Active and deactive modes of modified montmorillonite in p-cresol acylation

Article abstract of DOI:10.1016/j.molcata.2014.05.017

para-Toluene sulphonic acid (p-TSA)-treated montmorillonite clay used as heterogeneous catalyst in acylation of para-cresol (PC) with aliphatic carboxylic acids. Reactions were studied under microwave and conventional modes of heating and reaction conditions were optimized by varying mole ratio, temperature, amount of catalyst and reaction time. Under optimized conditions the reaction was carried out involving p-cresol and decanoic acid. The reaction involved two steps, O-acylation involving ester formation followed by the Fries rearrangement involving C-acylation resulting in ketone product. Microwave heating mode showed higher conversion and the catalytic activity almost retained in repeated use. On the other hand the catalytic activity dropped by more than 50% in the case of conventional heating indicating rapid deactivation. A change in the color of the used catalyst was more intense in the case of conventional than in the microwave heating. Used catalysts were characterized for surface area and pore volume by BET technique, acidity by FTIR spectroscopy and amount of coke by TGA. Further investigations on the catalyst used in conventional heating revealed that the deactivation occurred during the O-acylation and not in the subsequent Fries rearrangement. However, the catalyst in the microwave irradiated reaction, exhibited a retarded rate of formation of coke precursors on the surface during O-acylation, thus preventing any decrease in catalytic activity. Present study indicates that the technique chosen for heating the reaction medium plays an important role in suppressing deactivation.

Full text of DOI:10.1016/j.molcata.2014.05.017

Journal of Molecular Catalysis A: Chemical 392 (2014) 181–187
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Active and deactive modes of modified montmorillonite in p-cresol
acylation
N.J. Venkatesha, B.M. Chandrashekara, B.S. Jai Prakash, Y.S. Bhat∗
Chemistry Research Centre, Bangalore Institute of Technology, KR Road, Bangalore 560 004, India
a r t i c l e i n f o
a b s t r a c t
Article history:
para-Toluene sulphonic acid (p-TSA)-treated montmorillonite clay used as heterogeneous catalyst in acy-
lation of para-cresol (PC) with aliphatic carboxylic acids. Reactions were studied under microwave and
conventional modes of heating and reaction conditions were optimized by varying mole ratio, temper-
ature, amount of catalyst and reaction time. Under optimized conditions the reaction was carried out
involving p-cresol and decanoic acid. The reaction involved two steps, O-acylation involving ester forma-
tion followed by the Fries rearrangement involving C-acylation resulting in ketone product. Microwave
heating mode showed higher conversion and the catalytic activity almost retained in repeated use. On the
other hand the catalytic activity dropped by more than 50% in the case of conventional heating indicating
rapid deactivation. A change in the color of the used catalyst was more intense in the case of conventional
than in the microwave heating. Used catalysts were characterized for surface area and pore volume by
BET technique, acidity by FTIR spectroscopy and amount of coke by TGA. Further investigations on the
catalyst used in conventional heating revealed that the deactivation occurred during the O-acylation and
not in the subsequent Fries rearrangement. However, the catalyst in the microwave irradiated reaction,
exhibited a retarded rate of formation of coke precursors on the surface during O-acylation, thus pre-
venting any decrease in catalytic activity. Present study indicates that the technique chosen for heating
the reaction medium plays an important role in suppressing deactivation.
Received 27 February 2014
Received in revised form 9 May 2014
Accepted 17 May 2014
Available online 29 May 2014
Keywords:
Microwave irradiation
p-TSA clay
Catalyst deactivation
O-acylation
Fries rearrangement
C-acylation
© 2014 Elsevier B.V. All rights reserved.
1. Introduction
clays [9,10] and heteropoly acids [11,12]. It is also desirable to use
less hazardous acylating agents like carboxylic acids instead of acyl
The acylation of phenolic compounds is an important route for
the synthesis of aromatic ketones that are intermediates in the
manufacturing of fine and specialty chemicals. Direct acylation of
hydroxyl group yields ester, which converts to ketone by subse-
quent Fries rearrangement. Both ester and ketone formed in the
reaction are valuable intermediates in the synthesis of pharma-
ceuticals and fragrances [1–4]. The reaction occurs by interaction
of the phenolic compound with carboxylic acids in the presence of
an acid catalyst and involves an intermediate acylium ion generated
by protonation of acylating agent and subsequent dehydration [5].
In the present industrial practice, most of the acylation reactions are
carried out using corrosive Lewis acid catalysts, such as AlCl3, HF,
BCl3, FeCl3, ZnCl2, SnCl4 and TiCl4 which are required in stoichio-
metric amounts and generate substantial amount of waste products
[1,6–8]. Efforts have been made to replace these acids by reusable,
eco-compatible, heterogeneous solid acids like zeolites [1], acidic
halides and anhydrides.
Acidic zeolites, heteropoly acids, metal oxides and cation
exchanged clays are the most studied heterogeneous catalysts for
the direct acylation. Activity and selectivity of these catalysts have
been investigated in the direct acylation of several organic sub-
strates like toluene, biphenyl and naphthalene, aromatic ethers
like anisole, thioanisole, veratrole, and 2-methoxy naphthalene,
heterocyclic compounds like thiophene, benzofuran and phenolic
compounds like phenol, PC, naphthol and resorcinol [7,10,12,13].
Among various solid acid catalysts employed in organic synthe-
sis, acidic clays are exclusive because of their abundant availability
and easy modification. Acidity in clays can be generated mainly by
cation exchange and acid treatment. Exchange with several cations
and p-TSA treated montmorillonite clay possess strong Lewis and
Bronsted acidic sites compared to other solid acids [6,14]. One of
the major problems in acylation using clays is lower conversion and
longer duration in the liquid phase conventional heating process
[11].
Emphasis of the present work is to study the effect of mode of
heating on the mechanism of deactivation and on the chain length
∗
Corresponding author. Tel.: +91 80 26615865; fax: +91 80 26426796.
E-mail address: bhatys@yahoo.com (Y.S. Bhat).
http://dx.doi.org/10.1016/j.molcata.2014.05.017
1381-1169/© 2014 Elsevier B.V. All rights reserved.

182
N.J. Venkatesha et al. / Journal of Molecular Catalysis A: Chemical 392 (2014) 181–187
100
of the acylating agent for the formation of ester and ketone on a
p-TSA treated montmorillonite clay catalyst. This particular cata-
lyst was selected for its high acidity, large surface area and easy
accessibility to the active site.
90
80
70
60
50
40
30
20
10
0
Conversion of DA
Yield of Ester
Yield of ketone
2. Experimental
2.1. Catalyst and chemicals
Surface and acidity modification of montmorillonite clay was
carried out by treating with p-TSA using the procedure reported
earlier [6,14].
It involved treatment of the montmorillonite obtained from
Bhuj area, Gujarat, India. The clay was converted to sodium form by
1 M NaCl solution treatment. The composition was found using XRF
to be 42.86% SiO2, 16.64% Al2O3, 10.05% Fe2O3, 2.58% MgO, 1.92%
Na2O, 0.41% K2O and 1.79% CaO with idealized structural formula
Si4[Al1.348Fe0.386Mg0.266]O10(OH)2Na(0.324) [6].
0.0
0.2
0.4
0.6
0.8
1.0
1.2
Catalyst amount, g
Fig. 1. Effect of catalyst amount on the acylation of PC with DA. Reaction condition:
mole ratio PC:DA, 2:1; reaction time, 60 min; temperature, 463 K.
Chemicals used in this study were procured from SD fine chem-
icals, India. PC and carboxylic acids were distilled before use and
all other chemicals were used without further purification.
to extract adsorbed reactants and products and filtered to separate
the catalyst. Analysis of the ingredients before and after the reac-
tion was carried out using Chemito GC-1000 gas chromatograph
with TR-WAX capillary column (30 m length, thickness 0.32 mm
and 0.5 ␮m internal diameter) and flame ionization detector. The
products were also analyzed and confirmed by GC–MS [1,14].
Conventional heating. Same amount of reactants and catalyst
were taken as in microwave heating in a 50 ml vessel placed in
a hot-air oven at 463 K for about 16 h. After the completion of the
reaction, analysis was performed as given under the microwave
heating.
2.2. Characterization
The p-TSA treated clay catalyst samples were characterized for
acidity, surface area, X-ray diffraction (XRD) and thermogravimet-
ric analysis (TGA) before and after the reactions.
Acidity was measured by FT-IR spectroscopy using pyridine as
probe molecule. All the samples were activated by degassing at
383 K for 2 h and then saturated with pyridine. The catalyst sam-
ples were then evacuated at 423 K for 2 h to remove physisorbed
pyridine. FT-IR spectra of the samples were then recorded in the
range 1400–1600 cm−1 using Bruker model Alpha-P IR spectropho-
tometer having resolution of 4 cm−1 fitted with a diamond ATR cell
[15,16].
Surface area measurements were carried out using Quanta
chrome Nova-1000 surface analyzer under liquid nitrogen temper-
ature [6,14].
The catalyst samples used in reactions under microwave and
conventional heating were also characterized by TGA to study the
deactivation by coke precursors. TGA studies were carried out using
a Mettler Toledo 851e stare 7.01 TGA–DTA system. The tempera-
ture was ramped from 303 to 873 K at the rate of 5 K min−1 under
flowing air [17,18].
3. Results and discussion
In order to optimize the reaction conditions, various parameters
like effect of reaction temperature, catalyst amount, mole ratio of
the reactants and reaction time for acylation of PC with DA over p-
TSA clay were studied. Under optimum conditions, acylation of PC
with different carboxylic acids was investigated under microwave
as well as conventional heating over p-TSA clay as catalyst.
3.1. Acylation of PC with DA under microwave heating
3.1.1. Effect of catalyst amount
No reaction was observed between PC and carboxylic acids in
the absence of catalyst. Under the specified conditions, increase in
the conversion of DA was observed with the increase in the cata-
lyst amount from 0.1 to 0.5 g and thereafter it decreased slightly.
At low catalyst amounts, O-acylation (ester) dominated with small
amount of C-acylated (ketone) product. Fig. 1 shows that the con-
version of acid to ester improved with the increase in catalyst
amount and reached a maximum at 0.5 g. When the ester con-
centration increases and reaches an optimum value, formation of
ketone increases through the rearrangement of the ester. This reac-
tion takes place at an enhanced rate with catalyst quantity higher
than 0.5 g as more number of acid sites are available. There is
a simultaneous hydrolysis of ester forming DA and p-cresol thus
showing a lower conversion of DA.
Structural integrity of the catalyst samples after the reaction was
checked by powder XRD. The data were recorded by step scanning
at 2Â = 0.020◦ per step from 3◦ to 80◦ on Shimadzu MAXima X XRD-
7000 X-ray diffractometer with graphite monochromatized Cu K␣
˚
radiation (k = 1.5406 A) [15,16].
2.3. Catalytic tests
Microwave heating. Ten millimoles of PC and 5 mmol of aliphatic
carboxylic acid were mixed with 1 g catalyst in a microwave reactor
vessel with magnetic stirring. Reactor vessel was kept in microwave
lab station ‘START-S’ having software that enables the on-line con-
trol of temperature of the reaction mixture with the aid of infrared
sensor by regulation of microwave power output in such a way
that the reaction mixture was exactly in line with infrared sen-
sor that monitors the temperature. Variable power up to 1200 W
was applied by microprocessor-controlled single-magnetron sys-
tem. The maximum irradiation power of 1200 W was used in the
initial 1 min of the reaction to attain the temperature of 463 K, and
then, variable power (500–700 W) from the reactor auto-maintains
that temperature. The reaction mixture was then cooled for 10 min,
followed by addition of 10 ml of toluene, stirred for about 30 min
3.1.2. Effect of temperature
Temperature has a significant effect on the catalytic acylation of
PC with DA. Effect of temperature was studied under the same con-
ditions by varying temperature from 403 to 463 K. The results are
shown in Fig. 2. It was observed that at lower temperatures (433 K),
DA conversion is more selective toward ester and the selectivity
toward ketone increased as the temperature was raised beyond

N.J. Venkatesha et al. / Journal of Molecular Catalysis A: Chemical 392 (2014) 181–187
183
70
70
65
60
55
50
45
40
35
30
25
20
15
10
5
Conversion of DA
Yield of Ester
Yield of Ketone
60
50
40
30
20
Conversion of DA
Yield of Ester
10
0
Yield of Ketone
0
400
410
420
430
440
450
460
470
480
0
20 40 60 80 100 120 140 160 180 200 220 240 260
Reaction time, minutes
Temperature, K
Fig. 2. Effect of temperature on the acylation of PC with DA. Reaction condition:
catalyst amount, 1 g; mole ratio PC:DA, 2:1; reaction time,60 min.
Fig. 3. Effect of reaction time on the acylation of PC with DA. Reaction condition:
catalyst amount, 1 g; mole ratio PC:DA, 2:1; temperature, 463 K.
by the direct acylation of aromatic ring, but forms through Fries
rearrangement of primary ester product.
450 K. This suggests that ketone formation takes place through the
Fries rearrangement of ester and is favored at higher temperature
[1].
Earlier studies have reported that acid treated clays are good
catalysts for the Fries rearrangement [12,19]. Conversion of ester
to ortho-hydroxyketone was favored owing to the presence of
strong ortho directing hydroxyl group and to the formation of
intra-molecular hydrogen bonding between carbonyl oxygen and
hydrogen of hydroxyl group [20]. Possible reaction mechanism is
given in Scheme 1.
3.1.3. Effect of mole ratio
The molar ratio of PC to DA was changed from 3:1 to 1:3 with
1 g of clay catalyst at reaction temperature of 463 K. Table 1 sum-
marizes results of these experiments. When the mole ratio of PC:
DA was 1:1, 40% conversion of DA was observed with 75% and 25%
selectivity toward ester and ketone, respectively. Further increase
in PC shows an increase in conversion of DA. At 2:1 mole ratio of
PC: DA, 52% of conversion of DA with 38% selectivity for ester and
62% selectivity for ketone were observed. Generally, acylation reac-
tion occurs through protonation of carbonyl oxygen of carboxylic
group by the Bronsted acid sites forming an acylium ion which gets
attacked by the readily available PC in the reaction mixture [17].
On the other hand, conversion of PC decreased to 36% when the
amount of DA was doubled. This indicates the inhibitive effect of
carboxylic acid on the reaction [1].
3.2. Acylation of PC with various carboxylic acids under MW and
conventional heating
Under optimized conditions, the effect of different modes of
heating on reaction time was studied for acylation of PC with
acetic, propionic, butyric, hexanoic, octanoic and decanoic acids
at 463 K with 1 g of catalyst and carboxylic acid:PC mole ratio of
1:2. Reaction time was varied from 5 to 240 min and 1 to 40 h
under microwave and conventional heating respectively. Conver-
sion of different carboxylic acids as a function of reaction time
under microwave and conventional heating are shown in Fig. 4(a)
and (b) respectively. The increase in conversion from acetic acid to
decanoic acid is due to the enhanced inductive effect with increase
in carbon chain length.
Difference in conversion was observed between two modes
of heating. Under conventional heating, reaction proceeded very
slowly and at the end of 10 h, all the acids showed conversion
between 12 and 20%. On the other hand, under microwave heat-
ing, conversion of different acids was in the range of 40–58% in
60 min.
Microwave heating showed more conversion toward ester of all
the acids in the beginning. Further increase in the parameters like
temperature, catalyst amount, reaction time and mole ratio alters
the ratio of ester to ketone; a maximum of 42% ketone was observed
under microwave heating. In the case of conventional heating, the
conversion of acid showed a maximum of 35%, in which 30% got
converted to ester and the rest of it to ketone. Beyond this point
any increase in temperature, time and the catalyst amount had no
effect on the ratio of ester to ketone.
3.1.4. Effect of reaction time
Reaction time as a function of conversion of DA under simi-
lar reaction conditions are shown in Fig. 3. Initially, conversion of
DA was observed to be 52% with 85% selectivity toward ester. As
the reaction proceeds, the conversion of DA gradually increased
and attained thermodynamic equilibrium at 240 min with a max-
imum of 62% in. The selectivity toward ketone was as low as 15%
in the first 10 min and as the reaction proceeds, selectivity toward
ketone increased to 63% with a simultaneous decrease in selec-
tivity toward ester (37%). This suggests that ester was the primary
product and enhancement in the concentration of ketone with con-
comitant decrease of ester indicated that ketone is not obtained
Table 1
Effect of mole ratio on the acylation of PC with DA.
PC:DA (mmol)
Conversion
of DA (%)
Conversion
of PC (%)
Reactant conversion
toward
Ester (%)
Ketone (%)
15:5
10:5
5:5
5:10
5:15
53
52
40
–
–
–
–
36
32
23
20
30
25
23
30
32
10
11
9
3.3. Catalyst deactivation
Higher catalytic activity and selectivity were observed in the
microwave heating with shorter reaction times compared to
the conventional heating mode. Maximum yield of the products
observed in the case of MW irradiation was not achieved in case
–
Reaction condition: catalyst amount 1 g; reaction time 60 min and temperature
463 K under MW irradiation.

184
N.J. Venkatesha et al. / Journal of Molecular Catalysis A: Chemical 392 (2014) 181–187
Clay-
H +
H
O +
H O
R
H O
+
O
H
O
O
O
R
O
R
Clay-
H +
Clay-
H
O
O +
R
H
R
O
O H
O
O
H
H
+
R
R
Clay-
Scheme 1. Possible reaction mechanism with catalyst for the acylation of carboxylic acid with PC.
of conventional heating after 40 h. The color of recovered cata-
lysts after the reaction under conventional and microwave heating
were observed to be dark brown and pale green respectively, possi-
bly due to formation of coke precursors. Further investigation was
carried out on the recovered catalyst samples from the reaction
between PC and DA for 180 min in the case of MW irradiation and
for 40 h in the case of conventional heating. These were respectively
designated as p-TSAC(MW) and p-TSAC(CH). The catalyst recovery
was done by separating the used catalyst from the reaction mixture
followed by washing with 15 g of toluene, filtered and then dried
in hot air oven for 3 h at 393 K.
The recovered catalysts were again used in the reaction between
PC and DA under optimized conditions in the conventional as well
as microwave heating. The results are presented in Table 2.
The catalyst p-TSAC(MW) when used under both microwave and
conventional heating showed almost same activity as fresh p-TSAC,
but p-TSAC(CH) catalyst when used under conventional heating
shows less than 8% conversion. On the other hand, p-TSAC(CH)
when used in MW heated reaction, showed similar activity as p-
TSAC(MW) used in conventional heating mode. The results clearly
demonstrate that microwave mode of heating suppressed the
deactivation maintaining the same activity as the fresh catalyst.
However, conventional heating accelerated the deactivation pro-
cess resulting in lower catalytic activity.
80
70
p-TSAC(MW) (a)
60
Further the recovered catalysts p-TSAC(CH) and p-TSAC(MW)
were characterized using TGA–DTA, pyridine FT-IR, XRD and for
BET surface area to investigate the changes brought about in the
catalytic characteristics after the reaction by the two modes of
heating.
BET surface area was measured for both the catalyst samples
before and after the reaction in the two modes of heating. Both sam-
ples, fresh and the used one in microwave heating showed almost
the same specific surface area of about 130 m2. On the other hand,
there was a substantial decrease (to the extent of 30%) in the sur-
face area of the used catalyst sample in the conventional mode of
heating compared with the fresh one and also the one used in MW
heated reaction. The decrease in surface area of catalyst sample p-
TSAC(CH) showed that deactivation of catalyst did occur mainly in
conventional mode of heating. Negligible change in the surface area
50
40
Acetic acid
Propionic acid
Butyric acid
Hexanoic acid
Octanoic acid
Decanoic acid
30
20
10
0
0
20 40 60 80 100 120 140 160 180 200 220 240
Reaction time, min
80
70
60
50
40
30
20
10
0
Acetic acid
Propionic acid
Butyric acid
Hexanoic acid
Octanoic acid
Decanoic acid
p-TSAC(CH) (b)
Table 2
Comparison of conversion of DA with the fresh and used catalyst samples recovered
after the reaction.
Catalyst
Mode of
heating
Conversion
of DA (%)
Reactant conversion
toward
Ester (%)
Ketone (%)
p-TSAC (fresh)
p-TSAC (MW)
p-TSAC (CH)
p-TSAC(fresh)
p-TSAC (MW)
p-TSAC (CH)
Microwave
Microwave
Microwave
Conventional
Conventional
Conventional
57
53
21
26
24
8
22
21
16
19
18
6
35
32
5
7
6
0
5
10
15
20
25
30
35
40
45
Time, hours
2
Reaction condition: catalyst amount, 1 g; mole ratio PC:DA, 2:1; temperature, 463 K.
Microwave heating; reaction time, 120 min; conventional heating; reaction time,
40 h.
Fig. 4. Conversion of different carboxylic acids versus reaction time in the acylation
of PC under (a) microwave heating and (b) conventional. Reaction condition: catalyst
amount, 1 g; mole ratio PC: carboxylic acid, 2:1; temperature, 463 K.

N.J. Venkatesha et al. / Journal of Molecular Catalysis A: Chemical 392 (2014) 181–187
185
100
95
90
85
80
75
70
p-TSAC(MW)
p-TSAC(CH)
a
p-TSAC Fresh
b
c
1550
1445
1490
c p-TSAC Fresh
b p-TSAC(MW)
a p-TSAC(CH)
0
50 100 150 200 250 300 350 400 450 500 550 600 650
Temperature, 0C
1600 1580 1560 1540 1520 1500 1480 1460 1440 1420 1400
Fig. 5. TGA–DTA plots of p-TSAC fresh, p-TSAC(MW) and p-TSAC(CH).
Wavenumber, cm-1
between the fresh p-TSAC and p-TSAC (MW) catalyst samples leads
to the conclusion that the surface is not significantly covered by the
species responsible for the deactivation.
Fig. 7. Pyridine FT-IR spectra of p-TSAC fresh, p-TSAC(MW) and p-TSAC(CH).
precursors in high concentration. The loss of Bronsted acidity in
the case of p-TSAC(CH) was clearly responsible for its low activity
in the acylation reaction. On the other hand, small changes in the
Bronsted acidity in p-TSAC(MW) explains its retention of high activ-
ity. In both the used catalysts, additional peak around 1510 cm−1
was attributed to the adsorbed species on the catalyst.
The TGA plots of fresh p-TSAC, p-TSAC(MW) and p-TSAC(CH)
are shown in Fig. 5. Initial weight loss up to 150 ◦C was attributed
to the loss of water molecules coordinated to interlayer cations
[6]. p-TSAC(MW), in general, showed almost similar weight loss as
that of fresh p-TSAC indicating negligible presence of the species
responsible for the color change. On the other hand, p-TSAC(CH)
showed a considerable weight loss (about 15%) than fresh p-TSAC
evidently due to presence of coke precursor in the former catalyst.
XRD pattern of fresh p-TSAC, p-TSAC(MW) and p-TSAC(CH) is
shown in Fig. 6. The patterns clearly show no significant change
in structural integrity of the catalyst samples before and after the
reaction conducted in different heating modes.
Pyridine-FT-IR spectra of p-TSAC, p-TSAC(MW) and p-TSAC(CH)
are shown in Fig. 7. The fresh p-TSAC showed three different peaks
at 1550, 1445 and 1490 cm−1 due to Bronsted, Lewis and both Lewis
and Bronsted type respectively. Acylation reaction is a Bronsted
acid catalyzed reaction. There is negligible change in the Bron-
sted acidity before and after the reaction under microwave heating.
Whereas the catalyst used under conventional heating showed sig-
nificant changes in the FT-IR pattern. The peak corresponding to
Bronsted acidity almost disappeared and the one corresponding to
Lewis acidity showed broader peak. However, there is some change
in Lewis acidity in the catalyst used under microwave heating. This
could be due to some adsorbed carbonaceous species in very low
concentration as evident by the change in color of the catalyst
from off white to pale green. While in conventional heating color
of the catalyst becomes dark brown due to the presence of coke
3.4. Role of catalyst in Fries rearrangement
Reaction of PC with carboxylic acid takes place in two steps
first through direct O-acylation resulting in ester, followed by Fries
rearrangement of ester to ortho-hydroxyketone. In order to under-
stand the effect of microwave and conventional heating on the
catalyst deactivation in both the steps, direct Fries rearrangement
of the ester, p-cresyldecanoate, was investigated with fresh p-TSAC
under both the modes of heating. The results obtained are shown
in Fig. 8(a) and (b) respectively. The trend of the reaction shows
conversion of the ester into PC, DA and ketone in both cases [21].
However, in the case of microwave mode of heating, the conversion
of the ester to ketone is found to be higher at any given point of time
Fig. 8(a). This indicates that the Fries rearrangement is more facile
in presence of microwave than in the conventional mode of heat-
ing. For example, at 180 min under microwave heating, the ester
showed a total conversion of 69% out of which 31%was found to
be toward ketone and 38% toward DA and PC whereas, in conven-
tional heating 15% of ketone with 50% of DA and PC were found to be
formed in 40 h. Further, in both cases, the conversion of the reactant
ester was high and observed to be almost same (about 65%) which
shows that there is no catalyst deactivation in both the modes of
heating. This leads to the conclusion that the deactivation of the
catalyst in acylation reaction under conventional mode of heating
is mainly due to the ester formation reaction and not due to the
Fries rearrangement.
a
b
Chandrashekara et al. [1] have reported that the coke formation
in acylation reactions is primarily due to the intermediate acylium
ions formed during the esterification step. In the present reaction
studied with p-TSAC, acylium ions are expected to be formed dur-
ing the esterification step which apparently causes the catalyst
deactivation.
c
3.5. Retardation of catalyst deactivation in microwave irradiation
5
10
15
20
25
30
35
40
45
50
55
60
65
70
75
80
In acylation reaction, acylium ion is generated by the pro-
tonation and dehydration of carboxylic acid. This acylium ion
oriented on the polar acid sites of catalyst in a specific way leads to
2θ (degree)
Fig. 6. XRD pattern of (a) p-TSAC fresh, (b) p-TSAC(MW) and (c) p-TSAC(CH).

186
N.J. Venkatesha et al. / Journal of Molecular Catalysis A: Chemical 392 (2014) 181–187
Table 4
Conversion of Ester
Effect of heating rate on the conversion of decanoic acid.
70
60
50
40
30
20
10
0
Ketone
PC
(a)
Time to reach
463 K
Reaction time
at 463 K (min)
Conversion of
DA (%)
DA
123 s
5 min
10 min
30 min
60 min
10
10
10
10
10
50
50
50
50
50
Reaction condition: catalyst amount, 1 g; mole ratio PC:DA, 2:1; temperature, 463 K.
a preset temperature followed the order, decanoic acid > octanoic
acid > hexanoic acid > butyric acid > propanoic acid > acetic acid > p-
cresol > water. However time required for the reaction mixture
consisting p-cresol, carboxylic acid [1] and p-TSA clay was almost
same irrespective of the carboxylic acids. It is important to note
that p-cresol with decanoic acid along with the catalyst required
58 s and 40 min in microwave and conventional heating mode
respectively. Heating time to reach reaction temperature of 463 K
was varied from 123 s to 60 min in microwave mode. After reaching
463 K the reaction mixture was maintained at this temperature
for 10 min in all the cases. Results are given in Table 4. It is
quite evident from this table that irrespective of heating time
required to achieve the pre-set reaction temperature, conversion
of the reactant decanoic acid remained the same (50%) indicating
suppression of coke precursors from acylium ion.
According to Kappe et al. [22], in the majority of cases the rea-
son for the observed rate enhancements is a purely thermal/kinetic
effect, that is, a consequence of the high reaction temperatures
that can rapidly be attained when irradiating polar materials in
a microwave field. Present study, however, reveals yet another
aspect of the effect of microwaves on the reaction path wherein
the formation of coke precursors, in some way, is retarded by the
microwaves. The acylium ions adsorbed on the polar acid sites of
the clay have less time to orient themselves to generate coke pre-
cursors but enough time for the interaction with PC to form the
ester. Apparently, microwaves cause the polar acid sites to change
their orientations with the magnetic field not allowing the acylium
ions to position themselves to facilitate the formation of coke pre-
cursors.
0
20
40
60
80
100 120 140 160 180 200
Reaction time, min
80
70
60
50
40
30
20
10
0
Conversion of Ester
Ketone
PC
DA
(b)
0
4
8
12 16 20 24 28 32 36 40 44
Reaction time, hours
Fig. 8. Fries rearrangement of p-cresyl decanoate versus reaction time under (a)
MW heating, and (b) conventional heating. Reaction conditions: catalyst amount,
0.5 g; p-cresyldecanoate, 10 mmol; temperature, 463 K.
4. Conclusions
formation of ketene and polymerization of ketene leads to higher
hydrocarbon, which covers the active sites and deactivates the
catalyst [1,23]. Although acylium ions are formed in microwave
heating, their effect in causing deactivation probably is reduced
in presence of microwaves. The microwave radiations apparently
favor the non-promotion of coke precursors being formed from
the acylium ions favoring the esterification and further conver-
sion of ester to ketone. Influence of microwave heating mode
in suppression of coke precursor formation was confirmed by
measuring the time required to reach the reaction temperature
by individual reactants and reaction mixture. Measured values are
given in Table 3. The values showed that the time required to reach
Higher catalytic activity was observed in the microwave irra-
diated reaction with a significantly short reaction time compared
with conventional heating mode in all aspects of the solvent-less
acylation of PC with DA over p-TSAC. The reaction mechanism
was similar between the two modes of heating but exhibiting an
increased rate of reaction in microwave heating mode. Lower con-
version observed under conventional heating was attributed to
catalyst deactivation caused by the acylium ions accelerating for-
mation of coke precursors.
Several reports on microwave induced reactions suggest that
the enhancement in the rate is a consequence of rapid attainment
of high temperatures in the reaction medium comprising polar
materials and is purely a thermal/kinetic effect. Present study on
acylation reveals yet another aspect of the effect of microwaves on
the reaction path wherein the formation of coke precursors, in some
way, is retarded by the microwaves. The acylium ions adsorbed
on the polar acid sites of the clay probably have less time to ori-
ent themselves for the formation of coke precursors but enough
time for the interaction with PC to form the ester. Apparently,
microwaves cause the polar acid sites to change their orientations
with the magnetic field not allowing the acylium ions to position
themselves to facilitate the formation of coke precursors.
Table 3
Time required reaching 373 K, when 30 mmol of individual compound was heated
at a constant power of 1200 W under microwave heating.
Compound
Time taken to
reach 373 K (s)
Compound
Time taken to
reach 373 K (s)
Acetic acid
20
25
31
49
80
Decanoic acid
p-Cresol
Water
p-TSA clay (4 g)
p-Cresol + decanoic
acid + p-TSA clay
116
17
12
94
57
Propanoic acid
Butyric acid
Hexanoic acid
Octanoic acid

N.J. Venkatesha et al. / Journal of Molecular Catalysis A: Chemical 392 (2014) 181–187
187
Acknowledgements
[7] S.K. Jana, Catal. Survey Asia 10 (2006) 98–109.
[8] J. Kaur, J. Catal. 208 (2002) 448–455.
[9] B.M. Choudary, M. Sateesh, M.L. Kantam, K.V.R. Prasad, Appl. Catal. A 171 (1998)
155–160.
[10] J. Farkas, S. Bekassy, B. Agai, M. Hegedus, F. Figueras, Syn. Commun. 30 (2000)
2479–2485.
[11] G. Wine, E. Vanhaecke, S. Ivanova, R. Ziessel, C.P. Huu, Catal. Commun. 10 (2009)
477–480.
[12] E. Kozhevnikova, Appl. Catal. A 245 (2003) 69–78.
[13] B.M. Choudary, M. Sateesh, M.L. Kantam, K.V.S. Ranganath, K.V. Raghavan, Catal.
Lett. 76 (2001) 231–233.
The authors thank Prof. P. Vishnu Kamath, BU for providing TGA
and ATR facilities and Mr. Raghvendra, RVCE for helping in XRD
analysis of the clay samples. The authors would like to extend their
thanks to the Principal and Governing Council of Bangalore Insti-
tute of Technology for the facilities provided and VTU for financial
support.
[14] S. Ramesh, B.S. Jai Prakash, Y.S. Bhat, Appl. Catal. A 413 (2012) 157–162.
[15] O.S. Ahmed, D.K. Dutta, J. Mol. Catal. A 229 (2005) 227–231.
[16] A.K. Panda, B.G. Mishra, D.K. Mishra, R.K. Singh, Colloid Surf. A: Physiochem.Eng.
Aspects 363 (2010) 98–104.
[17] J. Ravichandran, B. Sivasankar, Clays Clay Miner. 45 (1997) 854–858.
[18] F. Kooli, W. Jones, Clay Miner. 32 (1997) 633–643.
[19] E.F. Kozhevnikova, E. Rafiee, I.V. Kozhevnikov, Appl. Catal. A 260 (2004) 25–34.
[20] L.M. Jackman, M.M. Petrei, B.D. Smith, J. Am. Chem. Soc. 113 (1991) 3459–3466.
[21] G.D. Yadav, G. Ginish, J. Mol. Catal. A 292 (2008) 54–61.
[22] C.O. Kappe, Angew. Chem. Int. Ed. 43 (2004) 6250–6284.
[23] E.A. Gunnewegh, R.S. Downing, H. van Bekkum, L. Bonneviot, S. Kaliaguine,
Stud. Surf. Sci. Catal. 97 (1995) 447.
References
[1] B.M. Chandrashekar, B.S. Jai Prakash, Y.S. Bhat, J. Catal. 290 (2012) 101–107.
[2] G.A. Olah, Friedel-Crafts and Related Reactions: General Aspects, vol. 4, Wiley
Interscience New York, New York, 1963.
[3] P. Metivier, R.A. Sheldon, H.V. Bekkum, Fine Chemicals through Heterogeneous
Catalysts, Wiley-VCH, Weinheim, 2001.
[4] E.G. Derouane, G. Crehan, C.J. Dillon, D. Bethell, H. He, S.B.A. Hamid, J. Catal. 194
(2000) 410–423.
[5] F. Jayat, M.J. Sabater Picot, M. Guisnet, Catal. Lett. 41 (1996) 181–187.
[6] S. Ramesh, Y.S. Bhat, B.S. Jai Prakash, Clay Miner. 47 (2012) 231–242.