Structure, activity and selectivity relationships in propane ammoxidation to acrylonitrile on V-Sb oxides .3. Modifications during the catalytic reaction and effect of feed composition

The change in surface reactivity up until steady-state behavior is reached in propane ammoxidation of a series of V-Sb-oxide catalysts with Sb:V ratios in the range 1-10 and prepared either by calcination or heat treatment in vacuum at 600 degrees C is reported and analyzed in terms of the change in the structural features of the catalyst determined as a function of the time on stream by IR spectroscopy, X-ray diffraction and chemical analysis data. The results indicate that during the catalytic reaction, V5+ oxide when present, quickly reduces forming a V4+O2/VSbO4 solid solution with an increase in the selectivity to propene, but not to acrylonitrile, An increase in the selectivity and productivity to acrylonitrile occurs when an Sb-rich approximate VSbO4 phase forms ('VSbO4'). This phase, however, is partially metastable, decomposing to 'VSbO4' + Sb2O4 when Sb5+ ions are reduced and not rapidly reoxidized. V5+ ions on the surface of the rutile phase stabilize the Sb-rich 'VSbO4' phase, and catalyze the reoxidation of Sb3+. This side oxidation of ammonia to nitrogen competes for the reduction of these V5+ ions and influences the above redox and solid-state reactions. Therefore, a considerable dependence of the surface reactivity on the feed was observed. The optimal catalytic behavior determined for the series of catalysts studied was found to depend on the feed composition indicating that in the analysis of the structure, activity and selectivity relationships in propane ammoxidation the concentration of reactants in the feed plays a specific important role.