Abstract
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 (C(2)H(6)) and ethylene (C(2)H(4)) 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 C(2)H(4) and C(2)H(6), 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 C(2)H(4) to "open" in C(2)H(6). This led to a C(2)H(6)/C(2)H(4) 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.