AMMOXIDATION OF PROPANE OVER ANTIMONY VANADIUM-OXIDE CATALYSTS

Catalysts belonging to the Sb-V-O system were prepared with various Sb/V ratios and were used for propane ammoxidation to acrylonitrile. XRD patterns of freshly prepared samples show those with excess vanadia to consist Of V2O5 and SbVO4, while SbVO4 and alpha-Sb2O4 are constituents in the samples with a Sb/V ratio above unity. High rate and selectivity for propylene formation at low conversion are characteristic for samples with excess vanadia and considering XRD, Raman, infrared, and XPS results, this is explained by formation of amorphous vanadia spread over the surface of SbVO4. Catalysts with both alpha-Sb2O4 and SbVO4 phases are the most selective for acrylonitrile formation, a function that is linked to their ability to selectively transform intermediate propylene. XPS data suggest this function to be associated with the formation of suprasurface antimony sites on SbVO4 as a result of migration of antimony from alpha-Sb2O4 during the catalytic process. Raman and infrared spectral features revealed that compared with SbVO4, the samples with both alpha-Sb2O4 and SbVO4 are more efficiently reoxidised during propane ammoxidation. Rate dependences on the partial pressures of reactants over a sample with excess alpha-Sb2O4 show that the adsorption of propane is the rate limiting step for propylene formation, and that acrylonitrile and carbon oxides are predominantly formed from the intermediate propylene in routes comprising nonequilibrated steps. Addition of water vapour results in an increase of rate and selectivity for acrylonitrile formation. The kinetic dependences indicate that for acrylonitrile formation it is advantageous to have a feed rich in propane and to use recirculation for obtaining high productivity. (C) 1994 Academic Press, Inc.