A Study of Propane and Propene Ammoxidation over Antimony-Vanadium-Oxide Catalysts at Steady-State and Transient Conditions

Sammanfattning: Propane ammoxidation catalysts of differing Sb/V ratio and consisting of differing amounts of the V2O5, alpha-Sb2O4, and Sb0.92V0.92O4 phases were prepared. In the first paper, the structure of Sb0.92V0.92O4 was determined by thermogravimetry, X-ray microanalysis, X-ray powder diffraction, and neutron powder diffraction. Sb0.92V0.92O4 was found to have a defective rutile structure involving cation vacancies. In the second, third, and fourth papers, the initial rates and selectivities of these catalysts were determined when used in the ammoxidation of propane and propene, respectively. The main products obtained in propane ammoxidation were acrylonitrile, acetonitrile, carbon oxides and propene. Two synergy effects were revealed; catalysts rich in vanadium showed a maximum in total reaction rate and catalysts rich in antimony a maximum in selectivity to acrylonitrile formation. The main products obtained in propene ammoxidation were acrylonitrile, acetonitrile, carbon oxides and acrolein. The catalysts were characterised by XRD, XPS, FTIR, and FT-Raman measurements before and after their use in propane and propene ammoxidation, respectively. In the course of ammoxidation, catalysts with an excess of alpha-Sb2O4 were found to be enriched at the surface with antimony, creating a surface that was selective for nitrile formation. This enrichment appeared to be caused by the migration of antimony from alpha-Sb2O4 to the surface of Sb0.92V0.92O4. No evidence that antimony migrates from the bulk of Sb0.92V0.92O4 up to the surface of it was obtained. The fifth and sixth papers concerned the transient response method. Differential equations describing the consumption of reactants and the formation of products were solved numerically for a step change from inert to reactant feed. Different types of propene/propane oxidation/ammoxidation mechanisms were considered. In comparing the experimental and the simulated responses, it was found that the adsorption of propane was rate limiting for its consumption in the ammoxidation of it. However, the propene/acrylonitrile product ratio was governed by the step in which water desorbs. These results were not contradicted by the results reported in paper four, in which dependencies of the rate on the partial pressure of the reactants was investigated under steady-state conditions.