Effects of acidity and immiscibility of lactam-based Bronsted-acidic ionic liquids on their catalytic performance for esterification

Article abstract of DOI:10.1039/b921081d

Several lactam-based Bronsted-acidic ionic liquids with different acidities were synthesized and applied to the esterification of carboxylic acids with alcohols. High conversion and perfect selectivity were obtained under mild conditions. Among the ionic liquids investigated, those having a methyl sulfonate anion (which has weaker acidity than those with a tetrafluoroborate anion) afforded the highest activity for esterification. The results indicated that the acidity and immiscibility of Bronsted-acidic ionic liquids has a synergistic effect on their esterification performance. Furthermore, after removal of water under vacuum, such ionic liquids could be reused several times without substantial loss of activity.

Full text of DOI:10.1039/b921081d

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www.rsc.org/greenchem | Green Chemistry
Effects of acidity and immiscibility of lactam-based Brønsted-acidic ionic
liquids on their catalytic performance for esterification†
Hancheng Zhou, Jing Yang, Linmin Ye, Haiqiang Lin and Youzhu Yuan*
Received 12th October 2009, Accepted 20th January 2010
First published as an Advance Article on the web 19th February 2010
DOI: 10.1039/b921081d
Several lactam-based Brønsted-acidic ionic liquids with different acidities were synthesized and
applied to the esterification of carboxylic acids with alcohols. High conversion and perfect
selectivity were obtained under mild conditions. Among the ionic liquids investigated, those
having a methyl sulfonate anion (which has weaker acidity than those with a tetrafluoroborate
anion) afforded the highest activity for esterification. The results indicated that the acidity and
immiscibility of Brønsted-acidic ionic liquids has a synergistic effect on their esterification
performance. Furthermore, after removal of water under vacuum, such ionic liquids could be
reused several times without substantial loss of activity.
zolium hydrosulfate ([BMIm][HSO4]),20 N-propanesulfonic
Introduction
acid pyridinium tetrafluoroborate ([PSPy][BF4]),21 N-methyl-
Many organic esters, ranging from aliphatic to aromatic, are
2-pyrrolidonium methyl sulfonate ([NMP][CH3SO3]),22
produced in high volume for their extensive application in
triphenyl(propyl-3-sulfonyl)phosphonium
para-toluene-
pharmaceuticals, agrochemicals, foods and fine chemicals.1 They
are mainly manufactured by the fundamental esterification
reaction. However, esterification, which was formerly catalyzed
by inorganic acids, e.g. sulfuric acid,2 faced the unavoidable
problems of deactivation of catalyst,3 side reaction and corrosion
of equipment and tedious isolation of catalyst-product. To
overcome the drawbacks caused by homogeneous catalysts, a
number of other catalyst systems such as resins,4 supported
mineral acids,5 heteropoly acids,6 and zeolites7 have been
developed. These catalysts, due to the tendency to deactivation,
operation loss, and high transfer resistance, could not be used in
large-scale production.8 Therefore, the development of a highly
efficient, easily separable catalyst system for the esterification of
carboxylic acid with alcohol is still highly desired.
Room-temperature ionic liquids (ILs), owing to their
negligible volatility, selective solubility and tunable structures,9-11
have been successfully applied in several classical organic
reactions.12-15 Protic ionic liquids are an interesting subset of
ILs. One of the key properties of such ILs is that the proton
transfer from the acid to the base leads to the presence of
proton-donor and -acceptor sites, which can be used to build
up a hydrogen-bonded network.16,17 Efficient esterification
can be obtained by use of ILs, specifically protic Brønsted-
acidic ILs,18-24 such as 1-butyl-3-methylimidazolium para-
toluenesulfonicacid ([BMIm][PTSA]),18 1-methylimidazolium
tetrafluoroborate ([HMIm][BF4]),19 1-butyl-3-methylimida-
sulfonate ([Ph3P(CH2)3SO3H][PTSA]),23,24 etc. The catalytic
activities varied considerably depending on the properties of
ILs. For instance, a satisfactory yield for the esterification of
acetic acid and 1-butanol was obtained with both [HMIm][BF4]
and [NMP][CH3SO3], but the catalyst [NMP][CH3SO3] showed
higher activity.19,22 In the case of using [HMIm][BF4] as a
catalyst, very high yields could be achieved when the volume
of the IL catalyst was equal to carboxylic acid and the
◦
reaction temperature was as high as 110 C.19 In the case of
[NMP][CH3SO3], however, the reaction proceeded smoothly by
using a molar ratio of IL catalyst to substrate of 25% based on
carboxylic acid (used throughout this investigation) at room
temperature.22 In order to better understand the essence of
such difference and to construct better Brønsted-acidic ILs
for esterification, it is necessary to explore the effect of acidity
and immiscibility of Brønsted-acidic ILs on their catalytic
performance.
In this work, N-methyl-2-pyrrolidone (NMP) and caprolac-
tam (CP) were chosen as precursors to synthesize lactam-based
Brønsted-acidic ILs with different Hammett acidity function
(H0) (Scheme 1). The results showed that the acidity and
immiscibility of such lactam-based Brønsted-acidic ILs had a
synergistic effect on their esterification performance.
State Key Laboratory of Physical Chemistry of Solid Surfaces, National
Engineering Laboratory for Green Chemical Productions of
Alcohols-Ethers-Esters, College of Chemistry and Chemical
Engineering, Xiamen University, Xiamen, 361005, P. R. China.
E-mail: yzyuan@xmu.edu.cn; Fax: +86 592-2183047; Tel: +86
592-2181659
† Electronic supplementary information (ESI) available: NMR, TG-
DTA, acidity, UV-vis and solubility data. See DOI: 10.1039/b921081d
Scheme 1 The synthesis of lactam-based Brønsted-acidic ILs.
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Table 1 Thermal stability of lactam-based Brønsted-acidic ILs
IL
[CP][BF4]
168 ◦
[CP][CH3SO3]
186 ◦
[CP][NO3]
100 ◦
[CP][TFA]
87 ◦
[NMP][BF4]
130 ◦
[NMP][CH3SO3]
160 ◦
[NMP][NO3]
78 ◦
[NMP][TFA]
75 ◦
C
a
Td
C
C
C
C
C
C
C
a Td was recorded with 10% of mass loss of IL.
Methyl yellow (MY) (15 mg L-1, pKa=3.3), 4-
phenylazodiphenylamine (PADA) (10 mg L-1, pKa=1.5)
and 2-nitrophenylamine (NPA) (5 mg L-1, pKa=-0.2) in
dichloromethane were chosen as the basic indicators. The
concentration of each lactam-based Brønsted-acidic IL was set
at 80 mmol L-1 in dichloromethane.
Experimental
General remarks
All commercial chemicals were used as received. NMR measure-
ments were performed on a Bru¨ker AV-400 Fourier transform
NMR spectrometer using an inner capillary filled with CDCl3
1
for H and 13C NMR. Chemical shifts were reported in parts
Esterification procedure
per million (ppm, d). Thermal gravimetric analysis (TGA) was
performed with a Simultaneous Thermal Analysis-STA 409EP.
The thermal decomposition temperature (Td) was recorded with
10% mass loss of lactam-based Brønsted-acidic ILs, with a
scan rate of 10 C min-1 under a N2 atmosphere. UV-visible
spectroscopy was conducted on a Shimadzu 2100 UV-visible
spectrophotometer.
The reaction was carried out in a glass reactor with a reflux
condenser and oil-bath. The acid and alcohol were added
to the lactam-based Brønsted-acidic IL. The esterification
was performed under different profiles of reaction time and
temperature with vigorous stirring. Qualitative and quantitative
analyses were conducted with an Agilent 7890A/5975C GC/MS
and a capillary GC (Rtx-1: 30 m ¥ 0.25 mm ¥ 0.25 mm) equipped
with a flame ionization detector, respectively. The concentration
of reactant and product were calculated using butyl butyrate as
an internal standard. After the reaction, the ester and IL were
separated by decantation. The IL was reused after removal of
water under 1–5 mmHg vacuum at 110 ◦C for 1 h.
◦
Preparation and characterization
Two series of lactam-based Brønsted-acidic ILs [NMP][X] and
[CP][X] (X = BF4, NO3, CH3SO3 and TFA (trifluoroacetate))
were synthesized according to the procedure previously reported
in the literature.25,26 As an example, the Brønsted-acidic IL
caprolactam trifluoroacetate ([CP][TFA]) was synthesized as
following.
Results and discussion
Benzene (30 mL) was added to a 100 mL flask containing
11.32 g of CP (0.10 mol) with stirring. Then, 11.40 g triflu-
oroacetic acid (0.10 mol) was added dropwise into the above
flask, which was immersed in an ice bath, within ca. 20 min.
After 4 h at room temperature, benzene was removed under
reduced pressure. The resultant was further dried at 90 ◦C under
1–5 mmHg for 1 h, affording the IL [CP][TFA].
The lactam-based Brønsted-acidic ILs were synthesized with
high yields of 92–98%. Most of them were miscible with polar
organic solvents such as 1-butanol, acetonitrile, and acetic acid
but insoluble or partly soluble in non-polar organic solvents such
as benzene, ethyl ether, and n-butyl acetate. The solubility of ILs
in reaction mixture can be measured according to the procedure
described in the literature using UV-visible spectroscopy.28 For
instance, the solubility of IL [CP][CH3SO3] in n-butyl acetate
was as low as 0.54 g per 100 mL, while the solubility was 194 g
per 100 mL in acetic acid and 170 g per 100 mL in 1-butanol. The
thermal stability of these Brønsted-acidic ILs was characterized
by the thermal decomposition temperature (Td) in Table 1 and
was found to be in the range 75–186 ◦C.
1
The NMR data for the IL [CP][TFA] were as follows: H
NMR (CDCl3): 1.67–1.79 (m, 6H), 2.50 (q, J = 5.2, 2H), 3.28
(q, J = 5.6, 2H), 8.43 (s, 1H), 14.81 (s, 1H). 13C NMR (CDCl3):
22.38, 28.07, 29.94, 34.83, 43.13, 110.83–119.39 (q, CF3, J =
288.0), 160.80, 181.57. Td ([CP][TFA]) = 87 ◦C.
Acidity measurement
The acidities of these Brønsted-acidic ILs, characterized as
the value H0, are reported in Table 2. The results indicated that
among the lactam-based Brønsted-acidic ILs studied, the IL
[NMP][BF4] had the strongest relative acidity of H0 = -0.25,
while the IL [CP][TFA] showed the weakest relative acidity of
H0 = 4.56. The relative acidities of the Brønsted-acidic ILs
had the following order of H0 value: [NMP][BF4] (-0.25) >
[CP][BF4] (-0.22) > [NMP][CH3SO3] (0.95) > [CP][CH3SO3]
(0.98) > [NMP][NO3] (2.83) > [CP][NO3] (2.94) > [NMP][TFA]
(4.46) > [CP][TFA] (4.56). Clearly, the anions played a crucial
role in defining the acidity of the Brønsted-acidic ILs. Changing
the anions in the Brønsted-acidic ILs could bring about large
differences in the H0 value. For example, the H0 values of ILs
[CP][BF4], [CP][NO3] and [CP][TFA] were -0.22, 2.94 and 4.56,
respectively. However, the effect of cations on the H0 value was
The acidic scales of the lactam-based Brønsted-acidic ILs were
measured by UV-vis spectra with basic indicators according to
the procedure reported in the literature.25,27 Based on eqn (1),
where H0 is the Hammett acidity function which represents the
relative acidity of IL; [I] and [IH+] are the molar concentrations
of, respectively, the unprotonated and protonated forms of the
indicator. Based on the Beer–Lambert law the absorption is
proportional to the concentration of absorbing species in the
material, and on condition of the same light path length, the
ratio of [I]/[IH+] can be calculated by the absorbance difference
of basic indicator measured after addition of lactam-based
Brønsted-acidic ILs.
H0 = pK(I) + log([I]/[IH+])
(1)
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Table 2 Calculation of Hammett function for different lactam-based
Brønsted-acidic ILs
Table 4 Results of esterification of acetic acid with 1-butanol catalyzed
by different lactam-based Brønsted-acidic ILs
Substance
Amax
[I]%
[HI]%
H0 ( 0.05)
Entry
IL
Acid conv. (%)
Ester selectivity (%)
NPA
2.46
1.16
1.21
2.44
0.57
0.53
2.71
0.69
0.82
2.52
2.57
100
47
49
100
89
96
100
77.9
79.9
94.8
93.0
0
—
1
2
3
4
5
6
7
8
[CP][TFA]
[CP][NO3]
69.4
89.0
94.8
92.8
74.1
89.5
95.5
93.0
100
100
100
100
100
100
100
100
[NMP][BF4]a
[CP][BF4]a
PADA
53
51
0
11
4
-0.25
-0.22
—
0.98
0.95
—
2.83
2.94
4.46
4.56
[CP][CH3SO3]
[CP][BF4]
[NMP][TFA]
[NMP][NO3]
[NMP][CH3SO3]
[NMP][BF4]
[NMP][CH3SO3]b
[CP][CH3SO3]b
MY
0
[NMP][NO3]c
[CP][NO3]c
[NMP][TFA]c
[CP][TFA]c
22.1
20.1
5.2
7.0
Reaction conditions: 20 mmol acetic acid, 20 mmol 1-butanol and
4 mmol lactam-based Brønsted-acidic IL at 30 ◦C for 4 h.
a NPA (2-nitrophenylamine) was used as basic indicator. b PADA (4-
phenylazodiphenylamine) was used as basic indicator. c MY (methyl
yellow) was used as basic indictor.
such as ethers were not detected by capillary GC. Combined
with the above-mentioned acidity order of these Brønsted-
acidic ILs ranked by H0, it was obvious that the acidity had
a great effect on the catalytic performance of the esterification.
The catalytic activities of these catalysts with the same anions
but different cations were of an equal level. For example,
in the presence of catalysts [CP][TFA] and [NMP][TFA], of
which the acidity was in the weakest level among the catalysts
investigated, the conversion of acetic acid was only 69.4% and
74.1%, respectively (Table 4, entries 1 and 5). The catalysts with
stronger acidity, either with nitrate anion, such as [CP][NO3] and
[NMP][NO3], or with tetrafluoroborate anion, such as [CP][BF4]
and [NMP][BF4], showed higher activity for the esterification
(Table 4, entries 2 and 6, 4 and 8). Compared with the catalysts
having a nitrate anion, higher activity was obtained with use of
[CP][BF4] or [NMP][BF4]. For instance, when the esterification
was catalyzed by [CP][BF4] or [NMP][BF4], the conversion of
acetic acid reached 92.8% and 93.0%, while about 89.0% and
89.5% was achieved in the cases of [CP][NO3] or [NMP][NO3],
respectively. The above results revealed that the acidity of the
lactam-based Brønsted-acidic ILs played a vital role in their
catalytic performance for the esterification. The activities of the
catalysts with the same anion but different cations were quite
close to each other. This may be due to the feature of acid
catalysis for the esterification.
The catalysts having a methyl sulfonate anion afforded the
highest catalytic activity among those investigated. When the ILs
[CP][CH3SO3] or [NMP][CH3SO3] were employed as catalysts
for the esterification under identical conditions, the conversion
of acetic acid reached 94.8% and 95.5%, respectively (Table 4,
entries 3 and 7). However, since the esterification is a class of
typical reversible reaction, the influence of ILs on the reaction
rate is in large part attributable to an expected Le Chatelier effect
in which a greater solubility of the IL catalyst in the reaction
mixture slows down the reaction rate. For instance, the catalyst
[CP][CH3SO3] was miscible with 1-butanol and acetic acid but
almost immiscible with n-butyl acetate, so the ester formed
could be separated from the layer of catalyst [CP][CH3SO3] and
reaction mixture promptly, resulting in a faster esterification
rate. However, the catalyst [CP][BF4] was miscible with the ester
and reaction mixture due to its good solubility in substrates and
also in ester (142 g per 100 mL in n-butyl acetate), so it showed
a slightly slower reaction rate than [CP][CH3SO3], although its
acidity was stronger than [CP][CH3SO3] (Table 4, entries 3 and
4). Therefore, the acidity of lactam-based ILs having methyl
Table 3 Results of esterification of acetic acid with 1-butanol catalyzed
by [CP][CH3SO3] under different conditions
Acid : butanol : Temp. Time Acid
Ester selectivity
Entry IL (molar ratio) (◦C)
(h)
conv. (%) (%)
1
2
3
4
5
6
7
8
5 : 5 : 1
5 : 5 : 1
5 : 5 : 1
5 : 5 : 1
5 : 6 : 1
5 : 5 : 2
8 : 8 : 1
5 : 5 : 1
30
30
30
30
30
30
30
100
0.5
2
4
6
4
4
4
4
67.4
91.6
94.8
97.2
98.5
95.1
91.1
100
100
100
100
100
100
100
100
95.0
Reaction conditions: 20 mmol acetic acid, 20 mmol 1-butanol and
4 mmol [CP][CH3SO3].
insignificant. For example, the H0 values for ILs [CP][NO3] and
[NMP][NO3] were 2.94 and 2.83, respectively.
Before investigating the acidity effect of lactam-based
Brønsted-acidic ILs on their catalytic performances in the
esterification, IL [CP][CH3SO3] was selected to optimize the
reaction conditions, due to its immiscibility with ester and higher
apparent viscosity. Conditions optimized included the molar
ratio of reactants, reaction temperature and time profile. The
results are listed in Table 3. It can be seen that 94.8% of acid
conversion with 100% of ester selectivity could be obtained with
20% molar ratio of the catalyst to substrate at 30 ◦C for 4 h.
Better results with 97.2% and 98.5% of ester yield could be
obtained when the reaction time was prolonged to 6 h (Table 3,
entry 4) or the concentration of 1-butanol was increased (Table 3,
entry 5), respectively. It is also noteworthy that no significant
increase either in acid conversion or ester selectivity occurred
as a result of the increase of molar ratio of catalyst to substrate
from 20% to 40% (Table 3, entries 3 and 6). A 100% conversion
was obtained at the cost of the ester selectivity by increasing the
reaction temperature to 100 ◦C (Table 3, entry 8). Therefore, in
consideration of efficiency, we decided to carry out the reaction
using a molar ratio of acid:alcohol:IL = 5 : 5 : 1 at 30 ◦C for 4 h.
Next, a set of lactam-based Brønsted-acidic ILs with different
anions and cations were employed as catalysts for the esterifica-
tion of acetic acid with 1-butanol. The results are summarized in
Table 4. It can be seen that these catalysts showed moderate to
high conversion and perfect selectivity at 30 ◦C. The by-products
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Table 5 Esterification of different carboxylic acids with alcohols catalyzed by IL [CP][CH3SO3]
Temp. (◦C) Time (h) Acid conv. (%)
IL solubility in ester
Entry Carboxylic acid
Alcohol
Ester selectivity (%) (g per 100 mL)a
1
2
3
4
5d
6
7
8
Acetic acid
Lauryl acid
Stearic acid
Acetic acid
Oxalic acid
Cyclopropanecarboxylic acid Ethanol
Undecenoic acid
Benzoic acid
1,4-Butanediol 60
4
8
8
4
4
8
8
8
93.6
92.1
91.8
83.8
93.1
87.0
89.4
84.0
100b
100
100
100
100
100
100
100
5.2
1.3
2.9c
4.1
8.0
8.6
3.9
8.0
Methanol
Methanol
30
60
Benzyl alcohol 60
1-Butanol
60
30
30
60
Methanol
Ethanol
Reaction conditions: 20 mmol carboxylic acid, 20 mmol alcohol and 4 mmol [CP][CH3SO3]. a The solubility of [CP][CH3SO3] in corresponding ester
was determined by UV-visible spectroscopy at 30 ◦C. b The total selectivity of mono- and diester. c The solubility was measured at 60 ◦C. d Using
20 mmol of oxalic acid and 40 mmol of 1-butanol and the total selectivity of dibutyl oxalate and butyl oxalate.
sulfonate anions and their immiscibility with esters may have a
synergistic effect on the catalytic performance for esterification.
To further verify the synergistic effect, the esterification
of acetic acid and 1-butanol was carried out in ILs 1-butyl-
3-methylimidazolium methanesulfonate ([BMIm][CH3SO3])
When the reaction time was prolonged to 8 h, the conversion
increased to 84.0% (Table 5, entry 8). Therefore, the lactam-
based ILs having a methanesulfonate anion are versatile catalysts
with high activity for the esterification of aliphatic or aromatic
carboxylic acids.
and
1-(propyl-3-sulfonyl)-3-methylimidazolium
trifluo-
The reusability of the catalyst [CP][CH3SO3] is illustrated in
Fig. 1. It can be seen that there was no obvious loss of catalytic
activity for the esterification of acetic acid with 1-butanol after
running consecutive recycling runs ten times. The conversion
of acetic acid remained >93.0% and the selectivity to n-butyl
acetate remained at 100%. The results indicate that lactam-based
Brønsted-acidic ILs having a methanesulfonate anion are a class
of robust and reusable catalysts for esterification under mild
conditions.
romethanesulfonate ([MIm(CH2)3SO3H][TfO]). The catalyst
[BMIm][CH3SO3] which shows non-acidity but good biphasic
behaviour with n-butyl acetate (1.44 g per 100 mL) had negligible
activity for the esterification, indicating that the acidity was the
key for the activity of catalysts for the esterification. As for the
catalyst [MIm(CH2)3SO3H][TfO], it possessed stronger acidity
(H0 = -1.8) and good biphasic behavior with the resultant
esters (0.39 g per 100 mL). Accordingly, the higher conversion
(98.5%) of acetic acid was obtained in a shorter reaction time
of 1 h. Consequently, as a whole, as in inorganic acid catalysts,
the stronger the acidity, the higher the catalytic activity of
the lactam-based Brønsted-acidic ILs in the esterification.
Furthermore, the synergistic effect of the acidity of ILs and
their immiscibility with esters could enhance the catalytic
activity of esterification.
The synergistic effect of the acidity of lactam-based ILs having
a methanesulfonate anion and their immiscibility with esters
was also valid in a variety of esterifications. Table 5 reports the
esterification of different carboxylic acids with several alcohols
catalyzed by the catalyst [CP][CH3SO3]. Since some typical
carboxylic acids are solid at room◦temperature, the reaction
temperature for these was set at 60 C. Three typical aliphatic
acids, namely acetic acid, lauryl acid and stearic acid, which
have different chain lengths, were examined. Satisfactory results
(Table 5, entries 1–4) were obtained, despite their different chain
lengths, with prolonged reaction times. Oxalic acid, as a dibasic
aliphatic acid, was esterified with two equivalents of 1-butanol
Fig. 1 The recycling of [CP][CH3SO3] IL for esterification. Reaction
conditions: 60 mmol acetic acid, 60 mmol 1-butanol, and 12 mmol
[CP][CH3SO3] at 30 ◦C for 4 h.
◦
at 60 C for 4 h, and a conversion of 93.1% and selectivity of
100% to dibutyl oxalate was obtained (Table 5, entry 5). Alicyclic
and unsaturated aliphatic acids, being different from the above
saturated aliphatic acids, were also tested, with good results
obtained. The conversions of cyclopropanecarboxylic acid and
undecenoic acid reached 87.0% (Table 5, entry 6) and 89.4%
(Table 5, entry 7) with 100% selectivity to the corresponding
esters, respectively. For a typical aromatic acid, benzoic acid, a
moderate result was obtained. When the reaction was carried out
at 60 ◦C for 4 h, the conversion of benzoic acid was only 62.0%.
Conclusions
In conclusion, several lactam-based Brønsted-acidic ILs were
synthesized. These ILs were potentially efficient catalysts for
esterification under mild reaction conditions without addition
of any organic solvents or cocatalysts. The catalytic performance
was dependent on the synergistic effect of the acidity of
Brønsted-acidic ILs and their immiscibility with esters. The
lactam-based Brønsted-acidic ILs having a methanesulfonate
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anion showed the best catalytic performance, with excellent
reusability and stability.
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Acknowledgements
The authors gratefully acknowledge the financial supports from
the NSFC (20873108), the 973 program (2009CB939804) and
the Key Scientific Project of Fujian Province (2009HZ0002-1).
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