New insights into catalysis for Heck reactions with fine supported Pd particles

Abstract

We have more deeply studied SiO₂-, Al₂O₃-, MgO-, SBA-15-, TiO₂- and ZrO₂-supported Pd^II systems derived by calcination of supported Pd(acac)₂ in air and their respective supported Pd⁰ systems prepared by reduction of supported Pd(acac)₂ in H₂ for the Heck coupling of PhBr and styrene. The supported Pd^II systems turn out to exhibit incomparable catalytic performance. The advantage of the supported Pd^II systems is elucidated in terms of the dependence of catalytic activity on the concentration of soluble active Pd species measured by selective scavenging of soluble molecular Pd species over a supported Pd system. Under Heck reaction conditions used, a supported Pd^II system is able to produce a larger amount of fine Pd particles by in situ reduction of the supported Pd^II, which results in a higher fraction of molecular Pd species in solution owing to an Ostwald ripening effect. The correlation that the smaller the size of supported Pd particles, the higher the catalytic activity is further suggested for homogeneous catalysis in nature. The catalyst recyclability of the supported Pd^II systems under equivalent reaction conditions is dependent on the size of the supported Pd particles and the interaction between the Pd particles and support. The supported Pd^II systems having a Pd loading of 1% or above can give rise to a strong catalyst recyclability. At the end of the Heck reaction, the contamination level of residual Pd to products can be controlled to <5 ppm with the supported Pd^II systems having a loading of 1% Pd, but increases at a higher Pd loading. Properly selecting the Pd loading allows a compromise between strong catalyst recyclability and low Pd contamination to products for a supported Pd^II system.