Base catalyzed glycerolysis of benzyl acetate

Article abstract of DOI:10.2174/157017811796504936

Glycerol was successfully used as a green solvent and as an acyl acceptor in the transesterification of benzyl acetate using representatives' soluble and solid base catalysts. It was found that increasing the reaction temperature, the reaction time, the s

Full text of DOI:10.2174/157017811796504936

504
Letters in Organic Chemistry, 2011, 8, 504-508
Base Catalyzed Glycerolysis of Benzyl Acetate
Adi Wolfson*, Eran Azran, Christina Dlugy and Dorith Tavor
Green Processes Center, Chemical Engineering Department, Sami Shamoon College of Engineering, Bialik/Basel Sts.
Beer-Sheva, 84100 Israel
Received February 15, 2011: Revised March 17, 2011: Accepted May 17, 2011
Abstract: Glycerol was successfully used as a green solvent and as an acyl acceptor in the transesterification of benzyl
acetate using representatives' soluble and solid base catalysts. It was found that increasing the reaction temperature, the
reaction time, the substrate concentration or the catalysts loading increased the yield of benzyl alcohol. Using glycerol as
a solvent also enabled the separation of product by simple extraction with diethyl ether and catalyst recycling.
Keywords: Base catalyst, glycerol, green solvent, transesterification.
1. INTRODUCTION
glycerol as a green solvent and as an acyl acceptor (Fig. 1).
The effects of reaction conditions and catalyst type and load-
ing on benzyl alcohol yield were studied. In addition, prod-
uct separation procedure and catalyst recycling were also
examined.
The hydrolysis of esters is a basic organic transformation
[1, 2]. Traditionally, the reaction is performed in an acidic or
basic aqueous solution under mild conditions yielding the
corresponding carboxylic acid and alcohol. Alternatively,
due to low miscibility of most esters in water and since
acidic or basic conditions lead to equipment corrosion, cata-
lytic transesterification of ester in the presence of an alcohol
that removes the carboxylic group from the ester and releases
the corresponding alcohol (alcoholysis) was also extensively
studied. The alcoholysis of ester is an equilibrium reaction,
which usually requires excess amounts of alcohol to yield
high conversion, and the presence of catalyst that can be
either homogenous or heterogeneous [3-5]. Solid acids and
bases as well as immobilized lipase are often employed for
this purpose as they have the advantage of being easily sepa-
rated from the reaction mixture, recycled and reused.
2. EXPERIMENTAL
In a typical procedure, 0.1 g of benzyl acetate was added
together with 0.01 g of catalyst to a vial with 5 g of alcohol
(all purchased from Aldrich). The mixture was placed in a
preheated oil bath and heated to the required temperature
(45-100 °C) after which it was magnetically stirred for 1-5 h.
At the end of the reaction, the reaction mixture was cooled
and extracted with 3ꢀ10 mL diethyl ether. The organic phase
was concentrated under reduced pressure, and the resulting
crude product was analyzed by GC analysis using an HP-5
column (30 m ꢀ 0.25 mm, 0.25 μm thick).
Various alcohols can be employed for the alcoholysis of
an ester. We recently showed that glycerol can be success-
fully used as both a solvent and an acyl acceptor in the ki-
netic resolution of ester racemates via transesterification
using immobilize Candida antarctica lipase B (CAL-B) as a
catalyst [6]. Glycerol triacetate (triacetin) was also used si-
multaneously as a solvent and an acyl donor in the kinetic
resolution of racemic mixture of alcohols and in the produc-
tion of isoamyl acetate, a characteristic banana flavor ester
used in the food industry, via teansesterification, using CAL-
B or acidic ion exchange as a catalyst [7, 8]. Besides its
green character, the use of triacetin allowed easy product
work-up and catalysts recycling. Both, glycerol and triacetin
are renewable, recyclable and nontoxic green solvents that
can be used as an alternative reaction medium for various
organic reactions [9-11].
Two catalyst recycling methods were tested. The first re-
action cycle in both methods was run as follows: 1 g of ben-
zyl acetate and 0.1 g of solid catalyst were added to a vial
with 10 g of glycerol. The vial was then heated at 75 °C for 1
h. For the first catalyst recycling method, the catalyst was
filtrated at the end of the reaction and the product was ex-
tracted by 5ꢀ10 mL diethyl ether and analyzed by GC. The
filtrated catalyst was then added to a fresh mixture of benzyl
acetate in glycerol, and the reaction was repeated. For the
second method, the product was extracted by 5ꢀ10 mL di-
ethyl ether from the glycerol-catalyst mixture without the
filtration of the catalyst, to which was added fresh benzyl
acetate.
Microwave assisted reactions were conducted at atmos-
pheric pressure in a domestic microwave (Crystal WP900,
900W) in a vial, which was covered with a watch glass. The
substrate was dissolved in 5 g glycerol followed by addition
of the catalyst. After the vial was covered with the watch
glass, the reaction mixture was heated in the microwave
oven at low intensity from 26 °C to 56 °C for duration of 40
s and at full intensity from 26 °C to 61 °C for 5 s. At the end
of the reaction the vial was cooled to room temperature in
ice, and the reaction mixture was extracted with petroleum
ether for GC analysis.
In this paper we report on our study about the glyceroly-
sis of benzyl acetate, using sodium hydroxide and magnesia
as representative soluble and solid base catalysts, using
*Address correspondence to this author at the Green Processes Center,
Chemical Engineering Department, Sami Shamoon College of Engineering,
Bialik/Basel Sts. Beer-Sheva, 84100 Israel; Tel: 972-8-6475766; Fax: 972-
8-6475636; E-mail: adiw@sce.ac.il
1570-1786/11 $58.00+.00
© 2011 Bentham Science Publishers

Base Catalyzed Glycerolysis of Benzyl Acetate
Letters in Organic Chemistry, 2011, Vol. 8, No. 7
505
OAc
O
OH
OH
OH
OH
Base
O
OH
OH
Fig. (1). Transesterification of benzyl acetate in glycerol.
3. RESULTS AND DISCUSSION
and an acyl acceptor it may yield glycerol monoacetate,
glycerol diacetate, and/or triacetin as by-products. However,
the high boiling points and high solubilities of these potential
by-products in glycerol allow the product to be easily sepa-
rated from the reaction mixture.
The synthesis of benzyl alcohol is usually produced via
synthesis of benzyl chloride by chlorination of toluene that is
then hydrolyzed under alkali conditions [12, 13]. This multi-
stage process includes many steps of separation and purifica-
tion after the respective reactions. Thus it is not
advantageous economically and environmentally. Moreover,
the hydrolysis step requires an excess amount of alkali base
resulting in a large amount of a salt solution contaminated
with organic compounds that has to be treated. However,
benzyl alcohol can also be produced by oxyacetoxylation
from toluene, acetic acid, and oxygen to benzyl acetate
followed by hydrolysis of the ester. As ester alcoholysis can
be accomplished using heterogeneous acids and bases and
alcohol as an acyl acceptor it is more favored to avoid by-
product salts formation as in aqueous hydrolysis [14, 15].
Magnesia is commonly used solid base catalyst. It is well
known that the basic sites on magnesia surface adsorb oxy-
gen and moister from air that affect catalytic performance.
Hence, the effect of pretreatment temperature and time on
magnesia performance in the transesterfication of benzyl
acetate was first examined (Fig. 3). As illustrated in Fig. (3),
both pretreatment time and temperature affect the product
yield, and increase in oven temperature decreases the re-
quired time to release the adsorbed molecules and activate
the basic sites on the surface, thereby increasing the product
yield. Therefore, based on energy efficiency measurements,
the catalyst was preheated at 300 °C for 2 h for all subse-
quent experiments of the study.
As previously mentioned, esters can be alcoholized
through transesterification using alcohol, which can simulta-
neously be used as a solvent and as an acyl acceptor, using
various homogeneous and heterogeneous catalysts [3-5].
Hence, the investigation began by testing the progress of
benzyl alcohol yield with time in glycerol in several tem-
peratures (Fig. 2). In general, as expected, increasing the
reaction time or temperature in the range of 1-5 h and 45-100
°C respectively, increased the product yield. Yet, it can be
seen from the results in Fig. (2) that increasing the tempera-
ture behind 75 °C did not significantly change the product
yield. At the end of the reaction, benzyl acetate was easily
separated from the reaction mixture by simple extraction
with diethyl ether. As glycerol was used as both a solvent
The progress of benzyl alcohol yield with time in glyc-
erol in several temperatures was also studied over magnesia
(Fig. 4). As previously detected with NaOH, increasing ei-
ther the reaction temperature or the reaction time increased
the yield of benzyl alcohol, and again, at temperature above
75 °C the effect of temperature was negligible after 2 h. As
the active sites of the two representatives’ homogeneous and
heterogeneous catalysts are different by strength and amount
the comparison of the performance of the two catalysts was
done on equal mass basis. It was found that the soluble base
is more active than the solid base and using NaOH yielded
37% yield after 1 h at 55 °C while employing MgO required
100
90
80
70
60
50
40
30
20
10
0
0
1
2
3
4
5
6
Time (h)
Fig. (2). Effect of reaction temperature on the reaction progress with time of benzyl acetate in glycerol using NaOH as catalyst. Reaction
conditions: 0.1 g benzyl ester, 0.01 g NaOH, 5 g glycerol. (ꢀ) 45°C; (ꢁ) 55°C; (ꢂ) 75°C; (x) 100°C.

506 Letters in Organic Chemistry, 2011, Vol. 8, No. 7
Wolfson et al.
38
36
34
32
30
28
26
24
22
20
0
0.5
1
1.5
2
2.5
3
3.5
4
Activation time (h)
Fig. (3). Effect of MgO activation conditions on the yield of benzyl acetate in glycerol. Reaction conditions: 0.1 g benzyl ester, 0.01 g MgO,
5 g glycerol. (ꢀ) 80°C; (ꢁ) 300°C; (ꢂ) 450°C.
80
70
60
50
40
30
20
10
0
0
1
2
3
4
5
6
Time (h)
Fig. (4). Effect of reaction temperature on the reaction progress with time of benzyl acetate in glycerol using MgO as catalyst. Reaction con-
ditions: 0.1 g benzyl ester, 0.01 g MgO, 5 glycerol. (ꢀ) 45°C; (ꢁ) 55°C; (ꢂ) 75°C; (x) 100°C.
75 °C to reach 33 % yield after an hour. Yet, though the per-
formance of the soluble base was higher than this of the solid
one, solid catalyst is advantageous from industrial point of
view, as it can be easily separated from the reaction mixture
by filtration and recycled.
second catalytic cycle by extraction of the product from the
glycerol-catalyst mixture without the filtration of the cata-
lyst, to which was added fresh benzyl acetate (Table 1).
Thus, to test the viability of MgO reuse, the first reaction
cycle was run for 1h at 75 °C after which the catalyst was
filtrated and the product and the residual substrate were fully
extracted by diethyl ether (Table 4, entry 1). After the first
reaction cycle, the catalyst was filtrated and added to a fresh
mixture of benzyl acetate in glycerol and the reaction was
run again under similar conditions. Two cycles of catalyst
reuse were done by this method resulting in a small decrease
of yield in each cycle (Entries 2 and 3), probably due to the
catalyst lost during recycling procedure. Since glycerol was
used both as a solvent and an acyl acceptor, recycling of the
glycerol together with the heterogeneous catalyst was also
Increasing the substrate to catalyst ratio (S/C), by either
increasing benzyl acetate amount or decreasing MgO
loading, increased the initial reaction rate, which was
calculated by the amount of reacted substrate per g of
catalyst divided by reaction time (Fig. 5).
The recycling of magnesia was also tested using two dif-
ferent methods: running the first reaction cycle and filtration
of the catalyst that was subsequently employed in second
cycle with fresh benzyl acetate and glycerol, or running the

Base Catalyzed Glycerolysis of Benzyl Acetate
Letters in Organic Chemistry, 2011, Vol. 8, No. 7
507
40
35
30
25
20
15
10
5
0
0
20
40
60
80
100
120
S/C (g/g)
Fig. (5). Effect of S/C on initial reaction rate in the transesterification of benzyl acetate in glycerol using MgO as catalyst. Reaction condi-
tions: 0.1 g benzyl ester, 0.01 g MgO, 5 g glycerol, 75°C, 1h.
tested by extraction of the product and the residual substrate
from the glycerol-MgO mixture with diethyl ether followed
by the addition of fresh benzyl acetate. This recycling proce-
dure yielded similar product yield as the first reaction cycle
(Table 1, entries 4 and 5), supporting our previous assump-
tion that the loss of catalyst during filtration was the reason
for the decrease in the product yield in the former recycling
procedure.
for microwave-promoted organic synthesis under atmos-
pheric pressure [10].
4. CONCLUSIONS
Glycerol can be successfully used as a green solvent and
an acyl acceptor in the transesterification of benzyl ester to
benzyl alcohol by using soluble and solid base. Using glyc-
erol in the reaction facilitated efficient separation of the
product from the catalyst and recycling of the catalyst, and it
allowed the conventional heating method to be replaced with
the more efficient microwave-promoted heating. The yield of
benzyl alcohol was increased by increasing the reaction time
and the temperature while maintaining the loading of the
catalyst. Heating the reaction in an unmodified home micro-
wave instead of using conventional heating resulted in higher
reaction rate.
Table 1. Recycling of MgO in the Glycerolysis of Benzyl
Acetatea
Entry
Procedure
Conversion
1
2
3
4
5
First cycle
33
27
25
32
32
Second cycle of the catalyst, fresh glycerol
Third cycle of the catalyst, fresh glycerol
Second cycle of the catalyst and glycerolb
Third cycle of the catalyst and glycerolb
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ous separation of the catalyst.
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