Mechanism for incorporation of oxygen in vapour-phase selective oxidation of isobutene, butadiene and furan over various bismuth catalysts

Abstract

The selectivity to methacrylaldehyde and the yield ratio (methacrylaldehyde/aromatics) in the oxidation of isobutene over bismuth salts of oxyacids increased on increasing the strength and amount of acid existing on the surface, whereas such a correlation and the formation of aromatics were not observed over bismuth molybdate catalysts. Aromatics were formed in the reduction of the catalysts with isobutene and the yield ratio decreased with increasing degree of reduction. The latter catalysts produced maleic anhydride in the oxidation of butadiene and of furan, in contrast to the former ones, but the selectivity to anhydride decreased with increasing content of Bi₂O₃. These results are discussed in terms of the type of oxygen species incorporated into the products. The surface bridging oxygen bonded to Bi³⁺ has the ability to add to a positively charged hydrocarbon intermediate formed from isobutene by hydrogen abstraction, but not to the adsorbed butadiene and furan, even when they have high positive charge. In contrast, the terminal oxygen bonded to molybdenum ion is thought to be able to add to these adsorbed molecules and to the hydrocarbon intermediate. This difference is attributed to the different reactivity of the adsorbed hydrocarbon species to which lattice oxygen adds.