Position-dependent carboxyl functionalization in covalent organic frameworks for selective photocatalytic CO₂...

Photocatalytic carbon dioxide (CO₂) reduction offers a promising route for converting greenhouse gas into value-added solar fuels under mild conditions. However, the rational regulation of product selectivity, especially the directional conversion of CO₂ toward either shallow reduction products such as carbon monoxide (CO) or deep reduction products such as methane (CH₄), remains a major challenge. In this study, two carboxyl-position isomeric covalent organic frameworks covalent organic frameworks (COFs), TpBdda and TpBdad, were constructed to investigate how subtle differences in functional-group spatial arrangement affect CO₂ photoreduction pathways. Although the two COFs possess similar framework compositions and overall morphologies, the different distribution of carboxyl groups leads to distinct structural features, pore environments, surface chemical properties, and photoinduced reaction behaviors. Photocatalytic CO₂ reduction tests show that TpBdda mainly produces CO, with a CO evolution rate of approximately 5.2 μmol g^-1 h^-1 and nearly complete CO selectivity. In contrast, TpBdad exhibits a pronounced deep-reduction tendency, delivering CH₄ as the dominant product with a CH₄ evolution rate of approximately 1.8 μmol g^-1 h^-1 and a CH₄ selectivity of about 90%. In situ diffuse reflectance infrared Fourier transform spectroscopy further reveals that TpBdda favors the formation and desorption of *CO-related intermediates, whereas TpBdad promotes the stabilization and subsequent hydrogenation of key intermediates, especially methoxy-related species, thereby facilitating CH₄ formation. These results demonstrate that carboxyl-group spatial arrangement can effectively modulate intermediate evolution and product selectivity in COF-based photocatalytic CO₂ reduction, providing a molecular-level strategy for designing selective organic photocatalysts.