Electronic structure and microenvironment modulation of Pd@UiO-66 enhances direct CO esterification to dimethyl carbonate

Abstract

While the microenvironment around metal catalytic sites is recognized to be crucial in heterogeneous catalysis, its roles in CO esterification to dimethyl carbonate remain subtle. Herein, Pd nanoparticles are confined into the metal–organic frameworks with TiIV metal substitutions and functional linker groups, namely Pd@Ti-Zr-UiO-66-X (X = H, NH2, NO2). The partial substitution of metal nodes by Ti species can dramatically enhance the activity of CO, and synergistically improves catalytic performance with electron-withdrawing functionalized nitroxide linkers. As a result, Pd@Ti-Zr-UiO-66-NO2 exhibits superior conversion of CO (67.5%), selectivity for DMC (83.5%, based on all products) and WTY for DMC (1725 g·kgcat−1·h−1) much higher than those of Pd@Zr-UiO-66-H with conversion of CO (46.4%), selectivity for DMC (87%) and WTY for DMC (1175 g·kgcat−1·h−1). Based on results of experimental and DFT calculation, the synergistic effect of highly electronegative Ti species and nitro-functional linker groups with electron-withdrawing ability realizes the precise control of the electronic structure and microenvironment of Pd NPs. The number of charges transferred from Pd NPs to Zr-UiO-66 and Ti-Zr-UiO-66-NO2 increased from 1.83 to 2.69 e, respectively. The microenvironment of electron-deficient palladium species and Ti-Zr-UiO-66-NO2 promotes the activation of MN and CO, which exhibits a superior performance in direct CO esterification to dimethyl carbonate. This study not only provides a new approach for rational modulation of the chemical microenvironment of metal centers and optimization of catalytic performance but also offers important insights for the design of heterogeneous catalysts.