Preparation of Highly Dispersed Chromium Oxide Catalysts Supported on Mesoporous Silica for the Oxidative Dehydrogenation of Propane Using CO2: Insight into the Nature of Catalytically Active Chromium Sites

Highly dispersed chromium oxide catalysts supported on mesoporous silica (Cr-MSU-x) were prepared via a ((NMn+)-M-0)I-0 pathway with the goal of achieving the high performance oxidative dehydrogenation of propane (ODHP) reaction. The resulting materials exhibited a mesopore structure resembling 3D wormhole-like holes, as characterized by N-2 adsorption-desorption isotherms and RR-TEM. Catalytic experimental results revealed that the catalyst with a 0.028 Cr/Si molar ratio showed the highest catalytic activity among the catalysts studied. Two types of chromium species, isolated Cr(VI) and polymeric Cr(VI) species, were observed, as evidenced by H-2-temperature-programmed reduction. They were designated as "hard Cr(VI)" and "soft Cr(VI)" sites, respectively. The initial composition of the soft Cr(VI) in the total Cr(VI) is a major determinant factor in the ODHP performance.