Robust {Cd4}-Organic Framework for Efficiently Catalyzing CO2 Cycloaddition and Knoevenagel Condensation

The high-value-added carbonates generated from CO2 have attracted the attention of more and more researchers because of which the optimization of metal–organic framework (MOF)-based catalysts has seen a considerable upsurge at present. The scarcely reported cadmium(II)-based MOFs inspire us to explore CdOFs with excellent catalytic activity and high reusability. Herein, the unification of the unreported {Cd4(μ3-OH)2(CH3CO2–)} cluster and 2,6-bis(2,4-dicarboxylphenyl)-4-(4-carboxylphenyl)pyridine (H5BDCP) led to a highly robust nanoporous crystalline material of {(Me2NH2)5[Cd4(BDCP)2(μ3-OH)2(CH3CO2)(H2O)2]·3DMF·2H2O}n (NUC-67) with 57.4% void volume. Structural analysis displays that the inner surface of channels in activated NUC-67a is functionalized by Lewis acid sites of unsaturated Cd2+ ions and Lewis base sites of μ3-OH– groups, CH3CO2– anions, free pyridine, and C═O groups. Under solvent-free conditions, NUC-67a exhibits high catalytic performance on the cycloaddition of CO2 with epoxides; for instance, the conversion rate of propylene oxide (PO) into propylene carbonate (PC) with 1 atm CO2 can reach 99% within 6 h at 55 °C, resulting in a 660 turnover number and 110 h–1 turnover frequency. Moreover, Knoevenagel condensation reactions of aldehydes and malononitrile can be efficiently catalyzed by activated NUC-67a. Encouragingly, NUC-67a shows strong structural stability and good reversible cyclicity in the above two organic reactions with metal leaching below 8 ppb. Hence, this work proves that the optimization of MOF-based catalysts should focus on the design and selection of organic ligands, which plays a decisive role in structural regulation, such as cluster-based nodes, high defect of metal sites, unexpected insertion of Lewis base sites, and high-porosity channels.