Tetracarboxylate Linker-Based Flexible Cu^II Frameworks: Efficient Separation of CO₂ from CO₂/N₂ and C₂H₂ from C₂H₂/C₂H₄ Mixtures

We report the synthesis, structure, and adsorption properties of two new metal–organic frameworks (MOFs) {[Cu₂(bpp)₃(L1)]·(bpp)·(4H₂O)} (1) and {[Cu₂(bipy)₂(L2)­(H₂O)₂]·(bipy)·(5H₂O)} (2) obtained from two different flexible tetracarboxylate linkers (L1 and L2) of variable lengths and flexibility. While 1 comprising Cu^II, L1, and 1,3-bis­(4-pyridyl) propane (bpp) is a 2D MOF with a cage-type structure, 2 consisting of Cu^II, L2, and 4,4′-bipyridine (bipy) has a 3D twofold interpenetrated structure. Both frameworks manifest permanent porosity, as realized from CO₂ adsorption at 195 K. 2 shows excellent CO₂/N₂ and C₂H₂/C₂H₄ adsorption selectivity at 298 K. This has been established by using 2 as a separating medium in a breakthrough column for separating mixtures of CO₂/N₂ (15:85, v/v) and C₂H₂/C₂H₄ (1:99, v/v). The selectivity of 2 toward CO₂ over N₂ and C₂H₂ over C₂H₄ is governed by favorable thermodynamic interactions owing to its structural flexibility, unsaturated metal sites, and polar carboxylate groups. Thus, 2 proves to be an extremely efficient material for specific gas separation.