SULFUR LIGAND-STABILIZED PALLADIUM AGGREGATES PRODUCED ON THE SURFACEOF BENZOYLTHIOUREA-FUNCTIONALIZED SILICA XEROGELS

Two benzoylthiourea-functionalized silica xerogels, 4.5SiO(2) . SiO3/( 2)(CH2)(3)NHC(S)NHC(O)Ph (XGbztu) and SiO3/2(CH2)(3)NHC(S)NH-C(O)Ph (X Gbztu), prepared by the set-gel process, are able to bind palladium b y reaction with Pd(OAc)(2), giving the hybrid materials Pd/XGbztu and Pd/XGbztu. The ligand CH3(CH2)(2)NHC(S)NHC(O)Ph (Hbztu), a model of t he surface-binding function, on reaction with Pd(OAc)(2), afforded the mononuclear complex [Pd(bztu)(2)] and a sulfur-containing metal collo id. Diffuse reflectance FTIR (DRIFT) experiments have shown that Pd/XG bztu is able to fix CO at room temperature, giving a three nu(CO) fre quency pattern, which changes with increasing temperature. Pd/XGbztu a nd Pd/XGbztu are effective catalysts in the hydrogenation of phenylac etylene to give styrene or ethylbenzene, depending on the reaction tim e. Nanoscale palladium aggregates have been evidenced by transmission electron microscopy (TEM) investigations on the materials after the hy drogenation processes; energy-dispersive Xray (EDX) analysis has revea led a significant presence of sulfur in the vicinity of the particles. X-Ray photoelectron spectroscopy (XPS) data have shown that Pd2+ and Pd-o are simultaneously present in Pd/XGbztu recovered after the catal ytic cycle.