Arene hydrogenation using supported rhodium metal catalysts prepared from [Rh(COD)H]4, [Rh(COD)2]+BF4−, and [Rh(COD)Cl]2 adsorbed on SiO2 and Pd-SiO2

Abstract

Abstract Four different rhodium precursors, [Rh(COD)H]4, [Rh(COD)2]+BF4−, [Rh(COD)Cl]2, and RhCl3·3H2O, adsorbed onto silica (SiO2) or onto palladium supported on silica (Pd-SiO2), were examined for their catalytic arene hydrogenation activities. Rates of toluene hydrogenation were compared when the catalysts were pretreated with hydrogen as follows: none, 40 °C for 24 h, and 200 °C for 4 h. With no hydrogen pretreatment, the rhodium precursors adsorbed on Pd-SiO2 showed higher activities than the corresponding precursors on only SiO2. As the hydrogen pretreatment temperature was increased, the activities of the catalysts on SiO2 increased; the highest activity occurred with catalysts pretreated at 200 °C. Conversely, milder pretreatments were more effective for the catalysts on Pd-SiO2; higher temperature pretreatments reduced activities. The catalysts were characterized by their activities, diffuse reflectance infrared Fourier transform spectroscopy (DRIFT), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). All of the studies indicate that the active catalytic species is rhodium metal. On silica, H2 reduction of the rhodium complex to Rh metal under arene hydrogenation conditions was slow in all cases except [Rh(COD)H]4. However, on Pd-SiO2, the palladium accelerated the reduction of all of the Rh complex precursors to rhodium metal under the same conditions. The resulting Rh-Pd-SiO2 bimetallic catalysts exhibited higher activities than catalysts of the same composition prepared by classical methods.