Magnetic superhydrophobic polymer nanosphere cage as a framework for miceller catalysis in biphasic media

Article abstract of DOI:10.1002/cctc.201400009

This study demonstrated the use of a magnetic superhydrophobic polymer nanosphere cage derived from a Pickering emulsion as a framework for miceller catalysis in biphasic media. We designed this system through the covalent stabilisation of active site (2,2,6,6-tetramethylpiperidine 1-oxyl)-grafted amphiphiles onto the magnetic hydrophobic core to form a micelle-like architecture and used it as a Pickering emulsifier to develop a new kind of micelle-catalysed biphasic system. The catalysis results reveal that these fixed micelle-tethered 2,2,6,6-tetramethylpiperidine 1-oxyl catalysts demonstrate much higher activity than their soluble counterparts in the biphasic Montanari oxidation of alcohols, which could be due to the unique properties of Pickering emulsion and miceller catalysis. In addition, the excellent magnetic response ensures the efficient recycling of the catalyst by magnetic decantation. The catalyst can be recycled at least 10 times without significant loss of activity or degradation of magnetic susceptibility. Moreover, this study demonstrates that this new Pickering emulsion-based and micelle-catalysed biphasic system is technically simple and practically applicable. Free from the cage: The use of a magnetic polymer nanosphere cage derived from a Pickering emulsion as a framework for micelle-catalysed biphasic systems has been described. The catalysis results reveal that these fixed micelle-tethered catalysts self-assembled at the water-oil interface demonstrate much higher activity than their soluble counterparts. Moreover, the simple strategy applied for emulsion demulsifying (sonication or magnetic decantation) and re-emulsifying (magnetic stirring) makes this new system technically simple and practically applicable.

Full text of DOI:10.1002/cctc.201400009

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DOI: 10.1002/cctc.201400009
Magnetic Superhydrophobic Polymer Nanosphere Cage as
a Framework for Miceller Catalysis in Biphasic Media
Zhi Zheng, Jianli Wang,* Hualiang Chen, Linbin Feng, Ren Jing, Meizhen Lu, Bao Hu, and
Jianbing Ji[a]
This study demonstrated the use of a magnetic superhydro-
phobic polymer nanosphere cage derived from a Pickering
emulsion as a framework for miceller catalysis in biphasic
media. We designed this system through the covalent stabilisa-
tion of active site (2,2,6,6-tetramethylpiperidine 1-oxyl)-grafted
amphiphiles onto the magnetic hydrophobic core to form a mi-
celle-like architecture and used it as a Pickering emulsifier to
develop a new kind of micelle-catalysed biphasic system. The
catalysis results reveal that these fixed micelle-tethered 2,2,6,6-
tetramethylpiperidine 1-oxyl catalysts demonstrate much
higher activity than their soluble counterparts in the biphasic
Montanari oxidation of alcohols, which could be due to the
unique properties of Pickering emulsion and miceller catalysis.
In addition, the excellent magnetic response ensures the effi-
cient recycling of the catalyst by magnetic decantation. The
catalyst can be recycled at least 10 times without significant
loss of activity or degradation of magnetic susceptibility. More-
over, this study demonstrates that this new Pickering emul-
sion-based and micelle-catalysed biphasic system is technically
simple and practically applicable.
Introduction
So far, in many cases, cross-linked polymer, carbon, silica gel,
and metal oxide have been widely used as insoluble supports
for biphasic catalysis, which thus allows a catalyst recycling of
the reaction media through simple filtration.[1,2] Unfortunately,
they often result in low catalytic activity because of the weak
affinity between mutually immiscible substrates as well as low
accessibility of active sites to reactants; therefore, it would be
extremely desirable if one could not only recycle the catalysts
but also fine-tune their desired catalytic activity.[3,4]
molecules from emulsified oils to micelles in water.[5g] Other-
wise, the amount of the hydrophobic substance usually ex-
ceeds the solubilisation capacity of the micelle; in this case,
the amphiphilic molecules mainly serve as emulsifying agents
and thus help in the dispersion of excess hydrophobic droplets
rather than form micelles in water.[8] Moreover, the problems of
product isolation and catalyst recycling from stable emulsion
limit the scope of this approach.
Herein, we described a Pickering emulsion-based and mi-
celle-catalysed biphasic system, which is different from the ex-
isting dynamic miceller catalysis in biphasic media. In Pickering
emulsions (water-in-oil or oil-in-water),[9] as driven by the re-
duction of total surface energy, the nanometre- to micrometre-
sized particles (SiO2, CdSe, Au, Fe3O4, polystyrene nanosphere,
microgel, etc.) can energetically self-assemble at the water–oil
interface to form particle framework capsules. This unique
property of the Pickering emulsion makes it potentially attrac-
tive as a new platform for the micelle-catalysed biphasic
system with high efficiency; the potential advantages of Pick-
ering emulsions are as follows: 1) solid Pickering emulsifiers
can be rationally designed through the covalent stabilisation
of amphiphiles onto the hydrophobic polymer nanosphere
(core) to form a micelle-like architecture, and this fixed micro-
environment composed of the hydrophobic core and the am-
phiphilic shell can perform similarly as a micelle;[10,11] 2) the
highly dispersed micelle-like nanoparticles adsorbed at the
water–oil interface well alleviate the particle agglomeration
problem, affording plentiful active interspace for potential mi-
celler catalysis; and 3) Pickering emulsions can be demulsified
by centrifugation or sonication and re-emulsified by stirring
the mixture. Even though Pickering emulsions have these ad-
vantages, the use of Pickering emulsions to develop a new mi-
To solve this issue, one elegant and general strategy is to
graft the catalyst onto amphiphiles and subsequently form mi-
celles in water.[5] In certain instances, the presence of dynamic
micelles can have an accelerating effect through the solubilisa-
tion of the hydrophobic reagent in water and through the acti-
vation of the substrate by the assembled catalyst on the sur-
face or in the interior of micelles.[5g,6] However, along with
these advantages, there exists a defect with respect to the dy-
namic nature of micelles. The stability of micelles depends
highly on the factors such as amphiphilic structure, reagent
polarity, critical micelle concentration, and temperature.[7] In
addition, the miceller catalysis in biphasic media is complicated
because of the interphase transport barrier of hydrophobic
[a] Z. Zheng, Prof. J. Wang, H. Chen, L. Feng, R. Jing, M. Lu, Prof. B. Hu,
Prof. J. Ji
Zhejiang Province Key Laboratory of Biofuel
State Key Laboratory Breeding Base of
Green Chemistry-Synthesis Technology
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.201400009.
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celle-catalysed biphasic system with high efficiency has been
scarcely investigated so far.
To verify these advantages, we first design a new and simple
strategy by using magnetic recyclable, cross-linked polystyrene
nanosphere (Fe3O4/PS NP) as a hydrophobic core for potential
miceller catalysis and choosing 2,2,6,6-tetramethylpiperidine 1-
oxyl (TEMPO) as a model oxidation catalyst[12] to test its catalyt-
ic behaviour in biphasic media. As depicted in Scheme 1, the
Figure 1. Size distribution of Fe3O4/chloromethyl PS.
Scheme 1. a) Evolution of the structure of magnetically fixed micelle teth-
ered with TEMPO and b) its miceller catalysis with the magnetic polymer
nanosphere cage derived from the Pickering emulsion.
covalent stabilisation of TEMPO-grafted amphiphiles [Mim-
CnTEMPO]Cl (n=4, 6, and 10) onto the Fe3O4/PS NP surface
could form a micelle-like architecture and used as Pickering
emulsifiers in biphasic Montanari media for potential miceller
catalysis; the magnetic Fe3O4 in the PS core domain could
ensure the quantitative recycling of the catalyst in situ by
simple magnetic decantation. Although fluorous/triazole-
tagged TEMPO has been demonstrated to promote the forma-
tion of an emulsion in Montanari media, the use of a Pickering
emulsion to design a new Montanari oxidation system has
never been reported.[13] This study demonstrates that this new
Pickering emulsion-based and micelle-catalysed biphasic
system is technically simple and practically applicable.
Figure 2. FTIR spectra of a) Fe3O4/chloromethyl PS, b) Fe3O4/PS-C0TEMPO,
c) Fe3O4/PS[im-C4TEMPO]Cl, and d) im-C4TEMPO.
analysis (EA), TEM, CA measurements, and vibrating sample
magnetometer (VSM). FTIR data (Figures 2 and S1) reveal that
the ÀCH2Cl peaks disappear at 1260 and 680 cmÀ1 and new
peaks appear at 1362, 1177, and 1103 cmÀ1 attributed to the
presence of NÀO, CÀN, and CÀO bonds, respectively, which in-
dicates the linkage of im-C4TEMPO to the Fe3O4/PS NP surface.
This result is in good agreement with TG-FTIR data (Figures 3
and S2–S4). Volatile products released at the temperature of
maximum weight loss show peaks at 1360, 1200, and
1100 cmÀ1, which are due to the degradation of the im-
C4TEMPO moiety. Moreover, these data can be further deter-
mined by XPS measurement of Fe3O4/PS[im-C4TEMPO]Cl (Fig-
ures 4, S5, and S6). N1s peaks at 402.3, 401.2, and 400.2 eV are
associated with NÀC and NÀO bonds (Figure 4b), and the ab-
solute intensity ratio of 1.0:0.98:0.98 shows good agreement
with the expected value of 1.0:1.0:1.0, that is, two N atoms in
the imidazolium ring and one in the TEMPO moiety; the Cl2p3
spectrum is assigned to two peaks at 199.1 and 197.1 eV, and
the peak at 197.1 eV is consistent with the reference value of
Cl in the anion form (Figure S6). The TEMPO loading on the
Fe3O4/PS NP surface was determined from quantitative EA, and
the results are presented in Table S1. The TEM analysis of
Fe3O4/PS[im-C4TEMPO]Cl (Figure 5A) demonstrates the struc-
Results and Discussion
Catalyst preparation and characterisation
The structure of [im-CnTEMPO]Cl (n=4, 6, and 10) amphiphiles
stabilised on the Fe3O4/PS NP surface was similar to that devel-
oped in our previous work,[14] and the catalysts were prepared
by using a concise route outlined in Scheme S1. Fe3O4/chloro-
methyl PS NPs (divinylbenzene content: ꢀ3%; chlorine con-
tent: ꢀ1.5 mmolCl gÀ1; mean size: ꢀ60 nm; Figure 1) were pre-
pared by applying the miniemulsion polymerisation tech-
nique.[15] im-CnTEMPO (n=4, 6, and 10) species were synthes-
ised according to the work of Griffin and co-workers[16] and
verified by NMR spectroscopy. Then, Fe3O4/chloromethyl PS
NPs were directly functionalised with excess im-CnTEMPO (n=
4, 6, and 10) to afford the corresponding catalyst Fe3O4/PS[im-
CnTEMPO]Cl (n=4, 6, and 10) by heating to reflux in toluene.
The Fe3O4/PS-C0TEMPO catalyst was prepared according to our
previous report.[14] The resulting catalysts were characterised
by using FTIR, thermogravimetry coupled to FTIR spectroscopy
(TG-FTIR), X-ray photoelectron spectroscopy (XPS), elemental
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Figure 3. A) TG and derivative TG curves of Fe3O4/PS[im-C4TEMPO]Cl and
B) the corresponding FTIR spectra of volatile products in TG analysis at the
temperature of maximum weight loss.
Figure 4. A) XPS spectrum of Fe3O4/PS[im-C4TEMPO]Cl and B) XPS spectra of
N 1s peaks of a) Fe3O4/PS-C0TEMPO and b) Fe3O4/PS[im-C4TEMPO]Cl.
ture of the catalyst with a mean diameter of approximately
60 nm. CA measurements of the catalyst surface were per-
formed by using the drop shape method, in which a drop of
water or benzyl alcohol was allowed to contact the Fe3O4/
PS[im-CnTEMPO]Cl (n=4, 6, and 10) surface (Figures 6A and B,
10, 11, S7, and S8). VSM data (Figures 14 and S9) reveal that
all synthesised particles have superparamagnetic properties
(coercivity: ꢀ1.1–2.9 G) with large saturation magnetisation
(ꢀ10.41 emugÀ1), which indicate that these catalysts can be
strongly attracted to magnetic fields with a magnet (Fig-
ure 5B).
Pickering emulsion-based Montanari oxidation of alcohols
The catalytic activity of Fe3O4/PS[im-C4TEMPO]Cl was evaluated
by performing the Montanari oxidation of alcohol. For Monta-
nari oxidation, we used the household bleach NaClO in combi-
nation with 10 mol% NaBr and 1 mol% TEMPO in the H2O–
CH2Cl2 biphasic system; Montanari oxidation has been widely
investigated for the oxidation of alcohols owing to its low tox-
icity, reversible redox behaviour, as well as high efficiency and
selectivity.[17] To investigate the potential miceller catalysis with
Figure 5. A TEM image (scale bar: 50 nm) and B photograph of magnetic
property of Fe3O4/PS[im-C4TEMPO]Cl.
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Figure 6. CA of A) water and B) benzyl alcohol on the surface of Fe3O4/
PS[im-C4TEMPO]Cl. Photographs of C) TEMPO and D) Fe3O4/PS[im-
C4TEMPO]Cl (ꢀ0.14% (w/v)-mediated Montanari oxidation of benzyl alcohol
(Table 1, entry 1).
Figure 7. Optical microscopy image of Fe3O4/PS[im-C4TEMPO]Cl (ꢀ0.14%
(w/v))-stabilised water droplets (scale bar: 300 mm).
a Pickering emulsion in Montanari oxidation, the efficiency of
catalyst composites to form a Pickering emulsion was investi-
gated in the aqueous NaClO–CH2Cl2 (3.75:2.5) solution. In
a Pickering emulsion, one of the main factors that affect the
formation of emulsions is the wettability of particles; therefore,
the interfacial behaviour of the catalyst was measured. We
found that the contact angle (CA) of water on the Fe3O4/PS[im-
C4TEMPO]Cl surface was approximately 908 (Figure 6A), which
indicates that the catalyst surface domain can be equally
wetted by oil and water (Figure S10). In addition, as stated by
Binks et al.,[18] the strength with which a solid particle is held
at a water–oil interface becomes maximum at a CA of 908,
which leads to the strong attachment of catalyst particles at
the water–oil interface. Thus, as envisioned in Scheme 1, the
catalyst particles can reside at the water–oil interface and sta-
bilise the water droplets to form a quasi-stable emulsion under
vigorous stirring. As shown in Figure 6D, these particle-
stabilised emulsions (in the upper layer) contain most of the
catalyst particles and aqueous NaClO–NaBr droplets with a
diameter ranging from approximately 100 to 300 mm
lysts in the Pickering emulsion system was much higher than
that of the soluble counterparts TEMPO (Table 1) and [Mim-
C4TEMPO]Cl in the classical biphasic system (Table S2). For ex-
ample, the Fe3O4/PS[im-C4TEMPO]Cl catalyst demonstrated
more than 99.9% conversion of benzyl alcohol, which is higher
than that demonstrated by the soluble catalysts TEMPO
(37.8%Æ1.1%; Table 1, entry 1) and [Mim-C4TEMPO]Cl
(88.7%Æ1.2%; Table S2, entry 1). To make a more obvious
comparison, the relationship of benzyl alcohol conversion
versus time was elucidated (Figure 8). This comparison, al-
though not fully extended to other alcohols, still demonstrates
that our Pickering emulsion-based system demonstrates
a higher activity.
Evidence for Pickering emulsion-based miceller catalysis
We then focused our attention on another role played by the
catalyst at the water–oil interface. Yang et al. demonstrated
(Figure 7). Furthermore, these emulsions are too
Table 1. Catalytic activity comparison between TEMPO- and Fe3O4/PS[im-C4TEMPO]Cl-
stable to collapse (Figure S11); this finding is consis-
tent with carboxylic acid-grafted PS particle-stabilised
aqueous NaCl–C16H34 emulsions.[18a]
mediated Montanari oxidation of alcohols to the respective carbonyl derivatives.[a]
Substrate
Conversion [%]
Fe3O4/PS[im-C4TEMPO]Cl
TEMPO[b]
1[c]
2[c]
3[c]
4
5
6
benzyl alcohol
37.8Æ1.1
30.7Æ3.2[d]
18.2Æ2.6
39.3Æ2.7
15.1Æ1.6
57.6Æ1.1
>99.9
Catalytic activity evaluation of the Pickering emul-
sion-based Montanari oxidation of alcohols
4-methoxybenzyl alcohol
4-nitrobenzyl alcohol
2-phenylethanol
diphenylmethanol
pentanol
91.2Æ0.9[e]
>99.9
>99.9
42.7Æ1.3
>99.9
To demonstrate the improvement in Montanari oxi-
dation with the resulting Pickering emulsion, we
compared the Pickering emulsion-based Montanari
oxidation of alcohols with the classical biphasic
system under the same conditions. Similar to fluo-
rous/triazole-tagged TEMPO-stabilised emulsion,[13]
because of the sharp increase of the catalytic inter-
face, the activity of the Fe3O4/PS[im-C4TEMPO]Cl cata-
[a] Reaction conditions: alcohol (1 mmol), 10 mol% NaBr (0.1 mmol), 1.5 equiv. NaClO
(1.5 mmol, 3.75 mL, pH ꢀ9.1), 1 mol% catalyst (0.01 mmol), CH2Cl2 (2.5 mL), T=108C,
t=30 s; conversions and selectivities were determined from GC analysis, all selectivi-
ties were >99%; [b] The same number of catalytic sites as those in Fe3O4/PS[im-
C4TEMPO]Cl; [c] The reaction was conducted in 15 s; [d] The selectivity was >97%;
[e] The selectivity was >98%.
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Table 2. Contribution of the miceller structure in the TEMPO-mediated
Montanari oxidation of benzyl alcohol to benzaldehyde.[a]
Entry
Substrate
Conversion [%]
1
2
3
4
5
6
7
8
9
TEMPO
Fe3O4/PS-C0TEMPO
72.1Æ1.4
50.0Æ1.8
81.2Æ1.7
92.1Æ1.5
87.2Æ0.9
18.4Æ1.1
22.0Æ1.8
22.8Æ3.2
79.8Æ1.2
Fe3O4/PS[im-C4TEMPO]Cl
Fe3O4/PS[im-C6TEMPO]Cl
Fe3O4/PS[im-C10TEMPO]Cl
Fe3O4/SiO2[im-C4TEMPO]Cl
Fe3O4/SiO2[im-C6TEMPO]Cl
Fe3O4/SiO2[im-C10TEMPO]Cl
Fe3O4/propyl SiO2[im-C4TEMPO]Cl
Figure 8. The catalytic activity comparison between Fe3O4/PS[im-C4TEMPO]Cl
and TEMPO in the Montanari oxidation of benzyl alcohol (experimental
detail: see Table 1; conversions and selectivities were determined from GC
analysis, all selectivities were >99.9%).
[a] Reaction conditions: benzyl alcohol (3 mmol), 10 mol% NaBr
(0.3 mmol), 1.5 equiv. NaClO (4.5 mmol, 11.25 mL, pH ꢀ9.1), 0.1 mol%
catalyst (0.003 mmol), CH2Cl2 (7.5 mL), T=108C, t=60 s; conversions and
selectivities were determined from GC analysis, all selectivities were
>99%.
that the micelle-like Pd/SiO2 catalyst with a hydrophobic core
and a hydrophilic shell functions as a micelle in the hydrogena-
tion reaction, which drives the solubilisation of organic reac-
tants in the water phase to alleviate the kinetic barriers caused
by the transport of organic substrates to catalytic sites.[11a]
Herein, to demonstrate the miceller acceleration of the catalyst
at the water–oil interface in Pickering emulsions, the CA of
benzyl alcohol was measured on the catalyst surface. The
result indicates that the Fe3O4/PS[im-C4TEMPO]Cl catalyst dem-
onstrates super-wettability of benzyl alcohol (CAꢀ08; Fig-
ure 6B). On the basis of this result, we hypothesised that the
CH2Cl2 solution of benzyl alcohol can be directly assimilated
from oil droplets in the PS core domain of the swollen Fe3O4/
PS[im-C4TEMPO]Cl catalyst at the H2O–CH2Cl2 interface, which
thus provides billions of micelle-like reactors to further acceler-
ate the reaction.
To test this hypothesis, we scaled down the catalyst addition
to a certain amount (0.1 mol% to benzyl alcohol, ꢀ0.015% (w/
v); Table 2, entries 3–5) so that the particles residing at the
water–oil interface are far from enough to form Pickering
emulsions (Figure 6A and D versus Figure 9); therefore, if the
reaction activity is still higher, it is mainly due to miceller catal-
ysis. Under this condition, the Fe3O4/PS[im-CnTEMPO]Cl (n=4,
6, and 10) catalyst demonstrated much higher conversions of
benzyl alcohol (Table 2, entries 3–5) than did the soluble cata-
lyst TEMPO (72.1%Æ1.4%; Table 2, entry 1). To our knowledge,
this is the first example of miceller catalysis obtained from the
self-assembly of catalyst particles at the water–oil interface in
the Pickering emulsion.[5 g] The potential advantages are as fol-
lows:[8] 1) the dynamic micelle-catalysed biphasic system can
be simplified, and thus the interphase transport barrier of hy-
drophobic molecules from emulsified oil droplets to micelles in
the water phase is alleviated; 2) even at far below critical mi-
celle concentration, all the amphiphiles pre-stabilised on the
Fe3O4/PS NP surface can play dual roles of emulsifers and mi-
celler catalysts; and 3) the magnetic response ensures the recy-
Figure 9. Photograph of A) soluble TEMPO, B) Fe3O4/PS[im-C4TEMPO]Cl
(ꢀ0.014% (w/v)), C) Fe3O4/PS[im-C6TEMPO]Cl (ꢀ0.015% (w/v)), and
D) Fe3O4/PS[im-C10TEMPO]Cl (ꢀ0.017% (w/v))-mediated Montanari oxidation
of benzyl alcohol (Table 2, entries 1 and 3–5).
cling of the miceller catalyst by magnetic decantation. There-
fore, these advantages essentially make our Pickering emul-
sion-based and micelle-catalysed biphasic system technically
simple and practically applicable.
To further verify the miceller acceleration of the catalyst in
the Pickering emulsion at the microstructure level, we synthes-
ised a set of counterpart catalysts with different core and shell
domains and used them for the Montanari oxidation of benzyl
alcohol. A synergistic effect was observed in all instances: that
the catalyst activity was found to be directly correlated with
the hydrophobic core and the amphiphilic shell (Table 2, en-
tries 2–9). For example, at the catalyst loading of 0.1 mol%,
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the Fe3O4/SiO2[im-CnTEMPO]Cl (n=4, 6, and 10) catalyst (
Table 2, entries 6–8) with the hydrophilic core (CAꢀ308; Fig-
ure S7) demonstrated much lower activities than the Fe3O4/
PS[im-CnTEMPO]Cl (n=4, 6, and 10) catalyst (Table 2, entries 3–
5) with the hydrophobic core (CAꢀ908; Figure S7) as well the
soluble catalyst TEMPO (Table 2, entry 1). Thus, we considered
that the increase in the activity of Fe3O4/SiO2[im-C4TEMPO]Cl at
a higher catalyst loading (1 mol% to benzyl alcohol; Table S2,
entry 1) was only due to the acceleration of Pickering emulsifi-
cation (Figure S12). To clarify the contribution of the hydropho-
bic core, we immobilised [Mim-C4TEMPO]Cl onto relatively hy-
drophobic Fe3O4/chloropropyl SiO2 NPs (Table 2, entry 9; for
details of surface regulation, see Section 2.5 in the Supporting
Information).[19] As expected, the resulting Fe3O4/propyl
SiO2[im-C4TEMPO]Cl catalyst (mean size: ꢀ20 nm; Figure S13)
showed much improved affinity for non-polar reagents (Fig-
ure S8) and similar catalytic activity to Fe3O4/PS[im-C4TEMPO]Cl
(Table 2, entry 3) because of their similar surface properties
(Figure 6A and B versus Figure 10).
Figure 10. CA of A) water and B) benzyl alcohol on the surface of Fe3O4/
propyl SiO2[im-C4TEMPO]Cl (Table 2, entry 9).
Moreover, the amphiphilic shells reflect the activity of these
catalysts (Table 2, entries 2–5). For example, the activity of
Fe3O4/PS[im-C6TEMPO]Cl (92.1%Æ1.5%; Table 2, entry 4) was
much higher than that of Fe3O4/PS-C0TEMPO (50.0%Æ1.8%;
Table 2, entry 2), probably owing to the optimised hydrophilic–
lipophilic balance value of the catalyst surface. Dynamic CA
measurements (Figure 11) show that Fe3O4/PS[im-CnTEMPO]Cl
(n=4, 6, and 10) catalysts are relatively more wettable than
Fe3O4/PS-C0TEMPO because of the ionic liquid (IL) moiety with
an appropriate alkyl chain length, which facilitates the admis-
sion of both hydrophilic and hydrophobic molecules.
Figure 11. Dynamic CA of water on the surface of Fe3O4/PS-C0TEMPO and
Fe3O4/PS[im-CnTEMPO]Cl (n=4, 6, and 10).
dation of benzyl alcohol (scale: 3 mmol; catalyst loading:
0.5 mol% to benzyl alcohol) through successive ultrasonic de-
mulsification and magnetic decantation steps, followed by
washing thoroughly with CH2Cl2, acetone, and water in this
order. These catalyst particle-stabilised emulsions can be
Substrate scope of Pickering emulsion-based Montanari
oxidation
Notably, the Fe3O4/PS[im-C6TEMPO]Cl catalyst also demon-
strates excellent activity for the selective oxidation of various
types of alcohols without substrate limitation. As
shown in Table 3, the primary benzylic, aliphatic, or
Table 3. Fe3O4/PS[im-C6TEMPO]Cl-mediated Montanari oxidation of alcohols to the re-
heteroatoms containing alcohols and secondary ben-
zylic or aliphatic alcohols can be quantitatively con-
verted into the corresponding aldehydes and ketones
within minutes. To our knowledge, these results were
far more superior to those reported in the case of re-
cyclable nitroxyl radicals in the Montanari oxidation
of alcohols.[12d,17,20] For example, the activity of Fe3O4/
PS[im-C6TEMPO]Cl (Table S3, entries 1, 2, and 4) was
increased up to approximately 500-fold in compari-
son with TEMPO immobilised on carbon-coated Co
nanoparticles (Table S3, entry 3)[21] and 100-fold in
comparison with TEMPO immobilised on bare Fe3O4
nanoparticles via click chemistry (Table S3, entry 5).[22]
spective carbonyl derivatives.[a]
Substrate
Catalyst loading
[mol%]
t
Selectivity Conversion
[min] [%] [%]
1[b] benzyl alcohol
0.01
0.1
0.1
60
3
3
3
3
3
3
3
3
>99
96
2
benzyl alcohol
>99
>99
96
>99
>99
>99
99
99
99
>99
>99
>99
99
>99
>99
>99
>99
>99
>99
>99
>99
97
3
4-nitrobenzyl alcohol
4
4-methoxybenzyl alcohol 0.5
5
6
7
1-phenethyl alcohol
3-pyridinemethanol
1-pentanol
0.25
0.25
0.5
0.5
0.5
0.5
0.5
0.5
1
8
1-hexanol
9
1-octanol
10
11
12
13
1-nonanol
3
2-phenethyl alcohol
diphenylmethanol
cyclopentanol
10
10
6
>99
98
Catalyst recyclability
[a] Reaction conditions: alcohol (3 mmol), 10 mol% NaBr (0.3 mmol), 1.5 equiv. NaClO
(4.5 mmol, 11.25 mL, pH ꢀ9.1), CH2Cl2 (7.5 mL), T=108C; conversions and selectivities
were determined from GC analysis, all the results were obtained by at least three re-
petitive reactions; [b] The reaction was conducted in 30 mmol scale.
We finally turned our attention to the catalyst recycle
scope. The recyclability of Fe3O4/PS[im-C6TEMPO]Cl
was evaluated by isolating it from the Montanari oxi-
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broken by magnetic decantation or sonication. Because the
particles do not adsorb at the interface of the droplets, water
droplets collapse; thus, the two immiscible liquids are separat-
ed and the catalysts are recycled. For the next cycle, the fresh
reactants and recycled catalysts can be re-emulsified by stir-
ring. As shown in Figure 12, the Fe3O4/PS[im-C6TEMPO]Cl cata-
lyst can be recycled overnight by applying a magnetic field or
within 30 min after ultrasonic demulsification for 10 s. Of fur-
ther interest is the good recyclability of Fe3O4/PS[im-
C6TEMPO]Cl; the Fe3O4/PS[im-C6TEMPO]Cl catalyst could be re-
cycled 10 times, with the conversion of benzyl alcohol remain-
ing 94% or more (Figure 13); the catalyst weight was slightly
declined from 13.7 to 11.3 mg. VSM data reveal that the mag-
netic susceptibility of the catalyst did not decrease significantly
(from 10.4 to 9.3 emugÀ1; Figure 14), which indicates the high
efficiency of magnetic separation of this catalyst.
Figure 14. Magnetisation curves of Fe3O4/PS[im-C6TEMPO]Cl (fresh and 10th
recycle).
Conclusions
We demonstrated the use of magnetic polymer nanosphere
cage derived from a Pickering emulsion as a framework for mi-
celler catalysis, which can be used as an efficient platform for
accelerating biphasic catalysis. The significant increase in the
activity of 2,2,6,6-tetramethylpiperidine 1-oxyl after it is immo-
bilised onto the magnetic recyclable, cross-linked polystyrene
nanosphere surface is attributed to the dual accelerating effect
of Pickering emulsification and miceller catalysis of the catalyst
at the water–oil interface. This finding can also be extended to
some synthetically elaborate, stimuli-sensitive (e.g., pH and
temperature), and core or shell cross-linked micelles[23] for the
design and preparation of catalysts to accelerate other bipha-
sic catalyses. Moreover, this study not only affords a new op-
portunity to develop new micelle-catalysed biphasic systems
with high efficiency and practicability but also extends the ap-
plication fields of Pickering emulsions.
Figure 12. Photographs of magnetic separation of Fe3O4/PS[im-C6TEMPO]Cl
in the Montanari oxidation of benzyl alcohol (the excess hypochlorite was
destroyed by drops of saturated Na2SO3).
Experimental Section
Chemicals and reagents
All reagents were of analytical grade and used as purchased with-
out further purification.
Preparation of im-CnTEMPO (n=4, 6, and 10)
A solution of 4-OH-TEMPO (1.73 g, 10 mmol) in THF (10 mL) was
added dropwise to a mixture of Br(CH2)nBr (n=4, 6, and 10;
30 mmol), Bu4HSO4 (0.136 g, 4 mol%), and aq NaOH (50 wt%,
10 mL), and the resulting mixture was stirred overnight at RT. Then,
the mixture was poured into ice water and extracted with EtOAc.
The organic layer was washed with saturated NaCl, dried over an-
hydrous MgSO4, evaporated by rotary evaporation in vacuum, and
finally purified by using silica column chromatography (petroleum
ether–EtOAc: 8:1), which gave red viscous oil Br(CH2)n-O-TEMPO
(BrCnTEMPO). In addition, a solution of BrCnTEMPO as-synthesised
Figure 13. Recyclability of Fe3O4/PS[im-C6TEMPO]Cl in the Montanari oxida-
tion of benzyl alcohol (experimental detail: see Section 3 in the Supporting
Information).
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above (n=4, 6 and 10; 10 mmol) in THF (10 mL) was added drop-
wise to a mixture of imidazole (2.04 g, 30 mmol), Bu4HSO4 (0.136 g,
4 mol%), and aq NaOH (50 wt%, 10 mL), and the resulting mixture
was stirred vigorously overnight at RT. Then, the mixture was
poured into ice water and extracted with EtOAc. The organic layer
was washed with saturated NaCl, and the water layer was again ex-
tracted with EtOAc. The combined organic solution was dried by
anhydrous MgSO4, evaporated by rotary evaporation in vacuum,
and finally purified by using silica column chromatography
(CH2Cl2–CH3OH: 9:1), which gave red viscous oil im-CnTEMPO (n=
4, 6, and 10). Before NMR analysis, the paramagnetic centre of ni-
troxyl radical residues was reduced in situ by phenylhydrazine to
the corresponding hydroxylamine species. All these products were
characterised by applying 1H NMR, 13C NMR, FTIR, and EA
techniques.
1H), 3.41 (t, J=6.7 Hz, 2H), 1.95–1.89 (m, 2H), 1.77–1.69 (m, 2H),
1.58–1.50 (m, 2H), 1.47 (t, J=11.8 Hz, 2H), 1.30 (dd, J=29.1,
10.4 Hz, 12H), 1.21 (s, 6H), 1.16 ppm (s, 6H); 13C NMR (126 MHz,
CDCl3): d=20.49, 25.81, 26.09, 28.63, 28.95, 29.03, 29.05, 29.80,
30.56, 31.61, 44.48, 46.53, 58.99, 67.86, 70.07, 118.43, 127.96,
136.71 ppm; FTIR (CaF2 optical window, selected data): 1649.8,
1507.8, 1463.2, 1357.4, 1283.2, 1228.4, 1177.6, 1098.0 cmÀ1; elemen-
tal analysis calcd (%) for C22H40N3O2: C 69.76, H 10.58, N 11.06.
Preparation of Fe3O4/PS[im-CnTEMPO]Cl (n=4, 6, and 10)
Fe3O4/chloromethyl PS NPs (2.0 g, ꢀ3 mmol Cl) were suspended in
toluene (50 mL) before the addition of im-CnTEMPO (n=4, 6, and
10; 4.5 mmol). The reaction mixture was heated to reflux for 48 h
under an atmosphere of nitrogen. The solid material was collected
by magnetic decantation, washed thrice with THF and thrice with
deionised water, and finally freeze dried in vacuum.
1
BrC4TEMPO: H NMR (500 MHz, CDCl3): d=3.60 (tt, J=11.0, 4.1 Hz,
1H), 3.52–3.39 (m, 4H), 2.04–1.93 (m, 4H), 1.73 (dt, J=13.4, 6.2 Hz,
2H), 1.52 (t, J=11.9 Hz, 2H), 1.27 ppm (s, 6H), 1.21 (s, 6H);
13C NMR (126 MHz, CDCl3): d=20.70, 28.58, 29.64, 31.77, 33.69,
44.58, 59.38, 66.94, 70.40 ppm; FTIR (CaF2 optical window, selected
data): 1460.6, 1375.5, 1362.2, 1244.2, 1177.8, 1103.0 cmÀ1; elemental
analysis calcd (%) for C13H25NO2Br: C 50.89, H 8.10, N 4.60.
Characterisation of Fe3O4/PS[im-CnTEMPO]Cl (n=4, 6, and
10)
The particle size of Fe3O4/PS NPs was measured with a Zetatrac
U2561ZS (Microtrac, US). The NMR spectra were recorded with an
AVANCE III 500 MHz NMR spectrometer (Bruker, Switzerland) using
TMS as internal standard. The FTIR spectra (200–4000 cmÀ1) were
recorded by diffuse reflectance with a Tensor 27 FTIR spectrometer
(Bruker, Switzerland). Powder samples were pressed into KBr disc,
liquid samples were dried on a CaF2 optical window, and then the
spectra were recorded. Thermal stability measurements of these
catalysts were performed on a TG 209 F3 Tarsus (Netzsch, Germa-
ny) for TG analysis coupled with a Tensor 27 FTIR (Bruker, Switzer-
land) spectrometer; the samples were heated from 30 to 7008C
(heating rate: 108CminÀ1) under an atmosphere of nitrogen and
held at 1058C for 10 min. The XPS spectra were performed on
a RBD upgraded PHI-5000C ESCA system (PerkinElmer, US) using
MgKa radiation (h=1253.6 eV) or AlKa radiation (h=1486.6 eV). EA
(C, H, N) was performed on a Vario MACRO cube elemental analy-
ser (Elementar, Germany) to determine the total millimoles of nitro-
gen per gram of particles. TEM images were recorded on a JEM-
3010 transmission electron microscope (JEOL, Japan) operating at
an acceleration voltage of 300 kV. CAs were measured with
a DSA10MK2G140 (Kruss, Germany) apparatus operating at 168C
and 32% humidity. Catalysts were pressed into disc-shaped sam-
ples with 12 mm diameter and 0.3 mm thickness (10 MPa pressed
pressure, 60 mg catalyst). The magnetic measurements of these
catalysts were performed with a XL-7 Magnetic Property Measure-
ment System (Quantum Design, US). The optical microscopy image
of the reaction emulsion was obtained on a Winner100 dynamic
particle image analyzer (Jinan Winner Particle Instruments, China).
1
BrC6TEMPO: H NMR (500 MHz, CDCl3): d=3.62 (tt, J=11.1, 4.1 Hz,
1H), 3.52–3.47 (m, 2H), 3.47–3.40 (m, 2H), 2.00 (dd, J=12.8, 4.0 Hz,
2H), 1.90 (t, J=14.2 Hz, 2H), 1.72–1.56 (m, 2H), 1.56–1.47 (m, 4H),
1.47–1.38 (m, 2H), 1.28 (s, 6H), 1.23 ppm (s, 6H); 13C NMR
(126 MHz, CDCl3): d=20.57, 25.21, 27.76, 29.76, 31.86, 32.50, 33.76,
44.71, 59.07, 67.79, 70.30 ppm; FTIR (CaF2 optical window, selected
data): 1460.9, 1375.4, 1361.8, 1242.5, 1177.8, 1102.9 cmÀ1; elemental
analysis calcd (%) for C15H29NO2Br: C 53.69, H 8.70, N 4.18.
1
BrC10TEMPO: H NMR (500 MHz, CDCl3): d=3.67–3.60 (m, 1H), 3.50
(t, J=6.7 Hz, 2H), 3.44 (t, J=6.9 Hz, 2H), 2.01 (dd, J=12.7, 4.0 Hz,
2H), 1.95–1.82 (m, 2H), 1.70–1.59 (m, 2H), 1.53 (t, J=11.9 Hz, 2H),
1.47 (dd, J=14.3, 7.4 Hz, 2H), 1.44–1.33 (m, 12H), 1.28 (s, 6H),
1.24 ppm (s, 6H); 13C NMR (126 MHz, CDCl3): d=20.53, 25.99, 27.93,
28.51, 29.14, 29.21, 29.24, 29.98, 31.93, 32.60, 33.88, 44.78, 58.97,
68.07, 70.23 ppm; FTIR (CaF2 optical window, selected data):
1462.7, 1375.1, 1361.7, 1242.8, 1177.6, 1100.6 cmÀ1; elemental anal-
ysis calcd (%) for C19H37NO2Br: C 58.38, H 9.49, N 3.54.
im-C4TEMPO: 1H NMR (500 MHz, CDCl3): d=7.47 (s, 1H), 7.03 (s,
1H), 6.87 (s, 1H), 3.91 (t, J=7.2 Hz, 2H), 3.53 (tt, J=11.1, 4.1 Hz,
1H), 3.42 (t, J=6.1 Hz, 2H), 1.94–1.87 (m, 2 H), 1.86–1.78 (m, 2H),
1.51 (dt, J=15.5, 6.1 Hz, 2H), 1.45 (t, J=11.8 Hz, 2H), 1.21 (s, 6H),
1.15 ppm (s, 6H); 13C NMR (126 MHz, CDCl3): d=20.58, 26.84, 28.06,
31.87, 44.51, 46.66, 59.01, 67.11, 70.56, 118.55, 128.11, 136.88 ppm;
FTIR (KBr, selected data): 1660.6, 1567.2, 1514.8, 1461.9, 1374.0,
1241.8, 1177.7, 1104.2 cmÀ1
C16H28N3O2: C 65.33, H 9.46, N 14.24.
; elemental analysis calcd (%) for
im-C6TEMPO: 1H NMR (500 MHz, CDCl3): d=7.45 (s, 1H), 7.06 (s,
1H), 6.87 (s, 1H), 3.86 (t, J=7.1 Hz, 2H), 3.57 (tt, J=11.1, 4.1 Hz,
1H), 3.42 (t, J=6.5 Hz, 2H), 1.95 (ddd, J=12.7, 6.0, 1.6 Hz, 2H),
1.78–1.68 (m, 2H), 1.59–1.51 (m, 2H), 1.48 (t, J=11.8 Hz, 2H), 1.42–
1.33 (m, 2H), 1.32–1.26 (m, 2H), 1.23 (s, 6H), 1.19 ppm (s, 6H);
13C NMR (126 MHz, CDCl3): d=20.52, 25.46, 26.06, 29.67, 30.64,
31.84, 44.58, 46.61, 58.93, 67.60, 70.30, 118.48, 128.05, 136.78 ppm;
FTIR (CaF2 optical window, selected data): 1650.1, 1508.2, 1462.2,
1358.1, 1283.2, 1233.9, 1178.5, 1092.4 cmÀ1; elemental analysis
calcd (%) for C18H32N3O2: C 67.22, H 9.96, N 13.13.
General method for the Fe3O4/PS[im-C6TEMPO]Cl-mediated
Montanari oxidation of alcohols
The alcohol substrate (3.0 mmol), Fe3O4/PS[im-C6TEMPO]Cl
(0.003 mmol radical, 2.60 mg), and CH2Cl2 (7.5 mL) were placed in
a 50 mL three-necked, round bottom flask. The mixture was soni-
cated for 0.5 min and then cooled to 108C. NaBr (1m, 0.3 mL,
0.3 mmol) and NaClO (0.4m, 11.25 mL, 4.5 mmol, pH ꢀ9.1) were
added sequentially. The resulting mixture was magnetically stirred
at 108C. After a desired time, the quasi-stable reaction emulsion
was immediately destabilised and quenched by the destroying of
excess hypochlorite with saturated Na2SO3 (7.5 mL), the catalyst
im-C10TEMPO: 1H NMR (500 MHz, CDCl3): d=7.47 (s, 1H), 7.03 (s,
1H), 6.87 (s, 1H), 3.88 (t, J=7.2 Hz, 2H), 3.55 (tt, J=10.9, 4.0 Hz,
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Keywords: alcohol oxidation
·
heterogeneous catalysis
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Received: January 3, 2014
Revised: March 7, 2014
Published online on && &&, 0000
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ChemCatChem 0000, 00, 1 – 10
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These are not the final page numbers!

FULL PAPERS
Z. Zheng, J. Wang,* H. Chen, L. Feng,
R. Jing, M. Lu, B. Hu, J. Ji
Free from the cage: The use of a mag-
netic polymer nanosphere cage derived
from a Pickering emulsion as a frame-
work for micelle-catalysed biphasic sys-
tems has been described. The catalysis
results reveal that these fixed micelle-
tethered catalysts self-assembled at the
water–oil interface demonstrate much
higher activity than their soluble coun-
terparts. Moreover, the simple strategy
applied for emulsion demulsifying (soni-
cation or magnetic decantation) and re-
emulsifying (magnetic stirring) makes
this new system technically simple and
practically applicable.
&& – &&
Magnetic Superhydrophobic Polymer
Nanosphere Cage as a Framework for
Miceller Catalysis in Biphasic Media
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemCatChem 0000, 00, 1 – 10
&10
&
ÞÞ
These are not the final page numbers!