Tailoring porphyrinic MOF-545 performances for CO₂ photocatalytic reduction through linker chlorination

The development of technologies for CO₂ sequestration and its conversion into value-added chemicals has received considerable and growing attention. However, achieving high conversion efficiency and product selectivity under mild conditions remains a major challenge. This study investigates the photocatalytic CO₂ reduction activity of Zr-based porphyrinic MOF-545 derivatives synthesized by varying the percentages of two porphyrin linkers: tetrakis(4-carboxyphenyl)porphyrin (TCPP) and its β-pyrrolic chlorinated analogue (TCPPCl₈). A comprehensive set of spectroscopic and analytical techniques was employed to characterize the materials, revealing the impact of linker chlorination on the optical band gap, particle size and crystallinity. The incorporation of chlorine-substituted linkers significantly enhanced the photocatalytic activity. Notably, under visible light irradiation and using triethanolamine (TEOA) as a sacrificial electron donor, the MOF-545 derivative containing 50% TCPPCl₈ achieved the highest formate production, with a rate 2.6 times greater than that of pristine MOF-545, with a production rate of 625 μmol g^-1 h^-1 after 2 h. Density Functional Theory (DFT) calculations were also performed to gain insight into the electronic properties of the chlorinated porphyrinic materials. These calculations showed the stabilization of both the HOMO and LUMO energy levels upon chlorination of the porphyrin linkers, with a more pronounced stabilisation of the LUMO, leading to a smaller band gap, in line with optical measurements. Additionally, the stabilisation of the HOMO level is expected to increase the oxidizing power of the photogenerated holes, thus facilitating TEOA oxidation and enhancing the overall photocatalytic activity, and this rationalizes the experimental observations.