Catalysis by heteropoly compounds Part 39 The structure and redox behaviour of vanadium species in molybdovanadophosphoric acid catalysts during partial oxidation of isobutane

Abstract

The states and roles of vanadium of 11-molybdo-1-vanadophosphoric acid (H₄PMo₁₁VO₄₀, PMo11V) catalyst in the partial oxidation of isobutane (2-methylpropane) were analysed by EPR, ⁵¹V and ³¹P NMR, IR spectroscopy, and redox titration, and compared with dodecamolybdophosphoric acid (H₃PMo₁₂O₄₀, PMo12) catalyst. Thermal treatment of PMo11V at 623 K in O₂ caused the elimination of V from the Keggin anion and formed undefined polymeric and a monomeric V species. It was shown based on the redox titration and EPR spectra that the average valency of V in used catalysts significantly changed with the oxygen partial pressure, while that of Mo remained almost unchanged for both PMo11V and PMo12. The selectivities to methacrylic acid (MAA) and methacrolein (MAL) extrapolated to zero conversion were similar for both catalysts, but PMo11V showed a significantly higher selectivity as the conversion increased. This is because the secondary reaction, that is, oxidative decomposition of MAL and MAA, was slower over PMo11V than over PMo12. This was consistent with the marked difference in the reaction order in oxygen pressure, and corresponded to the differences in the above redox properties.