Facile synthesis of litchi shaped cuprous oxide and its application in the aerobic oxidative synthesis of imines

Article abstract of DOI:10.1039/c4ra14890h

In the present work, uniform litchi shaped cuprous oxide (Cu₂O) nanoaggregates were synthesized via a facile method by employing copper(ii) chloride, sodium hydroxide, ethylene glycol and ascorbic acid in the absence of surfactants at room temperature. With further increase of the reaction temperature, broken hollow copper nanoaggregates were obtained. The structure of a Cu₂O nanoaggregate was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), Transmission Electron Microscopy (TEM), Brunauer-Emmett-Teller (BET) analysis and High Resolution Transmission Electron Microscopy (HRTEM). The as-obtained Cu₂O nanoaggregates showed efficient catalytic activities in the aerobic oxidative synthesis of imines.

Full text of DOI:10.1039/c4ra14890h

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Facile synthesis of litchi shaped cuprous oxide and
its application in the aerobic oxidative synthesis of
imines†
Cite this: RSC Adv., 2015, 5, 10341
ab
and Zheng Dangc
*
Lei Bai
In the present work, uniform litchi shaped cuprous oxide (Cu2O) nanoaggregates were synthesized via a
facile method by employing copper(II) chloride, sodium hydroxide, ethylene glycol and ascorbic acid in
the absence of surfactants at room temperature. With further increase of the reaction temperature,
broken hollow copper nanoaggregates were obtained. The structure of a Cu2O nanoaggregate was
characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron
spectroscopy (XPS), Transmission Electron Microscopy (TEM), Brunauer–Emmett–Teller (BET) analysis
and High Resolution Transmission Electron Microscopy (HRTEM). The as-obtained Cu2O nanoaggregates
showed efficient catalytic activities in the aerobic oxidative synthesis of imines.
Received 19th November 2014
Accepted 7th January 2015
DOI: 10.1039/c4ra14890h
www.rsc.org/advances
preparation of Cu2O nanostructures can more precisely control
the growth of nanocrystals as well as modify the surfaces of the
crystals to be t for dispersing in different solvents.14 On the
other hand, surfactant-free synthesis of nanomaterials could
provide crystals having a relatively clean surface but with a large
size.3c,d Thus, the synthesis of uniform nanostructures of Cu2O
in several hundred nanometers (below 200 nm) by facile
methods with low-cost of materials and low energy-
consumption is still a hot topic from the view of both the
fundamental research and applications in potential.
Herein, we report the synthesis of a novel litchi shaped
Cu2O nanoaggregates (ꢀ100 nm) by the use of copper(II)
chloride, sodium hydroxide, ethylene glycol (EG) as solvent
and ascorbic acid (AA) as the reducing agent at room
temperature without employing any polymers or surfactants.
Furthermore, by increasing the temperature, broken hollow
Cu nanoaggregates are obtained possibly due to the further
reduction of Cu2O. The structure of the nanoaggregates is well
characterized and nally, the obtained Cu2O nanoaggregates
have been found inspiring applications in the aerobic oxida-
tive synthesis of imines.
Introduction
Nanomaterials, as well recognized, show enhanced properties
and have a variety of potential applications in catalysis,
biosensors and nanoelectronics. Besides, the use of nano-
materials could be affected by their size, morphology and
structure.1,2 To obtain nanomaterials with controllable struc-
tures, various methods have been developed.3 Generally, both
surfactants such as polyvinylpyrrolidone and hard templates
such as ordered structural carbon have been adopted in order to
optimize the dimension, direction and morphology.4,5 In the
meantime, self-assembled nanomaterials also attract a great
deal of interest.3c
Cuprous oxide (Cu2O), which is a non-stoichiometric defect
p-type semiconductor, has been widely applied in several elds
such as CO oxidation, organic synthesis, antibacterial activity,
photocatalysis, gas sensors, solar driven water splitting and
solar energy conversion.6–9
Size- and morphology-dependent properties of Cu2O such as
the size-dependent color exhibition require better controlling
the growth of nanocrystals in the synthesis strategy.10 Nano-
cubes, nanowires, nanospheres, hollow structures and various
polyhedra (with different number of facets) of Cu2O have been
successfully obtained with the assistance of polymers and
surfactants.11–14 The presence of surfactants and polymers in the
Experimental
Synthesis of Cu2O nanoaggregates
All the reagents were used as received without further puri-
cations. In a typical synthesis, 1 mmol CuCl2$2H2O and 8 mmol
sodium hydroxide were added into 10 mL EG and stirred until a
dark blue solution was obtained. Then, 2 mmol of AA solid were
added into the above solution directly and kept stirring for
desired time at room temperature. Finally, the yellow product
was washed by distilled water and ethanol and dried under
vacuum for further use.
aCentre of Advanced Nanocatalysis, University of Science and Technology of China,
Hefei, 230026, Anhui, China. E-mail: leibai@ustc.edu.cn; baileixy2007@163.com
bDepartment of Chemistry, University of Science and Technology of China, Hefei,
230026, Anhui, China
cTechnique Centre of Hai Nan Entry-Exit Inspection and Quarantine Bureau, China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c4ra14890h
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Characterizations
X-ray diffraction (XRD) patterns were recorded on MiniFlex 600,
Rigaku. X-ray photoelectron spectroscopy (XPS) analysis was
performed with a Thermo ESCALAB 250 instrument with a
monochromatic Al Ka (hv ¼ 1486.6 eV) X-ray source. Field
emission scanning electron microscopy (FE-SEM) analysis was
performed on JEOL JSM-7500B at 10 kV. The morphologies of
nanocrystals were examined by transmission electron micros-
copy (TEM), using a JEOL JEM-2010 LaB6 high-resolution
transmission electron microscope operated at 200 kV. The
electron diffraction and HRTEM were performed on JEOL
JEM-ARM200F, which was operated at 200 kV.
Brunauer–Emmett–Teller (BET) nitrogen-sorption data were
obtained with a Micromeritics Tristar 3000 automated gas
adsorption analyzer. Nuclear magnetic resonance hydrogen
spectra (1H NMR) were recorded on Avance III plus 400 MHz.
Ultraviolet-visible spectra were recorded on SHIMADZU
UV-3600 spectrophotometer.
Fig. 2 SEM (A), TEM (B) images of Cu2O nanoaggregates and size
distribution of Cu2O nanoparticles (C) in the nanoaggregates.
of Cu2O nanoaggregates and this phenomenon is also
observed in the case of Cu2O–GNS.15 However, the XRD
pattern conrms that the sample still remains in Cu2O phase
as discussed above.
Furthermore, the size and morphology of the samples are
characterized by SEM and TEM. As displayed by Fig. 2A, it is
shown that by using EG as solvent, the Cu2O nanoaggregates
have a litchi shaped structure and it is suggested from the
images that the surface of the nanoaggregates, with a ꢀ100 nm
in diameter, is coarse and the litchi shaped nanoaggregate is
composed by small particles (inset of Fig. 2A). TEM image in
Fig. 2B further conrms that the nanoaggregates are composed
by a different amount of small particles. The result of particle-
size analysis shows that the small particles are about 5–11 nm
as displayed in Fig. 2C based on the calculation of one hundred
particles. In addition, the BET specic surface area of nano-
aggregates is 28.2 m2 gꢂ1 (see Fig. S1†).
Synthesis of imines
In our case, for a typical reaction, 1 mmol of aromatic alcohol,
1.2 mmol of amine, 5 mmol% of Cu2O with 1.5 mmol of KOH
were added into 25 mL reactor pre-charged with 1 mL toluene
and the reaction was kept at 343 K for 19 h. The conversion was
conrmed by 1H NMR with 1,4-dioxane as an internal standard.
Results and discussions
In Fig. 3A, the electron diffraction of a nanoaggregate (shown
in the inset) is displayed, indicating that the nanoaggregates are
polycrystalline. The formation of polycrystalline Cu2O nano-
aggregates is possibly due to the fact that in the absence of
surfactants, small Cu2O nanoparticles form in EG and assemble
with each other quickly into nanoaggregates without a prefer-
ential growth face. The growth process of Cu2O nanoaggregates
would be studied in the following. A HRTEM image is displayed
in Fig. 3B and a part of the nanoaggregate shows a square
pattern, indicating that it is aligned to [001] direction. The (200)
and (110) plane spacings are measured to be 0.214 and
0.302 nm respectively, which are very close to the previously
reported values,16 further conrming the existence of Cu2O.
The inuence of the reaction time on the structures of Cu2O
nanoaggregates is also investigated. When the synthesis was
First of all, aer washing and dispersing in ethanol, a brown-
yellow sample is obtained as displayed in Fig. 1. The sample is
then dropped on a piece of glass, dried and characterized by
XRD.
As shown in Fig. 1A, the peaks at 29.5, 36.4, 42.1, 61.2 and
73.4ꢁ corresponding to the crystal face of (110), (111), (200),
(221) and (311), clearly suggests the formation of Cu2O (ICSD
no. 063281).
In order to further determine the chemical composition of
the Cu2O nanoaggregates, XPS is performed and the XPS
spectrum of Cu 2p exhibits two peaks at 932 and 934.5 eV in
Fig. 1B, corresponding to the Cu 2p3/2 spin–orbit peaks of Cu+
and Cu2+, respectively. As displayed in the gure, most species
of the nanoaggregate are Cu+ species. The specie of Cu2+
possibly occurred due to the oxidation of Cu+ on the surface
Fig. 1 Representative XRD pattern of Cu2O specie ((A) inset, the photo Fig. 3 Electron diffraction (A) and high resolution TEM image (B) of the
of the sample) and the XPS spectrum of Cu2p (B).
Cu2O nanoaggregate.
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Scheme 1 Proposed mechanism of the formation of Cu2O and Cu
nanoaggregates.
room temperature. With the enhanced temperature, the AA
Fig. 4 TEM images of Cu2O nanoaggregates obtained at different molecules adsorbed on Cu2O nanoaggregates or in the solution
reaction time (A) 5 and (B) 60 min.
further reduce the nanoaggregates, which leads to a corrosion
of Cu2O as well as the formation of copper specie.
Aer the investigation of the formation of Cu2O nano-
stopped at 5 min, the TEM image of the product in Fig. 4A
suggests that the nanoaggregates formed by small particles with
obvious voids are obtained with a relatively large range of
diameter. With the increase of the reaction time, for the sample
taken at 60 min (Fig. 4B), its structure is similar as that obtained
at 120 min, suggesting that the particles are assembled with
each other to form relatively solid spherical aggregates.
According to the above observations Fig. 4, aer adding AA, due
to the steric effect of EG molecules, spherical Cu2O nano-
aggregates with many voids in the interior of spheric structure
form quickly to reduce the surface energy. These aggregated
Cu2O nanostructures are not in thermodynamically equilibrium
status because of the quite large surface energy.12a Thus, aer a
certain time, relatively solid nanoaggregates are obtained and
no obvious changes in the size of nanoaggregates are noticed
between 60 min and 120 min.
aggregates, the nanoaggregates are employed as a catalyst in the
aerobic oxidative synthesis of imines. Imines with a reactive
C]N bond can undergo different types of reactions such as
reduction, cycloaddition and addition as well as be versatile
intermediates for nature products and biochemical active
compounds.17–19
As shown in Table 1, when Cu2O and KOH is used separately
in the reaction with benzyl alcohol and benzylamine, the
conversions of 74% and 18% imines are observed, indicating
the catalytic ability of Cu2O. In the same reaction, the conver-
sion of the formation of imine is increased to 95% in the
presence of both Cu2O and KOH (entry 3). Moreover, the reuse
of the catalyst in entry 3 with KOH, the conversion is still up to
78% for imines formation (entry 4). From these data, we reason
that the Cu2O nanoaggregate is an efficient catalyst for
promoting the formation of imines.
More interestingly, when the reaction is further carried out
at 353 K for 120 min, broken litchi shaped copper specie is
formed as displayed in Fig. 5 and also conrmed by XRD
(see Fig. S2†). The enhanced temperature increases the
reducing ability of AA and it is shown that the increase of
reaction temperature also enlarge the size of the aggregates up
to 330 nm due to the absence of stabilizers.
With the obtained results, we then try to explore some other
substrates and the results are listed in Table 2. For aniline and
benzyl alcohol, the conversion of the imine formation is 85%
(entry 1 of Table 2). The inuence of functional groups of
aniline as an example is investigated when benzyl alcohol is
xed.
As shown in Table 2, for p-methoxyaniline and o-methoxy-
The whole process of the synthesis is summarized in Scheme
1. It is proposed that under strongly basic conditions, a part of
EG molecules can be transformed into anions that coordinated
with copper ions to form Cu2+–EG complexes as well as play a
aniline,
a big difference in conversion (44%) of its
Table 1 The performances of Cu2O nanoaggregates on aerobic
role in controlling the aggregation. By introducing AA, this kind oxidative synthesis of iminesa
of complex is reduced and transformed into Cu2O species at
Entry
Conv. (%)
1
2
3
4
KOH only
Cu2O only
KOH and Cu2O
Reused of 3
18
74
95
78
a
Reaction conditions: benzylalcohol (1 mmol), benzylamine
(1.2 mmol), KOH (1.5 mmol) and Cu2O (5 mmol%) were added into
toluene (1 mL) in a reactor and stirred for 19 h with an oxygen
balloon. The formation and conversion of imines were conrmed by
1H NMR and based on the consumption of aromatic alcohol.
Fig.
5 SEM (A) and TEM (B) images of copper nanoaggregates
obtained at 353 K for 2 h.
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Table 2 Extension of the catalytic abilities of Cu2O nanoaggregates
species has been oxidized into CuO species as displayed in
Fig. S3† aer comparing with that of Cu2O. Taking into account
of the catalytic results (entry 1, 2, 3 as well as 4 in Table 1), it is
reasonable that Cu2O is the active phase for the synthesis of
imines and more efficient than CuO. In addition, the leaching
of the catalyst during the reaction is also investigated by
extracting and analyzing the reaction solution. As shown by
UV-Vis spectra in Fig. S4,† in contrast with the spectrum of CuII
solution, no CuII species are detected from the reaction solu-
tion. Based on the observations from Fig. S4† and the leaching
experiments, it is conrmed that the transformation of catalysts
occurs and no loss of copper is noticed. Thus, according to the
above results, the further work could be focused on the
comparison of the effect between CuO and Cu2O on the aerobic
synthesis of imines.
on aerobic oxidative synthesis of iminesa
Entry
R1
R2
Conv. (%)
1
C6H5
C6H5
C6H5
C6H5
C6H5
C6H5
C6H5
C6H5
C6H5
85
94
96
50
84
84
54
74
91
80
74
24
2
4-MeOC6H4
4-MeOC6H4
2-MeOC6H4
4-MeC6H4
3-MeC6H4
2-BrC6H4
CH3(CH2)2
CH3(CH2)10
C6H5CH2
C6H5CH2
C6H5CH2
3b
4
5
6
7
8
9
C6H5
10
11
12
4-MeC6H4
4-ClC6H4
3-Py
Conclusions
In conclusion, we had developed litchi shaped Cu2O nano-
aggregates by using EG and AA at room temperature in the
absence of any templates or surfactants. The nanoaggregate was
composed by ultra-small particles (5–11 nm in size) as
evidenced by SEM and TEM results. The structure of the
nanoaggregates was characterized by XRD, XPS, HRTEM and
BET. Interestingly, broken litchi shaped Cu nanoaggregate was
obtained at a higher temperature. The as-obtained Cu2O was an
efficient catalyst in the aerobic oxidative synthesis of imines.
This work could provide a new insight for the synthesis of
nanomaterials and their applications in organic synthesis.
a
Reaction conditions: alcohol (1 mmol), amine (1.2 mmol), KOH
(1.5 mmol), Cu2O (5 mmol%) were added into toluene (1 mL) in a
reactor and stirred for 19 h with an oxygen balloon. The formation
and conversion of imines were conrmed by 1H NMR and based on
the consumption of alcohol. b Another batch of Cu2O.
corresponding imine is observed by the results in entry 2 and 4,
suggesting that the steric effect has a signicant inuence on
the conversion. While for p-methoxyaniline and p-toluidine, the
difference in imine conversion could be resulted from the
different ability in electron donating. A close conversion of the
formation of imines between m- and p-toluidine is found in
comparison with the aniline itself. In addition, for electron
attractive group such as bromine, a low conversion (54%) which
is close to that of o-methoxyaniline is obtained as displayed in
entry 7, possibly due to that bromine is a smaller group. For
aliphatic amines, conversions of 78 and 91% for the formation
of imines between propylamine, laurylamine and benzyl alcohol
are also observed, respectively. On the other hand, as for the
modication of benzyl alcohol, good conversions of imines are
obtained for 4-chlorobenzyl alcohol and 4-methylbenzyl
alcohol. However, it is worth noting that this catalyst is not
efficient when pyridin-3-ylmethanol is used, possibly due to the
difficulty in oxidation of pyridin-3-ylmethanol (entry 12).
For the formation of imines, it is proposed that rst the
aromatic alcohol is oxidized into aldehyde from previous work20
and then condensation reaction between the two regents occurs
as proposed in Scheme 2.
Acknowledgements
Professor Y. D. Li and Professor Y. B. Kang are greatly appreci-
ated for the nancial support and useful advice. China
Postdoctoral Science Foundation (no. 2014M561829) was
greatly appreciated for the nancial support. Hai Nan Natural
Science Foundation (no. 213028) was also appreciated for the
nancial support.
Notes and references
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It is known that Cu2O is easy to be oxidized as evidenced by
XPS results above. Aer the reaction in our experiments, the
catalyst, Cu2O nanoaggregate is investigated by XRD to nd the
changes of its structure and it is suggested that the Cu2O
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Scheme 2 Possible reaction path of the formation of imines.
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