Ionic Liquids / Deep Eutectic Solvents-Based Hybrid Solvents for CO 2 Capture

: The CO 2 solubilities (including CO 2 Henry’s constants) and viscosities in ionic liquids (ILs) / deep eutectic solvents (DESs)-based hybrid solvents were comprehensively collected and summarized. The literature survey results of CO 2 solubility illustrated that the addition of hybrid solvents to ILs / DESs can signiﬁcantly enhance the CO 2 solubility, and some of the ILs-based hybrid solvents are super to DESs-based hybrid solvents. The best hybrid solvents of IL–H 2 O, IL–organic, IL–amine, DES–H 2 O, and DES–organic are [DMAPAH][Formate] (2.5:1) + H 2 O (20 wt %) (4.61 mol / kg, 298 K, 0.1 MPa), [P 4444 ][Pro] + PEG400 (70 wt %) (1.61 mol / kg, 333.15 K, 1.68 MPa), [DMAPAH][Formate] (2.0:1) + MEA (30 wt %) (6.24 mol / kg, 298 K, 0.1 MPa), [TEMA][Cl]-GLY-H 2 O 1:2:0.11 (0.66 mol / kg, 298 K, 1.74 MPa), and [Ch][Cl]-MEA 1:2 + DBN 1:1 (5.11 mol / kg, 298 K, 0.1 MPa), respectively. All of these best candidates show higher CO 2 solubility than their used pure ILs or DESs, evidencing that IL / DES-based hybrid solvents are remarkable for CO 2 capture. For the summarized viscosity results, the presence of hybrid solvents in ILs and DESs can decrease their viscosities. The lowest viscosities acquired in this work for IL–H 2 O, IL–amine, DES–H 2 O, and DES–organic hybrid solvents are [DEA][Bu] + H 2 O (98.78 mol%) (0.59 mPa · s, 343.15 K), [BMIM][BF 4 ] + DETA (94.9 mol%) (2.68 mPa · s, 333.15 K), [L-Arg]-GLY 1:6 + H 2 O (60 wt %) (2.7 mPa · s, 353.15 K), and [MTPP][Br]-LEV-Ac 1:3:0.03 (16.16 mPa · s, 333.15 K) at 0.1 MPa, respectively. mol / kg). The CO 2 solubility in [TMGH][Im] + H 2 O at water content range of 1–7 wt %, and then, it decreased when the water content was larger than 7 wt % in [TMGH][Im], resulting in the best absorption capacity of 4.23 mol / kg for [TMGH][Im] + H 2 O (7 wt %). viscosity of [DEA][Bu] + H 2 O decreases with the increase of water content and temperature. Yasaka et al. found that the viscosities of [P 4444 ][HCOO] + H 2 O decreased with water contents from 25 (356 mPa · s) to 91 mol% (14.4 mPa · s), corresponding to an increase of the CO 2 solubility between the water conetnt of 25 and 50 mol%, and then, they decrease from 50 to 91 mol%, which is regarded as the typical property of carboxylate ILs [32]. Aghaie et al. measured the viscosities of [HMIM][Tf 2 N], [HMIM][FAP], and [BMIM][Ac] aqueous solutions at 298–333 K, 2 MPa, and water mass percentages of 0.1, 1, 2, 5, and 10 wt %, respectively [35]. The result indicates that water has a signiﬁcant e ﬀ ect on [BMIM][Ac] viscosity, e.g., the viscosity of [BMIM][Ac] decreased from 47.64 (pure IL) to 3.77 (10 wt % H 2 O) mPa · s at 333 K. However, for [HMIM][Tf 2 N] and [HMIM][FAP], their viscosities only slightly decrease at 0.1–10 wt % water. For example, the viscosity of [HMIM][FAP] at 333 K is 20.72 mPa · s, while it is 20.47 mPa · s for [HMIM][FAP] + H 2 O (10 wt %). Additionally, increasing the water amount in these three ILs results in the decrease of CO 2 solubility.


Introduction
CO 2 emission is an urgent issue due to its main contribution to global warming [1]. It has been evidenced that CO 2 capture is a promising route to mitigate CO 2 emissions, and in general, cost, energy demand, and environmental impact need to be considered when for selecting the potential CO 2 capture technologies [2]. At present, the absorption technology with 30 wt % MEA aqueous solution is the commercialized one. However, this technology with the corresponding solvent has the drawbacks of high energy demand (4.2 GJ/t CO 2 ), high cost ($0.19-1.31/t CO 2 ), low thermal and chemical stability, and high volatility and corrosion [3][4][5][6], which underlines the necessity for developing greener and more efficient solvent for CO 2 capture. combined with property measurements, making it necessary to update the latest research progress. Meanwhile, to the best of our knowledge, there is no review article for the DES-based hybrid solvents.
To fulfill this gap and to promote the technology development on CO 2 capture in IL/DES-based hybrid solvents, this review summarizes the CO 2 solubilities (including Henry's constants) and viscosities of IL-based hybrid solvents since 2016 and DES-based hybrid solvents since 2014 to avoid the repetition of the published reviews. Finally, the best candidates for IL/DES-based hybrid solvents were obtained and compared with each other.

ILs-Based Hybrid Solvents
Regarding the CO 2 Table 3. The full names of ILs-based hybrid solvents are displayed in Table S1.

IL-H 2 O
The effect of the addition of H 2 O in [DMAPAH][Formate] (0.5:1, 1.0:1, 2.0:1, 2.5:1) was studied by Vijayaraghavan et al. [30]. Except for the molar ratio of amine to acid of 0.5:1, CO 2 solubility initially increased up to a certain water amount of 20 wt % ( Figure 1); then, it steadily decreased as the H 2 O concentration increased. As shown in Figure 1, the best candidate for CO 2 [12]. The result indicates that CO 2 solubility first increased at a water content range of 1-7 wt %, and then, it decreased when the water content was larger than 7 wt % in [TMGH][Im], resulting in the best absorption capacity of 4. 23 [35]. The result indicates that the CO 2 solubility reduced by 45% in these three ILs aqueous solutions compared to the solubility of CO 2 in pure IL at 298 K and water content of 10 wt %. [Octanoate] were synthesized with different molar ratios of base to acid (0.5:1, 1.0:1, 2.0:1, and 2.5:1, respectively) and hybrid with MEA for CO 2 capture, respectively [30]. According to Table 1 and Figure 2, the hybrid solvents of the synthesized ILs with an additional MEA showed enhanced CO 2 solubility, which agrees with the studies from Zeng et al. [ In conclusion, (1) a certain amount of water in ILs (mainly for chemical-based ILs) can enhance the CO 2 solubility, due to the decrease in viscosity and the formation of new products (e.g., carbamate and bicarbonate). However, excess water in ILs corresponds to a low ILs concentration and results in the decrease of CO 2 solubility; (2) the IL-organic and IL-organic aqueous solution as absorbents exhibit remarkable CO 2 capture performances, including high absorption capacity and low desorption enthalpy. The organic molecular weight, type, and water content in ILs can affect their CO 2 capture performance. Based on the summarized result, the organic solvent with low molecular weight together with a certain amount of water is beneficial for capturing CO 2 ; (3) IL-MEA shows better CO 2 capture performance than that of IL-MDEA and IL-DMEE; additionally, the IL-amine based hybrid solvent has higher CO 2 solubility than that of IL-H 2 O and IL-organic hybrid solvents. The viscosities of IL-H2O hybrid solvents are given in Table 3. Figure 3  N] + PEO1000 was proposed for CO 2 capture due to the higher CO 2 solubility of the pure PEO1000 [43]. The viscosities for 18 kinds of ILs (imidazolium-and phosphonium-based) + TG at different of mole ratio of each of these ILs at 278.15-323.15 K and 0.1 MPa were studied [18], evidencing that the presence of TG can significantly decrease the viscosity, resulting in about 50 mPa·s for these hybrid solvents (Table 3).

IL-Amine
As shown in Table 3 The CO 2 solubility unit with mole scale for reference [45] is mol CO 2 /mol absorbent, while in [52] is mol CO 2 /mol amine.

DESs-Based Hybrid Solvents
The CO 2 solubility data for 33 kinds of DESs-based hybrid solvents, together with viscosities for six types of DESs-based hybrid solvents since 2016, and Henry's constants for 21 kinds of DES-based hybrid solvents since 2013 have been reported, as summarized in Tables 4 and 5. The full names of the studied components of DESs are given in Table S1.

DES-Organic
The CO 2 solubilities in DESs-organic hybrid solvents are given in  [25]. The result indicates that all of these three superbases can enhance the CO 2 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 CO 2 solubility. For the effect of these three superbases on different male ratio of DESs, it is found that   The ratio of DES-organic for reference [9] is mole ratio, while in [25] is volume ratio. The Henry's constant unit in reference of [54] is MPa, while it is kPa·m 3 ·kmol −1 for reference [55].

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 CO 2 . As shown in Table 4, using water as a hybrid solvent in glycerol-based DESs can significantly decrease the viscosity of the DES [9]. 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.

Comparison of CO 2 Solubility and Viscosity
The obtained best candidates of IL-H 2 O, IL-organic, IL-amine, DES-H 2 O, 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 CO 2 solubilies are higher than their pure IL/DES under the same condition.  (2.5:1). This result indicates that IL/DES-based hybrid solvents are remarkable ones for CO 2 capture. Additionally, the IL-based hybrid solvent shows better CO 2 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 CO 2 .

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.  (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.
Viscosity is the key factor for impeding the mass transfer of gas in absorbent [58].

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 www.mdpi.com/xxx/s1, Table S1: Full names and abbreviations of ILs, components of DESs and hybrid solvents.

Conflicts of interest:
The author declares no conflict of interest. Viscosity is the key factor for impeding the mass transfer of gas in absorbent [58]. [Cl]-GLY 1:2 not only decreases the viscosity but also improves the CO 2 solubility due to the increase of the mass transfer [61], while excess water in [BTMA][Cl]-GLY 1:2 results in a decrease of CO 2 solubility, which is in agreement with Li et al. [12].

Conclusions
This review summarizes the research work on developing ILs/DESs-based hybrid solvents (i.e., IL-H 2 O, IL-organic/organic aqueous solution, IL-amine, DES-H 2 O, and DES-organic) for CO 2 capture, including CO 2 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 CO 2 solubility. IL-amine based hybrid solvents are super to IL-H 2 O 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 CO 2 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 CO 2 capture. To develop the efficient IL/DES hybrid solvents for CO 2 capture, the following aspects are suggested for consideration to decrease the viscosity and increase the CO 2 solubility: (1) hybrid of functional ILs/DESs that have high CO 2 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 CO 2 capture capacity to ILs and DESs.
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.