Evaluation of Schiff-Base Covalent Organic Frameworks for CO₂ Capture: Structure–Performance Relationships, Stability, and Performance under Wet Conditions

Abstract

Covalent organic frameworks (COFs) have been considered promising adsorbent materials for postcombustion CO2 capture due to their high porosity, tunable functionalities, and excellent framework stability. Nevertheless, few research studies have systematically investigated the structure–performance relationships and the effect of moisture on CO2 capture performance of COFs. In this study, a series of Schiff-base COFs with different functionalities, pore sizes, and framework dimensions are prepared and evaluated for potential applications in postcombustion CO2 capture. Gas sorption isotherms and ideal CO2/N2 sorption selectivity calculations reveal the following: (1) COFs undergoing enol-to-keto transformations outperform other studied COFs with imine functionalities and similar pore sizes. (2) CO2 uptake capacity of a COF is not necessarily a function of its pore aperture and specific surface area. TpPa-1 with keto-enamine moieties exhibits an impressive CO2 uptake of 0.6 mmol g–1 and a CO2/N2 sorption selectivity of 114. Dynamic breakthrough experiments of wet CO2/N2 mixed gas (17% relative humidity) indicate that both keto-COFs studied, NUS-2 and TpPa-1, retain about 70% of their dry CO2 adsorption capacities, which can be attributed to the moderately hydrophobic pore environment of the COFs. Considering the noticeable cost of flue gas desiccation, our study suggests that COFs with moderate hydrophobicity would be promising adsorbent candidates for practical postcombustion CO2 capture.