Identifying the Unique Properties of α-Bi2Mo3O12 for the Activation of Propene

Abstract

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.