Graphene-supported Cu2O nanoparticles: An efficient heterogeneous catalyst for C-O cross-coupling of aryl iodides with phenols

Article abstract of DOI:10.1039/c4cy01048e

Cu₂O/graphene as a heterogeneous catalyst can effectively ignite and catalyze the Ullmann C-O cross-coupling of aryl iodides with phenols under mild conditions. The yield of diphenyl ether from the cross-coupling of phenol and iodobenzene can reach up to 96% at 150 °C in 3 h, and the turnover frequency can be as high as 1282 h^-1. Meanwhile, the catalyst exhibits activity for varieties of C-O cross-coupling of aryl iodides, bromides and chlorides with phenol derivatives to form the corresponding aryl ethers. This journal is

Full text of DOI:10.1039/c4cy01048e

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Graphene-supported Cu O nanoparticles: an
2
efficient heterogeneous catalyst for C–O
Cite this: DOI: 10.1039/c4cy01048e
cross-coupling of aryl iodides with phenols†‡
Received 12th August 2014,
Accepted 3rd September 2014
ab
a
a
a
Zhaoyang Zhai, Xiaoning Guo, Zhifeng Jiao, Guoqiang Jin
a
and Xiang-Yun Guo*
DOI: 10.1039/c4cy01048e
www.rsc.org/catalysis
Cu
2
O/graphene as a heterogeneous catalyst can effectively ignite
the availability of nanostructures and nanoporous solids with
very large surface areas, which lead to increased yields.
Indeed, more and more studies suggest that the coupling of
phenols with aryl halides could be achieved by heterogeneous
4
and catalyze the Ullmann C–O cross-coupling of aryl iodides with
phenols under mild conditions. The yield of diphenyl ether from
the cross-coupling of phenol and iodobenzene can reach up to
9
6% at 150 °C in 3 h, and the turnover frequency can be as high as
catalytic routes. Kim et al. reported that Cu O nanocubes with
a size of ~45 nm can catalyze the formation of biaryl ethers.
2
−
1
5
1
282 h . Meanwhile, the catalyst exhibits activity for varieties of
C–O cross-coupling of aryl iodides, bromides and chlorides with
Ling et al. reported that CuO supported by solid oxides
phenol derivatives to form the corresponding aryl ethers.
can catalyze the coupling reaction under ligand-free condi-
tions. Saberi et al. prepared carbon nanotube-supported
6
α-Fe O @CuO composites and found that the magnetic feature
2
3
Introduction
7
of composites helped facile separation of the target ethers.
These heterogeneous catalysts usually achieve a relatively low
Diaryl ethers are widely employed as intermediates in organic
synthesis, and thus their synthesis has attracted considerable
attention for decades. Traditionally, they are prepared
through the Ullmann coupling of phenols with aryl halides
promoted by copper reagents, which usually are stoichiomet-
ric and result in low or moderate yields. Homogeneous cata-
−
1 7
turnover frequency (TOF) of less than 1 up to 50 h .
Cu O, CuO and Cu were found to be active for the
1
2
5
,6,8
Ullmann coupling reactions.
Buchwald and Bolm, trace amounts of Cu O in salts of other
According to the study of
9
2
2
metals can significantly increase the yield of the target prod-
lytic routes using palladium or copper complexes can greatly
improve the efficiency of the coupling reactions. Taillefer
2
uct, and therefore, Cu O could be the catalytically active
3
phase for the reactions. It is well known that nanoparticles
will have significantly increased surface area and thus cata-
lytic activity when their size is reduced to several nanometers.
et al. observed that the efficiency of the copper-catalyzed
arylation of phenols from aryl chlorides could be greatly
increased by addition of 2,2,6,6-tetramethyl-3,5-heptanedione
Unfortunately, nanosized Cu O particles are easily oxidized
2
3
c
in air or moist conditions. Our recent work shows that
as the ligand. However, the catalysts employed in the above
routes are homogeneous and require ligands, and these lead
to the difficulty in recycling and reuse of catalysts and
2
graphene, a two-dimensional grid of sp -bonded carbon
atoms with highly delocalized electrons, can prevent the oxi-
3
e
10,11
dation of Cu O and Cu nanoparticles.
The graphene-
increase the production cost. Therefore, more economical
and flexible routes are still required to improve the synthesis
processes. Compared with the homogeneous processes, the
routes employing recyclable heterogeneous catalysts would be
efficient and environmentally friendly. Additionally, the
improved technologies in materials chemistry have furnished
2
2
supported Cu O or Cu nanoparticles can exert stable catalytic
functions for oxygen reduction reaction and coupling of
nitrobenzene, respectively. It is therefore expected that nano-
2
sized Cu O particles may have higher activity for this type of
Ullmann coupling reactions. Herein we report that the Cu O
2
nanoparticles supported on graphene exhibit tremendous cat-
alytic activity and recyclability for the Ullmann coupling reac-
tions of phenols with aryl halides.
a
State Key Laboratory of Coal Conversion, Institute of Coal Chemistry,
Taiyuan 030001, China. E-mail: xyguo@sxicc.ac.cn; Fax: +86 351 4065282
b
University of the Chinese Academy of Sciences, Beijing 100039, China
The Cu O/graphene catalyst with a Cu O loading of 5 wt.%
2
2
†
2
‡
The work was financially supported by SKLCC (ref. 2013BWZ006 and
was prepared via a two-step liquid-phase method using
diethylene glycol as the reducing agent (see the ESI‡). Trans-
014BWZ006).
Electronic supplementary information (ESI) available. See DOI: 10.1039/
c4cy01048e
mission electron microscopy (TEM) images of the Cu O/
2
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−
1
graphene catalyst are presented in Fig. 1A and B. Homoge-
highest TOF of 49.73 h for the cross-coupling of iodo-
7
neous Cu O nanoparticles with mean diameter of about
benzene and phenol. Under the same reaction conditions,
2
8
nm are uniformly dispersed on the surface of graphene
2
the TOF of Cu O/graphene catalyst is about 25 times higher
sheets. The lattice fringe of the nanoparticles has an inter-
planar spacing of 0.25 nm, which is in agreement with the
than that of the nanocomposite. No product was detected
in the absence of Cs CO . It is mainly because that Cs CO
2
3
2
3
(
(
111) plane of cubic Cu
XRD, Fig. 1C), all of the strong diffraction peaks can be
2
O. In the X-ray diffraction patterns
can enhance the nucleophilicity of phenols and accelerate
the detachment of H species in phenols and therefore pro-
mote the coupling reaction.
indexed to the (111), (200), (220), (311) lattice planes of
Cu O, confirming that Cu exists on the graphene sheets pri-
marily as the cuprous phase. X-ray photoelectron spectros-
copy (XPS) of the catalyst can further confirm it. In Fig. 1D,
the binding energies of Cu 2p3/2 at around 932.8 eV and Cu
2
To explore the influence of different substituents on the
coupling reactions of aryl halides with phenols, we employed
various phenols to react with different iodobenzene deriva-
tives. The presence of electron-donating groups such as
methoxy or methyl in iodobenzene can slightly decrease the
reaction yield (Table 1, entries 2–4). In contrast, the electron-
withdrawing groups such as acetyl or nitro in iodobenzene
can increase the reaction yield (entries 5–6). However, the
influences of electron-donating (entries 7–8) and electron-
withdrawing groups (entries 9–10) in phenol are opposite to
those in the aryl halides. When the electron-donating groups
are simultaneously presented in iodobenzene and phenol,
the reaction yield will further decrease (entry 11).
+
0
2
p1/2 at 952.2 eV can be attributed to the Cu and Cu states,
1
2
respectively. The existence of metallic phase Cu indicates
that the catalyst precursor was over-reduced in the prepara-
tion process. Our previous work has demonstrated that
metallic Cu nanoparticles can stably exist on the graphene
sheets. The XPS results presented here further suggest that
some of the metastable nanoparticles can be stabilized by
using graphene as the support.
1
1
We tested several Cu
2
O/graphene catalysts with different
Cu O loadings and found that the 5 wt.% Cu O/graphene
showed the best catalytic activity for the cross-coupling of
phenol and iodobenzene. Therefore, all of the reactions in
Besides aryl iodides, the catalyst can also catalyze the cou-
pling reactions of aryl bromides and chlorides with phenol
derivatives to produce the corresponding aryl ethers. From
the results shown in Table 2, the catalyst showed lower activ-
ity for aryl bromides and chlorides due to their low reactivity
2
2
this work were performed over the 5 wt.% Cu O/graphene
2
catalyst. The reaction results for the cross-coupling of phe-
nols and iodides are summarized in Table 1. All of the reac-
tions were conducted in a sealed reactor under the protection
of argon. The yield of diphenyl ether from the cross-coupling
of phenol and iodobenzene is 96% and the TOF is high, up
4
(entries 1–4).
For comparison, we also prepared graphene-supported Cu
and CuO nanoparticles as the catalysts for the cross-coupling
reaction of phenol and iodobenzene. The CuO/graphene was
prepared by a facile impregnation method. The Cu/graphene
catalyst here was completely the same as that used in our
−
1
to 1282 h (Table 1, entry 1). Saberi et al. collected the TOF
results over different catalysts and found that the carbon
1
1
nanotube-supported α-Fe
2
O
3
@CuO nanocomposite has the
previous work. Under the same reaction conditions, the
TOF values over 5 wt.% Cu/graphene and 5 wt.% CuO/
graphene are 291 (49% iodobenzene conversion) and 393
(
53%), respectively. When only graphene was used as the cat-
alyst, only 3% yield of diphenyl ether was detected. These
results suggest that graphene-supported Cu or CuO nano-
particles also possess considerable catalytic activity for the
cross-coupling. However, the TOF values of both catalysts are
further lower than that of the Cu O/graphene catalyst. There-
2
fore, the active component in the catalyst for the Ullmann
cross-coupling should be Cu O.
2
We investigated the yield of diphenyl ether from the
reaction of phenol and iodobenzene at different times and
found that the reaction just reached its equilibrium at 3 h.
Therefore, the product yield at 3 h was monitored for five suc-
cessive rounds to test the recyclability of the Cu
catalyst. A slight decrease in the catalytic activity was detected
Fig. 2A). The product yield in the first round was 96%, and it
2
O/graphene
(
decreased to 94.4% after 5 rounds. The TEM image (Fig. 2B)
of the catalyst after 5-round usage shows no obvious change
in morphology or aggregation of the Cu
2
O nanoparticles. The
mean diameter of the Cu O particles is still about 8 nm. The
2
Fig. 1 TEM images (A, B), XRD pattern (C) and XPS profile (D) of the
wt.% Cu O/graphene catalyst, and the size distribution of Cu
above results indicate that the Cu O/graphene catalyst has
5
2
2
O
2
nanoparticles (inset of A).
excellent catalytic stability in the reaction.
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Table 1 C–O cross-coupling of aryl iodides with phenols over the Cu
Communication
O/graphene catalysta
2
b
−1
Entry
1
Phenols
Aryl halides
Product
Yield (%)
TOF (h
1282
)
96
90
87
2
3
1202
1162
4
5
6
7
87
98
99
97
1162
1309
1322
1295
8
9
98
85
83
82
1309
1135
1108
1095
1
1
0
1
a
The reactions were conducted in a sealed argon atmosphere at 150 °C for 3 h using 0.014 mol of aryl iodides, 0.014 mol of phenols, 10 mL of
CO , and 10 mg of 5 wt.% Cu O/graphene catalyst. Conversions were based on the amount of
substituted aryl halides used. TOF was calculated based on the amount of Cu
tetrahydrofuran (THF), 1.5 equiv. of Cs
2
3
2
b
2
O. Obtained from a Bruker GC-MS.
Table 2 C–O cross-coupling of aryl bromides and chlorides with phenols over the Cu
2
O/graphene catalysta
b
−1
Entry
1
Phenols
Aryl halides
Product
Yield (%)
TOF (h
347
)
26
2
57
761
3
4
20
5
267
67
a
b
The reaction conditions were the same as those in Table 1. Obtained from a Bruker GC-MS.
which can efficiently catalyze the C–O cross-coupling
reactions of aryl halides and phenols based on the Ullmann
Conclusions
In summary, the present study demonstrates a new class of
method under mild conditions. Cu O nanoparticles with a
2
heterogeneous catalysts, graphene-supported Cu O nanoparticles,
mean size of 8 nm are homogeneously dispersed on the
2
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2
3
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Fig. 2 Recyclability of the 5 wt.% Cu O/graphene for the cross-
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2
−1
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2
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Catal. Sci. Technol.
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