One-Pot Synthesis of Hydrophobic Porphyrin Zirconium-Based MOFs for the Photoreduction of CO₂ to Formate

Using zirconium tetrachloride as a metal source and tetra(4-carboxyphenyl)porphyrin as a ligand and by in situ introducing octadecylphosphonic acid (OPA), three hydrophobic porphyrin zirconium metal-organic frameworks (MOFs) with different structural topologies were constructed, where these MOFs are labeled as OPA@PCN-222, OPA@PCN-223, and OPA@PCN-224. Compared with the original Zr-MOFs without the modification of OPA, the modified porphyrin Zr-MOFs show excellent hydrophobic properties and can maintain excellent stability in a long-term humidity environment. Meanwhile, OPA@PCN-222, OPA@PCN-223, and OPA@PCN-224 exhibit wide absorption of visible light and steadfast and expeditious photocurrent response by leveraging the properties of porphyrin ligands. When illuminated by visible light, the hydrophobic Zr-MOFs demonstrate an efficient reduction of CO₂ to HCOO^-, achieving average reaction rates of 330, 260, and 258 μmol·h^-1·g^-1 for OPA@PCN-222, OPA@PCN-223, and OPA@PCN-224. These rates are 1.13-1.41 times higher than that of the original porphyrin Zr-MOFs. The mechanism study shows that both the porphyrin ligands and the Zr-O clusters serve as catalytically active sites, enabling the conversion of CO₂ to HCOO^-. This research shows that the introduction of hydrophobic alkyl chains can effectively enhance the stability of MOFs under a humid environment while maintaining their catalytic activity, which provides a reference for improving the comprehensive performance of MOF catalysts.