Magnetic Core-Shell to Yolk-Shell Structures in Palladium-Catalyzed Suzuki-Miyaura Reactions: Heterogeneous versus Homogeneous Nature.

Abstract

This study describes a comparative investigation on the heterogeneous versus homogeneous nature of the Pd-catalyzed Suzuki-Miyaura cross-coupling reaction mechanism with specific magnetic hierarchical core-shell and yolk-shell structures. The hierarchical core-shell Fe3 O4 @SiO2 -Pd@mCeO2 (m=mesoporous) catalyst contains a core of nonporous silica-sheltered magnetite (Fe3 O4 ) nanoparticles (NPs), a transition layer of active palladium (Pd) NPs, and an outer shell of porous ceria (CeO2 ). The magnetic yolk-shell Fe3 O4 @h-Pd@mCeO2 (h=hollow) catalyst was prepared by selectively etching the nonporous silica interlayers. Notably, the results of the hot-filtration heterogeneity test, the effect of Pd concentration, and solid-phase poisoning, indicate that the two kinds of catalysts function in Pd-catalyzed Suzuki-Miyaura cross-coupling reactions through different catalytic mechanisms. Moreover, both catalysts demonstrated better catalytic activity than the Fe3 O4 @SiO2 -Pd catalyst. This finding can be ascribed to the outermost CeO2 shell having a high concentration of trivalent cerium and oxygen vacancies, which gives rise to the increased electron density of Pd NPs, and a faster rate-determining step in the oxidative addition reaction for the Suzuki reaction. In addition, we propose a feasible mechanism elucidating the synergistic effect between the supporting CeO2 and active species.