Isomeric Covalent Organic Frameworks for High-Efficiency Photocatalytic CO2 Reduction: Substituent Position Effect

Abstract

The exploration of covalent organic frameworks (COFs) for high-efficiency photocatalytic CO₂ reduction is urgently demanded. Herein, COF-based catalysts are constructed for the selective photoreduction of CO₂ to CO via delicately designed isomeric monomers with substituent at the 4,5,9,10- positions (K) or 1,3,6,8-positions (A) of pyrene knots. The distinct substituted regions significantly affect the planarity of pyrene knots, resulting in COFs with different microstructures and photocatalytic activities. While employing a 5 W LED white-light as the light source, the single atomic Co contained A-Py-Bpy-COF-Co showcased a moderate CO evolution rate of 2174.4 µmol g⁻¹ h⁻¹. In sharp contrast, K-Py-Bpy-COF-Co reveals a considerable CO photo-reduction rate of 12 476.4 µmol g⁻¹ h⁻¹ (5.7 times higher than A-Py-Bpy-COF) with a selectivity up to 93.3%. Remarkably, the excellent photocatalytic activity of K-Py-Bpy-COF-Co can be maintained for at least 5 cycles without obvious decay. The distinct photocatalytic properties of the two isomeric COFs can be attributed to the larger steric-hindrance of K-Py-4CHO which enlarges the interlayer distances to inhibit exciton quenching and electron-richer nature of monatomic Co in K-Py-Bpy-COF-Co. This work provides a new protocol to explore COFs with boosted photocatalytic performance via isomeric design from refined modulation of reported COFs.