Neutral, supported ionic liquid catalysts

Bhanage and coworkers [62] investigated the reaction in polyethylene glycol and used the recycled rhodium phosphinite catalyst up to five times. Wasserscheid s group performed HAM in a continuous reactor operating with supported ionic liquid phase (SILP) catalysts [49]. A particular feature was that, by using a SILP catalyst based on neutral oxide and porous carbon supports and ILs of low basicity, aldol condensation could be fully suppressed. Alternatively, the reaction has been run with the assistance of a rhodium catalyst immobilized in a sol-gel matrix [73]. 

This type of co-catalytic influence is well loiown in heterogeneous catalysis, in which for some reactions an acidic support will activate a metal catalyst more efficiently than a neutral support. In this respect, the acidic ionic liquid can be considered as a liquid acidic support for the transition metal catalysts dissolved in it. 

This type of co-catalyhc influence is well known in heterogeneous catalysis, in which for some reachons an acidic support will achvate a metal catalyst more efficiently than a neutral support In this respect, the acidic ionic liquid can be considered as a liquid acidic support for the transihon metal catalysts dissolved in it As one would expect, in those cases in which the ionic liquid acts as a co-catalyst, the nature of the ionic liquid becomes very important for the reactivity of the transihon metal complex. The opportunity to opdmize the ionic medium used, by vari-ahon of the halide salt, the Lewis acid, and the ratio of the two components forming the ionic liquid, opens up enormous potential for ophmizahon. However, the choice of these parameters may be restricted by some possible incompahbilihes with the feedstock used. Undesired side reachons caused by the Lewis acidity of the ionic Hquid or by strong interaction between the Lewis acidic ionic Hquid and, for example, some oxygen funchonalihes in the substrate have to be considered. 

Due to the good solubility of organometallic compounds, ionic liquids have been used as reaction media, replacing traditional molecular solvents, or as the catalyst-supporting phase in a biphasic system. Influences of the ionic liquid on the reaction rate and selectivity can mostly be explained by the reactivity of the anion, which can be noncoordinating or coordinating as well as Lewis-acidic, Lewis-basic or neutral. The cation, in contrast, is considered to be essentially noncoordinating and innocent. 

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