Effect of ammonia chemisorption on the surface reactivity of V-Sb-oxide catalysts for propane ammoxidation

The influence of ammonia on the reactivity of a V-Sb-oxide catalyst in propane ammoxidation to acrylonitrile (ACN) was studied by infrared spectroscopy and flow reactor tests. Ammonia coordinates as such or as ammonium ions on the catalyst surface in relative amounts which depend on both the temperature at the time of contact and the hydroxylation state of the catalyst. The two chemisorbed species influence both the surface reactivity and the pathways of the reaction in propane ammoxidation. NH, chemisorbed on Lewis acid sites inhibits the activation of propane differently from that chemisorbed on Bronsted sites, The formation of the latter species, however, inhibits the reactivity of the Bronsted sites in catalysing various side reactions which decrease the selectivity to ACN. The results of the infrared study are in good agreement with the change in surface reactivity observed in flow reactor tests as a function of the concentration of ammonia in the feed. Both the nature and the amount of the chemisorbed ammonia species thus determine the intrinsic surface reactivity and selectivity of V-Sb-oxide catalysts in the synthesis of acrylonitrile.