Leaf-like CuO nanosheets on rGO as an efficient heterogeneous catalyst for Csp-Csp homocoupling of terminal alkynes

Article abstract of DOI:10.1016/j.catcom.2020.106260

In this work, the economic and well-defined leaf-like CuO nanosheets on rGO (CuO nanosheets/rGO) was synthesized by a convenient hydrothermal method. The morphology and chemical composition of CuO nanosheets/rGO were confirmed by XRD, SEM-EDS, TEM, HR-TEM, and XPS techniques. The CuO nanosheets/rGO was successfully applied as a high-performance heterogeneous catalyst in the homocoupling of 12 terminal alkynes, and the isolated yield of each product was more than 80%, except for propargyl alcohol. This catalyst could be reused five times with little activity loss. Thus, it is beneficial for green and sustainable development of organic synthetic chemistry.

Full text of DOI:10.1016/j.catcom.2020.106260

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Leaf-like CuO nanosheets on rGO as an efficient heterogeneous
catalyst for Csp-Csp homocoupling of terminal alkynes
Bin Wang, Lingfeng Gao, Gengxiu Zheng
PII:
S1566-7367(20)30336-8
DOI:
https://doi.org/10.1016/j.catcom.2020.106260
CATCOM 106260
Reference:
To appear in:
Catalysis Communications
Received date:
Revised date:
Accepted date:
10 August 2020
25 November 2020
27 November 2020
Please cite this article as: B. Wang, L. Gao and G. Zheng, Leaf-like CuO nanosheets on
rGO as an efficient heterogeneous catalyst for Csp-Csp homocoupling of terminal alkynes,
Catalysis Communications (2020), https://doi.org/10.1016/j.catcom.2020.106260
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2020 Published by Elsevier.

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Leaf-like CuO nanosheets on rGO as an efficient heterogeneous
catalyst for C -C homocoupling of terminal alkynes
sp
sp
a
a,*
a,*
Bin Wang , Lingfeng Gao , Gengxiu Zheng
a
School of Chemistry and Chemical Engineering, University of Jinan, 336
Nanxinzhuang West Road, 250022, Jinan (China)
Abstract
In this work, the economic and well-defined leaf-like CuO nanosheets on rGO (CuO
nanosheets/rGO) was synthesized by a convenient hydrothermal method. The
morphology and chemical composition of CuO nanosheets/rGO were confirmed by
XRD, SEM-EDS, TEM, HR-TEM, and XPS techniques. The CuO nanosheets/rGO was
successfully applied as a high-performance heterogeneous catalyst in the homocoupling
of 12 terminal alkynes, and the isolated yield of each product was more than 80%,
except for propargyl alcohol. This catalyst could be reused five times with little activity
loss. Thus, it is beneficial for green and sustainable development of organic synthetic
chemistry.
Keywords: Homocoupling; Terminal alkynes; CuO nanosheets; rGO; Heterogeneous
catalysis
Corresponding authors: Tel.: +86053182765841
*
E-mail address: gaolf108@mail.ustc.edu.cn; chm_zhenggx@ujn.edu.cn
1

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1
. Introduction
Graphene is a type of carbon nanomaterial that possesses a single layer of sp -bonded
2
[
1]
carbon atoms. During the past few years, most of the chemical engineering processes
utilizing graphene-based nanomaterials are catalytic reactions to meet the increasing
[
2]
demand of sustainable development. Naturally, the progress of graphene-based
nanomaterials was motivated by its advantages, including large specific surface area,
[
3]
excellent thermal stability, and interesting chemical and electronic properties. Most
importantly, the oxygen-containing functional groups of reduced graphene oxide (rGO)
can interact strongly with the supported metal nanomaterials, thereby realizing
heterogeneous catalysis.
The synthesis and application of size- and morphology-controlled Cu-based
nanomaterials have always been recognized as important subjects in the fields of both
chemistry and materials science. In particular, CuO-based nanomaterials offer distinct
advantages such as ease of synthesis, low cost, and intrinsic stability, which makes it a
[
4-5]
promising candidate for practical applications.
achieved in the development of CuO/rGO nanomaterials
heterogeneous catalytic organic reactions need further exploration.
Although significant progress was
[
3,4-5,6-10]
, their applications in
[
3,4-5,7-8]
Thus, CuO
nanomaterials with well-defined morphology on rGO are required to investigate its
catalytic properties and establish a highly efficient heterogeneous organic catalytic
system.
1
,3-Diyne, a type of carbon skeleton with high rigidity, has been well utilized in
many fields, involving fluorescent probes, organic building blocks, pharmaceuticals,
[
11-14]
and organogelators.
As known, Glaser coupling is a good approach for the
[
15]
preparation of symmetrical 1,3-diynes. In recent years, several catalytic systems were
2

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[
16-34]
reported for the C -C homocoupling of terminal alkynes.
Among these previous
sp
sp
works, a series of efficient and recyclable catalysts were explored, such as Cu (BTC)2
3
[
16]
[21]
[23]
[26]
[27]
MOF , meso Cu/MnOx , metallomicelles , Cu/C N4 , MCM-41-2N-CuI
,
3
[
28]
[29]
CuO/TiO2 , and CuOx NPs . Nevertheless, there are still some disadvantages such
[
19-20, 22]
as the use of noble metals as the catalytic centre
, which are difficult to recycle as
[
17, 24-25, 31-34]
the catalyst
activated by Cu(II) species through the coordination
nanomaterials can be an economic catalytic center for the C -C homocoupling of
. By contrast, the C≡C bond of terminal alkynes can be well
[
16, 18, 31-35]
, which indicates the CuO
sp
sp
terminal alkynes. Thus, the exploration of a mild catalytic method for homocoupling of
terminal alkynes that is catalysed by CuO nanomaterials on rGO is highly important not
only for the development of organic synthetic chemistry but also for carbon
nanomaterials.
In the present work, well-defined leaf-like CuO nanosheets/rGO was prepared
through a simple hydrothermal method. The nanocomposites were characterized by the
XRD, SEM-EDS, TEM, HR-TEM and XPS techniques to explore its morphology and
chemical composition. The excellent synergy between CuO nanosheets and rGO made
the CuO nanosheets/rGO an economic and efficient heterogeneous catalyst for the Csp-
C homocoupling of various terminal alkynes. Furthermore, this catalyst was reused
sp
five times with little loss of its catalytic activity. All these indicate that this work will
expand the application scope of CuO nanosheets/rGO and promote green and
environmentally friendly development of organic synthetic chemistry.
2
. Experimental
2
.1. Synthesis of graphene oxide (GO)
3

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[
S1]
GO was prepared through a modified Hummer’s method.
The manufactured GO
was subjected to a freeze-drying process and yielded a dark brown solid. Then, the
obtained GO was preserved for subsequent characterization and usage.
2
.2. Synthesis of CuO nanosheets/rGO
In the present work, the CuO nanosheets/rGO was prepared using the hydrothermal
method. In a typical synthesis of CuO nanosheets/rGO, 100 mg GO was added to 80 mL
purified water and sonicated (3 h) to obtain a homogeneous suspension at 20 °C. Next, 1
mL CuSO solution (0.125 g/mL) and 50 mg sodium glutamate were added to the above
4
suspension. Subsequently, the pH of the system was adjusted to 11 using NH ·H O and
3
2
stirred for 4 h. The suspension was then transferred into a 100 mL Teflon-lined
autoclave and hydrothermally treated at 100 °C. Nine hours later, the system was
centrifuged and washed with ethanol and purified water for three times. Finally, the
CuO nanosheets/rGO was obtained using a freeze-drying process and preserved
carefully for further characterization and application.
In addition, CuO nanosheets without rGO was also synthesized using the same
method for further comparison (Supporting Information).
3
. Results and discussion
3
.1. Characterization of CuO nanosheets/rGO
[
Insert Fig. 1]
The synthesized CuO nanosheets/rGO was characterized by powder X-ray diffraction
(XRD) to perform crystal structure and phase analysis. The XRD patterns of GO, rGO,
CuO nanosheets and CuO nanosheets/rGO are shown in Fig. S1b and Fig. 1a. In Fig.
S1a, graphite showed a characteristic sharp reflection peak centered around 2θ = 26°
[
S2]
(002 graphitic plane).
After the oxidation, a new peak centered at 10.8° appeared,
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which is the expanded plane (001) of GO due to the intercalation of oxygen-containing
[
S2]
functional groups between the graphene layers (Fig. S1b).
When the hydrothermal
treatment for GO was completed, a broad peak centered at 24.5° was detected and rGO
[
S2]
was formed.
8.7°, 48.7°, 53.5°, 58.2°, 61.6°, 66.2°, 68.2°, 72.4°, and 75.1° corresponded to the
110), (-111), (111), (-202), (020), (202), (-113), (-311), (220), (311), and (004) planes
of monoclinic CuO (JCPDS 48-1548), respectively. The sharp diffraction peaks of XRD
For the prepared CuO nanosheets (Fig. 1a), the peaks at 32.4°, 35.5°,
3
(
[
3,5]
pattern also indicated the high crystalline structure of CuO nanosheets.
Similarly, the
XRD data of CuO nanosheets/rGO (Fig. 1a) agreed with the XRD data of CuO
nanosheets but without the peak at 24.5° (002 plane) from rGO. This was because of the
growth of CuO nanosheets on the rGO sheets at functionalized sites, which mitigated its
restacking during the freeze-drying process; hence, the interplanar diffraction signal of
rGO was too weak to observe. All these results proved that CuO was formed and GO
was reduced to rGO during the hydrothermal process.
The surface chemical composition and bonding information of the CuO
nanosheets/rGO were obtained by X-ray photoelectron spectra (XPS) analysis (Fig. 1b,
S2). The measured XPS spectrum (Fig. S2a) confirmed that CuO nanosheets/rGO
contain the elements of Cu, O and C, which is consistent with the EDS mapping results
(Fig. S5d). The Cu element belonged to CuO nanosheets, C was attributed to rGO, and
O was mainly ascribed to the oxygen-containing group on rGO and the incorporated
CuO nanosheets. The survey spectrum of CuO nanosheets/rGO revealed the presence of
Cu 2s, Cu 2p, Cu LLMd, Cu LLMb, Cu LLMc, Cu LLMa, O 1s, C 1s, Cu 3s, and Cu 3p
[
3, S3-S4]
photoelectrons (Fig. S2a).
As shown in C 1s spectrum (Fig. S2b), three types
fitted peaks located at 284.8, 285.9, and 288.6 eV were observed and were assigned to
5

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[
S5]
C=C, C-O, and O-C=O bond, respectively.
This further proved the existence of an
oxygen-containing functional group on the surface of rGO. The high-resolution
spectrum of Cu 2p showed the presence of Cu 2p1/2 and Cu 2p3/2 peaks with binding
energies of 953.5 and 933.6 eV, respectively, accompanied with their corresponding
[
S5]
satellite peaks (Fig. 1b). The distance between Cu 2p1/2 and Cu 2p3/2 peak was 20
[
S5-S9]
eV, which is in agreement with the standard XPS spectrum of CuO.
Furthermore,
the binding energy of 568.8 eV in Cu LMM spectrum proved the characteristic Cu(II)
[
S4, S10-S12]
oxidation state of CuO nanosheets/rGO (Fig. S2d).
The O 1s spectrum can be
deconvoluted into four peaks at 529.8, 531, 531.8, and 533.1 eV (Fig. S2c), which are
assigned to the C=O, Cu-O, O=C-OH and C-O bonds, respectively. Therefore, the XPS
results fully illustrated the chemical status of CuO nanosheets/rGO. ICP-OES was used
to determine the weight percentage of Cu in CuO nanosheets/rGO catalyst, and it was
found to be 18.45 wt%.
[
Insert Fig. 2]
As shown in Fig. 2, the morphology of as-prepared CuO nanosheets/rGO was
characterized using SEM and TEM techniques. The rGO showed a classic wrinkle
structure (Fig. 2a and S3). The well-defined leaf-like CuO nanosheets was distributed
uniformly on the surface of rGO (Fig. 2a, S4), which indicated the oxygen functional
groups of rGO could effectively hinder the excessive recrystallization of the CuO
[
5]
nucleus. The elemental mapping and energy-dispersive X-ray (EDS) elemental
analysis (Fig. S5) of CuO nanosheets/rGO were performed to evaluate the distribution
of elements. Fig. S5a-c shows the homogeneous distribution of C, O, and Cu elemental
signals and good spatial correspondence within the CuO nanosheets/rGO. Likewise,
Fig. S5d shows the distribution of C, O, and Cu elements, which were consistent with
6

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the elemental mapping results. The TEM images of CuO nanosheets/rGO (Fig. 2b, S6)
also shows that leaf-like CuO nanosheets were uniformly dispersed on rGO and they
were well consistent with the SEM images. After the loading of leaf-like CuO
nanosheets (Fig. 2b, S6a), the highly wrinkled transparent surface of the rGO
nanosheets could still be observed. The HR-TEM image (Fig. S6b) shows the (020) face
of CuO nanosheets with a lattice space of 0.17 nm, suggesting crystal growth along that
[
5]
direction. All these characterizations demonstrated that the CuO nanosheets/rGO was
successfully prepared by the hydrothermal method.
3
.2. Catalytic homocoupling of terminal alkynes
After the characterization of the CuO nanosheets/rGO, it was then used as a catalyst
for the C -C homocoupling of terminal alkynes. Phenylacetylene (1a) was treated as a
sp sp
model substrate to obtain the optimal reaction conditions (Table S1, 1). The progression
of the reaction was determined by thin-layer chromatography.
For the initial investigation, various catalysts including GO, rGO, bulk CuO powder,
different copper salts, and CuO nanosheets were first tested (Table S1). The reaction
hardly proceeded without any catalyst (entry 1, Table S1). The performances of GO and
rGO were also quite poor (entries 2 and 3, Table S1). The common copper salts and
bulk CuO powder also provided a fair yield (entries 4-8, Table S1). Though the CuO
nanosheets catalyzed the reaction to some extent and higher than the abovementioned
copper salts under the same conditions, the yield were also far from satisfactory (entry
9
, Table S1). However, the excellent results obtained by CuO nanosheets/rGO illustrate
[
19, S13-S14]
the synergic catalytic effect of CuO and rGO at the interface.
This showed that
CuO nanosheets/rGO was an efficient catalyst for the homocoupling of
phenylacetylene.
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[
Insert Table 1]
To estimate the catalytic performance of CuO nanosheets/rGO under the designated
reaction conditions, different kinds of solvents were first investigated (Table 1). An
exciting aspect was that the isolated yield of 2a reached 98% in ethanol and 96% in
isopropanol with NaOH as the base, which was higher than other solvents (entries 1-
1
0). Toluene and some polar solvents, such as 1,4-dioxane, CH CN and DMSO, were
3
[
18,26]
also examined and lower yields were obtained (entries 4-6 and 8, Table 1).
The
good solubility of NaOH in alcoholic solvents probably facilitated this reaction.
Inhibition was almost observed when the reaction was carried out in water (entry 7,
Table 1). The effect of various bases on the performance of CuO nanosheets/rGO was
also studied. Different yields of 2a were obtained under these bases, while trace amount
of 2a was produced without any bases (entries 9 to 14, Table 1). This revealed that the
[
34]
existence of bases was necessary to promote the homocoupling reaction. Though a
moderate yield was achieved by NaOtBu, it remained lower than NaOH. Moreover, the
[
16]
poor yield of 2a (entry 15) indicated that the homocoupling reaction is aerobic.
Likewise, obvious effects of reaction temperature and time were observed on the
homocoupling reaction (Table S2 and S3). Lastly, the hot filtration and leaching study
of CuO nanosheet/rGO showed it was stable and catalyzed the homocoupling reaction
in a heterogeneous manner. Hence, the combination of CuO nanosheets/rGO (3 mg)
with NaOH (1.2 equivalent) in ethanol and reaction at 80 °C for 12 h was considered as
the optimized conditions for the homocoupling reaction.
[
Insert Table 2]
Having established the optimized conditions, terminal alkynes with different
substituents were evaluated to verify the versatility of CuO nanosheets/rGO (Table 2,
8

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eq.2). Generally, the homocoupling products could be easily isolated. The
homocoupling of phenylacetylene with electron-donating/withdrawing substituents at
para-position could be proceeded efficiently (entries 2, 3, 5, and 6, Table 2). For larger
substituents such as n-butyl, n-pentyloxy, and t-butyl, a good result could be achieved
after the reaction time was prolonged to 18 h (entries 4, 7, and 8, Table 2). Interestingly,
in the case of meta-position substituted phenylacetylenes (entries 9 and 10, Table 2), a
good yield was also realized. The yield for the homocoupling products of o-
chlorophenylacetylene was 81% (entry 11, Table 2). Hexa-2,4-diyne-1,6-diol was also
obtained, and the yield was 43% (entry 12, Table 2). Moreover, no dehalogenation was
detected for both m-chloro and o-chlorophenylacetylene. Thus, CuO nanosheets/rGO
could be considered as a versatile catalyst for the C -C homocoupling of terminal
sp
sp
alkynes.
Generally, the sustainability of one catalyst was determined by its reusability during
the heterogeneous catalytic process. For this assessment, CuO nanosheets/rGO was
reused for catalyzing the homocoupling of phenylacetylene under the optimized
conditions. In Fig. S7, a mild decrease (9%) in the yield was observed after five runs.
The TEM technique was used to characterize the morphology of the recycled CuO
nanosheets/rGO (Fig. S8). By comparing the TEM image of the recycled catalyst with
the fresh one, the morphology and structure of the recycled catalyst showed no obvious
change. These indicated that CuO nanosheets/rGO shows excellent recyclability for the
homocoupling of phenylacetylene.
[
Insert Fig. 3]
Based on the experimental results and by analogy with previous reports, a possible
pathway (Fig. 3) was proposed for the C -Csp homocoupling of terminal alkynes
sp
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[
30-34, S15-S22]
catalyzed by CuO nanosheets/rGO.
First, alkynes were adsorbed on the
active sites of the CuO nanosheets/rGO, and the π−π stacking between alkynes and rGO
increased its concentrations. The C≡C bond was activated by Cu(II) species, and
deprotonation of terminal alkynes then occurred with the assistance of bases (I).
Consequently, the C≡C bond could interact with Cu(II) species to yield the key
[
31-34, S15-S21]
intermediate copper acetylide (II).
underwent a dimerization process (III).
The copper acetylide species then
The dimeric species underwent the
[
16, 31-34]
electron transfer process promoted by CuO nanosheets/rGO and C-C bond formation
step to yield the homocoupling product. During the reaction, the transformation of
Cu(II) species was achieved by O (IV). Finally, the CuO nanosheets/rGO was
2
regenerated and a new cycle was started.
4
. Conclusions
The CuO nanosheets/rGO was successfully prepared through a facile hydrothermal
method. The well-defined CuO nanosheets/rGO showed the layer structure of rGO with
an evenly distributed leaf-like CuO nanosheets. More importantly, the CuO
nanosheets/rGO was successfully applied as an economic and efficient heterogeneous
catalyst for the C -C homocoupling of terminal alkynes. This catalyst showed little
sp
sp
activity loss after reuse for five times, and its morphology showed no obvious change.
Thus, this work broadens the application of CuO/rGO nanocomposites and promotes
green and sustainable development of organic synthetic chemistry.
Acknowledgment
This work was financially supported by the Natural Science Foundation of Shandong
Province (2019GGX104021).
Appendix A. Supplementary data
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Supplementary data to this article can be found online at: please add the website.
1
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List of Tables & Figures
Table captions
Table 1. Screening condition for estimating the catalytic performances of CuO
a
nanosheets/rGO
a
Table 2. Substrate scope
Figure captions
Fig. 1. (a) XRD patterns of CuO nanosheets and CuO nanosheets/rGO; (b) The high-
resolution XPS survey spectrum of Cu 2p.
Fig. 2. (a) SEM and (b) Low-magnification TEM images of CuO nanosheets/rGO.
Fig. 3. A possible mechanism for the homocoupling of terminal alkynes catalyzed by
CuO nanosheets/rGO.
1
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Fig. 1.
Fig. 2.
Fig. 3.
1
6

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Table 1. Screening condition for estimating the catalytic performances of CuO
a
nanosheets/rGO
b
Entry
Solvent
Base
NaOH
NaOH
NaOH
NaOH
NaOH
NaOH
NaOH
NaOH
/
2a Yield (%)
c
1
2
3
4
5
6
7
8
9
CH OH
76
98
3
C H OH
2
5
(CH ) CHOH
96
3
2
Toluene
24
1,4-Dioxane
68
CH CN
52
3
H O
2
Trace
49
DMSO
C H OH
Trace
53
2
5
1
1
1
1
1
1
0
1
2
3
4
5
C H OH
Na CO3
2
5
2
C H OH
K CO3
2
74
2
5
C H OH
NaOtBu
DBU
84
2
5
C H OH
69
2
5
C H OH
N(C H )
5 3
60
2
5
2
d
C H OH
NaOH
trace
2
5
a
Reaction condition: CuO nanosheets/rGO (3 mg), solvent (2 mL), 1 mmol 1a, base
b
c
d
(
1.2 mmol), 80 °C, 12 h, O (a balloon filled with oxygen). Isolated yield. Reflux.
2
Under a nitrogen atmosphere.
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a
Table 2. Substrate scope
b
Entry
1
Substrate
Yield(%)
98
2
3
91
89
83
88
92
84
81
c
4
5
6
c
7
c
8
9
86
87
1
0
1
1
1
2
81
43
a
Reaction conditions: alkynes (1 mmol), CuO nanosheets/rGO (3 mg), NaOH (1.2
b
c
mmol), EtOH (2 mL), 80 °C, O (a balloon filled with oxygen), 12 h. Isolated yield.
2
1
8 h.
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Highlights
The economic and well-defined leaf-like CuO nanosheets/rGO was synthesized.
The CuO nanosheets/rGO catalyzed the homocoupling of terminal alkynes efficiently.
The well synergy between CuO nanosheets and graphene.
This catalyst was reused five times with little activity loss.
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Author Contributions
Bin Wang: Conceptualization, Data curation, Investigation, Methodology,
Writing-original draft.
Lingfeng Gao: Formal analysis, Funding acquisition, Methodology,
Project administration, Supervision, Writing-review & editing.
Gengxiu Zheng: Funding acquisition, Project administration, Resources,
Validation, Writing-review & editing.
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Declaration of interests
The authors declare that they have no known competing financial interests or
☒
personal relationships that could have appeared to influence the work reported in this
paper.
☐
The authors declare the following financial interests/personal relationships which may be
considered as potential competing interests:
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