Acid catalyzed organic transformations by heteropoly tungstophosphoric acid supported on MCM-41

Article abstract of DOI:10.1016/j.apcata.2011.10.024

In this work, solid acid catalysts of the Keggin-type 12-tungstophosphoric acid (H₃PW₁₂O₄₀, HPW) incorporated within the mesochannels of MCM-41 are prepared through a simple and effective impregnation method. The catalysts are characterized by various techniques such as XRD, FTIR, TEM, N₂ adsorption and thermal analysis. The surface acidities are measured by non-aqueous titration of n-butyl amine in acetonitrile and FTIR spectra of chemisorbed pyridine. The acidity and the textural parameters of the nanocomposites can be controlled simply by changing the loading of HPW on the MCM-41. The results indicate that the surface saturation coverage of MCM-41 is reached with 60 wt% HPW. The high saturation coverage is indicative of the well-dispersion of HPW within the mesochannels of MCM-41. The catalytic activities of the HPW/MCM-41 catalysts for the Pechmann, esterification reaction and Friedel-Crafts acylation reactions are studied in detail. Both the surface acidity and the catalytic activity sharply increase with the modification of MCM-41 by HPW. The sample with 60 wt% HPW shows the highest acidity and catalytic activity. The reusability tests of the catalysts show that the catalysts can be used several times without significant loss in activity. The HPW/MCM-41 catalysts have great potential for applications as commercial catalysts in promoting acid-catalyzed organic transformations under environmental friendly conditions and processes.

Full text of DOI:10.1016/j.apcata.2011.10.024

Applied Catalysis A: General 411–412 (2012) 77–86
Contents lists available at SciVerse ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Acid catalyzed organic transformations by heteropoly tungstophosphoric acid
supported on MCM-41
Abd El Rahman S. Khdera,b,∗, Hassan M.A. Hassana,c, M. Samy El-Shalla,∗∗
a
Department of Chemistry, Virginia Commonwealth University Richmond, VA 23284-2006, United States
Department of Chemistry, Mansoura University, Mansoura, Egypt
Department of Chemistry, Faculty of Science, Suez Canal University, Suez, Egypt
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 23 July 2011
Received in revised form 10 October 2011
Accepted 15 October 2011
Available online 20 October 2011
In this work, solid acid catalysts of the Keggin-type 12-tungstophosphoric acid (H PW12O40, HPW) incor-
porated within the mesochannels of MCM-41 are prepared through a simple and effective impregnation
3
method. The catalysts are characterized by various techniques such as XRD, FTIR, TEM, N2 adsorption
and thermal analysis. The surface acidities are measured by non-aqueous titration of n-butyl amine in
acetonitrile and FTIR spectra of chemisorbed pyridine. The acidity and the textural parameters of the
nanocomposites can be controlled simply by changing the loading of HPW on the MCM-41. The results
indicate that the surface saturation coverage of MCM-41 is reached with 60 wt% HPW. The high saturation
coverage is indicative of the well-dispersion of HPW within the mesochannels of MCM-41. The catalytic
activities of the HPW/MCM-41 catalysts for the Pechmann, esterification reaction and Friedel–Crafts acy-
lation reactions are studied in detail. Both the surface acidity and the catalytic activity sharply increase
with the modification of MCM-41 by HPW. The sample with 60 wt% HPW shows the highest acidity and
catalytic activity. The reusability tests of the catalysts show that the catalysts can be used several times
without significant loss in activity. The HPW/MCM-41 catalysts have great potential for applications as
commercial catalysts in promoting acid-catalyzed organic transformations under environmental friendly
conditions and processes.
Keywords:
Solid acid catalyst
MCM-41
Heteropoly acid
Pechmann
Esterification
Friedel–Crafts acylation reactions
©
2011 Elsevier B.V. All rights reserved.
1
. Introduction
[6–8]. For MCM-41 to be used as a catalyst or catalyst support in
acid-catalyzed reactions Brønsted acid sites need to be created and
the acid strength must be enhanced. The acidity of MCM-41 can
be increased by surface modification, through the introduction of
strong acid species such as sulfate ions [9], sulfated zirconia [10] or
heteropoly acids with Keggin-type structures [11,12], either on the
surface or within the inner channels of MCM-41. This can result in
creating Brønsted acid sites which can lead to a significant improve-
ment in the acid strength of MCM-41.
In recent years, environmental considerations have raised
strong interest in the development of economically feasible mate-
rials and processes to eliminate the use of harmful substances
and the generation of toxic waste materials. In this respect, het-
erogeneous catalysis can play a key role in the development of
environmentally benign processes particularly in the petroleum
and chemical industries. For example, the substitution of liquid
acid catalysts by efficient solid materials could contribute towards
this goal. Highly porous molecular sieves such as MCM-41 pro-
vide an attractive possibility for the development of highly active
solid acid catalysts [1,2]. MCM-41 is a promising support because
Heteropoly solid acids with stable and strongly acidic prop-
erties, such as 12-tungstophosphoric acid (H PW12O40, HPW)
3
have attracted much attention because of their strong acidity,
high oxidation potential and redox characteristics which offer
applications as Brønsted acid and redox catalysts [13–22]. Indeed,
HPW supported on various materials has demonstrated high
catalytic activity as an acid and oxidation catalyst for a variety
for organic transformations including oxidative dehydrogenation
of alkanes, olefin oxidation to epoxides, hydroisomerization and
esterification reactions [13,16–18,23,24]. The choice of the support
can overcome the major limitations of HPW which include low
2
of its large surface area (∼1000 m /g), high thermal stability and
large pore size (1.5–8 nm) [1–5]. However, MCM-41 lacks Brønsted
acid sites and exhibits only weak hydrogen-bonded type of sites
∗ Corresponding author at: Department of Chemistry, Mansoura University, Man-
soura, Egypt. Tel.: +20 010 6491649.
∗
2
surface area of 5–10 m /g, low thermal stability and poor porosity
∗ Corresponding author at. Tel.: +1 804 828 3518.
E-mail addresses: askhder2244@yahoo.com (A.E.R.S. Khder), mselshal@vcu.edu
[
13–16]. For example, HPW clusters with diameters ∼1.2 nm can
(
M.S. El-Shall).
be introduced inside the MCM-41 large pores of 2–8 nm, thus
0
926-860X/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.apcata.2011.10.024

7
8
A.E.R.S. Khder et al. / Applied Catalysis A: General 411–412 (2012) 77–86
◦
allowing for a significant increase in the surface area of the HPW
with a possible increase in its thermal stability, in addition to
creating Brønsted acid sites within the MCM-41 support [4]. The
large pore sizes of MCM-41 play a major role in overcoming the
problem of the obstruction of bulky organic molecules resulting
from the small pore volume of HPW. Therefore, HPW supported
on MCM-41 can provide a potential solid acid catalyst with high
activity and thermal stability.
under static air at 80 C for 12 h. The mesoporous material was
◦
finally obtained by calcination of the hybrid structure at 550 C for
4 h.
HPW was supported on MCM-41 by an impregnation method.
One gram of calcined MCM-41 was dispersed under vigorous stir-
ring into a solution of the desired amount of HPW in 20 ml of water
for 4 h. After removing the water by evaporation, the sample was
dried at 110 C over night in an oven and subsequently calcined at
350 C for 4 h in a muffle furnace.
◦
◦
Here, we report the preparation of MCM-41 with different
loadings of the Keggin-type 12-tungstophosphoric acid (HPW)
nanocrystals incorporated within the mesochannels of MCM-41
through a simple and effective impregnation method. The resulting
catalyst materials have been characterized by various techniques
including XRD, FTIR, TEM, thermal analysis, N2 adsorption, and
non-aqueous titration. The activity of these catalysts towards three
different acid-catalyzed organic reactions has been extensively
investigated under solvent-free conditions. The studied reactions
include the Pechmann, esterification and Friedel–Crafts acylation
reactions. The Pechmann reaction is one of the most widely used
methods for the preparation of substituted coumarins since it pro-
ceeds from simple starting materials and results in good yields of
a variety of substituted coumarins [25,26]. These compounds are
important for a wide range of applications including fragrances,
pharmaceuticals, food additives, cosmetics, tunable dye lasers, and
others. Esterification is one of the fundamental reactions in organic
syntheses. However, most of the reported procedures for the syn-
thesis of esters require the use of sulfuric acid, hydrochloric acid,
and toxic chemicals such as dimethyl sulfate, and/or methyl iodide
which are environmentally hazardous [27,28]. Replacement of
these acids by a solid acid catalyst such as HPW supported on MCM-
2
.2. Characterization
X-ray powder diffraction patterns of samples were determined
using an X’Pert Philips Materials Research Diffractometer. The pat-
terns were run with copper radiation (Cu K␣, ꢀ = 1.5405 A˚ ) with the
second monochromator at 45 kV and 40 mA with a scanning speed
of 2 in 2ꢁ/min. FT-IR spectra of calcined samples were recorded
by using Nicolet-Nexus 670 FTIR spectrophotometer (4 cm reso-
lution and 32 scans) in dried KBr (Sigma) pellets and a measuring
range of 400–4000 cm . Transmission electron microscope (TEM)
images and the particle size were obtained using a Jeol JEM-1230
operated at 120 kV. For TEM images the sample powder was dis-
persed in methanol by using ultrasonic radiation for 10 min and a
drop of the suspension was placed onto the carbon-coated grids.
The thermal stability of the samples was studied using TA thermal
analyzer (DSC Q200 and TGA Q5000). The adsorption isotherms
and the specific surface area (SBET) of the various catalysts were
◦
−
1
−
1
◦
determined from nitrogen adsorption studies conducted at −196 C
using Quantachrom Nova Sorbimetric system. The total acidity
of the solid samples was measured by means of potentiometric
titration [32,33]. The solid (0.05 g) was suspended in acetonitrile
4
1 would result in a simplified product recovery and elimination
or reduction of undesirable waste streams. Friedel–Crafts acylation
of aromatic compounds is an important reaction which is usu-
ally used in fine chemical and pharmaceutical industries [29,30].
The development of active and selective heterogeneous catalysts
for these reactions is an important goal since the current efficient
catalysts suffer from leaching of the active metal and behaving
similar to homogeneous catalysts which cannot be regenerated. In
this work, we demonstrate the successful applications of the pre-
pared HPW–MCM-41 catalyst for three classes of acid-catalyzed
reactions, and provide detailed information on the stability of the
catalyst and the effect of reaction parameters on conversion and
product selectivity. The results suggest that highly efficient HPW
catalysts supported on MCM-41 can provide significant advances
in the development of environmentally benign processes in the
chemical industry.
(
Merck), and agitated for 3 h. Then the suspension was titrated
with 0.05 N n-butylamine (Merck) in acetonitrile at 0.05 ml/min.
The electrode potential variation was measured with an Orion 420
digital A model by using a double-junction electrode. Lewis and
Brønsted acid sites present on the surface of the catalyst were
determined with FT-IR spectra of adsorbed pyridine. Prior to the
pyridine adsorption [34], small portions of the calcined samples
◦
were degassed under vacuum at 200 C for 3 h followed by sus-
pending in dry pyridine. Then, the excess pyridine was removed by
evaporation. The FT-IR spectra of the pyridine-adsorbed samples
were carried out using Nicolet-Nexus 670 FTIR spectrophotometer
by mixing 0.005 g of the sample with 0.100 g KBr.
2
.3. Catalytic activity
◦
In all experiments the catalysts were activated at 200 C for 2 h.
2
. Experimental
The reactions were carried out in a 25 ml round bottom flask fitted
with a condenser. The temperature of the reactions was maintained
using an oil bath.
2.1. Preparation of catalysts
The original method for the preparation of MCM-41 was first
proposed by Beck et al. [1]. However, the method used here is based
on the modifications introduced in the original method to con-
duct the synthesis in mild conditions in terms of temperature and
amount of surfactant amount. In a typical synthesis [31], 1.988 g
of cetyl trimethyl ammonium bromide (CTAB, 98%, Sigma) was
dissolved in 120 g of water at room temperature. After complete
dissolution, 8 ml of aqueous NH3 (32% in water, Merck) was added
to the above solution. Then 10 ml of tetraethyl orthosilicate (TEOS,
2.3.1. Synthesis of 7-hydroxy-4-methyl coumarin by Pechmann
reaction
In a typical run, a mixture of resorcinol (10 mmol), ethyl acetoac-
etate (20 mmol) and activated catalyst (0.1 g), were magnetically
◦
stirred and heated to attain the reaction temperature (120 C) for
1 h. After completion of the reaction, the reaction mixture was
transferred into 10 ml of ethanol and stirred for 15 min. The catalyst
was removed from the reaction mixture by simple filtration. The
product sample was identified by GC–MS (HP GCD system equipped
with EID) analysis. The ethanolic solution was heated to evaporate
the solvent and subsequently the residue is recrystallized to afford
7-hydroxy-4-methyl coumarin. The yield of 7-hydroxy-4-methyl
coumarin was obtained as follows:
9
9%, Flouka) was added to the solution under vigorous stirring.
The hydrolysis of TEOS takes place during the first 2 min at room
temperature (the solution becomes milky and slurry) whereas the
condensation of the mesoporous hybrid material is achieved after
1
h of reaction. The material was then filtered and allowed to dry

A.E.R.S. Khder et al. / Applied Catalysis A: General 411–412 (2012) 77–86
79
Yield (wt%) = (Obtained weight of product) × 100/(Theoretical
weight of product).
(A) (100)
MCM-41 Calcined
MCM-41 as prepared
20wt.% HPW
30wt.% HPW
40wt.% HPW
70wt.% HPW
2.3.2. Esterification of acetic acid with n-octanol
The esterification reaction was carried out using 0.05 g of the
catalyst. In a typical run, a mixture of 2 mmol of acetic acid (Aldrich,
9
9.8%) and 2 mmol of n-octanol (Aldrich, 98%) were magnetically
◦
stirred and heated to attain the reaction temperature (80 C) for
1
h. Then the reaction mixture was filtered and the products were
(110)
(200)
analyzed by means of GC–MS (HP GCD system equipped with EID).
The yield was defined as the percentage of octyl acetate formed and
calculated based on gas chromatographic analysis.
(210)
2
3
4
5
6
7
2.3.3. Friedel–Crafts acylation of anisole with acetic anhydride
In a typical run, a mixture of anisole (20 mmol), acetic anhy-
2θ
dride (5 mmol) and activated catalyst (0.1 g), were magnetically
◦
stirred and heated to attain the reaction temperature (120 C) for
(
B)
60wt.% HPW/MCM-41
0wt.% HPW/MCM-41
Pure HPW
1
h. After completion of the reaction, the catalyst was removed from
7
the reaction mixture by centrifugation. Then, the reaction mixture
was analyzed using GC–MS (HP GCD system equipped with EID).
The main products are the para-methoxyacetophenone (p-MAP)
and the ortho-methoxyacetophenone (o-MAP).
The conversion was calculated based on gas chromatographic
analysis of the unconsumed acetic anhydride. The selectivity to
each isomer was calculated based on the following:
%
Selectivity to p-MAP = 100 × (peak area of p-MAP)/(peak area
of p-MAP + peak area of o-MAP).
3
. Results and discussion
8
10
12
14
16
18
20
3.1. Characterization of HPW catalyst supported on MCM-41
2
θ
Fig. 1(A) displays the XRD patterns of the as-prepared and
Fig. 1. (A) Low-angle XRD diffraction patterns of MCM-41 and HPW/MCM-41 sam-
ples. (B) XRD diffraction patterns of pure HPW, 60 wt% HPW/MCM-41, and 70 wt%
HPW/MCM-41.
calcined MCM-41 before and after the incorporation of different
weight percentage loadings of HPW namely, 20%, 30%, 40% and 70%.
The XRD pattern of MCM-41 has four peaks that could be indexed
to (1 0 0), (1 1 0), (2 0 0) and (2 1 0) reflections, which correspond
to a well ordered hexagonal pore system [1,2]. The intensities of
the diffraction peaks increase significantly after calcination, which
must be related to the removal of the occluded surfactant molecules
during the calcination process resulting in the enhancement of
the structural ordering [35]. The 100 reflection of the MCM-41 is
still observed after the HPW loading although the intensity of the
diffraction peak becomes broader and weaker as the HPW loadings
increases up to 60 wt%. This suggests that the mesoporous structure
of the MCM-41 remains almost unchanged upon the HPW load-
ing but the long-range order is significantly decreased. Although,
the presence of HPW usually decrease long-range order of MCM-
the support. This result is not surprising since the MCM-41 has a
pore size higher than HPW nanocrystals, HPW are expected to be
present inside the pores as well as on the MCM-41 surface. How-
ever, with loadings above 60 wt%, the reflections from the HPW
◦
◦
◦
crystals at 2ꢁ = 10.3 , 14.5 and 18.0 are observed indicating poor
dispersion or agglomeration of the HPW crystals at loading above
60 wt%.
TEM images of MCM-41 with different magnifications are shown
in Fig. 2(A). Periodic structures can be observed in the high magni-
fication images indicating the high quality and long range order
of the hexagonal MCM-41 materials. TEM images of the 20 wt%
and 70 wt% HPW/MCM-41 are shown in Fig. 2(B). In the presence
of HPW, darker spots appear in the TEM images of the MCM-41
which could be attributed to some HPW nanocrystals within the
pores and/or on the surface of the MCM-41 crystals. No evidence for
aggregation of the HPW nanocrystals could be found in the MCM-
41 samples with HPW loading percentage <60 wt%. However, in the
case of 70 wt% HPW/MCM-41 larger dark spots are observed on the
surface which could be due to formation of multilayers of HPW on
the surface of the MCM-41 crystals consistent with the XRD results
shown in Fig. 1(B).
4
1 [36,37], but little explanation regarding this phenomenon was
discussed. We studied the stability of MCM-41 under acidic con-
ditions, in water and in presence of WO ; the calcined MCM-41
3
material was stirred overnight in 0.1 M HCl, water and ammonium
para tungstate (50 wt% WO /MCM-41) solutions followed by dry-
3
◦
ing and calcination at 350 C. The obtained XRD scans are shown
in Supplementary Fig. S1. The main reflection at (d(1 0 0)) of MCM-
1, under all conditions was decreased. However, in presence of
4
WO3 much intensity decrease and broader reflection of (d(1 0 0)
)
peak was observed. The results proved that MCM-41 is readily
hydrolyzed under acidic conditions and becomes broader due to
radiation absorption by the tungsten oxide. It is assumed that the
severe reflection of MCM-41 in presence of HPW gradual hydroly-
sis in acidic medium and is mainly due to radiation absorption by
the tungsten element present in HPW.
The absence of diffraction peaks due to the crystalline HPW
phase in the diffraction patterns of the MCM-41 with HPW loadings
up to 60 wt% (Fig. 1(B)) suggests that HPW is well dispersed within
The supported HPW-MCM-41 samples were analyzed by FTIR
in order to confirm the presence of the Keggin anion on the
3−
MCM-41. The PW12O40 Keggin anion structure consists of a PO4
tetrahedron surrounded by four W O9 groups formed by edge-
3
sharing octahedra. These groups are connected to each other by
corner-sharing oxygen [38]. This structure gives rise to different
types of oxygen atoms, being responsible for the fingerprint IR
−
1
bands of the Keggin anion between 1100 and 500 cm . The FTIR

8
0
A.E.R.S. Khder et al. / Applied Catalysis A: General 411–412 (2012) 77–86
Fig. 2. TEM images of (A) pure MCM-4 and (B) 20 wt% HPW/MCM-41 (left), 70 wt% HPW/MCM-41 (right).
spectra of HPW with different loading percentages on MCM-41 are
presented in Fig. 3. Bulk HPW shows five characteristic bands in
not observed if the crystalline organization of MCM-41 is lost [37].
−
1
Another band at 807 cm is due to the symmetric stretching vibra-
−1
−1
the region 1100–500 cm , observed at 1081, 982, 889, 797 and
tion of the rocking mode of Si–O–Si [39]. The band at 970 cm is
−
1
5
95 cm , which can be assigned to the stretching vibrations of
assigned to the presence of Si–OH symmetric stretching vibration.
The spectra displayed in Fig. 4 show clearly that the small bands
P–O, W Ot, W–Oc–W, W–Oe–W and the bending vibration of P–O,
respectively. The MCM-41 is characterized by a broad band around
−
1
−1
at 982 cm and 889 cm became more evident with increasing
the HPW loading on MCM-41. Moreover, the intensity of the MCM-
−
1
1
300–1000 cm
assigned to an asymmetric stretching mode of
−
1
−1
Si–O–Si. This strong framework band of MCM-41 at 1090 cm is
41 band at 807 cm gradually increases with the HPW loading due
−1
to the overlap with the HPW band at 797 cm . These characteristic
peaks of the Keggin ion confirm the presence and the stability of
HPW into the MCM-41 framework.
e
d
The N adsorption isotherms and pore size distributions of pure
2
MCM-41 and several HPW samples with different loading percent-
ages on MCM-41 are shown in Fig. 4(A) and (B). Table 1 lists the
textural properties of pure MCM-41 and the supported HPW/MCM-
4
1 [36,40]. These properties include d(1 0 0) (the d-spacing of the 100
diffraction in Å), I(1 0 0) (the intensity of the 100 diffraction), ao (unit
cell in nm), SBET (BET surface area in m /g), pore volume in cc/g, and
c
2
pore diameter in nm.
The adsorption isotherm of the parent MCM-41 mesoporous
material displayed in Fig. 4(A) could be assigned to type IV isotherm.
The HPW supported samples show a drop in the adsorption con-
densation region, at P/P = 0.2–0.4. The pore size distribution of pure
and loaded MCM-41 shows a unique peak centered at about 35 A˚
diameters. It is evident that the pore volume and the specific surface
area of the loaded sample are much lower compared to that of the
parent MCM-41. This is consistent with previous results which indi-
cate that the surface area of MCM-41 continuously decreases with
b
a
o
2
500
2000
1500
1000
500
-1
Wavenumber (cm )
Fig. 3. FTIR spectra of (a) MCM-41, (b) 50 wt% HPW/MCM-41, (c) 60 wt% HPW/MCM
1, (d) 70 wt% HPW/MCM-41 and (e) bulk HPW.
4

A.E.R.S. Khder et al. / Applied Catalysis A: General 411–412 (2012) 77–86
81
Table 1
Textural properties of the HPW/MCM catalysts.
2
d(1 0 0)(Å)
I(1 0 0)
Unit cell, ao (nm)
SBET (m /g)
Pore volume (cc/g)
Pore diameter (nm)
Dried MCM-41
Calcined MCM-41
40.04
34.29
31.19
34.25
33.09
33.18
34.03
31.61
482
1311
310
252
147
68
4.62
3.96
3.60
3.95
3.82
3.83
3.93
3.65
–
982
846
789
723
604
472
219
–
–
0.85
0.81
0.70
0.59
0.52
0.35
0.22
3.5
3.5
3.6
3.6
3.5
3.4
3.5
2
3
4
5
6
7
0 wt% HPW/MCM-41
0 wt% HPW/MCM-41
0 wt% HPW/MCM-41
0 wt% HPW/MCM-41
0 wt% HPW/MCM-41
0 wt% HPW/MCM-41
164
33
◦
increasing the heteropolyacid content [36,40]. The reduction in the
pore volume and surface area after loading could be due to the fact
that the HPW is deposited inside the mesochannels and is well-
dispersed on the surface of the hexagonally ordered mesoporous
MCM-41 support.
(c) a peak in the range of 370–580 C (1.42%) due to the loss of
1.5 H O molecules corresponding to the loss of all acidic protons
and the beginning of decomposition of the Keggin structure [41].
These results indicate that the hydrated HPW contains 21 H O
molecules per Keggin unit. Another exothermic effect at 605 C
2
2
◦
The thermal stability of HPW and HPW loaded on MCM-41 sam-
ples was studied using DSC of the uncalcined samples. The catalytic
activity and structure of the heteropolyacids mainly depend upon
its degree of hydration. Thus, TGA and DSC scans were obtained
for the hydrated bulk HPW sample as shown in Fig. 5(A). The
bulk hydrated H PW12O40·xH O shows three stage weight loss pro-
is due to the decomposition of Keggin anion (PW12O38.5) to form
both phosphorous and tungsten oxides [42]. HPW showed that the
decomposition takes place without appreciable weight loss, which
may support the decomposition of Keggin anion (PW12O38.5) to the
corresponding oxides (0.5 P O5 and 12 WO ).
2
3
Fig. 5(B) displays some selected DSC curves of the prepared
samples. The samples show endothermic peaks between 150 and
3
2
◦
cesses: (a) the first peak observed at a temperature below 140 C
◦
(
9.11%) corresponds to the loss of 15 molecules of the physisorbed
260 C, corresponding to removal of the adsorbed water molecules.
◦
water; (b) a peak in the temperature range of 150–280 C (2.52%),
The significant result is the absence of the exothermic peak due to
the decomposition of HPW in the supported MCM-41 samples with
up to 60 wt% HPW. This may be due to the high dispersion of HPW
into MCM-41 pores. On the other hand, a weak exothermic peak
which accounts for the loss of 6 H O molecules per Keggin unit;
2
600
400
200
0
◦
(
A)
is observed at 605 C in the DSC curve of the 70 wt% HPW loaded
on MCM-41 indicating the decomposition of the aggregated HPW
crystals on the surface of the support.
1
2
(
A)1
00
DSC
95
90
85
80
75
3
1
2
3
- MCM-41
- 30 wt % HPW/MCM-41
- 70 wt % HPW/MCM-41
TGA
0
.0
0.2
0.4
0.6
0.8
1.0
o
p/p
0
0
0
0
0
0
0
.12
.10
.08
.06
.04
.02
.00
100 200 300 400 500 600 700
MCM-41
(
B)
o
3
7
0 wt % HPW/MCM-41
0 wt % HPW/MCM-41
Temperature ( C)
(B)
d
c
b
a
1
0
20 30 40 50 60 70 80 90 100
ο
100 200 300 400 500 600 700
Pore Width (Α)
o
Temperature ( C)
Fig. 4. (A) Adsorption–desorption isotherms of N2 at −196 ◦C over MCM-41, 30 wt%
HPW/MCM-41 and 70 wt% HPW/MCM-41. (B) Pore size distribution curves of MCM-
Fig. 5. (A) TGA–DSC profiles of pure HPW. (B) DSC scans of: (a) 50 wt% HPW/MCM-
4
1, 30 wt% HPW/MCM-41 and 70 wt% HPW/MCM-41.
41, (b) 60 wt% HPW/MCM-41, (c) 70 wt% HPW/MCM-41 and (d) bulk HPW.

8
2
A.E.R.S. Khder et al. / Applied Catalysis A: General 411–412 (2012) 77–86
800
600
400
200
0
MCM-41 Calcined
g
f
20wt.% HPW/MCM-41
30wt.% HPW/MCM-41
40wt.% HPW/MCM-41
50wt.% HPW/MCM-41
60wt.% HPW/MCM-41
70wt.% HPW/MCM-41
e
-200
d
c
0.0
0.5
1.0
1.5
2.0
2.5
3.0
V (ml) n-butylamine added
Fig. 6. Potentiometric titration curves of n-butyl amine in acetonitrile for the pre-
pared catalysts.
b
Generally, the thermal stability of heteropoly acids loaded on
different supports depends on the type of the support, the HPW
loading and the conditions of pretreatment. Acidic or neutral car-
riers such as SiO2 [43], active carbon [44], and acidic ion exchange
resin [45] are suitable as supports. Basic solids such as MgO [46]
tend to decompose HPW due to the strong interaction with HPW.
Wu et al. reported that the thermal stability of 12-tungstogermanic
acid loaded on silica is lower than that of pure 12-tungstogermanic
acid due to strong interactions with the support [47]. On the other
a
1700
1600
1500
1400
1300
-1
Wavenumber (cm )
Fig. 7. FTIR spectra of pyridine adsorbed on: (a) MCM-41, (b) 20 wt% HPW/MCM-41,
(c) 30 wt% HPW/MCM-41, (d) 40 wt% HPW/MCM-41, (e) 50 wt% HPW/MCM-41, (f)
60 wt% HPW/MCM-41, and (g) 70 wt% HPW/MCM-41.
hand SiO -supported molybdenum HPWs, retain the Keggin struc-
2
ture at high loadings but decompose at very low loadings due to
strong interactions with surface silanol groups [48]. In the present
work, with loadings up to 60 wt% of HPW on MCM-41, HPW forms
finely dispersed nanocrystals inside the pores and on the MCM-41
surface. However, when the HPW content exceeds the monolayer
coverage (as evident from XRD), this leads to the formation of
polylayers or agglomeration of HPW on the MCM-41 surface. The
surface excess concentration of HPW is accompanied by an exother-
the increase of TPA content up to 60 wt%, then decreases with fur-
ther increase in HPW content. Additionally, the acid strength of
the catalysts changes in the same way. Pure MCM-41 surface has
Si–OH (silanol groups) which are weakly acidic in nature. When
these silanol groups replaced with stronger acidic sites of HPW the
number and strength of the acid sites will increase. As the cover-
age of MCM surface increases by HPW, the number and strength of
acid sites will increase. The sample with the highest HPW content
(70 wt% above surface saturation coverage) shows loss of acidity
and acid strength probably due to agglomeration (Keggin–Keggin
interactions) of the crystalline HPW on the surface of the support
as detected by XRD and TEM.
◦
mic effect at 605 C observed in the sample loaded with 70 wt%
HPW. This indicates that the thermal stability of HPW on MCM-41
seems to be comparable to that of pure HPW. This may be due to
weak interactions between HPW and the support as a result of the
weakly acidic nature of MCM-41.
The Brønsted (B) and Lewis (L) acidic sites in the supported HPW
catalysts can be characterized using pyridine as a probe molecule.
The FTIR spectra of pyridine adsorbed on the catalysts are shown in
Fig. 7. All the catalysts contain Lewis (L) acid sites and Brønsted (B)
3
.2. Surface acidity measurements
The acidity of the solids was characterized by potentiometric
titration with n-butylamine. Using this technique, it is possible to
estimate the strength and the total number of acid sites present in
the solids. As a criterion to interpret the results, it was suggested
that the initial electrode potential (E ) indicates the maximum acid
strength of the sites and the value of mequiv. amine/g solid where
the plateau is reached indicates the total number of acid sites
−
1
acid sites, as indicated by the adsorption bands at 1444 cm (L),
− −1 −1
1
1
600 cm (L), 1542 cm (B) and 1640 cm (B) [52]. In addition,
−
1
the band at 1488 cm indicates the formation of the adjacent L and
B acid sites. In comparison with the support (MCM-41), the nature
of the acidity of the HPW supported catalysts shows the following
changes:
i
[
49,50]. Nevertheless, the end point of the titration given by the
inflexion point of the curve is a good measure to carry out a com-
parison of the acidity of the different samples. On the other hand,
the acid strength of acid sites may be classified according to the fol-
lowing scale: E > 100 mV (very strong sites), 0 < E < 100 mV (strong
(1) The FTIR spectrum of MCM-41 after the adsorption of pyri-
−
1
dine shows bands at 1600 and 1448 cm in agreement with
results previously reported [53]. These bands are assigned to
the adsorption of pyridine on weak L acid sites or to weakly
hydrogen-bonded pyridine.
i
i
sites), −100 < E < 0 mV (weak sites) and E < −100 mV (very weak
i
i
sites) [51].
The titration curves of the catalysts are displayed in Fig. 6. While
(2) Incorporation of HPW on MCM-41 and the adsorption of pyri-
dine give rise to the development of both B and L acid sites.
Bands assigned to the pyridinium ions are observed at 1542
the MCM-41 support exhibits acid sites with an E value of 7 mV, the
i
supported HPW samples exhibit much higher values (as expected)
in the range of 374–813 mV (Table 2). The number of acid sites
determined by potentiometric titration increases in parallel with
−
1
and 1640 cm and those associated with pyridine coordinated
−
1
to Lewis (L) acid sites are observed at 1444 and 1600 cm . In

A.E.R.S. Khder et al. / Applied Catalysis A: General 411–412 (2012) 77–86
83
Table 2
−
1
−1
Acidic properties of the HPW/MCM-41 catalysts. Ei, initial electrode potential in mV, I(B)1542 and I(L)1444 are the intensity of the IR bands at 1542 cm and 1444 cm
corresponding to pyridine adsorbed on Brønsted and Lewis acid sites, respectively.
−
1
Ei
No. of acid sites in mequiv. g
I(B)1542
I(L)1444
I(B)/I(L)
Dried MCM-41
–
–
–
–
–
Calcined MCM-41
+7
0.25
0.47
0.93
1.48
1.96
2.99
2.81
–
0.41
1.10
0.81
0.85
0.80
2.01
1.72
–
20 wt% HPW/MCM-41
30 wt% HPW/MCM-41
40 wt% HPW/MCM-41
50 wt% HPW/MCM-41
60 wt% HPW/MCM-41
70 wt% HPW/MCM-41
+374
+537
+603
+617
+813
+778
0.44
0.69
0.73
1.32
4.72
3.62
0.40
0.85
0.86
1.65
2.35
2.10
addition, the band at 1488 cm−1 indicates the formation of the
adjacent L and B acid sites.
ethyl acetoacetate on the Brønsted acid sites of the catalyst [54].
This is followed by a nucleophilic attack by resorcinol to form
coumarin. A close examination of the B/L ratios (Table 2) and the
catalytic activity results (Fig. 8) shows clearly that the sample with
higher B/L ratio exhibits the maximum catalytic activity. The direct
correlation between surface acidity (Brønsted acidity) and catalytic
activity explains the role of HPW in enhancing the surface acidity
and consequently the catalytic activity of the catalysts. Compared to
other published work [54–56], our catalyst exhibits higher catalytic
activity in a shorter reaction time, lower temperature and smaller
amount of the catalyst. The reusability of the 60 wt% HPW/MCM-41
sample was checked with two consecutive syntheses of 7-hydroxy-
4-methyl coumarins by using recovered samples after reactivation
(
3) The acidity varies depending on the amount of HPW loaded on
the support. The intensity of B and L acid sites, obtained from
−1
the absorbance at 1542 and 1444 cm and their corresponding
B/L intensity ratios [51] are shown in Table 2. At low loading,
the catalyst showed both Brønsted and Lewis acidities; with an
increase in loading both Brønsted and Lewis acidities increase.
On the other hand, the increase in the Brønsted acidity is much
more than the Lewis acidity up to 60 wt% HPW at which the
Brønsted acidity reaches a maximum (B/L = 2.35).
From the results of the acidity measurements it can be con-
◦
cluded that, the incorporation of HPW enhances the surface acidity
and acid strength up to the monolayer coverage. Increasing in the
HPW loading above 60 wt% decreases the surface acidity and acid
strength due to agglomeration of the crystalline HPW, which pre-
vents the accessibility of pyridine to the active sites.
at 200 C. The product yield in the two consecutive runs (95.4% and
93.8%, respectively) indicates excellent reusability with very little
activity loss from the first run (96%).
3.3.2. Esterification reaction
The esterification reaction between acetic acid and n-octanol
3
.3. Catalytic activity
◦
was carried out using 0.05 g of the catalyst at 80 C for 1 h reac-
tion time. The main products of the reaction were octylacetate
with high selectivity and a mixture of 2- and 4-octene with low
selectivity. As shown in Fig. 9, the most active catalyst (60 wt%
HPW/MCM-41) shows 100% yield with nearly 99% selectivity to
octylacetate. Generally Brønsted acid sites catalyze esterification
reactions, but according to other work esterification can also be
catalyzed by Lewis acid sites typically using metal ions in low coor-
dination states [57,58]. However, most of the literature work shows
that both Brønsted and Lewis acid sites are responsible for cat-
alyzing the esterification reaction [59,60]. Moreover, the formation
of alkenes as side products of the reaction supports the idea that
the mechanism of the reaction proceeds via a protonated alco-
hol intermediate [61]. This means that the reaction proceeds by
The HPW/MCM-41 catalysts prepared were used as solid acid
catalysts in three acid-catalyzed organic reactions namely, the
Pechmann, esterification and Friedel–Crafts acylation reactions.
3
.3.1. The Pechmann reaction
The reaction between resorcinol and ethyl acetoacetate was car-
ried out using 0.1 g of the HPW supported catalyst (20–70 wt%) at
1
high catalytic activity under the reaction conditions used. For the
most active catalyst (60 wt% HPW), more than 96% of 7-hydroxy-
4
◦
20 C for 1 h. As shown in Fig. 8, the prepared catalysts exhibit
-methyl coumarin was obtained with 100% selectivity. Sudha
et al. reported that, the formation of 7-hydroxy-4-methyl coumarin
takes place through the chemisorption of the carbonyl group of
100
100
8
6
4
2
0
0
0
0
0
80
60
40
20
0
%
Yield of octylacetate
% Selectivity of octylacetate
% Selectivity of 2 and 4-octene
0
10
20
30
40
50
60
70
0
20
40
60
Amount of HPW loading (wt. %)
Amount of HPW loading (wt. %)
Fig. 9. Influence of HPW content in HPW/MCM-41 catalysts on yield and selectivity
Fig. 8. Influence of HPW content in HPW/MCM-41 catalysts on yield of 7-hydroxy-
of octyl acetate and on selectivity to octyl acetate or to 2 and 4 octene (reaction
condition: acetic acid/n-octanol, 1:1; reaction time, 1 h; temperature, 80 C; catalyst
weight, 0.05 g).
◦
4
-methyl coumarin (reaction condition: resorcinol/EAA, 1:2; reaction time, 1 h;
◦
temperature, 120 C; catalyst weight, 0.1 g).

8
4
A.E.R.S. Khder et al. / Applied Catalysis A: General 411–412 (2012) 77–86
1
20
00
%
%
%
conversion of acetic anhydride
Selectivity to p-MAP
Selectivity to o-MAP
120
100
80
60
40
20
0
% Conversion of acetic anhydride
% Selectivity to o-MAP %
Selectivity to p-MAP %
%
1
8
6
4
2
0
0
0
0
0
0
10
20
30
40
50
60
70
60
80
100
120
140
160
o
Amount of HPW content (wt%)
Reaction temperature ( C)
Fig. 10. Influence of HPW content in HPW/MCM-41 catalysts on conversion of acetic
anhydride (in %) and % selectivity to p-MAP and o-MAP (in %) (reaction condi-
tion: anisole/acetic anhydride, 4:1; reaction time, 1 h; temperature, 120 C; catalyst
Fig. 11. Influence of reaction temperature on conversion of acetic anhydride (in %)
and selectivity to p-MAP and o-MAP (in %) over 60 wt% HPW/MCM-41 catalyst (reac-
tion condition: anisole/acetic anhydride, 4:1; reaction time, 1 h; catalyst weight,
◦
weight, 0.1 g).
0.1 g).
1
00
the adsorption of n-octanol over Brønsted and/or Lewis acid sites
forming a protonated alcohol intermediate, which then reacts with
acetic acid to form the corresponding ester and water.
9
0
The reusability of 60 wt% HPW/MCM-41 sample was also
checked with two consecutive reactions by using the recovered
sample after reactivation at 200 C and resulting overall yield was
80
◦
9
8.7 and 97.1%, respectively.
70
60
50
3.3.3. Friedel–Crafts acylation reaction
4
6
0 wt.% HPW/MCM-41
0 wt.% HPW/MCM-41
The Friedel–Crafts acylation of anisole using acetic anhy-
◦
dride was carried out using 0.1 g of the catalyst at 120 C
for 1 h (the optimum conditions). The main products obtained
are para-methoxyacetophenone (p-MAP, major product) and the
ortho-methoxyacetophenone (o-MAP, minor product).
70 wt.% HPW/MCM-41
1
:1
2:1
3:1
4:1
5:1
6:1
Anisole: Acetic anhydride molar ratio
Fig. 10 displays the catalytic activity of the catalysts prepared
with different loadings of HPW (20–70 wt%) at 120 C. It should be
Fig. 12. Variation of the conversion of acetic anhydride with the anisole/acetic anhy-
dride molar ratio HPW/MCM-41 catalysts (reaction time, 1 h; temperature, 120 C;
◦
◦
noted that pure MCM-41 did not show any catalytic activity for
the acylation of anisole with acetic anhydride. However, loading
MCM-41 with of 20 wt% HPW results in a sudden increase in the %
conversion of acetic anhydride to around 70%. The % conversion of
acetic anhydride further increases as HPW content increase up to
maximum limit at 60 wt% HPW with 99.2%. A slight decrease in the
catalyst weight, 0.1 g).
100
80
%
conversion of acetic anhydride is observed for the 70 wt% HPW
catalyst. It is important to note that in all the different HPW load-
ings, the selectivity towards p-MAP between 98.1 and 99.5% in all
the catalysts, which may indicate that this selectivity is indepen-
dent of the number and strength of acidic sites.
6
0
40
Fig. 11 displays the effect of reaction temperature effect on
the acetic anhydride conversion using the 60 wt% HPW/MCM-41
catalyst. Significant increase in the overall yield is observed with
40 wt.% HPW/MCM-41
60 wt.% HPW/MCM-41
70 wt.% HPW/MCM-41
2
0
◦
increasing temperature until 120 C after which a slight decrease
0
in the yield is observed. The slight decrease in conversion at high
temperatures is probably due to the inhibiting effect of the p-MAP,
which can be strongly adsorbed on the catalyst at higher con-
version. A decrease in conversion at high temperatures was also
observed in the acylation of toluene with acetic anhydride due to
the strong adsorption of methylacetophenones [62].
0
20 40 60 80 100 120 140 160 180 200
Reaction time (min.)
Fig. 13. Effect of reaction time on conversion of acetic anhydride (%) (reaction con-
◦
dition: anisole/acetic anhydride, 4:1; temperature, 120 C; catalyst weight, 0.1 g).
The effect of the anisole to acetic anhydride molar ratio on the
acetic anhydride conversion was studied using 0.1 g of 40, 60 and
acetic acid and p-MAP formed through the catalyst pores by the
excess anisole as anisole is a self-solvent [63].
◦
7
0 wt% HPW loaded MCM-41 at 120 C for 1 h, and the results are
shown in Fig. 12. The conversion of acetic anhydride increases with
increasing the concentration of anisole up to four times that of
acetic anhydride and thereafter remains constant. Increasing con-
version of acetic anhydride is attributed to the facile desorption of
The effect of reaction time on the % conversion of acetic anhy-
dride over 0.1 g of 40, 60 and 70 wt% HPW loaded MCM-41 keeping
anisole: acetic anhydride molar ratio at 4:1 at 120 C is shown
◦
in Fig. 13. It was found that the percentage of conversion of

A.E.R.S. Khder et al. / Applied Catalysis A: General 411–412 (2012) 77–86
85
O
O
C
C H 3
C H 3
C at-H+
+
C H 3CO OH
+
+ C
C H3
O
C
O
-
H O
2
C H3
C H3
+O
C H3
O
O
C
O
O
+
C H3
H
C
O
+
C
CH 3
CH 3
Minor product
C H3
O:
C H3
O
Lewisacidsite
∂
+
O
+
+
C
C H3
C
C H3
O
Main product
Scheme 1. Proposed reaction mechanism for the acylation of anisole with acetic anhydride in presence of Brønsted and Lewis acid sites.
acetic anhydride becomes maximum after 1 h and then remains
almost constant with further increase of reaction time more
than 3 h.
4. Reaction mechanism
The acylation reaction is an electrophilic substitution reaction
where Brønsted acid sites are responsible for the generation of an
R–CO+ ion from the actyling agent. Thus the catalytic activity is
expected to depend on the number and strength of the Brønsted
acidity sites. The proposed mechanism (Scheme 1) for the acy-
lation reaction implies the formation of an adsorbed acylium ion
adsorbed on the active site by interaction of the acetic anhydride
with a Brønsted acid site [64,65]. Then, the reaction between the
adsorbed acylium ion and anisole in the liquid phase occurs to give
the final products. Generally the acylation of anisole with acetic
anhydride gives p-MAP with a high selectivity and o-MAP with a
low selectivity. The formation of p-MAP with high selectivity could
be due to steric hindrance which makes p-MAP more favorable than
o-MAP. On the other hand, the Lewis acid sites may also play an
Recycling of the catalyst is an important aspect of any indus-
trial process. For this purpose, reusability of the catalyst is tested
◦
by carrying out repeated runs of the reaction at 120 C, keep-
ing the reactants molar ratio (anisole/acetic anhydride) at 4:1. It
was observed that no significant loss in the activity of the 40 wt%
HPW/MCM-41 with increasing the number of cycles of the reaction.
This could probably be due to irreversible adsorption of methylace-
tophenones on the catalyst active sites or loss of catalyst due to
filtration and washing processes (Fig. 14).
1
20
00
% Conversion of acetic anhydride
%
%
Selectivity to p-MAP %
Selectivity to o-MAP %
important role as shown in Scheme 1. The Brønsted acid sites are
1
+
responsible for the CH CO formation while the Lewis sites favor
3
the formation of the para substituted product [66]. The stronger the
8
6
4
2
0
0
0
0
0
Lewis acid site the more electron deficient the ortho position and
+
this could restrict the attack of the electrophile (CH CO ) mostly to
3
the para position thus leading to higher para selectivity.
5. Conclusions
The results of this study show that HPW loading has a notable
effect on the acidity and catalytic activity of MCM-41. The meso-
porous structure of MCM-41 is not affected much with the HPW
loading up to 70 wt%. Higher loadings result in structural distor-
tion of MCM-41 with considerable loss of the long range order.
Both the surface acidity and the ratio of Brønsted to Lewis acidic
sites of the HPW catalyst attain the maximum value at 60 wt%
HPW loading. The catalytic activity of these catalysts in Pechmann,
Run (1) Run (2) Run (3) Run (4) Run (5)
Number of runs
Fig. 14. Variation of conversion of acetic anhydride (%) or selectivity to p-MAP and
o-MAP (%) with reusability, studied over 40 wt% HPW/MCM-41 catalyst (reaction
condition: anisole/acetic anhydride, 4:1; reaction time, 1 h; temperature, 120 C;
◦
catalyst weight, 0.1 g).

8
6
A.E.R.S. Khder et al. / Applied Catalysis A: General 411–412 (2012) 77–86
esterification and Friedel–Crafts acylation reactions is significantly
high thus suggesting the HPW/MCM-41 materials are potentially
promising catalysts for acid-catalyzed organic transformations in
environmentally friendly processes to supersede the use of con-
ventional homogeneous HPW catalysts.
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