Identification of active oxide ions in a bismuth molybdate selective oxidation catalyst

Abstract

Direct structural identification of catalytically active oxide ions for selective olefin oxidation has been achieved using in situ Raman spectroscopy. Spectroscopic examination of Bi₂MoO₆ reduced with select probe molecules such as but-1-ene, propene, methanol and ammonia, when reoxidized with oxygen-18, establishes the existence of the multifunctional nature of catalyst active site for propene oxidation and ammoxidation, wherein α-H abstraction occurs by oxide ions bridging bismuth and molybdenum atoms (Bi—O—Mo), and oxygen or NH insertion into the π-allylic intermediate occurs at centres associated with molybdenum. Sites for O₂ chemisorption, reduction and dissociation are likely to be associated with the directed lone pairs of electrons of the Bi—O—Bi species in the structure. Pulse kinetic experiments reveal that diffusion of the oxide ions through the bulk of the catalyst is the rate-limiting step in catalyst reoxidation. This process is rapid when an oxide ion of only one functional type, i.e.α-H abstraction or oxygen insertion, is removed. The process becomes less facile when oxide ions associated with both α-H abstraction and oxygen insertion are removed from the catalyst. The relative rates reflect the extent of active-site reconstruction achieved by diffusion of oxide ions from the bulk to the surface of the catalyst.