Construction of a Multi-Cage-Based MOF with a Unique Network for Efficient CO₂ Capture

As a kind of MOFs, cage-based MOFs usually carry large voids and small windows, which are advantageous to the storage of small molecules that remain kinetically trapped inside the cages (confinement effect). By adjusting the size of windows via reticular synthesis, the cage-based MOFs can selectively capture and separate the suitable size molecules. Here, considering angle-directed and face-directed strategies, a novel multicage-based MOF NUM-3 with a new (3,4,5)-connected topology was successfully constructed in the mixed-ligands assembly. In the framework of NUM-3, there exist four different kinds of cages, which exhibit diverse polyhedral configurations. The four kinds of cages in the order ABCDDCBA as the minimum repeat unit form a 1D tortuous channel along the c axis. Based on the structure characteristics that the 1D channel exhibits different inner diameter (from 4.0 to13.0 Å), NUM-3a (actived NUM-3) can capture CO₂ over C₂H₄ and C₂H₆ by the size selectivity (the empirical kinetic diameters: CO₂ < C₂H₄ < C₂H₆). In addition, it also exhibits commendable selectivity for CO₂ over N₂ and CH₄.