CrN/Sb mixed oxides, catalysts for the ammoxidation of propane to acrylonitrile -: Part II.: Effect of catalyst composition on catalytic performance

Rutile-type, Cr/V\/Sb mixed oxides having different atomic ratio between components were studied as catalysts for propane and propylene ammoxidation to acrylonitrile. Catalysts were more active than Cr/Sb and V/Sb mixed oxides; this was attributed to (i) the higher specific surface area of Cr/Sb/O and CrN/Sb/O with respect to V/Sb/O, and (ii) the preferential formation of V4+ in CrN/Sb/O. The nature of the V species and the catalytic performance of CrN/Sb/O samples were functions of the (Cr + V)/Sb atomic ratio. When the latter was higher than approximate to1, the prevailing species was V4+; the catalysts were very active but poorly selective to acrylonitrile (selectivity lower than 20%) because of the prevailing formation of carbon oxides and propylene. This was due to the absence of sites able to transform the unsaturated intermediate to acrylonitrile. When the (Cr + V)/Sb ratio was between approximate to1 and approximate to0.5, catalysts reached a selectivity to acrylonitrile between 20 and 30%, and to propylene lower than 10%. In these samples, the presence of intra-crystalline Sb gradients in the rutile lattice provided a Sb surface enrichment and the development of allylic ammoxidation sites, able to transform the unsaturated intermediate to acrylonitrile. When the (Cr + V)/Sb ratio was lower than approximate to0.5, i.e. in systems having excess Sb, the prevailing species was V3+; the selectivity to acrylonitrile was higher than 30%, with low formation of carbon oxides and of propylene. In this case additional sites for allylic ammoxidation were provided by excess antimony oxide dispersed over the rutile surface. (C) 2003 Elsevier B.V. All rights reserved.