Water-dispersible and magnetically separable gold nanoparticles supported on a magnetite/s-graphene nanocomposite and their catalytic application in the Ullmann coupling of aryl iodides in aqueous media

Article abstract of DOI:10.1039/c4ra04479g

Water-dispersible sulfonated graphene (s-G) was synthesized by anchoring sulfonic acid groups on graphene sheets. Subsequently, magnetically separable Fe₃O₄/s-G was synthesized from the Fe₃O ₄ nanoparticles decorated on s-G sheets by the co-precipitation method of iron ions. Finally, Fe₃O₄/s-G was successfully decorated with gold nanoparticles in a facile route by reducing chloroauric acid in the presence of sodium dodecyl sulfate, which is used as both a surfactant and reducing agent. The obtained Au/Fe₃O₄/s-G nanocomposite remained soluble in water, but could be easily separated from reaction solutions by an external magnetic field and then used as a heterogeneous catalyst for the Ullmann coupling reaction in water. The catalytic activity reduction was not significant even after five consecutive reaction runs due to the efficient magnetic separation, the high dispersion and stability of the catalyst in aqueous solution.

Full text of DOI:10.1039/c4ra04479g

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Water-dispersible and magnetically separable gold
nanoparticles supported on a magnetite/
s-graphene nanocomposite and their catalytic
application in the Ullmann coupling of aryl iodides
in aqueous media†
Cite this: RSC Adv., 2014, 4, 39428
a
Minoo Dabiri, Monire Shariatipour,a Siyavash Kazemi Movaheda
*
and Sahareh Bashiribodb
Water-dispersible sulfonated graphene (s-G) was synthesized by anchoring sulfonic acid groups on
graphene sheets. Subsequently, magnetically separable Fe3O4/s-G was synthesized from the Fe3O4
nanoparticles decorated on s-G sheets by the co-precipitation method of iron ions. Finally, Fe3O4/s-G
was successfully decorated with gold nanoparticles in a facile route by reducing chloroauric acid in the
presence of sodium dodecyl sulfate, which is used as both a surfactant and reducing agent. The
obtained Au/Fe3O4/s-G nanocomposite remained soluble in water, but could be easily separated from
reaction solutions by an external magnetic field and then used as a heterogeneous catalyst for the
Ullmann coupling reaction in water. The catalytic activity reduction was not significant even after five
consecutive reaction runs due to the efficient magnetic separation, the high dispersion and stability of
the catalyst in aqueous solution.
Received 13th May 2014
Accepted 18th August 2014
DOI: 10.1039/c4ra04479g
www.rsc.org/advances
catalyzed reactions.4,12,14 Graphene is an ideal candidate
because of its a two-dimensional sheet of sp2 bonded carbon
Introduction
atoms, which can be viewed as an extra-large polycyclic
aromatic molecule and large specic surface area.15 Loading
metal NPs onto graphene can not only prevent graphene sheets
from restacking but also improve the catalytic performance
owing to the strong synergistic interaction between the two
components.16 Therefore graphene as a polycyclic aromatic
molecule is a potential candidate as both support and stabilizer
of Au NPs for chemical transformations.
Recently, much attention has been paid on the synthesis of
Fe3O4 NPs/graphene as a new kind of hybrid material, owing to
wide-ranging applications such as immobilizing bioactive
substances, energy storage and environmental remediation.16,17
The unique properties of Fe3O4 NPs/graphene hybrids,
combining characteristics of graphene as a polycyclic aromatic
molecule, which has high conductivity, low price, high chemical
inertness, and large specic surface area,15 and Fe3O4 NPs, with
high magnetism, low expense, and environmentally benign
nature,18 open a new window for fabricating highly stable
multifunctional nanomaterials using these hybrids as support
materials. In addition, Fe3O4 has already been introduced as a
suitable support for preparing highly active metal catalysts, and
immobilizing noble metal nanocatalysts on magnetic Fe3O4
support prevents agglomeration of the catalyst particles during
recovery and can increasing catalyst durability.19 Thus, a
Noble metal nanoparticles such as Au, Pd, Ru, Rh have attracted
considerable attention in a wide variety of applications due to
their unique physicochemical properties, especially their cata-
lytic activity in a number of chemical reactions.1 To enhance
their stability and catalytic activity, they are oen deposited
onto supporting materials forming catalytic systems.2 Recent
years witness a tremendous growth in the number of supported
gold nanoparticles (Au NPs) catalysing highly selective chemical
transformations.3 The catalytic performance of Au NPs-support
strongly depends on the size and shape of Au NPs, the nature of
the support, and the Au NPs-support interface interaction.4
Supporting carriers may function by dispersing and xing the
Au NPs. Supports such as oxides and mixed oxides (CeO2,5 SiO2,6
Al2O3,7 TiO2,8 Mg–Al–O9 and Ga–Al–O10), polymers (PVP11 and PS
derivatives12) and ordered mesoporous carbon13 have been used
for supporting Au NPs. Among these, p-interaction of aromatic
rings with gold nanoparticles leads to nanoparticle stabilization
and improve the catalyst performance in a variety of gold-
aFaculty of Chemistry, Shahid Beheshti University, Tehran, Islamic Republic of Iran.
E-mail: m-dabiri@sbu.ac.ir; Fax: +98 21 22431661; Tel: +98 21 29903255
bDepartment of Marine Biology, Faculty of Biological Sciences, Shahid Beheshti
University, G. C., Evin, Tehran, Iran
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c4ra04479g
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combination of Fe3O4 NPs and graphene may optimize both
dispersion and catalytic activity of metal NPs.
However, due to the weak dispersity of graphene in water
and organic solvents, it is difficult to gra foreign nano-
structures (such as NPs) on the graphene surface. In order to
enhance the dispersity of graphene for facilitating the subse-
quent functionalization, various functionalized graphene
sheets were synthesized. It is well-known that the s-G is water
dispersible without the need for any polymeric or surfactant
ꢀ
stabilizers. The negatively charged SO3 units prevent the
graphitic sheets from aggregating in solution thereby yielding
isolated sheets of s-G with improved water dispersity.20
Aryl–aryl bond formation reactions are one of the most
important reactions in organic synthesis as they give rise to
many naturally occurring biologically and pharmaceutically
active products. The original and most widely used route to
produce biaryls is via the Ullmann reaction, the copper-
mediated homo-coupling of aryl halides. The need to avoid
the harsh conditions typically required for Ullmann couplings
(>140 ꢁC, stoichiometric amounts of copper and selective halide
substrates) have motivated researchers to nd milder varia-
tions. Although, signicant progress in this area has been
achieved with a variety of Pd, Cu, and Ni-based catalysts, the
majority of reports about Ullmann protocols are still homoge-
neous and successful examples using heterogeneous and recy-
clable catalyst systems are limited.21 However, only a few studies
have involved the application of free or supported Au NPs as
catalyst in Ullmann type reaction of aryl iodides.22–24 Further-
more, literature reports involving homocoupling of aryl iodides
catalysed by Au NPs in water are very scarce.25
Scheme 1 Schematic illustration of the preparation procedure of Au/
Fe3O4/s-G nanocomposite.
Herein, we report a novel and easy approach to homoge-
neously immobilize Fe3O4 and Au nanoparticles on sulfonated
graphene sheets (s-G). The catalyst is designed with the aim of
combining the excellent supporting property of graphene
effectively immobilizing and stabilizing Fe3O4 and Au nano-
particles with the magnetic property of the Fe3O4 nanoparticles
for easy separation of catalyst and therefore improving their
reusability. Additionally, this nanocomposite is shown to act as
an efficient heterogeneous catalyst for the Ullmann homocou-
pling reaction in aqueous solution under aerobic condition and
could be efficiently reused while keeping the inherent catalytic
activity.
Fig.
1 Raman spectra of s-G, Fe3O4/s-G and Au/Fe3O4/s-G
nanocomposites.
The Raman spectra of the prepared s-G, Fe3O4/s-G and Au/
Fe3O4/s-G are shown in Fig. 1. The characteristic D and G bands
of carbon materials were observed around 1301 and 1386 cmꢀ1
,
respectively in Raman spectrum of s-G (Fig. 1). The D band is
characteristic of a breathing mode for k-point phonons of A1g,
while the G band is the result of the rst-order scattering of the
E2g mode of sp2 carbon domains.29 The Raman spectrum of
Fe3O4/s-G composite displays signals at 200–700 cmꢀ1, which
are due to the Fe3O4 NPs, and slightly split signal centred at
1293 cmꢀ1, which represents the overlap of two peaks, one from
Fe3O4 at 1280 cmꢀ1 and a typical graphene D band peak at 1301
Results and discussion
The process for the preparation of Au/Fe3O4/s-G nanocomposite
is schematically described in Scheme 1. The s-G nanocomposite
was synthesized by Samulski method.20 Subsequently, the
magnetic separable Fe3O4/s-G was synthesized from the Fe3O4
nanoparticles decorated on s-G sheets by the co-precipitation
method of FeSO4 and FeCl3 as iron ions in pH ¼ 8–9.26
Finally, Fe3O4/s-G was successfully decorated with gold nano-
particles in a facile route by reducing chloroauric acid (HAuCl4)
in the presence of sodium dodecyl sulfate (SDS), which is used
as both a surfactant and reducing agent.27
cmꢀ1 30 Another typical graphene signal is also observed at 1573
.
cmꢀ1, which is identied as the G band of graphene. The
Raman spectrum of Au/Fe3O4/s-G shows signals of Fe3O4 NPs
and graphene. Additionally, aer conjugation of the Au NPs
onto the Fe3O4/s-G sheets, the intensity of the Raman signals of
graphene (D and G bands) were enhanced relative to Fe3O4/s-G,
because of the surface enhanced Raman spectroscopy (SERS) of
Au NPs. SERS were obtained via an electromagnetic enhance-
ment (excitation of localized surface plasmons involving a
Raman spectroscopy is a very useful tool for investigating the
electronic and phonon structure graphene-based materials.28
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Fig. 3 Scanning electron micrograph of (a) Au/Fe3O4/s-G nano-
composite and corresponding quantitative EDS element mapping of
(b) Au, (c) Fe and (d) S.
Fig. 2 (a) Full range XPS spectrum of Au/Fe3O4/s-G nanocomposite.
(b) S 2p, (c) Fe 2p, and (d) Au 4f core level regions XPS spectra of Au/
Fe3O4/s-G nanocompoiste, respectively.
X-ray spectroscopy (EDS) mapping. As is seen in Fig. 4b–d,
rather than only located at the edges of s-G sheets, the elements
Au, Fe, and S are found to be uniformly dispersed on the whole
surface of Au/Fe3O4/s-G nanocomposite.
Fig. 4a and c show a comparison between the morphologies
of Fe3O4/s-G, and Au/Fe3O4/s-G nanocomposites investigated
using transmission electron microscope (TEM). As Fig. 5a
physical interaction) or chemical enhancement (formation of
charge-transfer complexes involving chemical interaction) with
enhancement factors of ꢂ1012 and ꢂ10 to 100, respectively. The
low enhancement factor for the Au/Fe3O4/s-G nanocomposite
indicates the presence of a chemical interaction or bonding
between Au NPs and Fe3O4/s-G.31 The enhanced broad peak in
the frequency region of 400–800 cmꢀ1 is due to the amorphous
sp3 bonded carbon in Au/Fe3O4/s-G nanocomposite.32 To be
assured, we synthesized a sample without magnetic nano-
particles (Au/s-G) and the similar peak was observed in this area
(Fig. S1†).
The electronic properties of Au/Fe3O4/s-G nanocomposite
was probed by X-ray photoelectron spectroscopy (XPS) analysis.
As shown in Fig. 2a, the peaks corresponding to Au 4d & 4f, C 1s,
Fe 2p & 2s, O 1s, and S 2p & 2s are clearly observed in the XPS full
spectrum. The S 2p spectrum of Au/Fe3O4/s-G nanocomposite is
shown in Fig. 2b. The S 2p signal for Au/Fe3O4/s-G nano-
composite includes two components. The rst one is consti-
tuted by two doublets, situated at 166.54 and 168.13 eV,
respectively, attributable to SO3H groups on s-G sheets.33 The
second component located at 169.52 and 171.88 eV is attribut-
able to the sulfur atoms of the dodecyl sulfate anions as
surfactant of Au NPs.34 From the Fe 2p XPS scan shown in
Fig. 2c, the two peaks at 726.28 and 711.99 eV, are assignable to
Fe 2p1/2 and Fe 2p3/2 for Fe3O4, respectively.35 The XPS spectrum
of Au 4f core for Au NPs-RGO level displays main peaks at 83.79
and 87.56 eV which correspond to the binding energy of Au0
4f7/2 and Au0 4f5/2, respectively (Fig. 2d).36
Fig. 3a depicts the scanning electron microscope (SEM)
micrograph of Au/Fe3O4/s-G sample. Several graphene layers
and folds in their planes are visible. The density and distribu-
tion of elements of the Au, Fe, and S on the Au/Fe3O4/s-G
nanocomposite are evaluated by quantitative energy dispersive
Fig. 4 (a) TEM of Fe3O4/s-G nanocomposite (b) SEAD pattern of
Fe3O4/s-G nanocomposite, (c–e) TEM of Au/Fe3O4/s-G nano-
composite and (f) SEAD pattern of Au/Fe3O4/s-G nanocomposite.
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Table 1 Screening of the reaction conditionsa
Entry
Base
Time (h)
Yieldb (%)
1
2
3
K2CO3
NaOH
KOH
K3PO4
K3PO4
K3PO4
48 h
48 h
48 h
48 h
48 h
48 h
27
50
68
95
0
4
5c
6d
0
a
Phenyl iodide (1.0 mmol), base (3.0 mmol), Au/Fe3O4/s-G (2 mol% of
b
Au), 110 ꢁC, and H2O (2 ml). GC yield, n-dodecane was used as an
internal standard. c s-G (100 mg). d Fe3O4/s-G (100 mg).
s-G nanocomposite was found to be 10.06 emu gꢀ1 as measured.
This value is smaller than the reported value of Fe3O4 bulk of
Fig. 5 The digital images of (a) water soluble Au/Fe3O4/s-G nano-
composite, (b) easy separation of Au/Fe3O4/s-G nanocomposite by an
92 emu gꢀ1
.
This could be attributed to the presence of
37
external magnet field, and (c) distribution of Au/Fe3O4/s-G nano- magnetically inactive layers at nanoparticle surfaces. This effect
becomes more pronounced as particle size decreases.38 Addi-
composite in a biphasic water–n-hexane system.
tionally, the relatively low amount of Fe3O4 loaded on s-G, which
is estimated to be 15.23 wt% calculated from the content of Fe
by inductively coupled plasma-optical emission spectrometry
(ICP-OES).
The catalytic activity of Au/Fe3O4/s-G nanocomposite was then
tested in the Ullmann coupling reaction. We chose the homo-
coupling phenyl iodide as a model reaction in H2O as solvent at
110 ꢁC in the presence of K2CO3 as base and with a catalyst
loading of 2 mol% of Au. Under these conditions, we found that
the cross coupling reaction proceeds well, affording the prom-
ising low yields (27%) of the corresponding biphenyl (Table 1,
entry 1). Various bases such as K3PO4, NaOH, and KOH, were also
screened for their effect on the reaction in H2O as solvent at
ꢁ
110 C. A superior yield was obtained when K3PO4 was used as
the base (Table 1, entries 1–4). It is also noteworthy that, when
this reaction was carried out with s-G or Fe3O4/s-G, we failed to
isolate any coupled product (Table 1, entries 5 and 6).
Fig. 6 The VSM curve of Au/Fe3O4/s-G nanocomposite.
With the optimized reaction conditions at hand, we next
managed to examine the scope and limitation of Ullmann
shows, the surfaces of s-G are covered with good dispersion of
Fe3O4 NPs with an average size of 10–20 nm. Fig. 4b shows
selected area electron diffraction (SAED) pattern of Fe3O4/s-G
nanocomposite. Fig. 4c–e shows the large-sized particles and
no agglomeration of Au NPs in Fe3O4/s-G sheets. Fig. 4f shows
SAED pattern of Au/Fe3O4/s-G nanocomposite. Additionally, Au
and Fe3O4 NPs are not found outside of the s-G sheets.
As shown in Fig. 5a the Au/Fe3O4/s-G nanocomposite provide
a homogeneous and stable suspension aer dispersing in water
due to the presence of SO3H groups in its surface. Moreover, the
strong magnetic property of the prepared nanocomposite was
revealed by complete and easy attraction by an external magnet
eld (Fig. 5b).
The magnetic properties of Au/Fe3O4/s-G nanocomposite
was investigated by a vibrating sample magnetometer (VSM) at
room temperature in external magnetic elds ranging from
ꢀ8000 to 8000 Oe. As illustrated in Fig. 6, the magnetization
curve of the prepared material has little hysteresis, remanence,
and coercivity, which demonstrates their superparamagnetic
characteristics.16 The saturation magnetization of the Au/Fe3O4/
Table 2 Au/Fe3O4/s-G nanocomposite catalyzed Ullmann coupling
Entry
R
X
Yielda (%)
1
2
3
4
5
6
7
8
9
10
H
4-OMe
4-Me
4-MeCO
4-NO2
2-Me
1,3,5-Trimethyl
H
I
I
I
I
I
I
I
Br
Br
Br
95
84
75
98
91
53
36
Trace
Trace
Trace
4-Me
4-MeCO
a
Isolated yields.
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Table 3 Reusability of the Au/Fe3O4/s-G nanocomposite in the Ull-
amount of gold that was originally present in the fresh nano-
composite. Finally, the magnetic nanocomposite was easily and
completely separated from the aqueous phase by an external
magnetic eld (Fig. 5b).
mann coupling reaction of phenyl iodidea
Reaction cycle
1st
95
2nd
94
3rd
89
4th
82
5th
78
Yieldb (%)
The heterogeneous nature of the catalysis was proved using a
hot ltration test, atomic absorption spectroscopy (AAS) anal-
ysis and Hg0-poisoning experiment. To determine whether the
catalyst is actually acted in a heterogeneous manner or whether
it is merely a reservoir for more active soluble gold species, we
performed a hot ltration test in the Ullmann coupling reaction
of phenyl iodide aer ꢂ50% of the coupling reaction was
completed. The hot ltrates were then transferred to another
ask containing K3PO4 (3 equiv.) in H2O (2 ml) at 110 ꢁC. Upon
further heating of catalyst-free solution for 48 h, no consider-
able progress (ꢂ9% by GC analysis) was observed. Moreover,
applying AAS to the same reaction solution at the midpoint of
completion indicated that no signicant quantities of gold are
le to the reaction liquors during the process. To determine
a
Phenyl iodide (1.0 mmol), K3PO4 (3.0 mmol), Au/Fe3O4/s-G (2.0 mol%
b
of Au), 48 h and H2O (2 ml). GC yield, n-dodecane was used as an
internal standard.
coupling reaction with various types of aryl halides derivatives
(Table 2). The aryl iodides bearing electron-donating and
electron-withdrawing groups reacted well and gave good yields
(Table 2, entries 1–4). The aryl iodide possessing electron-
withdrawing group (p-COCH3 and p-NO2) exhibited higher
yield compared to an aryl iodide possessing electron-donating
groups (p-OMe and p-Me) (Table 2, entries 2–4). The hindered
2-iodotoluene and 2-iodo-1,3,5-trimethylbenzene substrates
converted to the corresponding homocoupling product with
lower yield (Table 2, entries 6–7). Under the same reaction
conditions the homocoupling of bromobenzene and aryl
bromide bearing electron-donating and electron-withdrawing
groups failed to form of homocoupled products (Table 2,
entries 8–10).
whether the catalyst is heterogeneous or homogeneous,41
a
mercury poisoning experiment was performed. With standard
reaction conditions for the Ullmann homo-coupling reaction,
an experiment with Au/Fe3O4/s-G nanocomposite was started as
described above: phenyl iodide (1.0 mmol), K3PO4 (3 mmol),
and Au/Fe3O4/s-G (2.0 mol% of Au) in H2O (2 mL) were reacted
in a stainless-steel bomb. Aer about 24 h, the reaction was
stopped, and the excess of Hg0 (60 mmol) was added, and the
reaction was then restarted. The yield for 24 h was 46% and for
an additional 24 h aer the addition of mercury it was 47%. This
result clearly demonstrates the heterogeneous nature of the
catalyst.
Table 4 compares efficiency of Au/Fe3O4/s-G nanocomposite
(size of Au NPs, reaction conditions, time, and yield) with effi-
ciency of other reported heterogeneous gold nanoparticles
catalysts in Ullmann homocoupling reaction of phenyl iodide.
These result indicated that the size of Au NPs does not change
the reactivity in Ullmann homocoupling
Recently, Karimi presented
a new strategy (double-
separation technique) that includes easy separation of the
catalyst by an external magnetic eld as well as low solubility
catalyst in organic solvents.39 The presence of hydrophilic SO3H
in the surface of magnetic nanocomposite provides a means of
complete dispersion of the catalyst into the aqueous phase as
far as the nanocomposite have no affinity to the organic phase.
Considering this property, aer the rst use of the catalyst in
the above mentioned Ullmann coupling reaction, the product
was simply extracted with n-hexane while Au/Fe3O4/s-G nano-
composite remained in the aqueous phase (Fig. 5c). In the next
stage, the aqueous phase containing Au/Fe3O4/s-G nano-
composite was recharged with phenyl iodide and K3PO4 (ref. 40)
for the next run without any washing and purication of the
catalyst. The results indicate that this simple separation
method could be repeated for ve consecutive runs and the
recovered aqueous phase containing the Au/Fe3O4/s-G nano-
composite showed remarkably constant catalytic activity in all
of 5 cycles (Table 3). Additionally, it was observed that the yield
of the product gradually dropped with each reaction cycle. To
verify whether any leaching occurs, the gold content in the used
Au/Fe3O4/s-G nanocomposite (aer 5 cycles) was analyzed by
ICP-OES and it revealed the loss of about 5.4% of the initial
Conclusions
In conclusion, we demonstrated that Au/Fe3O4/s-G nano-
composite as a new highly water-dispersible/magnetically
separable semi heterogeneous for catalysis of organic reaction
in aqueous medium. This nanocomposite is highly active
catalyst for Ullmann coupling of aryl iodides in water. The
catalyst exhibits extremely low solubility in organic solvents, the
recovered aqueous phase containing the catalyst can be easily
Table 4 Comparison of efficiency of various gold nanoparticle catalysts in Ullmann homocoupling reaction of phenyl iodide
Catalyst
Size of Au NPs
Condition
Yield
Time
Ref.
Au/Fe3O4/s-G
Au NPs-RGO
Au@PMO
Au NP
Au25(SR)18/CeO2
NAP–Mg–Au
20–35 nm
10–20 nm
3–15 nm
1 nm
1.3 nm
5–7 nm
K3PO4, H2O, 100 ꢁC
95%
97%
95%
98%
99.8%
92%
48 h
6 h
16 h
7 h
48 h
48 h
This work
K3PO4, NMP, 100 ꢁ
K3PO4, NMP, 100 ꢁ
C
C
24
14
25
22
23
H2O/TBAOH, glucose, 100 ꢁ
C
K2CO3, DMF, 130 ꢁ
K2CO3 DMF, 140 ꢁ
C
C
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