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

Abstract

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.