Activity coefficients at infinite dilution of organic solutes in methylphosphonate based ionic liquids using gas-liquid chromatography

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Highlights

Abstract

Activity coefficients at infinite dilution γ12 were determined for organic solutes in two ionic liquids 1-ethyl-3-methylimidazolium methylphosphonate [EMIM][(MeO)(H)PO2] and 1.3-dimethylimidazolium methylphosphonate [DMIM][(MeO)(H)PO2] by gas-liquid chromatography at temperatures within the range (313.15 to 373.15) K. The values of the selectivity and capacity for few separation problems cyclohexane/benzene, cyclohexane/thiophene and cyclohexane/pyridine were calculated from activity coefficients at infinite dilution data and compared to literature values for other ionic liquids and some industrial solvents such as sulfolane, N-methyl-2-pyrrolidone (NMP) and N-formylmorpholine (NFM). The selectivity values of both ILs investigated in this study are greater than that for NMP, NFM, and sulfolane. These results indicate the potential use of the two ILs in extractive separation processes.

Introduction

In recent years, research on ionic liquids (ILs) is one of the most rapidly growing fields as novel prospective materials for a variety of innovative applications [1], [2], [3], [4], [5], [6], [7]. Due to their unique properties, such as negligible vapor pressure at room temperature, stable liquid phase over a wide temperature range and thermal stability at high temperatures, ionic liquids are creating an continuously growing interest to use them in synthesis and catalysis as well as extraction processes for the reduction of the amount of volatile organic solvents used in industry. Ionic liquid are alternative green solvents to common organic solvents in diverse applications such as organic synthesis, separation processes, catalysis and electrochemistry [8], [9], [10], [11].

It is now well established that ILs may be used in various a large number of applications such as liquid-liquid extraction, catalysis, synthesis and gas separations [12], [13], [14]. Among others, extractive desulfurization using ionic liquids (ILs) is regarded as a promising process: it has a high sulfur removal ratio and a great selectivity under mild operating conditions, moreover, it is safe, simple but also reproducible. The use of such solvents in the field of extractive desulfurization presents a great potential due to their thermodynamic properties: negligible vapor pressure, great thermal chemical stabilities [15], [16]. In recent work, Hassan et al. [17], [18] have shown that alkylphosphonate based ILs could be used for the extraction of carbohydrates and cellulose. Recent studies on the dissolution of cellulose in 1-butyl-3-methylimidazolium chloride [BMIM][Cl] and dimethylimidazolium methylphosphonate [DMIM][MPh] indicate that the anion of the IL acts as a hydrogen bond acceptor which interacts with the hydroxyl groups of the cellulose [19]. The Kamlet–Taft parameters α: hydrogen bond acidity, β: hydrogen bond basicity and π: polarity show that [DMIM][MPh] and [BMIM][Cl] ILs displayed high basicity and polarity values compared to classical solvents. Methylphosphonate based ILs could be a good candidate for different problems of separation but there is still a lack of information concerning the behavior of this family of ILs with organic compounds.

Gas chromatography is a good tool to understand the behavior of the solutes and the stationary phase through the measurements of partition coefficients or activity coefficients at infinite dilution. [1], [20], [21], [22], [23], [24]. Similar approaches based on gas chromatography technique were proposed to quantify various intermolecular solute-IL interactions. Among others, Abraham et al. have developed the Linear Solvation Energy Relationship model (LSER) allowing to correlate thermodynamic properties of phase transfer processes [25], [26], [27], [28]. Abraham solvation parameter model for both the gas-to-solvent partition coefficient, KL, and the water-to-solvent partition coefficient, P have the following expression:LogKL=c+eE+sS+aA+bB+lL,LogP=c+eE+sS+aA+bB+vV.The dependent variables in equations (1), (2) are solute descriptors as follows: E and S refer to the excess molar refraction in units of (cm3 · mol−1)/10 and a dipolarity/polarizability description of the solute, respectively, A and B are measures of the solute hydrogen-bond acidity and basicity, V is the McGowan volume in units of (cm3 · mol−1)/100, and L is the logarithm of the gas-to-hexadecane partition coefficient at T = 298 K. The coefficients c, e, s, a, b and l (or v) are not simply fitting coefficients, but they reflect complementary properties of the solvent phase.

The system constants are identified as the opposing contributions of cavity formation and dispersion interactions, l, the contribution from interactions with lone pair electrons, e, the contribution from dipole-type interactions, s, the contribution from the hydrogen-bond basicity of the stationary phase (because a basic phase will interact with an acid solute), a, and b the contribution from the hydrogen-bond acidity of the stationary phase. The system constants are determined by multiple linear regression analysis of experimental logSP(logKL in this work) values for a group of solutes of sufficient number and variety to establish the statistical and chemical validity of the model.

In this work, we present experimental measurements of γ12 for selected organic solutes (alkanes, alkynes, cycloalkanes, alcohols, aromatics and ketones) in 1-ethyl-3-methylimidazolium methylphosphonate [EMIM][(MeO)(H)PO2] and 1.3-dimethylimidazolium methylphosphonate [DMIM][(MeO)(H)PO2] from T = (313.15 to 373.15) K using gas-liquid chromatography.

Section snippets

Materials and reagents

The ionic liquids 1-ethyl-3-methylimidazolium methylphosphonate [EMIM][(MeO)(H)PO2] and 1.3-dimethylimidazolium methylphosphonate [DMIM][(MeO)(H)PO2] were purchased from Solvionic with a purity of 98% by mass. The structures of investigated ionic liquids are presented in figure 1. The ionic liquids were dried for more than 24 h at T = 323.15 K under reduced pressure to remove volatile impurities and trace water. Most of the solutes were purchased from Sigma Aldrich and Fluka and the purities were

Activity coefficients and selectivity at infinite dilution

The values of γ12 of different organic solutes (alkanes, cycloalkanes, 1-alkynes, benzene, thiophene, alcohols, ether, acetonitrile, pyridine and 1-nitropropane) in 1-ethyl-3 methylimidazolium methylphosphonate [EMIM][(MeO)(H)PO2] and 1.3-dimethylimidazolium methylphosphonate [DMIM][(MeO)(H)PO2] obtained at different temperatures are listed in TABLE 1, TABLE 2. Each experiment was repeated twice or more for some solute to check the reproducibility. Retention times were generally reproducible

Conclusions

Activity coefficients at infinite dilution of selected organic solutes in two ionic liquids 1-ethyl-3-methylimidazolium methylphosphonate [EMIM][(MeO)(H)PO2] and 1.3-dimethylimidazolium methylphosphonate [DIMIM][(MeO)(H)PO2] were measured by (gas-liquid) chromatography at the temperatures range (313.15 to 373.15) K. The selectivities for a few separation problems as cyclohexane/thiophene and cyclohexane/benzene and cyclohexane/pyridine were calculated from γ12 obtained in this study and

Acknowledgment

The present work has been done in the framework of the international project CMEP/Tassili (CMEP 12MDU875 – Egide 26311TA).

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