Hierarchical zeolite Beta: An efficient and eco-friendly nanocatalyst for the Friedel-Crafts acylation of toluene

Article abstract of DOI:10.1166/jnn.2013.7153

P-Methyacetophenone, the acylated product of toluene finds a wide range of applications in the flavors and fragrance industry. It is typically produced on an industrial scale by Friedel-Crafts acylation of toluene with acetic anhydride using homogeneous, corrosive and polluting acid catalysts such as aluminium chloride. The pollution problems related to this process such as the disposal of catalyst and treatment of acidic effluent needs to be replaced by a green process. The current work reports on the activity of hierarchical zeolite Beta in the liquid phase acylation of toluene with acetic anhydride. The liquid phase reactions were carried out in the temperature range of 60-140 °C in an autoclave. The effect of various reaction parameters such as time-on-stream (TOS), mole ratio of reactants, catalyst loading, and reaction temperature on the rates of reaction has been investigated. Under the optimum reaction conditions the performance of hierarchical zeolite Beta was compared with nanocrystalline zeolite Beta. It was found that hierarchical zeolite Beta catalyst exhibit higher activity, which is due to the hierarchical porosity and to the nano size of the Beta zeolite catalyst particles allows faster diffusion of the products out of the catalyst. Copyright

Full text of DOI:10.1166/jnn.2013.7153

Copyright © 2013 American Scientific Publishers
All rights reserved
Printed in the United States of America
Journal of
Nanoscience and Nanotechnology
Vol. 13, 4415–4420, 2013
Hierarchical Zeolite Beta: An Efficient and Eco-Friendly
Nanocatalyst for the Friedel–Crafts Acylation of Toluene
Rosilda Selvin1ꢀ∗, L. Selva Roselin2, Suzan A. Khayyat1, and Ahmad Umar3ꢀ4
1Department of Chemistry, King Abdulaziz University (Girls Campus), Jeddah 21589, Kingdom of Saudi Arabia
2Department of Chemistry, Periyar Maniammai University, Periyar Nagar, Vallam, Thanjavur, Tamil Nadu 613403, India
3Faculty of Arts and Sciences, Department of Chemistry, Najran University, Najran-11001, Kingdom of Saudi Arabia
4Promising Centre for Sensors and Electronic Devices, Najran University, Najran-11001, Kingdom of Saudi Arabia
P-Methyacetophenone, the acylated product of toluene finds a wide range of applications in the
flavors and fragrance industry. It is typically produced on an industrial scale by Friedel–Crafts acy-
lation of toluene with acetic anhydride using homogeneous, corrosive and polluting acid catalysts
such as aluminium chloride. The pollution problems related to this process such as the disposal of
catalyst and treatment of acidic effluent needs to be replaced by a green process. The current work
reports on the activity of hierarchical zeolite Beta in the liquid phase acylation of toluene with acetic
anhydride. The liquid phase reactions were carried out in the temperature range of 60–140 ꢀC in an
autoclave. The effect of various reaction parameters such as time-on-stream (TOS), mole ratio of
reactants, catalyst loading, and reaction temperature on the rates of reaction has been investigated.
Under the optimum reaction conditions the performance of hierarchical zeolite Beta was compared
with nanocrystalline zeolite Beta. It was found that hierarchical zeolite Beta catalyst exhibit higher
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activity, which is due to the hierarchical porosity and to the nano size of the Beta zeolite catalyst
IP: 63.142.228.125 On: Wed, 28 Oct 2015 11:00:27
particles allows faster diffusion of the products out of the catalyst.
Copyright: American Scientific Publishers
Keywords: Zeolite, Beta Zeolite, Hierarchical Zeolite, Nanozeolite, Acylation, Toluene.
1. INTRODUCTION
such as zeolites, clays and heteropolyacids.4–11 Attempts
to replace homogeneous acids with their heterogeneous
counterparts in fine-chemicals synthesis has recently been
reported by Rhodia, which has established the first indus-
trial application of zeolites for the acylation of anisole and
veratrole, with HY and H-ꢁ zeolites as catalysts in a fixed
bed process.12–13
It is evident from the literature that the zeolite cat-
alyzed acylation of phenols and aromatic ethers is car-
ried out with very good yields and selectivitys.14–16 On the
other hand, acylation of low activated aromatic rings like
toluene and benzene are poorer using less reactive acy-
lating agents, such as carboxylic acids and their anhy-
drides on zeolites.17ꢀ18 Thus, there still exists scope to
develop better catalysts which would catalyze the acyla-
tion of toluene with excellent conversion and selectivity at
comparatively lower temperatures.
Friedel–Crafts acylation reactions are of great importance
in the industrial production of aromatic ketones, and are
used extensively in the production of fine and specialty
chemicals as well as pharmaceuticals.1ꢀ2 Conventional acy-
lations have been carried out by using homogeneous acid
catalysts such as AlCl3, FeCl3, ZnCl2, HF etc.3 The use
of these catalysts leads to many problems concerning han-
dling, safety, corrosion and waste disposal. Indeed the
major drawbacks linked with the effective utilization of
these catalysts are environmental concerns due to the
excessive generation of wastes along with corrosion prob-
lems, and the impossibility of regeneration. In addition,
the formation of complexes between the acid catalysts and
the produced ketone leads to a gradual loss of activity
and a separation post-reaction is further needed to recover
the formed ketone. Recently, the development and use of
solid acid catalysts simplified the purification processes
involved; therefore, considerable efforts have been made to
develop heterogeneous processes using solid acid catalysts
Zeolites are currently studied for applications in the syn-
thesis of organic compounds and fine chemicals. Zeolite
materials are attractive as catalysts for the acylation reac-
tions due to their acidity, shape selectivity and regenera-
bility properties, etc.19–21 Zeolite beta (BEA), a large-pore
high-silica zeolite with a three dimensional (3-D) pore
∗Author to whom correspondence should be addressed.
J. Nanosci. Nanotechnol. 2013, Vol. 13, No. 6
1533-4880/2013/13/4415/006
doi:10.1166/jnn.2013.7153
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Hierarchical Zeolite Beta: An Efficient and Eco-Friendly Nanocatalyst for the Friedel–Crafts Acylation of Toluene
Selvin et al.
ꢀ
system has been demonstrated to be an active and selective
heterogeneous catalyst for several liquid-phase reactions.
Unfortunately, slow diffusion of reactants through com-
mercial micro-porous zeolites makes them relatively poor
catalysts in liquid phase.
Zeolites with a small particle size have exhibited attrac-
tive performances in a series of catalytic reactions such as
hydro cracking of crude oil, hydroxylation of phenol, iso-
merization of xylene, and oligomerization of ethylene.22–25
The decrease in the crystal size of zeolites can effectively
improve their catalytic activity and stability.
template approach under controlled steaming at 200 C.
Tetraethyl ammoniumhydroxide (TEAOH) was used as
a structure directing agent and styrene butadiene rubber
(SBR) latex particles was used as macro pore template.
In a typical synthesis, a clear solution of zeolite
was prepared by mixing tetraethyl ammonium hydrox-
ide (TEAOH) and aluminium isopropoxide (AIP) and to
the clear solution, tetraethylorthosilicate (TEOS), and DI
water were added with stirring for 2 hrs. The molar
composition of the resulting sol was 0.27 TEAOH/1
TEOS/0.02 AlPO/4 EtOH/15 H2O. The pH of the final
solution was 11.02. The clear sol was concentrated in a
This suggests that both activity and stability may be
improved by using the zeolites of small crystal size and
large external surface as catalysts in the acylation reac-
tion. Nevertheless, nanocrystalline zeolite particles provide
large external surface areas and shortening of the diffusion
distances in the channels of the particles, thereby exposing
the active surface for reaction and reducing deactivation.
However, the crystals often easily aggregate and the mass
transfer to and from the active sites located within the
micropores can be slow, depending on diffusivities. It is
thus advantageous to introduce meso- and even macro-
pores into the zeolite materials.26ꢀ27 The introduction of
mesopores or macropores into zeolites has recently been
achieved using various methods.28–34 The porosity of hier-
archical materials is primarily controlled by varying the
size of the nanocrystals. Therefore good control of nano-
rotary evaporator at 80 C as described in the literature.32
ꢀ
The concentrated precursor sol was divided into two por-
tions. To one portion of the concentrated sol, 0.3 g of SBR
latex solid content was added and dried at room tempera-
ture. The other portion of the concentrated sol was dried
at room temperature. The samples were steamed at 200 ꢀC
for 24 hrs. The steamed samples were calcined in air at
ꢀ
ꢀ
550 C (5 h, 2 C/min).
2.3. Characterization
Powder X-ray diffraction patterns were recorded on a
Regaku 2000 diffractometer using Cu-Kꢂ radiation from
2ꢃ = 2ꢄ5 to 50ꢀ at a scan rate of 2ꢀ/min with a step
size of 0.04ꢀ. Morphology and particle size of the zeo-
lite nanocrystals were examined with Transmission elec-
tron microscopy (TEM; JEM-2010, 200 kV) and scanning
electron microscopy (SEM; JEOL JSM-6500f) and. For
TEM analysis, a drop of the diluted solution (0.0005 wt%
g/water) was placed (placed a drop of ethanol and dried
before placing sample) on carbon coated copper grid.
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crystal size is a pre-requisite to control the porosity of the
IP: 63.142.228.125 On: Wed, 28 Oct 2015 11:00:27
hierarchical materials. The macro pore size of the zeolite
material can be flexibly tailored through a rational choice
of the macro template.
Copyright: American Scientific Publishers
The current study deals with the acylation of toluene
using hierarchical zeolite Beta catalyst under autogenous
pressure. The reaction parameters such as, time-on-stream,
toluene:acetic anhydride molar ratio, catalyst quantity and
reaction temperature were optimized. The influence of
hierarchical zeolite Beta pore size on the catalyst perfor-
mance has been studied.
3. RESULTS AND DISCUSSION
3.1. Morphological and Structural Characterizations
The morphologies of hierarchical zeolite Beta and
nanocrytalline zeolite Beta were analyzed by TEM and
SEM and results are presented in Figure 1. Figure 1(a)
shows the typical TEM image of synthesized nanocry-
talline zeolite Beta. It is seen from the TEM images that
the nanocrystals are grown in high density and possess-
ing almost spherical shaped morphologies. The sizes for
most of the nanocrystals are similar; however some big-
ger and smaller sized nanocrystals are also seen in the
micrograph. The average particle size was found to be
around 40 nm. Figures 1(b) and (c) exhibit the typical
SEM images of hierarchical zeolite Beta. It is very clear
from the SEM image that the zeolite mixed with SBR
produced long-range ordered and well distributed macro-
pores along the material (Fig. 1(b)). This was due to the
fact that the zeolite precursors possess much silanol groups
and they can interact more with the SBR and form a long-
range macropore order material. The average size of the
2. EXPERIMENTAL DETAILS
2.1. Chemicals
Aluminium isopropoxide (AIP), Tetraethoxysilane
(TEOS), tetraethylammonium hydroxide (TEAOH, 35%
aq), were commercial samples from Merck and were used
without further purification. Styrene butadiene rubber
latex was purchased from Apar Industries, India.
2.2. Synthesis of Hierarchical Zeolite Beta and
Nanocrystalline Zeolite Beta Catalyst
In order to synthesize of hierarchical zeolite Beta and
nanocrystalline zeolite Beta, a hydrolysed clear sol of
TEAOH-AlP-TEOS-H2O was prepared at room tempera-
ture. The hierarchical zeolite Beta was fabricated using dual
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Selvin et al.
Hierarchical Zeolite Beta: An Efficient and Eco-Friendly Nanocatalyst for the Friedel–Crafts Acylation of Toluene
(b)
(a)
0
5
10
15
20
25
30
35
40
2 Theta (degrees)
Fig. 2. Typical XRD patterns for (a) hierarchical zeolite Beta and
(b) nanocrystalline zeolite Beta.
of the micrometric zeolite reported in the literature which
provide a clear indication that the crystallites of the pre-
pared crystalline materials are in nanoscale.
3.2. Catalyst Activity
Acylation of toluene by acetic anhydride was studied sys-
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tematically using the synthesized hierarchical zeolite Beta
IP: 63.142.228.125 On: Wed, 28 Oct 2015 11:00:27
samples as catalyst. The catalyst sample was activated
Copyright: American Scientific Publishers
ꢀ
before the reaction by heating for 3 h at 200 C under N2.
The acylation of toluene catalyzed by hierarchical zeolite
Beta catalysts were carried out in a Teflon autoclave under
ꢀ
autogeneous pressure at 120 C for 100 min. A mixture
of 200 mmol of toluene and 10 mmol of acetic anhy-
dride was added. The freshly activated catalyst (0.5 g)
was added in an autoclave. The autoclave was placed in
air oven and theꢀcontents were heated at a constant tem-
perature at 120 C. The products were analyzed by gas
chromatographic analysis (Hewlett Packard 5890), using
OV-101 column (2 m length, 1/8 inch dia) and FID detec-
tor. The major product of acylation of toluene was found to
be p-Methylacetophenone (MAP) and only 1 to 3% of the
ortho-substituted product was formed. Scheme 1 represents
the reaction pathway of the acylation of toluene with acetic
anhydride. The effect of various reaction parameters such
as time-on-stream (TOS), mole ratio of reactants, catalyst
loading, and reaction temperature on the rates of reaction
has been investigated. Under the optimum reaction con-
ditions the performance of hierarchical zeolite Beta was
compared with nanocrystalline zeolite Beta.
Fig. 1. Typical (a) TEM image of nanocrystalline zeolite Beta and
(b) and (c) SEM images of hierarchical zeolite Beta.
hierarchical zeolite Beta is about 80 nm, which is almost
double of nanocrytalline zeolite Beta (Fig. 1(c)).
The crystallinity and crystal phases of prepared hierar-
chical zeolite Beta and nanocrystalline zeolite Beta was
observed by X-ray diffraction (XRD). The XRD analy-
sis results of sample are shown in Figure 2. Figure 2(a)
exhibits the typical XRD pattern for hierarchical zeo-
lite Beta while Figure 2(b) shows the XRD pattern for
nanocrystalline zeolite Beta. All the diffraction reflections
and the interplanar spacings calculated from the XRD pat-
terns are comparable and well matched with the reported
values for zeolite Beta.33 Interestingly, it is seen that the
diffraction reflections in the XRD patterns for the prepared
materials are broad compared to the diffraction reflections
3.2.1. Effect of Time on Stream on
Catalytic Activity (TOS)
In the first step the influence of time-on-stream (TOS) on
the conversion was studied using the catalyst hierarchical
J. Nanosci. Nanotechnol. 13, 4415–4420, 2013
4417

Hierarchical Zeolite Beta: An Efficient and Eco-Friendly Nanocatalyst for the Friedel–Crafts Acylation of Toluene
Selvin et al.
CH3
100
90
80
70
60
50
CH3
(CH3CO)2O
CH3COOH
COCH3
Scheme 1. Acylation of toluene with acetic anhydride.
zeolite Beta. Figure 3 shows the conversion of acetic anhy-
dride converted into the acylated product with the reaction
temperature at a time on stream (TOS) of 140 min. It was
found that the conversion first increases with increasing
TOS but remains practically constant after 100 min. Hence
all further experiments were carried out for 100 min.
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
Catalyst weight (g)
Fig. 4. Effect of catalyst quantity on catalytic activity of toluene with
acetic anhydride (Ac2O) over hierarchical zeolite Beta (TOS, 100 min;
molar ratio of toluene: Ac2O, 20:1; reaction temperature, 120 ꢀC).
3.2.3. Effect of Catalyst Quantity
3.2.2. Effect of Mole Ratio
The effect of catalyst quantity was studied oꢀver a range
of 0.1–0.7 g for acylation of toluene at 100 C (Fig. 5).
The conversion of acetic anhydride was found to increase
with increase in catalyst amount, which is due to the pro-
portional increase in the number of active sites and the
increase becomes less significant beyond 0.5 g. This indi-
The effect of toluene to acetic anhydride mole ratio was
studied at 10:1, 15:1, 20:1, 25:1 and 30:1 by keeping the
total volume constant (Fig. 4). The conversion of toluene
was found to increase till the toluene to acetic anhydride
mole ratio was 20:1 and thereafter remained constant.
This is probably due to the inhibiting effect of methy-
lacetophenone, which can be strongly adsorbed on the
catalyst.18 The increasing conversion of acetic anhydride
is attributed to the easy desorption methylacetophenone
formed through the zeolite pores by the excess toluene.
Further increase in molar ratio has no appreciable effect
in Ac2O conversion. Therefore, all reactions were studied
by using a toluene to acetic anhydride mole ratio of 20:1.
cates that beyond 0.5 g, the additional active sites do not
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increase the adsorption of reactants at a given concentra-
Copyright: American Scientific Publishers
tion and so the conversion of acetic anhydride does not
increase much. Hence all further experiments were carried
out using 0.5 g catalysts.
3.2.4. Effect of Reaction Temperature
The effect of temperature was studied under otherwise
similar reaction conditions in the range of 60–140 ꢀC
80
70
60
50
40
30
20
10
0
100
80
60
40
20
0
5
10
15
20
25
30
35
0
20
40
60
80
100
120
140
Toluene/Ac2O molar ratio
Reaction time (min)
Fig. 3. Effect of time-on–stream (TOS) on catalytic activity of toluene
with acetic anhydride (Ac2O) over hierarchical zeolite Beta (reaction tem-
perature, 120 ꢀC; catalyst, 0.5 g of hierarchical zeolite Beta; molar ratio
of toluene: Ac2O, 20:1).
Fig. 5. Effect of molar ratio of toluene: Ac2O on catalytic activity
of toluene with acetic anhydride (Ac2O) over hierarchical zeolite Beta
(catalyst, 0.5 g of hierarchical zeolite Beta; TOS, 100 min; reaction
temperature, 120 ꢀC).
4418
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Selvin et al.
Hierarchical Zeolite Beta: An Efficient and Eco-Friendly Nanocatalyst for the Friedel–Crafts Acylation of Toluene
100
the acylation of toluene can escape from the macro pores
of zeolites much more easily and prevents the heavy prod-
ucts from occupying the active sites.
80
60
40
20
4. CONCLUSIONS
The present work covers the use of hierarchical zeolite
Beta and nano-sized zeolite Beta catalysts for acylation of
toluene with acetic anhydride. The catalysts were charac-
terized for their structure by XRD, TEM and SEM. The
effects of various parameters were studied on the conver-
sion of acetic anhydride under otherwise similar condi-
tions. The small crystal size of nano-sized zeolites may
0
40
60
80
100
120
140
160
180
200
bring on more accessible active sites and then enhance the
catalytic activity. Catalytic activity is strongly influenced
by the pore size of the crystal too. The best catalyst for
toluene acylation under study is hierarchical zeolite Beta
catalyst. It can be concluded from the present investiga-
tions that the exposed pore openings in nano-sized zeolites
allow a fast desorption of heavy products from the catalyst
and can then reduce the occupancy of active sites by the
adsorption of products. The present study shows that of
hierarchical zeolite Beta catalysts has a potential replace-
ment for conventional Lewis acid or mineral acid catalysts
in a number of reactions.
Temperature (ºC)
Fig. 6. Effect of temperature on catalytic activity of toluene with acetic
anhydride (Ac2O) over hierarchical zeolite Beta (TOS, 100 min; catalyst,
0.5 g of hierarchical zeolite Beta; molar ratio of toluene: Ac2O, 20:1).
(Fig. 6). The conversion had been found to inꢀcrease signif-
icantly with increasing temperature till 120 C and after-
wards a slight decrease in conversion of acetic anhydride
was noticed due to the inhibiting effects of methylace-
tophenone which can be strongly adsorbed on the cata-
lyst at higher temperature. A decrease in conversion at
high temperature was also observed for the acylation of
toluene with acetic anhydride due to the strong adsorp-
Thus, 120 C was found
an optimum temperature.
Acknowledgments: Rosilda Selvin greatly acknowl-
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edges the research support of Lunghwa University of
tion of methylacetophenones.18 IP: 63.142.ꢀ228.125 On: Wed, 28 Oct 2015 11:00:27
Copyright: American SSciceiennticfiec PanudbliTsehcehrnsology. Ahmad Umar would like to
acknowledge the support of the Ministry of Higher Edu-
cation, Kingdom of Saudi Arabia for granting a Promising
Centre for Sensors and Electronic Devices (PCSED) dated
24/3/1432 H, 27/02/2011. Ahmad Umar also acknowl-
3.2.5. Influence of Zeolite Properties
Table I compares the performance of two Beta catalysts
having different particle sizes and pore sizes in the conver-
sion of acetic anhydride for acylation of toluene. Although
the particle size of the nanocrystalline zeolite Beta is
smaller than that of the hierarchical zeolite Beta, the later
exhibited higher catalytic activity for acylation of toluene.
This can be explained on the basis of the macropore
present in the nanocrystalline zeolite Beta catalyst.
edges Promising Centre for Sensor and Electronic Device
project (PCSED-03-11) for support.
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Hierarchical Zeolite Beta: An Efficient and Eco-Friendly Nanocatalyst for the Friedel–Crafts Acylation of Toluene
Selvin et al.
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Delivered by Publishing Technology to: Deakin University Library
Received: 3 June 2012. Accepted: 19 December 2012.
IP: 63.142.228.125 On: Wed, 28 Oct 2015 11:00:27
Copyright: American Scientific Publishers
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J. Nanosci. Nanotechnol. 13, 4415–4420, 2013