A hierarchical Fe3O4@P4VP@MoO2(acac) 2 nanocomposite: Controlled synthesis and green catalytic application

Article abstract of DOI:10.1016/j.molcata.2013.04.019

Magnetite/poly(4-vinylpyridine) composite nanospheres with multiple Fe ₃O₄ cores embedded in a P4VP shell were first synthesized by miniemulsion polymerization and then used as recoverable supports for MoO₂(acac)₂ complex. The supported complex, together with superparamagnetism and strong magnetic response, showed high efficiency for the catalytic epoxidation of cis-cyclooctene with aqueous H₂O₂ in ethanol. The response strongly facilitated the easy recovery of the catalyst, which was stable in recycling tests.

Full text of DOI:10.1016/j.molcata.2013.04.019

Journal of Molecular Catalysis A: Chemical 378 (2013) 344–349
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
A hierarchical Fe O @P4VP@MoO (acac) nanocomposite: Controlled synthesis
3
4
2
2
and green catalytic application
a
a,∗
a
a
a
a
a
b
Wanchun Guo , Ge Wang , Qian Wang , Wenjun Dong , Mu Yang , Xiubing Huang , Jie Yu , Zhan Shi
a
School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, PR China
State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, Changchun 130012, PR China
b
a r t i c l e i n f o
a b s t r a c t
Article history:
3 4
Magnetite/poly(4-vinylpyridine) composite nanospheres with multiple Fe O cores embedded in a P4VP
Received 31 October 2012
Received in revised form 22 March 2013
Accepted 18 April 2013
shell were first synthesized by miniemulsion polymerization and then used as recoverable supports for
MoO2(acac)2 complex. The supported complex, together with superparamagnetism and strong magnetic
response, showed high efficiency for the catalytic epoxidation of cis-cyclooctene with aqueous H2O2 in
ethanol. The response strongly facilitated the easy recovery of the catalyst, which was stable in recycling
tests.
Available online 25 April 2013
Keywords:
Magnetite/poly(4-vinylpyridine)
nanospheres
©
2013 Elsevier B.V. All rights reserved.
Epoxidation
Catalysis
Magnetic recovery
Superparamagnetism
1
. Introduction
in inorganic or polymeric matrixes have been developed to immo-
bilize catalyst for application in liquid reaction [5–7].
High dispersion and easy recoverability of heterogeneous
However, there are still some problems such as the low mag-
netic content and high remanence of most magnetic composite
supports, as reported previously. A low magnetic content of com-
posite supports results in a weak response affecting the recovery
efficiency of nanocatalysts, while a high remanence of those leads
to their possible coagulation during the catalytic transforma-
tion. As a result, how to obtain nanocomposite supports with
a strong magnetic response and superparamagnetism as well is
still a challenge. Superparamagnetic inorganic/polymer composite
nanospheres, which are usually composed of abundant magnetic
cores to ensure a strong magnetic response and polymeric shells
to provide favourable functional groups and get rid of magnetic
particle aggregation, may provide an opportunity to solve this
problem. Maintaining their high activity, homogeneous catalysts
anchored on composite nanospheres may be recovered easily
and rapidly upon a moderate magnetic field. Superparamagnetic
composite nanospheres (e.g., Fe O /polystyrene) with narrow size
distribution, magnetic cores homogenously distributed into poly-
mer matrixes and a high magnetite fraction (from 70%, up to 80%,
respectively), have been prepared successfully by miniemulsion
polymerization [8,9], but further surface modification of com-
posite nanospheres under harsh conditions may destroy their
magnetism before firm immobilization of transition-metal com-
plex catalysts [10]. This problem may be avoided by synthesizing
catalyst in liquid-phase reaction are of great importance in
order to achieve high efficiency and effective reuse. Classical
micrometre-size heterogeneous catalysts are recoverable [1], but
have significantly reduced reaction activity or selectivity due to low
dispersion in the liquid-phase systems, the low concentration of
active sites, and difficult access for reactants and products to active
sites deep inside the support [2]. With the development of nano-
technology during the last decade, various nanoscale polymeric and
inorganic supports with a large surface area have been synthesized
to raise homogeneous catalyst loading and catalytic activity [3,4].
Especially, complex catalysts grafted on nanoparticles in liquid-
phase system exhibit excellent activity similar to the equivalents
since the active sites located on the surface of the nanoparticles
are easily accessible to reactants at a molecular level. Unfortu-
nately, conventional separation techniques such as filtration and
centrifugation may become insufficient for support particles less
than 100 nm in diameter. Magnetic nanoparticles incorporated into
supports provide a practical route to overcome this drawback. Vari-
ous composite supports with magnetic nanoparticles encapsulated
3
4
∗ Corresponding author. Tel.: +86 010 62333765; fax: +86 010 62327878.
E-mail address: gewang@mater.ustb.edu.cn (G. Wang).
1
381-1169/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molcata.2013.04.019

W. Guo et al. / Journal of Molecular Catalysis A: Chemical 378 (2013) 344–349
345
superparamagnetic inorganic/functional polymer nanospheres via
the miniemulsion polymerization. It is well known that polyvinyl
pyridine can serve as a catalyst scaffold to immobilize transition
metal complexes such as Re, Mo, Au, Cr by strong electrostatic
interaction [11,12].
Ka radiation. Gas chromatography–mass spectrum was recorded
using Agilent 7890A/5975C. HP-5MS column was used in GC, the
injection temperature was 50 C, and helium gas was used as
◦
carrier gas. Products were determined by GC–MS using internal
standard technique, and nitrobenzene was used as an internal
standard. Particle diameter and size distribution of samples were
measured using a Malvern Zetasizer 3000HS at a fixed scattering
In the present work, superparamagnetic Fe O /P4VP compos-
3
4
ite nanospheres with multiple Fe O4 cores embedded in a P4VP
3
◦
shell were synthesized successfully via miniemulsion polymeriza-
tion. The composite nanospheres possess rich N-based functional
groups without any surface modification and abundant Fe O cores
angle of 90 . Mo content of supported catalysts was measured by
inductively coupled plasma-atomic emission spectrometry (Varian
VISTA-MPX).
3
4
with a strong magnetic response to enhance separation efficiency.
This structure is extremely beneficial to the immobilization and
rapid recovery of homogeneous complex catalysts. The specific
synthetic route is shown in Scheme 1. 4-Vinylpyridine monomer
2
.3. Synthesis of OA modified Fe O nanoparticles
3 4
Fe O nanoparticles were synthesized according to an
3
4
3
was polymerized on the surface of oleic acid (OA) modified Fe O4
improved co-precipitation procedure [10]. FeCl ·6H O (25.3 g) and
3
2
nanoparticles to form Fe O /P4VP composite nanospheres in which
3
4
FeCl ·4H O (8.6 g) were dissolved in deionized (400 mL) water
2
2
the Fe O nanoparticles were homogenously dispersed into the
◦
3
4
under nitrogen gas with vigorous stirring at 80 C. First, 28 mL
of ammonium hydroxide aqueous solution was rapidly added
into the solution. Then OA (8 mL) were added dropwise into
the above suspension within 16 min. The suspension was kept
P4VP backbone. The Fe O /P4VP composite nanospheres were used
3
4
as support to immobilize MoO (acac) . To prepare olefin epoxide,
2
2
a key chemical intermediate, with environment-friendly chemical
methods, epoxidation of cis-cyclooctene with oxidant of aqueous
◦
reacting at 80 C for 1 h. After cooling to room temperature, the
Fe O nanoparticles were successively washed several times with
deionized water and ethanol and then dried in a vacuum for 12 h
at 40 C.
H O in the solvent of ethanol was chosen as a model system [4]
2
2
3
4
to illustrate the use of our recoverable Fe O /P4VP-Mo catalyst.
3
4
To the best of our knowledge, there has been no report about the
preparation of composite nanospheres with multiple Fe O cores
◦
3
4
embedded in a P4VP shell and its application as the catalyst support
in a green epoxidation system.
2
.4. Synthesis of Fe O /P4VP composite nanospheres
3 4
Fe O /P4VP nanospheres were synthesized via a miniemulsion
3
4
2
. Experimental
polymerization method. In a typical procedure, n-hexane-based
magnetite ferrofluid (4.8 g) with as-synthesized magnetite weight
content of 40% was added into aqueous solution (110 g) containing
SDS (0.14 g). The mixture was treated by 300 W ultrasound in an
ice-water bath for 20 min to obtain miniemulsion and then trans-
ferred to a 250 mL four-neck flask with a stirrer, condenser and N2
inlet. The miniemulsion was stirred for 30 min at 300 rpm in nitro-
gen atmosphere, followed by the addition of the mixture of 4-VP
(1.32 mL) and DVB (a certain amount) under stirring for 120 min to
allow the swelling of 4-VP and DVB into the miniemulsion. Then
2.1. Materials
Oleic Acid (90%), 4-vinylpyridine (4-VP, 96%), divinyl-benzene
(
(
DVB, 80%), cis-cyclooctene (96%), styrene (96%), 2-methyl styrene
98%), nitrobenzene (98%), and MoO (acac) (99%) were purchased
2
2
from Alfa Aesar. Ferric chloride hexahydrate (FeCl ·6H O, AR), fer-
3
2
rous chloride tetrahydrate (FeCl ·4H O, AR), ammonium hydroxide
2
2
aqueous solution (28%), n-hexane (AR), sodium dodecyl sulfate
◦
(
(
SDS, AR), potassium persulfate (KPS, AR), ethanol (AR) and H O2
30%) were purchased from Beijing Chemical Reagents Company.
the reaction system was heated to 70 C and 0.4 wt% KPS aqueous
2
solution (10 mL) was added rapidly to initiate the polymerization.
After polymerization had proceeded for 8 h, the formed Fe3O4/P4VP
nanospheres were washed with ethanol six times and dried in a
4
-VP and DVB were distilled under reduced pressure before use.
All other chemicals were used without further purification.
◦
vacuum for 12 h at 40 C.
2.2. Characterization
2
.5. Immobilization of MoO (acac) on Fe O /P4VP nanospheres
2 2 3 4
(
Fe O /P4VP-Mo)
3 4
Scanning electron microscope (SEM) images of samples were
obtained with a ZEISS SUPRA55 instrument operated at an
acceleration voltage of 10 kV. Transmission electron microscopy
MoO (acac)2 (1.0 g) was dissolved in ethanol (340 mL) under
2
mechanical stirring. Fe O /P4VP composite nanoparticles (1.0 g)
(
TEM) images were obtained using a JEOL JEM-100CX II electron
3
4
were then added into this solution and the system was refluxed
microscope operating at 200 kV. High-resolution transmission elec-
tromicroscopy (HRTEM) images were taken with a JEM-2010 at
◦
at 70 C for 72 h. The product (Fe O /P4VP-Mo) was magnetically
3
4
separated, washed with ethanol several times and then dried in a
2
00 kV. Fourier-transformed infrared spectra (FTIR) were recorded
◦
vacuum for 12 h at 40 C.
with a NICOLET 6700 infrared spectrophotometer using KBr pellet
samples. The magnetization curves of samples were measured by a
MPMS-XL superconducting quantum interference device (SQUID)
at room temperature. The magnetite content of a dried sam-
ple was measured by a TGA instrument (Netzsch, model STA
f409) with platinum pans, where about 40 mg of dried sample
was placed in platinum pans and heated from room temperature
2.6. Catalytic epoxidation of cis-cyclooctene
In a typical procedure, the Fe3O4/P4VP-Mo catalyst (0.1 g), H2O2
(22 mmol), cis-cyclooctene (3.5 mmol), nitrobenzene (0.2 mmol)
and ethanol (5 mL) were added into a 50 mL round bottom flask and
◦
◦
◦
to 900 C at a heating rate of 10 C/min under nitrogen atmo-
sphere (flow rate 20 mL/min). X-ray photoelectron spectroscopy
data was gained with PHI Quantera SXM. All spectra including
C 1s, N 1s and Mo 3d were curve fitted and referenced to the
C1s neutral peak at 284.8 eV. Powder X-ray diffraction (XRD)
data were collected with a DSADVANCE diffractometer using Cu
put in a 60 C Water Bath Shaker for 12 h under continuous shaking.
After the reaction the catalyst was quickly recovered with a magnet,
rinsed with ethanol, and dried in a vacuum for 12 h for the next cycle
of catalysis under identical conditions. Products were quantified by
gas chromatography-mass spectrum on Agilent 7890A/5975C, and
nitrobenzene was used as an internal standard.

3
46
W. Guo et al. / Journal of Molecular Catalysis A: Chemical 378 (2013) 344–349
Scheme 1. Schematic illustration of the synthesis of composite Fe3O4/P4VP nanospheres and the immobilization of MoO2(acac)2.
Fe O /P4VP composite nanospheres were prepared via
3
4
miniemulsion polymerization. The nanospheres had unique
core/shell architecture with multiple strong magnetic responsive
cores and a functional polymeric shell. Abundant pyridine func-
tional groups of P4VP may immobilize catalyst and promote
catalytic activity as well due to their Lewis base charac-
teristic for some specific reactions, such as epoxidation of
olefins [4,11,14].
As shown in Fig. 2b, multiple black Fe O4 nanoparticles were
3
homogeneously embedded in the P4VP nanosphere, in which
Fe O nanoparticles corresponding to particles of ca.12 nm (Fig. 2a)
3
4
were consistent with the result calculated according to Scherer’s
equation. The average size of the composite nanospheres with
narrow size distribution was ca.180 nm (Fig. 2b). IR spectrum of
the as-synthesized polymer composite nanospheres (Fig. S2b) also
Fig. 1. Magnetization curves of samples at 300 K: the OA modified Fe3O4 (a),
Fe3O4/P4VP nanospheres synthesized by various DVB concentrations from 0/8 (b),
1
/8 (c), 2/8 (d), 4/8 (e) (molar ratio) based on 4-VP monomer, and Fe3O4/P4VP-Mo
confirmed that OA-modified Fe O4 nanoparticles were completely
catalyst (f). Photos of the insets depict magnetic recycling of Fe3O4/P4VP-Mo cat-
alyst (a: before applying the external magnetic field; b: ca. 5 s after applying the
external magnetic field).
3
wrapped into cross-linked P4VP backbone. Absorption bands at
−
1
−1
−1
−1
1597 cm , 1540 cm , 1414 cm , 580 cm were assigned to the
N, C C vibration modes of pyridine and benzene [15], and Fe
C
O
−
1
of Fe O , corresponding to an absorption peak at 576 cm [16] in
3
4
3
. Results and discussion
FTIR spectra of OA-modified Fe3O4 nanoparticles (Fig. S2a), respec-
tively.
3
.1. Synthesis of Fe O /P4VP composite nanospheres
The magnetization curves of synthesized samples are shown in
Fig. 1. The magnetization curves of composite nanospheres exhibit
no remanence at room temperature. Again, the results indicate
their superparamagnetism, which is crucial for controllable floc-
culation and dispersion in solution in an external magnetic field
[17]. The saturation magnetization value of composite nanosphere
of 54.6 emu/g (Fig. 1c) ensures its rapid recoverability as catalyst
support under an external magnetic field, and is much higher than
that of other reported magnetic composite supports [5,18]. How-
ever, the value is slightly lower than that of OA-modified Fe3O4
nanoparticles (60.8 emu/g) (Fig. 1a) due to the capsulation of mag-
netic components into the P4VP.
3
4
OA modified Fe O nanoparticles were synthesized via an
3
4
improved co-precipitation method reported previously [10]. The
modification of Fe O with OA may enhance the compatibility of
the nanoparticles with organic monomer, which favours the encap-
sulation of polymer by miniemulsion polymerization [8,9]. XRD
spectra (Fig. S1) indicate that the as-synthesized Fe O nanoparti-
cles have a cubic spinel structure. The average grain size calculated
by Scherer’s equation is about 11.5 nm, smaller than the critical size
of Fe O (ca. 30 nm) [13], which confirms the superparamagnetic
3
4
3
4
3
4
nature of the as-synthesized Fe O4 nanoparticles (Fig. 1a).
3
Fig. 2. HRTEM image of the OA modified Fe3O4 nanoparticles (a), TEM image of Fe3O4/P4VP nanospheres (b).

W. Guo et al. / Journal of Molecular Catalysis A: Chemical 378 (2013) 344–349
347
as-synthesized polymer composite nanospheres with molar ratio
of DVB/4-VP at 1/8 were chosen preliminarily as complex catalyst
support.
3.1.2. Effect of the concentration of magnetite ferrofluid on the
structure of composite Fe O /P4VP nanospheres
3
4
The concentration of magnetic fluid has an important impact on
the magnetite content of as-synthesized Fe O /P4VP nanospheres
3
4
for miniemulsion polymerization [9,21]. A higher concentration of
magnetic fluid may produce Fe O /P4VP composite nanospheres
3
4
with higher magnetite content, which will enhance the magnetic
response of the composite nanospheres. As expected, according
to the TGA results (Fig. S4), the magnetite content went up from
6
4.90%, 67.85%, to 72.44%, with an increase in concentration of
magnetic fluid from 20%, 30%, to 40%. However, with a further
increase in the concentration of magnetic fluid to 50%, there was
no corresponding increase in the magnetite content of the as-
synthesized composite nanospheres, which remained at 72.11%
Fig. 3. TGA curves of the OA modified Fe3O4 nanoparticles (a), Fe3O4/P4VP
nanospheres synthesized by various DVB concentrations from 0/8 (b), 1/8 (c), 2/8
(
d), 4/8 (e) (molar ratio) based on 4-VP monomer, and Fe3O4/P4VP-Mo (f).
(
Fig. S4d). With concentrations of magnetic fluid up to 50%, there
To optimize synthesis parameters, the effects of the ratio
of DVB/4-VP, the concentration of magnetite ferrofluid, and the
were too many OA-modified Fe O4 nanoparticles to disperse into
n-hexane in preparation for magnetic fluid due to a minor discrep-
ancy in the solubility parameter between OA(17.38(MPa) ) [22]
and n-hexane(14.9(MPa) ) [22]. Because the magnetic response of
3
amount of the surfactant SDS on the structure of Fe O /P4VP com-
1/2
3
4
posite nanospheres are discussed in detail as follows.
1/2
as-synthesized composite nanospheres correlated positively with
their magnetite content [8], composite nanospheres with stronger
magnetic response are more suited to recovering the magnetic
composite catalyst.
3
.1.1. Effect of the ratio of DVB/4-VP on the structure of
Fe O /P4VP composite nanospheres
3
4
It is well known that a cross-linking agent has a critical influence
on the strength, elasticity and solvent resistance of polymer struc-
ture [19]. The effect of cross linker DVB amount on the structure,
Fe O content and magnetic response of as-synthesized magnetic
3.1.3. Effect of the concentration of SDS on the structure of
composite Fe O /P4VP nanospheres
3
4
3
4
polymer nanospheres was investigated in detail.
In order to optimize preparation parameters, the effects of
SDS concentration on the structure of Fe O /P4VP composite
With the increase of DVB content, the Fe O content and
3
4
3
4
corresponding magnetic response of the composite nanospheres
decreased significantly. Fig. 3b–e reveals the TGA results of
Fe O /P4VP composite nanospheres synthesized by various DVB
nanospheres were further investigated. The SEM images (Fig.
S5a–d) indicate the Fe O /P4VP nanospheres were all synthesized
3
4
in various SDS concentrations, and DLS results show the mean parti-
cle diameter of as-synthesized Fe O /P4VP nanospheres decreased
3
4
contents from 0/8 (b), 1/8 (c), 2/8 (d), 4/8 (e) (molar ratio) based
3
4
◦
◦
on 4-VP monomer. Weight loss between 200 C and 850 C can
be attributed to the decomposition of the OA and polymer struc-
ture. According to the residual weight of the TGA samples, the
Fe O content of as-synthesized composite nanospheres decreases
dramatically from 188.7 nm, 173.3 nm, 142.5 nm, to 124.9 nm as
the SDS concentration was increased. However, the polydisper-
sity index of the as-synthesized Fe O /P4VP nanospheres went up
3
4
from 0.04, 0.03, 0.07, to 0.08 as the SDS content was increased,
which means that the particle size distribution of the composite
nanospheres became wider. The broad size distribution may lead
to the loading of uneven amounts of catalyst on the composite
nanospheres and an uneven magnetic response of the composite
nanospheres. Considering that catalyst support demands a high
specific surface area of composite nanospheres and consistent mag-
netic response demands a narrow size distribution, 2.92 wt% of
SDS content based on magnetic fluid was chosen as the optimal
synthesis parameter.
3
4
from 77.13–72.44% to 62.31–45.66% as the DVB content gradually
increases, consistent with the reduction trend in the saturation
magnetization of those composite nanospheres. Similar to the
magnetic hysteresis loop of OA-modified Fe O nanoparticle, as-
3
4
synthesized composite polymer nanospheres with various DVB
concentrations keep their superparamagnetism (Fig. 1), which
can be attributed to weak interaction between Fe O4 nanoparti-
3
cles evenly distributed into the polymer composite nanosphere
framework. The saturation magnetization of the composite poly-
mer nanospheres decreases from 58.5, 54.6, 45.6, to 33.4 emu/g
(Fig. 1b–e) as DVB content is increased. This is due to the incor-
3.1.4. Synthesis mechanism of Fe O /P4VP composite
3
4
poration of more diamagnetic components into the composite
nanospheres [8].
nanospheres via miniemulsion polymerization
Monodispersed composite nanospheres with a strong magnetic
response were obtained via miniemulsion polymerization. In the
synthesis process, n-hexane-based magnetite ferrofluid dispersed
into aqueous SDS solution was broken up into numerous nanoscale
drops under strong ultrasonication. Magnetite content in the drops
grew with the increase of magnetic fluid concentration, and the
particle size of the resulting composite nanospheres decreased as
TEM images of Fe O /P4VP nanospheres (Fig. S3) indicate that
3
4
OA-modified Fe O nanoparticles are all well distributed in the
3
4
P4VP framework. However, when the molar ratio of DVB/4-VP
increases to 4/8 (Fig. S3d), significant aggregation is observed
among as-synthesized composite nanospheres. High DVB content
causes the degree of crosslinking of polymer chains to increase and
endows the surface of the polymer nanospheres with excessive
double bonds to enhance the probability of conglomeration and
crosslinking between the composite polymer nanospheres [20].
Considering the further decrease in the magnetic response of
composite nanospheres immobilizing catalysts and facile swollen
nature of noncrosslinked polymer in organic solvent leading
to leakage of magnetic cores from the polymer framework,
SDS concentration increases. OA modified on the surface of Fe O4
3
nanoparticle acted as an ultrahydrophobe to suppress Ostwald
ripening to an extent, which makes nanoscale drops stable for a long
time against their fusion.[9] The monomer 4-VP and crosslinker
DVB were able to swell into those above droplets due to the similar-
ity of their solubility parameters (22.5(MPa)1 [23], 17.39(MPa)
/2
1/2
1
/2
[24], respectively) with that of n-hexane(14.9(MPa) ). Then the

3
48
W. Guo et al. / Journal of Molecular Catalysis A: Chemical 378 (2013) 344–349
Table 1
The catalytic activity for epoxidation of cis-cyclooctene using H2O2 in ethanol cat-
a,b
alyzed by Fe3O4/P4VP-Mo catalyst.
Time (h)
3
6
9
12
Scheme 2. Schematic illustration of catalytic epoxidation of cis-cyclooctene using
Fe3O4/P4VP-Mo catalyst.
Conv. (%)c
Sel. (%)c
79.9
99
89.9
99
93.8
99
94.5
99
a
Reactions were conducted using cis-cyclooctene (3.50 mmol), 30 wt% H2O2
◦
nanoscale miniemulsion droplets containing 4-VP and DVB as
major nucleation sites were in situ polymerized to corresponding
(22 mmol), catalyst (0.10 g, 0.098 mmol Mo), and ethanol (5.0 mL) at 60 C.
b
TOFs, described as mol epoxide/mol catalyst/time (h), can be calculated to
−
1
−1
9
.7 h
after 9 h reaction time and 39 h after 10 min reaction time. Reactions
Fe O /P4VP composite nanospheres with the same size and narrow
3
4
were conducted using cis-cyclooctene (3.50 mmol), 30 wt% H2O2 (22 mmol), 1 mol%
size distribution as the droplets. The increase in DVB content makes
more nonmagnetic components swell into miniemulsion droplets,
thus leading to the reduction of magnetite content in the resulting
composite nanospheres.
◦
catalyst, and ethanol (5.0 mL) at 60 C.
c
The conversion and selectivity were determined by GC–MS with respect to an
internal standard (nitrobenzene).
Considering generally the effects of the DVB content, the SDS
concentration and the concentration of magnetic fluid on the struc-
ture of the composite nanospheres, as-synthesized Fe O /P4VP
composite nanospheres, with 1/8 molar ratio of DVB/4-VP, 2.92 wt%
of the SDS concentration and 40% of the concentration of magnetic
fluid, were chosen as catalyst support.
epoxidation [5,18,30]. However, it is rare for magnetic catalyst to
be used for the above field with both H O and ethanol. Fe O /P4VP
2
2
3
4
3
4
nanospheres as complex catalyst-scaffold provide a potentially
extensive application for more liquid-phase chemical reactions
using green solvents.
As shown in Table 1, Fe O /P4VP-Mo catalyst exhibits high
3
4
conversion (94.5% at 12 h), lower than that of its corresponding
homogenous counterpart (95% at 6 h), and excellent selectivity
3
.2. Immobilization of MoO (acac)2 on Fe O /P4VP composite
2 3 4
nanosphere
(
99%) for this green epoxidation system. TOFs of the nanocata-
−
1
lyst has been also determined to 9.7 h after 9 h reaction time
and 39 h after 10 min reaction time with 1 mol% catalyst, respec-
tively. After the reaction is finished, Fe O /P4VP-Mo catalyst can
ICP-AES results also reveal that the Mo content of composite cat-
alyst is 0.975 mmol/g, consistent with the TGA result (Fig. 3) that
the residue weight of Fe O /P4VP-Mo (Fig. 3f, 82.55%) is higher
than that of Fe O /P4VP (Fig. 3c, 72.44%), indicating the deposi-
tion of Mo species on Fe O /P4VP nanosphere support. Compared
with the Fe O /P4VP nanospheres (Fig. S2b), the IR spectrum of
Fe O /P4VP-Mo (Fig. S2c) exhibits the absorptions at 944, 905 cm
of vibration of Mo O of MoO (acac) [25], and the blue shift of C
vibration absorption from 1598 to 1630 cm [15], which confirms
that MoO (acac) was introduced to the Fe O /P4VP composite
−
1
3
4
3
4
be magnetically (ca. 5 s) recovered from the reaction system (inset
in Fig. 1), which exhibits a strong response of this superpara-
magnetic catalyst. The specific saturation magnetization of this
nanocomposite catalyst (48.3 emu/g, Fig. 1f) is far higher than that
of magnetic composite catalyst reported in most previous literature
3
4
3
4
3
4
−
1
3
4
N
2
2
[
5,6,18,30,31].
−
1
Eight recycling tests of Fe O /P4VP-Mo catalyst for epoxidation
3
4
2
2
3
4
of cis-cyclooctene were carried out. As shown in Table 2, 99% of
selectivity of epoxidation retained and conversion of epoxidation
decreased slightly over the eight consecutive runs, which confirms
that Fe O /P4VP–Mo maintains its initial activity well. The content
support.
The XPS spectra of samples (Fig. S6) also provide valuable infor-
mation. Different from a single symmetrical N1s peak at binding
energy 399.9 eV in Fe O /P4VP nanospheres (Fig. S6a), the N 1s
3
4
3
4
of Mo species in the recovered catalyst is 0.928 mmol/g, slightly
lower than that in fresh catalyst (0.975 mmol/g), indicating that
the loss of Mo active sites bound to P4VP is minimal. In view of
similar morphologies (Fig. S5e–f) and IR spectra (Fig. S2c and d) for
spectrum of Fe O /P4VP-Mo catalyst (Fig. S6c) consists of three
3
4
peaks at 398.4 eV, 399.6 eV, and 401.5 eV, corresponding to N atoms
coordinated with Mo species [26], neutral N atoms of pyridine
ring, and protonated N species [26], respectively. The Mo 3d spec-
trum of Fe O /P4VP-Mo catalyst (Fig. S6d) exhibits similar double
peaks with that of MoO (acac) (Fig. S6b). The above results confirm
fresh and recovered catalyst, Fe O /P4VP composite nanospheres
3
4
3
4
further turn out to be outstanding support for MoO (acac) .
2
2
2
2
Fe O /P4VP-Mo catalyst was further applied to the catalytic oxi-
3
4
that Mo species was successfully immobilized on the Fe O /P4VP
3
4
dation of aromatic olefins such as styrene and 2-methylstyrene
under the same reaction conditions (Table 3). The main products
were benzaldehyde and 2-methybenzaldehyde, respectively, with
selectivity of more than 90%. The reason may be that styrene and
nanospheres, as shown in Scheme 1.
.3. Catalytic epoxidation of cis-cyclooctene
3
As illustrated in Scheme 2, to evaluate the performance of
Fe O /P4VP-Mo catalyst, green epoxidation of cis-cyclooctene,
Table 2
3
4
Recycling tests of Fe3O4/P4VP-Mo catalyst in the epoxidation of cis-cyclooctene with
H2O2 in ethanol.
with aqueous H O as green oxidant and ethanol as environ-
2
2
a
mentally benign solvent, was chosen as a probe reaction. Organic
oxidants such as t-butyl hydroperoxide [27] and noxious solvents
such as acetonitrile [28] were often used for epoxidation of olefin,
but the side effects of the reaction on the environment were much
more difficult to eliminate. An alternative way of circumventing
Cycle
Time (h)
Catalytic activityb
Conv. (%)b
Sel. (%)b
1
2
st
nd
12
12
12
12
12
12
12
12
94.5
92.1
91.6
91.3
90.7
90.5
90.5
90.4
99
99
99
99
99
99
99
99
this drawback was to use aqueous H O2 as green oxidant [29] and
3rd
4th
2
ethanol as an environmentally benign solvent. The hydrophilicity
of P4VP in Fe O /P4VP nanospheres makes them easily disperse
5
6
7
th
th
th
3
4
in green polar solvents (e.g. ethanol and water), which renders
MoO (acac) firmly anchored on the surface of the nanospheres
2
2
8th
catalyze substrates efficiently at a molecular level. At the same
time, the strong magnetic response of Fe O /P4VP nanospheres
ensures easy recoverability of heterogeneous catalyst. So far,
some magnetic catalysts have been applied in the field of olefin
a
Reactions were conducted using cis-cyclooctene (3.5 mmol), 30 wt% H2O2
3
4
◦
(
22 mmol), catalyst (0.1 g, 0.098 mmol Mo), and ethanol (5.0 mL) at 60 C.
b
The conversion and selectivity were determined by GC–MS with respect to an
internal standard (nitrobenzene).

W. Guo et al. / Journal of Molecular Catalysis A: Chemical 378 (2013) 344–349
349
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Epoxidation of olefin with H2O2 in ethanol catalyzed by Fe3O4/P4VP-Mo catalyst.
Entry
Time (h)
Alkene
Catalytic activityb
(
3
(
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Conv. (%)
Sel. (%)
1
2
3
12
12
12
Cis-cyclootene
Styrene
2-Methyl-styrene
94.5
83.3
91.7
99
92
95
J. Am. Chem. Soc. 128 (2006) 14685–14690;
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(
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Reactions were conducted using cis-cyclooctene (3.50 mmol), 30 wt% H2O2
Chem. 308 (2009) 25–31;
◦
(
22 mmol), catalyst (0.10 g, 0.098 mmol), and ethanol (5.0 mL) at 60 C.
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The conversion and selectivity were determined by GC–MS with respect to an
internal standard (nitrobenzene).
c
The main product is benzaldehyde.
The main product is 2-methyl benzaldehyde.
d
[
[
[
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(
(
3
(
4
. Conclusions
In summary, we have synthesized novel superparamagnetic
composite nanospheres with multiple Fe O cores embedded
(
3
4
Rev. 111 (2011) 3036–3075.
in P4VP shell by miniemulsion polymerization. The magnetic
response of composite nanospheres was tuned precisely by chang-
ing the DVB content or magnetic fluid concentration, and the
size could be varied by SDS concentration. Both strong magnetic
response and abundant pyridine rings enabled them to serve
as easily recyclable support for the immobilization of catalysts.
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[
Fe O /P4VP composite nanospheres supported MoO (acac) and
3
4
2
2
(
exhibited good catalytic activity for green cis-cyclooctene epox-
idation system and strong magnetic response (48.3 emu/g), and
basically maintained initial catalytic activity after eight runs. While
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complex on Fe O /P4VP nanospheres, the basic design principles
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described here may not be limited to MoO (acac) but should
2
2
6
be expanded to other transition metal systems and noble metal
nanoparticles; that is, carefully tuned superparamagnetic compos-
ite nanospheres could potentially play a crucial role as general
easily recoverable support to anchor a wide variety of transition
metal complex catalysts and noble metal nanoparticles.
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[
[
[
[
[
[
[
Acknowledgements
We are grateful to the planned Science and Technology Project
of Beijing, China (grant no. Z111103066611011), and the co-build
project of Beijing Municipal Commission of Education for financial
support.
[
[
[
[
[
Appendix A. Supplementary data
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3
68 (2009) 139–145;
Supplementary data associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/
j.molcata.2013.04.019.
(
b) A.C. Coelho1, S.S. Balula1, S.M. Bruno1, J.C. Alonso, N. Bion, P. Ferreira, M.
Pillinger, A.A. Valente, J. Rocha, I.S. Gon c¸ alves, Adv. Synth. Catal. 352 (2010)
1759–1769.
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