Continuous synthesis of glycerol acetates in supercritical carbon dioxide using Amberlyst 15

Article abstract of DOI:10.1039/b815674c

Continuous esterification of glycerol with acetic acid was investigated in supercritical carbon dioxide (scCO₂) using Amberlyst 15 as a heterogeneous catalyst. The effect of pressure at (65-300) bar on the substrate conversion and the reaction yield and selectivity was studied. With increasing pressure, the percent of total yield and conversion remain almost unaffected and the selectivity of monoacetin synthesis increases while the selectivity for triacetin stays relatively unchanged. The effect of temperature on the yield, conversion, and the selectivity at (100-150) °C was also investigated. With increasing temperature from 100 to 140 °C, the selectivity for monoacetin decreases while for tri- and diacetin slightly increases. In contrast, with further increase in temperature, from 140 °C to 150 °C, the selectivity of monoacetin synthesis increases while that of diacetin decreases. By increasing the molar ratio of acetic acid to glycerol to 24, a selectivity of 100% was achieved for 2 h while the yield was 41% for the continuous triacetin synthesis in scCO₂. When neat scCO₂ as solvent with no catalyst was used, only monoacetin with 29% conversion was synthesized. The catalyst durability was also studied by monitoring the reaction for 25 h. The results show that the catalyst retains its activity even for 25 h but the selectivity for triacetin synthesis declines from 100% to about 60%.

Full text of DOI:10.1039/b815674c

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Continuous synthesis of glycerol acetates in supercritical carbon dioxide
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using Amberlyst 15ꢀ
Marzieh Rezayat and Hassan S. Ghaziaskar*
Received 9th September 2008, Accepted 14th January 2009
First published as an Advance Article on the web 18th February 2009
DOI: 10.1039/b815674c
Continuous esterification of glycerol with acetic acid was investigated in supercritical carbon
R
dioxide (scCO2) using Amberlyst 15ꢀ as a heterogeneous catalyst. The effect of pressure at
(65–300) bar on the substrate conversion and the reaction yield and selectivity was studied. With
increasing pressure, the percent of total yield and conversion remain almost unaffected and the
selectivity of monoacetin synthesis increases while the selectivity for triacetin stays relatively
unchanged. The effect of temperature on the yield, conversion, and the selectivity at (100–150) ◦C
was also investigated. With increasing temperature from 100 to 140 ◦C, the selectivity for
monoacetin decreases while for tri- and diacetin slightly increases. In contrast, with further
increase in temperature, from 140 ◦C to 150 ◦C, the selectivity of monoacetin synthesis increases
while that of diacetin decreases. By increasing the molar ratio of acetic acid to glycerol to 24, a
selectivity of 100% was achieved for 2 h while the yield was 41% for the continuous triacetin
synthesis in scCO2. When neat scCO2 as solvent with no catalyst was used, only monoacetin with
29% conversion was synthesized. The catalyst durability was also studied by monitoring the
reaction for 25 h. The results show that the catalyst retains its activity even for 25 h but the
selectivity for triacetin synthesis declines from 100% to about 60%.
glycerol is the main by-product.12,13 It is reported that 1 kg of
Introduction
crude glycerol is formed for every 9 kg of bio-diesel produced.14
Esters have a wide variety of applications as solvents, as
It is therefore a reasonable scientific and technological task to
emulsifying and stabilizing compounds, and as raw materials in
find new or improved methods for the conversion of this huge
food, and the cosmetic and pharmaceutical industries.1 Acetins
are mono-, di-, and tri-esters of glycerol acetates. Monoacetin or
glycerol monoacetate are used in the manufacture of explosives,
pile of relatively low price glycerol to valuable products. For
example, Suppes et al. have reported dehydration of glycerol
via catalytic reactive distillation to produce acetol.14 Also the
in tanning, and as solvents for dyes. Diacetin or glycerol diacetate
synthesis of acrolein from dehydration of glycerol in sub- and
is used as a plasticizer and softening agent and solvent.2 Glycerol
supercritical water has been reported.15,16 Indeed, some articles
triacetate or triacetin is used as a solvent for dissolving or
have focused on the esterification of glycerol with fatty acids
diluting drugs and organic compounds. Also it is used as
using a catalyst, especially, heterogeneous catalyst to get the
an antimicrobial and emulsifying agent in cigarette filters.3,4
desired esters.17–19 Mota et al. have done the esterification of
Triacetin is used especially in the pharmaceutical industries as
glycerol with acetic acid in the presence of different solid acid
a skin pH controller for treatment of skin disorders and some
catalysts in a free solvent medium under reflux.20 According
pathological diseases and is used as a drug delivery compound.5,6
Glycerol acetates have been synthesized via esterification of
glycerol with acetic acid or acetic anhydride with or without a
to their report, the esterification achieved 97% conversion at
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30 min of reaction time in the presence of Amberlyst 15ꢀ with
selectivity for mono-, di-, and triactin of 31%, 54% and 13%,
homogeneous or heterogeneous catalyst using an organic solvent
respectively.
and in batch or continuous processes.7–9 Usually, the produced
Other researchers have resorted to supercritical carbon diox-
esters are accompanied with some by-products, which a change
ide (scCO2) as a solvent to replace organic ones. Esterification
in their color and odor so that it makes their purification difficult
of glycerol with lauric acid over a heterogeneous catalyst has
and costly. As a result, selective synthesis of the esters with high
excellent conversion in scCO2 compared to the use of mesitylene
purity has been a great challenge for some researchers.10,11
as solvent.1 Sugi et al. have explained that it is mainly due
In recent years, considerable attention has been paid to
to the decrease in coke formation and removal of the water
replacing fossil fuels with bio-diesel, because of its presumed en-
formed from the catalyst acidic sites by scCO2. Because scCO2
vironmental and economic benefits compared to petroleum. Bio-
is non-flammable, inexpensive, and environmentally safe and it
diesel is produced by transesterification of vegetable oil where
◦
has moderate critical conditions (tc = 31.2 C, pc = 73.8 bar),
scCO2 is a preferred solvent.21
There are only a few published papers involving continuous
acid-catalyst reactions in scCO2. Most researchers have used this
system when one or more of the substrates are gaseous, such as in
Department of Chemistry, Isfahan University of Technology, Isfahan,
84156-83111, I. R. Iran. E-mail: ghazi@cc.iut.ac.ir;
Fax: +98-311-3913260
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hydroformylation,22 hydrogenation,23,24,25 and partial oxidation
of alcohols.26 The main reason is the greater miscibility of gases
in scCO2 leading to higher reaction rates.
of our experiments was a 10 mm (i.d.), 316-stainless steel tubing
with an internal volume of about 19 mL, containing about 9.5 g
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of dried catalyst (Amberlyst 15ꢀ), placed in an air oven with a
However, investigations have shown that tuning of the pres-
sure of the reactions, when none of the substrates are gases, could
change the selectivity and the yield to a high extent. For example,
temperature controller ( 1 ◦C) throughout the experiment.
To be sure that the catalyst is completely dried, a flow of scCO2
◦
at 100 C was passed over the catalyst for about 1 h.8 When
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etherification of n-terminal diols in scCO2, over Amberlyst 15ꢀ
the system was set at the desired pressure and temperature, a
HPLC pump (PU-980), continuously and at a constant flow rate,
pumped a homogenized mixture of liquid glycerol and acetic
acid, premixed in the desired molar ratio. Meanwhile another
HPLC pump introduced liquid CO2 into the system at a set flow
rate. The two flows met each other in a 1/8 inch (o.d.) tubular
mixer, 1.8 m long, filled with glass beads (mesh of 20–40) and
then the mixture passed over the catalyst bed. Each sample was
collected in a cold trap, at different time intervals and analyzed
by GC-FID.
as a solid catalyst, could switch dramatically between mono-
and bis-ethers, while the pressure was increased from 50 bar to
200 bar.27 Also, Han et al. reported esterification of ethylene
glycol with propionic acid in scCO2 in the presence of p-toluene
sulfonic acid as a catalyst in a batch mode. They showed that
with increasing pressure, the yield and selectivity of the diester
increased, while those of the monoester decreased. They claimed
that the distribution of reactants and products between the vapor
and liquid phase at high pressure is the main reason.28
In this work, we have investigated the synthesis of glycerol
esters in scCO2 by a continuous esterification of glycerol with
Analytical method
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acetic acid in the presence of Amberlyst 15ꢀ as a strong solid-
Analysis of the samples was carried out using a GC-FID (Agilent
Technologies model 6890 N). The carrier gas was helium and
the capillary column of HP-5 (with 30 m length, 0.25 mm i.d.,
and 0.25 mm of film thickness) was used.
acid catalyst, as shown in Scheme 1. It should be noted that
the boiling points of mono-, di-, and triacetin are close to each
other. Specially, the boiling points of di- and triacetin are very
similar, making their separation by conventional processes very
difficult and costly.
The temperature program was used for the analysis. The GC
injection port and the detector temperature were set at 240 and
◦
260 C, respectively. The initial column temperature was set at
70 ◦C for 2 min and programmed from 70 ◦C t◦o 150 ◦C for
◦
◦
1.5 min at the r◦ate of 45 C min-1 and from 150 C to 180 C
at the rate of 8 C min-1 and from 180 to 240 ◦C at the rate of
35 ◦C min-1. The quantification was performed by injecting some
standard ester solutions containing an internal standard and
integrating their peak areas to establish the calibration curve.
Since we could not find commercial monoacetin, it was synthe-
sized via a previously reported method.30 The identification of
the products in all experiments was carried out by GC-MS (Trio
1000, Fisons Instruments, model 8060).
Scheme 1 Synthesis of glycerol esters in three reversible steps.
The yield, conversion, and selectivity for each sample are
calculated as follows:
Experimental
Total moles of detected esters
Yield =
¥ 100
As shown in Fig. 1 a continuous flow apparatus was used to
carry out the esterification reaction. The details of the system
are described elsewhere.29 The catalytic bed reactor. used in most
Total moles of glycerol in the feed solution
Total moles of detected esters
Conversion =
¥ 100
Moles of detected esters and glycerol in exit flow
Moles of each ester
Selectivity =
¥ 100
Total moles of detected esters in exit flow
Materials
Carbon dioxide with a purity of 99.95% was supplied from
ZamZam Co. Ltd (Isfahan, Iran). Acetic acid (purity > 99%)
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and Amberlyst 15ꢀ was purchased from Merck. The catalyst
capacity26 was calculated as 4.64 meq g-1. Triacetin (glycerol tri-
acetate, purity > 99%) and diacetin (glycerol diacetate, purity =
50% verified by GC analysis) were purchased from Fluka.
Absolute ethanol was purchased from Temad Co. (purity > 99%,
Fig. 1 The schematic diagram of the setup used for the esterification
reaction; 1, molecular sieve trap; 2, liquid pump; 3, check valve; 4, needle
valve; 5, air oven; 6, preheating coil; 7, static mixer; 8, T connector; 9,
catalytic bed reactor; 10, back pressure regulator.
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Tehran, Iran). Glycerol was purchased from Hopkin & Williams;
1-hexanol was purchased from Riedel-deHae¨n (purity > 98%).
Results and discussion
The esterification reaction of glycerol with acetic acid proceeds
in three consecutive reversible steps as shown in Scheme 1.
The effect of pressure, temperature, and molar ratio of acetic
acid to glycerol has been investigated for the reaction over
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Amberlyst 15ꢀ as a heterogeneous strong acid-catalyst in scCO2.
In addition, we studied the catalyst reusability and the reactor
length at two different molar ratio of substrates. The reaction was
also performed in scCO2 in the absence of the catalyst and only
the reactor was packed with crushed glass with almost the same
grain size as the catalyst. The conversion, yield, and selectivity
of the reactions have been reported. All data reported in Fig. 5,
6, and 7 are the equilibrium data, taken after the continuous
system has reached a steady state that is about 120 min after the
start of the reaction.
Fig. 3 Variation of the diacetin selectivity in continuous esterification
of glycerol with acetic acid versus time using Amberlyst 15ꢀR (9.5 g) as
catalyst at a temperature of 100 ◦C and acid/glycerol ratio of three at
different pressures.
Effect of pressure
The effect of pressure on the esterification of glycerol with
acetic acid and the formation of tri-, di-, and monoacetin in a
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continuous flow reactor at (65–300) bar using Amberlyst 15ꢀ is
shown in Fig. 2–4 at different time from the start of the reaction.
All the experiments were carried out at 100 ◦C. The molar ratio
of acetic acid to glycerol was equal to 3 and the flow rate of liquid
CO2 (at 0 ◦C) and substrates was 1.2 mL min-1 and 0.2 mL min-1,
respectively.
Fig. 4 Variation of the monoacetin selectivity in continuous esterifica-
tion of glycerol with acetic acid versus time using Amberlyst 15ꢀR (9.5 g)
as catalyst at a temperature of 100 ◦C and acid/glycerol ratio of three at
different pressures.
products in scCO2 at high pressures drives the esterification
towards the selective synthesis of triacetin. In addition, dryness
of the catalyst at the beginning of the reaction helps the
forward reaction proceed to completion and triacetin is formed
selectively. As time passes, since the synthesis of one mole
triacetin is associated with three moles of water, the catalyst
absorbs water and as a result, the reverse reaction to monoacetin
lowers the selectivity.
The data reported in Fig. 5 is the equilibrium data shown
in Fig. 2–4 and taken after the continuous system has reached
to the equilibrium state where the selectivity does not change
versus time. It seems the esterification reaction was found to
be relatively unaffected by changes in the system pressure. As
shown in Fig. 5 the total yield was not seen to be very sensitive
to change in the system pressure. Moreover, the conversion was
about 92% at different pressures. With increasing the pressure,
a slight increase in monoacetin formation and a slight decrease
in the diacetin formation are observed. Increasing the system
pressure increases the scCO2 density and as a result, the solubility
of the monoacetin (i.e. more polar) in scCO2 increases. The
produced monoacetin at this condition could be extracted by
scCO2 from the catalyst bed where the esterification takes place.
Fig. 2 Variation of the triacetin selectivity in the continuous esterifica-
tion of glycerol with acetic acid versus time using Amberlyst 15ꢀR (9.5 g)
as catalyst at a temperature of 100 ◦C and acid/glycerol ratio of three at
different pressures.
These data show that the effect of pressure on the selectivity
of tri- and monoacetin is more significant at the beginning of
the reaction, where the selectivity of diacetin has less variation in
time at different pressures. The highest variation takes place in
the first 60 min. In addition, with increasing the system pressure,
a longer time is needed for the esterification reaction to reach
equilibrium. Due to higher solubility of the products at the
higher pressures, scCO2 can extract products from the catalyst
bed and postpone the equilibration time.28 This phenomena
also affects the selectivity of produced esters at different times
before the equilibrium is established. The greater solubility of
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The change in selectivity could be related to the hydrolysis
of diacetin to monoacetin as a decrease in the solubility of the
produced esters in the presence of the water formed in scCO2
when the system temperature is increased.
Effect of molar ratios of the substrates
In order to investigate the effect of molar ratio of acetic acid to
glycerol on the yield and selectivity of the produced esters, the
molar ratio was varied from 1.5 to 24 while other variables were
kept constant (200 bar and 110 ◦C). The results are shown in
Fig. 7.
Fig. 5 The effect of increasing pressure on the conversion, yield, and
product selectivity in the continuous esterification of glycerol with acetic
acid at a temperature of 100 ◦C using Amberlyst 15ꢀR (9.5 g) as catalyst.
Effect of temperature
The effect of temperature on the esterification of glycerol with
acetic acid in the range of (100–150) ◦C at 200 bar, with a molar
ratio of acetic acid to glycerol of 3 and a flow rate of liquid CO2
◦
(at 0 C) and substrates of 1.2 mL min-1 and 0.2 mL min-1,
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respectively, in the presence of Amberlyst 15ꢀ, is shown in
Fig. 6. The conversion was nearly constant at 91%. But the total
yield of the esterification decreased continually with increasing
temperature as shown in Fig. 6.
Fig. 7 The effect of increasing acetic acid to glycerol molar ratio on
the conversion, yield and product selectivity in continuous esterification
reaction using Amberlyst 15ꢀR (9.5 g) at 110◦C and 200 bar.
Except for the molar ratio of 1.5, the conversion of esterifica-
tion reaction was 100 percent in all molar ratios studied. On the
other hand, the total yield appears to have a maximum at molar
ratio of 4.5 and a further increase in acetic acid concentration is
accompanied by a decrease in the total reaction yield. The rise in
total yield observed when the molar ratio is increased from 1.5
to 4.5 is thought to be related to the role of acetic acid, not only
as an excess reactant, forcing the esterification reaction forward,
but as a co-solvent in scCO2. With further increase of the molar
ratio, the selectivity for triacetin increases while it significantly
decreases for monoacetin and diacetin. Fig. 6 shows that at
the molar ratio of 24 the yield reaches a low level. However, the
higher acetic acid concentration drives the esterification reaction
towards triacetin production with 100% selectivity up to 120 min
after the start of the reaction.
Fig. 6 The effect of increasing temperature on the conversion, yield,
and products selectivity in the continuous esterification of glycerol with
acetic acid using Amberlyst 15ꢀR (9.5 g) as catalyst at a pressure of
200 bar.
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More information about the Amberlyst 15ꢀ performance in
The decrease in the total yield with raising the system
temperature might be due to the reduction of the catalyst
activity that is happening via lose of the catalyst active sites
at high temperatures by desulfonation of the catalyst.31 It has
this reaction could be found by comparing the yield with the
conversion data in all the experiments. The difference between
these data could be related to the amount of the fed glycerol
that is not taking part in the reaction. It seems that there is
an interaction between glycerol and the catalyst active sites
causing a decrease in the number of free glycerol molecules
for the reaction with the protonated acetic acid over the
catalyst surface.18 Since the catalyst capacity for the adsorption
of glycerol and the amount of the catalyst used in all the
experiments are more or less the same, the yield is decreased
while the molar ratio of the substrates increased.
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been reported that the Amberlyst 15ꢀ activity at 200 bar and
150 ◦C in an alkylation reaction in scCO2, was about 10% lower
compared to the untreated one. Morover, at high temperatures
scCO2 density is reduced and as a result the amount of water that
could be extracted out of the catalyst bed is diminished. Since
the process is reversible and coproduced water would affect the
equilibrium the yield of the reaction is lowered at the higher
temperatures.
The influence of the acetic acid concentration on the selectivity
of the esters formed shows the esterification equilibrium of
At temperatures above 140 ◦C, the selectivity for monoacetin
formation increases while that for diacetin formation decreases
and remains nearly constant for triacetin.
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the glycerol with acetic acid using Amberlyst 15ꢀ in scCO2
could be progressed in three consecutive reversible reactions.
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Table 1 Influence of the reactor length on the conversion, yield, and
Table 2 Influence of the catalyst and scCO2 alone on the yield,
the product selectivity of the esterification of glycerol with acetic acid in
conversion, and product selectivity of the esterification of glycerol with
scCO2 using Amberlyst 15ꢀR
acetic acid at scCO2
a
a
Reactor
Condition Conversion (%) Yield (%) TA (%) DA (%) MA (%)
length (cm) Conversion (%) Yield (%) TA (%) DA (%) MA (%)
1b
2c
35
100
29
41
0
100
0
0
100
0
25
100
100
41
48
100
82
0
19
0
0
100
a At 200 bar, 110 ◦C; the flow rate of scCO2 and substrates was 1.2
and 0.2 mL min-1, respectively; the acid/glycerol ratio was 24. b The
catalyst reactor was loaded with crushed glass or finely crushed ceramic
Raschig rings (16/20 mesh). c The catalyst reactor was loaded with dried
Amberlyst 15ꢀR .
a At 200 bar, 110 ◦C; the flow rate of scCO2 and reagents was 1.2 and
0.2 mL min-1, respectively; the acid/glycerol ratio was 24.
Therefore, the greater acetic acid concentration as a substrate
could promote diacetin to triacetin conversion. Further studies
are going on in our research group to increase the yield of
the esterification reaction while keeping the high selectivity of
triacetin synthesis at 100% for a longer time.
Table 3 Influence of the catalyst reusability on the yield, conversion,
and product selectivity of the esterification of glycerol with acetic acid
a
in scCO2
Times catalyst
recycled
Conversion (%) Yield (%) TA (%) DA (%) MA (%)
Effect of the reactor length
One of the ways considered to maximize the yield was to increase
substrate–catalyst contact time. Table 1 presents the effect of
increasing the length of the reactor tube on the yield and the
selectivity of products while maintaining the same amount of
the catalyst, the volume of the reactor, and the flow rate of
scCO2. At a four-fold reactor length, the selectivity of triacetin
synthesis was unexpectedly lower while the total yield increased
17% as illustrated in Table 1. It seems that difficulty in removal of
water formed in the esterification reaction when a longer reactor
is used enhances the reverse hydrolysis reaction of triacetin to
diacetin.
3b
3c
100
100
82
49
27
92
42
8
31
0
a At 200 bar, 110 ◦C; the flow rate of scCO2 and reagents was 1.2
and 0.2 mL min-1, respectively. b The acid/glycerol ratio was 6.0 c The
acid/glycerol ratio was 24.
We also evaluated the reusability of the catalyst (Amberlyst
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15ꢀ). Since the substrates and products have a good solubility
in absolute ethanol, this solvent was used for removal of the
unreacted substrates from the catalyst surface using a Soxhlet
extraction apparatus. Analysis of the extract by GC-FID and
FT-IR spectroscopy confirmed that nearly all of the materials
desorbed from the catalyst surface were glycerol. The washed
catalyst was subsequently re-used in its dried form to investigate
the reusability of the catalyst for the esterification reaction.
Table 3 shows that the recycled catalyst, after three times of
usage, has clearly retained its activity in this reaction. However,
the selectivity of triacetin synthesis for the molar ratio of 24 was
decreased to 91.6%.
Finally as shown in Fig. 8, the catalyst stability was in-
vestigated by following the reaction at the optimum reaction
condition for 25 h. The results revealed that the catalyst was
still active over a long period. Nevertheless, after about 5 hours
the system reaches a steady state in which the selectivity for
triacetin declines to about 60% while that of diacetin rises to
about 40% with no monoacetin among the products. Even at
Effect of the catalyst and its reusability
Finally, the esterification of glycerol with acetic acid at 200 bar
and 110 ◦C was performed in scCO2 as solvent with and without
the catalyst when the molar ratio of the acid to the alcohol
was 24. The results are shown in Table 2. When scCO2 is used
as solvent in the absence of the catalyst, the total yield was
29%, but the selectivity of monoacetin synthesis was 100%. In
fact, CO2 can function in this reaction, in addition to all the
factors already mentioned, as a Lewis acid, even a Brønsted
acid in high pressures and in the presence of water to catalyze the
reaction.32 Alternatively, the reaction under the same conditions
in the presence of the catalyst, leads to 100% selective synthesis
of triacetin. This confirmed the importance of the catalyst in
this esterification reaction.
Fig. 8 Time variation in percent yield and selectivity of tri- and diacetin over a period of 25 h in continuous esterification of glycerol with acetic
acid over Amberlyst 15ꢀR in scCO2, at a pressure of 200 bar, a temperature of 110 ◦C, and acid/glycerol ratio of 24.
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this stage the selectivity for triacetin in the reaction performed
in scCO2 is about 4.6 times the reaction under reflux using the
same catalyst.33
4 CN Pat., 1 194 997, 1998.
5 US Pub., 2004/0091506A1, 2004.
6 US Pat., 5 601 839, 1997.
7 J. B. Senderens, Chem. Abs., 1914, 8, 17522.
8 US Pat., 4 381 407, 1983; G. Janzso, L. Cegledi, L. Manya and J.
Kocsis, Chem. Abs., 1992, 116, 173576j.
9 US Pat. 6 586 609 B2, 2003; JP Pat. 8 027 061, 1996; B. Ding and
Z.-guo Wang, Chem. Abs., 2003, 138, 273232s.
10 US Pat., 5 434 280, 1995.
11 F. H. Mattson and R. A. Volpenhein, J. lipid Research, 1962, 3, 281–
296.
12 H. E. Hoydonckx, D. E. De Vos, S. A. Chavan and P. A. Jacobs,
Topics in Catalyst, 2004, 27, 83–96.
13 A. Demirbas, Energy Convers. Manage., 2006, 47, 2271–2282.
14 C. W. Chiu, M. A. Dasari, G. J. Suppes and W. R. Sutterlin, AIChE,
2006, 52, 3543–3547.
15 M. Watanabe, T. Iida, Y. Aizawa, T. M. Aida and H. Inomata,
Bioresour. Technol., 2007, 98, 1285–1290.
16 Y. Pouilloux, S. Me´tayer and J. Barrault, C. R. Acad. Sci. Paris, Se´rie
IIc, Chimie/Chem., 2000, 3, 589–594.
17 I. D´ıaz, F. Mohino, T. Blasco, E. Sastre and J. Pe´rez-Pariente,
Micropor. Mesopor. Mater, 2005, 80, 33–42.
18 Y. Pouilloux, S. Abro, C. Vanhove and J. Barrault, J. Mol. Catal. A-
Chem., 1999, 149, 243–254.
19 I. D´ıaz, F. Mohino, J. Pe´rez-Pariente and E. Sastre, Appl. Catal. A:
Gen., 2003, 242, 161–169.
20 V. L. C. Gonc¸alves, B. P. Pinto, J. C. Silva and C. J. A. Mota, Catal.
Today, 2008, 133–135, 673–677.
21 R. S. Oakes, A. A. Clifford and C. M. Rayner, J. Chem. Soc. Perkin.
Trans., 2001, 1, 917–941.
22 O. Hemminger, A. Marteel, M. R. Mason, J. A. Davies, A. R. Tadd
and M. A. Abraham, Green Chem., 2002, 4, 507–512.
23 M. G. Hitzler, F. R. Smail, S. K. Ross and M. Poliakoff, Org. Process
Res. Dev., 1998, 2, 137–146.
24 P. Stephenson, P. Licence, S. K. Ross and M. Poliakoff, Green Chem.,
2004, 6, 521–523.
Conclusion
The results show that the use of scCO2, the molar ratio of the
substrates, and the catalyst play a major role in tailoring the yield
and selectivity in the esterification reaction of glycerol with acetic
acid. However, the pressure and temperature in the synthesis
of the acetins do not have a significant contribution. The
continuous flow esterification of glycerol with acetic acid using
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Amberlyst 15ꢀ while the molar ratio, pressure, and temperature
are 24, 200 bar and 110 ◦C, respectively, has shown the real
advantage of 100% selectivity for triacetin synthesis for about
120 min. However, the selectivity of triacetin was diminished
when the reaction is performed for a longer time. Moreover, the
results of the catalyst reusability show that, the adsorption of
glycerol over the catalyst was due to a physical interaction.
This work has demonstrated that the esterification could be
performed without the catalyst at the molar ratio, pressure, and
temperature of 24, 200 bar, and 110 ◦C, respectively, with 100%
selectivity of the reaction towards monoacetin synthesis.
In short, the controlling of the yield and selectivity of the
produced esters in the continuous esterification reaction of
glycerol with acetic acid using Amberlyst 15ꢀ under scCO2 was
affected by Lewis acidity of CO2, removal of the formed water
from the catalyst surface, the amount of the product solubility
in scCO2, and the adsorption of glycerol by the active sites of
the catalyst.
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25 J. R. Hyde, P. Licence, D. Carter and M. Poliakoff, Appl. Catal. A
Gen., 2001, 222, 119–131.
26 G. Jenzer, D. Sueur, T. Mallat and A. Baiker, Chem. Commun., 2000,
22, 2247–2248.
27 P. Licence, W. K. Gary, M. Sokolva and M. Poliakoff, J. Am. Chem.
Soc., 2005, 127, 293–298.
28 J. Zhang, T. Jiang, B. Han, A. Zhu, X. Ma, Y. Zhou, L. Bai, H. Xiang
and Y. Li, J. Phys. Chem., 2007, 111, 5322–5325.
29 H. S. Ghaziaskar, A. Daneshfar and L. Calvo, Green Chem., 2006,
8, 576–581.
Acknowledgements
This work has been supported by IUT. The comments and
critical review of Professor M. Yalpani is greatly acknowledged.
30 C. C. Yu, Y. S. Lee, B. S. Cheon and S. H. Lee, Bull. Korean Chem.
Soc., 2003, 24, 1229–1231.
31 R. Amandi, P. Licence, S. K. Ross, O. Altonen and M. Poliakoff, Org.
Process Res. Dev., 2005, 9, 451–456.
32 E. J. Beckman, Chem. Commun., 2004, 1885–1888.
33 V. L. C. GonC¸ alves, B. P. Pinto, J. C. Silva and C. J. A. Mota, Catalysis
Today, 2008, 133–135, 673–677.
Notes and references
1 A. Sakthivel, R. Nakamura, K. Kamura and Y. Sugi, J. Supercrit.
Fluids, 2007, 42, 219–225.
2 J. C. Kern, Encyclopedia of Chemical Technology, 3rd ed., 1978,
Vol. 11, p. 931.
3 US Pat., 5 777 157, 1998.
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