Rational Design of Covalent Organic Framework‐Based Membranes with Pore‐in‐Pore Structure for Efficient Mass Transport

ABSTRACT Covalent organic framework (COF) membranes have garnered widespread attention for applications in molecular sieving and water purification. How to fully leverage the advantages of COF materials while mitigating issues such as brittleness and interfacial defects remains a challenge. Herein, we develop a pore engineering strategy that integrates supramolecular confinement with chemical crosslinking. This approach enables the transformation of small 2D COF nanosheets into larger and thicker carboxymethyl‐β‐cyclodextrin (CM‐β‐CD) @COF nanosheets through chemical crosslinking, thereby enhancing their stacking density and reducing interfacial defects in the resulting laminar membranes. Additionally, the inner diameter of COF pores can be precisely matched with the outer cavity size of CM‐β‐CD, forming a “CM‐β‐CD‐in‐COF” hierarchical pore structure, which can further improve the selectivity of the separation membrane. Leveraging the synergistic role of size sieving and Donnan effect, the fabricated CM‐β‐CD@COF/aramid nanofibers (ANF) nanofiltration membranes demonstrate an ultrafast water permeability of 439 L·m ‒2 ·h ‒1 and the high rejection more than 97% for those dyes whose sizes exceed 1.0 nm. Moreover, the CM‐β‐CD@COF/ANF nanofiltration membranes also exhibit excellent durability, superior antifouling property, and strongly mechanical stability. This pore engineering strategy offers a novel pathway for tailoring the pore structure and dimensions of COFs to achieve precise mass separation.