3.1 CO2 Capture Performance and Regeneration
The equilibrium absorption of CO2 in carbanion-based ILs
and DESs was first investigated as a function of partial pressure and is
shown in Figure 1a(the
detailed solubility data are given in Table S1 ). The same
methodology as in our previous work was used for the measurements20,40,41. In general, the basicity of a species has a
positive influence on its uptake of CO2. Due to the fact
that [CH(CN)2]- is the
supernucleophilic anion with high basicity (pKa=11)24, the two ILs,
[N2222][CH(CN)2] and
[N1111][CH(CN)2], are endowed
with high absorption capacities as expected (3.44 and 3.07 mol/kg at
303.2 K and 1.0 bar, respectively).
[N2222][CH(CN)2] has an
apparently larger uptake of CO2 than
[N1111][CH(CN)2], primarily due
to the fact that the larger spatial structure of
[N2222]+ cation provides more free
volume for the accommodation of CO2.42However, when an even larger cation such as
[P66614]+ is used, the absolute
absorption of CO2 in
[P66614][CH(CN)2] decreases to
2.65 mol/kg 24, even though equimolar absorption (a
relative value) is approximately realized. It demonstrates that the
cation has also an important influence on the absorption of
CO2 in the carbanion-based ILs, and that
[N2222]+ is better than
[P66614]+ and
[N1111]+ in improving the absolute
solubility.
In comparison with the carbanion-based ILs, the five carbanion-based
DESs behave quite differently in the absorption of CO2,
and the nonionic components paired with
[N2222][CH(CN)2] in the DESs
play the key roles. 1-ethylimidazole (Eim), as the hydrogen-bond
acceptor (HBD) of weak basicity (pKa=7.1), enables the
best
absorption of CO2 in
[N2222][CH(CN)2]-Eim
(3.06 mol/kg at 1.0 bar and 303.2 K), whereas imidazole (Im), as the
species of both acidity and basicity, behaves the worst in absorbing
CO2 into
[N2222][CH(CN)2]-Im (2.19
mol/kg). 2-(2-dimethylamino-ethoxy)-ethanol (DMEE) is the tertiary amine
(pKa=8.95) that can assist
[N2222][CH(CN)2] to absorb
CO2 satisfactorily (2.77 mol/kg), better than the two
neutral species, ethylene glycol (EG) and sulfolane (SUL), in the
absorption (2.59 and 2.50 mol/kg). The uptakes of CO2 in
the five DESs follows the order of Eim>DMEE>
EG>SUL>Im that cannot be interpreted only from
the basicity of the nonionic components.