Bonding of Polyethylenimine in Covalent Organic Frameworks for CO₂ Capture from Air

We have developed a polyethylenimine-functionalized covalent organic framework (COF) for capturing CO₂ from the air. It was synthesized by the crystallization of an imine-linked COF, termed imine-COF-709, followed by linkage oxidation and polyamine installation through aromatic nucleophilic substitution. The chemistry of linkage oxidation and amine installation was fully characterized through Fourier transform infrared spectroscopy, elemental analysis, and solid-state nuclear magnetic resonance (ssNMR) spectroscopy. Sorption isotherms and dynamic breakthrough were applied to study the sorption behavior of the resulting sorbent (COF-709). The COF exhibited a CO₂ capacity of 0.48 mmol g^-1 under dry conditions and 1.24 mmol g^-1 under 75% relative humidity, both from simulated air containing 400 ppm of CO₂ at 25 °C. The CO₂ capacity and adsorption rate of COF-709 showed a strong relationship with the relative humidity in the environment, in accordance with the CO₂ adsorption mechanism revealed by ssNMR. The chemical stability of C-S bonds utilized to covalently install the polyamine in COF pores prevented its amine loss and hydrolysis, giving COF-709 an excellent cycling stability, which was confirmed by applying 10 adsorption-desorption cycles under simulated direct air capture conditions, showing no uptake loss.