A porous rhodium(III)-porphyrin metal-organic framework as an efficient and selective photocatalyst for CO2 reduction

A rhodium(III)-porphyrin zirconium metal-organic framework (Rh-PMOF-1(Zr)) has been prepared from the self-assembly of a Rh-based metalloporphyrin tetracarboxylic ligand Rh(TCPP)Cl (TCPP = tetrakis(4-carboxyphenyl)porphyrin) with ZrCl4. The framework of Rh-PMOF-1 is stable up to 270 °C as disclosed by the variable-temperature powder X-ray diffraction (VT-PXRD) measurements, and possesses good chemical stability over a wide range of solvents including water. The single-crystal structural analysis reveals that Rh-PMOF-1 contains 3-D channels (1.9 × 1.9 nm2), and the Rh-porphyrin units are exposed to the cavities. The calculation based on the N2 adsorption at 77 K shows Rh-PMOF-1(Zr) has a high BET surface area (3015 m2g−1). The luminescence decay of Rh-PMOF-1 is well fitted to a tri-exponential curve featuring a long average lifetime of 207 μs at 298 K under vacuum, which represents a rare example of room-temperature phosphorescence of Rh-porphyrin complexes. Under 1 atm, it displays CO2 uptake up to 42, 53 and 98 cm3g−1 at 308, 298 and 273 K, respectively. Catalytic results show that, under the visible light (≥400 nm) irradiation without any additional photosensitizer, Rh-PMOF-1 is powerful to catalyze CO2 reduction to the formate ion with up to 99% selectivity, and can be recycled and reused for 3 runs. Theoretical study was further carried out to reveal the energy levels of the frontier orbitals of Rh-PMOF-1 and the preferred binding sites of CO2 in the framework.