Activity of chromium oxide deposited on different silica supports in the dehydrogenation of propane with CO2 – A comparative study

Four series of chromium oxide-based catalysts containing 0.7–7 wt.% of Cr were prepared by incipient wetness impregnation of conventional amorphous silicas (SiO2-p; SBET = 261 m2 g−1 and SiO2-a; SBET = 477 m2 g−1) and mesoporous siliceous sieves with cubic (SBA-1; SBET = 1181 m2 g−1) and hexagonal (SBA-15; SBET = 750 m2 g−1) pore structure. The combination of different techniques (chemical analysis with Bunsen–Rupp method, ICP, XRD, UV–vis DRS and quantitative/qualitative H2-TPR) in the characterization of the calcined catalysts revealed that the chromium species anchored on the surface of mesoporous supports show structural properties similar to those on the conventional silicas, but a higher dispersion of chromium species could be achieved using mesoporous supports due to their much higher SBET. This reflects in higher content of Cr6+ species stabilized in comparison with conventional silicas. The Cr6+ species was found to be crucial for high activity in the dehydrogenation of propane with CO2 (DHP–CO2). The rate of propene formation increases almost proportionally to the concentration of Cr6+ species in the calcined catalysts. In situ UV–vis DRS measurements during DHP–CO2 process evidences that the Cr6+ species are reduced rapidly (in a stream of CO2 + propane) to Cr3+ and Cr2+ species indicating that the Cr6+ species are rather precursor than active sites, similar as in nonoxidative dehydrogenation of propane (DHP). The reduction of Cr6+ species generates dispersed Cr3+ and Cr2+ sites at the beginning of the DHP–CO2 that participate in nonoxidative pathway of propene formation. In the presence of CO2, Cr3+ and Cr2+ sites, may participate additionally in an alternative oxidative pathway of propene formation and in a consumption of hydrogen produced in the DHP by reverse water-gas shift reaction.

Graphical abstractDehydrogenation of propane to propene in the presence of CO2 on different silica-supported chromium oxide catalysts was studied to probe effects of support properties on catalytic activity. A correlation between the content of Cr6+ in fresh catalysts (precursor of active Cr3+/Cr2+ sites) and the initial rate of propene formation was found.Figure optionsDownload full-size imageDownload high-quality image (263 K)Download as PowerPoint slideHighlights► Chromium oxide dispersed on commercial silicas and siliceous mesoporous materials as a catalysts for dehydrogenation of propane with CO2 (DHP–CO2). ► Content of Cr redox species increases with the rise of specific surface area of the support. ► Propene formation rate is proportional to the number of Cr redox species in the catalysts. ► Presented in the fresh catalyst Cr(VI) species are reduced rapidly to Cr(II)/Cr(III) species at the beginning of the DHP–CO2 process.