Selective Oxidation Catalysis: An Organic Chemist’s View of Mechanism

Abstract

The mechanism of the commercially significant oxidation and ammoxidation of propylene to acrylonitrile using heterogeneous bismuth molybdate catalysts has been extensively studied and is believed to occur via a Mo π-allylic intermediate formed in the rate determining step, followed by nitrogen insertion via reversible formation of σ-allylic species. The active surface site is likely composed of two Mo and one Bi centers, where Bi is required for H-abstraction, and Mo for O or N-insertion. Valuable insights have been provided by the computational chemistry of Goddard including the favorable thermodynamics associated with the “spectator oxo” effect, which provides a driving force for the collapse of the initially formed π-allyllic species onto a Mo-dioxo species. It also provides a plausible explanation for the N-insertion to be favored over O-insertion by reaction with a Mo-oxo-imido species, thus leading to acrylonitrile in the presence of ammonia. Key functions of H-abstraction/O-insertion and re-oxidation can be applied to other selective (amm) oxidation catalysts. Bi-Mo-O catalysts are also useful for selective (amm) oxidation of other allylic/benzylic hydrocarbons.