This review highlights our studies of the nature of the true catalytic species in the Heck coupling of bromobenzene and styrene with conventionally prepared SiO2-supported Pd particles. Approaches including kinetic test, filtration test and catalyst poisoning were used to probe the behavior and nature of the possible active Pd species in combination with transmission electron microscopy and elemental analysis. We found that the catalytic activity of soluble Pd is suppressed in the presence of the supported Pd particles. Through a study of dependences of reaction rate on Pd loading and on leached Pd concentration, we demonstrated that the leached soluble Pd is responsible for the catalysis irrespectively of the supported Pd particles. With the aid of catalyst poisoning, we further illustrated that Pd clusters are inactive for the homogeneous Heck reaction, molecular Pd being solely active. We also discuss Pd scavenging from solution by solidbound thiols during the Heck reaction. In addition, we propose a ligand-free homogeneous Heck reaction mechanism with the generation of soluble molecular Pd0 by Pd leaching from conventionally prepared supported Pd particles.
Keywords: C-C coupling, Heck reaction, leaching, molecular Pd, Pd clusters, silica, supported Pd particles, true catalytic species