Ionic Liquids in Catalysis

Abstract

Ionic liquids (ILs), salts with melting points below 100°C, represent a fascinating class of liquid materials typically characterized by an extremely low vapor pressure. Besides their application as new solvents or as electrolytes for electrochemical purposes, there are two important concepts of using ILs in catalysis: Liquid-liquid biphasic catalysis and IL thin film catalysis. Liquid-liquid biphasic catalysis enables either a very efficient manner to apply catalytic ILs, e.g. in Friedel-Crafts reactions, or to apply ionic transition metal catalyst solutions. In both cases, phase separation after reaction allows an easy separation of reaction products and catalyst re-use. One problem of liquidliquid biphasic catalysis is mass transfer limitation. If the chemical reaction is much faster than the liquid-liquid mass transfer the latter limits the overall reaction rate. This problem is overcome in IL thin film catalysis where diffusion pathways and thus the characteristic time of diffusion are short. Here, Supported Ionic Liquid Phase (SILP) and Solid Catalyst with Ionic Liquid Layer (SCILL) are the two most important concepts. In both, a high surface area solid substrate is covered with a thin IL film, which contains either a homogeneously dissolved transition metal complex for SILP, or which modifies catalytically active surface sites at the support for SCILL. In each concept, interface phenomena play a very important role: These may concern the interface of an IL phase with an organic phase in the case of liquidliquid biphasic catalysis. For IL thin film catalysis, the interfaces of the IL with the gas phase and with catalytic nanoparticles and/or support materials are of critical importance. It has recently been demonstrated that these interfaces and also the bulk of ILs can be investigated in great detail using surface science studies, which greatly contributed to the fundamental understanding of the catalytic properties of ILs and supported IL materials. Exemplary results concerning the IL/vacuum or IL/gas interface, the solubility and surface enrichment of dissolved metal complexes, the IL/support interface and the in situ monitoring of chemical reactions in ILs are presented.

Ionic Liquid as Reaction Media for the Production of Cellulose-Derived Polymers from Cellulosic Biomass

“…It is worth noting that the label "green solvent" does not mean that all ILs are non-toxic [29]. An update on ILs use as solvents in synthesis and catalysis has been published by Hallett and Welton [30] and by Steinrück and Wasserscheid [31]. The structures of the main cations and anions described in literature are shown in Figure 2.…”

Thermal stability of imidazolium-based ionic liquids

“…[1,9] They have consequently been considered for a variety of applications such as separation technology, [10,11] catalysis, [12][13][14] biomass treatment [15,16] and energy storage. [1,9,17,18] For industrial applications of IL as alternative green solvents, the determination of both decomposition and melting temperatures is of paramount importance.…”

Perspective: Chemical reactions in ionic liquids monitored through the gas (vacuum)/liquid interface

“…57 In SILP, 54,[123][124][125] transition metal (TM) complexes are commonly employed as catalytically active species, with a thin IL film serving as solvent for both reactants and metal complex. An interesting situation arises when the ILs provide ligands themselves, with much higher concentrations than achieved in traditional molecular solvents.…”

“…We are particularly interested in applications in heterogeneous catalysis, where ILs have led to completely new catalyst concepts such as the Supported Ionic Liquid Phase (SILP) and Solid Catalyst with Ionic Liquid Layer (SCILL) technologies. [54][55][56][57][58] Both SILP and SCILL profit from the non-volatile IL character which allows for running reactions in gas streams without losing the liquid layer by evaporation. In the SILP system, an inert support material is coated with a very thin IL layer with the homogenously dissolved catalyst.…”