Surface chemistry of V–Sb–oxide in relation to the mechanism of acrylonitrile synthesis from propane. Part 1.—Chemisorption and transformation of possible intermediates

Abstract

The mechanism of propane ammoxidation to acrylonitrile on V–Sb–oxide is analysed by IR spectroscopy in three papers dealing with the study of (i) the reaction network in propane conversion, (ii) the relationship between ammonia chemisorbed species and catalytic behaviour and (iii) the effect of ammonia chemisorption on the surface reactivity. This first part deals with the study of the nature of surface adspecies formed by chemisorption of acetone, isopropyl alcohol, acrylic acid, ally alcohol, propene, propionic acid, acetic acid, propane and acrylonitrile on a vanadium antimonate catalyst (V/Sb = 1.0) and subsequent thermal treatment. The results indicate that the catalyst is characterized by multifunctional properties (H-abstraction, O-insertion and oxidative cleavage properties), the relevance of which for the mechanism and catalytic behaviour in acrylonitrile synthesis from propane is discussed. A surface reaction network in propane oxidation is also proposed involving multiple possible pathways of transformation. The main route of propane conversion is through the formation of propene as an intermediate, although a side reaction of acrylate formation via a propionate intermediate is possible. Propene may be oxidized according to two routes, the first leading to acetone which undergoes quick oxidative cleavage to an acetate species and a C₁ fragment, and the second to the formation of an allyl alcoholate adspecies which transforms to acrolein which quickly further converts to an acrylate species. The acrylate species strongly interacts with the surface and is thermally stable up to relatively high temperatures.