Hybrid polyoxotungstate/MlL-101 materials: Synthesis, characterization, and catalysis of H2O2-based alkene epoxidation

Article abstract of DOI:10.1021/ic902459f

Polyoxotungstates [PW₄O₂₄ ]³(PW ₄) and [PW₁₂O₄₀]³- (PW₁₂) have been Inserted Into nanocages of the metal organic framework MIL-101. The hybrid materials PW_x/MIL-101 (x= 4 or 12) containing 5-14 wt % of polyoxotungstate have been obtained and characterized by elemental analysis, N₂ adsorption, FT-IR, Raman, and ³¹P NMR MAS spectroscopic techniques. Their catalytic performance was assessed In the selective oxidation of alkenes with aqueous hydrogen peroxide under mild reaction conditions ([H₂O₂] = 0.1-0.2 M, 50 °C, MeCN). PW _x/MIL-101 enclosing 5 wt % of polyoxotungstate demonstrated fairly good catalytic activities in the epoxldation of various alkenes (3-carene, llmonene, α-pinene, cyclohexene, cyclooctene, 1-octene), the turnover frequencies (TOF) and alkene conversions were close to the corresponding parameters achieved with homogeneous PW_x. For the oxidation of substrates with aromatic groups (styrene, cis- and trans-stilbenes), a higher level of olefin conversion was attained using PW₁₂/MIL-101. Moreover, confinement of PW₁₂ within MIL-101 nanocages allowed us to reach higher epoxide selectlvitles at higher alkene conversions. The hybrid PW _x/MIL-101 materials were stable to leaching, behaved as true heterogeneous catalysts, were easily recovered by filtration, and reused several times with the maintenance of the catalytic performance.

Full text of DOI:10.1021/ic902459f

2920 Inorg. Chem. 2010, 49, 2920–2930
DOI: 10.1021/ic902459f
Hybrid Polyoxotungstate/MIL-101 Materials: Synthesis, Characterization, and
Catalysis of H2O2-Based Alkene Epoxidation
Nataliya V. Maksimchuk,† Konstantin A. Kovalenko,‡ Sergey S. Arzumanov,† Yurii A. Chesalov,†
Maxim S. Melgunov,† Alexander G. Stepanov,† Vladimir P. Fedin,‡ and Oxana A. Kholdeeva*,†
†Boreskov Institute of Catalysis, Lavrentiev Avenue 5, Novosibirsk, 630090, Russia, and ‡Nikolaev Institute of
Inorganic Chemistry, Lavrentiev Avenue 3, Novosibirsk, 630090, Russia
Received December 11, 2009
Polyoxotungstates [PW4O24]3- (PW4) and [PW12O40]3- (PW12) have been inserted into nanocages of the metal organic
framework MIL-101. The hybrid materials PWx/MIL-101 (x = 4 or 12) containing 5-14 wt % of polyoxotungstate have been
obtained and characterized by elemental analysis, N2 adsorption, FT-IR, Raman, and 31P NMR MAS spectroscopic
techniques. Their catalytic performance was assessed in the selective oxidation of alkenes with aqueous hydrogen peroxide
under mild reaction conditions ([H2O2] = 0.1-0.2 M, 50 °C, MeCN). PWx/MIL-101 enclosing 5 wt % of polyoxotungstate
demonstrated fairly good catalytic activities in the epoxidation of various alkenes (3-carene, limonene, R-pinene, cyclohexene,
cyclooctene, 1-octene), the turnover frequencies (TOF) and alkene conversions were close to the corresponding parameters
achieved with homogeneous PWx. For the oxidation of substrates with aromatic groups (styrene, cis- and trans-stilbenes), a
higher level of olefin conversion was attained using PW12/MIL-101. Moreover, confinement of PW12 within MIL-101
nanocages allowed us to reach higher epoxide selectivities at higher alkene conversions. The hybrid PWx/MIL-101 materials
were stable to leaching, behaved as true heterogeneous catalysts, were easily recovered by filtration, and reused several
times with the maintenance of the catalytic performance.
Introduction
tation of catalytic processes which eliminate the use of
hazardous reactants and reduce generation of waste is an
important goal.6-8
Hydrogen peroxide has attracted great attention as
an environmentally benign oxidant because it is quite
cheap, readily available, easy to handle, and gives water as
the only co-product.10-13 Tungsten compounds are well-
known as active and selective catalysts for alkene epoxi-
dation with aqueous H2O2.8,14-21 In the mid of 1980s,
highly effective systems based on the use of [PW4O24]3-
Alkene epoxidation is one of the basic reactions in
industrial organic synthesis because epoxides are key
intermediates for the manufacture of a wide variety of
valuable products.1-8 The selective epoxidation of alkenes
is widely used in the fine chemicals industry and can be
successfully achieved by using stoichiometric amounts of
peroxy acids.9 However, the development and implemen-
*To whom correspondence should be addressed. E-mail: khold@
catalysis.ru. Fax: þ(7)-(383)-330-9573.
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Published on Web 02/18/2010
2010 American Chemical Society

Article
Inorganic Chemistry, Vol. 49, No. 6, 2010 2921
(PW4) and [PW12O40]3- (PW12) polyoxometalates (POMs)
were developed independently by the Venturello22-23 and
Ishii24-26 groups. The major drawback of the Venturello-
Ishii epoxidation systems is the use of toxic chlorocarbons
as solvents to obtain high reaction rates and yields of
epoxides. An improved ternary catalytic system for the
selective epoxidation of simple alkenes has been suggested
by Noyori and co-workers.27,28 This entirely free of or-
ganic solvents and halides system consists of Na2WO4,
(aminomethyl)phosphonic acid and QþHSO4-. The use of
lipophilic quaternary ammonium hydrogen sulfate significantly
enhances the yield of epoxide. However, this catalytic system
can not be employed for the production of acid sensitive
epoxides because epoxide ring-opening occurs under acidic
conditions. Furthermore, both the Venturello-Ishii and
Noyori systems have drawbacks typical of homogeneous cat-
alysts, such as complexity of the catalyst separation from the
reaction mixtures and contamination of products with traces of
transition metals (even if two-phase conditions are employed).
Immobilization of active homogeneous catalysts on solid
supports has attracted a lot of research interest because solid
catalysts have the advantages ofbeing easier to recoverand to
recycle. Many research papers were dedicated to immobiliza-
tion of the 12-tungstophosphoric acid, H3PW12O40, on
different solid supports, such as amorphous silica,29-41
mesostructured silicates,31,42-47 layered double hydroxides
(LDH),48,49 active carbon,50 metal oxides,29,51-54 and poly-
mers.55,56 Silica has been widely used as supporting material
for PW12 since it interacts weakly with the PW12 anion,
thereby preserving its structure in the supported form. How-
ever, PW12 immobilized by weak interaction is easily leached
out of the support in polar reaction media.39-41 On the
contrary, strong interaction, for example, with alumina, may
result in PW12 destruction in the course of immobilization.54
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catalysts,44,45,47,53 but a few examples of using immobilized
PW12 in alkene epoxidation with H2O2 were also reported.
Thus, LDH intercalated PW12 was found to be almost
inactive in cyclohexene epoxidation with 10% H2O2.49 On
the other hand, the catalyst prepared by supporting PW12 on
mesoporous silica preliminary modified with Ph3SiOEt and
Me3NCH(OCH2Ph)2 groups produced epoxides from differ-
ent alkenes with the yields up to 97% using aqueous 15%
H2O2 without an organic solvent.33 However, no data on the
catalysts stability, recyclability, metal leaching, and the
nature of catalysis were reported for this system.
Different low nuclearity oxo- and peroxotungstic species,
such as WO4 , , , ,
- 57 WO53- 58-61 W7O246- 62 [W2O3(O2)4]2- 63
and [(HPO4){WO(O2)2}2]2- (PW2),64-66 were immobilized
on solid supports by anion exchange with LDH57,62 or
Amberlyst A26,65 adsorption onto silica supports64-66 and
binding with organophosphorous groups on the surface of
functionalized silica,58,63 resin,59 hydroxyapatite59 or porous
polymers.61 A mixed-valence oxotungsten-silica mesoporous
W-SBA-15 material prepared by the sol-gel technique
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2922 Inorganic Chemistry, Vol. 49, No. 6, 2010
Maksimchuk et al.
were found to be liable to tungsten leaching. The W-SBA-15
catalyst was reused successfully just once;67 moreover, the
“hot catalyst filtration test”68 was not reported, and, hence,
the nature of catalysis remained unclear.
immobilization of homogeneous catalysts.77-87 In 2005,
ꢀ
Ferey and co-workers reported the synthesis of the mesopor-
ous chromium terephthalate coordination polymer MIL-101
which possesses a rigid zeotype crystal structure, extremely
large surface area and quasi-spherical cages of two modes
(free internal diameters are close to 29 and 34 A, while the
cages are accessible through microporous windows of about
12 and 16 A).88 Importantly, this material is resistant to
air, water, common solvents, and thermal treatment (up to
By far, only few true heterogeneous and recyclable cata-
lysts have been published for H2O2-based alkene epoxida-
tion, and the elaboration of such catalysts is still a challenging
goal. Several attempts to heterogenize the Venturello com-
plex, PW4, have been performed, including anion exchange
with Amberlite IRA-900,15,69-71 electrostatic binding to a
silica xerogel covalently modified with phenyl and quatern-
ary ammonium groups,72 and “solvent-anchored” support-
ing technique when covalently attached to silica surface
polyethers acted as a solvent and/or complexing agents for
POM.73 The “solvent-anchored” and xerogel-attached PW4
catalyzed selectively cyclooctene epoxidation with 30% aqu-
eous H2O2 without organic solvent at room temperature.72,73
The PW4-Amberlite catalyst showed fairly good selectivity
in alkene epoxidations with H2O2 (93 and 97% selectivities
were reported for limonene and 3-carene at 84 and 55%
conversions, respectively, after 24 h at 38 °C).69 Both the
“solvent-anchored” and PW4-Amberlite catalysts were re-
cycled several times without loss of the activity, and the
results of the hot catalyst filtration experiments pointed out
true heterogeneous nature ofcatalysis. However, it was found
that epoxide product deactivates the PW4-Amberlite cata-
lyst and totally inhibits the reaction at 160 TON.71
320 °C).76,88 Ferey and co-workers demonstrated a successful
ꢀ
incorporation of the lacunary heteropolytungstate K7PW11O39
within nanocages of MIL-101.88
Recently, we have first preparedcomposite materialsbased
on the MIL-101 matrix and redox active Keggin heteropo-
lyanions, [PW11CoO39]5- and [PW11TiO40]5-, and explored
their catalytic performances in the allylic oxidation of alkenes
with O2 and H2O2, respectively.89 Both composite materials
were proven to be stable to POM leaching, behaved as true
heterogeneous catalysts, and could be used repeatedly with-
out suffering a loss of the activity and selectivity under mild
reaction conditions. Recently, MIL-101 was used as a stable
support for palladium in the Knoevenagel condensation,90
Heck coupling reaction,86 and alkene hydrogenation.91
SO3H-functionalized MIL-101 was active as acid catalyst
in the esterification of acetic acid with ethanol,86 while
L-proline-modified MIL-101 showed remarkable catalytic
activities in asymmetric aldol reactions.92 Yet, MIL-101 with
coordinatively unsaturated chromium sites (after heating
under vacuum at 150 °C) was found to be active in cyanosily-
lation,91 selective oxidation of tetralin to 1-tetralone with
TBHP,93 and sulfoxidation of thioethers to sulfoxides with
H2O2.94 Very recently, a direct encapsulation of phospho-
tungstic acid into MIL-101 by the addition of PW12 during
the synthesis of MIL-101 was reported.95 These materials
were found to be active in acid-catalyzed reactions, such as
the Knoevenagel condensation of benzaldehyde with ethyl
cyanoacetate, esterification of n-butanol with acetic acid, and
dehydration of methanol to dimethylether.
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and so forth.74-77 All these properties allow considering
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Article
Inorganic Chemistry, Vol. 49, No. 6, 2010 2923
epoxidation with aqueous H2O2 and compared with the
catalytic performance of the corresponding homogeneous
POM catalysts. The critical issues on the catalysts stability,
reusability, and the nature of catalysis have been addressed.
Method B (with Catalyst Pre-Treatment). A mixture contain-
ing PW12/MIL (28 mg, 4.7 ꢀ 10-4 mmol [PW12O40]3-), H2O2
(0.2 mmol, 510 equiv with respect to PW12), internal standard
(biphenyl), and acetonitrile (1 mL) was stirred for 1 h at 50 °C.
Then the oxidation reaction was started by the addition of
cyclohexene (0.1 mmol).
Experimental Section
Method C (One-Pot Catalyst Preparation and Catalytic Oxi-
dation). To a solution of H3PW12O40 (1.4 mg, 4.7 ꢀ 10-4 mmol)
in MeCN (1 mL) was added MIL-101 (28 mg) under vigorous
stirring. After stirring the solution at 25 °C for 30 min, the
oxidation reaction was started by adding cyclohexene (0.1
mmol) and H2O2 (0.2 mmol) followed by heating the reaction
mixture to 50 °C.
Aliquots of the reaction mixture were withdrawn periodically
during the reaction course by a syringe through a septum. Each
experiment was reproduced at least 2-3 times. After the reac-
tions, catalysts were filtered off, washed with acetonitrile,
methanol, dried in air at room temperature overnight, and then
reused.
Materials. Hydrogen peroxide was used as 30 wt % solution
in water; its concentration was determined iodometrically prior
to use. Heteropolyacid H3PW12O40 6H2O was a commercial
3
product and was purified by extraction with diethyl ether. The
compound purity was confirmed by 31P NMR (a single peak at
-15.0 ppm in H2O). All other reactants were obtained commer-
cially and used without further purification.
Synthesis of MIL-101. MIL-101 was prepared by a hydro-
thermal reaction of terephthalic acid with Cr(NO3)3 9H2O, HF,
3
and H2O following the procedure reported by Ferey et al.88 In a
ꢀ
typical synthesis, chromium nitrate (1.2 g, 3 mmol), terephthalic
acid (500 mg, 3 mmol), 3 M HF (1 mL, 3 mmol), and H2O (15
mL, 0.833 mol) were mixed in a Teflon linear and kept at 220 °C
during 6 h. The crystalline MIL-101-as product in the solution
was twice filtered off using a glass filter with the pore size 160 μm
to remove free terephthalic acid and then through a paper filter.
The purification of the rude MIL-101-as product was carried
out in four steps: double treatment in N,N-dimethylformamide
(DMF) at 60 °C during 6 h and then double treatment in hot
ethanol at 70 °C during 6 h. After drying in air at 70-80 °C
overnight, the X-ray diffraction (XRD) analysis confirmed the
structure of pure MIL-101. MIL-101-B with increased surface
area was prepared by solvothermal treatment of the rude MIL-
101-as with ethanol at 80 °C for 24 h.90,94
Instrumentation and Methods. The reaction products were
identified by GC-MS and quantified by GC using an internal
standard. GC analyses were performed using a gas chromato-
graph “Tsvet-500” equipped with a flame ionization detector
and a quartz capillary column (30 m ꢀ 0.25 mm) Agilent DB-
5MS. GC-MS analyses were carried out using a gas chromato-
graph Agilent 6890 (quartz capillary column 30 m ꢀ 0.25 mm/
HP-5 ms) equipped with a quadrupole mass-selective detector
Agilent MSD 5973.
Nitrogen adsorption at 77 K was studied using an ASAP-2020
instrument within the partial pressure range 10-6-1.0. Before
measurements, samples were degassed at 150 °C under vacuum
during 48 h. The specific surface area and pore volume were
measured by the comparative method.96 XRD measurements
were performed on a high-precision X-ray diffractometer Philips
APD1700 using cupric radiation (CuKR1;2 1.54060; 1.54439). FT-
IR spectra were recorded using pellets containing 2 mg of a sample
and 500 mg of KBr (for PW12-containing samples) and as Nujol
mull between CsI windows (for PW4-containing samples) on a
BOMEM-MB-102 spectrometer in the 250-4000 cm-1 range. The
Raman spectra were recorded at room temperature in the range of
100-3600 cm-1 using FT-Raman spectrometer RFS 100/S BRU-
KER. The excitation source was the 1064 nm line of the Nd:YAG
laser operating at power level of 100 mW. UV-vis spectra were
recorded using an Agilent 8453 spectrophotometer (λ = 250-255
nm, l = 10 mm). 31P NMR MAS spectra were recorded on a
Bruker Avance-400 spectrometer. The spectrum frequency was
161.97 MHz, and the rotation speed of a 7 mm rotor was 6.5 kHz.
The duration of the π/2 pulse was 7 μs, and the delay for spectrum
acquisition was 20 s. Spectra were referenced to 85% H3PO4
as external standard. Elemental analyses were performed using
atomic emission spectroscopy.
PW4 Synthesis. The Venturello complex [PW4O24]3- (PW4)
was prepared by the peroxide mediated decomposition of
H3PW12O40 following the slightly modified published proce-
dure.26 To a solution of H3PW12O40 (720 mg, 0.25 mmol) in of
30% H2O2 (15 mL, 150 mmol) was added 5 M aqueous solution
of H3PO4 (0.1 mL, 0.5 mmol). The solution was stirred for 4 h at
40 °C to give PW4, which was confirmed by 31P NMR spectro-
scopy (δ 4.4 ppm in H2O/H2O2 solution).
Immobilization of PWx on MIL-101. Adsorption/desorption
measurements were carried out using UV-vis spectroscopy as
described previously.89 The composite PW12/MIL-101 was
synthesized by dissolving H3PW12O40 (5 mg, 0.002 mmol) in
water (1.5 mL, pH 2.05), adding MIL-101 (100 mg), stirring for
3 h at room temperature, filtering off, thorough washing with
water and acetone, and drying at room temperature. Additional
drying at 150 °C for 4 h led to about 5% of weight loss.
PW4/MIL-101 was prepared by adding MIL-101 (100 mg) to
0.005 M PW4 solution (1 mL) obtained by decomposition of
H3PW12O40 with excess of H2O2 (vide supra), stirring at room
temperature for 3 h, filtering off, washing with water and
acetone, and drying at room temperature.
Results and Discussion
The POM loading was determined by elemental analysis. The
catalysts were also characterized by N2 adsorption, FT-IR,
Raman, and 31P NMR MAS spectroscopy.
Treatment of the PW12/MIL-101 (150 mg) with 1 M solution
of Bu4NClO4 in MeCN (3 mL) was carried out at room
temperature under vigorous stirring for 1 h followed by filtra-
tion and drying at room temperature.
ꢀ
Catalysts Preparation and Characterization. Ferey et al.
estimated that about five [PW11O39]7- polyanions can be
incorporated within a large cage of MIL-101.88 Recently,
we have shown that only one Keggin anion per nanocage
(this corresponds to 7-10 wt % of POM in the material) is
irreversibly adsorbed from acetonitrile solution by MIL-
101.89 An electrostatic character of POM binding to the
MIL surface has been suggested based on ion exchange
experiments.
Catalytic Oxidations. Catalytic experiments were carried out
in thermostatted glass vessels under vigorous stirring (900 rpm)
at 50 °C. Alkene oxidations over PWx/MIL-101 were carried
out following three different modes designated as methods A, B,
and C.
In this work, we prepared PW12/MIL-101 hybrid ma-
terials following a similar methodology, but adsorption
Method A. Reaction was started by adding H2O2 (0.1-0.2
mmol) to a mixture containing alkene (0.1 mmol), PWx/MIL-
101 catalyst (28 mg; 5.8 10-3 mmol W in PW12/MIL-101 or
3
4.9 10-3 mmol W in PW4/MIL-101) prepared as described
above, an internal standard (biphenyl), and acetonitrile (1 mL).
(96) Fenelonov, V. B.; Romannikov, V. N.; Derevyankin, A. Yu. Micro-
porous Mesoporous Mater. 1999, 28, 57–72.
3

2924 Inorganic Chemistry, Vol. 49, No. 6, 2010
Maksimchuk et al.
indication of the maintenance of the POM structure after
immobilization.
Since FT-IR bands of the MIL-supported PW4 were
rather weak because of the low POM loading, we used
Raman spectroscopy to characterize this material. The
Raman spectra of the aqueous solution of PW4 and
MIL-supported PW4 are shown in Figure 4. They
exhibit principal bands of the peroxotungstate: 965
ν(WdO), 853 ν(O-O), and 563 ν(W(O2)), thus indi-
cating preservation of the PW4 structure after immobi-
lization.
Figure 1. Adsorption/desorption isotherms (MeCN, 25 °C) for
H3PW12O40 (adsorption and desorption curves are marked by b and O,
respectively) on (a) MIL-101 and (b) MIL-101-B.
31P NMR MAS technique is a useful tool to probe the
state of a POM after immobilization.29-32,43,50 Figure 5
shows 31P NMR MAS spectra of PW12/MIL-101 samples
after different treatments. The 31P NMR MAS spectrum
of the initial PW12/MIL-101 exhibits three main peaks at
-16.4 (form I), -14.9 (form II), and -11.7 ppm (form
III). Since the FT-IR spectra indicated no noticeable
change in the POM structure after deposition of PW12
onto MIL-101 (see Figure 3), the presence of several
peaks in the 31P NMR MAS spectrum might imply
different modes of interaction between PW12 and the
MIL-101 surface.
of heteropolyacid PW12 was carried out from an aqueous
solution. The Venturello complex, PW4, was generated in
situ from PW12, H3PO4, and an excess of H2O2, following
a standard procedure described by several research
groups26,64,94 and immobilized on MIL-101 directly from
the H2O/H2O2 solution. The complete transformation of
PW12 (δ -15.0 ppm) to PW4 (δ þ4.4 ppm) was confirmed
by 31P NMR spectroscopy. The adsorption/desorption
process was monitored by UV-vis spectroscopy. The
amount of irreversibly adsorbed PW12 and PW4 typically
achieved 4-5 wt % (the adsorption/desorption curves are
shown in Figure 1a).
The elemental analysis data for the prepared PWx/
MIL-101 samples along with the P/W/Cr values calcu-
lated for POM loadings acquired from the adsorption
measurements are given in Table 1. The results of the
adsorption study are in good agreement with the elemen-
tal analysis data.
Textural properties of the initial MIL-101 and hybrid
PWx/MIL-101 materials assessed from N2 adsorption
isotherms are also presented in Table 1. One can observe
some decrease (ca. 20 %) in both specific surface area (A)
and pore volume (Vp) after POM insertion. A similar
trend was observed earlier for immobilization of PW11M
on MIL-101.89 XRD patterns of MIL-101 and PWx/
MIL-101 materials are shown in Figure 2. The XRD data
confirmed the preservation of the MIL-101 structure after
the immobilization of the POMs.
Earlier, deposition of PW12 onto silica29-32,35,42,43,46
and carbon50 was found to result in a signal shift, line-
broadening and splitting in the 31P NMR MAS spectra,
indicating a distortion of the PW12 structure compared
with the bulk PW12. Different peaks were associated with
different types of interaction between the Keggin anion
and surface. According to Lefebvre,32 the signal at about
-15 ppm is due to PW12 having no interaction with silica
surface, while the peak at -14.6 ppm can be related to a
polyanion which undergoes a slight electrostatic inte-
raction with a support via the formation of (tSi-
OH2)þ(H2PW12O40)- species. It was also supposed that
a low field shift in the 31P NMR MAS signal can be due to
partial dehydration of the heteropolyacid during the
impregnation process.35,43 Indeed, for dehydrated bulk
H3PW12O40 a sharp 31P NMR MAS peak was observed at
-11.0 ppm, while H3PW12O40 6H2O showed a peak at
3
-15.6 ppm.97 Additionally, 31P NMR spectra of CsxH3-x
salts of PW12 were found to display four resonances
which have been assigned to the polyanions associated
with 0, 1, 2, and 3 protons, respectively.98 Hence, a 31P
NMR MAS signal drift could arise also from a different
protonation state of the POM. Kozhevnikov et al. as-
sumed that a 31P NMR MAS signal shift and splitting
might be due to partial decomposition of PW12 producing
lacunary species, like PW11 or P2W17, during the prepara-
tion of supported catalysts.31
ꢀ
Recently, Ferey and co-workers reported a few meth-
ods for MIL-101 purification which allowed enlarging
significantly its surface area and pore volume.90,94 We
have also found a significant increase of the surface area
(from 2200 to 3900 m2 g-1, see Table 1) after treating
MIL-101 with hot EtOH. In turn, this led to a significant
augmentation of the maximal amount of POM accom-
modated within MIL-101 and allowed us to introduce
irreversibly up to 14 wt % of PW12 into MIL-101 (see
Figure 1b and Table 1).
FT-IR and Raman spectroscopic studies were per-
formed to check whether the POM and MIL-101 struc-
tures are preserved in the hybrid materials. As one can
judge from the FT-IR spectra of MIL-101 and PWx/MIL-
101 presented in Figure 3a (the most intensive peaks
belong to MIL-101), the matrix structure certainly re-
tained upon POM immobilization. Figure 3b shows FT-
IR spectra of the bulk and MIL-supported PW12 after
subtraction of the spectrum of MIL-101. The IR spec-
tra of the PW12/MIL composite exhibit principal bands
of the PW12 Keggin unit: 1081 νas(PO4), 985 νas(WdO),
895 and 820 cm-1 νas(W-O-W), which is a definite
We have found that the ratio of forms I-III changed
after drying the PW12/MIL-101 sample at 150 °C
(Figure 5B). This implies that, in effect, the forms could
be related to different hydration states of the POM.
Previously, electrostatic binding between POM and
MIL-101 surface has been hypothesized based on the
ion exchange experiments.89 We performed three conse-
cutive treatments of the sample 5% PW12/MIL-101 with
(97) Uchida, S.; Inumaru, K.; Misono, M. J. Phys. Chem B 2000, 104,
8108–8115.
(98) Okuhara, T.; Nishimura, T.; Watanabe, H.; Misono, M. J. Mol.
Catal. 1992, 74, 247–256.

Article
Inorganic Chemistry, Vol. 49, No. 6, 2010 2925
Table 1. Physicochemical Properties of Hybrid PWx/MIL-101 Catalysts
sample
P/W/Cra (wt %)
P/W/Crb (wt %)
Ac (m2 g-1
)
Vpd (cm3 g-1
)
MIL-101
MIL-101-B e
5% PW12/MIL-101
-/-/13
2200
3900
1700
1700 f
600 g
1800
500 h
2900
1.1
2.1
0.04/3.1/13
0.05/3.0/12
0.05/3.0/12 f
0.05/3.2/13 g
0.1/2.8/14
0.8
0.8 f
0.4 g
0.8
5% PW4/MIL-101
0.1/2.6/13
0.1/2.7/14 h
0.13/9.4/12
0.3 h
1.2
14% PW12/MIL-101-B e
0.15/10.7/12
a Evaluated from the adsorption data. b Based on the elemental analysis data. c Specific surface area. d Mesopore volume. e After treatment with hot
EtOH (see Experimental Section). f After one cycle of cyclohexene oxidation; reaction conditions as in Table 2. g After five cycles of cyclohexene
oxidation. h After four cycles of cyclohexene oxidation.
Figure 2. XRD patterns for (A) MIL-101, (B) PW12/MIL-101, (C)
PW4/MIL-101, and (D) PW12/MIL-101 dried at 150 °C for 4 h.
Figure 4. Raman spectra of (A) PW4 (0.05 M solution in H2O-H2O2),
(B) MIL-101, (C) PW4/MIL-101, and (D) PW4/MIL-101 after 4 cycles of
cyclohexene oxidation.
immobilized PW12 (ca. 1 wt %) remained after three
consecutive treatments with Bu4NClO4. Hence, forms
I and II can be removed using Bu4NClO4, thus pointing
to the electrostatic character of binding between these
forms and positively charged surface of the MIL-101
nanocage.
It should be mentioned that the ratios of forms I-III in
the samples 5% PW12/MIL-101 and 14% PW12/MIL-101-
B were different (compare Figure 5 A and G). While in the
former sample form I predominated (ca. 50%), the latter
was enriched with form III (56%). At the moment, the na-
ture of form III is not understood completely. Most likely, it
derives from a stronger interaction of dehydrated PW12 and
the MIL surface. It seems reasonable that purification of
MIL and increasing surface area would enhance this inter-
action. Yet, the formation of lacunary POM species can not
Figure 3. (a) FT-IR spectra of (A) MIL-101, (B) PW12/MIL-101, (C)
PW4/MIL-101, (D) PW12/MIL-101 after treatment with 0.2 M H2O2 for 1
h at 50 °C, (E)-(G) PW12/MIL-101 after 1, 3, and 5 cycles of cyclohexene
oxidation, and (H) PW4/MIL-101 after 4 cycles of cyclohexene oxidation.
(b) FT-IR spectra of (A) PW12, (B) PW12/MIL-101, (C) PW12/MIL-101
after treatment with 0.2 M H2O2 for 1 h at 50 °C, and (D)-(F) PW12
/
MIL-101 after 1, 3, and 5 cycles of cyclohexene oxidation; in (B)-(F) the
spectra of MIL-101-supported PW12 are given after subtraction of the
spectrum of MIL-101. For reaction conditions, see Experimental Section,
method A.
be ruled out completely.31 The minor broad signal in the 31
P
NMR MAS spectrum at about -7.1 ppm could be tenta-
tively assigned to such species.
Catalytic Oxidation. The catalytic performance of the
PW4/MIL-101 and PW12/MIL-101 hybrid materials was
first assessed in cyclohexene oxidation with aqueous
H2O2 in acetonitrile medium. The results of the catalytic
runs along with blank experiments and experiments
performed in the presence of the corresponding homo-
geneous POMs are presented in Table 2. Both PW4 and
PW12 incorporated within the MIL-101 matrix showed
fairly good catalytic activity, while the matrix alone was
quite inert. The catalytic activities of 5% PW12/MIL-101
and 5% PW4/MIL-101 referred to one W atom were
similar (compare entries 4 and 6 in Table 2). It is worth
noting that the activities of PWx/MIL expressed in TOF
values were just slightly lower than the activity of homo-
geneous PWx (0.36 vs 0.34 min-1 for PW4 and 0.41 vs 0.39
min-1 for PW12). This implies that immobilization of
1 M solution of Bu4NClO4 in MeCN. The corresponding
31P NMR MAS spectra are presented in Figure 5 C-E.
One can see that after the first treatment the 31P NMR
MAS peak at -16.3 ppm decreased markedly (Fig-
ure 5C), while after the second treatment it disappeared
completely along with the signal at -15.0 ppm (Fig-
ure 5D). According to the elemental analysis data, about
one-third of the immobilized PW12 was transferred into
solution after the first treatment with Bu4NClO4. The 31
P
NMR measurements confirmed that the wash-out solu-
tion contained only intact Keggin PW12 (Figure 6A).
After the third treatment, the 31P NMR MAS spectrum
(Figure 5E) was similar to that run after the second
treatment, indicating the presence of solely form III.
Elemental analysis indicated that about one-fifth of the

2926 Inorganic Chemistry, Vol. 49, No. 6, 2010
Maksimchuk et al.
Figure 5. 31P NMR MAS spectra of 5% PW12/MIL-101: (A) initial, (B) dried at 150 °C for 4 h, (C)-(E) after 1-3 consecutive treatments with 1 M
Bu4NClO4 in MeCN, and (F) after 5 cycles of cyclohexene oxidation; (G) 31P NMR MAS spectrum of 14% PW12/MIL-101-B.
POM on MIL-101 causes no deactivation and allows to
keep the catalytic activity, at least, at the same level.
With 2 equiv of H2O2, cyclohexene was converted
mainly to its epoxide over 5% PWx/MIL-101. The
allylic oxidation products, 2-cyclohexene-1-ol and
2-cyclohexene-1-one (totally ca. 10%) were identified
as the main byproduct, while the product of epo-
xide ring-opening, 1,2-trans-cyclohexane diol, was
not found in the reaction mixture. This contrasts with
the homogeneously catalyzed oxidation in the presence
of PWx where the diol was the main byproduct. The
reaction over PW4/MIL-101 gave epoxide with 74%
selectivity at 76% substrate conversion after 3 h; a
similar result was acquired over PW12/MIL-101 (entry
6, Table 2). For comparison, cyclohexene (3 mmol)
oxidation with H2O2 (3 mmol) over PW4-Amberlite
(0.016 mmol PW4) at 32 °C gave epoxide with only 57%
selectivity at a similar conversion after 24 h.70 In turn,
5% PW12/NH2-SBA-15 produced mainly trans-cyclo-
hexane diol under the same reaction conditions (entry
13, Table 2).
It has been established by several research groups
that the catalytic activity of the PW12/H2O2 system in
alkene epoxidation is caused by in situ formation of
Figure 6. 31P NMR spectra of: (A) washout solution after treatment of
PW12/MIL with 1 M Bu4NClO4 in MeCN and (B) PW12 after homo-
geneous cyclohexene oxidation.

Article
Inorganic Chemistry, Vol. 49, No. 6, 2010 2927
Table 2. Cyclohexene Oxidation with H2O2 in the Presence of PWx/MIL-101
Catalystsa
cyclohexene
conversion
(%)
epoxide
TOFb selectivity c
entry
catalyst
(min-1
)
(%)
d
1
2
3
4
5
6
7
8
9
-
12
11
90
76
72
72
71
75
84
68
67
78
67
64
77
74
78
76
77
77
80
81
83
36
17
MIL-101 e
PW4
5% PW4/MIL-101
PW12
f
0.36
0.34
0.41
0.39
0.39
0.41
0.40
0.32
0.32
0.42
0.19
f
5% PW12/MIL-101
5% PW12/MIL-101 g
5% PW12/MIL-101 h
5% PW12/MIL-101 i
Figure 7. Cyclohexene conversion (9, 0), epoxide yield (b, O) and
selectivity (2, 4) versus time in the oxidation with H2O2 over PW12/
MIL-101 (method A - 9, b, 2 and method B - 0, O, 4). Reaction
conditions as in Table 2.
10 1% PW12/MIL-101 j
11 5% PW12/MIL-101 k
12 14% PW12/MIL-101-B l
13 5% PW12/NH2-SBA-15 m 48
a 0.1 M cyclohexene, 0.2 M H2O2, 28 mg of catalyst (5.8 ꢀ 10-3 and
4.9 ꢀ 10-3 mmol W in PW12/MIL-101 and PW4/MIL-101, respectively),
method A, 1 mL of MeCN, 50 °C, 3 h. b TOF = (moles of substrate
consumed) (moles of W)-1 time-1; determined by GC from the initial
3
3
rates of cyclohexene oxidation. c GC yield based on the substrate
consumed. d No catalyst was present. e MIL-101 matrix without POM.
f Homogeneous catalyst (5.8 ꢀ 10-3 mmol W for PW12 or 4.9 ꢀ 10-3
mmol W for PW4). g Method B. h Method C. i 60 mg of the catalyst (1.2 ꢀ
10-2 mmol W). j 99 mg of PW12/MIL-101 (4.1 ꢀ 10-3 mmol W) after 3
consecutive treatments with 1 M Bu4NClO4 in MeCN. k PW12/MIL-101
l
dried at 150 °C for 4 h. 14 mg of the catalyst (8.2 ꢀ 10-3 mmol W).
Allylic oxidation products, cyclohexenol/-one (ca. 5%), along with
trans-cyclohexane diol (24%), 2-hydroxycyclohexanone, and adipic
aldehyde were also found. m trans-Cyclohexane diol (67%) and cyclo-
hexenol/-one (15%) were also found.
lower nuclearity peroxotungstates, such as PW4 and
PW2.99-104 In water, PW12 completely transforms to
PW4 and other peroxotungstates after 1 h treatment with
50 equiv of H2O2 at room temperature.100 However, the
use of organic medium (acetophenone, DMF) enables
increasing POM stability with respect to solvolytic de-
struction in the presence of aqueous H2O2, which leads to
incomplete and reversible PW12 transformations under
the conditions of H2O2 excess (400 equiv, 2 h, 90 °C).102
Indeed, we found no PW4 and only 3% of PW2 (small
signal with δ 0.4 ppm in the 31P NMR spectrum shown in
Figure 5B)105 in the acetonitrile solution of PW12 after
homogeneous cyclohexene oxidation under turnover con-
ditions (500 equiv of H2O2). We had a thought that
immobilization on MIL-101 could additionally increase
stability of the polyanion structure.
Figure 8. Raman spectra of (A) PW4 (0.05 M solution in H2O-H2O2),
(B) solid PW12, (C) MIL-101, (D) PW12/MIL-101, (E)-(G) PW12/MIL-
101 after 1, 3, and 5 cycles of cyclohexene oxidation, respectively.
Reaction conditions as in Table 2 (method A).
conversion and epoxide yield versus time showed no induc-
tion period, and the initial rate areas coincided in methods
A and B (Figure 7). In turn, the 31P NMR MAS spectrum
of 5% PW12/MIL-101 remained practically unchanged
after five cycles of cyclohexene oxidation (see Figure 5A
and F). Hence, we may conclude that if the transformation
of the MIL-immobilized PW12 to the lower nuclearity
species occurs in the course of the catalytic oxidation under
the conditions employed, such transformation is appar-
ently reversible. Yet, one can not exclude that the active
species responsible for catalysis is present in minor
amounts, not detectable by 31P NMR MAS.
According to the FT-IR and Raman spectra (Fig-
ures 3b and 8, respectively), MIL-101 supported PW12
does not lose its Keggin structure after 3 cycles of
cyclohexene oxidation with H2O2, but begins to trans-
form to the peroxotungstate species PW4 after five reuses
(i.e., after about 770 TON). The FT-IR band at 1081
cm-1 broadens after several cycles of cyclohexene oxida-
tion because of anion symmetry lowering (Figure 3b). As
was mentioned above, homogeneous PW12 partially
transformed to PW2 peroxotungstate already after the
first cycle of cyclohexene oxidation (155 TON) under the
conditions employed. Thus, we may conclude that im-
mobilization onto MIL-101 does increase stability of
PW12 toward destruction in the presence of H2O2.
Indeed, we performed cyclohexene oxidation over
PW12/MIL-101 after a preliminary treatment of the
catalyst with a 500-fold excess of 30% H2O2 (see the Ex-
perimental Section, method B) but found no changes
in both activity and selectivity (compare entries 6 and 7
in Table 2). Additionally, the curves of cyclohexene
ꢀ
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2928 Inorganic Chemistry, Vol. 49, No. 6, 2010
Maksimchuk et al.
We also examined a “one-pot” procedure which com-
bined the preparation of the PW12/MIL-101 catalyst and
cyclohexene oxidation (see the Experimental Section,
method C). Importantly, following such a simple proto-
col we managed to obtain cyclohexene epoxide with the
yield even superior to that found using conventional
method A (compare entries 6 and 8 in Table 2).
One cansee fromthe datashownin Table 2andFigure 7
that cyclohexene conversion was incomplete over PWx/
MIL-101 catalysts despite a 2-fold excess of H2O2 was
employed. This could be due to partial unproductive
decomposition of the oxidant on MIL-101. Previously,
we revealed that MIL-101 does possess some activity in
H2O2 dismutation at 50 °C.89 Indeed, the presence of
some amount of cyclohexene allylic oxidation products
supports the involvement of radical species in the oxida-
tion process. Meanwhile, we found that about 25% of the
oxidant was still present in the reaction mixture at the end
of the catalytic reaction. So, we believe that another
reason of the incomplete cyclohexene conversion could
be blockage of active sites by the reaction products.
Indeed, by increase of the catalyst amount (entry 9,
Table 2) we managed to achieve higher cyclohexene
conversion (84%) and epoxide yield (66%).
The catalyst prepared using MIL-101 with enlarged
surface area and pore volume (14% PW12/MIL-101-B)
also revealed a higher conversion of cyclohexene (entry 12,
Table 2). However, the selectivity to epoxide was lower for
this catalyst compared to 5% PW12/MIL-101 because of
subsequent transformations of the epoxide to trans-1,2-
cyclohexanediol, 2-hydroxycyclohexanone, and adipic al-
dehyde. Hence, we may conclude that 5 wt % is an optimal
PW12 loading requiredfor the selectivealkene epoxidation,
and higher loadings should be avoided.
To clarify the catalytic behavior of forms I-III which
are present in PW12/MIL-101 catalysts, we compared
the performance of the parent 5% PW12/MIL-101 sample
(entry 6, Table 2) with the performances of the sample
dried at 150 °C (entry 11, Table 2) and the sample
obtained after three consecutive treatments with 1 M
Bu4NClO4 (entry 10, Table 2) in cyclohexene oxidation
with H2O2. The latter two materials contained mostly
form III and were found to be less active (TOF 0.32
min-1) but more selective (81-83%) toward epoxide
formation compared to the initial 5% PW12/MIL which
included mainly form I. The higher selectivity of form III
in the epoxidation reaction corroborated its assignment
to dehydrated PW12. Interestingly, form III also predo-
minated in 14% PW12/MIL-101 according to its 31P
NMR MAS spectrum (Figure 5G), but as we mentioned
above, the selectivity over this sample was rather low
because high POM loading favors epoxide ring-opening
and/or overoxidation processes.
(octene-1). Both homogeneous PW12 and heterogeneous
PW12/MIL-101 revealed a very similar range of the max-
imal (3-carene) and minimal (octene-1) TOF values,
which may indicate the absence of diffusive limitations
in the alkene epoxidation over PW12/MIL-101. It is worth
noting that activity of PW12/MIL expressed in TOF
values is significantly higher than the activity of pre-
viously reported PW4-Amberlite catalyst at the same
reaction conditions. For example, TOF for limonene
epoxidation over PW4-Amberlite calculated on the basis
of reported data is equal to 0.01 min-1 71
.
Importantly, PW12/MIL-101 is able to catalyze selec-
tively epoxidation of acid sensitive terpenes, and 70-96%
selectivities can be achieved depending on the specific
alkene structure and reaction conditions. Oxidation of
styrene and stilbenes proceeded less efficiently than oxida-
tion of electron-rich alkenes, but note that substrate con-
versions and epoxide selectivities were higher with PW12/
MIL-101 than with homogeneous PW12. Thus oxidation of
styrene with 1 equiv of H2O2 over PW12/MIL-101 gave
24%substrateconversionand 54%epoxideselectivityafter
3 h at 50 °C, while the corresponding parameters for
homogeneous PW12 were 20 and 40% (see Table 3). This
effect can be rationalized if we take into account the affinity
of the MIL-101 framework for benzene-like molecules.86
It is worth noting that for the majority of alkene
substrates an increase of H2O2/alkene molar ratio led to
both higher substrate conversions and epoxide selectiv-
ities (Table 3). With 2 equiv of H2O2 the selectivity of
epoxidation attained 96, 89, and 99% for 3-carene,
limonene and cyclooctene, respectively. These values
exceed the selectivities acquired using homogeneous
PW12. The same tendency was observed earlier for
caryophyllene epoxidation with H2O2 over Ti-POM/
MIL-101.89 Note that maintenance of selectivity with
growing conversion under excess of H2O2 is a quite
unusual phenomenon for alkene epoxidation,69 for which
an excess of olefin is usually employed to achieve a high
selectivity.23 The epoxide selectivity typically tends to
decrease with conversion because of consecutive epoxide
ring-opening and overoxidation processes. This is exactly
what happens in the presence of homogeneous PW12 (see
examples in Table 3). We suppose that the non-typical
behavior that we revealed for the hybrid PW12/MIL-101
catalysts might originate from an ability of MIL-101 to
adsorb preferably H2O2 and alkene substrate and not to
adsorb water because of hydrophobicity of the organic
linker. This agrees with the recently published data on the
water sorption on MIL-101, which showed that H2O
uptake starts at rather high partial pressures (p/p0 =
0.4).106 On the basis of these results, one might expect that
MIL-101 surface would behave as a hydrophobic one at
rather low water concentration in an organic solvent.
Catalyst Stability and Recycling. While studying liquid
phase processes over solid catalysts it is extremely im-
portant to address the issues of stability of the active
component with respect to leaching and maintenance of
the catalyst porous structure under the reaction condi-
tions. The nature of catalysis and recycling behavior are
crucial points as well. Following the methodology suggested
To assess the scope and limitations of the PWx/MIL-
101 catalysts, the activity of the optimal sample, 5%
PW12/MIL-101, was tested in the oxidation of alkenes
with different steric and electronic structures, including
natural terpenes (R-pinene, 3-carene, and limonene),
cyclooctene, octene-1, styrene, and cis-/trans-stilbenes.
The results are presented in Table 3. The rank of the
catalytic activity expressed in TOF values was typical
for an electrophilic oxygen transfer process: R2CdCHR
(3-carene) > RCHdCHR (cyclooctene) > RCHdCH2
€
€
(106) Kusgens, P.; Rose, M.; Senkovska, I.; Frode, H.; Henschel, A.;
Siegle, S.; Kaskel, S. Microporous Mesoporous Mater. 2009, 120, 325–330.

Article
Inorganic Chemistry, Vol. 49, No. 6, 2010 2929
Table 3. Alkene Oxidation with H2O2: Homogeneous PW12 versus 5% PW12/MIL-101a
a Reaction condition: 0.1 M substrate, 28 mg of catalyst 5% PW12/MIL-101 or 1.4 mg of PW12 (5.8 ꢀ 10-3 mmol W), method A, 1 mL of MeCN,
3
50 °C, 3 h. b TOF = (moles of substrate consumed) (moles of W)-1 time-1; determined by GC from the initial rates. c GC yield based on the substrate
3
3
consumed. d Limonene 1,2-epoxide. e 5 h. f Benzaldenyde was also formed. g cis-Stilbene epoxide.
by Sheldon and co-workers,68 we performed fast hot cata-
lyst filtration tests and found no further substrate conver-
sion in the filtrate after the removal of both PW12/MIL-101
and PW4/MIL-101 (Figure 9). This proves the true hetero-
geneous nature of the oxidation catalysis over the PWx/
MIL-101 catalysts. Additionally, elemental analysis data
(see Table 1) confirmed that no evident tungsten or chro-
mium leaching occurred under the conditions employed.
The recycling behavior of the PWx/MIL-101 catalysts
is shown in Figure 10. One can see that the performance of
PW4/MIL-101 was quite stable during four consecutive
runs, but the epoxide yield slightly decreased after the
third run (Figure 9a). In turn, PW12/MIL-101 kept its
activity and selectivity during four catalytic cycles (600
TON) and then the activity decreased (Figure 10b).
The Raman spectrum of PW4/MIL-101 run after four
cycles of cyclohexene oxidation (Figure 4D) revealed the
principal bands of the peroxotungstate structure. Hence,
we may suppose that PW4 supported on MIL-101 kept
its structure during four cycles of cyclohexene oxidation
(250 TON). Hill and co-workers have reported that
deactivation of homogeneous PW4 is caused mainly by
the reaction product, epoxide.100,107 The PW4-Amberlite
(107) Hill C. L. In Comprehensive Coordination Chemistry II; Wedd, G. A.,
Ed.; Elsevier Science: New York, 2004; Vol. 4, p 679-759.

2930 Inorganic Chemistry, Vol. 49, No. 6, 2010
Maksimchuk et al.
surface of the mesoporous zeotype coordination polymer
MIL-101withretentionofboth POMandMOFstructures.
The hybrid PW4/MIL-101 and PW12/MIL-101 materials
demonstrate similar catalytic activities in H2O2-based
alkene epoxidation, comparable to the activity of the
corresponding homogeneous PWx and significantly higher
than activities of the known supported PW4 catalysts.71 In
turn, the epoxide selectivities achieved over the PWx/MIL-
101 catalysts are close15,69,72,73 or even superior70 to those
reported for PW4 immobilized by other techniques. The
selectivity strongly depends on the POM loading, the
optimal value of which is about 5 wt %. A simple one-
pot procedure which involves both the catalyst preparation
and oxidation reaction can be used to achieve high yields of
epoxides. In contrast to the homogeneous systems, the use
of a higher H2O2/alkene molar ratio allows increasing
simultaneously both the alkene conversion and the epoxide
selectivity. This unusual behavior, most likely, results from
the specific sorption properties of MIL-101. Immobiliza-
tion within the MIL-101 matrix allows increasing consider-
ably the stability of PW12 toward solvolytic destruction in
the presence of H2O2.
Figure 9. Hot catalyst filtration experiments for cyclohexene oxidation
with 0.2 M H2O2 over (a) PW12/MIL-101 (methods A-b and C-() and
(b) PW4/MIL-101.
Both PW4/MIL-101 and PW12/MIL-101 materials be-
have as true heterogeneous catalysts and can be recycled
several times without loss of activity and selectivity with
retention of both POM and MIL-101 structures; how-
ever, some deterioration of the catalytic properties may
occur after several reuses because of partial destruction of
the MIL-101 matrix and/or filling pores by the reaction
products.
Figure 10. Catalyst recycling in cyclohexene oxidation with 0.2 M
H2O2: (a) PW4/MIL-101 and (b) PW12/MIL-101 (method A). Reaction
conditions as in Table 2 (method A).
catalyst was also reported to be poisoned by the epoxide
product.71 We believe that some deactivation of PWx/
MIL-101 might be caused by destruction of the MIL-101
matrix and/or filling pores by the reaction products.
Indeed, both specific surface area and pore volume
decreased after 4-5 cycles of cyclohexene oxidation over
PWx/MIL-101 (see Table 1). In turn, the XRD technique
showed some amorphization of the MIL-101 structure
because of its partial decomposition.
Acknowledgment. We thank Dr. R. I. Maksimovskaya
for the 31P NMR measurements in solution and Mr. V. A.
Utkin for identification of the oxidation products by GC/
MS technique. The research was partially supported by
the Russian Foundation for Basic Research (RFBR
Grants 09-03-93109, 09-03-12112, and 09-03-93113) and
by Siberian Branch of the Russian Academy of Sciences
through Interdisciplinary project (Grant 107).
Conclusions. Polyoxotungstates [PW4O24]3- and
[PW12O40]3- have been electrostatically attached to the