A one-step method to produce graphene-Fe3O4 composites and their excellent catalytic activities for three-component coupling of aldehyde, alkyne and amine

Article abstract of DOI:10.1039/c2ta00485b

The construction of reduced graphene oxide or graphene oxide (GO) with magnetic nanoparticles has gained more and more attention due to its promising and wide applications in catalysis, photoelectric materials, biomedical fields and so on. The synthesis o

Full text of DOI:10.1039/c2ta00485b

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A one-step method to produce graphene–Fe3O4
composites and their excellent catalytic activities for
three-component coupling of aldehyde, alkyne and
amine†
Cite this: J. Mater. Chem. A, 2013, 1,
651
a
a
Xing Huo, Jian Liu, Baodui Wang, a Haoli Zhang,b Zhengyin Yang,a Xuegong Sheb
*
and Pinxian Xia
The construction of reduced graphene oxide or graphene oxide (GO) with magnetic nanoparticles has
gained more and more attention due to its promising and wide applications in catalysis, photoelectric
materials, biomedical fields and so on. The synthesis of reduced graphene oxide (RGO) or graphene
magnetic nanoparticle nanocomposites with well-dispersed decorated particles is still a challenge.
Herein, we first report a simple method to prepare graphene–Fe3O4 with uniform Fe3O4 NPs based on
decomposition of Fe(CO)5 on the surface of graphene oxide. The main novelty of this work is that the
decomposition products of Fe(CO)5 reacted with GO leading to the formation of graphene–Fe3O4. The
resulting sample can be easily manipulated by an external magnetic field and exhibits excellent catalytic
activity in the A3-coupling reaction. A diverse range of propargylamines were obtained in a moderate to
high yield under mild conditions. The separation and reuse of graphene–Fe3O4 were very simple,
effective and economical.
Received 28th September 2012
Accepted 18th October 2012
DOI: 10.1039/c2ta00485b
www.rsc.org/MaterialsA
two-dimensional material with large surface area,5 has shown
various exceptional electronic,6,7 mechanical8 and thermal
1 Introduction
properties.9 It has recently been recognized as a suitable
support material for metal and metal oxide nanoparticles.10–12
Especially, GO–Fe3O4 and graphene–Fe3O4 nanocomposites
have been proven to possess new and/or enhanced functionality
that cannot be realized by either component alone, leading to
promising and wide applications in catalysis, photoelectric
materials, biomedical elds and so on.13
At present, a variety of methods, such as solvothermal reac-
tion,14 the reduction of GO and Fe3+ in a NaBH4 solution,15 have
been applied to prepare graphene–Fe3O4. Generally, the above
preparation protocol is relatively complex, and the size and
coverage density of NPs on graphene were hardly controlled.
However, some research group improved the above synthesis
method and produced graphene–Fe3O4 with uniform Fe3O4
crystal NPs.16
However, to the best of our knowledge, the preparation of
graphene–Fe3O4 with uniform and larger sized Fe3O4 NPs only
using Fe(CO)5 and GO as starting materials via one step reaction
has not been exploited until now. Similarly, there are no any
studies that report on the catalytic activity of these nano-
composites in the aldehyde–alkyne–amine coupling (A3-
coupling).
In this study, we rst report a simple method to prepare
graphene–Fe3O4 by a one-step thermal decomposition reaction
of Fe(CO)5 in a GO dispersion system. Graphene oxide was
Nowadays, constructing nanocatalysts with well-dened struc-
tures, environmentally benign, high catalytic activity, good
magnetic separation performance and high chemical stability is
highly desirable to address industrial and environmental
concerns.1 Superparamagnetic nanoparticles, having remark-
able magnetic properties, have gained intensive research
interest in the above eld.2 However, these superparamagnetic
nanoparticles (diameter ꢀ 10 nm) without surface modication
have a great tendency to deform and aggregate during the
course of chemical reactions. This limitation hampers the
general application of these superparamagnetic nanoparticles
in advanced applications. To deal with this issue, one way is by
surface modication or immobilization of these magnetic
nanoparticles using appropriate capping agents such as poly-
mers to prevent aggregation.3 However, these organic capping
agents have a great tendency to decompose at high temperature
and consequently metal nanoparticles can deform and aggre-
gate easily. Very recently, graphene,4 an emerging fascinating
aKey Laboratory of Nonferrous Metal Chemistry and Resources Utilization of Gansu
Province, Lanzhou University, Lanzhou, 730000 P.R. China. E-mail: wangbd@lzu.
edu.cn
bState Key Laboratory of Applied Organic Chemistry, Lanzhou University, Lanzhou,
730000 P.R. China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c2ta00485b
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patterns of the particle were recorded on a Bruker AXS D8-
˚
Advanced diffractometer with Cu Ka radiation (l ¼ 1.5418 A). X-
ray photoelectron spectroscopy (XPS) measurements were
carried out on a PHI-5702 multifunctional spectrometer using
Al Ka radiation. Raman spectra were obtained using a confocal
microprobe Raman system (Renishaw, RM2000). Magnetic
properties were studied using a Lakeshore 7404 high sensitivity
vibrating sample magnetometer (VSM) with elds up to
1
1.5 Tesla at room temperature. H and 13C NMR spectra were
recorded on a Bruker AM-400 MHz instrument and spectral data
were reported in ppm relative to tetramethylsilane (TMS) as an
internal standard. Chemical shis are reported as d values
1
relative to CDCl3 (d ¼ 0 for H NMR and 77.0 for 13C NMR). IR
spectra were recorded on a Nicolet FT-170SX spectrometer.
HRMS data were obtained on a Bruker Daltonics APEXII 47e FT-
ICR spectrometer.
2.3. Synthesis of graphene–Fe3O4
50 mg of GO was dispersed in a solution of oleylamine (OAM)
(4 mL) in benzyl ether (16 mL) by sonication to obtain a light
brown dispersion and the solution was heated at 120 ꢁC under
N2 protection and kept at this temperature for 1 hour. The
solution was further heated to 220 ꢁC when 0.05 mL of Fe(CO)5
was injected. Aer 10 minutes, the mixture solution was heated
to reux for 1 hour. Aerwards, it was cooled down to room
temperature and exposed to air for oxidation. The particles were
precipitated by adding ethanol to the solution and collected by
centrifugation. The product was washed twice with a 1 : 1
mixture of ethanol and hexanes. The nal product was re-
dispersed in CHCl3.
Fig. 1 Illustration of the synthesis procedure of graphene–Fe3O4, TEM images of
GO (A) and graphene–Fe3O4 (inset: selected area electron diffraction (SAED)
pattern) (B), particle size distribution of Fe3O4 on a graphene sheet (C), and room-
temperature magnetization hysteresis loops of graphene–Fe3O4 (inset: the
behaviour of the composites under an external magnetic field) (D).
reduced to graphene by an endogenous reducing agent (CO)
from thermal decomposition of Fe(CO)5 and the Fe3O4 nano-
particles (NPs) were grown on the carbon basal planes simul-
taneously (as shown in Fig. 1). The products reveal good
distribution of Fe3O4 and a considerable saturation magneti-
zation. Such a novel composite with high specic surface area
and strong magnetic sensitivity exhibits excellent catalytic
activity in the A3-coupling reaction.
2.4. Typical procedure for the A3-coupling reaction
In an argon atmosphere, a sealable reaction tube with a Teon-
coated screw cap equipped with a magnetic stir bar was charged
with aldehyde (1.0 mmol), amine (1.2 mmol), alkyne
(1.5 mmol), catalyst (0.05 mmol) and CH3CN (2.0 mL). The
reaction vessel was placed in an oil bath at 80 ꢁC and the
mixture was stirred for 24 h, then it was cooled to room
temperature, the solvent was ltered and concentrated under
reduced pressure, and the residue was puried by ash chro-
matography on silica gel (eluant: petroleum/ethyl acetate) to
give the corresponding A3-coupling product.
2 Experimental
2.1 Materials
Fe(CO)5, graphite, benzyl ether and oleylamine were purchased
from Sigma-Aldrich. All chemicals were used without further
purication. For the A3-coupling reactions, all reactions were
carried out under argon atmosphere. Solvent THF was reuxed
with Na and CH3CN was reuxed and freshly distilled prior to
use. All reactions under standard conditions were monitored by
thin-layer chromatography (TLC) on gel F254 plates. The silica
gel (200–300 mesh) was used for column chromatography and
3 Results and discussion
3.1 Characterization of graphene–Fe3O4
GO was prepared via oxidizing graphite by using a modied
Hummer’s method,17,18 and then it was dispersed in a solution
of oleylamine (OAM) in benzyl ether by sonication to obtain a
light brown dispersion, followed by ushing with nitrogen, and
ꢁ
the distillation range of petroleum ether was 60–90 C.
ꢁ
then heating to 120 C at which point Fe(CO)5 was added. The
2.2. Characterization and measurements
reaction mixture was heated to 220 ꢁC and kept under reux for
Samples for TEM were prepared by drying the particle solutions one hour.
on an amorphous carbon coated copper grid and imaged by
The morphology of graphene–Fe3O4 was determined by
using a Tecnai-G2-F30 (FEI) transmission electron microscope TEM. As Fig. 1B shows, the surfaces of graphene are densely
at an acceleration voltage of 300 kV. X-ray powder diffraction covered by narrowly distributed Fe3O4 NPs with an average size
652 | J. Mater. Chem. A, 2013, 1, 651–656
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of 27 nm (Fig. 1C). The distribution of Fe3O4 NPs on each gra- peak centered at 2q ¼ 11.2ꢁ, corresponding to the (002) inter-
phene sheet is even, and no big conglomeration of Fe3O4 NPs or planar spacing of 0.87 nm. Another weak peak appears around
large vacancy on graphene is observed. The selected area elec- 2q ¼ 24ꢁ, which is the characteristic peak of unoxidized residual
tron diffraction (SAED) pattern also shows that the interplanar graphite. In graphene–Fe3O4, the peak at 2q ¼ 24.2ꢁ can be
spacings are consistent with Fe3O4 NPs and graphene (inset of observed, with the disappearance of 2q ¼ 10.2ꢁ, indicating the
Fig. 1B). XPS was employed to analyze GO and the product of reduction of GO in the reaction.27,28 The pattern of graphene–
graphene–Fe3O4 composites. The C1s spectrum of GO contains Fe3O4 displayed obvious diffraction peaks of Fe3O4 and the peak
three components (Fig. 2A): the nonoxygenated ring
C
positions and relative intensities match well with the standard
(286.7 eV), C–O species (280.9 eV) and C]O species (287.8 eV). XRD data for magnetite (JCPDS card, le no. 19-0629). The
Aer the CO reduction, the relative contribution of the observation of XRD and XPS conrms the successful prepara-
components associated with oxygenated functional groups tion of graphene–Fe3O4 by using a one-step Fe(CO)5 decompo-
disappeared (Fig. 2B), indicating the deoxygenation of GO sition reaction.
under CO reduction.19 In the Fe 2p high resolution XPS spec-
trum of graphene–Fe3O4 (Fig. S2†), the binding energy peaks at tigated using a vibrating sample magnetometer. Fig. 1D shows
711.7 and 725.3 eV correspond to Fe 2p3/2 and Fe 2p1/2 the room-temperature magnetization hysteresis loops of the
The magnetic property of the obtained samples was inves-
,
respectively, which are in good agreement with the known graphene–Fe3O4 composites with a starting Fe(CO)5 amount of
values of Fe(2p3/2) and Fe(2p1/2) of Fe3O4, respectively.20 It is 0.05 mL. The saturation magnetization is 10.15 emu gꢂ1, which
noteworthy that the charge transfer satellite of Fe 2p3/2 was not is lower than that of pure Fe3O4 nanocrystals reported in
detected at around 720 eV, revealing the formation of mixed previous literature,29 mainly attributed to the presence of gra-
oxides of Fe(II) and Fe(III) such as Fe3O4.21
phene and OAM. The composites can be easily manipulated by
Fig. 3(A) shows the Raman spectra of graphene oxide and an external magnetic eld (inset of Fig. 1D), which is important
graphene–Fe3O4. The Raman spectrum of graphene oxide for the magnetic separation of graphene–Fe3O4 from the reac-
exhibited two intense peaks at 1361 and 1608 cmꢂ1, corre- tion mixture.
sponding to the D and G bands, respectively.22,23 The D and G
peaks of graphene–Fe3O4 blue-shied to 1306 and 1602 cmꢂ1
.
3.2 The catalytic activity of graphene–Fe3O4 to A3-coupling
The ID/IG ratio of graphene@Fe3O4 was found to be higher than
that of initial GO, revealing a substantial reduction in the
content of the sp3-bonded carbon atoms and the oxidized
molecular defects.24 The blue-shi of the D and G peaks indi-
cates the reduction of GO and the existing interaction force
between graphene and Fe3O4 NPs.22,25,26 Fig. 3(B) displays the
XRD patterns of GO and graphene–Fe3O4. GO shows a strong
reaction
Multicomponent reactions of simple molecules provide a very
versatile and efficient method to construct desired molecules.
Propargylic amines, products of the A3-coupling, are important
synthetic intermediates for potential therapeutic agents and
natural products.30 Some literature reported that single Fe3O4
nanoparticles could catalyze A3-coupling.31 As a comparison, we
Table 1 Three-component coupling of cyclohexanecarbaldehyde, piperidine,
and phenylacetylene catalyzed by the graphene–Fe3O4 catalysta
Entry
Solvent
Isolated yieldb (%)
Fig. 2 C1s XPS spectra of GO (A) and graphene@Fe3O4 (B).
1
2
3
THF
CH3CN
DMF
85
90
40
4
5
6
7
CH3OCH3
CH2Cl2
CH3CH2OH
H2O
78
68
57
66
8
9
10
11
CH3COOEt
CH3CN
CH3CN
CH3CN
60
72c
48d
58e
a
For a detailed experimental operation, see ESI.† All reactions were
carried out on a 1 mmol scale with aldehyde–amine–alkyene ¼
b
Fig. 3 (A) Raman spectra of GO and graphene@Fe3O4 and (B) XRD patterns of
GO and graphene–Fe3O4.
1 : 1.2 : 1.5, 5% catalyst. Isolated yields based on the aldehyde.
Fe3O4 (10 nm). d g-Fe2O3. e Graphene.
c
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Table 2 Three-component coupling of cyclohexanecarbaldehyde, piperidine,
used graphene–Fe3O4 as a catalyst to test the catalytic perfor-
mance of these composites for A3-coupling reaction. In order to
search for a suitable solvent, cyclohexanecarbaldehyde, piperi-
dine, and phenylacetylene were used as model substrates
(Table 1). Among the solvents tested, acetonitrile was the most
effective reaction solvent for this A3-coupling reaction (Table 1,
entry 2). Slightly lower yields were obtained when using tetra-
hydrofuran or diethyl ether as the solvent (Table 1, entries 1 and
4). N,N-Dimethylformamide (DMF), ethanol, dichloromethane,
water, acetone and ethyl acetate afforded the products in
moderate yields (Table 1, entries 5–8). It is worth noting that the
isolated yields using graphene–Fe3O4 are higher than yields
using Fe3O4 NPs (27 nm) in acetonitrile (Table S1†). Similarly,
our isolated yields are higher than Zeng’s report31 when using
tetrahydrofuran as the solvent. The possible reason might be
explained on the basis of the highly dispersed Fe3O4 (having
high surface area) on the surface of graphene (Fig. 1B) and a
synergistic effect that comes from graphene (exhibiting a
certain catalytic activity, Table 1), which allow more molecules
to be in contact with the surface of Fe3O4 NPs. Similarly, g-Fe2O3
nanoparticles (<50 nm) also catalyzed the reaction, affording
the corresponding propargylamine products in a lower yield
(Table 1, entry 10).
a
and phenylacetylene catalyzed by graphene–Fe3O4
To expand the scope of this A3-coupling, various aldehydes,
alkynes and amines were used as substrates under the opti-
mized reaction conditions. As shown in Table 2 (entry 15), only a
trace of A3-coupling product was observed by NMR. However,
aliphatic aldehydes such as cyclohexanecarboxaldehyde, iso-
butyraldehyde, 2-ethylbutanal, formaldehyde and heptanal all
give both higher conversions and greater yields (Table 2, entries
1–6). It was important to notice that formaldehyde provides
high yield (Table 2, entry 4). Good yields were observed when
cyclic dialkylamines such as pyrrolidine and morpholine were
used (Table 2, entries 6 and 7). We also found that a variety of
other alkynes could be effectively reacted with cyclo-
hexanecarboxaldehyde and pyrrolidine under the present reac-
tion conditions (Table 2, entries 9–14). Note that the reactions
proceeded smoothly to give the corresponding propargylamines
in a good yield. The presence of electron-rich groups on the
benzene ring (entries 9–13) increased the reactivities of the
alkynes whereas electron-withdrawing groups (Table 2, entry 14)
on the benzene decreased the yield.
The recyclabilities of graphene–Fe3O4 were then evaluated.
Aer each run, ethyl acetate was added to dilute the reaction
mixture and the organic layer was simply decanted with the
Table 3 The reuse of the catalyst in A3-couplinga
Cycle
Yield (%)
Cycle
Isolated yieldb (%)
1
3
5
7
90
68
54
51
2
4
6
8
79
63
54
50
a
For a detailed experimental operation, see ESI.† All reactions were
carried out on a 1 mmol scale with aldehyde–amine–alkyene ¼
a
b
All reactions were carried out on a 1 mmol scale with aldehyde–amine–
1 : 1.2 : 1.5, 5% catalyst. Isolated yields based on the aldehyde.
b
c
alkyene ¼ 1 : 1.2 : 1.5, 5% catalyst. Isolated yields based on the
CH3CN : THF ¼ 1 : 1 as solvent.
aldehyde.
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assistance of a small magnet. The catalyst was quickly concen-
trated to the side wall of the reaction vial once a magnet is
placed nearby. Aer a simple wash using ethyl acetate and air-
drying, the catalyst was reused for the next run. It was shown
that the catalyst could be recovered and reused at least 8 times
without signicant loss of catalytic activity (Table 3). Further,
aer the sixth catalytic cycle the graphene–Fe3O4 nanocatalyst
was characterized by TEM studies (Fig. S1†). The Fe3O4 NPs were
well dispersed on the surface of the graphene sheet suggesting
the very high chemical stability of our graphene–Fe3O4 nano-
catalyst and thus it is suitable for many catalytic cycles.
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4 Conclusion
In summary, we have successfully developed, for the rst time, a
one-step reaction to prepare graphene–Fe3O4 based on decom-
position of Fe(CO)5 on the surface of graphene oxide. During
the process of decomposition, one decomposition product led
to the reduction of the graphene oxide, and another decompo-
sition product led to the in situ formation of Fe3O4 NPs on
graphene sheets. The resulting sample can be easily manipu-
lated by an external magnetic eld. A highly efficient graphene–
Fe3O4-catalyzed A3-coupling of aldehydes, alkynes, and amines
has been achieved. The process was simple and generated a
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Acknowledgements
The work was supported by the Specialized Research Fund for
the Doctoral Program of Higher Education (20100211120010),
the National Basic Research Program of China (973 Program)
no. 2012CB933102, the Gansu NST (1107RJZA255 and 13 Y. Zhang, B. Chen, L. Zhang, J. Huang, F. Chen, Z. Yang,
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