CO2 Behavior in a Highly Selective Ultramicroporous Framework: Insights from Single-Crystal X‑ray Diffraction and Solid-State Nuclear Magnetic Resonance Spectroscopy

Metal–organic frameworks (MOFs) are a class of porous materials that have attracted much attention for gas adsorption applications. The ultramicroporous SIFSIX-3-Zn MOF, made up of zinc and pyrazine sheets pillared into a three-dimensional framework by hexafluorosilicate ions, has displayed remarkable carbon dioxide (CO2) capture abilities. Even though the CO2 adsorption capability of SIFSIX-3-Zn is known, a detailed experimental description of the CO2-loaded SIFSIX-3-Zn structure and the behavior of the adsorbed CO2 guests is yet to be reported. In this work, CO2 adsorption in SIFSIX-3-Zn is examined at the molecular level. Single-crystal X-ray diffraction (SCXRD) was employed to identify the CO2 adsorption sites. The crystal structure of CO2-loaded MOF was subsequently compared to that of the as-made SIFSIX-3-Zn, which shows that the crystal symmetry of the MOF is guest-dependent, although the framework topology remains the same. 13C, 19F, 29Si, and 67Zn solid-state nuclear magnetic resonance (SSNMR) spectroscopy was also performed to investigate the as-made, activated, and CO2-loaded SIFSIX-3-Zn. The SSNMR results reveal new insights into the effect of guest inclusion on the structure of the three SIFISIX-3-Zn phases. We have also examined CO2 dynamics in SIFSIX-3-Zn via variable-temperature 13C SSNMR spectroscopy. Adsorbed CO2 displays unusually hindered mobility, which is affected very little by temperature. This unique motional behavior is attributed to the strong CO2–MOF interactions imposed by the ultramicroporosity of the framework. The combination of SCXRD and SSNMR results clearly show that the synergistic interactions of framework hydrogen and fluorine atoms with CO2 inside the MOF is a major factor that leads to the excellent CO2 adsorption capability of SIFSIX-3-Zn.