HEXAIRIDIUM CLUSTERS SUPPORTED ON GAMMA-AL2O3 - SYNTHESIS, STRUCTURE,AND CATALYTIC ACTIVITY FOR TOLUENE HYDROGENATION

[Ir(CO)(15)](2-) was formed by surface-mediated synthesis on gamma-Al2 O3 powder by treatment of adsorbed [Ir(CO)(2)(acac)] in CO at 100 degr ees C and 1 atm. The supported clusters were characterized by infrared and extended X-ray absorption fine structure (EXAFS) spectroscopies a nd by extraction into solution by cation metathesis. Treatment of gamm a-Al2O3-supported [Ir-6(CO)(15)](2-) in He at 300 degrees C led to dec arbonylation without disruption of the cluster frame, giving Ir-6/gamm a-Al2O3, as indicated by EXAFS data determining a first-shell Ir-Ir co ordination number of 4.05 +/- 0.07 (where the error bounds represent p recision, not accuracy), matching that of the supported [Ir-6(CO)(15)] (2-), With a first-shell Ir-Ir coordination number of 4.07 +/- 0.03. T he supported Ir-6 clusters were found to be catalytically active for t oluene hydrogenation at temperatures in the range of 60-100 degrees C; the turnover frequency at 60 degrees C with a toluene partial pressur e of 50 Torr and a H-2 partial pressure of 710 Torr was 1.7 x 10(-3) s (-1). The catalyst was stable in operation in a flow reactor, and cons istent with this observation, EXAFS results for the used catalyst indi cated that the nuclearity of the supported Ir-6 clusters was essential ly unchanged during catalysis. The gamma-Al2O3-supported Ir-6 catalyst is an order of magnitude less active (per total Ir atom) for toluene hydrogenation than a catalyst consisting of small aggregates of iridiu m (with an average of about 50 atoms each) supported on gamma-Al2O3. B ecause toluene hydrogenation is known to be a structure-insensitive ca talytic reaction, the data suggest that the concept of structure insen sitivity in catalysis does not extend to clusters as small as Ir-6. Ho wever, the relatively low activity of the Ir-6 clusters may be more an indication of an effect of the support as a ligand changing the elect ronic properties of the iridium in the cluster than an effect of the c luster size.