Tuning Functionalized Ionic Liquids for CO2 Capture

The increasing concentration of CO2 in the atmosphere is related to global climate change. Carbon capture, utilization, and storage (CCUS) is an important technology to reduce CO2 emissions and to deal with global climate change. The development of new materials and technologies for efficient CO2 capture has received increasing attention among global researchers. Ionic liquids (ILs), especially functionalized ILs, with such unique properties as almost no vapor pressure, thermal- and chemical-stability, non-flammability, and tunable properties, have been used in CCUS with great interest. This paper focuses on the development of functionalized ILs for CO2 capture in the past decade (2012~2022). Functionalized ILs, or task-specific ILs, are ILs with active sites on cations or/and anions. The main contents include three parts: cation-functionalized ILs, anion-functionalized ILs, and cation-anion dual-functionalized ILs for CO2 capture. In addition, classification, structures, and synthesis of functionalized ILs are also summarized. Finally, future directions, concerns, and prospects for functionalized ILs in CCUS are discussed. This review is beneficial for researchers to obtain an overall understanding of CO2-philic ILs. This work will open a door to develop novel IL-based solvents and materials for the capture and separation of other gases, such as SO2, H2S, NOx, NH3, and so on.


Introduction
According to the report "State of the Global Climate 2021" recently published by the World Meteorological Organization (WMO), CO 2 mole fraction reached new high (413.2 ± 0.2 ppm in 2020, while pre-industrial mole fraction of 278 ppm [1]. The increasing concentration of CO 2 in the atmosphere during these centuries, especially since the 20th century, leads to the greenhouse effect and global climate change. A large amount of all human-produced CO 2 emissions come from the burning of fossil fuels, such as coal, natural gas, and oil, including gasoline. In recent decades, carbon capture, utilization, and storage (CCUS) has become one of the important technologies to reduce CO 2 emissions [2]. For carbon capture, the common CCUS technologies are based on chemical sorption, physical sorption, membrane separation, calcium looping, etc. For example, aqueous monoethanolamine (30 wt%) process is the current CO 2 capture technology in industry via carbamate mechanism. Although the chemical reaction methods are more efficiency, the regeneration energy consumption of these methods is high [3]. For carbon utilization, the most effective strategy is CO 2 convention, including thermocatalysis, photocatalysis, or electrocatalysis, of CO 2 cycloaddition reaction, CO 2 reduction reaction (CO 2 RR), etc. [4,5]. For example, the final products of the CO 2 RR are widely distributed from C 1 (carbon monoxide, formic acid, methane) to C 2+ (ethylene, ethanol, acetone, etc.) [6]. However, the CO 2 conversion via CO 2 RR approach is still steps away from widespread commercialization. For carbon storage, the widely used way to store captured CO 2 is in deep geological formations, such as oil fields, gas fields, coal seams, and saline aquifers [7]. However, the process increases the amount of energy Int. J. Mol. Sci. 2022, 23, 11401 2 of 20 required by power plants. Therefore, alternative CCUS technologies with high efficiency and low energy-consumption are highly desired.
Ionic liquids (ILs) are composed of organic cations and organic or inorganic anions with melting points below 100 • C [8][9][10][11]. Their excellent properties, including extremely low vapor pressure, high thermal and chemical stability, wide liquid temperature range, high electrical conductivity and wide electrochemical window, and good solubility for both polar and non-polar compounds, make it possible for ILs to be designed according to needs. Thus, ILs are widely used as green solvents and catalysts in such fields as energy and environment [12][13][14], chemistry and chemical synthesis [15,16], sorption and separation [17], and pharmaceutics and medicine [18,19]. Compared with conventional methods, ILs, especially functionalized ILs, have been used in CCUS with great interest all over the world due to the advantages of fast absorption, high capacity, low energyconsumption, good stability, and good recyclability.
Several interesting reviews for CO 2 capture by ILs have been published during the last few years. For example, Zhang et al. [20] reviewed the IL-based CO 2 capture systems from structure and interaction to process. Zhang and Ji et al. [21] reported the reviewing and evaluating of ionic liquids/deep eutectic solvents for CO 2 capture. However, it is crucial to review this developing field from a viewpoint of functionalization of ILs with active sites, which is beneficial for researchers to obtain an overall understanding of CO 2 -philic ILs and grasp the development direction.
In this critical review, we mainly focus on the development of functionalized ILs for CO 2 capture in the past 10 years (2012~2022). The main contents include three parts, cation-functionalized ILs, anion-functionalized ILs, and cation-anion dual-functionalized ILs for CO 2 capture ( Figure 1). Besides, classification, structures, and synthesis of functionalized ILs are also summarized. Finally, future directions, concerns, and prospects for functionalized ILs in CCUS are discussed. oil fields, gas fields, coal seams, and saline aquifers [7]. However, the process increases the amount of energy required by power plants. Therefore, alternative CCUS technologies with high efficiency and low energy-consumption are highly desired. Ionic liquids (ILs) are composed of organic cations and organic or inorganic anions with melting points below 100 °C [8][9][10][11]. Their excellent properties, including extremely low vapor pressure, high thermal and chemical stability, wide liquid temperature range, high electrical conductivity and wide electrochemical window, and good solubility for both polar and non-polar compounds, make it possible for ILs to be designed according to needs. Thus, ILs are widely used as green solvents and catalysts in such fields as energy and environment [12][13][14], chemistry and chemical synthesis [15,16], sorption and separation [17], and pharmaceutics and medicine [18,19]. Compared with conventional methods, ILs, especially functionalized ILs, have been used in CCUS with great interest all over the world due to the advantages of fast absorption, high capacity, low energy-consumption, good stability, and good recyclability.
Several interesting reviews for CO2 capture by ILs have been published during the last few years. For example, Zhang et al. [20] reviewed the IL-based CO2 capture systems from structure and interaction to process. Zhang and Ji et al. [21] reported the reviewing and evaluating of ionic liquids/deep eutectic solvents for CO2 capture. However, it is crucial to review this developing field from a viewpoint of functionalization of ILs with active sites, which is beneficial for researchers to obtain an overall understanding of CO2-philic ILs and grasp the development direction.
In this critical review, we mainly focus on the development of functionalized ILs for CO2 capture in the past 10 years (2012~2022). The main contents include three parts, cation-functionalized ILs, anion-functionalized ILs, and cation-anion dual-functionalized ILs for CO2 capture ( Figure 1). Besides, classification, structures, and synthesis of functionalized ILs are also summarized. Finally, future directions, concerns, and prospects for functionalized ILs in CCUS are discussed.

Classification and Structures of Functionalized ILs
Functionalized ILs (task-specific ILs, or functional ILs) can be simply classified into three categories according to the locations of active sites, including cation-functionalized ILs, anion-functionalized ILs, and cation-anion dual-functionalized ILs. The cation-functionalized ILs and anion-functionalized ILs can be divided into two categories according to the number of functional groups and the mechanism of CO2 capture, including singlesite functionalized ILs and multiple-site functionalized ILs. It is clear that cation-anion dual-functionalized ILs are multiple-site functionalized ILs. The main reaction groups

Classification and Structures of Functionalized ILs
Functionalized ILs (task-specific ILs, or functional ILs) can be simply classified into three categories according to the locations of active sites, including cation-functionalized ILs, anionfunctionalized ILs, and cation-anion dual-functionalized ILs. The cation-functionalized ILs and anion-functionalized ILs can be divided into two categories according to the number of functional groups and the mechanism of CO 2 capture, including single-site functionalized ILs and multiple-site functionalized ILs. It is clear that cation-anion dual-functionalized ILs are multiple-site functionalized ILs. The main reaction groups with active sites are listed in each category, such as amino, carboxylate, alkoxide, phenolate, and azolate. The structures of cations and anions for synthesis of functionalized ILs are collected in Figure 2.
with active sites are listed in each category, such as amino, carboxylate, alkoxide, pheno late, and azolate. The structures of cations and anions for synthesis of functionalized IL are collected in Figure 2.

Synthesis of Functionalized ILs
Generally, the synthesis of functionalized ILs includes several separated unit operations, such as quaternization, anion-exchange, acid-base neutralization, coordination, etc. According to the structure of functionalized ILs, the methods or pathway for the synthesis

Single-Site Mechanisms
It is known that the most studied cation-functionalized ILs for CO2 capture should be amino-functionalized ILs, which were first reported by Davis et al. [22] in 2002, two decades ago. They showed that 0.5 mole of CO2 per mole of IL was captured by 1-propylamide-3-butyl imidazolium tetrafluoroborate ([apbim][BF4]) via a carbamate mechanism (2 amino: 1 CO2). Compared with conventional alkanolamine aqueous solution (30 wt% monoethanolamine) for CO2 capture, amino grafted on cations of ILs showed high thermostability [23,24], while amino grafted on ILs showed high capture capacity compared It is known that the most studied cation-functionalized ILs for CO 2 capture should be amino-functionalized ILs, which were first reported by Davis et al. [22] in 2002, two decades ago. They showed that 0.5 mole of CO 2 per mole of IL was captured by 1-propylamide-3-butyl imidazolium tetrafluoroborate ([apbim][BF 4 ]) via a carbamate mechanism (2 amino: 1 CO 2 ). Compared with conventional alkanolamine aqueous solution (30 wt% monoethanolamine) for CO 2 capture, amino grafted on cations of ILs showed high thermostability [23,24], while amino grafted on ILs showed high capture capacity compared with conventional ILs [25]. Subsequently, a number of amino-grafted cation-functionalized ILs were reported for efficient CO 2 capture [26][27][28][29][30]. The mechanisms of amino-CO 2 reaction in ILs are similar to those in aqueous alkanolamine solutions. Compared with primary and secondary amines, tertiary amine is considered unreactive with CO 2 under anhydrous conditions ( Figure 4). However, He et al. [31] reported that tertiary amino-containing Li with conventional ILs [25]. Subsequently, a number of amino-grafted cation-functionalized ILs were reported for efficient CO2 capture [26][27][28][29][30]. The mechanisms of amino-CO2 reaction in ILs are similar to those in aqueous alkanolamine solutions. Compared with primary and secondary amines, tertiary amine is considered unreactive with CO2 under anhydrous conditions (Figure 4). However, He et al. [31] reported that tertiary aminocontaining Li-

Multiple-Site Mechanisms
Multiple functional sites on the cations are multiple amino groups. For example, Zhang et al. [32] reported CO 2 capture by a dual amino-containing cation-functionalized IL, 1, 3-di (2 -aminoethyl)-2-methylimidazolium bromide (DAIL), via a 2:1 carbamate mechanism (amino: CO 2 ). However, the synthesis of DAIL was not easy. Therefore, other kinds of polyamine-based ILs were developed through acid-base neutralization or metal coordination. Clyburne et al. [33] and Meng et al. [34] [37] reported the tuning of stability constants of metal-amine complexes for efficient CO 2 desorption.
The comparison of the absorption capacities, including molar capacities and corresponding mass capacities, of typical cation-functionalized ILs for CO 2 capture are listed in Table 1.

IL
T ( • C) P (bar) M w (g mol -1 ) a n CO 2 /n IL n CO 2 /kg IL b g CO 2 /g IL b Ref. [

Anion-Functionalized ILs for CO 2 Capture
Compared with only amino-grafted cations for efficient CO 2 capture by ILs, there are numerous kinds of functional groups grafted on anions for efficient CO 2 capture. According to their reaction mechanism with CO 2 , the anion-functionalized ILs can be classified into two categories, including single-site mechanisms and multiple-site mechanisms.

Single-Site Mechanisms
Functionalized ILs with single-site on anions include amino anions, carboxylate anions, alkoxide anion, phenolate anions, and azolate anions. The typical mechanisms for the reaction of non-amino anion-CO 2 can be found in Figure 5a.
The comparison of the absorption capacities, including molar capacities and corresponding mass capacities, of typical cation-functionalized ILs for CO2 capture are listed in Table 1.

Anion-Functionalized ILs for CO2 Capture
Compared with only amino-grafted cations for efficient CO2 capture by ILs, there are numerous kinds of functional groups grafted on anions for efficient CO2 capture. According to their reaction mechanism with CO2, the anion-functionalized ILs can be classified into two categories, including single-site mechanisms and multiple-site mechanisms.

Single-Site Mechanisms
Functionalized ILs with single-site on anions include amino anions, carboxylate anions, alkoxide anion, phenolate anions, and azolate anions. The typical mechanisms for the reaction of non-amino anion-CO2 can be found in Figure 5a.      [39,40] for equimolar absorption of CO 2 via a 1:1 mechanism. In order to understand the CO 2 absorption mechanisms with AAILs, Xing et al. [41] showed that the actual mechanism went beyond the apparent stoi-chiometry. Take [Gly] and [Met] anions as the examples, although the apparent chemical stoichiometry approached 1:1 and the absorption was previously considered to simply follow the 1:1 mechanism, their results indicated that more than 20 % of the CO 2 still was absorbed in the 1:2 reaction mechanism. Recently, Mehrdad et al. [42]  ) for CO 2 capture via physical and chemical sorption mechanism. However, the hydrogen bond in these AAILs resulted in high viscosity, and the viscosity increased dramatically after the absorption of CO 2 . Therefore, other AAILs supported on porous materials [43][44][45][46] or mixed with liquids [33,34] were reported; (2) Carboxylate anion-functionalized ILs (O-site) From the 1:1 mechanism of AA ILs with CO 2 , the carboxylate in the AA anions provides a negative charge but seemed to not interact with CO 2 . Through tuning the structure of carboxylate ILs, Ils can also chemically react with CO 2 . 1-Butyl-3-methylimidazolium acetate ([Bmim][Ac]), reported by Maginn et al. [47], was the first carboxylate IL example for efficient CO 2 capture. The reported mechanism of N-heterocyclic carbene−CO 2 was verified by NMR. However, Steckel et al. [48], Shi et al. [49], and Ruiz-López et al. [50] studied the mechanism via ab initio calculations. The results indicated that for glycinate anion, interactions with the amino and carboxylic moieties involved comparable energetics. For example, Tao et al. [ [54] showed that acetylacetonate ILs could also chemically absorb CO 2 ; (3) Alkoxide anion-functionalized ILs (O-site) The alkoxide is an anion that forms when we remove the hydrogen atom from the -OH group of an alcohol. It was known that switchable solvents, a liquid mixture of an alcohol (e.g., pKa of ethanol in DMSO is 29.8) and a strong organic base (e.g., 1,8-diazabicyclo- [5.4.0]undec-7-ene, DBU), could chemically bind CO 2 to form an alkylcarbonate salt through proton transfer from alcohol to superbase [55,56]. Thus, alcohols with the appropriate acidity can be used to synthesize alkoxide ILs through dehydrogenation. Dai et al. [57] reported a series of superbase-derived protic ILs with trifluoroethanolate (TFE, pKa = 23.5), 1-phenyl-2,2,2-trifluoroethanolate (TFPA, pKa = 23), and 2,2,3,3,4,4-hexafluoro-1,5-pentanediolate (HFPD, pKa = 23.2) anions for equimolar CO 2 capture. Subsequently, Liu et al. [58] used [DBUH][TFE] (1.01 mol CO 2 per mol IL) to catalyze CO 2 conversion into quinazoline-2,4(1H,3H)-diones; (4) Phenolate anion-functionalized ILs (O-site) With appropriate acidity (pKa = 10 in water), the phenol could be used to form the functional anion, phenolate (or phenoxide), via removal of the hydrogen atom from the -OH group of a phenol to prepare ILs for efficient CO 2 capture. For example, Wang et al. [59] studied a series of phenolate anion-functionalized ILs for CO 2 chemisorption. Through tuning the structure of phenolate anion with different substituents, CO 2 absorption performance could be further regulated.  [64] constructed several phenolate chelate ILs for CO 2 chemisorption via coordination of phenolate alkali metal salts with crown ethers. With 15-crown-5-coordinated Na + as the cation, the CO 2 uptake capacity of the phenolate anion decreased in the following order: Azoles, such as imidazole (Im), pyrazole (Pyrz), 1,3,4-trizole (Triz), tetrazole (Tetz), etc., are a kind of five-membered heterocycles. These azoles with high basicity were used by Dai and Wang et al. [57,65] [70] were in the rage of 0.8~1.0 at room temperature and atmospheric pressure. Different from aprotic cations or protic cations with a strong base, Oncsik and MacFarlane et al. [72] and Yang et al. [73] reported N,N-dimethylethylenediaminium azolate ILs for CO 2 capture via forming carbamate species. The former authors believed that CO 2 reacted with azolate anions, while the latter authors found that CO 2 reacted with the cations. Thus, it is well understood that (1) the anion but not the cation has the key function in CO 2 capture and (2) the mechanism follows a 1:1 stoichiometry. On the other hand, the absorption molar ratios of CO 2 to IL were affected by the basicity of ILs. Wang et al. [65] [74,75].
The substituents on the heterocycles will affect the performance of CO 2 capture by azolate ILs. Wu et al. [76] studied the reactivity of azolate anions with CO 2 from the density functional theory (DFT) perspective. It was studied that the absorption capacity by imidazolate [66,69], pyrazolate ILs [77,78], and indazolate ILs [79] was affected by the substituents on the heterocycles. For example, Wang et al. [60] showed that the molar ratio of CO 2 to [P 66614 ][4-CHO-Im] could reach 1.24 at 20 • C and 1 bar via the interactions of imidazolate-CO 2 and hydrogen bonding. Later, they also concluded that [P 66614 ][4-Br-Im] was an ideal substituent imidazolate IL with desirable absorption enthalpy (−61.4 kJ mol −1 ), basicity (pKa is 12.2 in H 2 O), and CO 2 capacity (0.87 mol CO 2 per mol IL) [66]. It is also clearly that the substituents resulted in different basicity of azolate ILs. Recently, a lightresponsive 1,3,4-trizolate IL was reported by Wang et al. [80], and the decreased capacity of CO 2 was found when the IL converted from the trans to cis state. They confirmed that the entropy change was the key influencing factor. Additionally, different from the aminofunctionalized ILs with the viscosity increasing during the CO 2 absorption, the viscosity of the viscosity of [P 66614 ][Im] was found to decrease from 810.4 cP to 648.7 cP after absorption of CO 2 . Jiang et al. [81] revealed the microscopic origin for the decrease in viscosity after CO 2 absorption by [P 66614 ][Im] via molecular dynamics (MD) simulation. Rogers et al. [76] reviewed the ILs with azolate anions due to their desired properties, including a diffuse ionic charge, tailorable asymmetry, and synthetic flexibility. Azolate anionfunctionalized ILs are also reported with the name "aprotic heterocyclic anion" ([AHA]) based ILs. Maginn et al. [82] reported [P 66614 ][2-CN-Pyr]) and [P 66614 ][2-CF 3 -Pyra] could obtain~0.9 mol CO 2 per mol IL via a 1:1 mechanism. Brennecke et al. [83,84] investigated the influence of substituent groups on the reaction enthalpy of CO 2 -[AHA], and the estimated values range between −37 and −54 kJ mol -1 , lower than that of CO 2 -MEA (−85 kJ mol -1 ). The structure and mechanism of azolate-CO 2 was systematically studied via DFT calculations, [82] ab initio MD simulation [85][86][87], Monte Carlo simulation [88], first principles simulations [89,90], and other computer calculations [91,92].
Another reaction pathway was reported. It should be noted that when the anion has a certain basicity for CO 2 capture, the basicity of the anion can cause it to pull out an active hydrogen atom on the imidazolium or phosphonium cation to form a carbene or zwitterionic compound (ylide), which could subsequently interact with CO 2 to form a carbene-CO 2 or ylide-CO 2 complex, respectively. Brennecke et al. [93] [94] showed that the more basic [AHA] anion would form carbene-CO 2 via DFT. As the formed carbene-CO 2 resulted in the reduced efficiency of anions, Wang et al. [95] investigated that substituted imidazolium reduced the amount of carbene-CO 2 and increased the amount of azolate-CO 2 . For the ylide-CO 2 pathway, Brennecke et al. [78,96] investigated the cationanion and [AHA] -CO 2 interactions and the quantification of ylide-CO 2 in phosphonium ILs. In addition to the azolate anions, the phenolate anions and carboxylate anions can also result in carbene-CO 2 in imidazolium ILs [97][98][99] or ylide-CO 2 in phosphonium ILs [100], respectively.
The comparison of the absorption capacities, including molar capacities and corresponding mass capacities, of typical anion-functionalized ILs for CO 2 capture via single-site mechanisms are listed in Table 2. Table 2. Typical anion-functionalized ILs for CO 2 capture via single-site mechanisms.

IL
T ( • C) P (bar) M w (g mol -1 ) a n CO 2 /n IL n CO 2 /kg IL b g CO 2 /g IL b Ref. [

Multiple-Site Mechanism
It is known that single-site in ILs result in up to a 1:1 stoichiometry absorption capacity. However, multiple-site in ILs may not result in doubled capacity. For multiple sites sharing one negative charge, the efficiency of a site may be decreased. Besides, even if the two sites are independent or each has a negative charge, the absorption capacity may not double, due to the complex interactions in ILs. The typical multiple-site mechanisms can be found in Figure 5b.
(1) Multiple same groups in anion-functionalized ILs As amino group is a functional group for CO 2 1.68, and 1.62, respectively, via the reaction mechanism of 1:1. Different from two amino groups in one anion, CO 2 capacities of several dicationic ILs with two amino acid anions [105] or azolate anions [106,107] were reported nearly twice that of the monocationic analogues. Additionally, CO 2 absorption capacity of a superbase-derived diolate IL [MTBDH] + 2 [HFPD] 2− reported by Dai et al. [57] was more than 2.04 mol CO 2 per mol IL because of two alkoxide groups. Wang et al. [108] investigated the CO 2 capture by a pillar [5]arene-based −10 valent carboxylate anionfunctionalized phosphonium IL, [P 66614 ] 10 [112] investigated a series of hydroxypyridine ILs with different kinds of cations, and the enhanced CO 2 capacity up to 1.83 mol CO 2 mol −1 IL could be obtained at 20 • C and 1 bar via reducing cation-anion interactions. Xu et al. [113] reported that the CO 2 [114,115]; (3) Imide anion-functionalized ILs Imide and amide anions reported to have nucleophilic reactivities [116]. In order to improve CO 2 capacity under low concentration CO 2 (10 vol%), Cui and Wang et al. [117]  Further studies showed that the electro-withdrawing phenyl group on the anion, [Ph-Suc], reduced the CO 2 absorption capacity, while the electro-donating cyclohexyl group on the anion, [Cy-Suc], increased the CO 2 absorption capacity (1.76 mol CO 2 mol −1 IL for 10 vol% and 2.21 mol CO 2 mol −1 IL for 100 vol%) via enhanced cooperation and physical interaction [118]. Additionally, the obtained imide-based ILs are stable in water, and the CO 2 absorption could be improved under low water content [119]. The results of thermodynamic studies showed that the absorption was an enthalpy-driven process [120]. Wang et al. [121] reported an aminomethyl-functionalized tetrazolate IL, [P 66614 ][MA-Tetz], with a CO 2 capacity of 1.13 mol CO 2 per mol IL, due to the interaction of one amino group (H-N-H) with two molecules of CO 2 ; (4) Other multiple-site anion-functionalized ILs When multiple sites are independent in an anion, they give an opportunity for improving CO 2 capture. As amino and carboxylate could both interact with CO 2 efficiently, Tao et al. [122]  On the other hand, when multiple sites are dependent in an anion, their interactions may lead to mutual restraint for CO 2 capture. For example, Liu et al. [125] synthesized a -3 valent carboxylate-hydroxypyridinate-containing anion IL, [P 4444 ] 3 [2,4-OPym-5-Ac] and found that a CO 2 capacity of 1.46 mol CO 2 per mol IL could be obtained, lower than the theoretical value. However, Luo and Lin et al. [124] reported a −2 valent IL, [P 66614 ] 2 [Am-iPA], with amino functionalized dicarboxylate anion. Their results showed that CO 2 capture capacity of this IL was 2.38 mol CO 2 per mol IL at 30 • C. Besides, multiple sites dependently shared one negative charge, resulting in decreased efficiency of CO 2 capture. Wang and MacFarlane et al. [126] studied CO 2 capture by an amino-containing hydroxypyridinate anion-functionalized ILs with the capacity of 0.87~0.99 mol CO 2 per mol IL. The NMR results indicated the primary reaction of amino-CO 2 and the lesser reaction of phenolate-CO 2 . Tao et al. [127] reported CO 2 capture by amino-functionalized triazolate anion ILs, [Bmim][ATZ] and [Emim][ATZ], with a capacity as low as 0.14 and 0.13 mol CO 2 per mol IL, respectively, via physical interaction.
The comparison of the absorption capacities, including molar capacities and corresponding mass capacities, of typical anion-functionalized ILs for CO 2 capture via multiplesite mechanisms are listed in Table 3. Table 3. Typical anion-functionalized ILs for CO 2 capture via multiple-site mechanisms.

IL
T ( • C) P (bar) M w (g mol -1 ) n CO 2 /n IL n CO 2 /kg IL g CO 2 /g IL Ref. [

Cation-Anion Dual-Functionalized ILs for CO 2 Capture
It is clear that functional groups on cations are mainly amino groups. Thus, with the combination of functional cations and functional anions, kinds of dual-functionalized ILs with multiple sites were developed for CO 2 capture.

Amino-Based Cation and Amino-Based Anion
The early dual-functionalized ILs were ILs with amino-functionalized cations with amino acid anions.  [132,133] were found to capture CO 2 via 2:1 mechanism (amino:CO 2 ). Jing et al. [134] used quantum chemical simulation for screening of multi-amino-functionalized ILs for CO 2 capture. Their experimental results confirmed the predictions and the absorption capacities of [TETAH][Lys] (5 amino groups) and [DETAH][Lys] (4 amino groups) were 2.59 and 2.13 mol CO 2 per mol IL, respectively, via 2:1 zwitterionic mechanism.

Amino-Based Cation and Phenolate Anion
Based on the high reactivity of phenolate anions for CO 2 capture, Ye and Li et al. [135] reported that the CO 2

Amino-Based Cation and Azolate Anion
Based on the high reactivity of azolate anions for CO 2 capture, Ye and Li et al. [135] reported that the CO 2  The comparison of the absorption capacities, including molar capacities and corresponding mass capacities, of typical cation-anion dual-functionalized ILs for CO 2 capture are listed in Table 4. Table 4. Typical cation-anion dual-functionalized ILs for CO 2 capture.

IL
T ( • C) P (bar) M w (g mol -1 ) a n CO 2 /n IL n CO 2 /kg IL b g CO 2 /g IL b Ref. [

Conclusions and Outlook
It is known that functionalized ILs started in 2002, and it has been just two decades. Due to the designable and tunable structures of ILs, functionalized ILs have developed rapidly in the past ten years (2012-2022). CO 2 -philic active sites can be tethered to the cations and anions, forming cation-functionalized ILs, anion-functionalized ILs, and cationanion dual-functionalized ILs. Compared with conventional ILs for physisorption of CO 2 , functionalized ILs or task-specific ILs could chemically absorb CO 2 through single-site mechanisms or multiple-site mechanisms. Based on the research results, we can safely conclude that efficient absorption of CO 2 with a high capacity, low energy consumption, and high reversibility could be reached through tuning the structures of functionalized ILs and regulating the interactions between active sites and CO 2 . Nonetheless, for largescale industrial application of IL-based CCUS technology, we also need to consider the following issues: (1) Reaction mechanism of functionalized IL-CO 2 needs to be investigated further; (2) A large amount of CO 2 absorption experiments was tested at room temperature and atmospheric pressure, but the temperature of flue gas is high (50~80 • C) and the concentration of CO 2 is low (10~15 vol%), there is still a big gap between laboratory research and industrial application; (3) The selective capture of CO 2 and the deactivation of functionalized ILs under other gases conditions (H 2 O, SO 2 , NOx, etc.) should be studied; (4) Compared with conventional absorbents such as alkanolamine aqueous solutions, pure functionalized ILs have higher viscosity and cost; (5) It is important to investigate capture efficiency in mass absorption capacity or gravimetric capacity in order to better comparison and realize the competitive ILs. Thus, functionalized ILs with high mass absorption capacity should be developed. (6) The regeneration of the ILs is also important and related to energetic consume and the absorption cost. Thus, the absorption enthalpies should be investigated.
Here are some suggestions or strategies to address the aforementioned issues: (1) A combination of NMR and IR analysis and chemical calculations can be used to investigate the absorption mechanisms of active sites on the ILs with CO 2 ; (2) The performance of CO 2 capture is affected by absorption temperature and CO 2 partial pressure. Due to the tunable structure and property of ILs, design functionalized ILs with high active sites is an efficient way to help ILs applicate in industry; (3) H 2 O, SO 2 , NOx, etc. will lead to a decrease in the activity of ILs, especially ILs with strong basicity. Thus, these impurities should first be removed. For example, ILs with weak basicity for SO 2 or NOx removal and ILs with strong basicity for CO 2 removal; (4) Functionalized ILs with a low viscosity could be synthesized through tuning the structures of cation and anion. Besides, the viscosity of amine-containing functionalized ILs or protic ILs were reported to be increased during the absorption of CO 2 , while for amine-free functionalized ILs and aprotic ILs no obvious change during CO 2 capture was reported due to the absence of strong hydrogen bonded networks in these ILs (Table 5); (5) Aqueous monoethanolamine (30 wt%) process is the current CO 2 capture technology in industry with a mass capacity of~7 wt%. It can be found in Tables 1-4 that functionalized ILs with a high molecular weight resulted in a high molar capacity but a low mass capacity. Functionalized ILs with a high molar capacity open the door to developing functionalized ILs with a high mass capacity via combining functional sites and a small molecular weight; (6) High regeneration or reversibility of the ILs for CO 2 capture needs weak interactions or low absorption enthalpies, which always results in low efficiency. Thus, functionalized ILs is always accompanied by high energy consumption. However, the results from CO 2 capture by preorganized imide-based ILs indicate that multiple weak interactions also lead to strong adsorption and high capacity, even under low concentrations of CO 2 .  [137] Therefore, continuously developing novel functional IL-based CO 2 -philic solvents or sorbents and systematically studying the reaction mechanism of CO 2 with active sites under different conditions are the main concern of IL-based CCUS technologies in order to realize large-scale, rapid, economical, efficient, and reversible absorption of CO 2 in the flue gas.