VOx species on alumina at high vanadia loadings and calcination temperature and their role in the ODP reaction

Vanadium species formed on the alumina support after calcination between 773 and 973 K at vanadium loadings between 4.45 and 13.2 wt.% were characterized with different methods (V-51 NMR, ESR, H-2-TPR and in situ UV-vis-DRS) and tested in the oxidative dehydrogenation of propane. By calcination at 873 K, beside dispersed isolated and polymerized V5+ surface species mainly in tetrahedral coordination, a crystalline AIVO(4) phase was formed. The portion of vanadium present as AIVO(4) increases with increasing vanadium loadings (e.g. about 60 at 13.2 wt.%). The domain size of the polymerized V5+ species is only weakly influenced by the vanadium loadings at calcination up to 873 K. It is suggested that both isolated and polymerized tetrahedrally coordinated V5+ species are active in propane dehydrogenation. The AIVO(4) phase and bulk-like V2O5 have only a low catalytic activity. The highest propene selectivity was obtained for the sample with the highest dispersion of the isolated and polymerized tetrahedrally coordinated V5+ species. (C) 2004 Elsevier B.V. All rights reserved.