A Fine-Tuned Fluorinated MOF Addresses the Needs for Trace CO<sub>2</sub> Removal and Air Capture Using Physisorption

The development of functional solid-state materials for carbon capture at low carbon dioxide (CO<sub>2</sub>) concentrations, namely, from confined spaces (<0.5%) and in particular from air (400 ppm), is of prime importance with respect to energy and environment sustainability. Herein, we report the deliberate construction of a hydrolytically stable fluorinated metal–organic framework (MOF), <b>NbOFFIVE</b>-1-Ni, with the appropriate pore system (size, shape, and functionality), ideal for the effective and energy-efficient removal of trace carbon dioxide. Markedly, the CO<sub>2</sub>-selective <b>NbOFFIVE</b>-1-Ni exhibits the highest CO<sub>2</sub> gravimetric and volumetric uptake (ca. 1.3 mmol/g and 51.4 cm<sup>3</sup> (STP) cm<sup>–3</sup>) for a physical adsorbent at 400 ppm of CO<sub>2</sub> and 298 K. Practically, <b>NbOFFIVE</b>-1-Ni offers the complete CO<sub>2</sub> desorption at 328 K under vacuum with an associated moderate energy input of 54 kJ/mol, typical for the full CO<sub>2</sub> desorption in conventional physical adsorbents but considerably lower than chemical sorbents. Noticeably, the contracted square-like channels, affording the close proximity of the fluorine centers, permitted the enhancement of the CO<sub>2</sub>–framework interactions and subsequently the attainment of an unprecedented CO<sub>2</sub> selectivity at very low CO<sub>2</sub> concentrations. The precise localization of the adsorbed CO<sub>2</sub> at the vicinity of the periodically aligned fluorine centers, promoting the selective adsorption of CO<sub>2</sub>, is evidenced by the single-crystal X-ray diffraction study on <b>NbOFFIVE</b>-1-Ni hosting CO<sub>2</sub> molecules. Cyclic CO<sub>2</sub>/N<sub>2</sub> mixed-gas column breakthrough experiments under dry and humid conditions corroborate the excellent CO<sub>2</sub> selectivity under practical carbon capture conditions. Pertinently, the notable hydrolytic stability positions <b>NbOFFIVE</b>-1-Ni as the new benchmark adsorbent for direct air capture and CO<sub>2</sub> removal from confined spaces.