Reduction potential, UV-visible absorption edge energy, and oxidation catalysis of niobium-containing H_(3+x)PW_(12-x)Nb_xO_(40) Keggin and H_(6+x)P2W_(18-x)Nb_xO_(62) Wells-Dawson heteropolyacid catalysts

摘要

Niobium-containing H_(3+x)PW_(12-x)Nb_xO_(40) (x=0,1,2 and 3) Keggin and H_(6+x)P2W_(18-x)Nb_xO_(62) (x=0,1,2 and 3) Wells-Dawson heteropolyacids (HPAs) were prepared in this work to explore their redox properties and oxidation catalysis. Reduction potentials and absorption edge energies of H_(3+x)PW_(12-x)Nb_xO_(40) and H_(6+x)P2W_(18-x)Nb_xO_(62) HPA catalysts were measured by an electrochemical method and UV-visible spectroscopy, respectively. The trend of reduction potential is well consistent with the trend of absorption edge energy with respect to niobium substitution in both series of H_(3+x)PW_(12-x)Nb_xO_(40) and H_(6+x)P2W_(18-x)Nb_xO_(62) HPA catalysts. Absorption edge energy of H_(3+x)PW_(12-x)Nb_xO_(40) and H_(6+x)P2W_(18-x)Nb_xO_(62) HPA catalysts shifted to lower value with increasing reduction potential of the HPA catalysts, regardless of the identity of HPA catalysts; an HPA catalyst with higher reduction potential exhibited lower absorption edge energy. In order to probe oxidation catalysis of H_(3+x)PW_(12-x)Nb_xO_(40) and H_(6+x)P2W_(18-x)Nb_xO_(62) HPA catalysts, vapor-phase benzyl alcohol oxidation was carried out as a model reaction. Yield for ben-zaldehyde increased with increasing reduction potential and with decreasing absorption edge energy of H_(3+x)PW_(2-x)Nb_xO_(40) and H_(6+x)P2W_(18-x)Nb_xO_(62) HPA catalysts.