Ammoxidation of Propane on Nickel Antimonates

Abstract

The catalytic behaviour of Ni–Sb mixed oxides doped by vanadium has been investigated for the ammoxidation of propane and propene to acrylonitrile. The binary nickel antimonates, with 1:1<Ni:Sb<1:3, were found to be active and selective in the ammoxidation of propene to acrylonitrile (selectivity >80%) but they showed no activity in propane ammoxidation till 470°C. The activity/gram and the yield in acrylonitrile (ACN)/gram presented a maximum at Ni:Sb 1:2 due to a balance between the surface area and the doping effect of antimony. With the addition of vanadium to the Ni–Sb system, the activity and productivity of the catalysts were increased markedly, both in propane and propene ammoxidation. The optimum vanadium loading in terms of ACN yield was found for NiSb2O6to be V:Ni 0.2:1 atomic ratio, a compromise between activity and selectivity. It was found that sites containing vanadium are involved in the selective nitrogen insertion step in propene ammoxidation, as well as in the activation of propane. The ammoxidation of propane is a cleaner reaction than the ammoxidation of propene, as smaller amounts of hydrogen cyanide (HCN) and acetonitrile (AceN) were formed for the same yield of acrylonitrile. X-ray analysis revealed the presence of NiSb2O6and free αSb2O4in all samples. In the Ni–Sb vanadium doped oxides the FTIR characterisation showed that up to a V:Ni ratio of 0.2, vanadium species different from V2O5, and very likely interacting with the NiSb2O6, were formed; these species are the ones involved in propane activation. With higher loadings of vanadium, V2O5species formed which are responsible for the lowering of acrylonitrile selectivity.