Pickering emulsion stabilized by catalytic polyoxometalate nanoparticles: A new effective medium for oxidation reactions

Article abstract of DOI:10.1002/chem.201201799

Decyl-, dodecyl-, and tetradecyltrimethylammonium cations were combined with the catalytic polyoxometalate [PW₁₂O₄₀]^3- anion to give spherical and monodisperse nanoparticles that are able to stabilize emulsions in the presence of water and an aromatic solvent. This triphasic liquid/solid/liquid system, based on a catalytic surfactant, is particularly efficient as a reaction medium for epoxidation reactions that involve hydrogen peroxide. The reactions proceed at competitive rates with straightforward separation of the phases by centrifugation. Such catalytic "Pickering" emulsions combine the advantages of heterogeneous catalysis and biphasic catalysis without the drawbacks (e.g., catalyst leaching or separation time). Mixing it up a bit: Spherical nanoparticles of decyl- and dodecyltrimethylammonium cations with [PW₁₂O₄₀] ^3- formed stable Pickering emulsions in the presence of an aromatic solvent and water. This system is an efficient medium for epoxidation (see figure). Copyright

Full text of DOI:10.1002/chem.201201799

FULL PAPER
DOI: 10.1002/chem.201201799
Pickering Emulsion Stabilized by Catalytic Polyoxometalate Nanoparticles:
A New Effective Medium for Oxidation Reactions
Loꢀc Leclercq,[a] Adrien Mouret,[a] Anna Proust,[b] Vꢁronique Schmitt,[c]
Pierre Bauduin,[d] Jean-Marie Aubry,[a] and Vꢁronique Nardello-Rataj*[a]
Abstract: Decyl-, dodecyl-, and tetra-
decyltrimethylammonium cations were
combined with the catalytic polyoxo-
metalate [PW12O40]3À anion to give
spherical and monodisperse nanoparti-
cles that are able to stabilize emulsions
in the presence of water and an aro-
matic solvent. This triphasic liquid/
solid/liquid system, based on a catalytic
surfactant, is particularly efficient as a
reaction medium for epoxidation reac-
tions that involve hydrogen peroxide.
The reactions proceed at competitive
rates with straightforward separation of
the phases by centrifugation. Such cata-
lytic “Pickering” emulsions combine
the advantages of heterogeneous catal-
ysis and biphasic catalysis without the
drawbacks (e.g., catalyst leaching or
separation time).
Keywords: epoxidation · nanoparti-
cles · oxidation · Pickering emul-
sions · polyoxometalates
Introduction
have been explored, including biphasic catalysis,[3,4] ionic liq-
uids,[5] mesoporous materials,[6] dendrimers,[7] and hyper-
branched polymers.[8] In water/oil biphasic media, the cata-
lytic performance of POMs critically depends on the solubil-
ity of the peroxoPOMs in the organic phase. To facilitate
mass transfer of the anionic reactants or the catalysts be-
tween the two phases and to enhance the water/oil interfa-
cial area on stirring, cationic phase-transfer catalysts
(PTCs), such as hydrophobic onium salts, have been
added.[3,4] Venturello and co-workers and Ishii and co-work-
Many industrial processes still involve the use of stoichio-
metric reagents, which generate considerable amounts of
chemical waste. With regard to the growing demand for sus-
tainable chemistry, there is a strong need for the replace-
ment of these reagents by catalytic systems that exhibit both
high selectivity and high activity.[1] Polyoxometalates
(POMs) have been found to be efficient as catalysts for the
selective oxidation of substrates by hydrogen peroxide.[2]
Over the last three decades, several elegant oxidation reac-
tions with hydrogen peroxide that use POMs as catalysts
ers[9]
have
described
the
water-soluble
ionic
[PO4{W(O)(O2)2}4]3À catalyst, which has been extensively
applied to the oxidation of various organic substrates (e.g.,
alkenes, alcohols, diols, alkynes, amines, and sulfides) during
the last two decades.[10] Despite the numerous advantages of
phase-transfer catalysis, two serious drawbacks can be point-
ed out: 1) The recovery of the PTC and of the catalyst from
the reaction mixture is often troublesome and 2) the possi-
ble formation of a stable emulsion at the end of the reaction
complicates the separation of the two phases. One way to
circumvent these drawbacks is to immobilize the catalyst.[11]
However, this strategy remains marginalized and requires
several synthetic steps. Herein, as an alternative to conven-
tional phase-transfer catalysis, we propose an original ap-
proach based on emulsions that are stabilized by solid cata-
lytic nanoparticles. The major advantage of this strategy lies
in the possibility of obtaining stable micrometric droplets
that can be destabilized on demand by centrifugation. These
catalytic nanoparticles then become readily recoverable by
filtration after completion of the reaction. Depending on
their wettability at the water/oil interface, the particles tend
to stabilize the so-called water-in-oil (W/O) or oil-in-water
(O/W) Pickering emulsions.[12] Recently, Crossley et al. have
designed catalytic nanoparticles in emulsions by deposing
palladium onto carbon nanotubes.[13] Herein, we report the
[a] Dr. L. Leclercq, A. Mouret, Prof. Dr. J.-M. Aubry,
Prof. Dr. V. Nardello-Rataj
Universitꢀ Lille Nord de France
Universitꢀ Lille 1, ENSCL, EA 4478
Chimie Molꢀculaire et Formulation
59655 Villeneuve dꢁAscq Cedex (France)
Fax : (+33)320-470-599
E-mail: veronique.rataj@univ-lille1.fr
[b] Prof. Dr. A. Proust
UPMC Univ Paris 06, UMR CNRS 7201
Institut Parisien de Chimie Molꢀculaire
4 Place Jussieu, Case 42, 75005 Paris (France)
[c] Dr. V. Schmitt
Centre de Recherche Paul Pascal
Universitꢀ de Bordeaux, UPR 8641-CNRS
115 Avenue du Dr. Albert Schweitzer
33600 Pessac (France)
[d] Dr. P. Bauduin
Institut de Chimie Sꢀparative de Marcoule
UMR 5257, CEA/CNRS/UM2/ENSCM
BP 17171 CEA Marcoule
F-30207 Bagnols-sur-Cꢂze (France)
Supporting information for this article, including general information
and characterization data, is available on the WWW under http://
dx.doi.org/10.1002/chem.201201799.
Chem. Eur. J. 2012, 00, 0 – 0
ꢃ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
1
&
ÞÞ
These are not the final page numbers!

first example of easily accessible Pickering emulsions as a
catalytic oxidizing reaction medium. The amphiphilic POM
nanoparticles are obtained by electrostatically coupling
[PW12O40]3À anions to alkyltrimethyl ammonium cations,
hereafter abbreviated as [Cn] (with C6 to C16 alkyl groups).
The usefulness of such stabilized emulsions as reaction
media is illustrated in the catalytic epoxidation of several al-
kenes by hydrogen peroxide.
water molecules, which indicates that the nanoparticles
remain hydrated despite the lyophilization. In other words,
the nanoparticles should include structuring water mole-
cules. In aqueous solutions, [PW12O40]3À anions have a limit-
ed stability range. Indeed, at pH>3, they are reversibly and
quickly transformed into various lacunar species. Herein,
the [PW12O40]3À anions remain intact owing to the precipita-
tion of [C12]3ACTHNUTRGNEUNG
[PW12O40].[14] Moreover, for the following study,
we must bear in mind previous reports that have indicated
that the addition of an organic solvent has a stabilizing
effect on the POM anions.[15] For the dodecyltrimethylam-
Results and Discussion
monium species, [C12]3ACHTUNGTRNE[GNU PW12O40], the size of the particles as
The POM nanoparticles were prepared by simple cation ex-
change. An aqueous solution of H3ACHTUNGTRNE[UNG PW12O40] (1 equiv) was
estimated from the TEM images was 35(Æ5) nm. This value
was confirmed by dynamic light scattering (DLS) experi-
ments, that is, 30(Æ8) nm, which also shows that the nano-
particles are monodisperse (see the Supporting Information,
Figure S1). It is noteworthy that the speeds of addition and
stirring have no influence on the size of the nanoparticles.
However, to obtain monodisperse nanoparticles, the concen-
added dropwise to an aqueous solution of alkyltrimethylam-
monium hydroxide [Cn][OH] (3 equiv) at 258C under vigo-
rous magnetic stirring (1500 rpm). A colorless precipitate
was formed within a few minutes, which was separated and
lyophilized, thereby providing spherical and fairly monodis-
perse [Cn]
N
tration of H3ACTHNGUTERNNUG
[PW12O40] must be lower than ꢄ10À3 m. At
It is noteworthy that the 31P NMR, Raman, and IR spec-
tra are typical of the [PW12O40] anion (see the Supporting
Information). Moreover, elemental analysis shows that the
nanoparticles are formed from a combination of three or-
ganic cations to one [PW12O40] anion and that no lacunar
POM structures are present; there are only some residual
higher concentrations, the nanoparticles self-assemble into
very compact and polydisperse aggregates (see the Support-
ing Information, Figure S2). The structure is very different
from that obtained with [NH4]3ACTHNUTRGNEUNG
[PW12O40],[16,17] in which the
[PW12O40]3À units are connected by the acidic protons; in
this case, each POM is maintained by van der Waals interac-
tions between the alkyl tails of the ammonium cations. In
other words, we suppose that the alkyl chains of the cations
are overlapped to generate a lamellar-packing arrangement
of the POM units. These interactions ensure cohesion be-
tween the nanoparticles. Finally, the influence of the alkyl
chain length on the size of the [Cn]3ACHTUNGRTNEUNG[PW12O40] nanoparticles
has been investigated (Figure 1e). The size is around 40 nm
for 6ꢀnꢀ12 and increases up to 96 nm for the
ACHUNTGERNUN[G C16]3AHCTUGNTREN[NUGN PW12O40] nanoparticles.
To confirm the internal structure of the [C12]
nanoparticles, small-angle X-ray scattering (SAXS) mea-
AHCTUNGTREGsNNUN urements were performed (Figure 2). In the low wave-
3ACHTGNURTEN[NUNG PW12O40]
vector (q) range, a decrease in the scattered intensity, which
follows a qÀ4 dependence, that is, the so-called Porodꢁs
regime, is attributed to nanoparticles with diameters of
around 30–40 nm (Figure 1). At higher q values, the scatter-
ing spectrum shows two broad and intense peaks at qmax
=
Figure 1. SEM (a) and TEM images (b,c) of the [C12]3ACHTNUTGRNE[GNU PW12O40] nanopar-
ticles; d) schematic representation of the lamellar organization of
[PW12O40]3À anions in the nanoparticle; and e) the effect of alkyl chain
length on the size of the [Cn]
DLS.
G
Figure 2. Small-angle X-ray scattering (SAXS) spectra of powders of
[C12]3A[PW12O40] (grey line) and H3A[PW12O40] (black line).
N
CHTUNGTRENNUNG
&
2
&
www.chemeurj.org
ꢃ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 0000, 00, 0 – 0
ÝÝ
These are not the final page numbers!

Polyoxometalate-Nanoparticle-Stabilized Pickering Emulsion
FULL PAPER
2.20 and 6.36 nmÀ1, which correspond to the internal struc-
ture of the nanoparticles. These peaks correspond to distan-
ces in real space (2p/qmax) of 2.85 and 1 nm, respectively.
The second peak can be attributed to the intramolecular
structure of the POM because the [PW12O40]3À anion is
roughly spherical, with a diameter of 1 nm. The SAXS spec-
trum of H3ACHTUNGTRENNUNG[PW12O40] is provided for comparison. This spec-
trum shows sharp scattering peaks in the range qꢁ6 nmÀ1,
which are due to the longer ordering of the POM in the
crystal. The first peak for [C12]3ACHTUNGTRNE[UNG PW12O40] provides informa-
tion on the intermolecular distance between the POM units
inside the nanoparticles. This distance (2.85 nm) corresponds
to the length of the surfactant, that is, around 1.9 nm in an
extended conformation, which is added to the diameter of
the POM (1 nm). A peak of lower intensity is observed at
q=4.4 nmÀ1, which corresponds to exactly twice the qmax
value of the first peak (2.2 nmÀ1). This peak likely corre-
sponds to the second order of a local lamellar arrangement
of the POM layers inside the nanoparticles. Consequently,
the nanoparticles are composed of closed-packed
Figure 3. a) Microphotograph that was taken after dilution in toluene of
the water/toluene (3:1 v/v) emulsion that was stabilized by 1.7 wt.%
[C12]3ACTHUNGTREN[GNU PW12O40] nanoparticles (11500 rpm, 60 s). b) Droplet-size-distribu-
tion profile, as estimated from the microphotograph. The average droplet
size is shown by the arrow after fitting with the log-normal function (Ori-
ginPro 8).
Table 1. Average droplet size of the Pickering emulsions as a function of
the aromatic solvent.[a]
Solvent
Diameter [mm]
Solvent
Diameter [mm]
ACHTUNGTRENNUNG[C12]3ACHTUNGTRENNUNG[PW12O40] units and the alkyl chains of the surfactant
moiety adopt an extended conformation.
Contrary to previously reported catalytic surfactants that
were based on the dimethyldioctylammonium and molyb-
benzene
toluene
o-xylene
33
20
12
m-xylene
p-xylene
mesitylene
21
13
17
[a] Obtained by using ImageJ software (National Institutes of Health,
date dianion, [C8C8]
three-liquid-phase microemulsion systems in the presence of
water and solvent,[18] the[Cn]
3A[PW12O40] nanoparticles, with
2ACHTUNGTNER[NUNG MoO4], which spontaneously formed
USA) and fitted with the log-normal function; S.D.=10%.
CTHUNGTRENNUNG
n=6–16, are neither soluble in water nor in typical organic
solvents, such as alkanes, aromatic compounds, chloroform,
dichloromethane, or acetonitrile. However, by screening a
series of water/organic-solvent biphasic systems in the pres-
All of the aromatic solvents behave in a similar manner,
thereby giving a monomodal but slightly polydisperse emul-
sion. Some discrepancies can be observed between the aro-
matic solvents because benzene provides the largest droplets
whereas the smallest ones are obtained with o-xylene. Pre-
sumably this difference in droplet size results from the
greater or lesser ability of the solvent to penetrate the inter-
facial nanoparticles; the penetration degree of the solvent
across the organic matrix depends on its molecular structure
and its polarity.
The stability of the emulsion has been investigated by
multiple light scattering coupled with vertical scanning to
identify and quantify the destabilization phenomena.[19] Be-
cause the backscattering intensity is directly proportional to
the volume fraction of the dispersed phase and to the drop-
let size, local changes in concentration (e.g., creaming and
sedimentation) and global changes in size (e.g., flocculation
and coalescence) can be detected (Figure 4). The evolution
of the diffuse reflectance at the mid-height of the tube (red
arrow) over time indicates a slight increase in droplet size
and the clear decrease in the backscattering at the top of
the sample (black arrows) is characteristic of the formation
of a small upper water phase. This evolution is characteristic
of a slow limited-coalescence process.[20] It is noteworthy
that temperature increases the rate of coalescence but the
emulsion remains globally stable for over 24 h (Figure 4b).
After 6 months, a small upper water phase that corresponds
to 8% of the initial total volume is observed. The other part
of the sample is composed of the Pickering emulsion. Al-
though the emulsion exhibits long-term stability, breaking-
ence of [Cn]3ACHTUNGTRENNUNG[PW12O40] nanoparticles as a function of n, we
found that water/aromatic-solvent (e.g., toluene, xylenes,
etc.) systems that were based on [C10], [C12], or [C14] ammo-
nium POM nanoparticles gave stable whitish emulsions
after stirring at 11500 rpm for 60 s. The low electrical con-
ductivity of these emulsions indicates the presence of an ex-
ternal oil phase. For a water/aromatic-solvent ratio of 3,
W/O-stable emulsions can be obtained with 0.5 wt.% nano-
particles. It is noteworthy that the viscosity of the emulsion
increases when the amount of nanoparticles increases.
Moreover, no phase inversion occurs on changing the water/
oil volume ratio. Nanoparticles with ammonium chain
lengths of 6, 8, and 16 lead to less stable emulsions with
excess water and solvent phases (see the Supporting Infor-
mation, Figure S3).
The microstructure of the water/toluene emulsion that
was obtained with [C12]3ACHTUNGTRNEUNG[PW12O40] nanoparticles was exam-
ined by optical microscopy after dilution in toluene. Based
on emulsion microphotographs, the droplet-size distribution
of stable W/O Pickering emulsions that were prepared with
other aromatic solvents, that is, benzene, ortho-, meta-, and
para-xylenes, and mesitylene, has been determined. The dis-
tribution for the water/toluene emulsion that was stabilized
by [C12]3ACHTUNGTRENNUNG[PW12O40] nanoparticles is shown in Figure 3 and
the mean diameters of the emulsion droplets with the vari-
ous investigated aromatic solvents are reported in Table 1.
Chem. Eur. J. 2012, 00, 0 – 0
ꢃ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
&
3
&
ÞÞ
These are not the final page numbers!

V. Nardello-Rataj et al.
Figure 4. Evolution of the residual diffuse reflectance (DR) versus the
Figure 5. a) Macroscopic views of the water/toluene/ACHTUNRTGENNU[G C12]3ACHTUNGTRENNUNG[PW12O40]
height of the sample and time of the water/toluene emulsion (3/1 v/v)
that was stabilized with [C12]3ACHTUNGTRNEUNG[PW12O40] nanoparticles (1.7 wt.%) after
emulsification (11500 rpm, 60 s): a) at 258C and b) at 608C. Inset: repre-
sentation of the emulsion-limited-coalescence process (T: transmitted
light, BS: backscattered light).
system before emulsification, during the reaction, and after centrifugation
(from left to right). b) Schematic representation of the catalytic epoxida-
tion of olefins inside this emulsion.
soluble catalytic particles remain stable compared to the
corresponding solvated species. On the other hand, it has
been reported that the addition of phosphoric acid can im-
prove catalyst performance through interactions with the
polyoxotungstate.[21] Hence, dodecyltrimethyl ammonium di-
up of the droplets and ultimate coalescence can be readily
induced on demand by centrifugation (4000 rpm, 5 min),
thereby providing a system that is composed of three well-
separated phases: water, toluene, and solid amphiphilic
POM nanoparticles at the bottom of the tube.
hydrogen phosphate, [C12]ACHTNUGTRNEUNG[H2PO4], was added to the system
to provide a pH-buffering capacity to the system (pHꢁ6.5),
thereby preventing hydrolysis of the epoxide. It is notewor-
thy that the addition of this water-soluble amphiphile does
not modify the stability of the emulsion.
The optimal conditions for the reaction were determined
by using cyclooctene as a model substrate because of its
easy and selective conversion into its corresponding epoxide
(Table 2).
The physicochemical properties of the Pickering emulsion
that is stabilized by [C12]3ACHTUNGTRENUNG[PW12O40] nanoparticles (i.e., the
increase in the W/O interface and in emulsion stability),
combined with its potential catalytic activity and its facile
breaking by centrifugation, make such a system of particular
interest for oxidation reactions that involve hydrogen perox-
ide as the oxidant (Figure 5).
Most hydrogen-peroxide-based epoxidation reactions that
are catalyzed by H3PW12O40 are usually performed in bipha-
sic systems by using chlorinated solvents or acetonitrile in
the presence of various PTCs (e.g., methyltrioctylammoni-
um chloride, cetylpyridinium chloride, tetrabutylammonium
bromide, etc.), which complicates the recovery of the prod-
ucts and the catalysts.[2b,4,11c] Moreover, as pointed out by
Hill and co-workers, “the use of chlorocarbons largely de-
feats the significant environmental and economic advantages
of using H2O2 as the oxidant”.[4a] The same authors have
also reported that catalyst inactivation can take place, owing
to decomposition of the PTC. In fact, the reaction is per-
formed at relatively high temperatures (>658C) and the re-
action medium is highly acidic (pHꢁ1). In this case, the in-
In the presence of toluene and water, only the amphiphil-
ic nanoparticles [C12]3ACHTUNGTRNE[NUG PW12O40] provide both a stable emul-
sion system and the best catalytic performances (Table 2,
entry 4). When the acid form of the catalyst, H3PW12O40, is
used, a biphasic water/toluene medium is obtained and the
epoxidation of cyclooctene at 658C proceeds very slowly
(TOF0 <0.8 hÀ1; Table 2, entry 1). The addition of stoichio-
metric amounts of [C12][Br] (Table 2, entry 2) or [C12][OH]
(Table 2, entry 3; molar ratio: 3:1) increases the catalytic ac-
tivity up to 11 hÀ1 but the conversion is only 35% after 3 h.
Under these conditions, solid particles are generated in situ,
thereby providing an emulsion with some excess water but
the emulsion is rapidly destabilized into a biphasic and inef-
ficient system. This result is probably due to partial cation
&
4
&
www.chemeurj.org
ꢃ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 0000, 00, 0 – 0
ÝÝ
These are not the final page numbers!

Polyoxometalate-Nanoparticle-Stabilized Pickering Emulsion
FULL PAPER
Table 2. Catalytic oxidation of cyclooctene in water (W)/organic solvent
does not increase the TOF0 value). Moreover, at 358C, the
reaction also gives a relatively good activity (conversion:
38%, TOF0 =13.9 hÀ1). Because the [PW12O40]3À anion is
known to form the Venturello catalyst ([PW4O24]3À) in the
presence of H2O2, we also performed the reaction with
(toluene T, acetonitrile A, and chloroform C).[a]
[c]
Entry Solvent Emulsion Catalyst
Conversion TOF0 [hÀ1
[%][b]
]
1[d]
2[e]
W/T
W/T
no
unstable 3[C12][Br]/H3-
[PW12O40]
unstable 3[C12][OH]/H3-
[PW12O40]
[C12]3A[PW12O40]
unstable H3PW12O40
H
G
<3
33
<0.8
10.8
H CHTUNTGNERUG[N PW4O24] and [C12]3CAHTUNGTRENNUNG
3A
As expected, the replacement of
[PW4O24] provides a high TOF0 value (58.6 hÀ1) but the con-
[PW4O24] (Table 2, entries 8 and 9).
AHCTUNGTRENNUNG
H3ACHUTGNTRN[EUNNG PW12O40] by H3-
3[e]
W/T
36
11.4
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
4
W/T
W/T
W/T
stable
N
98 (38)[f]
4
73
32.3 (13.9)[f]
1.2
24.4
version reaches a plateau plateaus at 61% after 3 h (Table 2,
entry 8). This latter value is improved in the presence of the
amphiphilic Venturello [C12]3ACHTNUGTREN[UNG PW4O24] species because a
conversion of 93% after 3 h is obtained (Table 2, entry 9).
However, despite its very low solubility in water and in aro-
matic solvents, [C12]3ACHTNUGTRNE[UGN PW4O24] does not provide any emul-
sion, probably because the formation of the nanoparticles is
hindered by the non-spherical geometry of the anion. We
also notice that the wettability of the solid by water and tol-
uene is bad. Moreover, although it is known to be a very
active intermediate,[2] the Venturello catalyst suffers from
several drawbacks: 1) it requires prior preparation, 2) it
shows some leaching (only 50% of the catalyst could be re-
cycled and the conversion was only of 63% after 3 h), and
3) mass-transfer limits the reaction rate (i.e., without stir-
ring, the conversion was 20% after 3 h, whereas it reached
5[g]
6[e,g]
stable
3[C12][OH]/H3-
[PW12O40]
[C12]3A[PW12O40]
3A[PW4O24]
[C12]3A[PW4O24]
[C12]3A[PW12O40]
[C12]3A[PW12O40]
AHCTUNGTRENNUNG
7[g]
W/T
W/T
W/T
W/A
W/C
stable
no
U
N
91
61
93
15
32
30.4
58.6
56.1
3.4
8[d]
9[d]
no
N
CHTUNGTRENNUNG
10[h]
11[h]
no
N
CHTUNGTRENNUNG
no
G
U
16.2
[a] Reaction conditions: catalyst (30 mmol), [C12]ACHTNUTRGNEG[UN H2PO4] (30 mmol), water
(4.5 mL), organic solvent (1.5 mL), cyclooctene (3 mmol), H2O2
(3 mmol), 658C. [b] Determined by GC (Agilent 6890N, HP-1, 30 mꢄ
0.32 mm) after 3 h, epoxide>99%. [c] Initial turnover frequency (mole
of alkene converted per mole of [C12]
[d] Without [C12][H2PO4], 1000 rpm. [e] [C12][Br] or [C12][OH] (90 mmol)
and H3A[PW12O40] (30 mmol), 1000 rpm. [f] T=358C. [g] Silica (hydrophilic
3ACTHUGNTREN[NNUG PW12O40] nanoparticles per hour).
ACHTUNGTRENNUNG
CHTUNGTRENNUNG
fumed Silica Aerosil 150; 200 mg), 1000 rpm. [h] 1000 rpm. All of the sys-
tems were emulsified with an Ultra-turrax at 11500 rpm for 60 s before
heating at 658C.
98% with [C12]
son, the activity of the [C12] CTHUNGTRENNUNG
[PW12O40]; see above). Finally, for compari-
3A[PW12O40] nanoparticles was
exchange, thereby leading to a mixture of different species
with different interfacial activities. In fact, under the same
also investigated in acetonitrile and chloroform (Table 2, en-
tries 10 and 11). In both of these media, the catalytic activity
was much weaker, owing to the absence of a stabilized
emulsion system. All of these findings clearly show that the
conditions, H
provide stable emulsions. On the contrary, with
[C12]3A[PW12O40] nanoparticles, the conversion reaches 98%
ACHTUNGTRENNUNG[C12]2ACHTUNGTRENNUNG[PW12O40] and H2AHCUTNGETRG[NNUN C12]ACHTNUGTERN[UNGN PW12O40] do not
A
U
[C12]3ACHTUNGTREN[NUG PW12O40] nanoparticles act as both emulsion stabiliz-
after 3 h and the TOF0 value is multiplied by a factor of
three (Table 2, entry 4). To estimate the catalytic properties
of the [C12]3ACHTUNGTRENNUNG[PW12O40] nanoparticles compared to the free
ers (as in the case of the Pickering emulsions) and as effi-
cient catalysts because of their interfacial properties and ac-
tivities.
polyoxometalate, at similar emulsion stabilities, we per-
formed some control experiments with silica nanoparticles
Raman spectroscopy was performed on the solid
ACHUTNGERN[UNG C12]3AHCTUNTGNERN[UNG PW12O40] nanoparticles before and after the reaction.
(Table 2, entries 5–7). With acidic catalyst H
[PW12O40], the
As shown in the Supporting Information (Figure S4), the
FTIR, Raman, and 31P NMR MAS spectra were identical:
only [PW12O40]3À anions were detected at the beginning and
end of the reaction.[11c,22] Moreover, during the reaction,
analysis of the separated aqueous and organic phases of the
emulsion by these two techniques did not show the presence
conversions and TOF0 values are similar to those without an
emulsion (cf. Table 2, entries 1 and 5). In contrast, with the
addition of stoichiometric amounts of [C12][OH] (Table 2,
entry 6; molar ratio: 3:1), the catalytic activity increases up
to 24.4 hÀ1 for a conversion of 73% after 3 h. Therefore, the
presence of a stable emulsion is essential and increases the
catalytic activity by a factor of two (cf. Table 2, entries 3 and
6). Finally, the addition of silica to the emulsion that is
of the Venturello [C12]3ACHTUNGTRENUNG[PW4O24]. As a very active inter-
mediate, we cannot completely exclude its involvement in
the process. However, we can assert that, if it is formed, it
does not exceed 0.2% otherwise it would have been detect-
ed. Finally, the TEM and SEM images before and after the
reaction reveal that the morphology of the nanoparticles re-
mains unchanged. These observations are indicative of the
stability of the nanoparticles under the catalytic conditions
and support the literature data, which claim that the addi-
tion of an organic solvent has a stabilizing effect on the
Keggin anions.[11] This assumption has also been confirmed
by recycling the nanoparticles. Indeed, the already-used
emulsified reaction medium was centrifuged, water and tol-
uene were removed, and the catalytic nanoparticles were
charged again with water, toluene, cyclooctene, H2O2, and
based on [C12]
better results (cf. Table 2, entries 4 and 7). Hence, we have
demonstrated that amphiphilic [C12]3A[PW12O40] nanoparticles
3ACHTUNGTRENNUNG[PW12O40] nanoparticles does not provide
CTHUNGTRENNUNG
are the most efficient system, owing to the combination of
emulsification and catalytic properties. Indeed, the good ac-
tivity of [C12]3ACHTUNGTRENNUNG[PW12O40] nanoparticles results from the mi-
crostructured reaction medium, that is, the emulsion pro-
vides a much larger water/oil interfacial area, where the cat-
alytic nanoparticles are localized. Moreover, it is also rele-
vant to note that these catalytic performances are obtained
without stirring the Pickering emulsion, thus indicating that
the process is only driven by the catalytic cycle (the stirring
Chem. Eur. J. 2012, 00, 0 – 0
ꢃ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
&
5
&
ÞÞ
These are not the final page numbers!

V. Nardello-Rataj et al.
[C12]
G
Conclusions
selectivity of the recovered catalyst were the same after at
least five consecutive cycles.
Solid amphiphilic catalytic polyoxometalate nanoparticles
that are made of [C12]3A[PW12O40] form Pickering emulsions
To extend the scope of this oxidizing microheterogeneous
catalytic system, the epoxidation reactions of cyclohexene,
limonene, and 1-octene have been carried out; the results
are reported in Table 3. The reactivity of cyclohexene is sim-
ilar to that of cyclooctene (TOF0 >30 hÀ1) and complete
CHTUNGTRENNUNG
(as do silica nanoparticles) in the presence of water and an
aromatic solvent. These new reaction media are particularly
efficient for the epoxidation of olefins, thereby leading to
competitive reaction rates, good yields, and good selectivity.
Such catalytic emulsions combine the advantages of biphasic
catalysis and heterogeneous catalysis because they allow
easy separation of the products and the catalysts without
their drawbacks (e.g., separation time, catalyst leaching). In
comparison with the immobilization technique on silica, the
preparation of this catalyst is much simpler. Moreover, this
type of emulsion, which is based on 75% water and only
25% organic solvent, allows for the recovery of catalyst and
product(s) in two distinct phases and the catalyst can be
readily recycled. Its efficiency in terms of reactivity has
been demonstrated in the selective preparation of epoxides
on a preparative scale (0.5m). Owing to the current interest
in new catalytic processes, further development of these cat-
alytic Pickering emulsions is expected for other catalytic
processes that involve hydrogen peroxide and other poly-
Table 3. Epoxidation of cyclooctene, cyclohexene, limonene, and
1-octene in
a
water/toluene emulsion that was stabilized by [C12]3-
ACHTUNGTRENNUNG
[PW12O40] nanoparticles.[a]
[c]
Substrate
H2O2
[equiv]
Dt
[h]
Conversion[b]
[%]
Yield of
epoxide [%]
TOF0
[hÀ1
]
1
1
3
3
98
95
>99
32.3
34.8
>99
1
1.5
6
6
83
97
>95[d]
>95[d]
27.6
32.9
AHCTUNGTRENNUNG
6
17
6
49
92
63
96
88
59
74
42
1
2
11.1
13.4
15
Experimental Section
[a] Reaction
conditions: [C12]
[PW12O40]
nanoparticles
(30 mmol),
A
ACHTUNGTRENNUNG[H2PO4] (30 mmol), water (4.5 mL), toluene (1.5 mL), olefin
General procedure for the biphasic epoxidation experiments:
[C12]3A[PW12O40] nanoparticles (30.0 mmol), [C12][H2PO4] (30 mmol), and
N
G
ACHTUNGTRENNUNG
CTHUNGTRENNUNG
H2O2 (3 mmol) were dissolved in water (4.5 mL), the olefin (3 mmol) was
dissolved in toluene (1.5 mL), and the two solutions were charged in a
glove bag into a 10 mL reactor that was emulsified by using an Ultra-
turrax (11500 rpm, IKA, T 10 basic). The reactor was sealed and heated
at 658C under magnetic stirring (1150 rpm). The reaction medium was
sampled during the reaction for GC analysis of the organic phase after
decantation. The results presented are the average of at least 3 runs
under each set of conditions.
cles per hour). [d] Limonene-1,2-epoxide. All of the systems were emulsi-
fied with an Ultra-turrax at 11500 rpm for 60 s before heating at 658C.
conversion is obtained within 3 h with 1 equivalent of H2O2
(compared to the substrate). For the two other substrates, a
slight excess of H2O2 is needed to obtain a satisfactory con-
version. In the case of 1-octene, despite an increase in the
kinetic rate, a significant decrease in the selectivity for the
epoxide is observed over time, in favor of the corresponding
diol. In all cases, the TOF0 values were higher than—or
equivalent to—those of previously reported biphasic reac-
tions that were performed with a PTC.[4,11c,23] For instance,
Maksimchuk et al. reported a TOF0 value of 24.6 hÀ1 for the
homogeneous epoxidation of cyclohexene in acetonitrile
Acknowledgements
Support for this work was provided by the ANR Catasurf (Project ANR-
10-CD2I-01). We are most grateful to Ahmed Addad for performing the
TEM experiments, Jean-Noꢅl Staelens for performing the FTIR experi-
ments, and Sꢀverine Renaudineau for her helpful discussions.
with the H3ACHTUNGTRENNUNG
[PW12O40] catalyst.[11c] However, the conversion
was only 72% and the selectivity for the epoxide was 78%
after 3 h. Moreover, the catalyst and products could not be
separated easily and the reaction required 2 equivalents of
H2O2 per substrate. Similar results have been reported for
cyclooctene, limonene, and 1-octene (TOF0 =24.0, 27.6, and
7.8 hÀ1, respectively).[11c] In each case, the conversion and
the selectivity were lower than those that were obtained
with the Pickering emulsion system that was based on cata-
lytic nanoparticles.
[1] C. D. Frohning, C. W. Kohlpainter in Applied Homogeneous Cataly-
sis with Organometallic Compounds (Eds.: B. Cornils, W. A.Herr-
mann,), Wiley-VCH, Weinheim, 1996, p. 29.
[2] a) C. Venturello, R. D’Aloisio, J. Org. Chem. 1988, 53, 1553–1557;
b) Y. Ishii, K. Yamawaki, T. Ura, H. Yamada, T. Yoshida, M.
Ogawa, J. Org. Chem. 1988, 53, 3587–3593; c) C. L. Hill, C. M.
Prosser-McCartha, Coord. Chem. Rev. 1995, 143, 407–455; d) T.
Okuhara, N. Mizuno, M. Misono, Adv. Catal. 1996, 41, 113–256;
e) B. Notari, Adv. Catal. 1996, 41, 253–334; f) R. Neumann, Prog.
Inorg. Chem. 1998, 47, 317–370; g) T. Sakamoto, C. Pac, Tetrahe-
dron Lett. 2000, 41, 10009–10012; h) R. G. Finke in Polyoxometalate
Chemistry (Eds.: M. T. Pope, A. Mꢆller,), Kluwer Academic Publish-
&
6
&
www.chemeurj.org
ꢃ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 0000, 00, 0 – 0
ÝÝ
These are not the final page numbers!

Polyoxometalate-Nanoparticle-Stabilized Pickering Emulsion
FULL PAPER
er: Netherlands, 2001. pp. 363–390; i) I. V. Kozhevnikov, Catalysis
by Polyoxometalates, Wiley, Chichester, 2002; j) M. Klawonn, M. K.
Tse, S. Bhor, C. Dçbler, M. Beller, J. Mol. Catal. A Chem. 2004, 218,
13–19; k) N. Mizuno, K. Yamaguchi, K. Kamata, Coord. Chem. Rev.
2005, 249, 1944–1956; l) Y. Ding, B. Ma, D. Tong, H. Hua, W. Zhao,
Aust. J. Chem. 2009, 62, 739–746; m) K. Kamata, K. Yonehara, Y.
Nakagawa, K. Uehara, N. Mizuno, Nat. Chem. 2010, 2, 478–483.
[3] a) R. Neumann, A. M. Khenkin, J. Org. Chem. 1994, 59, 7577–7579;
b) A. Lambert, P. Plucinski, I. V. Kozhevnikov, Chem. Commun.
2003, 714; c) S. Zhang, G. Zhao, S. Gao, Z. Xi, J. Xu, J. Mol. Catal.
A Chem. 2008, 289, 22–27; d) J. Gao, Y. Zhang, G. Jia, Z. Jiang, S.
Wang, H. Lu, B. Song, C. Li, Chem. Commun. 2008, 332–334; e) Q.
Wang, X. Zhang, L. Wang, Z. Mi, J. Mol. Catal. A Chem. 2009, 309,
89–94; f) J. Xue, A. Wang, H. Yin, J. Wang, D. Zhang, W. Chen, L.
Yu, T. Jiang, J. Ind. Eng. Chem. 2010, 16, 288–292.
[4] a) D. C. Duncan, R. C. Chambers, E. Hecht, C. L. Hill, J. Am.
Chem. Soc. 1995, 117, 681–691; b) Y. Sasson, R. Neumann, Hand-
book of Phase Transfer Catalysis, Blackie Academic & Professional,
New York, 1997; c) S. G. Casuscelli, M. E. Crivello, C. F. Perez, G.
Ghione, E. R. Herrero, L. R. Pizzio, P. G. Vazquez, C. V. Caceres,
M. N. Blanco, Appl. Catal. A 2004, 274, 115–122; d) Y. Ding, Q.
Gao, G. Li, H. Zhang, J. Wang, L. Yan, J. Suo, J. Mol. Catal. A
Chem. 2004, 218, 161–170; e) R. Tayebee, Asian J. Chem. 2008, 20,
8–14.
c) N. V. Maksimchuk, K. A. Kovalenko, S. S. Arzumanov, Y. A.
Chesalov, M. S. Melgunov, A. G. Stapanov, V. P. Fedin, O. A. Khol-
deeva, Inorg. Chem. 2010, 49, 2920–2930.
[12] a) S. Pickering, J. Chem. Soc., Trans, 1907, 91, 2001–2021; b) P. Wal-
stra in Fundamentals of Interface and Colloid Science, Vol. 5, (Ed. J.
Lyklema,), Elsevier, Amsterdam, 2005, pp. 8.1–8.94; c) F. Gautier,
M. Destribats, R. Perrier-Cornet, J.-F. Dechꢀzelles, J. Giermanska,
V. Hꢀroguez, S. Ravaine, F. Leal-Calderon, V. Schmitt, Phys. Chem.
Chem. Phys. 2007, 9, 6455–6462; d) V. O. Ikem, A. Menner, A. Bis-
marck, Angew. Chem. 2008, 120, 8401–8403; Angew. Chem. Int. Ed.
2008, 47, 8277–8279; e) D. J. Cole-Hamilton, Science 2010, 327, 41–
42.
[13] S. Crossley, J. Faria, M. Shen, D. E. Resasco, Science 2010, 327, 68–
72.
[14] a) M. T. Pope, Heteropoly and Isopoly Oxometalates, Springer,
Berlin, 1983; b) Z. Zhu, R. Tain, C. Rhodes, Can. J. Chem. 2003, 81,
1044–1050; c) S. Himeno, M. Takamoto, T. Ueda, Bull. Chem. Soc.
Jpn. 2005, 78, 1463–1468.
[15] a) M. Fournier, R. Thouvenot, C. Rocchiccioli-Deltcheff, J. Chem.
Soc. Faraday Trans. 1991, 87, 349–356; b) L. Pizzio, P. Vꢇzquez, C.
Cꢇceres, M. Blanco, Catal. Lett. 2001, 77, 233–239.
[16] K. Okamoto, S. Uchida, T. Ito, N. Mizuno, J. Am. Chem. Soc. 2007,
129, 7378–7384.
[17] a) N. Mizuno, M. Misono, Chem. Lett. 1987, 967–970; b) T. Oku-
hara, H. Watanabe, T. Nishirama, K. Inumaru, M. Misono, Chem.
Mater. 2000, 12, 2230–2238.
[5] L. Liu, C. Chen, X. Hu, T. Mohamood, W. Ma, J. Lin, J. Zhao, New
J. Chem. 2008, 32, 283–289.
[6] M. V. Vasylyev, R. Neumann, J. Am. Chem. Soc. 2004, 126, 884–890.
[7] C. Jahier, L. Plault, S. Nlate, Isr. J. Chem. 2009, 49, 109–118.
[8] A. Haimov, H. Cohen, R. Neumann, J. Am. Chem. Soc. 2004, 126,
11762–11763.
[9] a) C. Venturello, E. Alneri, M. Ricci, J. Org. Chem. 1983, 48, 3831–
3833; b) C. Venturello, R. DꢁAloisio, J. C. J. Bart, M. Ricci, J. Mol.
Catal. 1985, 32, 107–110; c) Y. Ishii, K. Yamawaki, T. Yoshida, M.
Ogawa, J. Org. Chem. 1988, 53, 5549–5552.
[10] a) J.-M. Brꢀgeault, M. Vennat, L. Salles, J.-Y. Picquemal, Y. Mahha,
E. Briot, P. C. Bakala, A. Atlamsani, R. Thouvenot, J. Mol. Catal. A
Chem. 2006, 250, 177–189; b) K. Kamata, T. Hirano, S. Kuzuya, N.
Mizuno, J. Am. Chem. Soc. 2009, 131, 6997–7004.
[18] V. Nardello-Rataj, L. Caron, C. Borde, J.-M. Aubry, J. Am. Chem.
Soc. 2008, 130, 14914–14915.
[19] a) O. Mengual, G. Meunier, I. Cayre, K. Puech, P. Snabre, Colloids
Surf. A 1999, 152, 111–123; b) C. Lemarchand, P. Couvreur, M. Bes-
nard, D. Costantini, R. Gref, Pharm. Res. 2003, 20, 1284–1292.
[20] S. Arditty, C. P. Whitby, B. P. Binks, V. Schmitt, F. Leal-Calderon,
Eur. Phys. J. E 2003, 11, 273–281.
[21] Y. Mahha, L. Salles, J.-Y. Piquemal, E. Briot, A. Atlamsani, J.-M.
Brꢀgeault, J. Catal. 2007, 249, 338–348.
[22] Y. Guo, K. Li, X. Yu, J. H. Clark, Appl. Catal. B 2008, 81, 182–191.
[23] C. Aubry, G. Chottard, N. Platzer, J.-M. Brꢀgeault, R. Thouvenot, F.
Chauveau, C. Huet, H. Ledon, Inorg. Chem. 1991, 30, 4409–4415.
[11] a) R. Neumann, M. Cohen, Angew. Chem. 1997, 109, 1810–1812;
Angew. Chem. Int. Ed. Engl. 1997, 36, 1738–1740; b) K. Inumaru, T.
Ishihara, Y. Kamiya, T. Okuhara, S. Yamanaka, Angew. Chem. 2007,
119, 7769–7772; Angew. Chem. Int. Ed. 2007, 46, 7625–7628;
Received: May 22, 2012
Published online: && &&, 0000
Chem. Eur. J. 2012, 00, 0 – 0
ꢃ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
&
7
&
ÞÞ
These are not the final page numbers!

V. Nardello-Rataj et al.
Polyoxometalates
L. Leclercq, A. Mouret, A. Proust,
V. Schmitt, P. Bauduin, J.-M. Aubry,
V. Nardello-Rataj* ........... &&&& — &&&&
Pickering Emulsion Stabilized by
Catalytic Polyoxometalate Nanoparti-
cles: A New Effective Medium for
Oxidation Reactions
Mixing it up a bit: Spherical nanoparti-
cles of decyl- and dodecyltrimethylam-
monium cations with [PW12O40]3À
the presence of an aromatic solvent
and water. This system is an efficient
medium for epoxidation (see figure).
formed stable Pickering emulsions in
&
8
&
www.chemeurj.org
ꢃ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 0000, 00, 0 – 0
ÝÝ
These are not the final page numbers!