Scalable production of Cu@C composites for cross-coupling catalysis

Article abstract of DOI:10.1016/j.materresbull.2015.04.043

A novel Cu@C core-shell microstructure was prepared by reduction of [Cu(NH₃)₄]²⁺ with glucose using a mild hydrothermal process. The carbon shell of such Cu@C composite can be tuned to different carbonization degrees just through varying the calcination conditions. The structural properties of as-prepared Cu@C were investigated in detail by scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), transmission electron micrographs (TEM) and Raman spectra. In addition, these Cu@C composites were firstly used to catalyze the CN cross coupling of amines with iodobenzene. Among them, the catalytic ability of Cu@C composites increased as their surface carbon's carburization degree improved.

Full text of DOI:10.1016/j.materresbull.2015.04.043

Materials Research Bulletin 70 (2015) 163–166
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Materials Research Bulletin
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Short communication
Scalable production of Cu@C composites for cross-coupling catalysis
Lijuan Bu a, Hai Ming b,
*
a
Key Laboratory of Chemical Biology and Traditional Chinese Medicine Research (Ministry of Education), College of Chemistry and Chemical Engineering,
Hunan Normal University, Changsha 410081, PR China
b
College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, PR China
A R T I C L E I N F O
A B S T R A C T
A novel Cu@C core–shell microstructure was prepared by reduction of [Cu(NH3)4]2+ with glucose using a
mild hydrothermal process. The carbon shell of such Cu@C composite can be tuned to different
carbonization degrees just through varying the calcination conditions. The structural properties of
as-prepared Cu@C were investigated in detail by scanning electron microscopy (SEM), energy-dispersive
X-ray spectroscopy (EDX), transmission electron micrographs (TEM) and Raman spectra. In addition,
these Cu@C composites were firstly used to catalyze the CꢀꢀN cross coupling of amines with
iodobenzene. Among them, the catalytic ability of Cu@C composites increased as their surface carbon’s
carburization degree improved.
Article history:
Received 4 May 2014
Received in revised form 14 March 2015
Accepted 19 April 2015
Available online 21 April 2015
Keywords:
Copper
Carbon
Microstructure
Cross coupling
Catalyst
ã 2015 Elsevier Ltd. All rights reserved.
1. Introduction
advantages of easy separation, quick recycling and minimization
of metal trace in the product. However, to the best of our
Micro- and nano-structured metal have received great
attention because of their potential utilizations in a wide range
of advanced applications in the field of sensor, catalytic,
environment and energy storage [1–3]. Among them, the metal
of copper is of particularly interest due to its fascinating properties
while much lower cost compare to other noble metals of Pd, Pt and
Au [4]. Until now, numerous approaches have been reported for
preparing micro-/nano-structured Cu with different morphologies
of cube, rod, disk and wire [5–7], as well as other Cu-based
composites such as Cu@Si, Cu@CNTand Cu@TiO2 [8,9]. As reported,
these metal-particles with unique small size could be utilized as
effective catalysts with high activities [11].
However, a formidable problem always remains, that is, the
metal-particles have a strong trend to agglomerate in the processes
of preparation and utilization by the strong surface tension [12].
One of the most promising strategies to this problem is the
immobilization of catalysts on certain stable matrix. To date, the
way of coating metal particle with a layer of carbon is of great
interest, and the inert carbon resultant metal@C composite
could exhibit excellent catalytic activities in many cases of
photooxidation of alkanes [13], electrooxidation of methanol
[14], lithium–oxygen battery [15], and so on. Distinct from
homogenous catalyst, such heterogeneous catalysts have the
knowledge, the synthesis of Cu@Carbon (i.e., Cu@C) with a
core@shell microstructure is quite limited, especially processed
in a one-step with a large-scale production [10].
To study this catalyst system better in light of the respective
merits of metal particles and carbon structures, herein we present
a facile one-pot hydrothermal procedure to synthesis Cu@C
structure and also the carburization degrees of carbon shell were
well controlled. Considering the copper-mediated Ullmann
condensation of aryl halides with amines is the most frequently
used pathway for coupling reactions [16,17]; therefore herein the
coupling reaction of iodobenzene and aniline was applied with
Cu@C as catalyst at 383 K to evaluate its catalytic performance.
Promisingly, we found that the conversion yield was significantly
improved as the graphitization degree of the carbon shell
improved. Noteworthy, the yield was high to 73%, which is much
higher than other similar nano/micro-particle catalyst [16,17].
2. Experimental
2.1. Preparation of Cu@C composites
In a typical procedure, solution A was prepared by adding
1.225 g CuSO4
and solution B was prepared by mixing 5 mL of NH3
35 mL of water. Then, solution A was added into solution B slowly
under the stirring for 30 min. After that, the blue aqueous solution
was transferred into stainless-steel autoclaves with Telfon. The
ꢁ
5H2O and 2 g of glucose into 30 mL of distilled water,
ꢁ
H2O (25%) with
* Corresponding author. Tel.: +86 512 61769645; fax: +86 512 61769645.
E-mail addresses: lunaticmh@163.com, 158686250@qq.com (H. Ming).
http://dx.doi.org/10.1016/j.materresbull.2015.04.043
0025-5408/ã 2015 Elsevier Ltd. All rights reserved.

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L. Bu, H. Ming / Materials Research Bulletin 70 (2015) 163–166
autoclave was sealed and maintained at 453 K for 8 h, and then cool
down to room temperature naturally. Finally, a red-dark product
was collected by centrifugation and wash by deionized water and
alcohol for several times. Vacuum dry the sample at 353 K for 24 h
and it was named as pristine Cu@C composite.
To obtain composites with different carbonization degree of
carbon shell, we further calcined the pristine Cu@C composite at
573, 873 and 1173 K under the N2 atmosphere, under which the
heating rate was 5 K minꢀ1 and the heating time was 4 h. The
products of Cu@C-573 K, Cu@C-873 K, and Cu@C-1173 K were
obtained, respectively, in which the acronym of 573 K means
calcinations the sample at 573 K.
with CuK
a
radiation
(l= 0.154178 nm). Scanning electron
microscopy (SEM) images and energy-dispersive X-ray
spectroscopy (EDX) were collected under a FEI-quanta 200F
scanning electron microscope with acceleration voltage of 20 kV.
Fourier transform infrared spectroscopy (FTIR) was investigated by
Hyperion 3000 (Bruker). Raman spectra were collected on a Laser
confocal micro Raman spectrometer (HORIBA Jobin Yvon).
Transmission electron micrographs (TEM) were taken on
FEI-Tecnai F20 (200 kV) transmission electron microscope.
a
3. Results and discussion
3.1. Formation of Cu@C composites
2.2. Catalytic experiments
A typical XRD pattern of the as-prepared Cu@C composite was
shown in Fig. 1a. Three slightly broadened diffraction peaks can be
indexed to the cubic phase of Cu, in accordance with the reported
data (JCPDS No. 85-1326, a = 0.3615 nm). The broad peaks arising
from 22ꢂ are attributed to the thin amorphous carbon layer on the
surface of Cu. There was no characteristic peaks of impurities of Cu
(OH)2, CuO, and/or Cu2O, suggesting that the CuSO4 was fully
deoxidized by glucose and generated elementary Cu. In addition, it
also indicated that such Cu nanoparticles are very stable in air due
to the protection of carbonaceous shells. The SEM image (Fig. 1b)
provides direct information about the size and typical core@shell
structure of the as-synthesized Cu@C composites. This core@shell
structure is also confirmed by the sharp contrast between the pale
edges and the dark centers under the TEM, as shown in Fig. 1c and
d. The thickness of colorless colloidal carbon shell reached to
50 nm, and the diameter of Cu core ranging from 200 to 400 nm.
The HRTEM image (Fig. S1) reveals that the Cu particles were
indeed coated by a layer of amorphous carbon. However, we could
not observe the clear lattice fringe image because the thick
The coupling reaction of aniline and benzene halides using the
as-prepared Cu@C composites (i.e., pristine Cu@C, Cu@C-573 K,
Cu@C-873 K, and Cu@C-1173 K) as catalyst, DMSO as solvent, and
KOH as alkali, was performed under N2 protection. The reaction
equation was shown insert in Fig. 4. Typically, the mixture was
heated at 383 K for 18 h and the process of reaction was monitored
by thin-layer chromatography (TLC). The reaction mixture was
cooled to room temperature, and treated with 5 mL ethyl acetate
and 2 mL water. The aqueous layer was separated and extracted
with ethyl acetate. The combined organic solution was dried with
Na2SO4 and purified on silica–gel column chromatography, using
ethyl acetate and hexane as eluent. The conversion yields of
product were calculated.
2.3. Characterization
The X-ray powder diffraction (XRD) patterns were recorded by
using a X’Pert-ProMPD (Holand) D/max-gAX-ray diffractometer
Fig. 1. (a) XRD pattern, (b) SEM, (c) and (d) TEM images of Cu@C composite.

L. Bu, H. Ming / Materials Research Bulletin 70 (2015) 163–166
165
Fig. 2. (a) XRD patterns and SEM images of (b) Cu@C-573 K, (c) Cu@C-873 K, and (d) Cu@C-1173 K, the scale bar is 1 mm.
thickness of carbon and narrow lattice distance of Cu (e.g., only
3.2. Catalytic ability of Cu@C composites
0.2086 nm for (111) and 0.1806 nm for (2 0 0)). Additionally, the
EDX spectrum and mapping images (Fig. S2) of Cu@C composite
indicated that three kind elements of Cu, C, and O contained in
composite and they have a uniformly distribution, in which the
trace element of C is from the carbonaceous shell. This result was
also confirmed by the FTIR spectrum (Fig. S2), in which a large
Here, we select the coupling reaction of iodobenzene and
aniline as the standard substrates to evaluate the catalytic
properties of Cu@C composites, and the catalytic data are
summarized in Fig. 4. The resultant product has been confirmed
by 1H NMR, 1H NMR: (400 MHz, CDCl3),
d 7.236–7.274 (m, 4H),
numbers of functional groups such as ꢀꢀOH and C
¼
O groups
7.074–7.054 (d, 4H), 6.940–6.904 (m, 2H) and 5.773 (s, 1H).
Detailed information is shown in Fig. S5. The catalysts of Cu@C,
Cu@C-573 K, Cu@C-873 K, and Cu@C-1173 K composites are named
as A, B, C and D, respectively. It could be find that the Cu@C-1173 K
has the best catalytic ability may due to the higher graphitization
degree of the carbon shell, in which the yield of the product can be
as high as 73%. The reason should be ascribed to the follow aspects:
(i) carbon shell could stabilize Cu particles and prevent them from
aggregation; (ii) the synergistic effect of Cu and carbon may have a
positive effect of the catalytic reaction; (iii) the higher
carbonization degree and porous structure of carbon are helpful
for the sufficient contact of catalyst and reactant and (iv) more
existed, and it might improve the linkage of bio-molecules or
catalytic species to the surface in the future application.
In order to increase the carbonization degree of shell, the
sample of Cu@C was calcined at different temperatures of 573,
873 and 1173 K under the N2 atmosphere for 4 h. From the XRD
patterns (Fig. 2a), it was confirmed that there is no crystalline
phase change during the calcinations process. These samples
almost keep the same morphologies after the calcinations as
observed under the SEM images (Fig. 2b–d), and the only
difference is that the carbon shells become thinner as the calcined
temperature enhanced. The thickness of carbon shell is also
confirmed by TEM images (Fig. S3), and the thickness of carbon
shell decreased as the temperature increased. Note that for the
sample of Cu@C-1173 K (Fig. 3d), there is a porous structure formed
in the carbon shell, and it may benefit for the contact of Cu catalyst
with the reactant. In addition, the high temperature calcinations
give rise to better carbonization degree of carbon shell, as
confirmed by the Raman spectra (Fig. 3), in which the intensity
of the overall Raman spectra increased with increasing the
temperature. Meanwhile, the graphitization degree of the carbon
shell is also increased. In detail, the sample of Cu@C composite
heated at 1173 K has the highest intensity ratio of the D and G band
(ID/IG), which is a measurement of the order extent, as well as the
ratio of sp2/sp3 carbon structure. Therefore, the higher calcination
temperature endows the carbon shell more sp2 carbon structure,
D
G
Cu@C-1173K
Cu@C-873K
Cu@C-573K
Pristine Cu@C
25-010
1000
1500
2000
3000
Raman shift (cm
)
Fig. 3. Raman spectrum (lex = 633 nm) of these Cu@C composites calcined at
different temperatures.
indicating more
p–p chemical bonds formed [18].

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L. Bu, H. Ming / Materials Research Bulletin 70 (2015) 163–166
Fig. 4. The product yields of these Cu@C composites in different carburization degree.
p
–
p
chemical bonds in higher graphitized carbon ensures a
Appendix A. Supplementary data
stronger electron field for accelerating this catalytic reaction
[9,16,17]. The mechanism of this reaction is best viewed from the
perspective of the Cu@C catalyst, as shown in Fig. S6. The first step
is the oxidative addition of Cu@C to the halide 2 to form the Cu@C
species 3. Reaction with base gives intermediate 4, which via
transmetalation with complex 6 forms the Cu@C species 8.
Reductive elimination of the desired product 9 restores the
original Cu@C catalyst 1.
Supplementary data associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/j.
materresbull.2015.04.043.
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Acknowledgments
Financial supports from the Nature Science Foundation of China
(Nos. 20873089 and 20975073), Nature Science Foundation of
Jiangsu Province (No. BK2011272), Industry-Academia Cooperation
Innovation Fund Projects of Jiangsu Province (No. BY2011130),
Project of Scientific and Technologic Infrastructure of Suzhou (No.
SZS201207), Graduate Research and Innovation Projects in Jiangsu
Province (No. CXZZ13_0802) and Hunan Provincial Innovation
Foundation For Postgraduate (No. CX2012B206) are gratefully
acknowledged.
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