Effect of ionic liquid 1-methylimidazolium chloride on the vapour liquid equilibrium of water, methanol, ethanol, and {water + ethanol} mixture

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Abstract

Measurements of vapour pressure data were conducted using a quasi-static ebulliometer for systems containing water, methanol, ethanol, and a mixture of {water + ethanol} in the presence of an ionic liquid (IL), namely, 1-methylimidazolium chloride ([MIm]Cl), wherein the IL-content ranged from w2 = (0.10 to 0.50). The vapour pressure data of IL-containing binary systems were correlated by the NRTL model with an overall average absolute relative deviation (AARD) of 0.0103, and the resulting binary parameters were used to predict the vapour pressures of a ternary system {water + ethanol + [MIm]Cl} with an AARD less than 0.0077. Further, the isobaric vapour liquid equilibria (VLE) for the ternary system {water + ethanol + IL} with IL-content of w3 = (0.10, 0.30, and 0.50) for [MIm]Cl and x3 = 0.15 for [MIm]Cl, [C4MIm]Cl, and [C6MIm]Cl were predicted at 101.3 kPa, respectively. It is indicated that [MIm]Cl presents the strongest ability to enhance the relative volatility of ethanol to water in the mixture of {water + ethanol} than that of [C4MIm]Cl and [C6MIm]Cl, which is consistent with the cationic sizes and hence the ionic hydration ability. Therefore, distillation separation of the azeotrope of {water + ethanol} can be sufficiently facilitated by the addition of [MIm]Cl at a specified content.

Highlights

Vapour pressure data for three binary systems and a ternary system were measured. ► Water, ethanol, methanol, and 1-methylimidazolium chloride were studied. ► The vapour pressure data can be well correlated by the NRTL model. ► The isobaric (vapour + liquid) equilibria were predicted by the NRTL model. ► The salt effect of ILs on the VLE of {water + ethanol} mixture was investigated.

Introduction

Ionic liquids (ILs) being entirely made of ions are liquid at temperatures around 373.15 K or below. ILs are gaining wide recognition in both academia and industry in recent years due to their unique properties, such as negligible vapour pressure, tunable miscibility with polar and apolar solvents, electrolytic attributes, and easy regeneration. There was no large-scale industrial application of ILs until the BASIL (biphasic acid scavenging using ionic liquids) process was announced by BASF in March 2003, which involved a neutralization of 1-methylimidazole by hydrochloric acid from the product of alkoxyphenylphosphine and formed an IL 1-methylimidazolium chloride (noted as [MIm]Cl hereinafter) [1].

The application of ILs as an alternative kinds of entrainer attracts much attention recently, especially in the separation of azeotropes and close boiling mixtures [2], [3], [4], wherein alkylimidazolium-based ILs with varying length of alkyl substitutes and different kinds of anions have been comprehensively studied, e.g. 1-alkyl-3-methylimidazolium tetrafluoroborate [5], [6] and 1-alkyl-3-methylimidazolium chloride [7], [8], [9], [10]. It is showed that both cation and anion can affect the salt effect of ILs on VLE of azeotropes and show a definite order, e.g. Cl > [N(CN)2] > [BF4] [11] for anions, and [C2MIm]+ > [C4MIm]+ > [C8MIm]+ [12] for the imidazolium cations. On this basis, it is inferred that IL [MIm]Cl may present the strongest salt effect on VLE of azeotropic mixture of (water + ethanol) due to its favourable combination of anion and cation with the smallest sizes and accordingly the strongest ionic solvating abilities. Furthermore, in order to enhance the relative volatility of ethanol to water, it is preferred that the IL added has a stronger affinity to the water component. On this account, [MIm]Cl is preferable since chlorine anion is of hydrophilicity, and the smallest alkyl chain length of cation is also helpful to increase the hydrophilicity, e.g. [MIm]Cl can strongly uptake a large amount of moisture in air within a short time [13].

In view of the great importance of vapour pressure data of IL-containing systems in assessing intermolecular interactions between IL and solvent, screening appropriate entrainer for special rectification as well as developing thermodynamic models specific to IL-containing systems, the effects of [MIm]Cl on the vapour pressure of water, methanol, ethanol, and {water + ethanol} mixture at differing temperature and IL-content were investigated in this work. The experimental data for the binary systems were correlated using the nonelectrolyte NRTL model, and the resulting binary parameters were used to predict the vapour pressures of the ternary system {water + ethanol + [MIm]Cl}. Besides, the isobaric vapour liquid equilibria (VLE) at 101.3 kPa for the ternary system {water + ethanol + [MIm]Cl} with IL-content of w3 = 0.10, 0.30, and 0.50 were predicted, respectively. As a case study, the isobaric VLE of {water + ethanol + IL} with IL-content of x3 = 0.15 at 101.3 kPa were predicted for [MIm]Cl and compared with two ILs of the same series reported in the literatures, viz. [C4MIm]Cl and [C6MIm]Cl. The results indicate that the relative volatility of ethanol to water is enhanced by the addition of [MIm]Cl, and [MIm]Cl presents the most significant ability to shift the equilibrium line upward for the mixture of {water + ethanol} than ILs [C4MIm]Cl and [C6MIm]Cl.

Section snippets

Materials

The chemical reagents used in this work were distilled-deionized water, ethanol, methanol, 1-methylimidazole (MIm), hydrochloric acid (HCl), and [MIm]Cl. Ethanol and methanol were both purchased from Beijing Red Star Co. Ltd., China, and dried for 48 h prior to use the with commercial molecular sieves ZMS-5, with the purity of w  0.997 as analyzed by gas chromatography (SHIMADZU GC2010 equipped with a FID detector and FFAP capillary column). MIm was purchased from Zhejiang Linhai Kaile Chemical

Vapour pressure for binary systems

Vapour pressure data for three binary systems {water (1) + [MIm]Cl (2)}, {methanol (1) + [MIm]Cl (2)}, and {ethanol (1) + [MIm]Cl (2)} with IL-content of w2 = (0.10, 0.20, 0.30, and 0.50) at varying temperature were measured and listed in TABLE 1, TABLE 2, TABLE 3, respectively.

The effect of IL on the nonideality of a solution can be expressed by the activity coefficient of component i, γi, which can be calculated by the following phase equilibrium equation, equation (1):γi=yiϕˆiVp/(xiϕiSpiS),where p

Conclusions

Vapour pressure data were measured using a quasi-static ebulliometer for three binary systems and a ternary system containing water, methanol, ethanol, {water + ethanol} and a hydrophilic IL [MIm]Cl at varying temperature and IL content. The results indicate that the vapour pressures of all solvents studied show a negative deviation from the Raoult’s law, and the relative volatilities of ethanol to water in {water + ethanol} mixture can be enhanced by the addition of [MIm]Cl. The hydrophilic halide

Acknowledgements

The authors are grateful to the financial support from National Natural Science Foundation of China (21076005) and Research Fund for the Doctoral Program of Higher Education of China (20090010110001).

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