Chemical and physical characterization of alumina-supported chromia-based catalysts and their activity in dehydrogenation of isobutane

Dehydrogenation catalysts based on chromium oxide supported on commercial alumina which contained increasing amounts of chromium oxide were prepared and characterized with different techniques: XPS, UV-Vis-NIR diffuse reflectance spectroscopy, XRD, and chemical analysis. Potassium-doped catalysts containing increasing amounts of alkali metal also were prepared and characterized. The reactivity of the samples in isobutane dehydrogenation was tested, and the effect of chromium and potassium loading was examined. The results were analyzed and compared with data from the scientific literature in order to draw up a model of this catalytic system. It is proposed that in samples calcined at 600 degrees C an initial coverage of the alumina support occurs, constituted of Cr6+ species anchored to the support, and of dispersed Cr6+ and Cr3+ oxide species. This latter Cr6+ species could be removed by a treatment with water, and thus was not grafted to the support. Crystalline alpha-Cr2O3 was found only for the higher chomium oxide loading. In spent catalysts all the chromium was present in the Cr3+ form; the Cr3+ species formed by reduction in the reaction environment of the Cr6f species were less active than the Cr3+ species in dispersed Cr3+ oxide. The activity was found to be proportional to the overall amount of chromium, except for the highest CrO3, loading (15.3 wt% CrO3) which showed a lower activity. The addition of potassium led to the formation of a potassium chromate phase, the formation of which occurred at the expense of the dispersed Cr3+ oxide and of alpha-Cr2O3. The catalytic activity was generally inhibited by the addition of potassium, except for the catalyst containing 15.3 wt% CrO3. In this case an increase in activity was achieved by addition of amounts of potassium lower than 1 Wt% K2O. (C) 1996 Academic Press, Inc.