Surface chemistry of V-Sb-oxide in relation to the mechanism of acrylonitrile synthesis from propane .2. Reactivity towards ammonia and relationship with catalytic behaviour

In this second part of a series of three papers on the surface chemistry of V-Sb-oxide for propane ammoxidation, the nature and amount of ammonia chemisorbed on the catalyst as a function of the Sb/V ratio and catalyst preparation method have been studied by IR spectroscopy. The IR data were then used to analyse the catalytic behaviour in propane ammoxidation of the same samples. Ammonia is chemisorbed on Bronsted- and Lewis-acid sites in relative amounts which depend on (i) the temperature, (ii) presence of water, (iii) Sb/V ratio and (iv) method of catalyst preparation. The amount of chemisorbed ammonia was found to be correlated with the catalytic behaviour and in particular, a linear relationship between amount of ammonium ions and selectivity to acrylonitrile was observed. The activity of the catalyst was also found to parallel the number of Lewis-acid sites, although the strong chemisorption of ammonia on such sites suggests competitive chemisorption of ammonia and propane during the catalytic reaction. The reactivity of ammonia towards catalyst structural oxygens was also analysed. It was found that two vanadium species, characterized by different rates of reduction and reoxidation, react with ammonia to form N-2 and water.