## 4 Solvents for CO\({}_{2}\) Capture
### Chemical Solvents
#### 4.2.4 Ionic Liquids  
Ionic liquids (ILs) often refer to compounds resembling the structure of molten salts which are found as liquids at temperatures below 100 \({}^{\circ}\)C. In contrast to amine solvents, ILs are composed of two parts, an anion and a cation which can either be of organic or inorganic nature. With a large number of available ionic species that act as \"synthesis blocks\" to choose from, the number of potential synthesized ILs is vast [82], with the most common ions being the tetrafluoroborate ([BF\({}_{4}\)]\({}^{-}\)) and acetate ([Ac]\({}^{-}\)) as well as the imidazolium ([R\({}_{1}\)R\({}_{2}\)R\({}_{3}\)Im]) and phosphonium ([P]) cations. ILs have very low vapor pressure, resulting in negligible losses due to thermodynamic equilibrium mandates and high thermal stability that generally leads to reduced requirements for their regeneration as well as tunable CO\({}_{2}\) capacity [83, 84, 85]. On the other hand, their fabrication as a combination of bulky ions creates large molecules that result in highly viscous fluids which increase pumping energy requirements. Their production cost is also higher than that of conventional solvents and their toxicity and environmental impact is still obscure due to being relatively new. All of the above properties are directly dependent on the nature of the ionic groups comprising the IL and as such, they can be tailored according to the needs of a given process. For solvent-based CO\({}_{2}\) capture, one such property is evidently centered on the rate of reaction. To this end, imidazolium carboxylate ILs are the most prominent ones [86, 87]. Such ILs follow a 1:2 stoichiometry when reacting with CO\({}_{2}\)[88] and their heats or reaction are in the range of \(-\)35 to \(-\)39 kJ/mol, less than half of that of MEA [89]. However, they show a high viscosity [90] and decompose at temperatures higher than 100 \({}^{\circ}\)C [91]. Another IL category that has received attention is the amino acid ILs (AA-ILs) which show reduced toxicity and have been reported as good CO\({}_{2}\) capture candidate solvents [92, 93]. These ILs are believed to react with CO\({}_{2}\) with a mechanism similar to the zwitterton mechanism of primary and secondary alkanolamines and show very low heats of absorption in the range of \(-\)15 to \(-\)35 kJ/mol. However, their very high viscosity leads to low reactions rates and thus the need for modification in the form of support or encapsulation is recommended [94]. Similar to AA-ILs, aprotic heterocyclic ionic liquids (AHA-ILs) have shown promising properties for CO\({}_{2}\) capture. These include their high reaction rates and high CO\({}_{2}\) solubility as well as high thermal stability [95, 96, 97]. Unlike carboxylate or AA-ILs, AHA-ILs follow a 1:1 stoichiometry and have high decomposition temperatures. However, they have relatively high heats of absorption (\(-\)37 to \(-\)54 kJ/mol) and medium to high viscosity [96]. Such issues may be partially alleviated by supporting or encapsulating AHA-ILs using co-solvents [98]. A number of process simulation studies exist for assessing the performance of ILs and evaluating the cost of the overall CO\({}_{2}\) capture process as well as operating parameters such as the regeneration demands. [PERSON] et al. [99] compared a mixed aqueous solution of MEA and six different ILs, concluding that ILs based on the 2-cyanopyrrole anion showed the lowest circulation rate and lowest regeneration energy requirements. This energy was estimated at 3.6 GJ/t CO\({}_{2}\) by [PERSON] and [PERSON] [100]. [PERSON] et al. [101] studied CO\({}_{2}\) capture using the imidazolium-based [BMIM]Ac, which showed a better performance (\(-\)20% energy requirements for regeneration) compared to the benchmark amine solvent MEA. The energy requirements could be as low as 1.4 GJ/t CO\({}_{2}\) according to [PERSON] et al. [102] for mixtures of tetraglyme and AHA-ILs.