Catalysts based on bismuth molybdate are used extensively for the oxidation of propene to acrolein and the ammoxidation of proene to acrylonitrile. We have carried out experimental and theoretical studies aimed at elucidating the role of catalyst composition on the activity of propene and iso-butene oxidation, as well as the oxidative dehydrogenation of 1- and 2-butene. We have observed for all of the reactions studied that cleavage of the weakest C-H bond in the reactant is the rate limiting step. This has led us to ask how the activity and selectivity of the catalyst depend on the composition and structure of the catalyst. We have found empirically that the activation barrier for C-H bond cleavage, the rate-limiting step, scales with the band gap of the oxide and the strength of the C-H bond. These findings are supported by DFT studies of the reaction pathway and estimates of the band gap. Born-Haber analysis of the C-H bond activation process reveals that the component of the activation barrier most sensitive to catalyst composition is the energy for ligand-to-metal charge transfer, which in the case of oxides involves the transfer of an electron from the HOMO of O atoms to the LUMO of metal atoms. DFT studies also provide an explanation for how the band gap changes with oxide composition and crystal structure. A further finding of this work is that low band gap materials, while being more active, exhibit poor selectivity to desired product and higher selectivity to products of complete combustion. By contrast, the O atoms present at the surface of high band gap materials are insufficiently active to promote C-H activation, which now becomes increasingly dependent on adsorbed O<sub>2</sub>. We will also discuss our preliminary work on the ammoxidation of propene to acrylonitrile, and will compare and contrast the mechanism of this reaction with that for propene oxidation.

Mechanism and kinetics of propene oxidation and ammoxidation on bismuth molybdate-based catalysts

249th National Meeting (2015)

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