SO3H-functionalized Bronsted acidic ionic liquids as efficient catalysts for the synthesis of isoamyl salicylate

Article abstract of DOI:10.1039/c3ra45921g

Six Bronsted acidic ionic liquids (BAILs) composed of [HSO ₄] were prepared, characterized, and used as catalysts of low dosage in the synthesis of isoamyl salicylate. The effects of various parameters such as the kind of BAILs, temperature, catalyst loading, and molar ratio of the reactants on the conversion of salicylic acid were also examined in detail. The results suggested that the catalytic performances of BAILs were of close relevance to their Hammett acidities. The SO₃H-functionalized BAILs 1-(4-sulfonic acid) butyl-3-methylimidazolium hydrogen sulfate ([BSmim][HSO ₄]) and N-(4-sulfonic acid) butyl triethylammonium hydrogen sulfate ([BSEt₃N][HSO₄]) of strong acidities exhibited excellently catalytic activities and selectivities in the esterification of salicylic acid with isoamyl alcohol. The fully optimized geometries of [BSmim][HSO₄] and [BSEt₃N][HSO₄] further manifest that their strong acidities are derived from the strong interactions between the anion with the sulfonic acid group. In addition, it was found that [BSmim][HSO₄] could be also recovered easily and used repetitively at least six times without obvious decline in activity and quantity.

Full text of DOI:10.1039/c3ra45921g

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SO3H-functionalized Brønsted acidic ionic liquids
as efficient catalysts for the synthesis of isoamyl
salicylate†
Cite this: RSC Adv., 2014, 4, 1
*
Duan-Jian Tao, Jing Wu, Zhen-Zhen Wang, Zhang-Hui Lu, Zhen Yang
*
and Xiang-Shu Chen
Six Brønsted acidic ionic liquids (BAILs) composed of [HSO4] were prepared, characterized, and used as
catalysts of low dosage in the synthesis of isoamyl salicylate. The effects of various parameters such as
the kind of BAILs, temperature, catalyst loading, and molar ratio of the reactants on the conversion of
salicylic acid were also examined in detail. The results suggested that the catalytic performances of BAILs
were of close relevance to their Hammett acidities. The SO3H-functionalized BAILs 1-(4-sulfonic acid)
butyl-3-methylimidazolium hydrogen sulfate ([BSmim][HSO4]) and N-(4-sulfonic acid) butyl
triethylammonium hydrogen sulfate ([BSEt3N][HSO4]) of strong acidities exhibited excellently catalytic
activities and selectivities in the esterification of salicylic acid with isoamyl alcohol. The fully optimized
geometries of [BSmim][HSO4] and [BSEt3N][HSO4] further manifest that their strong acidities are derived
from the strong interactions between the anion with the sulfonic acid group. In addition, it was found
that [BSmim][HSO4] could be also recovered easily and used repetitively at least six times without
obvious decline in activity and quantity.
Received 18th October 2013
Accepted 5th November 2013
DOI: 10.1039/c3ra45921g
www.rsc.org/advances
their industrial applications in the synthesis of isoamyl salicy-
late. Therefore, there is an urgent need to develop environ-
Introduction
mentally friendly, high efficient and easily reusable catalysts for
the isoamyl salicylate synthesis process.
Isoamyl salicylate is an important intermediate and ne
chemicals in the elds of food, pharmaceuticals, avors, and
perfumes, owing to its distinctive fragrance of clover.1 At
present, isoamyl salicylate is mostly synthesized by using
concentrated sulfuric acid as homogenous catalyst through the
esterication reaction of salicylic acid with isoamyl alcohol in
the spice industry. However, salicylic acid with the structure of
hydroxyl and carboxyl groups was easy to oxidize because of the
strong oxidization of concentrated sulfuric acid, which led to
increase the side-effects and decrease the esterication rate.
Besides, conventional mineral acids would cause severe equip-
ment corrosion and environmental pollution. Although solid
acid catalysts such as mesoporous materials,2,3 metal oxides,4,5
and zeolites6,7 overcame some shortcomings compared with
traditional concentrated sulfuric acid in a certain extent, there
were also other defects such as relatively low activity, high mass
transfer resistance, uneven acid strength distribution, easy
carbon deposition on the surface and so on, which all limited
In recent years, ionic liquids (ILs) are regarded as new type of
green efficient catalyst and reaction media. They present a
broad application prospect and have been successfully intro-
duced into the eld of catalysis owing to their unique chemical
properties such as negligible vapor pressure, excellent thermal
stability, remarkable solubility, and effortless reusability.8–10 Till
now, ILs has been extensively used in alkylation,11 esterica-
tion,12 acetalization,13 ketalization,14 nitration,15 and so on,
which obviously exhibits high catalytic activity and selectivity.
Thus, acid-catalyzed esterication reactions based on ILs have
received more and more attention.16–20 In 2001, Deng et al.12
reported their pioneering work on the esterication of
carboxylic acids with alcohols in the presence of acidic
chloroaluminate ILs as catalyst. The catalyst ILs can be recycled
and reused by decanting simply the produced ester that existed
mainly in the upper phase of the resulting mixture. Aer that,
Cole et al.16 had rstly synthesized SO3H-functionalized
Brønsted acidic ionic liquids (BAILs) and used them for the
esterication of acetic acid with ethanol. They found that these
BAILs showed a good catalytic performance and reusability in
the esterication reactions. Yu et al.18 had further illustrated the
merits of using SO3H-functionalized BAILs for the synthesis of
plasticizer ester. Li et al.19 reported the synthesis of methyl
salicylate in BAILs under microwave irradiation. Despite the
Jiangxi Inorganic Membrane Materials Engineering Research Centre, College of
Chemistry and Chemical Engineering, Jiangxi Normal University, Nanchang 330022,
People's Republic of China. E-mail: djtao@jxnu.edu.cn; cxs66cn@jxnu.edu.cn; Fax:
+86-791-88120843; Tel: +86-791-88121974
† Electronic supplementary information (ESI) available: Details on the fully
optimized geometries structures and molecular coordinates of BAILs. See DOI:
10.1039/c3ra45921g
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researches on BAILs used in the esterication reaction had been 1-Methylimidazole (0.1 mol) and 1,4-butane sultone (0.1 mol)
reported a lot, the study on synthesis of isoamyl salicylate using were charged into a 100 mL round-bottom ask. Then, the
BAILs as catalysts has not been seen. In addition, despite the mixtures were stirred at 80 ꢀC for 12 h. The white solid zwit-
excellent yields, the microwave-accelerated strategy oen terion was washed repeatedly with ether (2 ꢁ 5 mL) to remove
formed side products (such as phenol) and decreased the any unreacted starting materials and dried in vacuum. Then, a
selectivity of methyl salicylate.19 Herein, we believe that further stoichiometric amount of concentrated sulfuric acid was added
investigation is needed to explore the synthesis of isoamyl and the mixture stirred for six hours at 60 ꢀC during which time
salicylate in BAILs with high activity and selectivity.
the solid zwitterion dissolved/liqueed, resulted in the forma-
Therefore, several BAILs had been prepared and their cata- tion of [BSmim][HSO4]. The BAIL phase was then washed
lytic activities for the synthesis of isoamyl salicylate were repeatedly with dichloromethane and ether to remove non-ionic
investigated in this present work. The reaction parameters such residues, and dried in vacuum. Then [BSmim][HSO4] of yellow
as kind of BAILs, temperature, catalyst loading, and molar ratio viscous liquid was obtained with a yield of 90%. 1H NMR
of reactants were explored in detail to obtain the optimum (300 MHz, D2O-d2, 25 ꢀC, TMS); d (ppm) 1.605 (m, 2H), 1.888 (m,
conditions. For comparison, the catalytic activities of the two 2H), 2.809 (t, 2H), 3.754 (s, 3H), 4.110 (t, 2H), 7.305 (s, 1H), 7.364
conventional heterogeneous catalysts (phosphorictungs acid (s, 1H), 8.60 (s, 1H).
and Amberlyst-15 resin) in the synthesis of isoamyl salicylate
N-(4-Sulfonic acid) butyl triethylammonium hydrogen
were also carried out. Additionally, the Hammett acidity and sulfate ([BSEt3N][HSO4]): the synthesis of [BSEt3N][HSO4] was
reusability of BAILs was studied, and their acidity–catalytic carried out according to the procedure described for [BSMim]-
activity relationships were further discussed.
[HSO4] from triethylamine and 1,4-butane sultone. [BSEt3N]-
[HSO4] of colorless viscous liquid was obtained with a yield of
1
ꢀ
92%. H NMR (300 MHz, D2O-d2, 25 C, TMS): d 1.122 (t, 9H),
1.679 (m, 4H), 2.825 (t, 2H), 3.08 (t, 2H), 3.178 (q, 6H).
1-Methylimidazole hydrogen sulfate ([Hmim][HSO4]): the
preparation method was according to the previous ref. 22 the
1-methylimidazole (0.1 mol) was charged into a 100 mL round-
bottom ask in a water bath. Sulfuric acid (0.1 mol) aqueous
solution was added slowly into the reaction solution at 0 ꢀC
under stirring. The mixture was stirred for 10 h to complete the
reaction. The resulting solution was then dried in vacuum at
95 ꢀC to remove the water until the weight of the residue
remained constant. The yield of [Hmim][HSO4] was 98%. 1H
NMR (300 MHz, D2O-d2, 25 ꢀC, TMS): d (ppm) 3.563 (s, 3H),
7.083 (s, 2H), 8.307 (s, 1H).
N-Triethylammonium hydrogen sulfate ([Et3NH][HSO4]): the
preparation process was similar to the procedure of [Hmim]-
[HSO4].23 The yield of [Et3NH][HSO4] was 96%. 1H NMR (300
MHz, DMSO-d6, 25 ꢀC, TMS): d (ppm) 1.17 (t, 9H), 3.08 (m, 6H),
8.89 (s, 1H).
1-Butyl-3-methylimidazolium hydrogen sulfate ([Bmim]-
[HSO4]): the synthetic method was in accordance with the
previous literature.24 1-Methylimidazole (0.1 mol), 1-bromobu-
tane (0.1 mol) and ethanol (50 mL) were charged into a 100 mL
Experimental
Materials
1-Methylimidazole (purity $99%), and 1,4-butyl sultone (purity
$99%) were purchased from Aladdin Chemical Reagent Co. Ltd.
(Shanghai, China). The ion-exchange resin Amberlyst-15 was
obtained from Rohm and Haas. Other reagents such as salicylic
acid, ethanol, n-propanol, n-butanol, n-pentanol, isoamyl alcohol,
n-hexanol, sulfuric acid, phosphorictungs acid, triethylamine,
1-bromobutane, sodium hydrogen sulfate, and acetone were of
analytical grade and used without any further purication.
Preparation and characterization of BAILs
Six BAILs composed of [HSO4] anion and different cations
(Fig. 1) were synthesized by a two-step method, which is similar
to the procedure described in our previous work and other
ref. 21–25. The detailed preparation procedure and 1H NMR
spectrum (Bruker DPX-300) of these BAILs were as follows:
1-(4-Sulfonic acid) butyl-3-methylimidazolium hydrogen
sulfate ([BSmim][HSO4]): the preparation of this BAIL was
similar to those used in the previous literature.21
Fig. 1 Structure of six Brønsted acidic ionic liquids.
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round-bottom ask. Then, the mixtures were vigorous stirring at
80 ꢀC for 6 h. Aer that, the solvent and unreacted components
were removed by rotary evaporation, and [Bmim][Br] (about 20 g)
white solid was obtained aer being dried at 60 ꢀC under vacuum
for 5 h. Then, a stoichiometric amount of sodium hydrogen
sulfate was added to the mixture solution of [Bmim][Br] and
anhydrous acetone. The metathesis reaction was stirred for 24 h
at room temperature and the white solid formed was then
ltered off. The volatiles were nally removed in vacuum, and the
BAIL [Bmim][HSO4] of light yellow liquid was obtained with a
yield of 85%. The concentration of Brꢂ in [Bmim][HSO4] was
measured by Mohr titration, and the related impurity was less
than 0.02 wt%. 1H NMR (300 MHz, D2O-d2, 25 ꢀC, TMS); d (ppm)
1.058 (t, 3H), 1.656 (m, 2H), 1.921 (m, 2H), 3.785 (s, 3H), 4.144 (t,
2H), 7.327 (s, 1H), 7.398 (s, 1H), 8.637 (s, 1H).
Computational methods
Density functional theory (DFT) was employed to perform the
geometry optimizations at the 6-31++G** level using the
Gaussian 09 program package.28 Each nal optimized structure
of BAILs was checked to be a true minimum through frequency
calculation at the corresponding levels.
General procedure for the esterication reaction
In a typical run, the esterication reaction was performed in a 25
mL round-bottom ask with a magnetic stirrer. The temperature
of the reaction mixture was controlled using a temperature
controller with an accuracy of ꢃ0.1 ꢀC. Weighed amounts of sal-
icylic acid, isoamyl alcohol and catalyst (BAILs, phosphotungstic
acid or Amberlyst-15 resin) were mixed and typically allowed to
proceed for 0.5–2.5 h with the vigorous stirring and heating at the
designed temperature. About 0.2 mL of liquid sample was taken
out from the reactor at regular intervals, and the composition of
products was analyzed by gas chromatography-ame ionization
detector (GC-FID) (Agilent 6890, HP-1 column, 30 m ꢁ 0.32 mm ꢁ
Butyltriethylammonium hydrogen sulfate ([N2224][HSO4]):
the preparation method is referenced to our previous work.25
the synthesis of [N2224][HSO4] was carried out according to the
procedure described for [Bmim][HSO4] above from triethyl-
amine and 1-bromobutane. [N2224][HSO4] of colorless viscous
liquid was then obtained with a yield of 88%. The concentration
of Brꢂ in [N2224][HSO4] was measured by Mohr titration, and the
1mm)usingN2 asthecarriergasataowrateofabout3mLminꢂ1
.
The GC-FID conditions were as follows: detector temperature, 250
ꢀC; injection port temperature, 200 ꢀC; oven temperature, 180 ꢀC.
Then the conversion and selectivity were calculated according to
the area of chromatograph peak. Aer the reaction, the esteri-
cation system oen forms a liquid–liquid biphase. The upper
phase, containing mainly the desired ester, can be isolated simply
by decantation, whereas the lower phase composed of BAIL and
water can be easily recovered and reused aer treatment under
vacuum to remove water. Aer that, the as synthesized isoamyl
salicylate was characterized by 1H NMR (Bruker DPX-300) and FT-
IR(ThermoNicolet870): 1HNMR (300MHz, CDCl3-d, 25 ꢀC, TMS);
d (ppm) 1.01 (d, 6H), 1.67 (q, 2H), 1.80 (m, 1H), 4.38 (t, 2H), 6.87 (t,
1H), 6.96 (d, 1H), 7.44 (t, 1H), 7.82 (d, H), 10.86 (s, 1H). IR n ¼ 3185,
2960, 2873, 1676, 1614, 1486, 1302, 1089, 756 cmꢂ1. Therefore, it
was validated from these characterization results that isoamyl
salicylate was successfully synthesized by using BAILs as catalysts.
1
related impurity was less than 0.02 wt%. H NMR (300 MHZ,
D2O-d2, 25 ꢀC, TMS): d (ppm) 0. 81 (t, 3H), 1.09 (t, 9H), 1.26 (m,
2H), 1.49 (m, 2H), 3.00 (m, 2H), 3.14 (q, 6H).
UV-vis determination of acidity
The Brønsted acidity is an essential property of BAILs, and
Hammett function (H0) is oen an effective way to assess the
acidity of BAILs in organic solvents, which is calculated by
evaluating the protonation extent of indicator base using an
Agilent 8453 UV-visible spectroscopy.19,26,27 In our experiment,
the Hammett acidity of six BAILs in ethanol solutions (80 mmol
Lꢂ1) had been investigated at 25 ꢀC, and 4-nitroaniline was used
as indicator (0.06 mmol Lꢂ1). The maximal absorbance of
unprotonated form of 4-nitroaniline was observed at 371 nm,
and the pK(I)aq. value of 4-nitroaniline is 0.99. The results are
then listed in Table 1, and the acidity order is as follows:
[BSmim][HSO4] > [BSEt3N][HSO4] > [Hmim][HSO4] > [Et3NH]-
[HSO4] > [Bmim][HSO4] > [N2224][HSO4].
Results and discussion
Catalyst performance
The esterication reaction of salicylic acid with isoamyl alcohol
in the presence of six BAILs composed of various cations was
conducted to assess their catalytic activities at the temperature of
130 ꢀC, with salicylic acid to isoamyl alcohol molar ratio of 1 : 2,
and catalyst loading of 20 wt% (based on the mass of salicylic
acid). The results are then summarized in Table 2. Combined
with the Brønsted acidity values from Table 1, it was obvious that
these six BAILs catalysts exhibited different catalytic activities,
Table 1 Hammett acidity function (H0) values of H2SO4 and different
BAILsa
Entry
IL
Amax
[I] (%)
[IH+] (%)
H0
1
2
3
4
5
6
7
—
1.03
100
51.5
52.2
90
93.5
94.8
96.1
0
—
1.02
[BSmim][HSO4]
[BSEt3N][HSO4]
[Hmim][HSO4]
[Et3NH][HSO4]
[Bmim][HSO4]
[N2224][HSO4]
0.530
0.538
0.927
0.963
0.976
0.990
48.5
47.8
10
6.5
5.2
3.9
1.03 and the catalytic performance of a BAIL was mainly relevance to
1.94
2.15
2.25
its Brønsted acidity. The BAIL possessing strong acidity shows
the relatively high catalytic activity.27,29 For example, [BSmim]-
[HSO4] and [BSEt3N][HSO4] beared with an butyl sulfonic acid
2.38
exhibited the strongest acidities among all the BAILs investi-
a
The Hammett function (H0) is written as: H0 ¼ pK(In)aq. + log([I]/[IH+]),
where pK(I)aq. is the pKa value of the indicator referred to an aqueous
solution. [I] and [IH+] are the molar concentrations of the
unprotonated and protonated forms of the indicator, respectively.
gated. Thus, the SO3H-functionalized BAILs [BSEt3N][HSO4] and
[BSmim][HSO4] catalyzed the esterication reaction to have high
conversions of 90.5% and 90.8% at 2 h, respectively. By
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Table 2 Results of the esterification reaction of salicylic acid with
isoamyl alcohol in the presence of different catalystsa
functionalized BAILs. This is because that phosphorictungs
acid can be dissolved in water or alcohol and act as homoge-
neous catalyst in the esterication reaction. Nevertheless, the
heterogeneous catalysis of Amberlyst-15 resin existed high
transfer resistance and resulted in relative lower catalytic
performance. Thus, [BSEt3N][HSO4] and [BSmim][HSO4] are
considered to be promising substitutes in the synthesis of iso-
amyl salicylate, and then they were chosen as the catalysts for
the following factor experiments.
Conversion of
Entry
Catalyst
salicylic acid (%)
1
2
3
4
5
6
7
8
9
[BSmim][HSO4]
[BSEt3N][HSO4]
[Hmim][HSO4]
[Et3NH][HSO4]
[Bmim][HSO4]
[N2224][HSO4]
Amberlyst-15
Phosphorictungs acid
Blank
90.8
90.5
25.6
20.3
17.4
18.7
55.1
87.2
1.2
Optimization of reaction conditions
The investigation on the effect of temperature is very important
because this information is essential to the synthesis of isoamyl
salicylate. Thus, the esterication reaction of salicylic acid was
performed using [BSEt3N][HSO4] and [BSmim][HSO4] as catalysts
at 110 ꢀC, 120 ꢀC, 130 ꢀC and 140 ꢀC, with salicylic acid to isoamyl
alcohol molar ratio of 1 : 2, and catalyst loading of 20 wt%. As
a
Reaction conditions: the esterication reaction was conducted at the
temperature of 130 ꢀC, with salicylic acid to isoamyl alcohol molar
ratio of 1 : 2, and catalyst dosage of 20 wt% for 2 h. No byproducts
were found by GC.
comparison, in the presence of weak acidic catalyst [Hmim]- seen from Fig. 2, the conversion of salicylic acid increased rapidly
[HSO4] and [Et3NH][HSO4], the conversions of salicylic acid at 2 h with the increase in the reaction temperature. For [BSmim]
were only 25.6% and 20.3%, respectively, being signicantly less [HSO4], the conversion of salicylic acid at 2 h increased obviously
than those of [BSEt3N][HSO4] and [BSmim][HSO4] under iden- from 71% to 90% with the rise of temperature from 110 ꢀC to 130
tical conditions. [Bmim][HSO4] and [N2224][HSO4] also induced ꢀC. This means that high temperature is facilitated to improve the
very low conversions of salicylic acid at 2 h, due to their neutral or conversion of salicylic acid. However, while the temperature was
weakly acidic cations. Therefore, it was demonstrated that further increased to 140 ꢀC, the conversion of salicylic acid at 2 h
varying the cationic structure of BAIL can change its acidity was improved with a limited extent, due to the limitation of
conveniently, and the strong acidity enables the BAIL to perform chemical reaction equilibrium. This phenomenon was also
excellent catalytic activities in esterication.
similarly observed in the esterication system containing
In addition, in order to understand the role of cations in the [BSEt3N][HSO4] catalyst. In addition, side product (such as
acidities of BAILs, DFT calculations were performed using the phenol) would be probably produced, whilst the temperature was
Gaussian 09 program. The fully optimized geometries of BAILs higher than 150 ꢀC.2,6 This illustrated that the over high reaction
were then presented in Fig. S1–S4 in the ESI.† As seen from temperature was negative to the conversion of salicylic acid and it
calculation results, the SO3H-functionalized BAILs [BSmim]- was unnecessary for the esterication reaction. Therefore, an
[HSO4] and [BSEt3N][HSO4] are found to have much stronger optimized reaction temperature would choose at 130 ꢀC for the
intramolecular hydrogen bonds than [Hmim][HSO4] and SO3H-functionalized BAILs to reach the best catalytic activity.
[Et3NH][HSO4]. For example, the strong interactions between
The catalyst loading was expressed as the weight ratio of the
the anion with the sulfonic acid proton exhibit in [BSmim]- catalyst to the weight of salicylic acid. Catalyst loading was
ꢀ
˚
[HSO4], with H24/O23 distance of 1.607 A and H35/O25 distance varied from 10 wt% to 25 wt% at a temperature of 130 C and
˚
of 1.638 A (Fig. S1†). As such, the O/H distances of hydrogen with salicylic acid to isoamyl alcohol molar ratio of 1 : 2. The
˚
bonds H37/O35 and H45/O38 in [BSEt3N][HSO4] are 1.722 A and effect of different catalyst loadings on the conversion of salicylic
˚
1.652 A, respectively (Fig. S2†). Due to these strong interactions, acid was then given in Table 3. It was demonstrated that, with
the H–O bond of the sulfonic acid group has been lengthened an increase in the relative amount of BAILs, the total number of
and thus the SO3H-functionalized BAILs are inclined to exist in a acid sites available for the reaction increased and thus the rate
form of the zwitterion and H2SO4.30 As a result, the SO3H-func- of esterication reaction was enhanced, resulting in a higher
tionalized BAILs containing two adjacent acid sites show strong conversion of salicylic acid. For example, the conversion of
acidities as well as H2SO4 does, and then they catalyze the salicylic acid increased from 77.4% to 90.8% with the increase
esterication reaction to have the considerable conversions.
in the amount of [BSmim][HSO4] from 10 wt% to 20 wt%.
For comparison, the esterication reaction without catalyst However, only a fair change was obtained in the conversion of
or in the presence of Amberlyst-15 resin and phosphorictungs salicylic acid when the amount of [BSmim][HSO4] catalyst was
acid was also investigated. As seen from Table 2, it was indi- further added to 25 wt%, 30 wt%, and 35 wt%, respectively.
cated that the conversion of salicylic acid at 2 h in the blank Beyond a certain catalyst loading, the conversion of salicylic
reaction was less than 5%, showing that the esterication acid has not a large degree of improvement. Therefore, it
reaction is very difficult to occur without catalyst. In addition, it suggests that too much dosage of BAILs is not very necessary for
was found that the conversion of salicylic acid in the presence of the synthesis of isoamyl salicylate. In this work, considering the
Amberlyst-15 resin was lower than that in the cases of [BSEt3N]- reaction rate and the cost of BAILs, 20 wt% was taken as the
[HSO4] and [BSmim][HSO4], whereas phosphorictungs acid optimum catalyst loading and used in most of the esterication
showed the catalytic activity almost as high as those two SO3H- experiments.
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Fig. 2 Effect of temperature on the conversion of salicylic acid with time using (a) [BSmim][HSO4] and (b) [BSEt3N][HSO4] as catalysts at (-)
110 ꢀC; (C) 120 ꢀC; (:) 130 ꢀC; (;) 140 ꢀC, with salicylic acid to isoamyl alcohol molar ratio of 1 : 2, and catalyst loading of 20 wt%.
Table 3 Effect of catalyst loading on the conversion of salicylic acida
Conversion of salicylic acid (%)
Entry
Catalyst loading
[BSmim][HSO4]
[BSEt3N][HSO4]
1
2
3
4
5
6
10 wt%
15 wt%
20 wt%
25 wt%
30 wt%
35 wt%
77.4
85.1
90.8
91.7
91.8
91.4
79.1
86.9
90.5
91.3
91.5
91.1
a
Reaction conditions: the esterication reaction was conducted at the
temperature of 130 ꢀC with salicylic acid to isoamyl alcohol molar
ratio of 1 : 2 for 2 h. No byproducts were found by GC.
Fig. 3 Effect of molar ratio on the conversion of salicylic acid in the
presence of [BSmim][HSO4] at the temperature of 130 ꢀC with catalyst
loading of 20 wt% for 2 h.
Fig. 3 represents the effect of salicylic acid to isoamyl alcohol
molar ratio on the esterication reaction in the presence of
[BSmim][HSO4] as catalyst. It was illustrated that as the amount
of isoamyl alcohol increased, the conversion of salicylic acid
was rapidly improved. For example, the conversion was obvi-
Esterication of salicylic acid with other alcohols
ously increased from 76% to 91% in 2 h, while the initial molar Using the above-mentioned optimized conditions, we had
ratio of salicylic acid to isoamyl alcohol increased from 1 : 1 to further investigated the esterication of salicylic acid with other
1 : 2. Higher conversions were obtained equally with the molar alcohols in the presence of the BAILs [BSmim][HSO4] and
ratio of 1 : 3 and 1 : 4 in comparison to the case of 1 : 1. Thus, it [Bmim][HSO4] as catalysts, and the experimental results are
was demonstrated that increasing the amount of isoamyl given in Table 4. It was showed that [Bmim][HSO4] was of very
alcohol was benecial to the forward reaction since the esteri- poor activity in these other esterication reactions, whereas
cation of salicylic acid with isoamyl alcohol is a reversible [BSmim][HSO4] performed considerable conversions owing to
reaction. In addition, it was also found that the reaction rates in its strong acidity. For example, the conversions of salicylic acid
the initial acid to alcohol ratios of 1 : 3 and 1 : 4 were just in the esterication systems containing n-butanol, n-pentanol,
comparable to the case of 1 : 2, indicating that a further and n-hexanol were 89.5%, 89.6%, and 80.6%, respectively. In
increase in the amount of isoamyl alcohol can not continue to addition, it was also found that the conversion of salicylic acid
improve the conversion of salicylic acid. This means that an increases as the carbon chain of alcohol prolongs (entries 1–6).
initial excess of isoamyl alcohol is not necessary for the esteri- This was because the change in the formation of liquid–liquid
cation of salicylic acid. In summary, the optimal molar ratio of biphases, which was pointed out in our previous work.25 For
salicylic acid to isoamyl alcohol was preferred to be 1 : 2.
example, in the cases of ethanol, iso-propanol, and n-propanol,
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Table 4 Results of the esterification reaction of salicylic acid with
be easily separated from the reaction mixture by simple
decantation. Then the catalyst BAIL could be recovered and
reused in the next run aer removing the water under vacuum
at 95 ꢀC for 12 h. The conversions of salicylic acid from six
consecutive runs thus are as shown in Fig. 4. The experimental
results implied that the BAIL [BSmim][HSO4] exhibited nearly
no obvious change in activity and 88.3% conversion was
maintained aer six recycle, which shows that the BAIL catalyst
is stable enough and the presence of water produced in the
esterication reaction has less impact on the catalytic activity of
[BSmim][HSO4]. The slight decrease in the activity may be
ascribed to the slight loss of [BSmim][HSO4] due to the trans-
ferring of samples during the regeneration.
various alcoholsa
Conversion of salicylic acid (%)
Entry
Alcohol
[BSmim][HSO4]
[Bmim][HSO4]
1
2
3
4
5
6
Isopropyl alcohol
Ethanol
n-Propanol
n-Butanol
n-Pentanol
n-Hexanol
51.1
63.4
73.5
89.5
89.6
80.6
1.2
4.3
6.8
10.1
11.6
6.5
a
Reaction conditions: [BSmim][HSO4] and [Bmim][HSO4] as catalysts
with a dosage of 20 wt% at 130 ꢀC for 2 h, molar ratio of salicylic acid
to alcohol ¼ 1 : 2. No byproducts were found by GC.
Conclusions
both of them have better solubility in the mixture of reactants
and products owing to their good hydrophilicity. These esteri-
cation systems then are difficult to form phase splitting,
leading to the decrease in the reaction rates and thus the
conversions. By contrast, the alcohols n-butanol, n-pentanol,
and n-hexanol have relatively long carbon chain and are partly
immiscible in the BAIL catalyst and water. Thus, their esteri-
cation systems could induce fast phase splitting. The product of
ester forms a light phase, while water and BAIL are combined to
be a heavy phase. Subsequently, the formation of liquid–liquid
biphases facilitates the shi of chemical equilibrium to the
product side and thus high conversions.25,31
Six BAILs composed of [HSO4] had been prepared successfully
and applied in the synthesis of isoamyl salicylate in this study.
The experimental results demonstrated that the SO3H-func-
tionalized BAILs [BSmim][HSO4] and [BSEt3N][HSO4] exhibited
extremely strong acidities and played excellently catalytic
activities and selectivities in the esterication of salicylic acid
with isoamyl alcohol and other alcohols such as n-butanol,
n-pentanol, and n-hexanol. The fully optimized geometries of
[BSmim][HSO4] and [BSEt3N][HSO4] also manifest that their
strong acidities are derived from the strong interactions
between the anion with the sulfonic acid group. In addition, it
was found that increasing the reaction temperature, catalyst
loading and the amount of isoamyl alcohol could improve the
conversion of salicylic acid effectively, and the optimal condi-
tion was set as follows: temperature of 130 ꢀC, initial molar ratio
of 1 : 2, and catalyst loading of 20% w/w. [BSmim][HSO4] also
could be recovered easily and used repetitively at least six times
without signicant losses of activity and quantity. Therefore,
the catalyst SO3H-functionalized BAILs prepared in this study
was proved to be an excellent recyclable homogeneous catalyst
for the synthesis of isoamyl salicylate and showed potential
application in industry.
Catalyst recycling test
The recovery and reuse of catalysts is highly preferable for a
greener chemical process. Thus, the reusability of the BAIL
[BSmim][HSO4] catalyst was investigated in the esterication
reaction of salicylic acid with isoamyl alcohol (temperatures of
130 ꢀC, initial molar ratio of 1 : 2, catalyst loading of 20% w/w,
and reaction time of 0.5 h and 2 h). In each cycle, the esteri-
cation system oen forms a liquid–liquid biphase at the end of
the reaction. The lower phase composed of BAIL and water can
Acknowledgements
This work was supported by the National Natural Science
Foundations of China (no. 21206063), the National High
Technology Research and Development Program of China (no.
2012AA03A609), and the Science & Technology Supporting
Programs and the International Technological Cooperation
Programms of Jiangxi Provincial Department of Science and
Technology (no. 20123BBE50081 and 20132BDH80003).
Notes and references
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Fig. 4 The recycle test of [BSmim][HSO4] catalyst (red column for 0.5
h and green column for 2 h).
6 | RSC Adv., 2014, 4, 1–7
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