ASPECTS OF SELECTIVE OXIDATION AND AMMOXIDATION MECHANISMS OVER BISMUTH MOLYBDATE CATALYSTS

The interaction of allyl radicals, generated in situ by thermal nitrogen extrusion from 3,3′-azopropene, was studied at 320°C. The catalyst systems were MoO3, Bi2O3·nMoO3 (where n = 1, 2, and 3), a multicomponent Bi-molybdate, and Bi2O3. Allyl radicals gave significantly more byproducts (e.g., CO2, acetaldehyde, benzene) than did allyl iodide or propylene under corresponding reaction conditions. When allyl radicals were generated from azopropene over MoO3, and in the presence of ammonia, substantial yields of acrylonitrile were obtained. In the absence of ammonia, acrolein was the major useful product. The results for propylene oxidation and ammoxidation over Bi-molybdates are interpreted in terms of a concerted chemisorption, α-hydrogen abstraction mechanism, with subsequent oxygen (or NH) insertion, which does not proceed via a free allyl radical as the major pathway. Allyl radicals can, however, favorably compete with ammonia for oxygen (or NH) insertion sites over MoO3 at 320°C. The proposed mechanism involves the formation of a π-allylic Mo (V) intermediate as the rate-determining step, in rapid equilibrium with a σ-bonded Mo (V) ester, the acrolein precursor. Acrylonitrile formation is explained by reaction of the initially formed π-allylic intermediate with ammonia, loss of water, and subsequent H-abstraction from the thus formed σ-bonded Mo (V) amide. © 1979 Academic Press, Inc. All rights reserved.