Hydrogenation of Arenes over Silica-Supported Catalysts That Combine a Grafted Rhodium Complex and Palladium Nanoparticles:  Evidence for Substrate Activation on Rh_single-site−Pd_metal Moieties

The complex Rh(cod)(sulfos) (Rh^I; sulfos = ^-O₃S(C₆H₄)CH₂C(CH₂PPh₂)₃; cod = cycloocta-1,5-diene), either free or supported on silica, does not catalyze the hydrogenation of benzene in either homogeneous or heterogeneous phase. However, when silica contains supported Pd metal nanoparticles (Pd⁰/SiO₂), a hybrid catalyst (Rh^I−Pd⁰/SiO₂) is formed that hydrogenates benzene 4 times faster than does Pd⁰/SiO₂ alone. EXAFS and DRIFT measurements of in situ and ex situ prepared samples, batch catalytic reactions under different conditions, deuterium labeling experiments, and model organometallic studies, taken together, have shown that the rhodium single sites and the palladium nanoparticles cooperate with each other in promoting the hydrogenation of benzene through the formation of a unique entity throughout the catalytic cycle. Besides decreasing the extent of cyclohexa-1,3-diene disproportionation at palladium, the combined action of the two metals activates the arene so as to allow the rhodium sites to enter the catalytic cycle and speed up the overall hydrogenation process by rapidly reducing benzene to cyclohexa-1,3-diene.