(330d) A DFT+U Investigation of Propene Oxidation Over Bismuth Molybdate

Catalysts based on a bismuth molybdate active phase have been used industrially for over 60 years for the selective conversion of propene to acrolein and acrylonitrile. Yet despite this long and successful history, there remains much that is not yet known about the detailed reaction mechanisms taking place on these catalysts. In particular, the structure of the active site and the identities of the reaction intermediates produced after the initial rate-determining step have so far eluded experimental determination. In order to better understand these mechanistic details, we have employed the DFT+U flavor of density functional theory. Our results suggest that the initial, rate-determining hydrogen abstraction step takes place at a molybdenyl oxygen, not at a Bi-O-Mo site as literature has proposed. Bismuth is neither oxidized nor reduced during the reaction cycle. However, bismuth still plays an essential role in enabling the initial hydrogen abstraction, by a mechanism which this talk will clarify. H-abstraction generates a symmetric allyl intermediate that is not bound to a particular metal center, but can move freely across the catalyst surface. This radical eventually attacks a second molybdenyl oxygen, generating an allyl-alcohol-like intermediate. A second hydrogen abstraction produces acrolein in a kinetically irrelevant step. With the better understanding of the active site and reactive intermediates provided by these calculations, it is possible to rationally formulate a more active, more selective propene oxidation catalyst.