Heteropoly tungstate supported on tin oxide catalysts for liquid phase benzylation of anisole with benzyl alcohol

Article abstract of DOI:10.1016/j.apcata.2010.06.015

A series of 12-tungstophosphoric acid supported on tin oxide catalysts were prepared and characterized by FT-Infra red, X-ray diffraction, Laser Raman spectroscopy and temperature programmed desorption of ammonia. The characterization results suggest the presence of Keggin ions and the generation of strong acidic sites on the support. The catalysts were evaluated for benzylation of anisole with benzyl alcohol as benzylating agent. The benzylation activity depends on the content of TPA on tin oxide; the catalyst with 15 wt% loading of tungstophosphoric acid showed highest activity. The structural and surface modifications and their influence on benzylation activities were studied by treating the catalysts at different calcination temperatures. The benzylation activity varied with the change in stability of the Keggin ion structure, which depended upon treatment conditions. The alcohol conversion and selectivity were also dependent on the anisole-to-benzyl alcohol ratio, reaction temperature, reaction time and catalyst weight.

Full text of DOI:10.1016/j.apcata.2010.06.015

Applied Catalysis A: General 384 (2010) 101–106
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Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Heteropoly tungstate supported on tin oxide catalysts for liquid phase
benzylation of anisole with benzyl alcohol
∗
Ch. Ramesh Kumar, P.S. Sai Prasad, N. Lingaiah
Catalysis Laboratory, Inorganic and Physical Chemistry Division, Indian Institute of Chemical Technology, Hyderabad 500607, India
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of 12-tungstophosphoric acid supported on tin oxide catalysts were prepared and characterized
by FT-Infra red, X-ray diffraction, Laser Raman spectroscopy and temperature programmed desorption of
ammonia. The characterization results suggest the presence of Keggin ions and the generation of strong
acidic sites on the support. The catalysts were evaluated for benzylation of anisole with benzyl alcohol
as benzylating agent. The benzylation activity depends on the content of TPA on tin oxide; the cata-
lyst with 15 wt% loading of tungstophosphoric acid showed highest activity. The structural and surface
modifications and their influence on benzylation activities were studied by treating the catalysts at dif-
ferent calcination temperatures. The benzylation activity varied with the change in stability of the Keggin
ion structure, which depended upon treatment conditions. The alcohol conversion and selectivity were
also dependent on the anisole-to-benzyl alcohol ratio, reaction temperature, reaction time and catalyst
weight.
Received 9 February 2010
Received in revised form 4 June 2010
Accepted 9 June 2010
Available online 16 June 2010
Keywords:
Tungstophosphoric acid
Tin oxide
Benzylation
Anisole
Benzyl alcohol
©
2010 Elsevier B.V. All rights reserved.
1
. Introduction
Tin oxide is an important material for various technological
applications such as gas sensors and conductive coatings but it
has received little attention as a carrier for supported catalysts. In
recent times it also received considerable interest in preparation of
SnO2 based solid acid catalysts [19].
Usage of solid acid catalysts in organic synthesis has received a
lot of interest as these catalysts offer high activity and reusability.
Among the different available solid acid catalysts, heteropoly acids
have attracted considerable attention in recent times [1,2]. Conse-
quently, acid catalysis and catalytic oxidation are the two major
areas of catalysis by HPAs [3,4]. HPAs have been proved to be one
of the alternative to traditional acid catalysts due to their strong
acidity, as they possess readily available protons in their Keggin
structure.
The Friedel–Crafts reaction is widely used for the production of
alkylated aromatic compounds. Alkylated anisole compounds are
constituents of lubricants with improved properties, for example
in thermal oxidation and hydrolytic stabilities [20]. In general, acid
catalysts such as AlCl , H SO , HF or H PO have been used for
3
2
4
3
4
Friedel–Crafts reactions, in spite of their low selectivity and high
corrosiveness [21].
The major disadvantages of HPA as catalyst lie in their low ther-
2
mal stability and low surface area (1–10 m /g). The acidity of the
HPAs can be enhanced by exchanging their protons partially with
metal ions such as Cs [5] or by supporting them on acidic sup-
The universal tendency to decrease the pollutant effects of
industrial processes has stimulated research on heterogeneous
catalysis since they are easier to separate from the reaction medium
and are more reusable and less corrosive than homogeneous cata-
lysts.
Benzylation of anisole with benzyl chlorides produces HCl, an
environmentally undesirable product. Use of benzyl alcohol instead
of benzyl chloride is advantageous due to the formation of only
water as side product.
Friedel–Crafts reaction using HPAs catalysts has been attract-
ing considerable attention in recent years [22]. Shimizu et al.
reported polyvalent-metal salts of heteropolyacids as catalysts for
Friedel–Crafts alkylation of aromatics with alcohol [23]. Polymer
supported 12-tungstophophoric acid catalysts were studied for
benzylation of aromatics [24]. In most of these studies, the role
of surface and structural characteristics of HPA are not studied in
+
ports. Supporting the HPAs on acidic supports not only enhances
the acidity but also the thermal stability. Moreover, the HPAs can
be made insoluble by supporting on solid support. The support pro-
vides an opportunity for HPAs to be dispersed over a large surface
area, which results in increased catalytic activity. Various supports
like silica [6,7], titania [8,9], active carbon [10], niobia [11,12], zirco-
nia [13,14], alumina [15], MCM-41 [16,17] and acidic ion exchange
resins [18] have been used for supporting HPAs.
∗ Corresponding author. Tel.: +91 40 27193163; fax: +91 40 27160921.
E-mail address: nakkalingaiah@iict.res.in (N. Lingaiah).
0
926-860X/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.apcata.2010.06.015

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02
Ch.R. Kumar et al. / Applied Catalysis A: General 384 (2010) 101–106
detail. The relations between activity and modification of surface
and structural features of HPAs are important. The present study
focused on the variation of activity with change in the surface and
structural chemistry of supported heteropoly tungstates.
In this work, tin oxide supported heteropoly tungstate catalysts
are prepared and evaluated for benzylation of anisole with ben-
zyl alcohol. The prepared catalysts were characterized by different
spectroscopy methods and the derived properties are correlated
with the observed catalytic activity. The surface and structural
features of the catalysts are studied by treating the samples at
different temperatures. Reaction parameters such as temperature,
molar ratio of the reactants, catalyst weight and effect of catalyst
calcination temperature were also evaluated.
2
. Experimental
2.1. Catalyst preparation
The chemicals 12-tungstophsphoric acid and tin oxide were
obtained from Aldrich Chemicals. A series of catalysts with vary-
ing amounts of 12-tungstophsphoric acid supported on tin oxide
were prepared by an impregnation method. In a typical procedure,
a required amount of TPA was dissolved in water and this solu-
tion was added to tin oxide with continuous stirring. The resultant
mixture was allowed to stand for 3 h and the excess water was
evaporated on a water bath. The dried catalyst masses were kept
Fig. 1. FT-IR spectra of heteropoly tungstate supported on tin oxide catalysts. (a)
SnO2, (b) 5% TPA/SnO2, (c) 10% TPA/SnO2, (d) 15% TPA/SnO2, (e) 20% TPA/SnO2 and
(
f) 25% TPA/SnO2.
The products were identified by GC–MS (SHIMADZU-2010)
analysis after the separation of products on a DB-5 column with
He as carrier gas.
◦
overnight for further drying in an air oven at 120 C and finally cal-
◦
cined at 300 C for 2 h. The catalysts are denoted as x% TPA/SnO2
3. Results and discussion
where x indicates the content of TPA.
3.1. Characterization of catalysts
2.2. Characterization of catalysts
FT-IR patterns of SnO -supported TPA catalysts are shown
X-ray powder diffraction patterns were recorded on a Rigaku
Miniflex diffractometer using Cu K␣ radiation (1.5406 Å) at 40 kV
2
in Fig. 1. The catalysts showed four bands in the region of
−
1
−1
1
100–500 cm ; the main bands at 1081, 986, 890, and 800 cm
and 30 mA and secondary graphite monochromatic. The measure-
◦
can be assigned to the stretching vibrations of P–O, W = Ot,
ments were obtained in steps of 0.045 with count times of 0.5 s, in
the 2Â range of 10–80 .
◦
c e
W–O –W, W–O –W, respectively, related to Keggin ion [12]. The
FT-IR results indicate that the Keggin structure of heteropoly
tungstate remained unaltered when supported on SnO2.
X-ray diffractograms of the catalysts are shown in Fig. 2. The
SnO2 patterns are predominant in all the catalysts. The diffrac-
tion lines suggest the presence of tetragonal cassiterite structure of
The FT-IR spectra were recorded on a Bio-Rad Excalibur series
spectrometer using the KBr disc method.
Temperature programmed desorption of ammonia (TPD) was
carried out on a laboratory-built apparatus equipped with a gas
chromatograph using a thermal conductivity detector. In a typical
experiment, about 0.1 g of the oven dried sample was taken in a
◦
quartz tube and treated at 300 C for 1 h by passing pure helium
(
99.9%, 50 ml/min). After pretreatment, the sample was saturated
with anhydrous ammonia (10% NH –90% He mixture gas) with a
3
◦
flow of 50 ml/min at 100 C for 1 h and was subsequently flushed
with He at the same temperature to remove physisorbed ammonia.
The process was continued until a stabilized base line was obtained
in the gas chromatograph. Then the TPD analysis was carried out
◦
by programming the temperature from 100 to 800 C at a heating
rate of 10 C/min.
◦
2.3. General alkylation procedure
The alkylation reaction was carried out in a 50 ml three necked
round bottom (RB) flask provided with a reflux condenser, a nitro-
gen gas inlet and a septum for sample removal. In a typical run,
1
0 g of anisole and 3.376 g of benzyl alcohol (15:5 molar ratio)
were taken in a RB flask and 0.1 g catalyst was added. The reaction
◦
was carried out at a reaction temperature of 120 C. The reaction
mixture was withdrawn at different intervals and analyzed using
a gas chromatograph (VARIAN GC-3800) equipped with a SE-30
column and flame ionization detector. The oven temperature was
Fig. 2. XRD patterns of heteropoly tungstate supported on tin oxide catalysts. (a)
◦
◦
programmed from 100 to 250 C at 20 C/min with N as carrier gas
2
SnO , (b) 5% TPA/SnO , (c) 10% TPA/SnO , (d) 15% TPA/SnO , (e) 20% TPA/SnO and
2
2
2
2
2
to separate the products.
(f) 25% TPA/SnO2.

Ch.R. Kumar et al. / Applied Catalysis A: General 384 (2010) 101–106
103
Table 1
Acid strength distribution of TPA/SnO2 catalysts.
Catalyst Acidity (mmol/g)
Weak
Moderate
Strong
Total
5% TPA/SnO2
10% TPA/SnO2
15% TPA/SnO2
20% TPA/SnO2
25% TPA/SnO2
0.058
0.009
–
–
–
–
0.049
0.030
0.056
0.037
0.031
0.107
0.087
0.131
0.133
0.120
0.048
0.075
0.096
0.089
◦
of TPA. Further strong acid sites are observed at 650 C, which might
be related to SnO2 support. The intensity of these sites decreased
with increase in TPA content as seen in the decrease in the amount
of strong acidity related to support. The increase in acidity with TPA
content related not only to the increase in amount of TPA but also
to the high dispersion of TPA on support. The absence of XRD pat-
terns related to Keggin ions for the catalysts with low TPA content
suggest the high dispersion of TPA on SnO . The decrease in the
2
◦
intensity of weak and strong acidic sites (peak at 300 and 650 C)
with increase in TPA loading further suggests that these sites are
related to SnO2.
The catalysts with 15 wt% TPA/SnO2 showed the presence of
moderate and strong acidic sites generated by interaction of TPA
with support.
3
3
.2. Catalytic activity
.2.1. Effect of TPA content
The catalytic activities of TPA/SnO2 catalysts are presented in
Table 2. The support SnO2 showed very low activity for the benzy-
lation of anisole. The impregnation of TPA on SnO2 results in the
increase of catalytic activity. The alkylation activity increased as a
function of active component loading, reached to a maximum for
the catalyst with 15 wt% TPA, and remained almost constant with
further increase.
Fig. 3. Temperature programmed patterns of the catalysts. (a) 5% TPA/SnO2, (b) 10%
TPA/SnO2, (c) 15% TPA/SnO2, (d) 20% TPA/SnO2 and (e) 25% TPA/SnO2 catalysts.
SnO2 [25]. The weak peaks related to Keggin ion are visible for the
catalysts with TPA content above 15 wt%. The XRD results suggest
that TPA is highly dispersed on SnO2 at low content of TPA.
The characterization results well support the observed catalytic
activity. The catalyst with 15% TPA/SnO2 showed highly dispersed
TPA on SnO2 as it does not show any diffraction peaks related to TPA.
The XRD patterns of TPA that are seen beyond 15 wt% suggest the
monolayer coverage of TPA on SnO2 at this loading. The alkylation
activity results can be correlated with the observed acidity of the
catalyst. The catalysts, which showed the presence of moderate to
strong acidic sites, associated with TPA, exhibited high activity. The
15% TPA/SnO2 catalyst showed strong acidic sites with high amount
of total acidity, as shown in Table 1. The constant activity at high
TPA content might be related to the presence of the same amount
of total acidity. The results suggest that moderate to strong acidic
sites are responsible for benzylation activity.
Ammonia adsorption–desorption technique usually enables one
to determine the strength of acid sites present on the catalyst sur-
face together with the total acidity. The TPD of NH3 profiles of the
catalysts are shown in Fig. 3. The distribution of acidity was cal-
culated based on their strengths from TPD profiles and values are
shown in Table 1. Three types of desorption peaks in the range of
◦
3
00–650 C are present for these catalysts. The peaks observed at
low temperature are related to weak acidic sites on the support. The
◦
moderate to strong acidic sites in the range 400–500 C correspond
◦
to moderate to strong acidic sites. The peak at 300 C was predom-
inant in the catalysts with low TPA content. This desorption peak
corresponds to the weak acidic sites related to SnO . A desorption
peak centered at around 500 C observed for all the catalysts and
3.2.2. Effect of catalyst calcination temperature
2
◦
In order to understand the surface and structural characteristics
and their influence on benzylation activity, we subjected the most
active 15% TPA/SnO2 catalyst to calcination at different tempera-
its intensity increased with increase in TPA content on SnO . This
2
peak is related to strong acidic sites generated due to the presence
Table 2
Effect of TPA loading on tin oxide on the benzylation of anisole.
Catalyst
Conversion of benzyl alcohol (%)
Yield (%)
o-
Selectivity (%)
p-
0.58
4.61
34.01
39.69
36.66
40.19
Ether
o-
p-
Ether
SnO2
1.63
14.1
85.17
95.13
89.21
95.12
0.53
3.78
27.56
33.98
22.92
31.93
0.52
5.71
23.61
21.46
22.63
22.99
32.59
26.83
32.36
35.72
33.53
33.56
35.52
32.71
39.93
41.72
41.09
42.26
31.71
40.48
27.72
22.56
25.36
24.17
5
1
1
2
2
%TPA/SnO2
0% TPA/SnO2
5% TPA/SnO2
0% TPA/SnO2
5% TPA/SnO2
◦
Reaction conditions: anisole (10 g), benzyl alcohol (3.376 g), catalyst weight (0.1 g), reaction temperature (120 C) and reaction time (1.5 h).

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Ch.R. Kumar et al. / Applied Catalysis A: General 384 (2010) 101–106
Fig. 4. Effect of calcination temperature of 15% TPA/SnO2 catalyst on benzylation
activity.
Fig. 6. X-ray diffraction patterns of 15% TPA/SnO2 catalyst calcined at different
temperatures. (a) 300 C, (b) 400 C, (c) 500 C, (d) 600 C and (e) 750 C.
◦
◦
◦
◦
◦
tures. The effect of catalyst calcination temperature on benzylation
of anisole was studied and the results are shown in Fig. 4. The
results suggest that the benzylation activity depends on the calci-
nation temperature of the catalyst. The conversion of benzyl alcohol
X-ray diffractograms of the catalysts obtained at different cal-
cination temperatures are shown in Fig. 6. The samples exhibited
the main patterns related to support SnO2. However, small pat-
terns corresponding to Keggin ions are clearly seen for the catalyst
◦
increased with calcination temperature up to 400 C and decreased
◦
◦
◦
drastically thereafter. The catalyst calcined at 400 C showed max-
calcined up to 400 C. The catalyst calcined beyond 400 C showed
◦
imum conversion with high selectivity. In order to determine the
reasons for variation in activity with catalyst calcination tempera-
ture, we rather characterize the catalysts.
peaks at 2Â of 23.12, 23.60, 24.38 , which are related to WO3 phase
[26]. The formation of WO3 is expected as heteropoly tungstate
decomposes at high temperature.
Fig. 7 shows the Raman spectra of 15% TPA/SnO2 catalyst cal-
cined at different temperatures. Raman analysis helps to identify
clearly the presence of metal oxide phases when heteropoly Keg-
3
.3. Characterization of 15% TPA/SnO2 catalyst calcined at
different temperatures
◦
gin ions are decomposed. The catalysts calcined at 300 and 400 C
−
1
showed the peak at 1006 cm related to asymmetric W = Ot vibra-
FT-IR patterns of 15% TPA/SnO2 catalyst calcined at different
◦
tion of Keggin ions. The catalysts calcined at 500–750 C clearly
showed intense peaks at 805, 706 and 273 cm . The 805 and
temperatures are shown in Fig. 5. The catalysts calcined at 300 and
4
teristic peaks of Keggin ions were absent for the catalyst calcined
at higher temperatures. This indicates that the decomposition of
TPA into its constituent metal oxides occurred when calcined above
−
1
◦
00 C showed the bands related to Keggin ions [12]. The charac-
−
1
7
06 cm
bands are related to stretching vibrations of W–O–W
−
1
and the 273 cm band (not shown) was the characteristic bend-
ing mode of WO3 [27–29]. The Raman spectra clearly suggest the
formation of WO3 due to the decomposition of TPA.
◦
4
00 C.
Fig. 5. FT-IR spectra of 15% TPA/SnO2 catalyst calcined at different temperatures.
Fig. 7. Raman spectra of 15% TPA/SnO2 catalyst calcined at different temperatures.
◦
◦
◦
◦
◦
◦ ◦ ◦ ◦ ◦
(a) 300 C, (b) 400 C, (c) 500 C, (d) 600 C and (e) 750 C.
(
a) 300 C, (b) 400 C, (c) 500 C, (d) 600 C and (e) 750 C.

Ch.R. Kumar et al. / Applied Catalysis A: General 384 (2010) 101–106
105
Fig. 9. Influence of reaction temperature on the conversion of benzyl alcohol.
yield of benzylated products increased with increase in reaction
temperature. The formation of benzylated product increased with
temperature and reached a maximum at high temperature. The
selectivity towards ether is observed when the reaction is carried
out at low temperature.
◦
Fig. 8. TPD patterns of 15% TPA/SnO2 calcined at different temperatures. (a) 300 C,
◦
◦
◦
◦
3.5. Effect of catalyst weight
(
b) 400 C, (c) 500 C, (d) 600 C and (e) 750 C.
The effect of catalyst weight on the benzylation reaction was
studied and the results are presented in Fig. 10. From the figure
one can observe that the yield of benzylated product increased
with increase catalyst weight up to 0.1 g and that there is little
The TPD of NH profiles of 15% TPA/SnO2 catalyst calcined at
3
◦
3
00–750 C are shown in Fig. 8. The catalyst calcined at low temper-
◦
atures (300 and 400 C) showed ammonia desorption peaks related
to strong acidic sites of the catalysts. The catalysts calcined at high
temperature did not show the presence of so much acidity com-
pared to the low temperature calcined catalysts. The catalysts that
contain intact Keggin structure exhibited strong acidity. When the
samples were calcined at high temperature the Keggin structure
decomposed into its constituents such as WO and the samples did
3
not show high acidity. The characterization studies reveals that the
degradation of Keggin ion of heteropoly tungstate occurs when the
materials are subjected to high temperature treatment.
The benzylation activity of the catalysts calcined at different
temperatures can be directly correlated to the physico-chemical
properties of these samples derived from XRD, FT-IR, Laser Raman
and TPD measurements of NH . The catalytic activity is related to
3
the presence of acidity, which in turn is related to the existence of
intact Keggin ions of heteropoly tungstate on tin oxide.
3.4. Effect of reaction temperature
The reaction parameters are studied to optimize the benzyla-
tion activity. The benzylation of anisole was carried out at different
◦
temperatures ranging from 80 to 140 C and the results are pre-
sented in Fig. 9. It is observed from the figure that the percentage
Fig. 10. Effect of catalyst weight on the benzylation of anisole.
Table 3
Effect of anisole-to-benzyl alcohol molar ratio on benzylation of anisole.
Anisole to benzyl alcohol ratio
Conversion of benzyl alcohol (%)
Selectivity (%)
o-
p-
Ether
1
3
5
.5
21.24
72.55
99.88
20.96
31.19
56.86
29.58
38.17
43.14
49.44
30.65
–
1
◦
Reaction conditions: catalyst weight (0.1 g), reaction temperature (120 C) and reaction time (1 h).

1
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Ch.R. Kumar et al. / Applied Catalysis A: General 384 (2010) 101–106
improvement with further increase in catalyst weight. The max-
imum conversion is reached even with low catalyst amount. The
selectivity remained almost identical with variations in catalyst
amount.
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[
Acknowledgment
8
[
[
Ch. Ramesh Kumar thanks Council of Scientific and Industrial
Research (CSIR), India, for financial support in the form of a Junior
Research Fellowship.
(
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