Homogeneous Transition-Metal Catalysis in Molten Salts

Molten salts may be classified (1) into simple ionic salts (e.g. the alkali and alkaline earth metal halides), simple and polymeric oxyanionic salts (e.g. metal nitrates, carbonates, phosphates. 

Oftentimes the melt is called upon to play more than one role in a chemical process. It may, for example, simply act as a solvent for reactants and products it may be a catalyst it may supply one or more of the reactants, as well as aid in controlling the heat of reaction. 

Molten salts have already found numerous technical applications in industrial catalytic processes. Examples include the Deacon and Transcat process for generating chlorinated hydrocarbons as well as SO2 oxidation processes (1,2). 

This review illustrates the diverse uses of melt catalysis with solubilized (homogenous) transition-metal derivatives, in applications that include  

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HOMOGENEOUS TRANSITION-METAL CATALYSIS IN MOLTEN SALTS... 

The first example of homogeneous transition metal catalysis in an ionic liquid was the platinum-catalyzed hydroformylation of ethene in tetraethylammonium trichlorostannate (mp. 78 °C), described by Parshall in 1972 (Scheme 5.2-1, a)) [1]. In 1987, Knifton reported the ruthenium- and cobalt-catalyzed hydroformylation of internal and terminal alkenes in molten [Bu4P]Br, a salt that falls under the now accepted definition for an ionic liquid (see Scheme 5.2-1, b)) [2]. The first applications of room-temperature ionic liquids in homogeneous transition metal catalysis were described in 1990 by Chauvin et al. and by Wilkes et ak. Wilkes et al. used weekly acidic chloroaluminate melts and studied ethylene polymerization in them with Ziegler-Natta catalysts (Scheme 5.2-1, c)) [3]. Chauvin s group dissolved nickel catalysts in weakly acidic chloroaluminate melts and investigated the resulting ionic catalyst solutions for the dimerization of propene (Scheme 5.2-1, d)) [4]. 

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