Asymmetric Ligands of a Metal-Organic Framework on Enhanced Photocatalytic CO 2 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 CO 2 reduction in MOFs is first proposed. MOF-303(Al) with asymmetric ligands (pyrazolyldicarboxylic acid) exhibits synergistic catalytic effects. Specifically, pyrazoles participate in CO 2 activation; i.e., pyrazole and μ 2 -OH form hydrogen bonds with CO 2 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 CO 2 and H 2 O adsorption to avoid competition, respectively. Owing to its advantages, MOF-303-modified g-C 3 N 4 achieves nonsacrificial and transition-metal-free photocatalytic CO 2 reduction to CO of 16.19 μmol·g -1 ·h -1 , significantly higher than that of g-C 3 N 4 . This work provides fresh insights into asymmetric ligands in photocatalytic CO 2 reduction.