Participation of double bond type lattice oxygen in vapor-phase catalytic oxidation of olefins

The reactivity of double bond type lattice oxygen Mo6+O in supported molybdena catalyst has been investigated by means of ESR and ir spectroscopy. It has been shown that the double bond oxygen is similar to a free radical oxygen in nature, in contrast to σ-bond oxygen MoOMo. This character increases as a result of electron transfer from an adsorbed olefin species to a nonbonding atomic orbital of Mo6+ with some decrease in the strength of MoO bond during vapor-phase oxidation of lower olefins. The reactivity of the double bond oxygen as a radical increases inversely with the electronegativity of the molybdenum ion as modified with an oxide of a VA group element or of an alkali metal. This was supported by studies of the catalysis of the oxidation of propylene to acrolein, of butadiene to maleic anhydride and the oxidative dehydrogenation of trans-2-butene to butadiene. On the other hand, the activity for these reactions of catalyst modified with a high concentration of alkali metal oxide increases with the electronegativity of the alkali metal. This different effect due to the concentration of the alkali metal oxide is also discussed.