1. INTRODUCTION
Carbon dioxide (CO2) is known as the main cause of global warming1,2, and the key to mitigating climate change is to cut off or capture its emission3,4. It remains a challenge to capture CO2 in a more effective and energy-efficient way5, and one common thought within the industry is to develop new agents to facilitate the process of CO2 capture6-8. Since specific ionic liquids (ILs) were validated as ideal CO2 capture agents 9 in 1999, ILs have become a focused research field in gas capture because of their unique characteristics10, such as negligible vapor pressure11, wide-range temperature steadiness, low corrosiveness 12 and physicochemical adjustability13-15.
Davis and his coworkers reported the first example of CO2 chemisorption using amino-functionalized ILs in their work 16. More amino-functional ILs were reported recently 17,18, such as imidazolium-based19, amino acid-based 20 and choline-based ILs 21. ILs with oxygen-containing functional groups have also attracted interests among the fields22, and some ILs with aldehydes and phenolic compounds as anions have been developed successively to possess a fairly good solubility of CO2 23. To the best of our knowledge, Dai group 24 reported the first and only example of using supernucleophilic carbanion as the interacting site of CO2 in November 2022. Malononitrile is deprotonated to be the carbanion in their paper, and the ILs so derived have a maximum CO2 uptake of 2.65 mol/kg at 298.2 K and 1.0 bar, close to the equimolar absorption of CO2. In fact, before the publication of Dai’s paper we had also been working on carbanion-based ILs using malononitrile as the starting material for more than four months. Even though the cations of our carbanion-based ILs are different, a similar absorption mechanism exists as in Dai’s work. However, the carbanion-based ILs are found to have large viscosities, especially during the absorption of CO2. The absorption of CO2 is also quite slow due to the high viscosity 25,26 and the ILs after absorption turn into gel at 1.0 bar, suggesting that the pure carbanion-based ILs are not practical as absorbents, even though they have the excellent absorption mechanism resulting from the supernucleophilic nature of carbanion site.
As an environmental benign analogue of ILs, deep eutectic solvents (DESs) can overcome the shortcomings of ILs due to the presence of nonionic components in DESs 27,28. To date, more DESs with N and/or O as the interacting site(s) for CO2 have been reported 29,30. Among them, Yan et al. reported that [HDBU][Im]/ethylene glycol (EG) with a molar ratio of 7:3 had a CO2 uptake of 3.2 mol/kg at 313.2 K and 1.0 bar, and demonstrated that the absorption process in [HDBU][Im]/EG has a synergistic action of N and O sites, producing a mixture of carbamate and carbonate products 31. In comparison with monoethanolamine (MEA) and ethylenediamine (EDA)32, the desorption of CO2 from [HDBU][Im]/EG is superior although the absorption capacity is slightly lower. The efforts above demonstrate that DESs have their own extraordinary features, such as negligible volatility33, high CO2 affinity34, low viscosity 35,36, and biocompatibility 37 owing to their unique hydrogen bonding network structure 38. Hence the question is, can the carbanion-based ILs be developed into carbanion-based DESs for efficient CO2 capture? The answer is positive.
In this paper, a promising energy-efficient CO2 capture system is proposed using carbanion-based ILs for the construction of DESs. The essence of our strategy is to exploit the supernucleophilic nature of carbanion for siting and attaching to CO2, and the rich hydrogen bonding network in the DESs for easy stabling and decomposing of DES-CO2 adducts, so that fast and high absorption/desorption of CO2 can be realized in an energy-saving manner.