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.