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

Abstract

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% Cr 2 O 3 on the Al 2 O 3 , TiO 2 , SiO 2 , and SiO 2 –Al 2 O 3 supports, respectively. In situ Raman and DRS characterization studies under dehydrated conditions revealed the presence of dispersed surface Cr 6+ oxide species for samples below monolayer loadings. Additional Cr 5+ and δ-Cr 3+ species were detected by EPR. From the TPR studies, the reducibility of the surface chromium oxides ( T max ) was decreasing in the order TiO 2 <SiO 2 –Al 2 O 3 ∼Al 2 O 3 <SiO 2 . The reactivity studies using the oxidative dehydrogenation of propane revealed that the activity and propene selectivity of the supported chromia catalysts depend on the loading and the oxide support used. The highest activity was obtained on the Cr 2 O 3 /Al 2 O 3 sample, since the largest amount of chromium oxide could be dispersed on the supports. The selectivity to propene at monolayer coverages was similar for the CrAl and CrTi samples, since at these coverages exposed support sites are unavailable for degrading propene. For the CrSi and CrSiAl samples, however, a significant amount of support is exposed for degrading propene. The intrinsic activity of the surface chromium oxide species, the TOF values, follows the trend CrTi≅CrSi>CrSiAl≥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.