The Effect of Palladium Confinement and Support Functionalization in the Suzuki Reaction

Abstract

A series of silica, i.e., Aerosil-380, SBA-15 and P.SBA-15, were functionalized with thiol groups (-SH) and then impregnated with palladium (Pd) to obtain supported Pd catalysts for the Suzuki reaction. The structures and chemical compositions of the catalysts were investigated by nitrogen physisorption, X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), inductively coupled plasma-atomic emission spectroscopy (ICP-AES) and atomic adsorption spectroscopy (AAS). Then, the effect of confinement structure and the influence of functional groups on the catalytic activity, recyclability and reaction kinetics were investigated by applying catalysts into different Suzuki reactions and by studying their stability. From catalytic tests and stability tests, it was found that the S/Pd ratio and the surface density of –SH, rather than confinement structure, are the dominant factors which influence the catalytic activity, recyclability as well as the stability of Pd nanoparticles on the silica. However, due to the plugs in the channels of P.SBA-15, P.SBA-15 retained its 2D hexagonal pore structure, while the structure of SBA-15 with open mesopores totally collapsed during catalysis at 130 ˚C. Moreover, by combining the results from recyclability tests and catalyst poisoning tests, it was concluded that the supported Pd catalyst serves as metal reservoir for active homogeneous Pd species.