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

Abstract

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+)2M18O62](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 H6P2W18O62·24H2O, phospho-molybdic H6P2Mo18O62·nH2O and arsenic-molybdic H6As2Mo18O62·nH2O) 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.