Hydrophobic Porphyrin Titanium-Based MOFs for Visible-Light-Driven CO₂ Reduction to Formate

Three hydrophobic porphyrin titanium-based metal-organic frameworks (MOFs) (HPA/DGIST-1, DPA/DGIST-1, and OPA/DGIST-1) were synthesized through a postsynthetic coordination reaction by using alkylphosphonic acid of different lengths (HPA, hexylphosphonic acid; DPA, dodecylphosphonic acid; OPA, octadecylphosphonic acid). Compared with the hydrophilic DGIST-1, modified DGIST-1 exhibits excellent hydrophobicity and presents good stability in humid atmospheres. Due to the introduction of porphyrin ligands, HPA/DGIST-1, DPA/DGIST-1, and OPA/DGIST-1 showed good visible-light absorption (380-700 nm) and sensitive photogenerated charge responses. When acted as catalysts, these hydrophobic Ti-MOFs can selectively reduce CO₂ to HCOO^- under visible-light irradiation with average reaction rates of 150.9, 178.5, and 228.3 μmol·h^-1·g^-1, where these values are 1.3-2.0 times higher than the system mediated by the initial porphyrin Ti-MOF catalyst. ¹³C NMR spectroscopy demonstrates that the catalytic product HCOO^- anion originates from the reactant CO₂. The photocatalytic experiments, electron paramagnetic resonance, and photoluminescence spectra tests showed that porphyrin ligands and Ti-O units can act as catalytic activity centers to realize the conversion of CO₂ to HCOO^-. This work demonstrated that the combination of porphyrin titanium-based MOF and alkyl hydrophobic groups is an effective way to enhance the stability of titanium-based MOFs and maintain their high photocatalytic performance.