Introduction of a Small Amount of Chromium to Enhance the Catalytic Performance of SBA-15 for the Oxidative Dehydrogenation of Isobutane to Isobutene

Abstract

Various solid oxide catalysts are active for the oxidative dehydrogenation of propane, but achieve yields of less than 2% when these were used for the oxidative dehydrogenation of isobutane to isobutene. An improved isobutene yield of 8% was obtained in the oxidative dehydrogenation of isobutane by using a folded-sheet mesoporous material (FSM-16) and mesoporous molecular sieve (MCM-41), with or without chromium via impregnation or template-ion exchange methods. Further activity enhancement, however, could not be attained. In the present study, chromium cations were introduced into the framework of SBA-15 using a direct synthesis method by mixing chromium nitrate with tetraethylorthosilicate in an aqueous solution with an initial pH of 1.5 and subsequent hydrothermal treatment. Loading the resultant catalyst, SBA-15, with a small amount of chromium (1.84 wt% Cr-SBA-15) increased the isobutene yield (15.4%) at 723 K, while a previous report showed that chromium oxide supported on SBA-15 (CrOx/SBA-15) prepared via incipient wetness impregnation had a lower isobutene yield (11%) at 813 K. To explain this improvement, the catalysts were characterized using X-ray diffraction, transmission electron microscopy, N2 adsorption-desorption, and NH3 temperature-programmed desorption. Cr-SBA-15 was found to have a large specific surface area (1,610 m2/g), although the structural and acidic nature of the catalyst was similar to the general properties of other mesoporous silicas. The conversion of Cr3+ species into Cr6+ on Cr-SBA-15 with the large specific surface area could affect the improvement of the oxidative dehydrogenation of isobutane to isobutene.