Esterification of salicylic acid using Ce4+ modified cation-exchange resin as catalyst

Article abstract of DOI:10.4067/S0717-97072012000400029

The esterification of salicylic acid with methanol was carried out over a series of Ce⁴⁺ modified cation-exchange resins. The effect of different reaction conditions was studied on the conversion of salicylic acid, and the optimal reaction parameters were obtained. The experimental results indicated that Ce(SO₄)₂/001×7 was an effective catalyst for the synthesis of methyl salicylate. The conversion of salicylic acid could reach 93.3% while its selectivity was more than 99.0%. SEM-EDS and TG-DSC analysis were employed to characterize the structure and property of the catalyst. Besides, the catalytic performance of Ce(SO₄) ₂/001×7 in the esterification of salicylic acid with different alcohols was compared. The reusability of Ce(SO₄)₂/ 001×7 was also studied by using salicylic acid and methanol as model substrates. The mechanism was proposed for the esterification of salicylic acid with methanol over Ce⁴⁺ modified cation-exchange resins.

Full text of DOI:10.4067/S0717-97072012000400029

J. Chil. Chem. Soc., 57, Nº 4 (2012)
4+
ESTERIFICATION OF SALICYLIC ACID USING Ce MODIFIED CATION-EXCHANGE RESIN AS CATALYST
*
MING ZHANG, WEN SHUAI ZHU, HUA MING LI , HUA SHI, YONG SHENG YAN, ZHI GAO WANG
School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang 212013, P. R. China
(Received: March 3, 2011 - Accepted: January 24, 2012)
ABSTRACT
4
+
The esterification of salicylic acid with methanol was carried out over a series of Ce modified cation-exchange resins. The effect of different reaction conditions
was studied on the conversion of salicylic acid, and the optimal reaction parameters were obtained. The experimental results indicated that Ce(SO ) /001×7 was
4
2
an effective catalyst for the synthesis of methyl salicylate. The conversion of salicylic acid could reach 93.3% while its selectivity was more than 99.0%. SEM-
EDS and TG-DSC analysis were employed to characterize the structure and property of the catalyst. Besides, the catalytic performance of Ce(SO ) /001×7 in the
4
2
esterification of salicylic acid with different alcohols was compared. The reusability of Ce(SO ) /001×7 was also studied by using salicylic acid and methanol as
4
2
4
+
model substrates. The mechanism was proposed for the esterification of salicylic acid with methanol over Ce modified cation-exchange resins.
Keywords: Salicylic acid, Esterification, Metal modified, Cation-exchange resin
INTRODUCTION
2. Sample preparation
Modified cation-exchange resin Ce(SO ) /001×7 was prepared as follows:
4
2
Organic esters which represent peculiarly medicinal and chemical
characteristics have been extensively used in the synthesis of drugs, plasticizers,
food preservatives, pharmaceuticals, solvents, perfumes, cosmetics, and chiral
1 mL of 98 wt.% sulfuric acid was added into 20 mL of distilled water. And
then under vigorous stirring, 1.22 g of Ce(SO ) · 4H O was dissolved in the
4
2
2
above sulfuric acid solution. Finally, 4.0 g of cation-exchange resin 001×7
with pre-treatment was added and stirred for 3 h, aged for 3 d, filtered at room
temperature and activated at 100ºC in a vacuum. The content of ceric sulfate in
1
auxiliaries . Conventional mineral catalysts for esterification reaction included
H SO , HF, H PO , NaHSO and so on. However, these catalysts often suffered
2
4
3
4
4
4
+
from inherent problems of corrosiveness, more byproducts, environmental
catalyst was 20%. For comparison, other Ce modified cation-exchange resins
were synthesized with the similar method.
2
hazards, difficulty in catalyst recovery and reuse, susceptibility to water . A
variety of materials such as clays, zeolites, mesoporous materials, sulfated
metal oxides and heteropolyacids have been developed as solid acid catalysts
3. Catalyst characterization
Thermogravimetric and differential scanning (TG-DSC) analysis was
carried out on a Model STA-449C Jupiter apparatus (Netzsch Corporation,
Selb, Germany). The testing process was 30-1000ºC at a heating rate of 10ºC/
3
, 4
in esterification reactions . In esterification reactions, their main advantages
in comparison to conventional mineral catalysts were non-corrosive and easy
5
to be separated from the reaction mixture for reuse . However, these catalysts
min under a N atmosphere.
2
had their own shortcomings such as the hybrid acid sites and deconcentrated
acidity, leading to low selectivity toward target product . To solve these
SEM-EDS measurements were conducted on catalysts using a field-
emission scanning electron microscope (Hitachi, S-4800) equipped with
an energy dispersive X-ray spectroscope (EDS) operated at an acceleration
voltage of 10 kV.
6
problems, ionic liquids (ILs) have been explored as a new green reaction media
owing to their negligible volatility, excellent thermal stability, and the variety
7
-9
10-
of structures available . Hence, studies about ILs in esterification were hot
.
4. Catalytic activity investigation
13
However, the high cost restricted the application of ILs in esterification
The esterification of salicylic acid (SA) with methanol was carried out
as the model reaction to investigate the catalytic activity. This reaction was
performed at atmospheric pressure and refluxed in a single-neck flask with a
condenser. The reactor was heated in a temperature-controlled oil bath with a
magnetic stirrer. In a typical procedure, the reaction substrates containing 0.01
mol of SA, some methanol and catalyst were heated for hours. After reaction,
diethyl ether was added to dissolve the unreacted SA. The mixture was divided
into two phases. The upper liquid phase consisting of the produced ester and
some unreacted SA was separated by decantation, while the lower solid phase,
the catalyst, could be reused after removal of water.
industry.
Ion-exchange resins, which have a bead structure, the advantages of
solid acid catalysts, and the ability to ion-exchange or chelate metal ions,
have been widely applied in many fields, such as purification of water and
wastewater, separation of heavy metal ions, esterifiation and etherification
14-18
19
20
reactions . Besides, Yonemoto and Li have reported on the biodiesel
production using anion-exchange and cation-exchange resins as heterogeneous
catalysts respectively. Furthermore, several references in the form of research
2
1
articles were available on the application of metal-doped resins. Arnold ,
22
23
Drelinkiewicz , and Kralik reported that metal-doped resins were efficient for
epoxidation, carbon–carbon coupling, hydrogenation, and hydroformylation
reactions. Within the application of esterification, many studies have focused
primarily on the kinetic study of cation-exchange resins catalyzed in the
The composition of products was analyzed by gas chromatography-
flame ionization detector (GC-FID) (Agilent 7890A, HP-5 column, 30 m
× 0.32 mm i.d. × 0.25 μm film thickness). The GC-FID conditions were as
follows: detector temperature, 300ºC; injection port temperature, 300ºC; oven
temperature program, 70-170ºC at a 15ºC/min gradient; injection volume of
sample, 0.4 μL. The products were identified by comparing with the standards.
GC results showed that the major product was methyl salicylate and the minor
side product was phenol. The conversion and selectivity were calculated
2
4-26
synthesis of various esters . A careful survey of literature showed that
there were few detailed studies about metal modified cation-exchange resins
catalyzed in the esterification of salicylic acid.
In the present work, modified cation-exchange resin Ce(SO ) /001×7 was
4
2
2
7
synthesized, characterized and used in the synthesis of salicylate. The catalytic
performance of Ce(SO ) /001×7 in the esterification of salicylic acid with
according to the area of chromatograph peak using the following expressions :
4
2
different alcohols was compared. The reusability of Ce(SO ) /001×7 was also
4
2
examined using salicylic acid and methanol as model substrates. An attempt
was also made to study the mechanism of the esterification reaction.
EXPERIMENTAL
1
. Materials
Cation-exchange resins (D001-cc, 001×7, D61, D72, NKC-9) were
purchased from the Chemical Plant of Nankai University, China. These resins
2
0
were pretreated using methods in the previous literature . All reagents (A.R.)
were purchased from China National Pharmaceutical Group Corporation
(Shanghai) and treated according to standard methods.
e-mail address: lhm@ujs.edu.cn
1477

J. Chil. Chem. Soc., 57, Nº 4 (2012)
RESULTS AND DISCUSSION
1
. TG-DSC analysis
As is described in Fig. 1, TG-DSC analysis was carried out to investigate
the thermal stability of Ce(SO ) /001×7.
4
2
Figure 2 SEM photomicrographs of the surface of the catalysts: (a) resin
01×7, (b) fresh Ce(SO ) /001×7 , and (c) Ce(SO ) /001×7 after the first
0
4
2
4 2
recycling reaction. Magnification: 150.
Figure 1: TG and DSC curves of Ce(SO ) /001×7.
4
2
In the range of 30-100ºC, the weight loss may be caused by the physically
absorbed water in the catalyst. From 100 to 310ºC, the weight loss may be duo
to the partial carbonization of the resin. By further heating (from 310 up to
5
10ºC), a two step endothermic process happened. The weight loss between 310
and 420ºC may be attributed to the loss of the sulfonic groups and short alkyl
28
groups in resin . The weight from 420 to 510ºC was lost in the transformation
of Ce(SO ) into Ce O (SO ) and SO . From 510-700ºC, the decomposition of
4
2
3
2
4
4
3
styrene-divinylbenzene copolymer took place, and finally Ce O (SO ) turned
3
2
4 4
29
into CeO and SO3
.
2
2
. SEM-EDS analysis
SEM photomicrographs of 001×7 and Ce(SO ) /001×7 are presented in
4
2
Fig. 2 and Fig. 3. EDS analysis is carried out on fresh Ce(SO ) /001×7 in the
4
2
SEM equipped with EDS (Fig. 3d).
1
478

J. Chil. Chem. Soc., 57, Nº 4 (2012)
Figure 4 Esterification of salicylic acid with methanol, effect of molar
ratio of salicylic acid to methanol. Reaction conditions: catalyst weight, 0.4 g;
time, 12 h; temperature, 368 K.
Figure 3 SEM photomicrographs of the surface of the catalysts: (a) resin
01×7, (b) fresh Ce(SO ) /001×7, (c) Ce(SO ) /001×7 after the first recycling
0
4
2
4 2
reaction, and (d) EDS of fresh Ce(SO ) /001×7. Magnification: 5000.
4
2
In Fig. 5, the esterification reaction was executed with different amounts
As is shown in Fig. 2 and Fig. 3, SEM photographs are under the
magnification of 150 and 5000, respectively. In Fig. 2a, the surface of resin
of Ce(SO ) /001×7 under the same reaction condition. As the amount of the
4
2
catalyst increased from 0.1 g to 0.5 g, the conversion of SA was improved from
46.7% to 94.1%. This could be attributed to the number of Lewis acid sites
increasing as the catalyst amount increased. When the catalyst concentration
exceeded 0.4 g, the increase of SA conversion was not obvious. In all cases, the
selectivity for methyl salicylate was more than 99.0%.
0
01×7 was smooth like most of gel resins. After the introduction of Ce(SO ) ,
4 2
the surface of Ce(SO ) /001×7 (Fig. 2b) changed obviously in comparison to
4
2
0
01×7. At high magnifications, the surface of Ce(SO ) /001×7 (Fig. 3b) was
4 2
rough, whereas the surface of 001×7 (Fig. 3a) was relatively plain. Compared
with Fig. 2b and Fig. 3b, Fig. 2c and Fig. 3c showed that there were no obvious
changes on the surface of Ce(SO ) /001×7 after esterification reaction. EDS
4
2
analysis also indicated Ce(SO ) existed on the surface of the resin. The Na
4
2
signal in the EDS was from the rudimental sodium ion in the resin after pre-
treatment, and the Au was sprayed when preparing the sample (Fig. 3d).
3
. Effect of different reaction systems
As could be indicated in Table 1, cerium sulfate was effective for the
esterification of salicylic acid. It was clear that catalytic performance of the resin
was obviously improved after modified with Ce(SO ) , and Ce(SO ) /001×7
4
2
4 2
was relatively active. With the incorporation of Ce(SO ) , the total acid sites
4
2
of the catalysts greatly increased and the catalytic activity could be effectively
enhanced. Besides, the selectivity for methyl salicylate was more than 99.0%,
which was closely related to the strong acid strength and the distribution of the
30
catalyst . Ce(SO ) /001×7 was used as a typical catalyst to obtain the optimal
4
2
reaction parameters for the synthesis of methyl salicylate.
a
Table 1: Effect of different esterification systems
SA conversion of different esterification systems (%)
Type of
resins
Entry
Modified with
Ce(SO4)2
Without
Ce(SO4)2
Ce(SO ) only
4
2
1
2
3
4
001×7
D061
93.3
90.6
88.2
81.4
83.5
55.7
48.1
34.3
42.7
52.5
Figure 5 Esterification of salicylic acid with methanol, effect of catalyst
weight. Reaction conditions: molar ratio of SA to methanol, 1:3; reaction time,
2 h; reaction temperature, 368 K.
1
D001-cc
D72
7
6.2
As is shown from Fig. 6, the esterification system composed of salicylic
acid (SA), methanol and the catalyst Ce(SO ) /001×7 was used to research the
4
2
5
NKC-9
influence of various reaction time and temperatures. The conversion of SA
increased at different temperatures with the increase of time. For instance, the
conversion of SA at 368 K increased from 75.1% to 93.3% when the reaction
time was changed from 6 h to 12 h. However, the conversion of SA increased
slowly to 93.8% when reaction time reached 14 h. Besides, the results displayed
that the reaction activity was the best at 368 K for 12 h, and the conversion of
SA could reach 93.3%. However, at 373 K and 363 K, it only got to 91.5% and
a
Reaction conditions: molar ratio of SA to methanol, 1:3; temperature, 368
K; time, 12 h; catalyst weight, 0.4 g.
4
. Optimization of reaction conditions
The esterification reaction was carried out using different molar ratios of
salicylic acid (SA) to methanol varied from 1:2 to 1:6 under the same reaction
condition (Fig. 4). When the molar ratio reached a critical value (1:3), the
conversion of SA for the catalyst Ce(SO ) /001×7 was up to the top level 93.3%.
8
7.7%, respectively.
4
2
In the case of Ce(SO ) /D061 and Ce(SO ) /D001-cc, for the same molar ratio
4
2
4 2
from 1:2 to 1:6, the conversion levels were lower than that of Ce(SO ) /001×7.
4
2
The selectivity for methyl salicylate in all cases remained above 99.0%.
1
479

J. Chil. Chem. Soc., 57, Nº 4 (2012)
Figure 7 Esterification of salicylic acid with methanol, the reusability of
catalyst Ce(SO ) /001×7. Reaction conditions: molar ratio of SA to methanol,
Figure 6 Esterification of salicylic acid with methanol, effect of
temperature and reaction time. Reaction conditions: molar ratio of SA to
methanol, 1:3; catalyst weight, 0.4 g.
4 2
1
:3; temperature, 368 K; time, 12 h; catalyst weight, 0.4 g.
7
. Mechanism of the esterification system
The esterification of salicylic acid with methanol is a nucleophilic
substitution. This reaction was so slow that it needed to be activated by high
From the above study, the suitable condition for the reaction could be
found as follows: molar ratio of SA to methanol, 1:3; temperature, 368 K; time,
3
4, 35
temperature or a catalyst to facilitate the overall reaction
. A possible
1
2 h; catalyst weight, 0.4 g. The selectivity for methyl salicylate in all cases
remained above 99.0%.
. Esterification of different alcohols catalyzed by Ce(SO ) /001×7
mechanism of methyl salicylate formation was illustrated in Scheme 1, which
2
7
involved the nucleophilic attack by methanol on carboxyl . The observed
activity could be explained as follows: On the surface of the resin, SA was
absorbed and activated on the acid sites by its carbonyl oxygen, and then the
formation of methanol attacking the unstabilizing cationic intermediate took
place. After electron transfer reaction happened, the reaction product methyl
salicylate could be obtained.
5
4
2
Data in Table 2 illustrated the catalytic performance of Ce(SO ) /001×7
4
2
in the esterification of salicylic acid with methanol and other kinds of alcohols
under the same condition. For methanol, the SA conversion was the highest,
9
3.3%. Besides, the SA conversions for ethanol, butanol, pentanol, hexanol,
and octanol were much lower, which were 71.5%, 70.6%, 46.0%, 61.3%, and
6.0%, respectively. These results showed that the SA conversion of alcohols
4
3
1
was related to the length of carbon chain and the dielectric constant . Alcohols
with shorter carbon chain gave higher conversion. It was due to the increase
of steric hindrance when the carbon chain grew longer. This would hinder the
esterification reaction to the direction of ester formation. The dielectric constant
indicated how easy a material could be polarized by imposition of an electric
field on an insulating material. The dielectric constants of methanol, ethanol,
butanol, pentanol, hexanol, and octanol were 33.0, 25.3, 17.8, 13.9, 13.3, and
3
2
8
.1 at room temperature respectively . Alcohols with higher dielectric constant
gave higher SA conversion. Hence, the length of carbon chain and the dielectric
constant may have synergistically effect on the esterification of salicylic acid.
a
Table 2 Esterification of different alcohols catalyzed by Ce(SO ) /001×7
4
2
b
Entry
Alcohol
Methanol
Ethanol
Butanol
Pentanol
Hexanol
Octanol
Conversion (%)
Selectivity (%)
99.9
1
2
3
4
5
93.3
71.5
70.6
46.0
61.3
46.0
99.9
90.1
99.9
99.4
6
91.8
a
Reaction conditions: molar ratio of SA to alcohol, 1:3; temperature, 368
K; time, 12 h; catalyst weight, 0.4 g.
b
Selectivity for esters (base on salicylic acid).
6
. Recycling capability of Ce(SO ) /001×7
4 2
The reusability of Ce(SO ) /001×7 was examined using salicylic acid and
4
2
methanol as model substrates. After reaction, Ce(SO ) /001×7 was readily
4
2
separated and washed with diethyl ether for three times. After removal of water
at 80ºC for 6 h, it could be directly used for the next cycle. As is shown in Fig.
7
, the conversion of SA decreased from 93.3% to 80.0% after reusing for 5
Scheme 1 Mechanism for methyl salicylate formation.
times. The decrease of the reusability may be attributed to the swelling effect
of the resin and the loss of active sites after reaction . Besides, the selectivity
for methyl salicylate of all cases was more than 99.0%.
3
3
1
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J. Chil. Chem. Soc., 57, Nº 4 (2012)
CONCLUSION
32. C. Wohlfarth in CRC Handbook of Chemistry and Physics, 90th Edition,
D. R. Lide, W. M. Haynes eds. Taylor and Francis, Boca Raton, 2010; pp.
Section 6, 148-169.
33. Y. Zhang, L. Ma, J. C. Yang, React. Funct. Polym. 61, 101, (2004)
34. Y. M. Liu, W. L. Feng, T. C. Li, H. Y. He, W. L. Dai, W. Huang, Y. Cao,
K. N. Fan, J. Catal. 239, 125, (2006)
From the detailed studies on the esterification of salicylic acid with
methanol, the following conclusion could be derived. A series of Ce
4
+
modified cation-exchange resins were successfully synthesized and showed
high selectivity for methyl salicylate through the esterification of the –COOH
group of salicylic acid while the –OH group remaining nearly unreacted. With
the incorporation of Ce(SO ) , the catalytic activity of the caion-exchange
35. A. Corma, V. Fornes, M. T. Navarro, J. Perez-Pariente, J. Catal. 148, 569,
(1994)
4
2
resins were greatly improved. Besides, the optimal reaction parameters for
the synthesis of methyl salicylate were obtained. A mechanism that involved
salicylic acid being activated on acid sites via its carbonyl oxygen and
nuleophilic attack by methanol on the carboxyl atom was proposed.
ACKNOWLEDGMENTS
This work was financially supported by the National Nature Science
Foundation of China (Nos. 20876071, 21076099, 21106055). Specialized
Research Fund for the Doctoral Program of Higher Education Of China (No.
2
0103227110016). The Natural Science Foundation of Jiangsu Province (No.
BK2011506). Industry Key Item Fund of Zhenjiang City (GY2007031).
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