Solubilities of CO2, H2, N2 and O2 in ionic liquid 1-n-butyl-3-methylimidazolium heptafluorobutyrate

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Abstract

Ionic liquids (ILs) are promising in the capture and separation of CO2. In this work, IL 1-n-butyl-3-methylimidazolium heptafluorobutyrate, [C4mim][CF3CF2CF2COO], has been synthesized and characterized by 1H, 13C and 19F NMR spectra. Solubilities of CO2, H2, N2 and O2 in this IL have been determined at the temperature range from (303.15 to 333.15) K and the pressure up to 8.9 MPa. From these data, the Henry’s constant, the standard state solution Gibbs energy, standard state solution enthalpy and standard state solution entropy of these gases in [C4mim][CF3CF2CF2COO] were derived and analyzed from molecular interactions. It was shown that the solubility of CO2 is obviously higher than that in [C4mim][CF3COO] and [C4mim][CF3SO3] reported in literature, which makes this IL an attractive solvent for gas separation processes. Furthermore, these solubility data were well correlated by Pitzer model.

Highlights

Ionic liquid [C4mim][CF3CF2CF2COO] was synthesized and characterized. ► Solubilities of CO2, H2, N2 and O2 in the IL were determined from 303.15 to 333.15 K and up to 8.9 MPa. ► CO2 solubility is 6–10 times that of the other three gases in this IL. ► The dissolution of CO2, O2, N2 and H2 in [C4mim][CF3CF2CF2COO] was controlled by entropy term. ► These solubility data were correlated with high accuracy by the Pitzer model.

Introduction

Nowadays, carbon capture technology has attracted increasing attention of governments, scientists and engineers worldwide, since CO2 is one of the major contributors to the greenhouse effect. Among the different technologies to capture post-combustion carbon dioxide, those based on chemisorption by aqueous alkanolamines are considered to be the most efficient and relatively least expensive [1], [2], [3]. However, amine-based aqueous processes have several drawbacks such as reduced CO2 absorption capacity, potential toxic byproduct release to atmospheric environment as a result of their volatileness, operational instability and frequent equipment maintenance due to excessive corrosion of the amine system. Consequently, the discovery of materials which can selectively and efficiently absorb CO2 from flue gases is essential to realizing practical carbon capture and sequestration (CCS).

Ionic liquids (ILs) are promising in this regard because of their unusual properties such as no-volatility, non-flammation, recyclability, high thermal stability and strong solubility capacity [4]. These properties make ILs a good potential to replace conventional organic solvents (aqueous solutions of alkanolamines). ILs are also termed designer solvents due to the tunability of their molecular structures and physicochemical properties. Accordingly, a great deal of works has been carried out to explore the possibilities of CO2 capture by using ionic liquids.

It has been reported that solubility of carbon dioxide is high in certain ILs, especially in those having imidazolium cations [5], [6], [7], [8], [9]. The increase in length of alkyl chains with branching or ether linkages in ILs can improve CO2 solubility [10], [11], [12], [13] although viscosities of these ionic liquids generally increase with increasing number and length of alkyl substituents [14]. More interestingly, the increase in the number of fluorine atoms in ILs could greatly increase the uptake of CO2 [11], [15], [16], [17], especially in the anion dominating interactions [18], [19], [20]. For example, Almantariotis et al. [21], [22] determined the values of low pressure solubility of CO2 in (1-methyl-3-3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluorooctyl)imidazolium bis[(trifluoromethyl)sulfonyl]amide ([C8H4F13mim][Tf2N]) and 1-alkyl-3-methylimidazolium tris(pentafluoroethyl)-trifluorophosphate [Cnmim] [eFAP] (n = 2, 4, 6) ionic liquids. Their results showed that the presence of fluoroalkyl substituents significantly enhanced the absorption of carbon dioxide, which was attributed to more favorable interactions between CO2 and the polar region of the ionic liquids. In addition to these, introducing carbonyl groups might be a good choice to improve CO2 solubility through increasing the CO2-philic property of ILs. There is plenty of evidence in the literature for Lewis acid/Lewis base (A/B) interaction between CO2 and the carbonyl group [23], [24], [25], where CO2 acts as a Lewis acid and the acidic central carbon interacts with the charged or uncharged Lewis bases [26].

In order to develop more efficient ILs for CO2 capture, ionic liquid 1-n-butyl-3-methylimidazolium heptafluorobutyrate ([C4mim][CF3CF2CF2COO]) was synthesized and characterized in the present work. The solubilities of CO2, N2, O2 and H2 in this IL were measured in the temperature range from 303.15 to 333.15 K and the pressure up to 8.9 MPa. The solubilities of CO2 have been compared with those of some ILs available in literature to examine the anionic effect. The Henry’s constant, standard state solution Gibbs energy, standard state solution enthalpy and standard state solution entropy of these gases in [C4mim][CF3CF2CF2COO] were derived and analyzed from the interactions between the IL and the gases. Thus obtained information was used to interpret the solubility selectivity of the IL for CO2. Additionally, the experimental solubility data were successfully correlated using the Pitzer model.

Section snippets

Materials

[C4mim][BF4] (CAS Registry No. 174501-65-6) and [C4mim][Tf2N] (CAS Registry No. 174899-83-3) were purchased from Shanghai Cheng Jie Chem. Co. Ltd, with nominal purity of 99 %. Both of them were used after degassed and dried under vaccum. N-methylimidazolium and 1-chlorobutane were purchased from Linhai Kaile Chem. Co. and Shanghai Hushi Chemical Factory, respectively, which were distilled twice before use. Amberlite IRA-400 (Cl) ion exchange resin (CAS Registry No. 9002-24-8) was obtained from

Experimental solubilities and anionic effect of the ILs

To validate the procedure applied for the present solubility measurements, the solubilities of CO2 in 1-n-butyl-3-methylimidazolium terafluoroborate ([C4mim][BF4]) and 1-n-butyl-3-methylimidazolium bis(trifluoromethyl)sulfonylimide ([C4mim][TF2N]) were measured at T = 323.15 K. The results were shown in table 1 and illustrated in figure 2a and b, respectively, as a function of CO2 pressure. For the sake of comparison, the data available in literature were also included in these figures. It can be

Conclusions

In the present work, the solubilities of CO2, O2, N2 and H2 in [C4mim][CF3CF2CF2COO] were determined from 303.15 to 333.15 K and up to 8.9 MPa. Then, Henry’s constants, standard state solution Gibbs energy, standard state solution enthalpy and standard state solution entropy of these gases in the IL were derived and analyzed from molecular interactions. It was shown that because of more fluorine atoms and/or the carbonyl group presented in the anions, [C4mim][CF3CF2CF2COO] has a better ability to

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grant No. 20977025) and the Innovation Scientists and Technicians Troop Construction Projects of Henan Province (No. 092101510300).

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