Supported CuO/γ-Al2O3 as heterogeneous catalyst for synthesis of diaryl ether under ligand-free conditions

Article abstract of DOI:10.1016/j.molcata.2012.01.028

CuO/γ-Al₂O₃ was employed as catalyst in the coupling reaction of iodobenzene with m-cresol to give 3-methyldiphenyl ether under ligand-free conditions. It performed more efficiently than other supported CuO catalysts (CuO/SBA-15, CuO/ZrO₂, CuO/CeO₂, CuO/La₂O₃). Reaction conditions were investigated and the yield reached 76% under the optimized terms. As a heterogeneous catalyst, CuO/γ-Al₂O₃ could be reused for three cycles with moderate to good yields. According to the results of XRD, TPR and XPS, the catalytic activities of supported CuO catalysts were due to the highly dispersed Cu(II) species and independent of the electronic state of these copper species.

Full text of DOI:10.1016/j.molcata.2012.01.028

Journal of Molecular Catalysis A: Chemical 357 (2012) 112–116
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Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Supported CuO/␥-Al2O3 as heterogeneous catalyst for synthesis of diaryl ether
under ligand-free conditions
Pengxiang Ling, Dao Li∗, Xingyi Wang
Key Laboratory for Advanced Materials, Research Institute of Industrial Catalysis, East China University of Science and Technology, Shanghai 200237, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
CuO/␥-Al2O3 was employed as catalyst in the coupling reaction of iodobenzene with m-cresol to give 3-
methyldiphenyl ether under ligand-free conditions. It performed more efficiently than other supported
CuO catalysts (CuO/SBA-15, CuO/ZrO2, CuO/CeO2, CuO/La2O3). Reaction conditions were investigated
and the yield reached 76% under the optimized terms. As a heterogeneous catalyst, CuO/␥-Al2O3 could
be reused for three cycles with moderate to good yields. According to the results of XRD, TPR and XPS,
the catalytic activities of supported CuO catalysts were due to the highly dispersed Cu(II) species and
independent of the electronic state of these copper species.
Received 5 November 2011
Received in revised form 9 January 2012
Accepted 28 January 2012
Available online 6 February 2012
Keywords:
Ullmann coupling reaction
Heterogeneous catalysis
CuO/␥-Al2O3 Catalyst
Active species
© 2012 Elsevier B.V. All rights reserved.
1. Introduction
Recently, Cu, CuO and CuI nanoparticles as well as Cu2O
nanocube were explored for the coupling of aryl iodides and aryl
Diaryl ethers are important structures in numerous biologi-
cally active compounds, natural products and polymers [1]. The
traditional synthetic method for the synthesis of diaryl ethers
was classic Ullmann coupling reaction by using aryl halides
and phenols as substrates and copper as catalyst. However, this
application was limited by harsh reaction conditions such as
high temperature (>200 ◦C) and stoichiometric amount of copper
reagent which consequently led to the problem of waste disposal
[2–5].
bromides with phenols [12–16]. The reactions took place without
ligand and the catalysts were reusable. However, the production
and the separation of nanomaterials in industrial-scale process still
remain problem.
Due to easy separation and efficient recyclability, immobi-
lized catalysts received much attention in industrial applications.
Recently, Miao and Wang [17] developed a catalytic system for
the preparation of diaryl ethers by using organic–inorganic (sil-
ica) hybrid materials-supported copper as catalyst. The drawback
of this procedure is the intricacy of the synthesis of silica-
supported copper. Lipshutz et al. [18] reported a system based
on the use of charcoal as support. To construct diaryl ethers
with Cu/C catalyst, they explored the reaction of aryl bro-
mides and phenols in aid of microwave which greatly reduced
the reaction time. Nonetheless, ligand was required to achieve
high yield. Taillefer et al. [19,20] devised sol–gel immobilized
copper catalysts which were applied for the C O coupling of
aryl iodides and aryl bromides with phenols. However, the
preparation of this sol–gel immobilized copper catalyst is compli-
cated.
In order to develop an efficient and easy handling catalyst for
Ullmann coupling reaction, we investigated supported CuO cata-
lysts for the synthesis of diaryl ether under ligand-free conditions.
The support effect and reaction conditions on catalytic performance
of supported catalysts were explored. Characterizations of XRD, TPR
and XPS were carried out to discuss the active species of supported
CuO catalyst for Ullmann coupling reaction.
To overcome these limitations, much attention was paid
to the improvement of Ullmann-type coupling reaction and
significant progress has been achieved. In recent years,
a
large number of highly efficient ligands were successfully
developed for copper-catalyzed arylations, e.g., 1-naphthoic
acid [6], N,N-dimethylglycine [7], 8-hydroxyquinoline [8],
pyrrolidine-2-phosphonic acid phenyl monoester [9] and 2,2,6,6-
tetramethylheptane-3,5-dione [10,11]. These elaborate ligands
indeed promoted the cross-coupling reaction, but increased the
cost. Meanwhile, the procedures proceeded homogeneously were
problematic in terms of recovery/recycling of catalyst. From a
sustainable point of view, developing of inexpensive, ligand-free
and recyclable heterogeneous catalysts is highly desired.
∗
Corresponding author. Tel.: +86 21 64253372; fax: +86 21 64253372.
E-mail address: lidao@ecust.edu.cn (D. Li).
1381-1169/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2012.01.028

P. Ling et al. / Journal of Molecular Catalysis A: Chemical 357 (2012) 112–116
113
2. Experimental
3-methyldiphenyl ether and the results are summarized in Table 1.
Under heterogeneous conditions, steric hindrance of methyl on
m-cresol was thought to be a disadvantage for Ullmann cou-
pling reaction [13]. In our experiment, no product was detected in
absence of copper catalyst (Table 1, Entry 1). With pure CuO as cat-
alyst, reaction afforded low yield of 28% (Table 1, Entry 2). When
SBA-15 and ZrO2 were employed as supports, lower yields of 8%
and 19% were obtained and no improvement was achieved com-
pared with pure CuO (Table 1, Entries 3 and 4). The reaction yields
increased dramatically when CeO2, La2O3 and ␥-Al2O3 were used
as supports (Table 1, Entries 5–7). Amongst the catalysts screened,
CuO/␥-Al2O3 gave the highest yield of 76%.
2.1. Catalyst preparation
ZrO2, La2O3, and ␥-Al2O3 were purchased from Sinopharm
Chemical Reagent Co. Ltd. and used without further treatment.
SBA-15 was purchased from Shanghai Boyle Chemical Co. Ltd.
and used without further treatment. The support of CeO2 was
obtained by calcination of Ce(NO3)3·6H2O at 550 ◦C for 6 h. Prepa-
ration of supported CuO catalysts: 1 g support was impregnated
with Cu(NO3)2·3H2O aqueous solution (0.304 g Cu(NO3)2·3H2O
in a certain volume of water). The sample was placed at ambi-
ent temperature for 12 h, heated in static air at 80 ◦C for 6 h and
calcined at 650 ◦C for 6 h. CuO catalyst was obtained by calci-
nation of Cu(NO3)2·3H2O at 650 ◦C for 6 h. The copper content
in supported CuO catalysts (CuO/SBA-15, CuO/ZrO2, CuO/CeO2,
CuO/La2O3, CuO/␥-Al2O3) was calculated as 8 wt%.
The structural characteristics (BET surface area, pore volume
and average pore size) of supported CuO catalysts were measured
(listed in Table 1). The results showed that there was no correlation
between these properties and catalytic activity.
To optimize the reaction conditions, CuO/␥-Al2O3 was used
as catalyst and the effect of base, solvent, substrates mole ratio
and reaction time were tested (see Table 2). Obviously, base
showed a significant influence on the coupling reaction. The use
of carbonates (K2CO3, Cs2CO3, Na2CO3) did not lead to high yield
of 3-methyldiphenyl ether (Table 2, Entries 1–3). Cs2CO3 as a
strong base, though it was remarkable in homogeneous reaction
[6,7,9–11], only afforded 19% yield in heterogeneous reaction.
K3PO4, on the contrary, exhibited its effectiveness to give 76% yield
of product in combination with CuO/␥-Al2O3 heterogeneous cata-
lyst (Table 2, Entry 4), which was consistent with previous reports
[21–24]. In order to investigate the effect of solvent on the coupling
reaction, DMSO and N,N-dimethyl formamide (DMF) were taken
into account (Table 2, Entries 4 and 5). It can be seen that a lower
yield (15%) was obtained in DMF than in DMSO (76%). This might be
due to the unstability of DMF under high temperature and strong
basic circumstance. As to the mole ratio of iodobenzene to m-cresol,
61% yield was obtained with stoichiometric ratio (Table 2, Entry 8).
Increasing m-cresol to the mole ratio of iodobenzene to m-cresol
of 1:1.5, improvement of yield presented only 4% (Table 2, Entry
9). The further increasing m-cresol to the mole ratio of iodoben-
zene to m-cresol of 1:2 resulted in the slight decrease of the yield
(Table 2, Entry 10). However, yield reached 76% when iodobenzene
increased to the mole ratio of 1.5:1 and remained unchanged with
the mole ratio of 2:1 (Table 2, Entries 4 and 7). But further changing
of the ratio to 3:1, a lower yield of 72% was obtained (Table 2, Entry
6).
2.2. Catalyst characterizations
The Brunauer–Emmett–Teller (BET) specific surface area, pore
volume and average pore size were determined by physical adsorp-
tion of N2 at −196 ◦C in an automatic volumetric system using
Quantachrome NOVA instrument.
X-ray diffraction (XRD) spectra of catalysts were recorded in a
RINT2000 vertical goniometer, using Cu-K␣ radiation. Spectra were
registered between 20◦ and 80◦ (2Â) with an increment of 0.02◦ and
a scan-speed of 6◦/min.
Temperature programmed reduction experiments were carried
out with H2 (H2-TPR) in a Micromeritics device. 50 mg of catalyst
was heated at 10 ◦C/min from 30 ◦C to 600 ◦C under a 5% H2/N2
flow (45 ml/min), and the H2 consumption was monitored with a
thermal conductivity detector (TCD).
X-ray photoelectron spectrometer (XPS) characterization was
carried out in a Thermo ESCALAB 250 electron spectrometer
using Al-K␣ (1486.6 eV) radiation source. To obtain the XPS spec-
tra, the pressure of the analysis chamber was maintained at
5 × 10−10 mbar. The binding energy (B.E.) scale was adjusted by
setting the C 1s transition at 285.0 eV.
2.3. Catalytic activity measurement
A mixture of iodobenzene (3 mmol), m-cresol (2 mmol), catalyst
(0.032 g, 0.04 mmol Cu), K3PO4 (4 mmol) and dimethyl sulfoxide
(DMSO) (4 ml) in a sealed tube was stirred at 150 ◦C under N2
atmosphere for 10 h. Subsequently, the reaction was allowed to
cool to room temperature and the catalyst were filtered off and
washed with ethyl acetate (EtOAc). The excess EtOAc was evapo-
rated followed by adding tridecane (0.6 mmol) as internal standard.
All samples were analyzed by gas chromatography with flame ion-
ization detector (FID).
It was generally reported that it would take a long time to
accomplish Ullmann coupling reaction in homogeneous conditions
[7,11,21–24]. But, in our experiments, yield reached 65% within 4 h
and 74% at 8 h, which is much shorter than those in homogeneous
conditions (Table 2, Entries 11 and 12). After that, 76% yield was
obtained at 10 h and no increase could be observed at extended
time (Table 2, Entries 4 and 13).
3.2. Structure and surface characterization
2.4. Recycle of catalyst
XRD patterns of the supported CuO catalysts are shown in
Fig. 1. The diffraction peaks corresponding to copper-containing
phases could not be observed for CuO/␥-Al2O3 and CuO/La2O3
catalysts, while the intense reflections corresponding to CuO
at 35.5◦ and 38.8◦ were clearly presented for the catalysts of
CuO/ZrO2 and CuO/SBA-15. For CuO/CeO2 catalyst, the CuO diffrac-
tion peaks were relatively weak. The absence of CuO diffraction
peaks on CuO/␥-Al2O3 and CuO/La2O3 catalysts indicated the
formation of highly dispersed copper species on the supports
surfaces, which were not detectable by XRD. The diffraction
peaks of CuO on CuO/ZrO2 and CuO/SBA-15 manifested the for-
mation of bulk CuO on these catalysts surfaces. As to CuO/CeO2
catalyst, the relatively weak diffraction peaks could attribute to
After the completion of coupling reaction of iodobenzene with
m-cresol, the reaction mixture was filtered. The residue was
washed with EtOAc, ethanol and water. The solid was dried at 80 ◦C
for 6 h and calcined at 650 ◦C for 2 h. The obtained catalyst was used
for the next reaction of iodobenzene with m-cresol.
3. Results and discussion
3.1. Catalyst activity
CuO catalysts with different supports were tested with
the coupling reaction of iodobenzene and m-cresol to give

114
P. Ling et al. / Journal of Molecular Catalysis A: Chemical 357 (2012) 112–116
Table 1
Catalyst screening for Ullmann coupling reaction of iodobenzene with m-cresol.a
.
I
OH
O
CH3
Catalyst , K3PO4
DMSO , 150 , 10h
CH3
Entry
Catalysts
BET surface area (m2 g−1
)
Pore volume (cm3 g−1
)
Pore size (nm)
Yield (%)b
1
2
3
4
5
6
7
–
CuO
–
–
–
–
–
–
0
28
8
19
64
71
76
CuO/SBA-15
CuO/ZrO2
CuO/CeO2
CuO/La2O3
CuO/␥-Al2O3
278.3
3.3
45.5
15.9
133.7
0.359
0.019
0.145
0.048
0.424
3.5
2.5
7.8
12.0
7.9
Entries in bold are the best item (highest yield, optimized reaction conditions and high recyclability).
a
Reaction conditions: PhI (3 mmol), m-cresol (2 mmol), K3PO4 (4 mmol), catalysts (0.032 g, 0.04 mmol Cu), in DMSO (4 ml) at 150 ◦C under N2 for 10 h.
GC yield determined with tridecane as internal standard.
b
250oC
330oC
: CuO
CuO/SBA-15
CuO/SBA-15
CuO/ZrO2
CuO/ZrO2
CuO/CeO2
CuO/CeO2
CuO/La2O3
CuO/La2O3
CuO/ -Al2O3
CuO/ -Al2O3(after first cycle)
CuO/ -Al2O3
500 600
100
200
300
400
10
20
30
40
50
60
70
80
Temperature (oC)
2
Fig. 2. H2-TPR profiles of supported CuO catalysts.
Fig. 1. XRD patterns of supported CuO catalysts.
small CuO particles on CeO2 surface. Consequently, the disper-
sion of Cu species on different supports decreased in order of
CuO/␥-Al2O3 ≈ CuO/La2O3 > CuO/CeO2 > CuO/ZrO2 ≈ CuO/SBA-
15, which agreed with the change of catalyst activities.
Therefore, it can be assumed that highly dispersed copper
species are more active than bulk ones for Ullmann coupling
reaction.
of CuO/SBA-15 catalyst showed one peak centered at 330 ◦C, which
was attributed to the reduction of bulk CuO [25]. Compared with
CuO/SBA-15 catalyst, the reduction of CuO/ZrO2 appeared at lower
temperature (260 ◦C), which was definitely not caused by the dis-
persion according to the result of XRD. The reason for enhanced
reducibility of Cu2+ species might be caused by the redox prop-
erty of ZrO2 and the metal-support interaction between CuO and
ZrO2 [26]. On the contrary, the lower reduction peak of CuO/␥-
Al2O3 at 250 ◦C should be caused by the dispersion of copper
TPR experiments were conducted to study the reducibility of
CuO catalysts and the profiles are presented in Fig. 2. The profile
Table 2
Effect of reaction conditions on Ullmann coupling reaction of iodobenzene with m-cresol using CuO/␥-Al2O3 as catalyst.
Entry
Base
Solvent
Mole ratio of iodobenzene to m-cresol
Reaction time (h)
Yield (%)a
1
2
3
4
5
6
7
8
9
10
11
12
13
K2CO3
Cs2CO3
Na2CO3
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
DMSO
DMSO
DMSO
DMSO
DMF
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
1.5:1
1.5:1
1.5:1
1.5:1
1.5:1
3:1
10
10
10
10
10
10
10
10
10
10
4
35
19
11
76
15
72
76
61
65
62
65
74
76
2:1
1:1
1:1.5
1:2
1.5:1
1.5:1
1.5:1
8
24
Entries in bold are the best item (highest yield, optimized reaction conditions and high recyclability).
a
GC yield determined with tridecane as internal standard.

P. Ling et al. / Journal of Molecular Catalysis A: Chemical 357 (2012) 112–116
115
Table 3
Binding energy and relative ratios of surface Cu species derived from XPS results.
Entry
Samples
B.E. (eV)
Cu0/Cu1+
Ratio (%) of Cu0/Cu1+:CuO:Cu2+
CuO
Cu2+
1
2
3
4
5
6
7
CuO
CuO/SBA-15
CuO/ZrO2
CuO/CeO2
CuO/La2O3
–
–
933.9
933.8
933.7
933.7
933.7
933.7
933.7
–
0:100:0
0:69:31
2:23:75
9:30:61
21:79:0
2:20:78
4:32:64
936.1
934.9
935.1
–
935.2
935.0
932.5
932.5
932.2
932.5
932.5
CuO/␥-Al2O3
CuO/␥-Al2O3 (after first cycle)
species in accordance with XRD result, since ␥-Al2O3 does not show
redox property. The profile of CuO/La2O3 catalyst presented two
reduction peaks (250 ◦C and 325 ◦C), indicating that CuO species
were partially aggregated. However, the aggregated CuO was not
big enough to exhibit CuO reflection peaks with XRD. Similar to
CuO/La2O3, CuO on CuO/CeO2 was partially aggregated with big-
ger crystal size that exhibited weak CuO diffraction peaks with
XRD. The lower reduction peaks (136 ◦C and 179 ◦C) were due to
the strong metal-support interaction and redox property of CeO2
via electron transfer between the oxides [27]. Due to the different
properties of supports, the copper species showed different redox
capabilities. Obviously, there is no direct relationship between the
redox property and catalytic performance for supported CuO cata-
lysts.
Commonly, Cu+ was believed to be active in homogeneous
synthesis of diaryl ethers. However, Cu0, Cu+ and Cu2+ showed
capabilities to catalyze Ullmann coupling reaction in heterogeneity
without specific explanation. Here, XPS was performed to explore
the electronic state of catalytic centers of supported CuO cata-
lysts.
Fig. 3 shows the Cu 2p3/2 spectra of supported CuO catalysts.
The binding energy and relative ratios of surface Cu species derived
from XPS results are listed in Table 3. As reported [28–30], the peak
at 935 eV indicated the appearance of Cu2+ species strongly inter-
acting with support, while the peak at 933.7 eV was assigned to CuO
species and the one at 932.5 eV was related to low valence copper
species (Cu+/Cu0). For most supported CuO catalysts, Cu+/Cu0 were
the minor species (lower than 10%), suggesting that high valence
copper species played a crucial role in Ullmann coupling reaction.
As to catalysts with low catalytic activity, CuO/ZrO2 contained Cu2+
as the major species, while CuO dominated on CuO/SBA-15. On the
other hand, CuO/CeO2, CuO/La2O3 and CuO/␥-Al2O3 presented rel-
atively high catalytic activity. For CuO/CeO2 and CuO/␥-Al2O3, Cu2+
was presented as the dominant copper species. Nonetheless, XPS
of CuO/La2O3 showed that CuO was the main species on the cat-
alyst. In combination with the catalytic activity results, it can be
figured out that both Cu2+ strongly interacted with support and
CuO could catalyze Ullmann coupling reaction under heteroge-
neous conditions, and electronic state of copper species was not
the key to the catalytic performance of supported CuO catalysts.
3.3. Recycle of catalyst
As a heterogeneous catalyst, the recyclability of CuO/␥-Al2O3
catalyst in the coupling reaction of iodobenzene with m-cresol was
examined and the results were listed in Table 4. The yield for the
second cycle (61%) was lower than the first one (Table 4, Entry 3).
For the third cycle, nearly no loss of activity was observed (Table 4,
Entry 3). Compared with fresh CuO/␥-Al2O3 catalyst, XRD of used
CuO/␥-Al2O3 did not exhibit CuO diffraction peaks yet (see Fig. 1).
The XPS spectra indicated that CuO/␥-Al2O3 catalyst contained
Cu2+ as the main copper species before and after the reaction (see
Fig. 3c). The XRD and XPS results suggested that the structure and
Fig. 3. Cu 2p3/2 XPS spectra of supported CuO catalysts: (a) CuO/ZrO2 and CuO/SBA-
15; (b) CuO/CeO2 and CuO/La2O3; (c) fresh and used CuO/␥-Al2O3.

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P. Ling et al. / Journal of Molecular Catalysis A: Chemical 357 (2012) 112–116
Table 4
Recycle of catalyst.a
Entry
Catalyst
Yield (%)b
Cu leaching after first cycle (%)c
Run-1
Run-2
Run-3
1
2
3
CuO/CeO2
CuO/La2O3
CuO/␥-Al2O3
68
71
76
58
56
61
39
37
60
19
26
20
Entries in bold are the best item (highest yield, optimized reaction conditions and high recyclability).
a
Reaction conditions: PhI (3 mmol), m-cresol (2 mmol), K3PO4 (4 mmol), catalysts (0.04 mmol Cu), in DMSO (4 ml) at 150 ◦C under N2 for 10 h.
GC yield determined with tridecane as internal standard.
The content of copper leaching was determined by ICP-AES.
b
c
electronic state of surface copper species did not change in the reac-
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In summary, we developed an easy handling catalytic system for
the synthesis of diaryl ether by using supported CuO catalysts under
ligand-free conditions. The supported Cu-catalysts have a potential
to be used in coupling reactions. CuO/␥-Al2O3 was proved to be the
most effective catalyst for the coupling reaction and it could be
reused for three cycles providing the product in moderate to good
yields. Highly dispersed copper species on the supports showed
higher catalytic activity than bulk CuO. Meanwhile, electronic state
of Cu(II) did not affect catalytic performance of heterogeneous cata-
lysts. Namely, Cu2+ or CuO was contained as the main active copper
species of supported CuO catalysts in heterogeneous catalytic sys-
tem. In addition, the redox property of supported CuO catalysts did
not affect the catalyst activity directly. Further investigations on
optimization of the supported CuO catalyst and reaction conditions
are under study in our laboratory.
Acknowledgments
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We would like to acknowledge the financial support from
National Basic Research Program of China (Nos. 2010CB732300,
2011AA03A406) and National Natural Science Foundation of China
(No. 20977029).
[30] http://www.lasurface.com.