Unique acid catalysis of heteropoly compounds (heteropolyoxometalates) in the solid state

Abstract

Fundamental and superior characteristics of heteropoly compounds (heteropolyoxometalates) in the solid state that make them suitable for catalyst design at the atomic/molecular levels are described, together with important principles required for the understanding and design of solid heteropoly catalysts. First, the molecular nature of heteropolyanions (metal oxide clusters), which can be preserved in the solid state, enables control of the acid and redox properties over a wide range. Second, the presence of hierarchical structures (primary, secondary and tertiary structures) can lead to three catalysis modes—surface-type, pseudoliquid (or bulk-type I) and bulk-type II. Precise control of pore size is possible through the understanding of the microstructure, which results in unique shape selectivity observed for various reactions. Heteropoly compounds are green catalysts functioning in a variety of reaction fields and efficient bifunctional catalysts when combined with other components. The elucidation of catalytic processes is also possible at the atomic/molecular level due to their molecular nature. The positions and dynamic nature of protons as well as organic reaction intermediates in the pseudo-liquid phase can be clarified by spectroscopic techniques. Various reactions promoted by solid heteropoly catalysts are collected from recent publications to illustrate the usefulness of the above ideas and of heteropoly catalysts themselves.