| Abstrakt: |
Vanadium-antimony-oxides are known to be active and selective catalysts for the ammoxidation of propane to acrylonitrile. The performance of these catalysts can be enhanced by doping with different metal ions. However, the catalytic function of these promoters is still unknown. In the present work the influence of promoters (W, Mg, Nb, Te, Ga, Pb, Sm) on the surface acidic and bulk redox properties and on the catalytic performance of VSb5Ox/Al2O3 catalysts was investigated. The freshly prepared catalysts exhibited only small differences in their acidic and redox properties, while dramatic changes occurred under reaction conditions in the first hours-on-stream. The selectivity to acrylonitrile of the modified catalysts in the final state (after 5 h on-stream) increased in the following order of dopants: Sm, Pb < Ga < unpromoted < Nb, Te < Mg < W. In the same order the surface Lewis-acidity increased, while basicity decreased. The catalysts exhibiting higher selectivity to acrylonitrile also showed a higher degree of reduction of vanadium. In the W modified catalyst only the redox pair VIV/VIII could be detected. It is concluded that in the Me promoted catalysts which showed better performance than the unpromoted catalyst (MeW, Mg, Nb and Te) the role of the promoters is to catalyze the formation of the active and selective form of the vanadium, most probably a VIVSbO4 phase, or to stabilize this phase under reaction conditions. |
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