Partitioning COF Membrane Channels into Ultramicroporous and π-Electron-Rich Compartments for Multicomponent Ion Separations

Covalent organic frameworks (COFs) build a versatile platform for constructing separation membranes, affording them disruptive candidates for nanoscale molecule/ion sieving. However, the reported aperture of COFs mainly concentrates on 0.8–10.0 nm, leaving a daunting challenge in fabrication of ultramicroporous (<0.7 nm) COF membranes. Herein, the channel partition of COF membranes is elaborately implemented by taking advantage of the copper-catalyzed azide–alkyne cycloaddition (CuAAC) based on click chemistry. Two kinds of novel alkynyl COF membranes bearing symmetric alkynyl monomers are predesigned and synthesized. The alkynyl groups can react with dual-azide molecules to form triazole ring bridge, which allows the partition of hexagonal or tetragonal channels. Moreover, the π-electron-rich structures of triazole ring offer cation-philic or anion-repulsive region, showing great potentials in precise ion separations. Consequently, the superior performance is obtained in separating ternary/binary ion mixtures: Li⁺/Mg²⁺/(K⁺) and F⁻/SO₄²⁻/(Cl⁻). Example of dealing with a K⁺/Li⁺/Mg²⁺ ternary mixture, the optimum p-TpPa-C≡CH membrane exhibits an actual selectivity of 101.2 for K⁺/Mg²⁺ and 30.9 for Li⁺/Mg²⁺. The strong adaptability and wide range of application scenarios of our COF membranes exceed the ever-reported membranes. This work provides a fundamental to comprehending the channel manipulation mechanisms and advances the development of high-performance ion separation membranes.