Behavior of active sites on Cr-MCM-41 catalysts during the dehydrogenation of propane with CO2

M-MCM-41 catalysts (M: V, Cr, Mn, Fe, Co, Ni, and Ga) prepared by direct hydrothermal (DHT) synthesis have been tested for dehydrogenation of propane with carbon dioxide. The synthesized materials were characterized by X-ray diffraction (XRD), N2 adsorption (77 K), diffuse reflectance UV–vis, UV-Raman, and X-ray absorption (XANES and EXAFS) spectroscopic measurements. Cr-MCM-41 showed the highest activity among M-MCM-41 catalysts tested, resulting in the production of propene with a conversion of 30% and a selectivity above 90%. The rate of carbon monoxide formation increased together with that of propene, while the rate of hydrogen formation stayed at almost constant value, with increasing the partial pressure of carbon dioxide. The mechanism of catalyst deactivation as well as regeneration was discussed based on the structure–catalytic property relationships. It is suggested that Cr(VI) in tetrahedral coordination formed as an active monochromate species and reduced to Cr(III) in octahedral coordination as a less active polychromate species during the reaction. Deactivated catalyst was regenerated by a treatment with gaseous oxygen, during which a reoxidation of the Cr(III) species to the Cr(VI) species was observed. Not only oxygen but also carbon dioxide could regenerate Cr(VI)O4 tetrahedra from reduced Cr(III)O6 octahedra, even though the efficiency of carbon dioxide was lower than that of oxygen. It is concluded that during the reaction propane is dehydrogenated to propene by Cr(IV)O4 tetrahedra, which is simultaneously reduced to Cr(III)O6 octahedra. The reduced Cr(III)O6 octahedra can be reoxidized to Cr(VI)O4 tetrahedra by carbon dioxide, and thus the reduction–oxidation cycle between Cr(VI)O4 tetrahedra and Cr(III)O8 octahedra has an important role in the dehydrogenation of propane with carbon dioxide over Cr-MCM-41.