MECHANISM OF PROPENE OXIDATION

Much work has been invested to reveal the mechanism by which propylene is catalytically oxidized to acrolein over the heterogeneous catalyst surface. Isotope labeling experiments by Sachtler and DeBoer revealed the presence of an allylic intermediate in the oxidation of propylene to acrolein over bismuth molybdate. In these experiments, propylene was tagged once at C_l, another time at C₂ and the third time at C₃.

The formed acrolein was photochemically degraded to ethylene and carbon monoxide. It has been found that radioactivity was exclusively associated with ethylene when propylene tagged with ¹⁴C at C₂ was used.

Also, carbon monoxide was found to be free from radioactivity:

When propylene tagged with ¹⁴C at either C_l or C₃ was oxidized to acrolein and then degraded, both CH₂=CH₂ and CO were radioactive, and the ratio of radioactivity was 1.

A proposed mechanism for the oxidation of propylene to acrolein is by a first step abstraction of an allylic hydrogen from an adsorbed propylene by an oxygen anion from the catalytic lattice to form an allylic intermediate:

The next step is the insertion of a lattice oxygen into the allylic species.  This creates oxide-deficient sites on the catalyst surface accompanied by a reduction of the metal. The reduced catalyst is then reoxidized by adsorbing molecular oxygen, which migrates to fill the oxide-deficient sites. Thus, the catalyst serves as a redox system.