12-heteropolymolybdates as catalysts for vapor-phase oxidative dehydrogenation of isobutyric acid: I. Alkali and alkaline-earth metal salts

Abstract

The effects of cations and heteroatoms on the catalytic activity and selectivity of 12-heteropolymolybdates, as catalysts, have been investigated in the vapor-phase oxidative dehydrogenation of isobutyric acid. The conversion of isobutyric acid at 300 °C increased with decreasing electronegativity of the cations (H +, alkali, and alkaline-earth metal ions) and the heteroatoms (P 5+, As 5+, and Si 4+), whereas it decreased with the change in the nature of the cations at 250 °C. The yield ratio (acetone/methacrylic acid) at 300 °C showed a behavior similar to the conversion, but the ratio was maximum over lithium and strontium salts. On the other hand, the electron affinity of Mo 6+ in these 12-heteropolymolybdate-type catalysts paralleled the electronegativity of the cations and the heteroatoms, and the reactivity of Mo 5+ to gaseous oxygen increased with decreasing electronegativity of these catalyst components (cations and heteroatoms). These results are discussed as the effects of these catalyst components on the electrochemical properties of the molybdenum atom, and the catalytic results obtained are explained on the basis of the exclusive participation of the lattice oxygen, the strength of molybdenum-oxygen bond, and the acid-base properties of the catalyst. It is shown that the molybdenum-oxygen bond is weakened as the catalyst components become less electronegative.