Tailoring flexibility of nanofluidic membranes for efficient separation of gases with similar kinetic diameters

Conventional nanofluidic membranes often exhibit low selectivities for efficient separation of gases with similar kinetic diameters. Soft nanofluidic membranes overcome this challenge through a combination of selective binding sites and tunable pore structures, creating an on-demand separation switch that enables adaptive pore opening for enhanced gas separation. Herein, three different nanofluidic membranes of soft covalent organic frameworks (named S-COF1, S-COF2, and S-COF3) with varied flexibility levels were synthesized for similar-sized gas separation using ethane (C₂H₆) and ethylene (C₂H₄) as model gases. The flexibility was precisely tuned by introducing varying numbers of functionalized -OH linkers to form intramolecular [-O-H⋯N[double bond, length as m-dash]C] hydrogen bonding. Highly flexible S-COF1 and S-COF2 demonstrated similar pore behavior for C₂H₄ and C₂H₆, resulting in poor separation efficiency. In contrast, S-COF3, with enhanced rigidity due to the addition of the highest amount of -OH linkers, exhibited distinct pore switching from "close" in C₂H₄ to "open" in C₂H₆. This led to a C₂H₆/C₂H₄ selectivity of 18.2, which is superior to that of most of the reported membranes. This work establishes a functionalized -OH linker strategy to precisely tune COF flexibility, revealing its critical role in gas separation and advancing the design of dynamic porous membranes.