Redox properties and oxidation catalysis of group 5 metal (V, Nb, Ta)-containing Keggin and Wells–Dawson heteropolyacid (HPA) catalysts Original Research Article

Group 5 metal-containing H4PW11M1O40 (M = V, Nb, Ta) Keggin heteropolyacids (HPAs) and H7P2W17M1O62 (M = V, Nb, Ta) Wells–Dawson HPAs were prepared in this work. Reduction potentials and UV–visible absorption edge energies of H4PW11M1O40 (M = V, Nb, Ta) and H7P2W17M1O62 (M = V, Nb, Ta) HPA catalysts in solution were measured by an electrochemical method and UV–visible spectroscopy, respectively. It was observed that reduction potentials of H4PW11M1O40 (M = V, Nb, Ta) and H7P2W17M1O62 (M = V, Nb, Ta) HPA catalysts increased with decreasing absorption edge energies of the HPA catalysts. In order to correlate the redox properties (reduction potential and absorption edge energy) with the oxidation catalysis of H4PW11M1O40 (M = V, Nb, Ta) and H7P2W17M1O62 (M = V, Nb, Ta) HPA catalysts, we carried out vapor-phase benzyl alcohol oxidation as a model reaction. Yield for benzaldehyde increased with increasing reduction potential and with decreasing absorption edge energy of the HPA catalysts. Reduction potentials of H4PW11M1O40 (M = V, Nb, Ta) and H7P2W17M1O62 (M = V, Nb, Ta) HPA catalysts measured by an electrochemical method and absorption edge energies of the HPA catalysts measured by UV–visible spectroscopy could be utilized as probes of oxidation catalysis of the HPA catalysts.