376715 Ammoxidation of Propene to Acrylonitrile over Bismuth Molybdate
Multicomponent bismuth molybdenum oxide catalysts have been used industrially for over 60 years for the oxidation and ammoxidation of light olefins. Despite extensive research on bismuth molybdates since their discovery in the late 1950s by SOHIO, the mechanistic details of these related reactions have not been fully established. In particular, most of the research in this area has focused on the simpler oxidation reaction of propene to acrolein, despite the fact that these catalysts are generally used for the ammoxidation of propene to acrylonitrile.
The ammoxidation and oxidation reactions are thought to have the same rate-determining step, namely a hydrogen radical abstraction from propene to produce an allyl radical. The ammoxidation reaction is a 6-electron transfer process that requires 3 hydrogen abstractions and a nitrogen insertion, compared to the oxidation reaction that involves a 4 electron transfer, 2 hydrogen abstractions, and an oxygen-insertion from the metal oxide surface. Recent experimental  and theoretical  investigations in our group have elucidated a likely mechanism for the oxidation reaction over alpha phase bismuth molybdate. The active site for the rate-determining step was found to be an oxygen atom doubly bound to a molybdenum center that is perturbed by the presence of a nearby bismuth atom. Only molybdenum reduces during reaction, however the bismuth alters the band energies around the Fermi level, allowing the reaction to happen.
While the oxidation mechanism is fairly well understood, the ammoxidation reaction remains almost uninvestigated. Researchers have often surmised that findings regarding the oxidation reaction can be extrapolated to the related ammoxidation reaction, however there is little data in the literature to support or refute that claim. Specifically, it is not known how ammonia incorporates into the catalyst, the nature of active site for the rate-determining step, or at what point in the reaction mechanism nitrogen-containing species insert into the hydrocarbon.
In this work, we present a rigorous investigation of the kinetics for the ammoxidation reaction of propene to acrylonitrile over alpha-phase bismuth molybdate. By varying the partial pressures of oxygen, ammonia and propene in the feed and analyzing the ratio of acrolein to acrylonitrile produced, we are able to determine the relative rates of these parallel, competing reactions. From these kinetic studies, we propose a mechanism for the formation of acrylonitrile from propene over bismuth molybdate that builds on that of the oxidation reaction. In addition, we hypothesize a mechanism based on experimental findings and Density Functional Theory calculations for how ammonia inserts into the surface of bismuth molybdate to generate active mono- and di-imido-substituted molybdate species.
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