Emission of CO2 is continuously increasing since the industrial revolution. Due to a suspected relation between the green-house effect and the increased CO2 concentration in the atmosphere, the development of the methods for CO2 capture and storage is of great interest. The current available process is based on aqueous amine solutions containing for example monoethanolamine (MEA). This poses a couple of problems. First, a large enthalpy of reaction between CO2 and amines, demands a huge amount of heat to release the captured CO2 for regeneration. Furthermore, this process is cost intensive and the used amines are highly corrosive and volatile. That is why huge efforts have to be put into finding alternatives for capturing and storing CO2 effectively.
In this context ionic liquids (ILs) attract increasing more interest. Their benefits for this application are clear: with their ultra-low vapour pressure the problem of releasing volatile compounds to the atmosphere can be minimized. Appropriate ILs exhibit good thermal and chemical stability in an acidic or oxidative gas environment so a continuous solvent substitution and the addition of antioxidants is not necessary. Furthermore, with specially designed IL candidates, the corrosion of plant components can be reduced and, at the same time, an elevation of absorption/stripping rates and CO2 loading can be achieved.
For this reason several ionic liquids were examined as solvents for CO2 capture. The pure forms as well as binary and tertiary mixtures with water and amines were tested, whereas the amines were monoethanolamine (MEA), diethanolamine (DEA), and methyldiethanolamine (MDEA).
Fig.1. Chemical structure of amines.
The ionic liquids used were 1-ethyl-3-methylimidazolium acetate EMIM OAc, two lactam cation based-ILs, i. e. pyrrolidinium-2-one bis(trifluoromethylsulfonyl)imide BHC BTA and trifluoroacetate BHC TFA, two amino acid-based ILs 1-ethyl-3-methylimidazolium lysinate EMIM Lys and serinate EMIM Ser, and the 1-alkyl-3-methylimidazolium tricyanomethanides RMIM TCM (R= ethyl, butyl, hexyl, octyl).
Fig. 2. Chemical structure sof tested ionic liquids.
By use of the gravimetric technique the CO2 absorption capacity and kinetics were measured at different temperatures, pressures, and water contents.2
Even though none of the nine tested ILs can compete alone with amine solutions different interesting results came up. Diluting the ILs with water reduced the viscosity and thereby accelerated the CO2 diffusivity. With the exception of EMIM Lys - which showed good CO2 binding capacity but isn’t stable - the capture performance stayed below 30 %. In case of the BTA-, TFA- and acetate anion-ILs the addition of water dropped CO2 absorption capacity and increased corrosiveness. On the other hand, a mixture composed of EMIM Lys, BHC] BTA, and DEA was found to be the least ecotoxic among the other solvent compositions, but showed a high corrosiveness.2
For an industrial application ionic liquids need to reveal some features that facilitate the replacement of a significant fraction of amine content: the absorption of such ILs has to be enhanced by addition of water, the ILs have to show physical sorption, and should act as corrosion inhibitors. Ionic liquids that seem to meet these requirements are the TCM-based ILs. A mixture from three such ILs - bearing different chain lengths in the cation - in combination with MDEA showed good capture performance, less toxicity and reduced corrosiveness. This mixture appears to be a promising solution for the capture of CO2 in industrial scale.