Capture of Acidic Gases from Flue Gas by Deep Eutectic Solvents

: Up to now, many kinds of deep eutectic solvents (DESs) were investigated for the capture of acidic gases from ﬂue gases. In this review, non-functionalized and functionalized DESs, including binary and ternary DESs, for SO 2 , CO 2 and NO capture, are summarized based on the mechanism of absorption, physical interaction or chemical reaction. New strategies for improving the absorption capacity are introduced in this review. For example, a third component can be introduced to form a ternary DES to suppress the increase in viscosity and improve the CO 2 absorption capacity. DESs, synthesized with halogen salt hydrogen bond acceptors (HBAs) and functionalized hydrogen bond donors (HBDs), can be used for the absorption of SO 2 and NO with high absorption capacities and low viscosities after absorption, due to physicochemical interaction between gases and DESs. Emphasis is given to introducing the absorption capacities of acidic gases in these DESs, the mechanism of the absorption, and the ways to enhance the absorption capacity.


Introduction
Acidic gases, such as sulfur dioxide (SO 2 ), nitric oxide (NO) and carbon dioxide (CO 2 ), are the main pollutants of fuel gases, which are mainly released from the combustion of fossil fuels (coal, oil, and natural gas), posing a significant threat to the environment by forming acid rain, ozone destruction, global warming and so on. Due to the high hazards of these acidic gases, various methods have been developed to reduce their emissions, such as lime−limestone flue gas desulfurization process for SO 2 capture [1], carbon dioxide capture and sequestration process for CO 2 capture [2], selective catalytic reduction, and selective non-catalytic reduction for NO conversion [3,4]. Nevertheless, the drawbacks of them are obvious, such as volatility of absorbents, catalyst poisoning, substantial energy consumption and corrosiveness, the generation of huge amounts of byproduct and wastewater, which may generate secondary pollution to the environment if not properly treated. Therefore, it is necessary to capture the acidic gases with high efficiency, low cost, no by-products and high sustainability. Some work has been focused on the solutions to the above problems, among them one of the most effective and feasible ways is to develop novel recyclable green absorbents with high absorption efficiency and low desorption energy consumption.
Over the past two decades, ionic liquids (ILs) have received the most extensive attention in the field of gas absorption, owing to their unique characteristics. The promising advantages of ILs that can be used for gas capture are their negligible vapor pressures and their design ability by changing the structures of their cations and anions to meet different needs. The lower vapor pressure cannot only ensure no contamination of the gas stream but also lower the energy consumption and solvent regeneration. Hence, considerable efforts have been made to design novel ILs able to solubilize acidic gases physically and/or chemically [5][6][7]. For non-functionalized ILs that are able to dissolve acidic gases physically,

Non-Functionalized DESs for SO 2 Capture
The S atom in SO 2 is sp2 hybridized to form two σ bonds with two O atoms, which is a V-shaped geometric structure. The uneven distribution of positive and negative charges between the sulfur atom and the two oxygen atoms resulting in a great net dipole, which means that its dipole-dipole interaction is very strong. According to the "law of similarity and intermiscibility" principle, SO 2 can be easy absorbed by polar solvents through physical interaction. Thus, selecting polar organics as HBDs or HBAs is one way to improve the absorption capacity of DESs.

Caprolactam-Based DESs
In 2010, Duan et al. [29] synthesized a series of DESs by mixing caprolactam (CPL) and tetrabutyl ammonium bromide (TBAB) with different molar ratios. The solubility of SO 2 in CPL:TBAB DES (1:1) is about 0.1 g SO 2 /g DES at 298.2 K and 101 kPa. They found the solubility of SO 2 decreased as both temperature and the molar ratio of CPL to TBAB increased, due to CPL is a weakly polar molecule. Then, Liu et al. [30] investigated a series of CPL-based DES with more polar organic acids and bases, such as acetamide (AA), furoic acid, benzoic acid, o-toluic acid. CPL: AA DES with a molar ratio of 1:1 presents the highest SO 2 solubility of 0.497 g SO 2 /g DES at 303.15 K and 101 kPa. 1 H NMR analysis confirmed that CPL: AA DES absorbed SO 2 through physical interaction. The SO 2 absorption capacity of CPL:AA DES is higher than that of CPL:organic acids DESs. After that, they developed an inorganic thiocyanate-based DESs paired with organic bases to improve the SO 2 absorption capacity [31], based on the report that thiocynate and dicyanide anions in ILs play an important role in the physical capture of SO 2 [32,33]. CPL:KSCN DES (3:1) possessed SO 2 solubility of 0.607 g SO 2 /g DES at 293.15 K and 101 kPa. The DESs could be easily regenerated under vacuum at 343.15 K and recycled for five times without an apparent decline of the absorption efficiency.

DESs Containing Halogen Anion
Simulating works were conducted to find a criterion that can explain the solubility order of SO 2 at 101 kPa in ILs. Elliott et al. [34] suggested an SO 2 affinity order of Br − ≈ [NO 3 ] − > [NTf 2 ] − > [BF 4 ] − > [PF 6 ] − with a cation of [C 4 C 1 im] + . On the basis of the solvation free energy and binding energies of SO 2 in ILs, Mondal et al. [35] inferred the SO 2 solubility order as Cl − > [BF 4 ] − > [NO 3 ] − > [PF 6 ] − > [NTf 2 ] − . Notably, simulation results suggested that halogen anions have stronger potential to physically capture SO 2 than any other anions.
Han et al. [36] investigated the SO 2 absorption capacities of ChCl:glycerol(Gly) DESs with different molar ratios (from 1:1 to 1:4). The absorption amount is decreased with the increase of the molar ratio of Gly to ChCl, which implies that ChCl is a major component of absorption. For example, the solubility of SO 2 in ChCl:Gly DES (1:1) is 0.678 g SO 2 /g DES, which is much higher than that of ChCl:Gly DES (1:2, 0.482 g SO 2 /g DES), at 293. 15 K and 101 kPa. The entire absorption process was completed in 10 min and the absorbed SO 2 could be easily released at a relatively low temperature of 323.15 K within 20 min. 1 H and 13 C NMR analyses were conducted to investigate the mechanism of absorption. The investigation concluded that ChCl: Gly DESs interacted with SO 2 physically, as there is no new peak and no significant peak shift after SO 2 capture. However, it still retained the absorptive capacity of 0.153 g SO 2 /g DES at 10 kPa, which is much better than that of most physical absorption DESs and attracted extensive attention.
Ab initio molecular dynamics (AIMD) and density functional theory (DFT) have been used to investigate the interaction between SO 2 molecules and ChCl:Gly DESs [37,38]. A strong interaction between Cl − and SO 2 was observed by different methods. The results showed that an electron is transferred from ChCl: Gly DES to SO 2 during the direct interaction between S and Cl atoms, and accompanied a disruption of the anion-OH network and nonpolar-nonpolar network, which can explain the dramatic drop in viscosity. Charge transfer interaction is believed to play an important role in SO 2 absorption. The interaction energies were calculated by the different models and the value ranged from −28.45 to −60.25 kJ/mol, which is a moderate interaction for SO 2 absorption. It means that the interactions are very sensitive to the solvent structure and easy to regenerate, which is consistent with the low regeneration temperature and high desorption rate in Han's work [36].
Due to the excellent property and high capacity of SO 2 , more and more DESs containing halogen anions were designed and synthesized. Wei et al. [39] (1:2) was more than two times higher than that of ChCl:thiourea DES (1:1), which means that hydroxyl functional groups are more conducive to absorbing SO 2 under a low pressure. Deng et al. [40] further improved the absorption capacity of DESs by selecting a more polar organic acid levulinic acid (LA) as HBD compared to polyols, paired with a variety of HBAs. Such as choline acetyl chloride (ChAC), tetraethylammoniumchloride (TEAC), tetrabutylammonium chloride (TBAC), 1-ethyl-3-methylimidazolium chloride (EmimCl). The effect of these HBAs on absorption capacity was ordered as TEAC > ChAC > ChCl > TBAC.

Imidazolium-Based DESs
It is reported that imidazolium-based ILs exhibited considerable SO 2 absorption capacity. Therefore, Yang et al. [41][42][43] designed a series of EmimCl-based DESs, including EmimCl:EG DES (1:1), EmimCl:triethylene glycol(TEG) DES (1:1), and EmimCl:succinonitrile (SN) DES (1:1), for efficient SO 2 capture and obtained the corresponding absorption capacity of 1.03, 0.91, and 1.13 g SO 2 /g DES, respectively, at 293.15 K and 101 kPa. It showed that the efficiency of imidazolium cation is better than that of ammonium. This can be interpreted as the π· · · π interaction between SO 2 and imidazole ring. The effect of HBD in DESs for SO 2 capture was ordered as succinonitrile > EG > TEG. The SO 2 solubility in EmimCl:TEG DESs with different molar ratios (from 6:1 to 1:1) were determined, as shown in Figure 1. The SO 2 uptake in EmimCl:TEG DESs increased by~24% from 1:1 to 4:1, whereas only 4% increment was observed at a 6:1 ratio. When the molar ratio is larger than 1:4, the absorption capacity remains almost unchanged. It was demonstrated that there is a limit to increase the absorptive capacity by the increase of the molar ratio of effective absorption components. Considering the cost and absorptive capacity, there is an optimal molar ratio. With the optimal molar ratio, the absorption capacities of EmimCl:EG DES (1:2) and EmimCl:TEG DES (1:4) were 1.03 and 1.20 g SO 2 /g DES, respectively, at 293.15 K and 101 kPa. It is especially noted that EmimCl:SN DES (1:1) can capture 0.120 g SO 2 /g DES at 2000 ppm, while EmimCl:EG DES (1:1) can only absorb 0.047 g SO 2 /g DES at the same conditions. Furthermore, all the SO 2 captured by EmimCl:EG DES (1:2) can be desorbed at 313.15 K within 50 min, which is lower than the desorption temperature of EmimCl:SN DES (353.15 K) and EmimCl:TEG (373.15 K). The effect of HBAs and HBDs in DESs was further investigated by Tantai et al [44]. As shown in Figure 2, the gravimetric SO 2 absorption capacities of ethylenurea (EU) and 1,3dimethylurea (DMU)-based DESs with various HBAs displayed the same trend: EmimCl > BmimCl > BmimBr > TBAC~P 4444 Cl. Among them, the DESs with imidazolium salts as HBAs exhibited lower viscosities than those containing TBAC or P 4444 Cl. After SO 2 absorption, the viscosities of all DESs (188~389 mPa·s) decreased significantly to below 8 mPa·s, which can be attributed to the disruption of the anion-OH network and nonpolarnonpolar network by the strong interaction between SO 2 and Cl − . Moreover, it was also noted that when the SO 2 partial pressure decreased to 2 kPa at 298.15 K, the SO 2 absorption capacities of EU:BmimCl DES (1:2) could still be retained at 0.25 g SO 2 /g DES. Using the unit of mol SO 2 /mol DES, some intrinsic conclusions could be drawn. For instance, the gravimetric absorption capacities of SO 2 in EU-based DESs increased, following the order of EU:BmimCl (1:2) > EU:BmimCl (1:1) > EU:BmimCl (2:1) with the values of 1.18, 1.07, and 0.94 g SO 2 /g DESs, respectively, while the molar capacities of SO 2 in EU:BmimCl DES (1:2), EU:BmimCl DES (1:1), and EU:BmimCl DES (2:1) were 8.03, 4.38, and 5.10 mol SO 2 /mol DESs, respectively. It could be concluded that both HBDs and HBAs took an active part in SO 2 capture. The effect of the alkyl chain length of imidazole-based DESs on SO 2 absorption capacity was investigated by Hu et al. [45]. The result showed that the absorption capacity of SO 2 in a mass unit decreased with the increase of alkyl chain length, which may be due to the increase of molecular weight. However, this order would be totally different if the unit was changed into mol SO 2 /mol DES, i.e., EmimCl: AA > BmimCl:AA > HmimCl:AA with the values of 1.25, 1.17, and 1.02 g SO 2 /g DESs, corresponding to 4.01, 4.28, 4.16 mol SO 2 /mol DES. EmimCl:AA performed the highest gravimetric absorption capacity and BmimCl:AA performed the highest molar absorption capacity. However, the effect of alkyl chain length on absorptive capacity became a minor factor with the decrease of SO 2 partial pressure. The absorptive capacities of SO 2 in EmimCl:AA DES (1:1), BmimCl:AA DES (1:1), and HmimCl:AA DES (1:1) were 0.46 (1.48), 0.44 (1.61), 0.38 (1.55) g SO 2 /g DESs (mol SO 2 /mol DES) at 101 kPa. A series of DESs paired EmimCl with different HDBs was investigated [46,47], including EmimCl:DMU, EmimCl:N-methylurea (MU), EmimCl:thioacetamide(TAA), EmimCl:CPL, and EmimCl:N-Formylmorpholine (NFM). The absorption properties of SO 2 in terms of gravimetric absorption are very close in those DESs, distributed from 0.36 to 0.45 g SO 2 /g DES at 10 kPa and from 1.09 to 1.26 g SO 2 /g DES at 101 kPa, 293.15 K, as shown in Table 1. In general, HBAs of EmimCl-based DESs have a much stronger influence on the solubility of SO 2 than the HBDs, which played a vital role in absorption and leaded to the similar absorption capacity of EmimCl-based DESs. Kang et al. [48] developed a new type of DES formed by EmimCl and N-ethylpyridinium bromide (EPyBr), in which halogen anions were introduced into both HBA and HBD. Although the viscosity of EmimCl:EPyBr was 2260 mPa·s before absorption, the viscosity dropped rapidly from 2260 to 6 mPa·s with the dissolution of SO 2 , owing to the strong charge transfer interactions between SO 2 and the anions Cl − and Br − . The SO 2 absorption capacity of EmimCl:EPyBr with a molar ratio of 3:1 was 0.698 g SO 2 /g DES at 293.15 K and 10 kPa, which was higher than the results of most DESs and ILs reported previously. Moreover, it could still absorb 0.170 g SO 2 /g DES at 293.15 K and an SO 2 concentration of 2000 ppm and the regeneration of EmimCl:EPyBr could be achieved at 363.15 K within 45 min by blowing N 2 .

Functionalized DESs for SO 2 Capture
There are two methods to obtain functionalized DESs for SO 2 capture. Due to the strong acidity of SO 2 , it is expected to reach a high capacity by selecting organic compounds with alkaline functional groups as HBA or HBD, such as alcohol amines, polyamines, and azoles. Using conjugate bases of weak acids as HBAs, such as carboxylated and phenolated ones, is another way to enhance SO 2 solubility. Basicity is significant during the structure property designing of DESs for enhancing SO 2 solubility. It is reported that SO 2 capture by DESs could be tuned by the basicity of components. Recently, by mixing the halogen salt HBAs which can efficiently absorb SO 2 through physical interaction mentioned above and functionalized HBDs can achieve higher absorption capacity and desorption rate than DESs, which relies mainly on physical or chemical absorption. The reported functionalized DESs for SO 2 capture and their SO 2 capacities at different conditions are included in Table 2.

Azole-Based Functionalized DESs
Azoles, such as imidazole (Im), triazole (Tri), tetrazole (Tetz), are definitely able to interact with SO 2 based on Lewis acid-base reaction in a form of N··S bonding. Due to the excellent property and high capacity of SO 2 capture, numerous azole-based functionalized DESs were designed and synthesized. In 2013, Liu et al. [29] reported CPL:Im DES (1:1). The solubility of SO 2 in CPL:Im DES (1:1) is 0.624 g SO 2 /g DES at 303.15 K and 101 kPa. During the absorption process, the transplant and colorless CPL:Im DES (1:1) turned to a yellowish paste, which can be interpreted by the chemical reaction between imidazole and SO 2 . Deng et al. [49] synthesized the azole-based DES with acetyl choline chloride (ACC) as HBA. The results showed that the structure of azole has evident influence on the SO 2 absorption capacity. As shown in Table 2, ACC:Im DES possessed higher absorption capacity of SO 2 than ACC:Tri DES, which is related to the higher basicity of the imidazole than that of 1,2,4-triazole. ACC:Im DES with molar ratio of 1:3 exhibited the highest absorption capacity of 0.383 g SO 2 /g DES at 0.1 bar and 303.15 K. The higher absorption capacity of ACC:Im than that of Im:Gly [50] could be attributed to the stronger physical absorption of chloride anion. However, the two DES performed differently in desorption experiments. For the DES of ACC:Tri (1:1), the absorbed SO 2 could be completely stripped out by bubbling N 2 at 363.15 K, while there was approximately 0.025 g SO 2 /g DES still remained after regeneration for ACC:Im DES (1:2).
The effect of substituents on imidazole ring was studied by Wu et al. [50]. They designed Im:Gly, 2-methylimidazole (2-Me):Gly, 2-ethylimidazole (2-Et):Gly and 2-propylimidazole (2-Pr):Gly for efficient SO 2 capture at 313.15 K, 2 kPa and obtained the corresponding absorption capacity of 0.253, 0.246, 0.241, 0.225 g SO 2 /g DES. The result demonstrated that the alkyl connected to the N(3) atom has almost no effect on the molar absorption capacities which reach 1:1 stoichiometry. However, with the increase of the molecular weight of HBAs, the mass absorption decreased. The SO 2 absorption mechanism was proposed after the saturated system was characterized by 1 H NMR and FTIR, the acidic SO 2 reacted with the basic N atom in Im through strong acid-base reaction, which leaded to an increase in viscosity from 149 mPa·s to 470 mPa·s for Im:Gly DES (1:2) at 313.15 K and incomplete desorption. Considering that flue gas contains a large amount of water (e.g., 8 vol %), which does not just act as a hydrogen bond donor such as Gly, but also can increase the absorption capacity per mass unit and significantly reduce the viscosity due to its low molecular weight, Wu et al. [51] mixed 2-Me:Gly, 2-Et:Gly and 2-Pr with H 2 O to synthesize three kinds of cheap and low viscosity functional DESs. The absorption results showed that the mole absorption capacities of three DESs were all about 1 mol SO 2 /mol HBA at 313 K and 2 kPa. Im:H 2 O DES (1:4) showed a mass absorption capacity of 0.331 g SO 2 /g DES, which is higher than that of most DESs reported in the literature.

Other Kinds of Functionalized DESs
Cui et al. [54] designed a series of functionalized DESs based on piperazinium cation, which were prepared by mixing 1-hydroxyethyl-1,4-dimethyl-piperazinium bromide (PPZBr) with Gly at different molar ratios. The absorption capacities of PPZBr:Gly with different molar ratios were ranged from 0.08 to 0.1 g SO 2 /g DES at 293.15 K and 1 kPa, corresponding to 0.96~0.99 mol SO 2 /mol DES, which indicated that the molar ratio did not significantly affect the adsorption capacity under low SO 2 partial pressures. The change of viscosity along with the absorption of SO 2 under atmospheric conditions was investigated, as shown in Figure 4. It can be seen that the viscosity of each PPZBr:Gly DES dramatically increased in first 5 min and then decreased sharply with a further increase of SO 2 dissolution. This phenomenon is similar to the finding of Wu et al. [55] and Zhang et al. [56] on SO 2 absorption by ILs. The results of viscosity experiments proved that PPZBr:Gly efficiently absorbed SO 2 through a combination of chemical and physical interactions. Wu et al. [57] reported a new type of internal salt type DES based on betaine (Bet) or L-carnitine (L-car), with the ability to chemically absorb SO 2 by strong acid-base reactions between the acidic SO 2 and Lewis-based COO − . The absorption capacity of L-car:EG with a molar ratio of 1:3 was 0.151 g SO 2 /g DES at 2 kPa, 313.15 K. However, due to the strong acid-base reactions between the acidic SO 2 and Lewis-based COO − , the absorbed SO 2 cannot be completely released.

Non-Functionalized DESs for CO 2 Capture
In 2006, Zhu et al. [24] reported that ChCl:urea DES supported on molecular sieves could effectively catalyze the reactions of CO2 and epoxides. After that a significant number of studies were conducted on the capacity of the choline chloride-based DES for CO 2 capture [58][59][60]. Lu et al. [61] reviewed the choline-based DESs for CO 2 separation recently. Thus, CO 2 capture by choline chloride-based DESs is not included in this review. Since CO 2 is a linear non-polar molecule, it is difficult for CO 2 to dissolve in DESs at atmospheric pressure. According to the reported results, high capacity of CO 2 physical absorption in non-functionalized DESs is favored with lower temperatures and higher pressures. Later, different combinations of HBDs and HBAs components were tried to optimize the CO 2 uptake capacity in DESs but the capacities of all the tested DESs remained in the range of 0.009-0.85 mol CO 2 /kg DES.
Ali et al. [62] studied 17 different types of DESs based on different ammonium and phosphonium salts with five different HBDs, namely EG, Gly, diethanolamine (DEA), triethanolamine (TEA), and monoethanolamine (MEA), for CO 2 capture. The solubility values for CO 2 in DESs at 298.15 K and pressures up to 1.0 MPa remained in the range of 0.013-0.0749 g CO 2 /g DES and 0.0211-0.1441 mol CO 2 /mol DES, as shown in Table 3. The experimental results demonstrated that the DESs have intricate physical behaviors compared to pure HBA or HBD. Thus, the absorption capacity cannot be simply predicted by considering the contribution effect of its components. For instance, ChCl:EG DESs had lower CO 2 solubility compared to pure EG, while the CO 2 solubility in glycerol-based DESs was found to be higher than that in pure glycerol. For ChCl:MEA DES (1:6), the solubility was 0.1096 mol CO 2 /mol DES, which was nearly two times higher than that in a 30 wt% MEA aqueous solution at 298.15 K and 1 MPa. However, the gravimetric absorption of DES was 0.0749 g CO 2 /g DES, which was much lower than 0.62 g CO 2 /g DES in the 30 wt% MEA aqueous solution. Moreover, high-performance liquid chromatography (HPLC) analysis results showed that only 10% of amine reacted with CO 2 while for the aqueous solution of MEA, all of amine reacted with CO 2 . This means that the strong intermolecular hydrogen bonding was formed between Cl anions and MEA, which may hinder MEA from reacting with CO 2 .
Sarmad et al. [63] synthesized 35 DESs, in order to understand how HBD, HBA and other factors affect the properties of DESs at 298.15 K and pressures of up to 2 MPa, as shown in Table 3. For all DESs, the solubility increased with the increase of pressure, as is typically expected for the physical solubility of gases in liquids, i.e., the CO 2 solubility is proportional to its partial pressure. The effect of different types of HBD with hydroxyl group or carboxylic group on the CO 2 solubility was ordered as: .584 mol CO 2 /kg DESs, respectively, and they followed the trend: TBAB:MEA > TPAC:MEA > ChCl:MEA. Additionally, increasing the alkyl chain length of HBA or HBD could lead to an increase of CO 2 solubility. For example, the solubilities of CO 2 in TEAC:AC DES (1:2) and TBAC:AC DES (1:2) were 1.177 and 1.411 mol CO 2 /kg DESs, which increased as the alkyl chain length increased from ethyl to butyl. By increasing the alkyl chain length from acetic acid (AC) to octanoic acid (OCT) in TEAC:AC DES and TEAC:OCT DES, the solubility of CO 2 increased from 1.230 to 1.390 mol CO 2 /kg DESs. It is plausible that the length of the alkyl chain had a significant effect on the free volume within the DESs.
Zubeir et al. [64] investigated the CO 2 solubility and Henry's constants in six hydrophobic DESs, which were prepared by combining decanoic acid (DecA) with five quaternary ammonium salts. The results show that the effect of the halide anions chloride and bromide of the quaternary ammonium salts on the CO 2 solubility is negligible. The effect of alkyl chain length is consistent with the research by Sarmad [60]. Among the DESs, TBAC:DecA DES (1:2) shows the highest CO 2 solubility of 1.52 mol CO 2 /kg DESs at 298.15 K and 1990 kPa.

Amino-Based Functionalized DESs
Amines are commonly used for the chemical absorption of CO 2 . Generally, two equivalents of amine can react with one equivalent of CO 2 to form one equivalent of carbamate under anhydrous conditions, whereas an increase in viscosity is usually accompanied by the absorption process. MEA aqueous solution is widely used in industry for CO 2 absorption. However, the solvent degradation and evaporation, corrosive to equipment limited the application of MEA. Zhang et al. [65] synthesized several BmimCl-based DESs with MEA as HBD. The CO 2 uptake of BmimCl:MEA increased from 8.4% to 21.4% with the increase of the molar ratio of MEA from 1:1 to 1:4 at 298.15 K and 101 kPa, which was much higher than ChCl:MEA DES (1:6) (7.49%) at 298.15 K and 1 MPa. The different performance of ChCl:MEA and BmimCl:MEA can be attributed to the ability of cations to form hydrogen bonding with Cl − . According to the results of 1 H NMR, the C2 hydrogen in the DESs shifted to downfield as the composition changed with the addition of MEA, which implied that both of them formed hydrogen bonds with HBA as hydrogen bond donors. The acidic C2 hydrogen of imidazolium can formed a strong localized and highly directional hydrogen bond, which weakens the hydrogen bond between MEA and Cl − , resulting in an increase in absorption capacity.
Choi et al. [66] synthesized four dual amino-functionalized DESs by reacting various amino compounds, such as MEA, TEA, UE, TAA, ethylendiamine (EDA) with HCl, and then mixed with EDA. The CO 2 uptake capacity was ordered as: MEAC:EDA > TEAC:EDA > UEC:EDA > TAAC:EDA. The gravimetric uptake of MEAC:EDA DES with a molar ratio of 1:3 is 31.5 wt% correspond to 0.502 mol CO 2 /mol DES for 3h, while the viscosity increased sharply from 21.6 mPa·s to 3995 mPa·s after 2.5 min of CO 2 absorption. Excessive viscosity hindered the mass transfer process of CO 2 in DES and is not conducive to the progress of absorption. To facilitate atmospheric regeneration and avoid high viscosity, MEAC:EDA DES (1:3) was diluted to be 30 wt% in EG. The CO 2 desorption was achieved by heating the solution at 373.15K for 2.5 h. After that Shukla et al. [67] studied different types of polyamines and alcohol amines DESs formed between HBAs such as MEAC, HmimCl and TBAB and HBDs such as EDA, diethylenetriamine (DETA), tetraethylenepentamine (TEPA), pentaethylenehexamine (PEHA), 3-amino-1-propanol (AP) and aminomethoxypropanol (AMP). The results showed that the increase of the number of secondary amines in the molecule can hardly help to increase the mole absorption capacity of DES, but it will significantly reduce the mass absorption of DES, as shown in Table 4.  [TETA]Cl 3 :EG DES. As we know, there is always an amount of water in flue gas which can be absorbed by DESs during the absorption process. Although water content has a slight effect on absorptive capacity, it may increase the energy cost during the desorption process. Wu et al. [69] synthesized a new kind of hydrophobic functionalized DES formed by polyamine hydrochloride and thymol, which is hydrophobic before and after CO 2 absorption. The absorption capacity of [TETA]Cl:thymol DES (1:3) was 0.09 g CO 2 /g DES at 313.15 K and ambient pressure, corresponding to 1.298 mol CO 2 /mol DES, which indicated that thymol could not enhance the capacity of CO 2 in DESs.
Li et al. [70] synthesized a series of DESs based on ethanolamine and quaternary ammonium salt. From the results, the gravimetric absorption capacities vary with the length of the alkyl chain of the quaternary ammonium salt as follows: ChCl ≈ TMAC >TEAC > TEAB > TBAC > TBAB. The effect of alcohol amine HBD with different numbers of substituents on absorption was ordered as: MEA > DEA > MDEA > TEA. ChCl:MEA DES (1:5) exhibited the best performance with the capacities of 0.2523 g CO 2 /g DESs. Considering the cost and absorptive capacity, there is an optimal molar ratio of 1:5. The desorption experiment requires higher temperatures (373.15-413.15 K) and longer time (5~6 h) due to high viscosity and strong interaction between DES and CO 2 .

Azole-Based Functionalized DESs
Azole-based functionalized DESs are another kind of functionalized DESs used for the capture of CO 2 . Yang et al. [ 2) formed is thermally stable and has low viscosity DES, which shows absorption capacities of 10.3 and 11.4 wt% at 0.005 bar and 1 bar of CO 2 , respectively, at 298.15 K. The absorbed CO 2 at 298.15 K can easily be released under N 2 at 313.15 K. Multiple species were found in the product of CO 2 absorption. On the basis of NMR and FTIR analysis, they proposed four possible routes, as shown in Figure 5. Route 1 is the complexation of CO 2 with the pyrrolide anion that forms carbamate (N−COO). Route 2 is the deprotonation of the imidazolium cation by the anion that forms a carbene zwitterion, which then binds with CO 2 to form carboxylate (−C−COO). Route 3 is the protonation of the anion by EG, resulting in deprotonated EG that subsequently reacts with CO 2 to form carbonate(−O−COO). In the presence of water, bicarbonate (HO-COO) also forms via Route 4. (Reprinted with permission from Ref. [74]. Copyright (2021) American Chemical Society).

Super Base Functionalized DESs
Another covalent strategy for reversible CO 2 capture is to combine organic compounds containing alcohol functionalized groups with superbases. Superbases (SBs), neutral organic bases with proton affinities so high that their protonated conjugate acids (BH + ) cannot be deprotonated by the hydroxide ion, play a key role as proton acceptors in the preparation of supported protic ionic liquids (PILs) with exceptional thermal stability [75,76]. Due to the many similarities between DESs and ILs, Baker et al. [77] for the first time added super base into ChCl:Gly DES to form ternary DESs. The structures of DES components and various superbases are shown in Figure 6A. The result demonstrated the addition of super base enabled ChCl:Gly DES to chemically absorb CO 2 by deprotonating the hydroxyl group in choline chloride or glycerol to form the negative oxygen which acted as chemical absorption site to react with CO 2 and generate carbonate under ambient conditions ( Figure 6B). The effect of three different types of SBs on CO 2 absorption capacity was investigated under ambient conditions, and obtained the corresponding absorption capacity of 0.103, 0.100, 0.035 g CO 2 /g DES for ChCl:Gly:1,5-diazabicyclo  2:6), respectively. DBN leaded to optimal CO 2 capture, due to its higher basicity and less steric hindrance. As a result of chemical absorption, the viscosity of all ternary DES systems increased with the increase of absorbed amount. After absorption, ChCl:Gly:DBN DES (1:2:6) showed a viscosity ranged from 5450 to 34,613 mPa·s that is 1 to 2 orders of magnitude higher than traditional ILs and separated into two phases. Such a viscosity increase negatively affects mass transfer and capture kinetics, hindering further CO 2 capture. However, the absorbed CO 2 could be released completely within 35 min at 333.15 K, while a noticeable capacity decreased can be observed after three cycles during the cycling experiments due to slight evaporative losses of the superbase component. (B) Proposed reaction scheme for superbase-promoted CO 2 capture using a task-specific DES. In this scheme, choline chloride and the superbase DBN are shown for illustration; however, a similar reaction is believed to be operative for the other superbases as well as for CO 2 binding to the multiple alcohol functionalities of glycerol (Reprinted with permission from Ref. [77]. Copyright (2014) American Chemical Society).
Afterward, Zhang et al. [78] paired DBN with EU, 1,3-dimethylurea (DMU), and dimethylolurea (DMLU) to synthesize a series of novel superbase/acylamido-based DESs with low viscosity. Most of these DESs with the viscosity no more than 12 mPa·s exhibited excellent gravimetric absorption capacity. For example, the CO 2 gravimetric absorption capacities of DBN:EU DES (2:1), DBN:DMU DES (2:1), and DBN:DMLU DES (2:1) were 23.02, 17.34, and 4.27 wt%, respectively. Even though it is inevitable that the viscosity increases with the increase of the amount of CO 2 absorbed. Owing to the coaction of reaction dynamics and reaction thermodynamics, the absorption capacity and the rate of the DBN:EU DES (2:1) increased with the increase in temperature from 298.15 K to 318. 15  :EG (0.108 g CO 2 /g DES) with a mass ratio of 7:3 at 313.15 K and 100 kPa, which is consistent with the basic order of DESs. The viscosities of all three DESs after absorbing CO 2 increase slightly, while the three pure ILs after CO 2 absorption become a gelatinous state. The absorbed CO 2 could be completely released under 343.15 K with N 2 purging, and CO 2 absorption capacity remained nearly unchanged. Two kinds of possible mechanism of [HDBU][Im]:EG DES for CO 2 absorption were proposed. One pathway was that the H atom on EG transfers to the electronegative N of the IL to obtain the final product of carbonate, and the other was that IL reacts with CO 2 directly to generate carbonate. Therefore, the process of IL-based DES for CO 2 absorption was proved to be the synergistic interaction between EG, IL, and CO 2 , resulting in the mixed product of carbamate and carbonate.  1 Data in brackets are the molar fraction. 2 The CO 2 concentration was 14%. 3 The CO 2 concentration was 0.5%.

Non-Functionalized DESs for NO Capture
In 2011, Duan et al. [81] synthesized three kinds of DESs using CPL as HBD and tetrabutylamine halide as HBAs to absorbed NO. The solubility of NO with various halide anions displayed the following trend: F − > Br − > Cl − , which means that halogen anions play an important role in the absorption process. It can be seen that the solubility of NO in the DESs increased as the temperature increased from 308.15 to 363.15 K and molar ratio affected the absorption rate. The solubility of NO in CPL:TBAB DESs (4:1 and 6:1) increased sharply with an increase in temperature. The highest mole fraction solubility of NO was 0.170 mol NO/mol DES with a molar ratio of 4:1 at 353.15 K. The results of 1 H NMR spectra showed that NO was physically absorbed by DESs. It is worth noting that the colorless TBAB:CPL DES changed to pale yellow after absorbing NO. It is well-known that the appearance of a yellowish to orange color in the sulfur dioxide solvates of halides and pseudohalides are assigned to a charge transfer interaction, where SO 2 acts as an electron acceptor species [82,83]. It is reported that NO can also act as an electron acceptor [84,85]. Therefore, the color change of TBAB:CPL DES may be due to the charge transfer between Br − and NO.

Functionalized DESs for NO Capture
In recent years, considerable efforts have focused on the experimental and theoretical studies in the field of acidic gas separation and absorption by ILs. However, it was difficult for NO to form the hydrogen bonding with ILs, owing to its weak chemical polarity. Therefore, DESs, with chemical absorption capacity, are highly desirable. In 1960, Drago et al. [84] for the first time reported that NO could reacted with secondary amines to form the product Et 2 NH 2 + Et 2 N 2 O 2 − . However, the mechanism of whether the nitric oxide molecule dimer added base or whether nitric oxide added base and then reacted with another nitric oxide molecule have not been clarified. Until 2008, Laali et al. [83] explored the kinetics and mechanism of the reaction between secondary amines and NO. The mechanism is interpreted in terms of competitive addition of amine to either NO or its dimer. The NO solubility with the scale of mol mol −1 for the studied DESs are given in Table 5.

Amino-Functionalized DES
Tantai et al. [86,87] synthesized a series of DMU and 1,3-dimethylthiourea (1,3-DMTU)based DESs, paired with tetrabutylphosphine halides and tetrabutylamine halides, such as P 4444 Cl, P 4444 Br, TBAC, and TBAB. The result showed that the DESs containing chlorine salts as HBAs performed much better absorption capacity than those containing bromine salts, which is consistent with previous reports, as shown in Figure 7a. Moreover, the NO absorption capacity increased with the increase of the molar ratio of 1,3-DMTU in the DESs (Figure 7b), which indicated that the primary amine of 1,3-DMTU is the main site for chemical absorption of NO. For example, the absorption capacity of P 4444 Cl:DMTU increased from 2.10 to 4.25 mol NO/mol DES at 303.15 K and 101.3 kPa, as the molar ratio increased from 1:1 to 3:1. Additionally, 1,3-DMTU exhibits better performance than DUM as HBA in absorption capacity. The solubility of NO in P 4444 Cl:1,3-DMTU DES (1:1) is 1.13 mol NO/mol DES at 303.15 K and 101.3 kPa, which is much higher than 0.663 mol NO/mol DES in P 4444 Cl:DMTU DES (1:1). The results can be interpreted as sulfur atom is more electronegative than oxygen atom, so more of the charge from nitrogen is transferred to sulfur atoms, resulting in a drop in the electron cloud density of a nitrogen atom. As a consequence, the amino groups on 1,3-DMTU are prone to deprotonated than the amino groups on DMU, which is more conducive to the progress of the reaction. Nevertheless, the desorption residue P 4444 Cl:1,3-DMTU DES (1:3) was about 1.3 mol NO/mol DES, while the absorbed NO by P 4444 Br: DMU DES (1:3) can be completely desorbed, due to the stronger interaction between NO and 1,3-DMTU. Considering the effect of deprotonation on absorptive capacity, they chose azole-derived as HBDs paired with P 4444 Cl as HBA to synthesize DESs for further research [88]. The absorption capacity with different azole-derived HBDs was ordered as Tetz:P 4444 Cl > Triz:P 4444 Cl > Im:P 4444 Cl, and the same trend was observed in ChCl-based DESs (Figure 8a). Please note that this order was opposite to the basic order of azole derivatives. As shown in Figure 8b, the effect of NO partial pressure on the NO absorption capacity of Tetz:P 4444 Cl was studied. Notably, under low partial pressures ranging from 2 to 12 kPa, the NO absorption capacities increased sharply from 0.39 to 1.01 mol NO/mol DES, which confirmed the efficient chemical absorption of NO under low partial pressures. After that it increased linearly to 2.10 mol NO/mol DES when the partial pressure of NO increased from 12 to 101 kPa at 303.15 K, which was a physical absorption process. The experiment corroborated that Cl − can significantly absorb NO through physical interaction. The highest absorption capacity of Tetz:P 4444 Cl is equivalent to that of 1,3-DMTU:P 4444 Cl under the same conditions. Although Tetz:P 4444 Cl possessed desirable reusability for NO absorption and could maintain high NO absorption after recycling five times. About 0.72 mol NO/mol DES remained after desorption, indicating a desorption rate of nearly 66%. However, the DESs mentioned above were only performed to capture pure NO. In the light of the reaction of NO with the primary amine of dipropylamine (DPA) to form dipropyldiazeniumdiolate [83], Wu et al. [89] synthesized a series of polyamine-based DES and successfully used them for the absorption of low concentrations. Triethylenetetramine chloride([TETA]Cl) and tetraethylenepentamine chloride ([TEPA]Cl) were chosen as HBAs and 1,3-propanediol (1,3-PDO), Gly, polyethylene glycol (PEG), and EG were chosen as HBDs.
[TEPA]Cl:EG DES with 1:3 molar ratio exhibited the best performance with the capacity of 4.52 mol NO/mol DES (0.33 g NO/g DES) at 303 K and 101 kPa. It is worth noting that even though EG is not the main absorption component, increasing the molar ratio of EG can still increase the gravimetric absorption capacity, which demonstrated that EG can promote the absorption process enhancing the basicity of [TEPA]Cl though hydrogen bonding interactions.

Regeneration
The regeneration of absorbents is an important property that determines the production cost and the sustainability of equipment investment in industrial processes. Experimentally, the regeneration of an absorbent was conducted by bubbling N 2 at a high flow rate into the saturated absorbent to sweep out the absorbed gas or reducing the pressure of absorbents at a desired temperature until the weight of the absorbent maintains constant. The whole process of absorption and desorption is repeated at least five times to verify the reusability of DESs.
The strength of the interaction between an acidic gas and a DES determines the conditions required for desorption. Weak interaction between the gas and the non-functionalized DES makes the desorption process easy and fast. For example, SO 2 absorbed by ChCl:Gly DES (1:1) could be completely released by bubbling N 2 at 60 mL min −1 and 323.15 K within 20 min [36]. For other non-functionalized DESs with ChCl as an HBA, such as ChCl:EG (1:2), ChCl:MA (1:1), ChCl:UE (1:2), and ChCl:thiourea (1:1), absorbed SO 2 could be completely released at 250 mL min −1 and 353.15 K with 20 min [39]. Yang et al. [41] reported that the molar ratio of HBA to HBD could also affect the desorption behavior. About 96% SO 2 captured by EmimCl:EG DES (2:1) can be released at 353. 15 [57] and Im:Gly DES (1:2) are about 70% and 80% at 363.15 K and 373.15 K, respectively. As the partial pressure of an acidic gas increases, the saturated absorption capacity increases, which leads to an increase in the desorption rate. The desorption rate of EU:4-CH 3 -Im:BmimCl (1:2:1) and EU:4-CH 3 -Im:BmimCl (1:1:1) for 0.2 vol % SO 2 could reach 74.6% and 72.1%, respectively, at 100 mL/min and 353.15 K, while the desorption rates were calculated to be 92.1% and 92.7%, respectively, based on the saturated absorption capacity of pure SO 2 at the same conditions [53]. Deng et al. [49] reported that the different desorption behaviors of ACC:Im DES (1:2) and ACC:Tri DES (1:1) were related to the basicity of DESs. For ACC:Tri DES (1:1), absorbed SO 2 could be completely stripped out by bubbling N 2 at 363.15 K. However, approximately 6% SO 2 still remained after regeneration in ACC:Im DES (1:2) at the same condition. The residue of desorption was attributed to different basicity of DES. It is well-known that the pK a values of Im and Tri are 18.6 and 14.8, respectively. Thus, the basicity of the former is stronger than that of the latter. The interaction between ACC:Im DES (1:2) and SO 2 is stronger than that between ACC:Tri DES (1:1) and SO 2 , which make the desorption harder.

Conclusions and Outlook
In summary, DESs can effectively absorb SO 2 , CO 2 and NO of low concentrations in flue gas, and generally have lower viscosity before absorption compared to ILs. Especially for DESs used for SO 2 capture, the absorption capacity of non-functionalized DESs with BmimCl as HBA can be comparable to that of functionalized DESs at low pressures (≤ 10 kPa), but the DESs with BmimCl as HBA shows fast desorption rate, low desorption temperature and no desorption residual problem. The synthesis process of DESs is simple and does not require further purification. The absorption capacity and viscosity can be adjusted by changing the molar ratio of HBD to HBA, which makes a great room for cost adjustment. However, the study on the capture of acidic gases from flue gas by DESs still has a lot of challenges in the near future. First, although DESs used for the absorption of SO 2 and NO can inhibit the significant increase in the viscosity during the absorption process by introducing halogen anions, the significant increase in viscosity caused by the chemical interaction between CO 2 and DESs is still remains a challenge. This may limit the application of functionalized DESs in industrial scale. Second, the density, viscosity, solubility enthalpy, heat capacity, toxicity and the long-time stability (> 6 months) are important for applications, but they are seldom reported in the literature. Third, because of the limitation of the difficult complete desorption of functionalized DESs, except for changing the type of HBD or HBA, there is no effective method to remove residual gas. The interaction mechanism between residual gas and DESs remains to be studied. Fourth, the molar ratio of HBA to HBD is a very important factor for DESs. In general, HBA and HBD can form a DES at molar ratios from 1:1 to 1:7, and the absorption capacity and viscosity can be tuned by changing the molar ratio. Generally, for DESs with chloride salt as an HBA paired with different types of HBDs, the highest mass absorption occurs between the molar ratios of 1:3 to 1:5 (HBD:HBA). However, considering the cost and absorptive capacity, the optimal molar ratio is usually different from that at the highest mass absorption. The influence of molar ratio on absorptive capacity has been widely discussed in previous reports, but it is too dispersed to be studied systematically and lacks in-depth studies. More importantly, the flue gas is made of a variety of gases, and hence it is necessary to study the effects of mixed gases on absorption and regeneration performance. DESs containing polyamine, imidazole and triazole can absorb SO 2 , CO 2 and NO simultaneously by the free amino groups in DESs with similar mechanism. The competitive absorption between acidic gases and the competitive desorption of the acidic gases are inevitable problems in industrial applications, but no research in this area has been published. Although there are above challenges, functionalized DESs, especially functionalized DESs with strong physical absorption, show enormous potentials for gases capture with easy preparation, low cost, biodegradability, and nontoxicity. Acknowledgments: The long-term subsidy mechanism from the Ministry of Finance and the Ministry of Education of PRC (BUCT) is acknowledged.

Conflicts of Interest:
The authors declare no conflict of interest.