Zirconia supported undecatungstophosphate: Synthesis and characterization of a bifunctional catalyst

Article abstract of DOI:10.1039/b804788j

The synthesized supported undecatungstophosphate has been proven to be successful for acid catalyzed as well as oxidation reactions, especially, in obtaining 98.5% conversion of styrene and 100% selectivity for benzaldehyde.

Full text of DOI:10.1039/b804788j

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COMMUNICATION
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Zirconia supported undecatungstophosphate: synthesis and characterization
of a bifunctional catalyst†
Pragati Shringarpure and Anjali Patel*
Received 20th March 2008, Accepted 6th May 2008
First published as an Advance Article on the web 3rd June 2008
DOI: 10.1039/b804788j
The synthesized supported undecatungstophosphate has
been proven to be successful for acid catalyzed as well as
oxidation reactions, especially, in obtaining 98.5% conversion
of styrene and 100% selectivity for benzaldehyde.
(Fig S3 (b), ESI†) shows bands at 746 cm−1, 952 cm−1, and at
1097 and 1048 cm−1 corresponding to the symmetric stretching of
=
=
W–O–W, W O and P O bonds respectively. The positions are
in good agreement with those of LPW, which has been reported
earlier7d confirming the presence of these groups in the synthesized
materials.
In recent times, the use of polyoxometalates (POMs) has become
The 31P MAS NMR for LPW2/ZrO2 reveals two resonances, one
at −3.98 ppm and another at −11.95 ppm. The peak at −3.98 ppm
reveals the presence of a strongly adsorbed highly fragmented
Keggin unit, while peak at −11.95 ppm indicates the presence of
an adsorbed, partially fragmented Keggin unit. This observation
agrees well with the related previous report.9
(n+4)−
a very important research area. Lacunary POMs (XM11O39
,
XM10O32(n+5)−, where X = Si, P; n = 4, 3; M = Mo(VI), W(VI)), which
are formed by the removal of one or more of the MO octahedra
from the fully occupied POMs (XM12O40n−) have also gained much
attention in the area of acid and oxidation catalysis.
The catalytic evaluation of the lacunary POMs has been
reported by Neumann et al.1 as well as Hill and co-workers.2
Especially, detailed studies have been carried out on oxidation
reactions3–6 using lacunary silicotungstates by Mizuno et al.
The acid catalysis over silicotungstates has also been reported
by the same group.6 Thus, most of the work has been re-
ported on lacunary silicotungstates, and studies on the lacunary
phosphotungstates7 are very scarce. Moreover, it has been also
observed that no literature is available on the catalytic aspects of
supported lacunary polyoxometalates.
TGA of LPW shows 8% weight loss in the temperature range
100–150 ◦C due to the loss of crystalline water. It also shows 2%
weight loss at 400 ◦C. This may be due to the decomposition
of the Keggin structure. TGA of LPW2/ZrO2 shows 16% weight
◦
loss in the temperature range 70–100 C due to loss of adsorbed
◦
water. It does not show any weight loss up to 475 C, indicating
the synthesized catalyst is stable up to 475 ◦C. The DSC of
LPW2/ZrO2 shows an endothermic peak at 100 ◦C indicating
the loss of adsorbed water. It also shows an exothermic peak at
470 ◦C which may be due to some phase change or decomposition
of the Keggin ion. The TGA and DSC studies show an increase in
the stability of LPW after being supported on ZrO2.
In the present communication, for first time, we report the
synthesis, characterization and bifunctional catalysis of novel
mono lacunary undecatungstophosphate, (PW11O39)7− (LPW),
supported on zirconia (ZrO2). Its bifunctional catalytic activity
has been evaluated for acid catalyzed as well as oxidation reactions.
The synthesized sodium salt of LPW has been characterized
by TGA, DSC, FT-IR and 31P solution as well as 31P MAS
NMR.‡ The frequencies of the FT-IR bands (Fig. S3, ESI†) are in
The XRD pattern of LPW2/ZrO2 shows the amorphous nature
of the materials indicating that the crystallinity of the LPW is lost
on supporting it onto ZrO2. Further, it does not show any diffrac-
tion lines of lacunary LPW indicating a very high dispersion of so-
lute as a non-crystalline form on the support surface. The increase
in the value of the BET surface area for the LPW2/ZrO2 (260.6 m2
g−1) as compared to that of ZrO2 (170.0 m2 g−1) is as expected.
The synthesized catalyst was evaluated for esterification as well
as for oxidation reactions. The esterification reaction of n-butanol
with acetic acid (mole ratio 1 : 4.4) was carried out in a 50 mL
glass reactor provided with a double walled air condenser, Dean–
Stark apparatus, magnetic stirrer and a guard tube. The resultant
mixture was heated at 80 ◦C for 4 h. Dean–Stark apparatus
was attached to a round bottom flask to separate the water
formed during the reaction. The same reaction was carried out
by changing the corresponding acid concentration with different
amounts of the catalyst. For esterification of isobutanol, 2-butanol
and cyclohexanol the used mole ratio of alcohol to acid was 1 : 4.4,
1 : 4.4 and 1 : 3, respectively. The obtained esters were analysed on a
Gas Chromatograph (Nucon-5700) using a Carbowax 20 column.
Fig 1 shows the results for the % conversion of n-butyl acetate.
LPW indicates the amount of the active species on the catalyst,
which is 25 mg, 41.6 mg and 81.3 mg for the corresponding 0.15 g,
0.25 g and 0.5 g catalyst. It is seen that LPW2/ZrO2 is the best of
good agreement with those previously reported.7d The solution 31
P
NMR spectra of the undecatungstophosphate shows a chemical
shift value of −10.6 ppm which is in good agreement7a with the
formula [PW11O39]7− and indicates the formation of the Keggin
structure with a vacancy created by the removal of one of the
corner as well as the edge shared tungstate octahedra. 31P MAS
NMR spectra for the LPW shows an intense peak at −11.30 ppm.
The observed shift in MAS NMR as compared to that of solution
NMR is as expected.
The synthesized catalyst was studied for its chemical stability
and it did not show any leaching. The FT-IR spectra for the ZrO2,
LPW, LPW2/ZrO2 are shown in Fig. S3 of the ESI.† The FT-
IR spectra for the ZrO2 (Fig. S3 (a), ESI†) show all the expected
bands as reported earlier by us.8 The FT-IR spectra of LPW2/ZrO2
Department of Chemistry, Faculty of Science, M. S. University of Baroda,
Vadodara, 390 002, India. E-mail: aupatel_chem.@yahoo.com
† Electronic supplementary information (ESI) available: Fig. S1: TGA;
Fig. S2: DSC; Fig. S3: FT-IR; Fig. S4: solution 31P NMR; Fig. S5: 31P
MAS NMR; Fig. S6: XRD. See DOI: 10.1039/b804788j
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all. The study on the effect of mole ratio of alcohol to acid was
also carried out and it was found that the 1 : 4.4 mole ratio gives
optimum results. The esterification of all other alcohols has been
carried out under optimized conditions. The % yield for all esters
is presented in Table 1.
A study has also been carried out to see the effect of temperature
on the catalyst structure as well as the catalytic activity. The
catalyst was calcined and characterized for FT-IR to see any
structural change of supported LPW species as well as the
effect of calcinations on catalytic activity. The FT-IR spectra
of LPW22/ZrO2, LPW23/ZrO2, LPW24/ZrO2 are also shown in
Fig. S3 (b), ESI.† The position of all the bands is almost at the
same frequency as in catalyst LPW2/ZrO2 indicating that the LPW
keeps its Keggin type structure up to 400 ◦C. In order to see
the effect of calcination on catalytic activity all calcined catalysts
have been evaluated for esterification of n-butanol and acetic acid
under optimized conditions. The %yield is 75.4%, 73% and 72.6%
Fig. 1 Percentage (%) conversion of butyl acetate with different catalysts
with different amounts and % loading. Mole ratio of alcohol to acid
is 1 : 4.4. Z= ZrO2, HW = LPW, ZHW2= LPW2/ZrO2, ZHW3
=
LPW3/ZrO2, ZHW4 = LPW4/ZrO2, ZHW5 = LPW5/ZrO2.
Table 1 Catalytic activity evaluation for LPW2/ZrO2
Acid catalyzed esterification reactiona
Acid
Alcohol
Ester
%Yield/TON
68.24/8384.47
76.31/9376.36
50.35/6186.60
41.56/5106.56
73.71/9056.90
66.42/8161.15
Oxidation of alkenesb
Substrate
Products
% Conversion
% Selectivity/TON
Styrene
Benzaldehyde
—
—
98.50
100.0/7273.37
—
—
Cyclohexenec
cis-cyclooctenec
No significant conversion
No significant conversion
a Amount of the catalyst = 0.15 g; alcohol : acid ratio = 1 : 4.4; reaction temperature = 80 ◦C; reacti◦on time◦= 4 h; amount of the active LPW on the
support = 0.025 g. b Amount of the catalyst = 25 mg; alkene : H2O2 = 1 : 3; reaction temperature = 80 C. c 50 C; reaction time = 48 h; amount of active
LPW on the support = 4.16 mg.
Table 2 % Yield of different esters with the recycled catalysts
% Yield
Catalyst
Butyl formate Butyl acetate Butyl propionate Isobutyl acetate Sec-butyl acetate Isoamyl acetate
Cyclohexyl acetate
Fresh catalyst
1st cycle
2nd cycle
3rd cycle
68.24
67.90
65.1
65.06
65.00
76.31
76.04
73.4
73.00
73.02
50.35
49.91
46.7
46.03
46.07
73.71
73.22
70.61
70.06
70.01
41.56
40.94
38.38
38.13
38.11
66.42
66.11
63.05
63.01
63.01
43.20
43.02
40.10
40.05
40.03
4th cycle
3954 | Dalton Trans., 2008, 3953–3955
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for LPW22/ZrO2, LPW23/ZrO2 and LPW24/ZrO2 respectively. No
appreciable change in % yield indicates the stability of the catalyst
up to 400 ◦C.
hyde. Further, the amount of active species required is very small
(4.16 mg) for the same reactions. The introduced catalyst is not
only selective, for oxidation of styrene, but is also a promising
alternative for traditional acid catalysts.
In order to investigate the details of the deactivation, repeated
use of the catalyst was examined. The catalyst was separated from
the reaction mixture by simple filtration, washed with conductivity
water till the filtrate was free from the acid, dried at 100 ◦C in an
oven for 5 h and the recovered catalyst was charged for the further
run. The obtained results are as shown in Table 2. It can be seen
that the catalyst can be used for up to four cycles without any
appreciable change in the % yield.
Acknowledgements
Ms Pragati Shringarpure is thankful to University Grants Com-
mission, New Delhi for providing financial support.
Notes and references
In order to study the bifunctional nature of the synthesized
catalyst, the oxidation reaction was considered to be an ideal
one. Oxidation of alkenes was carried out in a three necked flask
provided with a double walled condenser containing catalyst,
alkene (10 mmol) and aqueous H2O2 (30 mmol) at 80 ◦C with
constant stirring for 48 h. The temperature was maintained at
80 ◦C in an oil bath. The reaction was carried out by varying
different parameters such as mole ratio of alkene to H2O2,
amount of catalyst, temperature and reaction time to optimize
the conditions. After completion of the reaction, the catalyst
was removed and the product extracted with dichloromethane.
The product was dried with magnesium sulfate and analyzed on
a Nucon Gas Chromatograph using a SE-30 column. Products
formed after completion of reactions were analyzed by gas
chromatography using a SE-30 column. Product identification
was done by comparision with authentic samples and finally by
combined gas chromatography mass spectrometry. The conversion
as well as selectivity under optimized conditions is presented in
Table 1. The present study indicates that the synthesized catalyst
is selective for oxidation of styrene.
The Na salt of LPW is expected to exist in the dissociated form
in aqueous solution i.e. as Na+ and anionic LPW. This Na+ is
replaced by H+ from the aqueous medium in situ. Apart from H+,
tungsten can also contribute to the acid catalyzed reactions. The
role of tungsten in acid catalyzed reactions has also been reported
by Mizuno et al.6
In conclusion, we have introduced a novel bifunctional solid
catalytic system comprising mono lacunary undecaphospho-
tungstates and zirconia. The catalyst has been proved to be
successful for acid catalyzed as well as oxidation reactions. The
superiority of the present catalyst lies, especially, in obtaining
98.5% conversion of styrene and 100% selectivity for benzalde-
‡ Catalyst synthesis has been carried out in three steps. The first and second
step involves the synthesis of the mono lacunary undecatungstophosphate
(LPW)7c and hydrous zirconia (ZrO2)8 following the methods reported
previously. The third step involves the supporting of the LPW on
ZrO2. A series of catalysts containing 20–50% LPW were synthesized by
impregnating ZrO2 (1 g) with an aqueous solution of LPW (0.2–0.5 g in
20–50 mL of conductivity water) and dried at 100 ◦C for 10 h. The obtained
materials were designated as LPW2/ZrO2, LPW3/ZrO2, LPW4/ZrO2
and LPW5/ZrO2. (The numerical subscript after W corresponds to
the % loading of LPW onto the surface of support, e.g. LPW2/ZrO2
means 20% loading of LPW onto zirconia.) The selected best catalyst
(LPW2/ZrO2) was calcinated at 200, 300 and 400 ◦C in air for 5 h and
designated as LPW22/ZrO2, LPW23/ZrO2 and LPW24/ZrO2 respectively.
The synthesized LPW as well as the catalysts have been characterized by
TGA, DSC, FT-IR, solution 31P NMR and 31P MAS NMR, BET surface
area and XRD.
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The Royal Society of Chemistry 2008
Dalton Trans., 2008, 3953–3955 | 3955
©