(308f) Performance Testing and Structural Characterization of Gamma-Al2o3-Supported Rhenium Clusters and Particles for Methylcyclohexane Conversion

Highly stable γ-Al₂O₃-supported rhenium clusters were prepared by adsorption of H₃Re₃(CO)₁₂ followed by treatment for 2 h in flowing H₂ at 450 °C. The resultant supported species were characterized by EXAFS spectroscopy and found to have an average Re?Re coordination number of 1.7 at a distance of 2.69 Å. These results indicate that the triangular Re₃ framework remained essentially intact after removal of the CO ligands. The Re edge position in the absorbance spectrum indicates that the rhenium is cationic, with the oxidation state estimated to be between +4 and +7. For comparison, an γ-Al₂O₃-supported rhenium catalyst was prepared conventionally from aqueous (NH₄)ReO₄ and reduced in H₂, under conditions that have been shown to produce small rhenium metal particles on the support. The two catalysts were used for the conversion of methylcyclohexane at 500°C and atmospheric pressure in a once-through plug-flow reactor. The product analysis indicated the occurrence of dehydrogenation (to form toluene) accompanied by isomerization and other reactions. The conversion data indicate that the catalyst prepared from the H₃Re₃(CO)₁₂ precursor was initially more active than the conventional catalyst. Furthermore, although the conventionally prepared catalyst underwent rapid deactivation, the catalyst derived from the trinuclear cluster exhibited an increase in activity with time on stream. The used catalyst prepared from the trirhenium clusters was characterized by EXAFS spectroscopy. The Re?Re coordination number was found to be approximately 1.5, indicating little if any change in the cluster size and suggesting that the Re₃ framework was largely maintained during the catalytic reaction.