Kinetics and mechanisms in the ammoxidation of toluene over a V 2O 5 catalyst. Part 2: Non-selective reactions

Abstract

In the ammoxidation of toluene over a V 2O 5 catalyst, the variation of initial rates with partial pressures of oxygen, toluene and ammonia was determined for formation of carbon oxides. Dependencies obtained were analyzed and expressed in terms of rate equations. It was found that CO and CO 2 are initial products formed at different sites, none of which are involved in selective reactions. The mechanisms derived for the formation of the two oxides have many features in common. In each mechanism there are two parallel routes originating from the same active site, which is suggested to be an ensemble exposing vanadium ions and electrophilic oxygen species. One of the routes proceeds without participation of ammonia, while in the other route ammonia is adsorbed. In both routes, the rate-determining step can be considered to be one of the steps in a stepwise reoxidation process. The rates for formation of carbon oxides decrease strongly with increasing partial pressure of ammonia, which is due to a combined effect of introduction of a new reaction pathway and competitive adsorption between oxygen and ammonia.