Oxidation of geraniol and other substituted olefins with hydrogen peroxide using mesoporous, sol-gel-made tungsten oxide-silica mixed oxide catalysts

Article abstract of DOI:10.1016/j.jcat.2004.07.006

The preparation of a series of mesoporous tungsten oxide-silica mixed oxides by sol-gel methods under basic conditions is reported. Surface modification with methyl and 3-chloropropyl groups is possible in an amount between 10 and 40 mol% with respect to the silane precursor. The amount of polar organic functional groups controls the surface area, the porosity, and the catalytic activity of the solids in the oxidation of different substrates with hydrogen peroxide. The oxidation of geraniol is studied in detail. The catalysts are active and produce epoxides in good yields. The latter are influenced by the presence of polar organic groups. The preparation method allows the preparation of catalysts that are resistant to leaching and can be recycled several times without appreciable loss of activity.

Full text of DOI:10.1016/j.jcat.2004.07.006

Journal of Catalysis 227 (2004) 344–351
www.elsevier.com/locate/jcat
Oxidation of geraniol and other substituted olefins
with hydrogen peroxide using mesoporous,
sol–gel-made tungsten oxide–silica mixed oxide catalysts
Filippo Somma, Giorgio Strukul ∗
Dipartimento di Chimica, Università di Venezia, and Consorzio INSTM, Dorsoduro 2137, 30123 Venezia, Italy
Received 17 May 2004; revised 30 June 2004; accepted 2 July 2004
Available online 26 August 2004
Abstract
The preparation of a series of mesoporous tungsten oxide–silica mixed oxides by sol–gel methods under basic conditions is reported.
Surface modification with methyl and 3-chloropropyl groups is possible in an amount between 10 and 40 mol% with respect to the silane
precursor. The amount of polar organic functional groups controls the surface area, the porosity, and the catalytic activity of the solids in the
oxidation of different substrates with hydrogen peroxide. The oxidation of geraniol is studied in detail. The catalysts are active and produce
epoxides in good yields. The latter are influenced by the presence of polar organic groups. The preparation method allows the preparation of
catalysts that are resistant to leaching and can be recycled several times without appreciable loss of activity.
2004 Published by Elsevier Inc.
Keywords: Oxidation; Allylic alcohols; Hydrogen peroxide; Sol–gel; WO –SiO catalysts
3
2
1. Introduction
bic organics inside the pores of the solid. The possibility of
modifying the surface of these systems is an important as-
pect for improving their adsorption properties with respect to
organic reactants [3,4]. Moreover, when metal complex cata-
lysts or metal oxides are immobilized onto silica, the surface
hydrophobizationwith methyl, phenyl, and other groups pre-
vents active metals from leaching when H2O2 is used as
oxidant [3a,3b].
To date, most of research work in this area has focused on
supported TiO2 as the active phase. Unfortunately, all stud-
ies dealt mainly with t-butylhydroperoxide as the oxidant,
because the activity observed with H2O2 was rather modest,
mainly because TiO2 crystallites decompose the oxidant.
We have recently reported some ZrO2–SiO2 mesoporous
and microporous mixed oxides that have been employed for
the oxidation of variety of olefins with hydrogen peroxide,
leading mainly to the formation of diols, and show a high
efficiency in H2O2 use [4].
The oxidation of organic compounds with hydrogen per-
oxide is very important for environmentally friendly indus-
trial technologies. Hydrogen peroxide is a very attractive
oxidant: it is cheap, environmentally clean, and easy to han-
dle, but it is poorly active without a catalyst. Zeolites and
crystalline silicate-based systems are active and selective
epoxidation catalysts with hydrogen peroxide [1] but their
small pore dimensions constitute also their main limitation
when relatively large organic molecules are used. In recent
years amorphous titania–silica mesoporous materials (aero-
gels and xerogels) have emerged as promising catalysts with
organic hydroperoxides [2].
One of the problems when working with mixed oxides in-
stead of zeolites in the liquid phase is their generally high
hydrophilicity, which hampers the diffusion of hydropho-
It is well known that tungsten-based homogeneous cata-
lysts show excellent activity and selectivity in epoxidation
reactions with H2O2 [5]. On the other hand, examples of
*
Corresponding author. Fax: +39 041 234 8517.
E-mail address: strukul@unive.it (G. Strukul).
0021-9517/$ – see front matter
doi:10.1016/j.jcat.2004.07.006
2004 Published by Elsevier Inc.

F. Somma, G. Strukul / Journal of Catalysis 227 (2004) 344–351
345
supported WO3 as catalysts for epoxidation reactions with
Table 1
Preparation features of different WO –SiO (1% W) catalysts
3
2
H2O2, where the active phase does not dissolve during the
reactions, are relatively rare [6,7]. In fact, WO3 dissolves
in hydrogen peroxide solutions to give the so-called “per-
tungstic acid” that is an excellent soluble epoxidation cata-
lyst [8]. Deng and co-workers [6a,6b] have recently reported
a series of WO3–SiO2 catalysts made by impregnation of
a sol–gel-made silica with ammonium tungstate, in which
leaching from the matrix was prevented by calcination at
550 ◦C. During the use of these materials in the oxidation
of cyclopentene to glutaraldehyde with hydrogen peroxide,
extensive sintering could be observed (leading to deactiva-
tion of the catalyst) that could be reverted by calcination.
In this work we report the preparation and characteri-
zation of mesoporous heterogeneous WO3–SiO2 catalysts
made by sol–gel techniques that allow the incorporation of
W in the silica network, thereby preventing the problems de-
scribed above. These materials are very interesting catalysts
for the selective epoxidation of allylic alcohols. In addition,
being allylic alcohols moderately hydrophilic molecules, we
report also the effect on the activity and selectivity of the sur-
face modification of the catalysts with apolar (methyl) and
moderately polar (3-chloropropyl) organic groups that are
introduced to improve the accessibility on the active sites
to molecules that are so different in terms of hydrophilicity
properties such as hydrogen peroxide and an olefin.
Sample
Heating
Amount
functional
group (%mol)
temperature
◦
( C)
WO –SiO -110
110
200
500
700
110
110
110
200
200
200
–
–
–
3
2
WO –SiO -200
3
2
WO –SiO -500
3
2
WO –SiO -700
–
3
2
WO –SiO -10ClPr
10
20
30
20
40
–
3
2
WO –SiO -20ClPr
3
2
WO –SiO -30ClPr
3
2
WO –SiO -20Me
3
2
WO –SiO -40Me
3
2
WO –SiO -Aero
3
2
2.3. Preparation of xerogels
The following general preparation procedure was used
in all cases. To a 250-ml beaker were added TMOS and
MTES or 3CPTMES in the appropriate amount (see Table 1
for composition) when necessary. In a typical experiment
4.7 ml TMOS (or the appropriate silane precursors mix-
ture) were dissolved in 14 ml EtOH. The mixture was stirred
at room temperature for 5 min. After this time, 1.05 ml of
a 30% ammonia solution was added. Hydrolysis occurred
within 5 min. Over the prehydrolyzed mixture, 1.17 ml of a
W(OPr)6 solution in propanol (5 g/100 ml) was added, in
order to obtain a final composition of 1 wt% W/SiO2. Gela-
tion was completed in 5 min. Gels were normally transparent
and were aged for 72 h at room temperature. The different
xerogels were dried at 110 ◦C overnight. Where necessary
the samples were calcined at 200, 500, and 700 ◦C in air for
2 h: gas flow, 30 ml/min; heating ramp 3 ◦C/min.
2. Experimental
2.1. Materials
The following metal alkoxides were used: tetramethoxy-
silane (TMOS) and methyltriethoxysilane (MTES), 3-chlo-
ropropyltrimethoxysilane (3CPTMES) (both from Aldrich),
tungsten hexa-iso-propoxide(Chemat Technology inc.). The
oxidants was 35% hydrogen peroxide (Fluka). Substrates
were purchased from Fluka. All chemicals were purum or
puriss grade and used without further treatment.
2.4. Preparation of the aerogel
A modified tungsten alkoxide precursor was synthesized
according to the procedure reported in Ref. [9]. A solu-
tion of acetylacetone (acacH), i-PrOH (3 ml), and W(OR)6
(acac/W(OR)6 1/1) was refluxed for 1 h under N2 with stir-
ring. The modified tungsten precursor and TMOS (9.8 ml)
were dissolved in i-PrOH (10 ml). A hydrolyzing solution
consisting of distilled water (6 ml) and nitric acid (0.27 ml)
diluted in i-PrOH (6 ml) was added dropwise to the alkox-
ides solution under vigorous stirring. After 90 min, 40 ml
i-PrOH was introduced to the system. The corresponding
molar ratios H2O/alkoxide/acid were 5/1/0.09. The mix-
ture was transferred to a stainless-steel autoclave.
The conditions for obtaining a supercritical drying of the
solvent are: Vc = 48 ml; Pc = 48 atm; Tc = 235 ◦C. The
high-pressure system was flushed with N2, pressurized to
7 atm, and heated at 240 ◦C. The autoclave was kept at
the final temperature for 60 min to ensure complete ther-
mal equilibration. The final pressure was about 80 atm. The
pressure was then released at constant temperature and fi-
nally the system was flushed with N2 and cooled to room
2.2. Methods
BET surface areas and pore size were determined with
N2 at 77 K on a Micromeritics ASAP 2000 apparatus. X ray
powder diffraction analysis was performed using a X’Pert
diffractometer operating with Cu-Kα Ni-filtered radiation,
a graphite monochromator, and a proportional counter with
a pulse height discriminator. The diffraction patterns were
measured step by step (0.05 ◦ in 2θ). Differential thermal
analysis (DTA) was performed on a Netzsch instrument in
flowing air with temperature rate set at 10 ◦C/min in the 25–
700 ◦C temperature range. GC-MS measurements were per-
formed on a Hewlett–Packard 5971 mass-selective detector
connected to a Hewlett–Packard 5890 II gas chromatograph.
ICM–MS measurements were carried out on a Agilent 7500
instrument.

346
F. Somma, G. Strukul / Journal of Catalysis 227 (2004) 344–351
temperature. The sample was calcined at 200 ◦C in air for
2 h: gas flow, 30 cc/min; heating ramp 3 ◦C/min.
A summary of the preparation parameters is reported in
Table 1.
2.5. Catalytic oxidation procedures
Catalytic reactions were performed in 10-ml glass vials
equipped with a small magnetic bar. The catalyst (10 mg),
substrate (1 mmol), and H2O2 (1 mmol) were placed in the
vial under N2 flow. The reaction vessels were sealed and
placed in an oil bath at constant temperature, while an exter-
nal magnetic stirrer ensured agitation. At different reaction
times the mixtures were cooled down to room temperature,
diluted with MeOH (2 ml), and analyzed by GC. Prod-
uct identification was performed by GC-MS analysis and
by comparison with authentic samples. Conversions and se-
lectivities were determined by analysis of the relative peak
areas of reactants and products. Residual H2O2 was deter-
mined by iodometric titration.
Fig. 1. Powder X-ray diffraction profiles for WO –SiO -500 (upper) and
3
2
WO –SiO -Aero (lower).
3
2
Leaching of W was determined in randomly selected cat-
alytic runs by analyzing the reaction solution with ICP-MS
technique.
3. Results and discussion
3.1. Preparation
All xerogels were prepared via a sol–gel method under
basic conditions. Under the conditions reported under Exper-
imental, gelation for the individual oxides occurs in 4–5 min.
The simultaneous introduction of W precursor and the silica
precursors is not possible because of the very different gela-
tion rates that would lead to segregation of WO3 in the silica
matrix. This problem was solved by adding the W precur-
sor in the prehydrolyzed silicon alkoxide solution. This is an
important point for the achievement of mixed oxides with
high homogeneity and regular dilution of W centers within
the silica matrix. The preparation of highly transparent ho-
mogeneous gels is generally a good indication of a high W
dispersion, which is a key step for stabilizing the catalyst
against leaching.
All gels were aged in air for 72 h to yield vitreous xe-
rogels. They were subsequently dried in the oven at 110 ◦C
overnight to eliminate organic and inorganic solvents.
In the preparation of the aerogel sample, the prepara-
tion mixture was introduced into the autoclave without any
apparent gelation. In this case, supercritical solvent evapora-
tion is a fundamental step for obtaining a transparent gel.
All samples that did not contain surface organic groups
were also calcined at different temperature (200, 500,
700 ◦C).
Fig. 2. Typical DTA analyses for some representative samples: A, WO –
3
SiO -500; B, WO –SiO -40Me; C, WO –SiO -30ClPr.
2
3
2
3
2
3.2. Characterization
X-ray powder diffraction analysis indicated that all xe-
rogels were completely amorphous, even those calcined at
700 ◦C, a feature that is a reasonable indication that WO3 is
well dispersed in the silica matrix [3a,3b]. On the contrary,
the aerogel sample showed the presence of hexagonal WO3
(peaks at 14◦, 23◦, 28◦, 36◦, 51◦, and 56◦ in 2θ) in the amor-
phous matrix of silica (Fig. 1). This is a clear indication that
the low amount of W (1%) is not a limitation to detection, if
the WO3 phase is crystalline.
Some DTA profiles are shown in Fig. 2. For each sam-
ple we can roughly divide the explored temperature range
into two regions. The first one, from 50 to 250 ◦C, is dom-
inated by an exothermic, broad peak, most likely due to
loss of organic solvents; Fig. 2 indicates also that there are
no significant differences among the various samples. The
second region ranges from 250–300◦C to 600 ◦C and is
dominated by an exothermic band only in the case of the

F. Somma, G. Strukul / Journal of Catalysis 227 (2004) 344–351
347
Fig. 3. (A) N adsorption isotherms for some representative samples; (B) BJH pore-size distributions for some representative samples. (◦) WO –SiO -500;
2
3
2
(ꢀ) WO –SiO -40Me; (ꢀ) WO –SiO -30ClPr.
3
2
3
2
Table 2
between modified and nonmodified samples. An increase of
the chloropropyl content increases the amount of microp-
ores (in Fig. 3B sample WO3–SiO2-30ClPr is shown, i.e.,
the most microporous), while an increase of methylation de-
gree gives the opposite effect and the isotherm shifts the
hysteresis loop toward larger P/P0 values (in Fig. 2A WO3–
SiO2-40Me is shown). Also the aerogel sample showed an
isotherm typical of mesoporous materials. However, its pore-
size dimensions, while still in the range of mesopores, show
a very large distribution (50–800 Å).
As can be seen from Table 2, in the 3-Cl-propyl-modified
samples the overall surface area is strongly dependent on
the amount of organic functional groups. On the other hand,
their presence does not change the diameter of residual
mesopores (the only ones that can be revealed by the BJH
method), but decreases their amount. Fig. 3B shows that at
30% chloropropylation the amount of residual mesopores
is drastically decreased. Conversely, the presence of methyl
groups increases greatly the average pore diameter (41 to
108 Å) while at the same time decreasing the surface area
(820 to 661 m2/g).
Surface area and average pore diameter for different WO –SiO samples
3
2
Sample
Heating
BET
BJH average
pore diameter
(Å)
temperature
surface area
◦
2
( C)
(m /g)
WO –SiO -110
110
200
500
700
110
110
110
200
200
200
550
820
528
540
511
373
113
830
661
1434
41
41
41
40
38
37
42
53
108
206
3
2
WO –SiO -200
3
2
WO –SiO -500
3
2
WO –SiO -700
3
2
a
a
a
WO –SiO -10ClPr
3
2
WO –SiO -20ClPr
3
2
WO –SiO -30ClPr
3
2
WO –SiO -20Me
3
2
WO –SiO -40Me
3
2
WO –SiO -Aero
3
2
a
For these samples only the diameter of the residual fraction of meso-
pores is shown.
surface-modified samples; the latter band is most likely due
to decomposition of the surface organic groups. Interest-
ingly, the nonmodified sample (curve A) does not show any
evidence of the exothermic peak at ∼ 600 ◦C found by Deng
and co-workers in their samples [6a] and attributed to the
amorphous/crystalline transition.
The calcination temperature does not modify the average
pore diameter but shows a high influence on the surface area.
As can be seen from Table 2 there is a maximum for calci-
nation at 200 ◦C. Indeed a surface area of ∼ 800 m2/g is
not surprising for sol–gel silicas calcined at low tempera-
tures [13] and it is conceivable that it should be probably
even higher for WO3–SiO2-110. However, in view of the
DTA analysis reported in Fig. 2, the lower value observed
for WO3–SiO2-110 could be due to a less effective surface
solvent removal at 110 ◦C. The decrease in surface area ob-
served for samples calcined at higher temperatures is just a
normal effect due to surface hydroxylscondensation. Similar
effects have been previously observed in ZrO2–SiO2 mixed
oxides prepared by sol–gel methods [4a]. On the same DTA
Surface area and pore-size distribution were determined
from N2 adsorption/desorption isotherms and some typical
ones are shown in Fig. 3A. Those of the nonsurface-modified
and methyl-modified samples are type IV isotherms accord-
ing to IUPAC [10] and they show a hysteresis loop typical
of mesoporous solids. On the contrary, the chloropropyl-
modified samples are extensively microporous. Fig. 3B
shows also some typical pore size distributions for surface-
modified and nonmodified WO3-containing xerogels. This
is generally rather narrow and falls into the range of meso-
pores.
The BET [11] surface areas of the samples, along with
their average pore diameter calculated according to BJH
method [12] are reported in Table 2. We will now distinguish

348
F. Somma, G. Strukul / Journal of Catalysis 227 (2004) 344–351
Table 3
Reactivity of different WO –SiO catalysts in the oxidation of geraniol
3
2
with hydrogen peroxide
Entry Sample
Conversion (%)
Selectivity Spec.
epoxide
(%)
conversion
(%/m )
Geraniol H O
2
2
2
1
2
3
4
5
6
7
8
9
WO –SiO -110
69
61
68
79
83
100
100
100
95
95
90
92
75
18
90
94
80
31
44
52
88
55
92
0.125
0.074
0.128
0.146
0.117
0.163
0.504
0.097
0.092
0.044
3
2
WO –SiO -200
3
2
WO –SiO -500
3 2
WO –SiO -700
3 2
WO –SiO -10ClPr 60
3 2
Chart 1.
WO –SiO -20ClPr 61
3
2
WO –SiO -30ClPr 57
3
2
WO –SiO -20Me
81
64
64
3
2
WO –SiO -40Me
3
2
10
WO –SiO -Aero
90
3
2
Reaction conditions: catalyst, 10 mg; geraniol, 1 mmol; 35% H O ,
2
2
◦
1 mmol; temperature, 70 C; reaction time, 210 min.
all giving the corresponding diols upon hydrolysis. In all
cases we have observed only the oxidation of the 2–3 double
bond with the formation of the corresponding epoxyalcohol
and the subsequent ring opening due to hydrolysis and for-
mation of the corresponding diol. This behavior is typical
for tungsten and titanium chemistry [14,15], where the –OH
functionality is known to coordinate to the transition metal
center and olefin epoxidation occurs preferentially at the al-
lylic position. The reaction was carried out in the absence of
solvent, a condition that may be of interest with respect to
environmental issues.
Scheme 1.
basis, calcination of the surface-modified samples at temper-
atures higher than 200 ◦C is not recommended.
3.3. Oxidation of geraniol
Initial tests were carried out on plain WO3–SiO2 sam-
ples at 70 ◦C. In all cases reaction products were only the
epoxyalcohol and the corresponding diol (actually a triol);
no other products were observed. The catalysts calcined at
the different temperatures were tested. In Table 3 their con-
versions after 210 min, the selectivity to epoxide, and hydro-
gen peroxide consumption are reported. Conversion vs time
profiles are shown in Fig. 4A. As can be seen an increase
The oxidation of geraniol (Chart 1) with 35% hydrogen
peroxide was chosen as the reference reaction to compare
the behavior of the different catalysts. Geraniol shows inter-
esting properties, as it has two different double bonds: one is
in allylic position with respect to the –OH group, while the
other one (the 6–7 double bond) is isolated. In principle, dif-
ferent epoxidation products can be observed (see Scheme 1),
Fig. 4. (A) Conversion vs time plots for the oxidation of geraniol with hydrogen peroxide catalyzed by samples calcined at different temperatures. (B) Selec-
tivity to epoxide vs time plots for the oxidation of geraniol with hydrogen peroxide catalyzed by samples calcined at different temperatures.

F. Somma, G. Strukul / Journal of Catalysis 227 (2004) 344–351
349
in the initial rate with increasing calcination temperature is
Table 4
Effect of catalyst recycling in the oxidation of geraniol with hydrogen per-
observed, while from Table 3 it appears that there is also a
moderate tendency to increase the conversion. This is evi-
dent also if one considers the specific conversion calculated
per square meter of surface, a parameter that is more in-
dicative of the intrinsic catalytic properties of each sample.
Sample WO3–SiO2-110 is odd in this respect, as is indi-
cated by both Fig. 4A and Table 3. Again with the exception
of WO3–SiO2-110, in all cases the maximum conversion
occurs after about 120–130 min, while after this time the
reaction is practically finished as indicated by the full con-
sumption of H2O2. The change in selectivity to epoxide with
time is shown in Fig. 4B. As can be seen, in samples cal-
cined at 200 ◦C or above, a moderate decrease within the
first 100 min is observed and then the selectivity stabilizes
in the range 80–95%. The sample dried at 110 ◦C is again an
exception as the selectivity to epoxide decreases constantly
and after 210 min it is as low as 18% (Fig. 4B and Table 3).
The anomalous behavior of this sample in all respects could
probably be due to incomplete drying at 110 ◦C.
oxide catalyzed by WO –SiO -700
3
2
Recycle
Conversion
(%)
Epoxide
selectivity
(%)
Fresh
First
Second
Third
71
70
71
70
81
91
89
94
Reaction conditions: catalyst, 10 mg; geraniol, 1 mmol; 35% H O ,
2
2
◦
1 mmol; temperature, 70 C; reaction time, 120 min.
The presence of 3-Cl-propyl groups on the surface does
not result in significant improvements on the activity and
hydrogen peroxide consumption, if compared to the corre-
sponding nonmodified sample (i.e., WO3–SiO2-110). More-
over, an increase in the 3-Cl-propyl content does not seem
to have any influence on the conversion and hydrogen per-
oxide consumption (Table 3). Conversely, an analysis of the
specific conversion indicates a significant improvement as
the chloropropyl content increases (Table 3, entries 5–7),
as does the selectivity to epoxide which increases from 31
to 52%. Although the effect is moderate, it is in line with
previous findings on the oxidation of olefins with ZrO2–
SiO2 modified with methyl groups [4a] and is indicative
of a more favorable environment for the reaction to take
place.
If we now compare the behavior of the two methylated
samples with the corresponding unpromoted sample (Ta-
ble 3, entries 2, 8, and 9), it is clear that an improve-
ment in activity and selectivity can be achieved only at a
moderate methylation degree (20%, entry 8). The specific
conversion is practically similar in both methylated sam-
ples.
Overall, the results obtained in the epoxidation of geran-
iol seem to show that only a moderate hydrophilicity of the
surface is beneficial to achieve good conversions and se-
lectivities. These catalyst surface conditions can be better
achieved either by chloropropylation or by simple calcina-
tion. Indeed, 30% chloropropylation leads to higher specific
conversions but at the expenses of a drop in surface area and
a lower selectivity to epoxide. This observation partly differs
from the previous findings obtained with simple olefins [4],
but is in line with the moderate hydrophilicity of the sub-
strate which contains a –OH functional group. Therefore, the
general principle that “like adsorbs like” seems to hold even
in this case, at least in qualitative terms.
Fig. 5. Effect of WO –SiO -Aero catalyst recycling on the oxidation of
geraniol with hydrogen peroxide.
3
2
Some tests to check the dissolution of WO3 into the re-
action medium were carried out on some randomly selected
reaction mixtures after a 210-min reaction time. The catalyst
was filtered and new reagents were added to the solution.
The activity observed was negligible (1–2% conversion af-
ter 60 min) and this was confirmed also by ICP-MS analysis
that revealed only 2–3 ppm of W in the solution after filtra-
tion.
3.4. Catalyst recycling
Tests on catalyst recycling were carried out on WO3–
SiO2-700. The oxidation of geraniol was tested with a fresh
sample followed by a series of recycling using the solid iso-
lated from the previous reaction mixture by simple centrifu-
gation. The recovered solid was dried for 5 h in the oven at
110 ◦C and reused. A summary of the results obtained in the
different cycles is collected in Table 4. As can be seen, after
4 cycles there is no loss of activity (conversion), while the
selectivity to epoxide seems to benefit to some extent from
recycling.
A comparison with the aerogel sample (WO3–SiO2-
Aero) seems interesting. From Table 3, it appears that this
catalyst (entry 10) shows comparable catalytic performance
with the other nonmodified samples, but it loses very much

350
F. Somma, G. Strukul / Journal of Catalysis 227 (2004) 344–351
Table 5
Oxidation of allylic alcohols and olefins with hydrogen peroxide using dif-
ferent WO –SiO catalysts
3
2
Sample
Substrate
Reaction Con-
Epoxide Spec.
time
(min)
version selectiv- conversion
2
(%)
ity (%) (%/m )
WO –SiO -700
Nerol
120
210
210
210
63
60
53
55
84
30
32
40
0.117
0.117
0.142
0.487
3
2
WO –SiO -10ClPr Nerol
3
2
WO –SiO -20ClPr Nerol
3
2
WO –SiO -30ClPr Nerol
3
2
WO –SiO -110
Prenol
Prenol
60
40
60
60
60
47
28
37
34
36
73
43
71
56
37
0.085
0.052
0.072
0.091
0.318
3
2
WO –SiO -700
3
2
WO –SiO -10ClPr Prenol
3
2
WO –SiO -20ClPr Prenol
3
2
WO –SiO -30ClPr Prenol
3
2
WO –SiO -700
Cyclooctene
8
8
8
8
8
8
3
4
8
15
6
14
100
100
100
100
100
100
0.005
0.008
0.021
0.133
0.007
0.021
3
2
WO –SiO -10ClPr Cyclooctene
3
2
Fig. 6. Conversion/selectivity to epoxide vs time plots in the oxidation of
WO –SiO -20ClPr Cyclooctene
3
2
prenol with hydrogen peroxide using WO –SiO -700 as catalyst.
3
2
WO –SiO -30ClPr Cyclooctene
3
2
WO –SiO -20Me Cyclooctene
3
2
WO –SiO -40Me Cyclooctene
3
2
in activity upon recycling (Fig. 5). This corresponds to a
significant loss of tungsten (∼ 60%) during the first cycle.
Leaching of W is probably due to the presence of crystalline
WO3 in the structure as shown by XRD analysis (Fig. 1).
This result seems to support the idea that leaching can be
avoided only when the active component is well dispersed
and tightly held within the matrix. This is also in agreement
with the findings of Deng and co-workers [6a,6b] who ob-
served catalytic activity only when WO3 is amorphous and
well dispersed. It should be pointed out that the catalysts
used by the latter authors were prepared by simple impreg-
nation of Na2WO4 on silica and calcined at 550 ◦C or above.
This procedure prevents tungsten from leaching, but leads
to sintering of the amorphous WO3 particles during catalyst
use. This behavior was evident especially upon catalyst re-
cycling. This leads to a loss of activity and selectivity that
could be reverted by recalcination of the catalyst at 550 ◦C
[6a]. The preparation method reported in the present paper
seems to give more stable catalysts as WO3 entities are co-
valently bound to the support. In this way they can neither
dissolve in the reaction medium nor migrate to form larger
clusters.
Reaction conditions: catalyst, 10 mg; substrate, 1 mmol; H O , 1 mmol;
2
2
◦
reaction time, 120 min. Temperatures: nerol and cyclooctene, 70 C; prenol,
◦
40 C.
The epoxidation of nerol showed similar results with re-
spect to geraniol. No significant effects could be attributed
to the different structures of the two isomeric substrates.
The oxidation of prenol was carried out at a lower temper-
ature (40 ◦C) and for shorter times with respect to the other
allylic alcohols, because the selectivity to epoxide drops
quite dramatically with time (Fig. 6). After 80–100 min
the corresponding glycol is practically the only product ob-
served. The effect on selectivity is very similar to the one
shown in Fig. 4B for WO3–SiO2-110. Interestingly in this
case, the latter catalyst seems to be more selective than
WO3–SiO2-700, i.e., the opposite of what was observed
with geraniol. Also odd is the effect of chloropropylation
on the selectivity: While with geraniol or nerol an increase
in chloropropyl groups content increases the selectivity to
epoxide, with prenol the opposite occurs (Table 5). A possi-
ble rationale for this behavior might be found in the different
structure of the allylic alcohols; i.e., prenol does not have a
long aliphatic chain and therefore is less hydrophobic than
geraniol or nerol. This might explain why better results are
observed with the least hydrophobic catalyst surface, i.e.,
WO3–SiO2-110. The lack of reactivity with allyl alcohol
could be simply due to the fact that the absence of alkyl sub-
stituents at the C=C double bond makes it too electron poor
(insufficiently activated) for the reaction to proceed. This re-
sult, as well as the lack of activity toward 1-octene, is in line
with the well-known behavior observed in the oxidation with
W(VI) complexes and, more generally, d0 transition metal
peroxo complexes in solution [16]. In this case, the metal-
centered transformation consists in the nucleophilic attack
of the olefin onto an electron-poor peroxy oxygen formed by
interaction of the oxidant with the d0 transition metal center.
Support for this view is provided by the nature of the epoxy
3.5. Oxidation of other olefins
Other allylic alcohols (nerol, prenol, allyl alcohol) and
simple olefins (cyclooctene and 1-octene, all shown in
Chart 1) were tested in the oxidation with hydrogen per-
oxide. The catalysts chosen were WO3–SiO2-700 because
of its good results shown in the epoxidation of geraniol,
the chloromethylated samples because of the effect shown
on the selectivity, and the methylated samples, only in the
case of the most hydrophobic substrates (cyclooctene and
1-octene). Results are summarized in Table 5. Allyl alcohol
and 1-octene gave no conversion under the usual experimen-
tal conditions.

F. Somma, G. Strukul / Journal of Catalysis 227 (2004) 344–351
351
alcohols obtained from geraniol and nerol. As indicated by
Chart 1, the latter substrates are isomers and the correspond-
ing epoxides are also isomers. A metal-centered oxidation
similar to that observed with soluble species should lead,
with either substrate, to the formation of a single epoxide
isomer, not to a mixture of both as observed with a radical-
type mechanism. This is indeed the case as the trans olefin
(geraniol) gives the trans epoxide, while the cis olefin (nerol)
gives the cis epoxide.
In the oxidation of cyclooctene, epoxide is the only prod-
uct. This is no surprise as cyclooctene epoxide is quite stable
to hydrolysis. Although conversions are very low, the in-
fluence of the functional groups (3-Cl-propyl and methyl)
is clear and similar to previous observations with surface-
modified ZrO2–SiO2 samples [4a].
Physical Chemistry, University of Venice for X-ray diffrac-
tion analysis.
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Acknowledgments
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We thank MIUR, Rome (PRIN 2003), and Consorzio
INSTM, Firenze, for financial support for this work. We also
express our gratitude to Dr. Patrizia Canton, Department of
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