Surface chemistry of V–Sb–oxide in relation to the mechanism of acrylonitrile synthesis from propane Part 3.—Influence of ammonia on the competitive pathways of reaction

Abstract

The effect of ammonia chemisorption on the surface reactivity of V–Sb–oxide catalysts with Sb/V=1.0 and the role of ammonia adspecies on the pathways of propane conversion to acrylonitrile are discussed in this third part of a series of three articles on the surface chemistry and reactivity of V–Sb–oxides for the synthesis of acrylonitrile from propane. Ammonia chemisorbed on Lewis-acid sites inhibits the activation of propane differently from that chemisorbed on Brønsted-acid sites, and the two ammonia adspecies play different roles in the further mechanism of transformation of the chemisorbed intermediates to acrylonitrile. Ammonium ions react with acrylate species to give acrylonitrile via acrylamine intermediates, whereas activated ammonia species formed by heterolytic splitting of coordinated NH ₃ react with an allyl alcoholate intermediate to give an acrylimine adspecies further transformed to acrylonitrile. The second route is faster than the first. The conversion of Brønsted sites to ammonium ions by reaction with ammonia inhibits their negative action in catalysing side-reactions which decrease the selectivity to acrylonitrile. In particular, Brønsted-acid sites catalyse (i) a carbon chain degradation pathway leading to C ₁ and C ₂ species and (ii) the hydrolysis of an amide species, formed by reaction of ammonium ions with acrylate adspecies, with formation of free, weakly chemisorbed acrylic acid, the latter is probably an intermediate to the formation of carbon oxides. The overall reaction network in propane ammoxidation is also discussed with particular attention directed to the presence of multiple pathways of reaction from propane to acrylonitrile characterized by different rates and intrinsic selectivities.