Ionic multiphasic reaction

Obviously, there are many good reasons to study ionic liquids as alternative solvents in transition metal-catalyzed reactions. Besides the engineering advantage of their nonvolatile natures, the investigation of new biphasic reactions with an ionic catalyst phase is of special interest. The possibility of adjusting solubility properties by different cation/anion combinations permits systematic optimization of the biphasic reaction (with regard, for example, to product selectivity). Attractive options to improve selectivity in multiphase reactions derive from the preferential solubility of only one reactant in the catalyst solvent or from the in situ extraction of reaction intermediates from the catalyst layer. Moreover, the application of an ionic liquid catalyst layer permits a biphasic reaction mode in many cases where this would not be possible with water or polar organic solvents (due to incompatibility with the catalyst or problems with substrate solubility, for example). 

Notwithstanding their very low vapor pressure, their good thermal stability (for thermal decomposition temperatures of several ionic liquids, see [11, 12]) and their wide operating range, the key property of ionic liquids is the potential to tune their physical and chemical properties by variation of the nature of the anions and cations. An illustration of their versatility is given by their exceptional solubility characteristics, which make them good candidates for multiphasic reactions (see Section 5.3.4). Their miscibility with water, for example, depends not only on the hydrophobicity of the cation, but also on the nature of the anion and on the temperature. 

Table 5.3-2 Different technologies for multiphasic reactions making use of ionic liquids.

A summary of the research activities of the last four years reveals three different important trends (a) The design of new ionic ligands for excellent catalyst immobilisation in ionic liquids and high regioselectivity (b) the successful application of cheap, halogen-free ionic liquids in the biphasic Rh-catalysed hydroformylation (c) the successful development of unusual multiphasic reaction concepts for Rh-catalysed hydroformylation, namely catalysis in ionic liquid/supercritical C02 and SILP-catalysts. 

What we highlight here are some new recent multiphasic reaction systems for catalysis. The systems described here have in common a catalyst-philic phase, which contains, or coats a catalyst (mainly heterogeneous), or in some instances is the catalyst itself (PTC). There are two or three separate phases, and a general composition that can be summarized as liquid-liquid-solid (L-L-S), or liquid-liquid-liquid-solid (L-L-L-S). One of the Ls indicates the liquid-ionic/hydrophilic... 

Ionic Liquids in Multiphasic Reactions 271 Turnover frequency [(mol of aldehydes produced)(mole Rh) (min) ]... 

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