Local protons enhance photocatalytic CO₂ reduction by porphyrinic zirconium-organic frameworks

The immobilization of molecular catalysts based on porphyrin fragments within metal-organic frameworks (MOFs) offers a promising approach for achieving sustainable and stable photocatalytic activity. In this study, we presented the synthesis of a phenolic hydroxy-modified iron-porphyrinic zirconium-based MOF, Zr6O4(OH)(4)(FeTCBPP-OH)(3), named MOF-OH (FeTCBPP-OH = iron 5,10,15,20-tetrakis[4-(4 '-carboxyphenyl)-2,6-dihydroxylphenyl]porphyrin), through post-synthetic modification of a precursor MOF called MOF-OCH3 (Zr6O4(OH)(4)(FeTCBPP-OCH3)(3), FeTCBPP-OCH3 = iron 5,10,15,20-tetrakis[4-(4 '-carboxyphenyl)-2,6-dimethoxyphenyl]porphyrin). Initially, we attempted the direct assembly of Zr4+ centers and FeTCBPP-OH ligands; however, this approach was unsuccessful in obtaining MOF-OH. This perhaps resulted from the high number of hydroxyl groups on the polyphenolic porphyrinic fragments, which exhibited a stronger binding affinity towards zirconium centers. Consequently, we achieved MOF-OH by selectively modifying the partial methoxy positions of the FeTCBPP-OCH3 fragments in MOF-OCH3 through demethylation. To evaluate the photocatalytic performance of MOF-OH, we conducted CO2 reduction experiments without any additional photosensitizer. Remarkably, after 72 hours, the yield of CO reached a high value of 26.8 mmol g(-1). Notably, the CO production of MOF-OH was significantly higher than that of MOF-OCH3, possibly due to the presence of phenolic hydroxyl substituents, which led to higher local proton concentrations. Furthermore, MOF-OH exhibited excellent stability, as demonstrated by the consistent CO production observed during four consecutive runs of CO2 reduction. To gain insights into the photocatalytic CO2 reduction process, we conducted a comprehensive series of characterizations and density functional theory calculations, which provided a deeper understanding of the mechanism involved.