抄録
Vapor-phase oxidation of butadiene over supported molybdena catalysts has been investigated with reference to the nature of oxygen and adsorbed butadiene species selective for maleic anhydride formation. A double bond type lattice oxygen Mo5+=O, which is formed by electron transfer from the adsorbed butadiene species to a nonbonding atomic orbital of Mo6+ in Mo6+=O, has been found to be the selective oxygen species and to behave like a radical oxygen.
It has also been shown that maleic anhydride was formed from the butadiene species adsorbed on Mo5+ in contrast to that on Ti4+. Mo5+ is softer than Ti4+, and for different dienes a diene for which the energy level of the lowest vacant antibonding moleculer orbital is lower gave a higher rate of anhydride formation, thus providing powerful evidence for the effect of haredness and softness of the metal ion on the selectivity. The results are discussed with respect to the reaction mechanism for maleic anhydride formation, and 2, 5-dihydrofuran has been presumed to be the first intermediate compound for the anhydride formation.
