Microwave-induced deactivation-free catalytic activity of BEA zeolite in acylation reactions

Article abstract of DOI:10.1016/j.jcat.2012.03.005

Solventless liquid-phase acylation of p-cresol with different aliphatic carboxylic acids like acetic, propionic, butyric, hexanoic, octanoic, and decanoic acids was investigated over BEA zeolite under conventional as well as microwave heating. An unanticipated huge difference in activity was observed between two modes of heating. Under conventional heating, conversion of all the acids was less than 20%, while under microwave heating, the conversion was in the range of 50-80%. Ester formed through O-acylation and ortho-hydroxyketone formed through Fries rearrangement of the ester were the only products. Conversion of carboxylic acid increased with chain length up to hexanoic acid and then it showed a decrease in the trend. With all the acids, O-acylation occurred rapidly followed by slow conversion to ortho-hydroxyketone. The ketone/ester ratio increased with catalyst amount, temperature, and reaction time. Used catalyst samples were characterized by TGA, XRD, and IR studies to understand lower activity and deactivation behavior under conventional heating. The results showed absence of coke precursor/coke on the catalyst used in microwave-irradiated reactions in contrast to catalyst used in conventionally heated ones. Higher yield in the case of microwave-assisted reactions is attributed to the prevention of coke precursor/coke on the active sites by microwaves.

Full text of DOI:10.1016/j.jcat.2012.03.005

Journal of Catalysis 290 (2012) 101–107
Contents lists available at SciVerse ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Microwave-induced deactivation-free catalytic activity of BEA zeolite
in acylation reactions
⇑
B.M. Chandra Shekara, B.S. Jai Prakash, Y.S. Bhat
Department of Chemistry, 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:
Solventless liquid-phase acylation of p-cresol with different aliphatic carboxylic acids like acetic, propi-
onic, butyric, hexanoic, octanoic, and decanoic acids was investigated over BEA zeolite under conventional
as well as microwave heating. An unanticipated huge difference in activity was observed between two
modes of heating. Under conventional heating, conversion of all the acids was less than 20%, while under
microwave heating, the conversion was in the range of 50–80%. Ester formed through O-acylation and
ortho-hydroxyketone formed through Fries rearrangement of the ester were the only products. Conver-
sion of carboxylic acid increased with chain length up to hexanoic acid and then it showed a decrease
in the trend. With all the acids, O-acylation occurred rapidly followed by slow conversion to ortho-hydrox-
yketone. The ketone/ester ratio increased with catalyst amount, temperature, and reaction time. Used
catalyst samples were characterized by TGA, XRD, and IR studies to understand lower activity and deac-
tivation behavior under conventional heating. The results showed absence of coke precursor/coke on the
catalyst used in microwave-irradiated reactions in contrast to catalyst used in conventionally heated ones.
Higher yield in the case of microwave-assisted reactions is attributed to the prevention of coke precursor/
coke on the active sites by microwaves.
Received 25 January 2012
Revised 6 March 2012
Accepted 7 March 2012
Available online 18 April 2012
Keywords:
Microwave irradiation
BEA
Catalyst deactivation
Acylation
P-cresol
Ó 2012 Elsevier Inc. All rights reserved.
1. Introduction
molecules is a potential solid acid for various organic transforma-
tions [5,6]. It has been studied as a catalyst in the acylation and re-
The acylation of phenolic substrates is an important organic
reaction in the fine and specialty chemical industries. Direct acyl-
ation of hydroxyl group yields ester, which can be converted to
ketone by subsequent Fries rearrangement. Both ester and ketone
formed in the reaction are valuable intermediates in the synthesis
of pharmaceuticals and fragrances [1]. Acylation reactions are gen-
erally catalyzed by strong protonic acids and Lewis acids such as
AlCl3 and BF3. These catalysts, besides being highly corrosive, are
often required to be used in more than stoichiometric amounts.
They are difficult to handle, not reusable, and the work-up proce-
dure generates large amounts of polluting wastes [2]. Efforts have
been made to replace these acids by reusable, eco-compatible het-
erogeneous solid acids like zeolites, acidic clays, and heteropoly
acids [3]. It is also highly desirable to use less hazardous acylating
agents like carboxylic acids instead of acyl halides and anhydrides.
Acidic zeolites are the most studied heterogeneous catalysts for the
acylation, and in an important development, Rhodia company has
established the first industrial-scale application of zeolites for the
acylation of anisole and veratrole [4]. BEA zeolite with tunable
acidity and three-dimensional large pore accessibility to reacting
lated Fries rearrangement of several organic substrates. Acylation
of arenes like toluene, biphenyl, and naphthalene, aromatic ethers
like anisole, thioanisole, veratrole, and 2-methoxy naphthalene,
heterocyclic compound like thiophene, and phenolic compounds
like phenol and resorcinol have been investigated using BEA zeolite
as catalyst [7–17]. Major problems faced in the acylation over zeo-
lites are lower conversions and longer reaction times, especially
when less-reactive acylating agents like carboxylic acids are used
[18]. Main reason for lower rate is the initial inhibition of the reac-
tion due to preferential adsorption of carboxylic acids and deacti-
vation of the catalyst [19]. Acidity, hydrophobicity, and crystallite
size of BEA zeolite were found to be the other important factors
influencing the conversion and catalyst deactivation [20].
Microwave-assisted organic synthesis has attracted a consider-
able interest in recent years. Principally, in all type of thermally
driven chemical reactions, use of microwave irradiation as a source
of heat can profoundly reduce the reaction times [21–23]. Even
though there is lot of scope to improve the efficiency of acylation
reactions over zeolites under microwave heating, reports of these
are scarce in the literature [24,25]. Acylation of phenolic substrates
using carboxylic acids over zeolites is one such reaction, where
microwave could play a greater role because all the reactants
and intermediates are polar compounds, which are very good
⇑
Corresponding author. Tel.: +91 80 26615865; fax: +91 80 26426796.
E-mail address: BHATYS@yahoo.com (Y.S. Bhat).
0021-9517/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved.
http://dx.doi.org/10.1016/j.jcat.2012.03.005

102
B.M. Chandra Shekara et al. / Journal of Catalysis 290 (2012) 101–107
absorbers of microwave radiation. The main objective of the pres-
ent study is to investigate the acylation of p-cresol with different
aliphatic carboxylic acids mainly under microwave irradiation
and to compare with conventional thermal heating. Further objec-
tive is to study the behavior of catalyst in the reactions carried out
under microwave and conventional heating.
performed by Chemito GC1000 gas chromatograph using BP-20
capillary column (30 m ꢁ 0.32 mm) and flame ionization detector.
The products were also analyzed and confirmed by GC-MS.
3. Results and discussion
In order to find the optimum reaction conditions, effect of
various parameters like reaction temperature, catalyst amount,
reactant mole ratio, and reaction time was studied for acylation
of p-cresol (PC) with hexanoic acid (HA) over BEA zeolite under
microwave heating. Under optimum conditions, acylation of p-cre-
sol with different carboxylic acids was studied under microwave as
well as conventional heating over BEA zeolite.
2. Experimental
2.1. Catalysts and chemicals
BEA zeolite in NHþ4 form (Si/Al = 30) was gifted by Süd-Chemie
India Ltd. It was calcined in dry air for 8 h at 813 K to obtain pro-
tonic form of BEA zeolite. All other chemicals used in this study
were procured from SD fine chemicals, India. p-Cresol and carbox-
ylic acids were distilled before use, and all other chemicals were
used without further purification.
3.1. Acylation of p-cresol with hexanoic acid under microwave heating
3.1.1. Effect of catalyst amount
Results of the effect of catalyst amount on the acylation of PC
with HA are shown in Fig. 1. Under the conditions mentioned, PC
did not react with HA in the absence of catalyst. Initially, conver-
sion of HA increased with the catalyst amount, reached maximum
with 0.5 g, and thereafter, it decreased slightly. Product distribu-
tion also varied with the amount of the catalyst. With lower cata-
lyst amount, O-acylation was predominant, resulting in the
formation of ester with only small amounts of C-acylated product
(ketone). The amount of ester increased with the increase in cata-
lyst amount, reached a maximum, and then decreased with con-
comitant increase in ketone concentration. Increase in conversion
of HA and shift in selectivity toward ketone with increase in cata-
lyst amount are apparently due to increased number of catalyti-
cally active sites for the O-acylation of PC and subsequent Fries
rearrangement of ester to ketone.
2.2. Characterization
Acidity of the fresh BEA sample and of those used in the reac-
tions under conventional and microwave heating was measured
by IR spectroscopy using pyridine as probe molecule. All the sam-
ples were activated by degassing at 383 K for 2 h and then satu-
rated with pyridine. The catalyst samples were then evacuated at
423 K for 2 h to remove physisorbed pyridine. IR spectra of the
samples were then recorded in the range 400–4000 cmꢀ1 using
Bruker model Alpha-P IR spectrophotometer having resolution of
4 cmꢀ1 fitted with a diamond ATR cell. The catalyst samples used
under microwave and conventional heating were also character-
ized by thermogravimetric analysis (TGA) to study the deactivation
by coke precursors. TGA studies were carried out using a Mettler
Toledo 851e stare 7.01 TGA/SDTA system. The temperature was
ramped from 303 to 873 K at the rate of 5 K minꢀ1 under flowing
air. Structural integrity of the catalyst samples after the reaction
was checked by powder XRD. The data were recorded by step-
scanning at 2h = 0.025° per step from 3° to 40° on Philips X’pert
3.1.2. Effect of temperature
The influence of temperature on the acylation of PC with HA
was studied under otherwise similar conditions by varying tem-
perature from 403 to 463 K, and the results are shown in Fig. 2.
Temperature showed a marked effect on the conversion of HA
and distribution of products. Conversion of HA increased with in-
crease in temperature indicating endothermic nature of the reac-
tion. At lower temperature, HA was more selective toward ester,
but with increase in temperature, selectivity toward ketone
increased. This suggests that O-acylation is favoured at lower tem-
perature, and C-acylation, probably occurring through Fries rear-
rangement of the ester formed, is favoured at higher temperatures.
PRO X-ray diffractometer with graphite monochromatized Cu K
radiation (k = 1.5405 Å).
a
2.3. Catalytic reaction
Microwave-heated reactions were carried out in a 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. All the reactions
were carried out in a 50-mL closed glass vessel. All the reactions
were stirred with the help of built-in automatic magnetic stirrer
using Teflon stirring bar. Reactor vessel was kept in such a way that
the reaction mixture was exactly in line with infrared sensor that
monitors the temperature. Variable power up to 1000 W was ap-
plied by microprocessor-controlled single-magnetron system. The
maximum irradiation power of 1000 W was utilized in the initial
1.0 min of the reaction to attain the temperature of 463 K, and
then, lower power was used to maintain that temperature. Reac-
tions under conventional heating were carried out in stainless steel
autoclave fitted with 50-mL Teflon vessel. Autoclave was heated in
a hot air oven with microprocessor-based temperature controller.
In a typical reaction procedure, 20 mmol of p-cresol, 10 mmol of
carboxylic acid, and 0.5 g of catalyst were taken in a reaction ves-
sel. The reaction was carried out at desired temperature without
using solvent. After the reaction, reaction mixture was dissolved
in 15 g of toluene, stirred for about 30 min to extract adsorbed
reactants and products, and filtered to separate the catalyst. Anal-
ysis of the reaction ingredients before and after the reaction was
80
70
60
50
40
30
20
Conversion of HA
Yield of Ester
10
Yield of Ketone
0
0.0
0.2
0.4
0.6
0.8
1.0
Catalyst amount, g
Fig. 1. Effect of catalyst amount on the acylation of p-cresol with hexanoic acid.
Reaction condition: mole ratio PC: HA, 2:1; reaction time, 60 min; temperature,
463 K; max MW power, 1000 W.

B.M. Chandra Shekara et al. / Journal of Catalysis 290 (2012) 101–107
103
80
70
60
50
40
30
20
10
0
Conversion of HA
Yield of Ester
80
60
40
20
Yield of Ketone
Conversion of HA
Yield of ester
Yield of ketone
400
410
420
430
440
450
460
470
0
Reaction temperature, K
0
20
40
60
80
100
120
140
160
180
200
Fig. 2. Effect of temperature on the acylation of p-cresol with hexanoic acid.
Reaction condition: catalyst amount, 0.5 g; mole ratio PC:HA, 2:1; reaction time,
60 min; max MW power, 1000 W.
Reaction time, min
Fig. 3. Effect of reaction time on the acylation of p-cresol with hexanoic acid.
Reaction condition: catalyst amount, 0.5 g; mole ratio PC: HA, 2:1; temperature,
463 K; max MW power, 1000 W.
3.1.3. Effect of mole ratio of reactants
Effect of mole ratio of reactants on conversion and selectivity
was studied by varying the mole ratio of PC: HA from 3:1 to 1:3,
other conditions being similar. Results are tabulated in Table 1.
When the mole ratio of PC: HA was 1:1, 52% conversion of HA
was observed with 58% and 42% selectivity toward ester and
ketone, respectively. But the conversion of HA increased drastically
to 77% with increased selectivity toward ketone (57%) when the
amount of PC in the reaction mixture was doubled. Generally, acyl-
ation reaction occurs through formation of an acylium intermediate
by protonation of nucleophilic carbonyl oxygen of carboxylic acid
by Bronsted acid site of catalyst and subsequent attack of substrate
by acylium ion [26]. Increased conversion of HA with increase in the
amount of PC may be attributed to ready availability of PC for elec-
trophilic attack by acylium ion. On the other hand, when the
amount of HA was doubled, conversion of PC decreased to 45% with
decreased selectivity toward ketone, which indicates the inhibitive
effect of carboxylic acid on the reaction.
the hydroxyl group and stabilization of transition state and the
product ketone by intramolecular hydrogen bonding between the
hydrogen of hydroxyl and oxygen of the carbonyl group [27]. Possi-
ble mechanism of the reaction consistent with the above made
observations is given in Scheme 1.
3.2. Acylation of p-cresol with different carboxylic acids under
microwave and conventional heating
Effect of reaction time on the conversion of different carboxylic
acids in the acylation of PC with acetic, propionic, butyric, hexa-
noic, octanoic, and decanoic acid was studied under optimum reac-
tion conditions of temperature 463 K, mole ratio of PC: carboxylic
acid 2:1 and catalyst amount of 0.5 g. Reaction time was varied
from 3 to 240 min under microwave heating and 1–30 h under
conventional heating. Conversion of different carboxylic acids as
a function of reaction time under microwave and conventional
heating is shown in Figs. 4 and 5, respectively.
3.1.4. Effect of reaction time
Conversion of HA as a function of reaction time under otherwise
similar conditions is shown in Fig. 3. Conversion of HA was rapid in
the beginning; 37% conversion with 100% selectivity toward ester
was observed in first 3 min of the reaction. However, as the reaction
proceeds, conversion increased slowly to a maximum of 80% at
180 min with decreased selectivity toward ester (37%), accompa-
nied by an increase in ketone selectivity (63%). The initial selectivity
toward ketone was zero, suggesting that it was not a primary prod-
uct. The decrease in the concentration of ester with concomitant in-
crease of the ketone suggests that it was not formed through direct
acylation of aromatic ring, but through the Fries rearrangement of
the primary product ester. BEA zeolite has been reported as a good
catalyst for this type of rearrangement [17]. Conversion of ester to
o-hydroxyketone seems to be favoured by ortho-directing effect of
Large difference in conversion was observed between two
modes of heating. Under conventional heating, reaction proceeded
very slowly, and at the end of 30 h, all the acids showed conversion
of 10–20%. On the other hand, under microwave heating, conver-
sion of different acids was in the range of 50–80%. The ester formed
through O-acylation and ortho-hydroxyketone formed through
Fries rearrangement of the ester were the only products. With all
the acids, ester was initially formed followed by conversion to
ortho-hydroxyketone. Selectivity of different acids toward ester
and ortho-hydroxyketone after 240 min of reaction time is shown
in Fig. 6.
The ketone/ester ratio increased with reaction time, and the
equilibrium ratio in all the cases was higher than 1. Conversion
of carboxylic acid increased with chain length up to hexanoic acid,
Table 1
Effect of mole ratio of reactants on the acylation of p-cresol with hexanoic acid.
Mole ratio (PC: HA)
Conversion of HA (%)
Conversion of PC (%)
Yield of ester (%)
Yield of ketone (%)
3:1
2:1
1:1
1:2
1:3
78
77
52
–
–
–
–
45
42
34
33
30
32
30
44
44
22
13
12
–
Reaction condition: Catalyst amount, 0.5 g; reaction time, 60 min; temperature, 463 K; max MW power, 1000 W.

104
B.M. Chandra Shekara et al. / Journal of Catalysis 290 (2012) 101–107
Zeolite -
H+
Zeolite -
O
H
H
H
O
O
R
+
H
O
R
O
+
O
O
+
R
H
O
O
O
R
-H2O
R
R= Alkyl group
Ester
Zeolite -
H
O
O
+
H
H
O
+
H
O
+
O
O
O
R
O
R
R
H
R
ortho-hydroxy ketone
Zeolite -
Scheme 1. Possible reaction mechanism for the acylation of p-cresol with carboxylic acid.
but decreased in the case of octanoic and decanoic acids. Variation
90
80
70
60
50
40
30
20
10
0
of conversion with the chain length of the carboxylic acid could be
attributed mainly to two factors that operate simultaneously, but
in opposite directions. First factor is the stability of intermediate
acyl cation, which increases with chain length of acid due to elec-
tron-donating effect of carbon chain, and the second factor is diffu-
sion constraint, which increases with chain length of acid and
limits the conversion [28,29]. It appears that up to hexanoic acid,
the positive inductive effect of carbon chain predominates, and
in case of octanoic and decanoic acids, diffusion limitations
become a major factor.
Acetic acid
Propionic acid
Butyric acid
Hexanoic acid
Octanoic acid
Decanoic acid
3.3. Catalyst deactivation and reusability
Reactions under microwave heating were not only faster but
also showed higher conversion and selectivity when compared to
conventional heating. Under conventional heating, conversion
was very low initially, and with increase in reaction time, it in-
creased gradually before reaching a value that is very much lower
than the thermodynamic equilibrium value. This is an indication of
catalyst deactivation. Color of the catalyst used under conventional
0
20 40 60 80 100 120 140 160 180 200 220 240 260
Reaction time, min
Fig. 4. Conversion of different carboxylic acids versus reaction time in the acylation
of p-cresol under microwave heating. Reaction condition: catalyst amount, 0.5 g;
mole ratio PC: carboxylic acid, 2:1; temperature, 463 K; max MW power, 1000 W.
100
80
60
50
40
Acetic acid
Propionic acid
Butyric acid
Hexanoic acid
Octanoic acid
Decanoic acid
60
40
20
0
30
Towards ester
Towards ketone
20
10
0
0
5
10
15
20
25
30
Reaction time, h
Fig. 5. Conversion of different carboxylic acids versus reaction time in the acylation
of p-cresol under conventional heating. Reaction condition: Catalyst amount, 0.5 g;
mole ratio PC: carboxylic acid, 2:1; temperature, 463 K.
Fig. 6. Selectivity of different carboxylic acids toward acylated products under
microwave heating. Reaction condition: catalyst amount, 0.5 g; Mole ratio PC:
carboxylic acid, 2:1; temperature, 463 K; reaction time, 240 min.

B.M. Chandra Shekara et al. / Journal of Catalysis 290 (2012) 101–107
105
heating showed 42% conversion of HA. Thus, the activity of
BEAR(CH) sample under microwave heating was found to be much
higher than that exhibited by fresh BEA zeolite under conventional
heating. This shows that the active sites of BEAR(CH) sample were
not completely deactivated by coke precursors. This was further
investigated by acidity measurements of the recovered catalyst
samples by IR spectroscopy in ATR mode using pyridine as probe
molecule. Chemisorbed pyridine on the Lewis and on the Bronsted
acid sites of zeolite BEA is revealed by characteristic bands at 1450
and 1545 cmꢀ1, respectively. Intense peak at 1491 cmꢀ1 is ascribed
to the superposition of signals of Lewis and Bronsted adsorbed spe-
cies [31]. In the spectra displayed in Fig. 8, Bronsted acid peak of
BEAR(CH) sample was broadened with decreased intensity when
compared to BEA and BEAR(MW) samples, suggesting that the
Bronsted acid sites of this deactivated sample are decreased. Slight
shift in the 1545 cmꢀ1 peak suggests the presence of coke precur-
sors on the Bronsted sites. In order to check the regenerability,
BEAR(CH) sample was calcined at 813 K for 8 h and tested in the
reaction. Activity of this calcined sample was found to be similar
to fresh BEA zeolite. The structural integrity of the sample after cal-
cination was checked by X-ray diffraction. The XRD patterns of the
BEA, BEAR(CH), and BEAR(MW) catalysts as shown in Fig. 9 indi-
cate that the structure of the zeolite remains intact even after
the reaction.
The foregoing studies show that the microwave heating is supe-
rior to conventional heating in all aspects of the reaction. Micro-
wave irradiation not only accelerates the reaction rate but also
enhances the activity and selectivity of BEA zeolite toward acyla-
tion and stabilizes the catalytic activity. On the other hand, under
conventional heating, reactions were found to be sluggish from the
beginning and finally resulted in very low yields due to catalyst
deactivation. Similar lower conversions have been reported under
conventional heating in the zeolite-catalyzed acylation of organic
substrates with carboxylic acids [9]. These reactions are known
to be inhibited initially by strong adsorption of reactants on the
surface, followed by decomposition of resulting acylium ion to ke-
tene, which can condense to diketene, and further polymerize to
high-molecular-weight compounds ultimately blocking the active
sites leading to deactivation of the catalyst [32].
100
90
80
70
60
BEA
BEAR(MW)
BEAR(CH)
300
400
500
600
700
800
900
Temperature, K
Fig. 7. TGA plots of fresh and used BEA samples.
heating changed to dark brown, but the catalyst used under micro-
wave changed to pale yellow at the end of the reaction. For further
investigations, catalyst samples from the reaction of PC with HA
under microwave and conventional heating were recovered after
4 and 30 h of the reaction and were, respectively, designated as
BEAR(MW) and BEAR(CH). To recover the catalyst, reaction mix-
ture was stirred with 15 g of toluene for 30 min; the solid phase
was separated from the reaction mixture by filtration and dried
at 393 K for 4 h before further use. Mass of catalyst recovered from
the conventionally heated reaction, BEAR(CH), was found to be
higher than the mass of catalyst sample used in the reaction, indi-
cating coke/coke precursors built up on the surface. However, neg-
ligible change was observed in the mass of catalyst recovered from
microwave-heated reaction, BEAR(MW). The recovered catalyst
samples were characterized by TGA, IR, and XRD. Thermogravimet-
ric analysis (TGA) plots of BEA, BEAR(MW), and BEAR(CH) samples
are shown in Fig. 7. Initial weight loss of the samples up to 423 K is
mainly due to removal of adsorbed moisture, whereas the weight
loss above this temperature could be attributed to coke precursors
[30]. It can be seen that at 873 K, percent weight loss of BEAR(MW)
sample was just 2% higher than that of BEA sample, while that of
BEAR(CH) sample was more than 20%. This clearly shows the pres-
ence of coke precursors on BEAR(CH) sample. Reusability of these
samples was checked in the reaction of PC with HA under micro-
wave as well as conventional heating, and the results are shown
in Table 2. BEAR(MW) catalyst when reused under microwave
heating showed similar conversion as that of fresh BEA zeolite,
which shows that the catalyst was not deactivated in the micro-
wave-heated reaction. But, BEAR(MW) catalyst when reused
showed lower conversions similar to fresh BEA under conventional
heating. BEAR(CH) when reused under conventional heating also
showed very low HA conversion (5%), but under microwave
Reaction of PC with carboxylic acid takes place in two steps first
through direct O-acylation resulting in ester next by Fries rear-
rangement of ester to ortho-hydroxyketone. In order to understand
the effect of microwave and conventional heating on these two
reactions separately, direct Fries rearrangement of the ester, p-cre-
syl hexanoate, was investigated over BEA zeolite under microwave
as well as conventional heating. The results obtained are shown in
Figs. 10 and 11, respectively. Even though reaction proceeded at a
faster rate under microwave heating, no difference in final conver-
sion and selectivity was observed between the two modes of heat-
ing. Yield of ortho-hydroxy ketone obtained at the end of the
reaction was found to be same as that obtained in direct reaction
Table 2
Reusability of fresh and used BEA catalysts.
Catalyst
Mode of heating
Conversion of HA (%)
Yield of ester (%)
Yield of ketone (%)
BEA
Microwave
Microwave
Microwave
Microwave
Microwave
Conventional
Conventional
Conventional
80
77
79
42
78
16
17
5
28
28
29
18
29
6
52
49
50
24
49
10
11
2
BEAR(MW)
BEAR(MW) calcined
BEAR(CH)
BEAR(CH) calcined
BEA
BEAR(MW)
BEAR(CH)
6
3
Reaction condition: Catalyst amount, 0.5 g; Mole ratio PC: HA, 2:1; temperature, 463 K. Microwave heating; reaction time, 240 min; max MW power, 1000 W. Conventional
heating; reaction time, 30 h.

106
B.M. Chandra Shekara et al. / Journal of Catalysis 290 (2012) 101–107
a
b
c
1.00
1450
1545
1491
1350
1380
1410
1440
1470
1500
1530
1560
1590
1620
Wavenumber, cm-1
Fig. 8. IR spectra of pyridine adsorbed on (a) BEA, (b) BEAR(CH), and (c) BEAR(MW) samples.
70
60
50
40
30
20
10
0
Conversion of ester
Yield of Ketone
HA formed
c
PC formed
b
a
0
20
40
60
80
100 120 140 160 180 200
Reaction time, min
Fig. 10. Fries rearrangement of p-cresyl hexanoate versus reaction time under
microwave heating. Reaction condition: catalyst amount, 0.5 g; p-cresyl hexanoate,
10 mmol; temperature, 463 K; max MW power, 1000 W.
0
5
10
15
20
25
30
35
40
45
2θ
Fig. 9. XRD patterns of (a) BEA, (b) BEAR(CH), and (c) BEAR(MW) samples.
reactants was found to be in the order water < p-cresol < acetic
acid < propanoic acid < butyric acid < hexanoic acid < octanoic
acid < decanoic acid. Heat-up time of carboxylic acids increased
with increase in chain length. However, the heating rate of reaction
mixture containing p-cresol, carboxylic acid, and BEA zeolite was
found to be same as that of p-cresol irrespective of the carboxylic
acid. This mixture was heated to 373, 403, and 463 K in 17, 28, and
98 s, respectively, while the conventionally heated reaction
required 35 min to reach 463 K. This difference in heating rate be-
tween two modes of heating was thought to be the reason for the
lower conversion and formation of coke precursors in conventional
heating. To check the effect of heating rate on conversion of HA, a
set of reactions were conducted with different microwave pro-
grams, so that the desired temperature (463 K) was reached in dif-
ferent times, and the results are shown in Table 4. Same conversion
was seen, and no deactivation of the catalyst samples was noticed
irrespective of the heat-up time. This indicates that initial inhibi-
tion of O-acylation and subsequent formation of coke precursors
under conventional heating are not due to slow heating of reaction
mixture. But, microwave irradiation was found to have perceptible
influence on the reaction rate. One important difference in the two
methods of heating is generation of coke material on the catalyst
of PC with HA under microwave heating. This clearly shows that
the lower conversions observed under conventional heating in
the reaction of PC with carboxylic acids are mainly due to inhibi-
tion of O-acylation reaction. Hence, the advantageous effects of
microwave heating in the acylation of PC with HA must be due
to its ability to prevent the initial inhibition of O-acylation that
was observed under conventional heating.
3.4. Promoting action of microwaves
In the reactions conducted under microwave irradiation, heat-
ing of reaction mixture depends on the ability of chemical materi-
als used in the reaction to absorb microwave energy and
subsequent release of this as heat energy. Heat-up time of different
reactants and catalyst used in the reaction was measured in terms
of time taken by a known quantity of compound to heat up to a
particular temperature with constant microwave power of
1000 W. Time required to reach 373 K by different reactants, BEA
zeolite, and water are given in Table 3. Heat-up time of different

B.M. Chandra Shekara et al. / Journal of Catalysis 290 (2012) 101–107
107
4. Conclusions
70
60
50
40
30
20
10
0
Microwave heating was found to be superior to conventional
heating in all aspects of the solventless acylation of p-cresol with
different carboxylic acids over BEA zeolite. Microwave-heated reac-
tions resulted in higher conversions of carboxylic acids with the for-
mation of ester through O-acylation as the primary product
followed by the Fries rearrangement to form ortho-hydroxy ketone.
Present study showed that lower conversion observed under con-
ventional heating was due to initial inhibition of O-acylation and
catalyst deactivation. p-Cresol shows low heat-up times in the
MW-heated reaction and is chiefly responsible for heating up of
the reaction mixture. The preferential adsorption of carboxylic acid,
which is known to be responsible for coke precursor formation, is
probably affected by the heat-up energy released by p-cresol to
the medium. This might have prevented the initial inhibition of O-
acylation reaction as well as catalyst deactivation and allowed the
catalyst to function with maximum activity and increased stability.
Conversion of ester
Ketone
HA formed
PC formed
0
2
4
6
8
10
12
Reaction time, h
Fig. 11. Fries rearrangement of p-cresyl hexanoate versus reaction time under
conventional heating. Reaction condition: catalyst amount, 0.5 g; p-cresyl hexano-
ate, 10 mmol; temperature, 463 K.
Acknowledgements
The authors thank the Principal and the Governing Council of
Bangalore Institute of Technology for the facilities provided.
Thanks are due to Süd chemie India Pvt Ltd. for providing zeolite
sample. The authors would like to extend their thanks to Prof.
PVK, BU for TGA and ATR facilities.
Table 3
Time required to reach 373 K when 30 mmol of compound was heated at a constant
MW power of 1000 W.
Compound Time taken to
reach 373 K (s)
Compound
Time taken to
reach 373 K (s)
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Decanoic acid
p-Cresol
116
17
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microwave heating leads one to say that the interaction of reactant
molecules on the surface of the catalyst is certainly influenced by
the method of heating.
In conventionally heated reaction, initial inhibition and coke
formation are primarily observed in the O-acylation reaction,
where both p-cresol and carboxylic acid are involved. It is reported
that one of the main reasons for initial inhibition of the reaction is
due to preferential adsorption of carboxylic acid [33]. p-Cresol is
chiefly responsible for heating up of the reaction mixture as no-
ticed by its low heat-up time. Consequently, the heat-up energy
of p-cresol subsequently released to the medium might be affect-
ing the adsorption of the carboxylic acid. This is probably respon-
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surface of the catalyst. Due to this, the catalyst exhibits deactiva-
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ther investigation.