Acylation of anisole over 12-heteropolyacid of tungsten and molybdenum promoted zirconia

Article abstract of DOI:10.1016/j.molcata.2008.09.018

The liquid-phase acylation of anisole using acetic anhydride was performed over 12 heteropoly acid of tungsten and molybdenum (PWM) modified zirconia (ZPWM). Catalysts with different PWM loadings (3-20 wt.%) were prepared by the incipient wetness impregna

Full text of DOI:10.1016/j.molcata.2008.09.018

Journal of Molecular Catalysis A: Chemical 297 (2009) 93–100
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Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Acylation of anisole over 12-heteropolyacid of tungsten and molybdenum
promoted zirconia
K.M. Parida∗, Sujata Mallick, G.C. Pradhan
Colloids and Materials Chemistry Department, Institute of Minerals and Materials Technology, Bhubaneswar 751013, Orissa, India
a r t i c l e i n f o
a b s t r a c t
Article history:
The liquid-phase acylation of anisole using acetic anhydride was performed over 12 heteropoly acid of
tungsten and molybdenum (PWM) modified zirconia (ZPWM). Catalysts with different PWM loadings
(3–20 wt.%) were prepared by the incipient wetness impregnation method using mesoporous zirconia
and 12-heteropolyacid of W and Mo. The samples calcined at 500 ◦C were characterized by chemical
analysis, nitrogen adsorption–desorption, surface acid sites, powder X-ray diffraction patterns, Fourier
transform infrared spectroscopy, UV–vis diffuse reflectance spectroscopy, 31 P nuclear magnetic resonance
spectroscopy and electron probe microanalysis The sample promoted by 15 wt.% PWM was found to be
the most active in the acylation reaction giving 89% conversion with 97% para-selectivity. The effect of
temperature, molar ratio and catalyst amount on the conversion of acetic anhydride and anisole were
studied in detail to optimize the process parameters. Reusability of the catalyst was also appraised.
© 2008 Elsevier B.V. All rights reserved.
Received 13 February 2008
Received in revised form
11 September 2008
Accepted 11 September 2008
Available online 25 September 2008
Keywords:
Zirconia
12-Heteropoly acids of Mo and W
Acylation
Anisole
1. Introduction
In the present work, we have studied acylation of anisole using
acetic anhydride over 12-heteropoly acid of molybdenum and
Acylation reactions are largely employed in fine chemical indus-
try to produce a variety of synthetic fragrances and pharmaceuticals
[1–3]. The acylation of aromatics has been generally carried out
with homogeneous Lewis acid-type catalysts (anhydrous metal
halides such as AlCl3 and FeCl3) and acyl halides as acylating agents
[4–6]. The overall process produces a significant amount of unde-
sirable products and destroys the catalyst. The use of solid acids
such as zeolites, sulfated zirconia, ZSM-5, Ga-doped SBA-15, meso-
porous aluminosilicates synthesized from zeolite seeds etc. [7–11]
represents an attractive alternative route with economic and envi-
ronmental advantages. Among these catalysts, zeolites with shape
selectivity are the most widely studied due to their strong and
well-documented acid properties.
tungsten modified zirconia. The characterizations of the modified
catalysts have also been co-related with the catalytic activity.
2. Materials and methods
2.1. Preparation of zirconia
Zirconium hydroxide gel was prepared from aqueous solution
of zirconium oxychloride (Fine Chemical) by dropwise addition
of ammonium hydroxide solution (25% ammonia) (Merck) till it
attained pH 9.5. The hydrogel was refluxed for 24 h, filtered and
washed with deionized water and dried in an oven at 120 ◦C for
24 h. The finely grounded sample was calcined at 500 ◦C for 5 h
(named Z hereafter).
The Keggin-type heteropoly acids typically represented by the
formula H8−X [XM12O40] where X is the hetero atom, x is its oxida-
tion state, and M is the addenda atom (usually Mo6+ or W6+), are the
2.2. Preparation of 12-heteropoly acids of tungsten and
molybdenum
most important solid acid catalysis [12]. The HPAs like H3PW12O40
,
H3PMo12O40, H4SiW12O40 and H4SiMo12O40 have been widely used
as acid and oxidation catalysts in the neat or supported form for
organic synthesis [13,14]. We have also published a number of
papers using these materials [15,16]. However, only a few studies on
the use of 12-heteropoly acids of W and Mo have been reported [17].
Na2WO4·2H2O
(45.1 g),
Na2MoO4·2H2O
(33 g)
and
Na2HPO4·2H2O (8.15 g) were dissolved in 200 ml of deionized
water. The solution was kept at 80 ◦C for 3 h with agitation and
then concentrated to 80 ml by the use of an evaporator. Then 100 ml
of 24% HCl was added to it. The yellow crystals were obtained by
extraction with ether from a 50% aqueous solution. The contents
of P, Mo and W in PWM were analyzed by inductively coupled
plasma (ICP) and found to be in the ratio 1:6:6.
∗
Corresponding author. Tel.: +91 674 2581636x425; fax: +91 674 2581637.
E-mail address: kmparida@yahoo.com (K.M. Parida).
1381-1169/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2008.09.018

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K.M. Parida et al. / Journal of Molecular Catalysis A: Chemical 297 (2009) 93–100
Scheme 1. Schematic representation acylation of anisole.
2.3. Preparation of catalyst
and selectivity. To confirm the active species as solid acids, the reac-
tion filtrate was tested for the presence of metal ions. Standards
containing 1–6 wt.% PMA in water were prepared. To 10 ml of the
solution 1 ml of 10% ascorbic acid was added. Then it was diluted
to 25 ml. The resultant blue coloured solution was scanned at ꢀmax
784 cm−1. A standard calibration curve was obtained by plotting
values of absorbance with percentage solution. About 1 g of 12 wt.%
ZPWM with 25 ml n-butanol was refluxed for 4 h. Then 10 ml of
the superintendent solution was treated with 10% ascorbic acid.
The same procedure was repeated with the filtrate of the reaction
mixture after completion of the reaction.
A series of catalysts with 3–20 wt.% loading was synthe-
sized by impregnating 2 g of Z with an aqueous solution of
12-heteropoly acid of tungsten and molybdenum (PWM) (0.06 g
to 0.30 g/10–50 ml of conductivity water) under constant stirring
followed by heating till complete evaporation of water takes place
(4 h). Then it was dried in an oven at 110 ◦C for 24 h. Catalysts
with different PWM loading from 3 to 20 wt.% were then calcined
at 500 ◦C. The catalysts will be hereafter referred to as xZPWM
(x = 3–20 wt.%).
2.4. Physico-chemical characterization
3. Results and discussion
The BET surface area and pore size distribution were deter-
mined by multipoint N2 adsorption–desorption method at liquid
N
3.1. Characterization
2 temperature (−196 ◦C) by a Sorptomatic 1990 instrument (Ther-
Table 1 shows the BET surface area of zirconia and ZPWM with
different weight percentages. Surface area of zirconia is found to be
142 m2/g. With the increase in PWM loading from 0 to 15 wt.%, the
surface area increases from 142 to 291 m2/g. However with further
increase in PWM beyond 15 wt.%, the surface area decreases. This
implies that the presence of PWM plays a role in making the mate-
rial porous. The added PWM forms a surface overlayer that reduces
the surface diffusion of zirconia and inhibits sintering. It stabi-
lizes the tetragonal phase of zirconia, which leads to an increase
in surface area (cf. XRD pattern). However, when the PWM content
increases beyond 15 wt.%, pore blocking takes place due to the pres-
ence of an excess amount of 12-heteropoly acids of molybdenum
and tungsten.
moQuest, Italy). Prior to analyses, all the samples were subjected
to vacuum at 200 ◦C to ensure a clean surface.
Surface acidity was determined spectrophotometrically on the
basis of irreversible adsorption of organic bases such as pyridine
(PY, pKb = 3.5) and 2, 6-dimethyl pyridine (DYPY, pKb = 8.7) [18].
The X-ray powdered diffraction pattern was recorded on a
Philips PW 1710 diffractometer with automatic control. The pat-
terns were run with a monochromatic CuK␣ radiation at a scan rate
of 2◦ min−1
The FTIR spectra were taken using a Jasco FTIR 5300 spectrom-
eter in KBr matrix in the range of 400–4000 cm−1
.
.
The UV–vis spectra of the samples were recorded in a Varian
UV–vis-DRS spectrophotometer fitted with Carry 100 software. The
spectra were recorded against the boric acid background.
The 31P NMR spectra were recorded on a Varian Mercury
300 MHz NMR spectrometer, 31P chemical shifts are referenced to
85% H3PO4 as an external standard, and the time of scan was 380.
Micrographs showing X-ray image mapping of different ele-
ments of 12-heteropoly acid of tungsten and molybdenum
supported zirconia were taken using a Japanese Model (JXA-8100)
EPMA.
Table 1
Surface area of various catalysts.
Catalysts
Surface area (m2 g−1
)
Z
142
156
176
189
211
254
239
3 ZPWM
6 ZPWM
9 ZPWM
12 ZPWM
15 ZPWM
20 ZPWM
2.5. Catalytic activity towards acylation of anisole
The acylation reaction was carried out in liquid phase in a 50 ml
three-necked round bottom flask fitted with a thermometer, reflux
condenser with CaCl2 tube and a magnetic stirrer bar. A mixture of
100 mmol of anisole and 10 mmol of acetic anhydride were added to
the flask along with 0.1 g of n-tridecane, which was used as an inter-
nal standard for GC analysis. The catalyst was added after adjusting
the temperature to 70 ◦C. The reaction mixture was separated from
the catalyst after 2 h and analyzed gas chromatographically, using
capillary column (Scheme 1).
Table 2
Acid sites of various catalysts.
Catalysts
Acid sites (␮mol/g)
PY
2,6 DMPY
3 ZPWM
6 ZPWM
9 ZPWM
12 ZPWM
15 ZPWM
20 ZPWM
121
154
167
199
243
225
43
54
72
86
106
95
Addition of p- and o-methoxyacetophenone to the reaction mix-
ture was done to know the influence of the product on conversion

K.M. Parida et al. / Journal of Molecular Catalysis A: Chemical 297 (2009) 93–100
95
Fig. 1. (a) N2 adsorption–desorption isotherm of zirconia. (b) N2 adsorption–desorption isotherms of 12-heteropoly acid of W and Mo supported on zirconia.
Table 2 shows the surface acid sites of zirconia and zirconia
impregnated PWM samples. Adsorption of pyridine (PY) mea-
sures the total acidity where as 2 6-dimethylpyridine (DMPY) gives
Brønsted acid sites. It is found that with an increase in the PWM
content from 0 to 15 wt.%, the acidity gradually increases from 156
to 310 ␮mol/g, and thereafter decreases to 276 ␮mol/g for 20 wt.%
ZPWM. The increase in surface acidity, with an increase in PWM
loading may be due to the formation of monolayer coverage of PWM
on zirconia. The decrease in surface acidity at high PWM concen-
tration is probably due to the formation of multilayer coverage of
PWM on zirconia, which decreases the number of Brønsted acid
sites and consequently that of total strong acid sites.
Fig. 2 shows the BJH pore size distribution of zirconia and 15 wt.%
ZPWM. From the figure one can see that there is no appreciable
difference in pore diameter between zirconia and 15 wt.% ZPWM.
The XRD patterns of zirconia and ZPWM samples are presented
in Fig. 3. It was observed that pure zirconia contains a mixture
of tetragonal and monoclinic phases with the latter as the major
constituents. However, XRD of samples with low PWM loadings
is similar to that of the support. As percentage loading of PWM
increases, it shows increase in the tetragonal phase while at 15 wt.%
loading ZPWM is fully tetragonal. But on further increase in PWM
loading, it shows the monoclinic phase along with the tetragonal
one.
The nitrogen adsorption–desorption isotherms of parent zirco-
nia and 15 wt.% ZPWM are shown in Fig. 1a and b. The isotherms
are of the type IV isotherm according to the IUPAC classification of
adsorption isotherms [19], which is a typical characteristic of meso-
porous solids. Inflections at P/P0 0.7–1.0 for zirconia and at 0.8–1.0
for 15 wt.% ZPWM are seen in the figure; these are ascribed to the
spontaneous filling of mesopores due to capillary condensation.
The FTIR spectra of zirconia and zirconia impregnated with 12-
heteropoly acid of tungsten and molybdenum (15 wt.% ZPWM) are
presented in Fig. 4a and b. The FTIR spectrum of zirconia shows a
broad band in the region of 3410 cm−1 due to asymmetric stretching
of OH group and two bands at 1621 and 1386 cm−1 which are due
to bending vibration of –(H-O-H)- and –(O-H-O)-bond. The band
at 503 cm−1 resulted from the existence of both tetragonal and

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K.M. Parida et al. / Journal of Molecular Catalysis A: Chemical 297 (2009) 93–100
Fig. 4. FTIR spectra of (a) Z and (b) 20 wt.% ZPWM.
M atom, c for the corner sharing oxygen atom connecting M3O13
units and e for the edge sharing oxygen connecting Ms.
Fig. 5 shows the background subtracted UV–vis-DRS spectra of
(a) zirconia, (b) 15 wt.% ZPWM calcined at 500 ◦C. The zirconia sam-
ple (Fig. 5a) exhibits a strong absorption band at 230 nm, which
may be attributed to the charge transfer from oxide species to
zirconium cation (O− → Zr4+). In contrast, the spectra of 15 wt.%
ZPWM broad showing bands at 220 nm and 265 nm match well
with the literature [20], suggesting thereby the presence of unde-
graded H3PW6M6O40 species.
Fig. 2. Distribution of pore size as a function of pore radius of (a) zirconia and (b)
20 wt.% ZPWM.
monoclinic zirconia. The spectra at 580 and 730 cm−1 are attributed
to the presence of the Zr-O bond (Fig. 5a). In addition to these bands,
the FTIR spectra of 3–20 wt.% of ZPWM samples show bands at
1072, 972, 877 and 790 cm−1 due to PWM Keggin species. The FTIR
spectrum of 15 wt.% ZPWM is shown as an example (Fig. 4b). The
characteristic bands found at 1068, 890, 792 cm−1 are assigned to
the symmetric stretching of P-O, M-Oc-Mo, and M-Oe-M respec-
tively and a weak shoulder at 962 cm−1 due to M-Ot confirmed
the presence of the undegraded 12-heteropoly acid of tungsten and
molybdenum. Here t stands for the terminal oxygen bonding to one
The 31P chemical shift (␦) provides important information con-
cerning the structure, composition and electronic states of HPA. The
crystal of H3PW6M6O40 has the Keggin structure based on a central
PO4 tetrahedron surrounded by 12MO6 octahedral M3O13 group.
The groups of M3O13 are linked by sharing corners to each other
and to the central PO4 tetrahedron. Mo and W are crystallographi-
cally disordered, each M = 1/2Mo + 1/2W, statistically occupying in
the crystal.
The 31P NMR spectrum of H3PW6M6O40 exhibits a single line at
−1.3 ppm due to the P-O bond (Fig. 6a). In the case of H3PW6M6O40
Fig. 3. XRD patterns of (a) Z, (b) 9 wt.% ZPWMA, (c) 12 wt.% ZPWM, (d) 15 wt.%
ZPWM, (e) 20 wt.% ZPWM and (f) 25 wt.% ZPWM (calcined at 500 ◦C).
Fig. 5. UV–vis spectra of (a) Z and (b) 20 wt.% ZPWM.

K.M. Parida et al. / Journal of Molecular Catalysis A: Chemical 297 (2009) 93–100
97
Fig. 6. 31 P NMR spectra of (a) H3PMo6W6O40 and (b) 20 wt.% ZPWM.
Fig. 8. Scanning electron micrograph of 20 wt.% ZPWM.
tungsten loadings, 3–20 wt.% ZPWM catalysts are used in acyla-
tion of anisole with acetic anhydride at 70 ◦C. The results show
that all the catalysts were efficient for catalyzing the reaction. The
selectivity of para- and ortho-methoxyacetophenone is shown in
Table 3. Among the catalysts with different PWM loadings, the
15 wt.% ZPWM catalyst gave the highest conversion (89%). Further,
increase in the PWM loading decreases the acetic anhydride conver-
sion. The sample containing 20 wt.% ZPWM shows 72% conversion.
The activity of the catalysts has been found to be related to the num-
ber of Brønsted acid sites. The correlation of catalytic activities with
number of acid sites and strength of Brønsted acid sites have been
reported by Gauthier et al. [21]. Since no metal ion was present in
the reaction filtrate, it is presumed that the true active species is the
solid acid, which developed due to chemical interaction between
the HPA and zirconia. This also supports that the catalyst is stable
under reaction conditions.
Anisole is an ortho/para directing substrate for electrophylic
substitution reactions, presenting a relatively high susceptibility
to such reactions by means of the release of electron from the
methoxy-oxygen atom to the aromatic ring, constituting itself a
very feasible substrate for the synthesis of substituted ketones. For
the acylation of anisole by acetic anhydride, Corma et al. [22] have
suggested that an acylium cation is formed by the protonation of
an acyl species interacting with the proton of the zeolite, and that
the acylium species attaches to the nucleophilic aromatic ring to
form the acylated product. On the contrary Bonati et al. [23] have
suggested that the acylating agent is ketene, which is formed in
situ by the decomposition of acetic anhydride into acetic acid and
ketene. The electrophilic ketene then reacts with the anisole in a
loaded zirconia catalyst the chemical shift occurred at −1.3 ppm
suggesting the presence of undegraded H3PW6M6O40
.
Electron micrographs of zirconia and 15 wt.% ZPWM samples are
presented in Figs. 7 and 8, showing the secondary electron image of
haphazardly arranged zirconia crystallites and zirconia supported
12-heteropoly acid of W and Mo.
Fig. 9 illustrates the X-ray image maps of W, Mo, P, and Zr ele-
ments in a selected area (SL) showing their level of concentration
through clustering of pixels. It is found that W and Mo are adsorbed
in Zr grain more than that of phosphorus. This is obvious since in
original PWM, the tungsten and molybdenum contents are 6 times
higher than the phosphorus one.
3.2. Acylation of anisole
The acylation of anisole with acetic anhydride gave ortho-
methoxyacetophenone and para-methoxyacetophenone as the
acylated products. The conversion is expressed as the percentage
of acetic anhydride converted into the acylated product. In order to
investigate the effect of 12-heteropoly acids of molybdenum and
Table 3
Conversion and selectivity of various catalysts towards acylation of anisole.
Catalyst
Conversion (%)
Selectivity
p-MAP
o-MAP
Z
45
57
65
74
83
89
72
79
82
85
89
92
97
93
21
18
15
11
8
3 ZPWM
6 ZPWM
9 ZPWM
12 ZPWM
15 ZPWM
20 ZPWM
3
7
Anisole: 100 mmol, acetic anhydride: 10 mmol, catalyst amount: 0.1 g, temperature:
70 ◦C, and time: 2 h.
Fig. 7. Scanning electron micrograph of zirconia.

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K.M. Parida et al. / Journal of Molecular Catalysis A: Chemical 297 (2009) 93–100
Fig. 9. Scanning electron micrograph of 20 wt.% ZPWM (A) X-ray image map of Si. (B) Secondary electron image of morphological view of 20 wt.% ZPWM, (C) X-ray image
map of Zr, (D) Morphological view of 20 wt.% ZPWM, (E) X-ray image map of Mo, and (F) X-ray image map of W.
Scheme 2. Mechanism for acylation of anisole.

K.M. Parida et al. / Journal of Molecular Catalysis A: Chemical 297 (2009) 93–100
99
Fig. 10. Effect of reaction temperature on acylation of anisole. Catalyst amount 0.1 g,
reaction temperature 70 ◦C, anisole: acetic anhydride ratio 10:1, time = 2 h.
Fig. 12. Effect of molar ratio of anisole/acetic anhydride on acylation of anisole
catalyst amount 0.1 g, reaction temperature 70 ◦C, time = 2 h.
rather similar manner to the acylium species. In this study the pos-
sible pathways for the production of methoxyacetophenones in the
Friedel–Crafts acylation of anisole with acetic anhydride catalysed
by heteropoly acids are shown below:
The interaction of anhydride molecule with Brønsted acid
site of PWM acid generates acylium ion, which subsequently
attacks the ␲-electrons of anisole to form methoxyacetophenones
(Scheme 2).
version increased to 89%. It was observed that the conversion
decreased by adding p- or o-methoxyacetophenone in the reaction
mixture. This is probably due to the inhibiting effect of methoxy-
acetophenone, which can strongly adsorb on the catalyst surface.
This inhibiting effect would be less significant for mixtures richer
in anisole, because the excess of anisole acts as a solvent for
the ketone produced, product inhibition reduced, and therefore
the acetic anhydride conversion is higher at higher anisole-to-
acetic anhydride molar ratios [25,26]. The inhibiting effect was
also confirmed by p-methoxyacetophenone in the reaction mix-
ture.
3.2.1. Influence of reaction temperature
The influence of reaction temperature on the acylation of anisole
is shown in Fig. 9. The reaction was carried out in the tempera-
ture region 50–90 ◦C over 15 wt.% ZPWM as catalyst with the other
parameters fixed. At 50 ◦C, acetic anhydride conversion is 65% and it
increases to 91% at 80 ◦C. Further increase in reaction temperature
to 90 ◦C has no appreciable effect on the acetic anhydride conver-
sion. Similar observation has also been made by Devassy et al. for
benzoylation of veratrole with benzoic acid [24].
3.2.4. Influence of time
The effect of time on conversion and selectivity for p-
methoxyacetophenone over 15 wt.% ZPTM is shown in Fig. 12. It was
found that the percentage of conversion increased from 45 to 93%
with an increase in reaction time from 0.5 to 3 h and then remains
almost constant with further rise of reaction time up to 4 h.
3.2.2. Influence of catalyst amount
The effect of the catalyst amount (0.05, 0.1, and 0.15 g) was inves-
tigated at an anisole-to-acetic anhydride mole ratio of 10:1 at 70 ◦C
(Fig. 10). The conversion of anisole increases from 61 to 91% with
an increase in the catalyst amount from 0.05 to 0.15 g.
3.3. Recyclability of the catalyst
The catalyst with 15 wt.% loading was used for recycling exper-
iments. In order to regenerate the catalyst after 2 h of reaction, it
was separated by filtration, washed several times with conductiv-
ity water and dried at 110 ◦C. The material calcined at 500 ◦C was
used in the acylation of anisole with a fresh reaction mixture. In the
regenerated sample after two cycles, the yield decreased by 3%. The
activity loss observed with the regenerated catalyst could be due to
partial loss of acid sites of the catalyst during reaction/regeneration
(Fig. 13).
3.2.3. Influence of molar ratio
The effect of molar ratio was studied at anisole-to-acetic anhy-
dride ratios of 1:1, 5:1 and 10:1 at a temperature of 70 ◦C over
15 wt.% ZPWM catalyst and the results are shown in Fig. 11. The
conversion was found to increase with an increase in the anisole-
to-acetic anhydride molar ratio. At a molar ratio of 1, the conversion
was 13% and with an increase in the molar ratio to 10, the con-
Fig. 11. Effect of catalyst amount on acylation of anisole. Reaction temperature 70 ◦C,
anisole: acetic anhydride = 10:1, time = 2 h,.
Fig. 13. Effect of reaction time on acylation of anisole. catalyst amount = 0.1 g, reac-
tion temperature 70 ◦C anisole: acetic anhydride ratio 10:1.

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K.M. Parida et al. / Journal of Molecular Catalysis A: Chemical 297 (2009) 93–100
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•
The 12 heteropoly acid of W and Mo supported on zirconia are
found to be an efficient, stable and reusable solid acid catalyst
towards anisole acylation reaction.
The nitrogen adsorption–desorption studies have revealed that
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•
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There is no appreciable change in pore diameter after impregna-
tion of heteropoly acid in zirconia.
FTIR and 31PNMR spectra confirmed that the heteropoly acids of
•
W and Mo retain their Keggin-type structure when supported on
zirconia even at 500 ◦C.
•
EPMA studies revealed that the dispersion of PWM anion into the
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•
The catalyst is good for the conversion of anisole (89%) to para-
methoxyacetophenone (97%).
This catalyst system is easy to prepare and the deactivated cata-
lyst can be reused after regeneration by calcinations.
•
•
PWN/ZrO2 is an ecofriendly catalyst for use in acylation reactions.
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The authors are thankful to Prof. B.K. Mishra, Director, Institute
of Minerals and Materials Technology (CSIR), Bhubaneswar for the
constant encouragement and permission to publish this paper. One
of the authors Mrs. Sujata Mallick is grateful to CSIR for the award
of SRF. The financial support from DST is acknowledged.
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