Esterifications of carboxylic acids and alcohols catalyzed by Al 2(SO4)3 · 18H2O under solvent-free condition

Article abstract of DOI:10.1134/S0023158410040154

Esterifications of equimolar mixture of carboxylic acids and alcohols can be effectively catalyzed by Al₂(SO₄)₃ · 18H₂O under solvent-free condition. The esterification catalyzed by Al₂(SO₄)₃ · 18H₂O is a promising green method thanks to no need of organic solvent, no pollution, no causticity and ease to handle after reaction. This catalyst is of a low toxicity (usually it is used as purifying agent for drinking water), low-cost compound and is easily separated from the reaction mixture by simple filtration. Moreover, the catalyst can be recycled for the further esterification and the conversion does not evidently decrease.

Full text of DOI:10.1134/S0023158410040154

ISSN 0023ꢀ1584, Kinetics and Catalysis, 2010, Vol. 51, No. 4, pp. 559–565. © Pleiades Publishing, Ltd., 2010.
Esterifications of Carboxylic Acids and Alcohols Catalyzed
by Al2(SO4)3
·
18H2O under SolventꢀFree Condition1
Li Gang and Pang Wenhui
College of Chemistry and Chemical Engineering, Anyang Normal University, China
eꢀmail: Ligang@aynu.edu.cn
Received July 31, 2009
Abstract—Esterifications of equimolar mixture of carboxylic acids and alcohols can be effectively catalyzed
by Al2(SO4)3 · 18H2O under solventꢀfree condition. The esterification catalyzed by Al2(SO4)3 · 18H2O is a
promising green method thanks to no need of organic solvent, no pollution, no causticity and ease to handle
after reaction. This catalyst is of a low toxicity (usually it is used as purifying agent for drinking water), lowꢀ
cost compound and is easily separated from the reaction mixture by simple filtration. Moreover, the catalyst
can be recycled for the further esterification and the conversion does not evidently decrease.
DOI: 10.1134/S0023158410040154
1
Esters are essential fine chemicals used widely in
Esterifications of carboxylic acids and alcohols catꢀ
industry and agriculture production and daily life. alyzed by solid Lewis acids have also been reported in
They can be used as pharmaceuticals, flavors, polyꢀ a few examples [21, 22]. Yamamoto et al. reported
merization monomers, plasticizers in rubber and plasꢀ direct esterifications of equimolar mixture of carboxyꢀ
tic industry, emulsifiers in the food and cosmetic lic acids and alcohols catalyzed by hafnium(IV) and
industry and solvent in chemistry industry due to a low zirconium(IV) salts (ZrCl4, Zr(iꢀPrO)4, HfCl4, Hf(tꢀ
toxicity [1–3]. Recently, long chain alkyl carboxylic BuO)4) with azeotropic reflux to remove water from
acids esters were used as biosdiesel to substitute for system [23]. Juan et al. researched Zr(SO4)2·4H2O
decreasing fossil fuels [4, 5].
Traditional esterifications of carboxylic acids and
alcohols are catalyzed by liquid strong acids, such as
catalyzed esterifications of fatty acids and alcohols
[24]. Besides those, Sugi et al. reported that various
multivalent metal chlorides catalyzed esterifications of
long chain aliphatic acids and long chain alcohols in
mesitylene as solvent [25]. Although Hf(IV) and
Zr(IV) salts are highly effective for esterifications of
carboxylic acids and alcohols, they are enough expenꢀ
sive compounds.
sulfuric, phosphoric or
pꢀtoluenesulfonic acids [6–9].
Although these homogeneous liquid Br
ø
nsted acids
are highly efficient for esterifications, the reactions
catalyzed by them have a lot of deficiencies: coꢀprodꢀ
ucts, strong causticity to equipment, and a great deal
of waste water in the process of dealing with product
ester and so on. Therefore, chemists are always gropꢀ
ing for an ideal esterification catalyst to substitute for
strong liquid acid. Recently, it was continually
reported that various kinds of novel solid acid catalysts
were used to catalyze esterifications of carboxylic acids
and alcohols, such as ion exchange resins [10, 11],
mesoporous molecular sieves [12–14], solid superacꢀ
ids [15] and heteropolyacids (salts), lipase, sulfonic
acids supported on molecular sieves, active carbon or
polymers and so on [16–20]. Although these solid catꢀ
alysts possess high activity and can be easily separated
from reaction system, they have also a row of shortꢀ
comings: ion exchangeꢀresins easily deactivate at high
temperature for a long lime and show poor reusability.
For solid supported catalysts, their application is limꢀ
ited on a large scale because of troubled preparation
and high cost.
In this work, we describe esterifications of various carꢀ
boxylic acids and alcohols catalyzed by Al2(SO4)3 ·
18H2O under solventꢀfree conditions. Al2(SO4)3
·
18H2O not only possesses high catalytic activity for
esterification, but also is a cheap, low toxic comꢀ
pound. Moreover, it can be easily recovered by simple
filter and reused to the further reaction. So Al2(SO4)3 ·
18H2O should be a promising green catalyst for esteriꢀ
fications of carboxylic acids and alcohols.
EXPERIMENTAL
Materials
Ethanol, diethyl ether, 1ꢀbutanol, KOH, caprylic
acid, chloroactic acid, and benzoic acid were purꢀ
chased from Tianjin Fuyu Fine Chemicals Industry
Co., Ltd (China); cyclohexanol, 2ꢀbutanol,
n
ꢀbutyric acid and acetic acid—from Tianjin Yuanli
1
The article is published in the original.
Chemical Reagent Co., Ltd (China). MgSO4 · 7H2O,
559

560
LI GANG, PANG WENHUI
Table 1. Esterification of equimolar mixture of stearic acid
and 1ꢀoctanol catalyzed by various metal salts under solventꢀ
free conditions
phthalein solution in ethanol (10 g/L) as indicator was
added. The titration was performed by means of about
0.1 mol/L KOH solution in ethanol. The volume of
KOH solution consumed was recorded, and the perꢀ
centage conversion of carboxylic acids was calculated
using the following expression:
Entry
Catalyst
Conversion, %
1
2
Al2(SO4)3 · 18H2O
Al2(SO4)3
94.6
95.3
63.5
43.3
41.1
68.1
40.9
55.9
69.9
75.0
52.7
40.8
⎡
⎤
V
V M
1
Conversion to carboxylic acid =
,
×100%
1 −
⎢
⎥
⎦
3
CuSO4 · 5H2O
ZnSO4 · 7H2O
MgSO4 · 7H2O
KAl(SO4)2 · 12H2O
Na2SO4 · 10H2O
NiSO4 · 6H2O
LnNO3 · 6H2O
AlCl3 · 6H2O
SnCl4 · 5H2O
None
⎣
1
4
where V represents the volume in mL of reaction mixꢀ
ture consumed after completion of the esterification,
V1—the volume of KOH solution in mL consumed by
1 g of carboxylic acid, which was titrated by KOH–
ethanol solution solely according to the same method,
M1(g)—the weight of carboxylic acid added into flask.
After the titration, lowꢀboiling solvents and water
in the reaction mixture were removed by a rotary evapꢀ
orator, and the pure esters were obtained by decomꢀ
pression distillation. The product ester structures were
confirmed by IR (Varian 800 FTꢀIR). The IR spectra
of some products are presented in Appendix.
The analytical procedures are described in publicaꢀ
tions [26–29].
5
6
7
8
9
10
11
12
Note: Reaction condition: molar ratio of stearic acid to 1ꢀoctanol
1 : 1, amount of catalyst 10 mol %, reaction time 24 h, temperature
80 C.
°
Reuse of the Catalyst
ZnSO4 · 7H2O, CuSO4 · 5H2O, Na2SO4 · 10H2O
.
3ꢀphenylpropionic acid, isopentanol, isobutanol, and
octanoic acid were obtained from Sinopharm Chemiꢀ
cal Reagent (China); 1ꢀoctanol, 1ꢀpentanol, cinꢀ
The recycle performance of catalyst Al2(SO4)3 ·
18H2O was researched by esterification of stearic acid
and 1ꢀoctanol under identical conditions. The
Al2(SO4)3 · 18H2O catalyst filtered was reused to cataꢀ
lyze the further esterification without any further
treatment. The same amount of carboxylic acid, alcoꢀ
hol and catalyst were used for each of the recycled
esterification.
namic acid,
βꢀphenethyl alcohol, LnNO3 · 6H2O,
SnCl4 · 5H2O, NiSO4 · 6H2O, KAl(SO4)2 · 12H2O,
AlCl3 · 6H2O, and Al2(SO4)3 · 18H2O, from Beijing
Chemicals Reagent Co., Ltd (China); oleic acid,
linoleic acid, stearic acid and palmitic acid, from
Tianjin Damao Chemicals Reagent Industry Co., Ltd
(China). Al2(SO4)3 was prepared from Al2(SO4)3 ·
18H2O by calcining to 300°C for 2 h. All chemicals
used were of analytical grade and were used without
further purification.
RESULTS AND DISCUSSION
Catalytic Activities of Various Metal Salts
in Esterification
The results of testing of various metal salts catalysts
in the esterification of stearic acid with 1ꢀoctanol as
typical substrates are summarized in Table 1. To
develop green chemistry, stearic acid and 1ꢀoctanol
reacted without using any solvent and agent. By comꢀ
paring various metal salts, it can be found that
Al2(SO4)3 · 18H2O and Al2(SO4)3 are the most efficient
catalyst to esterification. Furthermore, catalytic activꢀ
ity of Al2(SO4)3 appreciably excel over Al2(SO4)3 ·
18H2O (95.3 and 94.6%, respectively). We consider
that the distinction should be attributed to Al2(SO4)3
absorbability to water. Anhydrous Al2(SO4)3 can
adsorb water formed during esterification from prodꢀ
uct ester and shift the equilibrium towards product
side. However, conversions on other metal salts such as
Esterification
The esterifications were carried out at atmospheric
pressure in a 50 mL twoꢀneck roundꢀbottom flask
equipped with a reflux condenser and a thermometer.
An equimolar mixture of carboxylic acid and alcohol
and 10 mol % catalyst were added into the flask withꢀ
out any solvent and dehydrating agent. The reaction
mixture was continuously stirred using a teflonꢀcoated
magnetic stirrer for 24 h at 80°C. Paraffin liquid was
used as heat conduction medium.
Analysis
After completion of the reaction, the percentage CuSO4 · 5H2O, ZnSO4 · 7H2O, MgSO4 · 7H2O,
conversion of carboxylic acids was measured by stanꢀ KAl(SO4)2 · 12H2O, Na2SO4 · 10H2O, NiSO4 · 6H2O,
dard titration. The catalyst was separated from product LnNO3 · 6H2O, AlCl3 · 6H2O, and SnCl4 · 5H2O are
by simple filter. Then the product was dissolved in ethꢀ evidently low comparing to that on Al2(SO4)3 · 18H2O
anol–diethyl ether mixture (1 : 2, v/v) and phenolꢀ and Al2(SO4)3 under the same conditions. Although
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ESTERIFICATIONS OF CARBOXYLIC ACIDS AND ALCOHOLS CATALYZED
561
Table 2. Al2(SO4)3 · 18H2O catalyzed esterifications of linear carboxylic acids and alcohols
Entry
Carboxylic acid
Alcohol
Conversion, %
1
2
3
4
5
6
7
CH3(CH2)16COOH
CH3(CH2)16COOH
CH3(CH2)14COOH
CH3(CH2)16COOH
CH3(CH2)6COOH
CH3COOH
CH3(CH2)6CH2OH
CH3CH2CH2CH2OH
CH3(CH2)6CH2OH
CH3CH2OH
94.6
90.2
91.6
82.7
86.5
83.2
76.1
CH3(CH2)6CH2OH
CH3(CH2)6CH2OH
CH3CH2OH
CH3COOH
Note: Reaction condition: molar ratio of carboxylic acid to alcohol 1 : 1, amount of catalyst 10 mol %, reaction time 24 h, temperature
80 C.
°
both Al2(SO4)3 and Al2(SO4)3 · 18H2O have excellent
catalytic activity, the preparation of Al2(SO4)3 is trouꢀ
Esterifications of Substrates with Functional Groups
Additionally, we investigated the Al2(SO4)3 · 18H2O
catalyzed esterifications of substrates with functional
groups such as chlorines, double bonds, and phenyl
groups under solventꢀfree condition as shown in Table 4.
For esterifications of benzoic and cinnamic acids
(entries 1, 2), conversions are only 50.3 and 65.6%,
respectively. The low conversions should be attributed
to the conjugation effect of the carbonyl and ethylene
or phenyl groups, which increase stability of the carꢀ
bonyl and go against formation of tetrahedral intermeꢀ
diate. However, for 3ꢀphenylpropionic acid in which
carboxylic carbonyl can not conjugate with phenyl,
the conversion is over 90% (entries 3, 4). Therefore,
esterificaton of 3ꢀphenylpropionic acid is not affected
bled and it is more expensive than Al2(SO4)3 · 18H2O
.
So we deep investigate Al2(SO4)3 · 18H2O catalyzed
esterifications of carboxylic acids and alcohols.
Esterifications of Linear Carboxylic Acids and Alcohols
The reaction was examined with structurally
diversed fatty carboxylic acids and alcohols as shown
in Table 2. For linear carboxylic acids and alcohols,
esterifications catalyzed by Al2(SO4)3 · 18H2O can be
smoothly carried out with moderate to excellent yield.
Long chain fatty compounds generally prepared using
acyl chlorides [30]. They are esterified with over 90%
conversion (entries 1–3). Conversions of short chain
linear substrates are relative low compared to that of
long chain fatty substrates (entries 4–7).
by phenyl group. Esterifications of stearic acid with ꢀ
β
phenethyl alcohol and benzyl alcohol give over 90%
conversions (entries 5, 6). It indicates that phenyl also
does not affect on esterification activity of alcohol in
which hydroxyl group does not directly link phenyl
group. Esterifications of oleic acid and linoleic acid
(entries 7, 8), where double bonds arc away from the
carboxylic acid group, with 1ꢀoctanol give excellent
conversion under the same condition. It shows that the
effect of the double bond on the esterifications is negꢀ
ligible. Chloroactic acid can smoothly react with 1ꢀ
octanol and gives 89.2% conversion (entry 9). The
Esterification of Hindered Alcohols
and Carboxylic Acids
The esterifications of various hindered alcohols
with stearic acid CH3(CH2)16COOH catalyzed by
Al2(SO4)3 · 18H2O are also investigated under the same
condition as shown in Table 3. The results indicate
that the conversion of hindered alcohols (entry 1),
especially tertiary alcohols, are distinctly low as comꢀ
pared to that of primary alcohol. The conversion of
stearic acid and tertiary butyl alcohol is only 9.4% even
after prolonged reaction time 48 h (entry 3). The low
conversion should be attributed to the alcohols’ strong
steric hindrance. The conversion of cyclohexanol and
octanoic acid is up to 67.8% for 24 h (entry 2), which
is better than that of 2ꢀbutanol because the close alkyl
Table 3. Al2(SO4)3 · 18H2O catalyzed esterification of hinꢀ
dered alcohols with stearic acid
Reaction
time, h
Conversion,
%
Entry
Alcohol
1
2
24
24
CH3CH2CH(OH)CH3
54.2
67.8
OH
chain reduces steric hindrance. The conversion of iso
ꢀ
3
4
48
24
(CH3)3COH
9.4
amyl alcohol catalyzed by Al2(SO4)3 · 18H2O can be
up to 91.3% under the same condition (entry 4), and it
indicates that hindrance effect of the methyl group is
not obvious to esterification.
(CH3)2CH2CH2CH2OH
91.3
Note: Reaction condition: molar ratio of carboxylic acid to alcoꢀ
hol 1 : 1, amount of catalyst 10 mol %, temperature 80 C.
°
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LI GANG, PANG WENHUI
Table 4. Al2(SO4)3 · 18H2O catalyzed esterifications of substrates with functional groups
Entry
1
Carboxylic acid
Alcohol
Conversion, %
50.3
CH3(CH2)3CH2OH
COOH
2
3
4
5
6
CH3(CH2)6CH2OH
CH3(CH2)6CH2OH
65.6
90.6
91.2
93.6
90.1
C C COOH
H H
CH2CH2COOH
CH2CH2COOH
CH2CH2OH
CH2CH2OH
CH2OH
CH3(CH2)16COOH
CH3(CH2)16COOH
7
8
CH3(CH2)7CH=CH(CH2)7COOH
CH3(CH2)4CH=CHCH2–CH=CH(CH2)7COOH
ClCH2COOH
CH3(CH2)6CH2OH
CH3(CH2)6CH2OH
CH3(CH2)6CH2OH
ClCH2CH2OH
91.2
92.0
89.2
68.7
9
10
CH3(CH2)16COOH
Note: Reaction condition: molar ratio of carboxylic acid to alcohol 1 : 1, amount of catalyst 10 mol %, reaction time 24 h, temperature 80°C.
excellent conversion should be attributed to the pull stearic acid and 1ꢀoctanol under the same condition
inductivity of the chlorine, which increase positive was conducted. The same amounts of oleic acid and 1ꢀ
charge of the carbonyl carbon and favor form of tetraꢀ octanol were used for each of the recycled esterificaꢀ
hedral intermediate. For 2ꢀchloroethanol (entry 10), tions. As shown in Table 5, in the first recycle converꢀ
the 68.7% conversion of stearic acid is attributed to the sion decrease from 94.6 to 85.3%. It shows that the
pull inductivity of the chlorine, which decrease the activity of Al2(SO4)3 · 18H2O catalyzed esterification
alcohols’ nucleophilicity.
slightly declines. The conversion does not further
decrease in the second and third recycles. The catalyst
Al2(SO4)3 · 18H2O is easily separated from the reaction
system by simple filtration. These results indicate that
Al2(SO4)3 · 18H2O can be expediently recycled without
special treatment, such as calcination or rinse.
Reusability of Catalyst Al2(SO4)3 · 18H2O
In order to investigate the recycle performance of
Al2(SO4)3 · 18H2O catalyst, a recycle esterification of
Effect of Various Reaction Conditions on Esterifications
Table 5. Reusability of catalyst Al2(SO4)3 · 18H2O for esterꢀ
ification of equimolar mixture of stearic acid and 1ꢀoctanol
In order to research effect of reaction variables on
the esterifications and optimize reaction condition,
the solventꢀfree esterification of stearic acid and 1ꢀ
octanol as typical substrates catalyzed by Al2(SO4)3 ·
18H2O was carried out under various conditions.
Number of cycle
Conversation, %
Fresh catalyst
94.6
85.3
85.2
85.3
As shown in Table 6, when the amount of alcohol is
doubled regarding to carboxylic acid, conversion
increases slightly (entry 2). It shows that effect of
molar ratio of carboxylic acid and alcohol on esterifiꢀ
cation is insignificant. The esterifications can
smoothly process in equimolar mixture of carboxylic
acid and alcohol.
1
2
3
Note: Amount of catalyst 10 mol %, reaction time 24 h, temperaꢀ
ture 80 C.
°
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ESTERIFICATIONS OF CARBOXYLIC ACIDS AND ALCOHOLS CATALYZED
Table 6. Effect of various reaction conditions on esterifications catalyzed by Al2(SO4)3 · 18H2O
Carboxylic acid : alcohol Catalyst amount,
563
Entry
Temperature,
°
C
Time, h
Conversion, %
(mol
/
ratio)
mol %
1
2
3
4
5
6
7
8
1 : 1
1 : 2
1 : 1
1 : 1
1 : 1
1 : 1
1 : 1
1 : 1
10
10
10
10
5
80
80
70
90
80
80
80
80
24
24
24
24
24
24
18
30
94.6
95.2
79.7
96.1
72.4
95.5
86.3
95.9
15
10
10
It is well known that temperature is a key factor for
organic chemical reaction. Similarly, temperature can
evidently influence on the esterifications: when reacꢀ
tion temperature is 70°C (entry 3), conversion
(79.7%) is obviously lower than at 80°C (94.6%).
When temperature is 90°C, the conversion only adds
up to 96.1% (entry 4). It indicates that 80°C is approꢀ
priate reaction temperature. When temperature is
below 80°С, conversion decreases; when temperature
is over 80°C, energy is waste.
We further investigated effect of reaction time on
the conversion under fixed other conditions (entries 7,
8). The conversion evidently increases from 86.3 to
94.6% when reaction time is prolonged from 18 to 24 h.
However, when reaction time is prolonged from 24 to
30 h, the increasing of conversion is only 1.3%. It
shows that the esterification is basically close to balꢀ
ance for 24 h. So, we think that optimum condition of
the esterification should be as follows: 1 : 1 molar ratio
of carboxylic acid to alcohol, 10 mol % of Al2(SO4)3 ·
18H2O catalyst, 24 h reaction time and 80°C reaction
temperature.
Solid Lewis acids such as Cu2+, Mg2+, Al3+, and
Zr2+ have been used as catalysts for esterification in a
few works [31–33]. Barbosa et al. provided a plausible
mechanism for esterification reaction on SiO2/ZnCl2
[21]. Based on previous works, we presume the mechꢀ
anism of Al2(SO4)3 · 18H2O catalyzed esterifications of
carboxylic acids and alcohols as shown in Scheme 1.
Effect of catalyst amount on reaction is also invesꢀ
tigated. When amount of catalyst Al2(SO4)3 · 18H2O
used is 5, 10, and 15 mol %, reaction conversions are
72.4, 94.6, and 95.5%, respectively (entries 5, 1,
and 6). The increasing of amount of catalyst used over
10 mol %, weakly raise the conversion. When amount
of catalyst used is below 10 mol %, decreasing of reacꢀ
tion conversion is evident. 10 mol % should be an
appropriate amount.
· 18H2O
Al2(SO4)3
O
O
+ Al2(SO4)3 · 18H2O
R
C
R C
OH
OH
R
R'
R
R'
H
· 18H2O
Al2(SO4)3
O−
Al2(SO )
4 3 · 18H2O
O+
C
O
C
O
OH
H HO
R
C
O
–H2O
O−
Al2(SO4)3 · 18H2O
+ Al2(SO4)3 · 18H2O
OR'
R'O
C
R
OH+2
Scheme 1. Proposed mechanism of esterification catalyzed by Al2(SO4)3 · 18H2O.
KINETICS AND CATALYSIS Vol. 51
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564
LI GANG, PANG WENHUI
This mechanism is described as follows: the carbonyl double bond. The esterification, does not demand any
oxygen of the carboxylic group combines with
Al2(SO4)3 · 18H2O to form an activated complex,
which is attacked by the nucleophile alcohol to form a
tetrahedral intermediate. The product ester is
obtained after the intermediate eliminates water.
The results of our research show that Al2(SO4)3 ·
18H2O can effectively catalyze esterifications of variꢀ
ous carboxylic acids and alcohols, including substrate
solvent and drier and one of the reactants excess. The
Al2(SO4)3 · 18H2O catalyst is a low toxicity, lowꢀcost
compound, and is easily separated from the reaction
mixture by simple filtration. Moreover, the catalyst
can be recycled and the conversion does not evidently
decrease. So Al2(SO4)3 · 18H2O compound should be
a promising green catalyst for esterification of carboxꢀ
with functional group, such as phenyl, halogen and ylic acids and alcohols.
APPENDIX
IR Spectra of Part Product Esters
Transmittance, %
100
Transmittance, %
100
90
80
70
60
50
40
30
20
80
60
40
20
0
4000 3500 3000 2500 2000 1500 1000 500
10
4000 3500 3000 2500 2000 1500 1000 500
1
Wavenumber, cm−
1
Wavenumber, cm−
Fig. 1. IR spectrum of 1ꢀamyl benzoate.
Fig. 2. IR spectrum of 1ꢀoctyl chloroacetate.
Transmittance, %
100
Transmittance, %
100
90
80
70
60
50
40
80
60
40
20
0
4000 3500 3000 2500 2000 1500 1000 500
30
4000 3500 3000 2500 2000 1500 1000 500
1
Wavenumber, cm−
1
Wavenumber, cm−
Fig. 3. IR spectrum of 1ꢀoctyl stearate.
Fig. 4. IR spectrum of 1ꢀoctyl oleate.
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ESTERIFICATIONS OF CARBOXYLIC ACIDS AND ALCOHOLS CATALYZED
565
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80
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0
4000 3500 3000 2500 2000 1500 1000 500
1
Wavenumber, cm−
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ACKNOWLEDGMENTS
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KINETICS AND CATALYSIS Vol. 51
No. 4
2010