Acidic functionalized ionic liquids as catalyst for the isomerization of α-pinene to camphene

Article abstract of DOI:10.1007/s11164-015-2041-2

An acidic functionalized ionic liquids (ILs) [HSO₃-(CH₂)₃-NEt₃]Cl-ZnCl₂ was synthesized and used to catalyze the isomerization of α-pinene in a homogeneous system. The optimum conditions for isomerization were obtained as follows: n(α-pinene):n(ILs) = 9:1, reaction temperature 140 °C, and reaction time 4 h, α-pinene 0.04 mol. Under the optimal conditions, the conversion of α-pinene was 97.6 % and the selectivity for camphene could reach 64.8 %. In addition, the catalyst could be easily separated by centrifugation after the isomerization completely finished. When the ILs were repeatedly used for four times, the conversion of α-pinene and the selectivity for camphene were still excellent, indicating the superb recycle ability of the acidic functionalized ILs catalyst.

Full text of DOI:10.1007/s11164-015-2041-2

Res Chem Intermed
DOI 10.1007/s11164-015-2041-2
Acidic functionalized ionic liquids as catalyst
for the isomerization of a-pinene to camphene
Yue Liu1 Lu Li2 Cong Xia Xie1
•
•
Received: 27 January 2015 / Accepted: 2 April 2015
Ó Springer Science+Business Media Dordrecht 2015
Abstract An acidic functionalized ionic liquids (ILs) [HSO3-(CH2)3-NEt3]Cl-
ZnCl2 was synthesized and used to catalyze the isomerization of a-pinene in a
homogeneous system. The optimum conditions for isomerization were obtained as
follows: n(a-pinene):n(ILs) = 9:1, reaction temperature 140 °C, and reaction time
4 h, a-pinene 0.04 mol. Under the optimal conditions, the conversion of a-pinene
was 97.6 % and the selectivity for camphene could reach 64.8 %. In addition, the
catalyst could be easily separated by centrifugation after the isomerization com-
pletely finished. When the ILs were repeatedly used for four times, the conversion
of a-pinene and the selectivity for camphene were still excellent, indicating the
superb recycle ability of the acidic functionalized ILs catalyst.
Keywords Acidic functionalized ionic liquids Á Catalyst Á a-Pinene Á
Isomerization
Introduction
The isomerization of a-pinene can produce camphene, which is an intermediate in
the synthesis of camphor and widely used in fragrances and the pharmaceuticals
industry. In the isomerization of a-pinene, as catalysts, aqueous mineral acids are
playing important roles. However, these aqueous mineral acids have many
inevitable shortcomings, such as serious corrosion of equipment, complicated
&
Cong Xia Xie
xiecongxiaqust@163.com
1
Key Laboratory of Eco-chemical Engineering, Ministry of Education, College of Chemistry and
Molecular Engineering, Qingdao University of Science and Technology, Qingdao 266042,
People’s Republic of China
2
College of Chemical Engineering, Qingdao University of Science and Technology,
Qingdao 266042, People’s Republic of China
123

Y. Liu et al.
separation procedures, environmental pollution, non-recyclability, and so forth. In
order to solve these problems, as the primary object, solid acids, such as PW/SBA-
15 (SBA-15 supported H3PW12O40), AlCl3/c–Al2O3 and AlCl3/SiO2, MSU-S(BEA)
,
have become more and more significant in the catalysts field during the last
10 years. Yao et al. [1] reported that, in the catalysis process of a-pinene, the
reaction conditions of PW/SBA-15 (SBA-15 supported H3PW12O40) were listed:
reaction temperature 130 °C, w(PW/SBA-15):w(a-pinene) = 2 %, and reaction
time 2 h. Based on these conditions, the conversion of a-pinene was 94.5 % and the
selectivity for camphene was 48.5 %. After AlCl3 was immobilized on c-Al2O3, the
AlCl3/c–Al2O3 catalyst had high activity leading to the conversion concerning a-
pinene of 95.5 % and the selectivity of 40.6 % for camphene. When AlCl3 was
immobilized on SiO2, the AlCl3/SiO2 catalyst gave only 28.5 % selectivity for
camphene and 98.4 % conversion of a-pinene [2]. When MSU-S(BEA) was used as
catalyst (molar ratio of Si/Al is 67), the conversion of a-pinene was 97.0 % and the
selectivity was just 48.0 % for camphene [3]. Burcu et al. [4] reported the
isomerizaton of a-pinene over H3PW12O40 catalysts was supported on natural
zeolite, the selectivity for camphene was about 40.0 %, and the conversion of a-
pinene was 99.4 %. SO42-/ZrO2 catalyzed the isomerization of a-pinene, the
conversion of a-pinene was 96.4 %, and the selectivity of camphene was 49.7 %
[5]. Thus, although solid acidic catalysts could avoid the shortcomings of mineral
acids, they also had some drawbacks, such as low selectivity for camphene,
heterogeneous reaction.
According to the above analysis, the present catalytic systems have a tedious post
processing for the mass transfer between interphase and low selectivity for
camphene, and cannot provide efficient and green technology for the isomerization
of a-pinene. Therefore, it has become a vital aspect for promoting the isomerization
of a-pinene to avoid the shortcomings of the present technology to synthesize one
kind of perfect catalyst and further to design a novel catalytic system. In recent
years, as environmentally friendly catalysts, Brønsted and Lewis acidic ILs have
received much attention from researchers, which showed excellent selectivity and
yield in several organic reactions, such as esterification [6–9], carbonylation [10,
11], polymerization [12, 13] by Friedel–Crafts [14–19], Beckmann [20, 21], and
Diels–Alder [22]. In our laboratory, studies about the catalysts in the isomerization
of a-pinene by both traditional catalysts and Brønsted and Lewis acidic function-
alized ionic liquids catalysts have been performed at the same time. Compared with
traditional catalysts, our results indicated that acidic functionalized ILs catalysts
exhibited many outstanding advantages, such as simplicity, efficiency, and easy
product isolation.
Therefore, in this work, a new kind of stable Brønsted–Lewis acidic function-
alized ILs [HO3S-(CH2)3-NEt3]Cl-ZnCl2 was synthesized, characterized, and used
in the isomerization of a-pinene. The catalyst showed excellent reuse performance
and a superior catalytic activity in the homogeneous reaction and could readily
separate after the complete isomerization. The results revealed that Brønsted–Lewis
acidic functionalized ILs catalyzing the isomerization of a-pinene, the technology
was efficient and environmentally-friendly.
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Acidic functionalized ionic liquids as catalyst for the…
Experimental
Chemicals and instruments
The a-Pinene (99.8 %), deionized water, 1,3-propane sultone (99.7 %, Wuhan
Fengfan Chemicals Co. Ltd., China), and other chemicals (analytic purity) such as
hydrochloric acid, triethylamine, zinc chloride, anhydrous magnesium sulfate,
sodium chloride, and sodium carbonate were all purchased from Aldrich. All
materials were directly used without further purification.
Fourier transform infrared spectroscopy (FT-IR) was carried out on a Nicolet
510P FT-IR spectrometer in the range of 4500–400 cm-1. The NMR spectra were
taken on a Bruker AV500 Fourier-transform spectrometer with reference to SiMe4
using solvent dimethyl sulphoxide (DMSO) containing a 5 % sample.
Preparation of catalysts
Synthesis of [HO3S-(CH2)3-NEt3]Cl-ZnCl2 ILs
Firstly, under vigorous stirring, 12.2 g (0.1 mol) 1,3-propanesultone was dissolved
in 50 mL ethyl acetate, then 10.1 g (0.1 mol) triethylamine was dropped slowly into
the solution at 50 °C. The mixture was subsequently stirred for 2 h, and the resultant
mixture was filtered to get a white precipitate, which was washed with 30 mL ethyl
acetate for three times and dried at 100 °C for 2 h to get 20.6 g white powder NET3-
PS and the yield was 92.4 %. Secondly, 11.15 g (0.05 mol) NET3-PS was dissolved
in 15 mL water, and 4.9 g (0.05 mol) hydrochloric acid was slowly dropped into the
solution at room temperature (RT). After dropping, the mixture was stirred at RT for
30 min, heated in an oil bath at about 90 °C for 2 h, and the water under reduced
pressure at 90 °C. We got 12.6 g of a white viscous liquid [HO3S-(CH2)3-NEt3]Cl at
RT. Finally, under the protection of nitrogen, [HO3S-(CH2)3-NEt3]Cl reacting with
various amounts of ZnCl2 at 60 °C for 2 h to get a viscous liquid of [HO3S-(CH2)3-
NEt3]Cl-ZnCl2 at RT. The reaction formula of [HO3S-(CH2)3-NEt3]Cl-ZnCl2
(x = 0.83) is shown in Scheme 1.
[HO3S-(CH2)3-NEt3]Cl-ZnCl2: IR (KBr): m = 3351, 2989, 1622, 1486, 1458,
1
1228, 1151, 1043, 952, 816, 794, 735, 694, 600, 518 cm-1; H NMR (500 MHz,
D2O): d = 1.15 (t, 2H), 1.92 (m, 2H), 2.81 (q, 3H), 3.16 (t, 2H), 4.67 (t, 1H), 2.85 (t,
J = 7.5 Hz, 2H), 2.25 (m, 2H); 13C NMR (500 MHz, D2O): d = 6.75, 17.28, 47.73,
52.12, 54.23.
Scheme 1 Synthesis of ILs [HO3S-(CH2)3-NEt3]Cl-ZnCl2 (x = 0.83). X: the molar fraction of ZnCl2
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Y. Liu et al.
Synthesis of [mim][XCl] ILs
The [mim][XCl] ILs was prepared as follows:
Firstly, N-methylimidazole was reacted with equal-mole hydrochloric acid at
50 °C for 5 h to get [mim]Cl. After the reaction finished, the mixture was washed
three times with ethyl acetate and dried under reduced pressure at 90 °C for 4 h.
Secondly, Lewis acidic ILs were synthesized through the reactions between the
above products and different metal chlorides, including AlCl3, FeCl3, ZnCl2, TiCl4
and SnCl4, until these two materials completely mixed at 60 °C under the protection
of nitrogen.
Isomerization of a-pinene
The formula on isomerization of a-pinene is shown in Scheme 2. The reactions
were achieved in a glass reactor provided with a magnetic stirrer, sampling port,
temperature bath, and condenser. Then 0.04 mol a-pinene was loaded into the
reactor, stirred, and warmed to the desired temperature. Once the desired
temperature was attained, the catalyst was loaded into the reactor. Then, the
solvent system became a homogeneous phase. The reaction products were analyzed
by gas chromatography by taking samples at 120, 180, 240, 300, and 360 min after
adding the catalyst, respectively. When the reaction finished, the products cooled to
RT at the end of the reaction. When the temperature fell down to RT, the catalyst
can separate by centrifugation from the reaction system and easily be recovered.
Scheme 2 The isomerization of a-pinene
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Acidic functionalized ionic liquids as catalyst for the…
Reaction liquid was washed to neutrality with a Na2CO3 solution and a saturated
NaCl solution, then immediately analyzed with a GC17 Shimadzu gas chro-
matograph equipped with a FID and a capillary DB1 column (60 m in length). The
reaction products were identified by comparing the retention times of terpene
standards or using their Kovats index with confirmation by mass spectroscopy. The
compounds present in the samples were identified in a Thermofinigan GC Top
8000/Voyager MS Gas chromatograph/mass spectrometer system. The column used
was an All Tech AT-5 capillary column 30 m in length and 0.25 mm in internal
diameter. The temperatures of the injector and the transference line were maintained
constant at 260 °C. The initial temperature of the furnace was maintained at 80 °C
for 2 min. Then it was increased at the rate of 4 °C/min up to 160 °C, and kept for
2 min. Finally, the temperature was raised at a rate of 10 °C/min up to 250 °C,
maintained for 2 min.
Results and discussion
Effect of different catalysts on the isomerization of a-pinene
The catalytic performance of different catalysts on the isomerization of a-pinene is
given in Table 1. First of all, we tested a blank in the experiment and the conversion
of a-pinene was 4.9 % and the selectivity for camphene was 15.4 % (Entry 1).
When ZnCl2 was chosen as the catalyst, the conversion of a-pinene was 94.6 %, and
the selectivity for camphene was 50.4 % (Entry 2). But it was found that ZnCl2 had
some drawbacks limiting its further application, for example, its equipment
Table 1 Effect of the types of catalysts on the results of the reaction
Entry Catalyst
Components of product (%)
C (%) S (%)
camphene limonene terpinolene
1
No catalyst
1.23
47.67
7.73
4.97
2.66
2.26
1.31
18.02
10.46
10.02
11.76
10.63
10.31
8.31
1.91
14.51
21.56
17.56
11.21
14.56
10.25
10.25
15.61
9.43
4.9
94.6
85.0
84.3
98.7
90.5
98.3
43.2
66.4
80.2
98.9
15.4
50.4
9.1
2
ZnCl2
3
[mimH]ZnCl3
[mimH]FeCl4
[mimH]SnCl5
[mimH]AlCl4
4
5.9
5
2.7
6
2.5
7
[HSO3(CH2)3-NEt3]Cl-ZnCl2(x = 0.83) 27.92
[HSO3(CH2)3-NEt3]Cl-ZnCl2(x = 0.50) 15.85
[HSO3(CH2)3-NEt3]Cl-ZnCl2(x = 0.67) 21.85
[HSO3(CH2)3-NEt3]Cl-ZnCl2(x = 0.80) 24.22
[HSO3(CH2)3-NEt3]Cl-ZnCl2(x = 0.86) 21.07
28.4
36.7
32.9
30.2
21.3
8
9
11.05
9.05
10
11
11.22
12.31
Reaction conditions: Entry 1: a-pinene 5.44 g. Entry 2: a-pinene 5.44 g, w(ZnCl2):w(a-pinene) = 3:100,
reaction temperature 140 °C, and reaction time 10 h. Enter (3–11): a-pinene 0.04 mol, n(a-pinene):
n(catalyst) = 1:1, reaction temperature 140 °C, and reaction time 4 h
C conversion of a-pinene, S selectivity for camphene
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Y. Liu et al.
corrosion, tedious work-up procedures, environmental problems, and also it cannot
be reused. Then, we chose [ZnCl3]- as the anion to design and synthesize
[mimH][ZnCl3] ILs as a Lewis acid, and investigated the catalytic efficiency, but the
conversion of a-pinene was 85.0 % and the selectivity for camphene was only
9.1 % (Entry 3). At the same time, when the [SnCl5]- and [AlCl4]- were as the
anions, the conversion of a-pinene was excellent . Nevertheless, the selectivity for
camphene was just 2.5 % (Entry 5–6).
Therefore, to get new catalysts that are of excellent catalytic activity and
selectivity for camphene, Bronsted-Lewis acidic functionalized ILs [HSO3-(CH2)3-
NEt3]Cl-ZnCl2 was designed and synthesized. As shown in Table 1, when the molar
fraction of ZnCl2 was 0.83, the conversion of a-pinene was 98.3 % and the
selectivity for camphene was 28.4 % (Entry 7). When the Bronsted acid was
introduced to the ILs, compared with the Entries (3–6), the conversion of a-pinene
had great growth, and the selectivity for camphene was better . With the increase of
the molar fraction of ZnCl2, the conversion of a-pinene correspondingly improved,
but the selectivity for camphene oppositely decreased. For example, when the molar
fraction of ZnCl2 was 0.50, the conversion of a-pinene was 43.2 % (Entry 8), which
was lower compared with that of 0.86 of the molar fraction of ZnCl2 (98.9 %, Entry
11), and the selectivity for camphene was 36.7 % (Entry 8), which was higher than
that of 0.86 of the molar fraction of ZnCl2 (21.3 %, Entry 11), respectively. This
may be ascribed to the different acidity of ILs by the molar fraction of ZnCl2. To
characterize the Lewis acidity of the prepared catalyst, acetonitrile, a weak Lewis
base, was used as the probe for FTIR analysis. It was reported that acetonitrile had
two characteristic absorption peaks around 2290 and 2250 cm-1, and if the reactive
group of CN reacted with Lewis acid, thus, it would be a new absorption peak at
2200–2400 cm-1 [23]. As shown in Fig. 1, the characteristic absorption peaks of
acetonitrile/[HSO3-(CH2)3-NEt3]Cl-ZnCl2 (x = 0.5) were nearly the same as that of
acetonitrile, indicating that [HSO3-(CH2)3-NEt3]Cl-ZnCl2 (x = 0.5) was not of
Fig. 1 FT-IR spectra of ILs
using acetonitrile as probe.
a Pure acetonitrile;
b Acetonitrile/[HO3S-(CH2)3-
NEt3]Cl-ZnCl2 (x = 0.5);
c Acetonitrile/[HO3S-(CH2)3-
NEt3]Cl-ZnCl2 (x = 0.8);
d Acetonitrile/[HO3S-(CH2)3-
mim]Cl-ZnCl2 (x = 0.83).
Acetonitrile is 1/2 in volume in
the samples of (b–d)
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Acidic functionalized ionic liquids as catalyst for the…
Lewis acidity. In the spectra of acetonitrile/[HSO3-(CH2)3-NEt3]Cl-ZnCl2 (x = 0.8)
and acetonitrile/[HSO3-(CH2)3-NEt3]Cl-ZnCl2 (x = 0.83), besides the two peaks
mentioned above, new absorption peaks at 2318 and 2326 cm-1 appeared. This
demonstrated that both [HSO3-(CH2)3-NEt3]Cl-ZnCl2 (x = 0.8) and [HSO3-(CH2)3-
NEt3]Cl-ZnCl2 (x = 0.83) had Lewis acidity. Moreover, the peak of [HSO3-(CH2)3-
NEt3]Cl-ZnCl2 (x = 0.83) was higher than that of [HSO3-(CH2)3-NEt3]Cl-ZnCl2
(x = 0.8), which displayed that the Lewis acidity of [HSO3-(CH2)3-NEt3]Cl-ZnCl2
(x = 0.83) was stronger than that of [HSO3-(CH2)3-NEt3]Cl-ZnCl2 (x = 0.8). It was
found that the conversion of a-pinene could be increased in Lewis acidity [24].
However, the selectivity for camphene decreased to 21.3 %, when the Lewis acidity
of ILs was too strong (x = 0.86). Therefore, taking account of all considerations, we
concluded that the Brønsted–Lewis acidic ILs [HSO3-(CH2)3-NEt3]Cl-ZnCl2
(x = 0.83) could give excellent catalytic performance for the isomerization of a-
pinene. Next, the process optimization on the isomerization of a-pinene using
[HSO3-(CH2)3-NEt3]Cl-ZnCl2 (x = 0.83) as catalysts is discussed in detail.
The optimization of reaction conditions
The performance of the catalyst has a great influence on the conversion of a-pinene
and the selectivity for camphene in the isomerization of a-pinene. However, the
performance of the catalyst will be affected by many factors. Thus, we investigated
the related condition parameters affecting the performance of acidic functionalized
ILs in the isomerization of a-pinene, including of the amount of catalyst, reaction
temperatures, and reaction time.
Effect of the amount of ILs
Table 2 shows the effects of different amounts of catalyst [HO3S-(CH2)3-NEt3]Cl-
ZnCl2 (x = 0.83) on the isomerization of a-pinene. With increasing the amount of
[HO3S-(CH2)3-NEt3]Cl-ZnCl2 (x = 0.83), the conversion of a-pinene did not
obviously change, but the selectivity for camphene apparently increased and then
decreased. As we observed from Table 2, when n(a-pinene):n(ILs) was 9:1, the
Table 2 Effect of different amounts of ILs on the results
n(a-pinene):n(ILs)
Components of product (%)
C (%)
S (%)
Camphene
Limonene
Terpinolene
11:1
10:1
9:1
42.22
50.97
63.24
55.07
53.43
7.86
8.79
7.24
8.59
96.4
96.9
97.6
98.7
98.4
43.8
52.6
64.8
55.8
54.3
8.83
11.88
20.21
21.37
8:1
12.92
15.32
7:1
Reaction conditions: a-pinene 0.04 mol, reaction temperature 140 °C, reaction time 4 h
C conversion of a-pinene, S selectivity for camphene
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Y. Liu et al.
selectivity for camphene could reach a maximum value of 64.8 %. The result may
be explained by the increase of ILs, the superficial area of catalyst became larger,
and the activation performance became powerful, so the conversion of a-pinene
became increscent. But the strong catalyst activation leads to increasing side effects.
Over all considerations, the optimal dosage of catalyst was n(a-pinene):
n(ILs) = 9:1.
Influence of reaction temperatures
The influence of different reaction temperatures on the isomerization of a-pinene
was investigated by monitoring the conversion of a-pinene and the selectivity for
camphene through varying the reaction temperature (120, 130, 140, 150, 160 °C)
(Table 3). From Table 3, with the enhancement of temperature, the conversion of a-
pinene dramatically increased, but the selectivity for camphene decreased. When
the reaction temperature was 140 °C, the selectivity for camphene could reach
64.8 %. The reason might be that, the higher the temperature, the greater the
electron shaking, the conversion of a-pinene increased and reduced the selectivity
of the reaction by increasing the number of accessible reaction pathways [25].
Therefore, 140 °C was the optimal reaction temperature.
Optimization of reaction time
We investigated the effect of reaction time (2, 3, 4, 5, 6 h) on the isomerization of a-
pinene as shown in Table 4. The conversion of a-pinene increased with the aid of
reaction time. However, the selectivity for camphene obviously improved and
decreased. At the reaction time of 4 h, the selectivity for camphene could reach
64.8 %, consistent with that of the amount of catalyst (9:1) and the reaction
temperature (140 °C). Therefore, the optimal reaction time of 4 h was chosen.
Based on these optimal reaction conditions, with the synthesized catalyst of
acidic functionalized ILs [HO3S-(CH2)3-NEt3]Cl-ZnCl2 for the isomerization of a-
pinene, we obtained 97.6 % of the conversion of a-pinene and 64.8 % of the
selectivity for camphene, which was better than previous reported [3].
Table 3 Influence of reaction temperatures on the results
Reaction temperature (°C)
Components of product (%)
C (%)
S (%)
Camphene
Limonene
Terpinolene
120
130
140
150
160
15.30
28.07
63.24
46.51
41.06
3.17
4.91
8.83
7.43
5.86
5.93
14.86
11.88
29.56
26.24
22.9
56.7
97.6
97.1
98.7
66.8
49.5
64.8
47.9
41.6
Reaction conditions: a-pinene 0.04 mol, n(a-pinene):n(ILs) = 9:1, reaction time 4 h
C conversion of a-pinene, S selectivity for camphene
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Acidic functionalized ionic liquids as catalyst for the…
Table 4 Effect of reaction time on the results
Reaction time (h)
Components of product (%)
C (%)
S (%)
Camphene
Limonene
Terpinolene
2
3
4
5
6
39.24
47.70
63.24
56.66
48.46
10.31
3.74
8.83
8.89
9.13
8.31
10.54
11.88
17.36
22.14
77.1
86.1
97.6
98.2
98.7
50.9
55.4
64.8
57.7
49.1
Reaction conditions: a-pinene 0.04 mol, n(a-pinene):n(ILs) = 9:1, reaction temperature 140 °C
C conversion of a-pinene, S selectivity for camphene
Table 5 The results of repeated use of [HO3S-(CH2)3-NEt3]Cl-ZnCl2 (x = 0.83)
Times
C (%)
S (%)
1
2
3
4
97.6
95.3
94.7
93.2
64.8
62.6
60.3
59.7
Reaction conditions: a-pinene 0.04 mol, n(a-pinene):n(ILs) = 9:1, reaction temperature 140 °C, reaction
time 4 h
C conversion of a-pinene, S selectivity for camphene
The reusability of the ILs
When the reaction finished and the reaction temperature declined below 40 °C, the
ILs became solid in the lower layer and the product was in the top layer, and we
obtained the ILs in the low layer after centrifugation, and the ILs could be entirely
separated from a-pinene and easily be recovered. The reusability of [HO3S-(CH2)3-
NEt3]Cl-ZnCl2 (x = 0.83) was investigated (Table 5). It could be seen that the
conversion of a-pinene and the selectivity for camphene was almost unchanged
after [HO3S-(CH2)3-NEt3]Cl-ZnCl2 (x = 0.83) was repeatedly used four times,
which indicated that the Brønsted–Lewis acidic functionalized ILs had excellent
recyclability in the isomerization of a-pinene. This result was explained in terms of
two aspects. First, in the [HO3S-(CH2)3-NEt3]Cl-ZnCl2 (x = 0.83), an alkane
sulfonic acid group was catalytically active for the isomerization, which was
covalently tethered to the ILs cation, and this group could not be easily lost. Second,
the [ZnCl3]- anion of the ILs, which was inert and stable in water or Brønsted acid
and its activity was also not easily lost, was the site of catalytic activity for the
isomerization. At the same time, acid strength was also tested. The FT-IR spectra of
ILs using acetonitrile as the probe is shown in Fig. 2. It can be clearly observed the
reused (four times) ILs acid strength was nearly unchanged compared with that of
the fresh ILs, which indicated that the structure of the reused ILs was retained and
the synthesized ILs exhibited superb recycling ability.
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Y. Liu et al.
Fig. 2 FT-IR spectra of ILs
using acetonitrile as probe
(a New ILs, b Reused ILs for
four times)
Conclusions
The Brønsted–Lewis acidic functionalized ILs of [HO3S-(CH2)3-NEt3]Cl-ZnCl2
was synthesized, characterized, and used in the isomerization of a-pinene.
Compared with traditional catalysts, the Brønsted–Lewis functionalized acidic ILs
exhibited many outstanding advantages, such as simplicity, efficiency, and easy
product isolation. The optimum conditions for isomerization were obtained as
follows: n(a-pinene):n(ILs) = 9:1, reaction temperature 140 °C, and reaction time
4 h. Under the optimal conditions, the conversion of a-pinene was 97.6 % and the
selectivity for camphene was 64.8 %. The [HO3S-(CH2)3-NEt3]Cl-ZnCl2 catalyst
could be recycled up to four times and nearly no loss in activity was observed. Thus,
more researches should be done to improve the catalytic performance of the
Brønsted–Lewis acidic functionalized ILs.
Acknowledgments The financial support provided for this research by the National Natural Science
Foundation of China (31000275) and Postdoctoral Science Foundation of China (201104582) is gratefully
acknowledged.
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