Tetracarboxylate Linker-Based Flexible Cu(II) Frameworks: Efficient Separation of CO(2) from CO(2)/N(2) and C(2)H(2) from C(2)H(2)/C(2)H(4) Mixtures

[Image: see text] We report the synthesis, structure, and adsorption properties of two new metal–organic frameworks (MOFs) {[Cu(2)(bpp)(3)(L1)]·(bpp)·(4H(2)O)} (1) and {[Cu(2)(bipy)(2)(L2)(H(2)O)(2)]·(bipy)·(5H(2)O)} (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 CO(2) adsorption at 195 K. 2 shows excellent CO(2)/N(2) and C(2)H(2)/C(2)H(4) adsorption selectivity at 298 K. This has been established by using 2 as a separating medium in a breakthrough column for separating mixtures of CO(2)/N(2) (15:85, v/v) and C(2)H(2)/C(2)H(4) (1:99, v/v). The selectivity of 2 toward CO(2) over N(2) and C(2)H(2) over C(2)H(4) 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.