AMMOXIDATION OF PROPANE ON NICKEL ANTIMONATES - THE ROLE OF VANADIUM AS PROMOTER

The catalytic behaviour of Ni-Sb mixed oxides doped by vanadium has be en investigated for the ammoxidation of propane and propene to acrylon itrile. The binary nickel antimonates, with 1 : 1 < Ni:Sb < 1 :3, were found to be active and selective in the ammoxidation of propene to ac rylonitrile (selectivity >80%) but they showed no activity in propane ammoxidation till 470 degrees C. The activity/gram and the yield in ac rylonitrile (ACN)/gram presented a maximum at Ni:Sb 1:2 due to a balan ce between the surface area and the doping effect of antimony. With th e addition of vanadium to the Ni-Sb system, the activity and productiv ity of the catalysts were increased markedly, both in propane and prop ene ammoxidation. The optimum vanadium loading in terms of ACN yield w as found for NiSb2O6 to be V:Ni 0.2:1 atomic ratio, a compromise betwe en activity and selectivity. It was found that sites containing vanadi um are involved in the selective nitrogen insertion step in propene am moxidation, 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 t he presence of NiSb2O6 and free alpha Sb2O4 in 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 lik ely interacting with the NiSb2O6, were formed; these species are the o nes involved in propane activation. With higher loadings of vanadium, V2O5 species formed which are responsible for the lowering of acryloni trile selectivity. (C) 1997 Academic Press.