Separation of acetonitrile and methanol azeotropic mixture using imidazolium-based ionic liquids as entrainers

Accepted Manuscript Separation of acetonitrile and methanol azeotropic mixture using imidazolium-based ionic liquids as entrainers Zhigang Zhang, Ru Yang, Wenxiu Li, Tao Zhang, Debiao Zhang, Qinqin Zhang PII:

S0378-3812(18)30327-3

DOI:

10.1016/j.fluid.2018.08.009

Reference:

FLUID 11923

To appear in:

Fluid Phase Equilibria

Received Date: 24 May 2018 Revised Date:

10 August 2018

Accepted Date: 11 August 2018

Please cite this article as: Z. Zhang, R. Yang, W. Li, T. Zhang, D. Zhang, Q. Zhang, Separation of acetonitrile and methanol azeotropic mixture using imidazolium-based ionic liquids as entrainers, Fluid Phase Equilibria (2018), doi: 10.1016/j.fluid.2018.08.009. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT Separation of acetonitrile and methanol azeotropic mixture using imidazolium-based ionic liquids as entrainers Zhigang Zhang, Ru Yang, Wenxiu Li **, Tao Zhang, Debiao Zhang, and Qinqin Zhang *. Liaoning Provincial Key Laboratory of Chemical Separation Technology, Shenyang University of Chemical Technology, Shenyang 110142, China.

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Abstract:

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In this work, three imidazolium-based ionic liquids (ILs) with different anions, namely 1-butyl-3-methylimidazolium chloride ([BMIM][Cl]), 1-butyl-3-methylimidazolium bromide ([BMIM][Br]) and 1-butyl-3-methylimidazolium acetate ([BMIM][OAc]), were tested as entrainers to separate acetonitrile and methanol mixtures. Isobaric vapor-liquid equilibrium (VLE) data of the acetonitrile (1) + methanol (2) + IL ternary systems were measured at 101.3 kPa. Addition of ILs produced a salting-out effect on acetonitrile, thus leading to the increase of the relative volatility of acetonitrile to methanol. As IL content increased to a certain value, the azeotropic point of acetonitrile and methanol could be totally eliminated. In addition, the separation performance of ILs was closely related to anion structure, with [BMIM][OAc] producing the most significant salting-out effect orderly followed by [BMIM][Cl] and [BMIM][ Br]. Finally, the measured ternary VLE data were correlated with the nonrandom two-liquid (NRTL) model, and σ-Profiles were used to explain the separation performance differences of the three ILs. Keywords:

1. Introduction

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Vapor-liquid equilibrium; ionic liquids; acetonitrile; methanol; NRTL model; σ-profiles.

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Acetonitrile and methanol are important solvents, and have been extensively used in chemical and pharmaceutical processes.[1,2] For example, they are used in reverse phase high performance liquid chromatography[3], and also used as raw material in organic synthesis to produce acetamidine hydrochloride which is widely served as a synthetic pharmaceutical intermediate of 4,6-Dihydroxy-2-methylpyrimidine. [4,5] Moreover, mass spectrum and chromatography techniques also produce some mixtures of them.[6-8] Those procedures inevitably produce a large quantity of mixtures of methanol and acetonitrile. Unfortunately, the binary system of methanol and acetonitrile forms an azeotrope at atmospheric pressure, and this azeotrope cannot be separated by conventional distillation process.[1] Extractive distillation, which combines the virtues of extraction and distillation, is an effective special distillation and is widely adopted in industry to separate azeotropic or close-boiling mixtures. [9-11] the key to extractive distillation is the selection of competent entrainers. Yang Yu et al. [12] reported isobaric VLE data for the system of acetonitrile and methanol using traditional organic solvents (N,N-dimethylformamide and aniline) as entrainers. However, the traditional organic solvents used in extractive distillation have some disadvantages such as environment pollution caused by volatile emission and difficulty in recycling.[13] Therefore, new entrainers which are eco-friendly and easy to be recycled are urgently desired. ILs, which are totally composed of ions, are regarded as green and designable solvents. They have many unique physic-chemical properties such as high thermal and chemical stability, excellent dissolving capacity

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and nonvolatility.[14,15] These properties make ILs attractive alternatives for extractive distillation. As far as we know, there have been massive studies about separating azeotropic mixtures with ILs as entrainers. Furthermore, many review articles have summarized this topic [16-19]. Briefly speaking, for the separation of polar-polar systems, such as water/alcohol, water/nitrile and alcohol/nitrile mixtures, imidazolium and alkyl quaternary ammonium cations and anions such as hydrogen sulfate, halogen and acetate were commonly used. For nonpolar-nonpolar systems, such as alkane/alkene and aliphatics/aromatics mixtures, ILs composed of cations such as imidazolium and pyridine and anions such as tetrafluoroborate (BF4), bis(trifluoromethylsulfonyl)imide (NTf2) and dicyanamide (DCA) were extensively adopted. For the separation of polar-nonpolar systems, for example, alcohol/aliphatic mixtures, the most commonly used separation methods were liquid-liquid extraction with ILs consisted of imidazolium cation and sulfate anion as entrainers. So far, there have been several reports concerning with the use of ILs as entrainers to separate acetonitrile and methanol system. Qing Li et al. [20] reported the effects of 1-octyl-3-methylimidazolium tetrafluoroborate([OMIM][BF4]) and 1-ethyl-3-methylimidazolium tetrafluoroborate([EMIM][BF4]) on the VLE behavior of the methanol and acetonitrile system and found that [OMIM][BF4] produced a stronger salting-out effect on methanol than [EMIM][BF4]. However, those ILs with [BF4]- anion could easily release corrosive hydrogen fluoride upon contacting with water and are also difficult to be synthesized.[13] Yuxin Zhang et al. [21] investigated the separation performance of 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM][NTf2]) and 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([BMIM][NTf2]) on methanol and acetonitrile mixtures and found that both [EMIM][NTf2] and [BMIM][NTf2] could eliminate azeotropic phenomenon when their mole fractions reached 0.05. But, those ILs containing [NTf2]- anion have the disadvantage of high cost.

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In our previous work [22], separation of acetonitrile and ethanol mixture was investigated with dialkylphosphate ILs as entrainers. As the polarity of methanol is greater than that of ethanol, the VLE behavior of acetonitrile and methanol mixture must be different from that of acetonitrile and ethanol. In this work, three imidazolium-based ILs containing different anions, namely [BMIM][Cl], [BMIM][Br] and [BMIM][OAc], were tested an entrainers to separate acetonitrile and methanol mixture. Isobaric VLE data of the ternary systems of acetonitrile (1) + methanol (2) + [BMIM][Cl], acetonitrile (1) + methanol (2) + [BMIM][Br], and acetonitrile (1) + methanol (2) + [BMIM][OAc] were measured at 101.3 kPa. The effects of the three ILs on the VLE behavior of the methanol + acetonitrile system were discussed and compared. The nonrandom two-liquid (NRTL) model [23] was used to correlate the experimental data. Finally, the separation performance difference of the three ILs investigated in this work was explained by the σ-profiles [24, 25] of solvents and ILs in the view of the strength of the hydrogen bond interactions between solvents and ILs. 2. Experimental 2.1. Chemicals In the present study, methanol (HPLC grade >99.9wt%) and acetonitrile(AR grade >99.0wt%) were supplied by Sinopharm Group CO. Ltd. Methanol and acetonitrile were directly used without further purification since no impurities were detected by gas chromatography. [BMIM][Cl] and [BMIM][Br] were prepared in our laboratory according to a method in literature [26, 27], and [BMIM][OAc] was purchased from Yulu Chemical. The mass fraction purity of ILs was measured by liquid chromatography. ILs were

ACCEPTED MANUSCRIPT dried by heating at T=363.15 K under high vacuum (0.08 kpa) for 48 h before use. The water content, determined by Karl Fischer titration, was less than 100 ppm for all chemicals. [BMIM][Cl] and [BMIM][Br] are solid at room temperature, but [BMIM][OAc] is liquid. The specifications of chemicals used in this work are summarized in Table 1. Table 1 Specification of chemical samples Source

acetonitrile

Sinopharm Group

0.990

methanol

Sinopharm Group

0.999

[BMIM][Cl]

Synthesized own

[BMIM][Br] [BMIM][OAc]

(Mass)

Purification method

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Purity

Chemical name

Analysis method GCa

None

GCa

0.980

Vacuum desiccation

KFb and LCc

Synthesized own

0.980

Vacuum desiccation

KFb and LCc

Yulu Chemical

0.990

Vacuum desiccation

KFb and LCc

GC = Gas chromatography. KF = Karl Fischer titration. c LC = Liquid chromatography. b

2.2. Apparatus and procedure

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The VLE data were measured in a circulation vapor-liquid equilibrium still (a modified Othmer still). The apparatus has been described in detail in literature [28, 29]. For convenience of the readers, the apparatus is briefly explained here. The equilibrium still consists of six main parts: vapor liquid boiling chamber, heating rod, liquid sampling point, ball condenser, thermometer and vapor sampling point. The total volume of the equilibrium still is 100 cubic centimeters. The equilibrium temperature was measured by a precise and calibrated thermometer with a standard uncertainty of 0.05 K. The system pressure was controlled by a gas buffer connected with the still with a standard uncertainty of 0.1 kPa. The experimental procedure is as follows: Initially, the vapor-liquid boiling chamber was filled with a sample solution. A heating rod was used to heat the solution to boil. After a while, the steam went into the vapor chamber, and was condensed by the ball condenser, and then returned to the vapor-liquid chamber for continuous circulation. Equilibrium was assumed when the system temperature remained unchanged more than 30 minutes. Then, the condensed vapor and liquid samples were taken out and analyzed. In this experiment, every gas and liquid samples of acetonitrile and methanol were placed in a headspace sampler (G1888 Network headspace sampler, Agilent Technologies) and then analyzed by a gas chromatography (Agilent GC7890A) with a thermal conductivity detector (TCD) and a DB-WAXETR capillary column (30m × 0.32mm, 1µm in thickness). The temperatures of column, injector and detector were 338K, 473K and 473 K, respectively. Since ILs have the property of nonvolatility, the contents of ILs in liquid samples were measured by comparing the mass of liquid samples before and after being dried under vacuum desiccation at 393 K for 48 hours. The standard uncertainty of IL content in liquid phases was 0.001 in mole fraction. For the binary system of acetonitrile (1) + methanol (2), each experimental point was obtained by adding different amounts of methanol to the initial sample of acetonitrile. For the ternary systems of acetonitrile (1) + methanol (2) +ILs(3)，the acetonitrile + IL mixture and methanol + IL mixture with the same mole fraction of ionic liquids were first prepared. Then, different quantities of methanol + IL mixtures were added to the acetonitrile + IL mixture in an attempt to keep the mole fraction of IL constant in each series. All the samples were gravimetrically prepared with a digital balance (ACCULAB ATL-224-1 China, standard

ACCEPTED MANUSCRIPT uncertainty = 0.0001g). The unimolecular quantum chemically based Conductor-like Screening Model for Real Solutes (COSMO-RS) has developed into an independent approach to the simulation of fluid phase equilibria. In this work, the COSMO-therm X16（C30_1701）program, an efficient and flexible implementation of the COSMO-RS method, was used to calculate the σ-profiles of acetonitrile, methanol and the ions of ILs. [24, 25] More specifically, the σ-profiles were obtained by importing the conformations of them from the purchased COSMO database (parameterization BP_TZVP_1701). Results and discussion

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3.

3.1. Experimental data

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The VLE data of the acetonitrile and methanol binary system were measured at 101.3 kPa. Table S1 lists the experimental data which are compared with the literature data [20, 21, 30-32] in Fig.S1 to verify the reliability of the method and apparatus used in this work. It is obvious that the experimental data obtained in this work is consistent with the reported data, proving that the method and apparatus employed in this study are reliable.

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The VLE data of the acetonitrile (1) + methanol (2) + [BMIM][Cl] (3) and acetonitrile (1) + methanol (2) + [BMIM][OAc] (3) ternary systems were measured at 101.3 kPa by keeping the mole fractions of ILs approximately constant at 0.05, 0.10 and 0.15, while that of the acetonitrile (1) + methanol (2) + [BMIM][Br] (3) system was obtained with the mole fraction of IL nearly constant at 0.05, 0.10, 0.15 and 0.20. All the ternary VLE experimental data are listed Tables 2-4, where T is the equilibrium temperature, x1 represents the mole fraction of acetonitrile in the liquid phase containing ILs, x3 is the mole fraction of IL in the liquid phase, x1’ represents the mole fraction of acetonitrile in the liquid phase expressed on an IL-free basis, y1 represents the mole fraction of acetonitrile in the vapor phase, and α12 is the relative volatility.

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Table 2 Isobaric VLE data for the ternary system of acetonitrile (1) + methanol (2) + [BMIM][Cl] (3) at 101.3 kPa.a x1

x3

x1 ’

y1

α12

T/K

x1

x3

x1 ’

y1

α12

339.05 338.54 338.50 339.41 340.16 341.16 342.80 344.46 345.33 347.43 351.20 353.74 340.42 339.94 340.04 340.86 341.82

0.033 0.114 0.200 0.331 0.406 0.479 0.598 0.682 0.718 0.779 0.864 0.912 0.038 0.127 0.210 0.331 0.410

0.050 0.050 0.053 0.051 0.052 0.054 0.050 0.052 0.050 0.050 0.049 0.050 0.100 0.101 0.113 0.104 0.100

0.035 0.120 0.210 0.352 0.433 0.512 0.634 0.719 0.755 0.820 0.910 0.960 0.042 0.141 0.233 0.368 0.456

0.050 0.141 0.217 0.322 0.373 0.423 0.506 0.581 0.617 0.685 0.823 0.917 0.060 0.170 0.250 0.349 0.426

1.451 1.204 1.043 0.871 0.778 0.697 0.593 0.540 0.523 0.478 0.459 0.459 1.443 1.247 1.095 0.921 0.884

345.04 346.19 348.91 350.30 352.10 354.99 342.08 341.57 341.72 342.61 343.58 346.00 348.16 350.38 352.67 354.27 357.44

0.575 0.617 0.700 0.736 0.779 0.842 0.032 0.101 0.212 0.276 0.350 0.472 0.551 0.618 0.697 0.735 0.803

0.100 0.101 0.113 0.104 0.100 0.100 0.151 0.151 0.150 0.150 0.150 0.151 0.151 0.152 0.150 0.150 0.150

0.639 0.685 0.777 0.817 0.865 0.935 0.038 0.118 0.250 0.329 0.417 0.562 0.656 0.736 0.820 0.865 0.945

0.576 0.624 0.718 0.760 0.829 0.923 0.060 0.161 0.297 0.375 0.446 0.584 0.670 0.771 0.844 0.893 0.967

0.770 0.762 0.729 0.707 0.757 0.836 1.630 1.426 1.270 1.227 1.127 1.098 1.067 1.206 1.187 1.308 1.708

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0.470

0.100

0.522

0.475

0.830

358.82

0.829

0.150

0.975

0.987

1.964

Standard uncertainties u are u(T) = 0.05 K, u(P) =0.1 kPa, u (x3) =u (x1) =u (x1’) = u (y1) = 0.001.

Table 3 Isobaric VLE data for the ternary system of acetonitrile (1) + methanol (2) + [BMIM][Br] (3) at 101.3 kPa.a x3

x1 ’

y1

α12

T/K

x1

339.06 338.85 338.79 338.89 339.13 340.36 341.61 343.03 344.49 348.21 351.28 353.82 340.75 340.40 340.61 341.87 342.50 343.28 344.81 346.78 347.50 350.12 352.91 354.63

0.071 0.112 0.174 0.237 0.305 0.461 0.566 0.653 0.719 0.811 0.876 0.919

0.061 0.060 0.061 0.060 0.058 0.055 0.050 0.046 0.043 0.050 0.050 0.050 0.115 0.114 0.115 0.121 0.115 0.109 0.100 0.105 0.100 0.102 0.101 0.100

0.076 0.119 0.185 0.252 0.324 0.488 0.596 0.684 0.752 0.854 0.922 0.967 0.055 0.189 0.256 0.397 0.468 0.538 0.641 0.709 0.746 0.826 0.896 0.934

0.095 0.141 0.194 0.249 0.287 0.388 0.462 0.527 0.580 0.705 0.818 0.915 0.072 0.209 0.260 0.370 0.431 0.471 0.550 0.608 0.639 0.731 0.827 0.886

1.276 1.214 1.063 0.985 0.840 0.664 0.580 0.514 0.456 0.409 0.381 0.365 1.340 1.135 1.021 0.896 0.861 0.766 0.686 0.635 0.605 0.575 0.558 0.553

341.83 341.56 341.75 342.24 343.05 344.19 345.47 347.78 349.48 354.01 356.02 358.78 343.55 343.42 343.79 344.44 345.32 346.80 347.84 350.36 352.37 356.63 358.64 361.48

0.053 0.136 0.204 0.271 0.341 0.411 0.472 0.560 0.613 0.726 0.768 0.822 0.055 0.156 0.217 0.276 0.333 0.405 0.447 0.530 0.585 0.683 0.723 0.774

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0.049 0.167 0.227 0.349 0.414 0.479 0.577 0.635 0.671 0.743 0.806 0.840

x3

x1 ’

y1

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x1

0.150 0.151 0.151 0.152 0.150 0.154 0.152 0.150 0.151 0.149 0.150 0.151 0.200 0.200 0.201 0.202 0.201 0.200 0.201 0.203 0.200 0.201 0.200 0.200

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T/K

0.063 0.159 0.240 0.319 0.401 0.483 0.555 0.659 0.721 0.854 0.904 0.967 0.069 0.195 0.272 0.345 0.416 0.506 0.558 0.662 0.731 0.854 0.904 0.967

0.088 0.193 0.267 0.329 0.395 0.454 0.521 0.620 0.680 0.829 0.889 0.963 0.103 0.250 0.319 0.387 0.456 0.535 0.583 0.682 0.750 0.880 0.927 0.978

α12 1.443 1.258 1.148 1.047 0.976 0.888 0.872 0.845 0.825 0.829 0.850 0.896 1.553 1.374 1.258 1.200 1.179 1.123 1.106 1.095 1.100 1.258 1.346 1.511

Standard uncertainties u are u(T) = 0.05 K, u(P) =0.1 kPa, u (x3) =u (x1) =u (x1’) = u (y1) = 0.001.

Table 4 Isobaric VLE data for the ternary system of acetonitrile (1) + methanol (2) + [BMIM][OAc] (3) at 101.3 kPa.a T/K 339.00 338.68 338.73 339.14

x1 0.062 0.119 0.230 0.308

x3 0.050 0.051 0.052 0.051

x1 ’ 0.065 0.125 0.242 0.324

y1 0.094 0.161 0.262 0.319

α12 1.493 1.348 1.112 0.978

T/K 343.62 346.15 347.91 350.62

x1 0.470 0.566 0.640 0.721

x3 0.106 0.107 0.106 0.103

x1 ’ 0.526 0.633 0.716 0.807

y1 0.528 0.630 0.692 0.784

α12 1.010 0.984 0.891 0.868

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0.051 0.050 0.051 0.052 0.053 0.051 0.050 0.050 0.106 0.106 0.105 0.103 0.105 0.106

0.415 0.521 0.625 0.755 0.801 0.865 0.925 0.965 0.050 0.096 0.157 0.241 0.337 0.433

0.378 0.445 0.518 0.631 0.681 0.762 0.856 0.928 0.087 0.149 0.219 0.298 0.385 0.458

0.853 0.736 0.643 0.555 0.530 0.500 0.478 0.467 1.811 1.645 1.511 1.340 1.231 1.110

353.67 356.15 343.32 342.85 342.74 342.95 343.99 345.32 347.21 349.06 350.05 352.96 356.69 358.06

0.798 0.854 0.062 0.109 0.174 0.258 0.344 0.427 0.511 0.582 0.615 0.698 0.785 0.812

0.105 0.106 0.153 0.153 0.154 0.152 0.155 0.154 0.155 0.153 0.153 0.152 0.153 0.153

0.894 0.955 0.073 0.129 0.206 0.304 0.407 0.505 0.605 0.687 0.726 0.824 0.926 0.958

0.879 0.952 0.137 0.220 0.322 0.421 0.520 0.600 0.689 0.750 0.790 0.876 0.956 0.977

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0.395 0.495 0.594 0.717 0.761 0.822 0.879 0.917 0.045 0.086 0.140 0.215 0.301 0.387

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339.87 341.10 342.79 345.78 347.18 349.44 352.06 354.04 341.35 340.97 340.72 340.82 341.41 342.41

0.869 0.930 2.001 1.906 1.828 1.663 1.582 1.473 1.450 1.365 1.419 1.513 1.745 1.866

Standard uncertainties u are u(T) = 0.05 K, u(P) =0.1 kPa, u (x3) =u (x1) =u (x1’) = u (y1) = 0.001.

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3.2. Correlation of VLE Data

Because the experiments in this work were carried out at atmospheric pressure and the vapor pressures of ILs can be neglected, thus vapor phase only consists of volatile components and can be assumed to be ideal vapor phase. The activity coefficient of component i, γi, and the relative volatility of acetonitrile (1) to methanol (2), α12, could be calculated by the following equations (1) and (2), respectively. (1)

⁄ ⁄

=

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=

=

(2)

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Where γi represents the activity coefficient of component i, xi and yi represents the mole fraction of component i in the vapor and liquid phase, respectively, Pio is the saturated vapor pressure of component i at the VLE temperature and can be calculated by the Antoine equation which were taken from the literature [30], P denotes the vapor pressure (atmospheric pressure) of the system. In this work, the binary and ternary VLE data were correlated with the non-random two-phase (NRTL) model due to its excellent applicability for IL-containing systems. [23, 33-36] For ternary systems, the expression of the model is given as follows: =

∑

∑

+

∑

−

∑

∑

( = 1,2,3) (3)

with = # $%−

&,

=

∆( ( −( = ( = 1,2,3) (4) )* )*

Where △gij is the cross interaction energy parameter between the two components i and j, αij is the

ACCEPTED MANUSCRIPT non-randomness parameter of the ternary system, R is the ideal gas constant and T is the VLE temperature. According to the NRTL model, there are nine adjustable parameters (α12, α13, α23, △g12, △g21, △g23, △g32, △g13, △g31) for a ternary system. Among these parameters, α12, △g12, and △g21 were obtained from binary VLE data, and the other six parameters were regressed from ternary data using Levenberg-Marquardt method by minimizing the following objective function (see Eq.(5)). 1

:

1

23456

−

78679

23456

1 × 100 (5)

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ARD(%) =

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Where n represents the total number of points in our experiment, γiexptl and γicalcd are the experimental and calculated activity coefficients of component i, respectively. The correlation was carried out for each ternary system and the regressed NRTL model parameters (αij, ∆gij and ∆gji ) are listed in Table 5. Table 5 Estimated values of non-randomness factors, αij, and energy parameters ∆gij and ∆gji for NRTL model ∆gij / (J/mol) 869.51a -13521.3 -25562.1 -20610.8 -30897.7 -23120.1 -33706.6

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αij 0.30a 0.33 0.28 0.31 0.26 0.40 0.33

∆gji / (J/mol) 1846.81a 2470.0 2502.3 -29.4 3651.6 -206.1 -4072.9

ARD % 1.04 1.15 0.93 1.02

Parameters obtained from Ref[21].

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Component j Methanol [BMIM][Cl] [BMIM][Cl] [BMIM][Br] [BMIM][Br] [BMIM][OAc] [BMIM][OAc]

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Component i Acetonitrile Acetonitrile Methanol Acetonitrile Methanol Acetonitrile Methanol

Fig.1. Isobaric VLE diagram for the ternary system of acetonitrile (1) + methanol (2) + [BMIM][Cl] (3) at 101.3 kPa: dotted line, IL- frees system; ●, x3≈0.05; ▲, x3≈0.10;■, x3≈0.15; solid line, correlated using NRTL model.

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Fig.2. Isobaric VLE diagram for the ternary system of acetonitrile (1) + methanol (2) + [BMIM][Br] (3) at 101.3 kPa: dotted line, IL- frees system; ●, x3≈0.05;▲, x3≈0.10;■, x3≈0.15; , x3≈0.20; solid line, correlated using NRTL model.

Fig.3. Isobaric VLE diagram for the ternary system of acetonitrile (1) + methanol (2) + [BMIM][OAc] (3) at 101.3 kPa: dotted line, IL- frees system; ●, x3≈0.05; ▲, x3≈0.10;■, x3≈0.15; solid line, correlated using NRTL model. To make it easy to observe the effects of ILs on acetonitrile (1) + methanol (2) binary system, the x1＇–y and x１＇– α12 diagrams for acetonitrile (1) + methanol (2) + IL (3) ternary systems were shown in Figs.1-3 and Figs. 4-6, respectively. As can be seen in Figs.1-3, addition of ILs produces a remarkable salting-out effect on acetonitrile, which enhances the content of acetonitrile in vapor phase. This can be seen more obviously from Figs. 4-6 which show that the relative volatility of acetonitrile to methanol increases with addition of ILs. Meanwhile, this salting-out effect becomes stronger as the content of ILs gradually increases, indicating that the greater the mole fraction of ILs, the larger the content of acetonitrile in vapor phase and the relative volatility of acetonitrile to methanol. Moreover, the azeotropic point moves towards right with addition of ILs. For [BMIM][Cl] and [BMIM][OAc], the azeotropic point disappears when the mole fraction of ILs increases to 0.15, which is 0.2 for [BMIM][Br] in order to achieve the same effect. Based on the NRTL model, the minimum mole fractions of [BMIM][Br], [BMIM][Cl], and [BMIM][OAc]

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required to eliminate the azetrope point are 0.183, 0.138 and 0.116, respectively, proving that the separation performance of the three ILs investigated in this work decreases in the order of [BMIM][OAc] > [BMIM][Cl] > [BMIM][Br]. This can be clearly observed in Figs.4-6 with [BMIM][OAc] giving the greatest α12 orderly followed by [BMIM][Cl] and [BMIM][Br].

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Fig.4. Relative volatility of acetonitrile (1) + methanol (2) with different mole fraction of [BMIM][Cl] at 101.3 kPa: dotted line, IL- free system; ●, x3≈0.05; ▲, x3≈0.10;■, x3≈0.15; solid line, correlated using the NRTL model.

Fig.5. Relative volatility of acetonitrile (1) + methanol (2) with different mole fraction of [BMIM][Br] at 101.3 kPa: dotted line, IL- free system; ●, x3≈0.05;▲, x3≈0.10;■, x3≈0.15; , x3≈0.20;solid line, correlated using the NRTL model.

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Fig.6. Relative volatility of acetonitrile (1) + methanol (2) with different mole fraction of [BMIM][OAc] at 101.3 kPa: dotted line, IL- free system; ●, x3≈0.05; ▲, x3≈0.10;■, x3≈0.15; solid line, correlated using the NRTL model.

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3.3.Analysis of the σ-profiles

Fig.7. σ-Profiles for acetonitrile (1) and methanol (2).

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Fig.8. σ-Profiles for [Cl]-, [Br]- , [OAc]-, [BF4]- and [NTf2]-

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In this work, the σ-Profiles calculated by COSMO-thermX16（C30_1701） model were used to analyze the different hydrogen bond interactions between solvents and ILs and then explain the different effect of [BMIM][Cl], [BMIM][Br] and [BMIM][OAc] on the VLE behavior of acetonitrile and methanol. The σ-Profiles of acetonitrile and methanol are shown in Fig. 7, where the two vertical dashed lines denotes the cutoff values for hydrogen bond. σ< -0.0082 e/Å2, -0.0082 e/Å2<σ<0.0082, and σ>0.0082 e/Å2 represent hydrogen bond donator region, non-polar region, and hydrogen bond acceptor region, respectively. [24,25,37] There are distinct peaks both in the hydrogen bond acceptor region and hydrogen bond donator region for acetonitrile and methanol, indicating that acetonitrile and methanol have not only hydrogen bond acceptor ability but also hydrogen bond donator ability. The farther the peak from the cutoff values of σ, the stronger the hydrogen bond acceptor and donator ability. [38] Thus, it is clear that methanol has stronger hydrogen bond acceptor and donator ability than acetonitrile. For the three ILs in this work, they are composed of the same imidazolium cation which has hydrogen bond donator ability resulting from the hydrogen atoms of the imidazolium ring and different anions which own hydrogen bond acceptor ability as indicated in Fig. 8. Thus, both methanol and acetonitrile could form hydrogen bond with the cations and anions of ILs, but the hydrogen bond interactions between methanol and ILs are stronger than that between acetonitrile and ILs as the former has stronger hydrogen bond acceptor and donator ability as indicated in Fig.7. This will make acetonitrile more volatile compared to IL-free systems, thus improving the relative volatility of acetonitrile to methanol. The differences of the three ILs lie in their anions, the hydrogen bond acceptor ability of which follow the order: [OAc]- > [Cl]- > [Br]- by comparing the distance of the peak apart from the right cutoff value of 0.0082 e/Å2. Therefore, the hydrogen bond interaction between methanol and [BMIM][OAc] is the strongest, orderly followed by [BMIM][Cl] and [BMIM][Br], indicating their separation performance follows the order of [BMIM][OAc]> [BMIM][Cl]> [BMIM][Br]. As mentioned above, [EMIM][BF4], [OMIM][BF4], [EMIM][NTf2] and [BMIM][NTf2] were employed as entrainers to separate acetonitrile and methanol system [20, 21]. Obviously, the effects of ILs investigated in this work differ from those in literature. The structure differences between them mainly lie in anions. For ease of comparison, the σ-profiles of [BF4]- and [NTf2]- were also illustrated in Fig. 8. It can be clearly seen that the hydrogen bond acceptor ability of [BF4]- and [NTf2]- is weaker than [OAc]-, [Br]- and [Cl]- by

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In this study, [BMIM][Cl], [BMIM][Br] and [BMIM][OAc] were tried as entrainers to separate acetonitrile and methanol. The VLE data of the acetonitrile and methanol system containing [BMIM][Cl] or [BMIM][Br] or [BMIM][OAc] were measured at 101.3 kPa. The experimental results showed that all three ILs produced obvious salting-out effect, and significantly enhanced the relative volatility of acetonitrile to methanol. The more is the amount of ILs, the greater is the salting-out effect. The separation performance of the three ILs is [BMIM][OAc]> [BMIM][Cl]> [BMIM][Br]. The experimental VLE data were well correlated with the NRTL model and the minimum mole fractions of [BMIM][Br], [BMIM][Cl] and [BMIM][OAc] needed to eliminate the azeotrope were calculated to be 0.183, 0.138 and 0.116, respectively. Through the σ-profiles of solvents and ILs, the separation effect differences of the three ILs could be attributed to their different hydrogen bond interactions with methanol.

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Acknowledgments

This work is financially supported by National Science Foundation of China (Project No. 21076126), National Science Foundation of China (Project No. 21576166), Liaoning Province Science Foundation of China (Project No. 2014020140), and Liaoning Provincial Committee of Education Science Foundation (Project No. L2016015).

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List of symbols

Mole fraction of solvent i in the liquid phase

xi’

The liquid-phase mole fraction of component i excluding IL

yi T ∆gij

Mole fraction of solvent i in the vapor phase Equilibrium temperature Binary energy parameter of NRTL model

yicalcd P

Mole fraction of solvent i in the vapor phase calculated with the NRTL model Total pressure in the equilibrium system

Pi0

Saturated vapor pressure of component i at equilibrium temperature

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xi

Greek letters α12 Relative volatility of component 1 to component 2 αij

Non-randomness parameter of NRTL model

γi

Activity coefficient of component i

γiexptl

The activity coefficient of component i measured by experimental data

γicalcd

The activity coefficient of component i calculated with the NRTL model

σ

surface screening charge density, e/Å2

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