Solubility of carbon dioxide and ethane in three ionic liquids based on the bis{(trifluoromethyl)sulfonyl}imide anion

doi:10.1016/j.fluid.2007.05.002

Fluid Phase Equilibria

Volume 257, Issue 1, 15 August 2007, Pages 27-34

https://doi.org/10.1016/j.fluid.2007.05.002 Get rights and content

Abstract

The objective of this work was to study the influence of changing the cation of the ionic liquid (IL) on gas solubility. For this purpose, the low-pressure solubility of carbon dioxide and of ethane in three ILs based on the bis{(trifluoromethyl)sulfonyl}imide anion ([NTf₂]⁻) was determined experimentally. Solubility data is reported for 1-ethyl-3-methylimidazolium ([C₁C₂Im]⁺), 1-butyl-1-methylpyrrolidinium ([C₁C₄pyrr]⁺) and propylcholinium ([N_1132-OH]⁺) bis{(trifluoromethyl)sulfonyl}imide ILs between 300 and 345 K. These data are precise to within ±1% and accurate to within ±5%. In these ILs, carbon dioxide (mole fraction solubility between 1 and 3 × 10⁻², molarity between 0.03 and 0.1 mol L⁻¹) is one order of magnitude more soluble than ethane. The effect of changing the cation is small but significant. Changing the cation has a similar effect on both gases even if the differences are more pronounced in the case of ethane with the order of solubility [C₁C₄pyrr][NTf₂] > [C₁C₂Im][NTf₂] > [N_1132-OH][NTf₂]. For all the systems, the solubility decreases with temperature corresponding to exothermic processes of solvation and negative enthalpies and entropies of solvation were calculated. The properties of solvation of the two gases in [C₁C₄pyrr][NTf₂] do not vary significantly with temperature while important variations are depicted for both gases in [C₁C₂Im][NTf₂].

Introduction

The aim of this work is to contribute to the study of the interactions between ionic liquids (ILs) containing the anion bis{(trifluoromethyl)sulfonyl}imide ([NTf₂]⁻) and two gaseous solutes, carbon dioxide and ethane through the experimental determination of gas solubility. Gas solubilities were measured as a function of temperature and at pressures close to atmospheric, using an isochoric saturation method. The data obtained provides important information for the characterization of solute–solvent interactions and so contribute to understand the mechanisms of dissolution. From a practical point of view, gas solubility can be useful in the calculation of (vapour + liquid) equilibria and thus pertinent to the development of new reaction and separation processes.

Our research group has previously studied the solubility of a selection of gases in two ILs namely 1-butyl-3-methylimidazolium tetrafluoroborate [C₁C₄Im][BF₄] [1] and 1-butyl-3-methylimidazolium hexafluorophosphate [C₁C₄Im][PF₆] [2] as a function of temperature from 303 to 343 K. In these two liquids, carbon dioxide was found to be the most soluble of the gases investigated, followed by ethane and methane. Other solutes including oxygen, argon, nitrogen, hydrogen and carbon monoxide showed much lower solubility. We have also studied the influence of changing the anion in 1-butyl-3-methyl-imidazolium-based ILs on the solubility of gaseous solutes [3]. It was observed that the gas solubilities in [C₁C₄Im][BF₄] and [C₁C₄Im][PF₆] are significantly lower than those in [C₁C₄Im][NTf₂].

The influence of changing the cation on the solubility of gases has also been previously investigated [4] through the study of carbon dioxide and hydrogen in three ILs composed of [NTf₂]⁻ with either an imidazolium cation (1-ethyl-3-methylimidazolium, [C₁C₂Im]⁺ and [C₁C₄Im]⁺) or an ammonium cation (butyl-trimethylammonium, [N₄₁₁₁]⁺). In contrast with variation with the IL anion, the cation influence was found to be much smaller. Both gases studied were found more soluble in the ammonium-based IL studied than in the two imidazolium-based ILs. Moreover, a slight increase of the solubility was systematically observed when increasing the chain length on the imidazolium cation.

Herein, it is our aim to continue the study of the influence of changing the cation of the IL on the solubility of gaseous solutes. We have chosen three ILs based on the [NTf₂]⁻ anion combined with either an aromatic ring ([C₁C₂Im]⁺) or a cycle (1-butyl-1-methylpyrrolidinium, [C₁C₄pyrr]⁺) and hydroxyl functionalised acyclic (propylcholinium [N_1132-OH]⁺) tetraalkylammonium cation as shown in Fig. 1.

The solubility of carbon dioxide in a variety of ILs has been determined at low pressures [1], [2], [3], [4], [5], [6], [7], [8], [9], [10] and high pressures (up to 73 MPa) [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24]. In particular, the system (carbon dioxide + [C₁C₂Im][NTf₂]), also investigated in this work, was studied by several authors [7], [8], [9], [10], [14], [17]. Camper et al. [7], [8], [10] have determined the solubility of carbon dioxide experimentally in this IL from 303 to 343 K and at low pressures by isochoric pVT measurements. The reported Henry's law constants (4.0 MPa at 303 K and 7.7 MPa at 343 K) increase when increasing temperature corresponding to a decrease of the solubility with temperature. Morgan et al. [9], whose main focus was to determine the experimental diffusivity of carbon dioxide in a variety of ILs, also measured the solubility of this gas by passing it through a supported IL film and measuring the gas pressure after the membrane at equilibrium. At 313 K, the solubility of carbon dioxide in [C₁C₂Im][NTf₂] was found to be 0.1 mol L⁻¹ atm⁻¹ which corresponds to a Henry's law constant of 4.1 MPa. Cadena et al. [14] used a gravimetric microbalance to determine the solubility of carbon dioxide in [C₁C₂Im][NTf₂] between 283 and 323 K and at pressures up to 1.4 MPa. Their measured Henry's law constants increase with temperature and vary from 2.53 to 5.15 MPa. Finally, Kim et al. [17] have measured the solubility of carbon dioxide in [C₁C₂Im][NTf₂] at 298.15 K and up to 1 MPa with a method similar to that of Cadena et al. [14]. Therein, the Henry's law constant calculated by extrapolating their data at atmospheric pressure was nearly 30% higher than that reported by Cadena et al. [14] at the same temperature. To the best of our knowledge, only Anthony et al. [12] have measured the solubility of carbon dioxide in [C₁C₄pyrr][NTf₂] and reported Henry's law constants at 283, 298 and 323 K of 3.02, 3.86 and 5.61 MPa, respectively.

In comparison, the solubility of ethane in ILs has been far less studied than the solubility of carbon dioxide [1], [2], [8], [10], [11]. Camper et al. [8], [10] have studied the system ethane + [C₁C₂Im][NTf₂] which is also reported herein. Henry's law constants of 10.7 MPa at 303 K and of 16.8 MPa at 313 K were found with the solubility decreasing with increasing temperature.

The solubility of carbon dioxide and ethane has been investigated in a wide range of imidazolium-based ILs including [C₁C₂Im][NTf₂] [8], [10], [C₁C₄Im][NTf₂] [22], [C₁C₄Im][BF₄] [1], [10], [C₁C₄Im][PF₆] [2], [10], [11], 1-ethyl-3-methylimidazolium dicyanamide ([C₁C₂Im][dca]) [10] and 1-ethyl-3-methylimidazolium triflate ([C₁C₂Im][OTf]) [10]. In all these ILs, carbon dioxide exhibits a higher solubility than ethane.

In general, changing the anion of the IL has a larger impact on the gas solubility than modifying the cation [3], [6], [12], [14], [15]. The presence of the [NTf₂]⁻ anion systematically increases the solubility of carbon dioxide [3], [6], [12], [14] this anion showing a larger affinity for this solute regardless of whether the liquid is based on the imidazolium, pyrrolidinium or ammonium cations [12]. The effect of changing the cation of the IL on the solubility of gases is less significant [12], [15]. A slight increase of the solubility of carbon dioxide when increasing the alkyl side chain length in the imidazolium cation was observed [6], [15], [23]. In addition, Anthony et al. [12] compared the solubility of carbon dioxide in three ILs containing the anion [NTf₂]^- and either an imidazolium, an ammonium or a pyrrolidinium cation. Replacing the imidazolium cation with either the tetraalkylammoinum-based cations resulted in a decrease in the solubility of carbon dioxide of between 15 and 30%. Interestingly, Baltus et al. [6] have observed, using a quartz crystal microbalance, a significantly greater carbon dioxide solubility in imidazolium-based ILs having fluorine-substituted cation as compared with the corresponding IL with a nonfluorinated cation. In this case the Henry's constant differs by a factor of seven between [C₁C₈Im][NTf₂] and the corresponding fluorine-substituted IL [C₁(C₈F₃)Im][NTf₂]). This observation is compatible with the known affinity of carbon dioxide by fluorinated solvents [37].

In the present work, the selected ILs with the exception of [C₁C₂Im][NTf₂] are not commonly studied. The solubility of carbon dioxide and ethane, was precisely determined as a function of temperature from 303 to 343 K and at pressures close to atmospheric using a high precision isochoric saturation method. The knowledge of the variation with temperature of the solubility allows the calculation of the thermodynamic properties of solvation.

Section snippets

Experimental

The IL [C₁C₂Im][NTf₂] was prepared at the QUILL research center at Belfast, using the standard synthesis procedures described by Bonhôte et al. [25]. The bromide content of this IL sample was determined using ion chromatography (IC) [26] and was found to be below the 5 ppm detection limit. The water content was determined before and after the solubility measurements by volumetric Karl-Fisher titration using a Mettler Toledo DL31 titrator. A reference value of 45 ppm (w/w) of water was obtained

Results and discussion

For each system, multiple experimental data points were obtained between 293 and 343 K in steps of approximately 10 K. The experimental solubilities of carbon dioxide and ethane in [C₁C₂Im][NTf₂], [C₁C₄pyrr][NTf₂] and [N_1132-OH][NTf₂] are reported in Table 1. The solubility results are expressed as mole fractions of solute and as Henry's law constants. The mole fraction solubilities, at the reference pressure of 0.1 MPa, are calculated from the experimental data on K_H (at slightly different total

Conclusions

In this paper, the effect of changing the cation of the IL on the low-pressure solubility of two gaseous solutes, carbon dioxide and ethane, was investigated. The solubility of these gases was measured as a function of temperature using an accurate isochoric saturation method in [C₁C₄pyrr][NTf₂], [C₁C₂Im][NTf₂] and [N_1132-OH][NTf₂]. Carbon dioxide with mole fractions solubility between 1 and 3 × 10⁻², was found to be one order of magnitude more soluble than ethane. These data have been shown to

Acknowledgements

MD thanks QUILL, the Royal Society and EPSRC for funding through the Portfolio Partnership.

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Solubility of carbon dioxide and ethane in three ionic liquids based on the bis{(trifluoromethyl)sulfonyl}imide anion

Abstract

Introduction

Section snippets

Experimental

Results and discussion

Conclusions

Acknowledgements

J. Chem. Thermodyn.

Fluid Phase Equilib.

Fluid Phase Equilib.

Thermochim. Acta

J. Electrochem. Soc.

J. Phys. Chem. B

Ind. Eng. Chem. Res.

Ind. Eng. Chem. Res.

Ind. Eng. Chem. Res.

Ind. Eng. Chem. Res.

J. Phys. Chem. B

J. Phys. Chem. B

J. Chem. Eng. Data

J. Am. Chem. Soc.

J. Phys. Chem. B

J. Chem. Eng. Data