Effect of Sb/V ratio and of Sb+V coverage on the molecular structure and activity of alumina-supported Sb-V-O catalysts for the ammoxidation of propane to acrylonitrile

The effect of the total coverage of Sb+V oxides and the effect of the Sb/V atomic ratio on the ammoxidation of propane to acrylonitrile on alumina-supported V and Sb oxide catalysts is reported. The fresh and used catalysts are characterized by XRD and in situ Raman spectroscopy. Comparison with binary V-Al-O and Sb-Al-O catalysts shows that the presence of both Sb and V oxides strongly enhances the rate of propane ammoxidation to acrylonitrile on alumina-supported Sb-V oxide catalysts. The stability and structural changes during on-stream operation prior to reach steady-state operation originates from a close interaction between Sb and V oxides. The Sb-V interaction depends on total Sb+V coverage on alumina. Below the dispersion limit, SbVO4 phases are not stable under reaction and break into the individual oxides. At Sb+V loading beyond dispersion limit, SbVO4 phases are stable under reaction conditions while Sb and V oxides that did not combine during calcination of the precursor recombine into SbVO4 phases. This solid-state reaction accounts for a higher propane conversion and selectivity to acrylonitrile. Comparison of the performance and molecular structures of fresh and used catalysts further suggests that Sb-V-O phases are necessary for this reaction. The specific formation of acrylonitrile per vanadium site reaches a maximum at an atomic Sb/V ratio of 2. It is likely that a moderate excess of antimony may be necessary for an efficient ammoxidation of propane to acrylonitrile. (C) 2002 Elsevier Science (USA).