n-Butyl Acetate Synthesis in the Presence of Pyridinium-Based Acidic Ionic Liquids: Influence of the Anion Nature

Article abstract of DOI:10.1007/s10562-017-2135-0

Abstract: The catalytic behavior of three Br?nsted acidic ionic liquids: pyridinium hydrogen sulfate ([H–Pyr]⁺[HSO₄]^?), pyridinium dihydrogen phosphate ([H–Pyr]⁺[H₂PO₄]^?) and pyridinium nitrate ([H–Pyr]⁺[NO₃]^?) in the esterification of acetic acid with n-butanol was compared in the current paper. The alteration in the values of acetic acid conversion and esterification rate were investigated as a function of the process parameters: catalyst nature, catalyst loading, initial molar ratio n-butanol:acetic acid and reaction temperature. The anion variety modified the acidity of the obtained samples and effected their catalytic activity as follows: [H–Pyr]⁺[NO₃]^? < [H–Pyr]⁺[H₂PO₄]^? < [H–Pyr]⁺[HSO₄]^?. The increasing of both catalyst loading and reaction temperature led to higher values of acetic acid conversion and esterification rate. The existence of a positive linear relationship between the esterification rate and acetic acid concentration was observed. The increasing of the n-butanol concentration negatively affected the rate of esterification. The acetic acid conversion was improved as the initial molar ratio n-butanol-to-acetic acid was raised. Graphical Abstract: [Figure not available: see fulltext.].

Full text of DOI:10.1007/s10562-017-2135-0

Catal Lett
DOI 10.1007/s10562-017-2135-0
n-Butyl Acetate Synthesis in the Presence of Pyridinium-Based
Acidic Ionic Liquids: Influence of the Anion Nature
1
1
1
1
Ivaylo Tankov · Magdalena Mitkova · Radoslava Nikolova · Anife Veli ·
2
Dicho Stratiev
Received: 4 April 2017 / Accepted: 29 June 2017
©
Springer Science+Business Media, LLC 2017
Abstract The catalytic behavior of three Brønsted
Graphical Abstract
acidic ionic liquids: pyridinium hydrogen sulfate ([H–
+
−
Pyr] [HSO ] ), pyridinium dihydrogen phosphate ([H–
4
+
−
+
−
Pyr] [H PO ] ) and pyridinium nitrate ([H–Pyr] [NO ] )
2
4
3
in the esterification of acetic acid with n-butanol was com-
pared in the current paper. The alteration in the values of
acetic acid conversion and esterification rate were inves-
tigated as a function of the process parameters: catalyst
nature, catalyst loading, initial molar ratio n-butanol:acetic
acid and reaction temperature. The anion variety modi-
fied the acidity of the obtained samples and effected
+
−
their catalytic activity as follows: [H–Pyr] [NO ] <[H–
3
+
−
+
−
Pyr] [H PO ] <[H–Pyr] [HSO ] . The increasing of both
2
4
4
Keywords Pyridinium · Ionic liquid · Brønsted acidity ·
Batch esterification · n-Butyl acetate
catalyst loading and reaction temperature led to higher val-
ues of acetic acid conversion and esterification rate. The
existence of a positive linear relationship between the ester-
ification rate and acetic acid concentration was observed.
The increasing of the n-butanol concentration negatively
affected the rate of esterification. The acetic acid conver-
sion was improved as the initial molar ratio n-butanol-to-
acetic acid was raised.
1
Introduction
Esterification of carboxylic acids is an important reaction
in chemical and pharmaceutical industries [1]. The simplest
route to produce esters with high yield is the direct esterifi-
cation of acids with alcohol in the presence of homogene-
ous or heterogeneous catalyst.
Esterification is a very slow and highly reversible reac-
tion. Consequently, the conversion of the reactants is deter-
mined by the equilibrium [2–4]. Since, the equilibrium
constant of esterification reaction is in the range of 1–10,
considerable amounts of reactants exit in the equilibrium
mixture. Being a reversible reaction, the reagent conversion
during esterification can be generally improved by the fol-
lowing methods: (1) using alcohol in large excess and (2)
removal of water by physical means such as distillation.
Taking into account that the limiting step in the esterifica-
tion reaction is the protonation (activation) of carboxylic
*
Ivaylo Tankov
igtankov@yahoo.com
1
2
University “Prof. Dr. Assen Zlatarov” Burgas, Burgas 8010,
Bulgaria
LUKOIL Neftohim Burgas, Burgas 8010, Bulgaria
Vol.:(0
1
123456789)
3

I. Tankov et al.
acid, both homogeneous and heterogeneous acids can
accelerate the reaction.
synthesis in the presence of ionic liquids with different
+
+
+
+
cations ([N ] , [BSEt N] , [H–Pyr] , [Et N–H] and
2
224
3
3
+
The conventional industrial esterification of acetic acid
with n-butanol for production of n-butyl acetate (Fig. 1) is
carried out in batch process using mineral acids (sulphuric,
hydrofluoric, phosphoric) as catalysts [5]. However, this
process leads to some serious problems such as side reac-
tions (mainly dehydration of n-butanol to dibutyl ether),
corrosion of equipment and hard isolation of product. To
overcome these drawbacks, solid heterogeneous catalysts
such as heteropoly acids [6], molecular sieves [7], resins [1,
[H–Mim] ) was extensively studied by Tao et al. [2]. How-
ever, the published studies focused on different anion nature
for promoting of acetic acid esterification with n-butanol
are limited. Hence, the investigation of the catalytic activ-
ity of ionic liquids with variety of the anion seems to be
necessary. On the other hand, the industrial application
of a process requires the usage of an inexpensive catalyst
that can be easily regenerated. Therefore, the current paper
reported the synthesis of low-cost ionic liquids containing
+
−
8
, 9] and supported metal oxides [10, 11] are extensively
studied.
Brønsted acidic ionic liquids (BAILs), compounds
traditional cation [H–Pyr] and different anions ([H PO ] ,
2 4
−
−
[HSO ] and [NO ] ) for esterification of acetic acid with
4
3
n-butanol. Infrared spectroscopy and thermogravimetric
analysis were used as characterization methods. In addi-
tion, the influence of the process parameters on the acetic
acid conversion and esterification rate was also discussed.
which possess negligible volatility, excellent thermal stabil-
ity, remarkable solubility and structural variety [7–9], are
thought to be promising catalysts for esterification of acetic
acid with various alcohols [12–17]. For example, Bourahla
et al. [12] investigated the catalytic performance of BAILs,
−
containing hydrogen sulphate anion ([HSO ] ) and dif-
2 Experimental
2.1 Chemicals
4
ferent cation (l-butyl-3-methylimidazolium hydrogen
sulphate, l-hexyl-3-methylimidazolium, l-butyl-3-meth-
ylimidazolium dihydrogen phosphate and l-[2-(2-hydroxy-
ethoxy)-ethyl]-3-methylimidazolium hydrogen sulphate) in
esterification of acetic acid with C –C and C alcohols.
Acetic acid (99.95 wt%), n-butanol (99 wt%), pyridine
(99 wt%), sulphuric acid (98 wt%), phosphoric acid
(85 wt%) and nitric acid (65 wt%) are purchased from
Merck. Diethyl ether is obtained from Sigma-Aldrich,
while hexane (99.4 wt%) and heptane (99.8 wt%) are sup-
plied by Riedel-de Haën. All chemicals are used without
any further purification.
5
7
10
Similar to Bourahla et al. [12], Tao at al [13]. presented the
esterification of acetic acid with C –C alcohols in the pres-
1
4
ence of 1-methylimidazole hydrogen sulfate, N-triethylam-
monium hydrogen sulfate, butyl triethylammonium hydro-
gen sulfate and N-(4-sulfonic acid) butyl triethylammonium
sulfate. Authors reported that the esterification process
depends mainly on both the acidity and hydrophilic nature
of the catalysts.
2.2 Catalysts Preparation
The effect of cation variety on the catalytic behavior
was also determined by Zhao et al. [15] for esterification
of acetic acid with 1-pentanol in the presence of 3-methyl-
The current article presents the synthesis of three ionic
+
liquids, composed of [H–Pyr] cation and different ani-
−
−
−
ons ([NO ] , [HSO ] and [H PO ] ) (Fig. 2). Prepara-
3
4
2
4
+
−
1
1
1
-imidazole)-ethylene hydrogen sulphate, 3-methyl-
-imidazole)-propylene hydrogen sulphate and 3-methyl-
-imidazole)-butylene hydrogen sulphate.
tion procedure of the acidic ionic liquid [H–Pyr] [NO ] ,
3
+
−
+
−
[H–Pyr] [HSO ] and [H–Pyr] [H PO ] is similar to
4
2
4
those described by Tao et al. [13] and Duan et al. [14].
Pyridine (51.06 g, 0.6455 mol) and ultra clean water
(34.86 g, 1.9365 mol) were charged into a 0.250 L three-
necked flat bottom reactor, equipped with a magnetic stir-
ring, reflux condenser and thermometer. The aqueous
It was generally accepted that higher acidity of the anion
in the ionic liquid results in higher yield of the ester [13,
1
8, 19]. In the light of this, Duan et al. [14] showed that the
esterification of acetic acid with cyclic olefins, catalyzed by
ionic liquid (2-methylpyridinium trifluoroacetate, 2-meth-
ylpyridinium trifluoromethanesulfonate, 2-methylpyridin-
ium methanesulfonate) proceeds with high selectivity and
product yield. In addition, the kinetics of n-butyl acetate
solution of pyridine (C =7.4195 mol/L) was stirred at
pyr
ambient temperature for 30 min, where after the tempera-
ture was reduced up to 2–8°C. Then nitric acid (40.67 g,
0.6455 mol) was added dropwise over a period of 90 min
Fig. 1 Esterification of acetic
acid with n-butanol in the pres-
ence of acidic catalyst
1
3

n-Butyl Acetate Synthesis in the Presence of Pyridinium-Based Acidic Ionic Liquids: Influence…
Fig. 2 Synthesis of pyridinium-
based ionic liquids
under vigorous stirring. The reaction mixture was stirred at
reflux for an additional period of 5 h at 80°C.
and thermometer. Mixing of the reaction fluid was per-
formed by a magnetic stirrer.
For solvent removing, the aqueous solution of pyridin-
ium nitrate was distillated at 60°C and reduced pressure
(
0.050 MPa) and the product was dried under the same
2.6 Catalytic Test
conditions for 8 h until the weight of the residue remained
constant. After cooling, the resulting white-colored solid
A typical procedure for the esterification of acetic acid
with n-butanol (Fig. 2) was as follows. Acetic acid
(0.2778 mol) and ionic liquid (5, 10 or 15 wt% with
respect to the mass of acetic acid) were placed in the reac-
tor. Hexane (0.2778 mol) or heptane (0.2778 mol) was
added to the reaction mixture for water removal in the
form of azeotrope mixture during the reaction progress.
Once the desired reaction temperature (80 and 100 °C)
was attained, a known quantity of n-butanol (0.2778,
0.8834, 1.3890 and 1.9446 mol to reach an n-butanol-to-
acetic acid molar ratio (N /N ) of 1/1, 3/1, 5/1 and 7/1,
+
−
(
[H–Pyr] [NO ] ) was washed with diethyl ether 0.100 L
3
for three times and dried in vacuum (0.050 MPa) at
8
0°C for 12 h. Preparation protocol of the ionic liquids
+
−
+
−
[
H–Pyr] [HSO ] and [H–Pyr] [H PO ] did not differ
4
2
4
+
−
from that of [H–Pyr] [NO ] .
3
2
.3 Infrared Spectroscopy
Infrared
spectra
of
pure
pyridine,
and
+
−
+
−
[
H–Pyr] [NO ] ,
[H–Pyr] [HSO ]
4
3
B
A
+
−
[
H–Pyr] [H PO ] samples were recorded using Nicolet
respectively), separately heated to the set temperature
was added into the reactor. This time was considered as
the zero reaction time (t = 0).
2
4
iS 50 Thermo Scientific FT-IR spectrophotometer in the
−
1
frequency region of 4000–400 сm . Infrared spectra of
+
−
pure pyridine and [H–Pyr] [HSO ] were measured by
The samples (1 ml) were withdrawn at regular inter-
vals of time (20 min) and immediately cooled in an ice
bath. The acetic acid concentration was analyzed using
GC 7890A (Agilent Technologies), equipped with a flame
ionization detector (FID) and capillary column HP-INNO-
WAX, 30 m×0.32 mm×0.25 µm. High purity helium was
used as a carrier gas at a flow rate of 0.0015 L min. The
oven temperature was programmed at 70°C for 2 min and
then it was raised from an initial value of 70–160°C at a
ramp rate of 5°C/min. The injector and detector tempera-
ture were maintained at 250 and 300°C, respectively. The
acetic acid conversion was calculated according to Eq. 1:
4
+
−
means of KBr pellets, while those of [H–Pyr] [NO ] and
3
+
−
[
H–Pyr] [H PO ] using Nujol.
2 4
2
.4 Thermal Gravimetric Analysis
+
−
Thermal
stability
of
[H–Pyr] [NO ] ,
3
+
−
+
−
[
H–Pyr] [HSO ] and [H–Pyr] [H PO ] ionic liquids was
4
2
4
investigated in NETZSCH STA 449 F3 instrument. The
samples are heated from 25 to 700°C at 10°C/min under
stream of nitrogen (0.020 L min).
ꢀ
ꢁ
2
.5 Apparatus
X = C − CA ∕CAo
A
(1)
A0
where X is the acetic acid conversion (%); C is the initial
The experiments were carried out in a three-necked
round bottom flask of 0.250 L capacity, equipped with
condenser, Dean–Stark trap, a port for sample withdrawal
A
A0
acetic acid concentration (mol/L) and C corresponds to the
A
acetic acid concentration (mol/L) at reaction time t (min).
1
3

I. Tankov et al.
2
.7 Kinetic Model
3 Results and Discussion
3.1 Infrared Spectroscopy
In this work, the esterification of acetic acid with
+
−
n-butanol in the presence of [H–Pyr] [HSO ] ,
4
+
−
+
−
[
H–Pyr] [H PO ] and [H–Pyr] [NO ] catalysts was
Infrared spectra of the obtained ionic liq-
2
4
3
+
−
+
−
proposed as a second-order reversible reaction. Based
on this, a pseudo-homogeneous kinetic model was pre-
sented (Eq. (2)).
uids
[H–Pyr] [HSO ] ,
[H–Pyr] [H PO ]
and
4
2
4
+
−
−1
[H–Pyr] [NO ] in frequency region of 2000–400 cm are
3
shown in Fig. 3. For comparison, the infrared spectrum of
pure pyridine was also included.
dCA
−
rA = −
= k C C − k C C
B
(2)
It was observed that the infrared spectrum of pure
pyridine contains characteristic bands at 607, 702, 749,
1
A
Y
2
Z
dt
−1
where C , C , C and CZ describe the concentration
993, 1028, 1070, 1147, 1217, 1437, 1483 and 1579 cm
A
Y
B
−
1
(
mol/L) of acetic acid, n-butanol, ester and water at reac-
(Fig. 3). The peaks at 607, 707 and 1028 cm are due to
bending vibrations in aromatic ring, while those at 749,
tion time t (min), respectively.
−
1
Assuming that the reaction products (B and Z) do not
present in the initial reaction mixture and the esterifica-
tion reaction is conducted via reactive distillation pro-
cess (expressed by continuous removal of water in the
form of azeotrope), Eq. (2) becomes as follows:
993, 1070, 1147 and 1217 cm represent the wagging
vibrations of C–H bonds in –CH groups [20]. The charac-
2
−
1
teristic bands 1437, 1483 and 1579 cm denote a variety
in С=C and С=N bonds length in aromatic core.
Results showed that the interaction between pyridine and
+
−
ꢀ
ꢁ
sulphuric acid (synthesis of ionic liquid [H–Pyr] [HSO ] )
4
M − XA
1
ln
= k1t
(3)
leads to a significant alteration in the infrared spectrum
of pyre pyridine (Fig. 3). The latter was confirmed by
a disappearance of the characteristic bands at 1437 and
C (M − 1)
A0
M(1 − XA)
where M denotes the initial reactants molar ratio. The left
−
1
1
579 cm in infrared spectrum of pyre pyridine and devel-
hand side of the Eq. (3) has dimension L/mol. The plot of
ꢀ
ꢁ
1
M−XA
oping of new ones at 576, 883, 1001, 1064, 1173, 1546 and
ln
versus time (t) is a straight line pass-
CA0(M−1)
M(1−XA)
−1
+
−
1
639 cm in infrared spectrum of [H–Pyr] [HSO ] . The
4
−
1
ing through origin. The slope of the line represents the
intensive peaks at 576 and 1173 cm , and the bands at
−
1
kinetic constant (k) value with dimension L/mol.min.
1001 and 1064 cm characterize bond vibrations in hydro-
−
gen sulphate anion ([HSO ] ). The first two peaks are due
4
to symmetrical vibrations of S=O bond, while the latter
two ones denote S–OН stretches [21, 22].
+
−
−
+
−
Fig. 3 Infrared spectra of pure pyridine and the ionic liquids [H–Pyr] [HSO ] , [H–Pyr]+[H PO ] and [H–Pyr] [NO ]
4
2
4
3
1
3

n-Butyl Acetate Synthesis in the Presence of Pyridinium-Based Acidic Ionic Liquids: Influence…
−1
The characteristic bands at 1546 and 1639 cm in infra-
+
−
red spectrum of [H–Pyr] [HSO ] illustrate a vibration
4
+
of N–H bond in [H–Pyr] cation [23, 24]. In other words,
−1
the bands at 1546 and 1639 cm indicate the formation of
pyridinium cation.
Figure
3
2
presents the infrared spectrum of
+
−
[
H–Pyr] [H PO ] sample. It contains bands at 492,
4
5
1
34, 889 (shoulder), 979, 1054, 1110, 1250, 1530 and
−
1
−1
636 cm . The bands at 492, 534, 1054 and 1110 cm
−
characterize the О=P=O group in anion [H PO ] . In
2
4
−1
details, the peak at 492 cm was associated with out-of-
plane bending vibration, while those at 492, 534, 1054 and
−1
1
110 cm were due to rocking, symmetric and asymmet-
ric vibration, respectively [25]. The out-of-plane and in-
plane P–OH deformation vibrations were registered by the
−
1
appearance of a shoulder and a peak at 889 and 1250 cm ,
−
1
respectively. The band at 979 cm shows a stretching
vibration of HO–P–OH atomic group [26]. The vibration of
+
N–H bond in pyridinium cation ([H–Pyr] ) is presented by
−
1
peaks at 1530 and 1636 cm .
+
−
The infrared spectrum of [H–Pyr] [NO ] illustrates
3
the appearance of peaks at 672, 826, 1055, 1376 and
−1
1
758 cm (Fig. 3). The peaks of low intensity at 672, 826
−1
and 1758 cm are attributed to vibrational stretching of
−
−1
O=N–O group, while the prominent band at 1376 cm
+
−
Fig. 5 TGA curves and weight loss of [H–Pyr] [HSO ] (solid line),
4
characterizes an asymmetric valence vibration in nitrate
+
−
+
−
[
H–Pyr] [H PO ] (dot line) and [H–Pyr] [NO ] (dash line)
2 4 3
−
anion ([NO ] ).
3
Batsanov [27] investigated the crystal structure of
pyridinium nitrate at 25°C and showed the existence
of a strong hydrogen bond between the pyridinium cat-
measured by TG includes three main regions of weight
loss: (i) from room temperature to 125°C, (ii) from 167
to 256 °C and (iii) between 273 and 397 °C. The weight
loss (3.47%) in the first temperature region can be
assigned to the elimination of physically adsorbed water,
while the second step of weight loss (6.50%) was corre-
lated with the removal of crystalline water. The weight
loss (85.02%) determined in the last region was con-
nected with molecular destruction of the sample [13].
+
ion ([H–Pyr] ) and nitrate anion (Fig. 4). The latter was
−1
detected by the presence of a peak at 1055 cm in infra-
+
−
red spectrum of [H–Pyr] [NO ] (Fig. 3) [28]. Similar to
3
+
−
[
H–Pyr] [H PO ] sample, infrared spectrum of ionic
2 4
+
−
liquid [H–Pyr] [NO ] also contains bands (at 1530 and
3
−1
1
636 cm ) due to N–H bond vibration in pyridinium
cation.
Thermogravimetric
analysis
showed
that
+
−
3
.2 Thermogravimetric Analysis
[H–Pyr] [H PO ] possesses higher thermal stability
2 4
+
−
compared to that of [H–Pyr] [HSO ] . Two temperature
4
+
−
TGA curves of ionic liquids [H–Pyr] [HSO ] ,
areas of weight loss were detected: a wide one from 129
to 528 °C and a sharp one from 588 to 701 °C. In the first
temperature region the weight of sample decreases with
4
+
−
+
−
[
H–Pyr] [H PO ] and [H–Pyr] [NO ] are shown in
2 4 3
+
−
Fig. 5. The thermal behavior of [H–Pyr] [HSO ] sample
4
4
1.53% due to elimination of physically adsorbed water
and partial decomposition processes. The sharp tempera-
ture region (588–701 °C) showed that the mass of sam-
ple decreases up to 19.32% due to thermal destruction.
Consequently, the complete thermal decomposition of
+
−
ionic liquid [H–Pyr] [H PO ] occurs at temperatures
2
4
higher than 700 °C. Figure 5 clearly indicates that among
+
−
all samples, [H–Pyr] [NO ] possesses the lowest ther-
3
+
−
mal stability. Derivatogram of [H–Pyr] [NO ] contains
Fig. 4 Formation of hydrogen bond between pyridinium cation ([H–
3
+
−
+
−
Pyr] ) and nitrate anion ([NO ] ) in ionic liquid [H–Pyr] [NO ]
a sharp region of weight loss between 122 and 251 °C,
3
3
1
3

I. Tankov et al.
+
where complete thermal decomposition of the sample
was observed.
Taking into account that the cation ([H–Pyr] ) in all
samples does not differ (Fig. 2), it becomes clear that the
acetic acid conversion depends strongly on the anion nature.
At first sight, it seems that the highest acidity (the largest
It can be summarized that the ionic liq-
+
−
+
−
uids
[H–Pyr] [HSO ] ,
[H–Pyr] [H PO ]
and
4
2
4
+
−
+
−
[
H–Pyr] [NO ] usage as catalysts for esterification cata-
number of protons) possesses [H–Pyr] [H PO ] sam-
3
2 4
+
−
lysts requires temperatures lower than 120°C.
.3 Catalytic Activity
The catalytic activity
ple, and the lowest acidity—[H–Pyr] [NO ] one. Conse-
3
quently, the acetic acid conversion should reach (i) a maxi-
3
mum value when esterification process is carried out in the
+
−
presence of [H–Pyr] [H PO ] catalyst and (ii) the lowest
2
4
+
−
+
−
of
[H–Pyr] [NO ] ,
value when [H–Pyr] [NO ] is used. However, a maximum
3
3
+
−
+
−
+
−
[
H–Pyr] [H PO ] and [H–Pyr] [HSO ] samples was
X value of 76% was obtained when [H–Pyr] [HSO ] is
A 4
2
4
4
+
−
evaluated by the variation of acetic acid conversion (X )
used as catalyst, whereas [H–Pyr] [H PO ] sample gaves
2 4
A
during esterification with n-butanol (Fig. 6). For compari-
conversion of 52% (Fig. 6). In addition, the presence of
+
−
son, the value of acetic acid conversion for non-catalyzed
[H–Pyr] [NO ] catalyst increased the acetic acid conver-
3
(
blank) reaction was also included.
sion with 20%, from 12 (non-catalyzed reaction) to 32%.
In order to explain the observed absence of correlation
between experimental data and proposed catalytic activity
It is well known that a crucial step in the esterification
of carboxylic acids is the formation of active carbonyl com-
plex (pronated acid) as a result of carboxylic acid–catalyst
interaction [29, 30]. The number of those active complexes
is directly proportional to the extent of catalyst acidity. In
other words, the concentration of pronated acid is corre-
lated to the catalyst capacity (expressed by acid dissocia-
+
−
+
−
of [H–Pyr] [HSO ] and [H–Pyr] [H PO ] samples, the
4
2
4
−
−
electronic effects in anions [HSO ] and [H PO ] were
4
2
4
discussed.
Figure 7 shows that the structure of hydrogen sul-
−
phate anion ([(HO)SO ] ) contains three oxygen atoms,
3
tion constant K ) to produce hydrogen cations (protons,
which are not attached to hydrogen atoms (in the form of
A
+
−
H ) [2, 31, 32].
HO group). Since the oxygen atom possesses the second
highest electronegativity of any atom (3.44 according to
Pauling scale) [33], the electron density is drawn toward
those atoms by an inductive effect. The inductive effect pro-
duced by the three oxygen atoms causes electron density on
the sulphur atom to decrease and as a result a slight posi-
δ+
tive charge (S ) is formed. The latter is partially compen-
sated by shifting of electron density from the O–H bond.
The electron density shift OH → S → O makes the O–H
bond more polar and more suitable for cleavage to proton
+
−
(
H ). Unlike [(HO)SO ] , dihydrogen phosphate anion
3
−
(
[(HO) PO ] ) possesses two oxygen atoms, unbounded to
2
2
hydrogen atoms (Fig. 7). It would mean that polarization of
−
OH groups in [(HO) PO ] would be less pronounced than
2
2
−
that in (HO)SO ] .
3
Based on the observed electronic effects in ani-
−
−
ons [HSO ] and [H PO ] , it could be concluded
4
2
4
+
−
that [H–Pyr] [HSO ] sample may generate a greater
4
Fig. 6 Effect of different catalyst on the of acetic acid conversion: a
+
−
+
−
non-catalyzed reaction; b [H–Pyr] [NO ] ; c [H–Pyr] [H PO ] ; d
3
2
4
+
−
−
−
[
H–Pyr] [HSO ] . T=80°C; t=2 h; N /N =3/1; C 0.0700 mol/L
Fig. 7 Structure of anions [HSO ] and [H PO ]
4
B
A
cat
4
2
4
1
3

n-Butyl Acetate Synthesis in the Presence of Pyridinium-Based Acidic Ionic Liquids: Influence…
number of hydrogen cations during the esterification
+
−
reaction in comparison with [H–Pyr] [H PO ] . In
2
4
other words, the acetic acid conversion during esteri-
fication could increase more rapidly in the presence of
+
−
+
−
[
H–Pyr] [HSO ] catalyst than of [H–Pyr] [H PO ] .
4 2 4
+
−
The greater capacity of [H–Pyr] [HSO ] to produce pro-
4
+
tons is confirmed by the K values of cation [H–Pyr]
A
−
−
and anions [HSO ] and [H PO ] . The dissociation con-
4
2
4
+
−6
stant value of [H–Pyr] is 6.7×10 (pK = 5.17), while
A
−
−
−2
it values for anions [HSO ] and [H PO ] are 1.0×10
4
2
4
−
8
(
[
[
pK = 2.00) and 6.2×10 (pK = 7.21), respectively
A
A
+
−
34]. The valued kinetic constant for [H–Pyr] [HSO ] ,
4
+
−
+
−
H–Pyr] [H PO ] and [H–Pyr] [NO ] at given reaction
2
4
3
−
4
conditions were19.38± 2.53(L/mol.min,×10 );10.12± 0.83
−
4
−4
(
L/mol.min,×10 ) and 5.28± 0.59 (L/mol.min,×10 ),
+
−
respectively. Since, [H–Pyr] [HSO ] sample possesses
4
+
−
a higher catalytic activity than [H–Pyr] [H PO ] and
2
4
+
−
[
H–Pyr] [NO ] , it was preferred as a catalyst for further
3
studies.
.4 Effect of Catalyst Concentration
3
The amount of catalyst plays a major role in the conver-
sion of carboxylic acid to ester (esterification reaction). In
a batch process the interaction between acid and catalyst is
much more pronounced at the start of the reaction since the
concentration of former is high. However, during reaction
progress the acid concentration decreases which leads to
poorer acid–catalyst interaction. At this point, the amount
of catalyst plays a major role in increasing the overall con-
version. In this study, the effect of catalyst concentration
Fig. 8 Effect of catalyst concentration on the acetic acid conversion.
C
: filled left pointing triangle 0.0000 mol/L (non-catalytic reac-
cat
tion); Filled triangle 0.0350 mol/L; diamond 0.0700 mol/L and filled
squared 0.1050 mol/L. T=80°С; N /N = 3/1
B
A
obtained for the blank (non-catalyzed) reaction and
the esterification carried out using 5, 10 and 15 wt%
(
C ) on the conversion value of acetic acid to n-butyl ace-
cat
+
−
tate was studied with respect to time (Fig. 8). The acetic
acid conversion at non-catalytic reaction is also added to
the data in Fig. 8.
[H–Pyr] [HSO ] catalyst, respectively.
4
The observed alteration in acetic acid conversion can
be confirmed by calculating the rate of reaction as a func-
tion of different catalyst loading. In terms of acetic acid
conversion [Eq. (1)], the kinetic model [Eq. (2)] can be
written as:
It was observed that the value of acetic acid conver-
sion during esterification in the absence of catalyst (non-
catalyzed reaction) is 12% at 120 min reaction time. The
percentage conversion gradually increases as the catalyst
ꢀ
ꢁ
−
rA = −CAo dX ∕dt
A
(4)
loading is increased. When C increases from 0.0350 to
cat
After rearranging and integration, Eq. (4) becomes as
0
.1050 mol/L, X varies from 66 to 78% (at 2 h). Zonder-
A
follows:
van et al. [35] investigated the esterification of myristic
acid with propanol using p-toluene sulphonic acid as cat-
alyst and showed that the increasing of catalyst loading
ꢀ
ꢁ
r = C X ∕t
A
(5)
A0 A
+
Substituting Eq. (1) in Eq. (5):
enhanced the number of H ions available for acid activa-
ꢀ
ꢁ
tion. Thus, higher conversion value (78%) at catalyst con-
centration of 0.1050 mol/L, with respect to the conver-
r = C − CA ∕t
A
(6)
A0
The effect of catalyst loading on the reaction rate is
sion value (66%) at C = 0.0350 mol/L was related to a
cat
shown in Fig. 9. Results demonstrated that the reaction
rate is directly proportional to catalyst concentration,
greater number of formed active carbonyl complexes. The
−
4
rate constant values of 2.12 ± 0.14 (L/mol.min, ×10 );
i.e. with increasing of C , the esterification rate also
−
4
cat
1
3.18 ± 0.98 (L/mol.min, ×10 ); 19.38 ± 2.53 (L/mol.
increases.
−
4
− 4
min, ×10 ) and 47.31 ± 3.11 (L/mol.min ×10 ) were
1
3

I. Tankov et al.
Fig. 9 Effect of catalyst concentration on the reaction rate. Ccat
:
filled triangle 0.0350 mol/L; filled squared 0.0700 mol/L; filled circle
0
.1050 mol/L. T=80°С; N /N = 3/1; t=2 h
B A
Fig. 10 Effect of molar ratio of n-butanol-to-acetic acid on the
of acetic acid conversion. N /N : filled left pointing triangle 1/1;
3
.5 Effect of Molar Ratio
B
A
filled circle 3/1; diamond 5/1; filled squared 7/1. T=80°С; C
=
cat
0
.0700 mol/L; t=2 h
The alteration of acetic acid conversion as a function of
molar ratio of n-butanol-to-acetic acid (N /N = 1/1–7/1)
B
A
was established at temperature of 80 °С, catalyst concentra-
of acetic acid conversion at given reaction conditions is in
range from 5/1 to 7/1. The results showed that the kinetic
constant value depends highly on the reactants molar
ratio. Accordingly, for N /N = 1/1, 3/1, 5/1 and 7/1 the
tion 0.0700 mol/L and reaction time 2 h (Fig. 10).
Experimental data indicated that N /N value signifi-
B
A
cantly affects the acetic acid conversion to n-butyl acetate.
The variation of molar ratio value from 1/1 to 3/1 slightly
increased the conversion of acetic acid from 72 to 76%
B
A
−
4
kinetic constant values of 14.16± 0.83 (L/mol.min, ×10 );
−
4
19.38± 2.53 (L/mol.min, ×10 ); 34.28± 5.41 (L/mol.
−
4
−4
(
Fig. 10). It was evident that with increasing of N /N up
min, ×10 ) and 27.08± 2.73 (L/mol.min, ×10 ) were
obtained, respectively.
B
A
to 5/1, the acetic acid conversion notably enhances and
reaches a value of 96%. At a molar ratio of 7/1, the highest
acid conversion (98%) was observed.
3.6 Effect of Reactant Concentration
Figure 10 clearly presents that the conversion of acetic
acid decreases by reduction of the molar ratio value. In
other words, when the initial concentration of n-butanol in
reactor decreases (molar ratio becomes lower), acetic acid
conversion diminishes. This observation was not surpris-
ing, considering the fact that low concentration of reactants
shifts the reaction equilibrium toward the right side of stoi-
chiometry equation [36, 37]. It was mentioned above that
the acetic acid conversion at molar ratio of 5/1 and 7/1 is 96
and 98%, respectively. Therefore, it can be concluded that
the ratio of alcohol-to-acid required to attain a maximum
The reaction rate (calculated from Eq. 5) was determined
by varying the concentration of reactants (acetic acid, C ,
A
and n-butanol, C ). The results of these variations are
B
shown in Fig. 11a, b.
Figure 11a represents the existence of a positive lin-
2
early relationship (R =0.9939) between the reaction rate
and concentration of acetic acid. It was found that the
increasing of acetic acid concentration from 1.2057 to
3.5661 mol/L resulted in increment of reaction rate from
0.0098 to 0.0214 mol/L.min. A possible explanation of
1
3

n-Butyl Acetate Synthesis in the Presence of Pyridinium-Based Acidic Ionic Liquids: Influence…
Fig. 11 Effect of acetic acid and n-butanol concentration on the reac-
tion rate. a Filled left pointing triangle 1.2057 mol/L; Filled circle
b Filled left pointing triangle 8.4401 mol/L; Filled circle
7.7382 mol/L; diamond 6.4755 mol/L; Filled squared 3.5661 mol/L.
1
.5476 mol/L; diamond 2.1585 mol/L; Filled squared 3.5661 mol/L.
T=80°С; C = 0.0700 mol/L; t=2 h
cat
the observed tendency could be the high concentration
of acetic acid, which favors the forward reaction [35]. In
other words, the high concentration of acetic acid shifts
the chemical equilibrium to reaction products. During the
esterification progress the concentration of acetic acid
diminishes and hence the reaction rate decreases.
3.7 Effect of Reaction Temperature
The effluence of reaction temperature on the acetic acid
conversion and reaction rate was established at 80 and
100°С using inert solvents, hexane and heptane, respec-
tively (Fig. 12a, b).
In contrast to the effect of acetic acid concentration,
the increasing of n-butanol concentration from 3.5661 to
The experimental data in Fig. 12a showed that the effect
of reaction temperature on both the conversion of acetic
acid and rate of esterification is significant. For example, at
temperature of 80°С the conversion of acetic acid was 76%
and it increases up to 98% when the reaction temperature
goes up to 100 °С. The latter was due to a greater number
of effective impacts between reactants, a consequence of
the acquired kinetic energy. The observed alteration in ace-
tic acid conversion (from 76 to 98%) indicates that at reac-
tion temperature of 100 °С the acetic acid concentration
should decrease more rapidly, compared to the decreas-
ing of acetic acid concentration at 80 °С. The latter means
that at temperature of 100°С the reaction rate decreases
faster than the rate at 80 °С, according to Eq. 5. Calcula-
tions showed that the rate of esterification at temperature of
100 °С deceases with 46% (from 0.0344 to 0.0185 mol/L.
8
.4401 mol/L caused the rate of esterification to decrease
2
linearly (R =0.9940) from 0.0214 to 0.0098 mol/L
(
Fig. 11b). This phenomenon suggests that a certain (criti-
cal) value of acetic acid concentration was achieved. It has
been already mentioned that the maximum conversion of
acetic acid (96–98%) was obtained when molar ratio var-
ies from 5/1 to 7/1 (see pt. 3.5). At those values of N /N ,
B
A
the acetic acid concentration was changed from 1.5476 to
.2057 mol/L. Above a critical value of n-butanol con-
centration (in the current paper it varies from 7.7381 to
1
8
.4401 mol/L at N /N = 5/1–7/1), a decreasing in overall
B A
acetic acid conversion due to a dilution effect was observed.
The latter reduces the rate of esterification process, accord-
ing to Eq. 5.
1
3

I. Tankov et al.
Fig. 12 Effect of temperature on the acetic acid conversion and reaction rate. diamond 80°С; Filled squared 100°С. C = 0.0700 mol/L; N /NA
cat
B
=
3/1
min), while at temperature of 80 °С—with 31% (from
.0201 to 0.0139 mol/L.min) (Fig. 12b). The kinetic con-
stant value at reaction temperature of 100, 80 and 60 °С
The acetic acid conversion increased proportionally to cata-
lyst concentration due to a greater number of formed active
carbonyl complexes. It was established that the molar ratio
of alcohol-to-acid required to attain a maximum of ace-
tic acid conversion is in range from 5/1 to 7/1. While the
increase of acetic acid concentration enhanced esterifica-
tion reaction rate, the augmentation of n-butanol concen-
tration diminished it. It was proposed that critical values
of acetic acid and n-butanol concentration are achieved.
Above a critical value of n-butanol concentration a decreas-
ing in overall acetic acid conversion due to a dilution effect
was observed. The acetic acid conversion increased with an
increasing in temperature, which confirmed that the reac-
tion is kinetically controlled.
0
−
4
was 31.43± 5.64 (L/mol.min, ×10 ); 19.38± 2.53 (L/mol.
−
4
−4
min, ×10 ) and 7.27± 0.65 (L/mol.min, ×10 ), respec-
tively. Based on the Arrhenius law, values of 77.25 kJ/mol
4
and 767.39 kmol/min ×10 were obtained as activation
energy and pre-exponential factor, respectively.
4
Conclusions
+
−
Three
acidic
2
ionic
liquids:
[H–Pyr] [HSO ] ,
3
4
+
−
+
−
[
H–Pyr] [H PO ] and [H–Pyr] [NO ] were synthe-
4
sized and tested as catalysts for direct esterification of ace-
tic acid with n-butanol to produce n-butyl acetate. It was
found that the obtained ionic liquids are thermally stable
up to 140°C. Above that temperature they undergo ther-
mal decomposition. Therefore their use as catalysts for
esterification requires reaction temperatures lower than
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