Selective oxidation of cyclopentene to glutaraldehyde by H2O2 over the WO3/SiO2 catalyst

Article abstract of DOI:10.1006/jcat.2001.3319

A novel WO₃/SiO₂ was prepared by incipient wetness impregnation of the SiO₂ support synthesized by the xerogel method with the W-containing salt solution. The as-prepared WO₃/SiO₂ catalyst exhibited a very high yield of glutaraldehyde in the liquid phase cyclopentene oxidation by aqueous H₂O₂ and the leach of WO₃ species during the reaction could be neglected. As a heterogeneous catalyst, it seems more suitable for the industrial process than those homogeneous catalysts owing to its easy separation from reaction products, which makes it possible to use the catalyst repetitively. According to the XRD patterns, the WO₃ was present in amorphous state due to its high dispersion on the SiO₂ support. These amorphous WO₃ species were proved to be the active sites since the crystallization at high temperature caused a considerable deactivation. The lifetime of the catalyst was measured and its regeneration method was proposed. Effects of various factors on the catalytic behaviors, such as the WO₃ loading, the calcination temperature, and the reaction media, were also investigated and discussed based on the characterizations of BET, XRD, DSC, TEM, EXAFS, and Raman spectra.

Full text of DOI:10.1006/jcat.2001.3319

Journal of Catalysis 203, 75–81 (2001)
doi:10.1006/jcat.2001.3319, available online at http://www.idealibrary.com on
Selective Oxidation of Cyclopentene to Glutaraldehyde by H2O2
over the WO3/SiO2 Catalyst
Ronghua Jin, Hexing Li, ,1 and Jing-Fa Deng†
Department of Chemistry, Shanghai Normal University, Shanghai 200234, P. R. China; and †Department of Chemistry,
Fudan University, Shanghai 200433, P. R. China
Received December 4, 2000; revised May 31, 2001; accepted June 14, 2001
tained from the byproducts of C5 fraction presented in re-
fining oils (10, 11). Recently, several W-containing homoge-
neous catalysts have been reported which allowed us to use
the environmentally friendly aqueous H2O2 as the oxidant
for the CPE selective oxidation to GA (12–14). Although
the high GA yield was obtained, their application in in-
dustrial processes seems impractical since the separation of
these homogeneous catalysts from the reaction products is
very difficult. On one hand, product cleanup is evitable. On
the other hand, the catalyst cannot be used repetitively and
regenerated. One of the most promising ways is to design
the W-containing heterogeneous catalysts by depositing the
WO3 species on the suitable supports. As we know, how-
ever, no such a work has been reported so far, possibly due
to the poor catalytic efficiency of the heterogeneous cata-
lysts in comparison with the corresponding homogeneous
catalysts. Our previous studies demonstrated that the pore
size of the support played a key role in determining the per-
formance of the supported heterogeneous catalysts, since
the large pore size of the support is necessary to ensure
the oxidation of bulky cycloalkenes over those catalysts
(15, 16). In the present paper, we report a novel WO3/SiO2
with larger pore size based on the combination of both the
xerogel method and the incipient wetness impregnation.
The as-prepared WO3/SiO2 catalyst exhibited a very high
GA yield in liquid phase CPE oxidation by aqueous H2O2,
almost the same as that obtained by using the tungstic acid
as a homogeneous catalyst. The effects of various factors on
its catalytic behaviors were determined and well discussed
according to various characterizations.
A novel WO3/SiO2 was prepared by incipient wetness impreg-
nation of the SiO2 support synthesized by the xerogel method with
the W-containing salt solution. The as-prepared WO3/SiO2 catalyst
exhibited a very high yield of glutaraldehyde in the liquid phase cy-
clopentene oxidation byaqueousH2O2 and the leach of WO3 species
during the reaction could be neglected. As a heterogeneous catalyst,
it seems more suitable for the industrial process than those homoge-
neous catalysts owing to its easy separation from reaction products,
which makes it possible to use the catalyst repetitively. According
to the XRD patterns, the WO3 was present in amorphous state
due to its high dispersion on the SiO2 support. These amorphous
WO3 species were proved to be the active sites since the crystalliza-
tion at high temperature caused a considerable deactivation. The
lifetime of the catalyst was measured and its regeneration method
was proposed. Effects of various factors on the catalytic behaviors,
such as the WO3 loading, the calcination temperature, and the re-
action media, were also investigated and discussed based on the
characterizations of BET, XRD, DSC, TEM, EXAFS, and Raman
c
spectra.
2001 Academic Press
Key Words: WO3/SiO2 catalyst; selective oxidation; cyclopen-
tene; H2O2; glutaraldehyde; xerogel method.
INTRODUCTION
WO3-based catalysts are important not only in selective
reduction of NH3 (1–3) but also in epoxidation of unsatu-
rated compounds (4). Supported tungsten oxide catalysts
are very efficient for various acid catalyzed heterogeneous
reactions. However, almost no attention has been devoted
to their application in the oxidative cleavage of carbon–
carbon double bonds with aqueous H2O2 to produce di-
aldehydes, which is now mainly prepared by ozonization of
olefins (5, 6) or other synthetic methods (7, 8). Glutaralde-
hyde (GA) has been used extensively for disinfection and
sterilization in many areas (9). An important way to pro-
duce GA is the selective oxidative cleavage of cyclopentene
(CPE), since a great quantity of CPE could be easily ob-
EXPERIMENTAL
Catalyst Preparation
The SiO2 support was prepared by an alkoxide-sol-gel
method (17–20). In general, a certain amount of ethanol
was added to 150 ml tetraethylsilicate (TEOS) solution.
After being stirred for 30 min, water and HCl solu-
tion were added to the above solution, in which the
molar ratio of H2O : TEOS : HCl was adjusted to be
1 To whom correspondence should be addressed. E-mail: HeXing-
Li@shtu.edu.cn.
75
0021-9517/01 $35.00
c
Copyright
2001 by Academic Press
All rights of reproduction in any form reserved.

76
JIN, LI, AND DENG
5 :1 :0.09. The gel was kept at 353 K for 24 h and then chromatograph with TCD detector by using cyclopentane
at 373 K for another 24 h, which resulted in a dried gel. as an internal standard. The yield of GA was measured by
The powder of dried support was then ground with mortar gas chromatograph with FID detector by using an external
and pestle and sieved to 80–100 meshes. Then, the sample standard method. The products were analyzed by GC-MS.
was heated programmed at 5 K/min in a nitrogen flow of The H2O2 was measured by standard iodometric titration.
500 ml/min and kept at 473 K for 2 h. After cooling down
to 353 K, it was calcined in an air flow of 500 ml/min at
RESULTS AND DISCUSSION
Comparison of Different Catalysts
a programmed temperature with the speed of 5 K/min up
to 823 K and kept at that temperature for another 4 h to
remove most of the organic residues. The WO3/SiO2 was
obtained by an incipient wetness impregnation of the as-
prepared SiO2 support with appropriate amounts of ammo-
nium tungstate solution (2, 21–23), which was then dried at
393 K for 16 h following a calcination in air flow at 823 K
for another 16 h.
The WO3/SiO2 catalysts with different WO3 loadings ex-
hibited different activities and selectivities to GA during
the CPE oxidation. As shown in Table 1, the GA yield first
increased rapidly with the increase of the WO3 loadings up
to 15 wt% and then kept nearly constant when the WO3
loadings further increased, indicating the optimum WO3
loading was 15 wt% . Although the GA yield over 15 wt%
WO3/SiO2 catalyst was slightly lower than that on the cor-
responding homogeneous catalyst obtained by dissolving
WO3 H2O in the reaction solution, the WO3/SiO2 cata-
lyst, as a heterogeneous catalyst, was more suitable for the
industrial process since it could be used repetitively during
the reaction owing to its easy separation from the reaction
mixture. After reaction for three times, significant decrease
in the GA yield was observed, showing the occurrence of
the catalyst deactivation. Such a deactivation was possibly
due to the covering of the catalyst surface with the organic
species since the activity could be recovered mostly by cal-
cining the catalyst at 823 K for 6 h.
Catalyst Characterization
Specific surface areas (BET) and mean cylindrical pore
diameters were determined by nitrogen physisorption at
77 K using a Micromeritics ASAP 2000 instrument.
X-ray powder diffraction (XRD) patterns were obtained
on a Bruker D8 Advance X-ray generator using Cu K
˚
radiation ( = 1.54 A) at 40 kV and 40 mA.
Differential scanning calorimetry (DSC) was conducted
under nitrogen (99.99% ) atmosphere on a Dupont 9900
computer-thermal analysis system at the heating rate of
1
10 K min
.
Transmission electron microscope (TEM) was obtained
on a Hitachi H600 scan-transmission electron microscope.
Extended X-ray absorption fine structure (EXAFS) was
performed. The absorption data of W k-edge were collected
at the 4W1B beamline in Beijing Synchrotron Radiation Fa-
cility, China. The electron beam energy is 22.0 GeV and the
stored current is in the range of 30–50 mA. The monochro-
mator is a channel-cut Si(111) crystal monochromator;
d = 0.31355 nm. The data were collected in the transmission
mode using ion chambers of nitrogen (75% ) argon (25% )
mixed gas at room temperatures from 8050 to 9400 eV. We
registered data three times for estimating the deviation.
Data were processed by using the program package
FXEA.
According to the ICP analysis, only 2.3 ppm W-species
was detected in the solution after reaction for 24 h with
15 wt% WO3/SiO2 catalyst, showing that the loss of
W-species could be neglected. To make sure whether the
heterogeneous WO3 on the SiO2 support or the dissolved
homogeneous WO3 was the real catalyst responsible for
TABLE 1
Structural Properties and Catalytic Performance of Different
WO3/SiO2 Catalystsa
Conversion
(% )
Raman spectra were recorded with a Superlab Ram
Raman spectrometer with a resolution of 2 cm 1. The laser
power at the sample location was set to 15 mW. The excita-
tion line of the Raman scattering was 632.817 nm.
WO3 loading
(wt% )
BET
Vp (N2)
dp
Yield
1
1
(m2
g
)
(cm3
g
)
(nm) CPE H2O2 (% ) GA
5
10
15
20
569
539
522
475
—
0.28
0.28
0.27
0.27
—
1.9
1.9
1.9
1.8
—
95.5
97.8
100
100
100
1.5
98.6
99.1
100
100
100
0.1
46.3
54.9
59.9
60.0
62.3
0
Activity Test
WO3 H2Ob
In a typical run, the oxidation experiment was carried
out in a sealed 10-ml glass reactor in which 0.5 ml of
CPE (Fluka), 5 ml of t-BuOH (as the solvent), and 0.2 g
catalyst were mixed at 308 K with vigorous stirring. The
reaction was started by adding 0.7-ml 50% aqueous H2O2
solution (industrial grade) into the mixture and was kept
for 24 h. The conversion of CPE was measured by gas
c
WO3
15d
—
502
—
0.28
—
1.8
98.3
100
57.5
a Reaction conditions: 308 K, 0.2 g catalyst (calcined at 823 K), 0.5 ml
CPE, 0.7 ml 50% H2O2, 5 ml t-BuOH, reaction for 24 h.
b Homogeneous catalyst.
c Insoluble WO3 obtained after WO3 H2O was calcined at 673 K.
d The catalyst after regeneration.

OXIDATION OF CPE TO GA OVER WO3/SiO2
77
SCHEME 1. The reaction route of cyclopentene (CPE) oxidation.
the present oxidation, the following procedure, proposed
by Sheldon et al. (24), was carried out. After reaction for
4.5 h in which the CPE conversion reached nearly 61.6% ,
the reaction mixture was filtered and then allowed the
mother liquor (filtrate) to react for another 16 h at the same
reaction conditions. No significant activity was observed,
demonstrating that the active species are not the dissolved
WO3 leached from WO3/SiO2. Therefore it is reasonable to
suggest that the present catalysisisheterogeneousin nature.
According to the GC-MS analysis, besides GA as the
main product, a variety of the byproducts, such as ox-
ide (CPO), cyclopentanone, 2-cyclopenten-1-one, trans-1,2-
cyclopentandiol and its mono ether, were observed, indicat-
ing that the reaction was very complex. Therefore, a possi-
ble reaction scheme was described as follows (Scheme 1).
Over the 15 wt% WO3/SiO2 catalyst with WO3 loading
of 15 wt% , the dependence of the contents of CPE, GA,
and various byproducts on the reaction time is illustrated
in Fig. 1. As the CPO produced rapidly at the beginning
and then consumed progressively with the increase of GA,
one can conclude that CPO was possibly a main interme-
diate from which GA produced via its further oxidative
cleavage.
FIG. 2. XRD patterns of (a) anhydrous WO3, and the 15% WO3/SiO2
sample calcined at 823 K, (b) before reaction, (c) after regeneration,
(d) after reaction for three times (72 h), and (e) the 15% WO3/SiO2 sample
calcined at 973 K.
After WO3 H2O was calcined at 673 K for 12 h, an anhy-
drous WO3 sample was obtained which is insoluble in the
aqueous H2O2. Against expectation, almost no activity was
observed over such an anhydrous WO3 catalyst, as shown
in Table 1.
The above results could be explained based on the XRD
analysis. As shown in Fig. 2, the anhydrous WO3 exhib-
ited a typical crystalline structure while the fresh WO3/SiO2
exhibited a typical amorphous structure. According to the
activity test, as shown in Table 1, the anhydrous WO3 ex-
hibited almost no activity during the reaction. Therefore,
one can conclude that the WO3 species in amorphous state
were the active sites in the present catalysis. At lower WO3
loadings (<20 wt% ), the WO3 species were well dispersed
on the support surface without significant crystallization.
Thus, the GA yield increased with the increase of WO3
loadings since the number of surface active WO3 sites in-
creased. However, partial crystallization occurred at higher
WO3 loadings (>20 wt% ), which was responsible for the
decrease in its activity. The crystallization of WO3/SiO2 at
higher WO3 loadings was possibly due to the gathering of
the WO3 particles since the BET surface area decreased
rapidly with the increase of WO3 loadings, as shown in
Table 1. After reaction for three times, GA yield decreased
since partial crystalline WO3 appeared. The activity could
be recovered mostly by calcining the catalyst at 823 K for
6 h since the amorphous WO3 species were obtained again
after its regeneration.
The relationship between the crystallization of WO3
species and its dispersion on the SiO2 support was fur-
ther illustrated by the TEM. As shown in Fig. 3a, the fresh
WO3/SiO2 sample exhibited a well dispersed morphology
comprising very small particles. These particles are spher-
ical with an average size around 10–60 nm. After reac-
tion for three times, big lumps with an average size larger
than 100 nm appeared owing to the gathering of the WO3
FIG. 1. Dependence of distribution of CPE and the products on the
reaction time. A =CPE, B =CPO, C =GA, D =trans-1,2-cyclopentandiol,
E = 1,2-cyclopentandiol mono ether. Reaction conditions: 308 K, 0.2 g
15 wt% WO3/SiO2 catalyst calcined at 823 K, 0.5 ml CPE, 0.7 ml 50%
H2O2, 5 ml t-BuOH.

78
JIN, LI, AND DENG
FIG. 3. TEM photograph of the as-prepared 15% WO3/SiO2 sample calcined at 823 K. (a) before reaction, (b) after reaction for three times (72 h),
and (c) after regeneration.
particles, as shown in Fig. 3b. After the regeneration of nation temperature was lower than 823 K. However, when
the catalyst, these big lumps disappeared and the well- the WO3/SiO2 catalyst was treated at higher temperatures
dispersed morphology similar to that in Fig. 3a appeared from 823 to 1073 K, several diffraction peaks appeared.
again (see Fig. 3c). One can see that the particle size was Those peaks were assigned to the crystalline WO3 phases
around 30–100 nm, which was slightly larger than that in since no significant diffraction peaks of SiO2 support were
the fresh WO3/SiO2 sample.
observed; even it was treated at 1073 K. Considering the
change of catalytic performance, as shown in Table 2,
the CPE conversion remained at 100% while the GA
yield increased from 41.4 to 59.9% when the calcinations
temperatures increased from 673 to 823 K. However, both
the activity and GA yield decreased dramatically when
the calcination temperatures further increased from 823 to
1073 K. This clearly demonstrated that only the amorphous
WO3 phase could serve as the active sites during the present
catalysis. The crystallization process of 15 wt% WO3/SiO2
sample could also be analyzed by DSC, as shown in Fig. 5.
The single exothermic peak on the DSC spectra suggested
that the WO3/SiO2 sample transferred from the amorphous
state to its crystallized state at around 873 K without any
Influence of the Calcination Temperature
To further confirm the correlation of the activity of
WO3 to its structure, the in situ XRD patterns of 15 wt%
WO3/SiO2 catalyst treated at elevated calcination temper-
atures were determined and their corresponding activity
and GA yield were measured. As shown in Fig. 4, the
in situ XRD patterns revealed that the WO3 was present
in an amorphous state on the SiO2 support when the calci-
TABLE 2
Influence of the Calcination Temperature on the Structural and
Catalytic Properties of the WO3/SiO2 Catalyst with 15 wt% WO3
Loadinga
Conversion
(% )
Temperature
(K)
BET
(m2 g
Vp (N2)
dp
Yield
1
1
)
(cm3 g
)
(nm) CPE H2O2 (% ) GA
673
823
973
610
522
367
0.29
0.27
0.19
1.7
1.9
2.2
100
100
48.7
100
100
52.4
41.4
59.9
19.7
FIG. 4. In situ XRD patterns of 15 wt% WO3/SiO2 sample calcined
at 673, 823, and 973 K, respectively.
a Reaction conditions are as the same as those given in Table 1.

OXIDATION OF CPE TO GA OVER WO3/SiO2
1
79
270 cm corresponding to the symmetric stretching mode
–
–
of W O, bending mode of W O, and the deformation mode
– –
of W O W, respectively, were observed (see Fig. 6c) sim-
ilar to those found in the good crystalline WO3 samples
(25, 26), showing the occurrence of the crystallization.
Meanwhile, the amorphous structure of the WO3/SiO2 sam-
ple after reaction for three times could be regenerated after
being treated at 823 K for 6 h since all the peaks disap-
peared, as shown in Fig. 6b. These results were in good
agreement with the aforementioned activity test. A similar
Raman spectrum was also obtained when the WO3/SiO2
sample was calcined at 973 K, as shown in Fig. 6d.
The RDF curves of the anhydrous WO3 and WO3/SiO2
samples obtained from Fourier transforms of their EXAFS
k2 x at the W L3 edge are shown in Fig. 7. For the anhydrous
˚
WO3, as shown in Fig. 7a, the peaks between R = 0.6 2.5 A
˚
–
FIG. 5. DSC curve of the 15% WO3/SiO2 sample calcined at 823 K.
were assigned to W O bonds and the peaks at R = 3.7 A to
–
the W W bonds, respectively (27). RDF curves similar
to the WO3/SiO2 catalyst after reaction for three times
or the WO3/SiO2 catalyst after being calcined at 973 K
were also obtained. In comparison with that of anhydrous
other intermediate phases. As well known, the higher
calcination temperatures could increase the interaction
between WO3 and the SiO2 support and in turn inhibit the
leaching of the active WO3 during the reaction. Therefore,
823 K was chosen as the optimum calcination temperature
of 15 wt% WO3/SiO2 catalyst because at that temperature
no significant crystallization occurred. It should be noted
that the crystallization temperature changed with the WO3
loading. Lower calcination temperatures should be em-
ployed with higher WO3 loadings to avoid crystallization.
The Raman spectra provided additional information
about the structure of the WO3/SiO2 samples, as shown
in Fig. 6. One can see that the WO3/SiO2 calcined at 823 K
was present in a typical amorphous state since no signif-
icant peaks appeared, as shown in Fig. 6a. After reaction
for three times, various strong bands around 800, 720, and
˚
WO3, a new peak around 3.3 A appeared on the RDF
FIG. 6. Raman spectra of the 15% WO3/SiO2 sample calcined at
823 K. (a) before reaction, (b) after regeneration, (c) after reaction, and
(d) the 15% WO3/SiO2 sample calcined at 973 K.
FIG. 7. RDF curves of W k edge in (a) anhydrous WO3 and (b) 15%
WO3/SiO2 sample calcined at 823 K.

80
JIN, LI, AND DENG
TABLE 3
CONCLUSION
Influence of Various Solvents on the Performance of the
WO3/SiO2 Catalyst with 15 wt% WO3 Loadinga
The following conclusions can be drown from this study:
1. The WO3/SiO2 catalyst is one of the powerful hetero-
geneous catalysts for the liquid phase cyclopentene oxida-
tion by H2O2, which exhibits high activity and excellent
selectivity to GA. The as-prepared catalyst seems more
suitable for the industrial process than those homogeneous
catalysts owing to the convenience in the separation of the
catalyst from the reaction products, which makes it possi-
ble to use the catalyst repetitively and to regenerate the
deactivated catalyst.
Solvent
Conversion of CPE (% )
Yield of GA (% )
MeOH
EtOH
i-PrOH
t-BuOH
MeCN
THF
93.1
91.5
95.7
100
74.7
78.1
12.1
38.5
57.4
59.9
44.4
47.1
a Reaction conditions: 5 ml each of the solvents, other conditions are
as the same as those given in Table 1.
2. In the WO3/SiO2 catalyst, the amorphous WO3 species
are determined to be the active sites. The anhydrous WO3
exhibited almost no activity because of its good crystalline
structure. The optimum WO3 loading is determined as 15
wt% to ensure the largest number of active sites but without
significant crystallization. The optimum calcination temper-
ature is determined as 823 K to guarantee the strongest
interaction and support under the condition that no sig-
nificant crystallization occurs, which can effectively inhibit
the leaching of the active sites during the reaction. The t-
BuOH is proved to be the best solvent in the present ox-
idation reaction owing to the promoting effect of TBHP
formed through the reaction between t-BuOH and H2O2
on WO3/SiO2 as an acid catalyst.
3. Under the present conditions, the 15 wt% WO3/SiO2
catalyst can be used repetitively for three times (72 h).
After reaction for 72 h, significant decrease in the activ-
ity of WO3/SiO2 catalyst was observed, possibly due to the
structural conversion of WO3 from the amorphous state
to the crystalline state. The deactivated catalyst could be
regenerated easily by calcining it at 823 K for 6 h.
curve of WO3/SiO2 sample, as shown in Fig. 7b, which
–
was possibly attributed to a W Si bond resulting from
WO3 anchoring onto the SiO2 surface. A new shoulder
˚
peak around 2.1 A with a high Debye–Waller factor
–
was presumably corresponding to the W O bonds in the
– –
– –
– –
W O W and W O Si bridges (28). Such W O Si bridges,
as confirmed by Raman spectra (29), have been claimed to
be favorable for the selective oxidation reaction (30–33).
The calculation from the EXAFS data also revealed that
the coordination number of W in the above-mentioned
bridges was 4.8. Such unsaturated W sites on the surface
(25) favored the adsorption of reactants and in turn,
increased the catalytic activity of the WO3/SiO2 catalyst.
Effect of the Solvents
It is well known that the solvent plays a very important
role in determining the catalytic activity and selectivity in
many catalytic oxidations by H2O2 (34). Therefore, the
effects of various common solvents on the catalytic perfor-
mance of the 15 wt% WO3/SiO2 catalyst during the CPE
oxidation to GA were investigated. As shown in Table 3,
higher conversion and better selectivity to GA were ob-
tained in the tert-butanol (t-BuOH) medium than those in
other media, such as methanol, ethanol, iso-propanol, ace-
tonitrile, and tetrahydrofurian. According to the GC-MS
analysis, the promoting effect of t-BuOH on the CPE
oxidation to GA could be attributed to the formation of
tert-butyl hydroperoxide (TBHP) through to the reaction
between H2O2 and t-BuOH with WO3/SiO2 as an acid
catalyst (Scheme 2), which has been proved to be an excel-
lent oxidant for the selective CPO to GA in liquid phase
(15, 24).
ACKNOWLEDGMENTS
This work was supported by the National Natural Science Foundation of
China and SINOPEC. We are also grateful to the Committee of Shanghai
Education for providing financial support for this study.
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