CYCLOHEXENE HYDROGENATION CATALYZED BY MGO-SUPPORTED TETRAIRIDIUM CLUSTERS

Tetrairidium clusters supported on MgO powder were prepared by decarbonylation of supported [HIr4(CO)11](-) by treatment in He followed by H-2 at 300 degrees C. Infrared spectroscopy showed that the CO ligands were fully removed, and extended X-ray absorption fine structure (EXAFS) spectroscopy indicated that the tetrahedral metal frame was retained. Thus the sample is represented as Ir-4/MgO. H-2 chemisorption measurements indicate that the clusters adsorb less H-2 than supported metallic crystallites; the H:Ir ratio was only 0.25. Temperature-programmed desorption indicated strong interactions between adsorbed hydrogen and iridium, represented by the major desorption peak at about 440 degrees C. Ir-4/MgO was tested as a catalyst for cyclohexene hydrogenation in the presence of liquid-phase reactants at 25 degrees C and 1 atm. The used catalyst was characterized by EXAFS spectroscopy and found not to have changed significantly during the catalytic reaction. The first-shell Ir-Ir coordination number was 3.3, the same, within experimental error, as that of the MgO-supported [HIr4(CO)(11)](-) precursor and the sample formed by its decarbonylation. The turnover frequency was found to be 0.018 +/- 0.006 s(-1), which is two orders of magnitude less than that of conventional highly dispersed supported metallic catalysts. It is inferred that the small supported clusters are quasimolecular and catalytically distinct from supported metallic clusters. (C) 1995 Academic Press, Inc.