Coordinated assembly of a new 3D mesoporous Fe3O4@Cu2O-graphene oxide framework as a highly efficient and reusable catalyst for the synthesis of quinoxalines

Article abstract of DOI:10.1039/c5cc00250h

A new three-dimensional (3D) mesoporous hybrid framework was synthesized by coordinated layer-by-layer assembly between nanosheets of reduced graphene oxide and Fe₃O₄@Cu₂O. This 3D mesoporous framework shows an excellent c

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ARTICLE TYPE
Coordinated Assembly of A New 3D Mesoporous Fe3O4@Cu2OꢀGraphene
Oxide Framework As Highly Efficient and Reusable Catalysts for the
Synthesis of Quinoxalines
Zhiyi Wang, Guowen Hu, Jian Liu, Weisheng Liu, Haoli Zhang, and Baodui Wang*
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX DOI: 10.1039/b000000x
nanocrystals (NCs) into functionalized reduced GO (rGOꢀSꢀDIB)
interlayer to form uniformly layered structure, as depicted in
Scheme 1. It is found that the Fe3O4@Cu2O NCs are partially
inserted between two sheets of GO, with the remaining part
exposed in the mesopore channels. This unique pore structure not
only allows the mass transportation but also provides the NCs
with enough protection, preventing them from leaching, sintering,
and aggregating during catalytic reactions. The native magnetic
properties of Fe3O4@Cu2OꢀrGO nanocomposite enable this
catalyst to be easily separated from the reaction very quickly. All
these features enable the proposed system to be very active in
catalysis reaction. To the best of our knowledge, it is the first time
to prepare Fe3O4@Cu2O core@shell structure and its
corresponding mesoporous assembly framework. We find that,
compared with the homogeneous catalyst and supportless
Fe3O4@Cu2O, Fe3O4@Cu2OꢀrGO nanocomposite exhibits not
only an enhanced activity but also an excellent reusable
performance in the synthesis of quinoxalines using a variety of
substituent (electronꢀrich and electron withdrawing groups) oꢀ
phenylenediamine and terminal alkyne. This phenomenon is due
to its desirable properties, including high surface area, large pore
volume, unique semiexposed structure of Fe3O4@Cu2O NCs, and
enhanced mass transport. In addition, the obtained Fe3O4@Cu2Oꢀ
rGO nanocomposite can be easily recycled by the simple
application of an external magnetic field and subsequently reused
without significant loss of its catalytic activity after recycling
nine times.
A new threeꢀdimensional (3D) mesoporous hybrid framework
was synthesized by coordinated layerꢀbyꢀlayer assembly
between nanosheets of reduced graphene oxide and
Fe3O4@Cu2O. This 3D mesoporous framework show an
excellent catalytic performance with a remarkable activity,
selectivity (>99%), and strong durability in synthesis of
quinoxalines.
Despite intensive efforts to synthesize heterogeneous catalysts by
incorporating various functionalities into nanostructures to mimic
an enzyme,1 the organization of different nano objects into
integrated uniform porous heterogeneous catalysts that resemble
a reactor on the nanoscale is still a significant challenging task. In
the past two decades, porous materials such as zeolites,2 carbons,3
silica,4 and metalꢀorganic frameworks (MOFs)5 have been used to
prepare nanoreactors and the reactants get into their porous
channels where the catalyst is located. Recently, threeꢀ
dimensional (3D) grapheneꢀbased frameworks (3DꢀGFs) with
porous carbon materials have been emerged as promising support
for accommodating of catalysts due to their continuously
interconnected macroporous structures, low mass density, large
surface area, and high electrical conductivity.6 However, major
challenges remain towards the fabrication of wellꢀdefined microꢀ
or nanoꢀporous 3D graphene structures. In addition, there have
been no reports of controllable assemble of core−shell structure
metal oxide nanoparticles (NPs) with magnetic properties and
catalytic activity into graphere oxide (GO) interlayer to form
metal oxideꢀgraphere oxide frameworks (MOꢀGOFs) as
organic synthesis reaction catalysts.
As a heterogeneous catalyst, copper oxide (Cu2O) has been
widely used due to its high activity in heterogeneous catalysis,
such as synthesis of methanol, methanol steam reforming,7
oxidation of CO,8 deoxidization of NOx,9 dimethyl ether (DME)
synthesis10 and waterꢀgas shift (WGS) reaction.11 Recently, it was
reported that Cu@Cu2O core@shell microspheres exhibited a
better catalytic performance than the physically mixed Cu and
Cu2O microspheres.12 However, NPs too small in size tend to
agglomerate, which results in the limited practical use of
Cu@Cu2O NPs. Recently, one of most promising strategies for
tacking the aggregation problem of NPs is to disperse them on the
surface of GO sheet.13 Although the obtained catalysts have a
higher catalytic activity, they usually exhibit low selectivity to
desired products or a high deactivation rate due to the gradual
migration and agglomeration of active phase during the reaction.
In addition, most catalysts reported so far are recovered by
tedious filtration or centrifugation. Thus, the design and synthesis
of more efficient catalytic system remain challenging.
Scheme 1. Illustration of the preparation of Fe3O4@Cu2OꢀrGO
nanocomposite.
Scheme 1 and Scheme S1 show schematic illustrations of
the synthesis route for the Fe3O4@Cu2OꢀrGO nanocomposite. In
a typical synthesis process, the core@shell Fe3O4@Cu2O NCs
were synthesized by thermal decomposition of Fe(acac)3 and
Cu(acac)2 in the presence of oleylamine and diphenyl ether. GO
was synthesized using modified Hummer’s method starting from
graphite powder.14 Reduced graphene oxide (rGO) was prepared
Herein, we have developed an interesting approach to directly
assemble the asꢀsynthesized core@shell Fe3O4@Cu2O
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DOI: 10.1039/C5CC00250H
by NaBH4 reduction of GO15 and then dispered in pyridine/DMF
(Vpyridine/VDMF=1:5). Nꢀmethylꢀglycine and 3,4ꢀ
histograms are given in Figure 1A (inset), indicating that the
average diameters of the prepared Fe3O4@Cu2O NCs display a
very narrow size distribution of 16±0.3 nm. As shown in Figure
S1, the energyꢀdispersive Xꢀray (EDX) analysis on selected
different areas of an individual Fe3O4@Cu2O sphere
demonstrates clearly that the coreꢀshell spheres contain Cu, O and
Fe elements. The elemental mapping results indicate that the Fe
(Figure 1F) is localized in a core area, while Cu (Figure 1E) is
localized in a shell configuration. The dense O signal (Figure 1G)
is found throughout the nanoparticles, from the inner core to the
periphery, possibly owing to the Fe3O4 and Cu2O. The typical Xꢀ
ray photoelectron spectroscopy (XPS) (Figure S2), and the
formation mechanism (Figure S3ꢀ5 and Scheme S2) of
core@shell Fe3O4@Cu2O NCs are given in the supporting
information.
dihydroxybenzaldehyde were added to the DMF phase, and then
0
the mixture was refluxed at 145 C and kept there for 96 h.16 The
obtained rGOꢀSꢀDIB composites were isolated by filtration,
washed with DMF, ethanol and dichloromethane, and finally
dried in air. Subsequently, the asꢀprepared rGOꢀSꢀDIB
composites were reacted with the asꢀsynthesized Fe3O4@Cu2O
NCs by coordination reaction between catechol group modified
on the rGO and Cu+ ion exisited on the surface of Fe3O4@Cu2O.
During the coordination process, the Fe3O4@Cu2O NCs were
gradually deposited and embedded in the rGO networks.
Figure 2. (A) XRD pattern of Fe3O4@CuO NCs (red line) and Fe3O4@CuOꢀ
rGO nanocomposite (black line); (B) Nitrogen adsorption and desorption
isotherms of the Fe3O4@CuOꢀrGO framework. The inset shows the pore size
distribution of the Fe3O4@CuOꢀrGO framework.
Figure S6 and Figure 1C show the TEM images of the rGOꢀSꢀ
DIB and Fe3O4@Cu2OꢀrGO nanocomposite. A TEM image of the
obtained rGOꢀSꢀDIB shows transparent sheets with folded
boundaries (Figure S6). However, when rGOꢀSꢀDIB is
assembled with Fe3O4@Cu2O NCs, the rGO sheets packed into
several nanometers, indicating that the rGO and Fe3O4@Cu2O
NCs undergo layer by layer assembly. TEM images also show
that the Fe3O4@Cu2O NCs are embedded between two sheets of
rGO with
morphology
simulation diagram
a
homogeneous dispersion and semiexposed
in
the
of
mesoporous
forming
composites.
The
Fe3O4@Cu2OꢀrGO
nanocomposite is showed in Figure 1D. In the FTꢀIR spectrum,
the absorption peak of phenolic ʋ(C–O) vibration in rGOꢀSꢀDIB
is observed at 1174 cmꢀ1. After forming Fe3O4@Cu2OꢀrGO
nanocomposite, the peak of the phenolic ʋ(C–O) at 1174 cmꢀ1 is
not observed. However, a new absorption peak of C–O–Cu
vibration at 1095 cmꢀ1 appears, indicating that the phenolic
hydroxyl group of rGOꢀSꢀDIB are bound to the surface of
Fe3O4@Cu2O NCs (Figure S7).18,19 The XRD patterns of
Fe3O4@Cu2OꢀrGO nanocomposite (Figure 2A) also show the
lattice parameters of both Fe3O4@Cu2O NCs at (311), (400),
(422), (511), (440), and (533)and rGO at (002).20,12,21 The
e3O4@Cu2OꢀrGO nanocomposite exhibits superparamagnetic at
room temperature (Figure S8). Therefore, this nanocomposite can
be rapidly and conveniently separated from toluene for reuse
under an applied magnetic field (inset in Figure S8).
Figure 1. (A) TEM images of coreꢀshell Fe3O4@Cu2O NCs. The inset
shows the size distribution histograms and selected area electron
diffraction (SAED) pattern of the coreꢀshell Fe3O4@Cu2O NCs. (B)
HRTEM image of coreꢀshell Fe3O4@Cu2O NCs; (C) TEM images of
Fe3O4@Cu2OꢀrGO nanocomposite. The inset shows the SAED pattern of
Fe3O4@Cu2OꢀrGO nanocomposite. (D) The simulation diagram of
forming Fe3O4@Cu2OꢀrGO nanocomposite. The corresponding element
mapping images of (E) Cu, (F) Fe, and (G) O. The scale bars of the (f)ꢀ(h)
are 30 nm.
Figure 1A shows a TEM image of coreꢀshell structured
Fe3O4@Cu2O NCs with the darker region in the center being 10
nm Fe3O4 and the lighter ring being 3 nm Cu2O (Figure 1B). The
different contrast between the core and shell region is due to the
difference in electron scattering power when the electron beam
strikes on its surface. The detailed structure of a single
Fe3O4@Cu2O NC is characterized with HRTEM, as shown in
Figure 1B. The lattice fringes of the core are clearly shown in the
image with an adjacent fringe spacing of 0.301 nm,
corresponding to (311) lattice planes for Fe3O4. The lattice
fringes in the Cu2O shell are clearly seen in the HRTEM. The
interfringe spacing is 0.252 nm, close to the interplane distance of
the (111) planes in the face centered cubic (fcc) Cu2O, indicating
that the Fe3O4 nanoparticles are coated with a layer (3 nm) of
crystalline Cu2O.17 The corresponding sizeꢀdistribution
The porosity of Fe3O4@Cu2OꢀrGO framework is characterized
by N2 adsorption measurements at 77 K after heating the samples
0
at 100 C for 8 h to remove any moisture and solvent molecules
presented in the pores. As shown in Figure 2B, the specific
surface area and total pore volume of Fe3O4@Cu2OꢀrGO
framework estimated by the BrunauerꢀEmmettꢀTeller (BET)
method are 175.6 m2/g and 0.7 cm3/g, respectively. According to
the BarrettꢀJoynerꢀHalenda (BJH) model, Fe3O4@Cu2OꢀrGO
framework has a relatively broad mesopore diameter with a
maximum frequency near 30.0 nm, and the average pore width is
15.6 nm (inset in Figure 2B).
2
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Table 1. Reactions of oꢀPhenylenediamine (1a) with various Terminal
Alkynes (2aꢀf) a.
4ꢀNitroꢀoꢀphenylenediamine with electronꢀaccepting group did
not produce the target product (Table 2, entry 4). In all alkynes
used for comparison, Fe3O4@Cu2OꢀrGO nanocomposite showed
higher product yields than other reported catalytic system.22
Notably, substrates substituted by OCH3, CH3 and Br groups only
gave one isomer, which was better than Chen’ report catalytic
system.22 This result demonstrated that the Fe3O4@Cu2OꢀrGO
catalyst had a high selectivity for catalyzing substituted oꢀ
phenylenediamine with phenylacetylene to form quinoxalines
products.
Entry
1
Alkyne
(2a)
Product
Yield b (%)
92 (86) c
2
99 (99) c
Table 2. Reactions of substituted oꢀPhenylenediamine (1bꢀf) with
phenylacetylene (2a) a.
(2b)
(2c)
3
4
80 (52) c
80 (49) c
Entry
R
Product
Yield b (%)
95 (89) c
1
CH3 (1b)
2
OCH3
(1c)
94 (87) c
(2d)
5
65 (54) c
3
4
Br (1d)
65 (31) c
N.R.
(2e)
NO2 (1e)
N.R.
a All of the reactions were carried out in sealed tubes using 0.25 mmol of 1a,
1 mmol of 2bꢀf, 1 mol% of Fe3O4@Cu2OꢀrGO, and 3 equiv of each base in
each solution at 70 for 8 h. b Isolated yields. c Reported yields.21
a
All of the reactions were carried out in sealed tubes using 0.25 mmol of
1bꢀf, 1 mmol of 2a, 1 mol% of Fe3O4@Cu2OꢀrGO, and 3 equiv of each
base in each solution at 70 for 8 h. b Isolated yields.c Reported yields.22
Given the Fe3O4@Cu2OꢀrGO nanocomposite contains Cu2O, it
can be utilized as an active catalyst for synthesizing of quinoline
compounds. Quinoxalines and their derivatives have attracted
wide attention because they are not only useful intermediates of
other organic cyclic compounds20 but also possess significant
biological activities including antiviral, antibacterial, and antiꢀ
inflammatory.21 In this study, the effects of different different
catalysts were examined. The reaction using rGO and Fe3O4 NPs
did not proceed (Table S3, entry3 and 4). Fe3O4@Cu2O NCs and
.
Cu(OAc)2 H2O were found to be moderate yield (Table S3, entry
2, 5). Other catalysts such as Cu(OTf)2 and CuCl2 gave low yields
(Table S3, entry 6 and 7). However, the prepared catalyst showed
the highest activity (Table S3, entry 1). We proposed that the
high specific surface area of mesoporous rGO layer adsorbed
more reactant molecules so as to increase the reactant
concentrations within the rGO channels, leading to higher
production yields (92%). Also, we found that this reaction could
not work only in the presence of organic base (Table S2, entry 4).
In the organic base containg DMAP, Pyridine and Et3N, only the
DMAP get the best yield (Table S2, entry 3 and 7). Among
inorganic bases tested, Cs2CO3 turned out to be the best (Table S2,
entry 7).
Under the optimal reaction condition, oꢀphenylenediamines
and a variety of different substituted phenylacetylenes were
examined, and the results were summarized in Table 1. A variety
of aromatic alkynes afforded quinoxalines products in a high
yields (Table 1, entries 1ꢀ5). Notably, aliphatic alkyne provided
corresponding quinoxalines product in 80% yield (Table 1, entry
3). Also, the reaction scope of oꢀphenylenediamine was studied
(Table 2). 4ꢀmethylꢀoꢀphenylenediamine and 4ꢀmethoxyꢀoꢀ
phenylenediamine with electronꢀdonating group formed the
products in high yields (Table2, entry 1 and 2). 4ꢀbromoꢀoꢀ
phenylenediamine with electronꢀaccepting group afforded the
target products in moderate yields (Table 2, entry 3). Meanwhile,
Figure 3. Catalytic cycle of Cu(OAc)2·H2O (blue column), Fe3O4@Cu2O
NCs (green column) and Fe3O4@Cu2OꢀrGO nanocomposite (red column).
Isolation and reuse of the catalyst are crucial requirements for
any practical applications in terms of cost and environmental
protection. As shown in Figure 3 and Table S4, Fe3O4@Cu2Oꢀ
rGO catalyst could be reused at least 9 times without significant
decrease in catalytic activity, while the catalytic activities of
Fe3O4@Cu2O catalyst decreases very quickly after four cycles
.
(red column in Figure 3), and the Cu(OAc)2 H2O only can be
used one time (blue column in Figure 3). This could be attributed
to the incorporation of Fe3O4@Cu2O NCs into the GO interlayer,
which effectively inhibits the leaching, sintering, and aggregating
of the Fe3O4@Cu2O active (green column in Figure 3) and also
enhances the hydrothermal stability.23 After each run, the
Fe3O4@Cu2OꢀrGO catalysts can be easily recovered by an
external magnet, and the TEM image (Figure S11) of the
Fe3O4@Cu2OꢀrGO catalyst after nine cycles was consistent with
those of the asꢀprepared one, suggesting that the Fe3O4@Cu2O
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DOI: 10.1039/C5CC00250H
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NCs incorporated into the GO interlayer could enhance the
hydrothermal stability of the mesoporous structure.
In summary, we have successfully fabricated 3D mesoporous
hybrid framework via coordinated layerꢀbyꢀlayer assembly
between nanosheets of reduced graphene oxide and Fe3O4@Cu2O.
Compared with the Fe3O4@Cu2O NCs, this 3D mesoporous
framework, due to the introduction of rGO, exhibits good
structural stability, high surface area, and large amount of
mesopores.
In
this
mesoporous assembly
framework,
Fe3O4@Cu2O NCs are embedded between two sheets of rGO and
partially exposed to the pore channels. Such features ensure
special catalytic reactions performed only in nanochannels and
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nanostructures recovery using an outer magnet is simple and
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†Electronic Supplementary Information (ESI) available: [Experimental details,
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‡
This work was supported by the National Natural Science Foundation of
China (21271093, and 21431002), the National Basic Research Program
of China (973 Program) No. 2012CB933102, the Program for New
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