In general, the presence of nonionic components in the DESs usually leads to the decrease in the solubility of CO2 due to the dilution effect, e.g., the uptake of CO2 (3.44 mol/kg) in pure [N2222][CH(CN)2] at 1.0 bar is larger than those in the five DESs (2.19 to 3.06 mol/kg). However, when the partial pressure is lower than 0.15 bar, it is interestingly noted that the absolute absorption of CO2in [N2222][CH(CN)2]-Eim is even slightly larger than that in pure [N2222][CH(CN)2]. In addition, when the partial pressure exceeds 0.2 bar, the absorption of CO2 in [N2222][CH(CN)2]-Eim levels off with the pressure, in contrary to the continuing absorption behavior of the pure IL. The two facts above imply that [N2222] [CH(CN)2], as the main absorbent in the DES, can react more efficiently with CO2 at low pressures (<0.15 bar) under the assistance of Eim. It is deduced that the efficient chemical absorption at low pressures may be due to the cooperative interactions among [N2222][CH(CN)2], CO2, and Eim, that requires careful investigation. It should be also pointed out that the DES [N2222][CH(CN)2]-Eim is particularly suitable for the capture of CO2 from flue gas due to its efficient chemical absorption at low partial pressures 38.
FIGURE 1. (a) Solubility of CO2 in [N1111][CH(CN)2], [N2222][CH(CN)2] and [N2222][CH(CN)2]-based DESs as a function of pressure at 303.2 K; (b) Dynamic absorption of CO2 under 1.0 bar of initial pressure and 303.2 K; (c) Five consecutive cycles of CO2absorption/desorption in [N2222][CH(CN)2] and [N2222][CH(CN)2]-Eim (absorption under 1.0 bar and 303.2 K, and desorption under 0.025 bar and 353.2 K).
The dynamic absorption of CO2 in the ILs and DESs is also investigated under 1.0 bar of initial pressure, 303.2 K, and constant gas-liquid area (Figure 1b, the detailed solubility data are given in Table S2 ). Impressively, the three Eim-, DMEE-, and SUL-based DESs, absorb CO2 quite faster (<20 min to reach absorption equilibrium) than the pure IL [N2222][CH(CN)2] (>70 min to reach equilibrium), and other two EG- and Im-based DESs (>50 min to reach equilibrium). In particular, the initial apparent absorption rate constant, K , in [N2222][CH(CN)2]-Eim is calculated to be the largest, about 9 times of that in [N2222][CH(CN)2] (2.35 vs. 0.26 min-1, see Table S3 ), and about 18% and 56% larger than those in [N2222][CH(CN)2]-DMEE and [N2222][CH(CN)2]-SUL, respectively (2.35 vs. 1.99 and 1.5 min-1). It indicates again that the cooperative chemical interactions exist in [N2222][CH(CN)2]-Eim-CO2. It is reasonable to postulate that Eim as a special hydrogen bond acceptor (HBA) can stabilize the system by extending the hydrogen bond network of the DES to include CO2.
In fact, the absorption rate is greatly influenced by the solubility of CO2 and the viscosity of absorbents. The larger solubility of CO2 can provide a better concentration difference to drive the dynamic absorption, and this can be used to explain the absorption rates in the DESs decreasing in the order of Eim>DMEE>SUL>Im (3.06, 2.77, 2.50, and 2.19 mol/kg corresponding to K values of 2.35, 1.99, 1.5 and 0.42 min-1). However, when the solubilities of CO2 in [N2222][CH(CN)2]-SUL and [N2222][CH(CN)2]-EG differ very little (2.50 vs. 2.59 mol/kg), the larger absorption rate in the former DES than that in the latter (1.50 vs. 0.61 min-1) has to be attributed to the smaller viscosity of the former DES (16.8 vs. 21.0 mPa·s). It should be particularly pointed out that the influence of viscosity on the rate prevails mainly during the absorption. At the beginning of the absorption, the pure IL and all DESs have in general small viscosities ranging from 27.6 to 12.7 mPa·s (see Table S3 ). However, the viscosity in the pure IL [N2222][CH(CN)2] increases very fast during the absorption so that a very high viscosity value (5023 mPa·s at 303.2 K) is obtained at the end of the absorption (see Table S4 ). Even though the elevation of temperature can dramatically lower the solution viscosities (Figure S2 and Table S4 ), the fast increase in the viscosity during the absorption is still a problem, i.e., the viscosity rises still from 13.6 to 575.6 mPa·s at the temperature as high as 333.2 K. This is why the absorption rate in [N2222][CH(CN)2] is the lowest in comparison with those in all DESs, even though [N2222][CH(CN)2] has the largest solubility of CO2. In contrary, the viscosities of [N2222][CH(CN)2]-Eim before and after absorption are 12.7 and only 115.7 mPa·s at 303.2 K,and 5.5 and only 24.4 mPa·s at 333.2 K (Table S4 ), justifying the fastest absorption rate in [N2222][CH(CN)2]-Eim due to the lowest viscosity.
Multiple cycles of CO2 absorption (under 1.0 bar and 303.2 K) and desorption (under 0.025 bar and 353.2 K) in [N2222][CH(CN)2] and [N2222][CH(CN)2]-Eim are investigated as examples to show the reversibility or recyclability of the absorbents (Figure 1c, the detailed data are given inTable S5 ). Since the viscosity of [N2222][CH(CN)2] after absorbing CO2 is as high as 5023 mPa·s, the desorption of CO2 is difficult, and the regeneration efficiency decreases gradually with the number of cycles, so that the absorption capacity after 5 cycles is only about 80% of its original value. As a comparison, the regeneration of [N2222][CH(CN)2]-Eim is much easy, and the absorption capacity in each cycle keeps nearly constant at about 92% of the primitive. The majority of 8% of capacity loss occurs in the first cycle, mainly due to the incomplete desorption of CO2 under the given conditions. In general, [N2222][CH(CN)2]-Eim has high regeneration efficiency and excellent reversibility, much better than the pure IL.