Reduction potentials of H3+xPMo12–xVxO40 and H6+xP2Mo18−xVxO62 heteropolyacid (HPA) catalysts and their catalytic activity for the vapor-phase oxidative dehydrogenation of isobutyric acid

Vanadium-containing H6+xP2Mo18−xVxO62 (x = 0, 1, 2, and 3) Wells-Dawson heteropolyacid (HPA) catalysts were prepared for use in the oxidative dehydrogenation of isobutyric acid (IBA) to methacrylic acid (MAA). Vanadium-containing H3+xPMo12−xVxO40 (x = 0, 1, 2, and 3) Keggin HPA catalysts were also investigated for the purpose of comparison. The reduction potentials of H3+xPMo12−xVxO40 and H6+xP2Mo18−xVxO62 catalysts were determined by temperature-programmed reduction (TPR) measurements. The reduction potentials of H3+xPMo12−xVxO40 and H6+xP2Mo18−xVxO62 catalysts showed volcano-shaped curves and exhibited the same trend with respect to vanadium substitution. The conversions of IBA over H3+xPMo12−xVxO40 and H6+xP2Mo18−xVxO62 catalysts also showed volcano-shaped curves and exhibited the same trend with respect to vanadium substitution. However, the H6+xP2Mo18−xVxO62 catalysts showed a higher reduction potential and a higher conversion of IBA than the H3+xPMo12−xVxO40 catalysts at the same level of vanadium substitution. A correlation between reduction potential and catalytic activity of the HPA catalysts clearly demonstrated that the conversion of IBA increased monotonically with increasing reduction potential of the HPA catalysts, across both families of vanadium-containing HPA catalysts examined in this work.