Ordered Mesoporous Pd/Silica−Carbon as a Highly Active Heterogeneous Catalyst for Coupling Reaction of Chlorobenzene in Aqueous Media

Abstract

Heterogeneous palladium catalysts, which are supported on ordered mesoporous silica−carbon nanocomposites, have been applied in water-mediated coupling reactions of chlorobenzene without assistance of any phase-transfer catalysts. Characterization by XRD, TEM, N₂ sorption, FT-IR, TG, XPS, and H₂ chemisorption techniques reveals the highly ordered mesostructure, high surface areas (∼345 m²/g), large pore volumes (∼0.46 cm³/g), uniform mesopore sizes (∼6.3 nm), hybrid silicate and carbonaceous compositions, and a high dispersion of palladium nanoparticles (about 3 nm) in the mesopores. The catalyst exhibits a high yield for trans-stilbene (∼60%) in the Heck coupling reaction of chlorobenzene and styrene at 100 °C and for biphenyl (46%) in the Ullmann coupling reaction of chlorobenzene at 30 °C, using water as a solvent. When substituted aryl chlorides (hydroxyl, methoxyl, and methyl) are involved in the Ullmann reaction, the yields of symmetrical substituted biphenyl are also higher than 44% (this value reaches 86% for the coupling reaction of 4-chlorophenol) at a low temperature of 30 °C. This heterogeneous catalyst is stable, which shows negligible metal leaching, and can be reused more than 20 times. For comparison, the catalytic activities for Pd catalysts supported on pure mesoporous polymeric, carbonaceous, and silicate frameworks are also investigated. The results clearly indicate that the pore wall nature shows great influence on the dispersion of metallic Pd species and, in turn, the catalytic performance.