Propane ammoxidation on an Al-Sb-V-W-oxide catalyst. A mechanistic study using the TAP-2 reactor system

The reaction mechanism of propane ammoxidation was studied on an Al-Sb-V-W-oxide catalyst using a TAP-2 reactor system. Analyses of the responses from both high-speed pulse transients with reactants and TPD experiments were performed. Since the ammoxidation process with three reactants proceeds from propane to acrylonitrile over propylene as an intermediate, the experiments comprised oxidation of propylene, oxidation of ammonia, ammoxidation of propylene, oxidation of propane, and ammoxidation of propane. The results show that propane is irreversibly adsorbed at the surface forming propylene, which desorbs. Propylene then readsorbs forming an intermediate allyl species, which reacts with lattice oxygen to give acrolein. Acrolein is unstable and some of it reacts further to produce either carbon oxides, or, acrylonitrile. Formation of the nitrile occurs by adsorbed acrolein reacting with an NH, species. The latter species is short-lived and reacts competitively to form N-2, N2O and NO. Lattice oxygen plays an important role in the pathway to acrylonitrile. However, weakly adsorbed oxygen species are also present at the catalyst surface, and these species participate in degradation routes producing waste products. Consideration of the mechanistic scheme which is derived from the experimental results shows the possibility to achieve improvement of the ammoxidation process by using either a recirculating solids reactor, or a high propane/oxygen ratio in the feed. (C) 1999 Elsevier Science Ltd. All rights reserved.