Coupling Ligand Tuning With Polymer‐Induced Aperture Regulation for Molecular Sieving in Flexible MOFs

ABSTRACT Framework flexibility fundamentally limits molecular sieving in many metal–organic frameworks (MOFs) by enlarging their effective pore apertures beyond crystallographic constraints. This challenge is pronounced in zeolitic imidazolate framework‐8 (ZIF‐8), where dynamic aperture expansion prevents CO 2 /N 2 size‐sieving despite an ideally positioned nominal aperture size. Here, we show that polymer infiltration enables in situ regulation of dynamic apertures, converting ZIF‐8 from a nonselective adsorbent into a CO 2 /N 2 molecular sieve. During mixed‐matrix membrane (MMM) formation, poly(ethylene glycol) diacrylate (PEGDA) partially infiltrates ZIF cavities and suppresses framework dynamics, as supported by solid‐state NMR ( 1 H‐ 13 C HetCor) and spin–lattice relaxation. Consequently, the CO 2 /N 2 diffusion selectivity of ZIF‐8‐NH 2 with 6 mol% 2‐aminobenzimidazole increased from 0.58 (polymer‐free) to 26.6 within the polymer environment, enabling CO 2 /N 2 size sieving. Membranes with 20 wt% filler achieved a CO 2 permeability of 261.7 Barrer and a CO 2 /N 2 selectivity of 73.9. This strategy is generalizable to other flexible ZIFs, such as ZIF‐7 and ZIF‐67, establishing polymer infiltration as a simple and versatile route for controlling dynamic MOF apertures for effective gas separation.