Heteropoly acid encapsulated into zeolite imidazolate framework (ZIF-67) cage as an efficient heterogeneous catalyst for Friedel-Crafts acylation

Article abstract of DOI:10.1016/j.jssc.2015.11.014

A new strategy has been developed for the encapsulation of the phosphotungstic heteropoly acid (H₃PW₁₂O₄₀ denoted as PTA) into zeolite imidazolate framework (ZIF-67) cage and the PTA@ZIF-67(ec) catalysts with different PTA content were prepared. The structure of the catalysts was characterized by XRD, BET, SEM, FT-IR, ICP-AES and TG. The catalytic activity and recovery properties of the catalysts for the Friedel-Crafts acylation of anisole with benzoyl chloride were evaluated. The results showed that 14.6-31.7 wt% PTA were encapsulated in the ZIF-67 cage. The PTA@ZIF-67(ec) catalysts had good catalytic activity for Friedel-Crafts acylation. The conversion of anisole can reach ~100% and the selectivity of the production can reach ~94% over 26.5 wt% PTA@ZIF-67(ec) catalyst under the reaction condition of 120 °C and 6 h. After reaction, the catalyst can be easily separated from the reaction mixture by the centrifugation. The recovered catalyst can be reused five times and the selectivity can be kept over 90%.

Full text of DOI:10.1016/j.jssc.2015.11.014

Journal of Solid State Chemistry 233 (2016) 303–310
Contents lists available at ScienceDirect
Journal of Solid State Chemistry
journal homepage: www.elsevier.com/locate/jssc
Heteropoly acid encapsulated into zeolite imidazolate framework
(
ZIF-67) cage as an efficient heterogeneous catalyst for Friedel–Crafts
acylation
n
Muhammad Ammar, Sai Jiang, Shengfu Ji
State Key Laboratory of Chemical Resource Engineering Beijing University of Chemical Technology, 15 Beisanhuan Dong Road, P.O.Box 35, Chaoyang District,
Beijing 100029, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A new strategy has been developed for the encapsulation of the phosphotungstic heteropoly acid
Received 15 May 2015
Received in revised form
(H PW12O40 denoted as PTA) into zeolite imidazolate framework (ZIF-67) cage and the PTA@ZIF-67(ec)
catalysts with different PTA content were prepared. The structure of the catalysts was characterized by
XRD, BET, SEM, FT-IR, ICP-AES and TG. The catalytic activity and recovery properties of the catalysts for
the Friedel-Crafts acylation of anisole with benzoyl chloride were evaluated. The results showed that
3
5
November 2015
Accepted 6 November 2015
Available online 7 November 2015
14.6–31.7 wt% PTA were encapsulated in the ZIF-67 cage. The PTA@ZIF-67(ec) catalysts had good catalytic
Keywords:
Heteropoly acid
Encapsulate
Zeolite imidazolate framework
Catalyst
Friedel–Crafts acylation
activity for Friedel-Crafts acylation. The conversion of anisole can reach ꢀ100% and the selectivity of the
production can reach ꢀ94% over 26.5 wt% PTA@ZIF-67(ec) catalyst under the reaction condition of
120 °C and 6 h. After reaction, the catalyst can be easily separated from the reaction mixture by the
centrifugation. The recovered catalyst can be reused five times and the selectivity can be kept over 90%.
2015 Elsevier Inc. All rights reserved.
&
1
. Introduction
nts employ for the Friedel–Crafts acylation of aromatic compounds.
However, there is ambit to develop heterogeneous acid catalysts for
the Friedel–Crafts acylation reaction. Solid catalysts are secure, rea-
sonable and benign. Furthermore the methods using solid catalysts
have advantages for instance better selectivity, moderate reaction
conditions, minimize waste and cheap construction of material [9,10].
Acetic acid, acetic anhydride and acetyl chloride as acylating agents
have been utilized for the acylation of activated aromatic compounds
in presence of zeolites [10], acid-treated metal oxides [11] and het-
eropoly acids [12]. However, heteropoly acids (HPAs) have been used
in excess of organic reactions [9,12]. Nevertheless heteropoly acids in
unsupported form present poor stability, fast deactivation and low
efficiency. A variety of supports, as mesoporous alumina silicate, me-
soporous silica, carbon, alumina, zirconia and metal-organic frame-
works (MOFs) have been employed to build up the stability and ef-
fectiveness of heteropoly acids [13]. Indeed, the catalytic activity and
acidity of the supported heteropoly acids rely mainly on the loading
and the nature of carrier, i.e. strong interaction with activated carbons
cause low catalytic activity than that of the heteropoly acid itself, while
dramatic leaching cause by low interactions of heteropoly acid with
support. Direct synthesis of the Keggin structures inside the zeolite
cavities (FAU) has been used to achieve the encapsulation of hetero-
poly acids. This method is revealed to solve the leaching problem,
since the windows of the zeolitic cavities are smaller than the het-
eropoly acid clusters, but only very low loadings can be utilized
(o5 wt%) if diffusion limitations are to be avoided [14].
The production of various pharmaceutical, pesticides, dyes and
agrochemicals at industrial scale involves the synthesis and fur-
ther transformation of aromatic ketones. The Friedel–Crafts acy-
lation of aromatic compounds is an essential route for the synth-
esis of the aromatic ketones that are intermediates in the manu-
facturing of fine and special chemicals as well as pharmaceuticals
[1]. Traditionally Lewis acid catalysts i.e. ZnCl
2
, AlCl
3 3
, FeCl and
Brönsted acid catalysts i.e. HCl, HF and H SO
2
4
had been sig-
nificantly used for the acylation of aromatic compounds [2]. Uti-
lization of homogeneous acid catalyst for the acylation reactions
caused a number of issues such as produced impurities along with
desired products, production of large amounts of toxic waste, in-
crease in expenditures, and utilization of catalyst more than stoi-
chiometric quantity, and difficulty in recovery. For Friedel–Crafts
reactions, incessant investigation for appropriate heterogeneous
catalyst had led to growth of catalysts for example Re–Br(CO)
LiClO -acyl anhydride complex [3], acid-treated clays [4], HZSM-5
zeolite [5], cation exchange resins [6], Ln-(OTf)
heteropoly acids supported catalyst [8,9].
5
,
4
3 4
-LiClO [7], and
The literatures until now supply an array of catalysts and reage-
n
Corresponding author. Fax: þ86 10 64419619
E-mail addresses: jisf@mail.buct.edu.cn, 2071658081@qq.com (S. Ji).
http://dx.doi.org/10.1016/j.jssc.2015.11.014
022-4596/& 2015 Elsevier Inc. All rights reserved.
0

304
M. Ammar et al. / Journal of Solid State Chemistry 233 (2016) 303–310
Metal-organic frameworks (MOFs) consist of metallic nodes
2.2.2. PTA@ZIF-67(ec) Preparation
bonded by organic linker, are currently receiving significant at-
tention because of their versatile properties [15–17].They have
been extensively utilized as gas storage [18], separation [19], ion
exchange [20], sensor drug delivery [21] and catalysis [22,23].
MOFs as compared to traditional microporous and mesoporous
inorganic materials reveal advantages such as tunable pore sizes,
high surface area, structural diversity, flexibility and geometrical
control by the functionalization and controlling the size of organic
linkers [24,25]. However, MOFs used in catalysis are limited be-
cause of limited thermal and moisture stability including often
completely blocked metal sites by the organic linker or solvent,
leaving no free position available for substrate chemisorption’s
The encapsulation of the phosphotungstic acid in ZIF-67 has
been done by the same procedure as follow in the synthesis of ZIF-
67 with same synthesis mixture. But the phosphotungstic acid
(PTA) was dissolved in 11 mL deionized water and added to Co
(NO
3
)
2
ꢁ 6H
2
O solution. Different loadings of phosphotungstic acid
in ZIF-67 structure were achieved (14.6 wt% to 46.4 wt% according
to ICP-AES), depending on the amount of phosphotungstic acid
introduced into the synthesis mixture.
2.3. Catalyst characterization
The power X-ray diffraction patterns of all samples were re-
[
26]. Zeolite imidazolate frameworks (ZIFs) classified as a novel
corded at room temperature using a Cu K
¼1.54056 Å), operating at 40 kV and 50 mA using a Rigaku D/
Max 2500 VB2þ/PC diffractometer. The BET surface areas and pore
size distributions were obtained from N adsorption–desorption
α
radiation source
subclass of MOFs, have fascinated important attention as they
unite advantages from both zeolite and conventional MOFs
[
(λ
27,28]. In the past decade, research works have directed on syn-
2
thesizing new ZIFs and utilizations in different fields. In the lit-
erature, there are a small number of applications of ZIFs as catalyst
or catalyst supports for transformation of organic compounds as
compare to conventional MOFs [29–31].
isotherms measured on the Micrometrics ASAP 2020 adsorption
analyzer at 77 K. Structure and morphology of samples were
characterized by scanning electron microscopy (SEM) on a Zeiss
Supra55. Fourier transform infrared (FT-IR) spectra of the samples
were collected on a Bruker Tensor 27, by dispersing the samples on
In this work, we wish to report a direct, synthetic encapsulation
of active species such as phosphotungstic acid (PTA) into MOFs.
We demonstrate that incorporation of highly dispersed PTA into
zeolite imidazolate frameworks-67 (ZIF-67) cage is possible to
achieve by following the one-pot synthesis approach. As far as we
know, the Friedel–Crafts acylation reaction catalyzes by PTA en-
capsulated in ZIF-67 are not previously mentioned in the litera-
ture. The novel composite material PTA@ZIF-67(ec) has been uti-
lized as an efficient heterogeneous catalyst for Friedel–Crafts
acylation reaction of anisole with benzoyl chloride. High conver-
sions have been achieved with PTA@ZIF-67(ec) catalyst without an
inert atmosphere. PTA@ZIF-67(ec) catalyst was easily separated
from the reaction mixture and reused without significant de-
gradation in activity.
ꢂ1
KBr pellets in the range of 600–4000 cm . Thermogravimetric
analysis (TGA) measurements were collected on a SETSYS Evolu-
tion from Setaram Instrumentation with a heating rate of 10 °C
/min in air up to 800 °C. Elemental analysis was performed by
means of Inductively Coupled Plasma Atomic Emission Spectro-
scopy (ICP-AES). The samples were investigated with SPECTRO
Analytical Instruments GmbH in order to calculate the amount of
PTA encapsulated into the ZIF-67 structure. Gas chromatographic
(GC) analyses were carried out using a Beijing Beifenruili SP-2100
with flame ionization detector (FID). GC–MS analyses were carried
out using a Shimadzu GCMS-QP5000.
2.4. Catalytic performance testing
The Friedel–Crafts acylation of anisole with benzoyl chloride
using PTA@ZIF-67(ec) catalyst was carried out in the magnetically
stirred round-bottom flask. In a typical reaction, a mixture of an-
isole and benzoyl chloride with molar ratio 1:2, n-Dodecane as an
internal standard was charged into a 50 mL flask containing the
PTA@ZIF-67(ec) catalyst. The reaction mixture was then heated to
120 °C with continuously stirring for 6 h. After 6 h, the reaction
mixture was quenched with an aqueous NaOH solution (1%,
0.15 mL). The organic components were extracted using diethyl
2
. Experimental
2.1. Materials
Cobalt nitrate hexahydrate (99%), 2-methylimidazole (98%) and
phosphotungstic acid were purchased from Aladdin Chemical Co.,
Ltd., China. Anisole (Z98%) was purchased from Sinopharm
Chemical Reagent Co., Ltd., China. Benzoyl Chloride (98%) was
purchased from Tianjin Fuchen Chemical Reagents, China. Diethyl
ether and methanol were purchased from Beijing Yili Fine Che-
mical Co. Ltd., China. Hydrogen gas and nitrogen gas were pur-
chased from HaiPu Gas Industry Co. Ltd., China. All these reagents
were used as received without further purification.
2 4
ether (2 mL), dried over anhydrous Na SO and the product was
analyzed by GC. The structure of the product was defined by GC–
MS. For the investigating the reusability of the catalyst, the
PTA@ZIF-67(ec) catalyst was separated by simple centrifugation
from the reaction mixture, washed with abundant amounts of
dichloromethane (DCM), dried under vacuum at room tempera-
ture for 6 h and reused in reaction.
2.2. Catalyst Preparation
2.2.1. Zeolite imidazolate framework (ZIF-67) preparation
For the synthesis of ZIF-67, 0.722 g of Co (NO
was dissolved in 25 mL methanol and 1.629 g of 2-methylimidazole
19.84 mmol) was dissolved in 25 mL methanol separately, were stir-
3
)
2
ꢁ 6H
2
O (2.48 mmol)
3. Results and discussion
(
3.1. XRD and BET
red until dissolved. When both reagents were entirely dissolved in
methanol, the solution consists of 2-methylimidazole was slowly ad-
Fig. 1 illustrates the XRD patterns of bare ZIF-67 and PTA@ZIF-
67(ec) samples. The XRD patterns of synthesized samples in this
work were in good agreement with simulated single crystal
structure patterns in the Cambridge Structural Database (CCDC
code¼GITTOT). In the bare ZIF-67 sample, a very sharp peak was
detected at 7.3° on the XRD pattern, demonstrating that the ma-
terial was obtained, is highly crystalline material.
ded to the solution of Co (NO
3
)
2
ꢁ 6H
2
O. The solution became purple
immediately, and the resultant mixture was stirred for 2 h at room
temperature. The solids were collected by centrifugation for 10 min
and were washed with methanol for several times to remove excess
2-methylimidazole present on the surface and pores. Then solids were
dried overnight in air oven, at room temperature. Finally, the sample
was dried under vacuum at 130 °C for 2 h.
In fact, whenever MOF-based materials are synthesized, highly

M. Ammar et al. / Journal of Solid State Chemistry 233 (2016) 303–310
305
67 as shown in Fig. 2B. With the inclusion of PTA, the pore size and
pore volume were substantially decreased. The result is evidence
that the highly accessible structure of the ZIF-67 is maintained,
even at high PTA loading.
Table 1 presents the results of BET surface area, pore size and
total pore volume. The BET surface area of the ZIF-67 encapsulated
phosphotungstic acid decreases from 1390.45 to 600.27 m /g with
(
f)
2
( 1 1 2 )
addition of PTA. The total pore volume of the ZIF-67 encapsulated
phosphotungstic acid also decreases from 0.71 to 0.29 cm /g.
(
0 0 2 )
3
(e)
(d)
(c)
(b)
(a)
3.2. Structure and morphology
Fig. 3 shows the SEM micrographs of ZIF-67 and different
PTA@ZIF-67(ec) samples, synthesized at room temperature. Ac-
cording to SEM micrographs, ZIF-67 revealed that well-shaped,
hexagonal nano-crystals were obtained having an average particle
size of about 200 nm (Fig. 3a) and as the amount of PTA en-
capsulated into the cage of ZIF-67 (14.6–31.7 wt%) keep up the
crystal shape and structure (Fig. 3b and c). But the higher amount
of PTA about 46.4 wt% could not encapsulate PTA in the cage and
demolished the crystal structure as shown in Fig. 3d.
1
0
20
30
40
50
2
Theta (degree)
Fig. 1. XRD patterns of simulated ZIF-67 obtained from Cambridge Structural Da-
tabase(a), ZIF-67 synthesized in this work(b), 14.6 wt% PTA@ZIF-67(ec)(c), 26.5 wt%
PTA@ZIF-67(ec)(d), 31.7 wt% PTA@ZIF-67(ec)(e) and 46.4 wt% PTA added in synth-
esis mixture caused to demolish the crystal structure(f).
3.3. FT-IR spectroscopy
crystalline material is always expected. However, the PTA@ZIF-67
(
ec) sample patterns showed hardly any difference and good
FT-IR spectroscopic studies were carried out to confirm whe-
agreement with the peaks of bare ZIF-67 up to a certain amount of
PTA added in the synthesis mixture (14.6–31.7 wt% PTA@ZIF-67
ther PTA and ZIF-67 structures were preserved in the novel com-
posite materials PTA@ZIF-67(ec). FT-IR spectra of bare ZIF-67 and
(
ec)). While, the presence of higher amount of PTA (46.4 wt% PTA
different PTA@ZIF-67(ec) samples are shown in Fig. 4.
ꢂ1
added in synthesis mixture) caused to lower the pH of the
synthesis mixture. Low pH prevented ligand from coordination
with metal due to protonation and resulted into demolishing the
crystal structure. Apart from the fact that the crystal structure
appeared to remain unchanged during the PTA encapsulation
while the intensity of the peak at 7.3° decreased for the PTA en-
capsulated samples.
The bands at 693 and 753 cm
in the spectral region are as-
sociated with out-of-plane bending of the imidazole ring, while
ꢂ1
peaks in the region of between 900 and 1350 cm are assigned as
ꢂ1
in-plane bending. The peaks at 1576 and 1663 cm are attributed
to the stretching and bending N–H vibration of the imidazole ring,
respectively. The intense and convoluted bands at 1350–1500 cm
ꢂ1
are associated with the entire ring stretching, whereas two peaks at
ꢂ1
Fig. 2A shows the N
2
adsorption–desorption isotherms of bare
2922 and 3131 cm
are attributed to the aliphatic and the aro-
ZIF-67 and PTA@ZIF-67(ec). The isotherms revealed type-I with
sharply increased adsorption at low relative pressure, which is
typical for microporous materials. No indication of pore blocking
was observed in the isotherms. The pore size distribution curves of
bare ZIF-67 and PTA@ZIF-67(ec) were transformed using the
Horvath–Kawazoe (HK) method, which further supported the
presence of two kinds of micropore in bare and encapsulated ZIF-
matic C–H stretch of the imidazole, respectively. As one can judge
from the FT-IR spectra of bare ZIF-67 and PTA@ZIF-67(ec) presented
in Fig. 4, the most intensive peaks belong to ZIF-67 and the matrix
structure certainly retained upon PTA encapsulation in ZIF-67. The
presence of strongly bonded water in the PTA@ZIF-67(ec) samples
can easily be observed as shown in Fig. 4(A) (range between 800
ꢂ1
and 4000 cm ), from a broad absorption band between 3200 and
5
4
3
2
1
00
00
00
00
00
0
4
3
2
1
0
(a)
(b)
(c)
(d)
(
(
(
(
a)
b)
c)
d)
0
.0
0.2
0.4
0.6
0.8
1.0
0.5
1.0
1.5
2.0
Relative Pressure (P/Po)
Pore Width (nm)
2
Fig. 2. N adsorption–desorption isotherms(A) and pore size distribution curves(B) of ZIF-67 synthesized in this work(a), 14.6 wt% PTA@ZIF-67(ec)(b), 26.5 wt% PTA@ZIF-67
(
ec)(c) and 31.7 wt% PTA@ZIF-67(ec)(d).

306
M. Ammar et al. / Journal of Solid State Chemistry 233 (2016) 303–310
Table 1
Structural characteristics of ZIF-67 and PTA@ZIF-67(ec) samples.
2
3
Samples
ZIF-67
BET Surface area (m /g)
Pore size (nm)
Total pore volume (cm /g)
PTA (wt%)
ZIF-67 (wt%)
1390.45
994.85
647.42
600.27
2.04
2.21
1.98
1.95
0.71
0.55
0.32
0.29
–
100.00
85.40
73.50
68.30
1
4.6 wt%PTA@ZIF-67(ec)
6.5 wt%PTA@ZIF-67(ec)
1.7 wt%PTA@ZIF-67(ec)
14.60
26.50
31.70
2
3
Fig. 3. SEM micrographs of ZIF-67 synthesized(a), 14.6 wt% PTA@ZIF-67(ec)(b), 26.5 wt% PTA@ZIF-67(ec)(c) and 46.4 wt% PTA added in synthesis mixture caused to demolish
the crystal structure(d).
(
a)
(
a)
b)
(
(b)
(c)
(
c)
d)
e)
(
(d)
(
e)
(
4
000 3500 3000 2500 2000 1500 1000
1800 1600 1400 1200 1000 800
-
1
-1
Wavenumber (cm )
Wavenumber (cm )
Fig. 4. FTIR spectra of the phosphotungstic acid (PTA)(a), ZIF-67 synthesized(b), 14.6 wt% PTA@ZIF-67(ec)(c), 26.5 wt% PTA@ZIF-67(ec)(d) and 31.7 wt% PTA@ZIF-67(ec)(e).

M. Ammar et al. / Journal of Solid State Chemistry 233 (2016) 303–310
307
1
00
100
(
c)
(
(
b)
e)
(
d)
8
6
4
0
0
0
9
9
9
9
8
6
4
2
(
d)
a)
(
(
c)
b)
(
(
a)
20
0
1
00 200 300 400 500 600 700 800
0
2
4
6
8
10
o
Temperature ( C)
Catalyst Content (wt%)
Fig. 5. TGA analysis of the ZIF-67 synthesized(a), 14.6 wt% PTA@ZIF-67(ec)(b),
6.5 wt% PTA@ZIF-67(ec)(c) and phosphotungstic acid (PTA)(d).
Fig. 6. Effect of catalyst content on the reaction conversion in the presence of
catalyst: ZIF-67 synthesized(a), 14.6 wt% PTA@ZIF-67(ec)(b), 26.5 wt% PTA@ZIF-67
2
(
ec)(c), 31.7 wt% PTA@ZIF-67(ec)(d), and 46.4 wt% PTA added in synthesis mixture
ꢂ1
ꢂ1
3
700 cm
and the broad band centered at 1305 cm
[32]. The
caused to demolish the crystal structure(e).
ꢂ1
stretching band at 1305 cm
is also present in the bare ZIF-67
sample; however the width and intensity is increased in PTA en-
3.5. Catalytic performance testing
capsulated samples.
ꢂ1
When focusing on the 800–1100 cm
range (Fig. 4(B)), after
3.5.1. Screening of catalysts
The performance of the PTA@ZIF-67(ec) as a heterogeneous
catalyst in the Friedel–Crafts acylation of anisole with benzoyl
chloride was assessed (Scheme 1). The benzoyl chloride was used
as an acylating agent to form p-benzoylanisole as the major pro-
duct and o-benzoylanisole as the minor product.
subtraction the bare ZIF-67 FT-IR spectra from PTA@ZIF-67(ec)
sample spectra. The FT-IR spectra of novel composite materials
PTA@ZIF-67(ec) exhibit the principal stretching modes of the PTA
Keggin units such as W–O–W, W ¼O and PO
4
. The PO
4
stretching
ꢂ1
ꢂ1
appeared at 1060 cm . The bands at 956 and 888, 823 cm
,
In this work, it was achieved that the Friedel–Crafts acylation of
anisole with benzoyl chloride can take place in the presence of
bare ZIF-67 and PTA@ZIF-67(ec) as a catalyst. The reaction was
carried out at 120 °C for 6 h with anisole:benzoyl chloride molar
ratio 1:2 and the results were graphically summarized in Fig. 6.
The bare ZIF-67 gave hardly 45% conversion with higher
amount of catalyst. PTA@ZIF-67(ec) samples were found much
more active catalysts as compared to bare ZIF-67 under the same
condition. A conversion of 100% was achieved in the presence of
clearly present in all PTA@ZIF-67(ec) samples, are related to the
W¼O and W–O–W stretching, respectively. This definitely in-
dicated that the heteropolyanion structure was maintained after
encapsulation.
3.4. Thermal stability analysis
Thermal degradation of solid materials has significant im-
portance for their application because most of them depend on
their thermal stability. Fig. 5 shows the TGA curves of PTA, ZIF-67
and PTA@ZIF-67(ec) samples. It can be seen that PTA revealed a
one-step degradation process; bare ZIF-67 and PTA@ZIF-67(ec)
samples followed two-step degradation process. The TGA curve of
PTA only illustrated a weight loss at about 160 °C, perhaps as a
result of dehydration of water from crystalline structure, and no
more considerable weight loss was observed until 800 °C.
26.5 wt% PTA@ZIF-67(ec) catalyst under reaction condition with-
out an inert atmosphere. As it can be observed from Fig. 6, the bare
ZIF-67 did not reveal good catalytic activity, most probably caused
by the lack of active sites on which the Friedel-Crafts acylation
reaction could take place. On the other hand, the encapsulated PTA
into the ZIF-67 cage gave dramatic increment of strongly active
sites on PTA@ZIF-67(ec), which resulted in a boost in the reaction
conversion.
The TGA curve of bare ZIF-67 revealed that only tiny weight
loss up to 350 °C, concerning to the removal of guest molecules
and un-reacted species, while the thermal stability of PTA@ZIF-67
Furthermore, it was also observed that the catalytic activities of
26.5 wt% PTA@ZIF-67(ec) and 14.6 wt% PTA@ZIF-67(ec) catalysts were
better than the 31.7 wt% PTA@ZIF-67(ec) catalyst under the same
conditions, that can be caused by active sites strongly bonded with
carrier and could not take part in the reaction efficiently in 31.7 wt%
PTA@ZIF-67(ec) catalyst. The demolish sample of ZIF-67 caused by the
PTA, showed higher conversion than 31.7 wt% PTA@ZIF-67(ec) catalyst
may be because of some trace amount of PTA present with demol-
ishing structure after washing. But, further investigation would be
required to examine the mechanism thoroughly.
(
ec) samples were reduced in the range 200–350 °C owing it to the
introduction of PTA. Therefore, when comparing the residual
content of bare ZIF-67 and PTA@ZIF-67(ec) samples, it can be
observed that TGA curves showed high residual content system-
atically with the increase of PTA content. From TGA, it can be seen
that encapsulation of the PTA into ZIF-67 structure did not badly
damage the thermal stability.
Scheme 1. Friedel–Crafts acylation of anisole with benzoyl chloride using PTA@ZIF-67(ec) catalyst.

308
M. Ammar et al. / Journal of Solid State Chemistry 233 (2016) 303–310
Table 2
1
00
80
Conversion
p-isomer
Effects of 26.5 wt% PTA@ZIF-67(ec)catalyst content on reaction conversion and
a
yield .
2
6
6.5 wt% PTA@ZIF- PTA Con- ZIF-67
7(ec) Content
Conversion Selectivity Yield
tent
(wt%)
Content
(wt%)
(%)
(%)
(%)
(
wt%)
6
4
0
0
2
4
6
8
0.53
1.06
1.59
2.12
2.65
1.47
2.94
4.41
5.88
7.35
44.31
82.14
95.75
99.21
99.99
92.53
93.51
93.03
93.88
94.47
40.99
76.81
89.08
93.14
94.46
10
a
Reaction condition: anisole, 0.006 mol; benzoyl chloride, 0.012 mol; tem-
perature, 120 °C; time, 6 h.
20
0
1
00
110
120
130
140
3
.5.2. Effect of catalyst content
Table 2 shows the effect of PTA@ZIF-67(ec) catalyst content on
o
Temperature ( C)
reaction conversion, selectivity and yield. In the literature, it was
reported that the Friedel-Crafts acylation reaction can be needed
an extra stoichiometric amount of the Lewis acid due to complex
formation with the oxygen atom of aroyl products [33,34]. The
catalyst concentration can be diminished dramatically with var-
ious solid catalysts for the Friedel–Crafts acylation reaction such as
beta zeolite [35,36], metal triflate loaded SBA-15 [37], hybrid
zeolite-mesostructured material [38] and mesoporous superacid
catalyst [39,40]. In fact, the Friedel–Crafts acylation reaction can
need less than 1 wt% to more than 10 wt% catalyst, depending on
the nature of the catalyst as well as the substrate.
In this work, it was observed from the preliminary investiga-
tions that catalyst content is a significant factor that can con-
siderably influence the reaction conversion as well as yield of
product. The reaction rate was found directly proportional to the
6.5 wt% PTA@ZIF-67(ec) catalyst content. 44% to 100% conver-
sions were achieved in the presence of 2 wt% to 10 wt% of
6.5 wt% PTA@ZIF-67(ec) catalyst. Moreover, the reaction conver-
Fig. 7. Effect of temperature on the reaction conversion and p-isomer selectivity
over 26.5 wt% PTA@ZIF-67(ec) catalyst. Reaction condition: anisole, 0.006 mol;
benzoyl chloride, 0.012 mol; catalyst amount, 0.1 g; time, 6 h.
able to give a significant increase in reaction conversions at 130
and 140 °C (Fig. 7) because the reaction reached almost the equi-
librium under 140 °C. The reaction selectivity was found almost
unchanged at ꢀ94% to p-benzoylanisole (Fig. 7).
Whenever a solid catalyst is used, a vital issue is the risk that
some active sites can transfer to the liquid phase from the solid
support and so on; these leached species can be a significant factor
for the reaction conversion [45]. In order to find out either
leaching of active sites occurred in Friedel–Crafts acylation reac-
tion using PTA@ZIF-67(ec) catalyst or not. An experiment was
accomplished to assess the contribution of leached active species
to the reaction conversion. A simple centrifugation was carried out
during the conduct of the reaction to separate the solid catalyst
from the reaction mixture. If the catalytic reaction still carried on
after the separation of solid catalyst, this would indicate that the
real active species was leached acid rather of the solid PTA@ZIF-67
2
2
sion observed for the 26.5 wt% PTA@ZIF-67(ec) catalyst was higher
than several previously reported Lewis acid catalysts, where
higher catalyst loading and/or longer reaction time was needed for
the same reaction [41]. The selectivity was found almost the same
for all cases with ꢀ94% for p-benzoylanisole.
(
ec) catalyst. Within experimental error, after the solid catalyst
was separated from the organic phase, no further reaction was
found. Consequently, it can be concluded that there were no lea-
ched active species in the organic phase, and reaction could only
be carried out in the presence of PTA@ZIF-67(ec) catalyst.
Choudhary and co-workers previously investigated by using a
zeolite-based catalyst and reported that the rate of the Friedel–
Crafts acylation reaction was drastically enhanced in the presence
of new Brönsted acid sites developed by the interaction of the
Lewis acid sites of the catalyst with moisture [43]. Lewis acid sites
combine with Brönsted acid sites could devote to the catalytic
activity of solid catalyst in the Friedel–Crafts acylation reaction.
Previously, Firouzabadi and co-workers used aluminum dode-
catungstophosphate for Friedel–Crafts acylation of anisole with
benzoyl chloride, as heterogeneous catalyst and achieved 80% se-
lectivity to the p-benzoylanisole [42].Chourdhary and co-workers
used zeolite based catalyst for the same reaction and ameliorated
the selectivity of p-isomer to 90% [43]. Kemnitz and co-workers
utilized borate zirconia solid acid as a heterogeneous catalyst and
enhanced the selectivity to 93-95%, but the reaction was carried
out in the presence of nitrobenzene as solvent [44]. Therefore, the
selectivity of 26.5 wt% PTA@ZIF-67(ec) catalyst in the Friedel-
Crafts acylation of anisole with benzoyl chloride was comparable
with those previously reported in the literature.
3
.5.4. Effect of electron donors and withdraws as substitution group
The study was then expanded to investigate the effect of dif-
ferent substrates on the Friedel–Crafts acylation reaction using
PTA@ZIF-67(ec) catalyst. The reaction was accomplished by using
3
.5.3. Effect of reaction temperature
With such results in hand, we then decided to investigate the
effect of reaction temperature on conversion. The Friedel–Crafts
acylation reaction was performed using 4 wt% of 26.5 wt% PTA@-
ZIF-67(ec) catalyst at 140, 130, 120, 110 and 100 °C, respectively.
Aliquots from the reaction mixtures were withdrawn after 6 h and
analyzed by GC to provide kinetic data for the course of the re-
action. After 6 h, a conversion of 82% was achieved at 120 °C re-
action temperature. A considerable drop in the reaction rate was
observed by decreasing the reaction temperature, with 71% and
4
wt% of 26.5 wt% PTA@ZIF-67(ec) catalyst with substrate:benzoyl
chloride molar ratio 1:2 for 6 h. Fernandes and co-workers re-
ported that the most excellent yields of Friedel–Crafts acylation
were observed in the presence of electron donor groups in the
aromatic ring [33]. In this work, the results illustrated that the
addition of an electron donor group on the aromatic ring increased
the acylation reaction conversion as well as yield while the addi-
tion of an electron withdraw groups significantly decreased them.
The acylation reaction of anisole with benzoyl chloride gave 82%
conversion at 120 °C after 6 h, whereas 73% and 69% conversions
57% conversions being achieved after 6 h at 110 and 100 °C, re-
spectively. In fact, increasing the reaction temperature was not

M. Ammar et al. / Journal of Solid State Chemistry 233 (2016) 303–310
309
Table 3
4. Conclusion
a
Effect of electron donating and withdraw substitution group on acylation reaction .
The Friedel–Crafts acylation of anisole with benzoyl chloride is
investigated over novel PTA@ZIF-67(ec) catalyst. The PTA@ZIF-67
(ec) catalyst has been synthesized by direct addition of phospho-
tungstic acid to the synthesis mixture of ZIF-67 at room tem-
perature. An excellent dispersion of phosphotungstic acid over the
ZIF-67 sample is achieved. Different amount of phosphotungstic
acid has been encapsulated in a ZIF-67 structure (14.6–31.7 wt%)
depending on the amount of phosphotungstic acid introduced in
the synthesis mixture. Among the various (14.6–31.7 wt%) PTA@-
ZIF-67(ec) catalyst, 26.5 wt% PTA@ZIF-67(ec) catalyst reveals high
activity, stability and reusability for acylation reaction. The effects
of catalyst content, reaction temperature and electron donor and
withdraw as substitution group on reaction conversion and yield
have been discussed in detail. In conclusion, we have utilized ZIF
structure as a support for heteropoly acid and develop a highly
active, stable, reusable and environmentally friendly hetero-
geneous catalyst for the Friedel–Crafts acylation.
Conversion (%) Selectivity (%)
p-isomer o-isomer
Yield (%)
Substrate
R¼OCH
R¼CH
R¼C
3
82.14
73.04
69.11
45.64
40.41
93.51
88.54
87.63
69.75
74.43
6.49
11.46
12.37
30.25
25.57
76.81
64.67
60.56
31.83
30.08
3
2
H
5
R¼Br
R¼CHO
a
Reaction condition: substrate, 0.006 mol; benzoyl chloride, 0.012 mol; cata-
lyst amount, 0.1 g; temperature, 120 °C; time, 6 h.
1
00
Conversion
p-isomer
80
60
40
20
0
Acknowledgment
This work was carried out with financial support from
the National Natural Science Foundation of China (Grant nos.
21136001 and 21173018).
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