Locking of phase transition in MOF ZIF-7: Improved selectivity in mixed-matrix membranes for O₂/N₂ separation

The framework flexibility of metal-organic frameworks (MOFs) is beneficial for gas storage and adsorption but is disadvantageous to the separation process based on the size-extrusion mechanism. Herein, the framework flexibility of ZIF-7, a typical MOF famous for its phase transition between wide-pore (ZIF-7-I) and narrow-pore (ZIF-7-II) forms on desolvation, was successfully hindered by embedding MOF nanocrystals in the rigid polymer matrix. For the first time, it was experimentally demonstrated that ZIF-7 could be "locked" in the wide-pore phase (ZIF-7-I) even after complete desolvation. A mixed-matrix membrane containing ZIF-7-I nanocrystals and rigid polyimides showed O₂ permeability of 3 Barrer and O₂/N₂ selectivity of 15, surpassing the state-of-the-art upper limit for O₂/N₂ and falling in the commercially attractive region. In view of the weak competitive interaction between O₂/N₂ and hybrid membrane materials, similar permeation results were also found for the separation of equal-molar O₂/N₂ binary mixtures.