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Deep Eutectic Solvents as Azeotrope Breakers: Liquid-Liquid Extraction and COSMO-RS Prediction Andreia S. L. Gouveia, Filipe S. Oliveira, Kiki Adi Kurnia, and Isabel M. Marrucho ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b01542 • Publication Date (Web): 31 Aug 2016 Downloaded from http://pubs.acs.org on September 3, 2016

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ACS Sustainable Chemistry & Engineering

Deep Eutectic Solvents as Azeotrope Breakers: Liquid-Liquid Extraction and COSMO-RS Prediction

Andreia S.L. Gouveia,1,2 Filipe S. Oliveira,1 Kiki A. Kurnia,3 Isabel M. Marrucho1,2*

1

Instituto de Tecnologia Química e Biológica António Xavier, Universidade Nova de

Lisboa, 2780-157 Oeiras, Portugal. 2

Centro de Química Estrutural, Instituto Superior Técnico, Universidade de Lisboa,

Avenida Rovisco Pais, 1049-001 Lisboa, Portugal. 3

Department of Chemical Engineering, Universiti Teknologi PETRONAS, Seri Iskandar

32610, Perak, Malaysia.

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ABSTRACT The efficient and sustainable separation of azeotropic mixtures remains a challenge in chemical engineering. In this work, the performance of benign solvents, namely Deep Eutectic Solvents (DES) in the separation of aromatic-aliphatic hydrocarbons azeotropic mixtures via liquid-liquid extraction (LLE) is evaluated. The DES studied in this work were based on different ammonium salts (Cholinium Chloride, [Ch]Cl, BenzylCholinium chloride, [BzCh]Cl, and Tetrabutylammonium chloride, [N4444]Cl) as Hydrogen Bond Acceptor (HBA) and one organic acid (levulinic acid, LevA) as Hydrogen Bond Donor (HBD), always in the mole ratio of 1 HBA : 2 HBD. The thermophysical properties, namely density and viscosity, of the three used DES were measured in the temperature range of T = (293.15 up to 353.15) K and at atmospheric pressure. The phase equilibria diagrams of all ternary systems were determined at T = 298.15 K and at atmospheric pressure using 1H-NMR spectroscopy. The results showed that the introduction of a more hydrophobic HBA in the DES promotes the improvement of the distribution coefficient, while playing with the aromaticity of the DES leads to higher selectivity. In addition, the performance of the predictive Conductor-like Screening Model for Real Solvent (COSMORS) model in the description of these systems was also evaluated. COSMO-RS is capable to quantitatively predict the phase behavior and tie lines for ternary mixtures containing DES as well as to estimate the trend of distribution ratio and selectivity.

KEYWORDS: Deep Eutectic Solvents, Aromatic-Aliphatic Azeotropic Mixtures, LiquidLiquid Extraction, COSMO-RS.

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INTRODUCTION In many areas of industry, particularly in petrochemical industry, solvent mixtures accumulate due to recycling difficulties. Therefore, their efficient separation into pure components is mandatory so that they can be reused. However, many of these solvent mixtures contain azeotropes, whose separation is still an issue in chemical engineering. In particular, the separation of aromatic hydrocarbons, namely toluene, benzene, xylenes and others, from its mixture with aliphatic hydrocarbons, still remains an enormous challenge since these compounds have close boiling points and several combinations of them can form azeotropes.1 The conventional industrial processes commonly used in the separation of aromatic and aliphatic hydrocarbon mixtures are azeotropic or extractive distillation and liquid-liquid extraction (LLE). LLE presents economic advantages compared to the other processes since it requires low energy demand. However, the conventional extraction solvents typically used in industry are organic solvents, such as sulfolane,2-4 tetraethylene glycol5 and Nmethylpyrrolidone (NMP),6 which require additional investments and energy consumption for their recovery and are considered toxic, volatile and/or flammable.7 Consequently, their replacement by greener alternatives has been a topic of interest over the past several years. In this context, ionic liquids (ILs), which are known for their exceptional combination of properties, such as negligible vapour pressure, low flammability and high solvation capacity for a wide range of inorganic and organic materials, have been proposed as candidates for the replacement of conventional organic extractants. Due to their properties, it is expected that the use of ILs in the extraction of aromatics from aromatic/aliphatic mixtures leads to more sustainable separation with less energy consumption and smaller number of process steps. A large number of articles have reported the use of different 3 ACS Paragon Plus Environment


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families of ILs, such as imidazolium, pyridinium or ammonium-based ILs. In particular, imidazolium-based ILs combined with several anions, such as [OAc]-, [SCN]-, [NTf2]-, [HSO4]-, [CH3SO4]-, [C2H5SO4]-, [CF3SO3]- and others, have been deeply explored as suitable extraction solvents in the separation of different aromatic/aliphatic hydrocarbon mixtures, and some ILs showed higher separation factors than those of the conventional sulfolane.8-17 Nevertheless, and despite their promising and clear advantages, the environmental impact of ILs has been shown as mostly dependent on the chemical structure of the IL, which means that these fluids are not universally green.18 Moreover, the complex ILs synthesis and purification lead to disadvantageous high cost of production of some ILs. Deep eutectic solvents (DES) have been recently emerged as potential alternatives to ILs since their physicochemical properties are similar to those of ILs.19, 20 DES are mixtures of an hydrogen-bond acceptor (HBA), such as halide salts, and, typically, an hydrogen bond donor (HBD), viz. carboxylic acids, alcohols or amines, that present a much lower melting point than the pure components. In addition, DES can be synthesized from non-toxic, cheap, biodegradable and biocompatible materials, using simple methods or preparation, heating and mechanical stirring or grinding, without the need of further purification steps. This fact is probably the main advantage of DES over ILs. Recently, DES have been tested as extraction solvents for the separation of aromatic/aliphatic mixtures. For instance, DES based on tetrabutylphosphonium bromide and ethyltriphenylphosphonium iodide as HBA and ethylene glycol or sulfolane as HDB in the separation of toluene from n-heptane were reported by Kareem et al..1,

21

The same group also tested DES based on

methyltriphenylphosphonium bromide as a salt and also ethylene glycol as HBD for the separation of benzene from hexane.22 Generally, it was observed a similar or higher performance of DES compared to conventional solvents or ILs. Moreover, Mulyono et al.23 4 ACS Paragon Plus Environment

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studied DES based on an ammonium salt and sulfolane as HBD in the LLE of BTEX aromatics (benzene, toluene, ethylbenzene and xylenes) from n-octane. DES based on choline chloride and three different HBD, such as ethylene glycol, levulinic acid and glycerol, were already reported by our group as promising alternatives to ILs in the separation of ethanol/n-heptane mixtures, since remarkably high selectivities and distribution coefficients were obtained.24 In this context, in this work we explore the performance of sustainable DES based on three different salts, namely choline chloride, benzylcholine chloride and tetrabutylammonium chloride, and levulinic acid as hydrogen bond donor, in the separation of toluene from n-heptane via liquid–liquid extraction at T = 298.15K. The thermophysical properties (density and viscosity) were also measured and discussed since they influence the mass transfer operations in extraction processes. In addition, the capability of Conductor-like Screening Model for Real Solvent (COSMO-RS) to predict the phase equilibrium of ternary mixture containing DES was evaluated. COSMO-RS is a quantum chemical based prediction method25, 26 that has been widely used to predict liquid-liquid phase equilibria of binary and ternary mixtures containing ionic liquids and hydrocarbon.27, 28 However, to our knowledge, this work is the first to address the capability of the model to predict ternary phase diagrams containing DES.

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EXPERIMENTAL SECTION Materials. Choline Chloride ((2-hydroxyethyl)trimethylammonium chloride, ≥ 98 wt%), BenzylCholine Chloride (Benzyl(2-hydroxyethyl)dimethylammonium chloride, ≥ 97 wt%), [N4444]Cl (tetrabutylammonium chloride, ≥ 97 wt%) and Levulinic Acid (98 wt%) were purchased from Sigma-Aldrich. The chemical structures, respective acronyms and mole ratio of the DES used in this work are presented in Table 1. The components of the azeotropic mixture, n-heptane and toluene, were purchased from Carlo Erba (99 wt%) and Sigma-Aldrich (99.9 wt%), respectively, and were used without any further purification.

Synthesis and Characterization of DES. In this work, three DES, namely [Ch]Cl:LevA (DES1), [BzCh]Cl:LevA (DES2) and [N4444]Cl:LevA (DES3) were synthesized in a mole ratio of 1 HBA : 2 HDB using the grinding method,29 in which both components were first mixed and then grinded in a mortar with a pestle at room temperature in order to obtain a homogeneous liquid. In order to reduce the water and other volatile substances contents, the DES were dried under vacuum (1 Pa) at room temperature for at least 3 days. The water contents of the dried DES were measured by Karl Fischer coulometric titration and all the DES contained less than 0.5 wt% H2O. The thermophysical properties, namely density, ρ (kg·m-3), and viscosity, η (Pa·s) of the synthesized DES were measured within the temperature range (293.15 and 353.15) K and at atmospheric pressure using an SVM 3000 Anton Paar rotational Stabinger viscometerdensimeter. The standard uncertainty for the temperature is 0.02 K. The repeatability of density and viscosity of this equipment is 0.5 kg·m−3 and 3.5·10-6 Pa.s, respectively. Triplicates of each sample were performed to ensure accuracy and the reported results are


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average values. Furthermore, the relative standard uncertainty in density and viscosity was calculated by the ratio of the standard deviation and the average of the three replicates of each sample, where the highest relative standard uncertainty registered for the density and dynamic viscosity measurements was 0.2 and 2·10-5, respectively.

LLE Measurements. The LLE consisted of two steps: in the first step, preparation of calibration curves to determine the concentration of toluene in n-heptane or DES phase was carried out, while in the second step the tie-lines for each one of ternary mixtures were determined. The calibration curves were constructed using binary mixture of n-heptane + toluene and toluene + DES at different mole fractions, depending on the solubility of toluene in the respective solvent. All the binary mixtures were stirred using a magnetic stirrer for at least 1h. After the equilibrium, each sample was analyzed using 1H-NMR spectroscopy (Bruker Avance II+ 500 MHz NMR spectrometer) in order to obtain the calibration curves, integration of the area under the peak vs. mole fraction. For the NMR analysis, the samples were places in NMR spectroscopy sealed tubes with dimethyl sulfoxide-d6 (DMSO-d6) as solvent. The deuterated solvent was not mixed with the sample but kept inside a sealed capillary tube, which was then introduced inside the NMR tube, minimizing the amount of solvent required. The DMSO-d6 was used for the external lock of the NMR magnetic field and the greatest quantitative agreement was found when selecting, for [C2mim][OAc], the peaks corresponding to the imidazolium cation (for instance, the methyl group bonded to the nitrogen atom of the imidazolium ring) and for the studied DES, the peaks corresponding to their HBA (for DES1 and DES2, the peaks of the methyl groups bonded to the nitrogen atom and for DES3, the peaks of the terminal methyl groups). 7 ACS Paragon Plus Environment


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After the calibration curves were established, the next step was determination of tie-lines for each ternary mixture. In this step, ternary mixtures of known composition were prepared in vials and were vigorously stirred by magnetic agitation for 1h and left to reach equilibrium for at least 24h in an incubator at 25ºC to ensure the complete phase separation. This period of time was checked to be sufficient for reaching the equilibrium. Then, each phase was carefully sampled to avoid cross contamination of the phases, and both phases were analysed by 1H-NMR spectroscopy using the capillary method previously described to establish the calibration curves. The composition of each phase was calculated by the previously obtained calibration curves.

COSMO-RS prediction. Conductor-like Screening Model for Real Solvent developed by Klamt and co-worker,30 is a predictive model that integrates the concepts of quantum chemistry, dielectric continuum models, electrostatic interactions, and statistical thermodynamic. Numerous theoretical documentation and evaluation of many COSMO-RS application performance can be found in literature. In general, the model has been proven to correctly describe the solvent-solute solvation behaviour in a real solvent system qualitatively and to some extent, quantitatively.28 At present time, COSMO-RS represents one of the most efficient models to predict the thermodynamic properties of pure compound, such as ionic liquids, and its mixture with organic compounds. In this work, we used this model to predict the LLE behaviour between the studied DES, aromatic, and aliphatic compounds and to better understand the mechanism underlying these separations. The first step for COSMO-RS calculation is to produce the required .cosmo file for all compounds present in the ternary mixtures. In this step, the continuum solvation COSMO calculations of electronic density and molecular geometry were performed using the 8 ACS Paragon Plus Environment

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TURBOMOLE 6.1 program package on density functional theory level, utilizing the BP functional B88-P86 with a triple-ζ valence polarized basis set (TZVP) and the resolution of identity standard (RI) approximation.31 After the calculation was finished, the produced .cosmo file was then used as input for the next calculation. In the second step, the estimation of liquid-liquid equilibrium of the studied ternary mixtures was performed with COSMOtherm program utilizing the parameter file BP_TZVP_C30_1302 (COSMOlogic GmbH & Co KG, Leverkusen, Germany). The equilibrium for a ternary liquid-liquid system is defined by the following equation:

xiI γiI = xiII γiII

(1)

where γi is the activity coefficient of component i in a phase I or II, and xi is the mole fraction of component i in phase I or II. Both γi and xi are solely predicted using COSMORS. It should be highlighted that during all calculation, the DES is treated as complex neutral molecule.

RESULTS AND DISCUSSION Density and Viscosity Measurements. The thermophysical properties, namely density and viscosity, are significant parameters that influence the selection of proper extraction solvent in LLE, since, for instance, high viscosities lead to mass transfer limitations and also operational costs related to liquid pumping and dispersion problems. Bearing this in mind, the thermophysical characterization of the studied DES was performed in this work, except for DES1, which thermophysical properties were previously reported by our group.32 The density, ρ (kg·m-3), and viscosity, η (Pa·s) values of the three DES studied in this work are given in Table S1 in the Supporting Information. Since the studied DES are composed by



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the same HBD (Levulinic acid), it allows us to study the impact of HBA chemical structures toward density and viscosity of the obtained DES. As can be seen from Figure 1a, DES2 has the highest density, closely followed by DES1, while DES3 has the lowest density. The observed difference on the density of the DES might be attributed to the different HBA. DES1 and DES2 are formed by HBAs that contain one hydroxyl group that enables the further establishment of hydrogen bonds with HBD, leading to more close arrangement between HBA and HBD, and ultimately, to higher density. On the other hand, the lack of hydroxyl group of the HBA in DES3 is responsible to its lower density when compared to the other two DES. Similar behaviour was also observed previously by Kroon et al.33 for DES based on choline chloride and tetramethylammonium chloride as HBAs. Thus, the presence of the hydroxyl group in the HBA forming the DES might have significant impact in its density. The density of the studied DES decreased with increasing temperature – a general trend observed for organic compounds and ILs. The density decrement shows a linear behaviour with the temperature increment, and thus the density values were fitted as a linear function of temperature, T (K), by the least squares method using the linear expression given by Equation 2:

= +

(2)

where a and b are adjustable parameters and ρ is density. The reliability of Equation 1 was evaluated based on the coefficient determination, R2, and the relative standard deviation, σ, which was calculated using the following Equation:


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= ∑

, , ,

(3)

where ρexp,i is the experimental data, ρcalc,i is calculated from the calculation and N is the total number of experimental points. The a and b parameters, along with the R2 and σ values are given in Table 2. From the correlation coefficient, R2, listed in Table 2, and also the standard deviation values, it can be concluded that the use of a linear function adequately describes the measured density data for the studied temperature range. From the temperature dependence of density, the molar volumes (Vm) of the studied DES were calculated using Equation 4: =

(4)

where ρ corresponds to the density (g·cm-3) and M is the molar mass (g·mol-1) of the DES. The calculated molar volume values are given in Table S1 in the Supporting Information and are depicted in Figure 1b.

In general, the molar volumes of all DES slightly increase

with increasing temperature. Within the studied temperature range, the molar volume can be ranked as: DES1 < DES2 < DES3. This rank of molar volumes of the studied DES is similar to the rank of their molar mass, where DES1 (123.95 g·mol-1) < DES2 (149.32 g·mol-1) < DES3 (170.05 g·mol-1). Thus, it is interesting to note that the presence of different HBA plays minimal role in molar volume, since the differences between the molar volumes of the studied DES are similar to those observed for the molar mass. Regarding the viscosity of the studied DES, from Figure 1c, it can be seen that DES2 presents the highest viscosity, followed by DES3 and DES1 has the lowest viscosity values. The highest viscosity of DES2 in comparison to DES1 might be attributed to the presence of the aromatic ring in its structure that, besides largely increasing the DES molar volume,



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might promote additional π-π interactions. Unlike density, it seems that the presence of hydroxyl group in the HBA does not have significant impact on viscosity, since the viscosity of DES3 lays in between that of DES1 and DES2. These intermediate viscosity values might be attributed to the longer alkyl chains of ammonium cation in the HBA of DES3, since and as already observed in the literature,34 an increase in the alkyl chain length of the quaternary ammonium cation, increases the area of contact between the molecules, which also leads to an increase of the dispersive forces between the molecules and consequently, high viscosities. Similar to density, the viscosity of the studied DES decreases with increasing temperature. A very significant decrement of viscosity is observed for DES2, from 2.026 Pa·s (at 293.15 K) to 0.04 Pa·s (at 353.15 K) in the studied temperature range. The experimental viscosity values were fitted as a function of temperature, using the Vogel−Fulcher−Tammann (VFT) model described in Equation 5: &'

ln # = $% + ()

'

(5)

where η is the viscosity in mPa·s, T is the temperature in K, and Aη, Bη, and Cη are adjustable parameters. The adjustable parameters, which were determined from the fitting of the experimental data, are listed along with the relative standard deviation, σ, and the energy barrier of a fluid to shear stress, E, (kJ·mol-1) at T = 298.15 K, in Table 3. The σ values were calculated using Equation 3 and the energy barrier was determined based on the viscosity dependence with temperature using Equation 6, as follows:35 * = +.

-./ % 1 -0 3 2

= +. 4

&'

67 ' 76' 5 7 8 9 2 2

:

(6)


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where η is the viscosity, T is the temperature, Bη and Cη are the adjustable parameters obtained from Equation 5, and R is the universal gas constant. The higher the energy barrier value, the more difficult it is for the DES molecules to flow. This fact can be a direct consequence of the size or entanglement of the molecules and/or of the presence of stronger interactions, for instance, H-bonding interactions and π-π interactions, in the fluid. From Table 3, it can be observed that DES2, containing [BzCh]Cl as HBA, presents a higher E value, due to the presence of the aromatic ring in its structure, than DES1 that contains [Ch]Cl as HBA. The E values of the three DES studied can be ordered as follows: DES1 < DES3 < DES2, which are in agreement with the experimental viscosity values obtained.

LLE measurements. The use of 1H-NMR spectroscopy as quantification method for other ternary systems containing ionic liquids was already tested and reported in literature.8, 9, 36, 37

To further validate the NMR method used in this work, the tie-lines of the ternary system

n-heptane + toluene + [C2mim][OAc] measured in this work were compared to literature data that was obtained using a conventional method.16 Figure 2 shows a comparison between the literature and experimental (this work) tie-lines of this ternary system at T = 298.15 K. As it can be seen from Figure 2, the tie-lines determined by 1H-NMR spectroscopy are in a good agreement with those reported in the literature determined by the conventional method at the same temperature (rmsd = 0.9%), which allow us to validate 1H-NMR spectroscopy as quantification method. In this context, the performance of DES1, DES2 and DES3 in the specific separation of toluene from n-heptane was studied using the validated methodology. 13 ACS Paragon Plus Environment


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For all the studied ternary systems, the composition of each one of the DES in the nheptane-rich phases could considered to be negligible, since 1H-NMR spectra showed no detectable signals corresponding to DES. The 1H-NMR spectra of a typical upper (heptanerich phase) and bottom phases (DES-rich phase) of the studied ternary systems are provided in Supporting Information (Figures S1-S6). From these spectra, it can also be observed that the proportion of all the DES is maintained for the three ternary systems and also for all the studied compositions. This leads to the conclusion that, contrary to what happens in aqueous systems,38,

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dissociation of the DES in these organic solvent mixtures is not

observed and thus, DES are suitable solvents to carry out the proposed application. The experimental liquid-liquid phase equilibria data of the three ternary systems were measured at T = 298.15 K and atmospheric pressure and are presented in Table 4 and plotted through a ternary diagram in Figures 3, 4 and 5. It should be highlighted that the compositions of both phases in equilibrium were calculated using calibration curves established in this work. From Figures 3 and 4, it can be seen that DES1 and DES2 are nearly immiscible to nheptane. Also, although the solubility of these two DES in toluene is very similar, DES2 presents a slightly higher solubility value, due to the presence of the aromatic ring in the HBA. Moreover, a different behaviour was found when the HBA was changed to a quaternary ammonium salt with higher hydrophobicity, [N4444]Cl. DES3 containing this salt as HBA presents higher solubility in toluene compared to DES1 and DES2, as it can be seen in Figure 5. The very low solubility of the studied DES1 and DES2 in the heptane-rich phase minimizes the loss of the DES and consequently, the contamination of the refined stream. However, the cross contamination in DES3 is very high, thus requiring a difficult


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solvent recovery stage. The recovery of entrainers is usually carried out using distillation. Therefore, thermal studies will be required before the introduction of DES in a refinery. It can also be observed from Figures 3-5 that the tie-lines obtained for all the studied ternary systems exhibit negative slopes, leading to distribution coefficients lower than the unity and consequently, high amount of DES will be required for this extraction. Despite the fact that DES are cheap and easy to synthesize, this drawback can be overcome by recovering and reusing the extraction solvent. Distribution coefficient, β, and selectivity, S, are the two crucial parameters broadly used in the evaluation of extraction solvent suitability to perform LLE. Thus, these two parameters were calculated from the experimental data as follows: ; =

=>?@ AB

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