Esterification of stearic acid by isomeric forms of butanol in a microwave oven under homogeneous and heterogeneous reaction conditions

Article abstract of DOI:10.1039/a702219k

In this paper the influence of conventional and dielectric heating on the esterification of stearic acid by butanol has been studied. The influences of different isomeric forms of butanol, a catalyst and conditions of reaction (homogeneous versus heterogeneous) upon esterification have been investigated. The difference between conventionally or dielectrically heated reactions has been studied. For heterogeneous reactions a significant temperature enhancement effect is observed. This effect can be enlarged by the application of heat captors, which have been mixed or impregnated with catalyst. Further rate enhancements are realised by the use of a combination of microwave heating and ultrasound for heterogeneously catalysed reactions.

Full text of DOI:10.1039/a702219k

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Esterification of stearic acid by isomeric forms of butanol in a
microwave oven under homogeneous and heterogeneous reaction
conditions
Farid Chemat,a Martine Pouxa and Saskia A. Galema*,b
a Ecole Nationale Superieure d’Ingenieurs de Genie Chimique de Toulouse,
18, Chemin de la Loge, 31078 Toulouse, CEDEX 4, France
b Unilever Research Laboratory, Olivier van Noortlaan 120, 3133 AT Vlaardingen,
The Netherlands
In this paper the influence of conventional and dielectric heating on the esterification of stearic acid by
butanol has been studied. The influences of different isomeric forms of butanol, a catalyst and conditions
of reaction (homogeneous versus heterogeneous) upon esterification have been investigated. The
difference between conventionally or dielectrically heated reactions has been studied. For heterogeneous
reactions a significant temperature enhancement effect is observed. This effect can be enlarged by the
application of heat captors, which have been mixed or impregnated with catalyst. Further rate
enhancements are realised by the use of a combination of microwave heating and ultrasound for
heterogeneously catalysed reactions.
OH
Introduction
In the past ten years since the publication of Gedye et al.1 there
O
has been considerable interest in the application of microwave
heating upon chemical reactions. After much debate2 there
OH
seems to be general agreement that in most cases microwave
[catalyst]
heating can only give rise to different temperature regimes
which can be used in a profitable way, for instance when reac-
tions need to be rapid as in the case of the synthesis of radio-
pharmaceuticals3 or when high temperatures need to be
O
reached for the preparation of some inorganic compounds.4 In
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Scheme 1 The esterification of stearic acid with n-butanol
addition, microwave heating is ideal for solvent-free reaction
systems: so called ‘dry reactions’.5 Temperature effects are also
the origin of the fact that a power input change can cause a
difference in selectivity.2a,6 In this report, rates of esterification
of stearic acid during microwave heating under homogeneous
and heterogeneous conditions are determined. Specific focus is
on whether claimed temperature enhancement effects can be
used advantageously for improving yields of reactions or for the
acceleration of reactions. Butyl esters of stearic acid are com-
pounds which can be applied as a solvent, spreading or soften-
ing agent in polymers (plastics). In addition, they are used in
the textile, cosmetic and rubber industries.7
Effect of catalyst
In the case of the uncatalysed reaction, a second-order depend-
ence of the rate of esterification on the acid concentration was
observed, due to autocatalysis of the esterification by the acid.
When the concentration of alcohol is in tenfold excess of the
concentration of acid, the rate for the uncatalysed reaction is
given by eqn. (1). kobs,uncat can be measured and would be a
v = kuncat [alcohol][acid]2 = kobs,uncat[acid]2
(1)
pseudo-second-order rate constant, because the acid is not
only a reactant but is also an autocatalyst.
The rate of reaction, v, for the catalysed reaction is given by
eqn. (2), in which kcat is the rate constant of the catalysed
Results and discussion
General strategy
The reaction studied is the esterification of stearic acid with
butanol. In Scheme 1, the reaction is shown for n-butanol. The
influence of various reaction conditions has been examined: the
reactions can be carried out under homogeneous conditions
with TPSA (toluene-p-sulfonic acid) as a catalyst, or a hetero-
geneous catalyst can be used. One objective is to investigate
whether reactions under different conditions (homo- or hetero-
geneous media) would be influenced differently by the heating
mode.
For reactions under homogeneous reaction conditions the
effects of temperature variation and concentration of catalyst
on reaction rates have been investigated. For reactions under
heterogeneous conditions the effect of the nature of the catalyst
and the effect of the simultaneous use of microwave and ultra-
sound irradiation have also been studied. The results of these
studies are reported below.
v = kcat [alcohol][acid] = kobs,cat [acid]
(2)
reaction and kobs,cat is the pseudo-first-order rate constant for
the catalysed reaction.
Table 1 shows that the activation energy Ea for the esterifi-
cation under homogeneous conditions is influenced by the
presence of catalyst. The catalysed reaction has a lower acti-
vation energy than the uncatalysed reaction. However, there is
no difference in activation energies with respect to the heating
mode. The values of the activation energy are in accordance
with literature values.8 The activation energies for both the
esterification reactions (catalysed and uncatalysed) are the same
irrespective of the heating mode when they are carried out
under homogeneous reaction conditions. The fact that the heat-
J. Chem. Soc., Perkin Trans. 2, 1997
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Table 1 Activation energies for the catalysed and the uncatalysed
esterification reaction between stearic acid and n-butanola
Table 3 Reactions with graphite (heat captor) and catalysta
Catalyst
t/min
Yield (%)
Reaction
conditions
Ea (conventional)/
Ea (dielectric)/
kJ molϪ1
kJ molϪ1
KF
Fe2(SO4)3
TPSA
5
5
5
82
75
95
No catalystb
TPSAc
85
69
84
70
a T = 250–300 ЊC, [butanol] = 10 [acid] = 100 [catalyst]. In each experi-
ment 2 g of graphite was used.
a Error in the activation energies is approximately 3%, the rate constants
from which the activation parameters were calculated are available as
supplementary material from the British Library (SUP 57296, 2pp.). For
details of the supplementary publications scheme, see ‘Instructions for
Authors’, J. Chem. Soc., Perkin Trans. 2, Issue 1, 1997. b [Butanol] = 10
[acid]. c [Butanol] = 10 [acid] = 100[TPSA].
Table 2 Influence of the heating mode on reaction yields for different
esterification catalysts
c
d
Catalyst
Reaction time/min Y(d)
Y(c)
Y(d)/Y(c)
TPSAa
Fe2(SO4)3
60
120
120
120
120
91
71
78
85
95
89
50
60
64
82
1.0
1.4
1.3
1.3
1.2
b
b
TiBu4
Fig. 2 Yields of esterifications with montmorillonite KSF catalyst at
140 ЊC for conventionally heated (᭿), microwave heated (᭜) and
microwave–ultrasound heated (᭹) reactions14
KFb
Montmorillonite KSFb
a Homogeneous reaction conditions. b Heterogeneous reaction condi-
tions. c Y(d): yield (dielectrically heated). d Y(c): yield (classically heated).
heterogeneous reaction conditions were carried out for 2 h at
140 ЊC. In Table 2 the results of this study are assembled.
It can be seen from Table 2 that the yields of the esterification
reaction under homogeneous conditions are similar, irrespect-
ive of the heating mode. Under heterogeneous reaction condi-
tions the dielectrically heated reaction shows a higher yield,
after 2 h, than does the conventionally heated reaction mixture.
The increase in yield after 2 h of heating can be expressed by a
term Y(d)/Y(c): the yield of the dielectrically heated reaction over
the conventionally heated reaction. The effect is greatest for the
catalyst iron() sulfate (1.4). For TiBu4 and for KF a value of
1.3 was observed, and the smallest effect of dielectric heating
was observed for montmorillonite (Y(d)/Y(c) = 1.2). Interest-
ingly, the catalyst which gives rise to the lowest overall yield
under conventional heating conditions seems to profit the most
from dielectric heating. However, in all cases the heterogeneous
catalysts do not surpass the homogeneous catalyst with respect
to yield, except for montmorillonite KSF, but only after a pro-
longed reaction time. Differences due to dielectric heating only
seem to be observed under heterogeneous reaction conditions.
Fig. 1 Extent of esterification for the n-butyl (᭿), sec-butyl (᭜) and
tert-butylalcohol (᭹) as a function of reaction time; all reactions
have been carried out at 140 ЊC
ing mode does not alter the rate of reaction has been observed
previously.2c–e,9
Further use of temperature effects
Although the highest yields were observed for the homo-
geneously catalysed esterification reaction, the yields are
greatly improved due to the heating mode when heterogeneous
catalysts were used. The advantage of dielectric heating is
caused by the fact that temperature enhancement effects can be
realised.11 In order to show the profitable use of microwave
heating it would be required to demonstrate that temperature
enhancement effects can be exploited. Therefore, a further
study was carried out on catalysis in the presence of heat
captors. Heat captors are materials which are known to be able
to generate heat very efficiently and rapidly when heated by
dielectric means. The use of these types of materials is interest-
ing because very hot zones can be created at the surface of the
solid particles without inducing a degradation of the products.
The bulk temperature is kept constant if the concentration of
the heat captor is low.
Effect of isomer
Under homogeneous reaction conditions the rate of reaction of
the three butanol isomers has been followed, using either con-
ventional or dielectric heating. The results of this study are
depicted in Fig. 1. Under both conventional and microwave
heating conditions the reactivity order is n-butyl > sec-
butyl > tert-butyl alcohol. There is no increased reactivity of
the isomeric forms of butanol under dielectric heating condi-
tions, illustrating that the difference in reactivity towards esteri-
fication is, as in conventional heating, due to variation of the
hydrocarbon moiety of the alcohols. The so-called antenna
group effect2b (specific activation of OH groups which could
counteract the difference in reactivity) is therefore not
apparent.
In Table 3 yields of the esterification reaction of stearic acid
by n-butanol catalysed by TPSA, by iron() sulfate or by KF in
the presence of graphite are shown. Rates of reaction increase
dramatically when this heat captor is used in conjunction with
microwave heating. Within 5 minutes, maximum yields of 75–
95% of ester are observed. Again TPSA seems to be the most
efficient catalyst. These high yields are caused by the fact that
Reactions under homogeneous and heterogeneous conditions
Reactions under heterogeneous reaction conditions have been
conducted at 140 ЊC only. It has already been established that
reactions under microwave conditions are faster than those
heated conventionally.10 The rate increases are most probably
caused by a temperature enhancement effect. Reactions under
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Table 4 Catalysts used for the esterification reaction
Catalyst
Amount
TPSA
Fe2(SO4)3
TiBu4
KF
0.032 g (0.0002 mol)
0.16 g
0.14 g
0.1 g
Montmorillonite KSF
0.8 g
the reaction mixture reaches very high temperatures during
microwave dielectric heating. These temperatures have been
measured during the experiment (see Fig. 3) and vary between
250–300 ЊC. Under conventional heating conditions these temp-
eratures cannot be reached, even when heat captors are applied.
Fig. 3 Schematic representation of the experimental design of the
reaction vessel used for this study
Simultaneous use of microwave heating and ultrasound
When reactions are carried out with the reaction mixture sub-
jected to ultrasound, a mechanical effect is developed on the
surface of the catalyst and this improves the mass transfer and
hence higher yields of reactions can be observed.12 If reaction
mixtures were to be heated dielectrically while applying ultra-
sound, a further positive influence on the yields could be
expected. This is a consequence of the concerted surface effects
of both microwave heating (temperature effect at surface) as
well as the mechanical effect of ultrasound on the surface of the
catalyst. In Fig. 2 the extent of reaction as a function of time
for the esterification of butanol with stearic acid with con-
ventional heating, microwave heating and simultaneous micro-
wave heating and sonication are shown, with montmorillonite
as the catalyst. The rate of reaction shows the sequence MW–
US > MW > conventional.
connected to a regulator. The regulator could control the tem-
perature by choosing a high or low intensity of the microwave
field (in commercial microwave ovens the temperature is con-
trolled by an ‘on–off’ mechanism). In addition to a gas therm-
ometer, the temperature was also monitored by a fibre optic
thermometer. The experimental setup in the microwave oven
is shown in Fig. 3. The degree of esterification was obtained
by determination of the remaining fatty acid by a method
described in ‘AFNOR T 60–112’†. The AFNOR method has
been validated by checking the amount of ester formed at the
end of the reaction. When the reaction mixture was heated con-
ventionally, this was carried out in a Pyrex reaction tube
immersed in a thermostatted oil bath, and fitted with a reflux
condenser. The temperature was monitored with the aid of a
thermocouple. The ultrasound–microwave system was
described in detail by Chemat et al.12
Conclusion
Rates of esterification are influenced by the presence of a
catalyst. In the absence of a catalyst, the rate of esterification
has a different dependence on the concentration of acid due to
autocatalysis. The yields of esterification are different for
the different butanol isomers, when the reaction mixtures are
conventionally or dielectrically heated. The application of
microwave heating does not alter the reaction rates for different
isomers. Therefore there can be no specific activation under
homogeneous reaction conditions, a finding noted earlier for
esterifications.
Hence the explanation is that only heat effects can be
exploited when dielectric heating is applied. This can be realised
by carrying out reactions under heterogeneous reaction condi-
tions. The yields of reaction using a heterogeneous catalyst such
as Fe2(SO4)3, TiBu4, KF or montmorillonite KSF do not com-
pare positively with the homogeneously catalysed reaction.
However, yields are improved by application of dielectric heat-
ing compared with conventional heating. The optimal use of
the different temperature regime can be further improved by the
use of a heat captor and microwaves or heterogeneous catalysts
and microwaves in conjunction with ultrasound.
Reagents
Stearic acid, butanol (normal, secondary and tertiary) and
hexane were purchased from Merck and were all 99% ϩ grade.
TPSA (toluene-p-sulfonic acid) and graphite were purchased
from Prolabo. Fe2(SO4)3 and KF were obtained from Merck. In
addition, montmorillonite KSF was obtained from Aldrich and
tetrabutyl orthotitanate was purchased from Fluka. In all
experiments the concentration of alcohol was chosen to be ten
times that of the acid in order to maintain pseudo-first-order
reaction conditions. A typical reaction mixture consisted of
0.02 mol of stearic acid (5.69 g) and 0.2 mol of butanol (14.8 g).
All isomeric forms of butanol were used. Table 4 shows the
catalysts and the typical amounts used in the reaction.
† AFNOR T 60–112 is an abbreviation of the standard method which
has been used in the institute in Toulouse to determine the amount of
free fatty acid.
References
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positive effect by causing increased mass transfer at the catalyst
surface. Application of both techniques has a positive influence
on the rate of esterification under heterogeneous reaction
conditions.
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Paper 7/02219K
Received 2nd April 1997
Accepted 14th July 1997
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