Iron(III)-substituted polyoxotungstates immobilized on silica nanoparticles: Novel oxidative heterogeneous catalysts

Article abstract of DOI:10.1016/j.catcom.2010.11.005

Silica nanoparticles supporting polyoxometalates (POMs), namely an iron(III) mono-substituted Keggin-type polyoxotungstate of formula α-[PW₁₁Fe^III(H₂O)O₃₉] ^4- and a sandwich-type tungstophosphate with the formula B-α-[(PW₉O₃₄)₂Fe^III ₄(H₂O)₂]^6- were synthesized. The POM/SiO₂ nanocomposites were obtained by alkaline hydrolysis of tetraethoxysilane using a reverse micelle and sol-gel technique. The spectroscopic studies suggest that the POMs were successfully immobilized on the silica nanoparticles. The catalytic activity of POM/SiO₂ nanomaterials was tested in the epoxidation of geraniol using H ₂O₂ as oxygen donor. The α-[PW₁₁Fe ^III(H₂O)O₃₉]^4-/SiO₂ nanocomposite was the most efficient catalyst with high geraniol conversion and good regioselectivity for 2,3-epoxygeraniol.

Full text of DOI:10.1016/j.catcom.2010.11.005

Catalysis Communications 12 (2011) 459–463
Contents lists available at ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
Iron(III)-substituted polyoxotungstates immobilized on silica nanoparticles: Novel
oxidative heterogeneous catalysts
a
b
b
b
Joana L.C. Sousa , Isabel C.M.S. Santos , Mário M.Q. Simões , José A.S. Cavaleiro ,
a,
a,
Helena I.S. Nogueira ⁎, Ana M.V. Cavaleiro ⁎
CICECO, Department of Chemistry, University of Aveiro, 3810-193 Aveiro, Portugal
a
b
QOPNA, Department of Chemistry, University of Aveiro, 3810-193 Aveiro, Portugal
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 19 July 2010
Received in revised form 22 October 2010
Accepted 3 November 2010
Available online 11 November 2010
Silica nanoparticles supporting polyoxometalates (POMs), namely an iron(III) mono-substituted Keggin-type
III
4−
polyoxotungstate of formula α-[PW11Fe (H
2
O)O39
]
and a sandwich-type tungstophosphate with the
nanocomposites were obtained by
III
6−
formula B-α-[(PW
9
O
34
2
) Fe
4
(H
2
O)
2
]
were synthesized. The POM/SiO
2
alkaline hydrolysis of tetraethoxysilane using a reverse micelle and sol–gel technique. The spectroscopic
studies suggest that the POMs were successfully immobilized on the silica nanoparticles. The catalytic activity
of POM/SiO
2
2 2
nanomaterials was tested in the epoxidation of geraniol using H O as oxygen donor. The α-
Keywords:
III
4−
[PW11Fe (H
2
O)O39
]
2
/SiO nanocomposite was the most efficient catalyst with high geraniol conversion and
Polyoxometalates
Silica nanoparticles
Heterogeneous catalysis
Epoxidation
good regioselectivity for 2,3-epoxygeraniol.
© 2010 Elsevier B.V. All rights reserved.
Hydrogen peroxide
Geraniol
1
. Introduction
ces for several applications [8–21], including heterogeneous oxidative
catalysis using silica nanoparticles [10,11,14]. However, to our
knowledge, only our group has prepared silica nanoparticles as a
support for POMs by the present method [16,21].
Following our previous studies in oxidative catalysis with
hydrogen peroxide using transition metal-substituted POMs as
homogeneous catalysts [22–25], we report here the development of
The most important reasons for the on-going research on
oxidation reactions are the effective transformation of cheap natural
compounds into valuable intermediates for organic synthesis as well
as the development of environmentally benign oxidation procedures
[1]. Recently, there has been an increase in research for new catalytic
processes for the production of fine chemicals from renewable
resources [2]. Geraniol is an example of a monoterpene available
from essential oils of some plants, widely used as a fragrance in
several commercially available cosmetics and toiletries [3]. This allylic
alcohol offers several possible sites of oxidative attack, namely at the
two double bonds, at the allylic carbon centres and at the carbon of
new heterogeneous catalysts based on POM/SiO
polyoxometalates investigated were an iron(III) mono-substituted
Keggin-type polyoxotungstate of formula α-[PW11Fe (H
2
nanomaterials. The
III
4−
2
O)O39]
(abbreviated to PW11Fe) and a sandwich-type tungstophosphate
III
6−
with the formula B-α-[(PW
(PW Fe ). The catalytic activity of these nanocomposites was
studied in the epoxidation of geraniol using H as the oxidizing
agent and CH CN as solvent, in a heterogeneous system.
9
O
34
)
2
Fe
4
(H
2
O)
2
]
(abbreviated to
9
)
2
4
2
the CH OH group.
2 2
O
Transition metal-substituted POMs are efficient catalysts for the
oxidation of organic compounds in homogeneous conditions [4–6].
The immobilization of POMs in solid supports brings advantages to
the oxidation reactions, in particular the easier catalyst recovery and
its possible reutilization in new catalytic cycles. Silica is one of the
most attractive supports, because it is chemically inert, thermally
stable, harmless and inexpensive [7]. Some recent papers report the
incorporation of transition metal-substituted POMs into silica matri-
3
2. Experimental
2
2.1. Synthesis of POM/SiO nanocomposites
The potassium salts of the POMs (K-POM) were prepared by
III
4−
adaptation of known procedures: [PW11Fe (H
2
O)O39
[27] (Appendix A).
The following procedure for the preparation of POM/SiO
]
PW11Fe [26]
III
6−
and [(PW O
9 34
)
2
Fe
4
(H
2
O)
2
]
9 2 4
(PW ) Fe
2
⁎
Corresponding authors. Nogueira is to be contacted at Tel.: +351 234 370 727; fax:
nanocomposites was adapted from the literature [21]. A microemul-
sion containing 2.38 g of Triton X-100, 1.88 g of 1-octanol, 7.25 g of
cyclohexane, 233 μL of TEOS (tetraethoxysilane) and 60 mg of K-POM
+351 234 370 084. Cavaleiro, Tel.: +351 234 370 734; fax: +351 234 370 084.
E-mail addresses: helenanogueira@ua.pt (H.I.S. Nogueira), anacavaleiro@ua.pt
(A.M.V. Cavaleiro).
1566-7367/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2010.11.005

460
J.L.C. Sousa et al. / Catalysis Communications 12 (2011) 459–463
Table 1
was recovered, washed with ethanol or acetonitrile and dried under
vacuum.
For the reactions in homogeneous phase, a typical oxidation
procedure was performed as follows: the catalyst (TBA-PW11Fe,
Spectroscopic data (FT-IR and FT-Raman) for the POMs used as starting materials and
the corresponding silica nanocomposites.
Compound
Vibrational spectra (cm−1)a
3
9 2 4
.82 μmol or TBA-(PW ) Fe , 2.08 μmol; TBA- indicates that the
ν(P―O)
ν(W=O)
ν(W―O―W)
tetrabutylammonium salt of the POM, prepared as in Appendix A,
was used) dissolved in acetonitrile (1.8 mL), substrate (1.2 mmol)
and 30 wt.% H O (3.6 mmol) were placed in a closed reactor, and the
2 2
mixture stirred at r.t. in the absence of light. After 3 or 5 h of reaction,
an aliquot of 1 μL was taken directly from the reaction mixture with a
microsyringe and injected into the GC.
K-PW11Fe
1086 s, 1058 s
961 vs
994 vs, 980 vs
950 m
892 s, 808 vs, 731 vs
906 m, 848 w, 805 sh
880 m, 737 m
870 m, 860 m, 804 sh
917 sh,722 sh
883 sh, 864 sh, 807 sh
875 vs, 805 vs, 734 vs
896 m, 884 sh, 808 vw
879 m, 738 m
1
053 vw
PW11Fe/SiO
PW11Fe/SiO
2
2
969 sh, 949 vs
954 m
965 vs, 948 sh
951 vs
970 vs
952 m
966 vs
952 m
969 vs
after catalysis
K-(PW Fe
)
9 2
4
1054 vs, 1035 sh
1
044 vw
(PW
9
)
2
Fe
4
/SiO
2
2
3. Results and discussion
1
045 vw
880 w, 800 vw
878 m, 738 m
887 w, 810 vw
(PW
9
)
2
Fe
4
/SiO
2
3.1. Preparation and characterization of POM/SiO nanomaterials
after catalysis
a
Raman data in italic; v: very, s: strong, m: medium, w: weak, sh: shoulder.
The silica nanoparticles supporting the potassium salts of the
III
4-
III
6-
polyoxoanions [PW11Fe (H
2
O)O39
]
9 34 2 4 2 2
and [(PW O ) Fe (H O) ]
(
potassium salt of the respective POM) dissolved in 1.16 mL of water
were prepared by alkaline hydrolysis of TEOS, using the sol–gel
method within reverse micelles. This is a system based on water-in-oil
(w/o) microemulsions [21], each microemulsion being composed by
was prepared. Another microemulsion containing 2.38 g of Triton X-
1
2
00, 1.88 g of 1-octanol, 7.25 g of cyclohexane and 233 μL of aqueous
5 wt.% NH was also prepared. After vigorous stirring for 30 min, the
3
3
an aqueous phase (NH 25 wt.% solution in water or an aqueous
two microemulsions were mixed and the resulting mixture was
stirred at r.t. for 48 h. Acetone was then added and a precipitate was
obtained. The mixture was kept standing for 24 h. The solid obtained
solution of polyoxoanion and TEOS), a surfactant (Triton X-100), a co-
surfactant (1-octanol) and an organic phase (cyclohexane) [28].
Acetone was added in order to disaggregate the micelles and to
promote the precipitation of a solid with variable colour depending on
the supported POM. These nanosized materials were characterized
by FT-IR and FT-Raman spectroscopy and by transmission electron
microscopy (TEM).
2
was centrifuged, washed with ethanol and ethanol:H O (4:1 v/v), and
dried under vacuum.
Silica nanoparticles containing 43.6 wt.% of K-PW11Fe and 41.3 wt. %
of K-(PW Fe were obtained by this method.
)
9 2
4
The presence of polyoxotungstates in the POM/SiO
sites is shown by several bands in the 700–1100 cm
2
nanocompo-
range in the
−
1
2
.2. Oxidation procedure
infrared and Raman spectra (Table 1, and Figs. 1 and 2). The infrared
spectrum of nanosized silica (Fig. 1c) prepared in the absence of POM
Geraniol and 30 wt.% H
2 2
O were purchased from Aldrich and
Riedel-de Häen, respectively, and used as received.
(by the same method as the POM/SiO
characteristic bands at 1095, 957, 798 and 468 cm assigned to νas
(Si―O―Si), ν(Si―OH), ν (Si―O―Si) and δ(Si―O―Si), respectively
[29]. The infrared spectra of the POM/SiO nanomaterials are
2
nanocomposites) shows four
−1
Heterogeneous reactions were performed as follows: POM/SiO
2
nanomaterials (25 mg) dispersed in acetonitrile (1.8 mL), substrate
1.2 mmol) and 30 wt.% H (3.6 mmol) were placed in a closed
s
(
O
2 2
2
reactor, and the mixture stirred at r.t. in the absence of light. At regular
intervals, the reaction mixture was centrifuged and an aliquot of 1 μL
was taken directly from the supernatant with a microsyringe and
injected into the gas chromatograph (GC). At the end of the reaction,
the reaction mixture was centrifuged and the heterogeneous catalyst
dominated by the intense and broad bands of silica, which prevent
the observation of most of the POM characteristic vibrational modes,
as shown in Fig. 1b. The opposite is observed in the Raman spectra
(Fig. 2), in which the bands of the POM supported on silica are clearly
visible and silica does not show any signal in the 1100–100 cm−
1
2 9 2 4
Fig. 1. FT-IR spectra of K-POM (a), POM/SiO nanocomposite (b) and nanostructured silica (c), for the POMs PW11Fe and (PW ) Fe .

J.L.C. Sousa et al. / Catalysis Communications 12 (2011) 459–463
461
2 9 2 4
Fig. 2. FT-Raman spectra of K-POM (a) and corresponding POM/SiO nanomaterial (b), for the POMs PW11Fe and (PW ) Fe .
region. The POM bands are assigned to the terminal W=O stretches
materials before and after catalysis (Fig. 4A and Table 1) are very
similar, which indicates that the materials are not destroyed during
−1
(
WO
949–994 cm ) and W―O―W stretching modes in edge-shared
−1
6
octahedra (701–894 cm ) [30] as shown in Table 1. Overall,
catalysis neither by leaching of the POM from SiO
decomposition. This is confirmed by the infrared spectra (Fig. 4B), and
also by the TEM image of the (PW Fe /SiO nanocomposite after
2
support or by POM
these results suggest that the polyoxotungstates are effectively
immobilized on silica nanoparticles. Structure maintenance of the
POM on the silica support could not be completely demonstrated by
9
)
2
4
2
catalysis (Fig. 4C) that shows that the morphology is identical to that
of the starting material (Fig. 3A).
the vibrational spectra. Though in the case of (PW
strong similarities in the Raman spectra of both starting POM and
POM/SiO material (Fig. 2), for PW11Fe there is a shift in the position
9 2 4
) Fe there are
2
3.2. Catalytic studies
of the most intense bands.
Fig. 3 shows TEM images of the two silica nanomaterials with
The properties of the two prepared POM/SiO
catalysts were evaluated in the oxidation of geraniol with H O
2
nanocomposites as
. It was
immobilized (PW
particles have a spherical form and similar diameters (between 25
and 35 nm). In the nanocomposites (PW Fe /SiO (Fig. 3A) the POM
is assigned to the inner dark core, which is encapsulated by the silica
forming core/shell nanoparticles. In PW11Fe/SiO the POM is scattered
in the nanoparticles (Fig. 3B). Energy dispersive X-ray spectroscopy
EDX) confirmed the presence of tungsten in the two POM/SiO
nanomaterials.
Raman spectroscopy was also used to characterize the recovered
material after catalysis. The Raman spectra for both POM/SiO
)
9 2
Fe
4
or PW11Fe, respectively. These silica nano-
2 2
found by GC analysis that geraniol is preferentially epoxidized at the
2,3-position yielding 2,3-epoxygeraniol as the main product. Other
minor products were observed, namely the 6,7-epoxide and the
diepoxide (Scheme 1). The results are presented in Table 2. The
highest catalytic activity was obtained in the presence of the PW11Fe/
9
)
2
4
2
2
(
2
SiO
81% of conversion after 5 h of reaction was obtained with (PW
SiO . In the absence of catalyst but in the presence of the support
(nanostructured silica), the epoxidation of geraniol led to 12% of
2
nanomaterial (96% of conversion after 3 h of reaction), whereas
)
9 2
Fe /
4
2
2
9 2 4 2 2
Fig. 3. TEM images of (PW ) Fe /SiO (A) and PW11Fe/SiO (B) nanocomposites.

462
J.L.C. Sousa et al. / Catalysis Communications 12 (2011) 459–463
Fig. 4. FT-Raman (A) and FT-IR (B) spectra of (PW
)
9 2
Fe
4
/SiO
2
before (a) and after (b) the first catalytic cycle, and TEM image (C) of this material after catalysis.
Scheme 1.
conversion after 3 h of reaction. The selectivity obtained for the 2,3-
2
epoxygeraniol was 88% and 91% for PW11Fe/SiO and (PW Fe /SiO ,
respectively. Catalyst nanomaterials were reused through two other
catalytic runs and continued to show catalytic activity.
This heterogeneous catalyst presents a core/shell structure (Fig. 3A),
which remained unchanged after the catalytic reactions (Fig. 4C). The
spectroscopic studies (Figs. 4A and B) suggest that the POM in the
silica nanocomposites was not destroyed and there is no evidence of
POM leaching during the oxidation process under the conditions used
in our catalytic studies.
2
9
)
2
4
The consumption of H
presented in Table 2. Overall, about 60–70% of theinitial added H
not consumed. However, when the stoichiometric quantities of H
2
O
2
during the oxidation of geraniol is also
2 2
O
was
O
2 2
In Fig. 5, the evolution of geraniol epoxidation in the presence
were used, the conversion of geraniol was very low, reaching conversion
values of 11% and 20% after 3 h and 5 h of reaction, respectively.
There are only a few reports on the oxidation of geraniol in the
presence of transition metal-substituted POMs, where 2,3-epoxygeraniol
is generally obtained, along with high conversion values [22,31–35].
It is possible to compare the catalytic results obtained in this
heterogeneous system with the results achieved with the tetrabuty-
lammonium (TBA) salts of the same anions in homogeneous systems,
in identical reaction conditions. In the case of the heterogeneous
2
of the PW11Fe/SiO catalyst is compared with that observed after
removal of the catalyst after 15 min of reaction. There is a large
Table 2
Catalytic results for the epoxidation of geraniol with H
2
O
2
a.
Catalyst
Time Conversion 2,3-epoxide
2 2
H O
Consumed
(%)
b
(h)
(%)
Selectivity
(%)
catalyst (PW
9
)
2
Fe
4
/SiO
2
, a significant selectivity improvement oc-
PW11Fe/SiO2
TBA-PW11Fe
3
3
3
5
5
5
3
5
3
5
96
98
11
20
81
99
12
17
12
21
88
83
100
100
91
81
69
73
68
30.9± 1.9
33.5± 2.2
curred over the behaviour of the corresponding TBA salt (Table 2).
To evaluate the possible effect of oxygen in the head-space reactor,
tests under argon atmosphere were carried out in the reaction
catalysed by PW11Fe/SiO . Similar results were obtained, in compar-
2
ison with those in the presence of air. Thus, the possibility of
autoxidation appears to be ruled out. The epoxidation of geraniol by
c
PW11Fe/SiO
2
–
28.1± 0.5
35.5± 1.1
51.1± 1.4
–
–
–
–
(
PW
9
)
2
Fe
4
/SiO
Fe
2
TBA-(PW
None
9
)
2
4
(
CH
None
SiO +CH
3
CN+geraniol+H
2
O
2
)
2 2 2
H O catalysed by PW11Fe/SiO does not seem to be a radical process,
(
2
3
CN+geraniol+H
2
O
2
)
77
because the addition of about 1.0 mmol of 2,6-di-tert-butyl-4-
methylphenol (BHT), a radical scavenger, did not inhibit the
formation of any product in the epoxidation of geraniol.
a
Reaction conditions: heterogeneous (POM/SiO
PW11Fe, 3.82 μmol; or TBA-(PW Fe , 2.08 μmol) catalyst, CH
1.2 mmol), 30 wt.% H (3.6 mmol), were stirred at r.t. and in the absence of light.
Based on the amount of consumed substrate;
2
, 25 mg) or homogeneous (TBA-
9
)
2
4
3
CN (1.8 mL), substrate
(
2 2
O
The solid recovered after the first catalytic cycle with (PW
)
9 2
4
Fe /
b
c
SiO was characterized by TEM, FT-Raman and FT-IR spectroscopy.
2
2 2
H O /substrate molar ratio of 1.

J.L.C. Sousa et al. / Catalysis Communications 12 (2011) 459–463
463
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Acknowledgments
Thanks are due to the University of Aveiro and FCT/FEDER for
funding CICECO and QOPNA.