Activity Coefficients at Infinite Dilution of Various Solutes in

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

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Activity Coefficients at Infinite Dilution of Various Solutes in Tetrapropylammonium Bromide + 1,6-Hexanediol Deep Eutectic Solvent Nkululeko Nkosi,† Kaniki Tumba,*,‡ and Suresh Ramsuroop† †

Department of Chemical Engineering, Durban University of Technology, Steve Biko Campus, Durban, 4001, South Africa Department of Chemical Engineering, Mangosuthu University of Technology, uMlazi, Durban, 4031, South Africa

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‡

ABSTRACT: The present work focuses on the application of a deep eutectic solvent (DES) to the separation of liquid mixtures. Experimental activity coefficients at infinite dilution γ∞ 13 of 23 organic solutes in tetrapropylammonium bromide +1,6-hexanediol DES were measured by gas−liquid chromatography (GLC) at four temperatures ranging from (313.15 to 343.15) K and atmospheric pressure. The hydrogen bond acceptor-to-hydrogen bond donour molar ratio was 1:2. The effect of molecular structure on the values of γ∞ 13 was also examined. From experimental limiting activity coefficients, values of partial molar excess enthalpy at infinite dilution (ΔHE,∞ i ) were obtained using the Gibbs−Helmholtz equation over the same ∞ temperature range. Furthermore, infinite dilution selectivity (Sij,s ) and capacity (k∞ ) for various practical separation problems were calculated from experimental γ∞ j,s 13 and were compared with those of previously investigated ionic liquids and popular industrial solvents. It was observed that the investigated deep eutectic solvent would be an effective separation agent for mixtures of alkanes and thiophene or pyridine, as well as those involving alcohols and cycloalkanes or aromatics or ketones. regarded as “designer” solvents, due to their adjustable properties such as density, viscosity, solubility, and refractive index by altering the cation or the anion involved in the molecular structure.19,20 However, high viscosity, a complicated synthesis process, poor biodegradability, unknown toxicity, and the cost of their starting materials still hinder their application in industrial processes.7 Over the past decade, deep eutectic solvents (DESs) which are green solvents emerged as alternatives to conventional organic solvents and ILs.21 DESs are defined as mixtures of two or more compounds which exhibit a significant decrease in melting point to the extent of having the resulting melting point far below that of the constituting compounds.22−24 These solvents present excellent solvation properties as they involve a hydrogen bond acceptor (HBA) and a hydrogen bond donour (HBD). Additionaly, DESs are known to share similar advantages to ILs while being prepared to overcome their disadvantages.25 They are generally less costly than ILs.26 Recently, DESs such as tetramethylammonium chloride + glycerol,27 choline chloride + glycerol,28 and tetramethylammonium chloride + ethylene glycol29 have been investigated as separation agents for various liquid mixtures, on the basis of experimental limiting activity coefficient data. It was concluded that the investigated DESs would be effective replacements to

1. INTRODUCTION Because of the rapid increase of energy cost and concerns over environmental pollution, the chemical and petrochemical industries have to develop new advanced methods to enable improvements in industrial processes.1−3 In relation to solventenhanced separation processes, organic liquids such as sulfolane, N-formyl-morpholine (NFM), glycols, and Nmethyl-pyrrolidine (NMP) have adverse effects on the environment and human health. Furthermore, organic solvents are often toxic, corrosive, and cause crystallization problems.4,5 Hence, a search for alternative environmentally friendly and cost-effective solvents is imperative. Ionic liquids (ILs) as organic salts with a melting point below 100 °C,6 have become a subject of intensive studies in both academic and industrial fields in recent years.7−12 They offer interesting physicochemical properties such as low vapor pressure, a wide electrochemical window, excellent solvation, negligible volatility, nonflammability, good thermal conductivity, and high thermal and chemical stability.7,13−15 Additionally, ILs are widely promoted because they are easily recovered in separation processes. Owing to these desirable properties, ILs have been regarded as attractive alternatives to popular organic solvents in many fields of chemistry and chemical engineering.6,16−18 The investigation of imidazolium, thiazolium, morpholinium, and pyridinium-based ILs gave promising results in separation processes. Many ILs exhibited the ability to separate azeotropic as well as close-boiling binaries by liquid−liquid extraction or extractive distillation under realistic conditions. They are © XXXX American Chemical Society

Received: July 12, 2018 Accepted: October 26, 2018

A

DOI: 10.1021/acs.jced.8b00600 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

Article

currently used industrial solvents in various separation problems. However, there is still scarcity of infinite dilution activity coefficients data for systems involving DESs. This article is part of our ongoing study aimed at generating limiting activity coefficient data for systems involving deep eutectic solvents.27,29 These data will be helpful to understand the effect of structure on the ability of various deep eutectic solvents to separate various liquid mixtures through liquid− liquid extraction or extractive distillation. Furthermore, they can be instrumental in developing or refining some thermodynamic models for the aforementioned systems. In the present study, tetrapropylammonium bromide + 1,6hexanediol ([4C3N]+[Hdiol]) is investigated for better interpretation of γ∞ data of the three DESs with similar features reported in the literature. They are all ammoniumbased and contain at least a halogen atom in their HBA and at least two hydroxy groups in their HBD structure. It should be noted that the selection of [4C3N] + [Hdiol] at the molar ratio of [1:2] was informed by a previous study30 establishing its liquid state at room temperature. The present study offers among other things the opportunity to examine the effect of hydrogen bonding sites on interactions between DESs and various organic solutes. The information contained in this study, along with similar data to be published in a near future will elucidate the structure−property relationship in connection with γ∞ as well as the separation performance of DESs. Additionally, preliminary solvent design in separation processes can be based on the knowledge of γ∞ 13 as this fundamental thermodynamic property provides insights into the behavior of liquid mixtures.31 Activity coefficients data can further be used to assess the separation ability of a solvent by determining selectivities and capacities at infinite dilution in relation to binary mixtures. The higher are these separation indices, the better is the solvent for the investigated separation problem. There are three major techniques for the experimental determination of γ∞ 13. These are gas−liquid chromatography (GLC), inert gas stripping, and differential ebulliometry. In this work, an apparatus based on the GLC technique was utilized as it is suitable for solvents that have low vapor pressure.32−35 The aim of this study was to assess the DES comprising tetrapropylammonium bromide (HBA) and 1,6hexanediol (HBD) as a separation agent for various binary liquid mixtures. The DES was prepared in the HBA-to-HBD molar ratio of 1:2. Infinite dilution activity coefficient (IDAC) measurements were performed for 23 organic solutes in the DES. The latter was benchmarked against other DESs, ILs, and currently used industrial solvents.

Table 1. Suppliers and Quoted Purity of Materials Used in This Study compound n-hexane n-heptane n-octane cyclopentane cyclohexane cycloheptane cyclooctane hex-1-ene hept-1-ene hept-1-yne oct-1-yne methanol ethanol propan-1-ol propan-2-ol benzene toluene ethylbenzene acetone butan-2-one thiophene pyridine Methyl acetate ethyl acetate dichloromethane helium tetrapropylammonium bromide 1,6-hexanediol

2. EXPERIMENTAL SECTION 2.1. Materials. All solutes used in the experiments described in this work are listed in Table 1, along with the components of the DES and the carrier gas. No attempt was made to purify these chemicals before use.36 They were purchased with a stated minimum purity corresponding to 0.98 mass fraction. The DES was prepared and purified according to the procedure described by various researchers.25,28,37 The structure of its components is shown in Figure 1. 2.2. Experimental Procedures. 2.2.1. DES Preparation and Characterization. The investigated DES was prepared by mixing tetrapropylammonium bromide (1 mol) with 1, 6hexanediol (2 mol) at room temperature. The mixture was placed in a tightly sealed beaker and heated on a hot plate at a

supplier Merck SigmaAldrich SigmaAldrich Fluka ACE ACE SigmaAldrich SigmaAldrich Fluka SigmaAldrich SigmaAldrich Macron SigmaAldrich Lab Scan Lab Scan SigmaAldrich SigmaAldrich SigmaAldrich SigmaAldrich SigmaAldrich SigmaAldrich SigmaAldrich Capital Lab ACE SigmaAldrich Afrox-SA SigmaAldrich SigmaAldrich

purity (mass fraction)

CASRN

≥99.0 ≥99.9

110-54-3 142-82-5

≥99.9

111-65-9

≥98.5 ≥99.0 ≥99.9 ≥99.5

287-92-3 11-82-7 291-64-5 292-64-8

≥98.0

592-41-6

≥98.0 ≥99.9

592-76-7 628-71-7

≥99.9

629-05-0

≥99.9 ≥99.9

67-59-1 64-17-5

≥99.5 ≥99.5 ≥99.9

71-23-8 67-63-0 71-43-2

≥99.9

108-88-3

≥99.9

100-41-4

≥99.9

37-64-4

≥99.7

78-93-3

≥99.9

110-02-1

≥99.9

100-86-1

≥98.0 ≥99.5 ≥99.9

79-20-9 141-78-6 75-09-2

≥99.99 ≥99.9

7440-49-7 1941-30-6

≥99.0

629-11-8

Figure 1. Structure of the components comprising the DES: tetrapropylammonium bromide (HBA) and 1,6-hexanediol (HBD).

higher temperature until all crystals were completely dissolved. Subsequently, the HBA and HBD were vigorously mixed by means of a magnetic stirrer bar at 393.15 K. The preparation process was allowed to run for approximately 4 h when the mixture perfectly formed a clear colorless homogeneous liquid. The obtained DES was subjected to a temperature of 373.15 K B



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component, assuming negligible solubility of air in the stationary phase. Solute retention times were measured with an uncertainty of ±0.05 min. Chromatograms associated with solutes were collected and processed using TotalChrom Workstation software. The injector and detector temperatures were kept at 523.15 K which was higher than the boiling point of all organic solutes investigated. The pressure drop across the column varied from (14 to 60) kPa depending on the system temperature and the nature of the solute. The uncertainty on both inlet and outlet pressures was estimated as ±5 Pa. Moreover, activity coefficients at infinite dilution of various solutes in hexadecane were measured at various temperatures to confirm the reliability and accuracy of the experimental procedure used in this study. Results deviated within 2.63% from data already available in the literature.39−44 In relation to data reported in the present study, the estimated uncertainty of γ∞ 13 was ±10% of the measured values. These were found using the calculation procedure described by Bahadur and coresearchers.44 Equations developed by Everett45 and Cruickshank et al.46 were used to compute γ∞ 13 values along with other correlations described in the literature.38 Critical pressures, critical volumes, ionization energies, and constants used in Antoine equation for vapor pressure calculations were taken from the literature.47,48 Mixed virial coefficients were calculated by means of the equation suggested by McGlashan and Potter49 while mixing rules proposed by Hudson and McCoubrey50 were used to estimate the critical properties and the ionization energy of mixtures. Pure component properties and mixed virial coefficients relevant to this study are provided in Tables 3 and 4.

in an oven for more than 8 h to remove any traces of volatile impurities. The final DES sample was kept in an airtight container to avoid exposure to moisture. To characterize the DES, its density, refractive index, and composition were measured. The refractive index and density were determined at 313.15 K using an Atago refractometer model RX-7000 (with an uncertainty of ±0.001) and the Anton Paar density and sound velocity meter, model DSA 500 M (with an uncertainty of ±0.001 g/cm3). Density and refractive index data are given in Table 2. After purification in Table 2. Experimental Refractive Index (r) and Density (ρ) of Tetrapropylammonium Bromide +1,6-Hexanediol DES (HBA-to-HBD Molar Ratio = 1:2) at 313.15 K and 101.3 kPaa

a

solvent

r

DES

1.47923

ρ (g·cm−3) 1.04225 3

Standard uncertainties u are u(ρ) = 0.001 g/cm , u(r) = 0.001, u(T) = 0.02 K, and u(p) = 2 kPa. The uncertainty on DES composition was 0.001 mole fraction.

the oven, nuclear magnetic resonance analyses confirmed no notable change of DES composition (HBD-to-HBA molar ratio of 1:2 on a water free basis) while the water content determined by Karl Fischer titration was 2.3 × 10−2 wt %. 2.2.2. Activity Coefficient at Infinite Dilution Measurements. In this sudy, infinite dilution activity coefficients in the DES were measured following the procedure explained in detail in previous publications.32−34 For this reason, only essential information is provided in this section. Experiments were undertaken with two different solvent column loadings, namely (27.41 and 29.04) wt %. Each column was made of 1 m long stainless steel and had an internal diameter of 4.1 mm. The solid support (Chromosorb W-HP 80/100 mesh) was purchased from Sigma-Aldrich in South Africa. Dichloromethane was allowed to dissolve and coat the DES onto chromosorb. Dichloromethane was subsequently evaporated using a vacuum pump at a moderate vacuum of 60 kPa. The use of two separate columns was justified by the need to confirm the reproducibility and accuracy of experimental values reported in the present study. The uniform packing of the columns was achieved with the aid of a vacuum pump along with vibration The uncertainty of the DES loading on the column was ±2.10 × 10−7 mol. Prior to loading, care was taken to clean each column. This was done by repeatedly rinsing with hot soapy water followed by cold water before drying with acetone. Experimental runs were undertaken by means of a gas chromatograph (Shimadzo GC-2014) equipped with a thermal conductivity (TCD) and flame ionization (FID) detector. The helium (carrier gas) flow rate which varied from (15 to 40) mL/min was measured with a bubble soap flow meter fitted at the TCD vent. Its uncertainty was estimated as ±0.02 mL/ min. This flow rate was corrected for water vapor pressure associated with bubbles in the flow meter. The size sample of injected solutes ranged from (0.1 to 0.2) μL, complying with the requirement for the infinite dilution region. At least three injections were carried out for each data point to check reproducibility as well as the stability of experimental conditions. It was determined that retention times were reproducible within (10−3 to 10−2) min. The dead time, tG, was determined by using air as the unretainable

3. RESULTS AND DISCUSSION 3.1. Infinite Dilution Activity Coefficients. Experimental γ∞ 13 values as obtained in this study are provided in Table 5. They were in fact averages of those obtained using the two columns for the 23 organic solutes in the DES determined at different temperatures. Both columns gave similar limiting activity coefficient values and the average relative deviation between data from the two columns was ±2.13%. This indicates the absence of interfacial adsorption in both columns used in this study.51 Hence, the reported results are not only accurate but also reproducible. Figures 2 to 4 (Plots obtained from data reported in Table 5) suggest a notable dependence of γ∞ 13 on temperature in most cases. A linear relation between the natural logarithm of γ∞ 13 and the inverse of temperature expressed in K. Therefore, infinite dilution activity coefficient values can easily be estimated at various other temperatures through interpolation from data obtained in this study. The γ∞ 13 values for all investigated organic compounds decreased with an increasing system temperature, except those associated with heterocyclics (i.e., pyridine and thiophene) as well as methyl acetate. It is an indication that for all investigated solutes, except pyridine, thiophene, and methyl acetate, an increase in temperature enhanced solubility in the DES as low limiting activity coefficients imply strong solute−solvent interactions. A comparison between γ∞ 13 values across homologous series is shown in Figure 5 in terms of ln γ∞ 13 as a function of the carbon chain length. The linear data regression was of excellent quality. It emerged that limiting activity coefficient values decreased in the following order: n-alkanes > alk-1-enes > cycloalkanes > alkylbenzenes > ketones > alk-1-ynes > alkanols C



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Table 3. Vapour Pressure (P1*), Molar Volume (V1 *) and Mixed Virial Coefficient Data Used in the Calculation of Infinite Dilution Activity Coefficients at Various Temperatures (T). Subscripts 1 and 2 Refer to the Solute and Helium, Respectively

n-hexane

n-heptane

n-octane

cyclopentane

cyclohexane

cycloheptane

cylooctane

hex-1-ene

hept-1-ene

hept-1-yne

oct-1-yne

methanol

ethanol

propan-1-ol

propan-2-ol

T

P1*

V1*

B11

B12

K

kPa

cm3·mol−1

cm3·mol−1

cm3·mol−1

313.15 323.15 333.15 343.15 313.15 323.15 333.15 343.15 313.15 323.15 333.15 343.15 313.15 323.15 333.15 343.15 313.15 323.15 333.15 343.15 313.15 323.15 333.15 343.15 313.15 323.15 333.15 343.15 313.15 323.15 333.15 343.15 313.15 323.15 333.15 343.15 313.15 323.15 333.15 343.15 313.15 323.15 333.15 343.15 313.15 323.15 333.15 343.15 313.15 323.15 333.15 343.15 313.15 323.15 333.15 343.15 313.15 323.15

37.29 54.08 76.42 105.47 12.34 18.90 28.06 40.52 4.14 6.71 10.48 15.86 73.99 103.93 142.66 191.79 24.64 36.26 51.90 72.47 6.08 9.54 14.47 21.32 1.72 2.87 4.59 7.12 45.06 64.71 90.61 124.01 14.93 22.60 33.22 47.55 35.44 55.56 84.52 125.15 17.90 29.48 46.90 72.35 17.90 29.48 46.90 72.35 35.44 55.56 84.52 125.15 7.00 12.18 20.33 32.69 14.23 23.93

133.79 135.83 137.98 140.26 149.47 151.49 153.62 155.84 166.15 168.21 170.35 172.59 96.75 98.17 99.65 101.23 110.43 111.80 113.24 114.74 123.78 125.10 126.46 127.88 137.96 139.26 140.61 142.00 130.15 132.16 134.29 136.54 143.42 145.38 147.43 149.59 129.02 130.71 132.48 134.34 151.74 153.39 155.10 156.88 39.81 40.44 41.11 41.82 54.95 55.84 56.78 57.78 71.00 72.04 73.14 74.29 72.45 73.66

−1650.50 −1512.98 −1392.65 −1286.72 −2474.07 −2252.23 −2059.51 −1891.11 −3599.35 −3256.52 −2960.35 −2702.97 −1101.26 −1013.43 −936.23 −867.99 −1767.06 −1612.59 −1478.11 −1360.34 −2943.30 −2662.52 −2420.03 −2209.38 −4612.79 −4143.14 −3739.76 −3391.28 −1558.80 −1429.47 −1316.24 −1216.53 −2326.22 −2118.26 −1937.55 −1779.59 −2318.76 −2108.41 −1925.89 −1766.58 −3810.44 −3437.55 −3116.19 −2837.61 −339.87 −321.20 −304.04 −288.21 −541.32 −507.11 −476.24 −448.26 −870.73 −808.73 −753.60 −704.32 −769.99 −716.44

53.85 54.46 55.02 55.55 59.90 60.56 61.17 61.74 66.26 66.96 67.61 68.22 41.39 41.94 42.45 42.93 46.40 47.00 47.56 48.08 51.15 51.81 52.43 53.01 56.04 56.75 57.42 58.04 52.78 53.37 53.91 54.43 58.11 58.75 59.34 59.90 54.04 54.68 55.27 55.83 60.17 60.87 61.52 62.14 24.08 24.49 24.87 25.23 30.29 30.76 31.19 31.60 36.29 36.81 37.30 37.77 36.87 37.38

D



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Table 3. continued

isobutylalcohol

benzene

toluene

ethylbenzene

acetone

butan-2-one

thiophene

pyridine

methyl acetate

ethyl acetate

T

P1*

V1*

B11

B12

K

kPa

cm3·mol−1

cm3·mol−1

cm3·mol−1

333.15 343.15 313.15 323.15 333.15 343.15 313.15 323.15 333.15 343.15 313.15 323.15 333.15 343.15 313.15 323.15 333.15 343.15 313.15 323.15 333.15 343.15 313.15 323.15 333.15 343.15 313.15 323.15 333.15 343.15 313.15 323.15 333.15 343.15 313.15 323.15 333.15 343.15 313.15 323.15 333.15 343.15

38.79 60.80 38.48 70.97 124.01 206.70 24.41 36.25 52.34 73.65 7.89 12.29 18.55 27.20 2.87 4.69 7.40 11.31 56.62 81.95 115.61 159.37 23.64 35.57 51.96 73.94 207.27 310.62 452.32 641.87 59.78 95.54 147.58 221.11 540.87 791.07 1125.97 1564.06 251.11 380.38 558.89 799.12

74.94 76.30 88.77 90.01 91.32 92.69 91.27 92.37 93.52 94.72 109.08 110.29 111.55 112.86 125.96 127.27 128.63 130.04 75.82 76.97 78.19 79.49 94.18 95.46 96.81 98.22 77.49 78.37 79.28 80.23 86.86 87.75 88.66 89.60 81.56 82.83 84.17 85.60 67.88 69.90 72.19 74.80

−668.66 −625.82 −1261.10 −1162.95 −1076.52 −999.97 −1537.89 −1402.27 −1284.30 −1181.07 −2423.19 −2194.64 −1997.06 −1825.24 −3580.40 −3224.41 −2918.04 −2652.83 −730.83 −680.01 −634.67 −594.01 −1174.77 −1084.25 −1004.44 −933.68 −1181.17 −1086.23 −1002.93 −929.42 −1851.09 −1686.54 −1543.57 −1418.61 −792.57 −737.51 −688.39 −644.32 −360.06 −335.38 −313.24 −293.27

37.85 38.30 42.34 42.91 43.45 43.96 40.57 41.13 41.65 42.14 47.14 47.75 48.32 48.85 53.19 53.84 54.45 55.03 35.72 36.21 36.68 37.11 42.15 42.70 43.22 43.70 36.07 36.60 37.10 37.57 39.32 39.91 40.47 40.99 37.74 38.26 38.75 39.20 31.64 32.04 32.41 32.76

strongly interacted with DES. The lowest IDAC values were observed for heterocyclics (i.e., pyridine and thiophene) which strongly interacted with the studied DES. For alkylbenzenes, it was observed that the introduction of an alkyl (i.e., ethyl or methyl) radical to the benzene ring resulted in an increased γ∞ 13 values, caused by the change in polarizable π-electrons density and the decrease in polarity. Additionally, alkanols with the same number of carbons (i.e., propan-1-ol and propan-2-ol) had different IDAC values due to the difference in polarity between these isomers. The position of the hydroxyl group in the carbon chain influenced the values of γ∞ 13 which are, among other things, related to polarity. From Figure 5, it can also be observed that long carbon chains of the solute lead to high IDAC values, that is, weak solute−solvent interactions for all solutes, except pyridine, thiophene, and esters. It is worth noting that the various trends

> esters > heterocyclics. As expected, a nonpolar compound could not strongly interact with the DES, which comprised polar components. Low activity coefficient values exhibited by alkanols, esters, and heterocyclics can be attributed to the possibility of strong dipole−dipole interactions or additional hydrogen bonds occurring between these classes of solute and the DES components. It appeared that the presence of double or triple bonds in hydrocarbons significantly increased solute− solvent interactions, a reflection of increased solubility. This can be ascribed to an increasing number of polarizable electrons present in double or triple bonds, which lead to strong induced dipole−dipole interactions between these solutes and the DES. It also emerged that γ∞ 13 values for cycloalkanes and alk-1-enes were much lower compared to nalkanes, though still higher than alk-1-ynes. This can be attributed to π-electrons of cycloalkanes and alk-1-enes that E



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Table 4. Critical Temperatures, Volumes, and Pressures (Tc, Vc, and Pc) as Well as Activation Energies (I) Used in the Calculation of Infinite Dilution Activity Coefficients I solutes n-hexane n-heptane n-octane cyclopentane cyclohexane cycloheptane cyclooctane hex-1-ene hept-1-ene hex-1-yne hept-1-yne methanol ethanol propan-1-ol propan-2-ol isobutyl alcohol benzene toluene ethylbenzene acetone butan-2-one pyridine thiophene methyl acetate ethyl acetate

kJ·mol

Tc −1

977.4 957.1 947.5 1014.1 951.3 962 941.7 910.8 910.8 960 960 1046.9 1010.2 982.2 981.3 999.67 892.1 851 846.2 935.9 918.54 915.91 871.59 1009.41 985.84

Vc cm ·mol 3

K 507.9 540.2 568.7 511.6 553.5 604.3 647.2 504 537.3 539.3 551.6 512.64 513.9 536.8 508.3 547.0 562.2 591.8 617.2 508.1 536.8 620.0 580.0 506.8 402.4

368 428 492 260 308 359 410 355 409 331 377 118 167 219 220 273 256 316 374 209 267 254 219 228 164

Pc −1

kPa 3025 2740 2490 4508 4073 3840 3570 3143 2920 3762 3340 8097 6148 5175 4762 4300 4895 4108 3609 4700 2410 5670 5660 4694 5630

Figure 2. Plot of ln (γ∞ 13) against 1/T at 101.3 kPa for methanol (△); ethanol (■); propan-1-ol (●); propan-2-ol (×);methyl acetate (○); ethyl acetate ( ◇ ); pryidine ( ◆ ); thiophene ( □ ) (1) in tetrapropylammonium bromide + 1,6-hexanediol DES (HBA-toHBD molar ratio = 1:2) (3).

of IDAC variation with the nature of solute (polar and nonpolar solutes) are similar to those reported by other researchers.28−50 3.2. Partial Molar Excess Enthalpies at Infinite Dilution. In this study, experimental γ∞ 13 values were used to observe both effects during mixing (exothermic and endothermic) using the thermodynamic function referred to as partial molar excess enthalpy. According to the Gibbs−

Table 5. Activity Coefficients at Infinite Dilution (γ∞) for Selected Organic Solutes (1) in Tetrapropylammonium Bromide +1,6-Hexanediol DES (HBA-to-HBD Molar Ratio = 1:2) (3) at T = (313.15, 323.15, 333.15, and 343.15) K and 101.3 kPaa solute

T/K = 313.15

T/K = 323.15

T/K = 333.15

T/K = 343.15

n-hexane n-heptane n-octane cyclopentane cyclohexane cycloheptane cyclooctane hex-1-ene hept-1-ene hept-1-yne oct-1-yne methanol ethanol propan-1-ol propan-2-ol benzene toluene ethylbenzene acetone butan-2-one thiophene pyridine methyl acetate ethyl acetate

28.03 30.75 40.07 9.32 12.00 13.58 16.22 14.94 19.49 1.83 1.66 0.34 0.53 0.63 0.75 2.07 3.06 4.56 1.62 1.91 0.15 0.08 0.31 0.37

21.25 28.34 37.63 8.38 11.03 12.71 15.25 13.26 17.69 1.83 1.66 0.33 0.52 0.61 0.73 2.04 3.03 4.45 1.58 1.87 0.16 0.08 0.32 0.35

19.20 26.19 35.75 7.96 11.08 11.97 14.41 13.09 16.66 1.80 1.65 0.34 0.51 0.59 0.71 2.02 2.97 4.36 1.51 1.82 0.17 0.09 0.33 0.35

18.31 24.20 32.56 8.47 9.70 11.37 13.55 12.96 16.85 1.78 1.64 0.33 0.50 0.58 0.67 1.98 2.92 4.25 1.46 1.79 0.17 0.09 0.34 0.34

Standard uncertainties: u(T) = 0.1 K, and u(γ∞) = 10% of measured values. The uncertainty on DES composition was 0.001 mole fraction.

a

F



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Figure 5. Plot of ln (γ∞ 13) against Nc (number of carbon atoms in the solute) at 313.15 K and 101.3 kPa for alkanes (◆); alk-1-enes (◇); alk-1-ynes (×); cycloalkanes (□); alkanols (∗); alkylbenzenes (○); heterocyclics (●); ketones (+); and esters (△) in tetrapropylammonium bromide + 1,6-hexanediol DES (HBA-to-HBD molar ratio = 1:2) (3).

Figure 3. Plot of ln (γ∞ 13) against 1/T at 101.3 kPa for acetone (△); benzene (●); hep-1-yne (×); toluene (▲); butan-2-one (◇); ethylbenzene (◆); oct-1-yne (□) (1) in tetrapropylammonium bromide + 1,6-hexanediol DES (HBA-to-HBD molar ratio = 1:2) (3).

Table 6. Partial Excess Enthalpy of Mixing at Infinite Dilution for Selected Organic Solutes in Tetrapropylammonium Bromide + 1,6-Hexanediol DES (HBA-to-HBD Molar Ratio = 1:2) Obtained from the Gibbs−Helmholtz Equation

Figure 4. Plot of ln (γ∞ 13) against 1/T at 101.3 kPa for n-hexane (■); n-octane (●); n-heptane (▲); hex-1-ene (○); hept-1-ene (◆); cyclopentane (◇); cyclohexane (□); cycloheptane (△); cyclooctane (×) (1) in tetrapropylammonium bromide + 1,6-hexanediol DES (HBA-to-HBD molar ratio = 1:2) (3).

Helmholtz equation, the values for partial molar excess enthalpies ΔHE,∞ at infinite dilution presented in Table 6 i can be obtained as follows: E ,∞ ij ∂ ln γi∞ yz jj zz = − ΔHi jj zz R k ∂(1/T ) { p , x

(1)

where T and R represent temperature and the universal gas constant, respectively. This thermodynamic function provided further information on the extent of temperature dependence of IDAC values on temperature, in addition to heat effects associated with mixing. Negative values of ΔHiE,∞ were observed for heterocyclics (i.e., pyridine and thiophene) and methyl acetate. However, positiveΔHE,∞ values were observed i for n-alkanes, alk-1-enes, cycloalkanes, alkylbenzenes, ketones, alk-1-ynes, alkanols and ethyl acetate. Positive values of ΔHE,∞ i indicate weaker (exothermic dissolution) associative molecular interactions between solute and the DES than those associated

solute

ΔHE,i (∞)/(kJ mol−1)

n-hexane n-heptane n-octane cyclopentane cyclohexane cycloheptane cyclooctane hex-1-ene hept-1-ene hept-1-yne oct-1-yne methanol ethanol propan-1-ol propan-2-ol benzene toluene ethylbenzene acetone butan-2-one thiophene pyridine methyl acetate ethyl acetate

12.44 6.00 7.12 3.08 5.62 5.32 5.32 3.99 4.51 0.76 0.33 1.07 2.02 2.67 3.42 1.30 1.39 2.08 3.02 1.97 −4.27 −3.19 −2.45 2.46

with solute−solute pairs. Positive values imply an immiscible phase occurrence while negative values indicate the opposite. 3.3. Assessment of Tetrapropylammonium Bromide +1, 6-Hexanediol DES as Separation Agent. Measurements undertaken in this study were used to assess the performance of tetrapropylammonium bromide +1,6-hexanediol DES as solvent in selected separation problems. G


H

b

12.15/0.69 c

3.18/2.56 122.77/0.79 72.44/0.52 16.86/0.43 d

3.44/2.56 156/0.79 108.92/0.52

50.07/0.49 140.82/0.75 43.29/2.54 25.45/1.21 2.98/1.22 70.60/0.85 27.94/1.10 18.09/1.49 12.48/1.34

31.98/0.49 89.51/0.75 30.95/2.54 11.91/1.21 2.63/1.22 45.81/0.85 19.31/1.10 13.75/1.49 9.98/1.34

1.28/1.03 71.26/0.46 54.22/0.39

93.15/0.38

95.52/0.38

5.05/0.02 22.03/0. 05b 14.95/0.06b 25.95/0.40 41.28/0.35 20.84/1.71 12.45/1.26 2.77/1.28 30.98/0.58 16.96/0.97 13.00/1.41 9.13/1.23

173.85/6.13

heptane/ thiophene

130.37/6.13

hexane/ thiophene

10.42/0.49

hexane/ benzene

e

315.43/2.29 34.79/0.88

387.71/0.64 100.86/0.22c 75.40/0.32c 2.24/0.03 93.36/0.78 44.50/3.65 25.06/2.54 2.73/1.27 70.25/1.31 29.15/1.66

256.04/12.05

hexane/ pyridine

1.89/0.11

1.17/0.94 3.26/0.02 2.85/0.02

2.24/0.03 2.29/0.02 1.94/0.16 1.35/0.14 1.19/0.55 2.38/0.04 1.95/0.11 1.76/0.19 0.63/0.08

0.72/0.00

1.60/0.08

hexane/ hexene

0.86/1.03 20.28/0.46 23.91/0.39 8.34/0.43 7.29/0.69

4.45/0.02 12.18/0.05 5.71/0.06 13.00/0.40 16.17/0.35 9.64/1.71 12.48/1.26 2.14/1.28 13.70/0.58 10.05/0.97 8.19/1.41 5.84/1.23

5.41/0.49

cyclohexane/ benzene

0.36/0.72 0.56/0.37

2.39/5.06

5.85/6.04 5.53/2.54 2.72/1.07 1.16/0.50

3.85/6.58 0.40/0.51 0.68/0.88 2.18/1.26 0.80/0.77 0.76/1.07 0.75/0.92

27.46/4.10 0.10/0.43 0.64/0.84 0.29/0.59 0.28/0.66 0.25/0.80

40.48/0.81 34.75/1.64 16.25/1.04 2.90/1.16

6.18/3.03

benzene/ methanol

3.31/0.32 2.4/0.50 2.13/0.63 0.33/0.87

3.06/1.93

acetone/ ethanol

2.39/1.79 56.17/0.28 51.17/0.25 15.02/0.29 10.97/0.51

2.71/0.01 8.68/0.02d 6.60/0.03d 25.73/0.25 34.62/0.18 18.62/1.09 19.54/0.93 2.57/1.05 103.96/1.26 17.22/0.68 12.91/1.07 8.64/0.93

9.36/0.33

heptane/ toluene

4.20/5.06 64.81/1.47 43.94/0.72 7.20/0.37

23.11/4.10 4.30/0.43 1.20/0.71 19.92/0.84 6.14/0.59 3.81/0.66 3.78/0.80

70.10/0.32 95.0/0.50 55.63/0.63 28.34/0.87

21.33/1.93

cyclohexane/ ethanol

Reference

60 61 62 63 64

27 28 28 52 53 54 55 56 56 57 58 59

This work

Interpolated values. n-Octane/benzene. n-Octane/pyridine. n-Octane/toluene. [4C3NBr]+[Hdiol], tetrapropylammonium bromide + 1,6-Hexanediol; [4C1NCl]+[Gly], tetramethylammonium chloride + glycerol; [ChCl]+[Gly], choline chloride + glycerol; [N1,1,1,2OH][NTf2], choline bis{(trifluoromethyl)sulfonyl}imide; [EMPYR][Lac], 1-ethyl-1-methylpyrrolidinium lactate; [TDC]+[DCA]−, 1,2,3-tris(diethylamino) cyclopropenylium bis{(trifluoromethyl)sulfonyl}imide; [COC2N1,1,2][FAP], ethyl-dimethly-(2-methoxyethyl)ammonium trifluorotris(perfluoroethyl) phosphate; [C16MIM][BF4], 1-hexadecyl-3-methylimidazolium tetrafluoroborate; [EMIM][TCM], 1-ethyl-3-methylimidazoliu tricyanomethanide; [COC2N1,1,2][NTf2], N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium bis{(trifluoromethyl)sulfonyl}imide; [C5C1Pip][NTf2], 1-n-alkyl-1-methylpiperidium bis{(trifluoromethyl)sulfonyl}imide; [C6H13OCH2MIMM][NTf2], 1-hexyloxymethyl-3-methyl-imidazolium bis{(trifuoromethyl)sulfonyl}imide; [P14,4,4,4][DBS], tributyltetradecylphosphonium dodecylbenzenesulfonate; [BMIM][SCN], 1-butyl-3-methylimidazolium thiocyanate; N-C3OHmMOR][NTf2], 4(3hydroxy)-4-methylmorpholinium bis{(trifluoromethyl)sulfonyl}imide.

a

DES 1; [4C3NBr]+[Hdiol] [1:2] DES 2 [4C1NCl]+[Gly] DES 3; [ChCl]+[Gly] [1:1]a DES 4; [ChCl]+[Gly] [1:2]a [N1,1,1,2OH][NTf2]a [EMPYR][Lac]a [TDC]+[DCA]− [COC2N1,1,2][FAP]a [C16MIM][BF4] [EMIM][TCM]a [COC2N1,1,2][NTf2]a [C5C1Pip][NTf2]a [C6H13OCH2MIMM] [NTf2]a [P14,4,4,4][DBS]a [BMIM][SCN]a N-C3OHmMOR][NTf2]a sulfolanea NMP

solvente

∞ S∞ ij /kj

Table 7. Selectivity and Capacity at Infinite Dilution of Various Solvents for Common Industrial Separation Problems at T = 323.15 K

Journal of Chemical & Engineering Data Article



Article

Experimental γ∞ 13 values were directly used to determine selectivity (Sij∞) and capacity (kj∞) at infinite dilution, according to the following equations: Sij∞ =

k∞ j =

4. CONCLUSION Gas−liquid chromatography was used to measure activity coefficients at infinite dilution for 23 polar and nonpolar organic solutes in tetrapropylammonium bromide + 1,6hexanediol DES at various temperatures and atmospheric pressure. The change of IDAC with temperature and solute polarity followed the typical trend reported in the literature for previously investigated ionic liquids and deep eutectic solvents. Experimental IDAC data were further used to determine for the organic solutes. Selectivities and capacities at ΔHE,∞ i infinite dilution were also calculated from IDAC values for selected separation problems. It can be concluded that tetrapropylammonium bromide + 1,6-hexanediol DES may be an alternative solvent to organic solvents and ILs in fuel denitrification and desulfurization as well as cycloalkanes− alcohols, aromatics−alcohols and ketones−alcohols separation problems. The experimental results obtained in this study shed light on the nature of interactions between the investigated DES and organic compounds in the context of solvent-enhanced separation processes. Moreover, the reported results would be useful for developing, validating or refining the much needed thermodynamic models for IDAC in deep eutectic solvents. In-depth comparison between the performance of the examined DES and ILs with emphasis on the contribution of structure to the separation performance was not attempted. This was due to the scarcity of IDAC data for systems involving DESs. For this reason, there is still need for more experimental data to clarify the effect of composition on separation efficiency.

γi∞ γj∞

(2)

1 γj∞

(3)

where i refers to solutes andj to the component to be extracted. The performance of the studied DES was compared to that of other DESs, ILs, and popular industrial solvents (i.e., sulfolane and NMP) in Table 7. The temperature of 323.15 K was selected for limiting selectivity and capacity because most available data were obtained at this temperature. The typical separation problems examined were those related to mixtures of aliphatics−aromatics (represented by n-hexane−benzene mixture) or heterocyclics or alcohols, alkanes−alkenes, ketones−aromatics and esters−alcohols. As suggested by Table 7, obtained selectivity values for the separation of aliphatics−heterocyclics mixtures were quite high. This suggests that the investigated DES could be an effective solvent for these separations problems, outperforming most listed DESs, ILs, and molecular solvents. However, the separation of aliphatics−aromatics, aliphatics−alcohols, aliphatics−olefins, ketones−aromatics, and esters−alcohols did not present exceptional selectivity values (quite low) but were acceptable (S∞ ij > 1). Unfortunately, the selectivity value for the separation of esters from alcohols problem was too low and unacceptable. These observations support the high affinity displayed by DES for all polar compounds. In other terms, in the presence of two polar components, the DES cannot display adequate selectivity. values were In relation to limiting capacity, high k∞ j displayed by heterocyclics (i.e., thiophene and pyridine) compounds in the context of aliphatics−heterocyclics separation scenario. On the other hand, results for separation problems involving alcohols displayed low but still acceptable limiting capacity values (k∞ j > 1). Unfortunately, the values of k∞ j for other separation problems were not attractive as they ranged from 0.08 to 0.49. Such low capacities imply poor solubility in the DES and modest throughput of the separated components. As a matter of fact, higher values of both selectivity and capacity are desired for efficient separation under economic conditions. In this study, the selectivity and capacity of the investigated DES for the n-hexane−pyridine separation problem (S∞ ij = 256.04, k∞ = 12.05) were found to be the highest of the listed j values in Table 7. Interestingly, selectivity and capacity values which rank second and third place were observed for the n∞ hexane (S∞ ij = 130.37) or n-heptane (Sij = 173.85) from ∞ thiophene (kj = 6.13) separation problems. This makes the investigated DES a potentially good solvent for these separation problems which are relevant to the denitrification and desulfurization of fuels. Furthermore, the calculated selectivities and capacities also indicate that tetrapropylammonium bromide + 1,6-hexanediol DES could be used as an effective entrainer for the separation of cyclohexane−ethanol, benzene−methanol, and acetone−ethanol pairs.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Kaniki Tumba: 0000-0003-2685-3741 Funding

Funding for this research was provided by the National Research Foundation (NRF) in South Africa. Notes

The authors declare no competing financial interest.

■

REFERENCES

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K


Activity Coefficients at Infinite Dilution of Various Solutes in

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