Stabilized Cu/Cu2O nanoparticles on rGO as an efficient heterogeneous catalyst for Glaser homo-coupling

Article abstract of DOI:10.1016/j.catcom.2019.04.002

Stabilized Cu/Cu₂O nanoparticles on reduced graphene oxide (Cu/Cu₂O-NPs@rGO) was synthesized by one-step co-reduction and acted as a green and efficient non-noble metal heterogeneous catalyst for Glaser homo-coupling. Through the synergic catalytic effect of Cu/Cu₂O nanoparticles and graphene, the heterogeneous hybrid nanoparticles catalyst showed excellent catalytic performance for Glaser homo-coupling with the yield up to 99% of 1,4-diphenyl buta-1,3-diyne. And excellent functional group tolerance was obtained with oxygen as a green oxidant. Furthermore, the catalyst can be easily separated and recycled seven times without significant decline in its catalytic performance.

Full text of DOI:10.1016/j.catcom.2019.04.002

Accepted Manuscript
Stabilized Cu/Cu2O nanoparticles on rGO as an efficient
heterogeneous catalyst for Glaser homo-coupling
Weiyang Lu, Wei Sun, Xiaofeng Tan, Lingfeng Gao, Gengxiu
Zheng
PII:
S1566-7367(19)30110-4
DOI:
https://doi.org/10.1016/j.catcom.2019.04.002
CATCOM 5670
Reference:
To appear in:
Catalysis Communications
Received date:
Revised date:
Accepted date:
2 January 2019
30 March 2019
1 April 2019
Please cite this article as: W. Lu, W. Sun, X. Tan, et al., Stabilized Cu/Cu2O nanoparticles
on rGO as an efficient heterogeneous catalyst for Glaser homo-coupling, Catalysis
Communications, https://doi.org/10.1016/j.catcom.2019.04.002
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ACCEPTED MANUSCRIPT
Stabilized Cu/Cu O nanoparticles on rGO as an efficient
2
heterogeneous catalyst for Glaser homo-coupling
a
a
a
a,*
Weiyang Lu , Wei Sun , Xiaofeng Tan , Lingfeng Gao gaolf108@mail.ustc.edu.cn and
a,*
Gengxiu Zheng chm_zhenggx@ujn.edu.cn
a
School of Chemistry and Chemical Engineering, University of Jinan, Jinan,
Shandong 250022, China
Corresponding authors.

ACCEPTED MANUSCRIPT
Abstract
Stabilized Cu/Cu O nanoparticles on reduced graphene oxide (Cu/Cu O-NPs@rGO)
2
2
was synthesized by one-step co-reduction and acted as a green and efficient non-noble
metal heterogeneous catalyst for Glaser homo-coupling. Through the synergic
catalytic effect of Cu/Cu O nanoparticles and graphene, the heterogeneous hybrid
2
nanoparticles catalyst showed excellent catalytic performance for Glaser
homo-coupling with the yield up to 99% of 1,4-diphenyl buta-1,3-diyne. And
excellent functional group tolerance was obtained with oxygen as a green oxidant.
Furthermore, the catalyst can be easily separated and recycled seven times without
significant decline in its catalytic performance.
Keywords: Cu/Cu O nanoparticles; graphene; Glaser homo-coupling; heterogeneous
2
hybrid catalyst; oxygen

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1
. Introduction
,3-Diynes have both symmetric and unsymmetric structures with the properties
1
of anti-inflammatory, anti-bacterial, and anti-cancer activities. These compounds
widely exist in numerous pharmaceuticals and bioactive molecules[1-4]. They are
also useful precursors in the synthesis of functional materials because the unsaturated
alkynyl group could further involve in many kinds of reactions[5-7]. Therefore, it is
of great significance to develop an excellent catalyst for the synthesis of 1,3-diynes.
Glaser homo-coupling is one of the most classical reaction for the synthesis of
symmetric 1,3-diynes[8]. The reaction was generally carried out on homogeneous
systems, in which copper salts or palladium salts were frequently used as catalyst[9,
1
0]. Typical disadvantages of homogeneous systems were the difficulties in the
recyclability of catalyst and product contamination caused by heavy metal residue
resulting in their difficulties in industrial application. To overcome these problems,
some of efficient heterogeneous catalysts have been investigated, such as
Au/CeO [11], Au@NH -SBA-15[12], Au/La O [13], AgNPs@g-C N [14]. Most of
2
2
2
3
3
4
these catalysts used the noble metal with the high cost and multi-step synthesis
processes. Thus, developing efficient no-noble metal heterogeneous catalyst with the
simple synthetic process is of great value for practical applications in Glaser
homo-coupling reaction.
Among the transition metal nanomaterials, Cu-NPs (e.g., Cu, Cu O, or CuO)
2
materials have always attracted more attention as the catalyst than others because of
copper’s low cost, abundant reserves, outstanding physical and chemical
0
+
properties[15]. However, the inherent drawback of Cu /Cu -NPs was easily oxidized
under atmospheric conditions, limiting their practical application[16]. Graphene, as a
2
single layer of sp bonded carbon material[17], possesses many advantages such as

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superior electroconductive performances[18, 19], large surface area, excellent thermal
and chemical stability[20]. The unique properties facilitated its use in photoelectric
devices[21, 22], energy storage[23, 24], chemical sensor[25, 26] as well as an ideal
support for high performance catalysts[27, 28]. For the synthesis of highly active
catalysts, we combined both the advantages of Cu-NPs and graphene. The formation
of Cu/Cu O-NPs@rGO made the Cu/Cu O-NPs well dispersed and stabilized on the
2
2
surface of rGO. By this method, Cu/Cu O-NPs were endowed with great
2
anti-aggregation and excellent stability, then further applied in the field of organic
catalysis.
In our previous work, we have developed a green and economical Cu NPs-based
catalyst for the oxidation of alcohols to aldehydes with high conversion and
selectivity[29]. Then we further developed a sustainable heterogeneous catalyst
Cu/Cu O-NPs@rGO for Glaser homo-coupling. By the π-π stacking interaction
2
between phenylacetylene and rGO layers[30], the alkynyl groups were enriched
around the rGO to further accelerate the reaction rate. As a result, our new designed
hybrid Cu/Cu O nanoparticles on reduced graphene oxide (Cu/Cu O-NPs@rGO)
2
2
shows excellent catalysis for Glaser homo-coupling. Meanwhile, Cu/Cu O-NPs@rGO
2
could be easily separated by centrifugation and reused seven times without significant
decline in its catalytic performance.
2
. Experimental
2
.1. Graphene Oxide(GO) preparation
Graphene oxide (GO) was prepared by modified Hummers method through the
intense oxidation of natural graphite power and then ultrasonic stripping. The
prepared details are given in the Supporting Information.
2
.2. Synthesis of Cu/Cu O-NPs@rGO catalysts
2

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The Cu/Cu O-NPs@rGO catalysts were prepared as follow: The as-prepared GO
2
-1
was dispersed into deionized water getting GO solution of 2 mg·mL . 10 mL of
-1
-1
Cu(OAc) ·H O (0.2 mol·mL ) was mixed with 50 mL of GO (2 mg·mL ) under
2
2
constant magnetic stirring to get an evenly mixture. Subsequently, 10 mL of ascorbic
acid (VC) aqueous solution (0.4 mol/L) was added to the mixture slowly. Then, the
o
mixture was stirred for 2 h at 60 C. After the reaction was finished, the reaction
solution was cooled to room temperature naturally. The Cu/Cu O-NPs@rGO catalyst
2
was centrifuged out from the reaction solution, washed with water and ethanol, and
dried in a lyophilization step. The rGO was synthesized by the same way without
Cu(OAc) ·H O.
2
2
2
.3. Characterization
The prepared Cu/Cu O-NPs@rGO samples were characterized by XRD, XPS,
2
N -BET, ICP, SEM, TEM. More details about these characterizations are given in
2
Supporting Information.
2
.4. Typical experimental process for Glaser homo-coupling
Glaser homo-coupling reaction was evaluated in a 25 mL round bottomed flask
under constant magnetic stirring. Typically, a mixture of phenylacetylene (1 mmol),
catalyst, base and solvent (2 mL) were added into round the bottomed flask.
Subsequently O was introduced into the reaction system instead of air. The mixture
2
o
solution was stirred at 80 C for 8 h. After the reaction was finished, the reaction
solution was cooled down to room temperature naturally. The solid catalyst was
centrifuged recovered and organic phase was extracted with ethyl acetate to get crude
products. All crude products were purified by column chromatography to calculate the
1
13
yields and identified by H NMR and C NMR spectra.
. Results and discussion
3

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3
.1. Characterization of Cu/Cu O-NPs@rGO
2
[
Insert Fig. 1]
The typical XRD diffraction pattern of Cu/Cu O-NPs@rGO was presented in Fig.
2
1
a. The diffraction peaks at 29.6, 36.5, 42.4, 61.5 can be assigned to the (110), (111),
(
200), (220) planes of cubic Cu O crystal phase (JCPDS No. 65-3288). And strong
2
diffraction peaks at 43.3, 50.4, 74.1 were well consistent with the (111), (200), (220)
planes of cubic Cu crystal phase (JCPDS No. 04-0836) while no diffraction peaks of
other impurities were observed. The results indicated that the Cu/Cu O nanoparticles
2
had the high crystallinity. XPS was conducted to analyze the oxidation state of Cu on
the surface of graphene. The result was shown in Fig. 1 b. The binding energy located
0
+
at approximately 932.5 eV and satellite peaks at 952.5 eV can be assigned to Cu /Cu
0
because there is only a difference of 0.1-0.2 eV between the binding energies of Cu
+
and Cu [30]. The much lower peak at 934.9 eV and satellite peaks at 943.2 eV were
2
+
2+
attributed to Cu(OH) (Cu ) in Cu/Cu O-NPs@rGO[31] because a small part of Cu
2
2
was incomplete reduced[29]. And the XRD pattern clearly indicated that the
formation of Cu/Cu O crystals without the presence of CuO crystals. ICP-OES was
2
used to detect total amount of Cu in the catalyst and the result was 26.24 w. t. %. The
above characterization results indicated that Cu/Cu O nanoparticles were successfully
2
generated with highly crystalline.
[
Insert Fig. 2]
The morphology and structure of Cu/Cu O-NPs@rGO were characterized by
2
TEM and SEM. From the TEM image of Fig. 2 a, oxygen-containing groups (such as
hydroxyl, epoxy, and carboxyl group) on the surface of GO can act as anchoring sites
for the formation of metal nanoparticles[32]. And we could see that a lot of dark

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Cu/Cu O nanoparticles were distributed evenly on the surface of transparent
2
films-like rGO. Further, the typical wrinkle structures were still exhibited on the
surface of Cu/Cu O-NPs@rGO, which demonstrated that the layered structure of the
2
catalyst was unchanged by one-step co-reduction. From the SEM image of Fig. 2 b, it
could be seen that some bright nanoparticles supported well on the wrinkled surface
of the rGO. The consistency of both TEM and SEM images suggested that Cu/Cu O
2
nanoparticles were successfully loaded on the rGO. Moreover, the N -BET result was
2
shown in Fig. S4. and Fig. S5. the SBET of the Cu/Cu O-NPs@rGO was obtained as
2
2
-1
1
00 m ·g , and the average pore diameter of Cu/Cu O-NPs@rGO was 11.9 nm.
2
3
.2. Catalytic performance
At the beginning of the experiment, the Glaser homo-coupling was carried out in
o
EtOH with various catalysts under O at 80 C for 8 h. And the catalytic performance
2
is summarized in Table 1. In the absence of catalyst, the homo-coupling of
phenylacetylene was inhibited (Table 1, entry 1). It was indicated that the reaction
was very difficult to carry out without any catalyst. Several copper salts exhibited
moderate catalytic activities. (Table 1, entries 2, 3). Cu-NPs, Cu O-NPs and CuO-NPs
2
were also investigated in the experiment. It could be seen that Cu-NPs showed
relatively higher catalytic activity compared with Cu O-NPs and CuO-NPs at the
2
yield of 81% (Table 1, entries 5-7). The mixed catalyst of Cu-NPs and Cu O-NPs of
2
equal mass ratio only gave the yield of 80% (Table 1, entry 8). In addition, rGO
exhibited little catalytic activity (Table 1, entry 9). With the extension of reaction time,
the yield of 1,4-diphenyl buta-1,3-diyne was also increased (Table 1, entries 10-12,
for details, see Fig. S2). Notably, when the reaction time reached 8 hours,
Cu/Cu O-NPs@rGO gave excellent catalytic activity for the homo-coupling of
2
phenylacetylene with the yield up to 99% (Table 1, Entry 11). Besides, when the

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dosage of phenylacetylene was increased to 10 mmol, the yield was 96% and no
significant loss in the yield was observed (Table 1, Entry 13). It was proved that the
Cu/Cu O-NPs@rGO had outstanding potential in industrial application. The above
2
results indicated that the synergic catalytic effect of Cu/Cu O nanoparticles and rGO
2
significantly improved the catalytic activity for the Glaser homo-coupling.
[
Insert Table 1]
Then, various solvents were evaluated for the Glaser homo-coupling. And the
results were shown in Table S1. When some polar solvents such as DMF, DMSO,
CH CN were utilized as solvents, low yields were observed (6-42%, Table S1, entries
3
1
-3). Then some non-polar solvents were investigated in the reaction such as CH Cl ,
2 2
THF and toluene, low yields were also obtained (0-32%, Table S1, entries 4-6).
Through experiment and observation, we found that the Cu/Cu O-NPs@rGO had
2
poor dispersion in the solvents of CH Cl , THF and toluene. Because phenylacetylene
2
2
and H O were incompatible, the reaction was even almost inhibited in H O (Table S1,
2
2
entry 10). To our delight, when alcohols were used as solvent, the good to excellent
yields of the homo-coupling of phenylacetylene were obtained (Table S1, entries 7-9).
Especially in EtOH solvent, the yield was up to 99%. Based on the above results,
alcohols solvents were propitious to the Glaser homo-coupling reaction. For the
convenience of handing, EtOH (2 mL) was utilized as the solvent.
Moreover, Fig. S1 indicated that the optimum reaction temperature on the
o
homo-coupling of phenylacetylene over the Cu/Cu O-NPs@rGO was 80 C. Different
2
bases were also optimized for the Glaser homo-coupling and Cs CO is the best
2
3
choice (for details, see Table. S2). Thus, the optimal reaction parameters were
obtained as following: 1 mmol of phenylacetylene, 4 mg of Cu/Cu O-NPs@rGO and
2
o
1
.2 mmol of Cs CO in 2 mL EtOH at 80 C under O for 8 h.
2
3
2

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Under the optimum reaction conditions, we have investigated a series of
functional groups on phenylacetylene including electron-donating groups and
electron-withdrawing groups. All of them gave good to excellent yields. The
phenylacetylene bearing withdrawing groups F-, Cl- and Br- offered higher yields of
9
4%, 98%, 96% respectively(Table 2, entries 2-4). The phenylacetylene bearing
donating groups offered the high yields (CH CH -: 92%, (CH ) C-: 91%,
3
2
3 3
CH (CH ) CH -: 90%, CH O-: 94%, NH -: 85%) (Table 2, entries 5-9). The
3
2 3
2
3
2
heterocyclic substrates were also examined in the reaction condition, giving good
yields of 83% and 94% (Table 2, entries 10, 11). Hexadeca-7,9-diyne was also
investigated in the reaction with a moderate yield of 53% (Table 2, entry 12). The
above results revealed that the Cu/Cu O-NPs@rGO exhibited excellent catalytic
2
activity for Glaser homo-coupling of both aromatic and heterocyclic alkynes, and
possessed excellent functional groups tolerance.
[
Insert Table 2]
According to the previous reports[33-35] and our studies, a proposed mechanism
was shown in Fig. 3. Due to the strong electric conduction of graphene, the value
states of Cu/Cu O-NPs were easily changed by electron transfer. Also, the presence of
2
the huge surface of rGO and a number of exposed Cu/Cu O nanoparticles facilitated
2
the homo-coupling of the terminal alkynes. First, by the π-π stacking interaction
between aromatic ring and rGO layers, the alkynyl groups were enriched around the
rGO to coordinate with exposed Cu/Cu O nanoparticles on the rGO and generated I.
2
The inactive C-H bond was activated for active C-H bond. Under the alkaline
condition of Cs CO , I was deprotonated, C-Cu/Cu O-NPs@rGO bond was formed
2
3
2
obtaining II. II were dimerized affording III. Subsequently, III taken part in the
electron transfer promoted by good conductive Cu/Cu O-NPs@rGO and C-C bond
2

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was then formed to obtain the final product 1,3-diyne with realizing the
Cu/Cu O-NPs@rGO for the next catalysis cycle. Along with the catalysis process, the
2
transformation of Cu(0) and Cu (I) on the rGO was realized by the oxidation of O2.
[
Insert Fig. 3]
Recyclability is an imperative aspect of heterogeneous catalytic performance.
The recyclability of the Cu/Cu O-NPs@rGO was investigated with the
2
phenylacetylene as standard substrate under optimal conditions. Cu/Cu O-NPs@rGO
2
was recycled seven times after easily separated by centrifugation and washed with
ethanol and water. And the result was shown in Fig. S3. There was no evident decline
in its catalytic activities after seven times with the yield of 92%. Moreover, the XRD
pattern of the recycled Cu/Cu O-NPs@rGO was shown in Fig. S6. The diffraction
2
peaks of cubic Cu and Cu O were highly consistent with Fig. 1 a. It indicated that the
2
catalyst had no obvious change after recycled for seven consecutive times.
4
. Conclusions
In summary, we have developed an efficient heterogeneous catalyst
Cu/Cu O-NPs@rGO for Glaser homo-coupling. The Cu/Cu O-NPs@rGO was
2
2
prepared by simple chemical co-reduction and had some advantages such as green
catalytic system, high catalytic activity, easy separation and excellent recyclability.
After a series of characterizations of XRD, XPS, N -BET, ICP, SEM, TEM, Cu/Cu O
2
2
nanoparticles were proved to firmly support on the rGO. Cu/Cu O-NPs@rGO
2
successfully catalyzed the homo-coupling of various terminal alkynes with good to
excellent yields. Our results provide a reliable method for configuring copper-based
carbon catalysts and more opportunities for heterogeneous catalytic applications in
organic reaction.
Acknowledgements

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This study was supported by the Natural Science Foundation of Shandong Province
(ZR2017PB005) and the National Natural Science Foundation of China (Nos.
2
1601064).
Appendix A. Supplementary date
Supplementary data to this article can be found online at: please add the website.

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List of Tables & Figures
Table captions
Table 1. The catalytic performance of different catalysts for the Glaser
homo-coupling
Table 2. Cu/Cu O-NPs@rGO catalysed homo-coupling of various terminal alkynes
2
Figure captions
Fig. 1. Characterization of Cu/Cu O-NPs@rGO (a) XRD pattern; (b) XPS survey
2
spectra
Fig. 2. Characterization of Cu/Cu O-NPs@rGO (a) TEM image; (b) SEM image
2
Fig. 3. Proposed mechanism for Cu/Cu O-NPs@rGO catalyzed Glaser
2
homo-coupling
Fig. 1 Characterization of Cu/Cu O-NPs@rGO (a) XRD pattern; (b) XPS survey
2
spectra
Fig. 2 Characterization of Cu/Cu O-NPs@rGO (a) TEM image; (b) SEM image
2

ACCEPTED MANUSCRIPT
Fig. 3. Proposed mechanism for Cu/Cu O-NPs@rGO catalyzed Glaser
2
homo-coupling
Table 1 The catalytic performance of different catalysts for the Glaser
a
homo-coupling
b
Entry
Catalyst
blank
t(h)
8
Yield (%)
c
1
trace
2
3
5
6
7
CuBr
8
8
74
10
CuBr2
e
Cu-NPs
8
81
e
Cu O-NPs
8
70
2
e
CuO-NPs
8
68
d
8
Cu-NPs/Cu O-NPs
8
80
2
9
rGO
8
trace
83
1
0
1
Cu O-rGO
6
2
1
Cu O-rGO
8
99
2
1
2
Cu O-rGO
10
99
2
f
1
3
Cu O-rGO
8
96
2

ACCEPTED MANUSCRIPT
a
Reaction conditions: phenylacetylene (1mmol), catalyst (4 mg), Cs CO (1.2 mmol),
2
3
o
b
c
d
EtOH (2 mL), 80 C, O , 8 hours. Isolated yields. Catalyst-free. m(Cu-NPs) :
2
e
f
m(Cu O-NPs)=1 : 1. Commercial. Phenylacetylene (10 mmol).
2
a
Table 2 Cu/Cu O-NPs@rGO catalysed homo-coupling of various terminal alkynes
2
b
Entry
1
Substrate
Product
Yield (%)
99
2
3
4
5
6
7
94
98
96
92
91
90

ACCEPTED MANUSCRIPT
8
9
94
85
83
94
53
10
11
12
a
Reaction conditions: 1 (1 mmol), Cu/Cu O-NPs@rGO (4 mg), Cs CO (1.2 mmol),
2
2
3
o
b
EtOH (2 mL), 80 C, O , 8 hours. Isolated yields.
2

ACCEPTED MANUSCRIPT
Highlights




Non-noble Cu/Cu O-NPs@rGO catalyzed Glaser homo-coupling.
2
Great functional group tolerance.
The synergic catalysis of Cu/Cu O-NPs and graphene.
2
Great recyclability.