Vapour phase synthesis of salol over solid acids via transesterification

Article abstract of DOI:10.1007/s12039-010-0022-y

The transesterification of methyl salicylate with phenol has been studied in vapour phase over solid acid catalysts such as ZrO₂, MoO ₃ and SO^2-₄ or Mo(VI) ions modified zirconia. The catalytic materials were prepared and characterized for their total surface acidity, BET surface area and powder XRD patterns. The effect of mole-ratio of the reactants, catalyst bed temperature, catalyst weight, flowrate of reactants, WHSV and time-on-stream on the conversion (%) of phenol and selectivity (%) of salol has been investigated. A good yield (up to 70%) of salol with 90% selectivity was observed when the reactions were carried out at a catalyst bed temperature of 200°C and flow-rate of 10 mL/h in presence of Mo(VI)/ZrO ₂ as catalyst. The results have been interpreted based on the variation of acidic properties and powder XRD phases of zirconia on incorporation of SO^2-₄ or Mo(VI) ions. The effect of poisoning of acid sites of SO^2-₄ or Mo(VI) ions modified zirconia on total surface acidity, powder XRD phases and catalytic activity was also studied. Possible reaction mechanisms for the formation of salol and diphenyl ether over acid sites are proposed. Indian Academy of Sciences.

Full text of DOI:10.1007/s12039-010-0022-y

J. Chem. Sci., Vol. 122, No. 2, March 2010, pp. 193–201. © Indian Academy of Sciences.
Vapour phase synthesis of salol over solid acids via transesterification
a,
S Z MOHAMED SHAMSHUDDIN * and N NAGARAJU
b
a
Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560 012
b
St. Joseph’s Research Center, 46, Langford Road, Shanthinagar, Bangalore 560 027
e-mail: Em_Es@rediffmail.com
MS received 17 March 2009; revised 16 July 2009; accepted 7 September 2009
Abstract. The transesterification of methyl salicylate with phenol has been studied in vapour phase
2–
2 3 4
over solid acid catalysts such as ZrO , MoO and SO or Mo(VI) ions modified zirconia. The catalytic
materials were prepared and characterized for their total surface acidity, BET surface area and powder
XRD patterns. The effect of mole-ratio of the reactants, catalyst bed temperature, catalyst weight, flow-
rate of reactants, WHSV and time-on-stream on the conversion (%) of phenol and selectivity (%) of salol
has been investigated. A good yield (up to 70%) of salol with 90% selectivity was observed when the re-
actions were carried out at a catalyst bed temperature of 200°C and flow-rate of 10 mL/h in presence of
Mo(VI)/ZrO
2
as catalyst. The results have been interpreted based on the variation of acidic properties and
2–
powder XRD phases of zirconia on incorporation of SO
4
or Mo(VI) ions. The effect of poisoning of acid
2–
sites of SO
4
or Mo(VI) ions modified zirconia on total surface acidity, powder XRD phases and catalytic
activity was also studied. Possible reaction mechanisms for the formation of salol and diphenyl ether over
acid sites are proposed.
Keywords. Salol; solid acids; diphenyl ether; zirconia; vapour phase transesterification.
1
. Introduction
Salicylate esters are an important group of esters
used in perfumery, flavouring, sun tan preparations
11
Solid acids are safe alternatives for conventional and sweeteners. Phenyl salicylate ester (Salol),
liquid acid catalysts used in synthetic organic chem- which has got a pleasant taste and odour, is used in
istry in petroleum refineries, synthesis of fine sun-tan lotions, as an antiseptic, anodyne and pain-
1
chemicals, pharmaceuticals, etc. Due to the envi- relieving agent. It is obtained by esterification of
11
1
2
ronmental hazards and technical difficulties associ- salicylic acid with phenol. As the solubility of
ated with the use of liquid acids, extensive research salicylic acid in phenol is very low, this method
is in progress to phase out these catalysts from requires the use of the latter in higher molar ratios.
chemical industry. Solid acids like zeolites, cation Hence, the method of transesterification of methyl
exchange resins, oxides and their modified forms salicylate with phenol is employed wherein the reac-
and AlPOs have been used successfully as catalysts tants, being mutually soluble in one another, can be
for several acid-catalysed reactions such as isomeri- mixed in different molar ratios. Also, the solid acid
sation, catalytic cracking, oligomerisation, dehydra- catalysts that have been chosen for this work are
2–6
tion, acylation, etc.
Transesterification is an industrially important which get poisoned by water molecules produced
acid-catalysed reaction with wide range of applica- during the direct esterification reaction between
known to possess moisture-sensitive active centers,
7
–9
13
tions. It is worthwhile to explore the possibility of alcohol and the acid. Thus, the anhydrous condi-
finding a suitable environmentally benign solid cata- tion prevalent in transesterification of methyl salicy-
lyst for this reaction considering the fact that the ex- late with isoamyl alcohol is an added advantage.
isting procedures require expensive or corrosive Transesterification is an equilibrium-driven reaction.
acids and solvents which cannot be reused due to Hence, vapour-phase conditions of the experiment
1
0
tedious methods of isolation.
are better suited than liquid phase reaction condi-
tions. In a vapour-phase reaction, the products of the
reaction are not in contact with the catalysts during
*
For correspondence
1
93

1
94
S Z Mohamed Shamshuddin and N Nagaraju
the entire reaction period, thereby preventing back-
ward reaction.
ZrO is an interesting catalytic material and can • Mo(VI)/ZrO
form a paste which was then dried in an air oven
at 120°C for 12 h.
(MZ) containing 2% of Mo(VI)
was prepared by wet impregnating 3 g of hydrated
zirconia with 0⋅2 g of (NH Mo O. The
resulting wet mixture of the Mo(VI) salt and
2
2
be prepared in many ways in various modifications.
It possesses acidic, basic, oxidizing and reducing
properties on the surface and these four properties
including phase modifications (monoclinic or
tetragonal) change independently with the method of
4
)
6
7 2
O24⋅4H
2
0
hydrated zirconia was dried at 120°C for 12 h.
1
4,15
2–
preparation and heat treatment.
4
The acid–base The hydrated zirconia and its SO and Mo(VI) ions
properties of zirconia can be modified by the addi- modified forms were calcined to 550°C for 5 h in a
tion of anionic or cationic substances. Generally, it muffle furnace before their use as catalysts.
has been observed that the acidic properties of the
modified forms of ZrO
17
2
will be higher than ZrO
2
2
.2 Catalyst characterization
itself. It has been reported that when zirconia is
2–
modified with SO
4
or Mo(VI) or W(VI) ions, their
All the catalysts were analyzed for their total surface
acidity, BET surface area, and powder XRD. The
total surface acidity was measured by NH -TPD
1
6–18
acidic properties increase drastically.
The present paper contains the results of studies
on vapour phase transesterification of methyl salicy-
late with phenol in the presence of solid acids such
3
method by using AutoChem-2910 instrument and
also by n-butyl amine back titration method using
dry benzene as the solvent and bromothymol blue
2
–
2 3 4 2 2
as ZrO , MoO , SO /ZrO and Mo(VI)/ZrO as
catalysts. The catalytic activity of these materials
has been studied in the synthesis of salol by trans-
esterification method. The reaction conditions have
been optimized by varying the reaction parameters
such as, mole-ratio of the reactants, catalyst bed
temperature, catalyst weight, flow-rate of reactants,
WHSV and time-on-stream.
21
indicator. The BET surface area of the samples
was determined using nitrogen as the adsorbent in a
NOVA-1000 high speed gas sorption analyzer ver-
sion-3.70. The X-ray powder diffraction patterns of
all the samples were collected on Seimens-D5005
X-ray diffractometer with a Ni filtered Cu-Kα radia-
tion (1⋅5418 Å).
2
. Experimental
2
.3 Catalytic activity
2
.1 Preparation of catalysts
Catalytic activity of the catalyst samples was deter-
mined in vapour phase transesterification of methyl
a) Hydrated zirconia was obtained by the precipita- salicylate with phenol. The reactions were carried
tion method as follows. 25 g of ZrOCl ⋅8H O were out in a fixed bed continuous down flow glass tube
(
2
2
dissolved in 250 mL deionised water. To this clear reactor (i.d. 30 mm) heated by tubular furnace with
solution liquor ammonia (28%) was added drop- a temperature control. In a typical reaction, 1⋅0 g of
wise from a burette with constant stirring. Thus the catalyst (mesh 18–30) pre-calcined at 550°C was
obtained precipitate of Zr(OH)
oughly to remove the soluble ions and dried in an air 200°C. The reaction mixture containing methyl sali-
oven at 120°C for 12 h. cylate and phenol in 1 : 1 molar ratio was introduced
b) Molybdenum oxide (MoO ) was prepared by into the pre-heater maintained at 250°C by means of
mixing well 2 g of ammonium hepta molybdate an infusion pump before passing over the catalyst.
(NH Mo O] with 2 mL distilled water The reaction mixture was fed into the reactor using a
followed by drying the resulting paste in an oven at flow-rate controller. The liquid products were col-
4
was washed thor- placed over glass wool in the reactor maintained at
(
3
[
4
)
6
7 2
O24⋅4H
1
20°C for 12 h.
lected at the bottom of the reactor at time intervals
(
c) Sulfate ion and Mo(VI) modified zirconia of 15 min. Each reaction was carried out for 1 h.
were prepared as follows:
Further studies on the effect of mole-ratio of the
reactants, catalyst bed temperature, catalyst weight,
•
Sulfated zirconia (SZ) was obtained by impreg- flow-rate of the reactants, WHSV and time-on-
nating 3 g of hydrated zirconia with 1⋅5 mL of stream on the conversion (%) of phenol and selecti-
M H SO . The mixture was thoroughly mixed to vity towards salol was studied.
3
2
4

Vapour phase synthesis of salol over solid acids via transesterification
195
Table 1. Physico-chemical properties of solid acids used.
3
Acidity distribution obtained by NH -TPD (mmol/g)
BET surface
2
area (m /g)
Medium
Strong
Very strong
Catalyst
ZrO
(150–250°C) (250–410°C) (>410°C)
Total
2
71
79
0⋅40
0⋅46
–
–
–
0⋅40 (0⋅44)
0⋅51 (0⋅53)
1⋅15 (1⋅09)
1⋅00 (1⋅01)
0⋅35 (0⋅36)
0⋅32 (0⋅30)
MoO
SZ
3
0⋅05
0⋅89
0⋅91
0⋅26
0⋅29
–
149
116
146
113
0⋅26
–
MZ
0⋅09
–
SZ (poisoned)
MZ (poisoned)
0⋅09
–
0⋅03
Numbers in the parenthesis correspond to the total surface acidity values obtained by
n-butyl amine back titration method
The products obtained after the reactions were as ‘medium’, ‘strong’ and ‘very strong’. It has also
analysed quantitatively by gas chromatograph fitted been reported that pure ZrO contains ‘medium’
contains ‘strong’ and ‘very
column coupled with FID detector and qualitatively strong’ acidic sites and Mo(VI)/ZrO contains ‘me-
2
2
–
4 2
with a (10% SE-30 chromosorb w-AW, 3 m × 1/8″) acidic sites, SO /ZrO
2
2
3,24
by GC-MS (Varian).
dium’ and ‘strong’ acidic sites.
The specific surface area of SO
Mo(VI)/ZrO was significantly higher than that of
pure ZrO . According to Arata , higher surface area
/ZrO was found to be due to the cracking of
crystallites into fine particles upon treatment
with sulfate ions. Whereas in the case of Mo(VI)/
ZrO sample the observed increase in the specific
surface area was explained based on the formation
2
–
4
2
/ZrO and
2
3
. Results and discussion
1
6
2
2
–
of SO
4
2
3
.1 Catalyst characterization
ZrO
2
The values of BET surface area and total surface
acidity along with acid site distribution of all the
catalysts used in this investigation are presented in
table 1.
2
2
5
of Mo–O–Zr linkages resulting a porous material.
Typical powder X-ray diffractograms of pure ZrO
sulfate and Mo(VI) modified zirconia are presented
in figure 1. The powder XRD patterns of pure ZrO
showed the presence of both monoclinic (2θ = 24⋅7,
8⋅4, 31⋅6) and tetragonal phases (2θ = 30⋅3, 35⋅3,
0⋅7). When the powder XRD patterns of pure ZrO
being an amphoteric oxide showed least acidity and its sulfate and Mo(VI) ions modified forms were
2
2
,
The total acidity of the catalysts measured both by
NH
3
-TPD and n-butylamine titration method was
2
found to follow the order:
2
2 3
ZrO < MoO < MZ ≤ SZ.
5
2
ZrO
where as SO
showed higher surface acidity when compared with tetragonal phase than monoclinic. This was attrib-
2
–
4
2 2
/ZrO (SZ) and Mo(VI)/ZrO
(MZ) compared, the modified forms consists of more of
2
–
pure zirconia. When SO
ZrO
the basic sites are expected to be suppressed. ions with zirconia support inhibiting the growth of
This effect indirectly increases the number of acidic monoclinic phase of zirconia. No peaks corre-
sites which are known to be responsible for their sponding to either MoO or ZrMo in Mo(VI)/ZrO
4
ions are doped on hydrated uted to the strong interaction of sulfate and Mo(VI)
2
2
2
3
2
O
8
2
1
5
enhanced catalytic activity. Mo(VI)/ZrO
was found to have higher surface acidity than pure
zirconia due to electron deficient states formed by lation between the increases in surface acidity of
the introduction of Mo(VI) ions in the lattice of zir- ZrO on incorporation with sulfate or Mo(VI) ions
conia solid. Chary et al has reported that increase and a decrease in the monoclinic phase with the
in the acidity of MZ is due to molybdena phase. Fur- similar modification with a corresponding increase
ther, it is well-established that in NH
-TPD meas- in the tetragonal phase (figure 1). These observa-
urement, based on the temperature at which NH
tions indicate that the tetragonal phase of zirconia is
molecules desorb from the surface of a solid acid associated with acid sites that are catalytically active
catalyst, the strength of acid sites can be classified in bringing about the transesterification of methyl
2
catalyst sample were observed.
It is interesting to note that there is a good corre-
2
2
2
23
3
3

1
96
S Z Mohamed Shamshuddin and N Nagaraju
salicylate and phenol. In order to confirm the rela- neutralized by NH
3
to a certain extent in addition to
tion between surface acidity, powder XRD phases the acidic sites formed due to modification of ZrO
2–
2
and catalytic activity of ZrO
the following studies on poisoning of acid sites on that the powder XRD of poisoned SZ and MZ re-
SZ and MZ were taken up. sembled to those of pure ZrO i.e. former samples
At the first instance, when total surface acidity had got more of monoclinic phase than tetragonal
and powder XRD phases of calcined pure ZrO are phase (figure 2). When the intensities of powder
compared with that of calcined SZ and MZ, the sur- XRD lines corresponding to monoclinic phases of
face acidity values of latter are higher than pure poisoned SZ and MZ were compared with that of
ZrO and the powder XRD phases of SZ and MZ pure ZrO , poisoned SZ and MZ had more intense
have more of tetragonal phase than monoclinic phase. powder XRD lines corresponding to monoclinic phase
In order to evaluate this, the acidic sites on calcined than tetragonal. But, however not much change in
SZ and MZ were poisoned by exposing them to NH the BET surface area of poisoned SZ and MZ and
vapours for ~15 min and heating it to 120°C for 2 h the unpoisoned ones were observed which indicates
in an air oven. By this time all the acid sites on the that poisoning of acid sites does not affect surface
surface of the catalyst get neutralized by NH area. Poisoning only neutralizes the acid sites with-
2 4
and its modified forms, with SO or Mo(VI) ions. It is interesting to note
2
2
2
2
3
3
.
The total surface acidity of these poisoned SZ and out affecting the surface area of catalytic materials.
MZ samples were measured and their powder XRD
patterns were recorded. It was observed that the total
3
.2 Catalytic activity
surface acidity of the poisoned SZ and MZ was de-
creased to an extent that it was slightly less than the
total surface acidity of pure ZrO . This suggests that
the acidic sites arising due to only ZrO were also
In general, it was found that all the catalysts used for
the study exhibited good catalytic activity in trans-
2
2
Figure 2. Powder X-ray diffraction patterns of ZrO2
2–
(Z), poisoned-SO /ZrO (SZ poisoned) and poisoned-
4
2
–
Figure 1. Powder XRD patterns of ZrO
SZ) and Mo(VI)/ZrO
2
M-monoclinic ZrO and T-tetragonal ZrO .
2
(Z), SO
4
/ZrO
2
2
(
2
(MZ) samples calcined at 823 K: Mo(VI)/ZrO2 (MZ poisoned) samples: M, monoclinic
ZrO and T, tetragonal ZrO .
2
2
2

Vapour phase synthesis of salol over solid acids via transesterification
197
Table 2. Comparative catalytic activity of various catalysts in vapour phase transesterifi-
cation (molar ratio of methyl salicylate: phenol = 1 : 1; amount of catalyst = 1⋅0 g; flow-rate
of reactants = 10 mL/h; catalyst bed temperature = 200°C).
Yield of
salol
Conversion
Selectivity
Selectivity towards
Catalysts
ZrO
of phenol (%) towards salol (%) diphenyl ether (%)
2
39⋅06
41⋅86
59⋅04
63⋅00
36⋅66
37⋅60
42
46
72
70
39
40
93
91
82
90
94
94
6
8
MoO
SZ
3
18
10
5
MZ
SZ (poisoned)
MZ (poisoned)
4
esterification of methyl salicylate with phenol.
However, there was no activity towards transesteri-
fication when the reaction was conducted in the
absence of any catalyst indicating that the above
said reaction is a catalysed reaction.
The results pertaining to the catalytic activity of
2 3
the catalysts ZrO , MoO , SZ and MZ investigated
in vapour phase transesterification of methyl salicy-
late with phenol for the synthesis of salol are pre-
sented in table 2. It was observed that apart from the
desired product salol and primary by-product metha-
nol, diphenyl ether was also formed as the side-
product over all the catalysts. The high reaction
temperatures may facilitate the dehydration of phe-
nol and hence favour the ether formation. But over
SZ catalyst the formation of diphenyl ether was
more which may be attributed to the presence of Figure 3. Effect of mole-ratio of the reactants on the
phenol conversion (%) and selectivity of the products
%). (Amount of the catalyst (MZ) = 1.0 g; flow-rate of
the reactants = 10 mL/h; catalyst bed temperature =
very strong acidic sites which facilitate the dehydra-
tion of phenol and formation of diphenyl ether that
is found to be absent in ZrO , MoO and MZ. Opti-
mization of reaction conditions was carried out in
presence of Mo(VI)/ZrO catalyst because it showed
good conversion (%) of phenol with reasonably
good selectivity towards salol.
(
2
3
2
00°C; pre-heater temperature = 250°C.)
2
3
.2b Effect of catalyst bed temperature: Suitable
temperature conditions for transesterification reac-
tion was determined by varying catalyst bed tempe-
rature between 150°C and 300°C keeping the pre-
heater temperature fixed at 250°C. As can be seen in
figure 4, at temperatures less than 200°C, the forma-
tion of diphenyl ether was less. Further, at this tempe-
rature, the conversion of phenol was also low. When
the catalyst bed temperature was higher than 200°C,
the temperature was found to be too high because
the selectivity towards salol was decreased (from 10
to 14%) and the side-product (diphenyl ether) for-
mation was high. Traces of other impurities were
also formed at higher catalyst bed temperature and
by noticing the colour of the catalyst we can say that
the catalyst was getting deactivated might be due to
the polymerization of reactants or the products.
3
.2a Effect of mole-ratio of the reactants: The
mole-ratio of the feed mixture (methyl salicylate:
phenol) was varied from 2 : 1 to 1 : 2. It was ob-
served that an increase in the concentration of methyl
salicylate (MS) increases the conversion of phenol
(
figure 3). Since, transesterification is a reversible
reaction; excess of MS favours the forward reaction,
leading to formation of the desired product. How-
ever, increase in the concentration of phenol
decreased the formation of salol with a slight in-
crease in the formation of diphenyl ether. This may
be because excess of phenol favours the formation
of diphenyl ether. Further, it was also observed that
in the presence of excess of phenol catalyst was get-
ting deactivated.

1
98
S Z Mohamed Shamshuddin and N Nagaraju
Therefore, a catalyst bed temperature of 200°C was the results are presented in figure 5. When the amount
found to be suitable for reasonable conversion of of catalyst was less than 1⋅0 g, conversion of phenol
phenol with good selectivity towards salol. was less. As the amount of catalyst was increased
beyond 1⋅0 g, the conversion of phenol increased
.2c Effect of catalyst weight: The catalysts cal- and the selectivity towards salol decreased with an
3
cined to 550°C were used in the transesterification increase in the formation of diphenyl ether. This
reaction in amounts ranging from 0⋅1 g to 2⋅0 g and may be because, as the thickness of the catalyst-bed
increases, the product molecules may get trapped
within the catalyst layer leading to side-product
formation. Decrease in the selectivity of salol at
higher catalyst weights may also be to a certain
extent due to back-pressure generated due to fine
powder form of the catalyst.
3
.2d Effect of flow-rate (feed-rate) of reactants:
The transesterification reaction was studied at dif-
ferent flow-rates between 2⋅5 mL/h to 12⋅5 mL/h.
From figure 6, it is clear that the conversion of phe-
nol decreases with increasing flow-rate of the reac-
tants. However, at higher flow-rates selectivity of
salol was found to increase with a decrease in the
selectivity of diphenyl ether. This may be attributed
to the higher contact time of reactant and product
molecules on the catalyst when the flow-rate is less.
A higher contact time could probably result in fur-
ther reaction of reaction intermediates or products
leading to the formation of side-products.
Figure 4. Effect of catalyst bed temperature on the
phenol conversion (%) and selectivity of the products
%). (Molar ratio of the reactants = 1 : 1; amount of the
catalyst (MZ) = 1⋅0 g; flow-rate of the reactants =
0 mL/h; pre-heater temperature = 250°C.)
(
1
3.2e Effect of WHSV: Weight hourly space velocity
–1
(
WHSV) was varied from 11 to 43 g-cat.h.mol by
Figure 5. Effect of catalyst weight on the phenol con- Figure 6. Effect of flow-rate on the phenol conversion
version (%) and selectivity of the products (%). (Molar (%) and selectivity of the products (%). (Molar ratio of
ratio of the reactants = 1 : 1; catalyst bed temperature = the reactants = 1 : 1; amount of the catalyst (MZ) = 1⋅0 g;
2
00°C; flow-rate of the reactants = 10 mL/h; pre-heater catalyst bed temperature = 200°C; pre-heater tempera-
temperature = 250°C.)
ture = 250°C.)

Vapour phase synthesis of salol over solid acids via transesterification
199
keeping mole-ratio and reaction temperature con- ther reaction of reactants or reaction intermediates
stant (figure 7). With increasing WHSV there is an or products to produce more side-products.
increase in the conversion, which is due to the in-
creased residence time (contact time) of the reac- 3.2f Effect of time-on-stream: The time-on-stream
tants with the catalyst. However, selectivity towards experiments were carried over MZ catalyst at a cata-
salol decreased with increase in WHSV. This may lyst bed temperature of 200°C and the results are
be due to the higher contact time which leads to fur- represented in the form of a graph (figure 8). The
conversion of phenol stays fairly steady up to 4 h
but then decreases gradually up to 8 h. The selectiv-
ity of salol or diphenyl ether remained rather steady
throughout the time-on-stream experiment.
3
.2g Effect of poisoning of acid sites of SZ and MZ
catalysts on catalytic activity: As discussed in the
section ‘catalyst characterization’ under ‘results and
discussion’ the acid sites of SZ and MZ catalysts
3
were poisoned by NH vapours. It was observed that
total surface acidity and powder XRD phases of zir-
conia and its modified forms are related to each
other. In order to support this further, the poisoned
catalysts were used in the title reaction and the
results are included in table 2. The decrease in the
values of conversion (%) of phenol can be attributed
to the loss of acidic sites of SZ and MZ on poison-
ing. Hence, we can infer that the total surface acid-
Figure 7. Effect of WHSV on the phenol conversion ity, powder XRD phases and the catalytic activity of
(
%) and selectivity of the products (%). (Molar ratio of
zirconia and modified zirconia are linked with each
other.
the reactants = 1 : 1; amount of the catalyst (MZ) = 1⋅0 g;
catalyst bed temperature = 200°C; pre-heater tempera-
ture = 250°C.)
3.3 Mechanism for salol formation over an acid site
Transesterification takes palace between methyl
salicylate adsorbed on the acid site of catalyst form-
ing an electrophile, and phenol in the vapour phase
(
scheme 1) forming salol with subsequent removal
of a molecule of methanol.
3
.4 Mechanism for ether formation over an
acid site
Formation of diphenyl ether from phenol via dehy-
dration involves interaction between an acid site on
the catalyst surface and a molecule of phenol fol-
lowed by nucleophilic attack from a second phenol
molecule and subsequent removal of a water mole-
cule (scheme 2).
Figure 8. Effect of time-on-stream on the phenol con-
version (%) and selectivity of the products (%). (Molar
ratio of the reactants = 1 : 1; amount of the catalyst
4
. Conclusions
(
MZ) = 1⋅0 g; flow-rate of the reactants = 10 mL/h; cata-
Phenyl salicylate (salol) can be conveniently synthe-
sized by vapour phase transesterification of methyl
lyst bed temperature = 200°C; pre-heater tempera-
ture = 250°C.)

2
00
S Z Mohamed Shamshuddin and N Nagaraju
Scheme 1. Mechanism for salol formation over an acid site.
Scheme 2. Mechanism for ether formation over an acid site.
2
–
salicylate with phenol using ZrO
and Mo(VI)/ZrO as catalysts. Among all the cata- catalytic activity of the catalysts was observed.
lysts, Mo(VI)/ZrO showed good conversion (%) of
phenol with reasonably good selectivity towards
salol. Although, ZrO and MoO showed good selec-
tivity towards salol but their conversion (%) of phe- The authors gratefully acknowledge the help extended
2
, MoO
3
, SO
4
/ZrO
2
total surface acidity, powder XRD phases and the
2
2
Acknowledgements
2
3
2
–
4 2
nol is poor. Over SO /ZrO
catalyst even though the by the authorities of Indian Institute of Science
conversion (%) of phenol is high the selectivity (IISc), Bangalore for permitting the use of library
towards salol decreased due to the formation of facilities, Prof. M S Hegde for his useful discus-
more diphenyl ether. Catalyst bed temperature of sions, Dr. C Shivakumar, Indian Institute of Science
2
00°C with flow-rate of reactant mixture at 10 mL/h (IISc) for powder XRD analysis, Dr Jaiprakash Ban-
with 1⋅0 g of catalyst was found to be the optimal galore Institute of Technology (BIT), Bangalore, for
reaction condition to get good yield of salol with surface area analysis, Dr Joyce D’Souza, Indian In-
high selectivity. A linear correlation between the stitute of Technology Madras for TPD analysis. MS

Vapour phase synthesis of salol over solid acids via transesterification
201
1
1
2. Kuriakose G and Nagaraju N 2004 J. Mol. Catal.
A223 155
thanks HMS Institute of Technology (HMSIT),
Tumkur authorities for their part-financial help.
3. Sen S E, Smith S M and Sulivan K A 1998 Tetrahe-
dron 55 12657
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