Models for Supported Aqueous-Phase Catalysis

Abstract

Immobilized homogeneous catalysts constitute a class of catalysts,which attempt to combine the merits of classical homogeneous and heterogeneous catalysts, i.e. high selectivities and mild reaction conditions with an ease of catalyst separation and recycling [1]. One of the most recent techniques for the immobilization of homogeneous catalysts is to adsorb a thin layer of aqueous solution of a water soluble catalyst precursor onto a hydrophyllic support such as silica [2]. It has been shown that the thus formed immobilized catalysts, termed supported aqueous-phase (SAP) catalysts, are effective for a variety of catalytic reactions in solvents which are immiscible with water [3]. It has also been proven that when a hydrophobic organic solvent is used with SAP catalysts, the adsorbed thin layer of water provides hydrogen bridges between the polar functional groups of the catalyst complex and those of the silica support strong and mobile enough to give an active immobilized catalyst which does not leach into the organic phase [2–4]. However, the nature of SAP catalysis is not yet fully explored. For example, it is ambiguous where exactly the actual catalytic process takes place with an SAP catalyst.