Hybrid peroxotungstophosphate organized catalysts highly active and selective in alkene epoxidation

Article abstract of DOI:10.1016/j.catcom.2013.03.036

Heterogenization of homogeneous catalysts is still a challenge but can improve drastically the processability of these compounds. Hybridization of polyoxometalates offers an efficient heterogenization route of homogeneous epoxidation catalysts. The Keggin [PW₁₂O₄₀]^3- and the Ventruello {PO₄[WO(O₂)₂] ₄}^3- species were inserted by electrostatic interactions in a poly(ampholytic) polymeric matrix in order to prevent the leaching of these species. The presence of the polymeric matrix allows to tune the catalyst performance in cyclooctene epoxidation and to improve the selectivity to epoxide. Indeed, the hydrophobicity of the matrix induces a quick desorption of hydrophilic epoxide species. Their over-oxidation and the catalyst species deactivation by over-adsorption of epoxide are then avoided.

Full text of DOI:10.1016/j.catcom.2013.03.036

Catalysis Communications 37 (2013) 80–84
Contents lists available at SciVerse ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
Hybrid peroxotungstophosphate organized catalysts highly active and
selective in alkene epoxidation
⁎
Colas Swalus, Benjamin Farin, François Gillard, Michel Devillers, Eric M. Gaigneaux
Institute of Condensed Matter and Nanoscience (IMCN), Division Molecules, Solids and Reactivity (MOST), Université catholique de Louvain, 1348 Louvain-la-Neuve, Belgium
a r t i c l e i n f o
a b s t r a c t
Article history:
Heterogenization of homogeneous catalysts is still a challenge but can improve drastically the processability
Received 7 January 2013
Received in revised form 12 March 2013
Accepted 25 March 2013
Available online 6 April 2013
of these compounds. Hybridization of polyoxometalates offers an efficient heterogenization route of homoge-
3−
neous epoxidation catalysts. The Keggin [PW12O40
]
and the Ventruello {PO4[WO(O2)2]4}3− species were
inserted by electrostatic interactions in a poly(ampholytic) polymeric matrix in order to prevent the leaching
of these species. The presence of the polymeric matrix allows to tune the catalyst performance in cyclooctene
epoxidation and to improve the selectivity to epoxide. Indeed, the hydrophobicity of the matrix induces a
quick desorption of hydrophilic epoxide species. Their over-oxidation and the catalyst species deactivation
by over-adsorption of epoxide are then avoided.
Keywords:
Organic–inorganic hybrid composites
self-assembly
© 2013 Elsevier B.V. All rights reserved.
polyoxometalates
heterogeneous catalysis
epoxidation
1. Introduction
This tuned behavior of catalytic and physicochemical properties
could lead to highly selective heterogeneous catalysts. A self-assembled
Hybrid materials that allow to combine complementary characteris-
tics of individual constituents are becoming currently more and more at-
tractive [1–3]. The association of organic and inorganic components at
the nanosize level offers many benefits in countless applications. On
the one hand, the infinity of inorganic compounds with magnetic, dielec-
tric, mechanic, and catalytic properties allows to use such hybrid mate-
rials in various fields such as medicine [4], optics [5], electronics [6]
and catalysis [7–9]. On the other hand, the organic part brings additional
convenient processing and electronic or photoconductivity properties.
Furthermore, the use of a self-assembled organic matrix could induce
specific molecular arrangements in the hybrid that leads to a fine control
of its structural properties [10].
In catalysis, the main goal of hybridization usually consists in solid
catalyst synthesis through active species heterogenization. In the present
case, hybridization targets more valuable effects. First, the organization
of the organic part is assumed to confer new specificity properties. This
could for example occur via the organization of a polymeric partner
around the active sites to mimic the enzyme protein arms function.
This organization would impose geometrical constraints to control the
reaction of the catalytic inorganic partner. Second, the use of an organic
environment is expected to induce specific adsorption properties of the
reagents. Such a specificity is expected to be induced through the struc-
turing of hydrophilic/phobic environment around the active species.
amphiphilic poly(ampholytic) copolymer was selected here as an effi-
cient matrix to structure the space around an inorganic compound and
to form an organized hybrid with improved adsorption performance in
catalysis [11].
The inorganic compounds selected here are polyoxometalates
(POMs). POMs are inorganic early transition metal oxide clusters.
Their composition, structure, size, redox chemistry, charge versatility
and solubility in various media are the numerous reasons that boost
the POMs chemistry at the forefront of fundamental research today
such as catalysis [12], materials [13,14] and medicine [15]. Moreover,
in comparison to enzymes and organometallic complexes, POMs are
thermally and oxidatively stable [16]. In catalysis, POMs display simul-
taneously high Brønsted acidity and efficient redox behavior allowing
fast transformations under soft conditions. POMs are active in esterifica-
tion, hydrolysis, and alkylation but also in alcohol or sulfide oxidation
and alkene epoxidation [17,18].
Hybridization of POMs was already performed but only very few
applications are reported in catalysis [8,9,19]. Different possibilities
are allowed for the hybridization. A simple physical blending seems not
efficient enough to avoid leaching of catalytic species. Covalent bonding
of POMs with organic compounds was already achieved but is limited
to a few kinds of POMs [20,21]. Moreover covalent bonding can change
the POM structure and composition and induce changes in the catalytic
properties. Hybridization was thus achieved here by taking advantage
of electrostatic interactions between negatively charged POMs and a
poly(ampholytic) copolymer positively charged. The copolymer consists
of substituted diallylamines alternated with maleic acid monomers. The
amine monomer brings the positive charge and bears side alkyl chains
⁎
Corresponding author at: Université catholique de Louvain Croix du Sud, 2 bte L7.05.17,
1348 Louvain-La-Neuve, Belgium. Tel.: +32 10 47 36 65; fax: +32 10473649.
E-mail address: eric.gaigneaux@uclouvain.be (E.M. Gaigneaux).
1566-7367/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.catcom.2013.03.036

C. Swalus et al. / Catalysis Communications 37 (2013) 80–84
81
with eighteen carbon atoms (hereafter named C18). Hydrophobic in-
teractions between these alkyl side chains confer to the polymer a
self-assembled lamellar arrangement (Scheme 1). The periodic di-
mension of this comb-like structure is controlled by the length of
these chains [11].
vigorously stirred during 2 h. The solids are recovered by filtration,
washed three times with distilled water and dried overnight in vacu-
um at room temperature. The two hybrids were hereafter denoted
PW12–C18 and PW4–C18 respectively.
Herein, to probe the catalytic performance of hybrids, the epoxidation
of cyclooctene with Keggin H3PW12O40 species (hereafter named PW12)
and trianionic diperoxo species {PO4[WO(O2)2]4}3− (hereafter named
PW4) were selected. This cluster, the so-called Venturello complex,
results from the activation of the Keggin-type compound H3PW12O40
with hydrogen peroxide and H3PO4. PW4 clusters have been largely rec-
ognized for a long time as highly active species in catalytic oxidation
[22,23].
2.3. Characterizations of the samples
The weight percentages of W and P were measured by inductively
coupled plasma-atomic emission spectroscopy (ICP-AES) on an Iris
Advantage apparatus from Jarrell Ash Corporation. X-ray diffraction
(XRD) measurements were performed with a Siemens D5000 diffrac-
tometer using the Kα radiation of Cu (λ = 1.5418 Å). The small 2θ
range (1°–10°) was scanned at a rate of 0.002° s−1. The high 2θ range
(10°–60°) was scanned at a rate of 0.01° s−1. Fourier transformed infra-
red spectroscopy was performed with transparent discs of the samples
prepared after diluting them in KBr (>99%, Janssens Chimica) by a
weight factor of 100. The spectra were obtained by recording 100 scans
between 400 and 4000 cm−1 with a 4 cm−1 resolution. The IFS55
Equinox spectrometer (Bruker) used in the transmission mode was
equipped with a DTGS detector. Raman spectroscopy was performed
on a Bruker RFS 100/S apparatus equipped with a coherent Nd-YAG
laser (λ = 1064 nm, 100 mW). The resolution was 4 cm−1 and the
signal was recorded in the 3500–100 cm−1 range. For each reported
spectrum 50 scans were recorded and averaged.
2. Experiment
2.1. Matrix synthesis
C18 matrix was synthesized by a copolymerization of equimolar
amounts of N,N-diallyl-N-octadecylamine and maleic acid in a water/
ethanol (90:10 v/v) mixture as described elsewhere [11], and isolated
analogously by precipitation four times into cold acetone. The crude co-
polymer was dialyzed first in a mixture of ethanol/CH2Cl2/37 wt.% HCl
(60:39:1 v/v/v) for ten days, then in an ethanol/CH2Cl2 (70:30 v/v)
mixture for five days, changing the solvent every two days. After remov-
ing the solvent, the copolymer was dissolved in ethanol; an equimolar
amount of OH−-loaded strong basic exchange resin was added, and
the mixture was stirred for 2 h. The collected precipitate was dried at
60 °C in vacuum and gave the pure copolymer in the form of a white,
hygroscopic powder.
2.4. Catalytic experiments
The monophasic catalytic reaction was performed in a 5 ml round-
bottomed flask equipped with an oil bath, a magnetic stirring rod and a
septum. In a typical experiment, the catalyst (mass adjusted to always
have 0.03 mmol of W in the reactor), cyclooctene (0.5 mmol), acetoni-
trile (5 ml) and hydrogen peroxide (30% in water) (2.5 mmol) were
added in the flask. For homogeneous catalytic tests, namely with PW12
and PW4 alone, the addition order of the reaction mixture components
is inverted. In these cases, a pre-activation with H2O2 is achieved during
1 h followed by the addition of CH3CN and the alkene. These tests were
made with 0.0025 mmol of H3PW12O40 (named PW12-H2O2) and
0.005 mmol of H3PO4 (85%) (named PW12-H3PO4-H2O2) to form the
corresponding {PO4[WO(O2)2]4}3− species. The reaction was performed
at 50 °C and was monitored by GC analysis with the internal standard
method (dibutyl ether). Assignments of products were made by compar-
ison with authentic samples and by GC-MS. The catalyst was recovered
by centrifugation after reaction, washed with CH3CN three times, dried
overnight in vacuum at room temperature and reused.
2.2. Hybrid synthesis
The polyoxometalates used in this work were H3PW12O40 (PW12)
supplied by Aldrich. The two hybrids were synthesized by a two-phase
ion-exchange process. For the PW12 hybrids, a water solution of PW12
(2 g, 0.67 × 10−3 mol) was mixed with a chloroform solution containing
the organic matrix (0.94 g, 2.01 × 10−3 mol of repeated unit). For PW4
hybrids, the peroxotungstophosphate species (PW4) were synthesized
according to a protocol adapted from the literature [24]. Its corresponding
hybrid was made as follows: after 1 h of agitation, 250 ml H2O2
(30%) containing 5 g (1.67 × 10−3 mol) of H3PW12O40 and 0.229 ml
(3.35 × 10−3 mol) of H3PO4 (85%) were mixed with a chloroformic
solution containing the organic matrix (7.01 g, 15.03 × 10−3 mol of
repeated unit). The both biphasic mixtures are then magnetically
Scheme 1. Scheme of the supramolecular organization of the self-assembled amphiphilic poly(ampholytic) copolymer (inspired from [13]).

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C. Swalus et al. / Catalysis Communications 37 (2013) 80–84
3. Results and discussion
PW12 and PW4 hybrids were synthesized by a two-phase ion
exchange process of water and chloroform. Quantities were based
on an equivalent molar charge ratio between three times negatively
3−
charged [PW12O40
]
and {PO4[WO(O2)2]4}3− units and one time
positively charged polymer repeated unit. The solid hybrids are recov-
ered by filtration and then dried. Thermogravimetric analysis (TGA)
under air and ICP-AES analyses revealed that 51.9 wt.% of PW12 and
28.8 wt.% of PW4 were effectively hybridized in the matrix. The
obtained molar ratio POM/polymer repeated unit is 0.52/3 and 0.49/3
respectively.
Vibrational spectroscopies (IR and Raman) confirm the hybridization
of the Keggin PW12 and PW4 species. In IR, as presented in Fig. 1, the clas-
sical bands of Keggin structure at 1080 cm−1, 984 cm−1, 891 cm−1 and
810 cm−1 are observed and can be assigned to v(P–O), v(W = O) and
the asymmetric vibration of v(W–O–W) (corner and edge-sharing re-
spectively) [25]. Small shifts are observed and attributed to electrostatic
interactions between POM species and positive charges along the matrix
framework [26]. The peroxo species bonds are observed at 1030 cm−1
,
966 cm−1, 884 cm−1, 836 cm−1 and 560 cm−1 for v(P–O), v(W = O),
v(W–O–W), v(O–O) and v(W(O2)) respectively [27]. Raman confirms
the hybridization of peroxotungstophosphate anions and shows the
characteristic bands at 958 cm−1, 853 cm−1 and 561 cm−1 assigned to
v(W = O), v(O–O) and v(W(O2)) [28]. It should be pointed out that
the presence of other smaller peroxo species cannot be totally excluded
by the IR and Raman results.
X-ray diffraction at small angle reveals the nanoscale organization
of the pure C18 matrix and the hybrids (Fig. 2). The self-assembled la-
mellar organization of the polymer induces two diffraction peaks at
2.80° and 5.61°, which correspond to the first and the second order
reflection peaks of the matrix [11]. The hybridization of PW12 and
PW4 species leads to two different behaviors. In the first case, the ma-
trix organization is lost as the hybrid is formed. The broad large peak
obtained reveals an amorphous reorganization of the polymer chains
around the PW12 species likely randomly distributed in the matrix. In
the second case, the addition of PW4 species does not perturb the ma-
trix supramolecular organization. Besides, it causes a shift of the main
peak to lower diffraction angle (2.49°). According to the Bragg law,
the characteristic inter-reticular distance of the resulting structure
Fig. 2. XRD patterns at small angle of (a) the PW12–C18 hybrid, (b) the PW4–C18 hybrid
and (c) the pure C18 copolymer matrix.
increases from 3.15 nm to 3.55 nm. This phenomenon is assumed to
be due to the regular insertion of the PW4 species inside the matrix.
The opposite behavior of the PW12 and PW4 species could be attributed
to the difference of size between the two POM species. The 1 nm of di-
ameter of the Keggin PW12 structure appears too big to be inserted in
the matrix hydrophilic region without reorganization. The smaller size
of PW4 species sounds more favorable for an organized insertion [29].
The absence of crystalline peak at large angle (results not presented)
argues in favor of the absence of “pure” POM crystallized species, and
thus of a good dispersion of the polyoxometalates species through the
matrix.
Cyclooctene epoxidation managed in acetonitrile was selected to
measure the hybrid catalytic performance. Hydrogen peroxide was
used as oxidizing agent. Results are presented in Table 1 and Fig. 3. In
order to highlight the impact of hybridization, homogeneous catalytic
tests were also achieved. The PW12 species appear to be inactive in the
reaction but need a pre-activation by H2O2 before a test. This pre-
activation can be managed by changing the order of addition of reaction
mixture components as exposed in Table 1 [28]. After 1 h of reaction
with hydrogen peroxide, PW12 forms the active {PO4[WO(O2)2]4}3−
Venturello species and dianionic {[WO(O2)2(H2O)]2O}2− species [17].
In that case, a 95% conversion is achieved after 3 h. The conversion is
faster when H3PO4 is added to perfect the stoichiometric reaction of
[PW12O40]
3− to form the {PO4[WO(O2)2]4}3− species. This result con-
firms the better activity of Venturello clusters than the peroxotungstate
species [17,30].
To be active it appears that the PW12 species must be previously
chemically modified by H2O2 to form the active species PW4. Then,
the effect of the addition order of reaction mixture components will
only have an impact if PW12 species are used directly as catalysts.
In the case of the PW4–C18 hybrid the reaction order will not have
an influence on the catalytic performance (because PW4 are the al-
ready active species) and was thus not tested. Results on the effect
of reactants introduction order are exposed in Table 1 for the
PW12–C18. As suspected, no activity is observed for the C18–PW12
hybrid catalyst even if reaction with H2O2 during 1 h is performed.
The hybridization of PW12 species does not allow the polyoxometalates
transformation into active peroxo species. This may likely be explained
by the facts that (i) polyoxometalates are known to be more stable
when hybridized [31] and (ii) H2O2 has hindered access to PW12 be-
cause of the high hydrophobicity of the C18 matrix.
Fig. 1. IR spectra of hybrid materials in the low frequency region of polyoxometalate
species: (a) the pure C18 copolymer matrix, (b) the PW4–C18 hybrid, (c) the PW4–C18
hybrid recovered after four catalytic tests and (d) the PW12–C18 hybrid.

C. Swalus et al. / Catalysis Communications 37 (2013) 80–84
83
Table 1
Catalytic performance in cyclooctene epoxidation.a
Catalyst
Addition order of reaction mixture components
Reaction time
Conversion
[%]
Yield
[%]
Selectivity
[%]
TOFb [mol/h molW
]
PW12
PW12
PW12 + H3PO4
PW12–C18
PW12–C18
PW4–C18
CH3CN/cyclooctene/H2O2
H2O2/CH3CN/cyclooctene
H2O2 + H3PO4/CH3CN/cyclooctene
CH3CN/cyclooctene/H2O2
H2O2/CH3CN/cyclooctene
CH3CN/cyclooctene/H2O2
5
3
1.5
5
5
4
–
–
0.20
10.20
32.47
0.15
0.14
6.83
95
98
4
8
96
81
72
–
85
74
–
–
–
5
95
99
a
Reaction conditions: catalyst (0.03 mmol of W), hydrogen peroxide (30% in water) (2.5 mmol), cyclooctene (0.5 mmol) and acetonitrile (5 ml), 50 °C.
TOF = (moles of substrate consumed) × (moles of W)−1 × time−1; determined by GC from the initial rates of cyclooctene oxidation.
b
Although slightly less active than the homogeneous activated spe-
hydrophobic environments [32]. Moreover the hydrophobicity could
induce a better olefin adsorption. Inversely, the hydrophilicity of the
epoxide could reduce the contact time between the epoxide and the hy-
drophobic hybrid catalyst, preventing its non-desired over-oxidation
[33]. The combination of these two aspects brings about the improved
epoxide selectivity [34]. Such a combination of high conversion and
high selectivity with large excess of H2O2 is a quite remarkable
phenomenon in olefin epoxidation [9,35]. Finally a test with an
olefin/H2O2 ratio of 1/0.5 mol/mol was used to evaluate the H2O2
decomposition during the reaction. A conversion higher than 49%
of the olefin was reached while under these conditions a maximum
of 50% could be obtained (owing to the defect of H2O2), indicating
the excellent H2O2 efficiency.
Being an essential aspect of liquid phase heterogeneous catalysis,
the occurrence of leaching of hybridized PW4 species was also inves-
tigated. The recyclability was only evaluated for the active PW4
hybrid catalyst. The evolution of the conversion of a reaction medium
was monitored after catalyst removal by centrifugation at 40% conver-
sion (Fig. 4). It appears that no further conversion was observed in the
collected medium. The absence of leaching was confirmed through
recycling tests, namely using several times successively a given batch
of the hybrid catalyst in fresh reaction media. The corresponding results
are presented in Table 2. The latter has been tested four times. Consider-
ing 5 h of reaction, the conversion along these consecutive tests is the
same, suggesting that no leaching occurs. It appears clearly that electro-
static interactions are strong enough to avoid leaching of catalytic
species. According to these observations, the hybrid catalyst used in
this work can be assimilated to a heterogeneous catalyst without any
ambiguity. The absence of catalyst deactivation is also remarkable. In-
deed homogeneous PW4 species are usually deactivated by diols and
epoxide (via their over-adsorption) and are thus almost not reusable
[24,32]. It comes out that the PW4 immobilization inside the hydropho-
bic matrix induces an additional and highly interesting protection effect
of the PW4 species, namely it favors the quick desorption of deactivating
hydrophilic products.
cies, hybridized PW4 is efficient to epoxidize cyclooctene (Fig. 3). The
smaller activity may be linked to a lower accessibility of PW4 species
either by H2O2 or by the alkene molecule due to the hybridization.
Interestingly, the selectivities of the homogeneous and the PW4
hybrid catalysts are quite different. Remarkably, a constant >99% ep-
oxide selectivity is obtained with the hybrid. At the opposite, traces of
allylic oxidation products and 1,2-cyclooctanediol were detected for
the homogeneous catalysts which leads to diminish epoxy selectivity
(80%) (Fig. 3). These catalysts bring acidity induced by the activation
of H3PW12O40 by H2O2. This leads to the ring-opening of the epoxide
and likely explains the obtained lower selectivity. Our experiments
reveal that the hybridization prevents the acid-sensitive epoxide from
ring-opening. Moreover, despite a large excess of hydrogen peroxide
(olefin/H2O2 = 1/5 mol/mol), the hybrid catalyst also prevents the
over-oxidation process. This interesting property may be related to
the hybrid catalyst hydrophobicity. Indeed, some authors pointed out
the impact of the catalyst's hydrophobicity on epoxide hydrolysis or
over-oxidation [9]. It has been reported that epoxide is not stable in a
hydrophilic environment and that allylic oxidation is not significant in
Fig. 3. Catalytic performance of homogeneous and hybrid catalysts for cyclooctene
epoxidation in CH3CN. A better conversion is obtained for homogeneous catalysts
(a) but PW4 hybrid catalyst presents a higher selectivity in epoxycyclooctane (b).
Fig. 4. Hot catalyst filtration experiment on hybrid for cyclooctene epoxidation. (Δ)
Conversion without recovery and (■) test with catalyst recovery after 1 h by centrifu-
gation and filtration.

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C. Swalus et al. / Catalysis Communications 37 (2013) 80–84
Table 2
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In this study, the epoxidation of cyclooctene with homogeneous
and hybridized tungstophosphate species was achieved. It appears that
homogeneous [PW12O40]
3− species need a pre-activation step before a
catalytic test in monophasic H2O2/CH3CN conditions. Their direct hybrid-
ization as catalyst is then not efficient. Inversely hybrid catalyst with
{PO4[WO(O2)2]4}3− shows to be remarkably efficient in various points
of view. First, hybridization by electrostatic interaction is sufficient to
stabilize heterogenized active species that undergo no leaching under
epoxidation conditions. Second, protection against deactivation of PW4
species is brought by their immobilization inside the hydrophobic ma-
trix. Finally, the organic matrix allows to control the olefin and epoxide
adsorption so as to avoid epoxide ring-opening or overoxidation pro-
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and stable heterogeneous catalysts. Catalytic tests of hybrids with differ-
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C.S. thanks FRIA for his PhD student position. The Belgian laboratories
are partners of the “Inanomat” IUAP network and are indebted to the
‘communauté française de Belgique’ for financial support through the
ARC program (08/13-009).