Chapter 5 - Supported heteropoly acids

Abstract

While homogeneous catalysis paved the way for advanced technological processes using heteropoly acids, challenges regarding these reactions including precipitation, aggregation, loss of activity, and separation difficulties resulted in studies in which the use of heteropoly acids as heterogeneous catalysts was explored. Bulk solid polyoxometalates were initially used as heterogeneous acid and oxidation catalysts. During the past decade, owing to the challenges related to their precipitation, aggregation, and degradation under operational conditions and in order to extend their application range, many reviews have focused on the specialized aspects of the heterogenization of polyoxometalates. In order to deposit polyoxometalates as individual molecules, micro- or nanostructured particles, layers, or bulk materials on heterogeneous substrates, a wide variety of techniques have been developed. These techniques include dip-coating, solvothermal deposition, sonication deposition, layer-by-layer assembly, drop-casting, electrodeposition, entrapment, and impregnation and drying. Basically, three different modes of chemical binding are observed for POM-substrate composites. These modes include electrostatic anchoring, covalent anchoring, and supramolecular anchoring. The selection of the appropriate substrate for heteropoly acid deposition is principally ruled by the target application. Regarding the acid-, base-, or redox catalysis, particularly in liquid phase, inorganic substrates with large, accessible surface areas, like porous metal oxides and nanostructured carbons, have been used to support heteropoly acids. The hybrid species prepared by supporting a polyoxometalate onto a metal-organic frameworks or incorporating it into it enjoy unique properties. When the polyoxometalate is encapsulated inside an MOF as a host, the POM-based MOF hybrids may be applied in catalytic systems (e.g., electrocatalysis, organocatalysis, or photocatalysis).