Insertion of CO <sub>2</sub> in metal ion-doped two-dimensional covalent organic frameworks
Chengjun Kang, Zhaoqiang Zhang, Shibo Xi, He Li, Adam K. Usadi, David C. Calabro, Lisa Saunders Baugh, Yuxiang Wang, Dan Zhao
Abstract
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.