Alumina-Supported Trirhenium Clusters:  Stable High-Temperature Catalysts for Methylcyclohexane Conversion

Samples prepared from H3Re3(CO)12 adsorbed on porous γ-Al2O3 were decarbonylated at 773 K in flowing H2 and characterized by X-ray absorption spectroscopy (XAS). X-ray absorption near-edge spectra show that rhenium in the treated sample was cationic, and extended X-ray absorption fine structure spectra show a Re−Re first-shell coordination number of approximately 2, consistent with trirhenium clusters bonded to the support. The samples were tested as catalysts for the conversion of methylcyclohexane in the presence of H2 at atmospheric pressure and at 723 and at 773 K in a flow reactor. A range of hydrocarbon products was observed, indicating the occurrence of dehydrogenation, isomerization, ring opening, and hydrocracking reactions. The catalyst used at 723 K underwent deactivation over a period of several hours, during which the selectivity for the major dehydrogenation product (toluene) increased significantly. At 773 K, the catalyst underwent activation, during which the product distribution changed. This increase in activity was retained when the temperature was reduced to 723 K, resulting in higher activity and different selectivity relative to what had been observed before at this temperature. The fresh and used catalyst samples were characterized by X-ray absorption spectroscopy, which showed that the trirhenium framework remained intact after catalysis, although changes in the rhenium coordination were observed. The catalytically active species are inferred to be trirhenium.