Porphyrin-based metal–organic framework catalysts for photoreduction of CO2: understanding the effect of node connectivity and linker metalation on activity

Abstract

Three zirconium–porphyrin based MOFs with different linker connectivity, PCN-222, PCN-223 and PCN-224, and their Zn(II) ion metalated analogues, were prepared and used as catalysts for the photoreduction of CO₂ under visible-light irradiation. Before metalation with Zn²⁺ in the porphyrin linkers, the lowest linker-connected PCN-224 (6-fold) exhibited a higher formate yield rate of 45.2 μmol g⁻¹ h⁻¹ compared to PCN-222 and PCN-223. However, after the incorporation of Zn²⁺ into the porphyrin, PCN-222-Zn showed the highest formate yield rate of 120.2 μmol g⁻¹ h⁻¹. The density functional theory (DFT) computational results revealed that the catalytic activity correlated with the CO₂ adsorption affinity. The CO₂ adsorption energy on PCN-222-Zn (294.4 kJ mol⁻¹) was the highest among all of the samples. The overall catalytic performance was affected by the Zr₆-oxo cluster connectivity and the introduced dual reaction sites when the porphyrin linkers were metalated with Zn(II) ions.