Building Unit Engineering Toward COF Membranes with Controlled Stacking for H 2 Purification

Abstract Hydrogen purification by membrane technology offers a sustainable path to meet the escalating demands of green energy. However, conventional polymeric membranes are constrained by permeability‐selectivity trade‐off and instability under real‐world operating conditions. While covalent organic framework (COF) membranes hold promise, their overlarge pores and poor film‐processibility are to be imperatively solved. Herein, a ternary building unit system is designed for synthesizing imine‐based COF nanosheets with programmable interlayer offsets. By synergizing a planar aldehyde monomer as the basic structural unit and a none‐planar alkyl‐functionalized aldehyde monomer as the structure regulation unit, we induce layer distortion that disrupts π–π dominated AA stacking, enabling angstrom‐precise pore tuning (1.4–0.6 nm) via controlled transitions to AB stacking while retaining crystallinity. The mechanically robust nanosheets are easily assembled into large‐area membranes via a facile blade casting, overcoming the processability bottleneck associated with binary building unit systems. The resulting membranes demonstrate an exceptional H 2 /CO 2 selectivity of 60, surpassing existing benchmarks. When treating gas mixtures from methanol steam reforming, a two‐stage membrane process achieves 99.5% H 2 purity and 94.0% recovery. Precise modulation of pore architecture and mechanical flexibility through building units engineered stacking affords a platform for microporous organic membranes.