Covalent organic frameworks for carbon dioxide capture from air

Direct air capture of carbon dioxide is an imperative component of the efforts to limit global warming. Capturing CO₂ from such a dilute source requires new materials with reactive CO₂-specific sites, but also room to tune the energy consumption and stability in order to reduce the cost of operation. We report the use of porous covalent organic frameworks (COFs) as solid-state platforms onto which reactive aliphatic amine species could be covalently bound and used for CO₂ capture from air. This was achieved through the crystallization of an imine-linked COF, termed COF-609-Im, followed by the conversion of its imine linkage to base-stable tetrahydroquinoline linkage through aza-Diels-Alder cycloaddition, and finally, the covalent incorporation of tris(3-aminopropyl)amine into the framework. The obtained COF-609 exhibits a 1,360-fold increase in CO₂ uptake capacity compared to the pristine framework, and a further 29% enhancement in the presence of humidity. We confirmed the chemistry of framework conversion and corroborated the enhanced CO₂ uptake phenomenon with and without humidity through isotope-labeled Fourier transform infrared spectroscopy and solid-state nuclear magnetic resonance spectroscopy. With this study, we show that the use of COFs offers advantages for the precision design of frameworks and modification of the chemical environment within their pores, and establish a new synthetic strategy to access a class of chemisorbents characterized by high affinity to CO₂ in dilute sources, such as the air.