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, N 2 adsorption-desorption, and NH 3 temperature-programmed desorption. Cr-SBA-15 was found to have a large specific surface area (1,610 m 2 /g), although the structural and acidic nature of the catalyst was similar to the general properties of other mesoporous silicas. The conversion of Cr 3+ species into Cr 6+ on Cr-SBA-15 with the large specific surface area could affect the improvement of the oxidative dehydrogenation of isobutane to isobutene.

Effect of Introduction of Trace Amount of Chromium Species in Improving Catalytic Performance of MCM-48 in Oxidative Dehydrogenation of Isobutane

“…For example, the chromium species was either introduced or impregnated into various mesoporous silica catalysts possessing a uniform hexagonal structure such as FSM-16 (Sugiyama et al, 2013(Sugiyama et al, , 2015Ehiro et al, 2015) and MCM-41 (Ehiro et al, 2016a(Ehiro et al, , 2016b and great improvement in the yield of isobutene was recorded (≈8% at 723 K). Recently, a similar enhancement was observed during the ODH of isobutane using SBA-15 when a trace amount of chromium was introduced (Kato et al, 2018). In that study, however, the chromium species could not be characterized due to its small amount.…”

“…Table 1 shows the chromium content of MCM-48 and Cr-MCM-48 estimated using ICP-AES, surface areas, pore volumes and pore sizes. The surface area of MCM-48 (1,307 m 2 /g) was greater than that of other mesoporous materials such as FSM-16, MCM-41, and SBA-15 (871, 788, and 690 m 2 /g, respectively) (Ehiro et al, 2015(Ehiro et al, , 2016a(Ehiro et al, , 2016bKato et al, 2018). This larger surface area was observed in each example of Cr-MCM-48, and decreased when the amounts of chromium were increased.…”

“…Figure 1 shows the catalytic performances for MCM-48 and Cr-MCM-48 (Si/Cr ratios=10,000; 1,000; 100; and 40). Over MCM-48, isobutane was barely converted, as with other mesoporous materials such as FSM-16 and MCM-41 (the yields of isobutene were 1.9% for MCM-48, 1.3% for FSM-16, 0.9% for MCM-41, and 3.6% for SBA-15 at 6 h on-stream) (Sugiyama et al, 2015;Ehiro et al, 2016b;Kato et al, 2018). The conversion of isobutane and the selectiv-ity to isobutene were greatly improved by introducing only a small amount of chromium into MCM-48.…”