Selective oxidation and ammoxidation of propene on bismuth molybdates, ab initio calculations

In this paper we use first principles quantum mechanical methods (B3LYP fla vor of density functional theory) to examine the mechanism of selective oxi dation and ammoxidation of propene by BiMoOx catalysts. To do this we use f inite clusters chosen to mimic likely sites on the heterogeneous surfaces o f the catalysts. We conclude that activation of the propene requires a Bi(V ) site while all subsequent reactions involve di-oxo Mo(VI) sites adjacent to the Bi. We find that two such Mo sites are required for the most favorab le reactions. These results are compatible with current experimental data. For ammoxidation, we conclude that ammonia activation would be easier on Mo (IV) rather than on Mo(VI). Ammonia would be activated more easily for more reducing condition. Since ammonia and propene are reducing agents, higher partial pressures of them could accelerate the ammonia activation. This is consistent with the kinetic model of ammoxidation proposed by Grasselli and coworkers that imido sites (Mo=NH) are more abundant in higher partial pre ssures of feed. Our calculations also indicate that allyl groups produced a s a result of the hydrogen abstraction from propenes would be adsorbed more easily on imido groups (Mo=NH) than on oxo groups (Mo=O) and that the spec tator oxo effect is larger than spectator imido effect. Thus, we propose th at the best site for ammoxidation (at least for allyl adsorption) is the im ido group of the "oxo-imido" species.