Building Unit Engineering Toward COF Membranes with Controlled Stacking for H₂ Purification

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 H₂/CO₂ selectivity of 60, surpassing existing benchmarks. When treating gas mixtures from methanol steam reforming, a two-stage membrane process achieves 99.5% H₂ 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.