Photocatalytic CO₂ Reduction Based on a Re(I)-Integrated Conjugated Microporous Polymer: Role of a Sacrificial Electron Donor in Product Selectivity and Efficiency

Abstract

One of the major challenges in photocatalytic CO2 reduction is achieving control over the selective formation of a single product while maintaining a high conversion efficiency. Here, we report the synthesis and characterization of a conjugated microporous polymer (TEB-BPY) formed by C–C coupling between 1,3,5-triethynylbenzene and 5,5′-dibromo-2,2′-bipyridine. Further, [Re(CO)5Cl] covalently integrated with the polymer, and the resulting metalated Re@TEB-BPY polymer was used as a catalyst for the visible-light-driven CO2 reduction. Re@TEB-BPY displays photoconversion of CO2 to CO with a production rate of 91.7 μmol g–1 h–1 and a selectivity of ∼68% in the presence of triethylamine (TEA) as the sole sacrificial electron donor. Interestingly, CH4 is produced as a major product instead of CO when CO2 reduction was performed using 1-benzyl-1,4-dihydronicotinamide (BNAH) as a sacrificial electron donor in the presence of TEA as a base. In this reaction, Re@TEB-BPY produces CH4 as the major product with a rate of 2.05 mmol g–1 h–1 (selectivity of ∼96% and apparent quantum efficiency of 0.22%). From an in situ diffuse reflectance FTIR spectroscopy (DRIFTS) study together with DFT calculations, a possible catalytic cycle for CO2 reduction to CO or CH4 is constructed. Theoretical calculations along with control experiments further reveal that TEA acts mainly as a base in the presence of BNAH to suppress the back electron transfer process, resulting in enhanced photocatalytic activity.