Unsupported vanadium antimonate catalysts with Sb/V ratios of 1 and 5 and samples with the latter ratio supported on alumina were studied in toluene and propane ammoxidn. to benzonitrile and acrylonitrile, resp., and were characterized by XPS anal. before and after catalytic tests. Activity data for toluene ammoxidn. suggest that excess antimony with respect to the stoichiometric amt. required for forming the VSbO4 rutile phase affects the dispersion of the latter phase giving smaller particles. Vanadium sites are involved both in the activation of toluene and in the insertion of nitrogen in this reaction, whereas antimony does not play a specific role in the reaction mechanism. In propane ammoxidn., on the other hand, due to a higher reaction temp. with respect to toluene (500° vs. 370°), free vanadia on the surface of the catalyst has a neg. influence on the selectivity because it promotes the conversion of ammonia to nitrogen, decreasing the surface nitrogenous species required for the selective formation of acrylonitrile. Excess antimony is thus necessary for completing the reaction between antimony and vanadium oxides, but antimony also participates in the reaction mechanism. In propane ammoxidn., in fact, XPS data show that both vanadium and antimony sites are reduced. Tentatively, vanadium sites are involved in the activation of propane, while antimony sites insert nitrogen. The differences between the toluene and propane ammoxidn. mechanisms are interpreted to be primarily related to the different reaction temps. [on SciFinder(R)]
Surface characterization and reactivity in ammoxidation reactions of vanadium antimonate catalysts
CENTI, Gabriele;
1994-01-01
Abstract
Unsupported vanadium antimonate catalysts with Sb/V ratios of 1 and 5 and samples with the latter ratio supported on alumina were studied in toluene and propane ammoxidn. to benzonitrile and acrylonitrile, resp., and were characterized by XPS anal. before and after catalytic tests. Activity data for toluene ammoxidn. suggest that excess antimony with respect to the stoichiometric amt. required for forming the VSbO4 rutile phase affects the dispersion of the latter phase giving smaller particles. Vanadium sites are involved both in the activation of toluene and in the insertion of nitrogen in this reaction, whereas antimony does not play a specific role in the reaction mechanism. In propane ammoxidn., on the other hand, due to a higher reaction temp. with respect to toluene (500° vs. 370°), free vanadia on the surface of the catalyst has a neg. influence on the selectivity because it promotes the conversion of ammonia to nitrogen, decreasing the surface nitrogenous species required for the selective formation of acrylonitrile. Excess antimony is thus necessary for completing the reaction between antimony and vanadium oxides, but antimony also participates in the reaction mechanism. In propane ammoxidn., in fact, XPS data show that both vanadium and antimony sites are reduced. Tentatively, vanadium sites are involved in the activation of propane, while antimony sites insert nitrogen. The differences between the toluene and propane ammoxidn. mechanisms are interpreted to be primarily related to the different reaction temps. [on SciFinder(R)]Pubblicazioni consigliate
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