Oxidative Dehydrogenation of Propane over Supported Chromia Catalysts: Influence of Oxide Supports and Chromia Loading

Several loadings of chromium oxide ranging from sub- to above monolayer coverages were prepared on various oxide supports and characterized by various techniques. From a combination of Raman, XPS, and DRS characterization studies the monolayer surface coverages were determined to be ∼12, ∼4, ∼1, and ∼3% Cr2O3 on the Al2O3, TiO2, SiO2, and SiO2–Al2O3 supports, respectively. In situ Raman and DRS characterization studies under dehydrated conditions revealed the presence of dispersed surface Cr6+ oxide species for samples below monolayer loadings. Additional Cr5+ and δ-Cr3+ species were detected by EPR. From the TPR studies, the reducibility of the surface chromium oxides (Tmax) was decreasing in the order TiO2CrSiAl≥CrAl. There is a difference between the variation in TOF values and the reducibility of the supported chromium oxide catalysts. Thus, it appears that the reducibility of the catalyst is not the only factor that is important in determining the activity of the surface chromium oxide species. Furthermore, based on the dispersion amount of chromium oxide on the different oxide supports, the CrTi catalysts appear to be the most suitable among the catalysts studied for the ODH of propane.