Catalysts based on bismuth molybdates are used commercially to produce acrolein and acrylonitrile via propene oxidation and ammoxidation, respectively. The importance of these processes has stimulated a considerable interest in understanding the mechanism and kinetics of these processes and the particular role of catalyst composition and structure in defining catalyst activity and selectivity. This talk will review the findings of experimental sand theoretical studies based on density functional theory aimed at providing support for deductions drawn from experiments and, more importantly, providing information that cannot be accessed experimentally. We focus on a-Bi2Mo3O12, since it exhibits high activity and stability and has been investigated extensively by many researchers. We find that the most energetically demanding reaction for both propene oxidation and ammoxidation is C-H bond activation of a methyl group in propene. The transition state for hydrogen abstraction is similar in both energy and geometry to the product state, which is a high-energy p-allyl radical. The p-allyl radical is then stabilized by reaction with a Mo=O group to form an allyl alkoxide. This species can then either undergo abstraction of its alpha hydrogen to form a surface-coordinated acrolein or, in the case of propene ammoxidation, reaction with NH3 adsorbed on proximate Bi3+ cations, to form allylamine. The activation barrier for the latter process is significantly lower than for the former, consistent with our experimental observations. Subsequent reaction of allylamine occurs by sequential abstraction of the four hydrogen atoms in order of increasing bond dissociation energy, with stabilization of the intermediate species via bonding with a surface molybdenyl oxo species. Our theoretical work also explains the positive correlation of the apparent activation energy for propene oxidation to acrolein with the band gap for bismuth molybdate containing vanadium, tungsten, or iron. Finally, we show that Bi3+ cations in bismuth molybdate catalysts used for propene oxidation and ammoxidation are important because of their ability to adsorb propene near Mo=O groups involved in the abstraction of a hydrogen atom from the methyl group of adsorbed propene and stabilizing the resulting Mo-OH group.

Experimental and theoretical studies of propene oxidation and ammoxidation over bulk bismuth molybdate

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