Alkali Metal-Functionalized Covalent Organic Frameworks for CO₂ Adsorption and CO₂/N₂ Separation

Covalent organic frameworks (COFs) have become potential candidates for gas purification technology, especially in carbon capture and storage applications. By utilizing strategies such as pore structure design and functional modification, COFs have been developed to exhibit high selectivity and superior adsorption capacity for the effective capture and separation of CO2 from industrial waste gases. Notably, alkali metals exhibit strong interactions with the backbone of the COFs, which can enhance the CO2/N2 adsorption separation performance of these porous materials. This study details the synthesis of two-dimensional COF materials modified with acidic groups (TpPa-COOH and TpPa-SO3H) via mechanical grinding. Alkali metals such as Na, Li, and K were introduced into these groups through simple ion exchange reactions, thereby creating metal-modified COF materials. The impact of various alkali metals on the CO2/N2 adsorption separation performance of COF materials was investigated. Among the resulting products, lithiated TpPa-COOH showed the highest CO2 capture capacity of 2.65 mmol/g at 273 K and 1 bar. This may be due to the smaller atomic size of Li that offers larger adsorption space. Under the same conditions, the K-modified TpPa-SO3H exhibited the highest selectivity of 548, which surpasses most COFs reported in the literature. The superior performance was caused by the high polarization of the COF backbone triggered by the alkali metals. The inference was further confirmed by computational simulations. This work provides a facile method for the modification of porous materials with alkali metals to achieve superior gas separation performance.