Liquid phase transesterification of methyl salicylate and phenol over solid acids: Kinetic studies

Article abstract of DOI:10.1016/j.molcata.2007.03.066

The liquid phase transesterification of methyl salicylate and phenol to synthesize a value added ester, salol has been studied over the catalysts, ZrO₂ (Z), SO₄^2-/ZrO₂ (SZ) and Mo(VI)/ZrO₂ (MZ). Physical and chemical characterization of the catalysts in the form of sulfur and Mo(VI) contents, BET surface area, total surface acidity and powder XRD were conducted. Effects of various parameters, such as speed of agitation of reaction mixture, mole ratio of reactants, catalyst weight, reaction temperature and reaction time were studied to optimize the reaction conditions. ZrO₂ and Mo(VI)/ZrO₂ were 100% selective towards the formation of salol, but in the case of SO₄^2-/ZrO₂ diphenyl ether was also formed as a by-product. The conversion of phenol was correlated with surface acidity of the catalysts and the structure of ZrO₂. The conversion of phenol was found to increase with an increase in reaction temperature from 393 to 423 K and also increased with an excess of methyl salicylate in the reacting system. The kinetic studies suggested that the reaction follows the Eley-Rideal model and a possible reaction mechanism is proposed. The energy of activation of the catalysts in the reaction followed the order Z > MZ ≥ SZ.

Full text of DOI:10.1016/j.molcata.2007.03.066

Journal of Molecular Catalysis A: Chemical 273 (2007) 55–63
Liquid phase transesterification of methyl salicylate and
phenol over solid acids: Kinetic studies
S.Z. Mohamed Shamshuddina,∗, N. Nagarajub
a Department of Chemistry, University College of Education, P.O. Box 1933, Zanzibar, Tanzania
b St. Joseph’s College Research Center, 46 Langford Road, Shanthinagar, Bangalore 560 027, India
Received 17 November 2006; received in revised form 22 March 2007; accepted 23 March 2007
Available online 30 March 2007
Abstract
The liquid phase transesterification of methyl salicylate and phenol to synthesize a value added ester, salol has been studied over the catalysts,
ZrO2 (Z), SO42−/ZrO2 (SZ) and Mo(VI)/ZrO2 (MZ). Physical and chemical characterization of the catalysts in the form of sulfur and Mo(VI)
contents, BET surface area, total surface acidity and powder XRD were conducted. Effects of various parameters, such as speed of agitation of
reaction mixture, mole ratio of reactants, catalyst weight, reaction temperature and reaction time were studied to optimize the reaction conditions.
ZrO2 and Mo(VI)/ZrO2 were 100% selective towards the formation of salol, but in the case of SO42−/ZrO2 diphenyl ether was also formed as a
by-product. The conversion of phenol was correlated with surface acidity of the catalysts and the structure of ZrO2. The conversion of phenol was
found to increase with an increase in reaction temperature from 393 to 423 K and also increased with an excess of methyl salicylate in the reacting
system. The kinetic studies suggested that the reaction follows the Eley–Rideal model and a possible reaction mechanism is proposed. The energy
of activation of the catalysts in the reaction followed the order Z > MZ ≥ SZ.
© 2007 Elsevier B.V. All rights reserved.
Keywords: Solid acids; Salol; Zirconia and its modified forms; Transesterification; Kinetics and mechanism
1. Introduction
handling and transportation. Moreover, high product selectivity
and reusability of the catalysts cannot be expected. Hence, there
are numerous global efforts to replace these environmentally
harmful chemicals with more eco-friendly and less expensive
catalysts [5]. Solid acids, such as zeolites, cation exchange
resins, metal oxides and AlPOs and their modified forms have
been found to be safe alternatives for such conventional liquid
acid catalysts used in synthetic organic chemistry in petroleum
refining, fine chemicals synthesis, pharmaceutical industries,
etc. [6–8].
The salicylate esters are an important group of esters used
for the synthesis of drugs, food preservatives, plasticizers, phar-
maceuticals, solvents, perfumes and cosmetics [9]. Generally,
phenyl esters of salicylic acid, such as phenyl salicylate which
has a commercial name ‘salol’ are prepared under liquid phase
by refluxing the reactants (carboxylic acid and phenol) in the
presence of small amount of concentrated H2SO4, HCl, POCl3
or sulphonic acid as the catalysts [10]. Recently, we have used
zeolitesandsulfatedoxidesascatalystsinthesynthesisofphenyl
salicylate (salol) via esterification of phenol and salicylic acid.
The catalysts showed a decrease in their activity when they were
reused, due to the deactivation of the catalytic centers by the
Transesterification is one of the classic organic reactions that
have enjoyed numerous laboratory uses and industrial applica-
tions. Organic chemists make use of this reaction quite often as
a convenient means to prepare esters.
Transesterification is an equilibrium process, where a car-
boxylic ester is transformed into another through interchange
¨
of the alkoxy moiety in the presence of Bronsted acidic cata-
lysts, such as sulfuric, sulfonic, phosphoric, hydrochloric acids
or basic catalysts like sodium and potassium alkoxides, metal
acetates, metal carbonates, etc. [1–4].
Although many effective and reliable methods for the prepa-
ration of aromatic esters exist, there is still an ample scope for
research towards finding eco-friendly and economically viable
processes. The use of the above-mentioned liquid catalysts is
undesirable from the environmental point of view as these chem-
icals are corrosive and generally encounter the problems of
∗
Corresponding author.
E-mail address: Em Es@rediffmail.com (S.Z.M. Shamshuddin).
1381-1169/$ – see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2007.03.066

56
S.Z.M. Shamshuddin, N. Nagaraju / Journal of Molecular Catalysis A: Chemical 273 (2007) 55–63
water formed as the by-product during esterification reaction [8].
Thus, the anhydrous condition prevalent in transesterification of
methyl salicylate and phenol is an additional advantage.
(a) Sulfated zirconia was prepared by impregnating 3 g of
hydrated zirconia with 1.5 mL of 3 M H2SO4. The mixture
was thoroughly mixed to form a paste which was then dried
in an air oven at 393 K for 12 h [6].
A simple mechanism of transesterification of an ester with
phenol includes the following steps. In the first step, the ester
accepts a proton from the acid site of a solid acid. The sec-
ond step is the nucleophilic attack of a phenol molecule to the
carbonyl-carbon of ester to give an intermediate. In the final
step, a proton is transferred from one oxygen to another to form
another intermediate, which further loses a molecule of alcohol
and proton to give a phenyl ester. All these steps are reversible.
The goal of the present work was to develop an environ-
mentally benign catalyst system that combines good catalytic
performance with satisfactory recovery of the catalysts used.
ZrO2 is an interesting catalytic material and can be prepared in
manywayswithvariousmodifications. Itpossessesacidic, basic,
oxidizing and reducing properties on the surface and these four
properties, including phase modifications (monoclinic or tetrag-
onal) change independently with the method of preparation and
heat treatment. The acid–base properties of zirconia can be mod-
ified by the addition of anionic or cationic substances. Generally,
it has been observed that the acidic properties of the modified
forms of ZrO2 will be higher than ZrO2 itself [11].
Here, we report the use of zirconia and sulfate and molyb-
denum(VI) ions modified zirconia as catalysts in the liquid
phase transesterification of methyl salicylate with phenol. The
results obtained were very encouraging in the sense that the
solid acids not only showed ∼100% selectivity but also high
activity towards the formation of salol. The effect of molar
ratio of the reactants, catalyst weight, reaction temperature and
reaction time on the yield of salol has been studied. Kinetic
data on the transesterification reaction over solid acids are not
widely available. In this work, we have tried to obtain various
kinetic parameters in the transesterification of methyl salicylate
and phenol. The Langmuir–Hinshelwood (LH) and Eley–Rideal
(ER) models are commonly used to correlate the kinetic data for
the solid acid catalyzed reactions [12–14]. These two models
are derived based on the assumption that the rate limiting step
is the surface reaction between two adsorbed molecules (LH) or
between an adsorbed molecule and a molecule in the bulk (ER).
We have tried to fit the kinetic data into LH and ER models and
describe the reaction mechanism for the transesterification of
methyl salicylate and phenol based on the best fit.
(b) Similarly, 2%Mo(VI)/ZrO2 was prepared by impregnat-
ing 3 g of hydrated zirconia support with 0.2 g of (NH4)6
Mo7O24·4H2O [15].
The hydrated zirconia and their modified forms were calcined
to 823 K for 5 h in a muffle furnace before their use as catalysts.
Thus, prepared catalysts were denoted by (Z) for ZrO2, (SZ) for
SO42−/ZrO2 and (MZ) for 2%Mo(VI)/ZrO2.
2.2. Catalyst characterization
All the catalysts were analyzed for sulfur and molybde-
num(VI) content, BET surface area, total surface acidity and
powder XRD. The amount of sulfur in sulfated catalysts was
obtained by conducting elemental analysis using an Elementar
Vario EL III Carlo Erba 1108 instrument. The Mo(VI) con-
tent in MZ sample was determined by energy dispersive X-ray
(EDX) analysis using a Stereo scan 440 apparatus. The BET
surface area of the samples was measured using nitrogen as the
adsorbent in a NOVA-1000 high speed gas sorption analyzer
version 3.70. The total surface acidity was measured by NH3-
TPD measurement on an AutoChem-2910 instrument and also
by the n-butylamine back titration method [16], using dry ben-
zene as solvent and bromothymol blue as indicator. The X-ray
powder diffraction patterns of all the samples were collected on
a Siemens-D5005 X-ray diffractometer with a Ni filtered Cu K␣
˚
radiation (1.5418 A).
2.3. Catalytic activity studies
The catalytic activity of all the catalysts was determined in
liquid phase transesterification of methyl salicylate with phenol
in a 100 mL round bottomed (RB) flask fitted with water-cooled
condenser in an oil bath with continuous stirring. Methyl salicy-
late (MS), phenol (P) and the catalyst were taken in the RB flask
and heated for a definite period. The total volume of the reaction
mixture was kept constant at 15 mL in all the reactions. After a
definite period of time, the reaction mixture was cooled to room
temperature and filtered. The filtrate containing the reactants and
theproductswasanalyzedquantitativelybyagaschromatograph
fitted with a (10%SE-30 chromosorb w-AW, 3 m × 1/8 in.) col-
umn coupled with a FID detector and qualitatively by GC–MS
(Varian). The various parameters, such as molar ratio of the reac-
tants, amount of the catalyst, reaction temperature and reaction
time were varied to optimize the reaction conditions.
The kinetic studies were conducted in a temperature range
from 393 to 423 K. The reactions were carried out by follow-
ing the procedure as mentioned in the previous paragraph, by
varying the amount of catalysts (0.1–1.5 g) and reaction time
(0.5–12 h). The reusability of the catalysts was also studied by
using the spent catalyst in the next consecutive reaction cycles
after washing them with acetone and calcining in a furnace for
2 h at 823 K.
2. Experimental
2.1. Preparation of the catalytic materials
Hydrated ZrO2 was obtained by the following precipitation
method. Twenty-five grams of ZrOCl2·8H2O was dissolved in
250 mL deionised water. To this clear solution, aqueous ammo-
nia was added drop-wise from a burette with constant stirring.
Thus, obtained precipitate of Zr(OH)4 was washed thoroughly
to remove the soluble ions and dried in an air oven at 393 K for
12 h.
Sulfate or Mo(VI) ion modified zirconia was prepared as
follows:

S.Z.M. Shamshuddin, N. Nagaraju / Journal of Molecular Catalysis A: Chemical 273 (2007) 55–63
57
Table 1
Physico-chemical properties of solid acids
Catalyst
Total surface acidity
(mmol/g)
BET surface
area (m2/g)
Sulfur (or)
Mo(VI) (%)
Z
SZ
MZ
0.40 (0.44)
1.15 (1.09)
1.00 (1.01)
71
149
116
–
2.2
2.1
Note: Numbers in the parenthesis correspond to the total surface acidity values
obtained by n-butyl amine back titration method.
3. Results and discussion
3.1. Characterization of catalysts
The sulfur and Mo(VI) contents, BET surface area and total
surface acidity of all the catalysts used in this investigation are
presented in Table 1. The sulfur and Mo(VI) contents showed
good agreement with the theoretical values within an error of
2%. This error is due to the reduction in the weight as a result
of dehydration of the support upon calcination.
The specific surface area of pure ZrO2 was found to be
71 m2/g which was almost doubled on modification with sul-
fate or Mo(VI) ions. The increase in the surface area of sulfated
zirconia was attributed by Arata [17] to cracking of ZrO2 into
fine particles upon treatment with sulfate ions, whereas for the
MZ sample, the observed increase in the specific surface area
was explained based on the formation of Mo–O–Zr linkages
resulting in a porous material [18].
Fig. 1. Powder X-ray diffraction patterns of ZrO2 (Z), SO42−/ZrO2 (SZ) and
Mo(VI)/ZrO2 (MZ) samples calcined at 823 K: M—monoclinic ZrO2 and
T—tetragonal ZrO2.
Temperature programmed desorption (TPD) of ammonia is
a popular method for the determination of acidity of solid cata-
lysts. The total acidity (total acid amount) of the catalysts which
were used in our study follows the order; Z < MZ ≤ SZ. This
order is in good agreement with the already published results
[19]. The acid sites on zirconia have been classified as ‘medium’,
‘strong’ and ‘very strong’, based on the temperature at which
NH3 molecules desorbs from its surface [20]. It is observed that
incorporation of either SO42− or Mo(VI) ions on the surface of
ZrO2 results in a considerable increase in the total surface acid-
ity. This increase in the acidity may be due to the creation of new
acid sites with different strengths. It is also reported that pres-
ence of SO42− ions on ZrO2 results in the creation of acid sites
that belong to ‘strong’ and ‘very strong’ categories [19]. How-
ever, Mo(VI) promotes the formation of sites with ‘medium’ and
‘strong’ acid strengths [15].
lished polymorphs; monoclinic, tetragonal and cubic phases
[21]. The zirconia used in the present study found to exhibit
monoclinic and tetragonal phases (Fig. 1). The sharp diffrac-
tion lines at (2θ = 24.7, 28.4, 31.6) are due to the monoclinic
form. The sulfate- or Mo(VI)-modified forms consist of more of
the tetragonal phase than the monoclinic phase; it can be clearly
observed in the figure that the peaks representing the monoclinic
phase significantly decreases, whereas those of tetragonal phase
(2θ = 30.3, 35.3, 50.7) increases in intensity. We have reported
a detailed study on the powder XRD phases of zirconia and
its modified forms [22]. No diffraction lines corresponding to
MoO3 or any compound formation between MoO3 and ZrO2
(i.e., ZrMo2O8) are observed.
The powder X-ray diffractograms of Z, SZ and MZ are
presented in Fig. 1. Zirconia is known to exhibit three well estab-
3.2. Catalytic activity studies
The catalytic activity of the calcined Z, SZ and MZ was exam-
ined in the transesterification reaction of methyl salicylate with
phenol (Table 2). The GC and GC–MS analyses showed the
presence of only four components, i.e., methyl salicylate, phe-
nol, salol and methanol over Z and MZ catalysts. In the presence
of SZ, in addition to salol and methanol, diphenyl ether was also
formed as the by-product. The catalytic activity of a solid acid
catalyst in the formation of a definite product in an organic reac-
tion depends on the acid strength and concentration of active
sites present on the solid acid.
Table 2
Activity of different catalysts for the synthesis of ester and ether
Catalyst
Conversion of
phenol (%)
Selectivity for
salol (%)
Selectivity for
ether (%)
Z
SZ
MZ
36
54
50
100
91
100
0
9
0
Molar ratio of MS:P = 1:1; reaction temperature = 403 K; reaction time = 4 h;
amount of catalyst = 1.0 g.

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S.Z.M. Shamshuddin, N. Nagaraju / Journal of Molecular Catalysis A: Chemical 273 (2007) 55–63
Fig. 2. Effect of methyl salicylate:phenol molar ratio. Catalyst weight = 1.0 g; reaction temperature = 403 K; reaction time = 4 h. (ꢀ) Conversion of phenol; (ꢁ)
selectivity to salol; ( ) selectivity to diphenyl ether.
When the reaction was carried out without any catalyst,
no conversion of phenol and methyl salicylate into salol was
observed. This suggests that the reaction is a catalyzed reac-
tion. All the catalysts were found to be active towards the
are associated only with SZ are catalytically active for diphenyl
ether formation. As has already been reported [13,24], the for-
mation of ether by acid catalyzed dehydration of an alcohol
requires ‘very strong’ acid sites.
formation of salol. It was further observed that the presence
2−
of SO4
or Mo(VI) ions on ZrO2 has increased the con-
3.2.1. Effect of molar ratio of the reactants
version of phenol to salol. A correlation of the surface area
and surface acidity with the catalytic activity showed the same
trend.
The effect of molar ratio of reactants on the conversion of
phenol and selectivity towards salol was studied by varying the
molar ratio of methyl salicylate to phenol from 2:1 to 1:2. It was
found that an increase in the concentration of methyl salicylate
increased the conversion of phenol (Fig. 2). This may be due
to the fact that transesterification is a reversible reaction and
excess of methyl salicylate favors the forward reaction, leading
to the formation of the desired product. On the other hand, it was
found that an increase in the concentration of phenol decreases
its conversion into salol, possibly because of the deactivation of
the catalyst. Z and MZ catalysts were 100% selective towards the
formation of salol. But in the case of SZ catalyst the selectivity
towards salol was decreased accompanied with a decrease in the
selectivity for diphenyl ether. However, the effect was only to
an extent of ∼4%.
An attempt to identify the different phases of zirconia respon-
sible for the formation of salol indicated that both monoclinic
and tetragonal phases were associated with the catalytically
active sites responsible for the formation of salol. However, the
powder XRD suggests that an increase in the tetragonal phase of
zirconiaisresponsibleforhighertransesterificationactivity [23].
Formation of diphenyl ether indicates that SZ is associated with
active centers with ‘very strong’ acid strengths which are absent
in Z and MZ catalysts. Thus, these studies have indirectly pro-
vided information that the ‘medium’ and ‘strong’ acid sites are
present on the surface of Z, SZ and MZ, which are responsible
for the salol formation and that the ‘very strong’ acid sites that
Fig. 3. Effect of catalyst weight. MS:P = 1:1; reaction time = 4 h; reaction temperature = 403 K. (ꢀ) Conversion of phenol; (ꢁ) selectivity to salol; ( ) selectivity to
diphenyl ether.

S.Z.M. Shamshuddin, N. Nagaraju / Journal of Molecular Catalysis A: Chemical 273 (2007) 55–63
59
Fig. 4. Effect of reaction temperature. MS:P = 1:1; catalyst weight = 1.0 g; reaction time = 4 h. (᭹) Conversion of phenol; (ꢀ) selectivity to salol; (ꢂ) selectivity to
diphenyl ether.
3.2.2. Effect of amount of catalyst
in the conversion of phenol was observed on increasing the
reaction temperature over all the catalysts. Selectivity towards
salol remains 100% over both Z and MZ catalysts. However, in
the case of SZ catalyst, the selectivity towards salol decreased
from 96 to 88% accompanied with an increase in the selectivity
towards diphenyl ether from 4 to 12%. This suggests that over
SZ, high temperatures favor the ether formation.
The effect of catalyst weight on the conversion (%) of phe-
nol was checked. The amount of catalyst was varied from 0.1
to 1.5 g. As illustrated in Fig. 3, an increase in the amount of
catalyst substantially increased the conversion of phenol. The
higher the catalyst loading, the greater the conversion of phenol.
A maximum conversion of phenol (70%) with 100% selectivity
towards salol was observed with 1.5 g (MZ) of the catalyst in 4 h
at 403 K. In the case of SZ the conversion of phenol increased
from 7 to 71% on increasing the weight of SZ from 0.1 to 1.5 g.
But no reasonable change in the selectivity towards either salol
or diphenyl ether was observed, i.e., the selectivity towards salol
was ∼91% and selectivity towards diphenyl ether was ∼9%.
3.2.4. Effect of reaction time
Reactions were also carried out at different reaction (reflux-
ing) times. The reaction time was varied from 0.5 to 12 h. The
conversion of phenol increased rapidly in the beginning and
gradually leveled off after 4 h (Fig. 5). For example, over MZ, the
conversion increased from 15% in the first 0.5 h to around 54%
in the next 4 h and the conversion increased only to an extent
of 61% in 12 h. The selectivity towards the salol was 100%
with both Z and MZ catalysts. In the case of SZ, the conver-
sion increased from 16 to 55% in the first 4 h and the selectivity
3.2.3. Effect of reaction temperature
The effect of reaction temperature on the conversion of phe-
nol and the selectivity towards the ester or ether was studied over
the temperature range from 393 to 423 K (Fig. 4). An increase
Fig. 5. Effect of reaction time. MS:P = 1:1; reaction temperature = 403 K; catalyst weight = 1.0 g. (᭹) Conversion of phenol; (ꢀ) selectivity to salol; (ꢂ) selectivity
to diphenyl ether.

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S.Z.M. Shamshuddin, N. Nagaraju / Journal of Molecular Catalysis A: Chemical 273 (2007) 55–63
Fig. 6. Plot of first-order equation for the transesterification of methyl salicylate with phenol over ZrO2, SZ and MZ at 393 K (ꢃ), 403 K (᭹), 413 K (ꢂ) and 423 K
(ꢀ).
towards either salol or diphenyl ether remained almost the same.
The reaction period of 4 h seems to be good to get a reasonable
conversion of phenol to salol.
(and etherification in case of SZ only) to be a first-order reaction.
The first-order rate constants obtained from the slopes of these
plots, the energies of activation calculated from the Arrhenius
equation and the temperature coefficients are given in Table 3.
The energy of activation for transesterification are in the order;
3.3. Kinetic studies
Z > MZ ≥ SZ
Prior to the kinetic studies, experiments were conducted to
establish the effect of heat and mass transfer limitations during
the transesterification reaction.
Zirconia has the highest energy of activation among the cat-
alysts used in our case study. As far as SZ and MZ catalysts
are concerned, the energy of activation are almost identical,
which means that the reaction proceeds under the same kinetic
mechanism, including the active sites required for the formation
of salol. This indicates that transesterification is equally facile
over SZ and MZ. The energy of activation for the formation of
The effect of heat transfer limitations was studied by carrying
out the experiments at different agitation rates of the reaction
mixture. Not much change in the conversion of phenol was
observed when the rate of agitation was varied, i.e., conversion
of phenol was independent of the rate of agitation. This indicates
the absence of heat transfer limitation and therefore, it could be
suggested that any external resistance to its transfer from the
bulk liquid phase to the external surface of the catalyst is absent.
The reactions were carried out by varying the weight of cat-
alyst. An increase in the conversion of phenol with an increase
in the catalyst weight was observed (Fig. 3). This indicates that
the resistance to mass transfer is negligibly small between liquid
bulk of the reactant and the outer surface of the catalyst parti-
cles; according to Madon and Boudart [25] in the absence of
all transport limitations, the rate of a reaction is proportional to
the number of active sites, which in turn is proportional to the
weight of the catalyst.
diphenyl ether over SZ catalyst was found to be 91 kJ mol−1
.
Plots of −ln[1 − Yieldsalol] versus reaction time for the
transesterification reactions carried out at different reaction tem-
peratures ranging from 393 to 423 K over Z, SZ and MZ are
given in Fig. 6. Since diphenyl ether was also formed over SZ,
the transesterification and etherification were treated separately
to get the kinetic data (Fig. 7). The first-order plot for the ether-
ification in the presence of SZ is shown in Fig. 8. The plots are
nearly linear in all the cases, indicating the transesterification
Fig. 7. Plot of first-order equation for the etherification of phenol over SZ at
393, 403, 413 and 423 K.

S.Z.M. Shamshuddin, N. Nagaraju / Journal of Molecular Catalysis A: Chemical 273 (2007) 55–63
61
Fig. 8. Effect of methyl salicylate concentration on the initial reaction rate. Reaction temperature = 403 K; catalyst weight = 1.0 g.
Table 3
Reaction rate constants (10−3 min−1), energy of activation (kJ/mol−1) and temperature coefficients for transesterification and etherification
Catalyst
Rate constant (salol)
Rate constant (ether)
Energy of activation (Ea) for the formation of;
Temperature coefficient (k2/k1)
393 K (k1)
403 K (k2)
393 K (k1)
403 K (k2)
Salol
Ether
Salol
Ether
Z
SZ
MZ
1.1
2.1
2.3
1.7
2.9
3.1
–
0.1
–
–
0.2
–
57.31
37.87
39.31
–
1.54
1.33
1.35
–
2.00
–
91.28
–
MZ is a preferred catalyst for salol synthesis due to its 100%
selectivity towards the salol. This also suggests that, transesteri-
fication of methyl salicylate and phenol to prepare salol requires
‘medium’ or ‘strong’ acid sites which are present in all the
catalysts used for our work.
of phenol. On the other hand, an increase in the concentration
of methyl salicylate increased the yield of salol with negligible
deactivation (poisoning of active sites) of the catalyst. We did
not observe any deactivation or color change of the catalyst even
at a molar ratio of MS:P = 1:1. Thus, we can say that it is methyl
salicylate which is adsorbing on the active sites of the catalyst
and not phenol.
3.4. Mechanism of reaction
Since, there is no mass transfer limitation on the catalyst sur-
face as mentioned already, the chemical step was assumed to
be the rate limiting step. If the reaction proceeds through a LH
mechanism, then a plot of rate versus concentration must pass
through a maximum, according to Narayanan and co-workers
[27], while if it follows an ER mechanism, no such maximum
is encountered. Fig. 8 indicates that the initial reaction rate
increases linearly with methyl salicylate concentration. This
would suggest that the transesterification of methyl salicylate
with phenol follows an ER mechanism.
Based on the above assumptions a possible mechanism for
the transesterification of adsorbed methyl salicylate and phenol
over solid acid catalysts is given in Scheme 2, which is sim-
ilar to the mechanism of acid catalyzed Fischer esterification
[28].
In general, methyl salicylate reacts with phenol in the pres-
ence of an acid catalyst [X] to give the ester, salol as shown in
Scheme 1. This acid catalyzed transesterification is an equilib-
rium chemical reaction.
The transesterification reaction can take place by follow-
ing either a Langmuir–Hinshelwood model or an Eley–Rideal
model. The kinetics of these two mechanisms does vastly differ.
For example, LH kinetics involves a reaction between methyl
salicylate adsorbed on the catalyst surface and phenol which is
adsorbed on an adjacent site. In contrast, the reaction follows an
ER mechanism, when only one of the reactant species (methyl
salicylate or phenol) is bound on the catalyst surface and is con-
verted to product when the other impinges upon it from the gas
phase.
It has been reported in literature [26] that adsorbed phe-
nol does not react, only poisons the catalytic sites. Therefore,
it is assumed that phenol from the liquid phase reacts with
adsorbed methyl salicylate. Furthermore, a decrease in the rate
was observed with an increase in the concentration of phenol;
this can be attributed to the poisoning of the active sites of the
catalyst which was further proved by the change in the color of
the catalyst from white to grey on increasing the concentration
3.5. Reusability of the catalysts
In order to study the reusability of the catalysts, the solid
catalyst recovered after the reaction by filtration was washed
with acetone and calcined at 823 K in a furnace for 2 h. Thus,
recovered catalyst was reused in the transesterification reaction.
The experiments were repeated three times by following the
same procedure and the variation in the conversion of phe-
nol are given in Table 4. The data in the table shows that
there is no considerable change in the catalytic activity of
the Z and MZ catalysts. However, the SZ catalyst showed
a substantial decrease in its activity after each cycle of the
Scheme 1. Transesterification of methyl salicylate with phenol in presence of
an acid catalyst [X].

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S.Z.M. Shamshuddin, N. Nagaraju / Journal of Molecular Catalysis A: Chemical 273 (2007) 55–63
Scheme 2. Transesterification of methyl salicylate and phenol over solid acids (protonic acid sites).
Table 4
Eley–Rideal mechanism. A possible mechanism for the trans-
esterification of methyl salicylate and phenol over acid sites is
proposed.
Reusability of the catalysts in the transesterification reaction
Catalyst
Conversion (%) phenol
Fresh
First cycle
Second cycle
Third cycle
Acknowledgements
Z
SZ
MZ
36
54
50
34
50
47
33
45
47
33
36
46
The authors gratefully acknowledge the help received from
Bangalore Institute of Technology in the surface area analysis
of the samples, IISc authorities for lending their library facilities
and powder XRD analysis and one of the authors (MS) thank
HMSIT and UCEZ authorities for financial support.
Molar ratio of MS:P = 1:1; reaction temperature = 403 K; reaction time = 4 h;
amount of catalyst = 1.0 g.
reaction. Thus, we can conclude that the Z and MZ catalysts
are efficient and that they can be reused atleast upto three
cycles.
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