Why use Ionic Liquids as Solvents for Transition Metal Catalysis

Why use Ionic Liquids as Solvents for Transition Metal Catalysis  

Probably the most prominent property of an ionic Hquid is its lack of vapor pressure. Transition metal catalysis in ionic Hquids can particularly benefit from this on economic, environmental, and safety grounds. 

As in stoichiometric organic reacHons, the appHcation of nonvolatile ionic Hquids can contribute to the reducHon of atmospheric polluHon. This is of special relevance for non-continuous reactions, in which complete recovery of a volatile organic solvent is usuaUy difficult to integrate into the process. 

As weU as this quite obvious environmental aspect, the switch from a volaHle, flammable, organic solvent to an ionic Hquid may significantly improve the safety of a given process. This will be especially trae in oxidation reactions in which air or pure oxygen are used as oxidants the use of common organic solvents is often restricted due to the potential formation of explosive mixtures between oxygen and 

Obviously, the use of a non-volatUe ionic liquid simplifies the distillative work-up of volatile products, especially in comparison to the use of low boiling solvents, where this may save the distillation of the solvent during product isolation. Moreover, common problems related to the formation of azeotropic mixtures of a volatile solvent and the products/by-products formed, are avoided by using a non-volatile ionic liquid. In the Rh-catalyzed hydroformylation of 3-pentenoic acid methyl ester it was even found that the ionic liquid added could stabilize the homogeneous catalyst during the thermal stress of product distillation (Fig. 5.3-1) [30]. This option may be especially attractive technically, due to the fact that the stabilizing effects could already be observed with quite low amounts of ionic liquid added. 

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