A green strategy to enhance a liquid-liquid heterogeneous reaction with a magnetic recyclable Pickering emulsion

Article abstract of DOI:10.1002/cctc.201402856

This work described a general strategy to enhance a typical organic-aqueous heterogeneous green oxidation reaction by introducing surface tunable magnetic nanoparticles into liquids to form a controllable Pickering emulsion. We synthesized cross-linked polystyrene (PS)-covered Fe₃O₄ nanoparticles grafted with different N-alkyl imidazoles and systematically discussed the effect of surface properties on the formation of emulsions. By loading such particles, the dichloromethane-water-based 2,2,6,6-tetramethylpiperidine-1-oxyl-mediated Montanari oxidation system was converted from the unstable millimeter-scale dispersion into a stable micrometer-scale emulsion, which significantly reduced the reactant transfer distance and resulted in a significantly high overall reaction rate. This reaction efficiency is elucidated in terms of unique properties inherent in an emulsion, including the self-assembly of Fe₃O₄/PS and Fe₃O₄/PS[im-C_n]Cl (n=1, 4, 6, and 10) nanoparticles, well ensuring a large specific surface area of the water-oil interface. Pucker up for a Pickering emulsion: This work describes a general strategy to enhance a typical organic-aqueous heterogeneous green oxidation reaction, in which surface tunable magnetic nanoparticles are introduced into liquids to form a controllable Pickering emulsion. The catalysis results revealed that the stabilized micrometer-scale emulsion for biphasic Montanari oxidation resulted in a significantly higher reaction rate.

Full text of DOI:10.1002/cctc.201402856

DOI: 10.1002/cctc.201402856
Full Papers
A Green Strategy to Enhance a Liquid–Liquid
Heterogeneous Reaction with a Magnetic Recyclable
Pickering Emulsion
Linbin Feng, Jianli Wang,* Liang Chen, Meizhen Lu, Zhi Zheng, Ren Jing, Hualiang Chen,
and Xianbo Shen[a]
This work described a general strategy to enhance a typical or-
ganic–aqueous heterogeneous green oxidation reaction by in-
troducing surface tunable magnetic nanoparticles into liquids
to form a controllable Pickering emulsion. We synthesized
cross-linked polystyrene (PS)-covered Fe3O4 nanoparticles graft-
ed with different N-alkyl imidazoles and systematically dis-
cussed the effect of surface properties on the formation of
emulsions. By loading such particles, the dichloromethane–
Montanari oxidation system was converted from the unstable
millimeter-scale dispersion into stable micrometer-scale
a
emulsion, which significantly reduced the reactant transfer dis-
tance and resulted in a significantly high overall reaction rate.
This reaction efficiency is elucidated in terms of unique proper-
ties inherent in an emulsion, including the self-assembly of
Fe3O4/PS and Fe3O4/PS[im-Cn]Cl (n=1, 4, 6, and 10) nanoparti-
cles, well ensuring a large specific surface area of the water–oil
interface.
water-based
2,2,6,6-tetramethylpiperidine-1-oxyl-mediated
Introduction
Organic–aqueous biphasic reactions such as Suzuki coupling,
aldol condensation, hydroformylation, biomass conversion,
polymerization, and oxidation have been widely used in syn-
thetic organic chemistry for industrial research.[1] Aqueous re-
actants are especially useful in green chemistry because with
use of water, the need for organic solvents is reduced. Howev-
er, owing to the incompatibility in such a biphasic reaction
system, the reagents in the two phases cannot achieve a full
contact, which results in an improved reaction rate.[2] In such
a case, strong agitation, optimization of the reactor, ex-field as-
sistance (ultrasonic and microwave), and addition of a phase
transfer catalyst (PTC) can significantly increase the overall re-
action rate and reduce the operation time. The addition of
a PTC can increase the miscibility of the insoluble liquids and
favors the formation of an emulsion, and the PTC can some-
times act as a carrier to transfer reaction species. Consequently,
liquid–liquid heterogeneous reactions can be significantly ex-
pedited. However, the isolation of the PTC, such as quaternary
ammonium and phosphonium salts, from the reaction solution
is difficult owing to its amphoteric properties like a surfactant.
Moreover, the reuse and recycling PTCs is a challenge.
solid nanoparticles could assemble at the liquid–liquid inter-
faces driven by the reduction in interfacial energy and the
emulsion reaction medium could function as highly dispersed
microreactors.[3] The strong attachment of particles onto the
phase interface hinders the coalescence of liquid drops, and
thus more stable emulsions are formed.[4]
Pickering emulsions have been used in various applications
such as in the preparation of Janus materials, selectively per-
meable capsules, and core–shell structure materials.[5] The Pick-
ering emulsion technique could be a promising strategy to im-
prove the mass diffusion limitation in the liquid–liquid multi-
phase system, especially for an organic system in water. Picker-
ing emulsions are emerging as an interesting tool to increase
the reaction rate because they can offer a large area of the
water–oil interface.[6] Compared with conventional emulsions,
the solid particle emulsions irreversibly adsorbed at the liquid–
liquid interfaces and probably can be recycled.[7]
Several factors affect the formation and stability of Pickering
emulsions. It has been previously reported that the particle
concentration affects the emulsion droplet size because the in-
terfacial area is related to the amount of nanoparticles.[8] The
surface properties of nanoparticles also affect the properties of
Pickering emulsions.[9] Stiller et al. investigated the effect of the
wettability of different surface-modified titanium dioxides on
the type and stability of Pickering emulsions and found that
the stability of water-in-oil emulsions increased with increasing
apparent contact angle value against water.[10] Tarimala et al.
found that hydrophobic and hydrophilic solid particles could
assemble at the water–oil interface at the same time.[11] Yu
et al. changed the surface properties of Ru/CNT–TiO2 (CNT=
carbon nanotube) by modulating the amount of carbon nano-
tube component and found the increasing catalytic activity in
Fortunately, the Pickering emulsion technique gives us
a new way to increase the reaction rate of biphasic reactions.
A Pickering emulsion is an emulsion stabilized by solid parti-
cles that adsorb at the interface between the two phases. For
several decades, this emulsion technique showed that certain
[a] L. Feng, Prof. Dr. J. Wang, L. Chen, M. Lu, Z. Zheng, R. Jing, H. Chen, X. Shen
Zhejiang University of Technology
Hangzhou 310014 (P.R. China)
E-mail: wangjl@zjut.edu.cn
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201402856.
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the selective oxidation of benzyl alcohol to benzaldehyde in
the emulsion system.[12]
Although the formation of a Pickering emulsion has been in-
vestigated for many years, the use of the Pickering emulsion
concept to develop a system for increasing the reaction rate of
the organic–aqueous biphasic reactions has been rarely ex-
plored so far. Our previous work has reported the oxidation of
alcohols with use of 2,2,6,6-tetramethylpiperidine-1-oxyl
(TEMPO)-grafted nanoparticles as a catalyst system; this work
demonstrated a remarkably high selectivity and activity in the
oxidation of alcohols with a dichloromethane–water-based
Montanari oxidation system.[13] We also discussed a covalent
stabilization of TEMPO (active site)-grafted amphiphiles on the
magnetic hydrophobic core to form a micelle-like architecture
and used it as a Pickering emulsion to develop a new kind of
micelle-catalyzed biphasic system. The catalysis results revealed
that these fixed micelle-tethered TEMPO catalysts demonstrat-
ed a much higher activity than their soluble counterparts in
Scheme 1. Nanoparticle-stabilized emulsion for the CH2Cl2/H2O-based
TEMPO-mediated Montanari oxidation as a heterogeneous reaction.
size and the reaction efficiency of our biphasic reactions
(Scheme 1).
This study essentially demonstrates that this novel Pickering
emulsion-based system is technically simple and highly useful
for practical applications.
the biphasic Montanari oxidation of alcohols, which could be Results and Discussion
due to the unique properties of Pickering emulsions and micel-
lar catalysts.[14]
Preparation and characterization of nanoparticles
However, the reason why catalyst-grafted nanoparticles can
increase the reaction rate of the biphasic reaction system and
how it enhances the reaction are still not clear. Little research
has been performed to reveal the increase in activity of hetero-
geneous reactions contributed solely by the function of Picker-
ing emulsion and how the surface properties contribute to the
formation of such kind of emulsion. Herein, we designed and
synthesized a series of Fe3O4/PS (PS= polystyrene) nanoparti-
cles with modified surfaces but without grafted TEMPO and
discussed the stabilization of the corresponding Pickering
emulsion. We selected a typical liquid–liquid biphasic reaction
system, so-called Montanari oxidation system, that was exten-
sively used in the selective oxidation of an alcohol. This reac-
tion is performed with NaClO and NaBr as oxidants and
TEMPO as a catalyst. It is a typical biphasic reaction in which
the substrate alcohol is dissolved in an organic phase and the
oxidants NaClO and NaBr exist in water. We began our study
by comparing our Fe3O4/PS nanoparticle-stabilized Pickering
emulsion reaction system with the conventional reaction
system (Figure 1) and evaluated their reaction efficiency. More-
over, we explored the surface modification of Fe3O4/PS with
different N-alkyl imidazoles ([im-Cn]Cl; n=1, 4, 6, and 10),
systematically discussed how the surface properties affected
the formation of nanoparticle-stabilized Pickering emulsions,
and explained the relationship between the emulsion droplet
The [im-Cn]Cl (n=1, 4, 6, and 10) species stabilized on the
Fe3O4/PS nanoparticle surface were prepared according to the
work of Zheng et al,[15] and the surface-modified nanoparticles
were prepared by a concise route outlined in Schemes 2 and
S1. Fe3O4/chloromethyl PS nanoparticles (divinylbenzene con-
tent: ꢀ3%; chlorine content: ꢀ1.5 mmolCl gꢁ1; mean size:
ꢀ81 nm; Table 1) were prepared by applying the miniemulsion
polymerization technique. N-Alkyl imidazole im-Cn (n=4, 6,
and 10) species were synthesized according to our previous
work and was verified by NMR spectroscopy. Then, Fe3O4/
Scheme 2. Synthesis of Fe3O4/PS[im-Cn]Cl (n=1, 4, 6, and 10).
Table 1. Sizes of Fe3O4/PS[im-Cn]Cl (n=1, 4, 6, and 10).[a]
Entry
Substrate
Average size
[nm]
1
2
3
4
5
6
Fe3O4/PS
80
81
86
92
95
Fe3O4/PS–CH2Cl
Fe3O4/PS[im-C1]Cl
Fe3O4/PS[im-C4]Cl
Fe3O4/PS[im-C6] Cl
Fe3O4/PS[im-C10]Cl
100
Figure 1. Comparison of the conventional reaction system and the nanopar-
ticle-stabilized Pickering emulsion system.
[a] The size of nanoparticles was measured with a Zetasizer ZS90.
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chloromethyl PS nanoparticles were directly functionalized
with excess im-Cn species (n=1, 4, 6, and 10) to afford the cor-
responding Fe3O4/PS[im-Cn]Cl (n=1, 4, 6, and 10) nanoparticles
by heating to reflux in toluene. The Fe3O4/PS–TEMPO catalyst
was prepared according to our previous report.[13] The resulting
Fe3O4/PS and Fe3O4/PS[im-Cn]Cl (n=1, 4, 6, and 10) nanoparti-
cles were characterized by using Zetasizer ZS90, FTIR spectros-
copy, thermogravimetric analysis (TGA), elemental analysis (EA),
and contact angle measurements. The size of the nanoparticles
was measured with a Zetasizer ZS90, and the data reveal that
the average size of the nanoparticles is between 80 and
100 nm (Table 1). The FTIR spectra (Figure 2) show the disap-
Figure 2. FTIR spectra of a) Fe3O4/chloromethyl PS, b) Fe3O4/PS[im-C1]Cl,
c) Fe3O4/PS[im-C4]Cl, d) Fe3O4/PS[im-C6]Cl, and e) Fe3O4/PS[im-C10]Cl.
Figure 4. A) XP spectrum of Fe3O4/PS[im-C10]Cl. and B) XP spectra of
a) Fe3O4/PS[im-C10]Cl and b) Fe3O4/chloromethyl PS, showing Cl 2p3 peaks.
pearance of ꢁCH2Cl peaks at 1260 and 680 cmꢁ1, and the ap-
pearance of a new strong vibration peak at vCꢁN =1177 cmꢁ1 in
the FTIR spectra prove that [im-Cn]Cl (n=4, 6, and 10) species
have been bonded on the magnetic PS microspheres. Volatile
products released at 200–2508C are due to the degradation of
the im-Cn moiety (Figure 3). Moreover, these data can be fur-
ther determined from the X-ray photoelectron (XP) spectra of
Fe3O4/PS[im-Cn]Cl (Figures 4 and S1–S4). The new N1s peak of
Fe3O4/PS[im-C10]Cl at 402 eV is associated with NꢁC bonds in
the imidazolium ring (Figure 4A); the shift of the Cl2p3 peak
from 200.4 to 197.2 eV confirms that an N-alkyl imidazole spe-
cies has been bonded to the benzylic proton (ꢁCH2Cl) (Fig-
ure 4B), as the Cl2p3 peak of Fe3O4/PS[im-C10]Cl at 197.2 eV is
consistent with the reference value of Cl in an anion form. The
N-alkyl imidazole loading on Fe3O4/PS nanoparticle surface
were determined from quantitative EA, and the results are pre-
sented in Table 2. Contact angle measurements of the nano-
Table 2. Results of the elemental analysis of Fe3O4/PS[im-Cn]Cl (n=1, 4, 6,
and 10).
Entry
Nanoparticle
Elements [%]
N-Alkyl imidazole
loading [mmolgꢁ1
]
[C]
[H]
[N]
1
2
3
4
Fe3O4/PS[im-C1]Cl
Fe3O4/PS[im-C4]Cl
Fe3O4/PS[im-C6]Cl
Fe3O4/PS[im-C10]Cl
65.66
66.38
67.60
68.24
6.43
6.82
7.18
7.49
3.85
3.59
3.48
3.19
1.38
1.28
1.24
1.14
Figure 3. TGA of a) Fe3O4/PS[im-C10]Cl, b) Fe3O4/PS[im-C1]Cl, c) Fe3O4/PS[im-
C4]Cl, and d) Fe3O4/PS[im-C6]Cl.
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Figure 6. Position of nanoparticles at a droplet interface for a contact angle
less than 908, giving a solid-stabilized water-in-oil emulsion.
all;[17] therefore, we indirectly measure the contact angle
against water on the surface of Fe3O4/PS and Fe3O4/PS[im-Cn]Cl
(n=1, 4, 6, and 10). The values of the apparent contact angle
indicate different surface wettability of Fe3O4/PS and Fe3O4/
PS[im-Cn]Cl (n=1, 4, 6, and 10) with different N-alkyl imida-
zoles.
Although we cannot measure the three phase contact angle
of nanoparticles at the water–oil interface, the apparent con-
tact angle against water on the surface of nanoparticles allows
a satisfactory comparison of the surface properties of investi-
gated nanoparticles. We found that the apparent contact
angle of water on the surface of nanoparticles is 1048 for
Fe3O4/PS (Figure 5a), 1108 for Fe3O4/PS[im-C1]Cl (Figure 5b),
1158 for Fe3O4/PS[im-C4]Cl (Figure 5c), 1168 for Fe3O4/PS[im-
C6]Cl (Figure 5d), and 1308 for Fe3O4/PS[im-C10]Cl (Figure 5e).
The value of contact angle of water indicates that the surface
modification of Fe3O4/PS with different N-alkyl imidazoles has
a significant effect on the wetting properties of the measured
nanoparticles.
Figure 5. Contact angle for a water droplet on the surface of a) Fe3O4/PS
b) Fe3O4/[im-C1]Cl, c) Fe3O4/PS[im-C4]Cl, d) Fe3O4/PS[im-C6]Cl, and e) Fe3O4/
PS[im-C10]Cl.
particles were done by using the drop shape method, when
a drop of water was allowed to contact the surface of Fe3O4/PS
and Fe3O4/PS[im-Cn]Cl (n=1, 4, 6, and 10) (Figure 5).
To find the relationship between the surface properties of
the nanoparticles and the stability of the emulsions, photo-
graphs of emulsions stabilized by Fe3O4/PS and Fe3O4/PS[im-
Cn]Cl (n=1, 4, 6, and 10) were taken in the Montanari oxida-
tion of alcohols. For Montanari oxidation, we used the house-
hold bleach NaClO in combination with 10 mol% NaBr and
0.1 mol% TEMPO in the water (3.35 mL)–dichloromethane
(2.5 mL) biphasic system. As shown in Figures 7 and 8, Picker-
ing emulsions formed by different nanoparticles are quite dif-
Effect of particle surface properties on the emulsion droplet
size and stability for the Pickering emulsion-based Monta-
nari oxidation of alcohols
To reveal the relationship between the particle surface proper-
ties and the reaction efficiency of our biphasic reactions, we
first investigated the emulsion droplet size and stability for our
Pickering emulsion-based Montanari oxidation of alcohols.
Then, we calculated the energy required to desorb particles
from the water–oil interface by using Equation (1).[16]
2
Eg ¼ pr2gð1ꢁjcos qjÞ
ð1Þ
In which q is the contact angle of the particle (Figure 6), r is
the average diameter of particles and g represents the inter-
facial tension of the water–oil interface. This equation reveals
that the desorption energy reaches the highest value if q is
908, which means that the adsorption of particles at the inter-
face is more irreversible and the emulsion is more stable to
form emulsion droplets.
To explore the stability of nanoparticle-stabilized emulsions,
we need to measure the three phase contact angle of nano-
particles in water-in-oil emulsions. However, the direct way to
measure the three phase contact angle of a single nanoparticle
at the water–oil interface is extremely difficult if possible at
Figure 7. Photographs of a) soluble 0.1 mol% TEMPO, b) 0.1 mol%
TEMPO+5 mg Fe3O4/PS, c) 0.1 mol% TEMPO+5 mg Fe3O4/PS[im-C1]Cl,
d) 0.1 mol% TEMPO+5 mg Fe3O4/PS[im-C4]Cl, e) 0.1 mol% TEMPO+5 mg
Fe3O4/PS[im-C6]Cl, and f) 0.1 mol% TEMPO+5 mg Fe3O4/PS[im-C10]Cl taken
in the Montanari oxidation of 4-methoxybenzyl alcohol at RT for 1 h
(Table 4, entries 1–6).
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the combined results of Figures 7–9, it can be concluded that
the emulsion stabilized by Fe3O4/PS[im-C6]Cl is more stable
than other emulsions. The smaller the droplet, the more stable
the emulsion.
From the above results, we also found that if the contact
angle against water is 1168 for Fe3O4/PS[im-C6]Cl, the emulsion
shows the highest stability and the smallest droplet size. The
reason may be that the desorption energy of the particles
reaches the highest value if the contact angle against water is
approximately 1168. Making a surface modification of Fe3O4/PS
with different N-alkyl imidazoles can change the desorption
energy of nanoparticles in this reaction system. Because of dif-
ferent desorption energies at the water–oil interfaces, Fe3O4/PS
and Fe3O4/PS[im-Cn]Cl (n=1, 4, 6, and 10) nanoparticles can
form a different scale of the droplets of the emulsion.
Figure 8. Photographs of a) soluble 0.1 mol% TEMPO, b) 0.1 mol%
TEMPO+5 mg Fe3O4/PS, c) 0.1 mol% TEMPO+5 mg Fe3O4/PS[im-C1]Cl,
d) 0.1 mol% TEMPO+5 mg Fe3O4/PS[im-C4]Cl, e) 0.1 mol% TEMPO+5 mg
Fe3O4/PS[im-C6]Cl, and f) 0.1 mol% TEMPO+5 mg Fe3O4/PS[im-C10]Cl taken in
the Montanari oxidation of 4-methoxybenzyl alcohol at RT for 1 week
(Table 4, entries 1–6).
ferent and the Fe3O4/PS[im-C6]Cl-stabilized Pickering emulsion
shows the smallest emulsion droplet size and the highest
emulsion stability. It has been previously reported that the sur-
face properties of nanoparticles significantly affect the droplet
size of the emulsion. Therefore, we used an optical microscope
to measure the scale of single water-in-oil emulsion droplets.
As shown in Figure 9, the average droplet sizes of the emul-
sions are in the following order (the size distribution of the
droplet size is shown in Figures S5–S9): 295 mm (Fe3O4/PS[im-
C6]Cl)<330 mm (Fe3O4/PS[im-C4]Cl)<350 mm (Fe3O4/PS[im-
C10]Cl)<450 mm (Fe3O4/PS[im-C1]Cl)<759 mm (Fe3O4/PS). From
Catalytic activity evaluation of the Fe3O4/PS-stabilized
A Pickering emulsion system was obtained if the reaction
system was stirred in the presence of Fe3O4/PS. To demonstrate
the enhanced Montanari oxidation in this resulting Pickering
emulsion system, we compared the Montanari oxidation of al-
cohols in the Pickering emulsion-based system (with Fe3O4/PS
nanoparticles as emulsions) with that in the conventional bi-
phasic system (without Fe3O4/PS nanoparticles as emulsions)
under the same conditions. For the investigated alcohols, the
Pickering emulsion system afforded better conversion than the
conventional reaction system (Table 3). These comparisons suf-
ficiently demonstrated the effectiveness of the Pickering emul-
sion system. The improved reaction efficiency is probably due
to the self-assembly of particles at fluid interfaces, which was
driven by the reduction in interface energy. Nanoparticles can
be assembled at water–oil interfaces to form water-in-oil emul-
sions (Figure 7) that will create a large reaction area and
Table 3. Catalytic activity comparison of TEMPO only- and 5 mg Fe3O4/
PS+TEMPO reaction systems used in the Montanari oxidation of alcohols
to the respective carbonyl derivatives.[a] R1 is a phenyl derivative or alkyl
group, R2 is hydrogen or phenyl.
Entry
Substrate
Conversion [%]
TEMPO only
TEMPO +
5 mg Fe3O4/PS
1
2
3
4
5
6
7
benzyl alcohol
81.9ꢂ0.8
41.2ꢂ4.1[b]
66.8ꢂ2.3
79.0ꢂ2.2
22.4ꢂ3.1
85.4ꢂ3.5
79.7ꢂ1.7
98.9ꢂ0.4
64.1ꢂ2.7[b]
93.1ꢂ2.0
97.2ꢂ0.8
46.0ꢂ2.5
97.7ꢂ0.8
85.9ꢂ3.0
4-methoxybenzyl alcohol
4-nitrobenzyl alcohol
2-phenylethanol
diphenylmethanol
1-pentanol
1-nonanol
Figure 9. Optical micrographs of single water-in-oil emulsion droplets in
CH2Cl2/NaClO solution stabilized by a) 0.1 mol% TEMPO+5 mg Fe3O4/PS,
b) 0.1 mol% TEMPO+5 mg Fe3O4/PS[im-C1]Cl, c) 0.1 mol% TEMPO+5 mg
Fe3O4/PS[im-C4]Cl, d) 0.1 mol% TEMPO+5 mg Fe3O4/PS[im-C6]Cl, and
e) 0.1 mol% TEMPO+5 mg Fe3O4/PS[im-C10]Cl in the Montanari oxidation of
4-methoxybenzyl alcohol (Table 4, entries 2–6). Scale bar=300 mm.
[a] Reaction conditions: alcohol (1 mmol), 10 mol% NaBr (0.1 mmol),
1.5 equiv. of NaClO (1.5 mmol, 3.35 mL, pH ꢀ9.1), 0.5 mol% catalyst
(0.005 mmol), CH2Cl2 (2.5 mL), T=108C, t=60 s; conversions and selectivi-
ties were determined from GC analysis, and all selectivities were >99%;
[b] The selectivity was >97%.
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achieve a full contact of reagents in the two phases, resulting
in a high reaction rate.
Table 4. Contribution of nanoparticles in the TEMPO-mediated Montanari
oxidation of 4-methoxybenzyl alcohol to 4-methoxybenzaldehyde.[a]
Catalytic activity evaluation of the Fe3O4/PS[im-Cn]Cl (n=1,
4, 6, and 10)-stabilized Pickering emulsion-based Montanari
oxidation of alcohols
To understand the role of structure [im-Cn]Cl as emulsions, we
synthesized a set of counterpart nanoparticles with different
shell domains by grafting ionic liquids with different alkyl
chain lengths onto the Fe3O4/PS surface. From the previous re-
sults, we concluded that the Fe3O4/PS[im-Cn]Cl (n=1, 4, 6, and
10)-stabilized Pickering emulsion is more stable and more
likely to form smaller emulsion droplets (Figure 10). We rea-
Entry
Substrate
Conv.
[%]
1
2
3
4
5
6
7
TEMPO
TEMPO+5 mg Fe3O4/PS
20.7ꢂ1.0
31.7ꢂ2.0
52.9ꢂ2.9
76.9ꢂ1.1
79.0ꢂ2.7
73.5ꢂ1.5
41.5ꢂ2.5
TEMPO+5 mg Fe3O4/PS[im-C1]TEMPO]Cl
TEMPO+5 mg Fe3O4/PS[im-C4]TEMPO]Cl
TEMPO+5 mg Fe3O4/PS[im-C6]TEMPO]Cl
TEMPO+5 mg Fe3O4/PS[im-C10]TEMPO]Cl
TEMPO+5 mg CTAB (PTC)
[a] Reaction conditions: 4-methoxybenzyl alcohol (1 mmol), 0.1 equiv. of
NaBr (0.1 mmol), 1.5 equiv. of NaClO (1.5 mmol, 3.35 mL, pH ꢀ9.1),
0.001 equiv. of catalyst (0.001 mmol), CH2Cl2 (2.5 mL), T=108C, t=30 s;
conversions and selectivities were determined from GC analysis, and all
selectivities were >97%.
activities are in the following order: 79.0%ꢂ2.7% (Fe3O4/
PS[im-C6]Cl; Table 4, entry 5)>76.9%ꢂ1.1% (Fe3O4/PS[im-C4]Cl;
Table 4, entry 4)>73.5%ꢂ1.5% (Fe3O4/PS[im-C10]Cl; Table 4,
entry 6)> 52.9%ꢂ2.9% (Fe3O4/PS[im-C1]Cl; Table 4, entry 3)>
31.7%ꢂ2.0% (Fe3O4/PS; Table 4, entry 2). The reaction of
Fe3O4/PS[im-C6]Cl (79.0%ꢂ2.7%; Table 4, entry 5) as emulsifiers
with the smallest average droplet size (295 mm) gives the high-
est reaction rate. Interestingly, the reaction efficiency is inverse-
ly proportional to the scale of the droplets of the Pickering
emulsion stabilized by different nanoparticles, and their aver-
age droplet sizes are in the following order: 295 mm (Fe3O4/
PS[im-C6]Cl)<330 mm (Fe3O4/PS[im-C4]Cl)<350 mm (Fe3O4/
PS[im-C10]Cl)<450 mm (Fe3O4/PS[im-C1]Cl)<759 mm (Fe3O4/PS).
Smaller emulsion droplets can achieve a full contact by offer-
ing a larger mass transfer specific area of the water-oil inter-
face and reducing the diffusion distance by autodiffusion of
the reactant molecules. For the biphasic reaction, increasing
the mass transfer specific area of the reagent is a general way
to increase the reaction rate. The overall volumetric mass
transfer rate (N’) for biphasic reactions is calculated by using
Equation (2).
Figure 10. Comparison of Fe3O4/PS- and Fe3O4/PS[im-Cn]Cl (n=1, 4, 6, and
10)-stabilized Pickering emulsion systems.
soned that the high conversion of the oxidation reaction after
adding nanoparticles is probably related to the droplet size
and stability of the emulsions formed by the nanoparticles
with different surface properties, and we used them for the
Montanari oxidation of 4-methoxybenzyl alcohol.
N0 ¼ Kaowðcꢁc Þ
ð2Þ
*
To our delight, a high conversion of the reaction has been
achieved with a relatively low catalyst loading of 0.1%. If
Fe3O4/PS[im-Cn]Cl (n=1, 4, 6, and 10) was added to the reac-
tions, the reaction activity increased significantly. The conver-
sion rate of the conventional oxidation of 4-methoxybenzyl al-
cohol was only 20.7%ꢂ1.0% (Table 4, entry 1) when no nano-
particles were added as emulsions, whereas the conversion
rate of the reaction was doubled when nanoparticles were
added to form Pickering emulsions. Furthermore, Fe3O4/PS and
Fe3O4/PS[im-Cn]Cl (n=1, 4, 6, and 10) performed different activ-
ities because of their different surface properties. Their reaction
in which K is the overall mass transfer coefficient (msꢁ1), aow is
the mass transfer specific area (mꢁ1), and (cꢁc*) is the mass
transfer driving force of the solute (molLꢁ1). By using this
equation, we found that the larger mass transfer specific sur-
face area can increase the volumetric mass transfer rate and
result in a higher reaction conversion. Therefore, the Pickering
emulsion technique can be a universal strategy to increase the
overall reaction rate of biphasic reactions. To further describe
the catalytic efficiency of our surface-modified nanoparticles,
we compared the nanoparticle-stabilized Pickering emulsion
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system with the conventional PTC-enhanced reaction system.
We chose cetyltrimethylammonium bromide (CTAB) as our PTC
and used it in our biphasic reaction. We found that our sur-
face-modified Fe3O4/PS[im-C6]Cl-enhanced reaction system
show higher reaction conversion (79.0%ꢂ2.7%; Table 4,
entry 5) than the CTAB-enhanced reaction system (41.5%ꢂ
2.5%; Table 4, entry 7).
Recyclability of nanoparticles
Finally, we turned our attention to the scope of recycling the
nanoparticles. The recyclability of Fe3O4/PS[im-C6]Cl (5 mg) was
evaluated by isolating it from the Montanari oxidation of
benzyl alcohol reaction system (scale: 1 mmol; TEMPO:
0.5 mol% to benzyl alcohol) through successive ultrasonic de-
mulsification and magnetic decantation steps, followed by
washing thoroughly with water, acetone, and dichloromethane
in turn. These nanoparticle-stabilized emulsions can be broken
through magnetic decantation or sonication. Because the parti-
cles do not adsorb at the interface of the droplets, the water
droplets collapse; thus, the two immiscible liquids are separat-
ed and the nanoparticles are recycled. For the next cycle, the
fresh reactants and recycled nanoparticles can be reemulsified
by stirring. As shown in Figure 11, Fe3O4/PS[im-C6]Cl can be re-
cycled by applying a magnetic field or within 30 min of ultra-
Catalytic activity comparison of Fe3O4/PS–TEMPO and
TEMPO+nanoparticle reaction systems
To further understand the reaction efficiency of our Pickering
emulsion system and the reason why nanoparticles covalently
grafted with (active site) TEMPO can increase the reaction rate
of the biphasic reaction system and how it enhances the reac-
tion, we compared our reaction with that described in our pre-
vious work.[13] We have previously synthesized magnetic PS
nanosphere-immobilized TEMPO (Fe3O4/PS–TEMPO), which
shows the improved reaction efficiency compared with the
conventional reaction system. We now compared our Pickering
emulsion (nanoparticle+TEMPO) system with the Fe3O4–
TEMPO reaction system. The results in Table 5 indicate that the
reaction activity is the highest if the surface-modified Fe3O4/
PS[im-C1]Cl is added to the reactions, and it is higher than
those of Fe3O4/PS–TEMPO and TEMPO-only systems. Thus, we
concluded that the increased reaction activity of our Fe3O4/PS–
TEMPO system is due to the emulsifying properties of Picker-
ing emulsions that create a larger reaction area of the organic–
aqueous biphasic reaction system. Furthermore, the nanoparti-
cles grafted with (active site) TEMPO can increase the reaction
rate of the biphasic reaction system, which is mainly due to
the formation of such a Pickering emulsion system.
Figure 11. Photographs of magnetic separation of Fe3O4/PS[im-C6]Cl in the
Montanari oxidation of benzyl alcohol.
sonic demulsification for 10 s. Of further interest is
the good recyclability of Fe3O4/PS[im-C6]Cl; the
Fe3O4/PS[im-C6]Cl nanoparticles could be recycled
10 times, with the conversion of benzyl alcohol re-
maining more than 98% (Figure 12).
Table 5. Catalytic activity comparison of Fe3O4/PS–TEMPO- and TEMPO+nanoparticle
reaction systems used in the Montanari oxidation of benzyl alcohol to benz-
aldehyde.[a]
Conclusions
Entry
Substrate
Catalyst loading
[mol%]
T
[s]
Select. Conv.
[%]
[%]
We investigated the effect of wettability and nano-
particles on the droplet size of the emulsion and sta-
bility of water and dichloromethane in the Pickering
emulsion-based Montanari oxidation of alcohols. By
using contact angle measurements, we found the
surface wettability changes of Fe3O4/PS (PS=poly-
styrene) and Fe3O4/PS with different N-alkyl imida-
zoles ([im-Cn]Cl; n=1, 4, 6, and 10). The emulsion
showed the highest stability and the smallest droplet
size if the contact angle is 1168 for Fe3O4/PS[im-C6]Cl.
The reason is that the desorption energy reaches the
highest value if the contact angle against water is
1168.
1
2
3
4
5
6
7
8
10 mg Fe3O4/PS[im-C1]Cl+TEMPO 1.0
15
15
15
15
30
30
30
30
60
60
60
60
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
99.4
88.7
67.8
44.6
98.5
81.8
61.4
59.5
75.4
74.6
57.7
70.2
10 mg Fe3O4/PS+TEMPO
10 mg Fe3O4/PS–TEMPO
TEMPO
1.0
1.0
1.0
0.5
0.5
0.5
0.5
0.1
0.1
0.1
0.1
5 mg Fe3O4/PS[im-C1]Cl+TEMPO
5 mg Fe3O4/PS+TEMPO
5 mg Fe3O4/PS–TEMPO
TEMPO
9
1 mg Fe3O4/PS[im-C1]Cl+TEMPO
1 mg Fe3O4/PS+TEMPO
1 mg Fe3O4/PS–TEMPO
TEMPO
10
11
12
[a] Reaction conditions: benzyl alcohol (1 mmol), 0.1–1 mol% catalyst, 10 mol% NaBr
(0.1 mmol), 1.5 equiv. of NaClO (1.5 mmol, 3.35 mL, pH ꢀ9.1), CH2Cl2 (2.5 mL), T=
108C; conversions and selectivities were determined from GC analysis, and all the re-
sults were obtained by repeating the reaction at least three times.
We demonstrated a universal strategy to perform
organic–aqueous reactions with nanoparticle-stabi-
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13C NMR (126 MHz, CDCl3): d=13.44, 19.68, 33.03, 46.67, 118.72,
129.32, 137.02 ppm; MS (EI): m/z (%): calcd [M]+ 124.18; found:
124.1; FTIR (CaF2 optical window): n˜ =3105.1, 2959.9, 2871.5,
1508.0, 1461.5, 1375.4, 1281.7, 1227.8, 1108.6, 1078.3 ppm.
1
im-C6: H NMR (500 MHz, CDCl3): d=7.37 (s, 1H), 6.96 (s, 1H), 6.82
(s, 1H), 3.84 (s, 2H), 1.69 (s, 2H), 1.22 (s, 6H), 0.81 ppm (s, 3H);
13C NMR (126 MHz, CDCl3): d=13.84, 22.36, 26.10, 30.95, 31.13,
46.92, 118.68, 129.23, 136.96 ppm; MS (EI): m/z (%): calcd [M]+
152.23; found: 152.1; FTIR (CaF2 optical window): n˜ =3107.7,
2926.0, 2854.8, 1507.8, 1461.6, 1374.0, 1282.5, 1228.3, 1108.5,
1076.9 ppm.
1
im-C10: H NMR (500 MHz, CDCl3): d=7.47 (s, 1H), 7.06 (s, 1H), 6.91
(s, 1H), 3.93 (s, 2H), 1.77 (s, 2H), 1.26 (s, 14H), 0.88 ppm (s, 3H);
13C NMR (126 MHz, CDCl3): d=14.04, 22.61, 26.50, 29.02, 29.20,
29.38, 29.42, 31.03, 31.80, 46.99, 118.71, 129.26, 137.00 ppm; MS
(EI): m/z (%): calcd [M]+ 208.34; found: 208.2; FTIR (CaF2 optical
window): n˜ =3105.6, 2930.2, 2860.7, 1507.5, 1460.9, 1378.5, 1282.4,
1228.3, 1107.2, 1077.3 ppm.
Figure 12. Recyclability of Fe3O4/PS[im-C6]Cl in the Montanari oxidation of
benzyl alcohol (for experimental details, see Section 3 in the Supporting In-
formation).
Preparation of Fe3O4/PS[im-Cn]Cl (n=1, 4, 6, and 10)
The magnetic nanoparticle-supported N-alkyl imidazole was pre-
pared by using the procedure shown in Schemes 2 and S1. Mag-
netic nanoparticles, easily prepared by using the coprecipitation
method, were chosen as the support. Fe3O4 nanoparticles were
first protected with a layer of PS to prevent aggregation. And the
obtained Fe3O4/chloromethyl PS and 1-methylimidazole were
mixed in dry toluene (25 mL) and heated to reflux for 48 h to
afford MNP-1. The mixture was cooled to RT and washed at least
five times with toluene and then at least five times with deionized
water. The product was obtained under vacuum freezing condition.
Other MNP-supported ionic liquids were prepared using the similar
procedure with different N-alkyl imidazoles (Scheme 1, MNP-1,
MNP-2, MNP-3, and MNP-4).
lized Pickering emulsions. Compared with the conventional re-
action system, the Pickering emulsion system exhibits a much
higher reaction efficiency. The addition of Fe3O4/PS[im-C6]Cl
particles can achieve the optimal reaction efficiency owing to
their highest desorption energy on the liquid–liquid interface.
The reaction efficiency is inversely proportional to the scale of
the droplets of Pickering emulsions stabilized by different
nanoparticles. Smaller emulsion droplets create a larger reac-
tion area and a full contact of reagents in the two phases,
which results in a higher reaction rate. The high reaction effi-
ciency and facile recycling of our magnetic nanoparticles will
bring the biphasic reaction to a universal green and sustaina-
ble chemistry.
Characterization of Fe3O4/PS[im-Cn]Cl (n=1, 4, 6, and 10)
Experimental Section
The size of Fe3O4/PS and Fe3O4/PS[im-Cn]Cl (n=1, 4, 6, and 10)
nanoparticles was measured with a Zetasizer ZS90 (Malvern, UK).
The NMR spectra were obtained with an AVANCE III 500 MHz NMR
spectrometer (Bruker, Switzerland) using TMS as internal standard.
The FTIR spectra (200–4000 cmꢁ1) were recorded on a Tensor 27
spectrometer (Bruker). Powder samples were pressed into KBr
discs; liquid samples were dried on CaF2 optical windows; and the
spectra were recorded. The thermal stability of these nanoparticles
was measured with a TG 209 F3 Tarsus (Netzsch, Germany) for
TGA; the samples were heated in nitrogen from 30 to 7008C (at
a constant heating rate of 108Cminꢁ1; kept at 1058C for 10 min).
The XP spectra were recorded on a RBD upgraded PHI-5000C ESCA
system (Perkin–Elmer, USA) with MgKa radiation (hn=1253.6 eV) or
AlKa radiation (hn=1486.6 eV). EA (C, H, N) was performed on
a vario MACRO cube elemental analyzer (Elementar, Germany) to
determine the total millimoles of nitrogen per gram of particles.
Contact angles were measured with a DSA10MK2G140 apparatus
(Kruss, Germany) operating at 258C and 50% humidity. Nanoparti-
cles were compressed into disc-shaped samples with 12 mm diam-
eter and 0.3 mm thickness (10 MPa pressure, similar to preparing
a disc for a FTIR sample; 60 mg of nanoparticles). The optical mi-
croscopy image was obtained with an optical microscope (Olym-
pus BX41, Japan).
Chemicals and reagents
All reagents were of analytical grade and used as purchased with-
out further purification.
Preparation of N-alkyl imidazole im-Cn (n=4, 6, and 10)
CH3(CH2)nCl (n=3, 5, and 9; 40 mmol) was added to THF (10 mL).
Imidazole (8.16 g, 120 mmol), Bu4NHSO4 (0.543 g, 4 mol%), and aq
NaOH (50 wt%, 35 mL) were added to the above suspension and
then stirred at RT for 24 h. After the reaction, the mixture was ex-
tracted with EtOAc. The organic layer was washed with saturated
aqueous NaCl solution, dried over anhydrous Na2SO4, evaporated
by rotary evaporation in vacuum, and finally purified by using silica
column chromatography (petroleum ether/EtOAc=9:1). The pure
product was obtained after the mixture was dried over anhydrous
Na2SO4 and evaporated by rotary evaporation in vacuum. All these
products were characterized by 1H NMR, 13C NMR, MS, and FTIR
analysis.
1
im-C4: H NMR (500 MHz, CDCl3): d=7.43 (s, 1H), 7.02 (s, 1H), 6.88
(s, 1H), 3.90 (s, 2H), 1.73 (s, 2H), 1.29 (s, 2H), 0.91 ppm (s, 3H);
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bottom flask. The mixture was sonicated for 0.5 min and then
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·
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Received: October 23, 2014
Published online on && &&, 0000
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L. Feng, J. Wang,* L. Chen, M. Lu,
Z. Zheng, R. Jing, H. Chen, X. Shen
Pucker up for a Pickering emulsion:
This work describes a general strategy
to enhance a typical organic–aqueous
heterogeneous green oxidation reac-
tion, in which surface tunable magnetic
nanoparticles are introduced into liquids
to form a controllable Pickering emul-
sion. The catalysis results revealed that
the stabilized micrometer-scale emul-
sion for biphasic Montanari oxidation
resulted in a significantly higher reac-
tion rate.
&& – &&
A Green Strategy to Enhance
a Liquid–Liquid Heterogeneous
Reaction with a Magnetic Recyclable
Pickering Emulsion
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