Asymmetric Ligands of a Metal-Organic Framework on Enhanced Photocatalytic CO₂ Reduction

Metal-organic frameworks (MOFs) have been studied extensively in the catalytic field. However, the role of ligands in catalysis has been less well investigated. Here, an asymmetric ligand photocatalytic strategy for CO₂ reduction in MOFs is first proposed. MOF-303(Al) with asymmetric ligands (pyrazolyldicarboxylic acid) exhibits synergistic catalytic effects. Specifically, pyrazoles participate in CO₂ activation; i.e., pyrazole and μ₂-OH form hydrogen bonds with CO₂ to polarize C═O bonds. Furthermore, the lowest unoccupied molecular orbital (LUMO; A pyrazole) and highest occupied molecular orbital (HOMO; B pyrazole) act as the electron donor and acceptor to spatially separate the excited electron-hole, with A and B pyrazoles for CO₂ and H₂O adsorption to avoid competition, respectively. Owing to its advantages, MOF-303-modified g-C₃N₄ achieves nonsacrificial and transition-metal-free photocatalytic CO₂ reduction to CO of 16.19 μmol·g^-1·h^-1, significantly higher than that of g-C₃N₄. This work provides fresh insights into asymmetric ligands in photocatalytic CO₂ reduction.