Robust {Cd<sub>4</sub>}-Organic Framework for Efficiently Catalyzing CO<sub>2</sub> Cycloaddition and Knoevenagel Condensation
Shurong Liu, Tuoping Hu, Kun Yang, Xiutang Zhang
Abstract
The high-value-added carbonates generated from CO 2 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 {Cd 4 (μ 3 -OH) 2 (CH 3 CO 2 – )} cluster and 2,6-bis(2,4-dicarboxylphenyl)-4-(4-carboxylphenyl)pyridine (H 5 BDCP) led to a highly robust nanoporous crystalline material of {(Me 2 NH 2 ) 5 [Cd 4 (BDCP) 2 (μ 3 -OH) 2 (CH 3 CO 2 )(H 2 O) 2 ]·3DMF·2H 2 O} 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 Cd 2+ ions and Lewis base sites of μ 3 -OH – groups, CH 3 CO 2 – anions, free pyridine, and C═O groups. Under solvent-free conditions, NUC-67a exhibits high catalytic performance on the cycloaddition of CO 2 with epoxides; for instance, the conversion rate of propylene oxide (PO) into propylene carbonate (PC) with 1 atm CO 2 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.