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; S-BET = 261 m(2) g(-1) and SiO2-a; S-BET = 477 m(2) g(-1)) and mesoporous siliceous sieves with cubic (SBA-1; S-BET = 1181 m(2) g(-1)) and hexagonal (SBA-15; S-BET = 750 m(2) g(-1)) pore structure. The combination of different techniques (chemical analysis with Bunsen-Rupp method, ICP, XRD, UV-vis DRS and quantitative/qualitative H-2-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 S-BET. 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. (C) 2011 Elsevier B.V. All rights reserved.