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 rea ched 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 he at treatment in vacuum at 600 degrees C is reported and analyzed in te rms of the change in the structural features of the catalyst determine d as a function of the time on stream by IR spectroscopy, X-ray diffra ction 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 pro pene, but not to acrylonitrile, An increase in the selectivity and pro ductivity to acrylonitrile occurs when an Sb-rich approximate VSbO4 ph ase forms ('VSbO4'). This phase, however, is partially metastable, dec omposing 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 sid e oxidation of ammonia to nitrogen competes for the reduction of these V5+ ions and influences the above redox and solid-state reactions. Th erefore, a considerable dependence of the surface reactivity on the fe ed was observed. The optimal catalytic behavior determined for the ser ies of catalysts studied was found to depend on the feed composition i ndicating that in the analysis of the structure, activity and selectiv ity relationships in propane ammoxidation the concentration of reactan ts in the feed plays a specific important role.