Transfer Hydrogenation of Nitroarenes Catalyzed by CoCu Anchored on Nitrogen-doped Porous Carbon

Article abstract of DOI:10.1002/aoc.5438

The non-precious metal catalysts with high catalytic activity is extremely desirable but still full of challenges. In this paper, CoCu bimetal immobilized on nitrogen-doped porous carbon (CoCu-N-C) was prepared by an effective ligand-stabilized pyrolysis strategy. CoCu-N-C exhibited excellent catalytic efficiency for the transfer hydrogenation of nitroarenes with ammonia borane as hydrogen source, which can be ascribed to the well dispersed metal nanoparticles, the synergetic interaction of CoCu bimetal and nitrogen-doped carbon. The durability and recyclability experiments of the recycled CoCu-N-C catalyst indicated that no obvious change in catalytic performance was observed after five consecutive cycles. To gain insight into the catalytic mechanism of CoCu-N-C for the hydrogenation reaction, density functional theory calculations was also conducted. This work provides an universal approach for constructing highly efficient non-precious metal heterogeneous catalysts and which may find diverse high performance applications.

Full text of DOI:10.1002/aoc.5438

Received: 23 August 2019
DOI: 10.1002/aoc.5438
Revised: 8 November 2019
Accepted: 4 December 2019
F U L L P A P E R
Transfer Hydrogenation of Nitroarenes Catalyzed by CoCu
Anchored on Nitrogen-doped Porous Carbon
Xiaotong Liu1 | Longkang Zhang1
| Junmin Wang1 | Ningzhao Shang1
|
Shutao Gao1 | Chun Wang1
| Yongjun Gao2
1College of Science, Hebei Agricultural
University, Baoding, 071001, P. R. China
2College of Chemistry and Environmental
Science, Hebei University, Baoding,
071002, China
The non-precious metal catalysts with high catalytic activity is extremely desir-
able but still full of challenges. In this paper, CoCu bimetal immobilized on
nitrogen-doped porous carbon (CoCu-N-C) was prepared by an effective
ligand-stabilized pyrolysis strategy. CoCu-N-C exhibited excellent catalytic effi-
ciency for the transfer hydrogenation of nitroarenes with ammonia borane as
hydrogen source, which can be ascribed to the well dispersed metal
nanoparticles, the synergetic interaction of CoCu bimetal and nitrogen-doped
carbon. The durability and recyclability experiments of the recycled CoCu-N-C
catalyst indicated that no obvious change in catalytic performance was
observed after five consecutive cycles. To gain insight into the catalytic mecha-
nism of CoCu-N-C for the hydrogenation reaction, density functional theory
calculations was also conducted. This work provides an universal approach for
constructing highly efficient non-precious metal heterogeneous catalysts and
which may find diverse high performance applications.
Correspondence
Chun Wang, College of Science, Hebei
Agricultural University, Baoding 071001,
Hebei, China.
Email: chunwang69@126.com;
wangchun@hebau.edu.cn
Yongjun Gao, College of Chemistry and
Environmental Science, Hebei University,
Baoding 071002, China.
Email: yjgao@hbu.edu.cn
Funding information
Natural Science Foundation of Hebei
Province, Grant/Award Numbers:
B2015204003, B2016204131, B2018204145,
B2019204023; National Natural Science
Foundation of China, Grant/Award
Numbers: 21603054, 21773053
K E Y W O R D S
ammonia borane, CoCu bimetaldensity functional theory, Nitroarene, transfer hydrogenation
1 | INTRODUCTION
metals impedes their industrial applications. On the
other hand, the high catalytic activity of the noble metal-
Aromatic amine compounds are indispensable intermedi-
ates in the synthesis of various products including
agrochemicals, pigments, dyes, polymers and pharmaceu-
ticals.[1,2] A general synthetic route for aromatic amines
is the catalytic hydrogenation of the corresponding
nitroaromatic compound with hydrogen catalyzed by Pt,
Pd, Au or other noble metal based nanoparticles.[3–5]
Although hydrogenation with H2 is considered as one of
the most attractive approach, however, the reduction of
nitroaromatic compounds with H2 is usually performed
at high pressure and high temperature. Meanwhile, H2 is
explosive, flammable and needs special equipments for
production and transportation. Furthermore, not only
the expensive price, but the limited reserve of noble
based catalysts usually resulted in high conversion but
low selectivity.[6]
To avoid the employment of dangerous hydrogen, cat-
alytic transfer hydrogenation (CTH), a much safer and
environmental friendly method, has been proposed.[7]
Among the hydrogen donors employed in CTH,
ammonia-borane (AB) is a very potent hydrogen storage
compound due to the high hydrogen capacity (19.6 wt%),
excellent stability, low toxicity and high solubility in
polar solvents, such as water and methanol.[8,9] Non-
noble metal-based catalysts for CTH have received much
interest due to its earth-abundant property and compara-
ble catalytic performance with noble metal-based
catalysts.[10–12] For example, Co–Mo2C/AC developed by
Appl Organometal Chem. 2020;e5438.
wileyonlinelibrary.com/journal/aoc
© 2020 John Wiley & Sons, Ltd.
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https://doi.org/10.1002/aoc.5438

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LIU ET AL.
Guo et al exhibited unexpected catalytic performance for
the facile and chemoselective reduction of nitroarenes.[13]
Co/CN catalyst with Co2+ species on the catalyst surface
can be applied as efficient heterogeneous catalyst for the
transfer hydrogenation reactions with AB as hydrogen
donor.[14] CuNi nanoparticles encapsulated in MIL-101
was fabricated by a double solvent approach followed by
in situ reduction with NH3BH3, which exhibited good
catalytic performance for the cascade reactions of
nitroarene reduction.[15] Sun et al. assembled CuNi
nanoparticles on graphene, which also displayed well cat-
alytic activity for the nitroarene reduction[16]. Although
significant progress has been achieved, these catalytic
systems still have one or more limitations in terms of cat-
alytic activity, stability, dispersity of the metal
nanoparticles, recyclability and so on.[17–19] The develop-
ment of highly selective, active, and stable heterogeneous
catalysts for the catalytic transfer hydrogenation of
nitroarenes under mild conditions is still desired urgently
for practical use.
Transition metal supported on nitrogen-doped carbon
(M-N-C) are demonstrated as promising catalysts for
electrocatalysis and heterogeneous catalysis.[19–22] Nitro-
gen doping may serve as coordination sites for stabilizing
the small size metal nanoparticles. Furthermore, the
doped nitrogen may modulate the electronic structure of
the nitrogen-doping carbon, which may regulate and
improve the catalytic performance and selectivity of the
M-N-C catalysts.[20,21] M-N-C catalysts are commonly
prepared via pyrolyzing various nitrogen-containing pre-
cursors and metal salts at high temperature. However,
the direct carbonization approaches usually cannot
obtain highly dispersed metallic nanoparticles, which
may deteriorate their catalytic activity. Although hard-
template method may be a feasible process, however, it
suffers from the disadvantages such as tedious synthesis
processes and high cost. Moreover, it is usually required
to remove the templates with hazard and toxic reagents.
Another challenge is the large-scale production of the M-
N-C catalysts, which is crucial to their practical applica-
tions. The method for mass preparation should be low
cost, employing commercially available supports and
scalable process. Therefore, a facile strategy compatible
with mass production is still highly desirable to construct
M-N-C catalysts.
with AB as hydrogen donor. Density functional theory
calculations revealed the important role of M-Nx active
sites for the excellent catalytic performance.
2 | EXPERIMENTAL SECTION
2.1 | Preparation of the catalysts
Cu (NO3)2•3H2O(0.06 mmol), Co (NO3)2•6H2O
(0.15 mmol) and 2,20-dipyridyl (2 mmol) were dissolved
in ethanol (50 ml) at room temperature with stirring for
10 min. Afterwards, Vulcan XC72R carbon (230 mg) was
added into the above solution and the mixture was ultra-
sonicated for 30 min and stirred at 60 ꢀC for 4 hr. The sol-
vent was removed in vacuo. The dried sample was
grinded to a fine powder. The sample was placed in a 60
degree oven for drying. The obtained black solid powder
was sintered at 800 ꢀC for 4 hr under Ar (5 ꢀC/min ramp
rate). The product was named as Co5Cu2-N1-C (the mol
ratio of Co and Cu was 5:2). Co2Cu5-N1-C and Co1Cu1-
N1-C were fabricated with the above procedure except
that the mole ratio of Co and Cu was 2:5 and 1:1,
respectively.
Co5Cu2-N2-C and Co5Cu2-C was prepared by the
same method except that 1,10-phenanthroline was added
instead of 2,20-bipyridine and without addition of 2,20-
bipyridine, respectively. *Co5*Cu2-C was obtained with
the same method except that 0.15 mmol cobalt ace-
tylacetonate and 0.06 mmol copper acetylacetonate were
added instead of Cu (NO3)2•3H2O, Co (NO3)2•6H2O and
2,20-bipyridine.
2.2 | Catalytic reduction of nitro-
compounds
A mixture of nitro compound (0.2 mmol), ammonia-
borane (0.8 mmol, 24.7 mg), catalyst (10 mg) and solvent
(5 ml, EtOH:H2O = 3:2, containing 0.2 mol/l NaOH)
were added into a 25 ml round bottom flask. The flask
ꢀ
was placed in a water bath at 30 C with stirring and the
reaction time was recorded. The reaction was monitored
by TLC, upon completion of the reaction, the product
was extracted with dichloromethane (3 x 5 ml) and quan-
tified by gas chromatography (GC). The products were
identified by comparison for their retention time with
pure chemicals and further confirmed by GC–MS.
In continuation of our efforts in exploring efficient
heterogeneous catalysts,[22–25] we now report a versatile
CoCu bimetal immobilized on nitrogen-doped porous
carbon (CoCu-N-C) for the transfer hydrogenation reac-
tion of nitroarenes. CoCu-N-C was prepared via an effec-
tive ligand-stabilized pyrolysis strategy. The CoCu-N-C
catalyst displayed superior catalytic activity and stability
for the CTH of nitro-compounds under mild conditions
3 | RESULTS AND DISCUSSION
The catalyst preparation process was illustrated in
Scheme 1. Vulcan XC-72R, a commercial available

TRANSFER HYDROGENATION OF NITROARENES
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acetylaceton as the ligand, and without the addition of a
ligand, respectively (see details in Experiment section).
3.1 | Characterization of the materials
The structure and morphology of the as-obtained cata-
lysts were characterized by TEM. Figure 1a showed that
fine crystalline of CoCu nanoparticles in Co5Cu2-N1-C
sample were evenly dispersed on the support with an
average diameter of about 11 nm. As shown in Figure 1b
and 1c, the TEM images obvious revealed that CoCu
nanoparticles were encapsulated in the lattice fringes of
graphite carbon. The high-resolution TEM image
(Figure 1c) displayed that the lattice fringes of the encap-
sulated nanoparticles were 0.208 nm, which was attribut-
able to the (111) plane of fcc Co or Cu due to the
similarity of the lattice constants of Co and Cu[26]. It can
be seen from Figure 1c that the thickness of the carbon
shell was between 4 and 7 nm, and the lattice fringes of
0.338 nm matched the (002) plane of graphite carbon.[27]
Elemental mapping images of Co5Cu2-N1-C obviously
confirmed that Co, Cu and N were uniformly distributed
on the catalyst. There was some copper signal noise out-
side the metal particles. This was due to the copper mesh
SCHEME 1 Schematic preparation process of CoCu-N-C
carbon material, possesses large specific surface area and
high porosity, which make it an ideal candidate for cata-
lyst support. Firstly, with the assistance of the ligands,
the complex of Co (NO3)2•6H2O and Cu (NO3)2•3H2O
with 2,20-dipyridyl was anchored on the surface of Vulcan
XC-72R carbon. After the solvent was removed in vacuo,
the obtained composite was transferred to a ceramic boat
ꢀ
in tube furnace and sintered at 800 C for 4 hr under a
pure Ar atmosphere. During the pyrolysis at high temper-
ature, the chelating agent containing nitrogen can effec-
tively sequester the metal ions and prevent the metal
nanoparticles from aggregation. The well dispersed and
nitrogen-doped carbon shell encapsulated CoCu bimetal-
lic catalyst (CoCu-N1-C) was finally obtained.
For comparison, Co5Cu2-N2-C, *Co5*Cu2-C and
Co5Cu2-C were also prepared with 1,10-phenanthroline,
FIGURE 1 TEM images of Co5Cu2-N1-C catalyst (a, b and c) and corresponding size distribution of Co5Cu2 nanoparticles, element
mappings of Co, Cu and N in Co5Cu2-N1-C (d), TEM images of Co5Cu2-C catalyst (e and f) and corresponding size distribution of Co5Cu2
nanoparticles

4 of 10
LIU ET AL.
used as sample support for TEM analysis. (Figure 1d)[28]
.
probably because CoCu were covered by carbon shell.
The Co 2p emission spectrum of Co5Cu2-N1-C
(Figure 2b) indicated two main signals located at 780 and
796 eV, corresponding to Co 2p3/2 and Co 2p1/2, respec-
tively, and weak satellite peaks located at around 786 and
805 eV. The binding energies at 780.4, 781.9 and 786.8 eV
can be attributed to Co 2p3/2 peak. The binding energy of
780.4 eV was assigned to the cobalt coordinate with
pyridinic N to form catalytically active Co-Nx sites. The
energy at 781.9 eV was attributed to the metallic Co(0)
element.[30] The XPS peaks of Cu 2p3/2 and 2p1/2 at the
binding energy of 933 eV and 953 eV can be classified
into four types. The peaks observed at 932.9 eV and
952.6 eV were assigned to metallic Cu(0). The peaks at
In contrast, for Co5Cu2-C sample prepared without addi-
tion of a nitrogen-containing ligand, the size of the bime-
tallic nanoparticles is relatively large, and the diameter of
the metal particles was approximately 30 nm to 40 nm
(as shown in Figure 1e). No clear carbon layers over the
Cu-Co bimetallic nanoparticles can be observed
(Figure 1f). These results indicated that a graphite carbon
shell was formed in Co5Cu2-N1-C by direct pyrolysis of
the nitrogen-rich metal complex, which not only
prevented the aggregation of metal nanoparticles, but
also the nitrogen-containing species can stabilize the
small size metal nanoparticles. The content of cobalt and
copper in Co5Cu2-N1-C was 4.37 wt% and 1.78 wt%,
respectively, which were determined by inductively
coupled plasma atomic emission spectrometry (ICP-
AES).
934.6 eV and 954.5 eV were corresponding to Cu2+
-
[31,32]
Nx
.
However, compared with Co5Cu2-C and Co5Cu2-
To confirm the electronic structures of Cu-Co based
metal/metal oxide nanoparticles in Co5Cu2-N1-C, XPS
was carried out. Based on the XPS data, atomic informa-
tion of C, Co, Cu, and N can be obtained. The three peaks
centered at 289.0 285.5 and 284.6 eV were attributed to
C − N bonds, sp3-hybridized diamond-like carbon and
N2-C (with 1,10-phenanthroline as the ligand) catalysts,
the Co peak of Co5Cu2-N1-C shifted to a lower binding
energy due to the close coupling of Co and N atoms,
which indicated the electrons of N transferred to the
cobalt (Figure S1). Therefore, the electron distribution on
the catalyst surface was modulated, which was beneficial
to the catalytic reaction.[33] The N 1 s XPS spectrum of
the catalyst can be deconvoluted into three peaks at
sp2-hybridized
graphite-like
carbon,
respectively
(Figure 2a).[29] The signal of Co and Cu were weak,
FIGURE 2 XPS data of Co5Cu2-N1-C catalyst. High-resolution spectrum of C 1 s (a), Co 2p (b), Cu 2p (c) and N 1 s (d)

TRANSFER HYDROGENATION OF NITROARENES
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398.8, 399.6 and 401.3 eV, which were corresponded to
pyridine N, pyrrole N and graphite N, respectively. The
nitrogen species may coordinated with copper and cobalt.
Moreover, according to the previous reported results, pyr-
rolic N species had strong ability to absorb and activate
nitro compounds, and induce favorable energetic charges
in heterogeneous catalytic processes.[31,34]
72R and N1-C did not give
a
conversion of
4-nitrochlorobenzene, indicating that the support mate-
rial was not active against the reduction of
4-nitrochlorobenzene under the conditions studied.
(Table 1, entries 1 and 2). Co5Cu2-C and *Co5*Cu2-C pre-
pared without nitrogen-containing precursors exhibited
much low activity on the reaction (Table 1, entries 3 and
4). Adding nitrogen-containing ligands can significantly
improved the catalytic activity of the catalyst, Co5Cu2-
N1-C prepared with 2,20-dipyridyl as the ligand delivered
excellent catalytic performance for the CTH of
4-nitrochlorobenzene. The experimental results showed
that near complete conversion and 99% selectivity were
The XRD patterns of the samples were investigated, a
pronounced diffraction peak at 26ꢀ appeared in all sam-
ples due to the (002) reflection of the graphite type lattice
(Figure 3a). Sample presents three peaks at 2θ = 44.1ꢀ,
51.5ꢀ and 75.8ꢀ, which was assigned to the cobalt (111),
(200) and (220) diffraction peak (PDF#15–0806). The
main characteristic of copper (111), (200), and (220) were
observed at 43.2ꢀ, 50.4ꢀ, and 74.1ꢀ (PDF#04–0836). For
the Co5Cu2-N1-C sample, we can see the obvious peaks
of Co and Cu at 44.1ꢀ, 51.5ꢀ, 43.2ꢀ, 50.4ꢀ, indicating the
formation of CoCu composite, which was consistent with
the results of the electron micrograph. Compared with
the XRD patterns of Co5Cu2 alloy, the results also dem-
onstrated that CoCu in the Co5Cu2-N1-C sample was not
an alloy structure. Typical N2 adsorption–desorption iso-
therms of the samples were shown in Figure 3b. All sam-
ples displayed H3-type hysteresis loop and a type-IV
curve, indicating the mesoporous structure of the pre-
pared samples, which is confirmed by the pore size distri-
bution curve (Figure 3c). The surface area of Co5Cu2-C,
Co5Cu2-N1-C, XC-72R and N1-C were 207, 77, 253 and
106 m2 g−1, respectively.
ꢀ
obtained after 15 min at 30 C (Table 1, entry 5). Com-
pared with the reported catalysts in the literatures,
Co5Cu2-N1-C delivered the highest activity except the
precious metal catalyst (Table S1). However, the Co5Cu2-
N2-C catalyst prepared with phenanthroline as the ligand
can only achieve 69% selectivity after 30 min reaction
although 99% conversion can be achieved (Table 1, entry
6). These results indicated that the structure of the nitro-
gen precursor plays an important role in the catalytic per-
formance of the catalysts. For comparison, Cu-N1-C, Co-
N1-C, Co2Cu5-N1-C and Co1Cu1-N1-C catalysts were also
fabricated. However, the selectivity of Cu-N1-C and Co-
N1-C was relatively low (Table 1, entries 7–8). Compared
with Co5Cu2-N1-C, the catalytic selectivity of Co2Cu5-
N1-C and Co1Cu1-N1-C decreased slightly with increas-
ing the amount of Cu in the catalysts (Table 1, entries
9–10). The above results clearly demonstrated that the
synergetic interaction of Cu and Co also responsible for
the excellent activity of Co5Cu2-N1-C.
3.2 | The catalytic hydrogenation of
nitroarenes
We also studied the effects of the type of solvents on
the reaction. The results showed that the solvent has an
important influence on the catalytic reaction (Table 1,
entries 11–14). Due to the poor solubility of nitroarene in
water, the conversion of 4-nitrochlorobenzene was only
25% after 30 min reaction under the same reaction condi-
tions with water as solvent (entry 14). When MeOH,
In order to study the catalytic performance of the pre-
pared catalysts, the CTH of 4-nitrochlorobenzene with
AB as the hydrogen donor were used as the model
reaction. We screened the reaction conditions for the for-
mation of p-chloroaniline and listed them in Table 1. XC-
FIGURE 3 (a) XRD images of different catalysts, (b) The N2 adsorption–desorption isotherms of Co5Cu2-N1-C, Co5Cu2-C, N1-C and
XC-72R carbon

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LIU ET AL.
TABLE 1 Screening of catalysts
Entry
1
Catalysts
Solvents
Time (min)
Con (%)
Sel(a) (%)
XR-72R
EtOH:Water (3:2)
EtOH:Water (3:2)
EtOH:Water (3:2)
EtOH:Water (3:2)
EtOH:Water (3:2)
EtOH:Water (3:2)
EtOH:Water (3:2)
EtOH:Water (3:2)
EtOH:Water (3:2)
EtOH:Water (3:2)
EtOH
60
60
30
30
15
30
30
30
30
30
30
30
30
30
5
-
-
2
N1-C
-
-
3
*Co5*Cu2-C
Co5Cu2-C
3
87
79
99
69
77
70
95
82
94
56
88
99
99
99
99
73
4
4
5
Co5Cu2-N1-C
Co5Cu2-N2-C
Cu-N1-C
99
99
99
75
99
99
99
99
99
25
99
83
99
92
6
7
8
Co-N1-C
9
Co1Cu1-N1-C
Co2Cu5-N1-C
Co5Cu2-N1-C
Co5Cu2-N1-C
Co5Cu2-N1-C
Co5Cu2-N1-C
Co5Cu2-N1-C
Co5Cu2-N1-C
Co5Cu2-N1-C
Co5Cu2-N1-C
10
11
12
13
14
15b
16c
17d
18e
MeOH
MeOH:Water (3:2)
Water
EtOH:Water (3:2)
EtOH:Water (3:2)
EtOH:Water (3:2)
EtOH:Water (3:2)
30
5
30
*Reaction conditions: ammonia borane (0.8 mmol), 4-nitrochlorobenzene (0.2 mmol), catalyst (5 mol%), solvent (5 ml, containing 0.2 mol/l NaOH), 30 ꢀC.
bammonia borane(1.6 mmol), cammonia borane (0.4 mmol), dcatalyst (10 mol%), ecatalyst (2.5 mol%).
MeOH:Water (3:2), EtOH and EtOH:Water (3:2) were
employed as the solvent, 99% conversion was achieved
due to the good solubility of nitroarene in these solvents,
the selectivity was 56%, 88%, 94% and 99% (entries 5,
11–13), respectively. The difference of selectivity may
related to the polarity of the solvents and the solubility of
the products and the reaction intermediates. When the
dosage of the catalyst and ammonia borane increased,
the reaction can be completed in 5 min (Table 1, entries
15 and 17). When we reduced the amount of catalyst and
ammonia borane, only a conversion of 83% and 92% was
obtained after 30 min, respectively (Table 1, entries 16
and 18). Changing the concentration of sodium hydrox-
ide has a great influence on the reaction rate and selectiv-
ity. Perhaps the presence of hydroxide accelerates the
hydrogen production of AB and competes with transfer
hydrogenation (Figure S2). However, the reason for the
improved activity in the presence of extra OH− was still
unclear.[35]
catalyst
can
maintain
a
conversion
of
4-nitrochlorobenzene greater than 95% in 15 min under
the same reaction conditions even after seven runs
(Figure S3). The TEM image of the recycled catalyst
showed no significant agglomerate after seven runs com-
pare with the fresh catalyst and still maintained its excel-
lent dispersity (Figure S4). The XRD pattern showed no
obvious difference between the fresh catalyst and the re-
used catalyst up to 7 runs (Figure S5). The results clearly
demonstrated that the prepared catalyst has high durabil-
ity and stability.
To further elucidate the heterogeneous nature of the
catalyst, a hot leaching experiment was performed. After
the hydrogenation of 4-nitrochlorobenzene was con-
ducted for 5 min, the Co5Cu2-N1-C catalyst was filtered
out from the reaction solution and the filtrate was
allowed to react for 30 min, negligible changes in conver-
sion was observed (Figure S6). The experimental results
show that metal leaching did not occur and the reaction
was predominantly heterogeneous.
The reusability of Co5Cu2-N1-C was tested by using
CTH and AB of 4-nitrochlorobenzene as a model reac-
tion. The resulted showed that the recycled Co5Cu2-N1-C
In order to further prove the universal applicability of
the catalyst, various nitroaromatic hydrocarbons were

TRANSFER HYDROGENATION OF NITROARENES
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TABLE 2 Reduction of nitro compounds to corresponding amines
Entry
Substrates
Products
Time (min)
Con. (%)
Sel. (%)
1
2
99
99
99
99
99
2
3
4
1
2
2
99
99
99
5
6
2
2
99
99
99
99
7
1
99
99
8
2
99
99
99
99
99
99
9
15
30
10
11
12
13
30
30
2
99
99
99
99
79
99
14
15
4
2
99
99
99
99
(Continues)

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LIU ET AL.
TABLE 2 (Continued)
Entry
Substrates
Products
Time (min)
Con. (%)
Sel. (%)
16
5
99
99
17
4
99
99
18
19
30
30
99
99
75
88
*Reaction conditions: ammonia borane (0.8 mmol), nitroaromatics (0.2 mmol), catalyst (5 mol %), ethanol-water (3:2), (5 ml, containing 0.2 mol/l NaOH),
30 ꢀC.
subjected to CTH over Co5Cu2-N1-C catalyst under the
optimized reaction conditions. It is gratifying to find that
the catalyst has high activity not only for nitrobenzene
(Table 2, entry 1), but also for other nitro compounds
with electron donor substituents, such as -NH2, -CH3, -
OH, CH3O-, HOCH2- (Table 2, entries 2–8). These sub-
strates gave 99% yields and high selectivity in less than
2 min. The choice of substituents with steric hindrance
(Table 2, entries 3 and 6) didn't interfere with the activity
of the catalyst. It was noteworthy that the halogen-
substituted nitrobenzene was reduced to the
corresponding aniline in excellent yield without any
dehalogenation (Table 2, entries 9–12). It is worth noting
that even unstable labilebromide- and iodo-nitroarenes
were selectively hydrogenated with a selectivity of 99%
and 79% in 30 min, respectively (Table 2, entries 11 and
12). In particular, carbonyl functionalized nitroaromatics
(e.g., aldehydes and ketones) were successfully reduced
to the aniline product with a yield of up to 99% in 5 min
(Table 2, entries 13–17), but lost selectivity for
nitrobenzaldehydes in which aldehyde groups were also
reduced (Table 2, entries 13–14). The position of the alde-
hyde and ketone substituent does not significantly affect
the reactivity (Table 2, entries 14, 16 and 17). p-
Nitrobenzonitrile was reduced to the corresponding
amine products with 75% selectivity (Table 2, entry 18).
Finally,
we
tested
the
hydrogenation
of
1-nitronaphthalene and 88% selectivity was obtained
(Table 2, entry 18).
3.3 | DFT calculations
The high catalytic activity of CoCu-N-C can be also rev-
ealed from density functional theory (DFT) calculations.
The adsorption of nitrobenzene on CoCu-N-C catalyst
FIGURE 4 Summary of adsorption models of nitrobenzene on CoCu-N-C surface (a, b) and H atoms are located in the triangular voids
formed by CuCo nanocluster (c)

TRANSFER HYDROGENATION OF NITROARENES
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FIGURE 5 Proposed catalytic process
for hydrogenation of nitrobenzene catalyzed
by the CoCu-N-C catalyst
was verified and there are two adsorption site, CoNx sites
and nitrogen sites, on the surface of the catalyst. The
CoCu-N-C catalyst acts as an electron donor, and the oxy-
gen atom (O) in nitrobenzene acts as an electron accep-
tor. Figure 4 showed the optimized configurations for the
adsorption of nitrobenzene on the CoCu-N-C catalyst.
The adsorption ability of nitrobenzene on CoNx site of
CoCu-N-C was the strongest, the adsorption energy was
−0.95 eV. The length of N − O bond in nitrobenzene can
be extended to 1.281 Å compared with the intrinsic
length of N − O (1.237 Å) .
To illustrate the origin of the improved catalytic
activity, the electronic structures of the catalyst were
investigated. Bader charges of Co atom in CoNx were
listed in Table S2. The results indicated that there is
net 0.04e electron transfer when Co nanoparticles were
absorbed on CoNxC (CoNxC@Co), and 0.05e electron
transfer when Co5Cu2 nanoparticles absorbed on
CoNxC (CoNxC@CoCu), so there was additional 0.01e
electron transfer because of the introduction of copper
atom. There was 0.19e electron transfer from Co atom
to O atom when nitrobenzene was absorbed on the
catalyst.
According to the reaction mechanism of AB dehydro-
genation, AB was adsorbed on the catalyst firstly to pro-
duce H radical. To further investigate the activity origin,
the adsorption ability of the active sites for H radical were
calculated by DFT. Figure 4c and Figure S7 showed the
different atomic configurations of H radical on Co(111)-
N-C, Cu(111)-N-C, CoCu-N-C, respectively. Among
them, the bridge adsorption of H on CoCu-N-C catalyst
possessed the strongest adsorption ability, the shortest
distance (1.65 Å) between H to metal nanoparticles and
the highest adsorption energy (−2.91 eV). H atoms were
located in the triangular voids of CuCo nanocluster.
Based on the above investigations, we proposed a ten-
tative multiple-catalytic-sites mechanism for the reduc-
tion reaction of nitrobenzene catalyzed by the CoCu-N-C
catalyst, which was displayed in Figure 5. Firstly, nitro-
benzene preferred to bond on the CoNx site and dipyridyl
N accompanying with fast electron accumulation. Mean-
while, H was adsorbed on the multiple sites of CoCu-N-
C. The multiple reaction sites greatly reduced the migra-
tion distance for the active reaction species and thus
improved the catalytic performance of CoCu-N-C.
4 | CONCLUSIONS
In summary, CoCu bimetal immobilized on nitrogen-
doped porous carbon prepared by an effective ligand-
stabilized pyrolysis strategy. The optimized Co5Cu2-N1-C
nanocatalyst exhibited excellent activity for the CTH of
nitro compounds. Both the experimental and DFT calcu-
lation results revealed the important role of M-N active
sites for the excellent catalytic performance. This
research presented a feasible and universal strategy to
design high-performance non-precious metal catalysts for
the hydrogenation of nitro compounds, and will inspire
the development of non-precious metal-based heteroge-
neous catalysts.
ACKNOWLEDGMENTS
This work was financially supported by the National
Natural Science Foundation of China (21773053,
21603054), the Natural Science Foundation of Hebei

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How to cite this article: Liu X, Zhang L, Wang J,
et al. Transfer Hydrogenation of Nitroarenes
Catalyzed by CoCu Anchored on Nitrogen-doped
Porous Carbon. Appl Organometal Chem. 2020;
e5438. https://doi.org/10.1002/aoc.5438