Transition metal substituted 12-tungstophosphates:Characterization and evaluation of their performance as oxidation catalysts

Abstract

Tetrabutylammonium salt of [PW₁₂O₄₀]^3- and its transition metal (Mn²⁺, Fe²⁺, CO²⁺ and Cu²⁺) substituted compounds are characterized by elemental analysis, thermogravimetric and spectroscopic techniques. Thermal stability of these compounds decreases after metal ion substitution, but the cobalt substituted compound is relatively more stable. The cobalt substituted compound is found to be a good catalyst for the oxidation of cyclohexanol and the catalytic activity of the transition metal substituted compounds decreases in the order: CO²⁺>> Mn²⁺> Fe²⁺³ Cu²⁺. Detailed studies of the reaction in the presence of Co²⁺and Mn²⁺ substituted compounds reveal that the transition metal ions are initially oxidized to a higher oxidation state which then act as the catalytic centre for the reaction. The enhanced catalytic activity of Co²⁺ substituted compound is due its oxidation to Co²⁺ in the presence of H₂O₂ which is evidenced from EPR and UV-VIS spectroscopic studies.