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

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 CO₂/N₂ 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 CO₂/N₂ 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 (¹H-¹³C HetCor) and spin-lattice relaxation. Consequently, the CO₂/N₂ diffusion selectivity of ZIF-8-NH₂ with 6 mol% 2-aminobenzimidazole increased from 0.58 (polymer-free) to 26.6 within the polymer environment, enabling CO₂/N₂ size sieving. Membranes with 20 wt% filler achieved a CO₂ permeability of 261.7 Barrer and a CO₂/N₂ 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.