Preparation, characterization, and catalytic activity of chromia supported on SBA-15 for the oxidative dehydrogenation of isobutane

Mesoporous SBA-15 and 1-12 wt% CrO_x/SBA-15 have been prepared by triblock copolymer P123-templated hydrothermal synthesis and incipient wetness impregnation method, respectively. The materials were characterized by means of a number of analytical techniques and their catalytic activities were evaluated for the oxidative dehydrogenation (ODH) of isobutane. It is found that with rise of chromia loading, the morphology of SBA-15 changed from long interconnected chains to short banana-like rods, and finally changed to spheres with wormholes. The results of Raman and X-ray photoelectron spectroscopic investigations reveal that the surface Cr species are mainly Cr⁶⁺ in mono- and polychromate, with a minor amount of Cr³⁺ due to α-Cr₂O₃ formation. The H₂ temperature-programmed reduction study demonstrates that the catalysts at chromia loading of 6-10 wt% are more reducible. The 10 wt% CrO_x/SBA-15 catalyst exhibits the best activity, showing 79% C₄-olefin selectivity and 11% C₄-olefin yield at 540 °C. One monolayer CrO_x coverage on SBA-15 occurs at Cr surface density = 1.05-1.43 Cr-atom/nm². We conclude that factors such as (i) presence of active Cr⁶⁺ in mono- and polychromate, (ii) strong redox ability of Cr species, and (iii) good dispersion of CrO_x on banana-like rods of well-ordered mesoporous SBA-15 are responsible for the good performance of the SBA-15-supported chromia in isobutane ODH.