SURFACE-CHEMISTRY OF V-SB-OXIDE IN RELATION TO THE MECHANISM OF ACRYLONITRILE SYNTHESIS FROM PROPANE .2. REACTIVITY TOWARDS AMMONIA AND RELATIONSHIP WITH CATALYTIC BEHAVIOR

In this second part of a series of three papers on the surface chemist ry of V-Sb-oxide for propane ammoxidation, the nature and amount of am monia 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 a mmoxidation of the same samples. Ammonia is chemisorbed on Bronsted- a nd Lewis-acid sites in relative amounts which depend on (i) the temper ature, (ii) presence of water, (iii) Sb/V ratio and (iv) method of cat alyst preparation. The amount of chemisorbed ammonia was found to be c orrelated with the catalytic behaviour and in particular, a linear rel ationship between amount of ammonium ions and selectivity to acrylonit rile was observed. The activity of the catalyst was also found to para llel the number of Lewis-acid sites, although the strong chemisorption of ammonia on such sites suggests competitive chemisorption of ammoni a and propane during the catalytic reaction. The reactivity of ammonia towards catalyst structural oxygens was also analysed. It was found t hat two vanadium species, characterized by different rates of reductio n and reoxidation, react with ammonia to form N-2 and water.