# Ionic Liquids Hybridization for Carbon Dioxide Capture: A Review
## 2 ILs Utilization for CO\({}_{2}\) Capture
AAILs have emerged as a distinct class of ILs as they offer a promising platform for the development of biodegradable absorbent for CO\({}_{2}\) capture. Furthermore, the presence of amine and carboxylic functional groups in amino acid may facilitate strong interactions and selective binding with CO\({}_{2}\) molecules. The utilization of AAILs was firstly introduced in a study by [PERSON] and co-workers in 2005, in which they studied the application of phosphonium-based ILs coupled with several amino acids as anion supported porous silica for CO\({}_{2}\) capture [36]. This also serves as one of the first examples of IL hybridization process, even though the term \"hybridization\" may not have been widely recognized initially. The same group also performed the CO\({}_{2}\) absorption by using [P(C\({}_{4}\))\({}_{4}\)][Gly] that contained 1% mass fraction of water and the CO\({}_{2}\) uptake by the IL amounted to -0.12 mole fraction. However, it was not until 2015 and onwards that AAILs gained significant attention from researchers, as evidenced by the surge in publications related to AAILs for CO\({}_{2}\) capture and its mechanism [37; 38; 39; 19]. A study by [PERSON] et al. showed the CO\({}_{2}\) capture capacity of their AAILs were more than 0.5 mol CO\({}_{2}\)/mol IL [40]. It has also been observed that CO\({}_{2}\) absorption mechanism by AAILs was affected by the types of amino acid. For example, more than 95% of absorbed CO\({}_{2}\) from a total of CO\({}_{2}\) absorption capacity by [P\({}_{66614}\)][Pro] and [P\({}_{4444}\)][Pro] were achieved via 1:1 reaction mechanism in which 1 CO\({}_{2}\) molecule react with 1 amine molecule. Apart from [Pro] ILs, other ILs exhibited a contribution of over 20% to the CO\({}_{2}\) absorption capacity through the reaction involving 1 CO\({}_{2}\) molecules per 2 amines molecules (1:2 mechanism). Various works also demonstrated the amount of amine sites presence in ILs chemical structure was found to have an impact on the capacity of CO\({}_{2}\) absorption [19; 38; 41; 42]. For example, among the pyridinium-based ILswith different amino acid anions, such as arginate [Arg]\({}^{-}\), lysine [Lys]\({}^{-}\), aanate [Ala]\({}^{-}\), and thyrosinate [Thy]\({}^{-}\), the [B\({}_{4}\)MPyr][Arg] demonstrated remarkable CO\({}_{2}\) absorption capacity [38]. This can be attributed to the presence of two primary and two secondary amines in [Arg]\({}^{-}\) structure, thus providing more active sites to be interact with CO\({}_{2}\) molecules [38]. In the meantime, dual-functionalized ILs (DFILs) refers to the ILs class with two similar or different functional groups present in both of their cation and anion. Examples of dual-functionalized ILs are [C\({}_{1}\)ImPA][Gly], [TEPA][Im] and [TETAH][Pz] [43; 44; 45]. The main purpose in the synthesis of dual-functionalized ILs is to provide an additional active site for the reaction to occur. As stated by [PERSON] et al., the earliest type of dual-functionalized ILs used for CO\({}_{2}\) capture primarily comprises of a cation with amino-functionalized and an amino acid anion. [PERSON] and co-workers prepared an aqueous solution containing [C\({}_{1}\)ImPA][Gly] in which the CO\({}_{2}\) capacity recorded at 30 \({}^{\circ}\)C was 1.23 mol CO\({}_{2}\)/mol ILs [43]. Despite its excellent performance, it was reported that the absorption capacity of pure [C\({}_{1}\)ImPA][Gly] was higher than that of its aqueous solution. The presence of water in aqueous [C\({}_{1}\)ImPA][Gly] may negatively impact the CO\({}_{2}\) absorption due to the competition between water and CO\({}_{2}\) molecules for active sites in the ILs. However, pure dual-amino functionalized ILs are reported to be highly viscous, thus discouraging their neat use in CO\({}_{2}\) capture. Therefore, the incorporation of other functionalized groups such as ether is proposed to reduce the ILs viscosity. The rotational flexibility that is caused by the presence of oxygen atom is expected to reduce the ILs viscosity.