Room temperature ionic liquids (RTILs) are found to have numerous applications. Numbers of investigations have shown that CO2 is remarkably soluble in imidazolium-based RTILs and has a great miscibility with supercritical CO2. Those findings lead many scientists to investigate their potential application as green absorbents for the carbon dioxide separation for replacing the conventional amine-based CO2 scrubber. Despite excellent properties of RTIL, some existing problems, primarily the lower CO2 absorption capacity in compared with the amine solution and the cost-effective production of RTILs, remain challenging which make those RTILs still underdeveloped to be applied in industrial scale. This paper contains a mini review on the progress of the CO2 absorption process employing the RTILs and disclose a proposed modified system consisting of a cheap and simple RTIL plus an additive to enhance the CO2 solubility in RTILs which may compromise the high cost production of fluorinated RTILs.
In the search of alternative liquid phase absorbent for CO2 capture technology, scientists found that room temperature ionic liquids (RTILs) can be expected as good alternative candidates to replace the already established amine-based solution, such as MEA, MDEA or piperazine for CO2 absorption. Although amine-based scrubber is very effective in adsorbing CO2 through a chemical interaction and an intermediate carbamate formation, its application in industrial scale still faces some problems mainly the corrosion due to the strong basic property, the low thermal stability and the high energy demand for the CO2 desorption process. In that regard, a highly effective CO2 physical adsorption should be introduced as an alternative CO2 adsorption process.
RTILs are organic salts composed entirely of bulky univalent ions. They posses low melting points (generally below 253 K), negligible vapor pressure and high thermal stability. They are also commonly recognized as versatile alternatives and tailored-solvents to conventional organic solvents.
Initiated by a publication in Nature shows that CO2 is remarkably soluble in imidazolium-based ionic liquid  and later, supported by the finding of great miscibility of ILs and supercritical CO2, followed by an in situ ATR investigation of CO2 dissolved in two ionic liquids at high pressures , and other works related to CO2-ILs phase equilibria [3,4,5,6] have drawn many scientist’s interest to investigate further the potential applications of ILs for the CO2 separation process. The excellent physical and chemical properties along with the nature behavior to easily adsorb CO2 make these types of liquids as promising candidates for future CO2 separation, especially CO2 from flue gas which huge and hot gases are produced or CO2 in the natural gas sweetening [7,8,9,10].
Several papers reported that type of anion in RTILs play an important role in the CO2 absorption [2,7,11]. Accordingly, the CO2 interact with the anion through a weak Lewis-acid-base interaction. Gases with large dipole moments (e.g. water) or quadrupole moments (e.g. CO2 ad N2O), as well as those with the opportunity for specific interactions (e.g. hydrogen bonding) have the highest solubilities in the RTILs. Anions containing fluorine such as bis[(trifluoromethyl)-sulfonyl]imide, Tf2N- have great affinity toward CO2 . Changing the cation from imidazolium to quartenary ammonium or pyrrolidinium,, all with an equal anion, makes little difference in the CO2 and O2 solubilities. A previous studies showed CO2 solubility for 1-butyl- 3-methylimidazolium ([bmim]+) based ILs at 60 °C increased in the order nitrate ([NO3]-) < tetrafluoroborate ([BF4]-) < dicyanamide ([DCA]-) ~ hexafluorophosphate ([PF6]-) ~ trifluoromethanesulfonate ([TfO]-) < bis[(trifluoromethyl)-sulfonyl]imide ([Tf2N]-) < tris(trifluoromethylsulfonyl)methide ([methide]-). For the cation, the CO2 solubility increases with increasing chain length .
Some efforts to design task-specific ILs by tethering amine functional groups on the cation or the anion moiety was also done to dramatically improve the CO2 solubility [10,13,14]. However, those RTILs are still far from real industrial applications primarily due to the ineffective production cost. In the line of other similar investigations employing RTILs, motivated by the success of utilizing zinc tetrahalide ionic liquids as the catalyst for the coupling reaction of CO2 and epoxides to produce the corresponding alkylene carbonates [15,16], we propose to employ this kind of catalyst in conjunction with a simple, cheap and readily available RTIL for the CO2 mitigation and sequestration. The other additives can be broad types of metal salts or non volatile organics. Until recently, only very limited numbers of cheap RTILs are known and discovered. Among of them are consist of imidazole and tetraalkyl ammonium cations and alkyl sulfate, dicyanamide, BF4-, and carboxylates anions.
To screen the appropriate RTILs-based CO2 adsorption system, solubility measurement is essential to carry out. Various techniques have been introduced which fall into three different types, gravimetric method using microbalance , isochoric method based on pressure drop , and the latest one, using a transient thin-liquid-film method . The first method needs very expensive apparatus and somewhat tedious calibration step against buoyancy effect. Though the latest method seems to be easy to build and to operate, the RTILs density information has to be provided. Then, the isochoric method seems to be the most feasible and sufficiently reliable from those three methods because it only requires a precise pressure indicator and calibrated equilibrium reactor.
Separation of CO2 from a gas stream, using fiber membrane contactor, is a promising alternative to conventional techniques such as column absorption since the later technology is energy-consuming and not easy to operate because of the frequent problems including flooding, foaming, chanelling and entrainment. Hollow fiber membrane contactor (HFMC) is a promising alternative. Even though RTILs have potential application as CO2 absorbents, investigation on the membrane supported RTILs for CO2 separation is still rare [19,20,21].
Some primary problems regarding the application of RTILs for the CO2 absorption are their physical properties such as viscosity and their chemical purities [22,23,24]. Several well-known RTILs have very low melting points or only possess glass temperatures, such as imidazolium, pyrrolidinium, ammonium or phosphonium-based ILs containing dicyanamide ([dca]-), alkyl sulfate ([ROSO3]-), BF4-, PF6-, trifluoromethanesulfonate ([TfO]-), bis[(trifluoromethyl)-sulfonyl]imide ([Tf2N]-). The presence of water in [bmim]PF6 was mentioned to slightly or negligibly reduce the ability of the RTIL to absorb CO2 . On the contrary, another work reported that water-saturated various RTILs ([bmim]PF6, [emim]Tf2N, [emim]CF3SO3, [emim]N(CN)2, and [thtdp]Cl] markedly improved their CO2 absorption capacity, even, in the case of the last three RTILs, CO2 solubility is very high . In a real gas-separation process, water vapor is always present in the gas feed stream and removing it is impractical. Indeed, a small concentration of water actually can be regarded as an additive which reduces the viscosity of RTIL and is expected to increase the CO2 solubility.
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