Propene ammoxidation over Bi 2 Mo 3 O 12 was investigated to elucidate product (acrylonitrile, acetonitrile, HCN, acrolein, N 2 , etc.) formation pathways. Propene consumption rate is first order in propene and zero order in ammonia (for NH 3 /C 3 H 6 = 0-2) and oxygen (for O 2 /C 3 H 6 ≥ 1.5) partial pressures, with an activation energy (E a = 22 kcal/mol) comparable to that for propene oxidation, suggesting the same rate-limiting step for both reactions. We propose two N-containing species are relevant at ammoxidation conditions: adsorbed NH 3 on surface Bi 3+ ions that reacts with a propene derivative to form products with C-N bonds, and a few metastable M-NH x (M=Mo, Bi; x=1, 2) groups that are very sensitive to destruction by water, but that are responsible for NH 3 oxidation to N 2 . A proposed reaction mechanism and model that captures the experimental trends in product distribution as a function of partial pressures and temperature is presented.

a b s t r a c tA systematic investigation of the oxidative dehydrogenation of propane to propene and 1-and 2-butene to 1,3-butadiene, and the selective oxidation of isobutene to methacrolein was carried out over Bi 1Àx/3 V 1Àx Mo x O 4 (x = 0-1) with the aim of defining the effects of catalyst and reactant composition on the reaction kinetics. This work has revealed that the reaction kinetics can differ significantly depending on the state of catalyst oxidation, which in turn depends on the catalyst composition and the reaction conditions. Under conditions where the catalyst is fully oxidized, the kinetics for the oxidation of propene to acrolein and isobutene to methacrolein, and the oxidative dehydrogenation of propane to propene, 1-butene and trans-2-butene to butadiene are very similar-first order in the partial pressure of the alkane or alkene and zero order in the partial pressure of oxygen. These observations, together with XANES and UV-Vis data, suggest that all these reactions proceed via a Mars van Krevelen mechanism involving oxygen atoms in the catalysts and that the rate-limiting step involves cleavage of the weakest CAH bond in the reactant. Consistent with these findings, the apparent activation energy and pre-exponential factor for both oxidative dehydrogenation and selective oxidation correlate with the dissociation energy of the weakest CAH bond in the reactant. As the reaction temperature is lowered, catalyst reoxidation can become rate-limiting, the transition to this regime depending on ease of catalyst reduction and effectiveness of the reacting hydrocarbons as a reducing agent. A third regime is observed for isobutene oxidation at lower temperatures, in which the catalyst is more severely reduced and oxidation now proceeds via reaction of molecular oxygen, rather than catalyst lattice oxygen, with the reactant.

The mechanisms and energetics for the propene oxidation and ammoxidation occurring on the (010) surface of Bi2Mo3O12 were investigated using density functional theory (DFT). An energetically feasible sequence of elementary steps for propene oxidation to acrolein, propene ammoxidation to acrylonitrile, and acrolein ammoxidation to acrylonitrile are proposed. Consistent with experimental findings, the rate limiting step for both propene oxidation and ammoxidation is the initial hydrogen abstraction from the methyl group of propene, which is calculated to have an apparent activation energy of 27.3 kcal/mol. The allyl species produced in this reaction is stabilized as an allyl alkoxide, which can then undergo hydrogen abstraction to form acrolein or react with ammonia adsorbed on under-coordinated surface Bi 3+ cations to form allylamine. Dehydrogenation of allylamine is shown to produce acrylonitrile, whereas reaction with additional adsorbed ammonia leads to the formation of acetonitrile and hydrogen cyanide. The dehydrogenation of allyalkoxide species is found to have a significantly higher activation barrier than reaction with adsorbed ammonia, consistent with the observation that very little acrolein is produced when ammonia is present. Rapid reoxidation of the catalyst surface to release water is found to be the driving force for all reactions involving the cleavage of C-H or N-H bonds, since practically all of these steps are endothermic.