Highly Selective Adsorption of C 2 /C 1 Mixtures and Solvent-Dependent Thermochromic Properties in Metal–Organic Frameworks Containing Infinite Copper-Halogen Chains

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)X]·X·6H 2 O·0.25DMSO} ( FJU-53, Btz = 1,4′-Bis(4 H -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 C 2 H 2 /CH 4 separation at 296 K 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.