The state of the art on Wells-Dawson heteropoly-compounds - A review of their properties and applications

The scientific literature concerning the structure, hydrolytic stability in solution, thermal stability in the solid state, redox-acid properties and applications of heteropoly-compounds (HPCs) with Wells-Dawson structure is summarized in the present work. Wells-Dawson heteropoly-anions possess the formula [(Xn+)(2)M18O62]((16-2n)-) where Xn+ represents a central atom (phosphorous(V), arsenic(V), sulfur(VI), fluorine) surrounded by a cage of M addenda atoms, such as tungsten(VI), molybdenum(VI) or a mixture of elements, each of them composing MO6 (M-oxygen) octahedral units. The addenda atoms are partially substituted by other elements, such as vanadium, transition metals, lanthanides, halogens and inorganic radicals. The Wells-Dawson heteropoly-anion is associated with inorganic (H+, alkaline elements, etc.) or organic countercations forming hybrid compounds. Wells-Dawson acids (phospho-tungstic H6P2W18O62.24H(2)O, phospho-molybdic H6P2Mo18O62.nH(2)O and arsenic-molybdic H6As2Mo18O62.nH(2)O) possess super-acidity and a remarkably stability both in solution and in the solid state. These properties make them suitable catalytic materials in homogeneous and heterogeneous liquid-phase reactions replacing the conventional liquid acids (HF, HCl, H2SO4, etc.). Although, the application of the acids in heterogeneous gas-phase reactions is less developed, there is a patented method to oxidize alkanes to carboxylic acids on a supported Wells-Dawson catalyst that combines acid and redox properties. Wells-Dawson anions possess the ability to accept or release electrons through an external potential or upon exposure to visible and UV radiation (electro and photochemical reactions). Additionally, Wells-Dawson HPCs catalyze the oxidation of organic molecules with molecular oxygen, hydrogen peroxide and iodosylarenes, epoxidation and hydrogenation in homogeneous and heterogeneous liquid-phase conditions. The ability of transition metal substituted Wells-Dawson HPCs to be reduced and re-oxidized without degradation of the structure is promising in the application of those HPCs replacing metalloporphyrins catalysts in redox and electrochemical reactions. (C) 2003 Elsevier B.V. All rights reserved.