PROPANE DEHYDROGENATION AND COKE FORMATION ON CHROMIA-ALUMINA CATALYSTS - EFFECT OF REDUCTIVE PRETREATMENTS

Propane dehydrogenation and coke formation over a series of alumina-supported chromia catalysts with different chromia contents were examined. X-ray photoelectron spectroscopy (XPS) showed that the calcined catalysts displayed Cr(VI) and Cr(III) in proportions which depended on the overall chromia content Cr(VI) appeared to be reduced in the initial stages of the dehydrogenation reaction, for which important changes in the selectivity to propene and in the coking rate were noted. These changes were mainly related to the hydrocarbon combustion by the oxygen released from Cr2O3 and to the suppression of the strong acid sites by coking. Coke formation on CO-prereduced catalysts was fast and could be related to the level and stability of the conversion to propene. Both activity and selectivity of the catalysts were determined by the dispersion of Cr and by the effect of Cr content on the porous structure of the catalysts.