Solvent-free esterification catalyzed by surfactant-combined catalysts at room temperature

Article abstract of DOI:10.1039/b615448d

Solvent-free esterifications of various carboxylic acids and alcohols can be catalyzed by surfactant-combined catalysts dodecylbenzene sulfonic acid (DBSA) and copper dodecylbenzene sulfonate (CDBS) in moderate to excellent yield at room temperature. The esterification method has two notable advantages: first, there is no need for any solvent, even water, and secondly, no need for energy, the reaction can proceed smoothly at room temperature. The Royal Society of Chemistry and the Centre National de la Recherche Scientifique.

Full text of DOI:10.1039/b615448d

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LETTER
www.rsc.org/njc | New Journal of Chemistry
Solvent-free esterification catalyzed by surfactant-combined catalysts at
room temperaturew
Li Gang,*ab Li Xinzongab and Wumanjiang Elia
Received (in Montpellier, France) 24th October 2006, Accepted 9th January 2007
First published as an Advance Article on the web 26th January 2007
DOI: 10.1039/b615448d
7
Solvent-free esterifications of various carboxylic acids and
alcohols can be catalyzed by surfactant-combined catalysts
dodecylbenzene sulfonic acid (DBSA) and copper dodecylben-
zene sulfonate (CDBS) in moderate to excellent yield at room
temperature. The esterification method has two notable advan-
tages: first, there is no need for any solvent, even water, and
secondly, no need for energy, the reaction can proceed smoothly
at room temperature.
alkylation reactions. Esterifications of carboxylic acids with
alcohols catalyzed by para-dodecylbenzene sulfonic acid
(DBSA) have also been reported in a few examples. Saam
et al. and Kobayashi et al., respectively, reported polyester-
ification of nonanedioic acid and 1,10-decanediol at 80 1C and
direct esterifications of various carboxylic acids and alcohols
at 40 1C using 10 mol% dodecylbenzene sulfonic acid catalyst
8
in water, Although dehydration esterifications in water are
exciting and novel, the method lacks practicability in actual
production processes since a high amount of wastewater,
which is difficult and troublesome to deal with, is produced
during the reaction process.
Introduction
Direct esterification of organic acids and alcohols is one of the
most fundamental and important reactions in organic chem-
istry industry. These ester products are essential fine chemicals
used widely in the manufacturing of flavors, pharmaceuticals,
plasticizers, polymerization monomers, and as emulsifiers in
The focus of the present work is the direct esterification of
carboxylic acids with alcohols at room temperature in the
presence of 5 mol% surfactant-catalyst: para-dodecylbenzene
sulfonic acid or copper para-dodecylbenzene sulfonate, in the
absence of solvent. Furthermore, the catalysis esterification
mechanism is investigated.
1
the food and cosmetic industries. Recently, an increasing
interest in esterification has been stimulated by the great
Direct esterifications of various carboxylic acids and alco-
hols were catalyzed by 5 mol% DBSA and CDBS at room
temperature as shown in Table 1. It is readily seen that
conversions of carboxylic acids are dependent upon the solu-
bility of the reactants and product ester in water: the poorer
the solubility of reactants and product ester in water, the
higher the conversion to carboxylic acids. The conversions
were excellent for water-insoluble reactants or product esters
(entries 1–6). For acetic acid and ethanol, because they are
fully soluble in water (entry 10) and since the product ethyl
acetate is also partially soluble in water, the conversions to
acetic acid catalyzed by DBSA and CDBS were only 76.8 and
75.9%, much lower than the 98.7 and 97.2% of esterification
of oleic acids with 1-octanol (entry 1). There are mainly two
reasons for this: one is that most of the water formed is still
present in a continuous phase because of a degree of solubility
of the reactant and product in water, which results in hydro-
lysis of the product ester. The other is that the reactants partly
dissolve in water in the micelle interior. For the case where the
reactants are soluble in water but the products are not, the
conversions were good (entries 7–9). Although iso-alcohols
reacted smoothly (entry 11), esterification of secondary alco-
hols was very slow due to strong steric hindrance (entries 13
and 14). For acrylic acid (entry 12), the esterification was
slow due to the conjugation effect of carbonyl and ethylene.
Higher conversion could be obtained by heating or prolonged
reaction. Esterification at room temperature using DBSA or
CDBS as catalysts is also excellent for strongly hydrophobic
substrates.
promise shown by alkyl esters as biodiesel to substitute for
2
fossil fuels.
Catalysts are key factors of esterification. Traditional in-
dustry esterifications are catalyzed by use of concentrated
sulfuric acid in organic solvents. Water formed during the
process is removed continuously from the system by means of
azeotropic distillation. However, there are many serious dis-
advantages in such catalysis esterification: the processes are
accompanied with many co-reactions such as oxidation, sul-
fonation and etherization, and a high amount of energy is
required to remove water to promote the equilibrium process
to shift towards the product side. Therefore, many substitutes
for sulfuric acid to catalyze esterification reactions have been
3
reported in the literature, such as solid acid, ion exchange
6
4
resin, molecular sieve and biocatalyst lipase.
5
Surfactants are a class of chemicals comprised of a hydro-
philic polar head group and a hydrophobic chain and can
assemble automatically in water or organic solvents. Brønsted-
or Lewis acid-type surfactants have been used to catalyze
many organic reactions such as aldol, Mannich, Michael and
a
Xinjiang Technology Institute of Physics and Chemistry, The
Chinese Academy of Science, Urumqi, 830011, People’s Republic of
China. E-mail: ligang@ms.xjb.ac.cn; Fax: +86 991-3838957; Tel:
+
86 991-3858347
Graduate University of the Chinese Academy of Sciences, Beijing,
00049, People’s Republic of China
b
1
w Electronic supplementary information (ESI) available: IR spectro-
grams of part product esters. See DOI: 10.1039/b615448d
3
48 | New J. Chem., 2007, 31, 348–351
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Table 1 DBSA and CDBS catalyzed direct esterifications of various substrates
0
rt 5 mol%cat
0
RCOOH þ R OH
RCOOR þ H2O
2
4 h
Conversion (%)
0
Entry
R
R
DBSA
CDBS
1
2
3
4
5
6
7
8
9
CH
CH
CH
CH
CH
CH
CH
CH
CH
CH
CH
CH
3
3
3
3
3
3
3
3
3
3
3
(CH
(CH
(CH
(CH
(CH
(CH
(CH
2
2
2
2
2
2
2
)
)
)
)
)
)
)
7
7
7
7
7
6
2
CHQCH(CH
CHQCH(CH
CHQCH(CH
CHQCH(CH
CHQCH(CH
–
2
2
2
2
2
)
)
)
)
)
7
–
7
–
7
–
7
–
7
–
CH
CH
CH
CH
CH
CH
CH
CH
CH
CH
CH
CH
3
3
3
3
3
3
3
3
3
3
3
3
(CH
(CH
(CH
(CH
2
2
2
2
)
)
)
)
7
4
3
2
–
–
–
–
98.7
98.4
97.1
95.4
94.6
94.8
91.8
88.5
86.9
76.8
90.8
97.2
95.7
94.4
92.6
90.8
94.9
85.3
83.6
83.8
75.9
90.9
59.0
2
CH –
(CH
(CH
(CH
(CH
2
2
2
2
)
4
–
4
–
4
–
4
–
–
)
)
)
2
CH –
–
10
11
12
–
(CH
CH
CH(CH
(CH
2
–
2
)
6
–
3 2
)CH –
2
QCH–
2
)
7
–
71.3
8
a
a
6.7
78.6
1
1
3
4
CH
CH
3
(CH
(CH
2
)
)
6
6
–
–
CH
CH
3
CH
CH(CH
2
CH(CH
)–
3
)–
54.9
64.5
48.9
60.2
64.3
3
2
3
3
b
b
7
6.5
a
b
7
2 h. 60 1C.
To understand the mechanism of direct esterification of
cient catalytic esterifications of carboxylic acids with alcohols:
+
carboxylic acids with alcohols catalyzed by surfactant-com-
bined catalysts DBSA and CDBS, the catalysis results of
DBSA and CDBS were compared with those of other catalysts
(i) long-alkyl chain of surfactant; (ii) Brønsted acid H or
2
+
Lewis acid Cu . DBSA and CDBS fully meet these two
requirements.
(
5 mol%) in esterification of oleic acid with 1-octanol as
Most of the esterification catalysts used in organic synthesis
are Brønsted acids such as H SO , CH SO H or hetero-
typical substrates.
2
4
3
3
As shown in Table 2, when the esterification reaction of
oleic acid with 1-octanol was catalyzed by 5 mol% surfactant-
combined catalysts DBSA and CDBS at room temperature,
the conversions of oleic acid were excellent, up to 98.7 and
geneous solid acids. The mechanisms of esterification catalysis
b,9
2
have been deeply researched.
The mechanism of DBSA
catalyzed esterification is similar. DBSA promotes the proto-
nation of the carbonyl oxygen on the carboxylic group to
favor of nucleophilic attack by the alcohol to form a tetra-
hedral intermediate as shown in Fig. 1. Dehydration of the
intermediate leads to the product ester.
9
7.6%, respectively (entries 1 and 2). Comparatively, the
ordinary surfactant sodium para-dodecylbenzene sulfonate
SDBS) was hardly active towards esterification (entry 3)
(
2
+
2+
3+
though the only difference between SDBS and DBSA and
+
CDBS is the nature of the cations (Na for SDBS; Brønsted
Lewis acids such as Cu , Mg and Al have been used
10
as catalysts for esterification in a few reports. Barbosa et al.
+
acid H for DBSA; and Lewis acid Cu
2+
for CDBS). This
2+
provided a plausible mechanism for esterification reaction
11
+
showed that Brønsted acid H or Lewis acid Cu are the key
factors to catalyze esterification. With simple non-surfactant
using SiO /ZnCl in solid-phase catalysis. Based on these
2
2
studies, we presume the mechanism as shown in Fig. 2 for
CDBS catalyzed esterifications of carboxylic acids and alco-
hols. The mechanism of CDBS catalysis is described as
follows: the carbonyl oxygen of the carboxylic group combines
Lewis acids (CuCl
2 2
, Cu(OTs) ) and Brønsted acids (TsOH,
H
2
SO ), esterification under the same conditions (entries 4–7),
4
was much inferior to that of DBSA and CDBS. This fact
indicated that the long alkyl chains of DBSA and CDBS were
also vital for the efficient catalysis. However, the cation
surfactant cetyltrimethylammonium bromide (CTAB) was
hardly efficient for esterification (entry 8). These results de-
monstrate that two qualifications are indispensable for effi-
Table 2 Direct esterifications with various catalysts
Entry
Catalyst
Conversion (%)
1
2
3
4
5
6
7
8
DBSA
CDBS
SDBS
98.7
97.2
2.1
20.1
18.5
55.2
48.5
0.9
H
2
SO
4
CuCl
TsOH
2
Cu(OTs)
CTAB
2
Fig. 1 Mechanism of the DBSA catalyzed esterification.
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Fig. 2 Proposed mechanism of the CDBS catalyzed esterification.
Fig. 4 TEM images of the oleic acid/1-octanol/DBSA system.
2
+
with Cu
of CDBS to form an activated complex which is
attacked by the nucleophile alcohol to form a tetrahedral
intermediate. The product ester is obtained after the inter-
mediate eliminates water.
Formation of micelles by DBSA in the reaction system was
confirmed by TEM as shown in Fig. 4. The reverse micelles
resemble micro separators which automatically separate water
produced by esterification from the product ester. Thus neither
azeotropic distillation equipment, energy nor a drying agent is
required.
Significantly, DBSA and CDBS as surfactants in hydro-
phobic substrates can self-assemble reverse clusters, which
provide a large interface between the catalyst and reactant
where the reaction takes place, so greatly enhancing the
reaction rate. Also the formation of DBSA and CDBS clusters
+
2+
Conclusion
would congregate catalytic species H and Cu
at the
reaction interface, which could catalyze the esterification more
efficiently. The process of esterification catalyzed by DBSA
and CDBS is illustrated in Fig. 3. Furthermore, because of
hydrophobic property of the continuous substrate phase and
hydrophilicity of the cluster interior, water molecules pro-
duced could automatically enter the cluster interior while the
product ester enters the continuous phase. With progress of
the reaction, surfactant clusters are enlarged to form inverse
micelles with the aid of alcohol as a co-surfactant. Separation
of water from the ester avoids reverse reaction hydrolysis and
rapidly promotes the equilibrium reaction towards product.
This study indicates that the catalysts DBSA and CDBS show
a twofold function during the esterification. First, as Brønsted
and Lewis acids, they catalyze the esterification reaction.
Secondly, as surfactants, they can form reverse micelles in
substrates to separate water produced from the product ester.
The results further show that the conversions of carboxylic
acid are dependent on the solubility of the reactants and
product ester in water: the poorer the solubility of reactants
and product ester in water, the higher the conversion to
carboxylic acids. Also, the reaction can smoothly proceed at
room temperature with no need for energy imput or equip-
ment to remove produced water, which automatically enters
the micelle interiors owing to the system hydrophobicity.
Additionally there is no need for any solvent, even water.
Therefore, the esterification catalyzed by surfactant-combined
catalysts DBSA and CDBS should be promising in practical
production.
Experimental
Chemicals
Dodecylbenzene sulfonic acid (DBSA, 98% purity), sodium
dodecylbenzene sulfonate (SDBS, chemical purity), 1-octanol,
2
-butanol, n-butyric acid and para-toluenesulfonate acid
Fig. 3 Illustration of auto-isolation of water produced during the
(TsOH, chemical purity) were purchased from Sinopharm
Chemical Reagent Co., Ltd (China). Oleic acid, 1-propanol,
esterification process.
3
50 | New J. Chem., 2007, 31, 348–351
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1
-butanol, KOH, 1,3-propanediol, Copper oxide, cetyl-
ꢁ ꢁ1
1
trimethylammonium bromide (CTAB), propionic acid and
acetic acid were purchased from Shanghai Shanpu chemical
industry Co., Ltd (China). Ethanol, 2-propanol, sulfuric acid
1 2
V (ml mol ) and V (ml mol ) represent the volume of
KOH solution, respectively consumed by carboxylic acid per
gram and catalysis per gram, which were titrated by KO-
H–ethanol solution solely according to same method. V (ml)
represents the volume consumed by the reaction mixture after
(
H SO4 98%), isobutanol, diethyl ether, acrylic acid and
2
octanoic acid were obtained from Tianjin Chemical Reagent
Factory (China). Benzene sulfuric acid (BSA) were obtained
from Alfa Aesar Co., Ltd. Benzyl alcohol, 1-pentanol and
valeric acid were all from Tianjin Guangfu Institute of Fine
Chemical Industry Chemical Reagent Factory (China). All the
other chemicals used were of analytical grade and were used
without further purification.
1 2
completion of the esterification. M (mol) and M (mol)
represent carboxylic acid and catalyst added in reactor,
respectively.
After the titration, low-boiling point solvents and water in
the reaction mixture were removed by a rotory evaporator, and
the pure esters were obtained by decompression distillation. The
ester structures were confirmed by IR (Nicolet-5DX).
Preparation of catalysis
Acknowledgements
Copper para-dodecylbenzene sulfonate was synthesized from
copper oxide and DBSA in a 1 : 1 molar ratio. The detailed
process was as follows: 32.6 g of DBSA was taken in a 100 ml
glass reactor. 7.95 g of copper oxide powder was added slowly
within 30 min at room temperature and was mixed using a
magnetic stirrer (500 rpm). The mixture was heated slowly to
This work is financially supported by ‘‘XIBUZHIGUANG’’
Science Foundation of the Chinese Academy of Sciences.
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ꢂ 100%
6
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New J. Chem., 2007, 31, 348–351 | 351