High-efficiency catalyst for copper nanoparticles attached to porous nitrogen-doped carbon materials: Applied to the coupling reaction of alkyne groups under mild conditions

Article abstract of DOI:10.1002/aoc.6163

Supported catalysts have attracted extensive attention due to their excellent catalytic performance and reliability in heterogeneous catalysis. In this work, we report a general synthesis strategy that achieves the self-coupling reaction of acetylene deri

Full text of DOI:10.1002/aoc.6163

Received: 24 September 2020
DOI: 10.1002/aoc.6163
Revised: 22 December 2020
Accepted: 1 January 2021
F U L L P A P E R
High-efficiency catalyst for copper nanoparticles attached
to porous nitrogen-doped carbon materials: Applied to the
coupling reaction of alkyne groups under mild conditions
Lei Ma
| Pengbo Jiang
| Li Gong
| Kaizhi Wang
| Rong Li
| Xiaokang Huang
|
Ming Yang
State Key Laboratory of Applied Organic
Chemistry (SKLAOC), the Key Laboratory
of Catalytic Engineering of Gansu
Province, College of Chemistry and
Chemical Engineering, Lanzhou
University, Lanzhou, China
Supported catalysts have attracted extensive attention due to their excellent
catalytic performance and reliability in heterogeneous catalysis. In this
work, we report a general synthesis strategy that achieves the self-coupling
reaction of acetylene derivatives to 1,3-diyne efficiently under conditions of
copper catalyst impregnated on the precursor formed by acetone and urea.
The experiments were performed by screening the base, solvent, tempera-
ture, and so forth to determine the optimum reaction conditions and then
characterization and analysis of the catalyst. The results demonstrate that
the Cu/CuO@CN(8) exhibits extraordinary reactivity to the self-coupling
reaction and achieves a high turnover frequency (TOF = 96.8). Typically,
the conversion of phenylacetylene reaches 99.9% under the optimal reaction
Correspondence
Rong Li, State Key Laboratory of Applied
Organic Chemistry (SKLAOC), the Key
Laboratory of Catalytic Engineering of
Gansu Province, College of Chemistry and
Chemical Engineering, Lanzhou
University, Lanzhou, Gansu 730000,
China.
Email: liyirong@lzu.edu.cn
conditions of NaOH (2 mmol) and tert-butanol (2 ml) and O (1 atm) at
2
ꢀ
6
0 C for 1 h. Nevertheless, it is worth noting that Cu/CuO@CN(8) has a
2
−1
large specific surface area (626.07 m g ) and low metal loading (3.3%)
measured by Brunauer Emmett–Teller (BET) and ICP-OES, respectively.
Simultaneously, kinetics and mechanism are also discussed and analyzed,
−
1
and the thermodynamic energy value is calculated as 22.74 kJ mol .
Besides, the optimum catalyst can be reused five times under optimal
conditions without a significant decrease in reactivity.
K E Y W O R D S
1
,3-diynes, coupling reaction, heterogeneous catalysis, kinetic study, phenylacetylene
[
2]
1
| INTRODUCTION
electromagnetic materials, and optical devices. Because
the initial synthesis strategy has some shortcomings, such
as low atom utilization, failure to achieve reuse, and
Transforming aromatic alkyne into 1,3-diyne releases its
enormous value as an organic monomer. 1,3-diyne is a
kind of conjugated diyne compound with symmetric and
asymmetric structure and unique properties, which are
widely applied to material chemistry and synthetic chem-
istry because of its antibacterial, anti-inflammatory, and
[
3]
complex reaction conditions. Therefore, the conversion
from homogeneous catalysis to heterogeneous catalysis
and performance improvement of heterogeneous cata-
lysts are significant topics in the current research field
[
4]
(Table S1).
[1]
anticancer effects.
At the same time, 1,3-diyne is
Due to the maturity of the reactions and complex-
also an important part of photovoltaic materials,
ity of the products, the Glaser–Hay coupling reaction
Appl Organomet Chem. 2021;35:e6163.
wileyonlinelibrary.com/journal/aoc
© 2021 John Wiley & Sons, Ltd.
1 of 16
https://doi.org/10.1002/aoc.6163

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MA ET AL.
attracted the attention of many scientists in recent
years and had become one of the vital strategies for
2 | EXPERIMENTAL SECTION
2.1 | Reagents and chemicals
[5]
organic synthesis. In the past few decades, terminal
alkynyl groups showed outstanding talent in various
[6]
fields. Palladium-, copper-catalytic systems have been
successfully developed to achieve self-coupling of
All the reagents and chemicals used in the synthesis of
catalyst were purchased from commercial sources and
used without further purification, unless otherwise
noted.
Detailed description of the reagents and chemicals
will be displayed in the support information.
[7]
terminal alkynes. It is worth noting that copper is
considered cheaper and easier to obtain than palla-
[8]
dium at the same conversion rate. Besides, limited
research was conducted on the recyclability and high
efficiency of the catalysts used in this reaction, which
[9]
remains a hot topic in current research. In addition,
the copper (I) salt (e.g., CuI and CuCl) has been
reported in a large number of previous catalytic
systems, which is a challenge to overcome the short-
2.2 | Preparation of N-doped porous
carbon precursor (CN)
[10]
coming in the recycling of the catalyst.
To address
For the synthesis of porous carbon-based materials
(Scheme SCHEME 1), initially, acetone (40 ml) and
sodium hydroxide (8 g) were added to a one-necked
100 ml round-bottomed flask equipped with a mechani-
cal stirrer. Subsequently, the mixture will be sonicated
for 30 min at room temperature. Since then, the reaction
the above issues, various strategies have been dedicated
to achieve load reduction based on reducing the band
gap and accelerating the separation of electron–hole
[2a,10b]
pairs and increasing their catalytic activity.
With
the development of a series of related researches, its
synthetic process has become more and more simple
and efficient. Nitrogen-doped carbon materials are
considered as a well carrier with high surface area,
ꢀ
was carried out in a water bath at 40 C, which isolated
the air and stirred vigorously for 2 h. After the reaction, it
was gradually cooled to room temperature. The color of
the mixture changes from colorless to orange during
this reaction.
[11]
better stability, and electrical conductivity.
The lone
pair of nitrogen can modify the copper nanoparticles
on the surface of the support and fix the Cu
nanoparticles, which increases the recyclability of the
A mixture of urea (3 g) and carbon precursor (1 g) in
agate mortar was completely ground for 10 min and
followed transferred to a quartz boat. Later, the sample
[12]
catalyst.
In addition, Cu catalyst doped with N has
ꢀ
been extensively studied in the coupling reaction.
Therefore, research on supported metal catalysts has
important significance. Indeed, this type of catalysts
was reported by many researches because of its high
was calcined at 800 C for 2 h with a heating rate of
ꢀ
−1
3 C min in a tube furnace under N flow. After the end
2
of calcination, the system was cooled to room tempera-
ture naturally and the black powder was obtained.
Furthermore, the sample was washed several times with
deionized water and hydrochloric acid (1 M, 62 ml) until
the solution become neutral. Subsequently, the mixture
was separated by filtration. Finally, the precursor was
[10c,13]
efficiency, recyclability, and simplicity.
Herein, this work focuses on Glaser–Hay reaction,
and phenylacetylene is used as a starting material to
[14]
construct 1,4-diphenylbutadiyne.
A type of high-
ꢀ
surface-area polyporous and coral-like carbon materials
obtained by drying at 60 C for 24 h and named by CN.
[15]
was synthesized from acetone
and applied to
synthetic 1,4-diphenylbutadiyne under mild reaction
[16]
conditions.
Optimal reaction conditions were
2.3 | Preparation of the Cu/CuO@CN
obtained by analyzing the factors, such as alkali,
solvent, reaction temperature, and reaction time. Exper-
imental exploration shows the prepared catalyst has
catalyst
An interconnected solid matrix with polyporous was syn-
thesized directly by immobilizing the Cu salts (Cu (NO3)2
3H O, CuSO 5H O, Cu (CH COO) ꢁH O, CuCl ꢁ2H O)
2
−1
a high surface area (656 m g ), and the Cu
nanoparticles are uniformly dispersed on the surface of
the carrier. Moreover, the catalyst produces high
2
4
2
3
2
2
2
2
on the surface of CN (Scheme SCHEME 1). The prepara-
tory work before the synthesis was as follows: Different
copper salts (0.03 M) and deionized water (30 ml) were
added to a 100-ml beaker, and the mixture was stirred
continuously for 20 min to allow the solid precipitate to
be dissolved thoroughly. Subsequently, the prepared solu-
tion (1 M) was transferred to a 100-ml conical flask.
ꢀ
conversion (99.9%) under conditions of 60 C and O2
(1 atm). Compared with the activities of the copper salt
catalysts, the cycle performance can be maintained at
least five times without significant reaction decay.
Furthermore, the kinetics of the exploration also illus-
trates the rationality of the catalyst.

MA ET AL.
3 of 16
SCHEME 1 Preparation of
the carbon materials and
catalysts
Next, the CN (400 mg) was suspended in 35-ml
deionized water. The copper nitrate solution (1.65 ml)
was added gradually to the suspension and followed by
2.4 | Catalytic evaluation
According to a typical process, the coupling reaction was
carried out in a 10-ml grinding tube filled with oxygen
(1 atm). To the mixture were added catalyst (20 mg),
NaOH (80 mg, 2 mmol) and phenylacetylene (102 mg,
ꢀ
slight stirring at 100 C for 3 h in an oil bath. The reaction
was carried out until the deionized water was completely
evaporated. After removing deionized water in air, the
ꢀ
ꢀ
mixture was collected and dried at 60 C in a vacuum
1 mmol) in tert-Butanol (2 ml) was heated at 60 C for 1 h
oven. Latterly, the catalyst was first calcined under a N
with magnetic stirring. Subsequently, the sample was
extracted with deionized water (2 × 3 ml) and ethyl
acetate (1 × 1 ml). Later, the product was collected by
centrifugation then analyzed and determined by gas
chromatography–mass spectrometry (GC–MS) (Agilent
6890N/5937N). The activity of the catalyst was calculated
as follows: conversion % = 100 × (([C ]–[C ])/[C ]), yield
2
−
1
ꢀ
flow of 60 ml min at 600 C for 40 min in a tube
ꢀ
furnace, followed by increasing the temperature to 800 C
ꢀ
−1
for 2 h with a heating rate of 3 C min . After cooling
to room temperature, the Cu/CuO@CN catalyst was
obtained.
Preparation of Cu/CuO@C by the same synthesis
method as Cu/CuO@CN without N, and samples with
different Cu content were obtained with labeled as
Cu/CuO@CN (x%), where x stands for the Cu/CN mass
ratio in the composite. So the Cu/CuO@CN (6.5%),
Cu/CuO@CN (12%), and Cu/CuO @ CN (20%) are syn-
thesized in this experiment. In addition, Cu/CuO@CN
0
1
0
% = 100 × (2 × [C ]/[C ]), and TOF = ([C ] − [C ])× V/
2
0
0
1
(M )/t. Based on the initial concentration [C ] and the
Cu
0
remaining concentration [C ] of the reactant, as well
1
as the concentration of the resulting product [C2],
V represents the volume of the solvent in the system, and
the reaction time is represented by t. In addition, MCu is
expressed as the molar amount of Cu in the catalyst.
(6), Cu/CuO@CN (7), and Cu/CuO@CN (9) were also
prepared separately by converting the highest tempera-
ꢀ
ꢀ
ture of the second heating process to 600 C, 700 C, and
9
ꢀ
00 C. Moreover, other different types of copper salts
2.5 | Characterizations
were synthesized by the same method and named
Cu/CuO@CN (S), Cu/CuO@CN (C), and Cu/CuO@CN
To understand the physical and chemical properties of
the catalyst in more detail. Some measurement methods
(Cl), respectively.

4
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MA ET AL.
−
1
would be applied as follows: X-ray diffraction (XRD), X-
ray photoelectron spectroscopy (XPS), high-angle annular
dark-field scanning transmission electron microscopy
approximately 1360 and 1590 cm , respectively, illustrat-
ing the formation of structural defects in the graphitic
2
−
plane and the E2g vibrational mode of the sp bond gra-
phitic carbons. The intensity of the D and G bands (ID/
IG) indicates the degree of graphitization of the carbon
material. Furthermore, the I /IG value increased from
0.968 to 1.0243 with the temperature rising from 600 C to
800 C. However, when the temperature rose to 900 C,
the value drops to 1.0076. Therefore, it can be concluded
that the highest I /IG value can be attributed to
(HAADF-STEM), Brunauer Emmett–Teller (BET), ele-
mental analysis (EA), Fourier-transform infrared spec-
troscopy (FT-IR), Raman spectra, and inductively
coupled plasma optical emission spectrometer (ICP-
OES). A detailed description of the catalyst characteriza-
tion instrument would be provided in the supporting
information.
D
ꢀ
ꢀ
ꢀ
D
Cu/CuO@CN(8), and this indicates that its structure has
a large defect concentration. In addition, Raman of CN
and Cu/CuO@CN (8) are also shown in Figure S2, which
indicates that the degree of graphitization of the carbon
material is not significantly changed by the introduction
of the metal.
3
3
| RESULTS AND DISCUSSION
.1 | Characterization of catalysts
The crystal structure and phase composition of the cata-
lysts were systematically investigated by using XRD pat-
terns and Raman spectra. The XRD results of the
precursor and the catalyst are shown in Figure 1. Nota-
bly, the two broad smooth peaks are shown in the
nitrogen-doped precursor (CN). The two strong peaks are
orthorhombic of the catalyst, and the positions of peaks
In order to obtain further detailed information on the
morphology and microstructures of the CN, as well as
the dispersion of metal particles on the precursor, the
scanning electron microscope (SEM) technique is
employed. As shown in Figure 2a and 2b, the SEM
images demonstrates that the CN is a petal-like structure.
It can be confirmed that the precursor has a porous struc-
ture from Figure 2c. At the same time, the topography of
Cu/CuO@CN(8) is also shown in Figure 2d, and it can be
observed that the structure of the carbon material is
slightly altered by doping the metal, but the porous,
petal-like structure in the catalyst is still clearly visible.
This may be attributed to the second calcination. Besides,
it can be clearly seen that the metal particles are uni-
formly dispersed on the catalyst surface. In addition, the
SEM is also used for the catalyst and support after the
cycles (Figure S3). Figure S3a–S3c and Figure S3d–S3f
reflect the changes in the morphology of the catalyst after
two and five cycles, respectively. Through observation of
ꢀ
ꢀ
at 36 and 43 correspond to the (111) and (111) planes,
which shows a typical pattern of crystalline Cu particles
(
JCPDS no. 70–3038) and CuO particles (JCPDS
no. 3–884). The patterns of other types of Cu-based cata-
lysts by the same method show five peaks at 36 , 43 , 50 ,
6
ꢀ
ꢀ
ꢀ
ꢀ ꢀ
1 , and 74 , which correspond to the (111), (111), (200),
(220), and (220) planes, respectively (Figure S1).
At the same time, because the catalyst is a carbon-
based composite, the Raman scattering spectroscopy
shows detailed defect information for the CN and catalyst
(Figure 1b). The characteristic D-band and G-band
of graphitized carbon correspond to two peaks at
FIGURE 1 (a) X-ray diffraction (XRD) pattern of the CN and Cu/CuO@CN and (b) Raman spectra of different catalysts

MA ET AL.
5 of 16
FIGURE 2 (a–c) Scanning electron microscope (SEM) image of the CN and (d) scanning electron microscope (SEM) image of the
Cu/CuO@CN(8)
Figure S3a–S3c, which can be seen that the morphology
of the catalyst did not change significantly after two
cycles, it still appeared petal-like, with only a small
amount of flake structure broken (Figure S3c). Although
the petal-like structure gradually changed—a little col-
lapse and the active components in the catalyst fell off a
little, resulting in a decrease in the activation rate after
five cycles, the overall catalytic activity remains well—
the conversion rate is still above 90%.
concluded that the particle size of the Cu nanoparticles is
approximately 2–5 nm, and the size is 3–4 nm account
for 40% of all the particles. At the same time, the average
size of the nanoparticles is calculated to be approximately
3.8 nm (inset in Figure S4). The results of the high-
resolution TEM (HR-TEM) image (magnified image of
Figure S4) are Figure 3c, which further indicate that the
lattice fringe spacing is d (0.206 nm) and d (0.245 nm)
1
2
corresponds to the 111, 111 crystal planes of Cu, CuO in
the target catalyst, respectively. Furthermore, they are
Meanwhile, the transmission electron microscopy
ꢀ
ꢀ
(TEM) images (Figure 3a and 3b) also demonstrate that
matched to diffraction peaks at 36 and 43 for XRD
results, respectively. HAADF-STEM acts as a specific area
of the element mapping shown in Figure 3g. Besides, the
elemental mapping of the catalyst indicates that C, N,
and Cu are uniformly distributed over a particular region,
thereby inferring that the Cu nanoparticles are homoge-
neously dispersed on the N-doped carbon material
(Figure 3d–3f). However, the content of Cu (Figure 3f) is
less than of C and N, which is attributed to the lesser
amount introduced in the process of synthesizing the
catalyst. In addition, the optimal catalyst is also charac-
terized after the cycles (Figure S5). It can be observed
that the general morphology of the petals remains
unchanged after several cycles—the morphology of the
petals remains obvious. Figure S5b still shows that copper
the Cu/CuO@CN(8) catalyst is a kind of petal-like mate-
rial with a unique porous structure and the wall of hole
is composed of the nanosheets, which is consistent with
the SEM image of Cu/CuO@CN(8) (Figure 2d). TEM
image of the Cu/CuO@CN(8) is shown in Figure 3a and
3
b. From Figure 3a, the topography of the carbon mate-
rial is changed slightly when nitrogen is incorporated,
which is more like a kind of three-dimensional nanorod
structure. As shown in Figure 3b, the structure of the cat-
alyst is converted from a petal shape to a sheet shape by
calcination with supported metal. The particle size distri-
bution image is shown in Figure S4, and it can be
observed that the metal nanoparticles are uniformly
distributed on the precursor. In addition, it can be

6
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MA ET AL.
FIGURE 3 Effect of posttreatment on surface morphology, pore size, Cu loading, and distribution of catalyst. (a–c) Morphological
characteristics of transmission electron microscopy (TEM) images at different magnifications. (d–f) The mapping of C, N and Cu. (g) A high-
angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image of the Cu/CuO@CN(8)
particles are evenly dispersed on the carrier. Figure S5c
is the measurement of the lattice after the cycle, which is
consistent with the result of Figure 3c. The result of line
scanning in Figure S5d reflects the proportion of catalyst
Cu, C, and N content, which indirectly indicates that the
content of Cu in the catalyst is less. In Figure S5e–S5h
are images of C, N, Cu, and HAADF, respectively.
Additionally, owing to the surface area and pore size
of the catalyst was closely related to the activity of
the catalyst. Therefore, the characteristics of these
corresponding aspects of the catalyst are investigated by
BET. According to Figure 4, the micropore characteristics
are investigated by nitrogen adsorption–desorption, and
the particle size distribution of the catalyst and precursor
is also illustrated. It can be found that the catalyst and
the carrier have obvious hysteresis loops. In detail, the
significant mesoporous properties are confirmed to have
existed in CN and Cu/CuO@CN(8), and Figure 4a and
Table 1 show that the pore width has fluctuated between
amount at the beginning of adsorption is not 0, which is
due to the microporous nature of the carbon material
and catalyst. Besides, compared with Cu/CuO@CN(8),
the initial amount of CN adsorption is larger, indicating
that the CN material has a large pore volume, and the
incorporation of Cu reduces the pore volume. The aver-
age values of pore width, surface area, and pore volume
are also shown in Table 1, and the surface area of
2
−1
Cu/CuO@CN(8) is reduced by 34 m g compared to
CN, indicating that the doped of Cu decreases the surface
area. Both Cu/CuO@CN(8) and CN have considerable
specific surface area values, but the difference is small
because of the low copper loading. It can also be seen
3
−1
from Table 1 that the volume of CN is 0.75 cm g ,
3
−1
whereas the Cu/CuO@CN(8) is 0.60 cm g , which indi-
cates that the Cu nanoparticles are dispersed inside the
pores of the carrier. Furthermore, the result of pore size
analysis shows that a small amount of doped Cu has a
slight effect on the pore size. Generally, the precursor
and catalyst with large surface area, pore volume, and
3
and 6 nm. In addition, Figure 4b reveals that the initial

MA ET AL.
7 of 16
FIGURE 4 (a) Pore size distribution image of precursor and catalyst. (b) Nitrogen absorption–desorption isotherms of CN and
Cu/CuO@CN(8)
small pore width are synthesized. Indeed, the introduc-
tion of the Cu element mainly increases the active sites
on the catalyst, thereby significantly improving the
catalytic activity of the composite catalyst.
spectrum of N1s is illustrated in Figure 5c. It can be con-
firmed from the figure that there are three states of N in
the catalyst, and the corresponding binding energy posi-
[18a]
tions are 398.2, 399.8, and 401.3 eV, respectively.
The
XPS is employed to determine the elemental compo-
sition and electronic state on the surface of the catalyst.
The XPS full spectrum analysis of the Cu/CuO@CN
characteristic peaks of the low binding energy are
assigned to pyridinic-N and pyrrolic-N, respectively. The
peak at the higher binding energy (401.3 eV) is attrib-
uted to graphite-N, which existed in the π-conjugated
system of lone pair electrons in carbon mat-
(8) is shown in Figure 5a. It can be clearly verified that
four elements of C, N, O, and Cu have existed in the
catalyst. Distinctly, it is found that C, N, and O have
strong peak intensities, and the binding energies of the
corresponding peaks are close to 284.2, 394.7, and
[
2a,10b,18a]
erials.
Indeed, it has been confirmed that N
atoms can enhance electron transfer, thereby increasing
the activity and stability of the catalyst. Therefore, it can
be concluded that N atoms have been successfully doped
[2a,11]
5
28.55 eV, respectively.
Nevertheless, the peak
[17,18]
intensity of Cu is weak and only 1.04% of Cu can be
detected according to Table 2, which is mainly due to
the fact that Cu particles are partially wrapped by the
into the carbon material and formed a CN bond.
In
addition, Figure 5d shows the high-resolution spectra of
Cu. Although the peak intensity of Cu is relatively weak
in full spectrum, a fine spectrum of Cu can still be
obtained by analyzing. The characteristic peaks at the
binding energy of 953.4 and 933.0 eV correspond to the
2p1/2 and 2p3/2 energy levels of zero-valent copper,
respectively. Simultaneously, there is a satellite peak
corresponding to each, and the characteristic peaks at
954.8 and 935.2 eV are attributed to 2p and 2p3/2 of
2
precursor. In detail, C mainly exists in the form of sp -
hybridized graphite C and CN bonds after high-
temperature calcination, the high-resolution spectrum of
C1s is shown in Figure 5b, which is split into three
peaks of C–C (284.3 ± 0.2 eV), C=N (285.8 ± 0.2 eV),
[10b,17]
and C–N (287.6 ± 0.2 eV).
Because of the presence
of the C–N bond, it is again demonstrated that the doped
N atom has existed in the carbon matrix of the catalyst.
Because the N atoms have a lone pair of electrons, it is
theoretically possible to adjust the electron cloud density
on the surface of the carbon material and increase the
degree of disorder, which makes the CN bonds (C=N
1
/2
2+
[1f,2a,6]
the oxidized Cu , respectively.
This is the result
of oxygen in the air reacting with metal Cu in the cata-
lyst after the catalyst is prepared, resulting in a small
amount of elemental Cu being oxidized to CuO. Further-
more, this phenomenon is consistent with the XRD pat-
tern results of Figure 1a.
[10b,17,18]
and C–N) shift to a higher energy level.
The fine
TABLE 1 Nitrogen absorption-desorption isotherms of CN and Cu/CuO@CN(8)
Surface area (m g−1)
2
Pore volume (cm g−1)
3
Sample
Average pore width (nm)
CN
4.26
3.82
660.94
626.07
0.75
0.60
Cu/CuO@CN (8)

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FIGURE 5 (a) Full spectrum analysis of X-ray photoelectron spectroscopy (XPS) data for the catalyst. (b-d) The high resolution XPS
spectra corresponding to C 1s, N 1s, and Cu 2p in the catalyst, respectively
TABLE 2 The XPS analysis of the
Sample
Carbon (%)
89.22
Nitrogen (%)
4.85
Oxygen (%)
5.52
Copper (%)
0.42
element content in Cu/CuO@CN (8)
and Cu/CuO(12%)@CN (8)
Cu/CuO@CN (8)
Cu/CuO(12%)@CN (8)
71.82
7.62
14.23
6.33
Abbreviation: XPS = X-ray photoelectron spectroscopy.
3.2 | Performance of catalysts
different anions are contrasted as shown in Entries 9–12,
−
from which it is known that the introduction of NO3
This study evaluates the catalytic performance of differ-
ent catalysts for phenylacetylene coupling; a significant
reaction is applied to the synthesis of organic monomers
and fine chemicals. As illustrated in Table 3, Entries 1–4
demonstrate that the catalytic activity of the coupling
reaction to phenylacetylene remains unchanged as the
Cu content reduces. In detail, the 1,4-diphenylbutadiyne
is obtained with a higher conversion of 99.9% and the
turnover frequency (TOF) value of Cu/CuO@CN(8) cata-
lyst is 96.8 h . Entry 4 further indicates that low load-
ings (3.3%) can also satisfy catalytic activity of the
reaction as measured by ICP-OES. Entries 4–7 show the
comparison of the catalysts at different temperatures,
demonstrating that Cu/CuO@CN (8) has a remarkable
catalytic activity. In addition, copper salt catalysts of
contributes to the catalytic reaction. In contrast, the
influence of the catalyst activity with the N-based doped
on the reaction is not negligible, as shown in Entries
4 and 8. According to the literature report and the experi-
mental demonstration of the Entries 13 and 14, it is illus-
trated that Cu and CuO are also responsible for the
coupling activity of phenylacetylene. Also, as shown in
Entry 15, the blank experiment without adding catalyst
was implemented once again to demonstrate the fact that
Cu is the active center of the reaction.
At the same time, the bases are also one of the
influencing factors of the coupling reaction. Therefore,
different bases were compared to further explore the cata-
lytic activity of the catalyst in this reaction, as shown in
Table S2 in the supporting information. The presence of
−
1

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9 of 16
TABLE 3 Catalytic activities of different catalysts
ꢀ
a
TOF (h−1
)
Entry
Catalyst
Time (h)
Temperature ( C)
Conversion (%)
1
2
3
Cu/CuO@CN(20%)
Cu/CuO@CN(12%)
Cu/CuO@CN(6.5%)
Cu/CuO@CN(8)
Cu/CuO@CN(9)
Cu/CuO@CN(7)
Cu/CuO@CN(6)
Cu/CuO@C(8)
Cu/CuO@CN(S)
Cu/CuO@CN(C)
Cu/CuO@CN (Cl)
Cu/CuO@CN(8)
Cu
1
1
60
60
60
60
60
60
60
60
25
25
25
25
25
25
60
99.9
99.9
99.9
99.9
96
16
26.7
49.3
96.8
92.8
94.28
93.42
94.84
9.29
9.21
9.32
9.48
16.3
9.68
0
1
b
4
1
c
5
1
c
6
1
97.4
96.5
98
c
7
1
d
8
1
e
9
10
10
10
10
6
96
e
1
1
1
1
1
1
0
1
2
3
4
5
95.2
96.3
98
e
87
CuO
10
10
70
—
trace
2
Note: Reaction condition: phenylacetylene (1 mmol), NaOH (2 eq.), catalyst (20 mg) solvent (2 ml), and O balloon (1 atm).
Abbreviations: GC–MS = gas chromatography–mass spectrometry, ICP-OES = inductively coupled plasma optical emission spectrometer, TOF = turnover
frequency.
a
Determined by GC–MS.
The mass fraction of Cu was 3.3 wt% as measured by ICP-OES.
b
c
The different temperatures of calcination.
d
No nitrogen source added.
e
The different types of copper salts.
base is contributed to the detachment of the H protons in
the terminal alkyne, thereby facilitating the rapid pro-
gress of the reaction system. Moreover, the inorganic base
plays a major role in the experimental system under the
conditions of isopropanol (IPA) as a solvent. In detail, it
can be observed that the conversion rate increases with
increasing alkalinity, and this is mainly due to the
enhanced ability of the base to remove H protons. Then
it is found that the effect of KOH and NaOH on the con-
version of the reaction is not significant by comparing
Entries 3 and 4. Therefore, NaOH was used as a base in
this study. At the same time, we also did a blank compar-
ison experiment, and it can be seen in Entry 11 of
Table S2 that no base addition is inactive for the conver-
sion of phenylacetylene. Similarly, we found that it is dif-
ficult to induce reactions when molecular oxygen is not
introduced into the reaction system rather than relying
on oxygen in the air (Entry 12, Table S2). The results
show that the nitrogen heteroatom incorporated into car-
occurrence of this reaction. So molecular oxygen is also
one of the indispensable factors that affect the progress of
the reaction.
To investigate the effect of the solvent on the catalyst,
a large amount of polar or nonpolar organic liquid is used
as a solvent to conduct catalytic experiments (Table 4).
Importantly, the temperature is also selected as one of
the critical conditions to the reaction process. As detailed
in Entries 1–18 of Table 4, and the screening conditions
for the solvent are carried out under conditions of a small
ꢀ
amount of substrate (0.2 mmol), 10 h and 25 C. As well
know that polar solvents have a great influence on this
reaction, especially when alcohols are used as a solvent.
Especially, it can be observed that tert-butanol (Entry 15)
is considered as the most suitable solvent for the reaction
comparison with glycol, methanol, acetone, DMF,
CH Cl and the like. At the same time, tert-butanol not
2
2
only has distinguished sustainability but also has excel-
lent solvent action. It is further worth mentioning that
the conversion of tert-butanol and isopropanol to the
bon material facilitates the activation of O to trigger the
2

10 of 16
MA ET AL.
a
b
TABLE 4 Catalytic activities of Cu/CuO(3.3%)@CN(8) in different solvents and temperatures
ꢀ
c
TOF (h−1
)
Entry
Solvent
Isopropanol
Time (h)
Temperature ( C)
Conversion (%)
1
2
3
4
5
6
7
8
9
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
4
25
25
25
25
25
25
25
25
25
25
25
25
25
25
25
25
25
25
70
60
60
60
60
50
50
50
60
60
95
60
2.03
1.28
1.97
1.76
1.984
1.82
2.07
0.618
1.58
1.685
1.856
1.706
1.344
1.43
2.09
1.71
0
Glycol
Methanol
Acetone
Ethanol
DMF
92.4
82.6
93
85.3
97
n-propanol
DMSO
29
THF
73
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
CH
2
Cl
2
79
CHCl
3
87
1,4-dioxane
Toluene
80
63
Ethyl acetate
tert-Butanol
67
98
Hydrazine hydrate
80
H
2
O
trace
trace
99.9
99.9
99.9
99.9
99.9
99.9
99.9
98.2
trace
trace
—
0
b
b
b
b
b
b
b
b
d
e
tert-Butanol
tert-Butanol
tert-Butanol
tert-Butanol
tert-Butanol
tert-Butanol
tert-Butanol
tert-Butanol
tert-Butanol
tert-Butanol
24.218
24.218
48.43
64.58
96.87
24.218
48.43
95.22
0
4
2
1.5
1
4
2
1
1
1
0
Abbreviations: GC–MS = gas chromatography–mass spectrometry, TOF = turnover frequency.
a
Reaction condition: phenylacetylene (0.2 mmol), catalyst (20 mg), NaOH (20 mg), solvent (1 ml), and O
2
balloon (1 atm).
balloon (1 atm).
b
Reaction condition: phenylacetylene (1 mmol), NaOH (2 eq.), catalyst (20 mg), tert-Butanol (2 ml), and O
2
c
Determined by GC–MS.
d
No base.
e
2
No O .
reaction is 98% (Entry 15) and 95% (Entry 1) under the
same reaction conditions, which proves that tert-butanol
is superior to isopropanol. In addition, TOF is also calcu-
lated accordingly. It can be concluded that when tert-
butanol is used as a solvent, its TOF value is 2.07 (Entry
Hence, we can draw a general conclusion that the
electron-donating ability of tert-butyl is weaker than that
of isopropyl, but from the point of view of acidity and
alkalinity, the stronger the electron-donating ability of
the electron-donating group, the weaker the acidity. In
addition, considering that the experiment is implemented
7
) under the condition that phenylacetylene is 0.2 mmol.

MA ET AL.
11 of 16
TABLE 5 Coupling of different alkynes
a
Entry
Substrate
Product
Conversion (%)
1
97
2
96
3
4
99
96
99
b
5
6
7
8
98
99
99
9
72
ꢀ
2
balloon (1 atm) at 60 C for 2 h.
Note: Reaction condition: phenylacetylene (0.2 mmol), catalyst (20 mg), NaOH (20 mg), solvent (1 ml), and O
Abbreviation: GC–MS = gas chromatography–mass spectrometry.
a
Determined by GC–MS.
b
Reaction time: 1 h.

12 of 16
MA ET AL.
in an alkaline environment, so we make the final deci-
sion that uses tert-butanol as the best solvent and apply it
to the reaction.
A suitable temperature is essential in terms of reducing
reaction time and speeding up the reaction rate. In this
way, the effect of temperatures on the coupling of
phenylacetylene is also illustrated in Table 4 (Entries
reusability. The circulation capability of the catalyst is
one of the necessary factors for its industrial production.
Therefore, Cu/CuO@CN(8) was investigated in this
experiment for the recyclability of phenylacetylene
coupled to 1,4-diphenylbutadiyne, as shown in Figure 6.
For the cycle results, it is clear that the conversion of the
reaction exceeds 90% after five cycles of testing. In
detail, the catalyst and the reaction system were sepa-
rated by filtration after one reaction, and then the
recovered catalyst was washed several times with etha-
nol and deionized water. Finally, the washed catalyst
1
9–28). Comparing the experimental results, 99.9% conver-
sion can be achieved after 4 h at the initial set temperature
of 70 C. However, with the continuous shortening of the
ꢀ
reaction time and the lowering of the reaction temperature,
we can find that the same conversion rate (99.9%) can be
ꢀ
was thoroughly dried in a vacuum oven at 60 C for
the subsequent cycle.
ꢀ
achieved after 1 h at 60 C and its TOF value is 96.87 (Entry
2
3). The conversion rate will be greatly reduced when the
Furthermore, the filtered catalyst is collected after the
cycle, and its metal content is slightly reduced by the
ICP-OES, indicating that only a small amount of copper
is separated from the catalyst. In addition, The FT-IR is
implemented to structural analysis and identification of
the catalyst. Figure 7 shows the infrared adsorption spec-
trum of the precursor and Cu/CuO@CN(8) on
phenylacetylene. Clearly, it can be found that there are
reaction temperature is further reduced. Therefore, the
appropriate temperature for this experiment is 60 C.
By screening the above experimental conditions, the
optimal reaction conditions for the coupling of
phenylacetylene to 1,4-diphenylbutadiyne are Cu/CuO
ꢀ
(
(
3.3%)@CN(8) (20 mg), NaOH (2 eq.), tert-Butanol
2 ml), and O balloon (1 atm) at 60 C for 1 h. In order to
ꢀ
2
−
1
verify the general applicability of this experiment, some
similar acetylenes are investigated as substrates under
optimal experimental conditions and the results are
shown in Table 5. The significant conversion (>95%) of
most of the corresponding alkynes are achieved on the
catalyst. Further, a platform substance (3-alkynyltoluene,
two new peaks at 748.2 and 686.4 cm , indicating that
the phenylacetylene was well adsorbed on the catalyst.
Further, it is observed that the adsorption peak is slightly
shifted after five cycles, and its corresponding positions
−
1
are 756.0 and 693.1 cm , respectively. This indicates that
there is no significant change in the catalyst active after
the catalyst is recycled, which reflects the excellent recy-
clability of the prepared catalyst.
4
-propylbenyne, 4-alkynylanisole, etc.) derived from a
coupling substance (phenylacetylene) is carried out. As
shown in Entries 5–7 in Table 5, the corresponding con-
version rates are 99%, 98%, and 99%, respectively. Hence,
it can be seen that the catalyst has a prominent catalytic
effect on alkyne compounds with benzene ring. Care-
fully, it can be found that the catalytic coupling effect of
the substrate with electron-pushing group on the ben-
zene ring is more significant than that of the substrate
with electron-withdrawing group on the benzene ring, by
comparing Entries 3 and 4 and Entries 2 and 5. In addi-
tion, 2-ethynylthiophene also gave a good coupling yield
of 99% (Entry 8). However, the conversion of n-hexyne is
not satisfactory compared to the above substrates (only
3.4 | Kinetic study
To further understand the reaction mechanism, kinetic
experiments are performed to study the phenylacetylene
coupling reaction. In this experiment, the reaction
kinetics of the coupling of phenylacetylene to
1,4-diphenylbutadiyne is studied by the speed-limiting
step method. The specific conditions of the reaction are
as follows: phenylacetylene (3 mmol), NaOH (200 mg),
tert-butanol (6 ml), O (1 atm), catalyst (10 mg), and reac-
2
7
2%). This may be attributed to the fact that the catalyst
tion time (40 min). This experiment is studied by using
models and basic assumptions of kinetic theory in hetero-
geneous catalysis. The details are as follows: The total
reaction formula is defined as Equation (1), by which the
total reaction rate Equation (2) can be derived. The resid-
ual substrate concentration and oxygen pressure in
tends to form a π bond with benzene ring-containing sub-
stance, which facilitates electron transport and thereby
enhances the reaction rate. Generally, the prepared cata-
lyst of Cu/CuO@CN(8) has distinguished properties for
coupling other acetylene compounds.
Equation (2) are represented by C and P, respectively,
A
and the corresponding reaction order is represented by
3
.3 | Cycles of catalyst
b and a. Because the pressure of O is always maintained
2
at 1 atm and its content is far excessive as the reaction
proceeds, so P(O2) is regarded as a constant. Hence, the
total reaction rate equation can be approximated as
In addition to excellent catalytic activity, a good heteroge-
neous catalyst should also have good recyclability and

MA ET AL.
13 of 16
(
Figure 8b). Therefore, it can be concluded that the reac-
tion conforms to the quasi-first-order kinetic equation.
On the basis of the consistency of other conditions, the
corresponding reaction rate constant k can be obtained
only by changing the reaction temperature. Finally, the
Arrhenius equation (Equations 4 and 5) can be used to
determine the apparent activation energy activation
energy E required for the reaction.
a
kc = Ae−
Ea=RT
Ea
lnkc = −
+ lnA:
RT
FIGURE 6 The recyclability and reusability of Cu/CuO@C(8).
Reaction condition: phenylacetylene (1 mmol), NaOH (2 eq.),
A, R, and T in the above equations are expressed as
apparent frequency factors, molar gas constants, and
thermodynamic temperatures, respectively. As shown in
Figure 8c, a straight line with good fitting degree can be
obtained by plotting a series of (1/T, lnk) experimental
ꢀ
catalyst (20 mg), tert-Butanol (2 ml) and O
2
balloon (1 atm) at 60 C
for 1 h. The conversion is determined by gas chromatography–mass
spectrometry (GC–MS)
data, and the value of Ea can be deduced as
2.74 kJ mol− according to the slope and intercept of the
1
2
line. It is further proved that the conclusions of the
kinetic study are in good agreement with the results of
the reaction conversion rate. The above consequences
fully demonstrate that the best catalyst has the lowest
reaction activation energy, which again indicates that
Cu/CuO@CN(8) is the best catalyst for the realization of
phenylacetylene coupling.
3.5 | Mechanism research
Samples are characterized by the FT-IR and explained in
combination with the mechanism schematic diagram to
investigate the reaction mechanism of this experiment.
Generally, Figure 7 shows that the characteristic absorp-
tion peaks of different types of catalysts are between
FIGURE 7 The Fourier-transform infrared spectroscopy (FT-
IR) of catalyst and precursor
−
1 [10b]
500 and 3500 cm .
In detail, the catalyst
(
Cu/CuO@CN-Fresh) and the carrier (CN-Fresh) that are
Equation (3). And the total reaction rate constant is
expressed by kc.
not in contact with the reaction substrate have similar
characteristic absorption peaks, which indicates that the
doping of metals does not cause changes in functional
groups. In addition, this experiment further explored the
infrared spectra of the catalyst (Cu/CuO@CN-Absorbed),
the carrier (CN-Absorbed), and the catalyst after the cycle
1
2
K
2
½phenylaetyleneꢂ + O2 ! ½1,4−diphenylbutadiyneꢂ + H2O
,
a
b
r = k P
CA
ðO2Þ
(
Cu/CuO@CN-Cycles) in contact with the substrate. By
b
r = k C
c
A
comparison, it can be found that there are two new weak
−1
peaks in the range of 650–800 cm , which may be due
to the introduction of Cu improves the adsorption
capacity of the catalyst to the substrate molecule
(phenylacetylene). Therefore, it can be attributed to the
out-of-plane bending vibration peak of the aromatic ring.
Next, the relationship between the reaction time and
the conversion rate is shown in Figure 8a, and the value
of b is 1.02 by linearly fitting of the remaining substrate
concentration and reaction time in different time periods

14 of 16
MA ET AL.
FIGURE 8 Kinetic study on the coupling of phenylacetylene
Furthermore, all catalysts and precursors have three
strong absorption peaks. In detail, peaks with wave num-
shown in Scheme SCHEME 2. Initially, the H proton
of the terminal alkyne is combined with the base and
−
1
0
+
bers in the range of 500–1200 cm are assigned to C–C
then detached, while Cu is oxidized to Cu by
[
10b,19]
−1
+
and C–N.
In the region of 1500–1650 cm , the
oxygen. Next, the oxidized Cu is matched with the
peaks of stretching vibration are mainly attributed to the
two substrates that have lost the H protons, then the
reduction elimination is implemented with the produc-
[
10b,19,20]
C=N.
And the stretching vibration peak that
+
0 [1a,
can be assigned to the N–H bond in the region of
tion of products and the reduction of Cu to Cu .
−
1 [10b,19a,20]
e,10c]
3
400–3500 cm .
Ultimately, water is formed by the combination
In addition, the degree of graphitization is maxi-
of H protons and oxygen, accompanied by the oxida-
0
+
[3e,6,7e,10c]
mized (I /IG = 1.0243) when the calcination tempera-
tion of Cu to Cu to promote the next cycle.
D
ꢀ
2+
+
ture reaches 800 C, which promotes the electron
In the case of Cu , it is reduced to Cu after the
reduction elimination process is implemented. Subse-
quently, the oxidation reaction is carried out as Cu is
oxidized to Cu again. Moreover, the results of the
cycle experiments demonstrate that the catalyst has a
reliable ability to recycle.
transfer between the catalyst and the substrate
+
(Figure 1b). At the same time, a blank control experi-
2+
ment is conducted with nitrogen to study the role of
oxygen as a metal oxide during the reaction (Entry
2
8, Table 4). The detailed mechanism inference is
SCHEME 2 Proposed reaction mechanism

MA ET AL.
15 of 16
4
| CONCLUSIONS
Xiaokang Huang https://orcid.org/0000-0003-0339-
257
1
In summary, we demonstrate a mild and reliable metal
precursor combination strategy for the synthesis of the
flexible acetone carbon and full advantage of impregna-
tion. The synthesis route in this experiment is a precursor
formed by acetone and urea to immobilize the metal
particles in space, which opens up a potential and advan-
tageous way to realize the carrier of acetone. In addition,
analysis of the experimental results and characterization
indicates that the optimal catalyst Cu/CuO@CN
Ming Yang https://orcid.org/0000-0002-3964-9830
Li Gong https://orcid.org/0000-0001-7773-0958
Rong Li https://orcid.org/0000-0003-1037-0841
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How to cite this article: Ma L, Jiang P, Wang K,
et al. High-efficiency catalyst for copper
nanoparticles attached to porous nitrogen-doped
carbon materials: Applied to the coupling reaction
of alkyne groups under mild conditions. Appl
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