Reduction potentials of H_(3+x)PMo_(12-x)O40 and H_(6+x)P2Mo_(18-x)X_xO62 heteropolyacid(HPA)catalysts and their catalytic activity for the vapor-phase oxidative dehydrogenation of isobutyric acid

摘要

Vanadium-containing H_(6+x)P2Mo_(18-x)V_xO62(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 H_(3+x)PMo_(12-x)V_xO40(x=0,1,2,and 3)Keggin HPA catalysts were also investigated for the purpose of comparison.The reduction potentials of H_(3+x)Mo_(12-x)V_xO40 and H_(6+x)P2Mo_(18-x)V_xO62 catalysts were determined by temperature-programmed reduction(TPR)measurements.The reduction potentials of H_(3+x)PMo_(12-x)V_xO40 and H_(6+x)P2Mo_(18-x)V_xO62 catalysts showed volcano-shaped curves and exhibited the same trend with respect to vanadium substitution.The conversions of IBA over H_(3+x)PMo_(12-x)V_XO40 and H_(6+x)P2Mo_(18-x)V_xO62 catalysts also showed volcano-shaped curves and exhibited the same trend with respect to vanadium substitution.However,the H_(6+x)P2Mo_(18-x)V_xO62 catalysts showed a higher reduction potential and a higher conversion of IBA than the H_(3+x)PMo_(12-x)V_xO40 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.