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 is 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 Bi3+ 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, because practically all of these steps are endothermic. (Chemical Equation Presented).

Propene ammoxidation over Bi2Mo3O12 was investigated to elucidate product (acrylonitrile, acetonitrile, HCN, acrolein, N2, etc.) formation pathways. Propene consumption rate is first order in propene and zero order in ammonia (for NH3/C3H6 = 0-2) and oxygen (for O2/C3H6 ≥ 1.5) partial pressures, with an activation energy (Ea = 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 NH3 on surface Bi3+ ions that reacts with a propene derivative to form products with C-N bonds, and a few metastable M-NHx (M = Mo, Bi; x = 1, 2) groups that are very sensitive to destruction by water, but that are responsible for NH3 oxidation to N2. A proposed reaction mechanism and model that captures the experimental trends in product distribution as a function of partial pressures and temperature are presented.

In order to understand the remarkable activity of α-Bi2Mo3O12 for selective oxidation and ammoxidation of propene, the propene activation ability of four molybdenum-based mixed metal oxides - Bi2Mo3O12, PbMoO4, Bi2Pb5Mo8O32, and MoO3 - was investigated using density functional theory. Propene activation is considered to occur via abstraction of a hydrogen atom from the methyl group of physisorbed propene by lattice oxygen. For each material, the apparent activation energy was estimated by summing the heat of adsorption of propene, the C-H bond dissociation energy, and the hydrogen attachment energy (HAE) for hydrogen addition to lattice oxygen; this sum provides a lower bound for the apparent activation energy. It was found that two structural features of oxide surfaces are essential to achieve low activation barriers: under-coordinated surface cation sites enable strong propene adsorption, and suitable 5- or 6-coordinate geometries at molybdenum result in favorable HAEs. The impact of molybdenum coordination on HAE was elucidated by carrying out a molecular orbital analysis using a cluster model of the molybdate unit. This effort revealed that, in 5- and 6-coordinate molybdates, oxygen donor atoms trans to molybdenyl oxo atoms destabilize the molybdate prior to H addition but stabilize the molybdate after H addition, thereby providing an HAE ∼15 kcal/mol more favorable than that on 4-coordinate molybdate oxo atoms. Bi3+ cations in Bi2Mo3O12 thus promote catalytic activity by providing both strong adsorption sites for propene and forcing molybdate into 5-coordinate geometries that lead to particularly favorable values of the HAE. (Graph Presented).

Bismuth molybdate is known to be active for the oxidation of propene to acrolein and its activity can be altered by substitution of other elements (e.g. Fe, V, W) into the scheelite phase of α-Bi2Mo3O12. This work has further revealed that the apparent activation energy for acrolein formation correlates with the band gap of the catalyst measured at reaction temperature. It is, therefore, of interest to establish to how the crystal structure of the catalyst affects the activation energy. We report here an investigation of propene oxidation conducted over Bi, Mo, V oxides having the aurivillius structure with the composition Bi4V2-xMoxO11+x/2 (x = 0-1) and compare them with oxides having the scheelite structure with the composition Bi2-x/3MoxV1-xO12 (x = 0-1). The aurivillius-phase catalysts again show a correlation between the apparent activation energy and the band gap of the oxide, and the only difference being that for a given band gap, the apparent activation energy for the aurivillius-phase catalysts is 1.5 kcal/mol higher than that for the scheelite-phase catalysts. This difference is attributed to the lower heat of propene adsorption on the aurivillius-phase catalysts. A further finding is that for catalysts with band gaps greater than ∼2.1 eV, the acrolein selectivity is ∼75% for the conditions used and independent of the propene conversion. When the band gap falls below ∼2.1 eV, the intrinsic selectivity to acrolein decreases rapidly and then decreases further with increasing propene conversion. This pattern shows that when the activity of oxygen atoms at the catalyst surface becomes very high, two processes become more rapid - the oxidation of the intermediate from which acrolein is formed and the sequential combustion of acrolein to CO2.