Surface structures and catalytic hydroformylation activities of Rh dimers attached on various inorganic oxide supports

trans-[(RhCp*CH3)2(mu-CH2)(2)] (Cp* = pentamethylcyclopentadienyl) was chemically attached to inorganic oxides such as SiO2, Al2O3, TiO2, and MgO to prepare Rh dimer catalysts. The catalytic activities of the obtained supported Rh dimers for ethene hydroformylation depended on the kinds of supports. The SiO2-attached Rh dimers showed highest activity and selectivity for the hydroformylation. Surface structure and behavior of he Rh sites during CO insertion process were examined by in situ FT-IR and EXAFS techniques. Similarly to the previous observation that Rh-Rh metal bonding in the Rh dimers on SiO2 alternately broken and formed in the course of the catalytic hydroformylation, Rh dimers on TiO2 showed a reversible bond break formation, but CO-bridged Rh dimers inactive for the hydroformylation were produced unlike the case of Rh dimer/SiO2. On Al2O3 mast of Rh dimers were degraded to monomers which exhibited no activity for the hydroformylation. The Rh dimers on MgO were aggregated to metal clusters. These structural behaviors of Rh sites are discussed in terms of the interaction between the Rh dimer and the support.