Sulfur ligand-stabilized palladium aggregates produced on the surface of benzoylthiourea-functionalized silica xerogels

Abstract

Two benzoylthiourea-functionalized silica xerogels, 4.5SiO₂·SiO₃/₂(CH₂)₃NHC(S)NHC(O)Ph (XGbztu) and SiO₃/₂(CH₂)₃NHC(S)NH-C(O)Ph (XGbztu*), prepared by the sol–gel process, are able to bind palladium by reaction with Pd(OAc)₂, giving the hybrid materials Pd/XGbztu and Pd/XGbztu*. The ligand CH₃(CH₂)₂NHC(S)NHC(O)Ph (Hbztu), a model of the surface-binding function, on reaction with Pd(OAc)₂, afforded the mononuclear complex [Pd(bztu)₂] and a sulfur-containing metal colloid. Diffuse reflectance FTIR (DRIFT) experiments have shown that Pd/XGbztu* is able to fix CO at room temperature, giving a three v(CO) frequency pattern, which changes with increasing temperature. Pd/XGbztu and Pd/XGbztu* are effective catalysts in the hydrogenation of phenylacetylene to give styrene or ethylbenzene, depending on the reaction time. Nanoscale palladium aggregates have been evidenced by transmission electron microscopy (TEM) investigations on the materials after the hydrogenation processes; energy-dispersive Xray (EDX) analysis has revealed a significant presence of sulfur in the vicinity of the particles. X-Ray photoelectron spectroscopy (XPS) data have shown that Pd²⁺ and Pd⁰ are simultaneously present in Pd/XGbztu recovered after the catalytic cycle.