Heck Chemistry - a Highly Active Ligand-Free Metal Catalyst System in Situ Generated from PdII Supported on SiO2

A ligand-free Pd catalyst system derived from Pd(acac)2/SiO2 and its catalytic properties toward the Heck coupling of bromobenzene and styrene have been studied. Combined studies by X-ray photoelectron spectroscopy (XPS), ultraviolet and visible absorption spectroscopy (UV-vis), thermogravimetric analysis (TGA) and powder X-ray diffraction (XRD) indicate that calcination of Pd(acac)2/SiO2 in air at 400℃ produces PdII/SiO2 that contains isolated Pd2+ ions coordinated to the lattice oxygen as the dominant component and PdO particles as the minor component. XPS and UV-vis observations together with catalytic results reveal that most of PdII is reduced to Pd0 on SiO2 by the reaction media during the Heck reaction and that the resulting catalyst system is highly active. Transmission electron microscopy measurements show that the in situ prepared Pd0/SiO2 has much finer Pd particles than conventional Pd0/SiO2. The rapid growth of the supported Pd particles during the highly efficient Heck reaction suggests that the catalysis occurs via an Ostwald ripening mechanism. Resulting in excellent catalytic properties, PdII/SiO2 possesses incomparable advantages over conventional Pd0/SiO2 as a precatalyst in catalytic activity, catalyst recycling and product contamination control.