Effect of Hydrogen Bond Donor on the Physical ... - ACS Publications

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Effect of Hydrogen Bond Donor on the Physical Properties of Benzyltriethylammonium Chloride Based Deep Eutectic Solvents and Their Usage in 2‑Ethyl-Hexyl Acetate Synthesis as a Catalyst M. Bengi Taysun, Emine Sert,* and Ferhan S. Atalay Department of Chemical Engineering, Ege University, 35040 Iż mir, Turkey ABSTRACT: Deep eutectic solvents (DES) containing benzyl triethylammonium chloride (BTEAC) as a hydrogen bond acceptor (HBA) and p-toluene sulfonic acid (PTSA), citric acid (CA), and oxalic acid (OX) as a hydrogen bond donor (HBD) were formed at their respective eutectic points. The physical properties such as pH, ionic conductivity, viscosity, density, and refractive index were measured between 293 and 333 K. Viscosity values as low as 0.21 Pa·s and conductivity values as high as 8 mS/cm were achieved, where the pH values of each DES proved to be extremely low. The effect of HBD on the physical properties was investigated and was found to be very significant. Also, the catalytic application of BTEAC based DES in the esterification reaction of the acetic acid with 2-ethyl-hexanol was studied, and the activation energy was obtained using initial reaction rates. The results showed that very high initial reaction rates and low activation energy can be achieved when catalyzed by DES which was formed using BTEAC and PTSA.

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INTRODUCTION In recent years, deep eutectic solvents (DESs) have gained increasing importance due to their potential in the context of green chemistry. DESs are made by mixing two different chemicals, a hydrogen bond donor (HBD) and a hydrogen bond acceptor (HBA). Due to hydrogen bond interactions between them, DESs have lower freezing points than either HBD or HBA. When prepared from biocompatible chemicals, DESs have a big potential as environmentally friendly chemicals.1 DESs share many physical characteristics with ionic liquids. But unlike ionic liquids, which are comprised entirely of ions,2 they can be formed by using nonionic species.3 Also unlike ionic liquids, of which their production may have a great impact on the environment, the preparation of DES is simple.4 Simple heat mixing is enough to prepare DES. Also, by switching the HBA and HBD, it is easily possible to prepare purpose-oriented DESs or to prepare DESs with specific physical properties. The importance of determining the physical properties of DESs cannot be overstated. Density and viscosity are the two most important properties for flow behavior. DESs containing carboxylic acids such as HBD have relatively high viscosities and densities caused by complex hydrogen bond interactions between HBA and HBD. But with switching the HBA and HBDs some researchers recorded very low viscosities with DESs.5,6 In particular, PTSA and OX are strong candidates for low-viscosity DESs. The viscosity and density of choline chloride (ChCl) and OX based DES were researched by several researchers, and the viscosities were reported as low as the 0.1 Pa·s range.1,5,7 In a study by Zhao et al.,1 the density and © XXXX American Chemical Society

viscosity values for ChCl-PTSA based DES was reported around 0.18 Pa·s and 1.21 g/cm3. The same study showed a contrasting high viscosity of ChCl-CA based DES. Literature also includes studies focused on the HBD effect on the physical properties of DESs. One of the most important advantages of DES is the ability to achieve desired properties by changing HBD according to the application area. Many studies showed that the freezing points of corresponding DESs were strongly affected by the nature of the HBD.4 Yusof et al.8 studied the densities, ionic conductivities, and viscosities of DES composed of tetra butyl ammonium bromide (TBABr) coupled with ethylene glycol, 1,3-propanediol, 1,5-pentanediol, and glycerol as hydrogen bond donors. They showed that a DES composed of TBABr and glycerol has a higher viscosity, density, and lower ionic conductivity compared to other DESs which was caused by glycerol’s extra hydroxyl group. Also, comparing the viscosities of ChCl based DESs, it is clear that the viscosity of DES is also highly dependent on the HBD. While DESs based on the ethylene glycol exhibited low viscosities,9 carboxylic acid based DESs show higher viscosity values.9,10 The partial ionic nature and low viscosities of DESs can provide high ionic conductivities. In general, DESs have relatively low ionic conductivities corresponding to their high viscosities. But thanks to the easy-to-tailor nature of DESs, high ionic conductivity DESs were synthesized. Siongco et al.11 Received: June 13, 2016 Accepted: March 3, 2017

A

DOI: 10.1021/acs.jced.6b00486 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

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the chemicals were used as received. Table 1 shows the properties of the chemicals used in this study.

studied the electrical conductivities of ethylene glycol based DESs and reported the ionic conductivities in excess of 30 mS/ cm at room temperature. However, the carboxylic acid based DESs generally showed low conductivity values, especially at low temperatures.6 DESs also have a great potential as catalysts. But the testing of catalytic activity of DESs has very limited published information in literature. The HBA of this study, BTEAC, is often employed in various organic reactions as a phase transfer catalyst.12,13 But there is no any study of its employment as an HBA in DESs in literature. De Santi et al.14 studied the employment of a DES made up from quaternary ammonium methanesulfonate salts and PTSA on the esterification of several carboxylic acids with alcohols. They showed high activity and a reusability of the DES with a mild, safe, and ecofriendly method. Also a DES was employed by Alhassan et al.15 for a waste tire pyrolysis upgrade, and an improved product quality was achieved. There is a growing trend in catalytic usage of DESs with promising results.14−16 But there is no known study about the catalytic application of BTEAC based DESs. This study investigates the physical properties of BTEAC based three different DESs, namely, BTEAC-PTSA (DES A), BTEAC-CA (DES B), and BTEAC-OX (DES C), and also the temperature dependence of the physical properties. The catalytic application of synthesized DESs, which are shown in Figure 1, were tested in the liquid phase esterification of acetic

Table 1. Chemicals Used in the Study chemical name

initial mole fraction purity

purification method

Merck

0.99

none

ABCR

0.98

none

SigmaAldrich SigmaAldrich AKKIM ABCR ABCR

0.99

none

0.99

none

0.99 0.99 0.99

none none none

source

benzyl triethyl ammonium chloride (BTEAC) p-toluene sulfonic acid monohydrate (PTSA) citric acid monohydrate (CA) oxalic acid dehydrate (OX) acetic acid 2-ethyl-1-hexanol 2-ethyl-hexyl acetate

Preparation of DES and Determination of the Eutectic Point. HBA and HBD were weighted and placed in a flask. This mixture is thoroughly mixed using a vortex mixer, ensuring the homogeneity of the solid mixture. Then the mixture was heated at 353 K and stirred by a magnetic stirrer with temperature control. After the mixture transformed into a clear and homogeneous liquid structure, periodic weightings were performed to confirm the removal of the water content present in DES. The weight values were compared to the theoretical water amount of DES. After reaching theoretical water content, the preparation procedure was continued for an additional 2 h to ensure constant weight. The DES samples were then immediately placed into aluminum-Teflon capped glass airtight vials. The freezing points of the samples for the HBA molar concentration range of 20−70% by 10 molar percent intervals were measured. For the freezing point investigation, a special laboratory setup was used. The setup consisted of a modified temperature controlled alcohol−water bath. To determine the freezing points of the DESs, they were filled into airtight glass bottles with 3 mL of volume and placed in the alcohol bath. The temperature was lowered slowly, 2 K per hour, and the vials were taken out and placed horizontally at each half hour. The point at which flow is not observed within 5 s is considered as the freezing point of the DES. Measurement of Physical Properties. The detailed procedures and conditions for measurements of physical properties were stated in the previous study.17 Table 2 summarizes the measurement procedures. For the pH, viscosity, and conductivity measurements, the temperature variation was provided using WiseStir MSH-20D magnetic heater stirrer modified to operate as a temperature-

Figure 1. Molecular structures of the synthesized DESs.

Table 2. Equipment for the Measurements acid with 2-ethyl hexanol. Also, an activation energy and Arrhenius factor were determined using the initial reaction rates.

device

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density

EXPERIMENTAL STUDY Materials. Benzyl triethylammonium chloride was used as an HBA. As an HBD, p-toluene sulfonic acid monohydrate, citric acid monohydrate, and oxalic acid dihydrate was used. Acetic acid and 2-ethyl-1-hexanol were used as the reactants for the esterification reactions. For the gas chromatography analyses 2-ethyl-hexyl acetate (ABCR, 99%) was used. All of

viscosity ionic conductivity pH refractive index B

Anton Paar (model DMA 35) Fungilab (model Expert R) Hanna (model HI 9033) WTW (model pH7110) SOIF (model WAY2S)

calibration method using chemicals with known densities the built-in test function of the device using sample solutions with known conductivities using buffer solutions using deionized water and ethanol



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both DES B and DES C were found at the HBA:HBD mole ratio of 1:1. For DES A, the eutectic point was achieved at 3:7 HBA:HBD mole ratio. The freezing points of the DESs reflected their HBD’s freezing points suggesting a strong HBD influence on the phase behavior of the DESs. Table 4 shows the freezing points of the studied DESs with respect to the HBA content.

controlled oil bath. Uncertainties for temperatures and weightings were 0.1 K and 0.0001 g, respectively. For fresh and used eutectic solvents, 1H and 13C nuclear magnetic resonance spectroscopy (NMR) spectra were recorded in deuterium oxide on a 400 MHz high-performance digital FT-NMR. NMR spectroscopy measurements were achieved with Varian VNMR 400 model spectrometer. Frequencies for 1H and 13C measurements were 400 and 100 MHz, respectively, with deuterated chloroform used as solvent. Procedure. The esterification reactions were conducted in a three-necked glass flask reactor placed on the magnetic heater stirrer. A condenser was fitted to the reactor to prevent any possible loss of material during the experiments through evaporation. Stirring was kept at 1000 rpm for all reactions to circumvent any possible concentration gradients in the reactor content. At the start of the experiments alcohol was quickly brought up the reaction temperature, and at this point a catalyst was added. After stabilization of the temperature, preheated acid is added to the reactor, and the reaction is started. Great care was given to this procedure to prevent an autocatalytic reaction between the alcohol and the acid.18 Samples were taken at determined times, and an analysis was performed immediately. The operating conditions of the esterification reactions are summarized in Table 3.

Table 4. Freezing Points of the Synthesized DES with Respect to the HBA Contenta DES A

DES B

DES C

Table 3. Operating Conditions of the Esterification Reactions catalyst

catalyst loading (wt %)

DES selection study of reaction progress kinetic study

DES A, B, C DES A

10 10

DES A

10

reusability study

DES A

10

temperature (K)

alcohol to acid ratio 1:1

180

1:1

373, 363, 353 373

1:1, 1:2, 1:3 1:1

5, 60, 120, 180 5

freezing point (K)

20 30 40 50 60 70 30 40 50 60 70 30 40 50 60 70

292 275 279 289 293 314 308 301 299 300 330 301 284 278 306 335

a Standard uncertainties are u(T) = 0.1 K, u(x(BTEAC)) = 1.3 × 10−5, and u(p) = 0.5 kPa. All measurements were performed at 101.3 kPa.

duration (min)

373, 363, 353 373, 363, 353

mol % BTEAC

Characterization of DES. After deciding the eutectic point of each DES, the physical properties such as pH, viscosity, conductivity, refractive index, and density were measured. The pH is the most important property for the catalytic application. Esterification reactions require acidic catalysts to initiate the reaction. Just like other physical properties, the pH values were also measured at temperatures between 293 and 333 K. Due to the increasing dissociation of acid in elevated temperatures, a slight drop of the pH is expected. However, this expectation was not fulfilled, and all of the DESs showed almost constant pH values through the temperature range. The pH values for DES A, B, and C were found as −1.5, 1, and −0.8, respectively. Low pH values are noteworthy especially considering the catalytic applications of DESs on the esterification reactions which require acidic catalysts. Comparing three HBDs, PTSA is the strongest acid followed by OX and CA. This behavior is also observed for the pH values of the synthesized DESs, this observation shows a high HBD effect on the pH of the synthesized DES. The viscosity of the DES directly affects its hydrodynamic application. It is the most important parameter for flow behavior. Generally DESs exhibit relatively high viscosities, which can be attributed to the extensive hydrogen bonding network inside them causing a lower mobility for the free species.20 Among the studied DESs, DES B showed a very high viscosity. Due to the high viscosity of this DES, measurements could not be made below 313 K for this DES. DES B, with four hydroxyl groups of CA, produces a complex network of hydrogen bonds which strongly increases its viscosity. Although OX has a higher number of hydroxyl groups than PTSA, hence a higher number of hydrogen bonds within the resultant DES, the viscosity of DES C proved to be lower. This can be attributed to the neat and tidy molecular

180

Analysis. An Agilent 7890A model gas chromatograph was used to analyze the samples and determine the acetic acid conversion. The configuration of the gas chromatography device and operating conditions were explained in the previous study.17

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RESULTS AND DISCUSSION First, the eutectic points of synthesized DESs were determined by measuring the freezing points of the DES samples having different HBA:HBD ratios. The eutectic point describes the molar ratio of HBA to HBD which provides lowest freezing point. Then, the physical properties of the synthesized materials were measured at the eutectic point to characterize the DES. Then finally, the synthesized and characterized DES samples were used as a catalyst in the esterification reaction of acetic acid to investigate the catalytic activities. Eutectic Point Determination. The depression of freezing point is the defining property of the DESs. The lattice energy of the resulting HBA:HBD complex and the entropy changes in the liquefaction are important factors affecting the freezing point.1 Studies found in literature prove that the freezing point depression is strongly linked to the HBA:HBD mole ratio.4,14,19 The lowest freezing point achieved for DES A was 275 K, for DES B 299 K, and for DES C 278 K. The eutectic points for C



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structure of OX. In literature, the van der Waals and hydrogen bonding interactions are believed to control the viscosity of deep eutectic solvents. The increase of viscosity for some deep eutectic solvents compared to typical solvents was found to increased van der Waals forces relative to the hydrogen bonding interactions.21,22 These findings suggest that the molecular structure of HBD plays an important part on the resultant DES. Table 5 shows the viscosity values of the studied

the HBA:HBD ratio affects ionic conductivity, and in general increasing the salt content increases the ionic conductivity.9 The eutectic point of DES C was determined as an mole ratio of 1:1, whereas the eutectic point of DES A was found as an HBA:HBD ratio of 3:7, and it is clear it contains more salt. Higher salt content and the lowest viscosity values provided DES C with the highest ionic conductivity among the studied DESs. Table 7 shows the ionic conductivity values of the

Table 5. Viscosity Values of the Synthesized DESsa

Table 7. Conductivity Values of the Synthesized DESsa

η (Pa·s)

σ (mS/cm)

temperature (K)

DES A (30 mol % BTEAC)

DES B (50 mol % BTEAC)

DES C (50 mol % BTEAC)

temperature (K)




333.15 328.15 323.15 318.15 313.15 308.15 303.15 298.15 293.15

0.344 0.506 0.732 1.224 2.340 4.054 6.967 12.698 26.457

76.406 102.953 135.350 172.454 292.536

0.207 0.419 0.514 0.748 1.158 1.741 3.115 4.108 5.152

333.15 328.15 323.15 318.15 313.15 308.15 303.15 298.15 293.15

1.713 1.097 0.770 0.477 0.327 0.177 0.097 0.041 0.014

0.105 0.060 0.028 0.014 0.009 0.005 0.004 0.002 0.002

8.530 6.553 5.267 4.090 3.127 2.313 1.683 1.213 0.807

Standard uncertainties are u(T) = 0.1 K, u(x(BTEAC)) = 1.3 × 10−5, ur(η) = 0.1, and u(p) = 0.5 kPa. All measurements were performed at 101.3 kPa.

Standard uncertainties are u(T) = 0.1 K, u(x(BTEAC)) = 1.3 × 10−5, u(σ) = 0.03, 0.002, and 0.14 mS/cm for DES A, DES B, and DES C, respectively, and u(p) = 0.5 kPa. All measurements were performed at 101.3 kPa.

a

a

DES. Literature shows that the viscosity values of DESs follow an Arrhenius-like behavior.23,24 The results of fitting are shown in Figure 2 and Table 6. Considering the current and future

studied DESs. The effect of HBD on the conductivity showed a similarity to the viscosity measurements. DES C, with its lowest viscosity exhibited the highest conductivity, reaching above 8 mS/cm at 333 K. The conductivity of DES B is remarkably low, only reaching 0.1 mS/cm at 333 K. Similar to the viscosity values, conductivities can also be fitted to the Arrhenius like equation, and the results are shown in Figure 3 and Table 8.

Figure 2. Linearized viscosity values of the synthesized DESs.

Table 6. Viscosity Fitting Parametersa ln η = ln η0 + (Eη/RT) DES A DES B DES C a

η0

Eη

R2

3.521 × 10−15 1.402 × 10−7 1.460 × 10−11

8.885 × 104 5.563 × 104 6.528 × 104

0.9971 0.9806 0.9862

Figure 3. Linearized ionic conductivity values of the synthesized DESs.

Table 8. Ionic Conductivity Fitting Parametersa

η, η0 = Pa·s, Eη = J/mol, T = K.

ln σ = ln σ0 + (Eσ/RT) σ0

potential electrochemical applications of DESs, the ionic conductivity of the DES is a very important physical property to study. In general, the high viscosities of DESs cause low ionic conductivities. Like viscosity, ionic conductivity values also showed an Arrhenius like behavior which showed a strong correlation between the viscosity and ionic conductivity. Also

DES A DES B DES C a

D

R2

Eσ

8.791 × 10 9.130 × 1011 2.137 × 108 14

−9.320 × 10 −8.337 × 104 −4.708 × 104 4

0.9773 0.9642 0.9969

σ, σ 0 = mS/cm, Eσ = J/mol, T = K. DOI: 10.1021/acs.jced.6b00486 J. Chem. Eng. Data XXXX, XXX, XXX−XXX


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Table 9. Refractive Index Values of the Synthesized DESsa

The Walden plot can be used to express relations between the ionic conductivity and the viscosity for the ionic liquids. Since DESs are closely related to the ionic liquids same can also applied to the DESs. By comparing Walden plots of DESs to the 1 M aqueous KCI solution, which is representative of independent ions within the solution without interionic interactions, some conclusions can be made.3,25 Figure 4

temperature (K)




333.15 328.15 323.15 318.15 313.15 308.15 303.15 298.15 293.15

1.5352 1.5369 1.5386 1.5404 1.5423 1.5441 1.5456 1.5470 1.5484

1.5183 1.5198 1.5212 1.5233 1.5247 1.5264 1.5279 1.5293 1.5307

1.5042 1.5058 1.5074 1.5089 1.5108 1.5123 1.5140 1.5156 1.5172

Standard uncertainties are u(T) = 0.1 K, u(x(BTEAC)) = 1.3 × 10−5, u(nD) = 0.001, and u(p) = 0.5 kPa. All measurements were performed at 101.3 kPa. a

Table 10. Refractive Index Fitting Parameters nD = A + BT DES A DES B DES C

Figure 4. Walden plot of the KCl (aq) and DES A and DES C between the temperatures 293 and 333 K.

A

B

R2

1.647 1.623 1.613

−3.360 × 104 −3.153 × 104 −3.267 × 104

0.9972 0.9982 0.9997

Table 11. Density Values of the Synthesized DESsa ρ (g/cm3)

shows Walden plot and tendency of DES A and DES C to form ions. DES C is very close to the ideal line of the Walden plot, DES A which is composed of BTEAC and PTSA, is not located close to the KCl line. This can be attributed to the higher viscosity and lower ionic conductivity of DES A compared to DES C. If precisely known, the refractive index values can be used to characterize the DES. Also refractive index values can be used to determine the DES purity. Also, the refractive index values give information about the molecular interactions within the DES.26 The thermal expansion caused by the increasing temperature causes a density decrease which in turn gives a greater freedom of movement for the light rays passing through the medium. Due to this, the refractive index decreases as temperature increases. At the microscale, the electromagnetic wave passing through the medium creates a disturbance in the charges of the atoms in its way, proportional to the electrical susceptibility of the medium. This slows the electromagnetic wave’s phase velocity. Therefore, the refractive index values can reveal information about the degree of electrical polarizability. According to literature, the refractive index variation with respect to temperature is linear.26 The refractive index values and results of regression for the refractive index measurements can be seen in Table 9 and Table 10. Density is a very basic and important physical property for a compound. Precision knowledge about density is both crucial and necessary. Although methods to predict DES densities exists and provide accurate results,27,28 the determination of experimental data still holds the most important place. It is common knowledge that the density of most compounds increases with increasing temperature due to thermal expansion. Studied DESs showed similar behavior. DES A density values ranged from 1.1721 g/ cm3 to 1.1454 g/cm3 across the measurement range. In the same temperature range the density of DES C varied between 1.1767 g/cm3 to 1.1493 g/cm3. The DES B density could not be measured due to its very high viscosity. The recorded density values are shown in Table 11. Across the measurement

temperature (K)



333.15 328.15 323.15 318.15 313.15 308.15 303.15 298.15 293.15

1.1454 1.1485 1.1508 1.1543 1.1571 1.1605 1.1642 1.1684 1.1721

1.1493 1.1520 1.1549 1.1578 1.1609 1.1643 1.1680 1.1723 1.1767

a Standard uncertainties are u(T) = 0.1 K, u(x(BTEAC)) = 1.3 × 10−5, u(ρ) = 0.0015 and 0.0014 g/cm3 for DES A and DES C, respectively, and u(p) = 0.5 kPa. All measurements were performed at 101.3 kPa.

range the OX based DES showed higher viscosity values. This can be attributed to the molecular structure of OX. With the tidy and small molecular structure of the OX, the resultant DES has a higher density. Also as found in literature,4,8,27,29 the DES densities reflected their HBD’s density order. Experimental density values can be fitted into a linear equation. The linear equation and fitting results for density can be seen in Table 12. Catalytic Application of DES in the Esterification Reactions of Acetic Acid with 2-Ethyl Hexanol. DES Selection. The effects of temperature and time on the conversion of acetic acid were achieved using the most catalytically active DES. So, first the esterification of acetic Table 12. Density Fitting Parametersa ρ = A + BT A DES A DES B a

E

1.366 1.374

R2

B −4

−6.650 × 10 −6.773 × 10−4

0.9938 0.9910

ρ = g/cm3, T = K. DOI: 10.1021/acs.jced.6b00486 J. Chem. Eng. Data XXXX, XXX, XXX−XXX


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acid conversion values were achieved prior to 60th minute mark. This rapid reaction progresses show a great advantage of DESs compared to solid catalysts. Solid catalysts are subject to mass transfer limitations which can be circumvented by DESs due to their liquid nature. Determination of the Initial Reaction Rate. In the DES selection experiments, the highest conversion of acetic acid was achieved with DES A. Therefore, a kinetic study was performed using DES A. The activation energy and Arrhenius factor for the esterification of acetic acid with 2-ethyl hexanol were calculated using the initial reaction rates obtained at different temperatures. The synthesis reaction of 2-ethylhexyl acetate can be described by the following reaction;30

acid with 2-ethyl hexanol was conducted using the same conditions by changing the DES to select the most efficient DES. Reactions were conducted with the following conditions: catalyst loading of 10 wt %, alcohol to acid ratio of 1:1, temperatures of 353, 363, and 373 K, and a reaction time of 180 min. Experiments were also conducted without catalysts to see the performance of eutectic solvents in the esterification reaction. The conversion values of the acetic acid for each DES are given in Figure 5. The results show the superior catalytic

acetic acid + 2‐ethylhexanol ⇌ 2‐ethylhexyl acetate + water

Considering the rapid nature of the progress of the reaction, in the kinetic study, the initial reaction rate was calculated using the acetic acid conversion at the fifth minute. The rate of reaction according to acetic acid consumption based on the homogeneous reaction rate kinetics is as follows; ( −rA )0 = k(CA )0 (C B)0 ( −rA )0 =

Figure 5. Comparison of the catalytic activities of the synthesized DESs in the esterification of acetic acid with 2-ethyl hexanol.

dCA dX = (CA )0 A dt dt

(1)

where CA (mol/L) and CB (mol/L) are the concentrations of acetic acid and 2-ethyl hexanol, respectively, XA is the acetic acid conversion, −rA (mol/L min) is the reaction rate, and k is the reaction rate constant. The values of −rA and k were found using eq 1 and are shown in Figure 7 and Table 13. The temperature dependence of the reaction rate constant can be correlated using the Arrhenius equation;

activity of DES A at all temperatures. This result is confirmed by the highest acidity of DES A among the studied DESs. Further reactions were carried out catalyzed by DES A. During the esterification reaction between the acetic acid and 2-ethyl hexanol, samples were taken at every 60th minute up to 180th minutes. Analyses of these samples provided a comprehensive overview of the reaction progress. In addition to that at the fifth minute, a sample was taken for the determination of the initial reaction rate. Progress of the reaction in terms of acetic acid conversion is presented in Figure 6. Results showed that the

⎛ −E ⎞ k = k 0 exp⎜ a ⎟ ⎝ RT ⎠

Figure 6. Progress of the reaction between 2-ethyl hexanol and acetic acid catalyzed by DES A.

(2)

Figure 7. Variation of the initial reaction rate with temperature and reactant concentrations.

esterification reaction catalyzed by DES A proceeds with remarkable rapidness. This is a contrast to the autocatalyzed reaction which only in 180 min only managed to reach around third of the conversion values achieved by the DES A catalyst in 60 min. Using this DES as a catalyst, very high initial reaction rates were achieved, and initial reaction rates increased with increasing temperature. For all three temperatures, final acetic

Table 13. Reaction Rate Constants

F

temperature (K)

k (L/min mol)

R2

373 363 353

0.02863 0.02506 0.02293

0.9985 0.9998 0.9940



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where T is the temperature (K), R is the gas constant, Ea is the activation energy (kJ/mol), and k0 is the Arrhenius factor (L/ min mol). The Arrhenius plot of the reaction is shown in Figure 8. The activation energy and Arrhenius factor were

Figure 9. Reusability of the DES A catalyst.

Figure 8. Arrhenius plot for the esterification of acetic acid with 2ethyl-hexanol.

found as 12.126 kJ/mol and 1.417 L/min mol, respectively. Any study about the esterification reaction between 2-ethyl-hexanol and acetic acid could not be found in literature. Comparing this activation energy to the reported activation energies of some esterification reactions of acetic acid18,31−35 catalyzed by heterogeneous catalysts, it is clear that the activation energy is significantly low when DES A is used as catalyst. Besides high catalytic activity, one of the most important advantages of deep eutectic solvents is the recovery from the liquid reaction mixture and the reusability. Reusability of the Selected DES. The main disadvantage of the traditional homogeneous catalysts is that it cannot be used again and again. It is clear that any catalyst, however high its performance might be, must be reusable. This is dictated by the economic, environmental, and practical aspects of the catalytic processes. The reusability of DES A was studied for the esterification reaction between acetic acid and 2-ethyl hexanol using the following conditions: temperature of 373 K, catalyst loading of 10 wt %, and an alcohol to acid ratio of 1:1. When the experiments have been completed, the catalyst is easily separated from the reaction mixture via a separating funnel. The DES is not dissolved in the produced ester. The produced water and DES were separated, and the water was evaporated in a vacuum oven at 353 K for 4 h. Then the waterfree DES was reused in the same conditions. The results of the reusability study are shown in Figure 9. DES catalyst was used four times without any significant reduction of catalyst activity. To determine the condition of DES recovered after the reaction, NMR measurements were made, and the spectrum of both unused fresh DES and recovered DES can be seen on Figures 10 and 11. According to Abbot et al.9 DESs made by the quaternary ammonium salts and HBD are formed by the equilibrium shown below:

Figure 10. 13C NMR spectrum of (a) fresh and (b) used DES A.

Figure 11. 1H NMR spectrum of (a) fresh and (b) used DES A.

molecular structures. NMR spectrum data of BTEAC, PTSA, acetic acid, 2-ethyl hexanol, and 2-ethylhexyl acetate for comparing DES spectra were obtained from spectral database of National Institute of Advanced Industrial Science and Technology (AIST) of Japan.36 Also, NMR analysis of fresh and used DES A was conducted, and results were given in Figures 10 and 11. As shown from Figures 10 and 11, characteristic peaks of BTEAC and PTSA is present in both fresh and used DES A. Characteristic peaks of 2-ethylhexyl acetate present in the 13C spectrum of the recovered DES (δ = 175.35 ppm for CO resonance) and characteristic peaks of acetic acid present on the 1H spectrum of the recovered DES (δ = 8.50 ppm for −OH resonance).This shows that catalyst retains its chemical structure after usage and drying, and

[quaternary ammonium cation][Cl−] + HBD → [quaternary ammonium cation] + [HBD]Cl−

This reaction scheme suggests that both quaternary ammonium cation and anion along with the HBD retain their G



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subsequent drops in catalytic activity can be attributed to the presence of the reactants and products in the used DES.

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CONCLUSION Three DESs sharing common HBA and BTEAC were synthesized. The eutectic points of each DES were found. The physical properties of the pH, density, viscosity, refractive index, and ionic conductivity were measured between 293 and 333 K. The result showed an Arrhenius like behavior of the ionic conductivity and viscosity, whereas the density and refractive index values showed a linear relation with respect to temperature. The pH values of the studied DESs showed stability through the measurement range. The HBD effect of the DES’s physical properties were studied. A strong HBD influence on the density, viscosity, ionic conductivity, and pH was observed, and the information obtained is potentially very useful in tailoring DESs for specific purposes. Also the HBD effect on the catalytic activity of the DESs were found to be high. The kinetics of the PTSA-BTEAC DES catalyzed esterification reaction between acetic acid and 2-et-HeOH were investigated. The results showed the great effect of HBD on the physical properties.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Emine Sert: 0000-0002-0868-8597 Funding

This study was supported by TUBITAK (213 M 643), by Ege University 15MÜ H009 and EBIL̇ TEM 2015/BIL̇ /026 scientific research projects. Notes

The authors declare no competing financial interest.

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