Site isolation and cooperation effects in the ammoxidation of propane with VSbO and Ga/H-ZSM-5 catalysts

The performances of a conventional mixed oxide catalyst, VSbyOx, and of a novel zeotite-based catalyst, Ga-modified H-ZSM-5 (Ga2Z27), are compared for the ammoxidation of propane. In both cases, the effects of reaction temperature and ammonia to propane ratio were investigated. Increasing productivity to acrylonitrile is observed when the temperature is raised and the ammonia to propane ratio increased. The maximum yields in acrylonitrile are 8.5 and 11.4% for the unpromoted VSbyOx and Ga2Z27 catalysts, respectively, at comparable propane conversions (ca. 30%). The activity and productivity to acrylonitrile of Ga2Z27 are about 50% of those of VSbyOx, at 500 degreesC, in the absence of any additional promoters. As reported previously, propene and carbon oxides, CO and CO2, were observed to be major by-products when using VSbyOx catalysts, 10-20 and 20-40%, respectively. By contrast, the Ga2Z27 catalyst shows a very low selectivity to carbon oxides (1% or less) and a rather high and interesting selectivity to C-4 hydrocarbons, consisting mainly of isobutane and isobutene. These observations can be explained by differences in the reaction mechanisms describing the ammoxidation of propane on these two systems. For VSbyOx, the classical mechanism applies. Propene is the primary reaction product which is easily oxidised to carbon oxides. For Ga2Z27, propane is initially activated to a protonated pseudo-cyclopropane (PPCP) transition state on bifunctional sites involving dispersed gallium species and Bronsted acid sites of the zeolite. The PPCP transition state can evolve in several ways leading to propane, C-2-C-4 hydrocarbons, and N-containing compounds. Isolation of gallium species is necessary for Ga/H-ZSM-5 in order to, active sites, whereas vanadium sites isolation is required to avoid overoxidation for VSbyOx. Phase generate the bifunctional, Ga3+-H+ cooperation is necessary for the VSbyOx catalysts, as reported previously, whereas site cooperation is required for Ga-modified H-ZSM-5.