Role of Nb in rutile-type Cr/V/Sb/Nb mixed oxides, catalysts for propane ammoxidation to acrylonitrile

Rutile-type Cr/V/Sb/Nb mixed oxides were prepared by coprecipitation from ethanolic solutions and calcination at 700 °C. They were then tested as catalysts for the gas-phase ammoxidation of propane. The addition of increasing amounts of Nb to the rutile Cr/V antimonate led to a considerable increase in the selectivity to acrylonitrile and to a lower selectivity to N2 derived from ammonia overoxidation. But this effect was evident only when excess Sb was present with respect to the stoichiometric requirement for the formation of the rutile compound. Evidence was obtained for the development of rutile-type mixed Cr/V antimonate/niobate; this multicomponent rutile had a highly defective structure, with cationic vacancies in relation to excess Sb5+ with respect to the stoichiometric composition. The progressive increase of Nb concentration in this mixed oxide with increasing Nb loading led to a partial segregation of Sb oxide in the form of Sb2O4. The excess Sb in rutile provided the active sites for allylic ammoxidation on intermediate adsorbed propylene. The contemporaneous presence of Nb in the lattice improved the efficiency of these sites and was responsible for the better catalytic performance with respect to the Cr/V/Sb/O systems.