PNNL

Abstract

The covalent organic frameworks (COFs) possessing high crystallinity and capability to capture low-concentration CO2 (400 ppm) from air are still under-developed. The challenge lies in simultaneously incorporating high-density active sites for CO2 insertion and maintaining the ordered structure. Herein, a structure engineering approach was developed to afford an ionic pair-functionalized crystalline and stable fluorinated COF (F-COF) skeleton. The ordered structure of the F-COF was well maintained after the integration of abundant basic fluorinated alcoholate anions, as revealed by synchrotron X-ray scattering experiments. The breakthrough test demonstrated 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 study revealed the reaction enthalpy of about -40 kJ mol-1 between CO2 and the COF scaffolds. This implied weaker interaction strength compared with state-of-the-art amine-derived sorbents, thus allowing complete CO2 releasing with less energy input. The structure evolution study from synchrotron X-ray scattering and small-angle neutron scattering confirmed the well-maintained crystalline patterns after CO2 insertion. The as-developed proof-of-concept approach provides guidance on anchoring binding sites for direct air capture (DAC) of CO2 in crystalline scaffolds.