Synthesis of Nipagin esters using acidic functional ionic liquids as catalysts

Article abstract of DOI:10.1080/00397911003707022

(Chemical Equation Presented) Several Bronsted acidic functional ionic liquids (FILs) with an alkane sulfonic acid group were synthesized. These FILs as dual solvent-catalysts for Nipagin esterification reactions were investigated. The results indicated that [HSO₃-pMIM]HSO₄ has the best catalytic activity and recyclability among the various kinds of FILs investigated, and its structure was characterized by infrared and NMR. The [HSO₃-pMIM]HSO₄ could be easily separated from the reaction mixture and reused without noticeably decreasing the catalytic activity. Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/00397911003707022

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Synthesis of Nipagin Esters Using Acidic
Functional Ionic Liquids as Catalysts
Guohua Wang a , Lu Li b , Congxia Xie a , Shitao Yu b , Fusheng Liu b
& Xiaoling Ye a
a Key Laboratory of Eco-chemical Engineering, Ministry of Education,
College of Chemistry and Molecular Engineering , Qingdao University
of Science and Technology , Qingdao , China
b College of Chemical Engineering , Qingdao University of Science
and Technology , Qingdao , China
Published online: 03 Mar 2011.
To cite this article: Guohua Wang , Lu Li , Congxia Xie , Shitao Yu , Fusheng Liu & Xiaoling Ye
(2011) Synthesis of Nipagin Esters Using Acidic Functional Ionic Liquids as Catalysts, Synthetic
Communications: An International Journal for Rapid Communication of Synthetic Organic Chemistry,
41:7, 945-952, DOI: 10.1080/00397911003707022
To link to this article: http://dx.doi.org/10.1080/00397911003707022
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Synthetic Communications1, 41: 945–952, 2011
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397911003707022
SYNTHESIS OF NIPAGIN ESTERS USING ACIDIC
FUNCTIONAL IONIC LIQUIDS AS CATALYSTS
Guohua Wang,1 Lu Li,2 Congxia Xie,1 Shitao Yu,2
Fusheng Liu,2 and Xiaoling Ye1
1Key Laboratory of Eco-chemical Engineering, Ministry of Education,
College of Chemistry and Molecular Engineering, Qingdao University of
Science and Technology, Qingdao, China
2College of Chemical Engineering, Qingdao University of Science and
Technology, Qingdao, China
GRAPHICAL ABSTRACT
Abstract Several Brønsted acidic functional ionic liquids (FILs) with an alkane sulfonic
acid group were synthesized. These FILs as dual solvent-catalysts for Nipagin esterification
reactions were investigated. The results indicated that [HSO3-pMIM]HSO4 has the best
catalytic activity and recyclability among the various kinds of FILs investigated, and its
structure was characterized by infrared and NMR. The [HSO3-pMIM]HSO4 could be
easily separated from the reaction mixture and reused without noticeably decreasing the
catalytic activity.
Keywords Acidic functional ionic liquids; catalysis; Nipagin ester; synthesis
INTRODUCTION
Nipagin ester (also known as hydroxylbenzoate) has been considered a safe
and effective preservative and antibacterial agent. At present, studies of high-carbon
Nipagin esters have been focused on heptyl Nipagin and octyl Nipagin,[1,2] but these
products have not been used in industry, and series of such products has not been
researched.[3,4]
Nipagin esters were synthesized using sulfuric acid as catalyst in a
homogeneous reaction system. The sulfuric acid cannot be reused and has other
Received April 22, 2009.
Address correspondence to Congxia Xie, Key Laboratory of Eco-chemical Engineering, Ministry
of Education, College of Chemistry and Molecular Engineering, Qingdao University of Science and
Technology, Qingdao 266042, China. E-mail: xiecongxia@126.com
945

946
G. WANG ET AL.
Figure 1. Structure of FILs.
disadvantages such as equipment corrosion, more by-products, tedious workup pro-
cedure, and environmental problems. Although the corrosion and pollution of
organic acid[5,6] are less than that of sulfuric acid, wastes are still a big problem.
Organic acid also cannot be recycled like sulfuric acid. Solid catalysts[7–10] are non-
volatile materials, benign to the environment, and can be easily separated from the
product, but they have some shortcomings such as high molecular weight=active-site
ratios and rapid deactivation from coking, which limits their application.
Ionic liquids (ILs) have been considered as promising, environmentally benign
alternatives.[11–15] Since Cole et al.[11] first reported Brønsted acidic FILs that bear an
alkane sulfonic acid group in an imidazole and triphenylphosphine cation, more
attention has been focused on these FILs.[16–21] Such Brønsted acidic FILs have
shown very high activity and selectivity in many esterifications as dual solvent–
catalysts.[22–25] Moreover, compared with the traditional catalyst, the FILs are also
recyclable and are tunable to satisfy specific chemical tasks,[16,26,27] such as acid–
base, solubility, stability, and so on.
In this article, several novel SO3H-functional Brønsted acidic FILs (Fig. 1) that
bear an alkane sulfonic acid group in N-methylimidazole, pyridine, and triethylmine
cations are reported, and their catalytic activities for preparation of Nipagin esters
are also investigated.
EXPERIMENTAL
Chemicals and Instruments
The infrared (IR) spectra were run on a Nicolet 510P Fourier transform (FT)–
1
IR spectrometer and are expressed in centimetersꢀ1 (KBr). H NMR spectra were
recorded on a Bruker AV500 (500 MHz, D2O), and 13C NMR spectra were mea-
sured on a Bruker AV500 (75.5 MHz, D2O), spectrometer. Thermogravimetry
(TG) was run on a STA-449C DSC-TG apparatus.
All chemicals (analytical-reagent grade) were commercially available and used
without further purification.
Preparation of Acidic FILs
For this article, the general syntheses of acidic FILs involve a two-step process
as described in the literature.[11,28,29] Taking 1-methyl-3-propanesulfonic-imidazo-
lium hydrogensulfate [HSO3-pMIM]HSO4, for example, the zwitterionic-type

SYNTHESIS OF NIPAGIN ESTERS
947
Scheme 1. Synthesis of FILs.
Scheme 2. Esterification in FILs.
precursors (MIM-PS) were prepared through a one-step direct sulfonation reaction
(Scheme 1), then followed by a proton reaction using acid addition (Scheme 2).
In accordance with the same method, acidic FILs [HSO3-pMIM]pTSA, [HSO3-
pPy]HSO4, [HSO3-pPy]pTSA, [HSO3-pTEA]HSO4, and [HSO3-pTEA]pTSA were
prepared, and they are all pale yellow viscous liquids at room temperature. The FILs
were analyzed by NMR and IR, and the spectral data agreed with their structures.
Spectral data for MIM-PS. 1H NMR (500 MHz, D2O):d 2.26 (m, 2H), 2.86
(t, 2H), 3.83 (s, 3H), 4.30 (t, 2H), 7.38 (s, 1H), 7.46 (s, 1H), 8.70 (s, 1H); 13C
NMR (125 MHz, D2O):d 24.79, 35.46, 46.97, 47.41, 121.85, 123.50, 135.82; IR
(KBr, n=cmꢀ1): n 3450, 3154, 3110, 2962, 1640, 1575, 1461, 1424, 1195, 1043, 746,
622, 532.
Spectral data for [HSO3-pMIM]HSO4. 1H NMR (500 MHz, D2O): d 2.11
(m, 2H), 2.71 (t, 2H), 3.71 (s, 3H), 4.16 (t, 2H), 7.25 (s, 1H), 7.32 (s, 1H), 8.53 (s,
1H); IR (KBr, n=cmꢀ1): n 3437, 3157, 2970, 1722, 1653, 1574, 1462, 1425, 1230,
1203, 1172, 1078, 1055, 1029, 880, 853, 748, 698, 655, 582, 528; DSC-TG (N2,
10 ꢁC= min):single decomposition peak, Td 366.4 ꢁC, weightlessness 81.57%.
Nipagin Esterification
p-Hydroxylzoic acid, alcohol, and FILs were added to a 50-mL flask with a
thermometer, water segregator, and reflex condenser. Under vigorous stirring and
heating, the esterification reaction was carried out for the desired time. Through
crystallization, the produced esters can be gained (Scheme 2).
RESULTS AND DISCUSSION
Esterification with Acidic FILs
The esterifications of low-carbon alcohol Nipagin were carried out using the
synthesized FILs, traditional acid, and solid catalyst as catalysts, respectively, and

948
G. WANG ET AL.
Table 1. Synthesis of low-carbon Nipagin esters
ROH
Catalyst
Entry
n
acid:nalcohol:ncat
T (ꢁC)
Time (h)
Yield (%)
C2H5OH
H2SO4
p-CH3–phSO3H[5]
JK008 resin[7]
1
2
1:4.0:0.1
1:4
1:5.5
85–100
–
3
3.5
6
4
2
5
6
3
3
3
3
3
3
3
3
3
1
5
2
1
2
3
3
1
1
1
1
1
88.0
75.0
91.7
93.2
87.5
72.9
77.8
91.3
92.3
93.7
95.8
90.5
83.4
85.5
87.5
87.6
86.1
92.8
91.1
87.1
91.0
92.7
95.3
86.8
86.4
86.1
84.3
84.8
3
72–110
115–135
84–92
[8]
SO24ꢀ=TiO2
TiSiW12O40=TiO2
[HSO3-pMIM]HSO4
4
1:4
1:4
[9]
5
6a
7a
8
1:4.0:0.1
1:4.0:0.1
1:4.0:0.1
1:4.0:0.15
1:3.0:0.15
1:3.0:0.15
1:4.0:0.1
1:4.0:0.1
1:4.0:0.1
1:4.0:0.1
1:4.0:0.1
1:3.5:0.1
1:2:0.072
1:4
75
75
85–100
85–100
85–100
85–100
85–100
85–100
85–100
85–100
85–100
125–135
130–170
84–92
9
10
11b
12
13
14
15
16
17
18
19
20
21
22
23b
24
25
26
27
28
[HSO3-pMIM]pTSA
[HSO3-pPy]HSO4
[HSO3-pPy]pTSA
[HSO3-pTEA]HSO4
[HSO3-pTEA]pTSA
H2SO4
C4H9OH
NaHSO4-ꢂ H2O[10]
[9]
TiSiW12O40 =TiO2
[HSO3-pMIM]HSO4
1:3.5:0.1
1:3.5:0.1
1:3.5:0.1
1:3.5:0.1
1:3.5:0.1
1:3.5:0.1
1:3.5:0.1
1:3.5:0.1
1:3.5:0.1
125–135
125–135
125–135
125–135
125–135
125–35
125–135
125–135
125–135
[HSO3-pMIM]pTSA
[HSO3-pPy]HSO4
[HSO3-pPy]pTSA
[HSO3-pTEA]HSO4
[HSO3-pTEA]pTSA
aBenzene as water-segregated reagent.
bDouble the amount of reactants.
the results are listed in Table 1. As seen from Table 1, in the synthesis of ethyl
Nipagin (entries 1–16), the FILs showed better catalytic activity than a traditional
acid and solid catalysts. On the one hand, using FILs as catalyst, the produced esters
gained the same yields (about 85%) as using a traditional acid as catalyst (entries 1
and 2), and the color and purity of the produced esters are better than when using
traditional acid as catalyst. On the other hand, the FILs reported here have a certain
polarity and were dissolved in the reaction system so that esterification is homo-
geneous. Compared with the solid catalyst (entries 3–5), the FILs can increase reac-
tion rate, shorten reaction time, and reduce energy consumption. With the same
cation, the FILs with sulfuric anion and p-methyl benzenesulfonic anion revealed
the same catalytic activity and received a similar ester yield. However, with the same
anion, imidazole FILs (entries 8 and 12) gained the best ester yield ( > 90%), the
second is triethylamine FILs (entries 15 and 16), and the last is pyridine FILs (entries
13 and 14). This is because the catalytic activity of FILs is related to their Brønsted
acidic strength and the stability in esterifications.[29] The pyridine ring and imidazole
ring withdraw electrons, while the ethyl group donates an electron. Therefore, the

SYNTHESIS OF NIPAGIN ESTERS
949
density of positive charge on the nitrogens of pyridine and imidazole is larger than
on that of triethylamine. In comparison with pyridine FILs, the positive charge is
dispersed for the imidazole because of the conjugative effect. The Brønsted acidic
strengths of the FILs with different nitrogen groups are in the sequence
pyridine > N-methylimidazole > triethylamine, and the stability of the FILs is
triethylamine > N-methylimidazole > pyridine.[30] Similarly, in the synthesis of butyl
Nipagin (entries 17–28), imidazole FILs (entries 20 and 24) also showed better cata-
lytic properties and gained the same yield (87%) as sulfuric acid (entry 17) after 1 h.
Compared with solid catalyst (entries 18 and 19), using FILs as catalyst, the esters
were produced in higher yield and shorter reaction time. Finally, FIL [HSO3-
pMIM]HSO4 was used as catalyst for synthesizing a series of Nipagin esters. The
results of synthesizing high-carbon Nipagin esters are listed in Table 2. From
Table 2 we can see that in the synthesis of high-carbon Nipagin esters, the imidazole
FILs also showed the same predominances as in low-carbon Nipagin esters.
In addition, reaction temperature is one of the important factors for esterifi-
cation. At too low of a temperature, the reaction rate was slow and the esters were
produced in poor yield; at too high of a temperature, the esterification was prone to
form ether and carbonation, which brings difficulties to the separation and purifi-
cation. In the synthesis of ethyl Nipagin (Table 1, entries 1–16), according to the
ethanol–water azeotropic principle, the ester yield markedly improved to 91.3%
without any water-segregated reagent (Table 1, entry 8). Under the optimal reaction
conditions (Table 1, entry 10), the yield reached 93.7%. In the synthesis of butyl
Nipagin (Table 1, entries 17–28), the amount of n-butanol directly impacts the reac-
tion temperature. Under the optimal reaction conditions (Table 1, entry 22), the
yield was 92.7%. In the synthesis of octyl Nipagin (Table 2, entries 1–9), when the
reaction temperature rose above 135 ꢁC, crystallization went with the oil-like sub-
stance, while the yield was lower. Under the optimal reaction conditions (Table 2,
entry 7), the yield was 94.6%. Similarly, the yield of dodecyl Nipagin was 94.8%
when reacted for 4 h at 130–140 ꢁC (Table 2, entry 14).
Table 2. Synthesis of high-carbon Nipagin esters
ROH
Catalyst
Entry
nacid:nalcohol:ncat
T (ꢁC)
Time (h)
Yield (%)
C8H17OH
H2SO4
[HSO3-pMIM]HSO4
1
2
1:3.0:0.1
1:3.0:0.1
1:3.0:0.1
1:3.0:0.1
1:3.0:0.1
1:3.0:0.1
1:3.0:0.1
1:3.0:0.1
1:3.0:0.1
1:3.0:0.07
1:4.0:0.1
1:4.0:0.1
1:4.0:0.1
1:4.0:0.1
120–135
100–110
110–120
120–130
130–140
140–150
120–135
120–135
120–135
130–140
120–130
130–140
140–150
130–140
3
3
3
3
3
3
3
3
3
4
3
3
3
4
89.8
87.0
92.4
94.6
87.6
83.6
94.6
97.1
93.8
90.1
87.4
93.6
83.2
94.8
3
4
5
6
7
8a
9
[HSO3-pMIM]pTSA
p-CH3-phSO3H [6]
[HSO3-pMIM]HSO4
10
11
12
13
14
C
12H25OH
aDouble the amount of reactants.

950
G. WANG ET AL.
Table 3. Effect of reusing [HSO3-pMIM]HSO4 on results of esterification
No.
1
2
3
4
5
6
7
8
Yield of ester 1a (%)
93.7
97.9
93.6
96.8
93.4
94.7
93.1
91.6
92.8
89.5
92.5
87.4
92.0
84.2
91.3
80.0
Recovery yield of ILs for
synthesizing ester 1a (%)
Yield of ester 2b (%)
92.7
93.3
92.4
83.3
91.8
78.3
91.2
71.7
89.8
65.2
88.7
53.3
91.5
Recovery yield of ILs for
synthesizing ester 2b (%)
Yield of ester 3c (%)
94.6
84.1
94.0
73.3
93.6
60.5
92.0
46.8
89.6
Recovery yield of ILs for
synthesizing ester 3c (%)
aEster 1 ¼ ethyl Nipagin, the reaction condition: nacid:nalcohol:ncat ¼ 1:3.0:0.15, 85–100 ꢁC, 3 h.
bEster 2 ¼ butyl Nipagin, the reaction condition: nacid:nalcohol:ncat ¼ 1:3.0:0.10, 125–135 ꢁC, 3 h.
cEster 3 ¼ octyl Nipagin, the reaction condition: nacid:nalcohol:ncat ¼ 1:3.0:0.10, 130–140 ꢁC, 3 h.
Recyclability of [HSO3-pMIM]HSO4
The recycling performance of [HSO3-pMIM]HSO4 was investigated for synthe-
sizing a series of Nipagin esters (Table 3). [HSO3-pMIM]HSO4 could be reused at
least five times without noticeable lowering of the catalytic activity in the esterifi-
cation. In the synthesis of butyl Nipagin, after [HSO3-pMIM]HSO4 was reused six
times, the recovery yield of FILs was 53.3%. When the quality of [HSO3-
PMIM]HSO4 was added to the same amount used in the first time, the yield of ester
resumed. It showed that the decrease of the catalytic activity of [HSO3-pMIM]HSO4
was mainly due to the mechanical loss; [HSO3-pMIM]HSO4 itself did not change.
Thus it can be seen that FIL is a good catalyst for synthesizing Nipagin esters. As
the length of the alcohol carbon chain increases, the melting point of the Nipagin
ester decreases, and the crystallization becomes more difficult, which results in losing
FILs. This indicates that the recovery yield of ILs was much lower as catalysts for
synthesizing high-carbon Nipagin esters, although greater ester yield can be gained.
Therefore, this question needs further exploration.
CONCLUSION
In this paper, the six acidic FILs combining the advantageous characteristics of
solid acids and liquid acids were synthesized. They showed good use as dual solvent–
catalysts for Nipagin esterification with good catalytic performance under mild reac-
tion conditions. Furthermore, the synthesized FILs simply recycled and reused with-
out noticeably decreasing the catalytic activity.
ACKNOWLEDGMENTS
This work was supported by the National Natural Science Foundation of
China (30571463) and the Natural Science Foundation of Shandong Province
(Y2005B07).

SYNTHESIS OF NIPAGIN ESTERS
951
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