LiquidâLiquid Equilibria for Binary Azeotrope Mixtures of Benzene and

Article pubs.acs.org/jced

Cite This: J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Liquid−Liquid Equilibria for Binary Azeotrope Mixtures of Benzene and Alcohols Using Choline Chloride-Based Deep Eutectic Solvents Irfan Wazeer,† Hanee F. Hizaddin,‡,§ Lahssen El Blidi,† Emad Ali,† Mohd. Ali Hashim,‡,§ and Mohamed K. Hadj-Kali*,† †

Chemical Engineering Department, College of Engineering King Saud University, P.O. Box 800, Riyadh, 11421, Saudi Arabia Department of Chemical Engineering, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia § University of Malaya Center for Ionic Liquids (UMCiL), University of Malaya, 50603 Kuala Lumpur, Malaysia ‡

S Supporting Information *

ABSTRACT: In this study, the COSMO-RS approach was used to qualitatively and quantitatively screen five choline chloride-based deep eutectic solvents (DESs) to separate the azeotropic binary formed between benzene and either methanol or ethanol. The activity coefficient at infinite dilution was calculated to evaluate the capacity, selectivity, and performance index of each DES. The interactions between the different species were also analyzed by interpreting the σprofile and σ-potential of each component. Then, three DESs were selected for experimental validation. They were prepared by combining choline chloride with ethylene glycol, levulinic acid, and 1,2-propanediol. The best performance in terms of distribution ratio and selectivity was achieved with choline chloride/ethylene glycol DES with 1:4 molar ratio. The experimental tie-lines were successfully correlated using the NRTL model. Regardless of the system investigated, no DES was found in the raffinate phase, implying minimal cross-contamination. Finally, 2D NMR analysis was conducted to study the extraction mechanism of alcohol and its effect on the DES structure. This analysis revealed that, when the alcohol concentration exceeds 40 mol %, the DES’ hydrogen bonds are broken such that only one phase occurs and thus the separation becomes impractical.

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INTRODUCTION Over the past few decades, several studies have set out to develop novel processes based on greener technologies in response to increased environmental concerns, the establishment of new regulations, as well increases in standards of living. In modern chemical plants, it is necessary to separate solvent mixtures into pure components. However, these solvent mixtures are usually difficult to separate because of the closeness of the boiling points of the individual components and/or the existence of azeotropes. One example of such a difficult separation is the azeotropic binary formed between alcohols and benzene in several chemical industries. Indeed, benzene is used as an entrainer for separating ethanol from water (which forms an azeotrope with ethanol).1 The main difficulty centers on the separation of benzene (extraneous material) from the product so that it can be reused. The separation of such special mixtures is an “old” engineering issue that cannot be solved by conventional atmospheric distillation.2−4 Numerous inherent processes for the separation of azeotropic mixtures, including azeotropic distillation, extractive distillation, liquid−liquid equilibrium (LLE), and adsorption © XXXX American Chemical Society

membranes, have been applied in an attempt to selectively separate one of the constituents.2,5 Distillation processes require high pressures or high temperatures to obtain a single fluid-phase system, resulting in a high energy consumption. Alternatively, the LLE process is generally regarded as being a more attractive and environmentally friendly alternative to azeotropic distillation, since it offers a high level of energy efficiency and can be conducted under ambient conditions.6−9 In this process, an extractant is added to an azeotropic mixture so that separation can be carried out. Typical industrial solvents used for the extraction of aromatic compounds are organic solvents such as propylene carbonate, sulfolane, and tetraethylene.10 However, these solvents are generally toxic, volatile, and flammable and involve further processes for energy recovery. As a result, therefore, ionic liquids (ILs) have appeared as attractive substitutes, because of notable properties such as their high thermal stability, low vapor pressure, nonflammability, Received: September 17, 2017 Accepted: January 19, 2018

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DOI: 10.1021/acs.jced.7b00829 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

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Table 1. List of DESs Screened in This Research Using COSMO-RS salt

HBD

HBD (formula)

molar ratio (salt:HBD)

abbreviation

choline chloride (C5H14ClNO)

ethylene glycol levulinic acid 1,2-propanediol urea lactic acid

C2H6O2 C5H8O3 C3H8O2 CH4N2O C3H6O3

1:4 1:2 1:4 1:2 1:2

ChCl:EG (1:4) ChCl:LV (1:2) ChCl:PD (1:4) ChCl:UR (1:2) ChCl:LA (1:2)

Figure 1. Selectivity at infinite dilution, S∞, of methanol and ethanol with respect to benzene in each DES.

Screening of DESs Using the COSMO-RS Approach. Computational Details. Five DESs were screened using the COSMO-RS approach, as shown in Table 1. First, geometry optimization was performed on each species involved (i.e., the DES constituent, ethanol, methanol, and benzene) using the density functional theory (DFT) BP86 level and triple-ζ valence potential (TZVP) basis set. Thereafter, single-point calculations were performed to generate the .cosmo files for use in the COSMO-RS calculations. The basis set BP86 def2TZVPD with the novel fine-grid marching tetrahedron cavity, FINE, was used for the single-point calculations. TMoleX version 4.0, a graphical user interface enabled version of the TURBOMOLE quantum chemistry program package, was used to perform both the geometry optimization and the single-point calculations. The generated .cosmo files were then used in the COSMORS calculations to predict the activity coefficient at infinite dilution, γ∞, of ethanol, methanol, and benzene in each DES. The capacity and selectivity of the ethanol and methanol with respect to benzene in the DESs at infinite dilution, C∞ and S∞, respectively, were estimated on the basis of the values of γ∞, as described in our previous work18 and as given by eqs 1 and 2, below:

nonvolatile nature, high dissolving strength, and reusability in separation processes.11−13 Nevertheless, despite the clear advantages of ILs, most are too expensive to be used on an industrial scale. Furthermore, they are more difficult to synthesize than organic solvents and are not universally green.14,15 Over the past few years, deep eutectic solvents (DESs) have been examined as versatile substitutes for ILs and conventional solvents.16,17 A DES is usually a mixture of halide salts and a hydrogen bond donor (complexing agent) such as amides, amines, or carboxylic acids. DES has a melting point less than that of any of its individual components. Most DESs are nontoxic, biodegradable, and biocompatible, and their synthesis is very economical and easy. Unlike ILs, DESs do not require costly starting materials or purification steps during their synthesis. Experimental LLE data on the use of different solvents including ILs, DESs, and organic solvents for the extraction of alcohols from different azeotropic mixtures is given in Table S1. In the present study, the feasibility and ability of choline chloride-based DESs were investigated as a means of separating benzene from alcohols at 298.15 K and atmospheric pressure. The alcohols studied were methanol and ethanol. The conductor-like screening model (COSMO-RS) was used to assess the extraction efficiencies both qualitatively and quantitatively. Subsequently, three DESs, with ethylene glycol (EG), 1,2-propanediol (PD), and levulinic acid (LV) as complexing agents, were selected for experimental validation. LLE ternary diagrams were plotted with the best solvent, and the selectivity and distribution coefficient parameters were determined to study the extraction efficiency. To the best of our knowledge, this is the first study that shows the use of DESs for the separation of benzene from alcohols with preliminary COSMO-RS screening.

⎛ 1 ⎞ ∞ = ⎜⎜ ∞ ⎟⎟ Calcohol ⎝ γalcohol ⎠DES phase

(1)

⎛ γ∞ ⎞ ∞ ⎟⎟ = ⎜⎜ benzene Salcohol/benzene,max = Salcohol/benzene ∞ ⎝ γalcohol ⎠DES phase

(2)

where the subscripts “alcohol” and “benzene” represent ethanol (or methanol) and benzene, respectively. B



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Figure 2. Capacity at infinite dilution, C∞, of methanol and ethanol in each DES.

observed that, in the same way as for S∞, the values are higher for methanol than those for ethanol. The highest C∞ for methanol was obtained for ChCl:PD (1:4), for which the C∞ value is 5.56, followed by ChCl:EG (1:2) (2.76), ChCl:LA (1:2) (2.59), ChCl:LV (1:2) (2.08), and last ChCl:UR (1:2) (1.27). For ethanol, the orders of the DESs based on the value of C∞ are the same as that for methanol, with lower values of C∞ reported (ChCl:PD (1:4) (3.43), followed by ChCl:EG (1:2) (2.47), ChCl:LA (1:2) (1.61), ChCl:LV (1:2) (1.39), and last ChCl:UR (1:2) (0.57)). The σ-profiles of the DESs, benzene, methanol, and ethanol, shown in Figure 3, can be used to explain the obtained values of S∞ and C∞. The σ-profiles were split into two parts (a and b) to facilitate the visibility of the different species and therefore their comparison. As expected, as benzene is a nonpolar compound, it has a σ-profile centered around σ = 0 and no peaks in the hydrogen bond donor and acceptor regions. Methanol and ethanol have similar σ-profiles which only differ in height due to the extra −CH chain in the ethanol. The DESs ChCl:PD (1:4) and ChCl:LA (1:2) produce high values of C∞ but low values of S∞ for methanol and ethanol. On the basis of their σprofiles, both DESs have a significant nonpolar presence which makes their interaction with benzene more likely than other DESs such as ChCl:EG (1:4) and ChCl:UR (1:2). At the same time, the DESs ChCl:PD (1:4) and ChCl:LA (1:2) are also more likely to interact with both methanol and ethanol due to their σ-profiles being complementary to those of the alcohols. This explains the high values of C∞ but the low values of S∞ given by DESs ChCl:PD (1:4) and ChCl:LA (1:2). On the other hand, the σ-profile of the DES ChCl:EG (1:4) complements those of methanol and ethanol, while the DES does not preferentially interact with benzene. This explains the high values of both S∞ and C∞ given by ChCl:EG (1:4) for methanol and ethanol. We selected three DESs that exhibited the best performance in terms of capacity at infinite dilution in order to experimentally validate the screening results. Experimental Procedure. The three solvents investigated experimentally in the present study are listed in Table 2. Puregrade compound ChCl was purchased from Acros Organics

Comparisons of the performance of DESs for the separation of methanol and ethanol from benzene were made on the basis of the estimated values of C∞ and S∞. The COSMOthermX software package was used to perform the COSMO-RS calculations using parametrization file BP_TZVPD_FINE_C30_1401.ctd. An electroneutral approach was used to represent the DESs in the COSMO-RS calculations, as recommended by the COSMOtherm software developer for representing ILs. A detailed description of the representation of DESs using this approach is given in the literature.18 Other than an electroneutral approach, two other approaches could be used to represent DESs in COSMO-RS, namely, the ion pair and metafile approaches. However, the electroneutral approach adopted in the present study and in our previous works is believed to be able to describe both ILs and DESs as they actually exist in the bulk mixture.19 Indeed, it has been reported that DESs in liquid form consist of a complex formed between the HBD and the halide anions of the salt and this is a similar depiction to that of ILs in reality.20 The mole fractions obtained by COSMO-RS calculation using COSMOthermX with an electroneutral approach must be converted to reflect the actual experimental definition of the mole fraction. Details on the conversion of the mole fraction from the COSMOthermX definition to that of an actual experimental definition were also described in our previous work.18 COSMO-RS Screening Results. Figure 1 shows the values of selectivity at infinite dilution, S∞, of methanol and ethanol relative to benzene for each DES. We can see that the values of S∞ are higher for methanol than for ethanol for every DES. The highest value of S∞ is obtained for ChCl:UR (1:2) with a value of 21.00, followed by ChCl:EG (1:4) (14.83), ChCl:LA (1:2) (13.99), ChCl:PD (1:4) (10.42), and last ChCL:LV (1:2) (7.30). For ethanol, the highest S∞ was reported for ChCl:EG (1:4) (13.31) followed by ChCl:UR (1:2) (9.39), ChCl:LA (1:2) (8.73), ChCl:PD (1:4) (6.43), and then ChCl:LV (1:2) (4.90). The capacities at the infinite dilution, C∞, values for the alcohols in each DES are shown in Figure 2, where it can be C



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Figure 3. σ-Profiles of benzene, methanol, ethanol, and the DESs (a) ChCl:EG (1:4), ChCl:LV (1:2), ChCl:PD (1:4) and (b) ChCl:UR (1:2), ChCl:LA (1:2).

Table 2. Physical Properties of DESs Synthesized in This Work

a

salt

HBD

molar ratio

density, g/cm3 (at 20 °C)

viscosity, Cp (at 20 °C)

water content (%)

choline chloride

ethylene glycol levulinic acid 1,2-propanediol

1:4 1:2 1:4

1.117021 1.140422 1.066323

1921 171.322 a 64.2

0.69 0.94 0.54

Viscosity measured at 25 °C.

was placed in screw-capped bottles. The bottles were stirred in an incubating shaker until a homogeneous liquid without any solid particles was obtained. The shaker featured speed and temperature control, a rotational speed of 200 rpm and a temperature of 100 °C (±0.1 °C). The feed mixture was prepared by mixing the chemicals that had been weighed using an analytical balance (±0.0001 g). The feed and the DESs were then mixed, at a mass ratio of 1:1. The vials were positioned in an incubator shaker, and each set of experiments was conducted at room temperature (298.15 K). The shaking time was set to 2

(Belgium), ethylene glycol was purchased from AVONCHEM (England), levulinic acid was acquired from Sigma-Aldrich (USA), and 1,2-propanediol was purchased from Loba Chemie (India). All of these chemicals were of high purity (>98 wt %) and were used without any additional purification. Three ChCl-based DESsChCl:EG (1:4), ChCl:LV (1:2), and ChCl:PD (1:4)were synthesized according to the method described by Abbott et al.17 ChCl mixed at a certain molar ratio (1:2 or 1:4 in the present study) with a hydrogen bond donor (ethylene glycol, levulinic acid, or 1,2-propanediol) D



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Table 3. Comparison between Selected DESs’ and Their Individual Components’ Extraction Capabilities at 20 wt % Ethanol in the Feed Mixture with Benzenea raffinate phase benzene

DES-rich phase

ethanol

DES

DES

β

S

0.236

0.738

7.585

283.0

0.255

0.496

4.408

17.00

0.139 0.233 Ethylene Glycol 0.082 0.190 1,2-Propanediol 0.274 0.179

0.627

7.379

51.00

0.727

7.899

93.00

0.546

5.099

18.00

benzene

ethanol

ChCl:EG (1:4) 0.967

0.031

0.000

0.026 ChCl:LV (1:2)

0.942

0.058

0.000

0.249 ChCl:PD (1:4)

a

0.968

0.032

0.000

0.976

0.024

0.000

0.965

0.035

0.000

Standard uncertainties are u(T) = 0.1 K, u(P) = 0.1 kPa, and u(x) = 0.007.

Table 4. Composition of Experimental Tie-Lines (Molar Fraction) and the Distribution Ratio and Selectivity Data for the Ternary Systems at P = 1 atm and T = 25 °Ca top layer x1

a

x2

bottom layer x3

0.9937 0.9862 0.9437 0.8766

0.0063 0.0139 0.0566 0.1232

0 0 0 0

0.9956 0.9878 0.9701 0.9471

0.0044 0.0121 0.0299 0.0537

0 0 0 0

x1

x2

β

x3

Benzene (1) + Ethanol (2) + ChCl:EG (1:4) (3) 0.0525 0.1329 0.815 0.0774 0.2277 0.696 0.1031 0.2886 0.609 0.0987 0.3430 0.557 Benzene (1) + Methanol (2) + ChCl:EG (1:4) (3) 0.0154 0.1895 0.795 0.0207 0.3154 0.664 0.0140 0.4080 0.578 0.0172 0.4755 0.507

S

21.19 16.49 5.12 2.78

± ± ± ±

1.05 0.88 0.30 0.18

401.00 212.15 46.97 24.33

± ± ± ±

23.2 16.15 3.89 1.63

42.96 26.07 13.71 9.04

± ± ± ±

1.24 0.73 0.59 0.58

2776.99 1235.43 944.81 495.10

± ± ± ±

130.01 78.05 92.84 39.74

Standard uncertainties are u(T) = 0.1 K, u(P) = 0.1 kPa, and u(x) = 0.007.

found to be less than 1 wt % for each DES synthesized in the present study. The physical properties of the three DESs are listed in Table 2.21−23

h, followed by a settling time of around 2 h, to make sure that the equilibrium stage was fully achieved. Subsequently, samples were taken from both the bottom and top layers and were examined using the gas chromatography (GC). Samples from the bottom and top layers were taken using a microliter pipet and then diluted using toluene to analyze their composition. A trace GC Ultra (Thermo Scientific) system with a flame ionization detector (FID) was used to determine the compositions of the raffinate and DES phases. The operating conditions were as follows: • Column oven temperature: 308.2 K for 2 min • FID temperature: 584 K • Injector temperature 584 K • Temperature ramp: 358.2 K (rate = 10 K/min) • Carrier gas (helium) flow rate: 30 mL/min • Split ratio: 16 • Injection volume: 0.4 μL A calibration curve for each alcohol and benzene was plotted to measure the composition. Each sample was analyzed at least three times using the GC, and the reported molar composition was estimated to be ±0.006. To ensure that the presence of DES in the benzene-rich phases can be considered negligible, samples from these layers were examined using a 1H NMR spectrometer. A JEOL RESONANCE ECX-500 II spectrometer was used to record the 1H NMR and 2D NOESY spectra at 24 °C, using dimethyl sulfoxide (DMSO-D6) as the solvent. Furthermore, the water contents of the three DESs were also determined using Karl Fisher titration. The water content was

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RESULTS AND DISCUSSION Distribution Ratio and Selectivity. The suitability of a DES as a solvent in a liquid−liquid extraction process was determined using two standard solvent selection parameters, i.e., the distribution coefficient, β, and the selectivity, S. The ratio of the solute concentration in the bottom layer to that in the top layer is defined as the distribution coefficient. The selectivity is the ratio of the solute distribution coefficient to that of the carrier (benzene). The distribution coefficient illustrates the solute carrying ability of the solvent (DES) and permits the determination of the quantity of solvent needed for the extraction process. The selectivity values can be used to investigate the affinity of ethanol/methanol toward DES as a solvent. Higher values of selectivity imply that DES interacts more with the alcohols than with the benzene. Equations 3 and 4, below, are used to define β and S, respectively. ′ βAlc = wAlc wAlc ″

S=

βAlc βBen

=

′ wAlc ′ ″ wBen wAlc ″ wBen

(3)

=

′ wAlc w″ × Ben ″ ′ wAlc wBen

(4)

The distribution coefficient is a quantitative measure to indicate how a solute is distributed between the extract and E



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raffinate phases. A high distribution coefficient is desirable, as this means that there is a lower solvent to feed ratio while a smaller-diameter extraction column is required, thus lower the operating costs. Selectivity measures the affinity of a solute toward the solvent (DES) or the carrier. It is desirable to have DES with a high selectivity to alcohols which means that fewer stages are required to remove them. This would reduce the capital cost. Table 3 lists the values of β and S for each experimental tieline composition, along with the LLE data for 20 wt % ethanol in the feed mixture with benzene for three DESs. The values of β and S, along with the LLE data for the best DES (ChCl:EG (1:4)), are presented in Table 4. For each system, we investigated the extraction capability of ChCl:EG (1:4) DES with ethanol/methanol compositions ranging from 10 to 40 wt % in the feed mixture. At higher alcohol concentrations, the mixture exhibits only one phase. Figures 4 and 5 show the

⎛ 1 − wBen ⎛ 1 − wDES ″ ⎞ ′ ⎞ ln⎜ ⎟ = a + b ln⎜ ⎟ ″ ′ ⎝ wBen ⎠ ⎝ wDES ⎠

(5)

⎛ w″ ⎞ ⎛ w′ ⎞ ln⎜ Alc ⎟ = c + d ln⎜ Alc ⎟ ″ ⎠ ′ ⎠ ⎝ wBen ⎝ wDES

(6)

where wBen, wDES, and wAlc denote the concentrations of benzene, DES, and alcohols (ethanol or methanol), respectively. Here, a and b are the fitting parameters of the Othmer− Tobias correlation, while the fitting parameters of the Hand correlation are c and d. Superscripts ′ and ″ refer to the extract and raffinate phases, respectively. The parameters of the Hand and Othmer−Tobias correlations are listed in Table 5. The degree of consistency of the LLE experimental data is indicated by the linearity of each plot (the values of the regression coefficient R2 are close to unity). NRTL LLE Regression. The nonrandom two-liquid (NRTL) model was used for the regression of the experimental LLE data.26 The phase composition in a liquid−liquid equilibrium calculation was achieved by solving the isothermal liquid−liquid flash at a given pressure and temperature. Generally, the NRTL thermodynamic model is more appropriate for the regression of LLE data than a cubic equation of state because it takes into account the nonidealities of the liquid state and the nonrandomness of the molecules in a complex system. Moreover, this model is useful for the design of multistage LLE pilot plants in that no specific modifications are needed for the use of this model in systems containing ILs and DESs. The flash calculations are represented by the following equations: Material balance: xi − (1 − ω)xiI − ωxiII = 0,

i = 1, NC

(7)

Equilibrium equation: xiIγi I − xiIIγi II = 0,

i = 1, NC

(8)

Equation of summation:

∑ xiI − ∑ xiII = 0

Figure 4. Variation of the distribution ratio with alcohols’ mole fraction in the feed.

(9)

Here, xi is the composition of component i in the mixture, xji is the composition of component i in the liquid phase j, ω is the liquid−liquid splitting ratio, γji is the activity coefficient of component i in the liquid phase j, and NC is the number of constituents. For a multicomponent system, the NRTL equation yields the following expression of the activity coefficient:

distribution ratio and selectivity, respectively, as a function of alcohol concentration in the feed. The values of β and S decrease as the concentration of ethanol/methanol increases in the upper layer or feed, indicating that the removal of ethanol or methanol is satisfactory at lower concentrations. In addition, the selectivity values are always higher than unity, indicating that ChCl:EG (1:4) DES could be used as a promising solvent for the azeotropic separation of alcohols and benzene. Similarly, the distribution ratio values are also greater than unity, showing that smaller amounts of solvent are sufficient to achieve a high separation efficiency. With this DES, each sample was analyzed at least two times and the average was reported in Table 4. The average uncertainty does not exceed ±0.007. Details about uncertainty estimation and error propagation as well as the calibration of methanol/benzene and ethanol/benzene are provided in the Supporting Information. Consistency Test. Tests of the consistency of the experimental data were performed using the Othmer−Tobias and Hand correlations. The following two correlations were used to express the Othmer−Tobias24 and Hand25 equations, respectively

ln γi =

∑j τjiGjixj ∑j Gjixj

+

∑ j

⎛ ∑ τ G x ⎞ ⎜⎜τij − k kj kj k ⎟⎟ ∑k Gkjxk ⎠ ∑k Gkjxk ⎝ Gijxj

(10) gij − gii

Cij

with ln Gij = −αijτij; αij = αji; τij = RT = RT ; τii = τjj = 0, where τij and τji are the binary interaction parameters and αij is the nonrandomness parameter. In the present work, the model development was achieved within a Simulis Thermodynamics environment, a thermo-physical properties calculation server provided by ProSim and available as an MS-Excel add-in.27 In the present study, the nonrandomness parameter αij was assumed to be equal to 0.20 for all binary combinations, while the binary interaction parameters Cij and Cji were estimated at 298.15 K from experimental data points by minimizing the F



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Figure 5. Variation of selectivity with alcohols’ mole fraction in the feed.

systems.29,30 Thus, we have maintained the same value in this work. Table 6 lists the values of the binary interaction parameters obtained using the NRTL model with each ternary system. The

Table 5. Parameters of Othmer−Tobias and Hand Correlation for Each Ternary System (Benzene + Alcohols + ChCl:EG (1:4) DES) Othmer−Tobias

Hand

alcohols

a

b

R2

c

d

R2

ethanol methanol

−0.854 −2.122

3.073 2.001

0.988 0.984

1.351 −2.107

3.756 1.850

0.999 0.988

Table 6. NRTL Binary Interaction Parameters i−j 1−2 1−3 2−3

root-mean-square deviation (RMSD) between the calculated and experimental solubilities of each constituent in each phase m

RMSD (%) = 100

c

2

∑∑∑ k=1 i=1 j=1

(xikj − xik̂ j )2 2mc

1−2 1−3 2−3

(11)

where x is the concentration of species in a mole fraction and the subscripts i, j, and k designate the component, phase, and tie-line, respectively. In addition, m is the number of tie-lines, c is the number of components, and j refers to the phases. The parameter αij measures the nonrandomness of the mixture. The mixture is considered to be an ideal solution (i.e., completely random) when the value of αij is zero. The nonrandomness parameter enables the NRTL model to be applied for different binary and ternary mixtures because of the additional degree of freedom that it accords. In the derivation of the NRTL equation, Renon and Prausnitz suggested to relate this parameter to Guggenheim’s quasichemical approximation, where they showed that αij is proportional to the inverse of coordination number z.26 On the basis of this relation, the nonrandomness parameter is approximated to be between 0.1 and 0.3. The value of αij was later revised to be between 0.2 and 0.47. However, the parameter also may not follow the rules set out by Renon as the value αij = −1 works in many cases. Finally, the physical significance attributed to the parameter αij has been eliminated by Tassios (1976)28 who mentioned that this parameter is mainly an empirical parameter which is obtained by regressing the experimental data. In our previous work with DESs as extraction solvents, the value of 0.2 gave accurate fitting for the ternary LLE

Cij (cal mol−1)

Cji (cal mol−1)

Ethanol (1) + Benzene (2) + ChCl:EG (1:4) (3) 168.05 −845.95 −629.83 −1086.16 2577.70 812.92 Methanol (1) + Benzene (2) + ChCl:EG (1:4) (3) 1093.62 −823.45 −659.69 −1395.36 2577.70 812.92

αij 0.2 0.2 0.2 0.2 0.2 0.2

interaction between the benzene and ChCl/EG (1:4) DES was considered to be independent of the alcohol. Good agreement was obtained between the calculated and experimental tie-lines. Finally, in order to confirm the consistency of the binary interaction parameters obtained in this work, we have used the topological analysis of liquid−liquid equilibrium correlations designed recently by Marcilla et al.31 and made available online as a Matlab toolbox. According to this test, the parameters that we have obtained in this work (reported in Table 6) are very consistent and satisfy the Gibbs stability criteria while giving at the same time very good agreement with experimental tie-lines. In fact, according to our lab experiments, when mixing the alcohol (1) (methanol or ethanol) with either benzene (2) or ChCl:EG (1:4) DES (3), only one phase occurs. However, when mixing benzene with the same DES, two layers appear. The same results are obtained when calculating the binary Gibbs mixing energy (provided in the Supporting Information in Figures S3 and S4). The miscibility boundary in both cases confirms this results with two homogeneous regions for 1−2 and 1−3 and a single liquid−liquid region for the system (2− 3). Moreover, our model in both cases very well represents the ternary diagrams. The average RMSD value between the experimental and NRTL calculated tie-line is 0.20% with methanol and 1.29% with ethanol. G



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Figure 6. Experimental (●), COSMO-RS predicted (○), and NRTL (▼) tie-lines for the ternary system (a) benzene (1) + methanol (2) + ChCl:EG (1:4) (3) and (b) benzene (1) + ethanol (2) + ChCl:EG (1:4) (3).

COSMO-RS Predicted Tie-Lines. Using the same parametrization file BP_TZVPD_FINE_C30_1401.ctd and electroneutral approach to represent ChCl:EG (1:4), predicted ternary LLE tie-lines were generated using the COSMOthermX software package on the basis of the experimental feed composition of each ternary system. The predicted tie-lines, along with the experimental tie-lines, are shown in Figure 6. The average RMSD value between the experimental and COSMO-RS predicted tie-line is 4.47 and 1.55% for methanol and ethanol, respectively. Solvent Regeneration. The possibility of DES regeneration was also investigated by using a rotary evaporator. After

extraction, the two phases were separated using a separating funnel and the DES-rich phase was purified under a 70 mbar vacuum at 40 °C to remove the benzene and alcohol. Then, a 1 H NMR analysis was performed to control the quality of the recycled DES. Figure 7 shows the NMR spectra which indicate that the purities of the pure DES and recovered DES are similar. Figure 7 also shows that an NMR analysis of the benzene-rich phase after the extraction process proved the absence of DES in this phase. Thus, high-purity (97% molar) benzene was obtained according to the 1H NMR starting with benzene/ethanol (20% molar ratio). H



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Figure 7. 1H NMR spectra of ChCl-EG DES (1:4, molar ratio) in chloroform: (a) pure DES; (b) recycled DES; (c) benzene rich phase.

Effect of Alcohol Addition to the DES Structure. As mentioned earlier, we noticed that there is no separation when the quantity of alcohol exceeds 40% of the feed mixture. The same behavior was observed in a recent work related to the application of DESs for enhanced oil recovery in which the effect of water addition on the stability of ChCl:glycerol (1:3) DES was investigated using 2D NMR (NOESY and HOESY) analysis.32 That study revealed that, when the water content exceeds 50 wt %, it destroys the hydrogen bonds within the DES structure. Therefore, to understand this phenomenon and understand the methanol extraction mechanism, we conducted the following analysis. The miscibility of alcohols (methanol and ethanol) with benzene can be explained by the hydrophobic and electronic interactions between the hydroxyl group of the alcohol and the aromatic ring of the benzene. The DES extraction of alcohol from an alcohol−benzene azeotropic system suggests that the DES can break these interactions by forming new and strong interactions with alcohol. To study the mechanism of this separation and its limitations, three samples were analyzed by 2D NMR: (i) pure ChCl/EG (1:4) DES, (ii) the same DES with 40 wt % ethanol, and (iii) the same DES with 60 wt % ethanol. First, the NOESY spectrum of the pure DES, shown in Figure 8a, demonstrated the existence of strong interactions between both the protons of the methyl and hydroxyl groups of ChCl and the protons of the hydroxyl groups from EG. This means that hydrogen bonds were formed between the hydroxyl groups of EG and both the hydroxyl group and chloride ions from ChCl. However, when 40 wt % ethanol was added, the NOESY spectra (Figure 8b) exhibited a strong interaction between the hydroxyl groups from the ethanol and those from both ChCl and EG, as well as weak interactions between the methyl groups from ChCl and hydroxyl groups from ethylene glycol, implying that the ethanol formed new hydrogen bonds between both the ChCl and EG but that the DES still exists. Nevertheless, with 60 wt % addition of ethanol (Figure 8c),

only interactions between the ethanol and both ChCl and EG were revealed and no interactions between the DES components were observed. This clearly shows that all of the hydrogen bond interactions in the DES were broken at this concentration. Thus, the extraction of ethanol by ChCl:EG (1:4) DES is only possible when the ethanol concentration in the azeotropic mixture is below 50%. In this case, the DES structure is maintained and it is possible to obtain two stable phases at equilibrium. Effect of DES Amount on Extraction Performance. The results of the 2D NMR analysis of the DES stability in the presence of ethanol were confirmed by varying the DES amount and investigating the effect on the extraction performance. To this end, a feed mixture of benzene−ethanol (40 wt % ethanol) was prepared and mixed with different DES quantities. Figure 9 shows that the distribution ratio increases with the amount of DES with a clear improvement obtained with the 1/2 feed/DES mass ratio. This result confirms the effect of ethanol on the DES stability and suggests that the amount of DES should be at least doubled to attain a better extraction performance.

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CONCLUSION In the present study, a COSMO-RS approach was used to screen five choline chloride-based DESs by predicting the activity coefficient at infinite dilution, γ∞, of benzene, ethanol, and methanol in each DES. Then, we evaluated three DESs as extracting agents for the separation of the azeotropic systems {benzene + ethanol} and {benzene + methanol} for an experimental validation. The ChCl:EG (1:4) DES exhibited a higher level of performance in terms of selectivity and distribution ratio. The experimental LLE data were measured for the ternary systems at T = 298.15 K and atmospheric pressure. The ChCl:EG (1:4) DES was also selected for the generation of ternary diagrams covering an ethanol and methanol concentration range from 10 to 40 wt % in the feed composition, since only one phase occurred at higher I



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Figure 8. 2D NMR spectra and 1H−1H-nuclear Overhauser enhancement spectroscopy (NOESY) in DMSO for (a) pure ChCl:EG (1:4) DES, (b) ChCl:EG (1:4) DES with 40 wt % ethanol, and (c) ChCl:EG (1:4) DES with 60 wt % ethanol.

alcohol concentrations. In view of the results obtained, the three DESs were deemed to be suitable for the separation of

benzene from alcohols via liquid−liquid extraction. Moreover, minimum cross-contamination was detected between the J



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extract and raffinate phases because no DES was found in the raffinate phase. The best DES (ChCl:EG (1:4)) was also regenerated using a rotary evaporator, and to this end, the purity of the recycled DES was analyzed by means of 1H NMR analysis. The results of the 1H NMR analysis confirmed that the recovery of the best DES was also possible by the rotary evaporation of both the benzene and alcohols after extraction. Finally, it was observed that ChCl:EG (1:4) DES can be used for this extraction only for alcohol concentrations lower than 40 wt %. Indeed, a 2D NMR (NOESY and HOESY) spectra analysis proved that, when the alcohol concentration exceeded this amount, the DES’ hydrogen bonds between the salt and the hydrogen bond donor were broken such that only one phase occurred.

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jced.7b00829. GC calibration curve of ethanol/benzene (Figure S1); GC calibration curve of methanol/benzene (Figure S2); topological analysis of LLE NRTL correlation for methanol (1) + benzene (2) + DES (3) (Figure S3); topological analysis of LLE NRTL correlation for ethanol (1) + benzene (2) + DES (3) (Figure S4); experimental LLE data on the use of ILs, DESs, and organic solvents for the extraction of alcohols from the azeotropic mixtures (Table S1); standard deviation on measured solubilities (Table S2); and propagation error on distribution ratio and selectivity (Table S3) (PDF)

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REFERENCES

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Figure 9. Effect of DES’ mass ratio on the distribution ratio for the benzene−ethanol (40 wt %) system.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Mohamed K. Hadj-Kali: 0000-0002-1374-9825 Funding

This research was carried out with funding from the Deanship of Scientific Research at King Saud University through the Group Project No. RG-1438-073 in collaboration with University of Malaya under the HIR Grant No. HIR-MOHE (D000003-16001). Notes

The authors declare no competing financial interest. K



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