Ionic Liquids and Poly (Ionic Liquids) for CO₂ Capture: A Comprehensive Review

Abstract

The rising concentration of atmospheric carbon dioxide (CO₂), driven largely by fossil fuel combustion, is a major contributor to global climate change and ocean acidification. As conventional CO₂ capture technologies, primarily amine-based solvents, face challenges such as high energy requirements, volatility, and degradation, there is an urgent need for alternative materials that are both efficient and sustainable. Ionic liquids (ILs) and poly (ionic liquids) (PILs) have emerged as promising candidates due to their unique physicochemical properties, including negligible vapor pressure, high thermal and chemical stability, structural tunability, and strong CO₂ affinity. This review provides a comprehensive overview of recent advancements in the design, synthesis, and application of ILs and PILs for CO₂ capture. We examine the mechanisms of CO₂ absorption in IL and PIL systems, analyze the structure-property relationships influencing capture performance, and compare their advantages and limitations relative to conventional solvents. Special attention is given to the role of functional groups, anion/cation selection, and polymeric architectures in enhancing CO₂ uptake and reducing regeneration energy. Finally, the review highlights current challenges and future research directions for scaling up IL and PIL-based technologies in industrial carbon capture and sequestration systems.

1. Introduction

As of 2024, the global average atmospheric carbon dioxide (CO₂) concentration was 422.8 parts per million (ppm) [1]. This value is approximately 50% higher than the pre-industrial CO₂ level of 278 ppm in 1750 [2]. The last time CO₂ levels were this high prior to industrialization was during the mid-Pliocene Warm Period, approximately 3 million years ago [3]. CO₂ is not only a greenhouse gas but also contributes significantly to ocean acidification. As CO₂ dissolves in ocean water, it forms carbonic acid, which lowers the pH of the ocean. This process alters the delicate balance of marine environments, affecting calcifying organisms such as corals and shellfish and ultimately impacting the entire marine ecosystem. Since 1985, ocean pH has dropped from 8.11 to 8.05 as of 2021 [4]. Although this may appear to be a small change, the pH scale is logarithmic; thus, this shift represents a 15% increase in acidity since 1985 [1,4,5,6]. Since the pre-industrial era, ocean acidity has increased by 40% [4].

Roughly 80% of anthropogenic CO₂ emissions stem from fossil fuel combustion, including coal, oil, and natural gas. With the growing global energy demand, particularly in developing economies, the implementation of carbon capture and sequestration (CCS) strategies has become increasingly urgent. CCS is a suite of technologies designed for preventing the release of CO₂ into the atmosphere by capturing it from industrial sources or directly from the air, then transporting it to a storage site for long-term sequestration [7,8].

Currently, the most widely used solvents for CO₂ absorption are monoethanolamine (MEA) [9] and diethanolamine (DEA) [10], which are amine-based compounds [11]. These solvents work efficiently through chemisorption, reacting with CO₂ to form carbamates. However, they suffer from serious drawbacks, including high volatility, degradation during operation, high regeneration energy requirements, and equipment corrosion [12,13,14]. For example, MEA-based systems typically consume 3.5 GJ of energy per ton of CO₂ captured, substantially reducing the net carbon mitigation benefit of the process [15].

These limitations have driven the search for alternative materials and solvents that offer better stability, lower regeneration energy requirements, and minimal environmental impact. In recent years, a broader range of CO₂ capture methods has emerged, such as solid sorbents [16] including metal organic frameworks (MOF) [17], supramolecular organic frameworks (SOF) [18], zeolites [19] and porous organic polymers [20]; membrane-based separations [21]; and oxy-fuel combustion [22]. Ionic liquids (ILs) have emerged as promising candidates due to their unique physicochemical properties such as high thermal stability, negligible vapor pressure, tunable structure, and strong CO₂ affinity. In particular, ILs offer the ability to design task-specific solvents through modification of the cation–anion pair or by introducing functional groups, enabling improved performance for CO₂ capture under various conditions [15]. Bera et al. synthesized tetraethylammonium alaninate ([N₂₂₂₂][Ala]) and tetraethylammonium lysinate ([N₂₂₂₂][Lys])-based deep eutectic solvents (DES) with ethylene glycol as a hydrogen-bond donor. Direct Air Capture (DAC) tests showed remarkable uptake from ambient air, 1.06 mol CO₂/mol for the lysinate system and 0.47 mol/mol for alaninate [23]. These findings indicate that amino acid-functionalized IL-based DESs exhibit high CO₂ affinity even under atmospheric conditions, suggesting their strong potential for DAC applications.

2. Overview of Carbon Capture Technologies

A variety of CO₂ capture methods have been explored and broadly classified into solvent absorption, membrane separation, physical adsorption, cryogenic separation, and chemical looping combustion [24,25,26,27,28,29,30,31].

Solvent absorption [31], especially using aqueous amine solutions like MEA, remains the most established and widely applied technology for post-combustion CO₂ capture. In this method, the flue gas is contacted with the solvent in an absorber column, where CO₂ is chemically absorbed. The CO₂-rich solvent is then sent to a regenerator or stripper, where heat is applied to release CO₂ and regenerate the solvent. Despite its effectiveness, this method is energy-intensive and presents operational challenges, including solvent degradation, foaming, and corrosion of equipment.

Membrane separation [28] involves the use of semi-permeable membranes to selectively separate CO₂ from gas mixtures. Membranes offer advantages such as compact design, scalability, and the absence of phase changes. However, current membrane materials often suffer from trade-offs between permeability and selectivity and are still being optimized for commercial viability. Membrane technologies under investigation include polymeric membranes, facilitated transport membranes, and mixed matrix membranes.

Physical adsorption [29] uses solid adsorbents like zeolites, activated carbon, silica, and MOFs to capture CO₂ through physisorption. These materials typically exhibit high surface area and porosity, making them suitable for capturing CO₂ at low concentrations. However, issues related to cyclic stability, moisture sensitivity, and regeneration energy need to be addressed.

Cryogenic separation [30] relies on cooling flue gases to low temperatures to condense CO₂. This method is energy-intensive but can achieve high-purity CO₂. It is more suitable for gas streams with high CO₂ concentrations.

Chemical looping [27] combustion uses metal oxides as oxygen carriers to oxidize fuels, producing a pure stream of CO₂. The metal oxides are cyclically oxidized and reduced, effectively separating CO₂ from nitrogen. Although this process shows promise, it is still in the developmental stage.

Each of these methods has its strengths and limitations, and ongoing research focuses on improving energy efficiency, scalability, and environmental compatibility. Against this backdrop, ILs and PILs have attracted significant attention due to their tunable properties, which can be exploited to overcome many of the drawbacks associated with conventional technologies.

3. Properties and Chemistry of ILs and PILs

Ionic liquids (ILs) are organic salts that remain liquid below 100 °C and are often referred to as green solvents due to their favorable environmental properties [26,32,33]. They exhibit high thermal stability and can be tailored to specific applications by modifying the cation and anion pairs. Figure 1 illustrates the commonly used cations and anions in ILs for CO₂ absorption. Common IL cations include imidazolium, pyridinium, ammonium, and phosphonium, while common anions include tetrafluoroborate ([BF₄]⁻), hexafluorophosphate ([PF₆]⁻), bis(trifluoromethylsulfonyl)imide ([Tf₂N]⁻), acetate ([Ac]⁻), and amino acid-based anions. The properties of ILs can be finely tuned by altering the length of alkyl side chains, introducing electron-withdrawing or electron-donating groups, or incorporating functional groups such as amines, hydroxyls, or carboxylates [34].

ILs can be synthesized through metathesis reactions or via neutralization methods, where an acidic cation precursor reacts with a basic anion precursor to form the desired IL [35]. The resulting ILs possess high ionic conductivity, high density, and unique solvating properties. Due to their ability to form strong electrostatic and van der Waals interactions with CO₂, ILs often show high CO₂ solubility and selectivity over other gases such as N₂, CH₄, and H₂. Moreover, ILs exhibit low vapor pressure and require low regeneration energy, saving 36% to 74% energy compared to conventional solvents [25]. ILs can also dissolve biopolymers like cellulose, which are otherwise insoluble in common organic solvents. For example, Meli et al. (2010) [36] demonstrated that room temperature ILs could dissolve cellulose for fiber spinning applications. These characteristics further highlight their versatility [36].

Despite these advantages, ILs face challenges such as high viscosity, slow mass transfer rates, and potential toxicity, which limit their broader adoption. One strategy to mitigate these issues involves the development of supported ILs, where ILs are immobilized on porous supports such as MOFs, silica, or polymers to enhance stability and facilitate handling. Poly(ionic liquids) (PILs) represent the polymeric analogs of ILs, wherein ionic moieties are covalently tethered to polymer backbones. PILs can be synthesized by direct polymerization of IL monomers or by post-polymerization modification of commercial polymers [37]. Due to their macromolecular architecture, PILs combine the functional diversity of ILs with the mechanical robustness, thermal stability, and processability of conventional polymers [24].

PILs also exhibit interesting electrospinning behavior due to their inherent ionic nature. During electrospinning, the presence of mobile ions leads to significant repulsion along the polymer chains, leading to greater chain stretching and consequently, finer fiber diameters compared to neutral polymers. The electrostatic repulsion also affects the rheological properties of PILs, impacting solution viscosity and fiber morphology [38].

4. Mechanisms of CO₂ Capture Using ILs and PILs

The mechanism of CO₂ capture in ILs and PILs depends on a complex interplay of physical and chemical interactions. CO₂ is a quadrupolar molecule, which allows interaction with both polar and nonpolar species in ILs [39]. The nature of these interactions can be broadly classified into physisorption and chemisorption, as shown in Figure 2.

Physisorption is governed by weak forces such as van der Waals interactions, electrostatic attraction, and hydrogen bonding. It occurs mainly in ILs with non-basic anions and is strongly influenced by parameters such as porosity, surface area, and temperature. The energy requirement for regeneration of physiosorbed CO₂ is typically low, making these systems attractive for cyclic operations [40].

Chemisorption, on the other hand, involves the formation of covalent or ionic bonds between CO₂ and functional groups within the IL or PIL. Basic anions such as acetate, imidazolate, or prolinate can act as Lewis bases, donating electrons to CO₂ (a Lewis acid), thereby forming stable carbamate or bicarbonate species [41,42,43,44].

Figure 2. Possible CO₂ interactions in ILs [45] *. * Molecular Interactions in Ionic Liquids: The NMR Contribution towards Tailored Solvents by Mónica M. Lopes, Raquel V. Barrulas, Tiago G. Paiva, Ana S.D. Ferreira, Marcileia Zanatta, and Marta C. Corvo are licensed under CC BY 3.0.

Figure 2. Possible CO₂ interactions in ILs [45] *. * Molecular Interactions in Ionic Liquids: The NMR Contribution towards Tailored Solvents by Mónica M. Lopes, Raquel V. Barrulas, Tiago G. Paiva, Ana S.D. Ferreira, Marcileia Zanatta, and Marta C. Corvo are licensed under CC BY 3.0.

In functionalized ILs containing amino groups, CO₂ reacts to form zwitterionic intermediates and carbamates, as illustrated in Figure 3. This reaction leads to increased viscosity due to enhanced hydrogen bonding and ion pairing among the reaction products [46]. Increased viscosity, in turn, limits CO₂ diffusivity and transport, reducing overall sorption efficiency [47,48].

The electrostatic environment within ILs can be analyzed using electrostatic potential (ESP) maps, which highlight regions of electrophilicity and nucleophilicity. Liu et al. (2021) [35] introduced the ionic polarity index (IPI), which correlates the average ESP to the net charge of the ion. A larger IPI difference between the cation and anion increases the fractional free volume (FFV) within ILs, thereby enhancing CO₂ sorption [35].

The CO₂ capture performance of ILs and PILs is governed by various factors, including the nature of the cation and anion, alkyl chain length, porosity, surface area, viscosity, and the presence of reactive functional groups. These factors will be discussed in more detail in the subsequent sections.

5. Factors Affecting CO₂ Sorption

5.1. Effect of Cation

Ionic liquids (ILs) are typically characterized based on their cation species. Commonly used ILs for CO₂ capture are based on pyridinium [49], imidazolium [41], or phosphonium [46] structures. Among these, imidazolium-based ILs (often represented as [C_nmim]) are the most widely studied due to their tunable properties, high directional polarizability, and adjustable hydrophilicity or hydrophobicity [50]. Particularly, 3-methylimidazolium derivatives are frequently explored in literature. In one study, Philip and Henni encapsulated two task-specific ILs (amine-based ILs), 1-Ethyl-3-methylimidazolium amino-acetate Glycine [EMIM][Gly] and 1-Ethyl-3-methylimidazolium (S)-2-aminopropionate Alanine [EMIM][Ala] in a ZIF-8 MOF. The crystal structure of ZIF-8 remained unaffected by IL loading. However, CO₂ uptake increased significantly: from 0.12 mmol/g in pristine ZIF-8 to 0.76 mmol/g and 0.88 mmol/g at 0.1 and 0.2 bar, respectively, at 303 K, with 30% [EMIM][Gly] loading. Initially, IL interaction with CO₂ is the dominating factor. However, as pressure increases, surface area and pore volume become dominating factors along with CO₂ affinity. ZIF-8 alone exhibited a CO₂/N₂ selectivity of 5, which improved to 28 and 19 at 0.1 bar and 0.2 bar with 30% [EMIM][Gly], and to 18 and 8 with 30% [EMIM][Ala]. Selectivity decreases at higher pressure as CO₂ has chemical affinity to the NH₂ group in IL, whereas N₂ absorption is physical and depends on pore volume and surface area. At higher pressures, CO₂ affinity decreases, and pore volume becomes a dominating factor, thus decreasing the CO₂/N₂ selectivity [41].

Imidazolium-based PILs are often brittle and fragile; therefore, a porous polymer support to maintain mechanical stability is required. Most PILs studied are limited to polycations with free counter anions [39]. Kammakakam et al. presented novel anionic PILs with delocalized anions in the polymer chain and mobile counter cations. Two IL monomers, 1-Ethyl-3-methylimidazolium 1-[3-(Methacryloyloxy)propylsulfonyl]-(trifluoromethane-sulfonyl)imide (MIL-CF₃) and 1-Ethyl-3-methylimidazolium 1-[3-(Methacryloyloxy)propylsulfonyl]-(p-toluene-sulfonyl)imide (MIL-C₇H₇) were synthesized. The monomers were polymerized using photopolymerization in the presence of poly (ethylene glycol) diacrylate (PEGDA), producing soft, flexible, and free-standing PILs. Free [C₂mim][Tf₂N] IL was incorporated into the PIL to form composite membranes, yielding high CO₂ selectivity and permeability [39].

5.2. Effect of Cation Alkyl Chain Length

Alkyl chain lengths of cations critically influence the IL and PIL properties such as viscosity, stability, hydrophobicity, and glass transition temperatures (T_g) [15,51,52]. To investigate this effect of cation alkyl chain length on CO₂ absorption, Orhan synthesized solutions by combining different 1-alkyl-3-methylimidazolium-based ILs ([C_n-mim][Ac], [C_n-mim][Cl], and [C_n-mim][Tf₂N] with n = 2 or 4) with MEA and 1-hexanol solution. An increase in cation alkyl chain length led to higher CO₂ absorption, attributed to an increase in hydrophobicity [15]. In another study, Lai et al. synthesized polystyrene-based IL/ZIF-8 hollow fiber membranes. An increase in PIL chain length improved CO₂ permeance from 3.6 GPU to 4.6 GPU and decreased the CO₂/N₂ selectivity from 29 to 23. The reduced selectivity was attributed to inefficient packing, which increased free volume. Additional MOF loading rigidified the polymer matrix, reducing chain mobility and thus gas permeance [52].

The effect of alkyl chain lengths on a polyepichlorohydrin (poly(ECH))-based PIL with imidazolium cations was investigated by Bogoya et al. The lowest cation chain length (1-methylimidazolium) presented the highest CO₂ absorption (9.6 mg/g at 1.1 bar and 159.28 mg/g at 10 bar and 30 °C). The short alkyl chain was observed to induce strong repulsion between CO₂-philic groups in the IL, while longer alkyl chain length screened them. Therefore, despite having higher chain mobility in longer alkyl chains, the highest CO₂ absorption was observed in short alkyl chains [51].

5.3. Effect of Anion

Anions are more influential for the CO₂ absorption properties of ILs compared to cations [14,53]. It is observed that smaller anions can easily react with CO₂ to form carbamate compared to larger anions [54]. Higher CO₂ sorption can be achieved by tethering the amine to an anion instead of a cation [12]. Fluorinated anions also enhance absorption but may pose environmental concerns due to poor biodegradability [12,13]. Orhan observed that [Ac] anions showed the highest CO₂ loading, followed by [Tf₂N] and [Cl]. This was attributed to the CO₂-philic nature of fluorinated [Tf₂N] and the hydrophilic nature of [Cl]. Figure 4 shows the CO₂ loading on different anion/cation blends. Increased hydrophobicity in anions increases CO₂ absorption [12,15].

To study the effect of anions, Shahrom et. al. synthesized poly(vinylbenzyltrimethylammonium)-based PIL with different anions. Anions with multiple amine groups showed a lower degradation temperature due to their higher negative charge density. Anions also influenced T_g, with flexible, short-chain amine anions lowering the T_g. Polymerization of PILs also increased the degradation temperature and T_g, which can be associated with CO₂ absorption capacity [13,55,56]. PILs with multiple primary and secondary amines (e.g., [Arg]⁻) showed the highest CO₂ absorption (1.14 mol/mol), while those with single amines (e.g., [Ala]⁻) had the lowest. Increased amine count and functionality correlated directly with enhanced CO₂ capture. CO₂ absorption was higher in primary amines, followed by secondary and tertiary amines of anions [13]. A similar trend of CO₂ absorption dependence on the number and type of amines was also observed by Noorani et al. [57]

In another study testing the anion effect, Szala-Bilnik et al. simulated ionic polyamides (i-PI) and imidazolium-based IL composite materials. IL has an effect of lowering the T_g on i-PI, which was found to be strongly related to CO₂ absorption, and the T_g of the material is related to the viscosity of the neat ILs. Low amounts of ILs have a blocking effect in i-PI due to pore blocking; however, IL concentrations of more than 30% improved the CO₂ sorption. i-PI with three different anions [Tf₂N], [PF₆] and [BF₄] were simulated with three different ILs [C₄mim][Tf₂N], [C₄mim][PF₆] and [C₄mim][BF₄]. The [Tf₂N⁻] anion composite showed the highest T_g and lowest viscosity, whereas the lowest T_g and highest viscosity for found for [PF₆⁻] anion. T_g was found to be highly dependent on anion. A reduction in surface area with an increase in T_g was observed, which in turn affects the CO₂ absorption [55]. Bogoya et al. also evaluated the effect of different anions and ionization ratios on the synthesized poly(ECH)-based PIL. The higher CO₂ absorption was observed with bis(trifluoromethane) sulfonimide ([TFSI]) anion compared to [Cl], [BF₄], and [AcO]. The higher ionization ratio (94.6%) showed higher CO₂ absorption. These results were attributed to the high interchain d-spacing and accessibility to CO₂-philic sites [58].

PILs with inert polymer backbones such as amide, urethane, urea, or amidoxime can enhance CO₂ absorption. Additionally, the hydrogen bonding ability of these polymers improves CO₂ uptake. Morozova et al. developed polyurethane-based PIL with various cations and counter anions. Polyurethane ILs were developed using ammonium, diquinuclidinium, quinuclidinium, and imidazolium cation-based ionic diols. These were paired with 13 different counter anions, including lactate, acetate, tetracyanoborate, CF₂SO₂-N-CN, bis(pentafluoroethylsulfonyl)amide, and metal halides (halogens: Cl, BE; metals: Cu (II), Fe(II), Zn(II)). The PILs obtained had high thermal stability (275 °C) and glass transition temperatures between 30 °C and 78 °C. The highest CO₂ sorption of 18.25 mg/g and 24.76 mg/g at 273 K and 1 bar was obtained with diquinclidinium cation with CH₃COO⁻ and BF₂⁻ anions, respectively. Although the choice of diisocyanate and cation affected the CO₂ adsorption, the effect of the choice of anion was found to be far more significant [53].

Anion functionalization has emerged as a powerful strategy to enhance the CO₂ capture performance of ionic liquids (ILs), particularly by enabling chemisorption through direct interaction between CO₂ and nucleophilic sites on the anion. Anion functional groups are generally classified based on the reactive site into N-site, O-site, and C-site categories, each offering distinct CO₂ binding mechanisms and affinities. N-site functionalized anions, such as imidazolate, triazole, and amino acid derivatives, are capable of forming carbamate or zwitterionic complexes with CO₂ via Lewis base–acid interactions [8]. Similarly, O-site anions, such as acetate and phenolate, interact with CO₂ through hydrogen bonding or nucleophilic addition, forming bicarbonate-like structures [44,59]. Luo et al. reported that phenolate- (O-site) and imidazolate-based (N-site) ILs with pyridine moieties could achieve cooperative multi-site binding with CO₂, resulting in high absorption capacities of up to 1.60 mol CO₂ per mol IL under mild conditions [60]. C-sites in ILs also contribute to CO₂ binding through chemisorption of CO₂. Zhang et al. reported that the C2 position in the imidazolium ring can play a key role in CO₂ capture by undergoing carboxylation reactions, forming zwitterionic species that facilitate strong interactions with CO₂ molecules [61]. Recent studies have highlighted the pivotal role of azole-based anions (e.g., imidazolate, triazolate, pyrazolate) as high-affinity N-site chemisorption centers for CO₂ capture in functionalized ionic liquids. The basicity of azolate anions, which is influenced by the structure and the number of nitrogen atoms, directly correlates with CO₂ absorption efficiency [62]. Cui et al. found that as the pKa value of azoles decreases from 19.8 for pyrazolate-based DES to 8.2 for tetrazole-based DES, the CO₂ capture capacity decreases from 0.26 g/g to 0.02 g/g [62]. Wang et al. report a dual-tuning strategy for developing azolate-based ILs optimized for CO₂ DAC at atmospheric levels (0.4 mbar). By independently varying both the anion basicity (different azolate structures) and the cation identity, they synthesized IL formulations that achieve exceptionally high CO₂ uptake and reversible absorption behavior. The optimal IL exhibited a CO₂ capacity of 2.17 mmol/g and N-site efficiency of 0.59 mol/mol at 30 °C under ambient CO₂ partial pressure. Anion selection can be used for coarse tuning, and cation selection for fine tuning of CO₂ absorption [63].

The CO₂ sorption capacity of ILs and PILs is strongly influenced by the physicochemical properties of the anion. Key anion properties include size, structure, functionalization, basicity, hydrophobicity, and fluorination. Larger and more flexible anions, such as [Tf₂N]⁻, create greater free volume within the IL matrix, facilitating enhanced physical CO₂ absorption. Weakly coordinating anions also contribute to lower viscosity and reduced ion pairing, improving gas diffusivity. Fluorinated anions promote favorable van der Waals interactions with CO₂, enhancing solubility due to the gas’ quadrupolar nature. In contrast, amine-functionalized anions, especially those with primary or multiple amine groups (e.g., [Gly]⁻, [Arg]⁻), can chemically bind CO₂ via carbamate formation, increasing chemisorption capacity. Hydrophobicity of the anion also plays a role, as more hydrophobic environments tend to favor CO₂ solubility. Overall, the interplay between anion size, interaction strength, and chemical reactivity governs the efficiency and mechanism of CO₂ sorption in IL and PIL systems.

5.4. Effect of Viscosity

High viscosity of ILs is a huge drawback in CO₂ absorption as it reduces CO₂ transport and diffusion. For instance, [Bmim][PF₆] is 300 times more viscous than water. [48]. Therefore, the development of low-viscosity ILs is highly desirable. Amino acid functionalized ILs with higher amine sites show higher CO₂ sorption capacity; however, they have higher viscosities and lower stability [13]. Alternatively, some ILs that exhibit low viscosity are phenolate (PhO), pyridinolate(n-OP), aprotic heterocyclicanions (AHAs)/azolate, and carboxylate-based ILs [34].

There have been efforts to develop low viscosity IL solutions by encapsulating the ILs in polymers [48,64]. Gaur et al. developed IL in oil pickering emulsions stabilized by alkylated graphene oxide (GO) nanosheets and polyurea shells. 1-Butyl-3-methylimidazolium hexafluorophosphate ([Bmim][PF₆]) was mixed with diamine. Octane/GO solution was then added to the IL and emulsified. Diisocyanate-octane solution was added to the IL/diamine/GO/octane solution. After 72 h, polyurea capsules were formed due to interfacial polymerization between diamines in IL and diisocyanates in the continuous phase. The capsules contained 60–80 wt.% ILs and thermal stability up to 250 °C. Aliphatic polymers and residual amine groups in polyurea improved the CO₂ uptake performance due to improved CO₂ transport. A CO₂ uptake of 0.065 mol/kg was observed at 760 torr and 20 °C in capsulated IL compared to 0.025 mol/kg uptake of neat IL [48].

Viscosity also affects the encapsulation or loading of ILs onto the substrate. IL viscosity often depends on the alkyl chain length of the cation. In one study, Bernard et al. encapsulated three fluorinated ILs with different cation chain lengths, (1-Ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([Emim][Tf₂N]), 1-Butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([Bmim][Tf₂N]), and 1-Hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([Hmim][Tf₂N])) in polysulfone shells. The encapsulation capacity was observed to decrease with an increase in cation alkyl chain length due to an increase in viscosity. Regardless of alkyl chain length, encapsulation improved CO₂ sorption due to improved mass transfer rate and increased surface area. An increase in alkyl chain length generally increases the CO₂ absorption due to an increase in free volume; however, in this study, a decrease in CO₂ absorption is observed. This is attributed to a decrease in encapsulation capacity and an increase in viscosity with an increase in chain length [47]. In another study by the same group, Nisar et al. incorporated metal oxide into [Bmim][Tf₂N] encapsulated in polysulfone shells. At 2 bar and 45 °C, the addition of 2% iron oxide (Fe₂O₃) decreased the CO₂ sorption from 46.1 mg/g to 37.9 mg/g. However, the addition of 10% and 15% iron oxide increased the sorption to 52.9 mg/g and 57.4 mg/g, respectively. The addition of 20% copper oxide (CuO) or 1% titanium dioxide (TiO₂) showed a CO₂ sorption of 48.19 mg/g and 46.10 mg/g, respectively. Therefore, metal oxides we found to have a significant effect on the CO₂ sorption capacity of ILs. Simulation studies showed TiO₂ had the maximum interaction with CO₂, followed by Fe₂O₃ and CuO [64]. The encapsulation of ILs has been shown to improve the thermal stability of ILs [48,64]. A decrease in viscosity and increased mass transport in encapsulated ILs improve the CO₂ uptake [48,65]. However, the type of encapsulating polymer is also an important factor to consider, as initial contact of CO₂ is with the polymer core. Aliphatic polymers and residual amine groups are shown to have a positive effect on CO₂ absorption in the study by Gaur et al. [48].

The viscosity of ILs can also be reduced by solution formation with organic solvents. To synthesize low viscosity ILs for CO₂ sorption, Liu et al. synthesized tetraethylenepentamine 2-methylimidazolium ([TEPAH][2-MI])/N-propanol (NPA)/ethylene glycol (EG) solution. The viscosities of the solution before and after absorption were 3.55 mPa.s and 7.65 mPa.s, respectively. For an NPA/EG ratio of 8:2, the absorption rate and capacity were 0.118 mol/mol.min. and 1.72 mol/mol, respectively. This is a huge improvement compared to the 0.53 mol/mol capacity of the MEA/water solution commonly used [66]. IL viscosities can also decrease with increasing temperatures, which increases the CO₂ sorption; however, IL interaction with CO₂ decreases significantly with temperature [46]. To test the temperature response of ILs, Ye et al. synthesized dual functionalized ILs with phosphonium-based cation and three different anions (1,2,4-triazole [Triz], 2-hydroxypyridine [2-Op], and 2-aminopyridine [2-Np]) containing fewer hydrogens. The ILs were stable up to 192 °C. CO₂ uptake of 5.32 mmol/g was observed with [aP₄₄₄₃][2-Np], which changed to 3.19 mmol/g when IL was loaded onto MCM-41 (3.19 mmol of CO₂ for 0.3 g of IL). An increase in temperature reduced the viscosity, which increased CO₂ absorption; however, the overall effect is to reduce CO₂ absorption due to other factors [46].

5.5. Effect of Water Content

The presence of water can also affect the chemisorption of CO₂ in ILs. The water molecules are attached to the ILs through hydrogen bonds between the hydrogen (H) of water and the oxygen (O) or nitrogen (N) of IL anions. The basicity of the anion determines the hydrogen bond strength, which in turn determines the CO₂ sorption. An increase in hydrogen bond strength leads to higher CO₂ capture due to water activation [44].

Zanatta et al. tested the effect of intrinsic water in PIL-IL composite membranes on CO₂ sorption. Poly-1-vinyl-3-ethylimidazolium acetate or poly-1-vinyl-3-ethylimidazolium hydroxide as PILs were paired with 1-butyl-3-methylimidazolium acetate IL. The acetate anions provide physical and chemical sorption of CO_2, while the hydroxide anions react with CO₂ to form bicarbonate or carbonate. The hydroxide anion is more basic than the acetate anion, which leads to a stronger interaction between the CO₂ and the anion. The basicity of anions activates the water, leading to the formation of bicarbonate with CO₂. In ILs, an increase in water content up to a certain extent can increase the CO₂ physisorption and bicarbonate formation. In PIL-IL composites, an increase in water content improves the CO₂ physisorption but negatively affects the bicarbonate conversion of CO₂ due to a reduction in water activation, as shown in Figure 5. The pH of the material affects the reaction of CO₂ with PILs. Higher pH material prefers the formation of CO₃²⁻ (carbonate), while lower material pH prefers the formation of HCO₃⁻ (bicarbonate) [44]. Other studies have also observed a decrease in CO₂ absorption with an increase in water content. For example, in a study by Ye et. al., the addition of water to the gas mixture reduces the CO₂ absorption as water molecules form hydrogen bond networks with amino groups, blocking CO₂ diffusion [46]. Water is, however, important in CO₂ hydration reactions. Molina-Fernández et al. observed that water is an essential component for the CO₂ hydration reaction; therefore, more hydrophobic membranes performed poorly as catalysts for CO₂ hydration [67]. Therefore, for PILs to be used as CO₂ hydration catalysts, optimal water content to balance the CO₂ capture and conversion reaction is required.

5.6. Effect of Glass Transition Temperature (T_g)

The glass transition temperature (T_g) of the IL or PIL affects CO₂ absorption by affecting viscosity or chain flexibility. To test the effect of different crosslinkers on PIL and their corresponding glass transition temperatures, Yin et al. fabricated two amino-terminated linear PILs with different molecular weights using the Debus-Radziszewski multicomponent reaction. The PILs were then crosslinked using polyoxyethylene bis(glycidyl ether) (POBGE) and trimethylolpropane triglycidyl ether (TMPGE) crosslinkers. 1-ethyl-3-methylimidazolium bis[(trifluoromethyl) sulfonyl] imide ([emim][NTf₂]) IL was also incorporated into the composite during crosslinking. Low molecular weight PIL crosslinked with POBGE showed the best CO₂ permeability of 170 Barrer and CO₂/N₂ selectivity of 36. Molecular weight affects the diffusivity and selectivity of membranes. High molecular weight inhibits CO₂ diffusivity due to strong CO₂-PIL interaction. The diffusivity of PILs with TMPGE crosslinker was lower than POBGE crosslinker due to high T_g and lower chain flexibility. An increase in crosslinker content increased the solubility and diffusivity of CO₂ due to the presence of ether groups. CO₂ has a higher diffusion coefficient in amorphous ether containing polymer chains compared to N₂. Therefore, the addition of IL to PIL significantly increased the permeability to 2070 Barrer and 24.6 selectivity [56]. A study by Szala-Bilnik et. al. also observed a similar effect of an increase in CO₂ absorption of IL-PIL composite with a decrease in T_g, as shown in Figure 6 [55].

The T_g of the ILs or PILs also significantly depends on the anions. In one study, Hayano et al. reported successful quaternization of poly(ECH) with 1-methylimidazolium to form highly hydrophilic polyether PIL (poly(N-glycidyl-N′-methylimidazolium chloride). The chlorine anion can be further exchanged using lithium salts to obtain the desired anions. Other PILs were prepared using [TFSI]⁻ and [BF₄]⁻ anions instead of [Cl]⁻. PILs with [Cl]⁻ and [BF₄]⁻ anions were plastics with T_g of 92 °C (when dry) and 67 °C, respectively. PIL with anion [TFSI]⁻ had T_g of −12 °C, making it elastomeric with high hydrophilicity. The T_g of neat Poly(ECH) was −45 °C [68].

5.7. Effect of Feed Pressure

The absorption of CO₂ in ILs and PILs is significantly influenced by changes in pressure. As pressure increases, the solubility of CO₂ in ILs and PILs generally rises, consistent with Henry’s Law (Equation (1)), due to enhanced gas–liquid interactions [69].

$$k_H=\underset{x\to 0}{\mathit{lim}}\left({\displaystyle \frac{P_{C{O}_2}}{x}}\right)$$

(1)

where x is the mole of gas in liquid and P_CO2 is the partial pressure of gas. A decrease in Henry’s law coefficient represents an increase in the solubility of gas in the liquid. In the study by Osman et al., a systematic gravimetric analysis of twelve alkylimidazolium-based ILs demonstrated that CO₂ solubility increased progressively with pressure increments from 1 to 20 bar, highlighting the physical absorption behavior of these ILs [69]. In a study by Simon et al., the CO₂ permeability of room temperature PILs is also observed to increase with increasing feed pressure [70]. Philip et al. observed a significant increase in CO₂ uptake with increasing pressure in ZIF-8 MOFs incorporated with 1-ethyl-3-methylimidazolium amino-acetate glycine [EMIM][Gly] and 1-ethyl-3-methylimidazolium (S)-2-aminopropionate alanine [EMIM][Ala] [41]. A similar trend is also observed in other studies [47,71]. CO₂ selectivity, however, has been observed to decrease with increasing CO₂ partial pressure [41,72]. These findings underscore that pressure not only affects the quantity of CO₂ absorbed but can also impact the structural and transport properties of ILs.

At ultra-low CO₂ partial pressures relevant to DAC (~0.4 mbar), the absorption performance of ILs becomes highly dependent on the chemical nature and binding affinity of the sorption sites. Unlike post-combustion capture scenarios (where CO₂ is often present at 10–15% or higher), DAC operates under dilute conditions, necessitating ILs or PILs with high CO₂ sorption selectivity and strong binding affinity. Due to the diffuse nature of CO₂ in air, potential absorbents with a lower Henry’s law constant are needed [73].

Bera et al. reported DES combining imidazole-based ILs and hydrogen-bond donors that leverage imidazole N-H sites to achieve efficient and reversible CO₂ uptake via hydrogen bonding interactions, offering enhanced capacity at ambient CO₂ levels [23]. Zeeshan et al. achieved selective absorption of CO₂ (0.5 mmol/g at 30 °C) under DAC conditions by incorporating 1-ethyl-3-methylimidazolium 2-cyanopyrolide ([EMIM][2-CNpyr]) IL into ZIF-8 MOF [74]. Wang et al. developed highly efficient IL-based DES for CO₂ absorption under DAC by utilizing fine-tuning of imidazole N-H hydrogen bond strengths. Hydrogen bond tuning improved the thermal stability and improved regeneration under DAC conditions [75]. These studies indicate the importance of hydrogen bond tuning. A thermodynamic sensitivity analysis of aprotic N-heterocyclic anion (AHA) ILs explores performance across CO₂ partial pressures ranging from DAC (0.0004 bar) to pre-combustion (13 bar) conditions. The results demonstrate that reaction thermodynamics (reaction enthalpy-ΔH and entropy-ΔS) and Henry’s constant values critically determine cyclic absorption capacity across these pressure conditions. AHA-ILs exhibit high versatility, but DAC performance specifically demands optimized absorption enthalpy and entropy for reversible CO₂ capture at low partial pressures [76].

6. IL and PIL-Based Composites for Enhanced CO₂ Capture

PIL provides high stability, but solidification can reduce the gas permeability. Therefore, the addition of IL to PIL can increase permeability and provide properties combining the two for efficient CO₂ capture [32]. There has been extensive research on the development of membranes for CO₂ capture and separation. However, the mechanism of PIL/IL interactions with CO₂ is still unclear [44]. Incorporation of other substrates or particles such as MOFs, metal oxides, or polymers can further improve the mechanical and chemical properties of CO₂ sorbents [52,77].

The incorporation of MOFs can provide an increase in surface area and CO₂ affinity, complementing the properties of ILs and PILs. In a study, Yang et al. synthesized PIL (amine functionalized imidazolium-based)—MOF (Cu₃(BTC)₂) composite material for effective CO₂ absorption. A decrease in surface area of the MOF was observed after incorporation of PIL, and therefore, a reduction in CO₂ physisorption capacity was observed. This was made up for by an increase in chemisorption of CO₂ by the amino groups and 2-position carbon atoms in the imidazole ring of the PIL [78]. In another study by Lai et al., compared to PIL membranes, the PIL-MOF composite membrane had 33% higher CO₂ permeance as the dense structure of PILs decreased the gas permeance. A CO₂ permeance of 6 and selectivity of 30 were observed with the addition of 0.5% MOF, as MOF provides a transport pathway for CO₂ [52].

Rocky et al. developed a consolidated composite for CO₂ adsorption in an adsorption cooling system. The consolidated composite was prepared using Maxsorb (III) activated carbon powder with [VBMTA][Ala] PIL or PVA as a binder. The PIL composite showed a higher uptake of 1.41146 cm³/g compared to PIL, which showed 1.31311 cm³/g. This method can be incorporated into CO₂ capture, storage, and release in other applications [79]. Barrulas et al. obtained PIL-chitosan aerogels (AEROPILs) with glutaraldehyde crosslinker. These aerogels presented skeletal density of 1.248–1.421 g/cm³, extremely high porosity (94.6–97.0%) and surface area (270–744 m²/g). Aerogel containing poly(diallyldimethylammonium chloride) PIL showed the highest CO₂ capture of 0.70 mmol/g at 25 °C and 1 bar [77].

Most gas separation polymeric membranes rely on solution diffusion and often trade off permeability and selectivity. In supported IL membranes, leaching of ILs is an issue. The use of PIL–IL gel membranes is an alternative to avoid leaching ILs, where PIL provides stability and ILs provide transport carriers for CO₂ [72,80]. Lee and Gurkan synthesized polydiallyldimethylammonium 2-cyanopyrrolide (P[DADMA][2-CNpyr], PIL)-1-ethyl-3-methyl imidazolium 2-cyanopyrrolide ([EMIM][2-CNpyr], IL mobile carrier) gel immobilized within a graphene oxide nanosheet framework for CO₂/N₂ separation. This membrane was supported on a PES/PET substrate as shown in Figure 7. The IL used has low viscosity for better transport and CO₂ binding at low pressures. The interactions between IL, PIL, and graphene oxide avoid IL leaching. This membrane had high CO₂ permeability and selectivity in direct air conditions (3090 GPU and 1180 CO₂/N₂ selectivity) as well as cabin air (620 GPU and 250 CO₂/N₂ selectivity) and suppressed N₂ permeance [72].

In another study combining PIL and IL, Nabis et al. synthesized a mixed matrix membrane (MMM) for CO₂/H₂ separation through the solvent evaporation method. Nabis et al. synthesized a MMM consisting of poly(diallylmethylammonium)bis(trifluoromethylsulfonyl) imide a pyrrolidinium based PIL (Poly[Pyr11][Tf₂N]), 1-butyl-3-methylpyrrolidinium bis(trifluoromethylsulfonyl) imide ([C₄mpyr][Tf₂N]), and three different MOFs highly selective to CO₂ (Cu₃(BTC)₂, ZIF-8, and MIL-53(Al)). These MMMs were found to be thermally stable up to 573 K. An increase in CO₂/H₂ selectivity and CO₂ permeability is observed with an increase in MOF content compared to PIL/IL membranes. The highest CO₂ permeability is observed in MMM containing ZIF-8 due to higher pore volume and BET surface area [24]. Phase change absorbents are interesting due to their low energy requirements for regeneration. These absorbents undergo a phase change upon CO₂ absorption, forming a CO₂-rich and CO₂-poor phase. The regeneration energy required is low for these solvents as only the CO₂-rich phase has to be regenerated [81]. Chang et al. synthesized a liquid phase absorbent with tetraethylenepentamine imidazole ([TEPA][Im]), diethyleneglycol monomethylether (DM), and water. Upon CO₂ absorption, a phase change to either liquid or solid is observed depending on the water content. The CO₂-rich phase can then be separated from the CO₂-poor phase mechanically. A CO₂ absorption capacity of 2.08 mol/mol was observed at 298.15 K. Used as a catalyst, the solvent also showed 96% conversion yield from CO₂ to quinazoline-2,3(1H,3H)-Dione [81]. Song et al. developed a porous hyper-crosslinked IL-based polymer. Song et al. used three different IL monomers containing 1, 2, and 4 benzene rings. The monomers were crosslinked with crosslinkers to form hyper-crosslinked polymers. Although ILs had higher CO₂ affinity, the polymer network formed with only the crosslinker, and no IL showed much higher CO₂ absorption due to high surface area and high porosity. Thus, porosity is a very important factor to consider while developing materials for CO₂ absorption. However, the CO₂/N₂ selectivity of IL-containing polymers was almost twice that without ILs, as ILs enhance CO₂/N₂ selectivity through electrostatic attraction [82]. Common cation and anion abbreviations are included in

Table 1. Cation and anion abbreviations.

[aP4443]	(3-aminopropyl) tributylphosphonium
[Emim], [C2mim]	1-Ethyl-3-methylimidazolium
[Hmim]	1-Hexyl-3-methylimidazolium
[Bmim]	1-Butyl-3-methylimidazolium
[VEI]	1-vinyl-3-ethylimidazolium
[TEPAH]	tetraethylenepentamine
[VBMTA]	(p-Vinylbenzyl) trimethylammonium
[C2OHmim]	1-hydroxyethyl-3-methylimidazole chloride
[Tf2N], [TFSI]	Bis(trifluoromethylsulfonyl)imide
[Cl]	Chloride
[Ac]	Acetate
[Gly]	Glycine
[Ala]	Alanine
[2-MI]	2-methylimidazolium
[Lys]	Lysinate
[PF6]	Hexafluorophosphate
[BF4]	Tetrafluoroborate
[2-Cnpyr]	2-cyanopyrrolide
[aemmim]	1-aminoethyl-2,3-dimethylimidazolium
[tau]	taurine

.

Table 2. CO₂ uptake in ILs.

Cation	Anion	CO2 Absorption	Conditions	Processing	Reference
[aP4443]	1,2,4-triazole	4.02 mmol/g	30 °C		[46]
[aP4443]	2-hydroxypyridine	4.44 mmol/g	30 °C		[46]
[aP4443]	2-aminopyridine	5.32 mmol/g	30 °C		[46]
[aP4443]	2-aminopyridine	3.19 mmo/g	30 °C	30%IL on MCM-41	[46]
[Emim]	[Tf2N]	39.5 mg/g	1 bar 298.15 K	Polysulfone Encapsulated IL	[47]
		62.7 mg/g	10 bar 298.15 K		[47]
[Bmim]	[Tf2N]	37 mg/g	1 bar 298.15 K		[47]
		56.5 mg/g	10 bar 298.15 K		[47]
[Hmim]	[Tf2N]	34.7 mg/g	1 bar 298.15 K		[47]
		54.6 mg/g	10 bar 298.15 K		[47]
[Emim]	[Tf2N]	0.67 mol/mol	2 bar 303 K	Bend with MEA and 1-hexanol	[15]
[Emim]	[Cl]	0.52 mol/mol	2 bar 303 K		[15]
[Bmim]	[Tf2N]	0.77 mol/mol	2 bar 303 K		[15]
[Bmim]	[Ac]	0.89 mol/mol	2 bar 303 K		[15]
[Bmim]	[Cl]	0.55 mol/mol	2 bar 303 K		[15]
[Emim]	[Gly]	0.89 mmol/g	0.2 bar 303 K	30% loaded on ZIF-8	[41]
[Emim]	[Ala]	0.91 mmol/g	0.2 bar 303 K		[41]
[Emim]	[Gly]	0.45 mmol/g	0.2 bar 303 K	20% loaded on MOF-177	[42]
[Emim]	[Ala]	0.42 mmol/g	0.2 bar 303 K		[42]
[Bmim]	[Tf2N]	46.1 mg/g	4 bar 45 °C	Polysulfone Encapsulated ILs	[64]
[Bmim]	[Tf2N]	57.4 mg/g	4 bar 45 °C	Polysulfone Encapsulated + 15 wt% Fe2O3	[64]
[Bmim]	[Tf2N]	46.1 mg/g	4 bar 45 °C	Polysulfone Encapsulated + 1 wt% TiO2	[64]
[Bmim]	[Tf2N]	48.19 mg/g	4 bar 45 °C	Polysulfone Encapsulated + 20 wt% CuO	[64]
[TEPAH]	[2-MI]	1.72 mol/mol		solution with N-propanol (NPA)/ethylene glycol—8:2	[66]
		0.53 mol/mol		MEA/water	[66]
poly [VBMTA]	[Arg]	1.14 mol/mol			[13]
[VBMTA]	[Arg]	0.83 mol/mol			[13]
poly [VBMTA]	[Lys]	1.13 mol/mol			[13]
[VBMTA]	[Lys]	0.66 mol/mol			[13]
poly [VBMTA]	[Ala]	0.56 mol/mol			[13]
[VBMTA]	[Ala]	0.29 mol/mol			[13]
[Bmim]	[PF6]	0.065 mol/kg	760 torr 20 °C	oil pickering emulsions stabilized by alkylated graphene oxide nanosheets and polyurea shells.	[48]
[Bmim]	[BF4]	0.8 mol/mol		AI simulation, highest pressure and lowest temperature	[83]
[C2OHmim]	[Lys]	1.29 mmol/g	0.1 bar 40 °C	GDX-103 support, 60% IL	[40]
[aemmim]	[Tau]	0.9 mol/mol	1 bar 303.15 K		[33]
[Bmim]	[Ac]	0.83 mmol/g	0.2 bar 303.15 K	Impregnated onto ZIF-30	[84]
[Bmim]	[Ac]	0.85 mmol/g	0.2 bar 30 °C	60% impregnated onto MCM-41	[85]
[CelEt3N]	[PF6]	38 mg/g	0.1 MPa 298.15 K	Cationic cellulose-based IL	[71]
[CelEt3N]	[PF6]	168 mg/g	3 MPa 298.15 K	Cationic cellulose-based IL	[71]

and

Table 3. CO₂ uptake by PILs.

PIL	IL	CO2 Absorption	Conditions	Processing	References
Poly(N-glycidyl-N′-methylimidazolium) (TSFI)		159.3 mg/g	10 bar 35 °C	94.6% ionization ratio	[58]
poly-4,4′-bis(chloromethyl)-1,1′-biphenyl		100 mg/g	1 bar 273 K		[86]
poly-1-vinyl-3-ethylimidazolium hydroxide	[Bmim][Ac]	0.89 mmol/g	20 bar 298 K	7 wt% PIL, intrinsic water 13.5 wt%	[44]
poly-1-vinyl-3-ethylimidazolium hydroxide	[Bmim][Ac]	0.35 mmol/g	20 bar 298 K	7 wt% PIL, added water 71.2 wt%	[44]
	[Bmim][Ac]	0.96 mmol/g	20 bar 298 K	intrinsic water 9.7 wt%	[44]
Polyurethanes- diquinuclidinium cation (CH3COO)		18.25 mg/g	1 bar 273 K		[53]
Polyurethanes- diquinuclidinium cation (BF4)		24.76 mg/g	1 bar 273 K		[53]
poly(diallyldimethylammonium chloride)-chitosan aerogels		0.70 mmol/g	1 bar 25 °C	glutaraldehyde crosslinker	[77]
Methyl-diaminopyridinium bis(trifluoromethanesulfonyl)imide		8.1 mg/g	1 bar 273 K	polymerized using isophthaloyl chloride (IPC)	[87]
Methyl-diaminopyridinium bis(trifluoromethanesulfonyl)imide		13.9 mg/g	1 bar 273 K	polymerized using 2,6-pyridinedicarbonyl chloride (PDC)	[87]
(1,2,3,4,5,6-hexakis(1-(3-vinylimidazolium) bromide) benzene monomer and bipyridine		1.44 mmol/g	1 atm 273.15 K	Free radical copolymerization, post-synthetic metalation with Zn2+	[88]
vinylimidazolium salt-based PIL		1.28 mmol/g	1 bar	Coated on hydroxyl functionalized CNTs	[89]
3-(3-(phthalimide)propyl)-1-vinylimidazolium bromide		0.59 mmol/g	1 bar 25 °C	N-allylphthalimide building blocks, free radical polymerization	[90]

summarize the recent CO₂ capture performance using ILs and PILs, respectively.

Table 4. CO₂ Selectivity.

PIL	IL	CO2/N2	CO2/CH4	CO2/H2	Conditions	Permeance	Processing	References
	[Emim][Gly]	28			0.1 bar 313 K		30% loaded on ZIF-8	[41]
		19			0.2 bar 313 K			[41]
	[Emim][Ala]	18			0.1 bar 313 K			[41]
		8			0.2 bar 313 K			[41]
	[Emim][Gly]	13			0.2 bar 313 K		20% loaded on MOF-177	[42]
	[Emim][Ala]	11			0.2 bar 313 K			[42]
polydiallyldimethylammonium 2-cyanopyrrolide	[Emim][2-Cnpyr]	1180			Direct Air	3090 GPU	immobilized within a graphene oxide nanosheet	[72]
		250			Cabin Air	620 GPU		[72]
	No IL	27			CO2/N2 (15/85 cm3 (STP) min−1)	497 GPU	Pebax® thin film composite	[91]
	[Bmim][BF4]	29			CO2/N2 (15/85 cm3 (STP) min−1)	629 GPU	Pebax® thin film composite, 10%IL	[91]
	[Bmim][BF4]	25			CO2/N2 (15/85 cm3 (STP) min−1)	751 GPU	15% ZIF-8, Pebax® thin film composite, 10%IL	[91]
	[Bmim][BF4]	25			CO2/N2 (15/85 cm3 (STP) min−1)	891 GPU	15% ZIF-94, Pebax® thin film composite, 10%IL	[91]
poly(vinylimidazolium) and poly styrene	[C2mim][Tf2N]	34.4			20 °C, 100 kPa	24.5 barrer	30% IL	[92]
[Emim][NTf2]		36				170 Barrer	polyoxyethylene bis(glycidyl ether) crosslinker	[56]
[Emim][NTf2]		24.6				2070 Barrer	trimethylolpropane triglycidyl ether crosslinker, [Emim][NTf2] free IL	[56]
1-Ethyl-3-methylimidazolium 1-{3-(Methacryloyloxy)propylsulfonyl}-(trifluoromethane-sulfonyl)imide	[C2mim][Tf2N]	86.81	118.6	4.1	3 bar 20 °C		Anion-based polymer, free cation (1-Ethyl-3-methylimidazolium), PEGDA crosslinker	[39]
1-Ethyl-3-methylimidazolium 1-{3-(Methacryloyloxy)propylsulfonyl}-(p-toluene-sulfonyl)imide	[C2mim][Tf2N]	51.65	61.88	3.18	3 bar 20 °C		Anion-based polymer, free cation (1-Ethyl-3-methylimidazolium), PEGDA crosslinker	[39]
vinylimidazolium salt-based PIL		39					Coated on hydroxyl functionalized CNTs	[89]

summarizes the CO₂ selectivity over other gases using ILs and PILs.

7. PILs as Catalysts for CO₂ Conversion

CO₂ conversion to high-value chemicals is highly desirable. Current available technologies react CO₂ with aziridines or epoxides to form products such as oxazolidinones and cyclic carbonates, which are valuable in synthetic chemistry, lithium ion battery electrolytes, construction, and the production of engineering plastics [88,93,94]. CO₂ fixation generally involves the use of metal or halogen-containing catalysts, which have environmental concerns. These reactions happen via CO₂ cycloaddition. This is a three-step process, involving: ring opening on reactants via nucleophilic attack, insertion of CO₂, and closing of the ring with release of the nucleophile. The presence of halogen anions acts as a nucleophile, promoting the ring opening. Ring opening of epoxide is the rate-determining step [88,94]. Metal sites act as Lewis acids, which promote substrate polarization and stabilization. The electrostatic and van der Waals forces developed between IL and CO₂ increase the potential use of ILs as catalysts for CO₂ conversion [95]. Many anions in ILs can act as nucleophiles, thus providing the possibility of removing the halogen. Other functional groups can provide acidic sites for stabilization and basic sites for CO₂ attraction. Thus, these ILs can be developed to be multifunctional organocatalysts [59]. Catalysts for CO₂ conversion can be homogeneous or heterogeneous. Homogeneous catalysts include ILs, while heterogeneous catalysts include PILs, MOFs, covalent organic frameworks (COFs), covalent triazine frameworks (CTFs), and hyper-crosslinked PILs [94].

Many studies explore CO₂ cycloaddition with epoxides with PILs as catalysts. Zhou et al. developed a mesoporous PIL-based organocatalyst for CO₂ fixation. These PILs include an acidic carboxyl group, a basic amino group, and a carboxylate group acting as a halogen-free nucleophile, accelerating the cycloaddition reaction. Thus, developing a halogen and metal-free catalyst for efficient CO₂ fixation [59]. Fu et al. synthesized a bifunctional copolymer catalyst using (1,2,4,5-tetrakis(1-((4-vinylphenyl)-N, N-dimethylamine) bromide) benzene monomer (quaternary ammonium-based monomer) and bipyridine (DVPy) via copolymerization. Electrophilic zinc sites were introduced into the synthesized copolymer using post-synthetic metalation. CO₂ cycloaddition with epichlorohydrin presented a 98.9% conversion and selectivity of 96.9% [94].

Cai et al. synthesized a hyper-crosslinked IL-based porous polymer network, using IL monomer and 4,4′-bis(chloromethyl)-1,1′-biphenyl with an adsorption capacity as high as 100 mg/g at 1 bar and 273 K, as well as successful conversion of CO₂ upon reaction with epoxides. The material showed 99% yield, high cyclic stability, and recovery of material [86].

Kulshrestha et al. developed a porous liquid by dispersing hollow silica nanorods (SiNRs) surface modified with organosilane in a sterically hindered solvent made of cation-based phosphonium ionic liquid and long alkyl chain carboxylic acid anions (fatty acid). The SiNRs provided the permanent accessible mesoporosity required, and ILs provided high gas diffusion. These ILs exhibited low viscosity (1.8P.s) and high CO₂ absorption at low pressures (0.017 P/P₀). Further addition of carbonic anhydrase converted the CO₂ into bicarbonate ions, and addition of calcium chloride converted the bicarbonate ions into calcium carbonate, thus mineralizing the CO₂ [96].

Carbonic anhydrases (CA) are a catalyst for the hydration of CO₂. CA can be immobilized onto a hydrophobic surface for CO₂ reaction. Molina-Fernandez et al. developed a CA immobilized PIL for incorporation as a gas–liquid separator for CO₂ hydration. PILs with larger CO₂ affinity create a CO₂-concentrated microenvironment, accelerating the catalytic activity of CA. CA was immobilized on a thin PIL film coated on PVDF. Different anions were tested for a gas-liquid separator membrane containing Na₂CO₃ solvent recirculating. Low stability and activity were associated with the anion presenting the most hydrophobic support for CA, as water is essential for CO₂ hydration [67]. Xie et al. synthesized multifunctional mesoporous PIL with a high density of nucleophiles and electrophiles. PIL incorporating (1,2,3,4,5,6-hexakis(1-(3-vinylimidazolium) bromide) benzene monomer and bipyridine monomer was synthesized using free radical copolymerization, followed by supercritical CO₂ drying post-synthetic modification for introducing electrophilic zinc sites. Bromide sites in PIL act as nucleophilic sites. A high surface area of 483.8 m² g⁻¹ was obtained with 5% Zn composite with a large pore volume (1.17 m² g⁻¹) and an average pore size of 13.1 nm. High CO₂ absorption (1.44 mmol/g at 273.15 K and 1 atm) was observed. 94.5% catalytic conversion yield was observed at 1 atm and 120 °C, which was significantly high compared to the 9.8% yield without zinc sites [88].

In another study, the incorporation of H-bond donors improved catalytic activity for CO₂ conversion. Diffusion paths of the reactants in catalysts also affect the performance. Short diffusion paths are favorable and improve catalytic performance [89]. Wan et al. fabricated PIL (vinylimidazolium salt-based) coated CNTs functionalized with hydroxyl or carboxyl groups as hydrogen bond donors for improved catalytic activity. The IL monomers were polymerized in the presence of CNTs. Polymerization occurs on the outer surface of CNTs, while the polymerization inside CNTs is hindered. Surface areas up to 118.4 m²/g were obtained in the hybrid samples. This area is lower than the CNT surface but is higher than that of the PIL. An increase in the PIL/CNT pass ratio decreased the surface area. CO₂ uptake of up to 1.28 mmol/g was observed at 1 bar, which is significantly higher than PIL or CNT alone. Whereas N₂ uptake was very low (<0.05 mmol/g), indicating high CO₂/N₂ selectivity (39). These hybrid samples were stable up to 250 °C. CO₂ cycloaddition with epoxide yield of 94.5% was observed at 100 °C and 0.2 MPa. Figure 8 shows the cycloaddition of CO₂ with different epoxides using the PIL modified CNTs as catalysts [89]. Recent findings related to CO₂ conversion are summarized in

Table 5. CO₂ conversion.

PIL	Properties	Yield	Reactant	Condition	References
poly-4,4′-bis(chloromethyl)-1,1′-biphenyl		99%	Styrene oxide	373.2 K, CO2 1.0 MPa, 24 h, SO 10 mmol, catalyst 50 mg.	[86]
1,4-butanediyl-3,3′-bis-1-vinyl imidazolium bromide	imidazolium IL (Vim-COOH), copolymer	92.70%	Epichlorohydrin	70 °C 1.0 Mpa	[97]
(1,2,4,5-tetrakis(1-((4-vinylphenyl)-_N_, _N_-dimethylamine) bromide) benzene monomer (quaternary ammonium-based monomer) and bipyridine (DVPy)	copolymerization, post-synthetic metalation with Zn2+	98.90%	Epichlorohydrin	50 °C 0.1 bar	[94]
(1,2,3,4,5,6-hexakis(1-(3-vinylimidazolium) bromide) benzene monomer and bipyridine	Free radical copolymerization, post-synthetic metalation with Zn2+	94.50%	Epoxide	1 atm 120 °C	[88]
vinylimidazolium salt-based PIL	Coated on hydroxyl functionalized CNTs	94.50%	Epoxide	0.2 MPa 100 °C	[89]
Poly [VEI][Br] and atomically dispersed Al-O-C	In situ polymerization of [VEI][Br] with 1,2-divinylbenzene and 2,2′-azobis-isobutyronitrile	91%	Epichlorohydrin	1 bar 80 °C	[98]

.

8. Challenges and Future Directions

Despite the promising capabilities of ILs and PILs for CO₂ capture and conversion, several challenges must be addressed before these materials can achieve widespread commercial deployment.

8.1. Current Challenges

One of the foremost issues in IL and PIL-based systems is high viscosity, particularly in ILs with strong CO₂-binding functional groups such as amino or carboxyl moieties. Elevated viscosity limits gas diffusivity and reduces mass transfer rates, ultimately lowering the overall CO₂ capture capacity [47,48]. While blending with co-solvents or supporting ILs on porous substrates has shown improvement, maintaining performance while mitigating viscosity remains a central obstacle. Thermal and chemical stability is another critical concern. ILs and PILs must endure repeated absorption/desorption cycles and exposure to flue gas impurities (e.g., SOx, NOx, H₂O) without degradation. Although many ILs offer thermal resilience, chemical degradation through oxidative or hydrolytic pathways can still occur, especially for task-specific ILs with labile functional groups. Leaching of ILs from supported or blended systems presents environmental and economic issues [72,80]. Over time, loss of ILs from porous membranes or polymer matrices can diminish CO₂ capture performance and pose contamination risks. Developing leach-resistant systems or covalently bound PIL structures is an area requiring further innovation.

The cost and scalability of IL/PIL synthesis, especially when incorporating functional or bio-inspired groups, remain non-trivial. High-purity precursors and multi-step synthesis routes currently limit their viability for large-scale use, necessitating streamlined, sustainable synthetic methods. Environmental impact, particularly from fluorinated anions such as [Tf₂N]⁻ and [PF₆]⁻, is another pressing issue. These anions are often persistent in the environment and may be toxic. The toxicity of ILs depends on multiple factors, including alkyl chain length, anion type, and functional group. Toxicity studies have shown that the toxicity trend generally follows [Br] < [DCA]< [Cl] < [BF₄] < [PF₆] < [NTf₂] [99]. However, this trend can be altered based on the cation pairing. Toxicity is also shown to increase with an increase in alkyl chain length [99]. Moreover, halogens and highly branched alkyl chains are particularly persistent in the environment and resistant to degradation [100]. While ILs and PILs enhance CO₂ solubility, the trade-offs in environmental compatibility must be carefully weighed. Water sensitivity, both beneficial and detrimental, adds complexity in real-world applications. While small amounts of water may facilitate CO₂ reaction pathways, excessive water can hinder diffusion or lead to competing reactions. This sensitivity complicates operation under variable humidity or in direct air capture (DAC) scenarios.

8.2. Future Research Directions

Looking forward, multiple research avenues show promise in overcoming these challenges. The design of low-viscosity ILs and PILs, through molecular engineering or co-solvent integration, will be critical to improving mass transport without compromising CO₂ affinity. There is also growing emphasis on fluorine-free and biodegradable IL/PIL systems, aiming to balance high CO₂ solubility with improved environmental sustainability. Amino acid-based anions, sugar-functionalized ILs, and bio-derived cations represent promising directions.

Composite and hybrid materials will continue to play a pivotal role. By integrating ILs or PILs into MOFs, aerogels, carbon nanotubes, or porous organic polymers, researchers can tailor both sorption and mechanical properties, achieving synergistic effects for CO₂ capture under diverse conditions. In the context of catalytic conversion, multifunctional PILs that integrate CO₂ sorption and activation, especially in metal-free or enzyme-mimicking systems, can bridge the gap between capture and utilization. This dual functionality supports circular economic strategies and carbon valorization. Computational screening techniques such as Volume Based Thermodynamic (VBT), Quantum Structure-Property Relationship (QSTR) and Conductors like Screening Model for Realistic Solvation (COSMO-RS) have been used for the high-throughput prediction and evaluation of materials and solvents for CO₂ capture [35]. Emerging tools such as machine learning (ML) models, including artificial neural networks [101], recurrent neural networks [102], Monte Carlo tree search [102], and random forest [103], may accelerate the discovery of new IL and PIL chemistries. By predicting optimal combinations of cations, anions, and functional groups, these techniques could shorten the design cycle and guide experimental efforts more efficiently.

Finally, future work must consider technoeconomic analyses and life cycle assessments to evaluate the real-world feasibility of IL and PIL-based systems. This includes not only performance metrics but also synthesis costs, regeneration energy requirements, and system integration challenges. In summary, while ILs and PILs offer unique advantages in CO₂ capture and utilization, addressing these challenges through targeted research will be key to realizing their full potential in sustainable carbon management technologies.