Dehydrogenation of ethylbenzene with CO2 over Cr-MCM-41 catalyst

M-MCM-41 catalysts (M: V, Cr, Fe, and Ga) prepared by direct hydrothermal synthesis (DHT) have been tested for dehydrogenation of ethylbenzene with CO,. The synthesized materials were characterized by X-ray diffraction (XRD), N-2 adsorption (77 K), and diffuse reflectance UV-vis spectroscopic measurements. Cr-MCM-41 showed the highest activity among M-MCM-41 catalysts tested, resulting in the production of styrene with the conversion of 65% and the selectivity above 90%. The rate of styrene formation increased with increasing Cr loading up to 1.7 wt.%. It is suggested that Cr(VI)O-4 in tetrahedral coordination is formed as an active monochromate species and reduced to Cr(III)O-6 in octahedral coordination as a less active polychromate species during the reaction. Deactivated catalyst was regenerated by a treatment with gaseous oxygen or CO,, during which redistribution as well as reoxidation of polymeric Cr(III)O-6 octahedra to monomeric Cr(VI)04 tetrahedra was observed. The rate of CO formation increased together with that of styrene formation, while the rate of H-2 formation decreased, with increasing partial pressure of CO2. It was confirmed that reverse water-gas shift reaction took place over Cr-MCM-41 by a separate experiment. The rate of CO formation during the dehydrogenation of ethylbenzene with CO2 over Cr-MCM-41 was well accounted for by assuming parallel occurrence of two reactions, i.e., direct oxidative dehydrogenation of ethylbenzene with CO2 and simple dehydrogenation of ethylbenzene thermodynamically assisted by reverse water-gas shift reaction. (c) 2004 Elsevier B.V. All rights reserved.