An investigation of the Al-Sb-V-W-oxide system for propane ammoxidation

AlSb-V-W-oxide catalysts were prepared and used for propane ammoxidation to give acrylonitrile. The preparations were characterized, both as freshly prepared and after use in propane ammoxidation, using XRD, XPS, Raman spectroscopy, and transmission electron microscopy together with electron diffraction and X-ray microanalysis. Activity tests of the freshly prepared samples reveal a remarkable activation behaviour with time-on-stream under influence from the reaction medium. During the first 15-50 h on-stream, a considerable increase in the activity and the selectivity to acrylonitrile is observed in parallel with a decrease in the selectivity to waste products. The duration of the activation period depends on the W-content and increases with increase of the latter. Characterization with TEM, electron diffraction, and X-ray microanalysis show that the activation of the catalyst precursor under reaction conditions results in the formation of an approximate to Sb(V, W)O-4 phase of rutile-type, which is active and selective to acrylonitrile formation. The aluminum in the catalysts is present in form of delta-Al2O3, which serves as a catalyst support. Analysis of the catalysts and their precursors with XPS shows coverage of the alumina surface with oxides with Sb, V, and W, and the Raman spectra show the presence of tetrahedral and octahedral tungsten oxide species being attached to the alumina surface. Both XPS and X-ray microanalysis results indicate that the surfaces and the external layers of the precursors are enriched with antimony. Activity test of an Al-Sb-V-W-oxide catalyst with Sb:V:W = 5 : 1:2 and 50 wt% Al2O3, using a feed which is stoichiometric with respect to acrylonitrile formation, shows that the selectivity to acrylonitrile is 48% at 77% propane conversion, corresponding to a yield of 37%. Comparison with Sb-V-oxide and Al-Sb-V-oxide preparations reveals a substantial improvement of the catalytic properties with the addition of Al and W to Sb-V-oxide. (C) 1999 Academic Press.