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

Abstract

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 CO 2 -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 13 CO 2 -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 CO 2 (195 and 298 K), Ar (84 K), O 2 (90 K) , N 2 (77 K), and CH 4 (298 K) is confirmed by high-pressure coadsorption experiments of the CO 2 /CH 4 (75:25 v/v) mixture at different temperatures (288, 293, and 298 K) and in situ NMR studies of the coadsorption of 13 CO 2 / 13 CH 4 (50:50 v/v; 195 K).

Tuning Adsorption-Induced Responsiveness of a Flexible Metal–Organic Framework JUK-8 by Linker Halogenation

“…By the combination of computational modeling and Rietveld analysis for PXRD patterns of the desolvated phases, we solved and refined the crystal structures of all JUK - 8X ( cp ) MOFs except the iodine derivative (Figures and S8, Supporting Information). The compounds are isostructural to the closed-pore phase of JUK - 8 ( cp ) whose structure was previously determined from SC-XRD data; however, in contrast to this parental structure, the desolvation of halogen derivatives causes weakening of the (N–H) pip ···O oba hydrogen bond (Tables S3 and S4, Supporting Information). The meticulous comparison of the g@op and cp structures reveals that all eight subnetworks in each MOF shrink to the nonporous cp phase upon activation, but halogen substituents have a significant influence on these processes.…”

“…The adsorption-induced pore-opening transition ( cp → op ) of all but the fluorine derivative is preceded by an intermediate state CO 2 @JUK - 8X ( ip ) (X = H, Cl, Br, I). The detailed analysis of the isotherms indicates that the first stage of adsorption for JUK - 8Cl , JUK - 8Br , and JUK - 8I is comparable to that for JUK - 8 ( cp → ip ) . Carbon dioxide molecules diffuse to the voids localized between two zinc atoms.…”

“…In previous reports, , we have shown that JUK - 8 maintains the structure and properties upon applying mechanical force, boiling in water, as well as upon exposition to a repeatable adsorption stress. According to the experiment and theory, the cell volume of JUK - 8Br during CO 2 adsorption increases significantly (38.1%) compared to JUK - 8 , JUK - 8F , or JUK - 8Cl (17–23%).…”

Nuclear Magnetic Resonance Studies of Carbon Dioxide Capture

“…A number of initial efforts have been carried out to study mixed gas adsorption, including CO2/H2O mixtures, 23,29,65 as well as CO2/CH4 mixtures. 49,66 The role of H2O should be studied in particular, since various studies have highlighted that H2O can have a significant impact on adsorption thermodynamics. 65 We further note that few studies have investigated CO2 capture materials in the presence of O2, despite O2 often playing an important role in the oxidative degradation of many carbon capture materials.…”

X-ray diffraction for probing free energy profiles and self-diffusivity of gases in metal–organic frameworks

“…X-ray diffraction (XRD) conducted in situ under a target atmosphere has been demonstrated to be useful for the investigation of adsorption sites in MOFs as well as adsorption induced structural changes. [22][23][24] Recent advances in adsorption crystallography based on single-crystal [25][26][27][28][29][30][31][32] or powder [33][34][35] XRD have made it possible to locate adsorbed gases in the micropores of MOFs (Table S1 †). This technique provides valuable information pertaining to the distribution of adsorbates in MOFs.…”