Ionic Pairs-Engineered Fluorinated Covalent Organic Frameworks Toward Direct Air Capture of CO₂

The covalent organic frameworks (COFs) possessing high crystallinity and capability to capture low-concentration CO₂ (400 ppm) from air are still underdeveloped. The challenge lies in simultaneously incorporating high-density active sites for CO₂ insertion and maintaining the ordered structure. Herein, a structure engineering approach is developed to afford an ionic pair-functionalized crystalline and stable fluorinated COF (F-COF) skeleton. The ordered structure of the F-COF is well maintained after the integration of abundant basic fluorinated alcoholate anions, as revealed by synchrotron X-ray scattering experiments. The breakthrough test demonstrates its attractive performance in capturing (400 ppm) CO₂ from gas mixtures via O$-$C bond formation, as indicated by the in situ spectroscopy and operando nuclear magnetic resonance spectroscopy using ¹³C-labeled CO₂ sources. Both theoretical and experimental thermodynamic studies reveal the reaction enthalpy of ≈-40 kJ mol^-1 between CO₂ and the COF scaffolds. This implies weaker interaction strength compared with state-of-the-art amine-derived sorbents, thus allowing complete CO₂ release with less energy input. The structure evolution study from synchrotron X-ray scattering and small-angle neutron scattering confirms the well-maintained crystalline patterns after CO₂ insertion. In conclusion, the as-developed proof-of-concept approach provides guidance on anchoring binding sites for direct air capture (DAC) of CO₂ in crystalline scaffolds.. Authors: Qiu, Liqi [Univ. of Tennessee, Knoxville, TN (United States)]; Lei, Ming [Vanderbilt Univ., Nashville, TN (United States)]; Wang, Caiqi [Washington State Univ., Pullman, WA (United States)]; Hu, Jianzhi [Washington State Univ., Pullman, WA (United States); Pacific Northwest National Laboratory (PNNL), Richland, WA (United States)]; He, Lilin [Oak Ridge National Laboratory (ORNL), Oak Ridge, TN (United States)]. DOE Contract: AC05-00OR22725; AC05-76RL01830; SC0012704. Subjects: 37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; Carbon Dioxide; Covalent Organic Frameworks; Direct air capture; Ionic liquids; Ionic pairs