Extraordinary separation of acetylene-containing mixtures with microporous metal-organic frameworks with open O donor sites and tunable robustness through control of the helical chain secondary building units

Acetylene separation is a very important but challenging industrial separation task. Here, through the solvothermal reaction of CuI and 5-triazole isophthalic acid in different solvents, two metal–organic frameworks (MOFs, FJU-21 and FJU-22) with open O donor sites and controllable robustness have been obtained for acetylene separation. They contain the same paddle-wheel {Cu₂(COO₂)₄} nodes and metal–ligand connection modes, but with different helical chains as Secondary Building Units (SBUs), leading to different structural robustness for the MOFs. FJU-21 and FJU-22 are the first examples in which the MOFs’ robustness is controlled by adjusting the helical chain SBUs. Good robustness gives the activated FJU-22 a, which has higher surface area and gas uptakes than the flexible FJU-21 a. Importantly, FJU-22 a shows extraordinary separation of acetylene mixtures under ambient conditions. The separation capacity of FJU-22 a for 50:50 C₂H₂/CO₂ mixtures is about twice that of the high-capacity HOF-3 and its actual separation selectivity for C₂H₂/C₂H₄ mixtures containing 1% acetylene is the highest among reported porous materials. Based on first-principles calculations, the extraordinary separation performance of C₂H₂ for FJU-22 a was attributed to hydrogen-bonding interactions between the C₂H₂ molecules with the open O donors on the wall, which provide better recognition ability for C₂H₂ than other functional sites, including open metal sites and amino groups