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Ionic liquids have been proposed as an absorbent for use in carbon capture and sequestration. Though ionic liquids have been studied for over a century, recently, within the past 20 years, research interest has grown in their potential applications in separations and other chemical processes. Besides separations, ionic liquids can be used in molten salt nuclear reactors, batteries, and thermal storage, such as that used in solar-thermal power plants. The urgency of climate change has spurred research into these energy-related applications.

Carbon capture
Amines are the most prevalent absorbent in postcombustion carbon capture technology today. In particular, monoethanolamine (MEA) has been used in industrial scales in postcombustion carbon capture, as well as other CO2 separations, such as "sweetening" of natural gas. However, amines are corrosive and degrade over time, and require large industrial facilities. The most important property that ionic liquids have, however, is their low vapor pressure compared to volatile amines. This property results from ionic liquids' strong Coulombic attractive force and vapor pressure remains low through the substance's thermal decomposition point (typically >300°C). This simplifies their use and makes them a "green" alternative. Additionally, it reduces risk of contamination of the CO2 gas stream and of leakage into the environment. One computational chemistry simulation estimates that using 1-butyl-3-methylimidazolium acetate ([BMIM][Ac]) could result in a 16% energy savings and 11% capital cost savings compared to MEA.

The solubility of CO2 in ionic liquids is governed primarily by the anion, and secondarily by the cation. The hexafluorophosphate (PF6–) and tetrafluoroborate (BF4–) anions have been shown to be especially amenable to CO2 capture.

Other separations
In 1998, the first report of separation by ionic liquids was published, and ionic liquids have become the main solvents in liquid-liquid extraction processes involving the aqueous phase since then. Beside that, ionic liquids have replaced the conventional volatile solvents in industry such as absorption of gases or extractive distillation. Additionally, ionic liquid is also used as co-solutes for the generation of aqueous biphasic systems, or purification of biomolecules.

Interest has grown in ionic liquid for separation processes, as study has increased in real implementation in industry.

Process
A typical CO2 absorption process consists of a feed gas, an absorption column, a stripper column, and output streams of CO2-rich gas to be sequestered, and CO2-poor gas to be released to the atmosphere. Ionic liquids could follow a similar process to amine gas treating, where the CO2 is regenerated in the stripper using higher temperature. However, ionic liquids can also be stripped using pressure swings or inert gases, reducing their energy requirement. A current issue with ionic liquids for carbon capture is that they have a lower working capacity than amines. Task-specific ionic liquids which employ chemisorption and physisorption are being developed in an attempt to increase the working capacity. 1-butyl-3-propylamineimidazolium tetrafluoroborate is one example of a TSIL.

Tunability
Like all separation techniques, ionic liquids exhibit selectivity towards one or more of the phases of a mixture. 1-Butyl-3-methylimidazolium hexafluorophosphate (BMIM-PF6) is a room-temperature ionic liquid that was identified early on as a viable substitute for volatile organic solvents in liquid-liquid separations. Other [PF6]- and [BF4]- containing ionic liquids have been studied for their CO2 absorption properties, as well as 1-ethyl-3-methylimidazolium (EMIM) and unconventional cations like trihexyl(tetradecyl) phosphonium ([P66614]). Selection of different anion and cation combinations in ionic liquids affects their selectivity and physical properties. Additionally, the organic cations in ionic liquids can be "tuned" by changing chain lengths or by substituting radicals. Finally, ionic liquids can be mixed with other ionic liquids, water, or amines to achieve different properties in terms of absorption capacity and heat of absorption. This tunability has led some to call ionic liquids "designer solvents." 1-butyl-3-propylamineimidazolium tetrafluoroborate was specifically developed for CO2 capture; it is designed to employ chemisorption to absorb CO2 and maintain efficiency under repeated absorption/regeneration cycles. Other ionic liquids have been simulated or experimentally tested for potential use as CO2 absorbents.

Industrial applications
Industrial operations require an energy efficient and environmentally friendly process for CO2 capture. Currently, CO2 capture uses mostly amine-based absorption technologies, which are energy intensive and solvent intensive. Volatile organic compounds alone in chemical processes represent a multi-billion dollar industry. Therefore, ionic liquids offer an alternative that requires less energy. Due to the properties of ionic liquids, they have potential for large-scale implementation of post-combustion CO2 capture.

During the capture process, the anion and cation play a crucial role in the dissolution of CO2. Spectroscopic results suggest a favorable interaction between the anion and CO2, wherein CO2 molecules preferentially attach to the anion. Furthermore, intermolecular forces, such as hydrogen bonds, van der Waals bonds, and electrostatic attraction, contributes to the solubility of CO2 in ionic liquids. This makes ionic liquids promising candidates for CO2 capture because the solubility of CO2 can be modeled accurately by the regular solubility theory (RST), which reduces operational costs in developing more sophisticated model to monitor the capture process.

For example, due to their practical properties, ionic liquids have been shifting more from academic labs into industrial applications. Ionic liquids have been marketed as Gasguard Subatmospheric System by Air Products. This application was specifically for gas absorption, where it has proven to be twice as effective in performance as normal absorption techniques.

One of the main drawbacks of ionic liquids is their high viscosity, which complicates their use in industrial operations. Supported ionic liquid phases (SILPs) are one proposed solution to this problem.