Catalytic synthesis of butyl isovalerate by br?nsted acidic ionic liquid under microwave irradiation

Article abstract of DOI:10.14233/ajchem.2013.14565

Butyl isovalerate was synthesized from isovaleric acid and butanol with ionic liquid as catalysts under microwave irradiation. The systematic study was investigated including of the effects of molar ratio of acid to alcohol, microwave power, amount of catalyst, microwave time and microwave temperature on the process. The optimum operation conditions were as follows: n(butanol):n(isovaleric acid) = 1.6:1, microwave power 500 W, amount of catalyst of 6 wt % to isovaleric acid, microwave time of 0.5 h and microwave temperature of 110 °C. The yield of butyl isovalerate could reach 92.1 % under the optimal operation conditions mentioned above. The excellent durability and recycle ability of ionic liquids were also addressed even after five times reused.

Full text of DOI:10.14233/ajchem.2013.14565

Asian Journal of Chemistry; Vol. 25, No. 14 (2013), 7675-7678
http://dx.doi.org/10.14233/ajchem.2013.14565
Catalytic Synthesis of Butyl Isovalerate by Brønsted
Acidic Ionic Liquid under Microwave Irradiation
*
DU HUAN, HE YANFEI, TAN ZHONGQIN and XIAOXIANG HAN
Department of Applied Chemistry, College of Food Science & Biotechnology Engineering, Zhejiang Gongshang University, Hangzhou,
310035, P.R. China
*Corresponding author: Tel: +86 571 88071024-7581; E-mail: hxx74@126.com; han-xx74@163.com
(Received: 14 September 2012;
Accepted: 8 July 2013)
AJC-13785
Butyl isovalerate was synthesized from isovaleric acid and butanol with ionic liquid as catalysts under microwave irradiation. The systematic
study was investigated including of the effects of molar ratio of acid to alcohol, microwave power, amount of catalyst, microwave time and
microwave temperature on the process. The optimum operation conditions were as follows: n(butanol):n(isovaleric acid) = 1.6:1, micro-
wave power 500 W, amount of catalyst of 6 wt % to isovaleric acid, microwave time of 0.5 h and microwave temperature of 110 ºC. The
yield of butyl isovalerate could reach 92.1 % under the optimal operation conditions mentioned above. The excellent durability and
recycle ability of ionic liquids were also addressed even after five times reused.
Key Words: Ionic liquid, Butyl isovalerate, Esterification, Microwave irradiation.
acidic-functionalized ionic liquids show special advantages:
no measurable vapor pressure, excellent thermal stability,
potential recover ability, recycle ability and are able to dissolve
compounds with varying polarity8-10. So applications of ionic
liquids have been extensively studied with high yields, among
which esterification is hot topic. Since then, a series of reports
regarding synthesis of ester using acidic-functionalized ionic
liquids as catalysts have been published11-13. Mostly all
mentioned reactions have been carried out using reactors
accompanied with traditional heat-transfer equipment.
However, the reaction time is long.
INTRODUCTION
Butyl isovalerate, a transparent with colourless or tiny
yellow colour liquid, is of fragrance feature of banana and
blue chess. It's common used as perfumes for the preparation
of dairy products and cheese and also as flavor chemicals,
solvents and precursors for pharmaceuticals and fine chemi-
cals1. In traditional industries, butyl isovalerate is generally
synthesized from isovaleric acid and butanol by using homo-
geneous catalysts such as H2SO4. However, sulphuric acid
suffers from inherent drawbacks such as equipment corrosion,
more byproducts, environmental hazards, difficulty in catalyst
recovery and reuse ability. These make this process with envi-
ronmentally questionable. Therefore, the development of
heterogeneous catalysts systems aimed at overcoming these
limitations have proven to be a significant task. Supported
heteropoly acid2, ion exchange resin3, molecular sieve4 and
lipase5 have been carried out on the esterification of isovalerate
ester. However, all these processes and catalysts have their
own disadvantages such as higher cost, low activity, easy
deactivation, difficult separation from the products and catalyst
recyclability.
Microwave irradiation becomes a new green technology
instead of conventional heating. It is an internal heated method
and demonstrates many characteristics i.e., fast heating rate
with no lag effects, lower energy consumption, easy to oper-
ate and less pollution. In many cases, microwave-assisted
chemical reactions have showed amazing accelerations, higher
yields and selectivity and higher product purities under milder
reaction conditions14,15. Microwave heating has also been
applied to esterification, besides being environmentally
friendly, it is also marked by a considerable reduction in
reaction time16,17. The aim of the present work is to establish
the optimal conditions for the esterification of isovaleric acid
with butanol in a laboratory prepared Brønsted acid
functionalized ionic liquid catalyst. Additionally, the stability
and reuse performance of catalyst was also examined.
During the past few decades, ionic liquids (ILs) have
received considerable attention as eco-friendly and effective
reaction catalyst for a wide variety of organic reaction and
other applications in chemistry due to its adjustable physical
and chemical properties6,7. Indeed, ionic liquid especially

7676 Huan et al.
Asian J. Chem.
Effect of ratio of butanol to isovaleric acid: Butanol is
a major component in the pre-treatment process via esterifi-
cation reaction. Its molar ratio significantly affects the con-
version of isovaleric acid to butyl isovalerate during the course
of reaction. The molar ratio of butanol to isovaleric acid varied
from 1.2:1 to 2.2:1, Fig. 1 revealed the ratio effect and it is
observed that as the amount of butanol increased, the yield of
butyl isovalerate increased and reached a maximum of 92.1 %
with butanol to isovaleric acid mole ratio of 1.6:1. According
to the chemical equilibrium principle, the excess of one of the
reactants made the equilibrium shifting towards the products
side. However, when the mole ratio of butanol to isovaleric
acid exceeded 1.6:1, the yield of butyl isovalerate decreased.
It might be due to dilution phenomenon of the isovaleric acid
and [HSO3-pmim]HSO4 with excess butanol.
EXPERIMENTAL
Catalyst preparation and characterization: Brønsted
acidic ionic liquids were prepared and characterized by IR,
NMR, TG and acidity determination in the laboratory following
the procedure outlined in literature13,18.All the chemicals were
research grade and were used without further purification unless
otherwise stated. The catalysts were abbreviated as: [HSO3-
pmim]HSO4, [HSO3-pmim]PTSA, [HSO3-pmim]H2PO4 and
[HSO3-pmim]COOCF3, HSO3-pmim = 1-methyl-3- (propyl-
3-sulfonic group) imidazole.
Activity testing: In a typical procedure, isovaleric acid
(0.1 mol, 10.21 g), butanol (0.16 mol, 11.85 g) and ionic liquid
(0.78 g) were added into a three-necked round bottom flask.
The reactions were carried out in microwave synthesis system
equipped with a magnetic stirrer and a water-cooled condenser.
The reaction mixture was heated to appropriate temperature
for desired time with certain microwave power. Upon comple-
tion, the reaction mixture was cooled to room temperature
and the layers separated. The upper layer was ester. The lower
layer consisting of the ionic liquid was reused in a further run.
Chemical analysis of the products was performed by gas
chromatography Agilent 6890N GC, equipped with a FID
detector and HP-5 capillary column. Reactants and products
were identified by comparison with authentic samples. Methyl
laurate was used as an internal standard.
92
88
84
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RESULTS AND DISCUSSION
1.2
1.4
1.6
1.8
2.0
2.2
Alcohol/Acid molar ratio
Alcohol/acid molar ratio
Comparison of activity of the four ionic liquid cata-
lysts: The acidity strength order of ionic liquids is consistent
with their activity order in some acid-catalyzed reactions. It is
known that anions have strong interactions with the proton
donated from sulfonic acid. The anion is likely to serve as
available acid sites. Hence the acidities and catalytic activities
of ionic liquids always depend on the kinds of anions. The
catalytic performances of ionic liquid with different anion at
certain conditions were also given in Table-1. From Table-1,
we could find that ionic liquid with HSO4– showed the best
catalytic activity for esterification with 92.1 % yield of butyl
Fig. 1. Esterification reaction of butanol with isovaleric acid under different
molar reactant ratios
Effect of microwave power: For the purpose of compare
the effect of microwave power on the esterification rate, butyl
isovalerate was synthesized under different power of micro-
wave irradiation. The results were shown in Fig. 2. It could be
found that microwave power had a great influence on yield of
butyl isovalerate. The yield of butyl isovalerate increased with
increasing microwave power. When the microwave power was
500 W, the highest yield of butyl isovalerate was 92.1 % in
0.5 h at 110 ºC. However, when the microwave power exceeded
500 W, the yield of butyl isovalerate decreased. The reason
was that the reaction was not complete with low microwave
power owing to fewer amounts of the excited molecules. While
the yield decreased with high microwave power because the
collisions between the reactant molecules were too violent17
and the velocity of side effects also greatly accelerated. Addi-
tionally, high power of microwave accumulated energy over a
period of time and released it quickly, thus increasing the
instantaneous temperature vaporized partial butanol. It was
not beneficial for esterification.
–
isovalerate. Ionic liquid containing H2PO4 and CF3COO–
anions showed poorer activity as compared to the above. Such
a high activity of Brønsted ionic liquid with HSO4– anion has
also been reported in the Fischer esterification reaction with
other ionic liquids19. The order of esterification activity for
ionic liquids with different anions was [HSO4–] > [p-CH3C6H4SO3–
] > [H2PO4–] > [CF3COO–], which was in excellent agreement
with their acidity order18. Detailed investigations to optimize
various reaction parameters were conducted using [HSO3-
pmim]HSO4 as catalyst since it was also less toxic and cost
effective besides showing high product selectivity.
TABLE-1
RESULTS OF DIFFERENT IONIC LIQUIDS ON THE YIELD OF BUTYL ISOVALERATE
Alcohol/acid mole
ratio
Amount of catalyst
(wt %)
Microwave power
(W)
Reaction time
(min)
Yield
(%)
Ionic liquid
[HSO3-pmim]HSO4
[HSO3-pmim]PTSA
[HSO3-pmim]H2PO4
[HSO3-pmim]COOCF3
1.6:1
1.6:1
1.6:1
1.6:1
6
6
6
6
500
500
500
500
30
30
30
30
92.1
91.3
43.8
39.2

Vol. 25, No. 14 (2013)
Catalytic Synthesis of Butyl Isovalerate by Brønsted Acidic Ionic Liquid 7677
ratio of butanol to isovaleric acid 1.6:1 and a temperature of
110 ºC under 500 W microwave radiation. As we can see in
Fig. 4, we could divide the reaction into three phases appro-
ximately according to the reaction trend. In the first phase, the
yield of butyl isovalerate reached 90 % rapidly within 25 min.
It was because excess of butanol to react with isovaleric acid
make the esterification more easily. In the second phase, the
process of the reaction became slowly from 25 to 30 min and
the reaction approached equilibrium in this phase. The best
yield of butyl isovalerate (92.1 %) was obtained around 0.5 h.
In the last phase, yield of butyl butyrate decreased slightly
instead of an upward trend when the reaction time was prolonged.
This was because esterification was a reversible reaction, leading
the product partially hydrolyzed when reaction time was too long.
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84
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100
200
300
400
500
600
700
Microwave power (W)
Microwave power (W)
Fig. 2. Esterification reaction of butanol with isovaleric acid under different
microwave power
Therefore, the optimal reaction time should be around 0.5 h.
100
Effect of amount of [HSO3-pmim]HSO4: The amount of
ionic liquids played an important role during the esterification
reaction. Finding the optimal amount of catalyst had the
priority among all reaction parameters. Fig. 3 showed the effect
of the amount of ionic liquids on the yield of butyl isovalerate.
As the amount of ionic liquids increased, the conversion of
isovaleric acid also increased. When 6 wt % of [HSO3-
pmim]HSO4 was added, 92.1 % of isovaleric acid was
converted into butyl isovalerate in 0.5 h. There were enough
active sites for the reaction when the amount of catalyst was 6
wt %. However, the yield of butyl isovalerate decreased slightly
when the amount of catalyst exceeded 6 wt %. It might be due
to the growth of side effect resulted from the increasing amount
of ionic liquid meant increasing the acidity of the reaction
system. Moreover, it was also ascribed to the solubility of ionic
liquid. The ionic liquid can dissolve in butanol, but had little
solubility in butyl isovalerate. The solubility of ionic liquid
was also restricted by the degree of ester and butanol mixing.
Considering the conversion of isovaleric acid, the product
distribution and less cost effective, the optimal amount of
[HSO3-pmim]HSO4 should be around 6 wt %.
80
60
40
20
0
0
10
20
30
40
Microwave time (min)
Microwave time (min)
Fig. 4. Esterification reaction of butanol with isovaleric acid under different
microwave time
Effect of microwave temperature: We also examined
the effect of reaction temperature on the esterification of butyl
isovalerate, the results were shown in Fig. 5. When the reaction
temperature was 90 ºC, 78.2 % of isovaleric acid was converted
into butyl isovalerate.Yield of butyl isovalerate increased with
a rise in temperature, reaching a maximum 92.1 % at 110 ºC.
Then, the yield declined gradually with increasing the tempe-
rature. The reason was that more successful collisions occurred
at high temperature, it could contribute to the enhancement of
reaction rate and the yield of butyl isovalerate. When the
reaction temperature exceeded 110 ºC even above the boiling
point of butanol (117.7 ºC), butanol was easily vaporized, the
successful collision possibility reduced and the yield of butyl
isovalerate decreased.
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Recycling of ionic liquid [HSO3-pmim]HSO4: The main
advantage of the ionic liquid catalysts was that it could be
recyclable for further use. To make this method more suitable,
we examined the reusability of catalyst. [HSO3-pmim]HSO4
was investigated by performing several runs under the
optimized reaction conditions determined above. After each
cycling reaction, the ionic liquid was extracted with toluene
and vacuum dried for 5 h at 120 ºC. After vacuum drying, the
[HSO3-pmim]HSO4 was assessed by 1H NMR spectroscopy,
which showed no traces of reactants or products. The results
from Table-2 showed that [HSO3-pmim]HSO4 could be reused
3
4
5
6
7
8
Amount of ionic liquid
Amount of ionic liquid
Fig. 3. Esterification reaction of butanol with isovaleric acid under different
amount of catalysts
Effect of microwave time: Generally, extending reaction
time could make the reaction more completely. The influence
of reaction time on the esterification of isovaleric acid and
butanol was determined by using 6 wt % catalyst, a molar

7678 Huan et al.
Asian J. Chem.
of 92.1 % yield of the esterification was achieved in 0.5 h
under microwave irradiation. The reaction time was signifi-
cantly reduced. The fact indicated that the friendly catalyst
[HSO3-pmim]HSO4 showed excellent catalytic activity, stability
and reusability in this case and we received a satisfactory yield
for esterification in short time. Therefore, an efficient and
environmentally friendly catalyst and heating method were
provided for the synthesis of butyl isovalerate.
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