Worldwide, approximately 10 billion pounds of acrylonitrile are produced each year by the ammoxidation of propene over multi-phase, multi-component bismuth-molybdate-based catalysts. This ammoxidation process is a six-electron redox reaction requiring co-feeds of ammonia and oxygen to reoxidize the catalyst and release water (stoichiometry 1C3H6:1NH3:1.5O2). The mechanism for this reaction is thought to be similar to that of the oxidation of propene to acrolein over the same catalysts, which has been more extensively studied in the literature. However, there are still many open questions regarding the influence of ammonia, the nature of the active site, and the mechanism for acrylonitrile formation. In this work, we investigate the properties of propene ammoxidation over alpha-bismuth molybdate, a single-phase, distorted-scheelite material similar to the active phase in the commercial catalyst.
In Chapter 1, the kinetics of propene ammoxidation over Bi2Mo3O12 are presented 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 based on propene consumption (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 is presented.
In Chapter 2, 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, acetonitrile and HCN, 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 has a calculated apparent activation energy of 27.3 kcal/mol. The allyl species produced in this reaction are stabilized as 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 the allyl alkoxide species has a significantly higher activation barrier than the 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 reaction, since nearly all reactant conversion steps are endothermic.
In Chapter 3, the propene activation ability of four molybdenum-based mixed metal oxides – Bi2Mo3O12, PbMoO4, Bi2Pb5Mo8O32, and MoO3 – was investigated using DFT in order to understand the remarkable activity of Bi2Mo3O12 for selective oxidation and ammoxidation of propene. 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 geometry 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 on 4-coordinate molybdate oxo atoms. Bi3+ cations in Bi2Mo3O12 thus promote catalytic activity by providing both strong adsorption sites for propene, and by forcing molybdate into 5-coordinate geometries that lead to particularly favorable values of the HAE.
