Probing Induced-Fit Acetylene Adsorption and Separation in a Flexible Ultramicroporous Framework by Multinuclear SSNMR Spectroscopy: Insights Beyond Crystallography

This study investigates the molecular-level-induced-fit adsorption of acetylene (C 2 H 2 ) and carbon dioxide (CO 2 ) in sql-SIFSIX-bpe-Zn, a flexible, anion-pillared ultramicroporous metal–organic framework (MOF). Sql-SIFSIX-bpe-Zn undergoes distinct pressure-dependent phase transitions, enabling a high selectivity for C 2 H 2 over CO 2 and ethylene (C 2 H 4 ). However, the unclear thermodynamic and kinetic mechanisms underlying this behavior, as well as limitations in models derived from single-crystal X-ray diffraction (SCXRD), necessitate further investigation. Using in situ multinuclear solid-state NMR (SSNMR) spectroscopy with isotopically enriched C 2 D 2 and 13 CO 2, we directly probed gas adsorption, framework dynamics, and competitive uptake in sql-SIFSIX-bpe-Zn across 153–298 K. Variable-temperature 2 H SSNMR revealed a C 2 D 2 -induced pore opening between 253 and 233 K, marking a cooperative structural transformation process that maximizes acetylene adsorption. Complementary 13 C and 19 F SSNMR spectra show that both the SiF 6 2– pillars and bpe linkers participate in host–guest interactions through C–D···F hydrogen bonding and linker reorientation, while CO 2 remains bound near the linker region. In mixed-gas systems, CO 2 assists pore activation, but acetylene dominates adsorption, confirming its stronger affinity and selective uptake. These results provide a molecular-level mechanism for induced-fit adsorption and competitive selectivity in flexible hybrid ultramicroporous materials, guiding the design of next-generation adaptive MOF sorbents.