3.1.1. DES–H2O

The CO2 solubility of DESs using water as a hybrid solvent was investigated by Sarmad et al. at 298.15 K and pressure up to 2 MPa (Table 4) [9]. From this study, the CO2 solubility of DES-based hybrid solvents can be affected by four factors. (1) The first is pressure: the CO2 solubility increased with the increasing pressure. For instance, the CO2 solubility of [TEMA][Cl]-GLY-H2O 1:2:0.11 increased from 0.025 to 0.66 mol/kg at a pressure range of 0.14 to 1.74 MPa. (2) The second factor is the mole ratio of water: the CO2 solubility of [TEMA][Cl]-GLY-H2O 1:2:0.05 first decreased at pressure range of 0.23–0.85 MPa, and then, it increased from 1.25 to 1.98 MPa compared with [TEMA][Cl]-GLY 1:2. Upon increasing the mole ratio of water, the CO2 solubility of [TEMA][Cl]-GLY-H2O 1:2:0.11 significantly enhanced with respect to [TEMA][Cl]-GLY 1:2 and [TEMA][Cl]-GLY-H2O 1:2:0.05. This result agrees with the DES-based hybrid solvent of [BTMA][Cl]-GLY-H2O 1:2:0.011 and 1:2:0.05. (3) The third factor is the type of hydrogen bond acceptor (HBA)—for example, the CO2 solubility of [TEMA][Cl]-GLY-H2O 1:2:0.05 (1.98 MPa, 0.66 mol/kg) > [BTMA][Cl]-GLY-H2O 1:2:0.05 (2.02 MPa, 0.29 mol/kg).

Harifi-Mood et al. investigated the Henry's constants for [Ch][Cl]-EG aqueous solution at temperatures of 303.15–323.15 K [54]. As shown in Table 5, the results indicate that the Henry's constant of CO2 increases with increasing water amount in the absorbent, corresponding to a decrease of CO2 solubility. This result agrees with the measured Henry's constants of [Ch][Cl]-EG, [Ch][Cl]-GLY, and [Ch][Cl]-MA aqueous solution from 303.15 to 313.15 K by Lin et al. [55].

#### 3.1.2. DES–Organic

The CO2 solubilities in DESs–organic hybrid solvents are given in Table 4. The result indicates that the addition of 0.03 mol of acetic acid in [MTPP][Br]-LEV 1:3 significantly enhanced the CO2 solubility and decreased the viscosity compared with [MTPP][Br]-LEV 1:3 [9].

A superbase can participate in the reaction of DES and CO2, thus increasing the CO2 solubility. Bhawna et al. studied the CO2 solubility by three hybrid superbases of TBD, DBN, and DBU with DESs of [Ch][Cl]-Urea 1:2 and [Ch][Cl]-EG 1:2, respectively [25]. The result indicates that all of these three superbases can enhance the CO2 solubility, and among them, TBD has the highest capacity, followed by DBU and DBN. The further addition of glycerol in these hybrid solvents decreased the CO2 solubility. For the effect of these three superbases on different male ratio of DESs, it is found that [Ch][Cl]-MEA 1:2 + DBN (5.11 mol/kg) > [Ch][Cl]-MEA + TBD 1:4 (3.91 mol/kg) > [Ch][Cl]-MEA 1:2 + DBU (3.54 mol/kg). Additionally, the same phenomenon was observed that the addition of glycerol in these [Ch][Cl]-MEA-based hybrid solvents can decrease the CO2 solubility.

An imidazole (Im)-derived DESs of [BMIM][Cl]-Im was synthesized for CO2 capture by hybrid with DBN [56]. These hybrid solvents show remarkable CO2 capture capacity up to 1.00 mol/mol, following the order of DBN-[BMIM][Cl]-Im 1:1:2 > DBN-[BMIM][Cl]-Im 1:1:1 > DBN-[BMIM][Cl]-Im 1:2:1. The theoretical calculation indicates that DBN plays a key role in the absorption process by forming a strong hydrogen bond with the derived [BMIM+-COO−].

In conclusion, the obtained best DES–H2O hybrid solvent is [TEMA][Cl]-GLY-H2O 1:2:0.11 (0.66 mol/kg, 298 K, 1.74 MPa), while it is [Ch][Cl]-MEA 1:2 + DBN 1:1 (5.11 mol/kg, 298 K, 0.1 MPa) for DES–organic hybrid solvent. These values are lower than the best IL–H2O and IL–organic hybrid solvents, respectively.


CO2solubilitiesandviscositiesofdeepeutecticsolvent(DES)-basedhybrid


**Table5.**Henry'sconstant of DES–H2Ohybridsolvents.


#### *3.2. Viscosity*

The DESs consisting of glycerol as the hydrogen bond donor (HBD) exhibited high viscosity. Meanwhile, their viscosities increased considerably with an increase in the amount of dissolved CO2. As shown in Table 4, using water as a hybrid solvent in glycerol-based DESs can significantly decrease the viscosity of the DES [9]. For example, the viscosity of [BTMA][Cl]-GLY 1:2 decreased from 1017.67 to 70.76 mPa·s when adding a 0.05 molar ratio of water in [BTMA][Cl]-GLY 1:2 (i.e., [BTMA][Cl]-GLY-H2O 1:2:0.05), but limiting the contribution of H2O to CO2 solubility. Meanwhile, increasing the water content of 0.11 mol in [BTMA][Cl]-GLY 1:2 results in a considerably reduced viscosity, which agrees with the results in the [TEMA][Cl]-GLY-H2O system. Additionally, the addition of 0.11 mol of water to the DES of [BHDE][Cl]-GLY 1:3 decreased the viscosity from 32.76 to 17.11 mPa·s at 298.15 K and 0.23–2.02 MPa, and it increased the CO2 solubility from 0.037 to 0.21 mol/kg. The viscosity of the [L-Arg]-GLY 1:6 as a function of water content from 0 to 60 wt % was measured, which indicates that viscosity of the DES decreased sharply with the increase of water contents, giving an option to lower the viscosity [57].

In a word, adding water and organic solvents in DES can significantly decrease the viscosity.

#### **4. Comparison of CO2 Solubility and Viscosity**

The obtained best candidates of IL–H2O, IL–organic, IL–amine, DES–H2O, and DES–organic hybrid solvents were compared with each other and their pure ILs and DESs (Figure 4). As shown in Figure 4, for either the IL-based or DES-based hybrid solvents, their CO2 solubilies are higher than their pure IL/DES under the same condition. For example, the CO2 solubility of [DMAPAH][Formate] (2.5:1) + H2O (20 wt %) is 4.61 mol/kg at 298 K and 0.1 MPa, while it is 2.32 mol/kg for [DMAPAH][Formate] (2.5:1). This result indicates that IL/DES-based hybrid solvents are remarkable ones for CO2 capture. Additionally, the IL-based hybrid solvent shows better CO2 capture performance compared with the DES-based hybrid solvent, as shown in Figure 4. Figure 5 gives the comparison of viscosities for these IL/DES-based hybrid solvents and pure IL and DES at 333.15 K and 0.1 MPa. As shown in Figure 5, the addition of hybrid solvents can significantly decrease the viscosity compared to pure ILs and DESs, which are beneficial to accelerate mass transfer during capturing CO2.

**Figure 4.** Comparison of CO2 solubility for IL and IL-based hybrid solvent, as well as DES and DES-based hybrid solvent.

**Figure 5.** Comparison of viscosity for IL and IL-based hybrid solvent, as well as DES and DES-based hybrid solvent at 333.15 K and 0.1 MPa.

Viscosity is the key factor for impeding the mass transfer of gas in absorbent [58]. For example, Gómez-Coma et al. investigated the viscosity and mass transfer performance of [EMIM][Ac] + H2O for CO2 capture [59], finding that the viscosity of [EMIM][Ac] + H2O decreased from 150 to 20 mPa·s with the increase of water content from 0–40 wt %. For the mass transfer coefficient, it is first increased from 1.7 <sup>×</sup> <sup>10</sup>−<sup>5</sup> to 9.34 <sup>×</sup> <sup>10</sup>−<sup>5</sup> <sup>m</sup>·s−<sup>1</sup> with the increasing of water from 0 to 30 wt % in [EMIM][Ac] and then decreased to 6.81 <sup>×</sup> 10−<sup>5</sup> m·s−<sup>1</sup> when water content up to 40 wt %. Huang et al. evidenced that the low viscosity of IL–MEA aqueous solution corresponds to a high mass transfer performance [60], i.e., [EMIM][Br] (20 wt %) + MEA (5 wt %) + H2O (75 wt %) (11.57 <sup>×</sup> 10<sup>6</sup> mol·m−3·s−1·Pa−1, 1.23 mPa·s) > [BMIM][Br] (20 wt %) + MEA (5 wt %) + H2O (75 wt %) (11.04 <sup>×</sup> 10<sup>6</sup> mol·m−3·s−1·Pa<sup>−</sup>1, 1.3 mPa·s) > [EMIM][Br] (30 wt %) + MEA (5 wt %) + H2O (65 wt %) (9.86 <sup>×</sup> 106 mol·m−3·s−1·Pa−1, 1.42 mPa·s) > [BMIM][Br] (30 wt %) + MEA (5 wt %) + H2O (65 wt %) (9.67 <sup>×</sup> 10<sup>6</sup> mol·m−3·s−1·Pa−1, 1.6 mPa·s). A similar phenomenon can be found in DESs hybrid solvent. Ma et al. indicated that a small amount of water in [BTMA][Cl]-GLY 1:2 not only decreases the viscosity but also improves the CO2 solubility due to the increase of the mass transfer [61], while excess water in [BTMA][Cl]-GLY 1:2 results in a decrease of CO2 solubility, which is in agreement with Li et al. [12].

#### **5. Conclusions**

This review summarizes the research work on developing ILs/DESs-based hybrid solvents (i.e., IL–H2O, IL–organic/organic aqueous solution, IL–amine, DES–H2O, and DES–organic) for CO2 capture, including CO2 solubility, Henry's constant, and viscosity. The results illustrate that the addition of hybrid solvents to ILs and DESs can decrease the viscosity and enhance the CO2 solubility. IL–amine based hybrid solvents are super to IL–H2O and IL–organic/organic aqueous solution, and some of the IL-based hybrid solvents show better performance than that of DES-based hybrid solvents. Additionally, some of the IL/DES hybrid solvents have higher CO2 solubility compared to their pure IL/DES, indicating that the addition of hybrid solvent to IL/DES is possible to develop greener and more efficient absorbents for CO2 capture. To develop the efficient IL/DES hybrid solvents for CO2 capture, the following aspects are suggested for consideration to decrease the viscosity and increase the CO2 solubility: (1) hybrid of functional ILs/DESs that have high CO2 solubilities with a certain amount of water; (2) the addition of organic solvent which has a small molecular weight to the ILs/DESs; and (3) applying amine solvent which has good CO2 capture capacity to ILs and DESs.

**Supplementary Materials:** The following is available online at http://www.mdpi.com/2073-4352/10/11/978/s1, Table S1: Full names and abbreviations of ILs, components of DESs and hybrid solvents.

**Author Contributions:** Writing—original draft preparation, Y.L.; investigation, Z.D., writing—review and editing, F.D., conceptualization and supervision, X.J. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work is financially supported by Carl Tryggers Stiftelse foundation (No. 18:175). X.J. thanks the financial support from the Swedish Energy Agency (No. P47500-1) and K. C. Wang Education Foundation (No. GJTD-2018-04). F.D. thanks the financial support from the National Nature Science Foundation of China (21808223).

**Conflicts of Interest:** The author declares no conflict of interest.
