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 s pectroscopy 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 ammoxid ation. NH, chemisorbed on Lewis acid sites inhibits the activation of propane differently from that chemisorbed on Bronsted sites, The forma tion of the latter species, however, inhibits the reactivity of the Br onsted sites in catalysing various side reactions which decrease the s electivity to ACN. The results of the infrared study are in good agree ment with the change in surface reactivity observed in flow reactor te sts as a function of the concentration of ammonia in the feed. Both th e nature and the amount of the chemisorbed ammonia species thus determ ine the intrinsic surface reactivity and selectivity of V-Sb-oxide cat alysts in the synthesis of acrylonitrile.