Solubility of carbon dioxide in three [Tf2N] ionic liquids

doi:10.1016/j.fluid.2014.07.015

Fluid Phase Equilibria

Volume 380, 25 October 2014, Pages 39-47

https://doi.org/10.1016/j.fluid.2014.07.015 Get rights and content

Abstract

The solubility of carbon dioxide in three bis(trifluoromethylsulfonyl)imide-based ionic liquids: N-methyl-n-pentylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([p(5)mpyrr][Tf₂N]), 1-butyl-2,3-dimethylimidazolium bis(trifluoromethyl sulfonyl)imide ([bmmim][Tf₂N]), and tributylmethylphosphonium bis(trifluoromethyl sulfonyl)imide ([P₄₄₄₁][Tf₂N]) was measured at 298.15, 313.15, and 343.15 K and pressures up to about 1.9 MPa. The data were well correlated with the Soave–Redlich–Kwong equation of state (SRK-EoS). Henry's law constants for CO₂ in the three ILs ([p(5)mpyrr][Tf₂N], [bmmim][Tf₂N], and [P₄₄₄₁][Tf₂N]) at 298.15 K were 3.1, 3.5, and 3.3 MPa, respectively.

Introduction

Anthropogenic emissions of carbon dioxide, a serious greenhouse gas, contribute significantly to global warming [1]. Emissions of CO₂ from large stationary sources such as coal-fired power plants can be controlled by capturing CO₂ from the emission point and sequestering it in geological storages [2] or using it for enhanced oil recovery (EOR). At present, the use of chemical solvents for CO₂ removal is the most advanced technology for the separation of CO₂ from flue gases. The technique is still quite expensive due to the cost of energy-intensive regeneration of the solvent for continuous use [3]. The first large scale carbon capture plant associated to a power plant (Shand Carbon Capture Test Facility) is situated in southern Saskatchewan (near Estevan, Canada) and utilizes an aqueous chemical solution for separating CO₂ from flue gas stream.

Room temperature ionic liquids (ILs) were proposed as a novel class of non-volatile solvents for CO₂ capture [4]. Typical ILs consist of organic cations such as imidazolium, pyridinium, pyrrolidinium, ammonium, sulfonium, or phosphonium derivatives, and organic or inorganic anions [5]. A variety of ILs can be synthesized by altering the cation or anion or by chemical modification of both of the anion or/and the cation. In 1999, Blanchard et al. [4] demonstrated the high solubility of CO₂ in an ionic liquid, 1-butyl-3-methyl-imidazolium hexafluorophosphate, and sparked further interest in developing ILs with higher capture capacity. A number of conventional ionic liquids were subsequently investigated with structural modifications in the cation and anion to reach the limit of physical solubility. Later, to further enhance solubility, amine-functionalized ionic liquids were investigated that absorbed CO₂ through chemical reaction, and it was possible to capture 0.5 mol of CO₂ per mole of ionic liquid at atmospheric pressure [6]. More recently, Wang et al. [7] used equimolar mixture of hydroxyl-functionalized ionic liquids and a superbase bicyclic amidine to achieve nearly 1 mol of CO₂ per mole of superbase. Mixtures of ionic liquid with other ionic liquids or organic solvents or amines were also investigated. The solubility of CO₂ in pure ionic liquids and in mixed solvents was reviewed recently in 2012 by Zhao et al. [8] and Lei et al. in 2013 [9]. The solubility data of CO₂ in 136 ionic liquids of varieties of chemical structure was reviewed by Lei et al. [9].

CO₂ capture by ionic liquids (ILs) through physisorption is promising as it requires less energy for regeneration than alkanolamines [4]. Designing ILs with higher physical absorption capacity of CO₂ is of great interest. One way to increase solubility of CO₂ in ILs is the addition of fluoro atoms to the anion [10]. The solubility of CO₂ was measured in many ionic liquids with the fluorinated anion bis(trifluoromethylsulfonyl)imide ([Tf₂N]), such as [emim][Tf₂N] [11], [bmim][Tf₂N] [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [hmim][Tf₂N] [10], [11], [23], [24], [dmim][Tf₂N][11], [MeBuPyrr] [Tf₂N][12], [P₆₆₆₁₄][Tf₂N] [25], and [MeBu₃N][Tf₂N] [12]. We have selected three new [Tf₂N]-based ILs with three different cations based on imidazolium, pyrrolidinium and phosphonium precursors for the measurement of CO₂ solubility and which has not been previously presented in the literature.

Section snippets

Materials

The ionic liquids and carbon dioxide used in the current study are listed in Table 1 with suppliers, structures and purity. The ionic liquids were used without further purification. However, during the experiment, the ionic liquids were dried before isotherm measurement as described in Section 2.3.

Density measurement

Densities of the ionic liquids at temperatures from 278.15 to 353.15 K, and at atmospheric pressure were measured with an Anton Paar density meter (DMA 4500). The apparatus consists of a glass U tube

The Soave–Redlich–Kwong (SRK) equation of state (EoS)

The solubility of CO₂ in some [Tf₂N] containing ILs was previously modeled with cubic EoS [11]. The solubility data in the present work were also correlated with the Soave–Redlich–Kwong [26], [27] (Eq. (1)): $P = \frac{R T}{V - b_{m}} - \frac{a_{m}}{V (V + b_{m})}$ where a_m and b_m are the mixture attractive and co-volume parameters, respectively and related to the corresponding pure component parameters with the following van der Waals quadratic two parameter mixing rules: $a_{m} = \sum_{i = 1}^{2} \sum_{j = 1}^{2} x_{i} x_{j} a_{i j}$ $b_{m} = \sum_{i = 1}^{2} \sum_{j = 1}^{2} x_{i} x_{j} b_{i j}$ where,(4) $a_{i j} = (1 - k_{i j}) \sqrt{a_{i i} a}$

Density

The results of the experimental density are presented in Table 3. The density decreases linearly with increasing temperature. Corresponding molar volumes are shown in Fig. 1 as a function of temperature. The linear correlations of density of the ionic liquids with temperature are presented in Table 4.

Verification of the measurements

The solubility data of CO₂ in [bmim][PF₆] at 298.15 and 323.15 K were measured and compared to the available values published by Shiflett and Yokozeki [29] and Anthony et al. [30] in Fig. 2. The

Conclusions

Data for the solubility of CO₂ in three ionic liquids [bmmim][Tf₂N], [p(5)mpyrr][Tf₂N], and [P₄₄₄₁][Tf₂N] at temperatures 298.15, 313.15, and 343.15 K are reported at pressure up to 1.9 MPa. The Henry's law constants of CO₂ in these ILs at 298.15 K were 3.5, 3.1 and 3.3 MPa respectively, indicating negligible effect of cation. Methyl substitutions at C₂ position in the [bmim] cation slightly decrease the solubility of ILs with [Tf₂N] anion. On the other hand, an increase in alkyl chain length in

Acknowledgements

COSMOlogic is thanked for support.

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Solubility of carbon dioxide in three [Tf2N] ionic liquids

Abstract

Introduction

Section snippets

Materials

Density measurement

The Soave–Redlich–Kwong (SRK) equation of state (EoS)

Density

Verification of the measurements

Conclusions

Acknowledgements

Energy

Chin. J. Chem. Eng.

J. Chem. Thermodyn.

J. Supercrit. Fluids

J. Supercrit. Fluids

J. Chem. Thermodyn.

Chem. Eng. Sci.

Fluid Phase Equilib.

Science

Science

Nature

Eng. Chem. Res.

J. Am. Chem. Soc.

Angew. Chem. Int. Ed.

Chem. Rev.

J. Phys. Chem. B

J. Phys. Chem. B

J. Phys. Chem. B

Solubility of carbon dioxide in three [Tf₂N] ionic liquids