A Phenazine-Based Two-Dimensional Covalent Organic Framework for Photochemical CO2 Reduction with Increased Selectivity for Two-Carbon Products.

The reduction of carbon dioxide (CO2) into valuable products will contribute to sustainable carbon use. Here we report the photocatalytic reduction of CO2 to carbon monoxide, formate, and oxalate ions using a redox-active phenazine-based two-dimensional covalent organic framework (Phen-COF) and its phenazine monomer. Under similar irradiation conditions, Phen-COF produced 2.9 times more CO, 11 times more formate and 13 times more oxalate compared to equimolar amounts of the monomeric phenazine, demonstrating that the COF architecture enhances catalytic performance (TOFCOF: 10-7 s-1 CO, 10-8 s-1 formate, 10-11 s-1 oxalate). Structural analysis, including X-ray diffraction and N2 porosimetry, confirmed the COF's long-range order and porosity. Mechanistic studies suggest a sequential formate-to-oxalate pathway, with CO and formate acting as intermediates. These results demonstrate the potential of the COF architecture to improve the performance of metal-free, redox-active aromatic systems such as phenazines to facilitate efficient and selective CO2 conversion under mild conditions.