Determination of the thermodynamic parameters of ionic liquid 1-propyl-3-methylimidazolium bromide by gas-liquid chromatography

doi:10.1016/j.jct.2018.09.023

The Journal of Chemical Thermodynamics

Volume 129, February 2019, Pages 92-98

https://doi.org/10.1016/j.jct.2018.09.023 Get rights and content

Highlights

•
Measurements of activity coefficients at infinite dilution using GLC.
•
The (gas-liquid) partition coefficients were calculated.
•
The excess thermodynamic functions, enthalpies, entropies, and Gibbs energies were calculated.
•
The solubility parameter of the IL [PMIM][Br] was determined.
•
Selectivity and capacity for hexane (i)/benzene (j), cyclohexane (i)/benzene (j).

Abstract

The measurements of activity coefficients at infinite dilution ( $γ_{i}^{\infty}$ ) for 33 solutes including alkanes, cycloalkanes, alkenes, aromatic hydrocarbons, acetonitrile, acetone, tetrahydrofuran, ethyl acetate, 1,4-dioxane, chloromethanes, alcohols in the ionic liquid (IL) 1-propyl-3-methylimidazolium bromide, [PMIM][Br], were determined by gas-liquid chromatography at five temperatures at 10 K intervals in range of (323.15–363.15) K. The densities of [PMIM][Br] were measured at the temperature range from 323.15 K to 363.15 K. The gas-liquid partition coefficients, $K_{L}$ at infinite dilution from 323.15 K to 363.15 K and the fundamental thermodynamic functions, such as the partial molar excess enthalpies at infinite dilution ( ${\bar{H}}_{i}^{E, \infty}$ ), Gibbs free energies ( ${\bar{G}}_{i}^{E, \infty}$ ) and entropies ( $T_{ref} {\bar{S}}_{i}^{E, \infty}$ ) were calculated at a reference temperature $T_{r e f}$  = 298.15. The Hildebrand’s solubility parameters of the IL [PMIM][Br] were also determined by the regular solution theory (RST) combined with Flory ‘‘combinatorial’’ equation. The values of selectivity and capacity for n-hexane ( $i$ )/benzene ( $j$ ), cyclohexane ( $i$ )/benzene ( $j$ ) at T = 323.15 K were calculated from $γ_{i}^{\infty}$ and compared to literature values for [Br]-based ILs for the same separation problems.

Graphical abstract

Introduction

Green chemistry involves the development of new synthetic reactions and methods to replace volatile organic compounds with less polluted solvents and to eliminate or reduce environmental pollution fundamentally. As a new solvent for green industrial chemistry, ionic liquids (ILs) are expected to replace traditional organic solvents and organic catalysts to meet the needs of green raw materials and green catalysts [1], [2], [3], [4]. They have been successfully applied in research and production in the fields of chemical separation, organic synthesis, electrochemistry, catalysis and materials.

The further application of ILs in the separation process depends on the systematic study of IL extraction selectivity. In addition to the phase equilibrium data of IL mixtures, the determination of the activity coefficient at infinite dilution ( $γ_{i}^{\infty}$ ) of solutes in ILs is a simple and convenient method to study the extraction selectivity of ILs. The $γ_{i}^{\infty}$ represents the maximum non-ideal property of solution and reflects an extreme situation when solute molecules are completely surrounded by solvent molecules. It is one of the important physicochemical parameters which can describe the interaction between the two molecules. It not only reflects the dissolved ability of IL, but also describes the relationship between solute and IL [5], [6], [7]. Therefore, $γ_{i}^{\infty}$ is of great significance in theory. More importantly, it also has direct and extensive industrial application. In actual work, $γ_{i}^{\infty}$ is mainly used to predict gas-liquid equilibrium, select extractant, predict dynamic solvent effect and so on [8].

In this paper, the infinite dilution activity coefficients ( $γ_{i}^{\infty}$ ) and the gas–liquid partition coefficients ( $K_{L}$ ) of 33 organic solutes (alkanes, cycloalkanes, alkenes, aromatic hydrocarbons, acetonitrile, acetone, tetrahydrofuran, ethyl acetate, 1,4-dioxane, chloromethanes, alcohols) in IL 1-propyl-3-methylimidazolium bromide ([PMIM][Br]) have been measured at the temperatures from 323.15 K to 363.15 K by gas–liquid chromatography. By using the relationship between $γ_{i}^{\infty}$ and the temperature, the values of the partial molar excess enthalpies at infinite dilution ( ${\bar{H}}_{i}^{E, \infty}$ ) were obtained. The entropies ( $T_{r e f} {\bar{S}}_{i}^{E, \infty}$ ) and Gibbs energies ( ${\bar{G}}_{i}^{E, \infty}$ ) of organic solutes at a reference temperature $T_{r e f}$  = 298.15 K were also determined from the $γ_{i}^{\infty}$ values. These thermodynamic functions emphasize the interaction between solute and IL and are important information for the extraction of IL. The research results can provide basic data and theoretical basis for the further application of ILs as green solvents.

The selectivity is used to determine the ability of the ionic liquid to separate in the aromatics during the extractive rectification process. The selectivity ( $S_{ij}^{\infty})$ and capacity ( $k_{ij}^{\infty}$ ) of different separation processes were analysed by the activity coefficient at infinite dilution. $S_{ij}^{\infty}$ and $k_{ij}^{\infty}$ at T = 323.15 K for IL [PMIM][Br] have been also calculated for n-hexane ( $i$ )/benzene ( $j$ ), cyclohexane ( $i$ )/benzene ( $j$ ). The results were analysed in comparison to previously published literature data for other [Br]-based ILs for the same separation problems.

Section snippets

Experimental materials

The IL [PMIM][Br] was supplied by Shanghai Chengjie Chemical Co., Ltd. and had a purity of mass fraction >0.99, with water <10⁻³ mass fraction. Before use, the IL was subjected to vacuum evapouration at T = (323–333) K over 24 h to remove any volatile chemicals and water from the ionic liquid. Karl Fischer titration indicates that the concentration of water in ionic liquid is less than 4 × 10⁻⁴. The chemical structures of [PMIM][Br] is given in Fig. 1. The organic solutes were obtained from

Activity coefficients at infinite dilution ( $γ_{i}^{\infty}$ )

In (gas-liquid) chromatography, the activity coefficients at infinite dilution $γ_{i}^{\infty}$ were obtained by the equation proposed by Cruickshank et al. [15] and Everett [16]. $l n γ_{i}^{\infty} = l n (\frac{n_{3} R T}{V_{N} p_{i}^{0}}) - \frac{B_{ii} - v_{i}}{RT} p_{i}^{0} + \frac{2 B_{i 2} - v_{i}^{\infty}}{RT} J_{2}^{3} p_{0}$ where $γ_{i}^{\infty}$ is the activity coefficient of solute $i$ at infinite dilution in the stationary phase (3), $p_{i}^{0}$ is the vapour pressure of the pure liquid solute $i$ , $n_{3}$ is the number of moles of the stationary phase component on the column, and $V_{N}$ is the standardized retention volume obtained by Eq.

Activity coefficients at infinite dilution ( $γ_{i}^{\infty}$ )

The experimental activity coefficients at infinite dilution for various solutes in the column with 42 mass % of the support material at five experimental temperatures in a range of (323.15–363.15 K) are listed in Table 1. And the results for 10 selected solutes in the column with 32.41 mass % of the support material at 323.15 K and 333.15 K are listed in Table 4S in the Supplementary Materials. It has very similar results to that in the column of 42 mass %. The only disparity between both sets

Conclusions

In this work, the potential use of [PMIM][Br] is discussed as a solvent in two processes of separation, such as n-hexane ( $i$ )/benzene ( $j$ ) and cyclohexane ( $i$ )/benzene( $j$ ). For that purpose, activity coefficients at infinite dilution and the gas-liquid partition coefficients for various solutes in the IL [PMIM][Br] were measured using gas-liquid chromatography technique at the temperatures from (323.15–363.15) K. The thermodynamic functions, as the partial molar excess Gibbs energy with respective

Acknowledgment

This work was supported by Beijing Institute of Petrochemical Technology Training Program for Graduate Students' Innovative Activities and Practical Abilities in 2018 (Zhang Miao).

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Determination of the thermodynamic parameters of ionic liquid 1-propyl-3-methylimidazolium bromide by gas-liquid chromatography

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Experimental materials

Activity coefficients at infinite dilution (γi∞)

Activity coefficients at infinite dilution (γi∞)

Conclusions

Acknowledgment

J. Chem. Thermodyn.

Fluid Phase Equilib.

Fluid Phase Equilib.

J. Chem. Thermodyn.

J. Chem. Thermodyn.

J. Chem. Eng. Data

J. Chem. Eng. Data

Ind. Eng. Chem. Process Des. Dev.

J. Phys. Org. Chem.

Ind. Eng. Chem. Process Des. Dev.

J. Chem. Eng. Data

J. Chem. Eng. Data

J. Chem. Eng. Data

J. Chem. Eng. Data

J. Chem. Eng. Data

Activity coefficients at infinite dilution ( $γ_{i}^{\infty}$ )

Activity coefficients at infinite dilution ( $γ_{i}^{\infty}$ )