The use of ionic liquids in carbon capture is a potential application of ionic liquids as absorbents for use in carbon capture and sequestration. Ionic liquids, which are salts that exist as liquids near room temperature, are polar, nonvolatile materials that have been considered for many applications. The urgency of climate change has spurred research into their use in energy-related applications such as carbon capture and storage.
Carbon capture using absorption
Ionic liquids as solvents 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 in other CO2 separations, such as "sweetening" of natural gas. However, amines are corrosive, degrade over time, and require large industrial facilities. Ionic liquids on the other hand, have low vapor pressures . This property results from their strong Coulombic attractive force. Vapor pressure remains low through the substance's thermal decomposition point (typically >300 °C). In principle, this low vapor pressure simplifies their use and makes them "green" alternatives. Additionally, it reduces risk of contamination of the CO2 gas stream and of leakage into the environment. The solubility of CO2 in ionic liquids is governed primarily by the anion, less so by the cation. The hexafluorophosphate (PF6–) and tetrafluoroborate (BF4–) anions have been shown to be especially amenable to CO2 capture. Ionic liquids have been considered as solvents in a variety of liquid-liquid extraction processes, but never commercialized. Beside that, ionic liquids have replaced the conventional volatile solvents in industry such as absorption of gases or extractive distillation. Additionally, ionic liquids are used as co-solutes for the generation of aqueous biphasic systems, or purification of biomolecules.
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 the process 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 that 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.
Research In 2023, a research team composed of Chuo University, Nihon University, Kanazawa University, and the Research Institute of Innovative Technology for the Earth utilized electronic state informatics to design and synthesize ionic liquids. Subsequently, they conducted precise measurements of CO2 solubility and successfully developed ionic liquids with the highest physical absorption capacity for CO2 to date.
Drawbacks
Selectivity In carbon capture an effective absorbent is one which demonstrates a high selectivity, meaning that CO2 will preferentially dissolve in the absorbent compared to other gaseous components. In post-combustion carbon capture the most salient separation is CO2 from N2, whereas in pre-combustion separation CO is primarily separated from H2. Other components and impurities may be present in the flue gas, such as hydrocarbons, SO2, or H2S. Before selecting the appropriate solvent to use for carbon capture it is critical to ensure that at the given process conditions and flue gas composition CO2 maintains a much higher solubility in the solvent than the other species in the flue gas and thus has a high selectivity. The selectivity of CO2 in ionic liquids has been widely studied by researchers. Generally, polar molecules and molecules with an electric quadrupole moment are highly soluble in liquid ionic substances. It has been found that at high process temperatures the solubility of CO2 decreases, while the solubility of other species, such as CH4 and H2, may increase with increasing temperature, thereby reducing the effectiveness of the solvent. However, the solubility of N2 in ionic liquids is relatively low and does not increase with increasing temperature so the use of ionic liquids in post-combustion carbon capture may be appropriate due to the consistently high CO2/N2 selectivity. The presence of common flue gas impurities such as H2S severely inhibits CO2 solubility in ionic liquids and should be carefully considered by engineers when choosing an appropriate solvent for a particular flue gas.
Viscosity A primary concern with the use of ionic liquids for carbon capture is their high viscosity compared with that of commercial solvents. Ionic liquids which employ chemisorption depend on a chemical reaction between solute and solvent for CO2 separation. The rate of this reaction is dependent on the diffusivity of CO2 in the solvent and is thus inversely proportional to viscosity. The self diffusivity of CO2 in ionic liquids are generally to the order of 10−10 m2/s, approximately an order of magnitude less than similarly performing commercial solvents used on CO2 capture. The viscosity of an ionic liquid can vary significantly according to the type of anion and cation, the alkyl chain length, and the amount of water or other impurities in the solvent. Because these solvents can be "designed" and these properties chosen, developing ionic liquids with lowered viscosities is a current topic of research. Supported ionic liquid phases (SILPs) are one proposed solution to this problem.
Tunability
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