Role of Nb in Rutile-type Metal Antimonates for Propane Ammoxidation

Rutile-type Cr/Nb, V/Nb and 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 development of the rutile structure during the thermal treatment of V/Nb/O and Cr/Nb/O systems was investigated by in-situ TP-Raman spectroscopy. Cr and V niobates were active but non-selective to acrylonitrile, while the corresponding antimonates and mixed Cr/V antimonates were active and selective. The addition of increasing amounts of Nb to the mixed Cr/V antimonate led to a considerable increase of the selectivity to acrylonitrile; this effect was evident when excess Sb was present. The multi-component rutile was a highly defective structure containing Nb 5+ and excess Sb 5+ with respect to the stoichiometric composition; this may be one key to the high catalytic performance.