Tailoring the Cu2+ Coordination Microenvironment in Porous Organic Frameworks for Switchable CO2 Photoreduction to CO or CH4

Abstract

Solar-driven conversion of CO₂ to fuels addresses both energy and environmental crises. This study reports the rational design and synthesis of two heteroatom-enriched imine-based porous organic frameworks (POFs), NON-POF and NNN-POF, further functionalized with copper sites (Cu@NON-POF and Cu@NNN-POF) for photocatalytic CO₂ reduction under visible light. Notably, Cu@NNN-POF exhibits exceptional CO production with a remarkable yield of 4.86 mmol g⁻¹, whereas Cu@NON-POF enables an 8-electron reduction pathway, selectively producing CH₄ (0.18 mmol g⁻¹) alongside notable amounts of CO and H₂. The enhanced CH₄ selectivity in Cu@NON-POF arose from the distinct hydrogen-bonding interactions between its N-H moieties and CO₂ intermediates, which promotes multi-step hydrogenation. Spectroscopic and theoretical analyses reveal how structural and electronic properties govern catalytic activity and product selectivity. Operando extended X-ray absorption fine structure (EXAFS) studies provide crucial insights into the dynamic coordination environment, oxidation states, and structural evolution of Cu active sites under operational conditions, clarifying the mechanistic basis for product selectivity. This work highlights the potential of Cu-incorporated POFs as robust and cost-effective systems for solar-driven CO₂ reduction, advancing catalytic design strategies for clean energy production. Additionally, the mechanistic investigations emphasized the pivotal role of ligand architecture and metal coordination environments in tailoring product selectivity during photocatalytic CO₂ reduction.