Catalytic chemistry of supported heteropolyacids and their applications as solid acids to industrial processes

This article reviews recent research and development of supported heteropolyacid (HPA) catalysts; focusing on the acidic and catalytic properties. First the basic knowledge of solid HPAs is provided briefly to facilitate understanding of heterogeneous catalysis of HPAs. Secondly, the structure as well as the physical and chemical properties of supported HPA catalysts is described. Especially the layer structure of HPA dispersed on the surface of SiO(2) and the changes in the surface acidity with the loading level of HPA are discussed. For this purpose, temperature programmed desorption of benzonitrile devised by Okuhara and Kamiya's group is useful. This method is capable to assess the surface acidity of the supported HPA, which controls the catalytic activity for the reactions proceeding by surface-type catalysis. Then, two new industrial processes developed by Showa Denko K. K. are described; (i) production of ethyl acetate from ethylene and acetic acid and (ii) oxidation of ethylene to acetic acid. Supported HPA catalysts are utilized for both processes, which were recently much improved by finely controlling the catalysts and reaction conditions.