Solubility and selectivity of CO2 in ether-functionalized imidazolium ionic liquids

doi:10.1016/j.jct.2016.08.028

The Journal of Chemical Thermodynamics

Volume 103, December 2016, Pages 292-298

https://doi.org/10.1016/j.jct.2016.08.028 Get rights and content

Highlights

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Solubilities of CO₂, N₂ and O₂ in [EOMmim][PF₆] and [EOMmim][Tf₂N] were determined.
•
Introduction of ether group results in increase of CO₂/N₂ and CO₂/O₂ selectivity.
•
The solution of CO₂ in the ionic liquids is an exothermic and orderly process.
•
Experimental solubility data have been well correlated by Pitzer model.

Abstract

Ionic liquids are widely recognized new materials in carbon dioxide capture and separation technology. In this work, we synthesized and characterized two kinds of ether-functionalized imidazolium ionic liquids, 1-methoxyethyl-3-methylimidazolium hexafluoroborate ([EOMmim][PF₆]) and 1-methoxyethyl-3-methylimidazolium bis(trifluoro-methylsulfony)imide ([EOMmim][Tf₂N]). Solubility values of CO₂ in these ionic liquids were determined by isometric weight method at the temperatures from 298.15 K to 343.15 K and the pressure up to 5.185 MPa. Furthermore, solubilities of other flue gases, N₂ and O₂, in these two ionic liquids were also measured at 303.15 K. It was shown that little influence had been exerted on CO₂ solubility by the introduction of ether groups on the cation, but it decreased N₂ and O₂ solubility, resulting in the remarkable increase of CO₂/N₂ and CO₂/O₂ selectivity. In addition, the solubility data were well correlated by Pitzer model, and the standard state solution Gibbs energy, solution enthalpy and solution entropy of CO₂ in the two ionic liquids were reported. Regeneration characteristics of the investigated ionic liquids was also studied by vacuum desorption and atmospheric desorption, respectively.

Introduction

In recent years, many ionic liquids have been found to be able to absorb CO₂, particularly those with imidazolium cations. Indeed, a considerable amount of solubility data for CO₂ in imidazolium ionic liquids is available in the literature [1], [2], [3], [4]. Nevertheless, for a CO₂ capture process from flue gas, solubility data are not enough to assess the performance of an absorbent, the data for CO₂ separation selectivity are also important. Unfortunately, such data are very limited, except for those of some conventional imidazolium-based ILs such as 1-butyl-3-methylimidazolium terafluoroborate [Bmim][BF₄], 1-butyl-3-methylimidazolium bis(trifluoromethyl)sulfonylimide [Bmim][Tf₂N], 1-butyl-3-methylimidazolium nitrate [Bmim][NO₃], and 1-butyl-3-methylimidazolium dicyanamide [Bmim][N(CN)₂] [5], [6], [7], [8], [9]. For functionalized ionic liquids, Bara et al. [5] found that introduction of ether groups to an imidazolium cation of ionic liquids led to an increases in solubility selectivity of CO₂/CH₄ and CO₂/N₂ relative to analogous ionic liquids with similar length n-alkyl groups attached to the cation. Recently, zhang et al. [10] also found that the repulsion between the polar group like ether group and nonpolar gas like CH₄ was very effective for selective separation of CO₂ in ether-functionalized pyridinium ionic liquids. These results are very important for ionic liquids potentially used as selective separation absorbents for CO₂. In order to further ascertain this solubility selectivity issue, we have synthesized new ether-functionalized ionic liquids and carried out a series of systemic studies.

In this work, new solubility data for CO₂ in ether-functionalized imidazolium ionic liquids 1-methoxyethyl-3-methylimidazolium hexafluoroborate [EOMmim][PF₆] and 1-methoxyethyl-3-methylimidazolium bis(trifluoro-methylsulfony)imide [EOMmim][Tf₂N] were determined experimentally in the temperature range from (298.15 to 343.15) K and the pressure up to 5.185 MPa. The solubility values of N₂ and O₂ in these ionic liquids were also measured to investigate the selectivity of the ionic liquids for CO₂ relative to N₂ and O₂. From the solubility data, Henry’s constant, the standard state solution Gibbs energy, solution enthalpy and solution entropy of the CO₂-ILs systems were derived and analyzed. In addition, the experimental solubility data were successfully correlated by Pitzer model. The regeneration and reuse of the ether-functionalized ionic liquids were also studied by a series of experiments. To the best of our knowledge, no solubility selectivity of CO₂ relative to N₂ and O₂ has been reported in [EOMmim][PF₆] and [EOMmim][Tf₂N] to date.

Section snippets

Materials

The ionic liquids used in the current study were synthesized by using similar procedures reported by Branco et al. [11] and Bara et al. [5]. Firstly, we need to prepare [EOMmim][Cl]. To this end, 2-chloroethylmethyl ether (0.95 mol) and 1-methylimidazole (0.63 mol) were added to a round-bottomed flask fitted with a reflux condenser, and the mixture was refluxed for 24 h at 80 °C with stirring until two phases were formed. The top phase containing unreacted stating materials was carefully decanted,

Experimental solubility of CO₂ in the ionic liquids

Solubility data of CO₂ in [EOMmim][PF₆] and [EOMmim][Tf₂N] were determined in the temperature range from 298.15 K to 343.15 K and the pressure range from 1.052 MPa to 5.185 MPa. These data were listed in Table 2, Table 3. For the sake of easy understanding, solubility values of CO₂ in the ionic liquids were exhibited in Fig. 1, Fig. 2 as a function of pressure at different temperatures. It can be seen that absorption capacities of CO₂ in these ionic liquids increased almost linearly with increasing

Conclusions

In this work, solubilities of CO₂ in ether-functionalized ionic liquids [EOMmim] [PF₆] and [EOMmim] [Tf₂N] were reported in the temperature range from 298.15 K to 343.15 K and pressure up to 5.185 MPa. Solubilities of N₂ and O₂ in these ionic liquids were also reported at 303.15 K. It was found that the introduction of ether groups on the cation of the ionic liquids had no essential influence on CO₂ solubility, but it greatly improved the solubility selectivity for CO₂/N₂ and CO₂/O₂. Experimental

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grant No. 21377036) and the Innovation Scientists Projects of Henan Province (No. 144200510004).

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Solubility and selectivity of CO2 in ether-functionalized imidazolium ionic liquids

Highlights

Abstract

Introduction

Section snippets

Materials

Experimental solubility of CO2 in the ionic liquids

Conclusions

Acknowledgements

J. Chem. Thermodyn.

J. Chem. Thermodyn.

J. Chem. Thermodyn.

J. Chem. Thermodyn.

J. Chem. Thermodyn.

Fluid Phase Equilib.

J. Phys. Chem. B

Ind. Eng. Chem. Res.

Ind. Eng. Chem. Res.

J. Phys. Chem. B

Ind. Eng. Chem. Res.

Bioresour. Technol.

Energy Fuels

Chem. –Eur. J.

Solubility and selectivity of CO₂ in ether-functionalized imidazolium ionic liquids

Experimental solubility of CO₂ in the ionic liquids