Synthesis of gold nanoparticles decorated on sulfonated three-dimensional graphene nanocomposite and application as a highly efficient and recyclable heterogeneous catalyst for Ullmann homocoupling of aryl iodides and reduction of p-nitrophenol

Article abstract of DOI:10.1002/aoc.4189

Gold nanoparticles were decorated onto sulfonated three-dimensional graphene (3DG-SO₃H) through spontaneous chemical reduction of HAuCl₄ by 3DG-SO₃H. This nanocomposite exhibited excellent catalytic activity for the synthesis of symmetric biaryls via the Ullmann homocoupling of aryl iodides in an aqueous medium. Additionally, this nanocomposite was used as a catalyst for the reduction of p-nitrophenol to p-aminophenol. The catalyst could be used more than six times successively without significant deactivation.

Full text of DOI:10.1002/aoc.4189

Received: 9 September 2017
DOI: 10.1002/aoc.4189
Revised: 23 October 2017
Accepted: 24 October 2017
F U L L P A P E R
Synthesis of gold nanoparticles decorated on sulfonated
three‐dimensional graphene nanocomposite and application
as a highly efficient and recyclable heterogeneous catalyst
for Ullmann homocoupling of aryl iodides and reduction of
p‐nitrophenol
Minoo Dabiri
| Seyede Razie Banifatemi Kashi | Noushin Farajinia Lehi | Sahareh Bashiribod
Faculty of Chemistry, Shahid Beheshti
University, Tehran 1983969411, Islamic
Republic of Iran
Gold nanoparticles were decorated onto sulfonated three‐dimensional
graphene (3DG‐SO3H) through spontaneous chemical reduction of HAuCl4
by 3DG‐SO3H. This nanocomposite exhibited excellent catalytic activity for
the synthesis of symmetric biaryls via the Ullmann homocoupling of aryl
iodides in an aqueous medium. Additionally, this nanocomposite was used as
a catalyst for the reduction of p‐nitrophenol to p‐aminophenol. The catalyst
could be used more than six times successively without significant deactivation.
Correspondence
Minoo Dabiri, Faculty of Chemistry,
Shahid Beheshti University, Tehran
1983969411, Islamic Republic of Iran.
Email: m‐dabiri@sbu.ac.ir
KEYWORDS
gold, heterogeneous catalyst, homocoupling, reduction, three‐dimensional graphene
1 | INTRODUCTION
together to construct three‐dimensional porous networks,
alleviating the restacking of individual graphene sheets.
Graphene exhibits a unique combination of physical and
chemical properties such as large specific surface area,
low density, superior electrical conductivity, excellent
thermal stability with oxidation resistance temperature,
high thermal conductivity, remarkable mechanical
strength and excellent optical transmittance, and is
readily chemically functionalized.[1–14] Moreover, it
shows interesting electrochemical properties, including
wide electrochemical potential windows, low charge‐
transfer resistance, low cost of fabrication and excellent
electrochemical activity.[15–17] However, graphene sheets
tend to undergo irreversible restacking arising from the
strong π–π interactions and van der Waals forces, which
tremendously compromises the superiority of the inher-
ent high specific surface area, high conductivity and
mechanical strength of individual graphene sheets.[18–21]
Many attempts have been made to tackle this
challenge. One effective way is to engineer a graphene
material in which individual graphene sheets are bonded
Three‐dimensional graphene (3DG) is commonly
produced using sol–gel chemistry, which involves the
reducing of graphene oxide (GO) to form a highly cross‐
linked graphene hydrogel, followed by freeze or supercrit-
ical drying.[22,23] 3DG has excellent properties, including
an ultrahigh surface‐to‐volume ratio, high surface area,
high porosity, low density, good electric conductivity,
strong mechanical strength and fast mass and electron
transport kinetics, which make it an ideal material for
supercapacitors, catalysts, electrochemical biosensors
and anodes in lithium ion batteries.[24]
Additionally, the low water and organic dispersibility of
graphene materials has led to many attempts to increase the
solvent dispersibility of graphene. Recently, a facile route to
isolated and water‐soluble graphene involved the sulfona-
tion of graphene with p‐sulfobenzenediazonium salt.[25–29]
−
When graphene sheets incorporate SO3 moieties, the
graphitic sheets will separate from one another to some
distance due to electrostatic repulsion.[30]
Appl Organometal Chem. 2017;e4189.
wileyonlinelibrary.com/journal/aoc
Copyright © 2017 John Wiley & Sons, Ltd.
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https://doi.org/10.1002/aoc.4189

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DABIRI ET AL.
Gold nanoparticles (Au NPs) have been demonstrated
The pre‐oxidized graphite was then subjected to oxidation
using the Hummers method. The pre‐treated graphite
powder was put into cold (0 °C) concentrated H2SO4
(125 ml). Then KMnO4 (15 g) was added gradually under
stirring, and the temperature of the mixture was kept
below 20 °C by cooling. The mixture was then stirred at
35 °C for 4 h and then diluted with DI water (250 ml).
Because adding water to concentrated sulfuric acid
medium releases large amounts of heat, the dilution was
carried out in an ice bath to keep the temperature below
50 °C. After adding all of the 250 ml of DI water, the
mixture was stirred for 2 h, and then an additional
750 ml of DI water was added. Shortly thereafter, 20 ml
of 30% H2O2 was added to the mixture and the colour of
the mixture changed to brilliant yellow and it began bub-
bling. The mixture was filtered and washed with 0.5 M
HCl to remove metal ions, followed by 500 ml of DI water
to remove the acid. The resulting GO was dried in air.[36]
in past decades to be efficient catalysts for various reac-
tions, such as oxidation of alcohols, secondary amines
and carbon monoxide, hydrogenation of olefins and
nitroarenes, formation of nitrogen‐containing com-
pounds, etc.[31] In recent research, Au NP catalysts have
been utilized in carbon–carbon coupling reactions. These
coupling reactions have been established as convenient
and general approaches for the production of biaryls.
Biaryl scaffolds are ubiquitous building blocks in many
biologically active compounds, commercial dyes,
agrochemicals, natural products, conducting materials
and pharmaceutical organic compounds.[32]
In conjunction with our research aimed at the
preparation of metal–graphene composite catalysts for
organic reactions,[33–35] herein we report a novel protocol
for the Ullmann homocoupling reaction catalysed by Au
NPs@3DG‐SO3H composite in aqueous solution. The
preparation of the Au NPs@3DG‐SO3H nanocomposite
is described in Scheme 1. GO as a starting material was
synthesized from commercial graphite using the modified
Hummers method.[36] The 3DG was assembled with GO
sheets by hydrothermal treatment with the assistance of
thiourea.[37] The 3DG‐SO3H nanocomposite was synthe-
sized by anchoring sulfonic acid groups on 3DG. Then,
Au NPs were decorated onto the 3DG‐SO3H through
spontaneous chemical reduction of HAuCl4 by 3DG.
2.2 | Preparation of 3DG
The 3DG was synthesized by hydrothermal treatment of
GO suspension with the assistance of thiourea. Firstly,
thiourea (1 g) was added to dispersed GO (0.1 g,
100 ml), and then the mixture was sealed in a Teflon‐lined
stainless steel autoclave and maintained at 180 °C for
4.5 h. After that, the graphene assembly was dipped into
DI water for 24 h to remove of the residual thiourea.
Finally, vacuum freeze‐drying was used to afford the
3DG structure.[37] The loading of sulfur was determined
to be 1.82 mmol g−1 from elementary analysis (CHNS).
2 | EXPERIMENTAL
2.1 | Preparation of GO
Graphite powder (2.5 g) was first treated with a mixture of
12.5 ml of concentrated H2SO4 with 2.5 g of K2S2O8 and
2.5 g of P2O5. The mixture was kept at 80 °C for 6 h.
Subsequently, the mixture was cooled to room tempera-
ture and diluted with 500 ml of deionized (DI) water
and left overnight. The mixture was then filtered and
washed with DI water to remove the residual acid. The
product was dried under ambient conditions overnight.
2.3 | Preparation of 3DG‐SO3H
A 4‐sulfobenzenediazonium salt was used for sulfonation,
which was prepared from the reaction of sulfanilic acid
(2 mmol), NaNO2 (2.2 mmol), HCl solution (1 M, 10 ml)
and water (20 ml) in an ice bath. The diazonium salt solu-
tion was added to a dispersion of 3DG (40 mg, 20 ml) in
an ice bath under stirring, and was kept in an ice bath for
SCHEME 1 Preparation of au NPs@3DG‐SO3H nanocomposite

DABIRI ET AL.
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2 h. Then, the mixture was washed with H2O–EtOH (1:1).
Also, the acid capacity of the functionalized solid carbon
was measured according to standard acid–base titration of
ion‐exchanged materials in which 50 mg of catalyst was
suspended into 50 ml of NaCl (2 M) for 24 h[38] (acidity of
0. 71 mmol g−1)). The loading of sulfur was determined to
be 2.65 mmol g−1 from elementary analysis (CHNS).
1,1′‐Biphenyl (2a). White solid; m.p. 65–66 °C (lit.[39]
67–69 °C). H NMR (300 MHz, DMSO‐d6, δ, ppm): 7.68–
7.65 (m, 4H), 7.47 (t, J = 7.7 Hz, 4H), 7.39–7.34 (m, 2H).
1
4,4′‐Dimethyl‐1,1′‐biphenyl (2b). White solid; m.p.
117–119 °C (lit.[39] 118–120 °C). 1H NMR (300 MHz,
CDCl3, δ, ppm): 7.52–7.49 (m, 4H), 7.28–7.25 (m, 4H),
2.41 (s, 6H).
2.4 | Preparation of Au@3DG‐SO3H
3DG‐SO3H (200 mg) was dispersed in 20 ml of water.
Then, 0.065 ml of HAuCl4 (0.039 M) was added to the
3DG‐SO3H suspension under stirring and kept for 2 h at
room temperature with constant stirring. The resulting
mixture was washed with DI water and ethanol, and dried
at room temperature. Inductively coupled plasma analysis
gave the actual gold content as 0.20 wt% for the Au
NPs@3DG‐SO3H nanocomposite.
4,4′‐Dimethoxy‐1,1′‐biphenyl (2c). White solid; m.p.
170–172 °C (lit.[39] 173–175 °C). 1H NMR (300 MHz,
2.5 | General procedure for Ullmann
reaction
Aryl iodide (2 mmol), K3PO4 (6 mmol), water (4 ml) and
Au NPs@3DG‐SO3H nanocomposite (1 mol% Au) were
mixed in a 10 ml round‐bottom flask. The resulting mix-
ture was refluxed at 100 °C, and the reaction monitored
by TLC. After the completion of the reaction, the reaction
mixture was cooled to room temperature and ethyl acetate
was added to the flask. The organic phase was dried over
anhydrous MgSO4 and filtered. The crude reaction
mixture was concentrated under vacuum. The residue
was purified by silica gel TLC using n‐hexane–ethyl
acetate as eluent to afford the pure product.
2.6 | General procedure for reduction of
p‐Nitrophenol (p‐NP)
FIGURE
NPs@3DG‐SO3H
1
Raman spectra of 3DG, 3DG‐SO3H and Au
An amount of 30 ml of p‐NP (0.12 mM) was mixed with
30 ml of a freshly prepared aqueous NaBH4 solution
(0.17 M). Then, Au NPs@3DG‐SO3H nanocomposite
(1 mol% Au) was added to the resulting solution, and
the reaction conversion was determined using UV–visible
spectroscopy.
2.7 | Analytical data
Compounds 2a,[39] 2b,[39] 2c,[39] 2d,[40] 2e,[41] 2f[39] and
2 g[39] are known compounds and have been reported
previously.
FIGURE 2 TGA plots of 3DG, 3DG‐SO3H and Au NPs@3DG‐
SO3H

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DABIRI ET AL.
DMSO‐d6, δ, ppm): 7.54–7.51 (m, 4H), 7.00–6.96 (m, 4H),
3.76 (s, 6H).
1,1′‐([1,1′‐Biphenyl]‐4,4′‐diyl)bis(ethan‐1‐one) (2d).
White solid; m.p. 192–194 °C (lit.[40] 194–195 °C). 1H
NMR (300 MHz, CDCl3, δ, ppm): 8.10–8.07 (m, 4H),
7.76–7.73 (m, 4H), 2.69 (s, 6H).
4,4′‐Dimethyl‐1,1′‐biphenyl (2f). Oil.[39] 1H NMR
(300 MHz, DMSO‐d6, δ, ppm): 7.30–7.19 (m, 6H),
7.05–7.2 (m, 2H), 1.97 (s, 6H).
3 | RESULTS AND DISCUSSION
Raman spectroscopy is a useful tool for the characteriza-
tion of carbon materials and provides information on
the structure, quality and electronic properties of
graphene materials. The Raman spectra of 3DG, 3DG‐
SO3H and Au NPs@3DG‐SO3H are shown in Figure 1.
The intensity ratio of G‐band and D‐band (ID/IG) is a
general parameter, reflecting the carbon hybridization
state of materials and the degree of disorder.[42] ID/IG
decreased from 1.07 for 3DG to 1.04 for 3DG‐SO3H,
FIGURE 3 XRD patterns of 3DG, 3DG‐SO3H and Au NPs@3DG‐
SO3H
FIGURE 4 SEM image of (a) 3DG, (b) 3DG‐SO3H and (c) Au NPs@3DG‐SO3H; corresponding quantitative EDS elemental mapping of (d)
au, (e) S and (f) N

DABIRI ET AL.
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which can be explained the linkage of the arylsulfonic
acid on 3DG and the increasing sp2 carbons on 3DG‐
SO3H compared to 3DG. After decoration with Au
NPs, ID/IG increased to 1.11 that indicated the probable
chemical interaction or bonding between the Au NPs
and 3DG.[43]
Figure 2. The TGA curve of Au NPs@3DG‐SO3H
nanocomposite shows high thermal stability. The total
weight lost was only 7.5% at temperatures below 350 °C.
Additionally, the rate of weight loss for the Au
NPs@3DG‐SO3H nanocomposite increased giving 3DG‐
SO3H composite at temperatures greater than 400 °C. This
result shows the catalytic activity of Au NPs in the
oxidative degradation processes.
Thermogravimetric analysis (TGA) plots of 3DG,
3DG‐SO3H,and Au NPs@3DG‐SO3H are shown in
FIGURE 5 TEM images of au NPs@3DG‐SO3H nanocomposite
TABLE 1 Nitrogen adsorption–desorption analysis results for 3DG, 3DG‐SO3H and Au NPs@3DG‐SO3H
3DG
3DG‐SO3H
Au NPs@3DG‐SO3H
Surface area (m2 g−1
)
86.70
28.09
84.41
Average pore volume (cm3 g−1
)
0.22
14.8
0.09
13.3
0.21
13.9
Average pore size (nm)
FIGURE 6 Nitrogen adsorption–desorption isotherms of 3DG, 3DG‐SO3H and Au NPs@3DG‐SO3H

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DABIRI ET AL.
The X‐ray diffraction (XRD) patterns of 3DG, 3DG‐
SO3H and Au NPs@3DG‐SO3H are shown in Figure 3.
The broad peak at 2θ = 20–30° of the graphite plane [0
0 2] is observed in the pattern of 3DG that indicated the
framework of 3DG is composed of few‐layer stacked
graphene sheets.[44] The weak peak at 2θ = 37.90°
corresponds to the [1 1 1] lattice plane of Au(0). Also,
the peaks at 2θ = 44° and 64° correspond to the stainless
steel sample holder of the powder diffractometer.
The three‐dimensional morphology of 3DG was
confirmed using scanning electron microscopy (SEM)
characterization (Figure 4). 3DG, 3DG‐SO3H and Au
NPs@3DG‐SO3H exhibit typical three‐dimensional
porous networks. Additionally, energy‐dispersive X‐ray
spectroscopy (EDS) mapping shows that the elements
Au, S and N are found to be uniformly dispersed on the
whole surface of the Au NPs@3DG‐SO3H nanocomposite
(Figure 4d–f).
Transmission electron microscopy (TEM) images of
the Au NPs@3DG‐SO3H nanocomposite are shown in
Figure 5. The cross‐linked graphene sheets are also folded
or crinkled, as is evident from the TEM images. The
surface of 3DG‐SO3H is covered with a good dispersion
of Au NPs with an average size of ca 20 nm. Additionally,
Au NPs are not found outside of the 3DG‐SO3H sheets.
Nitrogen sorption isotherms were used to investigate
the surface areas and pore structures of the as‐prepared
composite. The Brunauer–Emmett–Teller surface area,
average pore volume and average pore size are summa-
rized in Table 1. After sulfonation, the surface area
markedly decreased from 86.70 to 28.09 m2 g−1 that can
be attributed to the sulfonic acid groups which are
capable of hydrogen bonding, promoting intermolecular
interactions between neighbouring sheets, and resulting
in a reduction in the free volume.[45] After decoration of
Au NPs on 3DG‐SO3H, the surface area increased to
84.41 m2 g−1, which can be attributed the Au NPs acting
as spacers in the 3DG network thereby preventing
agglomeration and hydrogen bonding between
neighbouring sheets.[35] As shown in Figure 6, the
nitrogen sorption isotherms of the three materials are of
type IV.
The chemical composition of the Au NPs@3DG‐SO3H
nanocomposite was characterized using X‐ray photoelec-
tron spectroscopy (XPS). As shown in Figure 7a, peaks
corresponding to Au 4f, C 1 s, N 1 s, O 1 s and S 2p are
observed in the XPS full spectra. The N 1 s core level
spectrum can be deconvoluted into four component peaks
at binding energy of about 398.64, 399.77, 400.68 and
402.05 eV, corresponding to pyridinic N, pyrrolic N, qua-
ternary N and N‐oxide of pyridinic nitrogen, respectively
(Figure 7b).[46] The core level region of S 2p includes four
components (Figure 7c). The contributions at binding
FIGURE 7 (a) Full range XPS spectrum of Au NPs@3DG‐SO3H
nanocomposite; (b) N 1 s, (c) S 2p and (d) Au 4f core level region
XPS spectra of au NPs@3DG‐SO3H nanocomposite

DABIRI ET AL.
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energies of around 163.61 and 164.57 eV can be attributed
to the bonding sulfur in C―Sn―C (n = 1 or 2) bonds and
conjugated ―C═S― bonds (should be from thiophene‐
type), respectively. The other two contributions at higher
and lower binding energies of around 167.97 and
161.48 eV can be assigned to the (―SO3) and reduced
(―SH) sulfur moieties, respectively.[47] The core level
region of Au 4f for the Au NPs@3DG‐SO3H nanocompos-
ite shows two peaks at 84.01 and 87.68 eV which are
however, the yield decreased when the amount of gold
was reduced to 0.5 mol% (Table 2, entries 5 and 6). The
effect of temperature on the reaction was studied. Several
experiments were performed at 80, 100 and 120 °C
(Table 2, entries 1, 7 and 8). The optimal reaction temper-
ature is 100 °C (Table 2, entry 4). It is also noteworthy
that, when this reaction was carried out with 3DG or
3DG‐SO3H, we failed to isolate any coupled product
(Table 2, entries 9 and 10).
attributed to the binding energy of Au 4f7/2 and Au 4f5/2
,
Having obtained the optimal reaction conditions, we
set out to explore the scope of the reaction with various
aryl iodides. Aryl iodides bearing electron‐donating
groups (p‐Me and p‐OMe) reacted to give higher yield
than aryl iodides possessing electron‐withdrawing groups
(p‐COMe and p‐NO2) (Table 3, entries 2–5). The hindered
substrate (2‐iodotoluene) converted to the corresponding
homocoupling product with moderate yield (Table 3,
entry 6). Furthermore, it was observed that Au
NPs@3DG‐SO3H could also act as an excellent catalyst
for the homocoupling of 1‐iodonaphthalene to yield 1,1′‐
binaphthyl with 92% conversion (Table 3, entry 7). Under
the same reaction conditions, the homocoupling of
bromobenzene and aryl bromide bearing electron‐donat-
ing and electron‐withdrawing groups failed to form
homocoupled products (Table 3, entries 8–10).
respectively (Figure 7d). This result shows the spontane-
ous reduction of HAuCl4 on 3DG surface, which involved
the pervious spontaneous reduction of HAuCl4 on
graphene sheets.[31]
The catalytic activity of the Au NPs@3DG‐SO3H
composite as a catalyst was tested in the Ullmann
homocoupling reaction. To optimize the reaction condi-
tions, the homocoupling of iodobenzene was used as a
model reaction in water as solvent at 100 °C with K3PO4
as the base and with a catalyst loading of 1 mol% of Au
(Table 2). Under these conditions, we found the
homocoupling reaction proceeds well, affording a promis-
ing high yield (93%) of the corresponding biphenyl
(Table 2, entry 1). Various types of bases such as K2CO3,
Cs2CO3 and KOH were tested, and they showed low effi-
ciency and afforded low yields (Table 2, entries 1–4).
Additionally, the loading of gold was optimized. An
amount of 2 mol% of gold did not change the yield;
Reusability is an important characteristic of heteroge-
neous catalysis which should be examined in catalytic
reactions. Therefore, we performed a reusability test for
TABLE 2 Effect of various bases on Ullmann coupling of iodobenzenea
Entry
Base
Temperature (°C)
Yield (%)b
1
K3PO4
100
93
2
K2CO3
Cs2CO3
KOH
100
100
100
100
100
80
21
3
58
4
74
5c
6d
7
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
94
59
62
8
120
100
100
93
9e
10ff
Trace
Trace
aIodobenzene (1.0 mmol), base (3.0 mmol), Au NPs@3DG‐SO3H (1.0 mol% Au), H2O (4 ml).
bGC yield; n‐dodecane used as an internal standard.
cAu NPs@3DG‐SO3H (2.0 mol% gold).
dAu NPs@3DG‐SO3H (0.5 mol% gold).
e3DG (50 mg).
f3DG‐SO3H (50 mg).

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DABIRI ET AL.
TABLE 3 Homocoupling of aryl halides catalysed by Au NPs@3DG‐SO3Ha
Entry
Aryl halide
Product
Yield (%)b
1
93
2
3
91
92
4
5
86
83
6
7
78
92
8
2a
2b
2e
Trace
Trace
Trace
9
10
aAryl halide (2.0 mmol), K3PO4 (3.0 mmol), Au NPs@3DG‐SO3H (1 mol%), H2O (4 ml), 100 °C, 18 h.
bIsolated yield.
the Au NPs@3DG‐SO3H composite in the Ullmann
homocoupling of iodobenzene under the optimized condi-
tions. After reaction completion, the catalyst was collected
by centrifugation. By adding a new batch of iodobenzene
and K3PO4, the recovered composite can be used directly
for the next run. This recycling protocol enabled Au
NPs@3DG‐SO3H to be used for six runs furnishing
quantitative conversion of the substrate without loss of
activity and selectivity. Figure 8a depicts results for the
recovery and reusability of the Au NPs@3DG‐SO3H
composite. Furthermore, no significant change was
observed in the morphology of the Au NPs@3DG‐SO3H
nanocomposite after the six runs, which was concluded
from the TEM image in Figure 8b. The heterogeneous

DABIRI ET AL.
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FIGURE 8 (a) Recycling of Au NPs@3DG‐SO3H composite in
Ullmann homocoupling of iodobenzene. (b) TEM image of the
composite after six runs
nature of the catalyst was examined using atomic absorp-
tion spectroscopy. The amount of gold leaching from the
supported catalyst was thus determined, and the result
indicated that gold concentration was less than 2.0 ppm
in the reaction solution. Inductively coupled plasma
analysis of the used Au NPs@3DG‐SO3H composite
catalyst indicated leaching of 1.9% of gold in the Ullmann
homocoupling of iodobenzene after the six cycle.
Additionally, the hot filtration test (ca 50% conversion of
FIGURE 9 (a) UV–visible spectra of formation p‐nitrophenolate
after addition NaBH4 solution to p‐NP solution. (b) Time‐
dependent UV–visible absorption for reduction of p‐NP in the
presence of Au NPs@3DG‐SO3H nanocomposite. (c) Plot of ln(Ct/
C0) versus reaction time for reduction of p‐NP catalysed by Au
NPs@3DG‐SO3H nanocomposite
TABLE 4 Comparison of efficiency of gold heterogeneous catalysts in Ullmann homocoupling of iodobezene
Catalyst
Size of Au NPs (nm)
20
Conditions
Time (h)
18
Yield (%)
93
Ref.
Au NPs@3DG‐SO3H
K3PO4, H2O, 100 °C
This work
[30]
Au/Fe3O4/s‐G
AuNPs‐RGO
Au@PMO
20–35
10–20
3–15
1.3
K3PO4, H2O, 100 °C
K3PO4, NMP, 100 °C
K3PO4, NMP, 100 °C
K2CO3, DMF, 130 °C
K2CO3, DMF, 140 °C
48
6
95
[31]
[48]
[49]
[50]
97
16
48
48
95
Au25(SR)18/CeO2
NAP‐mg‐au
99.8
92
5–7

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DABIRI ET AL.
TABLE 5 Reusability of Au NPs@3DG‐SO3H nanocomposite in
reduction of p‐NPa
Additionally, the catalyst was chemically stable and could
be recycled at least six times in the corresponding reaction
without a reduction in the catalytic activity and could be
separated by simple centrifugation and reused for further
reaction cycles.
Reaction cycle
1
2
3
4
5
6
Yield (%)b
98
98
97
95
95
92
a30 ml of 0.12 mM p‐NP, 30 ml of 0.17 M NaBH4 and Au NPs@3DG‐SO3H
nanocomposite (1 mol% gold).
bYield determined by GC analysis.
ORCID
Minoo Dabiri http://orcid.org/0000-0002-7086-4010
iodobenzene) was used. The hot filtrates were transferred
to another flask containing water at 100 °C. Upon further
heating of catalyst‐free solution for 16 h, no considerable
progress (ca 3% by GC analysis) was observed. These
results confirmed the heterogeneous character of the cat-
alytically active species in this reaction.
A comparison was made of the present method with
other reported heterogeneous gold catalytic systems in
the Ullmann homocoupling of iodobenzene and the
results are presented in Table 4. Clearly, the present
method represents a simple, green (using water as
solvent), highly effective, less time‐consuming method
for the Ullmann homocoupling reaction.
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Additionally, the catalytic activity of the Au
NPs@3DG‐SO3H nanocomposite was tested for the
reduction of p‐NP using NaBH4 as a reducing reagent.
After the addition NaBH4 solution to p‐NP solution, the
formation p‐nitrophenolate in the alkaline medium
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