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

Article abstract of DOI:10.1021/acs.jced.6b00486

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.

Full text of DOI:10.1021/acs.jced.6b00486

Article
pubs.acs.org/jced
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 Izmir, 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.
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.
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.
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
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
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
© XXXX American Chemical Society
A
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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
the chemicals were used as received. Table 1 shows the
properties of the chemicals used in this study.
Table 1. Chemicals Used in the Study
initial mole
purification
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 eco-
friendly 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
chemical name
source
Merck
fraction purity
method
benzyl triethyl ammonium
chloride (BTEAC)
0.99
0.98
0.99
0.99
none
none
none
none
p-toluene sulfonic acid
monohydrate (PTSA)
ABCR
citric acid monohydrate (CA) Sigma-
Aldrich
oxalic acid dehydrate (OX)
Sigma-
Aldrich
AKKIM
ABCR
acetic acid
0.99
0.99
0.99
none
none
none
2-ethyl-1-hexanol
2-ethyl-hexyl acetate
ABCR
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
calibration method
density
Anton Paar (model
DMA 35)
using chemicals with known
densities
EXPERIMENTAL STUDY
viscosity
Fungilab (model
Expert R)
the built-in test function of the
device
■
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
ionic
conductivity
Hanna (model HI
9033)
using sample solutions with known
conductivities
pH
WTW (model
pH7110)
using buffer solutions
refractive
index
SOIF (model WAY-
2S)
using deionized water and ethanol
B
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controlled oil bath. Uncertainties for temperatures and
weightings were 0.1 K and 0.0001 g, respectively.
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.
1
For fresh and used eutectic solvents, H 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.
1
Frequencies for H 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 Content
a
mol % BTEAC
freezing point (K)
DES A
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
DES B
DES C
Table 3. Operating Conditions of the Esterification
Reactions
a
Standard uncertainties are u(T) = 0.1 K, u(x(BTEAC)) = 1.3 × 10−5,
catalyst
loading
(wt %)
alcohol
to acid
ratio
temperature
(K)
duration
(min)
and u(p) = 0.5 kPa. All measurements were performed at 101.3 kPa.
catalyst
DES
selection
DES A,
B, C
10
373, 363,
353
1:1
180
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
study of
reaction
progress
DES A
10
373, 363,
353
1:1
5, 60,
120,
180
kinetic study DES A
10
10
373, 363,
353
1:1, 1:2,
1:3
5
reusability
study
DES A
373
1:1
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
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
a
a
Table 5. Viscosity Values of the Synthesized DESs
Table 7. Conductivity Values of the Synthesized DESs
η (Pa·s)
σ (mS/cm)
temperature DES A (30 mol % DES B (50 mol % DES C (50 mol %
temperature DES A (30 mol % DES B (50 mol % DES C (50 mol %
(K)
BTEAC)
BTEAC)
BTEAC)
(K)
BTEAC)
BTEAC)
BTEAC)
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
a
a
Standard uncertainties are u(T) = 0.1 K, u(x(BTEAC)) = 1.3 × 10−5,
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.
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.
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.
a
Table 6. Viscosity Fitting Parameters
ln η = ln η0 + (Eη/RT)
η0
Eη
R2
Figure 3. Linearized ionic conductivity values of the synthesized DESs.
DES A
DES B
DES C
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
a
a
Table 8. Ionic Conductivity Fitting Parameters
η, η0 = Pa·s, Eη = J/mol, T = K.
ln σ = ln σ0 + (Eσ/RT)
σ0
Eσ
R2
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
8.791 × 1014
9.130 × 1011
2.137 × 108
−9.320 × 104
−8.337 × 104
−4.708 × 104
0.9773
0.9642
0.9969
a
σ, σ = mS/cm, Eσ = J/mol, T = K.
0
D
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a
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
Table 9. Refractive Index Values of the Synthesized DESs
temperature DES A (30 mol % DES B (50 mol % DES C (50 mol %
(K)
BTEAC)
BTEAC)
BTEAC)
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
a
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.
Table 10. Refractive Index Fitting Parameters
nD = A + BT
A
B
R2
DES A
DES B
DES C
1.647
1.623
1.613
−3.360 × 104
−3.153 × 104
−3.267 × 104
0.9972
0.9982
0.9997
Figure 4. Walden plot of the KCl (aq) and DES A and DES C
between the temperatures 293 and 333 K.
a
Table 11. Density Values of the Synthesized DESs
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.
temperature
(K)
DES A (30 mol % BTEAC) DES C (50 mol % BTEAC)
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
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
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
a
Table 12. Density Fitting Parameters
ρ = A + BT
A
B
R2
DES A
DES B
1.366
1.374
−6.650 × 10−4
0.9938
0.9910
−6.773 × 10−4
a
ρ = g/cm3, T = K.
E
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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
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
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(CB)0
dCA
dt
dXA
dt
Figure 5. Comparison of the catalytic activities of the synthesized
DESs in the esterification of acetic acid with 2-ethyl hexanol.
(−rA)0 =
= (CA)0
(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
⎛
⎜
⎝
⎞
⎟
⎠
−Ea
RT
k = k0 exp
(2)
Figure 6. Progress of the reaction between 2-ethyl hexanol and acetic
acid catalyzed by DES A.
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
temperature (K)
k (L/min mol)
R2
373
363
353
0.02863
0.02506
0.02293
0.9985
0.9998
0.9940
F
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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 2-
ethyl-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.
Figure 10. 13C NMR spectrum of (a) fresh and (b) used DES A.
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 water-
free 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.
1
Figure 11. H 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
According to Abbot et al.9 DESs made by the quaternary
ammonium salts and HBD are formed by the equilibrium
shown below:
[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
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J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data
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vents (DESs) and Their Physical Properties. Molecules 2014, 19,
8011−8026.
subsequent drops in catalytic activity can be attributed to the
presence of the reactants and products in the used DES.
(9) Abbott, A. P.; Harris, R. C.; Ryder, K. S. Application of Hole
Theory to Define Ionic Liquids by Their Transport Properties. J. Phys.
Chem. B 2007, 111, 4910−4913.
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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Based Deep Eutectic Solvents. Asia-Pac. J. Chem. Eng. 2015, 10, 273−
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Heat Capacities and Electrical Conductivities of Two Ammonium-
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AUTHOR INFORMATION
Corresponding Author
*E-mail: emine.sert@ege.edu.tr.
■
(17) Taysun, M. B.; Sert, E.; Atalay, F. S. Physical Properties of
Benzyl Tri-Methyl Ammonium Chloride Based Deep Eutectic
Solvents and Employment as Catalyst. J. Mol. Liq. 2016, 223, 845−
852.
ORCID
Emine Sert: 0000-0002-0868-8597
Funding
(18) Liu, Y.; Lotero, E.; Goodwin, J. G., Jr Effect of Water on Sulfuric
Acid Catalyzed Esterification. J. Mol. Catal. A: Chem. 2006, 245, 132−
140.
This study was supported by TUBITAK (213 M 643), by Ege
University 15MUH009 and EBILTEM 2015/BIL/026 scientific
research projects.
̈
̇
̇
(19) Abbott, A. P.; Barron, J. C.; Ryder, K. S.; Wilson, D. Eutectic-
Based Ionic Liquids with Metal-Containing Anions and Cations. Chem.
- Eur. J. 2007, 13, 6495−6501.
Notes
The authors declare no competing financial interest.
́ ̂
(20) Zhang, Q.; De Oliveira Vigier, K.; Royer, S.; Jerome, F. Deep
Eutectic Solvents: Syntheses, Properties and Applications. Chem. Soc.
Rev. 2012, 41, 7108−7146.
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