A highly efficient nano-sized Cu2O/SiO2 egg-shell catalyst for C-C coupling reactions

Article abstract of DOI:10.1039/c7ra13490h

Mesoporous SiO₂-supported Cu₂O nanoparticles as an egg-shell type catalyst were prepared by impregnation method. The obtained Cu₂O/SiO₂ egg-shell nanocatalyst had a large surface area and narrow pore size distri

Full text of DOI:10.1039/c7ra13490h

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A highly efficient nano-sized Cu2O/SiO2 egg-shell
catalyst for C–C coupling reactions†
Cite this: RSC Adv., 2018, 8, 6200
b
Soohee Kim,a Shin Wook Kang,b Aram Kim,a Mohammad Yusuf,a Ji Chan Park
*
a
*
and Kang Hyun Park
Mesoporous SiO2-supported Cu2O nanoparticles as an egg-shell type catalyst were prepared by
impregnation method. The obtained Cu2O/SiO2 egg-shell nanocatalyst had a large surface area and
narrow pore size distribution. In addition, most of the Cu2O nanoparticles, with sizes around 2.0 nm,
were highly dispersed in the mesoporous silica. Accordingly, fast reactant diffusion to the active sites
would occur, especially when the active metal sites are selectively located on the outer part of the
support, i.e., the outer region of the egg shell. In solvent-free Sonogashira reactions for the synthesis of
ynones from acyl chlorides and terminal alkynes, this catalyst exhibited a very high catalytic activity. The
excellent catalytic performance can be attributed to the synergistic advantages of mesoporous structure
and monodispersed Cu2O nanoparticles.
Received 20th December 2017
Accepted 22nd January 2018
DOI: 10.1039/c7ra13490h
rsc.li/rsc-advances
these signicant properties, Cu nanoparticles have been used as
catalysts for a very broad range of organic transformations, such
as click reactions, A3 coupling, C–H functionalization, bor-
Introduction
Supported nanocatalysts exploiting robust metal-oxide struc-
tures have been effectively used for various heterogeneous
catalytic processes.1–3 In particular, mesoporous silica supports
have been widely used as catalyst support based on their high
thermal stability and controllable surfaces as well as their high
pore volumes and large surface areas.4–8 For example, many
types of metal/silica nanocatalysts, such as metal@silica yolk–
shell9 and core–shell catalysts,10,11 have been developed to
enhance the catalytic activity and stability against sintering or
agglomeration problems of active metal sites.
In recent years, silica-12,13 and alumina-based14–16 egg-shell-
type catalysts in millimeter scale have been prepared in order
to avoid the diffusional restrictions of the reactants for some
catalytic reactions, such as Fischer–Tropsch synthesis.17,18
Simply, fast reactant diffusion to the active sites would occur,
especially when the active metal sites are selectively located on
the outer part of the support, such as the outer region of the egg
shell. However, most egg-shell-type catalysts prepared using the
controlled penetration of molten salt have still been focused on
the millimeter scale, not the micrometer or nanometer scale.
Accordingly, we utilized the synthesized egg-shell nano-sized
particles in the catalytic system. The benets of the number of
potentially reactive small-sized atoms exposed on the surface
are expected to include high activity and selectivity.19 Due to
ylation, cross-coupling and so on.20–25
In this work, we set out to use the Sonogashira protocol,
which has emerged as one of the most straightforward and
powerful methods for carbon–carbon formation.26 The products
of ynones are a kind of conjugated alkynyl ketones which are
useful intermediates of heterocyclic compounds as building
blocks for natural products, pharmaceuticals, and molecular
organic materials.27–30 Herein, we report a new type of Cu2O/SiO2
egg-shell nanocatalyst as an active and stable catalyst for the
synthesis of 1,3-diphenyl-2-propyn-1-one from benzoyl chloride
and phenylacetylene.
Experimental
Materials
Copper(II) nitrate trihydrate (Cu(NO3)2$3H2O, $98%), tetraethyl
orthosilicate (TEOS, 98%) and hexadecyltrimethylammonium
bromide (C16TAB, $98%) were purchased from Aldrich.
Ammonium hydroxide (NH4OH, 28% in water) and ethanol
(99.9%) were obtained from Junsei and Baker, respectively. The
reagents were used as received without further purication.
Synthesis of mSiO2 egg-shell support
NH4OH aqueous solution (2.5 mL) was added to a mixture of
ethanol (100 mL) and distilled water (8.0 mL), and then stirred
for 5 min. TEOS (10 mL) was added to the prepared solution and
stirred for 2 h at room temperature. The resulting silica parti-
cles were precipitated by centrifugation at 8000 rpm for 20 min,
then washed thoroughly with ethanol (100 mL). The dispersed
aDepartment of Chemistry, Chemistry Institute for Functional Materials, Pusan
National University, Busan, 46241, Republic of Korea. E-mail: chemistry@pusan.ac.kr
bClean Fuel Laboratory, Korea Institute of Energy Research, Daejeon, 34129, Korea.
E-mail: jcpark@kier.re.kr
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c7ra13490h
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ꢀ
silica in ethanol (100 mL) was diluted with distilled water reaction tube. Aer reacting in argon atmosphere at 80 C for
(200 mL). A solution of C16TAB (1.2 g, 3.3 mmol) dissolved in 12.0 h, the reaction mixture was ltered and extracted with
a solvent mixture of distilled water (20 mL) and ethanol (10 mL) dichloromethane, then analyzed using a gas chromatograph
was injected into the diluted silica solution. This mixture was (SHIMADZU, GCMS-QP2010 SE).
vigorously stirred under ambient conditions for 30 min. Aer
30 min, TEOS (2.15 mL, 9.6 mmol) was added to the reaction
solution and stirred for 12 h. The resulting mSiO2 egg-shell
Results and discussion
Preparation of Cu2O/SiO2 egg-shell nanocatalyst
nanoparticles were precipitated by centrifugation at 8000 rpm
for 10 min, and then washed thoroughly with ethanol (150 mL)
and acetone (150 mL). Aer drying at 100 C in a drying oven,
Fig. 1 shows a brief synthetic scheme for the Cu2O/SiO2 egg-
shell nanocatalyst. The solid SiO2 cores within mesoporous SiO2
shells (mSiO2 egg-shell supports) were prepared by a modied
sol–gel method as reported in the literature.32 The transmission
electron microscopy (TEM) images show uniform silica nano-
spheres with average diameter of 305 ꢂ 18 nm (Fig. 2a and b).
Mesoporous silica shells were coated onto the silica nano-
spheres using the cationic surfactant C16TAB, both as a struc-
ture-directing agent and a sacricial porogen. The TEM
images show mSiO2 egg-shell supports around 435 nm, larger
than the initial SiO2 nanospheres (Fig. 2c). The thickness of the
uniformly coated mesoporous silica shell on the solid silica core
was observed as approximately 65 nm in the HRTEM image
(Fig. 2d). Ordered mesoporous channels in the silica shell
could be generated by thermal removal of C16TAB.
Next, the copper(I) oxide/silica catalyst (designated as Cu2O/
SiO2 egg-shell nanocatalyst), with controlled Cu content of
10 wt% on the basis of Cu converted from the copper nitrate salt
aer thermal treatment, was prepared by incorporating Cu2O
particles formed inside the pores of mSiO2 egg-shell supports.
Impregnation of copper nitrate and subsequent hydrogen
reduction yielded small and uniform Cu2O particles in channel-
like SiO2 pores. The chemical reaction for the thermal decom-
position of Cu(NO3)2 and Cu2O formation is proposed to be:
ꢀ
the white powder was placed in an alumina boat in a tube-type
furnace, heated at a ramp rate of 4 ꢀC minꢁ1 to 500 ꢀC, and
ꢀ
calcined at 500 C for 8 h in air.
Synthesis of nano-sized Cu2O/SiO2 egg-shell catalyst
For synthesis of the Cu2O/SiO2 egg-shell nanocatalyst containing
10 wt% Cu, aqueous copper nitrate solution (8.74 M, 0.1 mL) was
added dropwise onto the mSiO2 egg-shell support (0.5 g). Inl-
tration was performed by grinding the mixture under ambient
conditions for 10 min until the powder was homogeneously pale
blue. Next, the mixed powder was placed in an autoclave reactor
and aged in an oven at 120 ꢀC. Aer aging for 24 h, the sample was
cooled in ambient atmosphere and transferred into an alumina
boat in a tube-type furnace. Finally, the copper-incorporated silica
powder with pale blue color was slowly heated under H2 ow of
ꢁ1
200 mL minꢁ1, at a ramp rate of 2.7 C min , to 350 C. The
sample was thermally treated under continuous H2 ow at 350 ꢀC
for 4 h. Aer calcination, the resulting powder with pale green
color was cooled down to room temperature.
ꢀ
ꢀ
Characterization
High-resolution transmission electron microscopy (HRTEM)
was performed using a Tecnai TF30 ST and a Titan double Cs-
corrected TEM instrument (Titan cubed G2 60-300). Energy-
dispersive X-ray spectroscopy (EDS) elemental mapping data
were collected using a higher-efficiency detection system
(Super-X detector). High-power X-ray powder diffraction (XRD)
(Rigaku D/MAX-2500, 18 kW) was also used for the analysis. N2
sorption isotherms were measured at 77 K with a Tristar II 3020
surface area analyser. Before the measurement, the sample was
2Cu(NO3)2 (s) + H2 (g) / Cu2O (s) + 4NO2 (g) + H2O (g) + O2 (g)
The TEM image indicates that the Cu2O nanoparticles were
incorporated well into the mesopores of silica (Fig. 3a). The
ꢀ
degassed under N2 ow at 300 C for 4 h.
Activity tests
In a typical run of the solvent-free Sonogashira coupling reac-
tion,31 0.50 mmol phenylacetylene, 0.75 mmol benzoyl chloride,
4.0 equiv. triethylamine (base) and 1.0 mol% Cu2O/SiO2 egg-
shell nanocatalyst were mixed in a 25 mL oven-dried Schlenk
Fig. 2 (a and b) TEM images of silica nanospheres, and (c) TEM and (d)
HRTEM images of mSiO2 egg-shell support. The bars represent 2 mm
(a), 200 nm (b, c), and 20 nm (d).
Fig. 1 Schematic illustration of the synthesis of the Cu2O/SiO2 egg-
shell nanocatalyst.
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Fig. 4 (a) N2 adsorption/desorption isotherms and (b) pore size
distribution diagrams using the Barrett–Joyner–Halenda (BJH)
method from the adsorption branches of the mSiO2 egg-shell support
and the Cu2O/SiO2 egg-shell nanocatalyst.
228.9 m2 gꢁ1 for the Cu2O/SiO2 egg-shell nanocatalyst, respec-
tively. The total pore volume of the Cu2O/SiO2 egg-shell nano-
catalyst was found to be 0.19 cm3 gꢁ1, which is about 54% of the
pristine mSiO2 egg-shell support (0.35 cm3 gꢁ1). The signicant
decrease in pore volume of the Cu2O/SiO2 egg-shell nanocatalyst
was attributed to the occupied copper oxide nanoparticle in the
silica pore. The pore size distributions of the initial mSiO2 egg-
shell support and Cu2O/SiO2 egg-shell nanocatalyst were iden-
tically observed in the range of 2–3 nm, well reecting the
embedded Cu2O crystallite size (Fig. 4b).
Fig. 3 (a) TEM and (b) HADDF images, (c–e) scanning TEM image with
elemental mapping, and (f and g) HRTEM images with the corre-
sponding FT pattern (inset of g), and (h) XRD spectrum of Cu2O/SiO2
egg-shell nanocatalyst. The bars represent 200 nm (a–e), 20 nm (f),
and 2 nm (g).
high-angle annular dark-eld scanning transmission electron
microscopy (HAADF-STEM) image also shows regions of varying
brightness. Relatively bright spots correspond to Cu2O nano-
particles (Fig. 3b). In the elemental mapping, Cu (green) and Si
(red) are conrmed (Fig. 3c–e). The magnied TEM image
shows black dots in channel-like pores, indicating small Cu2O
crystals (Fig. 3f). The HRTEM image and the corresponding
Fourier-transform (FT) pattern indicate the single crystalline
nature of the Cu2O particle (Fig. 3g). The inside particle size was
observed to be between 2 and 3 nm. The lattice distance of
0.246 nm between neighbouring fringes corresponds to the
(111) lattice spacing in cubic phase Cu2O. In the XRD pattern,
Cu2O/SiO2 egg-shell nanocatalyst is also well matched with
cuprite (Fig. 3h, space group: Pn3m, JCPDS no. 77-0199). The
broad peak at 2q ¼ 36.5ꢀ corresponds to the (111) plane of Cu2O.
The mean particle size was estimated to be 2.0 nm using the
Debye–Scherrer equation from the full width at half maximum
(fwhm) of the (111) peak.
Sonogashira coupling reactions
We explored the catalytic performance of the synthesized Cu2O/
SiO2 egg-shell nanocatalyst toward Sonogashira coupling reac-
tions. It was found that supported copper nanoparticles effi-
ciently catalyzed the synthesis of ynones of acyl chlorides and
alkynes.33,34 In this type of reaction, Cu2O nanoparticles activate
phenylacetylene to form copper(I) acetylide compounds, which
subsequently react with acyl chloride.35,36 In addition, this
reaction does not require any ligand and palladium source
under solvent-free conditions.
The effect of several factors, such as reaction time, tempera-
ture, base, and amount of catalyst, was studied. The results are
listed in Table 1. Initially, it was demonstrated that changes in the
catalyst loading have marked impacts on the conversions. It was
shown that increasing the amount of catalyst loading from
1 mol% to 1.5 mol% resulted in higher conversion (86%) (Table 1,
entries 1 and 2). In order to improve the conversion of the
reaction, we increased the temperature to 80 ꢀC, which resulted
N2 sorption experiments for the pristine mSiO2 egg-shell
support and the Cu2O/SiO2 egg-shell nanocatalyst show type
IV adsorption–desorption hysteresis (Fig. 4a). The Brunauer–
Emmett–Teller (BET) surface areas were calculated to be
481.2 m2 gꢁ1 for the pristine mSiO2 egg-shell support and
ꢀ
in higher conversion than at 40 C (Table 1, entries 3 and 4).
Thereaer, when the reaction time was extended to 12 h, the
conversion of 99% was achieved (Table 1, entry 5). In addition,
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Table 1 Solvent-free Sonogashira reactions for the synthesis of ynones from acyl chlorides and terminal alkynes catalyzed by Cu2O/SiO2 egg-
shell nanocatalysta
Entry
Cat. (mol%)
Temp (ꢀC)
Base (eq.)
Time (h)
Conv. (%)
Select. (%)
1
2
3
4
5
6
7
8
1
1.5
1
1.5
1
—
1
1
40
40
80
80
80
80
80
80
80
80
80
80
80
80
3
3
3
3
3
4
—
4
4
4
4
4
4
4
8
8
8
8
66
86
93
100
99
8
99
98
97
98
97
0
12
12
12
12
12
12
12
12
12
12
5
0
100
100
98
94
76
100
7
99
99
99
99
99
91
0
9
Recover from #8
Recover from #9
Recover from #10
1
10
11
12b
13c
14
1
Recover from 13
a
Reaction conditions: phenylacetylene (0.50 mmol), benzoyl chloride (0.75 mmol), base Et3N, cat. Cu2O/SiO2 egg-shell nanocatalyst, Ar atmosphere.
b
Determined by using gas chromatography-mass spectrometery (GC-MS). Cat. commercial Cu2O powder purchased from Aldrich (no. 208825).
c
Cat. conventional SiO2-supported Cu2O catalyst.
when the amount of base was increased (4.0 equiv.), the phenylacetylene (0.5 mmol), benzoyl chloride (0.75 mmol),
conversion was 100% (Table 1, entry 8), and the selectivity was Cu2O/SiO2 egg-shell nanocatalyst (1 mol%), and Et3N (4 equiv.)
ꢀ
99% owing to Cu2O/SiO2 egg-shell nanocatalyst hindering the at 80 C for 12 h under Ar atmosphere. Furthermore, when we
homocoupling of benzoyl chloride and phenylacetylene. conducted the reaction without base or catalysts under the
Subsequently, the optimized reaction conditions were optimized reaction conditions, each reaction hardly proceeded
Table 2 Substrate study for the coupling reaction of alkynes and acyl chloridesa
Entry
1
Acyl chloride
Alkyne
Product
Conv. (%)
66
Select. (%)
100
2
3
98
88
99
87
4
5
89
98
98
85
6
100
100
a
Reaction conditions: alkyne (0.5 mmol), acyl chloride (0.75 mmol), Et3N (4.0 equiv.), 1.0 mol% Cu2O/SiO2 egg-shell nanocatalyst, 80 ꢀC, 12.0 h.
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(Table 1, entries 6 and 7). Thus, a basic medium is essential for structure and the monodispersed Cu2O nanoparticles. The egg-
these cross-coupling reactions. However, under the same shell structure acts as a “nanoreactor framework”, which
condition, but replacing the Cu2O/SiO2 egg-shell nanocatalyst contains sufficient space and catalytically active surface within
with commercial Cu2O powder as catalyst, the coupling reaction its structure. The egg-shell nanoparticles have potential for
was obtained with lower conversion and selectivity (Table 1, application as nanoreactors and catalysts, drug delivery
entry 12). Even though the conventional SiO2-supported Cu2O carriers, and surface-enhanced Raman scattering substrates.
catalyst showed similar catalytic activity, low reusability was
obtained due to decomposed catalyst structures (Table 1,
entries 13 and 14). The Cu2O/SiO2 egg-shell nanocatalyst was
Conflicts of interest
recycled up to three times without any loss of its initial high
activity (>94%) in subsequent experiments (Table 1, entries
There are no conicts to declare.
9–11). Therefore, this indicates that the high dispersion and
excellent accessibility of the Cu2O NPs cause the high efficiency Acknowledgements
of the Cu2O/SiO2 egg-shell nanocatalysts. Moreover, they
This research was supported by Basic Science Research Program
showed enhanced catalytic activity and facilitated considerably
through the National Research Foundation of Korea (NRF) and
positive synergistic effects with nanosized porous support
the Research and Development Program of the Korea Institute
substrates,37–40 as compared to Cu2O nanoparticles without the
of Energy Research (KIER) (No. B8-2461-01), and funded by the
mesoporous support.41 All reactions are carried out in the void
Ministry of Science, ICT
&
Future Planning (NRF-
inside the shell. In other words, the egg-shell structure acts as
a “nanoreactor framework”, which contains enough space
between the core and shell. Each of the active nanoparticles
experiences a homogeneous environment in a void surrounded
by the silica shell. We also studied the role of solvent system
under the optimized conditions (ESI, Table S1†).42 In addition,
when we carried out the reactions with dipolar aprotic solvents
such as dimethylformamide (DMF) and tetrahydrofuran (THF),
results were inferior because of the formation of the corre-
sponding anhydride as a by-product. However, the reaction
under nonpolar solvent, such as toluene, gave 84% conversion.
Therefore, all reactions were performed neat under anhydrous
conditions. Encouraged by the above results, with these opti-
mized reaction conditions, the scope of the developed protocol
was extended for the synthesis of ynone derivatives using
different substrates (Table 2). As shown in Table 2, most of the
substrates gave good conversions despite electron-donating
substituents (methyl, tert-butyl and methoxy groups) and
electron-withdrawing substituents (uoro, cyano groups).
Furthermore, benzoyl chloride substituted with a nitro group
still gave the corresponding ynones with good conversion rate
(Table 2, entry 6).
2017R1A4A1015533 and NRF-2017R1D1A1B03036303).
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