Redox Properties and Catalytic Oxidation Activities of Polyatom-Substituted H n PW11M1O40 (M = V, Nb, Ta, and W) Keggin Heteropolyacid Catalysts

Abstract

Redox properties and catalytic oxidation activities of polyatom-substituted H n PW11M1O40 (M = V, Nb, Ta, and W) Keggin heteropolyacids (HPAs) were examined. Reduction potentials and UV–visible absorption edge energies of H n PW11M1O40 (M = V, Nb, Ta, and W) HPA catalysts in solution were determined by an electrochemical method and UV–visible spectroscopy measurements, respectively. It was observed that reduction potentials of H n PW11M1O40 (M = V, Nb, Ta, and W) HPA catalysts increased and UV–visible absorption edge energies of the HPA catalysts decreased with decreasing electronegativity of substituted polyatom. It was also found that the lower absorption edge energy corresponded to the higher reduction potential of the HPA catalyst. Vapor-phase oxidation of benzyl alcohol was carried out as a model reaction to probe the redox properties of H n PW11M1O40 (M = V, Nb, Ta, and W) HPA catalysts. Yield for benzaldehyde increased with increasing reduction potential and with decreasing absorption edge energy of the HPA catalyst, and in turn, with decreasing electronegativity of substituted polyatom. Reduction potential of H n PW11M1O40 (M = V, Nb, Ta, and W) HPA catalysts measured by an electrochemical method and absorption edge energy of the HPA catalysts measured by UV–visible spectroscopy could be utilized as a probe of oxidation catalysis of the HPA catalysts.