Regioselective acylation of 2-methoxy naphthalene catalyzed by supported 12-Phosphotungstic acid

Article abstract of DOI:10.1016/j.apcata.2014.08.026

12-Phosphotungstic acid supported on silica gel, zirconium sulfate, and a combination of silica gel and zirconium sulfate (50% w/w) were employed as solid acid catalysts for regioselective acylation of 2-methoxynaphtalene with acetic anhydride. 1-(6-Methoxynaphthalen-2-yl)ethanone (2,6-AMN), a commercially important intermediate for production of Naproxen, was obtained with excellent selectivity (>98%) at 67-68% conversion using 12-phosphotungstic acid supported on silica gel 20% (w/w) in refluxing tetrachloroethane. The unreacted starting material can be easily separated from the product by a simple crystallization from nonane.

Full text of DOI:10.1016/j.apcata.2014.08.026

Applied Catalysis A: General 486 (2014) 55–61
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Regioselective acylation of 2-methoxy naphthalene catalyzed by
supported 12-phosphotungstic acid
d
a
b
Thenkrishnan Kumaraguru , Avala Vedamayee Devi , Vidavalur Siddaiah ,
c
d,∗
Kishor Rajdeo , Nitin W. Fadnavis
Department of Chemistry, University of Minnesota, Minneapolis, USA
Department of Organic Chemistry, School of Chemistry, Andhra University, Visakhapatnam 530 003, India
Polymer Science & Engineering, Chemical Engineering Division, National Chemical Laboratory, Pashan Road, Pune 411008, India
Natural Products Chemistry Division, Discovery Block, Indian Institute of Chemical Technology, Uppal Road, Hyderabad 500007, India
a
b
c
d
a r t i c l e i n f o
a b s t r a c t
Article history:
12-Phosphotungstic acid supported on silica gel, zirconium sulfate, and a combination of silica gel
and zirconium sulfate (50% w/w) were employed as solid acid catalysts for regioselective acylation
of 2-methoxynaphtalene with acetic anhydride. 1-(6-Methoxynaphthalen-2-yl)ethanone (2,6-AMN), a
commercially important intermediate for production of Naproxen, was obtained with excellent selec-
tivity (>98%) at 67–68% conversion using 12-phosphotungstic acid supported on silica gel 20% (w/w) in
refluxing tetrachloroethane. The unreacted starting material can be easily separated from the product by
a simple crystallization from nonane.
Received 18 February 2014
Received in revised form 21 August 2014
Accepted 23 August 2014
Available online 29 August 2014
Keywords:
Naproxen
Acylation
©
2014 Elsevier B.V. All rights reserved.
Phosphotungstic acid
Supported
Silica gel
Zirconium sulfate
1
. Introduction
-(6-Methoxy-naphthalen-2-yl) propanoic acid (1) (Naproxen)
example, benzoylation of veratrole with benzoic anhydride to 3,4-
dimethoxy benzophenone using zirconia supported silicotungstic
acid has been described by Devassi and Halligudi [19]. Khder and
co-workers have used phosphotungstic acid incorporated within
mesoporous MCM-41 for Friedel–Crafts acylation of anisole with
acetic anhydride [20]. Recently, Chen and co-workers have reported
one pot synthesis of 1-acetylpyrene over phosphotungstic acid cat-
alyst supported on oxides such as SiO , TiO , K-10 montmorillonite
2
is a widely used non-steroidal anti-inflammatory agent [1]. The
industrial scale synthesis of naproxen involves Friedel-Crafts acyla-
tion of 2-methoxynaphthalene (2-MN) (2) with acetic anhydride in
presence of aluminium chloride to get 1-(6-methoxynaphthalen-2-
yl)ethanone (2,6-AMN) (3) in the first major step. This step results
in a substantial amount of waste and release of HCl which causes
corrosion problems [2]. To replace aluminium chloride that acts
as a Lewis acid, considerable efforts have been made to explore
use of solid acid catalysts such as zeolites, clays, Amberlyst-15,
and MCM-41 type silica material [3]. Unfortunately, most of the
catalysts yield unwanted 1-(6-methoxynaphthalen-1-yl)ethanone
2
2
and ␥-Al O3 with 100% selectivity for 1-acetylpyrene and 91.8%
2
yield on SiO -supported phosphotungstic acid [21]. These reports
2
encouraged us to examine acylation of 2-methoxy naphthalene
with acetic anhydride catalyzed by supported phosphotungstic
acid.
(
1,6-AMN) (4) as a major product with exception of Zeolite Beta
which shows higher selectivity towards the required 2,6-isomer
2. Materials and methods
(
55%) [4] (Scheme 1).
Several studies have reported application of heteropoly acids
HPAs) for aromatic acylation via Friedel–Crafts reaction [5–21]. For
2.1. General
(
IR spectra were recorded on a Perkin–Elmer RX-1 FT-IR system.
HPLC analysis was carried out on Varian Pro Star HPLC unit. Surface
area analysis was carried out Quantachrome Instrument, Model-
Nova 2000e, surface area and pore size analyzer, with Autosorb
software, USA. X-ray diffraction (XRD) patterns were obtained on
∗ Corresponding author. Tel.: +91 40 27191631; fax: +91 40 27160512.
E-mail addresses: fadnavis@iict.res.in, fadnavisnw@yahoo.com (N.W. Fadnavis).
http://dx.doi.org/10.1016/j.apcata.2014.08.026
926-860X/© 2014 Elsevier B.V. All rights reserved.
0

5
6
T. Kumaraguru et al. / Applied Catalysis A: General 486 (2014) 55–61
a function of relative pressure. Total surface area was calculated by
multipoint BET method.
2.4. Acylation reaction
2
-Methoxy naphthalene 2 (1 g, 0.01 mol) and acetic anhydride
(0.66 mL, 0.015 mol) were dissolved in the solvent (10 mL), the cata-
lyst was added and the contents were refluxed for 3 h. After cooling
to room temperature, the catalyst was filtered off and washed with
the solvent. The solvent from combined washings was removed
under reduced pressure and the crude product was analyzed by
HPLC. The products were identified from NMR and their reten-
tion times in HPLC analysis. Conversion and product selectivity was
determined from their respective concentrations in the crude mix-
ture based on their peak areas and corresponding response factors.
2.5. HPLC analysis
Formation of 3 and 4 by acylation of 2 was monitored
by reverse phase HPLC. Column C-8 (250 × 5 mm), Chrompack,
The Netherlands. Mobile phase, 50% CH CN–water. Flow rate,
3
0
.7 mL/min. Detection wavelength, 254 nm. Retention times: 1-(6-
methoxynaphthalen-2-yl)ethanone (3): 16.79 min; 1-(6-Methoxy-
-naphthyl)-1-ethanone (4): 17.87 min; 2-Methoxy naphthalene
2): 24.21 min.
Scheme 1. Acylation of 2-methoxy naphthalene.
2
(
Seimens D 5000 (Cheshire, UK) X-ray diffractometer over a 2ꢀ range
◦
of 2–65 using CuK radiation (0.15406 nm). X-ray photoemission
␣1
spectroscopy (XPS) analyses of catalysts were performed on a VG
Scientific ESCA-3000 spectrometer using a non-monochromatized
Mg K␣ radiation (1253.6 eV). Binding energies (BE) were referenced
with the C1S line corresponding at 284.6 eV with an accuracy of
2
.6. Separation of 2,6-AMN and 2-MN
The combined product of 5 runs (5 g) consisting of 60:40 mixture
of 2,6-AMN and 2-MN was dissolved in hot nonane (50 mL), heated
to 80 C to obtain a clear solution and slowly cooled to 20 C. 2,6-
AMN crystallized out as white crystals. These were filtered, washed
∼
0.1 eV for the XPS analysis. Raman spectra were recorded on a
Horiba Jobin-Yvon LabRam HR 800 UV Raman spectrometer with
a 35 mW internal He-Ne laser source of excitation wavelength
◦
◦
◦
once with nonane (5 mL) and dried under vacuum at 50 C. Yield
5
1
14 nm. The TEM images were obtained on a Tecnai (model-F20)
20 kV field emission gun with tungsten filament as an electron
(
2.9 g, 96.6%) with 99% purity. The filtrate was concentrated under
reduced pressure to obtain unreacted 2-MN (1.95 g, 97.5%, purity
5% (HPLC).
generator source. The samples for TEM were prepared by dispers-
ing the synthesized catalysts in isopropanol and drop casted over
9
2
00 mesh size carbon-coated copper TEM grids.
-Methoxy naphthalene (2) and authentic samples
of 1-(6-methoxynaphthalen-2-yl)ethanone (3) and 1-(6-
methoxynaphthalen-1-yl)ethanone (4) were generous gift
2
3. Results and discussion
a
3.1. Catalyst characterization
from M/s Godavari Drugs, Hyderabad. 12-Phosphotungstic acid
hydrate (H PW) Cat. No. 40116 and zirconium IV sulphate
3.1.1. FT-IR spectra
3
tetrahydrate Cat. No. 41041 (ZS) were obtained from Alfa Aesar,
Hyderabad, India. Silica gel for column chromatography 60–120
mesh (SG) was obtained from ACME Synthetic Chemicals, Mumbai,
India. All other reagents and solvents were of analytical grade
obtained from Qualigens, India.
In FT-IR spectra, unsupported catalyst shows typical bands
−
1
−1
for absorptions of P−O (1080 cm ), W Ot (983 cm ), W−Oc−W
−
1
−1
(898 cm ) and W−Oe−W (797 cm ). These bands are preserved
on the supported samples, but they are broadened and partly
obscured because of the strong absorption of supports. These obser-
vations are similar to those reported by Dias and co-workers [11].
2.2. Preparation of supported catalysts
3
.1.2. XRD analysis
XRD patterns were obtained to confirm that H PW was effec-
Supported H PW catalysts were prepared by stirring the sup-
3
3
port (10 g) with an aqueous solution of H PW (100 mL). The mixture
tively supported on SG and ZS-SG. Unsupported H PW exhibits
sharp and narrow diffraction lines (Fig. 1b). In case of silica gel sup-
ported catalyst, the diffraction lines are not very prominent due to
strong binding of the catalyst. In case of H PW supported on ZS-SG
prepared by heating at 140 C, the diffraction lines can be observed,
3
3
was stirred for 6 h at room temperature followed by drying on
a rotary evaporator. Finally, the catalyst was dried by heating at
◦
1
40 C under vacuum (2 mmHg pressure) for 3 h.
3
◦
although with less intensities (Fig. 1c). These lines are also seen in
case of supported ZS-SG catalyst recovered after the acylation reac-
2.3. Surface area measurement
tion (Fig. 1d), and washing with CH Cl (Fig. 1e). The pattern is still
Surface areas were measured using multi point BET nitrogen
adsorption method. Weighed amounts of samples were placed in
2
2
◦
visible in case of catalyst calcined at 300 C (Fig. 2a) for removal
◦
of soft coke, and is completely lost when it is calcined at 400 C in
glass cells and degassed under reduced pressure with nitrogen at
◦
our attempts to remove the hard coke (Fig. 2b). These observations
6
0 C for 3 h. After degassing, the samples cells were immersed
◦
indicate that H PW has a strong interaction and good dispersion on
in liquid nitrogen at −196 C, and total surface area was obtained
3
◦
3
the support which is destroyed if heated beyond 370 C.
from the volume of nitrogen (cm /g) adsorbed onto the surface as

T. Kumaraguru et al. / Applied Catalysis A: General 486 (2014) 55–61
57
◦
Fig. 2. XRD patterns of 20% (w/w) H3PW on ZS-SG. (a) Calcination at 300 C, (b)
◦
calcination at 400 C.
3
.1.4. Surface area measurement
Fig. 1. XRD patterns of (a) Zs-SG support, (b) unsupported H3PW, (c) 20% (w/w)
H3PW/Zs-SG, (d) catalyst recovered after filtration, (e) recovered catalyst after wash-
ing with dichloromethane.
The crystalline materials, zirconium sulphate and H3PW have
2
very low surface areas (≈2 m /g). Silica gel support has a large
2
2
surface area (411 m /g) which decreases to 279 m /g after loading
of H PW due coverage of the surface by the catalyst. In a similar
3
.1.3. Surface acidity
Changes in surface acidity of the catalysts were followed by
3
fashion, deposition of zirconium sulphate on silica gel reduces the
2
surface area of ZS-SG catalyst to 120 m /g. Loading of H PW on this
temperature programmed desorption (TPD) of ammonia (Table 2).
Since the Keggin structure of H PW is lost above 400 C [10], we
have studied the changes below 400 C where weak and moder-
ate acidities can be measured. The catalyst with 20% loading of
3
2
◦
catalyst causes a further decrease to 94 m /g (Table 2). However,
3
◦
the surface areas do not change significantly after acylation reac-
tion and washing with dichloromethane indicating that H PW is
3
retained in these supports.
H PW on silica has acidity of 14.5 mL/g, mainly due to weak acidic
3
sites. Combining ZS with SG for supporting H PW increases the
3
medium acidic sites of the samples and total acidity of the sample
also increases (13.1 mL/g). A minor decrease in total acidity was
observed for the spent samples most probably due to deposition
non-removable hard coke.
3.1.5. Transmission electron microscopy
The surface morphology of the catalyst H PW supported on ZS-
SG was probed by transmission electron microscopy (TEM). The
unsupported catalyst (Fig. 3a) appears to be polydispersed with
3
Fig. 3. Representative TEM images of the catalysts. (a) Unsupported H3PW; (b) H3PW (20% w/w) supported on ZS-SG after reaction; (c) H3PW (20% w/w) supported on ZS-SG
after reaction and DCM wash.
Table 1
a
Phosphotungstic acid catalyzed acylation of 2-methoxynaphthalene with acetic anhydride in different solvents .
ꢀ
Entry
Solvent
Temperature ( C)
Conversion
2,6-isomer (3) (%)
1,6-isomer (4) (%)
1.
2.
3.
4.
5.
6.
Acetonitrile
60
60
84
140
140
140
84
55
62
78
15
7
20
22
38
69
48
42
56
8
Dichloromethane
Dichloroethane
Tetrachloroethane
Nitrobenzeneb
90
N-Methylpyrrolidinone
No reaction
a
2
-Methoxynaphthalene (6.3 mmol), acetic anhydride 0.71 g, 7 mmol, catalyst 182 mg (1 mol%), 10 mL solvent, reaction period 3 h.
b
Reaction accompanied by tar formation.

5
8
T. Kumaraguru et al. / Applied Catalysis A: General 486 (2014) 55–61
Table 2
a
Acylation of 2-methoxynaphthalene with various catalysts .
2
Support
H3PW
SG
H3PW loading (%, w/w)
Surface area (m /g)
Total acidity (mL/g)
Conversion (%)
78
2,6-AMN Selectivity (%)
28
100
2.12
0
411
0
0
47
68
46
0
b
10
20
30
nd
nd
14.5
nd
85.6
98.3
88.6
279
nd
ZS
0
0
0
0
2.60
nd
1.06
nd
nd
nd
25.5
nd
0
12
47
7
0
1
2
3
91.1
97.5
84.3
ZS-SG (1:1 w/w)
0
0
0
0
120
nd
94
nd
nd
13.1
nd
0
56
67
39
0
1
2
3
97.5
98.3
97.0
nd
a
◦
2
-Methoxynaphthalene (1 g, 6.3 mmol), acetic anhydride (0.71 g, 7 mmol), catalyst (1 g), solvent (10 mL), reaction period 3 h at 140 C in tetrachloroethane.
b
nd: Not determined.
10000
1400
1
1
200
000
8
6
4
2
000
000
000
000
0
(
a)
8
6
4
2
00
00
00
00
0
(c)
(b)
8
00
850
900
950
1000
1050
1100
-1
800
850
900
950
1000
1050
1100
Wavelength (cm )
-
1
Wavelength (cm )
Fig. 4. Raman spectrum of unsupported H3PW.
Fig. 5. Raman spectra of 20% (w/w) H3PW supported on ZS-SG (a) before reaction;
b) after reaction; and (c) after DCM wash.
(
rounded shape. The morphology of supported catalyst particles
shows that the catalyst particles are uniformly distributed in the ZS-
SG matrix. The morphology does not change significantly after the
reaction (Fig. 3b), and also after washing with dicholoromethane
binding energy peak of 4f7/2 electron of W appears at 38.2 eV which
is slightly deviated from that of the catalysts peak (Fig. 6 b). From
the XPS data it is confirmed that catalysts contains the W atoms
and there is no change in its electronic state as expected for a Lewis
acid catalyzed acylation reaction which does not involve electron
transfer.
(Fig. 3c).
3.2. Raman spectra
−
1
Pure H PW shows a sharp band at 1006 cm with shoulders
at 992 and 983 cm
3
−1
−1
(Fig. 4). The bands at 1006 and 992 cm
3.3. Solvent effects on acylation reaction
can be attributed to ꢁ(W O) symmetric and asymmetric stretch-
ing modes while the band observed at 983 cm can be assigned to
the ꢁ(W–O–W) asymmetric stretching mode. After being supported
−
1
Initially, the acylation of 2-methoxy naphthalene with acetic
anhydride using unsupported phosphotungstic acid as catalyst
(surface area 2.12 m /g) was carried out in different solvents at their
−
1
2
on ZS-SG, the position of ꢁ(W O) band at 1006 cm is shifted to
−
1
1
026 cm
The bands at 833 cm
(W–O–Zr) vibration and ꢁ(W–O–W) stretching mode, respectively
Fig. 5a) [19,22–24]. Interestingly, the recovered supported catalyst
indicating an increased interaction with the support.
reflux temperatures (Table 1). Tetrachloroethane was found to be
the most appropriate solvent. It is a neutral solvent having good
dielectric constant (7.096) and is reported to be capable of show-
ing specific interactions with aromatic hydrocarbons [25]. Also, the
−1
−1
and 920 cm are most probably due to
ꢁ
(
◦
shows a small decrease in the intensity of the peaks (Fig. 5b), which
is partly recovered after washing with dichloromethane (Fig. 5c).
boiling point (140 C) is high enough to carry out the reaction at a
fair rate and solvent recovery is highly efficient (>97%). Our find-
ings are in agreement with Chen and co-workers [21] and further
studies were made using tetrachloroethane as the solvent.
3.2.1. X-ray photoemission spectroscopy (XPS)
In Fig. 6a, XPS spectrum of 20% (w/w) H PW on SG clearly
3
shows peak of W 4f7/2 appearing at 39.8 eV. Normally, this peak
in XPS appears around 36–36.3 eV [24]. If there are more elec-
tronegative atoms in the vicinity of tungsten, the binding energy
values shift towards the high end. After the acylation reaction, the
3.4. Effect of temperature
The acylation reaction catalyzed by H PW was studied at tem-
peratures ranging from 30 C to 140 C. Surprisingly, acylation
3
◦
◦

T. Kumaraguru et al. / Applied Catalysis A: General 486 (2014) 55–61
59
Fig. 6. (a) XPS of 20% (w/w) H3PW supported on SG (before reaction). (b) XPS of 20% (w/w) H3PW supported on SG (after reaction).
reaction took place even at room temperature with 70% conver-
sion in 3 h. By increasing temperature to 140 C, the conversion
carried out using basic and neutral alumina, pure zirconium sulfate
◦
and silica gel without H PW (Table 2). Since it is well established
3
increased only to 80% but selectivity towards formation of 2,6-
AMN increased dramatically (Fig. 7). This increased selectivity with
increasing temperature is most probably due to comparative insta-
bility of the 1,6-isomer which deacylates to 2-methoxynaphthalene
at higher temperatures (Scheme 1) while the 2,6-isomer remains
unaffected [12].
that H PW loses its Keggin structure at high temperatures [10,14],
all catalysts were prepared by heating at 140 C under vacuum.
The results in Table 2 show that supporting the heteropoly acid
on another support bearing Lewis acid sites does result in steer-
ing the selectivity towards the required 2,6-AMN. It was confirmed
3
◦
from control experiments that the supports without H PW (neutral
3
and basic alumina, silica gel and zirconium sulfate) did not catalyze
the acylation reaction.
3
.5. Acylation with supported H PW
3
Kukovecz and co-workers have demonstrated the presence of
Lewis acid sites in silica supports [13] which are essential for
Friedel-Crafts alkylation and acylation reactions. It occurred to us
that introduction of extra Lewis acid sites might further improve
3.6. Effect of H PW loading
3
In order to optimize the catalyst loading, further studies were
carried out by varying the amount of loading of the catalyst on sup-
ports from 10 to 30% (Table 2). In all the three cases, both selectivity
and conversion increased on increasing catalyst loading from 10 to
20% (w/w) but further increase in catalyst loading to 30% caused
a drop in both selectivity and conversion. This can be understood
the selectivity in H PW catalyzed acylation of 2-MN. We have hence
3
carried out acylation reactions using phosphotungstic acid (H PW)
3
supported on various supports (10% w/w) such as zirconium sul-
fate (ZS), and silica gel (SG). A combination of 50% (w/w) zirconium
sulfate and silica gel (ZS−SG) was also tried. Control reactions were
as follows: as the concentration of H PW on support increases, the
3
synergistic action of the Lewis acidic sites of supports and H PW
produces more and more 2,6-AMN. This reaches maximum at 20%
3
1
00
9
8
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
0
0
loading. Further increase in H PW loading causes the catalyst to
3
Conversion
start resembling unsupported H PW and the selectivity starts drop-
3
ping. This is further supported by the observation that increasing
the amount of supported catalyst (20% H3PW on silica gel) in the
reaction flask from 500 mg to 1 g at fixed amounts of reactants, the
conversion remains unchanged at 68 (± 2%) while the selectivity
increases from 85% to 98% (Fig. 8).
3.7. Time course of reaction
2
,6-AMN Selectivity
The course of acylation reaction in presence of supported cat-
alysts with 20% (w/w) H3PW loading was studied to optimize
the reaction conditions further. In all the three cases, the reac-
tion reached maximum conversion in 3 h with 98% selectivity
2
0
40
60
80
100
120
140
(Fig. 9).
º
Temperature ( C)
Continuing the reaction for longer period caused the prod-
uct to decompose and produce 2-methoxynaphthalene. This was
Fig. 7. Effect of temperature on conversion and selectivity towards formation
of 2,6-AMN during acylation of 2-methoxy naphthalene catalyzed by H3PW in
tetrachloroethane. 2-Methoxynaphthalene (1 g, 6.3 mmol), acetic anhydride (0.71 g,
glaringly obvious in case of H PW-ZS catalyst where whole
3
of 2-methoxy naphthalene was recovered after an overnight
reaction.
7
mmol), catalyst (182 mg, 1 mol%), 10 mL solvent, reaction period 3 h.

6
0
T. Kumaraguru et al. / Applied Catalysis A: General 486 (2014) 55–61
1
00
70
60
50
40
30
20
10
0
9
9
8
8
7
7
6
6
5
0
5
0
5
0
5
0
20%-Si
2
,6-AMN Selectivity
2
0%-ZS-Si
Conversion
900
1.0
1.5
2.0
2.5
3.0
5
00
600
700
800
1000
Cycle No.
Wt of catalyst (mg)
Fig. 10. Reusability of the catalyst.
Fig. 8. Effect of catalyst amount on acylation. 2-Methoxynaphthalene (1 g,
◦
6
.3 mmol), acetic anhydride (0.71 g, 7 mmol), solvent (10 mL). Temp. 140 C. Reac-
◦
70 C [10] and this is probably the reason for loss of catalytic
activity. XRD data support this possibility. Fig. 2a shows the pres-
3
tion period 3 h.
ence of characteristic peaks in XRD patterns of H PW supported
3
◦
on ZS-SG calcined at 300 C which are lost when the catalyst is
calcined at 400 C (Fig. 2b). Coking and regeneration of the H PW
3.8. Reusability of the catalyst
◦
3
supported on Si has been well studied by Kozhevnikov et al. [14]
and we are currently exploring the different possibilities of catalyst
regeneration for the acylation reaction.
Reusability of the catalyst was studied for both SG and ZS-SG
supports. Reaction was carried out under standardized conditions
(
2-MN, 1 g; Ac O, 0.71 g; catalyst, 1 g; tetrachloroethane, 10 mL,
2
◦
temp. 140 C; reaction period 3 h). After the reaction, the prod-
uct was filtered, the catalyst was washed with dichloromethane
to remove soft coke [10,14] and reused. Silica gel supported cata-
lyst could be used twice with full activity which decreases by 30%
in the third cycle. In comparison, the ZS-Silica supported catalyst
loses 70% of its activity after a single use (Fig. 10).
4. Conclusion
Selectivity in acylation of 2-methoxy naphthalene catalyzed
by heteropoly acid, 12-phosphotungstic acid, was studied. The
catalyst was supported on silica gel, zirconium sulfate and a com-
bination of silica gel and zirconium sulfate. Effects of catalyst
loading, temperature, solvent and reaction period on selectivity
towards formation of commercially important intermediate 2,6-
AMN was studied in detail. The best catalyst was found to be 20%
(w/w) 12-phosphotungstic acid supported on silica gel. Under opti-
mum conditions, the catalyst shows excellent selectivity (>98%) at
67–68% conversion. The unreacted starting material can be easily
separated from product by a simple crystallization from nonane
and reused.
Observed deactivation of the catalyst is most probably due to
formation of coke. The extraction with CH Cl removes soft coke
2
2
but not the hard coke. This was also evident from the increasing
dark brown colour of the catalyst after each recycle. We have tried
to remove the hard coke by aerobic treatment at high temperatures
◦
(
500–550 C) [10]. However, such a treatment did not lead to recov-
ery of catalytic activity. Kozhevnikov et al. have reported that H PW
3
decomposes and loses its Keggin structure when heated beyond
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
Acknowledgement
2
0% H PW-(ZS-SG)
3
We thank CSIR New Delhi for financial support under XII 5
year plan CSC0108-ORIGIN, and Prof. Dr V Vishwanathan, Sreyas
Institute of Engineering and Technology, Hyderabad for helpful
discussions.
2
0% H PW-SG
3
Appendix A. Supplementary data
Supplementary data associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/j.apcata.
2014.08.026.
2
0% H PW-ZS
3
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