Tunable Methylation in Imide-Linked Covalent Organic Frameworks Enables Highly Selective SF₆ Capture

Rational design of covalent organic frameworks (COFs) for energy-efficient SF₆ capture remains hindered by insufficient mechanistic understanding of structure-property relationships. Herein, we report that strategic methyl-group engineering within imide-linked COFs induces a profound stacking transition, enabling highly efficient SF₆/N₂ separation. Systematic methylation of the triazine building units drives a structural evolution from a conventional AA stacking to an ABC stacking mode. This transformation tailors the pore architecture at the molecular level, generating optimized ultramicropores (0.66 nm) that closely match the kinetic diameter of SF₆. Benefiting from this optimized pore environment, the dimethyl-functionalized NMUCOF-5 achieves an SF₆ uptake of 42.5 cm³·g^-1 and a high SF₆/N₂ selectivity of 126 (298 K, 100 kPa) despite a moderate BET surface area (751 m²·g^-1). Integrated breakthrough experiments and molecular simulations reveal that this performance originates from the synergistic effect of precise size-sieving within the tailored ultramicropores and enhanced C-H···F interactions afforded by the methyl-enriched pore walls. This work shows that methyl-directed stacking modulation can serve as an effective pore-engineering strategy in imide-linked COFs, providing mechanistic insight into how substituent-controlled packing can influence SF₆ capture and separation.