Separation of light hydrocarbon mixtures is a very important but challenging industrial separation task. Here, we have synthesized two isostructural cationic metal organic frameworks {(Cu(Btz)Xcenter dot X center dot 6H(2)O center dot 0.25DMSO} (FJU-53, Btz = 1,4'-Bis(4H-1;2,4-triazol-4-yl)benzene, X = Cl or Br, DMSO = dimethyl sulfoxide) containing infinite copper-halogen chains and first demonstrated that the adsorption selectivity toward C-2/C-1 mixtures in the charged MOFs can be improved by tuning counter-anions. FJU-53 exhibits the highest selectivity for C2H2/CH4 separation at 296 and 1 atm, and exceptional chemical stability in aqueous solutions with pH ranging from 1 to 13. In addition, FJU-53 also shows the attractive solvent- and halogen-dependent thermochromic behaviors. Its thermochromic mechanism is attributed to the thermally induced vibration of the infinite (CuX)(n)(n+) chains, remarkably different from that for the traditional copper(II) halide materials, the thermochromism for which comes from the coordination geometry transformation or Jahn-Teller distortion.
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