Insertion of CO(2) in metal ion-doped two-dimensional covalent organic frameworks

Carbon capture is one of the essential low-carbon technologies required to achieve societal climate goals at the lowest cost. Covalent organic frameworks (COFs) are promising adsorbents for CO(2) capture because of their well-defined porosity, large surface area, and high stability. Current COF-based CO(2) capture is mainly based on a physisorption mechanism, exhibiting smooth and reversible sorption isotherms. In the present study, we report unusual CO(2) sorption isotherms featuring one or more tunable hysteresis steps with metal ion (Fe(3+), Cr(3+), or In(3+))-doped Schiff-base two-dimensional (2D) COFs (Py-1P, Py-TT, and Py-Py) as adsorbents. Synchrotron X-ray diffraction, spectroscopic and computational studies indicate that the sharp adsorption steps in the isotherm originate from the insertion of CO(2) between the metal ion and the N atom of the imine bond on the inner pore surface of the COFs as the CO(2) pressure reaches threshold values. As a result, the CO(2) adsorption capacity of the ion-doped Py-1P COF is increased by 89.5% compared with that of the undoped Py-1P COF. This CO(2) sorption mechanism provides an efficient and straightforward approach to enhancing the CO(2) capture capacity of COF–based adsorbents, yielding insights into developing chemistry for CO(2) capture and conversion.