fig. 17. Mass transfer model for epoxidation in [BMI][PF6]/water PTC system (Q+ = dialkylimidazolium cation; M = sodium; X� = hexafluorophosphate anion).


The influence of the IL’s nature was studied on the phasetransfer-catalysed enantioselective Michael addition [362]. Thisreaction is promoted by a quaternary derived ammonium salt fromquinine (Scheme 48). In terms of yield and enantiomeric excess(ee), the best results were obtained using the more hydrophobic IL,[BMI][PF6]. Surprisingly, the ee obtained in ILs derived withimidazolium cation (½a�26

D <0) were reversed with those obtainedin [N-butylpyridinium][BF4] or classical organic solvents(½a�26

D >0). The factor responsible for the reversal of enantioselec-tivity was ascribed to the nature of the IL cation as previouslydemonstrated for the enantioselectivities of lipase catalysedtransesterification [363].

New candidates for phase transfer catalysis were described in2006 [364]. They consist of fluorous quaternary phosphonium saltsbearing four ponytails. After being applied in a model phasetransfer reaction (picrate extraction) to define their ability astransfer agent, they were applied for the nucleophilic substitutionof alkyliodide (Fig. 18). Using only 10% mol of the phosphoniumsalt [(CF3(CF2)7(CH2)2)3(CF3(CF2)5(CH2)2)P]+[I]� as the phasetransfer agent, conversion of compound 1 can reach 95% whereasno reaction occurred in the absence of the phosphonium salt.

Fig. 18. Substitution of alkyliodide usin

6. Overview of industrial applications and economic issues

Since the early days of ionic liquids in electrochemistry, thescope of their applications has been extended to many domainsand is now much broader than assumed. Following thistremendous development associated with the commercial avail-ability of ILs, the industrial applicability of ILs rapidly appeared asan important aspect as demonstrated by the accelerating numberof patents associated with the keyword IL. The patents (andpublications) often describe numerous applications such ascatalysis with increased rates and yields, recovery of catalyticsystems, use of ILs as solvents that can reduce environmentalimpacts and that lead to more energy-efficient separation. ILsappeared as novel solutions to the chemical industry. However,despite these significant benefits, their translation into viableindustrial processes is far from being obvious and the industria-lization of IL technologies is rather slow, particularly in the field ofcatalysis. For the industrial use of ILs, some major issues must beaddressed such as IL synthesis scale-up, purity, stability, toxicity,recycling, disposal and price and may constitute barriers to ILprocess commercialisation (see Section 6.2). Several pilots or

g a phosphonium salt as catalyst.

Fig. 19. Dimersol + Difasol package reaction: (1) Dimersol reactor(s), (2)

Vaporisation–condensation, (3) Difasol reactor.


industrial processes using ILs were nevertheless publiclyannounced. It is probable that some other processes have beendeveloped but the information has not been made public. In recentliterature, good reviews describe these examples in a fully detailedmanner [11,365].

6.1. Selected examples of industrial/pilot scale applications of ILs

In this paper, we have essentially mentioned the applicationsdeveloped in relation with catalysis. A non exhaustive list is givenbelow. Examples in the fields of electrochemistry, energy andengineering fluids are listed in a summary table with references tothe more representative work.

6.1.1. Dimerization and oligomerisation of olefins: IL as solvent and

Ni-co-catalyst

6.1.1.1. The Difasol process (Axens). Since the mid-1970s, IFPdeveloped the DimersolTM process (Dimersol-E, -G, and -X) thatupgrades light olefins by dimerization (respectively, ethylene,propylene and butenes). The Dimersol-XTM produces mixtures oflow branched octenes which are good starting materials forisononanol production (intermediates in the plasticizer industry).For this homogeneous process, the reaction is operated without asolvent in a unique liquid phase using a Ziegler-type catalyst basedon nickel and activated with an alkyl-aluminium co-catalyst [366].The DifasolTM process can be considered as the biphasic analogueof the Dimersol-XTM process. The reaction takes place with thesame nickel catalyst precursor using chloroaluminate ionic liquidsas the solvent (Eq. (1)). When associated with a chloroalkylalu-minium activator like EtAlCl2, this mixture can react to form ionicliquids presenting mixed anions (Eq. (2)) [252,367]. The ionicliquid can then act as both solvent and co-catalyst (Eq. (3)). As theactivity of the nickel system depends on the Lewis acidity, anaccurate adjustment of the EtAlCl2/AlCl3 is required to optimise theefficiency of the catalytic system. The best results were obtainedfrom [BMI][Cl]/AlCl3/EtAlCl2 (1:1.2:0.11) mixtures.

½BMI�½Cl� ÐAlCl3½BMI�½AlCl4� Ð

AlCl3½BMI�½Al2Cl7� (1)

½AlCl4�� þEtAlCl2 $ ½EtAl2Cl6�� (2)

NiCl2þ ½BMI�½Et2Al2Cl5� $ ½Ni-Et�½EtAlCl3� þ ½BMI�½AlCl4� (3)

Thanks to its ionic nature, the Ni catalyst is dissolved and remainsimmobilized in the IL, without additional ligand, where thereaction products are poorly soluble. The reactant’s miscibilityremains adequate to ensure reaction. No catalytic activity occurs inthe organic phase. No co-miscibility was observed between theproducts and the ionic liquids, product separation could beoperated by simple decantation of the two phases. In order todemonstrate the recyclability and the life time of the catalyticsystem, a continuous flow pilot plant was operated using a well-stirred reactor followed by a decanter [368,369]. The experimentswere run with a representative industrial C4 Raffinate-2 cutcomposed of 70% butenes (27% of which is 1-butene) and 1.5%isobutene (the remaining being n-butane and isobutane). The testwas conducted continuously over a period of 5500 h after which itwas deliberately stopped. No additional fresh ionic liquid wasrequired during the test. No ionic liquid can be detected in theproducts. This continuous pilot test definitely demonstrated thestability of chloroaluminates under dimerization conditions. Themain advantage of biphasic DifasolTM lies in the easy productseparation that can be performed in a subsequent step. Theproduct separation by settling does not require heating and resultsin energy savings plus reduced catalyst consumption. Another

interesting result is the excellent activity obtained when dilutedfeedstock is used. With Dimersol-XTM technology, olefin conver-sion is highly dependent on its concentration in the feed. On thecontrary, DifasolTM performance is maintained over a wide range ofbutene concentrations, with the same catalyst consumption. TheDifasol reaction section and settling sections can ideally beintegrated as a finishing reaction section after a first homogeneousDimersol reactor (Fig. 19). This configuration is particularlyadapted for the treatment of diluted feed for which the DifasolTM

efficiency has been demonstrated. This arrangement ensures moreefficient overall catalyst utilisation and an increase in the yield ofoctenes by about 10 wt% [369].

The main DifasolTM benefits can be summarised by thefollowing main points:

� the overall yield in C8 octenes can be 10% higher than in thehomogeneous process;� the nickel consumption is less than in the homogeneous process;� no ionic liquid can be detected in the products;� a much smaller reactor, operated with biphasic system, can give

the same throughput of octenes.

The DifasolTM process has been extended to the selectivedimerization of propene. In that case, the addition of bulky andbasic phosphine ligands such as triisopropylphosphine or tricy-clohexylphosphine is necessary to drive the reaction to theselective formation of 2,3-dimethylbutenes (2,3-DMB-1 and 2,3-DMB-2). 2,3-Dimethylbutenes are especially important since theycan be used as key starting olefins for fine chemical intermediates[370–372]. The reaction can be performed in acidic chloroalumi-nates with Ni(II) salt, trialkylphosphine and alkylaluminium as co-catalyst. The phosphine effect can be maintained providing a rightadjustment of the ionic liquid acidity [373,374]. The reaction wascarried out in a semi-continuous way for more than 50 h [375,376].The main issue was to maintain constant the 2,3-DMB selectivityover time, because of a competition for the basic phosphinebetween the ‘‘soft’’ Ni catalyst and the ‘‘hard’’ aluminium chloridepotentially present in the acidic ionic liquid. Aromatic hydrocarbonproved to be ideal basic additives to prevent the loss of 2,3-DMBselectivity. Due to their poor basicity, they do not strongly interferewith the Ni active centre and do not decrease the catalytic activity.They can be considered as buffers, thereby stabilising the‘‘phosphine effect’’. In that way, the hexene selectivity wasmaintained constant around 75–80 wt% hexenes/total productsand the 2,3-DMB-1 was maintained at 70–75 wt% relative to thetotal hexene content.

6.1.1.2. Oligomerisation of olefins for synthetic lubricants productio-

n. Alpha-olefins may be oligomerised to prepare syntheticlubricating oil base stocks which have desirable lubricatingproperties such as a low pour point and a high viscosity index.In 1997, BP Chemicals described a process for the oligomerisationof a mixture of a-olefins (typically C6-C10) using acidic ionicliquids [377]. Typical ionic liquids used are based on [EMI][Cl]/

Scheme 49. LAB production.


AlCl3 in a 1:2 molar ratio. In that case, kinematic viscosity at 100 8Cis usually below 20 cSt. Some years later, Chevron disclosed that itis possible to obtain oligomers with higher kinematic viscosity byperforming the oligomerisation reaction in the absence of organicdiluent [378–380]. By this way, polyalphaolefins having viscositiesin excess of 22 cSt and even 30 cSt may be readily prepared.Starting from 1-decene and [HNMe3][Cl]/AlCl3 in a 1:2 molar ratio,kinematic viscosity was 31.6 cSt. According to a recent informa-tion, this process would be applied at a pilot scale to producecommercial quantities of over 450 tons of polyalphaolefins [365].

6.1.2. Friedel-Crafts alkylation and acylation of aromatic

hydrocarbons: IL as solvent and catalyst

6.1.2.1. LAB production. Alkylation of benzene with linear olefins(C10–C14) is a well established industrial application (Scheme 49).The linear alkylbenzenes produced (LABs) are used as intermedi-ates in the manufacture of surfactants and detergents. Traditionalprocesses use acid catalysts such as AlCl3 and HF which suffer fromboth poor catalyst separation and recycling.

Akzo-Nobel developed specific ILs based on triethylaminehydrochloride and aluminium chloride (2AlCl3 + [HN-Me3][Cl]$ [HNMe3][Al2Cl7]). These ILs can be cheaper alternativeto imidazolium-based salts and can be applied in a similar manner[381]. These ILs were specially applied to the alkylation of benzenewith 1-dodecene [382]. It appeared that higher 2-dodecylbenzeneyields were obtained in the IL (46%/other monoalkylbenzeneisomers) than with the conventional HF process. The linearity ofthe alkylbenzenes is indeed an important parameter for thebiodegradability of the compounds.

One of the points of interest of operating the reaction in ILs isthat alkylbenzenes are poorly miscible in ILs. The reactionproceeds in a biphasic mode, thus making catalyst recovery andrecycling easier. In traditional processes, consecutive polyalkyla-tion reactions may occur since the alkylated benzene hydro-carbons are more reactive than the monoalkylated startingmaterial. In the biphasic IL mode, consecutive polyalkylationreactions are disfavoured since the alkylated benzenes are lesssoluble in the catalytic phase than the monoalkylated benzenes.Analogous ILs were also evaluated supported on solids like silicaalumina or zirconia with improved activity and selectivity formonoalkylated products [333]. This reaction has also beenperformed using protic ionic liquids that are free of Lewis acidity,such as [BMI][HSO4]/H2SO4. The difficulty in that case is to find theright acidity level to prevent isomerisation of the olefin doublebond.

6.1.2.2. Ethylbenzene production. Chloroaluminate ionic liquidswere also investigated by BP as liquid acid catalyst for thealkylation of benzene with ethylene to produce ethylbenzene[383]. The manufacture of this bulk chemical compound is

industrially dominated by the use of AlCl3-red oil as the acidcatalyst (red oil is defined as a mixture of AlCl3 with polyalkylatesuch as diethylbenzene). It has been proven that the liquid red oilforms a biphasic mixture with the reaction products. Unfortu-nately, during reaction some of the AlCl3 is gradually lost in thereaction product which rapidly renders the system monophasicand makes the catalyst recycling very complicated and noteconomically viable. The search for biphasic alternatives led tothe use of chloroaluminate ionic liquids based on imidazoliumcation (ex: [EMI][Cl]/AlCl3 or [HNMe3][Cl]/AlCl3 in a 1:2 molarratio). In a very detailed study based on bench-scale experiments[383], BP demonstrated the potential of ILs which comparedfavourably with the industrial red oil. One of the main advantagesof the IL system remains the biphasic character of the mixturewhich facilitates the recycling of the catalyst by gravity separation.However, the overall outcome of the laboratory BP evaluation isthat the high cost of producing and using ionic liquid catalyst couldonly be offset if certain technical targets could be met, such as atleast equivalent or superior conversion activity and ethyl benzeneselectivity to homogeneous AlCl3, a deactivation rate < 15% percycle, a production cost of ionic liquid catalyst � <$5000/t.

6.1.2.3. Friedel-Crafts acylation of aromatic hydrocarbons. ILs havebeen largely described as catalyst-solvents or solvents to performacylation reactions [384]. The main advantages of using ILs is theenhanced reaction rates, conversion and much higher selectivity.Solvation properties of ILs, which can aid in solubilising thereactants, can allow a reduction of the number of steps insynthesis. The acylium cation [RCO]+ is the key intermediate inthese reactions as it is the case in conventional solvents. WhenLewis acidic ILs are used a stable complex is formed between Lewisacid and the carbonyl oxygen of the product so that an excess ofcatalyst is need to achieve complete reaction. Even if ILs havedemonstrated their benefits, the main issue remains the separationof products from the ILs and the IL recycle.

6.1.3. Alkylation of olefins with isobutane: IL as solvent

and acid catalyst

Isobutane alkylation is one of the most important processes forproducing reformulated gasoline. Commercial alkylation plantsuse either sulphuric acid or hydrogen fluoride as catalyst. Both ofthem suffer from some limitations when we consider productivity,alkylate quality, safety aspects and operating costs [385,386]. Theuse of solid catalyst as alternative is an interesting issue butproblems of catalysts deactivation and regeneration must besolved before industrial application. Very early in the developmentof ionic liquids in catalysis, IFP applied acidic chloroaluminates forthe alkylation of isobutane with 2-butene or ethylene [387]. It hasbeen demonstrated, in a continuous-flow pilot plant operation,that [pyridine,HCl]/AlCl3 (1:2 molar ratio) IL was the bestcandidate in the case of ethylene [386]. The reaction can be run

Table 5Ionikylation pilot plant alkylate vs HF and H2SO4 processesa.

Yield (wt%) Process

Ionikylation HF H2SO4

C5 0.6 2.5 8.8

C6 1.0 1.9 4.9

C7 2.0 2.9 3.9

C8 95.6 90.1 80.7

C9+ 0.8 2.6 5.7

C8 components

TMPs 89.6 80.9 71.6

2,2,3-TMP 0.1 1.6 2.3

2,2,4-TMP 51.6 49.7 31.1

2,3,3-TMP 18.1 10.8 19.8

2,3,4-TMP 19.8 18.8 18.4

DMHs 6.0 9.2 9.0

2,3-DMH 1.3 – –

2,4-DMH 3.4 – –

2,5-DMH 1.3 – –

3,4-DMH 0 – –

TMP:DMH ratio 14.9 8.8 8.0

RON 100.1 97.3 97.6

MON 95.0 95.2 94.4

a Reproduced from ref. [389].


at room temperature and provides good quality alkylate (2,3-dimethylbutane is the major product) over a period of 300 h(MON = 90–94; MON = 98–101). When butenes are used instead ofethylene, a lower temperature and a fine tuning of the IL’s acidityare required to avoid cracking reactions and heavy by-productformation. The continuous butene alkylation has been performedfor more than 500 h with no loss of activity and stable selectivity(80–90% isooctanes are obtained containing more than 90%trimethylpentanes TMP; MON = 90–95; RON = 95–98). At thattime, it had been demonstrated that the addition of copper(-I)chloride to the acidic chloroaluminate improves the reactionperformance [388]. Some years later, Petrochina developed aprocess called ‘‘Ionikylation’’ for the alkylation reaction [389]. Theionic liquid used consists of a mixture of a conventionalchloroaluminate-IL with CuCl. In this composite IL, large quantitiesof mixed anion like AlCl4CuCl� were detected [390]. It is theorised

Table 6Alkylate from commercial H2SO4 unit before and after retrofita.

H2SO4

Flow ra

Feed C4 180

Product Light alkylate 125.9

Heavy alkylate 10.8

Gas 41.7

Loss 1.4

Total 180

Octane rating

RON 95.0

MON 93.0

AntiKnock index 94.0

Distillation (8C)

Initial boiling point 45.0

10% 77.0

50% 101.5

90% 108.5

Final boiling point 132.0

Actual gum (mg/100 mL) 0.9

Doctor test Pass

Copper corrosion at 50 8C,3 h, grade 1

a Reproduced from ref. [389].

that the addition of cuprous chloride enhances the acidity of the ILand when used for alkylation reaction inhibits undesirable sidereactions such as isomerisation and cracking. Petrochina demon-strated the high stability of this catalyst through an 8-monthageing test before a 60-day pilot scale operation. The alkylationreaction is performed at 15 8C and 0.4 MPa. During the pilot testperiod, olefin conversion was more than 99%. The C8 yield inalkylate gasoline was higher than 95% and the yield of TMP was90% (Table 5). It is interesting to note that the non corrosive natureof the IL-catalyst allows for the use of less costly material for thedesign of the pilot (carbon steel reactors, piping, tanks, pumps andvalves). Petrochina announced in 2006 that an alkylation processhas been retrofitted into an existing 65,000-tonne/year H2SO4

alkylation unit in China. Alkylation from the ionikylation processcompares favourably to alkylate from H2SO4 (Table 6). Improve-ments in terms of alkylate yield, process unit capacity and soeconomics were demonstrated.

6.1.4. Chlorination and fluorination reactions

6.1.4.1. Chlorination. For safety reasons, the substitution ofphosgene for industrial chlorination is a subject of major concern.In 2005, BASF reported the chlorination of diols (ex: 1,4-butanediol, Scheme 50) using HCl as the chlorinated agent incombination with an IL [391]. Compared with direct chlorinationof diol with HCl, the IL system produces almost pure 1,4-dichlorobutane (98% selectivity). The presence of IL in the reactionmixture seems to improve the HCl reactivity. Furthermore, whilethe reaction proceeds, the initial homogeneous single phasesystem (butanediol completely soluble in the IL) evolves througha biphasic mixture with the 1,4-dichlorobutane in the upperphase. The product as well as the chlorinating IL phase can beseparated off and recycled.

6.1.4.2. The liquid-phase HF fluorination: IL as catalyst. Chlorofluor-ocarbons which are still used as refrigerants in the air-conditioningand refrigeration industry will have to be replaced by chloride-freehydrofluorocarbons (HFCs). These HFCs can be produced by reactionof chlorinated hydrocarbons with HF through an acid catalysedreaction. The acid catalysts usually used for this chlorine/fluorine

alkylation Ionikylation

te (tpd) Yield (wt%) Flow rate (tpd) Yield (wt%)

– 248 –

70.0 186.1 75.0

6.0 8.2 3.3

23.2 53.7 21.7

0.8 – –

100 248 100

98.8

93.1

96.0

45.5

81.5

101.0

108.0

154.0

1.6

Pass

1

Scheme 50. Chlorination of 1,4-butanediol.

Scheme 51. Demethylation of 4-methoxyphenylbutyric acid.

Scheme 52. Synthesis of alkoxyphenylphosphines.


substitution are super acids based on antimony Lewis acid, such asSbX5 (X = F or Cl) associated with Brønsted acids. However, thesecatalytic systems suffer from a reductive deactivation with forma-tion of Sb(III) species. In industrial processes, this drawback can beovercome by the addition of Cl2 in order to re-oxidize Sb(III), but thisleads to a significant reduction of the yield in the desired productsdue to back chlorination of the fluorinated products. Arkema hasdemonstrated at a pilot stage that ILs based on [SbF6]� anion can beused as liquid catalyst phase in presence of HF [392]. Formation ofheteropolyanions were identified by both Raman spectroscopy and19F NMR. The pilot reaction has been running for more than 400 h forthe liquid fluorination of trichloroethylene, exceeding selectivities of99.5% to 1,2-dichloro-2,2-difluoroethane and 1-chloro-2,2,2-tri-fluoroethane, without any noticeable deactivation.

6.1.5. Ether cleavage

In 2004, Eli Lilly company reported the use of pyridiniumhydrochloride [PyrH][Cl] for the demethylation of 4-methoxyphe-nylbutyric acid (Scheme 51), a key feedstock for the synthesis of amedicine (LY518674) [393]. For preclinical evaluation, kilogramquantities of this compound are required. The classical approachesfor demethylation reaction (BBr3, BBr3/Me2S, BCl3 and many others),were not envisioned, each of them presenting undesirable features.

Eli Lilly researchers turned rapidly to the use of the meltedpyridinium hydrochloride, earlier reported for similar transforma-tions [393]. For preventing solidification of the high melting

Scheme 53. Alkoxyphenylphosphine

pyridinium hydrochloride (mp = 153 8C) during the post reactionsteps, a stoichiometric amount of HCl and dilution with water werenecessary. This way, a homogeneous non viscous catalytic solutionwas obtained at room temperature with no indication of anythermal hazards. Following the reaction, the products areextracted from the reaction mixture by solvent extraction. Thischemistry was implemented to a 190 L scale, affording 4-methoxyphenylbutyric acid in high yield and purity.

6.1.6. Acid scavenging

Alkoxyphenylphosphines are known as important raw materi-als for the manufacture of substances that are used as photo-initiators (BASF’s Lucirines1). These alkoxyphenylphosphines arecommonly produced by reaction of chlorophenylphosphines withalcohols. This reaction generates stoichiometric amounts of HClwhich must be trapped using a tertiary amine (Scheme 52).Unfortunately, the corresponding ammonium salts [HNR3][Cl]appear as a slurry, generating complicated work-up and sig-nificantly lowering the yield and capacity of the process.

In 2003, BASF announced the development of a new processcalled BASILTM (Basic Acid Scavenging utilising Ionic Liquids) toimprove the acid trapping in a more convenient way [394,395]. Itwas found that substitution of the classical tertiary amine by theliquid 1-methylimidazole reaches the stage of 1-methyl-imidazo-lium chloride [MI][Cl] production, a salt which remains liquidunder the reaction conditions (Scheme 53). More, this ionic liquid

s synthesis—The BasilTM route.

Scheme 54. Hydrosilylation of C55C double bond with Si-H-functional polydimethylsiloxanes.


forms a second lower phase with the pure product in the upper onegiving feasible the separation of products by simple decantation[396–398]. Interestingly, the 1-methyl-imidazolium chloride[MI][Cl] co-produced in the reaction can be treated with causticsoda to regenerate 1-methylimidazole.

In this work, 1-methylimidazole was also identified as anucleophilic catalyst. When used, it led to consirable improvementof the phosphorylation rate. Taking this effect into account, BASFdeveloped a specific reactor technology (jet reactor) whichincreases productivity by a factor of 80,000. The role of 1-methylimidazole as acid scavenging was further extended to othersimilar problems and revealed to be also very efficient [365]. TheBASILTM process is actually applied by BASF at a multi-ton scale.

6.1.7. Hydrosilylation: IL as solvent and nanoparticle stabiliser

Organomodified siloxanes account for approximately 15% of theentire silicone market and are accessible in a number of ways[399], including the catalysed hydrosilylation of C55C double bondcontaining compounds with Si-H-functional polydimethylsilox-anes (Scheme 54). Particularly interesting are polyethersiloxanes,constituting an important class of surface-active compoundswhich find use in a broad range of industrial applications.

The hydrosilylation reaction is usually performed in a single-phase homogeneous system with, nevertheless the recurrentproblem of catalyst and product separation after completion ofthe reaction. To overcome this limitation, Degussa introduced theuse of ionic liquid as a way of catalyst heterogenisation[331,400,401]. This work was directed by the need for using thestandard platinum catalyst without modification. Several ionic andneutral catalysts as well as ionic liquids were then evaluated. As animportant parameter of the study the separation behaviour of theionic liquid and the product were particularly studied. It turned outthat for successful recovery of the catalyst and its reusability ‘‘it is

crucial to find an appropriate combination of a catalyst and an ionic

liquid which has to be harmonised with the hydrophilicity/hydro-

phobicity of the product’’. For example of this fine tuning, it wasdemonstrated that hydrosilylation with ILs derived from 1,3-dialkylimidazolium salts did not give the desired polyethersilox-anes. The authors assumed that formation of NHC-carbenes whichcoordinate to the Pt centre is responsible of the catalyst deactivation.

Scheme 55. Isomerisation of 3,4-epox

One year after this ‘‘proof-of-concept’’, a more detailed study on thisreaction was performed with a view to finding a superior catalyst/ionic liquid system active and stable enough for industrialimplementation [330]. Depending on the catalyst precursors andreaction conditions, homogeneous or soluble nanoparticle catalystswere identified as the active species. Despite the large screeningconducted in this study, none of the catalyst combinations testedresulted in a superior catalytic system.

6.1.8. Isomerisation: IL as a solvent

Isomerisation of 3,4-epoxybut-1-ene to 2,5-dihydrofuran isdescribed in literature as a reaction step involved in the productionof tetrahydrofuran, a valuable compound useful as a chemicalprocess solvent, and as an intermediate in the preparation ofpolymers such as poly(tetramethyleneether)glycol. This isomer-isation process requires simultaneous activation by a Lewis baseand by Lewis acid being an organotin compounds. The EastmanChemical Company performed this reaction at an industrial scaleusing phosphonium iodide ionic liquid (Scheme 55). This ionicliquid was chosen for its high thermal stability, its highhydrophobic character, as well as its limited cost with respectto ammonium analogues. Advantages provided by this process alsoinclude milder reaction conditions, simplified product separationand the ability to remove and replenish the catalyst system. Thephosphonium-based IL is highly alkane soluble and can dissolvethe oligomer by-products. The 2,5-dihydrofurane (bp = 66 8C) andthe crotonaldehyde (bp = 104 8C) can then be separated from thereaction mixture by distillation due to their low boiling points,while the higher boiling oligomers remain in the IL-catalyst phase.This phase is further treated with an extractant solvent such asnaphta, to recover the oligomers and recycle the IL-catalyst. In thiscase, the IL is multi-task: solvent, catalyst and by-productextractant. The industrial plant was operated from 1996 to 2004with a reactor capacity of 1400 metric tons per year. It actuallyrepresents the largest industrial application involving ionicliquids.[402,403].

6.1.9. Methanol carbonylation

Industrial processes of methanol carbonylation are currentlyperformed in liquid phase using homogeneous catalyst systems

ybut-1-ene to 2,5-dihydrofuran.

Table 7Other examples of industrial/pilot scale applications of ILs.

Company Scale Ionic liquid Role of the IL Ref.

Separation/storage

EtOH/H2O or EtOH/THF separation BASF Pilot [RMI][BF4] Entrainer for breaking

azeotropes

[406,407]

Storage of gases Air products Commercial [RMI][BF4] [RMI][Cu2Cl3]

[RMI][Cu2Br3]

Liquid support [408,409]

Electrochemistry

Electroplating of Cr Scionix Pilot (1200 L) [HO(CH2)2NMe3][Cl] Electrolyte [410]

Electropolishing of stainless steel Scionix Pilot (50 L) – Electrolyte [79,125]

Energy

Dye Sensitized Solar Cells

(DSSC’s or Gratzel cells)

G24i Commercial – Electrolyte [411,412]

Engineering fluids

Hydraulic liquids Linde Pilot – Liquid piston [413–415]

Cleaning fluids Iolitec/Wandres Pilot – Cleaning agent [365]

Analytical chemistry

Commercial GC-stationary phase Supelco Commercial [1,9-di (3-vinyl-imidazolium)

nonane] [NTf2]

GC-stationary phase [416]


which are most commonly based on rhodium metal. In theseprocesses, the precipitation of rhodium during the flashing stepis a serious problem. Ionic liquids have been described assolvents of the anionic active rhodium species [Rh(CO)2I2]�

allowing catalyst stabilisation and recycling [404]. A continuousprocess, operated in gas phase, for the production of acetic acidfrom methanol and methyl iodide has also been proposed using anon-volatile catalyst phase composed of rhodium and an ionicliquid. As the reaction products are separated in the vapourphase, this process simplifies product purification while provid-ing high reaction rate and eliminating the addition of water tothe reactant stream [405].

6.1.10. Other examples

Taking into account the scope of this review, we didnot develop industrial/pilot scale applications of ionic liquidsin fields other than catalysis. However, it seems that veryattractive work is actually done for using ionic liquids aselectrolytes for electroplating, as hydraulic or cleaning fluids, orentrainers for breaking azeotropes. Some of these examples arelisted in Table 7 with references to the more representativework.

6.2. Main process engineering challenges and issues

As reported in the selected examples previously cited, it is nowobvious that ILs can contribute some significant improvements tovarious existing catalytic processes. However, to be applied in anindustrial process, ILs must meet a certain number of require-ments. Changing an established industrial process for a noveltechnology using ILs is not easy even if it can be more profitable inthe long term. It requires a willingness to accept the risk associatedwith the implementation of new approaches and products. Newenvironmental regulations and competition may force industry totake the plunge. The novelty and the price/production cost of ILsare probably one of the main barriers. The IL price must be relatedto the process performance and to overall economy. So the majorissues are IL life times e.g. their chemical and thermal stability,their loss in the process, their recovery and recycling. The noveltyof ILs raises a certain number of questions such as their toxicity,their reliable supply, their material compatibility and theirnotification requirements (REACH). Their reliable supply is todayno more a major concern as IL production and commercialisationare developed and have attracted the development of newcompanies or business.

6.2.1. IL stability, lifetime and recyclability

Most often, ILs display high thermal stability, the latter beingmostly dependent on the nucleophilic character of the anion.However, very often valuable information is missing concerningthe long-term stability of ILs. In some cases, TGA analysis providesthese data but the decomposition products are often not described.It has to be emphasised that the thermal and chemical stability ofILs depend also on the presence of impurities from their synthesisand on their exposure to the process conditions, reactants andproducts. A typical example is the decomposition of N,N-dialkylimidazolium cation to form N-heterocyclic carbenes underbasic conditions, or the hydrolysis of [PF6]� based ILs into HF andPOF3. In multiphase liquid–liquid systems, pre-treatment offeedstocks can be an important pre-requisite to protect thecatalytic system from polar impurities that can accumulate in theIL phase. For example in the Difasol ‘‘stand alone’’ process, thelower the content of feedstock pollutants, the longer thechloroaluminate ionic liquid lifetime will be. The best arrange-ment, to remove these poisons as thoroughly as possible, includes awater wash with condensed or feed boiling water, followed by awater removal device. The dry feed is then treated by the propermolecular sieves in order to remove both oxygenates and sulfurcompounds. It should be noted that all these feedstock treatmentsare also recommended to minimize classical DimersolTM catalystconsumption, but using standalone DifasolTM would lead to moresevere treatments and therefore higher investments and chemicalexpenses.

In multiphase processes, it is often reported that the reactionproducts are ‘‘simply’’ separated from the IL catalytic phase bysimple decantation which allows IL recycling. However, the partialmiscibility of ILs in organic reaction products, even at the tracelevel, can be at the origin of IL loss and product contaminations,which can be dramatic issues for the overall economy of theprocess and product quality. To improve the product-IL phaseseparation, different techniques have been envisaged such as theuse of an organic co-solvent, which can also be the non convertedrecycled reactants when they are non polar (e.g. hydroformyla-tion), or scCO2 but this renders the process scheme more complexand more energy consuming, and could probably be proposed onlyin sufficiently high valuable chemical production.

Supported IL processes, that use less IL immobilized on solidsupports, could be applied in some dedicated cases. However,these processes fit best with gas–liquid multiphase technology andtheir applicability remains limited to reactions involving lowvolatile reactants and products.

Scheme 56. Different possible routes to recover ILs.


Membrane separation technology could be used in the case ofseparation of non volatile products that are not suitable fordistillation or phase separation. However one must consider thetypically higher viscosity of ILs, and the treatment of membranes forIL recovery. Despite these limitations, a series of organic-solvent-stable nanofiltration membranes have been employed to recover ILsin Suzuki cross-coupling reaction mixtures [417]. The recovery ofhydrophilic ILs from aqueous solutions is still challenging [84].

The controlled decomposition of ILs has been proposed for ILrecycling (Scheme 56) [365]. This approach consists in transform-ing the IL into neutral molecules which can be recovered bydistillation. For example, protic ILs can be switched to the neutralamine or imidazole in the presence of a base. In the case of N,N-dialkylimidazolium, the alkylimidazole can be formed by heating,or relatively stable carbenes can be generated in presence of a base.The distillation of ILs, without decomposition, has also beenreported [47]. But this distillation requires high temperature undervacuum and is not realistic for large scale IL recovery.

6.2.2. Safety and environmental issues:

ILs have been claimed as being ‘‘green solvents’’ and possiblealternative to volatile organic solvents. This has been justified insome applications where ILs, because of their negligible vapourpressure and non-flammability, are used favourably instead ofchlorinated solvents. An example is given by the development of anoptimised process for degreasing and/or scouring metal, ceramic,glass, plastic composite material or semiconductor surface, bytreatment of the surfaces in a solution comprising an IL [418].Some ILs have been the subject of toxicity and ecotoxicity studies[419] and data are now available on a larger variety of organisms(bacteria, fungi, fish, algae.). Most studies have been carried out onimidazolium- and pyridinium-based ILs, with alkyl or alkoxy sidechains. The variety of anions studied is limited mainly to bromide,chloride, hexafluorophosphate and tetrafluoroborate. Much lessresearch has been devoted to the determination of the biodegrad-ability of ILs but the design of biodegradable ILs has been coveredin recent papers [420]. A high throughput screen based on the AgarDiffusion method was recently applied to test, in a first rapidapproach, the toxicity of ILs towards microorganisms and todistinguish toxic and biocompatible ILs [421].

Registration of ILs in inventories like EINECS are still rare. It isestablished that side chains on the imidazolium cations have astronger influence on IL toxicity. The longer and the more branchedthe side chain, the more toxic is the IL. Effects of anions aregenerally more complicated.

In evaluating a process (eco-efficiency evaluation), not onlyeconomics but also sustainability must be considered. Sustain-ability concerns the whole process from the raw materials, thecatalytic system, their manufacture, storage, transport, transfor-mation, separation and purification of the final products as well as

the by-products and disposal of waste. In this context, the wholelife cycle of ILs must be taken into account [422,423]. Disposal ofILs is the main concern and is generally not reported. Disposal of ILsby incineration can be applied.

7. ILs application in the biomass transformation intofuel and chemicals

With the declining fossil fuel resources, combined with thegreater demand for petroleum in emerging economies, thedemographic increase and the environmental concerns, thetransition to alternative energies, such as bio-fuels, is taking placein many countries. Discussions on current methods and futurepossibilities to transform the biomass into transportation fuels andvalue-added chemicals have recently emerged [424–429]. Up tonow, most attention has been focused on the transformation ofvegetable oils. But with the limited amount of triglyceridesavailable together with ethical reasons, lignocellulosic materials,rapidly appeared as one of the most interesting carbon neutral,renewable, cheap and abundant resource. For example, utilisationof wood-based biomass, mostly agricultural waste, attractedconsiderable interest as an important source of glucose. Thismaterial also has the advantage of being far less expensive thanother feedstocks (oil, corn, gas.). In a modern bio-refinery, one canexpect that lignocellulose would be converted through a number ofdifferent processes into mixture of products, including bio-fuels,valuable chemicals, heat and electricity. The biomass transforma-tions often use bio-chemical technologies such as fermentation orenzymatic procedures. Biological transformations account formain paths to convert feedstocks to building blocks. Chemicaltransformations predominate for the conversion of these buildingblocks to molecular derivatives and intermediates. Chemicaltransformations often include reduction, hydrogenation, andselective dehydration (acid-catalysed) reactions. The potential ofcatalysis in this area is important to progress towards moreenvironmentally processes or to achieve more efficient andselective synthesis under moderate operating conditions. Catalystdevelopment with improved stability and better tolerance tobiomass impurities would be necessary. However, the transforma-tion of carbohydrates is still challenging task because of their lowsolubilities in almost any solvent. Compared to common organicsolvents used in carbohydrate chemistry, such as LiCl/DMAc(dimethylacetamide), ILs display interesting properties anddifferent reviews describe a number of their advantages – to citejust a few – their reasonable chemical inertness, their rather goodthermal stability, their low volatility, their unique solvationabilities [8]. It can also be noted that the heterocyclic-based ILspresent cytotoxicities comparable to those of many classic solventsdespite many quaternary nitrogen compounds have antibacterialproperties [430]. So, ILs open a window of opportunities for the

Fig. 20. The cellulose network (A: Cellulose chain and B: inter and intra H-bonds present in cellulose).

Table 8Main solvent systems used in manufacturing cellulose and cellulose derivatives

[434].

Acronym Main systems used to dissolve cellulose

DMSO/TBAF Dimethyl sulfoxide/tetrabutylammonium fluoride

LiCl/DMAc Lithium chloride/dimethylacetamide

LiCl/DMI Lithium chloride/dimethylimidazolidinone

LiCl/NMP Lithium chloride/N-methyl pyrrolidine

N2O4/DMF Dinitrogen tetraoxide/dimethylformamide

DMSO/CH2O Dimethyl sulfoxide/paraformaldehyde

NMMO N-methylmorpholine-N-oxide monohydrate

Aqueous solutions metal complexes


dissolution of carbohydrates (Table 9), their recovery fromsolution, or their transformation into important derivatives,particularly esters and ethers. The evaluation of the potentialsand drawbacks of the use of ILs in bio-polymer industry has beenbriefly evaluated [431]. One interesting point to note is their lowvolatility which permits distillation of the volatile substances,thereby making IL recovery feasible. Their power to dissolve largeconcentrations of saccharides and carbohydrate polymers are nowwell-known and many papers discuss the design and optimalconditions for dissolving every type of cellulose and lignocellulosicmaterial. Concentrations up to 25 (wt%) of cellulose in imidazoliumtype ionic liquids were found. This ability to solubilise cellulose isuseful for acid/base catalytic reactions in homogeneous solutionsdirectly in the ILs or for direct enzymatic hydrolysis. Finally, theycan potentially be used as alternative solvents for sugartransformations into potential bio-fuels or bio-fuel intermediatesand for bio-transformations such as derivation of sugars ortransesterification of triglycerides. All these advantages make ILsplay an increasing role in the research and development of newtechnologies in the biomass area.

7.1. Processing of lignocellulosic and cellulosic materials

7.1.1. Direct solvent for dissolution of cellulose and sugars

Cellulose consists of b-(1! 4)-linked glucose repeating units(Fig. 20), it is the most abundant polymer on earth. It has been oneof the largest renewable biological resources (global volume ofcellulose is 700,000 billion tons, only 0.1 billion tons are used asfeedstock for processing of the 40 billion tons that naturerenewed), being used in industrial domains such as paper, fibres,polymers, textile, food. Natural cellulose is a highly crystalloid,containing strong intra and inter H-bonds and van der Waalsinteractions between the cellulose fibrils. It is insoluble in waterbut soluble in concentrated phosphoric acid, insoluble in conven-tional organic solvents, which is the main obstacle to the moreextensive development of its use. The research for new solvents,including ILs and phosphoric acid [432], for dissolving andprocessing cellulose have attracted increasing attention. Thesetreatments contribute to converting crystalline cellulose intoamorphous cellulose which can be practically hydrolysed or

transformed much faster. The pre-treatment of cellulosic materialscan affect its physical properties such as its degree of polymeriza-tion, its crystallinity and even the surface area of the substrateaccessible in the case a further enzymatic hydrolysis.

For this purpose cellulose solvents should have most of thefollowing features [433]: (i) able to dissolve cellulose (and wetcellulose) at low temperature: the IL ideally displays highdissolution capacity (>10 wt% of cellulose), melting point lowerthan 20 8C and high decomposition point (>200 8C); (ii) nonvolatile, non toxic and chemically stable, (iii) no cellulosedecomposition, (iv) easy cellulose regeneration (e.g. with water)and good fibre properties (at least as good as viscose process), (v)recyclable, (vi) cost effective and easy process an (vii) non toxic toenzymatic and microbial fermentation.

The typical commonly used solvents to dissolve cellulose, overthe past decades, were based on polar organic solvents such asDMF, DMAc, DMI or DMSO often added with charged compoundssuch as [NBu4][F] or LiCl (Table 8). For example, NMMO is used asone alternative to the carbon disulfide, sodium hydroxide andsulphuric acid in the fibre production (old viscose process). It canproduce solutions of cellulose of 10–15 wt% concentrations.However, these solvents suffer from their toxicity, high cost,dissolving capacity, difficult recycling and their thermal instabilityin process conditions [39]. Consequently, processing cellulose isoften complex and in many cases more expensive than comparablesynthetically-manufactured products such as polyester fibres.


The first report of dissolution of cellulose in an ‘‘IL’’ dates backfrom a US patent filed in 1934. The ionic solvent used was the [N-ethylpyridinium][Cl] in a presence of a nitrogen base such aspyridine but this system displayed a relatively high melting point(mp: 118–120 8C) [435]. In 2002, Rogers et al. report that cellulosecould be dissolved in ionic liquids [436]. They used several ILs,cellulose and operating conditions and concluded that the bestcases, with concentrations of cellulose in the 8–12 wt% range andup to 25 wt% by microwave activation, were found with 1-butyl-3-ethylimidazolium chloride (BMIC) as the solvent. But it can benoted that degradation under microwave irradiation seems to be

Table 9Examples of solubility of different bio-products in ILs and organic solvents.

substrate DP (cellulose) Solvent

Glucose 2-Methyl-2-propa

[BMI][N(CN)2]

[BMI][Cl]

Sucrose Acetone

Sucrose Pyridine

Sucrose [BMI][Cl]

Sucrose [BMI][Cl]

Sucrose [BMMI][Cl]

Cellulose 286 [BMI][Cl]


Cellulose �1000 [BMI][Cl]





Cellulose 225 [BMI][Fm]

Cellulose 250 [BMMI][Cl]

286 [BMI][Cl]

593 [BMI][Cl]

1198 [BMI][Cl]

Cellulose 650 [AMI][Cl]

1600 [AMI][Cl]

Cellulose 286 [AMMI][Br]

593 [AMMI][Br]

1198 [AMMI][Br]

Cellulose 250 [AMI][Fm]

250 [AMI][Fm]

Cellulose 286 [EMI][Cl]

593 [EMI][Cl]

1198 [EMI][Cl]

Cellulose 795 [EMI][Ac]

Cellulose 200–250 [EMI][PO2(H)(OM

Cellulose 200–250 [EMI][PO2(Me)(OM

Cellulose 200–250 [EMI][PO2(OMe)2

Cellulose 200–250 [EMI][PO2(OEt)2]

Cellulose 200–250 [MMI][PO2(OMe)2

Cellulose 286 [BMPy][Cl]

593 [BMPy][Cl]

1198 [BMPy][Cl]

Cellulose 200–250 [(HO(CH2)2)MI][C

Lignin [BMI][Cl]

[BMI][Cl]

Lignin [MMI][MeSO4]

Lignin [BMI][MeSO4]

Wood keratin fibres [BMI][Cl]

Eucalyptus pulp 569 [EMI][Ac]

Eucalyptus pulp 569 [EMI][Cl]

Eucalyptus pulp 569 [BMI][Ac]

Eucalyptus pulp 569 [BMI][Cl]

Starch [BMI][N(CN)2]

a Microwaves heating.

higher than under conventional heating conditions [437]. Theseresults opened up a new way of developing a class of cellulosesolvent systems and initiating an extensive research in this area.Overall, the ability of ILs to dissolve cellulose depends on thenature of the native cellulose (its degree of polymerization DP, andits crystallinity) on the operating conditions (temperature,reaction time, initial concentration of cellulose in the IL, activationwith microwaves) and presence of impurities, mostly water thatcan significantly change the result. Indeed, the use of non-dried ILscan affect the solubility of cellulose and it was demonstrated thatseverely dried ILs are indispensable for an optimal dissolution

Solubility T (8C) Ref.

nol 0.03 (wt%) 25 [444]

145 (g/L) 25 [444]

50 (g/L) 70 [445]

0.007 (wt%) 30 [430]

6.45 (wt%) 26 [430]

5 (wt%) 70 [445]

18 (wt%) 110 [446]

14 (wt%) 120 [446]

18 (wt%) 83 [447]

13 (wt%) 83 [447]

3 (wt%) 70 [436]

10 (wt%) 100 [436]

25 (wt%)a 110 [436]

10 (wt%) 83 [447]

6 (wt%) 80 [448]

8 (wt%) 110 [441]

4.5 (wt %) 110 [446]

9 (wt%) 80 [449]

6 (wt%) 80 [449]

4 (wt%) 80 [449]

14.5 (wt%) 80 [450]

8 (wt%) 80 [451]

12 (wt%) 80 [449]

4 (wt%) 80 [449]

4 (wt%) 80 [449]

10 (wt%) 60 [77]

22 (wt%) 85 [77]

12 (wt %) 80 [449]

6 (wt %) 80 [449]

4 (wt%) 80 [449]

20 (wt%) 80 [439]

e)] 10 (wt%) 45 [451]

e)] 10 (wt%) 55 [451]

] 10 (wt%) 65 [451]

14 (wt%) 100 [437]

] 10 (wt%) 100 [437]

39 (wt%) 105 [447]

37 (wt %) 105 [447]

12 (wt %) 105 [447]

l] 6.8 (wt%) 70 [452]

9.7 (wt%) 110 [446]

13.9 (g/L) 75 [453]

344 (g/L) 50 [453]

312 (g/L) 50 [453]

11 (wt%) 130 [454]

13.5 (wt%) 85 [455]

15.8 (wt%) 85 [455]

13.2 (wt%) 85 [455]

13.6 (wt%) 85 [455]

15 (wt%) 80 [456]


[437]. The main results on cellulose dissolution in ILs aresummarised in Table 9. Today more than 20 ILs are known fordissolving cellulose and are compared in terms of their dissolutionkinetics in a study [438]. A screening of different cations andanions was also undertaken by different groups [439] and recentlysome reviews on the cellulose solubilisation in ILs have appeared[39,430,431,440–443].

7.1.1.1. Anion effect in the dissolution. To dissolve carbohydrates, alarge number of ILs with different anions were screened. Rapidly[BF4]� and [PF6]� anions were eliminated due to their poorcapability to dissolve any kind of cellulose [457]. For the samereason [NTf2]� and [N(CN)2]� based ILs were rejected [441], also[N(CN)2]� are known as an enzyme denaturing anion. As presentedin Table 9 good dissolutions of cellulose may be obtained usinghalide based-ILs, especially with the chloride anion. It is wellknown that the higher the anion concentration, the better thesolubilisation; thus the small size and the strong electronegativityof the chloride are obvious advantages [458]. In addition, these ILsare cheaper than most ILs obtained by anion-exchange reactionsusing imidazolium halide salts as starting materials. However, itwas also demonstrated that the fairly high melting point of ILscontaining chloride anion (above 70 8C for [BMI][Cl]) could be atechnical drawback and possibly limit their practical application incellulose processing or homogeneous cellulose derivatisation [84].Relatively high dissolution temperatures (often above 80 8C) areoften required for dissolving cellulose, which possibly results incellulose modification by ILs themselves [459] and in thermaldecomposition of ILs [460] and produce some organohalogenides[461], which have uncertain toxicity and hazardousness. It shouldbe noted that in general, the high viscosity of the [BMI][Cl] andhigh hygroscopicity of halide ILs [441] make their handlingdifficult [437].

Some groups claim that we need a new class of ILs to replacethese current chloride salts, which can dissolve cellulose withlower viscosities and melting points but with a sufficient polarityto further process it [443]. Thus new classes of ILs wereinvestigated. Formate, acetate or phosphate based imidazoliumILs have been used and showed interesting potential to dissolvecellulose under mild conditions. ILs containing formate anions, forexample, were found to be good solvents for dissolving cellulose.Indeed allyl-methylimidazolium formate [AMI][Fm] dissolves upto 22% of cellulose, presenting a DP around 250, when [BMI][Cl]shows a solubilisation around 18% with the same substrate(Table 9). This result can be explained by the low viscosities ofthese types of salts due to the small ion size [77]. However formateILs generally exhibit low thermal stability, especially compared tothose based on [NTf2]� and [Cl]�, due to decarboxylation [451] andare known to be quite basic and unsuitable for enzymes [441].Acetate-based ILs such as [EMI][Ac] were found to be an interestingalternative due to the lower melting point, lower viscosity andtheir less toxic and corrosive character compared to chloride-basedILs [462]. Solutions containing as high as 20 wt% of cellulose couldbe obtained. In addition acetate-based ILs are more thermallystable than formate [441] and could dissolve cellulose without gelformation [439]. Acetate ions are considered as the choice anion byseveral groups [441].

Another alternative is the use of phosphate anions. Indeed, theethyl-methylimidazolium methylphosphonate [EMI][PO2(-H)(OMe)] IL allows the preparation of a 10 wt% cellulose solution(at 45 8C for 30 min) with stirring, or to dissolve 2–4 wt% cellulosewithout heating. Also, phosphate-based ILs present high thermalstability in the 260–290 8C range and low viscosities [451].

7.1.1.2. Cation effect in the dissolution. Although some simulationstudies [463,464] and some conclusions of several groups

[430,465] suggested that little or weak interaction between cationof ILs and cellulose existed, it can be admitted that cations wereinvolved in the dissolution process and their role in thedissolution mechanism should not be neglected [458,450,466].Thus, for a same chloride anion, increasing the alkyl chain on thedialkyl imidazolium cation leads to a decrease in cellulosesolubilisation. Introduction of a function such as an allyl groupon the imidazolium cation gave superior results, up to 14.5% ofcellulose can be dissolved using the allyl-methylimidazoliumchloride [AMI][Cl] when 13% of the same cellulose was dissolvedusing the [BMI][Cl]. Allyl based ILs generally showed lowerviscosity than those having propyl or propargyl groups [77]. Also,[AMI][Cl] can be viewed as a smaller cation because it containsonly three carbon atoms in the side chain, and the strong polar ofthe double bond seems to be essential [39]. Some results indicate arelatively high solubility of avicel1 cellulose (DP = 225) in[BMI][Fm] as high as 8 wt% when the solubility of the ammoniumformate salt [Bu4N][Fm] is only 1.5 wt% [441]. This correlates witha cation effect such as the dissolution results obtained with apyridinium chloride salt compared to the corresponding [BMI][Cl]salt [447].

7.1.1.3. Main properties involved in the dissolution process. Themain properties of ILs relevant to the dissolution and functionalmodification of cellulose and carbohydrates are their polarcharacter (see Kamlet-Taft parameter), the basicity of the ILanions and their ability to generate H-bonds. For example, thehydrogen bond basicity for the [BMI][Cl] is around 0.85 when thisvalue is 1.09 for the corresponding [BMI][Ac]. But the mostimportant contribution can be ascribed to hydrogen bondingability of the IL anion, such as chloride which gives H-bonding withthe hydroxyl groups of the bio-molecule (the solubility of bio-materials in [BMI][Cl] decreases after methylation of the hydroxylgroup of cyclodextrins) [467].

The ions of ILs are large and asymmetric, probably freer tointeract with OH groups of the cellulose than that of classicalchloride ions of LiCl in N,N-dimethylacetamide.

Higher concentrations of cellulose in acetate-based ILs com-pared to chloride-based ILs can be obtained due to the lowerviscosity of the solutions, providing promise for more efficientprocesses in cellulose dissolution and shaping for fibres manu-facture [462].

In a typical process for dissolving cellulose, the media has to beheated, so that in these conditions the thermal stability is also animportant aspect.

In Table 10, some melting points are considered, but thisproperty is highly dependent on the IL purity. For exampledifferent values for the same IL melting point (mp) can be found inthe literature, with the [BMI][Cl] the mp is found at 41 8C for agroup [468] when 73 8C is found for another one [447].

7.1.1.4. Mechanism of dissolution. Insight into the mechanism ofdissolution of cellulose in ILs has been achieved by applyingdifferent analytical methods. NMR spectroscopy and moleculardynamics simulations were applied to show that ILs act as nonderivatising solvents of cellulose [447]. It was shown that the anionof the IL acts as H-bond acceptor – or electron donor centre – whichinteracts with the hydroxyl group of cellulose in approximatelystoichiometric manner [469,470] to form a conceptual electron

donor-electron acceptor (EDA) complex (Scheme 57) [39]. It hasbeen thought that the anion was the major contributor and thecation did not play a significant role in the dissolution. Actually thecation, with its electron-rich aromatic p system, can be seen as anelectron acceptor centre via non-bonding or p electron interac-tions, and moreover can prevent the crosslinking of the cellulose[458].

Table 10Some parameters to consider regarding the solubility of bio-polymers in ILs.

IL cation IL anion Mp (8C) Viscosity

mPa s (at �RT)

Solvato-chromic

parameter b*

Interest/limitations for bio-polymer processing

[BMI]+ [Cl]� 66 11,000 0.83–0.87 Corrosiveness of the anion; possible degradation of cellulose

[AMI]+ [Cl]� 17 2090 0.83 Readily dissolve cellulose up to 14.5% (cotton linter)

[AEI]+ [Fm]� <�20 67 0.99 Low viscosity, high basicity of the anion

[AMI]+ [Fm]� �76 66 0.99 Solubilise 10 (wt%) cellulose at 60 8C[BMI]+ [Fm]� 156 1.01 Higher b value than chloride; but limited thermal stability

[BMI]+ [NTf2]� �2 52 Good thermal stability/bad dissolution

[BMI]+ [OTf]� 13 0.46

[BMI]+ [N(CN)2]� �6

[BMI]+ [PF6]� 0.21

[BMI]+ [Ac]� 646 1.09 Less corrosive than Cl�; non-toxic; but limited thermal

stability (110-120 8C) due to decarboxylation

[EMI]+ [Ac]� <�20 140 Concentrations as high as 20 wt%

[EMI]+ [PO2(H)(OMe)]� �86 107 1.00 Very low viscosity

[EMI]+ [PO2(Me)(OMe)]� �66 510 1.07

[EMI]+ [PO2(OMe)2]� �74 265 1.00

[EMI]+ [MeOSO3]� 0.61 Cellulose dissolution

[MeSO3]� 0.70

[BMPy]+ [Cl]� 95 Effective dissolution of cellulose; but high melting point

b H-bond basicity, solvatochromic dye: 2,6-dichloro-4-(2,4,6-triphenyl-1-pyridinio)phenolate.


Interactions between cellulose and ILs and formation of the EDAcomplexes are possible if the anion and cation are located closeenough. Then the hydrogen bonds network between the glucosidicmonomers in the cellulose are disrupted, resulting in thesolubilisation [39].

The effect of the temperature is also a non-negligibleparameter, indeed above the critical temperature, the ion pairsin the [AMI][Cl] IL dissociated to Cl� and AMI+ ions. Then free Cl�

ions associated with the cellulose hydroxyl protons and freecations complexed with cellulose hydroxyl oxygen, which led to aneasier dissolution of cellulose [443].

7.1.1.5. Precipitation–regeneration. Cellulose dissolved in ILs canbe precipitated from its solution by addition of a non-solvent suchas water, ethanol or acetone. The regenerated cellulose is separatedby filtration or centrifugation and due to its non-volatility, the ILcan be recovered after elimination of the anti-solvent throughdistillation. Recovery of the ILs is important for future cost-effective processing of cellulosic material. Studies on biphasicsystems for recovery have been investigated using IL/water[457,471], IL/alcohol [472], IL/supercritical CO2 [473] or twoimmiscible ILs [167]. Another approach has been examined usingsugars or sugar derivatives as water-IL solution additives forpreparing two-phase media. Sucrose is added to a solution of IL inwater to separate the IL, in moderate purity, from the aqueousphase [84,474]. But in any case the recovery of the IL is stillincomplete and needs to be improved through future research.

Regenerated cellulose could be obtained in different forms suchas monoliths, fibres and films. Compared to the native cellulose,

Scheme 57. Possible insertion of an imidazolium chloride in the cellulose matrix.

the regenerated sample can have the same degree of polymeriza-tion and polydispersity, although this much depends on theoperating conditions of the treatment. Generally macro- andmicro-structure especially the degree of crystallinity can bedrastically changed and modulated by changing the conditionsof regeneration. For example, cellulose reconstituted after beingdissolved in [AMI][Cl] and [BMI][Cl] had lower degrees ofcrystallinity than native cellulose [475]. It has been shown thatthe resultant regenerated cellulose is mostly amorphous withgreater accessibility of the polysaccharide chains to cellulase andexhibits improved enzymatic hydrolysis kinetics (Fig. 21)[458,466,476].

7.1.2. Treatment of lignocellulosic materials

Lignocellulosic biomass (Fig. 22) which comprises cellulose(33–45%), hemicellulose (20–30%) and lignin (15–30%) is perceivedas a future valuable resource but separation, recovery andprocessing of all its components in a cost-effective way representsa significant technical challenge. The effectiveness of lignocellulosepre-treatment is one of the keys to successful conversion of thisoriginally low-cost material into sugars and by enzymatic orcatalytic hydrolysis into biofuels or biofuel intermediates.

A number of lignocellulose pre-treatments are being investi-gated either at lab scale or in pilot plants, including variouschemical, physical or physico-chemical approaches such as acid-based methods (dilute or concentrated sulphuric acid), steam pre-treatments (steam explosion), solvent extraction methods (orga-nosolv) or ammonia methods (ammonia fibre explosion (AFEX),ammonia recycle percolation (ARR)) [477]. Many of thesetechniques suffer from relatively low sugar yields, high costsand fractionation and isolation of the major components remainsuncompleted. All these pre-treatments have the main objective ofseparating the cellulose for a future enzymatic conversion.However limiting factors exist and hinder the accessibility tothe enzymes. Generally these factors are divided into two groups:the biomass structural features, the so called biomass recalcitrants,and the enzyme mechanism. While enzymatic hydrolysis solelydepends on the accessibility, the structural feature is morecomplex [478].

Recalcitrances of lignocellulosic bio-materials to cellulaseenzymatic hydrolysis can be attributed to the accessible surfacearea, crystallinity, the protective lignin, degree of polymerization

Fig. 21. Enzymatic hydrolysis of cellulose [466].

Fig. 22. lignocellulose network.

Fig. 23. Example of wood chips dissolution in [EMI][Ac] as the ionic liquid

(reprinted from [438], Copyright 2009, with permission from Elsevier).


and biomass particle size [431]. Conventionally the presence of thelignin and the cristallinity are identified as the major factors. It hasbeen shown that removal the lignin has an important impact in thebiomass disgestibility [479]. Also, the presence of hemicellulose atthe surface of cellulose blocking its accessibility [480].

Lignin is an amorphous poly-phenolic polymer (the secondmost abundant on earth) which is co-produced during chemicalpulping under acidic or harsh basic conditions (approximately 26million tons of lignin are produced annually, globally from wood)[453]. Methods for removing this strong, three-dimensional,protective and highly complex network that envelops the cellulosefibres is explored but needs several improvements.

A study demonstrated that the methylsulfate imidazolium-based ionic liquids exhibit the best ability to dissolve softwoodkraft lignin [453] another study showed that extraction of ligninfrom bagasse using the ionic liquid ethyl-methylimidazoliumalkylbenzenesulfonate [EMI][ABS] was successfully achieved atatmospheric pressure with over 93% yield. However a number ofissues remain, such as simplication of ionic liquid and the hightemperatures (170–190 8C) involved [481]. When 40% of the ligninwas extracted, with a well chosen IL ([EMI][Ac]), the cellulosecrystallinity index dropped below 45 and the resulting hydrolysisof the wood flour cellulose is measured above 90%. This studyconcluded in a close correlation between lignin extraction andresidual cellulose crystallinity [475]. Finally another groupconcluded that delignification alone is sufficient for effectivehydrolysis over longer periods whereas for shorter hydrolysistimes the combination of delignification and decrystallizationshows great benefits [478].

The pre-treatment and eventual fractionation of lignocellulosewould permit (1) amorphous cellulose generation, (2) improvedenzyme use in the cellulose hydrolysis step with possible enzymerecovery leading to higher sugar yield with lower sugar degrada-tion and less inhibitor formation and (3) fractionation oflignocellulose for better economy and possible applicability to adiversity of biomass feedstock.

Choosing the appropriate pre-treatment for a biomass feed-stock is often a compromise between minimizing degradation ofthe hemicellulose and cellulose while maximising the ease ofenzymatic hydrolysis of cellulosic substrate.

7.1.2.1. Solubilisation. Some research on the solubilisation oflignocellulose with ILs has recently emerged [482–485] butremains less abundant than for cellulose dissolution. In thesereports it was shown that the mixture [BMI][Cl]/DMSO (85/15%)extracts both cellulose and lignin in an approximately equalmanner [483]. Apparently, two parameters are essential for thewood solubilisation. The first is the wood particle size, whichinvolves the complex and compact structure of the wood thatinhibits the diffusion of the ILs into its interior resulting in partialdissolution of wood chips. The second is the water content of thewood. Thus, water was found to reduce the solubility in ILs [483].The correlation between lignin solubilisation and wood dissolutionis not evident and some ILs can solubilise lignin without significanteffect on wood [475]. Thus the dimethylimidazolium methylsul-fate [MMI][MeSO4] dissolves >500 g of lignin per kg of IL and doesnot dissolve wood.

Best solvents for lignocellulosic materials are found to be the[BMI][Cl] and especially the [AMI][Cl] for hardwood and softwood[438,475], also [EMI][Ac] can be used for certain type of wood(Fig. 23) [438]. Dissolution of wood can be significantly increasedusing microwaves, thus 5 wt% of six biomass types were rapidlyand completely dissolved using microwave irradiation in [BMI][Cl][486].

7.1.2.2. Mechanism. As for cellulose, ILs are responsible for thehydrogen bonding disruption of the lignocellulosic complexmatrix. This is mostly attributed to IL anions, especially thechloride anion [438]. In the same way as cellulose, the cation can

Scheme 59. ILs used for catalytic hydrolysis of lignocellulose.

Scheme 58. Schematic lignocellulose hydrolysis.


play a non-negligible role in the solubilisation. Thus Kilpelainenet al. suggest that p–p interactions exist between the IL cation andthe aromatic compounds of lignin [482]. This hypothesis can beconfirmed by the dissolution obtained with the [AMI][Cl] IL, whichis a good solvent for every kind of wood and the only one whichexhibits p-electrons on the side-chain [438]. The possibility toacetylate the cellulose of solubilised biomass proved that the woodsolutions are real solutions and no gel or suspensions [482].

7.1.2.3. Remaining issues. The application of ILs can provide apotential new method of fractionation of lignocellulosic materials.But the fractionation using ILs faces some challenges to develop afeasible process: (i) the recovery and reuse of ILs. The cost of IL isstill high despite some development to decrease it [487,488], themajority of ILs remain expensive. (ii) The recovery of lignin andhemicellulose from the ILs after cellulose has been extracted. Inthis way lignin, which is mainly used as combustible, would bevalued and served as new starting material for different uses suchas dispersants or emulsifiers [489,490].

7.2. Applications of the use of ILs in the dissolution of

ligno-cellulosic materials

7.2.1. An improvement in the analysis of lignocellulosic material

Lignocellulosic materials have complex structures and compo-sitions. The analysis of such materials requires a series of reactionsand separation procedures. Dissolution of lignocellulose in ILscould make analysis simpler. High-resolution 13C NMR has beenused for determining the composition of cellulose-hemicellulose–lignin mixtures [483,491]. Complete phosphitylation of allcellulose hydroxyls can be obtained in [AMI][Cl] and for 31Plabelling. This can be useful for understanding of the interactionsoccurring in these solutions [492].

7.2.2. Transformation of poly-saccharides in sugars using ILs

7.2.2.1. Acid-catalysed hydrolysis of cellulose and lignocellulose in

ILs. Acid hydrolysis of cellulose has been known for a long time,but this method is not yet cost-effective for large-scale applica-tions. Several main issues have nevertheless been addressed suchas harsh conditions of temperature (>200 8C) and pressure fordiluted acid hydrolysis, formation of undesirable degradationproducts that lower the glucose yield and inhibit subsequentfermentation, requirement of corrosive resistant materials anddisposal problem for concentrated acid processes. Therefore,hydrolysis of lignocellulose or cellulose remains a challenge(Scheme 58). When cellulose (DP ranging from 100 to 450) isdissolved in an IL, such as [BMI][Cl], protons added in the solutioncan more easily access to the b-glucosidic bonds to perform thehydrolysis. Therefore it seems that a ‘‘physical’’ barrier can beovercome through the formation of the cellulose solution whichfacilitates the acid-catalysed hydrolysis at relatively low tempera-ture (100 8C) and lower catalyst loading. The strength of the acid inthe IL plays an important role with efficient hydrolysis obtainedwith H2SO4, 77% of total reducing sugars (TRS) and glucose yields isobtained with 0.11 acid/cellulose ratio at 100 8C [493].

Cellobiose was found to be a good model for determining theefficient conditions for cellulose hydrolysis; this model was thensuccessfully applied to hydrolysis of more complex polysacchar-ides in ILs. Optimised conditions have been thus determined: astrong acid, water content between 5 and 10% (w/w), carbohydratecontent less than 10% (w/w) and temperature between 80 and150 8C [494]. These conditions were also tested for miscanthusgrass and did not lead to an efficient cellulose hydrolysis (less than5%). For the lignocellulosic materials, it was proposed to combinean extraction of the lignin, as described elsewhere [475,481], andhydrolysis in these conditions in order to optimise yields [494].

Hydrolysis of lignocellulose from different origins was per-formed in [BMI][Cl] added with different Brønsted acids (Scheme59), and in parallel, for comparative purposes, in water underotherwise comparative conditions. The catalytic activity in[BMI][Cl] follows the order HCl > HNO3 > H2SO4 > maleic acid->H3PO4. Depolymerization of the polysaccharides occurs quicklyexcept with phosphoric acid yielding up to 70% TRS, when 4% TRSyield is obtained in water, according to the analytical methodwhich is solely a UV absorbance measurement [495]. Almostcomplete (97% after 2 h) hydrolysis of carbohydrate content of pinewood in [BMI][Cl] was found possible using trifluoroacetic acid(0.2 wt%) at 120 8C. But the non-selectivity (conversion of thecellulose into monosaccharides, hydroxymethylfurfural (HMF) andfurfural) observed remaining an issue [496]. A continuousextraction system can be envisaged to obtain higher yields becausecellulose degradation seems to be unavoidable [494].

Depolymerization/hydrolysis of cellulose over solid acids,which is not application to conventional slurry of cellulose inwater, becomes feasible in ILs. Different solid acids (Amberlyst,Nafion, alumina, sulfonated zircona, zeolithes) have been testedto hydrolyse either microcrystalline cellulose or a-cellulose in[BMI][Cl]. The hydrolysis of b(1!4) glycosidic linkages iscatalysed by the acid surface sites. Especially acid resins withlarge pores are found suitable for cellulose or wood depolymer-ization in [BMI][Cl] ionic liquid. The reaction proceedsquite selectively (no sugar dehydration products) and formscello-oligomers which can further be broken into sugarswith enzymes. Since monomer sugars are completelysoluble in the IL, it seems that stopping at the oligomer stagemay be an advantage for the separation of the products byadding water. It must be noted here that the further separationof the IL from water is not so easy and requires energy. Theseresults prove that inorganic solid acids can catalyse reactions inILs. It seems that possible interactions between [BMI][Cl] andmaterial surfaces do not poison the catalytic activity. On thecontrary, [BMI][Ac], which could have been more suitablebecause of its lower viscosity, led to rapid destruction of thecatalyst [497].

Table 11Results of homogeneous cellulose acetylation in ILs (acetic anhydride is used as

reagent).

Ionic liquid Reagent mol

per glucose unit

t (min)/T (8C) Degree of

substitution

Ref.

DMAc/LiCl 5 120/80 1–2.94 [504]

[BMI][Cl] 5 120/80 2.72 [447]

[EMI][Cl] 3 120/80 3 [449]

[BMMI][Cl] 3 120/80 2.92 [449]

[AMI][Cl] 5 15/80 0.94 [505]

[AMI][Cl] 5 480/80 2.49 [505]

[EMI][Ac] 3 15/25 2.31 [443]

[EMI][Ac] 5 15/25 3.00 [443]


7.2.2.2. Enzymatic transformation of carbohydrates in ILs. Since ILsdissolving carbohydrates are typically composed of anions thatform strong H-bonds with carbohydrates, they also present thedrawback of having a strong tendency in denaturing enzymes.Indeed, strong deactivation of cellulase was found with the widelyused [BMI][Cl] IL for cellulose hydrolysis [239].

A fine design of new ILs has been achieved that are able todissolve carbohydrates but do not considerably inactivate enzymessuch as lipases. The carbohydrate dissolution is made possiblethanks to the presence of oxygen-containing cation having lowbulkiness combined with hydrogen-bond forming anion. Theconcentration and the nature of anions are essential forcarbohydrate stabilisation [441]. Despite a study on the quaternaryammonium cations (such as the N,N-dimethylethanolammoniumseries) with acetate as the anion that shows a high tolerance ofcellulase for these kinds of ILs [486], it was shown that acombination of acetate anions and oxygen-containing cationscauses severe cellulase inactivation at low concentrations (2.0 and3.0 M) [498].

Design of new ILs for enzymatic transformations should be animportant future prospect, phosphate anion could be one of themas this anion shows high compatibility with enzymes and providesbetter cellulose conversion than for the corresponding acetateanion IL in the same conditions [499].

In conclusion, the transformation of poly-saccharides usingILs is still limited. First, enzymatic hydrolysis suffers of lowreaction rate and enzymes remain too expensive. More, acidhydrolysis remains difficult to control generating side products,some of them acting as inhibitors for further bio-transformationsteps.

7.2.3. Catalytic transformation of sugars

7.2.3.1. Dehydration of sugars in 5-hydroxymethylfurfural

(HMF). Glucose and fructose are potential interesting renewablebuilding blocks for the synthesis of chemical intermediates. Recentefforts have been devoted to convert these sugars to 5-hydroxymethylfurfural, a versatile intermediate which can serveto replace petroleum-based building blocks. However, the highproduction cost of HMF actually limits its availability. The currentprocesses use acid catalysts, they are limited to fructose asfeedstock and they produce side reactions and by-products, such aslevullinic acid, which are difficult to separate (Scheme 60). It wasknown that high yields in HMF from glucose could be obtained byusing strongly polar solvents such as DMSO, in organic–aqueousreaction media. By solubilising the sugar in well-selectedimidazolium-based ILs, it is now possible to convert glucose toHMF with a yield near 70% with negligible amounts of levulinicacid formation. This has been made possible thanks to the additionof metal salts, such as CrCl2 to the chloride based IL [500]. Thepossible formation of [EMI][CrCl3] was suggested. The CrCl3-anionwould play a role in proton transfer to facilitate the mutarotationof glucose (considered as the key step), leading to the isomerisationof glucose to fructose followed by dehydration to HMF.

Scheme 60. Conversion of fructose to

Methylimidazolium chloride [MI][Cl] ionic liquids have alsobeen used both as solvents and catalysts for the dehydration ofsucrose. The absence of HMF degradation products has beenascribed to the continuous separation of HMF with diethyl ether.Nevertheless, it would be very difficult to transpose thismethodology to a larger scale [501].

The dehydration reaction of fructose to HMF has been ascribedto the presence of acidic ionic liquids [502], but few studies havebeen completed to illustrate the relationship between the aciditiesof the ionic liquids and their activities in this reaction. It has beensuggested that the reaction performance of the dehydrationreaction of fructose to HMF is closely associated with both theacid strength and the acid type of the catalyst. Lewis acid ILs werefound to be better reaction media than Brønsted acidic one [503].

7.2.3.2. Derivation of (poly)saccharides. The solubilisation of car-bohydrates in ILs has enabled a number of chemical derivatisationsof these natural products in homogeneous solutions. The activity ofthe homogeneous solutions of cellulose in ILs has been studied fordifferent reactions such as carboxymethylation, etherification oresterification and used for several advantages: (i) no by-products,(ii) ILs can be repeatedly used, (iii) control degree of substitution.

One of the most important cellulose reactions is acetylation,with a wide range of applications for coatings, membranes ortextiles. Until now commercial cellulose acetate was produced inheterogeneous conditions with an excess of acetic anhydrides andin the presence of sulphuric acid and one drawback, among others,was the high energy demand required by this procedure. Since ILscan be a suitable solvent for cellulose, it was shown thatacetylation occurred readily in this media and a large number ofILs were tested (Table 11). [BMI][Cl] was found to be a goodreaction media, it was thus possible to form cellulose acetate witha degree of substitution (DS) value of 2.72 in one step procedure[447]. The reaction can be carried out rapidly when [EMI][Ac] isused as solvent, indeed at room temperature and in 15 mincellulose acetate with a DS between 2.31 and 3 is obtained [443].

Complete acetylation of wood was also possible using ILs, afteran addition of a mixture of acetic anhydride and pyridine, thereaction was characterized by disappearance of the hydroxyl

5-hydroxymethylfurfural [500].

Table 12Methanolysis of triglycerides using ILs.

Catalyst Ionic liquid Use Remarks Ref

Metal salts [BMI][InCl4] IL-catalyst Catalyst deactivation; no possible recycle;

leaching of catalyst

[512]

Organic base:

NaOMe, KOH, LDA. . .

[PR4][N(CN)2] Base immobilized in the IL 10 recycles by evaporation of MeOH, then separation

of the ester by decantation; increased reaction rate by

using ultrasounds; not compatible with the presence

of free fatty acid

[509]

Tin compound [BMI][PF6] Three-phase system Separation of the product by adding water to the

reaction mixture

[511]

Ester/water-glycerine/

IL-catalyst

Leaching of the catalyst

Lewis acid [BMI] based

chloroaluminate

IL-catalyst Chloroaluminates are water reactive; main issue is

the separation of the product from the IL-catalyst

[515]

Brønsted acid Sulfonated imidazolium Acid supported on

the BMI cation

Applied to treat waste oils [514]

Compatible with the presence of free fatty acids

Enzyme lipase [BMI][NTf2]/water Biphasic system Reuse of enzyme [508]


groups and appearance of strong C55O bands in IR [482]. The non-degradative nature of ILs and their high power to dissolve celluloseoffer an excellent platform for other reactions such as: (i)tosylation, which is made in [AMI][Cl] and permit degrees ofsubstitution around 1 [506] (ii) tritylation of cellulose in[EMI][Et2PO4] [437] (iii) esterification using fatty acid chlorideleading to cellulose laurates with DS from 0.34 to 1.54 [449], (iv)etherification for making carboxymethyl cellulose with DS of 0.49in [BMI][Cl] mixed with DMSO as a solvent and NaOH as a base[447], (v) encapsulation of biomolecules such as heparin on solidsupport or the synthesis of polymeric derivatives composed of in-situ polymerizable ILs and cellulose [507].

7.3. Transformation of vegetable oils

7.3.1. Transesterification of triglycerides: biodiesel production

Biodiesel is a C16–C18 fatty acid methyl ester which can beblended with conventional diesel to provide an alternative to thelatter [508]. Total EU27 biodiesel production for 2007 was over 5.7million metric tonnes, a 16.8% increase relative to 2006 figures.Biodiesel is produced by transesterification of vegetable oils,animal fats, or even recycled greases with methanol, and glycerol isformed as a co-product. Current processes mainly employtransesterification of triglycerides with methanol using NaOH orNaOMe or KOH as a base catalyst. The main drawback of thisprocess is the formation of soaps, which lead to separationdifficulties, and the great amount of salt and waste waterproduced. Acid catalysed homogeneous transesterification ispenalized by the need of higher temperature and methanol-to-oil ratio as well as the formation of by-product. To overcome theseproblems several alternative approaches have been developedsuch as heterogeneous catalysts. Enzymatic systems or the use ofsupercritical methanol are also currently under investigation butare disfavoured by high cost.

Several authors have used the versatile solvent properties ofionic liquids in order to improve the transesterification reaction(Table 12). Their first objective is to immobilize and recycle thecatalyst in the ionic phase.

When the reaction is catalysed by a base (MeONa) immobilizedin a phosphonium dicyanamide ionic liquid, a large excess ofmethanol (molar ratio methanol:oil = 30:1) is necessary to yield ahigh conversion. The reaction medium is monophasic at the end ofthe reaction and methanol evaporation is necessary to separate themethyl ester and ionic liquid phase. The latter can be recycled but arapid decay in catalytic activity is observed, probably due to

deactivation of methoxide by traces of free fatty acids or water[509]. Similar deactivation was observed for a catalytic systemcomposed of Cs2CO3 in [BMI][NTf2]. Although it was possible to usea lower excess of methanol with this latter system, the amount ofmonoglyceride in biodiesel was above the limit imposed byEuropean or American specifications [510].

Several Lewis acids were screened as catalyst in [BMI][PF6][511] or [BMI][InCl4] ionic liquids [512]. For most of the Lewis acidsbiodiesel yield were very low. The best results were obtained withSn(3-hydroxy-2-methyl-4-pyrone)2(H2O)2 but unfortunatelyrecycling of the catalyst was prevented by its decompositionunder the reaction conditions.

The use of sulphuric acid in [BMI][NTf2] allows the formation ofbiodiesel in high yield. The acid is almost completely retained inthe ionic liquid phase and the recovered IL can be reused at least sixtimes without any significant loss in biodiesel yield or selectivity.However, a large excess of methanol and long reaction times arerequired, and the amount of residual glycerides in biodiesel isabove specifications.

The transesterification reaction can also be catalysed byBrønsted or Lewis acid ionic liquids. SO3-H functional Brønstedacidic ionic liquids were described for the transesterification ofcottonseed oil or waste oil at 170 8C [513,514]. 1-(4-Sulfonic acid)butylpyridinium hydrogen sulfate showed the best catalyticperformance, which is nearly the same as that of concentratedsulphuric acid. With chloroaluminate ionic liquids [Et3NH][Cl-AlCl3] (x(AlCl3) = 0,7) it was possible to perform the reaction at alower temperature (70 8C) with good yield. The catalyst wasrecycled for six times and the yield remained unchanged. Howeverthe catalyst could be destroyed by the presence of water in thereactant [515].

Ionic liquids can also be used as a medium for lipase catalysedproduction of biodiesel [508,516]. The biodiesel is separated bysimple decantation and the recovered ionic liquid/enzyme systemcan be reused at least four times without loss of catalytic activityand selectivity.

Ionic liquids have also been considered for the purification ofbiodiesel. An equimolar mixture of triethylammonium chloridewith glycerol has been used to extract excess glycerol frombiodiesel formed from the reaction of soy bean oil with ethanol.The analysis of the biodiesel layer showed complete removal ofglycerol while the lower layer was enriched in glycerol. However,no simple method for recovering the glycerol and the quaternaryammonium salt from the washing eutectic is available at present[517].

Scheme 61. Ethenolysis of methyloleate.


7.3.2. Methyloleate metathesis

Methyl esters arising from the transesterification of vegetableoils can be converted into high value compound by a metathesisreaction. Ethenolysis of methyl oleate is expected to produce 1-decene and methyl-9-decenoate, two useful intermediates for theproduction of lubricants and polyesters (Scheme 61) [518].

The metathesis reaction catalysed by different Ru catalysts hasbeen reported in [BMI] or [BMMI][NTf2] ILs [519]. High conversionand selectivity were obtained with Hoveyda I catalyst but thereaction required a high catalyst loading (5 mol%). Complexesbearing ionic tags were found to be poorly recyclable underethenolysis reaction.

8. General conclusion and perspectives

The aim of this review was not to give an exhaustive, fulldescription of all the catalytic reactions that can be performed inILs. Our objective was rather to stand back from the huge quantityof publications and patents and to try to give a general overview ofwhat can be done. In this context, many topics have not beenbroached while being the object of much interest.

One can mention the increasing interest for the use of ILs inbiocatalysis reactions [34,35]. Surprisingly, enzymes of quitediverse types could stay active in ILs in presence of more or lesswater. ILs may offer advantages to this field in terms of bettersolubility of substrates compared to water, improved thermal andoperational stabilities, sometimes enhanced enzyme regio- orenantio-selectivity, and the high potential of integration of theseparation in the reaction, facilitated by the low vapour pressure ofILs. However, the properties of ILs that may affect the catalyticbehaviours of enzymes is not yet quite clear. Could the aqueoussolution of free enzyme be embedded in the IL nano-structurenetwork? What ‘‘quantitative’’ advantages can be expected fromusing ILs in bio-transformations? The data, extracted from openliterature, reveal the high degree of variation in the publishedresults which only permits semi-quantitative comparisons [520].

Ionic liquids present the potential to have a huge impact onorgano-catalysis [521]. This potential has been demonstrated inDiels-Alder reactions in which ILs can display interesting H-bonding with the reactants and then can direct the reactionselectivity. Imidazolium-derived organocatalysts which can berecycled because of their insolubility have also been reported.

Another interesting topic, not discussed in this review, is thecombination of ILs with an electrochemical process [29,522]. ILswere indeed first developed as low-temperature liquid electrolytesbecause of their good electrical conductivity. But so far, their usefor electrocatalysis or electrosynthesis is not as developed as itcould have been expected. For example, ionic liquids can offeradvantages because they combine different properties such asgood conductors of electricity, avoiding the use of supporting salts,and potential active species stabilisers such as Pd or Ptnanoparticles or others. The in situ electrochemical generationof H2O2 or the activation of oxygen for olefin epoxidation areexamples of the potential of ILs. The combination of ILs withsonochemistry can also be an interesting approach for processintensification provided that ILs remain stable under exposure toultra-sounds. In another approach, carbon ionic liquid compositeelectrodes (CILE) have recently been proposed and used asconvenient electrodes for different electrochemical applications[523]. It is well-known that electrochemical activation and

conversion of CO2 can provide an interesting solution to overcomeits thermodynamic stability and kinetic inertness. In the case ofelectro-reduction of CO2, the change of medium, whether or not itis aprotic, or the electrode can change the nature of the productsgenerated. In this context, ILs can open windows because of thepossibility to tune their polarity, miscibility with water and proticcharacter [524]. In this area, electrocatalytic synthesis of organiccarbonates from carbon dioxide and alcohols or phenols have alsobeen reported using ILs [525,526].

Another breakthrough concerns the preparation of newinorganic materials in purely ionic media. To give just oneexample, novel zeotype frameworks (aluminophosphates) couldbe synthesised with an IL serving as both solvent and templateagent [527]. ILs can offer very specific ways of interactions withtheir different degrees of order from liquid crystals to extendedhydrogen-bonding network, polar and non polar regions. They maybe new ‘‘all in one’’ systems acting together as solvent-template-reactants [528]. The new materials generated, with sometimesunprecedented and otherwise inaccessible structures, could openthe way to the discovering new catalytic supports. Finally theintroduction of cyclohexane on an imidazolium moiety can permitaccess to derivatives with very low vapour pressure, high densitychemical and thermal stability. Hydrocarbons can be considered asa liquid storage medium for H2 if they can be hydrogenated ordehydrogenated. These imidazolium salts can add reversibly 6–12hydrogens per ionic pair in the presence of classical hydrogena-tion/dehydrogenation nanoparticle catalysts based on Pd(0) orIr(0). Compressed hydrogen gas can only hold 15 g/L at 350 atm.These ILs could hold up to 30 g/L of H2 at atmospheric pressure.Despite the potential interest of this concept on paper, it is worthmentioning that the dehydrogenation process remains the keyfactor due to the endothermicity of alkane dehydrogenation. Thereaction occurs at T > 200 8C, under an inert atmosphere to avoidILs decomposition [529].

Acknowledgements

We thank J. Vedrine for his invitation to write this review. Wealso thank C. Vallee for his kind contribution to this review and forinstructive discussions. The authors are thankful for the manu-script’s anonymous reviewers contribution, for their constructivecomments and advices. The IFP is gratefully acknowledged forproviding us support and assistance.

We would like to dedicate this review to Y. Chauvin in honor ofhis pioneering contribution in the field of ionic liquids and hiscontinuous assistance.

References

[1] Ionic Liquids in Synthesis, Second, Completely Revised and Enlarged, WILEY-VCHVerlags GmbH & Co. KGaA, Weinheim, 2008.

[2] M. Smiglak, A. Metlen, R.D. Rogers, Acc. Chem. Res. 40 (2007) 1182.[3] J.S. Wilkes, Green Chem. 4 (2002) 73.[4] J.H. Davis Jr., Chem. Lett. 33 (2004) 1072.[5] T. Welton, Coord. Chem. Rev. 248 (2004) 2459.[6] Z.C. Zhang, Adv. Catal. 49 (2006) 153.[7] R. Giernoth, Top. Curr. Chem. 276 (2007) 1.[8] V.I. Parvulescu, C. Hardacre, Chem. Rev. 107 (2007) 2615.[9] S. Liu, J. Xiao, J. Mol. Catal. A: Chem. 270 (2007) 1.

[10] H.G. Joglekar, I. Rahman, B.D. Kulkarni, Chem. Ing. Tech. 30 (2007) 819.[11] N.V. Plechkova, K.R. Seddon, Chem. Soc. Rev. 37 (2008) 123.[12] R. Sebesta, I. Kmentova, S. Toma, Green Chem. 10 (2008) 484.[13] P. Kubisa, Prog. Polym. Sci. 29 (2004) 3.


[14] A. Riisager, R. Fehrmann, M. Haumann, P. Wasserscheid, Eur. J. Inorg. Chem.(2006) 695.

[15] J. Muzart, Adv. Synth. Catal. 348 (2006) 275.[16] S.G. Lee, Chem. Commun. (2006) 1049.[17] M. Haumann, A. Riisager, Chem. Rev. 108 (2008) 1474.[18] K.E. Johnson, R.M. Pagni, J. Bartmess, Monatsh. Chem. 138 (2007) 1077.[19] J. Durand, E. Teuma, M. Gomez, Compt. Rendus. Chim. 10 (2007) 152.[20] A. Paczal, A. Kotschy, Monatsh. Chem. 138 (2007) 1115.[21] K. Binnemans, Chem. Rev. 107 (2007) 2592.[22] X. Han, D.W. Armstrong, Acc. Chem. Res. 40 (2007) 1079.[23] P. Sledz, M. Mauduit, K. Grela, Chem. Soc. Rev. 37 (2008) 2433.[24] R. Singh, M. Sharma, R. Mamgain, D.S. Rawat, J. Braz. Chem. Soc. 19 (2008) 357.[25] S. Keskin, D. Kayrak-Talay, U. Akman, O. Hortacsu, J. Supercrit. Fluids 43 (2007)

150.[26] M.A.P. Martins, C.P. Frizzo, D.N. Moreira, N. Zanatta, H.G. Bonacorso, Chem. Rev.

108 (2008) 2015.[27] K. Bica, P. Gaertner, Eur. J. Org. Chem. (2008) 3235.[28] A. Winkel, P.V.G. Reddy, R. Wilhelm, Synthesis (2008) 999.[29] P. Hapiot, C. Lagrost, Chem. Rev. 108 (2008) 2238.[30] S. Park, R.J. Kazlauskas, Curr. Opin. Biotechnol. 14 (2003) 432.[31] F. van Rantwijk, R. Madeira Lau, R.A. Sheldon, Trends Biotechnol. 21 (2003) 131.[32] Z. Yang, W.B. Pan, Enzyme Microb. Technol. 37 (2005) 19.[33] N. Jain, A. Kumar, S. Chauhan, S.M.S. Chauhan, Tetrahedron 61 (2005) 1015.[34] S. Cantone, U. Hanefeld, A. Basso, Green Chem. 9 (2007) 954.[35] F. van Rantwijk, R.A. Sheldon, Chem. Rev. 107 (2007) 2757.[36] G. Imperato, B. Koenig, C. Chiappe, Eur. J. Org. Chem. (2007) 1049.[37] P.C.A.G. Pinto, M.L.M.F.S. Saraiva, J.L.F.C. Lima, Anal. Sci. 24 (2008) 1231.[38] P.D. de Maria, Angew. Chem. Int. Ed. 47 (2008) 6960.[39] L. Feng, Z. Chen, J. Mol. Liquids 142 (2008) 1.[40] I.J.B. Lin, C.S. Vasam, J. Organomet. Chem. 690 (2005) 3498.[41] C. Chiappe, D. Pieraccini, J. Phys. Org. Chem. 18 (2005) 275.[42] R. Glaser, Chem. Ing. Tech. 30 (2007) 557.[43] G. Keglevich, Z. Baan, I. Hermecz, T. Novak, I.L. Odinets, Curr. Org. Chem. 11

(2007) 107.[44] J. Lu, J. Yan, Prog. Polym. Sci. 34 (2009) 431.[45] B. Buszewski, S. Studzinska, Chromatographia 68 (2008) 1.[46] P. Majewski, A. Pernak, M. Grzymislawski, K. Iwanik, J. Pernak, Acta Histochem.

105 (2003) 135.[47] M.J. Earle, J. Esperanca, M.A. Gilea, J.N.C. Lopes, L.P.N. Rebelo, J.W. Magee, K.R.

Seddon, J.A. Widegren, Nature 439 (2006) 831.[48] S. Zhang, N. Sun, X. He, X. Liu, X. Zhang, J. Phys. Chem. Ref. Data 35 (2006) 1475.[49] H. Tokuda, S. Tsuzuki, M.A.B.H. Susan, K. Hayamizu, M. Watanabe, J. Phys. Chem.

B 110 (2006) 19593.[50] M. Smiglak, W.M. Reichert, J.D. Holbrey, J.S. Wilkes, L.Y. Sun, J.S. Thrasher, K.

Kirichenko, S. Singh, A.R. Katritzky, R.D. Rogers, Chem. Commun. (2006) 2554.[51] S. Schneider, T. Hawkins, M. Rosander, G. Vaghjiani, S. Chambreau, G. Drake,

Energy Fuels 22 (2008) 2871.[52] A.I. Bhatt, A.M. Bond, D.R. MacFarlane, J. Zhang, J.L. Scott, C.R. Strauss, P.I. Iotov,

S.V. Kalcheva, Green Chem. 8 (2006) 161.[53] A. Noda, K. Hayamizu, M. Watanabe, J. Phys. Chem. B 105 (2001) 4603.[54] R.A. Mantz, P.C. Trulove, in: P. Wasserscheid, T. Welton (Eds.), Ionic Liquids in

Synthesis, Wiley-VCH Verlag GmbH & Co., KGaA, Weinheim, 2005, pp. 72–88.[55] J. Jacquemin, P. Nancarrow, D.W. Rooney, M.F.C. Gomes, P. Husson, V. Majer,

A.A.H. Padua, C. Hardacre, J. Chem. Eng. Data 53 (2008) 2133.[56] A.P. Froba, H. Kremer, A. Leipertz, J. Phys. Chem. B 112 (2008) 12420.[57] C. Chiappe, M. Malvaldi, C.S. Pomelli, Pure Appl. Chem. 81 (2009) 767.[58] C. Reichardt, Green Chem. 7 (2005) 339.[59] F.V. Bright, G. Baker, J. Phys. Chem. B 110 (2006) 5822.[60] X. Wang, C.A. Ohlin, Q. Lu, Z.F. Fei, J. Hu, P.J. Dyson, Green Chem. 9 (2007) 1191.[61] W. Martino, F. Fernadez de la Mora, Y. Yoshida, G. Saito, J. Wilkes, Green Chem. 8

(2006) 390.[62] P. Wasserscheid, D. Gerhard, W. Arlt, Chem. Eng. Technol. 30 (2007) 1475.[63] D.R. MacFarlane, J.M. Pringle, K.M. Johansson, S.A. Forsyth, M. Forsyth, Chem.

Commun. (2006) 1905.[64] D.M. Chisholm, J.S. McIndoe, Dalton Trans. (2008) 3933.[65] W. Miao, T.H. Chan, Adv. Synth. Catal. 348 (2006) 1711.[66] S. Himmler, S. Horma, R. van Hal, P.S. Schulz, P. Wasserscheid, Green Chem. 8

(2006) 887.[67] E. Kuhlmann, S. Himmler, H. Giebelhaus, P. Wasserscheid, Green Chem. 9 (2007)

233.[68] S.H. Chung, R. Lopato, S.G. Greenbaum, H. Shirota, E.W. Castner, J.F. Wishart, J.

Phys. Chem. B 111 (2007) 4885.[69] S.F. Liu, T. Fukuyama, M. Sato, L. Ryu, Synlett (2004) 1814.[70] R.P. Swatloski, J.D. Holbrey, R.D. Rogers, Green Chem. 5 (2003) 361.[71] G.S. Fonseca, A.P. Umpierre, P.F.P. Fichtner, S.R. Teixeira, J. Dupont, Chem. Eur. J 9

(2003) 3263.[72] N. Ignat’ev, U. Welz-Biermann, A. Kucheryna, G. Bissky, H. Willner, J. Fluor. Chem.

126 (2005) 1150.[73] D. Bourissou, O. Guerret, F.P. Gabbai, G. Bertrand, Chem. Rev. 100 (2000) 39.[74] T. Ramnial, S.A. Taylor, M.L. Bender, B. Gorodetsky, P.T.K. Lee, D.A. Dickie, B.M.

McCollum, C.C. Pye, C.J. Walsby, J.A.C. Clyburne, J. Org. Chem. 73 (2008) 801.[75] A.I. Siriwardana, I.R. Crossley, A.A.J. Torriero, I.M. Burgar, N.F. Dunlop, A.M. Bond,

G.B. Deacon, D.R. MacFarlane, J. Org. Chem. 73 (2008) 4676.[76] W.L. Hough, M. Smiglak, H. Rodriguez, R.P. Swatloski, S.K. Spear, D.T. Daly, J.

Pernak, J.E. Grisel, R.D. Carliss, M.D. Soutullo, J.H. Davis, R.D. Rogers, New J. Chem.31 (2007) 1429.

[77] Y. Fukaya, A. Sugimoto, H. Ohno, Biomacromolecules 7 (2006) 3295.[78] Y. Fukaya, Y. Iizuka, K. Sekikawa, H. Ohno, Green Chem. 9 (2007) 1155.[79] A.P. Abbott, D. Boothby, G. Capper, D.L. Davies, R.K. Rasheed, J. Am. Chem. Soc.

126 (2004) 9142.[80] Y. Yu, X. Lu, Q. Zhou, K. Dong, H. Yao, S. Zhang, Chem. Eur. J. 14 (2008) 11174.[81] J.R. Harjani, R.D. Singer, M.T. Garcia, P.J. Scammells, Green Chem. 11 (2009) 83.[82] Y.S. Chun, J.Y. Shin, C.E. Song, S.G. Lee, Chem. Commun. (2008) 942.[83] T. Sasaki, M. Tada, C. Zhong, T. Kume, Y. Iwasawa, J. Mol. Catal. A: Chem. 279

(2008) 200.[84] B. Wu, W.W. Liu, Y.M. Zhang, H.P. Wang, Chem. Eur. J. 15 (2009) 1804.[85] P. Walden, Bull. Acad. Imp. Sci. (1914) 1800.[86] C.F. Poole, J. Chromatogr. A 1037 (2004) 49.[87] J.-F. Huang, G.A. Baker, H. Luo, K. Hong, Q.-F. Li, N.J. Bjerrum, S. Dai, Green Chem.

8 (2006) 599.[88] A. Fernicola, S. Panero, B. Scrosati, M. Tamada, H. Ohno, ChemPhysChem 8 (2007)

1103.[89] S. Guo, Z. Du, S. Zhang, D. Li, Z. Li, Y. Deng, Green Chem. 8 (2006) 296.[90] G. Driver, K.E. Johnson, Green Chem. 5 (2003) 163.[91] M. Yoshizawa, W. Xu, C.A. Angell, J. Am. Chem. Soc. 125 (2003) 15411.[92] D.R. MacFarlane, K.R. Seddon, Aus. J. Chem. 60 (2007) 3.[93] T.L. Greaves, C.J. Drummond, Chem. Rev. 108 (2008) 206.[94] W. Ogihara, T. Aoyama, H. Ohno, Chem. Lett. 33 (2004) 1414.[95] J.-F. Huang, H. Luo, C. Liang, I.-W. Sun, G.A. Baker, S. Dai, J. Am. Chem. Soc. 127

(2005) 12784.[96] Z.F. Fei, D.B. Zhao, T.J. Geldbach, R. Scopelliti, P.J. Dyson, Chem. Eur. J. 10 (2004)

4886.[97] A.E. Visser, R.P. Swatloski, W.M. Reichert, R. Mayton, S. Sheff, A. Wierzbicki, J.H.

Davis, R.D. Rogers, Chem. Commun. (2001) 135.[98] Z.F. Fei, T.J. Geldbach, D.B. Zhao, P.J. Dyson, Chem. Eur. J. 12 (2006) 2123.[99] H. Ohno, Bull. Chem. Soc. Jpn. 79 (2006) 1665.

[100] P.J. Dyson, T.J. Geldbach, Electrochem. Soc. Interface (2007) 50.[101] F.R. Trouw, D.L. Price, Ann. Rev. Phys. Chem. 50 (1999) 571.[102] P.C. Trulove, R.A. Osteryoung, Inorg. Chem. 31 (1992) 3980.[103] J.L. Campbell, K.E. Johnson, J. Electrochem. Soc. 141 (1994) L19.[104] Y. Wang, D. Jiang, L. Dai, Catal. Commun. 9 (2008) 2475.[105] G. Cheng, X. Duan, X. Qi, C. Lu, Catal. Commun. 10 (2008) 201.[106] D.C. Forbes, K.J. Weaver, J. Mol. Catal. A: Chem. 214 (2004) 129.[107] P. Nockemann, B. Thijs, S. Pittois, J. Thoen, C. Glorieux, K. van Hecke, L. van

Meervelt, B. Kirchner, K. Binnemans, J. Phys. Chem. B 110 (2006) 20978.[108] M. Feroci, I. Chiarotto, M. Orsini, G. Sotgiu, A. Inesi, Adv. Synth. Catal. 350 (2008)

1355.[109] E.D. Bates, R.D. Mayton, I. Ntai, J.H. Davis, J. Am. Chem. Soc. 124 (2002) 926.[110] J.M. Zhang, F.J. Zhang, K. Dong, Y.Q. Zhang, Y.Q. Shen, X.M. Lv, Chem. Eur. J. 12

(2006) 4021.[111] M. Yoshizawa-Fujita, K. Johansson, P. Newman, D.R. MacFarlane, M. Forsyth,

Tetrahedron Lett. 47 (2006) 2755.[112] Y. Cai, Y. Peng, G. Song, Catal. Lett. 109 (2006) 61.[113] W. Li, Z. Zhang, B. Han, S. Hu, J. Song, Y. Xie, X. Zhou, Green Chem. 10 (2008) 1142.[114] L. Xu, G. Ou, Y. Yuan, J. Organomet. Chem. 693 (2008) 3000.[115] C. Vallee, Y. Chauvin, J.M. Basset, C.C. Santini, J.C. Galland, Adv. Synth. Catal. 347

(2005) 1835.[116] H. Ohno, K. Fukumoto, Acc. Chem. Res. 40 (2007) 1122.[117] K. Fukumoto, H. Ohno, Chem. Commun. (2006) 3081.[118] B. Pegot, G. Vo-Thanh, D. Gori, A. Loupy, Tetrahedron Lett. 45 (2004) 6425.[119] B. Gadenne, P. Hesemann, V. Polshettiwar, J.J.E. Moreau, Eur. J. Inorg. Chem.

(2006) 3697.[120] L. Phan, J.R. Andreatta, L.K. Horvey, C.F. Edie, A.L. Luco, A. Mirchandani, D.J.

Darensbourg, P.G. Jessop, J. Org. Chem. 73 (2008) 127.[121] L. Phan, D. Chiu, D.J. Heldebrant, H. Huttenhower, E.A. John, X. Li, P. Pollet, R.

Wang, C.A. Eckert, C.L. Liotta, P.G. Jessop, Ind. Eng. Chem. Res. 47 (2008) 539.[122] P.G. Jessop, B. Subramaniam, Chem. Rev. 107 (2007) 2666.[123] J.F. Huang, H.M. Luo, C.D. Liang, D.E. Jiang, S. Dai, Ind. Eng. Chem. Res. 47 (2008)

881.[124] Y. Hou, Y. Gu, S. Zhang, F. Yang, H. Ding, Y. Shan, J. Mol. Liquids 143 (2008) 154.[125] A.P. Abbott, G. Capper, D.L. Davies, R.K. Rasheed, V. Tambyrajah, Chem. Commun.

(2003) 70.[126] H.Y. Liang, H. Li, Z.X. Wang, F. Wu, L.Q. Chen, X.J. Huang, J. Phys. Chem. B 105

(2001) 9966.[127] Y.S. Hu, H. Li, X.J. Huang, L.Q. Chen, Electrochem. Commun. 6 (2004) 28.[128] H.R. Jhong, D.S.H. Wong, C.C. Wan, Y.Y. Wang, T.C. Wei, Electrochem. Commun.

11 (2009) 209.[129] T.J. Geldbach, Organomet. Chem. 34 (2008) 58.[130] V. Lecocq, A. Graille, C.C. Santini, A. Baudouin, Y. Chauvin, J.M. Basset, L. Arzel, D.

Bouchu, B. Fenet, New J. Chem. 29 (2005) 700.[131] R. Srivastava, S.I. Fujita, S. Okamura, M. Arai, React. Kinet. Catal. Lett. 96 (2009)

55.[132] A.P. Abbott, G. Capper, D.L. Davies, R.K. Rasheed, V. Tambyrajah, Green Chem. 4

(2002) 24.[133] P.J. Dyson, J.S. McIndoe, D. Zhao, Chem. Commun. (2003) 508.[134] R. Balasubramanian, W. Wang, R.W. Murray, J. Am. Chem. Soc. 128 (2006) 9994.[135] A.P. Abbott, K.J. McKenzie, Phys. Chem. Chem. Phys. 8 (2006) 4265.[136] K. Binnemans, Chem. Rev. 105 (2005) 4148.[137] M. Smiglak, J.D. Holbrey, S.T. Griffi, W.M. Reichert, R.P. Swatloski, A.R. Kattrtzky,

H. Yang, D. Zhang, K. Kirichenko, R.D. Rogers, Green Chem. 9 (2007) 90.[138] J.M. Leveque, J. Estager, M. Draye, G. Cravotto, L. Boffa, W. Bonrath, Monatsh.

Chem. 138 (2007) 1103.


[139] J.D. Oxley, T. Prozorov, K.S. Suslick, J. Am. Chem. Soc. 125 (2003) 11138.[140] S. Horikoshi, T. Hamamura, M. Kajitani, M. Yoshizawa-Fujita, N. Serpone, Org.

Process Res. Dev. 12 (2008) 1089.[141] S. Himmler, A. Koenig, P. Wasserscheid, Green Chem. 9 (2007) 935.[142] N. Paape, W. Wei, A. Bosmann, C. Kolbeck, F. Maier, H.P. Steinruck, P. Wassersc-

heid, P.S. Schulz, Chem. Commun. (2008) 3867.[143] A. Koenig, M. Stepanski, A. Kuszlik, P. Keil, C. Weller, Chem. Eng. Res. Des. 86

(2008) 775.[144] A. Burell, R.E. Del Sesto, S.N. Baker, T.M. McCleskey, G.A. Baker, Green Chem. 9

(2007) 449.[145] M.J. Earle, C.M. Gordon, N.V. Plechkova, K.R. Seddon, T. Welton, Anal. Chem. 79

(2007) 758.[146] G.B. Appetecchi, S. Scaccia, S. Tizzani, F. Alessandrini, S. Paserini, J. Electrochem.

Soc. 153 (2006) A1685.[147] F. Endres, S.Z. El Abedin, N. Borissenko, Zeitschrift fuer Physikalische Chemie 220

(2006) 1377.[148] B.R. Clare, P.M. Bayley, A.S. Best, M. Forsyth, D.R. MacFarlane, Chem. Commun.

(2008) 2689.[149] R. Lungwitz, S. Spange, New J. Chem. 32 (2008) 392.[150] A. Stark, P. Behrend, O. Braun, A. Muller, J. Ranke, B. Ondruschka, B. Jastorff,

Green Chem. 10 (2008) 1152.[151] Y. Yasaka, C. Wakai, N. Matubayasi, M. Nakahara, Anal. Chem. 81 (2009) 400.[152] R. Ge, R.W.K. Allen, L. Aldous, M.R. Bown, N. Doy, C. Hardacre, J.M. MacInnes, G.

McHale, M.I. Newton, Anal. Chem. 81 (2009) 1628.[153] F. Maier, J.M. Gottfried, J. Rossa, D. Gerhard, P.S. Schulz, W. Schwieger, P.

Wasserscheid, H.P. Steinruck, Angew. Chem. Int. Ed. 45 (2006) 77.[154] J.M. Gottfried, F. Maier, J. Rossa, D. Gerhard, P.S. Schulz, P. Wasserscheid, H.P.

Steinrueck, Zeitschrift fuer Physikalische Chemie 220 (2006) 1439.[155] C. Wakai, A. Oleinikova, H. Weingartner, J. Phys. Chem. B 2006 (2006) 11.[156] C. Wakai, A. Oleinikova, M. Ott, H. Weingartner, J. Phys. Chem. B 109 (2005)

17028.[157] H. Weingartner, P. Sasisanker, C. Daguenet, P.J. Dyson, I. Krossing, J.M. Slattery, T.

Schubert, J. Phys. Chem. B 111 (2007) 4775.[158] G. Ranieri, J.P. Hallett, T. Welton, Ind. Eng. Chem. Res. 47 (2008) 638.[159] C. Reichardt, Chem. Soc. Rev. 94 (1994) 2319.[160] C. Reichardt, Pure Appl. Chem. 80 (2008) 1415.[161] J.-M. Lee, S. Ruckes, J.M. Prausnitz, J. Phys. Chem. B 112 (2008) 1473.[162] G. Angelini, C. Chiappe, P. de Maria, A. Fontana, F. Gasparrini, D. Pieraccini, M.

Pierini, G. Siani, J. Org. Chem. 70 (2005) 8193.[163] B.R. Mellein, S.N.V.K. Aki, R.L. Ladewski, J.F. Brennecke, J. Phys. Chem. B 111

(2007) 131.[164] C. Hardacre, J.D. Holbrey, M. Nieuwenhuyzen, T.G.A. Youngs, Acc. Chem. Res.

(2007) 1146.[165] L. Crowhurst, P. Mawdsley, J.M. Perez-Arlandis, P.A. Salter, T. Welton, Phys.

Chem. Chem. Phys. 5 (2003) 2790.[166] P. Kolle, R. Dronskowski, Inorg. Chem. 43 (2004) 2803.[167] A. Arce, M.J. Earle, S.P. Katdare, H. Rodriguez, K.R. Seddon, Chem. Commun.

(2006) 2548.[168] B. Bini, O. Bortolini, C. Chiappe, D. Pieraccini, T. Siciliano, J. Phys. Chem. B 111

(2007) 598.[169] S. Tsuzuki, H. Tokuda, K. Hayamizu, M. Watanabe, J. Phys. Chem. B 109 (2005)

16474.[170] S. Begel, P. Illner, S. Kern, R. Puchta, R. van Eldik, Inorg. Chem. 47 (2008) 7121.[171] D. Nama, P.G.A. Kumar, P.S. Pregosin, T.J. Geldbach, P.J. Dyson, Inorg. Chim. Acta

359 (2006) 1907.[172] P.S. Schulz, N. Muller, A. Bosmann, P. Wasserscheid, Angew. Chem. Int. Ed. 46

(2007) 1293.[173] J. Dupont, J. Braz. Chem. Soc. 15 (2004) 341.[174] M. Deetlefs, C. Hardacre, M. Nieuwenhuyzen, A.A.H. Padua, O. Sheppard, A.K.

Soper, J. Phys. Chem. B 110 (2006) 12055.[175] D. Xiao, J.R. Rajian, A. Cady, S. Li, R.A. Bartsch, E.L. Quitevis, J. Phys. Chem. B 111

(2007) 4669.[176] A. Mele, G. Romano, M. Giannone, E. Ragg, G. Fronza, G. Raos, V. Marcon, Angew.

Chem. Int. Ed. 45 (2006) 1123.[177] A. Triolo, O. Russina, H.-J. Bleif, E. di Cola, J. Phys. Chem. B 111 (2007) 4641.[178] J.N. Canongia Lopes, J. Deschamps, A.A.H. Padua, J. Phys. Chem. B 108 (2004)

2038.[179] J.N. Canongia Lopes, A.A.H. Padua, J. Phys. Chem. B 110 (2006) 19586.[180] J.N. Canongia Lopes, A.A.H. Padua, K. Shimizu, J. Phys. Chem. B 112 (2008) 5039.[181] A.A.H. Padua, M.F. Costa Gomes, J.N. Canongia Lopes, Acc. Chem. Res. 40 (2007)

1087.[182] J.N. Canongia Lopes, A.A.H. Padua, J. Phys. Chem. B 110 (2006) 3330.[183] J.L. Anderson, D.W. Armstrong, Anal. Chem. 75 (2003) 4851.[184] A. Aggarwal, N.L. Lancaster, A.R. Sethi, T. Welton, Green Chem. 4 (2002) 517.[185] J. Horwarth, K. Hanlon, D. Fayne, P. McCormac, Tetrahedron Lett. 38 (1997) 3097.[186] A. Vidis, C.A. Ohlin, G. Laurenczy, E. Kusters, G. Sedelmeier, P.J. Dyson, Adv. Synth.

Catal. 347 (2005) 266.[187] R. Bini, C. Chiappe, V. Llopis-Mestre, C.S. Pomelli, T. Welton, Org. Biomol. Chem. 6

(2008) 2522.[188] L. Crowhurst, R. Falcone, N.L. Lancaster, V. Llopis-Mestre, T. Welton, J. Org. Chem.

71 (2006) 8847.[189] C. Betti, D. Landini, A. Maia, Tetrahedron 64 (2008) 1689.[190] J.P. Hallett, C.L. Liotta, G. Ranieri, T. Welton, J. Org. Chem. 74 (2009) 1864.[191] T.P. Wells, J.P. Hallett, C.K. Williams, T. Welton, J. Org. Chem. 73 (2008) 5585.[192] J.N. Canongia Lopes, M.F. Costa Gomes, A.A.H. Padua, J. Phys. Chem. B 110 (2006)

16816.

[193] L. Cammarata, S.G. Kazarian, P.A. Salter, T. Welton, Phys. Chem. Chem. Phys.(2001) 5192.

[194] G. Dimitrakis, I.J. Villar Garcia, E. Lester, P. Licence, P. Kingman, Phys. Chem.Chem. Phys. 10 (2008) 2947.

[195] W. Jiang, Y. Wang, G.A. Voth, J. Phys. Chem. B 111 (2007) 4812.[196] Y. Danten, M.I. Cabaco, M. Besnard, J. Phys. Chem. A 113 (2009) 2873.[197] E. Amigues, C. Hardacre, G. Keane, M. Migaud, M. O’Neil, Chem. Commun. (2006)

72.[198] V.A. Farmer, T. Welton, Green Chem. 4 (2002) 97.[199] S. Doherty, P. Goodrich, C. Hardacre, H.K. Luo, D.W. Rooney, K.R. Seddon, P.

Styring, Green Chem. 6 (2004) 63.[200] S. Dorbritz, W. Ruth, U. Kragl, Adv. Synth. Catal. 347 (2005) 1273.[201] J.D. Holbrey, W.M. Reichert, M. Nieuwenhuyzen, O. Sheppard, C. Hardacre, R.D.

Rogers, Chem. Commun. (2003) 476.[202] J. Lachwa, J. Szydlowski, A. Makowska, K.R. Seddon, J.M.S.S. Esperanca, H.J.R.

Guedes, L.P.N. Rebelo, Green Chem. 8 (2006) 262.[203] J.C. Ma, D.A. Dougherty, Chem. Rev. 97 (1997) 1303.[204] T. Gutel, C.C. Santini, A.A.H. Padua, B. Fenet, Y. Chauvin, J.N. Canongia Lopes, F.

Bayard, M.F. Costa Gomes, A.S. Pensado, J. Phys. Chem. B 113 (2009) 170.[205] R. Gausepohl, P. Buskens, J. Kleinen, A. Bruckmann, C.W. Lehmann, J. Klanker-

mayer, W. Leitner, Angew. Chem. Int. Ed. 45 (2006) 3689.[206] T. Robert, H. Olivier-Bourbigou, L. Magna, B. Gilbert, ECS Trans. 3 (2007) 71.[207] C. Thomazeau, H. Olivier-Bourbigou, L. Magna, S. Luts, B. Gilbert, J. Am. Chem.

Soc. 125 (2003) 5264.[208] G.V.S.M. Carrera, L.C. Branco, J. Aires-de-Souza, C.A.M. Afonso, Tetrahedron 64

(2008) 2216.[209] A.R. Katritzky, M. Kuanar, I.B. Stoyanova-Slavova, S.H. Slavov, D.A. Dobchev, M.

Karelson, W.E.J. Acree, J. Chem. Eng. Data 53 (2008) 1085.[210] R.L. Gardas, J.A.P. Coutinho, Fluid Phase Equilib. 266 (2008) 195.[211] R.L. Gardas, J.A.P. Coutinho, Fluid Phase Equilib. 265 (2008) 57.[212] P. Hunt, I.R. Gould, B. Kirchner, Aust. J. Chem. 60 (2007) 9.[213] B.L. Bhargava, S. Balasubramanian, J. Phys. Chem. B 11 (2007) 4477.[214] B.L. Bhargava, M.L. Klein, S. Balasubramanian, ChemPhysChem 9 (2008) 67.[215] J.N.C. Lopes, A.A.H. Padua, J. Phys. Chem. B 110 (2006) 7485.[216] C.G. Hanke, A. Johansson, J.B. Harper, R.M. Lynden-Bell, Chem. Phys. Lett. 374

(2003) 85.[217] C. Cadena, J.L. Anthony, J.K. Shah, T.I. Morrow, J.F. Brennecke, E.J. Maginn, J. Am.

Chem. Soc. 126 (2004) 5300.[218] B.L. Bhargava, S. Balasubramanian, M.L. Klein, Chem. Commun. (2008) 3339.[219] N. Sieffert, G. Wipff, J. Phys. Chem. B 111 (2007) 4951.[220] Z. Lei, B. Chen, C. Li, H. Liu, Chem. Rev. 108 (2008) 1419.[221] J. Slattery, C. Daguenet, P.J. Dyson, T.J.S. Schubert, I. Krossing, Angew. Chem. Int.

Ed. 46 (2007) 5384.[222] F. Bessac, F. Maseras, J. Computat. Sci. (2008) 892.[223] M.J. Earle, S.P. Katdare, K.R. Seddon, Org. Lett. 6 (2004) 707.[224] A.M. O’Mahony, D.S. Silvester, L. Aldous, C. Hardacre, R.G. Compton, J. Chem. Eng.

Data 53 (2008) 2884.[225] K.R. Seddon, A. Stark, M.J. Torres, Pure Appl. Chem. 72 (2000) 2275.[226] P.J. Dyson, D.J. Ellis, W. Henderson, G. Laurenczy, Adv. Synth. Catal. 345 (2003)

216.[227] A. Stark, M. Ajam, M. Green, H.G. Raubenheimer, A. Ranwell, B. Ondruschka, Adv.

Synth. Catal. 348 (2006) 1934.[228] M. Picquet, I. Tkatchenko, I. Tommasi, P. Wasserscheid, J. Zimmermann, Adv.

Synth. Catal. 345 (2003) 959.[229] C. Daguenet, R. Scopelliti, P.J. Dyson, Organometallics 23 (2004) 4849.[230] C. Daguenet, P.J. Dyson, Organometallics 23 (2004) 6080.[231] C. Daguenet, P.J. Dyson, Organometallics 25 (2006) 5811.[232] F. D’Anna, R. Noto, Tetrahedron 63 (2007) 11681.[233] M. Meciarova, M. Cigan, S. Toma, A. Gaplovsky, Eur. J. Org. Chem. (2008)

4408.[234] M. Meciarova, S. Toma, Chem. Eur. J. 13 (2007) 1268.[235] P. Dash, R.W.J. Scott, Chem. Commun. (2009) 812.[236] J. McNulty, J.J. Nair, S. Cheekoori, V. Larichev, A. Capretta, A.J. Robertson, Chem.

Eur. J. 12 (2006) 9314.[237] D.B. Williams, M. Ajam, A. Ranwell, Organometallics 26 (2007) 4692.[238] D. Chen, M. Schmitkamp, G. Francio, J. Klankermayer, W. Leitner, Angew. Chem.

Int. Ed. 47 (2008) 7339.[239] M.B. Turner, S.K. Spear, J.G. Huddleston, J.D. Holbrey, R.D. Rogers, Green Chem. 5

(2003) 443.[240] G. Hinckley, V.V. Mozhaev, C. Budde, Y.L. Khmelnitsky, Biotechnol. Lett. 24

(2002) 2083.[241] F. Ganske, U.T. Bornscheuer, Org. Lett. 7 (2005) 3097.[242] B.K. Liu, N. Wang, Z.C. Chen, Q. Wu, X.F. Lin, Bioorg. Medicinal Chem. Lett. 16

(2006) 3769.[243] T. Itoh, S. Han, Y. Matsushita, S. Hayase, Green Chem. 6 (2004) 437.[244] W.Y. Lou, M.H. Zong, H. Wu, R. Xu, J.F. Wang, Green Chem. 7 (2005) 500.[245] B.K. Liu, Q. Wu, J.M. Wu, L. Xian-Fu, Chem. Commun. (2007) 295.[246] A. Babai, A.V. Mudring, Z. Anorg. Allg. Chem. 634 (2008) 938.[247] S. Tang, A. Babai, A.V. Mudring, Angew. Chem. Int. Ed. 47 (2008) 7631.[248] D.B. Williams, M.E. Stoll, B.L. Scott, D.A. Costa, W.J. Oldham, Chem. Commun.

(2005) 1438.[249] A.V. Mudring, A. Babai, S. Arens, R. Giernoth, Angew. Chem. Int. Ed. 44 (2005)

5485.[250] R. Bini, C. Chiappe, E. Marmugi, D. Pieraccini, Chem. Commun. (2006) 897.[251] V. Lecocq, H. Olivier-Bourbigou, Oil Gas Sci. Technol. 62 (2007) 761.[252] B. Gilbert, H. Olivier-Bourbigou, F. Favre, Oil Gas Sci. Technol. 62 (2007) 745.


[253] W. Zawartka, A.M. Trzeciak, J.J. Ziolkowski, T. Lis, Z. Ciunik, J. Pernak, Adv. Synth.Catal. 348 (2006) 1689.

[254] R.T. Carlin, J. Wilkes, J. Mol. Catal. 63 (1990) 125.[255] I. Raabe, K. Wagner, K. Guttsche, M. Wang, M. Gratzel, G. Santiso-Quinones, I.

Krossing, Chem. Eur. J. 15 (2009) 1966.[256] P.G. Gassman, P.A. Deck, Organometallics 13 (1994) 1934.[257] A. Nafady, T.T. Chin, W.E. Geiger, Organometallics 25 (2006) 1654.[258] S. Diez-Gonzalez, S.P. Nolan, Coord. Chem. Rev. 251 (2007) 874.[259] R.H. Crabtree, Oil Gas Sci. Technol. 62 (2007) 739.[260] J. Mo, L.J. Xu, J.L. Xiao, J. Am. Chem. Soc. 127 (2005) 751.[261] F. McLachlan, C.J. Mathews, P.J. Smith, T. Welton, Organometallics 22 (2003)

5350.[262] C.J. Mathews, P.J. Smith, T. Welton, Organometallics 20 (2001) 3848.[263] L. Magna, Y. Chauvin, G.P. Niccolai, J.M. Basset, Organometallics 22 (2003) 4418.[264] N.D. Clement, K.J. Cavell, C. Jones, C.J. Elsevier, Angew. Chem. Int. Ed. 43 (2004)

1277.[265] D. Bacciu, K.J. Cavell, I.A. Fallis, L.L. Ooi, Angew. Chem. Int. Ed. 44 (2005) 5282.[266] U. Hintermair, T. Gutel, A.M.Z. Slawin, D.J. Cole-Hamilton, C.C. Santini, Y.

Chauvin, J. Organomet. Chem. 693 (2008) 2407.[267] L.S. Ott, M.L. Cline, M. Deetlefs, K.R. Seddon, R.G. Finke, J. Am. Chem. Soc. 127

(2005) 5758.[268] J.D. Scholten, J. Dupont, Organometallics 27 (2008) 4439.[269] C. Vallee, C. Valerio, Y. Chauvin, G.P. Niccolai, J.M. Basset, C.C. Santini, J.C. Galland,

B. Didillon, J. Mol. Catal. A: Chem. 214 (2004) 71.[270] J.D. Scholten, G. Ebeling, J. Dupont, Dalton Trans. (2007) 5554.[271] J.-C. Hsu, Y.-H. Yen, Y.-H. Chu, Tetrahedron Lett. 45 (2004) 4673.[272] J. Mo, L. Xu, J. Xiao, J. Am. Chem. Soc. 127 (2005) 751.[273] X. Wu, J. Mo, X. Li, Z. Hyder, J. Xiao, Progress Nat. Sci. 18 (2008) 639.[274] T. Gutel, P. Campbell, C.C. Santini, F. Lefebvre, C. Lucas, Y. Chauvin, Chim. Oggi-

Chem. Today 27 (2009) 48.[275] K. Anderson, P. Goodrich, C. Hardacre, D.W. Rooney, Green Chem. 5 (2003) 448.[276] L.A. Blanchard, J.F. Brennecke, Ind. Eng. Chem. Res. 40 (2001) 287.[277] R.M. Lynden-Bell, N.A. Atamas, A. Vasilyuk, C.G. Hanke, Mol. Phys. 100 (2002)

3225.[278] D. Semeril, H. Olivier-Bourbigou, C. Bruneau, P.H. Dixneuf, Chem. Commun.

(2002) 146.[279] P. Illner, S. Begel, S. Kern, R. Puchta, R. van Eldik, Inorg. Chem. 48 (2008) 588.[280] L. Magna, in: B. Cornils, W.A. Herrmann, I.T. Horvath, W. Leitner, S. Mecking, H.

Olivier-Bourbigou, D. Vogt (Eds.), Multiphase Homogeneous Catalysis, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, 2005, pp. 477–494.

[281] P. Migowski, J. Dupont, Chem. Eur. J. 13 (2006) 32.[282] G. Yanlong, L. Guangxing, Adv. Synth. Catal. 351 (2009) 817.[283] J. Kramer, E. Redel, R. Thomann, C. Janiak, Organometallics 27 (2008) 1976.[284] J. Dupont, D. de Oliveira, in: D. Astruc (Ed.), Nanoparticules and Catalysis, Wiley-

VCH, Weinheim, 2007, p. 195, Chapter 6.[285] X.D. Mu, D.G. Evans, Y. Kou, Catal. Lett. 97 (2004) 151.[286] J. Huang, T. Jiang, B. Han, H. Gao, Y. Chang, G. Zhao, W. Wu, Chem. Commun.

(2003) 1654.[287] B. Leger, A. Icourt-Nowicki, A. Roucoux, H. Olivier-Bourbigou, Adv. Synth. Catal.

350 (2008) 153.[288] B. Leger, A. Icourt-Nowicki, H. Olivier-Bourbigou, A. Roucoux, Inorg. Chem. 47

(2008) 9090.[289] B. Leger, A. Icourt-Nowicki, H. Olivier-Bourbigou, A. Roucoux, ChemSusChem 1

(2008) 984.[290] H.S. Schrekker, M.A. Gelesky, M.P. Stracke, C.M.L. Schrekker, G. Machado, S.R.

Teixeira, J.C. Rubim, J. Dupont, J. Colloid Interface Sci. 316 (2007) 189.[291] T. Gutel, J. Garcia-Anton, K. Philippot, C.C. Santini, Y. Chauvin, B. Chaudret, J.M.

Basset, J. Mater. Chem. 17 (2007) 3290.[292] X. Xie, J. Lu, B. Chen, J. Han, X. She, X. Pan, Tetrahedron Lett. 45 (2004) 809.[293] Y. Liu, M. Li, Y. Lu, G.H. Gao, Q. Yang, M.Y. He, Catal. Commun. 7 (2006) 985.[294] C. Chiappe, D. Pieraccini, D. Zhao, Z. Fei, P.J. Dyson, Adv. Synth. Catal. 348 (2006)

68.[295] C.C. Cassol, A.P. Umpierre, G. Machado, S.I. Wolke, J. Dupont, J. Am. Chem. Soc.

127 (2005) 3298.[296] M.T. Reetz, J.G. de Vries, Chem. Commun. (2004) 1559.[297] J. Durand, E. Teuma, M. Gomes, Eur. J. Inorg. Chem. (2008) 3577.[298] N. Yan, X. Yang, Z. Fei, Y. Li, Y. Kou, P.J. Dyson, Organometallics 28 (2009) 937.[299] R. Giernoth, D. Bankmann, Eur. J. Org. Chem. 21 (2005) 4529.[300] J. Durand, F. Fernandez, C. Barriere, E. Teuma, K. Gomez, G. Gonzalez, M. Gomez,

Magn. Reson. Chem. 46 (2008) 739.[301] J. Kiefer, K. Obert, S. Himmler, P.S. Schulz, P. Wasserscheid, A. Leipertz, Chem-

PhysChem 9 (2008) 2207.[302] F. Hebrard, P. Kalck, L. Saussine, L. Magna, H. Olivier-Bourbigou, Dalton Trans.

(2007) 190.[303] J. Esser, P. Wasserscheid, A. Jess, Green Chem. (2004) 316.[304] X. Jiang, Y. Li, Z. Wang, Fuel (2008) 79.[305] W. Zhu, H. Li, X. Jiang, Y. Yan, J. Lu, J. Xia, Energy Fuels 21 (2007) 2514.[306] W. Zhu, H. Li, X. Jiang, Y. Yan, J. Lu, L. He, J. Xia, Green Chem. 10 (2008) 641.[307] E. Ito, J.A.R. van Veen, Catal. Today 116 (2006) 446.[308] D. Zhao, R. Liu, J. Wang, B. Liu, Energy Fuels 22 (2008) 1100.[309] L. Lu, S. Cheng, J. Gao, G. Gao, M.Y. He, Energy Fuels 21 (2007) 383.[310] Y. Nie, C. Li, A. Sun, H. Meng, Z. Wang, Energy Fuels 20 (2006) 2083.[311] N.H. Ko, J.S. Lee, E.S. Huh, H. Lee, K.D. Jung, H.S. Kim, M. Cheong, Energy Fuels 22

(2008) 1687.[312] H. Olivier-Bourbigou, D. Uzio, L. Magna, F. Diehl, Institut Francais du Petrole,

2004, US 2004045874.

[313] R.A. Sheldon, Chem. Commun. (2008) 3352.[314] T. Baker, S. Kobayashi, W. Leitner, Adv. Synth. Catal. 48 (2006) 1337.[315] B. Cornils, W.A. Herrmann, I.T. Horvath, W. Leitner, S. Mecking, H. Olivier-

Bourbigou, D. Vogt, Multiphase Homogeneous Catalysis, Wiley-VCH VerlagGmbH & Co. kGaA, Weinheim, 2005.

[316] B. Cornils, W.A. Herrmann, I.T. Horvath, W. Leitner, S. Mecking, H. Olivier-Bourbigou, D. Vogt (Eds.), Multiphase Homogeneous Catalysis, Wiley-VCHVerlag GmbH & Co. KGaA, Weinheim, 2005, pp. 405–605.

[317] P.J. Dyson, G. Laurenczy, C.A. Ohlin, J. Vallance, T. Welton, Chem. Commun.(2003) 2418.

[318] J. Jacquemin, P. Husson, V. Majer, M.F.C. Gomes, Fluid Phase Equilib. 240 (2006)87.

[319] L. Magna, S. Harry, D. Proriol, L. Saussine, H. Olivier-Bourbigou, Oil Gas Sci.Technol. 62 (2007) 775.

[320] Y.T. Vigranenko, V.A. Rybakov, V.S. Fedorov, R.B. Borisov, V.M. Gavrilova, G.N.Gvozdovskii, Petroleum Chem. 36 (1996) 56.

[321] R. Iwanaga, Bull. Chem. Soc. Jpn. 35 (1962) 865.[322] M.F. Sellin, P.B. Webb, D.J. Cole-Hamilton, Chem. Commun. (2001) 781.[323] P.B. Webb, T.E. Kunene, D.J. Cole-Hamilton, Green Chem. 7 (2005) 373.[324] M.R. Damen, R.W. Brand, S.C. Bloem, E. Pingen, K. Steur, C.J. Peters, G.J. Witkamp,

M.C. Kroon, Chem. Eng. Process 48 (2009) 549.[325] P.G. Jessop, R.R. Stanley, R.A. Brown, C.A. Eckert, C.L. Liotta, T.T. Ngo, P. Pollet,

Green Chem. 5 (2003) 123.[326] A. Bosmann, G. Francio, E. Janssen, M. Solinas, W. Leitner, P. Wasserscheid,

Angew. Chem. Int. Ed. 40 (2001) 2697.[327] M.T. Reetz, W. Wiesenhoefer, G. Francio, W. Leitner, Adv. Synth. Catal. 345

(2003) 1221.[328] F. Zayed, L. Greiner, P.S. Schulz, A. Lapkin, W. Leitner, Chem. Commun. (2008) 79.[329] P. Wasserscheid, M. Eichmann, Catal. Today 66 (2001) 309.[330] T.J. Geldbach, D. Zhao, N.C. Castillo, G. Laurenczy, B. Weyershausen, P.J. Dyson, J.

Am. Chem. Soc. 128 (2006) 9773.[331] B. Weyershausen, K. Hell, U. Hesse, Green Chem. 7 (2005) 283.[332] N. Hofmann, A. Bauer, T. Frey, M. Auer, V. Stanjek, P.S. Schulz, N. Taccardi, P.

Wasserscheid, Adv. Synth. Catal. 350 (2008) 2599.[333] C. de Castro, E. Sauvage, M.H. Valkenberg, W.H. Holderich, J. Catal. 196 (2000) 86.[334] T.M. Jyothi, M.L. Kaliya, M.V. Landau, Angew. Chem. Int. Ed. 40 (2001) 2881.[335] M.H. Valkenberg, E. Sauvage, D.C.-M. Christovao Paulo, W.H. Holderich, Imperial

Chemical Industries, 2001, WO 01/32308.[336] M.H. Valkenberg, C. de Castro, W.H. Holderich, Topics Catal. 14 (2001) 139.[337] C.P. Mehnert, R.A. Cook, N.C. Dispenziere, M. Afeworki, J. Am. Chem. Soc. 124

(2002) 12932.[338] C.P. Mehnert, E.J. Mozeleski, R.A. Cook, Chem. Commun. (2002) 3010.[339] C.P. Mehnert, Chem. Eur. J. 11 (2005) 50.[340] A. Riisager, P. Wasserscheid, R. van Hal, R. Fehrmann, J. Catal. 219 (2003) 452.[341] A. Riisager, K.M. Eriksen, P. Wasserscheid, R. Fehrmann, Catal. Lett. 90 (2003)

149.[342] U. Hintermair, G. Zhao, C.C. Santini, M.J. Muldoon, D.J. Cole-Hamilton, Chem.

Commun. (2007) 1462.[343] A. Riisager, B. Jorgensen, P. Wasserscheid, R. Fehrmann, Chem. Commun. (2006)

994.[344] J. Joni, M. Haumann, P. Wasserscheid, Adv. Synth. Catal. 351 (2009) 423.[345] B. Gadenne, P. Hesemann, J.J.E. Moreau, Chem. Commun. (2004) 1768.[346] M. Trilla, G. Borja, R. Pleixats, W.C.M. Man, C. Bied, J.J.E. Moreau, Adv. Synth.

Catal. 350 (2008) 2566.[347] D.W. Kim, D.J. Hong, K.S. Jang, D.Y. Chi, Adv. Synth. Catal. 348 (2006) 1719.[348] J. Fraga-Dubreuil, M.H. Famelart, J.P. Bazureau, Org. Process. Res. Dev. 6 (2002)

374.[349] C. Wu, J. Peng, J. Li, Y. Bai, Y. Hu, G. Lai, Catal. Commun. 10 (2008) 248.[350] L. Wang, Y. Zhang, C. Xie, Y. Wang, Synlett (2005) 1861.[351] P.G. Jessop, D.J. Heldebrant, C. Xie, C.A. Eckert, C.L. Liotta, Nature 436 (2005)

1102.[352] T. Yamada, P.J. Lukac, M. George, R.G. Weiss, Chem. Mater. 19 (2007) 967.[353] V. Blasucci, C. Dilek, H. Huttenhower, E.A. John, V. Llopis-Mestre, P. Pollet, C.A.

Eckert, C.L. Liotta, Chem. Commun. (2009) 116.[354] A. Behr, G. Henze, R. Schomacker, Adv. Synth. Catal. 348 (2006) 1485.[355] K. Fukumoto, H. Ohno, Angew. Chem. Int. Ed. 46 (2007) 1852.[356] A. Riisager, R. Fehrmann, R.W. Berg, R. van Hal, P. Wasserscheid, Phys. Chem.

Chem. Phys. 7 (2005) 3052.[357] P.J. Dyson, D.J. Ellis, T. Welton, Can. J. Chem. 79 (2001) 705.[358] J.J. Peng, J.Y. Li, Y. Bai, W.H. Gao, H.Y. Qiu, H. Wu, Y. Deng, G.Q. Lai, J. Mol. Catal. A:

Chem. 278 (2007) 97.[359] J. Peng, J. Li, Y. Bai, H. Qiu, K. Jiang, J. Jiang, G. Lai, Catal. Commun. 9 (2008) 2236.[360] B. Tan, J. Jiang, Y. Wang, L. Wei, D. Chen, Z. Jin, Appl. Organomet. Chem. 22 (2008)

620.[361] B. Wang, Y.-R. Kang, L.-M. Yang, J.-S. Suo, J. Mol. Catal. A: Chem. 203 (2003) 29.[362] R.T. Dere, R.R. Pal, P.S. Patil, M.M. Salunkhe, Tetrahedron Lett. 44 (2003) 5351.[363] R.A. Sheldon, R.M. Lau, M.J. Sorgedrager, F. van Rantwijk, K.R. Seddon, Green

Chem. 4 (2002) 147.[364] C. Emnet, K.M. Weber, J.A. Vidal, C.S. Consorti, A.M. Stuart, J.A. Gladysz, Adv.

Synth. Catal. 348 (2006) 1625.[365] M. Maase, in: P. Wasserscheid, T. Welton (Eds.), Ionic Liquids in Synthesis,

WILEY-VCH Verlags GmbH & Co. KGaA, Weinheim, 2008, pp. 663–687.[366] Y. Chauvin, H. Olivier, in: B. Cornils, W.A. Herrmann (Eds.), Applied Homoge-

neous Catalysis with Organometallic Compounds, WILEY-VCH Verlag GmbH,Weinheim, 2000, pp. 258–268.

[367] B. Gilbert, Y. Chauvin, I. Guibard, Vibr. Spectrosc. (1991) 299.


[368] F. Favre, F. Hugues, H. Olivier-Bourbigou, J.A. Chodorge, Petrole et Tech. 441(2003) 104.

[369] H. Olivier-Bourbigou, in: P. Wasserscheid, T. Welton (Eds.), Ionic Liquids inSynthesis, WILEY-VCH Verlag GmbH & Co.KGaA, Weinheim, 2008, pp. 464–488.

[370] H. Sato, H. Tojima, K. Ikimi, J. Mol. Catal. A: Chem. 144 (1999) 285.[371] H. Sato, H. Tojima, Bull. Chem. Soc. Jpn. 66 (1993) 3079.[372] M. Itagaki, G. Suzukalo, K. Nomura, Bull. Chem. Soc. Jpn. 71 (1998) 79.[373] Y. Chauvin, S. Einloft, H. Olivier, Ind. Eng. Chem. Res. 34 (1995) 1149.[374] H. Olivier-Bourbigou, E. Pellier, A. Forestiere, Institut Francais du Petrole, 2007,

WO2007/080287A1.[375] H. Olivier-Bourbigou, F. Favre, A. Forestiere, F. Hugues, in: R.H. Crabtree (Ed.),

Handbbok of Green Chem, Volume 1: Homogeneous Catalysis, Wiley-VCH,Weinheim, 2009, pp. 101–124.

[376] A. Forestiere, H. Olivier-Bourbigou, L. Saussine, Oil Gas Sci. Technol., in press.[377] M.P. Atkins, M.R. Smith, B. Ellis, BP Chemicals (1997), EP 0 791 643.[378] K.D. Hope, Chevron Phillips Chemical Company (2-11-2006), US 2006247482.[379] K.D. Hope, Chevron Phillips Chemical Company (26-5-2005), US 2005113621.[380] K.D. Hope, M.S. Driver, T.V. Harris, Chevron Chemicals (2002), US6395948.[381] K.D. Hope, D.A. Stern, D.W. Twomey, Akzo Nobel (2004), US 2004 0005985.[382] F.G. Sherif, C. Greco, L.J. Shyu, A.G. Talma, C. Lacroix, Akzo Nobel (1998), W0

1998/03454.[383] M.P. Atkins, C. Bowlas, B. Ellis, F. Hubert, A. Rubatto, P. Wasserscheid, in: R.D.

Rogers, K.R. Seddon, S. Volkov (Eds.), Green Industrial Applications of IonicLiquids, Kluwer Academic Publishers, Dordrecht, 2002, pp. 49–66.

[384] Z.C. Liu, X.H. Meng, R. Zhang, C.M. Xu, Pet. Sci. Technol. 27 (2009) 226.[385] L.F. Albright, Oil Gas J. (1990) 79.[386] H. Olivier-Bourbigou, in: B. Cornils, W. Herrmann, I.T. Horvath, W. Leitner, S.

Mecking, H. Olivier-Bourbigou, D. Vogt (Eds.), Multiphase Homogeneous Cata-lysis, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim, 2005, pp. 511–524.

[387] Y. Chauvin, A. Hirschauer, H. Olivier, J. Mol. Catal. 92 (1994) 155.[388] Y. Chauvin, A. Hirschauer, H. Olivier, Institut Francais du Petrole (1996), EP

0709356A1.[389] Z. Liu, R. Zhang, C. Xu, R. Xia, Oil Gas J. 104 (2006) 52.[390] Y. Liu, R. Hu, C. Xu, H. Su, Appl. Catal. A 346 (2008) 189.[391] V. Stegmann, K. Massonne, BASF (2005), WO 2005/026089 A2.[392] P. Bonnet, E. Lacroix, J.-P. Schirmann, ATOFINA (2001), WO 01/81353 A1.[393] C.R. Schmid, C.A. Beck, J.S. Cronin, M.A. Staszak, Org. Process Res. Dev. 8 (2004)

670.[394] M. Freemantle, Chem. Eng. News 81 (2003) 9.[395] R.D. Rogers, K.R. Seddon, Science 302 (2003) 792.[396] M. Maase, K. Massonne, K. Halbritter, R. Noe, M. Bartsch, W. Siegel, V. Stegmann,

M. Flores, O. Huttenloch, M. Becker, BASF (2003), WO 2003 062171.[397] M. Volland, V. Seitz, M. Maase, M. Flores, R. Papp, K. Massonne, V. Stegmann, K.

Halbritter, R. Noe, M. Bartsch, W. Siegel, M. Becker, O. Huttenloch, BASF (2003),WO 03/062251.

[398] M. Maase, O. Huttenloch, BASF (2005), WO 2005/061416A1.[399] S. Stadtmuller, Polym. Polym. Compos. 10 (2002) 49.[400] B. Weyershausen, K. Hell, U. Hesse, Organosilicon Chemistry VI: From Molecules

to Materials, [European Silicon Days], 2nd, Munich, Germany, Sept. 11–12, 2003.1 (2005) 424.

[401] B. Weyershausen, K. Hell, U. Hesse, ACS Sympos. Ser. 902 (2005) 133.[402] S.N. Falling, S.A. Godleski, L.W. McGarry, Eastmann Kodak Company (1993), US

5,238,889.[403] G.W. Phillips, S.N. Falling, S.A. Godleski, J.R. Monnier, Eastmann Kodak Company

(1994), US 5,315,019.[404] D. Commereuc, S. Harry, L. Magna, H. Olivier-Bourbigou, Institut Francais du

Petrole (2004), US2004059153.[405] G. Charles, R. Michelle, Eastman Chemical Co. (2003), US2003212295.[406] C. Jork, M. Seiler, Y.-A. Beste, W. Arlt, J. Chem. Eng. Data 49 (2004) 852.[407] W. Arlt, M. Seiler, C. Jork, T. Schneider, BASF (2002), WO 2002 074718.[408] D.J. Tempel, P.B. Henderson, J.R. Brzozowski, R.M. Pearlstein, Air Products and

Chemicals Inc. (2006), US2006060818.[409] D.J. Tempel, P.B. Henderson, J.R. Brzozowski, Air Products and Chemicals Inc.

(2006), US 2006 060817.[410] A.P. Abbott, G. Capper, D.L. Davies, R.K. Rasheed, Chem. Eur. J. 10 (2004) 3769.[411] Better plastic solar cells: http://www.g24i.com/news,technology-review-bet-

ter-plastic-solar-cells,145.html.[412] G24 innovations and BASF sign joint Development Agreement: http://

www.g24i.com/press,g24-innovations-and-basf-sign-joint-development-agreement,18.html.

[413] R. Adler, H. Mayer (2006), DE 102005026916.[414] M. Kompf, Linde Technol. (2006) 24.[415] R. Adler, Linde Technol. (2006) 27.[416] J.L. Anderson, D.W. Armstrong, G.-T. Wei, Anal. Chem. (2006) 2893.[417] S. Han, H.T. Wong, A.G. Livingston, Chem. Eng. Res. Des. 83 (2005) 309.[418] S. Fantini, F. Malbosc, Solvionic and Institut Francais du Petrole (2009),

FR2918998A.[419] J. Ranke, S. Stolte, R. Stoermann, J. Arning, B. Jastorff, Chem. Rev. 107 (2007) 2183.[420] S. Morrissey, B. Pegot, D. Coleman, M.T. Garcia, D. Ferguson, B. Quilty, N.

Gathergood, Green Chem. 11 (2009) 475.[421] M. Rebros, H.Q.N. Gunaratne, J. Ferguson, K.R. Seddon, G. Stephens, Green Chem.

11 (2009) 402.[422] Y.I. Zhang, B.R. Bakshi, E.S. Demessie, Environ. Sci. Technol. 42 (2008) 1724.[423] D. Reinhardt, F. Ilgen, D. Kralisch, B. Konig, G. Kreisel, Green Chem. 10 (2008)

1170.[424] G.W. Huber, S. Iborra, A. Corma, Chem. Rev. 106 (2006) 4044.

[425] A. Corma, S. Iborra, A. Velty, Chem. Rev. 107 (2007) 2411.[426] M. Strocker, Angew. Chem. Int. Ed. 47 (2008) 9200.[427] J.N. Chheda, G.W. Huber, J.A. Dumesic, Angew. Chem. Int. Ed. 46 (2007) 7164.[428] G.W. Himmel, S.-Y. Ding, D.K. Johnson, W.S. Adney, M.R. Nimlos, J.W. Brady, T.D.

Foust, Science 315 (2007) 804.[429] R. Kumar, S. Singh, O.V. Singh, J. Ind. Microbiol. Biotechnol. 35 (2008) 377.[430] O.A. El Seoud, A. Koschella, L.C. Fidale, S. Dorn, T. Heinze, Biomacromolecules 8

(2007) 2629.[431] S.D. Zhu, Y.X. Wu, Q.M. Chen, Z.N. Yu, C.W. Wang, S.W. Ding, Y.G. Wu, Green

Chem. 8 (2006) 325.[432] Y. Zhang, S. Ding, J. Mielenz, J. Cui, R. Elander, M. Laser, M. Himmel, J. McMillan, L.

Lynd, Biotechnol. Bioeng. 97 (2007) 214.[433] Y.P. Zhang, J. Ind. Microbiotechnol. 35 (2008) 367.[434] T. Heinze, T. Liebert, Prog. Polym. Sci. 26 (2001) 1689.[435] C. Graenacher (1934), US 1,943,176.[436] R.P. Swatloski, S.K. Spear, J.D. Holbrey, R.D. Rogers, J. Am. Chem. Soc. 124 (2002)

4974.[437] J. Vitz, T. Erdmenger, C. Haensch, U.S. Schubert, Green Chem. 11 (2009) 417.[438] M. Zavrel, D. Bross, M. Funke, J. Buchs, A.C. Spiess, Bioresour. Technol. 100 (2009)

2580.[439] F. Hermanutz, F. Gahr, E. Uerdingen, F. Meister, B. Kosan, Macromol. Symp. 262

(2008) 23.[440] S. Murugesan, R.J. Linhardt, Curr. Org. Synth. 2 (2005) 437.[441] H. Zhao, G.A. Baker, Z. Song, O. Olubajo, T. Crittle, D. Peters, Green Chem. 10

(2008) 696.[442] H. Ohno, Y. Fukaya, Chem. Lett. 38 (2009) 2.[443] Y. Cao, J. Wu, J. Zhang, H. Li, Y. Zhang, J. He, Chem. Eng. J. 147 (2009) 13.[444] Q. Liu, M.H.A. Janssen, F. van Rantwijk, R.A. Sheldon, Green Chem. 7 (2005) 39.[445] D.A. Fort, R.P. Swatloski, P. Moyna, R.D. Rogers, Chem. Commun. (2006) 714.[446] V. Egorov, S.V. Smirnova, A.A. Formanovsky, I.V. Pletnev, Y.A. Zolotov, Anal.

Bioanal. Chem. 387 (2007) 2263.[447] T. Heinze, K. Schwikal, S. Barthel, Macromol. Biosci. 5 (2005) 520.[448] K. Schlufter, H.-P. Schmauder, S. Dorn, T. Heinze, Macromol. Rapid Commun. 27

(2006) 1670.[449] S. Barthel, T. Heinze, Green Chem. 8 (2006) 301.[450] H. Zhang, J. Wu, J. Zhang, H. Jiasong, Macromolecules 38 (2005) 8272.[451] Y. Fukaya, K. Hayashi, M. Wada, H. Ohno, Green Chem. 10 (2008) 44.[452] H.M. Luo, Y.Q. Li, C.R. Zhou, Polym. Mater. Sci. Eng. 21 (2005) 233.[453] Y. Pu, N. Jiang, A.J. Ragauskas, J. Wood Chem. Technol. 27 (2007) 23.[454] H. Xie, S. Li, S. Zhang, Green Chem. 7 (2005) 606.[455] B. Kosan, C. Michels, F. Meister, Cellulose 15 (2008) 59.[456] A. Biswas, R.L. Shogren, D.G. Stevenson, J.L. Willett, P.K. Bhowmik, Carbohydr.

Polym. 66 (2006) 546.[457] K.E. Gutowski, G.A. Broker, H.D. Willauer, J.G. Huddleston, R.P. Swatloski, J.D.

Holbrey, R.D. Rogers, J. Am. Chem. Soc. 125 (2003) 6632.[458] A.P. Dadi, S. Varanasi, C.A. Schall, Biotechnol. Bioeng. 95 (2006) 904.[459] G. Ebner, S. Schiehser, A. Potthast, T. Rosenau, Tetrahedron Lett. 49 (2008) 7322.[460] B. Jastorff, R. Stormann, J. Ranke, K. Molter, F. Stock, B. Oberheitmann, W.

Hoffmann, J. Hoffmann, M. Nuchter, B. Ondruschka, J. Filser, Green Chem. 5(2003) 136.

[461] D.J. Gorman-Lewis, J.B. Fein, Environ. Sci. Technol. 38 (2004) 2491.[462] L. Feng, Z. Chen, Cellulose 15 (2008) 59.[463] B. Derecskei, A. Derecskei-Kovacs, Mol. Simulat. 32 (2006) 109.[464] T.G.A. Youngs, C. Hardacre, J.D. Holbrey, J. Phys. Chem. B 111 (2007) 13765.[465] R.C. Remsing, R.P. Swatloski, R.D. Rogers, G. Moyna, Chem. Commun. (2006)

1271.[466] A.P. Dadi, C.A. Schall, S. Varanasi, Appl. Biochem. Biotechnol. 136 (2007) 407.[467] D. Amstrong, L. He, Y. Liu, Anal. Chem. 71 (1999) 3873.[468] C.P. Fredlake, J.M. Crothwaite, D.G. Hert, S.N.V.K. Aki, J.F. Brennecke, J. Chem. Eng.

Data 49 (2004) 954.[469] J.S. Moulthrop, R.P. Swatloski, G. Moyna, R.D. Rogers, Chem. Commun. (2005)

1557.[470] R.C. Remsing, G. Hernandez, R.P. Swatloski, W. Massefski, R.D. Rogers, G. Moyna,

J. Phys. Chem. B 112 (2008) 11071.[471] J.G. Huddleston, R.D. Rogers, Chem. Commun. (1998) 1765.[472] J.M. Crosthwaite, S.N.V.K. Aki, E.J. Maginn, J.F. Brennecke, J. Phys. Chem. B 108

(2004) 5113.[473] S.N.V.K. Aki, B.R. Mellein, E.M. Saurer, J.F. Brennecke, J. Phys. Chem. B 108 (2004)

20355.[474] B. Wu, Y.M. Zhang, H.P. Wang, J. Chem. Eng. Data 53 (2008) 983.[475] S.H. Lee, T.V. Doherty, R.J. Linhardt, J.S. Dordick, Biotechnol. Bioeng. 102 (2009)

1368.[476] L. Liu, H. Chen, Chin. Sci. Bull. 51 (2006) 2432.[477] H. Huang, S. Ramaswamy, U. Tschirner, B. Ramarao, Sep. Purif. Technol. 62

(2008) 1.[478] L. Zhu, J.P. O’Dwyer, V.S. Chang, C.B. Granda, M.T. Holtzapple, Bioresour. Technol.

99 (2008) 3817.[479] V.S. Chang, M.T. Holtzapple, Appl. Biochem. Biotechnol. 5 (2000) 84.[480] K. Grohmann, R. Torget, M. Himmel, Biotechnol. Bioeng. Symp. 15 (1985) 59.[481] Y. Tan, D.R. MacFarlane, J. Upfal, L.A. Edye, W.O.S. Doherty, A.F. Patti, J.M. Pringle,

J.L. Scott, Green Chem. 11 (2009) 339.[482] I. Kilpelainen, H. Xie, A. King, M. Granstrom, S. Heikkinen, D.S. Argyropoulos, J.

Agric. Food Chem. 55 (2007) 9142.[483] D.A. Fort, R.C. Remsing, R.P. Swatloski, P. Moyna, D. Moyna, R.D. Rogers, Green

Chem. 9 (2007) 63.[484] H. Miyafuji, K. Miyata, S. Saka, F. Ueda, M. Mori, J. Wood Sci. 55 (2009) 215.

http://www.g24i.com/news,technology-review-better-plastic-solar-cells,145.html

http://www.g24i.com/news,technology-review-better-plastic-solar-cells,145.html

http://www.g24i.com/press,g24-innovations-and-basf-sign-joint-development-agreement,18.html




[485] N. Sun, M. Rahman, Y. Qin, M.L. Maxim, H. Rodriguez, R.D. Rogers, Green Chem.11 (2009) 646.

[486] S. Rayne, G. Mazza, Nature Precedings. hdl:10101/npre.2007.632.1 (2007).[487] J. Estager, J.M. Leveque, G. Cravotto, L. Boffa, W. Bonrath, M. Draye, Synlett 13

(2007) 2065.[488] D.A. Waterkamp, M. Heiland, M. Schluter, J.C. Sauvageau, T. Beyersdorff, J.

Thoming, Green Chem. 9 (2007) 1084.[489] F.S. Chakar, A.J. Ragauska, Ind. Crop. Prod. 20 (2004) 131.[490] J.F. Kadla, S. Kubo, R.A. Venditti, R.D. Gilbert, A.L. Compere, W. Griffith, Carbon 40

(2002) 2913.[491] S. Zhu, J. Chem. Technol. Biotechnol. 83 (2008) 777.[492] A.W.T. King, I. Kilpelainen, S. Heikkinen, P. Jarvi, D.S. Argyropoulos, Biomacro-

molecules 10 (2009) 458.[493] C. Li, Z.K. Zhao, Adv. Synth. Catal. 349 (2007) 1847.[494] L. Vanoye, M. Fanselow, J.D. Holbrey, M.P. Atkins, K.R. Seddon, Green Chem. 11

(2009) 390.[495] C. Li, Q. Wang, Z.K. Zhao, Green Chem. 10 (2008) 177.[496] C. Sievers, M.B. Valenzuela-Olarte, T. Marzialetti, I. Musin, P.K. Agrawal, C.W.

Jones, Ind. Eng. Chem. Res. 48 (2009) 1277.[497] R. Rinaldi, R. Palkovits, F. Schuth, Angew. Chem. Int. Ed. 47 (2008) 8047.[498] H. Zhao, C.L. Jones, G.A. Baker, S. Xia, O. Olubajo, V.N. Person, J. Biotechnol. 139

(2009) 47.[499] N. Kamiya, Y. Matsushita, M. Hanaki, K. Nakashima, M. Narita, M. Goto, H.

Takahashi, Biotechnol. Lett. 30 (2008) 1037.[500] H. Zhao, J.E. Holladay, H. Brown, Z.C. Zhang, Science 316 (2007) 1597.[501] C. Moreau, A. Finiels, L. Vanoye, J. Mol. Catal. A: Chem. 253 (2006) 165.[502] S. Hu, Z. Zhang, Y. Zhou, B. Han, H. Fan, W. Li, J. Song, Y. Xie, Green Chem. 10

(2008) 1280.[503] Q. Bao, K. Qiao, D. Tomida, C. Yokoyama, Catal. Commun. 9 (2008) 1383.[504] P. Wasserscheid, W. Keim, Angew. Chem. Int. Ed. 39 (2000) 3772.[505] J. Wu, J. Zhang, H. Zhang, J. He, Q. Ren, M. Guo, Biomacromolecules 5 (2004) 266.

[506] M. Granstrom, J. Kavakka, A. King, J. Majoinen, V. Makela, J. Helaja, S. Hietala, T.Virtanen, S.-L. Maunu, D.S. Argyropoulos, I. Kilpelainen, Cellulose 15 (2008) 481.

[507] M.A. Murakami, Y. Kaneko, J.I. Kadokawa, Carbohydr. Polym. 69 (2007) 378.[508] M. Gamba, A.A.M. Lapis, J. Dupont, Adv. Synth. Catal. 350 (2008) 160.[509] C. Reddy, V. Reddy, J.G. Verkade, Preprints of Symposia-American Chemical

Society, Division of Fuel Chemistry, vol. 51, 2006, p. 324.[510] A.A.M. Lapis, L.F. de Oliveira, B.A.D. Neto, J. Dupont, ChemSusChem 1 (2008) 759.[511] F.R. Abreu, M.B. Alves, C.C.S. Macedo, L.F. Zara, P.A.Z. Suarez, J. Mol. Catal. A:

Chem. 227 (2005) 263.[512] B.A. Silveira Neto, M.B. Alves, A.A.M. Lapis, F.M. Nachtigall, M.N. Eberlin, J.

Dupont, P.A.Z. Suarez, J. Catal. 249 (2007) 154.[513] Q. Wu, H. Chen, M. Han, D. Wang, J. Wang, Ind. Eng. Chem. Res. 46 (2007) 7955.[514] M. Han, W. Yi, Q. Wu, Y. Liu, Y. Hong, D. Wang, Bioresour. Technol. 100 (2009) 2308.[515] X. Liang, G. Gong, H. Wu, J. Yang, Fuel 88 (2009) 613.[516] S.H. Ha, M.N. Lan, S.H. Lee, S.M. Hwang, Y.M. Koo, Enzyme Microb. Technol. 41

(2007) 480.[517] A.P. Abbott, P.M. Cullis, M.J. Gibson, R.C. Harris, E. Raven, Green Chem. 9 (2007) 868.[518] J.C. Mol, Top. Catal. 27 (2004) 97.[519] C. Thurier, C. Fischmeister, C. Bruneau, H. Olivier-Bourbigou, P.H. Dixneuf,

ChemSusChem 1 (2008) 118.[520] C. Roosen, P. Mueller, L. Greiner, Appl. Microbiol. Biotechnol. 81 (2008) 607.[521] S. Toma, M. Meciarova, R. Sebesta, Eur. J. Org. Chem. (2009) 321.[522] J. Yoshida, K. Kataoka, R. Horcajada, A. Nagaki, Chem. Rev. 108 (2008) 2265.[523] A. Safavi, N. Maleki, F. Farjami, E. Farjami, J. Electroanal. Chem. 626 (2009) 75.[524] H. Toulhoat, F. Ropital, Institut Francais du Petrole (2008), US2008096146.[525] J.J. Peng, Y. Deng, New. J. Chem. 25 (2001) 639.[526] H. Yang, Y. Gu, Y. Geng, F. Shi, Chem. Commun. (2002) 274.[527] E.R. Cooper, C.D. Andrews, S. Wheatley, P.B. Webb, P. Wormald, R.E. Morris,

Nature 430 (2004) 1012.[528] P.A. Taubert, Z. Li, Dalton Trans. (2007) 723.[529] M.P. Stracke, G. Ebeling, R. Cataluna, J. Dupont, Energy Fuels 21 (2007) 1695.

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Ionic liquids and catalysis