CHEMISTRY IN CAGES - ZEOLITE-ENTRAPPED HEXAIRIDIUM CLUSTER CATALYSTS

Iridium carbonyl clusters in the cages of NaY zeolite were prepared from adsorbed [Ir(CO)2(acac)]. Reductive carbonylation of this precursor in CO at 1 atm and 50-degrees-C gave [Ir4(CO)12], which was converted in CO at 125-degrees-C and 1 atm to the isomer of [Ir6(CO)16] with edge-bridging ligands and in CO + H-2 at 250-degrees-C and 20 atm to the isomer of [Ir6(CO)16] with face-bridging ligands, both identified by infrared and extended X-ray absorption fine structure (EXAFS) spectra. The zeolite containing the latter catalyzed the CO hydrogenation reaction at 20 atm and 250-degrees-C, giving a high selectivity to propane and being stable for a number of days in a flow reactor. Either isomer of [Ir6(CO)16] in the zeolite could be decarbonylated in H-2 at 300-degrees-C and 1 atm and recarbonylated to give either isomer of [Ir6(CO)16]. The decarbonylated cluster in the presence of H-2 is inferred from EXAFS spectra to be predominantly Ir6 with an octahedral structure and an average Ir-Ir bond distance of 2.71 angstrom. These results illustrate the potential for preparation of supported metal clusters with controlled nuclearities and open the door to precise characterization of the structures and properties of uniform, highly dispersed supported metal catalysts.