# Ionic Liquids Hybridization for Carbon Dioxide Capture: A Review
## 2 ILs Utilization for CO\({}_{2}\) Capture  
The application of ILs for CO\({}_{2}\) capture was initially reported by [PERSON] and co-workers involving the synthesis of conventional RTILs, [BMIM][PF\({}_{6}\)] for high-pressure CO\({}_{2}\) absorption [18]. [BMIM][PF\({}_{6}\)] displayed a promising outcome as it recorded 0.6 mol fraction of CO\({}_{2}\) at 80 bar. The same research group also performed a study on phase behavior of ILs/CO\({}_{2}\) system at high pressure in which the result showed that [BMIM][PF\({}_{6}\)] achieved 0.72 mol fraction of absorption capacity at 40 \({}^{\circ}\)C, and 93 bar [22]. Since then, numerous ILs have been screening while simultaneously studying the effect of anion and length of alkyl chain towards CO\({}_{2}\) capture. [PERSON] et al. presented experimental and simulation studies for CO\({}_{2}\) absorption using imidazolium-based ILs coupled with several anions, namely, hexafluorophosphate [PF\({}_{6}\)]\({}^{-}\), tetrafluoroborate [BF\({}_{4}\)]\({}^{-}\) and bistrifluoromethyl(sulfonyl)imide [Tf\({}_{2}\)N]\({}^{-}\)[23]. The result demonstrated ILs with [Tf\({}_{2}\)N]\({}^{-}\) displayed the best affinity towards CO\({}_{2}\) compared to other anions. The same group also studied the effect of methyl group on C2 of imidazole cation of [EMMIM][Tf\({}_{2}\)N]. It has been found that the CO\({}_{2}\) absorption capacity of [EMMIM][Tf\({}_{2}\)N] and [EMIM][Tf\({}_{2}\)N] were approximately 0.26 and 0.28 mol fractions, respectively, thus suggesting a minor influence of the cation on the absorption process. While numerous publications confirm the significant impact of anions on CO\({}_{2}\) absorption, whether through experimental or simulation data [24; 25], the study on the effect of ILs alkyl chain demonstrated that the CO\({}_{2}\) absorption increased as the alkyl chains becomes longer [26; 27]. As the field progressed, Task-Specific ILs (TSILs) were designed to tailor the properties of absorbent exclusively for enhancing CO\({}_{2}\) capture performance. Normally, TSILs comprise of amine moiety, which is expected to assist in improving CO\({}_{2}\) absorption through chemisorption mechanism. A study by [PERSON] et al. demonstrated the CO\({}_{2}\) absorption capacity by [2-AEmim][Tf\({}_{2}\)N] reached 0.49 mol CO\({}_{2}\)/mol IL at 1.6 bar under 30 \({}^{\circ}\)C [28]. The research revealed that the CO\({}_{2}\) absorption process in amine-functionalized ILs follows a chemisorption mechanism. The analysis of the functional groups of [2-AEmim][Tf\({}_{2}\)N]-CO\({}_{2}\) indicated the presence of new peaks at 1666 cm\({}^{-1}\), which can be attributed to the carbamate (=C=O) group. This finding was supported by \({}^{13}\)C NMR spectroscopy, where new peaks at 162.3 and 56.5 ppm were observed, corresponding to the carbamate carbon and the methylene group of the carbamate nitrogen atom, respectively. Besides that, the work revealed ILs with [Tf\({}_{2}\)N]\({}^{-}\) recorded higher CO\({}_{2}\) absorption capacity than [BF\({}_{4}\)]\({}^{-}\) and dicyanamide [DCN]\({}^{-}\). Additionally, Hussain synthesized pyridinium-based ILs consisting of ether-functionalized groups combined with cyano-based anions, thiocyanate [SCN]\({}^{-}\), [DCN]\({}^{-}\) and tricyanamide [TCN]\({}^{-}\)[29]. This result demonstrated that the solubility of CO\({}_{2}\) increased with an increasing number of cyanide groups. On the other hand, the anion provided a significant effect towards CO\({}_{2}\) absorption though with the presence of functionalized groups. Apart from that, ILs functionalization is not only limited to the cation site but also encompasses the anion as well. Up to the present, numerous studies have been conducted to explore the potential of ILs containing carboxylate, azolate, phenolate and alkoxide anions for CO\({}_{2}\) absorption [30]. A study by [PERSON] et al.