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.