Performance Testing and Structural Characterization of Gamma-Al2o3-Supported Rhenium Clusters and Particles for Methylcyclohexane Conversion
Rodrigo J. Lobo1, Alper Uzun1, Simon R. Bare2, Michael J. McCall2, and Bruce C. Gates1. (1) Chemical Engineering and Materials Science, University of California at Davis, One Shields Avenue, Davis, CA 95616, (2) UOP LLC, 25 E. Algonquin Rd., Des Plaines, IL 60017
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.