Separation of binary mixtures based on gamma infinity data using [OMMIM][NTf2] ionic liquid and modelling of thermodynamic functions

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Highlights

Abstract

Activity coefficients at infinite dilution (γi) and (gas-liquid) partition coefficients (KL) for organic compounds (alkanes, alkenes, alkyl benzenes, acetonitrile, acetone, tetrahydrofuran, ethyl acetate, 1,4-dioxane, chloromethanes, and alcohols) in the ionic liquid (IL) 1-octyl-2,3-dimethylimidazolium bis(trifluoromethylsulfonyl)imide ([OMMIM][NTf2]) have been measured by the gas-liquid chromatographic(GLC) method in the temperature range of (313.15 to 353.15) K. Density, ρ as a function of temperature for [OMMIM][NTf2] at pressure p=101.3kPa were measured. The values of the partial molar excess enthalpies at infinite dilution (H¯iE,) were derived from the temperature dependence of the γi values. The entropies (TrefS¯iE,) and Gibbs energies (G¯iE,) of organic solutes in [OMMIM][NTf2] at a reference temperature Tref=298.15K were also calculated from the γi values. The Hildebrand’s solubility parameters of the IL [OMMIM][NTf2] were also determined by the regular solution theory (RST) combined with Flory “combinatorial” equation. The linear free energy relationship (LFER) analysis of the results was performed to disclose molecular interactions operating between the IL and the individual solutes. Selectivity (Sij) and capacity (kj) at infinite dilution at 323.15 K have been determined for n-hexane (i)/benzene (j), cyclohexane (i)/benzene (j) and compared to literature values for [NTf2]-based ILs for the same separation problems. For three isomeric xylenes separation problems, Sij at 323.15 K was also obtained from the γi values.

Introduction

Activity coefficients at infinite dilution (γi) are important thermodynamic parameters in various fields, which describe the degree of non-ideality for species i in a mixture and are useful in many chemical applications. For example, in chemical engineering, they usually can be used for the pre-screening of solvents for separation processes such as applied to extractive distillation, liquid-liquid extraction and absorption in the design. In addition, the use of infinite dilution information can be directly designed stripping operation, dilution material extraction and other types of phase separation. γi data gives information about the interactions between solvent and solute and could be used for the selection of selective solvents in the separation processes and is the key parameter to evaluate the performance of the solvent separation. Therefore, the solubilization ability of the stationary phases for different kinds of solutes could be characterized through this determined. Also, γi data provides valuable information for testing predictive models or computer simulation methods [1], [2], [3], [4], [5], [6].

Ionic liquids (ILs) as a new green solvent have become a subject of intensive studies in recent years, due to their unique physical and chemical properties, such as negligible vapour pressures, high thermal stability, wide range of solubility, non-flammability, high conductivity properties, and wide electrochemical window. They have emerged as potential substitutes for conventional organic solvents and are useful in chemical separation process applications. The thermodynamic properties and phase equilibrium of the ionic liquids were studied by measuring the infinite dilution activity coefficient of ILs, to fully exploit the ILs potential as solvents in separation processes [7], [8], [9], [10], [11], [12]. At present, the methods of measuring the γi include the retention time method (gas-liquid chromatography, GLC), ebulliometry, static methods, and the dilutor technique. Since ILs has a negligible vapour pressure, GLC is a suitable method for measuring the γi [13], [14], [15]. GLC is simpler and more accurate compared with other methods, and has been considered to be a particularly convenient and quite reliable technique for the determination of the γi [5], [6]. Until now, many research groups including our group have measured γi of various solutes in a number of ILs using the gas-liquid chromatograph. This work is a continuation of our investigation on ILs to determine of γi of various solutes (i) in ILs with a 2,3-dimethylimidazolium cation. The effect of introducing another methyl group in the imidazolium ring was further studied by increasing the aromaticity of IL and improving the selectivity or extraction ability during the separation process [8], [16], [17].

In this paper, γi and KL have been measured for 33 organic solutes: alkanes, cycloalkanes, alkene, aromatic hydrocarbons, alcohols, acetone, esters, acetonitrile, tetrahydrofuran and chloromethanes in [OMMIM][NTf2] by the GLC method in the temperature range of (313.15–353.15) K. The values of the partial molar excess enthalpies at infinite dilution (H¯iE,) were derived from the temperature dependence of the γi values. The entropies (TrefS¯iE,) and Gibbs energies (G¯iE,) of organic solutes at a reference temperature Tref=298.15K were also determined from the γi values. The solubility parameters of [OMMIM][NTf2] were calculated as a function of temperature with the regular solution theory and the linear free energy relationship (LFER) salvation model was applied to correlate the KL values. Furthermore, in separation processes both properties of extractant are important, namely Sij (Sij=γi,IL/γj,IL), and capacity,kj (kj=1/γj,IL), which can be directly calculated from γi for different separation problems and evaluate the IL as an entrainer for common separation problems [7], [14], [18]. Sij and kij at T = 323.15 K for ILs [OMMIM][NTf2] have been also calculated for n-hexane (i)/benzene (j), cyclohexane (i)/benzene (j). For three isomeric xylenes separation problems, selectivity at 323.15 K was also obtained from the γi values.

Section snippets

Chemicals and materials

The ionic liquid [OMMIM][NTf2] was purchased from Shanghai Chengjie Chemical Co., Ltd. and had a purity of >0.99 mass fraction according to manufacturer’s specifications, with the following certified mass fraction of impurities: w(Cl) < 5·10−4, water <10−3. The chemical structures of [OMMIM][NTf2] is presented in Fig. 1. Before use, the ILs were subjected to vacuum evapouration at T= (323 to 333) K over 24 h to remove possible traces of solvents and moisture. The water content was <5·10−4 mass

Activity coefficients at infinite dilution

In (gas-liquid) chromatography, the γi were obtained by the equation proposed by Cruickshank et al. [24] and Everett [25].lnγi=lnn3RTVNpi0-Bii-viRTpi0+2Bi2-viRTJ23p0,where γi is the activity coefficient of solute i at infinite dilution in the stationary phase (3), pi0 is the vapour pressure of the pure liquid solute i, n3 is the number of moles of the stationary phase component on the column, and VN is the standardized retention volume obtained by Eq. (2),VN=(J23)-1U0(tr-tG)TcolTf1-pw0po,

Activity coefficients at infinite dilution (γi)

Experimental results of γi values for 33 solutes in [OMMIM][NTf2] in the column with 45.23 mass % of the support material in the temperature range of (303.15–353.15) K are presented in Table 2. The measurement results for 10 selected solutes in the column with 34.62 mass % of the support material at 323.15 K and 343.15 K are listed in Table 4S in the Supplementary materials. It gave similar results to that in the column of 45.23 mass %. The difference between both sets of γi values is of the

Conclusion

In this work, we determined the γi of organic solutes with IL [OMMIM][NTf2] through GLC measurements. The KL have been calculated for a series of polar and nonpolar organic solutes at the temperature range from (313.15 to 353.15) K. The partial molar excess enthalpies at infinite dilution of the ionic liquid were obtained for the solutes from the temperature dependence of the experimental γi. The solubility parameters of [OMMIM][NTf2] were also determined by the regular solution theory and

Acknowledgments

This work was supported by China National Science & Technology Pillar Program (2015BAK16B03), 2016 Beijing Institute of Petrochemical Technology Graduate Program for Innovation Activity.

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