Active Sites Characterization in Mixed Vanadium and Iron Antimonate Oxide Catalysts for Propane Ammoxidation

Ammoxidation of propane was investigated over FeVSbO mixed oxide catalysts. The results obtained show that in VSbO4 mixed oxide catalyst systems, substitution of vanadium by small quantities of iron has a strongly positive effect on the activity of the catalysts. With such a system, the selectivity to acrylonitrile decreases while the selectivity to propene increases. At high levels of vanadium substitution by iron, the activity was low but the selectivity towards acrylonitrile was enhanced. The characterization of the catalysts carried out using several techniques, such as X-ray diffraction, Mössbauer spectroscopy, X-ray absorption, and electron microscopy, has shown that a continuous solid solution was formed between VSbO4 and FeSbO4. At low iron loading, Fe3+ substituted for V4+ in the cationic-deficient structure V4+0.64V3+0.28Sb5+0.92♢0.16O4, leading to a structure with both cationic and anionic vacancies, Fe3+xV3+0.33V4+0.41−xSb5+0.92♢0.18O4−x, and generating very active sites for alkane activation. At high iron loading, only V3+ was present in the bulk of the solids and only Fe3+ contributed to the isolation of the vanadium sites, which are selective to acrylonitrile.