Cupric oxide nanowires on three-dimensional copper foam for application in click reaction

Article abstract of DOI:10.1039/c7ra00014f

Readily prepared CuO nanowires (CuO NWs) have been found to be effective heterogeneous catalysts for the 1,3-dipolar cycloaddition (CuAAC) reaction without using any additional support and bases. Examination of CuO nanowire catalysts synthesized at variou

Full text of DOI:10.1039/c7ra00014f

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Cupric oxide nanowires on three-dimensional
copper foam for application in click reaction†
Cite this: RSC Adv., 2017, 7, 9567
a
a
a
a
a
Chunxia Wang,‡ab Fan Yang,‡ Yan Cao, Xing He, Yushu Tang and Yongfeng Li
*
*
Readily prepared CuO nanowires (CuO NWs) have been found to be effective heterogeneous catalysts for
the 1,3-dipolar cycloaddition (CuAAC) reaction without using any additional support and bases. Examination
of CuO nanowire catalysts synthesized at various temperatures showed that the use of CuO-600 catalyst
was the best choice for the success of the present click reaction. Not only aromatic and heteroaromatic
alkynes but also alkylalkynes were catalyzed regioselectively, affording triazoles in excellent yields.
Furthermore, we have identified that the reaction proceeded in a heterogeneous manner. It is
noteworthy that the CuO-600 catalyst can be easily recovered and reused nine times.
Received 1st January 2017
Accepted 27th January 2017
DOI: 10.1039/c7ra00014f
rsc.li/rsc-advances
development of high catalytic activity catalysts application in
click chemistry but also simplied for recovery of catalyst project.
Introduction
Cupric oxide (CuO) are promising materials for gas sensors,
heterogeneous catalysts, photovoltaics, infrared detectors, eld
emission emitters and lithium ion electrodes applications due to
their unique properties such as direct band gap (1.2–1.7 eV), non-
toxicity, chemical stability, abundant availability and low
production cost.31–38 To date, most of the ways used to generate
CuO nanowires (CuO NWs) are physical and chemical routes, for
instance, precursor methods, hydrothermal reaction, anodiza-
tion, electrospinning, and seed-mediated growth solution.39–43
Compared with these methods, the formation of CuO NWs by
direct thermal oxidation of Cu has been recently given consid-
erable attention due to its simplicity and large-scale growth.44
Herein, we report an efficient approach for the synthesis of CuO
NW and its remarkable catalytic properties in click chemistry
without using any additional supports and bases. To the best of
our knowledge, this is the rst report on a monolithic CuO NWs
catalysed organic molecular transformation.
1,2,3-Triazoles are one of the most vital class of N-containing
heterocyclic compounds that have received a much attention,
not only as biological active drugs but also as functional mate-
rials.1–9 In addition, these compounds are attracting much
attention mainly due to the fact that they are used in numerous
elds, such as organic semiconductors, dyes, dehydroannulenes,
and electroluminescent materials.10–21 Due to their intrinsic
importance, many methodologies have been developed for the
preparation of 1,2,3-triazoles. Among them, Huisgen's 1,3-
dipolar cycloaddition reaction of terminal alkynes and organic
azides, catalysed by homogeneous CuI salts (CuAAC), was re-
ported independently by Sharpless22 and Meldal23 (the so-called
click reaction), which is known as one of the most efficient
synthetic application tools in organic synthesis due to its 100%
atom economy, mild reaction conditions, wide substrate scope
and exclusive regioselectivity in the formation of the 1,4-disub-
stituted 1,2,3-triazoles. The click reaction can be catalysed also by
heterogeneous copper, which strategy is immobilization of
copper nanoparticles onto various heterogeneous organic and
inorganic supports.24–28 However, copper metal alone catalysed
this click reaction is relatively slow and requires high catalyst
loading with bases. Recently, Jin and co-workers developed an
interesting monolithic nanoporous Cu catalysts, which nano-
porosity structure of Cu surface led to a signicant enhancement
of catalytic activity in click chemistry without using any addi-
tional supports and bases.29,30 Therefore, design of nanostructure
on the surface of monolithic copper is not only feasible for
Experimental
General information
All the chemicals are used as received without further puri-
cation: foam Cu, porosity $ 98%, purity $ 99, PPI ¼ 110,
thickness ¼ 1.5 mm, bulk density ¼ 0.4–0.6 g cmꢀ3, average
pore size ¼ 200–300 mm (Kunshan desco electronics Co., Ltd);
benzyl azide (Aladdin Industrial Corporation); the terminal
alkyne, benzyl bromide (J&K Chemical); sodium azide (Micxy
Reagent); t-BuOH, toluene, hexane, THF, HCl (Beijing Chemical
Works). Deionized water was used in this study.
aState Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing,
102249, China. E-mail: yangfan@cup.edu.cn; yi@cup.edu.cn
bInstitute of New Energy, China University of Petroleum, Beijing, 102249, China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c7ra00014f
Synthesis of CuO NWs
In a typical procedure, a 3D CuO NWs network can be facile
prepared by one step thermal oxidation of Cu foam in air. The
‡ These authors contributed equally to this work.
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Cu foam substrate was cleaned in an aqueous 1.0 M HCl solu-
tion for about 20 s, followed by repeated rinsing with distilled
water. Aer it had been dried under a nitrogen gas ow, the Cu
foam was placed in an alumina boat and immediately heated to
the set-point temperature at ambient pressure in an oven and
annealed in air for 4 hours. The growth process was accompa-
nied with a change of colour from reddish to black, indicating
the forming of CuO NWs. We have tested a number of
temperatures: 400, 500, 600 and 700 ꢁC, and the respective
materials called CuO-n, (n ¼ 400, 500, 600, 700).
Results and discussion
High-density CuO NWs were rst grown by heating copper foam
substrate in air for 4 h at a suitable range of temperature
ꢁ
ꢁ
ꢁ
ꢁ
(400 C, 500 C, 600 C, 700 C), and the respective materials
called CuO-n, (n ¼ 400, 500, 600, 700). The morphology and
structure of all CuO-n catalysts have been examined by scanning
electron microscopy (SEM) (Fig. 1). For the Cu foam, the crack
on the surface of the skeleton with 3D structure was distinctly
observed as seen in Fig. 1a and b. When the Cu foam heated at
ꢁ
400 C for 4 h in air, only the sparse nanowires related to CuO
were formed on the surface of Cu foam (Fig. 1c and d). It was
quite obvious that CuO NWs of CuO-500 were formed denser
than that of CuO-400 (Fig. 1e and f). While the growth
temperature enhanced to 600 ꢁC, the density and length of CuO
NWs were perfectly formed compared with CuO-400 and CuO-
500 samples (Fig. 1g and h). When the temperature was
higher than 700 ꢁC, no CuO NWs were found and the growth of
CuO NWs will be terminated, which is due to the free energy of
CuO changes from negative to positive state,38 as seen in Fig. 1i
and j.
XRD is a useful method to characterize the crystal structure
of these catalysts, and the crystal information of CuO-400, CuO-
500, CuO-600 and CuO-700 are shown in Fig. 2. The character-
ized peaks locate at 2q value of 43.5ꢁ and 50.6ꢁ were corre-
sponding to the metal Cu (JCPDS PDF#85-1326). The
characterized peaks locate at 2q value of 36.6ꢁ, 42.5ꢁ, 61.6ꢁ and
Characterization
X-ray diffraction (XRD, Bruker D8 Advance Germany) was
applied to characterize the crystal structure of the hybrid
materials, and the data were collected on a Shimadzu XD-3A
diffractometer using Cu Ka radiation. The X-ray photoelectron
spectroscopy (XPS, Thermo Fisher K-Alpha American with an Al
K a X-ray source) was used to measure the elemental composi-
tion of samples. Transmission electron microscopy (TEM, Tec-
nai G2, F20) combined with an energy dispersive X-ray
spectroscopy (EDS) at an acceleration voltage of 200 kV were
used to measure the size, morphology, size distribution and
element content of nanoparticles.
Representative procedure for the Cu NWs catalysed click
reaction
To a H2O/t-BuOH (0.66 ml/0.33 ml) solution of 2 mg CuO NW (5
mol%, calculated the amount of catalyst basis of CuO) was
added phenylacetylene 1a (0.5 mmol, 51 mg) and benzyl azide
2a (0.5 mmol, 67 mg) in a V-shaped reactor vial. The reaction
mixture was stirred at room temperature for 12 h by using
a bulky round-shaped magnetic stirring bar. Aer consumption
of 1a and 2a which were monitored by TLC, the CuO NWs was
picked out of the reaction mixture by tweezers. The recovered
CuO NWs catalyst was washed with acetone and dried under
vacuum to use for the next run. Aer concentrated of the ltrate,
the white solid was puried via short silica gel chromatography
by using a 3 : 1 mixture of hexane and ethyl acetate as an eluent,
to afford 1-benzyl-4-phenyl-1H-1,2,3-triazole 3a as a white solid,
yield: 116 mg (99%).
Fig. 2 The XRD patterns of CuO-400, CuO-500, CuO-600 and CuO-
700.
Fig. 1 SEM images of (a and b) Cu foam, (c and d) CuO-400, (e and f) CuO-500, (g and h) CuO-600, (i and j) CuO-700.
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Table 1 Click reaction under different conditionsa
Entry
Cat.
Solvent
Yieldb (%)
Fig. 3 XPS survey of CuO-n samples (a). Cu 2p scans of CuO-n (b).
1
2
3
4
5
6
7
8
9
CuO-400
CuO-500
CuO-600
CuO-700
Cu foam
CuO-600
CuO-600
CuO-600
CuO-600
H2O
H2O
H2O
H2O
68
83
92
30
18
99
93
89
91
74.2ꢁ were attributed to the Cu2O (JCPDS PDF#73-2076). The
peaks locate at 2q value of 32.7ꢁ, 35.7ꢁ, 38.9ꢁ, 48.9ꢁ, 53.7ꢁ, 58.4ꢁ,
66.5ꢁ and 68.2ꢁ correspond to the CuO (JCPDS PDF#80-1917). It
was obvious to observe that the CuO-n were a mixture of Cu2O,
CuO and metal Cu skeleton. Moreover, the Cu characterized
peaks were disappeared and the Cu2O characterized peaks were
reduced along with the temperature increasing, which were in
accordance with the two steps growth mechanism of CuO NWs:
the rst one is forming of a Cu2O layer on the surface of copper,
and then CuO NWs were subsequently grown on the Cu2O
layer.44–46
H2O
H2O : t-BuOH
Toluene
Hexane
THF
a
Reaction conditions: phenylacetylene 51 mg (0.5 mmol) and benzyl
azide 67 mg (0.5 mmol), cat. (5 mol%), and in 1 ml solvent at room
temperature. b Isolated yield.
morphology of the CuO NWs on the Cu foam. In order to
determine the solvent system most suitable for the catalyst,
several experiments were conducted. Quantitative yield was
obtained with a solvent mixture of t-BuOH and H2O (2 : 1) (entry
6). The reactions were carried out in different solvents including
toluene, hexane and THF, no better results were obtained
compared with the mixture solvent of t-BuOH and H2O. These
results indicated that solubility and hygroscopic properties are
all important factors for CuO-600 catalysis of click reactions.48
The scope extension of the protocol to other alkynes and
benzyl azide was also investigated under the previously opti-
mized reaction conditions (Table 2). Phenyl acetylenes with
electron-donating and electron-withdrawing groups such as 1-
ethynyl-4-methylbenzene, 1-ethynyl-4-methoxybenzene and 1-
chloro-4-ethynylbenzene reacted readily with benzyl azide. The
corresponding triazoles 3b–3d were obtained in high yields
(entries 1–3). Heteroaromatic alkynes also gave the expected
adducts 3e and 3f as single regioisomers in high yields (entries
4 and 5). Hydroxy-substituted alkynes such as propargyl
alcohol and pent-4-yn-1-ol also gave the expected adducts (1-
benzyltriazol-4-yl) methanol 1g and 3-(1-benzyl-1H-1,2,3-triazol-
4-yl)propan-1-ol 1h as single regioisomers in good to high yields
(entries 6 and 7). While acetylenes conjugated with an ester
group such as phenyl propargyl ether and aliphatic alkynes such
as hex-1-yne and ethynylcyclohexane react with benzyl azide
produced the corresponding product with excellent yields
(entries 8–10). The reaction with alkynes containing electron-
withdrawing substituents and trimethyl silanes gave high
yields (entries 11 and 12). These results illustrated that not only
aromatic and heteroaromatic alkynes but also alkylalkynes were
catalyzed regioselectively, affording triazoles in excellent yields.
Under neat conditions, the catalytic loading can be
decreased to 1 mol% without any signicant inuence on the
reaction efficiency on the reaction temperature at 65 ꢁC. As
known to all, the CuO NWs are formed on the surface of the Cu
surface and the role of Cu foam is to support the CuO NWs
The chemical composition and electronic structure of the
CuO NWs were investigated by XPS. Fig. 3 depicts a high
sensitivity scan with the presence of Cu and O elements. No
other obvious impurities were found in the spectrum expect
low-intensity peak of carbon, which might result from the
surface contamination. The Cu 2p peak of CuO nanostructures
was illustrated in detail in Fig. 2b. The Cu 2p3/2 lies at 933.9 eV
with a shake-up satellite at about 944 eV and Cu 2p1/2 lies at
953.5 eV with a satellite at about 962.4 eV, which was in good
agreement with previously reported.45,47 The bonding energy
gap between Cu 2p1/2 and Cu 2p3/2 was about 20 eV, which was
in agreement with the standard spectrum of CuO. The O 1s peak
for CuO and Cu2O was at 530 eV and Cu2O was at 531.8 eV
(Fig. S1†), respectively.
The catalytic activities of various fabricated CuO-n samples
were examined in the click reaction of phenylacetylene (1a) and
benzyl azide (2a) in H2O at room temperature without using any
additional supports and bases (Table 1). The reactions using
CuO-n catalysts with calcination temperature at 400 ꢁC and
500 ꢁC (CuO-400 and CuO-500) gave moderate yields of the
corresponding triazole 3a (entries 1 and 2). Surprisingly, the use
of CuO-600 afforded 3a in almost quantitative yield (entry 3).
The reaction using CuO-700 as catalyst, resulted in a dramati-
cally decreased yield of 3a (entry 4). However, Cu foam was
inactive as a catalyst in current click reaction system (entry 5).
These results indicated that the annealing temperature has
a strong inuence on catalytic activity in the click reaction. The
remarkable difference catalytic activities are possibly ascribed
to the different morphology, density and surface component of
CuO NWs. Although, it can be found that the CuO component is
increased along with the temperature increasing in our XRD
results. It is hard to say which kind of Cu species (Cu2O and
CuO) mainly affect the catalytic activity in current stage, due to
the fact that both the morphology and density of the CuO NWs
are also inuential factors. Can be ascribed to the formation
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Table 2 Click reaction of benzyl azide with terminal alkynes in the
600 can be reached as high as 495 in current click reaction
system. In comparison with other Cu catalysts reported recently
(Table S1†), it is obvious that the CuO-600 catalyst demonstrates
superior or comparable catalytic activity. It is worthy to mention
that, the recyclability, separation and fabrication of CuO-600 are
much more advantageous than these Cu catalysts for practical
applications. Moreover, the CuO-600 catalyst demonstrated to
be very efficient in the multi-component 1,3-dipolar cycloaddi-
tion of phenyl acetylene and benzyl azide yielded in situ from
sodium azide and benzyl bromide, affording 1,2,3-triazoles 3a
in excellent yield eqn (2).
presence of CuO-600a
Entry
1
Alkyne
Product
Yieldb (%)
99
2
3
4
5
6
93
98
95
99
94
(1)
(2)
The reusability is the most important feature of a heteroge-
neous catalyst, which is superior to a homogenous one. First, to
conrm the reaction indeed catalysed by CuO-600 rather than
by homogenous CuO species, we have carried out the following
leaching experiments as shown in eqn (3). Aer the catalytic
click reaction of 1a and 2a was carried out for 4 h under the
standard conditions, the CuO-600 catalyst was removed from
the vessel by tweezers with 3a produced in 55% yield at this
time. No further reaction were took place aer removing the
catalyst, the CuO-600 catalyst was then put back into the
mixture. As a result, the click reaction was restarted, and
production 3a was obtained in 99% yield in 8 h. To assess
recyclability of CuO-600 catalyst, multiple phenyl acetylene 1a
and benzyl azide 1b click reactions were carried out, the
recovery of the heterogeneous catalyst was carried out by twee-
zers for the separation of the catalyst from the reaction mixture.
It was found that the catalytic activity was slightly reduced aer
the catalyst repetition four times at room temperature. The yield
of 3a can be improved by increasing the reaction temperature to
7
8
9
96
99
89
10
11
90
98
ꢁ
ꢁ
45 C. When the reaction temperature enhanced to 65 C, the
catalyst can be further recycled four times with producing 3a
excellent yield every time (Fig. 4a). Aer reaction, the catalyst
was again examined by SEM, and the image indicated that the
CuO NWs were covered by some organic compounds and some
12
88
a
Reaction conditions: acetylene (0.5 mmol) and benzyl azide 67 mg (0.5
mmol), cat. (5 mol%), and in 1 ml H2O : t-BuOH (2 : 1) at room
temperature. b Isolated yield.
catalytic species rather than the click reaction catalyst eqn (1).
Hence, we picked off the CuO NWs by ultrasonic in ethanol
solution, nding that no more than 20 wt% of the CuO NWs
were existed in CuO-600 catalyst. The TON value for the CuO-
Fig. 4 The yields for 10 cycles of CuO-600 catalysed click reaction (a),
and SEM image of Pd-3 catalyst after 10 cycles (b).
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of NWs were broke (Fig. 4b). However, the CuO NWs crystal
structure were not changed dramatically as shown in Fig. S3,†
which revealed that the CuO NWs was not transformed aer the
recycle test.
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Conclusions
In summary, CuO NWs can be synthesized by thermal oxidation
of 3D Cu foam in air at different temperatures. Among all of the
as-prepared catalysts, CuO-600 exhibits superior catalytic
activity for 1,3-dipolar cycloaddition of phenyl acetylene and
benzyl azide without using any additional supports and bases
compared with CuO-400, CuO-500 and CuO-700 catalysts. A
wide range of functional groups on terminal alkyne were
tolerated, affording the corresponding substituted 1,2,3-triazole
in good to high yields with a sole selectivity. Moreover, leaching
experiment revealed that the click reaction occurred by
heterogeneous way, and the CuO-600 catalyst can be effective
reused nine times. Further work is in progress to extend such
kind of catalyst for other applications.
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Acknowledgements
We gratefully thank for the National Natural Science Founda-
tion of China (No. 21576289), the Science Foundation of
China University of Petroleum, Beijing (No. 2462015YQ0306,
2462016YJRC027, 2462014QZDX01 and C201603), Thousand
Talents Program and National High-tech R&D Program of China
(863 Program, No. 2015AA034603).
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