Ligand Defect Density Regulation in Metal-Organic Frameworks by Functional Group Engineering on Linkers.

Defects in solid materials vitally determine their physicochemical properties; however, facile regulation of the defect density is still a challenge. Herein, we demonstrate that the ligand defect density of metal-organic frameworks (MOFs) with a UiO-66 structural prototype is precisely regulated by tuning the linker groups (X = OMe, Me, H, F). Detailed analyses reveal that the ligand defect concentration is positively correlated with the electronegativity of linker groups, and Ce-UiO-66-F, constructed by F-containing ligands and Ce-oxo nodes, possesses the superior ligand defect density (>25%) and identifiable irregular periodicity. The increase in ligand defect density results in the reduction of the valence state and the coordination number of Ce sites in Ce-UiO-66-X, and this merit further validates the relationship between the defective structure and catalytic performance of CO2 cycloaddition reaction. This facile, efficient, and reliable strategy may also be applicable to precisely constructing the defect density of porous materials in the future.

Atomically Dispersed High-Density Al-N4 Sites in Porous Carbon for Efficient Photodriven CO2 Cycloaddition.

Highly active catalysts that can directly utilize renewable energy (e.g., solar energy) are desirable for CO2 value-added processes. Herein, aiming at improving the efficiency of photodriven CO2 cycloaddition reactions, a catalyst composed of porous carbon nanosheets enriched with a high loading of atomically dispersed Al atoms (≈14.4 wt%, corresponding to an atomic percent of ≈7.3%) coordinated with N (AlN4 motif, Al-N-C catalyst) via a versatile molecule-confined pyrolysis strategy is reported. The performance of the Al-N-C catalyst for catalytic CO2 cycloaddition under light irradiation (≈95% conversion, reaction rate = 3.52 mmol g-1 h-1 ) is significantly superior to that obtained under a thermal environment (≈57% conversion, reaction rate = 2.11 mmol g-1 h-1 ). Besides the efficient photothermal conversion induced by the carbon matrix, both experimental and theoretical analysis reveal that light irradiation favors the photogenerated electron transfer from the semiconductive Al-N-C catalyst to the epoxide reactant, facilitating the formation of a ring-opened intermediate through the rate-limiting step. This study not only provides an advanced Al-N-C catalyst for photodriven CO2 cycloaddition, but also furnishes new insight for the rational design of superior photocatalysts for diverse heterogeneous catalytic reactions in the future.