Dual Site Lewis-Acid Metal-Organic Framework Catalysts for CO₂ Fixation: Counteracting Effects of Node Connectivity, Defects and Linker Metalation

Three Zr-oxo-cluster node and porphyrin-linker based MOFs, MOF-525, PCN-222 and PCN-224 exhibiting different linker connectivities of 12, 8 and 6 and their porphyrin-linker metalated analogues, were synthesized and tested as catalysts for CO₂ fixation by using the cycloaddition of CO₂ and propylene oxide to propylene carbonate as the test reaction. In general, the catalytic activity correlates with the connectivity of the Zr-oxo nodes. The lowest connected PCN-224 (6-fold) exhibits a superior catalytic activity in this series, while higher connected PCN-222 (8-fold) and MOF-525 (12-fold) are less active. Interestingly, the catalytic activity of the higher connected MOFs significantly depends on defects. The (ideally) 12-connected MOF-525, however exhibiting 16% of missing linker defects, features a higher catalytic activity compared to the 8-connected PCN-222 with less defects. The overall catalytic activity is increased in dual site catalysts when the porphyrin linkers are metalated with Mn(III) and Zn(II) centers, which are acting as additional Lewis acid sites. Here, the metalated MOFs with higher connectivity exhibit the highest activity.