Facile synthesis of hydrochar supported copper nanocatalyst for Ullmann C–N coupling reaction in water

Article abstract of DOI:10.1016/j.mcat.2019.110726

The exploration of inexpensive and stable heterogeneous catalysts and application of green solvents for Ullmann C–N coupling reaction remain challenging. We present a facile fabrication of copper nanoparticles on hydrochar as prepared from natural, inexpensive and renewable chitosan together with in-situ reduction of copper salt in a one-pot hydrothermal carbonization process. The copper nanoparticles were uniformly dispersed on hydrochar by choosing block copolymer F127 as surfactant. Moreover, maleic acid was introduced to improve the hydrophilicity of hydrochar. The most active copper nanocomposite catalyst, that is, Cu/HCS-MA-F127, exhibited excellent catalytic activity for Ullmann C–N coupling reaction in water. The nature of the Cu/HCS-MA-F127 was characterized by FTIR spectroscopy, TG, XRD, SEM and XPS. Moderate to excellent yields of aimed products were gained by using this catalytic strategy. Moreover, the Cu/HCS-MA-F127 catalyst can be reused by simple centrifugal recovery with a stable performance.

Full text of DOI:10.1016/j.mcat.2019.110726

MolecularCatalysisxxx(xxxx)xxxx
Contents lists available at ScienceDirect
Molecular Catalysis
journal homepage: www.elsevier.com/locate/mcat
Facile synthesis of hydrochar supported copper nanocatalyst for Ullmann
CeN coupling reaction in water
Xin Gea, Meng Gea, Xinzhi Chenb, Chao Qianb, Xuemin Liua,*, Shaodong Zhoub,*
a School of Chemical and Material Engineering, Jiangnan University, Wuxi, PR China
b Zhejiang Provincial Key Laboratory of Advanced Chemical Engineering Manufacture Technology, College of Chemical and Biological Engineering, Zhejiang University,
Hangzhou, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords:
The exploration of inexpensive and stable heterogeneous catalysts and application of green solvents for Ullmann
CeN coupling reaction remain challenging. We present a facile fabrication of copper nanoparticles on hydrochar
as prepared from natural, inexpensive and renewable chitosan together with in-situ reduction of copper salt in a
one-pot hydrothermal carbonization process. The copper nanoparticles were uniformly dispersed on hydrochar
by choosing block copolymer F127 as surfactant. Moreover, maleic acid was introduced to improve the hy-
drophilicity of hydrochar. The most active copper nanocomposite catalyst, that is, Cu/HCS-MA-F127, exhibited
excellent catalytic activity for Ullmann CeN coupling reaction in water. The nature of the Cu/HCS-MA-F127 was
characterized by FTIR spectroscopy, TG, XRD, SEM and XPS. Moderate to excellent yields of aimed products
were gained by using this catalytic strategy. Moreover, the Cu/HCS-MA-F127 catalyst can be reused by simple
centrifugal recovery with a stable performance.
Hydrochar
Nanocatalyst
Chitosan
Copper
eCN coupling reaction
In water
Introduction
(MNPs) as found for the organic coupling reactions, the nano-sized
metal catalysts on proper supports attract raising attention. [15–17]
Enormous efforts have been made for constructing a CeN bond,
which constitutes an important structural motif in various drugs, nat-
ural products and materials [1–3]. In 1990s, palladium-catalyzed re-
actions as explored by Buchwald and Hartwig [4,5] achieved a major
breakthrough in CeN coupling. After that, quite some protocols have
been exploited to improve the CeN coupling processes under mild re-
action condition. In particular, copper-catalyzed Ullmann reaction, in
which the toxic and expensive palladium is not involved, has proven to
be an efficient strategy and continues attracting broad attention [6,7].
For example, Buchwald [8] first disclosed the catalytic system of dia-
mine ligands for N-arylation; afterwards, Cristau and Taillefer [9] em-
ployed polydentate ligands like Schiff base and oxime in such reactions;
Ma [10] found that amino acids are particularly efficient for CeN
coupling. Though various ligands have been developed and used to
promote copper-catalyzed Ullmann reactions, these homogeneous sys-
tems suffer from well-known disadvantages, such as unrecyclable cat-
alysts, complex ligands and potentially toxic solvents (e.g., DMF,
DMSO, dioxane). [11–14] Thus, exploration of heterogeneous catalysts
and application of green solvents for Ullmann CeN coupling reaction
continue to be demanding.
Also, catalysts based on Cu, CuO and Cu2O nanoparticles have been
employed to catalyze the formation of CeN bonds. For example, Bai
et al. [18] reported that Cu0 anchored on nanofibers by hydrothermal
method for promoting Ullmann CeN coupling at 140 °C in DMF. Hyeon
et al. [19] prepared Cu2O coated Cu nanoparticles (NPs) as catalysts for
Ullmann type amination coupling in DMSO; S. Rawat et al. [20] re-
ported that copper NPs incorporated on alumina/silica support can
promote the cross coupling of aryl chlorides with amines at 150 °C in
DMF; Jafarzadeh et al. [21] loaded CuO NPs on MOFs for promoting
Ullmann coupling at 110 °C in DMSO. In spite of the above achieve-
ments, improvements are still required for the application of MNPs
regarding the use of organic ligands, multi-step synthetic process and
the emission of wastes.
So far as reported, the heterogeneous copper catalysts have been
prepared by entrapment and coordination of copper on various kinds of
supports, such as carbon [22], alumina [20], zeolites [23], polymers
[24,25] and magnetic materials [26,27], all of which exhibited re-
markable performance in catalyzing the Ullmann CeN coupling pro-
cesses. Due to the chemical inertness and economic accessibility of
carbon, MNPs fabricated on carbon (MNP/C) is an ideal approach.
Hydrothermal carbonization (HTC) processing of biomass turns out to
Recently, due to the high catalytic activity of metal nanoparticles
⁎ Corresponding authors.
E-mail addresses: lxm@jiangnan.edu.cn (X. Liu), szhou@zju.edu.cn (S. Zhou).
https://doi.org/10.1016/j.mcat.2019.110726
Received 7 August 2019; Received in revised form 25 October 2019; Accepted 25 November 2019
2468-8231/©2019ElsevierB.V.Allrightsreserved.
Pleasecitethisarticleas:XinGe,etal.,MolecularCatalysis,https://doi.org/10.1016/j.mcat.2019.110726

X. Ge, et al.
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Fig. 1. The prepartion of Cu/hydrochar catalyst.
be an effective strategy to prepare carbonaceous materials (often called
hydrochar) and metal-hybrid materials [28–30]. This method involves
thermal dehydration and transformation of biomass in water at low
temperatures [31,32]. Various carbonaceous structures and metal-hy-
brid materials (e.g., metal/C core-shell [33], metal/C nanocable [34]
and porous carbon [35]) were afforded using HTC; the latter thus ex-
hibits strong potential in many fields, such as adsorption [36], solid fuel
[37], energy storage [38] and catalysis [39,40]. On a different back-
ground, featured as biodegradable, renewable and inexpensive com-
pounds, polysaccharides have become the most promising biomass
[41]. Among these polysaccharides, chitosan (CS) possesses a large
amount of amino and hydroxyl groups, which would be retained in the
HTC process [42]. CS-derived hydrochar has strong metal-chelating
capability and are suitable support for MNP/C.
In this work, we presented a one-pot, in-situ HTC process using CS
and a solution of metal ions as the raw materials (Fig. 1). CS usually
serves as the reducing agents, while in this work it functions as both the
carbon source and the reductant. Consequently, the copper nano-
particles dispersed uniformly on hydrochar (Cu/hydrochar). To our
delight, the so-prepared Cu/hydrochar showed high catalytic activity
for the Ullmann CeN coupling. Moreover, such Cu/hydrochar species is
of strong hydrophilicity and thus able to promote the coupling reactions
in water. Herein, we describe in detail the preparation of copper NPs on
CS-derived hydrochar and its application in catalytic coupling of Ull-
mann CeN reaction.
vacuum at 50 °C overnight to give the CuSO4/CS catalyst. Similarly, the
Cu/CS catalyst was prepared [43]: 1 g chitosan and 0.3 g CuSO4·5H2O
were added into 30 ml of deionized water and stirred vigorously. Then
2 M NaOH was added dropwise to turn the pH value to 13. Next, 0.227
g NaBH4 was added to the above solution and continuously stirred for 2
h. The solid was separated by filtration, washed with water and ethanol,
dried under vacuum at 50 °C overnight.
The synthesis of Cu/HCS-MA-F127
Typically, 0.3 g CuSO4·5H2O and 0.2 g F127 were added into 30 ml
of deionized water. The mixture was stirred at room temperature for 30
min. Then 1 g chitosan and 0.2 g maleic acid were added to the above
mixture, and stirred for 30 min. Finally, the mixture was transferred
into a Teflon-lined stainless steel autoclave and heated at 180 °C for 10
h. After cooling naturally, the black brown carbonaceous material was
obtained. The material was washed with water and ethanol several
times, and dried in a vacuum at 80 °C overnight.
General procedure for the Ullmann reaction catalyzed by Cu/HCS-MA-F127
To a stirred solution of H2O (4 mL), aryl halide (1.0 mmol), nu-
cleophile (1.2 mmol), Cu/HCS-MA-F127 and K2CO3 (2 mmol) were
added at room temperature. Next, the reaction mixture was heated to
100 °C in air and stirred for 24 h. After cooling down to room tem-
perature, the catalyst Cu@HCS-MA-F127 was separated by centrifuga-
tion. The reaction mixture was partitioned by adding ethyl acetate (20
mL) and water (20 mL). Subsequently, the organic phase was separated
and the aqueous phase was extracted with ethyl acetate (20 mL) twice.
The combined organic phases were washed with brine, dried over
Na2SO4, and concentrated in vacuo. Finally, the crude product was
purified by column chromatography with silica gel, eluting with a
petroleum ether/ethyl acetate solvent mixture, to give the pure pro-
duct.
Experimental section
Materials and methods
Chitosan powder (MW: 10000–50000, deacetylation degree 95 %,
purchased from Aladdin reagent (Shanghai) Co., Ltd) was used without
further purification. Maleic acid (MA), acetic acid, sulfuric acid and
CuSO4·5H2O was purchased from Sinopharm Chemical Reagent Co. Ltd,
and Pluronic F127 was purchased from Energy Chemical. Aryl halides
and imidazole were purchased from Alfa Aesar. Nuclear magnetic re-
sonance (NMR) spectra were measured at 400 MHz (1H) or at 100 MHz
The characterization of catalysts
(
13C) with CDCl3 as the solvent on a Bruker Avance DRX-400 spectro-
The X-ray diffraction (XRD) pattern was recorded with a dif-
fractometer (Bruker D8 Advance) using Cu Kα radiation. The catalysts
were examined at room temperature and a range of 5-80° on 2θ. The
scanning electron microscopic (SEM) was equipped with a field-emis-
sion scanning electron microscope (S-4800, Hitachi) at 30 kV accel-
erating voltage to investigate the morphology of the catalysts. The
transmission electron microscopic (TEM) images were taken using a
JEM-2100plus at an acceleration voltage of 100 kV. The TEM samples
were prepared by dropping the catalyst suspension directly onto a
copper grid and allowed to dry. The fourier transform infrared spectra
(FTIR) were recorded in a range of 7800-350 cm−1 at a resolution of
meter. All reactions were monitored by analytical thin-layer chroma-
tography (TLC) from Merck with detection by UV. The products were
purified by column chromatography through silica gel (300–400 mesh).
All reagents and solvents were general reagent grade unless stated
otherwise.
The synthesis of CuSO4/CS and Cu/CS
Typically, 1 g chitosan and 0.3 g CuSO4·5H2O were added into 30 ml
of deionized water. The mixture was continuously stirred at room
temperature for 3 h. After adsorption of the copper, the solid was se-
parated by filtration, washed with water and ethanol, dried under
0.09 cm-1 using
Scientific, America). The thermogravimetric analysis (TGA) was
a “Nicolet 6700” spectrometer (ThermoFisher
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Table 1
Optimization of reaction conditionsa.
Entry
Catalyst
Base
Solvent
Temperature/oC
Yieldb/%
1
2
3
4
5
6
7
8
CuSO4/CS
Cu/CS
Cu/HCS
Cu/HCS-MA
Cu/HCS-AC
Cu/HCS-SA
Cu/HCS-MA-F127
Cu/HCS-MA-F127
Cu/HCS-MA-F127
Cu/HCS-MA-F127
Cu/HCS-MA-F127
Cu/HCS-MA-F127
Cu/HCS-MA-F127
Cu/HCS-MA-F127
Cu/HCS-MA-F127
Cu/HCS-MA-F127c
Cu/HCS-MA-F127d
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
NaOAc
KOAc
NaHCO3
KHCO3
Na2CO3
Cs2CO3
K2CO3
K2CO3
K2CO3
K2CO3
DMSO
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
110
110
100
100
100
100
100
100
100
100
100
100
100
90
70
60
75
88
80
80
98
60
70
80
86
90
92
90
70
80
95
9
10
11
12
13
14
15
16
17
80
100
100
a
Reaction conditions: 4-iodoanisole 1a (1.0 mmol), imidazole 2a (1.2 mmol), base (2.0 mmol) and the catalyst (100 mg) in the solvent (4 mL) for 24 h.
Isolated yield.
Hot-filtration test of the model reaction conducted under optimized conditions for 12 h.
Cu/HCS-MA-F127 was pretreated in water at 100 °C for 10 h.
b
c
d
performed on a METTLER instruments (TGA/DSC1/1100SF). TG curves
MA-F127 species; as a result, the latter exhibited even higher catalytic
activity in aqueous medium (Table 1, entry 7). Further, several bases
including K2CO3, NaOAc, KOAc, NaHCO3, KHCO3, Na2CO3 and Cs2CO3
were tested, and it turned out that K2CO3 performed best (Table 1,
entries 7–12). Then, as to an appropriate temperature (Table 1, entries
7, 14–15); the yield of the final product is up to 98 % at 100 °C. Finally,
to verify the heterogeneous nature of Cu/HCS-MA-F127, a hot filtration
test was performed. When the reaction proceeded for 12 h and the yield
was up to 80 % (Table 1, entry 16), the particles of Cu/HCS-MA-F127
were centrifuged out of the reaction mixture and the reaction result did
not change next 12 h. It turned out that the Cu is stable on the surface of
hydrochar and the catalyst is truly heterogeneous. To conclude, the
optional conditions for the CeN coupling reaction in water are: using
Cu/HCS-MA-F127 as catalyst, K2CO3 as base source, and controlling the
reaction temperature at 100 °C.
To further probe the nature of metal-hybrid carbonaceous materials
in a catalytic cycle, chitosan-derived hydrochar (HCS), Cu/HCS and Cu/
HCS-MA-F127 were characterized by X-ray diffraction (XRD). As shown
in Fig. 2, all carbonaceous materials had a broad diffraction peak at
around 20°, which is assigned to the (002) planes of carbon. Meanwhile,
both Cu/HCS-MA-F127 and Cu/HCS presented typical diffraction peaks
of 43.2°, 50.4° and 74.1° at 2θ, corresponding to the (111), (200) and
(220) planes of elemental copper, respectively (JCPDS File no.03-1018)
[45]. It was thus indicated that copper sulfate was reduced to elemental
copper, resulted from the strong reducibility of chitosan under the
hydrothermal condition [46]. More details about the morphology and
the microstructure of Cu/HCS-MA-F127 were investigated by scanning
electron microscopy (SEM) and transmission electron microscopy
(TEM). Fig. 3a showed the well dispersion of copper nanoparticles on
HCS. As shown in Fig. 3c, the particle size of the copper NPs ranges
from 50 to 100 nm with a spherical morphology, which is due to the
structure-directing agent (F127) tightly wrapping [47,48]. The average
size of spherical copper NPs is 61.11 nm (in Fig. 3d). Moreover, the
copper of the fresh Cu/HCS-MA-F127 was measured by ICP-AES and a
content of 1.1 mmol g−1 was detected.
were recorded from 40 to 800 °C under a N2 atmosphere with a heating
rate of 10 °C min−1. The X-ray photoelectron spectroscopic (XPS)
measurements were conducted on an AXIS Supra instrument.
Inductively coupled plasma atomic emission spectrometry (ICP-AES,
ThermoFisher IRIS Intrepid II, America) was used to determine the
elemental content of copper.
Results and discussion
Our initial experimental investigation focused on the influence of
metal-hybrid carbonaceous materials on the catalytic activity. The
coupling of 4-iodoanisole with imidazole was selected as the model
reaction to benchmark the performance of the metal-hybrid carbonac-
eous catalyst and to optimize the reaction condition; more details are
provided in Table 1. CuSO4 supported on chitosan (CuSO4/CS) was
prepared first as a reference to catalyze the model reaction in DMSO
(Table 1, entry 1), and the result was consistent with the previous study
[11]. The Cu particles supported on chitosan (Cu/CS) promote the
model reaction in H2O to give a yield of 60 % (Table 1, entry 2). When
we attempted one-pot, in-situ HTC of the precursor CS with CuSO4 so-
lution, copper NPs were dispersed on the hydrochar (Cu/HCS). Sur-
prisingly, the so-prepared Cu/HCS exhibited good catalytic activity for
the model reaction with 75 % yield (Table 1, entry 3); note that the
green and sustainable solvent, i.e. water, was employed here. Actually,
it has reported that different water-soluble vinyl monomers can be ef-
fectively combined with polymers under the HTC process to provide
larger surface and excellent mechanical properties, which implies a
cycloaddition mechanism between maleic acid and carbon framework.
[42,44] As introducing acid monomer into the system improves the
hydrophilicity of hydrochar, various acid monomers like. maleic acid,
acetic acid and sulfuric acid, were examined (Table 1, entries 4–6). All
acids were proven to improve the catalytic activity of metal-hybrid
carbonaceous materials, while obviously maleic acid outperformed the
others. Next, to enhance the uniformity of copper nanoparticle, block
copolymer F127 was chosen as the surfactant to prepare the Cu/HCS-
The FTIR spectra of Cu/HCS and Cu/HCS-MA-F127 were shown in
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Fig. 4. FTIR spectra of Cu/HCS and Cu/HCS-MA-F127.
Fig. 2. X-ray diffraction patterns of Cu/HCS-MA-F127 (a), Cu/HCS (b) and HCS
(c).
range of 50−100 °C, which might be caused by the evaporation of
adsorbed water; significant weight loss occur from 250 °C to 350 °C,
which can be attributed to the thermal decomposition of the materials.
Importantly, Cu/HCS-MA-F127 benefits from high thermal stability
that it can be used for reactions carried out at a temperature up to 200
°C.
X-ray photoelectron spectroscopy (XPS) experiments were carried
out to further analyze the elemental composition and chemical state of
the catalysts (Fig. 6). Fig. 6a is the full spectra of HCS and Cu/HCS-MA-
F127 in which the indication of copper elements is absent. This may be
caused by tight wrapping of the copper surface by F127 such that XPS
cannot detect the presence of copper. Moreover, the absence of 2p(Cu)
peak for fresh Cu/HCS-MA-F127 was consistent with the above
Fig. 4. For Cu/HCS, the characteristic absorption appears at 3400 cm−1
(O-H stretching vibration), 2873 cm−1 (C-H stretching vibration), 1575
cm−1 (N-H bending vibration), 1375 cm−1 (bending vibration of C-H),
1150 cm−1 (C-O-C stretching vibration) and 1025 cm−1 (C-O stretching
vibration). Compared with Cu/HCS, a new carboxyl peak emerges at
1700 cm−1 in the FTIR spectra of Cu/HCS-MA-F127. The new peak
implied that carboxyl groups were successfully incorporated into the
carbonaceous skeleton of the hydrochar. Moreover, the presence of a
large amount of carboxyl and hydroxyl groups constitutes the hydro-
philic surface of the hydrochar. Further, Fig. 5 exhibits the results for
TG analysis of HCS, Cu/HCS and Cu/HCS-MA-F127. Thus, all three
materials have two stages of mass loss: the initial weight loss is in the
Fig. 3. SEM images of Cu/HCS-MA-F127 (a), recovered Cu/HCS-MA-F127 after the second run (b), TEM images of Cu/HCS-MA-F127 (c) and particle size distribution
of Cu/HCS-MA-F127 (d).
4

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(Table 1, entry 17). After filtration and drying, the catalyst was ex-
amined and the yield of the final product was still up to 95 %. As shown
in Fig. 6c and d, the 1 s(N) peak of HCS and recycled Cu/HCS-MA-F127
after the second round were presented. Without chelation of copper, the
peaks of HCS at 398.2 eV and 399.5 eV (in Fig. 6c) can be attributed to
the N and -N-H- moieties of pyridine. When Cu/HCS-MA-F127 was
recylced after the second round, the 1 s(N) resonance can be divided
into three independent peaks at 398.4, 399.5 and 400.6 eV (in Fig. 6d),
owing to the N, -N-H and -N-Cu moieties of pyridine, respectively
[50–52].
As shown in Fig. 7, the C 1s and O 1s spectra of XPS were used to
further investigate the structural information of hydrochar. Fig. 7a is
the C 1s spectra of Cu/HCS, in which was fitted into two peaks at 284.4
and 286.8 eV, corresponding to the C-C, C-N and C-O bonds, respec-
tively [49,53,54]. In Fig. 7b, the C 1s spectrum of Cu/HCS-MA-F127
consists of various types of C functionalities, including C-C (284.5 eV),
C-N/C-O (285.9 eV), and C=O (287.4 eV) bonds, indicating that the
carboxyl groups were successfully incorporated into the carbon fra-
mework. As shown in Fig. 7c and d, the O 1s pattern of Cu/HCS and Cu/
HCS-MA-F127 were presented, The O 1s spectrum of Cu/HCS could be
deconvoluted into one peak, corresponding to C-O/O-H (532.6 eV). The
O 1s signal of Cu/HCS-MA-F127 at 530.8 eV and 532.3 eV, which were
contributed by C=O and C-O/C-H, respectively.
Further, in order to reveal the generality of this protocol, various
substrates were employed in Cu/HCS-MA-F127 catalyzed Ullmann CeN
coupling reaction in water; the associated details are summarized in
Table 2. Firstly, the catalytic activity was assessed with different aryl
halides using imidazole as the substrate. Substrates with all substitution
groups (such as ortho-, meta-, and para-substituted aryl halides, 3a-m)
Fig. 5. TG curves of HCS, Cu/HCS and Cu/HCS-MA-F127.
conjecture, as shown in Fig. 6b. Interestingly, the 2p(Cu) peak was
observed after the second round of cycling and the peaks of 2p1/2(Cu)
and 2p3/2(Cu) were 952.9 eV and 933.1 eV, respectively. In addition,
compared with Fig. 3a and b, the copper NPs are more clearly mono-
dispersed and immobilized on carbonaceous materials after the second
round of cycling. This is due to the fact that F127 was washed away and
the elemental copper was exposed after reaction [49]. In order to study
the stability of copper nanoparticles without F127, the Cu/HCS-MA-
F127 was pretreated in water at 100 °C for 10 h to get F127 wash away
Fig. 6. XPS spectra of HCS and Cu/HCS (a), Cu 2p peaks of Cu/HCS-MA-F127 and recovered Cu/HCS-MA-F127 after the second run (b), N 1s pattern of HCS (c) and
recovered Cu/HCS-MA-F127 after the second run (d).
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Fig. 7. C 1s pattern of Cu/HCS (a) and Cu/HCS-MA-F127 (b), O1s pattern of Cu/HCS (c) and Cu/HCS-MA-F127 (d).
were efficiently coupled with imidazole to afford the corresponding N-
aryl compounds with moderate to good yields (50–90 %). Moreover,
both aryl iodide and aryl bromide gave rise to good yields of desired
products (98 and 70 %), respectively. And the reactivity of different
aryl halides follows the order PhI > PhBr > PhCl (3a). As to the in-
fluence of substitution groups, the electron-donating ones, such as 4-
OMe (3b), 4-Me (3c), 4-OEt (3d) and 4-OCF (3e), result in slightly
higher yields than that given by the electron-withdrawing ones, such as
4-F (3 h), 4-Cl (3i), 4-Ac (3 j) and 4-NO2 (3k). Next, various nitride
substrates were examined during the reaction. All employed nitride
substrates (3n-t) could be efficiently coupled with 4-iodoanisole in
moderate to good yields (40–85 %).
Finally, we checked the recyclability of Cu/HCS-MA-F127 for aqu-
eous Ullmann CeN coupling systems. The separation of Cu/HCS-MA-
F127 from the aqueous medium was achieved by centrifugal filtration.
After that, the separated catalyst was washed several times with deio-
nized water to remove the base and dried at 50 °C under vacuum. The
catalyst was thus recycled and reused subsequently for the next round.
The recyclability of Cu/HCS-MA-F127 catalyst was examined with the
model reaction as mentioned in Table 1; the results were summarized in
Fig. 8. It was found that after five round the Cu/HCS-MA-F127 catalyst
kept its excellent catalytic activity and stability. Moreover, the SEM
image of Cu/HCS-MA-F127 after the second round of cycling was
shown in Fig. 3b, which clearly shows that copper NPs are mono-
dispersed and immobilized on carbonaceous materials. The copper
content of this recovered Cu/HCS-MA-F127 was determined as 0.8
mmol g−1 by ICP-AES.
Conclusions
In summary, we developed a facile method for fabrication of copper
NPs on the hydrochar by using natural, inexpensive and renewable
chitosan as the carbon source and reductant combined with in-situ re-
duction of copper salt in a one-pot hydrothermal carbonization process.
The copper NPs were uniformly dispersed on hydrochar, exhibiting
excellent catalytic activity for Ullmann CeN coupling reaction.
Moreover, Cu/HCS-MA-F127 possesses strong hydrophilicity and are
thus able to promote the coupling reaction in water with moderate to
excellent yields. Notably, the Cu/HCS-MA-F127 catalyst can be re-
cycled by simple centrifugal filtration and reused several times without
obvious decrease of activity.
Declaration of Competing Interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influ-
ence the work reported in this paper.
Acknowledgements
The authors are grateful for the financial support from the Natural
Science Foundation of China (21606104) and the National Key
Research and Development Program of China (2016YFB0301800).
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Table 2
Substrate scope for Ullmann CeN coupling reactiona.
a
Reaction conditions: aryl halides 1 (1.0 mmol), nitrogen nucleophiles 2 (1.2 mmol), K2CO3 (2.0 mmol) and Cu/HCS-MA-F127 (100 mg) in H2O (4 mL) at 100 °C
for 24 h.
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