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.

Covalent organic frameworks and multicomponent reactions: an endearing give-and-take relationship

“…COFs are endowed with fascinating properties including superb crystallinity, this important trait can be influenced by reaction conditions comprising the feeding rate of monomers, addition of a modulator, catalysis by acids or bases, and the aging process. 22 COFs with long-range order structures, superb visible light absorbance, large surface areas, and tunable band gaps have exhibited outstanding performance in diverse applications, 23 including membranes, 24 optoelectronic devices, 25 gas separation and storage 26 sensing, 27 catalysis, 28 and biomedical applications. 29 Our quest has led us to identify COFs as a promising class of materials, which can be accessed via inherently greener and sustainable MCRs and deployed in eco-friendly environmental technologies as photocatalysts, membranes for gas, and adsorbents for heavy metals and dyes.…”

Multi-Scale Computer-Aided Design of Covalent Organic Frameworks for CO2 Capture in Wet Flue Gas

“…Li and coworkers 18 studied water adsorption and the impact on CO 2 capture by Grand canonical Monte Carlo (GCMC) simulations of 18 chemically stable COFs, in which COF-300 exhibits excellent resistance to high relative humidity (p/p 0 = 0.8) when used to capture CO 2 . Zhao and co-workers 19 estimated a series of Schiff-base COFs for wet flue gas separation. It is indicated that keto-COFs of NUS-2 and TpPa-1 outperform the imine-COFs; moreover, they exhibit excellent hydrothermal and chemical stability.…”

“…However, hydrophobic pores without sufficient polarity usually show poor separation performance on CO 2 / N 2 . Zhao and co-workers 19 suggested that COFs with moderate hydrophobicity would be promising adsorbent candidates for practical postcombustion CO 2 capture. An in depth and meticulous scientific issue is that what structural units can commonly make COFs show both high CO 2 /N 2 selectivity and CO 2 uptake in humid environments.…”

Polyacrylonitrile-Derived N-Doped Nanoporous Carbon Fibers for CO2 Adsorption

“…Beyond these criteria, one important capability of the CO 2 capture sorbents is the selectivity of carbon gas toward other gases such as H 2 O and N 2 , in particular in the flue gas streams. To date, as a consequence of their high surface area and favorable mesoporous/microporous structures, various nanoporous solid-based sorbents such as metal–organic frameworks (MOFs), − zeolites, covalent–organic frameworks (COFs), , porous carbons, − and porous polymers − have been discovered and assessed in terms of both the carbon capture capacity and selectivity. Among those solid-based adsorbents, nanoporous carbons are undeniably favorable materials not only in CO 2 capture but also in water treatment, methane storage, and catalysis due to their outstanding textural properties, overall abundance, and low cost. − Although porous carbons possess advanced textural features such as a large specific Brunauer–Emmett–Teller (BET) surface area, a large pore volume, and tunable porosity, they, however, have limited performance in CO 2 adsorption capacity and selectivity efficiency owing to the lack of CO 2 -philic functionalities. , Thus, it is a necessity to decorate the porous carbons’ surface to enhance carbon capture and separation activity.…”

“…Beyond these criteria, one important capability of the CO 2 capture sorbents is the selectivity of carbon gas toward other gases such as H 2 O and N 2 , in particular in the flue gas streams. To date, as a consequence of their high surface area and favorable mesoporous/microporous structures, various nanoporous solid-based sorbents such as metal−organic frameworks (MOFs), 9−12 zeolites, 13 covalent−organic frameworks (COFs), 14,15 porous carbons, 16−20 and porous polymers 21−23 have been discovered and assessed in terms of both the carbon capture capacity and selectivity. Among those solid-based adsorbents, nanoporous carbons are undeniably favorable materials not only in CO 2 capture but also in water treatment, methane storage, and catalysis due to their outstanding textural properties, overall abundance, and low cost.…”