Redox properties of H3PMoxW12−xO40 and H6P2MoxW18−xO62 heteropolyacid catalysts and their catalytic activity for benzyl alcohol oxidation

Abstract

Abstract Reduction potentials of H 3 PMo x W 12− x O 40 ( x = 0, 3, 6, 9, and 12) Keggin heteropolyacid (HPA) and H 6 P 2 Mo x W 18− x O 62 ( x = 0, 3, 9, 15, and 18) Wells-Dawson HPA catalysts were measured by an electrochemical method in solution. UV–visible spectra of H 3 PMo x W 12− x O 40 and H 6 P 2 Mo x W 18− x O 62 catalysts in solution were then obtained in order to explore the reduction potentials of the HPA catalysts. Reduction potentials increased and UV–visible absorption edge energies shifted to lower values with increasing molybdenum substitution in both families of H 3 PMo x W 12− x O 40 and H 6 P 2 Mo x W 18− x O 62 catalysts. It was revealed that the lower absorption edge energy corresponded to the higher reduction potential of the HPA catalyst. In order to correlate the redox property (reduction potential and absorption edge energy) with the catalytic oxidation activity of H 3 PMo x W 12− x O 40 and H 6 P 2 Mo x W 18− x O 62 catalysts, vapor-phase oxidation of benzyl alcohol was carried out as a model reaction. Yield for benzaldehyde increased with increasing reduction potential and with decreasing absorption edge energy of the HPA catalyst, regardless of the identity of HPA catalyst (without HPA structural sensitivity). The absorption edge energies measured by UV–visible spectroscopy could be utilized as a correlating parameter for the reduction potentials (oxidizing powers) of H 3 PMo x W 12− x O 40 and H 6 P 2 Mo x W 18− x O 62 catalysts, and furthermore, as a probe of oxidation catalysis of the HPA catalysts.