Reviewing and screening ionic liquids and deep eutectic solvents for effective CO2 capture

CO2 capture is essential for both mitigating CO2 emissions and purifying/conditioning gases for fuel and chemical production. To further improve the process performance with low environmental impacts, different strategies have been proposed, where developing liquid green absorbent for capturing CO2 is one of the effective options. Ionic liquids (IL)/deep eutectic solvents (DES) have recently emerged as green absorbents with unique properties, especially DESs also benefit from facile synthesis, low toxicity, and high biodegradability. To promote their development, this work summarized the recent research progress on ILs/DESs developed for CO2 capture from the aspects of those physical- and chemical-based, and COSMO-RS was combined to predict the properties that are unavailable from published articles in order to evaluate their performance based on the key properties for different IL/DES-based technologies. Finally, top 10 ILs/DESs were listed based on the corresponding criteria. The shared information will provide insight into screening and further developing IL/DES-based technologies for CO2 capture.

possibility to produce more carbon-neutral transportation fuels, such as methanol, hydrogen, and synthetic hydrocarbons, where CO 2 removal is always required (Shahbaz et al., 2021). In the production of synthetic hydrocarbons with the Fischer-Tropsch technology, the inert CO 2 is removed to increase the efficiency and selectivity of C 5+ ; in the methanol production, CO 2 removal is made to obtain a favorable ratio of the gas mixture to increase the production yield; while in the hydrogen production, CO 2 separation is needed to purify hydrogen. In parallel, bio-methane production and its usage as vehicle fuels provide an alternative and important pathway to mitigate CO 2 emissions from the transportation sector. In general, the raw biogas generated in the anaerobic digestion contains CO 2 of around 30-50 vol%. To be used as vehicle fuels, CO 2 removal from the raw biogas (i.e., biogas upgrading) is needed to lower the CO 2 content to 2-6 vol% (Ahmed et al., 2021). Besides, bio-carbon capture, utilization and/or storage (bio-CCUS) from the bioenergy sector is of considerable interest towards negative carbon emissions, further highlighting the importance of CO 2 capture/separation. Currently, the available technologies for CO 2 separation mainly include absorption, adsorption, membrane, and cryogenic separation, and they are very costly ($50-100/ton-CO 2 ) combined with other challenges (Mondal et al., 2012). For example, the aqueous amine-based technology (absorption) is energy-intensive, and volatility (secondary pollution), degradation, and corrosion are other deficiencies; the selectivity of the membrane is often lost in the presence of H 2 ; adsorption is energy consuming and usually requires large-scale operations; the Rectisol process needs high power for refrigeration. Further development is needed to decrease the cost of CO 2 separation with marginal or no environmental influence, where solvent-based technologies have been suggested as the most suitable candidates considering the tolerance for the impurities from the gas streams.
To develop solvent-based technology for CO 2 separation, different kinds of absorbents have been proposed, where ionic liquids (ILs) have drawn significant attention because 1) they are nonvolatile, nonflammable, environmentally benign, and thermal stable; 2) their chemical and physical properties are tunable, making it possible to design a specific IL for a specific CO 2 capture process; and 3) relatively high CO 2 solubility and selectivity as well as relatively low energy-demand for solvent regeneration. Additionally, a new class of ILs or IL analogous, i.e., deep eutectic solvents (DESs), share similar properties to the conventional ILs together with other additional appealing properties, such as their simple preparation processes, low cost, biodegradability, and environmentally friendly materials used in synthesis (Sarmad et al., 2017a).
Owing to the unique properties of both ILs and DESs, intensive work has been conducted, and results have been reported all the time, calling for a new survey of the latest research. Also, the extensive work imposes the challenge to screen and suggest proper ILs and DESs for a specific purpose, which is related to the compositions and conditions of the streams needed for CO 2 separation as well as other available resources, such as waste-heat or the availability of steam, etc. To the best of our knowledge, it is still unclear which ILs and DESs within those developed are more desirable for a specific IL/DES-based technology. Indeed, process simulation and evaluation are conducted, but just after a very brief and quantitative comparison of some properties. Especially in some works, the important properties, e.g., viscosity and/or selectivity, are not included or concerned sufficiently, making it difficult to convince the reliability of the simulation results (Hospital-Benito et al., 2021). All these issues call for a proper screen, selection, and suggestion of promising ILs and DESs for further study and development.
To screen ILs/DESs, properties are needed, which can be determined experimentally or predicted theoretically. Theoretical models have been developed to predict properties of ILs/DESs qualitatively or quantitively, and the models include Quantitative Structure Property Relationship (QSPR) method (Eike et al., 2004), regular solution theory (Kilaru et al., 2008), Molecular dynamics (Kerlé et al., 2009), Monte Carlo (Shah and Maginn, 2005), group contribution method (Kim et al., 2005;Zhang et al., 2021b;Zhou et al., 2021), Conductor-like Screening Model for Real Solvents (COSMO-RS) method , statistical associating fluid theory (SAFT)-based equation of state (Ji et al., 2012;Ji and Zhu, 2013;Ji et al., 2014;Shen et al., 2015;Sun et al., 2019;Alkhatib et al., 2020;Sun, 2020), etc. Based on the properties, ILs and DESs are screened with different criteria, for example, thermodynamic CO 2 absorption capacity (CO 2 solubility and Henry's constant)  and (Henry's constant, selectivity, relative polarity, and molar volumes) (Sumon and Henni, 2011), thermodynamic and kinetic performances for CO 2 absorption (CO 2 absorption capacity, viscosity, melting point, and Henry's constant) (Farahipour et al., 2016;Zhang et al., 2021a) as well as CO 2 chemical absorption properties (chemical equilibrium constants, Henry's constants, and reaction enthalpies) (Moya et al., 2020), etc. Also, based on the thermodynamic analysis under specific operation conditions, screening has been conducted for both physical-based ILs and DESs (Zhang et al., 2016). In recent years, Zhou's group has developed data-driven machine learning models to predict both the thermodynamic and kinetic properties of CO 2 absorption with ILs/DESs (e.g., the CO 2 solubility, absorbent viscosity and CO 2 mass transfer property) , providing a more comprehensive route for screening the promising ILs or DESs. However, the models are dependent on a large number of reliable experimental data, while the widely used COSMO-based models have a prior feature, resulting in the robust application value, especially when experimental data are missing. Moreover, COSMO-RS can combine with process simulation via Aspen to further evaluate the performance and conduct techno-economic analysis (García-Gutiérrez et al., 2016;De Riva et al., 2018). Therefore, COSMO-RS is a powerful tool for prior screening ILs/DESs.
In this work, the latest developed ILs and DESs were surveyed and collected, which were further combined with those have been collected in other review articles to provide a complete database. The IL/DES-based technologies were introduced, and the important properties were identified for each technology, providing criteria for IL/DES screening. Because not all the properties of ILs/DESs were provided, COSMO-RS was combined to predict the properties required in screening. Subsequently, the desirable ILs and DESs were screened, selected, and suggested for different IL/DES-based technologies.
2 Technologies for CO 2 separation with ionic liquids/deep eutectic solvents 2.1 Ionic liquids/deep eutectic solvents as solvents Over the past two decades, ILs/DESs as absorbents for CO 2 capture have been widely studied (Smith et al., 2014;Zeng et al., 2017;Hansen et al., 2020). The extensive attention and research are based on their desirable properties, such as negligible vapor pressure, stable thermodynamic and chemical properties, tunable and designable structures, and good solubility for CO 2 . In particular, the negligible vapor pressure can simplify the CO 2 absorption process.  (Blanchard et al., 2001). Since then, research on the CO 2 capture by ILs as absorbents has been reported extensively. According to the difference in capture mechanisms, ILs can be treated as two classes, i.e., the physical-based absorbents and chemical-based absorbents. The former represents the conventional ILs being able to interact with CO 2 physically, while the latter represents the functionalized ILs being able to interact with CO 2 chemically. In conventional ILs, anions usually play a leading role in the CO 2 capture, while the capture effect is relatively less affected by cations (Anthony et al., 2005;Ramdin et al., 2012;Dai et al., 2017).

Ionic liquids as solvents
Similar to other physical absorbents, the CO 2 absorption capacity of these physical-based ILs is inferior to those of traditional organic amines. In order to further enhance and improve the CO 2 absorption capacity, researchers developed functionalized ILs with chemical interaction sites, such as carboxyl-based ILs, amino-based ILs, and amino acid-based ILs (Zeng et al., 2017). Bates et al. (2002) first prepared the amino-functionalized IL [NH 2 P-BIN] [BF 4 ] to capture CO 2 , with an absorption capacity up to 0.50 mol/mol. Xue et al. (2011) reported the CO 2 solubility in amine-functionalized IL [AemMIM][Tau] up to 0.90 mol/mol. Later, the functionalized ILs with the anions introduced by amines were proposed. For example, Gurkan et al. (2010)  , both of which exhibited CO 2 absorption capacity up to 0.90 mol/mol, near a 1:1 stoichiometry, and subsequently, an array of nonamino-functionalized ILs were developed (Lin et al., 2019b). To further improve the absorption capacity, Wang et al. prepared and synthesized the pyridine-based IL [P66614][3-OMe3-2-Op] (Luo et al., 2014), demonstrating the absorption capacity of up to 1.60 mol/mol under 20°C and 0.1 MPa because of the two interaction sites presented between oxygen and nitrogen in the anion and CO 2 .
Since the industry is more interested in the mass absorption capacity, researchers have made great efforts to improve and enhance the mass absorption capacity of ILs for CO 2 capture. For example, Zhang et al. (2013) developed a novel kind of bisaminofunctionalized IL, whose mass absorption capacity of CO 2 can reach 18.5%. Chen et al. (2016)  In a real process of CO 2 separation, in addition to the CO 2 absorption capacity (solubility), viscosity is also a key factor to consider. This is particularly essential for the IL-based technology, as the viscosity of most developed IL is much higher than the conventional solvents (Jiang et al., 2019a). Functionalized ILs have an even higher viscosity than conventional ones, and the absorption with CO 2 is, in general, accompanied by a chemical reaction that further increases the viscosity (Liu et al., 2009;Sistla and Khanna, 2015;Ma et al., 2019). Therefore, it is necessary to develop ILs with low viscosity. Recently, Zhang et al. found an effective method to reduce the viscosity of ILs by introducing ether groups into cations, and increasing the number of ether-oxygen bonds decreased the viscosity (Zeng et al., 2015). Adding molecular solvent to an IL-based hybrid is another effective method to reduce viscosity. For example, when an organic amine solvent is added to ILs, the viscosity is significantly reduced, while the CO 2 absorption capacity is not significantly weakened (Zhao et al., 2011).
Furthermore, in practical CO 2 capture processes, the CO 2 selectivity to other gases should be carefully considered (Ma et al., 2018). This is because ILs will serve as either a physical absorbent only, or the absorbent with both chemical and physical contributions (Ma et al., 2021) that is different from the conventional chemical absorbents. For instance, in the CO 2 capture from natural gas or biogas, Zeng et al. (2015) designed and synthesized ether-based functionalized ILs, and their selectivity of CO 2 /CH 4 was improved by 50% compared with non-functionalized ILs. Additionally, the interaction between IL and CO 2 , in general, is weak compared with the amine-based absorbents, and the focus has been specifically concerned on the desorption enthalpy in the research.

Deep eutectic solvents as solvents
As a new type of "green solvents," DESs can be regarded as IL analogous, and it has unique advantages such as lower preparation cost, biodegradability, and more eco-friendly properties when compared with ILs (Zhang et al., 2012;Smith et al., 2014). DESs have received wide attention for gas separation, especially CO 2 capture (Smith et al., 2014;Hansen et al., 2020). When DESs are used as absorbents for CO 2 capture, they can be divided into physical-and chemical-based absorbents, just like ILs. Most conventional DESs capture CO 2 by means of physical absorption, while functionalized DESs like superbase-based DESs can capture CO 2 by means of chemical absorption (Sarmad et al., 2017b). The study by Sarmad et al. (2017b) showed that DESs are promising alternatives to ILs because the solubility of CO 2 in DESs is even higher than the similar IL counterparts. Recently, Huang et al. (Liu et al., 2019) found that the CO 2 solubility in the DES (ChCl + urea) can be accommodated by the HBA/HBD molar ratios, following the order of 1:2 > 1:1.5 > 1:2.5, which leads to the fact that the selectivities of gas pairs for CO 2 /H 2 S and CO 2 /CH 4 can be tuned by controlling the HBA/HBD molar ratios. Their study also explored the mechanism of CO 2 capture at the molecular level, that is, it was dominated by the free volume inside DESs. However, the ChCl-based DESs have higher viscosities than the conventional ILs (Smith et al., 2014;Hansen et al., 2020).
In order to improve CO 2 absorption capacity, superbasebased DESs with chemical absorption have been developed. Wang et al. (2010a) used DESs/ILs and superbase mixtures to prepare the superbase-based DES/ILs. This study clarified the role of the superbase in CO 2 capture, that is, the superbase enables abstracting acidic hydrogens from the imidazolium ring of cation in ILs to enhance the affinity with CO 2 , increasing CO 2 capture performance. Other publications have also proved that adding superbases or basic groups into the DES/IL systems can evidently improve the CO 2 absorption capacity (Wang et al., 2010b;Wang et al., 2011). It should be noted that the addition of these superbases or basic groups always increases viscosity, which is unfavorable to the absorption and increases the energy usage for transporting absorbent in the CO 2 separation process.
In short, both ILs and DESs show strong potential for industrial application in CO 2 capture, thereby replacing traditional organic amine absorbents to overcome solvent volatility loss, equipment corrosion caused by organic amine absorbents as well as high energy usage for absorbent regeneration. Physical-based ILs show relatively low CO 2 solubility and selectivity of CO 2 to other gases while relatively low viscosity also. The task-specific functionalized ILs enhance CO 2 solubility and selectivity, but generally possess high viscosity, complex preparation steps, high synthesis cost, and high absorption enthalpy. In comparison, DESs have received extensive research attention with the simpler preparation process, lower synthesis cost, and greener biodegradability, but generally have higher viscosities than ILs.
When ILs/DESs are used as a liquid absorbent, and the physical absorption-based technology is chosen, the CO 2 mass absorption capacity is important, the viscosity needs a particular concern, and the selectivity is another important property when screening and suggesting ILs. Instead, for the chemical absorption-based technology, the CO 2 mass absorption capacity, the viscosity, including that after CO 2 absorption, and the desorption enthalpy are three primary properties, while the selectivity may need to concern a certain extent.

Nano-confined ionic liquids
In order to overcome the high viscosity for functionalized ILs and low selectivity for conventional ILs as liquid absorbents, the so-called nano-confined ILs, that is, ILs are confined in nanoporous matrices, such as metal-organic frameworks (MOFs), molecular sieves, silica, and porous carbons, with their spatial geometrical dimensions have been proposed. In this case, either or both the nanoporous matrix (rigid host) and IL (soft guest) can be suitably modified for task-specific separation processes. Nano-confined ILs can effectively reduce the amount of ILs and overcome their viscosity issues in the potential application, resulting in hybrid materials with distinct advantages like low energy usage for regeneration, high selectivity, and recoverable CO 2 sorption. It was found that the conventional nano-confined ILs in silica could enhance CO 2 sorption capacity and rate. Shi and Luebke (2013) studied CO 2 sorption performance in [HMIM][Tf 2 N] confined into silica slit pores with widths from 2.5 to 4.5 nm using molecular simulations. It was found that the CO 2 solubility in the confined IL is significantly higher than that in the relevant bulk IL due to the increased molar volume of confined ILs. In addition to the conventional ILs, confining functionalized ILs have also been reported for CO 2 capture. Zhang et al. (2006) first supported the tetrabutylphosphonium amino acid ILs on silica gels to achieve fast and reversible CO 2 sorption with the capacity of 0.50 mol/mol. It is worth noting that, when most of the pore surface of support is occupied by the immobilized IL, the CO 2 sorption capacity of nano-confined ILs is dominated by the immobilized ILs rather than solid adsorbent (Ren et al., 2012). Similar to the bulk ILs, the CO 2 sorption capacity is always dependent on the IL molecular structures. For instance, in terms of a list of amino acid-based ILs confined in porous silica, the CO 2 sorption performance was mainly dominated by anions, and the highest CO 2 sorption capacity was presented at the IL with the maximum number of amino groups. Also, the nano-confined Frontiers in Chemistry frontiersin.org IL with the longer cationic alkyl side chain corresponds to the lower CO 2 sorption capacity (Ruckart et al., 2015). These studies and findings indicate that it is essential to put the focus on ILs, and CO 2 absorption capacity of bulk ILs can be used as an index to screen and suggest ILs for confinement. In some cases, the performance of confined ILs depends on both ILs and supports. Research has demonstrated that MOF is a promising class of materials for confining ILs . Ban et al. (2015) confined [BMIM][Tf 2 N] into the nanocages of ZIF-8 to achieve a highly selective gas separation for CO 2 /N 2 and CO 2 /CH 4 by adjusting the molecular sieving properties of ZIF-8. The molecular simulation by Vicent-Luna et al. (2013) indicated that adding ILs into the pore of MOFs can intensify the CO 2 sorption capacity at low pressures. Whereas the sorption behavior of N 2 and CH 4 cannot be affected. These findings may be ascribed to the "like dissolves like" principle. The IL@ MOF composites can significantly increase the selectivity of CO 2 / N 2 , and a higher IL concentration in MOFs corresponds to a higher selectivity (Xue et al., 2016). Furthermore, the selectivity of CO 2 separation can be improved by achieving a better dispersion behavior of ILs in MOFs, which explains MOF is a better candidate than the covalent organic framework (COF) (Xue et al., 2016). Han et al. (2021) synthesized a novel IL-ZIF-IL composite with the shell-interlayer-core structure using [TETA] L and ZIF-8, and investigated its CO 2 capture performance. The IL-ZIF-IL composite exhibited a strong molecular sieving ability, and its performance can be adjusted by controlling the IL loading, thereby changing the thickness of the outer IL layer in the IL-ZIF-IL composite. Under the different operating conditions, the selectivity of CO 2 /CH 4 (50:50) and CO 2 /N 2 (15:85) mixtures was as high as 260-1990 and 1,688-5,572, respectively. This fully demonstrates the potential of nanoconfined ILs in ZIF-8 for the selective separation of CO 2 . It also indicates that the selectivity in the IL-confined technology can be tuned by adjusting other parameters than IL itself.
Zeolites or molecular sieves are also a class of promising porous materials to confine ILs for CO 2 capture (Yu et al., 2014;Ahmad et al., 2018). For example, Yu et al. (2014)  In addition, combining ILs and membrane materials to prepare IL-based membranes is another alternative strategy to avoid the high viscosity of ILs, and more importantly, the gas separation performance can also be efficiently improved. Here, most of the work is based on polyIL as the supported polymer membrane. Compared to a pure polyIL membrane, the polyIL-IL composite membrane containing free IL can increase the permeability of CO 2 , N 2 , and CH 4 by 300%-600% (Bara et al., 2008). For mixed matrix membranes (MMMs) containing polyILs or polymers, free ILs and porous particles, the presence of ILs can improve the interfacial adhesion between particles and organic polymers, increasing CO 2 permeability and selectivity (Hudiono et al., 2010;Hudiono et al., 2011). Hao et al. (2013)

Nano-confined deep eutectic solvents
DESs, as a novel class of IL analogous, have also been confined into solid materials to capture CO 2 . Lin et al. (2021) synthesized two-dimension materials of nano-confined DESs using DES (ChCl + ethylene glycol) and the nanoslits of titanium carbide (Ti 3 C 2 T x ), and the properties of this material were determined by hydrogen bonds (HBs) between DESs and solid materials. The nano-confined DES demonstrated good thermal stability, long-term durability, and relatively high selectivity of CO 2 separation, in which the selectivity of CO 2 / N 2 , CO 2 /CH 4 , and CO 2 /H 2 are as high as 319.15, 249.01, and 12.38, respectively. Amira et al. (2018) first reported the DESbased membranes by incorporating DES + ethylene glycol (1:3) into polyvinylidene fluoride-co-polytetrafluoroethylene to capture CO 2 . It was found that the CO 2 permeation was up to 25.5 × 103 GPU (gas permeation unit), but the selectivity of CO 2 / N 2 was only 2.0. (Lin et al., 2019a) further developed the DESbased membranes and prepared the graphene oxide (GO) supported DES membranes by confining ChCl + ethylene glycol with different HBA/HBD molar ratios into the GO nanoslits.
The composite membranes demonstrated remarkable permeation selectivity of CO 2 /N 2 (>400), CO 2 / CH 4 (>300), and CO 2 /H 2 (>20), superior thermodynamic stability, and long-term endurance. The intensifying mechanism of CO 2 capture was also investigated by using molecular dynamics simulations, that is, GO provided nanoconfined space for the DES, resulting in the unique spatial configuration, as well the interaction of DES-GO weakened the original interaction between DES molecules, which enlarges the free volume and prompts the gas diffusion.
It is evident that the nano-confined DESs/ILs can overcome the major shortages of bulk ILs/DESs e.g., fluidity, high viscosity, and slow diffusion. However, it should be noted that the nanoconfined DESs/ILs as a class of hybrid material consisted of nonfreely moving liquids and porous solids, and the loss of liquid phase cannot be negligible. Moreover, nano-confined DESs/ILs Frontiers in Chemistry frontiersin.org 05 as macroscopic solid materials may change the CO 2 capture process from absorption to adsorption (or membrane separation), which is suitable for the CO 2 streams with low CO 2 concentration and low gas flow rate, instated of those with high-concentration CO 2 capture at high gas flow rates. Therefore, IL/DES-based liquid absorbents still show their advantages, and different IL/DES-based technologies need to be developed for different cases.
Meanwhile, combining the features of ILs/DESs and IL/DESbased technologies, the key properties can be quite different. When ILs/DESs are used as a solvent, if it is a physical-based technology, CO 2 solubility in weight (mass)-basis, selectivity, and viscosity need to be considered simultaneously, while for the chemical-based one, CO 2 solubility in weight-basis, viscosity before and after CO 2 absorption as well as desorption enthalpy should be concerned. For the advanced nanoconfined ILs/DESs, CO 2 solubility in weight-basis is the primary one, and desorption enthalpy may need to be included, while concerns on viscosity and selectivity can be mitigated.
3 Literature survey, data collection, properties perdition, and ionic liquids/deep eutectic solvents screening Considering the importance of CO 2 capture and separation, a lot of ILs and DESs have been reported and summarized in different review articles, while more are continuously reported individually. To collect ILs and DESs that have been developed together with their properties, firstly, the published review articles were summarized to accumulate the ILs and DESs that were summarized, and then the recently developed or reported ILs and DESs were surveyed. All the reviewed and surveyed ILs and DESs were collected together with the properties, including CO 2 solubility, viscosity, selectivity/heat of absorption, and for those without providing such properties, COSMO-RS was used to predict for the promising ILs/DESs. Finally, top 10 ILs and DESs were suggested for different IL/DES-based technology developments.
During this procedure, the concern on selectivity depends on the gas streams for CO 2 separation. Since selectivity is more essential for the pure physical-based ILs and DESs, which are preferable for the gas streams with relatively high CO 2 content, such as biogas or natural gas (CH 4 +CO 2 ), synthesis gas (H 2 +CO 2 , H 2 +CO 2 +CO), here CH 4 , CO, and H 2 , together with N 2 , were also concerned in order to estimate the ideal selectivity.
It should be pointed out that corrosion is an important property, while to the best of our knowledge, the relevant study on ILs/DESs is still limited, and thus such property was excluded in screening ILs/DESs.

Review of ionic liquids and deep eutectic solvents developed since 2021
According to the survey, the developed ILs and DESs up to 2020 have been well collected and summarized, and thus, the newly published ILs and DESs since 2021 were surveyed and collected in this part based on the Web of Science (WOS) database.
For the new DESs, Pishro et al. (2021) synthesized ammonium-based ones using ethanolamine hydrochloride (EAHC) and tetraethylenepentamine (TEPA) with various molar ratios and the one with 1:9 (molar ratio) exhibited the highest solubility of 9.671 mol/kg at 303.15 K and 16.03 bar.

Chemical-based deep eutectic solvents
Nine studies have been focused on chemical-based DESs since 2021, as summarized in Table 4. Among them, three DESs are newly synthesized (Fu et al., 2021a;Ahmad et al., 2021;Qian et al., 2022), while others are the previously synthesized ones for various purposes, including the discussion about the water content (Liu et al., 2022) and estimating the effects of HBA:HBD molar ration, type of anion, and the alkyl chain (Cichowska-Kopczyńska et al., 2021). Ahmad et al. (2021)    Apart from two-component DESs, Qian et al. (2022) synthesized three-component ones using amino acids as HBA, amines as HBDs, and 10-30 wt% of ethylene glycol. In this case, the CO 2 solubility reached up to 5 mol/kg. Some DESs have been made by mixing ILs ethylene glycol and 4-methylimidazole (Fu et al., 2021b;Wang et al., 2021c;Lee et al., 2021;Wang et al., 2022a).
In summary, most of the work since 2021 is mainly on the synthesis of chemical-based ILs. Also, a great effort has been devoted to the imidazulim-, phosphonium-and ammonium-based ILs, which were previously synthesized.

Data collection, properties prediction, and ionic liquids/deep eutectic solvents screening
Based on the information from the previous reviews published in recent years as well as the new ILs and DESs that have been surveyed in this work, the available ILs and DESs that have been developed are summarized in Tables 1-4; Supplementary Table S1-S2. According to the survey and collection, the CO 2 solubility is the main concern during the development, while the viscosity, selectivity and/or heat of absorption may also be covered, which are also essential for developing different IL/DES-based technologies.
Based on the collected ILs and DESs, promising ones can be suggested for different IL/DES-based technologies based on different criteria. In this work, the screening was conducted based on two options, according to the characterizations of different IL/DES-based technologies for CO 2 separation.
3.3.1 Option 1: Top 10 ionic liquids/deep eutectic solvents based on the CO 2 absorption capacity For the technology of immobilization, i.e., nano-confined ILs/ DESs for CO 2 capture, the CO 2 absorption capacity is essential, while the challenges related to viscosity and selectivity can be mitigated compared to the technologies with bulk ILs/DESs. Therefore, according to the results listed in Tables 1-4; Supplementary Table   FIGURE 3 Viscosity of chemical-and physical-based ILs predicted by COSMO-RS.

FIGURE 4
Top 20 physical-based ILs based on their properties.

Frontiers in Chemistry
frontiersin.org S1-S2, top 10 ILs/DESs based on CO 2 absorption capacity were screened from all the studied ILs/DESs for each type, i.e., physical or chemical-based. The screened ILs/DESs are listed in Table 5.

Option 2: Top 10 ionic liquids/deep eutectic solvents based on a set of properties
In this option, top 10 ILs/DESs were screened based on a set of properties, including CO 2 absorption capacity, viscosity, and selectivity/ heat of absorption. For the solvent-based technologies with ILs/DESs, desirable properties more than CO 2 absorption capacity are needed. However, not all the properties are available for the collected ILs/DESs. To achieve the screening, the work was conducted step by step.
Step 1: top 20 ILs/DESs were screened based on the CO 2 absorption capacity, and other properties of these top 20 ILs/DESs were combined or predicted with COSMO-RS; Step 2: top 10 ILs/DESs were further screened based on the different sets of parameters, depending on the feature of ILs/DESs (chemical-based or physical-based).

Properties for the top 20 ionic liquids/deep eutectic solvents
According to the CO 2 absorption capacity of ILs and DESs collected in Tables 1-4; Supplementary Table S1-S2, top 20 ILs/DESs were selected. The chosen 20 ILs/DESs are listed in Supplementary  Table S3-S6. To conduct screening, as most important properties other than CO 2 absorption capacity are unavailable, theoretical predictions can be desirable. The COSMO-RS model, as a prior method, has been widely used to predict a variety of thermodynamic properties, such as the phase equilibrium, saturated vapor pressure, infinite dilution activity coefficient of systems containing ILs and DESs, and identify molecular mechanisms in chemical separation and reaction processes (Klamt, 1995;Zhou et al., 2012;Klamt et al., 2016;Yu et al., 2022), and the previous research has evidenced the reliability of COSMO-RS in screening ILs and DESs (Palomar et al., 2011;Sistla and Khanna, 2011;Hadj-Kali et al., 2020). In this work, the COSMO-RS model was employed to evaluate the CO 2 solubility, selectivity of CO 2 to other gases (i.e., CO, H 2 , CH 4 , and N 2 ) and CO 2 excess enthalpy in ILs and DESs as well as the viscosity of ILs by means of the COSMOThermX19 software. The selectivity of different gases was obtained from the corresponding Henry's constants.

FIGURE 5
Top 20 chemical-based ILs based on their properties.

FIGURE 6
Top 20 physical-based DESs based on their properties.

FIGURE 7
Top 20 chemical-based DESs based on their properties.

ILs DESs
Physical-based Chemical-based Physical based Chemical-based In the COSMO-RS calculation, the molecular geometry structures of HBAs and HBDs in DESs as well as cations and anions in ILs were optimized based on the density functional theory (DFT) at the calculation level of B3LYP/6-31+G (d, p) (Curtiss et al., 1995) by using the Gaussian 09 program (Frisch et al., 2009). Then, the COSMO file was obtained based on the most stable geometrical configuration by carrying out the calculation of COSMO continuum solution models at the theoretical level of BP86/TZVP (Perdew, 1986;Schäfer et al., 1994).
Using COSMO-RS, the viscosity, selectivity, and CO 2 excess enthalpy in ILs and DESs were estimated for the top 20 ILs/DESs. The results are summarized in Supplementary Table S7-S8, and depicted in Figures 1-3. Here, it should be mentioned that COSMO-RS cannot be used to predict the viscosities of DESs.
Generally, CO 2 excess enthalpy and selectivity for physical-based ILs/DESs should be lower than those of chemical-based. According to the prediction results of COSMO-RS (Figure 1), some physical-based ILs/DESs show even higher CO 2 excess enthalpies than those of chemical-based, and the selectivity of chemical-based ILs/DESs is not as high as we expected (Figure 2), especially for CO 2 over CO and CH 4 in chemical-based ILs. These observations indicate that the qualitative prediction of COSMO-RS may need to be verified and further improved. However, no work has been conducted to study the selectively for the chemical-based ILs/DESs, and the reliability of COSMO-RS prediction needs to be further verified. As shown in Figure 3, the viscosity of the chemical-based ILs is in general higher than the physical-based, which is consistent with the observation from experiments. For almost all the top 20 ILs/DESs, the properties other than CO 2 absorption capacity were not determined experimentally. In this work, COSMO-RS was thus used for a rough estimation for qualitative comparison. It should be mentioned here that, the molar ratios of some physical-based DESs were not mentioned in the literature, and thus the prediction with COSMO-RS cannot be performed. In this case, other DESs were added in the top 20 list to facilitate the utilization of COSMO-RS (see Supplementary  Table S6).

Top 10 ionic liquids/deep eutectic solvents
As discussed in the previous section, the CO 2 absorption capacity (mass basis), viscosity, and selectivity were used as the criteria to screen physical-based ILs/DESs, while the absorption capacity (mass basis), viscosity (before and after CO 2 absorption), and CO 2 absorption enthalpy were needed to screening chemical-based ILs/DESs. However, the viscosities of most DESs are not accessible, and it was excluded in the DES screening in this section. For the chemical-based ILs/DESs, the viscosity after CO 2 absorption is also important, which again is inaccessible for most ILs/DESs. To address this, CO 2 excess enthalpy in ILs and DESs was used as the index to reflect this. Also, CO 2 absorption capacity is another important issue, which was also reflected with the CO 2 excess enthalpy in ILs/DESs. Therefore, the top 20 ILs/DESs were further evaluated.
According to the results depicted in Figures 4-7, top 10 ILs/ DESs were screened, and the results are summarized in Table 6.

Conclusion
Owing to the unique properties of ILs and DESs, intensive research has been conducted to develop such novel liquids for CO 2 capture. In this work, different IL/DES-based technologies were summarized, and the important properties of each technology were identified to provide a guideline for screening the desirable ILs/DESs. Subsequently, the ILs and DESs studied in the recent 2 years were surveyed and then combined with those collected in the previous review articles to provide an extensive database. Furthermore, when the experimental data was not available, COSMO-RS was used to predict the properties that are needed for screening. Based on the established database, the identified important properties, and the predictions from the COSMO-RS, top 10 ILs/DESs were screened for different IL/DES-based technologies, being beneficial for further evaluation and development.
Author contributions SF, GY, and XJ: Writing-Review & Editing by.