Synthesis of terpinyl acetate using octadecylamine ethoxylate ionic liquids as catalysts

Article abstract of DOI:10.1007/s11164-012-0741-4

Terpinyl acetate, an important natural flavor, can be synthesized by esterification of α-pinene with acetic acid under acidic conditions. In this work, seven acidic ionic liquids were first synthesized, using octadecylamine ethoxylates (AC 1810, AC 1815, AC 1820, and AC 1830) as the cations, and characterized by FT-IR, ¹H NMR, and ¹³C NMR spectroscopy. The ionic liquids were then used as catalysts in the synthesis of terpinyl acetate. The effect of preparation conditions on catalyst performance was examined. It was found that the acidic ionic liquid 1-(3-sulfonic acid)propyl-3-poly(ethylene glycol) octadecylamine polyoxyethylene ether tetrafluoroborate ([PAC1815]⁺[BF₄]^-) was an excellent catalyst for synthesis of terpinyl acetate. The effects of the conditions used for preparation of terpinyl acetate were examined in detail, and the optimum preparation conditions were obtained. Under the optimum conditions, the yield of terpinyl acetate reached 35.70 %. When the catalyst was reused for the 5th time, the yield of terpinyl acetate was still 32.00 %. The results showed that effective separation and recycling of catalyst could contribute to developing a new strategy for synthesis of terpinyl acetate.

Full text of DOI:10.1007/s11164-012-0741-4

Res Chem Intermed (2013) 39:2095–2105
DOI 10.1007/s11164-012-0741-4
Synthesis of terpinyl acetate using octadecylamine
ethoxylate ionic liquids as catalysts
•
Lu Li Shiwei Liu Yanjing Shi Shitao Yu
•
•
•
•
Congxia Xie Chunjie Qi
Received: 25 June 2012 / Accepted: 19 July 2012 / Published online: 2 August 2012
Ó Springer Science+Business Media B.V. 2012
Abstract Terpinyl acetate, an important natural flavor, can be synthesized by
esterification of a-pinene with acetic acid under acidic conditions. In this work,
seven acidic ionic liquids were first synthesized, using octadecylamine ethoxylates
(AC 1810, AC 1815, AC 1820, and AC 1830) as the cations, and characterized by
1
FT-IR, H NMR, and 13C NMR spectroscopy. The ionic liquids were then used as
catalysts in the synthesis of terpinyl acetate. The effect of preparation conditions on
catalyst performance was examined. It was found that the acidic ionic liquid 1-(3-
sulfonic acid)propyl-3-poly(ethylene glycol) octadecylamine polyoxyethylene ether
tetrafluoroborate ([PAC1815]?[BF4]-) was an excellent catalyst for synthesis of
terpinyl acetate. The effects of the conditions used for preparation of terpinyl acetate
were examined in detail, and the optimum preparation conditions were obtained.
Under the optimum conditions, the yield of terpinyl acetate reached 35.70 %. When
the catalyst was reused for the 5th time, the yield of terpinyl acetate was still
32.00 %. The results showed that effective separation and recycling of catalyst
could contribute to developing a new strategy for synthesis of terpinyl acetate.
Keywords a-Pinene Á Acidic ionic liquid Á Esterification Á Terpinyl acetate Á
Octadecylamine ethoxylates
L. Li (&) Á S. Liu Á Y. Shi Á S. Yu Á C. Qi
College of Chemical Engineering, Qingdao University of Science and Technology, Qingdao, China
e-mail: zhanglilu@126.com
C. Xie
College of Chemistry and Molecular Engineering, Qingdao University of Science and Technology,
Qingdao, China
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Introduction
a-Pinene accounts for 58–65 % of turpentine, a mixture extracted from pine terpene
products. a-Pinene can be used as raw material to produce a variety of compounds
by use of acid catalysts. One example is terpinyl acetate [1], an important naturally
occurring compound widely used in chemical products and spices. In recent years it
has been synthesized by direct esterification of a-pinene with acetic acid. In the
esterification process, the selectivity of the catalyst is critical. Solid heteropoly acid
[2], solid acid [3, 4], and zeolite [5] have all been used as catalysts in the reaction.
However, these catalysts have many disadvantages, for example low catalytic
activity and selectivity, long reaction time, and short lifetime, which greatly limit
further development of the process. When seeking a new catalyst with high catalytic
activity, selectivity and reusability are crucial for promoting development of the
process.
Because ionic liquids can be used to design ‘‘environment-friendly’’ solvents and
catalysts, they are widely used in many esterifications [6–15]. As catalysts they
overcome the shortcomings of conventional catalysts and can be simply and easily
separated from the products under mild conditions. Imidazole-based ionic liquids
have usually been used as catalysts in esterification [16–20]. It has not been
discovered whether ionic liquids with fatty amine ethoxylates as cations can be used
as catalysts for esterification. Fatty amine ethoxylates when dissolved in alkali,
water, or another neutral medium are non-ionic, whereas in acidic media they are
cationic. Because of their excellent diffusion properties, they can greatly improve
reaction rate and conversion rate as catalysts [21]. In the work discussed in this
paper, seven acidic polyether ionic liquids were prepared and used in the
esterification of a-pinene. By studying the reaction conditions, for example the
nature and amount of the ionic liquid used, the ratio of reactants, and reaction
temperature, the optimum reaction conditions were determined. The reusability of
the catalyst was also investigated.
Experimental
Materials and instruments
All materials (a-pinene, acetic acid, deionized water, anhydrous magnesium sulfate,
octadecylamine polyoxyethylene (AC 1812, AC 1815, AC 1820, AC 1830), 1,
3-propanesulfonic acid lactone (1,3-PS), sulfuric acid, phosphoric acid, p-toluene-
sulfonic acid (PTSA), tetrafluoroborate) were purchased from Aldrich. All were
directly used after drying without further purification.
FT-IR spectra in the range 4,000–400 cm-1 were recorded with a Nicolet-510P
(American Nicolet Instrument Corporation). NMR spectra of the ionic liquids were
recorded with a 500 MHz Bruker spectrometer in DMSO or CDCl3 and calibrated
with tetramethylsilane (TMS) as internal reference. Gas chromatography–mass
spectrometry (Agilent 6890 N/5973 N) was used for analysis of the composition of
123

Synthesis of terpinyl acetate
2097
the product. Separations were achieved on a 50 m 9 0.25 mm 9 0.25 lm HP-5
column. The column temperature was increased from 80 to 240 °C at 4 °/min.
Synthesis of ionic liquids
A known amount of AC 18n was placed in a flask with stirrer, thermometer, Allihn-
type condenser, and constant-pressure spherical dropping funnel. The flask was
placed in a water-bath with an electrically operated agitator. 1,3-Propanesulfonic
acid lactone was dissolved in ethyl acetate and the solution was added dropwise to
an equimolar amount of AC 18n at 55 °C, and reaction was performed at 55 °C for
2 h. After reaction, ethyl acetate was removed under vacuum, and the red–brown
liquid 1-(3-sulfo)propyl-3-polyoxyethylene octadecylamine vinyl ether (PAC 18n)
was obtained. The PAC 18n was dissolved in water and an equimolar amount of
acid (sulfuric acid, phosphoric acid, p-toluenesulfonic acid, or tetrafluoroborate)
was slowly added dropwise to the PAC18n at 90 °C. Reaction was performed at
90 °C for 2 h. After removal of the water under vacuum 1-(3-sulfo)propyl-3-
octadecylamine polyoxyethylene salts ([PAC 18n]?[R]-) were obtained as brown
viscous liquids. The synthetic process is shown in Fig. 1.
The ionic liquids synthesized in this study were all liquids. They were characterized
to determine their structures. The results were as follows.
[PAC 1820]?[PTSA]-: FT-IR spectra (KBr liquid film, cm-1): 3,396 (–OH
stretching), 2,920 (saturated C–H stretching), 1,459 (C–H bending), 1,353 (N–CH2, C–
H deformation), 1,235 (S=O stretching), 1,112 (C–O–C stretching), 728 (–(–CH2)3– in-
plane rocking), 593 (S–O stretching). 1H NMR spectrum (solvent D2O, 500 MHz): d
7.54–7.05 (4H, CH 9 4), d 5.21–5.15 (1H, OH), d 4.65–3.24 (84H, CH2–O and NCH2),
d 2.96–2.17 (4H, CH2 9 2), d 1.89–1.85 (6H, CH2 9 3), d 1.40 (3H, CH3), d 1.17–1.00
(34H, CH2 9 17), d 0.77–0.76 (6H, CH2 9 3). 13C NMR spectra (solvent D2O,
500 MHz): d 165.50, d 129.13, d 125.47, d 71.70, d 69.56, d 64.23, d 63.77, d 60.30, d
O
O
(CH CH O) H
2
2
a
S
(CH CH O) H
2
2
a
O
N
N
(CH CH O) H
2 2 b
+
+
8
8
(CH CH O) H
2
2
b
CH CH CH SO
2
2
2
3
(CH CH O) H
a
2
2
(CH CH O) H
2
2
a
HR
-
N
(CH CH O) H
2 2 b
N
(CH CH O) H
R
2 2 b
8
8
CH CH CH SO
3
2
2
2
CH CH CH SO H
2
2
2
3
-
R=HSO4 -/ H2 PO4 -/ PTSA-/ BF4
Fig. 1 Synthesis of the ionic liquids [PAC 18n]?[R]-
a+b=n
123

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L. Li et al.
47.85, d 47.29, d 31.90, d 29.84, d 29.73, d 29.07, d 27.08, d 25.93, d 22.61, d 21.55, d
20.56, d 17.74, d 13.92.
[PAC 1820]?[H2PO4]-: FT-IR spectra (KBr liquid film, cm-1): 3,389 (–OH
stretching), 2,920 (saturated C–H stretching), 1,460 (C–H bending), 1,353 (N–CH2,
C–H deformation), 1,249 (S=O and P=O stretching), 1,108 (C–O–C stretching), 543
(S–O and P–O stretching). 1H NMR spectrum (solvent D2O, 500 MHz): d 5.21 (1H,
OH), d 4.65–3.09 (84H, CH2–O and NCH2), d 2.77–2.69 (4H, CH2 9 2), d 1.90–1.80
(6H, CH2 9 3), d 1.56 (3H, CH3), d 1.17–1.11 (34H, CH2 9 17), d 0.73–0.71 (6H,
CH2 9 3). 13C NMR spectra (solvent D2O, 500 MHz): d 130.04, d 129.37, d 71.68,
d 64.18, d 63.79, d 60.27, d 60.06, d 58.42, d 54.01, d 52.71, d 47.90, d 47.22, d 32.53,
d 31.86, d 29.80, d 29.35, d 26.96, d 26.29, d 25.96, d 23.01, d 22.56, d 17.66, d 13.87.
[PAC 1820]?[HSO4]-: FT-IR spectra (KBr liquid film, cm-1): 3,473 (–OH
stretching), 2,919 (saturated C–H stretching), 1,460 (C–H bending), 1,353 (N–CH2,
C–H deformation), 1,247 (S=O stretching), 1,110 (C–O–C stretching), 727 (–(CH2)3–
in-plane rocking), 585 (S–O stretching). 1H NMR spectrum (solvent D2O, 500 MHz):
d 5.21–5.15 (1H, OH), d 4.65–3.10 (84H, CH2–O and NCH2), d 2.79–2.70 (4H,
CH2 9 2), d 1.93–1.80 (6H, CH2 9 3), d 1.57 (3H, CH3), d 1.18–1.11 (34H,
CH2 9 17), d 0.73–0.71 (6H, CH2 9 3). 13C NMR spectra (solvent D2O, 500 MHz):
d 165.30, d 130.14, d 71.85, d 69.80, d 67.37, d 66.70, d 64.45, d 64.03, d 60.58,
d 60.26, d 58.70, d 58.45, d 54.36, d 53.10, d 48.14, d 47.86, d 47.51, d 32.59, d 31.94,
d 29.82, d 29.39, d 27.18, d 26.49, d 24.64, d 23.08, d 22.60, d 21.60, d 17.80, d 13.91.
[PAC 1820]?[BF4]-: FT-IR spectra (KBr liquid film, cm-1): 3,455 (–OH
stretching), 2,920 (saturated C–H stretching), 1,458 (C–H bending), 1,353 (N–CH2,
1
C–H deformation), 1,248 (B–F stretching), 1,110 (C–O–C stretching). H NMR
spectrum (solvent D2O, 500 MHz): d 4.64–3.10 (84H, CH2–O and NCH2),
d 2.77–2.70 (4H, CH2 9 2), d 1.97–1.87 (6H, CH2 9 3), d 1.57 (3H, CH3),
d 1.20–1.14 (34H, CH2 9 17), d 0.73–0.70 (6H, CH2 9 3). 13C NMR spectra (solvent
D2O, 500 MHz): d 71.71, d 64.20, d 63.82, d 60.33, d 58.43, d 52.75, d 47.87, d 47.30,
d 31.90, d 29.83, d 29.39, d 22.59.
[PAC 1810]?[BF4]-: FT-IR spectra (KBr liquid film, cm-1): 3,470 (–OH
stretching), 2,918 (saturated C–H stretching), 1,461 (C–H bending), 1,355 (N–CH2,
1
C–H deformation), 1,250 (B–F stretching), 1,108 (C–O–C stretching). H NMR
spectrum (solvent D2O, 500 MHz): d 4.60–3.12 (44H, CH2–O and NCH2), d
2.75–2.66 (4H, CH2 9 2), d 1.95–1.84 (6H, CH2 9 3), d 1.56 (3H, CH3), d
1.17–1.12 (34H, CH2 9 17), d 0.72–0.70 (6H, CH2 9 3). 13C NMR spectra (solvent
D2O, 500 MHz): d 72.09, d 70.07, d 69.89, d 69.69, d 64.43, d 64.03, d 60.62, d
58.71, d 55.58, d 54.26, d 53.11, d 48.18, d 47.51, d 31.98, d 29.95, d 29.81, d 29.45,
d 29.18, d 26.48, d 26.19, d 23.28, d 22.66, d 21.82, d 17.97, d 13.93.
[PAC 1815]?[BF4]-: FT-IR spectra (KBr liquid film, cm-1): 3,455 (–OH
stretching), 2,920 (saturated C–H stretching), 1,459 (C–H bending), 1,353 (N–CH2,
C–H deformation), 1,247 (S=O stretching), 1,109 (C–O–C stretching), 1,060 (B–F
stretching), 727 (–(CH2)3– in-plane rocking), 543 (S–O stretching). 1H NMR
spectrum (solvent D2O, 500 MHz): d 5.19 (1H, OH), d 4.64–3.07 (64H, CH2–O and
NCH2), d 2.78–2.71 (4H, CH2 9 2), d 1.98–1.74 (6H, CH2 9 3), d 1.55 (3H, CH3),
d 1.09 (34H, CH2 9 17), d 0.72–0.69 (6H, CH2 9 3). 13C NMR spectra (solvent
D2O, 500 MHz): d 71.90, d 68.67, d 64.24, d 63.85, d 60.39, d 58.45, d 58.19, d
123

Synthesis of terpinyl acetate
2099
52.83, d 47.90, d 47.34, d 31.92, d 29.86, d 29.71, d 29.40, d 29.09, d 26.93, d 26.34,
d 26.03, d 23.06, d 22.60, d 21.62, d 17.79, d 13.87.
[PAC1830]?[BF4]-:FT-IRspectra (KBrliquidfilm, cm-1):3,465(–OH stretching),
2,915 (saturated C–H stretching), 1,456 (C–H bending), 1,357 (N–CH2, C–H
1
deformation), 1,251 (B–F stretching), 1,106 (C–O–C stretching). H NMR spectrum
(solvent D2O, 500 MHz): d 4.63–3.12 (124H, CH2–O and NCH2), d 2.78–2.69 (4H,
CH2 9 2), d 1.95–1.84 (6H, CH2 9 3), d 1.59 (3H, CH3), d 1.24–1.12 (34H,
CH2 9 17), d 0.71–0.68 (6H, CH2 9 3). 13C NMR spectra (solvent D2O, 500 MHz):
d 71.75, d 69.62, d 64.26, d 63.88, d 60.53, d 60.35, d 58.50, d 54.33, d 52.84, d 47.90,
d 47.32, d 31.92, d 29.82, d 29.05, d 26.09, d 22.62, d 21.59, d 17.73, d 13.95.
The structures of the ionic liquids were confirmed by the results given above.
Determination of the acid value of the ionic liquids
Ionic liquid solutions of different concentration (0.005, 0.010, 0.015, 0.020, and
0.025 mol/L) were prepared. The ionic liquid solution (10 ml) was placed in a
100 ml tapered bottle and two drops of phenolphthalein indicator solution were
added. The solution was then titrated with 0.10 mol/L aqueous potassium hydroxide
standard solution until the indicator turned pink. Blank experiments were performed
by the same method.
Esterification of a-pinene
Known amounts of a-pinene, acetic acid, and ionic liquid were placed in a reaction
flask with an electronic stirrer. After reaction at a specific temperature for 10 h,
10 ml water was added in the mixture. The upper layer was collected and washed
with water to pH 7. The product was dried over anhydrous magnesium sulfate then
filtered to get obtain a clear organic solution. The solution was analyzed by GC. The
lower ionic liquid catalyst was recovered after dehydration and reused. The
mechanism of reaction [2] of a-pinene and acetic acid in the presence of a catalyst is
shown in Fig. 2.
The products were analyzed by GC–mass spectrometry (Agilent 6890 N/
5973 N). An HP-5 capillary column with a length of 50 m and an internal diameter
of 0.25 mm was used. The injector and transfer line temperatures were 280 °C. The
initial temperature of the column oven was maintained at 80 °C for 10 min then
increased at 4 °/min to 240 °C which was maintained for 10 min. The carrier gas
was N2. The sample size was 0.1 lL. The chromatogram obtained from the products
of esterification of a-pinene is shown in Fig. 3. The identities of the products were
determined by GC–MS. Rt (retention time): 12.42 min a-pinene; 12.89 min
camphene; 15.36 min dolcymene; 15.63 min limonene; 17.85 min terpinolene;
21.78 min a-terpineol; 22.85 min fenchyl acetate; 25.29 min isobornyl acetate;
25.40 min bornyl acetate; 25.70 min 4-terpinenyl acetate; 27.79 min terpinyl
acetate; 35.11 min terpinyl propionate.
123

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L. Li et al.
H
HOAc
B-
OAc
a
ring-expanding1
OAc
H
isomerization
HOAc
B-
III
VII
H
HOAc
B-
OAc
b
ring-expanding 2
VIII
isomerization
II
I
IV
IX
HOAc
B-
c
OAc
ring-opening
X
isomerization
+
V
XI
XII
Fig. 2 Reaction of a-pinene with acetic acid
9.96 - 52.53
27.79
0
5
0
5
0
5
0
5
0
5
0
5
0
5
0
5
0
5
0
5
0
12.42
12.89
15.63
17.85
35.11
22.82
25.29
21.78
15.36
16.57
25.40
25.70
32.90
30.54
36.14
37.44
20.27
19.88
44.93
43.97
46.17
15.02
11.87
47.83
28.77
42.80
41.57
51.4
25.02
32.14
10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50
5
Time (min)
Fig. 3 Gas chromatogram obtained from the products of esterification of a-pinene
123

Synthesis of terpinyl acetate
2101
Results and discussion
The effect of the type of catalyst on the reaction can be seen in Table 1. With the same
cation, the reaction was substantially affected by the anion species used. Table 1
shows that the yield of terpinyl acetate was highest when [PAC 1820]?[BF4]- was
used as catalyst, and the yield was the lowest when [PAC 1820]?[PTSA]-(PTSA: p-
toluene sulfonic acid) was used. The acid values of the different ionic liquids are listed
in Table 2. From this table it is apparent [PAC 1820]?[BF4]- had the maximum acid
value whereas that of [PAC1820]?[PTSA]- was lowest. It can, therefore, be
concluded that the yield of terpinyl acetate was determined by the acid value of ionic
liquids. In subsequent work we focused on studying the catalytic properties of [PAC
18n]?[BF4]-.
From Table 3 it is apparent that the degree of polymerization (n) affected the
yield of terpinyl acetate. With increasing n, the yield of terpinyl acetate first
increased and then decreased. When n was 15, the yield of terpinyl acetate was
maximum. This could be explained by its different acid value. As chain length
increased, the acid content per unit mass decreased. Therefore as the n value
increased, the amount of acid decreased (Table 4). When n was equal to 15, the
acidic strength was quite suitable for this reaction. Therefore, [PAC 1815]?[BF4]-
was used as the catalyst in the reaction.
Table 5 shows that the yield of terpinyl acetate increased with increasing reaction
time. When reaction time was 10 h the reaction basically achieved balance, and the
yield of terpinyl acetate reached its highest value of 35.70 %. When the reaction
time exceeded 11 h, the yield began to decrease, indicating that long reaction times
resulted in side effects and was of no benefit for generating terpinyl acetate. The
appropriate time was, therefore, 10 h.
The effect of reaction temperature on esterification is also shown in Table 5.
When the temperature was low, the rate of the reaction and the yield of terpinyl
acetate were both low. As the temperature was increased, the reaction rate increased,
and the yield of terpinyl acetate increased accordingly. When the temperature was
too high, however, a-pinene tended to isomerize rather than esterify. The results
showed the yield of terpinyl acetate first increased and then decreased with
Table 1 Effect of the type of catalyst on the reaction
Type of catalyst
Product components (%)
Yield of terpinyl
acetate (%)
a-
Pinene
Camphene Limonene Terpinolene Terpinyl
acetate
Blank
90.23
2.64
16.61
9.47
0.25
10.70
9.39
0.89
5.42
0.85
13.23
17.22
15.39
26.24
0.99
15.49
20.02
17.90
30.51
[PAC 1820]?[PTSA]- 48.90
[PAC 1820]?[H2PO4]- 47.76
[PAC 1820]?[HSO4]- 24.63
4.75
8.01
20.78
9.75
13.69
7.48
[PAC 1820]?[BF4]-
19.81
18.48
Reaction conditions: n(a-pinene):n(catalyst):n(acetic acid) = 1:0.06:4; a-pinene 0.05 mol, reaction
temperature 30 °C, reaction time 10 h
123

2102
L. Li et al.
Table 2 Effect of acid type on acid value
Concentration (mol/L) Volume of KOH solution consumed (V1 - V2)
[PAC
1820]?[PTSA]-
[PAC
1820]?[H2PO4]-
[PAC
1820]?[HSO4]-
[PAC
1820]?[BF4]-
0.005
0.90
1.80
2.72
3.61
4.50
70.8
0.95
1.90
2.80
3.70
4.65
77.2
1.05
2.10
3.16
4.22
5.20
87.4
1.45
2.80
0.010
0.015
4.12
0.020
5.50
0.025
6.40
Acid value (mg/g)
115.6
Table 3 Effect of degree of polymerization on the results of the reaction
Degree of
polymerization
Product component (%)
Yield of terpinyl
acetate (%)
a-
Pinene
Camphene
Limonene
Terpinolene
Terpinyl
acetate
10
15
20
30
16.77
7.13
20.72
17.23
18.48
15.81
10.33
16.55
11.62
12.51
9.30
13.99
10.89
6.66
28.45
30.70
26.24
25.07
33.11
35.70
30.51
29.20
19.81
25.81
Reaction conditions: n(a-pinene):n(catalyst):n(acetic acid) = 1:0.06:4, a-pinene 0.05 mol, reaction
temperature 30 °C, reaction time 10 h
Table 4 Effect of degree of polymerization on acid value
Concentration
(mol/L)
Volume of KOH solution consumed (V1 - V2)
[PAC
1810]?[BF4]-
[PAC
1815]?[BF4]-
[PAC
1820]?[BF4]-
[PAC
1830]?[BF4]-
0.005
1.42
2.72
1.40
2.71
1.45
2.80
1.40
2.80
4.16
5.36
6.60
87.1
0.010
0.015
3.91
3.71
4.12
0.020
5.00
5.10
5.50
0.025
5.95
6.20
6.40
Acid value (mg/g)
159.3
132.2
115.6
increasing temperature. The terpinyl acetate content reached a maximum at 30 °C,
which was, therefore, a suitable temperature.
When the amount of a-pinene was 0.05 mol, with increasing amount of acetic
acid the yield of terpinyl acetate first increased and then decreased (Table 5).
When the ratio n(a-pinene)/n(acetic acid) was 1:4, the yield of terpinyl acetate
reached a maximum. As the amount of acetic acid was increased, mutual
accessibility of acetic acid and a-pinene increased, so the yield of terpinyl acetate
123

Synthesis of terpinyl acetate
2103
Table 5 Effect of reaction conditions on the results of the reaction
t (h) T (°C) n(Acetic n(IL)
Product components (%)
Yield of
terpinyl
acetate
(%)
acid)
(mmol)
a-Pinene Camphene Limonene Terpinolene Terpinyl
acetate
(mol)
8
30
30
30
30
30
20
40
50
60
30
30
30
30
30
30
30
30
0.20
0.20
0.20
0.20
0.20
0.20
0.20
0.20
0.20
0.10
0.15
0.25
0.30
0.20
0.20
0.20
0.20
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
2.0
2.5
3.5
4.0
16.45
13.32
7.13
22.58
15.71
17.23
16.25
16.6
18.86
14.32
16.55
13.95
13.06
17.94
13.24
11.89
8.05
1.05
11.06
13.99
9.94
16.32
21.81
30.70
30.45
20.62
14.00
26.85
19.68
13.21
9.52
16.59
25.36
35.70
35.41
18.45
16.28
31.22
22.88
15.36
11.07
32.17
25.56
23.22
27.32
32.20
28.39
27.68
9
10
11
12
10
10
10
10
10
10
10
10
10
10
10
10
7.14
6.74
12.19
11.44
16.55
20.10
26.91
10.16
1.73
14.74
2.25
19.89
15.28
14.03
3.49
1.61
0.19
15.43
8.05
21.06
15.56
14.28
13.66
20.49
19.68
16.95
15.83
19.59
21.28
16.57
15.87
12.32
13.67
16.76
18.05
27.67
21.97
19.97
23.50
27.69
24.41
23.80
6.03
16.87
15.16
9.74
4.91
13.30
10.52
5.64
11.22
15.71
16.66
4.04
IL, ionic liquid; n(a-pinene) = 0.05 mol
increased [21]. When the amount of acetic acid was excessive, however, the amount
of catalyst per unit volume was too small for the esterification reaction. Therefore,
n(a-pinene)/n(acetic acid) should be controlled at approximately 1:4.
As is apparent from Table 5, as the amount of ionic liquid was increased, the
yield of terpinyl acetate increased, indicating increasing the quantity of ionic liquid
promoted this reaction. When n(a-pinene)/n(ionic liquid) was 1:0.06, the yield of
terpinyl acetate reached a maximum. If the amount of ionic liquid was increased
beyond this, however, the amount of esterification product decreased. This might
because the initial increase of amount of acid ionic liquid per unit volume was
beneficial to the reaction. When n(a-pinene)/n(ionic liquid) exceeded 1:0.06, the
viscosity and the mass transfer resistance of the system increased, contact of
a-pinene and acetic acid decreased, and this finally resulted in a decrease of the
yield of terpinyl acetate [22]. The viscosity of the reaction mixture was tested at
30 °C: n(a-pinene)/n(ionic liquid) = 1:0.05, viscosity = 1.95 mPa S; n(a-pinene)/
n(ionic liquid) = 1:0.06, viscosity = 2.05 mPa S; n(a-pinene)/n(ionic liquid) =
1:0.07, the viscosity = 2.25 mPa S. It is well known that with increasing viscosity
mass transfer resistance is enhanced [23]. Taken together, the appropriate n(a-
pinene)/n(ionic liquid) ratio was 1:0.06.
When the catalyst was reused for the 5th time, the yield of terpinyl acetate was
still 32.00 %, showing that effective separation and recycling of the catalyst could
123

2104
L. Li et al.
Table 6 Results from re-use of the catalyst
Cycle
Product components (%)
Yield of terpinyl
acetate (%)
a-Pinene
Camphene
Limonene
Terpinolene
Terpinyl
acetate
1
2
3
4
5
7.09
8.13
7.68
7.73
9.34
17.20
15.85
14.97
13.53
11.76
16.59
19.37
21.23
24.51
26.21
14.02
12.74
11.86
10.28
9.46
30.73
30.05
29.65
28.93
27.52
35.73
34.94
34.48
33.64
32.00
Reaction conditions: n(a-pinene):n(catalyst):n(acetic acid) = 1:0.06: 4, a-pinene 0.05 mol, reaction
temperature 30 °C, reaction time 10 h
be expected to contribute to the development of a new strategy for synthesis of
terpinyl acetate (Table 6).
Conclusions
The ionic liquid 1-(3-sulfonic acid)propyl-3-poly(ethylene glycol)-yl octadecylamine
polyoxyethylene ether tetrafluoroborate ([PAc 1815]?[BF4]-) was synthesized and
characterized. The ionic liquid had good catalytic activity in the esterification of a-
pinene, and the catalyst could be reused. The optimum conditions obtained were n(a-
pinene):n(ionic liquid):n(acetic acid) = 5:0.3:20, n(a-pinene) = 0.05 mol, reaction
temperature 30 °C, and reaction time 10 h. A new method is therefore available for
synthesis of terpinyl acetate.
Acknowledgments The authors are grateful to the National Natural Science Foundation of China
(31000275) and the Postdoctoral Science Foundation of China (20100471496) for financial support of this
work.
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