Synthesis of Porphyrinic Metal Organic Frameworks with High Robustness and Catalytic Activity

Abstract

MOFs are ideal platforms to immobilize porphyrins and their derivatives. MOF’s high surface areas and rigid structures not only make porphyrin moieties approachable substrates but also prevent the dimerization of their active centers. These advantages effectively enhance the reactivity and optical performance of porphyrins. Extensive study has been done to synthesize porphyrinic MOFs, however, many of the previously reported porphyrinic MOFs suffer from weak chemical stabilities, which severely hinders their wide utilization. My study is focused on the design and syntheses of porphyrinic MOFs with high robustness in various chemical environments. Three porphyrinic MOFs with excellent chemical stability, namely PCN-600, PCN-601 and PCN-602, were obtained, and their performances as catalysts in different reactions were explored.

PCN-600, a Fe-based mesoporous porphyrinic MOF was successfully synthesized through rational topological design and KTDA method. It exhibits high chemical stability in aqueous solutions with pH values ranging from 2 to 11. The catalytic activity of PCN-600(Fe) (with Fe³⁺ at the porphyrin center) was confirmed by its excellent performance in the co-oxidation of phenol and 4-aminoantipyrine (4-AAP) by H₂O₂.

Most of the reported stable porphyrinic MOFs, including PCN-22Xs (X = 2, 3, 4, 5) and PCN-600, are constructed by high-valent metal ions and carboxylate-based porphyrinic ligands. Though these materials have very high robustness in acidic solutions, they are relatively vulnerable in aqueous solutions containing some coordinating anions, such as OH⁻, F⁻, CO₃²⁻ and PO₄³⁻. To address these problems, two iso-structural porphyrinic MOFs constructed by pyrazolate-terminated ligands were synthesized, namely PCN-601 and PCN-602. These materials exhibit extraordinary stability in the solutions of NaOH, KF, Na₂CO₃ and K₃PO₄. Their chemical stabilities were rationalized from both thermodynamic and kinetic perspectives. The catalytic performance of PCN-601(Mn) and PCN-602(Mn) (with Mn³⁺ at the coordination centers of the porphyrinic ligands) in C-H halogenation reactions was explored. Compared with PCN-601(Mn), PCN-602(Mn) has a higher porosity and displays a better activity as a recyclable hetergeneous catalyst.