New insights into catalysis for Heck reactions with fine supported Pd particles
We have more deeply studied SiO2-, Al2O3-, MgO-, SBA-15-, TiO2- and ZrO2-supported PdII 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 PdII systems turn out to possess incomparable catalytic performance. The advantage of the supported PdII 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 PdII system is able to produce a larger amount of fine Pd particles by in-situ reduction of the supported PdII, 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 PdII systems under equivalent reaction conditions is dependent on the size of the supported Pd particles and the interaction between Pd particle and support. The supported PdII systems having a Pd loading of 1 % or above can give rise to a strong catalyst recyclability. At the end of Heck reaction, the contamination level of residual Pd to products can be controlled to < 5 ppm with the supported PdII systems having a loading of 1 % Pd, but increases at a higher Pd loading. Properly selecting Pd loading allows compromising between strong catalyst recyclability and low Pd contamination to products for a supported PdII system.