Insertion of CO₂ 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 CO₂ capture because of their well-defined porosity, large surface area, and high stability. Current COF-based CO₂ capture is mainly based on a physisorption mechanism, exhibiting smooth and reversible sorption isotherms. In the present study, we report unusual CO₂ sorption isotherms featuring one or more tunable hysteresis steps with metal ion (Fe³⁺, Cr³⁺, or In³⁺)-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 CO₂ between the metal ion and the N atom of the imine bond on the inner pore surface of the COFs as the CO₂ pressure reaches threshold values. As a result, the CO₂ adsorption capacity of the ion-doped Py-1P COF is increased by 89.5% compared with that of the undoped Py-1P COF. This CO₂ sorption mechanism provides an efficient and straightforward approach to enhancing the CO₂ capture capacity of COF-based adsorbents, yielding insights into developing chemistry for CO₂ capture and conversion.