Ammoxidation of propane to acrylonitrile on FeSbO4

The properties of FeSbO4 (Fe:Sb = 1:1 and 1:2) for propane (amm)oxidation have been investigated. Both catalysts showed little selective oxidation activity, however, both were active and selective for propane ammoxidation to acrylonitrile. The 1:2 catalyst showed increased selectivity, relative to the 1:1 catalyst, for acrylonitrile but at the expense of propane conversion. XRD and FT-IR showed the major phases present were rutile FeSbO4 and Sb2O4. XPS studies showed the surface of both catalysts to be antimony rich and TPD studies showed the major pathway to acrylonitrile was a two-stage process via propene and an allylic intermediate, although there is a direct contribution from a one-stage mechanism via an alkyl intermediate. It is postulated that the degree of surface oxidation is critical in determining the dominant surface intermediate. The rate limiting step in the ammoxidation reaction is considered to be dissociative adsorption of propane on the surface, the adsorption rate of propane being orders of magnitude lower than for propene, as manifested by TPD experiments. The role of Sb and Fe are discussed, together with the principal surface mechanisms.