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.