Recent Advances in the Synthesis, Application and Economic Feasibility of Ionic Liquids and Deep Eutectic Solvents for CO₂ Capture: A Review

Abstract

Global warming is one of the major problems in the developing world, and one of the major causes of global warming is the generation of carbon dioxide (CO₂) because of the burning of fossil fuels. Burning fossil fuels to meet the energy demand of households and industries is unavoidable. The current commercial and experimental techniques used for capturing and storing CO₂ have serious operational and environmental constraints. The amine-based absorption technique for CO₂ capture has a low absorption and desorption ratio, and the volatile and corrosive nature of the solvent further complicates the situation. To overcome all of these problems, researchers have used ionic liquids (ILs) and deep eutectic solvents (DESs) as a replacement for commercial amine-based solvents. ILs and deep eutectic solvents are tunable solvents that have a very low vapor pressure, thus making them an ideal medium for CO₂ capture. Moreover, most ionic liquids and deep eutectic solvents have low toxicity and can be recycled without a significant loss in their CO₂ capture capability. This paper first gives a brief overview of the ILs and DESs used for CO₂ capture, followed by the functionalization of ILs to enhance CO₂ capture. Moreover, it provides details on the conversion of CO₂ into different valuable products using ILs and DESs, along with an economic perspective on using both of these solvents for CO₂ capture. Furthermore, it provides insight into the difficulties and drawbacks that are faced by industries when using ILs and DESs.

1. Introduction

Long before the industrial revolution, the temperature of the Earth was controlled by greenhouse gases that naturally occurred, such as water vapor and clouds [1]. Greenhouse gases (GHGs) are transparent to sunlight, but they do not allow the heat radiated from the Earth to escape from the atmosphere and absorb a portion of it, thus keeping the temperature of the Earth uniform [2,3]. However, in the last few decades, the industrialization and burning of fossil fuels have generated anthropogenic GHGs, resulting in an imbalance in the temperature of the Earth, and researchers have coined the term climate change for this phenomenon [4]. The effect of climate change is different for different countries, and it is independent of the amounts of GHGs emitted by a particular country. The countries that are worst hit by climate change have qite small amounts of GHG emissions [5]. The major GHGs include carbon dioxide (75%), fluorinated gases (2%), methane (18%), and nitrous oxide (4%) [6]. Carbon dioxide (CO₂) is the major contributor to greenhouse gases because of fossil fuel burning in the transportation, heating, manufacturing, and electricity sectors [7,8]. The Intergovernmental Panel on Climate Change (IPCC) has shown that 79% of CO₂ produced from fossil fuels is used for power generation, and the main contributors are coal power plants, with a share of 60% [9,10]. Hence, it is highly necessary to devise an efficient strategy for CO₂ removal from waste gases and its utilization for the production of different chemicals [11].

The methods of CO₂ capture can be broadly classified into pre-combustion, oxy-combustion, and post-combustion processes [12]. In the process of pre-combustion, the fuel is liquified to generate syngas that contains H₂ and CO [13]. In the water–gas shift reaction, syngas is utilized to convert CO into CO₂ and to produce H₂ gas [14]. CO₂ can be captured, and H₂ gas can be used for power generation. The fuel that reacts with pure oxygen in oxy-combustion produces a very high concentration of CO₂ that is quite suitable for underground CO₂ storage [15]. The process of post-combustion CO₂ capture is the most widely used technique for CO₂ capture because, as compared to the pre-combustion process and oxy-fuel combustion, this process can be easily adapted to existing power plants [16]. The current techniques that are being used for CO₂ capture are shown in Figure 1.

These techniques include physical and chemical adsorption, absorption cryogenic, and membrane-based techniques [17]. Among the post-combustion techniques, chemical absorption using amines is widely used because amines have high thermal stability and reactivity, which facilitate their high absorption capacity for CO₂ [18]. Amine-based adsorption is basically carried out in a tall absorption column, which results in high capital costs. The other disadvantages of the amine process include the corrosion of the absorption column, higher regeneration costs, and the degradation of amines as a result of different impurities in flue gases, such as sulfur dioxide, oxygen, and nitrogen dioxide [19]. The cryogenic process is limited because of the exceptionally high cooling duty needed to capture CO₂ [20]. The adsorption process is used in the natural gas industry for CO₂ capture based on the difference in affinity among different hydrocarbons. The major limitations of the adsorption process include the low resistance to pressure changes, low adsorption capacity, and higher operating costs, along with the fire hazard in the adsorption process [21]. Membrane-based technologies are also widely used for CO₂ capture, but they have major issues related to selectivity, and higher costs are required for the pretreatment of the membrane [22].

From the above discussion, it can be concluded that absorption based on amines is one of the most effective approaches for capturing CO₂. However, the major problem in the amine process seems to be the use of the amine-based organic solvent and the regeneration process as a result of the chemical reaction. Thus, based on these conclusions, we need to devise a physical absorption process with some other solvents that could be used for CO₂ capture with a huge capacity. In a physical absorption process, we can easily regenerate the solvent. In the past few decades, researchers have used ionic liquids (ILs) and deep eutectic solvents (DESs) for different applications. In this review paper, we provide a detailed discussion regarding CO₂ capture and its conversion into useful products using DESs and ILs. A comparison of ILs and DESs with other absorbents is summarized in

Table 1. Comparison of several generations of CO₂ absorbents.

Absorbents	Advantages	Disadvantages	References
Biphasic solvents	Low energy consumption and low viscosity	Complex equipment	[23]
MEA	Low price	High volatility, corrosion, and high energy consumption	[24,25,26,27,28]
ILs	Nonvolatile, low corrosion, and high solubility	Biotoxicity, high price, and high viscosity	[29]
DESs	Nonvolatile, low corrosion, high solubility, nontoxic, low price, and biodegradable nature	High viscosity	[30]

With permission from Ref. [31]. Copyright 2020, Elsevier.

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2. Ionic Liquids

ILs are molten salts with a melting point lower than 100 °C and behave as a liquid at room temperature [32]. IL properties can be altered by the careful selection of the anion and cation [33,34]. ILs have bulky organic cations with low molecular symmetry and small organic/inorganic anions that are in their molten form in their pure states under ambient conditions [35].

Table 2. Structures of anions and cations commonly used for synthesis of ILs.

Cation	Structure	Anion	Structure
Pyridinium		Acetate
Imidazolium		Nitrate
Pyrrolidinium		Bis(trifluorophosphate)imide
Quaternary ammonium		Hexafluoro borate
Tetra alkyl phosphonium		Tetrafluoroborate
Piperidinium		Dicyanamide
Morpholinium		Triflate
Guanidinium		Alkylsulfate

gives examples of commonly used cations and anions to synthesize ILs. The strong affinity between anions and cations results in a negligible vapor pressure and nonvolatility at room temperature [36]. ILs are referred to as green solvents [37] due to their characteristics of inflammability, nonvolatility, and recyclability in comparison with organic solvents [38]. The nonvolatile behavior of ILs has been discussed as one of their fundamental properties and is necessary for the efficient capture of CO₂, with the benefit of lower energy consumption than other traditional solvents, such as amine scrubbing absorbents [39,40].

Figure 2 shows that the IL-CO₂ system’s physical behavior is incredible compared to conventional organic solvents (e.g., n-hexane and toluene). However, the absorption is still relatively low as compared to amines, which requires an increase in CO₂ absorption. Figure 2 illustrates conventional solvents’ critical-point phase behavior compared with IL systems. Most of the initial studies on capturing CO₂ with ILs were mainly focused on the CO₂ phase behavior with various physical (nonfunctionalized) ILs. In addition, different cations and anions were explored for CO₂ solubility in ILs [41].

Functionalization of ILs for CO₂ Capture

To enhance CO₂ absorption using ILs, researchers have come up with the functionalization of ILs for CO₂ capture, which facilitates the high absorption of CO₂ by ILs [42]. It is also reported that both physical and chemical adsorption is involved in the case of functional ILs [43]. A literature study also reveals that two molecules of ILs can absorb one molecule of CO₂ due to the chemical interaction following chemical equilibrium for the chemical absorption of CO₂ [44]. The functionalization of ILs was performed by introducing the CO₂-philic group to the anion/cation side of ILs or by introducing functional groups such as carbonyl, amine, or fluorine, which improves the adsorption affinity of CO₂ to ILs [45]. Figure 3 shows the effect of variations in the cation/anion and alkylation on the solubility behavior of CO₂.

Figure 3a,b illustrate that replacing hydrogen atoms with fluorine is a viable method to enhance CO₂ solubility [46]. These units can develop a network with CO₂ atoms to develop polar C-F bonds [47]. In Figure 3c, we can see that the cation is also one of the major players in CO₂ capture, and we can see that the cation [C₆H₄F₉mim] produces the highest solubility for CO₂. Similarly, in Figure 3d, it can be observed that the cation chain length also plays a vital role in CO₂ capture, and we can see that with the increase in the chain length, CO₂ capture is also enhanced [41].

Adding a carbonyl group is considered the most promising approach to enhancing the CO₂-selective behavior of ILs [24]. As far as the interaction mechanism is concerned, CO₂ completely behaves like a Lewis acid; however, carbon atoms saturated with low electron deficiency interact with the carbonyl group [48]. The interaction between the carbonyl group and CO₂ revealed that CO₂ could behave simultaneously as a Lewis acid and a Lewis base [49]. According to Muldoon et al., the addition of a butyl ester group to 1-butyl-nicotinic acid butyl ester bis (trifluoromethyl sulfonyl) imide [b2-Nic] [Tf₂N] has no significant effect on CO₂ solubility in low-pressure conditions. However, due to the interactions between the CO₂ molecule and the cation, [b2-Nic] [Tf₂N] enabled the better solubility of CO₂ at high pressures [50]. Yokozeki et al. reported the solubility of CO₂ in 18 distinct ILs. The ILs with [X-COO] were 1-butyl-3-methylimidazolium levulinate [C₄MIM] [LEV], 1-butyl-3-methylimidazolium trimethylacetate [C₄MIM] [TMA], 1-butyl-3-methylimidazoliumisobutyrate [C₄MIM] [IBS], 1-ethyl-3-methylimidazolium acetate [C₂MIM] [Ac], 1-butyl-3-methylimidazolium acetate [C₄MIM] [Ac], and 1-butyl-3-methylimidazolium prolinate [C₄MIM] [PRO], which resulted in strong chemical absorption and the high solubility of CO₂ [51].

Incorporating amine functional groups into IL structures also facilitates the chemical reactivity of ILs with CO₂ [52]. CO₂ is absorbed by amine-functionalized groups by following two paths: one is through carbonate formation, and the other is through carbamate [53]. The rate of CO₂ absorption using amines can be boosted by introducing either activators such as piperazine or a small proportion of primary and secondary amines [54]. Yamini’s group synthesized ILs based on an amino acid (aminate) as the anion. They evaluated and tested their capacity for CO₂ absorption. All ILs under investigation used the cation butyl methyl imidazolium ([Bmim]). The alginate ([ARG]) anion IL has demonstrated greater CO₂ absorption ability among all aminate-ILs examined, followed by lysinate ([LYS]) and histidinate ([HIS]), in terms of mole CO₂/mole IL. The order was prolinate [PRO] < alanine [ALA](1N) < valine [VAL](1N) < glycine [GLY](1N) < leucine [LEU](1N) < methionine [MET] (1N) < histidinate [HIS] (3N) < lysinate [LYS](2N) < arginine [ARG] (4N) for CO₂ molar absorption in the aminate-ILs investigated (1N). The FTIR spectra of CO₂-absorbed aminate-ILs confirm that CO₂ chemical absorption occurs through the production of carbamate species. Compared to amine-functionalized cation ILs and non-functionalized ILs, aminate-ILs demonstrated greater CO₂ solubility [26].

3. ILs Used for CO₂ Capture in the Last Five Years

The area of ILs is continuously expanding, and researchers are working on utilizing different ILs for CO₂ capture [55]. Table 3 shows the experimental CO₂ solubility in kgmol⁻¹ for the ILs used in the last five years. Benito et al. [56] used the Aspen Plus technique to investigate the CO₂ solubility behavior of ILs based on amino acids, carboxylate, and anionic-heterocyclic compounds. The ILs that were examined were 1-butyl-3-methylimidazoliumisobutyrate ([Bmim] [i-but]), trihexyltetradecylphosphonium-2-cyanopyrrole ([P₆₆₆₁₄] [CNPyr]), 1-butyl-3-methylimidazolium prolinate ([Bmim] [PRO]), and 1-butyl-3-methylimidazolium acetate ([P₂₂₂₈] [CNPyr]. [P₂₂₂₈] [CNPyr] showed excellent CO₂ absorption following thermodynamics, particularly at low pressure. The trend for CO₂ absorption capacity per mass of IL was as follows: [Bmim] [PRO] < [Bmim] [GLY] < [Bmim] [ibut] < [P₆₆₆₁₄] [CNPyr] < [Bmim] [acetate] < [P₂₂₂₈] [CNPyr] [56].

Chen et al. [57] used two kinds of amino acid ILs, namely, 1-ethyl-3-methylimidazolium alanine [EMIM] [Ala] and 1-ethyl-3-methylimidazolium glycine [EMIM] [Gly]. They compared them with 1-ethyl-3-methylimidazolium acetate [EMIM] [Ac], which had CO₂ solubilities in order of [EMIM] [Ac] < [EMIM] [Ala] < [EMIM] [Gly]. Somsak et al. [58] analyzed the CO₂ absorption phenomena in several ILs based on imidazolium-, pyridinium-, and tetraalkylammonium-based polyionic liquids (PILs). The highest CO₂ absorption was found in the PIL polyvinyl benzyl trimethylammonium hexafluorophosphate P[[VBTMA] [PF₆]], which exhibited an outstanding 77% (w/w) adsorption capacity. Jiang et al. [59] synthesized multi-amino-functionalized ILs and stated that ILs involving longer cationic chains, [TETAH] [Lys], performed better than [DETAH] [Lys]. The solubilities of functionalized ILs also rely on the choice of the anion and cation for the efficient absorption of CO₂ [60]. For some series of ILs, particularly ones containing ammonium- and phosphonium-functionalized cations, the influence of anions has been investigated ([DMAPAH] [61], [P₂₂₂₈] [62], [P₄₄₄₂] [63,64], [DETAH] [65], and [P₆₆₆₁₄] [60]). The top three ILs with functionalized anions and cations obtained CO₂ solubilities (mol kg⁻¹) of 10.15, 11.91, and 11.39 for [DETAH] [Gly], [DETAH] [Im], and [DETAH] [Py], respectively. The [DETAH]-based IL has a viscosity of about 1.02–0.54 mPa·s at a temperature of 303.15–333.15 K, whereas, in the case of [P₆₆₆₁₄]₂[Asp], it is 4905 mPa·s [64]. Luo et al. [66] examined the effect of various cations on viscosity and CO₂ absorption using 2-hydroxyl pyridium ([2-Op]) as the anion. They reported CO₂ solubilities in the order of [P₄₄₄₂OH] < [N₄₄₄₂] < [BMmim] < [P₄₄₄₂] < [Ph-C₈eim] < [BMPyr], whereas, for viscosity, the order was [Ph-C₈eim] >[BMmim] > [P₄₄₄₂OH] > [P₄₄₄₂] > [N₄₄₄₂] > [BMPyr] [66].

Table 3. CO₂ solubility in ionic liquids.

Ionic Liquid	Mw (g mol−1)	T (K)	P (kPa)	CO2 Solubility (mol kg−1)	References
[EMIM] [Ac]	170.2	313.2	100	1.65	[57]
[EMIM] [Ala]	200.26	313.2	100	1.89	[57]
[EMIM] [Gly]	186.3	313.2	100	2.32	[57]
[TETAH] [Lys]	266.43	313.15	100	9.72	[59]
[DETAH] [Py]	171.25	313.15	100	11.91	[65]
[DETAH] [Im]	171.25	313.15	100	11.39	[65]
[DETAH] [Lys]	277.39	313.15	100	7.68	[59]
[DETAH] [Gly]	178.24	313.15	100	10.15	[65]
[DETAH] [Tz]	172.23	313.15	100	10.1	[65]
[DMAPAH] [2F-PhO]	214.4	303.2	100	3.12	[61]
[DMAPAH] [3F-PhO]	21 4.4	303.2	100	3.39	[61]
[DMAPAH] [3,5F-PhO]	232.3	303.2	100	3.52	[61]
[DMAPAH] [4F-PhO]	214.4	303.2	100	3.99	[61]
[N1111] [Lys]	219	303	100	1.84	[64]
[P2228] [6-BrBnIm]	428.42	333.15	0–149.9	0–2.01	[62]
[P2228] [BnIm]	349.52	298.15	0–99.8	0–2.78	[62]
[P2228] [2CNPyr]	323.48	295.15	0–99.8	0–2.84	[62]
[BMIM] [2-Op]	233.31	303.15	100	4.37	[66]
[BMmim] [2-Op]	247.34	303.15	100	4.29	[66]
[BMPyr] [2-Op]	236.36	303.15	100	4.95	[66]
[N4442] [2-Op]	308.51	303.15	100	4.02	[66]
[P4442OH] [2-Op]	341.47	303.15	100	2.75	[66]
[P4442] [2-OP]	325.47	303.15	100	4.3	[66]
[Ph-C8eim] [2-Op]	379.54	293.15	100	4.45	[66]
[P4442] [DAA]	331.48	293.15	10	3.78	[63]
[P4442] [Suc]	329.46	293.15	10	5, 3.4	[63]
[P4442] [Ph-Suc]	329.47	293.15	100	4.25	[64]
[P4442] [Suc]	329.46	293.15	100	5.62	[64]
[P4442] [Cy-Suc]	383.56	293.15	100	5.76	[64]
[P66614] [Beta-Ala]	572.96	303.15	100	1.74	[60]
[P66614]2[Asp]	1068.83	303.15	100	1.83	[60]
[P66614] [MA-Tetz]	582.96	303.15	100	1.94	[60]
[P66614] [Gly]	558.93	303.15	100	2.15	[60]
[TMGH] [PhO]	209.29	313.15	100	0.24	[64]
[TMGH] [Im]	183.26	313.15	100	3.49	[64]
[TMGH] [Pyrr]	182.27	313.15	100	3.62	[64]
[VBTMA] [Ala]	294.44	298	100	0.98	[67]
[VBTMA] [Pro]	320.47	298	100	1.19	[67]
[VBTMA] [Ser]	310.44	298	100	1.26	[67]

With permission from Ref. [64]. Copyright 2020, Elsevier.

Table 3. CO₂ solubility in ionic liquids.

Ionic Liquid	Mw (g mol−1)	T (K)	P (kPa)	CO2 Solubility (mol kg−1)	References
[EMIM] [Ac]	170.2	313.2	100	1.65	[57]
[EMIM] [Ala]	200.26	313.2	100	1.89	[57]
[EMIM] [Gly]	186.3	313.2	100	2.32	[57]
[TETAH] [Lys]	266.43	313.15	100	9.72	[59]
[DETAH] [Py]	171.25	313.15	100	11.91	[65]
[DETAH] [Im]	171.25	313.15	100	11.39	[65]
[DETAH] [Lys]	277.39	313.15	100	7.68	[59]
[DETAH] [Gly]	178.24	313.15	100	10.15	[65]
[DETAH] [Tz]	172.23	313.15	100	10.1	[65]
[DMAPAH] [2F-PhO]	214.4	303.2	100	3.12	[61]
[DMAPAH] [3F-PhO]	21 4.4	303.2	100	3.39	[61]
[DMAPAH] [3,5F-PhO]	232.3	303.2	100	3.52	[61]
[DMAPAH] [4F-PhO]	214.4	303.2	100	3.99	[61]
[N1111] [Lys]	219	303	100	1.84	[64]
[P2228] [6-BrBnIm]	428.42	333.15	0–149.9	0–2.01	[62]
[P2228] [BnIm]	349.52	298.15	0–99.8	0–2.78	[62]
[P2228] [2CNPyr]	323.48	295.15	0–99.8	0–2.84	[62]
[BMIM] [2-Op]	233.31	303.15	100	4.37	[66]
[BMmim] [2-Op]	247.34	303.15	100	4.29	[66]
[BMPyr] [2-Op]	236.36	303.15	100	4.95	[66]
[N4442] [2-Op]	308.51	303.15	100	4.02	[66]
[P4442OH] [2-Op]	341.47	303.15	100	2.75	[66]
[P4442] [2-OP]	325.47	303.15	100	4.3	[66]
[Ph-C8eim] [2-Op]	379.54	293.15	100	4.45	[66]
[P4442] [DAA]	331.48	293.15	10	3.78	[63]
[P4442] [Suc]	329.46	293.15	10	5, 3.4	[63]
[P4442] [Ph-Suc]	329.47	293.15	100	4.25	[64]
[P4442] [Suc]	329.46	293.15	100	5.62	[64]
[P4442] [Cy-Suc]	383.56	293.15	100	5.76	[64]
[P66614] [Beta-Ala]	572.96	303.15	100	1.74	[60]
[P66614]2[Asp]	1068.83	303.15	100	1.83	[60]
[P66614] [MA-Tetz]	582.96	303.15	100	1.94	[60]
[P66614] [Gly]	558.93	303.15	100	2.15	[60]
[TMGH] [PhO]	209.29	313.15	100	0.24	[64]
[TMGH] [Im]	183.26	313.15	100	3.49	[64]
[TMGH] [Pyrr]	182.27	313.15	100	3.62	[64]
[VBTMA] [Ala]	294.44	298	100	0.98	[67]
[VBTMA] [Pro]	320.47	298	100	1.19	[67]
[VBTMA] [Ser]	310.44	298	100	1.26	[67]

With permission from Ref. [64]. Copyright 2020, Elsevier.

4. Conversion of CO₂ into Valuable Products Using ILs

The conversion of CO₂ into valuable products involves several major steps, such as CO₂ diffusion, adsorption, catalytic conversion, product distribution from the catalyst, and finally, the diffusion of the product to the bulk phase for solution separation [52,68]. Both the capture and conversion of CO₂ are essential features to achieve high-value products from CO₂ [69]. In photochemical, biochemical, and electrochemical reduction methods, ILs facilitate CO₂ conversion into quality products. This is the most reliable method for the electrochemical reduction of chemicals and CO₂. [70]. Both chemicals and epoxides were used for the conversion of CO₂ into linear and cyclic carbonates through cycloaddition reactions. Due to its remarkable ability for electrical energy storage from natural sources such as the sun, product conversion efficiency, and product selectivity, electrochemical CO₂ reduction is prevalent [71].

The cycloaddition of CO₂ with epoxides for its conversion via ILs is regarded as the most prominent approach toward the model conversion reaction. Peng et al. utilized ILs to facilitate the catalytic cycloaddition of CO₂ into epoxides without any involvement of organic solvents, even at 2 MPa and 110 °C [72]. Lkushima’s and Wang’s groups refined the rate of conversion and yield using supercritical CO₂ at pressures and temperatures of 8–14 MPa and 100–160 °C, respectively. The advantages of supercritical CO₂ comprise the rapid equilibrium and non-usage of organic solvents for the separation of reactants and products [73,74]. However, a lot of efforts have been made for the improvement and design of advanced IL synthesis via polymerization and functionalization [75] or through the use of support methods, such as MOF introduction [76], polymer support, and silica support [77]. Introducing metal elements into ILs by metal doping or immobilization also enhances their selectivity and efficiency in the cycloaddition of CO₂ [69,78]. ILs can act as catalysts and as solvents for the cycloaddition of CO₂ without involving the supercritical state, immobilization, or metals. Han et al. reported on the selectivity behavior and efficiency of task-specific ILs as solvents and catalysts at a pressure and temperature of 1 bar and 30–60 °C [79]. Many efforts have been made to reduce the viscosity of ILs to make them economical by improving their reaction time [80]. Wang et al. reported on the use of epoxide in propargylic alcohol for the cycloaddition of CO₂ to prepare α-alkylidene cyclic carbonate. The reaction was performed at a pressure and temperature of 1 bar and 60 °C, consuming 200 mol% of the IL without involving another solvent [81].

Zhang et al. reported that the superbase-derived PIL 1,8-Diazabicyclo[5.4.0]undec-7-ene acetate [DBU] [Ac] could catalyze carbamate formation from a solution of silica ester, amine, and CO₂ in acetonitrile, even at a pressure and temperature of 5 MPa CO₂ and 150 °C [82]. Amines based on aromatics showed lower activity than aliphatic amines because of the low PKa values of aromatic amines [82]. This is mainly attributed to the hydrogen-bonding interactions between aniline and the acetate anion of ILs. They require a protonated cation and basic anion to facilitate the efficient and smooth preparation of carbamate. The significant restrictions on using the results of this report were the presence of co-solvents (i.e., acetonitrile) and harsh conditions (such as 5 MPa and 150 °C). The synthesis of carbamate from CO₂ catalyzed by ILs could be improved even at low CO₂ pressure and mild temperature by designing new tasks for metal-free ILs [69].

In order to obtain C1-C21 hydrocarbons from CO₂ hydrogenation, Qadir et al. increased the conditions by raising the temperature to 150 °C and pressure to 6.8 MPa for H₂ and 1.7 MPa for CO₂ without any involvement of supporting solvents (such as DMSO and H₂O) using Ruthenium Iron (RuFe) as a catalyst and 1-butyl-3-methylimidazolium bis[trifluoromethyl)sulfonyl] imide [BMIM] [Tf₂N] as a solvent [83]. They used a hydrophobic IL, [BMIM] [Tf₂N], involving the same mechanism as that of the IL in Figure 4. Hydrocarbon production via media of non-basic ILs involves two steps: CO₂ conversion to CO by the reverse water–gas shift reaction on the surface of RuFe and subsequent chain circulation via Fischer–Tropsch synthesis [83].

In light of Qadir’s report, the involvement of the RuFe catalyst at a high temperature and pressure was a necessary condition to produce hydrocarbons. Melo et al. raised the systematic temperature above 150 °C to obtain CH₄ in a yield of 69%, maintaining a comparable pressure (4–6 MPa H₂ with a total H₂/CO₂ pressure of 8 MPa) through the involvement of Ru nanoparticles as the catalyst in 1-octyl-3-methyl-imidazolium bis(trifluoromethylsulfonyl) imide [OMIM] [Tf₂N]. The enhancement in the concentration of the catalyst and the temperature would also increase the CH₄ yield and TON [84].

Deng et al. successfully studied the fascinating combination of the reaction and separation using a system of cesium hydroxide (CsOH)-IL catalysts for the carbonylation of both aromatic and aliphatic amines with CO₂ at pressure and temperature of 6 MPa and 170 °C, respectively, to give rise to the formation of urea derivatives [85]. The product could also be recovered again using drying and filtration via the addition of water to the reaction mixture. The high pressure/temperature and the presence of CsOH would necessitate more energy [85]. Jing et al. used 1-butyl-3-methylimidazolium hydroxide [BMIM] [OH] by combining the benefits of ILs with the simplicity of CsOH to avoid the utilization of the relatively expensive and risky CsOH technique. Although the conditions for CO₂ conversion were still very tight (170 °C and 5.5 MPa), they expanded the range of amines to include benzylamine, cyclohexylamine, and aliphatic amines [86].

5. Economics of IL-Based CO₂ Capture

The cost of CO₂ capture using ILs is a challenging aspect of utilizing ILs at an industrial scale to attain the economic commercialization of this technology. However, significant research efforts in the past decades have been made to develop ILs for efficient CO₂ capture [87,88]. It was successfully achieved by improving the hydrothermal stability and the CO₂ capacity of ILs [89]. However, the overall cost includes the costs of the synthesis of ILs [90], CO₂ adsorption and desorption, and the regeneration of ILs [91]. The main problem is the high material costs and solvent requirements for synthesizing ILs. However, these costs are still significantly greater than those for conventional solvents and amine-based solvents [92,93]. Numerous experiments have been carried out to make ILs cost-effective for CO₂ capture and conversion [94]. The main benefit of ILs over conventional amine-based solvents during regeneration is their ability to absorb CO₂ [67] and other acidic gases, with the advantage of requiring minimum energy compared with conventional solvents based on amines [95,96]. Nevertheless, these systems may not be economically comparable to conventional solvents due to the current high demand for and price of solvents [56,88].

In terms of the techno-economic intentions of ILs in CO₂ absorption, Riva et al. proposed an alternative operative cost of 83 USD/t CO₂ using 1-ethyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]-imide ([Emim] [NTf₂]) for post-combustion CO₂ capture [97], whereas the cheapest possible cost previously attained by Martinez et al. was 90 USD/t CO₂ using 1-ethyl-3-methylimidazolium dicyanamide ([Emim] [DCN]) [98]. García et al. conducted a techno-economic evaluation for ILs such as 1-Hexyl-3-methylimidazolium bis[(trifluoromethyl)-sulfonyl]imide [Hmim] [NTf₂]), 1-ethyl-3-methylimidazolium bis (trifluoromethylsulfonyl)imide [Emim] [NTf₂], and trihexyl(tetradecyl)phosphoniumbis[(trifluoromethyl)-sulfonyl]imide ([P₆₆₆₁₄] [NTf₂]) for upgrading biogas, employing [Emim] [NTf₂] as a cost-effective alternative with a total cost of USD 271 per metric ton of CO₂ captured [99]. T. E. Akinola et al. investigated CO₂ removal using a solution of MEA and H₂O (30/30/40 wt%) with 1-butylpyridinium tetrafluoroborate, [Bpy] [BF₄], and found an expected cost (25 USD/t CO₂) to obtain a cost-efficient and energy-efficient gas separation technique [21]. This is due to the reduced operating cost of the [Bpy] [BF₄]-MEA-based process as a result of the low utility cost [100]. Y. Huang et al. performed a cost analysis for the feasibility of using [Bpy] [BF₄]-MEA-based solutions compared to the conventional MEA-based process and achieved a lower cost ranging from USD 70 to USD 60–62.5 per metric ton of CO₂ [101]. Shiflett et al. also performed an equilibrium-based simulation for 1-butyl-3-methylimidazolium acetate, [Bmim] [acetate], which showed an affinity toward CO₂ with a total cost of about 140 USD/t CO₂ [102].

One of the core issues that stands in the way of commercialisation is the high cost of ILs. However, in one of the recent modelling and simulation studies on the ILs production process shows that ILs can be produced at lower cost ($1.24 kg⁻¹), which is in comparison with most of the organic solvents such as acetone or ethyl acetate with a cost of $1.30–$1.40 kg⁻¹ [103]. Similarly, in another study, the extraction of aromatic hydrocarbon from aliphatic hydrocarbon with 4-methyl-N-butylpyridinium tetrafluoroborate was modelled using ASPEN resulting in a positive margin of about €20 million per year [104]. These results indicate that ILs are not necessarily expensive, and therefore large-scale ILs-based processes can become a commercial reality provided that some industry is ready to take up the project.

6. Difficulties and Drawbacks of Using ILs to Capture CO₂

ILs have been developed as potential sorbents for carbon capture and conversion operations. Despite the many advancements in ILs for carbon capture, there is still a need for significant advancements in ILs for the carbon capture and utilization (CCU) process [11]. To begin with, the majority of ILs used in the mature binding process are pyridine- or imidazole-based, which exhibits biological toxicity [105,106]. ILs disintegrate near the normal boiling point, and therefore, it is difficult to determine their critical properties. As a result, effective techniques for obtaining the essential properties of ILs must be developed [107].

One of the primary problems in CO₂ capture using ILs is their high viscosity, which leads to a decrease in CO₂ solubility in ILs. There are several approaches to this problem that need to be thoroughly explored and investigated. One approach is to use a combination of water and amines with ILs, for which the best composition and process conditions must be found [108]. The cost of ILs used for CO₂ capture is another drawback. In comparison to amines, ILs used in CO₂ capture processes are more expensive than conventional solvents. Although the cost of ILs on a large scale (less than USD 40/kg) can be substantially lower than the present lab-scale pricing (about USD 1000/kg), they are still 10–20 times more expensive than conventional solvents [107].

Researchers must develop easy and cost-effective synthesis methods for CO₂ capture using ILs as target solvents. To employ ILs as an absorption method on a wide scale, the right system design and operating conditions must be chosen [109]. Large-scale applications are one of the main problems with IL-based membranes. Lab-scale experiments are carried out by changing a single parameter (such as time) under ideal conditions. However, under actual conditions, the parameters vary at the same time, making the procedure extremely complicated [110]. For example, when technology is used to extract CO₂ from industrial exhaust gases, the conditions are vastly different from those seen in lab-scale research. This includes the flue gas composition, which contains SO₂, H₂O, N₂, CO₂, and O₂ and may contain ash, NO_x, CO, and other tiny particles. Another difficulty with flue gas streams is pressure loss. The loss of ILs from the surface of the membrane owing to dispersion, evaporation, and displacement causes the membrane to perform poorly [111].

Supported ionic liquid membranes (SILMs) have been developed to effectively separate CO₂ from various gas mixtures, particularly N₂ and CH₄. However, the stabilization of ILs on the membrane support, the degradation of membranes, and variation in membrane thickness are still the main challenges to obtaining better gas permeability and selectivity in real operating conditions [52]. However, research on process evaluations of CO₂ capture mediated by ILs is still limited to date, and more experimentally based and theoretical simulation techniques are encouraged to drive progress in IL-based technologies. The major difficulties in developing a proper CO₂ absorption system using ILs on an industrial scale are the high viscosity, availability, cost, compatibility, and purity of ILs [112].

7. Deep Eutectic Solvents

DESs are another class of task-specific solvents and are produced by combining a hydrogen-bond donor (HBD) and a hydrogen-bond acceptor (HBA) at a suitable molar ratio [113]. In most DESs, a quaternary ammonium halide, such as choline chloride (ChCl), is used as a hydrogen-bond donor. Figure 5 shows the synthesis of common DESs formed by combining ChCl with urea obtained from the amalgamation of a hydrogen-bond donor and acceptor. In contrast to ILs, they are not entirely composed of ions. Therefore, they have a low boiling point compared to the parent components [114]. They are nontoxic and non-flammable, and they have negligible vapor pressure and high thermal stability. In addition, they can be made from natural components and easily purified, thus ensuring biodegradability and significantly lower cost [115]. The structures of some common DESs with their names are shown in

Table 4. Common structures of halide salts and hydrogen bonds used for DES synthesis.

Halide Salt	Structure	Hydrogen Bond	Structure
Choline chloride		Lactic acid
Choline nitrate		Acetamide
Choline acetate		Glycerol
Ethyl ammonium chloride		Melic acid
N-Ethyl-2-hydroxy-N,N-dimethylethanaminium chloride		Oxalic acid
2-Chloro-N,N,N-trimethylethanaminium chloride		Phenol
Tetrabutylammonium bromide		Malonic acid
N,N,N-Trimethyl(phenyl)methenamine chloride		D-Glucose
Lithium bis[(trifluoromethyl)sulfonyl] imide		D-Fructose
1-Butyl-3 methyl-imidazolium chloride		D-Isosorbide
Methyltriphenylphosphonium bromide		Sorbitol
2-(Acetyloxy)-N,N,N-trimethylethanaminium chloride		Xylitol
Benzyltriphenylphosphonium chloride		Xylenol
1-Ethyl-3-butylbenzotriazolium hexafluorophosphate		Trifluoro acetamide
1-Butyl-3-methylimidazolium chloride		Glutaric acid
Tetra propylammonium bromide		O-Cresol
2-Fluoro-N,N,N-trimethylethanaminium bromide		Levulinic acid

.

CO₂ solubility in DESs is associated with factors such as temperature, pressure, alkyl chain length, and the molar ratio of hydrogen-bond donors [116]. It has been reported that the CO₂ separation performance of DESs increases with a decrease in temperature and pressure [117]. The negative effect of CO₂ separation is also associated with the humidity ratio in DESs. Water acts as an antisolvent on the solubility behavior of DESs [118]. The solubility behavior of CO₂ in DESs is associated with a molar ratio of hydrogen-bond donors of 1:2 to attain maximum CO₂ solubility [117].

Functionalization of DESs for CO₂ Capture

Regarding the chemical absorption of CO₂, amine functionalization in DESs has drawn a lot of interest. During amine functionalization, two equivalents of amines can react with one equivalent of CO₂ to form one equivalent of carbamate under anhydrous conditions. The most widely used solution for the industrial absorption of CO₂ is MEA. However, losses such as solvent degradation, equipment corrosion, and evaporation are also associated with MEA solutions, which limits their use for various applications [119]. Mukesh et al. [120] synthesized low-viscosity polyamine-based DESs to achieve high sorption efficiency with the advantage of low solvent losses. Their results revealed that the sorption of CO₂ using tetraethylenepentamine-based DESs was more stable as compared to mono ethylene amines at a temperature of 80 °C. Characteristics such as low viscosity, low solvent loss, and a high gas sorption rate not only promote CO₂ capture but also make them promising candidates as replacements for conventional amine-based technology [120].

Adeyemi et al. [121] used three ethanolamine-based DESs for CO₂ absorption in different molar ratios of 1:6, 1:8, and 1:10. They used monoethanolamine, diethanolamine, and methyl diethanolamine as primary, secondary, and tertiary amines as HBDs. The results revealed that the synthesized DESs showed high absorption capacity as compared to aqueous amine solutions (30 wt.%). Sarmad et al. [122] synthesized five new DESs via the functionalization of choline chloride–ethanolamine in a ratio of 1:7 (mol:mol) using diethanolamine, methyl diethanolamine, piperazine, and 1-(2-aminoethyl)piperazine. The solubility of CO₂ in the DES was measured at a temperature of 298.15 K and pressures up to 2 MPa. The results of the study revealed that the amines enhanced the absorption capacity of choline chloride–ethanolamine (1:7) in the order of piperazine > N-aminoethylpiperazine > N-methyldiethanolamine > N-diethanolamine [122]. Haider et al. [123] compared CO₂ absorption in amine-based DESs with glycol. The amine-based solvents showed higher CO₂ absorption as compared to glycol-based solvents. The highest absorption capacity was obtained for a tetra butyl ammonium bromide and methyldiethanol amine system, with 0.29 mol CO₂/mol DES at 1 MPa and 303.15 K [123]. In one of the studies by Zhang et al., they used BmimCl with MEA as a DES. It was found that they have exceptionally high CO₂ uptake as compared to ChCl with MEA-based DESs [124]. Choi et al. [118] studied the formation of dual amino-functionalized DESs by reacting mono ethanol amine (MEA) with hydrochloric acid (HCL), followed by mixing with ethylene diamine (EDA). However, the higher viscosity hinders CO₂ uptake, which was reduced by mixing the DESs in ethylene glycol, which enhances the absorption and desorption process [118].

Yang et al. [125] have also synthesized azole-based DESs, in which solid azole-based ILs and ethylene glycol were used. However, it was found that CO₂ does not interact directly with azolide, and it interacts with EG to form carbonate. Desorption takes places under a nitrogen atmosphere at 343.15 K [44,126]. In one of the other studies by Gurkan et al. [127], a low-viscosity and thermally stable DES was prepared from 1-ethyl-3-methylimidazolium 2-cyanopyrolide [Emim] [2-CNpyr] with ethylene glycol. This DES captures CO₂ via different routes, such as the complexation of CO₂ with the anion, the formation of carboxylate via the deprotonation of the cation by the anion to form carbene zwitterion, the protonation of the anion by ethylene glycol (EG) to form carbonate, and the formation of bicarbonate in the presence of water [127].

In one of the other functionalization strategies for DESs, superbases were combined with organic compounds with alcohol functionalization [128,129]. This in turn created ternary DESs, in which choline chloride and ethylene glycol were combined with different bases, such as 1,8-Diazabicyclo [5.4.0] undec-7-ene (DBU), 1,5-Diazabicyclo [4.3.0]non-5-ene (DBN), 7-Methyl-1,5,7-triazabicyclo[4.4.0]dec-5-ene (MTBD), and triazabicyclodecene (1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD). It was found that these ternary DESs chemically absorb CO₂ and form carbonate with a significant increase in viscosity, thus limiting the mass transfer. However, the absorbed CO₂ was easily released at 333.15 K in 35 min [130]. One of the other studies by Zhang et al. used DBN as a superbase with ethylurea (EU), 1,3-dimethylurea, and dimethylurea to form a low-viscosity DES with a viscosity lower than 12 mPa-s. The absorption rate initially increased with an increase in temperature from 298.15 K to 318.15 K, followed by a decrease in absorption from a temperature of 318.15 K to 338.15 K. Desorption was carried out at 353.15 and was in the range of 86–87% [131,132].

Zeng et al. prepared DESs from superbase ILs and combined them with ethylene glycol. The ILs used were based on the 1,8-diazabicyclo-[5.4.0]-undec-7-ene 2,2,3,3,4,4,5 [HDBU] cation, and three different anions were used: were imidazole [Im], indium [Ind], and triazole [Triz]. The highest CO₂ capture was found in DESs with the imidazolium anion, which has the highest basicity, followed by [Ind] and [Triz]. The viscosity of the DESs increased after the absorption of CO₂. CO₂ desorption took place at 343.15 K with nitrogen purging [133].

8. DESs Used for CO₂ Capture in the Last Five Years

Table 5 lists DESs that have undergone CO₂ capture testing since 2017, along with their related CO₂ solubilities (in mol/kg). The composition and mole ratios of the HBD substituents with HBAs are mainly responsible for the CO₂ solubility in DESs [134]. Abbott et al. combined ChCl and urea in a 1:2 molar ratio to create the first DESs [135]. Li et al. were the first to carry out an investigation on the potential of DESs for a CO₂ capture system utilizing eutectic mixtures of ChCl and urea in varied molar ratios of 1:1.5, 1:2, and 1:2.5 at various temperatures and pressures. At a temperature and feed pressure of 313.15 °C and 12 MPa, respectively, the highest CO₂ solubilities were found to be 0.201, 0.303, and 0.203. Their research demonstrated that variations in the molar ratio of the HBA to HBD affect CO₂ solubility in DESs [136].

Leron et al. investigated the CO₂ solubility behavior in different combinations of ChCl with glycerol, urea, and ethylene glycol. They found that the nature of the HBD plays a significant role in the CO₂ solubility behavior in DESs [137]. Sarmad et al. [125] synthesized DESs based on phosphonium or ammonium salts networked with different HBDs. The rate of CO₂ solubilities for DESs was measured at a pressure and temperature of 2 MPa and 298.15 K, respectively. In addition, viscosities were measured in the range of 293.15 to 333.15 K. Both the viscosities and CO₂ solubilities in synthesized DESs were compared to those in ILs. These synthesized DESs’ viscosities and CO₂ solubilities were compared to those of traditional ILs. Fifteen samples of these synthesized DESs were considered potential prospects because of their lower viscosities (below 200 mPas at 298.15 K) and CO₂ solubilities (>1 mol/kg DESs at a pressure of 2 MPa) compared to those of standard ILs. Vinyl acetate (v-AC), Tetraethylammonium chloride acetate (TEAC-AC), chlorine chloride ethyl acetate (ChCl-EA), benzyl trimethylammonium acetate (BTMA-AC), benzyltriethylammonium acetate (BTEA-AC), Methyltriphenylphosphonium-levulinic acetate (MTPP-LV-AC), Methyltriphenylphosphonium acetate (MTPP-AC), tetrabutylammonium bromide-ethyl acetate (TBAB-EA), tetrabutylammonium-chloride acetate (TBAC-AC), tetrabutylammonium-bromide acetate (TBAB-AC), tetramethylammonium-chloride acetate (TMAC-AC), tetrapropylammonium chloride-ethyl acetate (TPAC-EA), tetraethylammonium-chloride octyl (TEAC-OCT), tetrapropylammonium-chloride acetate (TPAC-AC), and guanidinium-ethyl acetate (Gua-EA) were among the promising DESs included [125,138].

Figure 6 compares CO₂ solubility in DESs with that in ILs from the literature. DESs based on phosphonium salt gain high CO₂ solubility as compared with DESs based on ChCl under the specified conditions. It can be seen in Figure 6 that, at 1500 kPa and 313.15 K, DESs show a good solubility for CO₂ of about 0.1950–0.2134 as compared to 1-Ethyl-3-methylimidazolium ethyl sulfate [Emim] [EtSO₄], N-butylpyridinium tetrafluoroborate [N-bpy]-[BF₄], and 1-butyl-3-methylimidazolium dicyanamide [Bmim] [DCA], with CO₂ solubilities of 0.1000, 0.1582, and 0.1440, respectively, and CO₂ shows good solubility in ILs based on imidazolium with anions of hexafluorophosphate [PF₆]⁻, tetrafluoroborate [BF₄]⁻, trifluoromethanesulfonate [TFO]⁻, and nitrate [NO₃]⁻ in the range of 0.1960–0.2340 [139].

The separation of CO₂ is significantly impacted by the insertion of subbases in DESs [140]. To date, a number of innovative superbase-based DESs have been studied. According to Wang et al. [141], DESs generated from superbase ionic liquids (ILs) and ethylene glycol (EG) containing biophenol have a high CO₂ capture affinity of up to 1.0 mol CO₂/mol DESs, which is higher than that of the parent ILs. Superbase-based DESs/ILs were created by Wang et al. [142] using DESs/ILs and superbase mixtures. The involvement of the superbase in CO₂ capture, which enables the removal of acidic hydrogens from the imidazolium cation ring in ILs and improves the affinity for CO₂, is a key finding in this study. Numerous studies have also shown that DESs/ILs can benefit from adding superbases or main groups [142]. Numerous studies have shown that improving DES/IL systems with superbases or primary groups improves CO₂ absorption [108]. In the CO₂ separation process, it is crucial to keep in mind that adding any superbases or primary groups will always raise the viscosity, which is unfavorable for absorption and increases energy consumption [143].

Table 5 shows that TBD-based DESs have superior CO₂ affinity compared to DBU-based DESs. At 298.15 K and 100 kPa, TBD-EG (1:4) exhibits a rather high absorption capacity of up to 12.9 mol kg-1. Additionally, the DBN-EU CO₂ molarity absorption increased as the DBN ratio dropped, demonstrating the importance of the molar ratio in CO₂ absorption. DBN also has the benefit of having relatively little steric restriction around the structure of the imine, which makes it a perfect super-foundation for creating DESs [144].

Table 5. CO₂ solubility in various DESs.

HBD	Salt	Molar Ratio	Solubility	Pressure (MPa)	Temperature (K)	References
1,4-Butanediol	ChCl	1:3	0.1624/mol·kg−1	0.5097	293.15	[145]
1,4-Butanediol	ChCl	1:4	0.1560/mol.kg−1	0.5134	293.15	[145]
2,3-Butanediol	ChCl	1:3	0.1501/mol.kg−1	0.5113	293.15	[145]
2,3-Butanediol	ChCl	1:4	0.1915/mol.kg−1	0.5085	293.15	[145]
1,2-Butanediol	ChCl	1:3	0.1827/mol.kg−1	0.5145	293.15	[145]
1,2-Butanediol	ChCl	1:4	0.1884/mol.kg−1	0.5015	293.15	[145]
Levulinic acid	ChCl	1:3	0.2549/mol.kg−1	0.57	303.15	[146]
Levulinic acid	ChCl	1:4	0.2700/mol.kg−1	0.5749	303.15	[146]
Levulinic acid	ChCl	1:5	0.2869/mol.kg−1	0.5667	303.15	[146]
Furfuryl alcohol	ChCl	1:3	0.1856/mol.kg−1	0.5828	303.15	[146]
Furfuryl alcohol	ChCl	1:4	0.2196/mol.kg−1	0.5815	303.15	[146]
Furfuryl alcohol	ChCl	1:5	0.2276/mol.kg−1	0.5774	303.15	[146]
Ethylene glycol	ChCl	1:2	3.1265/mol.kg−1	5.863	303.15	[137]
Phenol	ChCl	1:2	0.1945/mol.kg−1	0.4945	293.15	[147]
Phenol	ChCl	1:3	0.2052/mol.kg−1	0.5085	293.15	[147]
Phenol	ChCl	1:4	0.2108/mol.kg−1	0.5092	293.15	[147]
Diethylene glycol	ChCl	1:3	0.1687/mol.kg−1	0.5129	293.15	[147]
Diethylene glycol	ChCl	1:4	0.1852/mol.kg−1	0.5088	293.15	[147]
Triethylene glycol	ChCl	1:3	0.1909/mol.kg−1	0.504	293.15	[147]
Triethylene glycol	ChCl	1:4	0.1941/mol.kg−1	0.5135	293.15	[147]
Urea	ChCl	1:2	3.5592/mol.kg−1	5.654	303.15	[148]
Glycerol	ChCl	1:2	3.6929/mol.kg−1	5.863	303.15	[137,148]
Urea	ChCl	1:1.5	0.2010/mol.kg−1	11.84	313.15	[136]
Urea	ChCl	1:2	0.3090/mol.kg−1	12.5	313.15	[136]
Urea	ChCl	1:2.5	0.2030/mol.kg−1	12.45	313.15	[136]
Triethylene glycol	ChCl	1:4	0.0419/mol.kg−1	1	298.15	[149]
Ethylene glycol	ChCl	1:4	0.0230/mol.kg−1	1	298.15	[149]
Ethylene glycol	ChCl	1:8	0.0262/mol.kg−1	1	298.15	[149]
Urea	ChCl	1:4	0.0240/mol.kg−1	1	298.15	[149]
Urea	ChCl	1:2:5	0.0211/mol.kg−1	1	298.15	[149]
Glycerol	ChCl	1:3	0.0454/mol.kg−1	1	298.15	[149]
Glycerol	ChCl	1:8	0.0306/mol.kg−1	1	298.15	[149]
Ethanol amine	ChCl	1:6	0.1096/mol.kg−1	1	298.15	[149]
Diethanol amine	ChCl	1:6	0.0925/mol.kg−1	1	298.15	[149]
Glycerol	ChCl	1:12	0.0511/mol.kg−1	1	298.15	[149]
Ethylene glycol	BTPPC	1:12	0.0503/mol.kg−1	1	298.15	[149]
Ethanol amine	BTPPB	1:6	0.1441/mol.kg−1	1	298.15	[149]
Ethanol amine	MTPPB	1:7	0.1254/mol.kg−1	1	298.15	[149]
Ethanol amine	MTPPB	1:8	0.1189/mol.kg−1	1	298.15	[149]
Ethanol amine	MTPPB	1:6	0.1168/mol.kg−1	1	298.15	[149]
Diethanol amine	TBAB	1:6	0.1036/mol.kg−1	1	298.15	[149]
Triethanol amine	TBAB	1:3	0.0830/mol.kg−1	1	298.15	[149]
Lactic acid	TMACl	1:02	0.0588/mol.kg−1	1.992	308.15	[150]
Lactic acid	TEACl	1:02	0.0725/mol.kg−1	1.993	308.15	[150]
Lactic acid	TBACl	1:02	0.1272/mol.kg−1	1.992	308.15	[150]
Glycerol + DBN	ChCl	1:02:06 CH-Cl: gly:DBN	2.3−2.4 mol.kg−1	Ambient		[130]
Urea	ChCl	1:2	3.559 mol.kg−1	6	303.15	[148]
Ethylene glycol	ChCl	1:02	3.1265 mol.kg−1	5.863	303.15	[137]
Ethanolamine	ChCl	1:06	0.0749 mol.kg−1	1	298	[149]
Ethanolamine	MTPP_Br		0.0716 mol.kg−1	1	298	[149]
Ethanolamine	TBA_Br		0.0591 mol.kg−1	1	298	[149]
Triethylene glycol	ChCl	4:01	0.1941 mol.kg−1	0.5	293	[147]
Phenol	ChCl	4:01	0.2108 mol.kg−1	0.5	293	[147]
Diethylene glycol	ChCl	4:01	0.1852 mol.kg−1	0.5	293	[147]

With permission from Ref. [151] (Copyright 2015, American Chemical Society) and from Ref. [24] (Copyright 2017, John Wiley & Sons, Inc.).

Table 5. CO₂ solubility in various DESs.

HBD	Salt	Molar Ratio	Solubility	Pressure (MPa)	Temperature (K)	References
1,4-Butanediol	ChCl	1:3	0.1624/mol·kg−1	0.5097	293.15	[145]
1,4-Butanediol	ChCl	1:4	0.1560/mol.kg−1	0.5134	293.15	[145]
2,3-Butanediol	ChCl	1:3	0.1501/mol.kg−1	0.5113	293.15	[145]
2,3-Butanediol	ChCl	1:4	0.1915/mol.kg−1	0.5085	293.15	[145]
1,2-Butanediol	ChCl	1:3	0.1827/mol.kg−1	0.5145	293.15	[145]
1,2-Butanediol	ChCl	1:4	0.1884/mol.kg−1	0.5015	293.15	[145]
Levulinic acid	ChCl	1:3	0.2549/mol.kg−1	0.57	303.15	[146]
Levulinic acid	ChCl	1:4	0.2700/mol.kg−1	0.5749	303.15	[146]
Levulinic acid	ChCl	1:5	0.2869/mol.kg−1	0.5667	303.15	[146]
Furfuryl alcohol	ChCl	1:3	0.1856/mol.kg−1	0.5828	303.15	[146]
Furfuryl alcohol	ChCl	1:4	0.2196/mol.kg−1	0.5815	303.15	[146]
Furfuryl alcohol	ChCl	1:5	0.2276/mol.kg−1	0.5774	303.15	[146]
Ethylene glycol	ChCl	1:2	3.1265/mol.kg−1	5.863	303.15	[137]
Phenol	ChCl	1:2	0.1945/mol.kg−1	0.4945	293.15	[147]
Phenol	ChCl	1:3	0.2052/mol.kg−1	0.5085	293.15	[147]
Phenol	ChCl	1:4	0.2108/mol.kg−1	0.5092	293.15	[147]
Diethylene glycol	ChCl	1:3	0.1687/mol.kg−1	0.5129	293.15	[147]
Diethylene glycol	ChCl	1:4	0.1852/mol.kg−1	0.5088	293.15	[147]
Triethylene glycol	ChCl	1:3	0.1909/mol.kg−1	0.504	293.15	[147]
Triethylene glycol	ChCl	1:4	0.1941/mol.kg−1	0.5135	293.15	[147]
Urea	ChCl	1:2	3.5592/mol.kg−1	5.654	303.15	[148]
Glycerol	ChCl	1:2	3.6929/mol.kg−1	5.863	303.15	[137,148]
Urea	ChCl	1:1.5	0.2010/mol.kg−1	11.84	313.15	[136]
Urea	ChCl	1:2	0.3090/mol.kg−1	12.5	313.15	[136]
Urea	ChCl	1:2.5	0.2030/mol.kg−1	12.45	313.15	[136]
Triethylene glycol	ChCl	1:4	0.0419/mol.kg−1	1	298.15	[149]
Ethylene glycol	ChCl	1:4	0.0230/mol.kg−1	1	298.15	[149]
Ethylene glycol	ChCl	1:8	0.0262/mol.kg−1	1	298.15	[149]
Urea	ChCl	1:4	0.0240/mol.kg−1	1	298.15	[149]
Urea	ChCl	1:2:5	0.0211/mol.kg−1	1	298.15	[149]
Glycerol	ChCl	1:3	0.0454/mol.kg−1	1	298.15	[149]
Glycerol	ChCl	1:8	0.0306/mol.kg−1	1	298.15	[149]
Ethanol amine	ChCl	1:6	0.1096/mol.kg−1	1	298.15	[149]
Diethanol amine	ChCl	1:6	0.0925/mol.kg−1	1	298.15	[149]
Glycerol	ChCl	1:12	0.0511/mol.kg−1	1	298.15	[149]
Ethylene glycol	BTPPC	1:12	0.0503/mol.kg−1	1	298.15	[149]
Ethanol amine	BTPPB	1:6	0.1441/mol.kg−1	1	298.15	[149]
Ethanol amine	MTPPB	1:7	0.1254/mol.kg−1	1	298.15	[149]
Ethanol amine	MTPPB	1:8	0.1189/mol.kg−1	1	298.15	[149]
Ethanol amine	MTPPB	1:6	0.1168/mol.kg−1	1	298.15	[149]
Diethanol amine	TBAB	1:6	0.1036/mol.kg−1	1	298.15	[149]
Triethanol amine	TBAB	1:3	0.0830/mol.kg−1	1	298.15	[149]
Lactic acid	TMACl	1:02	0.0588/mol.kg−1	1.992	308.15	[150]
Lactic acid	TEACl	1:02	0.0725/mol.kg−1	1.993	308.15	[150]
Lactic acid	TBACl	1:02	0.1272/mol.kg−1	1.992	308.15	[150]
Glycerol + DBN	ChCl	1:02:06 CH-Cl: gly:DBN	2.3−2.4 mol.kg−1	Ambient		[130]
Urea	ChCl	1:2	3.559 mol.kg−1	6	303.15	[148]
Ethylene glycol	ChCl	1:02	3.1265 mol.kg−1	5.863	303.15	[137]
Ethanolamine	ChCl	1:06	0.0749 mol.kg−1	1	298	[149]
Ethanolamine	MTPP_Br		0.0716 mol.kg−1	1	298	[149]
Ethanolamine	TBA_Br		0.0591 mol.kg−1	1	298	[149]
Triethylene glycol	ChCl	4:01	0.1941 mol.kg−1	0.5	293	[147]
Phenol	ChCl	4:01	0.2108 mol.kg−1	0.5	293	[147]
Diethylene glycol	ChCl	4:01	0.1852 mol.kg−1	0.5	293	[147]

With permission from Ref. [151] (Copyright 2015, American Chemical Society) and from Ref. [24] (Copyright 2017, John Wiley & Sons, Inc.).

9. Conversion of CO₂ into Valuable Products Using DESs

The study of DESs is a significant research field with many applications, such as liquid extraction and CO₂ capture. In the last several decades, different researchers have applied several types of DESs for CO₂ capture, and their details are described in the following paragraphs.

The chemical fixation of CO₂ into cyclic carbonates has received significant attention because of its extensive application as an electrolyte for lithium-ion batteries [152], polar aprotic solubilizers [153], iso-cyanate-free synthetic polyurethane [154], and synthetic intermediates [155]. DESs are regarded as one of the most prominent candidates with great potential for the catalytic conversion of CO₂ [156], with advantages including their easy synthesis [157], availability [158], and outstanding catalytic activity. Cheng et al. [159] used catalysis for the conversion of CO₂ with chlorine chloride (ChCl) and ZnBr₂ into cyclic carbonates, which showed good agreement for CO₂ conversion. Liu et al. [160] revealed that the use of DESs based on phosphonium improves the efficiency in catalyzing CO₂ conversion into various cyclic carbonates, which promotes the activation of CO₂ and epoxides simultaneously. Furthermore, DESs modified by using heterogeneous lignin catalysts can be utilized for the cycloaddition of CO₂ and epoxides, with the advantage of attaining various cyclic carbonates at 1 MPa and 110 °C [161]. The above findings revealed that DESs are the most prominent candidates to be catalyzed for the cycloaddition of epoxides and CO₂. However, the CO₂ fixation catalyzed by DESs in cyclic carbonates under mild conditions is still underdeveloped.

The electro-enzymatic conversion of CO₂ is regarded as a magical approach toward the utilization of CO₂ with low efficiency in terms of its low conversion rate [162]. The conversion of CO₂ into valuable products such as fuels [163] or chemicals [164] is an effective strategy for reducing greenhouse effects and facilitating resource recycling to achieve sustainability in energy development [165]. To date, many efforts have been made to synthesize methanol from CO₂ using electrochemical, photochemical, and enzyme conversion methods [152].

However, the inertness of CO₂ [166] and high thermal stability [167] are factors that have made the conversion of CO₂ very challenging, along with the disadvantages of low selectivity, low efficiency, and high energy demand [168]. However, the enzymatic conversion of CO₂ is a promising approach, with the advantages of high selectivity and high efficiency, along with environmental friendliness, even under mild conditions [169]. The chronological enzymatic conversion of CO₂ to methanol follows the pathway of CO₂ → formic acid → formaldehyde → methanol, which can be favored by formaldehyde dehydrogenase, alcohol dehydrogenase, and formate dehydrogenase [170]. However, developing advanced electrolytes is a promising approach that can significantly improve the reaction efficiency by dissolving CO₂ via the transfer of electrons and the adsorption of CO₂ [171]. DESs are regarded as magical solvents that offer characteristics such as high conductivity [172], low vapor pressure [173], an inclusive electrochemical gate [174], biocatalysis, electrocatalysis [175,176], and energy storage [174], with the advantages of easy synthesis and high affinity for CO₂ capture [132], which may increase enzyme activity during biocatalysis [162]. The DESs created by Zeng et al. demonstrated strong CO₂ affinity with an adsorption capacity of up to 0.141 g-CO₂/g-DESs [171]. Copper selenide catalysts for the electroreduction of CO₂ to methanol were created by Yang et al. The current density reaches 41.5 mA/cm² with a Faradaic efficiency of 77.6% when supported by IL-based electrolytes. With their superior biocompatibility, natural deep eutectic solvents (NDESs) can significantly boost the yield by up to 181% [177,178]. In order to increase the electro-enzymatic conversion of glutamate to histidine, Zhang et al. produced four different NADESs, including glutamate glycerol (GluGly), [179], serine glycerol (SerGly) [180], arginine glycerol (ArgGly) [181], and histidine glycerol (HisGly) [182]. They employed them as co-electrolytes in the electro-enzymatic conversion of CO₂ and found that they have favorable biocompatibility. When these NADESs were tested in an enzymatic reaction, SerGly demonstrated the highest enzyme activity levels. Therefore, further research was conducted on the SerGly-based solution for the electro-enzymatic conversion of CO₂. It was discovered that the SerGly-based solution’s CO₂ solubility is 11 times greater than that of the traditional buffer, which helps to improve the enzymatic CO₂ conversion [162]. All of these studies demonstrated that using an electrolyte based on DESs effectively increases CO₂ conversion during electro-enzymatic catalysis.

10. Economics of DES-Based CO₂ Capture

The economic cost of DESs is essential for its implementation in an industrial process [183]. Compared with conventional solvents for capturing CO₂, DESs have numerous economical features, such as lower material costs [149], easy synthesis [116], no generation of by-products [161], low viscosity [184], nontoxicity [185], biodegradability [161], and environmental friendliness [186]. In addition, DESs show ionic and molecular solvent features that are not present in ILs or their molecular counterparts [187]. Generally, ILs are regarded as highly efficient solvents for CO₂ solubility, but their higher solution cost limits their excessive use for CO₂ capture [188]. During the amalgamation of ILs, many organic solvents and an immense amount of water are used, demonstrating the sizeable environmental cost of ILs [189]. However, in the case of DESs, the cost depends upon the formation of components for carbon capture [103,113]. The higher biocompatibility and biodegradability of DESs also positively affect their environmental cost, making them an economical solvent for CO₂ capture [190].

DESs are easy to prepare and typically involve a single-step synthesis without eliminating by-products and solvents [191]. Furthermore, biocompatible and biodegradable raw materials can be used instantaneously to synthesize DESs without altering their promising characteristics because of the weak interactions between components [192]. In the case of ILs, their tunable nature requires massive capital investment for bulk production [193]. Moreover, lower molar uptake values, inefficient energy consumption, higher viscosity, and capital costs also negatively affect the techno-economic efficiency of ILs for separation [194]. Apart from this, DESs maintain their performance under humidity [195] and attain quick regeneration, making them an economic candidate for a large variety of applications [196]. All of these advantages place DESs as an appropriate candidate over other conventional solvents, such as ILs, and simple alkanol amines commonly used for gas separation [197]. The recently published literature highlights the significance of utilizing DESs synthesized from ILs and supplementary HBDs [198]. These types of modified DESs not only enhanced CO₂ absorption but also improved many properties, such as heat capacity [199], density [200], thermal stability [201], and viscosity, compared to their essential structural components [202]. However, the enhanced CO₂ absorption mechanisms of DESs through modification in free volume and intramolecular/intermolecular interactions are under discussion to identify cost-saving substitutions for CO₂ capture [116].

11. Difficulties and Drawbacks of DESs for CO₂ Capture

DESs have shown promising industrial applications, especially in CO₂ capture. Important findings from research groups all over the world have demonstrated the importance of utilizing DESs for the CO₂ capture process [203]. Despite their excellent properties and good affinity toward carbon dioxide, they still possess certain limitations, and more investigations are needed in terms of their physical properties [149]. There has been very limited research reported on gas separation from flue gases using DESs. The lack of physical data on the high viscosity and corrosion behavior of DESs from pilot plant applications limits their utilization for industrial-based applications. Because of its uniqueness, this field requires additional research to develop neoteric DESs with a high efficiency of absorption, low economic cost, and low viscosity [116].

In addition, DESs are formed by mixing a volatile component and a nonvolatile component, resulting in a highly volatile compound. The highly volatile nature of DESs leads to difficulties during their application for CO₂ capture [204]. Because of the importance of DESs in CO₂ capture applications and the viscosity of the available DESs, they should be properly evaluated. DESs are typically quite viscous liquids [205], with a viscosity of 750 mPa s at 298.15 K for the traditional CHCl + urea (1:2 molar ratio). For certain DES families, this is significantly worse. For instance, recently proposed DESs based on sugar hydrogen-bond donors (HBDs) have extremely high viscosities (e.g., 34,400 mPa s for CH-Cl + glucose in a 1:1 molar ratio at 323.15 K) [206], and DESs that contain metallic compounds, such as a 1:2 molar ratio of CH-Cl + ZnCl₂, also have higher viscosities, such as 85,000 mPa s at 298.15 K. The higher viscosity of DESs hinders their implementation on an industrial scale due to poor mass and heat transfer, excessive pumping costs, and other problems. They create key processes in particular [207].

Additionally, prior research suggests that water molecules in DESs may behave like antisolvents and reduce CO₂ solubility. Su et al.’s research [208] reveals that when water concentrations in amino methanamide + 2-hydroxy-N,N,Trimethylethanaminium chloride (reline) climb to a mole ratio greater than 0.769, CO₂ absorption transitions from endothermic to exothermic [208]. The hygroscopic nature of DESs is referred to as a drawback, having a considerable impact on CO₂ solubility at low pressures. For industrial applications, CO₂ solubility at higher temperatures is particularly crucial [209]. Leron et al. [101] investigated the effect of temperature (303.15–343.15 K) on the solubility of CO₂ in mixtures of chloride-based DESs (CH-Cl + glycerol) and discovered that the DES mixtures diminish their CO₂ solubility when raising the temperature. Similar effects of CO₂ absorption mediated by DESs have also been investigated by other researchers with increased pressures and temperatures [137,210]. Very limited literature is available on DESs to be used for CO₂ capture. Therefore, detailed work is suggested to examine various types of DESs for CO₂ capture and observe their various influencing parameters.

12. Conclusions and Future Perspectives

CO₂ is a major environmental problem, and researchers are testing different procedures to control this issue and keep the environment clean. Different ILs and DESs are also used by various researchers to capture CO₂, and various advances have been made in this field. However, some difficulties are still faced in this research field. The main findings from this literature study are highlighted below:

❖

From the studied literature, it is concluded that various ILs and DESs, due to their attractive properties, are suitable candidates for CO₂ capture. Some researchers have reported ILs with better performance for CO₂ capture but reported DESs are more economical, especially on the basis of synthesis.

❖

Several functional groups, such as carbonyl, amine, and fluorine, as well as alkyl chain lengths, are incorporated in ILs because they improve the adsorption affinity of CO₂ toward ILs. In the case of DESs, some factors, such as temperature, pressure, and alkyl chain length, are important to examine. However, the amine functional group in DESs has drawn a lot of interest for CO₂ capture.

❖

From the reported literature, it is concluded that CO₂ could not only be captured through ILs and DESs from various resources but also possibly be converted into various valuable products.

❖

It is also concluded that the available data are not sufficient, and more ILs need to be explored, especially DESs, for CO₂ capture. The viscosity of ILs is the main problem for CO₂ capture, and highly viscous ILs are not desirable for such applications. Similarly, the IL purity is also very important. However, the lower viscosity of DESs is also favorable for CO₂ capture, and the literature reports that DESs with lower viscosity obtain good results for CO₂ capture. The hygroscopic nature of DESs has been referred to as a drawback of these mixtures, as it has a considerable impact on CO₂ solubility at low pressures.