Activity coefficients at infinite dilution for various organic solutes in the ionic liquid 1-(2-hydroxyethyl)-3-methylimidazolium hexafluorophosphate

J. Chem. Thermodynamics 140 (2020) 105867

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Activity coefficients at infinite dilution for various organic solutes in the ionic liquid 1-(2-hydroxyethyl)-3-methylimidazolium hexafluorophosphate Cheng Zhang ⇑, David Triger, Nicholas J. Ramer Department of Chemistry, Mathematics and Physics, College of Liberal Arts and Sciences, Long Island University (Post), 720 Northern Boulevard, Brookville, NY 11548, United States

a r t i c l e

i n f o

Article history: Received 22 March 2019 Received in revised form 13 July 2019 Accepted 15 July 2019 Available online 17 July 2019 Keywords: Activity coefficient at infinite dilution Ionic liquid (IL) 1-(2-Hydroxyethyl)-3-methylimidazolium hexafluorophosphate Gas-liquid chromatography Extractive distillation

a b s t r a c t Activity coefficients at infinite dilution (c1 i ) for a variety of polar and non-polar organic solutes (alkanes, cycloalkanes, alkenes, alkyl benzenes, alcohols, 1,4-dioxane, acetone, acetonitrile, tetrahydrofuran, chloroform and dichloromethane) in the ionic liquid (IL) 1-(2-hydroxyethyl)-3-methylimidazolium hexafluorophosphate ([C2OHmim][PF6]) have been determined by gas-liquid chromatography using the IL as the stationary phase. The measurements were conducted over the temperature range of 303–353 K. Þ; enthalpies (DHE;1 ) and entropies (DSE;1 Þ at infinite diluThe partial molar excess Gibbs energies (DGE;1 i i i tion of the 29 solutes in the IL have been calculated from the experimental c1 i values obtained over the 1 temperature range. The selectivity s1 ij and capacity kj at T = 323.15 K for aliphatic/aromatic hydrocarand compared to the bons and aliphatic/alcohol separation problems were calculated from the c1 i reported literature data for other [C2OHmim] based ILs. Ó 2019 Elsevier Ltd.

1. Introduction Activity coefficient at infinite dilution (c1 i ) is one of important thermodynamic parameters for a wide range of applications in chemical and production process [1,2]. It reflects the solubility and selectivity of solvent on solute and can be found in a wide variety of application such as in extractive distillation, liquid-liquid extraction and absorption in the chemical engineering process [3,4]. Activity coefficient at infinite dilution can illustrate the interaction between solute and solvent molecules and can be used to evaluate the performance of the solvent separation. Ionic liquids (ILs) as green solvents have emerged as potential substitutes for organic solvents and have found applications in organic synthesis, catalysis and separation technology due to their unique properties such as low vapor pressure, high thermal stability, wide range of solubility and non-flammability [5–8]. Studies on the determination of activity coefficients at infinite dilution using the gasliquid chromatography (GLC) method for various ILs have been widely reported [1–16]. However, no study has been reported so far for the IL 1-(2-hydroxyethyl)-3-methylimidazolium hexafluorophosphate ([C2OHmim][PF6]). This work is a continuation of

⇑ Corresponding author. E-mail address: [email protected] (C. Zhang). https://doi.org/10.1016/j.jct.2019.07.009 0021-9614/Ó 2019 Elsevier Ltd.

our studies on the determination of activity coefficients at infinite dilution for [C2OHmim] based ILs [17]. The selectivity (s1 ij ) at infinite dilution directly calculated from

the c1 i values offers an important means to assess the performance of ILs as solvents for different separation problems. The larger the selectivity value is, the more effective the separation is for the 1 components in the mixture. The capacitykj indicates the solubility 1

of the solutes in the solvent. The lower the value of the kj is, the smaller the amount of the solute dissolves in the solvent, causing the separation to be less efficient. Imidazolium-based ILs as a class of solvents for extraction and separation processes have been intensively studied in the literature [2–43]. Reviews of the properties and applications of such ILs are also available for greater detail [44,45]. In this work, we reported the activity coefficients at infinite dilution ðc1 i Þ for 29 solutes, including alkanes, cycloalkanes, alkenes, alkyl benzenes, alcohols, 1,4-dioxane, acetone, acetonitrile, tetrahydrofuran, chloroform and dichloromethane in the IL [C2OHmim][PF6] over the temperatures range of 303 to 353 K at 10 K intervals. The partial molar excess Gibbs energies (DGE;1 Þ;enthalpies (DHE;1 ) and entropies (DSE;1 Þ at infinite dilution i i i of the solutes in the IL were derived from the experimental c1 i values obtained over the temperature range. The selectivity s1 ij and 1

capacity kj at T = 323.15 K for IL [C2OHmim][PF6] were calculated

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for aliphatic/aromatic hydrocarbons and aliphatic/alcohol separation problems. The two sets of mixtures were chosen because they are usually difficult to separate from one another due to their azeo1 tropic behavior. Values of selectivity s1 ij and capacity kj calculated from this work were compared with the published literature data for other [C2OHmim]-based ILs. The purpose of this work was to contribute to the database with the view of extending the types of IL and the application of thermodynamic models as well as to understand the influence of the nature of ILs on selectivity and capacity in different separation problems. 2. Experimental 2.1. Materials The IL 1-(2-hydroxyethyl)-3-methylimidazolium hexafluorophosphate ([C2OHmim][PF6]) was synthesized from 1-(2-hydro xyethyl)-3-methylimidazolium chloride ([C2OHmim][Cl]) and potassium hexafluorophosphate (KPF6) according to a literature procedure [12]. The synthesized IL 1-(2-Hydroxyethyl)-3-methyli midazolium hexafluorophosphate was characterized by NMR spectroscopy. 1H and 13C NMR spectra were recorded on a Bruker Advance III 400 MHz NMR spectrometer with TMS as the internal standard at room temperature. The experimental peak integrals are reported as follow for 1-(2-Hydroxyethyl)-3-methylimidazo lium hexafluorophosphate: 1H NMR (400 MHz, d6-DMSO,dppm): 3.60–3.76 (t, 2H), 3.85 (s, 3H), 4.18–4.19 (t, 2H), 5.17 (s, 1H), 7.59 (s, 1H), 7.60 (s, 1H), 8.92 (s, 1H); 13C NMR (100 MHz, d6DMSO, d ppm): 35.90, 39.29, 39.71, 52.11, 59.73, 122.96, 123.61, 137.12. The synthesized IL contained less than 100 ppm of chloride ion impurity determined from a silver nitrate test. To remove organic solvents and water, the IL was purified and dried under high vacuum at 363 K for 24 h. The water content was determined by Karl-Fischer titration, and was found to be less than 400 ppm. The purity of the synthesized IL was >99%. All the analytical grade solutes were purchased from Sigma Aldrich and used without further purification. 101 AW (80/100 mesh, inert and white diatomite) was obtained from Shanghai Reagent Co. and used as a solid substrate for the ILs in the GC column. The sample description for the chemicals was provided in Table S1. Structures of investigated ILs are presented in Fig. 1. 2.2. Experimental procedure The detailed experimental procedure used in this work can be found in previous publications [4,9]. A GC column made of stainless steel with 2 m (length)  2 mm (inner diameter) was used. The detailed procedure to pack the column was as follow: Methanol was used as a solvent to coat IL onto the solid support 101 AW (80/100 mesh) to ensure the uniform spreading of the IL onto the surface of the support by a rotary evaporator. The solid support was weighed before and after the coating process. The mass of the stationary phase and of the solid support were weighed with

Fig. 1. The chemical structure of 1-(2-hydroxyethyl)-3-methylimidazolium hexafluorophosphate ([C2OHmim][PF6]).

a precision of ±0.0001 g. The methanol has a good solubility in the IL, and then the carrier was placed in this solution to be sufficiently dispersed. After the methanol was volatilized, the ionic liquid was well loaded on the surface of the carrier, and no IL remained in the beaker used. This operation has been also used in many literatures [46,47]. The solvent column packing varied from (45.0 to 50.0) mass fraction of the IL, which was large enough to prevent any potential solute residual adsorption on the column packing. The uncertainty in the mass of the IL packed on the solid support is about ±0.05%. Treatment before use of the column: Prior to the experiments, the column was conditioned by passing carrier gas at a high flow rate (about 2 cm3s1) and at high temperature (about 373 K) over a period of 8 h. The flow rate and temperature used had essentially no effect on the IL since the entire column was weighed again after the processing and there was basically no change in the weight of the just-filled material. Similar treatment has been reported in many literatures [47,48]. Carrier properties: 101 AW solid support is an inert and treated with dimethyl chloride silane (DMCS). The second column was used to check the reproducibility of results at a different column-packing level and performed at two different temperatures (323 and 353 K). Results from these two different columns were reproducible with errors less than 0.5%. Experiments were carried out on a GC-7900 gas chromatograph equipped with a heat-traced on-column injector and a flameionization detector. The flow rate of the carrier gas was determined using a GL-102B Digital bubble/liquid flow meter with an uncertainty of ±0.1 cm3min1, which was positioned at the outlet of the column. The flow rate of the carrier gas was adjusted to obtain adequate retention time. The outlet pressure, Po , was kept at atmospheric pressure. Depending on the flow rate of the carrier gas, the pressure drop (P i -Po ) varied between 45 and 180 kPa. The pressure drop was determined by a pressure transducer employed in the GC with an uncertainty of ±0.1 kPa. A membrane manometer with an uncertainty of ±0.2 kPa was used to measure the atmospheric pressure. Solute injection volumes, ranged from 0.2 ll to 0.5 ll, were considered to be at infinite dilution on the column. No significant differences in retention time, tr, were shown by injecting individual pure components or their mixtures. Experiments were conducted at the temperature range 313–364 K. At a given temperature, each experiment was repeated at least twice to ensure the reproducibility. The differences in the retention times of the two measurements were generally within 0.01–0.03 min. Absolute values of retention times varied between 0.5 and 30 min depending on the individual solute. At each temperature, values were measured for the dead time, tG, identical to the retention time of the nonretainable component. During all experiments, the injector and detector temperature were kept at 473 K and 523 K, respectively. The temperature of the oven was controlled within ±0.1 K. The GLC apparatus was tested for the system hexane in hexadecane as stationary phase at 298 K, and the obtained infinite-dilution activity coefficient of hexane in hexadecane (0.90 ± 0.04) is within 2.0% of the literature values [49]. To check the stability of the experimental conditions on the likely elution of the stationary phase by the stream of the carrier gas, the measurements of the retention times were repeated systematically on a daily base for hexane and benzene. No changes of the retention times were observed during the three months of nonstop operations. The uncertainty of c1 i values was obtained from the error propagation law. The following measured parameters were considered in the error calculations with their corresponding standard deviations: the flow rate of the carrier gas, (±0.0017 cm3s1); the inlet pressure, ±0.1 kPa, outlet pressure, ±0.2 kPa; the temperature of

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the oven, ±0.1 K; the uncertainty for the adjusted retention time tr, ±1%; mass of the stationary phase, ±0.05%. The mass of the stationary phase is the main source of uncertainty in the calculation of the net retention volume. The assessed uncertainty in determining the net retention volume V N is about ±2%. Considering that thermodynamic parameters are also exposed to errors, the overall error in c1 i was estimated to be accurate within ±4%. 3. Theoretical basis




   P B11 V 1 n3 RT P0 J 32 ð2B12  V 1 1 Þ  1 þ  RT V N P1 RT

"

1

Tcol P0 1 w Tf PO

#

ð2Þ

where t r is the retention time; t G is the dead time, U is the volumetric flow rate of the measured carrier gas by bubble flow meter at column outlet, T f is flowmeter temperature, T col is the column temperature, P o is the pressure at the column outlet, and Pow is the saturation vapor pressure of water at T f . The factor J 32 appearing in Eqs. (1) and (2) amends for the influence of the pressure drop alongside with the column given by Eq. (3) [56]

J 32 ¼

ð4Þ

where T is the column temperature. The correlation coefficients R2, the coefficient a and b, and the values of c1 i at 298.15 K calculated using Eq. (4) are given in Table 2. The correlation coefficients R2 lies between 0.990 and 0.999 indicating the excellent quality of the linear regression. Activity coefficients at infinite dilution are related with excess

2 ðPi =Po Þ3  1 3 ðPi =Po Þ2  1

DSE;1 : i

  E;1 DGE;1 ¼ RTln c1  T DSE;1 13 ¼ DH i i i

ð5Þ

The above equation can be rearranged in the following form:

ð1Þ

where n3 is the number of moles of solvent on the column packing; T is the column temperature; V N is the standardized retention volume of the solute; P1 is the saturated vapor pressure of the solute at temperature T; V 1 is the molar volume of the solute; P0 is the outlet pressure; V 1 1 is the partial molar volume of the solute at infinite dilution in the solvent (assumed  V 1 ); B11 is the second virial coefficient of the pure solute; and B12 (where 2 denote the carrier gas, nitrogen) is the cross second virial coefficient for the solute and the carrier gas. The values of B11 and B12 were computed using the McGlashan and Potter [51] equation for alkanes and Tsonopolous [52,53] equation for the rest of the solvents. By means of the Hudson and McCoubrey combining rules [54,55], critical parameters for mixtures were computed from the critical properties of the pure component. The net retention volume V N was calculated with the following usual relationship as shown in Eq. (2):

V N ¼ ðJ 32 Þ U ðtr  t G Þ

b T

thermodynamic functions at infinite dilution, DHE;1 , DGE;1 ; and i i

In this work, the equation developed by Everett [49] and Cruickshank et al. [50] was used to calculate the c1 i of solutes in the IL [C2OHmim][PF6] as shown in Eq. (1).

lnc1 13 ¼ ln

lnc1 i ¼ aþ

ð3Þ

where P o and Pi are the outlet and the inlet pressures of the GC column, respectively. The values of vapor pressure were computed using the Antoine equation, and the constants were obtained from the literature [57]. Critical data and ionization energies used to calculate Vc [55] were acquired from the literature [58,59]. Critical data needed to compute B11 and B12, and ionization energies used in the computation of Tc were obtained from the literature [51,52,55]. 4. Results and discussion The standard state of solute for the activity coefficient is hypothetical liquid at zero pressure. The values of c1 i for the 29 different organic solutes in the IL [C2OHmim][PF6] within the temperature range from T = 303 to 353 K are presented in Table 1. They were approximated by the linear regression, using Eq. (4):

  DHE;1 DSE;1 i ln c1  i 13 ¼ RT R

ð6Þ

where R is the gas constant. In combining Eqs. (4) and (6), the limat infiiting partial molar excess enthalpy, DHiE;1 , and entropy DSE;1 i nite dilution, can be obtained from the slope and the intercept,

DHiE;1 = bR, and DSE;1 = aR, respectively. According to Eq. (5), i DGiE;1 was therefore calculated at a reference temperature, Tref. for the solutes Table 2 lists the calculated DHiE;1 , DGiE;1 and DSE;1 i in the IL [C2OHmim][PF6] at Tref = 323.15 K. These thermodynamic functions have been widely utilized to disclose fundamental interactions between the solute and the IL [19,31,60–63,65]. The higher values of DHE;1 were observed for the linear alkanes, i alkene, and 1-alkyne and the values increase with increasing the = 8.05 kJ.chain length. The value for benzene in this work, DHE;1 i mol1 is larger than that for lower alkanols (methanol,

DHE;1 = 4.89 kJ.mol1; ethanol, DHE;1 = 6.68 kJ.mol1; 1-propanol, i i DHE;1 = 7.84 kJ.mol1) confirming that the p-p interactions in aroi matics are weaker than hydrogen bonds in the lower alkanols [64]. Negative values of DHE;1 were observed for 1,4-dioxane, acetonii trile, acetone and tetrahydrofuran indicating the intermolecular interactions between cations and/or anions of the IL and the polar groups in the above-mentioned solutes are stronger than for other solutes [26,65]. A similar effect was observed for DGE;1 . DGE;1 are i i positive for non-polar solutes such as alkanes, alkenes, alkynes were observed and aromatic hydrocarbons. Smaller values of DGE;1 i were observed for 1,4for alcohols, and negative values of DGE;1 i dioxane, acetonitrile and acetone due to the stronger hydrogen bonding formation between the IL and the solutes. T DSE;1 are posi itive for alkanes, alkenes, alkynes, alkanols, benzene, chloromethane and dichloromethane. The positive values of T DSE;1 for i alkanols indicate hydrogen bonds are breaking during the dissolution process. Negative values of T DSE;1 were observed for 1,4i dioxane, acetonitrile, acetone and tetrahydrofuran, and most aromatic hydrocarbons. This implies their reorganization inside the IL phase and the enhanced interaction between the abovementioned solute and the IL [5,19,31,61]. Figures 2–5 show the natural logarithm of the activity coefficients (lnc1 i ) in the IL [C2OHmim][PF6] as a function of the inverse absolute temperature (1000/T(K1)) for all investigated solutes. As revealed in Figs. 2–5, the values of c1 for the series of solutes i increase with an increase in chain length. Similar behavior was also observed for other reported alkyl (methyl-, ethyl-, butyl-, hexyl-) imidazolium cation-based ILs [29,30,35,36,44,45,65,66]. The higher values of c1 i indicate weaker interactions between the solute and the solvent. The values of c1 i for alkenes are lower than those for alkanes with the same carbon number, which is caused by the

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C. Zhang et al. / J. Chem. Thermodynamics 140 (2020) 105867

Table 1 Experimental activity coefficients at infinite dilution, ci1, for various solutes in the ionic liquid [C2OHmim][PF6] at different temperatures (303–353 K) and pressure P0 = 101.3 kPaa; solute standard state is hypothetical liquid at zero pressure.

a

Solute (i)

303 K

313 K

323 K

333 K

343 K

353 K

pentane hexane heptane octane nonane cyclohexane methyl cyclohexane 1-hexene 1-octene 1-decene 1-pentyne 1-hexyne 1-heptyne 1-octyne benzene toluene ethylbenzene o-xylene m-xylene p-xylene methanol ethanol 1-propanol 1,4-dioxane acetonitrile acetone tetrahydrofuran chloroform dichloromethane

22.4 65.8 163 461 536 72.8 130 32.0 164 350 11.6 26.1 58.2 116 8.00 14.8 33.0 22.4 29.3 27.3 1.55 2.81 5.29 0.840 0.570 0.750 2.08 5.78 3.11

16.5 48.0 103 322 448 52.5 97.1 24.3 133 319 9.56 20.0 45.4 93.3 7.43 14.1 30.6 21.2 27.8 25.6 1.48 2.66 4.83 0.920 0.580 0.800 2.19 5.40 2.92

13.1 35.6 73.8 240 322 39.1 70.9 17.8 101 274 7.29 16.1 37.2 74.2 6.85 12.9 27.6 19.8 25.4 23.8 1.41 2.47 4.40 0.990 0.590 0.830 2.24 4.96 2.63

10.3 27.4 55.1 197 256 29.7 52.6 13.5 74.4 234 5.95 12.5 29.3 57.7 6.09 11.9 25.1 18.5 23.6 21.9 1.32 2.29 4.04 1.05 0.610 0.860 2.29 4.54 2.43

8.3 21.3 40.8 150 199 22.5 39.7 10.3 56.0 199 4.78 10.2 22.6 47.7 5.60 11.3 22.3 17.2 21.5 20.3 1.26 2.11 3.73 1.11 0.600 0.870 2.23 4.12 2.22

6.72 16.4 30.0 115 154 17.5 29.8 7.90 41.3 175 3.99 8.21 17.3 38.1 5.15 10.4 20.7 16.0 19.8 18.4 1.18 1.94 3.38 1.17 0.600 0.870 2.16 3.72 2.00

Standard uncertainties u are u(T) = ±0.5 K, u(P) = ±2 kPa, ur(ci1) = ±4%.

Table 2 E;1 Coefficients a and b of Eq. (4), correlation coefficient R2, c1 ), Gibbs i at 298.15 K calculated using Eq. (4), values of the partial molar excess enthalpies at infinite dilution (DH i energies (DGE;1 Þ and entropies (T ref DSE;1 ) of organic solutes in the ionic liquid [C2OHmim][PF6] at a reference temperature Tref = 323.15 K. i i Solute (i) pentane hexane heptane octane nonane cyclohexane methylcyclohexane 1-hexene 1-octene 1-decene 1-pentyne 1-hexyne 1-heptyne 1-octyne benzene toluene ethylbenzene o-xylene m-xylene p-xylene methanol ethanol 1-propanol 1,4-dioxane acetonitrile acetone tetrahydrofuran chloroform dichloromethane

a 5.240 5.487 6.455 3.382 3.048 5.697 5.498 6.479 4.719 0.811 5.250 4.785 4.253 3.018 1.081 0.195 0.109 0.744 0.586 0.574 1.479 1.584 1.426 2.121 0.100 1.257 1.799 1.347 1.974

b K

R2

2526 2932 3485 2878 2813 3027 3151 3024 3001 1545 2346 2441 2533 2364 969 768 1039 727 859 840 588 803 943 686 196 461 316 953 954

0.999 0.999 0.998 0.996 0.994 0.999 0.999 0.999 0.992 0.991 0.998 0.999 0.992 0.997 0.993 0.993 0.994 0.994 0.991 0.991 0.993 0.991 0.997 0.99 0.993 0.986 0.956 0.990 0.991

c1 i

DHE;1 i kJmol

enhanced interactions between the double bond in alkenes and the polar IL. Alkynes and aromatic hydrocarbons have lower values of c1 than those for alkanes, cycloalkanes, and alkenes signifying i stronger interactions between the triple bond in alkynes and the

25.3 77.2 187.5 528.9 594.0 86.1 159.3 39.0 209.9 400.6 13.7 30.0 69.6 135.8 8.7 16.0 36.4 24.1 32.0 29.7 1.6 3.0 5.7 0.8 0.6 0.8 2.1 6.4 3.4

1

21.0 24.4 29.0 23.9 23.4 25.2 26.2 25.1 25.0 12.9 19.5 20.3 21.1 19.7 8.1 6.4 8.6 6.1 7.1 7.0 4.9 6.7 7.8 5.7 1.6 3.8 2.6 7.9 7.9

DGE;1 i kJmol

1

6.9 9.6 11.6 14.8 15.2 9.9 11.4 7.7 12.3 15.0 5.4 7.4 9.6 11.6 5.2 6.9 8.9 8.0 8.7 8.5 0.9 2.4 4.0 0.0 1.4 0.5 2.2 4.3 2.6

T ref DSE;1 i kJmol

1

14.1 14.7 17.3 9.1 8.2 15.3 14.8 17.4 12.7 2.2 14.1 12.9 11.4 8.1 2.9 0.5 0.3 2.0 1.6 1.5 4.0 4.3 3.8 5.7 0.3 3.4 4.8 3.6 5.3

six p-delocalized electrons in aromatics with the polar IL, respectively. Relatively much smaller values of c1 were observed for i alkanols, 1,4-dioxane, acetonitrile, acetone, tetrahydrofuran, chloroform and dichloromethane, which are attributed to the much

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C. Zhang et al. / J. Chem. Thermodynamics 140 (2020) 105867

Fig. 2. Plot of ln(ci1) for ionic liquid [C2OHmim][PF6] versus 1/T for the solutes: pentane, hexane, heptane, octane, nonane, cyclohexane, and methyl cyclohexane.

Fig. 4. Plot of ln(ci1) for ionic liquid [C2OHmim][PF6] versus 1/T for the solutes: 1hexene, 1-octene, 1-decene, 1-pentyne, 1-hexyne, 1-heptyne, and 1-octyne.

Fig. 3. Plot of ln(ci1) for ionic liquid [C2OHmim][PF6] versus 1/T for the solutes: benzene, toluene, ethylbenzene, o-xylene, m-xylene, and p-xylene.

Fig. 5. Plot of ln(ci1) for ionic liquid [C2OHmim][PF6] versus 1/T for the solutes: methanol, ethanol, 1-propanol, 1,4-dioxane, acetonitrile, acetone, tetrahydrofuran, chloroform, and dichloromethane.

stronger interactions between the polar groups (i.e., oxygen and chlorine atoms) of these compounds and the polar IL [C2OHmim] [PF6] and the special strength of ion-induced dipole interactions as well [26]. For alkanes, 1-alkenes, 1-alkynes, alcohols, alkyl benzenes, chloroform and dichloromethane, values of c1 decrease i with increasing temperature. For 1,4-dioxane, acetonitrile, acetone and tetrahydrofuran, values of c1 i slightly increase with increasing temperature. Compounds with stronger interaction with the IL have lower 1 lnc1 i values and smaller slopes in the plots. Therefore, the lnci values and the slope of the lnc1 /(1/T) plot follows the order of alcoi hols < aromatics < alkynes < alkenes < alkanes. This explains that the molecular interaction increases with the increasing polarity of the solutes, and the more polar solutes have better solubilities in the IL due to the attractive interaction of polar molecules with the charged ions of the IL [26]. This trend is also consistent with the reported solubility in the previous work of Zhang et al.

[67,68] and Su et al. [69]. The lnc1 i values in this work may be used to predict the effectiveness of separating different compounds. The selectivity at infinite dilution for the IL, which revealed suitability, s1 ij , of a solvent for separating mixtures of components i and j (where i and j refers to the solutes to be extracted), can be 1 calculated directly from experimental c1 i and cj values using Eq. (7) [49] 1 1 s1 ij ¼ ci =cj

ð7Þ 1 kj ,

The capacity, reported [70–72].

is defined as according to Eq. (8) as widely

1

kj ¼ 1=c1 j

ð8Þ ðs1 ij Þ

1 (kj Þ

Values of the selectivity and capacity for the separation of aliphatic/aromatic (hexane/benzene and cyclohexane/benzene) in the IL [C2OHmim][PF6] at infinite dilution at T = 323.15 K

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C. Zhang et al. / J. Chem. Thermodynamics 140 (2020) 105867

Table 3 1 Selectivityðs1 ij Þ and capacity ðK j ) for hexane (i)/benzene (j) and cyclohexane (i)/benzene (j) at infinite dilution for [C2OHmim] based ionic liquids at 323.15 K. K1 j

s1 ij

a b c

Ionic liquid

hexane (i)/benzene (j)

cyclohexane (i)/benzene (j)

benzene

Refs.

[C2OHmim][FAP] [C2OHmim][FAP] [C2OHmim][NTF2] [C2OHmim][BF4] [C2OHmim][C4F9SO3] [C2OHmim][PF6]

n/a 26.5 20.6 47.3 3.6 5.2

20.6 15.8 11.8 53.6 3.9 5.7

0.95 0.88 0.47 0.11 0.26 0.15

[43] [28]a [36]b [26,27]c [17] This work

Data was taken at 328.5 K. Data was taken at 322.55 K. Data was taken at 298.15 K.

Table 4 1 Selectivityðs1 ij Þ and capacity ðK j ) for benzene (i)/methanol (j) and hexane (i)/methanol (j) at infinite dilution for [C2OHmim] based ionic liquids at 323.15 K. K1 j

s1 ij

a b c

Ionic liquid

benzene (i)/methanol (j)

hexane (i)/methanol (j)

methanol

Refs.

[C2OHmim][FAP] [C2OHmim][FAP] [C2OHmim][NTF2] [C2OHmim][BF4] [C2OHmim][C4F9SO3] [C2OHmim][PF6]

1.5 1.6 2.4 9.9 4.1 4.9

n/a 42.8 49.0 471 14.7 25.2

1.44 1.43 1.12 1.09 1.08 0.71

[43] [28]a [36]b [26,27]c [17] This work

Data was taken at 328.5 K. Data was taken at 322.55 K. Data was taken at 298.15 K.

1

are listed in Table 3. Values of the selectivity ðs1 ij Þ and capacity (kj Þ for the separation of benzene/methanol, hexane/methanol and heptane/ethanol in the IL [C2OHmim][PF6] at infinite dilution at T = 323.15 K are listed in Table 4. The results are compared with previous work and reported literature data for other [C2OHmim] based ILs. For example, 1-(2-hydroxyethyl)-3-methylimidazolium tetrafluoroborate ([C2OHmim][BF4]), 1-(2-hydroxyethyl)-3-methy limidazolium trifluorotris(perfluoroethyl)phosphate ([C2OHmim] [FAP]), 1-(2-hydroxyethyl)-3-methylimidazolium nonafluoro-1butanesulfonate ([C2OHmim][C4F9SO3]), and 1-(2-hydroxyethyl)3-methylimidazolium bis(trifluoromethane sulfonyl)amide ([C2OHmim][NTF2]). The selectivity values presented in Tables 3 and 4 provide important comparisons and clearly indicate that the anion in the ILs containing the same cation plays a key role in affecting the separation efficiency in the mixtures of aliphatic/aromatic and alkane or aromatic/alcohol. For the separation of hexane/benzene and cyclohexane/benzene 1 system, as shown in Table 3, the s1 ij and kj using [C2OHmim][BF4] as the solvent are 47.3, 53.6 and 0.11 respectively [26]. The selectivity is high, indicating that [C2OHmim][BF4] could be an effective solvent to separate aliphatic from the aromatic compounds. However, the capacity (0.11) is very low. This implies a low solubility and therefore a low throughput of the separated component. The s1 ij using [C2OHmim][FAP] as the solvent at T = 328.5 K gives 26.5 1

and 15.8, and the kj is 0.88 [28], making it a more effective solvent for performing this separation. Orfao et al. reported corresponding data for cyclohexane/benzene separation at the reference temperature 323.15 K and clearly demonstrated the superior performance of FAP-based ILs for separation of aliphatic/aromatic hydrocarbons 1 [43]. From this work, the s1 calculated using [C2ij and the kj OHmim][PF6] as the solvent at T = 323.15 K are 5.2, 5.7 and 0.15 respectively. The selectivity is higher than that reported in the previous work using [C2OHmim][C4F9SO3] as the solvent at T = 323.15 K, which gives 3.6 and 3.9 respectively [17]. The selectivity is lower than the other reported data in Table 3 but high

enough to be acceptable. However, the capacity is most likely too low for effective separation of aliphatic from aromatic hydrocarbons by extractive distillation. For the system of benzene/methanol in [C2OHmim][PF6] at 1 T = 332.15 K, as shown in Table 4, the s1 calculated ij and the kj from this work are 4.9 and 0.71 respectively. The selectivity for this separation was higher than that reported for [C2OHmim][C4F9SO3] (4.1) [C2OHmim][FAP] (1.5, 1.6) [28,43] and [C2OHmim][NTF2] (2.4) [36] but not as high as that for [C2OHmim][BF4] (9.9) [27]. For the separation of hexane/methanol using [C2OHmim][PF6] as the solvent, the selectivity gives 25.2, which is higher than that for [C2OHmim][C4F9SO3] (14.7) but lower than other reported data in Table 4. The capacity value for methanol is 0.71, which is slightly lower than that for [C2OHmim][BF4] (1.09), [C2OHmim][C4F9SO3] (1.08), and [C2OHmim][NTF2] (1.12) [36]. This makes [C2OHmim] [PF6] a possible solvent for the separation of benzene/methanol and hexane/methanol by extractive distillation. When compared to the previous work using [C2OHmim][C4F9SO3] as a solvent [17], the relatively high selectivity obtained using [C2OHmim] [PF6] as a solvent for the investigated separation systems is in accordance with a relative bigger value of c1 i for aliphatic (hexane and cyclohexane) and aromatic (benzene) hydrocarbons caused by weaker interactions between the solute and the polar IL, and relative smaller value of c1 i for alkyl alcohols (methanol) caused by enhanced interaction in terms of hydrogen bonding between alkyl alcohol and polar ILs containing hydroxyl group in the cation structure. Apparently, the anion ([FAP], [NTF2], [BF4], and [C4F9SO3]) in the ILs containing the same cation ([C2OHmim]) plays a key role in affecting the separation efficiency in the mixtures of aliphatic/aromatic and alkane or aromatic/alcohol. More prominently, the data generated through this work, may be beneficial in understanding the nature of ILs and supplementing our knowledge for expanding and developing thermodynamic models involving mixtures containing ILs suitable for the separation process.

C. Zhang et al. / J. Chem. Thermodynamics 140 (2020) 105867

5. Conclusion Activity coefficients at infinite dilution (c1 i ) for 29 solutes in the ionic liquid 1-(2-hydroxyethyl)-3-methylimidazolium nonafluoro1-butanesulfonate ([C2OHmim][PF6]) were measured over the temperature ranges from 303 K to 353 K using GLC method. Values of the selectivity and capacity related to the separation of aliphatic hydrocarbons (hexane and cyclohexane) from aromatic hydrocarbon (benzene) and the separation of benzene from methanol and alkane (hexane) from alky alcohols (methanol) were calculated from the measured c1 i . These values of selectivity and capacity were compared to reported literature values for other [C2OHmim] based ionic liquids with different anions ([FAP], [NTF2], [BF4], and [C4F9SO3]). The results indicate that [C2OHmim][PF6] is not an ideal solvent for separating aliphatic from aromatic hydrocarbons due to the lower selectivity and capacity when compared to other reported [C2OHmim] based ILs. However, [C2OHmim][PF6] appears to be a possible solvent for the separation of benzene from methanol by extractive distillation, as values of the selectivity and capacity 1 are reasonably high ðs1 ij = 4.9, kj = 0.71) in comparison to those reported for [C2OHmim] based ILs except for [C2OHmim][BF4]. Acknowledgments

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The authors gratefully acknowledge the financial support from Long Island University (Post) Faculty Research Grant LIU 36037 (2016) and Undergraduate Research Grant LIU 36037 (2016–2018).

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Appendix A. Supplementary data

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JCT 2019-254