Facile fabrication of magnetic MoO2–Salen-modified graphene-based catalyst for epoxidation of alkenes

Article abstract of DOI:10.1002/aoc.3742

A facile, green and efficient method for the immobilization of MoO₂–Salen onto graphene hybridized with glucose-coated magnetic Fe₃O₄ nanoparticles is proposed to fabricate a magnetic organic–inorganic hybrid heterogeneous RGO/Fe₃O₄@C-Salen-MoO₂ catalyst for the epoxidation of cyclooctene and geraniol using tert-butyl hydroperoxide or H₂O₂ as oxidant. Carbon-coated Fe₃O₄ can improve the stability and add functional ─OH groups on the surface of Fe₃O₄. The fabricated composite exhibited good performance due to good dispersion of MoO₂–Salen active sites. The catalyst can be easily separated from the reaction system using a permanent magnet and used three times without significantly losing its catalytic activity and selectivity.

Full text of DOI:10.1002/aoc.3742

Received: 30 October 2016
DOI 10.1002/aoc.3742
Revised: 16 December 2016
Accepted: 23 December 2016
F U L L PA P E R
Facile fabrication of magnetic MoO –Salen‐modified
2
graphene‐based catalyst for epoxidation of alkenes
1,2
1
3
3
2
2
Zhifang Li | Changlong Yang | Yuanyuan Ma | Peng Li | Jingqi Guan
| Qiubin Kan
1
College of Materials Science and
A facile, green and efficient method for the immobilization of MoO –Salen
Engineering, Qiqihar University, Qiqihar
2
1
61006, China
onto graphene hybridized with glucose‐coated magnetic Fe O nanoparticles is
3
4
2
College of Chemistry, Jilin University,
proposed to fabricate a magnetic organic–inorganic hybrid heterogeneous
RGO/Fe O @C‐Salen‐MoO catalyst for the epoxidation of cyclooctene and
Changchun 130023, China
3
4
2
3
Key Laboratory of Fine Chemicals of
geraniol using tert‐butyl hydroperoxide or H O as oxidant. Carbon‐coated Fe O
College of Heilongjiang Province, Qiqihar
University, Qiqihar 161006, China
2
2
3 4
can improve the stability and add functional ─OH groups on the surface of Fe O .
3
4
The fabricated composite exhibited good performance due to good dispersion of
Correspondence
Jingqi Guan and Qiubin Kan, College of
Chemistry, Jilin University, Changchun
MoO –Salen active sites. The catalyst can be easily separated from the reaction
2
system using a permanent magnet and used three times without significantly losing
1
30023, China.
its catalytic activity and selectivity.
Email: guanjq@jlu.edu.cn; qkan@mail.jlu.
edu.cn
KEYWORDS
Funding information
core–shell catalyst, graphene, magnetism, cyclooctene
National Natural Science Foundation of
China, Grant/Award Number: 21303069;
Jilin Province, Grant/Award Number:
2
01105006; Science Research Project of
Heilongjiang Province Education
Department, China, Grant/Award Number:
1
2541861; Science Research Project of
Qiqihar University, Grant/Award Number:
35109303, 135109103; Civic Scientific and
1
Technological Project of Qiqihar, China,
Grant/Award Number: GYGG‐201405
1
| INTRODUCTION
carbonaceous layer or silica not only protects the magnetic
Fe O corefromleachinginanacidicenvironment,butalsoadds
3
4
[11]
There has been significant interest in more widespread use of
magnetic composites with a core–shell structure in the past
few decades due to their unique properties (easy prepara-
oxygen‐containing functional groups. A carbon layer shows
higher stability in alkaline media, high temperatures, etc., when
compared witha silicashell. Consequently, coatingofacarbona-
ceousshellontothe surface ofFe O has gainedparticular atten-
[
1–6]
[5–7]
tion),
especially in the catalytic reactions.
Among
3
4
these magnetic composites, Fe O composites are worth
tion. Coating of a carbonaceous shell is generally performed
3
4
exploring as ideal and widely used supports in catalysis due
to the properties of Fe O (low cost, low toxicity and ease of
from low‐cost glucose under hydrothermal conditions.
2
[12]
Graphene, two‐dimensional sp ‐hybridized carbon, has
3
4
[
8,9]
preparation).
And yet Fe O nanoparticles have few func-
attracted tremendous attention because of its fascinating chem-
3
4
[
13]
tional groups on their surface and are unstable in strong acidic
ical and physical properties and high surface area.
Of the
[
10]
solution. To overcome these problems, introducing a shell
of carbon or silica to the surface of core Fe O nanoparticles
family of graphene derivatives, graphene oxide (GO) is of par-
ticular interest.GOcanserveasanidealsubstratefor theimmo-
bilization of inorganic nanoparticles and allow them to
3
4
has been the subject of increasing attention, because the
Appl Organometal Chem. 2017;e3742.
wileyonlinelibrary.com/journal/aoc
Copyright © 2017 John Wiley & Sons, Ltd.
1 of 7
https://doi.org/10.1002/aoc.3742

2
of 7
LI ET AL.
participateinawiderangeofbondinginteractionsduetoalarge
number of oxygen‐containing functional groups and aromatic
2
[14]
sp domains. Chen and co‐workers reported the fabrication
of a catalyst of superparamagnetic Fe O nanoparticles on
3
4
[
15]
GO that was applied for controlled targeted drug delivery.
He and Gao deposited Fe O nanoparticles onto GO using a
3
4
[
16]
facile approach.
Li et al. decorated graphene sheets with
size‐ and density‐tunable Fe O nanoparticles for porcine pan-
3
4
[
17]
creatic lipase immobilization.
A reduced graphene oxide
(
RGO)/Fe O /Ag ternary composite was generated by a new
3 4
hybrid substrate of Fe O with RGO for the dispersion of noble
3
4
metal nanoparticles and exhibited excellent catalytic stability and
much higher catalytic activity than the corresponding RGO/Ag
[18]
for the reduction of 4‐nitrophenol to 4‐aminophenol. Further-
more, nanoparticle‐modified graphene can be used as an effec-
tive hybrid substrate for the synthesis of graphene‐based
composites. Fan et al. reported that the hybridization of graphene
sheets and carbon‐coated Fe O nanoparticles resulted in a cata-
3
4
SCHEME 1 Outline of covalent attachment of MoO
complex onto magnetic RGO/Fe @C support
2
–Salen
lyst showing great adsorption properties not only in water but
3 4
O
[9]
also in acidic environments. But beyond that, the dye removal
efficiency was 86 and 77% after five cycles in water and 1 M
HCl, respectively. The immobilization of Cu(II), Co(II), Fe(III)
or VO(IV) Salen complexes onto carbon‐coated Fe O nanopar-
ether, MgSO , NaHCO , ethylenediamine, styrene (98%),
4
3
isobutylaldehyde, FeCl ⋅6H O, CH COONa⋅3H O, glucose,
3
2
3
2
3
4
3
‐aminopropyltriethoxysilane (Aldrich), Fe(NO ) ⋅9H O,
3
3
2
ticles hybridized with graphene (RGO/Fe O @C) achieved
3
4
V O , acetylacetone, Co(NO ) ⋅6H O, Cu(CH COO) ,
2
5
3 2
2
3
2
core@shell structured magnetic nanocatalysts that exhibited
[19]
MoO , dimethylformamide and all organic solvents were of
3
high activity for the epoxidation of styrene.
Homogeneous molybdenum complexes demonstrated
AR grade. Dried toluene was obtained by treating with
Na/diphenylketone ketyl and distilling under nitrogen atmo-
sphere. Powder X‐ray diffraction (XRD) patterns were
recorded with a Shimadzu XRD‐6000 diffractometer using
[20]
excellent performance in oxidation of alkenes. And yet, the
difficulty of separating homogeneous catalysts from reaction
media restricts their applications. Heterogenization of homoge-
Ni‐filtered Cu K radiation scanning from 10° to 70° (2θ).
α
neous MoO –Salen complex with a magnetic composite sup-
2
Metal content was determined using inductively coupled
plasma (ICP) atomic emission spectroscopy conducted with
a PerkinElmer emission spectrometer. X‐ray photoelectron
spectroscopy (XPS) was performed using an ESCALAB
port to fabricate insoluble heterogeneous catalytic epoxidation
systems is an efficient strategy for retaining high catalytic activ-
ity on recycling, enhancing stability and enabling easy separa-
tion for reuse with an external magnet. We found that the
2
50 X‐ray electron spectrometer. Raman spectra were
immobilization of MoO –Salen onto SBA‐15 and GO catalysts
2
[
21,22]
obtained using a Renishaw Raman system model 1000 spec-
trometer with an excitation wavelength of 514 nm. Magnetic
hysteresis curves were obtained using a vibrating sample
magnetometer.
led to systems highly active in the epoxidation of alkenes.
In the work presented here, RGO/Fe O @C was utilized
3
4
as a support to prepare a magnetic tethered MoO –Salen com-
2
plex catalyst. The procedure for the covalent attachment of a
Salen complex of molybdenum(VI) dioxide to the magnetic
RGO/Fe O @C support to form RGO/Fe O @C‐Salen‐
3
4
3 4
2.2 | Synthetic procedures
MoO is depicted in Scheme 1. Catalytic performance was
2
evaluated in terms of the conversion of styrene, cyclooctene
and geraniol via their epoxidation with tert‐butyl hydroperox-
ide (TBHP) or H O to the corresponding epoxides.
The procedure for the covalent immobilization of MoO
2
–
Salen on the RGO/Fe @C magnetic support using a step-
O
3 4
wise procedure is presented in Scheme 1 and described
below.
2
2
2
2
| EXPERIMENTAL
2
.2.1 | Preparation of RGO/Fe O @C
3 4
.1 | Materials and instrumentation
GO was prepared according to a modified Hummers method.
RGO/Fe O was prepared as described in the following.
Graphite powder, H SO , KMnO , 30% H O , 5% HCl,
2
4
4
2
2
3 4
salicylaldehyde (99%), ZnCl , diethyl ether, petroleum
Typically, GO was dispersed in a mixture of diethylene
2

LI ET AL.
3 of 7
glycol (DEG) and ethylene glycol (EG) in an ultrasonic bath
for 2 h to produce solution A for later use. In the meantime,
FeCl ⋅6H O and CH COONa⋅3H O were dissolved in a
3 | RESULTS AND DISCUSSION
3
2
3
2
3.1 | Structure and morphology of supported
catalysts
mixture of DEG and EG to produce solution B. After that,
solution B was added into solution A, and then the mixture
was kept sonicated for 30 min and transferred into an
autoclave, heated at 220 °C for 10 h and cooled to room
temperature. The solid product of RGO/Fe O was obtained
The structures of the as‐prepared samples were determined
using XRD (Figure 1). A typical narrow peak at 11.2° is
observed intheXRD pattern ofGO (Figure 1 inset) clearlysug-
gesting that oxygenated functional groups had been introduced
3
4
by magnetic separation, washing with deionized water and
ethanol, and drying in a vacuum oven at 40 °C overnight.
As a control experiment, Fe O was synthesized under the
[
9]
into the carbon sheets. As shown in Figure 1(a), all diffrac-
tion peaks of Fe O can be assigned to face‐centered cubic lat-
3
4
3
4
[
24]
tice. All peaks of the prepared magnetic composites
Fe O @C and RGO/Fe O @C can be attributed to Fe O
3 4
same conditions without adding GO.
3
4
3
4
RGO/Fe O @C was prepared using a facile hydrother-
3
4
and amorphous carbon shell, except a large and obvious dif-
fraction peak appearing at 24.2° (indicating the reduction of
mal method. In brief, a mixture of RGO/Fe O and glucose
3
4
solution was transferred into a Teflon‐lined autoclave and
heated at 180 °C for 6 h. The resulting product of RGO/
Fe O @C was obtained by magnetic separation, washing
[
16]
[25]
GO inthe reaction process andthe presence ofgraphene
)
in the pattern of RGO/Fe O @C. The XRD patterns of the pre-
3
4
3
4
pared MoO –Salen‐supporting Fe O @C and RGO/
2
3 4
with deionized water and ethanol, and drying in a vacuum
oven at 40 °C overnight. Fe O @C was synthesized under
Fe O @C catalysts are analogous with the XRD patterns of
3
4
3
4
Fe O @C and RGO/Fe O @C, revealing that the structures
3
4
3 4
the same conditions.
of the supports are retained during the process of reaction.
Scanning electron microscopy (SEM) mapping and
transmission electron microscopy (TEM) images of RGO/
Fe O @C‐Salen‐MoO are shown in Figure S1 and Figure 2
2
.2.2 | Preparation of RGO/Fe O @C‐Salen‐
3
4
3
4
2
MoO
2
,
respectively. The results of the SEM mapping demonstrate
RGO/Fe O @C‐Salen was prepared successfully in our
3
4
that Mo element distributes uniformly on the support. In other
words, the homogeneous complex of molybdenum is evenly
distributed in the synthesized catalyst. Figure 2 exhibits
Fe O nanoparticles coated by carbonaceous shell.
[
19]
previous work.
Typically, a mixture of 1.0 g of RGO/
Fe O @C‐NH and 1.15 mmol of CM‐SalenH in pre‐dried
3
4
2
2
toluene was refluxed under nitrogen atmosphere for 24 h,
magnetically separated, washed with toluene, Soxhlet‐
extracted with CH Cl and dried.
3
4
2
2
MoO Cl (dmf) waspreparedandcharacterizedinour pre-
3.2 | Spectroscopic characterization
2
2
23]
2
[
vious work. RGO/Fe O @C‐Salen‐MoO was prepared as
3
4
2
Raman spectra of GO and RGO/Fe O @C‐Salen‐MoO were
3
4
2
follows. A mixture of RGO/Fe O @C‐Salen and a certain
3
4
obtained to investigate the chemical bonding and structural
amount of MoO Cl (dmf) in CHCl was stirred at room
2
2
2
3
temperature for 24 h. The resulting product was obtained by
magnetic separation, and was washed with toluene, Soxhlet‐
extracted with CH Cl and dried.
2
2
2
.3 | Catalytic tests
Epoxidation over the supported MoO –Salen catalyst was
2
carried out at atmospheric pressure by contacting 10 mg of
catalyst with 1 mmol of substrate (styrene, cyclooctene,
geraniol), TBHP or H O and 1 ml of solvent (CH CN or
2
2
3
CHCl ) in a magnetically stirred glass reactor, under reflux
3
(bath temperature) for a period of 8 h. At the end of the reac-
tion, the catalyst was separated from the reaction mixture by
magnetic separation. The reaction products were analyzed
with a GC‐8A with flame ionization detector using nitrogen
as a carrier gas. The used catalyst was washed with large
quantities of solvent and dried. After that, it was reused for
further epoxidation to test the catalyst reusability.
FIGURE 1 XRD patterns of GO (inset), (a) Fe
3 4 3 4
O , (b) Fe O @C,
(c) Fe O @C‐Salen‐MoO , (d) RGO/Fe O @C, (e) RGO/Fe O @C‐
3
4
2
3
4
3 4
Salen‐MoO
2

4
of 7
LI ET AL.
FIGURE 2 TEM image of RGO/Fe
3
O
4
@C‐Salen‐MoO
2
[17]
changes of the carbon framework.
As shown in Figure 3,
the D band and G band of GO and RGO/Fe O @C‐Salen‐
3
4
−
1
MoO can be seen at 1327 and 1583 cm due to the K‐point
2
3
phonons of the A ‐symmetry mode of sp carbon atoms and
1
g
2
the first‐order scattering of the E mode of sp carbon atoms,
2
g
[
17,26,27]
respectively.
3
.3 | X‐ray photoelectron spectroscopy
The XPS spectrum of RGO/Fe O @C‐Salen‐MoO is
3
4
2
displayed in Figure 4. Apparently, Mo, N, Si, C, O and Fe
signals can be observed in the XPS survey spectrum of
RGO/Fe O @C‐Salen‐MoO (Figure 4a), suggesting that
3
4
2
MoO –Salen has been successfully incorporated into the
2
FIGURE 3 Raman spectra of (a) GO and (b) RGO/Fe
MoO
3
O
4
@C‐Salen‐
magnetic RGO/Fe O @C support. It can also be noted that
3
4
2
the intensities of the characteristic Fe 2p peaks are very weak
and peaks of chlorine are still present, suggesting that
glucose was successfully coated onto RGO/Fe O and the
to 94.3% with decreasing ncyclooctene:nTBHP ratio from 1 to
1:2. However, the activity can be basically considered to
remain invariable when decreasing the ncyclooctene:nTBHP ratio
from 1:2 to 1:3 under the same operating conditions. There-
fore, it can be concluded that an ncyclooctene:nTBHP ratio of
1:2 is appropriate in the epoxidation of cyclooctene over
the RGO/Fe O @C‐Salen‐MoO catalyst.
3
4
chloromethyl did not completely react with amino groups. In
Figure 4(b), the binding energies of Mo 3d5/2 and Mo 3d3/2
peaks are located at about 232.9 and 236.1 eV in the catalyst,
which can be assigned to the presence of molybdenum in the
hybrid material.
3
4
2
In addition to the cyclooctene‐to‐TBHP ratio (Table 1),
the type of oxidant also affects the catalytic properties.
Table 2 shows the influence of oxidant on the catalytic activ-
ity of the RGO/Fe O @C‐Salen‐MoO catalyst. As evident
3
.4 | Catalytic ests
The performance of the RGO/Fe O @C‐Salen‐MoO
3
4
2
catalyst for the epoxidation of cyclooctene at a reaction
temperature of 70 °C operating with the accompanying
changes of ncyclooctene:nTBHP ratio is presented in Table 1. It
is evident from Table 1 that the RGO/Fe O @C‐Salen‐
3
4
2
from Table 2, the RGO/Fe O @C‐Salen‐MoO catalyst
3
4
2
shows activity with 94.3% conversion of cyclooctene using
−1
TBHP as an oxidant after 8 h (TOF = 65.1 h ), while a
3
4
−
1
MoO catalyst is active. The conversion increases from 50.7
lower activity with 22.2% conversion (TOF = 15.3 h ) is
2

LI ET AL.
5 of 7
styrene is 94.3 and 26.9%, respectively, which may be
because of the electron density.
The catalytic properties of the Fe O @C‐Salen‐MoO ,
3
4
2
RGO/Fe O @C and RGO/Fe O @C‐Salen catalysts were
3
4
3 4
also investigated under identical reaction conditions. It is evi-
dent from Table 2 that the fresh RGO/Fe O @C‐Salen‐
3
4
MoO catalyst exhibits higher activity than the homogeneous
2
VI [28]
catalyst MoO2 N,
Fe O @C‐Salen‐MoO , RGO/
3
4
2
Fe O @C and RGO/Fe O @C‐Salen. The enhanced activity
3
4
3 4
of the RGO/Fe O @C‐Salen‐MoO catalyst may also be due
3
4
2
to the isolation of the active species, the synergistic beneficial
catalytic interaction between MoO –Salen and the
2
RGO/Fe O @C magnetic support, and good dispersion
3
4
[
9,29]
of magnetic Fe O4 nanoparticles.
Meanwhile, the
3
graphene sheets prevent the agglomeration of Fe O .
3
4
In order to further verify whether the synthesized catalyst
is truly heterogeneous or not, a catalyst recycling experiment
was performed with the RGO/Fe O @C‐Salen‐MoO cata-
3
4
2
lyst. As evident from Table 2, the reuse of RGO/Fe O @C‐
3
4
Salen‐MoO (three times) does not significantly decrease
2
the conversion of cyclooctene and the yield of epoxide. ICP
analyses of the used catalysts (not shown) reveal no or little
leaching of molybdenum. This result confirms the heteroge-
neity of the majority of RGO/Fe O @C‐Salen‐MoO . TEM
3
4
2
was performed to investigate the RGO/Fe O @C‐Salen‐
3
4
MoO catalyst after catalytic reaction. TEM images of the
2
fresh RGO/Fe O @C‐Salen‐MoO catalyst and the RGO/
3
4
2
Fe O @C‐Salen‐MoO catalyst recovered after three cycles
3
4
2
are shown in Figure S2. Compared with the fresh catalyst,
the reused catalyst displays some agglomeration, which
explains the decrease of conversion as described above.
Furthermore, since Fe O nanoparticles are present in
FIGURE 4 XPS spectra of RGO/Fe
spectrum; (b) Mo 3d spectrum
3
O
4 2
@C‐Salen‐MoO : (a) survey
3
4
RGO/Fe O @C‐Salen‐MoO , it should have magnetic or
3
4
2
ferromagnetic property, which is beneficial for the separation
of the synthesized catalyst. Magnetic properties of the
composites catalyst were investigated using vibrating sample
magnetometry. As shown in Figure 5, the magnetic hysteresis
loop of the sample is an S‐like curve, illustrating that there is
almost no remaining magnetization when the external
magnetic field is removed, which indicates that the RGO/
Fe O @C‐Salen‐MoO catalyst exhibits superparamagnetic
obtained when using H O as the oxidant. Similar results are
obtained in the epoxidation of styrene and geraniol. From the
2
2
above discussion, it is evident that the RGO/Fe O @C‐
3
4
Salen‐MoO catalyst behaves well with TBHP as the oxidant
2
in CHCl compared with H O as the oxidant. This may be
3
2 2
due to H O decomposing into molecular oxygen and water.
2
2
Moreover, the conversion of various substrates
cyclooctene, geraniol and styrene) was also examined. From
3
4
2
(
behavior. The saturation magnetization of RGO/Fe O @C‐
3
4
−1
Table 2 it is evident that the conversion of geraniol is 97.6%
when using TBHP, while the conversion of cyclooctene and
Salen‐MoO (3.5 emu g ) shows a decrease compared with
2
Fe O , because of a decrease in the amount of magnetic
3
4
TABLE 1 Epoxidation of cyclooctene catalyzed by RGO/Fe
3
O
4 2
@C‐Salen‐MoO catalyst
a
−1 b
Entry
n
cyclooctene:nTBHP
Conversion (%)
Epoxide yield (%)
TOF (h )
1
2
3
1:1
1:2
1:3
50.7
94.3
94.6
50.7
94.3
94.6
35.0
65.1
65.3
a
Reaction conditions: catalyst (10 mg), cyclooctene (1 mmol), CHCl
3
(1 ml), TBHP, 8 h and 70 °C.
b
TOF (turnover frequency): moles of substrate converted per mole of metal ion per hour.

6
of 7
LI ET AL.
TABLE 2 Epoxidation catalyzed by various catalysts
a
−1 b
Catalyst
Oxidant
Solvent
Cycle
Substrate
Conversion (%)
Epoxide yield (%)
TOF (h )
RGO/Fe
RGO/Fe
RGO/Fe
RGO/Fe
RGO/Fe
RGO/Fe
RGO/Fe
RGO/Fe
3
O
O
O
O
O
O
O
O
4
4
4
4
4
4
4
4
@C‐Salen‐MoO
@C‐Salen‐MoO
@C‐Salen‐MoO
@C‐Salen‐MoO
@C‐Salen‐MoO
@C‐Salen‐MoO
@C‐Salen‐MoO
@C‐Salen‐MoO
2
2
2
2
2
2
2
2
TBHP
TBHP
TBHP
CHCl
CHCl
CHCl
3
3
3
1
2
3
1
1
1
1
1
1
1
1
1
1
1
Cyclooctene
Cyclooctene
Cyclooctene
Cyclooctene
Geraniol
94.3
86.0
80.1
22.2
97.6
51.2
26.9
11.2
86.0
19.7
94.2
22.3
6.5
94.3
86.0
80.1
22.2
87.2
51.2
26.9
—
65.1
—
3
3
3
3
3
3
3
58.9
15.3
67.4
35.4
18.6
7.7
H
2
O
2
CH
CHCl
CH CN
CHCl
CH CN
CHCl
CH CN
3
CN
TBHP
3
H
2
O
2
3
Geraniol
TBHP
3
Styrene
H
2
O
2
3
Styrene
Fe
3
Fe
3
Fe
3
Fe
3
O
O
O
O
4
@C‐Salen‐MoO
4
@C‐Salen‐MoO
4
@C‐Salen‐MoO
4
@C‐Salen‐MoO
2
2
2
2
TBHP
3
Cyclooctene
Cyclooctene
Geraniol
86.0
19.7
62.9
—
66.0
15.1
72.2
17.1
—
H
2
O
2
3
TBHP
TBHP
TBHP
TBHP
CHCl
CHCl
CHCl
CHCl
3
3
3
3
Styrene
RGO/Fe
RGO/Fe
3
O
O
4
@C
Cyclooctene
Cyclooctene
6.5
3
4
@C‐Salen
1.7
1.7
—
a
Reaction conditions: catalyst (10 mg), substrate (1 mmol), solvent (1 ml), oxidant (2 mmol), 8 h and 70 °C.
b
TOF (turnover frequency): moles of substrate converted per mole of metal ion per hour.
prevent Fe O nanoparticles from aggregating and carbon‐
3
4
coated Fe O4 nanoparticles can improve chemical and
3
mechanical stability, add functional groups and enhance the
specific surface area. Furthermore, the as‐prepared catalyst
showed high performance in the epoxidation of cyclooctene
and geraniol and could be easily recycled by simple magnetic
separation while maintaining catalytic activity and selectivity
without significant decrease even after three reaction cycles.
ACKNOWLEDGMENTS
This work was supported by the National Natural
Science Foundation of China (21303069), Jilin Province
(201105006), the Science Research Project of Heilongjiang
Province Education Department, China (12541861), the
Science Research Project of Qiqihar University
FIGURE 5 Magnetization curve of RGO/Fe
and optical photo (inset) of the catalyst in solution separated using
a magnet
3
O
4
@C‐Salen‐MoO
2
(135109303, 135109103) and Civic Scientific and Techno-
logical Project of Qiqihar, China (GYGG‐201405).
Fe O component in the composite. Separation using an
external magnetic field (inset of Figure 5) verifies that the
3
4
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[
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