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.