Chapter 2 - Heteropoly acids: An overview

Abstract

Polyoxometalates are members of a large group of nanosized metal‑oxygen cluster anions formed by a self-assembly process, typically in acidic aqueous solutions. In general, two types of polyoxometalates can be distinguished based on their chemical compositions: isopoly anions and heteropoly anions. Scores of structural types and stoichiometries have been identified for heteropoly anions (HPAs). The numerous and varied applications of polyoxometalates are primarily pivoted around their ionic charge, redox properties, conductivity, photochemical response, and ionic weights. A systematic nomenclature has been developed for HPAs that uses a labeling system for the metal atoms. Heteropoly acids are categorized based on the relative numbers of the central atoms and the metal addenda atoms in the surrounding octahedral units. On this basis, four different major classes can be recognized: 1:12 tetrahedral, 2:18 tetrahedral, 1:6 octahedral, and1:9 octahedral. Given the large possibilities of sizes, structures, and elemental compositions in which polyoxometalates can be produced, they may demonstrate a wide variety of properties, with a molecular identity maintained both in solutions and in the solid state. A variety of polyoxometalate catalysts have been developed for acid-catalyzed, photocatalytic, or oxidation reactions. Based on their structures, polyoxometalate base catalysts are categorized into four groups: (i) monomeric metalates, (ii) isopolyoxometalates, (iii) heteropolyoxometalates, and (iv) transition metal substituted polyoxometalates. Heteropoly acids are identified by a wide array of analytical techniques. This book focuses on speciation of heteropoly acids in aqueous solutions, in which many applications such as catalysis and biological processes are carried out.