Novel and highly efficient SnBr2-catalyzed esterification reactions of fatty acids: The notable anion ligand effect

Article abstract of DOI:10.1007/s10562-013-1089-0

In this work, the efficiency of Lewis acid catalysts, SnX₂ (X = F^-, Cl^-, Br^-, or ^-OAc), was investigated on the esterification reactions of fatty acids (i.e., myristic, palmitic, stearic, oleic, linoleic, and linoleic) with different alcohols (i.e., methyl, ethyl, propyl, isopropyl, and butyl alcohol). Tin(II) bromide was the most active catalyst in all reactions studied. We investigated the effects of main reaction parameters, such as catalyst concentration, temperature, and nature of alcohol and fatty acid. The highest activity of SnBr₂ catalyst was discussed in terms of its lower activation energy, higher Lewis acid strength, and higher softness of anionic ligand. Finally, the SnBr ₂ catalyst can be easily recovered and reused without loss of catalytic activity. Graphical Abstract: Effect of the tin catalyst nature on the oleic acid esterification with ethyl alcohol. [Figure not available: see fulltext.]

Full text of DOI:10.1007/s10562-013-1089-0

Catal Lett (2013) 143:1240–1246
DOI 10.1007/s10562-013-1089-0
Novel and Highly Efficient SnBr -Catalyzed Esterification
2
Reactions of Fatty Acids: The Notable Anion Ligand Effect
Arthur Batista Ferreira
M a´ rcio Jos e´ da Silva
•
Abiney Lemos Cardoso
•
Received: 11 June 2013 / Accepted: 17 August 2013 / Published online: 4 September 2013
Ó Springer Science+Business Media New York 2013
Abstract In this work, the efficiency of Lewis acid cata-
Nowadays, employing domestic waste, such as used
-
lysts, SnX (X = F , Cl , Br , or OAc), was investigated
-
-
-
cooking oil, and water waste generated by the food industry
and non-edible vegetable oil as feedstock is mandatory to
comply with the ecological and ethical needs for biofuels
[4, 5]. However, lipidic waste generally contains a high
amount of free fatty acids (FFA), which requires the
development of acid catalyzed-processes for its transfor-
mation into esters [6]. The use of acid solid catalysts is an
attractive alternative, which can minimize both the cost and
the environmental impact of processes [7]. However, the
development of stable solid acid catalysts that can be easily
recovered and repeatedly reused without losing activity is a
challenge yet to be overcome [8]. Generally, heterogeneous
metal-supported catalysts are synthesized from one
homogeneous metal catalyst, which should first be active in
the reaction-goal when in solution. Thus, catalysis studies
in homogeneous phase can contribute to improvement and
the production of more effective heterogeneous catalysts.
Due to the detrimental corrosive effects of Brønsted
acids, the use of Lewis acid catalysts has increased expo-
nentially in the fine and bulk chemical industries during the
last couple of decades [9]. However, important aspects,
such as the high cost, arduous manipulation, and the low
water tolerance of compounds traditionally used in organic
2
on the esterification reactions of fatty acids (i.e., myristic,
palmitic, stearic, oleic, linoleic, and linoleic) with different
alcohols (i.e., methyl, ethyl, propyl, isopropyl, and butyl
alcohol). Tin(II) bromide was the most active catalyst in all
reactions studied. We investigated the effects of main reac-
tion parameters, such as catalyst concentration, temperature,
and nature of alcohol and fatty acid. The highest activity of
SnBr catalyst was discussed in terms of its lower activation
2
energy, higher Lewis acid strength, and higher softness of
anionic ligand. Finally, the SnBr catalyst can be easily
2
recovered and reused without loss of catalytic activity.
Keywords Homogeneous catalysis Á Fatty acids Á
Acid catalysis Á Biomass conversion
1
Introduction
The inevitable exhaustion of fossil fuel reserves, and the envi-
ronmental impact generated by the green-house effect caused
by gas emissions in the combustion of diesel have provoked an
extensive search for renewable raw materials for clean energy
production [1, 2]. Biodiesel can answer this demand, emerging
as a viable alternative to petroleum diesel. Conventional bio-
diesel processes still suffer from problems associated with the
limitations imposed by the reversible chemical reaction and the
use of alkaline homogeneous catalysts, which lead to severe
economic and environmental drawbacks [3].
synthesis (i.e. BF ), does not favor the use of these liquid
3
Lewis acids in FFA esterification on an industrial scale
[10]. The literature has reported the successful use of
SnCl , a solid Lewis acid, as a catalyst in FFA esterifica-
2
tion [11]. Actually, SnCl is easy to handle, water tolerant,
2
and inexpensive [12]. In addition, though being used in the
homogeneous phase, SnCl catalyst can be reused and is
2
less corrosive than mineral acids [13].
A. B. Ferreira Á A. L. Cardoso Á M. J. da Silva (&)
Chemistry Department, Federal University of Vi c¸ osa, Vi c¸ osa,
Minas Gerais 36570-000, Brazil
In this work, we attempt to show that SnBr is also a
2
potential catalyst for esterification reactions with signifi-
cant advantages over Brønsted acid and other Lewis acid
e-mail: silvamj2003@ufv.br
1
23

Esterification Reactions of Fatty Acids
1241
catalysts. Herein, we assessed the activity of SnX salt
2
of alcohol (i.e., 170 mmol) and kinetic measurements were
obtained during the first hour of reaction. The catalyst load
ranged of 0.02–1.0 mmol, and the reaction temperature
was 333 K.
-
-
-
-
catalysts (X = F , Cl , Br , or OAc) in the esterification
reactions of saturated and unsaturated fatty acids. We
investigated the effects of substrate and catalyst concen-
tration, reaction temperature, and the nature of alcohol on
the esterification reactions. In the SnBr -catalyzed esteri-
2
2.4 Effect of Fatty Acid Nature on the SnBr -Catalysed
2
fication reactions, we determined both the activation
energy and the order in relation to catalyst concentration.
Esterification with Ethyl Alcohol
The highest activity of SnBr compared to other Sn(II) salt
2
Saturated and unsaturated fatty acids can have distinct
reactivity in esterification reactions. Herein, we have
assessed this effect as well as the effect of carbon chain
length of the saturated (i.e., palmitic, stearic, and myristic)
and unsaturated (oleic and linoleic) acids on the conversion
catalysts was discussed in terms of higher Lewis acidity
and the softness of more bulker anionic ligands, which
allow a more efficient activation of fatty acids.
and selectivity of SnBr -catalyzed esterification reactions.
2
2
Materials and Methods
Typically, fatty acid (1 mmol) and ethyl alcohol (170 mmol,
15 mL) were stirred and heated (ca. 333 K) for 5 mins;
2
.1 Chemicals
then, after the addition of the SnBr catalyst, the reaction
2
was started.
All chemicals were purchased from commercial sources
and used as received. Methyl, ethyl, propyl, isopropyl, and
butyl alcohols were acquired from Sigma-Aldrich
2.5 Effect of the Alcohol Nature on the SnBr2-
Catalysed Oleic Acid Esterification
(
99.9 %), as well as all the Sn(II) salts catalysts (i.e., SnF2,
SnCl Á2H O, SnBr ÁH O, and Sn(C H O ) ). Hexane was
The carbon chain length, as well as the steric hindrance on
the hydroxyl group, governs alcohol reactivity in esterifi-
cation reactions. To investigate the impact of these
parameters on the conversion and selectivity of SnBr2-
catalyzed oleic acid esterification, the following alcohols
were used: methyl, ethyl, propyl, isopropyl, and butyl
alcohol.
2
2
2
2
2 3 2 2
purchased from Vetec (S a˜ o Paulo, Brazil), and utilized
without prior handling. The fatty acids myristic (95 %),
palmitic (95 %), stearic (95 %), oleic (95 %), and linoleic
(
99 %) were acquired from Sigma-Aldrich and used as
received.
2
.2 SnX -Catalyzed Esterification Reactions of Oleic
2
Acid with Ethyl Alcohol: Effect of the Anionic
2.6 Effect of Temperature on the SnBr -Catalyzed
2
-
Ligand (X = F , Cl , Br or OAc)
-
-
-
Oleic Acid Esterification Reactions with Ethyl
Alcohol: A Kinetic Study
The catalytic tests were performed in a 50 mL three-
necked glass flask, equipped with a sampling system, a
reflux condenser, in thermostatic bath with magnetic stir-
rer, and a large excess of alcohol was employed to shift the
equilibrium towards esters formation. Typically, oleic acid
The kinetics of SnBr -catalyzed reactions was studied by
2
varying the reaction temperature in the range 303–343 K.
Typically, oleic acid (1 mmol) was solved in ethyl alcohol
(170 mmol), stirred, and heated at reaction temperature;
after the addition of the catalyst (1 mmol), the reactions
were started.
(
1 mmol) and ethyl alcohol (170 mmol, 15 mL) were
stirred and heated (ca. 333 K) for 5 mins; then, after the
-
addition of the SnX catalyst (X = F , Cl , Br , or
-
-
2
-
OAc), the reaction started.
2.7 Reaction Monitoring
2
.3 Effect of SnBr Concentration on the Esterification
2
The reaction progress was followed taking aliquots at
regular intervals and analyzing them via gas chromatog-
raphy (GC Varian 450, FID, fitted with Carbowax capillary
column (0.25 lm 9 0.25 mm 9 30 m). The temperature
program was as follows: 150 °C/1 min, 10 °C/min up to
240 °C, with a hold time of 5 min. Both the injector and
detector were kept at 250 and 280 °C, respectively. The
reaction yields were calculated by matching the areas of
the esters’ GC peaks into the corresponding calibration
curves.
Reactions with Ethyl Alcohol: Order in Relation
to Catalyst Concentration
To assess the effect of catalyst concentration on the reac-
tion rate and obtain order in relation to its concentration,
the kinetic measurements should be made under conditions
in which reaction rate dependence with regards the reac-
tants concentration (i.e., alcohol and fatty acid) is of
pseudo-zero order. For this reason, was used a large excess
123

1
242
A. B. Ferreira et al.
2
.8 Identification of the Reaction Products
change the efficiency of Sn(II) in polarizing fatty acids
carbonyl.
Reaction products were identified by analyses carried out
on a Shimadzu GC17A gas chromatograph coupled with a
MS-QP 5050A mass spectrometer (Tokyo, Japan) with a
DB5 capillary column (0.25 lm 9 0.25 mm 9 30 m) and
He as the carrier gas at 2 mL/min. The temperature profile
was as follows: 150 °C for 1 min, 10 °C/min up to 250 °C,
with a hold time of 5 min. The GC injector and MS ion
source temperatures were kept at 250 and 260 °C,
respectively. The MS detector was operated in the EI mode
at 70 eV, with a scanning range of m/z 50–400.
3.2 SnX -Catalyzed Esterification Reactions of Oleic
2
Acid with Ethyl Alcohol: Effect of the Anionic
-
Ligand (X = F , Cl , Br or OAc)
-
-
-
In general, the steric and electronic effects of ligand
coordinated to metal are crucial to make a catalyst less or
more active. Moreover, the hapticity and softness of
anionic ligand appears to have also affected the efficiency
of Sn(II) catalysts on the esterification reactions studied
herein (Fig. 2).
2
.9 Recovery/Reuse of SnBr Catalyst in the Oleic
2
The kinetic curves shown in Fig. 2 reveal that reactions
in the presence of tin catalysts containing the less labile
ligand and with strongest electron withdrawing character
Acid Esterification Reactions with Ethyl Alcohol
-
-
After reaction end, the solution was concentrated under
vacuum, recovering the ethanol excess for further utiliza-
tion. After cooling at neat remaining to room temperature,
it was added hexane in order to allow an easy remove. The
solid catalyst was recovered by filtration washing with
hexane and diethyl ether to remove of esters. After drying
at 373 K, it was weighted and reutilized in another cata-
lytic runs. This recovery/reutilization procedure was made
for five times.
achieved the lowest ester yield (i.e., F and OAc anions,
respectively). On the other hand, the catalysts containing
-
-
more bulky halides (i.e., Cl and Br ) and consequently
the more softness and stable anionic ligand were the most
active. These results may suggest that a mechanism in
which the anions are released throughout the reaction is
involved. Although bond length values of solid Sn(II)
halides are scarcely described in the literature, average
values obtained in the gas phase are reported [i.e., equal to
255 pm (SnBr ), 242 pm (SnCl ), and 206 pm (SnF )]
2 2 2
[
14]. Nevertheless, several works suggest that in most of
Sn(II)-catalyzed esterification reactions with either organic
i.e., alkil) or inorganic (i.e., acetate, halides) anions, these
3
Results and Discussion
(
3
.1 General Aspects
ligands remain coordinated to the tin catalyst throughout
the reaction [15, 16]. Consequently, discarding a possible
dissociation of ligands, we conclude that fatty acid car-
bonyl activation seems to be more favored when the Sn–X
bond length is the longest.
Esterification reactions are acid-catalyzed reversible pro-
cesses, where the alkyl group of alcohol shifts the acidic
hydrogen of carboxylic acid, resulting in water and ester as
products (Fig. 1).
Generally, the role of catalyst (i.e., Brønsted acid) is to
protonate the carboxylic acid carbonyl group, favoring its
attack by alcohol hydroxyl. However, when Lewis acid
metals are the catalysts, the activation process may occur
via carbonyl coordination of acid to the Lewis acidic center
100
8
6
4
2
0
0
0
0
SnBr2
SnCl2
SnF2
n?
of catalyst (i.e., formally a M cation). Although this
Sn(OAc)2
process may seem less effective than protonation, Lewis
acid catalysts, such as SnCl , have demonstrated efficiency
2
in these reactions [11–13]. The structural chemistry of
Sn(II) halides is affected by the nonbonding pair of elec-
trons; however, the different ligands anionic can notably
O
C
O
C
0
0
2
4
6
8
10
12
R
OH
H O
2
R´
OH
R´
OR
Fig. 2 Effect of the tin catalyst nature on the oleic acid esterification
with ethyl alcohol. Reaction conditions: ethyl alcohol:oleic acid:tin
catalyst molar ratio = 170:1:0.1; 333 K temperature
Fig. 1 Esterification reaction of carboylic acid with alcohol
1
23

Esterification Reactions of Fatty Acids
1243
Recently, Ramos et al. studied the activity of tin-based
catalysts (i.e., tin(II) acetate, tin(II) chloride, tin(II) hexa-
noate, and tin(II) stearate) in esterification and transeste-
rification reactions of acid vegetable oils [17]. Those
authors found that both SnCl and Sn(OAc) were more
1
00
8
0
0
0
0
0
2
2
6
4
2
active catalysts; however, they verified that catalyst activ-
ity was notably affected by solubility in reaction medium,
which was, in turn, dependent on the reaction temperature.
In general, the best results were achieved at 150 °C, where
-
2
-1
-1
-1
-1
-1
2
1
5
.0 x 10 mol.L
-2
.0 x 10 mol.L
-3
.0 x 10 mol.L
-
3
Sn(OAc) was the catalyst [17].
2.5 x 10 mol.L
-3
2
1
.0 x 10 mol.L
3
.3 Effect of SnBr Catalyst Concentration
2
on the Esterification Reactions with Ethyl Alcohol:
Order in Relation to Catalyst Concentration
0
2
4
6
8
2
Fig. 3 Effect of the SnBr catalyst concentration on the oleic acid
Kinetic measurements were obtained while the reaction
rate dependence in regards to reactants concentration (i.e.,
alcohol and fatty acid) was of pseudo-zero order. Thus,
despite the reactions being carried out until the conversions
remained constant, only the kinetic measurements obtained
within first 30 mins were used to determine the dependence
in relation to catalyst concentration. During this period,
both C H OH (used in large excess) and fatty acid have
esterification with ethyl alcohol. Reaction conditions: ethyl alco-
hol:oleic acid:tin catalyst molar ratio = 170:1:0.01–0.2; 333 K
temperature
2
5
their concentration almost constant; thus, we can assume
that only the catalyst concentration varies during this initial
period of each run.
Because the reactions were not conducted until the
equilibrium, increasing the catalyst concentration resulted
in an increase in the ester yields (Fig. 3). Notwithstanding,
a total conversion was reached using the two highest cat-
alyst concentrations. The linear plot shown in Fig. 4 was
achieved using the kinetic curves in Fig. 3, and allowed the
order in relation to SnBr to be determined.
2
The slope of the curve provided the order of reaction in
relation to SnBr catalyst concentration, which was equal
2
2
to 0.72. The high value of R confirmed the linearity of
2
Fig. 4 Order in relation to SnBr catalyst concentration
curves. The literature reports that, under almost similar
reaction conditions, the dependence in relation to SnCl2
concentration was equal to 0.87 [13]. The order in relation
concentration was confirmed [11–13]. Table 1 shows the
k values obtained for each temperature.
to SnBr -concentration is corroborated by their highest
2
In Table 1, linear plots of k values 9 log [oleic acid]t=ti
/
activity (Fig. 2).
[oleic acid]t=0 were built, and the first order in relation to
oleic acid concentration was confirmed (not shown herein
for simplification).
3
.4 Effect of Temperature on the SnBr -Catalysed
2
Oleic Acid Esterification Reactions with Ethyl
Alcohol: A Kinetic Study
Thus, it was possible to build an Arrhenius plot, which
allowed the activation energy value to be obtained (Fig. 6).
The activation energy for the SnBr -catalyzed reaction
2
-
was equal to 19.7 kJ mol . This value is considerably
1
The temperature effect was investigated in the range of
3
03–343 K, and the results are shown in Fig. 5.
lower than the one measured for SnCl -catalyzed esterifi-
2
From these data, it was possible to determine the acti-
cation where, under similar reaction conditions, a value
-
equal to 46.7 kJ mol was found [12]. The lowest acti-
1
vation energy for SnBr -catalyzed oleic acid esterification
2
with ethyl alcohol (Fig. 6). Firstly, as described in the lit-
erature, a first order dependence in relation to oleic acid
vation energy of the SnBr -catalysed esterification reac-
2
tions is in agreement with their highest activity (Fig. 2).
123

1
244
A. B. Ferreira et al.
1
00
Lewis acidity, such to increase its coordination number.
Thus, even though the ‘‘nonbonding’’ pair can act as a
donor to vacant orbitals, the ‘‘vacant’’ third up orbital and
8
0
0
0
0
0
5d orbitals can act as acceptors in forming further covalent
bonds [17]. Consequently, it can be hoped that this 5d
orbital will be more stabilized when tin is coordinated with
more electronegative anions, being thus more efficient in
activate the fatty acid carbonyl. However, our results
6
4
2
3
3
03 K
13 K
323 K
3
3
33 K
43 K
revealed that it does not affect the SnX -catalyzed esteri-
2
fication reactions.
Actually, it was observed that the catalysts with bulky
anions were the most effective: SnBr [ SnCl [ SnF .
2
2
2
This trend observed in the tin catalyst activity can be
reasonable explained if we take into account that, by
increasing the size of halide coordinate to the tin, its
capability of former species with higher coordination
number is also increased [18]. In a seminal work, Zubieta
and Zuckerman assessed this unexpected effect for
numerous tin compounds, and found that in solid phase,
both SnI and SnBr salts form structures with coordina-
0
2
4
6
8
2
Fig. 5 Effect of temperature on the SnBr -catalyzed oleic acid
esterification reactions with ethyl alcohol. Reaction conditions: ethyl
alcohol:oleic acid:tin catalyst molar ratio = 170:1:0.1; 303–343 K
temperature
2
2
-7.2
tion numbers higher than SnCl or SnF , respectively.
2
2
Thus, by analogy, it is possible that the coordination of
Y = -237.56 X – 0.35
-7.4
2
fatty acid to SnX catalyst, which results in an attack
2
R = 0.96
more effective of alcohol and consequently favor the
esterification is easier when a bulky halide ligand is
coordinate to the tin.
-7.6
Actually, experimental observations made in solid phase
-7.8
are not adequate to explain the behavior of the SnX cat-
2
alysts in solution; however, there is yet a powerful argu-
ment based on the effect of halide nature on the Lewis
acidity of tin-catalysts which can support our experimental
observations. Spencer et al. [19] assessed the effect of
halide on the Lewis acidity of organotin halides (i.e.,
general formula RSnX4-n, where R is methyl, ethyl, or
butyl, and X is Cl, Br, or I), using tributylphosphine to
former adduct with tin solved in different solvents. Among
other results, those authors verified that the overall
enthalpy for adduct formation increases (becomes more
negative) in the order Cl \ Br \ I [19]. This means that
Lewis acidity of these compounds also follows this ten-
dency. Even though that work focused on Sn(IV) catalysts,
it is reasonable to suppose that same anionic ligands have a
similar effect on the Lewis acidity of Sn(II) catalysts.
Finally, it is important to note that, despite the Sn(OAc)2
catalyst seeming to be an exception to the observed ‘‘ten-
dency’’, the catalysts with larger anions were the most
-
8.0
-8.2
0.0029
0.0030
0.0031
0.0032
0.0033
-
1
1
/T (K )
Fig. 6 Arrenhius plot ln k versus 1/T for SnBr
esterification reactions with ethyl alcohol
2
-catalyzed oleic acid
Table 1 k values obtained at
different temperatures on the
SnBr -catalyzed oleic acid
Temperature
K)
k values
-1
(s )
(
2
esterification reactions with
ethyl alcohol
-4
303
313
323
333
343
3.0 9 10
3.3 9 10
4.5 9 10
5.5 9 10
7.4 9 10
-
-
-
-
4
4
4
4
-
-
effective. Actually, OAc ion radiun is greater than F
anionic radiun, whilst Sn(OAc) was less active than SnF .
3
.5 Insights into the Catalytic Activity of SnX Salts
2
2
2
-
X = Br , Cl , F , or OAc)
-
-
-
(
However, it should mentioned that acetate is a bidentated
ligand that can obstruct the coordination of fatty acid to the
tin catalyst, compromising its activity. This effect probably
does not occur when higher temperatures are used (ca.
150 °C) [16].
The structural chemistry of Sn(II) halides is particularly
complex, partly because the nonbonding pair of electrons
may favor other reactions which are non-dependent on
1
23

Esterification Reactions of Fatty Acids
1245
3
.6 SnBr -Catalysed Esterification Reactions
2
100
with Ethyl Alcohol: Effect of the Fatty Acid Nature
8
6
0
0
The carbon chain length and the presence of double bonds
on fatty acid are features that affect ester yields on ester-
ification reactions. Figure 7 displays the kinetic curves
obtained on the esterification of saturated and unsaturated
fatty acids with ethanol catalyzed by SnBr2.
methyl alcohol
ethyl alcohol
40
propyl alcohol
butyl alcohol
isopropyl alcohol
In general, high yields were achieved after 8 h of
reaction; however, the fatty acids that had the longest
carbon chain were notably more reactive, regardless of
unsaturation number (i.e., C18:0, C18:1 and C18:2; stearic,
oleic, and linoleic acids, respectively).
2
0
0
0
2
4
6
8
3
.7 SnBr -Catalysed Oleic Acid Esterification
2
Reactions: Effect of the Alcohol Nature
Fig. 8 Effect of the alcohol nature on the SnBr -catalysed oleic acid
2
esterification reactions. Reaction conditions: ethyl alcohol:oleic
acid:tin catalyst molar ratio = 170:1:0.01–0.2; 333 K temperature
The kinetic curves shown in Fig. 8 reveal that the alcohols
with shorter carbon chains were the most reactive. More-
over, a poor yield of ester was only obtained in the iso-
propyl alcohol esterification, whose hydroxyl group is
more hindered.
Because the SnBr catalyst is insoluble in oleic acid or
2
ethyl oleate, it can be easily recovered from the reaction
medium after remove of ethyl alcohol. It is noteworthy that
excess of alcohol ethyl can be reused in other reaction.
These results are in agreement with the literature, which
described these effects for both Lewis and Brønsted acid
catalyzed-esterification reactions [20].
Thus, as described in the literature for the SnCl , the
2
recycling of the SnBr catalyst was easily performed after
2
3
.8 Recovery/Reuse of SnBr Catalyst in the Oleic
2
remove ethyl alcohol by vaporization and filtration of solid
catalyst remaining washing with hexane [12]. It was
achieved high recovery percent of SnBr2 catalyst; in
addition, its activity remained constant after reuse resulting
in yields of ethyl oleate ranging of 95–98 % in the reac-
tions performed.
Acid Esterification Reactions with Ethyl Alcohol
In the Fig. 9 is shown the data obtained after the successive
procedures of recovered/reuse of SnBr in the reactions of
2
oleic acid esterification with ethyl alcohol.
conversion
recovery
100
100
9
8
7
6
0
0
0
0
8
0
0
0
0
6
4
2
50
4
0
30
20
0
10
0
0
2
4
6
8
Time (h)
1
2
3
4
5
Fig. 7 Effect of the fatty acid chain on the SnBr
2
-catalysed
esterification reactions with ethyl alcohol. Reaction conditions: ethyl
alcohol:oleic acid:tin catalyst molar ratio = 170:1:0.01–0.2; 333 K
temperature
2
Fig. 9 Recovery/reuse of SnBr catalyst in the oleic acid esterifica-
tion with ethyl alcohol. Reaction conditions: ethyl alcohol:oleic
acid:tin catalyst molar ratio = 170:1:0.1; 333 K
123

1
246
A. B. Ferreira et al.
4
Conclusion
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-
-
The efficiency of Lewis acid catalysts, SnX (X = F , Cl ,
2
-
-
Br , or OAc), was investigated on the esterification reac-
tions of fatty acids (i.e., myristic, palmitic, stearic, oleic,
linoleic, and linoleic) with different alcohols (i.e., methyl,
ethyl, propyl, isopropyl, and butyl). Tin(II) bromide was the
most active catalyst in all reactions studied. The lowest
activation energy, in addition to the highest order in relation
to its concentration, corroborated these results. High ester
yields were achieved in all fatty acid esterification reactions;
however, the fatty acid with the longest carbon chain was
more reactive. Based on the literature and on the results
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more softness ligand anions were the most active (i.e.,
SnBr [ SnCl [ SnF [ Sn(OAc) , respectively). Finally,
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