Combining In Situ Techniques (XRD, IR, and ¹³C NMR) and Gas Adsorption Measurements Reveals CO₂-Induced Structural Transitions and High CO₂/CH₄ Selectivity for a Flexible Metal–Organic Framework JUK-8

Flexible metal-organic frameworks (MOFs) are promising materials in gas-related technologies. Adjusting the material to processes requires understanding of the flexibility mechanism and its influence on the adsorption properties. Herein, we present the mechanistic understanding of CO₂-induced pore-opening transitions of the water-stable MOF JUK-8 ([Zn(oba)(pip)]_n, oba^2- = 4,4'-oxybis(benzenedicarboxylate), pip = 4-pyridyl-functionalized benzene-1,3-dicarbohydrazide) as well as its potential applicability in gas purification. Detailed insights into the global structural transformation and subtle local MOF-adsorbate interactions are obtained by three in situ techniques (XRD, IR, and ¹³CO₂-NMR). These results are further supported by single-crystal X-ray diffraction (SC-XRD) analysis of the solvated and guest-free phases. High selectivity toward carbon dioxide derived from the single-gas adsorption experiments of CO₂ (195 and 298 K), Ar (84 K), O₂ (90 K)_, N₂ (77 K), and CH₄ (298 K) is confirmed by high-pressure coadsorption experiments of the CO₂/CH₄ (75:25 v/v) mixture at different temperatures (288, 293, and 298 K) and in situ NMR studies of the coadsorption of ¹³CO₂/¹³CH₄ (50:50 v/v; 195 K).