Versatile eco-friendly pickering emulsions based on substrate/native cyclodextrin complexes: A winning approach for solvent-free oxidations

Article abstract of DOI:10.1002/cssc.201300081

Solvent-less Pickering emulsions were developed and applied to catalytic oxidation. These systems are stabilized by inclusion complexes between cyclodextrins and substrates, forming a 3D network among the dispersed phase. In the presence of hydrogen peroxide as a green oxidant and [Na] ₃[PW₁₂O₄₀] as a catalyst, they provide particularly efficient reaction media for the oxidation of olefins, organosulfurs, and alcohols. The reactions proceed at competitive rates (up to 400 h^-1) with straightforward separation of the phases by centrifugation or heating. Moreover, these new eco-friendly systems work at a preparative scale (up to 2.5 M) and are recycled without loss of activity. Disperse and conquer! Solvent-less Pickering emulsions have been developed and applied to catalytic oxidation. The systems are stabilized by inclusion complexes between cyclodextrins and substrates forming a 3D network among the dispersed phase. In the presence of hydrogen peroxide as a green oxidant and [Na]₃[PW₁₂O₄₀] as a catalyst, they provide particularly efficient eco-friendly reaction media for the oxidation of olefins, organosulfurs, and alcohols. Copyright

Full text of DOI:10.1002/cssc.201300081

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DOI: 10.1002/cssc.201300081
Versatile Eco-friendly Pickering Emulsions Based on
Substrate/Native Cyclodextrin Complexes: A Winning
Approach for Solvent-Free Oxidations
Lo ¨ı c Leclercq, Roberto Company, Andrea Mꢀhlbauer, Adrien Mouret, Jean-Marie Aubry, and
[a]
Vꢁronique Nardello-Rataj*
Solvent-less Pickering emulsions were developed and applied
to catalytic oxidation. These systems are stabilized by inclusion
complexes between cyclodextrins and substrates, forming
a 3D network among the dispersed phase. In the presence of
hydrogen peroxide as a green oxidant and [Na] [PW O ] as
the oxidation of olefins, organosulfurs, and alcohols. The reac-
À1
tions proceed at competitive rates (up to 400 h ) with
straightforward separation of the phases by centrifugation or
heating. Moreover, these new eco-friendly systems work at
a preparative scale (up to 2.5m) and are recycled without loss
of activity.
3
12 40
a catalyst, they provide particularly efficient reaction media for
Introduction
During the last two decades, the green chemistry concept has
backs can be highlighted: 1) the recovery of the PTC and cata-
lyst from the reaction mixture is often troublesome, and 2) the
toxicity of the PTCs as cationic amphiphiles. Indeed, numerous
studies mentioned that PTCs resulted in skin irritation, hemoly-
[1]
emerged as a very important topic for sustainable chemistry.
It is framed by a set of principles, such as 1) the design of pro-
cesses to minimize the amount of byproducts and wastes,
[10]
2
3
) the use of safe and benign reactants and solvents, and
) catalytic processes. Generally, it is very difficult to reconcile
sis, environmental pollution, and so forth. One way to cir-
cumvent these drawbacks consists of immobilizing the catalyst
[5–7]
all these principles in a single process. For example, organic re-
actions often require organic solvents; thus the substitution of
hazardous solvents for eco-friendly ones, or even removing the
solvent completely, is a key point in the development of
a green process. In addition, to avoid chemical waste, the re-
placement of stoichiometric processes by catalytic systems
that exhibit both high selectivity and activity becomes more
on various supports.
However, this strategy requires several
synthetic steps. On the other hand, in 1991, Shimada et al. re-
[11]
ported that cyclodextrins (CDs) could be used as emulsifiers.
CDs are biodegradable and biocompatible cyclic oligomers of
d-glucopyranose and are described as shallow truncated cones
with a hydrophobic cavity that can be used to form inclusion
[12]
complexes (ICs; Figure 1). Recently, Saito et al. reported the
[
2]
and more clear. In this context, catalytic oxidation plays a cen-
[
3]
tral role in the fine-chemical industry. Polyoxometalates
POMs) are efficient catalysts for the selective oxidation of sub-
strates by hydrogen peroxide; a green reactant that generates
(
[
4]
only water as the byproduct. A large variety of processes
based on POMs and H O have been explored in recent de-
2
2
[
5]
[6]
cades that involved mesoporous materials, dendrimers, hy-
[
7]
[8]
Figure 1. Structure of the native CDs [n=6 (a-CD), 7 (b-CD), or 8 (g-CD)] and
their schematic representation.
perbranched polymers, phase-transfer catalysts (PTCs), and
[
9]
ionic liquids. In aqueous biphasic media, the catalytic perfor-
mance of POMs depends on the solubility of the peroxo-POM
species in the organic phase. To enhance the mass transfer of
the anionic POM catalyst between the two phases and to en-
hance the water/oil (W/O) interfacial area, cationic PTCs, such
formation of stable emulsions using CDs instead of the com-
[13]
monly and largely used surfactants. The presence of various
oil phases (e.g., n-alkane or n-alkanol) results in the formation
of oil/CD insoluble ICs. These solids, which are partially wetta-
ble by both water and oil, adsorb onto the W/O interface be-
tween the two phases and allow the formation of Pickering
[
8]
as hydrophobic onium, salts are added. Despite the numer-
ous advantages of phase-transfer catalysis, two serious draw-
[a] Dr. L. Leclercq, R. Company, A. Mꢀhlbauer, A. Mouret, Prof. Dr. J.-M. Aubry,
[13,14]
Prof. Dr. V. Nardello-Rataj
Universitꢁ Lille 1, EA 4478, Chimie Molꢁculaire et Formulation
F-59655 Villeneuve d’Ascq Cedex (France)
emulsions.
Despite their specific advantages, Pickering emulsions have
[15]
rarely been applied to catalysis. The few examples described
in the literature report the use of Janus silica nanoparti-
E-mail: veronique.rataj@univ-lille1.fr
Supporting Information for this article is available on the WWW under
http://dx.doi.org/10.1002/cssc.201300081.
[15a,b,c]
[15d]
cles,
amphiphilic POMs,
carbonaceous microcapsu-
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[
15e]
[15f]
les,
and polymersomes.
Herein, we report the first cata-
lytic Pickering emulsions based on CDs for the epoxidation of
alkenes and the oxidation of sulfides and alcohols. The concept
is based on a catalytic emulsion constituted of only the sub-
strate, which plays the role of the oil phase, water, and the CD.
In the present case, the catalyst is the water-soluble
[
Na] [PW O ] POM and the oxidizing agent is H O . The emul-
3 12 40 2 2
sion is stabilized by solid substrate/CD complexes. The solvent-
free microstructured reaction medium is particularly relevant in
terms of catalytic activity, workup, and recycling; this makes it
a particularly attractive eco-friendly system for oxidation at the
preparative scale.
Figure 2. TEM imaging the structural morphology of the b-CD/1 nanoparti-
cles separated from the 1/water/b-CD emulsion by centrifugation and their
mechanism of formation.
Results and Discussion
In the presence of a-, b-, and g-
CDs, and after emulsification
(11500 rpm during 60 s with an
Ultra-Turrax instrument), the cy-
clooctene (1)/water biphasic
system provides whitish, stable
emulsions for particular compo-
sitions. The electrical conductivi-
ty of the resulting emulsions in-
dicates a water external phase,
that is, O/W emulsions. More-
over, centrifugation (4000 rpm
for 5 min) allows rapid and dis-
tinct separation of the three
phases: water, 1, and a white
powder. After removal of the
two liquids, analysis of the dried
1
powder by H NMR spectroscopy
revealed that it was composed
Figure 3. Ternary phase diagrams of the 1/water emulsion stabilized by a-, b-, and g-CD at 258C. The composition
of an equimolar mixture of CD is represented in weight fractions. The white circles correspond to the composition used to perform the catalytic
reactions.
and 1, suggesting the formation
of 1:1 ICs. This is in good agree-
ment with the results reported
À1 [16]
by Yujuan and Runhua (Kb-CD/1 =706.5m ). TEM was used to
guest molecules into the cavity of CDs was studied by the
[17]
image the structural morphology of the precipitated fraction
PM7-DH+ method. From the minimum-energy structures of
the complexes shown in Figure S1 in the Supporting Informa-
tion, it can be explicitly understood that 1 fits the cavity size
of b-CD very well contrary to a- (the guest does not deeply
penetrate into the cavity) and g-CD (the cavity is too large to
achieve good stabilization of the guest). The calculated bind-
(
(
Figure 2). The structure reveals spherical nanoparticles
3.6 nm) formed from ICs (ꢀ46Æ20 ICs/particles). Based on
the results reported by Davarpanah and Vahabzade, we can
assert that these solid particles act as an emulsifier in our
[
14]
system.
À1
The ternary phase diagrams of the 1/water system stabilized
by a-, b-, and g-CDs are presented in Figure 3. Four different
types of behavior are observed, depending on the ratio
between the three components: 1) a stable O/W emulsion
ing energies are À71, À94, and À86 kJmol for the inclusion
of 1 into the a-, b-, and g-CDs, respectively, which are in good
agreement with the experimental observations. Because opti-
mal complexation is obtained with b-CD, this CD was further
investigated. It is noteworthy that, for a given CD percentage
in the O/Wstab region, the emulsion viscosity increases when
the 1/water ratio increases. Moreover, for a given 1/water ratio,
the emulsion viscosity increases when the amount of CD in-
creases. Finally, the best long-term stability is obtained for
high-viscosity emulsions. Therefore, the best compromise be-
tween stability and viscosity is obtained for 10 wt% CD supple-
mented with 45 wt% water and 45 wt% substrate, that is, 1 at
(
O/Wstab), 2) an unstable O/W emulsion with a separated water
phase (O/Wunstab +W), 3) an unstable O/W emulsion with a sepa-
rated oil phase (O/Wunstab +O), and 4) a two-liquid-phase region
plus a solid (O+W+S). It is also clear that the stable O/W
emulsion region is affected by the type of CD. Indeed, the
region for the stable emulsion phase extends in the following
order: a-CD<g-CD<b-CD. These results can be explained by
the variation in the cavity sizes. Indeed, the penetration of the
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a concentration of 4m. The phase diagrams of the other oil
phases investigated [i.e., cycloheptene (2), cyclohexene (3), 1-
methylcyclohexene (4), styrene (5), diphenylsulfide (6), thioani-
sole (7), and dibutylsulfide (8)] are presented in the Supporting
Information (Figures S2–S9).
birefringence of the droplets under crossed polarization was
due the adsorption of CD/1 ICs on the surface of the droplets.
The scope of the oils was then extended by replacing 1 with
other potentially oxidizable liquid substrates, notably alkenes
1–5 and sulfides 6–8 (see below and Table 1).
The emulsion stability was investigated by multiple light
[
18]
scattering at 608C (Figure 4). Because the backscattering in-
Table 1. Average droplet size of Pickering emulsions as a function of the
[
a]
oil phase.
[
b]
[b]
Oil phase
D
Oil phase
D
[
mm]
38
[mm]
1
2
6
7
25
28
23
3
4
5
33
34
35
8
9
9
27
59
59
[
a] Conditions: oil/water/b-CD (45/45/10 wt%), 11500 rpm, 60 s. [b] Ob-
tained from ImageJ (N.I.H., USA) and fitting with the log-normal function
Figure 4. Evolution of the residual diffuse reflectance, DR, at 608C versus
sample height and time for the cyclooctene 1/water/b-CD contact (45/45/
(standard deviation=10%).
10 wt%, 11500 rpm, 60 s; T=transmitted light, BS=backscattered light).
The physicochemical properties of the emulsion, which is
stabilized by the formation of insoluble ICs between native
CDs and the substrates that constitute the oil phase (i.e., O/W
interface increase and emulsion stability), combined with its
easy breaking by centrifugation make such systems of particu-
lar interest for catalytic oxidations involving H O as the oxi-
tensity is related to the volume fraction of the dispersed
phase, local changes in concentration and global changes in
size can be detected. As a function of time, we can see a back-
scattering increase at the bottom of the sample and a decrease
[15d]
at the top of the sample due to a sedimentation process.
2
2
Moreover, the evolution over time of the reflectance at mid-
height of the tube indicates only a slight increase in droplet
size. However, the variation of the diffuse reflectance remains
weak and no variation (i.e., phase separation) is visually notice-
able after six months. This behavior is characteristic of a slow
dant and the well-known [Na] [PW O ] water-soluble POM.
3 12 40
The optimal conditions for the reaction were determined by
using 1 as a model substrate because of its easy and selective
conversion into the corresponding epoxide (Table 2). It is note-
worthy that the addition of H O and catalyst does not disturb
2
2
[
19]
limited-coalescence process. Although the emulsion exhibits
long-term stability, ultimate coalescence can be induced on re-
quest by centrifugation (see above).
the stability of the emulsion system. The addition of H O
2 2
(0.27 equiv) allows a maximal conversion of 27% of the sub-
strate oil phase. The reactions were performed without stirring.
As a control experiment, the epoxidation at 658C in the
presence of [Na] [PW O ] and H O in a simple biphasic
The microstructure of the 1/water emulsion obtained with
b-CD was examined by optical microscopy and the droplet size
distribution of the stable O/W emulsion was determined; the
average droplet size is about 38 mm (Figure 5). Moreover, the
3
12 40
2
2
water/1 system free of b-CD gave no conversion (Table 2,
entry 1). This value is typical of POM-catalyzed oxidations with-
out PTC. In the presence of
0
1
.025 mmol of b-CD (i.e.,
8.5 mm in water) and
[
Na] [PW O ], no emulsion and
3 12 40
no catalytic activity are obtained
Table 2, entry 2). In contrast,
(
when 0.26 mmol was used,
a stable emulsion formed and
the conversion reached 24%
after 3 h (Table 2, entries 3 vs. 2).
This good activity clearly results
from the microstructured reac-
tion medium, that is, emulsion
provide a much larger W/O in-
Figure 5. Microphotographs observed after dilution of the O/W emulsion shown in Figure 4 in water (a) and
under cross-polarization (b), and the droplet size distribution profile (c).
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not alter the catalysis efficiency because all of these species
remain in the aqueous phase due to the absence of cationic
PTCs, which is also supported by the recycling of the catalytic
system after five consecutive runs (see below). Finally, we no-
ticed that the reaction was completely inhibited when the
starting materials contained traces of antioxidants because
they are often commercialized. Hence, all substrates were puri-
fied or synthesized. Then, to check the involvement of an anti-
2 2
Table 2. Catalytic epoxidation of 1 with H O in unstirred Pickering emul-
sion systems at 658C.
[
a]
[
b]
[c]
0
Entry
CD
mmol]
Emulsion
Catalyst
Conv.
[%]
TOF
À1
[
[h
]
1
2
3
0
no
no
Na
Na
Na
Na
Na
3
3
3
3
3
PW12
PW12
PW12
PW12
PW12
O
O
O
O
O
40
40
40
40
40
<1
<1
24
49
>99
20
–
–
0.025
0.26
0.26
1.3
0.26
0.26
0.26
yes
yes
yes
yes
yes
yes
42
400
372
38
7
[
d]
e]
4
5
6
7
8
[
À2
oxidant, a-tocopherol (9ꢃ10 mmol) was added to the cata-
H
3
PW12
O
40
24
40
lytic system. The finding clearly shows that the reaction does
not work at all (compare Table 2, entries 8 and 3). The addition
of radical scavengers (i.e., antioxidants) is often used to high-
light the formation of radical species during the oxidation pro-
cess. Moreover, the occurrence of a radical pathway often re-
sults in the formation of various byproducts, especially for 3.
Here, the selectivity in epoxide is very high for 3 (>99%;
Table 3). Therefore, we believe that another reason to account
Na
Na
3
PW
4
O
5
<1
[
f]
3
PW12
O
–
[
1
>
a] Conditions: 30 mmol of catalyst, 3.5 mmol of H
2
O
2
, 13 mmol of 1,
O, 3 h. [b] Determined by H NMR spectroscopy, epoxide
99%. [c] Initial turnover frequency. [d] 11 mmol of H , 8 h. [e] Condi-
, 6 mmol of 1, 1.35 mL of
1
.35 mL of H
2
2 2
O
tions: 30 mmol of catalyst, 6.3 mmol of H
H
2
O
2
À2
2
O, 2 mL of heptane, 30 min. [f] 9ꢃ10 mmol of a-tocopherol.
terfacial area (increase of ꢀ600). Moreover, the apparent acti-
vation energy of the 1/water/b-CD system is estimated to be
Table 3. Catalytic epoxidation of olefins in olefin/water emulsion stabi-
lized by b-CD/olefin complexes at 658C.
[a]
À1
2
5.4 kJmol . This activation energy is lower than that reported
for tungsten-catalyzed epoxidation (between 50 and
[b]
[c]
0
Substrate
Dt
[h]
Conversion
[%]
Epoxide
[%]
TOF
À1 [20]
À1
90 kJmol ). This suggests that the process is diffusion con-
[h
]
trolled. At 658C, it is also relevant to note that stirring
750 rpm) has no effect on the catalytic activity, indicating that
1
2
3
4
5
3
3
0.5
3
3
24
24
24
25
24
>99
>99
>99
>99
43
42
(
38
236
38
the viscosity is responsible for the diffusion limitation. In the
presence of 0.85 equivalents of H O relative to the substrate,
2
2
45
the conversion reaches 49% after 8 h and the turnover fre-
[
0
[
a] Conditions: 30 mmol of [Na]
3
[PW12
O
40],
3
2 2
to 3.5 mmol of H O ,
quency (TOF ) is multiplied by about 10 (Table 2, entry 4). The
0
.26 mmol of b-CD, 1.35 g of water, 1.35 g of olefin (12 to 14 mmol).
conversion does not reach 85% as expected, probably due to
blockage of interfacial active sites by the more polar epoxide,
compared with the substrate. This drawback can, however, be
overcome if required by replacing the “substrate” oil phase
with n-heptane. In this case, the catalytic activity increases up
1
b] Determined by H NMR spectroscopy. [c] Initial turnover frequency.
for the absence of the conversion of 1 when tocopherol is
added might be the blockage of active sites by the b-CD/a-to-
copherol complex. Indeed, in the presence of b-CD, a-toco-
pherol competes with the substrate, leading to fat- and water-
À1
to 372 h for a conversion of >99% after 0.5 h (Table 2,
entry 5). The possibility of using a solvent is also required for
solid substrates (see below, in the case of sulfide oxidation). In
the presence of H [PW O ], the conversion is less important
[22]
insoluble ICs. The b-CD/a-tocopherol ICs are located at the
1/water interface, leading to a reduction in the number of free
active sites.
3
12 40
than that with [Na] [PW O ] (Table 2, entries 6 vs. 3). With
3
12 40
[22b]
H [PW O ], the reaction medium becomes acidic, pHꢀ1, and
This finding indirectly confirms that the catalytic
3
12 40
under such conditions, at 658C, the CD starts to degrade, lead-
reaction takes place at the W/O interface (Figure 6).
[
21]
ing to a decrease of the catalytic activity.
Because the
The recycling of the catalytic system was then investigated:
the already used emulsified reaction medium was centrifuged
(4000 rpm for 5 min) and the upper organic phase was simply
removed and new batches of 1 (13 mmol) and H O (3.5 mmol)
3À
[
[
PW O ] anion is known to form the Venturello catalyst,
1
2
40
3
À
PW O ] , in the presence of H O , and thus, acting as a pre-
4
24
2
2
cursor of the active species, [Na] [PW O ] was also prepared
3
4
24
2
2
and investigated. Although it allows the formation of a stable
were added to the lower aqueous phase and the particles of
ICs. The reaction proceeded without any lost of activity (i.e.,
3À
3À
emulsion, the replacement of [PW O ] with [PW O ] , sur-
12
40
4
24
prisingly, provides a much lower catalytic activity in terms of
conversion and TOF ) and selectivity after at least five consecu-
0
both conversion and TOF (see Table 2, entry 7). Indeed, during
tive cycles. These observations are indicative of the stability
the particles of ICs under our catalytic conditions (see Fig-
ure S10 in the Supporting Information). Moreover, FTIR spec-
troscopy experiments were performed on the solid CDs after
decomplexation with tetrahydrofuran. As depicted in Fig-
ure S11 in the Supporting Information, the FTIR spectra were
identical before and after five runs. It is noteworthy that, under
the conditions described in Table 2, entry 3, the yield of isolat-
ed product was 23%, that is, about 1m.
0
3À
the decomposition of [PW O ] , several other catalytic spe-
12
40
3
À
cies are formed in addition to [PW O ]
(e.g.,
4
24
2À
[8d]
3À
[
W O (O ) (H O) ] ). These active [PW O ] decomposition
2
3
2 4
2
2
12 40
products (i.e., peroxotungstates), which are involved in cataly-
sis, are known to form in trace amounts in an irreversible way.
However, it seems that their combination is required because
the Venturello species alone exhibits a lower activity. Moreover,
in our system with [Na] [PW O ] as a “pre-catalyst”, this does
3
12 40
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actions with H [PW O ] proceed
3
12 40
in biphasic systems in the pres-
[27]
ence of cationic PTCs. Li et al.
reported an amphiphilic catalyst
based on [(n-C H ) N(CH ) ] -
18
37 2
3 2 3
[
PW O ] assembled in an emul-
12 40
[28]
sion in diesel for ODS at 608C.
This amphiphilic catalyst is effec-
tive for selective ODS because it
achieves high substrate conver-
sion (>95%), but the system
used, that is, the emulsion, does
not allow easy separation of the
products and catalyst. When the
Figure 6. Proposed mechanism for eco-friendly catalytic epoxidation in 1/water/b-CD Pickering emulsions.
Next, the epoxidation of 2, 3, 4, and 5 was carried out under
same reaction conditions (Table 3). It is noteworthy that control
experiments, performed without b-CD, gave no conversion
reaction proceeds rapidly (e.g., 0.5 to 1 h), a large excess of
H O (ꢁ10 equiv) and/or relatively high temperatures (ꢁ608C)
2
2
are needed, whereas lower H O to substrate ratios and/or
2
2
[
28,29]
after 3 h regardless of the substrate. The reactivities of 2, 4,
lower temperatures require longer reaction times (ꢁ5 h).
À1
and 5 were similar to that of 1 (TOF ꢀ40 h ) and the reaction
Acidic three-liquid-phase microemulsion systems based on
tungstate catalytic surfactants can be used to circumvent all
0
was complete within 3 h with 0.27 equivalents of H O relative
2
2
[30]
to the substrate. For 3, a very satisfactory conversion was ob-
these drawbacks.
tained within only 0.5 h and a clear increase of the kinetic rate
The catalytic Pickering emulsion described above was also
applied to the oxidation of 6, 7, 8, dibenzylsulfide (10), and di-
benzothiophene (11). These substrates are known to oxidize
into the corresponding sulfoxide, SO, and/or sulfone, SO2
(Table 4), although the selective formation of the sulfoxide is
still challenging.
À1
was observed (236 h ). It is noteworthy that the epoxide se-
lectivities were >99%, except for 5, for which a significant de-
crease was observed with time in favor of the corresponding
diol. Indeed, this epoxide is more readily hydrolyzed due to
the increase of its hydrophilic character and its favored confor-
mation at the interface compared with the epoxycycloalkanes.
However, in all cases, the catalytic activities were higher or at
least equivalent to that of biphasic reactions performed with
Table 4. Catalytic oxidation of organosulfurs (S) into sulfoxides (SO) and/
[
8b,d]
or sulfones (SO ) in Pickering emulsion stabilized by b-CD/substrates or b-
PTCs reported in the literature.
Under homogeneous condi-
2
[
a]
CD/heptane complexes at 658C.
tions with the H [PW O ] catalyst, Maksimchuk et al. reported
3
12 40
À1
[
b]
[c]
TOF values of 24.0, 24.6, and 5.4 h for the epoxidation of 1,
3
Substrate
Dt
[h]
Conversion
[%]
Selectivity [%]
TOF0
0
[
23]
À1
À1
, and 5, respectively, versus 42, 236, and 45 h reported
SO
SO
2
[h
]
[
d]
herein. Moreover, the reaction requires 2 equivalents of H O2
6
7
8
1.5
1
3
0.5
0.5
17 (<1)
75
87
73
83
54
25
13
27
17
46
28
141
42
45
43
2
[
d]
per equivalent of substrate for 1, 2, and 5 and the selectivities
in epoxides are lower than those obtained with the Pickering
emulsion systems, in particular, for substrates 1 and 3, that is,
25 (3)
21 (4)
[
d]
[
e]
[d]
[d]
1
0
95 (<1)
96 (<1)
[e]
11
9
5 and 78%, respectively. On the other hand, Gresley et al.
[
0
a] Conditions: 30 mmol of [Na]
3 2 2
[PW12O40], 2.0 to 3.0 mmol of H O ,
reported on the epoxidation of
2
and 3 with the
.26 mmol of b-CD, 1.35 g of water, 1.35 g of organosulfur (6.3 to
[(n-C H ) N] [PW O ] catalyst in a benzene/water biphasic
1
6
13 4
3
4
24
10.8 mmol). [b] Determined by H NMR spectroscopy. [c] Initial turnover
frequency. [d] Control experiment without b-CD. [e] Conditions: 30 mmol
of [Na] [PW O ], 0.95 mmol of H O , 0.26 mmol of b-CD, 0.67 mmol of
[
24]
system under reflux conditions (758C). The TOF values are
5
0
À1
9.0 and 17.0 h for 2 and 3, respectively. However, although
3
12 40
2
2
organosulfur, 1.35 g of water, 1.35 g of heptane.
the reactions are very selective in epoxides, the presence of
benzene is unacceptable for an industrial process and the cata-
lyst and products cannot be separated easily. In contrast, cata-
lytic solvent-free Pickering emulsions, in addition to their effi-
ciency in terms of catalytic activity, constitute a simple and
smart solution for the workup of the final reaction medium.
Fossil fuels, coal, petroleum, and natural gas contain undesir-
able pollutants (i.e., organosulfurs). Oxidative desulfurization
Substrates 10 and 11 were oxidized in heptane/b-CD/water
emulsions because they were solids. It is noteworthy that with-
out b-CD they gave no or very weak conversion after 3 h. The
oxidizing system is particularly efficient for all of the investigat-
ed organosulfur compounds. Indeed, as for 6, 7, and 8, more
than 17% of the starting materials are converted into the cor-
responding sulfoxide and/or sulfone with 0.27 equivalents of
H O within very short reaction times. Moreover, the catalytic
(ODS), based on environmentally friendly oxidizing agents,
such as H O , and combined with a catalytic process, is one of
2
2
the most promising alternatives to hydrodesulfurization be-
cause it generally requires milder conditions and lower operat-
2
2
system is very active in the heptane/b-CD/water emulsions
with 10 and 11 because conversions are higher than 95%.
In the literature, the prevalence of sulfoxides or sulfones is dic-
[
25]
ing costs. There have been recent reports on ODS processes
[
26]
involving H O and various catalytic POMs. For example, re-
2
2
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tated by the number of equivalents of H O . Indeed, with sub-
they can be readily destabilized at the end of the reaction by
heating instead of centrifugation. Thus a compromise must be
found between the reaction temperature, which must ensure
the stability of the emulsion system during the process and
good conversion of the starting substrate, and the tempera-
ture causing the separation of the emulsion on request at the
end of the reaction, which must allow good separation of the
emulsion for easy workup and recycling of the reaction
medium. For the alcohols 12–15, 40 and 658C are good com-
promises to ensure the stability associated with good catalytic
efficiency and the separation of the emulsion, respectively. It is
finally noteworthy that, as described in the literature, the oxi-
dation of 15 is often difficult to achieve because the alcohol is
2
2
stoichiometric amounts of H O , the sulfoxides are prevalent,
2
2
whereas the sulfones are obtained with surstoichiometric
[
31]
ratios of H O relative to the substrate. Here again, the reac-
2
2
tions are particularly efficient in terms of substrate conversion
compared with the amount of H O added. Moreover, for all of
2
2
the organosulfur compounds investigated, the reactions per-
formed in the CD-based Pickering emulsions are highly selec-
tive to the sulfoxide (>75%), except for 11.
Finally, to extend the scope of the system to other types of
oxidation reactions, in particular, to alcohol oxidations, 2-octa-
nol (12), cyclohexanol (13), menthol (14), and benzyl alcohol
(
15) were also investigated. These substrates were oxidized in
[33]
heptane/b-CD/water emulsions because using alcohols as the
solvent phase did not provide stable Pickering emulsions
above 358C. The reactions were conducted at 408C and the
products obtained were the corresponding ketones for 12, 13,
and 14, and aldehydes for 15 (Table 5).
primary. However, in the CD-based Pickering emulsion we
obtain a very good aldehyde selectivity, that is, no other oxida-
tion products; this is contrary to that reported in the literature
[34]
data.
Conclusions
Table 5. Catalytic oxidation of alcohols into ketones or aldehydes in Pick-
ering emulsion stabilized by b-CD/heptane complexes at 408C.
We explored and widened the scope of the solventless catalyt-
ic Pickering emulsion concept: a biphasic mixture of various
liquid substrates and water formed emulsions in the presence
of CDs through the formation of amphiphilic insoluble ICs with
the substrate. Such emulsions, in combination with the
[
a]
[
b]
Substrate
Conversion
%]
Selectivity
[%]
[
1
2
3
>99
>99
>99
>99
[
Na] [PW O ] catalyst, were highly efficient reaction media for
3 12 40
1
the oxidation of olefins, organosulfurs, and alcohols, leading to
competitive rates, good yields, and selectivities. Such catalytic
emulsions combine the advantages of biphasic catalysis, with-
out the drawbacks (e.g., separation time or catalyst leaching),
and heterogeneous catalysis because they allow easy separa-
tion of the products and catalysts by simple centrifugation or
heating. In comparison with immobilization on silica, this cata-
lytic system did not require synthetic steps. Moreover, the pro-
posed system is versatile, depending on the nature of the sub-
strate (solid or liquid) and it is eco-friendly because it does not
require any organic solvent, except when the substrate is solid,
nor a PTC and does not generate waste. Due to current inter-
est in new catalytic processes, further developments of these
catalytic Pickering emulsions are expected for other catalytic
processes.
1
1
4
5
>99
>99
>99
44
[a] Conditions: 30 mmol of [Na]
3 2 2
[PW12O40], 0.85 mmol of H O , 0.26 mmol
of b-CD, 0.67 mmol of alcohol, 1.35 g of water, 1.35 g of heptane, 24 h.
1
[b] Determined by H NMR spectroscopy.
According to reports in the literature, the oxidation of alco-
hols (12, 13, and 14) by the [PW O ] anion in a biphasic
system (water/chloroform) is not straightforward.
12
40
[
32]
Even al-
though it is easier under homogeneous conditions, a large
excess of H O (6 equiv), relatively high temperatures (828C),
2
2
and relatively long reaction times are needed to reach a com-
[
32]
plete conversion of the starting material. Li et al. proposed
Experimental Section
the use of [(n-C H )N(CH ) ] [PW O ] as a catalyst for the for-
1
8
37
3 3 3
28b]
12 40
[
mation of O/W emulsions.
This system works with a stoichio-
General information: [D ]DMSO and all other chemicals were pur-
6
chased from Aldrich. Distilled deionized water was used in all ex-
periments. All solvents and liquid reagents were degassed by bub-
bling nitrogen for 15 min before each use or by two freeze–pump–
thaw cycles before use. All reactions were performed in oven-dried
glassware under nitrogen by using standard Schlenk and vacuum-
line techniques. All measurements were taken in a thermostated
water bath. The temperature stabilization was estimated to be
better than Æ0.058C with a thermoregulated bath Lauda RC6. All
emulsifications were performed by using an Ultra-Turrax instru-
ment (11500 rpm, IKA, T 10 basic).
metric ratio of H O relative to the substrate, but complete
2
2
conversion is reached after 24 h at 808C and separation of the
emulsion can occur, depending on the nature of the reactants
[
28b]
and products.
see Table 5), reactions in the CD-based Pickering emulsions
are as fast and as selective as those reported in the litera-
For almost all of the alcohols investigated
(
[
8b,32]
ture,
but proceed at a lower temperature (408C), and offer
a simple and efficient separation method. Indeed, unlike the
emulsions based on olefins or sulfides described above, the
CD-based emulsions containing alcohols or their correspond-
ing oxidation products, that is, aldehydes and ketones, are
thermosensitive and provide an additional benefit because
1
13
31
NMR spectroscopic characterization: The H, C, and P NMR spec-
tra were recorded by using an Advance 300 Bruker spectrometer
at 300.13, 75.49, and 121.49 MHz, respectively. Chemical shifts are
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ChemSusChem 2013, 6, 1533 – 1540 1538

CHEMSUSCHEM
FULL PAPERS
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1
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4
24
ed with 50% hydrogen peroxide (4.2 mL, 70 mmol). After 20 min
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31
1
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[
D ]DMSO, 208C): d=1.7 ppm (d, J(W,P)=18.4 Hz).
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General biphasic oxidation experiments: [Na] [PW O ] (30.0 mmol)
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12 40
and H O (6 mmol) were dissolved in water (1.35 g). b-CD
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Keywords: cyclodextrins · nanoparticles · oxidation · Pickering
emulsions · polyoxometalates
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ChemSusChem 2013, 6, 1533 – 1540 1540